Aerosol and Air Quality Research, 11: 265–281, 2011
Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online
doi: 10.4209/aaqr.2011.02.0014
Decomposition of Methyl Tert-Butyl Ether by Adding Hydrogen in a Cold Plasma
Reactor
Lien-Te Hsieh
1,2*, Cheng-Hsien Tsai
3, Juu-En Chang
4, Meng-Chun Tsao
41 Department of Environmental Science and Engineering, National Pingtung University of Science and Technology, 1
Shuefu Fu Road, Pingtung 912, Taiwan
2 Emerging Compounds Research Center (ECOREC), National Pingtung University of Science and Technology, 1 Shuefu
Fu Road, Pingtung 912, Taiwan
3 Department of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807,
Taiwan
4 Department of Environmental Engineering, National Cheng Kung University, Tainan 701, Taiwan
ABSTRACT
Methyl tert-butyl ether (MTBE) is extensively used as an oxygenate and octane enhancer in gasoline. Its release to the environment has generated great public and governmental concern. In this study, we give a brief review of the decomposition of air toxics by the application of radio frequency (RF) plasma reactors and then present our study on decomposition of methyl tert-butyl ether by adding hydrogen in a cold plasma reactor. Based on our references, there are four types of the application in the RF plasma reactors are discussed, including: (i) Application I.: Converting methane, decomposing carbon dioxide, ethoxyethane, and ethylene oxide; (ii) Application II.: Decomposing methyl chloride, 1,1-C2H2Cl2, and CH2Cl2; (iii) Application III.: Decomposing dichlorodifluoromethane, CHF3, CH2F2, CCl2F2, and BF3;
(iv) Application IV.: Decomposing dichlorodifluoromethane, CH3SH, CS2, SF6, and SF6 + H2S mixture. Moreover, this
study demonstrates the feasibility of applying a radio frequency (RF) plasma reactor for decomposing and converting MTBE. Experimental results indicate that the decomposition efficiency (ηMTBE) and the fraction of total input carbon
converted into CH4, C2H2 and C2H4 (FCH4+C2H2+ C2H4) increased with the input power and decreased as both the H2/MTBE
ratio and the MTBE influent concentration in the MTBE/H2/Ar plasma environment increased. Interestingly, applying
radio frequency plasma to the decomposition of MTBE while adding hydrogen constitutes alternative method of decomposing and converting MTBE into CH4, C2H4, C2H2, iso-butane and iso-butene.
Keywords: Methyl tert-butyl ether (MTBE); Radio-frequency (RF); Plasma; Decomposition; Reaction.
INTRODUCTION AND A BRIEF REVIEW OF DECOMPOSITION OF AIR TOXICS IN RF PLASMA SYSTEM
Traffic-related pollutants have received a great deal of interest due to their inherent toxicities and possible heterogeneous reactions with other components in the atmosphere (Saunders et al., 1997; Cheng et al., 2010; Chuang et al., 2010a, b; Shen et al., 2010; Han et al., 2011; Ma et al., 2011). Cheng et al. (2010) investigated the gas/particle partitioning of dioxins in exhaust gases from automobiles. In their study, 6 sport utility vehicles (SUVs), 6 diesel passenger vehicles (DPVs), and 3 heavy duty diesel vehicle (HDDV) were examined using chasis dynamometer
Corresponding author. Tel.: 886-8-774-0521;
Fax: 886-8-774-0256
E-mail address: Lthsieh@mail.npust.edu.tw
tests for measuring vehicular dioxin emissions. They indicated the mean PCDD/F I-TEQ emission factors were 0.101, 0.0688 and 0.912 ng I-TEQ/km for the SUVs, DPVs and HDDV, respectively. Moreover, high concentrations of benzo(a)pyrene (BaP) and benzo(a)pyrene-equivalent carcinogenic power (BaPE) demonstrated the potential health risk of traffic-related PAHs at the roadside area in Xi’an. The ratio of between indeno(l,2,3-cd)pyrene (IP) and benzo(g,h,i)perylene (BghiP) is also used to test the primary source of PAHs. The ratio of IP/(IP + BghiP) and IP/BghiP indicated that the primary source of PAHs was gasoline vehicles at this roadside area (Shen et al., 2010).
Methyl tert-butyl ether (MTBE), an oxygenate, has become a common ingredient in gasoline, because of its excellent octane rating and positive contribution to air quality in large cities (U.S. EPA, 1988; 1993; Davidson and Creek, 2000). MTBE has been successfully added to gasoline to meet increasingly tough laws on liquid fuels. The positive effects of MTBE include a reduction in
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particulate emissions, unburnt hydrocarbons, CO and exhaust emissions. However, both the pollution of the environment by MTBE and the possible adverse effects of exposure to MTBE are of public concern (Johanson et al., 1995; Lacy et al., 1995; Brown, 1997; Lillquist and Zeigle, 1998; Nihlen et al., 1998; Hong et al., 1999; Deeb et al., 2001).
Johanson et al. (1995) measured the blood, water, and olive oil/air partition coefficients in vitro of MTBE, ETBE, TAME and tertiary butyl alcohol (TBA), a metabolite of MTBE and ETBE. Moreover, Johanson et al. (1995) exposed 10 healthy male volunteers to MTBE vapor at 5, 25 and 50 ppm for 2 h during light physical exercise. Their results show that the concentration of MTBE and TBA in blood was proportional to exposure level suggesting linear kinetics up to 50 ppm. The half life of 7–10 h in blood and urine indicates that TBA would be more suitable than the parent compound as a biomarker for MTBE exposure (Johanson et al., 1995). Additionally, MTBE was banned as a gasoline additive in California in 2003 and New York is likely to act similarly (Governor, 2003). Potential technologies for decomposing or removing MTBE have attracted much interest globally the world (Trotta and Miracca, 1997; Fields et al., 1998; Anderson, 2000; Chambreau et al., 2000). Plasma technology is preferred over biodegradation, which is time-consuming, for decomposing MTBE.
RF plasma is a branch of nonequilibrium plasma. Since both electrons and some activated ions therein have more kinetic energy than typical molecules, RF plasma is also called cold plasma (Eliasson and Kogelschatz, 1991). An advantage of using the RF plasma reactor is that a conventional reaction therein can be completed at a lower temperature elsewhere (Hsieh et al., 1998a). Many studies have demonstrated that the destruction or treatment of hazardous gases by applying RF plasma technology is very practical (Lee et al., 1996; Li et al., 1996; Hsieh et al., 1998a, b, c; Wang et al., 1999a, b; Liao et al., 2000; Wang
et al., 2000; Hsieh et al., 2001; Liao et al., 2001; Tsai et al.,
2001; Shih et al., 2002; Tsai et al., 2002; Shih et al., 2003; Tsai et al., 2003; Wang et al., 2003; Tsai et al., 2004; Liao
et al.,2005; Wang et al., 2005a, b; Hsieh et al., 2007; Tsai et al., 2007). The above mentioned cited articles (from
1996 to 2007) show that RF power is delivered through the power meter and the matching unit to an outer copper electrode that is wrapped around the reactor, the other electrode is earthed. The above mentioned system is generally inductively coupled type. It means that, in such system, the external electrode and the glass reactor wall beneath it, together with the conductive plasma inside the reactor, generated a capacitor that enabled capacitive coupling of RF power into the discharge. In order to insight the RF plasma technology, a brief review of the decomposition of air toxics by the application of radio frequency plasma reactors is presented as follows. Based on our references, there are four types of the application in the RF plasma reactors can be discussed, including: (i)
Application I.: Converting methane, decomposing carbon
dioxide, ethoxyethane, and ethylene oxide; (ii) Application
II.: Decomposing methyl chloride, 1,1-C2H2Cl2, and CH2Cl2;
(iii) Application III.: Decomposing dichlorodifluoromethane, CHF3, CH2F2, CCl2F2, and BF3; (iv) Application IV.:
Decomposing dichlorodifluoromethane, CH3SH, CS2, SF6,
and SF6 + H2S mixture.
Application I. Converting Methane, Decomposing Carbon Dioxide, Ethoxyethane, and Ethylene Oxide
Application of radio-frequency (RF) plasma as an alternative technology for converting methane or decomposing carbon dioxide, ethoxyethane, and ethylene oxide are demonstrated. Several research articles are discussed as follows:
(1) Converting Methane by Using an RF Plasma Reactor
Hsieh et al. (1998a) reported a radio-frequency (RF) plasma system was used to convert methane gas. The reactants and final products were analyzed by using an FTIR (Fourier transform infrared spectrometer). The effects of plasma operational parameters, including feeding concentration (C) of CH4 , operational pressure (P) in the
RF plasma reactor, total gas flow rate (Q) and input power wattage (W) for CH4 decomposition were evaluated. The
results showed that the CH4 decomposition fraction
increases with increasing power input, decreasing operational pressure in the RF plasma reactor, decreasing CH4 feeding concentration, and decreasing total gas flow
rate (Hsieh et al., 1998a).
(2) Decomposition of Carbon Dioxide in the RF Plasma Environment
Application of radio-frequency (RF) plasma as an alternative technology for the decomposition of carbon dioxide with methane gas is demonstrated (Hsieh et al., 1998c). The results revealed that in CO2/CH4/Ar plasma,
the best decomposition fraction of carbon dioxide was 60.0%, which occurs around 316°C in the condition designed for 5% feeding concentration of CO2, 5% feeding
concentration of CH4, 20 torr operation pressure, 100 sccm
total gas flow rate and 90 watts input power wattage. The CH, CH2 and CH3 radicals obtained from the destruction of
CH4 could result effectively in high decomposition of CO2
in the plasma reactor (Hsieh et al., 1998c). Hsieh et al. (1998c) show the optimal mathematical models based on the experimental data obtained and tested the proposal model by means of sensitivity analysis, which shows that the input power wattage (W) was the most sensitive parameter for the CO2 decomposition.
(3) Decomposition of Ethoxyethane in the Cold Plasma Environment
For ethoxyethane (EOE) contained gas, a radio frequency (RF) plasma system was also used to decompose the EOE contained gas (Liao et al., 2000). The reactants and final products were analyzed by using an FTIR (Fourier Transform Infrared) spectrometer. The effects of plasma operational parameters, including input power wattage (W), equivalence ratios (Φ), feeding concentration (C) of EOE and total gas flow rate (Q) for EOE decomposition were
Hsieh et al., Aerosol and Air Quality Research, 11: 265–281, 2011 267 also evaluated. Liao et al. (2000) gave the possible
reaction pathways for EOE decomposition and the formation of final products in the study. They suggest that at lower input power wattages, the creation of glow discharge is strongly dependent on the plasma production index (PPI). In their study, when input power wattages are smaller than 30 W, the minimum values of PPI to create glow discharge ranged between 18.2 and 19.0. Their result demonstrates that the decomposition fraction of EOE could reach 100% under most operational conditions in the RF plasma reactor (Liao et al., 2000).
(4) Decomposition of Ethylene Oxide in the RF Plasma Environment
For ethylene oxide (EO) contained gas, a radio frequency (RF) plasma system was also used to decompose the ethylene oxide (EO) contained gas in the EO/Ar, and EO/O2/Ar system, respectively. In the RF plasma system,
due to the importance of the high-energy electrons, the EO decomposition fraction in plasma reaction increased with decreasing operational pressure, while that of thermal reaction, reported by previous investigations, increased with increasing operational pressure (Liao et al., 2001). However, owing to the electrophilic characteristic of oxygen atoms in the EO molecule causing the effect of electron attachment, in conditions of higher EO feeding concentration, the pressure dependence became the same for both plasma- and thermal-reaction (Liao et al., 2001). The EO oxidation reaction has also been investigated; the result shows that EO almost completely oxidized at 600–692 K gas temperature. The main products for the EO/Ar system are CO, CH4, C2H6, C2H4, and C2H2, and those for the
EO/O2/Ar system are CO2 and H2O (Liao et al., 2001).
Liao et al. (2005) report an innovative method was used to simulate ethylene oxide (EO) oxidation in an RF plasma reactor. The objective of this work was to simulate the stable species mole fraction profiles measured in a flowing plasma system at constant temperature and pressure (Liao
et al., 2005). They concluded some crucial points and we
cited those important statements as follows.
● “The decomposition reactions for EO were changing
with varying O2/EO ratios in the complex plasma
system. The most important reaction with an O2/EO
ratio of zero was the electron dissociation reaction of EO, C2H4O + e- → CH3CHO + e-. However the most
significant reaction with an O2/EO ratio of 5.0 was
the formation reaction of HO2, which forms OH
radicals further, then enhances the decomposition of C2H4O by the reaction, C2H4O + OH = C2H3O +
H2O” (Liao et al., 2005).
● “The detail reaction pathways for decomposition of
EO at various O2/EO ratios in the RF plasma reactor
tell us that the CH3 radical goes around a reaction
loop to form significant amount of C2H6 at zero of
O2/EO. And the loop reaction for CH3 to form C2H6
has been stopped at 5.0 of O2/EO, and instead of
forming C2H6, CH3 forms the intermediate CH2O,
then it reacts further to become the final product CO2” (Liao et al., 2005).
Application II. Decomposing Methyl Chloride, 1,
1-C2H2Cl2, and CH2Cl2
Application of radio-frequency (RF) plasma as an alternative technology for the decomposition of methyl chloride, 1,1-C2H2Cl2 contained gas, and CH2Cl2 is also
demonstrated. Several research articles are discussed as follows.
(1) Decomposition of Methyl Chloride in the RF plasma environment
Application of radio-frequency (RF) plasma as an alternative technology for the decomposition of methyl chloride (CH3Cl) with oxygen is demonstrated by Hsieh et
al. (1998b). Hsieh et al. (1998b) concluded that in the
CH3Cl/O2/Ar plasma, the decomposition fraction of CH3Cl
was over 99.99%, which occurred around 440°C in the condition designed for 3% of CH3Cl feeding concentration,
1.0 of equivalence ratio (φ), 20 Torr of operation pressure, 100 sccm of total gas flow rate and 100 watts of input power wattage. They indicated that the CH3Cl
decomposition fractions increase when the equivalence ratio is raised to values around or below stoichiometry; and then decrease either sharply (50 W) or moderately (70 W) when the equivalence ratio goes up or down to values exceeding stoichiometry (Hsieh et al., 1998b). However, there is no significant difference for those in higher input power wattages (both 90 and 100 W). Moreover, at higher φ values, higher CH3Cl feeding concentrations, and higher input
power wattages, more soot formation and polymerization were also found in the plasma reactor. These resulted in a lower carbon balance in the effluent gas stream (Hsieh et al., 1998b). Both methyl chloride decomposition efficiency and fraction of total-carbon input converted into CO and CO2
were decreased by increasing the methyl chloride feeding concentration. Besides, input power wattage (W) is the most important parameter in governing the temperature in the plasma reactor because the plasma reaction is associated mainly with the energy provided (Hsieh et al., 1998b). Their study indicates the significance of sensitivity for the T (°C) in the plasma reactor is: W > CCH3Cl > φ.
(2) Decomposition of 1,1-C2H2Cl2 Contained Gas in an RF
Plasma Reactor
Lee et al. (1996) studied phosgene formation from the decomposition of 1,1-C2H2Cl2 contained gas in an RF
plasma reactor. The RF plasma system was used to decompose the 1,1-dichloroethylene (DCE) contained gas. The reactants and final products were analyzed by using an FTIR (Fourier Transform Infrared Spectroscopy). The possible reaction pathways for DCE decomposition and phosgene (COCl2) formation were built up and discussed.
Lee et al. (1996) revealed both DCE decomposition efficiency and the fraction of total carbon mass converted into CO2 and CO were decreased by the increasing DCE
feeding concentration. The DCE decomposition efficiency at varied equivalence ratios, ø (stoichiometric O2/actual
O2), was controlled by both oxidation and energy transfer
Hsieh et al., Aerosol and Air Quality Research, 11: 265–281, 2011
268
having an excess of oxygen, a larger amount of COCl2 was
formed due to a higher oxygen-feeding concentration. Moreover, higher input power wattage can increase both the DCE decomposition efficiency and the fraction of total-carbon mass converted into CO2 and CO, resulting in
the reduction of the COCl2 effluent concentration (Lee et
al., 1996).
(3) Decomposition of CH2Cl2 in the RF Plasma
Environment
In the investigation by Li et al. (1996), a RF plasma system was also applied to decompose a dichloromethane (CH2Cl2) containing gas. Analyses of the reactants and
final products were conducted by using Fourier Transform Infrared Spectroscopy. Moreover, the effects of plasma operation-parameters, including the gas flow rate, the feeding CH2Cl2 concentration, the equivalence ratio φ (=
stoichiometric O2 needed/actual O2 used) and the input
power wattage, for CH2Cl2 decomposition and for the
fraction of total carbon input converted into CO2 and CO
were investigated (Li et al., 1996). Mole fraction profiles for each experimental condition were determined for the reactants (CH2Cl2 and O2) and for CO, CO2, H2O, HCl,
CHCl3, CCl4, COCl2, C2HCl3 and C2Cl4 (Li et al., 1996).
Application III. Decomposing Dichlorodifluoromethane,
CHF3, CH2F2, CCl2F2, and BF3
Application of radio-frequency plasma as an alternative technology for the decomposition of dichlorodifluoromethane by adding hydrogen is also demonstrated. Moreover, the reaction mechanisms in both a CHF3/O2/Ar and CHF3/H2/Ar
RF plasma environment or in Both CCl2F2/O2/Ar and
CCl2F2/H2/Ar RF Plasma Environment are investigated
insightfully. Several research articles are discussed as follows.
(1) Decomposition of Dichlorodifluoromethane by Adding Hydrogen in a Cold Plasma System
For the destruction of chlorofluorocarbons (CFCs), it has drawn great attention because of its well-known depletion of the ozone layer at the stratosphere. In 1999, Wang et al. (1999a) studied the application of RF plasma for the decomposition and conversion of dichlorodifluoromethane (CFC-12 or CCl2F2). Wang et al. (1999a) mentioned that
the decomposition efficiency (ηCFC-12) and the fraction of
total carbon input converted into CH4 and C2H2 (FCH4+C2H2)
in H2 and Ar mixtures have been investigated as a function
of input power wattage, H2/CFC-12 ratio, operational
pressure, and CFC-12 feeding concentration by using response surface methodology and model sensitivity analysis (Wang et al., 1999a). Their results revealed that the ηCFC-12 is over 94% and the FCH4+C2H2 is over 80%
under the condition of 100 W of input power wattage, 7.0 of H2/CFC-12 ratio, 15 Torr of operational pressure, and
7.6% of CFC-12 feeding concentration (Wang et al., 1999a). They also use the method of sensitivity analysis to test the ηCFC-12 and FCH4+C2H2. It is indicated both the ηCFC-12
and FCH4+C2H2 are more sensitive to CFC-12 feeding
concentration and input power wattage. From this study,
we understand CFC-12 could be converted into CH4 and
C2H2 up to 80% of conversion in a hydrogen-based RF
plasma system.
(2) Reaction Mechanisms in both a CHF3/O2/Ar and
CHF3/H2/Ar RF Plasma Environment
Wang et al. (1999b) studied the decomposition of trifluoromethane (CHF3 or HFC-23) in a RF plasma
system. Wang et al. (1999b) obtained some important results and we cited them originally as follows.
● “The CHF3 decomposition fractions (ηCHF3) and mole
fractions of detected products in the effluent gas streams of CHF3/O2/Ar and CHF3/H2/Ar plasma
systems, respectively, have been determined. The effects of four experimental parameters, input power, O2/CHF3 or H2/CHF3 ratio, operational pressure,
and the CHF3 feeding concentration were
investigated” (Wang et al., 1999b)
● “The same species detected in the effluent gas
streams of both CHF3/O2/Ar and CHF3/H2/Ar plasma
systems were CH2F2, CF4, HF, and SiF4. However,
the CO2 and COF2 were detected only in the
CHF3/O2/Ar plasma system and the CH4, C2H2, and
CH3F were detected only in the CHF3/H2/Ar plasma
system” (Wang et al., 1999b).
● “The results of a model sensitivity analysis showed
that the input power was the most influential parameter for ηCHF3 both in the CHF3/O2/Ar and
CHF3/H2/Ar plasma systems” (Wang et al., 1999b).
● “The addition of hydrogen for CHF3 decomposition
can produce a significant amount of HF and the main carbonaceous byproducts were CH4 and C2H2. Even
though the ηCHF3 in the CHF3/H2/Ar plasma system is
lower than that in the CHF3/O2/Ar plasma system, but
due to the more advantages mentioned above, a hydrogen-based RF plasma system is a better
alternative to decompose CHF3” (Wang et al.,
1999b).
(3) Reaction Mechanism in both CCl2F2/O2/Ar and
CCl2F2/H2/Ar RF Plasma Environment
In 2000, Wang et al. (2000) presented the decomposition of dichlorodifluoromethane (CCl2F2 or CFC-12) in a radio
frequency (RF) plasma system. Here lists their major observations and cited them originally as follows.
● “The CCl2F2 decomposition fractions and mole
fractions of detected products in the effluent gas stream of CCl2F2/O2/Ar and CCl2F2/H2/Ar plasma,
respectively, have been determined. The experimental parameters including input power wattage, O2/CCl2F2
or H2/CCl2F2 ratio, operational pressure, and CCl2F2
feeding concentration were investigated” (Wang et al., 2000).
● “The main carbonaceous product in the
CCl2F2/O2/Ar plasma system was CO2, while that in
the CCl2F2/H2/Ar plasma system was CH4 and C2H2”
(Wang et al., 2000).
● “The results of the experiments showed that the
Hsieh et al., Aerosol and Air Quality Research, 11: 265–281, 2011 269
easily separate from the CCl2F2 molecule and
combine with the added reaction gas. This led to the reactions terminated with the CO2, CH4, and C2H2
formation, because of their high bonding strength. The addition of hydrogen would form a preferential pathway for the HCl and HF formations, which were thermodynamically stable diatomic species that would limit the production of CCl3F, CClF3, CF4,
andCCl4” (Wang et al., 2000).
● “The HCl and HF could be removed by neutral or
scrubber method. Hence, a hydrogen-based RF plasma system provided a better alternative to decompose CCl2F2” (Wang et al., 2000).
(4) Decomposition of Boron Trifluoride in the RF Plasma Environment
In 2003, a radio frequency (RF) plasma system used to decompose boron trifluoride (BF3) was also examined
(Wang et al., 2003). The BF3 decomposition fractions
(ηBF3) were determined in effluent gas streams of
BF3/CH4/Ar, BF3/O2/Ar and BF3/O2(glass)/Ar plasma
systems. The ηBF3 in the BF3/CH4/Ar plasma system was
49.8%, higher than that in the BF3/O2/Ar and
BF3/O2(glass)/Ar plasma system. The reaction in the
BF3/O2/Ar plasma system generated B2O3 fine particles
and led to the deposition of a white substance on the surface of the reactor. The ηBF3 was only around 25% for
mixing with O2, even when the input power exceeded 120
Watts, but the generation of fine particles in the system warrants much more investigation (Wang et al., 2003).
Application IV. Decomposing Dichlorodifluoromethane,
CH3SH, CS2, SF6, and SF6 + H2S Mixture
Application of radio-frequency plasma as an alternative technology for the decomposition of CH3SH, CS2, and SF6
is also demonstrated. Moreover, the Difference in conversions between dimethyl sulfide and methanethiol in a cold plasma environment are also investigated. Several research articles are discussed as follows.
(1) Decomposition of CH3SH in a RF Plasma Reactor:
Reaction Products and Mechanisms
Tsai et al. (2001) studied the application of the RF cold plasma method to the decomposition of methanethiol (methyl mercaptan, CH3SH) at different O2/CH3SH ratios
(0−4.5), with various input powers (20−90 W), and at constant operating pressure (30 Torr). The species detected in the CH3SH/O2/Ar RF plasma were SO2, CS2, OCS, CO,
CO2, CH4, C2H4, C2H2, H2, H2O, HCOH, and CH3OH.
However, CS2, CH4, C2H4, C2H2, H2, H2S, CH3SCH3
(DMS), and CH3S2CH3 (DMDS) were detected in the
CH3SH/Ar RF plasma (Tsai et al., 2001). Here lists their major observations and cited them originally as follows.
● In the CH3SH/Ar plasma, over 83.7% of the total
sulfur input was converted into CS2 at 60 W; this is
due to the lack of competition between O and S and the thermodynamic stability of CS2. (Tsai et al., 2001)
● In the oxygen-rich conditions of the CH3SH/O2/Ar
plasma, the most predominant sulfur-containing
compound was SO2. (Tsai et al., 2001)
● As the feed O2/CH3SH ratio was increased, MSO2
was increased, while MCS2 was decreased
simultaneously. MOCS was reduced by increasing either the O2/CH3SH ratio or the applied power.
(Tsai et al., 2001)
● From the decay of CS2 and the generation of CO at a lower O2/CH3SH ratio of 0.6, CS, CS2, and CO were suggested as the primary species to react with O, OH, O2, S, or S2 and then to form OCS. (Tsai et al., 2001) (2) Formation of Solid Sulfur by Decomposition of CS2 in
Plasma Environment
The conventional thermal processes for treating toxic carbon disulfide (CS2) gas were by the oxidation of sulfur
on CS2 to form SO2. Interestingly, in 2002, Tsai et al.
(2002) reported that using a radio-frequency cold plasma reactor in the oxygen-lean condition (O2/CS2 = R = 0.6) at
the applied power of 90 W, the decomposition fraction of CS2 could reach 88.2%, and there was 76.9% of input
sulfur mass from CS2 converted into solid sulfur with the
purity of 99.2% (Tsai et al., 2002). In this study, no solid sulfur was observed for the no-oxygen (R = 0) or oxygen-rich conditions (R = 3.0). Such results provided an approach for sulfur recovery to reduce the emissions of both CS2 and SO2 (Tsai et al., 2002).
(3) Decomposition of SF6 in an Plasma Environment
In general, sulfur hexafluoride (SF6)contained gas is a
common pollutant emitted during the etching process used in the semiconductor industry. Shih et al. (2002) studied the application of RF plasma in the decomposition of SF6.
The decomposition fraction of SF6 and the mole fraction
profile of the products were investigated as functions of input power and feed O2/SF6 ratio in a RF plasma reactor.
Their results revealed that at 40 W, SF6 exceeded 99%,
and the reaction products were almost all converted into stable compounds such as SiF4, SO2, and F2 with or
without the addition of oxygen (Shih et al., 2002). The results of this work can be used to design a plasma/chemical system for online use in a series of a manufacturing process to treat SF6 containing exhaust
gases (Shih et al., 2002).
(4) Decomposition of SF6 and H2S Mixture in Radio
Frequency Plasma Environment
Sulfur hexafluoride (SF6) is a gaseous pollutant generated
in manufacturing processes in the semiconductor industry. Shih et al. (2003) have applied the hydrogen sulfide (H2S),
as a reductant, to treat SF6 in a radio frequency (RF)
plasma system. Shih et al. (2003) mentioned that SiF4 and
SO2 were the two dominant species detected in the glass
reactor in the SF6/Ar plasma system; other detected species
were SO2F2, SOF2, and SOF4. In the SF6/H2S/Ar plasma
system, HF and elemental sulfur were the main produced species (Shih et al., 2003). Shih et al. (2003) also indicated that although the species SiF4, SO2, SO2F2, SOF2, and
SOF4 were detected in the SF6/H2S/Ar plasma system,
Hsieh et al., Aerosol and Air Quality Research, 11: 265–281, 2011 279 formation in the RF plasma reactor is different from the
particle depositions from others combustion process in atmospheric environment (Chiu et al., 2011; Kim et al., 2011; Ruttanachot et al., 2011)
In the 1970s, MTBE represented the great hope for replacing tetraethyllead as the main antiknock additive in gasoline. However, people are now trying to work MTBE out of the motor fuel systems. Applying RF plasma and adding hydrogen can convert MTBE into CH4, C2H4, C2H2,
iso-C4H8 andiso-C4H10. CONCLUSIONS
From many previously research studies, we understand that there are four types of the application in the RF plasma reactors can be discussed, including: (i) Application I.: Converting methane, decomposing carbon dioxide, ethoxyethane, and ethylene oxide; (ii) Application II.: Decomposing methyl chloride, 1,1-C2H2Cl2, and CH2Cl2;
(iii) Application III.: Decomposing dichlorodifluoromethane, CHF3, CH2F2, CCl2F2, and BF3; (iv) Application IV.:
Decomposing dichlorodifluoromethane, CH3SH, CS2, SF6,
and SF6 + H2S mixture. Up to now, the RF plasma reactor
is a potentially important technology for decomposition and gas treatment in many laboratories. In this study, the radio-frequency plasma reactor provides enough energy to break down MTBE and allows the plasma parameters to be controlled at an MTBE feeding concentration of up to 5%. In this study, the fraction of total input carbon converted into iso-butane and iso-butene (Fi-C4H10+i-C4H8) increases
with the H2/MTBE ratio and the influent concentration of
MTBE in the MTBE/H2/Ar plasma environment.
Experimental results indicated that ηMTBE exceeds 99.4%
and FCH4+C2H2+ C2H4 exceeds 39.5% at an input power of 50
W, H2/MTBE ratio of 7.5, an operational pressure of 20
Torr, and influent concentration of MTBE of 1%. Additionally, Fi-C4H10+i-C4H8 can reach 65.5% at an input
power of 50 W, an H2/MTBE ratio of 10, an operational
pressure of 20 Torr, and an influent concentration of MTBE of 5%. The results obtained herein show that MTBE can be significantly decomposed and converted into CH4, C2H2, and C2H4 in an MTBE/Ar/H2 mixture.
Moreover, in this study, the detected species formed at the exit of the RF reactor were classified into four groups. The first group, CxHy, includes CH4, C2H2, C2H4,i-C4H8, and
i-C4H10. The second group, Coy, includes CO and CO2.
The third group, R-CHO, includes HCHO and CH3CHO.
The fourth group, R-OH, includes only CH3OH. As far as
the mechanism of decomposition for MTBE, MTBE was immediately decomposed in the plasma reactor, and then CH3 free radicals were produced directly by breaking the
C-C bond. Then, in the plasma reactor, CH4 is formed and
decomposes. Besides, the production of C2H2 is associated
with the depletion of both C2H4 and CH4. In the reactor,
the important production of iso-butene may through several pathways, including: i-C4H8 + 4H2O = 4CO + 8H2; i-C4H8
+ 4CO2 = 8CO + 4H2; CH3OH + i-C4H8 = (CH3)3COCH3
(MTBE); (CH3)3COCH3 + hν → CH3OH + i-C4H8; and H
+ (CH3)3COCH3 → (CH3)3COCH3.
ACKNOWLEDGEMENTS
The authors wish to thank the reviewers for their thoughtful corrections and valuable suggestions. The authors would also like to thank the National Science Council of the Republic of China, Taiwan, for partly financially supporting this research under Contract No. NSC 89-2218-E-241-002 and NSC 90-2211-E-020-015.
NOTATION
ηMTBE MTBE decomposition efficiency (%);
WD Power density in the plasma (W/cm3);
CMTBE MTBE feeding concentration (%);
MCH4 Mole fraction of CH4 (%);
MC2H4 Mole fraction of C2H4 (%);
MC2H2 Mole fraction of C2H2 (%);
MCO Mole fraction of CO (%);
MCO2 Mole fraction of CO2 (%);
MCH3OH Mole fraction of CH3OH (%);
MHCHO Mole fraction of HCHO (%);
MCH3CHO Mole fraction of CH3CHO (%);
Mi-C4H8 Mole fraction of iso-butene (%);
Mi-C4H10 Mole fraction of iso-butane (%);
FCH4 Fraction of total input carbon converted into
CH4 (%);
FC2H4 Fraction of total input carbon converted into
C2H4 (%);
FC2H2 Fraction of total input carbon converted into
C2H2 (%);
FCO Fraction of total input carbon converted into
CO (%);
FCO2 Fraction of total input carbon converted into
CO2 (%);
FCH3OH Fraction of total input carbon converted into
CH3OH (%);
FHCHO Fraction of total input carbon converted into
HCHO (%);
FCH3CHO Fraction of total input carbon converted into
CH3CHO (%);
Fi-C4H8 Fraction of total input carbon converted into
iso-butene (%);
Fi-C4H10 Fraction of total input carbon converted into
iso-butane (%).
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Received for review, February 24, 2011 Accepted, March 14, 2011
