Mass Transfer
Fenske's equation for determining the minimum number of theoretical stages in distillation column holds good, when the

Relative volatility is reasonably constant
Multicomponent distillation is involved
Mixture (to be separated) shows negative deviation from ideality
Mixture (to be separated) shows positive deviation from ideality

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Mass Transfer
Channelling is most severe

In towers packed randomly with crushed solids
In dumped packing of regular units
At very high liquid flow rate
In towers packed with stacked packing

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