Irreversible heating

Hydrolysis in neutral aqueous solutions proceeds slowly at room temperature and more rapidly at acidic conditions and elevated temperatures. The hydrolysis—esterification reaction is reversible. Under alkaline conditions hydrolysis is rapid and irreversible. Heating the alkaline hydrolysis product at 200—250°C gives 4,4 -oxydibutyric acid [7423-25-8] after acidification (148). 

A process is thermodynamically reversible when an infinitesimal reversal in a driving force causes the process to reverse its direction. Since all actual processes occur at finite rates, they cannot proceed with strict thermodynamic reversibility and thus additional nonrevers-ible effects have to be regarded. In this case, under practical operation conditions, voltage losses at internal resistances in the cell (these kinetic effects are discussed below) lead to the irreversible heat production (so-called Joule heat) in addition to the thermodynamic reversible heat effect. 

The energy equation entails a detailed account of heat generation due to irreversible heat of the electrochemical reaction, reversible (or entropic) heat, and Joule heating. The heat generation term in a CFCD model must be unambiguous and location specific. More discussion is deferred to section 3.3. In addition, the heat accumulation in a porous material consisting of the matrix and fluid is given... 

The energy equation in Table 1 contains location-specific heat generation terms, including Irreversible heat of the electrochemical reaction, reversible en-... 

Among the three heat-generation terms, the irreversible and reversible heat sources of ORR are dominant. For a straight-channel cell shown in Figure 12, the total amount of heat release is 2.57 W, of which the irreversible heat is 55.3%, the reversible heat 35.4%, and the Joule heat only 9.3% The total heat released from the fuel cell can also be estimated from the overall energy balance, i.e. 

The ideal finite-time Rankine cycle and its T-s diagram are shown in Figs. 7.14 and 7.15, respectively. The cycle is an endoreversible cycle that consists of two isentropic processes and two isobaric heat-transfer processes. The cycle exchanges heats with its surroundings in the two isobaric external irreversible heat-transfer processes. The heat source and heat sink are infinitely large. Therefore, the temperature of the heat source and heat sink are unchanged during the heat-transfer processes. 

It was further stated that the energy of the shock.wave is continuously expended in the irreversible heating of a compressible substance and, therefore, a stationary shock... 

The heat sterilization of microorganisms and heat inactivation of enzymes are examples of first-order reactions. In the case of an enzyme being irreversibly heat-inactivated as follows ... 

An enzyme is irreversibly heat inactivated with an inactivation rate of k = 0.001 s at 80 °C. Estimate the half-life t of this enzyme at 80 °C. 

The third idealized process, illustrated in Figure 8, is a modification of the first to include irreversible heat transfer across finite temperature differences, and discharge of... 

Due to the irreversible heat exchange between the four heat reservoirs (for T0 = 300 K and T, = 600 K), the real maximum power is found to be close to a value of 0.3 rather than the ideal value of 0.5. The maximum power is found at an optimal flow rate (32 corresponding to an optimal set of temperatures T2 and T3/ satisfying the Carnot relation... 

The stability of both the wild-type and mutant proteins is expressed as the melting temperature, Tm, which is the temperature at which 50% of the enzyme is denatures during irreversible heat denaturation. To prevent the self-digestion of penicillolysin, heat treatment was carried out at pH 5.0 because the proteolytic activity is substantially reduced at this pH and the active form of the enzyme is stable. For the wild-type enzyme and the three mutants, the thermal stabilities and the 7m changes were assayed at pH 5.0 by measuring the far-UV CD spectrum at 222 nm as a function of temperature. For the wild-type penicillolysin, the Tm... 

Let us now consider a steady flow of heat dQ(irr) that occurs irreversibly between a phase at a high temperature T, and a phase at a low temperature T2 in a closed system as shown in Fig. 3.7. The phase 1 continuously receives heat dQ = T1dS1 in a reversible way from the surroundings at temperature Tl and the phase 2 continuously releases heat dQ - T2dS2 into the surroundings at temperature T2. In the steady state no change occurs in the state property of the system except an increase in entropy dSjrr due to the irreversible heat transfer dQ(irr) = dQ ... 

Funk, J.E. and Knoche, K.F., "Irreversibilities, Heat Penalties and Economics for the Methanol/Sulfuric Acid Processes", Proc. 12th Intersociety Energy Conversion Conference 1977, 1, Paper 779142, by ANS LaGrange Park, 111., pp. 933-938. 

Since TH > Tc, the total entropy change as a result of this irreversible process is positive. We note also that becomes smaller as the difference Th-Tc gets smaller. When TH is only infinitesimally higher than TCt the heat transfer is reversible, and AStotaI approaches zero. Thus for the process of irreversible heat transfer, AS,otal is always positive, approaching zero as the process becomes reversible. 

MCFCs and intermediate-temperature SOFCs can incorporate catalysed reform at their anodes, where the hydrogen electrochemical oxidation proceeds simultaneously, and heats the non-Faradaic and endothermic reform and shift reactions The latter process is immediately superior to a separate reformer, because it eliminates combustion reaction irreversibility. Heat produced at such an anode is given, in Appendix A, the title reversible heat , that is heat produced without the thermal degradation which occurs in the combustion reaction. 

The heat peaks of the nonreversing and reversing MDSC traces can be associated with nonreversible reactions such as a chemical reaction or cure and reversible reactions such as plasticizing processes in PU resin. Pad samples exposed to all tested media showed nonreversing heat peaks between 70 and 100 °C. Therefore, nonreversible chemical reactions are responsible for the pad softening. Endothermic irreversible heats reached their maximum value after approximately 180 h of exposure, as shown in Fig. 2.15. This suggests that chemical reactions that lead to pad softening are complete after approximately 180 h of exposure. 

Since Th > 7c,the total entropy changeasaresultoftliisirreversibleprocessis positive. Also, totai becomes smaller as tlie difference Th — Tc gets smaller. Wlien Th is only infinitesimally liigherthan Tc, the heat transferis reversible,and A5totai approaches zero. Thusfor the process of irreversible heat transfer, A Stotai is always positive, approacliingzero as the process becomes reversible. 

Phosphoenolpyruvate carboxylase (PEPC) of Thermus sp., was characterized and its mechanism of stabilization was studied by means of site-directed mutagenesis. A divergent sequence at a Gly-rich region of Thermus PEPC was revealed to contribute to the activity at hi temperature but not to the tolerance to the irreversible heat inactivation. 

The activity of Thermus PEPC was highest at 80° C when monitored with Vmax in the presence of 1 mM CoASAc (cf. Fig. 3). Above the optimum temperature, Thermus PEPC showed decreased activity. This decrease of activity was reversible and was not due to irreversible heat inactivation. Inactivation of Thermus PEPC at high temperature proceeded slowly even at 95°C(c/. Fig. 2) and the inactivation was negligible during measurement of enzyme activity which took about 5 min. Thus, the mechanism for loss of catalytic activity prior to irreversible heat denaturation remains to be elucidated. 

Rubin, M. (1980). Optimal configuration of an irreversible heat engine with fixed compression ratio. Phys. Rev. A. Vol. 22, pp 1741-1752. 

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