A mixture consisting of 1 mol% ethane, 5 mol% propane, 44 mol% n-butane and 50 mol% isobutane is brought to a condition of 70oF at pressure P. If the molar fraction of the system that is vapor is 0.20, what is the pressure P, and what are the compositions of the vapor and liquid phases ?
Hints:
Use the results from my solution to this problem in HW#2, based on the DePriester Charts, as a starting point for SRK. Solve for P, the 4 xi's, the 4 yi's and the two Z's (compressibility of the vapor and liquid phases). That is a whopping 11 unks. I reduced the problem to 9 unks by forcing the Σxi = 1 and Σyi = 1. The 9 eqns include 3 independent material balance eqns, 4 equilibrium eqns and 2 SRK eqns in terms of two Z's, A's and B's. Remember that A, B and Z are not the same in the liquid phase as they are in the vapor phase.
2 comments:
Do I actually use a flash calculation here? Or do I just do a mass balance? If so, I'm having a hard time getting 3 independent material balances.
Claire 12:30 AM:
There is no flash here, just bubble point and dew point calculations. But you could write 3 material balance equations, one on each species. These 3 equations are independent. But in this problem, you don't actually NEED to write any material balance eqns. The 5 eqns in this problem are the three Modified Raoult's Law (equilibrium) eqns and the 2 SRK (Z^3) eqns.
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