Series reaction networks

For the liquid-phase oxidation of anthracene (AN) described in problem 5-17, Rodriguez and Tijero (1989) obtained the following values for the rate constants at 95°C in the two-step series reaction network ... 

Table 18.1 Comparison of PFR and CSTR for series-reaction network A -4 B -+ C (isothermal, constant-density system K = kz/ki)...

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... 

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

For a series reaction network the most important variable is either time in batch systems or residence time in continuous flow systems. For the reaction system A - B - C the concentration profiles with respect to time in a batch reactor (or residence time in a PFR) are given in Figure 6. 

FIGURE 7 Catalyst performance as function of the Thiele modulus. Left the effectiveness factor right selectivity in a series reaction network. 

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. 

The simplest irreversible network is the series reaction network ... 

The same series reaction network, but considered under a commonly applied simplifying assumption, illustrates the central approximation used in the formulation of all reaction mechanisms the steady state approximation. This states that the net rate of change of the concentration of an unstable intermediate is zero. In the above network the intermediate B can be relatively stable, in which case its concentration increases to a maximum and then falls, or it can be unstable. If B is unstable, then soon after the reaction starts and a small amount of B is formed, any further B formed is immediately converted to C. Soon after the reaction begins therefore a steady state concentration of B is achieved where the net accumulation of B is effectively zero. We write this steady state condition as follows ... 

We can begin by computing the selectivities and yields for the series network in the CSTR versus the PFR first. Consider the simplest series reaction network ... 

Pore-diffusion limitations in porous catalysts affect catalyst and/or product selectivity. Two instances are considered here. The first involves a parallel reaction network in which the desired product is produced in one reaction and coke precursors form via the second reaction. Accumulation of the coke precursors in the catalyst leads to catalyst fouling and causes a continuous decline in catalyst activity. The second example is a series reaction network of the type A B C. Clearly, pore-diffiision limitations would hinder the removal of B (the desired product) from the catalyst, favoring further reaction to the ultimate product, C. Complementary experimental and theoretical investigations are presented below to show how pressure tuning with supercritical media may be exploited to stabilize catalyst activity and to enhance the selectivity of primary products. 

Series reaction networks are very important commercially. In many cases, the intermediate product, R, is desired and the terminal product, S, is undesired. 

Series reaction networks are not limited to two reactions. Butadiene (C4H6) is an important monomer that goes into a large number of elastomeric products, including automobile tires. Butadiene can be produced by the catalytic dehydrogenation of butane (C4H10) as shown below. 

Figure 7-2 illustrates a critical feature of series-reaction networks. The concentration of the intermediate product, R in this case, increases rapidly once the reaction has started. This concentration then goes through a maximum and declines. The value of the time at which R is maximum has been labeled optimum time or opt bi the figure. If the reaction that consumes R is irreversible, Cr will approach zero at very long times. 

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