Last month, we described the considerations associated with predicting filter service life and how total throughput can be estimated through experimentation. In this second installment, we will examine four filter selection strategies for maximizing service life in continuous-use applications. These aspects consider chemical compatibility, temperature, binding characteristics, and pore size.
At the most basic level, selecting filters that are compatible with the application's chemistry is the key to success. Filters with poor compatibility to the liquid being filtered typically fail well before normal clogging due to limitations associated with filter deterioration. The filter chosen must exhibit both good chemical compatibility with the fluid and good physical compatibility with the operating conditions. PTFE and polyether ether ketone (PEEK) filters exhibit some of the best resistance in applications using very harsh solvents. Nylon, polypropylene, and polyvinylidene difluoride (PVDF) filters have good chemical resistance and are more economical for less severe applications. Polyethersulfone (PES) membrane filters have very good resistance to pH extremes and some oxidizers. In addition to the primary fluid, the consumer must also consider compatibility with filter sanitization or cleaning regimens if they are used. The fluids used for sanitization or cleaning must be filtered to at least the same pore size as the filter being treated; otherwise particles in the cleaning fluid will reduce service life.
For applications with elevated operating temperatures, it is important to realize that chemical compatibility and resistance to differential pressure may be diminished. The filter's recommended maximum service temperature must be carefully considered. Pure PTFE, silver, glass fiber, alumina oxide, and quartz filters all have good resistance to very high temperatures.
In some life science filter applications, the fluids may contain high levels of suspended proteins. These fluids include tissue culture media, serums, lysates, and fermentation extracts. For maximum service life, the consumer should select membrane filters with inherent low protein binding characteristics, such as polyethersulfone (PES) or cellulose acetate (CA). Filters with surfaces that readily adsorb proteins, such as mixed cellulose esters (MCE) or nylon, may be rapidly clogged in these applications.
Finally, for maximum service life, the consumer should select the largest pore size rating that can reliably achieve the filtration goal. In general, as the pore size rating gets smaller, the particle loading capacity of a filter is reduced. It is a common temptation to select pore sizes smaller than necessary based on misconceptions that smaller pore sizes result in better quality filtrate, or that smaller pore sizes are indicative of better quality filters. Avoid this temptation and you can reduce filtration costs by using longer lasting filters. For applications requiring pore size ratings >1µm, consider using nonwoven fiber media filters, such as glass fiber filters or polypropylene fiber filters, as these will typically have better dirt holding capacity than conventional membrane filters.
In addition to selecting optimal filters based on the four criteria described above, there are other strategies that filter consumers can employ to maximize total throughput in continuous-use applications. Next month, in the third installment of this series, we will describe the use of pre-filters.
There are often other factors besides service life that must be considered when selecting filters. Sterlitech offers a guide for helping with filter selection at https://www.sterlitech.com/choosing-the-appropriate-membrane.
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