Low Temperature Process Design: Challenges and Approaches for using Exergy Efficiencies

Abstract

Abstract In the past few years, our research group has been working on the development of practical and efficient tools and methodologies for the design of Low Temperature Processes (LTPs). These methodologies (as any other) require an effective evaluator to validate each step in the design procedure. Since emphasis is on conceptual design, it has been decided to use thermodynamic attributes rather than cost or thermoeconomics to evaluate process performance, and the obvious choice is exergy efficiency. In LTPs, the use of exergy efficiency is even more obvious due to the work requirement in refrigeration cycles (i.e. work is pure exergy). However, current exergy efficiencies can be difficult or even impossible to utilize. These difficulties are a consequence of the lack of standardization and the incorrect understanding of the exergy transformation inside the process or unit operation. This paper describes the problem stated above and addresses it by decomposing the exergy and studying its components independently. It is also shown how these exergy components behave above and below ambient conditions. Besides, the sink and source scheme is utilized to describe the exergy transformation inside a process or unit. New mathematical representations are presented to illustrate the exergy sink and exergy source for some typical unit operations in LTPs.