Consecutive—parallel reactions

Ortho-xylene (A) is oxidised to phthalic anhydride (B) in an ideal, continuous flow tubular reactor. The reaction proceeds via the complex consecutive parallel reaction sequence, shown below. The aim of the reaction is to produce the maximum yield of phthalic anhydride and the minimum production of waste gaseous products (C), which are CO2 and CO. 

CONSECUTIVE-PARALLEL REACTIONS WITH REVERSIBLE STEPS... 

Therefore, parallel reactions implicitly affect proceeding kinetics of each other via the change of general reagent A concentration. Since consecutive-parallel reactions represent a combination of those mentioned above, the type of interaction between individual complex reactions will be similarly performed (see Figure 2.1). 

The reduced schemes (3.1)—(3.3) does not identify each separate conjugated reaction. The solution of the problem is also complicated by the possibility of using this scheme for a scheme of consecutive-parallel reaction, if one does not know that the scheme describes chemical induction. Meanwhile, schemes (3.1)—(3.3) possess an important advantage it shows the stages required for acceptor transformation. 

It is common knowledge that consecutive or consecutive-parallel reactions have intermediate products of two types ... 

However, the extrinsic analogy masks a principal difference consisting in the fact that in the case where chemical induction reactions are interrelated, consequently, they may not be described by two independent systems of differential equations, as for parallel reactions. If one makes an attempt to describe them by kinetic reactions for consecutive (or consecutive-parallel) reactions, similar to scheme (3.17), then conjugated reactions will be reduced to consecutive reactions, from which they principally differ. So what are the kinetic expressions that may show the individuality of chemical induction ... 

Mixing may occur on several scales on the reactor scale (macro), on the scale of dispersion from a feed nozzle or pipe (meso), and on a molecular level (micro). Examples of reactions where mixing is important include fast consecutive-parallel reactions where reactant concentrations at the boundaries between zones rich in one or the other reactant being mixed can determine selectivity. 

The following first-order consecutive-parallel reaction scheme is used, which is in accord with the oxidation of naphthalene (1) ... 

In all cases, B is the desired product. In the following derivations, only the consecutive-parallel reaction scheme is considered, since the other two are just special cases of this more general scheme. With the list of symbols given at the end,... 

This is a special case of the consecutive-parallel reaction,... 

In order to get a qualitative idea, Table 5.6 presents kinetic constants for the consecutive/parallel reaction scheme given in Figure 5.1 obtained with a Pd-type catalyst (Park et al. [15]). Hydrogen was in large excess so that first-order kinetics may be assumed. Note that kinetic constants are reported as the mass load W/Fphenoi. the phenol being produced by the evaporation of aqueous solutions. The nature of the support is the determinant for selectivity, but the activity is also affected. The most selective catalyst is Pd deposited on activated carbon (AC), but... 

Type 3. Consecutive-parallel reactions. In these examples (Scheme 2.70) we have to deal with the problems of the first two types combined. The starting compounds are polyfunctional. An initial reaction can occur at any of the available functions and therefore the reaction is likely to produce isomeric products. Then, as is the case with consecutive reactions, the intact functional groups still present in these products can be subject to additional transformations. 

When the catalyst is immobilized within the pores of an inert membrane (Figure 25.13b), the catalytic and separation functions are engineered in a very compact fashion. In classical reactors, the reaction conversion is often limited by the diffusion of reactants into the pores of the catalyst or catalyst carrier pellets. If the catalyst is inside the pores of the membrane, the combination of the open pore path and transmembrane pressure provides easier access for the reactants to the catalyst. Two contactor configurations—forced-flow mode or opposing reactant mode—can be used with these catalytic membranes, which do not necessarily need to be permselective. It is estimated that a membrane catalyst could be 10 times more active than in the form of pellets, provided that the membrane thickness and porous texture, as well as the quantity and location of the catalyst in the membrane, are adapted to the kinetics of the reaction. For biphasic applications (gas/catalyst), the porous texture of the membrane must favor gas-wall (catalyst) interactions to ensure a maximum contact of the reactant with the catalyst surface. In the case of catalytic consecutive-parallel reaction systems, such as the selective oxidation of hydrocarbons, the gas-gas molecular interactions must be limited because they are nonselective and lead to a total oxidation of reactants and products. For these reasons, small-pore mesoporous or microporous... 

Oxidation of aqueous phenol solutions was studied over various catalysts in a semibatch slurry and continuous-flow fixed-bed reactors at temperatures up to 463 K and pressures slightly above atmospheric. The results show that due to a complex consecutive-parallel reaction pathway and a heterogeneous-homogeneous free-radical mechanism both kinetics and reaction selectivity are strongly dependent on the type of reactor used. Although the catalysts employed were found to be active in converting aqueous phenol solutions to nontoxic compounds, neither metal oxides nor zeolites were stable at the reaction conditions. 

Under the experimental conditions used, the reaction was found to proceed, on all the catalysts tested, through a complex consecutive/parallel reaction network (Scheme 1), The reaction pathway involves the formation of 4-(hydroxyamino)-2-nitrotoluene (4HA2NT), 2-(hydroxyamino)-4-nitrotoluene (2HA4NT), 4-amino-2-nitrotoluene (4A2NT) and 2-amino-4-nitrotoluene (2A4NT) as relevant reaction intermediates. No significative formation of the hydroxyamino-aniinotoluene isomers was instead observed. Most likely, this is due to the high reactivity of these intermediates. 

Catalytic partial oxidation of hydrocarbons represents an important class among petrochemical reactions. Complete oxidation of hydrocarbons gives CO2, H2O. During the partial oxidation processes the conversion of a certain percentage of reactants and/or products to complete combustion products cannot be avoided. The main role of the catalyst in these reactions is to accelerate (at relatively lower temperatures) the reaction paths to the desired product without having the same effect on the paths to the complete combustion products. The partial oxidation reactions are usually consecutive or consecutive/parallel reactions with quite complex networks in many cases. 

Riser technology appears to be quite versatile. Patience and Mills [33] investigated propylene oxidation into acrolein and found that this technique has a potential for the commercial scale production of acrolein. Their kinetic model was based on a simplified single site redox mechanism involving consecutive-parallel reactions for the partial and complete oxidation of propylene. Its predictions of the performance of the reactor gave correct trends. 

This approach applied to the description of the hydrogenation reactions has been described in detail in our previous paper [13]. For the hydrogenation of 2,4-DNT over palladium on alumina catalyst the complex consecutive-parallel reaction pathways are shown in Fig 2. The appropriate rate equations can be written as follows [13] ... 

As example, we consider the following consecutive-parallel reactions, often found in organic chemistry processes, as nitration, alkylation, chlorination, etc ... 

Control of Reactant Concentration at the Catalyst - In a series of publications Harold and co-workers have attempted to improve the intermediate product yield of consecutive-parallel reaction schemes. They used... 

Imperfect mixing can also affect the selectivity for consecutive reactions of the following type, which are sometime called consecutive-parallel reactions ... 

Another conunon form of mixed consecutive-parallel reactions is the following ... 

This section analyses the consecutive parallel reactions... 

Fig. 3. Consecutive-parallel reactions in CSTR / Separator / Recycle a) Control structure b) Bifurcation diagram showing multiple steady states.

Several industrially important reactions follow a consecutive-parallel reaction scheme of the following type ... 

In a recent paper Cresswell jlSj discussed the effect of intraparticle convection on the yield of consecutive-parallel reactions. 

In another set of studies, this group studied the potentialities of the dense PCMRs for conducting consecutive-parallel reactions. In a theoretical study, considering non-isothermal conditions and perfectly mixed flow pattern, beyond other main assumptions already assumed in previous works and referred above, the authors analysed the hydrogenation pwpyne— pwpene pwpane in order to define in which conditions the... 

Sousa J M and Mendes A M (2006), Consecutive-parallel reactions in noniso-thermal polymeric catalytic membrane reactors , Ind Eng Chem Res, 45, 2094-2107. 

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