(257r) Solid-Liquid-Gas Phase Diagram of Palmitic Acid + Carbon Dioxide System

The research on supercritical fluids and their uses in industrial sectors such as extraction and chromatography operations are well documented in the literature. New processes have been suggested; however these cannot be implemented fully by the lack of experimental data on phase equilibria. Particle formation processes with supercritical fluids have applications in the pharmaceutical industry [1]. Knowledge of phase behavior and the solubility of compounds of interest in the supercritical fluid as a function of pressure, temperature and composition are important for the design, simulation and optimization.

 In the solid-liquid-gas phase transition for a binary mixture constituted by a solid and a supercritical fluid, the P-T diagrams are drawn by identifying a phase change. This kind of diagrams might happen if the melting point of the solid is greater than the critical temperature of the supercritical fluid and the solid is thermostable up to its melting point [2]. The static-visual method has been used for determining these diagrams; it involves visual observation of phase change. Phase transition may be from a fluid (s) phase (s) to solid phase (first freezing point technique) [2] or from solid phase to the first drops of liquid (first melting point technique) [3].

In this work the SLG phase diagram is reported for the palmitic acid (solute) + carbon dioxide (supercritical fluid) system in a wide range of temperature and pressure and using the first freezing point technique. This system is classified as type B-II behavior according to the tendency of SLG line [4]. The solid-liquid-gas line has a minimum temperature below triple point temperature of the solid and the temperature of a upper critical ending point, this happens for relevant effects to increased solubility and the increase in hydrostatic pressure, which exert influence on the behavior of the line [5]. Experimental data were represented using the Peng-Robinson EoS and the van der Waals mixing rules; for the representation of the solid phase, we use the Lemert-Johntson [6] proposal.

1. I. Kikic, M. Lora, A. Bertucco, Industrial & Engineering Chemical Research, 36, 12 (1997) 5507-5515.
2. C.A. van Gunst, F.E.C. Scheffer, G.A.M. Diepen, The Journal of Physical Chemistry, 57, 6 (1953) 578-581.
3. P.L. Cheong, D. Zhang, K. Ohgaki, B.C.-Y. Lu, Fluid Phase Equilibria, 29, 1, (1986) 555-562.
4. B.C.Y. Lu, D. Zhang, Pure & Applied Chemistry, 61, 6, (1989) 1065-1074.
5. Z. Knez, M. Skerget,  Journal of Supercritical Fluids, 20, 1 (2001) 131-144.
6. R.M. Lemert, K.P. Johnston, Fluid Phase Equilibria, 45, 1 (1989) 265-286.