Volumetric Properties of Propane, n-Octane, and Their Binary Mixtures at High Pressures

Density, isothermal compressibility, isobaric expansivity, thermal pressure coefficient, and excess volumes for binary mixtures of propane + n-octane are reported over a wide range of temperatures (from 320 to 440 K), and pressures (up to 400 bar) for mixture compositions with 0, 20.4, 42.4, 58.9, 79.7, and 100 wt % propane. Densities were determined using a fully computerized variable-volume view-cell system. A motorized pressure generator is used to bring about changes in the position of a movable piston in the cell at controlled and adjustable rates thereby bringing about changes in the internal volume and thus pressure. A long stroke-length linear variable differential transformer is used to continually monitor the position of the piston and thus the cell volume in real-time. Knowing the initial loading of the cell and the cell volume at any moment as pressure is altered permits generation of continuous density profiles along pressure scans in increasing (compression) or decreasing (decompression) direction of pressure at a given temperature. The density isotherms are readily correlated with polynomial equations and are used to generate the derived thermodynamic quantities such as the isothermal compressibility, isobaric expansivity, pressure coefficient, and excess volume. The results are discussed in terms of the effect of temperature, pressure and fluid composition for which there is no prior information in the open literature. In going from n-octane to propane, the data shows that compressibilities and expansivities increase, but the pressure coefficients tend to decrease. As examples, at 200 bar and 400 K, in going from n-octane to propane, values of compressibilities increase from about 2.0 x 10(-4) to 1.0 x 10(-3) bar(-1); isobaric expansivities increase from about 1.0 x 10(-3) to 2.5 x 10(-3) K-1, whereas thermal pressure coefficients decrease from about 4 to 3 bar/K. The excess volumes become more negative with increasing propane content, and the mixture with about 80 mol % (or 60 wt %) propane showing the largest negative excess volume of about -6.5 cm(3)/mol.