An apparatus for determining high-pressure liquid-vapour equilibrium data has been developed and proved to operate satisfactorily. The apparatus operates isothermally. Results have been obtained for the ethanolmethanol system at the following temperatures: loo", 120", 140" and 160". The results are shown to be thermodynamically consistent within the limits of the accuracy of their determination. The 'ideal activity coefficients' (i.e. calculated assuming that the gas phase behaviour is ideal) are correlated with the mole fraction of the component in the liquid phase by Margules-type equations (1) and (2). The accuracy of these equations is greater than predictions based on fugacities.From equations (5), (6) and (1) or (2) in the paper it is possible to estimate any pair of liquid-vapour equilibrium compositions for any specified pressure P and temperature T. Alternatively, for a specified P and liquid composition, the corresponding T and vapour composition may be calculated. IntroductionThe purpose underlying this work is to provide reliable equilibrium data for a number of relatively common binary systems over a wide pressure range above atmospheric pressure. These data are required primarily to evaluate the performance of a sieve tray in a 7-in. dia. high-pressure distillation column.Since it was intended that conditions in the equilibrium apparatus should be adjustable up to the values of temperature and pressure of the critical points of the components, the 'Static Method' as used by Verchoyle' and others2s3 was considered the best practical method. Recirculation stills have been used to obtain equilibrium data up to the critical region of some system^.^*^ However as the pressure of the system is increased, pv+pI and AH, is reduced. These tendencies encourage liquid entrainment in the recirculating vapour and flash evaporation of the condensate as it returns to the reboiler. It also becomes more difficult to prevent partial condensation on the walls of the vessel above the boiling liquid and/or total evaporation of liquid droplets splashing on to these walls.The usefulness of the 'Static Method' increases with increasing pressure. As py increases, the volume of the vapour phase required to produce a given mass of sample decreases, and the withdrawal of this sample causes a smaller thermal disturbance in the system with decreasing AH,. Finally, the mechanical design of such an apparatus is far simpler than that of a recirculation type of still designed to operate at similar pressures.The choice of the methanol-ethanol system for the first investigation which is reported here was influenced by the ease of obtaining materials of high purity of which the behaviour in solution might be anticipated to be nearly ideal and also the ease with which the system could be analysed. These considerations, together with those of thermodynamic consistency would provide a basis for confidence in the reliability of the data obtained from the system. The upper temperature of 160" represented the highest steam temperature then a...

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