Homogeneous Reactors for Radical Polymerization. Design and Operation

Radical polymerization is used to produce a wide variety of polymers (commodity, technical or specialty) on both laboratory and industrial scales. In a polymer manufacturing process, the role of the polymerization reactor is essential, since it is responsible for the structural characteristics of the macromolecules produced, and it is very difficult to modify them at a later stage. Polymers are thus called "process products" because of the close link between their application properties and the conditions under which they are produced. Engineers must therefore approach the usual questions of chemical reaction engineering (design, transposition, extrapolation, optimization of a polymerization reactor) by integrating all the consequences of this specificity.

The aim of this article is to review the state of the art in both free-radical polymerization and reactor design, focusing on processes involving a single-phase (homogeneous) reaction medium. It is therefore a general state of the art. The information provided should enable the reader to develop a relevant quantitative approach adapted to particular cases of reactors and monomers. The performance criteria of a polymerization reactor are defined in terms of both quantity and quality of the polymer produced, based on a description of the structural characteristics of the macromolecules. The kinetic scheme of radical polymerization in a single-phase medium is then described in detail, together with expressions for the rates of the elementary processes. Common approximations are presented and their validity discussed. The need for precise knowledge of the elementary processes taking place in the reactor is emphasized. Consideration of the choice of reactor on the polymer produced is presented using the method of moments, classical ideal reactor models, material balances (reactants and macromolecules) and the enthalpy balance on the reactor. Extension to non-ideal reactors is discussed. Finally, the dimensional analysis of a polymerization reactor is used to identify important parameters for extrapolation and transposition of results.