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Filtering Out Confusion
Select the right liquid filter media for the right process.
by Ronald Cox
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.
From largest to smallest particles, liquid filtration is divided into the following classes: Macro-filtration, micro-filtration, ultra-filtration, nano-filtration and reverse osmosis (RO).
Macro-filtration typically involves particles of 10 µm or more in size, as in metalworking, enzyme processing and sediment filtration applications. Due to the large particle sizes, a significant mass of particles may be removed from a fluid stream over time. For this reason, a variety of filtration techniques may be employed.
Very large particles may settle out of the stream in a settling tank. Particles between 100 and 500 µm may be removed by a washable screen or belt filter. Particles under 100 µm will typically require some type of media-based filtration. Various fluid chemistries make it crucial to check with filter manufacturers on the chemical resistance of their filters in your application to make sure that contaminants, binder chemistries or shed fibers are not leached out into the process stream.
Macro-filtration filters can take the form of filter disks, bags or cartridges. Bag filters are usually depth filters and fall into two broad classes: Nominal efficiency felt bags made of polypropylene or polyester with fiber diameters in the range of 20 to 40 µm, and much higher efficiency bags, often called absolute filter bags, made of a gradient density structure of meltblown polypropylene or polyester media with fiber diameters in the range of 1 to 5 µm.
Likewise, many types of cartridge filters are available from low-cost, string-wound and meltblown cartridges, followed by a wide variety of pleated cartridges (made of cellulose, cellulose/polyester, polypropylene or polyester media) and finally particulate-loaded cartridges, made with powder-activated carbon, phosphates and other types of media.
Cartridges can be custom-built to remove specific contaminants from a fluid stream. A wide variety of these filters are available at a relatively low cost. Even when finer filtration is required, macro-filters will often be employed upstream of the final filter to remove the large particle loads and extend life on more expensive finer filters downstream.
Micro-filtration generally involves particles ranging in size from 10 µm or below down to 0.05 µm. Examples of contaminants in this range include flour, giardia cysts, paint pigments, EDM process particles and coal dust. To remove these particles, very high-quality depth filters or fine-pore-surface filters are required. These filters are generally made of resin-bonded micro-fiberglass, cellulose or cellulose/synthetic blends, or ceramics (especially in high-temperature applications) and are more resistant to fluid flow, making it important to consider the quantity of fluid being filtered when sizing the system. Keep in mind that lower flow rates improve filter performance, because particles are more likely to be trapped by slower-moving fluid streams.
Ultra- and nano-filtration involves extremely small particles below 0.05 µm. Contaminants in this range include latex emulsions, carbon black and viruses. These particles must be removed by membrane filters, which can be made of cellulose acetone, polycelphon, PTFE or PVDF, among other materials. These membranes are semi-permeable and are constructed in cartridges of various forms, including tubular and spiral-wound membranes. An important consideration in employing membrane filters is membrane fouling, in which the small pores in surface-loading membranes become plugged, impeding the flow of the liquid through the filter. If a wide particle size distribution is present, a pre-filter may be advised.
The finest level of conventional liquid filtration today is RO that employs a non-porous membrane in a cartridge form. These filters are typically cross-fed under high pressure and rely on osmotic forces to move water molecules across the membrane. High pressure applied to the filtrate causes natural osmosis to be reversed; water molecules will move from an area of higher concentration to lower concentration. The cross flow helps prevent filter fouling by carrying away concentrated filtrate with the flow. In addition to their natural filtration properties, crossflow membrane filters can be employed to concentrate solutions by removing water from the feed.
RO is used in desalination of sea water, purification of potable water and in manufacturing processes in which high purity water is required.
Two primary methods of rating filter efficiency are nominal rating and beta ratio.
Nominal filtration rating generally refers to the efficiency of a filter at removing particles of a given size. Most manufacturers refer to nominal efficiency for removal of particles in the 95 to 98 percent range. However, efficiencies as low as 50 percent have been referred to as nominal.
A more rigorous method of reporting filter efficiency is Beta Ratio, which describes the ability of a filter to remove particles of a given size from a fluid stream. See Fig. 1 for the formula to derive efficiency from Beta Ratio.
Upgrading Your Filtration System
It’s important to consider liquid filtration as a necessity to ensure the integrity of your process, your equipment and your finished product. Don’t risk shutting down a multi-million-dollar piece of equipment because someone “skimped” on a $50 filter. Keep in mind that the “costs” associated with liquid filters include not only the filter itself, but also installation, operating and maintenance costs.
Start by quantifying and qualifying your filtration goals: Higher product quality, manufacturing cost improvement, environmental regulation compliance, reduced raw material consumption and reduced system wear and maintenance. Benchmarking the performance of your current filtration system before making changes is a good way to gauge improvement against goal. Because liquid filtration is such a complex process, it is important to seek help from a qualified expert. Filter manufacturers and distributors have much more expertise than even the most seasoned systems or maintenance engineer, and help from these experts is often available for free.
Ronald Cox is director of marketing for Kimberly-Clark Filtration Products (Roswell, Ga.) and can be reached at 770-587-7897 or firstname.lastname@example.org.