Question

n-butane vapor and liquid are in equilibrium (boiling) at 18.66 bars. Use the Peng-Robinson equation of state to determine the saturated liquid and saturated vapor molar volumes. Also determine the saturated liquid molar volume using the Rachett equation. In addition, use the Pitzer correlation for the Virial EOS to estimate the saturated vapor molar volume.

Answer 1

b) Rachett equation:

V_sat=Vc Zc (1-Tr)^2/7

Tr=T/Tc= 386/425=0.9082

= 255*0.274(1-0.9082)^2/7= 35.31 cm3/mol

c)

Pitzer type correlations:

Z=Z⁰+wZ¹

Pr= 18.66/37.96=0.4915 ,Tr= 0.9082

At these reduced conditions Z⁰+wZ¹ from Lee kesseler tables we have

Z⁰=0.5384   Z¹= -0.1161 and

Z=0.5384+(0.2*-0.1161)=0.51518

Z=PV/RT=V=RTZ/P=83.14*386*0.51518/18.66=886 cm3/mol

Calculation of saturated vapor volume cm3/mol using excel. Same procedure is adopted for saturated liquid volume.