Integrated exergy load distribution method and pinch analysis

Abstract

A new thermodynamic criteria of chemical process performance, intended exergy yield is proposed. The criteria is used for the simultaneous assessment of the internal and external exergy losses and of the performance yield of a chemical process. The link between the overall intended exergy yield of a process and exergy yields of the unit operations which constitute the process is established using the exergy load distribution method. The integration of pinch analysis to the thermodynamic analysis of a process through the exergy load distribution method is demonstrated in the case of a process improvement study. The process used for the illustration of this combined approach is a typical hydrogen production unit by methane reforming. Pinch analysis with exergy targeting is used to treat the heat exchanger network (HEN) as a single unit operation, thus greatly simplifying the graphical representation of the exergy analysis and necessary computations. It is shown that this method of exergy analysis helps to identify process changes which are likely to produce a desired effect with minimum computational efforts. For the case treated, it is found that a reduction of 15°C of the high temperature shift reactor and a 0.05% purity improvement of the absorber effluent produce a 2% increase in hydrogen yield at the expense of 1.4% of the exportable steam without equipment modifications.