Study On Refinery Of High-pure P-Cresol By Melt Crystallyzation Process

With the increasingly severe competition in chemical and pharmaceutical market and awareness of environmental protection,research, development and application of high efficiency separation and purification techniques have get attention of the industry. Melt crystallization is a technique used for separation and purification of mixtures of chemicals based on the different freezing point of mixtures. Compare with conventional distillation, melt crystallization process has advantages of low operation temperature, low energy consumption, high product purity and green environmental protection, etc. The researches of high purity chemical products prepared by melt crystallization become increasingly active. This using by P-cresol for object,the flow of melt crystallization of P-cresol in vertical tube was studied comprehensively in this paper.Bubble rise velocity and liquid slug length in crystallizer were measured by laser Doppler velocimeter and laser particle dynamic analyzer respectively to study on gas-liquid two phase flow in falling-film crystallizer. The results show: bubble rise velocity increased firstly and then kept invariant; with the increasing of superficial gas velocity, the rise velocity increased and axial distribution became unstable; liquid slug length in gas-liquid two phase slug flow accorded with logarithm normal distribution; gas and liquid velocities had little influence on liquid slug length. When bubble height is higher than a critical height, the effect of gas slug velocity on liquid slug length was slightly.Two phase flow model of falling-film melting system was built by analyzing the flow characteristic of gas slug in gas-liquid (p-cresol melt system) two phase flow. Gas-liquid two phase flow and crystal growth process of p-cresol melt crystallization process were simulated by CFD technique. The results show pulse-bubbling can intensify mass and heat transfer between melts-crystals wall laminar boundary layer and liquid bulk, and keep the stable of concentration and temperature gradient in the boundary layer, which were beneficial to p-cresol melt crystallization process. Crystal layer thickness increased with time and became flat finally. With the increasing of Ste number, driving force of heat transfer and the layer thickness largen. Crystal layer thickness decreased sharply and gradually stabilized when initial superheat degree increased.The optimal falling-film crystallization process conditions in p-cresol melt crystallization were as follows: (1) pulse gas flowrate is 90 L/h; (2) temperature drop rate is 0.6~0.8℃/min; (3) crystallization time is 40 min; (4) crystallization ratio is about 70 %; (5) the sweating heating rate is 0.2~0.3℃/min; (6) the sweating time is 40 min.The research results not only have important theoretical significance and practical value for studying on p-cresol melt crystallization purification process, and realistic significance to the development of mass transfer and separation engineering.