The limits of the ability of a distillation system to seperate a binary mixture are considered for two different cases. In the first case the heat supply solely takes place at the column bottom and the heat removal at the condenser. In the second case the heat supply and removal is distributed over the column height. For both cases, the limiting column capacity and the minimum heat consumption are related to the external stream compositions and to the heat and mass transfer coefficients. (C) 2017 Elsevier Ltd. All rights reserved.