This simulation computes and displays the Ponchon–Savarit (enthalpy–composition) diagram for a nonideal ethanol–water mixture at a pressure P that you select with the slider (50–1200 mmHg). The gas phase is assumed ideal; the liquid phase is nonideal. Enthalpy is expressed in kcal/kg and composition is expressed in mole fraction of ethanol.
Click "Ponchon-Savarit diagram" to show the saturated-liquid and saturated-vapor enthalpy curves together with tie lines (green) that connect the liquid and vapor compositions that are in equilibrium at a given temperature; each tie line is labeled with the boiling temperature in °C. The tie line at the azeotropic composition is shown in red and is vertical, since the equilibrium liquid and vapor compositions are equal there.
Click "equilibrium curve" to show the ethanol vapor–liquid equilibrium (x–y) curve on a mole-fraction basis, together with the y = x diagonal (green) and the azeotrope (red point).
Click "mole/mass" to toggle both graphs between expressing ethanol composition as a mole fraction and as a mass fraction; the composition axis labels update to match whichever basis is selected.
Click "download" to save a PDF with the Ponchon-Savarit diagram on top and the equilibrium curve below it, both drawn for the currently selected pressure and composition basis (mole or mass fraction).
Move the pressure slider to see how the enthalpy curves, the tie lines, the equilibrium curve, and the azeotrope change with pressure.
The Ponchon–Savarit diagram provides a graphical method for solving coupled material and energy balances in separation processes involving binary mixtures. This simulation illustrates how the Ponchon–Savarit diagram can be computed for a nonideal binary mixture that has an azeotrope.
Consider a binary mixture of ethanol and water at a low to moderate pressure, P, to be selected by the user. The gas phase is assumed ideal, but the liquid phase is nonideal. The simulation plots the Ponchon–Savarit diagram, which is also called the enthalpy–composition diagram. Enthalpy is expressed in kcal/kg and composition is expressed in mole fraction of ethanol. Also shown is the vapor equilibrium curve for ethanol–water.
Enthalpies for the liquid and vapor phases are computed using the following equations:
where T is the mixture temperature and xethanol is the ethanol mole fraction. Here, λi is the latent heat of vaporization of chemical species i at Tref = 273.15 K, and CP,L,i and CP,V,i are the heat capacities for the liquid and gas phases of species i, respectively; λi is a slowly-varying function of temperature and can be assumed constant [3]. The mole fractions of ethanol in the liquid and gas phases are denoted by xethanol and yethanol, respectively. Finally, HE is the excess enthalpy, which accounts for the nonideal behavior of the liquid mixture. The following correlation from [2] is used to compute the excess enthalpy as a function of composition and temperature:
Here, am(T) = bm + cmT + dmT2, where m = 0, 0.5, 1.5, 2.5, 4.5 and bm, cm, and dm are appropriate constants tabulated in [2].
The Ponchon–Savarit diagram for P = 760 mmHg is in full agreement with Figure 2-4 in [1].
The "tie lines", displayed in green, show the composition of the two phases that exist in equilibrium at the specified temperature. At the azeotropic composition the tie line is shown in red; it is vertical, confirming that xethanol = yethanol at the azeotrope. At P = 760 mmHg, the azeotrope composition in terms of ethanol mole fraction is 90.5% and the boiling temperature is T = 78.16 °C.
The vapor–liquid equilibrium data used to build both curves are obtained from a modified Raoult's law, with vapor pressures from the Antoine equation and liquid-phase activity coefficients from the van Laar model, solved at each liquid composition for the bubble-point temperature at the selected pressure.
[1] P. C. Wankat, Separation Process Engineering, 3rd ed., Upper Saddle River, NJ: Pearson, 2012.
[2] J. A. Larkin, "Thermodynamic Properties of Aqueous Non-Electrolyte Mixtures I. Excess Enthalpy for Water + Ethanol at 298.15 to 383.15 K," Journal of Chemical Thermodynamics, 7(2), 1975, pp. 137–148.
[3] R. H. Perry and D. W. Green, Perry's Chemical Engineers' Handbook, 7th ed., New York: McGraw–Hill, 1997.
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 is a JavaScript/HTML5 implementation of a Mathematica simulation by Housam Binous, Nadhir A. Al-Baghli, and Brian G. Higgins. It was prepared with financial support from the National Science Foundation (DUE 2336987 and 2336988). Address any questions or comments to LearnChemE@gmail.com.