Static Methods for Measuring Solubilities in Supercritical Fluids

STATIC METHODS FOR MEASURING SOLUBILITIES IN SUPERCRITICAL FLUIDS 

A data point is obtained in the following manner. At a fixed temper ure the mixture in the cell is compressed to a single phase at high pressures. The pressure is then slowly decreased until a second phase appears. Typically the image of the mixture in the cell is projected onto a video monitor using a borescope (Olympus Corp.) placed against the sapphire window and connected to a video camera. The reported solubility data point is in the pressure interval between the two-phase state and the single fluid-phase state. The heavy solute can be alternately solubilized and precipitated a number of times in an effort to decrease the pressure interval from two phases to one phase. The solubility in this pressure interval is known from the amounts of gas and solute loaded into the view cell. 

If a liquid solute is being studied, the vapor-liquid phase transition is determined in a similar manner (Occhiogrosso, 1985). The piston is slowly adjusted to lower the system pressure into the two-phase region. This decompression step is performed very slowly. If the pressure of the system is within 2 bar of the phase-split pressure, the rate of decompression is usually maintained at —0.03 bar/sec. The actual phase transition for the liquid solute is in the pressure interval between this two-phase state and the previous single, fluid-phase state. The entire procedure is then performed several times to decrease the pressure interval from two phases to one phase, so it falls within an acceptable range. The system temperature is now raised and the entire procedure is repeated to obtain more VLE information without having to reload the cell. In this manner, without sampling, an isopleth (constant composition at various temperatures and pressures) is obtained. 

When working with a liquid solute, a bubble point, dew point, or mixture critical point can be visually detected. A bubble point is defined as the 

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Static Methods for Measuring Solubilities in Supercritical Fluids 91... 

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