BACKGROUND There is a special type of azeotrope in the chemical industry. The components can form homoazeotrope under atmospheric pressure and azeotropic temperature but show partial miscibility when the temperature decreases. A two-column/decanter process was presented to separate the mixture. However, the energy consumption and the total annual cost are high. RESULTS Based on the decanter temperature, the operating parameters of the conventional process and the modified process were optimized to achieve the minimum total annual cost (TAC). Through comparing the two processes, the minimum TAC of the latter one is 14.2% lower than that of the former one when the decanters of the two processes are at 10 degrees C. A control structure, which can efficiently control the modified process, was also explored and verified. CONCLUSION For the conventional process and the modified process, the influences of the decanter temperature on the compositions of the layered liquid phases, the total reboiler duty of the columns, and the minimum TAC demonstrated that the decanter temperature has important impacts on energy cost and economy of the two processes. Furthermore, the modified process using two cooling media in combination can further reduce the TAC. (c) 2019 Society of Chemical Industry