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

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Mass Transfer
As the reflux ratio in a distillation coloumn is increased from the minimum, the

Liquid flow increases while the vapor flow decreases for a system
Slope of the operating line in stripping section decreases
Number of plates decreases very slowly first and then more and more rapidly
Total cost first decreases and then increases

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