Studies On Multi-Scale Characteristics And Coupling Method Of The Crystallization Processes

Crystallization is one of the basic unit operatings in chemical industry. Due to high purity, low energy consumption and low pollution, crystallization is widely used in the field of chemical, medicine, biological technology and environmental protection, etc. As most of the industrial crystallization involves solid and liquid phase flow, the quality of crystal products strongly depends on the fluid dynamics factors, state of liquid flow and crystal suspension, besides the crystallization kinetics factors, crystal growth and nucleation. But the researchs were mostly developed based on the fully mixed suspention ideal model in early time, which assumed the flow field of solid, liquid and different size of crystal in crystallizer is exactly the same, and equal to uniform value, that is to say ignored the influence of fluid dynamics. So it resulted in quite big discrepancies, when the results of theory or experiment were applied to the industrial crystallization amplification process. In order to more exactly describing the crystallization process, both the crystallization kinetics factors and fluid dynamics factors should be taken into consideration. But for the high-intensity turbulence in industrial crystallization process causes major difficulties for accurately describing the flow field. In recent years, with the rapid development of computer, the computational fluid dynamics simulation technology which used in the study of complex fluid, takes great convenience for industrial crystallization process study.The complicated industrial crystallization process has multi-scale characteristics. For realizing the analogue simulation of industrial crystallization process, this paper establishes a coupling model, by using the modern computing tools CFD. The research details are followed.According to the state of crystal suspension in crystallizer, the multi-scale coupling simulation model is established on the different levels of complex system. With the help of CFD software, the micro scale of crystallization process is mainly to investigate the flow field, especially the crystal flow field. The fluid dynamics information obtained at the micro scale will be used for the meso scale crystallization process simulation. At the meso scale, the effects of crystal suspension on crystallization process is simulated. Based on similar principle of the crystal suspension, the crystallizer is divided reasonably. Then the meso scale model is established on the basis of population balance equations and solute material balance equations, which fully considers the influence of crystal flow state in every zone. Finally taking the macro mix and external operating conditions into consideration, the overall macro simulation model of crystallization is build, and solved by the data analysis software.By using the multi-scale simulation model on the industrial crystallization process, the continuous cooling crystallization process of KCl-H2O system was simulated. The flow field, density of suspension and supersaturation were analysised at different inlet velocity or feed-in concentration. It provides a new simulation method for the industrial crystallization process.