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Properties of supercritical water oxygen mixtures Wang, Shuo


Supercritical water oxidation (SCWO) is a new process of waste treatment. The use of water and oxygen in SCWO makes it necessary to examine this mixture's thermodynamic properties, especially in the supercritical region around 25MPa. The Redlich-Kwong-Soave (RKS) and Hard-Sphere equations of state (EOS) are used to predict the fractions of the components and the constant pressure heat capacity of the supercritical water-oxygen mixture. The parameters in Hard Sphere EOS are adjusted for pressure of 25MPa while the parameters in the RKS EOS are taken from research papers (Abrams, D. S. and J. M. Prausnitz, AIChEJournal, Vol.21, No.l, 116-128(1975); Dahl, S. and M. L. Michelsen, AIChE Journal, Vol.36, No.12, 1829-1836(1990)). Phase boundaries vary with pressure and temperature. For phase boundary calculations, the results of both models are better for lower pressure. At the pressure of 50MPa, the Hard Sphere EOS has more accurate results than the RKS EOS. The RKS EOS predicts the heat capacity more accurately than the Hard Sphere EOS at the pressures of 24MPa and 26MPa. At 24MPa for 2% oxygen mixture, the error of the peak value is about 15% for RKS EOS and about 40% for the Hard Sphere EOS. For the RKS EOS, the peak position differences with experimental results are within 2-3°C. Specific volumes are needed to verify the equations of state. A venturi was used to measure the density of the mixture. At a fixed mass flow rate, the pressure difference is proportional to specific volume. The diameter of the tube in the SCWO pilot plant is 6.2mm. The throat diameter is 3mm. Results show that the sound speed is much higher than the velocity at the throat when the mass flow rate of 0.0367kg/s is applied. The pressure difference between the inlet and the throat of the venturi can be measured by the transducer currently available in the lab. Measurements in the supercritical region are more stable than those in the subcritical region. Discharge coefficients were applied according to the water flow rates. The density predicted by the RKS EOS has better agreement to the experimental data in the supercritical region than that in the subcritical region. Excess volume is also calculated. It has a similar behavior to the heat capacity in that it reaches a maximum near the critical point. The peak varies with pressure and temperature.

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