Catalytic reactions fluid-solid steps

For a catalytic reaction of a reactant from a single fluid phase (either gas or liquid) to take place on a solid catalyst, diffusion processes also play a role, so in the complete process the following steps can be distinguished ... 

Figure 1. Individual steps of a simple, heterogeneous catalytic fluid-solid reaction A — A2 carried out on a porous catalyst...

A heterogeneous catalytic reaction involves adsorption of reactants from a fluid phase onto a solid surface, surface reaction of adsorbed species, and desorption of products into the fluid phase. Clearly, the presence of a catalyst provides an alternative sequence of elementary steps to accomplish the desired chemical reaction from that in its absence. If the energy barriers of the catalytic path are much lower than the barrier(s) of the noncatalytic path, significant enhancements in the reaction rate can be realized by use of a catalyst. This concept has already been introduced in the previous chapter with regard to the Cl catalyzed decomposition of ozone (Figure 4.1.2) and enzyme-catalyzed conversion of substrate (Figure 4.2.4). A similar reaction profile can be constructed with a heterogeneous catalytic reaction. 

The assumptions inherent in this procedure should not be forgotten. They have been mentioned, but it is worthwhile to emphasize one, the con- stancy of C . This corresponds to an assumption that the maximum value of 9 is unity, or that the total number of active sites is constant. Also, note that we are considering a general approach to the formulation of rate equations for fluid-solid catalytic reactions. In several specific cases sufficient data have been obtained to permit a more detailed analysis of the mechanism and rates of the adsorption and surface-reaction steps. An example is the... 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

When components in solid and fluid phases react, the sequence of steps must be similar to those for fluid-solid catalytic reactions. In Sec. 8-2 catalytic reactions were explained in terms of a three-step process adsorption of the fluid molecule on the sohd surface reaction on the surface involving the adsorbed molecule, and desorption of product. We shall not be concerned here with the mechanism of these processes instead we shall start out on the basis that the rate equation is known from experimental measurements. Often observed data agree with a rate equation which is first order in concentration of the reactant injthe-fluid..phase and directly proportional to the surface of the reactant in the solid phase. For example, despite the stoichiometry of the reaction... 

In catalytic gas-solid reactions, the reaction takes place at catalytic sites on the surface of the solid. To obtain appreciable reaction rates, porous solids are used and the reactions take place on the surface of the pores in the interior of the particle. Hence, catalytic gas-solid reactions involve seven steps (1) transport of the reactant from the fluid bulk to the mouth of the pore, (2) diffusion of the reactant to the interior of the pore, (3) adsorption of the reactant to the surface of the solid,... 

The chemical steps for catalytic reactions adsorption, desorption and surface reaction, are covered in Chapter 5, These processes occur at a fluid-solid interface and the rates scale directly with the total number of surface, sites, Cm.< Table 7T. lists. some. of the. i.mportan.t. c.om.rae.rciai... 

The principles of homogeneous reaction kinetics and the equations derived there remain valid for the kinetics of heterogeneous catalytic reactions, provided that the concentrations and temperatures substituted in the equations are really those prevailing at the point of reaction. The formation of a surface complex is an essential feature of reactions catalyzed by solids and the kinetic equation must account for this. In addition, transport processes may influence the overall rate heat and mass transfer between the fluid and the solid or inside the porous solid, > that the conditions over the local reation site do not correspond to those in the bulk fluid around the catalyst particle. Figure 2.1-1 shows the seven steps involved when a molecule moves into the catalyst, reacts, and the product moves back to the bulk fluid stream. To simplify the notation the index s, referring to concentrations inside the solid, will be dropped in this chapter. 

Fluid-solid phase catalytic reactions are generally recognized to proceed according to the following seven (7) steps ... 

Heterogeneous catalytic reactions in supercritical solvents Obviously, a solid catalyzed reaction takes place only on the active sites of the porous catalyst with the implication of some mass and heat transport steps prior to and after the reaction. The first step is the diffusion of the reactants through the film surrounding the catalyst particle to the external surface of the catalyst, followed by diffusion of the reactants into the catalyst pore to the active site in the pores. These steps are limited by the dif-fusivity and viscosity of the reactants. In the case of a supercritical fluid phase reaction, the diffusivity is higher than the liquid diffusivity, viscosity is less than the liquid viscosity and therefore, the rate of transfer to the active site will be higher. After the adsorption, reaction and desorption steps, the products have to diffuse out of the pore, and again... 

The final step of the whole reaction process is the desorption of the products. This step is essential not only for the practical purpose of collecting and storing the desired output, but also for the regeneration of the catalytic active sites of the surface. Most reactions have at least one rate-hmiting step, which frequently makes the reaction prohibitively slow for practical purposes when, e.g., it is intended for homogeneous (gas or fluid) media. The role of a good solid-state catalyst is to obtain an acceptable... 

The process by which certain porous solids bind large numbers of molecules to their surfaces is known as adsorption. Not only does it serve as a separation process, but it is also a vital part of catalytic-reactionprocesses. As a separation process, adsorptionis used most oftenfor removal of lo w-concentrationimpurities and pollutantsfrom fluid streams. It is also the basis for cliromatography. In surface-catalyzedreactions, the initial step is adsorptionof reactant species the final step is the reverse process, desorption of product species. Since most industrially important reactions are catalytic, adsorption plays a fundamental role in reaction engineering. 

Step 1. Reactants enter a packed catalytic tubular reactor, and they must diffuse from the bulk fluid phase to the external surface of the solid catalyst. If external mass transfer limitations provide the dominant resistance in this sequence of diffusion, adsorption, and chemical reaction, then diffusion from the bulk fluid phase to the external surface of the catalyst is the slowest step in the overall process. Since rates of interphase mass transfer are expressed as a product of a mass transfer coefficient and a concentration driving force, the apparent rate at which reactants are converted to products follows a first-order process even though the true kinetics may not be described by a first-order rate expression. Hence, diffusion acts as an intruder and falsifies the true kinetics. The chemical kineticist seeks to minimize external and internal diffusional limitations in catalytic pellets and to extract kinetic information that is not camouflaged by rates of mass transfer. The reactor design engineer must identify the rate-limiting step that governs the reactant product conversion rate. 

For the heterogeneous catalytic process to be effective, the reactants present in the surrounding fluid phase must be transported to the surfece of the solid catalyst, and after the reaction, the products formed must be carried back from the surface to the bulk fluid. The path of the physical rate processes at the particle scale is divided into two parts, as depicted in the 7-step sequence of the continuous reaction model used in microkinetic analysis ... 

In this section, the catalyst particle or the catalytic surface is assumed to be nonporous, and the 7-step sequence is reduced to a 5-step sequence with steps 1, 3-4-5, and 7. In this case, the only mass transfer resistance involved is between the fluid and the outer surfece of the particle. The rates of external mass transfer depend on (i) the temperature, pressure, and physical properties of the fluid phase under these conditions, (ii) the gas velocity relative to the solid surface, and (iii) the intrinsic rate of the surface reaction. In other words, the rate at which mass is transported from the fluid to the surface is determined by the relative magnitudes of ... 

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