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

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

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Mass Transfer
McCabe-Thiele method

Uses molal units for material and energy balance
Can use any type of units
Is more accurate than Ponchan-Savarit method
Uses weight fractions to express liquid and vapour composition

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