Yields in multiple reactions

Semibatch reactors are especially important for bioreactions, where one wants to add an enzyme continuously, and for multiple-reaction systems, where one wants to maximize the selectivity to a specific product. For these processes we may want to place one reactant (say, A) in the reactor initially and add another reactant (say, B) continuously. This makes Ca large at all times but keeps Cg small. We will see the value of these concentrations on selectivity and yield in multiple-reaction systems in the next chapter. 

In the preceding chapter, the choice of reactor type was made on the basis of the most appropriate concentration profile as the reaction progressed, in order to minimize reactor volume for single reactions or maximize selectivity (or yield) for multiple reactions for a given conversion. However, there are still important effects regarding reaction conditions to be considered. Before considering reaction conditions, some basic principles of chemical equilibrium need to be reviewed. 

In Sect. 7.4.6, we discussed various stochastic simulation techniques that include the kinetics of recombination and free-ion yield in multiple ion-pair spurs. No further details will be presented here, but the results will be compared with available experiments. In so doing, we should remember that in the more comprehensive Monte Carlo simulations of Bartczak and Hummel (1986,1987, 1993,1997) Hummel and Bartczak, (1988) the recombination reaction is taken to be fully diffusion-controlled and that the diffusive free path distribution is frequently assumed to be rectangular, consistent with the diffusion coefficient, instead of a more realistic distribution. While the latter assumption can be justified on the basis of the central limit theorem, which guarantees a gaussian distribution for a large number of scatterings, the first assumption is only valid for low-mobility liquids. 

In multiple reactions the influence of the pressure on the rate of the various steps is mostly different. This makes the interpretation of the results of kinetic measurements more difficult. On the other hand, by the application of high pressure the ratio of the yield of a desired product to the conversion of initial reactants, the so-called selectivity, can be improved. 

Mixing effects on product distribution are of importance in multiple reactions because the impact of product distribution on design and economics can be profound. In such reactions, the desired product is one of two or more possible products. Economics is directly affected by the yield of the desired product and both design and economics are affected by downstream separation requirements. 

Quantum yield, in photochemical reactions, is the ratio of the number of chemical reactions caused by light to the number of photons absorbed by the chemical species. With the exception of some rare photochemical processes in bio-inorganic chemistry, in which chain reactions initiated by absorption of a single photon result in multiple catalytic events and hence quantum yield greater than unity, in the vast majority of photochemical reactions and in all known photobiological reactions such as photosynthesis, vision, and phototropism the quantum yield is less than 1.0. 

The selectivity and yield of a desired product is of major interest in multiple reactions. In order to assess the effects of transport processes and catalyst deactivation on the selectivity and yield, discussion shall be confined to simple intrinsic kinetics, for the qualitative behavior of complex reactions is often similar to that of, for instance, first-order reactions. This qualitative behavior of the selectivity and yield as affected by transport resistances and catalyst deactivation will be treated in the first part of this chapter with the understanding that the same approach, when coupled with numerical methods, can lead to quantitative results for any system. An excellent treatment of multiple reactions can be found in the book by Aris (1975). 

The relationship between reactants and products in addition reactions can be illustrated by the hydrogenation of alkenes to yield alkanes. Hydrogenation is the addition of H2 to a multiple bond. An example is the reaction of hydrogen with ethylene to form ethane. 

In the Monsanto/Lummus Crest process (Figure 10-3), fresh ethylbenzene with recycled unconverted ethylbenzene are mixed with superheated steam. The steam acts as a heating medium and as a diluent. The endothermic reaction is carried out in multiple radial bed reactors filled with proprietary catalysts. Radial beds minimize pressure drops across the reactor. A simulation and optimization of styrene plant based on the Lummus Monsanto process has been done by Sundaram et al. Yields could be predicted, and with the help of an optimizer, the best operating conditions can be found. Figure 10-4 shows the effect of steam-to-EB ratio, temperature, and pressure on the equilibrium conversion of ethylbenzene. Alternative routes for producing styrene have been sought. One approach is to dimerize butadiene to 4-vinyl-1-cyclohexene, followed by catalytic dehydrogenation to styrene ... 

Multiple pathways leading to the same product channel can also be observed in a reaction when there are a sufficient number of identical atoms, thereby allowing different intermediate structures to yield the same products. In these cases, the mechanisms in the two pathways are often quite similar, but involve differing positions of identical atoms on the reactants. The different pathways often involve formation of ring intermediates in which the rings have different sizes. A simple example of this class is the photodissociation of vinyl chloride [9]... 

Pyrrole-2-carboxylate is employed in the synthesis of various pharmaceuticals" and a potential herbicide. A number of organic syntheses have been described " However, they require multiple steps and result in low yields. Furthermore, the chemical carbonation of pyrrole with K2CO3 depends on high pressure and temperature. The one-step enzymatic conversion has advantages with regard to regiospecificity, yield, and mild reaction conditions. 

When dealing with multiple reactions, selectivity or reactor yield is maximized for the chosen conversion. The choice of mixing pattern in the reactor and feed addition policy should be chosen to this end. 

Multiple reactions in parallel producing by products. After the reactor type is chosen for parallel reaction systems in order to maximize selectivity or reactor yield, conditions can be altered further to improve selectivity. Consider the parallel reaction system from Equation 5.66. To maximize selectivity for this system, the ratio given by Equation 5.67 is minimized ... 

For reaction systems involving multiple reactions producing by products, selectivity and reactor yield can also be enhanced by appropriate changes to the reactor temperature, pressure and concentration. The appropriate choice of catalyst can also influence selectivity and reactor yield. The arguments are summarized in Figure 6.912. 

In discussions of systems in which only a single chemical reaction is involved, one may use the words yield and conversion as complementary terms. However, in dealing with multiple reactions, conversion refers to the proportion of a reagent that reacts, while yield refers to the... 

When multiple reactions are possible, certain of the products have greater economic value than others, and one must select the type of reactor and the operating conditions so as to optimize the product distribution and yield. In this subsection we examine how the temperature can be manipulated with these ends in mind. In our treatment we will ignore the effect of concentration levels on the product distribution by assuming that the concentration dependence of the rate expressions for the competing reactions is the same in all cases. The concentration effects were treated in detail in Chapter 9. 

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