Filtration for the Future

Chemical plants stand to benefit from several new and emerging filtration technologies

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To understand what new filtration products are emerging in the chemical process industry, you first need to distinguish between disruptive filtration technology and a simple tweak to or minor improvement of an existing product or process.

Disruptive technologies are commonplace. Instant photography, for example, was offered as a replacement for 35-mm film. It later was "disrupted" when digital cameras came along, which require no film at all. Manufacturers of fuel cells are developing disruptive technology to supplant the internal combustion engine. In chemical filtration, a similar step-jump must occur before the technology can be deemed "new and emerging."

This article highlights new or emerging filtration media and discusses their potential impact on some common chemical applications.

 

DURAPEX PET N2030 PM2.5 filter media (left) vs. 16-ounce-per-square-yard standard needfelt PET

Photo courtesy of Polymer Group Inc.

 

Cleaning exhaust air

Baghouse filters are used widely in many process industries to clean exhaust air in manufacturing environments. Among the many uses for these filter systems are to remove chemical dust in powder processing, metal fines in foundries, particles from abrasive processing applications and cement dust at cement manufacturing centers.

Woven fabrics from wool and cotton were the first primary filtration media. Later, rayon was used. Then, as synthetic fibers became available after World War II, polyester and nylon woven fabric filter bags captured the market. Needlefelt fabrics disrupted woven fabrics. In certain cases, polytetrafluoroethylene (PTFE) foamed coatings or microporous membranes laminated to needlefelt substrates then replaced needlefelt fabrics.

Each development was an improvement over the incumbent filtration medium and was disruptive to the manufacturers of the previous fabric media.

PTFE membranes and, to a lesser extent, PTFE and other foams laminated to or coated over needlefelt fabrics have eliminated the inherent problems caused by the large gaping holes in needlefelt fabrics. These membranes and coatings have become a huge success, but at a cost of up to 10 times the price of needlefelt fabrics. In spite of this high cost, membrane customers have been willing to take this route for improved product performance.

Another disruptive technology is now on the horizon. A new kind of nonwoven fabric is emerging that is not from needlefelt fabric or PTFE foam or membranes, but instead from hydroentangled nonwoven fabrics ," Durapex fabrics ," from an entirely new set of suppliers.

Hydroentangled nonwoven fabrics are constructed in a similar fashion to needlefelt fabrics, but with critical differences. Instead of barbed needles, which penetrate the web to entangle the fibers during the manufacture of needlefelt nonwoven fabric, very fine high-pressure water jets are used to produce hydroentangled nonwoven fabrics.

The water jets entangle the fibers, but unlike the hard metal needles used to construct needlefelt nonwoven fabrics, do not severely damage the fibers. Broken fibers in needlefelt constructions not only produce a weak fabric, but also cause premature bag failures. Broken fibers frequently break free and drift downstream whenever the baghouse filter is under any stress.

A second advantage of hydroentangled fabrics is that the hydroentangling process does not produce large needle holes or an uneven fiber distribution. Anyone who has examined the surface of a needlefelt fabric probably has observed this deficiency, which is represented by thousands of pot-marked holes across the surface. This moonscape-like surface causes the airflow in needlefelt fabric filters to be very uneven across and throughout the web. Preferential airflow naturally seeks the larger needle holes, while minimal airflow is available across those portions of the filter's surface without the needled holes.

This occurrence is problematic because the non-needled portions of the needlefelt fabric filter surface are intended to be the workhorse area. The mechanical needling that holds the filtration media together creates the holes or pores, which determine the efficiency and effectiveness of the filter itself. This phenomenon becomes a particular problem during filter system start-up and even more so later when the large particles, lodged in the needle hole pores, dislodge and off-load downstream.

Repeated cycles of loading and off-loading also contribute to premature filter failure. The endless loading and off-loading of particles also prevent the much denser non-needle-hole portions of the fabric from fully serving as the medium's intended filtering region. These areas are relatively dense and are not where air is first apt to flow, especially when large holes are readily available. In essence, the needle holes, not the pores between the non-needled portion of the fabric, control or limit the needlefelt's filtration capability.

It could be argued that the large holes eventually plug with dirt, and that those portions without the holes do the workhorse filtration. However, by the time this plugging occurs, the differential pressure of the media reaches such a high level that the useful life of the filter is all but spent.

Through another advance in the disruptive development chain, hydroentangled nonwoven fabrics are emerging that have 60 percent of the weight of needlefelt fabrics, but boast similar strength and other mechanical properties. Fiber distribution throughout the medium is incredibly uniform because the pores produced by the hydroentangling forming process are similar in size and configuration to the surrounding pores formed by the fibers themselves. In turn, airflow is even ," without the large gapping holes.

Hydroentangled nonwoven fabrics are reported to perform at levels near PTFE microporous membranes and foamed coatings in all regards, including efficiency and cloth-to-air ratios, but at a price similar to needlefelt nonwoven fabrics. End-users and plant maintenance crews now have a new tool to achieve high efficiency and uniformity at an affordable cost.

Difficult applications

The chemical industry has its share of tough filtration challenges. Most difficult are the three simultaneous requirements of high-temperature capabilities, chemical inertness and easy, rapid filter cleanability.

Individually, each need can be a challenge. However, when all three are needed at the same time, solutions are limited. This troika of capabilities is needed in many chemical processing, coal-fired power plant and other applications. It is a struggle for companies that design filtration systems to meet these challenges without the need for secondary or backup capability. A single stand-alone system continues to be the "holy grail" in these and other market segments.

To meet this challenge, one emerging technology ," the Thermapore technology ," appears particularly promising. This technology uses a filtration media from silicon carbide fibers that are 5 microns and/or 13 microns in diameter. The fibers either are wetlaid or otherwise made into a filtration medium with many possible designs, including a flat surface, pleated, spiral wrapped, rotary drum, through-wall or just about any conventional configuration required for use in a filter.

 

Shown here is a filter cartridge cleaning operation at 750C.

Photo courtesy of Industrial Ceramic Solutions.

Advantages of silicon carbide include its high temperature resistance and chemical inertness. When produced as a fiber and subsequent filter medium, filter and filtration system, silicon carbide appears to be a leading candidate and new disruptive technology worthy of in-depth investigation for potential wide use in demanding and stressful process environments.

Perhaps the best example of progress with silicon carbide filtration systems has been with diesel soot exhaust filters. Although many material candidates for this use have been explored, few, if any, fully meet all the requirements necessary to become a practical solution. Filters constructed of silicon carbide filtration media and support have been tested on dynamometers and through field trials and have been documented to remove 95 percent of 0.1-micron soot particles under a wide range of engine conditions, without adversely affecting the filter, the filtration system or vehicle performance.

Although these performance results are impressive, the most interesting part is that the filter has an associated magnetron (microwave device) that activates when the filter becomes contaminated with soot and reaches a predetermined differential pressure level. At this point, the magnetron is programmed to activate and heats the filter to a controlled temperature as high as 1,200C. At this temperature, the soot is vaporized instantly into a harmless exhaust plume, which is expelled to the atmosphere. The filter then is ready to collect more soot for another cycle. The entire purging cycle is performed in seconds.

Imagine the possibilities of rapidly eliminating large volumes of an unwanted organic substance in a chemical process in an environmentally beneficial manner. Of course, not every organic substance or circumstance will require a 1,200C burn-off to clean a filter. However, systems using silicon carbide Thermapore filters could be alternatives to widely available pressure and vacuum cake filtration devices, which typically use diatomaceous earth filter aids that often prove to be problematical in waste disposal.

Other new technologies

Other new and emerging filtration media technologies include:

Tensylon UHMWPE fabrics that have an extremely low elongation (2.8 percent) and creep (<5 percent with 20 percent load after almost a year). Tensylon has both a very high cut and abrasion resistance with tenacity at 17.0 grams per denier (gpd) and outstanding resistance to acids, bases and many other aggressive chemicals and fluids. The yarn has a surprising high temperature resistance for a polyethylene ," about 285F, a result of the long-chain molecules that make up the polymer. Filtration media from ultra-high strength Tensylon can be used as press cloths or belts under the most demanding applications.

MultiLobal micrometal fibers are metal filters made from stainless steel, nickel, titanium and other metals with fibers as fine as 1.5 microns in diameter. The media are capable of resisting heat as great as 500C, have high strength and modulus properties and are inert in most applications and through the pH range. When supported with wire cloth filter candles, cartridges or flat plates, they are rigid and capable of operating under high pressure.

EmmiTex PTFE yarns and fibers for filtration media are available as woven fabric and are emerging as wetlaid and hydroentangled nonwovens. PTFE has high surface release properties and temperature capabilities (to 250C) and is inert to most chemicals and through the pH range. These materials also are used as sewing threads in filter bags, belts and press cloths.

Halar ethylene-chlorotrifluoroethylene (E-CTFE) fluoropolymer filtration media are made into melt-blown fabrics. The media have very fine filtration capabilities, outstanding chemical properties and a wide range of pH capabilities. The media are emerging in demanding applications ," from coalescing and mist elimination to filtration of aggressive chemicals in cartridge and bag filter configurations.

Conclusion

New and emerging technologies ," disruptive filtration technologies ," will allow the chemical process industry to improve process applications and reduce costs. Disruptive filtration developments in the chemical process industry have not been a common occurrence. Disruptive technologies are not minor or incremental improvements, but wholesale developments that change the way products are produced and used.

Gregor is managing director of Edward C. Gregor & Associates LLC and is a co-founder of the American Filtration and Separations Society and Filtration Fellow. He is a consultant to the polymer, fiber and filtration industries and resides in Charlotte, N.C. Gregor can be contacted by e-mail at ecg@egregor.com or at (704) 442-1940.

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