Measurmement of high pressure phase equilibria. Open circuit methods

Since many processes are carried out at high pressures, it is essential to know how phases behave under these conditions. It is well known that pressure is one of the state variables conditioning thermodynamic equilibrium, and hence the composition of the phases present. As we saw in the [J 1 030] dossier, variations in pressure and/or temperature can induce phase separation. Knowledge of the effect of pressure is therefore of the utmost importance in the design and optimization of high-pressure chemical and separation processes, such as high-pressure reactions, polymer production, enhanced oil recovery, oil and natural gas processing, carbon dioxide capture and sequestration, supercritical fluid applications, refrigeration, etc. New avenues for high-pressure processes are now in the spotlight: ionic fluids and hydrates.

Nowadays, process design relies on simulators fed by thermodynamic equilibrium data, which must be as reliable and accurate as possible. Indeed, for delicate separations, with separation factors close to 1, even a slight inaccuracy in the equilibrium data can lead to serious design faults, with extreme impact on both the economics and specifications of the end products.

Although predictive numerical methods have made considerable progress, the time is still far off when they will be able to replace experimental determinations (see the quotations available on our website: http://www.drichon.wix.com/thermoadvices ). So, for the time being, we still have to rely on experimental work to provide data to theorists in charge of developing models and to industrialists in charge of designing their processes based on simulators. There is thus a synergy to be nurtured between experimenters, theorists and end-users, for example within the framework of an international network to be built up as a matter of priority. The end-user uses a simulator containing models developed and adapted by theorists using thermodynamic data from experiments.

Numerous experimental methods are described in the literature, each with its own advantages, disadvantages and specific details. We distinguish between closed-circuit methods (mixture contained in an equilibrium cell), cf.