Metal and Ceramic Media Compared for Hotgas Filtration
Cyclones, ceramic filters, metal powder filters and metal-fiber filters are typically used for hot-gas particulate filtration. Experiment results were summarized in Chemical Plants & Processing. Filter elements that can withstand temperatures between 300 and 900°C are required in the following applications: recovering precious material in chemical processes or in the production of catalyst; fluidized bed combustion and gasification in the chemical, metallurgical and waste treatment industries; and calcination kilns, where dust fouling of the heat recovery stages can be avoided by using clean hot flue gas.
Recently research was conducted by the Dept. of Chemical Engineering at the Catholic University of Leuven, Belgium, evaluating the performance of fibrous ceramic filters and sintered metal-fiber candles.
For its research the Catholic University used Cerafil porous, fibrous ceramic candles. These candles had an OD of 60 mm and a wall thickness of 10 mm. The standard length was 1 m. The average density was approximately 350 kg/m3. The porosity was 87.5 to 91 percent and the pore sizes were between 10 and 100 µm.
The University used Bekiflow HG elements as metal filters. These filters are constructed as rigid candles and consist of a rigid metal-fiber fleece, a weld-on carrier and a mounting flange. The length can vary from 0.3 to 3 m and the diameter from 30 to 180 mm. The metal fleece is composed of very thin metal fibers (2 to 30 µm), which are sintered in an inert gas atmosphere or in vacuum to transform the loose fiber web into a stable matrix. Metal-fiber fleece excels through its high porosity (85 percent) and its low specific weight and thickness (approximately 0.5 mm). These characteristics enable a small pressure drop in combination with good cleanability.
The inherent collection efficiency of the filters is so high that for practical purposes the precise level has not normally been the subject of much concern. The measured efficiencies range from 99.99 percent for sand, catalyst and quick-lime to 99.92 percent for TiO2 and 99.2 percent for carbonate particles. Particles larger than 3 µm are completely removed. No marked effect of temperature was noticed in the experiments.
The experiments demonstrated that the additional pressure drop caused by residual dust particles increases with the number of cleaning cycles and reaches a stable condition after N = 50 with ceramic candles or after N = 10 in the case of sintered metal-fiber candles. Baseline pressure drop (∆Pb) can be calculated for various surface velocities. The virgin pressure drop (∆Pv) is approximately 50 to 75 percent lower for sintered metal candles (Figure 2). One major advantage of sintered metal-fiber candles is the possibility of operating at velocities up to 0.1 m/s, while still achieving adequate pulse cleaning. With ceramic candles, the maximum velocity was approximately 0.06 m/s. With reactive or clogging dust, the baseline pressure drop (∆Pb) rises dramatically and chemical or thermal regeneration of the filter is necessary to return to the virgin state. Pulse cleaning is adequate for both filter types, although ceramic filters appear to be more prone to clogging, resulting in a continuous increase in ∆Pb.
FIGURE 2 The Virgin Pressure Drop ∆Pv is Lowered for Sintered Metal Candles
Inorganic chemical producer Tessenderlo Chemie has installed a metal fiber filter system from Bekaert at its Rotterdam plant in order to safely meet emission levels for process off-gas.
Before entering the filter, the off-gas has a temperature of around 120°C and typically contains 10 g/m3 of dust particles with diameters measuring between 0.2 and 20 µm. According to Bekaert, installing its Bekiflow® HG filter elements has enabled Tessenderlo to achieve emission levels of 4 mg/m3, which is well below its target of 10 mg/m3.