Methods for Measuring Solubilities in Supercritical Fluids

DYNAMIC METHODS FOR MEASURING SOLUBILITIES IN SUPERCRITICAL FLUIDS 

A flow apparatus representative of the type used to determine the solubility of a heavy liquid or solid in a supercritical fluid is shown in figure 4.1. The description of the apparatus and procedures for this system is taken from Van Leer and Paulaitis (1980). Although other flow methods are described in the literature. Van Leer s simplified technique embodies all the main features of a typical flow apparatus for obtaining solubility information (Prausnitz and Benson, 1959 Simnick et al., 1977 Kurnik, Holla, and Reid, 1981 Johnston 

After the saturated SCF-rich phase exits the second column, it is expanded to atmospheric pressure across a heated metering valve the heavy component falls out of solution and is collected in a cold trap. The temperature of the cold 

1990 Bolanos, Hochgeschurtz, and Thies, 1991). Either a pure component or a solution mixture can be delivered to the system using dual inlet pumps shown in the diagram. 

When determining the phase behavior of a ternary mixture, one of the pumps delivers the supercritical fluid of interest and the other delivers a binary mixture of fixed concentration. The overall loading of the solution delivered to the system is fixed by the flow rates of the pumps. The streams from each 

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Dynamic Methods for Measuring Solubilities in Supercritical Fluids 87... 

The NMR method we have developed gives a direct, in situ determination of the solubility and also allows us to obtain phase data on the system. In this study we have measured the solubilities of solid naphthalene in supercritical carbon dioxide along three isotherms (50.0, 55.0, and 58.5°C) near the UCEP temperature over a pressure range of 120-500 bar. We have also determined the pressure-temperature trace of the S-L-G phase line that terminates with the UCEP for the binary mixture. Finally, we have performed an analysis of our data using a quantitative theory of solubility in supercritical fluids to help establish the location of the UCEP. 

Kwiatkowski, J., Lisicki, Z., and Majewski, W., An Experimental Method for Measuring Solubilities of Solids in Supercritical Fluids, Ber. Bunsenges. Phys. Chem. 88 865-869 (1984)... 

Kwiatkowski, J., Z. Lisicki, and W. Majewski. 1984. An experimental-method for measuring solubilities of solids in supercritical fluids. Berichte der Bunsen-Gesellschc -Physical Chemistry Chemical Physics. 88, 865. 

Isobaric-isothermal methods are often also called dynamic methods. One or more fluid streams are pumped continuoirsly into a thermostated equilibriirm cell. The pressure is kept constant during the experiment by controlling an effluent stream, irsually of the vapor phase. One can distinguish between continuorrs-flow methods and semi-flow methods. In continuous-flow methods, both phases flow throrrgh the eqrrihbrirrm cell. They can be used only for systems where the time needed to attain phase equilibrium is sufficiently short. Therefore, such equipment is usually not applied to polymer solutions. In semi-flow methods, only one phase is flowing while the other stays in the equilibrium phase. They are sometimes called gas-saturation methods or pure-gas circulation methods and can be used to measure gas solubilities in liquids and melts or solubilities of liquid or solid substances in supercritical fluids. 

FIGURE 6 Variable-volume view cell for use in the static/synthetic method of solubility measurement. Components follow variable volume ceU (1), screw pump (2), magnetic stirrer (3), magnetic bar (4), upper sampling line (5), lower sampling line (6), sapphire window (7), thermostated bath (8), light som-ce (9), video camera (10), and monitor (11). (From Crampton, C., Charbit, G., and Neau, E., J. Supercrit. Fluids, 16(1), 11-20, 1999, reproduced with permission from Elsevier.)... 

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