Energy transfer diagram for site-wide analysis and application to a kraft pulp mill

• Energy degradation in industrial system can be represented with the energy transfer diagram.
• The diagram indicates the flow rate of transferred energy as a function of the level of energy quality.
• The level of energy quality is equal to the derivative of exergy with respect to enthalpy.
• The diagram has been applied for the site-wide analysis of a kraft pulp mill.

In industrial processes, heat is transferred from the heating utilities to the environment or converted to another form of energy. Process operations and heat exchanges decrease the level of energy quality; i.e., heat is cascaded to lower temperatures. The energy transfer diagram (ETD), which has been recently developed, indicates the flow rate of heat transferred from the utilities to the environment through each process operation and each heat exchanger as a function of temperature. This tool can be used to identify heat savings projects by modification of the process units and the heat-exchanger network (HEN). However, in a larger sense the diagram indicates the flow rate of energy transferred through any system as a function of the level of energy quality, and can be used for the analysis of a complete plant, including the thermal, chemical, mechanical and electrical transformations. This paper presents new developments to consider energy conversion with the ETD, the application to an entire kraft pulp mill, including the HEN, the process operations, and the utility system, and results from site-wide analysis.

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