Maximizing the Desired Product in Series Reactions

In Section 6.1, we saw that the unde.sired product could be minimized adjusting the reaction conditions (e.g., concentration) and by choosing t proper reactor. For series of consecutive reactions, the most important variat is time space-time for a flow reactor and real-time for a batch reactor. To ilti trate the importance of the time factor, we consider the sequence 

If the first reaction is slow and the second reaction is fast, it will be extremely difficult to produce species B, If the first reaction (formation of B is fast and the reaction to form C is slow, a large yield of B can be achieved. However, if the reaction is allowed to proceed for a long time in a batch reactor, or if the tubular flow reactor is too long, the desired product B wilt be converted to the undesired product C. In no other type of reaction is exactness in the calculation of the time needed to carry out the reaction more important than in series reactions. 

The oxidation of ethanol to form acetaldehyde is carried out on a catalyst of 4 wi % Cu-2 wt % Cr on AI-Oj. Unfortunately, acetaldehyde is also oxidized on this catalyst to form carbon dioxide. The reaction is carried out in a threefold excess of oxygen and in dilute concentrations fca. 0. ethanol, 1% Oi, and 98.9% N ). Consequently, the volume change with the reaction can be neglected. Determine the concentration of acetaldehyde as a function of space-lime, 

The reactions are irreversible and first order in ethanol and acetaldehyde, respectively. 

Because 0 is in excess, we can write the preceding series reaction as 

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Maximizing the Desired Product in Series Reactions 291 Algorithm for Solutionto Complex Reactions 295... 

A V - W where V is the desired product. These liquid phase reactions are to be carried out in a cascade of two equal volume CSTR s in series. If the reactors are to be sized so as to maximize the concentration of species V in the effluent from the second reactor, determine the reactor volumes necessary to process 500 gal/hr of feed containing 6 moles/gal of species A. No V or W is present in the feed. What fraction of the A ends up as V The rate constants kx and k2 are both equal to 0.5 hr - L... 

The evolution of concentration with time clearly indicates whether a particular component is an end product or an intermediate product that undergoes further reaction even as it is formed. The concentration-time (t or 7) profiles for the series, series-parallel, Denbigh, and triangular schemes mentioned above are sketched in Figure 5.4. An important feature of an intermediate product is that it has a maximum at a specific time If that compound happens to be the desired product, it is best to operate the reactor at / ax- Relationships for [Rjmax terms of the kinetic parameters of the reactions are important in maximizing production. They can be derived from the solutions given in Table 5.4, and are summarized in Table 5.5 for a few selected schemes in which R is produced as an intermediate. 

This discussion shows that the PFR is a better choice than a CSTR for series reactions where the desired product is an intermediate, such as R in this example, and where it is important to maximize the yield of the intermediate. 

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