Research On New Refining Technology For Separation Of Methylal (DMM) And Methanol Mixture

Methylal (DMM) is a kind of important industrial chemicals which may be used as diesel oil additive, organic synthesis intermediate and solvent. Additional, methylal, an air pollution environmental protection product using extremely widespread, may also substitute the fleon to reduce the volatile organic matter to the air. Usually, methylal was prepared through the reversible acetal chemical reaction by methanol and formaldehyde and a mixture of methylal, methanol and water were obtained. However, as methylal and methanol form azeotropic system, the purity of methylal can hardly be improved any higher and the high-purity product of methylal can not be got easily.The pressure swing distillation of double columns was used to refine methylal -methanol mixture and high purity products of methylal and methanol were obtained simultaneously. Firstly, the azeotrope of the methylal and methanol was measured under the experimental pressure and the result indicated that the azeotrope of methylal and methyanol changed obviously with the change of pressure, the content of methylal declined when the system pressure increased. The columns, made of stainless steel and packed with Dixon packing, were established and the experiment of the pressure swing distillation was carried out. The experiment contained two steps: the atmospheric pressure experiment and upper pressure (504.33kPa) experiment. The reflux ratio was investigated separately to study the effect of methylal and methanol system in the double columns and three repeated experiments have been done. The result of the atmospheric and upper pressure experiment indicated that the content of methylal of the product increased when the reflux ratio increased. The experimental result indicated the best reflux ratio was 5:1 at the higher pressure column and 4:1 at the atmospheric pressure column. The purity of methylal reached more than 99.9% and the purity of methanol reached 99%, the experiments repeated well.Finally, the simulation software Aspen Plus was applied with the equilibrium model. At first, the vapor-liquid equilibrium data of methylal and methanol system from literature under pressure closing to atmosphere was adopted for the regression and analysis comparison in the thermodynamic model. It was proved that the Wilson equation was fit better for the system of methylal and methanol compared with NRTL and UNIQUAC equations. The azeotropic compositions of methylal and methanol system under the experiment pressure were calculated simultaneously by Wilson and the result demonstrated that the experiment date was consistent with the computational data well. The experimental date was consistent with the computational date well through the simulation. Further more, the double-effect pressure swing distillation technology was proposed after energy conservation of the pressure swing distillation was analyzed and compared simultaneously. At last, the parameters of the simulation such as the reflux ratio, the theoretical plate number and the operation pressure have been optimized and the optimized results have been given. Finally, the feasibility of the pressure swing distillation was analyzed and discussed theoretically in the case of the separation of methylal and methonal and the result showed the phase equilibrium analysis was a new method to research on the feasibility. In addition, the double-effect pressure swing distillation was compared and analyzed with three kinds of extraction distillations.