I n d . Eng. Chem. Res. 1988,27, 1163-1168
1163
A Detailed Chemical Kinetic Mechanism for the Oxidative Pyrolysis of CH&l Sankaram B. Karra and Selim M. Senkan* Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616
A detailed chemical kinetic mechanism describing the oxidative pyrolysis of CH3C1, representing the second stage in the chlorine-catalyzed oxidative-pyrolysis (CCOP) process of converting CHI into higher molecular weight hydrocarbon products, is presented. Mole percent profiles for a variety of stable species calculated by using a one-dimensional flow reactor model were compared to those measured experimentally. The agreement between the model and experiment was generaliy satisfactory. The major reaction channels responsible for the formation and destruction of species have been identified from the calculation of individual reaction rates and by the sensitivity analysis. Chlorine-catalyzed oxidative-pyrolysis (CCOP) process was recently developed as a practical method to convert methane, the major component in natural gas, into more valuable products such as acetylene and ethylene (Senkan, 1987a). The CCOP process involves the initial chlorination of CHI and the formation of chlorinated methanes (CM) first, followed by the oxidative pyrolysis of CM and the formation of C2 and higher hydrocarbons and HC1 in the second step. The CCOP process ameliorates the problem of formation of carbonaceous solid deposits inherent with the earlier chlorine-catalyzed methane conversion processes which took place in the absence of oxygen (Gorin, 1943; Benson, 1980; Weissman and Benson, 1984). The HC1 produced can either be converted back to chlorine via the well-known Deacon reaction and recycled or can be used to oxychlorinate methane to form CMs, thus completing the catalytic cycle for chlorine (Senkan, 1987b). In this paper, a detailed chemical kinetic mechanism for the high-temperature oxidative pyrolysis of CH3Cl is presented since it forms the major chlorination or oxychlorination product of methane under conditions of interest to the CCOP process, i.e., when excess methane is present. The mechanism developed involves the participation of 37 species in 180 reversible elementary reactions and accounts for the available data obtained in flow reactor experiments with reasonable accuracy. I t is important to recognize that the detailed chemical kinetic mechanism presented here is a numerical tool of exceptional generality and of broad utility because of its comprehensive and fundamental nature. The mechanism is useful not only to correlate available experimental data but also to predict the behavior of the reaction system under a very broad range of conditions, including conditions under which the acquisition of experimental data may be impractical. In addition, by undertaking sensitivity and reaction path analysis, the size of the detailed mechanism can systematically be reduced, thereby rendering it useful for quick engineering calculations. However, this may not be desirable since reduced mechanisms inherently have a narrower range of applicability. Furthermore, with the availability of fast computers, the development and subsequent use of detailed reaction mechanisms no longer represents a costly proposition.
Reaction Mechanism An elementary reaction set describing the high-temperature oxidative pyrolysis of CH&1 is presented in Table I together with the rate parameters for the forward reaction paths. Reverse reaction rates were calculated from the
* To whom correspondence should be addressed.
considerations of the detailed balancing between the forward and reverse rates through the use of the equilibrium constant. This mechanism was cohstructed by systematically considering all plausible elementary reactions of CH3Cl and 02,and their daughter species consistent with experimental observations and the principles of physical organic chemistry, and by eliminating those reactions that did not contribute to mass flux and were determined to be unimportant by the sensitivity analysis. The mechanism was then combined with the chlorine-inhibited CO oxidation submechanism developed and tested previously (Chang et al., 1987). It should be pointed out that with the exception of H‘ and CH&P radicals, metathesis reactions resulting in C1 abstractions from chlorinated hydrocarbons, and in particular from CH,Cl, are highly endothermic. Consequently, the rates of these reactions are too slow under the conditions of interest to the CCOP process. For the case of CH2C1’ radical attack on chlorinated hydrocarbons, the situation is somewhat different. In this case we did not include C1 abstraction reactions into the mechanism, because their inclusion results in the formation of CH2C12. However, the available experimental data do not support the formation of CH2C12in the oxidative pyrolysis of CH&l (Granada et al., 1987). At present the mechanism extends only up to C2 compounds because accurate quantitative experimental data exist only for those species (Granada et al., 1987). In addition, the data up to C2hydrocarbons account for well over 90% of the carbon in the system. Furthermore, the development of a reaction mechanism containing higher molecular weight compounds would also be speculative because accurate data on reaction rate parameters and thermochemical properties, especially those of the chlorinated free radicals, are presently unavailable. The rate parameters for bimolecular reactions presented in Table I correspond to those reported in the literature whenever such information is available, and they are identified by letters which correspond to refrences listed at the end of the paper. However, for some of the proposed elementary reactions in the mechanism, such data presently do not exist. Consequently, they were estimated by using theoretical methods (Benson, 1976,1983),by analogy with similar reactions, and they are designated by “est”. It is particularly important to recognize that the recombination of CH,Cl’ and CH3’ radicals is responsible for the formation C2 hydrocarbons in the CCOP process as CH3C1is the only source of carbon. In addition, the unimolecular decomposition reactions C2 species formed are in the fall-off regime at the temperatures and pressures involved in the CCOP process. In this study, the chemically activated recombination reactions of CH2CPand CH;
0888-5885/88/2627-ll63$01.50/0 0 1988 American Chemical Society
1164 Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988 Table I. Chemical Kinetic Mechanism for the Oxidative Pyrolysis of CH&l at 515 Torr (k= A T nexp(-EIRT), in cm*cal*s*mol Units) reactionn Ab n E sourcee reactionD Ab n E sourcee
CH3C1= CH3' + CI' CH. = CH,' + H'
CH; + 0' CH3' 4 OH'CHI + O H = CH3' + H20 CHI + C10' = CH,' + HOCl CH&l+ C1' = CH&P + HC1 CH3Cl + H' = CHI' + HCl CHsCl + H' = CH2W + H2 CH3C1+ CH3' = CH,Cl' + CHI CH&1+ 02 = CHZCI' + HO,' CH3Cl+ HOC CHZC1' + H202 CH3C1 + 0' = CH2W + OH' CH3C1+ OH' = CH,CP + HzO CH3C1 + C10' = CH,C1' + HOCl CH3' + O2 = CH30' + 0' CH3' + 0 2 = CHpO + OH' CH3' + 0' = CHZO + H' CH3' + OH' = CH30' + H' CH3' + OH' = CHZO + H2' CH3' + ClO' = CHPO + HC1' CH3' + HO,' = CH30' + O H CH3' + CHzO = CH, + CHO'
'
CH3' + CH3' = C2H5' + H' CH3' + CH3' = C2H4+ H2 CH3' + CHpC1' = C2H5C1 CH3' + CH,CI' = C2H, + HC1 CH3' + CHZC1' = CzHS' + C1' CHZCP + CHZC1' = C2H4C1, CH,Cl' + CH,Cl' = CzH3Cl+ HC1 CH2C1' + CH,C1' = C2H4C1'+ C1' CH,' + CH3' = C,H4 + H' CH,' + CHpC1' = CzH4+ CI' CH,' + CH2' = C,H, + H' + H' CHZ' + CZHZO = CZH4 + CO CH; + C,HO' = CZH3' + CO CzHg = CH,' + CH3' CzHg + H' = C2H5' + Hz C,H. + C1' = CoH,' + HC1 CiH, + CH3' = k'c,' + CH, C,H6 + CH,C1' = C2H,' + CH3CI C,H, + 0' = C,H,' + OH' CiH, + OH' = C & 5 ' + HzO CzHB+ OH' = C2H5*+ H20 C2H5C1= CpH4+ HC1 C,H.CI' + H' = C,H,' + HC1 CiH,C1+ H' = C,H4k1 + HZ C2HKC1+ C1' = C,H4C1' + HC1 C2HKC1+ CH3' = C2H4C1'+ CHI C,H,Cl + CH&P = C,H,Cl' + CH,Cl C,H;Cl+ 0' = C2HIC? + OH' C2H5Cl+ OH' = CZH4C1'+ H20 C2H4C1,= CPH3Cl+ HC1 C2H4C12+ H' = C2H4CI'+ HCl C2H4C12 + C1' = CZH3ClC + HC1 CzH4ClZ + CH3' = CzH3C12' + CH4 CZH4C12 + CHZC1' = CZH3Clp' + CH3Cl C2H4C1, + 0' = CZH3C1,' + OH' C2H4Cl2 + OH' = CzH3Clp' + H20 C2H6' = C2H4+ H' C2H5' + H' = CzH4+ H, CoHz' + C1' = C,H, + HCl
CHO' CHO' CHO' CHO' CHO' CHO' CHO'
+ M = CO + H' + M
+ H' = CO + H, + OH' = CO + H20 + 0' = CO + OH' + 0, = CO + H02' + C1' = CO + HC1 + CHO' = CHpO + CO co + 0, = co, + 0' CO + OH' = CO, + H' CO + 0' + M = COP+ M CO + Hop' = CO, + OH'
9.98322 -2.4 l.lOE33 -5.9 5.50307 1.97 5.16306 2.11 7.94313 0.0 2.00313 0.0 1.20307 2.1 1.60E06 2.1 1.00E12 0.0 3.16313 0.0 3.72313 0.0 1.00E13 0.0 1.51E10 0.5 6.31313 0.0 3.00E13 0.0 1.30E13 0.0 2.54306 1.97 2.00312 0.0 1.50313 0.0 5.34313 0.0 1.26314 0.0 4.52314 0.0 4.00E12 0.0 6.31312 0.0 2.00E13 0.0 1.00310 0.5 7.80Ell 1.00E16 7.67335 2.31321 1.57E10 4.90338 8.25324 1.41E17 4.00E 13 5.00E13 1.00E14 1.00E12 3.00E13 2.00332 5.40302 4.64313 0.55E00 1.00E12 3.00E07 6.30306 6.30306
0.0 0.0 -7.06 -2.3 1.1
-8.03 -3.5 -1.0
0.0 0.0 0.0
6000.0 13039.0 31792.0 7600.0 4905.0 4171.0 9431.0 8088.0 9129.0
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
-5.0 3.5
92 2 2 5.0 5210.0 179.0 8294.0 8500.0 5115.0 645.0 645.0 57790.0 8600.0 8000.0 616.0 8500.0 9000.0 6600.0 4000.0 58000.0 8400.0 3100.0 8500.0 9000.0 7000.0 4000.0 42984.0
0.0
4.0 0.0
2.0 2.0 2.0 1.11314 -0.083 6.31313 0.0 1.00E13 0.0 1.41E13 0.0 1.00E12 0.0 3.16312 0.0 7.76313 0.0 5.00E13 0.0 6.61313 -0.084 6.31313 0.0 2.51E 13 0.0 1.00E12 0.0 3.16312 0.0 5.00E13 0.0 3.98313 0.0 7.00325 -4.1 1.90E12 0.0 2.00E12 0.0 2.00312 0.0 1.05E20 -2.36 3.16312 0.0 1.00E13 0.0 4.95320 -2.35 1.00E13 0.0 7.10E14 2.00E14 5.00E13 3.00E13 3.00312 1.00E14 2.00E13 5.00E13 4.40306 5.30313 1.50E14
91540.0 105150.0 11207.0 1580.0 55887.0 18000.0 7624.0 2462.0 7500.0 3300.0 9300.0 9000.0 9330.0 54000.0 16000.0 6900.0 1190.0 12000.0 28681.0 34574.0 2000.0 15500.0
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0
1.5 0.0 0.0
0.0
0.0 5000.0 22000.0 0.0
3000.0 20000.0 1000.0
C1 Kinetics a, ac CH,C1' + O2 = CH20 + C1' + 0' 1.50E13 b, ac CH,Cl' + 0, = CHClO + OH' 4.00313 1.00E14 C CHZC1' + 0' = CHZO + C1' CH,Cl' + OH' = CH,O + HC1 6.31312 d 5.00314 e CH,Cl' + OH' = CH,O + H' + C1' 6.31312 e CH2C1' + C10' = CHClO + HC1' CH,C1' + HO,' = CH,O + OH' + C1' 1.00E13 f 3.16Ell f CHzC1' + CHZO = CH&l+ CHO' est. CH30' + O2 = CH20' + HO,' 1.00E13 CH30' + M = CHzO + H' + M 2.00E14 g h CHzO + M = CHO' + H' + M 5.00E16 est. CHzO + H' = CHO' + H2 2.50309 CHzO + 0' = CHO' + OH' 1.70E06 g est. CH,O + OH' = CHO' + H 2 0 6.90304 est. CH20 + C1' = CHO' HC1 5.00E13 1.00E12 CHzO + HOB' = CHO' + HzOZ g 1.00E17 ad CHClO + M = CO + HCl+ M est. CHClO + H' = CHO' + HC1 2.00E13 CHClO + 0' = CO + CI' + OH' k 1.00E13 CHClO + OH' = CO + C1' + HpO k 1.00E13 f 1.00E13 CHClO + CY = CO + Cl' + HCl 1 1.30E13 CH,' + O2 = CO, + H' + H' m CHp' + 02 = CHpO + 0' 5.00313 est. 1.00314 CHZ' + 0 2 = CHO' + OH' CH,' + 0' = CO' + H' + H' 8.00E13 e e
+
C2 Kine!tics C2H4+ M = C2H3*+ H' + M k C2H4+ M = C2Hz+ Hz + M i C2H4+ C1' = C2H3' + HC1 CzH4+ H' = CzH3*+ H2 1 1 C2H4 + CH3' = CZH3. + CHI 1 C2H4+ CH2C1' = C2H3' + CH3C1 CzH4+ 0' = CH3' + CHO' 1 CzH4+ OH' = CzH3' + H20 1 CzH4+ OH' = CH3' + CH20 aa est. CzH4+ C10' = CH,C1' + CH,O ae C2H3C1= C2H2+ HC1 CzH3C1+ H' = C2H3' + HC1 ae ae C2H3C1+ H' = CpHzCP + Hz b C,H,Cl+ C1' = C2HZC1' + HC1 f CZHBCl+ CH3' = C2HzC1' + CHI 0 C,H&l+ CHZC1' = CZH2C1' + CH3C1 f CZH3CI + 0' = CHClO + CHZ' CZH3C1 + OH' = CH3' + CHClO est. CzH3Cl+ OH' = C2H2C1'+ H20 f C2H3C1+ C10' = CH,W + CHClO f C2H3*= C,H, + H' f C2H3' + H' = C2H2 + H2 P S C2H3' + C1' = C2H2 + HC1 est. CpH3' + O2 = C2H2+ HO,' 0 C2H3*+ O2 = CH20 + CHO' est. C2H3' + 0' = CzH20+ H' est. CZH, + CI' = CzHzC1' est. CZHpC1' + 0 2 = CHClO + CHO' est. CZHzC1' + 0' = CzH20 + C1' CzH2+ 0' = CH,' + CO P est. CzHp+ O'=C,HO'+ H' 0 CpHp + OH' = CzH20 + H' est. C2H2 + C10' = CpH20 + C1' est. CZHZO + H' = CH3' + CO est. CpHZO + C1' = CHZC1' + CO est. CzH2O + H'=CZHO*+ H2 b CzHzO+ C1' = C,H@ + HC1 CzHzO + M = CH2' + CO + M 1 est. CZHZO + 0' = CHO' + CHO' b C2H20 + 0' = CHZO + CO est. C2H20+ OH' = CH,O + CHO' est. C,H,O + OH' = C,HW + H20 S C,HO' + H' = CH,' + CO C,HO' + 0' = CO + CO + H' t est. C,HO' + 0, = CO + CO + OH' j
co and H2 Kinetics ab co + C10' = cop + c1' H' + 0, = 0' + OH' 0.0 ab 0.0 ab 0' + Hz = H' + OH' ab Hz + OH' = H, o + H' 0.0 0.0 ab 0' + H 2 0 = OH' + OH' H' + H' + M = H, + M 0.0 ab ab 0.0 0' + 0' + M = O z + M 63169.0 ab 0' + H' + M = OH' + M H' + OH' + M = H,O + M -740.0 ab -4538.0 ab H' + 0 2 + M = HO2' + M H' + Hop' = H, + 0, 23573.3 ab 16802.0
30300.0 est., af 34000.0 est. 1000.0 est. 0.0 est. 15000.0 est., af 0.0 est. 0.0 est., af 5000.0 est. 7170.0 e 0.0 0.0 19870.0 f 76480.0 f 0.0 1.27 11000.0 k 2.32 6200.0 k 2.65 -8000.0 k 500.0 n 0.0 7888.0 e 0.0 0.0 40000.0 est. 0.0 4500.0 est. 0.0 1000.0 est., ag 2000.0 est., ag 0.0 1000.0 est., ag 0.0 0.0 0.0 m 0.0 9000.0 m 3700.0 m 0.0 0.0 0.0 m 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
3.10E17 0.0 3.00E17 0.0 1.00E14 0.0 7.00E14 0.0 3.97311 0.0 2.00E12 0.0 1.60E09 1.2 3.50313 0.0 1.30312 0.0 5.00E12 0.0 2.75317 -1.3 1.00E14 0.0 1.00E13 0.0 1.00E14 0.0 1.00312 0.0 1.00E12 0.0 5.24311 0.0 5.00E12 0.0 5.00E13 0.0 5.00E12 0.0 9.30322 -3.7 1.00E13 0.0 1.OOE 13 0.0 1.60E13 0.0 4.00E12 0.0 3.00E13 0.0 5.19321 -3.48 2.00E12 0.0 3.00E13 0 0 4.10E08 1.5 4.00E14 0.0 3.00E12 0.0 3.00E12 0.0 2.00E13 0.0 5.00E13 0.0 3.00E13 0.0 5.00313 0.0 2.30E 15 0.0 2.00E13 0.0 2.00E13 0.0 1.00E13 0.0 1.00E13 0.0 1.50E14 0.0 1.00E14 0.0 1.46E12 0.0 1.00313 1.20E17 1.50E07 1.00E08 1.50310 6.40317 1.00E17 3.00E19 1.41323 7.00E17 2.50313
98160.0 79800.0 7000.0 14500.0 7988.0 12000.0 741.0 3012.0 -765.0 0.0 69312.0 4500.0 9000.0 5000.0 11000.0 12000.0 0.0 0.0
3000.0 0.0 37255.0
I
I
U I
f
est. V V
f
est. ac, w g est. est. est. est. est. est. est. est. b
0.0 I 0.0 est. 10400.0 f 0.0 X 0.0 m 0.0 Y, ac 0.0 est. 0.0 est.
1697.0 z 10660.0 z 1100.0 f 0.0 est. 0.0 ae 3000.0 est. 1434.0 m 1500.0 est. 57600.0 ae 2300.0 m 0.0 m 0.0 f
2630.0 m 0.0 z 0.0 m 2500.0 m
0.0 1000.0 ab -0.91 16504.0 ab 7547.0 ab 2.0 3296.0 ab 1.6 1.14 17244.0 ab -1.0 0.0 ab -1.0 0.0 ab -1.0 0.0 ab -2.0 0.0 ab -0.8 0.0 ab 0.0 693.0 ab
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1165 Table I (Continued) reaction" H' + HOz' = OH' + OH' OH' + HOC = HzO + 0 2 0' + HOZ' = Oz + OH' 0' + O H + M = HOZ' + M HOP' + HOP' = HZ02 + 02 H' + HZ02 = HzO + OH' HzO2 + M = OH' + OH' + M HzOz + Cl' = HOZ' + HC1 HC1+ M = H' + Cl' + M HC1+ H' = H2 + C1' o'+ HC1= O H + C1'
Ab
n
E
1.50E14 2.00E13 2.00E13 1.00E17 2.00E12 1.00E13 1.20E17 1.26313 4.36313 7.94312 3.16313
0.0 0.0 0.0
1003.0 0.0 0.0 0.0 0.0 3583.0 45379.0 2000.0 81760.0 3400.0 6700.0
0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0
source' ab ab ab ab ab ab ab ab ab ab ab
reaction'
Ab
n
E
+ HC1 = C1' + H20 0' + C10' = CI' + 02 C1' + HOZ' = HC1+ O2 C1' + HOz' = O H ' + Clo' C10' + HOz' = HOCl + Oz C10' + H2 = HOCl+ H' H' + HOCl = HC1+ O H Cl' + HOCl = HCl + Clo' 0' + HOCl = OH' + Clo' OH' + HOCl = H20 + C10' HOCl + M = OH' + C P + M
1.58E13 9.70312 1.08E13 2.47313 3.55311 1.00E13 1.00E13 1.00E13 5.00E13 1.80E12 1.00E18
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
1000.0 507.0 -338.0 894.0 1410.0 13500.0 1000.0 2000.0 1500.0 3000.0 55000.0
OH'
source' ab ab ab ab ab ab ab ab ab ab ab
"Chaperon efficiencies of third-body M: 0 2 = 0.35, H10 = 6.5, COP= 1.47, CO = 0.74, Hz = 1.0, Nz= 0.44. bThe notation xEn represents x X 10". 'Source: (a) Kondo et al., 1980; (b) Harris et al., 1986; (c) Schatz et al., 1984; (d) Heneghan et al., 1981; (e) Westbrook and Dryer, 19&4; (f) Warnatz, 1984; (9) Kerr and Moss, 1981; (h) Westanberg and de Haea, 1975; (i) Karra and Senkan, 1988; 6 ) Kiefer and Budach, 1984; (k) Hsu et ai., 1983; (1) Paczko et al., 1986; (m) Miller et al., 1982; (n) DeMore et al., 1985; (0)Wine and Semmes, 1983; (p) Haasler et al., 1966; (r) Olson and Gardiner, 1978; (s) Kondratiev, 1972; (t) Ashmore et al., 1982; (u) Weissman and Benson, 1984; (v) Hennessy et al., 1986; (w)Zabel, 1977; (x) Park et al., 1984; (y) Brunning and Stief, 1985; (z) Frank et al., 1988; (aa) Roth and Just, 1984; (ab) Chang et al., 1987; (ac) via fall-off analysis using Troe's formalism (Gardiner and Troe, 1984) or the QRRK method (Robinson and Holbrook, 1972); (ad) Jeong and Kaufman, 1982; (ae) Frank et al., 1986; (af)assuming fast decomposition of CHzC1O'; (ag) assuming fast decomposition of COCl'.
were treated by using the quantized RRK (Rice-Rampsperger-Kassel) or QRRK method (Karra and Senkan, 1988). In Table I we report rate parameters evaluated specifically at 515 Torr, the details of which are presented elsewhere (Karra and Senkan, 1988). Fall-off corrections for regular unimolecular decomposition reactions were made by using Troe's semiempirical formalism (Gardiner and Troe, 1984) or by the unimolecular QRRK method (Robinson and Holbrook, 1972).
Numerical Model The reaction mechanism developed was tested to simulate the flow reactor experiments reported recently by Granada et al. (1987). Simulations were made by assuming flat concentration profiles in the radial direction and by neglecting axial dispersion. That is, a unique relationship was assumed to exist between the reaction time and the axial position via the mean gas flow velocity through the reactor. This was a reasonable assumption because experimental measurements indicated that radial variations in species concentrations were less than 10%; thus the assumption of radially uniform concentration profiles results in an error which is about the same order of magnitude as other experimental errors. In addition, axial concentration gradients were too small to result in any appreciable diffusive fluxes when compared to convection. In the absence of diffusive fluxes, mass, momentum, and energy conservation equations represent a simple initial value problem. The model, therefore, involves the specification of an initial mixture composition and temperature, followed by the direct integration of the following conservation equations forward in time (or position): N
pCp dT/dt = -C W k M k h k k=l
(1)
where p is the mass density, C, is the mean specific heat of the mixture, T i s the temperature, t is the reaction time, wk is the net molar rate of generation of species k as a consequence of competition between various elementary reactions in the mechanism presented in Table I, Mk is the molecular weight, hk is the specific enthalpy, and yk is the mass fraction of species k. The Sandia CHEMKIN code was used for the calculations (Kee et al., 1980), in which the LSODE stiff differential equation integrator program was utilized (Hindmarsh, 1980). The thermochemical information, which includes the heats of formation, specific entropy, and the temperature-dependent specific heat data, was acquired from the JANAF Thermochemical Tables (Chase et al., 1985) NASA
Symbols: D a t a L i n e s : Model 0.4
I
C2H3C1 0
2
0
0
0
0
0
---()
Table 11. Conditions Investigated" apeciea initial mixture composition, mol % CH,Cl 0 2
Ar a
7.32 2.05 90.6
T = 980 "C, P = 515 Torr, u = 150 cm/s, mean residence time
range = 50-250 ma.
compilations (Bahn, 1973), Baulch et al. (1981), and Benson's work (Benson, 1976; Benson and Weissman, 1982; Shum and Benson, 1983; Weissman and Benson, 1984), whenever they were available. However, such data for some of the species in the mechanism are not documented; consequently, they were estimated by using theoretical methods (Benson, 1976).
Results and Discussion Before a detailed analysis of the numerical model is presented, a number of important points concerning the CCOP process must be noted. First, the oxidative py-rolysis of CH3Cl, in spite of the presence of O2 and high temperatures associated with the process, does not result in the formation of flames because the mixture is outside the limits of flammability. Second, the oxidative pyrolysis of CH3Cl is a short-chain process; i.e., the rates of formation of products are about a factor of 2-5 larger than the rate of decomposition of CH3C1. Third, the overall process is approximately thermoneutral; i.e., near isothermal conditions exist in the system. In Figure 1 calculated mole percent profiles (indicated by lines) for CH3C1,CzH4,C2H2,and CzH3Clare compared to those determined experimentally (indicated by symbols) for the conditions presented in Table 11. Similar profiles
CH3C1= CH,' + C1', respectively. The highly endothermic C12was unimportant under reaction C1' + CH3C1= CH,' these conditions and thus was removed from the mechanism. Upon its formation, HC1 rapidly converts the CH; radicals into CHI by CH,' + HC1 = CH4 + C1' and regenerates C1'. Consequently, CH2C1' becomes the primary C1 radical in the system under the conditions investigated (see Figure 3). Since the unimolecular decomposition of CH,C1 is in the fall-off regime in the CCOP process, the rate parameters given in the mechanism correspond to values specifically determined at conditions given in Table 11. The presence of oxygen in the system accelerates the rate of conversion of CH3C1because the reaction OH' + HC1= H20 C1' reliberates the C1' radical. In addition, CH3Cl + O2 = CH2C1' + HO; also contributes to the rate of consumption of CH3C1. These results are consistent with the experimental data for CH3C1pyrolysis obtained both in the presence and absence of O2reported previously (Granada et al., 1987). It is particularly important to note that CH2C1' and CH,' radicals formed are quite stable and that their reactions with 02,e.g., CH2C1' + O2 = CHClO + OH' and CH3' + O2 = CH20 OH', are too slow under the conditions of interest to the CCOP process. Consequently, CH2C1' and CH,' radicals undergo chemically activated recombination reactions, as opposed to oxidation reactions, and result in the formation of C2hydrocarbons. In fact, it is this feature of the CW,CI' and CH; radicals that renders the CCOP process practically feasible. Other important reactions of CH3C1are CH3C1+ H' = CH,' + HC1 and CH3C1 + H' = CH2C1' + H2, both of which remove the H' radicals from the system. These reactions are important because they inhibit the higher activation energy chain branching reaction H' + O2 = OH' + 0' from building up the OH' and 0' radical pool, thereby partly contributing to the prevention of flame formation in the system (Chang and Senkan, 1985; Chang et al., 1987; Westbrook, 1982). The HC1 formed in the CCOP process also behaves similarly, and this will be discussed further below. 02. Because of the unreactivity of CH2Cl' and CH,' radicals with O2 and the absence of flames, the extent of conversion of O2 is small and the mechanism accurately describes this phenomenon. Although the mole percentage of O2is sensitive to reaction CH3C1+ 0, = CH2Cl' + HO;, the principal reaction pathway for O2 is C2H3' + O2 = CH20 + CHO' because the rate of the former reaction is relatively low. The CH20 and CHO' formed are then rapidly converted into CO. The reactions H' + O2 = OH' + 0' and C2H3. O2 = C2H2+ H02' also contribute to the consumption of 02,but only to a minor extent. The reaction C2H3. + O2 = CH20 + CHO' also is the major route for the destruction of the C2H3' radical and is largely responsible for the suppression of formation of carbonaceous deposits in the CCOP process. This will be discussed further below. C2H4. As evident from Figure 2, model predictions for C2H4are in reasonable agreement with the experimental measurements. The appearance of CzH4 as a product is a consequence of a balance between its formation and destruction reactions. Formation of ethylene occurs almost exclusively by the chemically activated recombination of CH3' and CH2C1' radicals, Le., CH,' + CHzC1' = C2H4 + HC1, throughout the reaction period studied (Karra and Senkan, 1988). The early appearance of C2H4 noted in the experiments clearly supports this mechanism. The destruction of C2H4 occurs primarily by C1' and CH2Cl'
+
I
Symbols: Data L i n e s : Mode:
!
8.17
0.18
0.is
6.21
0.20
0.22
0.23
I
0.24
Time, z
Figure 2. Comparison of calculated and experimental species profiles for CO, CH,, HC1, and H2.
,
-2
C - 2' " _____________________I
-
I
-3
I
+
+
-6
1
tb-p7-j
__ _- -lrL _ _ _ ---- -_-_ -- ____. ___________ ____ cl_.__...____ _......... --.-.--. ...-_-.-*
~
-7 E I;
0 18
a
IO
0 20
e
2'
0 22
a
23
0 24
T ?e, i
Figure 3. Calculated profiles the major radical species.
for CO, CH4, 02,HC1, and H2 are presented in Figure 2. In addition, calculated free-radical concentration profiles are presented in Figure 3 to aid in the discussion of the mechanism. As seen in Figures 1 and 2, the agreement between model predictions and the experiment is generally good, which suggests that the major steps involved in the CCOP process are reasonably well described by the mechanism. Reaction pathways responsible for the formation and destruction of species were then determined from the calculations of individual reaction rates and first-order normalized sensitivity gradients ( S J . The SIJ'swere defined as the following:
s, = (k,/C,)[dC,/dk,I
= d[ln C,l/d[ln kJ]
(3)
where C, is the concentration of species i and k, is the rate coefficient of the forward branch of the jth elementary reaction. According to this definition, S, represents the fractional change in the concentration of species i upon a fractional increase in the rate parameter of any reaction j in the mechanism. Since the definition of S , involves the consideration of all the reactions in the mechanism, primary reaction channels as well as other influential elementary steps can be identified directly from the sensitivity analysis, without the requirement of the explicit presence of a particular species in the stoichiometry of a key elementary reaction. Sandia CHEMSEN computer code was used for the calculation of sensitivity gradients (Kramer et al., 1982). CH3Cl. As seen in Figure 1,model predictions describe the extent of conversion of CH3C1reasonably well. Most important reaction channels for CH3C1 are C1' + CHBCl = CH2C1' + HC1 and its unimolecular decomposition
+
Ind. Eng. Chem. Res., Vol. 27, No. 7,1988 1167 radical attack and to a lesser extent by its unimolecular decomposition into CzHzand HP Radical attack on CzH4 forms CzH3*,which is then converted into CO and C2H2, as noted above. It must be noted that uncertainties exist with respect to the relative rates of collisional stabilization and decomposition paths of the chemically activated adduct [C2H5C1]*,which form as a consequence of CHzC1' and CH3' radical recombination process, since direct measurements do not exist at these temperatures (Karra and Senkan, 1988). Consequently,model predictions presented in Figure 2 were obtained by adjusting the rate parameters of CH,' CHzC1*= CzH4 + HC1 within reasonable limits (see Table I). C2H2. For acetylene, model predictions also are in reasonable accord with the experimental observations. The appearance of CzHzas a product lags behind that for CzH4, and this is consistent with the stepwise mechanism for acetylene formation involving the recombination of CHzCY radicals and the formation of C2H3C1,followed by the decomposition of vinyl chloride via CzH3C1= C2H2+ HC1. The unimolecular decomposition of C2H3' as well as its reaction with O2 also contributes to C2H2formation via C2H3. = C2Hz+ H' and C2H3' Oz = CzH2 HOz*,respectively. According to the model, acetylene destruction occurs due to 0' and OH' radical attack. However, since the concentrations of both 0' and OH' radicals in the system are low (see Figure 3), CzHz destruction is relatively unimportant in the CCOP process. C2H3Cl. As noted above, vinyl chloride is the primary precursor for acetylene, and its concentration profile is predicted reasonably well by the model. Essentially all of the C2H3C1is produced via the chemically activated recombination of CHzC1' radicals discussed earlier (Karra and Senkan, 1988). Contributions from other reactions are negligible. The major path of destruction of CzH3C1 is its unimolecular decomposition to acetylene. Reactions with radicals such as CY and CH2CY also contribute to the destruction process of CzH3C1,but only to a minor extent. CHI. Experimental data indicate that methane is an inevitable byproduct of CH3C1pyrolysis (Granada et al., 1987), and the model accurately predicts this. Methane forms as a result of rapid reaction, CH3' + HCl = CHI + Cl'. Consequently, the concentration of CHI increases directly with the buildup of HC1 in the system. This reaction also competes with CH,' + CH2CY = C2H4 + HCl for the CH,' radicals, thus inhibiting the rate of formation of ethylene a t high HC1 concentrations. HCl. The calculated composition profiles for HC1 also are in good agreement with experimental data. The HCl mole fraction increases monotonically with increasing conversion of CH3C1 and in many respects follows the trends in CzHzand CzH4 within the range of conditions studied. However, since HCl is involved in a large number of reactions as shown in Table I, the composite behavior of these reactions is responsible for the net growth in HCl concentration. Therefore the correct prediction of the HCl profile is an encouraging sign for the reasonableness of the reaction mechanism and the thermochemical data used in the calculations. Initially, the formation of HC1 occurs via C1' + CH3CI = CHZCY HC1 and C&Cl+ H' = CH3' + HC1. However, it is also rapidly consumed by the CH,' radicals in forming CHI. Additional HCl formation takes place by the recombination of CHzCY radicals and via the decomposition of CzH3C1. Similar to CH3C1, HC1 also plays a crucial role in the system by suppressing the formation of flames, in partic-
+
+
+
+
ular at high conversions of CH3C1. That is, HC1 efficiently scavenges the H' radials via the fast reaction H' HC1 = H2 C1' and contributes to the inhibition of the chain branching reaction H' Oz = OH' 0' from building up the OH' and 0' radical pools in the system, thus helping to prevent the formation of destructive flames (Chang and Senkan, 1985; Chang et al., 1987; Westbrook, 1982). Hz. For Ha, model predictions are in good agreement with the experimental data in spite of the presence of considerable uncertainties in the latter. As discussed in Granada et al. (1987), mole percents for Hz were determined from hydrogen atom balances because of difficulties associated with the quantification of H2 by mass spectrometry. Consequently, Hz data possess large uncertainties because of the cumulative errors associated with the measurement of all the species used in the hydrogen atom balances. According to the proposed mechanism, hydrogen forms primarily by CH3C1 H' = CHzCI' H2 and HC1 H' = Hz + CY, which also competes with the chain branching reaction H' + O2 = OH' 0'. Secondary sources for H2 are H' + CzH4 = CzH3. + H2, and the unimolecular decomposition of C2H4 into CzHzand Hz. CO. In the proposed mechanism, CO forms primarily via CHO' M = CO + H' + M and to a lesser extent via CHO' O2= CO HO;, in which the major path for the formation of CHO' is through CzH3' + Oz = CHzO + CHO'. The CHzO formed also is rapidly converted into CHO' by CHzO C1' = CHO' + HCl. Although CO has a large sensitivity for CzH2+ 0' = CH; + CO, the rate of this reaction is too slow to contribute to CO formation because of low 0' concentrations (see Figure 3). Since the levels of OH' in the system are low, CO oxidation to C02,which occurs mainly by CO + OH' = COz H', is also suppressed. Therefore, CO forms as a major product in the CCOP process. As noted above, the formation of CO is closely coupled to the CzH3*radical, which is formed by C1' and CHzC1' radical attack on CZHb This is a significant result because C2H3. is also postulated to be an important precursor for the inception and subsequent molecular weight growth of high molecular weight hydrocarbons which ultimately yield to the formation of carbonaceous deposits (Weissman and Benson, 1984; Frenklach et al., 1985; Colket, 1988). In the presence of oxygen, CzH3' radicals are rapidly consumed via C2H3' Oz = CHzO + HCO', which subsequently lead to the formation of CO. Since Oz intercepts the CZH3. radicals, it suppresses the rate of formation of high molecular weight carbonaceous deposits in the system. This aspect of the mechanism is consistent with the experimental data on CH3Cl pyrolysis reported previously by Granada et al. (1987), in which the formation of visible carbonaceous deposits were suppressed by the presence of small amounts of Oz. In conjunction with the foregoing discussion, the major reaction channels associated with the CCOP process have been identified, and they are presented in Figure 4. In this figure arrows with varying line thicknesses are used as a visual guide to illustrate relative reaction rates. As evident from this diagram, the oxidative pyrolysis of CH3C1 can be described reasonably well by considering a significantly fewer number of reactions than those presented in the comprehensive mechanism of Table I. However, this must be done with care, as under different set of conditions different reactions are likely to be important in affecting the product distributions. Consequently, a model reduction process must be undertaken following the detailed reaction path analysis of the comprehensive reaction
+
+
+
+
+
+
+
+
+
+
+
+
+
+
1168 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 CH$L
1
02
‘\\, HYDROCARBON
\
CH20, CHO
n l CL
\
L POLYMER
co Figure 4. Major reaction pathways for the oxidative pyrolysis of CHSCl.
mechanism under a particular set of conditions.
Conclusions The detailed chemical kinetic mechanism presented in Table I provides an adequate description of the oxidative pyrolysis of CH3Cl as evidenced by the successful prediction of the experimentally measured species profiles reported previously. According to the mechanism, the Cz products CzH4,CzH2,and C2H3C1form as a consequence of chemically activated recombination of CH2Cl’and CH3’, as these radicals do not react with Oz under the conditions of interest to the CCOP process. The major impact of oxygen appears to be due to its interception of the CzH3’ radicals and converting them into CO, thereby preventing the inception and further molecular weight growth of Cz and higher hydrocarbons which ultimately result in the formation of carbonaceous deposits. In spite of the presence of oxygen and high temperatures associated with the CCOP process, the reactions proceed in a controllable manner without the formation of destructive flamesf since the mixture remains unflammable. Registry No. CH,Cl, 74-87-3.
Literature Cited Ashmore, P. G.; Owen, A. J.; Robinson, P. J. J . Chem. Soc., Faraday Trans. 1 1982, 78,677. Bahn, G. S. “Approximate Thermochemical Tables for some C-H and C-H-0 Species”. NASA Report CR-2178, 1973. Baulch, D. L.; Duxbury, J.; Grant, S. J.; Montague, D. C. J . Phys. Chem. Ref. Data 1981, 4(10), Suppl. 1. Benson, S. W. Thermochemical Kinetics; Wiley: New York, 1976. Benson, S. W. U S . Patent 4 199 533, 1980. Benson, S. W.; Weissman, M. Int. J. Chem. Kinetics 1982, 14, 1287. Benson, S. W. Can. J . Chem. 1983,61, 881. Brunning, J.; Stief, L. J. J . Chem. Phys. 1985, 83(3), 1005. Chang, W. D.; Senkan, S. M. Combust. Sci. Technol. 1985, 43, 49. Chang, W. D.; Karra, S. B.; Senkan, S. M. Combust. Flame 1987,69, 113. Chase, M. W.; Davies, C. A.; Downey, J. R.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. “JANAF Thermochemical Tables”. J . Phys. Chem. Ref. Data 1985, 14,Suppl. 1. Colket, M. B. 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p 851. DeMore, W. B.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. “Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling”. Evaluation No. 6, J P L Publication 85-37, 1985.
Frank, P.; Bhaskaran, K. A.; Just, Th. 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p 885. Frank, P.; Bhaskaran, K. A.; Just, Th. J . Phys. Chem. 1986,90,2226. Frenklach, M.; Clary, D. W.; Gardiner, W. C.; Stein, S. E. 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985, p 887. Gardiner, W. C.; Troe, J. “Rate Coefficients of Thermal Dissociation, Isomerization, and Recombination Reactions”. In Combustion Chemistry; Gardiner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984. Granada, A.; Karra, S. B.; Senkan, S. M. Ind. Eng. Chem. Res. 1987, 26, 1901. Gorin, E. U.S. Patent 2 320 274, 1943. Harris, S. J.; Weiner, A. M.; Blint, R. J.; Goldsmith, J. E. M. 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p 1033. Hassler, J. C.; Setser, D. W.; Johnson, R. L. J. Chem. Phys. 1966,45, 3231. Heneghan, S. P.; Knoot, P. A.; Benson, S. W. Int. J . Chem. Kinet. 1981, 13, 677. Hennessy, R. J.; Robinson, C.; Smith, D. B. 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p 761. Hindmarsh. A. C. “ L S O D E Livermore Solver for Ordinary Differential Equations”. Technical Report 3342, 1980; Lawrence Livermore Laboratory. Hsu, D. S. Y.; Shaub, W. M.; Creamer, T.; Gutman, D.; Lin, M. C. Ber. Bunsenges. Phys. Chem. 1983, 87, 909. Jeong, K.-M.; Kaufman, F. J. Phys. Chem. 1982,86, 1808. Karra, S. B.; Senkan, S. M. Ind. Eng. Chem. Res. 1988, 27, 447. Kee, R. J.; Miller, J. A.; Jefferson, T. H. “CHEMKIN: A GeneralPurpose, Problem-Independent, Transportable, FORTRAN Chemical Kinetics Code Package”. SAND80-8003, March 1980; Sandia National Laboratories, Livermore. Kerr, J. A., Moss, S. J., Eds. CRC Handbook of Bimolecular and Termolecular Gas Reactions; CRC: Boca Raton, FL, 1981; Vol. I. Kiefer, J. H.; Budach, K. A. Int. J . Chem. Kinet. 1984, 16, 679. Kondo, 0.;Saito, K.; Mukarami, I. Bull. Chem. SOC.Jpn. 1980, 53, 2133. Kondratiev, V. N. “Rate Constants of Gas Phase Reactions”. COM72-10014, National Bureau of Standards, Washington, D.C., 1972. Kramer, M. A.; Kee, R. J.; Rabitz, H. “CHEMSEN: A Computer Code for Sensitivity Analysis of Elementary Chemical Reaction Models”. SAND82-8230, August 1982; Sandia National Laboratories, Livermore. Miller, J. A.; Mitchell, R. E.; Smooke, M. D.; Kee, R. J. 19th SymDosium (International) on Combustion, The Combustion Institute, Pittsburgh, 1982, p 181. Olson. D. B.: Gardiner. W. C.. Jr. Combust. Flame 1978, 32, 151. Paczko, G.; Lefdal, P. M.;Peters, N. 21st Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p 739. Park, J. Y.; Heaven, M. C., and Gutman, D. Chem. Phys. Lett. 1984, 104, 469. Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; Wiley: New York, 1972. Roth, P.; Just, Th. 20th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, p 807. Schatz, G. C.; Wagner, A. F.; Dunning, T. H., Jr. J . Phys. Chem. 1984, 88, 221. Senkan, S. M. U S . Patent 4714796, 1987a. Senkan, S. M. Chem. Eng. Prog. 1987b, 83(12), 58. Shum, G. S.; Benson, S. W. Int. J . Chem. Kinet. 1983, 15, 341. Warnatz, J. “Rate Coefficients in the C / H / O System”. In Combustion Chemistry; Gardiner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984. Weissman, M.; Benson, S. Int. J. Chem. Kinet. 1984, 16, 307. Westbrook, C. K. 19th Symposium (International) on Combustion, T h e Combustion Institute, Pittsburgh, 1982, p 127. Westbrook, C. K.; Dryer, F. A. Prog. Energy Combust. Sci. 1984,10, 1. Westenberg, A. A.; de Haas, N. J . Chem. Phys. 1975,62, 3321. Wine, P. H.; Semmes, D. H. J. Phys. Chem. 1983,87, 3572. Zabel, F. Int. J. Chem. Kinet. 1977, 9,651.
Received for review August 3, 1987 Revised manuscript received J a n u a r y 28, 1988 Accepted February 12, 1988
