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The Chemical



48 (1992)



Development and verification of a simulation model for a nonisothermal water-gas shift reactor Riitta L. Keiski”, Tapio SaImib and Veikko J. Pohjola”
‘Department bDepatiment of Process Engineering, of Chemical Engineering, University of Ouh, SF-90570 Oulu (Finland) Abo Akademi, SF-20500 Turku (Finland..

(Received December 29, 1989; in final from July 5, 1991)


A fixed-bed test reactor suitable for studying non-isothermal reaction kinetics was developed. The reactor allows axial and radial temperature measurement a.s well as online gas analysis. The water-gas shift reaction over a commercial iron-based catalyst was chosen as the subject of a case study. A non-isothermal reactor operating at temperatures between 575 and 675 K and with feed compositions corresponding to industrial conditions was used. A method of sequential regression analysis was applied to determine the kinetic parameters from the temperature and conversion profiles of the bed. The experimental data could be fitted by a power-law type of reaction rate expression. The rate equation combined with a plug flow model of the bed was successfully used to predict the fixed-bed behaviour within large temperature, concentration and space velocity intervals.

1. Introduction

To study the kinetics of catalytic reactions several experimental techniques have been developed. All these techniques such as gradientless reactors, pulse microreactors, isotope labelling, IR microreactors, while good for fundamental kinetic studies, prove to be less useful iu providing information about the macrokinetics of a catalytic process. In many cases

the mass and heat transfer phenomena essential for the modelling of industrially operating reactors can be made by analysing the experimental concentration and temperature profiles. The water-gas shift reaction studied in the present work is a reversible exothermic reaction which is of particular interest owing to its industrial importance: CO+H,O M(298 = CO2 + H2 kJ mol-’ (I) K)= -41.09

sophisticated equipment cannot be used in studying a particular system because of problems in online analysis or in temperature and pressure control. Therefore for macrokinetic studies a simple tubular packed-bed reactor provides a very attractive alternative because of its easy set-up and operation. The main difficulty with a tubular reactor is the non-isothermality: temperature variations of several tens of degrees in the catalyst bed are normal. However, the kinetic parameters of a catalytic reaction can be determined using a packed bed operating in conditions close to those of the corresponding industrial process. With a reactor equipped with a device for sampling of the reaction mixture in various locations along the tube and for temperature measurements inside the tube it is possible to obtain detailed information of the concentration and temperature gradients of the catalyst bed. A quantitative description of the kinetics and

AS(298 K) = - 42.39 J K-’ mol-’ Iron oxides are used to catalyse the water-gas shift reaction in the temperature range 320-450 “C. The iron oxide catalyst contains chromium oxide as a stabilizer which retards sintering and loss of the surface area. The catalytically active component is believed to be magnetite: before use Fez03 is reduced to Fe304. Numerous studies of the kinetics of the water-gas shift reaction over iron oxide-chromium oxide high temperature catalysts (ferrochrome catalysts) have been reported in the past 20 years [ 11. There is, however, little or no agreement concerning either the precise form of the rate equation or the values of the rate constants or the activation energies. Both first-order [ 21 and second-order [3] kinetics have been suggested but many other more 0 1992 - Elsevier Sequoia. All rights reserved



R. L. Keiski et ul. / Water-gas

shifi reactor model

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