Horacio A. Duarte-Garza

Subatmospheric vapor pressures evaluated from internal-energy measurements

Vapor pressures were evaluated from measured internal-energy changes in the vapor+liquid two-phase region, ΔU(2). The method employed a thermodynamic relationship between the derivative quantity (ϖU(2)/ϖV)T and the vapor pressure (pσ) and its temperature derivative (ϖp/ϖT)σ. This method was applied at temperatures between the triple point and the normal boiling point of three substances: 1,1,1,2-tetrafluoroethane (R134a), pentafluoroethane (R125), and difluoromethane (R32). Agreement with experimentally measured vapor pressures near the normal boiling point (101.325 kPa) was within the experimental uncertainty of approximately ±0.04 kPa (±0.04%). The method was applied to R134a to test the thermodynamic consistency of a publishedp-p-T equation of state with an equation forpσ for this substance. It was also applied to evaluate publishedpσ data which are in disagreement by more than their claimed uncertainty.

Subatmospheric Vapor Pressures for Fluoromethane (R41), 1,1-Difluoroethane (R152a), and 1,1,1-Trifluoroethane (R143a) Evaluated from Internal-Energy Measurements

Vapor pressures were evaluated from measured internal-energy changes ΔU(2) in the vapor+liquid two-phase region. The method employed a thermodynamic relationship between the derivative quantity (∂U(2)/∂V)T, the vapor pressure pσ, and its temperature derivative (∂p/∂T)σ. This method was applied at temperatures between the triple point and the normal boiling point of three substances: fluoromethane (R41), 1,1-difluoroethane (R152a), and 1,1,1-trifluoroethane (R143a). In the case of R41, vapor pressures up to 1 MPa were calculated to validate the technique at higher pressures. For R152a, the calculated vapor pressure at the triple-point temperature differed from a direct experimental measurement by less than the claimed uncertainty (5 Pa) of the measurement. The calculated vapor pressures for R41 helped to resolve discrepancies in several published vapor pressure sources. Agreement with experimentally measured vapor pressures for R152a and for R143a near the normal boiling point (101.325 kPa) was within the experimental uncertainty of approximately 0.04 kPa (0.04%) for the published measurements.

PVT experiments for precise VLE data for mixtures of similar volatility

Abstract The separation of components of similar volatility is a major industrial problem requiring precise VLE data. Classical VLE experiments provide liquid and vapor compositions, x1 and y1 to ±0.2%, at best, when the important difference Δz ≡ y1 − x1 may be only 0.008 leaving the designer with 0.008 ± 0.004. That is, the uncertainties can fail to exclude an azeotrope for systems where none exist. The Burnett-Isochoric (B-I) method allows precise measurement of densities along isochores leading to the isopleth for mixtures of fixed overall composition. Determination of each isopleth by the change of isochoric slope method provides complete VLE information when a series of isopleths of different compositions are overlayed on the P/T diagram. Because both densities and dew-bubble loci are measured, these data can test the accuracy of an Equation of State - Mixing Combining Rules (EOS/MCR), both as to its ability to predict densities (vapor and liquid) as well as VLE. Further, such dual data allow experimental consistency checks through the Gibbs-Duhem equation. Starting with equations similar to those of Manley and Swift (1971) we here derive further equations valid only for narrow dew-bubble gaps in pressure for isopleths. One particularly simple and useful result is that (Δz)2 α BΔDP. The fractional error in Δz is thus one-half of that of the measured BΔDP; the absolute error e(Δz) for our apparatus is about 4 × 10−5 when BΔDP = 0.55 psi (3.8 kPa). This manuscript presents these derivations as well as an error analysis based upon simulated results.

Molar Heat Capacity at Constant Volume of Trifluoromethane (R23) from the Triple-Point Temperature to 342 K at Pressures to 33 MPa

Molar heat capacities at constant volume (Cv) of trifluoromethane (R23) have been measured with an adiabatic calorimeter. Temperatures ranged from the triple point to 342 K, and pressures up to 33.5 MPa. Measurements were conducted on the liquid in equilibrium with its vapor and on compressed liquid and gaseous samples. The samples were of high purity, as verified by chemical analysis. Calorimetric quantities are reported for the two-phase (C(2)v), saturated-liquid (Cσ or C′x), and single-phase (Cv) molar heat capacities. The C(2)v data were used to estimate vapor pressures for values less than 100 kPa by applying a thermodynamic relationship between the two-phase internal energy U(2) and the temperature derivatives of the vapor pressure. The triple-point temperature and the enthalpy of fusion were also measured. 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 C(2)v, and 0.7% for Cσ.

A technique for preparing thermodynamic property tables using incomplete data sets

Abstract Extended corresponding states (ECS) algorithms provide versatile predictions for the thermophysical properties of mixtures. The NIST14 computer database implements an ECS model and the resultant property calculations are remarkably accurate for a wide variety of mixtures. This model emphasizes single phase calculations for many systems and properties, and it has the usual difficulties with the phase envelope and the critical region. Thermodynamic property tables are calculated most accurately when data are available throughout the ranges of the tables. Unfortunately, data are often not available over the entire ranges or in particularly important regions. We have observed that the deviations of the NIST14 model from PVT data for several mixtures have the same basic shape for each mixture. A single, interpolative function can adjust the model to fit compression factors for several systems within ±0.1%. Utilizing this function along with NIST14 forms a technique employing available experimental data to calculate accurately (1.0–3.5%) energies, enthalpies and entropies even in regions which lack experimental data or are near the critical point. This is not a correction for NIST14, but a method using NIST14 along with data to create accurate property tables in regions which lack data or which present difficulties for models.