TECHNICAL PAPER # 41
UNDERSTANDING SOLVENT
EXTRACTION OF
VEGETABLE OILS
By
Nathan Kessler
Technical Reviewers
Dr. Earl Hammond
Jon E. McNeal
Robert Ridoutt
VITA
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 * Fax
703/243-1865
Internet: pr-info@vita.org
Understanding Solvent Extraction of Vegetable Oils
ISBN: 0-86619-253-0
[C]
1985, Volunteers in Technical Assistance
PREFACE
This paper is one of a series published by Volunteers in
Technical
Assistance to provide an introduction to specific
state-of-the-art
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
situations.
They are not intended to provide construction or
implementation
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
illustrated
almost entirely by VITA Volunteer technical experts on a
purely
voluntary basis.
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.
VITA Volunteer
Nathan Kessler is the Corporate Vice President of the Technical
Division of the A.E. Staley Manufacturing Company in
Decatur,
Illinois. The
reviewers are also VITA volunteers. Dr.
Earl Hammond
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,
Pennsylvania,
and has had several years' experience in extraction
technology.
VITA is a private, nonprofit organization that supports
people
working on technical problems in developing countries.
VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
their
situations. VITA
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
of
volunteer technical consultants; manages long-term field
projects;
and publishes a variety of technical manuals and papers.
UNDERSTANDING
SOLVENT EXTRACTION OF VEGETABLE OILS
By
VITA Volunteer Nathan Kessler
I. INTRODUCTION
Oil is extracted from seeds, beans, and nuts for use as
cooking
or salad oil; as an ingredient in paint, cosmetics, and
soap; and
even as fuel.
Historically, such oils have been extracted by wrapping
seeds(*) in
cloth, and then using devices operated by stones and levers
to
exert pressure on them.
An improved form of mechanical device, which allowed
considerably
more pressure to be exerted, involves the use of
hydraulically
operated rams: a
simple, hand-operated cylinder pump is used to
press flat plates or hollow cages attached to the hydraulic
ram
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
cake(**).
The next improvement in extracting oil was the screw press
or
expeller. Screw
presses use an electric motor to rotate a heavy
iron shaft, which has flights, or worms built into it to
push the
seeds through a narrow opening.
The pressure of forcing the seed
mass through this slot releases part of the oil, which comes
out
through tiny slits in a metal barrel fitted around the
rotating
shaft. Expellers
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
seed. To
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
all seeds,
beans, and nuts from which oil can be extracted.
(**) Terms in boldface are defined in the glossary at the
end of
this paper.
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
and
leave too much of the high value oil in the seed cakes,
methods
of extracting the oil with solvents were developed.
Seeds (like
soybeans) with low oil content are processed by solvent
methods
alone. In other
cases, presses are used first to extract part of
the oil; then solvents extract the oil that remains in the
seeds.
Because of their efficiency, processes employing solvents to
extract vegetable oils in large quantities are in wide use,
and
solvent extraction equipment is readily available
commercially.
The basic technology of solvent extraction is simple, but
great
care should be taken in deciding whether and where it can be
used.
Solvent extraction of vegetable oils, which recovers more
oil
than earlier methods and leaves more usable meal, begins to
be
economically attractive where large quantities of seed can
be
processed (at least 200 tons per day for continuous-feed
processes);
where storage, transportation, power, water, and solvent
supply are adequate; and where occupational safety and
training
standards can be enforced.
There are solvent extraction plants
with capacities of up to 4,000 tons per day.
II. OPERATING
PRINCIPLES
Solvent extraction is simple in principle, but complex in
operation.
See Figure 1.
32p03.gif (600x600)
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
the
oil out of them.
Solvents that boil at fairly low temperatures
(65'C) are used so that the solvent can be readily removed
from.
both the oil and the flakes.
Solvent extraction recovers almost
all the oil, leaving only one percent or less oil in the
flakes.
Unfortunately, most solvents are dangerous to handle, more
so
than gasoline.
They burn or explode very readily.
Therefore, the equipment that
extracts the oil and removes the solvent must be airtight
and
leakproof, and all motors and electrical switches, lights,
etc.
must be specially designed as vapor-explosion-proof (Class
I-D).
No matches, no smoking, and no cutting torches, welders
grinders, or other heat-producing or spark-producing devices
can
be permitted where such solvents are used.
Careless exposure to
sources of fire or sparks (including engines of trucks
driven too
close to extraction plants) have caused disastrous
explosions.
Attempts to find solvents that are not explosive and are
economical to use have not yet succeeded.
Chlorinated
hydrocarbons such as trichloroethylene worked well but were
found
to create a poisonous by-product in the extracted meal.
Solvent
extraction plants built in 1950 using trichloroethylene had
to be
discarded or converted to the commonly used explosive
solvent,
hexane. Today, all
commercial oilseed extraction plants utilize
hexane or a similar solvent.
III. DESIGN
VARIATIONS
Like pressing, solvent extraction can be done with equipment
that
processes the oilseed in batches, or with equipment that
processes it continuously.
A continuous extractor is not
considered economically practical unless it processes at
least
200 tons per day.
BATCH SOLVENT EXTRACTION
Batch solvent extraction is likely to be the appropriate
method
if you plan to process less than 200 tons of seed per day,
but
enough to yield oil in commercial quanitities.
Very few batch plants are in use in the United States
today. A
batch solvent extraction plant can be as simple as an
enclosed
steel tank with a false bottom made of screen or metal
slats. The
flakes are dropped into the tank, where they lie on the
false
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
extraction.
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
water-cooled
tubes. The solvent
is lighter than water, so it is readily freed
of water by standing in a tank from which water is decanted,
or
overflowed. The
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
later
batch. However,
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
the
solvent out of the mixture, recovering solvent for
reuse. The oil
then goes to a vacuum stripper, where it is heated to about
100'C
and steamed as it passes down through a series of steel
baffles
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
housings.
The conveyor may be an endless metal mesh belt or a series
of
sieve-bottom buckets attached to a traveling chain.
Another style uses vertical columns filled with solvent.
Flakes
are continuously fed at the top and removed from the bottom
by a
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
baskets or
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
prevents
solvent vapors from escaping out of the extractor into the
flake
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.
In this
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.
The
flakes are then drained (10 to 15 minutes), and dropped from
the
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
into a
desolventizer-toaster.
This is a steam-jacketed vessel, usually a
vertical set of kettles with gates that allow the flakes to
fall
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
vessel or
in a separate meal cooler.
As in the batch extractor system, the
solvent vapors flow to a condenser with water-cooled tubes,
and
the liquid solvent is separated from the water by decanting.
An older form of desolventizer employs a series of
steam-jacketed
paddle conveyors to evaporate most of the solvent.
The partially
desolventized flakes then crop into a larger conveyor, into
which
direct steam is blown, removing the rest of the
solvent. This
form of desolventization was improved by using super-heated
hexane vapor to quickly remove most of the solvent.
This first
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
to
be fed to nonruminant animals, a cooking or toasting stage
has
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
will be
cooked before being eaten.
Solvent in continuous systems is evaporated and recovered
from
the miscella in the same way as in batch systems.
However, when
solvent is removed from the flakes by the desolventizer-toaster
method, the hot vapors from the toaster can be used as the
heat
source in the first-stage miscella evaporator.
This results in
important energy savings.
For seed very high in oil, such as cottonseed groundnut or
peanut,
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.
this pre-pressing
is important in cottonseed also because it reduces the
antinutritional
gossypol material left in the meal.
Figure 2 illustrates
32p07.gif (600x600)
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
of a
technological and economic system that includes far more
than the
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,
and
of maintenance and storage facilities; and ability to find
and
train personnel and rigorously enforce safety
standards. Table 1
gives inforamtion about some of these requirements.
Table
1. Estimated Requirements for Solvent
Extraction of Vegetable Oils
Required
per ton
of seed
Batch
Continuous
processed
Units processing
processing
Steam
kilograms 700
280
Power
kilowatt hours 45
55
Water
cubic meters 14
12
Solvent
kilograms 5
4
Labor
person hours
0.8
0.5
Source: Ernesto
Bernadini, "Batch and Continuous Solvent
Extraction" Journal of the American Oil Chemists'
Society 53
(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
up to
200 tons per day), the choice is between batch solvent
extraction
and expeller (pressure extroaction) systems.
Batch solvent extraction
systems operate more slowly and less efficiently, are
more labor intensive and dangerous, and use greater
quantities of
solvent than properly designed continuous systems do.
Because of
these drawbacks, expellers are usually preferred for
installations
too small for continuous solvent systems.
However, there
are instances when expeller extraction is not suitable for a
small operation; in those cases, batch solvent extraction
may be
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.
Installation
of a plant without such engineering of details is a
dangerous
error.
COST
The cost of solvent extraction plants is much higher than
the
cost of expeller extraction plants, usually about double.
However, since a solvent plant recovers a greater proportion
of
the oil, it may still be the economically wiser choice.
For
example, solvent extraction should recover about 40 kilograms
more oil per ton from dry soybeans than expeller extraction
would.
PRODUCT QUALITY
Not only does solvent extraction yeild more oil, it avoids
the
overheating of the oil and meal that often occurs with
expeller
extraction.
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
operate
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.
Workers
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
essential
to be able to depend on a steady throughput.
Unscheduled interruptions
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
if
stroage arrangements are insufficient.
Unanticipated interruptions
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
food, it
is necessary to have a crude oil refinery that can handle
the
volume of oil produced by the extraction plant.
Food oil
refineries are more complicated to operate and more
expensive in
equipment costs than solvent extraction plants are.
For nonfood
uses, such as drying oil, a refinery is not necessary.
GLOSSARY
Expeller A
kind of screw press (see below)
Flakes
Thin, flat pieces of seed or press cake (see
below) prepared for solvent treatment.
Flights
Also termed worms--the screw threads in an
Expeller or screw press.
Miscella
Also termed wash--the liquid, containing oil
and solvent, drained after application of
solvent to flaked seeds.
Press cake
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
proteins.
Wash
Also termed miscella--the liquid, containing
oil and solvent, drained after application of
solvent to flaked seeds.
Worms In
a screw press the screw threads, or flights,
that guide and force seeds through a narrow
opening.
REFERENCES
Bibliography
American oil Chemists, society.
"Symposium: Papers
from the
Symposium on
Extraction Processes, presented at the 73rd
AOCS Annual
Meeting held in Toronto, Canada, May 2-6, 1982,"
Journal of the
American Oil Chemists' Society, 61, No.
8,
1358-1388,
August 1984.
Bernardini, Ernesto.
"Batch and Continuous Solvent Extraction,"
Journal of the
American Oil Chemists' Society, 53:
275-278.
1976.
United Nations Industrial Development Organization.
Guidelines
for the
Establishment and Operation of Vegetable Oil
Factories, 1977.
SUPPLIERS AND MANUFACTURERS
Sources of equipment and engineering of a complete plant
include:
French Oil Mill
Machinery Co.
Piqua, Ohio
45356
USA
Extraction technology:
DeSmet SA
Avenue Prince
Baudouin 265
B-2520 EDEGEM
ANVERS
BELGIQUE
Crown Iron Works
1229 Tyler
Street, East
Minneapolis,
Minnesota 55440
USA
Blaw-Knox Food
& Chemical Equipment Company
Box 1041
Buffalo, New
York 14240 USA
Construzioni
Meccaniche Bernardini Co.
00040 Pomezia
Rome, Italy
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