Evaluation of activated carbon filters for removal of ozone at the PPB level

Performance of filters for the removal of ozone at ambient concentration is characterized. The removal efficiency and pressure drop of 10 commercial filters--including 8 made of granule or powdered activated carbon, 1 activated carbon fiber filter, and 1 packed bed made of an ozone catalyst--were measured for an influent ozone concentration of 120 ppb at 50% relative humidity and 2.54 m/sec face velocity. Activated carbon filters can be very effective at ozone removal, although not indefinitely because chemical reactions of ozone and carbon change the carbon. Initial efficiencies of the 1.27-cm thick flat samples varied from 4.6 to 98.3%. Analysis of the structure and composition of the filters with scanning electron microscopy and X-ray photoemission spectrometry showed chemical reactions permanently changed the composition of the carbon and decreased the surface area. Consequently, removal efficiency decreased with use. Moreover, it was not feasible to regenerate the filters by simply removing them from ozone-laden air. Changes in relative humidity, from 20 to 80%, had no measurable effect on the performance of a granule activated carbon filter. However, because the rate of adsorption of water is faster and the pores are smaller in activated carbon fiber, efficiency of the fiber filter decreased when relative humidity was raised from 20 to 50%. A quality factor, equal to the ratio of a threshold breakthrough time and pressure drop, is used to compare filters. In general, those with higher carbon surface area per unit volume had higher efficiencies and greater pressure drops. Future work should address the removal of ozone in the presence of other gases.