Reverse osmosis membrane types

Reverse Osmosis. Membranes are used for the separation of smaller components (<500 daltons). They have smaller pore space and are tighter than those used for ultrafiltration. High pressure pumps, usually of the positive piston or multistage centrifugal type, provide pressures up to 4.14 MPa (600 psi). 

This method is used mainly to remove high-molecular-weight materials such as proteins, colloids, viruses, and bacteria. The same types of problem encountered with the use of reverse osmosis membranes are encountered here, and the proposed solutions are the same. 

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. 

The membranes used for pervaporation are similar to reverse osmosis membranes, i.e. both are composite membranes consisting of a very thin dense permselective film on top of a nonselective porous support. In pervaporation, however, the membrane is highly swollen at the feed side and relatively dry at the permeate side. Two different types of pervaporation membranes based on polymeric materials were developed at about the same time in the early 1980s [31] ... 

Figure 16. Reverse osmosis membrane exhibiting three types of macrovoids large (a), medium (b), and small (c). (A) Before testing and (B) after exposure to 13, atm hydraulic pressure. A longitudinal crack in the skin is designated by the per-...

Considerable activity has been generated on composite reverse osmosis membranes by Japanese researchers. Patent applications were recently published, for example, covering research at Teijin Ltd. on interfacially formed membranes prepared from polydiallylamines (17) and from amine adducts of trls-(glycidyl) isocyanurate (18). Both types of membranes were formed on micro-porous polysulfone supports. Kurihara and coworkers have developed a composite membrane, designated PEC-1000, which is formed by an... 

The chemical sensitivity or life expectancy of reverse osmosis membranes is very important for manufacturing application. Thus chlorine is the most well known reagent for water disinfection. Glaster et al. 61 inspected the influence of halogens on the performance and durability of reverse osmosis membranes. Cellulose acetate was unresponsive to halogen agents but polyamide-type membranes deteriorated rapidly when exposed to halogens. 

Kucera-Gienger, Studies on Halogen Interaction with Polyamide Type Reverse Osmosis Membranes, Master s Thesis, UCLA, March, 1984. 

Much effort has been expended in attempting to use membranes for separations. Reverse osmosis membranes are used worldwide for water purification. These membranes are based on size selectivity depending on the pores used. They do not have the ability to selectively separate target species other than by size. Incorporation of carrier molecules into liquid membrane systems of various types has resulted in achievement of highly selective separations on a laboratory scale. Reviews of the extensive literature on the use of liquid membrane systems for carrier-mediated ion separations have been published [15-20]. A variety of liquid membranes has been studied including bulk (BLM), emulsion (ELM), thin sheet supported (TSSLM), hollow fiber supported (HFSLM), and two module hollow fiber supported (TMHFSLM) types. Of these liquid membranes, only the ELM and TMHFSLM types are likely to be commercialized. Inadequacies of the remaining... 

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. 

Addition of a Nonsolvent to a Homogeneous Polymer Solution. This technique is widely used today for the preparation of symmetric microfiltration membranes as well as for manufacturing asymmetric "skin-type" ultrafiltration or reverse osmosis membranes (7). The preparation procedure can again be rationalized with the aid of a three-component isothermic phase diagram shown schematically in Figure 3. 

The development of composite reverse osmosis membranes is reviewed with emphasis on those types that have survived the selection for commercial development. 

Two types of composite reverse osmosis membranes, formed by the dip-coating approach, are known to be of commercial interest at the present time and are discussed below. One type consists of membranes made on a microporous support film by acid-catalyzed condensation of... 

During the 1960 s, the DuPont Company screened numerous polymers to determine the suitability of materials other than cellulose acetate for use in reverse osmosis desalination. The results of this work indicated that aromatic polyamides were the "choice as the best polymer type for use in the DuPont commercial permeators".7 The company was most successful in developing an asymmetric aromatic polyamide reverse osmosis membrane in a hollow fine fiber configuration which successfully competed with cellulose acetate in the market place. 

Various polyamines have been synthesized and evaluated in the fabrication of the NS-100 type of membrane. These various compositions are described in the patent literature. Some of these efforts have involved polymeric amines containing only secondary amino groups to reach a goal of improved chlorine resistance. Whether any of them have reached commercial status cannot be determined because of the current trend to avoid publication of the compositions of new commercial reverse osmosis membranes. 

Figure 5.16, adapted from Kurihara,79 80 shows a comparison of several types of commercial reverse osmosis membranes in terms of salt rejection and permeate flow rate under seawater test conditions (35,000 ppm, 800 psi, 25°C). This chart emphasizes the capability of PEC-1000 to provide complete single-stage seawater desalting. In a test at Toray s Ehime desalination test facility on 42,000 ppm seawater (equivalent to Red Sea salinity), PEC-1000 spiral elements operated at 35% recovery produced a permeate having an average salinity of only 220 ppm, well below WHO standards. Average salt rejection was 99.5%. 

Water flux through reverse osmosis membranes is considerably dependent on the hydrophilic character of the barrier layer. In the composite membrane approach, highly hydrophilic barrier layer compositions can be used, suitably insolubilized by crosslinking. To a large extent, water flux and salt rejection can be controlled by the type and extent of crosslinking. 

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