Mechanisms and experimental results of aqueous mixtures pervaporation using nanopore HS zeolite membranes

摘要

Dehydration of solvents using hydrophilic polyvinylalcohol pervaporation membranes is a well-established technology. However, these polymeric membranes may not be suitable for applications involving high water concentrations or applications containing harsh solvents like unsymmetrical dimethylhydrazine (UDMH) due to membrane stability problems and swelling effects. The recent development of solvent and temperature-resistant, hydrophilic zeolite Hydroxysodalite (HS) membranes has made it possible to overcome the above limitations of hydrophilic polymeric membranes. Zeolite membranes have uniform and molecular-sized pores, and they separate molecules based on differences in the molecules' adsorption and diffusion properties. Strong electrostatic interaction between ionic sites and water molecules (due to its highly polar nature) makes the zeolite HS membrane very hydrophilic. Zeolite HS membranes are thus well suited for separating liquid-phase mixtures by pervaporation. In this study, experiments were conducted with various UDMH-water mixtures (1-20 wt.%) at 25 °C. Total flux for UDMH-water mixtures was found to vary from 0.323 to 0.214 kg/m~2 h with increasing UDMH concentration from 1 to 20 wt.%. Ionic sites of the HS zeolite matrix play a very important role in water transport through the membrane. These sites act both as water sorption and transport sites. Surface diffusion of water occurs in an activated fashion through these sites. The precise nanoporous structure of the zeolite cage helps in a partial molecular sieving of the large solvent molecules leading to high separation factors. A comparison between experimental flux and calculated flux using Stephan Maxwell (S.M.) correlation was made and a linear trend was found to exist for water flux through the membrane with UDMH concentration.