Permeation flux membrane separation

In ultrafiltration and reverse osmosis, in which solutions are concentrated by allowing the solvent to permeate a semi-permeable membrane, the permeate flux (i.e. the flow of permeate or solvent per unit time, per unit membrane area) declines continuously during operation, although not at a constant rate. Probably the most important contribution to flux decline is the formation of a concentration polarisation layer. As solvent passes through the membrane, the solute molecules which are unable to pass through become concentrated next to the membrane surface. Consequently, the efficiency of separafion decreases as fhis layer of concentrated solution accumulates. The layer is established within the first few seconds of operation and is an inevitable consequence of the separation of solvent and solute. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

B.-B. Li et al. [64] have studied the separation of EtOH-H20 solutions by pervaporation (PV) using chitosan (CS), poly (vinyl alcohol)-poly(acrylonitrile) (PVA-PAN) and chitosan-poly(vinyl alcohol)/poly(acrylonitrile) (CS-PVA/PAN) composite membranes. It was found that the separation factor of the CS-PVA/PAN composite membrane increased with an increase of PVA concentration in the CS-PVA polymer from 0 to 40 wt%. With an increase in the membrane thickness from 12 to 18 pm, the separation factor of the CS-PVA/PAN composite membrane increased and the permeation flux decreased. With an increase of ethanol-water solution temperature, the separation factor of the CS membrane decreased and the permeation flux of the CS membrane increased while the separation factor and the permeation flux of PVA/PAN and CS-PVA/PAN composite membranes increased. 

To attain this goal, a pervaporation technique has been proposed, using a PVA composite membrane, made by casting of a mixture of PVA aqueous solution and a GA one on a polyethersulfone (PES) porous support, solvent evaporation and thermic curing [72], Excellent dehydration performance has been obtained (separation factor 320 and permeation flux 1.5 kg m 2 h 1, for 90 wt% TFEA in the feed and 80 °C). 

Solid PVA-Co2+ composite asymetric membranes have been prepared starting from PVA and two different salts Co(N03)2 and Co(CH3COO)2, respectively, in order to separate cyclohexene/cyclohexan mixtures. A facilitated transport mechanism has been evidenced, due to the capacity of Co2+ ions to coordinate the olefin molecules [82], The authors reported stronger complexation of Co2+ ions with cyclohexene in the case of PVA/ Co(CH3COO)2 mixtures then in the case of PVA/ Co(N03)2 mixtures. It was found that for a concentration ratio of ([Co2+]/[OH]) by 0.75 mol/mol, the permeation flux of PVA membrane containing Co2+ increases 2-3 times and the separation factor increses 50 times compared with pure PVA membrane. 

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Particular attention is addressed to the permeate flux and to this purpose some preliminary experiments were realized on a different configuration of membrane photoreactor with a submerged membrane module located separately from the photoreactor. Bubbled oxygen on the membrane surface has the roles to reduce the catalyst deposition, to increase the flux through the membrane and to facilitate the photocatalytic reaction. 

By following Cp vs. the reverse of the permeate flux, it is possible to quantify separately both part of the solute mass transfer occurring in NF convection and solvation (hydration)/diffusion as developed recently [9], The results are expected to be valid only in some limited domains of operating conditions (Jdiff and Cconv = Ctes) but may be useful for the comparison of the behaviour of different membranes. 

Cross-flow filter performance is often characterized by a flux rate, which equates to the permeate flow rate per unit area of membrane surface. The flux rate in most biological separations is reduced by a fouling phenomenon called gel polarization, which tends to concentrate material at the surface of membrane to impose an additional resistance to transmembrane flow. The deterioration in flux rate must be well characterized for a commercial bioseparation process to ensure the correct size for the cross-flow filtration unit and avoid hold-ups at this processing stage. 

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