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
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%.
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.