Environmental Engineering Reference
In-Depth Information
The biofuel is a gas, e.g., methane. This gas is present in the off-gas of the
fermentor mixed with the produced CO
2
. Theoretically, the energy to demix
these gases is the negative of the mixing energy, but in order to achieve practical
rates, the operation cannot be done close to equilibrium, and at least ten times the
mixing energy is required.
The biofuel is a water-miscible liquid such as ethanol. This liquid is dissolved in
the fermentation broth. A large amount of energy may be required for distillation.
The biofuel is a water-immiscible liquid such as pentadecane. If no stable emul-
sion is formed, this liquid can be recovered without much energy requirement.
The biofuel is a solid. Processing of solids is significantly more complicated than
processing of fluids. Their use as biofuel is not advocated.
The outcome of this analysis is that water-immiscible liquids are the most attractive
ones considering product recovery. Diesel-like products are good examples, since the
Gibbs energies of reaction for the formation of these products fromglucose are favorable
and, althoughyields are low, the energydensityof these compounds ishigh. This explains
the current attempts to produce diesel-like products by fermentation (see Section 13.5).
13.7 OUTLOOK
The (bio)fuel market is characterized by tight economic margins that require efficient,
low-cost processes. The routes described in this chapter concern pure cultures that
make use of conventional fermentation equipment and require sterilization of the
nutrients and inflow gases. A successful process would have, most likely, the follow-
ing characteristics:
Use of alternative feedstocks
. The same developments that are being made in the
production of lignocellulosic ethanol (see Section 13.3) can be applied in the
microbial production of other biofuels.
Sepsis and microorganism robustness
. Most pure cultures require sterilization of
the equipment, nutrients, and inflow gases in order to avoid contaminations. In
the ethanol production process, contaminations are naturally controlled since
most microorganisms cannot tolerate the ethanol levels in the fermentation
medium. As a result, low-cost (open) fermentors including cell recycle loops
can be used. In the production of diesel and jet fuel type of molecules, however,
suppression of growth of competing microorganisms by the product itself is not
expected. Hence, inherently robust microorganisms able to tolerate unfavorable
conditions for competing microorganisms are being used. Examples include
S. cerevisiae
, which tolerates low pH, and cyanobacteria, which tolerate high salt
concentrations. Another approach that is receiving much attention is the use of
mixed culture techniques. The production of medium-chain fatty acids has
already been accomplished (Steinbusch et al., 2011).
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