Methanol column sized by design spec — a NRTL process flowsheet
A methanol-water column sized by design specification rather than by trial and error — Aspen's Design Spec / SimCentral's Adjust, and a capability that had no curated example despite being solver-native.
The column is specified the way a shortcut (FUG) column is: light and heavy keys with their recoveries (98% of the methanol overhead, 2% of the water). What is not specified is the reflux ratio. Instead the flowsheet carries a design_specs entry — vary COL.reflux_ratio until the metric shortcut_n_stages equals 14 — and the solver root-finds it, re-solving the whole flowsheet each trial. Open the Design Spec panel to see it, or the solved result: reflux settles at 1.06 for exactly 14.00 stages.
Why that is the interesting question. Reflux and stages trade off against each other, and against energy. Left at the initial 1.6, this separation needs only 10.8 stages but 4.35 MW of reboiler duty; pulled down to 1.06 it needs 14 stages and just 3.49 MW — a 20% energy saving bought with three more trays. A design spec lets you state the column you can afford to build and have the solver tell you how to run it, instead of guessing a reflux ratio and reading off whatever height falls out.
The Sensitivity tab comes preloaded with that trade-off as a sweep (reflux ratio against reboiler duty) so the curve behind the single design-spec answer is one click away.
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Modeling assumptions & limitations
- 1** This is the Fenske-Underwood-Gilliland shortcut, not a rigorous stage-by-stage solve: `shortcut_n_stages` is Gilliland's correlation, so the 14 stages is a sizing estimate to take into a rigorous run, exactly as DSTWU feeds RadFrac. A design spec is an outer root-find over the whole flowsheet and runs in sequential / tear-EO modes only.
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- Rigorous NRTL thermodynamics, solved by the same engine every simulation runs on.
- 1 unit operations modeled: COL.
- Focus areas: Design spec, Shortcut column, FUG, Reflux vs stages.
- Thermodynamics
- NRTL
- Components
- methanol, water
- Unit operations
- COL
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("methanol-column-design-spec")
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
Acetone recovery — batch still vs. continuous column
A spent-solvent stream (60 mol% acetone, 40% water) recovered two ways, so the Analysis panel's Batch distillation (Rayleigh) tool has something to be compared against — it had no curated example, and a batch calculation only means anything next to the continuous alternative. The flowsheet is the continuous answer: a shortcut column recovering 98% of the acetone overhead and rejecting 98% of the water, giving 59.8 mol/s of distillate at high purity for 4.4 MW of reboiler duty. The batch answer (Analysis > Batch distillation). Charge 100 mol of the same mixture to a still and boil it down until the pot falls to 10 mol% acetone: you distil 67.4 mol and the average distillate is only 84.2 mol% acetone. That is the Rayleigh result and it is the whole lesson — a simple batch still has one theoretical stage, so the vapour it makes is enriched but never pure, and the composition drifts the entire time. Reaching column-grade purity in a batch needs reflux and cuts (a batch rectifier), or a second pass. Why the comparison is the point. Batch equipment is cheap, flexible, and right for small or campaign volumes; the column is right when the duty is continuous and purity matters. Having both numbers on the same feed is what makes that a decision rather than a preference. **.
Shortcut column sizing (Fenske-Underwood-Gilliland)
Before anyone builds a rigorous column they size it in about a second, and this is that step — Aspen's DSTWU, the calculation every distillation design starts from. State the two keys and how well you want them separated, and the shortcut answers the four questions that set the capital cost. For a depropaniser feed (30% propane, 40% n-butane, 30% n-pentane) at 10 bar recovering 98% of the propane overhead and 98% of the n-butane in the bottoms: Fenske gives 8.34 minimum stages — the count at total reflux, where you spend infinite energy to buy the fewest trays. Underwood gives a minimum reflux of 1.54 — the reflux at infinite stages, the opposite corner. Neither is buildable; the real column lives between them, and Gilliland interpolates: at 1.3x the minimum reflux you need 17.7 theoretical stages, with Kirkbride putting the feed on stage 7.85. That is a little over twice the minimum stage count for 1.3 times the minimum reflux, which is the trade every column is designed on. The energy follows: 1.67 MW reboiler against a 1.35 MW condenser. Bounded, and this is the important part. These are screening numbers, not a design. The shortcut assumes constant relative volatility and constant molal overflow — it cannot see a pinch, a tangent, an azeotrope, or a temperature profile. Its job is to give a rigorous MESH column a starting point that converges, not to replace it. Run the distillation unit op on 18 stages with a feed at 8 to check it.
Ethylene glycol plant: closed water loop + multi-effect evaporator dehydration
The water-integrated evolution of the fiber-grade MEG plant. Two changes turn the once-through EO/glycol chain into a real, water-economical process: (1) the recovered process water is RECYCLED back to the hydration reactors through a purge splitter. A pure recycle is inventory-singular (reactors are keyed on the shrinking EO pool, so water consumption is fixed regardless of how much water circulates), so an 8% purge pins the loop and makes it well-posed, cutting fresh water makeup from 10 to ~1.75 mol/s (a >80% reduction). (2) The bulk dehydration is done by a genuine TWO-EFFECT EVAPORATOR TRAIN (forward-feed, real steam economy: effect 1's low-temperature vapor is the heating steam for effect 2) doing rigorous (P,H)-flash water removal, rather than a single spec-based split. The recycle converges through the solver's Wegstein tearing (~15 outer passes) and MEG still comes out fiber-grade (>=99.9%). HONEST SCOPE: the reaction chemistry is rigorous stoichiometry (real atom balances, ~90/9/1 selectivity) and the evaporators are real energy-balanced flash effects — but flash evaporation CANNOT reach glycol dryness without slipping glycol into the overhead vapor (MEG has a real vapor pressure at 90 C), so the evaporators run cool and only pre-concentrate. An overhead knockout returns the ~1-2% slipped glycol to the product (no yield loss), and a final vacuum-refining polish (still a spec-based split, representing the refluxed dehydration column a flash cannot replicate) removes the last water. This mirrors a real MEG plant's multi-effect-evaporator + vacuum-refining dehydration section. The product columns remain spec-based (a converged 99.9% fiber-grade MEG column is not tractable under Peng-Robinson here — the MEG/DEG relative volatility is too narrow for the wide-boiling MESH path).
Pressure-swing ethanol dehydration (Gᴱ mixing rule)
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.
Industrial water reuse: RO + membrane distillation to near-ZLD
A chemical-site water-reuse train of the kind large industrial water users build when the intake is salinising and freshwater is getting scarce: reverse osmosis makes reuse-grade water, the RO concentrate is warmed by low-grade waste heat and pushed further by membrane distillation, and the MD concentrate is crystallised to salt. Overall water recovery comes out at 94.9%. The point of the flowsheet is why MD is there at all. RO is pressure-driven, so its ceiling is osmotic pressure — and this feed hits that wall hard: at 25 bar the same 75% recovery is infeasible, and the model says so with a number rather than a shrug (osmotic pressure 27.2 bar against a 25 bar feed, a structured SPEC_THERMODYNAMICALLY_IMPOSSIBLE error, which is why the feed here runs at 45 bar). Membrane distillation is driven by a vapour-pressure difference instead, so only vapour crosses and there is no osmotic ceiling at all: it takes the RO concentrate from 2.0 mol% to 9.7 mol% salt, recovering 81% of the water RO had to leave behind, and its distillate is solvent-only. The waste-heat coupling is not decoration. At the RO concentrate's own 25 C the MD flux is about 1 kg/m2/h and the module is pointless; warmed to 60 C it runs at 21.5 kg/m2/h, inside the 10-50 kg/m2/h band real DCMD modules achieve. That is the entire commercial case for MD — it is a heat-driven process, so it is only cheap where low-grade heat is already being vented.
Site water reuse: RO + EDI polish + MD to near-ZLD
The full hybrid a large chemical site builds when it is reusing its own effluent rather than buying river water: reverse osmosis does the bulk desalting, electrodeionisation polishes the permeate to boiler-feed quality, and the RO concentrate is warmed by waste heat and pushed to salt by membrane distillation and a crystalliser. It is the companion to industrial-water-reuse-ro-md-zld, which stops at reuse-grade water; the delta here is the polishing leg, and it is a genuinely different technology rather than a tighter membrane. Why EDI and not more RO. RO is pressure-driven and pushes water, so its energy scales with the volume produced. Electrodialysis drags ions out under a field and the water never crosses, so its energy scales with the salt removed — which is why it is hopeless on seawater and excellent on an RO permeate that is already 99% desalted. This model shows that rather than asserting it: at the same stack and current the moles removed are fixed by Faraday's law regardless of how salty the feed is. The reason the polisher is resin-filled is the interesting part. Plain electrodialysis has a limiting current density proportional to the diluate concentration, so as the water approaches pure the boundary layer runs out of ions to deliver and the salt flux plateaus — ED cannot reach ultrapure at any voltage, and the voltage runs away as the water stops conducting. Packing the diluate compartment with ion-exchange resin fixes both: the resin carries the current, and the water splitting that occurs above the classical limiting current — a fault in a solution-filled stack, causing a pH excursion and scale — is exactly what regenerates the resin in place. That is why an EDI polisher needs no chemical regeneration, and why the same operating point means opposite things in the two stacks.