Page 79 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 2.4 Reactors in Series 51

To achieve the Note again that for CSTRs in series the rate - rAl is evaluated at a conversion
same overall of 0.4 and rate -rAz is evaluated at a conversion of 0.8. The total volume is
conversion, the
total volume for V = VI + V2 = 364 dm3 (liters)
two CSTRs in
series is less than
that required for The volume necessary to achieve 80% conversion in one CSTR is
one CSTR
V= FAo(+)X = (0.867)(800)(0.8) = 555 dm3 (liters)

Notice in Example 2-5 that the sum of the two CSTR reactor volumes
(364 L) in series is less than the volume of one CSTR (555 L) to achieve the
same conversion. This case does not hold true for two plug-flow reactors con-
nected in series as shown in Figure 2-4. We can see from Figure 2-5 and from
the equation
I, -r, = I; ,+I,, '' dX
'' dX
dX
-r,
'I
that it is immaterial whether you place two plug-flow reactors in series or have
one continuous plug-flow reactor; the total reactor volume required to achieve
the same conversion is identical.

Xi = 0.4
FAo--
X, = 0.8
Figure 2-4 Two PFRs in series.

' 800 O 0 O I I
Makes no
difference if YOU
have two PmZs in
series or have one
PFR of the same
total volume.

Conversion, X
Figure 2-5 Levenspiel plot for two PFRs in series,