Design of optimal heat exchanger network with fluctuation probability using break-even analysis

Abstract

Heat exchanger network (HEN), which is designed to achieve the maximum energy recovery (MER) involves the integration and interactions of multiple process streams. In a plant, the system operation may experience various disturbances such as changes in supply temperature and flowrates. Small disturbances on one stream can affect other connecting streams. To manage these disturbances, the process to process and utility heat exchangers with bypass streams installation are typically overdesigned, leading to higher capital investment. This study presents the cost optimisation of flexible MER HEN design which considers the fluctuation probability using Break-Even Analysis (BEA). Stream data is extracted for the Pinch study and assessment for flexibility and MER was performed. The MER heat exchanger maximum size (MER-HEM) able to handle the most critical supply temperature fluctuations while minimising the utility consumption is calculated. However, the overdesign factor can affect the total annualised cost (TAC) at a certain probability of fluctuation occurrence. Besides that, the fluctuations experienced by the stream can result in the utility increasing or decreasing. Therefore, the MER heat exchanger original size (MER-HEO) is favoured when the fluctuation resulted in the utility cost increasing. The BEA is performed to determine the probability that results in high savings of the TAC and developed an optimal HEN design of MER-HEM or MER-HEO. The break-even point (BEP) from BEA indicate the exact fluctuation probability at which the TAC of MER-HEM and MER-HEO is the same. A case study with fluctuation probability over one-year operation is used to demonstrate the methodology. Application of the proposed methodology on the case study shows that the optimum size of heat exchanger can be determined and the additional savings of TAC can be achieved.