A method for improving equations of state near the critical point. (Volumes I and II)
Erickson, Dale Douglas
Ely, James F.
Doctor of Philosophy
This thesis develops better methods of predicting the thermodynamic properties of both pure components and fluid mixtures in the critical region. Accurate nonanalytic equations of state have been developed for carbon dioxide, methane, isobutane, butane and pentane for use as reference equations in the critical region, using a method originated by Fox which transforms any analytic equation of state into a nonanalytic equation of state as the critical point is approached. In the critical region, the resulting nonanalytic equations of state are more accurate for PVT, saturation and thermodynamic property estimation than the Schmidt-Wagner analytic equations of state developed at the National Bureau of Standards. Over the rest of the PVT surface, the nonanalytic equations of state are comparable to the Schmidt-Wagner equations of state. These accurate reference equations can be used to represent the thermodynamic properties of mixtures by using corresponding states methods, such as the van der Waals' one-fluid model. However, these corresponding states methods do poorly in representing the second virial contribution when there are large energy differences between the constituents which results in inaccurate thermodynamic property estimation. These methods fail, because the mixing rules are accurate to (1/T), where as the reference equations contain much higher orders of (1/T). A new corresponding states method was then developed which generates mixing rules in a generalized manner, but specifically for each individual reference equation. The basis of this new model is that each power of (1/T) in the reference equation requires its own mixing rule. This new method improves the representation of the second virial contribution while still behaving correctly at high densities. The improvement is most significant when there are large differences between the critical temperatures of the constituents. A new hard sphere expansion theory has been developed, referred to as hard sphere corrected theory. This theory uses the structure of the earlier hard sphere expansion developed by Mansoori and Leland, but is less sensitive to the diameter. For this reason, this new theory is able to accurately represent the properties of systems with large size ratios.
Chemical engineering; Mechanical engineering