Flux with Concentration Polarization

The effect of concentration polarization on retention was discussed in the previous section. In this section, it will be seen that concentration polarization can severely limit the flux. The control of polarization by proper fluid management techniques is essential to the economic feasibility of the process. 

Without the development of an anisotropic UF membrane, UF would not be a commercial process today. The thin skin minimizes the resistance to flow, and the asymmetry of the pores virtually eliminates internal pore fouling. However, the hydraulic permeability of these membranes also increases the convective ransport of solutes to the membrane surface. Consequently, the polarization modulus (defined as the ratio of the solute concentration at the membrane surface, Cs, to that in the bulk process stream, Cb) is higher than that experienced with lower permeability RO membranes. 

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Where the term vL/D is less than 2.0, solute diffusion has a significant effect on the rejection. Because diffusion lowers the rejection at low permeate flux and concentration polarization is important at high flux, the fraction rejected is predicted to go through a maximum with permeate flux. The diffusion effect should be quite pronounced if the active layer is very thin, say, 0.1 to 0.2 pm, but there are not enough data to confirm this. 

The analysis procedure developed in the previous section for gas permeation forms the basis for analyzing RO. However, the RO analysis is more complicated because of 1) osmotic pressure, which is included in Eq. fl7-12T and 2) mass transfer rates are much lower in liquid systems. Since the mass transfer rates are relatively low, the wt frac of solute at the membrane wall x will be greater than the wt frac of solute in the bulk of the retentate x, . This buildup of solute at the membrane surface occurs because the movement of solvent through the membrane carries solute with it to the membrane wall. Since the solute does not pass through the semipermeable membrane, its concentration will build up at the wall and it must back diffuse from the wall to the bulk solution. This phenomenon, concentration polarization, is illustrated in Figure 17-10. Concentration polarization has a major effect on the separations obtained in RO and UF (see next section). Since concentration polarization causes x > Xp the osmotic pressure becomes higher on the retentate side and, following Eq. fl7-12). the flux declines. Concentration polarization will also increase Ax in Eq. tl7-13 and flux of solute may increase, which is also undesirable. In addition, since concentration polarization increases solute concentration, precipitation becomes more likely. 

Major problems inherent in general applications of RO systems have to do with (1) the presence of particulate and colloidal matter in feed water, (2) precipitation of soluble salts, and (3) physical and chemical makeup of the feed water. All RO membranes can become clogged, some more readily than others. This problem is most severe for spiral-wound and hollow-fiber modules, especially when submicron and colloidal particles enter the unit (larger particulate matter can be easily removed by standard filtration methods). A similar problem is the occurrence of concentration-polarization, previously discussed for ED processes. Concentration-polarization is caused by an accumulation of solute on or near the membrane surface and results in lower flux and reduced salt rejection. 

Sea Salts - Most of the Na and Cl in both Greenland and Antarctica is of marine origin [1,13,21]. Near the ocean, sea salts may also account for most of the Mg, K, Ca, and S042-. Concentrations of Na and Cl display a maximum in winter Greenland precipitation which is coincident with the minimum oxygen isotope delta values [1,13,18]. The seasonal maximum in sea salt concentrations may be due to increased storminess, over the ocean in winter or to an increased poleward latent heat flux during the polar night [13]. 

Summarizing it can be stated that the separation by gas phase transport (Knudsen diffusion) has a limited selectivity, depending on the molecular masses of the gases. The theoretical separation factor is decreased by effects like concentration-polarization and backdiffusion. However, fluxes through the membrane are high and this separation mechanism can be applied in harsh chemical and thermal environments with currently available membranes (Uhlhorn 1990, Bhave, Gillot and Liu 1989). 

In Figure 50, the lower curve for E=3.9 v/cm shows a transition in slope. The flux decreases with decreasing Reynolds numbers until a point is reached where the convective transport of particles toward the membrane is just equal to the electrophoretic migration away from the membrane-i.e. the voltage is now the critical voltage. Further decreases in the Reynolds number will not decrease the flux as there is now no concentration polarization. 

Problems encountered with filtration ate that membrane fouhng can occur, which causes a decline in flux with time under constant operating conditions. Furthermore, concentration polarization, the effect that the increased concentration of components on the membrane surface reduces the flux due to the additional hydrodynamic resistance, is observed. This effect can be minimized in cross-flow filtration, by applying high flux rates across the membrane surface (Wang et al, 1979 Lee, 1989). 

Equation 8.7 [6] was obtained to correlate the experimental data on membrane plasmapheresis, which is the MF of blood to separate the blood cells from the plasma. The filtrate flux is affected by the blood velocity along the membrane. Since, in plasmapheresis, all of the protein molecules and other solutes will pass into the filtrate, the concentration polarization of protein molecules is inconceivable. In fact, the hydraulic pressure difference in plasmapheresis is smaller than that in the UF of plasma. Thus, the concentration polarization of red blood cells was assumed in deriving Equation 8.7. The shape of the red blood cell is approximately discoid, with a concave area at the central portion, the cells being approximately 1-2.5 pm thick and 7-8.5 pm in diameter. Thus, a value of r (= 0.000257 cm), the radius of the sphere with a volume equal to that of a red blood cell, was used in Equation 8.7. 

Two other major factors determining module selection are concentration polarization control and resistance to fouling. Concentration polarization control is a particularly important issue in liquid separations such as reverse osmosis and ultrafiltration. In gas separation applications, concentration polarization is more easily controlled but is still a problem with high-flux, highly selective membranes. Hollow fine fiber modules are notoriously prone to fouling and concentration polarization and can be used in reverse osmosis applications only when extensive, costly feed solution pretreatment removes all particulates. These fibers cannot be used in ultrafiltration applications at all. 

Gas separation Hollow fibers for high volume applications with low flux, low selectivity membranes in which concentration polarization is easily controlled (nitrogen from air)... 

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 most important effect of concentration polarization is to reduce the membrane flux, but it also affects the retention of macromolecules. Retention data obtained with dextran polysaccharides at various pressures are shown in Figure 6.12 [17]. Because these are stirred batch cell data, the effect of increased concentration polarization with increased applied pressure is particularly marked. A similar drop of retention with pressure is observed with flow-through cells, but the effect is less because concentration polarization is better controlled in such cells. With macromolecular solutions, the concentration of retained macromolecules at the membrane surface increases with increased pressure, so permeation of the macromolecules also increases, lowering rejection. The effect is particularly noticeable at low pressures, under which conditions increasing the applied pressure produces the largest increase in flux, and hence concentration polarization, at the membrane surface. At high pressure, the change in flux with... 

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. 

The processes of interest are NFand RO where the membranes are either nanoporous or essentially nonporous. In these processes the fouling is a surface layer, the effects of which maybe exacerbated by the high retention of solutes by the membrane. Operation is with crossflow and in industry fixed flux is commonly used. This section considers particulate fouling, biofouling and scale formation and then discusses the implications of cake enhanced concentration polarization on fouling outcomes. 

This effect, usually known as feed-side concentration polarization, may become particularly relevant for solutes with a high sorption affinity towards the membrane, which may lead to its depletion near the membrane interface if external mass-transfer conditions are not sufficiently good to guarantee their fast transport from the bulk feed to the interface [32, 36] (see Figure 11.3). As a consequence of their depletion near the interface the driving force for transport, and the resulting partial fluxes, become lower. 

Factors that affect fouling with NOM-calcium complexes include permeate flux and cross flow rate (see Chapters 3.3 and 9.4). At higher flux though the membrane, the concentration of calcium increases in the concentration polarization boundary layer at the membrane surface, as described above. Lower cross flow rates also increase the concentration of calcium in the boundary layer. The increases concentration of calcium at the membrane surface enhances the fouling of the membranes by the NOM-calcium aggregates.5... 

Figures 9.8 and 9.9 shows how Beta affects flux and salt passage (rejection), respectively, for two different brackish water concentrations (assumes membrane will deliver 20 gfd at 400 psi with a rejection of 99% at Beta equal to one (no concentration polarization)).8 From the Figures, it is shown that at Beta values greater than about 1.1, the water flux and salt passage (rejection) are significantly affected by Beta. Also shown is that the effect of Beta on performance is more pronounced at higher TDS feed water than with lower TDS feed water.

Many models have been published to calculate the microfiltration process. One important factor is the concentration polarization, which represents the most important limiting physical obstacle. At high particle concentration and with time, a layer is formed on the membrane. This layer is responsible for the flux reduction. A comprehensive overview on this technique is given by Ripperger52 and Staude.53 Often similar or identical module types are used in microfiltration and ultrafiltration. 

Thus a complicated interplay of forces and fluxes emerges diffusion, conduction and hydrodynamic flows. These will be treated in sec. 4.6, but in anticipation of this treatment we shall now emphasize on the (concentration-) polarization, mentioned under 1) and discuss some of Its consequences. From the outset it is Important to realize that quantities with different length scale Interact the double layer thickness is. and remains. 0(v ). but the polarization Jield, caused by the polarization of the particle, hcis a range 0(a). 

Another very important operating characteristics of inorganic membranes that is not shown in Table 1.4 has to do with the phenomena of fouling and concentration polarization. Concentration polarization is the accumulation of the solutes, molecules or particles retained or rejected by the membrane near its surface. It is deleterious to the purity of the product and the decline of the permeate flux. Fouling is generally believed to occur when the adsorption of the rejected componcni(s) on the membrane surface is strong enough to cause deposition. How to maintain a clean membrane surface so that... 

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