Biomedical Engineering Reference
In-Depth Information
reactor is also very high. This is not desirable operating strategy in practice as the conversion
is low. When the feed concentration is low, the conversion is high due to the very low concen-
tration of reactant in the effluent stream. Therefore, it is desirable to operate the reactor on the
lower branch of the curve. Fig. 16.7 shows similar phenomena when the feed parameter dilu-
tion rate D is altered. Therefore, our discussion on increasing feed concentration is applicable
to the increasing dilution rate (or feed rate).
To further our discussions on the effect of operating parameters on the steady-state oper-
ations of a CSTR, we focus on the change of effluent concentration C A with the dilution rate
(or feed rate). Fig. 16.8 is constructed directly from Fig. 16.7 . If we start with a very low dilu-
tion rate (or very small feed rate), i.e. D
D IG , the CSTR will be operated on the lower branch
(with high conversions). As we increase the dilution slowly, the effluent concentration
increases barely and thus the reactor capacity is increased. If the dilution rate increases to
D EX , any further increase in the dilution rate will cause the effluent concentration to increase
sharply to be nearly equal to the feed concentration. There is no gradual change in the
effluent concentration. Therefore, we call this point (corresponding to bifurcation point 2)
the extinction point. With the extinction point, we mean the reaction is slowed down to
a near stop. The steady-state operation is jumped to the upper steady-state branch.
If on the other handwe were operating the CSTRwith a very high dilution rate (or feed rate),
i.e. D
<
D EX , the reactant effluent concentration will be high (just slightly smaller than the feed
concentration). This indicates that the reactor capacity is lowand decrease the feed rate (or dilu-
tion rate) is desired. Decreasing the dilution rate slowly, we do not observe any significant
changes until D ΒΌ D IG . Further decrease the dilution rate to pass D IG , we observe as sudden
drop in the reactant effluent concentration. The CSTR steady operation jumped down to the
lower branch fromthe upper branch. The reactor operation is favorable as high capacity is expe-
rienced. This shift point correspond to the bifurcation point 1, which is different from the bifur-
cation point 2 for increasing dilution rate. We call this bifurcation point the ignition point,
meaning the reactor is started if we decrease the dilution rate from this point downward.
>
C A0
-r A
C A0 -
D =
C A
Bifurcation point 1
Bifurcation point 2
0
0
D IG
D EX
D
FIGURE 16.8 Hysteresis of steady-state operating effluent concentration C A as the dilution rate D (or feed rate)
is changed for a fixed feed concentration C A0 .
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