Ryo Akasaka

VAPOR–LIQUID EQUILIBRIUM MODELING FOR MIXTURES OF HFC-32 + ISOBUTANE AND HFC-32 + HFO-1234ze(E)

Vapor–liquid equilibrium (VLE) have been successfully modeled for the binary mixtures of difluoromethane (HFC-32) + isobutane and difluoromethane + trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)). These mixtures are considered as possible replacements for conventional refrigerants far from negligible global warming potential (GWP). A multifluid approach explicit in the Helmholtz free energy forms the basis of the model. The independent variables are the temperature, density, and composition. Accurate published equations of state for pure HFC-32, isobutane, and HFO-1234ze(E) are incorporated to calculate the Helmholtz free energy of each component. Typical uncertainties of bubble- and dew-point pressures calculated using the model are within 2%. Although adjustable parameters of the model are determined only from experimental VLE data, it is highly probable that the model reasonably predicts other thermodynamic properties such as enthalpy and heat capacities. Therefore, the model allows practical design and simulation of refrigeration systems using the mixtures as a working fluid.

Low-GWP refrigerants

We, the editors of this special issue on low-global warming potential (GWP) unsaturated halocarbon refrigerants, are pleased to present 16 articles with authors hailing from six countries. The special issue has been more than 1.5 years in the making and is the extension of and outcome from a long-standing collaboration among the editors spanning three continents and some 8 years. The authors represent industry (six authors), government (six authors), and academia (34 authors). We have specifically elected to focus this special issue on unsaturated halocarbon refrigerants to the exclusion of other low-GWP refrigerants, e.g., air, ammonia, carbon dioxide, hydrocarbons, and water, inter alia, primarily (1) because of the relatively recent, large amount of research and development regarding unsaturated halocarbon refrigerants, and (2) because there are other avenues (e.g., Gustav Lorentzen Natural Working Fluid Conferences; its 12th edition took place in August 2016) for publishing articles related to low-GWP natural refrigerants. Regardless, the editors of this special issue believe a future low-GWP special issue focused on natural refrigerants would be an important and worthy undertaking. Perhaps the primary impetus for the recent worldwide focus on low-GWP refrigerants is the Kyoto Protocol (1997) and ensuing legislation, primarily centered in Europe. In particular, in 2002, the European Union (EU) formerly adopted the Kyoto Protocol and followed this action with the adoption of the so-called F-Gas Regulations (2006) and the Mobile Directive (2006). More recently, the EU has repealed the 2006 F-Gas Regulations and passed more stringent F-Gas Regulations (2014). While the EU legislative initiatives are not the only ones in the world, the Mobile Directive led to enormous development efforts and monies being spent on developing low-GWP halocarbon refrigerants for automotive vehicle applications, with R1234yf (2,3,3,3-tetrafluoroprop-1-ene) having emerged as the “preferred solution” for meeting the directive. As a result of these automotive-focused development efforts and other legislative, regulatory, and taxing initiatives around the world, a number of other low-GWP unsaturated halocarbon refrigerants have been, and are being, investigated. While the definition of “low-GWP” is a qualitative one, is non-exact, and periodically changes based on historical circumstances, UNEP (2014) recently classified working fluids based on their 100-year time horizon GWP values relative to CO2. The UNEP taxonomy is provided in Table 1. Table 1. UNEP (2014) taxonomy of GWP values.

Recent trends in the development of Helmholtz energy equations of state and their application to 3,3,3-trifluoroprop-1-ene (R-1243zf)

Recent trends in the development of Helmholtz energy equations of state are briefly reviewed. Optimization procedures have been improved over the last two decades, and now nonlinear least-square fitting is effectively used in the optimization. This fitting technique makes it possible to develop a reliable equation of state for fluids with limited experimental data. The fitting is demonstrated for 3,3,3-trifluoroprop-1-ene (R-1243zf; CAS number 677-21-4). Experimental data for the critical parameters, vapor pressures, and liquid and vapor densities, including those at the saturation state, are available for this refrigerant. The fitting results in an equation with 17 terms. The equation of state is valid for temperatures from 234 to 376 K and for pressures up to 35 MPa. Comparisons to experimental data and verification of extrapolation behavior show that the equation of state has potential for most technical applications.

Application of the extended corresponding states model for prediction of the viscosity and thermal conductivity of cis-1,3,3,3-tetrafluoropropene (R1234ze(Z))

Prediction of the transport properties of low global warming potential refrigerants is a key issue for the refrigeration industry. In this work, the extended corresponding states models coupled with density-independent shape factors (empirical shape factors) or density-dependent shape factors (exact shape factors) are individually applied to predict the viscosity and thermal conductivity of 1,3,3,3-tetrafluoropropene (R1234ze(Z)). These models use 1,1,1,2-tetrafluoroethane (R134a) as a reference fluid. Empirical shape factors are obtained from the vapor pressures and saturated liquid densities of R1234ze(Z) and R134a. Exact shape factors are calculated from equations of state for these refrigerants. Predicted values with the extended corresponding states models are compared with experimental data, and the prediction capability of each model is discussed. In addition, viscosity and thermal conductivity shape factors are introduced to improve the prediction capability. With these additional shape factors, the extended corresponding states models represent the experimental data of the viscosity and thermal conductivity of R1234ze(Z) within ±2% and ±1%, respectively.