Reverse concentration polarization

Fig. 23. Two types of hollow-fiber modules used for gas separation, reverse osmosis, and ultrafiltration applications, (a) Shell-side feed modules are generally used for high pressure appHcations up to - 7 MPa (1000 psig). Fouling on the feed side of the membrane can be a problem with this design, and pretreatment of the feed stream to remove particulates is required, (b) Bore-side feed modules are generally used for medium pressure feed streams up to - 1 MPa (150 psig), where good flow control to minimise fouling and concentration polarization on the feed side of the membrane is desired.

J. Siler, "Reverse Osmosis Membranes-Concentration Polarization and Surface Fouling Predictive Models and Experimental Verifications," dissertation. University of Kentucky, Lexington, Ky., 1987. 

Electrodialysis Reversal. Electro dialysis reversal processes operate on the same principles as ED however, EDR operation reverses system polarity (typically three to four times per hour). This reversal stops the buildup of concentrated solutions on the membrane and thereby reduces the accumulation of inorganic and organic deposition on the membrane surface. EDR systems are similar to ED systems, designed with adequate chamber area to collect both product water and brine. EDR produces water of the same purity as ED. 

You may be surprised, but fouling is not always detrimental. The term dynamic membrane describes deposits that benefit the separation process by reducing the membrane s effective MWCO Molecular Weight cut-off) so that a solute of interest is better retained. Concentration polarization refers to the reversible build-up of solutes near the membrane surface. Concentration polarization can lead to irreversible fouling by altering interactions between the solvent, solutes and membrane. 

Dendrimers can also be prepared with an inverse relationship between their hydrophobic and hydrophilic constituents, i.e. with a hydrophobic periphery and a hydrophilic interior. They can then behave as reverse micelles and are able to concentrate polar molecules from solutions of nonpolar solvents. The shape of these molecules, when dissolved in a solvent that matches the hydrophobic nature of the periphery, is spherical with chain-ends extended towards the solvent. The interior may then collapse to a minimum volume, so that unfavourable interactions that might result from penetration by solvent molecules are minimized. 

The formation of new nuclei and of a fine-crystalline deposit will also be promoted when a high concentration of the metal ions undergoing discharge is maintained in the solution layer next to the electrode. Therefore, concentration polarization will have effects opposite those of activation polarization. Rather highly concentrated electrolyte solutions, vigorous stirring, and other means are employed to reduce concentration polarization. Sometimes, special electrolysis modes are employed for the same purposes currents that are intermittent, reversed (i.e., with periodic inverted, anodic pulses), or asymmetric (an ac component superimposed on the dc). 

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. 

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

Fig. Id represents the ionic changes and reversal of polarity of the membrane when the nerve is stimulated. Na+ ions enter the membrane ahead of the electrical charge and K+ ions pass out at the peak of the potential reversal.1 Fig. le shows how the ionic interchange is related to the action potential (or magnitude of polarity change). It must be stressed that the actual percentage changes of concentration are very small indeed. The exact nature of the restoration of the original concentration of ions is not completely known. Obviously a source of energy is required, and this is considered to be derived from the metabolism of the cell.

Useful PRO membranes do not require the very high permselectivity necessary in reverse osmosis, and a trade-off between flux and salt rejection in conventional RO membranes is possible. If the salt rejection is too low, however. Internal concentration polarization due to excessive salt leakage can limit the water flux. 

The apparent resistance due to the reduction in driving force from concentration polarization is generally small in most reverse osmosis systems and is neglected. The resistance can also be written in terms of a characteristic thickness and permeability... 

Measurement of Concentration Polarization Profile for Dissolved Species in Unstirred Batch Hyperfiltratlon (Reverse Osmosis)", Chem. Eng. Commun. 0-000. (1980)... 

Assessment of membrane damage was based on performance testing before and after chemical exposure. Testing was conducted in a small flat plate reverse osmosis unit designed to accommodate membrane discs of 45 mm diameter. Feed solution reservoir temperature was maintained at 25 1°C and the brine was continuously recirculated through a filter at the rate of 600 mL/min. Concentration polarization is considered negligible in this cell under these conditions. 

In reverse osmosis, where the solutes retained are relatively low in molecular weight and have a significant osmotic pressure, concentration polarization can result in osmotic pressures considerably higher than those represented by the bulk stream concentration. Higher pressures are required to overcome the osmotic pressure (Figure 6). 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

The effect of reversing the polarity of the current excitation signal on the concentration-distance profiles is shown in Figure 4.2C. The original reactant O is regenerated at a rate now determined by the flux of R back to the electrode. When the surface concentration of R becomes zero, a reverse transition time ir occurs. At this time, another species must undergo oxidation to enable the cell to accept the applied current. 

Figure 3.47 A cross-flow hollow fiber module used to obtain better flow distribution and reduce concentration polarization (the Tyobo Hollosep reverse osmosis module). Feed enters through the perforated central pipe and flows towards the module shell...

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. 

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 4.1 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,...

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. 

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)...

Figure 4.4 Salt concentration gradients adjacent to a reverse osmosis desalination membrane. The mass balance equation for solute flux across the boundary layer is the basis of the film model description of concentration polarization...

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