Question

One hundred mole/min of an equimolar mixture of 1-propanol and 2-propanol at 75C and 200kPa is sent to a flash distillation unit operating at 75C and 75 kPa.

a.) Assuming ideal liquid and ideal vapor mixtures, prove that the mixture will separate into liquid and vapor states at the flash operating condition.

b.) Determine the resulting composition and molar flow rate(s) of the exit stream(s).

c.) Comment on the appropriateness of the ideal liquid and vapor mixture assumptions for these two compounds.

Answer 1

Vapour pressure of pure 1-propanol at 75C is 300 mmHg or 40kPa and that for 2-propanol is 560mmHg or 75 kPa (taken from wiki)

P_tot = xP^o₁ + (1-x)P^o₂; x is the mole fraction of 1-propanol in vapour

a) since the external pressure is almost equal to the vapour pressure of 2-propanol and much higher than that of 1-propanol, the mixture will separate into liquid and vapour and the vapour predominantly contains 2-propanol.

b) the vapour contains, 75 = x.40 + (1-x)75 => x =0, vapour contains pure 2-propanol. Hence, the flow rate of vapour is 50 mole/min (note the it is a equimolar mixture)

c) This liquid mixture may not behave as ideal liquid and ideal vapour as there is an attraction force between molecules through H-bonds due to polar OH group.