Study On Pressurized Vapor-liquid Equilibrium Of Methanol-Water-Polyoxymethylene Dimethyl Ether

Polyoxymethylene dimethyl ether（PODE_n）with high oxygen content and high cetane number is a kind of high-quality diesel fuel additive.The basic unit of its synthetic raw material are formaldehyde providing in the main chain and methanol closuring chain.,In addition to the unreacted methanol,there will be a small amount of by-product water in the production process.And the azeotropic problem between methanol,PODE_n and water makes the separation and purification difficult.In this paper,the pressurized vapor-liquid equilibrium data of binary and ternary systems of methanol-PODE_n-water were measured,and the corresponding binary interaction parameters were obtained through correlation of three activity coefficient models,which provided data basis for separation operation.The VLE data of methanol-PODE₂ and methanol-PODE₃ binary systems at 0.7MPa were measured.The thermodynamic consistency was tested by Herington area integral method.The binary interaction parameters were obtained by Wilson,UNIQUAC and NRTL activity coefficient model regression.According to the experimental data and phase diagram of methanol-PODE₂,the azeotropic composition is between 0.93 and 0.95.The ternary vapor-liquid phase diagram of methanol-PODE₂-water system was drawn by measuring the ternary experimental data of methanol-PODE₂-water at four pressures of0.1MPa,0.4MPa,0.6MPa and 0.7MPa.It was found that the molar composition of water at the equilibrium point with higher vapor phase composition of PODE₂ increased compared with that at normal pressure,but the pressure continued to increase and the increase amplitude became smaller.It indicates that the increase of pressure was conducive to the separation of water and PODE₂.Three activity coefficient models were used to correlate the experimental data,and the average deviations of equilibrium temperature and vapor phase composition were not more than0.4k and 0.0076,which showed that the correlation values of the three models were in good a agreement with the experimental value.The binary interaction parameters of the ternary system were obtained by regression,which provided data reference for the separation of water-PODE₂.The ternary vapor-liquid phase diagram of methanol-PODE₃-water system was drawn by measuring the ternary experimental data of methanol-PODE₃-water at four pressures of 0.1MPa,0.4MPa,0.6MPa and 0.7MPa.It is found that the separation degree of water and PODE₃increases with the increase of pressure,and the equilibrium points with higher methanol content in the ternary vapor phase diagram of the system are dense,which indicates that methanol and PODE₃ are easy to separate.Three models were used to correlate the experimental data.The average deviations of equilibrium temperature were 0.4k,0.6k and 0.8k.The average deviations of vapor phase composition of methanol and PODE₃ were 0.0084,0.0099,0.0096 and 0.0043,0.0048 and 0.0060,which indicated that Wilson model had a good correlation result.The binary interaction parameters were obtained by three regression models,which provided data reference for the separation operation of the system.