© Copyright American Institute of Chemical Engineers. All rights reserved. Polycondensation is an important polymerization method for the synthesis of polyesters, polyamides, etc. A commercially important polycondensation product is polyethylene terephthalate (PET), which is mainly used in synthetic fibers, water bottles and molding materials. It is manufactured by a four-stage process which consists of transesterification, prepolymerization, melt polycondensation and solid state polycondensation. The main polycondensation reaction is reversible and thus equilibrium limited . In order to achieve a high molecular weight, byproduct (i.e., ethylene glycol) needs to be removed to favor the forward reaction by applying vacuum to the reactor in the finishing stage. Besides, the product in this stage determines the quality of final product, e.g. purity, melting point, average molecular weight, polydispersity index, etc. Therefore, control and monitoring of the finishing stage is of great importance.