What Is a Polyester Reactor: Polycondensation Process, Design and Applications Answering the core question: What is a polyester reactor? A polyester reactor is a heated, agitated, high-vacuum vessel in which purified terephthalic acid and monoethylene glycol are converted first to an oligomer and then to polyethylene terephthalate of commercial molecular weight. The process runs in three stages. Esterification at 240-270°C and 0.2-3 bar reacts the acid with glycol to form bis