Polybutylene Terephthalate Production

The process under analysis comprises two major sections: (1) Polymerization and (2) Recovery System.

Polymerization. Dimethyl terephthalate and butanediol (BDO) are mixed in the Paste System and, together with the catalyst, are fed into the Tower Reactor, in the Esterification section. The product from esterification and recovered BDO are fed into another section of the Tower Reactor, where post-esterification and pre-polymerization processes occur, through cascade reaction cups. The resulting pre-polymer is led to the Polycondensation reactor, where the catalyst and other additives are fed. Monomers, oligomers, and short-chain polymers are condensed in order to form long-chain Polybutylene Terephthalate (PBT), releasing BDO, recovered in a distillation column (BDO Recovery). All the light unreacted reagents and side products of esterification and polycondensation – mainly methanol, tetrahydrofuran (THF), and water – are sent to THF Recovery. After polymerization, PBT is sent to Finishing steps, in which it is pelletized, homogenized, and packed.

Recovery System. Products and unreacted reagents from polymerization are directed to a light-ends column present in the THF Recovery system. The light gas from the top of the column, containing THF, water, and methanol, passes through a second column operating at a higher pressure. Thanks to the change in pressure, the azeotrope properties are shifted, and high-purity THF is obtained as a bottom by-product. The new azeotrope is recycled to the first column of THF Recovery. At the bottom of this first column, methanol, BDO, and water are stripped off and directed to the BDO Recovery column, where BDO is recovered at the bottom. Finally, methanol is separated from water in the Methanol Purification column as an overhead product. Water and impurities leave the system as wastewater.