Preparation Of Highly Purified Methylal By Extractive Distillation

Methylal which has excellent physical and chemical properties, is widely used in cosmetic, pharmaceutical, household goods, pesticide and so on. As an environment-friendly product reducing gas pollution, methylal can replace Freon. Methylal and methanol form azeotrope, so highly purified methylal cannot be obtained by conventional distillation.This paper studied the preparation of highly purified methylal by extractive distillation using N,N-dimethylformamide(DMF) as the solvent.At first, methanol and formaldehyde chose as feeds and strongly acidic cation exchange resin chose as catalyst, crude methylal, which contains 94%(mass fraction) methylal, 5.6% methanol and 0.4% water, was obtained by batch reactive distillation. The solvent was selected by qualitative rules, and then the relative volatility was calculated by UNIFAC group contribution method. Finally, DMF was selected as proper solvent.The batch extractive distillation was carried to purify the crude methylal. The hydrodynamic parameters of the distillation column were determined, and also the number of theoretical plates at total reflux was determined. During the experiment, the effects of different reflux ratio and solvent flow rate on results were compared. And 99.9% methylal was obtained from distillate.Aspen Plus was used to simulate the process of continuous extractive distillation. Many technological parameters, such as the reflux ratio, feed stage of crude, feed stage of solvent and the mass of solvent in extractive distillation the column and the reflux ratio and feed stage in solvent recycling column, were optimized. At last, 99.9% methylal and 99.99% DMF were got respectively from the top of the extractive distillation column and the bottom of solvent recycling column, and the yield coefficients were 99.94% and 99.99% respectively.