Electron beam generated plasma decomposition of 1,1 ... - Springer Link

Plasma Chemistry and Plasma Processing, Vol. 16, No. 4, 1996

Electron B e a m Generated P l a s m a D e c o m p o s i t i o n of 1,1,1-Trichloroethane 1 S. A. Vitale, 2'3 K. Hadidi, 2'4 D. R. Cohn, 2'3 and P. Falkos 2 Received August 16, 1995; revised January 23, 1996

Dilute concentrations o f l,l,l-trichloroethane (TCA) in air were decomposed in an electron beam generated plasma reactor. The energy required for high levels of TCA decomposition (greater than 90%) was determined as a function o f inlet concentration. For 99% decomposition of TCA, e ~ 300 eV/molecule at 250 ppm inlet concentration, and e ~ lOO eV/molecule at 3000ppm. A radical reaction mechanism is proposed which accounts for the formation of the major reaction products." 1,1-dichloroethylene, HCI, chloroacetylchloride, CO,,, and COC12. A model is derived based on first-order inhibited kinetics: a fit of the data to the model shows that at high decomposition fractions, radical scavenging by reaction products is a significant inhibitor of TCA decomposition. KEY WORDS: Plasma-assisted decomposition;electron beam; plasma reactor;

1,1,l-trichloroethane.

1. I N T R O D U C T I O N l,l,l-Trichloroethane (CH3CCI3, TCA) is ubiquitous in the chemical process industries as a solvent, and in the metal machining industries as a degreasing agent. TCA and other volatile organic compounds (VOCs) are now found in hazardous concentrations at many industrial sites, and as contaminants in groundwaterJ ~) Effective methods of decomposing chlorinated organic molecules are vigorously being sought; some of the more widely used methods include: thermal incineration, 12) adsorption, ~'1 catalytic decomposition, ~3-5) thermal plasma processes, ~6-8)and bioreactorsJ 9) In addition to remediation efforts, novel methods of abating the release of these 'This work was supported by the Contaminant Plume Containment and Remediation Focus Area, Office of Environmental Management, U.S. Department of Energy. 2Plasma Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. 3Also affiliated with the Department of Nuclear Engineering. *To whom correspondence should be addressed. E-mail: hadid: @pfc.mit.edu. 651 0272-4324/96/1200-0651 $09.50/0 ~c' 1996 Plenum Publishing Corporation

652

Vitale, Hadidi, Cohn, and Falkos

compounds from current industrial streams are also being examined. Cost effective technology for the destruction of hazardous VOCs becomes increasingly important as restrictions on permissible emission limits become more and more stringent. This paper describes the use of an electron beam generated plasma reactor for the decomposition of low concentrations of 1,1,1-trichloroethane in atmospheric pressure air streams. Electron beams have been used for the decomposition of vinyl chloride, (l°) CC14, (11'12't3) TCE, (lj'j2,13) NOx, (14) SOx, ('5) and other compounds, tt6-2°) The electron beam generated plasma reactor creates a nonequilibrium cold plasma in the reaction chamber. The energy from the beam is directed preferentially toward the production of reactive radical species from the VOC molecules by two possible processes. In the first process, proposed by Slater, ('°) energy is transferred to the species in the gas with the lowest ionization potential through rapid charge transfer with the carrier gas ions, N2 + e-(fast) ~ N f + e-(slow) + e- (fast) N2+ + N2--+N4+

( 1) (2)

N4+ + X-*X + + 2 N2

(3)

X + + e-(slow)-+X'" + X""

(4)

Here, X is a species in the gas with lower ionization potential than the carrier gas (e.g., TCA), and X'" and X'" are reactive radicals produced by dissociative recombination, reaction (4). The second possible process, illustrated by Koch, (12) is one in which the reactive radicals are produced through dissociative electron attachment followed by charge exchange, N: + e-(fast) --+N2+ + e-(slow) + e-(fast)

(5)

X + e-(slow)---,X'" + X"-"

(6)

X"-'+ M~M- +X'"

(7)

Both dissociative recombination and dissociative electron attachment produce the same reactive radicals, so either process may initiate the decomposition reaction. The first mechanism, dissociative recombination, requires relatively fast electrons (>9 eV) to cause ionization of the VOC molecules. Electron attachment, on the other hand, occurs most effectively at low electron temperatures. For example, the dissociative electron attachment cross section for TCA is highest for electrons below 0.1 eV, (z') and the threshold energy for TCA ionization is 1 1.25 eV. (22) If dissociative recombination is the initiation step, then only the fast primary electrons can cause the reaction to occur. If electron attachment is the initiation step, then the reaction

Plasma Decomposition of Trichloroethane

653

proceeds mainly via the secondary electrons produced and via the primary electrons after they slow down by collisions. This study did not attempt to determine which mechanism is predominant in the reactor, but in previous studies,~ ~2.~3.23.24) the authors have proposed that dissociative electron attachment is the more likely mechanism. Since most VOCs have lower ionization potentials than nitrogen or oxygen molecules, and since their cross sections for dissociative electron attachment are higher than for nitrogen or oxygen molecules, the energy from the electron beam is directed preferentially towards dissociation of the VOC molecules into reactive radicals such as CI', no matter which initiation step is predominant. By directing the energy toward the VOC molecules, the cold electron beam generated plasma is more energy efficient for chlorinated VOC decomposition than thermal processes. 2. E X P E R I M E N T A L A P P A R A T U S

The electron beam generated plasma reactor has been described in detail previously. ~2'~23~ The reactor processes gases at atmospheric pressure to allow high throughput operation, and to reduce capital costs. The gas flowrate to the reactor used in this study was varied from 1 to 10 standard liters per minute. Cylinders of Matheson ultrahigh purity air (99.999%) were used to generate T C A inlet streams by passing the air through bubblers containing liquid TCA. This stream was then mixed with another air stream to achieve the desired T C A inlet concentration. The T C A inlet concentration was varied from 100 to 4000 p p m on a m o l / m o l basis. The flowrates of the streams were measured by rotameters calibrated with soap film flow meters. The exhaust gas from the reactor was analyzed using a Hewlett Packard 5890 gas chromatograph and a HP-5971-A mass spectrometer. The mass spectrometer was calibrated using certified calibration cylinders of T C A in air produced by Matheson. The mass spectrometer was recalibrated every few days to ensure that there was no change in the system. Based on the reproducibility of the results, the error in the measurement of T C A concentration is estimated to be +10%. The electron beam generated plasma reactor itself consists of a triode arrangement. The electrons are generated in a vacuum chamber by thermionic emission from a directly heated tungsten filament. The electrons pass through a control grid, which may be biased up to - 1 0 0 V with respect to the filament. The bias on the grid allows control over how many electrons pass through the grid, thus allowing control of the beam current. The electrons are then accelerated by an applied voltage of 100 kV from the control grid, through a 25/,tm aluminum foil window, into the reaction chamber through which the VOC-contaminated air stream flows at atmospheric

654

Hadidi, Cohn, and Falkos

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Electron Beam Dose (Mrad) Fig. I. Relative abundances f r o m mass spectrometer peaks o f the m a j o r reaction products o f T C A decomposition in the electron beam generated plasma reactor. The numbers on the Y-axis are a r b i t r a r y .

pressure. Each electron deposits approximately 17 keV of energy into the air stream before being dumped onto a titanium plate opposite the electron beam window. (231 These dosimetry calculations were performed using a Monte Carlo electron code from the ITS series: TM The maximum electron beam power to the plasma is approximately 25 W. The fast electrons from the beam ionize the nitrogen and oxygen molecules in the carrier gas, creating up to 1000 secondary electrons for each fast electron. The secondary electrons slow down by collisions with the VOC and carrier gas molecules, and initiate the decomposition of the VOC molecules. 3. E X P E R I M E N T A L RESULTS Several decomposition products of TCA were observed experimentally. The primary chlorine-containing product of TCA decomposition is HCI. The primary carbon-containing products of TCA decomposition in the plasma reactor are carbon dioxide, l,l-dichloroethylene, phosgene, and chloroacetylchloride. Several minor reaction products were also detected at the 10's of ppm level : chloroform, trichloroethylene, dichloroacetylchloride, trichl0roacetylchloride, and tetrachloroethane (both 1,1,1,2- and 1,1,2,2-isomers). The reaction mechanism proposed below accounts for the experimental observation of all of the major reaction products, and most of the minor products. The absolute concentrations of the intermediate species were not determined due to a lack of accurate calibration standards, but the relative ion abundances from the mass spectrometer peaks are presented for the major reaction products as a function of electron beam dose in Fig. 1.

Plasma Decomposition

of Trichloroethane

655

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