Study On Separation Process Of Polyoxymethylene Dimethyl Ethers(PODE_n) Synthesized By Formaldehyde And Methylal

Polyoxymethylene dimethyl ethers（PODE_n）is an important chemical material.Among different chain of PODE_n,PODE_3-5 is a kind of new diesel additive component.Besides,PODE₂ is a new green and environment-friendly aromatic free solvent.However,current PODE_n separation process is unable to recover formaldehyde solition by rectification.This thesis proposes the simultaneous extractive and azeotropic distillation process to separate formaldehyde solution.The process was simulated and optimized to obtain proper operations paramenters.Finally,a new separation process of PODE_n was estabilished.First,in order to extract the target product PODE_3-5,crude separation of PODE_nproducts were carried out in the first continuous distillation column and batch distillation of PODE_3-10 was conducted in the second distillation column.How to control temperature of cooling water to avoid formaldehyde precipitation was investigated in the first distillation column.The results show that two-step condensation can be used in the process,the first step is to use cooling water of 70℃,the second step is to use air as cooling medium.Through this way,formaldehyde won’t precipitate in the condenser.The light components of methylal,methanol,water,formaldehyde and PODE₂ can be separated in the top of the first distillation column.Pure PODE₃,PODE₄ and PODE₅ can be obtained by vacuum batch distillation of PODE_3-10,which is extracted from the bottom of the first distillation column.Then,in order to recover formaldehyde solution,methylal separation experiments was carried out in the third distillation column,and simultaneous extractive and azeotropic distillation process was conducted in the fourth distillation column by introducing supplementary water.Effects of reflux ratio and mass ratio of supplementary water to quaternary mixture were investigated in the process.The results showed that methylal and methanol binary mixture could be distillated in the third distillation column.Continuous distillation experiments of simultaneous extractive and azeotropic distillation showed that the overhead stream was methanol,water and PODE₂ ternary components while the bottom stream was convenient formaldehyde aqueous solution when the mass fraction of supplementary water to quaternary mixture was 0.5.Finally,in order to estabilish a complete process for separation of PODE_n,methylal and methanol binary mixture was separated by pressure swing distillation simulation.Strict mathematical model of simultaneous azeotropic and extractive process was set up.After comparision of experimental data with stimulated resuts and verification of the reliability of the model.Sensitivity analysis tools was used to analyze the effects of single factor,effects of important operation and design parameters on distillated FA and bottom PODE₂mass fraction and reboiler duty were investigated.The results showed that the total number of stages of 24,reflux ratio of0.2,supplementary water to quaternary mixture mass fraction of 0.45,the feed stage of quaternary mixture of 4,the feed stage of supplementary water of 3 were desirable to get high separation effects and low energy consumption.The results can provide the basis for the optimal design of the separation process of formaldehyde in PODE_nsystem.In the end,based on all experiments and simulations,a complete new separation process of PODE_n was developed.