Kinetic Analysis and Optimization for the Catalytic Esterification Step of PPT Polymerization Saptarshi Majumdar, 1 Kishalay Mitra,* 2 Gautam Sardar 2 1 Tata Research Development and Design Centre, 54B Hadapsar Industrial Estate, Pune 411013, India 2 Engineering and Industrial Services, Tata Consultancy Services, 54B Hadapsar Industrial Estate, Pune 411 013, India Fax: þ91 20 4042399; E-mail: kishalay.mitra@tcs.com Received: August 18, 2004; Revised: October 28, 2004; Accepted: November 8, 2004; DOI: 10.1002/mats.200400055 Keywords: kinetics; optimal control; optimization; polymerization; PPT; TBOT Introduction Poly(propylene terephthalate) (PPT) is coming up as a new semi-crystalline thermoplastic polyester. Earlier, 1,3-pro- panediol (PDO), one of the ingredients in PPT manufactur- ing, was not available in good amount. Recently, Du Pont and Shell have come up with some feasible processes to manufacture PDO industrially. This leads to a stiff growth in the PPT demand as it combines the advantageous properties of polyamides and polyesters. Analysis of PPT polymer- ization process through the route of the combination of terephthalic acid (TPA) and PDO with a suitable catalyst (Ti-based) has been considered here (in batch as well as semi-batch mode). The actual process consists of two steps: esterification and polycondensation. Acid-end groups of TPA can catalyze the reaction with an assumption of perfect mixing and constant melt density. This process should be isothermal in nature and normally generation of by-products increases with temperature. For poly(ethylene terephthalate) (PET), tetrabutoxytitanium (TBOT) reacts with trace quantities of aldehydic impurities produced in polymerization process to generate a yellow discoloration of the polymer. For PPT and poly(butylene terephthalate) (PBT), where color of polymer is not that critical, organic titanates are the catalysts of choice because of their greater reactivity than antimony or tin. It is worth- while to mention that phosphoric acid can control the color formation but normally slows down the rate of catalysis. There is not much modeling work available on PPT in the open literature except the recent work by Karayannidis et al. [1] Very similar research work has been done by Kang et al. [2] for PET polymerization with functional group analysis. With the modeling exercise, Karayannidis et al. [1] have done a lot of experiments with different catalysts and Summary: A well-validated kinetic scheme has been studied for PPT, poly(propylene terephthalate) polymerization pro- cess in batch and semi-batch mode with tetrabutoxytitanium (TBOT), a proven catalyst. Optimization study and analysis for PPT are rare, as the industrial relevance of PPT just became vibrant due to the commercial availability of one of its monomers in industrial scale in the recent past. Correct- ness of the analysis is checked by a new approach and parameters for the model are estimated from available ex- perimental data. Solubility of terephthalic acid (TPA) is less in reaction medium and this effect is also considered along with the reaction scheme. Several simulations have been performed to see various process dynamics and this ulti- mately helps in formulating optimization problems. Using recently developed and well tested real-coded non-domi- nated sorting genetic algorithm-II, a state-of-the art evolu- tionary optimization algorithm, a couple of three objective optimization problems have been solved and corresponding Pareto sets are presented. Results show remarkably promis- ing aspects of productivity enhancement with an improve- ment in product quality. Sensitivity analysis for relatively uncertain solubility parameter is also performed to estimate its effect over the proposed optimal solutions. Multiobjective Pareto front for 3 objectives: degree of poly- merization, time and (bTPA þ bPG). Macromol. Theory Simul. 2005, 14, 49–59 DOI: 10.1002/mats.200400055 ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper 49