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(252a) Modeling of Multiphase Reactors: A Mesoscale Perspective

Yang, N. - Presenter, Institute of Process Engineering, Chinese Academy of Sciences
Mesoscale transport phenomena and mechanisms are essential to achieve a more fundamental understanding on the mass, momentum and heat transfer in the classical study of transport phenomena and on the mixing, residence time distribution and rate-limiting analysis in chemical reaction engineering, yet they are now beyond the scope of classical textbooks of chemical engineering. We highlight a heuristic mesoscale modeling approach starting from a conceptual Energy-Minimization Multiscale (EMMS) model and ending at the stability-constrained multifluid CFD model. While the stability condition determines the direction of system evolution, the stability-constrained CFD further describes the dynamics of structure evolution. We establish the Dual-Bubble-Size (DBS) model, an extended EMMS approach for gas-liquid systems. Stability condition is formulated as the minimization of the sum of two energy dissipations, reflecting the compromise of a liquid-dominant regime at which smaller bubbles prevail and a gas-dominant regime favoring the existence of larger bubbles. It supplies a mesoscale constraint for conservation equations, and a mesoscale perspective to understand the macroscale regime transition. The model calculation for gas-liquid and gas-solid systems demonstrates the intrinsic similarity of the two systems: the system evolution at macroscale is driven by stability conditions. Theoretically stability condition may offer closure laws for CFD simulation, leading to the stability-constrained multifluid CFD model. While direct integration is difficult, we propose various simplified approaches to derive the closure models for drag, bubble-induced turbulence and correction factors for coalescence rate in population balance equations. The stability-constrained multifluid CFD model shows much advantage over traditional closure models. We will also show how this model is applied in complex liquid-solid flow of swelling particles in olefine polymerization and in predicting the drop size in liquid-liquid emulsification.