| Cryogenic distillation is the most widely adopted method to separateair components. Accurate thermophysical properties of air-product fluidsare the basis of designing device and process, which will certainlyinfluence the practicability and reliability of the system. As the demandson the amount of air products are significantly increasing, development ofair separation plantsis towards ultra-large scale and lower energyconsumption per unit of air product. Hence, it requires the support fromeven higher accuracy of thermophysical property data. In order tosupplement the experimental reference data of the thermodynamicproperties and improve the prediction accuracy in the critical region of theair component fluids, the present work focuses on the following sessions:1. Experimental data mining and disposing for thermodynamic propertiesSufficient reference data are necessary for developing empirical orsemi-empirical equations of state. p--T experimental data can reveal thebasic law of thermodynamic property of the fluids. It plays important rolein describing the existence state of fluids and in revealing the innerbehaviors of thermophysical properties. Experimental data, empiricalcorrelations and equations of state of pure nitrogen, oxygen and argonwere reviewed, upon which data distribution maps were plotted. Accordingto the uncertainty of the measurement reported in paper, this workanalyzed the average absolute deviation of these data.2. Development of program AirP for thermodynamic property calculationof Air fluidsBased on the Helmholtz free energy fundamental equation of stateand classical thermodynamics, a program for calculating air-componentsfluids’ thermodynamic properties was developed. AirP was coded inFortran for the physical and mathematical model, and in Visual Basic language for the graphical user interface （GUI）.3. Development of density measurement system based on capacitanceThe system consists of five sections: inflation and deflationconnecting pipe, capacitance measurement and acquisition system,temperature measurement and control system, pressure measurement andvacuum system. The temperature range is from80K to320K, andpressure up to3MPa. The total uncertainty in temperature and pressuremeasurement is5mK and350Pa respectively. This system could be usedto supplement the p--T experimental data in scanty area, and to study theliquid-vapor proportion in phase equilibrium state. Sophisticated densitymeasurement device could have important significance in describing theexistence state of new fluids.4. Experimental results and analysisComprehensive （p,, T） measurements on pure nitrogen, oxygen andargon have been carried out in the single-phase region and along the entirecoexistence curve. The results were listed in tables in this paper. Theexperimental results were compared with the results of previous workersand the calculation of fundamental equations of state. And then the relativeaverage absolute deviation were ascertained. Considering within theuncertainty in capacitance, temperature and pressure measurement, thetotal uncertainty of density measurements is evaluated to be about4.5×10-3.5. Research on critical thermodynamic propertiesThere exists large density fluctuations around the critical point, andmost thermal properties turned to be very complicated. However the meanfield theory and classical equations of state neglect these densityfluctuations. So if the critical phenomena was described by these theories,there would be great errors. Present work provided the accurate criticalindices by fitting the highest accurate experimental data to describe theproperties in the critical region. Grüneisen parameter was testified to beuseful in inspecting the validity of new equations of state around thecritical point. |
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