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Second Law Analysis to Improve Industrial Processes WILLIAM F. KENNEY Exxon Chemical Technology Department, Florham Park, NJ 07932
The methodology for Thermodynamic, or Second Law, Analysis of industrial processes has been discussed i n the previous ACS symposium and in a number of literature a r t i c l e s . This chapter w i l l attempt to provide some insight on using the data from such studies to achieve practical improvements i n the process indust r i e s . Examples w i l l be drawn from two recent presentations by the author (1-2) dealing with related aspects of finding and implementing economic improvements to plant energy efficiencies. The interaction of energy efficiency and capital cost w i l l be explored in some of these examples. The terms used i n this analysis are relatively conventional. A general thrust of the chapter w i l l be SIMPLICITY. Differences in terminology have been eliminated wherever possible. In this analysis A v a i l a b i l i t y , Available Energy, Exergy, and Work w i l l be used as equivalent. This means that kinetic and potential energy effects and the potential work to be derived from the diffusion of chemical species into equilibrium with the environment have been ignored. This simplification may introduce significant inaccuracies i n some studies, but is not important here. The intent is to demonstrate that simplified - perhaps even approximate - analysis can have valuable practical applications.
Table I. • • • • • • •
Methodology of Thermodynamic Analysis
Define Process Boundaries Obtain Consistent Heat and Material Balance Select Ambient Temperature, T Identify Consistent Source of Entropy Data Calculate Changes in Available Energy for the Overall Process Breakdown the Process into Subprocesses u n t i l Major Sources of Lost Work are Identified P r i o r i t i z e Areas for Improvement Q
0097-6156/83/0235-0051 $06.00/0 © 1983 American Chemical Society Gaggioli; Efficiency and Costing ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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SECOND LAW ANALYSIS OF PROCESSES
The methodology used was a l s o r e l a t i v e l y standard, but simplified. The steps i n the a n a l y s i s were c a r r i e d out i n the order above. On the assumption that the reader i s f a m i l i a r with the general methods of thermodynamic a n a l y s i s , these are presented without e x p l a n a t i o n . For f u r t h e r background, one can c o n s u l t reference (3). The c a l c u l a t i o n of process or equipment e f f i c i e n c i e s v i a second law a n a l y s i s w i l l not p l a y a l a r g e p a r t i n t h i s d i s c u s s i o n . In the authors' view, the r o l e of e f f i c i e n c y c a l c u l a t i o n s i s to help q u a n t i f y the p o t e n t i a l f o r improving a given segment of the process. By combining the magnitude of l o s s e s w i t h the e f f i c i e n c y of a given segment, a f e e l f o r the p r i o r i t y of studying that segment can be developed. For example, i n many cases there are l a r g e l o s s e s a s s o c i a t e d with major compressors i n i n d u s t r i a l s i t u a t i o n s . However, the e f f i c i e n c y of major machinery i s g e n e r a l l y very h i g h . Thus the p o t e n t i a l f o r s i g n i f i c a n t improvement i n the machinery s e c t i o n may be q u i t e low. In a l l c a l c u l a t i o n s only as much d e t a i l was developed as was needed to generate i n s i g h t . The process was broken down i n t o subs e c t i o n s to i d e n t i f y the sources of l o s s . Subsequent breakdowns continued only u n t i l a c l e a r p i c t u r e of the sources on i n e f f i ciency was developed. A check on accuracy was maintained by i n s u r i n g that the work balance, i . e . , W^ =W ^ - Wi
