An experimental study of the effect of intraparticle temperature gradients on catalytic activity (2023)

Cited by (27)

  • Effects of the collision integral, thermal diffusion, and the Prater number on maximum temperature in macroporous catalysts with exothermic chemical reaction in the diffusion-controlled regime

    2007, Chemical Engineering Science

    The classic Prater equation is useful to estimate intrapellet temperatures in packed catalytic tubular reactors when the dimensionless Prater number β is relatively small (i.e., the magnitude of β0.3). However, for strongly exothermic chemical reactions, both thermal diffusion and the temperature dependence of important physicochemical properties of reactive gas mixtures should be included in the analysis of coupled heat and mass transfer within macroporous catalytic pellets. In the diffusion-limited regime, intrapellet temperature increases could be much greater than those predicted by the Prater equation. The analysis of thermal diffusion in pseudo-binary Lennard-Jones gases with temperature-dependent physicochemical properties reveals that steady-state predictions for exothermic reactions might not be possible when the Prater number is on the order of unity, because core temperatures are more than one order of magnitude larger than temperatures on the external catalytic surface. For reference, the Prater equation predicts that the maximum intrapellet temperature is two-fold larger than that on the external catalytic surface when β=1, which severely underestimates realistic temperature increases by a factor of 5 or 6 (i.e., when β=1) for the synthesis of methanol from carbon monoxide and hydrogen. The largest increases in intrapellet temperature occur when all of the following conditions are satisfied; (i) chemical reactions are strongly exothermic, (ii) physicochemical properties of the reactive gas mixture exhibit temperature dependence, (iii) the Prater number approaches unity, and (iv) Soret diffusion enhances the molar flux of CO (i.e., MWCO<MWB in pseudo-binary mixtures) into the central core of macroporous catalysts as a consequence of negative thermal diffusion coefficients.

  • Measurement of gas composition at the center of a porous pellet during adsorption and catalytic reaction under dynamic conditions

    1997, Journal of Catalysis

    Gas composition is measured at the center of one-dimensional porous platinum/alumina pellets during carbon monoxide adsorption and catalytic oxidation. The apparatus is based on a single-pellet diffusion reactor that has been modified to allow continuous gas analysis and miniaturized in order to reduce the time constants of gas flow and mixing. Analysis of the CO adsorption response demonstrates that the centerplane volume and sample leak perturb the system only slightly and in a manner that can be accounted for during data analysis. A detailed kinetic model described previously is able to predict the qualitative features of the external concentration responses during carbon monoxide oxidation. However, the model is not able to predict major features of the responses measured at the pellet center, demostrating that the reactor is able to provide stricter tests of kinetic models than reactors in which only external compositions can be measured.

  • Nonuniform activity distribution in catalyst particles: Benzene hydrogenation on supported nickel in a single pellet diffusion reactor

    1995, Chemical Engineering Science

    A single pellet diffusion reactor (SPDR) of new design has been used to investigate the influence of nonuniform distributions of active ingredient on the Ni-catalyzed hydrogenation of benzene. Five different distributions, ranging from preferential shell-loading to uniform to preferential core loading, have been studied under isothermal conditions. Analysis of results has been carried out using a simple one-dimensional diffusion/reaction analysis, which the configuration of the SPDR is set to mimic. The rate of the reaction follows an Eley-Rideal model, effectively positive fractional-order under the experimental conditions, for which kinetics the nonuniform distribution problem has not been studied experimentally before. Preferential shell-loading yields the highest catalytic effectiveness but because of the complex interactions between the reaction kinetics and the activity profiles within the pellets, there is no simple relationship between diffusion lengths and effectiveness even for large values of the Thiele modulus.

  • Uniqueness of the steady state for an mth order reaction in a non-isothermal pellet with variable transport coefficients

    1985, Chemical Engineering Science

  • An unexpected temperature overshoot was found for a Pd on alumina catalyst pellet in its course towards a new steady state, after a change in concentration of one of the reactants. The reaction mixture consisted of ethylene, hydrogen and nitrogen as inert. A speculative model is introduced, which can explain these overshoots by a slow adsorption of one of the reactants on the active sites of the catalyst.

  • Un modèle simple de réaction-diffusion-conduction thermique dans et autour d'une particule poreuse. Partie II: Diffusion externe

    1976, The Chemical Engineering Journal

    On étudie le cas général où une réaction chimique, au sein d'une particule poreuse, est en compétition avec des phénomènes de diffusion externe et interne et avec des phénomènes de conduction thermique externe et interne.

    On écrit que la “résistance” globale est la somme d'une résistance externe et d'une résistance interne et on évalue la fraction externe φe de résistance.

    On définit une efficacité globale ηe, par rapport aux concentrations et à la température externes, mesurées, et on exprime ηe en fonction de l'efficacité interne ηi, définie dans la première partie. Cette relation générale entre ηe et ηi est valable pour une réaction chimique quelconque et elle implique seulement que les variables C et T soient séparables (cf. la première partie).

    Dans le cas particulier d'une réaction du premier ordre dans une particule isotherme, on étudie la multiplicité des points de fonctionnement dûe à l'échauffement dans la couche limite. On propose un critère de multiplicité, qui est en excellent accord avec celui proposé par Hlavacek et Kubicek.

    Comme exemple d'application de la méthode, on décrit la procédure à suivre pour déterminer la loi cinétique de vitesse d'une réaction quelconque, même dans le cas général où la mesure de cette vitesse est perturbée par des phénomènes diffusionnels externes et internes et par des effets thermiques.

    The general case of chemical reactions in a porous particle subject to competition between external and internal diffusion and thermal conductivity is studied.

    The overall “resistance” is expressed as the sum of the external and internal resistances and the fractional external resistance is calculated. An overall efficiency ηe is defined with respect to the measured values of the external temperature and concentration. ηe is expressed as a function of the internal efficiency which was defined in Part I. This general relationship between ηe and ηi is valid for any chemical reaction and only requires that the variables C and T be separable (cf. Part I).

    In the special case of an isothermal first order reaction in a porous particle the multiplicity of operating points due to the heating in the boundary layer is studied. A criterion of multiplicity is proposed which is in excellent agreement with that proposed by Hlavacek and Kubicek.

    To illustrate the use of the method the procedure of determining the kinetics of any reaction is described even in the general case where rate measurements are affected by internal diffusion and thermal effects.

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Copyright © 1967 Published by Elsevier Ltd.

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