Systematic Design and Optimization of a Membrane–Cryogenic Hybrid System for CO₂ Capture

CO₂ capture is a promising way of offloading the impact of fossil fuels on global warming. Although various techniques have been proposed for CO₂ capture, the main obstacle remains the economic performance. This paper focuses on designing a novel CO₂ capture process, the hybrid membrane–cryogenic system, and optimizing its economic performance. The optimization of such a hybrid separation system involves synthesis of separation sequences, design of heat exchanger networks, and optimization of compression/expansion networks. Each of the subsystems is a complex system, and there are tight interactions between the subsystems. Therefore, it is challenging to synthesize these subsystems. To optimize this complex hybrid carbon capture process, a novel two-step approach is proposed. The separation sequence, as well as energy consumption, is optimized first. Then, the remaining heat exchanger network is synthesized with flowsheet optimization simultaneously. An iterative procedure is proposed to build a solution pool, which serves as a bridge between the two steps. In addition, a discrete cutting method and a piecewise approximation approach are introduced to improve the reliability and validity of the two-step model. Case studies show that the optimized hybrid system can improve the economic performance for both precombustion and postcombustion carbon capture cases. It is especially suitable for precombustion CO₂ capture, where 33% of operating cost reduction and 28% total annualized cost reduction are achieved.