Development of a novel method to investigate the dew line of binary mixtures using a modified two-sinker densimeter
Final Report Abstract
The (p, ρ, T, x) behavior of three (methane + propane) mixtures was measured with a twosinker magnetic suspension densimeter over the temperature range of (248.15 to 373.15) K with maximum pressures of slightly transcending the dew point pressure at the measured temperature or the maximum pressure in the sample cylinder, whichever was lower. The compositions of the gravimetrically prepared mixtures were (0.74977, 0.50688, and 0.26579) mole fraction methane. For the measurements carried out with the original density sinkers the relative combined expanded uncertainty (k = 2) in density considering all effects, including the uncertainty in composition, was 0.05 % or lower for most points, except it was larger at densities less than 5 kg∙m–3. Comparisons to the GERG-2008 equation of state for natural-gas mixtures showed significant deviations in density (as large as –1.3 %) that increased with decreasing temperature, with increasing pressure, and with increasing propane fraction in the mixture. After a literature search these large deviations at the higher propane fractions were not unexpected because only high-quality data at high methane fractions (0.90 mole fraction methane and higher) were available during the fitting procedure of the GERG. The (p, ρ, T, x) data measured with the original set of sinkers was used to determine dewpoints. Comparing the experimental dew-point data with values calculated with the GERG-2008 equation, relative deviations in saturation pressure of about 0.7 % and in dew-point density of up to 0.15 % could be observed. Moreover, the experimental values were used to calculate interaction virial coefficients B12(T) for this system. The B12(T) agreed well with literature values. They were constant (within experimental uncertainty) with composition, as expected from theory. In contrast, the B12(T) calculated with the GERG-2008 equation of state varied with composition. Finally it can be stated that the densimeter became much more sensitive to the effects occurring on the sinker surfaces after modification of the densimeter by implementing a new set of sinkers optimized for dew-point measurements. Hence, the idea of using a special “sorption sinker” has been proven successful. Furthermore, due the use of a “sorption sinker” we observed that the density measurement at dew point conditions is much more complex than expected from the results when using the original densimeter sinkers. Without further understanding of the phenomena occurring close to the dew line it will be difficult to provide a sophisticated data analysis. Therefore, further detailed investigations are required.
Publications
- Vapor-Phase (p, ρ, T, x) Behavior and Virial Coefficients for the (Methane + Propane) System. J. Chem. Eng. Data, 2014, 59 (12), pp 4151–4164
Richter, M., McLinden, M. O.
(See online at https://doi.org/10.1021/je500792x) - Application of a two-sinker densimeter for phase-equilibrium measurements: A new technique for the detection of dew points and measurements on the (methane + propane) system. J. Chem. Thermodyn. Volume 99, August 2016, Pages 105-115
Richter, M., McLinden, M. O.
(See online at https://doi.org/10.1016/j.jct.2016.03.035)
