Heat Transfer in Industrial Polymerizations Free radical polymerizations are highly exothermic reactions, with adiabatic temperature rises for bulk monomers typically ~ 200 300 C (adiabatic temperature rise is the temperature increase that would occur on complete polymerization if no heat were removed from the system). If there is a process disturbance leading to a thermal runaway condition, the heat generation rate can exceed the heat removal In industrial polymerizations, the cooling system’s heat r emoval capacity often becomes a limiting factor. When a reaction is scaled up, the heat generation rate increases in direct proportion to the reaction volume, while the heat removal capacity increases in proportion to the surface area available for heat transfer. The ratio of surface area to reactor volume decreases as the reactor size increases, and therefore it can be expected that at some scale the cooling system’s safe operating capacity will be exceeded by the heat generation rate fr om the desired polymerization reaction. The use of a nonreactive component such as the aqueous phase in suspension or emulsion polymerization, or a solvent in solution polymerization, provides a‘‘heat sink’’ that absorbs some of the heat of reaction. This advantageous feature is offset by the reduction in reactor productivity caused by the relatively low overall monomer loading. Failure to adequately control temperature can have deleterious effects on the product quality and pose serious safety issues. As previously discussed, free-radical polymerizations are highly temperature-sensiti ve. Mixing Effects in Polymerization Reactors The importance of mixing, the c ontacting of fluid elements from different parts of the reaction vessel with each other, has been the subject of several studies. Mixing can directly affect the kinetics, molecular weight, and composition in radical polymerizations by homogenizing local concentration gradients, but can also indirectly play an important role through its role in reducing thermal gradients in a reactor. In a small laboratory reactor, good mixing is usually readily achieved and therefore the polymer properties and reaction rate are unlikely to be influenced by mixing effects. However, similar to thermal effects, mixing effects become more apparent as reactor size increases because effectively mixing the entire reaction mixture becomes more difficult. Within the chemical process industries, polymerization reactions offer a particularly challenging problem because of the large increase in viscosity accompanying the conversion from monomer ( ~1cP for liquids) to polymer (>105cP). Some processes are designed to not require mixing. For example, PMMA can be polymerizedin large sheets. By having large surface areas available for heat transfer, adequatetemperature control is achieved without the need to provide mixing during polymerization.
