Reaction system parallel reactions

In the following sections we will consider in detail only two sets of reactions, the parallel reaction system A — B and A C, and the series reaction system A —> B C. These... 

Various reactions or reaction systems (parallel or consecutive reactions) are influenced by alloying to a quite different degree. This should be kept in mind when attempting to find new or better alloy catalysts for a given reaction. 

The chemical compositions of many reacting systems can be expressed in terms of a single reaction progress variable. However, a chemical engineer must often consider systems that cannot be adequately described in terms of a single extent of reaction. In this chapter we are concerned with the development of the mathematical relationships that govern the behavior of such systems. We treat reversible reactions, competitive (parallel) reactions, and consecutive reactions, first in terms of the mathematical relations that govern the behavior of such systems and then in terms of the techniques that may be used to relate the kinetic parameters of the system to the phenomena observed in the laboratory. 

It is quite possible that a series of intermediate phases forms during the electrochemical process, their exact nature being controlled by the differences in the kinetics of the diffusion of the different ions or atoms in the system. Parallel reactions to products with similar thermodynamic stability may lead to a degradation of the reversible properties, if one of the products is not electrochemically reversible. Hence, knowledge of the real reaction partners and of their properties is the key for understanding the electrochemical processes in the system and elucidating the reaction mechanism. This is possible by making ex situ and/or in situ experiments with methods that supply information about the chemical composition, structural, and thermal properties of the compounds in the reaction mixture. A number of examples have been presented in literature based on the various methods as listed in Table 3.5. 

Multiple reactions in parallel producing byproducts. Rather than a single reaction, a system may involve secondary reactions producing (additional) byproducts in parallel with the primary reaction. Multiple reactions in parallel are of the tj ie... 

An example of a parallel reaction system occurs in the production of ethylene oxide ... 

Mixed parallel and series reactions producing byproducts. In more complex reaction systems, both parallel and series reactions can occur together. Mixed parallel and series reactions are of the type... 

Consider the system of parallel reactions from Eq. (2.4) with the corresponding rate equations. " ... 

Multiple reactions in parallel producing byproducts. Consider again the system of parallel reactions from Eqs. (2.16) and (2.17). A batch or plug-flow reactor maintains higher average concentrations of feed (Cfeed) than a continuous well-mixed reactor, in which the incoming feed is instantly diluted by the PRODUCT and... 

Figure 2.2 summarizes these arguments to choose a reactor for systems of multiple reactions in parallel. 

Mixed parallel and series reactions producing byproducts. Consider the mixed parallel and series reaction system from Eq. (2.10) with the corresponding kinetic equations ... 

Multiple reactions in parallel producing byproducts. Once the reactor type is chosen to maximize selectivity, we are in a position to alter selectivity further in parallel reaction systems. Consider the parallel reaction system from Eq. (2.20). To maximize selectivity for this system, we minimize the ratio given by Eq. (2.21) ... 

An example of such recychng in a parallel reaction system is in the Oxo process for the production of C4 alcohols. Propylene and synthesis gas (a mixture of carbon monoxide and hydrogen) are first reacted to ra- and isobutyraldehydes using a cobalt-based catalyst. Two parallel reactions occur ... 

In fact, it is often possible with stirred-tank reactors to come close to the idealized well-stirred model in practice, providing the fluid phase is not too viscous. Such reactors should be avoided for some types of parallel reaction systems (see Fig. 2.2) and for all systems in which byproduct formation is via series reactions. 

Multiple reactions. For multiple reactions in which the byproduct is formed in parallel, the selectivity may increase or decrease as conversion increases. If the byproduct reaction is a higher order than the primary reaction, selectivity increases for increasing reactor conversion. In this case, the same initial setting as single reactions should be used. If the byproduct reaction of the parallel system is a... 

The following details mathematical expressions for instantaneous (point or local) or overall (integral) selectivity in series and parallel reactions at constant density and isotliermal conditions. An instantaneous selectivity is defined as the ratio of the rate of formation of one product relative to the rate of formation of another product at any point in the system. The overall selectivity is the ratio of the amount of one product formed to the amount of some other product formed in the same period of time. 

Fournier, M.-C., Falk, L. and Villermaux, J., 1996. A new parallel competing reaction system for assessing micromixing efficiency - experimental approach. Chemical Engineering Science, 51, 5053-5064. 

The chemical reactions of XY can be conveniently classified as (a) halogenation reactions, (b) donor-acceptor interactions and (c) use as solvent systems. Reactions frequently parallel those of the parent halogens but with subtle and revealing differences. CIF is an effective fluorinating agent (p. 820) and will react with many metals and non-metals either at room temperature or above, converting them to fluorides and liberating chlorine, e.g. ... 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

In this chapter we will discuss the results of the studies of the kinetics of some systems of consecutive, parallel or parallel-consecutive heterogeneous catalytic reactions performed in our laboratory. As the catalytic transformations of such types (and, in general, all the stoichiometrically not simple reactions) are frequently encountered in chemical practice, they were the subject of investigation from a variety of aspects. Many studies have not been aimed, however, at investigating the kinetics of these transformations at all, while a number of others present only the more or less accurately measured concentration-time or concentration-concentration curves, without any detailed analysis or quantitative kinetic interpretation. The major effort in the quantitative description of the kinetics of coupled catalytic reactions is associated with the pioneer work of Jungers and his school, based on their extensive experimental material 17-20, 87, 48, 59-61). At present, there are so many studies in the field of stoichiometrically not simple reactions that it is not possible, or even reasonable, to present their full account in this article. We will therefore mention only a limited number in order for the reader to obtain at least some brief information on the relevant literature. Some of these studies were already discussed in Section II from the point of view of the approach to kinetic analysis. Here we would like to present instead the types of reaction systems the kinetics of which were studied experimentally. 

In contrast to consecutive reactions, with parallel competitive reactions it is possible to measure not only the initial rate of isolated reactions, but also the initial rate of reactions in a coupled system. This makes it possible to obtain not only the form of the rate equations and the values of the adsorption coefficients, but also the values of the rate constants in two independent ways. For this reason, the study of mutual influencing of the reactions of this type is centered on the analysis of initial rate data of the single and coupled reactions, rather than on the confrontation of data on single reactions with intergal curves, as is usual with consecutive reactions. 

The values of the rate constants and adsorption coefficients obtained by the study of isolated reactions agreed well with those obtained by the study of parallel reactions (Table V). The three values of the adsorption coefficient of each acid were obtained independently. In addition to one value from the study of isolated reactions, two additional values were determined by the study of the parallel system one from the kinetics of the consumption of the given acid by reaction (Vila) or (Vllb), and one from the kinetics of reaction (Vile). 

Experimental studies, combined with thermodynamic analysis, indicate that the CTA hydropurification process is a complex reaction system including both parallel and tandem reactions wherein 4-CBA hydrogenation is exothermic and its paralleled decarbonylation is endothermic. 

Das Chemidabor im Mikrochip, Blick durch die Wirtschafi, December 1997 Chemtel glass chip of Orchid Biocomputer, Princeton 144 cells for parallel processing matchbox-sized system with many devices micro pumps with no movable parts 10 nl internal volume carrying out of different reactions in parallel fashion complete chemistry laboratory en miniature 10 000 cells as future-development task [223],... 

Reactor 26 [R 26] Chip Micro Reaction System with Parallel Mixer-Reaction Channels... 

Reactor type Chip micro reaction system with parallel mixer-reachon channels Plate thicknesses 2 X 0.2 mm 5 X 0.7 mm 1x1 mm... 

As micro devices, mixers from various suppliers (]R 19], [R 17]) were used [37]. These devices were each connected to a PTFE tube of length up to 150 cm. In addition, a tailor-made micro reaction system with parallel channels and integrated cooling was used (]R 26]). 

The present book is devoted to both the experimentally tested micro reactors and micro reaction systems described in current scientific literature as well as the corresponding processes. It will become apparent that many micro reactors at first sight simply consist of a multitude of parallel channels. However, a closer look reveals that the details of fluid dynamics or heat and mass transfer often determine their performance. For this reason, besides the description of the equipment and processes referred to above, this book contains a separate chapter on modeling and simulation of transport phenomena in micro reactors. 

A system of parallel reactions as shown in Fig. 5.3-9 was studied by Paul et at. (1992). The reactions are an acid-base neutralization and a base-catalysed hydrolysis of product (C). The labile compound (Q is in solution in an organic solvent, and aqueous NaOH is added to raise the pH from 2 to 7. Enolization occurs under basic conditions and is accompanied by irreversible decomposition (ring opening), which is not shown in the figure. The system was studied in the laboratory using the 6-Iitre reactor shown in Fig. 5.3-10. 

The E-model was also applied to a system of parallel reactions (Baldyga and Bourne, 1990a). It was found that selectivity depends on compositions of both the initial reactor content and the stream added for chemically equivalent mixtures of three reactants (see reaction system given by Eqns. (5.4-143) and (5.4-144)). For an instantaneous reaction, the yield of 5 varies from 0 to 100 % depending on the mode of composing the feeding stream. 

Below some typical kinetic situations for the system of parallel reactions will be considered. 

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