Abstract
Work and heat are two significant forms of energy in chemical industries, and complex relationship exists in these strongly interacting properties (heat, work, temperature and pressure). Therefore, the research on the simultaneous integration of work and heat has a significant impact on the overall energy-efficiency improvement of the process. In this paper, a mixed-integer nonlinear programming (MINLP) model aiming at the minimum exergy consumption is formulated to obtain the optimal network configuration, which reveals the interactive mechanism between the expanders/valves placement and the heat exchanger networks synthesis. An enhanced stage-wise superstructure that involves pressure manipulation both in stages and between stages along with heat integration for each pressure-change sub-stream in stages is proposed in this paper. These sub-streams from one stream must concurrently perform pressure manipulation in the same stage and then return to their parent state at the end of the stage by adopting non-isothermal mixing. It is quite essential for the expanded streams to optimise the selection of end-heaters and end-coolers to meet their desired temperature targets. In addition, the multi-stream expansion problem can also be solved using the proposed method. A case study from the literature is used to demonstrate the proposed method. The results are consistent with the literature solutions. Moreover, the proposed method can successfully perform another case study with multi-stream expansion, which is not solved using the GCC-based graphical approach.
