Single-pellet Diffusion Reactor Research Articles

Diffusion and reaction in porous networks

The present paper reviews three-dimensional random network models of catalyst support structures. Multicomponent diffusion in the individual pores of the network is described by the dusty-gas approach. In contrast to previous publications, the present network model can be applied to any common reaction kinetics. This becomes inevitable in order to make three-dimensional network models applicable to practical problems in the industry. The present model is closely connected to measurable quantities which may be obtained by standard equipment. Examples of pore structure optimizations with respect to various optimization criteria are given. Investigations of depositions within the pores by percolation theoretical methods are described. Results of simulations are compared to measurements in a single-pellet diffusion reactor. The importance of surface diffusion is stressed.

Simulation and experiment of multicomponent diffusion and reaction in three-dimensional networks

A random three-dimensional pore network of interconnected pores was taken as a model of the porous structure of catalyst supports. Multicomponent Knudsen, molecular and surface diffusion were modelled according to the dusty gas model. In contrast to most previous publications, the present network model can be applied to any common reaction kinetics. This becomes quite inevitable in order to make three-dimensional network models applicable to practical problems in industry. To solve the model equations a globally convergent solver for nonlinear equations is introduced which may be transfered to many other problems in chemical engineering. The computations are compared with measurements in a single-pellet diffusion reactor. It turned out that surface diffusion may contribute significantly to the fluxes in a catalyst support.

Measurement of Gas Composition at the Center of a Porous Pellet during Adsorption and Catalytic Reaction under Dynamic Conditions

Gas composition is measured at the center of one-dimensional porous platinum/alumina pellets during carbon monoxide adsorption and catalytic oxidation. The apparatus is based on a single-pellet diffusion reactor that has been modified to allow continuous gas analysis and miniaturized in order to reduce the time constants of gas flow and mixing. Analysis of the CO adsorption response demonstrates that the centerplane volume and sample leak perturb the system only slightly and in a manner that can be accounted for during data analysis. A detailed kinetic model described previously is able to predict the qualitative features of the external concentration responses during carbon monoxide oxidation. However, the model is not able to predict major features of the responses measured at the pellet center, demostrating that the reactor is able to provide stricter tests of kinetic models than reactors in which only external compositions can be measured.

Nonuniform activity distribution in catalyst particles: Benzene hydrogenation on supported nickel in a single pellet diffusion reactor

A single pellet diffusion reactor (SPDR) of new design has been used to investigate the influence of nonuniform distributions of active ingredient on the Ni-catalyzed hydrogenation of benzene. Five different distributions, ranging from preferential shell-loading to uniform to preferential core loading, have been studied under isothermal conditions. Analysis of results has been carried out using a simple one-dimensional diffusion/reaction analysis, which the configuration of the SPDR is set to mimic. The rate of the reaction follows an Eley-Rideal model, effectively positive fractional-order under the experimental conditions, for which kinetics the nonuniform distribution problem has not been studied experimentally before. Preferential shell-loading yields the highest catalytic effectiveness but because of the complex interactions between the reaction kinetics and the activity profiles within the pellets, there is no simple relationship between diffusion lengths and effectiveness even for large values of the Thiele modulus.

Non-isothermal effects in a single pellet diffusion reactor

A theoretical analysis of an adiabatic non-isothermal single-pellet diffusion reactor shows that non-isothermal effects can alter the value of the centerplane concentration. Deviation from isothermal behavior is significant under exothermic conditions where multiplicity of solutions can lead to reactor instabilities or falsification of reaction parameters. Conversely, it is shown that the internal isothermal model is a good approximation to detect non-isothermal pellet behavior from centerplane concentration measurements.

Analysis of first-order reaction networks with a single pellet diffusion reactor

Analysis of first-order reaction networks with a single pellet diffusion reactor

Theoretical analysis of non-catalytic gas—solid in a single pellet diffusion reactor

Theoretical analysis of non-catalytic gas—solid in a single pellet diffusion reactor

Kinetics of deactivation of a reforming catalyst during methylcyclohexane dehydrogenation in a diffusion reactor

The deactivation of a commercial reforming catalyst having 0.6% Pt supported on γ-alumina was studied in a single pellet diffusion reactor using the dehydrogenation of methylcyclohexane to toluene reaction as a model of the reforming process. The reaction was studied in the temperature range 350–400 °C and methylcyclohexane and hydrogen partial pressure from 15–60 and 0–800 Torr, respectively. In this range of concentrations, the reaction is approximately first order with respect to MCH and H 2. Results of this work establish the sensitivity of the poisoning rate to hydrogen partial pressure in agreement with previous work. It is shown that only a part of the intermediate poison structures can be removed by hydrogenation under reaction conditions. A model of the poisoning process is developed which explains the data quantitatively, and agrees with previous work in important aspects.

Analytical solutions of limiting poisoning mechanisms for a zero-order main reaction

The single-pellet diffusion reactor has proven to be a particularly useful tool to diagnose the mechanism of catalyst poisoning. Previous work was done with main reactions of first, second and 1/2 order and behavior was found to be similar for reactions of different orders. However, a zero order main reaction in this reactor presents a singular behavior not observed previously. These singularities are illustrated using the limiting poisoning models referred to as pore-mouth, uniform and core poisoning: cases which can be solved analytically. From these analyses, it is clear that the single-pellet diffusion reactor can still be used to discriminate among the various poisoning mechanisms causing deactivation. However, the range of Thiele parameter over which the measured centerplane concentration gives useful additional information is reduced to values of the effective Thiele parameter less than √2.

Theoretical analysis of poisoning in a single-pellet diffusion reactor for main reactions not of first order

Theoretical analysis of poisoning in a single-pellet diffusion reactor for main reactions not of first order

On the deactivation of supported Pt catalysts during the hydrogenation of cyclopropane

Cyclopropane was hydrogenated to propane over an η-Al 2O 3 supported Pt catalyst between 50 and 75 °C, in a single-pellet diffusion reactor. The catalyst was slowly poisoned by a self-poisoning process, involving both cyclopropane and propane as poison precursors. Experiments showed a time delay in the poisoning if the catalyst was pretreated with H 2 for an extended time period, while the initial activity remained unchanged. Furthermore, part of the initial activity remained protected from poisoning, when a sintered catalyst was used, involving larger metal islands on the support.