Cellulose acetate membrane asymmetric membrane

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). 

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... 

In order to exclude disturbing substances, Newman (1976), Tsuchida and Yoda (1981), and Palleschi et al. (1986) covered the platinum electrode by a H2O2 selective asymmetric cellulose acetate membrane. The membrane (thickness, 15.3 pm) was prepared from acetyl cellulose... 

Kawada et al. (1987) have developed low-pressure composite membranes by using a skin layer of polyvinyl alcohol upon a polysulfone UF membranes. At operating pressure of 1 MPa, their membranes had permeation flux of 1-2 m fm. day with salt rejection of more than 90%. Resistance of these memberanes to chlorine was very good. As compared with conventional asymmetric cellulose acetate membranes, these membranes showed almost an order of magnitude higher fluxes at the same rejection, indicating the superiority of composite membranes. 

Astroquartz, fiber reinforcement for ceramic- matrix composite, 5 558t Asymmetric allylboration, 13 669-671 Asymmetric cellulose acetate membranes, 21 633... 

PERFORMANCE OF HOMOGENEOUS AND ASYMMETRIC CELLULOSE ACETATE MEMBRANES... 

Deterioration of Asymmetric Cellulose Acetate Membranes with NaOCl -------Structural and Chemical Change... 

Analysis of the Mechanisum of Deterlorationof Asymmetric Cellulose Acetate Membrane by Sodium Hypochlorite... 

The development of the Loeb-Sourlrajan asymmetric cellulose acetate membrane (1) has been followed by numerous attempts to obtain a similar membrane configuration from virtually any available polymer. The presumably simplistic structure of this cellulose acetate membrane - a dense, ultrathln skin resting on a porous structure - has been investigated by transmission and scanning electron microscopy since the 1960s (2,3). The discovery of macrovoids ( ), a nodular intermediate layer, and a bottom skin have contributed to the question of the mechanism by which a polymer solution is coagulated to yield an asymmetric membrane. 

The successful development of asymmetric cellulose acetate membranes by Loeb and Sourirajan in the early sixties, at the University of California, Los Angeles, has been primarily responsible for the rapid development of Reverse Osmosis (RO) technology for brack sh/sea water desalination. Reverse Osmosis approaches a reversible process when the pressure barely exceeds the osmotic pressure and hence the energy costs are quite low. Theenergy requirement to purify one litre of water by RO is only O.OO3 KW as against 0,7 KV required just to supply the vaporisation energy to change the phase of one litre of water from liquid to vapour by evaporation. Thus RO has an inherent capability to convert brackish water to potable water at economic cost and thus contribute effectively to the health and prosperity of all humanity. 

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... 

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

A great deal has been written about the mechanism involved in the formation of asymmetric cellulose acetate membranes (29). 

In an early application, an enzyme electrode system was reported for the determination of creatinine and creatine, using a combination of creatinine amidohy-drolase, creatine amidinohydrolase and sarcosine oxidase, co-immobilized on an asymmetric cellulose acetate membrane. Thus, the hydrogen peroxide produced was detected to give a quantitative measure of creatine and creatinine in biological fluids [70]. 

Asymmetric cellulose acetate membranes were developed in the early 1960s by Loeb and Sourirajan (2). For more than a decade, cellulose acetate and its blends were the only commercially available RO membranes. Improved membranes (with respect to operating pH, biodegradation, compaction, and organic compound rejection) were developed in the early 1970s (3). These membranes used aromatic... 

Since Loeb and Sourirajan 8) found how to cast asymmetric cellulose acetate membranes, which consist of a very thin surface layer, supported by a more porous thick layer in 1962, many workers have investigated the preparation and performance of cellulose acetate membranes. 

Asymmetric cellulose acetate membrane developed Loeb and Sourirajan -1962... 

A similar application is the processing of fuel gas, whose major components are hydrogen (about 80%) and methane (about 20%). Asymmetric cellulose acetate membranes have been used successfully to extract the more valuable hydrogen at high purity. New membrane materials more resistant to harsh conditions will accelerate the application of other H2 recovery schemes for... 

A decade after Dr. Hassler s efforts, Sidney Loeb and Srinivasa Sourirajan at UCLA attempted an approach to osmosis and reverse osmosis that differed from that of Dr. Hassler. Their approach consisted of pressurizing a solution directly against a flat, plastic film.3 Their work led to the development of the first asymmetric cellulose acetate membrane in 1960 (see Chapter 4.2.1).2 This membrane made RO a commercial viability due to the significantly... 

E. 1960 - Loeb and Sourirajan develop asymmetric cellulose acetate membrane at UCLA... 

H. Bokhorst, F. W. Altena and C. A. Smolders, Formation of asymmetric cellulose acetate membranes, Proc. Inst. Cong. Desalination and Water Re-use, Manama (1981), pp. 349-360. 

Schulz and Asumnaa (48), based on their SEM observation, assumed that the selective layer of an asymmetric cellulose acetate membrane for reverse osmosis consists of closely packed spherical nodules with a diameter of 18.8 nm. Water flows through the void spaces between the nodules. Calculate the water flux by Eq. (30) assuming circular pores, the cross-sectional area of which is equal to the area of the triangular void surrounded by three circles with a diameter of 18.8 nm (as shown in Eig. 8). 

The wet cellulose acetate membranes prepared for reverse osmosis purposes can be used for gas separation when they are dried. The water in the cellulose acetate membrane cannot be evaporated in air, however, because the asymmetric structure of the membrane will collapse. Instead, the multistage solvent exchange and the evaporation method is applied. In this method, the water in the membrane is first replaced by a water-miscible solvent such as ethanol. Then, the first solvent is replaced by a second volatile solvent such as hexane. The second solvent is subsequently air-evaporated to obtain a dry membrane. 

Figure 4 is a micrograph of the skin structure of an asymmetric polyamide and Figure 5 is a micrograph of the skin structure of an asymmetric cellulose acetate membrane. 

Table III illustrated this phenomenon, wherein a single test specimen (made with the piperazine trimesamide homopolymer) was sequentially exposed to feed solutions of sodium chloride, magnesium chloride, sodium sulfate and magnesium sulfate. The chloride salts were both poorly retained while retention of the sulfate salts was excellent. Thus, salt retention in the carboxylate-rich NS-300 membrane was controlled by the anion size and charge. This membrane could not distinguish between the univalent sodium ion and the divalent magnesium ion, which is the opposite of the behavior observed for asymmetric cellulose acetate membranes. Salt passage through the NS-300 membrane may be described as anion-controlled.

The functional stability of GOD membranes has also been enhanced by coupling with an asymmetric ultrafiltration membrane (Koyama et al., 1980). The GOD-cellulose acetate membrane used was prepared as follows 250 mg cellulose triacetate was dissolved in 5 ml dichloro-methane, the solution was mixed with 0.2 ml 50% glutaraldehyde and 1 ml l,8-diamino-4-amino methyl octane and sprayed onto a glass plate. After three days the membrane was removed from the support and immersed in 1% glutaraldehyde solution for 1 h at 35°C, rinsed with water and exposed for 2-3 h to phosphate buffer, pH 7.7, containing 1 mg/ml GOD. The membrane was then treated with sodium tetraborate, rinsed with water and stored at 4-lO°C until use. It was combined with the ultrafiltration membrane in the following way 20 mg cellulose diacetate was dissolved in 35 g formamide and 45 g acetone and cast on a glass plate. At room temperature the solvents evaporated within a few seconds and a membrane of about 30 pm thickness remained, which was kept in ice water for 1 h before application in the sensor. 

In the lactate analyzer HER-100 (Omron Tateisi, Japan) an asymmetric cellulose acetate membrane is used, bearing LOD covalently bound by -y-aminopropyl triethoxysilane and crosslinked by glutaralde-hyde (Tsuchida et al., 1985). The membrane is highly selective for hydrogen peroxide. The analyzer has been employed for lactate assay in... 

Typical data for asymmetric fibers for reverse osmosis applications are reported in Table 20.5-1. The ranges of these variables for as-spun and post reared cellulose acetate and polysulfone membranes currently used in gas separation are proprietary. Nevertheless, the surfnee porosity for such membranes is undoubtedly lower than for those described in Table 20,5-1, since, as indicated in Table 20,1-2, in their posttreated forms such membranes have seleclivities approaching the values or dense films. Porosities as high as those shown in Table 20.5-1 weuld produce unacceptably low seleclivities as a result of nondiscrirafimat pore flow,... 

The cellulose acetate membranes are asymmetric and fabricated from a single polymer. The use of electron microscopy in the 1960s demonstrated that the cellulose acetate membranes consisted of a relatively thin dense layer and a thicker porous layer of the same material. The membrane thickness is usually about 100 micrometers with the dense layer accounting for about 0.2% of the thickness and the remainder being an open cell porous matrix (see Figure 4.5). 

During the period of 1965 to 1972, the best data on flux and salt rejection for cellulose acetate membranes were exhibited by the composite membranes. However, these membranes never reached commercial viability efforts on them died out completely by 1975. Reasons for this appear to be threefold. First, composite cellulose acetate membranes were technically difficult to scale up. Second, the advent of noncellulosic composite membranes in 1972 (the NS-100 membrane) offered much more promise for high performance (salt rejection and water flux), especially for seawater desalination. Third, continual improvements in asymmetric cellulose acetate membrane casting technology (such as the development of swelling agents and of blend membranes) brought the performance of asymmetric membranes to full equality with composite cellulose acetate membranes. 

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