Selecting the Most Economic Cartridge Filter

Filtration is rarely the first concern when developing a new or improved process. Far more attention is given to product yield, minimizing unwanted byproducts, avoiding environmental problems and achieving favorable process economics. Although filtration rarely is considered early, it often significantly affects process economics -- both through the direct cost of filtration and through its impact on the acceptability of filtered fluid (filtrate) for the next process stage or final application. This article discusses key parameters for cartridge filter systems, to show how design choices affect costs and to identify various cost elements impacted by cartridge filter choice.

Filters are used to protect the process and the product. For instance, at refineries and natural gas processing facilities that use alkanolamines in gas-sweetening systems, removing contaminants from the recirculating amine stream can be important in minimizing operating problems, such as from foaming within gas/liquid contactors. Particles are known to stabilize foaming [1] that can reduce gas contactor throughput. Another example is the benefit that carbon columns receive from particulate filtration. So-called "mechanical" filters upstream of the columns protect the activated carbon granules from being plugged with debris that would impede fluid access to the activated carbon surface. Downstream, another set of particulate filters protects the fluid system from the fines generated by mechanical abrasion of granules within the carbon bed. Filters keep debris from continuing downstream to successive stages. When such carryover occurs, critical equipment, such as heat exchangers, may become fouled or the final product may be forced out of specification.

Filtration fundamentals
Several basic factors play key roles in selecting the right cartridge filtration system, including:

The nature of the contaminant. The contaminant is extremely important in determining how readily filtration can be accomplished. Rigid, angular particles are easy to remove from a fluid. Flexible contaminants, including fibers, aren't always easily removed because they sometimes can "snake" their way through the filter structure. The most difficult to remove are deformable gels. These have the ability to blind a filter by smearing over its surface. At high pressure drop, gels also can be extruded through the filter structure. They then emerge as a coalesced mass, possibly larger than pores within the filter, on its downstream side. Gels are best handled by reducing flux through the filter and operating at the lowest possible pressure drop across it. The use of filtration media with a graded porosity often is very effective in removing gels. Such media have progressively smaller pores going from upstream to downstream within the filter.

Types of filtration media. The work of separating contaminants from a fluid is done by filtration media. They can be grouped into three basic categories: surface, depth and adsorptive. Surface filters act like sieves. Particles too large to pass through the holes (pores) are retained on the surface. As contaminant cake builds on the surface, the degree of filtration often grows finer. A surface filter medium is generally thin. It commonly is pleated to maximize surface area packed into the filter cartridge, thereby increasing the amount of dirt that can be retained. In contrast, depth filters rely on a much thicker medium. Dirt particles must follow a tortuous path to pass through the filter cartridge. This complex path increases opportunities for particle capture within the filter matrix. The depth of such media leads to higher pressure drop across them than across a surface medium of comparable pore size.

 Adsorptive filter media are capable of retaining particles smaller than the rated filter pore size. This is possible in some systems because of surface charge modification of the filter media. The charge is most effective over short distances from the surface and falls off exponentially as distance increases. Accordingly, such filter structures are principally employed in sub-micron filtration.

The structure of the filter medium plays an important role in capturing and retaining contaminants. Filter media can be broadly categorized as non-rigid or rigid. Contaminants trapped within non-rigid structures are more likely to be released back into the fluid stream because such structures can deform. This effect is especially pronounced in the case of pulsating pressure associated with diaphragm pumps. Non-rigid filters, such as yarns wound around a core, often find use because of their wide availability and generally low purchase price. Rigid filter structures are better able to resist deformations and, thus, lessen the likelihood of releasing contaminants back into the fluid. Fluid bypass is also less likely with rigid filter cartridges because they don't collapse under increasing differential pressure across them, which could pose the danger of compromising the seal at the ends of the cartridge.

Flow rate. The rate at which fluid passes through a filter strongly influences its performance. A common experience is that doubling the number of filters used on a process stream results in more than twice the amount of fluid that can be processed before a filter change is needed. This nonlinear relationship has been expressed as [2]:

L₂ = L₁ [Q₁/Q₂]ⁿ                                                                          (1)