CO2/ethane extractive distillation with n-decane — a PENG-ROBINSON process flowsheet
CO₂ and ethane are close-boiling (both around 195 K at 1 atm) and form a near-azeotropic VLE at NGL-processing pressures, a known problem for straight distillation in CO₂-flood-associated gas and acid-gas-rich NGL streams. A heavy n-decane solvent shifts CO₂'s relative volatility enough to take it overhead in far fewer stages than an ordinary deethanizer would need; a second column then strips the solvent from the ethane bottoms for recycle. The extractive column runs at a moderately high reflux (R=4.5) — the operating point a genuinely close-boiling pair needs to hold a sharp CO₂/ethane split with a heavy solvent — and both columns converge.
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- Rigorous PENG-ROBINSON thermodynamics, solved by the same engine every simulation runs on.
- 1 unit operations modeled: 2× SR.
- Focus areas: Extractive distillation, CO2, NGL, Solvent.
- Thermodynamics
- PENG-ROBINSON
- Components
- co2, ethane, n_decane
- Unit operations
- 2× SR
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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("co2-ethane-extractive-distillation")
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
MCHT extractive distillation with phenol
Extractive distillation of methylcyclohexane (MCH) and toluene using phenol as the selective solvent, adapted from Tiverios and Van Brunt (Ind. Eng. Chem. Res. 2000, 39, 1614). Phenol raises toluene's relative volatility away from MCH enough to split an otherwise close-boiling pair; a solvent-recovery column then splits toluene from the phenol (reported as its own product rather than recycled).
Sulfolane extractive distillation of aromatics
Extractive distillation recovering benzene and toluene from a stabilized reformate's C₆-C₇ non-aromatics (represented by methylcyclohexane) using sulfolane, after Figure 10.2/10.6 of T. Brouwer (PhD thesis, TU Twente, 2021). A vacuum solvent-recovery column then splits the aromatics from the sulfolane (reported as its own product rather than recycled). Both columns converge (ED 44 iterations, SR 98, residuals ~1e-5): methylcyclohexane leaves overhead at 91.3 mol%, the aromatics product carries no sulfolane at all (0.00000 mole fraction) at 95.2% recovery of the feed's benzene and toluene, and the recovery column returns sulfolane 99.96% pure at 490 K. Both columns were originally mis-specified in the same way, and it is worth seeing once because the symptom looks like a thermodynamic failure rather than an arithmetic one. distillate_to_feed is a fraction of a column's TOTAL feed, and both were set above the amount of light key actually present, so each column was forced to drag its heavy key overhead to make up the flow. The ED asked for 0.15 of 140 mol/s = 21 mol/s of overhead when only 17.5 mol/s of non-aromatics exists, so 3.5 mol/s of aromatics had to come over — that was the 19.4% benzene lost to the raffinate. The recovery column asked for 0.42 of its bottoms against roughly a quarter of it being aromatics, so ~20 mol/s of solvent had to come over — that was the 42 mol% sulfolane in the product. Setting each to the light key's real share (0.125 and 0.265) removes both. The property method is the whole story here, and it is why this example ran unconverged for a long time. Extractive distillation exists because of solvent selectivity, so a cubic with van der Waals mixing rules — Peng-Robinson as this was originally written — cannot represent the one effect the column depends on; it was being asked to converge on a model that did not contain the physics. A plain activity model cannot be used either: sulfolane boils at 558 K, so a reboiler hot enough to strip it drives benzene past its 562 K critical point, where a gamma-phi formulation has no answer at all. What the system needs is both at once, which is exactly what a Ge mixing rule provides — PSRK is a cubic (no supercritical ceiling) whose mixing is driven by UNIFAC (real selectivity). pr-mhv1 converges here too, in 47 iterations.
Dilute acetic acid recovery by extractive distillation
Recovering acetic acid from a dilute aqueous stream (e.g. cellulose acetate or terephthalic acid plant wastewater) using diisobutyl ketone (DIBK) as an extractive entrainer. Because acetic acid and water form no azeotrope but have a relative volatility that flattens toward 1 as the mixture concentrates, straight distillation needs an impractically tall column and heavy reboil duty; the entrainer raises water's relative volatility so the extractive column takes water overhead in far fewer stages, and a second column then strips the entrainer from the acid bottoms for recycle.
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).
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).