Reaction light-off

There are several other aspects about CSTRs with exothermic reactions that should be mentioned at this point. The first involves the temperature of the feed. The colder the feed, the less heat must be transferred from the reactor. So control would be expected to be improved. However, as we will see in Chapter 3, a cold feed can produce some interesting dynamics for instance, an increase in feed flowrate initially decreases reactor temperature because of the sensible-heat effect. But as the reactant concentration in the reactor increases, the temperature eventually increases. A reactor temperature runaway can result if the cold feed quenches the reaction and reactant concentration builds to a very high level before the reaction lights off. ... 

FIGURE 12 WGS reaction light-off profiles for 1% Au on Ce02 nanorods ( ), cubes ( ), and polyhedra (A). Reprinted with permission from Si and Flytzani-Stephanopoulos (2008). Copyright 2008 Wiley-VCH. 

Samples containing Pd as a promoter with or without the addition of a-SiC had a higher selectivity and yield to MA. The optimum system investigated in this study consisted of 1.5 wt% Pd-10 wt% a-SiC/VPO. This newly developed catalyst formula lowered the reaction light-off temperature from around 420 to 300 °C as compared with bulk VPO resulting in a 84 % butane conversion to products at reaction temperature (420 °C) and a 25 % total product yield. 

In the CO-O2 reaction, light-off was revealed by a fast temperature rise near the reactor outlet in the same time, CO conversion jumped to about 100 %. Figure 1 shows the... 

Heat loss from the fuel processor has a significant effect on device performance. Thus the pure CPO process is unfavorable for high process efficiency in comparison with SR and OSR. However, there are more effects that should be considered. Microreactors that are operated autothermally with lower environmental losses achieve lower reaction light-off temperatures. For high-temperature reactions, the consider-... 

The reactor consists of a quartz tube (length = 62 cm, ID = 20 mm) placed in an oven for thermal insulation and reaction light-off. The monolithic honeycomb catalyst (diameter = 19 mm, variable length) is placed 230 mm downstream of the reactor inlet. For increased insulation, the part of the reactor tube, in which the catalyst is positioned, is wrapped with a 1-cm-thick layer of quartz wool. Uncoated monoliths up- and downstream of the catalyst are employed as heat shields (front heat shield (FHS) and back heat shield (BHS), respectively). The FHS also serves as support for the probe applied in the samphng technique. To prevent a gas by-pass between monoliths and quartz tube, the heat shields and the catalyst are wrapped with... 

Figure 18.6b, symbols, shows the results for the experiment run with 500 ppm of NO. The addition of NO to the reactant mixture significantly affected both the NH3 conversion activity and the products distribution. NH3 consumption reached 100 % already at 200 °C in the presence of NO (Fig. 18.6b), with a drop of about 50 °C of the reaction light-off in comparison with the NH3-O2 reacting system (Fig. 18.6a). At low T, the main reaction product was N2O. Above 200-250 °C, N2O started decreasing, while a modest production of N2 was visible together with the evolution of considerable amounts of NO and NO2.

The reaction gives off a brilliant white light, which served as a source of illumination in the headlights of early automobiles. 

Figure 8.62 shows the effect of temperature and of positive potential application on the reaction rates and on the nitrogen selectivity for the C3H6/N0/02 reaction.67,68 Electrochemical promotion significantly enhances both activity and N2 selectivity (e.g. from 58% to 92% at 350°C) and causes a pronounced (60°C) decrease in the light-off temperature of NO reduction in presence of 02. Positive potentials weaken the Rh=0 bond, decrease the O coverage and thus liberate surface sites for NO adsorption and dissociation. 

In contrast to NaZSM-5 zeolite, introduction of CoZSM-5 or HZSM-5 zeolite in the reaction system shifts the "light-off" temperature and modifies the chemistry now not only NO but Nj is formed. Hence, some intermediate species required for Nj formation must be stabilized on the catalyst surface. The "light-off"temperature shifts observed with CoZSM-5 and HZSM-5 catalysts may result from the enhanced redox capacity provided by these catalysts or from the NOj/NO equilibrium achieved more readily than with NaZSM-5. Moreover, equilibrium is approached at a somewhat lower temperature over CoZSM-5 than HZSM-5, and much lower than with the empty reactor (see Fig. 1 of Ref. lOl.The decomposition reaction of NOj into NO -t- occurs readily on these catalysts and the "light-off" temperature of both combustion and SCR is lower in comparison with that of the homogeneous reaction. 

Compared to CO, these reactions were much less studied over TW catalysts. Kobylinski and Taylor [67] have compared the NO reduction by CO and by H2. Their main results are summarized in Tables 8.10 (light-off activity) and 8.11 (selectivity). 

Figure 10.1. Light-off curves for the reaction of the standard mixtures (.R = 0.98, 5000 ppm H20) over Rh/Al203. (a) Conversions of CO, NO, C3H6 and C3H8 (b) production of N20 and NH3 (reproduced with permission from Ref. [21]). Reactions involved over three-way catalysts are oxidation, (1) CO + 7202 C02 (2) C Hy + (x + y/4) 02 -> xC02 + y/2H20, and reduction,...

Combustion is sometimes described as a chemical reaction giving off significant energy in the form of heat and light. It is easy to see that for this Arrhenius reaction, representative of gaseous fuels, the occurrence of combustion by this definition might be defined for some critical temperature between 600 and 1200 K. 

The direct reaction of methane partial oxidation always competes with total oxidation reactions, which are also responsible for O2 consumption, whereas steam and dry reforming and C-forming reactions are also to be considered. All reactions are catalyzed by the materials which are active in partial oxidation, but different scales of reactivity for the catalysts can be estimated from the experimental data. Total oxidation prevails at the light-off of the fuel-rich stream over most catalysts, but precious metals are more active than transition metals. 

Figure 7-18 Since tiie reactions in the ACC are sh ongly exothermic, hie catalyst can exhibit multiple steady states as hie catalyst suddenly lights off as the car is staled aid the terrqieratae rises to the ignition temperatiue, vdnch then causes a rapid increase in temperature.

Polymerization of PO with (NMTPP)ZnSPr (17,X=SPr) takes place very rapidly under the irradiation with visible light. For example, the reaction did not occur in the dark in 100 min ([PO]o/[17]o=40) (Fig. 49), while the polymerization was initiated rapidly upon irradiation and completed in 80 min. It should also be noted that the polymerization, once photoinitiated, did not subside upon turning the light off. As shown by the GPC profiles of the polymerization with the mole ratio [PO]q/[17]q of 430, initiated by irradiation for 40 min, the produced... 

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