Kinetic and catalytic aspects in melt transesterification of dimethyl terephthalate with ethylene glycol

The kinetics of melt transesterification of dimethyl terephthalate with ethylene glycol in the presence of zinc acetate as catalyst has been studied in semibatch conditions. We observed that this reaction occurs with the formation of many oligomers characterized from the terminal groups of the chains that can be hydroxyl–hydroxyl, methyl–hydroxyl or methyl–methyl. Experimental runs have been performed at different temperatures, initial reagents ratios, and catalyst concentrations, following the amount of methanol released during the time as well as the concentration of any kind of oligomer. All the oligomers have been identified and determined by HPLC analysis. A classic Kinetic model based on a complex reaction scheme containing four or five reaction sequences has been developed. The scheme with four sequences foresees 24 oligomeric species involved in 58 different reactions, while the scheme with five sequences has 48 oligomeric species involved in 228 reactions. Despite the large number of oligomers and occurring reactions, only two kinetic parameters and two equilibrium constants are necessary to simulate the kinetic behaviour of all the oligomers. A kinetic constant is related to the reaction of a methyl group with a hydroxyl of ethylene glycol, while the other corresponds to the reaction of a methyl group with a hydroxyl in a chain. Both kinetic constants show an activation energy of about 15 kcal/mol. We observed a nonlinear correlation between activity and catalyst concentration and interpreted this fact by assuming two different catalytic activity levels for a dissociated and an undissociated zinc ionic couple, respectively. © 1994 John Wiley & Sons, Inc.     

Kinetic and catalytic aspects in melt transesterification of dimethyl terephthalate with ethylene glycol in the presence of different catalytic systems

The kinetics of transesterification of dimethyl terephthalate with ethylene glycol performed in the presence of catalysts such as: Pb, Zn, Mg, Co, and Mn acetates and a mixture of Mg, Mn, and Zn acetates has been studied in semibatch conditions. The catalytic behavior of Sb₂O₃ has been proven, too. The performance of the different catalytic systems has been investigated following both the amount of methanol released during time and the evolution with time of the concentration of any kind of oligomer formed as consequence of the reaction. The oligomers obtained were separated, identified, and quantitatively determined by HPLC analysis. In this way, information have been achieved on both the activities and selectivities of the different catalysts. The experimental data have been interpreted through a classic kinetic model based on a complex reaction scheme. Despite the complexity of the model, only two kinetic parameters and two equilibrium constants are necessary to simulate the kinetic behavior of all the oligomers. A kinetic constant (K₁) is related to the reaction of a methyl group with a hydroxyl of ethylene glycol, while the other (K₂) corresponds to the reaction of a methyl group with a terminal hydroxyl of a growing chain. The Mn, Pb, and Zn acetates have shown comparable high catalytic activities; however, the Mn selectivity to give oligomers with hydroxyl-hydroxyl terminal groups is better and similar to that shown by Co and Mg acetate, at a lower activity. Sb₂O₃ has a very low activity in transes-terification but this activity could be important to eliminate the residual terminal methyl group during the polycondensation step. The catalytic activity of the mixture of Mg, Mn, and Zn acetate was greater than that shown by each component the mixture, while its selectivity was comparable with that of Mn and Mg acetate. © 1996 John Wiley & Sons, Inc.     

Transesterification of dimethyl terephthalate with ethylene glycol

Conclusion For the first time, data are given on the quantitative composition of the transesterification products of dimethyl terephthalate with ethylene glycol at various molar ratios of the starting materials.During the transesterification process, oligomerization reactions take place rather intensively. Thereupon the linear oligomers are formed both in the transesterification process of carbomethoxy groups and also as a result of the interaction of carbohydroxyethoxy groups. The ethylene glycol formed in the latter case ensures further transesterification of the carbomethoxy groups.It is possible in principle to reduce the molar ratio of ethylene glycol to dimethyl terephthalate in the transesterification stage. Thereupon a polyethylene terephthalate can be obtained which has properties similar to the serially produced product.Translated from Khimicheskie Volokna, No. 5, pp. 18–20, September–October, 1983.     

Kinetics of transesterification of dimethylene terephthalate with ethylene glycol

There is a disagreement about the order of reaction of transesterification of DMT with EG. The reactivity of the two ester groups is not compared yet. The transesterification of DMT and MHET is studied to establish the kinetics of the process. Attempts are made to treat the kinetic data on a mathematical model for third-order reaction.     

Studies on the formation of poly(ethylene terephthalate): 2. Rate of transesterification of dimethyl terephthalate with ethylene glycol

The transesterification of dimethyl terephthalate (DMT) with ethylene glycol (EG) was kinetically investigated in the presence of various catalysts at 197°C. The reaction was followed by the measurement of the quantity of methanol which distilled from the reaction vessel. This distillation made corrections of reactant and catalyst concentrations necessary. The transesterification was assumed to obey first-order kinetics with respect to DMT and EG, and a rate equation was derived. The reaction was found to be first order in catalyst concentration as well and when this finding was incorporated in the rate equation, excellent agreement between the observed and calculated values was recognized throughout the reaction. The first-order dependence on the catalyst concentration is valid below a critical concentration which was found to be dependent on the catalyst type. Above this concentration a lower reaction order was observed.     

Studies on the formation of poly(ethylene terephthalate): 3. Catalytic activity of metal compounds in transesterification of dimethyl terephthalate with ethylene glycol

The rate constant of transesterification of dimethyl terephthalate (DMT) with ethylene glycol (EG) in the presence of various metal compounds as catalysts at 197°C, calculated from the quantity of formed methanol, was used to evaluate each metal compound in its catalytic activity. First, in the case of highly basic metal salts the rate constants were found to be extremely large in the initial stage of the reaction. These values depended on the basicity of the metal salts, but decreased rapidly with the progress of the reaction, and reached values which were not appreciably dependent on the basicity. Secondly, in the case of metal salts of lower basicity such a phenomenon was not observed, and single rate constants were recognized throughout the reaction period. These rate constants were logarithmically correlated by a straight line (mountain-shaped) in a plot vs. stability constants (logβ₁) of dibenzoyl methane (DBM) complexes of the corresponding metal species. Also in the case of highly basic metal salts, the rate constants in the latter stage followed the same straight line. Consequently, the stability constant of DBM complex of each metal species was found very useful as a forecast of catalytic activity of the metal compound. The compound of metal species with values of logβ₁ ranging from 9 to 11 was most active as the catalyst.     

Modelling of poly(ethylene terephthalate) reactors. III. A semibatch prepolymerization process

A mathematical modelling of a semibatch prepolymerization process has been undertaken. Prepolymerization conducted at constant temperature and pressure and with a predetermined variation in temperature and pressure has been studied. Influence of DMT and TPA addition during prepolymerization has also been examined. The focus has been on investigation of the influence of processing and operational variables on productivity and side product formation, which controls product quality. The results of this investigation (which are applicable only up to DP ? to 30) are borne out by the limited experimental data available in the literature. Important pragmatic implications of the results of the work in terms of design and operation of prepolymerization reactors have been emphasized.     

The forced gas sweeping process for semibatch melt polycondensation of poly(ethylene terephthalate)

The experimental and modeling studies are presented on the melt polycondensation of poly(ethylene terephthalate) by a gas sweeping process. In this process, low molecular weight prepolymer is polymerized to a higher molecular weight polymer in a molten state at ambient pressure as ethylene glycol is removed by nitrogen gas bubbles injected directly to the polymer melt through a metal tube. In the temperature range of 260–280°C, the rate of polymerization by the gas sweeping process is quite comparable to that of conventional high vacuum process. The effects of nitrogen gas flow rate and reaction temperature on polymerization rate and polymer molecular weight were investigated. Polymer molecular weight increases with an increase in gas flow rate up to certain limits. A dynamic mass transfer–reaction model has been developed, and the agreement between experimental data and model simulations was quite satisfactory. The effect of ethylene glycol bubble nucleation on the polymerization has also been investigated. It was observed that the presence of nucleated ethylene glycol bubbles induced by the bulk motion of polymer melt has negligible impact on the polymerization rate and polymer molecular weight. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1388–1400, 2001     

Transesterification of dimethyl terephthalate with ethylene glycol. A kinetic model of the process

Conclusions The possibility of calculating and obtaining the values of the rate constants of reactions (4)–(6) in the process of transesterifying DMT with ethylene glycol over various catalytic systems at a 1.8 molar ratio of the starting components, EG:DMT, has been shown.The results presented permit one to solve the problem of optimizing the process of transesterifying DMT with ethylene glycol with the objective of reducing the molar ratio of the starting materials and cutting down energy outlays and starting material norms.Translated from Khimicheskie Volokna, No. 6, pp. 10–13, November–December, 1983.     

Modeling of poly(ethylene terephthalate) reactors. I. A semibatch ester interchange reactor

A comprehensive model for a semibatch ester interchange reactor has been developed with a view to investigate the effect of various process and operational variables on the DMT conversion rate as well as the by-product formation. The influence of important variables such as EG-to-DMT ratio, catalyst concentration, and operational variables such as temperature and pressure has been considered. Definite conclusions concerning the choice of the desirable range of process and operational variables to maximize productivity and minimize by-product formation have been reached.     

Modeling of poly(ethylene terephthalate) reactors. II. A continuous transesterification process

A comprehensive mathematical model for a continuous transesterification process has been built so as to enable prediction of the influence of different process and operational variables on productivity and by-product formation. The influence of temperatures and temperature profiles, of residence time and residence time distribution, and also of the number of reactors in series has been investigated. The modeling has been done as close to the industrial practice as possible. Important pragmatic implications from the point of view of operation of continuous transesterification are highlighted.     

Tendency modeling of semibatch reactors for optimization and control

Significant economical benefits can be derived from the optimal operation of semibatch reactors even when the detailed understanding of the chemical kinetics is not at hand. To realize these economical benefits a general adaptive optimal control strategy is needed that properly accounts for the lack of detailed knowledge and the great variety of semibatch processes and at the same time takes advantage of the plant data collected during the process operation.In the present paper a novel approach is introduced for the systematic development of approximate reaction networks that are the basis of the reactor model. Other simple kinetic models can be postulated on intuitive grounds. All such networks lump the known species as reactants, product and byproduct and along with a set of mass and energy balances for semibatch reactors constitute the tendency model.This modeling approach is applied to a complex kinetic network containing 20 parameters which is accurately approximated by two simple tendency models that have either 4 or 6 parameters related to the kinetic constants. Estimation of these parameters shows the accuracy of the derived models to be very satisfactory.     

Optimization of the transesterification stage of polyethylene terephthalate reactors

The tranesterification step of the polyethylene terephthalate (PET) formation consists of several side reactions in addition to the main ester interchange, transesterification, and polycondensation reactions. The side reactions considered in this work are acid end group, acetaldehyde, diethylene glycol, water, and vinyl end group formations. The objective function of the batch esterinterchange reactor is assumed to consist of maximizing the conversion and simultaneously minimizing the formation of side products. The control vector iteration procedure has been used to optimize the esterinterchange reactor and the temperature-time profile that gives the best performance has been found. It is found that the reactor should be operated at a high temperature initially to obtain high conversion of dimethyl terephthalate (DMT) first, but then it should be lowered to reduce the formation of side products.     

Activation of catalysts for the transesterification of dimethylterephthalate with ethylene glycol

Conclusions A possibility has been demonstrated of shortening the duration of the transesterification reaction between dimethyl terephthalate and ethylene glycol by preliminary treatment of the catalysts or by introducing investigated lower alcohols or acetone into the reaction mixture.Translated from Khimicheskie Volokna, No. 3, pp. 49–50, May–June, 1990.     

Optimization of the polycondensation step of polyethylene terephthalate formation in semibatch reactors

The polycondensation stage of polyethylene terephthalate (PET) formation is assumed to include side reactions leading to the formation of diethylene glycol, vinyl end groups, and acid end groups, in addition to the usual polymerization of bis (2-hydroxyethyl) terephthalate (BHET) in semi-batch reactors. A, flexible objective function has been proposed with temperature and pressure as control variables. Computations from the first variation technique show that the pressure should be reduced to the lowest limit under all possible conditions. Consequently, optimal temperature profiles in batch reactors are obtained for various lower limits of reactor pressures using the combination of first and second variation techniques. For the first variation technique, the vector iteration method of computation was used, and the near optimal profile so obtained was used as the initial guess for the second variation technique. The result of optimization shows that the lower limit of pressure and weighting parameters appearing in the objective function have profound effects on the optimum profiles. For higher pressures, it is shown that a high temperature must be used initially; but must be lowered later to minimize the formation of side products. However, for lower pressures, the temperature must be increased from a low value initially; but for large polymerization times, the temperature must be further reduced to minimize the formation of side products. It is thus seen that the optimum temperature profile for low pressures exhibits a broad maximum.