Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Heat Capacity of Saturated and Compressed Liquid Dimethyl Ether at Temperatures from (132 to 345) K and at Pressures to 35 MPa Jiangtao Wu# and Joseph W. Magee* Thermodynamics Research Center, Applied Chemicals and Materials Division (647), Material Measurement Laboratory, National Institute of Standards and Technology, Boulder, Colorado 80305-3337, United States ABSTRACT: Molar heat capacities at constant volume (Cv) of dimethyl ether have been measured with an adiabatic calorimeter. Temperatures range from the triple point to 345 K, and pressures up to 35 MPa. Measurements were conducted on liquid in equilibrium with its vapor and on compressed liquid samples. The samples are of high purity, as verified by chemical analysis. Calorimetric quantities are reported for the two-phase (Cv(2)), saturatedliquid (Cσ or Cx′), and single-phase (Cv) molar heat capacities. Low temperature Cv(2) data were employed to estimate vapor pressures for values less than 340 kPa by applying a thermodynamic relationship between the two-phase internal energy U(2) and the temperature derivatives of the vapor pressure. Vapor pressures were calculated at temperatures as low as the triple-point temperature. The principal sources of uncertainty are the temperature rise measurement and the change-of-volume work adjustment. The expanded relative uncertainty (with a coverage factor k = 2 and thus a two-standard deviation estimate) is estimated to be 0.7% for Cv, 0.5% for Cv(2), and 0.7% for Cσ.
1. INTRODUCTION Thermodynamic properties of a fluid may be calculated from a knowledge of its ideal gas properties and an accurate equation of state. Heat capacities derived in this manner, however, often lack sufficient accuracy since the calculation involves integration of the isochoric curvature (∂2p/∂T2)ρ as in the thermodynamic relation, C V − C V0 = −T
∫0
∂ 2p ⎞ dρ ⎜ 2⎟ 2 ⎝ ∂T ⎠ ρ ρ
range of temperature, would have a positive impact on an equation of state model. Both reviews mentioned above1,2 include a detailed review of the thermodynamic property literature and show the development of an equation of state model. Reference 1 also presents new experimental data for vapor pressure and density in wide ranges of temperature and pressure. Both reviews have important connections to the present study. The first review by Ihmels and Lemmon1 was done before the present work; hence, it does not include the experimental data from this study. The second review by Wu et al.2 does include all the present experimental data. In our numerical data comparisons in this work, we have used only the Ihmels and Lemmon model for calculations of heat capacity, vapor pressure, and density. In this way, we will show meaningful comparisons with a model that is based solely on prior studies. For very detailed comparisons of thermodynamic data (ca. 2011), including the present results, we would recommend consulting the review of Wu et al.2 In this paper, heat capacity and density data are reported for the compressed liquid region and the saturated liquid region from near the triple-point temperature to the upper limit (345 K) of the apparatus. In addition, vapor pressures evaluated from two-phase heat capacity measurements are reported from the triple point temperature to the normal boiling point temperature.
ρ⎛
(1)
where CV0 is the ideal-gas heat capacity. Since (∂2p/∂T2)ρ possesses small absolute values, except in the vicinity of the critical point, it is very difficult to measure accurately. To calculate Cv for compressed liquid states, additional data are required, including the vapor pressure and enthalpy of vaporization or heat capacity of the saturated liquid. Direct measurements of heat capacities will provide useful checks on calculated heat capacities when they are available along a path traversing the temperature range of interest. In many cases, such data are scarce. In two recent manuscripts, Ihmels and Lemmon1 and Wu et 2 al. reviewed published thermodynamic property measurements for dimethyl ether. Only five other sources of heat capacity data for dimethyl ether were found, and all five sources report values of heat capacity at constant pressure Cp. Just one of these studies reported saturated liquid heat capacities, and taken individually, each study covers a limited temperature range. As shown by ±8% deviations from their equation of state depicted in Figure 17 of ref 2, the five sources of Cp do not display mutual agreement. This finding makes it clear why another reliable set of heat capacity measurements, spanning a wide This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
Received: January 11, 2018 Accepted: April 5, 2018
A
DOI: 10.1021/acs.jced.8b00037 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 1. Two-Phase Heat Capacity Cv(2) and Heat Capacity of Saturated Liquid Cσ of Dimethyl Ether T/K 133.47 136.11 138.75 141.37 143.97 146.56 149.14 151.70 154.26 156.80 159.33 161.84 164.35 166.84 169.32 171.79 174.26 176.71 179.15 181.58 184.00 186.41 188.82 191.21 193.60 195.98 198.36 200.73 203.08 205.44 207.79 210.13 212.46 214.79 217.12 219.43 221.74 224.04 226.34 228.64 230.92 233.21 235.49 237.76 240.03 242.31 244.57 246.84 249.11 251.38 253.64 255.90 258.16 260.42 262.68 264.94 267.20 269.47 271.72 273.98 276.24
ρσa/mol·dm−3 19.100 19.031 18.963 18.895 18.827 18.759 18.691 18.624 18.557 18.490 18.423 18.357 18.290 18.224 18.158 18.093 18.027 17.962 17.896 17.831 17.766 17.701 17.636 17.572 17.507 17.442 17.377 17.313 17.248 17.183 17.119 17.054 16.989 16.924 16.859 16.794 16.729 16.664 16.599 16.533 16.468 16.402 16.336 16.270 16.204 16.137 16.071 16.003 15.936 15.868 15.800 15.731 15.663 15.593 15.524 15.453 15.383 15.312 15.240 15.168 15.095
Pσa/MPa −4
