Thin-Film Composite Membranes for Reverse Osmosis

High performance thin-film composite membranes for reverse osmosis applications were fabricated by coating solutions of a highly chlorine-tolerant disulfonated PAES [92,93]. As base monomers, 4,4 -dichlorodiphenyl sulfone and 4,4 -biphenol are used. 4,4 -dichlorodiphenyl sulfone is then directly sulfonated to get a disulfonated monomer, 3,3 -disutfonate-4,4 -dichlorodiphenyl sulfone. These monomers can be directly copolymerized on a commercially available porous polysulfone support. 

Thin-Film Composite Membranes for Reverse Osmosis... 

THIN-FILM COMPOSITE MEMBRANES FOR REVERSE OSMOSIS TABLE 1.1 Application of Reverse Osmosis Membrane Process... 

THIN-FILM COMPOSITE MEMBRANES FOR REVERSE OSMOSIS... 

A.K. Agarwal and R.Y.M. Huang, Studies on the Enhancement of Separation Characteristics of Sulfonated Poly (Phenylene oxide)/PolysuIfone Thin Film Composite Membranes for Reverse Osmosis Applications II. Effects of Nitromethane and the Chemical Treatment Combined with Gamma-Ray Irradiation, Angewandte Makromol. Chem., 1988,163, 15-21. 

This thin-film-composite membrane has been found to have appreciable resistance to degradation by chlorine in the feed-water. Figure 2 illustrates the effect of chlorine in tap water at different pH values. Chlorine (100 ppm) was added to the tap water in the form of sodium hypochlorite (two equivalents of hypochlorite ion per stated equivalent of chlorine). Membrane exposure to chlorine was by the so-called "static" method, in which membrane specimens were immersed in the aqueous media inside closed, dark glass jars for known periods. Specimens were then removed and tested in a reverse osmosis loop under seawater test conditions. At alkaline pH values, the FT-30 membrane showed effects of chlorine attack within four to five days. In acidic solutions (pH 1 and 5), chlorine attack was far slower. Only a one to two percent decline in salt rejection was noted, for example, after 20 days exposure to 100 ppm chlorine in water at pH 5. The FT-30 tests at pH 1 were necessarily terminated after the fourth day of exposure because the microporous polysul-fone substrate had itself become totally embrittled by chlorine attack. 

R.L. Riley, C.E. Milstead, A.L. Lloyd, M.W. Seroy and M. Takami, Spiral-wound Thin Film Composite Membrane Systems for Brackish and Seawater Desalination by Reverse Osmosis, Desalination 23, 331 (1977). 

Universal Oil Products (UOP) developed reverse osmosis equipment for demineralization of brackish and seawater using composite membranes with a polyamide as the functional coating. The UOP products carry a "TFC" registered trademark. Another good example of a thin-film composite membrane involving a thin film of polyamide as the functional coating is the FilmTec FT-30 membrane for RO (21). 

The list of polymer membrane materials is virtually endless insofar as possiljle chemical varieties are concerned (37 ). However, the number of fundamental physical structures into which they may be formed is much more limited. For present purposes, a distinction is made between skinned and skinless membranes. However, in view of the substantial and growing evidence cited above for the existence of pores in RO and UF membranes, even this is done with trepidation. Further subdivision results in three types of skinned membrane integrally-skinned ultragel3 integrally-skinned miorogels and nonintegrally-skinned miarogele (that Is, thin film composite membranes). Such skinned membranes are utilized in gas separations, reverse osmosis and ultrafiltration. 

We believe that for reverse osmosis new membranes with high chemical stability, high temperature resistance, and improved performance rates in respect to rejection characteristics and flux rates are coming on the market very soon in the form of improved thin-film composite membranes. 

The thin film composite membrane exhibited superior overall rejection performance in these tests, with ammonia and nitrate rejection showing an outstanding improvement. It has also been reported that silica rejection by the thin film composite membranes is superior to that of cellulose acetate. While the above data indicates a marginal improvement in the rejection of chemical oxygen demand (COD), which is an indication of organic content, other tests conducted by membrane manufacturers show that the polyurea and polyamide membrane barrier layers exhibit an organic rejection that is clearly superior to that of cellulose acetate. Reverse osmosis element manufacturers should be contacted for rejection data on specific organic compounds. ... 

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. 

In the market of reverse osmosis membrane today, thin film composite membranes have gotten a major position for their outstanding membrane performance and durability. In the usual thin film composite membrane, an ultra-thin salt barrier layer, formed by an in-situ inter cial polymerization reaction, covers the surface of a porous polysulfone substrate. Figure 1 illustrates a typical structure of the thin film composite membrane, TORAY "UTC-70". 

Lee CH, McCloskey BD, Cook J, Lane O, Xie W, Freeman BD, et al. Disulfonated poly(arylene ether sulfone) random copolymer thin film composite membrane fabricated using a benign solvent for reverse osmosis applications. J Membr Sci 2012 389 363-71. 

IP is one of the techniques used for preparing composite membranes, normally for the production of NF and reverse osmosis (RO) membrane. Historically, thin-film composite membrane prepared by IP was developed in order to overcome the limitations and the problems encountered by an asymmetric membrane constructed by the phase inversion method (Rao et al. 1997). IP was a breakthrough in the history of membrane technology and was developed by Cadotte at the North Star Research Institute for RO applications (Pinnau and Freeman 2000). It is based on the polymerization that occurs between two reactive monomers at the interface of two immiscible solvents an aqueous phase and an organic solvent such as hexane, as shown in Figure 5.1. 

Nataraj, S.K., Hosamani, K.M. and Aminabhavi, T.M. 2009. Nanofiltration and reverse osmosis thin film composite membrane module for the removal of dye and salts from the simulated mixtures, 249 12-17. 

Types of membranes for reverse osmosis. One of the more important membranes for reverse-osmosis desalination and many other reverse-osmosis processes is the cellulose acetate membrane. The asymmetric membrane is made as a composite film in which a thin dense layer about 0.1 to 10 pm thick of extremely fine pores supported upon a much thicker (50 to 125 pm) layer of microporous sponge with little resistance to permeation. The thin, dense layer has the ability to block the passage of quite small solute molecules. In desalination the membrane rejects the salt solute and allows the solvent water to pass through. Solutes which are most effectively excluded by the cellulose acetate membrane are the salts NaCl, NaBr, CaClj, and NajSO sucrose and tetralkyl ammonium salts. The main limitations of the cellulose acetate membrane are that it can only be used mainly in aqueous solutions and that it must be used below about 60°C. 

Many polymers have been screened for use as reverse osmosis membranes, but only a few have proved to be suitable. Examples of current interest include cellulose acetate, aromatic polyamides and certain thin-film composite membranes of polyamides. 

The performance of reverse osmosis in concentrating milk is limited by the osmotic pressure and most commercial modules have operating pressure limits of 30 to 40 bars, which limits the concentration of milk to a factor of three to four. In the production of skimmed or whole milk powder, the milk is usually concentrated to 45 to 50% total solids before spray drying. Thus reverse osmosis cannot substitute entirely for conventional evaporation, rather it is used as a pre-concentration step before evaporation, to reduce operating costs or to increase capacity of existing plant. The relative energy consumption of reverse osmosis and thermal concentration methods in the concentration of milk differ by an order of magnitude, and with thin film composite membranes the cost of reverse osmosis is lower still. 

K.-Y.C. Tsang, A Study of Reverse Osmosis Solute Separation and Production Rate for Sulfonated Poly(2,6-Dimethyl-l,4-Phenylene Oxide) Thin Film Composite Membrane in Different Tetraalkylammonium Forms , Fourth Year Thesis, Department of Chemical Engineering, University of Ottawa, 1995. 

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... 

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. 

Z.V.P. Murthy, S.K. Gupta, Sodium cyanide separation and parameter estimation for reverse osmosis thin-film composite polyamide membrane,... 

Kim, H.I. and Kim, S.S. 2006. Plasma treatment of polypropylene and polysulfone supports for thin film composite reverse osmosis membrane. 

Thin film composite polyamide membranes are used for reverse osmosis and nanofiltration... 

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