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.

这项研究阐明了在全尺寸吸附器-解吸器中的涂漆过程中，挥发性有机化合物（VOC）的循环吸附-解吸过程中导致脚跟形成的不同机制。研究了两批串珠状活性炭（BAC），其中一种进行了正常操作，另一种在解吸过程中无意中暴露在氧气中。这两个批次在早期循环中均表现出表观密度的急剧增加，这表明由于高能吸附位点的占据/受阻而形成高跟。热重分析和气相色谱-质谱分析结果确定了未解吸的吸附物，吸附物反应副产物和来自吸附物分解的炭。主要的脚跟形成机制包括物理吸附和化学吸附，然后进行热氧化，热解并最终形成炭。我们假设整个解吸器的热量和吹扫气体分布不理想是非解吸物质积累和脚跟形成的原因，从而缩短了吸附剂的使用寿命。除了在不存在氧气的情况下部分地导致脚跟形成的热解反应之外，热氧化产物的积累还导致在解吸过程中存在氧气的情况下吸附剂性能的下降。此外，未解吸的物理吸附和化学吸附的脚跟物质长时间暴露在重复的解吸循环中，最终通过聚合和形成焦炭将它们转变为永久性（不可解吸的）脚跟。因此，为减少脚跟堆积并最大化长期吸附性能，