Experimental determination of phase equilibria in systems containing a near-critical component

4 Experimental determination of phase equilibria in systems containing a near-critical component 

A variety of types of equipment have been reported in the literature for determining the relevant phase equilibrium compositions under given conditions and only a very discursive account can be given here. The types of method reported include 

Static methods. In which the system of interest is enclosed in a magnetically stirred variable volume cell [64, 76] which in some cases contains a window. The temperature and pressure within the cell are accurately metered. The cell volume may be changed either using a mercury piston or a mechanical piston and samples of the fluid phases present may be obtained, if required, under conditions of constant temperature and pressure by suitably reducing the cell volume. In the windowed cell version [76] sampling is unnecessary for binary systems since the cell may be charged with known amounts of the two components and conditions adjusted to obtain trace presence only of one of the phases. In this way the dew- and bubble-point curves for binary systems may be established and similarly the solubilities of solids in compressed fluids may be determined. 

Recirculation methods. When investigating equilibrium between fluid phases the system of interest is enclosed in a thermostatted stirred vessel with provision for recirculating both the phases [64]. The vapour phase is recirculated via a vapour sampling bomb and back to the equilibrium vessel where it bubbles up through the liquid. The liquid phase may also be recirculated via a liquid sample bomb. When it is considered that equilibrium has been reached, the pressure is noted and representative samples of the vapour and liquid phases are sealed off in the sample bombs, the contents of which are analysed. 

When investigating solid/vapour equilibrium by this technique, only the vapour phase is recirculated [64]. 

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