Preparation And Application Of NaA Zeolite Membranes And Zeolite/Carbon Composite Membranes

Pervaporation(vapor permeation) is a new-type technique used for separation of liquid mixture,which is high-efficient,energy-saving,environmental and economical compared with other separation techniques.At present the majority of pervaporation membranes are organic polymer membranes,but those application fields are confined because of some fatal shortcomings such as poor chemical and thermal stability,concentration polarization,low separation factor etc.Zeolite membranes have been widely applied in pervaporation in recent years because of those excellent chemical stability,thermal stability,mechanical stability,uniform molecular-sized pores and fast transferring for materials.NaA zeolite membranes,with the molar ratio Si/Al nearly equal to 1,three-dimensional open-framework and high hydrophilicity, have been found many potential applications on dehydration of organic by pervaporation (vapor permeation),and have become ideal materials for separation of small molecular gas mixture due to their uniform molecular-sized pores.So it is of paramount importance to study dehydration of organic by pervaporation using zeolite NaA membranes and gas separation using zeolite/carbon composite membranes.In this work,NaA zeolite membranes were hydrothermally synthesized on the outer surface of substrates by hot dip-coating method;the zeolite/carbon composite membranes were prepared on the alumina substrates and their permeability was also investigated;the NaA zeolite membranes were applied in the pervaperation dehydration of ethanol,where the molecular probe was used to study the permeability mechanism of dehydration of ethanol. The as-synthesized membranes were also applied in the dehydration of chlorinated methane, furfural and glycol dimethyl ether.The main results achieved were as follows:(1).NaA zeolite membranes were hydrothermally prepared on the outer surface of substrates by hot dip-coating method.It was showed that the growth rate of NaA zeolite on the alumina supports was two times larger than that on the hydrophobic carbon supports and stainless steel supports.Zeolite/carbon composite membranes were prepared by dip-coating method.The permeance of carbon dioxide on the as-synthesized NaA zeolite/carbon composite was 3.39×10^-7 mol·m^-2·s^-1·Pa^-1,which was two orders of magnitude higher than that on the carbon membranes.The separation factor of CO₂/N₂ and CO₂/CH₄ was 6.02 and 28.4 respectively.Gas adsorption experiments indicated when NaA zeolite had been added into the carbon materials,the selective adsorption of CO₂/N₂ to carbon materials was improved. Moreover,it was showed that permeance of composite membranes was directly proportional to the content of zeolite according to gas permeability of zeolite/carbon composite membrane.(2).The as-synthesized membranes were applied in the process of ethanol dehydration, and the effects of several factors,such as feed flow,feed temperature,ethanol content of feed and pressure of permeate side were investigated.The flux and separation factor on the dehydration of ethanol were 0.88 kg·m^-2·h^-1 and 15600.The permeability of dehydration of ethanol using NaA zeolite membranes was investigated by molecular-probe method.The results showed that there was grain boundary in the as-synthesized NaA zeolite membranes, whose separation factor exceeded 10000,and the size of the grain boundary was confirmed ranging from 0.82 nm to 1.02 nm,and thus the mechanism of dehydration separation of ethanol complied with adsorption-diffusion separation mechanism.(3).The trace water in the chloromethanes was removed with pervaperation(vapor permeation) on NaA zeolite membranes,the effect of feed temperature,feed flow and pressure difference of feed on the performance of dehydration was systematically investigated. The results indicated that:(a) the water content of product decreased from 2580 PPm to 50 PPm by sweep gas method in the water/chloromethane system and the separation factor of water/chloromethane was 74800;(b) the water content of product decreased from 2580 PPm to 36 PPm by vacuum method in the water/chloromethane system and the separation factor of water/chloromethane was 32500;(c) the separation factor and the flux of the dehydration of dichloromethane were 39900 and 0.172 kg·m^-2·h^-1,respectively;(d) the separation factor and the flux of the dehydration of trichloromethane at 50℃were 57300 and 0.564 kg·m^-2·h^-1, respectively;(e) the separation factor and the flux of the dehydration of carbon tetrachloride at 60℃were 68200 and 0.612 kg·m^-2·h^-1,respectively.(4).NaA zeolite membranes were successfully utilized in the preparation of the high-purity furfural and glycol dimethyl ether by pervaperation.The effect of feed flow, operation temperature and feed content on the dehydration of furfural was investigated.The furfural content of product increased from 89.6 wt.%to 99.52 wt.%under optimized conditions;the content of glycol dimethyl ether in the product increased from 81.5 wt.%to 99.8 wt.%under optimized conditions.