Reverse osmosis membranes layer

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

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. 

Mitrovic and Knezic (1979) also prepared ultrafiltration and reverse osmosis membranes by this technique. Their membranes were etched in 5% oxalic acid. The membranes had pores of the order of 100 nm, but only about 1.5 nm in the residual barrier layer (layer AB in Figure 2.15). The pores in the barrier layer were unstable in water and the permeability decreased during the experiments. Complete dehydration of alumina or phase transformation to a-alumina was necessary to stabilize the pore structure. The resulting membranes were found unsuitable for reverse osmosis but suitable for ultrafiltration after removing the barrier layer. Beside reverse osmosis and ultrafiltration measurements, some gas permeability data have also been reported on this type of membranes (Itaya et al. 1984). The water flux through a 50/im thick membrane is about 0.2mL/cm -h with a N2 flow about 6cmVcm -min-bar. The gas transport through the membrane was due to Knudsen diffusion mechanism, which is inversely proportional to the square root of molecular mass. 

Jackson, J.M., and Landolt, D., "About the Mechanism of Formation of Iron Hydroxide Fouling Layers on Reverse Osmosis Membranes," Desalination 12, 361-378 (1973). 

Since Loeb and Sourlrajan s discovery (1 ) of a workable asymmetric reverse osmosis membrane, a lot of work has been done in order to elucidate and control the formation, structure and properties of a "skin", the surface layer of an asymmetric membrane. 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

This implies that the selective layer of reverse osmosis membranes may have a different origin from that of the micelles. Such a case is clearly identified by examination of the skin structure of cellulose acetate/poly(bromophenylene oxide phosphonate) alloy membranes (1 ), which exhibit a high flux and high salt separation (Figure 13). The skin rests on an assembly of giant spheres (up to 1 pm in diameter) and is certainly originated by a different coagulation mechanism than that of the spheres. 

The origin of thin-film-composite reverse osmosis membranes began with a newly formed research institute and one of its first employees, Peter S. Francis. North Star Research and Development Institute was formed in Minneapolis during 1963 to fill a need for a nonprofit contract research institute in the Upper Midwest. Francis was given the mission of developing the chemistry division through support, in part, by federal research contracts. At this time the Initial discoveries by Reid and Breton ( ) on the desalination capability of dense cellulose acetate membranes and by Loeb and Sourlrajan (,2) on asymmetric cellulose acetate membranes had recently been published. Francis speculated that improved membrane performance could be achieved, if the ultrathin, dense barrier layer and the porous substructure of the asymmetric... 

The transition between reverse osmosis membranes with a salt rejection of more than 95 % and molecular weight cutoffs below 50 and ultrafiltration membranes with a salt rejection of less than 10% and a molecular weight cutoff of more than 1000 is shown in Figure 2.42 [74], The very large change in the pressure-normalized flux of water that occurs as the membranes become more retentive is noteworthy. Because these are anisotropic membranes, the thickness of the separating layer is difficult to measure, but clearly the permeability of... 

Rejections of other ions besides Na+ and Cl- are tunable characteristics of the reverse osmosis membranes that depend upon the intrinsic nature of polymer separating layer and how it has been processed. In general, bivalent ions like Ca2+ and SO4- are more easily rejected than are monovalent ones like Na+ and Cl-. 

Reverse osmosis membranes are characterized by an MWCO of -100 Da, and the process involves transmembrane pressures (TMP) of 10-50 bar (1000-5000 kPa), which are 5-10 times higher than those used in UF [11,36]. Unlike UF, the separation by RO is achieved not by the size of the solute but due to a pressure-driven solution-diffusion process [36]. Like UF membranes, RO membranes are uniquely stmctured films from synthetic organic polymers and consist of an ultrathin skin layer superimposed on a coarsely porous matrix [3]. The skin layer of the RO membrane is nonporous, which may be treated as a water-swollen gel, and water is transported across membrane by dissolving in this gel and diffusing to the low-pressure side... 

Summary of Physicochemical Parameters. In the previous section steric parameters 4 . and were introduced to describe the effective size of solution components and the average size of the transport corridor, respectively. A variety of quantities that could be used to represent

dense membranes and the skin layer of reverse osmosis membranes in which the transport corridors are beyond the resolution capabilities of modern instruments and may be dynamic in nature. Therefore, any discussion of membrane material selection based on steric considerations must be qualitative. 

The conclusion which can be drawn from the above studies is that the skin layer in integrally-skinned reverse osmosis membranes consists of a single layer of consolidated Sol 2 type micelles. 

The dense layer of reverse osmosis membranes transports water as if through pores, but no pores are observed in electron micrographs of the dried membranes. This Implies either that the pores are too small to be observed by electron microscopy or that they only exist in the presence of water, not in the dried polymer. 

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