Vapor pressures of 1,1,1,2,3,3,3-heptafluoropropane, 1,1,1,3,3,3-hexafluoropropane and 1,1,1,3,3-pentafluoropropane
Introduction
Hydrofluocarbons (HFCs) with zero ozone depletion potential (ODP) and lower global warming potential (GWP) are promising alternatives for chlorofluorocarbons (CFCs) and hydrochloroflurocarbons (HCFCs). During the past twenty years, the physical and chemical properties of fluorinated methanes and ethanes such as difluoromethane (HFC-32), pentafluoroenthane (HFC-125) and 1,1,1,2-tetrafluoroethane (HFC-134a) have been studied experimentally and theoretically throughout the world and have been used in various engineering fields [1], [2]. Fluorinated propanes have been studied in the past decade. Of the fluorinated propanes, the three most important are 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea), 1,1,1,3,3,3-hexafluoropropane (HFC-236fa), and 1,1,1,3,3-pentafluoropropane (HFC-245fa). HFC-227ea has already been produced commercially as a fire extinguishing agent [3]. HFC-236fa has been specified by the U.S. Navy as a replacement for CFC-114 in centrifugal chillers [4] and is also listed as an acceptable halon replacement in the EPA SNAP Program for Halon 1211 in portable fire extinguishers. HFC-245fa is a promising alternative to replace HCFC-141b in heat pumps and chemical blowing agents and is also used as the captured gas in UV-nanoimprint [5].
The vapor pressure is one of the most fundamental thermophysical properties of fluids with several laboratories publishing the experimental data for these three fluorinated propanes [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. The published vapor pressure data for the three fluorinated propanes is summarized in Table 1, Table 2, Table 3. However, obvious discrepancies for the vapor pressures of HFC-236fa and HFC-245fa in the literature data were found at temperatures near the critical region and below 270 K [15], [23]. Since there is better quality vapor pressure data of HFC-227ea, HFC-227ea was used as the test substance to check the experimental uncertainties. The experimental samples for each substance were supplied by two different suppliers and the samples were prepared carefully to remove impurities. The effect of the micro impurities on the fluid vapor pressure was studied using gas chromatography (GC) as well as gas chromatography with mass spectrum analysis (GC/MS) to analyze the samples.
Section snippets
Experimental
A Burnett apparatus was used to measure the vapor pressures. A diagram of the apparatus is shown in Fig. 1. The system has been rebuilt with improved thermostatic baths, temperature measurement system, pressure measurement system, vacuum system and data acquisition system. The system details were described by Feng et al. [24].
The thermostatic baths provide a uniform, stable temperature field. The temperature range for the thermostatic baths was from 233 to 453 K. The stability of the
HFC-227ea
The vapor pressures of HFC-227ea from two different samples were measured at temperatures from 234 to 371 K to obtain the sixty-one data points shown in Table 4 and Fig. 2. The uncertainties of the vapor pressures were estimated with the expression:where U(ps) is the vapor pressure uncertainty, U(pexp) is the uncertainty of the pressure measurement system, (dp/dT)s is the first derivative of the vapor pressure with respect to temperature and U(Texp) is the uncertainty
Conclusions
The vapor pressures of HFC-227ea were measured for temperature of 234–371 K, those of HFC-236fa were measured from 239 to 398 K and those of HFC-245fa were measured from 235 to 427 K with an uncertainty in the vapor pressure of ±300 Pa. The experimental data for each fluid was correlated using a Wagner-type vapor pressure equation. Critical and triple-point pressures, the normal boiling-point temperature and the acentric factor were calculated from the equations. The effects of impurities on the
Acknowledgments
We thank Dr. Gao Peng for his kind help during the GC/MS analysis. This work was supported by the National Natural Science Foundation of China (No. 50636020) and the National Key Technology R&D Program (2006BAF06B03).
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