Reactors Accomplishing Heterogeneous Reactions

1 I Homogeneous versus Heterogeneous Reactions in Tubular Reactors 

In earlier chapters, tubular reactors of several forms have been described (e.g., laminar flow, plug flow, nonideal flow). One of the most widely used industrial reactors is a tubular reactor that is packed with a solid catalyst. This type of reactor is called fixed-bed reactor since the solid catalyst comprises a bed that is in a fixed position. Later in this chapter, reactors that have moving, solid catalysts will be discussed. 

A complete and perfect model for a fixed-bed reactor is not technically possible. However, such a model is not necessary. Rather, what is needed is a reasonably good description that accounts for the major eifects. In this chapter, the fixed-bed reactor is analyzed at various degrees of sophistication and the applicability of each level of description is discussed. 

Illustrations of tubular reactors (a) unpacked tube, (b) packed tube. F, = C,v = uAcCc 

Effects of catalyst particle size on rj, and limits of flow rates. 

---

As mentioned above, the vast majority of examples of EPR spectroscopy in catalysis comprise heterogeneous catalytic reactions, mostly performed at elevated temperatures and under continuous flow of reactant gas mixtures. To realize these conditions, heatable flow reactors have to be placed directly into the cavity of the spectrometer. The design can be accomplished in various ways. 

A tubular reactor packed with a heterogeneous catalyst is accomplishing an oxidation reaction of an alcohol to an aldehyde. The reactions are ... 

The basic problem in the design of a heterogeneous reactor is to determine the quantity of catalyst and/or reactor size required for a given conversion and flow rate. In order to obtain this, information on the rate equaiion(s) and their parameter(s) must be made available. A rigorous approach to the evaluation of reaction velocity constants has yet to be accomplished for catalytic reactions at this time, industry still relies on the procedures set forth in the previous chapter. For example, in catalytic combustion leac-tioas, the rate equation is extremely complex and cannot be obtained either analytically or numerically. A number of equations may result and some simplification is often warranted. As mentioned earlier, in many cases it is safe to assume that the expression may be satisfactorily expressed by the rate equation of a single step. 

Heterogeneously catalyzed reactions are often accomplished in a packed bed reactor, which is essentially a tube packed with catalyst particles. In a packed bed reactor, the weight of catalyst takes the place of the residence time, and so we can show that the rate of the reaction is dependent on the flow rate of the reactant and the weight of catalyst ... 

---