Membrane desalination

Fuhy aromatic, thermally (up to 250°C) and hydrolyticahy resistant films of PODs have been reali2ed from polyhydra2ides (56). Films of these polymers are useful as seawater desalination membranes. 

Compaction of cellulose acetate desalination membranes, causing reduction in throughput and performance with time, can be significantly reduced by irrigation grafting of styrene onto the membrane (145). 

Desalination membranes with improved, rigid, and stable surfaces have been prepared from cellulose acetate propionate (170). These films are generally more resistant to hydrolysis than those from cellulose acetate. 

Elmer, T. H. 1978. Evaluation of porous glass as desalination membrane. Ceramic Bull. 57(11) 1051-53, 60. 

Pitera, E.W. and Middleman, S., "Convection Promotion in Tubular Desalination Membranes", Ind. Eng. Chem. Process Des. Develop, 1973, U, 52... 

S. Manjikian, Desalination Membranes from Organic Casting Solutions, Ind. Eng. Chem. Prod. Res. Dev. 6, 23 (1967). 

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...

Figure 5.2 Effect of pressure, feed salt concentration and feed temperature on the properties of good quality seawater desalination membranes (SW-30) [14]...

Seawater and brackish water desalination membranes operated with 0.5 to 5 wt% salt solutions at pressures of 200-1000 psi. 

The goal of most of the early work on reverse osmosis was to produce desalination membranes with sodium chloride rejections greater than 98 %. More recently membranes with lower sodium chloride rejections but much higher water permeabilities have been produced. These membranes, which fall into a transition region between pure reverse osmosis membranes and pure ultrafiltration membranes, are called loose reverse osmosis, low-pressure reverse osmosis, or more commonly, nanofiltration membranes. Typically, nanofiltration membranes have sodium chloride rejections between 20 and 80 % and molecular weight cutoffs for dissolved organic solutes of 200-1000 dalton. These properties are intermediate between reverse osmosis membranes with a salt rejection of more than 90 % and molecular weight cut-off of less than 50 and ultrafiltration membranes with a salt rejection of less than 5 %. 

F. H. Bntt, F. Rahman, and U. Baduruthamal, Characterisation of fonlants by autopsy of RO desalination membranes. Desalination 114, 51-64 (1997). 

Amphiphilic block copolymers containing a cationic block may carry as the second block a hydrophilic or a hydrophobic one. Those containing a hydrophilic second block belong to the so called double-hydrophilic block copolymers (DHBC), which have recently been reviewed [60]. This promising class of polymers has potential applications in drug-carrier systems, gene therapy, desalination membranes, as switchable amphiphiles, and others [45, 60, 77]. 

Developments in pervaporation were boosted by the improvements in membrane structure and membrane manufacture resulting from R D expenditures on desalination membranes in the 1970s. Two different types of pervaporation membrane were developed in the beginning of the 1980s ... 

Studies of the plasma polymerization of tetrafluoroethylene in such a capacltively coupled system are described in another paper presented at this symposium [ 9]. The apparatus has been used to coat polysulfone hollow fibers with pyridine and acetylene + nitrogen plasma polymer to form a composite reverse osmosis desalination membrane. Salt rejections of 90-93% have been achieved at fluxes of 1.5-2.0 g.f.d with a fiber take up rate of 50-100 cm/min. 

Explorations with homogeneous membranes quickly showed that the flux-selectivity requirements for water desalination membranes would demand more than a simple melt-spun hollow fiber. In fact, it has been necessary to work out structure-property relationships on all levels of structure to bring RO membrane technology involving aromatic polyamides to its current status. 

Non-celluloslc Membranes. While the development of CA gas permeation membranes can be directly attributed to the development of water desalination membranes, the Invention of modified silicone membranes and polysulfone membranes was more Influenced by the extension of knowledge of transport, sorption and diffusion of gases In polymers (24-27). In principle, rubbery polymers exhibit the highest gas permeabilities at the lowest selectlvitles, and. 

Table IV lists the best performance data obtained for piperazine oligomer membranes interfacially reacted with isophthaloyl chloride. The objective of these tests was to achieve single-pass seawater desalination membranes. As such, the presence of free carboxylate groups was avoided use was made of the trimesoyl chloride or alternate triacyl halides in the oligomer formation step, and diacyl chlorides in the interfacial reaction step. A few samples of seawater desalination membranes were obtained. Best results were seen for piperazine-cyanurate pre-polymers interfacially cross-linked by isophthaloyl chloride, but fluxes were low in view of the operating test pressure of 1500 psi (10 342 kPascal). Also, individual membrane results with piperazine oligomers were equally as erratic as those experienced for piperazine directly. The only notable advantage of the piperazine oligomer approach was the ability to incorporate...

The TFC-801 membranes did not exhibit any measurable CCRO effect under equivalent test conditions. These results are consistent with our previous work on osmotic membranes. (9,) Thin-film-composite desalination membranes derived from polyamine precursors (such as TFC-801) contain a gel layer within the porous sublayer formed by thermal crosslinking of the polyamine.(10) This gel layer retards the diffusion of ethanol into the membrane from the recirculation solution. Thus the ethanol concentration inside the porous sublayer is not increased effectively by increasing the concentration of the recirculation solution. By contrast, Loeb-Sourirajan-type asymmetric membranes or composite membranes derived from monomeric precursors do not contain any gel layer. Such membranes are better suited to use in the CCRO process. 

For most applications the product which may be obtained by the direct hyperfiltration is sufficiently desalinated and the protein loss of about 11% may be accepted. Preliminary results also seem to indicate a slight reduction in bitterness and soy off-flavour due to removal of very small bitter peptides and other flavour compounds in the permeate when this desalination membrane is used. 

The use of LLC-based membrane systems is still in its infancy. Promising early results indicate that research in this field will be expanded. The applications outlined here are just a few where functional LLC materials might be implemented. More challenging separation materials, such as water desalination membranes, selective proton-conducting membranes for fuel cells, catalytic membranes, chromatography, and chiral separation media are perhaps future candidates for new LLC membrane materials. 

Spectra of water in desalination membranes (celliilose acetate, polyimide, porous glass) show in contrast to biological Studies a smaller Av and indicate a weaker H-bonded system in these membranes compared with liquid water (j8, J9). 

Recent studies using Infrared Spectrocopy IR to characterize the state of water in desalination membranes have concluded that the water sorbed in these membranes has a low degree of association and that bonds between water and the membrane are considerably weaker than those in liquid water (1,2). These conclusions have been made for widely differing membrane materials such as cellulose acetate (1,2),polyimide, and porous glass and appear to contradict the conclusions obtained from pNMR differential scanning calorimetry (9-11) and transport... 

An explanation of the mechanism of solute selectivity and water transport in desalination membranes clearly depends on a resolution of the above apparent contradiction. A qualitative model of the state of water inside desalination membranes considering both the IR and pNMR results (including those presented here) will be proposed. 

Here we report results of the pNMR relaxation times of water adsorbed on the smallest (29 K) and the largest (189 A) pore size porous glass desalination membranes studied earlier (8) as a function of three coverages for the temperature range -80 to +90°C. Although the approach and the relaxation model used here is similar to that used by Belfort ) to study water adsorption at two... 

Clearly, the absolute amount and ratio of restricted water to bulk-like water will depend inversely on the pore-size of the porous glass desalination membrane (see column 6, Table I). By normalizing the samples, an average thickness of an annulus of the first adsorbed layer, t (in angstroms can be estimated by t = Vn, X lOVS 0CJ water Table I t (sample 2) = 0.024x10 /... 

The Fragmented Cluster Model proposed above is also Consistent with the IR results of Luck, et. al. for porous glass (X), and Toprak, et., for cellulose acetate (2). Indeed, the IR results confirm the original model of Belfort al. fo " explaining the desalting mechanism of porous glass desalination membranes... 

Unmodified porous glass desalination membranes that have shown moderate to hiah salt rejections have had measured pore diameters of 24 2 A, 41 3 A, and 50 5 A with approximate salt rejections of 65, 35 and 48%, respectively (6, 39). 

The present Fragmented Cluster Model predicts a priori the size range of the pore diameter below which bulk water is not able to establish itself. Above this size range, porous glass and dense cellulose acetate desalination membranes have not shown significant salt rejection. 

A variable temperature at constant adsorbed water coverage study was conducted for two different pore size glass desalination membranes at three different relative humidities. This compliments the earlier constant temperature and variable water coverage study (8). The following can be stated ... 

Luck, W.A.P. Schioberg, D. Sieman, U., "Infrared Investigations of Water Structure in Desalination Membranes" presented at the Faraday Society Meeting, "Transport Across Synthetic Membranes", Cranfield Institute, UK (March 1, 1979),... 

Osmotic phenomena have been observed since the middle of the eighteenth century. The first experiments were conducted with animal membranes and it wasn t unitl 1867 that artificial membranes were employed. In the early 1950 s, research workers at the University of Florida demonstrated, with thick films, that cellulose acetate possessed unique salt and water transport properties which made it potentially attractive as a reverse osmosis desalination membrane. During the 1960 s, Loeb and others at the University of California at Los Angeles developed techniques to prepare cellulose acetate membranes with an economical water flux and salt rejection at moderate driving pressures. With this development, reverse osmosis became a practical possibility. 

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