Activated carbon fiber cloth (ACFC), shown in the scanning electron microscope image above, is a novel adsorbent that can be used for volatile organic compound (VOC) control. One of the unique qualities that separates ACFC from the more conventional granular activated carbon (GAC) is that ACFC can be more readily regenerated using resistive heating. (GAC beds have contact resistance between each grain and thus have limited electrical conductivity while AFCF is a tightly woven material resulting in good bulk electrical conductivity.)
Comparing ACFC to GAC
Resistive heating provides the benefit that control of regeneration heating is independent of the regeneration carrier gas flow rate. This means that the carrier gas flow rate can be minimized such that the regeneration exhaust gas is highly concentrated with previously adsorbed material for cost effective disposal or reuse as feedstock (e.g., > 60% VOC). By contrast, GAC beds are typically regenerated by passing heated gas streams such as nitrogen or steam through the adsorbent bed, which dilutes the adsorbed material and may require additional treatment for reuse or disposal.
Consistency in ACFC electrical resistance values also provides the potential to use resistance as a sensor for select adsorbent properties. The recent Environmental Science & Technology paper, Monitoring and Control of an Adsorption System Using Electrical Properties of the Adsorbent for Organic Compound Abatement, about the gas recovery system, demonstrates how measured ACFC electrical resistance values can be used as a temperature sensor and an adsorbed mass sensor for real time control of cyclic VOC adsorption and resistive heating regeneration cycles.
This research is a collaboration between the Air Quality Engineering and Science research group at the University of Illinois at Urbana-Champaign (advised by Prof. Mark J. Rood) and China University of Petroleum (UPC), Qingdao, mainland China. Initial experiments were performed during the Ph.D. studies of yours truly (Dr. David Johnsen). Clean Air then provided air pollution control consulting, updates to system automation, and training so that UPC visiting scholar Ming-Ming Hu had the right tools to perform additional experiments to expand this research for cases with high humidity gas streams.
*The gas recovery system uses activated carbon fiber cloth to selectively remove dilute VOCs from gas streams and then electrothermal heating to regenerate the adsorbent and provide liquid VOCs as feedstock for reuse or lower-cost disposal (Patent US8500853 B2: Rood, Hay, Johnsen, and Mallouk). A method to control this system based on indirect electrical resistance measurements was later developed (Patent US8940077 B2: Rood and Johnsen). Hu expanded on the indirect electrical resistance method for cases with high humidity gas streams.