Fluid-solid catalytic systems

It is for these reasons that the editor decided to organise a NATO Advanced Study Institute covering all aspects, with the ultimate aim of an overview of the landscape to identify features that provide orientation. After many discussions with Professors W.-D. Deckwer, P.V. Danckwerts, C. Hanson and M.M. Sharma, it vias decided to limit the ASI to (1) gas-liquid, (2) liquid-liquid, and (3) gas-liquid-solid systems. Thus, the only really important area left out was fluid-solid systems, part of which v/as hov/ever dealt with in another NATO Advanced Study Institute on "Analysis of Fluid-Solid Catalytic Systems" under the directorship of Prof. G.F. Froment. The originally planned date for the Institute had to be postponed for one year in order to prevent a clash with another NATO Advanced Study Institute. 

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 ... 

Mass transfer with chemical reaction in multiphase systems" covers, indeed, a large area. Table 1 shows a general classification of the systems encountered. From the possible two-phase systems, solid-solid reactions, liquid-solid (reactive or catalytic) and gas-solid (reactive or catalytic) reactions are not discussed here. The first one was reviewed by Tamhankar and Doraiswamy (2) and gas-solid (reactive) systems, such as, coal gasification, calcination of limestone, reduction of ores, etc. have been treated in some detail in recent reviews (3-5). The industrially important fluid-solid catalytic processes were the topic of a previous Advanced Study Institute (6) and have been also discussed authoritatively elsewhere (5,7). Concerning solid (reactive)-liquid two-phase systems, only some interesting examples are presented in Table 2 (1). 

As indicated above, the principal difference between reactor design calculations involving homogeneous reactions and those involving catalytic (fluid-solid) heterogeneous reactions is that for the latter the reaction rate is based on the mass of solid, W, rather than on reactor volume V. For a fluid—solid heterogeneous system the rale of reaction of species A is then defined as — with the aforementioned units of (mols A reacted)/(mass catalyst) (time). 

A similar nonlinear equation for heterogeneous catalytic systems was developed empirically by Olaf Hougen and Kenneth Watson and derived on a more scientific basis by Irving Langmuir and Cyril Hmshelwood. WTien applied to fluid reactants and solid catalysts, the nonlinear equation m its simplest form becomes... 

In conventional solid-liquid or solid-gas heterogeneous catalytic systems, the catalyst is conveniently separated from the fluid-phase reaction product. When an ionic liquid is used as a phase to isolate a catalyst, the catalyst is fully dispersed and mobile and may be fully involved in the reaction. When a homogeneous catalyst is isolated by anchoring onto the surface of a solid support (e.g., by reaction with OH groups), the result may be a stable catalyst that is not leached into the reactant... 

In the case of two fluids, two films are developed, one for each fluid, and the corresponding mass-transfer coefficients are determined (Figure 3.2). In a fluid-solid system, there is only one film whereas the resistance within the solid phase is expressed by the solid-phase diffusion coefficient, however, in many cases an effective mass-transfer coefficient is used in the case of solids as well. Consider the irreversible catalytic reaction of the form... 

It should be noted here that while in catalytic systems the rate is based on the moles disappearing from the fluid phase - dddt, and the rate has the form ( —ru) = f(k, C), in adsorption and ion exchange the rate is normally based on the moles accumulated in the solid phase and the rate is expressed per unit mass of the sohd phase dqldt where q is in moles per unit mass of the solid phase (solid loading). Then, the rate is expressed in the form of a partial differential diffusion equation. For spherical particles, mass transport can be described by a diffusion equation, written in spherical coordinates r ... 

It should be noted here that while in catalytic systems the rate is based on the moles disappearing from the fluid phase -rm, in adsorption and ion exchange, the rate is normally based on the moles accumulated in the solid phase rm, and the rate is expressed per unit mass of the solid phase as... 

The various volumetric mass-transfer coefficients are defined in a manner similar to that discussed for gas-liquid and fluid-solid mass transfer in previous sections. There are a large number of correlations obtained from different gas-liquid-solid systems. For more details see Shah (Gas-Liquid-Solid Reactor Design, McGraw-Hill, 1979), Ramachandran and Chaudhari (Three-Phase Catalytic Reactors, Gordon and Breach, 1983), and Shah and Sharma [Gas-Liquid-Solid Reactors in Carberry and Varma (eds.), Chemical Reaction and Reactor Engineering, Marcel Dekker, 1987],... 

Consider a physical system shown schematically in Figure 1. A fluid stream containing reactant A is moving upwards in plug flow with a constant velocity U. The reactant is adsorbed by a stream of solid catalytic particles falling downwards with a constant velocity V and occupying the void fraction of 1 - e. On the surface of catalyst an irreversible chemical reaction A - B is occurring and the product B is then rapidly desorbed back into the fluid phase. Instantaneous adsorption equilibrium for the species A is assumed. 

In catalytic systems morphological changes of the pore structure, brought upon by the reaction and sorption processes, typically result in a reduction of the available pore volume. In some instances the internal pore structure is eventually blocked and becomes completely inaccessible to transport and/or reaction. In the field of noncatalytic fluid-solid reactions and acid rock dissolution, on the other hand, the chemical reaction consumes the solid matrix of the porous medium leading eventually to fragmentation and... 

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. 

We first present general criteria for the rational use of MSRs on the basis of fundamentals of chemical reaction engineering [21-24], The main characteristics of MSRs are discussed, and the potential gain in reactor performance relative to that of conventional chemical reactors is quantified (Section 2). Subsequently, the most important designs of fluid-solid and multiphase reaction systems are described and evaluated (Sections 3 and 4). Because microstructured multichannel reactors with catalytically active walls are by far the most extensively investigated MSRs for heterogeneous catalytic reactions, we present their principal design and recent synthetic methods separately in Section 5. 

Fluid-fluid (gas-liquid and liquid-liquid) reactions are of great industrial importance and include gas-liquid, liquid-liquid, gas-liquid-solid (noncatalytic and catalytic), and solid-liquid systems. 

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 ... 

Mass and Heat Transfer Effects on Heterogenous Catalytic Reactions 83 Table 2.1 Physical properties of fluid/solid systems [13]. 

Fluid-fluid-solid systems 1. Gas-1iquid-sparingly soluble reactive solid, 2.6as-liquid-insoluble reactive solid, 3.6as-liquid-catalytic solid... 

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 multi-phase catalytic reactor, the catalyst is usually a solid phase in contact with the liquid phase. Figure 4.1 shows a typical multi-phase catalytic system, where one fluid phase (gas or liquid) is dispersed in a liquid phase which contains porous catalyst particles. The reactants need to diffuse from their respective phases to the catalytic site where reaction products are formed and then they can diffuse back to one or both fluid phases. The overall reaction rate of the process will be affected by the inter-phase mass transfer rates near the gas-hquid and the liquid-solid interfaces, as well as by the intra-phase mass transfer rate competing with the intrinsic reaction rate inside the catalyst structure. 

A variety of gases are used to transfer solids from one location to another nitrogen, air, chlorine, and hydrogen. When properly fluidized, solids respond like fluids. Solid transfer requires small, granular, porous solids that respond positively to aeration. Several examples of industry processes that use this procedure are modern plastics manufacturing (granules, powder, flakes), catalytic cracking units, and vacuum systems. 

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