Lost work thermodynamic efficiency

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

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 ... 

The work lost in the process is Wlost = T0Sgen and is calculated to be 930J/mol. If the thermodynamic efficiency is calculated from... 

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. 

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... 

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

Alter the design of the cyclohexane process in Example 9,5 to reduce the lost work and increase the thermodynamic efficiency. Use a simulation program to complete the material and energy balances, and compute the entropies and availability functions for all of the streams, as well as the lost work for each piece of equipment. 

For a detailed analysis of the reboiler-flashing configuration, which is usually the most financially attractive of the three configurations, the reader is referred to Example 9.4, in which the lost work and thermodynamic efficiency are computed for the separation of propylene and propane. Note that these configurations are most attractive for the separation of... 

Before proceeding with a discussion of the second-law thermodynamic efficiency in the next section, two examples are provided to illustrate the calculation of lost work for chemical processes. 

The thermodynamic efficiency of an operation or an entire process depends on its main goal and the work lost in accomplishing that goal. Goals differ from application to application. For... 

In Sections 9.6 and 9.7, the total rate of lost work and overall thermodynamic efficiency of a propane refrigeration cycle, shown in Figure 9 20. is calculated. Now, consider this cycle in detail... 

Although the lost work is much lower than the value of 1,902.58 kW computed for the system in Figure 9,22. the thermodynamic efficiency is still low. The two cases are not really comparable because the product conditions are not the same. 

---