Multi-objective optimization of a Rectisol process integrated with compression-absorption cascade refrigeration system and ORC for waste heat recovery

The Rectisol is an essential component of coal to synthetic natural gas (SNG)/ methanol process, and its efficient utilization of energy is crucial for enhancing the energy efficiency and reducing the energy consumption in the entire coal to SNG/ methanol process. Therefore, a novel waste heat recovery system consisting of compressionabsorption cascade refrigeration system (CACRS), organic Rankine cycle (ORC) and heat exchanger network (HEN) is proposed for Rectisol process to achieve the efficient utilization of energy. To realize the optimal design for the waste heat recovery system of Rectisol process, a multi-objective optimization model is established with the aim of minimizing the total annualized cost (TAC) and total exergy destroy, while simultaneously optimizing the integration configuration and operating parameters. To overcome the solving difficulties caused by large scale model and multiple optimization variables, a decomposition strategy is proposed and applied in this paper. The optimal design of a Rectisol process including 13 hot streams and 13 cold streams is conducted. Compared with the optimal schemes given in the cited literature, the total amount of waste heat recovery is increased by 19.2 %, the cooling water consumption and low-temperature cold utility requirement are reduced by 100 % and 1.7 %, respectively, demonstrating the superiority of the proposed method in efficiency improvement and energy saving. With the implementation of multi-objective optimization, the optimal scheme with both good thermodynamic and economic performances is obtained with TAC of 11,082,365 $/y and Extotal