Para-xylene Melt Crystallization Process

Para-xylene(PX)is an important basic chemical raw material for the production of polyester fibers and resins,coatings,dyes and pesticides.At present,the external dependence of this product is nearly 50%,and the production technology is monopolized by foreign companies.China’s PX refining and separation technology is almost entirely based on technology introduction,and there is no relevant core technology in China.In order to realize the localization of PX high-efficiency separation technology,the melt crystallization technology was studied in detail.In this dissertation,the thermodynamic data of xylene solid-liquid equilibrium given by the predecessors are synthesized and evaluated.Based on this,the static crystallization process is studied first.In order to obtain the optimized process parameters,this paper investigated the effect of cooling rate on the crystallization process and sweating endpoint temperature on the sweating process.The experimental results show that static crystallization is more suitable for high PX concentration feed.A product having a purity of more than 99.8wt% can be directly obtained by sweat process.In this paper,the crystal growth rate under optimal conditions and the partition coefficient during sweating are fitted,which can provide guidance for subsequent industrial production.In the study of the suspension crystallization process,the influence of the crystallization parameters on the crystal product was investigated.It has been found that the addition of seed crystals can effectively reduce the moisture content of the product.The cooling rate has little effect on the crystallization,and the effect of the stirring rate on the moisture content is complicated.The filter cake obtained by the optimum process has a moisture content of about 12%,and the product of the target purity cannot be obtained.Compared to static crystallization,suspended crystallization has a time and cost advantage in the treatment of low concentration feeds.In order to study the sweating process in detail,this dissertation uses the fractal theory of porous media to model the sweating process.By introducing the heat transfer and seepage theory,the model fits well the changes in sweat quality and sweat composition during the sweating process.Through simulation and experiment,it is found that the sweating process is mainly controlled by heat transfer,and the concentration of sweat and the concentration of sweat conform to the solid-liquid equilibrium phase diagram.The establishment of this sweat model has guiding significance for the study of the mechanism of sweating and the control of sweating process.