External mass transfer, catalytic wall

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). 

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] ... 

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. 

In the present chapter we focus our discussion on external mass transfer in MSR. Thus, we assume that the reactions occur on the outer surface of the catalyst particle or of the wall catalytic layer. The observable effective reaction rate is determined by the ratio of the characteristic mass transfer time, t, and the characteristic reaction time, commonly known as second Damkohler number Dull (see Section 2.6.1). 

The knowledge of this entrance length is important for the design of inlet sections or to define the validity of simplified models derived on the basis of fully developed laminar flow conditions. In practice, an inlet presection in the channel (with an uncoated/inert wall) can be used to allow for flow development before the fluid reaches the catalytically active region. A similarity between the entrance length of velocity and concentration/temperature profiles can be found, particularly when the wall temperature can be assumed to be uniform or severe external mass transfer control [42]. In Lopes et al. [43], the thickness of this region is discussed for the mass transfer problem with a finite wall reaction. 

Endothermic reactions, such as steam reforming, are usually carried out in long narrow tubes filled with catalysts and externally heated by flames. The heat could be provided more uniformly and more accurately at the necessary level by a combustion catalyst coated on the outside of the tubes, and heat transfer rates could be further improved by coating the endothermic reaction catalyst on the inner wall of the tube. In this way, the heat of combustion is transferred to the heat sink (the endothermic reaction) through the solid wall, avoiding solid-gas heat transfer resistances. However, the tubular geometry is not most efficient for this application because of the difficulty to coat the inside of the tubes and the need to include static mixers to facilitate mass transfer to the catalytic surfaces. 

Two types of catalyst systems are commonly used (1) packed beds and (2) monolith blocks. The first type is a perforated container containing spherical beads coated with catalytic ingredients it allows the reactants to flow through the catalytic beds. It has high external surface area and high mass transfer efficiency between reactants and catalyst. The second type has the walls of monolithic honeycomb wash coaled with the catalyst ingredients and allows the reactants to flow through the channels in the monolith. 

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... 

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