Process Development Of The1000t/a Paraxylene Pilot-Plant

Paraxylene(PX) is an important organic chemical material that can be used to produce terephthalic dihydrazide(PTA), dimethylterephthalate, and polyethelene terephthalate(DMT). Nowadays in production of PX, two methods are usually used:toluene disproportion and xylene isomerization, both of them has a complex flowsheet and low conversion of toluene. Recently a new method with high toluene conversion and high toluene utilization is proposed, i.e. toluene alkylation with methanol, methanol is used as one of the reactant which fills the blank of Cl compound’s usage.This work chose the shape selectivity zeolite catalyst of ZSM-5type as the alkylation catalyst, developed a the process of toluene alkylation with methanol for paraxylene using process simulation software HYSYS. The process can be divided into three parts:(1) Reaction part. Among the reactant and carrier gas, first mix toluene and methanol to get a good solubility and good effect of heat transfer, then send the steam, hydrogen and toluene-methanol gas mixture together into the reactor. To promote toluene conversion and take side reaction into account at the same time, three reactors are installed in series, cold methanol are supplied between reactors as reactant and cool down the product temperature at the same time.(2) Primary separation part. This part aims to separate the carrier gas and unreacted toluene, and reuse them. The product from reaction part is cooled down to25℃and sent to the flash drum. The vapor phase from the flash drum is hydrogen whose molarity is over97%. The liquid phase from flash drum is sent to the decanter, getting water with molarity over99.9%and oil phase that consists of toluene and xylene. After distillation, oil phase can be divided into toluene with molarity of94%from the top and xylene from the bottom.(3) PX purification part. In this work, melt crystallization is used to purify PX. Finally get PX with molarity of99.5%and mother liquor which is mainly xylene isomers.Getting the primary process, software ASPEN ENERGY ANALYZER is used to optimize the heat exchange net. Letting product from reactor exchange heat with reactant, water, mixture of toluene and methanol orderly, raw PX exchange with mixture of toluene and methanol, can utilities consumption be decreased a lot.To reduce the hot utility consumed to vaporize water, heat pump is used in the process. The product from reaction part should be compressed to2.4MPa to raise the temperature after heating reactant and water, then exchange heat with water again to vaporize water. With the combination of heat and work, the latent heat of vaporization of water is recovered, the utilities is saved by284Y/tPX.At last, proposed a method to design trays of batch distillation column with an example of hydraulic design of a batch column which is used to separate benzene from toluene. Studies show that the process developed to produce PX of scale1000t/a is reasonable and feasible, can be used to build pilot plant.