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TECHNICAL PAPER # 41
Dr. Earl Hammond
Jon E. McNeal
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 * Fax
Understanding Solvent Extraction of Vegetable Oils
1985, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
They are not intended to provide construction or
details. People are
urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
almost entirely by VITA Volunteer technical experts on a
Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Gerald Schatz as
editor, Suzanne Brooks handling typesetting and layout, and
Margaret Crouch as project manager.
The author of this paper is a VITA volunteer.
Nathan Kessler is the Corporate Vice President of the Technical
Division of the A.E. Staley Manufacturing Company in
reviewers are also VITA volunteers. Dr.
is a Professor of Food Technology at the University of Iowa
in Ames, Iowa. Jon
E. McNeal is an analytical chemist with the
United States Department of Agriculture in Washington, D.C.
Robert Ridoutt is employed with Heinz, USA in Pittsburgh,
and has had several years' experience in extraction
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
SOLVENT EXTRACTION OF VEGETABLE OILS
VITA Volunteer Nathan Kessler
Oil is extracted from seeds, beans, and nuts for use as
or salad oil; as an ingredient in paint, cosmetics, and
even as fuel.
Historically, such oils have been extracted by wrapping
cloth, and then using devices operated by stones and levers
exert pressure on them.
An improved form of mechanical device, which allowed
more pressure to be exerted, involves the use of
operated rams: a
simple, hand-operated cylinder pump is used to
press flat plates or hollow cages attached to the hydraulic
against a fixed-position ram.
This type of press developed into a motorized hydraulic pump
system that pressed the seed bag and then released a press
The next improvement in extracting oil was the screw press
presses use an electric motor to rotate a heavy
iron shaft, which has flights, or worms built into it to
seeds through a narrow opening.
The pressure of forcing the seed
mass through this slot releases part of the oil, which comes
through tiny slits in a metal barrel fitted around the
have a continuous flow of seed through the
machine in contrast to the hydraulic system described above,
which uses small, individual packages or batches of
release as much oil as possible, the seeds must be dried to
rather low moisture content and exposure to high temperature
causes darkening of the oil.
It also causes some scorching or
(*) The term seed,
or seeds, will be used in this report to include
beans, and nuts from which oil can be extracted.
(**) Terms in boldface are defined in the glossary at the
overheating of the meal.
The meal contains protein which, if
undamaged, may be used for either human food, soy flour for
example, or animal feed such as soybean meal.
Because most press or expeller processes overheat the meal
leave too much of the high value oil in the seed cakes,
of extracting the oil with solvents were developed.
soybeans) with low oil content are processed by solvent
alone. In other
cases, presses are used first to extract part of
the oil; then solvents extract the oil that remains in the
Because of their efficiency, processes employing solvents to
extract vegetable oils in large quantities are in wide use,
solvent extraction equipment is readily available
The basic technology of solvent extraction is simple, but
care should be taken in deciding whether and where it can be
Solvent extraction of vegetable oils, which recovers more
than earlier methods and leaves more usable meal, begins to
economically attractive where large quantities of seed can
processed (at least 200 tons per day for continuous-feed
where storage, transportation, power, water, and solvent
supply are adequate; and where occupational safety and
standards can be enforced.
There are solvent extraction plants
with capacities of up to 4,000 tons per day.
Solvent extraction is simple in principle, but complex in
See Figure 1.
The seed is prepared by being cracked into chips.
These chips are
warmed and passed through smooth flaking rolls.
The flaking rolls
flatten the chips into paper-thin, flat flakes.
The thin flakes
can then be treated with solvent, which dissolves or washes
oil out of them.
Solvents that boil at fairly low temperatures
(65'C) are used so that the solvent can be readily removed
both the oil and the flakes.
Solvent extraction recovers almost
all the oil, leaving only one percent or less oil in the
Unfortunately, most solvents are dangerous to handle, more
They burn or explode very readily.
Therefore, the equipment that
extracts the oil and removes the solvent must be airtight
leakproof, and all motors and electrical switches, lights,
must be specially designed as vapor-explosion-proof (Class
No matches, no smoking, and no cutting torches, welders
grinders, or other heat-producing or spark-producing devices
be permitted where such solvents are used.
Careless exposure to
sources of fire or sparks (including engines of trucks
close to extraction plants) have caused disastrous
Attempts to find solvents that are not explosive and are
economical to use have not yet succeeded.
hydrocarbons such as trichloroethylene worked well but were
to create a poisonous by-product in the extracted meal.
extraction plants built in 1950 using trichloroethylene had
discarded or converted to the commonly used explosive
hexane. Today, all
commercial oilseed extraction plants utilize
hexane or a similar solvent.
Like pressing, solvent extraction can be done with equipment
processes the oilseed in batches, or with equipment that
processes it continuously.
A continuous extractor is not
considered economically practical unless it processes at
200 tons per day.
BATCH SOLVENT EXTRACTION
Batch solvent extraction is likely to be the appropriate
if you plan to process less than 200 tons of seed per day,
enough to yield oil in commercial quanitities.
Very few batch plants are in use in the United States
batch solvent extraction plant can be as simple as an
steel tank with a false bottom made of screen or metal
flakes are dropped into the tank, where they lie on the
bottom. The tank
inlet is closed, and solvent is pumped into
flood the bed of flaked oilseed.
The solvent is allowed to contact
the seed for 10 to 20 minutes; then the drain valve at the
bottom (under the false bottom) is opened to complete the
After the final extract has been fully drained, steam is
introduced into the bottom of the extractor.
This evaporates the
solvent out of the flakes.
This combination of steam and solvent
is piped as vapor into a condenser that contains
tubes. The solvent
is lighter than water, so it is readily freed
of water by standing in a tank from which water is decanted,
flakes now are nearly solvent free, but are wet
from the steam treatment.
They are conveyed out of the extractor
to a steam-heated dryer to reduce the moisture to about 12
percent for best storage quality.
Most of the washes, or miscellas, are saved and reused on a
fresh, oil-free solvent must be used for the
final wash of a batch.
And the first, oiliest miscella is pumped
to a steam-heated, tubular evaporator, which boils most of
solvent out of the mixture, recovering solvent for
reuse. The oil
then goes to a vacuum stripper, where it is heated to about
and steamed as it passes down through a series of steel
or a column of stoneware rings or saddles.
The purpose is to
expose every portion of the oil to steam, which is needed to
remove the last 5 to 10 percent of the solvent from the oil.
CONTINUOUS SOLVENT EXTRACTION
Continuous extractors use conveyors inside vapor-tight
The conveyor may be an endless metal mesh belt or a series
sieve-bottom buckets attached to a traveling chain.
Another style uses vertical columns filled with solvent.
are continuously fed at the top and removed from the bottom
vertical mass-flow elevator.
Fresh solvent enters at the bottom,
and oily miscella overflows from the top.
Still another style
uses a rotating carousel arrangement of the extraction
buckets as in the Rotocel:
this French Oil Mill Machinergy
Company stationary extractor rotates the inlet and outlet
assembly above and below stationary sieve-bottom baskets.
In all of these extractors, flaked seeds are conveyed continously
into the extractor through a vapor lock or seal which
solvent vapors from escaping out of the extractor into the
conveyor. The flakes
are sprayed or wet with miscella as they
enter the extractor, and receive several washes with successively
more dilute (less oily) miscella.
These miscellas drain down
through the flakes and through the sieve bottom or belt into
pans, which drain into pumps.
The pumps transfer the miscella to
the next state, from less oily to more oily flakes.
continuous countercurrent, the oldest solvent miscella (the
solvent miscella with the highest oil content), contacts the
fresh incoming flakes.
The final wash uses oil-free hexane.
flakes are then drained (10 to 15 minutes), and dropped from
belt or the basket into a spent-flake hopper.
From here a mass-flow conveyor lifts the still solvent-wet
flakes (containing 35 percent moisture) and delivers them
This is a steam-jacketed vessel, usually a
vertical set of kettles with gates that allow the flakes to
from one kettle into the next below while being treated with
direct steam. The
lower kettles act as dryers to bring the moisture
content down to proper levels.
Air is drawn to cool the
dried hot flakes, either in the lower part of the same
in a separate meal cooler.
As in the batch extractor system, the
solvent vapors flow to a condenser with water-cooled tubes,
the liquid solvent is separated from the water by decanting.
An older form of desolventizer employs a series of
paddle conveyors to evaporate most of the solvent.
desolventized flakes then crop into a larger conveyor, into
direct steam is blown, removing the rest of the
form of desolventization was improved by using super-heated
hexane vapor to quickly remove most of the solvent.
step is followed by a steam treatment.
However, neither of these
methods cooks soybean flakes thoroughly enough to eliminate
trypsin inhibitors. For this reason, if the flakes are going
be fed to nonruminant animals, a cooking or toasting stage
to be added: the
flakes are heated to about 125"C, reducing their
moisture to 18 percent or less.
When the flakes are intended for
human consumption, this step is not necessary, since they
cooked before being eaten.
Solvent in continuous systems is evaporated and recovered
the miscella in the same way as in batch systems.
solvent is removed from the flakes by the desolventizer-toaster
method, the hot vapors from the toaster can be used as the
source in the first-stage miscella evaporator.
This results in
important energy savings.
For seed very high in oil, such as cottonseed groundnut or
or sunflower, low-pressure expellers are ususally used to
remove part of the oil at reduced cost.
This is follwed by flaking
and solvent extraction as described above.
is important in cottonseed also because it reduces the
gossypol material left in the meal.
Figure 2 illustrates
the process for sesame seed processing.
IV. FACTORS TO
CONSIDER IN PLANNING AN OIL EXTRACTION SYSTEM
Solvent extraction of vegetable oils should be seen as part
technological and economic system that includes far more
extraction plant itself.
Factors affecting the operation of a
solvent extraction plant include:
potential markets; nature,
timeing, size, and reliability of seed and solvent supply;
adequacy and reliability of power, water, and transportation,
of maintenance and storage facilities; and ability to find
train personnel and rigorously enforce safety
standards. Table 1
gives inforamtion about some of these requirements.
1. Estimated Requirements for Solvent
Extraction of Vegetable Oils
kilowatt hours 45
cubic meters 14
Bernadini, "Batch and Continuous Solvent
Extraction" Journal of the American Oil Chemists'
(Hybe 1976): 278.
SIZE OF OPERATION
The size of the operation is the most important factor in
determining wich kind of process will be used.
For intermediate-scale operations (operations that process
200 tons per day), the choice is between batch solvent
and expeller (pressure extroaction) systems.
Batch solvent extraction
systems operate more slowly and less efficiently, are
more labor intensive and dangerous, and use greater
solvent than properly designed continuous systems do.
these drawbacks, expellers are usually preferred for
too small for continuous solvent systems.
are instances when expeller extraction is not suitable for a
small operation; in those cases, batch solvent extraction
the only practical way to proceed.
Continuous solvent extraction should be considered only for
systems that will treat 200 tons or more of seed per day.
SITE AND DESIGN
Solvent extraction plants are complex systems that must be
carefully engineered for safety because of their special
hazards. Because of
the danger of explosion, solvent extraction
plants need to be located a safe distance away from populous
areas, and to be designed by experienced engineers.
of a plant without such engineering of details is a
The cost of solvent extraction plants is much higher than
cost of expeller extraction plants, usually about double.
However, since a solvent plant recovers a greater proportion
the oil, it may still be the economically wiser choice.
example, solvent extraction should recover about 40 kilograms
more oil per ton from dry soybeans than expeller extraction
Not only does solvent extraction yeild more oil, it avoids
overheating of the oil and meal that often occurs with
Solvent-extracted meal can be toasted to optimum food
or feed quality.
PERSONNEL AND SAFETY
It takes less labor but more sophisticiation to maintain and
operate a solvent extraction plant than to maintain and
an expeller plant.
Two people per shift are required for the
former, compared to three for the latter.
The dangers of solvent
explosion make tightly controlled procedures necessary.
must be trained to have a wholesome fear of exposure to the
solvent and of solvent leakage.
RELIABILITY OF THROUGHPUT
For continuous solvent installations especially, it is
to be able to depend on a steady throughput.
of production, or discontinuities because of the inability
to transport the finished product, for example, mean that
seed will pile up somewhere and possibly spoil, expecially
stroage arrangements are insufficient.
of seed supply may cause buyers of oil and meal to
turn to more reliable sources.
Both batch solvent and expeller
operations are less vulnerable to the effects of such
interruptions than continuous solvent operations are.
INTENDED USE OF THE OIL
Since crude oil is usually refined before being used as
is necessary to have a crude oil refinery that can handle
volume of oil produced by the extraction plant.
refineries are more complicated to operate and more
equipment costs than solvent extraction plants are.
uses, such as drying oil, a refinery is not necessary.
kind of screw press (see below)
Thin, flat pieces of seed or press cake (see
below) prepared for solvent treatment.
Also termed worms--the screw threads in an
Expeller or screw press.
Also termed wash--the liquid, containing oil
and solvent, drained after application of
solvent to flaked seeds.
Seed residue left after pressing.
Screw press A
press that uses a screw to guide and force
seeds through a narrow opening.
Trypsin Inhibitors Enzymes that prevent the breaking down of
Also termed miscella--the liquid, containing
oil and solvent, drained after application of
solvent to flaked seeds.
a screw press the screw threads, or flights,
that guide and force seeds through a narrow
American oil Chemists, society.
Extraction Processes, presented at the 73rd
Meeting held in Toronto, Canada, May 2-6, 1982,"
Journal of the
American Oil Chemists' Society, 61, No.
"Batch and Continuous Solvent Extraction,"
Journal of the
American Oil Chemists' Society, 53:
United Nations Industrial Development Organization.
Establishment and Operation of Vegetable Oil
SUPPLIERS AND MANUFACTURERS
Sources of equipment and engineering of a complete plant
French Oil Mill
Crown Iron Works
& Chemical Equipment Company
York 14240 USA
Meccaniche Bernardini Co.