Complex sequential-parallel reaction

A cascade of 3 tanks in series is used to optimise the selectivity of a complex sequential-parallel reaction. Depending on the kinetics, distributing the feed of one reactant among the tanks may lead to improved selectivity. 

Kinetic models which consider demetallation as a complex reaction network of consecutive and parallel reactions taught by model compound studies have been recognized with real feedstocks. Tamm et al. (1981) suggest a sequential mechanism where the metal compounds are activated by H2S. Model compound reaction pathway studies in the absence of H2S, discussed in Section IV,A,1, and experiments in which H2S was present in excess (Pazos et al., 1983) indicate that sequential reactions are inherent to the chemistry of the metal compounds irrespective of the presence of H2S. However, it is possible that both mechanisms contribute to metal removal. 

When a reaction rate is measured in a chemical reactor, the reaction is generally a composite reaction comprised of a sequence of elementary reactions. An elementary reaction is a reaction that occurs at the molecular level exactly as written (Laidler, 1987). The mechanism of the reaction is the sequence of elementary reactions that comprise the overall or composite reaction. For example, mineral dissolution reactions generally include transport of reactant to the surface, adsorption of reactant, surface dilfusion of the adsorbate, reaction of the surface complex and release into solution, and transport of product species away from the surface. These reactions occur as sequential steps. Reaction of surface complexes and release to solution may happen simultaneously at many sites on a surface, and each site can react at a different rate depending upon its free energy (e.g., Schott et al., 1989). Simultaneous reactions occurring at different rates are known as parallel reactions. In a series of sequential reactions, the ratedetermining step is the step which occurs most slowly at the onset of the reaction, whereas for parallel steps, the rate-determining step is the fastest reaction. 

Most chemical reactions in nature include several mutually associated mono- or bimolecular steps and for this reason have a complex, often reversible character. The entire sequence and interconnection of elementary acts of the complex reactions is called reaction mechanism. These mechanisms differ not only in the number of elementary acts but also in the nature of their sequence and direction. Most common in hydrochemistry are sequential, parallel, and reversible mechanisms. 

These types of equations can be applied to more complex reaction schemes such as sequential reactions,parallel reactions and solid state reactions.Table 1 shows some of the reaction schemes where such equations can be written. 

Models for LLPTC become even more complicated for special cases such as PTC systems that involve reactions in both aqueous and organic phases, systems with a base reaction even in the absence of PT catalyst, or other complex series-parallel multiple reaction schemes. For example, Wang and Wu (1991) studied the kinetics and mass transfer implications of a sequential reaction using PTC... 

Ag(in)(H3l06)(H20)2] is the reactive silver(III) species involved in the diper-iodatoargentate(in) oxidation of nitrilotriacetic acid (NTA) in a mildly basic medium to produce formaldehyde and ammonia. NTA binds to the silver(III) complex in an axial fashion and two electrons are transferred from bound NTA to the silver(III) centre sequentially. The latter stages of the reaction consist of a composite process involving the oxidation of NTA and its products in parallel reactions. ... 

Fibrous fluorosilicate formation within the temperature range of 600-1000°C was demonstrated to result from complex non-stoichiometric reactions developing through sequential and parallel stages with the contribution of liquid and gaseous phases. In this, the process in the first 3h was limited by proper chemical interaction, while its further development was restricted by diffusion. 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

A complex reaction is run in a semi-batch reactor with the purpose of improving the selectivity for the desired product, P. The kinetics are sequential with respect to components A, P and Q but parallel with respect to B. The relative orders of the reactions for the reactions determine the feeding policy. 

Production of phenol and acetone is based on liquid-phase oxidation of isopropylbenzene. Synthetic fatty acids and fatty alcohols for producing surfactants, terephthalic, adipic, and acetic acids used in producing synthetic and artificial fibers, a variety of solvents for the petroleum and coatings industries—these and other important products are obtained by liquid-phase oxidation of organic compounds. Oxidation processes comprise many parallel and sequential macroscopic and unit (or very simple) stages. The active centers in oxidative chain reactions are various free radicals, differing in structure and in reactivity, so that the nomenclature of these labile particles is constantly changing as oxidation processes are clarified by the appearance in the reaction zone of products which are also involved in the complex mechanism of these chemical conversions. 

Although the simple rate expressions, Eqs. (2-6) and (2-9), may serve as first approximations they are inadequate for the complete description of the kinetics of many epoxy resin curing reactions. Complex parallel or sequential reactions requiring more than one rate constant may be involved. For example these reactions are often auto-catalytic in nature and the rate may become diffusion-controlled as the viscosity of the system increases. If processes of differing heat of reaction are involved, then the deconvolution of the DSC data is difficult and may require information from other analytical techniques. Some approaches to the interpretation of data using more complex kinetic models are discussed in Chapter 4. 

A multielectron electrode reaction may also occur by a number of mechanistic routes including sequential and parallel pathways, which in complex electrokinetics may also be analysed individually in terms of elementary chemical and electrochemical steps. Figure 7 depicts plots of log j vs. Tj for (a) sequential and (b) parallel paths for multielectron transfer reactions. It is apparent that, at a given electrode potential, in... 

However, the latter type of metal-catalyzed cascade reactions turns out to be even more challenging since issues of selectivity and efficiency are crucially dependent on the particular catalyst structure. This type can either be performed in a parallel or sequential fashion [16,21], Whereas parallel catalysis is significantly more difficult to develop, sequential catalysis offers the possibility of altering reaction conditions and additives from step to step in the sense of bi- or multicatalytic one-pot processes, assisted tandem catalysis, or auto tandem catalysis [1]. Therefore, a demanding goal is the development of one-catalyst multireaction sequences that set the stage for new reactions in diversity-oriented syntheses of complex molecular structures (for reviews on diversity-oriented syntheses see [27-33]). 

Future development work is needed to prove the possibilities of new concepts that have been created during recent years. The investigation of systems which work homogeneously but which can be separated easily [22] must be mentioned in this context (cf Section 3.1.1.1). The introduction of multidentate ligands and the evaluation of polymer systems containing more than one metal complex site, which cooperate eventually in the catalysis of parallel or sequential reactions [23-25], are among these concepts. In addition, the formation of new or normally unstable catalytic species [26, 27] attached to polymer supports, or the formation of immobilized metal complexes as intermediates of defined metal clusters [28], should be studied further (cf. Section 3.1.1.5). 

Most gas-phase reactions of interest, however, have a complex product distribution, can be sequential, and are to some extent catalyzed by many different catalysts examples are the partial oxidation reactions. For this class of reactions, a parallel testing would be much better than cocktail screening, as in this case promising catalysts could be directly identified. The drawback of parallel screening, however, is the more complex reactor design and the fact that a parallel analysis or, at least, a relatively fast sequential analysis is necessary. 

Due to the number of experiments necessary in such studies which take typically times of several hours to several days, there is a strong incentive to work on acceleration of synthesis experiments in zeolite science. On the other hand, there are also major obstacles to overcome in such an endeavour. Zeolite syntheses are often carried out at elevated pressure, reaction media are typically corrosive, precursor solutions and the synthesis gel itself may be highly viscous and difficult to handle, the synthesis is very sensitive to preparation conditions, such as sequence of reagent addition, stirring (difficult for small volumes) may be necessary for some formulations, aging conditions may differ from batch to batch, if automated sequential preparation is chosen, the work-up involving many steps is complex, and the resulting materials are often not phase-pure and difficult to characterize. Nevertheless, in spite of these problems some of the earliest examples for the synthesis of catalytically relevant solids in parallelized and - some points -automated equipment were reported for zeolites [4]. 

As mentioned above, zeolite catalyzed reactions are complicated by the fact that deactivation easily occurs. This can only be fully solved by truly parallelized analysis, such as provided by the FPA IR systems. Alternatively, if sequential analysis is used, sequential start of the reaction in the different channels is an alternative, so that all catalysts are compared at the same time on stream. However, this requires rather complex valve schemes which are difficult to implement. As a simpler solution, the catalysts performance is analyzed several times in different cycles, so that for each catalyst an activity pattern over time is obtained. 

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