The kinetics of the selective oxidation of ethene in air over an industrial silver on a-alumina catalyst were studied. Special attention was paid to the influence of the reaction products on the reaction rates of epoxidation and complete combustion. Kinetic data were obtained in two different types of internal recycle reactor and in a cooled tubular reactor, and were fitted separately to several reaction rate expressions based on different kinetic models. A Langmuir-Hinshelwood mechanism, in which adsorbed ethene reacts with adsorbed molecular oxygen, was chosen as the best kinetic model. The reaction products compete for adsorption on the active sites and reduce the rates of both reactions. Carbon dioxide enhances the selectivity towards ethene oxide, whereas water has almost no influence on the selectivity. The fitting of the three individual data sets obtained in the three reactors results in accurate, but different, reaction rate expressions, whereas the fitting of the three data sets simultaneously results in less accurate reaction rate expressions. The systematic deviations found may be explained, to some extent, by differences in the operating regimes in each reactor. The main reason for the deviations is probably the different catalyst activities in the three reactors caused by poisoning. The effect of the addition of products to the feed on the behaviour of the cooled tubular reactor can be described reasonably well by a mathematical model in which the kinetic equations obtained in the laboratory reactors are incorporated. The results of these simulations are sensitive to the kinetics used.

Abstract--The selective oxidation of ethene over a silver on cx-alumina catalyst was studied in a pilot plant with a wall-cooled tubular packed bed reactor. Gas and solid temperatures in the catalyst bed were measured at different axial and radial positions as well as concentrations at different axial position. Total pressure, inlet mole fraction ethene, mass flow rate and wall temperature were varied_ The experiments are compared with model calculations. A pseudo-homogeneous model and a heterogeneous two-dimensional model were used for the calculations. Kinetics and heat transport parameters in the model have been determined independently in separate studies_ The work is distinctive in the accuracy paid to a large number of experimental details. The models correspond reasonably well with the experiments at all operating conditions. The main problem in the description of the reactor is the inability of the kinetic expressions to predict the reaction rates over the entire range of partial pressures. As a result some systematic deviations are found, in particular in the predicted temperature profiles. The heterogeneous model gives better agreement with the experiments although systematically larger temperature differences are calculated on the basis of available correlations in literature than has been measured. On the basis of this work it is difficult to determine directly whether an influence of chemical reaction is present on the values of the heat transport parameters.

Several problems and pitfalls in the use of laboratory reactors for the determination of the kinetics of ethene epoxidation over industrial silver on E-alumina catalyst are discussed. Also, commonly used methodologies for kinetic studies are dealt with because of the general nature of some problems. Some advice is given in choosing and using the appropriate reactor type. Further, a method is discussed to determine kinetics in a cooled tubular reactor without having to use heat transport relations.The activation and deactivation of the silver catalyst have been studied in a Berty-type reactor, in a novel internal recycle reactor and in a cooled tubular reactor. It was found necessary to activate the silver catalyst for approximately 170 reaction hours under reaction conditions to obtain a stable and reproducible catalyst activity. Thermal sintering was probably of importance in experiments at the maximum temperature of 543 K. Deliberate addition of small amounts of 1,2-dichloroethane resulted in rapid deactivation of the catalyst. The activity could be restored by addition of small amounts of ethane to the feed. Also, fluorine and silica have been shown to poison the catalyst. Differences in the behaviour of the catalyst in the three reactors may be attributed to the sensitivity of the catalyst towards tiny amounts of poisons present in the reactors and feed mixtures used.

The signijkance of the statistical character of wall-cooled packed-bed reactors was studied by measuring angular temperature variations, which result @om the random nature of the packing. These are neglected in present-day mathematical models designed to describe the reactor behavior. The amplitude of these variations was determined experimentally as a function of the mass flux and the position in the tube, under conditions of heat transfer as well as of reaction. Angular variations are significant whenever radial temperature gradients are large.Different methods used to average observed variations were surveyed. For the system presented, the angular variations have only a limited influence on the reaction rates if the angular averaged temperature is used. Thus, a two-dimensional deterministic continuum model can be used, notwithstanding the statistical character of the packed bed.