Environmental Engineering Reference
In-Depth Information
are potentially achievable in the final fermentation broth. Preliminary experimental
data from fed-batch solid-state fermentation (SSF) confirm this assumption [20].
3.2.2.3
Supercritical Water Treatment
Water near or above its critical point (374°C, 218 atm) is attracting increased atten-
tion as a medium for chemical processes. Most of this new attention is driven by
the search for more 'green' or environmentally benign chemical processes. Water
near its critical point possesses very different properties from those of ambient
liquid water. The dielectric constant of sub- and supercritical water (SCW) is much
lower, and the number and persistence of hydrogen bonds are both diminished. As
a result, high-temperature water behaves like many organic solvents in that organic
compounds enjoy high solubilities in near-critical water and complete miscibility
with SCW. Gases are also miscible in SCW, so employing a SCW reaction environ-
ment that provides an opportunity to conduct chemistry in a single fluid phase that
would otherwise occur in a multiphase system under more conventional condi-
tions. The advantages of a single supercritical phase reaction medium is that a
higher concentration of reactants can often be attained and there is no interphase
mass transport processes which hinder reaction rates [21].
Hydrothermal processing of biomass offers a number of potential advantages over
other biomass processing methods, including high throughputs, high energy and
separation efficiency, the ability to use mixed feedstocks such as wastes and ligno-
cellulose, the production of direct replacements for existing fuels, and no need to
maintain specialized microbial culture or enzymes. In addition, because of the high
temperature involved, biofuels produced would be free of biologically active micro-
organisms or compounds including bacteria, viruses, and even prion proteins [22].
Applications of water as a process media of biomass range from subcritical water
extraction to supercritical water oxidation (SCWO). SCWO is a rapidly developing
technology for the destruction of organic wastes. In 1994, the world's first commer-
cial SCWO facility for treating industrial waste water became operational. Ongoing
catalytic SCWO studies have demonstrated the benefit of utilizing heterogeneous
catalysts for reducing energy and processing costs. The low dielectric constant and
low polarity of SCW are closer to the properties of non-polar solvents; SCW is
therefore capable of dissolving the majority of organic compounds and gases. This
characteristic provides an advantage over traditional biomass wet oxidation pro-
cesses, where oxidation rates are most likely mass-transfer limited due to the low
solubility of oxygen and organic compounds in the liquid phase of water [23].
One of the biggest disadvantages of high-temperature water treatment is corro-
sion, which is a problem for all subcritical and supercritical water systems. Special
materials for vessel linings and tubing are needed to resist the highly reactive
chemical species generated during the process. These challenges demand superior
engineering and design expertise for all system components. The water and pro-
cess streams must both be pumped to high initial pressures under exact flow and
