Thermoneutral point

The thermoneutral point (of oxygen-to-carbon ratio) is where the enthalpy of the reaction is zero, (AHf 298 = 0). 

Equation 9-4 and related heats of reaction can be manipulated to show that the maximum efficiency is a state point function, regardless of path (steam reforming, partial oxidation, or autothermal reforming), and is achieved at the thermoneutral point. In practice, x is set slightly higher than the thermoneutral point so that additional heat is generated to offset heat losses from the reformer. Table 9-1 presents efficiencies at the thermoneutral point for various hydrocarbon fuels. 

The fuel conversion efficiency for methane conversion to hydrogen is 93.9 percent at the thermoneutral point, x = 0.44 (an ATR reaction) and 91.7 percent at x = 0 (the SR reaction). The difference between the two efficiency values is exactly equivalent to the loss represented by the latent heat of vaporization of the H2O that escapes with the combustions products in the SR burner exhaust. The concentration of hydrogen is 53.9 percent at x = 0.44 (ATR) and 80 percent at X = 0 (SR). 

The energy efficiency will only be 100% at the thermoneutral point, so we can see the advantage in seeking to find this point, if possible and pertinent i, as for SO technology. Above or below this point, the energy efficiency will be less than 100%. From the point of view of the system, the energy efficiency is the only value that is important. 

As explained earlier, in practice, PEMWE and electrolyzers will exhibit exothermic function with their operational current densities. No external heat source will be used far from it - in fact, a coohng system will need to be employed. Equations [2.13] and [2.14] will therefore be used. The optimum energy efficiency (100%) will actually be attained with operation at the thermoneutral point, corresponding to very low current densities for PEMWE and AWE technologies. If these hypotheses are true, the electrical efficiency will at best be equal to Erev/Etn -83% (liquid water, 25°C, 1 bar). 

Remark 2.1- in the existing body of literature, we can find examples of efficiencies greater than 100%. Thus, if we apply equation [2.13] to voltages below the thermoneutral point Etn, the efficiency will artificially be made greater than 100% because we ignore the heat supplied from outside. Strictly speaking, there would be no reason for this to be trae if the formulae were correctly applied. 

What is the efficiency of an electrolyzer functioning at its thermoneutral point ... 

With this type of question, we first need to accurately locate the thermoneutral point. In the operating conditions specified, it is approximately = 1.291 V. This... 

Therefore, for a thermoneutral reaction, the intersection point between the bond order profiles for the bond making and the bond breaking processes coincides with the TS the reactivity of the two reacting atoms also equalizes at the TS, as can be seen from the intersection of their IT profiles. These intersection points of the associated bond orders and condensed FFs he toward the left (right) of the TS for an endothermic (exothermic) reaction, in agreement with the Hammond postulate. 

Figures 4.6—4.8 are the results for the stoichiometric propane-air flame. Figure 4.6 reports the variance of the major species, temperature, and heat release Figure 4.7 reports the major stable propane fragment distribution due to the proceeding reactions and Figure 4.8 shows the radical and formaldehyde distributions—all as a function of a spatial distance through the flame wave. As stated, the total wave thickness is chosen from the point at which one of the reactant mole fractions begins to decay to the point at which the heat release rate begins to taper off sharply. Since the point of initial reactant decay corresponds closely to the initial perceptive rise in temperature, the initial thermoneutral period is quite short. The heat release rate curve would ordinarily drop to zero sharply except that the recombination of the radicals in the burned gas zone contribute some energy. The choice of the position that separates the preheat zone and the reaction zone has been made to account for the slight exothermicity of the fuel attack reactions by radicals which have diffused into...

The reaction becomes less endothermic with increasing oxygen. It becomes thermoneutral" at X = Xo (0.44 for natural gas). Above this point, the reaction becomes increasingly exothermic. At X = 1, the pure POX reaction, the feed contains sufficient oxygen to convert all of the carbon in the fuel to CO2. No water needs to be added. The equation is a mix of the steam reforming reaction and the POX reaction at values of x between 0 and 1. 

In contrast to phosphines, dialkyl sulfides are relatively stable towards autoxidation. As we have pointed out elsewhere (25), the simplest explanation is an energetic one the over-all transfer of an oxygen atom from carbon to phosphorus (Reaction 11) is exothermic by some 33 kcal. per mole. Transfer to sulfur, in contrast, is approximately thermoneutral... 

From my estimates on the thermodynamic properties of peroxy and polyoxide molecules and radicals, we can estimate that the bond dissociation energy of the tetroxide is about 5 kcal. Thus, at room temperature, or even at dry ice temperature, the tetroxide is extremely unstable and should redissociate into the more stable (from a thermodynamic point of view) peroxy radicals. The competing step would be a concerted decomposition into an RO and an R03 (Step 14) radical, which would be uphill by 20 kcal., or else a concerted decomposition into 2 RO radicals and 02 (Step 14 ). The latter is almost thermoneutral. If we take the current data at face value, it provides, from the reported activation energy at least, strong evidence that the propagating interaction of two alkylperoxy radicals proceeds in a concerted fashion. 

CLs), resulting in a drastic drop in cell performance [17], Figure 3.13 also shows the difference between the theoretical cell potential (1.23 V) and the thermoneutral voltage (1.4 V), which represents the energy loss under reversible conditions (the reversible loss) [18], Very often, polarization curves are converted to power density versus current density plots by multiplying the cell voltage by the current density at each point of the curve. 

At this point little was known about the thermodynamics of this cycloreversion process. Some hints existed that it was close to thermoneutral, in that ketenes gave a net [3 + 2] cycloaddition to form a metallalactone structure, confirmed by X-ray crystallography.76 Similar behavior is seen for (N N)MeReOj complexes.77... 

Let us start with the three isomeric C-monomethylated species (the toluidines or methyl-benzenamines) and accept the recent experimental measurements and analysis17. This source points out the contradictory and incomplete measurements for these species and uses the expression in disarray to describe the literature. From thermoneutral equation 2, we would predict enthalpies of formation of —12, —5 and 55 kJ mol-1 for all three solid, liquid and gaseous toluidines, in reasonable agreement with the experimental values. We note that the o-isomer is slightly more stable than its m- and p-isomers. In comparison, the spread of enthalpy of formation values for the isoelectronic and isosteric xylenes is but 1 kJ mol 1 and for the so-related methylphenols3 (cresols) is but 7 kJ mol 1. As an additional check for thermochemical accuracy, consider the formal methylation reaction 4. For Ar = Ph, the enthalpies of reaction are —36.6 (lq) and —32.2 (g) kJ mol 1. For Ar = C6H4NH2, the ortho, meta and para gas-phase enthalpies of reaction are the... 

Every step in the propagation part must be exothermic or nearly thermoneutral. If a particular step is endothermic, then radicals will accumulate at that point, and they will react with one another to terminate the chain. 

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