For the distillation of a binary system, if the relative volatility is constant and the feed is a saturated liquid, show that the minimum reflux ratio is given by the following equation: $q=1$

$R_{\text{m}\text{i}\text{n}}=(\frac{1}{-1})(\frac{x_D}{z_f}-(\frac{1-x_D}{1-z_f}))$