Simple and controlled extrapolation of vapor pressures toward the triple point

Abstract

Although experimental vapor pressures are abundant in the medium-pressure range (psat , their values are scarce and uncertain at lower pressures due to experimental difficulties in determining the data. Reliable values are, however, required by technology (separation processes dealing with mixtures containing high molar mass compounds) and environmental concerns (transport and fate of high-boiling pollutants). The ability of frequently used vapor-pressure equations to extrapolate the data at medium pressures toward the triple-point temperature was tested on three groups of compounds (polar and nonpolar organic chemicals, n-alkanes, and 1-alkanols) for which reliable experimental data in the low-pressure range were available. Results of a simple extrapolation were compared with those from a simultaneous correlation of vapor pressures in the medium-pressure range and the differences between heat capacity of an ideal gas and that of the liquid available at lower temperatures. The latter is generally more reliable, as it is controlled by the exact thermodynamic constraints linking vapor pressures and the thermal data. The extent of improvement when using simultaneous correlation was examined, as well as the influence of the quality of input data on the results of extrapolation. Extrapolated values were compared with experimental vapor pressures and calorimetric enthalpies of vaporization that were selected from literature and pee of any important systematic errors. to accurately determine psat at low pressures is of prime importance from an environmental point of view.Reliable vapor-pressure data in the low-pressure range can be used for determining enthalpies of vaporization AHvap of high boiling compounds at 298.15 K, the vaporization data play an important role in various thermochemical and thermodynamic calculations for converting different enthalpic quantities between the gaseous and liquid states. Good-qual-

Recommended vapor pressures for thiophene, sulfolane, and dimethyl sulfoxide

“…H and C are auxiliary quantities which can be calculated using Eqs. (1) and (2) either from the vapor pressure correlating equation (by substituting the derivative dln p/dT into Eqs. (1) and (2) by combining heat capacity of ideal gas C g0 p,m and calorimetric values of heat capacity of liquid C p,m .…”

“…(1) and (2) either from the vapor pressure correlating equation (by substituting the derivative dln p/dT into Eqs. (1) and (2) by combining heat capacity of ideal gas C g0 p,m and calorimetric values of heat capacity of liquid C p,m . The possibility to calculate H and C both from the experimental thermal data and vapor pressure data and from a vapor pressure correlating equation means that after selecting a suitable relationship describing p vs. T it is possible to correlate simultaneously experimental vapor pressures p, enthalpies of vaporization g H m and heat capacity difference g C 0 p,m as a function of temperature.…”

Thermodynamic properties of dimethyl phthalate along the (vapour + liquid) saturation curve

“…It should be noted, however, that the Antoine equation is unsuitable for extrapolation below the lower limit of experimental data due to its low flexibility. (16) The three-parameter Cox equation was recommended by Růžička and Majer (16) on the basis of an analysis of the performance of different vapour pressure equations with regard to their ability to describe simultaneously vapour pressure and related thermodynamic properties as a function of temperature down to the triple point. An arbitrarily chosen temperature T 0 , and the corresponding vapour pressure p 0 , are used as a reference state.…”

Mercury CEM Calibration

“…Numerous expressions have been used to represent the vapor pressure of a pure fluid; many are reviewed in Růžička and Majer [108]. Equations of the general form…”

“…where τ = 1-T/T c , are attributed to Wagner [109][110][111][112] Since there is a lack of high-quality experimental vapor-pressure data in the low temperature region (T < 285 K), liquid heat capacity measurements at low temperatures can be used to supplement the vapor-pressure data [108,114,119]. This permits the simultaneous regression of heat capacity and vapor-pressure data to determine the coefficients of a vapor pressure equation that is valid down to the triple point.…”