Catalyst beds temperature effects

Figure 3.16 Size effect of Au nanoparticles on the reaction of propylene with 02 and H2 catalyst, Au/Ti02 (P25) 0.5 g catalyst bed temperature, 353 l< feed gas, C3He/02/H2/Ar = 10 10 10 70 space velocity, 4000h-1 mLgca, 1 [121].

When gum formation proceeds, the minimum temperature in the catalyst bed decreases with time. This could be explained by a shift in the reaction mechanism so more endothermic reaction steps are prevailing. The decrease in the bed temperature speeds up the deactivation by gum formation. This aspect of gum formation is also seen on the temperature profiles in Figure 9. Calculations with a heterogenous reactor model have shown that the decreasing minimum catalyst bed temperature could also be explained by a change of the effectiveness factors for the reactions. The radial poisoning profiles in the catalyst pellets influence the complex interaction between pore diffusion and reaction rates and this results in a shift in the overall balance between endothermic and exothermic reactions. 

Operational Considerations. The performance of catalytic incinerators (28) is affected by catalyst inlet temperature, space velocity, superficial gas velocity (at the catalyst inlet), bed geometry, species present and concentration, mixture composition, and waste contaminants. Catalyst inlet temperatures strongly affect destmction efficiency. Mixture compositions, air-to-gas (fuel) ratio, space velocity, and inlet concentration all show marginal or statistically insignificant effects (30). 

In the SCR process, ammonia, usually diluted with air or steam, is injected through a grid system into the flue/exhaust stream upstream of a catalyst bed (37). The effectiveness of the SCR process is also dependent on the NH to NO ratio. The ammonia injection rate and distribution must be controlled to yield an approximately 1 1 molar ratio. At a given temperature and space velocity, as the molar ratio increases to approximately 1 1, the NO reduction increases. At operations above 1 1, however, the amount of ammonia passing through the system increases (38). This ammonia sHp can be caused by catalyst deterioration, by poor velocity distribution, or inhomogeneous ammonia distribution in the bed. 

Zabor et al. (Zl) have described studies of the catalytic hydration of propylene under such conditions (temperature 279°C, pressure 3675 psig) that both liquid and vapor phases are present in the packed catalyst bed. Conversions are reported for cocurrent upflow and cocurrent downflow, it being assumed in that paper that the former mode corresponds to bubble flow and the latter to trickle-flow conditions. Trickle flow resulted in the higher conversions, and conversion was influenced by changes in bed height (for unchanged space velocity), in contrast to the case for bubble-flow operation. The differences are assumed to be effects of mass transfer or liquid distribution. 

Fig. 14. Influence of inlet SO2 concentration on behavior and performance in adiabatic, packed-bed SOj converters operating under periodic flow reversal. Simulation results for t = 30 min, SV = 514 h 1, Ta = 25°C (a) effect of inlet SO2 vol% on the temperature profile in the catalyst bed, (b) influence of inlet S02 on converter performance and the velocity of the temperature front. (Figure adapted from Xiao and Yuan, 1996, with permission of the authors.)...

Then the reduction of stored NO with hydrogen was addressed. The stability/reactivity of the NO adsorbed species was analysed under different atmospheres (inert and reducing) both at constant temperature and under temperature programming. The bulk of data pointed out that in the absence of significant thermal effects in the catalyst bed, the reduction of stored nitrates occurs through a Pt-catalysed surface reaction that does not involve the thermal desorption of the stored nitrates as a preliminary step. A specific role of a Pt-Ba interaction was suggested, which plays a role in the NO storage phase as well. 

The effect of the H-Beta ratio (y in wt%) in the dual-bed Pt/Z12(x) HB(y) catalyst system on the benzene purity at a reaction temperature (Tr) of 623 K is shown in Fig. 1. It is evident that the benzene purity gradually increased with increasing H-Beta ratio (Fig. la), eventually reaching a plateau value which meets the industrial specification of 99.85% at y 40 wt%. The effects of catalyst bed ratio on product yields are shown in Fig. lb. Comparing to the single-bed catalyst Pt/Z 12 (i.e., y = 0), the overall premium product yields of benzene and xylene (A68 yield) over the dual-bed catalyst Pt/Z12(x) HB(y) system reached an maximum at y 10 wt%. That the A68 yield dwindled and tetramethylbenzene (TEMB) increased with further increase in the H-Beta ratio may be attributed to the larger pore opening possessed by the bottom (H-beta) catalyst, which may provoke disproportionation of TMB to form tetramethylbenzene (TEMB) [8],... 

The main effect of MW irradiation on the graphite- and charcoal-supported catalysts is to reduce the average temperature required for the reaction to occur. The authors believe this is the result of hot spots formed within the catalyst bed (Sect. 7.4.2). Graphite-supported catalysts, moreover, seem to be more selective than the equivalent charcoal-supported catalysts, especially under the action of MW irradiation - 83.6-97.7% compared with 68.4-86.3%. This might be because of the hydrophobic nature of the graphite which directs the reaction away from the production of water by dehydration of the alcohol. 

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