Parallel reactions desired products

The competitive and non-competitive inhibitors are easily distinguished in a Lineweaver-Burk plot. The competitive inhibitor intercepts on the Mv axis whereas the non-competitive inhibitor intercepts on the 1/5 axis. The reaction of inhibitors with substrate can be assumed as a parallel reaction while the undesired product is formed along with desired product. The reactions are shown as ... 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

One might then expect that the yield of the desired product would correlate well with the local rate of energy dissipation. Villermaux et al. (1994) studied a system of the following parallel competing reactions ... 

To maximize the production of the desired P the ratio r /r2 must be maximized for parallel reactions... 

Parallel reactions, oai = om2, a i = am = 0, Ei > E2. The. selectivity to the desired product increases with temperature. The highest allowable temperature and the highest reactant concentrations should be applied. A batch reactor, a tubular reactor, or a cascade of CSTRs is the best choice. 

Parallel reactions, omi = oa2, aai = am = 0, E < E2. The selectivity of the desired product decreases with temperature. However, a low temperature disfavours the reaction rate. A nonuniform temperature-time profile should be applied to maximize the reactor productivity (see Fig. 5.4-70). Initially the temperature should be low to avoid the formation of too much unwanted product. The temperature is gradually raised with time to increase the reaction rate until the maximum allowable temperature is reached. At T u the reaction is completed. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

Multiple reaction selectivity can be defined similarly as the ratio of the rate of formation of the desired product to the formation rate of an undesired product as in a parallel reaction... 

Here Sb,c is selectivity of the desired product B to unwanted product in this parallel reaction. 

A semi-continuous reactor is used to carry out the parallel reaction shown below. It is of interest to investigate how the amount of desired product, P, depends on the differing orders of the two reactions and on the feeding rate. 

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. 

It is proposed to react 1 t-h-1 of a pure liquid A to a desired product B. Byproducts C and D are formed through series and parallel reactions ... 

The possibility of a species reacting by parallel paths to yield geometric isomers or entirely different products is often responsible for low yields of a desired product. If circumstances are such that the orders of the desired and unwanted reactions are different with respect to one or more species, it is possible to promote the desired reaction by an appropriate choice of reactor type and reaction conditions. 

Illustration 9.1 indicates how the principles enunciated above may be used in optimizing the yield of a desired product when dealing with parallel reactions. 

The bottleneck of conventional parallel/combinatorial synthesis is typically optimization of reaction conditions to afford the desired products in suitable yields and purities. Since many reaction sequences require at least one or more heating steps for extended time periods, these optimizations are often difficult and time-consum-... 

For illustration, we consider a simplified treatment of methane oxidative coupling in which ethane (desired product) and CO, (undesired) are produced (Mims et al., 1995). This is an example of the effort (so far not commercially feasible) to convert CH, to products for use in chemical syntheses (so-called Q chemistry ). In this illustration, both C Hg and CO, are stable primary products (Section 5.6.2). Both arise from a common intermediate, CH, which is produced from CH4 by reaction with an oxidative agent, MO. Here, MO is treated as another gas-phase molecule, although in practice it is a solid. The reaction may be represented by parallel steps as in Figure 7.1(a), but a mechanism for it is better represented as in Figure 7.1(b). 

Parallel reactions involve simultaneous coupling of two or more reactions to form different sets of products from a common set of reactants. If one of these product sets is desirable and the others are not, we need to examine conditions that tend to promote the desired reaction and suppress the others. This may be viewed through the relative rates of the parallel processes and the means of increasing a desired rate relative to others. 

Micromixing may also have a major impact upon the yield and selectivity of complex reaction networks. Consider, for example, the following parallel reaction network, where both a desired product (D) and an undesired product (U) may be formed ... 

Extension of the Kunii-Levenspiel bubbling-bed model for first-order reactions to complex systems is of practical significance, since most of the processes conducted in fluidized-bed reactors involve such systems. Thus, the yield or selectivity to a desired product is a primary design issue which should be considered. As described in Chapter 5, reactions may occur in series or parallel, or a combination of both. Specific examples include the production of acrylonitrile from propylene, in which other nitriles may be formed, oxidation of butadiene and butene to produce maleic anhydride and other oxidation products, and the production of phthalic anhydride from naphthalene, in which phthalic anhydride may undergo further oxidation. 

It should be noted that there are cases in which some selectivity will be lost in choosing a semi-batch mode over a simple batch reactor. If the desired product decomposes by a consecutive reaction, the yield will be higher in the batch reactor [177]. If, on the other hand, the reactants are producing by-products by a parallel reaction, the semi-batch process will give the higher yield. In any case, if the heat production rate per unit mass is very high, the reaction can then be run safely under control only in a semi-batch reactor. 

Obviously, reaction selectivity will depend on temperature. For isothermal operation, the temperature can be chosen to maximise the yield of desirable product. For example, consider the simultaneous parallel reactions (12). If the rate coefficients conform to the Arrhenius expression, e.g. 

Attention will be focussed on three typical chemical reaction schemes. For the first illustration, two parallel competing reactions are considered. For instance, it may sometimes be necessaru to convert into a desired product only one component in a mixture. The dehydrogenation of six-membered cycloparaffins in the presence of five-membered cycloparaffins without affecting the latter is one such example of a selectivity problem in petroleum reforming reactions. In this case, it is desirable for the catalyst to favour a reaction depicted as... 

These networks can be analyzed by breaking them down into their simple series and simple parallel components. For example, for the following elementary reactions, where R is the desired product, the breakdown is as follows ... 

When selectivity and yield of a given product need to be maximized, the design issues become more complicated. While rninimum T is frequently desired, it is usually more important to obtain maximum selectivity to a desired product and niinimiim selectivity to undesired products. For simple series and parallel reaction systems, we can fairly easily summarize the choices. 

Oxidation of mono-cysteine peptides to the dimer is a straightforward reaction that can produce only the desired product. In the case of bis-cysteine peptides statistically the oxidation leads to the homodimers in parallel and antiparallel orientation as well as to the disulfide-bridged monomer and oligomers. When the two cysteine residues are placed in the adjacent position formation of homodimers is highly favored over the cyclic monomer (Section 6.1.5.1) and the product distribution depends strongly on the peptide concentration. Such a type of intermolecular disulfide bridging is present in bovine seminal ribonuclease, where an antiparallel alignment occurs at the interface of the dimer. 97 ... 

If a second reactant B is involved in a system of parallel reactions, then the same principles apply to B as to A. The rate equations are examined to see whether the order of the desired reaction with respect to B is higher or lower than that of the undesired reaction, and to decide whether high or low concentrations of B favour a high yield of desired product. 

It is rare that a catalyst can be chosen for a reaction such that it is entirely specific or unique in its behaviour. More often than not products additional to the main desired product are generated concomitantly. The ratio of the specific chemical rate constant of a desired reaction to that for an undesired reaction is termed the kinetic selectivity factor (which we shall designate by 5) and is of central importance in catalysis. Its magnitude is determined by the relative rates at which adsorption, surface reaction and desorption occur in the overall process and, for consecutive reactions, whether or not the intermediate product forms a localised or mobile adsorbed complex with the surface. In the case of two parallel competing catalytic reactions a second factor, the thermodynamic factor, is also of importance. This latter factor depends exponentially on the difference in free energy changes associated with the adsorption-desorption equilibria of the two competing reactants. The thermodynamic factor also influences the course of a consecutive reaction where it is enhanced by the ability of the intermediate product to desorb rapidly and also the reluctance of the catalyst to re-adsorb the intermediate product after it has vacated the surface. 

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