VLE and flash calculations
After midterm · Session 1 of 6 · 180 minutes
30 September 2026
The question for today
How does pressure change the phase split at fixed temperature?
Before class: 3-page reading · 15–20 minutes
Bring the three preparation answers. Revisit your prediction after one controlled comparison and an independent check.
The 180-minute class
| Opening question and essential concepts |
30 |
| One hand calculation |
20 |
| One lab demonstration |
20 |
| Break |
10 |
| Student pair investigation |
70 |
| Discussion, exit ticket and feedback |
30 |
The 70-minute investigation is student work. Final block: discussion 20 min, exit ticket 5 min, feedback 3 min, closing 2 min.
Read the phase diagram first
Predict the phase at 110 kPa before calculating.
Boundaries and amounts answer different questions
P_{bubble}=\sum_i x_iP_i^{sat},\qquad \frac{1}{P_{dew}}=\sum_i\frac{y_i}{P_i^{sat}} z_i=(1-\beta)x_i+\beta y_i,\qquad y_i=K_ix_i
The boundary equations assume ideal phases at fixed T. A TP flash supplies T, P and z, then finds compositions and amounts.
In a nonideal liquid, K depends on the unknown liquid composition.
A reproducible ideal baseline
Lab 01 synthetic binary at 350 K: P_1^{sat}=150 kPa and P_2^{sat}=60 kPa. Choose ideal liquid, z_1=0.5, P=95 kPa.
| Bubble P at x₁=0.5 |
105 kPa |
| Dew P at y₁=0.5 |
85.7143 kPa |
| Flash x₁, y₁ |
0.388889, 0.614035 |
| Vapor fraction β |
0.493506 |
The values are synthetic definitions, not real-fluid measurements.
An independent check
Independently close both component balances and confirm that all phase compositions sum to one. Explain the phase state using bubble and dew pressures.
One demonstration · 20 minutes
Open the core lab
Use the synthetic ideal mixture at T=350 K, P=95 kPa and z1=0.5. Save the baseline, then change only P to 110 kPa. Predict the phase before calculating.
Keep one saved baseline for the pair investigation.
Pair investigation · 70 minutes
- 10 min: predict and record the baseline.
- 25 min: change one input and calculate.
- 20 min: exchange roles and check the result by hand.
- 15 min: prepare one plot or table and a short explanation.
In Lab 01, save one ideal two-phase baseline and one changed pressure at the same T and z. Predict the change, report phase amounts and check one component balance by hand.
Discussion · 20 minutes
How does pressure change the phase split at fixed temperature?
Compare results and discuss unexpected behavior. Use one numerical check to support your explanation.
Exit ticket + feedback · 8 minutes
5 min · Explain and check
Answer the session question in the form. Give one result or independent check from today’s lab, with units.
3 min · Help improve the next class
Rate the pace, report lab access, and suggest one improvement.
Thai or English. No name, ID or email requested.
Wait for “Response received.” We will use the final 2 minutes to close the class.
