How to simulate 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.

Frequently asked questions

What does the Shortcut column sizing (Fenske-Underwood-Gilliland) model simulate?

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.

Which thermodynamic method does it use?

The PENG-ROBINSON property package, over propane, n_butane, n_pentane — 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. Shortcut column sizing (Fenske-Underwood-Gilliland) runs entirely in your browser on MaximaLabs — free, no install, no license. Open the model to load it live and run the deterministic solver.