Study On Vapor-liquid Equilibrium Of Methanol-Formaldehyde-Polyoxymethylene Dimethyl Ethers

Polyoxymethylene dimethyl ethers(PODE_n)is a kind of compound with high oxygen content,which is developing to be a high quality diesel additive in recent years.At present,the raw materials for the production of PODE_n are mainly formaldehyde,methanol,methylal and paraformaldehyde.The reaction product contains unreacted methanol,formaldehyde and other substances.In order to obtain the target product of higher purity(PODE_2-5),it is necessary to adopt The rectification separation technique separates by-products and unreacted starting materials from the target product.However,there are not many relevant vapor-liquid equilibrium data that can be checked at present,and the basic data of the ternary system is less,especially for the ternary system containing the formaldehyde system,so it is necessary to supplement a large amount of vapor-liquid equilibrium data.Therefore,in this paper,the vapor-liquid equilibrium data of formaldehyde-containing,formaldehyde,and PODE_n-related systems are measured and simulated correlation is performed on the measured data.The vapor-liquid equilibrium data of the methanol-formaldehyde-PODE_n ternary system was first determined using a modified vapor-liquid equilibrium double-cycle ROSE kettle at a constant pressure of 101.3 k Pa.According to the practical problem of industrial synthesis,formaldehyde is used as a concentration gradient in the case where the formaldehyde content is relatively small.Based on maximum-likelihood method,the measured ternary vapor-liquid equilibrium data were correlated by Wilson,NRTL,and UNIQUAC activity coefficient model witn Aspen Plus software.And the corresponding binary interaction parameters and simulated calculation values of the three models are obtained respectively.The actual reliability of the established model is verified by comparing the experimental values with the simulated values.By comparing the simulated value with the experimental value,the average absolute deviation of the temperature and gas phase composition of the simulated value and the experimental value is obtained,and the applicability of the correlation model established by the experimental data regression binary interaction parameters to the corresponding system is confirmed.For the methanol-formaldehyde-PODE₁ ternary system,the NRTL and UNIQUAC model equations have good correlation results.By comparing the simulated value with the experimental value,the average absolute deviations of the equilibrium temperature and the gas phase composition of methanol and PODE₁ are obtained,which are less than 0.50K,0.0100 and0.0110,respectively.For the methanol-formaldehyde-PODE₂ ternary system,the correlation results of the three models are applicable to this system.By comparing the simulated value with the experimental value,the average absolute deviations of the equilibrium temperature and the gas phase composition of methanol and PODE₂ are obtained,which are less than 1.10K,0.0250and 0.0240,respectively.Therefore,the binary interaction parameters of the relevant ternary system obtained by regression experimental data can be applied to the rectification design of the corresponding methanol-forkmaldehyde-PODE_n ternary system,which lays a data foundation for industrial separation optimization of related systems.