Fractional distillation separates multicomponent liquid mixtures based on differences in their thermodynamic relative volatilities. Inside the vertical column, ascending vapor generated by the bottom reboiler contacts descending reflux liquid across staged trays. At each equilibrium stage, mass and heat transfer occur: the more volatile component is stripped into the vapor phase, while the less volatile component concentrates in the liquid phase, producing high-purity overhead distillate and bottoms residue.