Citation: ROBINSON, J.P. ... et al, 2004. Solvent flux through dense polymeric nanofiltration membranes.Journal of Membrane Science, 230 (1-2), pp. [29][30][31][32][33][34][35][36][37] Additional Information:• ABSTRACTThis work examines the flux performance of organic solvents through a polydimethylsiloxane (PDMS) composite membrane. A selection of n-alkanes, i-alkanes and cyclic compounds were studied in deadend permeation experiments at pressures up to 900 kPa to give fluxes for pure solvents and mixtures between 10 and 100 l m -2 h -1. Results for the chosen alkanes and aromatics, and subsequent modelling using the Hagen-Poiseuille equation, suggest that solvent transport through PDMS can be successfully interpreted via a predominantly hydraulic mechanism. It is suggested that the mechanism has a greater influence at higher pressures and the modus operandi is supported by the non-separation of binary solvent mixtures and a dependency on viscosity and membrane thickness. The effects of swelling that follow solvent-membrane interactions show that the relative magnitudes of the Hildebrand solubility parameter for the active membrane layer and the solvent(s) are a good indicator of permeation level. Solvents constituting a group (e.g. all n-alkanes) induced similar flux behaviours when corrections were made for viscosity and affected comparable swelling properties in the PDMS membrane layer.

The separation of organic solutes in organic solvents was assessed using dense poly(dimethylsiloxane) (PDMS) membranes with different degrees of cross-linking and varying thickness of the dense PDMS layer. The predominant rejection mechanism for low-polarity organic solutes is shown to occur via size exclusion, with the rejection also being dependent on the degree of membrane cross-linking, the swelling propensity of the membrane−feed stream, and the transmembrane pressure. It is postulated that the size-exclusion mechanism arises as a consequence of the relatively large degree of swelling of the PDMS material (up to 300%), which induces appreciable regions between the polymer chains for solvent and solute transport to take place. The degree of swelling governs the relative size of the transport regions within the membrane and, hence, the overall solvent flux and solute rejection characteristics. It is shown that solvent−solute coupling plays a major role in solute transport, with the convective element of solute flow increasing as the degree of swelling increases and the solute size decreases. Despite the existence of a size-exclusion mechanism, it is difficult to rule out the solution-diffusion model as an interpretation of the data; however, it is also demonstrated that models based on pore flow can adequately define the experimental data. The similarities between the two approaches are discussed, and potential evidence of a transition between solution diffusion and pore flow is introduced.

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The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied using mixtures comprising xylene, cyclohexane or n-heptane with oxygenate components at concentrations up to 75%. The effects of polarity on flux and rejection performance were determined through a test matrix of solvent type, concentration, filtration pressure, crossflow rate and the degree of membrane crosslinking.In all cases involving alcohols, the more polar compound in the feed mixture was partially rejected by the membrane and the extent of rejection was dependent on the polarity as quantified by solubility parameter. The rejection-concentration profiles for several alcohol/solvent mixtures exhibited a maximum, with the highest rejection around 30%. Mixtures containing MTBE did not separate, i.e. no rejection was observed.Rejection increased with increasing pressure and crossflow rate but was largely unaffected by the degree of membrane crosslinking. Component flux was affected by the oxygenate concentration in the mixture, which was attributed in part to changes in the degree of membrane swelling with composition. Experimental findings suggest that the separation is primarily governed by multicomponent solvent/oxygenate/membrane swelling equilibria, and results compare favourably with swelling isotherms available in the open literature.

This article provides an insight into the mechanisms affecting solvent flux through dense membranes, and forms a basis for rejection studies of organic solute compounds from organic solvents. ABSTRACTTransport mechanisms and process limitations are relatively well understood for aqueous nanofiltration systems. Much work has also been done on the use of membranes for the removal of suspended matter from organic solvents. The removal of organic solute compounds from organic solvents using membrane technology has been addressed by very few workers, and little is known of the fundamental transport and separation mechanisms.The work aims to enhance the understanding of non-aqueous nanofiltration by focusing on the flux performance of organic solvents through a dense 2 μm polydimethylsiloxane composite membrane. The flux of alcohols, n-alkanes, i-alkanes and cyclic compounds were studied in deadend mode, at pressures of 10-900 kPa. Fluxes of 10-80 l/m 2 h were obtained for alkanes and cyclic compounds, whereas alcohol flux was around two orders of magnitude lower. The results suggest that the solvent flux through polydimethylsiloxane takes place via two distinct mechanisms -namely hydraulic and chemical transport. Hydraulic transport appears to dominate at pressures above 300 kPa, whereas chemical transport becomes more apparent at lower pressures.Comparison of the hydraulic transport data with a Hagen-Poisuelle model gives good agreement for similar solvents. Swelling effects caused by solvent-membrane interactions are identified as playing a major role in solvent flux behaviour, and compressibility effects are also thought to account for deviations from the Hagen-Poisuelle model. Viscous flow was verified by a nonseparation of mixtures of n-alkane and cyclic compounds, which suggests that the polydimethylsiloxane layer cannot sustain a dense structure when used in organic solvent nanofiltration applications.