Industrial membrane elements

Some typical industrial membrane elements are illustrated in Figure 4.25, which shows pleated sheet membrane and lenticular elements. 

Many methods have been proposed to address this issue (see Chapter 9). Beside thermal and chemical resistances of the sealing materials other issues need to be considered as well. One such important issue is the mismatch of the thermal expansion coefficients between the membrane element and the sealing material or joining material. While similar material design and engineering problems exist in ceramic, metal and ceramic-metal joining developmental work in this area is much needed to scale up gas separation units ot membrane reactors for production. The efforts are primarily p ormed by the industry and some national laboratories. 

FIGURE 6.3 Ceramic membrane elements with a flower-like geometry, from TAMI industries. 

Abolmaali, B. Yassine, I. Capone, P. Water recovery from an aluminum can manufacturing process using spiral wound membrane elements. In Membrane Technologies for Industrial and Municipal Wastewater Treatment and Reuse Water Environment Federation Alexandria, VA, 2000 51-56. 

R.O. systems utilizing externally wound tubular membrane element in modular assemblies have been used in the desalination of brackish and sea waters, the treatment and/or concentration of industrial waste waters, the separation/concentration of fluid food, pharmaceuticals and chemical solutions, and the manufacture of water purifiers for domestic use. Generally, externally wound tubular membrane systems have been found to be highly suitable for ultrafiltration applications in the processing Industry and in water pollution control applications. 

The initial projections of 20 years ago have proven to be unrealistic in that reverse osmosis has not caused deserts to bloom, nor does every household contain a reverse osmosis unit to improve the tap water. Yet, the process has been of economic value in providing process water to industry, potable water to high income arid regions and a method of reclaiming municipal and industrial wastes. As of 1985, it was estimated that the worldwide market for reverse osmosis membrane elements (not total systems) was about 50 million. 

Although there are a number of materials with the desired pore structure, for instance silicone rubbers, hydrocarbon rubbers, polyesters, polycarbonates and others, their use for industrial applications is limited to polysulfones and cellulose acetates. While the latt have been used with good success for dehydration, technical gas separation relies exclusively on polysulfones which can be used up to approximately 70 °C (their melting point is around 200 °C) and at pressures between IS and 140 bar. The lowest pressure differential between the feed gas side and the permeate gas side is 3 1 and this differential pressure determines the wall thickness of the membranes. Figure 2.8 shows the design of a membrane element developed by Monsanto Company, USA and marketed by the name of Prism separator. Each of these elements or modules contains thousands of hollow fibres packed to a density of approximately 1(X) per cm. ... 

Single-stage SWRO systems are widely used for production of drinking water. However, these systems have found limited industrial application mainly because of the water quality limitations of the produced permeate. Even if using the highest rejection RO membrane elements commercially available today (nominal minimum rejection of 99.75%), the single-stage SWRO desalination systems typically cannot consistently yield permeate with TDS concentration lower than 200 mg/L, chloride level of less than 100 mg/L, and boron concentration lower than 0.5 mg/L. 

By 1960, the elements of modem membrane science had been developed, but membranes were used in only a few laboratory and smaU, specialized industrial appHcations. No significant membrane industry existed, and total annual sales of membranes for aU appHcations probably did not exceed 10 million in 1990 doUars. Membranes suffered from four problems that prohibited their widespread use as a separation process they were too unreHable, too slow, too unselective, and too expensive. Partial solutions to each of these problems have been developed since the 1960s, and in the 1990s membrane-based separation processes are commonplace. 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

A bipolar pilot membrane cell with six cell elements each of 1.8 m2 has been installed and run with industrial electrolyte. 

The use of a multichannel support made of a sintered oxide carrying a separation layer deposited on the surface of the channels was not a new concept. This was described in the patent literature as far back as the 1960s (Manjikian 1966). The multichannel geometry is particularly attractive in terms of its sturdiness, lower production cost compared to the single tube or tube-bundle geometry and lower energy requirement in the cross-flow recirculation loop. However, Ceraver was the first company to industrially produce multichannel membranes. Since 1984 these membranes, which have 19 channels per element with a 4 mm channel diameter are sold under the trademark Membralox. ... 

The structural elements of commercial inorganic membranes exist in three major geometries disk, tube or tube bundle, and multichannel or honeycomb monolith. The disks are primarily used in laboratories where small-scale separation or purification needs arise and the membrane filtration is often performed in the flow-through mode. The majority of industrial applications require large filtration areas (20 to over 200m ) and, therefore, the tube/tube bundle and the multichannel monolithic forms, particularly the latter, predominate. They are almost exclusively operated in the cross-flow mode. 

The realization of the MEA is a crucial point for constructing a good fuel cell stack. The method currently used consists in hot-pressing (at 130 °C and 35 kg cm ) the electrode structures on the polymer membrane (Nafion). This gives non-reproducible results (in terms of interface resistance) and this is difficult to industrialize. New concepts must be elaborated, such as the continuous assembly of the three elements in a rolling tape process (as in the magnetic tape industry) or successive deposition of the component layers (microelectronic process) and so on. 

Ogure K., Kawami Y., Takanama H., An actuator element of polymer electrolyte gel-membrane-electrode composite Bull. Government Industrial Research Institute Osaka, 43 (1992) 21. 

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