Pervaporation of solvent mixtures using polymeric and zeolitic membranes: Separation studies and modeling

The separation characteristics of binary alcohol-water mixtures were studied over a wide range of feed concentration and temperature using polymeric and zeolitic pervaporation membranes. For the hydrophilic PVA membrane, the total flux (at 55°C) for the ethanol-water system decreased from 0.45 to 0.05 kg/m2/hr as the feed ethanol concentration was increased from 30 to 95 wt.%. The separation factor (water/ethanol) was found to increase by about 100 times for the same range of concentration. The UNIQUAC theory was used to predict the activity of binary alcohol-water mixtures in the PVA membrane. The UNIQUAC theory successfully takes into account the nonidealities present in the alcohol/water-PVA membrane system. The transport of water and alcohol species through the PVA membrane was modeled using the UNIQUAC theory in conjunction with the conventional activity driving force model. Using the model and the experimental pervaporation data, the diffusivity correlations and concentration profiles for various species through the membrane were developed. Based on the developed diffusivity correlations, the water and alcohol fluxes through the PVA membrane were predicted at 80°C.; Experiments were also conducted on the water selective zeolite (type NaA) membrane using various alcohol-water mixtures and with dimethylformamide-water mixture over a wide range of temperatures (25 to 70°C) and solvent concentrations (0 100 wt.%). The total flux for the ethanol-water mixture was found to decrease from 2 to 0.05 kg/m2/hr at 60°C as the feed ethanol concentration was increased from 0 to 100 wt.%. Both, the water to ethanol and water to isopropanol separation factors were observed to lie between 1000 and 5000 over a wide range of solvent concentrations. The Maxwell-Stefan theory was used to model the permeation of water through zeolite NaA membranes. The precise micropore structure of the zeolite cage helps in a partial molecular sieving of the large solvent molecules leading to high separation factors. The zeolite membrane active layer may contain certain non-zeolitic interstitial pores with preferential water sorption. A high degree of hydrophilicity of the zeolite membrane is suggested from a pure water sorption value of 0.6 gm/gm zeolite. The detailed interpretation of this result, however, requires consideration of both true zeolitic microcavity uptake as well as interstitially held water between crystallites. The use of pervaporation for volume reduction and solvent recovery applications in the pharmaceutical industry has been demonstrated.