One mole feed of a binary mixture of a given composition is flash vaporised at a fixed P and T. If Raoult's law is obeyed, then changing the feed composition would effect
A. The product composition but not the fraction vaporised
B. The product composition as well as the fraction vaporised
C. The fraction vaporised but not the product composition
D. Neither the product composition nor the fraction vaporised
Answer: Option C
This Question Belongs to Chemical Engineering >> Mass Transfer
The correct answer is:
B. The product composition as well as the fraction vaporised
✅ Explanation:
In flash vaporization at fixed pressure and temperature, the feed is partially vaporized and reaches vapor-liquid equilibrium (VLE).
If Raoult’s Law is obeyed, the system is ideal, and equilibrium compositions depend on:
Temperature
Pressure
Feed composition
When you change the feed composition:
The equilibrium shifts, so:
The vapor and liquid compositions (product compositions) will change.
The fraction vaporized also changes because:
The relative volatilities change with composition.
The lever rule (used on a T-x-y or P-x-y diagram) shows that the fraction of vapor depends on the feed composition.
Therefore:
Changing feed composition at constant P and T affects:
The vapor and liquid product compositions
The fraction of the feed that vaporizes
✅ Correct Answer: B. The product composition as well as the fraction vaporised