What is operating line in distillation?

In distillation, the operating line represents the relationship between the liquid and vapor compositions in a distillation column, typically depicted on a McCabe-Thiele diagram. It reflects the mass balance and equilibrium stages in the column, illustrating how the concentration of the more volatile component changes as it ascends through the column. The slope of the operating line is determined by the reflux ratio and helps in designing and analyzing the efficiency of the distillation process. The intersection of the operating line with the equilibrium curve indicates the stages required for the desired separation.