Heterogeneous catalyzed reactions problems

This study was carried out to simulate the 3D temperature field in and around the large steam reforming catalyst particles at the wall of a reformer tube, under various conditions (Dixon et al., 2003). We wanted to use this study with spherical catalyst particles to find an approach to incorporate thermal effects into the pellets, within reasonable constraints of computational effort and realism. This was our first look at the problem of bringing together CFD and heterogeneously catalyzed reactions. To have included species transport in the particles would have required a 3D diffusion-reaction model for each particle to be included in the flow simulation. The computational burden of this approach would have been very large. For the purposes of this first study, therefore, species transport was not incorporated in the model, and diffusion and mass transfer limitations were not directly represented. 

In the previous sections, only simple, irreversible reactions have been considered whose kinetics were assumed to obey a simple power rate law of the type r = kc". The reason for this assumption was to have analytical solutions for most of the important problems in order to demonstrate the key effects in a clear manner. Moreover, many heterogeneously catalyzed reactions, although not strictly obeying a power rate law, can nevertheless be described by this kind of rate expression for practical purposes, at least when the concentration range to be covered is not too wide. 

Therefore, it is important to determine a reaction rate. Moreover, it is important to know how to determine it because the reaction rate is influenced by several factors, hampering the comparison of reactions, reactors and catalysts. The next sections will shortly introduce the basic concepts for the description of the kinetics of a heterogeneously catalyzed reaction and their practical applications followed by three real-research examples demonstrating the application and possible problems in detail. 

A further problem arises from the fact that catalysts Vork by forming chemical bonds with the substrate molecules. Thus metals form hydrides with H2 and H donors such as CH4 or NH3, or oxides with O2 and 0 donors such as CO2 or NO2. It is presumably these surface compounds that play the role of active intermediates in the catalytic reaction. Under slightly different conditions, however, it is possible to extend the catalyst-adsorbate reaction to produce bulk compounds, e.g., hydrides and oxides. In view of this ability of the surface phase to propagate into the bulk in many instances, it is not at all clear that only the surface of the solid catalyst is active in the reaction. Such complexities add enormously to the difficulty of interpreting the kinetic data from heterogeneously catalyzed reactions. 

A catalyst is a substance that increases the rate at which a chemical reaction approaches equilibrium without, itself, becoming permanently involved in the reaction. The key word in this definition is permanently since there is ample evidence showing that the catalyst and the reactants interact before a reaction can take place. The product of this interaction is a reactive intermediate from which the products are formed. This substratexatalyst interaction can take place homogeneously with both the reactants and the catalyst in the same phase, usually the liquid, or it can occur at the interface between two phases. These heterogeneously catalyzed reactions generally utilize a solid catalyst with the interaction taking place at either the gas/solid or liquid/solid interface. Additional phase transport problems can arise when a gaseous reactant is also present in the liquid/solid system. 

Hinshelwood kinetics have played an important role in developing an understanding of heterogeneously catalyzed reactions and that they have been useful in solving reactor and process design problems but there is no reason why these basic concepts caimot be modified to give a more accurate assessment of the actual processes taking place on a catalyst surface. 

This book was written to overcome this problem by providing the synthetic chemist with sufficient information to understand the effect that the different reaction variables can have on the outcome of a heterogeneously catalyzed reaction. While the complete coverage of these factors would require several volumes, it is felt that sufficient information is given here so a rational approach can be applied in selecting the reaction conditions needed to optimize the product yield. For those readers requiring more information, references to reviews and the original literature are provided. 

We may first divide tubular reactors into those designed for homogeneous reactions, and therefore basically just an empty tube, and those designed for a heterogeneously catalyzed reaction, and hence to be packed with a catalyst. Both types can of course be operated adiabatically, and it was the simplest model of these that we discussed in the last chapter. If the temperature of the reactor is to be controlled this is through the wall, and the associated problems of heat transfer now arise. These include transfer at the wall and subsequent radial diffusion across the flowing reactants. In the empty tubular reactor there may be considerable variations in flow rate across the tube. For example, in the slow laminar flow the fluid... 

There are also many technical problems in applying HTE approaches to catalyst development. A catalyst, especially one for a heterogeneously catalyzed reaction, is an extremely complex system (Fig. 15.1). Looking only at the active phase, the nature of this is... 

The introduction of the catalyst presents one of the main problems in using MSRs for heterogeneously catalyzed reactions. There are some examples of reactors that are constructed directly from the catalytically active material. Kestenbaum et al. [145] used silver foils for the construction of a microchannel reactor for the partial oxidation of ethene to oxirane. A similar concept was proposed by Fichtner et al. [91,146], These authors used a microstructured rhodium catalyst for the partial oxidation of methane to syngas. This reaction can be considered as a coupling of the exothermic oxidation and the endothermic reforming of methane, which occur at different reaction rates. In such a case, the formation of a pronounced axial temperature profile can be avoided through the use of a material with high thermal conductivity. The reactor... 

Homogeneously catalyzed reactions with dissolved transition metal complexes are generally carried out in the usual two-phase reactors for gas-liquid systems. The standard reactor is the batch or continuous stirred tank. Since diffusion problems are rarely encoimtered in homogeneous catalysis, the reaction engineering is much simpler than for heterogeneously catalyzed reactions. 

A proper description of heterogeneously catalyzed oxidation reactions must treat several difficult problems simultaneously. First is the characterization of the solid surface in its reactive state. What oxygen species exist on this surface and what reactions does each species undergo What other sites for adsorption are present Second is the problem of reaction path. What steps are involved in the reaction What are the structures and relative energy contents of the intermediates Third is the problem of reaction velocity, a general and difficult problem in all chemistry. What are transition states, activation energies, and reaction probabilities for the various steps ... 

The use of reactive distillation for reactions that rely on a solid catalyst was developed in the early 1970s [4], Heterogeneously catalyzed reactive distillation poses the additional problem of how to place the solid cata-... 

In addition to these mass transport steps, heat conduction can also be important in heterogeneously catalyzed processes. For exothermic reactions the heat generated at the catalytic site must be dissipated away from the catalyst and into the reaction medium while heat must be supplied to the active sites for endothermic reactions. In liquid phase processes heat transport is generally not a significant factor since the liquid tends to equalize the temperature throughout the reaction medium and, thus, facilitate temperature control. In vapor phase processes, however, heat transport can be a significant problem. 

Useful multicomponent catalyst systems as well as multifunctional catalysts both offer new possibilities for the performance of catalytic processes this potential, however, can hardly be used as yet. One of the reasons for this difficulty stems from the fact that the preparation of such catalytic systems requires highly selective as well as sufficiently active catalytic components which, in addition, all reach their optimal catalytic properties for the same reaction conditions. This demand can be fulfilled by the use of tailor-made, catalytically active, transition metal complexes. The problem, however, is that these catalysts normally work via a relatively complex catalytic cycle. In a one-pot reaction system, therefore, a large number of different chemical species must be expected. Such a complex structured system can lead to several problems since it cannot be assumed that in a homogeneously catalyzed reaction system all components do not negatively interact. Even if a sufficiently stable catalyst system can be found by applying one or more of the different heterogenization techniques, this type of problem is hard to solve be-... 

A major problem in using microstructured reactors for heterogeneously catalyzed gas-phase reactions is how to introduce the catalytic active phase. The possibilities are to (i) introduce the solid catalyst in the form of a micro-sized packed bed, (ii) use a catalytic wall reactor or (iii) to use novel designs. Kiwi-Minsker and Renken [160] have discussed in detail these alternatives. 

In heterogeneously catalyzed gas-phase reactions, one of the problems encountered in the first publication was the fact that only relatively large thermal signals could be detected. This problem was solved by the work of Holzwarth et al. [18], who used a background subtraction technique to reduce the detection limit to differences of about 0.1 K. With this set-up it was possible to analyze the activity of several metal-doped, amorphous, mixed metal oxides in total oxidation reactions of hydrocarbons. 

Step 11. Write all the boundary conditions that are required to solve this boundary layer problem. It is important to remember that the rate of reactant transport by concentration difhision toward the catalytic surface is balanced by the rate of disappearance of A via first-order irreversible chemical kinetics (i.e., ksCpJ, where is the reaction velocity constant for the heterogeneous surface-catalyzed reaction. At very small distances from the inlet, the concentration of A is not very different from Cao at z = 0. If the mass transfer equation were written in terms of Ca, then the solution is trivial if the boundary conditions state that the molar density of reactant A is Cao at the inlet, the wall, and far from the wall if z is not too large. However, when the mass transfer equation is written in terms of Jas, the boundary condition at the catalytic surface can be characterized by constant flux at = 0 instead of, simply, constant composition. Furthermore, the constant flux boundary condition at the catalytic surface for small z is different from the values of Jas at the reactor inlet, and far from the wall. Hence, it is advantageous to rewrite the mass transfer equation in terms of diffusional flux away from the catalytic surface, Jas. 

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