A Kinetic Study on the Static and Falling-Film Melt Crystallization and the Sweating Process

Xylenes have very broad applications in chemical industries. The mixed xylenes produced mainly consist of p-xylene (PX), m-xylene (MX), o-xylene (OX) and ethylbenzene (EB). Due to their similar molecular structures, these isomers have close boiling points (e.g., PX  138.37C , MX  139.12C , OX  144.41C and EB  136.19C ). Therefore, it is very difficult to separate them from each other by distillation. The current separation processes for these isomers are exploited based on the differences in freezing points and adsorption characteristics. The cost for separation and purification of these isomers is generally very high. Melt crystallization is a separation process which has increasingly gained importance in the last decades. The high separation efficiency of crystallization, especially if used to obtain ultrapure organic chemicals, makes this technology an economically attractive alternative for numerous applications.Sweating is a temperature-induced purification step based on a partial melting of crystals by heating the cooled surface closely up to the melting point of the pure component. Consequently, the impurities adhering to the crystal surface and those contained in the pores of the crystal layer melt and are then discharged under gravity. In this three-year proposal, both static and dynamic melt crystallization will be studied both experimentally and theoretically to separate PX from the mixed xylenes. The effects of process conditions and impurities on crystal purity and recovery ratio will be investigated. Sweating process will be applied after melt crystallization to further purify the obtained PX. The mathematical model for static and dynamic melt crystallization will be developed to describe the heat and mass transfer in the studied system. In the first year, a static melt crystallization experimental apparatus will be designed to study the effects of process conditions on the resulting PX crystals. The effects of impurities on crystal purity and recovery ratio will be investigated. The mathematical model for static melt crystallization will be developed to describe the heat and mass transfer in the studied system. In the second year, a dynamic melt crystallization experimental apparatus will be designed to study the effects of process conditions on the resulting PX crystals. The effects of impurities on crystal purity and recovery ratio will be investigated. The mathematical model for dynamic melt crystallization will be developed to describe the heat and mass transfer in the studied system. In the third year, sweating process will be applied after the static and dynamic melt crystallization to further purify the obtained PX. The effects of process conditions on crystal purity and recovery ratio will be investigated. The mathematical model for sweating process will be developed to describe the heat and mass transfer in the studied system.

Project ID:PB10608-3650
External Project ID:MOST106-2221-E182-053