When faced with having to clean up or recover products from hot gas streams, the options are clear: Either cool the gas sufficiently (below 250 [degrees] C) to avoid damaging standard filter media or look for temperature-resistant filtration media that can take the heat. Many ceramic and metallic-based filter elements have been designed to operate at temperatures of 250-1,000 [degrees] C and higher.
Put off by the higher cost of these filters, many customers in the chemical process industries (CPI) have opted to continue to use fabric fibers. But as the list of hot-gas filter suppliers get longer, more experienced and more competitive, more chemicals producers are discovering the advantages offered by hot-gas filters.
Able to withstand high temperatures to 1,600 [degrees] C, these new gas-cleaning systems operate under high pressures and in chemical environments that would destroy most fabric bags. And since these system operate at elevated temperatures, they avoid or reduce some of the incidentals associated with using fiber bags, such as cooling the hot gas by diluting it with air. The condensation and sublimation that cause fouling are also avoided, and in cases of incineration, there is no dioxin formation.
Today, there are some 30-35 companies in the world that offer hot-gas filter media, says Lutz Bergmann, president of Filter Media Consulting, Inc. (FMC; La Grange, Ga.). Most economical and least expensive are low-density ceramic candles, followed by glass or ceramic tubes, which are offered in different shapes and may have lower temperature tolerances. Stainless-steel sintered fleece, sintered powder-metal elements and rigid ceramic candles tend to be the most expensive, but are also the most versatile filters in terms of strength and cleanability, says Bergmann.
He puts the value of global market at $30-40 million, with products aimed at several key applications: catalyst and precious-metal recovery, waste incineration, advanced coal-based power generation, calcination, catalytic cracking and refining, and product recovery.
Better cleaning
Hot-gas filter systems that use jet-pulse cleaning require pressures of 0.5-1 MPa for atmospheric-pressure filter systems. And high-pressure systems require pressure that is two times higher than system pressure. However, even higher cleaning intensities with less cleaning pressure are offered in a coupled pressure-pulse (CPP) system developed by Research Center Karlsruhe (FZK, Karlsruhe, Germany) and USF-Schumacher Umwelt- und Trenntechnik GmbH (Crailsheim, Germany).
CPP (figure, above) requires cleaning-gas pressure that is only 0.05-0.1 MPa higher than system pressure. It consists of a vessel divided into raw and clean-gas sides by a tube sheet to which the …
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