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A new design of the high-pressure and high-temperature optical cell has been developed for precise measurements of the vapor–liquid equilibrium (VLE) properties of multicomponent mixtures at pressures up to 50 MPa and at temperatures from room to 450.15 K. The present design of the VLE optical cell allows avoidance of the separation of the binary system out of the sampler (in the sampling tube) and accurate determination of the concentrations of the coexisting phases without disturbing the phase equilibrium conditions during the high-pressure sampling procedure (thermodynamically consistent PTxy data). A sufficiently large volume (117 cm3) of the measuring cell allows extraction of a relatively large sample volume (about 4.1 cm3) from the liquid and the vapor phase independently without breaking the phase equilibrium (PT) conditions during the sampling procedure from each phase. This allows a considerable increase of the accuracy of the concentration determination of the equilibrium phases using the gravimetric method, especially the vapor phase. The gravimetric sampling method provides a precise analysis of the coexisting phase concentrations without using expensive chromatographic columns, which are not always suitable for the analysis of various complex multicomponent mixtures. The accuracy, reliability, and correct operation of the new design of the high-pressure VLE cell was verified by measuring the phase equilibrium properties of pure (propane) and the binary (propane + n-butane) mixture with well-known VLE properties. Also, the method was applied for the new, previously unexplored, ternary mixture of supercritical solvent (0.527 propane + 0.473 n-butane) + acetophenone. The values of the critical parameters for the ternary mixture have been estimated from the experimental VLE data near the critical point. The combined expanded uncertainty of the temperature, pressure, and concentration measurements at 0.95 confidence level with a coverage factor of k = 2 is estimated to be 0.15 K, 0.0022, and 0.03, respectively.

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