Pressure-swing ethanol dehydration (Gᴱ mixing rule) — a PR-MHV1 process flowsheet
Ethanol–water is the classic azeotrope, and pressure-swing distillation breaks it without an entrainer: the azeotrope moves with pressure, so a low-pressure column and a high-pressure column pass each other's azeotropic distillate and each recovers a pure product. The whole process only works if the property package tracks that shift — which is exactly where a conventional package choice falls between two chairs.
This flowsheet runs the high-pressure column at 15 bar on pr-mhv1: Peng-Robinson with an MHV1 excess-Gibbs mixing rule, so the cubic equation of state gets its attraction parameter from NRTL's excess Gibbs energy instead of from a single binary interaction constant. Switch the thermo package (Solver menu) and compare the predicted azeotrope:
| package | 1 atm | 15 bar | valid at 15 bar? |
|---|---|---|---|
| NRTL | 0.891 | 0.802 | no — γ-φ is a low-pressure formulation (~10 bar) |
| Peng-Robinson (kij) | 0.586 | 0.613 | yes, but a kij cannot represent this azeotrope |
| pr-mhv1 | 0.949 | 0.798 | yes |
(mole fraction ethanol; the repo's DECHEMA-validated 1 atm anchor is 0.894.) At 15 bar pr-mhv1 lands within 0.005 of NRTL while remaining a genuine equation of state, whereas plain Peng-Robinson is off by ~0.19 and puts the azeotrope in the wrong place entirely. Selecting nrtl here also trips the applicability guard, which warns that the activity model is past its pressure ceiling and names the fix.
The flowsheet demonstrates the mechanism on itself. Drop the column pressure to 1 atm and re-run, changing nothing else: the solve fails with SPEC_THERMODYNAMICALLY_IMPOSSIBLE, because at atmospheric pressure the requested bottoms purity sits beyond the azeotrope and no column can reach it. At 15 bar the same specification converges and the bottoms leaves at x_EtOH ≈ 0.924 — past the atmospheric azeotrope of 0.894, which is precisely the composition an atmospheric column cannot cross.
Modeling assumptions & limitations
- 1`pr-mhv1` is *less* accurate than NRTL at 1 atm (0.949 vs 0.891) — MHV1 is the right tool for the pressurized column, not a replacement for a fitted activity model at ambient pressure; multi-package flowsheets (`thermo_overrides`) let a real pressure-swing pair use each where it belongs.
- 2Only the high-pressure column is drawn here — the low-pressure column and the recycle that close the loop are left out to keep the thermodynamic point legible.
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- Rigorous PR-MHV1 thermodynamics, solved by the same engine every simulation runs on.
- 2 unit operations modeled: PUMP, Hpcol.
- Focus areas: Azeotrope, Pressure swing, Gᴱ mixing rule, MHV1, High pressure.
- Thermodynamics
- PR-MHV1
- Components
- ethanol, water
- Unit operations
- PUMPHpcol
Opens in a new tab, loaded straight into the app — no setup.
Read the step-by-step guideReproduce this exact result from Python — the real client.get_example() → run_and_wait() path, not a mockup.
from flowsim.sdk import FlowSimClient
client = FlowSimClient()
example = client.get_example("pressure-swing-ethanol-ge-mixing")
sim = client.create_simulation(example["title"], example["flowsheet"])
result = client.run_and_wait(sim["id"])
print(result["status"]) # "converged"
streams = client.streams(sim["id"])Related models
Ethanol–water distillation
An 8-stage column concentrating ethanol overhead toward the azeotrope (the headline demo).
Ethanol dehydration (pervaporation)
A hydrophilic pervaporation membrane pulls water out of near-azeotropic ethanol — crossing the 89 mol% ethanol-water azeotrope that ordinary distillation cannot. Permeate is ~95% water; the retentate is dried past the azeotrope.
Extractive distillation (ethanol/water with ethylene glycol)
Near-azeotropic ethanol/water (85 mol% ethanol) can't be pushed past the 89 mol% azeotrope by ordinary distillation. A heavy, high-boiling entrainer (ethylene glycol) fed near the top of the column raises water's relative volatility enough to pull overhead ethanol past the azeotrope in one pass; a second column then strips water overhead from the glycol, regenerating it as the bottoms product. The classic extractive-distillation textbook case (Luyben, Ind. Eng. Chem. Res. 2006, 45, 4625).
Pressure-swing azeotropic distillation (methanol/acetone)
Methanol and acetone form a minimum-boiling azeotrope whose composition shifts noticeably with pressure — enough that two ordinary columns run at different pressures can cross the azeotrope without an entrainer. Column 1 (low pressure) takes pure acetone bottoms and a near-azeotropic overhead; that overhead is compressed into Column 2 (high pressure), whose new, shifted azeotrope composition lets pure methanol come off the bottoms (the recycle of Column 2's now off-azeotrope overhead back to Column 1 is reported as a product here rather than closed, the same open-loop simplification the acetone-water example already uses). Luyben, Ind. Eng. Chem. Res. 2008, 47, 2696.
Waste solvent recovery — extractive distillation (IPA/water)
A pharmaceutical waste isopropanol/water stream (near the ~68 mol% IPA azeotrope) is fed to an extractive column with a high-boiling entrainer feeding in a few stages above it; a second column strips the entrainer back out of the bottoms for reuse.
Anhydrous ethanol — extractive distillation past the azeotrope (NRTL)
Producing fuel-grade anhydrous ethanol from a near-azeotropic ethanol/water feed using ethylene glycol as the extractive entrainer. Ordinary distillation is capped at the 89.4 mol% ethanol azeotrope; the high-boiling glycol raises water's relative volatility so ethanol crosses its azeotrope and leaves the extractive column essentially pure, while a vacuum solvent-recovery column strips the water and recycles the glycol. This is modeled with the NRTL activity package on pinned DECHEMA binaries for every pair (ethanol/water, ethanol/glycol, water/glycol) — the non-ideal, azeotrope-crossing VLE a cubic equation of state structurally cannot reproduce (Peng-Robinson stays trapped at the ~89% azeotrope).