Temperature Gradients with Catalytic Reactions

Example 3.7.a-l Temperature Gradients with Catalytic Reactions 

The maximum external temperature difference is then estimated, from Eq. 3.7.b-16, 

The actual value of 11°C indicates that at the high reactant concentration of yg = 0.195, the internal pellet concentration was not quite zero, as for maximum heat release conditions. The maximum overall temperature difference is estimated, from Eq. 3.7.b-l 1 

Finally, the internal temperature difference could be 38 — 36 = 2 K the value can also be estimated from Eq. 3.7.b-14 or Eq. 3.7a-5, the latter using the measured surface temperature. Thus, 

These results are also good estimates of the mqieiimeiitally measured values. 

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For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

The rate and selectivity of a surface-catalyzed reaction can be affected by the existence of concentration or temperature gradients in the quiescent layer of fluid which surrounds the catalyst particle or is contained within its pores upon whose surface much of the exposed active material is distributed. The reactants in the bulk phase reach the reactive sites by diffusion through these regions of the fluid, and they affect the kinetics when the rates of the surface-catalyzed reactions are fast relative to the rate of transport of reactants to the catalytic sites. In the hydrogenation of unsaturated liquids or compounds in solution, the agitation of the liquid-catalyst mixture must be adequate to assure that the solution remains saturated with hydrogen. 

Lateral transport of heat and reactants across the diameter of the reactor This feature is important when the reaction has a high heat effect or when insufficient mixing of the reactants would have a strong adverse effect on the performance. Examples are selective oxidation processes (highly exothermic), where lateral or radial temperature gradients would decrease the selectivity of the conversion. Another example is the selective catalytic reduction process of nitric oxide with ammonia, where mixing of ammonia with the flue gas is often a point of great concern. Because the void space in a BSR is continuous... 

Historically, the use of microreactors dates back to the 1940s when they were developed to measure kinetics of catalytic reactions.One of the key early findings was Denbigh s 1965 observation that if a reactor were made small enough, temperature and concentration gradients with the reactor would be negligible, so that differential (i.e., gradientless) behavior would be observed. This allowed much more accurate kinetic... 

In Chapter 1, Fyfe, Mueller, and Kokotailo describe the applications of solid-state NMR to the study of zeolite molecular sieve catalysts and related systems. Zeolites provide an apt arena in which to demonstrate the capabilities of modern techniques such as sample spinning, cross-polarization, and multidimensional correlation spectroscopy. In Chapter 2, Karger, and Pfeifer consider the question of molecular diffusion in catalyst systems. Applications of NMR techniques such as imaging, lineshape analysis, relaxation, pulsed field gradient echo spectroscopy, and NMR tracer exchange are described and compared with other, more traditional techniques such as radioactive tracing. In Chapter 3, Haw discusses the use of NMR to probe catalytic processes, showing how the combination of temperature control with novel NMR probes makes it possible to elucidate reaction mechanisms in situ. 

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