Exergy Availability Analysis

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

THERMOECONOMICS is the branch of thermal sciences that combines a thermodynamic (exergy) analysis with economic principles to provide the designer or operator of an energy-conversion system with information which is not available through conventional thermodynamic analysis and economic evaluation but is cmcial to the design and operation of a cost-effective system. Thermoeconomics rests on the notion that exergy (available... 

Of different nature is the entropy balance dealt with in Chapter 6. It is not a balance in the proper sense because no quantity is conserved. In fact it is a method of thermodynamic analysis of the processes in the system. More familiar to engineers is the notion of exergy (availability). The (necessarily positive) production of entropy in any node is proportional to the loss of exergy in the node the goal is to reduce the loss as far as possible. The chapter gives a number of examples of such thermodynamic analysis, along with critical comments. 

Accessible work potential is called the exergy that is the maximum amount of work that may be performed theoretically by bringing a resource into equilibrium with its surrounding through a reversible process. Exergy analysis is essentially a TA, and utilizes the combined laws of thermodynamics to account the loss of available energy. Exergy is always destroyed by irreversibilities in a system, and expressed by... 

M. V. Sussman, Availability (Exergy) Analysis (Lexington, MA Mulliken House, 1981). 

Part II, "Thermodynamic Analysis of Processes" (Chapters 6 through 8), discusses the thermodynamic efficiency of a process and how efficiency can be established and interpreted. A very useful thermodynamic property, called exergy or available work, is identified that makes it relatively easy to perform and integrate the environment into such an analysis. Some simple examples are given to illustrate the concept and its application in the thermodynamic or exergy analysis of chemical and nonchemical processes. 

We wish to alert the reader that in the analyses presented above, the results were essentially independent of the type of fuel used. From an efficiency point of view, this may be true, but from a sustainability point of view, it is not. In general, gas is a much cleaner burning fuel than coal and requires less pre- and posttreatment. Even though the standard power generation plants can be made more efficient using thermodynamic analysis (lost work, availability, or exergy analysis), we note that power generation based on fossil fuels is not sustainable since the combustion of these fuels leads to increased... 

Next, Cornelissen extended the LCA study to include the effect of depletion of natural resources making use of ELCA, the exergetic life cycle assessment. In this analysis a full mass and energy balance was made, that is, a first law analysis. Exergy values for all mass and energy streams were included in accordance with the Tables 6.1, 6.2, 6.3, 7.1, 7.2, 7.3 and 7.4 in Chapters 6 and 7. This analysis clearly showed where work was available in inputs and outputs and where it was lost. He could show that the cup scored less favorable than the mug in terms of depletion of natural resources (817 MJ vs. 442 MJ). 

In 2002, Wassenaar [67] made an analysis of the fossil fuel requirements of the Dutch potato industry and its main products. The products are of course, in terms of their "energy" that is, available work or exergy, of solar origin. Therefore, he introduced the concept of "fossil load factor," defined as the number of fossil fuel exergy units per exergy unit of potato-based useful... 

A general thrust of the chapter will be SIMPLICITY. Differences in terminology have been eliminated wherever possible. In this analysis Availability, Available Energy, Exergy, and Work will 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 in some studies, but is not important here. The intent is to demonstrate that simplified - perhaps even approximate - analysis can have valuable practical applications. 

A simple thermodynamic analysis provides considerably more data to work with. The required task is to separate propylene from propane. On a theoretical basis the ideal work (the minimum availability change) required for this separation is about 400 k BTU s/hr, of which an appreciable fraction is needed to raise the temperature of the products to the final values shown. The available energy (availability, exergy) supplied to this process from the condensing low pressure (20 psig) steam is 18.6 M BTU s/hr. 

Tsatsaronis, P. Schuster, H. R6rtgen "Exergy Analysis of the Nuclear Coal Hydrogasification Process , Thermodynamic Availability Symposium, AICHE Annual Meeting, Detroit, Michigan, August 16-19, 1 81. 

Sussman, M. V., "Availability (Exergy) Analysis A Self-Instruction Manual," M. V. Sussman, Publisher, Department of Chemical Engineering, Tufts University, Medford, MA (1980). 

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