Biomedical Engineering Reference
In-Depth Information
1
-r A
CSTR
PFR
0
C Ae
C A0
0
C A
FIGURE 5.9 Schematic diagram of the different reactor volume requirement for a CSTR and a PFR. The volume
of CSTR required is proportional to the whole rectangular area, whereas the volume of a PFR required is
proportional to the area underneath the curve only.
Substituting in the volumetric flow rate, rate constant, and conversion, we obtain
s
0:02 s 1
10 L
=
0:5
1 0:5 ¼ 500 L
V ¼
Compared with the 346.57 L required for a PFR at the same reaction conditions, CSTR is
much bigger.
Example 5-3 shows that the required reactor volume for a CSTR is bigger than a PFR
(Example 5-1) for the same reaction conditions and reactor throughput. The difference is
due to the level of concentration inside the reactor. For a PFR, the concentration changes
from inlet to outlet with reactant concentration being highest at the inlet, thus faster reaction
near inlet when the reactor is run under isothermal conditions. However, for a CSTR, the
concentrations are uniform in the reactor and are identical to those in the outlet. Therefore,
the reactant concentration is at lowest in the CSTR. This is shown in Fig. 5.9 . The CSTR reactor
volume required is proportional to the area of the rectangle, whereas the required reactor
volume for a PFR to achieve the same conversion is proportional to the area under the curve.
Comparing examples E5-1 and E5-3, we observe that the solution to a CSTR is easier to
achieve than to a PFR. For a CSTR problem, it reduces to a (set of) algebraic equation(s).
For simple kinetics, this solution is shown in Table 5.4 .
The solution to the molar balance equation for a CSTR at steady state, Eqn (5.40) , can be
visually illustrated by rearranging Eqn (5.40) to give
F A 0 F A
V ¼r A
(5.53)
which is the molar change rate of A in a CSTR per reactor volume. One can observe that the
left-hand side is the molar rate of A fed subtracted the molar rate of A letting out of the
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