Dear Lorenzo Caminiti
For distillation of realtive ideal mixtures (similar components)
we have approximately that
(see
www.chembio.ntnu.no/users/skoge/publications/1999/DistillationTheory.pdf
ln a = dHvap/RTb dT/Tb
where
a - relative volatility
dHvap - heat of vaporization [J/mol]
R = 8.31 J/J mol
Tb - geometric mean boiling point for the two components
dT - boiling point difference
Typical value according to Troutons rule:
dHvap/RTb = 85 / 8.31 = 10.2
Example 1. Methanol (L) -Ethanol (H)
TbL = 337.8K, dHvapL=35.2 kJ/mol
TbH = 351.5K, dHvapH=40.7 kJ/mol
Get
Tb = sqrt(Tbl*TbH) = 344.6K, dHvap = 37.0 kJ/mol -> dHvap/RT = 12.90
and
a = exp (dHvap/RTb dT/Tb) = exp (12.90 * 13.7/ 344.6) = exp(0.51) = 1.67
whereas the xperomental value is about 1.73
Example 2. Want to find a mixture with a = 1.5.
Assuming Troutons rule holds this means that we want the relative boiling
difference point to be approximately
dT/Tb = ln(1.5) / 10.2 = 0.040
For example, if Tb=350K then the boiling point difference should be
dT = 0.040*350K = 13.9 K
(I have not tried to search for such components but there should be MANY
possibilities).
Best regards,
Sigurd Skogestad
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