Bubble Column Reactors

Quak Foo LeeDepartment of Chemical and Biological Engineering

The University of British Columbia

Topics Covered

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

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

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.

Bubble Column

Co-current

Counter-current

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

Gas-Liquid Mixing

A) Bubble column; B) Downflow bubble column; C) Jet loop reactor

Pilot Scale bubble Column

Gas Distributions The gas is dispersed to create small bubbles and

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 systems and in the homogeneous flow regime.

In principle, however, significant mass transfer can be obtained at the gas distributor through a high local energy-dissipation density.

Static Gas Spargers

Perforated ring

Dip tube Perforated plate

Porous plate

Dynamic Gas Spargers

Flow Regimes

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.

Fluid Dynamics

Radial distribution of liquid velocity in a bubble column

Cell Structure in BCs

Bubble Size

Sauter diameter dbS

(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

GG

LMbs ed

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.

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.

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, n is 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.

nGG

LG

GG

U

VV

V

~

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.

Bubble Column Modeling

Fluid Dynamics Reaction

Mass transferHeat transfer

Bubble breakageAnd 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

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)

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

When to use 2D Simulation?

Estimate liquid phase mixing and heat transfer coefficient.

Predict time-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.

When to use 3D Simulation?

Capture details of flow structures.

Examine the role of unsteady structure on mixing.

Evaluate the size and location of draft tube on the fluid dynamics of bubble column reactors.

Simulation Consideration For column walls, which are impermeable to fluids,

standard wall boundary conditions may be specified.

Use symmetry when 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 column as the solution domain.

Overall flow can be modeled using an axis-symmetric assumption.

2D Bubble Column

Plenum

Gas

Sparger

Only gas phase

Gas-liquid Dispersion(gas as dispersed phase)

Gas-liquidInterface(may not be flat)

Liquid drops mayGet entrained in overhead space

Open to surroundings

Ptop

Ps

gdzpH

GGLLh 0

P0 = Ptop + Ph

P0

Hydrostatic head above the sparger

Overhead pressure

2D and 3D ‘Instantaneous' Flow Field

Descendingflow region

First bubbleflow region

Vortical structures

Descendingflow region

2D 3D

Source: http://kramerslab.tn.tudelft.nl/research/topics/multiphaseflow.htm

Dispersion of Tracer in a Liquid

Verification and Validation Scale-down for experimental program.

Experiments are carried out in simple geometries and different conditions than actual operating conditions.

Available information on the influence of pressure and temperature should be used to select right model fluids for these experiments.

Detailed CFD models should be developed to simulate the fluid dynamics of a small-scale experimental set-up under representative conditions.

The computational model is then enhanced further until it leads to adequately accurate simulations of the observed fluid dynamics.

The validated CFD model can then be used to extrapolate the experimental data and to simulate fluid dynamics under actual operating conditions.

2-D CFD Simulation

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

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)

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)