Membrane manufacture

Membrane manufacturers require a standard test to maintain batch-to-batch quality. Few use proteins. Materials selected are ones for which the complications are minimized, the probe is simple, fast, and cheap to detecl , does not readily biodegrade, and gives results, whatever they are, which are reproducible. There is no standardization of these tests within the industry. 

Some OEMs produce many of the individual components required for their systems totally in-house, although there are also a large number of specialist equipment manufacturers for pressure vessel (PV) housings, high pressure pumps, and the like. World-class membrane manufacturers include Trisep, Koch, Fluid Systems, Hydranautics, Du Pont, Osmonics, and Dow. 

Traditionally, ultrafilters have been manufactured from cellulose acetate or cellulose nitrate. Several other materials, such as polyvinyl chloride and polycarbonate, are now also used in membrane manufacture. Such plastic-type membranes exhibit enhanced chemical and physical stability when compared with cellulose-based ultrafiltration membranes. An important prerequisite in manufacturing ultrafilters is that the material utilized exhibits low protein adsorptive properties. 

Membrane improvement (particularly in its ability to withstand dehydration) and improvement of membrane manufacturing techniques... 

A commonly used simple method for determining if there are any cracks or pinholes in microporous membranes is the so-caUed bubble point test. It has been used by many organic membrane manufacturers and users alike and is also being adopted by some inorganic membrane manufacturers. The method utilizes the Washburn equation... 

The unique properhes of zeolite materials combined with the conhnuous separahon properhes of membranes make zeolite membranes very attrachve for a wide range of separahon and catalysis applications. Zeolite membranes, however, have poor processability, poor mechanical stability and are much more expensive than the commercial polymer membranes with current state-of-the-art membrane manufacturing process. So far, the only large-scale commercial zeolite membrane is the A-type zeolite membrane and it has been used for dehydrahon of alcohols [22]. Further advancement in making thinner zeolite membranes and continuous improvement in membrane produchon techniques and reproducibility will make zeolite membranes more successful in commercial applicahons. 

The development of a successful zeolite/polymer mixed-matrix membrane with properties superior to the corresponding polymer membrane depends upon good performance match and good compatibility between zeolite and polymer materials, as well as small enough zeolite particle size for membrane manufacturing on a large scale. 

Elliott and co-workers performed a detailed SAXS investigation of the morphology of Nafion membranes that were subjected to uniaxial and biaxial deformation. For as-received membranes, manufactured by Du Pont using an extrusion process, the cluster reflection was shown to exhibit a limited degree of arching in the direction perpendicular to the machine direction. Upon uniaxial extension, this arching was observed to increase in a manner consistent with previous studies. This arching was rationalized on... 

The authors wish to acknowledge the support of this research provided by the Office of Water Research and Technology, U.S. Department of the Interior, Washington, D.C., under Grant No. 14-34-0001-7810. Partial support was also provided by the State of California Saline Water Research Funds administered by the Water Resources Center at the University of California, Davis, California. We also express our thanks to the five membrane manufacturers for their splendid cooperation in providing samples for this study. 

All work has been accomplished using perfluorosulfonate ion exchange membranes manufactured by E.l. duPont and sold under the trade name Nafion. Nafion is a copolymer of polytetrafluoroethy-lene (PTFE) and polysulfonylfluoride vinyl ether containing pendent sulfonic acid groups. The sulfonic acid groups are chemically bound to the perfluorocarbon backbone. 

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... 

Membranes manufactured by Spectrum Separations, Inc., a subsidiary of SEPAREX CORPORATION, are of the cellulose acetate type. They are similar to those made for reverse osmosis except they must be dried for gas separation use. A proprietary process is used to accomplish this so that the membrane does not collapse and lose its asymmetric character upon removal of the water. 

The bubble point test is simple, quick and reliable and is by far the most widely used method of characterizing microfiltration membranes. The membrane is first wetted with a suitable liquid, usually water for hydrophilic membranes and methanol for hydrophobic membranes. The membrane is then placed in a holder with a layer of liquid on the top surface. Air is fed to the bottom of the membrane, and the pressure is slowly increased until the first continuous string of air bubbles at the membrane surface is observed. This pressure is called the bubble point pressure and is a characteristic measure of the diameter of the largest pore in the membrane. Obtaining reliable and consistent results with the bubble point test requires care. It is essential, for example, that the membrane be completely wetted with the test liquid this may be difficult to determine. Because this test is so widely used by microfiltration membrane manufacturers, a great deal of work has been devoted to developing a reliable test procedure to address this and other issues. The use of this test is reviewed in Meltzer s book [3],... 

Recently, some membrane manufacturers have attempted to produce anisotropic microfiltration membranes in which the open microporous support is a built-in prefilter. Unlike most other applications of anisotropic membranes, these membranes are oriented with the coarse, relatively open pores facing the feed solution, and the most finely microporous layer is at the bottom of the membrane. The goal is to increase filter life by distributing the particle load more evenly across the filter than would be the case with an isotropic porous membrane. 

A wide variety of ion exchange membrane chemistries has been developed. Typically each electrodialysis system manufacturer produces its own membrane tailored for the specific applications and equipment used. An additional complication is that many of these developments are kept as trade secrets or are only described in the patent literature. Komgold [16] gives a description of ion exchange membrane manufacture. 

Current membrane manufacturing process, 50 meters/miri... 

Membranes are very finely porous structures and like all such porous structures used in an industrial context are susceptible to fouling caused by adhesion of components of the materials being processed. This fouling can be minimised or avoided if suitable polymers are used in membrane manufacture. However, the selection of membrane polymers suited to particular separations has until now been a matter of experience (and failure) rather than science. However, an AFM used with the colloid probe technique [23] can provide a rapid means of assessing the adhesion of solutes to membrane materials and is hence a powerful tool for the membrane technologist. 

The lower the SDI, the lower the potential for fouling a membrane with suspended solids. Membrane manufacturers require that the SDI15 be less than 5 to meet their warranty conditions. However, even though the SDI may be less than 5, the water may still be capable of fouling the membranes. In practice, an SDIl5 of less than 3 is usually necessary to reduce the potential for fouling to an acceptable level. 

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