<p>The theory of internal energy saving in the rectification process is presented. Formulas for the quantitative assessment of internal energy saving are provided for various energy potentials of the feed mixture introduced into the column. An analysis of the relationship between the vapor-to-bottoms ratio and the reflux ratio at different energy potentials of the feed mixture showed that the reflux ratio is the sole parameter determining the rectification process, specifically, heat consumption in the reboiler, cooling demand in the condenser, column dimensions, cost, and other factors. It is proven that increasing internal energy savings within the column reduces the total costs associated with the separation of liquid mixtures. The theory of internal energy saving supports the necessity of intermediate pump-around reflux in complex crude oil distillation columns and offers a means of its optimization.</p>