Mass Transfer
Steam distillation is not recommended to be used, if the

Material to be distilled is thermally unstable or has the tendency to react with other components associated with it, at the boiling temperature
Liquids decompose, if distilled directly at atmospheric pressure
Material can not be distilled by indirect heating even under low pressure, because of the high boiling temperature
Azeotropic mixture is to be separated and the final product is miscible with water

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Mass Transfer
To get high tray efficiency

Interfacial surface between liquid and gas phase should be large
Liquid entrainment should be severe
Gas velocity should be very low
Time of contact between the two phases should be less

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