Isomerization of n-Pentane Lucien H. Hosten and Gilbert F. Froment' Laboratorium voor Petrochemische Techniek, Rijksuniuersiteit, Gent, Krijgslaan 271, B-9000 Gent, Belgium
The isomerization of n-pentane on a commercial reforming catalyst (Pt/A1203) was studied in an isothermal, bench-scale, fixed-bed reactor. The influence of total pressure, molar ratio hydrogen/hydrocarbon, chlorine content in the feed, and space time was investigated in the temperature range of 375" to 425°C. Reaction rate equations of the Hougen-Watson type were derived on the basis of the generally accepted mechanism for skeletal isomerizations. Discrimination among rival models was based upon significance tests for the overall regression and for each parameter separately. The experimental data were treated by both the differential and the integral method of kinetic analysis. Linear and nonlinear least squares techniques were used in the parameter estimation.
T h e isomerization of n-pentane is one of the reactions of naphtha reforming. Also, in some countries, n-pentane is isomerized separately and added to gasoline to improve the octane number. Yet, in spite of its importance, the kinetic data for this reaction are scarce and fragmentary. The purpose of the work reported here was t o investigate the influence of the process variables and to set u p a kinetic equation for the reaction (Hosten, 1968). I t is generally accepted that reforming isomerizations proceed in three stages (Mills et al., 1953; Sinfelt et al., 1960, 1962; Weiss and Swegler, 1957): Adsorption of an n-paraffin molecule on a dehydrogenation-hydrogenation site, followed by dehydrogenation into a n-olefin; desorption of the n-olefin from the dehydrogenationhydrogenation site and diffusion to a skeletal rearranging site, which converts the n-olefin into an z-olefin via a carbonium ion mechanism (Schmerling, 1953); and, desorption of the i-olefin from the skeletal rearranging site and diffusion to a hydrogenation-dehydrogenation site where it is finally hydrogenated into an i-paraffin molecule. For an industrial, bifunctional reforming catalyst this mechanism can be represented schematically as follows: Pt
n-paraffin 2 n-olefin
AlLOi
Pt
i-olefin 2 i-paraffin
Rossini's data (1953) show that the pentane equilibrium mixture contains approximately 677c isopentane a t 425" C.
' T o whom correspondence should be addressed. 280
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 2, 1971
The reaction is slightly exothermic with a heat effect of 1.9 kcal per mole. Apparatus
A flow sheet of the apparatus is given in Figure 1. The reactor was made of chromium steel containing no nickel. Its length was 115 cm and its internal diameter, 3.5 cm. The flow of n-pentane was metered in a buret and fed to the reactor using a pump with a diaphragm head. Small amounts of water in the pentane were removed using sodium. To eliminate trace amounts of oxygen and water, the hydrogen first was led through an electrically heated oven, filled with catalyst particles, and then dried on Ascarite. The hydrogen was not recycled. The reactor was electrically heated and automatically controlled. The temperature of the catalyst bed was measured a t several points along the axis of the reactor by a sliding thermocouple. The catalyst section was diluted with inert packing so the bed was strictly isothermal. Deviations from the chosen temperature levels were limited to 1"or 2" C. The pressure regulating device a t the exit of the reactor was electrically heated to prevent condensation of pentane. The total exit flow rate-Le., the sum of the rates of the hydrogen and of the saturated hydrocarbon flowswas measured by a wet-gas meter. The effluent was analyzed on line by gas liquid chromatography. The column was filled with Chromosorb P as carrier and 20 wt % di-(2-ethyl-hexy1)sebacate as sta-
A -VENT
Figure 1. Apparatus flow sheet A. 8. C. D. E. F. G. H. I. J. K. L. M.
Oven Drying towers for hydrogen Pentane reservoir Metering buret Feed pump Mixing chamber Reactor Tube for sliding thermocouple Manometer for reactor pressure Pressure regulating device Reference pressure manometer Gas chromatograph Gas meter
tionary phase. The carrier gas was hydrogen, so that only hydrocarbons were detected. The catalyst contained 0.7 wt % Pt on A120Jand 0.85 wt CC chlorine. The density was 0.52 gram per cc. and the internal surface area, 190 m' per gram. The particles were spherical with diameter between 1 and 2 mm. The catalyst was very sensitive to trace impurities leading to deactivation. Consequently, the catalyst activity was frequently checked by means of a standard test. Kinetic experiments were conducted a t 7, 11, 16, and 21 a t m absolute and temperatures of 375", 389", 400", 410". and 425" C. The molar ratio, hydrogen/n-pentane, was varied from 2 to 10. To study the effect of chlorine concentration on the activity of the catalyst, chlorine was added to the pentane feed as CC1,. Three chlorine levels were investigated, namely 0.006, 0.0121, and 0.0242 mol 76 (expressed as percentage chlorine in the reactant feed stream). The chloride content on the catalyst was not determined. Great care was taken, however, to equilibrate the catalyst with respect t o the chlorine concentration in the feed stream.
the reaction rate a t a given conversion, are seen to be independent of total pressure. This is in agreement with observations of previous investigations (Sinfelt et al., 1960; Marechal et al., 1961) and is of particular importance for the selection of a kinetic model. Carr (1960, 1968) found bell-shaped curves for the initial rates as a function of total pressure. Unfortunately, the catalyst used is not specified in detail. The total pressure has a slight effect upon the hydrocracking reactions-the higher the pressure the faster the hydrocracking. Figure 4 shows that the conversion into light hydrocarbons a t 425°C is noticeably lower a t 7 than a t 21 atm. The increase of the rate of hydrocracking with pt is due to the increasing partial pressure of hydrogen. Influence of the Molar Ratio Hydrogen/Hydrocarbon. Hydrogen is required to prevent rapid deactivation of the catalyst by coke deposition. However, a decrease of the selectivity of the isomerization owing to an increase in the rate of hydrocracking has been observed when the molar ratio hydrogen/hydrocarbon is increased. Figure 5 is a plot of the experimental conversions as a function of W / F t , where F, represents the total feed rate of both pentane and hydrogen, and shows a unique 05
011
