Presentation on theme: "Bubble Column Reactors"— Presentation transcript:
1 Bubble Column Reactors Quak Foo LeeDepartment of Chemical and Biological EngineeringThe University of British Columbia
2 Topics Covered Bubble column fundamentals Type of bubble columns Gas SpargersBubble flow dynamicsCFD ModelingExperiments vs. Simulations
3 IntroductionBubble 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 ColumnsGas 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.
9 Gas DistributionsThe 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.
10 Static Gas Spargers Dip tube Perforated plate Perforated ring Porous plate
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 DynamicsRadial distribution of liquid velocity in a bubble column
16 Bubble SizeSauter diameter dbS (mean bubble diameter, calculated from the volume to surface ratio)This formula is based on Kolmogorov's theory of isotropic turbulence.
17 Bubble Size Distribution (BSD) Narrow BSDFor bubble columns with relatively low gas volume fraction.In homogeneous regime.Wide BSDAs gas velocity and therefore, gas volume fraction increases, a heterogeneous or churn-turbulent regime sets in.
18 Gas HoldupGas 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 HoldupGas 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.
20 Interphase Forces Drag force Virtual mass force Basset force Resultant slip velocity between two phases.Virtual mass forceArising from the inertia effect.Basset forceDue to the development of a boundary layer around a bubble.Transversal lift forceCreated by gradients in relative velocity across the bubble diameter, may also act on the bubble.
21 Bubble Column Modeling Mass transport mixingFluid propertiesReactionFluid DynamicsEnhancementPhase distribution transfer resistanceMass transferHeat transferLimitationGas hold-upInterfacial area driving force mixingBubble recirculationBubble breakageAnd coalescenceFluid propertiesTurbulence shear stress terminal velocity residence time
22 CFD Modeling of Bubble Columns Eulerian-Lagrangian approachTo simulate trajectories of individual bubbles (bubble-scale phenomena)Eulerian-Eulerian approachTo 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 Two-dimensional To avoid imposing symmetry boundary conditionsTwo-dimensionalConsider the whole domainThree-dimensionalUse a body-fitted grid, orUse modified conventional axis boundary conditions to allow flow through the axis
24 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.
25 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.
26 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.
27 2D Bubble Column Open to surroundings Overhead pressure Liquid drops mayGet entrained inoverhead spacePtopGas-liquidInterface(may not be flat)Gas-liquidDispersion(gas as dispersedphase)Hydrostatic headabove the spargerP0SpargerPlenumPsOnly gas phaseP0 = Ptop + PhGas
28 2D and 3D ‘Instantaneous' Flow Field Descendingflow regionFirst bubbleVortical structures2D3DSource:
30 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.
32 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): (1999)
33 Simulation and Experiment t = 0.06s t = 0.16s t = 0.26 s t = 0.36 sSimulation 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): (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): (1999)