Example 05.19 VLLE of n-Butanol - Water at 50°C Using UNIQUAC


Calculate the miscibility gap for the system n-butanol (1) water (2) at 50°C and additionally the corresponding pressure and vapor phase composition using the UNIQUAC equation with the help of the UNIQUAC-parameters fitted to VLE data*. An ideal vapor phase can be assumed

* J. Gmehling, U. Onken, et al. Vapor-Liquid Equilibrium Data Collection, DECHEMA Chemistry Data

Series, Vol. I, 37 parts, Frankfurt starting 1977





input data

Relative VdW volumes and surfaces:

UNIQUAC binary interaction parameters:

Pure component vapor pressures:



The following function is used to calculate activity coefficients using the UNIQUAC model:

To plot the activity of component 2 vs. the activity of component 1, the following two functions are defined:

For convenience, the representation in the following plot contains a curve, red "x" every 1 mol% and a blue "+" every 10 mol%.

The immiscible region should be between about x1'=0.405 and x1''=0.984.

The exact solution can be found by solving the isoactivity criterion for both components:

The result should be verified:

The values of the activity coefficients for the liquid-liquid phase boundaries are:

The total pressure above these mixtures is:

The vapor phase composition can be calculated via the following function:

Identical results are obtained for both mixtures. As these are in thermal, mechanical and thermodynamic equilibrium, they must be in equilibrium with the same vapor phase. As the vapor phase mole fraction of component 1 lies between its mole fractions in the two liquid phases, the calculation yields a heterogeneous azeotrop at a liquid and vapor mole fraction of 0.20346. Literature data from the DDB (Version 2011) are shown in the plot below. Please note the different order of the components. The calculated value closely matches the literature information.