Influence of reaction products on the selective oxidation of ethene

The kinetics of the selective oxidation of ethene in air over an industrial silver on ¿-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

A novel reactor for determination of kinetics for solid catalyzed gas reactions

A novel perfectly mixed laboratory reactor for determining kinetics of heterogeneously catalyzed gas-phase reactions has been developed. Perfect mixing is achieved by circulating the gas in the reactor using an axial flow impeller in a well streamlined enclosure. Pellets are fixed in a rectangular opening in the blades of the impeller. They rotate with the impeller, thus realizing high particle velocities in the reactor. Interparticle mass transfer was studied experimentally by vaporization of naphthalene pellets. The mass-transfer coefficient in the novel reactor was found to depend on the velocity of a particle in the reactor. Mass-transfer coefficients in an internal recycle reactor at equal impeller tip speeds are 4-6 times lower than those in the novel reactor, and conditions can be chosen easily where at higher rotational speeds the mass- and heat-transfer rates are 8-10 times higher than in classical recycle reactors. The recycle flow rate in a recycle reactor was found to depend strongly on the resistance to flow caused by the catalyst bed itself. The novel reactor was tested under reacting conditions using the hydrogenation of ethene

Determination of the kinetics of ethene epoxidation

Several problems and pitfalls in the use of laboratory reactors for the determination of the kinetics of ethene epoxidation over industrial silver on α-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.\ud \ud 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 kinetics of the methanol synthesis on a copper catalyst: An experimental study

The kinetics of the low pressure of methanol from feed gases containing solely CO and H2 were studied in an internally recycled gradientless reactor. As experimental accuracy impeded the application of high CO contents, the experimental range of mole fraction of CO was limited to 0.04 to 0.22. The total pressure was varied from 3 to 7 MPa and the temperature from 503 to 553 K. Residence time distribution experiments confirmed the assumption of perfect mixing on a macroscale. A maximum likelihood approach was used to fit possible kinetic equations. Although more accurate results and better fits—compared to previous experiments in a simple integral reactor—were obtained, no single rate expression could be selected as the most appropriate one. This was mainly attributed to the effects of small amounts of CO2 and H2O formed in the reactor. Three different reaction rate equations fit the experiments equally well. Arguments are given that we never can expect to elucidate the reaction mechanisms on the basis of kinetic experiments

Three-phase packed bed reactor with an evaporating solvent—II. Modelling of the reactor

In this paper two models are presented for a three-phase catalytic packed bed reactor in which in evaporating solvent is used to absorb and remove most of the reaction heat. A plug flow model and a model comprising mass and heat dispersion in the reactor are discussed. The results of both models are compared to each other and to experimental data obtained in a miniplant on the hydrogenation of 2,4,6-trinitrotoluene to triaminotoluene described in Part I. The influence of reactor pressure, feed temperature and the molar ratio of hydrogen to the reactant in the feed are discussed. It is concluded that both models can well describe the influence of the operating variables on the reactor behaviour and that a large part of the reaction heat can be removed by evaporation of the solvent. From a comparison with the experimental results it is concluded that the dispersion model can well describe the experimental data for a fresh catalyst. The agreement between the model and the experimental data for a deactivated catalyst is not good. As long as no quantitative description of the local deactivation in the packed bed is available, any model will fail to predict local concentrations in the reactor. Despite this drawback recommendations can be given how to use this reactor type of optimize the selectivity in producing an intermediate product

An experimental study for the selective oxidation of ethene in a wall cooled tubular packed bed reactor

The selective oxidation of ethene over a silver on ¿-alumina catalyst was studied in a wall cooled tubular reactor. Temperatures were measured inside the bed at different axial and radical positions as well as the overall conversion and selectivity. Locally measured temperatures vary after repacking the bed whereas the global properties do not vary. Angular variations in temperature cannot be described by present day models. The steady state temperature profiles in the packed bed and overall conversion and selectivity as function of different operating conditions are discussed

An experimental study of the selective oxidation of ethene in a wall cooled tubular packed bed reactor

The selective oxidation of ethene over a silver on ¿-alumina catalyst was studied in a wall cooled tubular reactor. Temperatures were measured inside the bed at different axial and radical positions as well as the overall conversion and selectivity. Locally measured temperatures vary after repacking the bed whereas the global properties do not vary. Angular variations in temperature cannot be described by present day models. The steady state temperature profiles in the packed bed and overall conversion and selectivity as function of different operating conditions are discussed