Selectivity parallel networks

The Diels Alder reactions of maleic anhydride with 1,3-cyclohexadiene, as well the parallel reaction network in which maleic anhydride competes to react simultaneously with isoprene and 1,3-cyclohexadiene [84], were also investigated in subcritical propane under the above reaction conditions (80 °C and 90-152 bar). The reaction selectivities of the parallel Diels-Alder reaction network diverged from those of the independent reactions as the reaction pressure decreased. In contrast, the same selectivities were obtained in both parallel and independent reactions carried out in conventional solvents (hexane, ethyl acetate, chloroform) [84]. 

In this chapter, we develop some guidelines regarding choice of reactor and operating conditions for reaction networks of the types introduced in Chapter 5. These involve features of reversible, parallel, and series reactions. We first consider these features separately in turn, and then in some combinations. The necessary aspects of reaction kinetics for these systems are developed in Chapter 5, together with stoichiometric analysis and variables, such as yield and fractional yield or selectivity, describing product distribution. We continue to consider only ideal reactor models and homogeneous or pseudohomogeneous systems. 

It is more difficult to develop general guidelines regarding the selection and design of a reactor for a series-parallel reaction network than for a parallel-reaction or a series-reaction network separately. It is still necessary to take into account the relative... 

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 ... 

The reaction network for isobutane selective oxidation catalyzed by POMs consists of parallel reactions for the formation of methacrolein, methacrylic acid, carbon monoxide, and carbon dioxide. Consecutive reactions occur on methacrolein, which is transformed to acetic acid, methacrylic acid, and carbon oxides. ° Methacrylic acid undergoes consecutive reactions of combustion to carbon oxides and acetic acid, but only under conditions of high isobutane conversion. Isobutene is believed to be an intermediate of isobutane transformation to methacrylic acid, but it can be isolated as a reaction product only for very low alkane conversion. ... 

Activity that leads to network selection and integration (e.g. 40 Hz syn-chronicity), distinct from the vast parallel array of neuronal networks involved in non-conscious processing. 

The dependence of ethene selectivity on the conversion of ethane for the better catalysts shown in Fig. 1 shows that the selectivity is high at low conversions and decreases as the conversion increases. This trend is consistent with a reaction pathway that consists of mostly sequential reactions [Eq. (3)]. Depending on the reaction temperature, the reaction network may involve two parallel reaction pathways shown below, which is modified from... 

Local Orientation. The most striking observation of this work is that the selected area diffraction patterns are not in general of a Debye-Scherrer type. Among the various hypotheses which can be drawn to understand such a fact, the most probable one is that the sections are not truly transverse ones indeed, if one supposes the existence of a cylindrical symmetry at the level of each selected area, 0.5 to 1 ym in diameter (the symmetry axis being always parallel to the fiber axis) the "detectable" network main planes have to be parallel to 1he "c" axis of the individual... 

In a parallel reaction network of first-order reactions, the selectivity does not depend upon reaction time or residence time, since both products are formed by the same reactant and with the same concentration. The concentration of one of the two products will be higher, but their ratio will be the same during reaction in a batch reactor or at any position in a PFR. The most important parameters for a parallel reaction system are the reaction conditions, such as concentrations and temperature, as well as reactor type. An example is given in the following section. 

Frequently, several reactions proceed simultaneously, and consequently selectivity and yield in networks of parallel and series reactions with respect to a certain desired target component D are essential quantities. 

Equation (19-22) indicates that, for a nominal 90 percent conversion, an ideal CSTR will need nearly 4 times the residence time (or volume) of a PFR. This result is also worth bearing in mind when batch reactor experiments are converted to a battery of ideal CSTRs in series in the field. The performance of a completely mixed batch reactor and a steady-state PFR having the same residence time is the same [Eqs. (19-5) and (19-19)]. At a given residence time, if a batch reactor provides a nominal 90 percent conversion for a first-order reaction, a single ideal CSTR will only provide a conversion of 70 percent. The above discussion addresses conversion. Product selectivity in complex reaction networks may be profoundly affected by dispersion. This aspect has been addressed from the standpoint of parallel and consecutive reaction networks in Sec. 7. 

Vivarelli et al. (1995) used a hybrid system that combined a local genetic algorithm (LGA) and neural networks for the protein secondary structure prediction. The LGA, a version of the genetic algorithms (GAs), was particularly suitable for parallel computational architectures. Although the LGA was effective in selecting different... 

In this example, the rates in the networks with and without by-product formation were found to be of same algebraic form and the yield ratio and selectivity to be concentration-independent. This is due to the concentration independence of the segment coefficients of the two parallel pathways, A2X and A2X and is not generally true even if the different pathways consist of strictly analogous steps. 

Derive the rate expression for this mixed-parallel series-reaction network and the expression for the percent selectivity to the epoxide. 

In Example 1.5.6, the expression for the maximum concentration in a series reaction network was illustrated. Example 1.5.8 showed how to determine the selectivity in a mixed-parallel series-reaction network. Calculate the maximum epoxide selectivity attained from the reaction network illustrated in Example 1.5.8 assuming an excess of dioxygen. 

A second application of IMRs consists of using the membrane to distribute a reactant to a fixed bed of catalyst packed on the opposite side (see Figure 10.21c). The most frequent case corresponds to a series-parallel reaction network where there is a favorable kinetic effect regarding the partial pressure of the distributed reactant. Thus, IMRs have been used successfully as oxygen distributors in many oxidations where not only greater selectivity with respect to conventional arrangements is obtained but also a safer operation where a reduced formation of hot spots, lower probability of runaway, and catalyst life enhancement are achieved. 

Effect of Intraparticle Diffusion for Reaction Networks For multiple reactions, intraparticle diffusion resistance can also affect the observed selectivity and yield. For example, for consecutive reactions intraparticle diffusion resistance reduces the yield of the intermediate (often desired) product if both reactions have the same order. For parallel reactions diffusion resistance reduces the selectivity to the higher-order reaction. For more details see, e.g., Carberry, Chemical and Catalytic Reaction Engineering, McGraw-Hill, 1976 and Fevenspiel, Chemical Reaction Engineering, 3d ed., Wiley, 1999. 

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