Intensified p-Xylene Production Process through Toluene and Methanol Alkylation

The production of p-xylene has received more and more attention, since it is widely used in chemical synthetic resin, pharmaceutical, chemical fiber, and pesticide industries. The p-xylene production through toluene alkylation is considered to be more promising due to high selectivity of p-xylene and little environmental impact compared to other methods. Although the existing p-xylene production process through toluene alkylation could achieve high selectivity of p-xylene, the methanol conversion is still as low as 70.0%, requiring methanol recovery and recycle system and resulting in additional loss of toluene in the downstream separation of light component, methanol, and toluene. On the basis of these findings, an intensified p-xylene production process through toluene alkylation with complete methanol conversion is proposed and simulated using Aspen Plus V8.4. The optimal operating conditions for the alkylation reactor are obtained using the sensitivity analysis tool and sequential quadratic programming (SQP) optimization solver in Aspen Plus V8.4. It is found that the methanol conversion could reach 98.0% with a p-xylene selectivity of 92.0% through increasing the reaction temperature to 442.5 °C and pressure to 4.0 bar compared to the existing process, resulting in the removal of methanol recovery and recycle system and less toluene loss in the downstream separation. The comparative evaluations demonstrate that the proposed process is more efficient than the existing process based on economic and environmental metrics. The overall TAC is reduced by 4.71% and CO2 emissions are decreased by 40.2% compared to the existing process without heat integration.