Effects of Adsorbate Properties on Heel Buildup on Activated Carbon Fiber Cloth during Electrothermal Regeneration

Abstract

Automotive painting booths are a major source of volatile organic compounds’ (VOCs) in the automotive manufacturing sector. Adsorption on activated carbon (AC) has been widely used as a low cost and energy efficient VOC control method. A major challenge associated with the cyclic adsorption/desorption of VOCs on AC is the unwanted accumulation of adsorbates known as irreversible adsorption or heel formation, which leads to significant decrease in adsorbent capacity and lifetime. The objective of this study is to identify the effects of adsorbate properties such as boiling point, kinetic diameter and thermal stability on heel buildup and adsorption capacity loss of AC. For this purpose, five cycle adsorption/desorption tests were completed using activated carbon fiber cloth (ACFC) as adsorbent and nine different alkylbenzenes (toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, isopropylbenzene, 1,2,4 trimethylbenzene (TMB), 1,3,5 TMB and neopentylbenzene) with different physical and chemical properties were used as adsorbates. The ACFCs loaded with various adsorbates were electrothermally regenerated at 400°C, with a heating rate of 70°C/ min and N2 purge flow of 0.1 standard liter per minute (SLPM). The obtained results indicate that heel formation was mainly affected by the thermal stability of the adsorbates, whereas the adsorption capacity loss was affected by heel formation and the kinetic diameter of the adsorbates. The principal mechanism of heel formation was found to be the thermal decomposition of the adsorbates due to high temperature desorption, while the adsorbates’ boiling points (or vapour pressure) and kinetic diameters controlled their exposure to high temperature during desorption. The conversion rates of the adsorbates in pyrolysis have been used as an indication of their thermal stability, and an increase in conversion rate from less than 1% to more than 90% increased heel formation from 8.9% to 17.2%, showing that lower thermal stability of adsorbates causes higher decomposition and therefore higher heel. Bulkier compounds caused higher adsorption capacity loss exhibiting an increase in capacity loss from 25.2% to 80.0% when kinetic diameter of adsorbate molecules increased from 5.9 Å to > 7.5 Å. The results of this work will help understand how different adsorbates in industrial air streams will affect heel formation on AC. This will help to take proper measures and optimize regeneration conditions to control heel formation and extend the lifetime of adsorbents.