Solubility of terephthalic acid in aqueous acetic acid from 423.15 to 513.15 K

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Abstract

Using the steady-state method, the solubilities of terephthalic acid(1) in binary acetic acid(2) + water(3) solvent mixtures in a specially contrived vessel have been measured as a function of temperature in the temperature range 423.15–513.15 K and solvent composition range from x2 = 1.000 to 0.3103 (molar fraction). The experimental solubilities are correlated with the Apelblat equation. The calculated results show good agreement with the experimental solubilities.

Introduction

The molecular structure of the terephthalic acid (TPA) under study is as follows:

Currently, polymerization-grade TPA is one of the main feedstocks for producing polyester, which plays an important role in our modern life. After oxidized by air in an oxidation reactor from 433.15 to 493.15 K, p-xylene (PX) is converted into TPA [1]. During the oxidization process, the solvent is acetic acid (HOAC). Water (H2O) and TPA are the main products. Subsequently, TPA must be purified from aqueous HOAC solution. Usually, crystallization is used to obtain products with a high purity [2]. Solubilities of TPA in aqueous HOAC solution become a crucial factor in designing separation equipment, as well as in controlling relevant operation conditions.

Although it is useful for the aforementioned reasons, very few data are available on the solubility of TPA in aqueous HOAC solution, especially for the industrial PX oxidation process. The solubility of TPA in aqueous HOAC solution from 46.8 to 100.0 M% was investigated by Chen and Ma [3] from 312.63 to 435.05 K, but it did not cover the temperature range required by the industrial PX oxidation. Marquering [4] published the solubility data of TPA in 73.0 M% aqueous HOAC solution from 293.15 to 523.15 K, but it could not be extrapolated to other solvent compositions and the data source was not given.

The lack of useful solubility data of TPA in aqueous HOAC solution is likely due to the experimental difficulties of sampling at high temperatures and the minor concentration determination of the saturated ternary solution of TPA + HOAC + H2O. In this study, an accurate and reliable procedure to measure TPA solubility in aqueous HOAC solution at high temperature has been developed and the solubility of TPA in aqueous HOAC solution has been measured from 423.15 K to 513.15 K. The experimental solubility data are correlated by the three-parameter solubility equation of Apelblat and Manzurola [6], [7], [8]. The solubility data can directly be used for the purification of TPA.

Section snippets

Materials

TPA (99.5%, HPLC) of analytical grade was supplied by Shanghai Chemical Reagent Co. HPLC grade methanol and acetonitrile from USA Tedia Company were used as the flow-phase in liquid-chromatographic analysis. De-ionized water was used throughout all the experiments. Acetic acid, dimethyl sulfoxide and isopropyl benzene of analytical grade were supplied by Hangzhou Chemical Reagent Co. All the chemicals were used as received without further purification. Mass fractions of impurities detectable by

Chromatography constant

The mass ratio of TPA and isopropyl benzene in each standard sample and the corresponding peak area ratio of the chromatography response are plotted in Fig. 3, and the typical chromatogram is shown in Fig. 4. The chromatography constant K was evaluated to be 0.0517 by linear regression, and the linearity was 0.9996.

Solubility data and correlation

The solvent composition is defined by the molar fraction of HOAC, which can be expressed as:x2=mHOACmHOAC+mH2Owhere mi is the molality of component i in the solution, and the

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