Environmental Engineering Reference
In-Depth Information
Anaerobic processes
. Performing a process under anaerobic conditions has the
potential advantage of reducing the investment and operating costs by increasing
the yield of product on substrate and eliminating the input of (compressed) air.
Additionally, the produced fermentation gases can be used for providing
sufficient mixing in the reactor. However, anaerobic product formation is not
inherent to the routes for diesel-like products described in Section 13.5, so
metabolic engineering will probably be required. Research is currently being
conducted in this area (Weusthuis et al., 2011).
High cell densities and cell reuse
. Since the substrate is one of the major cost
drivers in industrial fermentation, it should be directed optimally to product
formation. This can be achieved by separate cell growth and product formation
regimes and by extracellular production. In this way, high cell densities can be
obtained first (in the cell growth regime) and maintained by cell retention or
recycle (in the product formation regime). This approach has been proven
successful in ethanol production. Microorganism robustness is an important
element in this approach.
Simple product recovery
. One advantage that diesel-like molecules have over
short-chain alcohols such as ethanol and butanol is their very low solubility in
water, which could result in simple product recovery by, e.g., gravity separation.
The cells and other medium components, however, can act as surfactants,
hindering the coalescence of the product droplets and forming stable emulsions.
These emulsions may require intensive centrifugation and/or de-emulsification
techniques such as the use of additives and pH and temperature shifts. These will
obviously increase the investment and operating costs of the process. Research
is currently being performed on coalescence improvement by equipment and
medium design.
Tailored products
. As mentioned earlier, products that do not require further
conversion steps and that can use the existing infrastructure, i.e., drop-in biofuels
such as long-chain alkanes, are preferred.
CHAPTER SUMMARY AND STUDY GUIDE
This chapter focuses on the production of liquid biofuels by means of industrial
fermentation. The most successful example, ethanol, has been presented in detail.
First-generation ethanol processes have reached worldwide implementation mostly
based on feedstocks such as sugarcane and corn. Second-generation processes,
which consume nonfood sugar sources such as lignocellulosic material, require a
series of chemical, physical, and enzymatic steps for obtaining fermentable sugars.
An overview of the current developments in this area is provided. Next, developments
in other liquid biofuels such as butanol and diesel-like compound are also described.
Finally, a comparison of biofuel routes by industrial fermentation is provided on the
basis of stoichiometry and thermodynamic data.
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