Heel Buildup during Thermal Desorption of Volatile Organic Compounds off Beaded Activated Carbon in the Presence of Oxygen Impurity

The presence of low-level oxygen in the purge gas during thermal regeneration triggers chemical reactions resulting in heel formation, which reduces the adsorption capacity and lifetime of the adsorbent. In this study, the effect of regeneration purge gas oxygen impurity on adsorption/regeneration of live aliphatic and six aromatic volatile organic compounds on activated carbon was investigated. Compounds were tested in live-cycle adsorption/regeneration experiments, with regeneration at two different levels of oxygen in nitrogen (<= 5 and 10,000 ppm). Comparison of results for the different adsorbates showed that the effect of the oxygen content in the purge gas is dependent on the nature of the adsorbate. The increased oxygen concentration resulted in higher heel formation for alkyl-substituted aromatic adsorbates but less so for benzene and aliphatic adsorbates. Derivative thermogravimetric analysis of the regenerated samples showed extensive chemisorption of species for adsorbates with the highest heel formation, while physisorption was the dominant heel formation mechanism for the other adsorbates with less heel formed. The results suggest that the presence of oxygen in the regeneration purge gas at high temperature promotes chemical reactions between oxygen and the adsorbate, which adversely affect thermal regeneration efficiency, depending on the functionality and nature of the adsorbate.