Page 318 - Elements of Chemical Reaction Engineering 3rd Edition

290 MultiRle Reactions Chap. 6

(E6-2.3)

TO make SDU as large as possible, the reaction should be carried out at low concen-
trations of A and of B. Use:

A CSTR.
A tubular reactor in which there is a large recycle ratio.
A feed diluted with inerts.
Low pressure (if gas-phase).

I
Case ZV a1 < a,, p1 > pz . Let a = a2 - a1 and b = p1 - p2, where a and b are
positive constants. Using these definitions we can write Equation (E6-2.1) in the form

(E6-2.4)

To maximize S,, , run the reaction at high concentrations of B and low concentra-
tions of A. Use:

A semibatch reactor in which A is slowly fed to a large amount of B.
A tubular reactor with side streams of A.
A series of small CSTRs with fresh A fed to each rea%cx.

Another definition of selectivity used in the_ current literature, SDU, is
given in terms of the flow rates leaving the reactor. SDU is the overall selectivity.

FD - exit molar flow rate of desired product
Overall selectivity SDu = selectivity = - - (6- 12)

For a batch reactor, the overall selectivity is given in tems of the number
of moles of D and U at the end of the reaction the:

Two definitions for One also finds that the reaction yield, like the selectivity, has two defini-
selectivity and yield tions: one based on the ratio of reaction rates and one based on the ratio of
are found in the
molar flow rates. In the first case, the yield at a point can be defined as the