Measurements and modeling of high-pressure excess molar enthalpies and isothermal vapor–liquid equilibria of the carbon dioxide + N, N-dimethylformamide system

Abstract

Mixtures of supercritical CO 2 and N, N-dimethylformamide (DMF) are very often involved in supercritical fluid applications and their thermodynamic properties are required to understand and design these processes. Excess molar enthalpies ( H m E ) for CO 2 + DMF mixtures were measured using an isothermal high-pressure flow calorimeter under conditions of temperature and pressure typically used in supercritical processes: 313.15 and 323.15 K at 9.00, 12.00, 15.00 and 18.00 MPa and 333.15 K at 9.00 and 15.00 MPa. The Peng–Robinson and the Soave–Redlich–Kwong equations of state were used in conjunction with the classical mixing rules to model the literature vapor–liquid equilibrium and critical data and the excess enthalpy data. In most cases, CO 2 + DMF mixtures showed very exothermic mixing and excess molar enthalpies exhibited a minimum in the CO 2-rich region. The lowest H m E value (−4526 J mol −1) was observed for a CO 2 mole fraction value of 0.713 at 9.00 MPa and 333.15 K. On the other hand, H m E at 9.00 MPa and 323.15 and 333.15 K varies linearly with CO 2 mole fraction in the two-phase region where a gaseous and a liquid mixture of fixed composition are in equilibrium. The effects of pressure and temperature on the excess molar enthalpy are large. For a given mole fraction, mixtures become less exothermic as pressure increases or temperature decreases. These excess enthalpy data were analyzed in terms of molecular interactions, phase equilibria, density and critical parameters previously reported for CO 2 + DMF. All throughout this paper, the key concepts and modeling tools originate from the work of van der Waals: the paper is intended as a small piece of recognition of van der Waals overwhelming contributions to thermodynamics.