Environ. Sci. Technol. 2008, 42, 9317–9322
Concomitant Adsorption and Desorption of Organic Vapor in Dry and Humid Air Streams using Microwave and Direct Electrothermal Swing Adsorption Z A H E R H A S H I S H O , * ,† HAMIDREZA EMAMIPOUR,‡ M A R K J . R O O D , ‡,§ K . J A M E S H A Y , | BYUNG J. KIM,| AND D E B O R A H T H U R S T O N ‡,§ Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta T6G 2W2, Canada, Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, 61801, Industrial and Enterprise Systems Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, and Engineer Research and Development Center, Construction Engineering Research Laboratory, Champaign, Illinois 61826
Received May 9, 2008. Revised manuscript received August 12, 2008. Accepted October 20, 2008.
Industrial gas streams can contain highly variable organic vapor concentrations that need to be processed before they are emitted to the atmosphere. Fluctuations in organic vapor concentrations make it more difficult to operate a biofilter when compared to a constant vapor concentration. Hence, there is a need to stabilize the concentration of rapidly fluctuating gas streams for optimum operation of biofilters. This paper describes new concomitant adsorption desorption (CAD) systems used with variable organic vapor concentration gas streams to provide the same gas stream, but at a user-selected constant vapor concentration that can then be more readily processed by a secondary air pollution control device such as a biofilter. The systemsadsorborganicvaporfromgasstreamsandsimultaneously heat the adsorbent using microwave or direct electrothermal energy to desorb the organic vapor at a user-selected set-point concentration. Both systems depicted a high degree of concentration stabilization with a mean relative deviation between set-point and stabilized concentration of 0.3-0.4%. The direct electrothermal CAD system was also evaluated to treat a humid gas stream (relative humidity ) 85%) that contained a variable organic vapor concentration. The high humidity did not interfere with CAD operation as water vapor did not adsorb but penetrated through the adsorbent. These results are important because they demonstrate the ability of CAD to effectively dampen concentration fluctuation in gas streams.
* Corresponding author fax: 780-492-0249; e-mail: hashisho@ ualberta.ca. † University of Alberta. ‡ Department of Civil and Environmental Engineering, University of Illinois. § Industrial and Enterprise Systems Engineering, University of Illinois. | Construction Engineering Research Laboratory. 10.1021/es801285v CCC: $40.75
Published on Web 11/19/2008
2008 American Chemical Society
1. Introduction The emission of organic vapors is a serious environmental issue. The emissions of anthropogenic volatile organic compounds (VOCs) to the atmosphere in the United States were 1.5 × 1010 kg during 2002 (1). Regulations pertaining to VOC emissions encourage the development of new air pollution control technologies to more effectively remove VOCs from gas streams at lower costs. Organic vapor control methods are classified as destructive methods such as biofiltration and thermal oxidation, and recovery methods such as absorption, condensation, and adsorption. The choice of the VOC control method depends on the operating conditions and the physicochemical characteristics of the VOC (2). Adsorption with regeneration is a desirable means to control emissions of a wide range of VOCs and organic vapors as it allows for capture, recovery and reuse of those compounds (3). When recovery and reuse of organic vapors is not technically or economically feasible, then it might be more suitable to convert the vapors into more benign products with biofiltration. However, effective and efficient control of organic vapors emissions can be challenging due to the highly variable concentration that exists for nonsteady state operations (e.g., painting operations). Fortunately, these fluctuations in concentrations can be dampened with the use of adsorption upstream of a biofilter. Hence, the use of adsorption upstream of a biofilter can allow for more efficient disposal of organic vapors that are not recoverable for reuse based on technical or economic constraints. Approaches to dampen the fluctuations in organic vapor concentration before treatment of gas streams by biofiltration are classified as passive or active. Passive dampening does not use active control during the desorption cycle. An example of this approach includes the use of adsorbents such as granular activated carbon (GAC) as biofilter packing material on which biofilm is grown (4-6). However, limited dampening of concentration fluctuations occurred because of competitive adsorption caused by water present in the biofilter and the short gas phase contact time in the vessel (6). Another example of passive dampening of concentration fluctuations is to use an adsorbent in a separate vessel upstream of a biofilter (6-8). While such approach resulted in better dampening efficiency than using an adsorbent as packing material for the biofilter, the dampening efficiency decreased with increasing gas flow rate, (i.e., decreasing empty bed contact time (EBCT)), and increasing vapor concentration (8). The most effective dampening occurred at low concentration (210 ppmv) and high EBCT (10 s) and resulted in a negligible (
