United States
                    Environmental Protection
 Air and Energy Engineering
 Research Laboratory
 Research Triangle Park NC 27711   3
                    Research and Development
 EPA/600/S2-85/081 Aug. 1985
&ERA         Project  Summary
                    Pilot  Plant Evaluation  of
                    Critical   Fluid  Extractions  for
                    Environmental  Applications
                    George S. Kingsley
                      Liquefied gas solvents were used in a
                    pilot plant study to extract oil from mill
                    scale  (a steel mill by-product) and
                    bleaching clay (a vegetable oil filtering
                    media). The process, operated on a
                    semi-batch cycle, involved two extrac-
                    tors and a solvent recovery system. The
                    results of the extraction experiments
                    demonstrated the feasibility of con-
                    densed gas extraction. Preliminary
                    economics indicate attractive payback
                    on full-scale plants—about 23 months
                    for a mill scale extraction facility and 17
                    months for a bleaching clay plant.
                      This Project Summary was devel-
                    oped by EPA's Air and Energy Engineer-
                    ing Research Laboratory, Research Tri-
                    angle Park,  NC, to  announce key
                    findings of the research project that is
                    fully documented in a separate report
                    of the same title (see Project Report or-
                    dering information at back).

                      In recent years, several studies have
                    been undertaken by the U.S. Environ-
                    mental Protection Agency for uses of
                    critical-fluid extraction technology for
                    waste treatment. For solvent extraction,
                    the process has  unique advantages in
                    facilitating recovery of by-products and
                    minimizing solvent residues because of
                    the high solvent volatility. Prior studies
                    sponsored by EPA and others have fo-
                    cused  on coupling extraction with  ad-
                    sorption; i.e., using critical fluids to strip
                    and regenerate adsorbents which selec-
                    tively trap organic pollutants from  liq-
                    uid and vapor effluents.
  More recently, the direct extraction of
 liquid and solid wastes has been consid-
 ered. A range of applications of critical-
 fluid extraction  to environmental prob-
 lems have been evaluated by Critical
 Fluid Systems, Inc. under EPA sponsor-
 ship. In a recent EPA study, laboratory
 scale extractions were conducted on:
 1) mill scale from steel production/proc-
 essing, and 2) bleaching clays from veg-
 etable oil and specialty oil production.
 Both of these wastes contain significant
 amounts of oil and constitute major
 solid waste disposal problems. The lab-
 oratory studies  showed that extraction
 of oil from these wastes was feasible
 using Solvent-12 or propane; prelimi-
 nary economic evaluations were favor-
  The present study extends these eval-
 uations to the pilot scale. The residual
 oils on both mill scale and bleaching
 clays are soluble in Solvent-12 well
 below critical conditions. This allows
 processing at ambient  temperatures
 and at pressures that do not require ex-
 otic materials or methods of construc-
 tion. All of the economic advantages of
 supercritical extractions still exist (sim-
 ple solvent recovery, high purity, desol-
 ventized products) with the added bene-
 fit of lower compressor costs. In many
 situations, the compressor will be the
 only energy consumer in the process.
This energy consideration is the prime
factor in making  condensed gas solvent
extraction, as well as near- and super-
critical solvent extraction, so economi-
cally attractive.

Nature of the Wastes

Steel Mill Scale Oily  Wastes
  The steel industry is a major producer
of oil-containing solid wastes. A variety
of waste streams exist. The major prob-
lems stream is that of mill scale. Mill
scale is waste iron oxides contaminated
with lubricating oil. It results from sur-
face contamination of steel stock as
stock is formed into  product during
rolling and handling. Mill scale repre-
sents  1.5 to 10% of the total raw steel
production of a plant. As the percentage
increases, so does the  incentive to re-
cover this waste and turn it into saleable
product.  About half of the scale cur-
rently generated is in fact recycled to
the blast furnaces, most by way of a sin-
tering operation. The other half  is stock-
  Only half of the mill scale is repro-
cessed because the sintering operation
is adversely affected by high oil  content.
The oil is volatilized during sintering
and recondenses in the  plant  off-gas.
Without treatment this produces a visi-
ble plume and is an obvious source of
pollution. When bag  houses are used,
the oil condenses and  impairs bag
house operation.
  Several de-oiling methods have been
tested. Water washing with hot alkaline
solution can remove at best  90-95% of
the oil on mill scale fines at low cost.
The solids would then be suitable for
recycling, but the oily solid disposal
problem becomes an  oily water dis-
posal problem. Thermal incineration of
the oily mill scale is a very effective de-
oiling method, but the  energy  costs of
the incineration and the subsequent off-
gas treatment could quickly offset the
economic advantages of raw  material
recycling. A direct fired kiln has been
reported  in commercial use.
  Another possibility is liquid  solvent
washing  using a chlorinated hydrocar-
bon solvent. The solvent would  be used
to strip the oil off the mill scale fines,
making them suitable for the sintering
operation. The solids would have to be
thoroughly  desolventized,  however,
and the solvent would have to be recov-
ered from the  product oil in  order that
the oil could be recycled. The energy
costs for these operations would have
to be evaluated and carefully compared
to the recovered raw material value. The
costs of solvent makeup  must also be
taken into account.
  An alternative that appears economi-
cal exists, based on condensed gases.
The effectiveness of the conventional
solvent process can be coupled with the
economic attractiveness of critical fluid
extraction to yield a process that holds
the promise  of efficient extraction at
reasonable processing costs. This pro-
cess uses sub-cooled condensed gases
as the solvent. After extraction, the
clean scale is depressurized to  atmos-
pheric conditions. Residual solvent va-
porizes within a short time, leaving the
de-oiled scale ready for recycling. The
oil-laden solvent is sent to a still where
the oil is concentrated and the  solvent
vaporized for reuse.
  The energy for  this vaporization
comes from the superheated compres-
sor discharge, which is routed through
heat exchange coils in the base (reboiler
section) of the still. Thus the only en-
ergy input into the system  is the com-

Bleaching Clay Oily Residues

  The refining of a number of synthetic
and natural oils includes a processing
step to decolorize, or bleach, the refined
product using special clays. These clays
are composed of very fine (<400 mesh)
diatomaceous earth particles and any of
a number of additives. These materials
adsorb impurities from the product. The
clay is mixed with the oil, then the sus-
pension is filtered. The resulting clay fil-
ter cake contains 30 to 60% (by mass)
occluded oil, and thus represents a pol-
lution problem as well as a yield loss.
About 0.5 kg of clay is used per 100kg of
refined oil product in vegetable  oil pro-
  There are a number of incentives for
recovering this oil. The first of these in-
centives is the potential savings  in haul-
ing and landfill costs. The mass of mate-
rial handled would be substantially
lower and, with the waste clay itself be-
ing de-oiled, would require less  sophis-
ticated landfilling techniques for its dis-
posal. The cost per kilogram of disposal
would be lower as would the total
amount of waste to be disposed of.
  A second incentive is the recovery of
the bleached oil product. This product
could be added to either refined or
crude oil, depending on  its quality, and
thus improve overall plant yield.  The de-
oiled  clay may also be reusable if its
activity has not been entirely spent.
  A final incentive to de-oil  spent
bleaching clays is one  of  safety. The
stored oily clay waste can  undergo
spontaneous combustion. This possibil-
ity would be lessened, if not eliminated.
by de-oiling  the material. Currently,
water is added to the clays to inhibit
spontaneous combustion. This adds to
the mass to be disposed of and so to
disposal costs.
  There appear to  be no de-oiling
schemes in current industrial usage. As
with mill scale, incineration is an option,
but the higher oil content may incur
higher costs in off-gas cleanup. Conven-
tional halocarbon solvent extraction is a
possibility,  but removing solvent from
the de-oiled clays would be difficult due
to the small pores in the clays. These
pores would adsorb and hold solvent by
capillary condensation. The solvent-
bearing clay may present a worse pollu-
tion problem than the starting material.
  Sub-cooled condensed gas extraction
has shown promise as a de-oiling tech-
nique for  bleaching  clays. Laboratory
scale runs produced product  oil of a
somewhat  higher quality than  the  pro-
cess refined oil, presumably due to the
longer contact time of the occluded oil
with the still-active clay. Condensed gas
extraction of bleaching clays offers the
same process economic advantages
outlined for mill  scale de-oiling, and so
was chosen for further experimenta-

Pilot Plant Description

  The solids  extraction plant used for
this study  is a unit,  primarily carbon
steel, mounted on a steel framework
(skid) measuring 3 by 2.5  m.  The pri-
mary equipment in the plant is a bank of
three extractors, a reboiler/still unit, a
vertical surge vessel, a compressor and
compressed gas storage vessel,  a
pump, piping, and  valves. (See  Fig-
ure 1.)
  The solvent first passes through  a
mass flow meter. This is  used as an
input to a computer controller which, in
turn,  outputs a signal  to a transducer.
The pneumatic control signal from this
transducer is sent to  a valve that con-
trols flow.
  The solvent next enters the inlet line
to the extractor bank. The three extrac-
tors include two  15 cm dia x 1.5 m long
schedule 80 units with six inlet and six
outlet ports, configuration  allowed for
upflow, downflow, and crossflow ex-
perimentation. The third extractor  was
of the same length and schedule, but
had a diameter of 7.6 cm. This allowed
for higher  velocity experimentation in
either upflow or downflow modes. This
smaller column  was not equipped for
crossflow work.

                                                                                                   Surge     _  ,
                                                                                                   Vessel     So've"'
 Figure 1.    Simplified process flow diagram.
  The solvent is directed through a
loaded extractor, where it picks up oil
from the solid charge. Pressure is main-
tained by a backpressure control valve
at the column exit manifold. This con-
troller ensures that the solvent passes
through the extractor in a liquid phase.
The  solvent/oil mixture flows through
the pressure control  valve and into a
series of two vapor/liquid  separators.
These separators allow any solvent
vapor to  enter the still tower at a point
above the liquid solvent inlet, and keep
flow at the liquid solvent inlet smooth.
  The still/reboiler  unit is  a  stainless
steel unit capable of operating at pres-
sures in  excess of 10 mPa. (This work
was carried out at pressures of less than
700 kPa.) It is a 10 cm diameter tower,
2.5 m high and coupled at its base to a
kettle-type reboiler containing 13 m of
1.3 cm diameter  heat transfer coils.
These coils allow efficient heat transfer
from  the superheated compressor dis-
charge to the contents of the reboiler.
Solvent is  boiled off and the extracted
oil is concentrated in this reboiler. As oil
builds up in the reboiler, a level control
valve opens to allow an oil/solvent mix-
ture to flash into the product receiver. In
our pilot plant system, this solvent is
vented to the atmosphere. The still pres-
sure is maintained at a pressure about
250 kPa less than the extractor pressure
by another (independent) backpressure
control valve.  The solvent vapors from
the reboiler (and vapor/liquid separa-
tors) flow through this valve and into a
vertical compressor inlet surge vessel.
  The compressor inlet surge vessel is
another  15 cm diameter steel vessel,
1.5 m high. This acts as a trap to prevent
liquid carryover (if any) from the still
from reaching the compressor. It also
dampens out the pulsations of the com-
pressor, thus isolating the still  from
pressure variations. The solvent vapors
pass through a filter before entering the
  The compressor is an oilless piston
compressor capable of delivering  85
LPM (free air flow) and having a maxi-
mum discharge pressure of 1700 kPa.
The output of the compressor is sent to
a 110 L surge vessel.  The pressure in
this surge vessel is maintained at
1400 kPa by another backpressure con-
trol valve that vents excess compressor
capacity to the compressor inlet surge

  The superheated compressed solvent
gas flows into the coils of the  reboiler
and is there liquefied as it gives up its
latent heat to the solvent/oil mixture in
the reboiler. The coils are followed by
another vapor/liquid  separator. This
separator serves to collect  non-
condensable gases and ensure that only
liquid solvent flows to the flow meter,
and so through the system for  another
extraction cycle.

Pilot Plant Operation

  The first step in  the extraction experi-
ment was to load the extractor with a
known amount of feed. This was accom-
plished by weighing starting material in
a transfer vessel and pouring the mate-
rial into the extractor. A large funnel
kept transfer losses  to  a minimum.
Samples of the feed were taken at this
time. When the transfer was complete,
the transfer vessel was reweighed. The
charge to the extractor was then calcu-
lated and noted,  as was  the height of
feed in the extractor.
  The extraction plant was then started
up in a bypass mode. No solvent was
put through the bed of solids during this
start-up phase. The flow of solvent was
cycled through the piping until  pres-
sures and'flow rates  achieved steady
state. Not until this steady running con-
dition  was achieved would extraction
  The extraction cycle was begun by fill-
ing the column with solvent. This was
done by the pump from the solvent stor-
age  vessel or, if the solvent volume in
that vessel was inadequate, from  the
main solvent supply  tank. Filling  the
column generally  took about 5 minutes.
Pumping was continued until the pres-
sure in the column was higher than the
vapor pressure corresponding to ambi-
ent temperature,  typically greater than
500 kPa. A valve at the top of the column
was then cracked  to relieve any trapped
air. The column  was  ready for extrac-
  The inlet  and  outlet valve arrange-
ment for the given experiment was set
and the  bypass valve switched to ex-
traction. The solvent mass flow totalizer
was re-zeroed at  this time. The experi-
ment continued for the prescribed time,
and then flow was once again  diverted
to the bypass mode. The total solvent
throughput was noted, and the extrac-
tor drained of solvent.
  Removal of solids  was  straightfor-
ward. After a final check that there was
no pressure in the extractor, the  top
cover  was  opened. The  height of
residue was noted and the top of the
bed of  solids examined visually for evi-
dence of channeling or agglomeration.
A sample of the  top of the bed  was
  The bottom cover was next removed
and the solids collected in  a weighed
container. This procedure required care
in the  case of bleaching clay because
this material rarely poured  out of its
own accord. The solids would be prod-
ded from the bottom with a rod to break
up any bridging, and then prodded from
the top to start the flow. Any solids es-
caping the collection vessel were swept
up  and added to the  residue.  The
column sides were examined for adher-
ring residue and cleaned (scraped)
when necessary.  Samples of the  bulk
residue were taken by a cone and quar-
tering technique in an effort to get a rep-
resentative sample for moisture and oil

  Extraction rate  data were taken by
grab sampling the extract stream at var-
ious times during the extraction cycle.
The sample's solvent was vented, and
the  concentration of oil  in  the extract
was calculated. When these data were

       4 w-
     plotted against time, an exponential
     decay in concentration with time was
     observed (Figure 2).
       Extraction efficiency data were based
     on  Soxhlet  extraction feeds and  resi-
     dues. A composite sample of feed was
     taken as the column was loaded; a sim-
     ilar sample of residue was taken at the
     conclusion of the run. These samples
     were then analyzed for their oil content.
     We defined  extraction efficiency as the
     quantity of oil removed divided by the
     quantity of oil available for extraction as
     determined  by the Soxhlet analyses.
       Table 1 provides extraction efficiency
     data. As shown, the extraction for mill
     scale was quite efficient:  half the runs
     gave greater than 90% oil  extraction. On
     the other hand, the  bleaching clay was
     more difficult to extract: most values
     were less than 50%. Note  that, although
     the bleaching clay extractions were less
     effective, larger amounts  of oil were re-
     covered due to the much greater oil
     content of the bleaching  clay waste. In
     addition, these efficiencies are based on
     a limited  extraction period. Figures 3
     and 4 show the cumulative oil extracted
     as a function of time for  mill scale and
     bleaching clay, respectively. Note that,
     for mill scale (Figure 3), only 60 minutes
     was required to achieve  maximum ex-
20           30

   Time, min.
Figure 2.   Decay in extract oil concentration with time during mill scale extraction.

Table 1.    Extraction Efficiencies
Mill Scale

Bleaching Clay

Run No.
Oil in
Waste, %
Efficiency, %
traction.  For bleaching clay (Figure 4),
the extraction curves were still rising
when the run ended.
  The  overall  lower oil reduction  effi-
ciences for bleaching clays (when com-
pared  to mill scale data)  may  be at-
tributed to the  nature of the feed.
Bleaching clay is a very fine, porous ma-
terial. The presence of  fine  pores
throughout the particle adds another
term to the overall mass transfer coeffi-
cient of oil  from the particle to the sol-
vent.  In  addition  to particle-to-solvent
mass transfer through the boundary
layer  of  the individual particle, there
exists an intra-particle diffusional resis-
tance. The  oil trapped in internal pores
must diffuse through the particle matrix
in order to  be extracted. This resistance
term will be independent of flow condi-
tions as  long as the oil at the particle
boundary layer is removed as fast as the
internal pore diffusion brings oil to the
particle surface.

Preliminary Economic Analysis

Mill Scale

  A preliminary design was prepared
for a  mill scale de-oiling plant for the
purpose  of a process feasibility study.
The  proposed plant would process
80,000 metric tons of mill scale per year
with a nominal oil content of 5%.
  Capital costs, operating costs, and ex-
pected credits combine to provide a net
credit of over $900,000/yr. A dollar value
cannot, however, be attached to the
credit for the elimination of a hazardous
waste. This benefit of mill scale de-
oiling will become an overriding factor
in the implementation of de-oiling tech-
Bleaching Clay
  Preliminary design of a vegetable oil
extraction facility has been completed
for a 3,600,000 kg/yr plant.
  Capital costs, operating costs, and ex-
pected credits combine to provide a net
credit of over $800,000/yr.

  1.  Steel mill scales containing several
     percent residual oil can be de-oiled
     to levels acceptable for sinter-plant
   feed using condensed dichlorodi-
   flouromethane  (Solvent-12),  at
   ambient temperatures, and pres-
   sures of about 700 kPa. The maxi-
   mum pressure  requirement  of
   such a system is 1400 kPa.
2.  The rate of mill scale extraction is
   proportional to flow rate.  Ninety
   percent extraction occurred in 3C
   minutes at 1.6 kg/min and in 60
   minutes at 0.9 kg/min.
3.  The  rate of  mill scale extraction
   was  increased by a static soaking
   period before extraction.
4.  Preliminary economic evaluation
   indicates that  credits for de-oiled
   mill scale and  fuel quality oil pro-
   provide an attractive payout for a
   plant de-oiling 80 metric tons per
   year of 5% oil  mill scale feed.
5.  Bleaching clays used in vegetable
   oil processing can be de-oiled
   using the condepsed gas extrac-
   tion process.
6.  Extraction efficiency of  bleaching
   clay  is primarily a function of con-
   tact time.
7.  Preliminary economic analysis in-
   dicates that product recovery cred-
   its and  disposal cost reduction
   make condensed gas extraction at-
          O  Solvent Mass Flow Rate of 0.9 kg/min.. EPA 30

          D  Solvent Mass Flow Rate of 1.1 kg/min.. EPA 32
             with 30 min. Solvent Soak

          A  Solvent Mass Flow Rate of 1.6 kg/min., EPA 31

Figure 3.   Cumulative oil extracted from mill scale as a function of time.


O  EPA 54
A  f PA 56
O  EPA 57
G>  EPA 58
          0     20     40     60     80    100    120    140    160    180   200    220    240    260    280   300    320

                                                             Time, min.

Figure 4.   Cumulative oil extracted from bleaching clay as a function of time.
                                            G. S. Kingsley is with Critical Fluid Systems. Inc., Cambridge, MA 02140.
                                            Bruce A. Tichenor is the EPA Project Officer (see below).
                                            The complete report, entitled "Pilot Plant Evaluation of Critical Fluid Extractions
                                             for En vironmental Applications," (Order No. PB 85-233 484/A S; Cost: $11.50,
                                             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:
                                                    Air and Energy Engineering Research Laboratory
                                                    U.S. Environmental Protection Agency
                                                    Research Triangle Park, NC 27711
                                                                          •fr U. S. GOVERNMENT PRINTING OFFICE: 1985/559-111/20662

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