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    EFFECT OF INTERACTION AND MOBILITY ON FIXED-BED REACTOR PERFORMANCE L.K. Doraiswamya; S.D. Prasada a National Chemical Laboratory, Pune, India

    To cite this Article Doraiswamy, L.K. and Prasad, S.D.(1987) 'EFFECT OF INTERACTION AND MOBILITY ON FIXED- BED REACTOR PERFORMANCE', Chemical Engineering Communications, 58: 1, 63 — 88 To link to this Article: DOI: 10.1080/00986448708911960 URL: http://dx.doi.org/10.1080/00986448708911960

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  • Chem. Eng. Comm. 1987, vol58, pp. 63-88 Photocopying permitted by license only 0 1987 Gordon and Breach Science Publishers S.A. Printed in the United States of America

    EFFECT OF INTERACTION AND MOBILITY ON FIXED-BED REACTOR PERFORMANCE

    L.K. DORAISWAMY and S.D. PRASAD

    National Chemical Laboratory Pune 41 1 008, India

    (Received November 17, 1986; infinal form February 10, 1987)

    The roles of interaction and mobility in determining surface rates and hence reactor performance, as reflected in the space time values required for achieving a desired conversion or selectivity, are demonstrated. It is shown that localized adsorption models predict larger space times in comparison to the mobile models. Further, repulsive forces lead to higher space times in comparison to attractive forces, and the divergence between models is most marked for localized adsorption. KEYWORDS Adsorption Surface heterogeneity Adsorbate interactions Selectivity

    Catalytic reactions Surface nonideality

    INTRODUCTION

    Rapid progress in surface science has revealed a rich variety of phenomena, such as phase transitions among adsorbed species, adsorbate restructuring and island formation (Ertl, 1983; Woodruff et al., 1983; Madix, 1983; Somorjai, 1978), but these have not influenced reactor design concepts in any significant way. A wealth of new phenomena including the role of surface phase transformations in causing dynamic instability like oscillating rates have been uncovered (Ertl, 1985). Equally intriguing is the observation of island formation among adsorbed species (oxygen atoms in particular) as a prelude to catalytic reaction proper, observed in many oxidation reactions (Barteau et al. 1981; Gland et al., 1982). Computer simulations (Silverberg et a[ . , 1985) as well as direct experimental observations (Gland and Kollin, 1983; Shigeishi and King, 1978) have unequivocally demons- trated that non-classical rate behaviour will be exhibited by these systems, primarily as a result of interactions between adsorbed molecules.

    While it is impossible to model all.these effects in their totality, some attempts have been made in the past to account for the role of interactions in determining activity and selectivity (Benziger and Madix, 1979; Zhadnov, 1981; Bhat et al., 1984). The first paper (Benziger and Madix, 1979) analyzed the rate behavior using a first-order interaction model like the Fowler-Guggenheim model, the second dealt with a more refined quasi-chemical approximation model (Zhadnov, 1981), and in the third paper the important problem of selectivity patterns was analyzed by employing both these models (Bhat et al., 1984).

    A natural engineering consequence of these previous studies is the need to address the questions: How significant are these effects in determining reactor

    D o w n l o a d e d A t : 1 0 : 2 3 2 0 J a n u a r y 2 0 1 1

  • L.K. DORAISWAMY AND S.D. PRASAD

    performance? Do the gross rates (which are obtained by averaging the point rates over the entire reactor length) still reflect the subtleties of interaction or are they masked? Does refinement in accounting for interaction effects change the picture dramatically? The last question is of particular importance as the computational effort increases substantially with refinement to account for interactions.

    A closely related problem is that in many experimental systems the adsorbate phase becomes mobile at temperatures close to those at which catalysis occurs. Thus conventional engineering models (in which molecules are assumed to be nailed to the surface sites) would become unrealistic and it would be necessary in such cases to consider adsorbate mobility superimposed on interaction effects.

    The main objectives of the work are: (1) By using accurate numerical methods, to demonstrate the significant role of

    interactions (attractive and repulsive) in determining the reactor perfor- mance, as indicated by the space time needed for achieving a desired conversion.

    (2) To assess the necessity for refinements in accounting for interactions between adsorbed molecules and the consequent effects on reactor performance.

    Throughout the paper, the analysis will be confined to the simplest possible model-the plug flow reactor--so as to focus attention on the main theme of the work. Further, to make the analysis tractable only simple reaction schemes are used.

    FORMULATION

    In the presentation that follows, single step control (i.e. surface reaction control) will be assumed. It will also be assumed that the adsorption-desorption steps are so fast compared to surface reaction that adsorption equilibrium prevails between the gas and surface phases at all pressures. Thus the surface and gas concentra- tion can be related through adsorption isotherms, and expressions can be readily written for the surface rates. All the products of reaction are assumed to be weakly bound, so that they are readily desorbed under conditions of reaction.

    The reaction schemes chosen are presented in Table I. All the reaction schemes are important in analyzing hydrocarbon conversions (Davis and Somorjai, 1983). Reaction schemes I and I1 are first- and second-order, respectively, but occur without a net change in the overall number of molecules. In schemes I11 and IV, respectively, volume expansion and contraction occur. Hypothetical schemes I, 111, IV idealize key steps in metal catalyzed isomerization and acid catalyzed isomerization in reforming. In contrast to this, scheme I1 represents an acid catalyzed disproportionation reaction. In scheme I11 a net volume expansion occurs. This could correspond to a metal catalyzed dehydrogenation step.

    D o w n l o a d e d A t : 1 0 : 2 3 2 0 J a n u a r y 2 0 1 1

  • FIXED-BED REACTOR PERFORMANCE

    TABLE I

    Reaction schemes studied in the present work

    Scheme I , E = 0

    * I A, - E, + F,

    Scheme 111, E = 1

    *2 2A,- C,+ D,

    Scheme V, e = 0

    1 2 2.4, d C, + D,

    Scheme 11, E = 0

    Scheme IV, E = - 112

    The volume change factor E is defined as the ratio the net number of molecules produced to the number of reactant molecules. Thus for the reaction by stochiometry E is 3. Now if the mole fraction of the reactant is f, the actual volume change factor would be f ~ . Thus with (f = 113) the actual volume change factor is unity. A choice of unity for E is thus not unreasonable for real applications. However, for the reaction scheme 111, A, would represent an adsorbed cyclohexane molecule, E,, F, denote desorbed cyclohexene and hydrogen respectively. For this reaction, the stochiometric E is unity, and the pure feed (f = 1) is assumed for the computations. Scheme IV could, in a sense, represent a dimerization step through a carbonium ion mechanism. There is a net decrease in the volume in this case. Thus it will be an ideal counterpart to reaction scheme 111.

    Reaction scheme V is the simplest one can construct by combining the schemes mentioned above and wherein the question of selectivity figures. Thus in this scheme we look for divergence in the selectivity behavior of the various models in addition to that in the rate behavior. This is the simplest reaction scheme which is describable in terms of one-component adsorption theory and which in a transparent way demonstrates the novel features of reactor selectivity as a consequence of the nonidealities.

    D o w n l o a d e d A t : 1 0 : 2 3 2 0 J a n u a r y 2 0 1 1

  • 66 L.K. DORAISWAMY AND S.D. PRASAD

    THE ADSORPTION MODELS

    The adsorption models chosen in the pres