Environ. Sci. Technol. lQaQ,23, 666-671
Reaction of Chlorocarbons to HCI and Hydrocarbons in a Hydrogen-Rich Microwave-Induced Plasma Reactor Robert B. Barat” and Joseph W. Bozzelli
Department of Chemical Engineering and Chemistry, New Jersey Institute of Technology, Newark, New Jersey 07102 The separate reactions of chloroform (CHCl,), trichloroethylene (C2HClJ, and chlorobenzene (C6H6C1)with water vapor or molecular hydrogen have been studied in a low-pressure [ca. 5 Torr (0.67 kPa)] microwave plasma tubular flow reactor. The experimental apparatus included feed introduction systems, a microwave plasma reactor, and full product analyses by flame ionization and thermal conductivity gas chromatography, mass spectrometry, and specific ion or pH detection for hydrogen chloride (HCl). Conversions of the parent chlorocarbon in the range of 50 to almost 100% were achieved. The high temperatures required for adequate conversion in a conventional thermal detoxification system were not necessary in this study. Product analyses indicate conversion to HC1, light hydrocarbons, some nonparent chlorocarbons, and soot. For the H 2 0 case, carbon monoxide and trace carbon dioxide were produced in place of some light hydrocarbons and soot. A t least 85 mol % of the chlorine (Cl) from the converted parent forms thermodynamically stable HCl at parent conversions of 80% or more. The remaining chlorine was present as nonparent chlorocarbons. Reaction mechanisms are proposed for the conversion of the parent chlorocarbon and the formation of observed products.
H
Introduction
1. Oxidation/Reduction of Halocarbons. Many research efforts on chemical detoxification of chlorinated hydrocarbon wastes have focused on oxidation as a means to convert carbon species to COP. Such processes do not always provide a thermodynamically favorable sink for the C1 atoms of the parent compound. In the 02-rich environment, competition for H atoms to form 0-H [bond strength 104 kcal/mol (I)] will inhibit H atoms from reacting with C1-containing species to quantitatively form H-C1 (103 kcal/mol). The C-C1 bonds (ca. 80 kcal/mole remain the most stable alternative to the lower bond strengths of the C1-C1, C1-0, and Cl-N species (ca. 60 kcal/mol), and so may persist in these oxidizing systems. Chlorine-containing products usually found in these combustion systems include phosgene, chlorinated aldehydes and acids, dioxins, smaller chlorinated hydrocarbons, and HCl. Senkan et al. (2) observed COCl2,C2C12,CZCl4, Cl,, HC1, COX,CC4, CHCl,, H20, and C6C&in an oxygen-rich C2HCI3flame. In fact, chlorinated and brominated hydrocarbons form the basis for many fire retardants, demonstrating the ability of these halogens to compete with O2for H. The use of chlorinated wastes as a fuel “supplement” in a kiln operation requires low concentrations of these wastes for proper flame maintenance, or substantial hydrocarbon fuel addition (3). The favorable thermodynamics of C1 removal by hydrogen (i.e., a reducing environment) as HC1 is shown by the large equilibrium constants (K,)listed for selected global reactions: CC14 + 4H2 = 4HC1+ CH, *Current address: Dept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. 666
Environ. Sci. Technol., Vol. 23, No. 6, 1989
T,K
KP
300 800
3.2 x 1065 6.1 X 10%
CC14 + 2H20 = 4HC1+ C02 T,K
KP
300 800
3.8 X loa 7.6 X 10%
Reactions of chlorinated solvents with H2 in an atmospheric pressure tubular flow reactor at elevated temperatures (above 500 “C) by Chuang (4) have shown high conversion to HCl and lower hydrocarbons. The use of H20 vapor as a detoxification agent in reaction with halocarbons offers the dual potential of halogen removal via the hydrogen halide (HC1, HBr, HF) and carbon removal via carbon oxides (CO,). Early studies by Chaung (5) on the reactions of chlorinated solvents in a high-temperature tubular flow reactor with H 2 0 vapor have shown high conversion to HC1, lower hydrocarbons, and COX. 2. Microwave Plasma Reactors. An interesting approach to the reactions of halocarbons with other species is the application of a plasma discharge. Hertzler et al. (6) oxidized halocarbons with molecular oxygen directly in a low-preasuretubular flow microwave plasma discharge reactor. The present study combines the interesting features of a plasma discharge as both a free-radical generator and reactor with the thermodynamic advantages of reacting halocarbons in the presence of hydrogen into a single active system. In a microwave or radio frequency plasma, energetic electrons, oscillating at the frequency of the electromagnetic wave generator, can have average energies of 1->10 eV and densities of 109-1012/cm3(7). Collisions of such electrons with various reagent species in the reactor can rupture molecular bonds and/or create relatively high steady-state concentrations of free radicals and species in highly excited states. The electron temperature T,of the plasma characterizes the average electron translation energy. The gas temperature Tgis the usual measure of average thermal energy of gas molecules obtained with a conventional thermocouple measurement. In the plasma, the ratio of T,to T is in the range of 10-100 (7). The free radicals and excited species generated are important to the initiation of reactions and the formation of products. We have studied the conversion of chlorocarbons with H2 or H20 in a tubular flow low-pressure [ca. 5 Torr (0.67 kPa)] microwave induced plasma reactor. Complete plasma reactor effluent analysis showed the formation of light hydrocarbons, soot, and hydrogen chloride (HCl) when complete chlorocarbon conversion occurred. When H20 was used as hydrogen source, carbon oxides (CO,) were also formed. No molecular chlorine (Cl,) or oxychloro compounds were observed. Experimental Section
The experimental apparatus used in this study consists of a feed vapor generation, metering, and delivery manifold system, a quartz tubular plug flow reactor, a microwave
0013-936X/89/0923-0666$01.50/0
0 1989 American Chemical Soclety
100
0
SUM
0
Hydrocarbons + C(S)
0
0 80
H2 H2°
_-
u
CH4
+ Carbon Solids
4.
MIX
-
VAC PUMP
w
n
I
Flgue 1. Bkck diagram schematic of plasma reactor, flame ionization plus thermal conductivity gas chromatographic analysis and reagent Inlet. The mixing chamber was maintained above 100 O C for water vapor inlet.
Table I. Sample Reactor Operating Conditions
reagent
power, P, W Torr
CzHCls/H20
500
CzHCls/Hz
325
CHCl,/H2
325
1.373 1.635 2.027 1.439 2.354 2.027 1.929 3.14 2.75 2-48
feed ratid
space time: ms
chlorocarbon conversn, %
0.59 1.04 2.29 2.38 29.0 39.3 50.7 16.3 29.9 78.4
72.5 83.8 71.5 101.9 37.5 36.4 37.4 42.7 43.5 43.4
59.3 73.4 88.9 99.9 81.2 87.0 91.2 54.1 67.6 83.7
Molar ratio Hz or HzO/chlorocarbon. bBased on chlorocarbon feed at 25 "C and system pressure.
power generator with wave guide, and two reactor effluent sampling systems. Product analyses include flame ionization and thermal conductivity gas chromatography [GC/FID and GC/TCD], GC/mass spectrometry (GC/ MS), and specific ion or pH measurements. Grab samples are taken for separate analysis by GC/MS, while the GC/FID, GC/TCD, and pH analytical capabilities are on-line. A block diagram of the reactor system is shown in Figure 1. 1. Feed Preparation and Delivery. The reagent inlet metering and delivery systems permit input of premixed gas or vapor to the reactor flow tube at pressures between 0.5 and 10 mmHg (0.067-1.33 kPa). Gaseous species are easily introduced and metered. For liquids with relatively high vapor pressures such as CHC13, metered H2 gas is saturated in two chlorocarbon-filled impingers (in series) before entry into the flow system. For liquids with a relatively low vapor pressure, such as HzO, vapor is generated in constant-pressure heated liquid reservoirs and fed through heated lines to the flow system. For metering, an off-line evacuated volume is filled with vapor to a known pressure (less than the vapor pressure of the liquid at room temperature) in a measured time. 2. Microwave Plasma Reactor. The reactions between Hzor HzO vapor and the halocarbons occur in a 2.15 cm i.d. quartz plug flow reactor (8) within which a stable plasma is established and maintained. Typical reactor space times (analogous to residence time) range from 0.035 to 0.01 s (see Table I). The flow tube passes at a 30" angle through a wave guide in which a resonant wave pattern of microwave radiation is maintained. The microwave power to the plasma is generated by a 0.6-kW Mitsubishi magnetron with variable-power supply (half-wave rectified, 2.45 GHz). It is assumed that approximately 50% of the measured input power to the
0 -O50
.
.
Chlorocarbon Products
60
70 CHCl3 CONVERSION (%)
v
,,, 80
-X-Banzene 85
Figure 2. Product concentrations as a function of chloroform conversion in a hydrogen plasma.
magnetron is converted to plasma energy (9). Average gas temperatures Tgare estimated to be 460 and 500 K for the Hz and HzO systems, respectively (IO). 3. Product Analysis. The reactor effluent is analyzed with an on-line GC/FID for quantitative determination of the reactant halocarbon and product halo- and hydrocarbon vapors (except CHJ. The sample is collected by drawing a side stream of reactor effluent through a '/le in. (0.16 cm) i.d. stainless steel loop which is cooled in a liquid nitrogen (-196 "C) bath. After cryo-trapping for a fixed time, the sample collection loop is then isolated, pressurized, and heated with boiling water (100 "C). The sample is then injected into the GC apparatus. The analytical GC column, a (1.83 m X 3.18 mm) 10% SP 2100 on Chromasorb W or DC 200 on Gaschrom P, is normally operated isothermally at 55 "C or greater in a Carle 9500 instrument. A Hewlett-Packard 3390A integrator quantifies all GC signals. Consistently sharp and reproducible GC peaks indicated no species adsorption onto the walls of the transfer lines. The reactor effluent is also analyzed with an on-line GC/TCD for quantitative determination of product CHI and carbon oxides (CO,). An evacuated volume (15 mL) on a sidearm of the main flow tube is filled with effluent to reactor system pressure. It is then isolated, pressurized, and injected. The analytical column (1.83mm X 3.18 mm packed with Carbosieve B or S) is operated isothermally at 80 OC in a Carle 8700 GC instrument. Quantitative measurement of HC1 is performed by either pH or specific ion electrode analysis. A known evacuated volume is filled with reactor effluent and then pressurized with He. The mixture is then bubbled through a basic solution. The change in pH or chloride ion concentration in this aqueous solution confirms the presence of HC1 in the reactor effluent gas. Separate reactor effluent samples are collected in a 250-mL evacuated grab sample bulb for qualitative analysis (complete species identification) by GC/MS. After being filled at reactor pressure, the bulb is pressurized with He to 2 atm (202 kPa) and run on a Varian Mat 44 capillary column GC/MS apparatus. The vapor sample is inlet to a fused silica capillary (22 cm X 0.2 Mm) column. A 3-mL volume of sample is cryo-focused at the head of the column before temperature programming.
Resutts and Discussion Four reacting systems were separately studied Hz/ chloroform (CHC13), Hz/trichloroethylene (C2HC13), HZ0/CzHCl3,and H2/chlorobenzene (c6H&1). Typical plasma reactor operating conditions employed in this study appear in Table I. Typical product distributions for these Environ. Sci. Technol., Vol. 23, No. 6, 1989 667
-
Table 11. Sample Product Distributions C in feed, mol % HYDROCARBON PRODUCTS vs
1.6 1.6 5.0 1.0 1.3 2.4 0.5 0.5 1.0 1.1 1.0
21.7 3.2 1.5 2.2 0.2 1.2
0.3 0.9 5.5 0.1 0.5
3.3 7.3 3.0 6.6 1.6 1.1 1.1 1.8 1.7 3.6 0.4
3.7 1.2 0.2
n w
I
84.1 31.8
67.8 32.8
5.5 6.3 33.2e 3.3 2.9
20
i"
3.4
18.P
4.1 0.7 0.2 100
'80
1.1 0.1 3.1e 0.7 1.4 100
04
86
88
90
92
0.3O 100 0 'O0I 80
systems are shown in Table 11. H2/Chloroform. Figure 2 shows the relative distribution of reactor effluent components (molar disposition of carbon in feed) as a function of CHC13 conversion. The total hydrocarbon + soot C(, products account for greater than 90% of the converted parent, with the remainder as byproduct chlorocarbon. The product Cb) is a solid sootlike material composed of nearly all carbon. An accurate measurement of the amount of C(s)produced was not possible, as most of this material remained on the walls in the reactor tube. Use of isothermal GC separation effectively prevented the observation of any hydrocarbons greater than approximately C8. Therefore, C(s),determined by difference, is correctly taken to be Cs+. The relatively high vapor pressure of CHI at liquid nitrogen temperature [lo mmHg (1.3 kPa)] did not permit complete freeze-trapping of all product CHI in the sampling system for the GC/FID. Verification of CHI was obtained by GC/MS analysis. Due to the uncertainties involved, the sum of CHI and C(s)was determined by difference based on a carbon balance. The sampling system for GC/TCD analysis of CH4was used on a limited basis. At a CHC1, conversion level of 60%, the CHI + C(s)combined yield accounted for 35% of the carbon from the converted CHC13. CHI was estimated to account for 5 of the 35%. By difference, the remaining 30% is C(s)at these operating conditions. The amount of CHI formed is similar to that of other light hydrocarbons yields. Therefore, C(s) is a significant product. The disposition of C1 is important from the standpoint of detoxification. Over the conversion range studied,
w
Environ. Sci. Technol., Vol. 23, No. 6, 1989
82
C2H3CI CONVERSION (%) Figure 3. Product concentrations from trichloroethylene conversion in a hydrogen plasma.
"CHC13 conversion, 67.6%; CHC13 in feed, 3.2 mol %; 43.5 ms space time. bCzHCl, conversion, 81.2%; CpHCl, in feed, 3.3 mol %; 37.5 ms space time. 'CZHCl3 conversion, 97.0%; CzHC13 in feed, 39.2 mol %; 91.6 ms space time. dC6H6Clconversion, 99.7%; C,H,Cl in feed. 1.5 mol %. eUnconverted Darent chlorocarbon.
688
30
1.4 0.8
4.9
100
CONVERSION CHCL=CCE + H2
70
o / ' - o
CO
+ CISollds)
z
C2 and C3 Hydroc"s
C2H3CI CONVERSION (%)
Figure 4. Carbon mole balance versus trichloroethyleneconversion in a water vapor plasma.
greater than 97% of the feed C1 in the converted parent is removed as HC1. Independent analysis (11) for C1 in the C(s)residues showed no measurable C1 present in this reaction product. H2/Trichloroethylene. Figure 3 shows the relative distribution of products as a function of C2HC13conversion. Total hydrocarbons + CQ)account for about 8590% of the feed carbon from the converted parent, with the remainder as byproduct chlorocarbons. C4) and HC1 were quantified by difference. No CHI product was observed. As before, C1 is effectively removed as HCl. For parent conversions greater than 80%, approximately 90% of the feed C1 in the converted parent is removed as HCl. H20/Trichloroethylene. Figure 4 shows the relative distribution of products, based on a molar disposition of the carbon in the converted parent, as a function of C2HC13 conversion. Total hydrocarbons + CO + C,, range from 50 to 95% of the feed carbon of the converted parent over the observed conversion range. The remaining carbon from the converted C2HCl9 was present in byproduct chlorocarbons. The combined yield of CO C, was found by difference, based on carbon balance. Only traces of C02 were observed. The thermodynamic stability of HC1 as a sink for C1 waa again demonstrated. For C2HC13levels of approximately
+
70% or greater, HC1 accounts for 90% or more of the C1 in the converted parent. On a limited basis, CO was quantitatively determined by GC/TCD. For these points, only the C(a)yield was found by difference. For the case of 83.6% C2HC13conversion, the 89.1% C,) + CO yield broke down into 40.7% CO and 48.4% C(a).For this data point, the feed ratio M (molar H20/C2HC13)was 1.45. For a similar conversion of 88.9%, the C(a)+ CO yield of 90.2% was also similar. However, this yield broke down into 15.4% CO and 74.8% C(ak The M ratio for this run was 0.58. Clearly, for similar parent conversions, a higher initial ratio of H20to C2HC13 results in CO accounting for a higher portion of the combined C(II)+ CO product. H2/Chlorobenzene. As a first step toward a potential PCB feed, chlorobenzene (C6H5C1)was input with H2into the plasma reactor. The C$16C1 was fed by using H2 impingers, resulting in low feed concentrations. High conversions (>98%) were achieved. Typical reactor effluent for this reaction is shown in Table 11. The C(s)+ CH4 combined product, (calculated by difference) accounts for -63% of the feed carbon. The C2-C, yield accounted for -35%. Reactor effluent analysis showed that HC1 was the only C1-containing product from the CBH6C1/H2system. The lack of any nonparent chlorocarbon byproducts is likely due to the low c1/c ratio in C6H6C1. Proposed Reaction Mechanisms. The electrons and ions in a microwave plasma are accelerated to high translational kinetic energy by the strong oscillating radiation fields. These energetic electrons [e-(KE)] collide with neutral gas species, causing dissociation (and ionization) to occur. The resulting fragments react along the pathways leading to chlorocarbon conversion and stable product formation. At the input plasma powers used here, order-of-magnitude estimates indicate that ion-molecule reactions are less significant than free-radical-molecule reactions (12).Qpical reactions in the H2/CHC13system, which explain the observed products, are presented. A similar mechanism has been suggested for the C2HC13 system (with H2 or HzO) (10). There are a number of important e- impact dissociations in these systems that initiate the reactions; for example: e-(KE) + H2 H + H + e(1) e-(KE) + CHC13 C1+ CHC1, + e(2) e-(KE) + CHC13 H + CC13 + e(3) Subsequent reactiops leading to parent species conversion and observed product formation generally involve attack by radicals on molecules or fragments via abstraction or the formation of an activated complex [*I. This complex can be collisionally stabilized or undergo unimolecular dissociation to new molecules and/or fragments, with further reaction often strongly favored at the relatively low pressures in this study. Initially, H atoms, present in relatively high concentrations due to excess H2, will abstract C1 from CHC13. H + CHC13 HC1+ CHCl2 (4) At the present gas temperatures, though, abstraction is likely to be slow as compared to step 2. The CHC1, radical formed in steps 2 and 4 will combine with H to form a CH2C12activated complex. H + CHC12 [CH2C12]' (5) The activated complex formed in step 5 will either eliminate a C1, (6) [CH2C12]* -w C1 + CH&l
--
-
-
1-221
POTENTIAL ENERGY LEVEL DIAGRAM Flguro 5. Potential energy level diagram for the reaction of atomic hydrogen with dichloromethylradical illustratlng a major pathway to loss of chlorine by hydrogen addition-chemically activated complex formation and rapid loss of chlorine due to carbon-chlorine bond strength being lower than the carbon-hydrogen bond formed (low energy exit channel). The low pressure and high plasma temperatures wlll shift the reaction away from the stabilization channel. The chloromethyl radical would undergo similar reaction to methyl radicals plus CI.
Table IIIg H + CHClz * PRODUCTS (for Potential Energy Diagram, Figure 5) k
A
1 -1
1.OE + 14 6.OE + 15 1.OE t 16 1.OE + 13
2
3 ( v ) = 1116.3 cm-E LJ parameters: u = 4.77 A e/k = 420.3 K
E,, kcal/mol
ref
0.0 100.0
a b
82.8 109.8
C
d e
f
Reaction Rate Constants k Predicted by Bimolecular QRRK Chemical Activation Analysis (300-1000 K) A factor,
P, Torr 7.6 760 7.6
reactionh
mL/mol.s
n
H t CHClz = [CHZClzlo 8.932 + 20 -3.68 H + CHC12 = [CH&12]o 9.953 + 22 -3.69 1.18E + 14 -0.02 760 H t CHClz = CH&l + C1
E,, kcal/ mol 1.50 1.56 0.04
" A from ' / , A for H + CH3: Allara, D.; Shaw, R. J. Phys. Chem. Ref. Data 1980, 9, 529. bA-l from A, and thermodynamics, E, = AHm. A2 calculated from thermodynamics and reverse A of 1.9 X estimated from ' l z A for CH3 + 0 (15). * A from transitionstate theory (19) and E, = AHH,, + 3 8 Setser, D. W.; Lee, T. J. Am. Chem. SOC.1985,89, 5799. e Shimanouchi, T. Tables of Molecular Vibrational Frequencies-Consolidated, NBS Data Series, 1972; Vol. I, NSRDS-NBS 39. /Calculated from critical constants in: Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw Hill: New York, 1976. EArrhenius factors A+ A2, and AS, l/s; A,, mL/mol.s; k = A T" exp(-E,/RT). hBath gas, H,; HCl + CHCl product channel is not important.
collisionally stabilize, or decompose back to reactants. An energy diagram, typical for this type of process, is shown in Figure 5 for H + CHC12. Note that C1 elimination according to step 6 represents a lower energy exit channel for the complex. Specific reaction rate constants are calculated for this system as a function of temperature and pressure by a modified form of the energized complex quantum RRK theory (13-15). Table I11 lists the input data, rate constants, and molecular parameters required as input to the QRRK program for the H + CHC1, system. Figure 6a Envlron. Sci. Technol.. Vol. 23, No. 6, 1989 669
CH3 + CHzCl-
CHPCl + C1 CH2C12 STABILIZED
+ CHC12 -->
0=H
+ CHCIZ -->
0=H
,
I
-25
-30
,
I
-20
-15
,
-05
-10
00
I
05
10
1 15
LOG PRESSURE (ATM)
''4
"
n
m
h l
..
0 = H + CHC12 0 = H t CHC12
--> CHZCl + CI (0 01 ATM) --> CH2C12 STABILIZED
-+
+
,
I
,
25
20
15
10
,
30
I
35
T (K)
1000
Flgure 6. Results from the Chemical Activation QRRK calculation on reaction of H CHCI,. a, Plot of log k reaction to chloromethyl CI atom and to stabilized methylene chloride products versus log pressure. b, Plot of log k to chioromethyl radical CI and stabilized methylene chloride versus 1000/T (K).
+
+
+
shows calculated rate constants for the stabilization and the low energy exit channels in the H + CHC12 reaction at 500 K. Reaction to the lower energy exit channel, C1 + CH2C1, dominates up to pressures over 1 atm. The variation of the rate constanta at a fixed pressure (7.6 TOR) with temperature is shown in Figure 6b. Note the nonArrhenius behavior. The low energy exit channel (C1 leaving) rate constant is essentially independent of temperature, while the stabilization rate constant decreases with rising temperature. Continuing the process of H addition and C1 removal, H attacks CH2C1and forms an activated CH3C1complex
+
H CHzCl- [CH,Cl]' (7) which dissociates to methyl radical and C1. (8) [CH3C1]* CH3 + C1 Methyl combines with H and, upon collisional stabilization, forms CH4. CH3 + H + M CHI M (9) Subsequent reactions of the early radicals lead to the observed products. Two CH2Cl radicals combine to form an energized complex, which dissociates.
-
-
+
-
[CzH4C12]*
(10)
C0H2CH2Cl+ C1
(W
CHzCl+ CHzCl [CzH4C12]*
-
-*
[ C2H4C12]* C2H,C1+ HC1 The C'H2CH2C1radical then dissociates by 0 scission to form ethylene and C1.
C'HZCH2C1- CzH4 + C1 (12) Methyl combines with CH2Cl to form an energized C2H5C1,which either stabilizes or dissociates to ethyl and c1. 870
Environ. Sci. Technol., Vol. 23, No. 6, 1989
C2H5 + C1
(13) Reactions of ethyl proceed to the observed C4compounds. In the plasma, ethylene will undergo unimolecular dissociation via collision with e-(KE) or react with H or C1 to form vinyl radicals. e-(KE) + C2H4 C2H3+ H + e(14a) H + CZH, H2 + C2H3 (14~ C1+ CzH4 HC1+ C2H3 (14~) Vinyl radical can dissociate to H and acetylene. C2H3 H + C2H2 (15) Acetylene will react with H or e-(KE) to form acetyl radical. C2H2 + H C2H H2 (16) Vinyl, acetyl, and other carbon radicals will rapidly add to acetylene and other unsaturates to form higher chain hydrocarbons via activated complex reactions (16) of the type C2H C2H2 [HC&CH=C*H] * H + C4H2 (17) Concentrations of the multi-acetylenic compounds C4H2, CBH2,and CsH2 were clearly observed in the GC/MS analysis of the reaction products. The formation of benzene from C2 and C4 species (16) also occurs in this system. Benzene and C2Hzappear to be the important building blocks for molecular weight growth and soot formation (17). The C(a)in these systems is likely formed through a complex series of reactions involving free radicals and unsaturates in the gas phase and at the reactor wall. Frenklach et al. (18)observed higher hydrocarbon and soot formation in the shock tube pyrolysis of CH3C1 and CH2C12. The presence of the key species needed to form benzene as a building block to soot can be inferred from the significant amounts of C2H2and other unsaturated hydrocarbons observed (Table 11). Soot formation is thermodynamically favored in various high energy hydrogen/carbon reaction environments (fuel-rich flames, shock tube pyrolysis, thermal hydropyrolysis). It is not surprising, then, that C2H2and soot account for a large portion of the converted carbon in our hydrogen-rich chlorocarbon plasmas. It is felt that the activated complex reactions probably dominate in this low-pressure system because of relatively high A factors and low energy barriers to formation (0-4 kcallmol), whereas barriers to C1 abstraction by H are equal to the enthalpy of reaction (if endothermic) plus 6-8 kcal/mol(19). Unimolecular dissociation of the energized complexes, formed by H or hydrocarbon radical recombination with a chlorocarbon radical or addition to a chlorinated unsaturate (vinylic or acetylenic here), to C1 and a radical or unsaturate, as shown, is exothermic with respect to the energies of the combining species. Such dissociations often have high A factors (ca. 1015-1016/s) due to the large moment of inertia associated with the leaving C1 atom (loose transition state) in the complex (19). Atomic chlorine produced in this system primarily reacts with the excess H2 to form HC1. C1+ H2 HC1+ H (18) GC/MS analyses confirmed that no C12 or stable oxychloro species were produced in these systems, as suggested by the thermodynamic arguments given in the introduction. It should be emphasized that the above reactions are by no means exhaustive. They are examples presented here
-
' b 0
-
---
I ,
[C2H&1] *
+
-
+
-
to show the chemistry of these systems and to account for CHC13 conversion and the observed products.
Conclusions The experiments in this study demonstrate the applicability of reacting chlorinated hydrocarbons in a hydrogen-rich microwave plasma discharge reactor. Through such a scheme, conversion occurs by removal of C1 as thermodynamically favorable HC1. Reaction in a hydrogen-rich environment prevents the formation of undesirable byproducts, such as dioxins, often seen in oxygen-rich incineration of halocarbon wastes. The reactivity of the microwave plasma reactor was demonstrated by achieving conversions of the parent chlorocarbons over a range from approximately 50 to almost 100%. Reactions of the chlorocarbons in the presence of Hz yielded significant quantities of HC1, a range of C1-C7light hydrocarbons, small amounts of nonparent chlorocarbons, and soot. The soot and high degree of unsaturation in the hydrocarbon products resulted from the reactivity of Cl and Czradical species which are produced in the plasma and then not rapidly stabilized. In the presence of HzO vapor, the major sink for C1 remained HC1. Significant quantities of CO were produced. Higher mole fractions of HzO in the feed resulted in a shift in the product slate from hydrocarbons and soot to more CO. For all reacting systems studied, at least 85 mol % of the C1 in the converted chlorocarbon parent forms HC1 for parent conversion levels of 80% or more. Chlorocarbon byproducts account for a greater percentage of the converted parent at lower conversion levels. Higher parent conversions, and better C1 removal as HC1, occur with greater residence times and higher feed H2, H20/chlorocarbon ratios. No Clz or oxy-chloro compounds were ever observed. Bimolecular activated complex QRRK theory shows how H generated in the plasma will add to a C1-containing olefin or recombine with a C1-containing radical to create an energized complex. Because of the low pressures (
