Study On System Integration Of Methanol-Dimethyl Carbonate Azeotrope Separation Process

As a kind of green solvent and chemical intermediate,dimethyl carbonate(DMC)is an important extension of the downstream industrial chain of coal chemical industry,methanol(MeOH)chemical industry and CO2 chemical industry.However,the MeOH-DMC azeotrope separation with many disadvantages like high energy consumption,complicated operation has greatly restricted its industrialization economic benefits.Ionic liquids(ILs),as novel solvents,have excellent properties such as structure designable,low vapor pressure,easy separation and resuable.ILs can achieve selective bonding and separation with MeOH-DMC azeotrope through varing their spatial structures and functional groups,which enables it a wide application prospect in azeotrope separation.In this paper,the vapor-liquid phase equilibrium of MeOH-DMC with ionic liquid was studied,and the process and system integration of DMC production process was simulated and optimized.The main innovative work and achievements were as follows:1.A study on phase equilibria of IL-MEOH-DMC system was carried out with three kinds of imidazole ionic liquids(i.e.[Bmim][Tf2N],[Emim][Tf2N]and[Emim][PF6])as extractants,comprising different spatial orientations and polarity of anions and cations.The results showed that the order of interacting effects between three ionic liquids and azeotropes followed by[Bmim][Tf2N]>[Emim][Tf2N]>[Emim][PF6].[Bmim][Tf2N]with longer chain cation and larger polar anion showed a better separation performance.And the azeotrope point could be eliminated when the molar ratio of[Bmim][Tf2N]was 0.05.Furthermore,the interaction energy of anion and cation with MeOH and DMC were calculated.The results showed that the interaction of cations and azeotrope molecules was higher than that of anions,but the effects of different cations were near the same,so the bond cooperation between anions and azeotrope molecules was the key to achieve high efficiency separation of azeotropic system.The order of the interaction energy between cation or anion and the azeotrope components was[PF6]-<[Tf2N]-,[Emim]+<[Bmim]+ respectively,[Bmim][Tf2N]was proved to have the best separation performance.2.The process simulation and optimization of four kinds of azeotropic separation processes(i.e.ILs extractive distillation,ethylene carbonate(EC)extractive distillation,ethylene glycol(EG)extractive distillation and pressure swing distillation)were systematically carried out.The global sensitivity analysis method was used to determine the constraint conditions of the process with co-production of DMC,which were as follows:EC conversion>99.8%,recycled MeOH purity>90wt%,DMC purity>99.5wt%and EG purity>99wt%.The order of products energy consumption in azeotrope separation processes was:EG extractive distillation>EC extractive distillation>ILs extractive distillation>pressure swing distillation.3.The system integration of effective energy flow in the DMC production process was comprehensively studied with exergy analysis method.The results showed that the internal circulation exergy of azeotrope separation process was 1.55 times larger than that in the input total exergy,which means higher energy was needed to establish the initial cycle process and the steady state process was not easy to achieve.Also,the exergy loss in azeotrope mixture separation process was about 7.9%,which occupying more than 80%of total flow exergy loss.The integrated utilization of different grade heat energy played an important role in reducing the exergy loss.With the energy integration,the energy consumption of the pressure swing distillation method was reduced by more than 27%to 4.16 t/tDMC.