Experimental Study and CFD Modeling of Fluidized-Bed ...

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Experimental Study and CFD Modeling ofFluidized-Bed Reactors Combined with

Atmospheric-Pressure Plasma Jets for SurfaceTreatment of Particles

Seyedshayan Tabibian

To cite this version:Seyedshayan Tabibian. Experimental Study and CFD Modeling of Fluidized-Bed Reactors Combinedwith Atmospheric-Pressure Plasma Jets for Surface Treatment of Particles. Chemical engineering.Sorbonne Université, 2019. English. �NNT : 2019SORUS382�. �tel-03001253�

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Sorbonne Université

Thèse de doctorat de Génie Chimique & Procédés

Ecole Doctorale Chimie Physique et de Chimie Analytique de Paris Centre (ED 388)

Laboratoire Interfaces et Systèmes Electrochimiques (UMR 8235)

Présenté par :

Shayan TABIBIAN

Dirigée par :

Jérôme PULPYTEL

À soutenir 24 Juillet 2019

Mme. Farzaneh AREFI-KHONSARI

Professeur, Sorbonne Université Examinatrice

M. David DUDAY Chef de Projet, Institut de Technologie du Luxembourg

Rapporteur

M. Mohammed YOUSFI Directeur de Recherche, CNRS, Laplace, Toulouse

Rapporteur

Mme. Michèle SALMAIN Directrice de Recherche, Sorbonne Université

Examinatrice

M. Cédric GUYON Maitre de Conférences, Ecole chimie de Paris

Examinateur

M. Jérôme PULPYTEL Maitre de Conférences, Sorbonne Université

Examinateur

Etude expérimentale et modélisation de réacteurs à lit fluidisé

de type Wurster couplés à des jets de plasma à pression

atmosphérique pour le traitement de surface de particules

2

3

Table of Content

I: List of the Figures ................................................................................................... 6

II: Nomenclature....................................................................................................... 11

General Introduction ......................................................................................... 12

Chapter 1. (Bibliography)

Combination of Atmospheric Pressure Plasma and Wurster Fluidized-bed

Reactor for Polymer Powders Treatment ....................................................... 15

1- Introduction to Wurster Fluidized bed reactor .............................................................................. 16

2- Plasma treatment of polymer powders .......................................................................................... 19

3- Analytical methods for characterization of plasma treated polymer powders .............................. 21

4- Atmospheric pressure plasma systems for polymeric powder treatment ...................................... 22

5- Low-pressure plasma fluidized-bed systems for polymeric powder treatment ............................. 33

6- CFD Hydrodynamic modeling of fluidized bed reactor................................................................ 35

7- Conclusion and Prospects ............................................................................................................. 37

Chapter 2.

Experimental investigation of a Wurster type fluidized-bed reactor coupled

with an air atmospheric pressure plasma jet for the surface treatment of

polypropylene particles ...................................................................................... 40

1- Introduction ................................................................................................................................... 41

2- Analytical methods used to characterize plasma treated polypropylene particles ........................ 42

2-1- Water contact angle measurements for Zisman plot .............................................................. 42

2-2- ATR-FTIR ............................................................................................................................. 42

2-3- SEM ....................................................................................................................................... 43

2-4- XPS ........................................................................................................................................ 43

3- Experimental set-up ...................................................................................................................... 43

3-1- Combination of APPJ & Wurster fluidized-bed reactor (W-FBR) ........................................ 45

4- Characterization of Polypropylene particles ................................................................................. 46

5- Results and discussions ................................................................................................................. 47

5-1- Scanning electron microscope (SEM) ................................................................................... 47

5-2- Determination of particles circularity from image analysis ................................................... 48

5-3- Analysis of PP particles by FTIR spectroscopy ..................................................................... 49

5-4- Dispersion of polypropylene particles in water ..................................................................... 51

4

5-5- Dye adsorption test ................................................................................................................ 52

5-6- Determination of polypropylene surface tension by the Zisman method .............................. 53

5-7-Characterization of surface chemistry by XPS ....................................................................... 55

6- Conclusion .................................................................................................................................... 57

Chapter 3.

Hydrodynamic Comsol Multiphysics CFD modeling of different Wurster

fluidized bed reactors and comparison with experimental data ................... 58

1- Introduction ................................................................................................................................... 59

2- Mass and momentum conservation ............................................................................................... 61

2-1- Laminar flow ......................................................................................................................... 62

2-2- Turbulent flow ....................................................................................................................... 62

3- Comparison of different turbulent models of Comsol Multiphysics V5.3 with experimental

results ................................................................................................................................................ 66

4- Wurster fluidized-bed reactor geometry ....................................................................................... 67

5- Single Phase CFD Comsol Multiphysics modeling of Wurster fluidized-bed .............................. 68

6- Thermal characterization of Wurster fluidized-bed reactor .......................................................... 70

6-1- Experimental temperature profile measurements .................................................................. 70

6-2- Heat transfer modeling by Comsol Multiphysics V5.3 ......................................................... 73

7- Biphasic Eulerian-Eulerian CFD by Comsol Multiphysics V5.3 ................................................. 74

7-1- Biphasic Eulerian CFD mathematical equations ................................................................... 75

7-2- Model Conditions .................................................................................................................. 76

7-3- Meshing ................................................................................................................................. 77

7-4- Gidaspow Drag Force ............................................................................................................ 77

7-5- Comparison of biphasic turbulent CFD model developed by Comsol Multiphysics with high

speed imaging results .................................................................................................................... 78

7-6- Calculation of the effective treatment time of particles inside the reactor ............................ 81

7-7- Reynold number calculation .................................................................................................. 82

7-8- Particles Reynold number and drag force calculation ........................................................... 83

8- Hydrodynamic Modeling of a spouted Wurster fluidized-bed reactor with Silicon Carbide (SiC)

particles by Comsol Multiphysics ..................................................................................................... 84

8-1- Experimental Setup ................................................................................................................ 85

8-2- Gas velocity calculation ......................................................................................................... 86

8-3- Silicon carbide (SiC) particles characterization ..................................................................... 87

8-4- Reactor geometry design for Comsol Multiphysics Modeling .............................................. 89

8-5- Model Approach and boundary conditions ............................................................................ 89

8-6- Particles behavior inside the reactor ...................................................................................... 90

8-7- Hydrodynamic results of Model ............................................................................................ 91

5

8-8- Residence time of SiC particles inside Wurster-tube ............................................................ 92

9- Conclusion .................................................................................................................................... 93

Chapter 4.

Atmospheric pressure plasma reactors for black peppercorn microbial

decontamination ................................................................................................. 95

1- Introduction ................................................................................................................................... 96

2- Sample characterization and selection of spices ........................................................................... 98

2-1- Dilution procedure ................................................................................................................. 99

2-2- Aerobic Plate Count Method ................................................................................................. 99

2-3- Yeast and Mold measurement .............................................................................................. 100

2-4- Microbiological analysis of selected spices ......................................................................... 101

3- Determination of the percentage of spore-forming bacteria in black pepper .............................. 102

4- SEM of broken black peppercorn ............................................................................................... 104

5- Plasma rectors for peppercorn treatment ..................................................................................... 105

5-1- System A (DBD spouted Fluidized-bed) ............................................................................. 105

5-2- System B (Rotary air-APPJ) ................................................................................................ 106

6- Plasma treatment black peppercorns ........................................................................................... 108

6-1- Black peppercorn plasma treatment with system A ............................................................. 108

6-2- Microbiological Analysis Results (System A) ..................................................................... 108

6-3- Black peppercorn plasma treatment with system B ............................................................. 109

6-4- Effect of the Plasma Treatment on the appearance (System B) ........................................... 109

6-5- Microbiological Analysis Results (System B) ..................................................................... 109

6-6- Comparison of plasma active species in both discharge systems (A and B) ....................... 111

7- Plasma treatment of inoculated Petri dishes with a dilution of black pepper sample (natural

microorganisms) .............................................................................................................................. 114

8- Influence of petri dishes heating up on microbial decontamination ........................................... 115

9- Plasma treatment of French Black Peppercorn samples inoculated with E. coli or S. epidermidis

......................................................................................................................................................... 116

10- Conclusion ................................................................................................................................ 118

General Conclusion & Future Prospects ....................................................... 120

References ......................................................................................................... 124

6

I: List of the Figures

Chapter 1

Figure 1-1: Schematic of a bottom spray Wurster fluidized bed reactor

Figure 1-2: Typical configuration for circulating fluidized bed system

Figure 1-3: Geldart classification of powders

Figure 1-4: Experimental setup of Sach et al.

Figure 1-5: Wettability for ethanol of plasma treated and untreated PP

Figure 1-6: atmospheric pressure glow plasma fluidized bed reactor of Kogoma et al.

Figure 1-7: influence of plasma power and oxygen flow-rate on the O/C ratio

Figure 1-8: Plasma barrel reactor (a) and plasma fluidized-bed reactor (b) Abourayana et al.

Figure 1-9: Effect of oxygen flow rate on the WCA of silicone particles

Figure 1-10: Schematic of the Plasma Circulating Fluidized Bed Reactor

Figure 1-11: Normalized particle frequency distribution

Figure 1-12: Plasma circulating fluidized bed reactor of Nakajima et al.

Figure 1-13: a) non-treated PS powder, b) PS powders clung to electrode

Figure 1-14: Atmospheric pressure plasma jet system developed by Gilliam et al.

Figure 1-15: Effect of plasma treatment time on the WCA of polymers

Figure 1-16: Photography of the barrel reactor (left) and its schematic diagram reactor (right)

Figure 1-17: Carbon peaks from XPS analysis, left non-treated PE powder and right treated

Figure 1-18: Atmospheric pressure DBD plasma setup (left), Scaled-up downer reactor (right)

Figure 1-19: DBD atmospheric downer reactor developed by Pichal et al.

Figure 1-20: Modified powder capillarity changes during storage (Aging effect of stocking)

Figure 1-21: Experimental setup of Nessim et al. consists of three discharge zones (30cm total)

Figure 1-22: Variation of the incorporation of nitrogen and oxygen atoms on the surface of

LDPE powders after a nitrogen plasma treatment in the low pressure fluidized bed reactor

7

Chapter 2

Figure 2-1: Experimental set-up of Wurster-FBR combined with APPJ. (1) Wind-box, (2)

plasma nozzle, (3) distributor plate, (4) wurster-tube, (5) fluidized-bed, (6) expansion

Figure 2-2: wurster fluidized-bed plasma reactor and the specially designed plasma nozzle by

Plasmatreat

Figure 2-3: General schematic of the different regions inside the Wurster fluidized-bed reactor

Figure 2-4: Laser granulometry measurements before and after plasma treatments of PP

powders (about 10000 PP particles)

Figure 2-5: SEM photography of non-treated (left) and treated (right) of polypropylene

particles

Figure 2-6: Circularity measurement of treated and non-treated PP particles

Figure 2-7: Experimental curve obtained by J.P.Luongo

Figure 2-8: FTIR-ATR result of 120s plasma treated and non-treated samples

Figure 2-9: FTIR of treated and non-treated PP (zoomed in)

Figure 2-10: Dispersion of non-treated (left) and treated (right) PP in water

Figure 2-11: Dye adsorption test. Left side treated and right side non-treated PP

Figure 2-12: Dye adsorption after 7 days. Left side is non-treated and right side is treated PP

particle

Figure 2-13: Zisman plot of treated PP with 400 and 200W and non-treated one

Figure 2-14: Ageing effect and hydrophobic recovery of 400W plasma treated PP particles

Figure 2-15: XPS spectra of untreated (left) and plasma treated (right) PP particle

8

Chapter 3

Figure 3-1: Velocity profile (Comsol Models vs. Experimental data)

Figure 3-2: Difference between experimental and model values of velocity using k-ε and SST

models (Vexp – Vmodel)

Figure 3-3: Design of the Wurster-FBR combined with APPJ by AutoCAD. (1) Wind-box, (2)

plasma nozzle, (3) distributor plate, (4) Wurster-tube, (5) fluidized-bed, (6) expansion

Figure 3-4: Velocity field inside the reactor and streamlines at the entrance of the Wurster tube.

Figure 3-5: Pressure profile of Wurster-tube

Figure 3-6: Gas trajectory by lines

Figure 3-7: Schematic of Wurster tube with the measurement points for the temperature

Figure 3-8: Radial temperature profiles. (A) is section 1, (B) is section 2 (interaction point),

(C) is section 3, (D) is section 4

Figure 3-9: Schlieren photography of the APPJ

Figure 3-10: Thermal imaging of the Wurster tube

Figure 3-11: Gas temperature profile

Figure 3-12: Modeling and experimental temperature of Wurster-tube interior wall

Figure 3-13: Reactor walls, inlet and outlet

Figure 3-14: The two phases at initial time (t=0). The blue part is air and red part is the PP

particles with a volume fraction of 0.6

Figure 3-15: Triangular meshing of the Wurster fluidized-bed reactor

Figure 3-16: Dispersed phase development in function of time (logarithmic scale)

Figure 3-17: Axial gradient of volume fraction of the particles

Figure 3-18: High speed photography of the reactor. The dotted arrows indicate the circulation

of particles inside the reactor. The PP particles have been treated by plasma, colored by a dye

and re-introduced inside the reactor

Figure 3-19: Simplified geometry of reactor and calculation element inside the Wurster-tube

for particles flow rate and total cycle number calculation

Figure 3-20: Experimental setup of the spouted Wurster fluidized-bed reactor

Figure 3-21: Picture of the DBD plasma jet system

Figure 3-22: Laser analysis for SiC particle size distribution

Figure 3-23: SEM analysis of SiC particles

9

Figure 3-24: Geldart’s classification showing the SiC particles

Figure 3-25: Reactor schematic and dimensions developed by Comsol Multiphysics

Figure 3-26: Initial values and boundary conditions

Figure 3-27: Volume fraction evolution of SiC particles

Figure 3-28: Height of the fountain region with SiC particles (d = 150 µm) and an argon gas

velocity of 3 m/s A) Experimental result B) Comsol modeling result

Figure 3-29: A) Velocity profile of continuous phase, B) Velocity profile, dispersed phase, C)

Volume fraction of dispersed phase

Figure 3-30: Velocity profile 2D, dispersed phase

10

Chapter 4

Figure 4-1: Samples of Spices used: A) Black mustard seeds, B) White mustard seeds, C) Black

peppercorn (broken), D) White peppercorn (broken).

Figure 4-2: Comparison of the growth observed in the plates of Mesophilic Aerobic Organisms

for the different spice samples

Figure 4-3: Thermal destruction of bacterial vegetative forms present in suspensions of black

French peppercorn broken and whole (80°C/12 min).

Figure 4-4: SEM photography of broken black peppercorn

Figure 4-5: Spouted fluidized bed coupled with a DBD plasma system (system A)

Figure 4-6: The blown-arc rotating atmospheric pressure plasma torch (RD1004 from

Plsmatreat)

Figure 4-7: Rotary blown-arc atmospheric pressure plasma system (system B)

Figure 4-8: Effect of the air plasma treatment on the color of sifted broken French Black

Peppercorn samples (voltage = 270 V, frequency = 19 kHz, PCT = 40%)

Figure 4-9: Comparison of emission spectra of Ar + O2 DBD atmospheric plasma and air

blown-arc atmospheric pressure plasma jet (Arc zone and post discharge zone)

Figure 4-10: UV absorption of native DNA and glycated DNA in incubated in various solution

of ribose

Figure 4-11: Log reduction of broken black peppercorn contaminated with natural

microorganism

Figure 4-12: Log reduction of Petri dishes contaminated with natural peppercorn

microorganisms

Figure 4-13: Log reduction of peppercorn inoculated with E. coli and S. epidermidis

11

II: Nomenclature

AFM atomic force microscopy

ATR-IR attenuated total reflection infrared spectroscopy

DBD dielectric barrier discharge

CVD chemical vapor deposition

EDX energy dispersive X-ray

FTIR Fourier-transform infrared spectroscopy

HMDSO hexamethyldisiloxane

HDPE HDPE high density polyethylene

HV high voltage

LDA laser Doppler analysis

LDPE low density polyethylene

LMWOM low molecular weight oxidized material

MW microwave

PA polyamide

PDMS polydimethylsiloxane

PE polyethylene

PECVD plasma enhanced chemical vapor deposition

PET polyethylene terephthalate

PP polypropylene

RF radio frequency

RONS reactive oxygen and nitrogen species

sccm standard cubic centimeters per minute

SEM scanning electron microscopy

slm standard liters per minute

TGA thermal gravimetric analysis

UHMWPE ultra-high molecular weight polyethylene

XPS X-ray photoelectron spectroscopy

12

Experimental Study and Modeling of Wurster type Fluidized Bed Reactors

coupled with Atmospheric Pressure Plasma Jets (APPJ) for the Surface

Treatments of Powders

General Introduction

In this thesis we have developed several reactors by combining atmospheric pressure plasma

jets and different types of fluidized-beds for particle surface treatment applications. The

fluidized-bed is one of the most used reactors for physicochemical surface modification of

particles, owing to its homogeneous mass and heat transfer, high rate of mixing and ability to

work in different batch and continuous processes.

For the first time, we have combined the atmospheric pressure plasma jet with a Wurster-type

fluidized bed. Wurster fluidized bed is a special conception of the conventional fluidized bed

which thanks to its design, would lead to a more uniform and homogeneous plasma treatment

of particles without the need for particle/gas separation by cyclone. The presence of a riser, so

called Wurster-tube inside this type of fluidized bed reactor makes the effective treatment time

distribution much narrower compared to conventional fluidized bed.

The plasma reactors, developed during this thesis, were used to treat two types of material, i)

the industrial polypropylene powders in order to improve their hydrophilicity and wettability,

ii) the commercial peppercorns particles in order to remove vegetative and spore-forming

bacteria from their surface.

In parallel with these experimental studies, we have developed different biphasic numerical

CFD models by Comsol Multiphysics to investigate the hydrodynamics of the reactors to

improve their configurations and optimize the parameters. The importance of the hydrodynamic

modeling of the fluidized-bed is to obtain a better understanding of the interactions between the

different phases in order to to better control the process. On the other hand accurate

experimental data of solid-gas interactions is required to validate the numerical modeling

results.

Chapter one of this thesis is dedicated to represent the state of the art on different reactors used

for the surface treatment of particles and in particularly the Wurster fluidized bed reactor.

Among the different atmospheric pressure plasma reactors, developed for surface treatment of

particles by other groups, we have described the most important ones i.e. fluidized bed reactors,

13

circulating fluidized bed reactors, barrel reactors and downbed reactors. Furthermore the

experimental results of their studies for hydrophilicity improvement of particles are presented.

Chapter two of this thesis is dedicated to plasma treatment of polypropylene (PP) micro-

particles with a Wurster fluidized bed reactor coupled with an air blown-arc atmospheric

pressure plasma jet aiming at increase the hydrophilicity of the particles. A batch of 200 g PP

powders was plasma treated for a short time (120s). The plasma treated PP powders in this

reactor disperse completely in water. Different analytical techniques were used to characterize

the physical and chemical properties of particles before and after the plasma treatment. Laser

granulometry and SEM analyses were performed to characterize the morphology and physical

modification of plasma treated PP particles. FTIR and XPS analysis were performed to

characterize the surface chemistry. Dye adsorption test was performed to analyze the

homogeneity of the treatment process. Zisman method was applied to characterize the surface

energy and aging effects of particles.

The CFD hydrodynamic modeling of this Wurster fluidized bed reactor is presented in chapter

three. An isothermal, turbulent Euler-Euler two-phase model was used to understand the

behavior of the inside the reactor. The velocity profile of both phases, i.e. gas and polypropylene

(PP) particles, and the concentration of PP particles in different regions of the reactor were

obtained by this model. High speed imaging using a SONY RX 10 camera (1000 frames/s) was

carried out to observe the trajectory of the PP particles in particularly at the exit of Wurster-

tube where the concentration of solid particles is high. With these methods, one could estimate

the particle velocity to compare the latter with the modeling results.

A non-isothermal, turbulent Euler-Euler model was developed to characterize the heat transfer

phenomenon inside the reactor. In this model, two physics were considered, turbulent CFD and

heat transfer. The heat transfer modeling results are compared with experimental temperature

profile ones obtained by thermocouple measurements, infrared thermography and Schlieren

photography.

In the last section of chapter 3, a CFD model will be presented for a so-called spouted Wurster

fluidized-bed reactor. This reactor will be used to plasma treat black peppercorn particles for

antimicrobial applications in chapter 4. The Eulerian-Eulerian approach was used as in the

previous reactor. The gas phase is argon and the dispersed one is the silicon carbide (SiC). In

this system we have applied the laminar equations to characterize the circulation of the particles

as well as their velocity profiles.

14

The plasma reactors for the peppercorn decontamination process are presented in chapter four.

The aim of this chapter is to determine the efficiency of different plasma reactors in

decontaminating peppercorns supplied from France, which are naturally contaminated with

mesophilic aerobic micro-organisms, and which, as we will see, include spore-forming micro-

organisms as well. The bacterial spores are known to be difficult to remove. Basically we have

used two different plasma systems. The first one is a blown-arc rotary plasma jet at atmospheric

pressure which uses air to generate a relatively high temperature plasma. The second system is

a non-thermal DBD (dielectric barrier discharge) which uses mixtures of Ar with a small

amount of O2 (2%). 7 g of peppercorns was treated with these two plasma devices. Different

reactor constructions, working gas, treatment time and sample morphologies were investigated

with their efficiency on spore removal and microbial decontamination. SEM analyses were

performed to observe the bacteria on the surface of peppercorn. A dilution of natural

microorganism present on the surface of peppercorns was deposited on petri dishes and plasma

treated with both systems to characterize the influence of porosity and the complex structure of

peppercorn that would protect some colonies of bacteria during the plasma treatments.

In the last part of chapter four, the peppercorns were sterilized and then inoculated with E. coli

and S. epidermidis microorganisms aiming at the comparison of the resistance of spore-forming

bacteria present on the surface of peppercorns and other non-spore vegetative state of bacteria.

Both plasma devices were used to treat 7 g of each inoculated pepper sample. All the

operational parameters of plasma systems were the same as before.


15

Chapter 1

State of the Art (Literature) Combination of Atmospheric Pressure Plasma and

Wurster Fluidized-bed Reactor for Polymer Powders

Treatment


16

1- Introduction to Wurster Fluidized bed reactor

The fluidization phenomenon occurs when a gas or liquid is passed through a bed of particles

and the particles react like a fluid. A fluidized bed reactor is frequently used for chemical

reactions with solid and gas or liquid. Solid particles are placed on the porous plate in the

reactor, which is positioned vertically, and a gas is injected from the gas inlet at the bottom of

the reactor. The gas passes up through a bed of the particles. At more than a critical flow rate

of the gas stream the drag force and pressure drops on individual powder increases. As result,

the powders start to move and become suspended in the fluid. This state is called “fluidization”

and means the condition of fully suspended particles. High rate of transfer phenomena like mass

transfer and heat transfer makes the fluidized-bed an appropriate system to treat the heat

sensitive materials like the polymers. There are several factors that would influence the quality

of fluidization. Among these factors we can name the physical properties of fluid and solid

particles, gas flow rate, particles size distribution, bed geometry, porosity of the distributer etc.

The fluidized-beds are one of the most used reactors for physicochemical treatment of materials.

The reason would be attributed to the relatively homogeneous mass and heat transfer, high rate

of mixing and ability to work in different batch and continuous processes. Therefore it is widely

used in the chemical industry, metallurgy, oil and thermal power generation. A conventional

fluidized-bed is a two phase system. The solid phase or dispersed phase and the fluid phase or

continuous phase.

Wurster fluidized bed reactor is a special type of fluidized bed which is used for powder coating,

granulation and encapsulation of discrete particles especially in pharmaceutical industries.

However there are significant differences between these two schematically similar reactors.

Coatings are used to mask the taste, or to improve the stability of pharmaceutical products and

to protect active ingredient. The coating process, using a Wurster fluidized bed reactor doesn’t

contain any fluid-bed region in the traditional sense, as it’s a circulating fluidized bed process.

Five main zones can be identified within the equipment: The Wurster-tube region (riser), the

spray zone, the fountain region, the downbed region and the horizontal transport region. The

Wurster coating equipment was invented by D. Wurster [1]. The Wurster fluidized-bed reactor

is typically combined with a spray system at the bottom of the reactor for coating applications

(Figure 1-1). The precursor is introduced in the form of micro size droplets with generally a

high flow rate of air to the reactor. The powders are sucked up to the Wurster tube which is

placed at top of the spray nozzle. The pressure drop and venture effect inside the Wurster-tube


17

leads to the horizontal motion of powders at the bottom of the reactor. The Wurster-tube is the

main zone where the coating process takes place because of the high interaction between the

powders and precursor solution.

Figure 1-1: Schematic of a bottom spray Wurster fluidized bed reactor

During the coating phase several processes take place simultaneously i.e. atomization of the

precursor solution, transfer of the film droplets to the substrate, adhesion of the droplets to the

substrate, film formation, and the coating cycle of the substrate and the drying of the coated

film. The most important properties of powder substrates for circulation in Wurster fluidized

beds are the density of the particles, their diameter and their stickiness.

The process characteristics are very different in each zone of the Wurster fluidized bed. Air and

powders velocities are not uniform across the Wurster-tube. The most important parameters

that influence the homogeneity of the coatings on powders are, the pressure, the temperature,

air flow rate, spray rate and concentration, droplet sizes, powder type, powder size distribution,

air fluidization velocity, Wurster-tube dimensions, effective treatment time as well as the

drying time. The Wurster tube dimensions of three different Wurster fluidized bed reactor are

presented in Table 1-1. [2] The powders go through circulating paths in a Wurster fluidized bed.

The time which takes for a particle to go through a complete coating cycle depends on the

reactor dimensions, but can be ranged between 20 s up to 2 min for large scale Wurster fluidized

beds. Studies have demonstrated that a properly operated Wurster fluid bed coating process can


18

provide thin coatings by CVD of the highest quality and uniformity compared to other

techniques [3,4]. This uniformity can minimize the coating thickness requirements, further

reducing the amount of coating materials needed and the processing time.

Supplier Glatt Glatt Aerocoater

Equipment/scale GPCG 200

Industrial

GPCG 3

Laboratory

MP-1

Laboratory

Diameter of Wurster-tube (cm) 22 8 5

Height of Wurster-tube (cm) 76 17.5 18

Number of Wurster-tubes 3 1 1

Capacity (kg) 300 3-5 0.8-2

Table 1-1: Three available Wurster fluidized bed reactor

The Wurster fluidized bed reactor provides thin coatings around the particles more

homogenously than a conventional fluidized bed reactor thanks to its design. The narrow

effective treatment time distribution in a Wurster reactor makes it completely different from a

conventional fluidized bed. Among the advantages of Wurster fluidized bed reactor compared

to a circulating one, comes the lower gas consumption, shorter treatment time and no-need for

gas/solid separation unit by means of a cyclone for example. A typical configuration for a

circulating fluidized bed (CFB) reactor is shown schematically in Figure 1-2.

Figure 1-2: Typical configuration for circulating fluidized bed system


19

2- Plasma treatment of polymer powders

Plasma treatment is an appropriate approach for coating applications and surface treatment of

a wide range of materials. The plasma treatment of polymer powders is a successful method to

modify the surface characteristics without changing the bulk properties. [5-11]

Plasma is an ionized gas which consists of photons, ions, electrons and metastable. These active

species interact with the polymer surface. The interaction of these species with polymer

substrates depends on physical and chemical properties of polymer substrates and plasma

treatment process parameters such as gas flow rate, input power, pressure, type of gas, system

geometry, treatment time, etc.

75% of available polymers used for painting, coating, printing or bonding must be treated to

improve their surface properties due to their low surface energy which in the initial state leads

to low wettability and poor adhesion to other materials [12].

Among the different surface treatment methods for the polymers, wet-chemical treatments,

physical processes, ultraviolet light radiation, oxygen treatment processes, plasma treatment,

biological processes and mechanical methods are the most important used processes. [13-15]

Plasma processes, being a clean, dry and environmentally friendly process, present also

advantages such as relatively low consumption of energy and chemicals. Furthermore, plasma

treatment processes are easy to operate and control, present a high efficiency and low operating

cost. [16-17]

Plasma polymer interactions have been widely studied since 1970s [18,19] which lead to surface

etching (cleaning), cross-linking, and surface functionalization. The polymers in our modern

world have the potential to replace traditional materials such as metals owing to their low

production costs, the desired physical and chemical properties such as high strength-to-weight

ratio and their corrosion resistance.

The majority of polymer products are in the form of powder. Thousands of tons of polymer

powders are manufactured every year in the field of powder coating. The advantage of polymers

in the form of powder is their high specific surface area which provoke high reaction rates with

liquids and gasses. Also the possibility to adjust the accurate dosage of powders in comparison

with polymer films and pellets is another advantage of polymer powders.


20

The plasma treatment of flat polymers films is widely investigated [20-25] but little attention has

been devoted to plasma treatment of polymer powders. One of the challenges for polymer

powder treatment is to develop a system to treat the powders homogeneously. The polymer

powders show a strong tendency to form agglomerates which leads to poor flowability and

consequently non-homogenous treatment.

To achieve a homogenous surface treatment of polymer powders the concept and design of the

system has a great importance. The powders must be in suspension or in motion to be

completely plasma treated in all the edges. Various reactor concepts have been presented by

other groups to treat polymer particles, including rotation, conveying, vibration and the most

important, fluidization. All these systems have in common that the powders must be in motion

to ensure uniform and homogenous treatments.

The appropriate operating pressure, powder quantity, size of the plasma reactor, treatment time

and gas-solid mixing must be experimentally determined to achieve a homogenous and optimal

plasma treatment [26]. After plasma treatment of polymers, the storage time and conditions

should be controlled. The parameters such as environment temperature, humidity, light and

storage gas (air) after treatment are important. Indeed the polymers can in contact with air

undergo post-plasma oxidation and consequently lose the modified surface properties. [27- 29]

Von Rohr et al. [30] have recently published a review article for plasma treatment of polymer

powders from laboratory scale to industrial scale. Among the different plasma reactors for

polymer powder treatments presented in the review, the fluidized-bed reactors are the most

studied setups. The combination of fluidized-bed and plasma technology makes it possible to

have a homogenous plasma treatment of powders. High rate of powder mixing with gas, ease

of controlling the operational parameters and the possibility to scale-up the system for industrial

treatments are the advantages of these systems. Long treatment times and relatively high

consumption of gas are the main limitations of these systems. [31]

According to Geldart classification [32] the powders are grouped into four so-called Geldart

groups presented on the diagram (Figure 1-3). The polymer powders with the density of 0.8 to

2 kg/m3 and the particle diameter of 20 to 1000 µm are classified in the group A (fluidizable),

and B (sand-like). Group C represents the particles being very fin and cohesive and group D

represents the particles being very dense and very large, and therefore difficult to fluidize.

Powders classified as belonging to group A and B are easy to fluidize with a low gas flow-rate


21

in comparison to class C and D. Therefore it is important to choose an appropriate polymer

powder in terms of density and size to fit in the A and B groups to treat with a fluidized-bed.

Figure 1-3: Geldart classification of powders

3- Analytical methods for characterization of plasma treated polymer

powders

Among the different analytical methods to characterize the physical and chemical properties of

polymer powders, XPS, SEM, FTIR, ATR-IR, AFM and contact angle measurement are the

most common ones used. XPS (X-ray photoelectron spectroscopy) is an analytical method to

characterize the elemental and chemical bonds at the topmost surfaces of powders (<10nm).

.FTIR (Fourier-transform infrared spectroscopy) and ATR-IR (attenuated total reflectance

infrared spectroscopy) are used to characterize the chemical properties of polymer powders and

to identify the introduced functional groups.

SEM (Scanning electron microscopy) is used to analyze the morphology and physical surface

structure of polymer powders and AFM (Atomic force microscopy) is used to determine the

surface roughness.

DSC (Differential scanning calorimetry) and TGA (thermal gravimetric analysis) are two

methods to investigate the thermal properties of polymer powders. DSC is used widely to

determine the thermal transitions in polymer materials, namely the glass transition temperature

(Tg), crystallization temperature (Tc) and melting temperature (Tm). During TGA analysis the

mass of a sample is measured over time as the temperature changes. Such measurements

provide information about the phase transition of polymer powders.

https://en.wikipedia.org/wiki/Mass

https://en.wikipedia.org/wiki/Measurement

https://en.wikipedia.org/wiki/Time_in_physics

https://en.wikipedia.org/wiki/Temperature


22

The capillary penetration and the sessile drop methods are used to determine the wettability and

surface free energy of polymeric powders. The sessile drop method is an optical measurement.

The liquid droplet is deposited on the surface of the polymer powder pellet and the contact

angle in the static mode is measured thanks to a camera and ImageJ software. This method is

used for hydrophobic polymers presenting contact angles exceeding 90°. The capillary

penetration method (Washburn) is based on the mass changing of bulk of polymeric powders,

stock in a cylinder because of capillary forces and liquid adsorption.

4- Atmospheric pressure plasma systems for polymeric powder treatment

In this section, various atmospheric pressure designed plasma reactor setups are presented to

treat polymer powders. These include fluidized bed reactors, circulating fluidized bed reactors,

batch reactors, barrel reactors and downer reactors. Several authors have investigated the

combination of the fluidized-bed with atmospheric pressure plasmas for treatment of polymeric

powders.

Sach et al. [33,34] have combined a fluidized-bed reactor with an atmospheric pressure plasma jet

(Plasmatreat, Germany) for plasma treatment of PP, PA and HDPE powders with particle size

distribution of 50 to 100 µm (group A or B). Figure 1-4 shows the schema of their experimental

system. The fluidized-bed was made of stainless steel with inner diameter of 10 cm. The aim

of this study was to improve the wettability of polymeric powders to optimize the laser beam

melting process. They have used compressed-air and a mixture of Ar and O2 (5%) as operating

gas. The N2 was used as fluidization gas. The plasma jet was placed at the center-bottom of the

fluidized-bed. A DC power supply with a voltage of 280 V and a pulsing frequency of 21 kHz

for plasma ignition was applied. 750 g of PP powders were treated in a batch reactor with their

plasma fluidized-bed reactor. The wettability of plasma treated polymer powders was

characterize by ethanol capillary penetration method (Washburn). Figure 1-5 demonstrates the

ethanol adsorption rate of plasma treated and non-treated PP powders. The higher the squared

mass gain over time, the better is the wettability of the powder. So the higher gradient of the

mass gain, as well as the asymptote horizontal value of the treated powder reveals that a plasma

treatment for 120s leads to a significant gain in wettability of the particle surface regarding

ethanol.


23

Figure 1-4: Experimental setup of Sach et al. [21-22] Figure 1-5: Wettability for ethanol of plasma treated

and untreated PP

Kogoma et al. [35] have analyzed the surface modification of PE powders with an atmospheric

pressure glow discharge in He using a DBD configuration with an inner HV electrode in a

quartz tube covered with the ground electrode. This reactor was combined with a fluidized-bed

reactor. Figure 1-6 demonstrates the details of their experimental system. The effective

treatment time was estimated to be about 30ms. The plasma treated PE powders showed enough

hydrophilicity to disperse completely in water even though the effective treatment time was

short. They have shown that the degree of oxidation could be controlled by changing the plasma

power and oxygen flow rate.

An ultra-sonic horn was used in the reactor setup to crush the aggregated powders. The

cylindrical chamber of fluidized-bed was made of quartz. Both inner and outer electrodes of

plasma were cooled to prevent PE powders to melt in the discharge zone. The flow rate of

carrier He was 8 L/min which contained 10 to 50 mL/min of mixed O2 or NH3. An RF power

supply (13.56 MHz) was used as the generator. The mean diameter of PE particles was 12µm

with a geometry close to a spherical one.

The chemical properties of treated and non-treated PE powders was characterized by XPS. The

influence of plasma power and oxygen flow-rate on the O/C ratio is shown in Figure 1-7. The

power was varied between 1000 and 2500 W and the maximum value for O/C was around 0.4

obtained for 2kW power. According to their results the plasma treated PE powders contained

OH, COOH and C=O groups.


24

Figure 1-6: atmospheric pressure glow plasma

fluidized bed reactor of Kogoma et al. [35]

Figure 1-7: influence of plasma power and

oxygen flow-rate on the O/C ratio

In another study Abourayana et al. [36] made a comparison between the performances of a pulsed

plasma fluidized bed and barrel reactors for the plasma activation of silicon particles. Both

systems operated at atmospheric pressure conditions (Figure 1-8). Both helium and helium

/oxygen gas mixtures were used to generate the plasmas in the two reactors. The two reactors

were used to treat 0.5 g of silicone polymer particles. The silicone particles were 5mm in

diameter and 0.5mm thickness. The plasma was operating at a frequency 20 kHz and an input

voltage of up to 110 V with a maximum of 100 W output from the power supply.

Figure 1-8: Plasma barrel reactor (a) and plasma fluidized-bed reactor (b)

by Abourayana et al. [36]


25

The static water contact angle (WCA) decreased from 145° to 75° after 2 hours plasma

treatment with fluidized-bed reactor and by using He as the operating gas. The mixture of

helium (10slm) with a small amount of oxygen (1%, 0.1slm) showed much better results (Figure

1-9). The WCA decreased to less than 5° after 2 minutes treatment inside the barrel reactor.

Using the pulsed fluidized bed reactor under similar processing conditions, it was observed that

the WCA only decreased to 26°. The difference in the level of particle activation achieved using

the two reactor types was attributed to the intensity and better homogeneity of the plasma

generated in the barrel reactor.

Figure 1-9: Effect of oxygen flow rate on the WCA of silicone particles

(He flow rate 10 slm and operating voltage 100 V)

Oberbossel et al. [37] investigated the surface activation of HDPE powder using a circulating

fluidized bed plasma system (transported bed) at atmospheric pressure. The treatment zone of

the reactor (plasma zone) is composed of 64 dielectric barrier discharges of Ar admixture with

O2 and CO2. They have shown that the WCA of plasma treated powder samples decreased with

increasing treatment time. After experimental optimizations, the surface activation was most

effective for O2 admixture of 0.25 vol% and CO2 admixture of above 1 vol%. The experimental

setup for circulating fluidized bed is demonstrated in Figure 1-10. The cylindrical plasma

channel has the diameter of 10mm and height of 26mm.


26

Figure 1-10: Schematic of the Plasma Circulating Fluidized Bed Reactor

The gas flow rate was 40 slm and a corona generator with a sinusoidal voltage with an amplitude

of 5 kVpp at a frequency of 14.8 kHz was used. The mean diameter of the powders was 70 µm.

The circulating fluidized bed reactor allowed multiple passes of polymer powder inside the

discharge channel and it was able to operate at both continuous and batch systems. The mean

residence time of the powders in the discharge channel was 5ms for a batch system and 6.2ms

for a continuous one. The evolution of particle size distribution during conveying in the reactor

was monitored by laser diffraction. The results showed that after ten cycles all particles below

9µm and a fraction of particles with diameters ranging between 9 and 60 µm were lost (Figure

1-11). [38]

Figure 1-11: Normalized particle frequency distribution


27

Nakajima et al. [39] investigated a fluidized bed reactor combined with an atmospheric pressure

glow discharge DBD system in He for treatment of polystyrene powders for antifoaming

application. They treated 5 g of polystyrene powders with the diameter ranging between 32-63

µm. They used mixtures of He (5000 sccm) with O2 (20 sccm) or CF4 (20 sccm) as operating

gas to ignite the plasma. The treatment time was 20 min. The experimental device is

demonstrated in Figure 1-12. An ultrasonic homogenizer is placed at the bottom of reactor to

avoid particles agglomeration. An RF 13.56 MHz generator was used with an applied power of

1000 W. Both inner (stainless steel) and outer electrodes (copper) were cooled down with water

circulation. The system was equipped with a cyclone system to separate the particles from the

gas phase. By increasing the treatment time, some powders clung to the inner electrode. The

shapes of untreated polystyrene and most of plasma treated polystyrene powders were

heterogeneous. However, those PS particles which clung to the electrode were spherical. That

was probably due to the heating of the electrode by plasma energy, though it was cooled by

water. Figure 1-13 demonstrates the SEM photography of PS powders clung to the electrode

and non-treated ones.

Figure 1-12: Plasma circulating

fluidized bed reactor of Nakajima et al.

Figure 1-13: a) non-treated PS powder, b) PS powders

clung to electrode

Gilliam et al. [40] treated polymer micro particles with a continuous atmospheric plasma process,

resulting in an increased surface oxidation and hydrophilicity. They treated PMMA (mean

particle size of 250µm) and PP (mean particle size of 350µm), with an atmospheric pressure

pulsed plasma jet (supplied by Plasmatreat) using N2 or air as plasma gas. The experimental

setup is shown in Figure 1-14. The electrode which is placed at the center of plasma source, is

connected to an excitation frequency of 20 kHz and a voltage ranging between 235 to 350 V.


28

The plasma cycle time (PCT) was varied between 70% to 100%. The gas flow rate was 1800

L/h. The particle delivery system was based on gravity. A flow rate of 2-3 g/min of polymer

particles reached to the plasma zone.

Figure 1-14: Atmospheric pressure plasma jet system developed by Gilliam et al.

A vibration system was used to set a fixed flow rate of particles. The residence time of the

particles in the plasma zone was estimated less than 1s. The temperature of the plasma zone

was measured and ranged from 30 to 65°C. Water was sprayed at the interaction point of the

plasma jet and where the powders were delivered. This water precursor system was used to

increase the hydrophilic properties of polymer powders and to enhance the surface oxidation

rate. According to XPS results the C-O component of the C1s photoelectron peak increased

from 3% (non-treated PP) to 11% (plasma water treated PP). At the same time an increase of

the C=O component was reported from 2% (non-treated PP) to 7% (plasma water treated PP).

Because of this surface oxidation of powders the PP treated particles dispersed completely in

deionized water under agitation while the non-treated one didn’t show any tendency to water

dispersion.

Besides the barrel lab scale reactor reported in reference [36] Figure (1-8a), M. Abourayana et

al investigated in reference [41] a scalable barrel atmospheric plasma reactor for the treatment

of polymer particles. The plasma generated was the same as before in He or mixture of He with

O2 and different polymers such as PP, silicone, ABS and PET particles were treated to improve

their hydrophilicity. The polymer particles diameter ranged from 3 to 5mm. According to WCA

measurements, the optimized condition for PP particles treatment was He flow rate of 10 slm

and O2 flow rate of 0.05 slm. With this operating conditions the WCA decreased from 110° for

non-treated PP particles to 65° for plasma treated one after 5 minutes plasma treatment inside


29

the barrel reactor. The effect of plasma treatment time on WCA measurements of polymer

powders is shown in Figure 1-15.

Figure 1-15: Effect of plasma treatment time on the WCA of polymers

The experimental setup of barrel reactor is demonstrated in Figure 1-16. This reactor consisted

of a cylindrical quartz chamber (15cm long and 10cm of inner diameter) which was placed on

two fix parallel aluminum rods, one acting as a HV electrode and the other grounded. The

plasma operated at a frequency of 20 kHz, an input voltage of 110 V with a maximum of 100

W output from the power supply. In each experiment 20gr of polymer powder was placed inside

the chamber and the whole chamber was agitated by rotation 7 times per minute.

According to XPS analysis, the C:O ratio on non-treated PET powder was 4.7 and after 5

minutes plasma treatment this value dropped down to 1.8. The addition of O2 gas to the He

didn’t influence significantly the oxidation process.

Figure 1-16: Photography of the barrel reactor (left) and its schematic diagram reactor (right)

S.Put et al. [42] have used an atmospheric pressure DBD plasma jet (PlasmaSpot®) to treat

ultrahigh molecular weight polyethylene (UHMWPE) powders, with a mean diameter of


30

150µm. They have used N2 (100 slm) and N2/CO2 as operating gas. Each particle passes 10

times through the reactor, providing 25ms effective treatment time which was sufficient to

incorporate 7 at.% oxygen and 1 at.% nitrogen at the particle surface. XPS result showed

apparition of -COH group at the surface of particles (Figure 1-17). They showed that the whole

volume of the UHMWPE powders dispersed completely in water after plasma treatment.

Figure 1-17: Carbon peaks from XPS analysis, left is non-treated PE powder and right is treated one

The experimental setup is shown in Figure (1-18-left). The reactor consisted of a dielectric

aluminum tube, with an inner diameter of 11 mm, coated with a metallic layer which served as

a high voltage electrode. The grounded electrode was positioned at the center of the dielectric

tube. The applied power was 450 W. The outer diameter of the grounded electrode was 8 mm.

The length of the plasma zone in the reactor was 100 mm. Put et al. used a powder feeder to

convey the particles inside the plasma zone, which then fell down through the plasma chamber

because of gravity. The feeder used 3 slm air to inject the particles with a flow rate of 20 g/min.

The amount of incorporated oxygen could be increased by up to 10% when adding a small

amount of CO2 (1.5 slm) to the afterglow. The treating time is 20 min. However, the residence

time, i.e. the contact time between powder and plasma, is much lower as the powder spends

most of the time in the non-plasma area.

In order to increase the residence time of the powders in the plasma zone and consequently the

treatment efficiency, Put et al. scaled up this downer bed plasma reactor (Figure 1-18-right).

This way, the discharge gap is enlarged from 1.5 mm in the PlasmaSpot® reactor to 2 mm in

the scaled--up reactor. The power was set to 5kW and 66 kHz. In this scaled-up system the

outer diameter of central electrode was increased to 16mm and that of the inner diameter of

ceramic was enlarged to 20mm. The length of the plasma zone was increased from 100 to

700mm. N2 gas with a flow rate of 260 slm was used to treat UHMWPE particles. In this

scaled-up reactor the residence time of each particle in the plasma zone was estimated to be


31

around 18ms for one pass (cycle). After 10 passes the atomic concentration of oxygen and

nitrogen on the surface of the powder increased up to 12% and 2% respectively. The

temperature in the afterglow zone was measured and which increased up to 155°C. Since the

melting temperature of UHMWPE was 130°C, low total residence times in afterglow zone were

used for the surface treatment of particles, in order to avoid softening and melting of the

particles. However no further investigations have been reported on the change of the other

properties of the particles such as crystallinity, etc. The treatment in the modified PlasmaSpot®

reactor (10 passes) is found to be similar to about 3 passes in the scaled-up reactor. This could

be related to the power density which is 16 times lower in the enlarged reactor compared with

the smaller reactor.

Figure 1-18: Atmospheric pressure DBD plasma setup(left), Scaled-up downer reactor (right)

(S.Put et al. [42]).

Pichal et al. [43,44] designed a new DBD plasma reactor (Figure 1-19), at atmospheric pressure

for surface modification of PE powders (mean diameter 250 µm). They used air as the operating

gas, in order to decrease the operating costs of the system.

The system consisted of an adjustable discharge channel with a rectangular cross-section. The

particles fall down in discharge zone because of gravity force. The residence time of each

particle in the discharge zone was less than 1s. The particles flow rate was set to 70 g/s. To

increase the residence time in discharge zone, the powders sample were repeatedly conveyed

to top of the system and plasma zone.


32

The power was set to 150 mW and each particle passed 40 times through the discharge channel.

According to XPS analysis the oxygen uptake O/C was 0.09. The capillary penetration of benzyl

alcohol to the bulk of powders was increased after plasma treatment, which they attributed to

the introduction of new functionalities i.e. hydroxide (OH), carbonyl (C=O) and carboxyl

(COO) groups on the surface of PE particles. The authors observed an ageing effect of the

surface properties which lead to around 20% reduction in the capillary penetration after 1100

days of storage. As shown in Figure 1-20 the main ageing effect takes place in first 300 days of

storage.

The objective of this study was to improve the polymer adhesion to metallic surface. The tensile

strength of joints made of plasma treated UHMWPE particles increased from 2 MPa (for non-

treated PE) to 6MPa.

Figure 1-19: DBD atmospheric downer

reactor developed by Pichal et al.

Figure 1-20: Modified powder capillarity changes

during storage (Aging effect of stocking)

Nessim et al. [45,46] treated UHMWPE (mean diameter 60 µm) with a shell type DBD reactor

(Figure 1-21). The shell design used pairs of shell electrodes placed on the outside of a

cylindrical quartz tube. They used helium-air, helium-oxygen and helium-nitrogen discharges

for treatment of particles. Their design consisted of a quartz tube with an inner diameter of

24mm which provided a discharge gap of 24mm. Along the height of the quartz tube, three

separate discharge zones, each, 10 cm long were created. The powders were feed downwards

from the top of the quartz tube with a flow rate of 20g/min. The total residence time of each

particle in the discharge zone was 0.1s. The He, N2 and O2 flow rates were 9, 2 and 0.1 slm,

respectively. The power treatment of 3 W/cm3 lead to a decrease of WCA from values higher

than 90° down to 80° for UHMWPE powders. The results of the powder aging experiments


33

showed that the powders lost less than 20% of their hydrophilic properties after 60 days of

storage. The effect varied depending on their powder storage environment.

Figure 1-21: Experimental setup of Nessim et al. consists of three

discharge zones (30cm total)

5- Low-pressure plasma fluidized-bed systems for polymeric powder

treatment

Low-pressure plasma systems are wildly used to improve hydrophilic character and surface

functionality of polymer powders [47-53]. They have generally used nitrogen and ammonia as

plasma gas. It has been shown that the hydrophilicity and the nitrogen incorporation on the

surface of polymer powders increase after plasma treatment. The nitrogen containing groups in

these plasmas lead to decrease of water contact angle and more hydrophilic surface.

In this section, several fluidized-bed reactors coupled with low-pressure plasma systems

reported in the literature for polymer powder treatment and surface modification will be

discussed. An important challenge using the low-pressure systems is to provide an isolated

chamber to avoid leaking. Energy consumption of vacuum motors and pumps increases the

operating cost of this process.

Vivien et al. [47] have investigated the wettability improvement of PE powders by coupling

fluidized bed reactor and a cold remote nitrogen plasma system at low-pressure. The wettability


34

improvement was characterized by contact angle measurement with various liquids with their

surface tension ranging between 46.6 mN/m for ethylene glycol to 72.8 mN/m for water. The

best hydrophilic behavior of PE powders was obtained for the N2 flow rate of 500 sccm, pressure

of 3.2 hPa, power of 300 W and treatment time of 20 minutes. A wetting liquid with surface

tension of 56.9 mN/m had a contact angle of Ɵ=0° with PE powders treated with the above

mentioned conditions.

Arefi-Khonsari et al. [48] investigated the surface modification of LDPE particles by nitrogen

and ammonia low-pressure plasma in a fluidized bed reactor. WCA was decreased from 120°

for non-treated PE powders to 57° (using nitrogen as plasma gas) and 76° (using ammonia as

plasma gas) after 7min plasma treatment. Therefore using N2 as plasma gas had a better

influence on surface functionalization of polymer powder in comparison to ammonia. This

difference is because of higher nitrogen uptake of the surface with N2 plasma treatment. Indeed

this plasma treatment introduces a nitrogen percentage on the LDPE powders which is two

times higher than those obtained with ammonia with same treatment times. The evolution of

the oxygen and nitrogen uptake of the surface after the nitrogen plasma treatment are shown in

Figure 1-22. The O/C atomic ratio increased with the treatment time and reached a plateau at

14% after 100 s which showed that a non-negligible amount of oxygen atoms were incorporated

on the surface of the nitrogen plasma treated LDPE powders. The N/C atomic ratio increased

from 0% to 12.8% by increasing the treatment duration, a plateau was obtained for treatment

times above 100 s. The plateau obtained for both oxygen and nitrogen is the result of the

competition between the functionalization and degradation phenomena.

Figure 1-22: Variation of the incorporation of nitrogen and oxygen atoms on the surface of LDPE

powders after a nitrogen plasma treatment in the low pressure fluidized bed reactor


35

The authors used the Laser Doppler Anemometry method [49] to characterize the trajectory,

velocity and the concentration of the particles in the different zones of the fluidized bed reactor.

Jung et al. [50] have investigated the surface modification of HDPE particles in a circulating

fluidized bed reactor by using oxygen as the plasma gas. The hydrophilic character of polymer

powders was improved by formation of oxygen containing groups such as C=O and COO on

the surface of powders, which reach 12% and 8% respectively of the total C1s photo-electron

peak by the oxygen plasma treatment. Jung et al. concluded that the circulating fluidized bed

reactor provided a high gas-solid contact, less axial gas dispersion, easy control of circulation

of solids, smaller particle segregation and agglomeration compared to a bubbling fluidized bed.

Indeed the circulating fluidized-bed reactor required higher RF powers compared to a bubbling

fluidized bed because the plasma operating in circulating fluidized-bed required higher gas flow

rate for circulation of powders.

Song et al. [51] investigated a low-pressure fluidized-bed reactor coupled with a RF generator of

13.56 MHz for PS powder treatment. Inagaki et al. [52,53] analyzed the surface modification of

PE powder inside a plasma fluidized-bed reactor using oxygen. During plasma treatment the

operating pressure was set to 133 Pa using a mechanical booster pump and a rotary pump

connected in series. The WCA measurements were performed with the Washburn capillary

method. No water was adsorbed by the Washburn method, because of the hydrophobic

character of the non-treated PE powder (WCA>90°). The contact angle of water was 75°, 54°

and 51° for PE plasma treated for 2, 3 and 6 hours, respectively. According to these results the

plasma treatment for more than 3 hours didn’t influence the hydrophilicity of PE. They have

shown that oxygen plasma treatment of polymer powders led to the formation of oxygen

functionalities including C=O and COO- groups at the outermost layer of the powder.

According to XPS results the concentration of the C=O and COO- functionalities reached 10%

and 6% of the total carbon elements, respectively.

6- CFD Hydrodynamic modeling of fluidized bed reactor

Several groups have investigated the hydrodynamic modeling and simulation of fluidized-bed

reactors to improve their configurations and optimize the parameters [54-61]. The importance of

the hydrodynamic modeling of the fluidized-bed is to understand the interactions between the

different phases to better control the process. On the other hand accurate experimental data of

solid-gas interaction is required because they must valid the numerical modeling results.


36

The Computational Fluid Dynamic (CFD) is a method to solve the momentum and conservation

equations in multiphase condition. In the case of turbulent flow when the gas velocity is high,

the CFD method is able to describe the turbulences. Analysis of these turbulent areas is essential

because most of the reactions take place there and the reaction yield is higher.

The Lagrangian and Eulerian CFD models exist to describe the gas-solid interaction inside a

fluidized-bed reactor. The Eulerian model considers both phases as fluid and the momentum

and continuity equations are applied for both phases [62-66]. For the Lagrangian model the

Newton’s second law is developed for each particle. In this model the interaction between the

particles and all the forces acting on particles are considered. The Eulerian-Lagrangian

approach, so-called discrete element method (DEM) consider the fluid phase as a continuous

phase and the solid particles as dispersed phase. It applies Eulerian approach to describe the

continuous phase and Lagrangian approach to simulate the dispersed phase behavior.

Huilin et al. [67] applied the Eulerian-Eulerian approach to simulate gas-solid interactions and

hydrodynamics in a fluidized-bed reactor. Then they compared the modeling results with

experimental information to confirm the CFD model. Several groups have carried out the

Eulerian models to investigate the gas-solid behavior.

Vegendla et al. [68] carried out a comparison between Eulerian-Eulerian and Eulerian-

Lagrangian method to model the gas-solid two-phase fluids. They considered the gas phase as

continuous phase and solid phase as the dispersed one. They have modeled the volume fraction

of solid particles, velocity profiles and gas phase turbulent kinetic energy and its dissipation.

They have concluded that Eulerian-Lagrangian method fits better to the experimental data.

Some groups have applied the CFD approach to model and optimize the fluidized-bed

polymerization reactors [69-74]. Chen et al. [75] have used the CFD method to describe gas-solid

behavior in polymerization reactor by considering complete hydrodynamics of the FBR. Many

groups investigated the influence of reactor geometry, operation conditions, distributor,

particles size distribution, gas velocity and pressure on the hydrodynamic of the system to carry

out the accurate scale-up and design of the reactor. [76-78]


37

7- Conclusion and Prospects

Different atmospheric pressure and low-pressure plasma reactors were designed for polymer

powder treatments with the objective of improving the wettability and hydrophilic behavior of

polymer powders. Most of the studies in this domain are conducted with low-pressure plasma

systems. The low-pressure systems are more complex as compared to atmospheric pressure

systems due to vacuum devices and construction of an isolated chamber with minimum of

leakage. These lead to an increase of the operating cost of low-pressure systems. No-need of

vacuum device and possibility of continuous treatment of powders make atmospheric pressure

plasma systems appropriate for industrial applications.

Researchers have used different types of the reactors (fluidized-bed, circulating fluidized-bed,

barrel reactor, downer reactor and batch reactor) to optimize interaction between plasma and

polymer particles. In all these studies the polymer powders moved inside the treatment zone

(plasma) to ensure the homogeneity of the plasma surface treatment surrounding the particles.

The plasma fluidized-bed reactors are the most used system in literature. That would be

attributed to the high rate of heat and mass transfer in such reactors.

Polymer powders are heat sensitive materials. The specific surface area of bulk of the powders

is much bigger than flat samples. The contact time between plasma and powders must be

optimized to transfer enough amount of oxygen containing groups (or any other functional

groups) to the surface of polymer powder to improve the hydrophilicity and at the same time to

prevent powders melting or burning. Overheating polymer powders leads to powder

agglomeration and system clogging. Design of the reactor, treatment time, operating pressure

and temperature, plasma power, plasma gas type and its flow rate, type of polymer powders and

their size distribution are the most important parameters which influence the wettability

improvement process. The researchers have typically used contact angle measurement to

evaluate the wettability improvement, XPS to characterize the chemical properties and atomic

concentrations on the surface of polymer powders and SEM photography to analyze the

physical structure and morphology of powders.

PE and PP powders have been investigated more than any other polymer which is due to their

simple structure and their wide use. Plasma treated PE and PP waxes are used in paints and

varnishes, initial material of printer toners, 3D printing processes, machine aqueous dispersions,

pastes and hot melt adhesives. One of the most important applications of plasma treated PP and

PE powders is rotational molding for production of plastic parts. The car bumpers and


38

dashboards produced with rotational molding of plasma treated polymer powders have enough

high surface tension to enable direct painting and bonding. Table 2 presents a summary of

atmospheric pressure plasma systems to treat polymer powders. The majority of the published

results discuss the surface modification of PE particles with a DBD plasma. Most of the

described reactors use He and Ar as the plasma gas, which is more expensive compared to

treatments in air or N2.

Reference Plasma

type

Particles &

Geldart Classification

Reactor Capacity

Frequency Gas Treatment

time

Sachs et al. [33-34]

APPJ-FB

PP-PE-PA d=60μm

Geldart Classification A

750g 21 kHz Air + N2 120 s

Kogoma et al. [35]

Glow plasma - FB

PE d=12µm

Geldart Classification C

- 13.56 MHz He + O2 30ms*

Abourayana et al. [36]

DBD-FB DBD-Barrel

Silicone Polymer (d=5mm)

Geldart Classification D

0.5g 20 kHz

(100 W) He + O2 2 min

Oberbossel et al. [37-38]

PCFBR 64 DBDs

HDPE d=66.5μm


275 to 600 g/Batch

14.8 kHz Ar + O2 +

CO2

5 ms*/cycle Total cycle number=20

and 80

Nakajima et al. [39]

Glow plasma - FB

PS d=32-63 µm


5g 13.56 MHz (1000 W)

He + O2/ CF4

20 min

Gilliam et al. [40]

Continuous system

PMMA, PP d=250-350µm

Geldart Classification B

2-3 g powders/mi

n 20 kHz N2 or air less than 1s*

Abourayana et al [41]

DBD-Barrel Silicone/PP/ABS/PET Geldart Classification

A 20g 20 kHz

He And He +

O2

30 min

S.Put et al. [42]

DBD Continuous

system

UHMW PE (d=150μm)


20 g powders/mi

n

100 kHz (450 W)

N2 or N2 + CO2

25 ms*

Pichal et al. [43-44]

DBD PE


1000g 50 Hz Air 1s/transit

Nessim et al. [45-46]

shell design DBD

UHMW PE d=60 µm


20g powders/mi

n -

He + Air/O2/N2

0.1s*

Table 2: Summary of the different atmospheric pressure plasma reactors used for treatment of polymer particles. * denotes an effective treatment time


39

Finally, lack of modeling and simulation in this domain leads to costly optimization process

because all the operating parameters must be optimized experimentally. Models would provide

information about pressures, temperatures, particles trajectories, velocities and concentration

of particles in different parts of the reactor which contribute to a better understanding of the

fundamental processing reactions in different plasma reactor systems. Modeling would enable

to modify the reactor geometry and operating parameters easily. Modeling and simulations are

highly valuable to scale-up a laboratory plasma reactor to an industrial one.

Chapter 2.

40

Chapter 2

Experimental investigation of a Wurster type fluidized-bed

reactor coupled with an air atmospheric pressure plasma

jet for the surface treatment of polypropylene particles

Chapter 2.

41

1- Introduction

The use of polymer materials has several advantages as compared to metals such as reduced

weight and ease of machinability. However, the inert character of the material results in poor

adhesion characteristics with other materials as well as poor paintability and printability. In

order to improve these properties without changing the bulk properties of the polymeric

material, surface treatment can be the solution. In this way the surface energy of the polymer

materials can be tailored, opening a wide range of applications. Among the different surface

treatments of polymers, one can cite the wet chemical processes, thermal flame treatments,

mechanical treatments such as blasting and sanding, etc. [79-81] The plasma processes such as

corona discharges, as well as low pressure plasma processes, are versatile dry and

environmentally friendly processes which modify the surface properties without affecting the

intrinsic bulk properties.

Surface treatment technologies are among the most advanced equipment in the packaging

industry. [82,83] Plasma treatments are used to prepare the polymer surfaces for the adhesion of

inks, paints, and coatings (cleaning, coating, printing, painting, and adhesive bonding).

Moreover, the plasma surface modification of polymers is used for biomedical applications [84]

and textile industry as well as SLM (selective laser melting), SLS (selective laser sintering)

processes to print three-dimensional structures. [85]

Atmospheric pressure plasma treatments on polymer foils have already been investigated

thoroughly. [86–90] However, the application of these treatments to polymer particles is not

straightforward. Therefore, this chapter is focused on the treatment of polypropylene (PP)

particles with atmospheric pressure plasmas. Polypropylene can be used in a variety of

applications including packaging and labeling, textiles, plastic parts and reusable containers of

various types, laboratory equipment, automotive components, and medical devices. [91-94]

The problems of the modification of the particles are practically the same as in the case of foils

or flat substrates. However, the difficulties of the treatment of particles are related to their small

size, large specific surface, and the homogeneity of the surface treatment of given particles.

Moreover the problems encountered with manipulation of particles must not be neglected.

Plasma reactors reported for particle treatments can be classified into three types: static bed,

moving bed (i.e., barrel reactor), and fluidized-bed reactors (FBR) (cf. chapter 1). The latter has

been the most widely used since the last four decades, and involve the suspension of particles

Chapter 2.

42

in the discharge area. Its application on solid processing is the most successful for very large

industrial scale processes. The key features of FBR, which promoted their applications in

almost all kinds of industries, are associated with the high rate of mixing coupled with an

excellent heat and mass transfer.

In this thesis, an atmospheric pressure plasma jet combined with a Wurster fluidized bed reactor

(WFBR) was used for the surface modification of polypropylene particles using compressed

dry air as ionizing gas. The Wurster fluidized bed (cf. chapter 1) is a modified design of a

conventional fluidized bed by adding a riser, called Wurster-tube in the main chamber of the

fluidized bed. The WFBR concept was introduced by D.E. Wurster as a new and easily scalable

method to have a better control on the uniformity of formation of coatings on particles in a

fluidized bed reactor. Nowadays this method is widely used in the pharmaceutical industries

and several improvements have been proposed to avoid particle agglomeration inside the

Wurster tube by using a swirling gas flow or by using additional and tangentially gas flow above

the distributor plate. Although the objective of this thesis is to investigate for the first time the

use of a plasma jet inside a WFBR, further improvements could be proposed for future work.

2- Analytical methods used to characterize plasma treated polypropylene

particles

2-1- Water contact angle measurements for Zisman plot

Sessile drop water contact angles were measured with a DSA10 (Kruss instrument) using a

CCD camera and a horizontal light source to illuminate the liquid droplet. For each

measurement, a drop of bidistilled water with a volume of 6 μL was dispensed onto compressed

powders pellets and the contact angles were measured before and after plasma treatments. 10g

of polypropylene powder was compressed with a force of 1 tone, applied by compressed

machine to build the powders pellets. The sessile drop contact angle values were obtained using

the Laplace–Young curve fitting and were averaged over three values measured on different

samples from the same batch.

2-2- ATR-FTIR

The chemical composition of the PP particles was characterized using a Bruker VERTEX 70

FT-IR spectrophotometer equipped with a single reflection ATR accessory using a germanium

crystal as an internal reflection element. Infrared absorbance spectra were recorded in the 400–

Chapter 2.

43

4000 cm-1 range, with a resolution of 4 cm-1. Before each sample scan, the signal of the bare

substrate was taken as reference. Baseline correction was performed by OPUS 6.5 software

after 200 scans of each sample.

2-3- SEM

The morphology of PP particles was studied by Scanning Electron Microscopy (SEM). SEM

images were taken using Zeiss Ultra 55 FEG SEM with GEMINI Column on gold coated

surfaces prepared by sputtering (Cressinton sputter coater- 108 auto).

2-4- XPS

XPS surface analysis of the PP particles was performed on a XPS (Axis Ultra, Kratos, UK)

equipped with a monochromatic aluminum Kα X-ray source (1486.6 eV). OriginPro9.0

software was used to curve fit the high resolution C1s peaks. The hydrocarbon component of

the C1s spectrum (285.0 eV) was used to calibrate the energy scale. In a next step, the peaks

were deconvoluted using the Gaussian curve-fitting technique.

3- Experimental set-up

The plasma torch used in this work, and its interactions with various polymer foils or substrates

have been described elsewhere. [95,96] Figure 2-1 schematically shows the experimental set-up

of the Wurster-fluidized bed reactor (WFBR) combined with an atmospheric pressure plasma

jet (APPJ). This original plasma reactor with a specially designed plasma nozzle by Plasmatreat

(Figure 2-2) was developed in the first part of the thesis. The reactor was scaled to treat about

1 kg of powders per batch, depending on the material density. The plasma torch (Openair®

PFW10, Figure 2-2) is placed at the center-bottom of the system and passes through the wind-

box. The plasma nozzle comes out of the bronze distributor plate. The pressure drop through

the porous distributor is large enough to obtain a homogenous gas distribution. The fluidized-

bed reactor is a transparent glass tube with an inner diameter of 10 cm. At the top of the reactor

an expansion area decreases the velocity of the gas and particles to avoid particle loss from the

reactor. Nevertheless a paper filter has been placed at the exhaust. The metallic cylindrical

wind-box has the same diameter as the fluidized-bed and is fed with an air flow. The plasma

was generated by using a FG5001 DC generator. The pressure of the fluidization air must be

fixed at 2.5–3 bars to promote the transfer of polypropylene particles on the surface of the

Chapter 2.

44

distributor. The air fluidization of distributor is necessary to avoid stagnation of particles on the

surface of distributor.

Figure 2-1: Experimental set-up of Wurster-FBR combined with APPJ. (1) Wind-box, (2) plasma nozzle,

(3) distributor plate, (4) wurster-tube, (5) fluidized-bed, (6) expansion

Figure 2-2: wurster fluidized-bed plasma reactor and the

specially designed plasma nozzle by Plasmatreat

Figure 2-2: Experimental setup of

Openair plasma nozzle

Chapter 2.

45

As shown on Figure 2-1 and 2-2, a concentric tube called Wurster tube is located in the center

of fluidized-bed reactor. In our experimental set-up, the Wurster tube is placed exactly above

the plasma nozzle at the distance ranging from 0.5 to 2 cm from the distributor surface. The

high flow-rate of plasma jet causes the particles to be sucked up into the Wurster tube, raised

in the fluidized-bed to create a fountain region and again fall down on the distributor. With this

configuration, one can optimize the circulation of the particles in order to have a homogeneous

treatment for all the particles.

3-1- Combination of APPJ & Wurster fluidized-bed reactor (W-FBR)

As mentioned previously, the Wurster process is well-known for the granulation, coating, and

encapsulation of discrete particles in a fluidized bed using differential air flow to create a cyclic

movement of the powders inside the reactor. The location of the plasma nozzle at the bottom

of the fluidized bed of particles is what sets our Wurster process apart from other plasma

treatment methods. Differential air streams move the bed of particles upwards inside the

chamber where the atmospheric pressure plasma treatments occur. This configuration ensures

that the active species of plasma and coating material can be applied efficiently to individual

particles while avoiding agglomeration of the particles.

Studies have demonstrated that a properly operated Wurster fluidized bed coating process can

provide thin coatings by CVD of the highest quality and uniformity compared to other

techniques [97-99]. This uniformity can minimize the coating thickness requirements, further

reducing the amount of coating materials needed and the processing time.

The Wurster fluidized bed process can be applied to a range of core materials in numerous

particle sizes and shapes. This process is flexible enough to treat small particles for use in

capsules and tablets, as well as to coat entirely capsules and tablets. Coating capabilities extend

all the way to coating soft gel capsules, extruded materials, particles, crystals, and granules. No

matter the shape (spherical, crystalline, irregular, amorphous) the Wurster process is capable of

creating a unique formulation to achieve the desired properties. [100,101]

As shown in Figure 2-3, the Wurster fluidized bed consists of five principle zones. The first

zone is the plasma jet, the second zone is the Wurster tube where the plasma treatment of

particles occurs. The third zone is the fountain region where the speed of the particle tends to

zero. The fourth zone is where the particles fall down because of the gravity, this is also the

region where the temperature of the particles cools down; indeed as we will see later, the plasma

Chapter 2.

46

jet is relatively hot and therefore this region might play an important role to prevent the polymer

particles from excessive heating.

And finally, the fifth zone is where the particles move horizontally towards the Wurster tube

and the plasma nozzle. The role of the distributor plate and the fluidization gas make this

horizontal transfer easier.

Figure 2-3: General schematic of the different regions

inside the Wurster fluidized-bed reactor

4- Characterization of Polypropylene particles

The PP particles used in this work were Icorene® 4014 grade (A. Schulman, France)

characterized by a density of 0.9 g cm−3 and a mean Sauter diameter of 700 μm determined

from laser diffusion measurements (Figure 2-4) which correspond to the B group of Geldart's

classification , i.e.sandlike particles . 200 g of particles were loaded into the reactor per batch.

By trying different treatment times, we observed experimentally by testing the particles with a

dye (Rhodamine B) that about 120 s is required to treat all particles homogenously, therefore

the treatment time was set to 120 s for all experiments. Table 2-1 shows the general operating

conditions. Laser diffusion is a widely used particle sizing technique for materials ranging from

hundreds of nanometers up to several millimeters in size.

Pressure Voltage

(V)

Frequency

(kHz)

Plasma cycle time

(PCT)

Power

(W)

Treatment

time (s)

Particle

Quantity (g)

1 atm 270 19kHz 30% and 60 % 200 and 400 120 200

Table 2-1: Operating condition for plasma treatment of PP particles

Chapter 2.

47

On Figure 2-4, one can see some differences between the population of particles before and

after plasma treatment. The reason for the partial disappearance of the very fine particles (D <

100 µm) are probably due to the elutriation process where these particles are entrained by the

gas flow outside the reactor and/or particles sticking to the reactor wall, as observed

experimentally. Indeed, concerning the particles sticking to the wall, glass and polypropylene

are the opposite in terms of the triboelectric series (positive for glass and negative for PP),

therefore the friction of the particles on the reactor wall might trap the latter by electrostatic

force. In order to check this hypothesis, it would be interesting for further work to treat

polyamide-6 particles (PA-6) since it is situated just below glass in the triboelectric series.

Figure 2-4: Laser granulometry measurements before and after

plasma treatments of PP powders (about 10000 PP particles)

5- Results and discussions

5-1- Scanning electron microscope (SEM)

SEM analysis was performed to characterize the morphology and physical modification of

plasma treated PP particles. According to Figure 2-5, it seems that PP particles becomes

smoother after plasma treatment. Additionally, from image analysis one can conclude that the

number of fine particles decreases (dp << 150 μm) after plasma treatment, and this is in

agreement with laser diffusion measurements.

The reasons for particles smoothening (disappearance of sharp angles) might be the

consequence of plasma chemical etching due to the production of reactive oxygen and nitrogen

species (RONS) combined with an increase of the surface temperature. Although the surface

Chapter 2.

48

temperature of PP particles was not determined during the treatments, both experimental

measurements and modeling results indicate an average gas temperature in the Wurster tube

around 100 °C for a plasma power of about 400 W. Because of the high heat transfer typical

for fluidized bed reactor, one would suggest that the surface temperature of the particles could

be equal to the gas temperature, however the bulk temperature would remain much cooler. The

smoothening of the particles was also confirmed by circularity measurements based on image

analysis and this point will be addressed in the next part.

Figure 2-5: SEM photography of non-treated (left) and treated (right) of polypropylene particles

5-2- Determination of particles circularity from image analysis

One way to measure the shape of the particles is to quantify how close the shape is to a perfect

circle. Circularity is the ratio of the perimeter of a circle with the same area as the particle

divided by the perimeter of the actual particle image. Several definitions of circularity could be

used but for accuracy the software reports HS (high sensitivity) Circularity in addition to

circularity. HS Circularity has a squared term in the denominator to sensitize the parameter to

very subtle variation in the area-perimeter relationship. It is defined as follows:

HS Circularity = 4πA/P2

Where A is the particle area and P is the particle perimeter.

HS Circularity has values ranging from 0 to 1. A perfect circle has a circularity of 1 while spiky

or irregular objects have a circularity value close to 0. HS Circularity is sensitive to both overall

form and surface roughness. The shape and morphology of 5000 PP particles (treated and non-

treated) were analyzed by optical imaging microscopy and the HS Circularity of the particles

was measured. Figure 2-6 demonstrates the number percentage of particles as a function of HS

Circularity. These measurement shows the particles tend to become smoother after plasma

https://fr.wikipedia.org/wiki/Pi_%28lettre_grecque%29

Chapter 2.

49

treatment, as already observed by SEM analysis. However, one should take also into account

that many of the finest particles have disappeared after plasma treatments, therefore it is

difficult to conclude firmly on any clear particle smoothing.

Figure 2-6: Circularity measurement of treated and non-treated PP

particles

5-3- Analysis of PP particles by FTIR spectroscopy

Ben Salem et al. observed phase transition in plasma treated polyamide-6 substrates [95] using

the same plasma torch. In this work, the crystallinity of PP particles was assessed by ATR-FTIR

measurements before and after plasma treatments. The FTIR system consists of a single beam

optic and a Germanium splitter and the incident angle was 30°.

Polypropylene presents different crystalline structures such as atactic and isotactic structure.

The isotactic structure is able to form crystalline structures while atactic one is amorphous.

Luongo [102] conducted an empirical study of the correlation between the band ratio at 974 and

997 cm-1 observed in the infrared spectrum of polypropylene and the proportion of isotactic and

atactic phase in the material. The ratio of these infrared bands for different PP (0, 20, 40, 60,

80, and 100% isotactic) are plotted in Figure 2-7.

Non-treated and 120s plasma treated PP particles have been analyzed by FTIR in mode ATR.

Figure 2-8 shows the whole range (500-4000 cm-1) FTIR-ATR analysis of treated and non-

treated PP sample and Figure 2-9 shows the region between 966 and 1006 cm-1. At 997 cm-1

there is no significant modification of the intensity between treated and non-treated samples,

while at 974 cm-1 the intensity decreased. According to the correlation obtained by Luongo et

Chapter 2.

50

al. the atactic structure of PP is reduced after plasma treatment from 45 to 35% which means

an increase of 10% in isotactic structure.

Figure 2-7: Experimental curve obtained by

J.P.Luongo

Figure 2-8: FTIR-ATR result of 120s plasma treated and non-treated samples

Vas (methyl group -CH3) – 2970 cm-1 / Vs (-CH3) – 2910 cm-1 / Vas (methyl group -CH2- ) – 2870 cm-

1 / Vs (-CH2- ) – 2840 cm-1 / δas (-CH2- ) – 1460 cm-1 / δs (-CH3) – 1370 cm-1

Chapter 2.

51

Figure 2-9: FTIR of treated and non-treated PP (zoomed in)

It is important to mention that the crystallinity of PP particles was analyzed without considering

the benefits of atactic or isotactic structures of PP particles for different applications and

objectives.

Beside the relatively small modification of crystalline structure, there is no clear evidence of

surface functionalization by polar groups (e.g. C=O, COOH or OH) based on FTIR analysis as

shown on Figure 2-8. Additional measurements such as XPS and water contact angle

measurements will however confirm the presence of polar function in the next part.

5-4- Dispersion of polypropylene particles in water

200 g of PP particles were treated by an air plasma in the WFBR for 120 s and the results of the

hydrophilic character achieved by APPJ were investigated by the dispersing the particles into

distillated water. As shown in Figure 2-10, after stirring, the treated PP particles are completely

dispersed in water while non-treated PP ones remain agglomerated near the water surface

because of their lower density and hydrophobic properties. In the literature, the dispersion of

PP particles in water was attributed to the formation of the hydrophilic groups such as −OH,

C=O, COO−, −OOH and −COOH at the surface of the polymer [103,104]. As expected,

measurements depending on the surface wettability are much more sensitive than ATR-FTIR

analysis to assess any surface modification of powders.

Moreover the plasma treated PP particles recovered partially their hydrophobic character after

treatment. After 60 days, the aged treated polymer particles do not disperse as well as the

Chapter 2.

52

freshly-treated particles. The main reason for the hydrophobic recovery of plasma treated

polymer, which is a well-kwon phenomena, has been attributed in the literature to inward-

diffusion, agglomeration, or sublimation of LMWOM (low molecular weight oxidized

materials) species and reorientation of polymer chains. [105]

Figure 2-10: Dispersion of non-treated (left) and treated (right) PP in water

5-5- Dye adsorption test

In this thesis, one of the critical questions was the following: Is the plasma treatment of 200 g

of powders homogeneous in the reactor? In order to address this critical question, we were

looking for a macroscopic test for the powders. And, therefore, the dye adsorption test was

carried out, as a macroscopic but qualitative test, to characterize the hydrophilic properties of

the PP treated particles as well as the homogeneity of the treatments. The same amount of

treated and non-treated PP, approximately 10 g, were mixed with an aqueous solution of

Rhodamine B (20 ppm in distillated water) which is basically pink in two different beakers.

After three days at ambient temperature, without any specific storage conditions, plasma treated

PP were still wet and the reason for that could be the entrapment of water molecules in or

between the particles. Furthermore as observed on Figure 2-11, the volume of the particles

increased by a factor of 3 or 4 approximately, which confirms that the treated polymers swell

in water while the non-treated ones were completely dry and kept their original white color

because of their hydrophobic character.

Chapter 2.

53

Figure 2-11: Dye adsorption test. Left side treated and right side non-treated PP

After 7 days, the plasma treated PP colored by Rhodamine B were completely dry. As shown

on Figure 2-12, the treated PP particles are homogeneously colored which is an indication that

the plasma treatment of 200 g of powders during 120 s was quite homogeneous.

Figure 2-12: Dye adsorption after 7 days. Left side is non-treated and right side is treated PP particle

5-6- Determination of polypropylene surface tension by the Zisman method

The method proposed by Zisman is for investigating the wettability of a solid by determining

the critical surface tension from contact angle measurements. According to this method, the

critical surface tension (σcrit) is the surface tension corresponding to a perfect surface wetting

by a liquid (θ = 0). Different water/ethanol solutions were prepared and the surface energy of

the different mixtures is shown on Table 2-2. The cosine of the contact angle is then plotted as

a function of the solution surface tension in order to determine σcrit. The value of the surface

tension obtained from the regression equation for cos θ = 1 corresponds to the critical surface

tension, [106] which is often interpreted as the surface free energy of the solid.

Chapter 2.

54

Two plasma treated polypropylene samples with the same treatment time of 120 s but different

powers of 200W and 400W and non-treated sample were analyzed by the Zisman's method.

The plasma temperature at the exit of plasma nozzle for 200W and 400W was measured 360°C

and 520°C respectively, by a K-type thermocouple. Different ethanol and water solutions were

prepared as testing liquids (Table 2-2). Tablets of PP particles with the diameter of 2.5 cm and

weight of 10 g were prepared by using a compress machine (1 tone) to obtain reproducible

samples. 6 μL droplets of different liquids were deposited on the surface of the tablets and the

cosine of the contact angle θ was measured as a function of the surface tension of the different

liquids. According to Figure 2-13, the solution with 40% ethanol can completely wet the non-

treated PP particles, therefore the critical surface tension of raw PP particles is about 30.7

mN.m−1, which is in agreement with the literature, while the critical surface tensions of treated

PP particles for 200 and 400W were equal to 33.5 and 38.6 mN.m−1 respectively. In the work

of Noeske and al. [96], the authors treated PP flat substrates using a similar plasma torch with air

and observed an increase of 27 mN/m for raw PP to 52 mN/m for plasma treated one. In our

work, we observed an increase of only 8 mN/m after treatment, which is small as compared to

the increase obtained by Noeske and al. It is clear that treating powders is definitely different

from treating flat substrates. This is the reason why we developed some models in the next

chapter to understand the phenomena in the reactor.

Water (mass %)

Ethanol (mass %)

Surface Tension at 20°C

(mN/m)

Cos (Ɵ) 400W

(T0,plasma=520°C)

Cos (Ɵ) 200W

(T0,palsma=360°C)

Cos (Ɵ) Non-treated

100 0 72.75 0.17 0 -0.258

95 5 56.41 0.52 0.45 0.2

90 10 48.14 0.67 0.6 0.37

85 15 42.72 0.85 0.7 0.57

80 20 38.56 1 0.85 0.75

75 25 36.09 1 0.9 0.82

70 30 33.53 1 1 0.9

60 40 30.69 1 1 1

50 50 28.51 1 1 1

40 60 26.72 1 1 1

30 70 25.48 1 1 1

Table 2-2: Contact angle measurement (Zisman method)

Chapter 2.

55

Figure 2-13: Zisman plot of treated PP with 400 and 200W and non-treated one

The ageing effect and hydrophobic recovery of treated PP particles (400W) was analyzed after

60 days in room condition by Zisman method and was compared to fresh plasma treated sample

with same condition and non-treated one (Figure 2-14). The surface tension of plasma treated

PP particles is decreased to 33 mN/m from 38.6 mN/m after 60 days. The surface tension of

untreated PP particles was 30.7 mN/m

Figure 2-14: Ageing effect and hydrophobic recovery of 400W plasma treated PP particles

5-7-Characterization of surface chemistry by XPS

As discussed before, the measurements performed by ATR-FTIR are not sensitive enough to

characterize any clear surface modification of PP powders. Therefore, we investigated the

surface chemistry of PP surface after plasma treatment by high resolution XPS. The C1s high-

resolution XPS spectra of original and treated PP particles are shown in Figure 2-15, and the

Chapter 2.

56

deconvoluted peaks with Gaussian-Lorentzian fitting are shown in Table 2-3. The untreated PP

particles are characterized by two components with binding energies of 285.1 and 286 eV which

may be assigned to −C─C- and −C─O (and/or −C─OH) moieties respectively. The plasma

treated ones show two additional peaks at 287.1 and 288.7 eV, which may be attributed to −CO

and OC─O, respectively. As shown in Table 2-3, the C─C component decreases after plasma

treatment while the oxygen-containing polar group such as C=O and OC─O increases the

atomic concentration of oxygen on the surface of particles from about 5.2 to 9.1% after 120 s

of plasma treatment in our reactor.

Binding

Energy (eV)

Non-treated

(at. %)

Treated

(at. %)

Possible functional

groups

285.1 94.8 90.9 -C-C-

286 5.2 4.8 -C-O and/or -C-OH

287.1 - 3.3 -C=O

288.7 - 0.98 O=C-O and/or COOH

Table 2-3: Percent peak area of XPS C1s core level spectra

Figure 2-15: XPS spectra of untreated (left) and plasma treated (right) PP particle

Chapter 2.

57

6- Conclusion

In part, a Wurster fluidized bed reactor (WFBR) was coupled for the first time with an

atmospheric pressure plasma jet to treat 200 g of PP particles per batch. The physical and

chemical properties of the latter were characterized by different methods. According to the

results the surface energy of the particles was increased from about 30.7 mN/m to 38.6 mN/m

after 120s plasma treatment. The atomic concentration of oxygen on the surface of PP particles

was increased by about 5% after the plasma treatment. This increase is in agreement with the

dispersion of plasma treated PP particles into water. The dye adsorption test confirmed that the

PP particles were homogeneously treated in the reactor. The high temperature of the

atmospheric pressure plasma jet induced also some small modifications of the morphology and

crystalline structure of the particles. According to FTIR analysis, the isotactic structure of PP

particles was increased by about 10%. This modification is probably due to the fact that the

high gas temperature inside the central part of the Wurster tube which may lead to a fast

annealing and then quenching in the cooler parts of the fluidized bed leading to a

recrystallization of the polymer. The important conclusion is to point out that plasma treatments

in air will modify the surface chemistry of polymers (i.e. increase of surface energy) but

depending on the type of plasma and polymer it is also possible to modify the crystallinity of

the polymers as well, and the latter could have a significant positive or negative impact

depending on the applications.

Chapter 3.

58

Chapter 3

Hydrodynamic Comsol Multiphysics CFD modeling of

different Wurster fluidized bed reactors and comparison

with experimental data

Chapter 3.

59

1- Introduction

The fluidized beds are one of the most used reactors for physicochemical treatment of divided

materials. The reason is attributed to relatively homogeneous mass and heat transfer, high rate

of mixing and being able to work at different states like batch and continuous. Therefore it is

widely used in the chemical industry, metallurgy, oil and thermal power generation. A

conventional fluidized-bed is a two phase system. The solid phase or dispersed phase and the

fluid phase or continuous phase. In our study, the fluid phase is a gas (air). When the gas passes

through the bed, bubbles are formed and the concentration of solid particles inside these bubbles

is negligible and around each bubble there is a considerable amount of particles. This area is

called the wake.

Other groups have investigated modeling and simulation of fluidized-bed to improve their

systems and optimize the parameters. The importance of the hydrodynamic modeling of the

fluidized-bed is to understand the interactions between the two phases for a better control of the

process. However obtaining accurate experimental data of solid-gas interaction is necessary to

validate the modeling results.

Computational Fluid Dynamic (CFD) is a method to solve the momentum and conservation

equations in multiphase condition. In the case of turbulent flow when the gas velocity is high,

the CFD method is able to describe the turbulences. Analysis of these turbulent areas is essential

because most of the reactions take place there and the reaction yield is higher.

To develop the CFD model of a two phase fluidized-bed we have used the following general

methodology in Comsol Multiphysics V5.3. The first step was to build the reactor geometry.

CAD software such as SolidWorks and AutoCAD were used to define the geometry and

physical structure of the system. Then the volume occupied by fluid is divided into discrete

cells or mesh. The meshing structure could be hexahedral, tetrahedral, prismatic, and pyramidal.

Meshing is a critical step as it controls the model accuracy and convergence, as well as the

computational time and CPU resources. After meshing, one have to define the physical model.

During this step we used the equations for fluid motion and heat transfer. Then the boundary

and the initial conditions must defined according to hypothesis (e.g. slippery boundary) or

experimental data (e.g. initial gas velocity). Then the equations are solved in steady-state or

Chapter 3.

60

transient condition. The final results of modeling are analyzed and visualized by a post-

processing.

The CFD module of Comsol Multiphysics can model the fluid dynamic of compressible and

incompressible flows, laminar and turbulent flows, single phase and multi-phase flows. The

software solves the fluid equations at all the interfaces in steady state and time-dependent

(transient) conditions in 2D, 2D axisymmetric, and 3D. However, in our work the 3D model

did not converge as solving turbulent Navier-Stokes equation in three dimensions is extremely

difficult due to the non-linear terms in the equations.

The Lagrangian and Eulerian CFD models are able to describe the gas-solid interactions inside

a fluidized-bed reactor. The Eulerian model considers both phases as fluid and the Navier-

Stokes equations are solved for each phases [107-110]. For Lagrangian model, the Newton’s

second law is developed for each particle. In this model the interaction between the particles

and all the forces acting on particles are considered. For the Eulerian-Lagrangian approach, the

so-called discrete element method (DEM) considers the fluid phase as continuous phase and

the solid particles as dispersed phase. It applies Eulerian approach to describe the continuous

phase and Lagrangian approach to simulate the dispersed phase behavior.

To simulate turbulent flow, Comsol Multiphysics provide different models such as k-ε, k-ω and

SST (Shear Stress Transport). The SST model is a combination of the k-ε model in the free

stream and the k-ω model at wall interfaces. The models mentioned are so-called two-equation

models because they consider the same fluid flow equations for both phases (continuous and

dispersed).

To simulate the Wurster fluidized-bed reactor detailed in chapter 2, one has to consider that the

particles volume is very small as compared to computational domain (all the reactor volume)

and the number of particles is very large. Therefore using multiphase flow models of Comsol

is appropriate. These models keep track of the mass fraction of the different phases and the

influence of the dispersed particles on the transfer of momentum to the fluid.

The Euler-Euler approach is one the multiphase flow models available in Comsol. This model

is used for laminar and turbulent flows and can be used to tackle bubbly flows, emulsions, liquid

suspensions, aerosols, and solid particles suspended in gases like fluidized-bed reactors.

In order to develop the CFD numerical model for our Wurster fluidized-bed reactor the first

step was to simulate the gas flow coming out of the plasma nozzle. So the other parts of the

Chapter 3.

61

reactor (e.g. riser, distributor) were not considered. The experimental velocity profile of plasma

jet was measured by using a hot wire anemometer, then this profile was compared with different

turbulent models. The turbulent model which accurately described our experimental

measurements was the k-ε model.

In the next step the real geometry of Wurster fluidized-bed reactor was designed by AutoCAD

and was imported to Comsol Multiphysics. The first CFD model used in our reactor is a non-

isothermal model, turbulent k-ε , 2D axisymmetric, stationary and single phase (i.e. without

particles). This model was used to characterize the heat transfer phenomena inside the reactor.

Indeed, the knowing the temperature profile inside the reactor is critical because of thermal

sensitivity of the polymeric particles. The temperature profile obtained by Comsol Multiphysics

was compared with different experimental measurements inside the reactor.

Finally, an isothermal, turbulent, 2D, time-dependent and Euler-Euler two-phase model was

used to understand the particles behavior inside the reactor. It is important to mention that such

model can be used only for isothermal conditions. The velocity profile of both phases, i.e. gas

and polypropylene (PP) particles, and the concentration of PP particles in different region of

the reactor were obtained by this model. High speed imaging using a SONY RX 10 camera

(1000 frames/s) was carried out to observe the particles behavior especially at the out-put of

Wurster-tube where the concentration of solid particles is high. With these methods, one could

estimate the particle velocity to compare the later with modeling results.

In the last section of this chapter, we present a CFD model for a spouted Wurster fluidized-bed

reactor. This reactor was used to plasma treat black peppercorn particles for antimicrobial

applications (Chapter 4). The Eulerian-Eulerian approach was used as in the previous reactor.

The gas phase is argon and the dispersed one is the Silicon Carbide (SiC). In this system we

have applied the laminar equations to characterize the particles circulation and velocity profiles.

2- Mass and momentum conservation

Fluid mechanics is the study of mass and momentum transfer inside fluids. The equations

described in this part have been used to simulate the hydrodynamic of the plasma jet by

assuming the later as a single phase isothermal air flow.

Chapter 3.

62

2-1- Laminar flow

This model is used to calculate the velocity and pressure profiles in the case of low Reynolds

number. The equations, solved by this model, are the Navier-Stokes equation for mass (eq. 2)

and momentum (eq. 1) conservations.

ρ∂u

∂t+ ρ(u. ∇)u = ∇. [−PI + τ] + F Equation 1

∂ρ

∂t+ ∇. (ρu) = 0 Equation 2

τ = μ(∇u + (∇u)T) −2

3μ(∇ ∙ u)I Equation 3

Where u is the fluid velocity (m/s), ρ the density of fluid (kg/m3), μ the dynamic viscosity (Pa.s)

and P is the pressure (Pa). F represents the external volume forces (N/m3) such as drag force (in

the case of solid particles) and gravity is the viscous stress tensor (Pa) and I is the unit tensor.

2-2- Turbulent flow

This model is used to characterize the hydrodynamics of the system in the case of high Reynolds

number. In this work, three different turbulent models namely k-, k- and SST have been used

to calculate the velocity and pressure profiles. All models are suitable for incompressible flows

and compressible flows at low Mach number (typically less than 0.3). Additional equations and

variables are added in each case to the general equations of momentum and mass conservations.

Moreover, the equation of momentum is modified for turbulent models to add one more term

which is a function of the turbulent kinetic energy (k) as shown in equation 4.

ρ∂u

∂t+ ρ(u. ∇)u = ∇. [−PI + τ −

𝟐

𝟑𝝆𝒌𝑰] + F Equation 4

Finally, it is important to stress out that turbulent flows are characterized by large space and

time fluctuation which need an enormous computational cost. Therefore, it is usual for many

applications to solve the so-called Reynolds averaged Navier-Stokes equations, which calculate

only the average values of velocity and pressure, knowing that the mean values of fluctuations

are zero.

Chapter 3.

63

2-2-1- k-ε turbulent model

Turbulence effects are modeled by adding two transport equations and two variables, the so-

called turbulent kinetic energy (k) and turbulent dissipation rate ().Flow close to walls is

modeled using wall functions.

Transport of turbulent kinetic energy:

ρ∂k

∂t+ ρ(u. ∇)k = ∇. [(μ +

μT

σk) ∇k] + Pk − ρε Equation 5

Transport of turbulent dissipation rate:

𝜌𝜕𝜀

𝜕𝑡+ 𝜌(𝑢. ∇)𝜀 = ∇. [(𝜇 +

𝜇𝑇

𝜎𝑘) ∇𝜀] + Cε1

𝜀

𝑘 Pk - Cε2 ρ

𝜀2

𝑘 Equation 6

Turbulent viscosity:

𝜇𝑇 = 𝜌𝐶𝜇𝑘2

𝜀 Equation 7

Production term:

𝑃𝑘 = 𝜇𝑇 [∇𝑢: (∇𝑢 + (∇𝑢)𝑇) −2

3(∇ ∙ 𝑢)2] −

2

3𝜌𝑘∇ ∙ 𝑢 Equation 8

Where Cε1, Cε2, Cμ, σk, σε are the model constants, u the velocity (m/s), k the turbulent kinetic

energy (m².s-2), ε the turbulent dissipation rate (m².s-3), ρ the fluid density (kg/m3), Pk is the

production term of k (kg.m-1.s-3) and μT the turbulent viscosity (Pa.s). The values of the constant

parameters are given in the Table 3-1.

In this thesis, we also compared the k- model with the low Reynolds k-e model, also referred

as Abe, Kondoh and Nagano or AKN model. The terms “low Reynolds” refers to the region

close to the wall were the viscous effects dominate. Two corrective terms are therefore added

to the transport of and to the turbulent viscosity.

𝜌𝜕𝜀

𝜕𝑡+ 𝜌(𝑢. ∇)𝜀 = ∇. [(𝜇 +

𝜇𝑇

𝜎𝑘) ∇𝜀] + Cε1

𝜀

𝑘 Pk - Cε2 ρ

𝜀2

𝑘∙ 𝒇𝜺 Equation 9

𝜇𝑇 = 𝜌𝐶𝜇𝑘2

𝜀∙ 𝒇𝝁 Equation 10

Chapter 3.

64

𝑓𝜇 = (1 − 𝑒−𝑙∗/14)2

∙ (1 +5

𝑅𝑡3/4 ∙ 𝑒−(𝑅𝑡/200)2

) Equation 11

𝑓𝜀 = (1 − 𝑒−𝑙∗/3.1)2

∙ (1 − 0.3 ∙ 𝑒−(𝑅𝑡/6.5)2) Equation 12

𝑙∗ =𝜌𝑢𝜀𝑙𝑤

𝜇, 𝑅𝑡 =

𝜌𝑘2

𝜇𝜀, 𝑢𝜀 = (

𝜇𝜀

𝜌)

1/4

Equation 13

The values of the constant parameters for both k- models are given in the following table

Constant Cµ Cε1 Cε2 σk σε

Value for k- 0.09 1.44 1.92 1.0 1.3

Value for AKN 0.09 1.5 1.9 1.4 1.4

Table 3-1: Model constants for the AKN or low Reynolds k- model

2-2-2- k-ω turbulent model

Turbulence effects are modeled using the Wilcox revised transport equations where the

turbulent dissipation rate () is replaced by the specific dissipation rate ().

Transport of turbulent kinetic energy:

ρ∂k

∂t+ ρ(u. ∇)k = ∇. [(μ + 𝜇𝑇𝜎𝑘

∗)∇k] + Pk − 𝛽0∗ρωk Equation 14

Transport of specific dissipation rate:

𝜌𝜕𝜔

𝜕𝑡+ 𝜌(𝑢. ∇)𝜔 = ∇. [(𝜇 + 𝜇𝑇𝜎𝜔)∇𝜔] + −𝛽0𝜌𝜔2 + 𝛼

𝜔

𝑘𝑃𝑘 Equation 15

Turbulent viscosity:

𝜇𝑇 = 𝜌𝑘

𝜔 Equation 16

Where 𝜎𝑘∗, 𝛽0

∗, 𝛽0, 𝜎𝜔, 𝛼 are the model parameters, k the turbulent kinetic energy (m²/s²), ω the

specific dissipation rate of kinetic energy (1/s), ρ the fluid density (kg/m3), , μ the dynamic

viscosity (Pa.s). .

The values of the constant parameters are given in the following table

Chapter 3.

65

Constant 0* 0 σk* σ

Value 0.52 0.104 0.09 0.5 0.5

Table 3-2: Model constants for the k- model

2-2-3- SST turbulent model

Turbulence effects are modeled using the Menter Shear-Stress Transport (SST) equations. This

model in an interpolation between the k- (near the walls) and the k- model. The SST model

depends on the distance to the closest wall. The physics interface therefore includes a wall

distance equation.

ρ∂k

∂t+ ρ(u. ∇)k = ∇. [(μ + 𝜇𝑇𝜎𝑘)∇k] + P − 𝛽0

∗ρωk Equation 17

𝜌𝜕𝜔

𝜕𝑡+ 𝜌(𝑢. ∇)𝜔 = ∇. [(𝜇 + 𝜇𝑇𝜎𝜔)∇𝜔] +

𝛾

𝜇𝑇𝜌. 𝑃 − 𝛽0𝜌𝜔2 + 𝛼

𝜔

𝑘𝑝𝑘 + 2(1 − 𝑓𝑣1)

𝜎𝜔2𝜌

𝜔∇𝑘. ∇𝜔

Equation 18

∇G. ∇G + σωG(∇. ∇G) = (1 + 2σω)G4 Equation 19

μT = ρα1k

max (α1.ω,Sfv2) , S = √2S: S, S =

1

2(∇u + (∇u)T) Equation 20

P = min (Pk, 10β0ρωk) Equation 21

∅ = fv1∅1 + (1 − fv1)∅2, ∅ = β, γ, σk, σω Equation 22

Where 0*, 1, 2, 1, 2, 1, k1, k2, w1, w2 are the model parameters, k the turbulent kinetic

energy (m²/s²), ω the specific dissipation rate of kinetic energy (1/s), ρ the fluid density (kg/m3),

μ the dynamic viscosity (Pa.s).

The values of the constant parameters are given in the following table:

Constant 0* 1 2 1 2 1 k1 k2 w1 w2

Value 0.09 0.075 0.0828 5/9 0.44 0.31 0.85 1.0 0.5 0.856

Table 3-3: Model constants for the SST model

Chapter 3.

66

3- Comparison of different turbulent models of Comsol Multiphysics V5.3

with experimental results

Different 2D axisymmetric non-isothermal turbulent models were applied for the gas phase to

describe the hydrodynamics of our plasma jet. In the literature, turbulent jets are usually

simulated by using the k-ε and Shear Stress Transport (SST) models. In order to discriminate

the best CFD model described in the previous part, hot wire anemometry measurements were

carried out in the jet to determine the gas velocity profile. The anemometer was placed at

different distances from of the plasma torch nozzle. The gas velocity at output of plasma jet

was measured to be 50 m/s (V0). The experimental data are shown by black dots in Figure 3-1.

As shown on this figure, both k-ε and SST models are appropriate to describe the

hydrodynamics of the reactor, which is in agreement with the literature. Although significant

errors are obtained in both case in the region between approximately 2 and 5 cm. Figure 3-2

shows the differences between experimental and model values of velocity using k-ε and SST

models. Both of the models show less accuracy in the first 5cm of gas fluid modeling.

According to this figure after 5cm as distance the k-ε model is completely matched to

experimental data, while this distance for SST model is 7.5cm. As a conclusion, the k-ε model

was selected for further studies.

Figure 3-1: Velocity profile (Comsol Models vs. Experimental data)

Chapter 3.

67

Figure 3-2: Difference between experimental and model values of velocity using k-ε and SST models

(Vexp – Vmodel)

4- Wurster fluidized-bed reactor geometry

Figure 3-3 schematically shows the experimental set-up of the Wurster-fluidized bed reactor

combined with the atmospheric pressure plasma jet (APPJ). The plasma torch (Openair®

PFW10) is located at the center-bottom of the system and passes through the wind-box. The

plasma nozzle comes out of the bronze distributor plate. The fluidized-bed is a transparent glass

tube with an inner diameter of 10 cm. At the top of the system, an expansion area was added in

order to decrease the gas and particles velocity and to avoid particles exiting from the reactor.

Nevertheless a paper filter has been placed at the exhaust to prevent elutriation of the finest

particles from the reactor. The all-metallic cylindrical wind-box has the same diameter as the

fluidized-bed and was fed with an air flow. The pressure of the fluidization air was fixed at 2.5–

3 bar to obtain a bubbling bed and promote the polypropylene particles transfer on the surface

of the distributor.

As demonstrated in Figure 3-3, a concentric tube called Wurster tube is located in the center of

fluidized-bed reactor. In our experimental set-up this tube is placed exactly on top of the plasma

nozzle at the distance of 0.5–2 cm from the distributor surface. The high flow-rate of plasma

gas causes the particles to be sucked up into the Wurster tube, raised in the fluidized-bed to

create a fountain region and again fall down on the distributor when the gas velocity becomes

-20

-15

-10

-5

0

5

10

15

0 5 10 15 20 25

V(e

xp)

-V

(mo

del

)

Distance (cm)

V(exp)-V(k-ε) V(exp)-V(SST)

Chapter 3.

68

low enough. With this configuration, one can optimize the particles circulation in order to have

a homogeneous treatment for all the particles.

Figure 3-3: Design of the Wurster-FBR combined with APPJ by AutoCAD. (1) Wind-box, (2)

plasma nozzle, (3) distributor plate, (4) Wurster-tube, (5) fluidized-bed, (6) expansion

5- Single Phase CFD Comsol Multiphysics modeling of Wurster fluidized-

bed

Although the whole reactor geometry was built in 3D using AutoCAD, because of the enormous

computation time and the difficulty to converge, a 2D axisymmetric single phase turbulent

model was developed by Comsol Multiphysics V5.3 to characterize the hydrodynamics of

system. As discussed before, the k-ε model was used to describe the turbulence, because this

model was close to experimental data of gas velocity measurements. The gas (air) velocity

coming out of the plasma nozzle has a velocity of 50m/s and the fluidization gas (air) coming

out of the distributor has a velocity of 0.3m/s. The other model parameters, boundary

conditions, meshing and wall functions are explained with details in the multiphasic model

section. The gas velocity profile and Venturi effect at the entrance of Wurster-tube are shown

on Figure 3-4. The high velocity of the plasma jet induces a low-pressure region at the entrance

of the Wurster-tube, the so-called Venturi effect, which sucks up the particles inside the

Wurster-tube. Moreover, the additional flow through the gas distributor at the bottom of reactor

improves the particles fluidization. In these conditions the shear stress between the particles is

reduced and the horizontal transport of particles becomes easier.

Chapter 3.

69

Figure 3-4: Velocity field inside the reactor and streamlines at the entrance of the Wurster tube.

The gas pressure profile is shown on Figure 3-5. Inside the Wurster-tube and especially at the

bottom of the later where a low-pressure area causes the Venturi effect. In Figure 3-6 the white

lines show streamlines and the formation of a recirculation region (eddy) at the exit.

Figure 3-5: Pressure profile of Wurster-tube Figure 3-6: Gas trajectory by lines

Chapter 3.

70

6- Thermal characterization of Wurster fluidized-bed reactor

6-1- Experimental temperature profile measurements

A challenge in treating polymer particles is their heat sensitivity which limits the choice of

possible surface modification techniques. Therefore, the thermal characterization of the

Wurster-tube was carried out. Indeed, this region corresponds to the main treatment region and

the heat transfer between the plasma jet and the fluidized bed occurs in this part of the reactor.

While the other regions of the reactor act as a cooling system, reducing the particles temperature

inside the reactor, between each passes through the plasma region.

As shown in Figure 3-7, four different sections inside the Wurster tube were chosen. The first

section is just above the plasma nozzle. The second one is where the jet touches the inner wall

of the Wurster tube (interaction section) and this part is in principle the hottest part of the surface

of the Wurster tube. The third section is exactly in the middle of the tube and the last one is at

the exit of the latter. Radial temperature profiles were determined by using a K-type

thermocouple at seven different locations for each section, that is, five points inside the Wurster

tube and two points outside the tube near the surface of the tube. The location of temperature

measurements are depicted with blue circles in Figure 3-7.

The corresponding radial temperature profiles are shown in Figure 3-8. To ensure steady state

conditions, all temperature measurement were done after 10 min of ignition of plasma. The tube

is represented with blue rectangles in Figure 3-8.

Figure 3-7: Schematic of Wurster tube with the measurement points for the temperature

Chapter 3.

71

An important point is to find out the interaction section between the plasma jet and the inner

wall of the Wurster tube (section 2). In order to determine this position, Schlieren photography

was used to determine the expansion angle of gas coming out from the plasma nozzle. Schlieren

photography [111] is an optical process used to visualize the flow of fluids with varying refractive

index which depend on their densities. According to Figure 3-9, the expansion angle of the jet

is around 24–25°. From this value, one can determine the distance where the interaction section

is located. This distance was between 75 and 80 mm from the bottom of the reactor.

Complementary thermo-photography of the Wurster tube using a FLIRTG165 thermal camera

confirmed these measurements (Figure 3-10).

Figure 3-8: Radial temperature profiles. (A) is section 1, (B) is section 2 (interaction point), (C) is

section 3, (D) is section 4

Chapter 3.

72

Figure 3-9: Schlieren photography of the APPJ Figure 3-10: Thermal imaging of the Wurster tube

The melting point of polypropylene is around 160 °C. As demonstrated from thermal

characterization, only the hottest part of the reactor which consists of the plasma jet outlet and

a small region after the plasma jet (section 1) exceeds this temperature. Nevertheless, the short

residence time of the particles in this zone and the relatively high specific heat capacity of

polypropylene (1920 J kg−1 °C) allow the particles to remain below the melting point.

Furthermore, once the particles exit the Wurster tube, they are subjected to a cooling process in

the fluidization column. Indeed, the down flow region outside the Wurster tube is practically at

room temperature, which helps to decrease the particles temperature because, the air flow

introduced through the distributor plate to fluidize the particles improve this cooling process,

during the circulation of the particles through the reactor. To summarize, the Wurster fluidized

bed promotes the cooling of heat sensitive substrates, avoiding particles agglomeration and

clogging inside the system.

Chapter 3.

73

6-2- Heat transfer modeling by Comsol Multiphysics V5.3

The heat transfer module of COMSOL Multiphysics 5.3 was used to model the transfers by

conduction and convection in the fluid. The general mathematical equation is:

𝜌𝐶𝜕𝑇

𝜕𝑡− ∇. (𝑘∇𝑇) + 𝜌𝐶𝑢∇𝑇 = 0 Equation 23

Where,

T = fluid temperature (K)

ρ = fluid density (kg.m-3)

C = fluid heat capacity (J.kg-1.°C-1)

k = fluid thermal conductivity (W.m-1.K-1)

u = velocity field calculated from k-ε turbulent model (m.s-1)

For a steady-state model, variables are not time dependent and consequently the first term is

neglected and equation 23 is simplified to:

−∇. (k∇T) + ρCu∇T = 0 Equation 24

To set the physical properties of the fluid, the plasma jet was considered as an air jet at the gas

temperature. The inlet temperature of air was set to 520 °C which corresponds to experimental

measurements. The thermal conductivity of air is 0.025 W/(m.K), the density is 1.2 kg/m3 and

the specific heat capacity of air at atmospheric pressure is 1.005 kJ/kg.K . The gas temperature

profile was calculated all over the reactor (Figure 3-11). In this model, two physics are

considered, turbulent CFD and heat transfer. As mentioned before the k-ε model was used to

describe the fluid flow. The Kays-Crawford heat transport turbulence model was used to

describe the heat transfer phenomena in fluid. Focusing on the temperature inside the Wurster-

tube and in particularly the interior walls of the tube, a comparison was made between modeling

and experimental results as shown on Figure 3-12.

As discussed previously, experimental measurements by Schlieren photography and infrared

thermography, located the interaction section (section 2) between plasma and the Wurster-tube

occurs at about 75–80 mm from the distributor plate, where the temperature is a highest (see

Figure 3-10). From our model, this region is shifted at 95 mm from the distributor plate, which

represents a difference of about 16% from the experimental measurements, and the temperature

is slightly higher (+20°C). This difference is probably due to the fact that i) the plasma jet was

considered as an air flow at the same temperature, ii) the physical properties of air (k, and Cp)

Chapter 3.

74

were considered as constant with the temperature, which is strictly not correct. Finally, one can

conclude that our CFD non-thermal model is relatively in good agreement with the

experimental measurements.

Figure 3-11: Gas temperature profile Figure 3-12: Modeling and experimental

temperature of Wurster-tube interior wall

7- Biphasic Eulerian-Eulerian CFD by Comsol Multiphysics V5.3

We started our modeling work by validating an isothermal and single phase turbulent model.

Then heat transfers were added to obtain a non-isothermal which was in relatively good

agreement with the experiment measurements. Finally, a 2D isothermal Eulerian turbulent

biphasic model was added in order to determine the particle trajectories and particle distribution

inside the reactor and to calculate the hydrodynamics of the two-phase mixture containing a

continuous phase (air) and a dispersed phase (polypropylene particles). The Eulerian model

assumes that both phases are continuous, fully interpenetrating, and incompressible. Typical

applications for this model are fluidized beds (solid particles in gas), sedimentation (solid

particles in liquid), or transport of liquid droplets or bubbles in a liquid. Unfortunately, it was

not possible to implement the physics to calculate the temperature, i.e. non-isothermal

conditions. Therefore the gas temperature was considered as constant is this final model.

Chapter 3.

75

7-1- Biphasic Eulerian CFD mathematical equations

The Multiphysics interface solves two sets of Navier- Stokes equations, one for each phase, in

order to calculate the velocity field of each phase. The phases interchange momentum using a

drag force equation. The pressure is calculated from a mixture-averaged continuity equation

and the volume fraction of the dispersed phase is tracked with a transport equation. Two-phase

turbulence is modeled using the standard two-equation k-ε model with reliability constraints.

The flow close to the walls is modeled using wall functions.

For continuous phase (Air):

ρc

∂kc

∂t+ ρcuc + ∇kc = ∇. ((μC +

μT,c

σk,c) ∇kc) + Pk,c − ρcεc Equation 25

ρc

∂Ɛc

∂t+ ρcuc + ∇Ɛc = ∇. ((μC +

μT,c

σƐ,c) ∇Ɛc) + Cε1,c

εc

kcPk,c − Cε2,cρ

εd2

kd Equation 26

For dispersed phase (Polypropylene particles):

ρd

∂kd

∂t+ ρdud + ∇kd = ∇. ((μD +

μT,d

σk,d) ∇kd) + Pk,d − ρcεd Equation 27

ρd

∂Ɛd

∂t+ ρdud + ∇Ɛd = ∇. ((μD +

μT,d

σƐ,d) ∇Ɛd) + Cε1,d

εd

kdPk,d − Cε2,dρ

εd2

kd Equation 28

The c and d indices are respectively responsible of continuous and dispersed phases, 𝜇𝑇,𝑐 the

turbulent viscosity (Pa.s), 𝜎Ɛ,𝑑 the turbulent particle Schmidt number, 𝑃𝑘,𝑐 the production term

(kg.m-1.s-3), u the velocity (m/s), ρ the density (kg.m-3), Cε1,c and Cε2,c are the model parameters

for continuous phase and Cε1,d and Cε2,d are the model parameters for dispersed phase. The

model parameters for both phases are the same values. (Cε1=1.44, Cε2=1.92, 𝐶𝜇=0.09, 𝜎Ɛ=1.3,

𝜎𝑘=1)

With this k-ε Eulerian-Eulerian model, we developed a CFD model to simulate the 10 first

seconds of the reactor operation with a time step of 0.03 second. Indeed, these calculations

require several days of computing but 10 seconds are enough to reach the steady state conditions

inside the reactor. The reference pressure and temperature were respectively 1 atmosphere and

293.15K. The selected drag force model is the model of Gidaspow and the solid pressure model

is the model of Ettehadieh.

Chapter 3.

76

7-2- Model Conditions

The Wurster fluidized-bed reactor has two inlets (plasma gas and distributor gas for

fluidization) and one outlet. The gas velocity coming out of the distributor and plasma nozzle

are 0.3m/s and 50m/s respectively according to experimental measurements. However, Comsol

Multiphysics 5.3 did not convergence when the gas velocity was higher than 10 m/s.

To describe the turbulence conditions in Comsol, we have to specify the turbulent intensity

(IT,c= 0.05) and turbulence length scale (LT,c= 0.01). These parameters were used by Comsol

for k-ε calculation.

At the initial time (t=0) the reactor was divided in two main parts (Figure 3-14). The smaller

area which corresponds to the bottom of the reactor (red part) is the dispersed phase, i.e. the

powders bed. The largest area corresponds to other parts of the reactor (blue part). In our study,

we have treated 200gr of polypropylene particles per batch and the corresponding bed height

was 2cm as height for a section of 10cm (reactor diameter). We have considered the initial

volume fraction of the particles equal to 0.6. The dispersed phase (PP Particle) is characterized

by an average particles diameter of 700μm and a density of 950 kg/m3. For the continuous phase

(air) we specified the density at 1.18kg/m3 and dynamic viscosity at 1.8x 10-5 Pa.s. In Figure 3-

13, the reactor walls are colored in blue. The Slip mode (u 0) was applied at all walls surface

for both dispersed and continuous phases.

Figure 3-13: Reactor walls, inlet and outlet Figure 3-14: The two phases at initial time

(t=0). The blue part is air and red part is the PP

particles with a volume fraction of 0.6

Chapter 3.

77

7-3- Meshing

We have used triangular meshes in Comsol Multiphysics as shown on Figure 3-15. The total

number of elements for meshing whole the Wurster fluidized-bed was 5868. The normal

element size was selected in the options of meshing. The mesh is refine close to out-put of

plasma nozzle and around Wurster-tube (riser) to have a more accurate calculation in these

critical areas. The size of the elements has a significant influence for the total calculation time

and the convergence of CFD equations. In the reactor, two aluminum rings hold the Wurster-

tube at his position. These rings are considered as solid in our model and they are shown with

two squares at both sides of the Wurster-tube (Figure 3-15).

Figure 3-15: Triangular meshing of the Wurster fluidized-bed reactor

7-4- Gidaspow Drag Force

The Gidaspow drag model is a combination of the Wen and Yu drag model and the Ergun

equation. The Wen and Yu drag model uses a correlation from the experimental data of

Richardson and Zaki. This correlation is valid when the internal forces is negligible which

means that the viscous forces dominate the flow behavior. The Ergun equation is derived for a

dense bed and relates the drag to the pressure drop through porous media.

Chapter 3.

78

The Gidaspow drag model can be written as equation (29)

Fdrag,c = −Fdrag,d = βuslip , uslip = usolid − ugas Equation 29

Where uslip is the difference of velocity between two phases and β in the Gidaspow coefficient:

β =3∅c∅dρcCd

4dd|uslip|∅c

−2.65 Equation 30

To calculate β, the drag force coefficient (Cd) must be calculated according to equation 31:

Cd = {

24

ReP

(1 + 0.1ReP0.687) ReP < 1000

0.44 ReP > 1000

Equation 31

And to calculate the Reynold number of particles the equation 32 was applied:

ReP =∅cddρc|uslip|

μc Equation 32

Where dd is the average diameter of solid particles (m), ρc is the gas phase density (kg.m-3), ∅𝑐

is the volume fraction of continuous phase (gas), μc is the dynamic viscosity of gas (Pa.s) and

uslip is the difference of velocity between two phases (m.s-1).

7-5- Comparison of biphasic turbulent CFD model developed by Comsol

Multiphysics with high speed imaging results

The volume fraction of the dispersed phase inside the reactor is shown in Figure 10 at different

times between 0 and 8 s. The latter corresponds to the stabilization time of the system, i.e.

steady state.

According to the simulation results, once the stabilization of the system is reached, a “fountain”

is formed in which the particles are concentrated. As shown in Figure 3-16, this zone is located

at approximately 70 cm from the base of the reactor.

Chapter 3.

79

Figure 3-16: Dispersed phase development in function of time (logarithmic scale)

Figure 3-17 shows the axial gradient of the volume fraction of the particles inside the reactor.

At the Wurster tube outlet (25 cm), the particle volume fraction is about 0.0015 and at the

“fountain” zone (70 cm) is approximately 0.016. To check the accuracy of these simulation

results, a SONY RX10 high speed camera (1000 frames/second) was used to capture the steady

high resolution images of the moving particles. According to the high speed photography

(Figure 3-18), the height of the “fountain” zone is about 60 cm. which means that the simulation

is relatively in good agreement with the experimental results.

Figure 3-17: Axial gradient of volume fraction of the particles

Chapter 3.

80

Figure 3-18: High speed photography of the reactor. The dotted arrows indicate the circulation of

particles inside the reactor. The PP particles have been treated by plasma, colored by a dye and re-

introduced inside the reactor

Chapter 3.

81

7-6- Calculation of the effective treatment time of particles inside the reactor

According to high speed photography, the mean particles velocity at exit of the Wurster-tube

was about 6 m s−1. From these measurements, it is possible to estimate the total cycle number

for each particle inside the plasma zone, and therefore the effective treatment time. As a

simplifying assumption, we considered that each particle entering the Wurster-tube had

interaction with the plasma. According to CFD simulation results, it is possible to determine

the solid volume fraction and particles mean velocity at the outlet region of the Wurster tube,

to finally obtain the flow rate of the particles. Then, by considering the total mass of particle

per batch, the time required to treat all particles can be estimated.

The calculated particles mean velocity is consistent with Fast Imaging measurements around 6

m/s. By using equation 37, the particles flow rate is estimated at 5.72 g/s. Therefore, by

considering the total particle mass of 200 g, the time required to treat all particles, assuming an

ideal plug flow is about 35 s. Finally, for a total treatment time of 120 s, one can estimate that

in average each particle passed 3-4 times inside the Wurster-tube and therefore and was treated

3-4 times by the plasma. It is also important to mention that the mass flow rate calculated at the

end of Wurster tube must be constant inside the tube (conservation of mass), although the

particles speed and concentration may vary simultaneously. (Figure 3-19)

Volume element = 𝜋𝑟2. ℎ = 7.056 cm3

(33)

Particles Volume in the element = Particle volume fraction * Volume = 0.0106 cm3

(34)

Figure 3-19: Simplified geometry of reactor and calculation element inside the Wurster-tube for

particles flow rate and total cycle number calculation

Chapter 3.

82

Particles Mass in the element = Particles Volume * Particles Density = 0.00953

grams

(35)

Residence Time of each particle in element= Height of the element

Average Particle Speed = 0.001666 s

(36)

Particles Flow-rate at exit of Wurster tube = Particles mass

Residence time of each particle = 5.7249

(grams/s)

(37)

Time of one Cycle of all the Particles in Wurster-tube (One circulation) = Total Particles Mass

Particles Flowrate= 34.93 s

(38)

7-7- Reynold number calculation

The flow Reynold number is calculated according to equation below:

𝑅𝑒𝑔𝑎𝑠 𝑓𝑙𝑜𝑤 =𝜌𝑢𝑑

𝜇 Equation 39

Where is the gas density (kg.m-3), u is the gas velocity (m.s-1), d is the tube diameter (m) and

μ in the viscosity dynamic of the gas (Pa.s).

According to experimental anemometry measurement of gas velocity at exit of plasma nozzle,

the reactor is fed with 50m/s of air with a dynamic viscosity of 1.8x10-5 Pa.s, and a density of

1.18 (kg.m-3). The plasma nozzle diameter is 0.5cm, therefore the experimental Reynolds

number would be 16400 so completely turbulent situation.

As mentioned previously, the gas velocity more than 10m/s the Comsol Multiphysics solver did

not converge. With a gas velocity of 10 m/s at plasma nozzle the Reynold number would be

3300. That is still in transient domain and very close to turbulent condition.

Chapter 3.

83

7-8- Particles Reynold number and drag force calculation

As mentioned previously, the Gidaspow model was used as drag model in Comsol

Multiphysics. To calculate the drag force the first step is particle Reynold number calculation

according to equation 32 below:

ReP =∅cddρc|uslip|

μc Equation 32

∅c is the volume fraction of continuous phase (gas), dd is the particle average diameter (m), ρc

is the gas density (kg.m-3), uslip is the difference of velocity between two phases (m.s-1) and μc

is the dynamic viscosity of continuous phase (Pa.s).

According to high frame rate imaging at the exit of the Wurster-tube (riser) the particles velocity

was evaluated around 6m/s and according to anemometry measurements the gas phase velocity

is about 10m/s. which give uslip equal to 4m/s. According to Comsol simulation the volume

fraction of the particles at the exit of Wurster-tube is 0.0015. Therefore, by using equation 32,

the Reynold number of particles at the exit of Wurster-tube is equal to 183.

According to Gidaspow the drag force coefficient is calculated according to equation below:

Cd = {

24

ReP

(1 + 0.1ReP0.687) ReP < 1000

0.44 ReP > 1000

Equation 31

So the drag force coefficient is 0.6. Then this value is replaced in equation 30 where is the

Gidaspow coefficient.

β =3∅c∅dρcCd

4dd|uslip|∅c

−2.65 Equation 30

After replacing all the parameters the value of β is 1.13. According to Gidaspow the drag force

applied to a particle at exit of Wurster-tube is calculated by multiplying this value (β) by uslip:

Fdrag,c = −Fdrag,d = βuslip = 𝟒. 𝟓𝟐 𝐍 Equation 29

In our system when a particle is in suspension, there are two forces applied to each particle.

Drag force and the weight of the particle because of gravity. The average diameter of particles

is 700µm. By assuming the particles as perfect spheres with a density of 950 kg/m3 for

polypropylene particles, the mass of each particle is calculated to be about 0.17 µg which is

equivalent of 1.66 N. At the fountain region the balance of the forces applied to the particles is

Chapter 3.

84

zero because the particles have the velocity of zero. So the drag force is equal to weight of

particle which means 1.66 N. The height of the fountain region is about 60cm and the drag force

is decreased in comparison with Wurster-tube output area because the gas velocity is lower.

8- Hydrodynamic Modeling of a spouted Wurster fluidized-bed reactor with

Silicon Carbide (SiC) particles by Comsol Multiphysics

In this section, a new design spouted Wurster fluidized-bed reactor coupled with a DBD plasma

system was modeled by Comsol Multiphysics. We have used 10 g of silicon carbide particles

with an average diameter of 150 µm as a sample powder to characterize the hydrodynamics of

the system. The velocity profiles and volume fraction of both phases (gas and solid particles)

were calculated by using the two phase Euler-Euler model as previously. This reactor works at

atmospheric pressure and Argon gas was used to generate the plasma. The maximum capacity

of this reactor is 20 g of SiC particles. This reactor was used in the last chapter to decontaminate

peppercorn particles; however, the latter could be also used for polymers plasma treatment

application like surface activation and deposition of thin layers at atmospheric pressure.

Chapter 3.

85

8-1- Experimental Setup

Figure 3-20: Experimental setup of the spouted Wurster fluidized-bed reactor

As demonstrated in Figure 3-20 an Erlenmeyer flask in Pyrex with 1 liter volume was use as

main body of the reactor. At bottom of the reactor, the DBD plasma jet system was built with

an internal high voltage electrode and an external ground electrode and the glass tube works as

the dielectric between both electrodes (Figure 3-21). The argon flow was introduced through

the high voltage electrode. At the bottom of the Erlenmeyer, a conical piece designed by

Solidworks software and printed out by using a Formlabs 3D printer was installed. We have

used this conical piece instead of a gas distributer in order to obtain a conical spouted bed, i.e.

all the particles are directed to the center-bottom of reactor where the gas flow (plasma) is

coming. The Wurster tube was located at 13mm from the gas entrance inside the Erlenmeyer.

The SiC particles are sucked up inside this tube because of venturi effect and even in some

cases when the appropriate power and gas flowrate are applied, the argon plasma was

propagating inside the Wurster-tube probably because of (µ)discharges between SiC particles...

With this kind of plasma spouted Wurster fluidized-bed reactor one can have a good control on

particles circulation in the place of random fluidization. The homogeneity of treatment is

ensured and all the particles have more and less the same plasma treatment time.

Chapter 3.

86

Figure 3-21: Picture of the DBD plasma jet system

8-2- Gas velocity calculation

A rotameter was used to control the argon flow rate. The experimental appropriate flowrate

value to make a uniform SiC powder circulation was found equal to 1.6 l/min. So the argon

velocity could be calculated as follow:

𝑣 =𝐷

𝑆 Equation 40

Where:

𝑣: The gas velocity at entrance of reactor (m/s)

D: The gas flowrate at entrance of reactor (m3/s)

S: The entrance section area (m²)

The gas entrance diameter is 4mm, so:

𝑣 =1.6 ∗ 10−3

𝜋 𝑑2

4

∗1

60= 2.14 𝑚/𝑠

The rotameter was calibrated for nitrogen, therefore the argon flow rate was corrected according

to the following equation, with a gas conversion coefficient of 1.4 for nitrogen and 1 for argon:

𝑣 = 2.14 ∗Nitrogen Conversion coefficient

Argon Conversion coefficien= 3 𝑚/𝑠 Equation 41

https://fr.wikipedia.org/wiki/Pi_(lettre_grecque)

Chapter 3.

87

8-3- Silicon carbide (SiC) particles characterization

The geometry and morphology of SiC powders influence significantly the fluidization of these

powders in fluidized-bed. Laser diffraction analysis have been performed to characterize the

size distribution of particles. Scanning Electron Microscope (SEM) analysis were performed to

characterize the morphology and shape of these powders.

According to laser diffraction measurements (Figure 3-22) the majority of particles have an

average diameter of 150 µm. Figure 3-23 shows the SEM photography of SiC particles. The

sizes of some the particles is estimated by green lines as well.

Figure 3-22: Laser analysis for SiC particle size distribution

Chapter 3.

88

Figure 3-23: SEM analysis of SiC particles

According to Geldart classification of particles for fluidization (Figure 3-24), the SiC particles

with an average diameter of 150 µm and density of 3.21 gr/cm3 are in class B which means that

they are relatively easy to fluidize.

Figure 3-24: Geldart’s classification showing the SiC particles

Chapter 3.

89

8-4- Reactor geometry design for Comsol Multiphysics Modeling

Figure 3-25 shows the spouted Wurster fluidized-bed reactor geometry developed by Comsol

Multiphysics. We aimed to develop a two-dimensional numerical model. The white rectangles

present the Wurster-tube walls with an inner diameter of 5mm and a length of 6 cm. The gas

entrance diameter is 4 mm and the reactor height is about 25 cm. The red part is the conical

piece printed out by 3D printer, filled with powders, which has the height of 35 mm and same

diameter as Erlenmeyer entrance, 54 mm.

Figure 3-25: Reactor schematic and dimensions developed by

Comsol Multiphysics

8-5- Model Approach and boundary conditions

A two-phasic laminar flow model was developed using the Euler-Euler approach for argon gas

as the continuous phase and SiC solid particles as the dispersed one.

According to model, we obtained the volume fraction of SiC particles in different zones of the

reactor and velocity profiles of both phases as well. We have developed this model for a time

period of 5s with a time step of 0.06s because the SiC particles circulation stabilizes in less than

one second.

Chapter 3.

90

The initial conditions, boundary conditions and wall positions are described in Figure 3-26. The

whole spouted fluidized-bed reactor is divided to two parts. The first part is conic piece where

the SiC particles are present at t=0 with a volume fraction of 0.5. The second part is the

Erlenmeyer body which contains no solid particle at t = 0.

Continuous Phase

Argon gas

Density = 1.73 kg/m3

Dynamic viscosity = 2.23x 10-5 Pa.s

Dispersed phase

Silicon carbide SiC Solid particles

Density = 3 210 kg/m3

Particles average diameter = 150 µm

Figure 3-26: Initial values and boundary conditions

8-6- Particles behavior inside the reactor

At t = 0s, the SiC particles are stagnant at conical part of the reactor with a volume fraction of

0.5 at this region. Once the argon flow with a velocity of 3 m/s feeds the reactor, some of the

particles are sucked up inside the Wurster-tube because of drag force and Venturi effect. The

particles rise up inside the Wurster-tube and at t = 0.12 the first particles come out of tube and

start to make a fountain region. Figure 3-27 presents the volume fraction evolution of dispersed

phase at initial time.

Chapter 3.

91

Figure 3-27: Volume fraction evolution of SiC particles

As shown in Figure 3-28, at t = 0.78s the particles circulation is already stabilized. The fountain

region of the powders is located at z = 15cm from the bottom of the reactor. This value is in

good agreement with experimental one (z = 16 cm).

Figure 3-28: Height of the fountain region with SiC particles (d = 150 µm) and an argon gas velocity

of 3 m/s A) Experimental result B) Comsol modeling result

8-7- Hydrodynamic results of Model

Figure 3-29 shows the gas and solid velocity profiles and the volume fraction of the dispersed

phase through the spouted Wurster fluidized-bed. According to Figure 3-29A, the initial gas

velocity is 3m/s. At the Wurster tube entrance the maximum gas velocity is observed. The

reason is attributed to powders agglomeration in this part that leads to a thinner section for the

gas flow. At the output of Wurster-tube (7cm), the gas velocity decreases and at fountain region

(15cm) the particles velocity is zero. According to Figure 3-29B, the maximum velocity of SiC

particles reached about 1.5m/s at exit of the Wurster-tube. Figure 3-29C shows the volume

Chapter 3.

92

fraction of the dispersed phase at different height in the reactors. These data confirm that the

fountain region is located at z =15cm as well. Figure 3-30 shows the velocity profile of the

dispersed phase in the lower part of the reactor. The arrows show the particles trajectory inside

the reactor.

Figure 3-29: A) Velocity profile, continuous phase B) Velocity profile, dispersed phase

C) Volume fraction, dispersed phase

Figure 3-30: Velocity profile 2D, dispersed phase

8-8- Residence time of SiC particles inside Wurster-tube

As mentioned previously, the plasma propagates inside the Wurster-tube, therefore this whole

part is the main treatment zone. The residence time of the SiC particles is estimated as below:

𝜏 =𝑉

𝑄𝑉 Equation 42

Where τ is Particle residence time (s), 𝑄𝑉 dispersed phase flow rate (m3/s) rate and V the volume

of Wurster-tube (m3). We have considered 1.4 m/s as average dispersed phase velocity inside

Chapter 3.

93

the Wurster-tube with the diameter of 6 mm. The residence time is calculated 43 ms which

means every particle passes 43 ms inside the main treatment zone during each cycle.

9- Conclusion

An isothermal, turbulent Euler-Euler two-phase model in Comsol Multiphysics was developed

to understand the trajectory of the PP particles inside the reactor. The Multiphysics interface

solves two sets of Navier- Stokes equations, one for each phase, in order to calculate the velocity

field of each phase. The PP particles were considered as dispersed phase and air as the

continuous one. Among the different turbulent models such as k-ε, k-ω, k-ε low Reynold

number and SST (Shear Stress Transport) applied for plasma gas flow simulation we selected

the k-ε model because it was in good agreement with the experimental gas velocity profile

measured by hot-wire anemometry. The Gidaspow model was used to calculate the drag forces.

The velocity profile of both phases, i.e. gas and polypropylene (PP) particles, and the

concentration of PP particles in different regions of the Wurster fluidized bed reactor were

obtained by this model. According to the pressure profile result obtained by model there is a

low pressure zone at the vicinity of the entrance of the Wurster-tube where PP particles are

sucked up inside the plasma zone because of Venturi effect.

Relying on the CFD modeling results, the height of the “fountain” zone is about 70 cm. During

the plasma treatment of PP particles the height of the fountain zone was measured to be 70 cm

which means that the simulation is in relatively good agreement with the experimental results.

By combining the concentration profile of PP particles developed with Comsol Multiphysics

model and experimental velocity measurements with high speed photography, the calculated

particle flow rate inside the Wurster-tube (plasma zone) was 5.7 g/s. Finally, for a total

treatment time of 120 s, one can estimate that in average each particle passed 3 to 4 times inside

the Wurster-tube and therefore was treated 3 to 4 times by the plasma.

A non-isothermal, turbulent Euler-Euler model was developed to characterize the heat transfer

inside the reactor. The Comparison of the temperature profile calculated by the model and the

one measured by three different ways i.e. thermocouple, infrared thermography and Schlieren

photography represents 16% differences between modeling and experimental results. This

difference is probably due to the fact that i) the plasma jet was considered to be an air flow at

the same temperature, ii) the physical properties of air (k, λ and Cp) were considered as

remaining constant with the temperature, which is strictly not correct. Finally, one can conclude

Chapter 3.

94

that our CFD non-thermal model is in relatively good agreement with the experimental

measurements.

As future prospects for the research project, a precursor injection system will be added to the

plasma nozzle to deposit thin layers on the surface of micrometric particles by PECVD. The

other important prospect would be to use the plasma-treated polymeric powders with our

Wurster fluidized bed reactor as primary substance for rotational molding process and selective

laser melting process (SLM). Indeed the mechanical resistance of the objects built by these

technologies must be investigated by comparing non-treated and plasma treated polymer

powder.

A CFD isothermal two phase model was also developed for a small scale spouted Wurster

fluidized bed reactor. A reactor with a similar conception was used to plasmatreat peppercorn

particles for disinfection application, presented in Chapter 4. The Eulerian-Eulerian approach

was used as in the previous reactor. The gas phase is argon and the dispersed one is the silicon

carbide (SiC). In this system we have applied the laminar equations to characterize the

circulation of the particles and their velocity profiles. The Gidaspow model was used to

characterize the drag force. According to CFD hydrodynamic model the height of the fountain

region was 15 cm and there was a good agreement with the experimental results (about 16cm).

Chapter 4.

95

Chapter 4

Atmospheric pressure plasma reactors for black

peppercorn microbial decontamination

Chapter 4.

96

1- Introduction

In the previous chapters (2 and 3), we have introduced a new-designed fluidized bed combined

with an air blown-arc plasma at atmospheric pressure for the treatment of polymeric particles.

In this chapter, we will describe a different application of plasma fluidized bed which is the

surface treatment of different spices and in particularly peppercorns, for microbial disinfection.

Plasma treatment of spices in the form of powders has the same challenges and difficulties

compared to polymeric particles. We have to design an appropriate system to put the different

particles in suspension and to treat the latter by atmospheric plasma. The Wurster fluidized-bed

reactor presented in previous chapters would not be appropriate for peppercorn treatment due

to the big amount of particles required and the high temperature of the plasma jet. Therefore we

have designed smaller plasma reactors to treat less amounts of particles with a more moderate

temperature in comparison to the previous plasma.

Black peppers are considered as a basic spice in food industry and they are highly contaminated

by bacteria and microorganisms. This contamination will accelerate the spoilage of food [112].

The conventional disinfection processes that do not influence the color and aromatic properties

of peppers are ozone treatment, thermal inactivation, steam sterilization, radiation technologies

etc. Radiation technology is hard to be applied and expensive [113,114]. The anti-microbial

property of low temperature plasmas was investigated by different groups [115-119]. Kim et al.

[120] have studied the microbial decontamination of red pepper powder by cold plasma. They

used nitrogen as operating gas to generate plasma with a power of 900 W and operating pressure

of 667 Pa. After 20 minutes of plasma treatment of red pepper powder contaminated with

Aspergillus flavus and Bacillus cereus spores the number of A. flavus was reduced by 2.5 ± 0.3

log spores/g and B. cereus spores by 3.4 ± 0.7 log spores/g. Argyropoulos et al. [121] investigated

the decontamination of black peppercorn using a microwave-generated low pressure air plasma.

They showed that microwave plasma would be efficient for spice decontamination applications.

They used cellulose strips and glass plates homogeneously respectively sprayed with 106 and

107 CFU/g spores of Bacillus subtilis as testing samples. The operating pressure was 0.7 mbar

and treatment time was 40s. The microwave generator worked in a pulsed mode with a power

of 400 W. Aguirre et al. [122] investigated the effect of atmospheric pressure cold plasmas on

the inactivation of Escherichia coli in fresh products such as lettuce, carrots and tomatoes by

using argon plasma. They used SEM photography to investigate the influence of the surface

structure on bacterial inactivation by cold plasmas. Plasma treatment of peppercorn powders

would be more complicated in comparison to larger products because of the higher specific

Chapter 4.

97

surface of the powders. According to the microbial investigations of black peppercorn presented

in literature [124], high amount of microbial contamination on the surface of peppercorns was

reported (1.0*108 CFU/g). This is the total mesophilic aerobic count. The main spoiling

microorganisms on black peppercorns are aerobic spore-forming bacteria with a concentration

of 5.6*107 CFU/g with presumptive B. cereus spore with concentration of 5.6*107 CFU/g. The

human pathogens like C. perfringens, Staphylococcus and Salmonella were detected on

peppercorn. The results of microbial characterization of black peppercorn is presented in Table

4-1.

Microorganism Results (CFU/g)

Total mesophilic aerobic count Spores of mesophilic aerobes Presumptive B. cereus Clostridium perfringens Enterobacteriaceae Sulf-red anaerobes Escherichia coli Coagulase-positive Staphylococcus Molds and yeasts Salmonella (in 25g)

1.0*108 5.6*107

5.6*107

2.7*103 1.5*102 4.7*106 1.0*103 4.9*104 6.0*103

Detected

Table 4-1: Microbial characterization of whole black peppercorns

Hertwig et al. [123] investigated the influence of remote plasma treatment on natural microbial

load and parameters related to the quality of pepper seeds, crushed oregano and paprika powder.

They used air as operating gas. The remote plasma treatment reduced the native microbial flora

of the pepper seeds and the paprika powder by more than 3 log after 60 min of plasma treatment.

In another study Hertwig et al. [124] investigated the influence of two different atmospheric

pressure plasma systems on the microbial decontamination of whole black pepper. They used

two different plasma devices. A direct plasma treatment with a radio frequency (RF) plasma jet

and a remote treatment with a microwave generated plasma. The operating gases for RF plasma

and microwave devices were Ar (10 slm) and air (18 slm), respectively. The results of the direct

RF plasma treatment showed a much lower inactivation, probably due to different inactivation

mechanisms and to the complex surface structure of peppercorns. After 15 minutes of direct

plasma treatment with RF plasma jet, an inactivation of 0.7 log for the total mesophilic aerobic

count and 0.6 log for the total spore count was achieved. The S. enteric, B. subtilis spores and

B. atrophaeus spores were reduced to 4.1, 2.4, and 2.8 log, respectively after 30 min remote

microwave plasma treatment, whereas, direct RF plasma jet did not result in equivalent

Chapter 4.

98

inactivation levels. However, important other parameters of the peppercorn such as color,

piperine and volatile oil content were not significantly affected.

Atmospheric pressure plasma systems have lower costs in terms of starting equipment as

compared to low pressure plasma ones because in the case of the former there is no need for

vacuum devices such as pumps and vacuum vessels. Therefore atmospheric pressure systems

have shown more potential to be scaled up and to be used in various applications. In recent

years some groups have investigated the efficiency of DBD [125-137] and glow discharge plasmas

[138-148] at atmospheric pressure for bacterial deactivation. Hetal K Bhatt et al. [149] have

published a review article about microbial decontamination of fruits, vegetables and spices with

different non-thermal plasma systems at atmospheric pressure.

The aim of this chapter is to determine the efficiency of different plasma reactors in

decontaminating peppercorns supplied from France, which are naturally contaminated with

mesophilic aerobic micro-organisms, and which, as we will see, include sporulated micro-

organisms as well. The bacterial spores are known to be difficult to remove. Basically we have

used two different plasma systems. The first one is a blown-arc rotary plasma jet at atmospheric

pressure which uses air to generate a relatively high temperature plasma. The second system is

a non-thermal DBD (dielectric barrier discharge) which uses mixtures of Ar with a small

amount of O2 (2%). Different reactor constructions, working gas, treatment time and sample

morphologies were investigated with their efficiency on spore removal and microbial

decontaminations.

2- Sample characterization and selection of spices

L.H.McKee et al. [150] reported high amounts of bacteria and molds in spices without any

decontamination processes, especially for black pepper the total bacterial count can reach even

107 CFU/g. Therefore black pepper is one of the spices with the highest bacterial loads, being

also one of the most used worldwide one and an adequate sample for this research work. Along

with black pepper, mustard seed were also treated.

The other important points to be considered in designing fluidized beds for treatment of spices

are the shape and the density of the particles, to minimize elutriation by the fluidizing gas during

the treatment. Therefore for this study the black and white pepper, as well as white (or yellow)

and black mustard seeds spices shown in Figure 4-1 were chosen.

Chapter 4.

99

Spices were obtained in bulk with a local intermediary distributor (Gare du Nord, Paris, France)

and they were analyzed microbiologically in order to choose the most contaminated one.

Bacteria were determined as the total mesophilic aerobic organism with the Aerobic Plate Count

Method (APC) and Fungi were determined as Molds and Yeasts. All the biological analysis

were carried out at LRS (laboratoire de réactivité de surface), Sorbonne University.

Figure 4-1: Samples of Spices used: A) Black mustard seeds, B) White mustard

seeds, C) Black peppercorn (broken), D) White peppercorn (broken).

2-1- Dilution procedure

The total amount of each spice (200 g) was collected in a sterile flask or in a sterile plastic bag

and was then homogenized by agitation. Then 10g was withdrawn and placed in a sterile

sampling plastic bag. 90 mL of 0.1% sterile peptone water, a microbial growth medium

composed of peptic digest of animal tissue, was added to the weighed sample to achieve 10-1

dilution, then was homogenized by shaking the bag for 2 min. Then appropriate 1:10 dilutions

were made by adding 1 mL of the solution in tubes containing 9 mL of 0.1% peptone water

until a dilution of 10-6 was obtained.

2-2- Aerobic Plate Count Method

All diluted solutions were shaken 25 times in 30 cm circles during 7 s, and by avoiding sampling

foam. 1ml of each dilution was pipetted into separate, duplicates, appropriately marked petri

dishes. The dilution bottle was reshaken 25 times in 30 cm circles during 7 s if it was still more

than 3 min before it is pipetted into the petri dish. 20-25 ml plate count agar was added and

heated to 45 ± 1°C and added to each plate within 15 min from the original dilution. Agar and

dilution water controls were poured into plates for each series of samples. The sample dilutions

A B

C D

Chapter 4.

100

were immediately mixed with the agar medium thoroughly and uniformly by alternate rotation

and back-and-forth motion of plates on flat level surfaces. The agar was let to solidify, then the

solidified petri dishes were inverted, and incubated promptly for 48 ± 2 h at 35°C.

All aerobic plate counts were computed from duplicate plates containing at least 25 colonies

but no more than 250 as estimated counts. Counts outside the normal 25-250 range may give

erroneous indications of the actual bacterial composition of the sample. Dilution factors may

exagerate low counts (less than 25), and crowded plates (greater than 250) may be difficult to

count or may inhibit the growth of some bacteria, resulting in a low count. The counts less than

25 or more than 250 colonies were reported as estimated aerobic plate counts (EAPC). The

latter counts all the colonies, except those that correspond to molds, if they do appear in this

medium.

2-3- Yeast and Mold measurement

All dilutions should be shaken but in a way to avoid sampling foam. By using sterile cotton-

plugged pipet or sterile micropipette, 1.0 ml portions of sample dilution were placed into pre-

labeled 15 × 100 mm Petri plates (plastic or glass), and immediately 20-25 ml of tempered PDA

(Potato Dextrose agar, CONDA, Spain) was added to petri dishes containing sterile tartaric acid

(1.4 mL of tartaric acid 10% w/v by each 100 mL of medium, to get a pH approximately of

3.5). Acidic pH inhibits bacteria while promoting the growth of molds and yeasts. The contents

were mixed by gently swirling plates clockwise, then counterclockwise, taking care to avoid

spillage on dish lid. After adding the sample dilution, Agar was added within 1-2 min;

otherwise, dilution may begin to adhere to the bottom of the dish (especially if the sample

contains a high starch content and dishes are in plastic) and may not mix uniformly. Each

dilution was plated in duplicate. From the preparation of the first sample dilution to pouring or

surface-plating of final plate, no more than 20 min (preferably 10 min) should elapse. Petri

dishes were incubated for 3-5 days at 25°C. Colonies with creamy or moist appearance

corresponded to yeasts while mold colonies would present a filamentous and large.

Chapter 4.

101

2-4- Microbiological analysis of selected spices

Results shown (Table 4-2, Figure 4-2) that the highest contamination corresponds to black

pepper, which agrees with the range of 106 – 107 CFU/g reported by other authors [124,150] for

the mesophilic aerobic count for this spice. White pepper showed less contamination as

compared to the black one, which is quite normal since black peppercorns are picked from the

plant and sun-dried, while the white peppercorn is produced by removing the outer layer,

leaving only the inner seed.

Figure 4-2: Comparison of the growth observed in the plates of Mesophilic Aerobic Organisms for

the different spice samples (dilution 1: 100, Plate Count Agar). A) Black Pepper, B) White Pepper,

C) Black mustard seeds, D) White mustard seeds.

Although it was not the maximum amount reported for black pepper, the load found was

adequate to test a disinfection technique. On the other hand, no yeasts were found and the

amount of molds was low, so the effectiveness of the process could only be measured with

respect to the mesophilic aerobic count. Mustard seeds have regular shapes and an adequate

size but a low contamination for our purpose of our study.

Spice

Mesophilic Aerobic

Count

(CFU/g)

Molds

(CFU/g)

Yeasts

(CFU/g)

Broken black pepper

8.2 x 106 5 x 102 < 102

Broken white pepper

6.7 x 104 2 x 102 < 102

Black mustard seeds

9.0 x 102 8 x 102 < 102

White mustard seeds

5.5 x 102 < 102 < 102

Table 4-2: Results of microbiological analysis of Spices obtained in France. (< 102 means no

growth at the detection limit)

A B C D

Chapter 4.

102

3- Determination of the percentage of spore-forming bacteria in black pepper

Two grams of each pepper type (whole and broken pepper) were placed in sterile test tubes and

18 milliliters of sterile 0.1% peptone water was added. As shown in Figure 4-3 the tubes were

placed in water at 80°C for 12 minutes to destroy the bacterial vegetative forms [151].

After heat treatment, the tubes were quickly cooled down by placing them in cold water and

decimal dilutions (until 10-5) were made in sterile 0.1% peptone water. One milliliter of each

dilution was placed into separate, duplicates, appropriately marked petri dishes, 20-25 ml plate

count agar (Standard Methods Agar PCA, CONDA, Spain) cooled to 45 ± 1°C was added to

each plate within 15 min of original dilution. Sample dilutions and agar medium was

immediately mixed thoroughly and uniformly by alternate rotation and back-and-forth motion

of plates on flat level surface. Then the medium solidified petri dishes were inverted and

incubated promptly for 48 ± 2 h at 35°C. As a sterility control, a single agar plate and one with

one milliliter of peptone water and agar were also poured.

Figure 4-3: Thermal destruction of bacterial

vegetative forms present in suspensions of

black French peppercorn broken and whole

(80°C/12 min).

At the end of the incubation all the colonies on the plates were quantified. The growth

correspond to the spores that endure the heat treatment. In order to obtain the proportion of

spores in the sample, the determination of the total bacterial load (without heat treatment) of

the whole and broken pepper samples was carried out at the same time.

Chapter 4.

103

The results of the quantification of bacterial colonies corresponding to aerobic mesophilic

microorganisms in the samples treated with heat and without treatment were used to obtain the

percentage of spores present in the two types of black pepper (Table 4-3). The calculation was

made using the following formula:

% of spores = (N1x100) / N0

N0 is the number of aerobic mesophilic microorganism colonies in the samples without heat

treatment.

N1 is the number of aerobic mesophilic colonies found in the samples subjected to the heat

treatment, 80 ° C / 12 min.

Sample

Mesophilic

Aerobic Count

(CFU/g)

Mesophilic

Aerobic Count

Treated sample*

(CFU/g)

% spores

Broken Black Peppercorn

12.6 x 106 91 x 105

72.22

Whole Black Peppercorn

43 x 104 24 x 104 55.81

Table 4-3: Results of microbiological analysis of broken and whole French Black Peppercorn

samples with and without thermal treatment to determine the percentage of spores present.

*Treatment: 80°C/12 min

The results in Table 4-3 show that both whole and broken pepper have a high percentage of

spores (above 50%), which can be attributed to their origin and management, since they come

from plants that naturally have contact with soil, which is considered to be the largest reservoir

of spore-forming microorganisms. Furthermore a higher percentage of spores was found in the

broken pepper (72%), which is probably due to manipulation of the latter, already reported by

other authors [150-152]. The higher specific area of the broken pepper as compared to whole

pepper can explain the higher number of spores measured on the former.

In addition, the low water activity of spices in general (≤0.6) [153] as compared to other grains

can explain why more spores have been measured as compared to, other microorganisms, since

in dry media the sporulated form are favored. Water activity or aw is the partial vapor pressure

of water in a substance divided by the standard state partial vapor pressure of water. This

parameter has no unit. In the field of food science, the standard state is most often defined as

the partial vapor pressure of pure water at the same temperature. Using this particular definition,

https://en.wikipedia.org/wiki/Partial_pressure

https://en.wikipedia.org/wiki/Vapor_pressure

https://en.wikipedia.org/wiki/Vapor_pressure_of_water

https://en.wikipedia.org/wiki/Food_science

https://en.wikipedia.org/wiki/Temperature

Chapter 4.

104

pure distilled water has a water activity of exactly one. Higher aw substances tend to support

more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7.

4- SEM of broken black peppercorn

SEM analyses were performed to observe the bacteria on the surface of broken French

peppercorn. Microbial analysis of broken peppercorn showed a high microbial load (approx. 1

to 10 x106 CFU/g), so it could be possible to see the microorganisms on pepper surface. SEM

images (Figure 4-4) show many microorganisms on the pepper. There are different kinds of

bacilli and cocci of different sizes. Cocci bacteria with spherical irregular shape and bacilli

bacteria with cylindrical structures are observed on the surface of broken peppercorn. The

regular hexagonal prismatic patterns are the natural structure of peppercorn (Figure 4-4B). The

shrinkage of Cocci bacteria structure could be due to the dehydration step while the bacilli are

more resistant and can be seen in their normal shape.

A) a broken peppercorn B) Prismatic natural structure of peppercorn

C) Bacili bacteria B) Cocci bacteria

Figure 4-4: SEM photography of broken black peppercorn

https://en.wikipedia.org/wiki/Distilled_water

https://en.wikipedia.org/wiki/Microorganisms

https://en.wikipedia.org/wiki/Bacterium

https://en.wikipedia.org/wiki/Fungi

Chapter 4.

105

Standard SEM procedures for biological samples involves chemical fixation,

drying/dehydration, coating sample with a metal (e.g. chromium, gold, platinum, etc.) for

examination under a conventional SEM apparatus. The fixation, drying/dehydrating steps need

to be done as carefully as possible to reduce shrinkage while ensuring preservation of cell

structures as close to the natural shape as possible. To avoid this problem the CPD (critical

point drying) technic was developed and is the most commonly used dehydrating method for

biological sample preparation. This procedure removes liquids from the specimen and avoids

surface tension effects (drying artefacts) by never allowing a liquid/gas interface to develop.

The transition from liquid to gas at the critical point takes place without an interface because

the densities of liquid and gas are equal at this point [154].

5- Plasma rectors for peppercorn treatment

In this section two different plasma reactors are presented for peppercorn disinfection. The first

system, modeled in the last part of chapter 3, is a spouted fluidized bed combined with a DBD

plasma (system A) and the second system is a rotary blown-arc atmospheric pressure plasma

jet (system B). Black peppercorns were plasma treated in both systems and microbial analyses

were carried out to investigate the antibacterial effect.

5-1- System A (DBD spouted Fluidized-bed)

This reactor is a 1 liter Erlenmeyer, a plastic part printed with 3D printer (FormLab) which is

placed at the bottom of system to convey the pepper particles to the plasma zone, and a DBD

plasma system (Figure 4-5). We have used 25 slm Ar mixed with 2% O2 (500 sccm O2) to

generate the plasma and also fluidize the peppercorns. The plasma is generated at the bottom

of the spouted fluidized bed with a glass tube having a diameter of 8mm and two outer

electrodes around. Mixtures of Ar and O2 pass through this tube between the high voltage and

the grounded electrode. A 10 W power was applied using an AC HV (high voltage) power

supply (SG2 STT Calvatron), composed of a corona generator and a HV transformer, working

in the 15-50 kHz frequency range. Each copper electrode has a width of 5mm and the distance

between the electrodes is 1 cm. These dimensions are the optimized ones to obtain a long plasma

jet (about 3cm) and to avoid arc formation. The plasma jet can extend even inside the spouted

fluidized bed. At the top of the spouted fluidized bed, a grid is placed at the output to avoid

peppercorns loss.

Chapter 4.

106

Figure 4-5: Spouted fluidized bed coupled with a DBD plasma system (system A)

5-2- System B (Rotary air-APPJ)

System B is composed of a curve-shape container (500mL) and a rotary blown-arc atmospheric

pressure plasma jet (RD1004 from PLASMATREAT – Figure 4-6). The setup configuration is

demonstrated in Figure 4-7. The patented rotary system evenly distributes the plasma effect on

the materials to be treated. Here, the surface is swept several times briefly in a pulse-like

manner, which is a very effective form of cleaning and activation with very low heat input at

the same time. Compressed air was used as plasma gas. The plasma was generated by using a

FG5001 DC generator with the voltage of 270 V and frequency of 19 kHz. The plasma cycle

time (PCT) was adjusted to 40% to generate a pulsed plasma and minimize the gas temperature

which is important to prevent thermal degradation of the organoleptic properties of

peppercorns. According to factory calibration of this generator, the power corresponding to this

conditions would be about 300W. The rotary plasma torch was placed at top of the system and

covered by a grid to avoid any loss of particles out of the reactor. The glass reactor was washed

with ethanol. Furthermore to avoid cross contamination, between each treatment the reactor

was cleaned with compressed air.

Chapter 4.

107

Figure 4-6: The blown-arc rotating atmospheric

pressure plasma torch (RD1004 from

PLASMATREAT)

Figure 4-7: Rotary blown-arc atmospheric pressure plasma system (system B)

Chapter 4.

108

6- Plasma treatment black peppercorns

6-1- Black peppercorn plasma treatment with system A

7 g of broken peppercorn was loaded into the DBD spouted fluidized-bed reactor. The broken

black peppercorns were fluidized and treated by plasma for 0.5, 2, 5 and 10 minutes. Ar and Ar

+ O2 mixtures were used to generate the plasma. The treated samples as well as the control

samples (untreated ones) were analyzed microbiologically to determine the bacterial and mold

load.

6-2- Microbiological Analysis Results (System A)

The microbiological analysis results did not show significant differences between plasma

treated and non-treated peppercorns in term of microbial concentration (Table 4-4). The

maximum log reduction of MAC is 0.32 log which is obtained for 0.5 minutes of plasma

treatment while the maximum log reduction of Molds is 0.25 log for 10 minutes of plasma

treatment. Therefore system A seems to be inefficient for black peppercorn decontamination.

The main reason is attributed to the presence of a high amount of spore-forming bacteria on

black peppercorns. We have shown above in Table 4-3 that among all of the mesophilic aerobic

counts on broken black peppercorn, 72% are spore-forming bacteria. High resistance of these

spore-forming bacteria, because of their natural structure in comparison to vegetative state of

bacteria, made the decontamination process not efficient. The other reason for system A

inefficiency is attributed to low UV and ozone emission of Ar + O2 DBD plasma for spore-

forming bacteria removal. The emission spectra of this type of plasma will be shown in the next

section.

Sample

Gas

Plasma treatment

(min)

Mesophilic Aerobic Count

(CFU/g)

Molds (CFU/g)

Log reductions

MAC Molds

7 g Broken black

peppercorn

Ar 10 13 x 106 5 x 101 0 0.25

Ar + O2 10 12 x 106 5 x 101 0 0.25

Ar + O2 5 11 x 106 13 x 101 0 0

Ar + O2 2 12 x 106 6 x 101 0 0.17

Ar + O2 0.5 4.7 x 106 6 x 101 0.32 0.17

Control Broken black peppercorn

- - 10 x 106 9 x 101 - -

Table 4-4: Results of microbiological analysis of broken peppercorns by DBD spouted

fluidized-bed (system A)

Chapter 4.

109

6-3- Black peppercorn plasma treatment with system B

7.0 g of broken pepper was weighted. Each sample was carefully placed in the reactor and

subsequently treated with the plasma for the corresponding time. At the end of the treatment,

the reactor was opened and the sample was poured and collected into sterile plastic bags. This

procedure ensures the integrity of the samples and the effect of their processing.

7g of each sample of whole and broken peppercorns were treated with the rotary torch air

plasma reactor equipment for 2 and 4 minutes. The treated samples as well as the control

samples were analyzed microbiologically to determine the bacterial and mold load.

6-4- Effect of the Plasma Treatment on the appearance (System B)

As shown in Figure 4-8, the treatment affected the color of the broken pepper producing a

golden brown coloration more noticeable in the sample treated for a longer time (4 minutes).

This change can be observed specially in the internal parts of the grain because they have a

lighter color and are exposed after the pepper is broken. The blown arc rotary discharge in air

is characterized by i) a relatively high temperature as shown previously in chapter 3, ii) the

presence of NOx and other oxidative species. . The physicochemical properties of the plasma

induce the color change of pepper. However whole peppercorns showed no noticeable changes

in their color because of their dark outer layer.

A) Control sample, without treatment B) 2 minutes air-plasma treatment C) 4 minutes air-plasma treatment

Figure 4-8: Effect of the air plasma treatment on the color of sifted broken French Black Peppercorn samples

(voltage = 270 V, frequency = 19 kHz, PCT = 40%)

6-5- Microbiological Analysis Results (System B)

As shown in Table 4-3, whole pepper has a lower microbial load than broken pepper. The

treatments with air plasma have an antibacterial effect on black pepper. The treatment allowed

one log reduction for the treatment of 2 minutes and almost two log reduction in 4 minutes in

the broken pepper. For whole pepper the same tendency is observed, with 2 minutes of

Chapter 4.

110

treatment, approximately one log reduction was obtained, a value slightly higher than that

obtained in broken pepper.

Regarding molds, the greatest reduction was observed (1.63 Log) for the longest treatment (4

minutes) with broken pepper. In the case of whole pepper a total elimination of the molds was

observed for 2 minutes of treatment, probably favored by the lower initial quantity of molds in

this sample.

These results are interesting in the aspect of plasma treatment time. For example Hertwig et al.

[124] have demonstrated that within the first 15 min of plasma treatment a fast reduction of 1.7

log for the total mesophilic aerobic count and of 1.4 log for the total spore count was observed.

A final inactivation of 2.0 log respectively 1.7 log was achieved after 30 min of cold

atmospheric pressure plasma treatment. Our plasma system with 2 and 4 minutes plasma

treatment was sufficient to achieve quite similar results.

Sample

Sample weight

(g)

Plasma treatment

time (min)


(CFU/g)

Molds

(CFU/g)

Log reductions

MAC Molds

Broken black peppercorn

(particle size > 1.6 mm)

7.0

2

15 x 105

9 x 101

0.94 0.37

7.0

4

15 x 104

5

1.94 1.63

Whole black peppercorn

(average diameter 3mm)

7.0

2 34 x 103 0

1.1 1.39

7.0 4 28 x 102 0 2.18 1.39

Control Broken black peppercorn

-

-

12. x 106

21 x 101

-

Control Whole black peppercorn

-

-

43.2 x 104

2.5 x 101

-

Table 4-5: Results of microbiological analysis of whole and broken peppercorns by

rotary atmospheric plasma jet (system B)

Chapter 4.

111

6-6- Comparison of plasma active species in both discharge systems (A and B)

The objective of this section is to investigate the different species produced in both plasmas that

lead to the decontamination process, and to explain the different efficiencies. Chen et al. [155]

have investigated the emission spectroscopy of an air blown-arc atmospheric pressure plasma

jet (Arc zone) to characterize the active species in the range of 200-900nm. This plasma is

similar to our plasma system B. Sarani et al. [156] have investigated the emission spectra of an

Ar + O2 plasma to characterize the emission intensity in same range of 200-900nm which is

quite similar to our plasma system A.

The optical emission spectra of both plasmas are summarized in Figure 4-9. In the case of Ar +

O2 plasma the intensity of signal in the UV range from 200 to 280 nm was negligible and only

the band of OH radicals at 309nm was detected. Argon driven plasma jets emit a certain amount

of vacuum ultraviolet (VUV) light in the range of 110 to 200nm, however we were not able to

detect such photons with our spectrometer. The spectrum of air blown-arc atmospheric pressure

plasma showed emission in the UV range which could be a reason for the higher efficiency of

this plasma for bacterial decontamination process compared to previous systems. In particularly

the NO-band emission detected in the range of 200 to 290nm (UV) and atomic O, N at 500 to

900nm (Visible) would impact on microorganisms and can lead to their inactivation. Indeed,

the UV absorption of DNA has a maximum around 265 nm according to the literature as shown

on Figure 4-10 and UV-C and UV-B photons are very likely to damage the DNA of bacteria.[157]

Pulpytel et al. [158] characterized the post discharge zone of an air blown arc APPJ. They showed

that the UV emissions are substantially reduced compared to the arc zone spectrum The post

discharge of the air blown plasma jet was dominated by the intense NO2 chemiluminescence

continuum identified in the range of 450 to around 1000 nm. In the UV part of the spectra, one

can also clearly identify the presence of NO systems (α, β) and OH (A–X) radicals. NO2 can

lead to microbial inactivation via the production of nitrite and nitrate moieties which could

explain the main reason for the higher efficiency of this plasma for bacterial decontamination

compared to Ar + O2 DBD plasma system.

In air plasma different reactive species, such as nitrogen and oxygen species are generated

which have direct impact on microorganisms and can lead to their inactivation [159]. It is

necessary to distinguish between vegetative microorganisms and bacterial spores. Reactive

nitrogen species (NOx) can accumulate on the surface of vegetative microorganisms and diffuse

through the cell membranes resulting in the inactivation of the bacterial cell. Spore-forming

Chapter 4.

112

bacteria have multilayer structures and the low water content in the core of the spores makes

them extremely resistant to various chemicals, such as acids, bases and oxidizing agents. The

reactive nitrogen species might react with the small amount of free water in the core structure

leading to the formation of nitric and nitrous acid. Nitrous acid kills spores by DNA damage[160].

Figure 4-9: Comparison of emission spectra of Ar + O2 DBD atmospheric plasma and air blown-arc

atmospheric pressure plasma jet (Arc zone and post discharge zone)

Chapter 4.

113

Figure 4-10: UV absorption of native DNA and glycated DNA in incubated in various solution of

ribose. [157]

All the results of microbial decontamination of broken peppercorn with both plasma devices

are summarized in Figure 4-11. Fully cleaned peppercorn bar in this figure refers to the log

reduction necessary for 100% microbial decontamination of MAC. At this point of our study,

since the log reduction of MAC treated by both plasma devices was low, and in order to exclude

the pepper geometry and porosity, we decided to check the influence of the DBD plasma

(system A) and the arc air-blown discharge (system B) on the same microorganisms present on

the surface of pepper but inoculated on glass petri dishes.

Figure 4-11: Log reduction of broken black peppercorn contaminated with natural microorganism

0

1

2

3

4

5

6

7

8

Fully cleanedpeppercorn

System A,0.5min

treatment

System A,2min

treatment

System A,5min

treatment

System A,10min

treatment

System B,2min

treatment

System B,4min

treatment

Log

red

uct

ion

of

tota

l MA

C

Chapter 4.

114

7- Plasma treatment of inoculated Petri dishes with a dilution of black pepper

sample (natural microorganisms)

This test was performed to avoid the effect of black pepper geometry, morphology and porosity

on bacteria resistance against the plasma treatment. A dilution of the broken black Pepper

sample was prepared and 1 mL was placed on the surface of petri dishes. They were left to dry

in a laminar flow hood (biosafety cabinet) for 4 hours and stored in a hermetic container. The

initial concentration of the inoculum was measured and controls were carried out on petri dishes

and slides. The inoculated petri dishes were carefully handled to avoid contamination. These

inoculated petri dishes with natural bacteria of black pepper were plasmatreated with both

system A and system B. Plasma treatment of inoculated petri dishes is much easier than

peppercorns particles. There is no need of container in system B and also no need of spouted

fluidized-bed in system A. The plasma gas, power, flow rate, voltage, frequency and PCT are

the same values as used for peppercorn treatments. After treatment the microorganisms were

recovered from the surfaces using 10 mL of sterile diluting solution. In the petri dishes, a sterile

magnetic stirrer was placed and stirred for 10 minutes to detach the microorganisms. The

microbiological analysis results are shown in the Table 4-6. The rotary blown-arc APPJ (system

A) shows a total reduction of the microorganisms after 3 minutes of treatment in the petri dishes,

but it was also observed that the temperature of the bottom of the petri dish was high (approx.

120 ° C). The temperature was measured with an infrared thermometer camera for each

treatment time for both systems.

Plasma equipment

Gas

Plasma treatment

time (min)

Final Temperature of petri dish

(°C)


(CFU/surf)

Log reductions

System B

Air

3 120 0 4.77

1 70 4.5 x 102 2.12

0.5 40 8.5 x 103 0.85

0.25 30 27 x 103 0.34

System A Ar + O2 (2%) 5 45 15 x 103 0.6

2 35 40 x 103 0.17

Control sample (inoculated petri dish)

- - - 60 x 103 -

Table 4-6: Results of microbiological analysis of petri dishes inoculated with microorganisms from

French Black Peppercorn treated with both plasma systems

Chapter 4.

115

In the case of the DBD system (system A), a small extent of disinfection effect was observed

for the petri dishes, although the treatment time was longer (5 minutes). However, 3 minutes of

treatment with plasma system B (rotary air-APPJ) was enough to decontaminate completely the

surface of inoculated petri dishes. According to obtained results the log reduction of MAC was

4.77 log that is about 2 log higher than for peppercorn treated during 4 minutes with the same

system (2.18 log).

Both systems showed better decontamination results on petri dishes as compared to black

peppercorns particles. The reason is attributed to the complex structure of peppercorn that

would protect some colonies of bacteria during the plasma treatments. The other reason would

be the difference between the effective treatment times in these two cases. In the case of petri

dishes the substrate is fixed in front of plasma jet and the effective treatment time is equal to

the total treatment time, while in the case of particles the effective treatment time is much lower

(see chapter 3). This long plasma exposure (with system B) leads to the temperature increase

up to 120°C. Therefore both active species of the plasma as well as the higher temperature are

the reasons for the total disinfection observed.

8- Influence of petri dishes heating up on microbial decontamination

To distinguish the influence of plasma species and the temperature alone on microbial

decontamination, inoculated petri dishes were heated up to 120°C for 3 minutes. An electric hot

plate was used to maintain this temperature and the petri dish was placed on the plate. Once the

petri dish bottom temperature reached 120°C, measured by infrared camera, 3 minutes heating

up was performed. The microbiological analysis results obtained (Table 4-5) show that 3

minutes heating at 120°C removed 1.6 log of MAC present from the dilution of natural

contamination of black peppercorns. So as a conclusion, the combination of temperature as well

as the plasma active species makes the disinfection process more effective. All of the results

obtained with the two plasma devices as well as the heat treatment of inoculated petri dishes

are summarized in Figure 4-12.

Chapter 4.

116

Sample Heating time

(min)

Mesophilic Aerobic

Count (MAC)

(CFU/surface)

Log

reductions

MAC Reduction

(%)

Inoculated Petri

dish, heated up to

120°C

3 1.5 x 103 1.6 97.5

Control sample

Inoculated petri

dish, non-treated

- 60 x 103 - -

Table 4-5: Influence of petri dishes heating up on microbial decontamination

Figure 4-12: Log reduction of Petri dishes contaminated with natural peppercorn microorganisms

9- Plasma treatment of French Black Peppercorn samples inoculated with E.

coli or S. epidermidis

In order to compare the resistance of spore-forming bacteria present on the surface of French

black peppercorns and other non-spore vegetative state of bacteria, the black peppercorns were

sterilized and then inoculated with E. coli ATCC 25522 or S. epidermidis CIP 68.21. Both

plasma systems A and B were used to treat 7 g of each inoculated pepper sample. All the

operational parameters of plasma systems were the same as before. The microbial results of

inoculated peppercorns show higher decontamination effect (log reduction) compared to

peppercorns with natural microorganisms on the surface. The result of microbial analysis are

shown in Table 4-6. In the case of system B, 1 minute of plasma treatment was enough to

0

1

2

3

4

5

6

Fullycleaned

peppercorn

System A,2min

treatment

System A,5min

treatment

System B,0.25min

treatment

System B,0.5min

treatment

System B,1min

treatment

System B,3min

treatment

Heat up120°C,3min

Log

red

uct

ion

of

tota

l MA

C

Chapter 4.

117

reduce E. coli bacteria, a gram negative bacteria, to 99.95% which corresponds to 3.34 log

reduction, while S. epidermidis bacteria, gram positive one shows more resistance against the

decontamination process. Treatments with system A show lower decontamination effects. Five

minutes plasma treatment of peppercorns by system A leads to 100% reduction of E. coli. Both

systems showed more satisfying results for microbial disinfection of inoculated pepper samples

as compared to natural microorganisms of pepper. The reason is attributed to the complex

structure and self-protective properties of spore-forming bacteria present on black pepper

surface.

Plasma equipment Bacteria

inoculated

Plasma treatment

time (min)

Bacterial count

(CFU/g)

Log reductions

Reduction (%)

System B

E. coli 3 0 4.34 100

1 10 3.34 99.95

S. epidermidis 3 0 4.27 100

1 190 2.00 99.00

System A

E. coli 5 0 4.34 100

3 1 x 103 1.34 95.46

S. epidermidis 5 150 2.10 99.21

3 1. 4 x 103 1.17 92.64

Control sample Whole black peppercorn

E. coli - 22 x 103 - -

Control sample Whole black peppercorn

S. epidermidis - 19 x 103 - -

Table 4-6: Microbial results of plasma treatment of artificially inoculated black peppercorns

with E. coli or S. epidermidis

All the results for plasma treatment of black peppercorns inoculated with E.coli and S.

epidermidis with both plasma devices are summarized in Figure 4-13. S. epidermidis, the gram

positive microorganism, showed more resistance to plasma treatment in comparison to E. coli

which is a gram negative microorganism. The reason is attributed to different cell wall

structures of gram positive and negative microorganisms. Gram positive microorganisms have

a thicker cell wall compared to gram negative ones which make them more resistant to active

species of plasma [161,162].

Chapter 4.

118

Figure 4-13: Log reduction of peppercorn inoculated with E. coli and S. epidermidis

10- Conclusion

French black peppercorn was selected to be investigated because of its high natural microbial

contamination (about 107 CFU/g) in comparison to other analyzed alimentary grains. The

concentrations of mesophilic aerobic and spore-forming bacteria were measured for broken and

whole black peppercorns. The concentration of spore-forming bacteria in the peppercorn was

about 72% and 56% for broken and whole peppercorns, respectively.

7 g of French black peppercorns per batch were treated by two different plasma systems. The

first system was an Ar + O2 DBD atmospheric plasma jet (system A) and the second one was a

rotary air blown-arc atmospheric pressure plasma jet. System A was not efficient for broken

peppercorn decontamination and the best results achieved by system B, for 4 minutes of plasma

treatment, lead to a 2 log reduction of the total MAC. System B was much more efficient for

the decontamination process as compared to system A. The reason was explained as being a

combination of a higher emitting plasma in the UV region, the presence of NOx and other

oxidative species (such as OH) and a higher temperature during the treatments. NO2

chemiluminescence continuum identified by OES in the range of 450 to around 1000 nm,

presented in the post discharge plasma zone of system B was the main reason for the higher

efficiency of this system compared to system A. The reason for such a low decontamination

efficiency of both plasma treatments was attributed to the much higher resistance of spore-

0

1

2

3

4

5

Fully cleanedpeppercorn

System A, 3mintreatment

System A, 5mintreatment

System B, 1mintreatment

System B, 3mintreatment

Log

red

uct

ion

of

tota

l MA

C

Log reduction of peppercorn inoculated with E. coli and S. epidermidis

E. coli S. epidermidis

Chapter 4.

119

forming bacteria of MAC but also due to the porous structure of peppercorn surface which lead

to a difficult accessibility of plasma and active species to the bacteria colonies. Therefore in a

second step the petri dishes were inoculated with a dilution of microorganisms existing on the

broken black peppercorn in order to separate the effect of the morphology and the porosity of

peppercorns on the plasma treatment. These samples were plasma treated with both plasma

systems at the same operational conditions as before for peppercorn treatments. The log

reduction of microorganisms with system A, after 5 minutes treatment was only 0.6 while this

value for system B was 4.77 after 3 minutes treatment which corresponded to 100% reduction

of MAC. In order to understand the influence of the high temperature of the plasma in system

B, an inoculated petri dish was heated up to 120°C for 3 minutes. The heating process without

plasma treatment reduced the total MAC by 1.6log (97.5% reduction). Therefore, the

combination of heat transfer and active species of plasma made the decontamination process

efficient to remove all bacteria.

In the next step the black peppercorns inoculated with E. coli or S. epidermidis bacteria were

decontaminated with the two plasma systems. System B was able to decontaminate completely

(100% bacteria reduction) after 3 minutes plasma treatment. 5 minutes plasma treatment of

inoculated peppercorn with system A was enough to reduce E. coli by 4.34 log (100%

reduction) and S. epidermidis by 2.10 log (99.21% reduction). S. epidermidis (gram positive)

showed lower reduction during plasma treatment compared to E. coli (gram negative). The

reason is attributed to different cell wall structures of gram positive and gram negative bacteria.

The gram positive bacteria have a thicker wall cell compared to gram negative ones, making

the penetration of the disinfection agents more difficult in the bacterial cell. Both systems

showed promising results for decontamination of inoculated peppercorns. The reason is

attributed to lower resistance of E.coli and S. epidermidis organisms as compared to the native

spore-forming bacteria present on the surface of black peppercorn.

Due to the high contamination of broken black peppercorn, it is possible to observe a lot of

microorganism on the surface of peppercorns by SEM photography. The SEM analysis

demonstrated the presence of Bacilli and Cocci microorganisms on the surface of broken and

whole French black peppercorns.

As a perspective, combining the rotary plasma jet (system B) for spore-forming bacteria

decontamination, with a fluidized bed reactor would lead to a lower temperature increase of

peppercorns. In this case the plasma treatment time could be increased without changing the

organoleptic properties of peppercorn.

120

General Conclusion & Future Prospects

Plasma reactors are usually designed to treat flat surfaces. However, since several materials are

in the form of particles or powders, it is extremely important for many industrial applications,

to confer new surface properties to divided substrates. One can cite for example elaboration of

polymers, heterogeneous catalysis or more recently 3D printing. When treating flat substrates

(e.g. metal foils, glass sheets, silicon wafers etc.) or objects having more complex shapes (e.g.

smartphone cover, car bumper etc…), one can control exactly the treatment time and the

position of such objects with respect (exposed ) to the plasma. However, when treating

powders, like PP powders in this work, 200 grams of particles per batch correspond to roughly

1 million of particles; it impossible for each particle to control exactly the treatment time

and its position inside (or with respect to) the plasma. To treat powders/particles with

plasma, one needs first to design the reactor which enables to treat particles homogenously.

In this scope, we started first by developing a small reactor enabling to treat about 15 grams of

particles coupled with an argon DBD plasma jet. Some experiments were performed to

understand the behavior of the particles inside the reactor, and to develop the Euler-Euler

biphasic CFD model to describe the experimental results. Then, a Wurster fluidized bed reactor

was combined for the first time with an atmospheric pressure plasma jet to treat approximately

200 g of industrial polypropylene (PP) particles per batch characterized by a very broad

distribution, irregular shapes and a mean diameter of 700µm. Indeed, as compared to other

academic work, the particles treated in this thesis were far from having a monodisperse

distribution or spherical shapes.

The physical and chemical properties of PP powders were characterized before and after plasma

treatments by different methods. Being able to treat homogenously a batch of microparticles

was the main challenge in this thesis; however it is difficult to find quantitative measurements

to characterize the whole batch of treated particles. The dye adsorption test, which is a non-

quantitative method, confirmed that the PP particles were homogeneously treated in the reactor

because of the uniform adsorption of Rhodamine B by plasma treated particles. According to

wettability measurements based on the Zisman method, the surface energy of PP particles

increased from about 30.7 mN/m to 38.6 mN/m after 120s plasma treatment. This increase was

confirmed by XPS analysis of plasma treated PP powders which showed two additional peaks

at 287.1 and 288.7 eV, attributed to −CO and OC─O, respectively. The atomic concentration

of oxygen on the surface of polypropylene particles was increased by about 5% after the

121

plasma treatment. This increase of the oxygen content is in agreement with the dispersion of

plasma treated PP particles into water. According to laser diffusion technique a partial

disappearance of the very fine PP particles (dp < 100 µm) was observed after the plasma

treatment inside the reactor which was attributed to the elutriation process where these particles

were entrained by the gas flow outside the reactor and/or particles sticking to the reactor wall,

as observed experimentally.

The high temperature of the atmospheric pressure plasma jet induced also some small

modifications of the morphology and crystalline structure of the particles. According to

FTIR analysis, the isotactic structure of PP particles was increased by about 10%. This

modification is probably due to the fact that the high gas temperature inside the central part of

the Wurster tube is high enough may lead to a fast annealing/heating and then quenching in the

cooler parts of the fluidized bed leading to a recrystallization of the polymer. The important

conclusion is to point out that plasma treatments in air will modify the surface chemistry of

polymers (i.e. increase of surface energy) but depending on the type of plasma and polymer it

is also possible to modify the crystallinity of the polymers as well, and the latter could have a

significant positive or negative impact depending on the applications.

In order to understand the behavior of the particles inside the reactor, turbulent Euler-Euler two-

phase model in Comsol Multiphysics was developed to understand the flow of the particles

inside the reactor. We first considered isothermal flow to simplify the model, and then we added

heat transfer equations to simulate the non-isothermal conditions which were consistent with

experimental measurements. The Multiphysics interface solved two sets of Navier- Stokes

equations, one for each phase, in order to calculate the velocity field of each phase. The PP

particles were considered as dispersed phase and air as the continuous one. Among the different

existing turbulent models (k-ε, k-ω, k-ε low Reynold number and SST (Shear Stress Transport)

applied for plasma gas flow simulation, the k-ε model was selected because of the consistency

with the experimental gas velocity profile measured by hot-wire anemometry. The Gidaspow

model was used to calculate the drag forces. The velocity profile of both phases, i.e. gas and

polypropylene (PP) particles, the concentration of PP particles, and the temperature in different

regions of the reactor were obtained by this model. The experimental results were in relatively

good agreement with the model.

By combining the concentration profile of PP particles developed with Comsol Multiphysics

model and experimental velocity measurements with high speed photography, the evaluated

particle flow rate inside the Wurster-tube (plasma zone) was 5.7 g/s. Finally, for a total

122

treatment time of 120 s, one can estimate that in average each particle was treated 3 to 4 times

by the plasma. The total treatment time for a single particle is in the order of 10 ms only, which

may explain why the increase of surface energy is moderate.

In the last part of this thesis, we also studied the possibility to use non-equilibrium plasmas to

decontaminate particles in a fluidized bed. Indeed, the plasma used to treat PP is characterized

by the formation of Reactive Oxygen and Nitrogen Species (RONS) such as OH or NOx, by

the emission of UV and a relatively high temperature; the latter is obviously a problem for

plasma medicine, but is not a problem to treat non-living objects, specially containing highly

resilient microorganisms such as spores.

We designed different plasma reactors to study the influence of two different devices for

microbial decontamination of black peppercorns. The latter was chosen because of its high

natural microbial contamination (about 107 CFU/g) in comparison to other analyzed alimentary

grains. The concentrations of mesophilic aerobic and spore-forming bacteria were measured for

broken and whole black peppercorns. The concentration of spore-forming bacteria in the

peppercorn was about 72% and 56% for broken and whole peppercorns, respectively. The first

system was an Ar + O2 DBD atmospheric plasma jet (system A) and the second one was a

rotary air blown-arc atmospheric pressure plasma jet (system B). System A was not efficient

for broken peppercorn decontamination and the best results achieved by system B for 4 minutes

of plasma treatment lead to a 2 log reduction of the total mesophilic aerobic counts (MAC);

killing bacteria is a combination of chemistry (ROS, RONS, pH) and physical conditions

(UV radiation, temperature, electric field), therefore it is not surprising if these two very

different plasma systems were characterized by very different efficiencies.

As compared to the surface functionalization of PP, where oxygen containing groups are grafted

at the surface of the particles, bacteria are not only on the surface of peppercorn particles

but also inside their pores. This explains why we observed only a 2 log reduction even in our

best conditions. Therefore in a second step petri dishes were inoculated with a dilution of

microorganisms existing on the broken black peppercorn in order to separate the effect of the

morphology and the porosity of peppercorns on the plasma treatment. These samples were

plasma treated with both plasma systems at the same operational conditions as before for

peppercorn treatments. The log reduction of microorganisms with system A, after 5 minutes

treatment was only 0.6 while this value for system B was 4.77 after 3 minutes treatment which

corresponded to 100% disinfection. These results confirmed that treating porous particles by

plasma is challenging.

123

There are many perspectives for this the present research work. In the field of surface

engineering, a precursor injection system will be added to the plasma nozzle to deposit thin

layers from monomers on the surface of micrometric particles by Plasma Enhanced Chemical

Vapor Deposition (PECVD). Previous works at LISE were dedicated to the deposition of

titanium and siloxane layers for photocatalysis and adhesion or anticorrosion applications

respectively. The challenge would be to obtain uniform layers, with the same thicknesses, on

each particle. Indeed, as compared to surface functionalization, which is a self-limiting process

(i.e. a surface is always finite), thin layers can continuously grow on a surface under precursor

flow. Shadowing or sticking effects between particles would be critical as the latter will prevent

the formation of coatings on a large part of particles. In the case of polymers, the plasma-treated

polymeric powders treated with our Wurster fluidized bed reactor could be used as primary

substances for rotational molding process and selective laser melting process (SLM). Indeed

the mechanical resistance of the objects built by these technologies must be investigated by

comparing non-treated and plasma treated polymer powder.

Treat porous materials by plasma processes has always been a challenge; “Will the plasma

treatment penetrate inside the pores?” This question has been studied and debated in the plasma

community. The results obtained in this thesis, on the surface decontamination, suggested that

the efficiency of the plasma treatments inside the pores were more difficult as compared to the

surface of the particles. Many applications are related to treatments inside the pores of particles,

as for example, in the field of heterogeneous catalysis. It would be interesting for future

prospects to study in details this issue.

Finally, modeling was also an important part of this work. The CFD model has provided

interesting insights on the process. The model considered the flow of gas and particles in

turbulent conditions, but there was no plasma. It would be interesting to understand the role of

the plasma on the behavior of the particles and reciprocally. Indeed we observed experimentally

that particles, the gas glow and the plasma propagation were strongly interconnected, however

we were able to obtain a better understanding of the physics behind these observations. Further

work in this direction would be interesting to develop as well as developing more realistic

models.

124

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