Methanol column sized by design spec

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.

Honesty notes. 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.