Environ. Sci. Techno/. 1995, 29, 1961-1965
Acetone Conversion in a Low-Pressure Oxygen Surface Wave Plasma JOSEP A R N O AND J O H N w. BEVAN* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255
MICHEL MOISAN Departement de Physique, Universiti de Montrial, Montreal, Quebec H3C 3J7, Canada
Destruction and removal efficiencies for acetone have been studied in a low pressure (4-8 Torr) oxygen plasma sustained at radio frequencies (rf) by surface waves. The solvent was introduced into the tubular reactor either as a vapor or as a fine mist using an ultrasonic nebulizer. The discharge was generated by a wave launcher external to the reactor tube, and product analysis consisted of on-line flame ionization gas chromatography techniques. Conversion efficiencies approaching 100% were achieved using low powers, millisecond-range residence times, and high solvent concentrations, without the need of the high temperatures required in thermal processes. The dependence of acetone removal on molar ratio, absorbed rf power, and pressure were studied in order to postulate possible reaction pathways and to discern the optimum discharge conditions to further improve destruction and removal efficiencies.
Introduction It is well-known that electromagnetic energy can be used to sustain a discharge in atomic and molecular gases. Such ionized gases have been applied to chemical processes for over a century. Recently, growing concerns over environmental pollution and the subsequent increase of environmental regulations have expanded the use of plasma technology to the removal of chemical waste. Plasma applications for the treatment of hazardous waste include the destruction and removal of greenhouse gases ( I - @ , toxicchemicalagents (7-101, hydrogensulfide (111,nuclear waste calcines (121,and liquid and gas phase halocarbons and hydrocarbons (13-25). Thermal and Nonthermal Plasmas. In plasma processing, a distinction is made between thermal and nonthermal (cold) plasmas. Plasma arcs and atmospheric pressure inductively coupled plasmas (ICPs) are typical examples of the first type. The destruction efficiency of thermal plasmas like the combustion processes occurring in an incinerator, depend upon generating high enough temperatures to produce and propagate free radicals such as OH, 0,and H. These highly reactive species are known to efficientlybreak intramolecular bonds of organic species.
0013-936X/95/0929-1961$09.00/0
1995 American Chemical Societv
Unfortunately, very high temperatures are needed to promote effective combustion. This requires burning fuel, in the case of the incinerators, or alarge supply of electrical energy, in the case of thermal plasmas. Either method is an inefficient way to produce radicals. Nonthermal plasmas are an alternative way to create highly reactive species. In these “cold”plasmas, electrons are accelerated by the imposed electric field, transferring their energy through elastic and inelastic collisions with neutral molecules. The reactions that take place under these conditions are far from thermodynamic equilibrium, resulting in destructionkinetic rates associatedwith electron temperatures of over 10 000 K while the neutral gas remains near ambient room temperature. This mechanism concentrates the input energy in producing radicals and atomic and molecular excitedspecieswithout adding the enthalpy associated with heating the entire gas mixture. High-Frequency Plasmas. Radio (rf; 1-300 MHz) and microwave (300 MHz-300 GHz) frequencies have been consolidated and utilized as one of the leading energy sources for sustaining plasmas. We shall designate them collectively as high-frequency (hf) sustained discharges. Compared to various dc discharges,it is now realized that, for research and engineering purposes, hf discharges (especially in the microwave region) are generallyeasier to handle, more efficient, less expensive, and more reliable sources for both fundamental studies and technical applications (26,27).For decades, high-frequency discharges were mostly maintained either in resonant cavities,within coils,or between metal electrodes located inside or outside a discharge vessel. In all these plasma sources,the discharge takes place within the hf circuit acting as a field applicator, resulting in a localized active zone. Only recently has an innovative method of producing hf discharges been proposed that extends beyond the applicator region, hence increasing the available plasma volume. Such a plasma is sustained by the propagation of an electromagnetic wave that uses the plasma column and its surrounding dielectric envelope as the sole propagating medium. These waves are called surface waves (SWs) because they carry their energy predominantly within a small distance from the plasma-dielectric tube interface; these waves seem to be attached to the boundary along which they propagate, tending to follow this boundary even when it is curved. This type of traveling wave discharge produces stable plasmas for a large range of pressures (W4Torr to a few times atmospheric pressure depending on tube diameter), frequencies (1 MHz- 10 GHz), discharge tube diameters (0.5->150 mm), and hf powers (5->3 kw). The surface wave can be excited by very compact wave launchers developed and patented by Moisan and Zakrzewski (28341, which need only to surround a small portion of the discharge tube. The plasmas obtained are easily reproducible and quiescent with low electron density fluctuations. As already mentioned, since the launched wave propagates along the discharge axis, the plasma column produced can extend many times the launcher size, substantially increasing the residence time of the ionized gases traveling within. In spite of its recent development, surface wave discharges have found many applications including materials processing, ion sources, lasers, el-
VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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kHz, VCXGOO power supply manufactured by Sonic &
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FIGURE 1. Diagram of low-pressure wrhce wow p l m m mactor.
emental analysis,andlighting. Descriptive characterhtion ofsurfacewaveplasmasiswe~advanced, andmanyreviews of their properties and applications can be found (35-39). In recent years, theoretical collisional-radiative (C-R) models have been developed for a few simple gases such as He, AI,and 02 (40) in order to understand the mechanisms leading to ionizationsin hf discharges at intermediate (0.1to afewTorr)pressures. ThesetheoreticalSNdietudies have been confirmedby Granierand co-workers(41)using VW absorption techniques to measure the concentration of atomic, molecular, and ionic species created by a lowpressure oxygen surface wave discharge. These sNdies revealed that the concentrationsof metastable OAa'A) and atomic oxygen are on average 1000 times larger than the concentrations of ozone ( 0 3 ) and 0-and 10 000 times greater than concentration of O2(b'2l). It is therefore safe to assume that Oz(alA)and OeP) play an important role in the conversion of organic species into environmentally innocuous molecules. Inthis paperwewilldiscussthedestructionandremoval of acetone in a low-pressure surface wave radio frequency 02plasma Percent conversionoftheorganiccontaminant has been studied using different solvent introduction techniques,at differentreactant mole ratios, pressures,and absorbed hf powers.
Experimental Ssctioi The experimental apparatus used in this study can be divided into three separate components. The feed system consistedofeithervapororul~nicatomizingtechniques to introducetheorganicsolventandaflowmeteredoxygen feed. The high-frequency plasma was produced using a radio frequency wave generator, a power ampliIier, a matching box, and a surface wave launcher. Product analysisincluded an on-line gas duomatograph (0 loaded with automaticsampleloop injection,two columns, a dual flame ionization @ID)detector, and thermal conductivity detector. A block diagram of the reactor system is shown in Figure 1. GasandUquIdIntmduclion. Theeoxygenflowwasfixed throughout the sNdy at a rate of 2.4 L(STP/min) and controUedusingaMathewn603 flowmeter. In the mtset of experiments, gaseous acetone was introduced into a 13 mm outer diameter (o.d.), 10.5 m m inner diameter (id.). Pyrex Nbular reactor as a vapor via a 602 Matheson flowmeter. Inthesecondset ofexperimentsliquidacetone was nebulized into a 25 m m o.d., 22 mm id. Pyrexreactor tube usinganultrasonicatomizingprobebackedwitha20 'lest. ENVIRONMENTAL SCIENCE 5 TECHNOLOGY I VOL. 2% NO. 8.1986
Materialshc. The organicliquidwasfedintothenebulizing probe from a calibrated volumetric cylinder and its flow regulatedattheinletvalve.The actual flowwas determined by measuring the volume drop in a set amount of time. In all experiments,the ultrasonicnebulizer required less than 100 W to operate and its function was not affected by discharged rf power. Due to the relativelowgastemperatwesreachedinnlowpressure plasmas, total vaporization of the reactants is not always readily achieved (4.3. Smallvolumes of high vapor pressure organic liquids can be easily introduced into a low-pressure reactor as vapor without disturbing the discharge. Higher flow rates and low vapor pressure compounds can be nebulized into the reactor. However, the stability of the discharge depends on the droplet size of the particles flowingwithin the reactorvolume. We have studied and tested different large-volume liquid sample introductiontechniquesand will discuss the advantageous characteristics of two of them. On the one hand, a glass pneumatic nebulizer constructed with a spray chamber with a drop size selection of approximately 5 pm (43) resulted in an inexpensive and reliable sample introduction device. The major drawback of using this tecbnique is the interdependencebetween thevolume of liquid introduced, the reactor pressure, and the gas flow. We favoredthe use of an ultrasonic atomizer since it supplied a fine, dense, and reproducible aerosol maintaininga consistently small droplet size, while allowing an independent adjustment of the liquid flow rate. RadioPquencyPlasmaReaetor. The40MHzsurface wave was generated using a Hewlett Packard 3200 B Vhf oscillator, and its amplitude was magnified by a AlOOO broad-band rf power ampMier (Electronic Navigation Instruments) with the capability to produce up to 1.8 kW of power. The surfacewave was launched into the tubular flow reactor via an air-cooled Ro-Box (30). Both incident power to the launcher PI and the power reflected PRwere monitored using a 4527 Thruline wattmeter. The load impedance was matched with a tunable LC matching network (29).We assumed P,- PRas the power absorbed in the plasma. Under normal conditions, the measured reflected power remained between 0.1% and 1%of the delivered power, reqiliring minimal adjustment with changing conditions and between runs. The overall system was leak checked and kept under low pressure by utilizing a two-stage EM-175 Edwards pump. Product Analysis. The reactor effluent gases were analyzed using an on-line gas chromatograph for quantitative determination of acetone. The sample was collected bydrawinga'/Iin.sidestreamfromthereactorandcooling a previously evacuated (to a few milliTorr) 'IS in. stainless steel loop with a liquid nitrogen (77 K) bath. After cryotrapping for a k e d amount of time, the trap was pressurized with helium and heated to 373 K with boiling water. This sample coUection technique is similar to the one reported by Barat and Bozzelli (16)in their analysis of the reaction products of a low-pressure microwave discharge. The sample was then allowed to lillan evacuated 10 mL sample loop until it reached slightly over 1 a m and was injected into a 14-AShimatzu gas chromatograph. We used helium as carrier gas, and an air/Hz flame ionization detector. The products were separated using a 60180 Carbopack B/IW SP-1000, 3 m long, '/e in. diameter stainless steel column. The oven was programmed to
TABLE 1
Plasma Reactor Operating Conditions solvent intro, reactor 0.d.
pressure (Torr)
plasma length (cm)
acetone vapor flow (ml,/min)
energy density' (LJIL)
feed ratiob
residence timeC(ms)
YO acetone
(W) 210 210 210 220 220 220 220 340 340 340 340 220 220 220 220
2.5 2.6 2.7 3.4 3.9 4.2 4.1 4.1 4.4 4.4 4.6 7.9 8.0 8.0 8.0
36 34 34 51 41 36 20 51 48 48 41 41 38 33 28
12.5 30.0 59.9 183 682 1057 1280 305 701 792 1280 161 387 542 a07
5.22 5.18 5.12 5.11 4.28 3.81 3.59 7.5 6.6 6.39 5.54 5.15 4.74 4.49 4.12
188 78 39 12.8 3.4 2.2 1.8 7.7 3.4 3.0 1.8 14.5 6.1 4.3 2.9
2.0 1.9 2.1 22.1 17.2 14.2 7.4 25.5 23.1 22.1 17.8 41.9 36.5 29.9 23.2
>99.8gd >99.97d >99.99d 84.7 70.8 40.6 < 40 97.4 71.9 57.6 < 50 99.2 95.0 94.0 48.0
power
vapor, 13 mm atomized, 25 mm
atomized, 25 mm
atomized, 25 mm
Power/overall flow. Molar ratio O2/C3H8O. Computed average time a molecule spends in the plasma. detection limits of analytical instrumentration.
maintain 40 "C for 3 min and to ramp at 32 "Clmin until it reached 220 "C. All sample transport lines and loops were purged, heated, and evacuated to a few milliTorrusing a Welch dual-seal vacuum pump before filling them with the sample gas. Background samples were systematically run before each injection to ensure minimum adsorption of species emanating from the transfer lines. It is worth mentioning that the instrument was set up to simultaneously inject a second sample through a Carbosieve column and into a thermal conductivitydetector (TCD) in order to analyze permanent gases. Unfortunately, the low sensitivity of the detector, and the fact that most of these gases do not get trapped at liquid nitrogen temperatures, prevented us from quantitatively analyzing these products.
Results and Discussion Acetone destruction as a function of contaminant flow rate was studied using two distinct absorbed rfpowers (220and 340 W) and under different reactor gas pressures. Plasma reactor conditions employed in these studies are summarized in Table 1. As previously reported (291, the observed plasma column length increased with increasing input power and decreased with increasing pressures. Moreover, for a fured pressure and power, the plasma shortened as the acetone flow was increased. Residence times are computed as a function of plasma length and gas velocity. The latter was determined using the following expression (44): u, (m/s) = 4.25 x 10-4(QT/Ap)
where Q is the gas flow rate in standard cubic centimeters per minute, Tis the temperature in kelvin, and A and p are the area (cm2)and pressure (Torr) of the reactor, respectively. A calibration curve was plotted to ensure linearity between acetone concentration and detector response and used for further calculation of solvent removal. The percent acetone conversion (v)was determined using the following expression:
'
=
[area,,] - [area,,] x 100 [area,,]
convers
Remaining acetone was below the
where [arealis the normalized integrated acetone peak from the gas chromatograph and the on and off subscripts denote with and without the discharge, respectively. In all cases the formed byproduct concentrations detectable with the used analytical instrument (Le., C;! or larger) remained significantly less than 10% of the concentration of the remaining acetone. This fact was corroborated by previous work done in our group, where the liquid byproducts from oxygen/organic (ketones,aromatics, chlorohydrocarbons) surface wave plasmas were systematicallytrapped in liquid nitrogen and injected in a GUMS for identification. Furthermore, in these previous studies, gaseous products where analyzed using a Fourier transform infrared spectrometer (FTIR)showing high concentrations of COz,H20, and HCl when chlorinated reactants where used, and CO depending on the 0 2 concentration and organic reactant. The reactor used in this study demonstrates greater than 99.99% acetone removal efficiencies at low acetone flows (between 12 and 60 mL(STP)/min) using a 13 mm 0.d. reactor (see Table 1). In spite of the low residence times and the very low rf powers absorbed, the reactor's residual acetone concentrations remained below the detection limits of the analytical detector and we could only estimate the lower limits of conversion. At the relatively high flow volumes of liquid solvent introduced in the second part of our study, the data shown in the Table 1 have been fit satisfactorily using an exponential expression. The resulting analytical functions were used together with the data in Figures 2 and 3 to verify the decay of acetone conversion with decreasing molar ratios ( [ 0 2 1 / [acetone]), This can be explained by realizing that increasing the acetone flow at fured 0 2 concentration diminishes the residence time by both shortening the plasma column and accelerating the gas flow. Second, as the ratio between reactants approaches unity, the probability that electrons will effectivelycollide with 02(X3Z)to form the reactive species decreases. The next two sections will relate the effects that pressure and power have on the conversion efficiency of acetone. Effect of Pressure. Pressure in the reactor was increased by adjusting a flow-limiting valve located in front of the vacuum pump. This flux regulation decreased the velocity of the flowing gas, almost doubling its residence time. The VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1963
BO
4 torr
.-
E
60
I
40
x? -
1
1
3ot 20
I
lot
O
1
ol 0
5
20
15
10
Molar Ratio [02/C3H60] FIGURE 2. Acetone convarsion varsus molar ratio at different pressures in a surface wave plasma reactor. 100
90
$ 8
T
+
340
/-+
4 0 i
__ _ _ a--220 w
-
-
--
OI
30;
0(1
z10o ] 0
i
0
-
---
1 -
10
5
Molar
15
20
Ratio [02/C3H60]
FIGURE3. Acetone conversion varsusmolar ratio at different powers in a surface wave plasma reactor.
effect of doubling the reactor pressure on conversion rate is depicted in Figure 2. While the plotted results display similar acetone removals below molar ratios of 3 at lower solvent concentrations,the conversionsat reactor pressures of 8 Torr are on average 20%above (over 99%removal) the conversions when the reactor pressure is maintained at 4 Torr. For a fxed discharge frequency and reactor radius, increasing the pressure has essentially the effect of enhancing the electron density at the gap and the axialgradient of this density, resulting in a shorter plasma column. For the specific case of an oxygen surface wave discharge, it has been shown (40,411that the concentration of Oz(alA) is independent of the reactor pressure and remains about 12% of the initial 0 2 concentration. The same study concludes that higher pressure favors the dissociation kinetics of molecular oxygen (both alA and X3X),resulting in higher concentrations of atomic oxygen (ranging from 4% to 10%for pressures between 0.5 and 2 Torr in a 390 MHz, 84 W sustained plasma). Highly reactive O3remains nearly constant within that pressure range and its concentration 3 orders of magnitude below the concentration of O(3P). Assuming a 10%dissociation of 02,an 021C3HsO molar ratio of 10 would yield a 2:l 0(3P)/C3Hs0stoichiometrical ratio needed for a full acetone conversion following the reaction C,H,O
+ ~ o ( ~ +P excess ) 0, 3C0,
+ 3H,O + excess 0,
To a first approximation, these results are consistent with our experimental results and emphasize the importance of atomic oxygen in the process of organic removal. Undoubtedly,the real process is much more complex and must await further investigation. A more realistic global 1964
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8.1995
model has to take into account the effects of longer residence times achieved by decreasing the gas velocities within the reactor, the higher temperatures of both gases and reactor wall, and the electron collisions with organic molecules responsible for the generation of the small and yet detected organic byproducts. Effects of Power. Acetone removal has been studied at 220 and 340 W of absorbed radio frequencypower in order to investigate the effects of power on conversion efficiency. In this study, a power increase from 220 to 340 W resulted in a 15 to 50%extension of plasma column length depending on the molar feed ratios. Reactor conditions are summarized in Table 1 and compared in Figure 3. The results, which are similar to the previous section, reveal an increase of percent conversion at higher powers above a molar ratio of 4. As the molar ratio approaches unity, the curves decay rapidly regardless of the amount of power used. It has been demonstrated(39)that increasing the absorbed power in a surface wave plasma column extends its length while keeping the same axial electron density gradient down to the end of the plasma column. Higher electron densities raise the gas and reactor wall temperature and increase the electron concentration, improving the electron-molecule collision rate. Ferreira (40) and Granier et al. (41)determined that the concentration of singlet oxygen Oz(ald)in a surface wave discharge was independent on the amount of power delivered to the reactor and again remained close to 12% of the ground state oxygen concentration. The concentrationof atomic oxygen, on the other hand, almost increased 1 order of magnitude when the electron density increased from 2 x 1Oloto 2 x 10” ~ m - Our ~ . results appear consistent with these previous studies as an increase of power would generate more electrons to dissociate and excite larger concentrationsof molecular oxygen,which in turn would react with the target species. However, this effect would be in part offset by an increase in the recombination frequency of 0 on the reactor wall due to greater wall temperatures. Further studies are needed to ascertain the optimum reactor conditions in terms of plasma operating frequency (applicationof radio versus microwave frequencies),reactor pressure limit and its dependence on product distribution, and the effects of surface wave plasma’snonuniform radial electron energy distribution at high flow rates. Furthermore, detailed experimental and theoretical electron kinetic studies involving complex systems are needed in order to fullyunderstandthe processes that take place in nonthermal plasmas containing oxygen and organic waste molecules. The experiments in this study demonstrate the applicabilityof convertingorganic molecules in a low-pressure oxygen surface wave plasma. In spite of the large solvent concentrations introduced, the low powers consumed,and the small residence times, high destruction percentages were obtained. Moreover, the process did not require the addition of expensive rare gases (Le., argon or helium) to produce a low-power, stable plasma. These aforementioned characteristics make low-pressure surface wave discharges an attractive alternative to the remediation of organic solvent contaminated airstreams.
Acknowledgments We acknowledge the TexasAdvanced Technology Research Program (TATFP), the Office of the Vice President for Research and Associate Provost for graduate studies, through the Center for Energyand Mineral Resources,Texas
A&M University, and Rf Environmental Systems for providing the financial support for this research. We also acknowledge the Natural Sciences Engineering Research Council of Canada (NSERC) and FCAR: Fonds “Formation des Chercheurs et Aide la Recherche” (Qu6bec) for the funding of M.M. The authors are grateful to Dr. B. A. Wofford, R. Grenier, and G. Sauv6 for discussions and to J. B. Miller for technical assistance with the ultrasonic nebulizer.
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Applications to Waste Treatment Workshop, Idaho Falls, ID, 1990. (22) Rosocha, L. A,; McCulla, W. H. 44thAnnual Gaseous Electronics Conference, Albuquerque, NM, 1991. (23) Rosocha, L. A.; McCulla, W. H.; Anderson, G. K.; Coogan, J. J.; Kang, M.; Tennant, R. A,; Wantuck, P. J. Proceedings of the 11th International Incineration Conference. Albuquerque, NM, 1992; p 179. (24) Storch, D. G.; Kushner, M. J. J. Appl. Phys. 1993, 73(1), 51. (25) Rosocha, L. A,; Anderson, G. K.; Bechtold, L. A.; Coogan, J. J.; Heck, H. G.; Kang, M.; McCulla, W. H.; Tennant, R.A.; Wantuck, P. J. NATO ASI 1993, 281. (26) Techniques and Applications of Plasma Chemistry, Hollahan, J. R., Bell A. T., Eds.; J. Wiley & Sons: New York, 1974. (27) GaseousElectronics;Mac Donald A. D., Tentenbaum, S. J., Hirsch, M. N., Oskam, H. J., Eds.; Academic: New York, 1978. (28) Zakrzewski, Z.; Moisan, M.; Glaude, V. M. M.; Beaudry, C.; Leprince, P. Plasma Phys. 1977, 19, 77. (29) Moisan, M.; Zakrzewski, Z. 1.Phys D: Appl. Phys. 1991,24, 1025. (30) Moisan, M.; Zakrzewski, Z. Rev. Sci. Instrum. 1987, 58, 1895. (31) Moisan, M.; Zakrzewski, Z. Canadian Patent 1,246,762, 1988. (32) Moisan, M.; Leprince, P.; Beaudry, C.; Bloyet, E. US Patent 4,049,940, 1977. (33) Moisan, M.; Zakrzewski, Z. US Patent 4,810,933, 1989. (34) Moisan, M.; Zakrzewski, Z. US Patent 4,906,898, 1990. (35) Alves, L. L.; Gousset, G.; Ferreira, C. M. J. Phys. D: Appl. Phys. 1992, 25, 1713. (36) Rakem, Z.; Leprince, P.; Marec, J. J. Phys. D: Appl. Phys. 1992, 25, 953. (37) Moisan, M.; Ferreira, C. M.; Hajlaoui, Y.; Henry, D.; Hubert, J,; Pantel, R.; Ricard, A.; Zakrewski, Z. Rev. Phys. Appl. 1982, 17, 707. (38) Microwave Discharges; Moisan, M., Ferreira, C. M., Eds. NATO ASI Ser., Ser. B, 1993, 302. (39) MicrowaveExcitedPlasmas; Moisan, M., Pelletier, J., Eds.; Elsevier Science Publishers: Amsterdam, 1992. (40) Ferreira, C. M. In Microwave Discharges; Moisan, M., Ferreira, C. M., Eds.; NATO AS1 Series 302; Plenum Press: New York, 1993; p 313. (41) Granier, A.; Pasquiers, S.; Boisse-Laporte, C.; Darchicourt, R.; Leprince, P.; Marec, J. 1.Phys. D: Appl. Phys. 1989, 22, 1487. (42) Hieftje, G. M.; Norman, L. A. Int. J. Mass Spectrom. Ion Processes 1992, 1181119, 519. (43) Koropchak, J. A. Spectroscopy 1993, 8(8),20. (44) Granier, A. In Microwave Discharges; Moisan, M.; Ferreira, C. M., Eds.; NATO AS1 Series 302; Plenum Press: New York, 1993; 302, p 491.
Received for review October 20, 1994. Revised manuscript received March 16, 1995. Accepted April 10, 1995.@
ES940647H @Abstractpublished in Advance ACS Abstracfs, May 15, 1995.
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