Internal mass transfer, catalytic wall

External mass transfer In general, the thickness of the catalyst layer will be kept sufficiently small to avoid the influence of internal mass transfer on the kinetics. In this case, only the transfer of the reactants from the bulk of the fluid to the catalytic wall must be considered. The radial velocity profile in a single channel develops from the entrance to the position where a complete Poiseuille profile is established (provided that the flow is laminar). The length of the entrance zone depends on the Reynolds number and can be estimated from the following empirical relationship [85,86] ... 

Advances in the technology of microstructured catalytic reactors depend crucially on the ability to generate appropriate catalyst layers. The activity of the catalyst determines the thickness of the layer that needs to be deposited on the structured support or the walls of the MSR. Relatively thick layers of up to several hundred micrometers are necessary for moderate reaction rates to achieve good reactor performance, whereas thin layers are desirable for very fast catalytic reactions to avoid internal mass transfer limitations (Section 3.2.3). 

As an example of the decreasing efficiency caused by external and internal mass transfers, we consider an irreversible first-order reaction and a porous catalyst layer. The situation corresponds to a catalytic wall reactor. The relative importance between external and internal mass transfers is characterized by the ratio of the diffusion time in the porous layer tp and the characteristic time for external mass transfer called the Biot number, Bi = t /t - (L /DJk a for mass transfer. 

If the thickness of the catalytic layer on the reactor wall is sufficiently small, internal mass transfer resistances can be neglected and only external resistances in the fluid phases are considered. The reaction rate per unit of the outer surface of the catalytic layer is described by a pseudo first order reaction (mol m s" ) ... 

Therefore, microstructured multichannel reactors with catalytically active walls are by far the most often used devices for heterogeneous catalytic reactions. Advantages are low pressure drop, high external and internal mass transfer performance and a quasi-isothermal operation. In most cases the reactors are based on micro heat exchangers as shown in Figure 15.2. Typical channel diameters are in the range of... 

Owing to the comparatively small size of the pores (up to 100 p.m, compared to a pitch of a few millimeters for the honeycomb channels) and the small thickness of the catalyst layer (a few microns, compared to some tenths of a millimeter for the catalytic wall of the honeycomb channels), both internal and external mass transfer limitations to NO conversion in catalytic filters can easily be neglected. An efficiency factor equal to unity can thus be assumed with confidence for NO reduction, contrary to honeycomb catalysts, for which this parameter is hardly higher than 0.05% at the conventional operating temperatures (320-380 C). 

The catalytic wall reactor with channel diameter in the range of 50-1000 pm and a length dependent on the reaction time required circumvents the shortcomings of micro packed beds. This is discussed in more detail in Section 6.5.4. However, in most of the cases, the catalytic surface area provided by the walls alone is insufficient for the chemical transformation and, therefore, the SSA has to be increased by the chemical treatment of the channel walls, or by coating them with highly porous support layers. The thickness of the layer 5 3, depends on catalytic activity. In general, the layer thickness is sufficiently small to avoid internal heat and mass transfer influences. Catalytic layers can be obtained by using a... 

In heterogeneous catalysts, the tailored and pillared cavity is an essential property as it enables the movement of reactants to inner catalytic sites. The mass transfer of reactants and products inside the pores is mainly influenced by the interaction of the internal walls of the channel with organic molecules, and can consequently be controlled by the difference in polarities. To ensure such properties, anchored ruthenium hybrid zirconium phosphate-phosphonates coated with hydrophobic linear double-stranded polystyrene over the inner surface of the Zr layers were prepared by the first complexation of Ru and then molding of inorganic backbone method, and used as the catalyst in the ATH of o-, m- and p-substituted acetophenones (Fig. 42) [121]. This catalyst showed good catalytic activity and enantioselectivity (73.6-95.6 % ees) in the aqueous reduction with FA-TEA as the hydrogen donor, and could retain its catalytic properties after five runs in the case of acetophenone. 

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