Acetate, membrane technology

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). 

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

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 development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

There have been many studies on the application of membrane technology to food Industries. Few have, however, reached a commercial success except those of dairy processes (1) DAICEL has been studying since 1971 the application of its cellulose acetate RO membranes and polyacrylonitrile UF membranes to food, pharmaceutical, medical, paper and other industries. As to the use of membranes in food industries other than dairy processes, only two cases were developed to a semicommercial scale, that is, grape juice concentration for wine must and tomato juice concentration for processing and storage of the juice till next harvest. 

In 1969, I founded Osmonics, Inc. to carry the technology of reverse osmosis and ultrafiltration to the marketplace. We originally purchased membrane from Eastman Kodak Company and made our own spiral elements. We continued purchasing membrane until Kodak decided not to remain in the membrane business and we decided to begin the manufacture of membrane. By 1973, we were in full production manufacturing cellulose acetate membrane using the Loeb-Sourirajan approach. One year later, we were manufacturing polysulfone membrane for ultrafiltration. Last year, 1979, Osmonics manufactured over one million square feet of RO/UF membrane. 

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. 

GagHatdo P., Adham S., Tmssell R. (1997), Water purification using reverse osmosis thin film composite versus cellulose acetate membranes, Proc. AWW A Membrane Technolog Conference, New Orleans, Feb. 97, 597-608. 

New materials with improved CO2/CH4 separation selectivity and membrane stability under realistic NG conditions have been developed however, even after three decades of development, only three membrane material types have been commercialized cellulose acetate-based Separex (Honeywell s UOP), Cynara (Cameron) membranes, polyimide-based membranes from Medal (Air Liquide) and Ube, and per-fluoropolymer-based Z-top membranes from Membrane Technology and Research, Inc. (MTR). The key reasons for the selection of the desired polymer for commercialization are the cost of material, ease of fabrication into commercially viable form, effect of impurities on membrane performance, and gas selectivity under realistic feed conditions. 

Membrane technology has often been mentioned as the next technological generation for the prtrification of natural gases. Indeed, membrane systems are operated successfully for gas sweetening for decades. The best known examples include CO2 selective membranes that are based on pure polymers, e.g., cellulose acetate (Cynara membranes by Natco or Separex membranes by UOP) and polyimide (Ube). Despite their popularity, their performance at high pressures deteriorates as a result of CO2 induced plasticization. 

The developmental membranes reported here that have improved productivity and performance over conventional commercial cellulose acetate membranes indicate that there are avenues for improvement with new and future technologies. 

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