A shortcut method used to find the minimum number of theoretical trays in a distillation column required for the separation of two key components. It is calculated from the mole fraction, x, of the key components in the distillate, and in the bottoms where the relative volatility of the key components is reasonably constant. For a binary mixture of components a and b, the minimum number of trays is:

where N_mn is the total minimum number of theoretical trays, σ_ab is the relative volatility and the subscripts D and B represent the distillate and bottoms, respectively. The method was proposed in 1932 by American professor of chemical engineering Merrell Robert Fenske (1904–71).