Linking pinch analysis and bridge analysis to save energy by heat-exchanger network retrofit

Abstract Reduction of energy requirements in the process industries results in increased profitability and better environmental performance. Methods for heat exchanger network (HEN) retrofit are based on thermodynamic analysis and insights, numerical optimization, or combined approaches. Numerical optimization-based methods are highly complex and may not guarantee identification of the global optimum. Pinch analysis, which is an approach based on thermodynamic analysis and composite curves, is the most widely used in the industry. Its simplicity, the use of graphical tools, and the possibility for the user to interact at each step of the design process help identify solutions with consideration of practical feasibility. In the last few years, bridge analysis has been developed for HEN retrofit. It includes the following tools: (a) the definition of the necessary conditions to reduce energy consumption which are expressed in the bridge formulation, (b) a method for enumerating the bridges, (c) the representation of the flow rate of cascaded heat through each existing exchanger on the energy transfer diagram (ETD), and (4) the use of the Heat Exchanger Load Diagram (HELD) to identify a suitable HEN configuration corresponding to modifications. It has been shown that reducing energy consumption implies decreasing the flow rate of cascaded heat through the existing exchangers across the entire temperature range between the hot and cold utilities. The ETD shows all possibilities to reduce the flow rate of cascaded heat through a HEN. The objective of this paper is to link these tools with the composite curves and concepts from pinch analysis into a consistent method for HEN retrofit. First, the relations between the composite curves, the ETD and the HELD are described, and a method for HEN retrofit which combines the insights from pinch analysis with the recently developed tools is proposed. Then the method is used to reduce energy consumption in three case studies. Comparison between results from pinch analysis and bridge analysis shows that the latter identifies supplementary solutions to save energy. The proposed synthesis fills a gap in pinch analysis, and its concepts can be helpful in the development of software for HEN retrofit.

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