Page 116 - The engineering of chemical reactions

100 Single Reactions in Continuous Isothermal Reactors

Figure 3-5 Residence times in CSTR (shaded rectangle)
and PFTR (area under curve) from the l/r plot.
10
c-
= ‘CSTR

= ‘PFTR-

1.
TO
0 CACI
CAo-CA

From these graphs of l/r versus CAM - CA we can construct the residence times in
PFfR and CSTR, as shown in Figure 3-5.
In the CSTR the rectangle is drawn with a height equal to l/r evaluatedat theproduct
conversion, while in the PFTR the height varies from inlet to product conversion. The CSTR
rectangle obviously has a larger area as long as 1 /v is a monotonically increasing function of
CAM - CA (Y monotonically decreasing), while areas are equal for n = 0, and the rectangle
has a smaller area for n < 0.
This construction also shows why the CSTR becomes much less efficient (requires
much larger volume) at high conversions. The l/r curve increases rapidly to cc as CA + 0,
and therefore z in a CSTR becomes very large compared to rnmn.
Thus it is evident that a PFTR is always the reactor of choice (smaller V) for greater
than zero-order kinetics in an isothermal reactor. The CSTR may still be favored for n > 0
for cost reasons as long as the conversion is not too high, but the isothermal PFTR is much
superior at high conversions whenever n > 0.
The question of choosing a PFTR or a CSTR will occur throughout this book. From
the preceding arguments it is clear that the PFTR usually requires a smaller reactor volume
for a given conversion, but even here the CSTR may be preferred because it may have lower
material cost (pipe is more expensive than a pot). We will later see other situations where a
CSTR is clearly preferred, for example, in some situations to maximize reaction selectivity,
in most nonisothermal reactors, and in polymerization processes where plugging a tube
with overpolymerized solid polymer could be disastrous.

SEMIBATCH REACTORS
Semibatch reactors are commonly used for small-volume chemical production. This reactor
type is frequently used for biological reactions and for polymerization. In the batch reactor,