Graphical representation of separation performance and the exergy loss of all subprocesses in a distillation column on material-utilization diagram (MUD) are demonstrated to disclose behaviors of distillation columns. The diagram displays flowrates of liquid and gas on abscissa and logarithm of mole fractions, ln x and ln y, on ordinate. By using this diagram, separation performance, i.e., concentrations of light component in the distillate and of heavy component in the bottoms, can be elucidated. This diagram is also useful for qualifying and quantifying the exergy loss in a distillation column, since the four kinds of exergy losses, i.e., losses due to mixing in liquid phase, due to mixing in vapor phase, and due to evaporation of light components and condensation of heavy components through phase change, can be displayed compactly in a single figure. The effect of the change in feed flowrate is also demonstrated. (C) 2004 Elsevier Ltd. All rights reserved.