Biomedical Engineering Reference
In-Depth Information
quality control: Each batch can be analyzed and certified (or discarded), while contamination
in a continuous process will invariably lead to a lot of worthless product before certifiable
purity is restored. Food and beverages are still made in batch processes in many situations
because biological reactions are never exactly reproducible, and a batch process is easier to
“tune” slightly to optimize each batch. Besides, it is more romantic to produce beer by
“beechwood aging,” wine by stamping on grapes with bare feet, steaks by charcoal grilling,
and similar batch processes.
We will develop mass and energy balances in flow reactors. In one limit, the reactor is
stirred sufficiently to mix the fluid completely (continuous stirred-tank reactor [CSTR] or che-
mostat), and in the other limit, the fluid is completely unmixed (plug flow reactor [PFR]).
In any other situation, the fluid is partially mixed and one cannot specify the composition
without a detailed description of the fluid mechanics. However, we will assume the reactors
to be either completely mixed or completely unmixed in this chapter. As to any flow equip-
ment, there is a need of mechanical force to push the fluid mixture through. Thus, pressure
drop or mechanic energy balance is also needed in addition to the mass and energy balances.
5.1. FLOW RATE, RESIDENCE TIME, SPACE TIME,
SPACE VELOCITY, DILUTION RATE
Figure 5.1 shows a schematic of a flow reactor with an inlet and an outlet. In the inlet, we
feed in a reaction mixture containing reactants and solvents, with a concentration of C j 0 for
species j . The temperature and pressure of the inlet stream are measured at T 0 and P 0 . The
volumetric flow rate is Q 0 . When the reaction mixture passing through the flow reactor of
total effective volume V , chemical or biological reaction(s) occur and the flows are controlled.
It outflows the reaction mixture containing unreacted reactants, products, and solvents. For
any given species j , its concentration is a concentration of C je . The temperature and pressure
of the outlet stream are measured at T e and P e , and the volumetric flow rate is Q e .
The reactor residence time is defined as the average time any fluid particle spent in the
reactor, from inlet to outlet. As such, it can be shown that
d V
Q
d t ¼
(5.1)
Where t is the residence time. Since the volumetric flow rate can vary inside the reactor, the
residence time calculation is not straightforward. For constant-volumetric flow reactors,
Eqn (5.1) is reduced to
Z
V
d V
Q ¼
V
Q e ¼
V
Q 0
t ¼
(5.2)
0
F j 0 = Q 0 C j 0
V
Q e ,
T e ,
P e
Q 0 , T 0 , P 0
Flow Reactor
F j e =
Q e C j e
FIGURE 5.1 Schematic diagram of a flow reactor showing the inlet and outlet.
Search WWH ::




Custom Search