Numerical Simulation of Reactive Crystallization in Stirred Tank Reactors

Model development is essential for design and scale up of multiphase chemical reactors to provide a better understanding of physical and chemical phenomena between phases at different scales. Mathematical descriptions of the reactive crystallization include mass transfer, chemical reaction, crystallization kinetics, population balance and hydrodynamics, which consist of a set of partial differential equations with high nonlinearity. A full simulation with a commercial computational fluid dynamics (CFD) software is possible with existing computational resources but not desirable during the initial stage of reactor design. The purpose of this thesis is to explore the mechanism of multiphase reactive crystallization and develop a compartmental model to combine hydrodynamics and detailed reaction efficiently. The chemical system of CO2(G)-H2O(L)-Mg(OH)2(S) is chosen as the practical application to reveal mechanisms of multiphase reactive crystallization, which couple the reactive dissolution, chemical absorption and crystallization. As modeling of such complex system is challenging, the reactive dissolution of Mg(OH)2(S) in HCl(aq) and reactive crystallization of CaCO3(S) from CO2(G) and Ca(OH)2(aq) system are studied separately. The first part of the thesis introduces the gas-liquid and solid-liquid mass transfer models based on two-film theory and Nernst-Planck electroneutrality. In addition, enhancement factor is adopted to modify mass transfer fluxes when chemical reaction occurs in liquid film. Then, population balance model is presented along with several solution techniques to calculate particle size distributions of gas bubbles and final crystal products. The closure models include nucleation and growth of crystals, breakage, coalescence and agglomeration of gas bubbles and crystals. Finally, a compartmental model combining the flow field obtained by CFD simulation and reaction mechanisms is constructed to estimate the influence of flow field on multiphase crystallization. The compartmental modelling results show that heterogeneous mixing has a strong influence on local mass transfer rates and size distribution of final crystal products. By appropriate division of the fluid domain, compartmental model can offer a more efficient simulation for reactive crystallization without the limitation of chemical components and geometries of different reactors. This characteristic highlights the potential extensibility and portability of compartmental model in reactor design and scale up.