Speed-of-Sound Measurements and Derived Thermodynamic Properties of Liquid Isobutane

Abstract

This article reports comprehensive and accurate measurements of the speed of sound in liquid isobutane. The measurements were carried out by a double-path-length pulse-echo technique and cover the temperature range between 200 and 420 K with pressures of up to 100 MPa. The expanded measurement uncertainties (at the 0.95 confidence level) are 2.1 mK for temperature, 0.007% for pressure, and 0.009% for the speed of sound with the exception of a few state points at low pressures and in the vicinity of the critical point, where it increases up to 0.035%. Furthermore, densities and specific isobaric and isochoric heat capacities were derived from the speed-of-sound data in the temperature range between 200 and 340 K and up to 100 MPa by the method of thermodynamic integration. Very accurate results for the derived properties were obtained by determining values of the isobaric heat capacity on the initial isobar for the integration by a well-known thermodynamic relation among the isobaric heat capacity, density, and speed of sound from very accurate density data at low pressures of (Glos, S.; Kleinrahm, R.; Wagner, W. J. Chem. Thermodyn. 2004, 36, 1037−1059) and our speed-of-sound data. Moreover, these initial values were manually adjusted to enforce physically correct behavior of the calculated derivatives of the equation of state. Comparisons of the experimental speeds of sound and derived properties with the Helmholtz energy formulation of Bucker and Wagner for isobutane (Bucker, D.; Wagner, W. J. Phys. Chem. Ref. Data 2006Data , 35, 929−1019 and data from the literature demonstrate the high accuracy of the results and reveal potential to improve the formulation.

Speed-of-Sound Measurements in Liquid n-Pentane and Isopentane

“…The temperature is measured with a long stem 25.5 Ω standard platinum resistance thermometer calibrated on the ITS-90 in the wall of the pressure vessel with an expanded uncertainty (at the 0.95 confidence level) of 2.1 mK, while the pressure is measured with two calibrated nitrogen-operated pressure balances with pressure ranges of 5 and 100 MPa, which are coupled to the sample fluid by a differential-pressure null-indicator, with an expanded uncertainty (at the 0.95 confidence level) of 85 × 10 –6 · p below 0.5 MPa, 59 × 10 –6 · p between 0.5 and 1 MPa, 39 × 10 –6 · p between 1 and 5 MPa, and 45 × 10 –6 · p above 5 MPa. The uncertainty analysis of the temperature, pressure, and speed-of-sound measurements was carried out as described in our previous works. , Table provides a summary of the uncertainty budgets for the measurements in n -pentane and isopentane at 300 K and 50.1 MPa as examples.…”

“…The uncertainty analysis of the temperature, pressure, and speed-of-sound measurements was carried out as described in our previous works. 3,4 Table 1 provides a summary of the uncertainty budgets for the measurements in n-pentane and isopentane at 300 K and 50.1 MPa as examples.…”

“…In recent works, we measured comprehensive and accurate speed-of-sound data sets in the n -alkanes ethane, propane, and n -butane and the first branched alkane isobutane in large parts of the liquid and supercritical regions with pressures up to 100 MPa. This Article continues these works with a study of the speed of sound in liquid n -pentane and isopentane.…”

Thermodynamic Properties of Liquid Toluene and n-Butane Determined from Speed of Sound Data

“…Since only a few data of these seven data sets lie in the temperature range of the thermodynamic integration near or at the 1 MPa isobar and there are systematic differences between them, as will be shown in Sect. 5.1.2, the initial values for the isobaric heat capacity were derived from the density data of McLinden and Splett [5] and the speed of sound correlation for toluene by applying the procedure developed in our previous work on isobutane [2]. In this procedure, Eq.…”

“…We have recently applied the method of thermodynamic integration to determine the density and isobaric and isochoric heat capacity of isobutane from an accurate speed of sound data set, which was measured with a double-path-length pulse-echo apparatus in our laboratory [2]. In that work, it was shown that initial values of the isobaric heat capacity, which are often not available, can be derived from very accurate density data sets, which cover the temperature range of the speed of sound measurements at moderate pressures.…”