Intrinsic kinetics of nickel/calcium aluminate catalyst for methane steam reforming

Abstract

A new catalyst for steam reforming of methane based on nickel/calcium aluminate is prepared. The new catalyst has shown stability and high activity at low steam to methane ratios. In this paper the intrinsic rate equations are derived and parameters estimation made. The rate equations show non-monotonic dependence on steam partial pressure. The rate equations also show that the primary product is CO, while CO is formed via the reverse water-gas shift reaction. The mechanism proposed and the rate equations obtained indicate that it may be essential to propose specific rate models for any given catalyst rather than generalized mechanism and rate models.Recently Xu and Fromentg obtained a more general rate expression which in addition to reactions (1) and (2). also includes the water-gas shift reaction (3) in the reaction network :Many contradictions have appeared in the literature regarding the order of reaction with respect to the species involved in the reaction as well as to the activation energie~.'.~ However, the most interesting contradiction has been the fact that some authors obtained effective positive order dependence of the rate of reaction upon steam partial pressure"-" while others obtained effective negative order dependence."-lY In fact Al-Ubaid et ~1 . '~ have noticed that for Ni catalysts with acidic supports (Y-zeolite and Ni aluminate) positive order is obtained whereas with less acidic supports (calcium aluminate) the order is negative.Xu and Fromentg used the Langmuir-Hinshelwood Hougon-Watson (LHW-HW) approach for the derivation of the rate expression they obtained. Their model is based upon a 13 step mechanism with three rate determining steps. The analysis given by Elnashaie et d.'* proved that the functional form of the rate equation obtained by the Xu and Froment expression is a more general and reliable form than previous rate equations and that it helps to resolve many contradictions in the literature especially that related to the +ve and -ve effective order with respect to steam discussed earlier.The purpose of this part of the work is to study the kinetics of the steam reforming of methane catalyzed by the Ni/Ca aluminate spinel catalyst developed by Al-Ubaid et u/." and characterized by its superior activity and stability at low steam to methane ratios.Due to the small temperature range available for performing steam reforming experiments and due to the fact that the water-gas shift reaction is almost at equilibrium at high temperature some methanation 131 J. Chem. Tech. Biorechnd. 0268-2575/92/$05.00 0 1992 SCI. Printed in Great Britain

Methane Steam Reforming Over Ni/NiAl2O4 Catalyst: The Effect of Steam-to-Methane Ratio

“…Under conditions of thermodynamic equilibrium the question is irrelevant, however, far from equilibrium it grows in significance. Furthermore, the nature of the rate determining step (RDS) of the steam reforming and the role of the support [19,20] are key parameters to understand the reaction in further detail and facilitate comparison between different investigations. The classical SR catalyst is a metal (nickel) on a (usually basic) support, which is assumed to be inert [13] or affect the reaction [21].…”

Balance between model detail and experimental information in steam methane reforming over a Ni/MgO‐SiO2 catalyst

“…The models comprise a kinetic force, a driving force and an adsorption term, albeit sometimes in a modified form . Some highly cited examples are the models proposed by Bodrov et al, Khomenko et al, Xu and Froment, Soliman et al, and Hou and Hughes, see Supporting Information Table S2. In these models, it is typically assumed that, to a certain extent, hydrogen can inhibit the reaction rate .…”

“…The assessment of the Numaguchi model and the experimental data suggested that including competitive adsorption effects may be helpful to improve the model performance. Reactant adsorption is, hence, incorporated in the next iteration of the methodology, see steps (5) to (8) in Table , which contain both gas phase and surface species. The adsorption and desorption of methane and water are assumed to be quasiequilibrated based on the fact that methane dissociation is the most demanding and, hence, rate‐determining step.…”

On the Modeling of Steam Methane Reforming

“…A complex model developed by Avetisov et al [61] converges to the model devel oped by Xu and Froment [59] under usual operating conditions of SMR. Moreover, the same reaction path is successfully used to develop the kinetic rate expressions for supported nickel catalysts by many authors [62][63][64]. Model developed by Xu and Froment [59] is also used extensively for modeling of SMR process [65][66][67][68][69][70][71][72][73][74] and is used in this work also.…”