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Flow Apparatus, Determination of Critical Points

Measurement of Critical Data

A flow apparatus for the measurement of critical data of pure components and binary mixtures has been constructed. Fig. 1 shows the principle of this apparatus. Two syringe pumps deliver a flow of constant composition through a transparent saphire cell thermostated in an air bath. At the end of the apparatus a regulation valve allows the variation of density and pressure, respectively, inside the measurement cell. Starting from the two phase region the temperature and density is varied until the critical point is reached. The detection of the critical point is performed visually observing the disappearence of the phase boundary (critical opalescence). The temperature is measured with a platinum resistance thermometer (Pt 100) inside the cell and the pressure is measured at the outlet of the apparatus by a pressure sensor (Druck PDCR 910). The measurement cell is constructed out of a saphire tube mounted between two metal flanges and equipped with Teflonâ (up to 170 °C) or gold sealings. The range of applicability using gold sealings is up to 400 °C and 400 bar.

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Fig. 1 Principle of the flow apparatus for the measurement of critical points

picture of the device 

This principle of measuring critical data is well known in literature [1]. It can also be applied to substances which show a thermal decomposition because short retention time of the fluids inside the cell can be reached. This allows an extrapolation to zero retention time. Figure 2 shows some typical results together with data reported by other authors (Hicks and Young [2], Sadus [3]).

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Fig. 2 Typical results for the system Propane (1) + n-Butane (2)

Critical points of mixtures can be used to fit PSRK group interaction parameters if only a limited number of VLE data is reported in literature.


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