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 resins, pharmaceutical, chemical fiber, and pesticides 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. Based on these findings, an intensified p-xylene production process through toluene alkylation followed by complete methanol conversion is proposed and a complete process flow diagram is simulated using Aspen Plus. 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. It is found that the methanol conversion could reach 98.0 % with p-xylene selectivity of 92.0 % through increasing reaction temperature to 442.5 ᵒC 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 are carried out based on Aspen Plus V8.4® and the proposed process is proved to be more efficient than existing processes from views of economic metrics and environmental metrics. The overall TAC is reduced by 4.73 % and CO2 emissions are decreased by 40.2 % compared to the existing process without heat integration.