The thermally coupled distillation column systems can save energy and capital cost compared with traditional distillation columns. Since the thermally coupled column system was introduced, the design concerns have been peeped out due to more degrees of freedom. This paper introduces a new design method that can be used to determine the structure of thermally coupled distillation column systems, namely the number of stages in all sections of the column system. The design method employs the approximate group methods. To explore the design performance of the proposed design method, three feed systems are analyzed to investigate its usefulness. The design procedure gives the optimum structure for a given ternary separation; with given product specifications various design methods can yield approximately the same results. Like structure designs, the optimal internal flow distributions are examined. The results indicate that the method works well for a variety of process conditions. (C) 2014 Elsevier B.V. All rights reserved.