Page 347 - Elements of Chemical Reaction Engineering Ebook

31 8 Multiple Reactions Chap. 6

SUMMARY

For the competing reactions

Reaction 1: A + B ‘ +D ($6- 1 )

ku
Reaction2: A+B _c3 U (S6-2)
1. Qpical rate expressions are

(S6-3)

-EUIRT a2 fi2
ru = A,e cA cB (S6-4)
and the instantaneous selectivity parameter is defined. as

a. If ED>Eu, the selectivity parameter S,, will increase with
increasing tempratwe.
b. If al>a2 and p2>p1, the reaction should be carried out at high
concentrations of A and low concentrations of B to maintain the
selectivity parameter S,, at a high value. Use a senlibatch reactor
with pure A initially or a tubular reactor in which B is fed at differ-
ent locations down the reactor. Other cases discussed in the text are
a2>%, P2>P1 and %’%, P1>P2.
2. The instantaneous yield at a point is defined as the ratio of the rate of
formation of a specified product D to the rate of depletion of the key
reactant A:

‘D
y,=_,z rD (S6-6)
-‘A - ‘A1 - ‘A2

The overall yield is the ratio of the number of moles of a product at
the end of a reaction to the number of moles of the key reactant that
have been consumed:

($6-7)

For a flow system, this yield is