09.03 - Shortcut Estimation of Saturation Properties

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Shortcut estimation of thermodynamic properties (sample calculation) can be very quick and sometimes reasonably accurate.(6min, uakron.edu) As a follow-up exercise, it is suggested to adapt the shortcut vapor pressure equation in combination with Eqn. 2.45 and the pathway of Fig. 2.6c to rapidly estimate stream properties. Briefly, all you need is an "IF" statement that checks whether the T is less than Tsat at the given P. If so, then H=Href+CpΔT+Hvap. If not, then H=Href+CpΔT. This can be a quick and convenient method to estimate stream attributes of a process flow diagram. One equation per cell and you're done. This sample calculation illustrates the process for the heat duty of a butane vaporizer and compares the PREOS to the methods of Chapter 2 (ie. Eq. 2.45 etc.)

Comprehension Questions: Suppose you want to tabulate the entropy (S) of your stream attributes by this approach.
1. How would you compute the Sig(T,P)-Sig(Tref,Pref) contribution?
2. How would you compute ΔSvap?
3. Compute "S" for propane at 355K and 3MPa relative to the liquid at 230K and 0.1MPa by this approach.
4. Compute "H" for propane at 355K and 3MPa relative to the liquid at 230K and 0.1MPa by this approach.
5. Compute H and S for the same conditions/reference using the PREOS.
6. Explain the discrepancies between the two approaches. e.g. compare the Hvap values and the (HV-Hig) values, where HV represents the enthalpy of the vapor phase, not the heat of vaporization (Hvap).

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