Product Distribution in Multiple Reactions

More often than not, solid-catalyzed reactions are multiple reactions. Of the variety of products formed, usually only one is desired, and it is the yield of this material which is to be maximized. In cases such as these the question of product distribution is of primary importance. 

Here we examine how strong pore diffusion modifies the true instantaneous fractional yield for various types of reactions however, we leave to Chapter 7 

No resistance to pore diffusion. Consider the parallel-path decomposition 

Here the instantaneous fractional yield at any element of catalyst surface is given by 

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Product Distribution in Multiple Reactions 413 Table E18.3b Calculations Used for the Differential Analysis... 

Aold N, Hasebe S, Mae K (2004) Mixing in microreactors effectiveness of lamination segments as a form of feed on product distribution for multiple reactions. Chem Eng J 101 (l-3) 323-331... 

The addition of hydrogen bromide to 1,3-butadiene allows the illustration of another important aspect of reactivity—the way temperature affects product distribution in a reaction that can take multiple paths. In general ... 

For reactor design purposes, the distinction between a single reaction and multiple reactions is made in terms of the number of extents of reaction necessary to describe the kinetic behavior of the system, the former requiring only one reaction progress variable. Because the presence of multiple reactions makes it impossible to characterize the product distribution in terms of a unique fraction conversion, we will find it most convenient to work in terms of species concentrations. Division of one rate expression by another will permit us to eliminate the time variable, thus obtaining expressions that are convenient for examining the effect of changes in process variables on the product distribution. 

As pointed out in the introduction to Chapter 7, in multiple reactions both reactor size and product distribution are influenced by the processing conditions. Since the problems of reactor size are no different in principle than those for single reactions and are usually less important than the problems connected with obtaining the desired product material, let us concentrate on the latter problem. Thus, we examine how to manipulate the temperature so as to obtain, first, a desirable product distribution, and second, the maximum production of desired product in a reactor with given space-time. 

Multiple Reactions and Product Distribution in Fluidized Beds... 

When a reaction involves multiple bonding changes, a question may arise whether the bonding changes occur by a stepwise or concerted pathway. An answer to such a question based on the classical reaction theory is that the reaction proceeds by a concerted pathway, by a stepwise pathway, or by a mixture of the two separate pathways. However, if one takes into account dynamic effects, the answer to the question of concerted versus stepwise may be much more complex. It is interesting to point out here that the case reported by Singleton for the ene reaction affords a case, where stepwise mechanism can dynamically operate on a concerted PES. This contrasts with the reactions described in section Nonstatistical Product Distribution , in which the... 

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. 

The time-independent method can be used to calculate the complete product distribution in a system where multiple reactions take place, if a rate equation is available for each independent reaction. However, the real time or space time required to reach the calculated product distribution cannot be obtained via the time-independent method. 

T-HjCHD, Dj). An ion-beam target-gas study showed that the N4 +D2 >N2D + N2 + D channel dominates other available reaction channels in addition to N2D, only a comparatively small amount (<5%) of N2 from collision-induced dissociation was detected [5]. Drift tube studies showed significantly more (13%) [2] or only N4H (100%) [6] to be formed in N4+H2 reactive collisions. The difference in product distributions is attributed [5] to the single [5] and multiple collision [2, 6] conditions that were present. Thermal rate constants at 300 K for the N2H and N2D product channels in the reactions of N4 with H2... 

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. 

Unlike the behavior over 0.2% platinum/alumina, the main features of the labeled product distributions obtained over 10% platinum/alumina and over platinum film catalysts (Tables VI and VII respectively) cannot be explained in terms of a single dominant reaction pathway via an adsorbed C6 cyclic intermediate. Again, parallel, multiple-step reaction pathways are involved. The results from 2-methylpentane-2-13C have been qualitatively accounted for (84) by the pathways... 

Unlike hydrogen these reactions do not appear to be activated. In addition the products distributions observed indicate comparable rates for multiple adduct formation. The mass complexity, relatively high ionization potentials, and the known prevalent dissociative ionization of the fully saturated carbonyls(42) has possibly caused the failure of some initial saturation experi ments(43). The ability to synthesize the stable carbonyl complexes will help this field significantly due to the vast amount of information available, especially their structures. 

With complex or multiple reactions, what product distribution can be obtained in comparison with that from a flow reactor ... 

From previous chapters, it is clear that the fluid mixing pattern within a reactor of a given size will affect the conversion achieved from that reactor in the case of multiple reaction schemes, the product distribution, and hence yield and selectivity, will also be dependent on mixing and... 

In this chapter we deal with single reactions. These are reactions whose progress can be described and followed adequately by using one and only one rate expression coupled with the necessary stoichiometric and equilibrium expressions. For such reactions product distribution is fixed hence, the important factor in comparing designs is the reactor size. We consider in turn the size comparison of various single and multiple ideal reactor systems. Then we introduce the recycle reactor and develop its performance equations. Finally, we treat a rather unique type of reaction, the autocatalytic reaction, and show how to apply our findings to it. 

Design for multiple reactions, for which the primary consideration is product distribution, is treated in the next two chapters. 

The preceding chapter on single reactions showed that the performance (size) of a reactor was influenced by the pattern of flow within the vessel. In this and the next chapter, we extend the discussion to multiple reactions and show that for these, both the size requirement and the distribution of reaction products are affected by the pattern of flow within the vessel. We may recall at this point that the distinction between a single reaction and multiple reactions is that the single reaction requires only one rate expression to describe its kinetic behavior whereas multiple reactions require more than one rate expression. 

In any case, the use of the proper contacting pattern is the critical factor in obtaining a favorable distribution of products for multiple reactions. 

The third reason for using fluid-fluid systems is to obtain a vastly improved product distribution for homogeneous multiple reactions than is possible by using the single phase alone. Let us turn to the first two reasons, both of which concern the reaction of materials originally present in different phases. 

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