The single equilibrium stage
Distillation columns are tall stacks of equilibrium stages — but the idea starts with one. A flash drum holds a feed at a fixed temperature and pressure and lets it settle into a vapor and a liquid in equilibrium. The lighter (more volatile) components concentrate in the vapor; the heavier ones stay in the liquid.
Everything in vapor–liquid equilibrium — K-values, bubble and dew points, the whole basis of distillation — shows up cleanly in this one unit.
A flash drum is one equilibrium stage: feed in, vapor and liquid out, split by volatility.