Pervaporation of alcohol-water and dimethylformamide-water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results

Dehydration of solvents using hydrophilic polyvinylalcohol pervaporation me mbranes is a well-established technology. However, these polymeric membrane s may not be suitable for applications involving high water concentrations or applications containing harsh solvents like dimethylformamide due to mem brane stability problems and swelling effects. These applications are commo nly encountered in the pharmaceutical industry. The recent development of s olvent and temperature-resistant, hydrophilic zeolite NaA membranes has mad e it possible to overcome the above limitations of hydrophilic polymeric me mbranes. In this study, experiments were conducted with various alcohol-wat er (methanol-water, ethanol-water, isopropanol-water) mixtures and with dim ethylformamide-water mixture over a wide range of temperatures (25-70 degre esC) and solvent concentrations (0-100 wt.%). The total flux for ethanol-wa ter mixture was found to vary from 2 to 0.05 kg/m(2)/h at 60 degreesC as th e feed solvent concentration was increased from 0 to 100 wt.%. The total fl ux for methanol-water and isopropanol-water mixtures was observed to vary f rom 2 to 0.15 and 2 to 0.21 kg/m(2)/h, respectively, as the alcohol concent ration was changed from 0 to 100 wt.%. The total flux was also found to rem ain approximately constant up to 70 wt.% alcohol in the feed. Both water to ethanol and water to isopropanol separation factors were observed to lie b etween 1000 and 5000 over a wide range of solvent concentrations. The water to methanol separation factor was found to lie in the range of 500-1000. T he total flux behavior was also found to be very similar for the other solv ent-water mixtures such as acetone-water and ethyl acetate-water. The ionic Na+ sites in the NaA zeolite matrix play a very important role in the wate r transport through the membrane. These sites act both as water sorption an d transport sites. The surface diffusion of water occurs in an activated fa shion on these sites. The precise micropore structure of the zeolite cage h elps in a partial molecular sieving of the large solvent molecules leading to high separation factors. The zeolite membrane active layer may contain c ertain non-zeolitic interstitial pores with preferential water sorption. On e of the reasons for the high hydrophilicity of zeolite NaA is the strong e lectrostatic interaction between ionic sites and the water molecule (due to its highly polar nature). A high degree of hydrophilicity of the zeolite m embrane is suggested from a pure water sorption value of 0.6 g/g zeolite. T he detailed interpretation of this result, however, requires consideration of both true zeolitic microcavity uptake as well as interstitially held wat er between crystallites. Any molecule highly polar in nature (indicated by a high dipole moment or dielectric constant) is expected to strongly intera ct with the ionic sites in the cage. In fact, this effect was indirectly ob served for the dimethylformamide-water mixture. The water flux in this case , was found to be lower than that for alcohol-water mixtures thus indicatin g the possibility of competitive sorption of dimethylformamide molecules on the zeolite sites. A linear correlation was found to exist for the pure wa ter flux through the membrane and the partial pressure driving force of the water. The other alcohol-water mixtures followed this trend reasonably wel l. The use of zeolite membranes for dehydration of complex solvent streams has been actually demonstrated by experiments with a synthetic ethyl acetat e mixture. Thus, the use of inorganic zeolite membranes for difficult solve nt separations seems to be very attractive. (C) 2000 Elsevier Science B.V. All rights reserved.