Composite Reverse Osmosis Membranes

UF Membranes as a Substrate for RO An important use of UF membranes is as a substrate for composite reverse-osmosis membranes. After the UF membrane (usually polysulfone) is prepared, it is coated with an aqueous solution of an amine, then dipped in an organic solution of an acid chloride to produce an interfacially polymerized membrane coating. 

Figure 18. Cross-section scheme of a composite reverse osmosis membrane...

Thin-Film Composite Reverse-Osmosis Membranes Origin, Development, and Recent Advances... 

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... 

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... 

In 1977 the North Star membrane research group was spun off by Midwest Research Institute, forming FilmTec Corporation. Two new thin-film-composite reverse osmosis membranes have been under development at FilmTec Corporation since that time, the NS-300 and the FT-30 membranes. 

Jr. "Transport in Composite Reverse Osmosis Membranes," in "Membrane Processes in Industry and Biomedicine," Bier,... 

Yasuda, H. "Composite Reverse Osmosis Membranes Prepared by Plasma Polymerization," in "Reverse Osmosis and Synthetic Membranes," Sourirajan, S., Ed., National Research Council, Canada, Ottawa, 1977, p.263. 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

J.E. Cadotte, Evolution of Composite Reverse Osmosis Membranes, in Materials Science of Synthetic Membranes, D.R. Lloyd (ed.), ACS Symposium Series Number 269, American Chemical Society, Washington, DC, pp. 273-294 (1985). 

Y. Kamiyama, N. Yoshioka, K. Matsui and E. Nakagome, New Thin-film Composite Reverse Osmosis Membranes and Spiral Wound Modules, Desalination 51, 79... 

R.J. Petersen and J.E. Cadotte, Thin Film Composite Reverse Osmosis Membranes, in Handbook of Industrial Membrane Technology, M.C. Porter (ed.), Noyes Publications, Park Ridge, NJ, pp. 307-348 (1990). 

The deposition of a plasma polymer on an appropriate porous substrate to form a composite reverse osmosis membrane is a typical example of the second case. In both cases, however, the selection of the substrate membrane is the crucially important factor, particularly in the second case. In the application of nanofilm, the pore size of the substrate membrane must have the uniformity of pore size in nanometer scale, which is an extremely difficult requirement. 

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

The first composite reverse osmosis membrane reported in the technical literature was developed by Peter Francis of North Star Research Institute in 1964 (4). This membrane was formed by float-casting an ultrathin film of cellulose acetate (CA) upon a water surface, removing the membrane from the water surface by lamination onto a pre-formed microporous support film and drying to bond the membrane to the support. This float-casting procedure has since been described in the technical literature for both flat sheet and tubular membranes ( 5, 6, T). 

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... 

Graefe, A.F. "Development of a Composite Reverse Osmosis Membrane for Single Pass Seawater Desalting" NTIS Report No. PB-80 124852... 

M., "Recent Developments in Thin-Film Composite Reverse Osmosis Membrane Systems." Desalination. 1981, 36, 207-233. 

Cadotte, J.E., Evolution of composite reverse osmosis membranes, in ... 

Reverse osmosis membrane is produced in sheet form-up to 60 inches wide and lengths up to 1,500 feet-and as a hollow fine fiber. The asymmetric cellulose acetate was originally produced as a sheet and later as a hollow fine fiber. The asymmetric aromatic polyamide was originally produced as a hollow fine fiber and later in sheet form. The composite membranes with polyamide or polyurea membrane barrier layers are produced in sheet form as of the end of 1987, but research has been and will continue to be done to produce the composite reverse osmosis membranes as a hollow fine fiber. 

Thin Film Composite Reverse Osmosis Membranes... 

A thin film composite reverse osmosis membrane can be defined as a multilayer membrane in which an ultrathin semipermeable membrane layer is deposited on a preformed, finely microporous support structure. This contrasts with asymmetric reverse osmosis membranes in which both the barrier layer and the porous substructure are formed in a single-step phase inversion process and are integrally bonded. 

Fabrication of a thin film composite membrane is typically a more expensive route to reverse osmosis membranes because it involves a two-step process versus the one-step nature of the phase inversion film casting method. However, it offers the possibility of each individual layer being tailor-made for maximum performance. The semipermeable coating can be optimized for water flux and solute rejection characteristics. The microporous sublayer can be optimized for porosity, compression resistance and strength. Both layers can be optimized for chemical resistance. In nearly all thin film composite reverse osmosis membranes, the chemical composition of the surface barrier layer is radically different from the chemical composition of the microporous sublayer. This is a common result of the thin film composite approach. 

Figure 5.1 (a) Schematic diagram of the cross section of a composite reverse osmosis membrane (b) scanning electron microscope photograph of the cross section of 8 composite reverse osmosis membrane. 

There are several potential routes to the preparation of composite reverse osmosis membranes, whereby the ultrathin semipermeable film is formed or deposited on the microporous sublayer.1 2 The film can be formed elsewhere, then laminated to the microporous support, as was done in the earliest work on this membrane approach. Or it can be formed in place by plasma polymerization techniques. Alternatively, membrane polymer solution or polymer-forming reactants can be applied in a dipcoating process, then dried or cured in place. The most attractive approach from a commercial standpoint, however, has been the formation of the semipermeable membrane layer in situ by a classic "non-stirred" interfacial reaction method. Several examples of membranes made by this last approach have reached commercial status. 

The first composite reverse osmosis membrane to be developed and described consisted of an ultrathin film of secondary cellulose acetate deposited onto a porous Loeb-Sourirajan membrane.3 The ultrathin film of cellulose acetate was fabricated by a water surface float-casting technique. This has been described to some extent in the published technical literature,4 5 and in considerable detail in several reports on government-funded research projects.3 6 Figure 5.2 illustrates this process schematically. 

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