Bubble Column Reactors

bubble column reactor engineering

1. Bubble Column Reactors
Quak Foo Lee
ChmlTech Ltd.

2. Topics Covered
⚫Bubble column fundamentals
⚫Type of bubble columns
⚫Gas Spargers
⚫Bubble flow dynamics
⚫CFD Modeling
⚫Experiments vs. Simulations

3. Introduction
⚫Bubble columns are devices in which gas, in the
form of bubbles, comes in contact with liquid.
⚫The purpose may be simply to mix the liquid phase.
⚫Substances are transferred from one phase to the
other

4. Bubble Columns
⚫Gas is sparged at the bottom of the liquid pool
contained by the column.
⚫The net liquid flow may be co-current or counter-
current to the gas flow direction or may be zero.
⚫Spargers, like porous plates, generate uniform size
bubbles and distribute the gas uniformly at the
bottom of the liquid pool.

5. Bubble Column
Co-
current
Counter-
current

6. Type of Bubble Columns
A)Simple bubble column; B) Cascade bubble column with sieve trays;
B)C) Packed bubble column; D) Multishaft bubble column;
C)E) Bubble column with static mixers

7. Gas-Liquid Mixing
A) Bubble column; B) Downflow bubble column; C) Jet loop reactor

8. Pilot Scale bubble Column

9. Gas Distributions
⚫The gas is dispersed to create small bubblesand
distribute them uniformly over the cross section of
the equipment to maximize the intensity of mass
transfer.
⚫The formation of fine bubbles is especially desirable
in coalescence-hindered systemsand in the
homogeneous flow regime.
⚫In principle, however, significant mass transfercan
be obtained at the gas distributor through a high
local energy-dissipation density.

10. Static Gas Spargers
Perforated ring
Dip tube Perforated plate
Porous plate

11. Dynamic Gas Spargers

12. Flow Regimes

13. Fluid Dynamics
⚫Rising gas bubbles entrain liquid in their
wakes.
⚫As a rule, this upward flow of liquid is much
greater than the net liquid flow rate.
⚫Because of continuity, regions therefore exist
in which the liquid is predominantly moving
downward.

14. Fluid Dynamics
Radial distribution of liquid velocity in a bubble column

15. Cell Structure in BCs

16. Bubble Size
Sauter diameter d
bS
(mean bubble diameter, calculated from the volume to surface ratio)
This formula is based on Kolmogorov's theory of isotropic turbulence.25.0
5.0
6.0
4.0
2
















=
L
G
G
LM
bs
e
d






17. Bubble Size Distribution (BSD)
⚫Narrow BSD
⚫For bubble columns with relatively low gas volume
fraction.
⚫In homogeneous regime.
⚫Wide BSD
⚫As gas velocity and therefore, gas volume fraction
increases, a heterogeneous or churn-turbulent
regime sets in.

18. Gas Holdup
⚫Gas holdup is one of the most important
operating parameters because it not only
governs phase fraction and gas-phase
residence time but is also crucial for mass
transfer between liquid and gas.
⚫Gas holdup depends chiefly on gas flow rate, but
also to a great extent on the gas–liquid system
involved.

19. Gas Holdup
⚫Gas holdup is defined as the volume of the gas phase
divided by the total volume of the dispersion:
⚫The relationship between gas holdup and gas velocity is
generally described by the proportionality:
⚫In the homogeneous flow regime, nis close to unity. When
large bubbles are present, the exponent decreases, i.e., the
gas holdup increases less than proportionally to the gas
flow rate. n
GG
LG
G
G
U
VV
V
~

+
=

20. Interphase Forces
⚫Drag force
⚫Resultant slip velocity between two phases.
⚫Virtual mass force
⚫Arising from the inertia effect.
⚫Basset force
⚫Due to the development of a boundary layer around a
bubble.
⚫Transversal lift force
⚫Created by gradients in relative velocity across the bubble
diameter, may also act on the bubble.

21. Bubble Column Modeling
Fluid Dynamics
Reaction
Mass transfer
Heat transfer
Bubble breakage
And coalescence
Mass transport
mixing
Fluid
properties
Phase distribution
transfer resistance
Gas hold-up
Bubble
recirculation
Turbulence shear
stress terminal
velocity
residence time
Fluid properties
Interfacial area
driving force
mixing
Limitation
Enhancement

22. CFD Modeling of Bubble Columns
⚫Eulerian-Lagrangian approach
⚫To simulate trajectories of individual bubbles
(bubble-scale phenomena)
⚫Eulerian-Eulerian approach
⚫To simulate the behavior of gas-liquid dispersions
with high gas volume fractions (e.g. to simulate
millions of bubbles over a long period of time)

23. Simulation Objective
⚫Unsteady, asymmetric
⚫To avoid imposing symmetry boundary conditions
⚫Two-dimensional
⚫Consider the whole domain
⚫Three-dimensional
⚫Use a body-fitted grid, or
⚫Use modified conventional axis boundary
conditions to allow flow through the axis

24. When to use 2D Simulation?
⚫Estimateliquid phase mixing and heat transfer
coefficient.
⚫Predicttime-averaged liquid velocity profiles and
corresponding time-averaged gas volume fraction
profiles.
⚫Evaluate, qualitatively, the influence of different
reactor internals, such as drat tubes and radial
baffles, on liquid phase mixing in the reactor.

25. When to use 3D Simulation?
⚫Capturedetails of flow structures.
⚫Examinethe role of unsteady structure on mixing.
⚫Evaluatethe size and location of draft tube on the
fluid dynamics of bubble column reactors.

26. Simulation Consideration
⚫For column walls, which are impermeable to fluids,
standard wall boundary conditions may be specified.
⚫Use symmetrywhen long-time-averaged flow
characteristics is interested.
⚫When the interest is in capturing inherently unsteady
flow characteristics, which are not symmetrical, it is
essential to consider the whole columnas the solution
domain.
⚫Overall flowcan be modeled using an axis-symmetric
assumption.

27. 2D Bubble Column
Plenum
Gas
Sparger
Only gas phase
Gas-liquid
Dispersion
(gas as dispersed
phase)
Gas-liquid
Interface
(may not be flat)
Liquid drops may
Get entrained in
overhead space
Open to surroundings
P
top
P
s( )gdzp
H
GGLLh
+=
0

P
0 = P
top+ P
h
P
0
Hydrostatic head
above the sparger
Overhead pressure

28. 2D and 3D ‘Instantaneous' Flow
Field
Descending
flow region
First bubble
flow region
Vortical
structures
Descending
flow region
2D
3D
Source: http://kramerslab.tn.tudelft.nl/research/topics/multiphaseflow.htm

29. Dispersion of Tracer in a Liquid

30. Verification and Validation
⚫Scale-downfor experimental program.
⚫Experiments are carried out in simple geometriesand different conditions
than actual operating conditions.
⚫Available information on the influence of pressure and temperature
should be used to select right modelfluidsfor these experiments.
⚫Detailed CFDmodels should be developed to simulate the fluid dynamics
of a small-scale experimental set-up under representative conditions.
⚫The computational model is then enhancedfurther until it leads to
adequately accurate simulations of the observed fluid dynamics.
⚫The validated CFD model can then be used to extrapolate the
experimental dataand to simulate fluid dynamics under actual operating
conditions.

31. 2-D CFD Simulation

32. Experiments
Lateral movement of the bubble hose in the flat bubble column (gas flow rate 0.8 l/min)
Becker, et al., Chem. Eng. Sci. 54(12):4929-4935 (1999)
Meandering motions

33. Simulation and Experiment
t = 0.06s t = 0.16s t = 0.26 s t = 0.36 s
Simulation and experimental results of a bubble rising in liquid-solid fluidized bed.
Fan et al. (1999)

34. References:
⚫Becker, S., De Bie, H. and Sweeney, J., Dynamics flow behavior in bubble
columns, Chem. Eng. Sci., 54(12):4929-4935 (1999)
⚫Fan, L.S., Yang, G.Q., Lee, D.J., Tsuchiya, K., and Lou, X., Some aspects
of high-pressure phenomena of bubbles in liquids and liquid-solid
suspensions, Chem. Eng. Sci., 54(12):4681-4709 (1999)