Reactivity of Some Complex Hydrocarbons on Constant and Variable Activity Reforming Catalysts
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Date
1988-04
Authors
Onukwuli, D.O
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University of Lagos
Abstract
The product distributions, kinetics and catalyst mortality of the reforming reactions of n-octane, iso-octane and methylcyclophentane (MCP) were investigated on constant and variable activity Pt/A12O3 and Pt-Re/A12O3 catalysts. The Berty CSTR was used for data collection at total pressure of 1atm, various reactant and H2 diluent partial pressures, W/F and temperature, depending on the reactant. The dehydrocyclization of n-octane on 0.3% Pt/A12O3 catalyst was investigated at temperature between 4000C and 4600C in H2. The products of the reaction were: hydrocracked products, iso-octane, ethylbenzene, m-xylene, p-xylene, O-xylene and toluene. Experimental data showed that the yields of cracked products and aromatics generally increase with increase in W/F and temperature, but the aromatic yield decreased with increased in n-octane partial pressure. Results obtained with varying hydrogen partial pressures show that, at constant temperature, the total conversion and the yield of aromatics each passed through a maximum. Another n-octane isomer (1, 1,3-trimethyl-pentane was the sole product of the reaction) of 2,2,4-trimethyl pentane (iso-octane) on Pt/A12O3 catalyst at temperatures between 3900C and 4300C. The conversion of iso-octane on 0.6% Pt/A12O3 catalyst was found to be approximately the same as that obtained with 0.3% Pt/A12O3 catalyst at the same conditions.
The reactions of MCP on 0.3% Pt/A12O3 and 0.3 Pt-0.3% Re/A12O3 catalyst were studied at temperature between 3700C and 4100C and MCP partial pressures between 0.058atm and 0.1816atm. At these conditions, the products of reaction were hydrogenolysis products, cylohexane and benzene. The beneficial effect of rhenium in Pt-Re/A12O3 catalyst was clear from the total conversion and benzene yield obtained with Pt/A12O3 (DRIED) catalyst. Experimental results showed that the total conversion and benzene yield obtained with the Pt/A12O3 (DRIED) catalyst was about 1.5 times greater than that obtained when the bimetallic catalyst was not dried before reduction. In addition to the high conversion and high benzene yield, the dried bimetallic catalyst was found to be more stable than both the Pt-Re/A12O3 catalyst and Pt-Re/A12O3 (UNDRIED) catalyst.Mechanistic kinetic equations of these reactions were developed on both steady and unsteady state catalyst surfaces. The development of mechanistic rate equations for the surfaces state kinetics of n-octane conversion was based on the mechanisms obtained from the modified reaction network proposed by Ako and Susu1. Nineteen rate equations were derived and discrimination among rival models were based on positiveness of rate and equilibrium constants, on the goodness of fit and also on the increase of the value of the rate constants with increase in temperature. The rate models that best fitted the data were based on:
1. Dissociative adsorption of hydrogen and conversion of adsorbed n-octane to adsorbed iso-octane as rate limiting step (Eqn. 3.1.5).2. Dissociative adsorption of hydrogen and desorption of adsorbed iso-octane (Eqn. 3.1.6).3. Dissociative adsorption of hydrogen and conversion of adsorbed iso-octane to adsorbed ethylbenzene as rate limiting step (Eqn. 3.1.7).4. Dissociative adsorption of hydrogen and conversion of adsorbed iso-octane to adsorbed o-xylene as rate limiting step (Eqn. 3.1.8). 5. Molecular desorption of hydrogen and conversion of adsorbed n-octane to adsorbed iso-octane as rate limiting step (Eqn. 3.1.13) Good fit and positive values of rate and equilibrium constants were obtained when models eqns. (3.1.7) and (3.1.8) were used to predict the conversion obtained with varying hydrogen partial pressures at constant temperature.The kinetic rate equations for the isomerization of iso-octane in the absence of coking (steady-state kinetics) were derived on the basis of the generally accepted mechanism for skeletal isomerization. The rate models that best fitted the data were based on: 1. Reaction of adsorbed unsaturated iso-octane on the acidic site to adsorbed iso-octane as the rate determining step (Eqn. 3.1.25). 2. Desorption of iso-octane from the acidic site as the rate determining step (Eqn. 3.1.27). Kinetic rate equations for the aromatization of MCP in the absence of coking (steady state kinetics) were derived on the basis of the reaction network proposed by this author (see chapter 6, section 6.1.3). Eleven rate equations were derived and tested. Five out of the eleven models satisfied the set criteria when Pt-Re/A12O3 catalyst was used for the conversion of MCP. The five rate equations that best fitted the data were based on: 1. The rate model is based on the dehydrogenation of adsorbed methylcyclopentene as the rate determining step (Eqn. 3.1.44) 2. The rate model is based on the conversion of adsorbed methylcyclopentene to adsorbed olefinic hydrogenolysis products as the rate determining step. (Eqn. 3.1.46)
3. The rate model is based on the hydrogenation of adsorbed olefinic hydrogenolysis products to adsorbed hydrogenolysis products as the rate determining step (Eqn. 3.1.48) 4. The rate model is based on the hydrogenation of absorbed cyclohexane to adsorbed cyclohexane as the rate determining step (Eqn. 3.1.49)
5. The rate model is based on the desorption of hydrogenolysis products as the rate determining step (Eqn. 3.1.50) Using the Pt-Re/A12O3 (DRIED) catalyst, however, only two rate equations (Eqns 3.1.48 and 3.1.49) satisfied the set criteria.For deactivation kinetic studies of Pt-Re/A12O3 catalyst iso-octane and MCP were used as reactants while for the deactivation kinetic studies of Pt-Re/A12O3 catalyst only MCP reactant was used. The reactant and hydrogen partial pressures were varied. To describe the distribution of products with time, and activity an a deactivation function of the non-separable type used. Model equations 3.2.11 and 3.2.19 were used to evaluate the constants of deactivation at various values of n (0, 1, and 2).Catalyst mortality experiments were also carried out with all the reactants investigated. Seven deactivation - regeneration cycles were carried out using iso-octane on fresh Pt-/A12O3 while forty deactivation - regeneration cycles were carried out with MCP on the Pt-/A12O3 catalyst used previously for the mortality study with iso-octane. Twelve deactivation regeneration cycles were carried out on dried and undried Pt-Re/A12O3 catalyst using MCP. Two stability states, characterized by the difference in the coke levels, were established in the life of Pt-/A12O3 during the mortality investigation with MCP. The transition from the first state to the second state occurred in the 6th cycle. The coke level in the first state was about 0.045gC while the coke level in the second state was about 0.09gC. The coking levels of the reactants investigated were in the order MCP > N-Octane Iso-Octane.
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Keywords
Variable Activity , Reforming Catalysts , Complex Hydrocarbons
Citation
Onukwuli, D.O (1988) Reactivity of Some Complex Hydrocarbons on Constant and Variable Activity Reforming Catalysts.University of Lagos School of Postgraduate Studies Phd Chemical Engineering Thesis and Dissertation Abstracts, 516p.