
165Homogeneous Reactors
that is, 0
2
qC Vk C
Cf Bf
Cf Bf
2
(3.72)
Overall selectivity of B (desired product) Φ
B
is
Φ
B
Bf
Cf
C
=
=
1
2
τ
(3.73)
As space time τ increases, selectivity
B
decreases and conversion of A
C
Af Af AO
1(
/
increases
x
Af
=
1
(3.74)
Combining Equations 3.73 and 3.74, we can write overall selectivity
B
in terms of total
conversion x
Af
as
Φ
B
Af
=
1
(3.75)
Thus, selectivity
decreases with an increase in conversion x
Af
.
For the series reaction
carried out in a PFR, the equation for overall
selectivity derived for the batch reactor (Section 2.1.8) can be used for PFR after replacing