How to simulate rate-based distillation (maxwell-stefan, chemsep-style)
A depentanizer split (n-pentane overhead from an n-pentane/n-hexane/n-heptane feed) solved with a **rigorous rate-based (nonequilibrium) stage model** rather than the usual equilibrium-stage assumption. Every stage carries separate bulk vapor and liquid compositions with a vapor-liquid interface in equilibrium and finite **Maxwell-Stefan mass-transfer fluxes** across each film, and the per-stage transfer coefficients come from the real Chan-Fair (1984) tray-efficiency correlations off estimated tray geometry — the exact physics ChemSep and Aspen RateSep are built on. The result: real trays lag equilibrium, so the finite-transfer distillate is measurably less pure (~98.7% C₅) than an equilibrium-stage model predicts (~99.8%) on the identical column — roughly 5x more hexane slips overhead. Open the equivalent equilibrium column ('ethanol-water-distillation' or any 'distillation' node) to see the gap the equilibrium-stage assumption hides. As the mass-transfer coefficients grow the model collapses back onto the equilibrium column (the built-in validation limit).
- 1Open the ready-made model
Open the "Rate-based distillation (Maxwell-Stefan, ChemSep-style)" model in the MaximaLabs workspace — no install, no license. It loads live on the canvas, ready to edit and run.
- 2Confirm the thermodynamics
This process is modeled with the PENG-ROBINSON property package over n_pentane, n_hexane, n_heptane — already selected, so the phase equilibrium and enthalpy are physically consistent from the first run.
- 3Review the flowsheet
The flowsheet chains COL. Every block is a real, solvable unit op you can reconfigure on the canvas.
- 4Run the simulation
Click Run. The deterministic solver converges the material and energy balances (recycles included) and fills the live stream table — the AI never invents a number.
- 5Read the results and iterate
Inspect the converged streams, tweak a spec, and re-run — or ask the AI copilot to explain a result or diagnose a failed solve in plain English.
- Thermodynamics
- PENG-ROBINSON
- Components
- n_pentane, n_hexane, n_heptane
- Unit operations
- COL
Opens live on the canvas — free, no install.
Explore the model & flowsheetFrequently asked questions
- What does the Rate-based distillation (Maxwell-Stefan, ChemSep-style) model simulate?
- A depentanizer split (n-pentane overhead from an n-pentane/n-hexane/n-heptane feed) solved with a **rigorous rate-based (nonequilibrium) stage model** rather than the usual equilibrium-stage assumption. Every stage carries separate bulk vapor and liquid compositions with a vapor-liquid interface in equilibrium and finite **Maxwell-Stefan mass-transfer fluxes** across each film, and the per-stage transfer coefficients come from the real Chan-Fair (1984) tray-efficiency correlations off estimated tray geometry — the exact physics ChemSep and Aspen RateSep are built on. The result: real trays lag equilibrium, so the finite-transfer distillate is measurably less pure (~98.7% C₅) than an equilibrium-stage model predicts (~99.8%) on the identical column — roughly 5x more hexane slips overhead. Open the equivalent equilibrium column ('ethanol-water-distillation' or any 'distillation' node) to see the gap the equilibrium-stage assumption hides. As the mass-transfer coefficients grow the model collapses back onto the equilibrium column (the built-in validation limit).
- Which thermodynamic method does it use?
- The PENG-ROBINSON property package, over n_pentane, n_hexane, n_heptane — already selected. You can switch the method on the canvas before running.
- Which unit operations are in the flowsheet?
- It chains COL. Every block is a real, solvable unit operation you can reconfigure, add to, or remove.
- Do I need to install software or buy a license?
- No. Rate-based distillation (Maxwell-Stefan, ChemSep-style) runs entirely in your browser on MaximaLabs — free, no install, no license. Open the model to load it live and run the deterministic solver.
More guides like this
Ethanol–water distillation
An 8-stage column concentrating ethanol overhead toward the azeotrope (the headline demo).
Tray efficiency — real trays vs ideal stages
The same ethanol–water column solved with a Murphree vapor tray efficiency of 0.7 instead of ideal equilibrium stages. A real sieve/valve tray never reaches full vapor-liquid equilibrium — the vapor leaving it only partly approaches the equilibrium composition with the tray liquid, mixing in un-equilibrated vapor from the tray below: y = E·K·x + (1−E)·y_below (Murphree 1925). At E = 0.7 each of these 12 trays does 70% of an ideal stage's work, so the overhead ethanol is lower than an equilibrium column of the same tray count would predict — which is exactly why a real column needs more trays than a shortcut (ideal-stage) calculation says. Both HYSYS and Aspen RadFrac expose this per-tray efficiency; set murphree_efficiency back to 1.0 to recover the ideal-stage column. The efficiency auto-selects the component-flow Naphtali-Sandholm solver (the reduced-form solvers carry no explicit per-tray VLE row to apply an efficiency to).
Distillation column startup dynamics (feed-rate step)
An 8-stage ethanol-water column at a reduced startup feed rate. Solves the steady state normally; switch to the Dynamic solve mode with weir/level-controlled hydraulics enabled and step the feed rate up (e.g. 6 → 9 mol/s) to watch the bottoms draw genuinely rebalance to the new throughput as the tray inventories fill — a feed-rate disturbance no fixed-hydraulics dynamic model (incl. this same column's own default rigorous mode) can show at all. Honest bound: the vapor traffic is held at its steady-state value in this mode, so the distillate draw (condenser-level-controlled off vapor inflow) does not move for a feed-rate-only step — only the liquid/bottoms side responds.
Perry Ch.13 Example 3: butane/pentane splitter
A simple two-cut distillation splitting butane overhead from pentane bottoms, from Chapter 13 of Perry's Chemical Engineers' Handbook.
Perry Ch.13 Example 4: sloppy-butane three-cut splitter
A three-cut column with a side draw producing a deliberately 'sloppy' (off-spec, cheaper-to-make) butane cut between a propane overhead and a pentane-plus bottoms, from Chapter 13 of Perry's Chemical Engineers' Handbook.
Acetone–water distillation
A 12-stage column recovers acetone overhead from a dilute aqueous solvent-recovery feed — a common industrial acetone/solvent-recycling duty.