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Session 3213

Entropy: Esoteric Concept or Utility Infielder?

Rebecca K. Toghiani P.O. Box 9595 Department of Chemical Engineering Mississippi State University Mississippi State, MS 39762

Introduction

Entropy and the second law of thermodynamics are topics which are generally covered in the undergraduate thermodynamics courses in the chemical engineering curriculum, Heat engines and Carnot cycles are often used as examples and, while beneficial to understanding the overall implications of the second law of thermodynamics, do not serve to connect these important ideas to more realistic problems encountered by chemical engineers in other subject areas. The usefulness of entropy and the second law to aid in understanding chemical engineering unit operations is a topic which is often overlooked in the traditional undergraduate chemical engineering sequence.

The conservation of mass and conservation of energy are analogous to the pitcher and catcher on a baseball team: they each serve a single purpose. The second law, in comparison, is extremely versatile, and can be used for a variety of purposes, much like a utility infielder. One can use the second law to determine whether a process will operate as stated, to evaluate inefficiencies in process operation, and to determine process operating limitations among others. Unfortunately, the second law is often treated as a pinch hitter rather than a utility infielder: brought out of the dugout only when the starters do not yield the desired result. That the second law is sometimes not included as part of the starting team in routine evaluation of thermodynamics problems places students at a disadvantage.

The majority of general thermodynamics textbooks designed for undergraduate use [for example, Moran and Shapiro, 1995; Cengel and Boles, 1994] incorporate a significant amount of material on the second law and present a variety of applications of the second law through examples and chapter problems. In these texts, the examples and problems focus primarily on problems where the second law is utilized to analyze the irreversibility of a process or the performance of a device. The majority of these examples and problems deal with pure components. Mixtures are generally ideal gas mixtures and the examples and problems tend to be very similar in nature - evaluation of the entropy change of mixing or the entropy production when two pure gas species are mixed together.

The application of the second law of thermodynamics to separations problems is of particular interest to chemical engineers. Unfortunately, with currently available textbooks for instruction in undergraduate chemical engineering thermodynamics and mass transfer operations, this topic is often overlooked. There are advanced and/or graduate level texts where this material is discussed [King, 1980; Henley and Seader, 1981], Additionally, the second law is often presented in a manner such as to limit its application to realistic problems involving mixtures. It is in the application to these problems where the second law can provide enlightenment. Particularly

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Toghiani, R. K. (1996, June), Entropy: Esoteric Concept Or Utility Infielder? Paper presented at 1996 Annual Conference, Washington, District of Columbia. 10.18260/1-2--6035