Modeling of bubble column reactors: Progress and limitations

Bubble columns are widely used for carrying out gas-liquid and gas-liquid-solid reactions in a variety of industrial applications. The dispersion and interfacial heat- and mass-transfer fluxes, which often limit the overall chemical reaction rates, are closely related to the fluid dynamics of the system through the liquid-gas contact area and the turbulence properties of the flow. There is thus considerable interest, within both academia and industry, to improve the limited understanding of the complex multiphase flow phenomena involved, which is preventing optimal scale-up and design of these reactors. In this paper, the progress reported in the literature during the past decade regarding the use of averaged Eulerian multifluid models and computational fluid dynamics (CFD) to model vertical bubble-driven flows is reviewed. The limiting steps in the model derivation are the formulation of proper boundary conditions, closure laws determining turbulent effects, interfacial transfer fluxes, and the bubble coalescence and breakage processes. Examples of both classical and more recent modeling approaches are described, evaluated, and discussed. Physical mechanisms and numerical modes creating bubble movement in the radial direction are outlined. Special emphasis is placed on the population balance modeling of the bubble coalescence and breakage processes in two-phase bubble column reactors. The constitutive relations used to describe the bubble-bubble and bubble-turbulence interactions, the bubble coalescence and breakage criteria, and the daughter size distribution models are discussed with a focus on model limitations. The demand for amplified modeling, more accurate and stable numerical algorithms, and experimental analysis providing data for proper model validation is stressed.