Page 698 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Oxidation 653

The products leave the reactor losing heat to the first heat material that may withstand corrosive conditions (Gloyna
exchanger and then to a second that is a part of the and Li, 1995, p. 189). No known material will handle all of
steam generation subsystem. Additional heat is lost by means these conditions simultaneously. A way around this was to
of an air-cooler. The solids and off-gas are separated as the construct an inner reaction vessel to contain the reaction and
last step in the reactor effluent processing. Oxygen is preferred an outer vessel to handle the pressure (the stress within the
over air because the reactor volume may be less since nitrogen walls of the inner vessel may be zero if the pressure is the
takes up volume and has no role in the reaction (Sawicki and same between the two sides). The inner vessel is also insu-
Casas, 1993). lated from the outer vessel so that the temperature of the outer
vessel may be maintained at lower temperature. A patent for
20.2.3.5 Research in the 1990s the reactor was pending (in 1995) by Kimberly-Clark, Inc.
Studies by Li et al. (1993) involving dinitrotoluene (DNT)
20.2.3.7 Case Study: SCWO of Pulp and Paper
wastewaters, also containing 2-nitrophenol, 4-nitrophenol,
Mill Sludge
4,6-dinitro-o-cresol, phenol, and DNT, found 99% TOC reduc-
Blaney et al. (1995) described SCWO a pilot plant study of
tion at u reactor) 1 min, T(reactor) 4508C, p(reactor) 306
pulp and paper mill sludge that utilized the UT pilot facility
atm (310 bar or 31,000 kPa). Both hydrogen peroxide and
with Q ¼ 150 L=h (40 gal=h) capacity. About 64 million
oxygen were added to the reactor in trials. Lower temperatures,
metric tons=year (70 million U.S. tons) of such sludge is
that is, 2008C –3008C, were effective for H 2 O 2 , while oxygen
generated worldwide. The sludge contains cellulosic fibrous
produced better results at 2008C–5008C(T c ¼ 3748C). Bio-
fines and debris, inorganics such as fillers and clay, and small
logical sludges could provide the extra heat needed by the
amounts of de-inking chemicals. Chlorinated organics present
process. At p(reactor) > p c ¼ 218 atm, the higher pressures
had no effect on removal efficiency (pressures were varied,
138 < p(reactor) < 306 atm). Removal efficiencies for munici-
pal sludges were about 99.4% at T(reactor) ¼ 4508C and BOX 20.2 SCWO: DEVELOPMENT
p(reactor) ¼ 27,600 kPa, which was for TOC(in) 14,200 OF A TECHNOLOGY
and TOC(out) ¼ 84 mg=L (Gloyna and Li, 1995, p. 183).
Professor Michael Modell, a chemical engineering pro-
Industrial wastewater TOC concentrations were reduced from
fessor at the Massachusetts Institute of Technology
about 1840 to about 4 mg=Lat T(reactor) ¼ 5008C.
(MIT), began an odyssey of some 30 years in the field
of SWCO; the odyssey began in 1975 when he patented a
20.2.3.6 Design Factors process for reforming organics in SCW. In 1980, he
Design considerations for SCWO include (Gloyna and Li, extended the work to the destruction of toxic and haz-
1995, p. 2) ardous wastes and formed MODAR, Inc. to develop and
commercialize the process, resigning his tenured aca-
. Reactor detention time demic appointment in 1982. The development activities
. Reactor pressure and temperature at MODAR included proof of technical feasibility, kin-
. Materials of construction for each unit operation etics, process design, destruction efficiency, economic
. Control and removal of solids either from the SCF or evaluation, scale-up guidelines, inorganic precipitation,
the treated effluent corrosion studies, and demonstration tests. In 1990,
. Operation and maintenance of the facility MODAR was sold to General Atomics, which applied
. Design for safety (facilities, working conditions, the reactor technology to destruction of chemicals from
regulatory compliance) weapons.
. Provision for analytical support and regulatory mon- In 1987, Dr. Modell formed another company,
itoring MODEC, Inc., to develop a new reactor design, that
. Provision for disposal of residuals is, a tubular reactor, to overcome the deposition of salts,
to mitigate corrosion, and to collect the carbon dioxide
To give some idea of dimensions, the first commercial reactor product in liquefied form. This reactor was the basis
at Austin, TX, built in 1995, was 4.0 m tall and 2.0 m for further work by Gloyna et al. at University of Texas
diameter, with Q(reactor) ¼ 25 L=min. The waste flow con- (UT). Among Dr. Modell’s many publications is a text,
tained about 10% petrochemical organic compounds with Thermodynamics and Its Applications, first edition,
fraction removed > 0.995. Previously, the waste was trans- M. Modell and R.C. Reid, Prentice Hall, 1974, with a
ported to a hazardous wastes incinerator at a cost of $0.30– second edition in 1982, and a third in 1997 Tester and
$0.40 per liter, with total cost $1 million=year. Modell (1997), the latter by J. Tester and M. Modell.
Some of the limiting factors in reactor design have to do Among his 12 patents, 5 have been in the field of
with the containment of high pressure and high temperature in SCWO. Applications of the SCWO technology have
the context of reactions that yield corrosive products. Thus, included work for the National Aeronautics and
the reactor construction requires a cylinder designed for the
stress caused by the high-pressure high-temperature, and (continued )