By Ryan Pastrana, John Krogue and Martin Hatfield, Clarcor Ind. Air
Cartridge filters have become a common design in many applications, ranging from water filtration, various air filtering needs and with industrial process fluids. Cartridge designs are very advantageous for increasing the surface area of the filter by making use of a more compact design versus a flat panel filter. The strength of a cartridge filter also lends itself to applications that see high pressures.
The technology advancements in this field have progressed significantly. Filtration media and the basic structural designs have continued to evolve, but in addition, the construction of media composites with designed geometry arrangements that improve performance and surface modification techniques have progressed to give users of cartridge filters lower cost operational improvements. Two examples of such technology are discussed below, and how they specifically improve the performance of cartridge filters.
In 1996 PECOFacet, a CLARCOR company, introduced a unique technology called PEACH┬«, (PECO ENGINEERED APPLIED CONICAL HELIX). This technology allows PECOFacet engineers to create depth filters with a gradient density with a very rigid structure of almost infinite variety. It starts with a PECOFacet’s Engineered Media that is specifically designed to provide a basis for creating one of the most innovative filters in the industry, as it is helically wound and bonded into a rigid filter tube.
Over the years, the innate flexibility of this construction has allowed PECOFacet to create many different filter products with greater filter capacity than other depth products, and sometimes greater than pleated products when removing deformable or shear sensitive contaminants. This also provides a complex matrix that changes the flow pattern of the fluid from a radial flow, for standard filter products to a helical and axial flow. This unique flow pattern provides longer contact time in the filter, which increases efficiency without having to have same level of tightness in the media. This reduces pressure drop and increases filter life.
Permanently Hydrophilic Membranes
Expanded polytetrafluoroethylene (ePTFE) membrane is commonly used in microfiltration applications. ePTFE membrane is selected over other micro porous membranes when the user desires features of ePTFE such as pore structure, chemical inertness, low extractables and non-shedding performance. However, one operational limitation arises when ePTFE is used in applications involving high surface tension fluids such as filtration of an aqueous (water based) suspension of particles. Due to the hydrophobic nature of ePTFE the high surface tension fluid will not pass through the low surface energy pore structure of the membrane. When first used, or after any period where the media becomes dry, ePTFE cartridge filters must go through a pre-wetting procedure with a low surface tension fluid such as isopropyl alcohol (IPA). The IPA fills the pores of the ePTFE membrane (displaces the air) thereby allowing the high surface tension fluid (such as water) to flow through the cartridge. Then the filtration system must be flushed to clear the IPA from the filtration system prior to use. This adds cost to the operation due to the IPA cost, the loss of valuable product during the “flushing step” to remove the IPA, and the time to perform the procedure. One novel approach to overcome this limitation is to treat the ePTFE such that it will be permanently hydrophilic and thereby able to flow water without the need for the IPA wetting procedure.
While surface modification methods exist to give temporary hydrophilic properties to PTFE, they typically dissipate after use. This loss of hydrophilic performance is accelerated during steam sterilization cycles associated with these types of filtration applications. A permanently hydrophilic ePTFE material allows the cartridges to be reused without the IPA wetting process, even when the material has dried out after a process shutdown and after multiple steam sterilization cycles. A method to make ePTFE membranes hydrophilic has been developed that makes use of an electron beam (E-beam) as the energy source to create a chemical covalent bond between the PTFE and hydrophilic chemistry applied to the individual fibers within the ePTFE structure. The material then does not require the IPA wetting process prior to use from a dry state or after steam sterilization.
To achieve the desired properties, the fibrous structure within the ePTFE membrane is first coated using a wet dipping process followed by drying. The coating consists of applying a proprietary reactive species designed such that one end will react with the PTFE while the other end has an alcohol group for hydrophilic performance. The “treated membrane” now has the hydrophilic chemistry in integral contact with the PTFE fibers inside the membrane. The next step is to apply sufficient energy (higher than the “chemical activation energy”) via electron beam irradiation in order to react the hydrophilic species to the PTFE polymer chains. This chemical reaction involves removing a fluorine atom from the PTFE backbone so the reactive species can permanently link (covalently bond) to the PTFE polymer chains. This “chemically grafting” is what makes the hydrophilic properties remain permanent through wet/dry and steam sterilization cycles.
This process leads to the transformation of a material that is naturally hydrophobic to one which is now permanently hydrophilic. When applied to a liquid filtration cartridge, this property eliminates the operational restriction previously described while maintaining the desired properties such as pore size, chemical inertness and “non-shedding” performance. This product is in the final stages of scale-up and scheduled for release in 2016 under the aspire┬« trademark, a CLARCOR brand.
As illustrated by these two case studies, cartridge filter designs can benefit from application specific enhancements that result in superior performance or improved operational efficiencies. Whether it is from mechanical design of the media to optimize flow within the filter, or via chemical grafting of polymers to an existing material to change surface properties, they demonstrate that innovation in cartridge filter design is possible to deliver improved filtration performance.
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