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Directions

A pot containing a binary liquid mixture sits on a burner that supplies heat at a constant rate. The pot is open to the atmosphere, so as the liquid is heated it eventually boils and vapor escapes — the amount of liquid remaining in the pot decreases with time.

• Choose a chemical pair from the dropdown menu.
• Use the sliders to set the constant heat input rate, the initial amount of liquid, and the initial liquid composition (mole fraction of the more volatile component).
• Click Play to start. Reset returns to the chosen initial conditions.
• Watch the pot: the liquid level falls as it evaporates, and bubbles/steam appear once the liquid starts boiling. The thermometer shows the current liquid temperature.
• The top graph plots liquid temperature vs. time; the bottom graph plots the amount of liquid remaining vs. time.
• The heat input rate can be changed at any time while running. Changing the chemical pair, initial amount, or initial composition resets the simulation.

Watch how the temperature first rises quickly (sensible heating), levels off in slope once boiling begins, and then keeps slowly increasing during boiling — even though the heat input never changes — because the more volatile component evaporates preferentially, leaving behind a liquid that is richer in the higher-boiling component.

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Details

Assumptions

• The liquid and vapor behave ideally (Raoult's law) at a total pressure of 1 atm.
• The pot is well mixed: the liquid has a uniform temperature and composition at every instant.
• Heat losses are neglected; all of the burner's heat input, \(\dot Q\), enters the liquid.
• Before boiling begins, all of \(\dot Q\) raises the sensible temperature of the liquid at constant composition.
• Once the bubble point is reached, the liquid remains at its (slowly rising) bubble-point temperature, and essentially all of \(\dot Q\) vaporizes liquid — this is the differential (Rayleigh) distillation approximation. The much smaller amount of heat needed to raise the bubble-point temperature as the composition drifts is neglected.
• Heats of vaporization and liquid heat capacities are treated as constants, evaluated near each pure component's normal boiling point.

Vapor–liquid equilibrium

Each component's vapor pressure follows the Antoine equation (\(T\) in °C, \(P\) in mmHg):

The liquid's bubble-point temperature at composition \(x\) (mole fraction of the more volatile component) satisfies

and the vapor in equilibrium with it has composition

Sensible heating stage (liquid amount \(n\) and composition \(x\) constant):

Boiling stage — molar rate of vaporization and open-system material balances:

Because the more volatile component evaporates preferentially (\(y > x\)), \(x\) falls with time, so the bubble point — and hence \(T(t)\) — keeps rising during boiling even though \(\dot Q\) is held constant. Once essentially all of the more volatile component is gone, the remaining liquid boils at the normal boiling point of the less volatile component until the pot is empty.

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About

This simulation was created in the Department of Chemical and Biological Engineering at University of Colorado Boulder for LearnChemE.com by John L. Falconer using Claude AI. It was prepared with financial support from the National Science Foundation (DUE 2336987 and 2336988). Address any questions or comments to LearnChemE@gmail.com.