The behaviour of a single catalyst pellet for the selective hydrogenation of ethyne in ethene

The steady-state and dynamic behaviour of a single Pd---Al2O3 catalyst particle is studied for the selective hydrogenation of ethyne in the presence of ethene, without addition of carbon monoxide. The particle-to-gas heat transfer in the reactor is characterized. During selective hydrogenation, not only the ignition and the extinction phenomena but also oscillatory behaviour is observed. The nature of the single and multipeak oscillations is discussed. With a dynamic model, based on relatively simple kinetic equations and an additional slow mechanism, e.g. the formation of ethylidyne on the catalyst surface, the qualitative features of this system can be described

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

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

Mechanism and kinetics of the selective hydrogenation of ethyne and ethene

A review of the kinetics and mechanism for the selective hydrogenation of ethyne and ethene on palladium catalysts is presented. The progress made in the last fifteen years is mainly discussed. It has become clear that the classical view, where the selectivity of the reaction was believed to be due to the thermodynamic factor is an over-simplification. Currently, it is generally assumed that at least two different sites are active during the selective hydrogenation, one of these might possibly involve the support. Ethene hydrogenation also occurs in the presence of high ethyne concentrations, which cannot be explained by the classical theory. Besides the two main hydrogenation reactions and the oligomerisation, there exists a direct route from ethyne to ethane, which, however, is only of minor importance. Possibly due to the rather complex nature of the system, there have been relatively few kinetic studies presenting practical rate expressions

Kinetic research on heterogeneously catalysed processes: a questionnaire on the state-of-the-art in industry

On the initiative of the Working Party `Chemical Engineering in the Applications of Catalysis¿ of the European Federation of Chemical Engineering an assessment of the issues in the determination and application of kinetic data within the European industry was performed. The basis of the analysis consisted of a questionnaire put together by researchers from Dow, DSM, Shell and Eindhoven University of Technology. The 24 companies, which have responded to the questionnaire, can be classified into four groups: chemical, oil, engineering contractors and catalyst manufacturers. From the overall input it appears that there are three, equally important, utilisation areas for kinetic data: process development, process optimisation and catalyst development. There is a wide variety of kinetic data sources. Most of the respondents make use of test units which were primarily designed for development and optimisation. Avoiding transport limitation is, certainly in the case of short range projects or for complex feedstocks, not always taken care of. With respect to the modelling approaches, a common philosophy is `as simple as possible¿. Most of the respondents state that `in principle¿ one should strive for intrinsic kinetics, but the majority nevertheless does for various reasons not separate all transport phenomena from reaction kinetics. Kinetic models are mostly simple first or nth order or Langmuir-Hinshelwood type expressions. More complex kinetic models are scarcely used. Three areas were frequently identified to offer opportunities for improvement. Gathering of kinetic data is too costly and time consuming. There is no systematic approach at all for determination and application of kinetics in case of unstable catalytic performance. Furthermore, the software available for the regression of kinetic data to rate equations based on mechanistic schemes as well as software to model reactors are insufficiently user friendly. The majority of the respondents state that the problems indicated should be solved by cooperation, e.g., between companies, between industry and academia and between the catalysis and the chemical engineering community. A workshop on the above topics was held in December 1996 with 15 companies and 6 academics attending. More information can be obtained from the secretariat of the Working Party

Interfacial areas and gas hold-ups in gas-liquid contactors at elevated pressures from 0.1 to 8.0 MPa

Interfacial areas and gas hold-ups have been determined at pressures up to 8.0 MPa in a mechanically agitated gas—liquid reactor and a bubble column with a diameter of 81 mm for superficial gas velocites between 1 and 5 and 1 and 10 cm/s, respectively. The interfacial areas have been determined by the chemical method using the model reaction between CO2 and aqueous diethanolamine (DEA). Contrary to earlier reported results on interfacial areas in a mechanically agitated reactor at pressures up to 1.7 MPa, a positive influence of pressure on the interfacial areas has been observed for higher pressures and higher superficial gas velocities. The product of the gas density G and the superficial gas velocity at the orifice υG, or was found to be an important parameter for the manifestation of the pressure effect. For values of GυG, or larger than 200 kg/m2 s the interfacial areas increase with increasing reactor pressure. Below this value of 200 kg/m2 s no influence of pressure could be observed. The gas hold-ups in the bubble column in water as well as in an aqueous solution of DEA with antifoam increase with increasing pressure. This pressure effect on the gas hold-up in bubble columns originates from the formation of smaller bubbles at the gas distributor. The relative increase in the gas hold-ups is smaller in water and also if a porous plate instead of a perforated plate is used as gas distributor. The differences in the magnitude of the pressure effect are caused by differences in the coalescence behaviour of the gas bubbles in both liquids and by differences in the bubble formation process at the two types of gas distributors, respectively. The interfacial areas in the bubble column also increase with increasing pressure. The relative increase in the interfacial areas aP/aatm with increasing pressure may be as large as 200% for a pressure increase from P = 0.15 to 8.0 MPa, depending on the type of gas distribution and the superficial gas velocity used

Mechanism and kinetics of the selective hydrogenation of ethyne and ethene

A review of the kinetics and mechanism for the selective hydrogenation of ethyne and ethene on palladium catalysts is presented. The progress made in the last fifteen years is mainly discussed. It has become clear that the classical view, where the selectivity of the reaction was believed to be due to the thermodynamic factor is an over-simplification. Currently, it is generally assumed that at least two different sites are active during the selective hydrogenation, one of these might possibly involve the support. Ethene hydrogenation also occurs in the presence of high ethyne concentrations, which cannot be explained by the classical theory. Besides the two main hydrogenation reactions and the oligomerisation, there exists a direct route from ethyne to ethane, which, however, is only of minor importance. Possibly due to the rather complex nature of the system, there have been relatively few kinetic studies presenting practical rate expressions