Chemical reactors - Principles

Process engineering can be defined as the engineering science concerned with the optimal implementation of processes for the physical-chemical and/or biological transformation of raw materials into functional products. One of the most important branches of process engineering is chemical reaction engineering, which is concerned with methods for the rational implementation of chemical transformations, and in particular with the devices in which reactions are carried out: chemical reactors. Even if the reactor represents only a modest part of the investment in an industrial process, its operation largely determines the upstream installations (preparation of reagent charges, choice of temperature and pressure conditions) and the downstream installations (product separation devices in particular). Improving reactor efficiency by just a few units can therefore result in a significant reduction in investment costs and material and energy consumption. In this sense, the reactor is truly the heart of the process, requiring the full attention of the engineer.

Figure 1 shows, in the form of a system diagram, the main factors governing the operation of a chemical reactor. It is sufficiently self-explanatory that a lengthy commentary is unnecessary. The aim of chemical reaction engineering is to study the interactions between these factors.

The engineer may face a number of problems:

• design a reactor for industrial production to given specifications, based on a chemical transformation recognized as possible in the laboratory;

• run a discontinuous operation optimally ;

• transpose a discontinuous operation into a continuous operation ;

• define extrapolation criteria to enable a change of scale;

• optimize an existing reactor, diagnose its operation and automate it;

• give priority to environmental protection and operational safety by controlling reaction conditions.

This list is by no means exhaustive. In each case, the questions are: What data do we need? What laboratory experiments and industrial trials are needed? Conversely, the researcher interested in gaining fundamental knowledge of a reaction must consider the choice of reactor and experimental conditions that will provide the desired information: for example, how can a reaction rate be measured without being hampered by concomitant heat and mass transfer phenomena?

These are just some of the questions that chemical reaction engineering aims...