Environ. Sci. Technol. 2005, 39, 6851-6859
Microwave-Swing Adsorption To Capture and Recover Vapors from Air Streams with Activated Carbon Fiber Cloth ZAHER HASHISHO AND MARK ROOD* Department of Civil & Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue, Urbana, Illinois 61801 LEON BOTICH ADS Technologies, Inc., 25120 South Doolittle Drive, Monee, Illinois 60449
Adsorption with regeneration is a desirable means to control the emissions of organic vapors such as hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) from air streams as it allows for capture, recovery, and reuse of those VOCs/HAPS. Integration of activatedcarbon fiber-cloth (ACFC) adsorbent with microwave regeneration provides promise as a new adsorption/ regeneration technology. This research investigates the feasibility of using microwaves to regenerate ACFC as part of a process for capture and recovery of organic vapors from gas streams. A bench-scale fixed-bed microwave-swing adsorption (MSA) system was built and tested for adsorption of water vapor, methyl ethyl ketone (MEK), and tetrachloroethylene (PERC) from an airstream and then recovery of those vapors with microwave regeneration. The electromagnetic heating behavior of dry and vaporsaturated ACFC was also characterized. The MSA system successfully adsorbed organic vapors from the airstreams, allowed for rapid regeneration of the ACFC cartridge, and recovered the water and organic vapors as liquids.
1. Introduction The emissions of volatile organic compounds (VOC) and hazardous air pollutants (HAPs) are serious environmental issues. The Environmental Protection Agency (EPA) reported that 1.6 × 1010 kg of VOCs and 5.7 × 108 kg of HAPs were emitted to the atmosphere from anthropogenic sources during 2001 and 2002, respectively (1, 2). For example, during year 2002, 12.2 × 106 kg of methyl ethyl ketone (MEK) and 1.1 × 106 kg of tetrachloroethylene (also known as perchloroethylene or simply PERC) were emitted to the atmosphere. Also, 60% of the MEK and 62% of the PERC were from point sources that can readily allow capture and recovery of those pollutants by techniques such as adsorption (2). Five HAPs that can be readily adsorbed from air streams are also VOCs, and they constitute 21% of those emissions. The annual cost to meet regulations for VOC control in 2010 is estimated at $2.3 billion (3). Regulations pertaining to HAP/VOC emissions encourage the development of new air pollution control technologies to more effectively remove HAPs/VOCs from gas streams at lower costs. Adsorption with regeneration is a desirable means to control the emissions of organic vapors such as HAPs and VOCs from air streams as it allows for * Corresponding author phone: (217)333-6963; fax: (217)333-6968; e-mail:
[email protected]. 10.1021/es050338z CCC: $30.25 Published on Web 08/05/2005
2005 American Chemical Society
capture, recovery, and reuse of those compounds. Adsorption with regeneration and reuse decreases the consumption of natural resources and the emission of pollutants to the atmosphere, providing for a more sustainable form of development. Activated carbon fiber cloths (ACFCs) have been shown to be very effective adsorbents to capture and recover organic vapors from gas streams at wide range of concentrations (4, 5). A bench-scale adsorption system was tested with select organic compounds at concentrations ranging from 233 to 1020 ppmv (6). A pilot-scale adsorption system was tested at MEK concentrations ranging from 73 to 1006 ppmv (7). ACFCs are typically polymeric-based adsorbents that can be prepared from Novoloid, Polyacrylonitrile (PAN), pitch, and Rayon precursors (8). Activated carbon fibers (ACFs) are unique as compared to traditional activated carbon adsorbents such as GAC because of their larger surface area (10002400 m2/g) (8); larger adsorption capacities (as high as 250% of that of GAC) (5), faster heat and mass transfer properties (9), and absence of impurities such as ash that can catalyze oxidative reactions (10, 11). GAC is typically regenerated with steam. ACF has been regenerated with steam (12) and inductive heating (13) and direct resistive heating (4, 5, 14). Microwave energy has been used to heat ACFCs during preparation and treatment (15-17); however, it has not been used to regenerate vapor laden ACFC. A strong motive for promoting microwave regeneration of adsorbent is the potential for selectivity of microwave heating between the adsorbate and the adsorbent, as well as among the adsorbates in the case of a multicomponent vapor stream. Microwave heating has the potential to control the deposition and distribution of energy in the system during regeneration. Adsorbent material and fittings can consume a significant part of the energy during regeneration (4). Selective heating of the adsorbate in the pores of the adsorbent can reduce the energy needed for the regeneration. The selective heating of the adsorbates according to their dielectric properties can allow for the separation of the adsorbates during the desorption process. This could simplify post-regeneration processing when treating multicomponent vapor streams such as in the case of a binary stream of water vapor and a nonpolar adsorbate or in the case of a mixture of polar and nonpolar adsorbates. Microwaves refer to the electromagnetic spectrum between 300 MHz and 300 GHz, with a corresponding wavelength between 1 m and 1 mm, respectively. The two frequencies of primary interest for commercial applications are 915 and 2450 MHz (18, 19). Microwave heating results from its electric field polarizing the workload’s molecules and the inability of this polarization to follow the extremely rapid reversal of the electric field (19). Molecules with symmetric charge distributions, such as PERC, are nonpolar, have a very small dipole moment and loss factor (′′), and hence exhibit limited absorption of microwaves. In contrast, water and a wide range of organic compounds such as MEK are polar because they have charge asymmetry resulting in large dipole moments. An advantage of microwave regeneration is that heating is dependent on the dielectric properties of the workload (i.e., adsorbate and/or adsorbent) rather than the purge gas flow rate during regeneration. Microwave heating is also volumetric, whereby all of the infinitesimal volume elements within the object are heated. Finally, in contrast to surface heating such as hot gas or steam heating, the direction of the heat flux from microwaves is from the inside to the outside of the workload. These properties can result in selective heating of adsorbates with a large dielectric loss factor within the pores of a microwave transparent adsorbent. Such heating VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Properties of the Tested Chemicals (28, 29) property
water
MEK
PERC
chemical structure
molecular weight 18.02 liquid density (g/cm3) 1.00 boiling point (°C) 100.0 specific heat (cal/g-K) 1.00 heat of vaporization 583.1 (cal/g) dielectric constant, ε′ 76.70 dielectric loss factor, 12.04 ε′′ dipole moment (D) 1.85 a
72.1 0.80 79.6 0.53 115.2 18.51 NAa 2.78
165.8 1.62 121.3 0.21 57.2 2.28 0.0023 0.00
NA ) not available.
mechanism will allow for higher energy efficiency and more selective and rapid heating of the adsorbate and regeneration of the adsorbent. Microwave energy was used for the regeneration of silica gel (20), zeolites (20, 21), char (22), GAC (23-25), powder activated carbon (26), and polymeric bead adsorbents (21). In these studies, the adsorption and desorption reactors are separate, and typically require movement of the adsorbent into the microwave reactor for regeneration. In addition to system complexity, movement of the adsorbent results in its attrition. The proposed fixedbed adsorption and microwave regeneration system consists of a single adsorption/regeneration vessel with no moving parts except for valves to control the direction of gas flow and a fan to cool the magnetron. This research describes the development of a new microwave-swing adsorption (MSA) system to capture and recover organic vapors from airstreams for reuse. ACFC was used as the adsorbent to capture water vapor, MEK, or PERC from air. Microwave energy was used to regenerate the ACFC. This is the first reported effort to integrate an effective adsorbent, ACFC, with microwave energy for capture and recovery of water and organic vapors. These results are important because this technology can be used to improve ambient air quality, and the pollutants that are emitted to the atmosphere can be recovered for reuse until suitable substitutes for the pollutants are developed and made available as an economically competitive alternative.
2. Experimental Setup and Methodology The MSA system was tested with select compounds having a wide range of polarities and dielectric properties (Table 1). Water was chosen because it is commonly found in air streams, its thermodynamic and electric properties are well documented, and it is safe and easy to handle. MEK is a polar organic compound, which is commonly used as a paint solvent. PERC is a nonpolar compound, which is a common detergent used at dry cleaners. Approximately 90% of the dry cleaners in USA use PERC at their facilities (27). 2.1. Experimental Setup. A bench-scale MSA system was built to include a gas generation system, an adsorption/regeneration vessel with an integrated microwave generator, a gas detection system, and a data acquisition system (Figure 1A). The gas generation system consisted of an air compressor, a high efficiency particle air (HEPA) filter (Gelman Sciences), and mass flow controllers (FC 280, FC 261, Tylan Inc.). The air was dried with a fixed-bed of silica gel before it passed through a custom Goretex membrane-based humidifier to inject water vapor into the air stream for water vapor adsorption tests (30). However, a syringe pump (KDS 200, 6852
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KD Scientific) and a hypodermic needle (100 mL, Hamilton) were used instead of the humidifier to inject reagent grade MEK (Fisher Scientific Inc) or PERC (Aldrich Inc) into the air stream for the organic vapor adsorption tests. About 0.5 g of ACFC (ACC-5092-20, American Kynol Inc.) was attached to the end of the hypodermic needle to dampen fluctuations in the resulting organic vapor concentration. The vessel’s inlet airflow rate was 72 slpm during the adsorption tests, which resulted in an empty-bed residence time of 0.09 s. The adsorption/regeneration vessel was made of aluminum, with an outer diameter of 14.4 cm and a wall thickness of 0.7 cm. The ACFC adsorbent (ACC-5092-20, American Kynol Inc.) was rolled around a Teflon-coated glass fiber mesh to form an annular cartridge with an inner diameter of 6 cm and a length of 45 cm. The main components of the microwave generator consisted of a nominal 1 kW magnetron (2M244, Panasonic), a waveguide, and an aluminum conductor that was terminated by an aluminum plate. The conductor was located coaxially along the centerline of an annular ACFC cartridge with one end installed in the waveguide to allow the microwaves to propagate along the cartridge (Figure 1B), and the other end fixed to the aluminum plate at the bottom of the cartridge. The aluminum plate provided support and a seal for the cartridge. A custom Teflon fitting was used to isolate the vessel from the waveguide and prevent the gas stream from entering the waveguide. The temperature of the ACFC cartridge during regeneration was initially measured intermittently using thermocouples with a diameter of 1.6 mm (1/16”) (Type K, Omega, Inc.). Continuous measurement of the cartridge’s temperature during microwave heating was later achieved using fluoroptic temperature sensors (Metricor 2000, Photonetics). The fluoroptic temperature sensors do not disturb the electric field in the microwave cavity and are not heated by the microwaves. The operating temperature for the fluoroptic sensors (-50 to 200 °C) imposed a maximum temperature during regeneration. Type K thermocouples were also attached to the outer wall of the vessel, at the same vertical positions as the fluoroptic temperature sensors, to monitor the temperature of the vessel. The gas detection system consisted of two relative humidity (RH) sensors (HMT 360, HMP 233, Vaisala) for the case of water adsorption and a photoionization detector (PID) (Modurae PDM 10A, RAE Systems), to detect the organic vapor concentration. The RH meters measured the RH and temperature at the inlet and outlet of the vessel. The PID was used to sample the vessel inlet and outlet at a nominal PID sample flow rate of 140 mL/min. Pressure drop across vessel’s inlet and outlet was measured at select gas flow rates with and without installation of the ACFC cartridge using a differential pressure gauge with minimum division of 0.02 in H2O (Magnehelic 2001, Dwyer Inc.) for a large pressure drop and a differential pressure gauge with minimum division of 0.01 in H2O (Magnehelic 2300-0, Dwyer Inc.) for a low pressure drop. Net pressure drop caused by the ACFC cartridge was determined by subtracting the pressure drop of the vessel without the cloth from the pressure drop of the vessel with the ACFC cartridge. The superficial gas velocity was calculated using the total gas flow rate and the cross-sectional area of the cartridge, which was calculated on the basis of the exposed cartridge length and the average of inner and outer cartridge diameter without consideration of the porosity of the cloth. The outputs from the gas detection system and temperature measurement units were connected to a data acquisition system (Keithley Instruments, Inc.) allowing continuous recording of concentration and temperature measurements. Data were recorded using Labview software at a rate of 10/ min; however, the reported data are provided at a rate of 2/min to 1/10 min to make the plots more readily readable.
FIGURE 1. (A) Schematic of the bench-scale microwave-swing adsorption system. (B) Detailed schematic of the adsorption/regeneration vessel and the porous cartridge. 2.2. Methodology. Metallic thermocouples do not measure temperature reliably in a microwave field due to the heating of the thermocouple by microwaves. However, thermocouples can be used on an intermittent basis when no microwave power is applied. When the microwave power was turned on, the thermocouples were located outside of the vessel. Once the microwave power was turned off, the thermocouples were inserted inside the vessel and contacted with the ACFC. The procedure was repeated throughout the regeneration cycle. Later, cartridge temperature measurements during microwave heating were performed on a continuous basis using the fluoroptic temperature sensors. The RH meters were factory calibrated. The meter used to measure the vessel’s outlet RH was checked against a reference RH meter that was used to monitor the RH at the vessel’s inlet. Negligible (
