Exergy-Based Efficiency Analysis of Pyrometallurgical Processes
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PYROMETALLURGICAL processes consume natural resources and emit waste products on a large scale. However, these resources are not inexhaustible and the ecosystem’s capacity to absorb waste streams is limited.[1,2] Faced with major challenges such as peak oil and climate change, people become increasingly aware of the need for more sustainable industrial development. Reducing the ecological footprint of the pyrometallurgical industry is therefore a key challenge.[3] Recent research confirms that, under the right socioeconomic conditions, efficiency optimization of industrial processes can be an important step toward increased industrial sustainability.[4] In general, the efficiency g of a process corresponds to the ratio of the total amount of useful production over the total amount of input this production requires. This is depicted in Figure 1. However, a quantitative determination of this efficiency ratio is not straightforward for pyrometallurgical processes. Because these processes often handle both mass and energy streams, they are described with separate mass and energy balances. BART KLAASEN, Research Assistant, PETER-TOM JONES, and DIRK DURINCK, Researchers, and PATRICK WOLLANTS and BART BLANPAIN, Professors, are with the Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Heverlee (Leuven), Belgium. Contact e-mail: bart. [email protected] JO DEWULF, Professor, is with the Research Group Environmental Organic Chemistry and Technology, Universiteit Gent, B-9000 Ghent, Belgium. Manuscript submitted August 21, 2009. Article published online August 21, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
However, these descriptions do not allow an unambiguous determination of a relevant process efficiency for two reasons. First, there is a dimensional problem. Because mass streams and energy streams are expressed in different units (kg vs. J), the calculation of a single process efficiency is not possible. A second reason is that these balances are based solely on the laws of mass and energy conservation. The second law of thermodynamics, which imposes a theoretical upper limit on the efficiency of a process, is ignored. However, this law has important practical, economical, and ecological implications that have been discussed in detail by Ayres.[5] A process description based on the thermodynamic quantity ‘‘exergy’’ is more suited for process efficiency analysis. Basically, the exergy content of a process stream describes to what extent the stream can be used to drive the process, taking into account thermodynamic limitations as imposed by both the first and second law. In addition, using an exergetic approach solves the dimensional problem because exergy can be calculated and expressed in joules (J) for energy streams as well as for mass streams. In this way, it is possible to determine a single, relevant exergetic efficiency ratio that can be used for consistent process comparisons. Although the thermodynamic concepts behind exergy already were developed by Carnot and Gibbs in the 19th
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