Liquid purification or separation – Processes – Separating
Reexamination Certificate
1998-06-18
2003-02-25
Kim, John (Department: 1723)
Liquid purification or separation
Processes
Separating
C210S483000, C210S488000, C210S489000, C210S490000, C055S521000, C095S078000, C096S067000, C096S069000
Reexamination Certificate
active
06524488
ABSTRACT:
The present invention relates to a filtration media and device comprising at least a layer having a structured surface that defines highly ordered fluid pathways.
BACKGROUND
An important segment of filtration media and filtering device development for removing particles from a fluid stream has been in the nonwoven fiber technology area. From the use of webs derived from meltblown microfibers to that of microdenier staple fibers, the trend has been to decrease fiber size in order to increase available surface area per unit volume of web. These nonwovens are generally polymeric based, entanglement bonded, low density webs that incorporate micron or near micron size fibers.
The principle mechanisms that control particle removal from a fluid stream by a fibrous filter are direct interception, inertial impaction, diffusion, and electrostatic attraction. Particle collection by interception occurs when a particle following a gas streamline strikes and is captured by the filtering surface. Inertial impaction results when particles deviate from the fluid stream to strike the fibers. Impacted particles in both cases adhere to the fibers by forces such as Van der Waals' forces. Diffusional collection occurs when the Brownian motion of very small particles enhances the probability of their contact with the filtering surface. This motion causes the particles to deviate from fluid stream lines and collect on the individual filter fibers. Electrostatic collection is an important mechanism whereby charged particles are attracted to oppositely charged collection surfaces by coulombic attraction.
Fibrous fluid filters, especially gas filters, typically combine all four capture mechanisms. Nonwoven filters incorporate the advantages of these fibrous filters due to their inherent properties. However, limitations with nonwovens as filtration media also stem from their inherent properties. Nonwoven webs by definition are randomly formed structures that have limited geometric order. Limited order is caused by the variability between individual fibers and the degree of fiber to fiber conformation within the web. This limited order is manifested by gross irregularities caused by the formation of macrostructures known as shingles and fiber nests. Web macrostructures have local concentrations of fibers that cause pore size variability as well as mass variability across the webs. As a result, relatively large openings between the fibers allow particles through that should have been excluded, and small openings fill and become ineffective. In filter media design these limitations are moderated by the use of additional material at the cost of higher flow resistance across the filter. These effects can be compounded during use in filtration applications by the force of the applied fluid, which can alter the web structure and thus the efficacy of the filtration device. In addition, pressure loading of the web, wherein the web is mechanically formed into product, for example, a pleated structure, can also cause additional deformation of the fibers and web, resulting in a decrease in filtration efficacy.
Other limitations of high surface area nonwoven webs as filtration media occur when the filter employs thin flat layers of nonwoven web, such as in respirators, or the filter employs pleated layers in a more three-dimensional arrangement, such as in room, furnace or computer filters. Because of their respective usages, the fluid velocity across the face of the respirator type filters tends to be lower, whereas the fluid velocity across the face of circulating air filters, i.e. the room, furnace or computer filters, tends to be higher. In both situations, however, the nonwoven web material typically performs as a surface loading filter, thereby eventually resulting in surface blinding. In surface binding, the first encountered layers of filter material fill and clog with particulate matter removed from the fluid stream. Therefore, the filters are not effectively using the greater portion of the filter mass, and thus the filters' performance is limited based on filter surface area rather than filter volume.
The use of multiple layers to increase filter efficiency, especially in respirator type filters, can cause an increase in flow resistance across the media as the fluid passes through the filter layers. Flow resistance is a function of the gas face velocity and the relationship of the size, orientation, and number of torturous channels through the filter. Generally, a filter media with more uniformly distributed surface area will achieve greater overall filtration efficiency permitting the use of less material and, in turn, reduce flow resistance across the media.
Flow resistance across a filter media is a general design constraint for any filtration device. Flow resistance is particularly problematic in lower face velocity applications because the fluid velocity is low even before filtering, and any resistance to flow within the filter will have a dramatic effect on its output. This flow resistance can cause problems with the overall fluid handling system in which the filter is used.
Pleated structures of smaller fiber nonwoven webs are often used in the higher face velocity applications to reduce flow resistance and improve service life. This is because there is more filtering surface in a given volume, thereby increasing the percentage of surface openings per frame area of filter. When the nonwoven web is composed of microfibers, however, pleated structures can sometimes reduce web loft, (see U.S. Pat. No. 5,656,368 to Braun et al.) and may be limited by the size of the microfibers used because smaller fibers are more likely to cause surface blinding. Larger fibers may cause the filter to suffer from reduced overall filtering capacity due to a decrease in the actual fiber surface area.
Another means of improving filter efficiency is through treatment of the filter fibers to make them more attractive to the particles or the like to be removed from a fluid stream. Treatment methods include both passive and active electrostatic charging of the fibers, application of tacky material to the fibers, application of chemical additives such as catalysts or other reactive agents, as well as application of other types of additives, including deodorizers, drying agents, disinfectants, fragrances, and ozone removing agents. Although treatment methods can enhance particle capture by the fibers, the filters are still subject to the deficiencies associated with random media, such as surface blinding and the flow resistance limitations discussed above. Examples of treated filter media include commercial filter products known as electrets, such as those available from 3M Company under the trade designation “Filtrete”.
Other types of filter media available for particle removal from a fluid stream include woven and knit materials. These types of materials tend to have a more ordered structure, thereby making them less susceptible to the limitations inherent in nonwovens. These materials, however, have their own problems with controlling structures fidelity due to variability in constituent fiber material, fiber formation and web construction. In addition, other problems include limitations such as small enough pore formation, constituent material costs, and manufacturing costs.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages and shortcomings of the prior art by providing a filtration media or filtration device that is efficient, is capable of depth-bed loading, functions at a low flow resistance, and has a high collection capacity. More specifically, the present invention provides a filtration media comprising at least a layer having a structured surface that defines highly ordered fluid pathways. Preferably, the filtration media of the present invention comprises a stack of layers having structured surfaces defining a highly ordered array of filter openings and fluid pathways through the filtration media.
The structured surfaces of the layers may comprise features defining channels that fo
Insley Thomas I.
Johnston Raymond P.
3M Innovative Properties Company
Bond William J.
Griswold Gary L.
Kim John
Sprague Robert W.
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