Environmental Engineering Reference
In-Depth Information
TABLE 13.3 Calculated material balance of a bioethanol factory
Feed stream
kg.s
-1
kg.s
-1
Exit stream
Sugarcane
139.96018
Ethanol
9.95870
Imbibition water
38.39679
Fermentation: CO
2
+H
2
O vapor
10.07730
Wash water
363.17538
Used wash water
364.52082
Air
204.39191
Exhaust gas
247.87209
Lime
0.12559
Ash
0.66098
Flocculant
0.04616
Filter cake
5.69991
Dilute acid
25.92310
Vinasse
125.83721
Fermentation additives
a
0.24126
Fusel oil
0.61930
Dilution water
a
Yeast bleed
a
24.12937
0.55403
Dilution water
a
Condensate
a
2.26117
34.25009
Total in
798.6509
Total out
800.0504
a
For simplicity, these streams are not indicated in Figures 13.1-13.6. The condensate is composed of
water and can be recycled as wash water. The yeast bleed prevents accumulation of dead yeast, and in a
steady-state process, this bleed equals the growth of the yeast.
50 km might easily become uneconomical. Then, the available cane within, e.g.,
50 km determines the maximum plant size. Of course, the produced ethanol needs
to be transported, but storage and transport in tanks are easy, and ethanol occupies
only 2% of the volume of the sugarcane. The shipping costs, from Brazil to Europe,
are acceptable. The factories run during the harvest season, which may be half a year.
13.2.4 Material Balances of the Process
In 2005, in collaboration with Brazilian experts, a sugarcane process has been quanti-
fied with respect to size and composition of all process streams. That was a conceptual
process design and assumed to be representative for a modern plant. Table 13.3 shows
its overall material balance. In the calculated flow sizes, the number of significant digits
shown is much larger than the accuracy of measurements of flows would have been, if
these were available. However, keeping so many digits in the calculations facilitates
checks for consistency with subprocess mass balances such as shown later.
Most striking are the huge quantities of water needed for washing the sugarcane and
air for the incineration of the bagasse, also resulting in a large amount of exhaust gas.
Another observation is that only 1 kg ethanol is produced from 14 kg sugarcane;
the other 13 kg ends up as side products, mainly water, as the water content of the cane
is ~65 wt%.
13.2.5 Material Streams in the Fermentation
The fermentative conversion is the heart of the process. It occurs at 34
C and
atmospheric pressure. Table 13.4 gives the material streams for the fermentation,
including their composition.
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