Pervaporation concentration polarization

In any process, if one component is enriched at the membrane surface, then mass balance dictates that a second component is depleted at the surface. By convention, concentration polarization effects are described by considering the concentration gradient of the minor component. In Figure 4.3(a), concentration polarization in reverse osmosis is represented by the concentration gradient of salt, the minor component rejected by the membrane. In Figure 4.3(b), which illustrates dehydration of aqueous ethanol solutions by pervaporation, concentration polarization is represented by the concentration gradient of water, the minor component that preferentially permeates the membrane. 

In coupled transport and solvent dehydration by pervaporation, concentration polarization effects are generally modest and controllable, with a concentration polarization modulus of 1.5 or less. In reverse osmosis, the Peclet number of 0.3-0.5 was calculated on the basis of typical fluxes of current reverse osmosis membrane modules, which are 30- to 50-gal/ft2 day. Concentration polarization modulus values in this range are between 1.0 and 1.5. 

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. 

Figure 4.3 Concentration gradients formed as a result of permeation through a selective membrane. By convention, concentration polarization is usually represented by the gradient of the minor component—salt in the reverse osmosis example and water in the pervaporation example (dehydration of an ethanol solution)...

Depending on the enrichment term (E0) of the membrane, the modulus can be larger or smaller than 1.0. For reverse osmosis E0 is less than 1.0, and the concentration polarization modulus is normally between 1.1 and 1.5 that is, the concentration of salt at the membrane surface is 1.1 to 1.5 times larger than it would be in the absence of concentration polarization. The salt leakage through the membrane and the osmotic pressure that must be overcome to produce a flow of water are increased proportionately. Fortunately, modem reverse osmosis membranes are extremely selective and permeable, and can still produce useful desalted water under these conditions. In other membrane processes, such as pervaporation or ultrafiltration, the concentration polarization modulus may be as large as 5 to 10 or as small as 0.2 to 0.1, and may seriously affect the performance of the membrane. 

The membrane s intrinsic enrichment Ea also affects concentration polarization. If the membrane is completely unselective, Ea = 1. The relative concentrations of the components passing through the membrane do not change, so concentration gradients are not formed in the boundary layer. As the difference in permeability between the more permeable and less permeable components increases, the intrinsic enrichment E0 achieved by the membrane increases, and the concentration gradients that form become larger. As a practical example, in pervaporation of organics from water, concentration polarization is much more important when the solute is toluene (with an enrichment Ea of 5000 over water) than when the solute is methanol (with an enrichment Ea less than 5). 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

Figure 4.11 shows that ultrafiltration and pervaporation for the removal of organic solutes from water are both seriously affected by concentration polarization. In ultrafiltration, the low diffusion coefficient of macromolecules produces a concentration of retained solutes 70 times the bulk solution volume at the membrane surface. At these high concentrations, macromolecules precipitate, forming a gel layer at the membrane surface and reducing flux. The effect of this gel layer on ultrafiltration membrane performance is discussed in Chapter 6. 

In the case of pervaporation of dissolved volatile organic compounds (VOCs) from water, the magnitude of the concentration polarization effect is a function of the enrichment factor. The selectivity of pervaporation membranes to different VOCs varies widely, so the intrinsic enrichment and the magnitude of concentration polarization effects depend strongly on the solute. Table 4.2 shows experimentally measured enrichment values for a series of dilute VOC solutions treated with silicone rubber membranes in spiral-wound modules [15], When these values are superimposed on the Wijmans plot as shown in Figure 4.12, the concentration polarization modulus varies from 1.0, that is, no concentration polarization, for isopropanol, to 0.1 for trichloroethane, which has an enrichment of 5700. 

Figure 4.12 A portion of the Wijmans plot shown in Figure 4.7 expanded to illustrate concentration polarization in pervaporation of dilute aqueous organic solutions. With solutes such as toluene and trichloroethylene, high intrinsic enrichments produce severe concentration polarization. Concentration polarization is much less with solutes such as ethyl acetate (enrichment 270), and is essentially eliminated with isopropanol (enrichment 18) [15]...

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

Concentration polarization plays a dominant role in the selection of membrane materials, operating conditions, and system design in the pervaporation of VOCs from water. Selection of the appropriate membrane thickness and permeate pressure is discussed in detail elsewhere [50], In general, concentration polarization effects are not a major problem for VOCs with separation factors less than 100-200. With solutions containing such VOCs, very high feed velocities through... 

Figure 9.15 Once-through pervaporation system design. This design is most suitable for removal of VOCs with modest separation factors for which concentration polarization is not a problem...

For treating water containing VOCs with separation factors of more than 500, for which concentration polarization is a serious problem, feed-and-bleed systems similar to those described in the chapter on ultrafiltration can be used. For small feed volumes a batch process as illustrated in Figure 9.16 is more suitable. In a batch system, feed solution is accumulated in a surge tank. A portion of this solution is then transferred to the feed tank and circulated at high velocity through the pervaporation modules until the VOC concentration reaches the desired level. At this time, the treated water is removed from the feed tank, the tank is loaded with a new batch of untreated solution, and the cycle is repeated. 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

Figure 6.44 shows some results of a numerical optimization of pervaporation cascades.32 Assumptions (1) the selectivity is constant and equal for all stages (2) the fluxes are small enough to neglect concentration polarization. 

In vapor permeation, feed-side concentration polarization is much less prone to occur than in pervaporation, owing to the high mass-transfer rate of the solute in the vapor feed phase. In fact, this feature is one of the main factors that distinguish the two processes. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

Finally, the pervaporation flux could be also limited by concentration polarization and the advantages of doublesided membranes for increasing fluxes diminishing the concentration polarization have been reported [23,24,169],... 

Vapor permeation differs from pervaporation, as stated above, insofar as the feed mixture to be separated is supplied as a vapor. At least the more-permeable component is kept as close to saturation conditions as possible. Thermodynamically there is no difference between a liquid and ifs equilibrium vapor, the partial vapor pressure and thus the driving force for the transport through the membrane are identical and the same solution-diffusion mechanism is valid. However, the density of the vaporous feed and thus the concentration of molecules per volume is lower by two to three orders of magnitude than that of the liquid. As a consequence the membrane is usually less swollen than when in contact with a liquid feed. As the feed mixture getting in contact with the membrane is already in the vapor phase no phase change occurs across the membrane and thus no temperature polarization will be observed. Concentration polarization, however, is still an issue. Although the diffusion coefficient is much higher in a vapor than in a liquid, this is at least partially outbalanced by the lower density of the vapor, and therefore concentration polarization effects may be observed at all concentrations of the component to be removed. Minimum... 

With pervaporation membranes the water can be removed during the condensation reaction. In this case, a tubular microporous ceramic membrane supplied by ECN [124] was used. The separating layer of this membrane consists of a less than 0.5 mm film of microporous amorphous silica on the outside of a multilayer alumina support. The average pore size of this layer is 0.3-0.4 nm. After addition of the reactants, the reactor is heated to the desired temperature, the recyde of the mixture over the outside of the membrane tubes is started and a vacuum is apphed at the permeate side. In some cases a sweep gas can also be used. The pressure inside the reactor is a function of the partial vapor pressures and the reaction mixture is non-boiling. Although it can be anticipated that concentration polarization will play an important role in these systems, computational fluid dynamics calculations have shown that the membrane surface is effectively refreshed as a result of buoyancy effects [125]. 

The concentration polarization is a phenomenon, which influences the pervaporation efficiency. The boundary-layer resistance to the solute transport was reported by Psaume et al. [9] for the recovery of trichloroethylene in aqueous solutions. They showed that under certain operating conditions, transport through the membrane was determined by the hydrodynamic conditions in the feed-side, and thus the resistance of the membrane becomes relatively negligible. Colman et al. [77] showed that resistance to the transfer of the boundary layer is a limiting factor of the dehydration of isopropanol by pervaporation. Various others studies [16,78,79] highlighted the importance of the hydrodynamics on resistance to the solute mass transport in pervaporation. 

Psaume, R., Aptel, Ph., AureUe, Y., Mora, J.C., and Bersillon, J.L., Pervaporation importance of concentration polarization in the extraction of trace organics from water, J. Membr. Set, 36, 373-384, 1988. 

If selectivity and flux data are available, pervaporation units can be designed without knowing the details of the concentration polarization, diffusion, and evaporation steps. For a binary separation the selectivity, B is defined as,... 

The main problem in membrane processes, especially for UF and MF separations, is the decrease of permeate flux caused by concentration polarization and fouling, whereas other membrane processes such as gas separation and pervaporation are less affected. Different approaches have been studied to reduce fouling. Hybrid systems using different types of membrane operations (e.g., distillation, dialysis, NF, pervaporation, and osmosis) prevent microbial fouling, offering a strong potential for the use of new types of thin-film composite membranes. 

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