A Dual-Path Pulse-Echo Instrument for Liquid-Phase Speed of Sound and Measurements on p-Xylene and Four Halogenated-Olefin Refrigerants [R1234yf, R1234ze(E), R1233zd(E), and R1336mzz(Z)].

We describe an instrument to measure the speed of sound in liquids based on the dual-path, pulse-echo technique; it covers a temperature range of 228.15-423.15 K, with pressures of up to 93 MPa. It differs from similar instruments in the method of mounting the quartz-crystal transducer, a path-length ratio of 2.5:1, and automated data-collection protocols. The path-length difference was calibrated with measurements on high-purity propane. The performance of the instrument was verified by comparison with recent literature data on p-xylene. We present new liquid-phase measurements for the halogenated-olefin refrigerants 2,3,3,3-tetrafluoroprop-1-ene [R1234yf], trans-1,3,3,3-tetrafluoroprop-1-ene [R1234ze(E)], trans-1-chloro-3,3,3-trifluoroprop-1-ene [R1233zd(E)], and cis-1,1,1,4,4,4-hexafluorobut-2-ene [R1336mzz(Z)]. These measurements cover a combined temperature range of 230 to 420 K, with pressures of up to 50 MPa; these data are compared to literature data (where available) and multiproperty equations of state. The average relative expanded uncertainty in the speed of sound ranged from 0.035 to 0.088% for the different fluids.

The NIST REFPROP Database for Highly Accurate Properties of Industrially Important Fluids.

The NIST REFPROP software program is a powerful tool for calculating thermophysical properties of industrially important fluids, and this manuscript describes the models implemented in, and features of, this software. REFPROP implements the most accurate models available for selected pure fluids and their mixtures that are valid over the entire fluid range including gas, liquid, and supercritical states, with the goal of uncertainties approaching the level of the underlying experimental data. The equations of state for thermodynamic properties are primarily of the Helmholtz energy form; a variety of models are implemented for the transport properties. We document the models for the 147 fluids included in the current version. A graphical user interface generates tables and provides extensive plotting capabilities. Properties can also be accessed through third-party apps or user-written code via the core property subroutines compiled into a shared library. REFPROP disseminates international standards in both the natural gas and refrigeration industries, as well as standards for water/steam.

Liquid-Phase Speed of Sound and Vapor-Phase Density of Difluoromethane.

Difluoromethane (HFC-32, DFM), with a global warming potential (GWP) of 677, is of interest as a pure refrigerant and as a component in low-GWP refrigerant mixtures. Additionally, difluoromethane has recently been identified as a safe, liquefied-gas electrolyte material in batteries. Using state-of-the-art instruments for measurements, this paper presents new liquid-phase speed of sound and vapor-phase density data for difluoromethane. Two hundred and nine liquid-phase speed of sound values were measured using a dual-path pulse-echo instrument at temperatures from 230 to 345 K and pressures from 2.1 to 70 MPa. Accounting for all sources of uncertainty, the relative expanded combined uncertainty (k = 2) in the speed of sound ranged from 0.035 to 0.17%. One hundred and thirty-eight vapour-phase density values were measured using a two-sinker densimeter at temperatures from 240 to 340 K and pressures from 0.1 to 1.61 MPa with an uncertainty of 0.011 to 0.12%. These experimental data will be valuable in the ongoing development of a new fundamental thermodynamic equation of state for difluoromethane.

Survey of Data and Models for Refrigerant Mixtures Containing Halogenated Olefins.

We survey existing data for refrigerant blends containing halogenated olefins (hydrofluoroolefins (HFO), hydrochlorofluoroolefins (HCFO) and hydrochloroolefins (HCO)) in the open literature. The data are primarily taken from the NIST SOURCE database and are presented in graphical form to demonstrate the relative coverage of the data. The primary conclusion is that blends containing halogenated olefins remain only sparsely measured in experiments, and some classes of data (e.g., speed-of-sound data) are particularly sparse for blends containing halogenated olefins. The second part of this study compares the thermodynamic models in NIST REFPROP against the experimental datasets and identifies systems (of which there are many) where refitting of the thermodynamic model is required.

New refrigerants and system configurations for vapor-compression refrigeration.

The high global warming potential of current refrigerants in cooling equipment based on the vapor-compression cycle has triggered a major effort to find and implement more environmentally benign alternatives. Here, we review the basics of the vapor-compression cycle together with the safety, environmental, and thermodynamic constraints that have led to the current and next generation of refrigerants. The development of new fluids has focused on fluorinated olefins, known as hydrofluoroolefins (HFOs), and blends that contain HFOs. Many of these are slightly flammable, presenting trade-offs between safety and environmental considerations. Engineers also have options with a resurgence of the "natural refrigerants" (ammonia, carbon dioxide, propane, and isobutane). Innovative system designs that reduce the required quantity of refrigerant may allow a wider choice of refrigerants.

(R)Evolution of Refrigerants.

As we enter the "fourth generation" of refrigerants, we consider the evolution of refrigerant molecules, the ever-changing constraints and regulations that have driven the need to consider new molecules, and the advancements in the tools and property models used to identify new molecules and design equipment using them. These separate aspects are intimately intertwined and have been in more-or-less continuous development since the earliest days of mechanical refrigeration, even if sometimes out-of-sight of the mainstream refrigeration industry. We highlight three separate, comprehensive searches for new refrigerants-in the 1920s, the 1980s, and the 2010s-that sometimes identified new molecules, but more often, validated alternatives already under consideration. A recurrent theme is that there is little that is truly new. Most of the "new" refrigerants, from R-12 in the 1930s to R-1234yf in the early 2000s, were reported in the chemical literature decades before they were considered as refrigerants. The search for new refrigerants continued through the 1990s even as the hydrofluorocarbons (HFCs) were becoming the dominant refrigerants in commercial use. This included a return to several long-known natural refrigerants. Finally, we review the evolution of the NIST REFPROP database for the calculation of refrigerant properties.

The Status of Thermodynamic Data and Models for CF3I and its Mixtures ⋆.

There is a renewed interest in the use of the refrigerant CF3I for refrigeration and air-conditioning applications, driven by its low global warming potential (similar to that of CO2), low (but nonzero) ozone depletion potential, and its ability to suppress flammability. In this paper the existing thermodynamic data for this fluid are summarized. Furthermore, as CF3I has been proposed as a component in mixtures with other refrigerants, the emphasis of this paper is on the survey of existing mixture data and an assessment of the suitability of existing thermodynamic modeling approaches and estimation schemes to model the properties of these mixtures. For mixtures where sufficient data exist, interaction parameters have been optimized.

Composition Determination of Low-Pressure Gas-Phase Mixtures by 1H NMR Spectroscopy.

1H NMR spectroscopy was used to analyze gas-phase mixtures of methane and propane at pressures near 0.1 MPa. The mixtures were prepared gravimetrically and had low uncertainty in their composition. The primary mixture used for this work had a methane mole fraction of xmethane,grav = (0.506875 ± 0.00019) and a propane mole fraction of xpropane,grav = (0.493125 ± 0.00019). NMR samples were prepared in two types of commercially available sample tubes that seal with a PTFE piston. Sample pressures ranged from 0.02 to 0.5 MPa. An analysis of measurement uncertainty for the NMR method resulted in combined standard uncertainties that decreased from 0.0082 x to 0.0010 x, as the pressure increased from 0.02 to 0.5 MPa. The larger uncertainties at lower pressures were primarily caused by uncertainties associated with phasing and baseline correction. A key difficulty in working with gas-phase samples, especially at lower pressures, is that the spectral peaks are inherently broad. Consequently, peak overlap was problematic, and it was not always possible to integrate a high percentage of a peak's intensity. However, with corrections to the integrated areas, based on the assumption of ideal Lorentzian peak shapes, excellent agreement between the NMR analyses and the gravimetric composition was observed across the entire pressure range. These experiments demonstrate the potential of 1H NMR for quantitative composition determinations of low-pressure gas-phase mixtures.

The hunt for nonflammable refrigerant blends to replace R-134a.

We investigated refrigerant blends as possible low GWP (global warming potential) alternatives for R-134a in an air-conditioning application. We carried out an extensive screening of the binary, ternary, and four-component blends possible among a list of 13 pure refrigerants comprising four hydrofluoroolefins (HFOs), eight hydrofluorocarbons (HFCs), and carbon dioxide. The screening was based on a simplified cycle model, but with the inclusion of pressure drops in the evaporator and condenser. The metrics for the evaluation were nonflammability, low GWP, high COP (coefficient of performance), and a volumetric capacity similar to the R-134a baseline system. While no mixture was ideal in all regards, we identified 16 binary and ternary blends that were nonflammable (based on a new estimation method) and with COP and capacity similar to the R-134a baseline; the tradeoff, however, was a reduction in GWP of, at most, 54% compared to R-134a. An additional seven blends that were estimated to be "marginally flammable" (ASHRAE Standard 34 classification of A2L) were identified with GWP reductions of as much as 99%. These 23 "best" blends were then simulated in a more detailed cycle model.

Densimetry for the Quantification of Sorption Phenomena on Nonporous Media Near the Dew Point of Fluid Mixtures.

Phase equilibria of fluid mixtures are important in numerous industrial applications and are, thus, a major focus of thermophysical property research. Improved data, particularly along the dew line, are needed to improve model predictions. Here we present experimental results utilizing highly accurate densimetry to quantify the effects of sorption and capillary condensation, which exert a distorting influence on measured properties near the dew line. We investigate the (pressure, density, temperature, composition) behaviour of binary (CH4 + C3H8) and (Ar + CO2) mixtures over the temperature range from (248.15 to 273.15) K starting at low pressures and increasing in pressure towards the dew point along isotherms. Three distinct regions are observed: (1) minor sorption effects in micropores at low pressures; (2) capillary condensation followed by wetting in macro-scale surface scratches beginning approximately 2% below the dew-point pressure; (3) bulk condensation. We hypothesize that the true dew point lies within the second region.

Limited options for low-global-warming-potential refrigerants.

Hydrofluorocarbons, currently used as refrigerants in air-conditioning systems, are potent greenhouse gases, and their contribution to climate change is projected to increase. Future use of the hydrofluorocarbons will be phased down and, thus replacement fluids must be found. Here we show that only a few pure fluids possess the combination of chemical, environmental, thermodynamic, and safety properties necessary for a refrigerant and that these fluids are at least slightly flammable. We search for replacements by applying screening criteria to a comprehensive chemical database. For the fluids passing the thermodynamic and environmental screens (critical temperature and global warming potential), we simulate performance in small air-conditioning systems, including optimization of the heat exchangers. We show that the efficiency-versus-capacity trade-off that exists in an ideal analysis disappears when a more realistic system is considered. The maximum efficiency occurs at a relatively high volumetric refrigeration capacity, but there are few fluids in this range.

LOW-GWP REFRIGERANTS FOR MEDIUM AND HIGH-PRESSURE APPLICATIONS.

The merits of an alternative refrigerant are established based on many attributes including environmental acceptance, chemical stability in the refrigeration system, low toxicity, flammability, efficiency and volumetric capacity. In an earlier work, these criteria were used to screen a comprehensive database to search for refrigerants with low global warming potentials (GWP). The present paper summarizes the screening process and presents the performance of the 'best' replacement fluids for small and medium-sized air-conditioning, heating, and refrigeration applications. In addition to considering cycle calculations based only on thermodynamic properties, a simulation model that included transport properties and optimized heat exchangers was used to assess the performance potentials of the candidate fluids. The need for this more detailed modeling approach is demonstrated for systems relying on forced-convection evaporation and condensation. The study shows that the low-GWP refrigerant options are very limited, particularly for fluids with volumetric capacities similar to those of R 410A or R-404A. The identified fluids with good COP and low toxicity are at least mildly flammable. Refrigerant blends can be used to increase flexibility in choosing tradeoffs between COP, volumetric capacity, flammability, and GWP. The probability of finding 'ideal', better-performing low-GWP fluids is minimal.

Development of a new densimeter for the combined investigation of dew-point densities and sorption phenomena of fluid mixtures.

For the determination of dew-point densities and pressures of fluid mixtures, a new densimeter has been developed. The new apparatus is based on the well-established two-sinker density measurement principle with the additional capability of quantifying sorption effects. In the vicinity of the dew line, such effects cause a change in composition of the gas mixture under study, which can significantly distort accurate density measurements. The new experimental technique enables the accurate measurement of dew-point densities and pressures and the quantification of sorption effects at the same time.

Speed-of-Sound Measurements in (Argon + Carbon Dioxide) over the Temperature Range from (275 to 500) K at Pressures up to 8 MPa.

The speed of sound of two (argon + carbon dioxide) mixtures was measured over the temperature range from (275 to 500) K with pressures up to 8 MPa utilizing a spherical acoustic resonator. The compositions of the gravimetrically prepared mixtures were (0.50104 and 0.74981) mole fraction carbon dioxide. The vibrational relaxation of pure carbon dioxide led to high sound absorption, which significantly impeded the sound-speed measurements on carbon dioxide and its mixtures; pre-condensation may have also affected the results for some measurements near the dew line. Thus, in contrast to the standard operating procedure for speed-of-sound measurements with a spherical resonator, non-radial resonances at lower frequencies were taken into account. Still, the data show a comparatively large scatter, and the usual repeatability of this general type of instrument could not be realized with the present measurements. Nonetheless, the average relative combined expanded uncertainty (k = 2) in speed of sound ranged from (0.042 to 0.056)% for both mixtures, with individual state-point uncertainties increasing to 0.1%. These uncertainties are adequate for our intended purpose of evaluating thermodynamic models. The results are compared to a Helmholtz energy equation of state for carbon capture and storage applications; relative deviations of (-0.64 to 0.08)% for the (0.49896 argon + 0.50104 carbon dioxide) mixture, and of (-1.52 to 0.77)% for the (0.25019 argon + 0.74981 carbon dioxide) mixture were observed.

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.

We explore a novel method for determining the dew-point density and dew-point pressure of fluid mixtures and compare it to traditional methods. The (p, ρ, T, x) behavior of three (methane + propane) mixtures was investigated with a two-sinker magnetic suspension densimeter over the temperature range of (248.15 to 293.15) K; the measurements extended from low pressures into the two-phase region. The compositions of the gravimetrically prepared mixtures were (0.74977, 0.50688, and 0.26579) mole fraction methane. We analyzed isothermal data by: (1) a "traditional" analysis of the intersection of a virial fit of the (p vs. ρ) data in the single-phase region with a linear fit of the data in the two-phase region; and (2) an analysis of the adsorbed mass on the sinker surfaces. We compared these to a traditional isochoric experiment. We conclude that the "adsorbed mass" analysis of an isothermal experiment provides an accurate determination of the dew-point temperature, pressure, and density. However, a two-sinker densimeter is required.

Three-body nonadditive potential for argon with estimated uncertainties and third virial coefficient.

The three-body nonadditive interaction energy between argon atoms was calculated at 300 geometries using coupled cluster methods up to single, double, triple, and noniterative quadruple excitations [CCSDT(Q)], and including the core correlation and relativistic effects. The uncertainty of the calculated energy was estimated at each geometry. The analytic function fitted to the energies is currently the most accurate three-body argon potential. Values of the third virial coefficient C(T) with full account of quantum effects were computed from 80 to 10000 K by a path-integral Monte Carlo method. The calculation made use of an existing high-quality pair potential [Patkowski, K.; Szalewicz, K. J. Chem. Phys. 2010, 133, 094304] and of the three-body potential derived in the present work. Uncertainties in the potential were propagated to estimate uncertainties in C(T). The results were compared with available experimental data, including some values of C(T) newly derived in this work from previously published high-accuracy density measurements. Our results are generally consistent with the available experimental data in the limited range of temperatures where data exist, but at many conditions, especially at higher temperatures, the uncertainties of our calculated values are smaller than the uncertainties of the experimental values.

A Liquid Density Standard Over Wide Ranges of Temperature and Pressure Based on Toluene.

The density of liquid toluene has been measured over the temperature range -60 °C to 200 °C with pressures up to 35 MPa. A two-sinker hydrostatic-balance densimeter utilizing a magnetic suspension coupling provided an absolute determination of the density with low uncertainties. These data are the basis of NIST Standard Reference Material® 211d for liquid density over the temperature range -50 °C to 150 °C and pressure range 0.1 MPa to 30 MPa. A thorough uncertainty analysis is presented; this includes effects resulting from the experimental density determination, possible degradation of the sample due to time and exposure to high temperatures, dissolved air, uncertainties in the empirical density model, and the sample-to-sample variations in the SRM vials. Also considered is the effect of uncertainty in the temperature and pressure measurements. This SRM is intended for the calibration of industrial densimeters.