If you are interested in modeling the vapor-liquid equilibrium for the carbon dioxide (1) + ethane (2) at 243.15 K. At this temperature, the vapor pressures of the components are as follows: P1sat = 1.52 MPa; P2sat = 1.06 MPa. This system also forms an azeotrope at x1 = y1 = 0.62 at this temperature. Which modeling technique would be best to use?

Question

A. Since the pressure is large and the components are small in size, the ideal solution for the liquid and the vapor phase would work best here.
B. Raoult’s law, since the components are small in size and the pressure of the system is low.
C. A phi-phi modeling approach using the Peng-Robinson equation of state with a k12 to fit the azeotrope would be reasonable.
D. A phi-phi modeling approach using the van der Waals equation of state with a k12 to fit the azeotrope would be reasonable.
E. A gamma-phi approach with the 1-parameter Margules equation as the gamma model and the virial equation as the phi model.

Answer 1

A. Incorrect. While the pressure is large and the components are (relatively) small in size, this is not an ideal solution in either phase.
B. Incorrect. The system is not an ideal solution and the pressure is high, so Raoult’s law would be inappropriate.
C. Correct. The Peng-Robinson equation would provide a reasonable model for the vapor pressure of both components and the k12 would model the azeotrope.
D. Incorrect. As in Chapter 8, the van der Waals equation does not adequately model the vapor pressures of the n-alkanes. Thus, another choice would be more appropriate.
E. Incorrect. While a gamma-phi approach could work here, the 1-parameter Margules equation would be a poor model for the liquid. Another choice would be better.