Study On Melt Crystallization Of Durene

Durene (1,2,4,5-tetramethyl-benzene) is an important chemical raw material toproduce pyromellitic dianhydride. At present, the production of durene includeseparation and synthesis methods. The major source of durene is thetetramethyl-benzene distillation cut from catalytic reforming unit in which durene isseparated by freezing crystallization techniques. Then the freezing crystallizationmust be centrifuged to isolate filter cake from the mother liquor, after which durene isobtained from the filter cake by squeezer. The deficiency of the traditional separatingmethod is low yield and purity. So, it’s required to find a more efficient method topurify durene from C10.C10is mostly consisted of durene, its isomers and homologues. Since the boilingpoints of tetra methyl benzene are very close, distillation is not appropriate to separatethem. But durene has higher melting point than other impurities due to its specialstructure. So it’s feasible to adopt crystallization to separate and purify durene fromits impurities.The mathematical model of heat conduction with a moving boundary wasapplied to simulate the durene crystallization process which carried out outside acylindrical crystallizer. Considering the effects of the temperature of the cooling fluidon that of solid-liquid interface, the analytic iteration method was applied to solve themodel. The calculated results fit well with experimental results under both isothermaland non-isothermal crystallization process.For the crude durene obtained from traditional process, which purity is94.02%,the static melt crystallization was adopted to purify durene and the effects of thecooling rate，the final crystallization temperature，the sweating temperature and thesweating time were systemically studied. Besides, the orthogonal experimental designmethod was used to determine the optimal crystallization condition. The raw materialwas crystallized at73°C with cooling rate of0.03°C/min and sweated at77°C for30min. The result indicated that the purity of durene reached to99.06%with theyield of75.29%and the purification rate coefficient kpwas equal to1.39×10-3s-1. For the mixtures containing62.3%durene which comes from the catalyticreforming unit, bubbling crystallization was used to purify durene. The mainparameters were studied and the appropriate operating condition was that bubblingrate was90L/h, cooling rate was0.1°C/min, crystallization time was1.5h, heatingrate was0.1°C/min and sweeting time was2h. By doing experiments with durenesolution of different concentration, the empirical crystallization and sweetingequilibrium equations are developed.The62.3%durene solution was purified to morethan99%by three continuous crystallization, the yield was more than60%and thecalculated results fitted very well with experimental data. Purifying durene by meltcrystallization is an energy-saving and environmental technology which doesn’t needany solvents and the mother liquor can be charged to the last crystallization stage andthe sweat to the next one.