Reverse osmosis cellulosic membranes

Figure 2.15 Measurements of Rosenbaum and Cotton [20] of the water concentration gradients in a laminated reverse osmosis cellulose acetate membrane under applied pressures of 68 and 136 atm. Reprinted from Steady-state Distribution of Water in Cellulose Acetate Membrane, S. Rosenbaum and O. Cotton, J. Polym. Sci. 7, 101 Copyright 1969. This material is used by permission of John Wiley Sons, Inc.

Suppose we place a semipermeable membrane between a saline (salt) solution and pure water. If the saline solution is pressurized under a greater pressure than its osmotic pressure, the direction of flow can be reversed. That is, the net flow of water molecules will be from the saline solution through the membrane into the pure water. This process is called reverse osmosis. The membrane usually consists of cellulose acetate or hollow fibers of a material structurally similar to nylon. This method has been used for the purification of brackish (mildly saline) water. It has the economic advantages of low cost, ease of apparatus construction, and simplicity of operation. Because this method of water purification requires no heat, it has a great advantage over distillation. 

Cellulose acetate is the material for the first-generation reverse osmosis (RO) membranes. The announcement of cellulose acetate membranes for seawater desalination by Loeb and Sourirajan in 1960 triggered the applications of membrane separation processes in many industrial sectors. Cellulose acetate membranes are prepared by the dry-wet phase inversion technique. 

Enzymes or whole cells can be immobilized in ultrafiltration (UF) and reverse osmosis (RO) membranes by several methods. First, cellulose acetate or polysulfone are used to obtain asymmetric membranes by the phase Inversion technique. Albumin and glutaraldehyde are then used for cell Immobilization within the membranes via co-cross-llnklng methods (25,26). 

Preparation Procedures of Asymmetric Membranes. The development of the first asymmetric phase inversion membranes was a major breakthrough in the development of ultrafiltration and reverse osmosis. These membranes were made from cellulose acetate and yielded fluxes 10 to 100 times higher than symmetric structures with comparable separation characteristics. Asymmetric phase inversion membranes can be prepared from cellulose acetate and many other polymers by the following general preparation procedure 27... 

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. 

Cellulose acetate (CA), the acetate ester of cellulose, is one of the most commonly used biocompatible materials for the preparation of semi-permeable membranes to be used for dialysis, ultrafiltration, and reverse osmosis. CA membranes have very low absorption characteristics and thermal stability with high flow rates. Cellulose-based materials are also widely used in the bio-pharmaceutical industry as the matrix for adsorbent beads and membranes. Moreover, CA nanofibers can be used as carrier for delivery of vitamins or pharmaceutical products [15]. 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

Reverse Osmosis. This was the first membrane-based separation process to be commercialized on a significant scale. The breakthrough discovery that made reverse osmosis (qv) possible was the development of the Loeb-Sourirajan asymmetric cellulose acetate membrane. This membrane made desalination by reverse osmosis practical within a few years commercial plants were installed. The total worldwide market for reverse osmosis membrane modules is about 200 million /yr, spHt approximately between 25% hoUow-ftber and 75% spiral-wound modules. The general trend of the industry is toward spiral-wound modules for this appHcation, and the market share of the hoUow-ftber products is gradually falling (72). 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

Polymer Plasticizer. Nylon, cellulose, and cellulose esters can be plasticized using sulfolane to improve flexibiUty and to increase elongation of the polymer (130,131). More importantly, sulfolane is a preferred plasticizer for the synthesis of cellulose hoUow fibers, which are used as permeabiUty membranes in reverse osmosis (qv) cells (131—133) (see Hollow-FIBERMEMBRANEs). In the preparation of the hoUow fibers, a molten mixture of sulfolane and cellulose triacetate is extmded through a die to form the hoUow fiber. The sulfolane is subsequently extracted from the fiber with water to give a permeable, plasticizer-free, hoUow fiber. 

Cellulose acetate films, specially cast to have a dense surface and a porous substmcture, are used in reverse osmosis to purify brackish water (138—141) in hollow fibers for purification of blood (artificial kidney) (142), and for purifying fmit juices (143,144) (see Membrane technology). 

Low viscosity cellulose propionate butyrate esters containing 3—5% butyryl, 40—50% propionyl, and 2—3% hydroxyl groups have excellent compatibihty with oil-modified alkyd resins (qv) and are used in wood furniture coatings (155). Acetate butyrate esters have been used in such varied apphcations as hot-melt adhesive formulations (156), electrostatically spray-coated powders for fusible, non-cratering coatings on metal surfaces (157—159), contact lenses (qv) with improved oxygen permeabiUty and excellent wear characteristics (160—162), and as reverse-osmosis membranes for desalination of water (163). 

Cellulose acetate, the earhest reverse osmosis membrane, is still widely used. Asymmetric polyamide and thin-film composites of polyamide and several other polymers have also made gains in recent years whereas polysulfone is the most practical membrane material in ultrafiltration appHcations. 

In contrast to reverse osmosis, where cellulose acetate has occupied a dominant position, a variety of synthetic polymers has been employed for ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate. Polycarbonate resins, substituted olefins, and polyelectrolyte complexes have been employed among other polymers to form ultrafiltration membranes. 

R/0 unit Reverse Osmosis Unit for water purification in small aquariums and miniature yard-ponds, utilizes a membrane under pressure to filter dissolved solids and pollutants from the water. Two different filter membranes can be used the CTA (cellulose triacetate) membrane is less expensive, but only works with chlorinated water and removes 50-70% of nitrates, and the TFC membrane, which is more expensive, removes 95% of nitrates, but is ruined by chlorine. R/0 wastes water and a system that cleans 100 gallons a day will cost ft-om 400 to 600 with membrane replacement adding to the cost. A unit that handles 140 gallons a day will cost above 700,00. 

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

Generally, a distinction can be made between membrane bioreactors based on cells performing a desired conversion and processes based on enzymes. In ceU-based processes, bacteria, plant and mammalian cells are used for the production of (fine) chemicals, pharmaceuticals and food additives or for the treatment of waste streams. Enzyme-based membrane bioreactors are typically used for the degradation of natural polymeric materials Hke starch, cellulose or proteins or for the resolution of optically active components in the pharmaceutical, agrochemical, food and chemical industry [50, 51]. In general, only ultrafiltration (UF) or microfiltration (MF)-based processes have been reported and little is known on the application of reverse osmosis (RO) or nanofiltration (NF) in membrane bioreactors. Additionally, membrane contactor systems have been developed, based on micro-porous polyolefin or teflon membranes [52-55]. 

For purposes of illustration, the following discussion, unless otherwise specified, is limited to single-solute aqueous feed solutions, cellulose acetate membranes, and reverse osmosis systems for which osmotic pressure effects are essentially negligible. 

Figure 2a. Experimental data on the effect of operating pressure, average pore size on membrane surface, and feed concentration on solute separation and product rate for the reverse osmosis system cellulose acetate membrane-sodium chloride-...

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