Types of Heterogeneous Reactions

Gas-solid heterogeneous reactions may be noncatalytic. An example is the hydrofluorination of uranium dioxide pellets referred to in Sec. 7-1. Since one reactant is in the solid phase and is consumed, the rate of reaction varies with time. Hence such processes are basically transient, in comparison with the steady-state operation of gas-solid catalytic reactors. The process for smelting ores such as zinc sulfide, 

In many noncatalytic types a solid product builds up around the reacting core [for example, Na2S04(j) is deposited around the NaCl particles in the last illustration]. This introduces the additional physical processes of heat and mass transfer through a product layer around the solid reactant. A somewhat different form of noncatalytic gas-solid reaction is the regeneration of catalysts which have been deactivated by the deposition of a substance on the interior surface. The most common is the burning of carbon (with air) which has been gradually deposited on catalyst particles used in hydrocarbon reactions. Many of the physical and chemical steps involved here are. the same as those for gas-solid catalytic reactions. The chief difference is the transient nature of the noncatalytic reaction. This type of heterogeneous reaction will be considered in Chap. 14. 

Liquid-liquid reactions are sometimes encountered. Alkylation of hydrocarbons with aqueous solution of sulfuric acid as a catalyst is an example. As in liquid-solid systems, definition of the liquid phase containing the catalyst may be difficult, reducing the effectiveness of a fundamental analysis in terms of chemical and physical steps. 

Solid-solid noncatalytic reactions are important in ceramics manufacture. It appears that diffusion resistances may be important to some extent in all such systems. The diffusion process itself is hard to define in solid-solid systems, since at least two possibilities exist volume diffusion in the solid and surface diffusion along interfaces and crystal boundaries. Little is known about the kinetics of solid-solid reactions at the reacting interface because most measurements include diffusion effects. 

In a sketch similar to Fig. 7-1 show schematically the concentration profiles for a first-order reversible catalytic reaction. Consider three cases a) reaction control, b) diffusion control, and (c) an intermediate case. 

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The various types of heterogeneous reactions are shown in Table 3.3. They are broadly grouped as solid-gas, solid-liquid, solid-solid, liquid-gas, and liquid-liquid reactions. The different types included in each group are also shown in the compilation. Some representative processes have been indicated as examples. It may be pointed out that in the group of solid-liquid reactions a specific mention of what is known as autocatalytic reactions has not been made. The autocatalytic processes occur when the liquid product reacts further with the solid undergoing reaction. The dissolution of copper in dilute sulfuric acid (or aqueous ammonia) in the presence of oxygen may be cited as an example ... 

We shall consider different types of heterogeneous reactions and their possible radical mechanisms 10, 27). Consider a reaction between two molecules AB and CD, where A, B, C, D denote the separate atoms or atomic groups. 

A heterogeneous catalytic reaction occurs at or very near the fluid-solid interface. The principles that govern heterogeneous catalytic reactions can be applied to both catalytic and noncatalytic fluid-solid reactions. These two other types of heterogeneous reactions involve gas-liquid and gas-Hquid-solid systems. Reactions between gases and liquids are usually mass-transfer limited. 

Heterogeneous Reactions. Heterogeneous reactions are those in which more than one phase is involved. Some important types of heterogeneous reactions of industrial interest are ... 

Reactions are classified kinetically as homogeneous or heterogeneous. A homogeneous reaction occurs entirely in one phase a heterogeneous reaction occurs, at least in part, in more than one phase. A common type of heterogeneous reaction has a rate which... 

It is unfortunate that the usefulness of the fiction of homogeneity in this and other types of heterogeneous reaction system should have helped to foster the fallacy that chemical reactions are intrinsically scalar phenomena. 

Thus, the reaction order in the reactant itself is useful to evaluate. If the reactant is likely to be adsorbed significantly, then the reaction order must be interpreted, as in other types of heterogeneous reactions, with due attention to the type of adsorption isotherm and the extent of coverage. Since the kinetics of electrode reactions depend in a primary way on the potential (see Section 4), the reaction order must be specified and evaluated with respect to a constant electrode potential. Usually a supporting (electrochemically inactive) electrolyte is used so that effects of ion distribution and potential in the double layer remain approximately constant as other quantities are varied in the experimental analysis, viz., pH, reactant concentration. 

Gas-solid reactions are among the most common type of heterogeneous reaction processes. The platinum surface catalyzed oxidation of hydrogen, discussed in the previous example, is an excellent example of a heterogeneous gas-solid surface reaction process. In Chapter 5, we will smdy a number of different gas-solid kinetic processes in great detail. To prepare for those smdies, in this section we will discuss a few more simple gas-solid surface reaction processes. 

A fixed bed (packed-bed) reactor may be physically viewed as a tubular flow reactor which is packed with solid catalyst particles. This type of heterogeneous reaction system is most frequently used to catalyze gaseous reactions. The design equation... 

From the point of view of the processes, we will distinguish two types of heterogenous reactions ... 

In this section, we want to explore how the mechanism of an irreversible reaction can be inferred from the overall reaction rate, instead of the other way around. This exploration can be complicated, hampered by elaborate algebra. As a result, we consider only special cases of the two types of heterogeneous reactions shown in Fig. 16.4-1. These types differ in the products produced by the reaction. In some cases, these products are fluid, and hence diffuse away. More commonly, the products form a layer of ash around an unreacted core this second case is sometimes called a top-ochemical model. 

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