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A model is developed for industrial natural gas steam reformers with top‐fired and side‐fired furnance design. The one‐dimensional heterogeneous model for catalyst tubes takes into account the intra‐particle diffusion resistances. Two approximations are used for the computation of the effective diffusivities via the Stefan‐Maxwell equations. The two‐point boundary value differential equations for the catalyst pellets are solved by a novel efficient and modified orthogonal collocation technique. The performance of the model has been checked in the case of two industrial reformers used in the ammonia and methanol industries in Egypt and Saudi Arabia.

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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

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