Introduction Stirred tanks with gas liquid two-phase flow are very widely

used in chemical and biochemical engineering process

Stirred tanks are commonly used in reactors of

Detergent plants

Paint mixing units

Food processing plants

Many researchers carried out numerical simulations on this gas –liquid flow in a stirred tank, with many assumptions and predictions Ranade and Khopkar, Pinelli

Zhang

Introduction (Contd.) 3D CFD model of a gas-liquid two

phase stirred tank with 2 six blade turbines and 4 baffles were developed

Impeller rotation speeds and inlet gas flow rates are varied

Simulation, Analysis and Model predictions about gas holdup and liquid velocity distributions are carried out generally

Hydrodynamic Characteristics

Gas holdup A dimensionless key parameter for design purposes that

characterizes transport phenomena of bubble columns

Liquid velocity

Bubble size fractions Transient bubble diameter distributions

Inlet air flow rate

Impeller rotation speed

Computational Fluid Dynamics Branch of fluid dynamics,

used to solve nonlinear differential equations involving fluid flow using numerical methods and algorithms

Extensive applications Aerospace

Turbo machinery

Nuclear thermal hydraulics

Automotive etc NumericalAnalysis

Fluid Dynamics

Computer

Science

Why CFD ???? Less time consuming & less expensive compared to

experiments

Powerful visualization capabilities

Predicts performance before modifying or installing actual systems or a prototype

Predict which design changes are most crucial to enhance performance

Model Eulerian approach is adopted

Stationary frame of reference

Unstructured grid

Two phases Dispersed and continuous phases

Satisfies compatibility condition

Continuity balance equation for each phase

1 lg

0ug

gg

gg

t

0u l

ll

ll

t

Model (Contd.)

The momentum balance equation for each phase

Drag force exerted by dispersed phase on continuous phase

Lift force acting perpendicular to the direction of relative motion of two phases

Numerical Modeling

Finite Element Method with a multi-grid solver was adopted (CFX 10.0)

Computational domain of the stirred tank divided into Rotating impeller domain

Stationary tank domain

Solver run over 70 s of computed time

Unstructured grids with a number of 1944 for impellers and 97104 for the tank were implemented

Mesh partitions

Test Setup

5L distilled water is filled into the tank (fig shows the case when static water height was 20 cm while height of tank as 30 cm)

Operating conditions: Rotation speed : 200, 400, 600 rev/min

Inlet air flow rate : 4, 6, 8L/min

Each measurement repeated 3 times

Overall gas holdups measured from height fluctuations of the water after gas injection for specified operating conditions

Test Setup (Contd.)

Physical dimensions of the stirred tank reactor, side view and top view (unit: mm)

Results and discussions Simulations of the stirred tank were carried out for five

different operating conditions

Gas flow number and Froude number are dimensionless

Rotation speed is fixed (Rs = 400 rev/min) an inlet air flow rate is varied and vice versa (Qg = 6L/min)

Summary of operating conditions in this study

Case No. 1 2 3 4 5

Impeller speed (rpm) 400 400 400 200 600

Gas flow rate (L/min) 4 6 8 6 6

Gas flow number 0.060 0.090 0.120 0.180 .060

Froude number 0.249 0.249 0.249 0.062 0561

Gas holdup

The volume fraction of dispersed gas phase is referred to as the gas hold-up

Volume averaged overall gas holdup along time

Time averaged local gas holdups along transversal courses

Transient gas holdup distributions at horizontal and vertical positions were simulated and analyzed

Volume averaged overall gas holdups

Along time courses under different operating conditions A: Qg = 6 L/min, Rs = 200, 400, 600 rev/min

B: Rs = 400 rev/min, Qg = 4, 6, 8 L/min.

Model simulated time-averaged local gas holdups along transversal course

(X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5,30,65,100,125,160 mm) under different operating conditions(Qg = 6 L/min, Rs = 200, 400, 600 rev/min).

Model simulated time-averaged local gas

holdups along transversal course

(X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5, 30,65,100,125,160 mm) under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min).

Model predictions of transient gas holdup distributions at the vertical sections

(X = 0 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4, 6, 8

L/min) with t = 10, 40, 70 s.

(X = 0 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200, 400, 600

rev/min) with t = 10, 40, 70 s.

Model predictions of transient gas holdupdistributions at the horizontal sections

( Z = 5,30,65,100,125,160 mm)

under different operating conditions(Rs = 00 rev/min, Qg = 4,6,8 L/min) with t = 40 s.

(Z = 5, 30,65,100,125,160 mm) and under different operating conditions

(Qg = 6 L/min, Rs = 200, 400, 600 rev/min) with t = 40 s.

Liquid velocity

Axial liquid velocity is selected and simulated and experimentally measured to characterize liquid flow fluctuations

Volume averaged overall liquid velocity along time

Time averaged liquid velocity along transversal courses

Transient liquid velocity distributions along horizontal and vertical positions

Volume averaged axial liquid velocities

Along time course under different operating conditionsA: Qg = 6 L/min, Rs = 200,400,600 rev/min

B : Rs = 400 rev/min, Qg = 4,6,8 L/min

Model simulated and experimental measured Time averaged axial liquid velocities along transversal course

(X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5, 30,65,100,125,160 mm) and under different operating conditions (Qg = 6 L/min, Rs = 200,400,600 rev/min)

Model simulated and experimental measured Time

averaged axial liquid velocities along transversal course

(X = 0 mm, Y = 15–75 mm) at different vertical positions (Z = 5,30, 65,100,125,160 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min)

Model predictions of transient liquid velocity distributions at the vertical sections

(X = 0 mm)and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8

L/min) with t = 10,40,70 s.

X = 0 mm)and under different operating conditions(Qg = 6 L/min, Rs = 200,400,600

rev/min)with t = 10, 40,70s

Model predictions of transient liquid velocity distributions at the horizontal sections

(Z = 5,30,65,100,125,160 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min) with t = 40 s.

(Z = 5,30,65,100,125,160 mm) and under different operating conditions (Qg = 6 L/min,

Rs = 200,400,600 rev/min) with t = 40 s.

Bubble size distribution

The behavior of bubbles in gas–liquid stirred tanks are very important especially in supplying oxygen from gas phase into liquid phase

The CFD model developed in the current work was coupled by the MUSIG model

This model considered several bubble group diameters which can be represented with Sauter mean diameter

Bubbles are divided into twenty groups and then predicted the bubble Sauter mean diameter.

Diameters of 20 bubble group ranged from 0.375 to 14.625mm

Volume averaged bubble size fractions

Stirred tank was mainly occupied by small bubble groups (dia less than 4 mm)

Advantageous for fermentation process

Improved mass and heat transfer

Increase in specific area with small bubble diameter

Volume averaged bubble diameters

Bubble Sauter mean diameter ranged from 1 to 2.5 mm

Increase in rotation speed obviously made a decrease in bubble diameter

Change in inlet air flow rate under investigated range had little effect on bubble diameter

Model predictions of transient bubble diameter distributions at the vertical sections

(X = 0 mm) and under different operating conditions(Rs = 400 rev/min, Qg = 4,6,8

L/min)with t = 10, 40,70 s.

(X = 0 mm) and under different operating conditions (Qg = 6 L/min, Rs =

200,400,600rev/min) with t = 10,40,70 s.

Model predictions of transient bubble diameter distributions at the horizontal sections

(Z = 5,30, 65,100,125,160 mm) and under different operating conditions (Rs = 400 rev/min, Qg = 4,6,8 L/min) with t = 40 s.

(Z = 5,30, 65,100,125,160 mm) and under different operating conditions (Qg = 6 L/min,

Rs = 200, 400,600 rev/min) with t = 40 s.

Conclusion A full-flow field, 3D transient CFD model based on

Eulerian approach was developed for a gas-liquid two phase stirred tank with 2 six-blade turbines and 4 baffles MUSIG model for bubble size distribution considering coalescence

and breakup

Increase in inlet air flow rate and rotation speed increase in overall gas holdup

Increase in inlet air flow rate or decrease in rotation speed increase in volume-averaged axial liquid velocity

Gas accumulated mainly in regions between the two impellers, as well as between the upper impeller and the top surface when inlet air flow rate was large

Conclusion (Contd.) Increase in rotation speed made a more dispersed gas

distribution all over the whole tank

Vortices were also generated in regions of bottom of the tank

The tank was mainly occupied by small bubbles with diameters smaller than 4 mm

Larger bubbles accumulated in regions near the lower impeller

between the two impellers

between the upper impeller and the top surface

Smaller bubbles accumulated in regions near wall

Increase in rotation speed made a decrease in bubble diameter

References

Wang, H., Jia, X., Wang, X., Zhou, Z., Wen, J., Zhang, J.,CFD modeling of hydrodynamic characteristics of a gas–liquid two-phase stirred tank, 2014, J. Appl. Math. Mod.,Vol. 38, No. 1, pp. 63-92

Kantarci, N., Borak, F., Ulgen, K.O., Bubble columnreactors, 2005, J. Process Chemistry, Vol. 40, No. 7, pp.2263-2283

André Bakker, Modeling Flow Fields in Stirred Tanks,Reacting Flows - Lecture 7, http://www.bakker.org

Questions