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Important considerations in polymerization reactor design include heat removal, kinetics, and process dynamics, as well as polymer properties and product quality.
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Polymer reaction engineering is an interesting field, with many rich and rewarding problems to be solved. To be successful in this field, a chemical engineer needs to understand a few principles beyond the basics of reaction engineering. Building on concepts presented in an earlier CEP article on plastics (I), this article explores some of the most important of those principles related to selecting, sizing, and troubleshooting polymerization reactors.
Polymers tend to be viscous, which often imposes lower limits on their concentrations in diluents and on reaction temperatures. At very low temperatures, or in the absence of a solvent, the viscosity of a reacting mixture might be so high that it severely limits heat transfer. Worse yet, the polymer might crystallize and coat the catalyst- and monomerinjection systems.
One common approach is to carry out the reaction in a low-viscosity continuous phase that includes monomer and other important components, with the polymerization occur- ring in a discrete phase made up of viscous, and sometimes even solid, polymer particles (Figure 1). Such heterogeneous polymerization systems are often used in free-radical polymerization.
An interesting example of a heterogeneous system is olefin polymerization in fluidized-bed reactors. A supported catalyst in the form of very small particles, with diameters of only a few microns, is injected into the reactor. Monomer in the continuous gas phase forms chains at the catalyst active sites. As the cycle of chain birth, growth, and death repeats, catalyst particles fragment and create more surface area for the monomer to attach to; the polymer links the fragmented catalyst pieces together, forming polymer particles whose final diameters are easily ten times those of the original particles (Figure 2). The viscosity problem is avoided, but the high reaction rates can cause a different problem: Particles that are unable to transfer energy to the gas phase at the same rate they are generating it are at risk of overheating, melting, and sticking to each other or to solid surfaces (2).
Viscosity, like melt strength, is an end-use property - the kind of property that producers and their customers use to measure product...