Systems reactions

This function is served by compact, helically coiled tubes of a given diameter for convenient use. They are readily connected to the other elements of the system, so that void volumes are minimized. Occasionally, they are heated by means of electrical wires or a thermostated bath. 

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Most processes are catalyzed where catalysts for the reaction are known. The strategy will be to choose the catalyst, if one is to be used, and the ideal characteristics and operating conditions needed for the reaction system. Decisions must be made in terms of reactor... 

Before we can proceed with the choice of reactor and operating conditions, some general classifications must be made regarding the types of reaction systems likely to be encountered. We can classify reaction systems into five broad types ... 

Single reactions. Most reaction systems involve multiple reactions. In practice, the secondary reactions can sometimes be neglected, leaving a single primary reaction to consider. Single reactions are of the type... 

An example of a parallel reaction system occurs in the production of ethylene oxide ... 

An example of a series reaction system is the production of formaldehyde from methanol ... 

Mixed parallel and series reactions producing byproducts. In more complex reaction systems, both parallel and series reactions can occur together. Mixed parallel and series reactions are of the type... 

Consider now which of the idealized models is preferred for the five categories of reaction systems introduced in Sec. 2.2. 

If the reaction involves more than one feed, the picture becomes more complex. Consider the reaction system from Eq. (2.6) with the corresponding rate equations ... 

The arrangement which gives the highest overall selectivity can only be deduced by detailed sizing and costing calculations specific to the reaction system. 

The equilibrium conversion can be increased by employing one reactant in excess (or removing the water formed, or both). b. Inerts concentration. Sometimes, an inert material is present in the reactor. This might be a solvent in a liquid-phase reaction or an inert gas in a gas-phase reaction. Consider the reaction system... 

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) ... 

An example of such recychng in a parallel reaction system is in the Oxo process for the production of C4 alcohols. Propylene and synthesis gas (a mixture of carbon monoxide and hydrogen) are first reacted to ra- and isobutyraldehydes using a cobalt-based catalyst. Two parallel reactions occur ... 

Multiple reactions in series producing byproducts. For the series reaction system in Eq. (2.18), the series reaction is inhibited by low concentrations of PRODUCT. It has been noted already that this can be achieved by operating with a low conversion. 

Another way to keep the concentration of PRODUCT low is to remove the product as the reaction progresses, e.g., by intermediate separation followed by further reaction. For example, in a reaction system such as Eq. (2.18), intermediate separation of the PRODUCT followed by further reaction maintains a low concentration of PRODUCT as the reaction progresses. Such intermediate separation is most appropriate when separation of the product from the reactants is straightforward. 

An alternative way to improve selectivity for the reaction system in Eq. (2.27) is again to deliberately feed BYPRODUCT to the reactor to shift the equilibrium of the secondary reaction away from BYPRODUCT formation. 

The choice of reactor temperature depends on many factors. Generally, the higher the rate of reaction, the smaller the reactor volume. Practical upper limits are set by safety considerations, materials-of-construction limitations, or maximum operating temperature for the catalyst. Whether the reaction system involves single or multiple reactions, and whether the reactions are reversible, also affects the choice of reactor temperature, as we shall now discuss. 

Having considered reactor temperature and pressure, we are now in a position to judge whether the reactor phase will be gas, liquid, or multiphase. Given a free choice between gas- and liquid-phase reactions, operation in the liquid phase is usually preferred. Consider the single reaction system from Eq. (2.19) ... 

However, a note of caution should be added. In many multiphase reaction systems, rates of mass transfer between different phases can be just as important or more important than reaction kinetics in determining the reactor volume. Mass transfer rates are generally higher in gas-phase than liquid-phase systems. In such situations, it is not so easy to judge whether gas or liquid phase is preferred. 

Further consideration of the reaction system reveals that the ammonia feed takes part only in the primary reaction and in neither of the secondary reactions. Consider the rate equation for the primary reaction ... 

In fact, it is often possible with stirred-tank reactors to come close to the idealized well-stirred model in practice, providing the fluid phase is not too viscous. Such reactors should be avoided for some types of parallel reaction systems (see Fig. 2.2) and for all systems in which byproduct formation is via series reactions. 

In Fig. 8.3, the only cost forcing the optimal conversion hack from high values is that of the reactor. Hence, for such simple reaction systems, a high optimal conversion would he expected. This was the reason in Chap. 2 that an initial value of reactor conversion of 0.95 was chosen for simple reaction systems. 

If waste byproducts are formed, it may be possible to upgrade them by further reaction in a different reaction system. 

Reducing waste from multiple reactions producing waste byproducts. In addition to the losses described above for single reactions, multiple reaction systems lead to further waste through the formation of waste byproducts in secondary reactions. Let us briefly review from Chap. 2 what can be done to minimize byproduct formation. 

The individual reactions need not be unimolecular. It can be shown that the relaxation kinetics after small perturbations of the equilibrium can always be reduced to the fomi of (A3.4.138t in temis of extension variables from equilibrium, even if the underlying reaction system is not of first order [51, fil, fiL, 58]. 

Generalized first-order kinetics have been extensively reviewed in relation to teclmical chemical applications [59] and have been discussed in the context of copolymerization [53]. From a theoretical point of view, the general class of coupled kinetic equation (A3.4.138) and equation (A3.4.139) is important, because it allows for a general closed-fomi solution (in matrix fomi) [49]. Important applications include the Pauli master equation for statistical mechanical systems (in particular gas-phase statistical mechanical kinetics) [48] and the investigation of certain simple reaction systems [49, ]. It is the basis of the many-level treatment of... 

The importance of numerical treatments, however, caimot be overemphasized in this context. Over the decades enonnous progress has been made in the numerical treatment of differential equations of complex gas-phase reactions [8, 70, 71], Complex reaction systems can also be seen in the context of nonlinear and self-organizing reactions, which are separate subjects in this encyclopedia (see chapter A3,14. chapter C3.6). 

Wei J and Prater C D 1962 The structure and analysis of complex reaction systems Advances in Catalysis (New York Academic) pp 203-392... 

Energy redistribution is the key primary process in chemical reaction systems, as well as in reaction systems quite generally (for instance, nuclear reactions). This is because many reactions can be separated into two steps ... 

This equation results from the assumption that the actual reaction step in themial reaction systems can happen only in molecules (or collision pairs) with an energy exceeding some tlireshold energy Eq which is close, in general, to the Arrhenius activation energy defined by equation (A3.13.3). Radiative energization is at the basis of classical photochemistry (see e.g. [4, 3 and 7] and chapter B2.5) and historically has had an interesting sideline in the radiation... 

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intennoleciilar energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [U, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32] and the literature cited therein. These cover a variety of aspects of tire topic and fiirther, more detailed references will be given tliroiighoiit this review. We should mention here the energy transfer processes, which are of fiindamental importance but are beyond the scope of this review, such as electronic energy transfer by mechanisms of the Forster type [33, 34] and related processes. 

There is one special class of reaction systems in which a simplification occurs. If collisional energy redistribution of some reactant occurs by collisions with an excess of heat bath atoms or molecules that are considered kinetically structureless, and if fiirthennore the reaction is either unimolecular or occurs again with a reaction partner M having an excess concentration, dien one will have generalized first-order kinetics for populations Pj of the energy levels of the reactant, i.e. with... 

As a rule, in diemial unimolecular reaction systems at modest temperatures, is well separated from the other eigenvalues, and thus the time scales for incubation and relaxation are well separated from the steady-... 

The master equation treatment of energy transfer in even fairly complex reaction systems is now well established and fairly standard [ ]. However, the rate coefficients kjj or the individual energy transfer processes must be established and we shall discuss some aspects of this matter in tire following section. 

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