Long-Term Heel Transformation during Cyclic Adsorption-Desorption of Volatile Organic Compounds in a Full-Scale Adsorber-Desorber

This study elucidates the different mechanisms contributing to heel formation during cyclic adsorption-desorption of volatile organic compounds (VOCs) from a painting process in a full-scale adsorber-desorber. Two batches of beaded activated carbon (BAC), one subjected to normal operation and the other inadvertently exposed to oxygen during desorption, were investigated. Both batches displayed sharp increases in apparent density during early cycles, which indicate high heel formation due to occupation/blockage of high-energy adsorption sites. Thermogravimetric analysis and gas chromatography-mass spectrometry results identified non-desorbed adsorbates, adsorbate reaction by-products and char from adsorbate decomposition. The main heel formation mechanisms include physisorption and chemisorption followed by thermal oxidation, pyrolysis and eventually char formation. We hypothesize that non-ideal heat and purge gas distribution across the desorber is the reason for the accumulation of non-desorbed species and heel formation, shortening the lifetime of the adsorbent. In addition to pyrolysis reactions that partially contributed to heel formation in the absence of oxygen, accumulation of thermal oxidation products also contributed to the deterioration of adsorbent performance in the presence of oxygen during desorption. Moreover, extended exposure of the non-desorbed physisorbed and chemisorbed heel species to repetitive desorption cycles ultimately transformed them to permanent (non-desorbable) heel via polymerization and char formation. Therefore, to reduce heel buildup and maximize long-term adsorber performance, it is recommended to (i) ensure effective desorption conditions to prevent or minimize the accumulation of non-desorbed physisorbed species, and (ii) maintain sufficiently low oxygen levels in the desorption purge gas to minimize the detrimental effects of any species that do accumulate.