© 2005, Carl T. Lira, for educational use only.
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Determination of stability when searching for entropy values.
Often when solving process problems, the entropy and pressure of a state are known, and the
remaining state variables are to be found. If this is the case, it is helpful to keep in mind the
behavior of the isobars. Whenever multiple real roots exist, the fugacity is used to determine
which root is stable as will be explained in this handout.
Figure 1 shows the behavior of the entropy values for ethane at 0.1 MPa as calculated by the
PengRobinson equation of state using a real gas reference state of
T
= 298.15K and
P
= 0.1
MPa. The figure was generated from the spreadsheet PREOS.xls by adding the formula for
entropy departure for the center root and then changing the temperature at a fixed pressure of 0.1
MPa. The corresponding values of S were recorded and plotted.
Figure 1. Entropy values for ethane calculated from the PengRobinson equation
along an isobar at 0.1 MPa. The largest
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 Spring '11
 Ku
 Thermodynamics, 18.185 J, 21.3021 J, 100.955 J, 184.2K

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