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## Bubble Column Reactors

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**Bubble Column Reactors**Quak Foo Lee Department 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**• Simple bubble column; B) Cascade bubble column with sieve trays; • C) Packed bubble column; D) Multishaft bubble column; • E) Bubble column with static mixers**Gas-Liquid Mixing**A) Bubble column; B) Downflow bubble column; C) Jet loop reactor**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**Dip tube Perforated plate Perforated ring Porous plate**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**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.**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.**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**Mass transport mixing Fluid properties Reaction Fluid Dynamics Enhancement Phase distribution transfer resistance Mass transfer Heat transfer Limitation Gas hold-up Interfacial area driving force mixing Bubble recirculation Bubble breakage And coalescence Fluid properties Turbulence shear stress terminal velocity residence time**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**Open to surroundings Overhead pressure Liquid drops may Get entrained in overhead space Ptop Gas-liquid Interface (may not be flat) Gas-liquid Dispersion (gas as dispersed phase) Hydrostatic head above the sparger P0 Sparger Plenum Ps Only gas phase P0 = Ptop + Ph Gas**First bubble**flow region Descending flow region Descending flow region Vortical structures 2D 3D 2D and 3D ‘Instantaneous' Flow Field Source: http://kramerslab.tn.tudelft.nl/research/topics/multiphaseflow.htm**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 modelfluids 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.**Experiments**Meandering motions 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)**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)