Selectivity in multiple reactions

In the light of the previous discussion it is quite apparent that a detailed mathematical simulation of the combined chemical reaction and transport processes, which occur in microporous catalysts, would be highly desirable to support the exploration of the crucial parameters determining conversion and selectivity. Moreover, from the treatment of the basic types of catalyst selectivity in multiple reactions given in Section 6.2.6, it is clear that an analytical solution to this problem, if at all possible, will presumably not favor a convenient and efficient treatment of real world problems. This is because of the various assumptions and restrictions which usually have to be introduced in order to achive a complete or even an approximate solution. Hence, numerical methods are required. Concerning these, one basically has to distinguish between three fundamentally different types, namely molecular-dynamic models, stochastic models, and continuous models. 

Membrane Reactors to Improve Selectivity in Multiple Reactions... 

In addition to using membrane reactors lo remove a reaction product in order to shift the equilibrium toward completion, we can use membrane reactors to increase selectivity in multiple reactions. This increase can be achieved by injecting one of the reactants along the length of the reactor. It is particularly effective in panial oxidation of hydrocarbons, chlorination, ethoxylation. hydrogenation, nitration, and sulfunation reactions to name a few. ... 

In the preceding section, the choice of reactor type was made on the basis of which gave the most appropriate concentration profile as the reaction progressed in order to minimize volume for single reactions or maximize selectivity for multiple reactions for a given conversion. However, after making the decision to choose one type of reactor or another, there are still important concentration effects to be considered. 

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. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

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. 

The semibatch can also be used to control the kinetics in multiple reaction sequences. The selectivity may be shifted to one reaction by adding a reactant slowly. This keeps one reactant concentration high with respect to the other. 

The CSTR is particularly useful for reaction schemes that require low concentration, such as selectivity between multiple reactions or substrate inhibition in a chemostat (see Section IV). The reactor also has applications for heterogeneous systems where high mixing gives high contact time between phases. Liquid-liquid CSTRs are used for the saponification of fats and for suspension and emulsion polymerizations. Gas-liquid mixers are used for the oxidation of cyclohexane. Gas homogeneous CSTRs are extremely rare. 

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. 

A stirred tank may give better or worse selectivities than a tubular-flow unit in multiple-reaction systems. As usual, the key point is the relative values of the activation energies for the reactions. In particular, for a set of parallel reactions, where the desired product is formed by the reaction with the higher activation energy, the stirred tank is advantageous. The production of allyl chloride considered in Example 5-2 is a case in point. The performance of a stirred-tank reactor for this system is discussed next, and the results are compared with the performance of the tubular-flow reactor. 

As we will see in Chapter 6. this arrangement is often used to improve selectivity when multiple reactions lake place. Here B is usually fed unifonnly through the membrane along the length of the reactor. The balance on B is... 

Several caveats should be mentioned at this point. When there are multiple reactions, the half-life in Equation 1.54 is the shortest half-life that affects yield and selectivity. In complex reaction schemes, there may be some important fast reactions that are mixing dependent even thought the overall reaction is slow. An example is a catalyst that may react with itself if not rapidly diluted. Equations 1.59 and... 

Reactions with recycle are very useful for a number of reasons, most notably because they can be used to improve the selectivity of multiple reactions, push a reaction beyond its equilibrium conversion, or speed up a catalytic reaction by removing products. A recycle loop coupled with a reactor will generally contain a separation process in which unused reactants are (partially) separated from products. These reactants are then fed back into the reactor along with the fresh feed. 

Initially, let us define yield and selectivity for multiple reactions occurring simnl-taneously, for reactions in parallel, series, or mixed. When there is only one reaction step, the yield is confused with the conversion. 

Note that at this stage, the H2S formed can be used to precipitate metals as metal sulfides for metal recovery and removal from feed and waste streams. Moreover, controlling the pH and the redox potential allows for the selective recovery of metals. For example, it is possible to separate copper from zinc, arsenic from copper, and iron from nickel, in multiple reaction stages at different pH values. 

Table 19.1 Ions monitored for determination of niacin by isotope dilution mass spectrometry using positive electrospray ionization. Both natural and labelled nicotinic acid are protonated in positive ion electrospray ionization, giving quasi-molecular ions at mass-to-charge ratios (m/z) of 124 and 128, respectively, which can be monitored directly in selected ion recording (SIR) experiments and selected as the parent ions in multiple reaction monitoring (MRM) experiments. In MRM experiments protonated nicotinic acid can be induced to produce daughter ions at several other m/z values, but the given transitions are the ones with the highest signal intensity.

Imperfect micro-mixing in viscous reaction media can have a dramatic effect on the selectivity of multiple reaction systems, particularly in stirred reactors with separate feed streams. The reason is that the concentration ratio of the two reactants varies locally between 0 and so that higher order competitive... 

In situations where mass transfer limits the rate of a reaction, concentration gradients arise, and these will generally influence the selectivity of multiple reaction systems. However, in sections 5.3.2, 5.3.3 and 5.3.4 it was assumed that no perceptible chemical reaction takes place in the diffusion layers, at least as far as the main reaction is concerned. 

To improve the specificity while retaining the high sensitivity, a new method (i.e., selected or multiple reaction monitoring (SRM/MRM, Chapter 2)) is evolved from the SIM technique, which is only focused on one miz of a compound to the SRM that is focused on both a molecular (or precursor) ion and a fragment-ion resultant from the precursor ion. The specific experiment in known as a "transition" and is usually written as precursor-ion mass fragment-ion mass. The only requirement to perform this technique is that the mass spectrometer employed has to possess the capability to perform MS/MS. 

The tandem MS/MS spectra shown in Fig. 26 are typical product ion spectra. In most tandem MS/MS applications this scan mode is used to obtain sttuctural information of a selected (precursor) ion. A variation of product ion scans are used also in multiple reaction monitoring (MRM), which is a useful technique for quantitation and kinetic studies (see the following text). Other MS/MS scan types are also applied even though not all of them are easily available in all tandem MS/MS... 

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). 

Membrane reactors can be used to increase conversion when the reaction is thermodynamically limited, as well as to increase the selectivity when multiple reactions are occurring. Thermodynamically limited reactions are reactions where the equilibrium lies far to the left (i.e reactant side) and there is little conversion. If the reaction is exothermic, increasing the temperature will only drive the reaction further to the left, and decreasing the temperature will result in a reaction rate so slow that there is very little conversion. If the reaction is endothermic, increasing the temperature will move the reaction to the right to favor a higher conversion however, for many reactions these higher temperatures cause the catalyst to become deactivated. 

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) ... 

The selection of reactor pressure for vapor-phase reversible reactions depends on whether there is a decrease or increase in the number of moles and whether there is a system of single or multiple reactions. 

Most processes are catalyzed where catalysts for the reaction are known. The choice of catalyst is crucially important. Catalysts increase the rate of reaction but are unchanged in quantity and chemical composition at the end of the reaction. If the catalyst is used to accelerate a reversible reaction, it does not by itself alter the position of the equilibrium. When systems of multiple reactions are involved, the catalyst may have different effects on the rates of the different reactions. This allows catalysts to be developed which increase the rate of the desired reactions relative to the undesired reactions. Hence the choice of catalyst can have a major influence on selectivity. 

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