Chapter 8 - Exergy analysis of drying processes and systems

Abstract

Exergy analysis can help reduce irreversibilities and increase the efficiency for drying processes. Increased efficiency reduces the energy required by drying systems for the production, transportation, transformation, and distribution of various energy forms. Exergy analysis is a powerful tool for optimizing drying conditions, and is particularly important for large-scale high-temperature drying applications in industry. This chapter describes and illustrates the exergy analysis of a drying process applied here to moist solids. It defines exergy efficiency as a function of heat and mass transfer parameters. An illustrative example is presented to demonstrate the importance of exergy methods for the analysis and optimization of drying processes and also to highlight the sensitivities of the results to such parameters as drying air temperature, moisture content, humidity ratio, and specific exergy; the exergy difference between inlet and outlet products; and product mass. An important aspect of designing drying technology is the mathematical modeling of the drying processes and equipment. Accurate modeling allows design engineers to choose the most suitable operating conditions and then size the drying equipment and drying chamber accordingly to meet the desired operating conditions. The main exergy losses for drying are associated with irreversibilities and are described qualitatively for three different types of industrial drying methods, namely, air drying, drum drying, and freeze drying. The study presents energy and exergy analyses of drying processes. The systems are illustrated with input and output terms, where there are four major interactions. Exergy analysis is demonstrated to be a significant tool for the design and optimization of drying processes.