Calculation of catalytic reactors — Preliminary approach

Processes involving heterogeneous catalytic reactions are widespread. The multiphase nature of these reactors often makes them difficult to size. Reactor design must take into account a number of constraints linked to catalyst use: maintaining catalyst activity and selectivity, stability and regeneration.

Generally speaking, sizing a catalytic reactor requires modelling on two scales: firstly, at the level of a catalyst grain, taking into account the internal diffusional limitations of matter and heat, and secondly, at the level of the reactor, focusing on the external matter and heat transfer processes between the reaction fluid and the catalyst grains, as well as the heat exchange technology associated with the reactor.

The aim of this dossier is to provide the reader with a methodological approach to the pre-sizing of a catalytic reactor.

To achieve this, it is first necessary to gather the preliminary information needed to determine the catalyst's operating mode.

There are several points to consider:

• the role of internal diffusion on catalyst operation ;

• its stability over time through deactivation ;

• problems related to product distribution (selectivity) ;

• the thermal characteristics of the reaction.

Then, depending on the answers to the previous questions, we'll select the catalytic reactor that seems best suited to the reaction.

A comprehensive treatment of all catalytic reactors would be far too lengthy. Only the fixed-bed catalytic reactor, the one most widely used industrially, will be the subject of an in-depth study. We will confine ourselves to describing the approach that has gradually led to the definition of what this reactor should be.