Page 320 - Elements of Chemical Reaction Engineering 3rd Edition

292 Multiple Reactions Chap. 6

A&B&C
in which species B is the desired product.
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 will be converted
to 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 consecutive reactions.

1 Example 6-3 Maximizing the yield of the Intermediate Product

The oxidation of ethanol to form acetaldehyde is carried out on a catalyst of 4 wt %
Cu-2 wt % Cr on A1203 .3 Unfortunately, acetaldehyde is also oxidized on this cat-
alyst to form carbon dioxide. The reaction is carried out in a threefold excess of
oxygen and in dilute concentrations (ca. 0.1% ethanol, 1% 02, and 98.9% N,).
Consequently, the volume change with the reaction can be neglected. Determine the
concentration of acetaldehyde as a function of space-time,

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

Solution
Because 0, is in excess, we can write the equation above as

A k',B kz,C
1. Mole balance on A:

(E6-3.1)

a. Rate law:
-r; = k1C.A
b. Stoichiometry ( E 1):
FA = CAUO

c. Combining, we have

(E6-3.2)

Let t' = W/ uo = pbv/ 7-10 = pbz, where pb is the bulk density of the catalyst.

R. W. McCabe and P. J. Mitchell, Ind. Eng. Chem. Process Res. Dev., 22,212 (1983).