Vapor-Liquid Equilibria For Multi-component Mixed Refrigerants At Cryogenic Temperature Range

In recent years, there are many great achievements on the research of mixed-gases Joule-Thomson refrigerators (MJTRs). Driven by an ordinary air-conditioning compressor, the MJTR has become dominant for applications over a large temperature range from (80 to 230) K (defined as a cryogenic temperature range in this paper) due to its numerous merits, such as no moving parts at low temperatures, high reliability, high thermodynamic performance, and low manufacture and maintenance cost, etc, comparing with other coolers as GM and Stirling coolers. Between the normal boiling point of nitrogen and the lowest temperature of the traditional one-stage compression refrigeration cycle, the cryogenic temperature range is a large scope, which will be used widely in many fields, such as national defence, military industry, space technologies, high temperature superconduct, semiconductor manufacture, liquid natural gas and biomedicine engineering, etc.However, there are still many important and difficult scientific problems on MJTRs to be investigated. One of these problems is the characteristic for Vapor-Liquid Equilibria (VLE) of multi-component mixtures. In this paper, theoretical and experimental investigations were carried out on the VLE of mixed-gas in the cryogenic temperature range, on the basis of former research work.1. Analysis on the thermodynamics and theoretical models of phase equilibriaThe research development of basic theories and calculation methods for the thermodynamics of phase equilibria was reviewed detailedly. After analyzing the features of various theoretical models, and charactoristics for small molecule hydrocarbon and freons, PR and SRK cubic Equation of State (EoS), associated by van der Waals (vdW), Huron-Vidal (HV) and Panagiotopoulos-Reid's modification (M) mixing rules, were choosen to correlate the measured VLE data in this paper, and great fitting results were achieved.2. Investigation on the interaction coefficient and the new modelThe application backgroud and concept development of interaction coefficient were given, and the new GE-EoS model was developed, basing on the Group-Contribution method and EoS method. By the UNIFAC method, groups were redivided for the binary system CF4+C₂H₆, and CF4 is defined as a new group. Combining PR-HV model with UNIFAC model, the new one was set up, and two interaction coefficients for CF4 and CH3 were expressed as the functions of the temperatures. The new model was verified by comparing the calculated results with PR-vdW model and UNIFAC model.3. Experimental investigation on the VLE and analysis on the uncertaintyAfter reviewing the former VLE experimental investigation methods, the experimental rig, measuring system and experimental process were introduced in detail. To give the confidence of the VLE data, the uncertainty for the measurements of temperatures, pressures and compositions were analyzed.4. Experimental investigation on the saturated vapor pressure equation of CF430 vapor pressure data were measured in a temperature range from (139.53 to 217.53)K, by the experimental rig with vapor phase recirculation method. The Wagner equation and Zhu Mingshan equation were used to correlate the experimental data, and both have good correlation results. So, the new vapor pressure equation for CF4 was obtained. To test the new equation, the interpolated and extrapolated results with this equation were compared with former experimental data, and the accuracy of calculation meets the demand for engineering design.5. Experimental investigation on the VLE of the binary system of CF4+C₂H₆The VLE data of the binary non-azeotropic system of CF4+C₂H₆ were first measured at four temperatures from (179.68 to 210.03) K and correlated by PR-vdW, SRK-vdW, PR-HV and SRK-HV models, and interaction coefficients as the functions of the temperature were obtained. With the comparison of the phase curve by different models, PR-HV model had the highest accuracy and its calculation difference for pressure and composition of vapor phase were given.6. Experimental investigation on the VLE of the binary system CH4+CF4The measured VLE data of binary system CH4+CF4 at five temperatures from (159.61 to 178.93) K were correlated by PR-vdW, SRK-vdW, PR-M and SRK-M models, and interaction coefficients were attained. With the comparison of the phase curve, PR-M model had the highest accuracy in low temperatures and its detailed difference were given.7. Experimental investigation and prediction on VLE of the trinary system CH4+CF4+C₂H₆With the limitation of the experimental rig, only isothermal experiments can be done. To validate the reliability of VLE data for two binary systems, the VLE data of trinary system CH4+CF4+C₂H₆ were measured at five temperatures from (165.74 to 184.08)K. Because there were many measured VLE data for the binary system CH4+C₂H₆, they were not repeated in this paper. After verified by thermodynamics consistency, four isothermol VLE data in a temperature range from (158.15 to 189.65)K were selected from the DETHERM data base and used to obtain the interaction coefficients. After comparison of phase curves for PR-vdW and SRK-vdW models, PR-vdW model was choosen for its higher calculation accuracy. So, the pressure and composition of vapor phase for the trinary system were calculated by PR-vdW model, with the superposition of these three binary systems. In the end, by the interaction coefficients obtained above, the isothermal and isobaric phase curves were set up and the trend of deformation of three sections (including vapor, liquid and vapor-liquid coexisting sections) was found.