KINETICS OF FLUID-SOLID CATALYTIC REACTIONS

CHAPTER 9 KINETICS OF FLUID-SOLID CATALYTIC REACTIONS... 

This approach to kinetics of fluid-solid catalytic reactions was proposed by C. N. Hinshel-wood ( Kinetics of Chemical Change, Oxford University Press, London, 1940) and developed in detail by O. A. Hougen and K. M. Watson ( Chemical Process Principles, part 3, Kinetics and Catalysis, John Wiley Sons, Inc., New York, 1947). 

Chemisorption data often do. not fit Eq. (8-6). However, the basic concepts on which the Langmuir isotherm is based, the ideas of a dynamic equilibrium between rates of adsorption and desorption and a finite adsorption time, are sound and of great value in developing the kinetics pf fluid-solid catalytic reactions. Equations (8-4) to (8-6) form the basis for the rate equations presented in Chap. 9. 

A chemical reactor is a vessel in which reactants are converted to products through chemical reactions. This vessel takes many shapes and sizes depending upon the nature of the chemical reaction. The choice of a suitable laboratory reactor depends upon the nature of the reaction system (fluid-solid catalytic, fluid-solid noncatalytic, fluid-fluid, etc.), the nature of the required kinetic or thermodynamic data, or the feasibility of operation. The important parameters for a successful reactor design are the following ... 

These intriguing situations, which are similar to the so-called "diffusion falsification" regime of fluid-porous catalytic solid systems (5), can be successfully handled by the "theory of mass transfer with chemical reaction". Indeed, they can be deployed to obtain kinetics of exceedingly fast reactions in simple apparatuses, which in the normal investigations in homogeneous systems would have required sophisticated and expensive equipment. Further, it is possible, under certain conditions, to obtain values of rate constants without knowing the solubility and diffusivity. In addition, simple experiments yield diffusivity and solubility of reactive species which would otherwise have been - indeed, if possible - extremely difficult. 

In a fixed-bed catalytic reactor for a fluid-solid reaction, the solid catalyst is present as a bed of relatively small individual particles, randomly oriented and fixed in position. The fluid moves by convective flow through the spaces between the particles. There may also be diffusive flow or transport within the particles, as described in Chapter 8. The relevant kinetics of such reactions are treated in Section 8.5. The fluid may be either a gas or liquid, but we concentrate primarily on catalyzed gas-phase reactions, more common in this situation. We also focus on steady-state operation, thus ignoring any implications of catalyst deactivation with time (Section 8.6). The importance of fixed-bed catalytic reactors can be appreciated from their use in the manufacture of such large-tonnage products as sulfuric acid, ammonia, and methanol (see Figures 1.4,11.5, and 11.6, respectively). 

Figure 3. Transition from the kinetic regime to the diffusion-controlled regime of a heterogeneous catalytic fluid-solid reaction carried out on a porous catalyst.

Tank reactors for solid-catalyzed gaseous or liquid reactions are seen much less frequently than tubular reactors because of the difficulty in separating the phases and in agitating a fluid phase in the presence of solid particles. One type of CSTR used to study catalytic reactions is the spinning basket reactor, which has the catalyst embedded in the blades of the spinning agitator. Another is the Berty reactor, which uses an internal recycle stream to achieve perfectly mixed behavior." These reactors (see Chapter 5) are frequently used in industry to evaluate reaction mechanisms and determine reaction kinetics. 

This brief overview of sohd catalysts addresses only the nature, class, and catalytic properties of a variety of catalysts. Modeling the kinetics of a reaction occurring on a solid surface is a challenging task and is firmly rooted in the principles of surface science. As this is still an evolving area, empirical shortcuts are often invoked. Furthermore, for sohd catalysts, the reactant(s) must first diffuse into the solid, and product(s) must diffuse out of it. Also, the heat evolved or required must be transported between the solid and the fluid bulk. Hence, diffusion accompanied by reaction becomes a major consideration. These microenvironmental aspects of sohd catalysts are briefly described below. 

Chapter 10 begins a more detailed treatment of heterogeneous reactors. The discussion assumes the fluid phase is a gas since this is the predominant case. This chapter continues the use of pseudohomogeneous models for steady-state, packed-bed reactors but derives expressions for the reaction rate that reflect the underlying kinetics of surface-catalyzed reactions. The kinetic models are site competition models that apply to a variety of catalytic systems including the enzymatic reactions treated in Chapter 12. Here in Chapter 10, the example system is a solid-catalyzed gas reaction that is typical of the traditional chemical industry. A few important examples are as follows ... 

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. 

When studying the kinetics of heterogeneous reactions or when designing a large catalytic reactor, there are more factors to consider than when dealing with homogeneous reactions. For a solid-catalyzed reaction, the rate depends on the reactant concentrations at the catalyst surface, but these are not the same as the bulk concentrations, because some driving force is needed for mass transfer to the surface. If the catalyst is porous, as is usually the case, there are further differences in the concentration between the fluid at the external surface and the fluid in the catalyst pores. Models must be developed to predict the surface concentrations as functions of the partial pressures or concentration in the gas or liquid, and the rate expression can then be written in terms of the fluid concentrations. 

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. 

The performance of a chemical reactor, i.e., the relation between output and input, is affected by the properties of the reactional system (kinetics, thermodynamics,...) and by the contacting pattern. The importance of this contacting pattern is quite obvious in the case of multiphase reactors like trickle-bed reactors. Reactants are indeed present in both fluid phases and reactions occur at the contact of the catalytic solid phase. Unfortunately the description of the fluid flow pattern is very difficult because of the high number of intricate mechanisms that can control this pattern. The situation is so complex that, in many cases, we do not even know the essential hydrodynamic parameters that may affect the performance of the reactor. 

Esterification of organic compounds often involves multiphase catalytic reactions in which contact of liquid (organic substrate) and solid (catalyst) phases are involved. The most common esterification processes fall into the category of two phase (liquid-solid) reactions. Both slurry and fixed bed reactors can be used for ion exchange resin catalyzed esterification reactions. The overall performance of these reactors depends on the inter phase mass transfer, intrinsic kinetics of reaction, physicochemical properties and mixing of the fluid phases. For a continuous process, fixed bed reactors should be preferred, however, in fixed bed reactors small catalyst particles cause higher pressure drop. Special type of support trays may also be required to support small catalyst particles in fixed bed reactors. 

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