Global catalytic reactor

Direct experimental studies on catalytic reactions using real atmospheric aerosols are just beginning. Therefore, the anticipated role of such reactions in the Earth s atmosphere is based mainly on estimates from experiments made with model catalysts, together with known data that characterizes the atmosphere as a sort of global catalytic reactor. For example, a drastic acceleration of chemical transformations in the atmosphere after volcanos eruptions has been observed [1]. Also, the possible... 

The rates at which chemical transformations take place are in some circumstances strongly influenced by mass and heat transfer processes (see Sections 12.3 to 12.5). In the design of heterogeneous catalytic reactors, it is essential to utilize a rate expression that takes into account the influence of physical transport processes on the rate at which reactants are converted to products. Smith (93) has popularized the use of the term global reaction rate to characterize the overall rate of transformation of reactants to... 

Let us now turn our attention to the problem of determining the global reaction rate at some arbitrary point in a heterogeneous catalytic reactor from a knowledge of the following parameters. 

In 2001, Mirodatos et al. [89] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of microkinetic analysis together with transient experiments should allow the determination of the global catalytic conversion from elementary reaction steps. Prerequisite for such analysis is the correlation of experimental data with the data of a model. Compliance between the data helps to derive the reaction mechanism. 

Catalytic reactors can be classified globally according to their mode of operation under steady state or transient conditions, as indicated in Fig. 4, or according to the contacting/mixing mode, as indicated in Fig. 5 for... 

If we adopt this global view, another question arises. Catalysts, as they work in catalytic reactors, are just the intermediary product of a solid state reaction chain which starts with preparation and activation, before the catalysts is contacted with the feed, and is followed by deactivation and, ultimately, "death". The question is what are the relationships among solid-state reactions occurring during the initial activation, catalytic reaction and deactivation ... 

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. 

Example 11-7 illustrates one of the problems in scale-up of catalytic reactors. The results showed that for all but -in. pellets intrapellet diffusion significantly reduced the global rate of reaction. If this reduction were not considered, erroneous design could result. For example, suppose the laboratory kinetic studies to determine a rate equation were made with f-in. pellets. Then suppose it was decide tojise f-ih. pellets in the commercial reactor to reduce the pressure drop through the bed. If the rate equation were used for the -in. pellets without modification, the rate would be erroneously high. At the conditions of part b) of Example 11-7 the correct would be only 0.68/0.93, or 73% of the rate measured with -in. pellets. 

The selectivity at a position in a fluid-solid catalytic reactor is equal to the ratio of the global rates at that point. The combined effect of both external and internal diffusion resistance can be displayed easily for a set of parallel reactions. We shall do this first and then consider how internal resistance influences the selectivity for other reaction sequences. 

One of the most common catalytic reactors is the fixed-bed type, in which the reaction mixture flows continuously through a tube filled with a stationary bed of catalyst pellets. Because of its importance, and because considerable information is available on its performance, most attention will be given to this reactor type. Fluidized-bed and slurry reactors are also considered later in the chapter. Some of the design methods given are applicable also to fluid-solid noncatalytic reactions. The global rate and integrated conversion-time relationships for noncatalytic gas-solid reactions will be considered in Chap. 14. 

In Chap. 4 the plug-flow model was used as a basis for designing homogeneous tubular ow reactors. The equation employed to calculate the conversion in the effluent stream was either Eq. (3-13) or Eq. (4-5). The same equations and the same calculational procedure may be used for fixed-bed catalytic reactors, provided that plug-flow behavior is a vahd assumption. AH that is necessary is to replace the homogeneous rate of reaction in those equations with the global rate for the catalytic reaction. 

This expression still includes the effect of longitudinal dispersion. It is identical to Eq. (6-41), except that the rate for a homogeneous reaction has been replaced with the global rate XpPs per unit volume for a heterogeneous catal)dic reaction. In Sec. 6-9 Eq. (6-41) was solved analytically for first-order kinetics to give Eq. (6-45). Hence that result can be adapted for fixed-bed catalytic reactors. The first-order global rate would be... 

The kinetic model proposed, eq. (13), is suitable for the reactor simulation with the aim of optimizing the reaction step and the combined global process (reactor-regenerator) for directly upgrading aqueous ethanol by catalytic transformation. 

Two-phase flow in three-phase fixed-bed reactors makes the reactor design problem complex [12], Interphase mass transfer can be important between gas and liquid as also between liquid and catalyst particle. Also, in the case of trickle-bed reactors, the rivulet-type flow of the liquid falling through the fixed bed may result (particularly at low liquid flow rates) in only part of the catalyst particle surface being covered with the liquid phase. This introduces a third mass transfer process from gas to the so-called gas-covered surface. Also, the reaction rates in three-phase fixed-bed catalytic reactors are highly affected by the heat transfer resistances resistance to radial heat transfer and resistance to fluid-to-particle heat transfer. As a result of these and other factors, predicting the local (global) rate of reaction for a catalyst particle in three-phase fixed-bed reactors requires not only... 

Multiphase catalytic reactors are employed in nearly 80% of industrial processes with annual global sales of about 1.5 trillion, contributing around 35% of the world s GDP [17]. Microreactors for multiphase reactions are classified based on the contact principles of gas and liquid phases continuous-phase contacting and dispersed-phase contacting [18]. In the former type, the two phases are kept in continuous contact with each other by creating an interface. In the latter case, one fluid phase is dispersed into another fluid phase. In addition, micro trickle bed operation is reported following the path of classical chemical engineering. The study of mass and heat transfer in two-phase flow in micro trickle bed reactors still remains as a less... 

Introduces global rate equations and explicit design equations for a variety of non-catalytic reactors... 

Fixed catalytic bed reactors have significant advantages relative to other types of heterogeneous catalytic reactors (particularly the fluid bed unit) and, consequently, are employed much more widely in the chemical industry. This section provides details on the procedures involved in the design of those reactors. As with fluidized bed reactors, global reactions rate information are required to predict the extent of conversion for a given set of operating conditions. 

The global process by which a heterogeneous catalytic reaction takes place in a packed-bed reactor can be described in a succession of steps, where the rate of the reaction is equivalent to the rate of the slowest stage in the whole mechanism of the reaction. The stages of a catalytic reaction in a packed-bed reactor are (see Figure 9.17) [126,127]... 

Another type of reactor that may have considerable future potential for use in homogeneous catalytic reactions is called the membrane reactor. These reactors have been successfully used for the commercialization of manufacturing processes based on enzyme catalysis. In fact, 75% of the global production of l-methionine is performed in an enzyme reactor. A membrane is basically an insoluble organic polymeric film that can have variable thickness. The catalyst... 

Catadiene [Catalytic butadiene] Also spelled Catadien. A version of the Houdry process for converting mixtures of butane isomers into butadiene by dehydrogenation over an alumina-chromia catalyst. Another version converts propane to propylene. Rapid coking of the catalyst necessitates use of several reactors in parallel, so that reactivation can be carried out continuously. Developed by Houdry and first operated at El Segundo, CA, in 1944. By 1993, 20 plants had been built worldwide. Now licensed by ABB Lummus Global. 

In contrast to fixed- and fluidized-bed reactors, several physical processes must occur in series before a reactant gas can reach the catalytic surface. Some of these are likely to affect the global rate appreciably, and others are not. Various investigators have considered the relative importance of the several steps. With hydrogenation as an illustration, the process may be visualized as occurring in the following sequence (see Fig. 10-6) ... 

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