(37f) Evaluating and Fine Tuning Cubic Equations of State for Supercritical Water Processes

Numerous equations of state have been developed over the years and new ones are continuously being proposed. The performance of these models in terms of the accuracy of predicting the state of matters has improved over time. Nevertheless, no perfect model has been found that is capable of predicting the state of matter accurately under all circumstances. The application of equations of state always involves a compromise between the efficiency and the accuracy. More often than not, cubic equations of state, especially with three parameters, are chosen due to their simple mathematical form, being easy to apply, and reasonably good accuracy. In the present study, the performance of the well-known Peng-Robinson and Patel-Teja equations of state has been examined in terms of their ability to predict the behavior of water around the critical area, including the saturated region from 0.01°C to critical point and the superheated region (10-60 atm, 400-600°C) . The measures compared include the specific volume, the enthalpy departure and the entropy departure. Both EoS's work reasonably well in terms of predicting the specific volume with Patel-Teja's EoS being clearly the better of the two. However, in terms of predicting the enthalpy departure and entropy departure the two EoS's are much more matched in most cases. The highest discrepancy observed always occurs at the critical point, which is about 47% for the specific volume, 28% for the enthalpy departure and 43% for the entropy departure. Using an acentric factor of 0.5491 instead of its genuine value in Peng-Robinson EoS for water, reduces the discrepancies dramatically, generally to about one third or less. However, it is difficult to retune PT-EoS for water.