Regenerative cycle

Fig. 29. Theoretical regenerative cycle at 16 MPa (160 bar), showing reheat and efficiency improvement resulting from regeneration, where ( ) is the region removed from cycle ( ), vaporization (x x x), superheating ( " ) turbine expansion (----), heat rejection (-), reheating and ( ), regeneration.

The Reheat Cycle The regenerative cycle improves the efficiency of a gas turbine but does not provide any added work per pound of air flow. To achieve this latter goal, the concept of the reheat cycle must be utilized. The reheat cycle utihzed in the 1990s has pressure ratios of as high as 30 1 with turbine inlet temperatures of out 2100° F (1150° C). The reheat is done between the power turbine and the compressor trains. The reheat cycle, as shown in Fig. 29-35, con-... 

FIG. 29-34 Performance map showing the effect of pressure ratio and turbine inlet temperature on a regenerative cycle. 

This cycle produces an increase of 30% in work output, but the overall efficiency is slightly decreased as seen in Figure 2-15. An intercooling regenerative cycle can increase the power output and the thermal efficiency. This combination provides an increase in efficiency of about 12% and an increase in power output of about 30%, as indicated in Figure 2-16. Maximum efficiency, however, occurs at lower pressure ratios, as compared with the simple or reheat cycles. 

This cycle, as shown in Figure 2-23, is a regenerative cycle with water injection. Theoretically, it has the advantages of both the steam injection and regenerative systems reduction of NO emissions and higher efficiency. The work output of this system is about the same as that achieved in the steam injection cycle, but the thermal efficiency of the system is much higher. 

Fig. 11. Effective specific heat v. Temperature in a two-bed regenerative cycle...

Critoph, R.E., A forced convection regenerative cycle using the ammonia-carbon pair. In proceedings of Solid Sorption Refrigeration, Paris, HR, 1992, pp. 80 85. 

We adopt the nomenclature introduced by Hawthorne and Davis [1], in which compressor, heater, turbine and heat exchanger are denoted by C, H, T and X, respectively, and subscripts R and I indicate internally reversible and irreversible processes. For the open cycle, the heater is replaced by a burner, B. Thus, for example, [CBTX]i indicates an open irreversible regenerative cycle. Later in this book, we shall in addition, use subscripts... 

The CHAT cycle may be seen as a low loss evaporative development of the dry intercooled, reheated regenerative cycle [CICBTBTX]. It offers some thermodynamic advantage—increase in turbine work (and heat supplied ) with little or no change in the compressor work, leading to an increased thermal efficiency and specific work output. 

Table 2. Variation in cooling COP of a zeolite - water regenerative cycle with evaporating, condensing, adsorption rejection and maximum desorption temperatures 0°C, 40°C, 50°C and 350°C respectively.

The thermal efficiency of the Rankine cycle can be increased by the use of regenerative heat exchange as shown in Fig. 2.15. In the regenerative cycle, a portion of the partially expanded steam is drawn off between the high-and low-pressure turbines. The steam is used to preheat the condensed... 

Figure 5.5a depicts a combined plant in which a closed Brayton helium nuclear plant releases heat to a recovery steam generator, which supplies heat to a Rankine steam plant. The generator is provided with a gas burner for supplementary additional heat when the demand of steam power is high. The Rankine plant is a regenerative cycle. 

The indirect electrochemical reduction of alkyl halides is also possible by use of nickel(I) complexes which may be obtained by cathodic reduction of square planar Ni(n)-complexes of macrocyclic tetradentate ligands (Table 7, No. 10, 11) 2 4-248) Comparable to the Co(I)- and Ni(O)-complexes, the Ni(I)-species reacts with the alkyl halide unter oxidative addition to form an organo nickel(III) compound. The stability of the new nickel-carbon bond dominates the overall behavior of the system. If the stability is low, the alkyl group is lost in form of the radical and the original Ni(II)-complex is regenerated. A large number of regenerative cycles is the result. 

This is the case for secondary and tertiary alkyl bromides. If the stability is high, however, as, for example, with primary alkyl bromides, the organo nickel(III) complex is further reduced to an alkyl nickel(II) complex which loses the alkyl group in form of the alkyl anion. An electroinactive Ni(II) species remains. The number of regenerative cycles is consequently low. The structure of the ligand also influences the lifetime of the alkyl nickel(ni) complex thus, a less stable complex is formed in the case of [A,A -ethylene-bis(salicylidene-irainato)]nickel(II) ([Ni(salen)]) as compared with (5,5,7,12,12,14-hexamethyl-l,4,8,ll-tetraazacyclo-tetradecane)nickel(II) ([Ni(teta)] ), and hence the former complex favors the radical pathway even with primary alkyl halides. 

As mentioned above, it is difficult to find organic compounds which are suitable as redox catalysts for oxidations. This is the case because organic cation radicals, which are mostly the active forms in indirect electrochemical oxidations, are usually easily attacked by nucleophiles, thus eliminating them from the regenerative cycle. Therefore, the cation radicals must be stabilized towards the reaction with nucleophiles. Nelson et al. demonstrated that the cation radicals of triaryl amines and related compounds are very stable if the para positions of the aryl... 

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