Ultrapure Water by Membranes

Director - Application and Research, Omexell Inc., Stajford, Texas 77477 

Membrane-based technologies have become the industry standard for the ultrapure water systems in the semiconductor, pharmaceutical, and power industries. The seminal discovery that changed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourtrajan process for making defect-free, high-flux, anisotropic reverse osmosis membranes (Loeb and Sourirajan, 1963). The most important development in the 1980s was the innovation of industrial membrane gas separation process. 

Advanced Membrane Technology and Applications. Edited by Norman N. Li, Anthony G. Fane, W. S. Winston Ho, and T. Matsuura Copyright 2008 John Wiley Sons, Inc. 

A typical ultrapure water system for the semiconductor industry is illustrated in Eigure 13.1. The reverse osmosis (RO)/electrodeionization (EDI) system is gaining more and more in importance in a typical UPW system due to its contamination-free design. 

Several methods for removal of dissolved oxygen from UPW arc currently available. The most conventional ones are the thermal and vacuum degassing systems, which have inherent drawbacks in terms of both operating costs and bulky construction. Also, with these physical methods, it is difficult to reduce the dissolved oxygen concentration from the parts-per-million (ppm) level down to a few parts-per-billion (ppb) levels (Sato et al., 1991 Kasama et al., 1990 Imaoka et al., 1991). Most of the recent installations use microporous hydrophobic membrane modules for the removal of dissolved oxygen from UPW. Such membrane modules also simultaneously achieve an ultralow level of 

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Although the principal application of reverse osmosis membranes is still desalination of brackish water or seawater to provide drinking water, a recent, significant market is production of ultrapure water by filtration of municipal drinking water. Such water is used in the electronics indnstry, where huge amoimts of extremely pure water with a total salt concentration significantly below 1 ppb are required to wash silicon wafers. 

The protein A layer was incubated for 1 h to be cross-linked by glutaraldehyde, which was followed by transfer to a compartment containing ultrapure water for rinsing. The protein A molecular membrane was then transferred onto the surface of an HOPG plate by the horizontal method. The molecular imaging of the preparation was obtained by AFM in solution. 

There are five basic water purification technologies—distillation, ion exchange, carbon adsorption, reverse osmosis, and membrane filtration. Most academic laboratories are equipped with in-house purified water, which typically is produced by a combination of the above purifying technologies. For most procedures carried out in a biochemistry teaching laboratory, water purified by deionization, reverse osmosis, or distillation usually is acceptable. For special procedures such as buffer standardization, liquid chromatography, and tissue culture, ultrapure water should be used. 

Membrane distillation - photocatalysis To solve the problem of membrane fouling observed in the pressure-driven membrane photoreactor, Mozia et al. [90] studied a new type of PMR in which photocatalysis was combined with a direct contact membrane distillation (DCMD). MD can be used for the preparation of ultrapure water or for the separation and concentration of organic matter, acids and salt solutions. In the M D the feed volatile components are separated by means of a porous hydrophobic membrane thanks to a vapor-pressure difference that acts as driving force and then they are condensed in cold distillate (distilled water), whereas the nonvolatile compounds were retained on the feed side. 

Separation of isopropanol (IPA) and water by pervaporation has also reached production scale. Much of the current capacity is devoted to azeotrope breaking and dehydration during IPA synthesis. Recently, anhydrous isopropanol has become a preferred drying solvent in the semiconductor industry, where chip wafers are first washed with ultrapure water, then rinsed with the alcohol to promote uniform drying. The water-laden isopropanol generated can be conveniently reused after dehydration by pervaporation. Unlike with pressure-driven membrane processes such as RO or UF, particulates and nonvolatile substances such as salts are not carried over during pervaporation. This helps maintain the effectiveness of contamination control. 

A number of commercial applications of MCs have been already successfully realized. A bubble-free membrane-based carbonation line, using Liqui-Cel equipment, is in operation by Pepsi in West Virginia since 1993. MCs are also used in beer production the CO2 removal stage is followed by nondispersive nitrogenation to obtain a dense foam head. Another important field of application of MC is the production of ultrapure water for semiconductor manufacturing. 

The resistance of the membrane, (Rm) is easily determined by the resistance to flow observed with ultrapure water and should be a constant for a given membrane operating with a specified fluid and temperature. 

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