CFD SIMULATION OF MIXING WITH MAXBLEND IMPELLER IN

A LAB-SCALE ANAEROBIC DIGESTER SINGH Buta1

1 MSc, doctorate student

Institute of Energy Engineering and Chemical Machinery, University of Miskolc

ABSTRACT

Mixing is the most prominent factor, undeviatingly determines the consequences

of an anaerobic digester with higher solid content. This paper presents the

Computational Fluid Dynamic model using Ansys SC tetra software that

characterize the mechanical mixing by maxblend impeller in an anaerobic

digester. Effect of impeller geometry is studied on the flow pattern, dead volume

and particle velocity distribution. Geometry of maxblend is analyzed along with

varying mixing speeds of impeller. Mixing was analyzed at three different speeds

i.e. 40, 80 and 100 rpm. It was observed that higher mixing intensity resulted in

increased particle velocity. Uniform distribution of velocity was observed and

mixing speed of 80 and 100 rpm seems optimal. This paper recommends the

strategy for modelling mechanically mixed slurry at lab scale.

Keyword: anaerobic digestion, mixing, CFD, maxblend impeller

Acronyms

CFD Computational fluid dynamics K Consistency index

AD Anaerobic digestion MI Marine impeller

RPM Revolution per minute AI Anchor impeller

RT Rushton turbine PI Pelton impeller

HR Helical ribbon HEB High efficiency blade

DFB Disc mounted flat blade TS Total solid

1. INTRODUCTION

Biogas is a trending source of renewable energy in modern world. Hence, there is

great emphasis to improve the biogas production rates from biomass by

developing the existing technology[1][2]. Different types of shapes and sizes are

used for methane production. However, deposition and stratification can result in

failure of digester due to in efficient and insufficient mixing in a digester.

Agitation of an anaerobic slurry is vital to accomplish, primarily, the supply of

substrate to be distributed uniformly, secondly, to keep continuous contact between

the microorganisms and sludge, tertiary, the concentration of end product and

prohibited biological intermediates have to be maintained at minimum levels[3]. The

mixing can boost the homogeneous distribution of nutrients and micro-organisms and

can evade formation of surface crust and sedimentation[4][5].

The mechanical mixing is considered as the most effective mixing mode in terms of

power consumption apart from gas mixing and pumped circulation[6][7]. Various

MultiScience - XXXIII. microCAD International Multidisciplinary Scientific ConferenceUniversity of Miskolc, 23-24 May, 2019, ISBN 978-963-358-177-3

1 corresponding author: vegybs@uni-miskolc.hu

1 Institute of Energy Engineering and Chemical Machinery, University of Miskolc

DOI: 10.26649/musci.2019.010

different types of impellers has been used by researchers to optimize the mixing in

anaerobic digester[8][9]. Many studies published last years, have been dealing with

impact of mixing on biogas production using distinctive designs, positions and

configurations of impellers along with shape of digesters. There are various factors

which directly effects the mixing time and biogas production rates in a digester such

as impeller design, impeller bottom clearance and inter impeller clearance, impeller

eccentricity, baffles and presence of draft tube. There is variation in results on

effectiveness and efficiencies of different mixers due to different methods and setups

used for evaluation along with different substrates and their

concentration[10][11][12]. Choosing proper impeller is very important as choice of

impeller depends on various factors like liquid viscosity, the need for turbulent shear

flows and design of digester etc. Various technologies are available for mixing, but

the digester design is equally important. The main objective of any impeller is to

avoid stratification, dead zones and solid settling or even floating of substrate in a

digester.

Lebranchu et al [8] concluded that HR was more effective in mixing of slurry as

compared to RT impeller and produced more biogas and better distribution of

velocity and viscosity in the vessel. F.Battista et al[9] tested four different types of

impellers to know the mixing effect on this high viscosity fluid. The impellers were

a MI with three blades, AI, a RT impeller with 45o inclined blades and PI. After the

comparison it was observed that the marine impeller possesses good homogenization

in the digester due to both axial and radial moments given to fluid. 6-blade rushton

impeller with blade inclination of 45o performed much better than traditional rushton

impeller resulting in increase in biogas production containing methane content. Fei

Shen et al were investigated different blades including the HEB, pitched blade (PB),

DFB at stirring rate between 20 rpm to 160 rpm. It was noted that at stirring rate of

80 rpm complete mixing of rice straw in vertical column was achieved by PB and

HEB blades where flow velocity varied in range of 0-0.36 ms-1 whereas at same rpm

in the triple impeller combination the flow velocity vectors varied from 0-0.44 ms-1.

The basic problem lies in the fact that the slurry is a set of liquid-solid phases of fiber

from various agricultural and animal wastes. This results in a high viscosity and a

different behavior from Newtonian fluids. In the high-speed range, no significant gas

production can be experienced, as the resulting conditions degrade the quality of life

of the bacteria. The tests are performed at the bottom of the speed range used for

mixing. Using mixing with forced flow, the purpose of combining two or more

materials is to achieve a homogeneous distribution, even if the smallest volume

element is tested. The perfect final state is a homogeneous, heterogeneous system.

The objective of this article is to investigate the flow pattern and viscosity distribution

produced by maxblend impeller.

2. MATERIAL AND METHOD

Two 4.5 L cylindrical vessel was designed to compare the mixing at different TS

content. This digester is a continuous type digester with both input and output ports

on opposite sides of vessel. Digester is equipped with standard maxblend impeller

with specifications listed in table 1. The impeller mixing speed is 40, 80 and 100 rpm

to analyze the appropriate mixing. All the simulations were carried out at same rpm,

at the TS content 12.1%, and its effect was analyzed. Various rheological properties

are described in Table 1[13].

Table 1. Rheological properties of cow manure.

TS (%) K (Pa sn) n 𝜼min (Pas) 𝜼max (Pas) Density (kg/m3)

2.5 0.042 0.710 0.006 0.008 1000.36

5.4 0.192 0.562 0.01 0.003 1000.78

7.5 0.525 0.533 0.03 0.17 1000.00

9.1 1.052 0.467 0.07 0.29 1000.31

12.1 5.865 0.367 0.25 2.93 1000.73

Table 2 Geometrical specifications of standard impeller.

Parameter Dimensional formulae Dimension(mm)

Digester diameter (D) D 150

Bottom clearance (c) 0.1D 15

Impeller blade height (h) 1D 150

Impeller blade diameter (d) 0.6D 90

Shaft diameter (ds) 0.1D 15

The paddle height (h1) 0.33D 50

The grid height (h2) 0.46D 70

The grid diameter (d1) 0.45D 67.5

.

Table 3 Geometry of digester

Parameter Dimension

Total Volume (V) 4.5 L

Working volume 3.5 L

Diameter (D) 150 mm

Height (H) 250 mm

Liquid height 200 mm

Figure 1. Impeller geometry 2-D and 3-D

150 mm

250 mm

12V DC Motor

Biogas outlet pipe

Vessel Lid

Impeller

Shaft Feed input port

Feed outlet port

70 m

m

Figure 2. Lab-scale anaerobic digester.

3. GOVERNING EQUATIONS

From the previous literature it is clear that the digestate may show a shear-thinning

rheology, so it is also necessary to determine the apparent viscosity μa and thus the

apparent shear rate γa within the digester. Slurry can be considered as a pseudo-plastic

(shear thinning) fluid in which viscosity decreases with increasing shear rate. For

modelling purposes, the viscosity of shear-thinning fluids may be expressed using a

power law.

𝜏 = 𝐾 ∙ γ𝑛

Where 𝜏 is shear stress, n is flow index number and K is consistency index number

The apparent viscosity μa is defined as the viscosity calculated at the apparent shear

rate

μ𝑎 = K ∙ γ𝑎𝑛−1

Figure 3. Shear viscosity as a function of shear rate for a pseudo-plastic fluid.

Mixing inside anaerobic digester is very complex and it is governed by continuity,

momentum, turbulence and rheological equations. The homogeneous single phase,

laminar flow CFD model was selected for simulating flow pattern of sludge in

digester. The nature of flow is related to fluid physical properties (density and

viscosity).

Navier stokes transport equations were solved in transient regime. The rotation of the

impeller was modelled using a sliding mesh approach, which offers intrinsically a

better precision than multi reference frame in case of slow rotating impeller. The

simulation, a turbulent flow case was investigated with the Large Eddy Simulation

(LES) approach. LES is a mathematical model for turbulence in the science of fluid

dynamics. The simulation numerically solves the Navier-Stokes equation:

𝜌 (

𝜕𝑣

𝜕𝑡+ 𝑣∆𝑣) = −∇𝑝 + ∇𝑇 + 𝑓

(1)

Continuity equation:

𝜕(𝜌𝑢)

𝜕𝑥+

𝜕(𝜌𝑣)

𝜕𝑦+

𝜕(𝜌𝑤)

𝜕𝑧= 0

(2)

Laminar flow model was preferred over turbulent based on the assumptions of low

order Reynolds number:

𝑅𝑒 =

𝜌. 𝑈∞2−𝑛. 𝑑ℎ

𝑛

𝑘. (0.75 +0.25

𝑛)𝑛. 8𝑛−1

(3)

where 𝜌: density [Kg/m3], U∞: characteristic velocity [m/s], dh: hydraulic diameter

[m].

4. CFD SIMULATIONS

Computational fluid dynamics is a valuable and efficient tool to understand and

evaluate the fluid dynamics of flow system. It became an invaluable resource for

simulation of processes involving fluid flow, heat transfer and mixing inside an

anaerobic digester. The specifications of tank geometry, impeller geometry boundary

and initial conditions are required while the discretization of continuity, momentum

and turbulent equations are solved. In particular code for the resolution of these

equations uses the finite volume method on the discretization of governing

differential equations.

Determination of mechanical power

In order to quantify and compare performances of mixing with maxblend impeller,

power dissipation and mixing times have been determined for each experiment.

Power dissipation quantifies the power transferred by the stirrer to the liquid phase,

further dissipated by viscous friction, which prevails in laminar regime. The

mechanical power transferred to the liquid phase was calculated by:

𝑃𝐶𝐹𝐷 = 2𝜋𝑁𝐶𝐶𝐹𝐷 with PCFD (W) the power transmitted by the impeller to the fluid, N

(s−1) the agitation rate and CCFD (N m) the calculated torque on the stirrer.

5. RESULTS AND DISCUSSIONS

Hydrodynamics simulations of the digester equipped with the maxblend impeller

revealed overall better distribution of velocity and viscosity, whatever the agitation

rate. Large volume of vessel was swept by the impeller even at lower rotational

speeds due to a higher flow rate property. However, in-vessel velocities are, of course,

not necessarily an indicator of the degree of mixing. The sludge may be moving at a

particular speed, but if all sludge in the immediate vicinity is moving at the same

speed and in the same direction, then mixing is not occurring, rather the sludge is

simply being moved within the vessel. With increase in the agitation rate the particle

velocity also increased. For the agitation rate of 40 rpm the maximum velocity was

noted as 0.2565 m/s. Figure 3 shows the results of Cfd simulations at different

agitation rates. Two weak vortexes were observed at top and bottom of impeller, but

the movement of slurry was missing on the upper volume of vessel. At 80 and 100

rpm the maximum velocities were 0.47090 and 0.5596 m/s respectively. The top and

bottom vortexes were strong as compared to lower agitation rate. Homogenization at

bottom of an anaerobic digester id very important because it can lead to settling and

stratification which can result in digester volume. Mixing at bottom was observed

better as there was strong radial and axial effect. Effective mixing is an important

feature of any digestion facility, not just for the optimization of the biological

processes, but also to prevent deposition of grit and heavier material present in the

sludge. Starting torque at all agitation rates was higher and increased with rpm as

shown in Table 4. Similar type of trend was observed in all cases. Increasing the

solids content will reduce the ability of particles to move and hence be mixed within

the flow field. There is an increasing reduction in the lack of movement of each

particle with increasing solids content. Table 4. Comparison of results on various agitation rates.

Rpm Starting

torque (Nm)

Running

torque

(Nm)

Power

consumption

(W)

Maximum

velocity

(m/s)

Average

velocity

(m/s)

40 0.018164 0.001105 0.0069 0.2565 0.142

80 0.410200 0.022363 0.1404 0.4709 0.144

100 0.524896 0.032068 0.2050 0.5596 0.149

At 40 rpm At 80 rpm At 100 rpm

Figure 4. Cfd simulation results at different rpm of maxblend impeller in vessel at different planes

6. CONCLUSION

Cfd results demonstrates that maxblend impeller possess good characteristics of

uniform distribution of velocity and viscosity.

• Agitation rate of 80 and 100 rpm can be used for mixing in an anaerobic

digester. Further time of mixing operation can be optimized to check the

minimum power consumption.

• Experimental approach can be continued with maxblend impeller to determine

the effect of mixing intensity on methane production.

• Scaleup factor for large scale biogas plant is also very important aspect to be

considered during the designing of anaerobic digester.

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