Study of Heat and Mass Transfer Patterns in Evaporative Crystallization Systems

摘要

Evaporative crystallization is of the complicated multiphase mass and heat transfer process. Because of the fact that the crystal nucleation and growth is determined by mass and heat transfer, which influences the final crystal size distribution (CSD), the transfer pattern of the evaporation crystallization process was analyzed firstly and a transfer model was built. In the model, according to the two steps crystallization theory, the dynamics mechanism of crystal nucleation and growth was analyzed. On this basis, a model of crystal nucleation and growth has been set up, which can describe accurately the pattern of the crystallization. Through the calculation of some parameters of the model, the control step can be evident. NaCl evaporation crystallization in chlorine alkali industry, for example, is studied and applied for the model. The result showed that under the high temperature and low velocity of flow, NaCl crystallization is controlled by the course of diffusion. According to these, a function taking into account the operation parameters, such as temperature, velocity of flow, supersaturation and size of crystal, which can express accuracy the crystallization process, was built. By the calculation of mass balance, thermal balance and crystal population balance, the influence function of operation parameters on the CSD was established. The result of model calculation accords accurately with the experimentation data, which suggests that the mathematical model in tin's study can simulate the relationship between the operation and crystal size. The model can be used to predict optimal operation parameters in industry.