Mixtures of fluorinated ether and sulfur derivatives as chlorodifluoromethane (R-22) alternatives
Başlık çevirisi mevcut değil.
- Tez No: 400859
- Danışmanlar: DR. ADOLPH L. BEYERLEIN
- Tez Türü: Doktora
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2001
- Dil: İngilizce
- Üniversite: Clemson University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 178
Özet
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Özet (Çeviri)
The refrigerant R-22 is used worldwide in the refrigeration and air-conditioning industry because of its desirable chemical and physical properties for refrigerant applications. However, it is scheduled to be phased out by the year 2015 by the Clean Air Act Amendment due to its ozone depletion potential. Since hydrofluorocarbons (HFCs) do not deplete ozone, they are considered primary CFC or HCFC replacements. Although there are currently two HFC mixtures, R-410A and R-407-C, being sold commercially as R-22 replacements, both of these alternatives have shortcomings. The refrigerant R-410A requires high operating pressures and R-407C fractionates during the evaporation cycle. The task of finding a better replacement for R-22 with the same performance or better is difficult because no HFC compounds can match the wide spread between the boiling point (-40.1°C) and critical temperature (96°C) for R-22. Consequently the search for finding an alternative to R-22 is oriented to finding the best alternative based on how nearly its coefficient of performance (COP) value approaches that of R-22. The coefficient of performance is equal to the heat removed from the surroundings during the evaporation portion of the refrigeration cycle relative to the work done during a complete cycle. In view of the fact that no broadly accepted alternative for R-22 could be identified among HFCs or their mixtures, in this work we investigate mixtures of the following highly fiuorinated compounds: hexavalent sulfur compound, trifluoromethylpentafluorosulfur (CF3SF5), perfluorodimethylthioether (CF3SCF3), and fluorinated ether derivatives, [CF3OCH3 (RE-143a), CF3OCF2CF3 (RE-218), CF3OCF2H (RE-125)] with HFC ethane derivatives and fluorinated ethers of lower fluorine content. Investigations were conducted on 16 binary mixtures and 7 ternary mixtures. Consistent with our goals this class of chemicals and mixtures covers all systems which have potential as an R-22 alternative and have not been adequately investigated. These mixtures have large positive deviations from ideality and the potential to form minimum boiling azeotropic mixtures with boiling points below -30°C. Because the components of these mixtures have gas-liquid critical temperatures above 80°C, they are expected to perform better than currently accepted alternatives whose critical temperatures are below 80°C. In addition, these mixtures have low toxicity and since they have a substantial concentration of a highly fluorinated component are expected to have acceptable flammability properties. The thermodynamic investigations on these mixtures include vapor pressure measurements as function of temperature up to the critical temperature, measurements of the saturated liquid density as function of temperature, measurement of the critical density and temperature, estimation of the saturated vapor density by the modified corresponding states methods and estimation of the ideal gas heat capacity by the Benson et al. functional group method. Qualitative assessment of the mixtures is made from examination of the boiling point and critical temperature. Further quantitative assessments are made by estimating the coefficient of performance. The coefficient of performance for an ideal cycle is estimated for pure chemicals and mixtures relative to that of R-22. In order to facilitate the estimation of thermodynamic vapor pressure behavior of a variety of mixtures, deviations from ideality are predicted from the expansion of the free energy, AG6, about its symmetric approximation, using correlations established with the experimental vapor-liquid behavior of CFsSFs/Ethane derivative and CFsOCFiH/Ethane derivative mixtures. This estimation method was judged by comparison of predicted vapor pressure and boiling points with experimental values for several mixtures, CF3OCF2CF3/CH3CF2H, CF3OCF2CF3/CF3OCH3, CF3SCF3/CF3CH2F and CF3OCF2H/CF3OCH3. The results show that one can estimate the mixture boiling points at different mole fraction and azeotropic compositions with a relative error of 4 to 5%. This type of estimation is useful where the experimental data is not available or at least one of the components is commercially not available. The COP values were estimated for seven mixtures using the thermodynamic analysis of Morrison and McLinden (NBS Technical Note 1226) for the refrigeration cycle which accounted for the PVT behavior outside the liquid-vapor coexistence region using the Carnahan-Starling Equation of State. From these COP values a simplified mathematical model was developed which estimated the COP from vapor pressure data and the ideal gas heat capacity without the need for liquid or vapor density data. The root mean square (RMS) deviation of COP values predicted by this model from the COP values obtained from a complete thermodynamic analysis of the refrigeration cycle is about 5.9%. Thus, one can use this model without the considerable effort associated with density measurements. The COP values using this model were estimated for 22 mixtures, and two pure chemicals. The calculations revealed that compounds and mixtures with low heat capacity yielded the highest COP values. Compounds and mixtures with critical temperatures below 80°C yielded poor performance in higher temperature applications where the condensation temperature exceeded 47°C. This is primarily the reason R-410A, a commercial R-22 alternative does not perform well in higher temperature heat pump applications. In conclusion, we have identified 6 alternatives (CF3OCF2H, CH3CH2F, CF3SCF3/CH3CF2F, CF3OCF2H/CF2H2/CH3CFi2F, CF3OCF2H/CF2H2/CH3CF2H and CF3OCF2H/CF2H2/CF3CH2F) which have COP that exceed those of R-410A and R-407C, i.e. the COP exceeds values that are 75% of those of R-22.. The most notable mixture is CF3SCF3/CH3CF2H with a relative COP value of 89% that of R-22. This azeotropic mixture COP value is about 13.4% and 17.6% higher those of current commercial alternatives, R-410A and R-407C, respectively.
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