Ind. Eng. Chem. Process Des. Dev. 1902, 21, 67-73
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Reaction-Deactivation Kinetics of Methanol Oxidation over a Silica-Supported Fe,O,-Moo, Catalyst PI0 Forratti' and G. Buzrl-Ferrarls Istituto di Chimica Industriele del Politecnico, Piazza Leonard0 da Vinci 32, 20 133 Milano, Italy
The reaction-deactivation kinetics of methanol oxidation over silica-supported Fe203-Mo03catalyst at 370 OC is determined from 13 deactivation runs performed under integral reactor conditions. Two reliable reaction-deactivation models are derived on the basis of previous chemical investigation on fresh and deactivated catalysts and/or information on methanol oxidation kinetics over Fe203-Mo03based catalysts. The parameters in the models are estimated by an appropriate calculating procedure which allows for an analysis of integral reactor data without too great a computational effort,while avoiding decoupling of the kinetics of the main reaction and of the decay. The two models are shown to describe well the experimental data, although indication that one is preferable to the other exists. The significance of the individual parameter estimates in the two models is also discussed.
Introduction In a recent paper, Cairati et al. (1980) present a novel fluid bed process of methanol oxidation to formaldehyde. In industrial practice this reaction is accomplished in fixed bed reactors only, with Fe203-Mo03or Ag based catalysts (La Page et al., 1978). The interest of the fluid bed process lies in the challenge to overcome the main drawback of fixed bed operation when using metal oxide catalysts, Le., the development of hot spots in the bed which are critical for the catalyst life. Indeed fluidized bed reactors achieve quite a good temperature control. Catalysts suitable for fluid bed operation were prepared by supporting Fe203-Mo03on silica with very low surface area (Cairati et al., 1979). These catalysts combine good attrition characteristics and quite interesting formaldehyde yields at the laboratory scale. The effects of the key process variables were also investigated and the best range of potential conditions for fluidized bed operation indicated (Cairati et al., 1980). In view of the possible commercial interest of the fluid bed process, a kinetic study over the novel silica-supported Fe203-Mo03 catalyst was undertaken. Actually kinetic information is required for correct scaling-up together with information on the flow characteristics in the commercial unit or in the proper pilot plant unit. The kinetic study was accomplished in a fixed bed reactor. In order to test the relevance of possible catalyst deactivation, preliminary experiments were made at T = 370 "C, a temperature higher than those explored in the fluidized bed investigation (250-340 OC). It was immediately evident that the catalyst deactivates in the fixed bed reactor under the chosen conditions, while maintaining a residual activity after the deactivation process had been completed. So far, examples of deactivation processes with residual activity are quite scarce in the current literature. They encompass activity replacement due to the change in the chemical nature of the active catalyst components (Hegedus, 1975; Ostermaier et al., 1976) as well as residual activity due to selective poisoning of only a fraction of catalyst active sites (Pozzi and Rase, 1958; Ballivet et al., 1974). Similarly, either chemical investigations or quantitative treatments of the deactivation of metal oxide catalysts under oxidation-reduction conditions are also scarce (Corado et al., 1975; Bielanski et al., 1979; Gibson and Hightower, 1976; Popov et al., 1976; Burriesci et al., 1980; Carbucicchio et al., 1980). Generally speaking, the formulation of a reliable model adequate for the description of reaction deactivation data 0196-4305/02/ 1121-0067$01.25/0
represents quite a difficult problem in which the physicochemical aspects of the process and the statistical mathematical procedures are involved. In addition to the simplifying assumptions usually made in normal kinetic analyses, e.g., the existence of a rate-determining step or of most abundant surface intermediates, the assumption of correlating deactivation kinetics in terms of separable rate forms (Szepe and Levenspiel, 1971) is almost universally made. However, there is both theoretical and experimental evidence that this assumption is questionable in some cases (Butt et al., 1978; Bakshi and Gavalas, 1975; Corado et al., 1975; Weng et al., 1975). Theory can be applied to the derivation of reactiondeactivation kinetics, but it may happen that the same set of experimental data is well described by completely different rate equations. Furthermore, an interaction may arise between chemical kinetics and catalyst deactivation so that the same reactant conversion data are mathematically described by different combinations of kinetic and deactivation equations (Krishnaswamy and Kittrell, 1978). This possibility further increases the probability that the parameters estimated in a reaction-deactivation model, although adequate for describing the experimental data, have no physical meaning. In the authors' opinion, all these facts point to the need of chemical and mechanistic investigation of the reaction-deactivation phenomenon as a basis in the derivation of a reliable reactiondeactivation rate equation. Concerning the kinetics of methanol oxidation, this has already been investigated by Jiru et al. (1964) and by Dente et al. (1964) for unsupported Fe203-Mo03 based catalysts. A rate equation derived on the basis of a redox mechanism, previously reported by Mars and Van Krevelen (1954) for the oxidation of aromatic hydrocarbons on V205,was used in both cases. This equation and the associated mechanism consider only the influence of the reactants. However, Pernicone et al. (1968) proved that water acts as a strong inhibitor of the methanol oxidation, much more than formaldehyde(Perniconeet al., 1969). No kinetic equation which could account for this effect is presented by these authors; neither is it available in the up to date literature. This paper reports on a detailed study of the kinetics of methanol oxidation over silica-supported Fe203-Mo03 catalysts and of the associated catalyst decay at T = 370 "C. The kinetics for the fresh and the deactivated catalysts have been derived in the light of a chemical investigation on these catalysts already published by Carbucicchio et 0 1981 American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Val. 21, No. 1, 1982
n
Y I
MP
u
4
i
Figure 1. Experimental setup: r, rotameter, G1,G2, G3, preheaters; GR, methanol or methanol-water graduated reservoir; MP micropump; R, reactor; GC, gaschromatograph; AI, A2, absorbers.
al. (1980) and the inhibitory effect of water vapor is incorporated in the proposed equations. Two alternative expressions derived on the basis of a reliable deactivation mechanism are considered for the decay term. The problems associated with parameter estimation for reaction-deactivation kinetics and the possibility to decouple the problem of determining the kinetics for the main reaction and the decay are also considered.
Experimental Section Preparation of the Catalyst. The catalyst was prepared by impregnating Grace 951 silica calcined at 1200 OC for 2 h (resulting surface area
