Thin-film composite RO membranes

CADOTTE AND PETERSEN Thin-Film Composite RO Membranes... 

In composite RO membranes, the selective top layer and the porous support layer are usually made of different polymeric materials. The selective top layer is formed on the porous support in a second step, typically by an interfacial polymerization reaction. For example, a commercially available thin film composite RO membrane is made by coating a porous polysulfone support with a polyamide thin film formed by the interfacial reaction of m-phenylenediamine and 1,3,5-benzenetricarbonyl trichloride. Details regarding membrane structures can be found elsewhere in the... 

Like RO membranes, many NF membranes are polyamide thin film composite membranes. These membranes can be prepared by interfacial reaction of piperazine with 1,3,5-benzenetricarbonyl trichloride and/or isophthaloyl dichloride, or by treating polyamide thin film composite RO membranes with compounds such as mineral acids, to increase their flux and lower their salt rejection. A few ceramic NF membranes have also been developed. New methods... 

Rao, A.P., Desai, N.V. and Rangarajan, R. 1997. Interfacially synthesized thin film composite RO membranes for seawater desalination, 124 263-272. 

Fig. 12. A spinal-wound leveise osmosis membrane element (a) schematic depiction (b) cross section of a spinal-wound thin-film composite RO Filmtec...

Commercial interest in RO began with the first high-flux, high-NaCl-retention Loeb-Sourirajan anisotropic cellulose acetate membrane. Practical application began with the thin film composite (TFC) membrane and implementation for seawater desalination at Jeddah, Saudi Arabia [Muhurji et ak. Desalination, 76, 75 (1975)]. 

Nanofiltration (NF) is a pressure-driven membrane separation technology used to separate ions from solution. Nanofiltration membranes were widely available beginning in the 1980 s. This technology uses microporous membranes with pore sizes ranging from about 0.001 to 0.01 microns. Nanofiltration is closely related to RO in that both technologies are used to separate ions from solution. Both NF and RO primarily use thin-film composite, polyamide membranes with a thin polyamide skin atop a polysulfone support (see Chapter 4.2.2). 

Interfacial polymerization has become a very important and useful technique for the synthesis of thin-film composite RO and NF membranes [5, 13]. Polymerization occurs at the interface between two immiscible solvents that contain the reactants (Fig. 3.6-8). For instance, a UF membrane is immersed in an aqueous diamine solution. The excess of water is removed, and the saturated support is put in contact with an organic phase that contains an acyl chloride. As a consequence, the two monomers react to form a thin layer (1 to 0.1 pm) of PA on top of the U F membrane. 

Kwak SY, Kim SH, and Kim SS, Hybrid organic/inorganic reverse osmosis (RO) membrane for bactericidal antifouling. 1. Preparation and characterization of TiOj nanoparticles self-assembled aromatic polyamide thin-film composite (TFC) membrane, Environmental Science and Technology 2001, 35, 2388-2394. 

TFC Thin-film composite RO and NF membranes. A typical TFC membrane consists of three layers a polyester web structural support (120—150 pm thick), a micro-porous inter layer ( 40 pm thick), and an ultra-thin polyamide (or other polymer) top layer (0.2 pm thick). See Figure 6.15. 

One of the more interesting fronts of development includes the search for improved membrane materials. While no new polymeric RO membranes have been introduced commercially over the last 20 to 30 years, there have been developments in performance (see Figure 1.5). These improvements in performance were achieved via modifications to the membrane itself (surface modifications made possible due to more advanced membrane characterization techniques) and closer tolerances in the interfacial polymerization reaction to make the membrane, and enhancements of the module design. Membranes with these improvements are commercially available today. While work is continuing with modifications to the current thin-film composite polyamide membranes, researchers are looking toward additional materials that might be suitable for use as RO membranes. 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-film composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has little effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich solid phase that forms the membrane and a polymer-poor liquid phase that forms the membrane pores or void spaces. 

An excellent review of composite RO and nanofiltration (NF) membranes is available (8). These thin-film, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-film composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-immiscible solvent. 

Two different RO membrane types were evaluated in this study. The first was a standard cellulose acetate based asymmetric membrane. The second type, a proprietary cross-linked polyamine thin-film composite membrane supported on polysulfone backing, was selected to represent potentially improved (especially for organic rejection) membranes. Manufacturer specifications for these membranes are provided in Table III. Important considerations in the selection of both membranes were commercial availability, high rejection (sodium chloride), and purported tolerance for levels of chlorine typically found in drinking water supplies. Other membrane types having excellent potential for organic recovery were not evaluated either because they were not commercially... 

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