Hydroisomerization and Hydrocracking of Cyclohexane in the Presence of a Palladium-Hydrogen-Faujasite CataIyst Mamerto G. Luzarraga* Esso Research Laboratories, Baton Rouge, La. 70821
Alexis Voorhies, Jr. Louisiana State LTna’versity, Baton Rouge, L a . 70803
A kinetic investigation was conducted of the simultaneous hydroisomerization and hydrocracking of cyclohexane in the presence of a particular zeolite, namely, a palladium-hydrogen-faujasite of low sodium content. The palladium-hydrogen-faujasite catalyst employed in this study was found t o be very effective for hydroisomerizing cyclohexane in the temperature range 490450°F.It was demonstrated that as the severity of the operations was increased, b y increasing either temperature or contact time, the hydrocracking reactions became more pronounced. A kinetic model was developed for the simultaneous hydroisomerization and hydrocracking of cyclohexane. The model, which involved a first-order reversible hydroisomerization of cyclohexane t o methylcyclopentane accompanied by first-order irreversible hydrocracking of cyclohexane and methylcyclopentane, was found to be in close agreement with the experimental data. The activation energies for hydroisomerization and hydrocracking were 24 ( & 1 ) and 33 ( & 3) kcal/mol, respectively, and the rate constants for both reactions were compatible with a dual-site adsorption mechanism.
R e s e a r c h work on cyclohexane isomerization catalysis has been extensive. The catalyst types employed to isomerize cyclohexane can be grouped into four general categories: acidic halides (Condon, 1958; Stevenson and llorgan, 1948; Lien, et al., 1952; Pines, el al., 1950, 1953), e.g., AllC1, and HCl; acidic chalcides (Condon, 1958), e.g., WS,; dual-function metal on amorphous supports (Ciapetta and Hunter, 1953), e.g., Ki-SiO2-AI2O3; and dual-function metals on zeolites (Hopper and Voorhies, 1972; dllan and Voorhies, 1972), e,g., crystalline aluminosilicates. I n general, the same catalyst types eniployed for hydroisomerizing cyclohexane have been utilized t o study the hydrocracking or ring-opening reactions (Camley and Hall, 1944; Egan, et al., 1962; Hatcher and Voorhies, 1969). The acidic halide catalysts are effective a t temperatures below 350°F. On the other hand, t h e acidic chalcides arid the dual-function metal on amorphous support catalysts operate a t temperatures in excess of 600°F. Intermediate temperatures in the range 400-600°F are required for the zeolite catalysts. Little attention has been given to the competitive kinetics involved in the siniultaiieous hydroisomerization and hydrocracking of cyclohexane. The objective of this \york is to study the simultaneous hydroisonierization and hydrocracking of cyclohexane in the presence of a specific zeolite; namely, a palladium-hydrogen-faujasite of IOTT per cent
J
!w2 r W a
w x
0.1 0.1 I .o PREDICTED HYDROISOME~lZ_ATlONRATE CONSTANT, LL
GK-sE Figure 7. Hydroisomerization rate constant correlation
Figure 4. It appears t h a t the reaction rate constants are independent of fluid velocity for the feed rates commonly employed in the present study, namely, 16-60 cm3ihr. The intraparticle diffusion of the reacting molecules depends on the size of the catalyst particles or “macropore” accessibility. T o investigate the effect of intraparticle diffusion, a set of experiments was conducted with different particle sizes but otherwise constant operating conditions as presented in Figure 5. Intraparticle diffusion does not seem t o have a n effect on t h e reaction rate constants. However, the present test does not take into consideration t h e diffusion in t h e internal channels or “micropores” of t h e zeolite crystal. Activation Energy Measurements. T h e effect of temperature on t h e hydroisomerization and hydrocracking rate constants is shown in Figure 6. A t least-squares analysis of t h e experimental data revealed the activation energies shown in Table 111. Correlation of Rate Constants. T o account for t h e effects of pressure on t h e reaction rate constants, t h e LangmuirHinshelwood-Hougen-Katson adsorption theory (Hougen and Watson, 1947) was employed. The hydroisomerization rate constants were found to be compatible rrith a dual-site adsorption mechanism. The dualsite model was formulated in terms of t h e hydrogen and the hydrocarbon partial pressures. Temperature dependencies were tied up with the adsorption parameters. A summary of the correlations is presented in Figure 7. The dotted lines shown in Figure 7 represent t h e experimental uncertainties associatied with the hydroisonierization
rate constant (u = 8.2’%/value) as determined from a set of 12 duplicate runs. The 12 duplicate points were only obtained to represent the variance of the data (dotted lines). The duplicate experiments were not plotted in t h e figures. The points plotted, totaling about 60, correspond to rate constants a t a wide variety of experimental conditions. The hydrocracking rate constants were also found t o be compatible n-ith a dual-site adsorption mechanism. A t temperatures below 520°F, it !vas found that the hydrocracking rate constants depended on both the hydrogen and t h e hydrocarbon partial pressures. However, a t temperatures in excess of 520°F, t h e hydrocracking rate constant appeared to be mainly determined by the total pressure of t h e system or the hydrogen partial pressure since excess hydrogen was employed in these experiments. A summary of the correlations is presented in Figure 8. The dotted lines in Figure 8 correspond to the experimental uncertainties associated with the hydrocracking rate constants (U = 1?.1% value). Nomenclature
K hi, ki’
k? k3
+ +
ka kb
kc kd
kol, k 0 2 ,
KO KH KP
-1I
P Po
PH RH
R
T
Equilibrium constant Hydroisomerization rate constants, em3 g-’ sec-l Cyclohexane hydrocracking rate constant, em3 g-l see-’ Methylcyclopentane hydrocracking rate constant, em3 g-‘ sec-l kl k2, em3 g-I sec-l kl’ k3, em3 g-l sec-l ICl, em3 g-l see-’ ki’,em3 g-I sec-l Constants, cm3 psia g-’ sec-l Hydrocarbon adsorption constant, psia-’ Hydrogen adsorption constant, psia-l Constant, psia-l Molecular weight of hydrocarbon Total pressure, psia Hydrocarbon partial pressure, psia Hydrogen partial pressure, psia Moles of hydrogen per mole of feed Gas 1aTv constant Temperature, O F
k02’
Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No.
3, 1973
197
+
RH)-', contact time, sec n cm-3 (ka - kb -' 4 2 7 ( 2 k c ) - ' (ka - kb f 4 2 )(2kc) Weight hourly space velocity, (g of feed) hr-' (g of catalyst)-' Hydrogen-free mole fractions of cyclohexane, methylcyclopentane, and cracked products Hydrogen-free mole fractions of cyclohexane and methylcyclopentane a t t, = 0 Hydrogen-free mole fractions of cyclohexane and methylcyclopentane a t t, = m (ka - k b l 2 f 4kckd (ka kb f 6 ) / 2 (ka k b - %'%)/a iVP(RT)-l(3600)(W/Hr/W)-l(l
t.9
Ul
-'
w2
W/Hr/W
+
+
Hatcher, W. J., Voorhies, A., Jr., Ind. Eng. Chem., Prod. Res. Develop., 8, 361 (1969). Hooke, - R., Jeeves, T. A., J. Ass. Comput. Machinery, 8, 212 I_
(1Y)tjl).
Howwer. J. R.. Voorhies, A.. Jr.. Ind. Ena. Chem.. Prod. Res. Dev'eiop.; 11, 294 (1972).' Hougen, 0. A., Watson, K. M., "Chemical Process Principles," Part 111, Wiley, Xew York, N. Y., 1947. Lien, A. P., D'Ouville, E. L Evering, B. L., Grubb, H. M., I
,
Ind. Eng. Chem., 44, 351 (1952).
Pines, H., Aristoff, E., Ipatieff, V. N., J . Amer. Chem. SOC.,72, 4055 (1950).
Pines, H., Aristoff, E., Ipatieff, V. N., ibid., 75, 4755 (1953). Rossini, F. D., Pitzer, K. S., Arnett, R. L., Braun, R. M., Pimentel, G. C., "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds," Carnegie Press, Pittsburgh, Pa., 1953. Stevenson, D. P., Morgan, V. H., J . Amer. Chem. SOC.,70, 2773 (1948).
Wei, J., Prater, C. D., Advan. Catal., 13, 203 (1960).
literature Cited
Allan, D., Voorhies, A., Jr., Ind. Eng. Chem., Prod. Res. Develop.,
RECEIVED for review January 18, 1973 ACCEPTED June 18, 1973
11, 159 (1972).
Bryant, P. A., Voorhies, A., Jr., AIChE J., 14, 852 (1968). Cawley, C. M., Hall, C. C., J . SOC.Chem. Ind., London, Trans. Commun., 63 33 (1944).
Ciapetta, F. G., Hunter, J. B., Ind. Eng. Chem., 45, 147 (1953). Condon, F. E., Catalysis, 6, 43 (1958). Egan, C. J., Langlois, E. E., White, R. J., J . Amer. Chem. SOC., 84, 1204 (1962).
Presented at the AIChE 65th Annual Meeting, Xew York, N. Y., Nov 1972. The authors are indebted to the Esso Research and Engineering Co. for sponsoring this project and to the Department of Chemical Engineering of Louisiana State University for the use of their facilities.
Acrylonitrile-Styrene Copolymer Emulsions Raymond B. Seymour,* Don R. Owen, Muriel 1. McGee, Climaco 1. Losada, and Murray Clark Department of Chemistry, Cniversity of Houston, Houston,
Tex.77004
Polymeric products with different solubility properties have been obtained b y the emulsion polymerization of a 2:l molar ratio of acrylonitrile and styrene. While an acrylonitrile-rich copolymer was obtained when a mixture of the two monomers was polymerized simultaneously, polymeric products that may consist predominantly of a homopolymer core surrounded b y a shell of another homopolymer were obtained when the addition of one of the monomers was delayed. There was little addition of styrene to polyacrylonitrile observed with a low concentration of surfactant. However, a good yield of a dimethylformamide (DMF)- and benzene-soluble product was obtained when a higher concentration of surfactant was used. A good yield of polymeric product was obtained when acrylonitrile was added to polystyrene in the presence of just enough surfactant for critical micelle concentration (cmc). This product was more compatible with DMF than a product prepared under similar conditions but with 3 times the concentration of surfactant required for cmc. However, neither product was completely soluble in benzene. The presence of essentially a 2:l molar ratio of acrylonitrile to styrene was indicated b y infrared spectroscopy (ir) data. The presence of long sequences of acrylonitrile in both samples was evident from thermal data but these data showed that the composition of the two products were not identical.
T h e rapid polymerization of monomers (such as styrene) which are solvents for their polymers has been explained b y the Smith-Ewart theory (Smith and Ewart, 1948) in which initiation takes place in the aqueous phase and the rate of propagation occurring in the micelles is dependent only on the number of particles in the emulsion system. 198 Ind.
Eng. Chem. Prod. Res. Develop., Vol. 12,
No. 3, 1973
However, in spite of many subsequent investigations, the specific site of polymerization is still controversial and some of the abnormalities observed in emulsion copolymerization systems have not been explained adequately. According to Sheinker and Nedvedev (1954), the rate of polymerization of monomers (such as acrylonitrile), which are not
