Organic solvent vapors are typical examples of vapor organic compounds (VOCs) which are very well known to cause health problems. A large volume of air becomes contaminated by organics vapor (coming from organic solvents) produced in dry cleaning (tetra-chloro-ethelene, TCE), furniture finishing (methyl ethyl ketone, acetone, methylene chloride, toluene) and painting. The produced VOCs combine with NOx in the presence of sunlight and produce ground-level ozone. Exposure to ground-level ozone can affect human health and cause agricultural crop loss and damage of forest and ecosystems.
The existing methods for removing VOCs from air or gas stream include absorption, adsorption, condensation and incineration. Of these, adsorption is an efficient and economical method for moderate to low concentration streams. The adsorbates (VOCs) need to be removed periodically after they saturate the adsorbent. The two conventional regeneration processes are steam and hot gas regeneration. The first one leaves the adsorbent bed wet with condensate and needs additional time to dry out the bed for further use. The second process needs the unsaturated gas to heat the adsorbent to a temperature that is sufficient for desorption. This hot gas should also heat the purge (adsorbate) as it gets desorbed. Another factor needs to be considered here. An effective adsorbent might be difficult to regenerate because of its higher binding energy. As a result a longer regeneration time and a large purge gas volume might be required. Such complexity and size of current equipments and technology required for VOC control place small sources in a very disadvantageous position to remove VOCs from air.
Experimental studies are being conducted by researchers all over the world to use microwaves for regeneration of exhausted adsorbents in VOC removal process. Microwave heating is generated internally within the adsorbents instead of originating from an external heat source. As the heating is totally internal and volumetric, the thermal gradients and the flow of heat in microwave generated adsorbents differ from those in an adsorbent regenerated by conventional heating (steam or hot gas). Microwave processing makes it possible to regenerate adsorbents very rapidly. The feasibility of this idea (using microwaves in regeneration) has already been successfully studied by McCallen Air Force Base, USA (C.Y. Cha and C. T. Carlisle, J.Air and Waste manage. Assoc., vol.51, 2001) and P. Di and D.Chang (Department of Civil and Environmental Engineering, University of California, Davis).
A pilot plant was built and successfully operated in McCallen Air force Base for a period of three months. Microwaves were used not only to regenerate the adsorbent but also to destroy VOC without generating any secondary air pollutants. So far activated carbon, zeolite and polymers have been used as adsorbents in microwave induced regeneration process. It is seen that adsorbent degradation can be successfully avoided for a longer time compared to conventional heating processes. The ongoing success in this research has also influenced the feasibility study of this regeneration process in natural gas purification (to regenerate adsorbents during removal of CO2) which is now in focus for many researchers.
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Chowdhury, T., Application of Microwave in Regeneration of Adsorbents Used in Removal of Solvents from Contaminated Air, ChE Thoughts 1 (1), 8-98, 2011.