Identifying Azeotropes

When the liquid and vapor phase of most mixtures are in equilibrium, the composition of the liquid phase is different from the composition of the vapor phase. This difference is why we can separate mixtures using distillation.

For some mixtures, the compositions of the liquid phase and vapor phase at equilibrium are equal. The temperature, pressure, and compositions at which this equality occurs is called the azeotropic point and we say that the mixture forms an azeotrope.

For example, the black curve in the figure to the right shows equilibrium compositions of acrylonitrile (a) and cyclohexane (b) at temperatures ranging from the boiling point of acrylonitrile to the boiling point of cyclohexane. The x axis shows the mole fraction of acrylonitrile in the liquid phase. The y axis shows the mole fraction of cyclohexane in the vapor phase. The figure shows that if we have an equilibrium mixture with a liquid phase containing 0.20 mol fraction of acrylonitrile, then the vapor phase will contain approximately 0.45 mol fraction of acrylonitrile. (The red circle. Note that this point corresponds to a temperature of approximately 32 °C.)

However, the figure shows that when the liquid phase acrylonitrile mol fraction of 0.48, the equilibrium vapor phase will also have an acrylonitrile mol fraction of 0.48. This is thus the mixture's azeotropic point. (At 30.03 kPa.)

To determine if a mixture forms an azeotrope, we can compare the slopes at the beginning and end of the mixture's y-x compositions curve. If both slopes are either greater than one or less than one, then the curve must pass through a point where both slope equal one, i.e., an azeotropic point. (See below for a derivation of this test.)

The equations for the slopes are also given in the figure. These equations require values for the vapor pressure and activity coefficients of both components in the mixture. Note that these properties are evaluated at the infinite dilution endpoints. At the endpoints the temperatures correspond to the boiling points of components 'a' and 'b'.

Molecular Structure Entry

Use the structure editor to the right to enter the stucture of your chemical of interest. The structure will be sent to a WebServer Edition of Cranium running in the cloud. Cranium will calculate the entered chemical's boiling point, vapor pressures, and infinite dilution activity coefficients. These values will be returned to this webpage for display and for calculation of the two limiting slopes. The table below shows these values and the estimate of whether or not an azeotrope is formed with the listed solvent.

To estimate the physical properties of chemical, begin by drawing the chemical's molecular structure in the editor to the right. (Press the editor's demo button to view a short video explaining the editor's operation.) Once you have drawn the molecular structure, press the 'Get Estimates' button in the sections below. The entered structure will be sent to a WebServer Edition of Cranium running on the internet. Cranium will use group-contribution and equation-based techniques to generate physical properties estimates that will be returned to this webpage and shown below. (Note that the estimation process may take several seconds to complete depending upon the complexity of your molecular structure.)

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Calculating Properties for Azeotrope Determination
Solvent (a) Pvp (a) γ,inf (a) Slope (a) Pvp (b) γ,inf (b) Slope (b) Azeotrope
How It Works

The Get Estimates buttons above retrieve your input molecular structure, temperatures, and pressures, package these into a request string and then send it to an instance of our Cranium, Web Server Edition software product running on a Microsoft Azure virtual machine. Cranium processes the request - dissecting molecular structure and estimating physical properties. The resulting estimates are then sent back to this webpage for display.

Click here to learn more about how you can use our Cranium Web Server to distribute your company's physical property data, estimates, and knowledge throughout your organization or contact us for further details.