How to simulate 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.
- 1Open the ready-made model
Open the "Distillation column startup dynamics (feed-rate step)" 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 NRTL property package over ethanol, water — 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
- NRTL
- Components
- ethanol, water
- Unit operations
- COL
Opens live on the canvas — free, no install.
Explore the model & flowsheetFrequently asked questions
- What does the Distillation column startup dynamics (feed-rate step) model simulate?
- 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.
- Which thermodynamic method does it use?
- The NRTL property package, over ethanol, water — 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. Distillation column startup dynamics (feed-rate step) 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).
Feed-tank level control (live loop)
A feed surge tank holding water at a target level: a level transmitter reads the tank's live inventory and a PID controller trims the upstream feed rate to hold it. Solves steady-state normally (the level loop is a no-op there); open the pid_controller's Live tab ("From canvas") to run the closed loop in real time and watch a setpoint change drain or fill the tank against a live PID.
CSTR thermal runaway (Arrhenius exotherm)
Adiabatic ethylene oxide hydrolysis to ethylene glycol in a CSTR — the classic reactor-safety teaching case (Fogler). Solves to a safe steady conversion at the design feed temperature; switch to the Dynamic solve mode and step the feed temperature up a few degrees to watch the exotherm and the Arrhenius rate feed back on each other (thermal runaway), self-limited as the reactant depletes.
Heat exchanger startup thermal lag
A hot ethanol-water process stream cooled against cold cooling water in a single counter-current exchanger. Solves the design steady state; switch to the Dynamic solve mode and step the hot feed temperature to see the classic HX thermal-lag response — the exchanger's metal wall smooths and delays the outlet-temperature change instead of tracking the feed step instantly.
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.