Mass Transfer
The main reason for dividing a tall packed tower into series of small towers is to

Avoid channeling
Reduce liquid hold up
Avoid flooding
Minimise the overall pressure drop

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Mass Transfer
If the solubilities of different components (in a liquid-liquid extraction system) increase with rise in temperature, then the temperature above which they dissolve completely is known as the critical solution temperature (CST or consolute temperature). If solubilities increase with decrease in temperature, then CST is the temperature below which they dissolve completely. If a binary system has no critical solution temperature, it implies that

The system comprises of miscible liquids
The system comprises of partially mis-cible liquids
The system comprises of an azeotrope
On heating, a vapor phase will appear ; while on cooling, a solid phase will appear

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