Excess enthalpy measurements using a novel highly refined microflow calorimeter and the prediction of vapour-liquid equilibria from such data

Abstract

A novel differential microflow calorimeter has been developed for excess enthalpy measurement on endothermic liquid systems. The design incorporates precise temperature equilibration of the fluids before mixing, the elimination of flow-rate- and physical-property-dependent heat leaks and a differential mode of operation based on a careful analysis of entropy generation in flows with mixing and friction. The compensation for frictional heating and the lack of dependence of measurements on flow rate and mixture physical properties other than h E represents a major advance. Measurements for the well-tested hexane-cyclohexane system (adopted as a reference standard by IUPAC) had an average deviation of only 0.21 J gmol −1, surpassing in accuracy any previous measurements on a flow calorimeter. Heat-of-mixing data are presented for the n-hexane-toluene system at 25, 40 and 55°C. Isobaric and isothermal vapour-liquid equilibrium data are predicted from the measured h E data using a new modification of the UNIQUAC equation aimed at improving the latter's temperature dependence. The predicted VLE data are in very good agreement with those measured by Sieg, Saito and by Michishita et al.