Simulation of cyclics and degradation product formation in polyethylene terephthalate reactors

Abstract

The second stage of a batch polyethylene terephthalate (PET) reactor with a kinetic scheme consisting of side reactions like thermal degradation, redistribution, cyclization and reaction with monofunctional compounds (cetyl alcohol), in addition to the major polycondensation step, has been simulated. The cyclization equilibrium has been shown by Semlyen for various cyclic compounds to be chain-length-dependent. Mass balance for various side products and the polymer, PET, have been carefully worked out and the resulting set of differential equations have been solved for the molecular weight distribution (MWD) of the polymer. It is found that the cyclization reaction has considerable effect on the polydispersity index of the polymer even though the total quantity of cyclic compound is no more than 5%. The effect of variation of temperature and initial concentration of monofunctional compound was found to have little influence on the polydispersity index of the polymer. Since the removal of the condensation product does not become diffusion-controlled till the average chain length of the polymer reaches the value of 30, the reaction mass can be assumed to attain a uniform ethylene glycol concentration for a given applied vacuum and can be determined by the vapour-liquid equilibrium relation. The fractional level of ethylene glycol concentration, [G] (≡[EG]/[P₁]₀), has been taken as a parameter and the MWD results were found to vary the redistribution rate constant with sensitivity increasing with decreasing [G]. It was shown that the importance of the redistribution reaction is increased when the cyclization reaction also occurs.