Vapour—liquid phase equilibrium at atmospheric pressure

Abstract

This paper attempts to precisely define and elucidate the various relationships between the physical and thermodynamic properties of solutions in vapour—liquid phase equilibrium. A coefficient of deviation from Raoult's Law, κ i , is defined and identified as the correlating variable used by most experimenters when working with vapour—liquid equilibrium data. Limitations on the use of κ i as compared to the true activity coefficient, γ L , are discussed for both the isothermal and isobaric case. Under the assumption that Amagat's Law applies to the vapour, this paper presents equations relating the deviation from Raoult's Law coefficient, κ i , to the differential heat of condensation. The different heat of condensation is then related to the integral isobaric heat of vaporization. Based on these equations, a method for the testing of isobaric vapour—liquid equilibrium data for consistency with heat of vaporization data is proposed. This work is derived from the similar treatment given to the true activity coefficient. An apparatus and procedure for the simultaneous measurement of vapour—liquid phase equilibrium data and the integral isobaric heat of vaporization are described. The results of measurements with the above-mentioned apparatus on the system acetone—chloroform at one atmosphere are presented. These results show that the measured liquid or bubble point composition varies directly with the boil-up rate. From the accurately known dew-point composition, the measured dew-point temperature and the heat data, a trial and error method for the calculation of the bubble-point composition consistent with the dew-point data is described. At the proper lower boil-up rates comparison of the Raoult's Law deviation coefficients κ i calculated from the consistent bubble-point data with the same coefficients calculated from the measured bubble-point composition shows good agreement. It is therefore concluded that the apparatus described in this work is capable of measuring consistent vapour—liquid equilibrium data. Thus it is shown that equilibrium liquid compositions may be calculated from measured dew-point composition—temperature data. The same method of calculation could be more easily employed with a system whose mixing effects are negligible.