顆粒床過濾與負載特性

Abstract

The purpose of this study was to investigate the filtration and loading characteristics of granular bed filters. Stainless steel holders (diameter 71.6 mm, height 70 mm) were fabricated to accommodate 500g of zirconium oxide (ZrO2) beads, as the packed media of granular bed. Monodisperse ZrO2 granules (0.3, 0.8, 2 and 4 mm in diameter) were used to demonstrate the effect of the granule size and packing geometry (uniformity of 0, 30 and 45) on both pressure drop and aerosol penetration. The ZrO2 granular bed filter has a constant porosity of 0.48 and is independent of granule size. The size distributions and number concentrations upstream and downstream of the granular bed filter were measured using a scanning mobility particle sizer and an aerodynamic particle sizer. Face velocities, ranging from 0.58 to 14.8 cm/sec, were varied to study the flow dependency. The experimental data were compared with empirical models developed in previous studies. For aerosol loading, 10 micrometer monodisperse acrylic powders was used. A fibrous filter was used in loading test for comparison purpose. The penetration test results showed that aerosol penetration increased greatly with increasing granule size of the filter beads. For submicrometer-sized aerosol particles, the aerosol penetration increased with increasing face velocity due to shorter retention time within filter media. For micrometer-sized particles, the aerosol penetration decreased with increasing face velocity, apparently due to higher inertial impaction. From the filter quality perspective, the selection of the ‘best” filter is complicate. Assuming a low face velocity (e.g., 0.58 cm/sec), large granular size is more cost-effective because of the higher filter quality factor. The phenomenon implies that the gain in filtration efficiency due to larger surface area (of small granules in the filter) did not compensate for the increase in air resistance. In the case of high face velocity, the “best” filter is dependent on the size of the particles to be removed. In order to remove large particles, small granules should be used. Large granules are preferred for removing small aerosol particles. In comparison to the more familiar fibrous filter, the granular bed filter had very different loading characteristics. The pressure drops across both fibrous the granular bed filters increased with increasing mass loaded on the filter. After the cake formation point, the dust cake on glass fiber filter became compressed. This dust cake compaction caused the pressure to drop precipitously and intermittently. In contrast, the rate of increase in pressure drop of the dust cake formed on the granular bed filters decreased with time probably due to the pinhole leaks in the increasing mass loading. The size and density of the pinholes are determined by the granule size, the face velocity and the size of the challenge aerosols.