An enhanced "first freezing point" method for solid-liquid-gas equilibrium measurements in binary systems

This work presents an enhanced method for solid-liquid-gas (SLG) phase equilibrium measurements in binary systems. The method is mainly based on the observation of the first freezing point. It consists of the formation of first solids from fluid phase(s) by means of slow temperature decreases. In this work, additionally to the observation of the appearance of first solids, SLG equilibrium measurements were confirmed through a pressure change when the solids were formed while temperature was still decreasing at constant volume. A data acquisition system was built to allow registration of both temperature and pressure in the static-view cell in real time. The three-phase SLG equilibria for the well-studied carbon dioxide + naphthalene system were determined in order to validate our methodology. SLG equilibria were also measured for the binary systems constituted by carbon dioxide + hexadecanoic acid, octacosane, triacontane, or dotriacontane. The experiments were performed at pressures up to 39 MPa and temperatures below the melting point of the pure solute. The reported uncertainties in temperature and pressure are ±0.02 K and ±0.02 MPa.