The Application Of Computational-Fluid-Dynamics (CFD) To Mixing Process In DTB Crystallizer

Draft tube baffle (DTB) crystallizer is a kind of crystallizer with higher efficiency in the late1950s. DTB crystallizer can form typical internal circulation system with the draft tube, which has better mixing efficiency. The Mixed Suspension Mixed Product Removal (MSMPR) crystallizers are commonly used in most industrial crystallization research works. The MSMPR assumed that the crystallizer was operated with folly mixed suspension and fully mixed product removal. However, it is difficult to strictly fulfill the assumption, especially in large-scale crystallizers. Therefore, the results obtained in the laboratory scale crystallizer can not directly used in large scale crystallizer design. During last decades most of researchers have paid attention to parameters affecting the industrial crystallization such as the crystal growth and nucleating rate, a few works was focused on solid suspension effect on the crystallization. As reported, the operation condition such as the mixing intensity and product removal location has great effect on the product particle size distribution in a continuous crystallizer. The reason is that the particle distribution in the crystallizer is different for different mixing and different sizes of particle. However, these information were rarely reported because the particle distributions in the crystallizer was difficult to be experimentally measured, especially for the continuous operation. If crystals mixing condition is known clearly, accordingly, the process of crystallization and the choosing of optimal operation situation should be understood well.Computational Fluid Dynamics (CFD) uses computer as a tool to study the fluid flow in different processes and the effect on processes of mass transfer, heat transfer and other transformation, through the numerical method of motion equations. And now, CFD is considered as a powerful tool for simulating the process of industrial crystallization.The aim of this work is to study the particle distribution in crystallizer with CFD simulation under continuous operation. The contents of the work are summarized as follows:Based on Euler-Euler multiphase model, solution was defined as the continuous phase, solid particle was considered fluid, and defined as dispersed phase. In the work, standard k-ε model was employed to describe turbulent fluid flow, because it was widely used in the industrial cases. And the inter-phase forces between the dispersed phase and continuous phase, mainly refers to drag force here. was described by Schiller Naumann Drag Model.Continuous crystallization process was simplified into a simulation of suspension state of single dimension crystal in this research. During setup the parameter of computational fluid dynamics simulation, assumption is that feed contain with a special crystal instead of generating crystal in the crystallization process. The effect of particle size, mixing intensity and product remove location on liquid velocity and solid volume fraction were studied. The flow hydrodynamics, the suspension distribution and particle size distribution were obtained by the simulated results.The results showed that the particle distribution in a crystallizer was strongly dependent on the size of the particle and the mixing intensity. Whereas, the product remove. location did not affect much under the studied condition.