Environmental Engineering Reference
In-Depth Information
HO
H
H
O
Fermentation
HO
+ 2CO
2
2
OH
H
HO
H
OH
Ethanol
C
2
H
6
O
MW = 46 g/mol
OH
H
Glucose
C
6
H
12
O
6
MW = 180 g/mol
Atom eficiency: (2 × 46/180) × 100% = 51%
FIGURE 18.1
Atom efficiency of ethanol production.
production is given. The production of every molecule of ethanol is accompanied by
the production of one molecule of CO
2
. The total atom efficiency of the process
(defined as the weight of the atoms in the product divided by weight of the atoms
in the feed) is, therefore, only 51%. The atom efficiency is one of the twelve principles
of
green chemistry
(see Box 18.1) (Anastas and Warner, 2000). In general, high atom
efficiencies are inherent to a sustainable process. Of course, the atom efficiency is only
one aspect. A more practical number is the so-called E-factor (see Example 18.1),
which takes into account all waste produced by the plant (Sheldon, 2007). The
E-factor can range from
0.1 in oil refineries to >100 in the production of
pharmaceuticals.
Biodiesel is the second most abundant renewable liquid fuel. The production
capacity in Europe on July 1, 2012 was around 23 million tonnes (tinyurl.com/
mjhnewl). As can be seen from Table 18.1, it has a favorable carbon chain length,
thus providing an excellent basis for a high-performance biofuel. The starting vege-
table oils from which FAME is produced (see Figure 18.2) are too viscous and usually
have a too high free fatty acid content to be used directly. The first diesel engine ran on
peanut oil (Diesel, 1895). FAME is a product of transesterification. Transesterification
lowers the molecular weight and viscosity by converting triglyceryl esters of fatty
acids into their methyl esters and glycerol (see Figure 18.2). This technology is rather
straightforward: simply boiling a bottle of oil with methanol and some base or acid
(as catalyst) can produce biodiesel.
The evaluation of the production of biodiesel with respect to the 12 principles of
green chemistry (see Box 18.1) is not so easy. The production processes of biodiesel
have to be stringently judged. For instance, in case palm oil-based facilities produce
large amounts of residual biomass (waste biomass of lignocellulosic nature), one can
argue that the E-factor (see Example 18.1) for these types of plants is unacceptably
high (principles 1 and 2). Therefore, this process does not comply with these 12 prin-
ciples. Furthermore,
the use of catalysts is not common practice in biodiesel
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