For polyester synthesis—specifically via polycondensation—the reactor design must handle a significant transition in physical properties, as the reaction mixture moves from a low-viscosity monomer melt to a high-viscosity polymer. Unlike the fixed-bed reactors used for gas-phase catalysis, polyester synthesis requires a Stirred Tank Reactor (STR) designed for intensive mass transfer and heat management.

Because the viscosity of the melt increases exponentially as the degree of polymerization (DP) rises, the mechanical stirring system is the most critical component. (1) Impeller Types: In early phase, standard turbines or anchors are sufficient for mixing low-viscosity monomers. Later on, the High-viscosity "Helical Ribbon" or "Anchor" impellers are required to ensure axial flow and prevent stagnant "dead zones" near the reactor walls. (2) Mechanical Seal: A double-mechanical seal with a pressurized barrier fluid is standard. This prevents air ingress (which causes oxidative yellowing of the polyester) and ensures the system can maintain the deep vacuum required in the final stages of the reaction. (3) Torque Monitoring: The motor is often equipped with a torque sensor. Since viscosity is proportional to torque, this serves as a real-time proxy for the molecular weight of the polymer.

Polyesterification is typically an endothermic process initially, requiring significant heat, but temperature control is vital to prevent thermal degradation. (1) Heating Method: A jacketed vessel using high-temperature thermal oil (e.g., Dowtherm) is preferred over direct electric heating for better temperature uniformity. (2) Reflux & Condensing: The reactor must be equipped with a distillation column. In the first stage (esterification), water or methanol (the by-products) must be removed to drive the equilibrium forward:

References:

M. Lomelí-Rodríguez, et al., Optimum Batch-Reactor Operation for the Synthesis of Biomass-Derived Renewable Polyesters,  Ind. Eng. Chem. Res. 2017, 56, 2, 549–559

A. Gaikwad, et al., Kinetics and Dynamics of Enzymatic Degradation of Synthetic Polyester (PET) in Batch and Continuous Reactors, Ind. Eng. Chem. Res. 2026, 65, 11, 5913–5926.