Study On The Preparation And Water Vapor Permeation Behaviors Of Composite Membrane For Propylene Dehydration

Water content in polymer-grade propylene production have rigorous standard.Absorption with molecular sieve as the traditional method of dehydration ofpropylene has many disadvantages such as complex equipment, high energyconsumption, pollution environment. So the basic researches on dehydration ofpropylene with vapor permeation method not only help to prepare VP dehydrationmembrane and study permeation mechanism, but also develop a new process for theindustrial application of dehydration of propylene. Dehydration process based on hollow fiber composite membrane is one of thecrucial and effective methods to remove water vapor from industrial gases such aslight hydrocarbon, compressed air, natural gas and so on. Dehydration of propylenewith VP method was studied in the thesis. We prepared the PVA/PS,CS/PS andPVA-CS/PS hollow fiber composite membranes by spread method. The permeationbehavior of water vapor among coagulable light hydrocarbons through compositemembranes and asymmetric membrane was researched by vapor permeation method.Also a differential mathematical model was established which can describedehydration of gas phase propylene. Then we discussed the permeation principles ofthe water vapor in the crosslinking PVA dense membranes. The polysulfone hollow fiber ultrafiltration membrane used as the basemembrane of composite membrane, and the heat treatment temperature should lessthan 120℃. In this process of dehydration of propylene, the base membrane almosthad no effect on the performance of the composite membranes. After analyzing bySEM, we found the thickness of the active layer was uniform and less than 10μm andit was combined well with the porous layer. We investigated the effects of the concentration of PVA(CS), the amount ofcrosslinker, the heat cross-kinking temperature,the time of heat treatment and otherfactors which may affect the performances of the composite membranes, and then byanalyzing these factors, the best conditions for making membrane were fixed. Also weinvestigated the effects of the feed concentration, pressure, flow rate, temperature andthe flow rate of the sweeping gas on the performances of the membranes, then theperformances of the PVA/PS, CS/PS, PVA-CS/PS and PI membranes was compared. IIThe result indicated the PVA-CS/PS membrane with separation factor 1.18×105 andwater flux 34.8g/m2h was the best one. The sorption dynamic curves of water vapor with different activity incrosslinking PVA membranes were measured by the method of weight absorption.Some useful results were obtained. Taking into consideration that some specificbehaviors of water vapor through hydrophilic PVA dense membrane, themathematical model of water cluster influence upon vapor permeation throughhydrophilic dense membrane was established. This model makes sure the presence ofcluster and the size of cluster was calculated. The validation of this mathematicalmodel has been done comparing the permeability values calculated by this model withthose ones obtained by means of VP tests. They have the same quantitativeprogression and this show this model can describe the permeation behaviors of watervapor through hydrophilic PVA dense membrane well. For PVA-CS/PS hollow fiber membrane module, a countercurrent-flow designmodel was established which can describe dehydration of gas phase propylene withVP method. The simulation using the model is performed by changing the operatingconditions and the optimum operating factors and the required membrane area areinvestigated. Then the design proposal of this technique for industrial application wasbrought forward.