Analysis of Heat Cascade Through Process Components to Reduce the Energy Consumption in Industrial Systems

Energy savings result in economic and environmental benefits. Site-wide energy analysis of industrial systems considers all plant components, such as unit operations, heat exchangers, boilers, and turbines. This article presents the analysis of heat cascade through these components to reduce the overall energy consumption. As a consequence of the two first principles of thermodynamics, heat from hot utilities transferred to a process is gradually cascaded to the environment through process operations and exchangers. The enthalpy rate curves shift to ambient temperature through operations and exchangers; the difference between the outlet and inlet enthalpy rate curves of a system is equal to the flow rate of cascaded heat as a function of temperature. Reducing the energy consumption implies decreasing the flow rate of cascaded heat through process operations or exchangers in the entire temperature interval between the hot utility and the environment. Herein, the heat cascade is first analyzed to identify process operation modifications in order to reduce the utility consumption. Second, the heat cascade through a plant, including a boiler, a turbine, process operations, and exchangers, is represented on a single diagram for site-wide energy analysis. Third, the heat cascade through several processes is analyzed to integrate an industrial site. The proposed method provides benefits in comparison to foreground/background analysis and total site analysis, which are two commonly used approaches to identify energy-saving opportunities for industrial symbiosis. Analysis of heat cascade through individual operations and heat exchangers is simple and powerful, and its concepts can be used in software to identify energy-saving projects.

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