The economic feasibility of processes in the chemical industry is often determined by the energy and raw material demand, hence process efficiency is crucial. Most published methods to increase the efficiency of industrial processes are limited by a maximum possible degree of complexity of the respective processes. Thus, a systematic approach for the retrofit of existing processes is developed in the presented work, particularly for processes with industrial relevant complexity. The hybrid approach combines heuristic and mathematical methods. Due to recycle streams in the processes, retrofit modifications can affect the entire process, thus the operating points of all units have to be adapted. Therefore, a mathematical optimisation problem is formulated and a method based on the global optimisation algorithm molecular inspired parallel tempering (MIPT) is developed. In order to solve the optimisation problem, MATLAB (TM) and Chemcad (TM) are linked via an OPC interface. The developed method is successfully applied to an existing complex industrial world-scale process, which comprises a large number of components, including 18 unit operations and a considerable number of recycle streams. The presented retrofit leads to an increase of the efficiency of this complex process. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.