Optimization And Synthesis Research On Polyoxymethylene Dimethyl Ethers Preparation Process

Polyoxymethylene dimethyl ether(abbreviated as DMMn) can be used as diesel fuel additive because of its high oxygen content(42%-51%) and high cetane number(>30). It has the characteristics of improving the combustibility and thermal efficiency of diesel fuel, reducing the emission of NOx、COx and dust.On the one hand,its physical and chemical properties are similar to diesel fuel, so the engine equipment and fuel system can be used directly. On the other hand, owing to DMMn can be prepared from methanol or the downstream products of methanol, it not only can extend the methanol industry chain and solve the trouble of the excess consumption of methanol production capacity, but also can improve the economic and environmental benefits. Thus, DMMn is regarded as a very promising diesel fuel additive and has a broad application prospect. In this paper, a route of preparing DMMn which can improve the selectivity of the objective products and the conversion of the raw material effectively was established according to the product requirements of the enterprise.This paper mainly studied the affection of different raw material on the synthesis of polyoxymethylene dimethyl ether. The synthesis of DMMn using dimethoxy methane and either trioxymethylene or paraformaldehyde as raw materials was investigated by the single-factor and the orthogonal experimental methods to obtain the best experimental conditions. Moreover, the vapor liquid equilibrium data of the binary systems of methanol + methylal at atmospheric pressure(99.9k Pa) were measured. The data provide theoretical basis for engineering design and industrial production.First of all, the affection of different raw material on the synthesis of DMMn was investigated. The results showed that the synthesis process of DMMn from dimethoxy methane and trioxymethylene is a suitable reaction scheme under the USY catalyst based on the selectivity of DMM3-5 and DMM3-8. Co MParing with other raw material,trioxymethylene was easy to be transported and stored.Secondly, the technological conditions of preparing polyoxymethylene dimethyl ethers by using of dimethoxy methane and trioxymethylene were investigated. The effects of reaction time, temperature, mass ratio of dimethoxy methane to trioxymethylene, amount of catalyst and pressure on the reaction were investigated by the single-factor and orthogonal experimental methods. The results showed that the appropriate catalyst was USY, and the appropriate operating conditions were reaction time 80 min, temperature 110 ℃, mass of catalyst 2.0%, mass ratio of dimethoxy methane to trioxymethylene 1.5, system pressure 2MPa. The selectivity of DMM3-5and DMM3-8 were found to be 46.33% and 62.11%, the conversion of dimethoxy methane was obtained to be 82.46% under the experimental conditions.Then, the synthesis of DMMn using paraformaldehyde instead of trioxymethylene was studied. The results indicated that the appropriate catalyst was USY. The effects of different influence factors on the reaction were investigated under the USY catalyst by the single-factor experimental method. The results showed that the appropriate operating conditions were reaction time 5h, temperature 130℃,mass of catalyst 3.0%, mass ratio of dimethoxy methane to paraformaldehyde 3,system pressure 3MPa. The selectivity of DMM3-5 and DMM3-8 were 13.81% and14.65% respectively, the conversion of dimethoxy methane was 29.85% under the experimental conditions.Considering the economic cost and the production cost, we can get the conclusion that methylal and trioxymethylene were the suitable reaction material of preparing DMMn. Thus, the catalytic performance of USY catalyst was investigated under the best reaction conditions and the most suitable reaction scheme. The results showed that catalyst USY still had good catalytic performance after six repeated reactions. Besides, Specific surface area testing results showed that USY catalyst has larger specific surface area and more active center. Scanning electron microscopy(SEM) picture indicated that USY catalyst has the characteristics of narrow particle size distribution and small particle size. The fourier transform infrared(FT-IR)spectroscopy indicated that some substances attached to USY catalyst in the reaction process and can be removed at the temperature of 400℃. Moreover, the calcination process did not change the component content of catalyst USY. Thethermogravimetric analysis(TGA) showed that the substance attached to USY catalyst can be removed at the temperature of 100℃, besides, there was no thermal weightlessness under the high temperature of 100 ℃ ~ 500 ℃. Therefore, USY catalyst was suited for preparing DMMnFinally, the vapor liquid equilibrium(VLE) data of the binary systems of methanol + methylal at atmospheric pressure(99.9k Pa) were measured by the improved Rose still. The experiment data passed the integral thermodynamic consistent test(area test). The VLE data were correlated by NRTL and Wilson models.The result was satisfactory by co MParison experimental vapor mole fraction and equilibrium temperature which were calculated by Aspen Plus V7.0. For Wilson model, the average relative deviation in vapor mole fraction was 7.99%, the average deviation in temperature was 0.12 K. For NRTL model, the average relative deviation in vapor mole fraction was 7.66%, the average deviations in temperature was 0.22 K.The associated effect of models was within the engineering data error.