Estimation Methods Based on Cubic EOSs

FIGURE 10.8 Pressure-specific volume plot for propane, showing the vapor, liquid, and two-phase regions, and the 100°F equilibrium line (at 188.32 psia), as well as the 100 isotherm calculated by the BWR EOS. (Observe the logarithmic scale for specific volume and the arithmetic scale for pressure). See the text for a discussion of this figure. 

Once the idea that we could calculate pure species vapor pressures (i.e., pure species VLE) from an EOS was widely accepted, it was a fairly short step to attempting to calculate multispecies VLE from a single EOS [11, 12]. Most modern VLE calculation programs do that, using cubic EOSs. As discussed in detail in Appendix F, these are EOSs that can be written in the form z=/(T Py. and w) in fairly simple algebraic expressions, with z appearing only as z, z, and z. Cubic equations can be solved algebraically numerical solutions on spreadsheets are much quicker and easier. 

Example 10.3 Repeat Example 10.2, using the Soave-Redlich-Kwong (SRK) EOS. 

The necessary background for this calculation and the details of it are shown in Appendix F (Example F.5/10.3). The results are summarized and compared with those of Example 10.2 in Table 10.2.  

At this T and P pure ethane would be a gas and pure n-heptane a liquid. Neither could exist in the other state. 

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