Gas separation – Specific media material
Reexamination Certificate
2000-06-01
2001-11-13
Simmons, David A. (Department: 1724)
Gas separation
Specific media material
C055S527000, C055SDIG005, C264S006000, C264S012000, C264S112000, C264S518000
Reexamination Certificate
active
06315806
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to methods and apparatuses for producing fibrous media, and fibrous web material formed thereby, and in preferred embodiments, to methods and apparatuses for producing composite fibrous media composed of discontinuous fine fibers and controlled dispersions of discontinuous ultra-fine or sub-micron electrostatically charged or uncharged fibers. Further preferred embodiments relate to filtration media and absorbent media comprising such composite fibrous media.
BACKGROUND OF THE INVENTION
The increased recognition of the deleterious health effects of chemical and biological pollutants, which often are in the form of particles of less than one micron in diameter, has created an urgent requirement for filtration media capable of removing such particles in an efficient and economical manner. There is a wide range of filter media available for removal of particles from air used for breathing or other processes requiring specific air purity standards. However, most of these media lose efficiency rapidly or increase in price and investment and operating costs, as particle filtrate size decreases.
A major portion of such increased cost is due to costs associated with the increased resistance of the filter to air flow as the efficiency of particle removal is increased. This creates a higher pressure drop for movement of air through the filter thereby imposing a greater work requirement upon the filter fan. This, in turn, can require the use of larger and more costly fans which consume greater amounts of energy which can make the entire process economically prohibitive. Also, filters capable of removing sub-micron diameter particles generally tend to rapidly plug up, necessitating frequent and costly filter replacement.
The most common filter media for air filtration consists of fibrous webs of fine fibers laid flat so that most are perpendicular to the direction of flow of the air being filtered. In the absence of electrostatic charging effects, equations describing the laws of fluid dynamics for particulate solids in an air stream have been found to provide good predictions of filter performance. Such physical laws and associated equations have been described by L. B. Torobin in “Momentum Transfer in Solids-Gas Flow”, Ph.D. Thesis, McGill University, 1960 (“Torobin Thesis”), incorporated herein by reference.
The effect of fiber size on air or gas filter collection efficiency has been calculated by W. C. Hinds, in “Aerosol Technology”, Wiley, 1982, p 184, using the appropriate solids-gas fluid dynamics equations with parameters set for conditions typical in industrial, commercial, and domestic air filters. The calculations reveal that, if sub-micron diameter fibers could be incorporated into filtration media in a uniform manner, they would be very effective in removal of sub-micron diameter particles from air streams. For a typical pollutant particle of 0.3 micron diameters entrained in air flowing through a filter made up of 10 micron diameter fibers, approximately 30 percent of the pollution particles were not removed by the filter. When the fiber diameter was reduced to 2 micron diameters, which corresponds to the lower limit of most industrial filters available today, the amount of unfiltered particulate matter decreased to approximately 17 per cent. Hinds projects that, if the media could employ fibers with a diameter 0.5 micron diameter, the filter would trap all but 3 percent of the incoming pollutant particles, while requiring only a relatively low pressure drop to sustain the flow of the gas being purified through the filter.
A number of significant problems, however, have inhibited the use appreciable quantities of sub-micron fibers in filter media. For example, one problem has been the difficulty of achieving uniform deposition of sub-micron diameter polymeric fibers onto a collecting surface. This is due, in part, to their tendency to to fly about in the fiber formation process. Because of their small size and low density, the sub-micron diameter polymeric fibers are extremely light and are therefore readily entrained in the stray large turbulence scale air currents, which are induced by essentially all fiber formation and collection processes.
This is in keeping with the theoretical predictions which show that currents with only one tenth to one hundredth the velocities needed to entrain the fibers above one micron in size are enough to carry off the sub-micron fibers. (Torobin Thesis at 228.) The dimensions of these currents or eddies and their velocities increase as the production process size is increased from the laboratory scale to commercial scale, and as the production processes are adjusted to make finer fibers. Accordingly, by conventional filter production methods, many of the sub-micron diameter fibers are not incorporated into the fiber web. Further, the stray fibers can damage or impair the filter production equipment.
Another problem inhibiting the use of appreciable quantities of sub-micron diameter polymeric fibers in filter media results from the greater tendency of polymeric sub-micron diameter fibers to attach to each other in flight during formation, forming undesired loose clumps, “ropes” or other agglomerates. This is a consequence, in part, of the unusually high ratio of fiber surface area to fiber volume characteristic of sub-micron diameter fibers. When this is combined with the relatively low density of polymers, the fibers tend to adhere to each other on contact.
The agglomeration problem is exacerbated by the presence of twisting and recirculating vortices in the ambient air which tend to twist the agglomerates into ropelike structures. The agglomerates eventually settle onto the formed fiber web where they constitute a source of significant non-uniformity. Another problem inhibiting the use of appreciable quantities of sub-micron diameter polymeric fibers in filter media results from the tendency of sub-micron diameter fibers to exhibit poor overall web mechanical properties and lack of self-support. This follows from the low tensile strength and relatively short length of the sub-micron diameter fibers. Sub-micron diameter fibers tend to only poorly attach to each other and are thereby susceptible to being lifted off and blown away by air currents associated with fiber production. Also, when such fiber webs are used as air filters, significant quantities of the sub-micron fibers are carried away intermittently by the air being purified. This results in unacceptable contamination of the air stream flowing through the filter.
Yet another problem retarding the use of sub-micron diameter polymeric fiber filtration media is the difficulty of uniformly distributing the ultra-fine fibers in the fiber web. If the sub-micron fibers are not uniformly distributed, much of the air being treated by the filter will not be subjected to the high efficiencies imparted by the sub-micron diameter fibers. Moreover, because the higher surface area associated with each region of high concentrations of the sub-micron diameter fibers imparts to it a relatively high resistance to air flow compared to regions of low concentrations, the ultra-fine fibers in these regions tend to be bypassed by the air being filtered, thereby reducing their effectiveness.
In addition to non-uniform distribution over the length and width of the fiber web, there is also a tendency for the sub-micron diameter fibers to collect at a certain depth of the fiber web. This results in premature plugging of the filter, since most of the solids separated by the filter collect in a thin plane.
It was found that the aforementioned difficulties encountered with sub-micron diameter polymeric fibers could be avoided to some degree when the fibers were made from higher tensile strength and higher density materials such as glass. Consequently, many of the air filters used today include sub-micron glass fibers. Sub-micron diameter glass fibers have been produced and incorporated in filtration media, as described, for example, in U.S. Pat. No. 4,548
Findlow Richard C.
Torobin Leonard
Foley & Lardner
Pham Minh-Chau T.
Simmons David A.
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