United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/005 May 1987
v>EPA Project Summary
Near Critical C02 Extraction of
Hazardous Organics from
Acrylonitrile, Pesticide and
Steel Mill Wastes
Paul N. Rice, William E. McGovern, and George S. Kingsley
Near critical carbon dioxide was used
to extract hazardous organic chemicals
from three aqueous waste streams in a
pilot plant scale continuous liquid-liquid
extraction system. Extractions were
performed on waste streams repre-
senting actual streams from steel,
pesticide, and acrylonitrile manufac-
turing plants.
An extractor plate efficiency was
determined based on the Kremser-
Souder-Brown equation using the
results of the acrylonitrile waste runs.
This efficiency was used to design a
commercial scale extractor. A con-
ceptual design for a commercial scale
process is also presented.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH, to
announce key findings of (fie research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The use of near-critical and supercritical
fluids as solvents for extraction has been
of interest for at least 20 years. Success-
ful applications have been developed
where the economic gains offset the costs
of high pressure operation. Two com-
ponents of that gain are:
1. Credits for by-product recovery.
2. Reduced processing costs.
A "critical fluid" is a fluid near its
thermodynamic critical temperature and
pressure. At the critical point there is no
distinct liquid or vapor state. The various
properties of the fluid in this state (e.g.,
density, viscosity, diffusivity) are inter-
mediate between the corresponding
values of the fluid in the gas and liquid
states. Further, these properties are not
fixed. They can be varied by manipulating
temperatures and/or pressures. Not only
are these physical properties variable
above the critical point, so also are the
solubilities of organic solutes. This can
be observed in Figure 1, a solubility map
of naphthalene in near- and supercritical
carbon dioxide. This diagram is based on
published solubility data in the near-
critical region taken with data on the
solubility of naphthalene in saturated
CO2. We see in this diagram fairly con-
ventional solute behavior in the region of
0 to 25 C. Solubility increases with in-
creasing temperature and, at a given
temperature, however, we observe
markedly different behavior. At pressures
near the critical pressure (7.5 MPa) the
solubility sharply decreases with increas-
ing temperature. The effect becomes less
pronounced as pressure is increased, but
it is clear that near the critical pressure
solubility is a strong function of tem-
perature. Thus, we see that operation in
the near critical and supercritical regime
allows the process designer to tailor the
solvent to the particular extraction under
consideration.
A number of fluids have been proposed
for use as extraction solvents. Some of
these fluids have been listed in Table 1
along with their critical properties. Carbon
dioxide is a prime candidate for organic
aqueous waste extraction and was chosen
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Temperature I°C)
10 20 30
40
50
10'
10-'
u
?>
•§
w3
i
i
30
Vapor
7MPa
I 6.5 MPa. 25°C
I Tie Line
Figure 1.
(a) NCL = Near Critical Liquid
(b) SCF = Super Critical Fluid
Solubility of naphthalene in carbon dioxide near its critical point.
1 atm = . 1013 MPa
for this project. Carbon dioxide is in-
expensive, relatively safe and can be used
at near ambient temperatures.
The scope of the wastes examined on
this project was extremely broad. Waste
streams from acrylonitrile, steel and
pesticide manufacturing plants were ex-
tracted with critical carbon dioxide. The
common feature to the three wastes ex-
tracted was the presence of a 70% by
weight or greater water fraction. Two of
the wastes had solids present in the feed
which added to the complexity of opera-
tion and data interpretation. In all three,
the focus of experimentation was to ef-
ficiently recover specific organics from
the bulk of water present. The full report
tells of the results of this project and
suggests solutions to the problems of
treating these hazardous wastes using
critical carbon dioxide as an extractio
solvent.
Objective
The primary objective of this pilot-plar
program was to demonstrate the technics
feasibility of the critical fluid extraction c
hazardous organic chemicals from aque
ous waste streams. This was achieved b
gathering engineering data and the
evaluating the effectiveness of the ex
traction process.
Summary and Conclusions
Steel Mill Sludge Waste
The steel mill industry produces <
variety of wastes that are potentia
candidates for treatment with the critica
fluids extraction process. Sludge cominj
from the underflow of a two-stage clarifi
cation process which contains oil, water
iron fines and residues was one of thi
wastes run with carbon dioxide solven
in this project. The focus of this experi
mentation was to recover oil from th<
sludge by extraction.
Extracting oil from the steel mill sludge
waste with carbon dioxide in the pilot
plant extraction system was difficult
Major plant design modifications wer<
necessary to handle a waste feed with
such a high loading of abrasive solids.
Dilution of the feed before processing
was attempted with the hope of pre
venting valve blockage due to the 2-5"X
particulate loading. However, this was
found to have a negligible effect on the
performance of the extraction system
Solvent-to-feed ratio was varied to fine
an optimal oil recovery. Using a 4.6 to 1 .C
solvent-to-feed ratio by weight, an oi
reduction of 30% was achieved. Highei
solvent-to-feed ratios did not enhance oi
recovery significantly.
Commercial systems would have t<
operate at or above the 4 to 1 solvent-to
feed ratio to recover enough oil for ar
effective extraction with carbon dioxide
The cost of operating at this level o
incoming solvent must be examined
Maximizing the difference between the
*"3l value of oil recovered and the cost o1
fresh solvent is necessary to make the
process more economically viable. Finally
commercial systems would have to be
designed to handle the abrasive slurry, fi
mix settler or stirred tank extractor woulc
be more effective than a sieve-tray ex-
tractor for this type of feed.
Pesticide Waste
The second waste tested was from £
pesticide manufacturing plant. The waste
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Table 1. Critical Properties for Selected Fluids
Fluid
Pentane
Butane
Solvent-12
Propane
Ethane
Ethylene
Carbon Dioxide
Sulfur Dioxide
Ammonia
Water
Critical
Temperature
(Tc. °C)
296.7
152.0
112.0
96.9
32.3
9.9
31.1
157.6
132.4
374.3
Critical
Pressure
(Pc. MPa)
3.4
3.8
4.1
4.3
4.9
5.2
7.5
8.0
11.
22.
Critical
Density
(Pc. g/cm3)
0.232
0.228
0.558
0.220
0.203
0.227
0.468
0.525
0.235
0.326
was a slurry which when separated by
decantation contains a top layer (mostly
xylene), a middle aqueous layer and a
heavy bottom layer classified as a sludge.
The waste contains water, xylenes, car-
bon tetrachloride, solids fines, salts and
other insolubles. The purpose of these
experiments was to extract carbon te-
trachloride and the mixed xylenes from
the waste. The mixed xylenes could be
recycled, and the carbon tetrachloride is
an extremely hazardous chemical that
must be isolated.
Pesticide waste was run in the pilot
plant extraction system with carbon
dioxide as the solvent. Distribution coef-
ficient determinations previously made in
a laboratory stirred extractor system
indicated that the separation would be
easy. This, however, was not the case in
the pilot plant. The difference between
pilot plant and laboratory scale results
are probably due to the high shear mixing
present in the laboratory extractor during
the extraction process, while there is
none in the pilot plant extraction.
The multipass extraction experiment
resulted in an 80% overall reduction in
the carbon tetrachloride level in the waste.
The overall reduction was calculated
using the initial feed concentration before
the first pass and the final raffinate con-
centration after the last pass of a four-
pass experiment. The experiment was
run with primarily a solvent-to-feed ratio
of 1 to 1 at a high flow rate of 4.5 Ibs-per-
minute (used to enhance mixing in the
extractor). A significant increase in the
extractability of carbon tetrachloride was
found in an experiment done in a single
pass when the solvent-to-feed ratio was
doubled. A 41% reduction was observed
for a solvent-to-feed ratio of 2 to 1 and
only an 8% reduction for 1 to 1.
Removal of the xylenes was more dif-
ficult. At first it appeared that the con-
centration of xylene in the raffinate was
increasing during the multipass experi-
ment. The method employed in performing
the multipass extraction, however, led to
an explanation of this anomaly. The feed
for each pass through the extractor during
the multipass experiment is the same
body of liquid as the raffinate from the
previous pass. Two conclusions can be
drawn from this knowledge. First, since
the feed concentration of xylenes dropped
by 60% over the last three passes, then
the raffinate level must have as well.
Second, there must have been a decanta-
tion or entrapment of xylene in the system
at some point between the extractor and
the raffinate tank which led into the
raffinate sampling system.
A commercial process could focus
specifically on reducing the carbon
tetrachloride' level in the aqueous layer of
the waste. This portion is typically treated
in a biotreatment facility, and the bacterial
cultures are sensitive to chlorinated
hydrocarbons. Removal of carbon tetra-
chloride would significantly enhance the
biotreatment facilities overall perfor-
mance. A commercial process treating
the whole waste stream should probably
be equipped with a stirred tank extractor
due to the presence of multiple phases
and solids in the waste feed.
Dissolved Organic* Waste
The last waste treated was from an
acrylonitrile manufacturing plant. This
waste contains acetonitrile, acrylonitrile,
salts of sulfates, and water. The focus of
experimentation for this waste was to
extract acetonitrile and acrylonitrile with
carbon dioxide to both detoxify the waste
and to recover the acrylonitrile product.
A multipass extraction experiment was
conducted with a solvent-to-feed ratio of
1.5 to 1.0. Overall reductions of 99.4%
and 99.6% were achieved for acetonitrile
and acrylonitrile in just four passes
through the extraction system.
The extractor was analyzed using the
Kremser equation, and an average sieve
tray efficiency was determined (E = .44)
assuming a constant distribution coef-
ficient (m = .76). A material balance was
performed around the extractor. Closure
was basically complete (95%) for the first
pass of the experiment but became poor
(55%) for later passes due to magnification
of error in laboratory analysis at low
organic concentrations.
Given the high degree of extractability
of the organics from this waste feed at a
pilot plant level, a commercial scale ex-
tractor was designed and a total process
conceptual design was developed for a
more concentrated waste stream.
An analysis of the effect of a 10% error
in distribution coefficient on extractor
sizing indicated that for this system, a
corresponding 50% error in the number
of trays could be incurred. Future design
work must concentrate on accumulating
more extensive distribution coefficient
data and not assuming a constant dis-
tribution coefficient (i.e. using a McCabe-
Thiele analysis), when analyzing the
extractor's efficiency.
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Paul N. Rice, William E. McGovern, and George S. Kingsley are with Critical
Fluid Systems, Inc., Cambridge, MA 02140.
MarkJ. Stutsman is the EPA Project Officer (see below}
The complete report, entitled "Near Critical CQz Extraction of Hazardous
Organics from Acrylonitrile, Pesticide and Steel Mill Wastes," (Order No.
PB 87-145 314/AS; Cost: $13.95, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
EPA
Iff N<*»
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Official Business
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EPA/60O/S2-87/005
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