Basics of Cartridge Filtration
Published with permission from Pentiar, Inc.
Reasons for Filtration
Removal of Fluid Contaminants.
A properly designed cartridge filter system can eliminate many costly problems. The removal of contaminants from a fluid process stream makes that fluid more valuable and increases product yields. A dirty fluid stream can decrease productivity and lead to high rejection rates. A cartridge filter placed in a strategic location can alleviate such problems and also act as a monitor for the whole process. For example, a filter that plugs prematurely for no apparent reason suggests that there are improper conditions somewhere in the process. Cartridge filters can be used to protect critical orifices (i.e. an extruder) so that the openings do not become clogged and cause downtime. If the fluid in question is recirculating, reclaim value can also be increased by placing a cartridge filter in line. Removing a haze or classifying particles are other reasons for using cartridge filters. Properly dispersing a mixture, such as pigment/resin mixture, is an example of this.
Collection of Suspended Solids.
Many chemical processes require the use of catalysts in order to be functional. Cartridge filtration can recover the unused portions of the catalyst for reuse. If the catalyst is a precious metal, or if a precious metal is used in the actual reaction, cartridge filtration can recover unused portions and help reduce operating costs.
In pollution control, contaminant’s need to be recovered from waste effluents before the fluid is released into the environment. This can be accomplished by cartridge filtration.
Means of Retention
Mechanical retention occurs when a particle is mechanically restricted from passing through the filter medium. Direct interception, sieving, and bridging are mechanisms of capture that facilitate mechanical retention. Sieving is the most dependable under normal forward flow conditions. Particles captured by both bridging and direct interception are mechanically retained, but are more condition dependent than sieving. Pulsing or surging will dislodge a filter cake and/or small particles directly intercepted by media obstructions. However, if operating conditions are stable, particles held by mechanical retention should not be released.
Adsorptive retention refers to the adherence of a particle to the filter medium due to interactions between the particle and the surface of the medium. The particle “sticks” to the filter. Phenomena behind this adsorptive affect include electrical and hydrophobic interactions. Smaller particles adsorb more strongly than larger particles. The tendency of particles to adsorb, however, is very condition dependent. Adsorptive retention predominates for particles captured by inertial impaction, diffusion interception, and electro kinetic attraction.
The term “depth filtration” describes parameters of the particle size/pore size relationship present during the filtration process. Surface filters retain particles on the surface of the medium, where as depth filters retain particles throughout the medium. Although filters are often generalized as being depth filters, in reality, the label is inappropriate unless the particle size/pore size relationship is known.
A true surface filter can be thought of as a screen that is challenged with particles too large to pass through its openings. The particles will collect on the surface, forming a filter cake. Retention will be absolute since no particles will be able to penetrate through the surface. Note, however, that if the same screen was challenged with small enough particles, they will not be captured at the surface. Hence, the process of surface filtration is strictly dependent upon the particle size/pore size relationship.
Sieve Retention: Uniform Pore Size
Pleated filters are designed to enhance surface filtration when appropriately utilized. Micro-fiber sheet media has a narrow pore size distribution, favoring absolute sieving, in addition to a large surface area, increasing the capacity to retain particles at the surface. The medium is thin, permitting higher flows with lower pressure drops. These properties promote the formation of a filter cake, giving this type of filter a high dirt-holding capacity.