Shock-tube pyrolysis of chlorinated hydrocarbons: Formation of soot

Shock-tube pyrolysis of chlorinated hydrocarbons: Formation of soot

C O M B U S T I O N A N D F L A M E 6 4 : 1 4 1 - 1 5 5 (1986) 141 Shock-Tube Pyrolysis of Chlorinated Hydrocarbons: Formation of Soot M. FRENKLACH,...

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C O M B U S T I O N A N D F L A M E 6 4 : 1 4 1 - 1 5 5 (1986)

141

Shock-Tube Pyrolysis of Chlorinated Hydrocarbons: Formation of Soot M. FRENKLACH,* J. P. HSU,** D. L. MILLER,*** and R. A. MATULA College of Engineering, Louisiana State University, Baton Rouge, LA 70803

Soot formation in pyrolysis of chlorinated methanes, their mixtures with methane, and chlorinated ethylenes were studied behind reflected shock waves by monitoring the attenuation of an He-Ne laser beam. An additional single-pulse shock-tube study was conducted for the pyrolysis of methane, methyl chloride, and dichloromethane. The experiments were performed at temperatures 1300-3000K, pressures 0.4-3.6 bar, and total carbon atom concentrations (1-5) x 10 ~7atoms/cm 3. The amounts of soot produced in the pyrolysis of chlorinated hydrocarbons are larger than that of their nonchlorinated counterparts. The sooting behavior and product distribution can be generally explained in terms of chlorine-catalyzed chemical reaction mechanisms. The pathway to soot from chlorinated methanes and ethylenes with high H : CI ratio proceeds via the formation of C2H, C2H2, and C2H3 species. For chlorinated hydrocarbons with low H : CI ratio, the formation of C2 and its contribution to soot formation at high temperatures becomes significant. There is evidence for the importance of CHCI radical and its reactions in the pyrolysis of dichloromethane.

INTRODUCTION Safe destruction of hazardous materials has become one of the major concerns of our society. Of particular importance are chlorinated hydrocarbons (CHC), which are major constituents of many industrial wastes. An effective way for the destruction of chlorinated hydrocarbons is incineration, which has stimulated an increased interest in their combustion characteristics. One important phenomenon that should be taken into account in the design and operation of commercial incinerators is the * Present address: Fuel Science Program, Department of Material Science and Engineering, Pennsylvania State University, University Park, PA 16802. ** Present address: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. *** Present address: Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA 19104. Copyright © 1986 by M. Frenklach, J. P. Hsu, D. L. Miller, and R. A. Matula Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

increased formation of soot in chlorinated hydrocarbon flames [1-5]. Characterization of soot formation from chlorinated hydrocarbons is also of interest from a more fundamental point of view: understanding the general mechanism of soot formation in hydrocarbon combustion. It has been argued [6] that the general pathway to soot in the combustion environment is probably the same as in the pyrolytic one. Since the latter is simpler, it is instructive to investigate soot formation from chlorinated hydrocarbons under the conditions of pyrolysis. To our knowledge, there have been only two reports related to the subject. Weissman and Benson [7] observed (from carbon mass balances) formation of soot in flow-tube pyrolysis of methyl chloride at approximately 1300K. In a recent shock-tube pyrolysis study conducted in our laboratory [6], soot from chlorobenzene was

142 shown to form at lower temperatures than from benzene. The work reported here is a comparative study of soot formation in the shock-tube pyrolysis of chlorinated methanes, their mixtures with methane, and chlorinated ethylenes. During the course of our investigation, an anomalous sooting behavior was observed for dichloromethane. To shed more light on the phenomenon, the reaction products of dichloromethane pyrolysis were determined in additional single-pulse shock-tube experiments. Experiments with methane and methyl chloride were also conducted for comparison.

EXPERIMENTAL The experimental work was performed using the shock-tube technique. The formation of soot was studied behind reflected shock waves in a 7.62 cm i.d. stainless steel shock tube. The apparatus and procedures used in this part of the study were similar to those described in the previous works [6, 8, 9]. The appearance of soot was monitored by the attenuation of an H e - N e laser beam (632.8 nm) at approximately 10 mm from the end plate of the shock tube. The soot yields are presented in the arbitrary form used in the previous studies [6, 8, 9]. It should be pointed out that the term " s o o t " has been used in our work as a lumped property meaning "species absorbing at a 632.8 wavelength" and " s o o t yield" as its practical measure. This measure provides a relative tendency of a given fuel (or fuel mixture) to form polycyclic aromatic hydrocarbons and soot. Pyrolysis product distributions for selected mixtures were determined from experiments performed in a 2.54 cm i.d. single-pulse shock tube. A 2.9 m driven section of the shock tube is constructed primarily of Pyrex pipe with a stainless steel test section of 0.7 m. The driver section was evacuated to less than 2 x 10 -4 Torr before each experiment. The length of the driver section is adjustable; in this manner the reaction dwell times were kept approximately constant, 500 _+ 50/xs, in different experiments. The experimental dwell times and quenching rates were determined by a pressure transducer

M. FRENKLACH ET AL. located at approximately 5 cm from the end wall of the shock tube. After quenching, the reaction products and partially consumed reactants were withdrawn from the test section of the shock tube and analyzed by gas chromatography. Flame ionization and thermal conductivity detectors were employed in the analysis. The separation was achieved using stainless steel columns: Porapak N and Q in series (both 3 mm × 2 m ) - - f o r nonchlorinated hydrocarbons, 1% SP-1000 on Carbopak B (3 m m × 2.5 m ) - - f o r chlorinated hydrocarbons, and MS13X and Porapak N in series (both 3 mm x 2 m ) - - f o r N2. Nitrogen was used in these mixtures to provide an internal standard to calibrate for the dilution taking place during the quenching phase in the shock-tube runs. The state of the gas behind reflected shock waves was calculated, assuming full relaxation and no chemical reactions [10], from the measured incident shock velocity extrapolated to the end of the shock tube. The thermodynamic data for N2, CH4, C2H4, and C2H2 were taken from Burcat [11], for C12 and chlorinated methanes-taken from JANAF tables [12, 13], and for chlorinated ethylenes were computed based on the vibration frequencies and moments of inertia from [14-18] and the heats of formation from [191. The mixtures were prepared manometrically. Matheson helium (99.995%), argon (99.995%), methane (99.97%), methyl chloride (99.5%), ethylene (99.5%), acetylene (99.6%), vinyl chloride (99.5%) and chlorine (99.9%), and Liquid Carbonic nitrogen (99.999%) were used without further purification. Methylene chloride (Mallinckrodt, 99.97%), chloroform (MCB, 99.99 %), carbon tetrachloride (MCB, 99.94 %), 1,1-dichloroethylene (Aldrich, 99%), transdichloroethylene (MCB, 95%), and trichloroethylene (MCB, 99.92%) were purified by repeated freezing and evacuation. It was determined, by a gas chromatographic analysis, that the remainder of the trans-dichloroethylene was primarily cis-dichloroethylene. Further experi-

PYROLYSIS OF CHLORINATED

143

HYDROCARBONS TABLE 1

Summary of Experimental Conditions

Series A B C D E F~ G H 1 J K L M N O

Percent (vol.) in Argon 9.30% 9.30% 9.30% 4.65% 4.65% 4.65% 9.30%

CH4 CH3CI CH2C12 CH2C12 CH2C12 CH2C12 CHCI3

9.30% CC14

4.65 % C2H2 4.65% C2H4 4.65% C2H3C1 4.65% 1,2-C2H2CIz 4.65% 1,1-C2H2C1: 4.65% C2HC13 4.65 % CH3CI

T5 (K)

P5 (bar)

[Carbon]5 x 10-17 (atoms/cm 3)

2017-2753 1808-2601 1524-2784 1713-2910 1560-2995 1542-2556 1850-2842 1861-2972 1584-2690 2024-2874 1937-2773 1714-2756 1730-2834 1750-2753 1961-2811

1.50-2.03 1.35-1.89 1.22-2.06 1.28-2.16 0.40-0.88 0.46-0.76 1.37-2.11 1.38-2.23 1.14-2.02 1.50-2.11 1.43-2.04 1.17-2.02 1.06-2.02 1.28-2.02 1.44-2.08

4.93-5.01 4.85-5.01 4.89-5.54 2.49-2.51 0.85-1.01 1.00-1.01 4.96-5.03 4.94-5.08 4.83-5.11 4.86-5.03 4.95-5.02 4.61-5.05 4.92-5.04 4.86-5.10 4.93-5.01

1765-2816

1.31-2.08

4.88-5.02

1734-2844

1.29-2.10

4.96-5.03

1720-2873

1.29-2.14

4.90-5.10

1297-1891

1.00-1.44

2.53-2.63

1645-2106

1.24-1.62

2.50-2.58

1533-2513

2.73-3.57

3.33-3.90

1345-2472

2.40-3.60

3.33-3.90

1272-1620

2.63-3.32

4.37-4.74

4.65% CH 4

P Q R S T U V W

4.65% 4.65% 4.65 % 4.65% 4.65 % 4.65% 9.30% 4.65% 4.65% 4.65% 3.00% 10.0% 3.00% 10.0% 3.00% 10.0%

CH2C12 CH4 CHCI3 CH4 CCL CH4 C12 CH4 C12 CH4 CH4 Nz CH3CI N2 CH2C12 N2

For this series of experiments the listed ranges of pressure, temperature, and total number of carbon atoms refer to the conditions behind incident shock waves, i.e., P2, 7"2, and [carbon]2 X 10-iT, respectively.

m e n t a l d e t a i l s and p r o c e d u r e s can be f o u n d in [20, 21].

RESULTS T a b l e 1 p r e s e n t s the m i x t u r e s tested and the experimental conditions covered. Series A-T w e r e u s e d in the s o o t f o r m a t i o n e x p e r i m e n t s , and s e r i e s U - W in the s i n g l e - p u l s e s h o c k - t u b e

study. N i t r o g e n u s e d in the latter g r o u p o f m i x t u r e s was a d d e d , as d i s c u s s e d in the p r e v i o u s s e c t i o n , to p r o v i d e a c a l i b r a t i n g s t a n d a r d f o r gas chromatographic analysis. T h e p r o d u c t d i s t r i b u t i o n s o b t a i n e d in the single-pulse shock-tube experiments with series U - W are s h o w n in F i g s . 1 - 3 . T h e results o f m e t h a n e p y r o l y s i s ( F i g . 1) a r e in g e n e r a l a g r e e ment with a previous single-pulse shock-tube

1~

M. FRENKLACH ET AL. ~::~1 I

F -

I ~.7,--

I

I

I

I

.............

I

~--°~

~b

lti00

1600

1800

2000

2200

TEMPERATURE

2q00

2600

(K]

Fig. 1. Productdistribution in the pyrolysis of methane (series U). The dashed line is the total amount of carbon atoms recovered by gas chromatographic analysis. The reaction time is approximately 500/~s.

work of Skinner and Ruehrwein [22]: acetylene (and molecular hydrogen, which was not analyzed for in our work) is the dominant product at conditions tested with small amounts of ethylene and ethane formed. Figure 2 presents the main reaction products determined in the pyrolysis of methyl chloride. Not shown in this figure are vinyl chloride and chloroethane that were also identified in the analysis but only in minute quantities. The main pyrolysis products and the relative order of their concentrations are in qualitative agreement with the flow-tube results of Weissman and Benson [7]. The product distribution obtained in the pyrolysis of dichloromethane is shown in Fig. 3. Relatively large quantities of chlorinated compounds are formed in this case. It should also be mentioned that relatively large amounts of soot were deposited on the shock-tube walls during this series of experiments. The formation of high molecular weight hydrocarbons, chlorinated hydrocarbons, and soot is also indicated by the decrease in the total amount of carbon atom

measured by gas chromatographic analysis of the quenched gases. Figures 4-11 present the results of the sootyield study with series A-T. For clarity in presentation only cubic spline approximations to the data points are shown in these figures. The observed degree of scatter agrees with previous work [6, 8, 9]; e.g., data points are plotted on Fig. 9; the actual experimental data can be found elsewhere [21]. Most of the experimental series were performed with approximately the same total number of carbon atoms, 5 x 10 ~T atoms/cm 3, which provided a common ground for the comparison of the soot yields. Qualitatively similar results were obtained at all observation times available in our shock-tube experiments (usually up to approximately 2 ms); a reaction time of 1 ms in most of the figures (Figs. 4, 6, 8-11) is chosen arbitrari l y - t o allow comparison with our previous results [6, 8, 9]. The dwell time at the high temperatures was often shorter than 1 ms. To emphasize important experimental trends at these temperatures, se-

PYROLYSIS OF CHLORINATED HYDROCARBONS

145

~N

C_I

I--¢'r ¢vI.-7

A

CH 4

" m

A

O

C2 H4

__j

" I~00

1600

1800

2000

2200

2t~O0

2600

TEMPERATURE (K)

Fig. 2. Product distribution in the pyrolysis of methyl chloride (series V). The dashed line is the total amount of carbon atoms recovered by gas chromatographicanalysis. The reaction time is approximately 500 t~s.

lected results are also reported for a reaction time of 0.5 ms (Figs. 5 and 7). Figures 4 and 5 present the soot yields obtained in the pyrolysis of chlorinated methanes, and Figs. 6 and 7 of chlorinated ethylenes. Also shown are the results for methane, ethylene, and acetylene at comparable conditions. As can be seen in these figures, the chlorinated hydrocarbons soot more than their nonchlorinated counterparts and generally more than acetylene. The general trend is that chlorinated hydrocarbons with H to Cl ratio of 1, i.e., CH2C12 and C2H2C12, are the sootiest: their soot yields are large, comparable to the yields observed in the pyrolysis of aromatic hydrocarbons [6, 8], and the maximum values are attained at lower temperatures, again comparable to the case of aromatic hydrocarbons [6, 8]. More hydrogenated CHC, i.e., CH3CI and C2H3C1, are poor sooters. Less hydrogenated CHC, i.e., CHCI3, CC14, and C2HC13, soot remarkably at high temperatures. Trichloroethylene is somewhat of an exception. On the

one hand, its soot-yield dependence on temperature exhibits a maximum (Fig. 7), whereas it is not clear that such a maximum exists for chloroform. On the other hand, the soot-yield maximum of trichloroethylene has a higher value but located at higher temperatures than its counterparts for dichloroethylenes. The temperatures when CHC with low H : CI ratios form soot are shifted to higher values with decreasing H : C1 ratio of the molecule. This trend can be clearly observed comparing the results for CHC13 and CCl4 at 0.5 ms shown in Fig. 5. Interestingly, the soot produced in the pyrolysis of carbon tetrachloride had a light color, off-white-gray. This soot was subjected to elemental analysis on an ISI 60A scanning electron microscope equipped with an EDAX 9100 energy dispersive x-ray analyzer operating at 25 kV. A strong signal for C1 was present; unfortunately, the soot samples collected after the shock-tube runs were too small for the elemental analysis to be performed quantitatively.

146

M. FRENKLACH ET AL.

8

o

d N

-,-`4 ¢.,J

I

!1

I

I I

} I

I I

7

O

°

C 2H2

i

,4

Z tt,-n

o

~~~

1300

rAr-~ ~ ~-~ ~-'O'xD.-~ ~ JA~<-...L , ' CaHae, - 2- ~"~-..~ ' ~--

lq00

1500

TEMPERRTURE

1600

[K)

Fig. 3. Product distribution in the pyrolysis of dichlorome-

thane (series W). The dashed line is the total amount of carbon atoms recovered by gas chromatographic analysis. The reaction time is approximately 500 #s.

In Fig. 8 the soot yields obtained in the pyrolysis of dichloromethane are compared to those of dichloroethylenes. As follows from this figure, dichloromethane is significantly sootier than dichloroethylenes, regardless of the basis on which the comparison is made--initial molecular concentration (series D) or total number of carbon atoms (series C). Also shown in this figure is the effect of the change in the initial fuel concentration on soot formation from dichloromethane (cf. series C, D, and E). This effect appears to be nonlinear: a strong influence at lower concentrations and only a small change at higher ones. Further examination of Fig. 8 reveals that the soot yields obtained in the pyrolysis of 1,1- and 1,2-dichloroethylenes are

about the same, although the 1,1- isomer soots slightly more than the 1,2- isomer. A striking feature, which is particularly obvious in Fig. 8, is the increase in sooting at the high temperatures, above approximately 2300K in Fig. 8. The increase is not observed for all the species tested (e.g., CHaC1, Figs. 4 and 5). When, however, this phenomenon takes place, it seems to occur at the same temperature (cf. Figs. 4-8, 10, 11). Similar behavior was observed in the pyrolysis of acetylene [9]. It was thought then that this second increase of soot yield with temperature may have been an artifact of the reflected shock-wave technique: the reaction is initiated by the incident shock wave before the reflected shock wave reaches the reaction area. In the present work we tested this hypothesis. Two series of experiments, series E and F, were performed with dichloromethane choosing such initial parameters that produce similar conditions behind incident (series F) and reflected (series E) shock waves. The results obtained in these experiments (Fig. 9) show that the increase in soot yield at the high temperatures is observed for both incident and reflected shock-wave modes, thus indicating that this phenomenon is the real temperature dependence and not an experimental artifact. The higher soot yields obtained in series E compared with that in series F are probably due to additional hydrogen atoms formed behind reflected shock waves from impurities [23-25]. A recent computer modeling study [26] revealed the importance of hydrogen atoms in soot formation: their abstraction reaction with aromatic hydrocarbons is the driving force in polymerization kinetics of soot precursors. Figure 10 presents the results obtained in the pyrolysis of the mixtures of chloromethanes with methane. The trend toward increasing sooting tendency from CH3C1-CH4 to CC14-CH4 mixtures is apparent from this figure. An increase in soot formation when a chlorinated hydrocarbon is added to a nonchlorinated fuel is anticipated [1-5, 7]. What is unexpected, however, is the extent of the observed increase,

PYROLYSIS OF CHLORINATED HYDROCARBONS

147

¢P 0

,9.3DX Cfl2CI2

0 Q

LO X

J

LI.J >..

¢:) (,t')

°°

//

g.3oY

CHCI~

g. 30 Z

C%CI

9.30X CCl~ q,. 65 X C~H~ 9.30X CH~

~T

R200.

] t600.

I

2(~00.

21100.

I

2aO0.

3200.

TEMPERRTURE(K) Fig. 4. Soot yields versus temperature in the pyrolysis of chlorinated methanes at a reaction time of 1.0 ms.

particularly for the CCI4-CH4 series (Fig. 10). To test whether the effect is due to the increase in the total chlorine content of the mixture, experiments with C12-CH4 mixtures were performed at comparable conditions. The results of these experiments are reported in Fig. 11. Inspection of this figure indicates that soot yields obtained in the pyrolysis of a C12-CH4 mixture (series S) containing the same amount of chlorine as the CC14-CH4 mixture (series R) are shifted to lower temperatures with a reduction in their values compared with the CC14-CH 4 case. Another interesting observation is that CCI4-CH4 is the only mixture in Fig. 10 which exhibits the high-temperature soot-yield rise.

DISCUSSION The results obtained in a recent modeling study on the mechanism of soot formation in the pyrolysis of acetylene [26] indicate that sooting tendency of C~- and C2-hydrocarbons may be correlated with the ease in formation of CzH, C2H2, and C2H3 species, which would be the most efficient initiators of the route to soot at these conditions. This suggestion may be viewed as a generalization of the acetylene theory (for review and references on the acetylene theory see, e.g., [27-31]), which basically states that the formation of acetylene is a necessary intermediate step in soot formation. Although this may not be true for aromatic

148

M. FRENKLACH ET AL. 0 O

9. ~)OX CHICI i

.0

!



X.

CHCI s

IJJ x w _.j

""

LLI pI--

o

o=

CClq

o /

~200.

I t~00.

2000,

2gO0.

.9.30X CHIC1

2000.

3200,

TEMPER~qTuP,E"(K] Fig. 5. Soot yields versus temperature in the pyrolysis of chlorinated methanes at a reaction time of 0.5 ms.

hydrocarbons [8, 32-34], the conversion to C2H, C2H2, and C2H3 is likely to precede the formation of aromatics and consequently soot in the case of Ci- and C2-hydrocarbons. High sooting propensity of chlorinated hydrocarbons has been attributed to a weaker C-C1 bond compared with a C - H bond [3, 5-7]. The weaker C-CI bond implies a relatively high concentration of C1. Weissman and Benson [7] stated that "in a system containing CI atoms and hydrocarbons (RH), the most rapid reactions will be buffer reactions of the type CI + RH = HC1 + R." They suggested the following mechanism for the early stages of methyl chloride pyrolysis: CH3CI = CH3 + CI,

(R1)

CI + CH3CI = HCI + CH2C1,

(R2)

C1 + CH 4 = HC1 + CH3,

(R3)

CH3 + CH3 = C2H6,

(R4)

CH2C1 + CH2C1 = 1,2-C2H4C12,

(R5)

CH3 + CH2C1 = C2H5C1,

(R6)

C2H5C1 = C2H4 + HC1,

(R7)

1,2-C2H4C12 = C2H3CI + HCI,

(R8)

C2H3C1 = C2H2 + HC1,

(R9)

C1 +

C2H6 = H C 1 + C 2 H s ,

(R10)

C2H5 = C2H4 + H ,

(Rll)

H + HC1 = H2 + C1.

(R12)

The main mass flux in the mechanism is the formation of CH 3 and CHzC1 radicals (R1)(R3), their recombinations (R4)-(R6), decomposition of the recombination products via HCI elimination (R7)-(R9), and further dehydrogenation driven by H-abstraction reactions

PYROLYSIS OF CHLORINATED HYDROCARBONS

149

4,. 65 X C2HCI ~ 4.65% I,lmC2H2C1 ~ X

.q LU >--

~C3 D



o"

0"3

4 . S 5 % 1,2-C2H~C12

W.65X C2H3C 1 .

", C2H2 4 . B5 7.

1

=t200,

tsoo.

+

~ooo.

24oo.

~8oo.

C2H ~

3~oo.

TEMPERATUREtK] Fig. 6. Soot yields versus temperature in the pyrolysis of chlorinated ethylenes at a reaction time of 1.0 ms.

(R10), etc. A similar reaction pathway was also obtained in a detailed chemical kinetic modeling of a preignition oxidation of methyl chloride [35]. Reaction sequence (R 1)-(R 12), suggested by Weissman and Benson [7] to interpret their experimental observations, also explains the product distribution of methyl chloride pyrolysis determined in this work (Fig. 2). For the formation of soot, Weissman and Benson suggested that C2H3 formed in reaction CI + C2H4 = H C I + C2H 3

(R13)

leads to C2H2 via reaction C2H3 = C2H 2 + H

(R14)

and "the formation of aromatics comes from the addition of C2H3 to C 2 H 2 " [7]. This addition was also found to be a dominant reaction pathway for

the formation of the first aromatic ring in acetylene pyrolysis [26]. Thus, the observed higher sooting tendency of methyl chloride compared with methane (Figs. 4 and 5) is explained by the chlorine-catalyzed formation of C2H3 and C2H 2. Taking the main features of the (methyl chloride) mechanism discussed above as guidelines, the basic reactions for the dichloromethane pyrolysis can be suggested as CH2C12 = CH2CI + CI,

(R15)

C1 + CH2C12 = HC1 + CHCI2,

(R16)

CH2C1 + CH2C12 = CH3CI + CHCI2,

(R17)

CHC12 + CHC12 = 1,1,2,2-C2H2C14,

(R18)

CH2CI + CH2C1 = 1,2-C2H4C12,

(RI9)

CHCI2 + CH2C1 = 1,1,2-C2H3C13,

(R20)

150

M. FRENKLACH ET AL.

CI~

LLJ X

D ._] t

[

~

\~.$57.

1,!-C2H2C12

),it)

D C)

~

g ~200.

1§00.

q. 65 7. CaH~ EO00.

2 00.

2800.

3200.

TEMPERRTUBE tK) Fig. 7. Soot yields versus temperature in the pyrolysis of chlorinated ethylenes at a reaction time of 0.5 ms.

1,1,2,2-C2H2C14

= C2HC13 +

HC1,

(R21)

1,2-C2H4C1 2 = C2H3C1 + HC1,

(R22)

1,1,2-C2H3C13 = C2H2C12 + HCI,

(R23)

C2H3C1 = C2H2 + HCI.

(R24)

This reaction sequence is also supported by the computational results obtained in a detailed chemical kinetic modeling of the preignition oxidation of dichloromethane [35]. The reader is referred to Ref. [35] for the details on the mechanism and thermochemical data for this system. The fastest reaction in the dichloromethane system is (R16), as predicted by Weissman and Benson [7]. Thus, there are more radicals CHCI2, formed in reaction (R16), than radicals CHzCI, produced primarily in the C - C I bond cleavage reaction (R15). The recombination of the radicals, (R18)--(R20), results in chlorinated ethanes, which, in turn, rapidly decompose to HCI and chlorinated ethylenes, (R21)-(R23).

Since CHClz is more abundant than CH2C1, as discussed previously, one would expect [C2HC13] > [C2H2C12] > [C2H3C1 ] .

(1)

Inspection of Fig. 3 indicates that inequality (1) is met only partially: the concentration of C2H3C1 is lower than [C2HC13] and [I,2CzH2CIz], but higher than [1,1-C2H2C12]. In other words, there is a significant difference in measured concentrations of 1,1- and 1,2-isomers of dichloroethylene. Also, the concentrations of 1,2-C2H2C12 and C2HC13 are close to one another and, actually, the former is larger than the latter at lower temperatures, contrary to inequality (1). The latter fact is explained by the results of the computational study [35]: the CHC12 recombination, forward of (R18), is very fast--much faster than the other recombinations, (R19) and (R20); the fast forward rate results in a rapid increase in the rate of the reverse direction of (R18), thus bringing this reaction to a state of partial equilibrium early in the pyroly-

PYROLYSIS OF CHLORINATED HYDROCARBONS

151

2

:12 t~ X

w J ill

,

I[~

..

¢,O

g

,d.

~2oo.

t~oo.

2doo.

2~oo.

2~oo.

3200.

TEMPERATURE[K) Fig. 8. Comparison of soot yields from chlorinated methanes and ethylenes at a reaction time of 1.0 ms.

sis. As a result, the net rate of reaction (R18) becomes smaller than the net rates of reactions (R19) and (R20). This should have resulted in low concentrations of dichloroethylenes, which may explain the results for 1,1-C2H2C12 (Fig. 3). The high concentration of 1,2-C2H2C12 and also the high soot-yield values (the latter will be discussed later) obtained in dichloromethane pyrolysis necessitate consideration of additional reactions, namely, those of CHC1. Competing with dissociation (R15), particularly at high temperatures, is another initiation channel, CH2C12 = CHCI + HC1.

(R25)

Radical CHCI formed in this reaction will probably react rapidly with the most abundant radical, CHC12, producing 1,2-C2H2C12, CHC1 + CHC12 = 1,2-C2H2C12 + CI,

(R26)

which may explain the observed difference in the concentrations of the dichloroethylene isomers. Another possible reaction of CHC1 is CHC1 + CH2C1 = C2H3C1 + C1,

(R27)

which will supply additional vinyl chloride. The latter is relatively unstable and quickly decomposes via reaction (R24) to C2H: and HC1. Indeed, the inspection of the product distribution in Fig. 3 indicates that the concentration of C2H3C1 is not high and the decline in this concentration with temperature coincides with the increase in acetylene concentration. Growing along with the acetylene concentration is the concentration of C3-species (Fig. 3). A dominant channel for this growth may be reaction

152

M. F R E N K L A C H ET AL

/~.65Z

md

° °

200

i

CH~CI z

~.6S~ CHaCIa ( S e r i e s F}

1600

2000

2~00

TEMPERRTURE

2800

3200

[K]

Fig. 9. C o m p a r i s o n o f s o o l y i e l d s i n pyrolysisofdichloromethanedeterminedbehind incidentandreflectedshockwavesatarcactiontimeofl.0ms.

g

~.B5X CCI~+U..BSX CH~

g X

,

I

CHCI~+

I

'~oo.

lloo.

\ ~ /

2doo,

~.6Sz

2~oo.

CH2C12+

2~oo.

32oo.

TEMPERFITURE (K] Fig. I0. Soot yields versus temperature in the pyrolysis of methane-chlorinated methanes mixtures at a reaction time of 1.0 ms.

PYROLYSIS OF CHLORINATED HYDROCARBONS

153

a o

E ua X

5 Z CH~

8o

--.I ~--

~-

9;~00.

1800.

2000.

2L&O0.

2800.

3200.

TEMPERATURE (K) Fig. 11. Comparisonof soot yields in CCI4-CH4and CI:-CH4mixtures at a reaction time of 1.0 ms.

CHC1 + C2H2 = C3H3 + CI.

(R28)

For instance, a similar reaction, CH2 + C2H2 = C3H3 + H,

(R29)

has been reported to be extremely fast [36]. The decrease in concentrations of C:H2 and C3hydrocarbons at approximately 1500-1600K (Fig. 3) coincides with the onset of sooting in dichloromethane pyrolysis (Figs. 4, 5, 8). To conclude our discussion of the product distribution in Fig. 3, we should mention that measurable amounts of methyl chloride were obtained in the pyrolysis of dichloromethane, which are probably formed via reaction (R17). To explain the higher sooting tendency of dichloromethane compared with that of methyl chloride (Figs. 4 and 5), one may consider a higher concentration of C1 atoms and thus a higher rate of chlorine-catalyzed production of C2H2, C2H, and C2H3 in the case of the former. However, this would not explain the higher sooting tendency of dichloromethane compared

with that of dichloroethylenes (Fig. 8), because, as discussed earlier, the latter are also the intermediates of the route to soot in the pyrolysis of the former. A search for the explanation leads again to reaction (R28). Radical C3H 3 formed in this reactions was suggested to be the cause for the high propensity to soot of allene [9]. It is pertinent to note that allene is an outstanding aliphatic in this respect: its sooting tendency is rather comparable to aromatics than to other aliphatics [9, 37], including vinylacetylene [38]. Thus, the rapid formation of C3H3 in (R28) initiates an efficient pathway to soot, characteristic to the allene system, which explains the outstanding sooting propensity of dichloromethane. In the case of chlorinated hydrocarbons with low H : C1 ratio, the formation of soot can hardly proceed by the same mechanism as in the case of nonchlorinated hydrocarbons. The rate of soot formation, according to a recently postulated mechanism [26], is determined by a

154 lifetime of " u n s t a b l e " vinyl-type radicals. The chlorinated analogues of these radicals, because of the weak C-C1 bond, should be completely unstable. Nevertheless, the experiments show that soot is formed even in the pyrolysis of carbon tetrachloride (Figs. 4 and 5), which may indicate the existence of another mechanism for soot formation at very high temperatures. This mechanism is likely to proceed through C2, whose formation is preferred at high temperatures. A possible importance of C2 in the first steps of soot formation has been also suggested in the past [30, 39]. Also, carbon mass growth by the addition of C2 to hydrocarbons has favorable thermodynamics [40]. The C2 mechanism may then explain the high-temperature increase in soot yields, which invariably coincides with the sooting from CC14, CHCI3, and C2HC13 (Figs. 4-8, 10). In addition to the C2 pathway, the " c o n v e n t i o n a l " mechanism may also be operational in the pyrolysis of CHCI3 and C2HC13; the switch of the mechanisms is particularly obvious in the latter case (Fig. 7). This would explain why CHCI3 and C2HC13 soot at lower temperatures than CC14. Following the above discussion, the rest of the experimental results can be explained. For the comparative sooting of chlorinated ethylenes (Figs. 6 and 7): vinyl chloride rapidly decomposes to HC1 and C2H2 and thus its sooting pattern is similar to that of acetylene; dichloroethylenes exhibit a transition from the "conventional" to the C2 mechanism; so does trichlorethylene. It is a fine balance of C - H and C-C1 bond breaking, HC1 elimination, C2 formation, and carbon-carbon addition reactions that differentiates the C2H2C12 and C2HC13 cases. As for the dichloroethylenes themselves, the HC1 elimination from the 1,1- and 1,2isomers results in the same C2HCI molecule and, therefore, no significant difference in the sooting is expected. The increased sooting when methane is mixed with chlorinated methanes (Fig. 10) is due to the chlorine-catalyzed enhancement of pyrolysis. The catalytic effect is proportional to the chlo-

M. FRENKLACH ET AL. rine content, as witnessed by the results of CI2CH 4 mixtures (Fig. 11). Yet, the larger amounts of soot and produced at higher temperatures in CCI4-CH 4 series compared with that in CI2-CH 4 series indicate an interaction between chlorinated and nonchlorinated molecules even in the CC14-CH4 case. The occurrence of the C2 mechanism in the latter series is probably due to reactions Cfl4 = CC13 + C1,

(R30)

C1 + CH4 = CH3 + HCI,

(R31)

CH3 + CC13 = 1,1,1 ,-C2H3C13,

(R32)

followed by a sequential HC1 elimination from 1,1,1-trichloroethane. Similar considerations for the rest of the methane-chlorinated methane mixtures lead to the formation of C2H, C2H2, and C2H3 but not C2, which may explain the experimental observation (Fig. 10) that CC14CH4 is the only mixture with a significant contribution from the C2 mechanism at high temperatures. CONCLUSIONS The amounts of soot produced in the pyrolysis of chlorinated hydrocarbons are larger than that of their nonchlorinated counterparts. The sooting behavior and product distribution can be generally explained in terms of chlorine-catalyzed chemical reactions outlined by Weissman and Benson [7]. The pathway to soot from chlorinated methanes and ethylenes with high H : C1 ratio proceeds via the formation of C2H, C2H2, and C2H3 species. For chlorinated hydrocarbons with low H : C1 ratio, the formation of C2 and its contribution to soot formation at high temperatures becomes significant. There is an evidence for the importance of CHC1 radical and its reactions in the case of dichloromethane. We wish to thank Drs. H. C. Eaton and A . C. Chow f o r performing the elemental analysis o f the "'white" soot. The research described in this article has been f u n d e d in part by the United States Environmental Protection Agency through Cooperative A g r e e m e n t No. CR80914010 granted to the Hazardous Waste Research Center at Louisiana State

PYROLYSIS OF CHLORINATED HYDROCARBONS

University and NASA-Lewis Research Center, Grant No. N A G 3-477. Although this research has been funded by the USEPA, it has not been subjected to Agency review and therefore does not necessarily reflect the views o f the Agency and no official endorsement should be inferred. REFERENCES 1. 2.

3. 4.

5. 6.

7. 8. 9. 10.

11.

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14.

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Received 13 March 1985; revised 3 September 1985