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Thermal oxidation of chloroform
T h e r m a l O x i d a t i o n of C h l o r o f o r m J. C. LOU* AND Y. S. CHANG Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, 80424 Republic of China Incineration of CHCI 3 was investigated at a high temperature under fuel-lean, isothermal conditions. A detailed chemical kinetic mechanism describing the oxidation of CHCI 3 is presented. The species CC14, C2C14, CO, CO2, C12, and HC1 were observed as major products for partial conversion of CHCI 3. The experimental results agree with those from modeling at temperatures from 700 to 1280 K. An equation for the destruction efficiency of CHCI 3 was derived in terms of the residence time and the reaction temperature, expressed as an Arrhenius function. Analysis of the measured rates of production of key species was used to discuss the mechanism. The results show that when CHC13 is destroyed, it produces the radical CCI 3 and also COCI 2. These are important intermediates in the oxidation of CHCI 3. © 1997 by The Combustion Institute
NOMENCLATURE A Ai
cross-sectional area (m 2) pre-exponential factor in the rate constant of the ith reaction (cm 3 mol- 1 s- 1; see Table 2) Cin inlet concentration of chloroform (ppmv) Cout outlet concentration of chloroform (ppmv) Cp mean specific heat of the mixture at constant pressure (J mol-l K - l ) Cpi specific heat at constant pressure of the ith species (J mo1-1 K -1) D R E destruction efficiency of chloroform (%) Ei activation energy of the rate constant of reaction i (kJ mol-~; see Table 2) hi specific enthalpy of the species (J mol- ~) K rate constant for chloroform oxidation
(s k Mi M m ni
P R T t
thermal conductivity of the mixture (W m -1 K 1) molar mass of species i mean molar mass of mixture (g/mol) mass flow rate, = p u A (g/s) temperature constant in the rate constant of reaction i (dimensionless; see Table 2) pressure(bar) universal gas constant (J K -1 mo|-1) temperature (K) reaction time (s)
* Corresponding author.
u
v, x
Y,
fluid velocity ( m / s ) diffusion velocity of species i ( m / s ) distance along the flame direction (m) mass fraction of species i (dimensionless)
Greek symbols P
4,
mass density ( g / m 3) equivalence ratio (dimensionless) net molar rate of generation of species i (mol/s)
INTRODUCTION Incineration is an effective way of treating toxic chemical wastes in which chlorinated hydrocarbons (CHCs) are of serious concern Hence, an understanding of the detailed chemical mechanisms is important for designing av efficient incineration process to destroy CHCs Chloroform (CHCI 3) is a common industrial chemical and a toxic pollutant. Thermal oxida. tion of chloroform is of interest, as it is both carcinogen [1] and a constituent of hazardou, wastes combusted in incinerators. Detailec chemical kinetic models constitute a rational starting point to evaluate the combustion o: hazardous mixtures. Related research include, fundamental kinetic mechanisms of chlori nated organics in complicated hazardous mix tures [2-5]. Chlorinated hydrocarbons arc known to inhibit the combustion of hydrocar bon fuels [3, 6]; the presence of chlorine de COMBUSTIONAND FLAME 109:188-197 (1997
0010-2180/97/$17.00 PII S00 I0-2180(96)00148-4
© 1997 by The Combustion Institute Published by Elsevier Science Inc.
OXIDATION OF CHLOROFORM creases the rate of propagation by scavenging H radicals and thereby also inhibits the oxidation of CO to CO 2. Thermal degradation of mixtures of CHCI 3 and CH 4 in a flow reactor under both pyrolytic and oxidative conditions has been investigated by measuring the distributions of the products from pure CH3C1, CH2C12, CHC13, and CCI 4 [7]. The most stable product is CH3CI and CHC13 is the least stable product. All the chloromethanes are more easily destroyed in the presence of excess oxygen than under pyrolytic conditions [7]. These previous studies also revealed that, although understanding of the high-temperature reactions of CHCI 3 has been improved, it is still not possible to predict the combustion products under diverse operating conditions. Consequently, to utilize combustion more effectively and to assess better the applicability and limitations of incineration, the kinetics of CHC combustion need understanding in greater detail. Here a detailed mechanism for the hightemperature oxidation of CHCI 3 is reported. The mechanism involves 34 species participating in 70 reversible reactions. Experiments are described under fuel-lean conditions, following earlier work [5]. The concentrations of reactants, intermediate species, and major products were all measured. Experimental results were compared with mathematical models of the combustion of mixtures of CHCI 3 + 02 + Ar. The current mechanism for fuel-lean conditions can help one develop even more complex mechanisms in the future. This work is also a reference for incinerator design. EXPERIMENTAL
The thermal oxidation of CHC13 in excess oxygen in an Ar bath gas was studied at a total pressure of 1 atm in tubular flow reactors. The reaction systems were analyzed systematically over a temperature range of 700-1280 K, with average residence times ranging from 2.0 to 4.0 s. The residence times were determined by Chang and Bozzelli [14] and altogether three different residence times were used as listed in Table 1. A schematic of the experimental set up is shown in Fig. 1. The feed CHCI 3 was intro-
189 TABLE 1 Experimental Conditions of Reactant Run Mole Percent No. CHCI 3 0 2 Ar 1 2 3
1 1 1
20 20 20
79 79 79
Equivalence Ratio(~b)
Residence Time(s)
C1/H Ratio
0.05 0.05 0.05
2.0 3.0 4.0
3 3 3
duced by passing a portion of the Ar bath gas through a diffusion tube, which was kept at a constant temperature in a water bath (20 × 23 x 15 cm3). Make-up Ar was introduced after the diffusion tube to adjust the inlet concentration of CHC13. Feed 0 2 was added into the flow stream as required. Complete mixing of the inlet gas was achieved in a mixer, located upstream of the furnace and held at 423 K. The 0 2 and Ar used were at least 99.99% purity. The reaction tube was made of quartz (length 900 mm; 13 mm i.d. and 15 mm o.d.); the length of the zone of isothermal reaction was 60 _+ 3 cm ( + 5%). The reactor was heated externally. The axial profile of the temperature was uniform within +20 K throughout the temperature range studied, so conditions were assumed to be isothermal. The concentrations of chlorinated hydrocarbon species (CHCI3, CC14, C2C14) in the effluent from the reactor were measured by injecting samples automatically, with a sampling valve, into a gas chromatograph (Shimadzu 14A) equipped with a flame-ionization detector. The gas chromatography (GC) peak area corresponding to the inlet concentrations was determined by sampling a reactor bypass stream. A stainless-steel column (2 m × 3 mm, 3% SP 1500 on 80-120 mesh Carbopack B) was used for separation and analysis of volatile organic compounds isothermally at 100 C. However, four major products of oxidation (HCI, C12, CO, and CO 2) were sampled from the vent (see Fig. 1). In fact, the exhaust gas passed through three heated transfer lines (at 100°C) to the sampling ports for HC1, C12, and both CO and CO 2, and collected independently. The concentrations of HC1 were determined after absorption in a bubbler containing acqueous NaOH solution (0.1 N) and by measuring the resulting concentration of the chlo-
190
J . C . LOU AND Y. S. CHANG
~
0
Vent & Sampling Isothermal Reactor
I,,,, , II iiiiiiii!!
Water Tang Fig. 1. Schematic diagram of the experimental apparatus. Key: PR, two-stage pressure regulator; MC, mass control valve; SV, sampling valve; VT, variable transformer; GC, gas chromatograph; FID, flame ionization detector.
ride ion with a chloride-ion electrode. The average errors of interferences for other species, as well as HCI, were less than +5%. The concentrations of CI 2 were determined according to the methyl orange method (US EPA Method 202) [8]. The concentrations of CO and CO2 were determined using infrared monitors (API model 300 and Enerac model, respectively). The lower limits of detection were 1.0 ppm for CO and 0.1% for CO 2.
where P is the total pressure (1 bar in this work), A is the cross-sectional area of the stream tube enclosing the flame and all other symbols are defined in the list of nomenclature. In this work, the absence of equations for diffusion, mass, momentum, and energy conservation reduces the system to a simple initial-value problem. For a one-dimensional premixed, isothermal reaction under plug flow, the enthalpy and species balance equations are
CHEMICAL KINETIC MODEL
--
1
dx
+A
i=l
N
(1)
+ A ~ ¢bihig i = O, i=l
dY d rh---~ + --~( p A Y i V i) - A d J i M i = 0
(i = 1. . . . . N),
(3)
pCp i=1
Kinetics coupled with a plug-flow model for CHCI 3 oxidation are considered. The equations governing steady, isobaric, quasi-one-dimensional flame propagation [9] are
rhc, dx
N
~_~ hi~oiM i = O,
(2)
dY i --
dt
(oiMi = - -
p
(i=
1 .....
N).
(4)
Equations 3 and 4 were solved numerically with the diffusionless Sandia FLAME code [10] on an microcomputer (80386 processor and 80387 math coprocessor) and a Sun-OS work station. The software used was the Sandia FLAME code written in Fortran. Thermochemical information, including enthalpies of formation, entropies, and temperature-dependent specific heats, was acquired from the JANAF tables [11, 12]. Simulations of CHCI 3 oxidation were conducted to compare with the experimental re-
OXIDATION OF CHLOROFORM suits at the conditions of P = 1 bar, t = 2 s (residence time), and ~b = 0.05 (equivalence ratio). The temperature was controlled from 700 to 1280 K. The equivalence ratio ~b for a combustor is defined as the stoichiometric ratio of oxygen to fuel normalized with respect to the real oxygen ratio [13]. Combustion is complete under fuel-lean conditions (excess air, 4) < 1), but incomplete under fuel-rich conditions (lean air, ~b > 1). A set of elementary reactions that describe the high-temperature oxidation of CHC13 is presented in Table 2 with the rate parameters for forward reaction paths. Reverse reaction rates were calculated from considerations of detailed balance between forward and reverse rates through the equilibrium constants. The mechanism presented in Table 2 was constructed by systematically considering all plausible elementary reactions of CHC13 and 0 2, and their daughter species. The oxidation kinetics of CHCI 3 are intimately related to the kinetics of combustion of many other C1 chlorohydrocarbons, and to some CHCs of greater molar mass. RESULTS AND DISCUSSION
Experimental Results The measured mole fractions of CHC13 and the major products are plotted against temperature in Figs. 2, 3, and 4, respectively, for residence times of t = 2, 3, and 4 s. The mole fractions of CHCI 3, CCI 4, C2C14, CO, C12, HC1, and CO 2 are presented as a function of temperature for several residence times in the reactor with O2:CHCI3:Ar = 20:1:79. It was found that complete decay (99.99%) of the CHCI 3 occurs after a residence time of 2 s at 950 K for this reactant ratio. The major products for CHCI 3 decomposition as shown in Fig. 2 are C2C14, CO, C12, and HC1. The minor products (mole fractions below 0.1) include CC14 and CO 2. Similar behavior is also found in Figs. 3 and 4. The sum of the amounts of chlorine in the chlorocarbon products and the reactant CHC13 decreases with increasing temperature and residence time. It was also found that the mole fraction of the intermediate C2C14 rose to 0.5, whereas that of GEl 4 was
191 merely 0.1, indicating that recombination reactions occurred during decomposition of CHC13. However, these recombination processes were suppressed when the temperature exceeded 1200 K. The major products were always CO, C12, and HC1. When the reaction temperature reached 1250 K, each intermediate was completely decomposed and the mole fraction of CO reached unity. When the intermediates, such as CC12 and C2C14, were completely decomposed, the mole fraction of C12 was twice that of HC1, suggesting that CI atoms became CI 2 and HC1 after the decomposition of CHC13. The mass balances for the elements carbon and chlorine depend strongly on temperature. As shown in Fig. 5, the balance ratios (i.e., the fraction detected) for carbon and chlorine increase with temperature. The major products when CHCI 3 conversion is above 90% (above 920 K, 2 s residence time) are HC1 CI 2, C2C14, CC14, and nonchlorinated species CO and CO z. The mass balance determinations for carbon (C) and chlorine (CI) were within _+20 and + 10%, respectively, over a temperature range from 920 to 1280 K. When the conversion of CHCI 3 is close to 100%, almost all the carbon is present as CO and the chlorine is present as HC1 and CI 2. Hence after decomposition of CHC13, carbon atoms and oxygen combined somehow to form CO; the formation of CO: was inhibited because of the large proportions of C12 and HC1. The best operating conditions for CHC13 decomposition were obtained as follows. Temperature was the major factor to affect the oxidative decomposition of CHCI 3 and also the species formed from them. Too low a temperature not only decreased combustion efficiency, but also easily generated products of incomplete combustion or secondary pollutants. Residence time also affects the amount of CHC13 treated, the size of an incinerator, and the completeness of combustion. It can be seen from Figs. 2-4 that when the residence time was 2 s, the temperature for a 99.99% destruction efficiency of CHC13 was 950 K; moreover, it was about 925 K for the same destruction efficiency when the residence time was 3 or 4 s. Hence reaction attained a state of complete destruction of CHCI 3 when the residence time was 2 s and nothing remarkable occurred when
192
J . C . LOU AND Y. S. CHANG TABLE 2 Reaction Mechanism for Oxidation of Chloroform A i
Reaction 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55.
CHCI 3 = C H C I 2 ÷ C1 CHC13 + O = C O C I 2 + HC1 CHC13 + O=CC13 + O H CHCI 3 + C|=CHC12 + C12 CHC13 + CI = CC13 + HC1 CHC12 = CC12 + HC1 CHC12 + CC! 3 =C2C14 + HC1 CHC12 + 0 2 = C H C 1 0 + CIO CHCI 2 + O = CHCIO + CI CC13 + 0 2 = C O C I 2 + CIO CCI 3 + O=COC12 + El CCI 4 = C C I 3 + CI CCI 4 + O = C C I 3 + C10 CC| 3 + CC| 3 =C2CI 4 + CI 2 CCI 2 + 0 2 = C I O + CC10 CCI 2 + C! 2 = C C I 3 + CI CHCIO + M = C O + HCl + M* CHCIO + C I = C C I O + HC1 CHC10 + O H = C C I O + H 2 0 CHCIO + O = CCIO + O H CHC10 + O 2 = CCIO + H O E CHCIO + C I O = C C I O + HOCI C2C14 + O = C O C I 2 + EEl 2 C2CI 4 + CI=C2C15 C2C15 + C I = C C I 3 + CC13 C2C15 + CI=C2C14 + 0 2 H 2 + O=O H + OH OH + OH 2 =H 2 0 + H H + 0 2 + M=HO 2 + M H + 0 2 =OH + O O + H 2 =OH + H H + H + M=H 2 + M H + H + H 2 =H 2 + H 2 H + H + H20=H 2 + H20 H + H + CO2 = H 2 + CO2 H + OH + M =H 2 0 + M H + O + M=OH + M CCIO + M = CO + C1 + M CCIO + H = CO + HCI CCIO + O H = CO + HOCI CCIO + O = CO + CIO CCIO + 0 2 = C O + 0 0 2 CCIO + C I = C O + C12 C12 + M = C I + C1 + M HCI + M = H + CI + M HC1 + H = H 2 + CI CI 2 + H = H C I + CI O + H C I = O H + CI OH + HCI=C1 + H 2 0 CI + C I O = O + CI 2 O + CIO=CI + 0 2 OH + HO 2 =H 2 0 + 0 2 H20 + O =O H + OH H + HO 2 ~ O H + O H H + HO 2 = H 2 0 + O
Ei
(cm 3 tool- 1 s - 1) 2.51E + 1.00E + 2.88E + 1.00E + 6.92E + 5.21E + 2.34E + 1.00E + 1.00E + 1.00E + 1.00E + 7.41E + 3.00E + 2.24E + 1.00E + 5.01E + 1.00E + 2.00E + 7.50E + 8.80E + 4.50E + 1.11E + 1.00E + 2.63E + 1.70E + 1.26E + 1.70E + 1.00E + 7.00E + 1.20E + 1.51E + 1.00E + 9.20E + 6.03E + 5.50E + 1.41E + 6.17E + 2.00E + 1.00E + 1.00E + 1.00E + 3.16E + 1.26E + 2.34E + 2.75E + 7.94E + 2.51E + 3.16E + 2.24E + 1.05E + 5.75E + 2.00E + 1.50E + 1.50E + 3.00E +
27 11 12 14 12 12 20 13 14 13 14 35 11 26 13 12 17 13 12 12 12 13 13 35 27 27 13 08 17 17 07 18 16 19 20 23 16 14 14 14 14 12 13 13 13 12 15 13 12 12 13 13 10 14 13
ni
(kJ m o l - 1)
-4.0 0.0 0.0 0.0 0.0 0.0 -2.5 0.0 0.0 0.0 0.0 -6.5 0.0 -4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -7.7 -4.0 -4.7 0.0 1.6 -0.8 -0.9 2.0 -1.0 -0.6 -1.3 -2.0 -2.0 -0.6 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.1 0.0 0.0
333.5 16.7 20.9 87.9 13.8 215.5 26.8 117.2 0.0 117.2 0.0 315.5 18.3 37.7 4.2 12.6 167.4 12.6 5.0 14.6 174.9 2.1 20.9 22.2 50.6 37.2 201.3 13.8 0.0 69.0 31.8 0.0 0.0 0.0 0.0 0.0 0.0 27.2 0.0 0.0 0.0 8.4 2.1 196.6 342.3 14.2 5.0 28.0 4.2 38.0 1.7 0.0 72.1 4.2 0.0
OXIDATION OF CHLOROFORM
193 TABLE
2--(Continued)
Ai Reaction 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70.
(cm 3 mol
H + HO e =H 2 + 0 2 CO + HO 2 =CO 2 + OH CO + O + M=CO 2 + M C10 + H 2 =HOC1 + H H + HOCI=HCI + OH C1 + H O C I = H C 1 + C 1 0 C1 + H O C I = C I 2 + O H OH + HOCI=H20 + ClO H O C I + M = O H + CI + M C O C I 2 + M = C C I O + C1 + M C O C I 2 + M = C O + CI 2 + M C O C I 2 + C I = C C I O + CI 2 C O C I 2 + O H = C C I O + HOC1 C O C I 2 + H = C C I O + HCI COCI 2 + O=CCIO + C10
2.50E 1.50E 5.30E 1.00E 1.00E 1.00E 1.26E 1.82E 1.00E 1.00E 1.00E 3.16E 1.00E 1.00E 2.00E
the residence period was increased. It ts concluded that under the fuel-lean conditions (~b = 0.05), the preliminary operating conditions for the destruction of CHC13 were residence time 2 s and temperature 950 K. If complete destruction of intermediates is also taken into consideration, the best operating conditions become residence time 2 s and temperature
1250K.
According to the above results, the main factors affecting the decomposition of CHCI 3 were the equivalence ratio (~b), the residence time (t), and the temperature (T). When the equivalence ratio was fixed, the efficiency of destruction (DRE) of CHCI 3 is represented by a residence time and the reaction temperature 1.0
=
0.8 .~
~" o 0.4
:,!.
1000 1100 Temperature/K
1200
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0
L)
2.9 98.7 8.4 56.5 4.2 8.4 25.1 12.6 234.3 318.0 209.2 83.7 41.8 8.4 71.1
for a first-order reaction is expressed as
C°ut
+Aexp(-~-TE)t,
-e
(5)
Cin
DRE=
1
~i~ - 1 - e x p
-Aexp
-~
t ,
(6) where A is now the frequency factor and, E is the activation energy of the reaction. Equation 6 gives a linear plot of Ink (where k = Ae e / n r ) against 1/T, as illustrated in Fig. 6, from which the frequency factor and the activation energy are (1.65 _+ 0.05) × 1012 s i and 199 + 16 kJ mol -l, respectively. The linear relationship of Fig. 6 shows that under fuel-lean
0.8
CHC13
x •
CCI4 C2CI4
•0 []
CO C12 HCI
" C02
900
13 14 13 13 13 13 13 12 18 16 16 13 12 13 13
(kJmol
1.0
02
800
+ + + + + + + + + + + + + + +
ni
1)
~.
+
0.6
Ei Is
1300
Fig. 2. Profiles of mole fractions of C H C I 3, CC14, C2C14, CO, C12, HC1, a n d C O 2 measured at the outlet of the reactor, as a function of temperature for ~b = 0.05 and t = 2 s. Reactant ratios: O2:CHC13:Ar = 20:1:79.
g
0.6
-~
0.4
~ CHC13 x
CCI 4
• • 0 []
C2CI 4 CO CI 2 HCI CO 2
0.2 0.0 800
900
1000
It00
1200
1300
Temperature/K Fig. 3. Profiles of mole fractions of C H C I 3, CC14, C2C14, CO, C I : , HC1, a n d C O e measured at the outlet of the reactor as a function of temperature for ~b = 0.05 and t = 3 s. Reactant ratios: O 2 : C H C I 3 : A r = 20:1:79.
194
J. C. LOU AND Y. S. CHANG 2
1.0 0.8 ._~
0.6
~
0.4
0.0 800
+
CHCl 3
X
CC14
•
C2C14
•
CO
© []
CI2 HCI
•
CO 2
0 ~,
-2 -4
ii
-6
...... : 900 1 0 0 0 1100 1200 1300 Temperature/K
Fig. 4. Profiles of mole fractions of CHCI3, CC14, C2C14, CO, CI 2, HCI, and CO 2 measured at the outlet of the reactor as a function of temperature for ~b = 0.05 and t = 4 s. Reactant ratios: Oz:CHCls:Ar = 20:1:79.
conditions the overall conversion of CHC13 can be considered as a first-order reaction. Comparison of Experimental Results with those from Modeling Figure 7 compares experimental and modeling results for & = 0.05 and t = 2 s. It is seen that CHCI 3 started to decompose at 700 K and attained complete destruction at 950 K; the theoretical results resemble the experimental ones. The experimental and calculated concentrations of CC14, and C2C14 were similar. As for the concentration of CO, the experimental and theoretical results were similar when the temperature was less than 1050 K; when the temperature was above 1050 K, the experimental results exceeded the theoretical ones. At 1200 K nearly all the carbon was transformed
-8 0.0011
0.0012
0.0013
0.0014
0.0015
T-qKq Fig. 6. Temperature dependence of the rate coefficient for the destruction of CHC13.
into CO, as indicated by the experimental observation that little C O 2 w a s formed, whereas CO dominated generally. The experimental results for HC1 resembled the theoretical ones below 1200 K, but the latter slightly exceeded the former above 1200 K. The experimental [C12] were close to the predicted values at 1100 K. Generally speaking, computed amounts of C 2 C 1 4 , C O , and 0 2 were slightly smaller than those observed, but the trends were similar. One thus can be quite confident that the oxidation of CHCI 3 can be modeled by 70 reversible chemical reactions of 34 reacting species, as listed in Table 2. Discussion of Reaction Mechanisms A more detailed investigation of the rates production of the major species was also under-
,0
\
0.8
\
.
1.4
F1Fq
1.2 Perfect Balance
1.0 o
"= 0.8
g~
~
[]
t~{~ " . •
"o
~r.
[]
_o
+ cu% • C2CI4
14
0.4
• CO
o
0.6 m
,/oo / ch
OCi.z
]
/
~
~
H
C
I
wHCI
0.4
Carbon Balance ( [] ) Chlorine Balance ( • )
0.2 0.0 800
0.0
. . . . . . . . 600
800
1000
1200
1400
Temperature/K 9()0
10()0
1100
12'00
1300
Temperature/K Fig. 5. Mass balance on carbon and chlorine atoms; experimental conditions: 4, = 0.05 and t = 2 s.
Fig. 7. Comparison of the experimental data (symbols) with modeling predictions (curves) for CHCI3, CCI4, C2C14, CO, C12, HCI, and CO 2. Operation conditions: ~b=0.05andt=2s.
OXIDATION OF CHLOROFORM
195
taken for the model to identify the more important reactions in the oxidation of CHC13. The main species include CHCI3, CC14, C2C14, CO, CO 2, C12, and HCI. The initial decomposition of CHC13 proceeds through thermal, unimolecular decomposition to CHC1z and C1 radicals. The following six reversible reactions are the main ways to decompose CHCI3: CHC13 =CHC12 + C1,
(7)
CHCI 3 + O=COCI 2 + HCI,
(8)
CHC13 + O=CC13 + OH,
(9)
CHCI 3 + CI=CHCI 2 + C12,
(10)
CHCI 3 + CI=CC13 + HCI,
(11)
CHC13 =CC12 + HC1.
(12)
The destruction of CHC13 occurred mostly through reactions 8 and 9. Under fuel-lean conditions, oxygen plays a more significant role. Reaction 7 increases the concentration of C1 atoms, thereby accelerating reactions 10 and 11 and also the destruction of CHCI 3. At the same time, CC13 and HCI become major species (see reactions 8-11). The intermediate CC14 is initially formed mostly from recombination of the radicals CCI 3 and C1 in the reverse of CCI 4 =CC! 3 + CI
(13)
and also in the sequence CCI 2 + 0 2 =CIO + CCIO,
(14)
CCI 4 + O=CCI 3 + CIO.
(15)
In addition, C10 formed in reaction 14 can also react with CCI 3 radicals to form CC14, (see the reverse of reaction 15). Under fuellean conditions, CC12 can form CIO radicals by reacting with 0 2 (see reaction 14), thus increasing the formation of CC14. The reverse of reaction 15 is more rapid than the forward step. Therefore, CC14 is an intermediate in the decomposition of CHC13. At increased temperatures, the forward reactions become more rapid than the reverse steps in reactions 13 and 15, so that CC14 is destroyed again. Hence, if the temperature is high enough (i.e., 1250 K), CCI 4 is effectively destroyed (see Fig. 2).
When the reaction time exceeds 2 s, the formation and decomposition of CC14, tend to be in equilibrium (see Figs. 3 and 4). The major intermediate C2C14, is initially formed mostly from the free radicals CHC12 and CC13 in CHC12 + CC13 =C2CI 4 + HC1.
(16)
When the temperature is above 900 K, the bimolecular combination of CC13 radicals gradually dominates, so that C2C14 and CI 2 are formed in EEl 3 + CCI 3 = C2CI 4 + C1z,
(17)
C2Cl 4 + O = COC12 + CCI 2.
(18)
Then, C2C14 reacts with C1 radicals to form C2C15 in C2C14 + CI=C2CI 5.
(19)
The rates of production of C2CI 5 and C1 are smaller than those of other reactions, such as: C2C15 + CI=C2C14 + Cl 2.
(20)
This interpretation is justified by Fig. 7, in which the concentration of C2C14 increased at 900-1000 K, and the same for C12. The reaction of C2C14 can thus be described by the five reversible reactions 16-20. These reactions show that C2C14 is formed from the radicals CC13 and CHCI 2, which are produced during the decomposition of CHC13 (see reactions 16 and 17). Reaction 20 between CzCI5 and CI is less important. Therefore, the oxygen content does not have much effect on the concentration of C2C14. Both HCI and C12 are very stable products and do not react readily with other species; therefore, C2C14 is formed and attacked by oxygen atoms in reaction 18. In fact, C2C14 can be completely destroyed, as long as the temperature is high enough; COCI z formed under these conditions has a close relationship to the increase of CO and CI z concentrations. One product of incomplete combustion, CO, is important in this work. Nearly all the C atoms eventually became CO, and the formation of CO 2 was not at all conspicuous. Figures 2-4 show that the concentrations of CO and C12 increased with temperature, whilst that of CO 2 was apparently suppressed. This indicates
196
J . C . LOU AND Y. S. CHANG
that CO was formed primarily by radicals such as CCIO and COCI 2 in reactions 8 and 14, and C1 atoms in reactions 21, 22, and 24. The main reason for this is that the production of C1 atoms inhibits CO from being oxidized to CO 2. This experimental result was previously studied by Chang and Senkan [9]. Therefore, a large ratio of CI to H makes CO dominate; the formation of CO 2 is thus inhibited. The following five reactions are the main reactions of CO and CO2: CC10 + M = CO + CI + M,
(21)
CCIO + CI= CO + C12,
(22)
C O + O + M = C O 2 + M,
(23)
COC12 + M = CO + C12 + M,
(24)
CC10 + 02 = CO + C10 2 .
(25)
It is seen that CO formation (below 1000 K) is initially through COC12, produced from the decomposition of CHC13, via reaction 24. During the second stage (above 1000 K), CO is produced as a result of CC14 and C2C14 decomposing, as is evident from Fig. 2. When CCl 4 and C2C14 start to decompose, the rate of formation of CO increases again. This phenomenon is further illustrated by the following pathways: A: C2C14 decomposition is achieved through reaction 18 and the COCI 2 thus produced then generates CO through reaction 24. B: Decomposition of CC14 occurs in reactions 13 and 15. These two step produce CCI 3 radicals which continue to react with 0 2 to form COCI 2 in: CCI 3 + 0 2 = COCI 2 + C10.
(26)
Then COCI 2 reacts with M to produce CO through reaction 24. Reaction 24 is the most important way of producing CO. The formation of CI 2 relies mainly on the participation of CC13 and COCI 2. When the concentration of CI atoms is too large (CI/H > 1), CI 2 is transformed into stable products. In this work both C1 atoms and H atoms were provided by CHC13, without other auxiliary fuels. Hence, the whole reaction of C12 resembles that of CO (see reactions 17 and 24).
With the decomposition of CHCI3, shown in Fig. 7, the concentration of C2C14 and CI z started to increase; then reaction 24 became faster than and more important than reaction 17. Therefore, the formation of C12 during the first stages below 1000 K is mainly achieved via reaction 24. The second stage of formation of C12 above 1000 K primarily involves the decomposition of C2CI 4 and CCI 4. Such decompositions produce COCI 2 (reaction 18) and C f l 3 (reaction 13), which finally form C12 through the reverse of reactions 17 and 24. One of the most stable products of the combustion of chlorinated hydrocarbons is HC1; it is derived from the decomposition of CHCI 3. The procedure is as follows: CHCI 3 is decomposed into CHC12 and CI radicals; then CHCI 3 reacts with CI radicals to form HC1 and CCl 3 in reaction 11. With an increased temperature of reaction, CHCI 2 reacts with CC13 to form HC1 and C2C14 in reaction 16; CCl 3 reacts with CI radicals to form CCI 4 in reaction 27. Thus, the concentration of HC1 increases and both CC14 and C2C14 appear in the products. This phenomenon is identical to the results of Fig. 2 because of reactions 7, 11, 16, and CCI 3 + C1 = E E l 4.
(27)
The above sections describe the reaction pathways for the reactant CHC13 and the major products (CCI4, C2C14, CO, CO2, El2, and HC1). An outline of the whole oxidation reaction of CHCI 3 is further illustrated in Fig. 8, together with the intermediates CCl 4 and C2CI 4. The results show that under fuel-lean conditions the oxidation of CHCI 3 is mainly achieved by the combined reactions of the radicals O and CI. The CCI 3 radical is the major precursor of the intermediates CC14 and C2C14. Also C O C l 2 is the major precursor for the final products CO and C12 whereas HCI is also a final product that is easily formed and the most stable. CONCLUSION A kinetic mechanism has been developed to describe the oxidation of chloroform under fuel-lean conditions in an isothermal reactor to investigate the stable products of oxidation.
OXIDATION
OF CHLOROFORM
Species have been measuredin experiments T h e results show that CHC13 attains a decomposition efficiency of 99.99% u n d e r fuel-lean (~b = 0.05) conditions for a reaction time of 2 s and a t e m p e r a t u r e o f 950 K. During the destruction o f CHC13, the main intermediates are CCI 4 and C2C14 whereas CO, CO2, CI 2, and HCI are the m a j o r products. These stable species describe the oxidative destruction of C H C I 3. As for the complete decomposition o f intermediates the best operating conditions for destroying CHC13 are an equivalence ratio 0.05 a residence time of 2 s, and a t e m p e r a t u r e of 1250 K. T h e m a j o r products are CO, CI 2, and HCI, whereas CO2 was inhibited. C o m p a r i s o n s were m a d e between the model and experimental m e a s u r e m e n t s of CHC13 destruction. T h e m e c h a n i s m probably involves the participation o f 34 species in 70 reversible elementary reactions and describes the combustion of CHC13 u n d e r fuel-lean conditions (~b < 1). Principal by-products of C H C I 3 oxidation at high temperatures are due to the CC13 radical acting as the reaction intermediate, whereas C O C I 2 is the major species for the formation of final products. We t h a n k the National Science Council o f the Repubfic o f China (grant N S C 84-2211-E-110009) f o r financial support.
REFERENCES 1. Sax, N. I., Dangerous Properties of Industrial Materials, Litton, New York, 1979.
197
Fig. 8. Major pathways for the thermal oxidation of CHCI 3. 2. Bos. D., and Senkan, S. M., Combust. Sci. Technol. 35:187-195 (1983). 3. Miller, D. L., Senser, D. W., Cundy, V. A., and Matula, R. A., Hazardous Waste 1:1-12 (1984). 4. Hung, S. L., and Pfefferle, L. D., Combust. Sci. Technol. 87:91-107 (1992). 5. Lou, J. C., and Chang, Y. S., J. Chinese. Inst. Environ. Eng. 4:1-9 (1994). 6. Frenklach, M., Hsu, J. P., Miller, D. L., and Matula, R. A., Combust. Flame 64:141-150 (1986). 7. Lee, W. J., Clerk, B., and Senkan, S. M., Environ. Sci. Technol. 27:949-960 (1993). 8. James, P. L., Jr., Ed., Method of Air Sampling and Analysis. Lewis Publishers, Inc., MI, 1989, pp. 313-315. 9. Chang, W. D., and Senkan, S. M., Environ. Sci. Technol. 23:442-450 (1989). 10. Kee, R. J., Miller, J. A., and Jefferson, T. H., CHEMKIN: A General Purpose, Problem-lndependent, Transportable, Fortran Chemical Kinetics Code Package, Sandia National Laboratories, Report SAND 808003, 1987. 11. Chase, M. W., Curnutt, J. L., Downey, J. R., Mcdonald, R. A., Syverud, A. N., and Vatenzuela, E. A., Eds., JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data 2 (1974); 4 (1975); 7 (1978), 11 (1982); 14 (1985). 12. Stull, D. R., and Prophet, H., Eds., JANAF Thermochemical Tables, U.S. NBS Publication No. NSRDSNBS 37, Washington, DC, 1971. 13. Flagan, R. C., and Seinfeld, J. H. Fundamentals of Air Pollution Engineering. Prentice-Hall, Englewood Cliffs, NJ, 1988, p. 63. 14. Chang, S. H., and Bozzelli, J. W., AICHE J. 33:1207 (1987). 15. Thomson, M. J., Higgins, B. S., Lucas, D., Koshland, C. P., and Sawyer, R. F., Combust. Flame 98:350-360 (1994). Received July 3, 1995
NOMENCLATURE A Ai
cross-sectional area (m 2) pre-exponential factor in the rate constant of the ith reaction (cm 3 mol- 1 s- 1; see Table 2) Cin inlet concentration of chloroform (ppmv) Cout outlet concentration of chloroform (ppmv) Cp mean specific heat of the mixture at constant pressure (J mol-l K - l ) Cpi specific heat at constant pressure of the ith species (J mo1-1 K -1) D R E destruction efficiency of chloroform (%) Ei activation energy of the rate constant of reaction i (kJ mol-~; see Table 2) hi specific enthalpy of the species (J mol- ~) K rate constant for chloroform oxidation
(s k Mi M m ni
P R T t
thermal conductivity of the mixture (W m -1 K 1) molar mass of species i mean molar mass of mixture (g/mol) mass flow rate, = p u A (g/s) temperature constant in the rate constant of reaction i (dimensionless; see Table 2) pressure(bar) universal gas constant (J K -1 mo|-1) temperature (K) reaction time (s)
* Corresponding author.
u
v, x
Y,
fluid velocity ( m / s ) diffusion velocity of species i ( m / s ) distance along the flame direction (m) mass fraction of species i (dimensionless)
Greek symbols P
4,
mass density ( g / m 3) equivalence ratio (dimensionless) net molar rate of generation of species i (mol/s)
INTRODUCTION Incineration is an effective way of treating toxic chemical wastes in which chlorinated hydrocarbons (CHCs) are of serious concern Hence, an understanding of the detailed chemical mechanisms is important for designing av efficient incineration process to destroy CHCs Chloroform (CHCI 3) is a common industrial chemical and a toxic pollutant. Thermal oxida. tion of chloroform is of interest, as it is both carcinogen [1] and a constituent of hazardou, wastes combusted in incinerators. Detailec chemical kinetic models constitute a rational starting point to evaluate the combustion o: hazardous mixtures. Related research include, fundamental kinetic mechanisms of chlori nated organics in complicated hazardous mix tures [2-5]. Chlorinated hydrocarbons arc known to inhibit the combustion of hydrocar bon fuels [3, 6]; the presence of chlorine de COMBUSTIONAND FLAME 109:188-197 (1997
0010-2180/97/$17.00 PII S00 I0-2180(96)00148-4
© 1997 by The Combustion Institute Published by Elsevier Science Inc.
OXIDATION OF CHLOROFORM creases the rate of propagation by scavenging H radicals and thereby also inhibits the oxidation of CO to CO 2. Thermal degradation of mixtures of CHCI 3 and CH 4 in a flow reactor under both pyrolytic and oxidative conditions has been investigated by measuring the distributions of the products from pure CH3C1, CH2C12, CHC13, and CCI 4 [7]. The most stable product is CH3CI and CHC13 is the least stable product. All the chloromethanes are more easily destroyed in the presence of excess oxygen than under pyrolytic conditions [7]. These previous studies also revealed that, although understanding of the high-temperature reactions of CHCI 3 has been improved, it is still not possible to predict the combustion products under diverse operating conditions. Consequently, to utilize combustion more effectively and to assess better the applicability and limitations of incineration, the kinetics of CHC combustion need understanding in greater detail. Here a detailed mechanism for the hightemperature oxidation of CHCI 3 is reported. The mechanism involves 34 species participating in 70 reversible reactions. Experiments are described under fuel-lean conditions, following earlier work [5]. The concentrations of reactants, intermediate species, and major products were all measured. Experimental results were compared with mathematical models of the combustion of mixtures of CHCI 3 + 02 + Ar. The current mechanism for fuel-lean conditions can help one develop even more complex mechanisms in the future. This work is also a reference for incinerator design. EXPERIMENTAL
The thermal oxidation of CHC13 in excess oxygen in an Ar bath gas was studied at a total pressure of 1 atm in tubular flow reactors. The reaction systems were analyzed systematically over a temperature range of 700-1280 K, with average residence times ranging from 2.0 to 4.0 s. The residence times were determined by Chang and Bozzelli [14] and altogether three different residence times were used as listed in Table 1. A schematic of the experimental set up is shown in Fig. 1. The feed CHCI 3 was intro-
189 TABLE 1 Experimental Conditions of Reactant Run Mole Percent No. CHCI 3 0 2 Ar 1 2 3
1 1 1
20 20 20
79 79 79
Equivalence Ratio(~b)
Residence Time(s)
C1/H Ratio
0.05 0.05 0.05
2.0 3.0 4.0
3 3 3
duced by passing a portion of the Ar bath gas through a diffusion tube, which was kept at a constant temperature in a water bath (20 × 23 x 15 cm3). Make-up Ar was introduced after the diffusion tube to adjust the inlet concentration of CHC13. Feed 0 2 was added into the flow stream as required. Complete mixing of the inlet gas was achieved in a mixer, located upstream of the furnace and held at 423 K. The 0 2 and Ar used were at least 99.99% purity. The reaction tube was made of quartz (length 900 mm; 13 mm i.d. and 15 mm o.d.); the length of the zone of isothermal reaction was 60 _+ 3 cm ( + 5%). The reactor was heated externally. The axial profile of the temperature was uniform within +20 K throughout the temperature range studied, so conditions were assumed to be isothermal. The concentrations of chlorinated hydrocarbon species (CHCI3, CC14, C2C14) in the effluent from the reactor were measured by injecting samples automatically, with a sampling valve, into a gas chromatograph (Shimadzu 14A) equipped with a flame-ionization detector. The gas chromatography (GC) peak area corresponding to the inlet concentrations was determined by sampling a reactor bypass stream. A stainless-steel column (2 m × 3 mm, 3% SP 1500 on 80-120 mesh Carbopack B) was used for separation and analysis of volatile organic compounds isothermally at 100 C. However, four major products of oxidation (HCI, C12, CO, and CO 2) were sampled from the vent (see Fig. 1). In fact, the exhaust gas passed through three heated transfer lines (at 100°C) to the sampling ports for HC1, C12, and both CO and CO 2, and collected independently. The concentrations of HC1 were determined after absorption in a bubbler containing acqueous NaOH solution (0.1 N) and by measuring the resulting concentration of the chlo-
190
J . C . LOU AND Y. S. CHANG
~
0
Vent & Sampling Isothermal Reactor
I,,,, , II iiiiiiii!!
Water Tang Fig. 1. Schematic diagram of the experimental apparatus. Key: PR, two-stage pressure regulator; MC, mass control valve; SV, sampling valve; VT, variable transformer; GC, gas chromatograph; FID, flame ionization detector.
ride ion with a chloride-ion electrode. The average errors of interferences for other species, as well as HCI, were less than +5%. The concentrations of CI 2 were determined according to the methyl orange method (US EPA Method 202) [8]. The concentrations of CO and CO2 were determined using infrared monitors (API model 300 and Enerac model, respectively). The lower limits of detection were 1.0 ppm for CO and 0.1% for CO 2.
where P is the total pressure (1 bar in this work), A is the cross-sectional area of the stream tube enclosing the flame and all other symbols are defined in the list of nomenclature. In this work, the absence of equations for diffusion, mass, momentum, and energy conservation reduces the system to a simple initial-value problem. For a one-dimensional premixed, isothermal reaction under plug flow, the enthalpy and species balance equations are
CHEMICAL KINETIC MODEL
--
1
dx
+A
i=l
N
(1)
+ A ~ ¢bihig i = O, i=l
dY d rh---~ + --~( p A Y i V i) - A d J i M i = 0
(i = 1. . . . . N),
(3)
pCp i=1
Kinetics coupled with a plug-flow model for CHCI 3 oxidation are considered. The equations governing steady, isobaric, quasi-one-dimensional flame propagation [9] are
rhc, dx
N
~_~ hi~oiM i = O,
(2)
dY i --
dt
(oiMi = - -
p
(i=
1 .....
N).
(4)
Equations 3 and 4 were solved numerically with the diffusionless Sandia FLAME code [10] on an microcomputer (80386 processor and 80387 math coprocessor) and a Sun-OS work station. The software used was the Sandia FLAME code written in Fortran. Thermochemical information, including enthalpies of formation, entropies, and temperature-dependent specific heats, was acquired from the JANAF tables [11, 12]. Simulations of CHCI 3 oxidation were conducted to compare with the experimental re-
OXIDATION OF CHLOROFORM suits at the conditions of P = 1 bar, t = 2 s (residence time), and ~b = 0.05 (equivalence ratio). The temperature was controlled from 700 to 1280 K. The equivalence ratio ~b for a combustor is defined as the stoichiometric ratio of oxygen to fuel normalized with respect to the real oxygen ratio [13]. Combustion is complete under fuel-lean conditions (excess air, 4) < 1), but incomplete under fuel-rich conditions (lean air, ~b > 1). A set of elementary reactions that describe the high-temperature oxidation of CHC13 is presented in Table 2 with the rate parameters for forward reaction paths. Reverse reaction rates were calculated from considerations of detailed balance between forward and reverse rates through the equilibrium constants. The mechanism presented in Table 2 was constructed by systematically considering all plausible elementary reactions of CHC13 and 0 2, and their daughter species. The oxidation kinetics of CHCI 3 are intimately related to the kinetics of combustion of many other C1 chlorohydrocarbons, and to some CHCs of greater molar mass. RESULTS AND DISCUSSION
Experimental Results The measured mole fractions of CHC13 and the major products are plotted against temperature in Figs. 2, 3, and 4, respectively, for residence times of t = 2, 3, and 4 s. The mole fractions of CHCI 3, CCI 4, C2C14, CO, C12, HC1, and CO 2 are presented as a function of temperature for several residence times in the reactor with O2:CHCI3:Ar = 20:1:79. It was found that complete decay (99.99%) of the CHCI 3 occurs after a residence time of 2 s at 950 K for this reactant ratio. The major products for CHCI 3 decomposition as shown in Fig. 2 are C2C14, CO, C12, and HC1. The minor products (mole fractions below 0.1) include CC14 and CO 2. Similar behavior is also found in Figs. 3 and 4. The sum of the amounts of chlorine in the chlorocarbon products and the reactant CHC13 decreases with increasing temperature and residence time. It was also found that the mole fraction of the intermediate C2C14 rose to 0.5, whereas that of GEl 4 was
191 merely 0.1, indicating that recombination reactions occurred during decomposition of CHC13. However, these recombination processes were suppressed when the temperature exceeded 1200 K. The major products were always CO, C12, and HC1. When the reaction temperature reached 1250 K, each intermediate was completely decomposed and the mole fraction of CO reached unity. When the intermediates, such as CC12 and C2C14, were completely decomposed, the mole fraction of C12 was twice that of HC1, suggesting that CI atoms became CI 2 and HC1 after the decomposition of CHC13. The mass balances for the elements carbon and chlorine depend strongly on temperature. As shown in Fig. 5, the balance ratios (i.e., the fraction detected) for carbon and chlorine increase with temperature. The major products when CHCI 3 conversion is above 90% (above 920 K, 2 s residence time) are HC1 CI 2, C2C14, CC14, and nonchlorinated species CO and CO z. The mass balance determinations for carbon (C) and chlorine (CI) were within _+20 and + 10%, respectively, over a temperature range from 920 to 1280 K. When the conversion of CHCI 3 is close to 100%, almost all the carbon is present as CO and the chlorine is present as HC1 and CI 2. Hence after decomposition of CHC13, carbon atoms and oxygen combined somehow to form CO; the formation of CO: was inhibited because of the large proportions of C12 and HC1. The best operating conditions for CHC13 decomposition were obtained as follows. Temperature was the major factor to affect the oxidative decomposition of CHCI 3 and also the species formed from them. Too low a temperature not only decreased combustion efficiency, but also easily generated products of incomplete combustion or secondary pollutants. Residence time also affects the amount of CHC13 treated, the size of an incinerator, and the completeness of combustion. It can be seen from Figs. 2-4 that when the residence time was 2 s, the temperature for a 99.99% destruction efficiency of CHC13 was 950 K; moreover, it was about 925 K for the same destruction efficiency when the residence time was 3 or 4 s. Hence reaction attained a state of complete destruction of CHCI 3 when the residence time was 2 s and nothing remarkable occurred when
192
J . C . LOU AND Y. S. CHANG TABLE 2 Reaction Mechanism for Oxidation of Chloroform A i
Reaction 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55.
CHCI 3 = C H C I 2 ÷ C1 CHC13 + O = C O C I 2 + HC1 CHC13 + O=CC13 + O H CHCI 3 + C|=CHC12 + C12 CHC13 + CI = CC13 + HC1 CHC12 = CC12 + HC1 CHC12 + CC! 3 =C2C14 + HC1 CHC12 + 0 2 = C H C 1 0 + CIO CHCI 2 + O = CHCIO + CI CC13 + 0 2 = C O C I 2 + CIO CCI 3 + O=COC12 + El CCI 4 = C C I 3 + CI CCI 4 + O = C C I 3 + C10 CC| 3 + CC| 3 =C2CI 4 + CI 2 CCI 2 + 0 2 = C I O + CC10 CCI 2 + C! 2 = C C I 3 + CI CHCIO + M = C O + HCl + M* CHCIO + C I = C C I O + HC1 CHC10 + O H = C C I O + H 2 0 CHCIO + O = CCIO + O H CHC10 + O 2 = CCIO + H O E CHCIO + C I O = C C I O + HOCI C2C14 + O = C O C I 2 + EEl 2 C2CI 4 + CI=C2C15 C2C15 + C I = C C I 3 + CC13 C2C15 + CI=C2C14 + 0 2 H 2 + O=O H + OH OH + OH 2 =H 2 0 + H H + 0 2 + M=HO 2 + M H + 0 2 =OH + O O + H 2 =OH + H H + H + M=H 2 + M H + H + H 2 =H 2 + H 2 H + H + H20=H 2 + H20 H + H + CO2 = H 2 + CO2 H + OH + M =H 2 0 + M H + O + M=OH + M CCIO + M = CO + C1 + M CCIO + H = CO + HCI CCIO + O H = CO + HOCI CCIO + O = CO + CIO CCIO + 0 2 = C O + 0 0 2 CCIO + C I = C O + C12 C12 + M = C I + C1 + M HCI + M = H + CI + M HC1 + H = H 2 + CI CI 2 + H = H C I + CI O + H C I = O H + CI OH + HCI=C1 + H 2 0 CI + C I O = O + CI 2 O + CIO=CI + 0 2 OH + HO 2 =H 2 0 + 0 2 H20 + O =O H + OH H + HO 2 ~ O H + O H H + HO 2 = H 2 0 + O
Ei
(cm 3 tool- 1 s - 1) 2.51E + 1.00E + 2.88E + 1.00E + 6.92E + 5.21E + 2.34E + 1.00E + 1.00E + 1.00E + 1.00E + 7.41E + 3.00E + 2.24E + 1.00E + 5.01E + 1.00E + 2.00E + 7.50E + 8.80E + 4.50E + 1.11E + 1.00E + 2.63E + 1.70E + 1.26E + 1.70E + 1.00E + 7.00E + 1.20E + 1.51E + 1.00E + 9.20E + 6.03E + 5.50E + 1.41E + 6.17E + 2.00E + 1.00E + 1.00E + 1.00E + 3.16E + 1.26E + 2.34E + 2.75E + 7.94E + 2.51E + 3.16E + 2.24E + 1.05E + 5.75E + 2.00E + 1.50E + 1.50E + 3.00E +
27 11 12 14 12 12 20 13 14 13 14 35 11 26 13 12 17 13 12 12 12 13 13 35 27 27 13 08 17 17 07 18 16 19 20 23 16 14 14 14 14 12 13 13 13 12 15 13 12 12 13 13 10 14 13
ni
(kJ m o l - 1)
-4.0 0.0 0.0 0.0 0.0 0.0 -2.5 0.0 0.0 0.0 0.0 -6.5 0.0 -4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -7.7 -4.0 -4.7 0.0 1.6 -0.8 -0.9 2.0 -1.0 -0.6 -1.3 -2.0 -2.0 -0.6 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.1 0.0 0.0
333.5 16.7 20.9 87.9 13.8 215.5 26.8 117.2 0.0 117.2 0.0 315.5 18.3 37.7 4.2 12.6 167.4 12.6 5.0 14.6 174.9 2.1 20.9 22.2 50.6 37.2 201.3 13.8 0.0 69.0 31.8 0.0 0.0 0.0 0.0 0.0 0.0 27.2 0.0 0.0 0.0 8.4 2.1 196.6 342.3 14.2 5.0 28.0 4.2 38.0 1.7 0.0 72.1 4.2 0.0
OXIDATION OF CHLOROFORM
193 TABLE
2--(Continued)
Ai Reaction 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70.
(cm 3 mol
H + HO e =H 2 + 0 2 CO + HO 2 =CO 2 + OH CO + O + M=CO 2 + M C10 + H 2 =HOC1 + H H + HOCI=HCI + OH C1 + H O C I = H C 1 + C 1 0 C1 + H O C I = C I 2 + O H OH + HOCI=H20 + ClO H O C I + M = O H + CI + M C O C I 2 + M = C C I O + C1 + M C O C I 2 + M = C O + CI 2 + M C O C I 2 + C I = C C I O + CI 2 C O C I 2 + O H = C C I O + HOC1 C O C I 2 + H = C C I O + HCI COCI 2 + O=CCIO + C10
2.50E 1.50E 5.30E 1.00E 1.00E 1.00E 1.26E 1.82E 1.00E 1.00E 1.00E 3.16E 1.00E 1.00E 2.00E
the residence period was increased. It ts concluded that under the fuel-lean conditions (~b = 0.05), the preliminary operating conditions for the destruction of CHC13 were residence time 2 s and temperature 950 K. If complete destruction of intermediates is also taken into consideration, the best operating conditions become residence time 2 s and temperature
1250K.
According to the above results, the main factors affecting the decomposition of CHCI 3 were the equivalence ratio (~b), the residence time (t), and the temperature (T). When the equivalence ratio was fixed, the efficiency of destruction (DRE) of CHCI 3 is represented by a residence time and the reaction temperature 1.0
=
0.8 .~
~" o 0.4
:,!.
1000 1100 Temperature/K
1200
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0
L)
2.9 98.7 8.4 56.5 4.2 8.4 25.1 12.6 234.3 318.0 209.2 83.7 41.8 8.4 71.1
for a first-order reaction is expressed as
C°ut
+Aexp(-~-TE)t,
-e
(5)
Cin
DRE=
1
~i~ - 1 - e x p
-Aexp
-~
t ,
(6) where A is now the frequency factor and, E is the activation energy of the reaction. Equation 6 gives a linear plot of Ink (where k = Ae e / n r ) against 1/T, as illustrated in Fig. 6, from which the frequency factor and the activation energy are (1.65 _+ 0.05) × 1012 s i and 199 + 16 kJ mol -l, respectively. The linear relationship of Fig. 6 shows that under fuel-lean
0.8
CHC13
x •
CCI4 C2CI4
•0 []
CO C12 HCI
" C02
900
13 14 13 13 13 13 13 12 18 16 16 13 12 13 13
(kJmol
1.0
02
800
+ + + + + + + + + + + + + + +
ni
1)
~.
+
0.6
Ei Is
1300
Fig. 2. Profiles of mole fractions of C H C I 3, CC14, C2C14, CO, C12, HC1, a n d C O 2 measured at the outlet of the reactor, as a function of temperature for ~b = 0.05 and t = 2 s. Reactant ratios: O2:CHC13:Ar = 20:1:79.
g
0.6
-~
0.4
~ CHC13 x
CCI 4
• • 0 []
C2CI 4 CO CI 2 HCI CO 2
0.2 0.0 800
900
1000
It00
1200
1300
Temperature/K Fig. 3. Profiles of mole fractions of C H C I 3, CC14, C2C14, CO, C I : , HC1, a n d C O e measured at the outlet of the reactor as a function of temperature for ~b = 0.05 and t = 3 s. Reactant ratios: O 2 : C H C I 3 : A r = 20:1:79.
194
J. C. LOU AND Y. S. CHANG 2
1.0 0.8 ._~
0.6
~
0.4
0.0 800
+
CHCl 3
X
CC14
•
C2C14
•
CO
© []
CI2 HCI
•
CO 2
0 ~,
-2 -4
ii
-6
...... : 900 1 0 0 0 1100 1200 1300 Temperature/K
Fig. 4. Profiles of mole fractions of CHCI3, CC14, C2C14, CO, CI 2, HCI, and CO 2 measured at the outlet of the reactor as a function of temperature for ~b = 0.05 and t = 4 s. Reactant ratios: Oz:CHCls:Ar = 20:1:79.
conditions the overall conversion of CHC13 can be considered as a first-order reaction. Comparison of Experimental Results with those from Modeling Figure 7 compares experimental and modeling results for & = 0.05 and t = 2 s. It is seen that CHCI 3 started to decompose at 700 K and attained complete destruction at 950 K; the theoretical results resemble the experimental ones. The experimental and calculated concentrations of CC14, and C2C14 were similar. As for the concentration of CO, the experimental and theoretical results were similar when the temperature was less than 1050 K; when the temperature was above 1050 K, the experimental results exceeded the theoretical ones. At 1200 K nearly all the carbon was transformed
-8 0.0011
0.0012
0.0013
0.0014
0.0015
T-qKq Fig. 6. Temperature dependence of the rate coefficient for the destruction of CHC13.
into CO, as indicated by the experimental observation that little C O 2 w a s formed, whereas CO dominated generally. The experimental results for HC1 resembled the theoretical ones below 1200 K, but the latter slightly exceeded the former above 1200 K. The experimental [C12] were close to the predicted values at 1100 K. Generally speaking, computed amounts of C 2 C 1 4 , C O , and 0 2 were slightly smaller than those observed, but the trends were similar. One thus can be quite confident that the oxidation of CHCI 3 can be modeled by 70 reversible chemical reactions of 34 reacting species, as listed in Table 2. Discussion of Reaction Mechanisms A more detailed investigation of the rates production of the major species was also under-
,0
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.
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14
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Carbon Balance ( [] ) Chlorine Balance ( • )
0.2 0.0 800
0.0
. . . . . . . . 600
800
1000
1200
1400
Temperature/K 9()0
10()0
1100
12'00
1300
Temperature/K Fig. 5. Mass balance on carbon and chlorine atoms; experimental conditions: 4, = 0.05 and t = 2 s.
Fig. 7. Comparison of the experimental data (symbols) with modeling predictions (curves) for CHCI3, CCI4, C2C14, CO, C12, HCI, and CO 2. Operation conditions: ~b=0.05andt=2s.
OXIDATION OF CHLOROFORM
195
taken for the model to identify the more important reactions in the oxidation of CHC13. The main species include CHCI3, CC14, C2C14, CO, CO 2, C12, and HCI. The initial decomposition of CHC13 proceeds through thermal, unimolecular decomposition to CHC1z and C1 radicals. The following six reversible reactions are the main ways to decompose CHCI3: CHC13 =CHC12 + C1,
(7)
CHCI 3 + O=COCI 2 + HCI,
(8)
CHC13 + O=CC13 + OH,
(9)
CHCI 3 + CI=CHCI 2 + C12,
(10)
CHCI 3 + CI=CC13 + HCI,
(11)
CHC13 =CC12 + HC1.
(12)
The destruction of CHC13 occurred mostly through reactions 8 and 9. Under fuel-lean conditions, oxygen plays a more significant role. Reaction 7 increases the concentration of C1 atoms, thereby accelerating reactions 10 and 11 and also the destruction of CHCI 3. At the same time, CC13 and HCI become major species (see reactions 8-11). The intermediate CC14 is initially formed mostly from recombination of the radicals CCI 3 and C1 in the reverse of CCI 4 =CC! 3 + CI
(13)
and also in the sequence CCI 2 + 0 2 =CIO + CCIO,
(14)
CCI 4 + O=CCI 3 + CIO.
(15)
In addition, C10 formed in reaction 14 can also react with CCI 3 radicals to form CC14, (see the reverse of reaction 15). Under fuellean conditions, CC12 can form CIO radicals by reacting with 0 2 (see reaction 14), thus increasing the formation of CC14. The reverse of reaction 15 is more rapid than the forward step. Therefore, CC14 is an intermediate in the decomposition of CHC13. At increased temperatures, the forward reactions become more rapid than the reverse steps in reactions 13 and 15, so that CC14 is destroyed again. Hence, if the temperature is high enough (i.e., 1250 K), CCI 4 is effectively destroyed (see Fig. 2).
When the reaction time exceeds 2 s, the formation and decomposition of CC14, tend to be in equilibrium (see Figs. 3 and 4). The major intermediate C2C14, is initially formed mostly from the free radicals CHC12 and CC13 in CHC12 + CC13 =C2CI 4 + HC1.
(16)
When the temperature is above 900 K, the bimolecular combination of CC13 radicals gradually dominates, so that C2C14 and CI 2 are formed in EEl 3 + CCI 3 = C2CI 4 + C1z,
(17)
C2Cl 4 + O = COC12 + CCI 2.
(18)
Then, C2C14 reacts with C1 radicals to form C2C15 in C2C14 + CI=C2CI 5.
(19)
The rates of production of C2CI 5 and C1 are smaller than those of other reactions, such as: C2C15 + CI=C2C14 + Cl 2.
(20)
This interpretation is justified by Fig. 7, in which the concentration of C2C14 increased at 900-1000 K, and the same for C12. The reaction of C2C14 can thus be described by the five reversible reactions 16-20. These reactions show that C2C14 is formed from the radicals CC13 and CHCI 2, which are produced during the decomposition of CHC13 (see reactions 16 and 17). Reaction 20 between CzCI5 and CI is less important. Therefore, the oxygen content does not have much effect on the concentration of C2C14. Both HCI and C12 are very stable products and do not react readily with other species; therefore, C2C14 is formed and attacked by oxygen atoms in reaction 18. In fact, C2C14 can be completely destroyed, as long as the temperature is high enough; COCI z formed under these conditions has a close relationship to the increase of CO and CI z concentrations. One product of incomplete combustion, CO, is important in this work. Nearly all the C atoms eventually became CO, and the formation of CO 2 was not at all conspicuous. Figures 2-4 show that the concentrations of CO and C12 increased with temperature, whilst that of CO 2 was apparently suppressed. This indicates
196
J . C . LOU AND Y. S. CHANG
that CO was formed primarily by radicals such as CCIO and COCI 2 in reactions 8 and 14, and C1 atoms in reactions 21, 22, and 24. The main reason for this is that the production of C1 atoms inhibits CO from being oxidized to CO 2. This experimental result was previously studied by Chang and Senkan [9]. Therefore, a large ratio of CI to H makes CO dominate; the formation of CO 2 is thus inhibited. The following five reactions are the main reactions of CO and CO2: CC10 + M = CO + CI + M,
(21)
CCIO + CI= CO + C12,
(22)
C O + O + M = C O 2 + M,
(23)
COC12 + M = CO + C12 + M,
(24)
CC10 + 02 = CO + C10 2 .
(25)
It is seen that CO formation (below 1000 K) is initially through COC12, produced from the decomposition of CHC13, via reaction 24. During the second stage (above 1000 K), CO is produced as a result of CC14 and C2C14 decomposing, as is evident from Fig. 2. When CCl 4 and C2C14 start to decompose, the rate of formation of CO increases again. This phenomenon is further illustrated by the following pathways: A: C2C14 decomposition is achieved through reaction 18 and the COCI 2 thus produced then generates CO through reaction 24. B: Decomposition of CC14 occurs in reactions 13 and 15. These two step produce CCI 3 radicals which continue to react with 0 2 to form COCI 2 in: CCI 3 + 0 2 = COCI 2 + C10.
(26)
Then COCI 2 reacts with M to produce CO through reaction 24. Reaction 24 is the most important way of producing CO. The formation of CI 2 relies mainly on the participation of CC13 and COCI 2. When the concentration of CI atoms is too large (CI/H > 1), CI 2 is transformed into stable products. In this work both C1 atoms and H atoms were provided by CHC13, without other auxiliary fuels. Hence, the whole reaction of C12 resembles that of CO (see reactions 17 and 24).
With the decomposition of CHCI3, shown in Fig. 7, the concentration of C2C14 and CI z started to increase; then reaction 24 became faster than and more important than reaction 17. Therefore, the formation of C12 during the first stages below 1000 K is mainly achieved via reaction 24. The second stage of formation of C12 above 1000 K primarily involves the decomposition of C2CI 4 and CCI 4. Such decompositions produce COCI 2 (reaction 18) and C f l 3 (reaction 13), which finally form C12 through the reverse of reactions 17 and 24. One of the most stable products of the combustion of chlorinated hydrocarbons is HC1; it is derived from the decomposition of CHCI 3. The procedure is as follows: CHCI 3 is decomposed into CHC12 and CI radicals; then CHCI 3 reacts with CI radicals to form HC1 and CCl 3 in reaction 11. With an increased temperature of reaction, CHCI 2 reacts with CC13 to form HC1 and C2C14 in reaction 16; CCl 3 reacts with CI radicals to form CCI 4 in reaction 27. Thus, the concentration of HC1 increases and both CC14 and C2C14 appear in the products. This phenomenon is identical to the results of Fig. 2 because of reactions 7, 11, 16, and CCI 3 + C1 = E E l 4.
(27)
The above sections describe the reaction pathways for the reactant CHC13 and the major products (CCI4, C2C14, CO, CO2, El2, and HC1). An outline of the whole oxidation reaction of CHCI 3 is further illustrated in Fig. 8, together with the intermediates CCl 4 and C2CI 4. The results show that under fuel-lean conditions the oxidation of CHCI 3 is mainly achieved by the combined reactions of the radicals O and CI. The CCI 3 radical is the major precursor of the intermediates CC14 and C2C14. Also C O C l 2 is the major precursor for the final products CO and C12 whereas HCI is also a final product that is easily formed and the most stable. CONCLUSION A kinetic mechanism has been developed to describe the oxidation of chloroform under fuel-lean conditions in an isothermal reactor to investigate the stable products of oxidation.
OXIDATION
OF CHLOROFORM
Species have been measuredin experiments T h e results show that CHC13 attains a decomposition efficiency of 99.99% u n d e r fuel-lean (~b = 0.05) conditions for a reaction time of 2 s and a t e m p e r a t u r e o f 950 K. During the destruction o f CHC13, the main intermediates are CCI 4 and C2C14 whereas CO, CO2, CI 2, and HCI are the m a j o r products. These stable species describe the oxidative destruction of C H C I 3. As for the complete decomposition o f intermediates the best operating conditions for destroying CHC13 are an equivalence ratio 0.05 a residence time of 2 s, and a t e m p e r a t u r e of 1250 K. T h e m a j o r products are CO, CI 2, and HCI, whereas CO2 was inhibited. C o m p a r i s o n s were m a d e between the model and experimental m e a s u r e m e n t s of CHC13 destruction. T h e m e c h a n i s m probably involves the participation o f 34 species in 70 reversible elementary reactions and describes the combustion of CHC13 u n d e r fuel-lean conditions (~b < 1). Principal by-products of C H C I 3 oxidation at high temperatures are due to the CC13 radical acting as the reaction intermediate, whereas C O C I 2 is the major species for the formation of final products. We t h a n k the National Science Council o f the Repubfic o f China (grant N S C 84-2211-E-110009) f o r financial support.
REFERENCES 1. Sax, N. I., Dangerous Properties of Industrial Materials, Litton, New York, 1979.
197
Fig. 8. Major pathways for the thermal oxidation of CHCI 3. 2. Bos. D., and Senkan, S. M., Combust. Sci. Technol. 35:187-195 (1983). 3. Miller, D. L., Senser, D. W., Cundy, V. A., and Matula, R. A., Hazardous Waste 1:1-12 (1984). 4. Hung, S. L., and Pfefferle, L. D., Combust. Sci. Technol. 87:91-107 (1992). 5. Lou, J. C., and Chang, Y. S., J. Chinese. Inst. Environ. Eng. 4:1-9 (1994). 6. Frenklach, M., Hsu, J. P., Miller, D. L., and Matula, R. A., Combust. Flame 64:141-150 (1986). 7. Lee, W. J., Clerk, B., and Senkan, S. M., Environ. Sci. Technol. 27:949-960 (1993). 8. James, P. L., Jr., Ed., Method of Air Sampling and Analysis. Lewis Publishers, Inc., MI, 1989, pp. 313-315. 9. Chang, W. D., and Senkan, S. M., Environ. Sci. Technol. 23:442-450 (1989). 10. Kee, R. J., Miller, J. A., and Jefferson, T. H., CHEMKIN: A General Purpose, Problem-lndependent, Transportable, Fortran Chemical Kinetics Code Package, Sandia National Laboratories, Report SAND 808003, 1987. 11. Chase, M. W., Curnutt, J. L., Downey, J. R., Mcdonald, R. A., Syverud, A. N., and Vatenzuela, E. A., Eds., JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data 2 (1974); 4 (1975); 7 (1978), 11 (1982); 14 (1985). 12. Stull, D. R., and Prophet, H., Eds., JANAF Thermochemical Tables, U.S. NBS Publication No. NSRDSNBS 37, Washington, DC, 1971. 13. Flagan, R. C., and Seinfeld, J. H. Fundamentals of Air Pollution Engineering. Prentice-Hall, Englewood Cliffs, NJ, 1988, p. 63. 14. Chang, S. H., and Bozzelli, J. W., AICHE J. 33:1207 (1987). 15. Thomson, M. J., Higgins, B. S., Lucas, D., Koshland, C. P., and Sawyer, R. F., Combust. Flame 98:350-360 (1994). Received July 3, 1995