Process of <i>p</i>-Xylene Production by Highly Selective Methylation of Toluene

Abstract

p-Xylene is an important industrial compound, and its demand has been increasing in recent years. It is mostly produced from cracking of naphtha, but there is a need for new and cost-effective methods for the production. Toluene alkylation with methanol over an alumino-silicate zeolite catalyst, such as ZSM-5, produces a mixture of xylene isomers with low p-xylene selectivity. Due to the very close boiling points of xylene isomers, it is very expensive to separate them. There has been some success in enhancing p-xylene selectivity by modifying the ZSM-5 catalyst. This results in reduced separation cost, which makes toluene methylation a competitive process for p-xylene production. Based on these findings, a novel process for the production of p-xylenes by the catalytic methylation of toluene followed by reactive distillation for the separation of p-xylene (instead of the more costly conventional technique of separation based on crystallization and adsorption) is developed and a complete process flow diagram is simulated using Aspen Plus. Using the built-in optimization tool in Aspen Plus, we optimized reactor parameters for a maximum p-xylene selectivity of 97.7%. After separation, a p-xylene product stream purity of 99.7% is achieved. High p-xylene selectivity in the reactor and use of reactive distillation reduces the separation cost and renders the new process economically competitive.