Supercritical Fluid Properties are integral to the development of applications.  The Pressure, Volume, Temperature (PvT) relationship is the starting point.

The simplest PvT relationship is the Ideal Gas Law (suitable for gases at high temperatures and low pressures).  However, SCFs are distinctly not ideal.  Over the years, numerous more complex Equations of State (EOS) have been developed.  Peng-Robinson EOS (PR-EOS) is one cubic EOS that has considerable popularity driven by maintaining reasonable simplicity while providing reasonable accuracy.  For carbon dioxide, NIST (NIST Webbook) currently uses the Fundamental EOS (FEOS) developed by Span and Wagner (1996).  The FEOS is far from simple but it is highly accurate with densities accurate to within 0.03 % around the critical region while accuracy diminishes to 2 % at pressures in excess of 4000 bar. For comparison, at 310 K and 100 bar, supercritical carbon dioxide's (SC-CO2) density is calculated at 170.76, 622.25 and 685.77 kg/m3 for the ideal gas law, PR-EOS and FEOS, respectively.   

The PvT relationship is the starting point.  Many supercritical properties derive directly from PvT and some indirectly.  Enthalpy and entropy are directly derived from PvT relationships and these directly impact any analysis of supercritical energy cycles.  Properties of viscosity, diffusivity and solubility are often correlated to pressure, temperature and/or density.

Enthalpy and entropy are effectively derivative properties of PvT EOS and with only delta enthalpy and delta entopy values matter.  Thus, subtle errors in PvT relations and errors that differ at different conditions are magnified in any energy cycle analysis.  

SCF CO2 PvT Properties

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