Overall selectivity multiple reactions

In practice, most industrial processes are staged with multiple reaction processes and separation units as sketched in Figure 4-15. A is the key raw material and is the key product, it is clear that many factors must be included in designing the process to maximize the yield of E. The effectiveness of the separations are obviously critical as well as the kinetics of the reactions and the choice of reactor type and conversion in each reactor. If separations are perfect, then the yields are equal to the selectivities, so that the overall... 

The contents of the present contribution may be outlined as follows. Section 6.2.2 introduces the basic principles of coupled heat and mass transfer and chemical reaction. Section 6.2.3 covers the classical mathematical treatment of the problem by example of simple reactions and some of the analytical solutions which can be derived for different experimental situations. Section 6.2.4 is devoted to the point that heat and mass transfer may alter the characteristic dependence of the overall reaction rate on the operating conditions. Section 6.2.S contains a collection of useful diagnostic criteria available to estimate the influence of transport effects on the apparent kinetics of single reactions. Section 6.2.6 deals with the effects of heat and mass transfer on the selectivity of basic types of multiple reactions. Finally, Section 6.2.7 focuses on a practical example, namely the control of selectivity by utilizing mass transfer effects in zeolite catalyzed reactions. 

As a consequence of the different definitions for selectivity and yield, when reading literature dealing with multiple reactions, check careftilJy to ascertain the definition intended by the autoor. Prom an economic standpoint it is the overall selectivities, S, and yields, Y, that are important in determining profits. However, the rate-based selectivities give insists in choosing reactors and reaction schemes that will help maximize the profit. However, many times there is a conflict between selectivity and conversion (yield) because you want to make a lot of your desired product (D) and at the same lime minimize the undesired product (U). However, in many instances the greater conversion you achieve, not only do you make more D, you also form more U. 

In many catalytic systems multiple reactions occur, so that selectivity becomes important. In Sec. 2-10 point and overall selectivities were evaluated for homogeneous well-mixed systems of parallel and consecutive reactions. In Sec. 10-5 we saw that external diffusion and heat-transfer resistances affect the selectivity. Here we shall examiineHEieHnfiuence of intrapellet res ahces on selectivity. Systems with first-order kinetics at isothermal conditions are analyzed analytically in Sec. 11-12 for parallel and consecutive reactions. Results for other kinetics, or for nonisothermal conditions, can be developed in a similar way but require numerical solution. ... 

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... 

When consecutive or parallel reactions are carried out between a gas and a liquid, the concentration gradients near the interface may influence the selectivity as well as the overall rate of reaction. For chlorination or partial oxidation of hydrocarbons, several workers have reported that the yield of intermediate products was influenced by agitation variables [6,7] and was less than predicted from the kinetic constants. Rigorous analysis of multiple reactions is complex, but film theory can be used to show when mass transfer effects are likely to change the selectivity [8]. 

The requirement for stereospeciHc reactions and the presence of multiple hydroxyl groups of similar and relatively low reactivity (7, 8) complicates the chemical synthesis of these oligosaccharides. Hazardous heavy metal salts (e.g. Ag-triflate, Hg(CN)2) are often used as catalysts. To achieve good selectivity multiple protection and deprotection steps have to be carried out and the overall yields are often low. In particular, despite recent progress (8), the synthesis of important a-sialylated oligosaccharides is difHcult yields range from 20-30% and the unnatural P-sialoside is often formed and must be separated from the desired product. 

In cycles of type (ii), a step of one cycle is also part of other cycles and the reaction rate of this step is a linear combination of multiple reaction route rates. As a result, this reaction rate cannot be represented by the difference between a forward and reverse reaction rate. Other cycles influence the reaction rate of the selected step not only quantitatively but qualitatively as well. Indeed, other cycles can change the direction of the overall reaction corresponding to the selected cycle. This is the main difference between multiroute mechanisms of types (i) and (ii). Many examples of deriving such equations using graph theory can be found in the books by Yablonskii et al. (1991) and Marin and Yablonsky (2011). 

In the case of a single reaction carried out in a reactor, fractional conversion of the limiting reactant (xy ) achieved in the reactor is usually taken as the index of reactor performance. In the case of multiple reactions, as conversion of limiting reactant leads to the formation of multiple products, conversion alone cannot be taken as an indication of good performance. Performance is considered to be good if conversion of a certain quantity of reactant leads to the formation of a larger proportion of desired product and a relatively lower proportion of undesired product. Thus, for multiple reactions, performance is characterised by two more factors apart from conversion, namely, yield and selectivity. Overall yield y is defined as the fraction of the overall limiting reactant that is converted into the desired product. Overall selectivity O is defined as the ratio of the amount of desired product to the amount of undesired product produced. If B is the desired product and C is the undesired product of the series reaction A— then for any batch time t... 

Suppose also that, of a starting amount of 10 mol of A, four form the desired product B, two form the undesired product C, one forms the undesired product D, and three remain unreacted. While this process produces 4 mol of valuable product, it could have produced 10 if everything went the way a practicing engineer would want it to, that is, if all 10 mol of A reacted to form B. The ratio of the 4 mol of B actually produced, to the 10 it potentially could have produced, is called the yield (0.40 or 40%). By definition, the yield of a reaction is a measure of how much of the desired product is produced relative to how much would have been produced if only the desired reaction occurred and if that reaction went to completion. Obviously, the firactional yield must be a number between zero and unity. As noted earlier, another definition that has been used by industry is the ratio of product formed to initial (inlet) reactant. An additional term used in conjunction with multiple reactions is selectivity. Selectivity is a measure of how predominant the desired reaction is relative to one of the side reactions. The value of the selectivity is obtained by dividing the number of moles of a desired product actually generated by the number of moles of one of the undesired products produced by a side reaction. Obviously, selectivity becomes more important than conversion if undesired side reaction(s) take precedent over desired reac-tion(s). Other definitions of selectivity that have appeared in the literature include the popular ratio of the amount of desired products formed to the amount of all products formed. Definitions have also included rate of production of one product relative to another product point selectivity) and amount of one product formed relative to another product overall selectivity). In the example above, the selectivity of B over C is 2.0 and that of B over D is 4.0. These definitions are employed in several illustrative examples to follow. 

Up to scale, this is the dependence of overall reaction rate on concentration Cb in the assumption of constant temperature and concentrations c 2 and Cab- All figures in this chapter illustrate certain qualitative features of kinetic behavior, i.e. rate-limitation, vicinity of equilibrium, steady-state multiplicity, etc. Parameter values are selected to illustrate these qualitative features. Certainly these features could be illustrated with "realistic" kinetic parameters. 

In other words, if the C coefficients /iH+R+i /is are given the values determined by Eq. (19), then the total of the expressions in (17) will be a direct mechanism. Furthermore, if we go through this procedure for every selection of C columns in (17) such that the C x C matrix M is nonsingular, then we get every direct mechanism for the overall reaction (14). Altogether there are R + C undetermined coefficients fiH+15. .., fis in (17), the last C of which are determined for each direct mechanism. The remaining R parameters fiH + ,..., /iH + R are in the expression (14) for the overall reaction, which is of multiplicity R. Similarly each direct mechanism must be a function of these R parameters. 

Each system considered in this section has a space of overall reactions whose dimension exceeds one. In many industrial reactions involving organic substances a major product is formed, but a side reaction contributes to loss in selectivity or yield of the desired product. Such cases may be said to exhibit a multiple overall reaction, unless the ratio of desired product to by-product remains constant over a range of operating conditions, so that a simple chemical equation might be employed to express the stoichiometry. 

Examination of the matrix given in Table XV brings up an item of special interest. If the combination s4 of atomic oxygen were assumed not to occur, we would still be able to produce ethylene oxide by a combination of the first three steps. This scheme could place a lower limit on the selectivity at 6 7 or 85.7%, corresponding to a simple overall reaction rather than a multiple overall reaction. This serves to illustrate that we get fewer overall reactions than would be predicted by considering only the atom-by-species matrix, as a result of a more restricted choice of possible steps. 

FIG. 18. Reaction parameters for n-hexane conversion by Ni and Ni-Cu alloys at 330°C Ai = log rw (rate per gram of catalyst) A2 = log rs (rate per square centimeter of total surface) Eact is activation energy of the overall process S is the selectivity for producing Cg products M is a fission parameter whose value inversely reflects the degree of multiple fragmentation to methane (102). 

Table 12.1 shows that in more than 70% of CYP2C9 reactions, the first option selected by the methodology matches the experimental one. Moreover, in more than 16% of cases, the second atom is that which fits the experimental one. Therefore, in considering the overall ranking list for the single and multiple sites of metabolism, the methodology predicts the site of metabolism for CYP2C9 within the first two atoms selected in approximately 86% of the reactions, independent of the conformer used. 

Closer examination of Figure 2.4 shows that the total amount of intermediates to be prepared for Route 3 is essentially the same as that for the linear route. To execute the synthesis of the octapeptide in a nonlinear manner, one additional step is required for each convergent route, which increases the amount of intermediates prepared. The additional step is necessary because of the multiple functional groups of amino acids. There are greater savings in the overall amount of intermediates required in routes where the individual subunits do not have multiple reactivity and hence do not require additional steps for selective reactions. 

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