Computers and Chemical Engineering 25 (2001) 807 – 820 Design and synthesis of multicomponent thermally coupled distillation flowsheets Ben-Guang Rong *, Andrzej Kraslawski, Lars Nystro ¨m Department of Chemical Technology, Lappeenranta Uniersity of Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland Received 10 May 2000; accepted 5 January 2001 Abstract The design and synthesis of thermally coupled distillation flowsheets for separations of five-component mixtures are studied. Four types of possible configurations are identified when simple and complex columns are both considered in a flowsheet. A universal design procedure is developed for design of any types of the identified configurations based on the abstraction of the three basic units in the flowsheets. Two examples demonstrated that this shortcut design method can be used in design of any types of the identified multicomponent thermally coupled distillation flowsheets, as well as give very good initializations for rigorous simulation of such configurations. Moreover, with a proposed computer representation of all the types of the feasible configurations, a synthesis algorithm is developed for synthesizing of multicomponent complex distillation flowsheets with both simple and complex columns. It is practical by the proposed methods for optimal design of multicomponent distillation systems in an extended search space to include the complex distillation flowsheets for industrial problems. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thermally coupled distillation flowsheets; Multicomponent mixture; Design; Synthesis Nomenclature Variables bottom product flow rate (kmol/h) B complex distillation flowsheet CDF COC annual capital costs, 10 4 $ (per.annum) COP annual operating costs, 10 4 $ (per annum) column distillate flow rate (kmol/h) D feed flow rate (kmol/h) F liquid flow rate of rectifying section (kmol/h) L L c liquid flow rate of coupling stream (kmol/h) liquid flow rate of stripping section (kmol/h) L  total number of theoretical trays N t N rt tray number of stripping section P b column bottom pressure (atm.) column top pressure (atm.) P d pressures at the main column locations of thermal coupling streams P i P  i pressures at the side column locations of thermal coupling streams Q thermal quality of feed stream Qc heat duty of condenser (10 6 kcal/h) www.elsevier.com/locate/compchemeng * Corresponding author. Fax: +358-5-6212199. E-mail address: benguang.rong@lut.fi (B.-G. Rong). 0098-1354/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0098-1354(01)00673-1