Comparison of pressure driven transport of ethanol/n-hexane mixtures through dense and microporous membranes

The solvent flux was measured in binary mixtures of ethanol and n-hexane for nine solvent-stable polymeric membranes in range of reverse osmosis (RO) to ultrafiltration (UF) (GE AK Osmonics, Dow 102326, GE DK Osmonics, MPF-34, STARMEM™ 122, STARMEM™ 240, NF30, NTR7450, NF-PES-010). GC-analyses of feed and permeate samples showed a separation factor close to 1, which indicates the solvent transport occurs by convection or by coupled diffusion through the membranes. The effect of viscosity, surface tension, di-electric constant and solubility parameter of solvent on permeation rate was studied for four categories of membranes, i.e. RO membranes, dense nanofiltration (NF) membranes, semi-porous NF membranes and micro-porous NF membranes. While viscosity seems to be a main transport parameter (similar composition of feed and permeate), higher fluxes of ethanol compared to n  -hexane (with lower viscosity) confirmed that the transport may occur through coupled diffusion. The influences of the solvent–membrane interaction parameters such as surface tension, polarity and solubility parameters of solvent and membranes for dense membranes were investigated. The effect of solvent membrane interaction by means of solubility parameters was more pronounced compared to surface tension since the respective surface tensions of solvents are close to one another (γethanol=21.9,γn‐hexane=17.9). Partial permeabilities were studied as well to evaluate the influence of each component on permeation of the other. Unexpected results were observed for MPF-34, NF30 and NTR7450. Further investigation confirmed that their polymeric structure changed in contact with the solvents. Hydrophobic STARMEM™ membranes, which are expected to have a higher permeability for apolar solvents showed higher fluxes for ethanol compared to n-hexane. The similar solubility parameter of these membranes and ethanol may increase the permeation rate of ethanol molecules through membranes. For porous membranes viscosity was recognized as the key transport parameter, while affinity between membrane and solvent has a lower effect.