GREEK 8c 8s
=
fractional coverage of complex, CIS0
= fraction of unoccupied sites, SI&
SUBSCRIPTS A = alcohol C = complex E = ether 2 = general 0 = olefin S = unoccupied sites TY = water literature Cited (1) Brey, \Y. S.,Krieger, K. A., J . A m . Chem. SOC. 71, 3637 (1949). (2) Butt, J. B., Bliss, H., Walker, C . A., A.Z.Ch.E. J . 8, 42 (1962). f 3 1 Coue. C. S.. A.Z.Ch.E. J . 10. 277 11964). ( 4 j Grknler, K. G., J . Chem. Piys. 37, 2094 (1962). ( 5 ) Heiba, E. I., Landis, P. S., J . Catalysis3, 471 (1964). ( 6 ) Hougen, 0. A., IVilke, C. R., Trans. A.Z.Ch.E. 45, 445 (1945).
(7) Ipatieff, V., “Catalytic Reactions at High Temperatures and Pressures,” p. 552, Macmillan, New York, 1936. (8) Jain, J. R., Pillai, C . N., Tetrahedron Letters 11, 675 (1965). (9) Kabel, R. L., Johanson, L.N., A.Z.Ch.E. J . 8,621 (1962). (10) Knozinger, H., Z. Physik. Chem. ( N e w Folge) 48, 151 (1966). (11) Knozinger, H., Kohne, R., J . Catalysis 5 , 264 (1966). (12) Peri, J. B., J . Phys. Chem. 69,211 (1965). (13) Pillai, C. N., Pines, H., J . Am. Chem. Soc. 83, 3274 (1961). (14) Pines, € I , , Haag, W. O., Zbid., 82, 2471 (1960). (15) Shchekachikhin, Y . M., Makarov, A. D., Kinetika i Katalir 5 , 568 (19154). (16) Solomon, H. J., D. Eng. dissertation, Yale University, New Haven, Conn., 1966. (17) Stauffer, J. E., Kranich, W. L., IND. ENG. CHEM.FUNDAMENTALS 1. 107 (1962). (18) Topchikva, K.V.,’ Yun-Pin, K., Smirnova, I. V., Adoan. Catalysis 9, 799 (1957). (19) Whitmore, F. C., J . A m . Chem. SOC. 54, 3274 (1932). (20) Winfield, M. E., “Catalysis,” P. H. Emmett,. ed... Vol. VII. Chap. 2, Reinhold,’New York,.l960.
RECEIVED for review September 8, 1966 ACCEPTEDMarch 28, 1967
CATALYTIC ADDITION OF HYDROGEN CHLORIDE
T O VINYIL CHLORIDE Studies in a Stirred Reactor R O B E R T G. R I N K E R A N D W I L L I A M H. CORCORAN Chemical Engineering Laboratory, California Institute of Technology, Pasadena, Calif.
The catalyzed reaction between hydrogen chloride and vinyl chloride in a stirred reactor was studied at temperatures in the range of 1 6 4 ” to 299” F., mass-flow ratios of vinyl chloride to hydrogen chloride from 2.1 6 to 2.46, and catalyst exposure times up to 1 8 0 0 hours. Zinc chloride carried on Celite was the catalyst, and regimes of fouling and nonfouling were observed. For the nonfouling case the rate of formation, rd, of the 1 ,I -dichlciroethane was found to follow the equation: rd = C(ph p o - pd/KP), where Cis an empirically determined rate constant, K p is the equilibrium constant referred to partial pressures, and ph, po, and pd are the partial piressures, respectively, of hydrogen chloride, vinyl chloride, and 1,l -dichloroethane. A fourcenter reaction appeared to be the most probable mechanism.
et al. (1928, 1931, 1932) made a systematic study of the addition of hydrogen halides to vinyl halides. They used a tubular reactor packed with a solid catalyst and obtained conversion data in the temperature range of 25’ to 200’ C. The work showed that ferric chloride catalyzed the reaction of hydrogen chloride with vinyl chloride to give 1 , l dichloroethane but had a much lower activity than either HgC12 or ZnC12. There was no correction, however, for the loss of activity due to fouling. Independent of the temperature and type of catalyst, the only product of the reaction between hydrogen chloride and vinyl chloride was 1,l-dichloroethane. The result suggested that the course of the addition reaction was governed by directed adsorption on the catalyst surface and was not a chain reaction. Between 1928 and 1040 Kharasch e t al. (1928, 1931, 1933, 1934, 1937, 1939, 1940) conducted a long series of comparative experiments on the mechanisms of the addition reactions of hydrogen chloride, hydrogen bromide, and hydrogen iodide with vinyl chloride. Kharasch arrived a t several generalizations from the experiments. First, he concluded that the additions of hydrogen chloride and hydrogen iodide IBAUT
to double bonds, although different in rate, occur by similar mechanisms which do not involve chain reactions. O n the other hand, the addition of hydrogen bromide involves chains in which Br is the chain carrier. Secondly, the effect of peroxides and metallic halides on the mechanisms and rates of addition was explained. Particularly the addition of HC1, with or without added ZnC12 or FeC18, and the addition of hydrogen bromide, with added FeC13, to vinyl chloride were mechanistically equivalent to the normal, uncatalyzed addition of hydrogen bromide to vinyl chloride to give l-bromo-lchloroethane. Therefore, the addition of hydrogen chloride to any ethylene derivative with or without catalysis would give only the normal addition product. Extensive experiments on the rate of thermal decomposition of 1,l- and 1,2-dichloroethane in a nonpacked tubular flow reactor were conducted by Barton (1949). I n a series of reactions involving the addition of small amounts of oxygen or chlorine to the feed, he observed that the rate of pyrolysis of the 1,2-dichloroethane was greatly increased, whereas that of the 1,l-dichloroethane was unaffected. H e concluded that in the case of the former compound, the mechanism of VOL. 6
NO. 3
AUGUST 1967
333
the pyrolysis was of the radical chain type. Both reactions gave the same products-vinyl chloride and hydrogen chloride. Barton found that the decomposition of the 1,l-dichloroethane in the temperature range of 356' to 453' C. was first order. I n the pressure range of 20 to 200 mm. of Hg, the rate constant varied with temperature according to the equation :
k = 1.2 X 10l2e
--27,500 RT
set.-'
(I 1
Significantly, the frequency factor was close to the value 10'3 usually expected for unimolecular decompositions. The complete absence of any inhibiting effect by propylene in the pyrolysis of 1,l-dichloroethane, together with the absence of an induction period and the excellent adherence to first-order kinetics well beyond the 50% decomposition point, proved conclusively to him that the mechanism of the 1,l-dichloroethane decomposition was different from that of 1,2-dichloroethane-that it was not a radical chain reaction. Howlett (1 952) studied the homogeneous decomposition of 1,l -dichloroethane and like Barton concluded that the reaction was unimolecular. H e performed experiments in a batch reactor at 412', 433', and 499' C. and observed that the reaction was insensitive to the presence of small amounts of oxygen, chlorine, and propylene. These observations, along with the fact that there was no induction period, were again strong evidence in favor of a unimolecular reaction. The purpose of the present work was to obtain more quantitative knowledge of the rate and mechanism for the reversible reaction between hydrogen chloride and vinyl chloride catalyzed by zinc chloride. Of special interest was the nature of the reaction after the initial fouling of the catalyst. Previous work had been more concerned with the initial reactions with fresh catalysts rather than the nature of the reactions after extended use of the catalysts. Apparatus
A stirred reactor was used in the experimental work. I t was nominally a 6-liter, borosilicate glass vessel that consisted of two parts, a lower cylindrical shell with an inside diameter of 5l/2 inches and a length of 133/4 inches which served as the main body of the reactor and a lid with an axial length of 2 inches which contained ports for the feed and discharge streams, the stirrer, and thermocouple leads. The stirrer, which was operated at 300 r.p.m., had a shaft of 3/~6-inchglass rod on which were mounted three four-bladed impellers with 2-inch blades of 30' pitch at positions of 23/4, @/4, and 123/4 inches, respectively, from the bottom of the lower shell. The separate feed streams to the reactor entered the lid and were carried through glass tubes to the bottom of the reactor where they discharged into the system at a position 11/8 inches from the bottom of the lower shell. Discharge was through a n outlet tube mounted in the lid and with its entrance at a position 113/4 inches from the bottom of the shell. Trays to carry the catalyst mass were mounted in the body of the reactor. I n operation the reactor was immersed in an insulated oil bath having a capacity of about 10 gallons of Dow Corning No. 550 silicone oil. The temperature of the bath was automatically controlled to 10.02' F. Temperature within the reactor was measured by means of a thermocouple probe consisting of a 7-mm. borosilicate glass tube with five side arms, each 0.5 inch in length, extending 45' from the tube axis, and spaced evenly along the probe a t positions 11/8, 4 l / g , 7l/g, 101/g, and 13l/g inches, respectively, from the bottom of the shell. Each of the side arms enclosed a copperconstantan thermocouple constructed from No. 40 wire. The leads from the five thermocouples passed collectively through the main tube to the outside. Chemicals fed to the reactor-hydrogen chloride and vinyl chloride-were obtained as compressed gases in steel cylinders. The hydrogen chloride was scrubbed in a 5-liter glass tower containing approximately 3 liters of crystal oil. The vinyl 334
l&EC FUNDAMENTALS
chloride was scrubbed in a steel tower charged with 3 liters of an aqueous solution containing 20% by weight of sodium hydroxide. Impurities removed from the hydrogen chloride by absorption in the crystal oil were primarily organic in nature. The only impurities of consequence in the vinyl chloride were hydrogen chloride and a small amount of phenol which had been added as an inhibitor against polymerization. From the scrubbing towers the gases flowed to predryers filled with anhydrous calcium chloride. T o ensure complete drying, the gas streams were then directed into large drying tubes filled with additional calcium chloride. After drying, the gases, still separate, flowed through glass coils immersed in an isothermal water bath controlled a t 25' C. From there they passed through calibrated rotameters for measurement of flow rates. Analyses of the feed and discharge streams were determined by means of gas-liquid chromatography. The 4-foot column was packed with 60- to 80-mesh Dicalite, and the partition liquid was didecyl phthalate present to the extent of 35y0 by weight of the total column packing. Preliminary work showed that this arrangement allowed good separation of vinyl chloride, 1,l-dichloroethane, and carbon dioxide, the latter compound being a measure of hydrogen chloride which had reacted with 100-mesh sodium bicarbonate upstream from the chromatographic column. Helium was used as the carrier gas, and a thermal-conductivity cell was the detector instrument. Gas samples having a volume of 2 cc. at atmospheric pressure and room temperature were used in the analysis. Figure 1 is a schematic diagram of the arrangement of the stirred-reactor apparatus and auxiliary equipment. Catalyst
Selection of the catalyst for the kinetic studies was based upon the results obtained in qualitative, constant-volume experiments in ampoules. Of the systems studied, Celite pellets impregnated with zinc chloride gave the greatest catalytic effect. Catalysts tested were HgzC12, CoC12, FeC13, ZnClz, NiC12, MnC12, CdC12, and BiC13. Carriers were long- and shortfiber asbestos, 10- to 20-mesh silica gel, 60- to 80-mesh Dicalite, 6- to 12-mesh Celite or 3/1s-inch pellets, and '/h-inch spheres or '/*-inch pellets of alumina. For the experimental work, the catalyst was prepared by impregnating Celite pellets, nominally "16 inch in size, with zinc chloride in the ratio of 0.200 gram of ZnC12 to 1.00 gram of carrier. The extruded, cylindrical pellets had the properties noted in Table I. Preliminary experiments in the ampoules and in the reactor had shown that a t least 3 grams of catalyst were required per liter of reactor volume to give measurable reaction rates a t 100' C. for retention times less than 12 hours. Upon that basis, 120 grams of the impregnated Celite containing 20 grams of zinc chloride were placed in the 6-liter reactor and distributed evenly among the eight trays.
Table 1.
Typical Properties of Cylinders of Celite 406
Composit ion Weight % stituent Con-
Si02 Ti02
FezOs A1203
PaOs CaO MgO NazO KzO
90.2 0.2 1.6 4.9 0.4 0.8 0.6 0.6 0.7
Physical Properties Diameter Length Average pore size Packed density Surface area based on nitrogen adsorption
Reaction in water, slightly acid to neutral.
0.175 inch
0.15'5 inch 22,000 A. 36 Ib./cu. ft. 1 0 . 6 sq.
meters/g.
Figure 1 . 1. 2. 3. 4.
5. 6.
7.
a.
9.
10. 11. 12. 13. 14.
15.
Flow diagram of stirred reactor system
Cylinders of compressed gas Bubble towers Expansion chambers Dryers Temperaturse-conditioning coils Manometer!; for feed streams Thermocouples for feed streams Rotameters Liquid-nitrogen sampling traps for feed streams Stirred reactor Liquid-nitrogen waste traps for product stream Liquid-nitrogen sampling traps for product stream Mercury manometer on vacuum line McLeod ga4ge Vacuum purnp
Experimental Procedure
Studies were first made of the homogeneous reaction between the hydrogen chloride and the vinyl chloride. I n the absence of a solid catalyst, no measurable reaction to form 1 , l-dichloroethane occurred a t atmospheric pressure within the temperature limits of 200' to 400' F. and for retention times u p to 12 hours. At 400' F., however, some cracking of the vinyl chloride was observed, as indicated by a slight carboh deposit on the glass surfaces. Whether this was catalyzed by the glass was not invesiigated further. The range of experimental conditions for the study of the heterogeneous catalysis was as follows: Stirring rate. 300 r.p.m. Temperature. 165' to 300' F. Pressure. 0.967 to 0.981 atm. Flow rates of hvdrogen chloride. 0.60 X to 8.58 X lb./hr. Flow rates of vinyl chloride. 1.38 X to 18.32 X lb./hr. Input weight ratio of vinyl chloride to hydrogen chloride. 2.16 to 2.46. I
"
For runs 1 through 25 the average temperature was 215.0' F., and the average difference of temperature between the top and bottom of the reactor was 0.22' 0.06' F. For runs 26 through 28, the corresponding values were 163.9' f 0.1' and 0.15' i 0.01' F., and for runs 29 through 39 they were 299.0' =k 0.1' and 0.25' 0.03' F. Tests showed that for feed rates corresponding to a retention time of 45 minutes, a significant vertical temperature gradient, on the order of 1.0' F. per foot, developed within 3 or 4 minutes when the stirrer was stopped. The small temperature gradients were therefore an indication that the stirring rate was sufficiently high to obtain good mixing. The temperature of the oil bath in which the reactor was immersed differed from the average =t0.4'
*
*
temperature of the reactor a t any given time by only 1 0 . 2 ' F., which showed that no large gradients existed in the radial direction. For the maximum total feed rate, the pressure drop was 0.0076 p.s.i., and for the minimum, it was 0.0040 p.s.i. Therefore the pressure in the reactor was uniform and essentially equal to the absolute pressure of the surroundings. The feed rates of hydrogen chloride and vinyl chloride were recorded a t half-hour intervals during each run, which usually lasted from 15 to 18 hours. Between runs the flow rates were reduced to very low values, but the weight ratio of vinyl chloride to hydrogen chloride was kept within the range of 2.16 to 2.46 at a constant temperature. I n most cases, the rotameter readings varied over a range of A3yo even with the temperature carefully controlled at 25' C. For this reason the average flow rates for each run were determined by graphical integration. The capacity of the reactor without catalyst was 6020 13 cc. a t 25' C. and when charged was 5870 + 3 cc. The volume of the reactor for a given run was determined from these data a t 25' C. with an appropriate temperature correction. A combination of the reactor volume with the measured flow rates allowed computation of retention times. Even for the highest flow rate that was used, the retention time of 45 minutes was sufficient to ensure that gas-phase diffusion was not controlling in the process. For the maximum rates which lb. mole/(hr.)(lb. of catalyst), were in the region of 4 X the estimated pressure difference across the gas-solid interface for 1,l-dichloroethane was in the region of 10-4 atm. Diffusion in the catalyst pellets also was not a controlling step because of a very low rate of reaction on the surface of the catalyst combined with pore diameters of 22,000 A. Product analyses obtained with a gas chromatograph were checked by material balances which were based on the valid assumption that 1,l-dichloroethane exclusively was formed from hydrogen chloride and vinyl chloride. Therefore a computation of the feed ratio of vinyl chloride to hydrogen chloride required to give the products was compared with the experimental feed ratio. During the first 350 hours of operation a t 215' F., the deviation averaged -15.670, and during I n both the initial 200 hours at 299' F., it averaged -9.0%. cases, the fact that the computed values of the vinyl chloridehydrogen chloride ratio were lower than the experimental values showed that a side reaction was consuming vinyl chloride. From results of the batch experiments, it was reasonable to assume that the side reaction was involved in the fouling of the catalyst. After 350 hours a t 215' F., the percentage deviation in the material balance decreased sharply and attained an average of =t3.6%. Likewise, after 200 hours at 299' F., it became +=3.8%. At 164' F., however, it averaged *2.6y0 for the entire period of 65 hours, because this temperature followed the work a t 215' F., and essentially no additional fouling occurred. The runs a t 299' F. were made after those a t 164' F. Results
The equation used in the analysis of the experimental results was the following :
This equation assumes the ideal case of complete mixing wherein composition, temperature, and pressure are uniform VOL. 6
NO. 3
AUGUST 1967
335
throughout the reactor. Diffusion at the entrance and exit sections was neglected, and perfect gases were assumed. I n the investigation, n d , was zero, and for even the early or heaviest fouling period, f was negligible and amounted to about 0.57, of the feed rate. Also P V / R T d(M,,nd)/dt was found to be 2% or less of the quantity f i n d . Therefore a reasonable approximation of Equation 2 is: Fill, = rdrv (3) A knowledge of the total feed rate, the weight fraction of 1 , l dichloroethane in the product stream, and the weight of catalyst is then sufficient to compute the instantaneous rate of reaction with respect to 1,I-dichloroethane. A detailed tabulation of experimental data and computed results based on those data has been deposited with the American Documentation Institute. With fresh catalyst or catalyst that had not been a run at higher temperature, the initial part of a run was characterized by fouling. The catalyst was first used a t 215' F., and the conversion in terms of weight fraction of 1,l-dichloroethane in the discharge stream is plotted in Figure 2 against catalyst exposure time for fixed values of (FO,/F,JBVand (Ft/W')aV. Twenty-five runs were made at this temperature and represent the first 1110 hours of use of the 120 grams of catalyst mass. A few of the experimental points are shown. The plot shows that the conversion continually decreased for approximately 450 hours. At the end of that time it leveled off and remained reasonably constant for the remaining 650 hours. These results suggest that steady conditions with respect to fouling had been attained, but even more significant was the fact that the conversion had not decreased to zero. This result is evidence that the fouling mechanism involved irreversible fouling of one type of adsorption site. If fouling of all sites had prevailed, the conversion would have decreased asymptotically to zero. In runs 26 through 28 a t 164' F., which were made following the runs a t 215' F., no fouling was observed. This result was not surprising because in going from a higher to a lower temperature, the fouling rate which nearly reached zero at 215' F. would be expected to decrease even further a t the lower temperature, and the total fouling would be the asymptotic fouling obtained a t 215' F. For runs 29 through 39 conducted a t 299' F. in sequence after the runs at 164' F., a fouling period followed by a nonfouling period was again observed as would be expected. Results similar to those presented in Figure 2 were obtained and are shown in Figure 3. I n comparison to the fouling a t 215' F., the fractional increase in fouling at 299' F. was much less.
60
26 22
"0 18 x
2(4 10 6
134
138
142
146
150
154
158
162
166
170
174
fe,hr x 101 ' Figure 3. Conversion to 1,I-dichloroethane vs. catalyst exposure time at 299" F. in stirred reactor
Data from nonfouling periods were used to determine the order of the reaction. The form of the rate equation selected for the analysis was
\
(4)
Exponents a and b were assigned values of 0.5, 1.0, 1.5, and 2.0 in all possible combinations and were applied to nonfouling data at feed rates ranging from 7 X to 18 X lop3 pound per hour. The term pd/K, was relatively small and was neglected for these particular computations. I t \vas found that drift in the calculated values of C was a minimum when the exponents ofph andp, were equal to 1. I n further analyses of the data, therefore, the primary addition reaction was assumed to be second-order. This assumption was verified by subsequent correlation of data. I n an effort to determine the most plausible mechanism for the bimolecular reaction between hydrogen chloride and vinyl chloride, it was assumed that one chemical step was ratecontrolling. Upon this basis 10 different mechanisms seemed possible. The mechanisms chosen for the analysis in no way exhausted the possibilities but appeared to be the most logical steps for describing the catalyzed reaction. Their general form can be written as:
(5) For the conditions studied the activities may be replaced by partial pressures, and Equation 5 takes the form of Equation 4 with exponents a and b equal to unity. A close examination of the coefficients obtained in a leastsquares computation using nonfouling data developed at 21 5' F. and following a procedure described by Hougen and Watson (1947) indicated that the most probable rate equation had the form:
50 40 N
0 X
30
E" 20
IO '0
1
Z
a
4
6
6
7
8
9
IO
t,,hr xl0-2 Figure 2. Conversion to 1,l -dichloroethane vs. catalyst exposure time at 21 5" F. in stirred reactor 336
l&EC FUNDAMENTALS
Equation 6 applies to the mechanism in which the surface reaction between two adsorbed molecules is controlling. I n order to obtain additional proof for the validity of Equation 6, another method was used in analyzing the data for the nonfouling period at 215' F. Each equation of the type shown as Equations 5 was used for various combinations of four separate experiments in order to handle the four unknowns, K , previously having been determined as 17.5 atrn.-' by separate solution of Equation 4 in a least-squares analysis (Hougen and Watson, 1047), for the nonfouling portion of the work at 215' F. Values of K , at other temperatures were
obtained by use of the va.n't Hoff equation and the value of K , at 215" F. The resulting function developed by Rinker and Corcoran (1966) is
H
+ 1.t87 [- 4.900 In T + 14$35
In K , = 8.096
\
~
0.005400 T
=
- 21.091 ( 7 )
(0.59 f 10.3 e@('*)) X l o 2
=
(2.303)(0.00138 t ,
- 2.87 X 10-5 t,2 + 4.26 X 10-8 t,3 - 1.81 X lo-" t e 4 )
/
H
C----C
+
C1
/
'\\\
+ C1-
(13)
\
/
a
H
I +\
H
H
H
c1
H
\
/
. .
H
The classical carbonium ion of Equation 14 reacts with C1to give 1,l-dichloroethane by the following reaction,
I
For the case of the nonfouling periods a t 215' and 299' F . , values of c Jvere found to be 60 and 340 (lb. moles 1,l-dichloroethane) (cu. ft.)2/(hr.) 1:lb. catalyst((1b. moles)2, respectively. A value of c for the nonfouling period at 164' F. was obtained by extrapolation with a n Arrhenius-type equation and found to be 16.5. The temperature dependence of c for the nonfouling case is then given by the equation: RT
+
C1
H
--21,100
,,,'
The reaction shown in Equation 13 requires that the molecule attacking the olefin be polar in nature. Hydrogen chloride has a dipole moment of 1.03 X 10-18 e.s.u. The resultant bridged-carbonium ion acquires a net positive charge on the a-carbon relative to the p-carbon when it is attacked on the back side by a C1- ion because of the stabilizing effect of the C1 group on the a-carbon. Thus:
ivhere te is the catalyst iexposure time in hours. Equations 8 and 9 are valid only a t 215' F. Insufficient experimental data were obtained to develop a general relationship between temperature and the fouling factor. Fouling experiments would be required a t each temperature beginning with fresh catalyst in order to obtain a general temperature function. The final empirical equation which can be used to predict the amount of 1,l-dichloroethane formed a t 215' F. and a t atmospheric pressure with a mass feed ratio of vinyl chloride to hydrogen chloride in the range of 2.16 to 2.46 is given as:
4.1 X 108 e
;H\\
\
H P
H
(9)
c =
H
+ H + - C1-
\
H
(8)
The quantity p ( t e ) was determined by least squares to have the form
P(fe)
c1
/ C=C
Equation 7 gave values of 93 and 1.8 atm.-' for K,, respectively, a t 164" and 299" .F. The computed values o f K , were used with Equation 4, with a and b each taken as unity, to compute values of C for both fouling and nonfouling periods a t each temperature. Multiplication by ( R T ) 2 / M dconverted C into c for use in a rate equation with concentrations rather than pressures as the potentials. The relation which approximates c a t 215' F. is given as : G
free radicals. A possible nonradical mechanism could involve a bridged-carbonium ion (Hine, 1956), as a transition product and can be represented by the following steps:
(11)
Substitution of Equation 11 into Equation 10 gives the predicted conversion during the nonfouling period as:
Equation 12 is applicable under varying conditions of residence time for the temperature range of 164" to 299' F., for mass ratios of vinyl chloride to hydrogen chloride in the range of 2.16 to 2.46, and for catalyst exposure times u p to 1800 hours, but specifically in the region of nonfouling. Discussion
I n the present work the fact that only the 1 , l product was formed in the addition of hydrogen chloiide to vinyl chloride eliminated the possibility that the reaction was propagated by
c1
/
H-C-C
I +\
H
H C1
+ C1-
+
I 1 I /
H-C-C-H
H
H C1
I n the presence of an ion-solvating medium, the mechanism described by Equations 13, 14, and 15 can occur with relative ease because of the presence of H' and C1-. I n the gas phase or on a nonsolvating catalyst, that mechanism becomes less probable. The solvating effect of anhydrous ZnCls on adsorbed HC1 is not known, so that the validity of the mechanism proposed for the catalyzed addition reaction is uncertain. A second type of nonchain mechanism for addition reactions is frequently called a four-center reaction (Hine, 1956; Ingold, 1953). I n this case the atoms in the reactions change their configuration to that of the product without the formation or destruction of ions. I n the transition state resulting from the collision of an HC1 molecule with a vinyl chloride molecule, four atoms are each simultaneously forming a new bond and breaking an old one. I t is especially important to note here that this type of reaction requires a high degree of molecular orientation before reaction occurs. This reaction can be represented in the following manner:
H
c1
H
H
H
H .......-c1
H
I-I
c1
c1
H C1
C-H+H-C-C-H
I 1
The mechanism described by Equations 16 and 17 would be expected to give the lowest reaction rate of the mechanisms discussed because of the small steric factor. The very low rates encountered in the stirred reactor even with catalysis strongly suggest that the mechanism given by Equations 16 and 17 is primarily responsible in forming the VOL. 6
NO. 3 A U G U S T 1 9 6 7
337
product. Moreover, the empirical fit of the rate data to a second-order, bimolecular reaction is a good indication that the activated complex includes at least a single HC1 and vinyl chloride molecule. As for the fouling, the nature of the curve in Figure 2 suggests irreversible fouling of one type of site in a system of two types of sites. The site associated with fouling catalyzes either the primary reaction or the fouling reaction. The other site shows a specificity only for the primary reaction. Acknowledgment
I n the course of the work, one of the authors, R . G. Rinker, held fellowships from the Union Carbide Corp. and the Monsanto Chemical Corp. The interest and aid of all the companies are gratefully acknowledged. Nomenclature a c
= activity of a component = empirical rate constant, (lb. mole 1,l-dichloroethane)
(cu. ft.)2/(hr.) (Ib. catalyst) (lb. mole)2 empirical rate constant, lb. 1,l-dichloroethane/(hr.) (lb. catalyst) (atm.)2 = rate of deposit of fouling on catalyst, lb./hr. f F = flow rate, lb./hr. k = first-order, homogeneous, specific rate constant, set.-' = specific rate constant including certain catalyst charack teristics K = equilibrium constant M = molecular weight n = weight fraction, outlet stream unless otherwise designated p = partial pressure, outlet stream unless otherwise designated, atm. P = total pressure, atm. r = rate of formation of component, lb./(hr.)(lb. of catalyst) = gas constant, B.t.u. (lb. mole)(’ R.) R R = gas constant, (atrn.) (cu. ft.)/ (lb. mole) (” R.) t = time, hour T = temperature, O R . W = weight of catalyst, pound P ( t J = time function for catalyst fouling = function of rate constant, activity of catalyst, and (p adsorption equilibria C
=
SUPERSCRIPTS a = constant b = constant SUBSCRIPTS av = average d = 1,l-dichloroethane D = refers to adsorption equilibrium constant for 1,ldichloroethane
338
l&EC FUNDAMENTALS
e
h H
= = =
i p
= =
u
= =
V
refers to catalyst exposure time hydrogen chloride refers to adsorption equilibrium constant for hydrogen chloride input refers to equilibrium constant based on partial pressures vinyl chloride refers to adsorption equilibrium constant for vinyl chloride
literature Cited
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RECEIVED for review September 23, 1966 ACCEPTEDMay 11, 1967 Work supported mainly by funds from grants by the Shell Companies Foundation to the Chemical Engineering Laboratory. Support also given by E. I. du Pont de Nemours & Co., Inc. Material supplementary to this article has been deposited as Document No. 9479 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, It’ashington 25, D.C. A copy may be secured by citing the document number and by remitting $1.25 for photoprints or $1.25 for 35mm. microfilm. Advance payment is required. Make checks or money orders payable to Chief, Photoduplication Service, Library of Congress.
