Intrinsic kinetics of heterogeneous reactions

In the interpretation of the intrinsic kinetics of catalytic reactions, the simple scheme based on Langmuir s work including chemisorption of reactants, surface chemical reaction, and desorption of products provides the framework together with Taylor s physical surface model postulating a fixed number of active surface sites. This analysis has been successful in correlating a wide range of kinetic results and also in predicting possible effects of new reaction conditions. 

In order to proceed with the kinetic analysis, the a priori assumption that the number of active sites is a constant proportional to the mass of catalyst (So) is utilized in writing a site balance  

The three key principles used in the formulation of intrinsic rate equations originate from these surface model concepts [14]  

1 Constancy of the total number of active sites, which is a priori assumption based on the physical surface model 

2 Quasi- or pseudo-steady-state approximation, which assumes that concentrations of intermediate complexes formed on the surface are small and time invariant 

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Intrinsic Kinetics of Heterogeneous Reactions Involving Solids 255... 

INTRINSIC KINETICS OF HETEROGENEOUS REACTIONS INVOLVING SOLIDS... 

From the literature survey it was observed that the reaction has been studied using various ion exchange resins or homogeneous catalysis by strong acids, however, detailed studies on intrinsic kinetics of the reaction are very limited. Also, kinetics of homogeneous-heterogeneous esterification has not been considered. Hence, the present... 

In the above derivation a power-law kinetic expression was assumed for the intrinsic chemical reaction. For the systems where large concentration differences occur between the bulk fluid and the interior of the porous solid, the Langmuir-Hinshelwood type rate expression should be used because this provides a better description of the rate of heterogeneous reactions. 

It is a good idea to run the laboratory reactor without catalyst to check for homogeneous reactions. However, this method does not work when the homogeneous reaction involves reactants that do not occur in the feed but are created by a heterogeneous reaction. It then becomes important to maintain the same ratio of free volume to catalyst volume in the laboratory reactor used for intrinsic kinetic studies as in the pilot or production reactors. 

The pseudohomogeneous reaction term in Equation (11.42) is analogous to that in Equation (9.1). We have explicitly included the effectiveness factor rj to emphasis the heterogeneous nature of the catalytic reaction. The discussion in Section 10.5 on the measurement of intrinsic kinetics remains applicable, but it is now necessary to ensure that the liquid phase is saturated with the gas when the measurements are made. The void fraction s is based on relative areas occupied by the liquid and soUd phases. Thus,... 

Note that the results of our simulation via the pseudohomogeneous model tracks the actual plant very closely. However, since the effectiveness factors r]i were included in a lumped empirical fashion in the kinetic parameters, this model is not suitable for other reactors. A heterogeneous model, using intrinsic kinetics and a rigorous description of the diffusion and conduction, as well as the reactions in the catalyst pellet will be more reliable in general and can be used to extract intrinsic kinetic parameters from the industrial data. 

In summary, it can be seen for the three-step reaction scheme of this example that the net rate of the overall reaction is controlled by three kinetic parameters, KTSi, that depend only on the properties of the transition states for the elementary steps relative to the reactants (and possibly the products) of the overall reaction. The reaction scheme is represented by six individual rate constants /c, and /c the product of which must give the equilibrium constant for the overall reaction. However, it is not necessary to determine values for five linearly independent rate constants to determine the rate of the overall reaction. We conclude that the maximum number of kinetic parameters needed to determine the net rate of overall reaction is equal to the number of transition states in the reaction scheme (equal to three in the current case) since each kinetic parameter is related to a quasi-equilibrium constant for the formation of each transition state from the reactants and/or products of the overall reaction. To calculate rates of heterogeneous catalytic reactions, an addition kinetic parameter is required for each surface species that is abundant on the catalyst surface. Specifically, the net rate of the overall reaction is determined by the intrinsic kinetic parameters Kf s as well as by the fraction of the surface sites, 0, available for formation of the transition states furthermore, the value of o. is determined by the extent of site blocking by abundant surface species. 

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