SCM: COSMO-RS GUI Tutorial - 4.5: Binary mixture of Methanol and Hexane

4.5: Binary mixture of Methanol and Hexane

In this example a vapor-liquid diagram of Methanol and Hexane is calculated and compared to experiment.

First a binary mixture of Methanol and Hexane is calculated at a constant temperature of 333.15 Kelvin. Next this binary mixture is calculated at a constant pressure of of 1.01325 bar. Experimental pure compound properties are used.

Select View → Compounds
Click on the left side 'Methanol'
Enter '0.845' in the 'Pure compound vapor pressure:' field
Enter '333.15' in the 'at temperature:' field
Click on the left side 'Hexane'
Enter '0.77' in the 'Pure compound vapor pressure:' field
Enter '333.15' in the 'at temperature:' field
Select Properties → Binary Mixture VLE/LLE
Select 'Methanol' for the first compound
Select 'Hexane' for the second compound
Enter '100' in the 'Number of mixtures' field
Select Isotherm or isobar → isotherm
Enter '333.15' in the 'Temperature Kelvin' field
Select File → Run
Click 'Yes' when asked to 'Save changes (required to run)?'
Select View → Graph X Axes → x1, y1
Select View → Graph Y Axes → total vapor pressure

In case of a miscibility gap there are two molar fractions x1 and x1', for which both compounds have the same activities. In the calculation one can plot the activity a1 versus a2. If there is a closed loop, there is a miscibility gap:

Select View → Graph X Axes → a1: x1*gamma
Select View → Graph Y Axes → activities

Miscibility gap Methanol-Hexane at 333.15 K

Thus there is a calculated miscibility gap. If one investigates the point at which the curve crosses itself, one can deduce that the calculated miscibility gap is between approximately x1 = 0.275 and x1' = 0.811, with a calculated total vapor pressure of approximately 1.48 bar. Within the miscibility gap, the liquid mixture consists of 2 immiscible liquid phases, one is Methanol-rich, the other Hexane-rich. Note, however, that within the miscibility gap the COSMO-RS calculation further incorrectly uses a forced 1 liquid-phase instead of 2 immiscible liquid phases. Also note that a pressure-maximum azeotrope is in the miscibility gap.

Select View → Compounds
Click on the left side 'Methanol'
Enter '1.01325' in the 'Pure compound vapor pressure:' field
Enter '337.8' in the 'at temperature:' field
Click on the left side 'Hexane'
Enter '1.01325' in the 'Pure compound vapor pressure:' field
Enter '342' in the 'at temperature:' field
Select Properties → Binary Mixture VLE/LLE
Select 'Methanol' for the first compound
Select 'Hexane' for the second compound
Enter '100' in the 'Number of mixtures' field
Select Isotherm or isobar → isobar
Enter '1.01325' in the 'Pressure bar' field
Select File → Run
Click 'Yes' when asked to 'Save changes (required to run)?'
Select View → Graph X Axes → x1, y1
Select View → Graph Y Axes → temperature

There is a calculated miscibility gap between approximately x1 = 0.221 and x1' = 0.835, with a calculated temperature of approximately 323.3 Kelvin (50.1 °C). Within the miscibility gap, the liquid mixture consists of 2 immiscible liquid phases, one is Methanol-rich, the other Hexane-rich. Note, however, that within the miscibility gap the COSMO-RS calculation further incorrectly uses a forced 1 liquid-phase instead of 2 immiscible liquid phases. Also note that a temperature-minimum azeotrope is in the miscibility gap.

Experimental results for the Methanol-Hexane mixture were taken from Ref. [451]. These are compared with the calculated ones in the next graph. More experimental VLE data might also be found at Ref. [452].