Study On The Purification Of Para-xylene By Melt Crystallization

Para-xylene(PX)is an important organic chemical raw material in industry.Adsorption separation of mixed xylene is the main method to produce p-xylene.Generally,the p-xylene obtained by adsorption separation contains a small amount of toluene.The distillation is traditional separation method to produce high purity p-xylene,but its tower equipment is more complex.The fixed investment and maintenance of equipment costs are high.Distillation process also contains carbonization,coking and other issues.Due to some problems in distillation separation,a new melt crystallization method is used to separate p-xylene and toluene mixture in this paper.The main work is as follows:(1)The solid-liquid equilibrium phase between p-xylene and toluene was measured by differential scanning calorimetry(DSC).The results show that the binary system is a low eutectic system.Utilizing NRTL activity coefficient model established solid-liquid equilibrium correlations.(2)The dynamic mathematic model of liquid film crystallization was established.The model can calculate the experimental process accurately.Based on the model,the effects of feed superheat,cooling rate and spray density on the growth of crystal layer were studied respectively.These researches provide guidance for industrial design.(3)Based on the previous thermodynamics and kinetics researches,liquid film crystallization separation process of p-xylene experiment were studied.The effects of the crystallization and sweating operating process parameters on the separation of products were investigated.Using optimization method can obtain p-xylene products with the purity over 99.5%.The results indicate that it is feasible to separate p-xylene and toluene by melt crystallization.(4)Cost accounting about melt crystallization and distillation separation processes was carried out respectively.The results show that fixed investment of crystallization is much lower than the distillation and operation conditions are stable.Crystallization is easy to get high purity products,but its energy consumption is slightly higher than distillation.So the crystallization is suitable for small batch production,and the distillation is appropriate for high-volume production.In this paper,the separation of p-xylene and toluene by melt crystallization is simple and green.It can obtain high purity p-xylene products and has important industrial practical value.