Page 313 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 6.l Maximizing the Desired Product in Parallel Reactions 285
The rate of disappearance of A for this reaction sequence is the sum of the
rates of formation of U and D:

-rA = rD + ru (6-3)

where cy1 and a2 are positive constants. We want the rate of formation of D,
rD, to be high with respect to the rate of formation of U, rU. Taking the ratio
instantaneous of these rates [Le., Equation (6-1) to Equation (6-2)], we obtain a rate selectiv-
selectivity ity pammeter, S, which is to be maximized:

SDU = - - - c, (6-5)
rD - kD
ru k"

6.1.1 Maximizing the Rate Selectivity Parameter S for One Reactant

Maximize the rate In this section we examine ways to maximize S, which is sometimes
referred to as the instantaneous selectivity, for different reaction orders of the
parameter
desired and undesired products.

a1 is the order of Case 1: a, > a? For the case where the reaction order of the desired urocluct
the desired reaction; is greater ihan the reaction order of the undesired product, let a be a Gositive
a2, the undesired
of
reaction number that is the difference between these reaction orders:

Then

(61-6)

For cil > cy2, make Tal make this ratio as largg as possible, we want to carry out the reaction
C, as large as in a manner that will keep me cbncentration of reactant A as high as possible
possible during the reaction. If the reaction is carried out in the gas phase, we should
run it without inerts and at high pressures to keep C, high. If the reaction is in
the liquid phase, lbe use of diluents should be kept to a minimum.'
A batch or plug-flow reactor should be used in this case, because in these
two reactors, the cioncentration of A starts at a high value and drops progressively
during the course of the reaction. In a perfectly mixed CSTR, the concentration
of reactant within the reactor is always at its lowest value (ie., that of the outlet
concentration) and therefore not be chosen under these circumstances.

For a number of liquid-phase reactions, the proper choice of a solvent can enhance selec-
tivity. See, for example, Znd. Eng. Chem., 62(9), 16 (1970). In gas-phase heterogeneous
catalytic reactions, selectivity is an important parameter of any particular catalyst.