Filtration is a basic concept that’s integral to aseptic processing. Whether you’re talking about ultrapure water, high-purity products or process chemicals, the basic idea is to remove contaminants from the fluid stream.
Depending on the application, the product itself may be filtered, as is the case with milk processing and paint/ink manufacturing, or contaminants during the manufacturing process may be filtered out, as is the case with the manufacture of dielectric insulations, fuel additive processing and metalworking applications.
While the concept may be simple, the process of liquid filtration can be complicated. That’s because in liquid filtration, one encounters a wide variety of fluid chemistries. High fluid temperatures are common, as are high viscosity and flow rates. There is also a wide range of particle sizes. In addition, the filtration process may require separation of particles, gels and solutions. Finally, high-cost processes depend on filtration, adding a significant financial incentive to installing the right filtration system.
These variables are compounded by the fact that there are no uniform performance standards for filtration, except in cases such as hydraulic filtration and potable water. Other standards, such as 21 CFR 177 and USP Class VI, which are mentioned in food processing and biomedical filtration applications, speak more to the safety of the materials employed in the filter for the application than to the performance of the filter itself.
Know Your Variables
One of the first challenges in designing an effective liquid filtration system is to understand the size, shape and consistency of the particles that need to be separated from the fluid stream. In most liquid processes, these particles can range from more than 1,000 microns (µm), visible to the naked eye and can often be separated by settling, to 0.00005 µm, visible only to scanning electron microscopes and which must be separated by membrane filters. Remember that not all particles are perfectly spherical. Many have high aspect ratios and/or sharp, abrasive edges.
Contaminants might even take the form of gels, which can squeeze through filter pores (e.g., platelets in blood filtration). In fact, pore size and distribution are key variables in the primary mechanism of particle capture in liquid filtration: Sieving.
The next challenge is to quantify the concentration and volume of particles to be removed by filtration. If particulate loads are light, as in whole-house water filtration systems, relatively simple filtration systems may be employed. If particle loads are heavy, as in metalworking and enzyme processing applications, more capital-intensive filtration systems – including multi-stage filtration – may be required.
Depth filtration (a three-dimensional pre-filter) may be employed when particle loads are high, with a wide range of particle sizes, whereas surface filtration (a two-dimensional final filter that “polishes” the liquid stream) works best when particle loads are light, with a narrow range of particle sizes. Unfortunately, the lack of standards leads most filter manufacturers not to report the particle capacities of their filters.
The final variable to consider is the volume of liquid to be filtered. Liquid filters are naturally resistant to flow, and this resistance generally increases as filters become more efficient. That’s because a “dirt cake” can build up on the filter, reducing pore size so that more particles of a smaller diameter can be captured. High volumes of liquid, coupled with highly flow-resistant filters, can lead to an extremely large filter bank in order to handle the capacity of the process flow. Fortunately, most filter manufacturers do report flow resistance values for clean filters, and this information should be incorporated into the engineering designs for the filtration system.
The issue of compatibility between fluid chemistries and liquid filters requires more in-depth discussion. However, most filter manufacturers and/or distributors have guides indicating chemical and temperature resistance of their filters and filter media.