Page 314 - Elements of Chemical Reaction Engineering 3rd Edition

286 Multiple Reactions Chap. 6

Case 2: cy2 > cy1 When the reaction order of the undesired product is greater
than that of the desired product,

Let a = cy2 - cy1 , where a is a positive number; then

For the ratio rD/ru to be high, the concentration of A should be as low as
possible.
This low concentration may be accomplished by diluting the feed with
For a2 > aI use inerts and running the reactor at low concentrations of A. A CSTR should be
a CSTR and used, because the concentrations of reactant are maintained at a low level. A
the feed stream
recycle reactor in which the product stream acts as a diluent could be used to
maintain the entering concentrations of A at a low value.
Because the activation energies of the two reactions in cases 1 and 2 are
not kriown, it cannot be determined whether the reaction should be run at high
or low temperatures. The sensitivity of the rate selectivity parameter to temper-
ature can be determined from the ratio of the specific reaction rates,

where A is the frequency factor, E the activation energy, and the subscripts D
and U refer to desired and undesired product, respectively.

Case 3: ED > E, In this case the specific reaction rate of the desired reac-
tion kD (and therefore the overall rate rD) increases more rapidly with increas-
ing temperature than does the specific rate of the undesired reaction k,.
Consequently, the reaction system should be operated at the highest possible
temperature to maximize S,, .

Case 4: E, > ED In this case the reaction should be carried out at a low tem-
perature to maximize S,,, but not so low that the desired reaction does not
proceed to any significant extent.

Example 61 Minimizing Unwanted Products for a Single Reactant

Reactant A decomposes by three simultaneous reactions to form three products: one
that is desired, D, and two that are undesired, Q and U. These gas-phase reactions,
together with their corresponding rate laws, are: