Density measurements on a binary (0.88 methane + 0.12 propane) mixture were carried out to evaluate the impact of different liquefaction procedures on the composition of condensed gas mixture samples. For these experimental investigations, a low-temperature single-sinker magnetic-suspension densimeter was used. The gaseous mixture was liquefied by condensation and via a special supercritical liquefaction procedure. The densities measured after applying the supercritical procedure serve as reference densities. Based on a comparison of the results of both procedures at otherwise same p,T conditions, the changes in composition can be estimated by differences in density, and these changes are caused by the procedure used for condensation.
It was found that the condensed liquids show distortions in composition of up to 0.94 mole-% that cannot be neglected for the accurate measurement of thermophysical properties. But it was also shown that under appropriate conditions and by applying special condensation procedures, single state points in the homogenous liquid region can be approached with just a minor or no detectable distortion in composition. However, when multiple state points shall be measured over a larger pressure or temperature range, no appropriate and easy method has been found to change the state point of the fluid under survey without significantly changing the composition of the liquefied sample. Only a supercritical liquefaction procedure in combination with a special VLE-cell is suitable for this application.
Keywords cryogenic liquid mixtures, liquefaction techniques, density measurement, magnetic-suspension coupling, single-sinker densimeter