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.
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 SizeSauter diameter dbS(mean bubble diameter, calculated from the volume to surface ratio) 0.6 0.25 2 σ 0.5 ηG d bs = 0.4 ε G eM ρ L η LThis formula is based on Kolmogorovs 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: VG εG = VG + VL The relationship between gas holdup and gas velocity is generally described by the proportionality: εG ~ UG n 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 Fluid mixing properties Fluid Dynamics Reaction Enhancement Phase distribution transfer resistance Mass transfer Limitation Gas hold-up Heat transfer Interfacial area Bubble driving force recirculation mixing Fluid properties Turbulence shear Bubble breakage stress terminal velocity And coalescence 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 Ptop Get entrained in Gas-liquid overhead space Interface (may not be flat) Gas-liquid ( ρ Lα L + ρGα G ) gdz H Dispersion ph = ∫ 0 (gas as dispersed Hydrostatic head phase) above the sparger P0 Sparger Plenum Ps Only gas phase P0 = Ptop + Ph Gas
2D and 3D ‘Instantaneous FlowFieldDescending First bubble Descendingflow region flow region flow region Vortical structures 2D 3DSource: 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 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
Experiments Meandering motionsLateral 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 sSimulation 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)