Ammonia temperature effects

In the context of safety of the process of neutralisation of nitric acid with ammonia, the effects of temperature (160-230°C), pressure (2.3-9.8 bar), and concentrations of ammonium nitrate (86-94%) and of nitric acid (0-4%) upon decomposition rate were studied. 

The model predicts a red shift of the absorption spectrum with temperature for both eh and eam. However, in ammonia the effect is about four times bigger, because the volume expansion of the cavity is the main contributing factor. In water, the temperature variation of the... 

TEM image, 64 temperature, 224 temperature effect, 162 temperature-programmed desorption of ammonia (TDP), 77... 

Accordingly, we conclude that the dual-site MR approach is compatible with the ammonia inhibition effects observed during unsteady SCR experiments, as well as with the oxygen dependence of the SCR kinetics at low temperatures, and can be successfully applied to simulate the complex dynamic behavior of... 

The volume expansions of alkali metals in liquid ammonia are discussed in the light of the current available data. Special emphasis is made of the anomalous volume minimum found with sodium-ammonia and potassium-ammonia solutions. Recent studies of potassium in ammonia at —34° C. were found to exhibit a large minimum in the volume expansion, AV, vs. concentration curve. The results of these findings were compared with the previous results of potassium in ammonia at —45° C. The volume minimum was found to be temperature dependent in that the depth of the minimum increased and shifted to higher concentrations with increasing temperature. No temperature effect was observed on either side of the minimum. These findings are discussed in light of the Arnold and Patterson and Symons models for metal-ammonia solutions. 

Figure 1. A comparison of the temperature effect on the volume change for lithium in liquid ammonia as a function of concentration...

C. The same pattern holds for the data on ammonia yields, as one would expect from Equations 1 and 2. The increase at — 80°C. suggests an effect of phase change while the opposed temperature effect at — 196°C. suggests that a different reaction mechanism is controlling at — 196°C. consistent with observations from electron spin resonance studies that different stable-free radicals are observed below — 150°C. for glycine. The low carbonyl yields found for methionine and the peptides at — 80°C. indicate that the low temperature radicals may still be the stable forms at... 

The slight temperature effect was confirmed further by observing the behavior of methyl red in ammonium chloride solutions. Noyes (p. 21) has reported that the dissociation constant of ammonia is independent of temperature. Since Kw increases 100 times, the pH of the boiling solution must be considerably decreased, and the color of solution must of necessity be shifted to the acid side. This was confirmed experimentally as follows. Several drops of methyl red added to a 0.2 N ammonium chloride solution showed an intermediate color (pH = 5.1). Boiling produced a much more intense red, although not quite the color of methyl red at a pH of 4.2. After cooling, the color corresponded to the original pH. 

The effect of the gaseous atmosphere was studied by Tarasevich and Radyuskina who concluded that under N2, Ar, and He in the temperature range of 500-1,000 °C for different dwell times between 0.3 and 5.0 h, the activity and durability of the studied metalloporphyrins and metallo-phthalocyanines were similar [25]. This was a case for disagreement between different groups who observed dependence between the activity of heat-treated catalysts and the gas used during the heat treatment. For example, Dhar et al. showed the difference in the catalytic activity of heat-treated CoTAA under vacuum, N2, Ar, and N [26]. They concluded that the most active ORR catalyst was obtained under vacuum with the following decrease in activity vacuum > N2 > Ar > N (Fig. 8.2). Dodelet and his collaborators [27] showed that an iron tetra(methoxyphenyl) porphyrin is the most stable, but the least active when heat-treated in Ar, and that it becomes very active and loses its stability when treated in ammonia, an effect attributed to the concurrent increase in the microporous surface area, as well as the N and Fe surface content. The authors raised important questions about the catalyst design and optimum heat-treatment conditions for maximum stability and activity. 

The MR rate expression Eq. (10.34) differs from the Eley-Rideal rate Eq. (10.18) only in its denominator, which accounts both for the adverse kinetic effect of NH3 and for the favorable O2 dependence at low ammonia coverage (as, e.g., at high temperature), the denominator tends to unity and Eq. (10.34) formally reduces to Eq. (10.18). Indeed, this is consistent with the experimental indications discussed above the ammonia inhibiting effect is particularly evident at low temperatures, but tends to disappear in the runs performed at temperatures of 250 °C and above, where ammonia coverage becomes lower. Likewise, the oxygen dependence is reportedly most manifest at low temperatures. 

According to Table 8.3, to maintain the same ammonia concentration at the outlet, the active temperatures of different catalysts are shown in Table 8.5. The table shows that when achieving the same ammonia concentration at the outlet (16.68%), the active temperature of ZA-5 is 30°C or 40°C lower than that of ICI74-1 and AllO-2 respectively. The activity of ZA-5 at 400°C is equivalent to that of the AllO-2 at 440°C and ICI74-1 at 430°C respectively. In other words, if the reaction temperature of ZA-5 catalyst is reduced by say 40°C, it would still achieve the same ammonia concentration at the outlet as AllO-2 could at 440° C. We call this effect the temperature effect . The initial hotspot temperature (the highest temperature point in beds) of ZA-5 catalyst bed is 430-450°C, and the operational temperature range is 300 500°C. Therefore, ZA-5 is an excellent catalyst that could be used at a low temperature and within a wide temperature range. 

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