The Effect of Regeneration Purge Gas Impurity on Irreversible Adsorption of Organic Vapors

Irreversible adsorption or heel formation is a known challenge for organic vapors adsorption onto activated carbon as it decreases the adsorbent capacity and lifetime. The objective of this research is to investigate the effect of nitrogen purge gas purity on heel formation during thermal desorption of activated carbon. Kureha beaded activated carbon (BAC) was tested for 5 consecutive adsorption/regeneration cycles using a mixture of nine organic compounds typically emitted from automotive painting operations. Nitrogen, with different concentrations of oxygen (5, 625, 1250, 2500, 5000, and 10000 ppm) was used as purge gas during thermal desorption cycles. The mass balance cumulative heel was 9, 16, 24, 35, and 34% higher for samples exposed to 625 ppm, 1250 ppm, 2500 ppm, 5000 ppm, and 10000 ppm O₂, respectively, compared to sample exposed to 5 ppm O₂. Samples exposed to higher O₂ concentration (625 to 10000 ppm) during regeneration also lost their adsorption capacity at a faster rate (up to a 119% increase in capacity loss) compared to sample exposed to 5 ppm O₂. Derivative thermogravimetric (DTG) analysis showed that heel formation was due to strong physical adsorption for sample exposed to 5 ppm O₂ and a combination of strong physisorption and chemisorption for the other samples. In contrast to previously published results, less pore blockage occurred in samples exposed to higher O₂ concentrations most likely because of diffusion resistance due to chemisorbed adsorbates. BAC samples, which were exposed to 50 successive adsorption/regeneration cycles, were also characterized to provide further insight about the impacts of long term exposure to high O₂ concentration, showing trends consistent with short term exposure (5 cycles). The results from this study help explain the heel formation mechanism and how it relates to regeneration purge gas purity.