Page 229 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 4..8 Recycle Reactors 201

certann selectivity (see Section 5.4.4). They are also used extensively in bio-
chemical operations. To design recycle reactors, one simply follows the proce-
dure developed in this chapter and then adds a little additional bookkeeping. A
schernatic diagram of the recycle reactor is shown in Figure 4-15.

-
Fat, 1 FB2
"01, etc. FB3
'1 Fc3
I etc' -_
FD3
L v, , FAR, FBR, FCR, etc. I
Ft:?
Figure 4-15 Recycle reactor.

The recycled stream is drawn off at point Q and merged with the fresh
feed at point P. We shall define the recycle parameter R as the moles recycled
per mole of product removed at point Q:

'Two conversions are usually associated with recycle reactors: the overall
TWO converwms conversion, X,,, and the conversion per pass, X,:
X, and X,,
moles of A reacted in a single pass
xs = (4- 88)
mole of A fed to the reactor

moles of A reacted overall
x, = mole of fresh feed (4-89)

The oiily new twist in calculating reactor volumes or conversions for a recycle
reactor is a mole balance at the stream intersections (points P and Q) to
express properly the species concentrations as a function of conversion.
As shown in the CD-ROM, along with the overall conversion and the
conversion per pass are related by

x,,
x, = (4-90)
1 +R(1 -XJ
The PFR design equation for a recyrle reactor is also developed in the
CD-ROM.