An equation that relates the quantity to the concentration of the more volatile component in batch distillation. The equation is formulated from an unsteady-state material balance in which vapour leaving the still is in equilibrium with the liquid. As the vapour is richer in the more volatile component, the composition of the liquid and vapour are not constant:
where L1 and L2 are the initial and final number of moles in the still, x1 and x2 are the initial and final mole fractions of the more volatile liquid, and y is the mole fraction in the vapour phase. The integral can be integrated graphically or numerically using equilibrium data. It is named after Lord Rayleigh (1842–1919) who first derived it.