The pressure-composition phase diagram for two immiscible liquids, benzene and water, is at constant temperature. Set the temperature with a slider to change the saturation pressures. Set the overall benzene mole fraction with buttons on the right. The bar graph shows the moles of liquid water (blue), liquid benzene (orange) and vapor (green), which contains both components. The system contains one mole total. You can change the piston height to change the pressure and the amounts of each phase. At a given temperature, all three phases co-exist at only one pressure. When the piston height increases at this pressure, one of the liquid phases completely evaporates before the pressure decreases. Select "piston-cylinder" instead of "amounts of each phase" to view a representation of a piston-cylinder with the volume of each phase on a log scale.

Immiscible components do not mix in the liquid phase, and each exerts its own saturation pressure \(P_b^\text{sat}\) and \(P_w^\text{sat}\) when present as a liquid; the subscripts \(b\) and \(w\) refer to benzene and water. The total pressure \(P\) above the two immiscible liquids is equal to the sum of their saturation pressures: $$P_i^\text{sat}=10^{(A_i-\frac{B_i}{T+C_i})},$$ where \(A_i\), \(B_i\), and \(C_i\) are Antoine constants for component \(i=(b,w)\), and \(T\) is temperature (°C).

For the benzene-water system, for conditions where only benzene condenses, the dew point curve is: $$P=P_b^\text{sat}/y_b ,$$ where \(y_b\) is the mole fraction of benzene in the vapor phase.

For conditions where only water condenses, the dew point curve is: $$P=P_w^\text{sat}/y_w ,$$ where \(y_w\) is the mole fraction of water in the vapor phase.

This simulation was created in the Department of Chemical and Biological Engineering, at University of Colorado Boulder for LearnChemE.com by Rachael Baumann under the direction of Professor John L. Falconer and was converted to HTML5 by Drew Smith. This simulation was prepared with financial support from the National Science Foundation. Address any questions or comments to learncheme@gmail.com. All of our simulations are open source, and are available on our LearnChemE Github repository.

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