A Methodology for the a Priori Selection of Catalyst Particle Models

Abstract

Numerical simulation with catalytic reactor models demands the use of computationally efficient intraparticle models because of the need for repetitive evaluation of the effectiveness factors. Some of the most widely used models of this type are the flux and the dusty-gas models, based on the Stefan-Maxwell equation, and the effective diffusivity model. But these are known to have varying degrees of rigour and computational speed, and in the absence of any conclusive evidence about the relative efficacy of these models, one often tends to use complex models in situations when simpler models suffice. The present work addresses this problem and suggests a procedure for the a priori selection of a suitable intraparticle model in a given situation. This is based on the interactions among the various constituents making up the reaction system and the type of regime prevailing within the catalyst pellet. With the help of this methodology, it has been shown that the effective diffusivity model, in spite of its shortcomings, could be safely used even when the intraparticle regime is diffusion-controlled, if the flux interactions among the species are low. Its use should however be restricted to cases with reaction-rate-controlled regimes, if there are strong interactions.

Simulation of an industrial adiabatic multi-bed catalytic reactor for sulfur dioxide oxidation using the Maxwell–Stefan model

“…In the absence of any conclusive evidence about their relative efficacy, one often tends to use complex models in situations when simpler one suffices. Reddy and Murty [6] addressed this problem and recommended the method for the a priori selection of a suitable intraparticle model. It is based on the calculation of flux interactions among constituent species in a system, as also suggested by Krishna [5].…”

“…It is based on the calculation of flux interactions among constituent species in a system, as also suggested by Krishna [5]. The proposed methodology can be summarized considering the type of interactions between the constituent species in the reaction system, as follows [6]: (1) if these interactions are negligible, the effective diffusivity model can be safely used even when the intraparticle regime is diffusion-controlled; (2) if the estimated interactions are strong, the effective diffusivity model can be used only if the intraparticle regime is reaction-rate-controlled; (3) if there are strong interactions between the species and also if the intraparticle regime is expected to be lightly-to-highly diffusion-controlled, the Maxwell-Stefan model should be used; Finally, (4) if in addition to what is given in (3), there is a considerable increase or decrease in the number of moles due to the reaction, the dusty-gas model is recommended. Thus, the diffusion fluxes (J) for the M -1 independent components in a system can be explicitly expressed in the terms of concentration gradients of all the components, as follows:…”

“…Nevertheless, the basic problem is how to identify these a priori. In the present work, it was done using the procedure of Reddy and Murty [6]. Packed-bed oxidation reactors modeling: Young and Finlayson [7] and Rosendall and Finlayson [8] proposed a model including radial and axial dispersion, heterogeneity, and density variations.…”

“…However, a low amount of sulfur dioxide remains without conversion and is returned to the reactor through the fourth bed of catalyst and then the resulting gases, mainly sulfur trioxide, flow to the next stage of the process. In the view of the state of the art of previously proposed models, the following advances of this work can be pointed out: (i) the suitable model for diffusion and reaction in the catalyst pellet (vanadium (V) oxide supported on silica) was justified basing on the methodology proposed by Reddy and Murty [6] and confirmed by a comparison of the results obtained using the dusty-gas, the Maxwell-Stefan, and the effective diffusivity (Fick) models; (ii) the catalyst effectiveness factor was calculated and evaluated for each catalytic bed; (iii) model validation was done comparing the simulation results with plant data.…”