3 replies to this topic

[absolute0]

I have a project on ternary VLE calculations due soon. The information provided is: Pressure (1 atm) and the molar vapor phase composition for all three components. Additionally, all the values needed to calculate Psat and the activity coefficient are given. I am assuming the fugacity coefficient is negligible based on a previous project's results.

So for the equation y_i * P = x_i * γ_i* P_i^sat , y_i and P are known.

My question is whether or not my plan to solve for the liquid phase and Temp make sense.

1. I plan on guessing T, plugging that into the Antoine Equation to get P_i^sat.
2. With the P_i^sat I will calculate xi with the assumption of Raoult's law.
3. Knowing x_i, I can use our three parameter margules equation to solve for γ_i.
4. Using γ_i I will recalculate x_i using the top equation
5. Then again calculate γ_i.
6. Go through steps 4 and 5 until little difference in iterations is observed.
7. With γ_i, x_i, and P_i^sat (and y_i already known), I will calculate P.
8. Continue changing the temperature till calculated P equals real P (1 atm).

There are two issues with this. We are asked to use the SRK EOS to see how well the equation of state describes the vapor phase. Since yi is given, I am not sure how to do this. If I find correct temperatures using the above plan, I could then solve the SRK, but I would have three roots. How do I know which root is the volume of my vapor? And even if I find that volume, how do I check the calculated volume against the given molar vapor compositions?

The second issue is of the three parameter margules. We are told A_i = sum(x_j * A_ij) from j = 1 to 3, which means I will need to know a value for A_ii ( A₁₁, A₂₂, A₃₃). We are only supplied with A_ij(A₁₂, A₁₃, A₂₁, A₂₃, etc.) values. How do I calculate A_ii?

Edited by absolute0, 23 October 2011 - 09:46 PM.

[breizh]

Hi ,
Aii=o


Hope this helps

Breizh

[absolute0]

Would that be because interactions between molecules of the same component should have no effect on mixing (since that isn't mixing)?

[S.AHMAD]

For the SRK:
1. The z that give biggest volume shall be vapor.
2. The z value that give smallest shall be liquid
3. Discard the one in-between
4. Alternatively, you can re-arrange the SRK into volume and solve directly for Volume.

Edited by S.AHMAD, 23 October 2011 - 11:30 PM.