Study On Synthesis And Separation Of Polyoxymethylene Dimethyl Ethers

Exhaust emission is a very important source of air pollution.Reducing pollutant emissions from the root cause is an urgent problem to be solved.Polyoxymethylene dimethyl ethers(DMM_n)as a kind of low molecular acetal polymer with high oxygen content and high cetane number,when mixed into diesel oil in a certain proportion,can significantly improve the combustion performance of diesel,and reduce the emissions of particulate matter,carbon oxides and nitrogen oxides.So they are considered to be a new type of environmental protection diesel additive.In this paper,the process conditions of DMM_n synthesizing from four routes of methylal with paraformaldehyde,methanol with paraformaldehyde,methanol with formaldehyde aqueous solution,methylal with formaldehyde aqueous solution were studied respectively.Comprehensive consideration of the objective product(DMM_3-6)selectivity,conversion rate of raw materials and content of the by-products,the optimum reaction conditions are obtained under the reaction path.Finally,the reaction mechanism is deduced according to the experimental results.The reaction product was washed by sodium hydroxide solution or extracted by cyclohexane with salt,then distillation was carried out to obtain the objective product.In order to obtain the optimum conditions of alkali washing and extraction,the liquid-liquid equilibrium relationship between DMM_1-4 and water system at different concentrations of sodium hydroxide and sodium formate,and the liquid-liquid equilibrium between DMM_1-4 aqueous solution and cyclohexane system were studied.The NRTL and CHEN-NRTL models were used to correlate the measured liquid-liquid equilibrium data,and the interaction parameters of each model were obtained.Finally,the Aspen plus software was used to study the steady-state simulation of the rectification of the organic phase obtained by alkali washing with sodium hydroxide solution and extraction with cyclohexane.NRTL model was adopted for simulation,and the interaction parameters obtained in the experiment were used for correction.Other unmeasured parameters were obtained from Aspen database or estimated by UNIFAC model.Ultimately,the process conditions of high purity DMM_3-8 products were determined by Aspen steady-state simulation studies.