Electro convective liquid crystal membranes (ECLCMs)

Abstract

An electroconvective liquid membrane for gas separations was previously investigated with the liquid crystal N-(4-methoxybenzylidine)-4-butylaniline (MBBA), and polar liquids 2-ethylhexanol (2EH) and diethyl phthalate (DEP). Recently, propylene carbonate and N-methyl-2-pyrrolidinone (NMP) were investigated as candidate liquid mediums for the membrane. These two solvents have higher gas permeabilities and selectivities than the previously tested liquid mediums. Both propylene carbonate and NMP were successfully tested in electroconvective liquid membrane experiments following purification to reduce their conductivities. The CO2 permeance with propylene carbonate increased from 5.8·10−7 cm3 (STP)/cm2·s·cm Hg to 3.6·10−6 cm3 (STP)/cm2·s·cm Hg when a 1400 volt/cm ac electric field was applied to the electrodes. A similar increase in the CO2 permeance was observed when NMP was used as the liquid medium. In comparison, the CO2 permeance with DEP increased from 2.0·10−7 cm3 (STP)/cm2·s·cm Hg to only 1.4·10−6 cm3 (STP)/cm2·s·cm Hg when a 6300 volt/cm ac electric field was applied to the electrodes. A disadvantage of using propylene carbonate or NMP is that the higher CO2 permeances are obtained at the cost of a relatively high power usage compared to when DEP is used as the liquid medium. A visual observation technique was also developed to observe the electrohydrodynamic motion of candidate polar liquid mediums for the membrane. The observed threshold voltages for electrohydrodynamic motion were compared to threshold voltages calculated using electrohydrodynamic instability criteria available in the literature for conductive liquids. The observed threshold voltages for propylene carbonate, NMP, DEP, and 2EH were approximately two orders of magnitude lower than the predicted values.