Geoscience Reference
In-Depth Information
effective detoxification and disposal of problematic industrial, nuclear, military,
and municipal wastes. In supercritical water, at 450
700
C, many organic com-
pounds are rapidly (0.1
%) oxidized (super-
critical water oxidation or SCWO), with their carbon, hydrogen, and nitrogen,
almost completely converted to CO
2
,H
2
O (mineralization), and N
2
. These attri-
butes make supercritical water an attractive medium for chemical reactions and
physical separations, i.e., for hydrothermal processing. Environmental applications
include the rapid and efficient destruction of hazardous organic substances, e.g.,
aqueous wastes, and decontamination and/or separation of inorganic pollutants
[405
100 s) and efficiently (99.9
99.99
1
408]
. Thus, SCWO is an emerging technology for the treatment of aqueous
waste streams, so that they can be recycled as process streams and for the ultimate
destruction of organic wastes. Recycling of waste plastics, such as polyethylene,
polystyrene, polypropylene and polyethylene terephthalate, radioactive waste, and
concrete wastes, have received special attention
[409]
. Similarly, the decomposition
of chlorocarbons, chlorofluorocarbons, polymers, polymer additives, nitroaro-
matics, and so on, under HPHT conditions, have been well understood
[409,410]
.
The conventional method of treating most of these waste materials is oxidation
pyrolysis in incinerators, but this method is not effective as to the dechlorination of
chlorofluorocarbons, for example, they need high-temperature plasma destruction,
requiring large and expensive apparatus, and the reactors are easily corroded by
HCl or Cl
2
gas (the decomposed productions). Similarly, reductive decomposition
is not a cost-effective method, requiring expensive reductive agents.
Savage and coworkers have worked out the reaction models for SCWO pro-
cesses in detail, based on molecular dynamics studies of supercritical water, in
order to understand better the potential roles of water in influencing elementary
chemical reaction rates
[411]
. The study of hydrogen bonding in supercritical water
and its dependence on temperature and density has been carried out by many work-
ers to understand fundamental issues connected with structure dynamics and ther-
modynamics of pure water
[411
414]
.
Hydrothermal decomposition of organics or recycling of waste materials is usu-
ally carried out in small autoclaves, or Turtle cold-cone seal autoclaves or batch
reactors/flow reactors, depending on the experimental conditions and purpose.
Adschiri et al.
[415]
have studied the conversion of lignin, polystyrene, and poly-
ethylene under supercritical conditions of water and found that polystyrene could be
completely decomposed into ethylbenzene, toluene, benzene, styrene, and xylene in
5 min. However, the conversion yield of polyethylene was fairly low, even at a lon-
ger reaction time of 2 h at 35 MPa, 400
C but, by the addition of oxygen (about
0.013 mol), conversion increased to 60% at the same temperature. The advantages
of such conversion are that lesser char and more aldehyde, ketone, and acid produc-
tion in supercritical water are observed, as compared to neat pyrolysis reaction.
The above works clearly indicate that a new trend is being set in hydrothermal
technology. This technology is going to be the one used in materials processing in
the twenty-first century, and it is not only human friendly and environmentally safe
but also economical cost wise. Already, several groups have cropped up all over
the world to tackle the existing problems and search for new avenues in materials
