Homogeneous Kinetics of Methyl Chloride Chlorination Bruce E. Kurtz Syracuse Technical Center, Allied Chemical Corp., P.O. Box 6 , Solvay, N Y 13909
The commercially important homogeneous thermal chlorination of methyl chloride yields major amounts of higher chloromethanes (methylene chloride, chloroform, arid carbon tetrachloride) as primary products, minor amounts of chloroethanes as secondary products, and still more minor amounts of chloroethylenes as tertiary products. Using the rigorous reaction mechanism and a priori values for individual reaction rate constants, the products of reaction were calculated as a function of chlorine-methyl chloride ratio, number of reaction stages, and temperature. Over 200 simultaneous reactions were involved, and a computer program was developed to obtain the results. Experiments were carried out in a multistage tubular flow reactor, and the calculated and observed results compared. For primary and secondary products the agreement obtained was well within the accuracy of the rate constant data. Calculated amounts of tertiary products were about cne fourth of the observed amounts. The primary ptoducts depend largely on chlorinemethyl chloride ratio. The relative amounts of secondary products depend only on chlorine-methyl chloride ratio, while absolute amounts depend on temperature and amount of free chlorine.
T h e mechanisms by which homogeneous chlorination of aliphatic hydrocarbons proceeds are relatively well understood, and a substantial number of data on the rate constants for individual reactions are available. Because of this, and because of the commercial importance of the products derived by chlorination of aliphatic hydrocarbons, the development of a computer model of the reaction system comprising chlorine, methane, ethane, ethylene, and all of the chlorinat'ed derivatives was undertaken. This computer model has been used to simulate chlorinations of methane, methyl chloride, various mixtures of chloroet,hanes, ethane, and 1,2-dichloroethane. Some of these results as well as the details of the development of the computer model have been presented earlier (Kurtz, 1967). I n this article, results from the computer model are compared with experimental data obtained for the chlorination of methyl chloride (CHsC1). The primary products are, of course, methylene chloride (CH2C12),chloroform (CHCL) , and carbon tetrachloride (CClJ. The relative amounts of t'hese products are readily calculable from a knowledge of relative reaction rates (Fuoss, 1943; Potter and Ylacdonald, 1947; n'atta and llantica, 1952; Johnson et al., 1959; Scipioni and Rapisardi, 1961) without' recourse to the computer model employed here. However, in addition to the amounts of primary products, we wish to calculate the amounts of by-products (chloroethanes and chloroethylenes) resulting from interactions among the free radical reaction intermediates and compare them with the observed amounts. A detailed study of byproduct formation in the production of chloromethanes has not previously been published, although there is a large amount of experimental data on the primary products (McBee et al., 1942; Johnson et al., 1959; Werezak and Hodgins, 1968; Belenko et al., 1969). Mechanism of Reactions
Homogeneous chlorinat'ion of aliphatic hydrocarbons proceeds by a free radical mechanism involving chlorine 332
Ind. Eng. Chem. Process Des. Develop., Vol. 11,
No. 3, 1972
atoms and organic free radicals as alternate chain carriers. A chlorine atom abstracts a hydrogen atom from a saturated molecule or adds to the double bond of an unsaturated molecule, forming a n organic free radical. The free radical reacts with a chlorine molecule or splits off a chlorine atom (forming a double bond), thus regenerating a chlorine atom. The introduction of a single chlorine atom or organic free radical normally results in many chain-propagating reactions; that is, the reaction chain is very long. The overall reaction rate depends on the competibion between chain initiation reactions which form atoms or free radicals, and chain termination reactions which dest'roy them. The reaction scheme for chlorination of methyl chloride is shown by Figure 1. Primary products (chloromethanes) result from chain-propagating reactions involving t,he chloromethane molecules and free radicals. Secondary products (chloroethanes) result from free radical-free radical chainterminating reactions. Tertiary products (chloroethylenes) may result from chlorination and dehydrochlorination of the secondary products. Derivation of Rate Equations
For ease of computation, the four chain-propagating reactions (and the usually unimportant organic molecule dissociation) involving each free radical are grouped together as shown below. The reactions are designated by n where n = 1, 6, 11, , . . . For substitution of methane and chloromethanes : Reaction No.
RH
n
n n n n
Reactants
f + + +
l 2 3 4
i
i
+
...
RR Clz ...
C1 . . .
+
-
Products
k
R
+
...
...
+ R
R
+
, . .
I
C1 ...
+
+
HC1 ... R RC1 , . .
For substitution or addition of ethane, chloroethanes, ethylene, or chloroethylenes:
Prod ucts
Reactants
Reaction
No.
n
HRRCl
n+1
R=R
n+2
t
.
.
Clz
n+3 n+4
+ C1 + C1 ... + RRCl
RRCl
RRCl RRCl
+
+
-+
I
+
CI
+ HCl + ClRRCl
C1 C1
+
CH,Cl
I
k
I
i
+R=R
Thus, the reactions are grouped by fives, each group having a common free radical intermediate and ordered by type as shown. Chain initiation can occur by chlorine molecule dissociation or by organic molecule dissociation. Chlorine molecule dissociation is usually the only important initiating reaction HIGhEP CbLORINATED B Y - F X O U C T S kd
Clz +c 1
+ C1
Figure 1 . Mechanism for the chlorination of methyl chloride
Homogeneous chain termination can occur in three ways: kr
c1 + C1 ---f c1, R,
+ C1+
It is important to note that the only condition assumed by Equation 1 is that the rate of accumulation of molecular products is much greater than that of the chlorine atoms and free radical intermediates. If, in addition, vie assume that the concentrations of these intermediates are constant, that is:
II.
RiCl
As long as the reaction chain is very long the rate of accumulation of the molecular products will be much greater than that of the chlorine atoms and free radical intermediates (Gavalas, 1966). Hence the rates of the nth plus (n 1) reactions are very nearly equal to the rates of the ( n 3) plus (n 4) reactions and
+ +
+
B
= "R =
kn(Mn)
(CU
+
kn+I(Jfn+l)
kn+dC12)
+
+
n
(Rn)'
=
kd(C12)
- k~(C1)'
n
k n + ~ ( J I n )-
n
n
kn+P(-1fn) -
0
To solve Equations 1 and 2 , and the various reaction rate expressions, the values of the individual reaction rate constants are required. Many of the needed activation energies and frequency factors have been measured in recent photochlorination experiments by Chiltz et al. (1963), Martens (1964, 1966), and Cillien e t al. (1967), and in earlier thermal dehydrochlorination experiments b y Barton and Howlett (1949) and by Barton and Onyon (1950). For the chlorine dissociation reaction rate constant no measured values are available; however, the equilibrium constant for the dissociation-recombination has been calculated from thermodynamic data by Evans e t al. (1955) and recently by Lloyd (1971). From the equilibrium constant and published values for the various recombination reactions, the dissociation reaction rate constant can be calculated as: K k,(Jfz)
[
+ c k,,B,B, + c BI] -
The change in concentration with time for the chlorine atoms and organic free radicals can be determined by stepwise integration of Equation 3, using Equation 1 to determine Bn in each increment. This procedure eliminates the instability in the solution which occurs if the atom and free radical concentrations are determined by direct stepmise integration of the rate expressions, as discussed elsewhere (Sno~v,1966; Kurtz, 1967). Using the atom and free radical concentrations determined from Equations 1 and 3, the molecular concentrations are determined by stepwise integration of the rate expressions for the nth, (n l ) , (n 3), and (n 4) reactions.
+
z
23
(Jf
+
=
then Equation 3 reduces to the traditional steady state (Benson, 1960). The advantage of the condition of Equation 1 is that it allows us to calculate the concentration of the chlorine atom and free radical intermediates, and thus the by-products resulting from interactions among these intermediates, as functions of time.
kd =
+
constant and (Cl)'
Individual Reaction Rate Constants
Differentiating Equation 1 and substituting in Equation 2 for (Re)' we obtain: kd(Cl)2 - k~(C1)'f
=
(1)
kn+4
where AIn is the concentration of the appropriate organic molecule. The accumulation rate of the chlorine atoms and organic free radicals is given by the following expression, where the prime denotes the time derivative: (C1)'
B, n
1)
n
kn+z(-lfn+2)
( a ) (JfJ (4)
where K is the chlorine dissociation reaction equilibrium constant and Ifz is the total concentration of molecules. From Equation 4 it is evident that kd will not be affected b y any of the combination reactions which are much slower than the others. Using data on the recombination rate of chlorine atoms from several sources (Chiltz et al., 1962; Christie et al., 1959; Hiraoka and Hardwick, 1962; Martens, 1963; Bader and Ogryzlo, 1964; Hutton, 1964; Huttoii and Wright, 1965), Kurtz (1967) has shown that the chlorine atom recombination reaction is much slower than chlorine atom-free radical or free radical-free radical combinations, Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
333
Table 1. Numbers Assigned to Components in the Computer Model
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21
CH, CH3Cl CH2C12 CHC13 CCl4 CH~CHB CHICHzCl CH3CHC12 CHiClCHzCl CH3CC13 CH2ClCHC12 CH2ClCC13 CHC12CHC12 CHC12CC13 CC13CCl3 CI12=CH2 CH,=CHCl CH2=CC12 CHCl=CHCl CHCkCC12 CC12=CC12
22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 44 45 46 47 48 ~~
CH3 CHzCl CHC12 CC13 CHaCHz CH3CHC1 CH2CH2Cl CHZCHClz CH3CC12 CHzClCHCl CH2CC13 CHClCHC12 CH2CICC12 CHClCC13 CHC12CC12 CC12CC13 C3's C4's
c1, C1
HCl
Products
+ C1-R HRRCl + C1+ RH
01 02 03 04 06 07 07 08 08 09 10
47 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47
11
11 12 13 14
22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48
tog A
EA
+ HC1 RRCl + HCl 10 10 10 10 11
10 9 10 9 10 9 10 10 10 9 9
7 5 4 2 0 3 9 0 8 8 4 2 1
4 8 8
3 3 3 3 1 1 1 3 1 2 3 3 2 3 3 3
9 1 1 3 0 4 5 3 4 9 4 2 7 3 3 3
Table 111. Reaction Rate Data for Addition to an Unsaturate by a Chlorine Atom Reactants
16 17 17 18 18 19 20 20 21
Products
R=R 47 47 47 47 47 47 47 47 47
+ C1+ 28 29 31 32 34 33 35 36 37
log A
RRCl 10 2 9 8 10 0 8 9 9 7 10 0 9 1 9 7 9 4
EA
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
so that accurate dat,a on chlorine atom recombination is not needed to calculate the rate of chlorine dissociation. Table I shows the numbers assigned to components in the computer model of the reaction system comprising chlorine, methane, ethane, ethylene, and their chlorinat,ed derivatives. Tables 11-V list, respectively, the values of frequency factor and act,ivation energy used for abstraction of hydrogen 334
Reactants
+
46 46 46 46 46 46 46 46 46 46 46 46 46 46 46 46
+
Products
Cln R C1z RRCl 47 22 23 47 24 47 25 47 26 47 27 47 28 47 30 47 29 47 31 47 32 47 34 47 33 47 36 47 35 47 37 47
tog A
+C1 + RCl +C1 + ClRRCl 02 03 04 05 07 08 09 10 11 11 12 12 13 14 14 15
9 9 9 8 10 9 9 8 8 8 8 8 8 8 8 8
9 6 0 7 1 4 4 8 8 8 7 7 7 8 8 3
EA
2 3 4 6 1
3 0 0 0 0
1 0
1 1 1 1 2 2 2 5 5 5
0 0 0 0 5 5 5 2 2 4
~~
Table 11. Reaction Rate Data for Abstraction of Hydrogen by a Chlorine Atom Reactants
Table IV. Reaction Rate Data for Reaction of a Chlorine Molecule with an Organic Free Radical
Ind. Eng. Chern. Process Des. Develop., Vol. 11, No. 3, 1972
Table V. Reaction Rate Data for Loss of a Chlorine Atom by an Organic Free Radical Reactants
28 29 31 32 34 33 35 36 37
Products
47 47 47 47 47 47 47 47 47
log A
RRCl +C 1 + R=R 13.9 16 17 13.8 17 13.8 18 13.7 13.7 18 13.7 19 13.7 20 20 13.7 21 12.8
EA
23.6 21.2 23.8 20.1 23.0 20.9 19.2 18.2 16.8
by a chlorine atom, addition to an unsaturat,e by a chlorine at'oni, reaction of a chlorine molecule with an organic free radical, and loss of a chlorine atom by an organic free radical. Many of the values are taken directly from the compilation by Chiltz et al. (1963) and from work by Martens (1964, 1966); others have been derived from these results by Kurtz (1967). More recent data on the hydrogen abstraction reactions are available from Cillien et al. (1967), but are not much different from the earlier values. Tables VI and VI1 list, respect'ively, the values for free radical-free radical and free radical-chlorine atom combiiiation reactions. The act'ivation energies for such reactions are essentially zero. The frequency factors for combination reactions of like free radicals have been compiled by Chiltz et al. (1963). With the exception of methyl and ethyl (Heller, 1958) there are no data on frequency factors for combiiiation reactions of unlike free radicals. Frequency factors have been estimated by Kurtz (1967) using a collision frequency averaging method. The source of the equilibrium constant' values used by the computer model to calculate chlorine dissociation from the tabulated data for combination reactions was Evans et al. (1955). Induction Periods
A characteristic of chain reactions is the induction periodthat time during which the concentrations of the intermediates are building 1111 from the initial zero values. An equation
Table VI. Reaction Rate Data for Free Radical-Free Radical Combination Products
Reactants
log A
Reactants
Log A
Products
Reactants
Products
Log A
45 45 45 45 45 4 f5 45 45 45 45 43 45 4 t5 45 45 45 45 45 45 45 45 45 45 45 45 45 45 45 4 $5 45 45 45 45 45 45 45 49 45 45 45 45 45 43 45 45 45
9 6 9 8 9 8 9 8 9 7 9 7 9 7 9 6 9 0 9 4 9 8 9 8 9 7 9 7 9 7 9 6 9 6 9 4 9 8 9 7 9 7 9 7 9 6 9 6 9 4 9 5 9 5 9 5 9 4 9 4 9 2 9 5 9 5 9 4 9 4 9 2 9 5 9 4 9 4 9 2 0 3 9 3 9 1 9 3 9 1 8 7
R+R+RR 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 24 25 26 27 28 29 30 31 32 33 34 35 36 37
22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 24 24 24 24 24 24 24 24 24 24 24 24 24 24
10 5 10 1 10 1 10 0 10 5 10 4 10 4 10 3 10 3 10 3 10 1 10 1 10 1 10 1 10 1 10 0 9 6 9 5 9 3 10 1 9 9 9 9 9 7 9 7 9 7 9 6 9 6 9 6 9 5 9 5 9 3 9 4 9 1 10 1 9 8 9 8 9 6 9 6 9 6 9 5 9 5 9 5 9 4 9 4 9 1
06 07 08 10 44 44 44 44 44 44 44 44 44 44 44
44 9 11 12 44 44 44 44 44 44 44 44 44 44 44 44 13 14 44 44 44 44 44 44 44 44 44 44 44 44
25
25 25 25 25 25 25 25 25 25 25 25 25 26 26 26 26 26 26 26 26 26 26 26 26 2; 27 27 27 27 27 27 27 27 27 27 28 28 28 28 28 28 28 28 28
2,?I 26 27 28 29 30 31 32 33 34 35 36 37 26 27 28 29 30 31 32 33 34 35 36 37 27 2s 29 30 31 32 33 34 35 36 37 28 29 30 31 32 33 34 35 36
for the length of the induction period can be derived as follows: Substituting (R,)
=
(Cl)’
Bn(C1)in Equation 3 we have
= a
+ b(Cl)* + c(C1)
8 0 7 7 4 4 4 2 2 2 1 1 8 8 10 5 10 3 IO 3 10 2 10 2 10 2 10 1 10 1 10 1 10 1 10 1 9 9 10 1 10 1 10 0 10 0 I0 0 9 9 9 9 9 9 9 8 9 8 9 6 10 1 10 0 10 0 10 0 9 9 9 9 9 9 9 8 9F
45 45 45 45 45 45 45 45 45 45 45
45 45 45 45 45 45 45 45
45 45 45 45 45 45 45 45 45 45 45 45 45
R
2s
8 10 9 9 9 9 9 9 9 9 9 9
15 44 44 44 44 44 44 44 44 ‘14 44 44 44
37 29 30 3I 32 33 34 35 36 37 30 31 32 3:j 34 :? 5 36 37 31 32 33 34 35 36 37 32 33 34 35 36 37 33 34 35 36 37 34 35 36 37 35 36 37 36 37 37
29 29 29 29 29 29 29 29 29 30 30 30 30 30 30 30 30 31 31 31 31 31 31 31 32 32 32 32 32 32 33 33 33 33 33 34 34 34 34 35 35 35 36 36 37
+
+
R or R C1 combination reactions. Further, under usual conditions k,+? = 0, that is, the rate of organic molecule dissociation reactions is negligible. Then Equation 4 for the rate of dissociation of molecular chlorine becomes
(5)
where
+
a = [kd(Cl?)(*~t)
c =
C B‘n/(l n
+
n
n
kn+n(JIn)
Bn)
I t has been stated that k r
-
MOLS
C 1 2 REACTEDIt4IL
CH,CI
FED
fidence interval is shown for each observation. Relative amounts of secondary products depend only on the relative amounts of primary products, hence depend only on the ratio of chlorine reacted to methyl chloride fed. However, absolute amounts depend on absolute concentrations of free radicals, hence are affected by the amount of free chlorine and reactor temperatures. Therefore, while the relative amounts of secondary products shown by Figures 6-11 are generally applicable, the absolute amounts apply only to the particular reactor configuration (number of stages) and bath temperature employed here. The observed results for individual tertiary products (trichloroethylene and tetrachloroethylene), assumed to result from dehydrochlorination of secondary products, are plotted on Figures 12 and 13 with the calculated curves superposed. The relatively poor agreement may be attributed to heterogeneous catalysis of dehydrochlorination reactions, as the reactor walls were coated with a layer of finely divided carbon known to catalyze dehydrochlorinations (Ghosh and Rama Das Guha, 1951). An alternative explanation is that unsaturated by-products result not predominantly from dehydrochlorination of saturated by-products but from disproportionation in which chloroethylenes and hydrogen chloride are directly formed by reactions between free radicals (Hassler and Setser, 1966). Conclusions
Observed and calculated results for the amounts of primary products (chloromethanes) of methyl chloride chlorination agree to within the accuracy of the rate constant data. The primary product distribution is a function of the ratio of chlorine reacted to methyl chloride fed; i t is unaffected by stagewise addition of chlorine and little affected by temperature. Observed and calculated results for the amounts of secondary products (chloroethanes) of methyl chloride chlorination agree to within the accuracy of the rate constant data. Relative amounts of secondary products depend only on the ratio of chlorine reacted to methyl chloride fed; absolute amounts depend on temperature and amount of free chlorine. Observed amounts of tertiary products (chloroethylenes) exceed the calculated amounts by a factor of about 4. This may be attributed to heterogeneous catalysis by carbon deposited on the nickel reactor wall or to disproportionation between free radicals. The reaction mechanism assumed in setting up the rate expressions and the a priori values of the rate constants used (Tables 11-VII) are supported by the good agreement between observed and calculated results. This is especially remarkable considering that the rate constants used were
338
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
based on photochlorination experiments a t temperatures below 300”C, while the results reported here were obtained from thermal chlorination experiments a t temperatures over 400°C. Perhaps the most important implication of this work results from the fact that a very complex system of reactions could be simulated so accurately. This eliminates the need for a great deal of experimentation in the investigation of proposed aliphatic hydrocarbon chlorination processes and should result in a significant reduction in development costs. Acknowledgment
The author is indebted to A. J. Barduhn of Syracuse University for helpful discussions and to many persons a t the Syracuse Technical Center (Allied Chemical Corp.) for assistance in obtaining the experimental results. Literature Cited
Bader, L. W., Ogryzlo, E. A., Nature, 201,491-2 (1964). Barton, D. H. R., Howlett, K. E., J . Chem. Soc., 1949, 15564 (1949). Barton. D. H. R., Onvon, P. F., J. dmer. Chem. Soc., 72, 988-95 (1950). Belenko. Yu. G.. Berlin. E. R.. Flid. R . M..Enalin. - , 4. L., Russ. J . Phys.’Chem., 43, 1063-5 (1969). Benson, S. W., J. Chem. Phys., 20, 1605-12 (1952). Benson, S. IT., “The Foundations of Chemical Kinetics,” pp 49-54, McGraw-Hill, S e w York, YY, 1960. Benson, S. IT., Buss, J. H., J . Chem. Phys., 28,301-9 (1958). Chiltz, G., Goldfinger, P., Huybrechts, G., Martens, G., Verbeke, G., Chem. Rev., 63,355-72 (1963). Chiltz, G., Eckling, R., Goldfinger, P., Huybrechts, G., Martens, G., Simoens, G., Bull. SOC.Chim.Belg., 71,747-58 (1962). Christie. M. I., Roy, R. S., Thrush, B. A., Trans. Faraday SOC., 55, 1139-48 (1959). Christie, If.I., Roy, R. S., Thrush, B. A., ibid., 1149-52 (1959). Cillien. C., Goldfinger. P., Huybrechts, G., Martens, G., ibid., 63, 1631-5 (19677. ’ Evans. W.H.. Munson. T. R.. Waaman. - . , 0. 0.. J . Res. Nut. Bur.’ Stand.; 55, 147-64 (1955). Fuoss, R. hl., J. Amer. Chem. Soc., 65,2406-8 (1943). Gavalas, G. R., Chem. Eng. Sci., 21, 133-41 (1966). Ghosh, J. C., Rama Das Guha, S., Petroleum, October 1951, p p 261-4. Hassler, J. C., Setser, D. W., J. Chem. Phys., 45, 3237-45 (1966). Heller, C. A., ibid., 28, 1255-6 (1958). Hiraoka, H., Hardwick, R., ibid., 36, 1715-20 (1962). Hutton, E., Nature, 203, 835-6 (1964). Hutton, E., Wright, hf.,. Trans. Faraday SOC.,61, 78-89 (1965). ’ Johnson. P. R.. Parsons, J. L., Roberts, J. B., Ind. Ens. Chem.: 51, 499-506 (1959). Kurtz, B. E., P h D dissertation, Syracuse University, ,4pril 1967 (Universitv Microfilms 68-5470). Lloyd, .4.C., Int.“J. Chem. Kinetics, 3, 39-68 (1971). Martens, G. J., Discuss. Faraday Soc., 33, 297 (1963). Martens. G. J.. DA-91-591-EUC-3200 Sci. Note No. 1, June 15, 1964.’ Nartens, G. J., personal communications, 1966. McBee. E. T.. Hass. H. B.. Keher. C. AI., Strickland, H., Ind. -~ Eng.’Chem.j 34,296-300 (1942).’ n‘atta, G., Mantica, E., J . Amer. Chem. SOC.,74, 3152-6 (1952). Potter, C.,hlacdonald, IT. C., Can. J. Res., 25, 415-19 (1947). Scipioni, A,, Rapisardi, E,, Chim. Ind. (Milan), 43, 1286-93 (1961). SnoF, R . H., J . Phys. Chem., 70,2780-6 (1966). Werezak, G. X., Hodgins, J. W., Can. J . Chem. Eng., 46, 41-8 (1968). I
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RECEIVED for review August 21, 1970 ACCEPTED April 7, 1972
