Integration of a New Process Into an Existing Site: A Case Study in the Application of Exergy Analysis

This paper presents a case study that demonstrates how exergy analysis can help make intelligent process design decisions in a logical manner. A proposed, new nitric acid plant is to be intergrated into an existing facility wherein a steam system produces part of the facility’s power needs and all of the process steam, at three pressures. The objective is to minimize the expenditure for fuel and power under two scenarios: when the ratio R of power to fuel costs is 2.5, and when the ratio is 4. The procedure used was first to carry out exergy analyses of the three principal subsystems: (1) the process utilization, (2) the existing steam/power generation, and (3) the proposed HNO₃ plant. Secondly, improvements of each of these subsystems were developed, independently, by systematically eliminating unnecessary thermodynamic inefficiencies. (For the steam plant and for the nitric acid plant, alone, improved reconfigurations have been presented earlier: Sama et al., 1988, 1989, respectively.) Finally, in this article, further opportunities for reducing inefficiencies, by integration of the three subsystems, were exploited. In the contractor’s original conceptual design with integration, the facility needs to purchase 77 MW of fuel and 2.2 MW of power. Improved designs developed here save equivalent fuel of over 45 MW when R=2.5 and over 60 MW when R=4 (i.e., 4.5 and 6.0 million dollars per year at a fuel cost of $100/kW-yr or $3.35/10⁶ Btu).