Consider a distillation column with 10 stages, a partial reboiler, and a total condenser operating at . This column is used to separate a non-ideal mixture of ethanol and water. Deviation from ideal behavior in the liquid phase is taken into account by using the Wilson model and a modification of Raoult's law. The feed is a saturated liquid with a composition of 50 mole% ethanol. The feed is introduced at stage 8. You can change the values of the reboil and reflux ratios, and , respectively. This rigorous simulation computes the composition profile by solving both material and energy balance equations. For and , the results obtained are compared to those provided by HYSYS, a major chemical process simulator (http://www.aspentech.com/hysys/). HYSYS results for the composition of ethanol and water in the column are shown with open orange and blue circles, respectively. For such a feed (i.e. relatively rich in water) and for large values of and , the bottom is almost pure water and the distillate approaches the composition of the positive azeotrope between ethanol and water, which is equal to 89 mole% ethanol or 95 weight% ethanol.