Environmental Engineering Reference
In-Depth Information
In the designed process, three large mixed continuous reactors are used in series.
The first reactor consumes 60% of the sugar, and two smaller reactors each consume
20%. The intention of this configuration is a certain extent of plug flow, exposing
yeast in each next reactor to a higher concentration of ethanol. Higher concentrations
of ethanol inhibit the yeast, thus slowing down all reactions. This actual reaction
course is not covered by our simplistic model that assumes constant reaction rates,
but it leads to a reasonable approximation of the reactor sizes.
To distribute failure risk, two parallel cascades of 2265 m
3
each lead to the required
fermentation volume of 4529 m
3
. Consequently, the liquid volume of the fermentors
per section is 1359, 453, and 453 m
3
, respectively. Each fermentor is filled with liquid
for only 90% (ao, because CO
2
bubbles are liberated), and therefore, the required
vessels are even larger.
The residence time of liquid in, e.g., the 3rd fermentor is
Volume
Volume flow
=
Volume
Mass flow
τ
=
=
density
453 m
3
=
7000 s
2h
m
3
≈
≈
130 kg
:
s
−1
=
2 cascades
=
1000 kg
=
The rising CO
2
bubbles can lead to a reasonable extent of mixing, but some
additional stirring is used. For this, the exact geometry is important.
As compared to the complete microbial oxidation of sugars to CO
2
and H
2
O, the
formation of ethanol from sugars does not liberate much energy. However, heat
production should not be neglected at this large scale. To maintain the fermentation
at the desired 34
C, aqueous streams between the fermentors are cooled with cooling
water using heat exchangers.
Since the aeration requirement is very small (a detail of the growth reaction), stirring
is modest, and no cooling coils are required in the vessels, the required fermentors will
be much less expensive than for aerobic (O
2
-consuming) fermentations.
In general, cylindrical stainless steel vessels are used, with a height-to-diameter
ratio of about 3:1.
13.2.7.2 Distillation
Among all equipment, distillation is the largest consumer of
energy sources. The evaporation of ethanol and in particular water requires much
energy, much more than for heating the liquid to its boiling point (see Table 13.5).
This energy is to a large extent lost to the environment.
TABLE 13.5 Enthalpy of evaporation (
h
fg
), specific heat capacity, and enthalpy
for heating from 30 to 80
C the boiling point of ethanol (
Δ
H
heating
=c
p
Δ
T)
for pure ethanol and water
Δ
H
heating
(kJ.mol
-1
)
Δ
H
heating
(kJ.kg
-1
)
h
fg
(kJ.mol
-1
)
h
fg
(kJ.kg
-1
)
c
p
(kJ.kg
-1
K
-1
)
Ethanol
38.6
838
2.4
2.6
120
Water
40.6
2250
4.2
3.8
210
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