Superheated Steam Drying of Foods and Biomaterials

Sakamon DevahastinDepartment of Food Engineering

King Mongkut's University of Technology Thonburi (KMUTT)Bangkok, Thailand

International Workshop on Drying of Food and Biomaterials

June 6-7, 2011

Superheated Steam Drying of Foods and Biomaterials

OUTLINE

• Introduction to Superheated Steam Drying (SSD)

• Basic principles of SSD

• SSD of foods and biomaterials

• Low-Pressure Superheated Steam Drying (LPSSD)

SUPERHEATED STEAM DRYING (SSD)

• Proposed over 100 years ago; received serious attention only during the past 20 years

• Uses steam in place of hot air or combustion/flue gases in a direct dryer

• More complex than hot-air drying system

• Lower net energy consumption (if exhausted steam can be used elsewhere in the process)

• Better product quality (in most cases)

Closed steam drying system

Recycled steam

Fan/blower

Direct use of steam

Energy recovery via heat exchanger

Removal of condensate

Heaterpurged steam

steam from boiler

Typical SSD set-up

SUPERHEATED STEAM DRYER

Saturated Steam FeedSaturated Steam FeedAssume 100°C, 1 bar; H = 2,690 kJ/kg

Steam Superheater

Superheated SteamSuperheated SteamAssume 110°C, 1 bar; H = 2,720 kJ/kg

Drying chamber

Saturated Steam ExhaustSaturated Steam ExhaustBack to 100°C, 1 bar; H = 2,690 kJ/kg

Bleeding off for other uses

SSD & ENERGY

• If exhausted steam can be used elsewhere or can be recycled, latent heat is not charged

• Net energy consumption is 1000-1500 kJ/kg water removed

• Reduced net energy consumption is a clear advantage of SSD!

Change in Energy Use*

ConventionalConventional

Ann

ual E

nerg

y In

put (

GJ)

Ann

ual E

nerg

y In

put (

GJ)

0

80,000

SSDSSD

Thermal

Electricity

*Centra Gas Manitoba, Inc.

SOME ADVANTAGES OF SSD

• Dryer exhaust is steam so it is possible to recover all latent heat supplied to SSD

• No oxidative reactions possible due to lack of O2;color and some nutrients are better preserved

• Higher drying rates possible in both CRP and FRP depending on steam temperature (above the so-called inversion temperature SSD is faster than air drying)

• Toxic or organic liquids can be recovered easily

SOME ADVANTAGES OF SSD

• Casehardened skin is unlikely to form in SSD

• SSD yields higher product porosity due to evolution of steam within the product - bulk density is thus lower while rehydration behavior is better

• Sterilization, deodorization or other heat treatments (e.g. blanching, boiling, cooking) can be performed simultaneously with drying

SOME DISADVANTAGES OF SSD

• SSD system is more complex than its hot-air counterpart

• Initial condensation is inevitable – sometimes desirable though

• Products that may melt, undergo glass transition or be damaged at saturation temperature of steam cannot be dried in SSD

• Limited knowledge and experience on SSD

BASIC PRINCIPLES OF SSD

• Drying rate in CRP depends only on heat transfer rate since there is no resistance to diffusion in its own vapor

• If sensible heat effects, heat losses and other modes of heat transfer are neglected, CRP drying rate is then:

λλ)( surfacesteam TThqN

−==


• In hot-air drying ΔT is higher at low drying temperatures; reverse is true at higher drying temperatures

• These counter-acting effects lead to phenomenon of inversion; beyond inversion temperature SSD is faster than hot-air drying

CRP drying rate

Temp.inversion temp.

hot-air drying

SSD

An inversion phenomenon

λλ)( surfacemedium TThqN

−==

Air drying: Tsurface = Twet-bulbSSD: Tsurface = Tsaturation


• FRP drying rate of SSD is sometimes higher than that of hot air - mechanisms responsible are different, however!

• FRP drying rate of SSD is sometimes higher than air drying rate since product temperature is higher. Casehardening is unlikely to form and product is likely to be more porous as well

Prachayawarakorn et al., Drying Technol., 20, 669-684 (2002)

Drying rates of shrimp dried in superheated steam and hot air

Vacuum steam dryers for wood*

Vacuum steam dryers for silk cocoons**

Fluidized bed dryersfor coal*

Impingement and/orthrough dryer for textiles, paper***

Flash dryers for peat (25 bar)****

Conveyor dryers for beet pulp (5 bar)****

Fluidized bed dryers for pulps, sludges*

* Extensive commercial applications** Laboratory scale testing*** Pilot scale testing****At least one major installation

Superheated Steam Dryers

Near AtmosphericPressure High PressureLow Pressure

Classification of superheated steam dryers basedon their operating pressure

RotarySpray

Fluid bed

Flash

Conveyor

POSSIBLE TYPES OF SSDs• Flash dryers with or without indirect

heating of walls• FBDs with or without immersed heat

exchangers• Spray dryers• Impinging jet dryers• Conveyor dryers• Rotary dryers• Impinging stream dryers

Pressurized Steam FBD (Niro A/S)

PRESSURIZED STEAM FBD

• Closed system

• Used to dry materials produced in brewery, food and sugar processing, wood-based bio-fuels

• Close to 90% energy recovery as steam at 2-4 bar

• No product oxidation

Exergy Steam Dryer (GEA Exergy AB)

Exergy Steam Dryer with Backmixing (GEA Exergy AB)

• Residence time of 5-60 sec

• Generated excess steam at 1-5 bar - can be reused either directly or after re-boiling

• If there is no external use, excess steam can be recompressed to 10-20 bar and used as heating media. Power consumption is 150-200 kWh/ton evaporated water

• 70-90% energy recovery is possible

Exergy Steam Dryer (GEA)

SSD OF FOOD PRODUCTS

• Received serious attention during the past 10 years

• Possesses several advantages that are of special interest to food processors e.g. lack of oxidative reactions, ability to maintain color, nutrients, yields product of higher porosity

• Ability to inactivate microorganisms

• Many heat treatments can be performed simultaneously with drying

HIGH-PRESSURE SSD OF FOODS

• Drying of pressed beet pulp after extraction of sugar

• Operates at pressure ~ 5 bar

• Consumes 50% less energy than conventional air dryer

• Product quality i.e. appearance, texture, digestability by cattle is better than air drying

• Pilot tests with spent grain from brewery, alfalfa, fish meal, pulp from citrus, etc.

NEAR-ATM PRESSURE SSD OF FOODS

• Most SSDs operate in this range of pressure

• Wide variety of products dried successfully e.g. potato chip, tortilla chip, shrimp, paddy, soybean, noodles

• Better product quality (in some cases) than air drying

Experimental set up of Iyota et al. (2001)

Iyota et al., Drying Technol., 19, 1411-1424 (2001)

initial condensation

Drying curves for both SSD and hot air drying of potato slices

Iyota et al., Drying Technol., 19, 1411-1424 (2001)

SEM photos of cross section near the surface of potato slices

SSD

Hot air

SEM photos of cross section near the surface of potato slices

SSD

Hot air

second-layer crust

Microbial inactivation using saturated

steamHot air drying

Raw material Product

– Temp. 120˚C– 10 min

– Temp. 80˚C– Air velocity 2 m/s

SSD & FOOD SAFETY

Decontamination of pepper seeds (Method # 1 –Conventional industrial method)

Saturated steam treatment

Raw material 1.1 ± 0.1 × 1021.3 ± 0.1 × 1040.71 ± 0.0312.03 ± 0.07

n.d.n.d.*0.85 ± 0.0217.75 ± 0.69

Yeasts and MoldsTPCaw

Moisture content(% w.b.)

Microbial survival, moisture content and aw after saturated steam treatment

* Not detectable

Higher MC & aw after treatment means higher drying load!

n.d.n.d.*0.213 ± 0.0035.01 ± 0.17

Yeasts and MoldsTPCaw


Microbial survival, moisture content and aw after hot air drying

* Not detectable

Total drying time = 210 min and Total process time = 220 min (rather long??)

– Temp. 120, 130, 140˚C– 5, 10 and 15 min– Pressure ∼ 1 bar

Microbial inactivation using superheated

steamHot air drying


– Temp. 80˚C– Air velocity 2 m/s

SSD & FOOD SAFETY

Decontamination of pepper seeds (Method # 2)

Moisture content and aw after superheated steam treatment

7.87 ± 0.13

9.09 ± 0.04

10.61 ± 0.17

8.53 ± 0.04

9.25 ± 0.39

11.68 ± 0.27

10.35 ± 0.11

11.65 ± 0.10

12.92 ± 0.09

Moisture (% w.b.) aw*Time (min)Temp.

0.31 ± 0.0115

0.38 ± 0.0210

0.48 ± 0.025140°C

0.35 ± 0.0115

0.43 ± 0.0210

0.55 ± 0.015130°C

0.47 ± 0.0115

0.52 ± 0.0210

0.63 ± 0.025120°C

* Initial aw ~ 0.71

Microbial survival after superheated steam treatment

n.d.n.d.15

n.d.n.d.10n.d.n.d.5140°C

n.d.n.d.15

n.d.n.d.10n.d.3.1 ± 0.2 × 1025130°C

n.d.n.d.15

n.d.n.d.10n.d.5.0 ± 0.4 × 1025120°C

1.1 ± 0.1 × 1021.3 ± 0.1 × 104Raw material

Yeasts and Molds (CFU/g)

Total plate count (CFU/g)Time (min)Temp.

* Not detectable

Hot air drying time

90

130

200

140

180

200

190

210

290

Drying time(min)

15

10

5

15

10

5

15

10

5

SHS treatmenttime (min)

105

140

205140 °C

155

190

205130 °C

205

220

295120 °C

Total process time (min)Temp.

SSD & FOOD SAFETY

Decontamination of pepper seeds (Method # 3)

– Temp. 120, 130, 140˚C– Pressure ∼1 bar

Microbial inactivation and drying using SSD


Microbial survival, moisture content and aw after superheated steam drying

5.77 ± 0.01

5.67 ± 0.09

5.94 ± 0.08


0.245 ± 0.04

0.242 ± 0.02

0.227 ± 0.01

aw

n.d.

n.d.

n.d.*

TPC

n.d.

n.d.

n.d.

Yeasts and Molds

140°C

130°C

120°C

Temp.

* Not detectable

Total process time: 120oC = 180 min130oC = 65 min140oC = 30 min

Much shorter than that of the previous two methods! Taste, odor and color are comparable to conventionally treated product

OTHER FOODS DRIED IN SSD

• Potato chip, tortilla chip

• Shrimp, pork, chicken, fermented fish

• Sugar beet pulp, spent grain from brewery, okara

• Paddy, soybean, sunflower seed, cacao bean

• Asian noodles

• Vegetables, fruits, herbs - problems here!

IF YOU REMEMBER...

• Products that may be damaged at saturation temperature of steam cannot be dried in SSD

Need exists for a low-pressure superheated steam drying system for

heat-sensitive products

LOW-PRESSURE SSD (LPSSD)

• Combines ability to dry product at low temperature with some advantages of SSD

• Dryer is operated at reduced pressure

• Steam becomes saturated (and superheated) at lower temperature

• Suitable for heat-sensitive products, e.g., herbs, fruits and vegetables and other biomaterials

Devahastin et al., Drying Technol., 22, 1845-1867 (2004)

Photographs of carrot cubes underwent LPSSD and vacuum drying



Relationship between β-carotene content and

MC of carrot during drying

Suvarnakuta et al., J. Food Sci., 70, S521-S526 (2005)

Methakhup et al., Lebensm.-Wiss. u.-Technol., 38, 579-587 (2005)

Methakhup et al., Lebensm.-Wiss. u.-Technol., 38, 579-587 (2005)

Nimmol et al., J. Food Eng., 81, 624-633 (2007)

Léonard et al., J. Food Eng., 85, 154-162 (2008)

(a) (b)

(c) (d)

(a) Fresh sample(b) HAD(c) VD(d) LPSSD

SEM images of Salmonella on cabbage surfaces

MATH MODELING OF LPSSD

• No resistance to mass (moisture) transfer at product surface

• Concept of mass transfer coefficient is not valid

• Earlier model: Assuming that free MC equals zero at the surface – not appropriate!

• Initial condensation not considered


• Simple 3-D liquid diffusion model

• Pressure gradient is the driving force for external mass transfer

• Net rate of evaporation or condensation per unit droplet area was estimated by the modified Hertz–Knudsen Equation

Kittiworrawatt and Devahastin, Chem. Eng. Sci., 64, 2644-2650 (2009)


Energy equation

When Ts < Tsat:


Convective heat transfer neglected!


When Ts < Tsat (cont’d):


Condensing vapor is superheated steam!


When Ts = Tsat:



When Ts > Tsat:



Mass transfer equation

When Ts < Tsat:



When Ts = Tsat:


When Ts > Tsat:

1 3

2

Initial condensation (only Model 1 can capture this!)


132


IN SUMMARY...

There is still much room to play with...

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Superheated Steam Drying of Foods and Biomaterials

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