Thermodynamics lost work

Final Product Molecular Weight Raw Materials Data Taken From Technology Level Raw Materials Final Product Steam Credit Lost Work Thermodynamic Efficiency3 (%)... 

N. De Neveis and J. D. Sender, "Mechanical Lost Work, Thermodynamic Lost Work, and... 

The thermodynamic variables as well as the lost work associated with this process are given in Table 17.5. 

De Nevers, N., and Seader, J. D. 1980. Lost work A measure of thermodynamic efficiency. Energy 5 757-69. 

T0 is the temperature at which large amounts of heat can be discharged (heat sink). The derivation of Equation 23 is given in several books dealing with engineering applications of thermodynamics. %MaS is sometimes called lost work. In this form, the assumption is implicit that the only heat reservoir is the surroundings at T0. When heat is taken from reservoirs above the ambient, the following relation (3) holds ... 

This chapter establishes a direct relation between lost work and the fluxes and driving forces of a process. The Carnot cycle is revisited to investigate how the Carnot efficiency is affected by the irreversibilities in the process. We show to what extent the constraints of finite size and finite time reduce the efficiency of the process, but we also show that these constraints still allow a most favorable operation mode, the thermodynamic optimum, where the entropy generation and thus the lost work are at a minimum. Attention is given to the equipartitioning principle, which seems to be a universal characteristic of optimal operation in both animate and inanimate dynamic systems. 

Finally, we want to point out that the concept of the endoreversible engine is a simplification that leads to a quick and clear insight into the role and thermodynamic cost of transfer processes in reducing the Carnot efficiency into smaller and more realistic values. But for finding the real optimum conditions, the concept of endoreversibility has to be sacrificed. This will complicate the matter to some extent but will allow for including all contributions to the lost work ... 

In line with what was discussed in Chapter 6 with regard to the quality of the Joule, one can interpret Orwell [6], "All Joules are equal, but some Joules are more equal than others." This means that 1J of heat at 1000 K is more useful than, say, 1J of heat at 298 K. This is a direct consequence of the work available in these amounts of heat, as stated in Chapters 6 and 7, where precise definitions of physical and chemical exergy are given. A direct consequence of the second law of thermodynamics is that the available work (exergy) can never be utilized completely in real processes. Since all real processes are irreversible, every process step will produce a finite amount of lost work, thus diminishing the amount of useful work. 

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... 

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. 

Thus, LCA is a tool to evaluate and analyze the environmental burden of a product throughout all stages of its life. Thermodynamic analysis, on the other hand, is not restricted to a product, but is equally well applied in the analysis of processes. A product and a process are separate entities, but they are related since the purpose of a process is to manufacture a product. Because of this, thermodynamic analysis can easily be used for a product (e.g., lost work per unit weight product, as in the polyethylene (PE) case study, Chapter 11). 

The thermodynamic analysis of the process and several alternatives is given in Table II, based on T0 77°F (537°R). The sum of the ideal work and the lost work is close enough to the actual work (-4%) to proceed. 

A simplified flow diagram for this segment of the process is shown in Figure 4, and thermodynamic analysis in Table III. The refrigerated exchangers and cold box represent about 30% of the lost work of the process. However, the tower itself has a very high percentage of the lost work in the system. Thus the details of the tower heat and material balance were examined in search of ways to improve its efficiency. 

De-Nevers, Joel and Seader, J. D. "Lost Work A Measure of Thermodynamic Efficiency", Energy. Vol. 5, No. 8-9, August-Sept. 1980, pp 7859-769... 

The first term on the left of this equation is the thermodynamic force. The force is not necessarily constant when we have minimum lost work. The optimum force that gives the minimum total entropy production rate is obtained from... 

A new design that takes the varying vapour flows into account was proposed. The effect of the changing hydrodynamic conditions has not yet been explored. It is still too early to conclude on the precise outcome of these optimisation studies, since the assumption of equilibrium on each tray was also used in the model. Progress in the methods of Section 4 may lead to improvements in the future. It is documented that the lost work can be reduced, but the increased investment costs are not yet clarified. Nevertheless, it is important to understand the thermodynamic conditions for optimal performance, independent of technical-economic considerations. 

Calculate the net work consumption, the thermodynamic efficiency, and the lost work. Discuss possible means of improving the thermodynamic efficiency. 

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