Environmental Engineering Reference
In-Depth Information
TABLE 13.12 Main economic figures for ethanol production at a location
in Brazil (in 2005 US$)
First generation
Second generation
Total capital investment ($)
101,907,323
132,597,219
Total operating costs ($ per year)
76,464,948
106,393,893
Revenues ($ per year)
145,462,674
229,886,830
Production cost of ethanol ($.t
-1
)
221
168
Production cost of sugar ($.t
-1
)
107
107
13.4 BUTANOL
Already in 1861, Pasteur observed 1-butanol production by anaerobic bacterial
fermentation. In the beginning of the twentieth century, the so-called acetone
-
butanol
ethanol (ABE) fermentation using a
Clostridium
species was developed.
The products are produced in a molar ratio of about 3:6:1. Potato starch was the first
carbohydrate feedstock used on an industrial scale. The process was the main source
of 1-butanol and acetone, which were used as solvents and for the production of
other chemicals.
In the early 1960s, petrochemical production methods led to the decline of the ABE
fermentation industry, although production continued much longer in South Africa,
the former Soviet Union, and China. Nowadays, there is a renewed interest in
ABE fermentation for reasons of sustainability and economic opportunities due to
increasing oil prices.
The main incentive for focusing on butanol rather than ethanol is that on a mass
basis butanol has a 31% higher combustion value than ethanol, which is very impor-
tant for an automotive fuel. Also, the lower volatility and higher hydrophobicity of
butanol allow blending with conventional gasoline in any proportion, whereas the
use of ethanol can lead to corrosion and may require adaptation of transportation lines
and engines.
ABE fermentation is performed by a large variety of
Clostridia
strains. When only
butanol is desired, the ideal stoichiometry starting from hexose sugars is
-
C
6
H
12
O
6
!
C
4
H
10
O + 2CO
2
+H
2
O
ð
RX
:
13
:
4
Þ
This overall reaction can be coupled to the formation of 2 ATP, thus enabling
cell growth and maintenance. Metabolic engineering of
Clostridia
and many other
microorganisms is used to adapt the original butanol fermentation in order to:
Maximize butanol yield on carbohydrate (by minimizing acetone and ethanol
formation and minimizing the need for cell disposal and regrowth)
Ferment lignocellulosic sugars into butanol
Increase the rate of butanol production by the used microorganism
Increase the tolerance of the used microorganism to butanol
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