Multiple reactions parallel

Westerterp K.R. and Ptasinsky K.J. "Safe design of cooled tubular reactors for exothermic, multiple reactions. Parallel reactions. Development of criteria". Chem. Eng. Sci. in print. 

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... 

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. 

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) ... 

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... 

This reaction cannot be elementary. We can hardly expect three nitric acid molecules to react at all three toluene sites (these are the ortho and para sites meta substitution is not favored) in a glorious, four-body collision. Thus, the fourth-order rate expression 01 = kab is implausible. Instead, the mechanism of the TNT reaction involves at least seven steps (two reactions leading to ortho- or /mra-nitrotoluene, three reactions leading to 2,4- or 2,6-dinitrotoluene, and two reactions leading to 2,4,6-trinitrotoluene). Each step would require only a two-body collision, could be elementary, and could be governed by a second-order rate equation. Chapter 2 shows how the component balance equations can be solved for multiple reactions so that an assumed mechanism can be tested experimentally. For the toluene nitration, even the set of seven series and parallel reactions may not constitute an adequate mechanism since an experimental study found the reaction to be 1.3 order in toluene and 1.2 order in nitric acid for an overall order of 2.5 rather than the expected value of 2. 

Multiple reaction selectivity can be defined similarly as the ratio of the rate of formation of the desired product to the formation rate of an undesired product as in a parallel reaction... 

Multiple reactions in series producing byproducts. Rather than the primary and secondary reactions being in parallel, they can be in series. Multiple reactions in series are of the type ... 

Multiple reactions in parallel producing byproducts. Consider the system of parallel reactions from Equation 5.4 with the corresponding rate equations1-3 ... 

Multiple Reactions—Choosing a reactor type to obtain the best selectivity can often be made by inspection of generalized cases in reaction engineering books. A quantitative treatment of selectivity as a function of kinetics and reactor type (batch and CSTR) for various multiple reaction systems (consecutive and parallel) is presented in [168]. 

The application of multiple reaction parameters executed in a parallel array format has been used to expedite the identification of optimal conditions for the synthesis of a collection of almost 600 new interleukin-1/ converting enzyme inhibitors [89]. The reaction in question was the problematic conversion of a / -tert-butyl aspartic acid bromoethylketone to the corresponding acyloxyketone (Scheme 2.63). The study en-... 

For multiple reactions a change in the observed activation energy with temperature indicates a shift in the controlling mechanism of reaction. Thus, for an increase in temperature Eq s rises for reactions or steps in parallel, Eobs falls for reactions or steps in series. Conversely, for a decrease in temperature E s falls for reactions in parallel, E s rises for reactions in series. These findings are illustrated in Fig. 2.3. 

Since multiple reactions are so varied in type and seem to have so little in common, we may despair of finding general guiding principles for design. Fortunately, this is not so because many multiple reactions can be considered to be combinations of two primary types parallel reactions and series reactions. 

Most multiple-reaction systems are more comphcated series-parallel sequences with multiple reactants, some species being both reactant and product in different reactions. These simple rules obviously will not work in those situations, and one must usually solve the mass-balance equations to determine the best reactor configuration. 

At the same time, as a chemist I was disappointed at the lack of serious chemistry and kinetics in reaction engineering texts. AU beat A B o death without much mention that irreversible isomerization reactions are very uncommon and never very interesting. Levenspiel and its progeny do not handle the series reactions A B C or parallel reactions A B, A —y C sufficiently to show students that these are really the prototypes of aU multiple reaction systems. It is typical to introduce rates and kinetics in a reaction engineering course with a section on analysis of data in which log-log and Anlienius plots are emphasized with the only purpose being the determination of rate expressions for single reactions from batch reactor data. It is typically assumed that ary chemistry and most kinetics come from previous physical chemistry courses. 

Kinetic mechanisms involving multiple reactions are by far more frequently encountered than single reactions. In the simplest cases, this leads to reaction schemes in series (at least one component acts as a reactant in one reaction and as a product in another, as in (2.7)-(2.8)), or in parallel (at least one component acts as a reactant or as a product in more than one reaction), or to a combination series-parallel. More complex systems can have up to hundreds or even thousands of intermediates and possible reactions, as in the case of biological processes [12], or of free-radical reactions (combustion [16], polymerization [4]), and simple reaction pathways cannot always be recognized. In these cases, the true reaction mechanism mostly remains an ideal matter of principle that can be only approximated by reduced kinetic models. Moreover, the values of the relevant kinetic parameters are mostly unknown or, at best, very uncertain. 

Multiple reactions Series or parallel reactions that take place simultaneously in a reactor. For example, A -(- B C and A + D - E are parallel reactions, and A + B C + D E + F arc series reactions. 

Since the classic papers by Ingold and his co-workers,110, 111 nitration has for a long time been considered as the standard electrophilic substitution. Many orientation and relative rate data on the nitration of both carbocyclic and heterocyclic substrates have been accumulated and the results have been generalized as valid for all electrophilic substitutions. As a matter of fact, this popularity is partially undeserved nitration is a complicated reaction, which can occur by a multiplicity of parallel mechanisms.112 In particular, in the case of the very reactive substrates that five-membered heterocycles are, two complications may make meaningless both kinetic measurements and competitive experiments.113 (i) Due to the great reactivity of both partners the encounter limiting rate may be achieved in this case, of course, all the substrates react at the same rate and the effect of structure on the reactivity cannot be studied. (ii) Nitrous acid, always present in traces, may exert an anticatalytic effect in some cases and a markedly catalytic effect in others with a very reactive substrate, nitration may proceed essentially via nitrosa-tion, followed by oxidation. For these reasons, the nitration data must be handled with much caution. 

Additional aggravating circumstances arise from the fact that chemical steps which are transfer limited will proceed differently in the industrial plant than on laboratory-scale. The selectivity of multiple reactions such as competing consecutive and parallel reactions depend very much on the extent of micro-mixing in the system. These facts are well known from Chemical Reaction Engineering textbooks. Conversely, these reactions are carried out to obtain details about the extent of micro- and macro-mixing in stirring. 

EIS has the ability to distinguish between influences from different processes, especially when the system involves multiple-step reactions, parallel reactions, or additional processes such as adsorption. Generally speaking, the measurements and analysis of the EIS for a PEMFC are complicated compared with those of the polarization curve. However, the results from both methods are not insular, and some relationships exist between the complicated impedance spectrum and the simple polarization curve [22],... 

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