Thermodynamic inefficiency

Figure 8 shows the characteristic sawtooth temperature profile which represents the thermodynamic inefficiency of this reactor type as deviations from the maximum reaction rate. Catalyst productivity is further reduced because not all of the feed gas passes through all of the catalyst. However, the quench converter has remained the predominant reactor type with a proven record of reflabiUty. 

The first method is used most frequently. The next preference is for the last method, mostly used in small compressors due to problems with speed control of electrical motors. Other means of capacity control are very seldom utilized due to thermodynamic inefficiencies and design difficulties. Energy losses in a compressor, when capacity regulation is provided by lifting the suc tion valves, are due to fric tion of gas flowing in and out the unloaded cylinder. This is shown in Fig. 11-84 where the comparison is made for ideal partial load operation, reciprocating, and screw compressors. 

Even in smaller cars that use less petroleum, most of the energy released in combustion is wasted and only 12-15% is finally applied to move the car. The rest is lost due to the thermodynamic inefficiency of the en-... 

In this chapter, we examine the thermodynamic efficiency of the propane-propylene separation process by distillation. The tools necessary for this analysis are developed using the first and second laws of thermodynamics. Sources of thermodynamic inefficiency are pinpointed and, finally, some options are discussed to improve the efficiency of the separation. 

In an effort to explore this aspect further, a paper written by Gyftopoulos and Benedict concerning the maximum potential efficiency of an air separation plant provided some insight (4 ). Compressed air is separated by cryogenic distillation into oxygen and nitrogen. In a unique approach, the authors developed an idealized process wherein all thermodynamic inefficiencies which could be corrected by capital investment were eliminated. The losses in the distillation tower were not much affected by this approach. Their thermodynamic analysis for the practical and idealized processes are compared in Figure 7. 

Most of the efficiency loss in methanol production occurs in the reformer (]). This is because high grade fuel must be burned to supply the reforming heat load and combustion is a thermodynamically inefficient process. 

Biomass-to-hydrogen conversion is a thermodynamically inefficient path for using solar energy. 

The most general and convenient method for the metallation of porphyrins is the procedure employing iV,A-dimethylformamide (DMF) as the reaction medium. Previous methods employed either bases such as pyridine or organic acids such as acetic acid for the reaction medium. These methods are thermodynamically inefficient, as the base competes with the porphyrin for the metal in the former procedure and the protons of the solvent compete with the metal for the porphyrin in the latter. - The DMF method has been applied to the synthesis of a wide variety of materials with a considerable variation in both the chelating structure and the metal chelated. - - ... 

Steam reforming of small organic molecules, to facilitate indirect electrochemical oxidation via H2, involves some thermodynamic inefficiency as well as formation, usually, of some CO in the H2 produced. Special catalysts for the fuel-cell oxidation of the H2 thus formed are then necessary, namely, catalysts that can effect dissociative adsorption of H from H2 in the presence of small but significant concentrations of CO in the H2. In recent years, such catalysts have been engineered (95) that allow oxidation of H2 at rates of several amperes per square centimeter in the presence of traces of CO. Similarly, a variety of modified noble metal catalysts have been developed that allow CH3OH oxidation to proceed with improved performance with respect to avoidance of self-deactivation behavior. Doping of Pt by Sn02 or Ru has been effective in this direction (96. 97). 

One evolutionary strategy is to search for thermodynamic inefficiencies to attack, for example, through exergy analysis. Another strategy is to search for structural inefficiencies including design redundancies. In our first flowsheet, both flash columns have similar feed compositions, both overheads go to the same decanter, and both underflows go to wastewater treatment. Therefore, these... 

This reaction is thermodynamically inefficient, reflected in the complexity of the enzymic... 

If this were the only thermodynamic inefficiency, the loss in turbine power would equal the rate of loss of availability in the tower Q, given by... 

The jacketed reactor process also illustrates the principle. The big reactor has a lot of heat transfer area, so only a fraction of the available temperature difference between the inlet cooling water and the reactor is used. A thermodynamically reversible process has no temperature difference between the source (the reactor) and the sink (the inlet cooling water). So the big reactor is thermodynamically inefficient, but it gives better control. 

An exergy analysis identifies the location, the magnitude, and the sources of thermodynamic inefficiencies in a thermal system. This information, which cannot be provided by other means (e.g., an energy analysis), is very useful for improving the overall efficiency and cost effectiveness of a system or for comparing the performance of various systems (Bejan, Tsatsaronis, and Moran, 1996 Moran and Shapiro, 1998). 

An exergy analysis provides, among others, the exergy of each stream in a system as well as the real energy waste, i.e., the thermodynamic inefficiencies (exergy destruction and exergy loss), and the exergetic efficiency for each system component. 

The real thermodynamic inefficiencies in a thermal system are related to exergy destmction and exergy loss. All real processes are irreversible due to effects such as chemical reaction, heat transfer through a finite temperature difference, mixing of matter at different compositions or states, unrestrained expansion, and friction. An exeigy analysis identifies the system components with the highest thermodynamic inefficiencies and the processes that cause them. 

The objective of a thermodynamic optimization is to minimize the inefficiencies, whereas the objective of a thermoeconomic optimization is to estimate the cost-optimal values of the thermodynamic inefficiencies. 

The purpose of an exergy analysis is generally to identify the location, the source, and the magnitude of true thermodynamic inefficiencies in power plants. Exergy flow... 

Professor Nurnberg then referred to the speaker s remarks concerning the inefficiency of involving two Carnot cycles in the production and consumption of hydrogen. He remarked that this was a serious matter only if fossil fuels remained the primary power source. Where the primary source was solar power or nuclear power, then this consideration was less important. Apart from the attendant thermodynamic inefficiency, the involvement of fossil fuels perpetuates the problems of atmospheric pollution including that of carbon dioxide. These problems will only be solved by using the correct primary energy sources. 

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