Determination And Study On The Corresponding Phase Equilibria In Terephthalic Acid Manufacture

In order to give sufficient data support for the design of terephthalic acid (TA) manufacture, corresponding phase equilibria, including solid-liquid equilibrium (SLE) and vapor-liquid equilibrium (VLE), have been determined and studied.Improvements have been made on our former apparatus, which was used for the measurement of solid solubility, for the purpose of getting more precise experimental data in the range of lower solubility. Horizontal stir was changed to magnetic whirly one, which shortened the dissolving time remarkably. Moreover, judgement criterion for phase equilibrium was moved to the time when the transmitted laser light intensity during stirring is equal to that without stirring, which is more reasonable than the former criterion that named equilibrium only when the transmitted laser light intensity performs the highest one. With TA, p-toluic acid (PT-acid), 4-carboxyl-benzaldehyde (4-CBA) as solutes, water, acetic acid, aqueous acetic acid of several concentration, N,N-dimethylformamide, N,N-dimethylacemide, dimethyl sulphoxide, N-methl-2-ketopyrolidine as solvents respectively, SLE of 9 binary systems and 5 ternary systems were determined over the temperature range from 288.25 K to 444.85K. General phase equilibrium theories, including Wilson equation, Wilson-T equation (parameters in Wilson equation were associated to temperature), λh model, UNIFAC method and modified UNIFAC method, have been chosen to study SLE of all the 9 binary systems and the corresponding model parameters were given. New groups, such as TA, PT-acid and 4-CBA were defined for UNIFAC method and modified UNIFAC method. The average relative errors of modified UNIFAC method, which treated the solubility of TA in acetic acid separately in two regions split according to temperature, and λh model were below 2.5%, which means they were more suitable for SLE of the systems abovementioned than the other activity coefficient equations. It was also found that better correlation of the experimental data was obtained if all the interaction parameters were treated as function of temperature. Unsatisfactory prediction of the ternary systems with general phase equilibriumtheories showed that those theories are insufficient to describe the system containing acetic acid that have strong interaction between molecules. Thus, chemical theory together with association was introduced. This method performed very well. The relationship between association constant and temperature was presented which demonstrated that there was severe association among acetic acid molecules in liquid phase at lower temperature.A set of new apparatus for determining VLE at high pressure based on the principle of bubble-dew method has been designed and established. And the reliability of this equipment was verified by comparison of determined saturated pressure of pure substances with the ones calculated by Antoine equation. At certain temperature of 140℃, 150℃ and 160℃, VLE of acetic acid-PX binary system was determined. VLE data were tested through thermodynamic consistency by both integrate and differential methods. Wilson model, NRTL model and modified UNIFAC method with new defined group PX were used to study the VLE, and model parameters were listed.