Membranes concentration polarization

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

During reverse osmosis and ultrafiltration membrane concentration, polarization and fouling are the phenomena responsible for limiting the permeate flux during a cyclic operation (i.e., permeation followed by cleaning). That is, membrane lifetimes and permeate (i.e., pure water) fluxes are primarily affected by the phenomena of concentration polarization (i.e., solute build up) and fouling (e.g., microbial adhesion, gel layer formation, and solute adhesion) at the membrane surface [11]. 

Gekas, V. and Hallstrom, B. Mass transfer in the membrane concentration polarization layer under turbulent cross flow I. Critical literature review and adaptation of existing Sherwood correlations to membrane operations, J. Membr. Sci., 30, 153, 1987. 

Mass transfer in the feed and strip solutions is limited by the extent of concentration polarization. On the feed side of the membrane, concentration polarization refers to an increase in the concentration of solutes at and near the feed-membrane interface because of evaporation of water into the membrane pores (Fig. 1). The resulting solute concentration gradient between the membrane-feed interface, where the concentration is greatest, and the bulk solution induces diffusive transport of rejected solutes back through the concentration polarization boundary layer into the bulk stream. Bulk solution is simultaneously transported to the membrane wall by convection. When equilibrium has been established under a given set of operating conditions (stream flow rate, temperature, fluid dynamics imposed by membrane module design), the rate of back diffusion is equal to the rate at which the solutes are carried to the membrane surface by convective flow. ... 

On the strip side of the membrane, concentration polarization refers to an increase in the water concentration at and near the strip-membrane interface because of condensation of permeate into the strip solution. At equilibrium, the solutes (osmotic agent) diffuse from the bulk stream towards the membrane wall at the same rate as their concentration is reduced by permeate condensation. Water is transported away from the membrane by convection. 

Figure 6.8 Current-voltage relation during electrodialysis of a sodium chloride solution using an ion exchange membrane (concentration polarization). Ion exchange membrane cation exchange membrane (NEOSEPTA CL-25T) measured under natural convection of a 0.104 N NaCl solution at 25.0 °C.

V. Gekas, B. Hallstrom, Mass transfer in the membrane concentration polarization layer under turbulent crossflow. I... 

In desalination, localized concentrations of solute build up at the point where the solvent leaves the solution and enters the membrane. The solute accumulates in a relatively stable boundary layer (Fig. 13.9-3) next to the membrane. Concentration polarization, is defined as the ratio of the salt concentration at the membrane surface... 

As wastewater permeates through the membrane there is a tendency for a cake of solids to form on the feed side of the membrane. The thickness of the cake is typically limited by the shearing action of the cross-flow, such that the flux undergoes an initial decay, and then stabilizes to a nearly steady-state value. With continued use, the cake may begin to density, at which point the membrane typically requires cleaning to prevent further decline in the flux rate. If, however, some of the solids or dissolved chemicals in the feed water load the pore stmemre of the filters, the flux may decline in an unrecoverable manner. In addition to the tendency for the formation of a cake on the feed side of the membrane concentration polarization may also limit the water flux through the membrane. If the flux is held at a constant value, the transmembrane... 

Beta is not a property of the membrane it is an artifact of the system design that is selected. Specifically, Beta is a function of how quickly the influent stream is dewatered through the RO system. If water is removed too quickly from the influent stream. Beta will increase, as a relatively high volume of dissolved soHds is left behind on the membrane because of the high volume of water that permeates out through the membrane. Concentration polarization further exacerbates the problem because of the diffusional-only flow away from the membrane surface. See Chapter 9.6 for more information about Beta and its relationship with water flux and salt passage. 

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.

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

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 = 4.05 X lO " cm/(s-kPa)(4.1 X 10 cm/(s-atm)) and = 1.3 x 10 cm/s (4)//= 1 mPa-s(=cP), NaCl diffusivity in water = 1.6 x 10 cm /s, and solution density = 1 g/cm . Figure 4 shows typical results of this type of simulation of salt water permeation through an RO membrane. Increasing the Reynolds number in Figure 4a decreases the effect of concentration polarization. The effect of feed flow rate on NaCl rejection is shown in Figure 4b. Because the intrinsic rejection, R = 1 — Cp / defined in terms of the wall concentration, theoretically R should be independent of the Reynolds... 

Dynamic membranes are concentration—polarization layers formed in situ from the ultrafiltration of coUoidal material analogous to a precoat in conventional filter operations. Hydrous zirconia has been thoroughly investigated other materials include bentonite, poly(acryhc acid), and films deposited from the materials to be separated (18). 

Fig. 5. Concentration polarization = concentration at membrane wall, Cj, = bulk concentration, Cj,. = bulk concentration of species i, J = flux, and...

P. Dejmek, "PermeabiHty of the Concentration Polarization Layer in Ultrafiltration of Macro Molecules," Proceedings of the International Symposium, Separation Processes by Membranes, Paris, Mar. 13—14,1975. 

Polarization. When the appHed current density equals the AX membrane and the apparatus are said to be concentration polarized or simply polarized. At the fluid at the surface of the membrane is essentially depleted of electrolyte and the electrical resistance of the apparatus iacreases... 

Fouling is the term used to describe the loss of throughput of a membrane device as it becomes chemically or physically changed by the process fluid (often by a minor component or a contaminant). A manifestation of fouling in cross-flow UF is that the membrane becomes unresponsive to the hydrodynamic mass transfer which is rate-controlling for most UF. Fouling is different from concentration polarization. Both reduce output, and their resistances are additive. Raising the flow rate in a cross-flow UF will increase flux, as in Eq. 

Concentration polarization is a significant problem only in vapor separation. There, because the partial pressure of the penetrant is normally low and its solubihty in the membrane is high, there can be depletion in the gas phase at the membrane. In other applications it is usually safe to assume bulk gas concentration right up to the membrane. 

Electrodialysis can be applied to the continuous-flow type of operation needed in industry. Multi-membrane stacks can be built by alternately spacing anionic- and cationic-selective membranes. Among the technical problems associated with the electrodialysis process, concentration polarization is perhaps the most serious (discussed later). Other problems in practical applications include membrane scaling by inorganics in feed solutions as well as membrane fouling by organics. 

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. 

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. 

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. 

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