Insight into the transport of hexane-solute systems through tailor-made composite membranes

This work presents composite membranes comprising poly(acrylonitrile) (PAN) as the support and polydimethylsiloxane (PDMS) as the selective top layer. For sunflower oil/hexane and polyisobutylene (PIB)/hexane, the permeation characteristics of these membranes for various feed concentrations and pressures are studied. For each system, the effect of transmembrane pressure upon the flux and retention of the PAN/PDMS composite membrane is investigated. Osmotic phenomena similar to those of aqueous systems are observed and interpreted using the van¿t Hoff equation. The hexane flux increases linearly with the applied pressure and the solution-diffusion model seems to describe satisfactory the aspects of its transport to these systems. The hexane permeability decreases with the increase of the feed concentration. Its normalization by the viscosity inside the swollen membrane (according to the Stokes¿Einstein diffusion phenomenon) and the swelling degree of the membrane results in a constant value quantifying the hexane transport independent of the solute present in the feed mixture. The flux of the solute (oil or PIB) increases linearly with the applied pressure as well, especially at low feed concentrations when the membrane swelling is higher, indicating coupling of solute transport to solvent flux. For the same feed solution concentration, the effect of flux coupling (solvent-induced solute dragging) decreases with the molecular weight of the solute. Ultimately, when the applied pressure increases; the increase of hexane flux is much higher than the corresponding solute (oil or PIB) flux resulting to an increase of the membrane retention

Effect of PDMS cross-linking degree on the permeation performance of PAN/PDMS composite nanofiltration membranes

This work focuses on the effect of poly(dimethyl siloxane) (PDMS) cross-linking on the permeation performance of the poly(acrylonitrile) (PAN)/PDMS nanofiltration (NF) composite membrane. PDMS membrane of various cross-linking degrees could be obtained by changing the ratio of a vinyl-terminated pre-polymer over a hydride cross-linker, 10/0.7, 10/1 and 10/2.\ud \ud The hexane permeability (Phexane) through the PAN/PDMS composite membrane prepared at pre-polymer/cross-linker ratio of 10/0.7 is higher than at ratio of 10/1 (4.5 and 3.1 lm¿2 h¿1 bar¿1, respectively), due to the higher membrane swelling. The Phexane through the PAN/PDMS prepared at pre-polymer/cross-linker ratio of 10/2 is however higher than through the composite membrane prepared at 10/1 ratio (4.1 and 3.1 lm¿2 h¿1 bar¿1). This result is not consistent with the swelling findings of the dense, free-standing PDMS membranes and might be due to the lower pore intrusion of the composite membrane prepared at ratios of 10/2 compared to 10/1 and/or due to the heterogeneous quality of the silicone network. The ¿apparent¿ viscosity inside the membrane and the membrane swelling are the most critical factors affecting the hexane permeability through all composites. Nevertheless, the composite membranes prepared at various pre-polymer/cross-linker ratios have similar oil and/or PIB retention probably due to the high swelling of the silicone network

Observations on the permeation performance of solvent resistant nanofiltration membranes

This work presents a systematic study of the influence of membrane–solvent–solute interactions on the permeation performance of solvent resistant nanofiltration (NF) membranes. Two different tailor-made composite membranes are prepared by dip coating of a polymer onto a polyacrylonitrile (PAN) support: a hydrophobic (using polydimethylsiloxane (PDMS) polymer) and a new hydrophilic (using polyethylene oxide (PEO)–PDMS–PEO tri-block copolymer). The transport of various pure solvents through the PAN/PDMS and PAN/PEO–PDMS–PEO composite membranes is studied showing a reasonably linear relation between the solvent permeability and the ratio of membrane swelling/solvent viscosity.\ud \ud For the transport of sunflower oil/toluene mixtures through the PAN/PDMS composite membrane, the “apparent” viscosity inside the membrane and the membrane swelling seem to be the main parameters affecting the toluene transport. The oil/toluene system is ideal. Osmotic phenomena are observed which can be interpreted by the van’t Hoff osmotic pressure model. Furthermore, flux coupling between oil and toluene is significant resulting in rather moderate oil retention by the membrane (70–80%). The coupling between oil/toluene is stronger than between oil/hexane.\ud \ud For the transport of tetraoctylammonium bromide (TOABr)/toluene mixtures through the PAN/PDMS membrane the phenomena are more complex. The system is non-ideal and no osmotic phenomena are observed. This can be correlated with the formation of ion-pairs of TOABr in toluene. Furthermore, the concentration polarization phenomena are significant resulting in low toluene fluxes and 100% retention of TOABr by the membrane

Degumming, dewaxing and deacidification of rice bran oil-hexane miscella using ceramic membrane: pilot plant study

An indigenously developed low-cost clay-alumina-based ceramic microfiltration membrane of 19-channel configuration has been evaluated for degumming, dewaxing and deacidification of rice bran oil (RBO) miscella having different oil contents at pilot scale. Rice bran wax and soap particles in miscella will aggregate with changes in temperature. This suggests a technique for their effective separation. Low-temperature cross-flow membrane filtration was used for single-stage degumming-dewaxing and showed 70 % and 80 % removal of acetone insoluble residue from two RBO miscella samples, respectively. Color reduction was 50 %, and oryzanol retention was 70 %. NaOH was used for deacidification in a 10 % excess of that required based on the free fatty acid content in oil. This reduced free fatty acids to 0.2 %. Operating for 10 h with a 0.7 bar trans-membrane pressure, permeate fluxes of 15 and 8 L/m(2) hr were obtained for the degumming-dewaxing and deacidification operations, respectively. The process has advantages, such as high micronutrient content (1.56 % oryzanol) and negligible oil loss (2.6 %). Moreover, ceramic membrane processing of RBO miscella could be an effective pre-treatment step with respect to micronutrient enrichment, elimination of heating, neutral oil recovery and a viable option for solvent separation