Liquid purification or separation – Electrical insulating or electricity discharging
Reexamination Certificate
1999-12-14
2001-08-14
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
Electrical insulating or electricity discharging
C210S266000, C210S290000, C210S490000, C210S492000, C210S505000
Reexamination Certificate
active
06274041
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to filters. More particularly, the present invention relates to filters which are intended to remove contaminants from a fluid stream.
BACKGROUND OF THE INVENTION
In general, filtration involves the removal of suspended solids and/or dissolved impurities from a fluid by passing the fluid through a porous medium. For many applications, the porous medium traditionally has been a bed of particulate matter, such as activated carbon and diatomaceous earth, or a solid porous filter element composed primarily of activated carbon. Solid porous filter elements are especially desirable for ease of handling and ready disposability. They may be made by a batch process, although it is both time consuming and labor intensive. More recently, they may be formed continuously by extruding a mixture of a thermoplastic binder material and a powdered or granular primary material such as activated carbon.
Apertured films, woven fabrics, and nonwoven materials also have been used as filter materials for removing or separating particles from liquids. Generally speaking, such filter materials rely on some form of mechanical straining or physical entrapment. Such filter sheets can pose limitations when the size of the particle to be removed is small relative to the average pore diameter of the filter sheet. For nonwoven materials, this is particularly true for particles of less than one micrometer in diameter.
Improved filters have been developed with modified surface charge characteristics to capture and adsorb particles by electrokinetic interaction between the filter surface and particles contained in an aqueous liquid. Such charge-modified filters typically consist of microporous membranes or involve the use of materials which are blends of glass fibers and cellulose fibers or blends of cellulose fibers and siliceous particles. Such filters may also utilize packed beds of particles. Charge modification generally is accomplished by coating the membrane or at least some of the fibers with a charge-modifying agent and then treating with a separate crosslinking agent in order to ensure the durability of the coating.
While microporous membranes generally are capable of effective filtration, flow rates through the membranes typically are lower than for fibrous filters. Moreover, microporous membranes generally have higher back pressures during the filtration process than do fibrous filters.
With the exception of reverse osmosis and, to a limited extent, hollow fiber membranes, no current filtration technology can effectively remove harmful bacteria and viruses from water. Usually, chemical and ultraviolet disinfection are the only reliable methods for eliminating biological contamination.
Typical activated carbon filter elements, such as those described earlier, are very efficient at removing most aesthetic contaminants from water, e.g., sediment, residual chlorine, and other contaminants which affect taste and odor. Such filter elements also are capable of removing certain health-threatening substances, such as lead and volatile organic compounds. Moreover, recent improvements in the manufacture of block activated carbon elements now permit the efficient removal of certain biological contaminants of larger sizes, such as Cryptosporidium and Giardia. However, in almost all cases the smaller bacteria can readily pass through these filter elements, and those which are trapped then are able to reproduce within the filter element. The continuous or frequent filtration of biologically contaminated water can cause the bacterial levels within the filter to reach several orders of magnitude higher than the levels in the water being filtered. Unless care is taken, this bacterial overgrowth can be flushed directly into the water to be consumed.
Accordingly, there is a need for a filter which has the ability to remove all or significantly reduce contaminants from water, including sediment, organic compounds, odors, chlorine, lead, and bacterial and viral pathogens.
SUMMARY OF THE INVENTION
The present invention addresses some of the difficulties and problems discussed above by providing an integrated filter for removing impurities from a fluid stream. The filter includes a first element adapted to remove at least some of the impurities by physical adsorption, and a second element adapted to remove at least some of the impurities by electrokinetic adsorption. Either or both of the first element and the second element further may be adapted to remove at least some of the impurities by sieving.
The first element generally may be composed of loose particles or granules of an adsorbent, or the first element may be composed of a porous adsorbent block, wherein the block is permeable to fluids and has interconnected pores therethrough. For example, the first element may be composed of a granular adsorbent component and a thermoplastic binder component. By way of illustration only, the adsorbent may be activated carbon, activated alumina, activated bauxite, fuller's earth, diatomaceous earth, silica gel, or calcium sulfate. However, other adsorbents known to those having ordinary skill in the art may be employed.
In some embodiments, the first element may be composed of a coextruded block composite which includes a porous first block and a second block. The porous first block is permeable to fluids, is composed of a granular adsorbent component and a thermoplastic binder component, and has interconnected pores therethrough having a first average diameter. In general, at least one of the first block and the second block is continuous, and at least a portion of the second block is contiguous with at least a portion of the first block.
The second element generally is composed of a porous, charge-modified fibrous web or packed beds which include fiber particles prepared from a thermoplastic polymer. For example, the thermoplastic polymer may be a polyolefin. As another example, the porous, charge-modified fibrous web may be a nonwoven web, such as a meltblown web. In addition, a felted mat of microfiber glass may be employed.
The present invention also provides an integrated filter as described above in which the first element is composed of a porous block of an adsorbent, wherein the block is permeable to fluids and has interconnected pores therethrough, and the second element is composed of a porous, charge-modified fibrous web or packed bed as defined above. Again, either or both of the first element and the second element further may be adapted to remove at least some of the impurities by sieving.
The present invention further provides an integrated filter for removing impurities from a fluid stream. In this case, the filter includes a first element adapted to remove at least some of the impurities by electrokinetic adsorption, a second element adjacent to and contiguous with the first element and adapted to remove at least some of the impurities by physical adsorption, and a third element adjacent to and contiguous with the second element and adapted to remove at least some of the impurities by electrokinetic adsorption. By way of example, each element may be in the form of a sheet.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention.
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McGrath Kevin P.
Williamson Bruce S.
Drodge Joseph W.
Kimberly--Clark Worldwide, Inc.
Nelson Mullins Riley & Scarborough L.L.P.
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