Multi-objective Optimization And Assessment Of P-xylene From Coal-based Methanol Process

P-xylene(PX)is an important raw material for preparing p-phthalic acid,the demands of PX increased fast with the rapid development of the polyester industry,chemical products derived from coal are overcapacity,and their homogeneity is serious these years.Therefore,new technologies for the synthesizing xylene from coal-based methanol not only can solve the shortage of PX,but also solve the surplus of methanol.This work studies the multi-objective optimization of two separation processes including a classic two-column sequence process and a dividing wall column process in coal-based methanol to PX process,and techno-economic analysis,life cycle assessment of two coal-based methanol-to-PX routes.1.Due to the challenge of high energy consumption in the chemical separation process,a multi-objective optimization research was carried out in the separation process of non-aromatic hydrocarbons,benzene,toluene and xylene mixture using Aspen Plus and Matlab software in this paper.Respectively,the traditional two-column sequence separation process and the dividing wall column process have been established and optimized by NSGA-? algorithm.The total annual cost(TAC),the energy consumption per product flow rate(GEC),the productivity of benzene-toluene and the productivity of xylene were set as objective functions.The results showed that effect of the reflux ratio on TAC is critical.The two separation processes were compared based on the lowest TAC under the premise of satisfy the constraints.The results indicated that the lowest TAC,heat transfer cost and GEC of the dividing wall column process were reduced about 7.7 %,16.1 % and 26.3 % in comparison with the two-column sequence process,respectively.It was indicated that the dividing wall column process has more economical and lower energy,which would be the best sustainable process in xylene production.2.An overall life cycle analysis of coal-based methanol-to-PX(MTP)technologies have been studied in this work.Two production routes named MTP-I and MTP-II have been simulated,analyzed and discussed.Several conditions,including energy efficiency,capital investment,product cost,green degree and life cycle cost,were optimized and compared for the two routes.Life cycle cost model based on green degree method was proposed.The results indicated that the production cost comparison of the MTP-I process route and the MTP-II process route is 7481.0 RMB/t PX<9691.2 RMB/t PX,the life cycle cost comparison is 11924.1 RMB/t PX<16086.0 RMB/t PX,and the greenness comparison is 31869.6gd/t PX>49242.9gd/t PX.The MTP-I process shows more advantage in product cost and life cycle cost,while the MTP-II process is more environmentally friendly in production with a higher green degree value.The results identify MTP-I as the most suitable options for PX production from an economic perspective,but MTP-II turned out to be the best sustainable alternative route from an environmental perspective.