Abstract
The tremendous progress in the computing power of modern computers has in the last 20 years favored the use of numerical methods for solving complex problems in the field of chemical kinetics and of reactor simulations considering also the effect of mass and heat transfer. Many classical textbooks dealing with the topic have, therefore, become quite obsolete. The present work is a review of the role that heat and mass transfer have in the kinetic studies of gas–solid catalytic reactions. The scope was to collect in a relatively short document the necessary knowledge for a correct simulation of gas–solid catalytic reactors. The first part of the review deals with the most reliable approach to the description of the heat and mass transfer outside and inside a single catalytic particle. Some different examples of calculations allow for an easier understanding of the described methods. The second part of the review is related to the heat and mass transfer in packed bed reactors, considering the macroscopic gradients that derive from the solution of mass and energy balances on the whole reactor. Moreover, in this second part, some examples of calculations, applied to chemical reactions of industrial interest, are reported for a better understanding of the systems studied.
Highlights
When a reaction occurs inside a catalytic particle, the reagents are consumed, giving rise to products and, in the meantime, heat is released or absorbed according to whether the enthalpy of the reaction is positive or negative
If the particles are put inside a tubular reactor, macroscopic gradients arise as a consequence of the average rate of reaction in any single catalytic particle and the regime of mass and heat flow developed in the specific reactor
Each particle inside a reactor has its own history, and microscopic gradients are developed in conditions at the particle surface that are generally different from the internal particle conditions
Summary
When a reaction occurs inside a catalytic particle, the reagents are consumed, giving rise to products and, in the meantime, heat is released or absorbed according to whether the enthalpy of the reaction is positive or negative. Gas–solid catalytic processes inletare usually carried out in very large capacity equipment represented by packed bed reactors with productivity of thousands of tons per year Such reactors are arranged in a complex scheme containing all the auxiliary equipment air inlet necessary for feeding, cooling, heating, or pressurizing operations. The problem must to be solved simultaneously both at a microscopic local level, with the obtainment of the reagents and product concentration particle profiles, as well as of the effectiveness factor for all the occurring reactions, and at a macroscopic level, reproducing all the long-range concentration and temperature profiles This specific situation requires an evaluation of the catalyst effectiveness factor in each position in the catalytic bed, considering the conditions we have at any instance in that point. We consider, first of all, the mass and heat transfer occurring in a single catalytic particle, and we will treat the macroscopic gradients related to the whole fixed-bed reactor
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