Solution of terephthalic acid in the reaction system terephthalic acid-ethylene glycol-esterification products

Abstract

UDC 678.674'524'420.03 An investigation of the solution of terephthalic acid (TPA) in ethylene glycol (EG) and their reaction products is of considerable interest because the solution is a necessary precondition for the reaction of direct esterification. It has been established [1] that at high temperatures (->250°C) the esterification proceeds in the diffusion region, i.e. the process rate is limited by the solution process of the TPA. The published information about the influence of the reaction medium on the solubility and esterification of TPA is highly contradictory. Some workers researching the solubility of TPA in organic solvents do not relate the solubility index to the concrete properties of the solvent [2] or relate it to its proton-acceptanc e capacity [ 3]. Krumpole et al. [4] cite data about the solubility of TPA in EG and draw the conclusion that the solubility of TPA does not depend on the presence of reaction products. Other workers [ 5; 6] offer evidence that the presence of diglycol terephthalate (DGT) in the system TPA-EG stimulates the reaction rate but fail to disclose the nature of this effect of the medium. Kodaira et al. [7] also report the favorable influence of DGT on the esterification rate and ascribe it to the entropy effect which increases with the DGT concentration. Other workers [ 8] state, however, that the solubility of TPA is higher in EG than in a mixture of mono- and diesters and show that DGT has an adverse effect on the reaction rate because its basicity characteristics enable it to combine with the TPA over hydrogen bonds resulting in the formation of inactive associated products. In view of these facts it was deemed relevant to investigate the solution of TPA in EG and in their reaction products and the effect of the chemical reaction on the solution process, and to determine the temperature boundary between the purely physical solution process and the onset of the chemical reaction. The experiments were carried out with TPA with a specific surface Ssp of 0.5, 1.5, and 2.9 m2/g (Ssp was determined by the BlOT adsorption method). All experiments were carried out with an EG/TPA ratio of 10 by weight. The COOH groups were determined in specimens from which the solid TPA had first been filtered out. The content of the products of the chemical reaction in the specimens was determined by polarography and thin-film chromatography [9]. To begin with, the influence of the speed of stirring on the rate of TPA solution in EG was investigated. It was found that the solution rate increases with an increase in the stirrer speed to 700 rpm after which it remains unchanged. The influence of the other diffusion factor, viz. Ss_, on the solution rate was therefore investigated at a stirrer speed of 700 rpm. The results are set out in Table 1. According to the theory of diffusion processes, the rate of the solution process described by Shchukarev's equation [ 10] always increases with the specific surface of the substance being dissolved: