Biomedical Engineering Reference
In-Depth Information
The residence time varies with temperature and pressure, besides the feed flow rate. There-
fore, the residence time is not an easy parameter to use when designing and/or operating
a reactor.
To simplify the calculation of a time scale, a space time
is defined. The space time is
the time needed to feed a reactor full of reaction mixture through the reactor. It is thus
defined as the total volume of the reactor divided by the inlet volumetric flow rate.
That is,
s
V
Q
0
s
¼
(5.3)
The space time is especially useful for liquid reactors. Since the density of liquids is relatively
constant, we can assume that the flow rate remains nearly constant. In this case, the space
time is nearly the same as the residence time.
Figure 5.2
shows the space time scale for a variety of important industrial reactions. One
can observe that the required space time is nearly inversely exponentially related to the reac-
tion temperature. Higher temperature requires significantly shorter reaction time.
Alternatively, one can define the frequency of reaction mixture being treated based on the
reactor. This is commonly called space velocity. In bioreactions, the biocatalysts are kept in
10
6
Fermentation
10
5
10
4
Pulping
Hot-Water Extraction
10
3
10
2
Hydrogenation
Fischer-Tropsch
CH
3
OH
10
1
EO
FCC
Maleic anhydride
Steam Reforming
10
0
C
4
dehydrogenation
Automotive converter
CH
4
coupling
10
-1
HCHO synthesis
10
-2
NH
3
oxidation
10
-3
0
200
400
600
800
1000
1200
T,
°C
FIGURE 5.2
Nominal space time required for several important industrial reactors vs the nominal reactor
operating temperatures. Time scale goes from days (fermentation or bioreactions) down to milliseconds (for
ammonia oxidation to form nitric acid). The low-temperature (
T
200
C) long-time (
10 min) processes involve
liquids and/or solids as substrates, while the high-temperature short-time processes involve gases, usually at high
pressures.
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