Characterization of relaxation phenomena and permeation behaviors in sodium alginate membrane during pervaporation separation of ethanol-water mixture

Abstract

The pervaporation separation of a concentrated ethanol–water mixture with 90 wt % of ethanol content through a sodium alginate (SA) membrane was carried out to investigate both the relaxation process and permeation behavior of the membrane in pervaporation. From the swelling measurement of the membrane in 90 wt % of ethanol content of the aqueous solution, it was observed that SA has an excellent permselectivity toward water and a high solubility of water: about 1000 of sorption selectivity and 21 wt % of water content in the swollen membrane at 40°C. Its excellent sorption properties could result in the outstanding pervaporation performance for the aqueous solution; higher than 10,000 of separation factors and 120–290 g/(m2h) of fluxes, depending on the operating temperature. However, a serious flux decline took place with operating time due to the occurrence of a significant relaxation process in SA. The flux decline was remarkable at the beginning stage and then mitigated with operating time. The fluxes were reduced by about 40–50% because of the relaxation process during the measurement. To analyze these phenomena in more detail, the hysteresis behavior of membrane performance along with an operation temperature cycle was investigated. During the heating process in the cycle, the relaxation process was important to affect membrane performance while the formation of excess stress in the membrane might be a crucial factor during the cooling process. It was postulated from the experimental observations that in the heating process an increase in flux with temperature is restricted by a more rapid relaxation process at a higher operation temperature, while in the cooling process, the excess stresses in the membrane are formed and accumulated in and near the transition zone and near the area in the membrane because of slower relaxational consolidation in unequilibrium cooling than in relaxational dilation in the heating process, resulting in flux more sensitive to temperature than in the heating process. These phenomena were found diminished as the relaxation time increased with aging time. The relaxation phenomena were discussed through an analysis on permeation activation energy data obtained from the pervaporation experiments. A qualitative model was established to describe the relaxation phenomena in the membrane material during the pervaporation process by using the basic principles of polymeric relaxation and the experimental observations obtained in this study. © 1996 John Wiley & Sons, Inc.