Cross-flow membrane filtration
09/01/2001
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The cleanroom market crystal ball (see "The truth about fabric filters," CleanRooms, August 2001, p. 10) continues to make filtration market predictions. This time the forecast concerns cross-flow filters.
A recent study indicates that more than $5 billion will be spent this year for hardware and membranes used in reverse osmosis, ultrafiltration and microfiltration cross flow systems. Despite a downturn in the semiconductor segment, the industry will see growth of over $400 million from last year. The report also states that desalination, biotechnology and municipal drinking water are the major contributors to this growth."
While most of this current cross-flow filter activity is not directly related to cleanroom operations, cross-flow filters have been, and will continue to be, important in semiconductor ultrapure water plants and thus to semiconductor manufacturing. Consequently, a revisit to membrane filters in the cross-flow configuration and their role in cleanroom operations is in order.
First of all, what is a cross-flow filter and how does it differ from other filters? The figure schematically illustrates the cross-section of a simple cross-flow filter.
The permeate stream is made up of water that has penetrated through the pores in the walls of the cylinder-the membrane component of the filter. The concentrate stream is water that has been rejected by the membrane filter.
A major advantage of this configuration is that much of the rejected material in the water gets swept away in the concentrate stream rather than accumulated on the surface of the filter media as is true of the conventional straight-through filter configuration. This action minimizes scaling and filter clogging.
Species separated from the feed water include even nonparticle contaminants (contaminants not detected by a particle counter), such as dissolved solids and ions, so that the permeate stream is much purer that the feed water. According to manufacturers, membrane cross-filters used in reverse osmosis units can remove contaminating species of molecular weight as low as 100 daltons. To achieve such impressive separation efficiency, reverse osmosis membranes operate with very high-pressure drops
Cross-flow membranes come in a variety of shapes and compositions. One of the more easily visualized shapes is the hollow fiber structure. As the name implies, an element of a membrane filter having this construction consists of a hollow fiber which separates the feed water from the permeate, very like the cross-section depicted in the figure. A bundle of such hollow fibers makes up a membrane filter. However, the more commonly used membrane filter configuration, better suited for operation at high pressure, consists of membrane sheets wound in a spiral with ends selectively sealed to isolate the feed channel from the permeate stream. This configuration is typical of the reverse osmosis and microfiltration units used in ultra-high purity water (UPW) systems.
The two most common membrane compositions used in reverse osmosis units are cellulose acetate and the thin-film composite. The latter composition is now favored in UPW systems because of its somewhat higher rejection of contaminants. However, a number of manufacturers supply the industry and subtle, proprietary differences are touted as customized for specific applications so that membrane selection is not necessarily cut and dried.
Reverse osmosis units are the workhorses of semiconductor UPW systems and without membrane filters the UPW delivered to manufacturing fabs would be of lower quality and more costly. It is also worth noting that improvements in membrane technology and lowering of membrane costs are leading to more widespread installation of water desalination plants.
Membranes perform important separation functions in addition to filtration, such as degasification and distillation. In these roles, the membrane must be hydrophobic, rejecting liquid water from its surface but allowing gases or vapors to penetrate. An important UPW role of such hydrophobic membranes is water degasification. A degasification membrane allows dissolved gases in water to escape through the membrane wall and be removed. Similar action is the basis for achieving selectivity in some commercial total organic carbon monitors and for separating inorganic carbon (at pH less than about 4, virtually all inorganic carbon in water is present as carbon dioxide) from the organic carbon in water. Membranes used in water distillation reject liquid water but allow water vapor to pass through the membrane. Water temperature in membrane distillation units can be much lower than in conventional distillation and have been successfully powered by just solar ponds.
Like fibrous filters, membranes are simple structures that perform surprisingly useful functions despite their mundane appearance. The varieties of roles played by membranes are likely to continue to grow in number and importance.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L&M Technologies Inc. in Albuquerque, NM.