Concentration polarization, phenomenon

To achieve a high membrane rejection towards the substrate it is important that the pore size of the membrane is smaller than the size of the molecules to be retained. Nevertheless, other factors influence the separation properties of a membrane, such as the shape and flexibility of the substrate and its acid-base properties, as well as the concentration-polarization phenomenon and the membrane fouling. 

This finding depends on different factors and, as previously described, was partially solved by enhancing the turbulent flow on the membrane surface. In this way, the deposition of the substrate and the catalyst was limited, avoiding the concentration polarization phenomenon that also affects the water flux across the membrane. This aspect is currently under study and some preliminary results obtained in a study on the Gemfibrozil degradation in the described PMR are reported in Table 15.4. 

Scaled membranes exhibit lower productivity and lower salt rejection. This lower salt rejection is a function of the concentration polarization phenomenon (see Chapter 3.4). When membranes are scaled, the surface of the membrane has a higher concentration of solutes than in the bulk solution. Since the membrane rejects when the membrane "sees," the passage of salts will be higher, even though the absolute or true rejection stays constant. 

Concentration polarization Convective transport and retention of solutes by the membrane results in an accumulation of species at wall. Local concentrations, C , are higher than in the bulk, Cb, and a back-diffusion from near the wall into the bulk liquid phase takes place. This is the so-called "concentration polarization" phenomenon (Fig. 12.1). A simple mass balance leads to the classical equation ... 

To illustrate the concentration polarization phenomenon, we consider an infinitely long electrodialysis cell pair having parallel channels in which the flow is fully developed and laminar. The qualitative behavior of the development of the salt concentration and potential distributions along the channels of a dialysate and concentrate cell pair are shown schematically in Fig. 6.2.3 for the case where the inlet salt concentrations are the same in both channels (Probstein 1972). 

Other researchers used CFD calculation with complex 3D models to study reactive and separating systems with Pd-based membranes " to take into account the concentration polarization phenomenon. However, their attention was strongly directed towards a specific process and, thus, it is very difficult to achieve and extrapolate general considerations about it. 

The concept of concentration polarization was given earlier and a simple model based on the him theory was developed to describe the phenomenon mathematically. In this chapter an attempt is made to develop a differential equation of the solute material balance, the solution of which will rigorously establish a concentration profile of the solute in the vicinity of the solution-membrane boundary. It is hoped that this approach will furnish a deeper understanding of the concentration polarization phenomenon. 

The separation process in UF and MF systems is realized in proper devices known as membrane modules. A membrane module must be able to support the membrane, to minimize the concentration polarization phenomenon and to provide a large surface area in a compact volume. 

A pressure of at least twice the osmotic pressure must be exerted on the must to force the water of the must to cross the membrane. This concentrates various must constituents. During water transfer, molecules and ions retained by the membrane are apt to accumulate on its surface, increasing the real must concentration to be treated and thus the required pressure. To limit this concentration polarization phenomenon, the filtering side of the membrane must be cleaned to minimize the thickness of this accumulating limit layer and to facilitate the retrodiffusion of the retained solutes. Reverse osmosis can be placed in the same category as tangential hyperfiltration. The equipment is comparable and only differs by the nature of the membrane. 

The phenomenon of concentration polarization, which is observed frequently in membrane separation processes, can be described in mathematical terms, as shown in Figure 30 (71). The usual model, which is weU founded in fluid hydrodynamics, assumes the bulk solution to be turbulent, but adjacent to the membrane surface there exists a stagnant laminar boundary layer of thickness (5) typically 50—200 p.m, in which there is no turbulent mixing. The concentration of the macromolecules in the bulk solution concentration is c,. and the concentration of macromolecules at the membrane surface is c. 

A phenomenon that is particularly important in the design of reverse osmosis units is that of concentration polarization. This occurs on the feed-side (concentrated side) of the reverse osmosis membrane. Because the solute cannot permeate through the membrane, the concentration of the solute in the liquid adjacent to the surface of the membrane is greater than that in the bulk of the fluid. This difference causes mass transfer of solute by diffusion from the membrane surface back to the bulk liquid. The rate of diffusion back into the bulk fluid depends on the mass transfer coefficient for the boundary layer on feed-side. Concentration polarization is the ratio of the solute concentration at the membrane surface to the solute concentration in the bulk stream. Concentration polarization causes the flux of solvent to decrease since the osmotic pressure increases as the boundary layer concentration increases and the overall driving force (AP - An) decreases. 

LIF is used to filter any large molecules (e.g., proteins) present in a solution by using an appropriate membrane. Although the driving potential in UF is the hydraulic pressure difference, the mass transfer rates will often affect the rate of UF due to a phenomenon known as concentration polarization (this is discussed later in the chapter). 

If the flux is perpendicular to the membrane surface, solute tends to accumulate at the membrane surface up to the solute solubility limit. This phenomenon, called concentration polarization, often results in solute leakage and in reduced driving force owing to increased osmotic pressure. 

The second approach to concentration polarization, and the one used in this chapter, is to model the phenomenon by assuming that a thin layer of unmixed fluid, thickness S, exists between the membrane surface and the well-mixed bulk solution. The concentration gradients that control concentration polarization form in this layer. This boundary layer film model oversimplifies the fluid hydrodynamics occurring in membrane modules and still contains one adjustable parameter,... 

Figure 4.1 shows the concentration gradients that form on either side of a dialysis membrane. However, dialysis differs from most membrane processes in that the volume flow across the membrane is usually small. In processes such as reverse osmosis, ultrafiltration, and gas separation, the volume flow through the membrane from the feed to the permeate side is significant. As a result the permeate concentration is typically determined by the ratio of the fluxes of the components that permeate the membrane. In these processes concentration polarization gradients form only on the feed side of the membrane, as shown in Figure 4.3. This simplifies the description of the phenomenon. The few membrane processes in which a fluid is used to sweep the permeate side of the membrane,...

As described above, the initial cause of membrane fouling is concentration polarization, which results in deposition of a layer of material on the membrane surface. The phenomenon of concentration polarization is described in detail in Chapter 4. In ultrafiltration, solvent and macromolecular or colloidal solutes are carried towards the membrane surface by the solution permeating the membrane. Solvent molecules permeate the membrane, but the larger solutes accumulate at the membrane surface. Because of their size, the rate at which the rejected solute molecules can diffuse from the membrane surface back to the bulk solution is relatively low. Thus their concentration at the membrane surface is typically 20-50 times higher than the feed solution concentration. These solutes become so concentrated at the membrane surface that a gel layer is formed and becomes a secondary barrier to flow through the membrane. The formation of this gel layer on the membrane surface is illustrated in Figure 6.6. The gel layer model was developed at the Amicon Corporation in the 1960s [8],... 

In a pressure-driven membrane process the molecules are generally rejected by the membrane and therefore their concentrations in the permeate are lower than those in the feed solution. However, an accumulation of excess particles can occur at the membrane surface with the creation of a boundary layer. This phenomenon, called concentration polarization, causes a different membrane performance. In particular, with low molecular weight solutes the observed rejection will be lower than the real retention or, sometimes, it could be negative. 

Concentration polarization is a phenomenon common to all electrolytical processes. As long as abnormally high current densities or too diluted solutions are not applied, this kind of polarization will not occur to a considerable extent in industrial electrolyses. This is due to the fact that the concentration differences which initiate this kind of polarization are usually not too high. 

Now, consider flow along the surface of a membrane. The same boundary layer forms as with flow through a pipe. However, with a membrane system, because there is a net flow out through the membrane, there is convective flow to the membrane, but only dif-fusional flow away from the membrane. Since diffusion is slower than convection, solutes rejected by the membrane tend to build up on the surface and in the boundary layer. Thus, the concentration of solutes at the membrane surface is higher than in the bulk solution. This boundary layer is called "concentration polarization."2 The phenomenon is shown in Figure 3.3. 

Relatively Important is the phenomenon of dijfusiophoresis and its counterpart plug or capillary osmosis. For both the driving force is a concentration gradient, either of an electrolyte or of a non-electrolyte. Consider for instance fig. 4.40. The presence of the gradient leads to at least an (osmotic) pressure gradient p(0) around the particle in the double layer. Moreover, by specific adsorption it can also lead to concentration polarization and x" may depend on 9. In this way driving forces are established to induce the particle to move. 

Permeate flux decreases with an increase in feed concentration. This phenomenon can be attributed to the reduction of the driving force due to decrease of the vapor pressure of the feed solution and exponential increase of viscosity of the feed with increasing concentration. The DCMD flux gradually increases with an increase in temperature difference between feed and cooling water. Lagana et al. [63] reported that the viscosity of apple juice at high concentration induces severe temperature polarization. It may be noted that temperamre polarization is more important than concentration polarization, which is located mainly on the feed side. 

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