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Aerosol collection in granular beds

University of British Columbia

The filtration of aerosols using granular beds was studied to determine the feasibility of using such devices as high efficiency particle collectors. Based on the experimental data, it was attempted to derive expressions for predicting the aerosol removal efficiency of the granular bed. Granular beds composed of fairly uniform, spherical nickel shot were employed in a 7.4 cm diameter copper column to collect solid, monodispersed, polystyrene latex aerosols. The collection efficiency of the granular bed was determined as a function of several variables, viz., aerosol diameter (0.109 to 2.02 urn); bed particle diameter (100 to 600 μm); bed depth (0.3 to 19 cm); superficial gas velocity (5 to 67 cm/sec); and flow direction (upflow and downflow). The monodispersed, latex aerosols were generated by atomizing dilute hydrosols of aerosol particles. The aerosol number concentrations were measured at the inlet and outlet of the granular bed (using light scattering techniques), from which the bed collection efficiency was determined. Using the concept of an isolated bed particle it was possible to quantitatively predict the collection efficiency of the bed. The collection of an aerosol by an isolated bed particle can be attributed to the following mechanisms:- inertial impaction, direct interception, diffusional deposition, gravitational deposition and electrostatic effects. In the present study electrostatic effects were eliminated by grounding the equipment and neutralizing the aerosol. Equations based on individual collection mechanisms and combinations were fitted to the experimental data by multiple regression analysis. An empirical model was developed, which gave good predictions of the experimental bed collection efficiency. The single collector efficiency (EB) was calculated using the following empirical equation: EB = 1.0 St + 150,000 NR[sup 4/3] Pe[sup -2/3] + 1.5 NG and the overall bed collection efficiency (EBT) was calculated using the following theoretical equation: [equation not included]. The difference between the experimental and calculated bed efficiencies were generally less than ten percentage points. Experimental results indicate that high collection efficiencies can be achieved with relatively shallow fixed beds of granular material. Inertial impaction was considered to be the dominant collection mechanism at high gas velocities, whilst diffusion and, to a lesser extent, gravity were considered dominant at low gas velocities. For all the experimental conditions studied, interception was shown to be insignificant.

Text

2010-02-26

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http://hdl.handle.net/2429/21154

Chemical and Biological Engineering

University of British Columbia

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