yoil and fat technology lectures iii crude oil production
TRANSCRIPT
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Extraction of Vegetable Oils
Basic approaches :
Mechanical Oil Extraction
- cold pressing means no heat applied
- hot pressing - external heat is applied
Solvent Extraction
- organic solvent (hexane, isopropyl alchool)
- supercritical solvent (carbondioxide)
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Mechanical oil extraction
Mechanical oil extraction (expression) is a solid-
liquid phase seperation method which is applied
to cooked seed flakes.
It can be executed by batch, mainly hydraulically,
and by continous, mainly mechanically, working
presses.
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Screw presses;
In oil industry, screw presses (expellers) aremostly utilized for expression. The main
parts of continous-screw press are;
Seed feeder,
Cone-shaped cage
Adjustable cone for press-cake outlet
Worm (pressure and feed)
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Screw pressCooked seed flakes
Crude oil
Main worm shaft
knife
cake
Adjustable cone for press-
cake outlet
Cone shaped pressure
cage
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The seeds enter the barrel and falls on the helical
pressure worm.During movement in the barell,
between worm and cage is gradually reduced andthe seed flakes are subjected to increasing
pressure.The cage is made of a number of special
stell bars which let liquids pass through. The oil
passes between the bars an flows out of the cage.The cone moves along the shaft of the expeller and
the space between the worm and cone can be
regulated.This permits easy control of the
thichnesses of cakes and of the degree of pressure
to which the cooked flakes are subjected.
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Screw Press
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Shaft Arrangement--
Screw Press
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Cage Arrangement
French Press
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Advantages and disadvantages of theexpeller process;
Expellers can be used with almost any kind of oilseeds and nuts. Theprocess is relatively simple and not capital-intensive. While thesmallest solvent extraction plant would have a processing capacityof 100-200 tons per day, expellers are available for much smallercapacities, from a few tons per day and up.
The main disadvantage of the screw-press process is its relatively lowyield of oil recovery. Even the most powerful presses cannot reducethe level of residual oil in the press-cake below 3 to 5%. In the caseof oil-rich seeds such as sesame or peanuts this may still beacceptable. Furthermore, most of the oil left in the cake can berecovered by a stage of solvent extraction. Such two stage
processes (pre-press/solvent extraction) are now widely applied . Inthe case of soybeans, however, a 5% residual oil level in the cakerepresents an oil loss of about 25%. Solvent extraction of the cakewould not be economical, because of the bulk of material which mustbe processed.
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The quality of the meal is therefore a factor of
particular importance in the selection of aprocessing method for soybeans. In thisrespect, the expeller process has severaldisadvantages. The first is the poor storage
stability of the press-cake, due to its high oilcontent. Furthermore,the extremetemperatures prevailing in the expeller mayimpair the nutritive value of the meal protein,mainly by reducing the biological availability ofthe amino acid lysine. At any rate, expellerpress-cake is not suitable for applicationsrequiring a meal with high protein solubility.
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Crude oil production
mechanical expression)
Cooked flakes
Screw press
Crude oil + seed particles Cake
(4-6%oil)
Crude oil seed particles
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Crude oil production
pre-pressing extraction + solvent extraction)
Cooked flakes
Screw press
Crude oil Oily cake
(10-16% oil)
Cake (0.5% oil) Solvent extraction
Crude oil
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Solvent extraction
solid-liquid extraction-leaching)
The lowest levels of residual oil after
pressing are 3-8%; exhaustive removal of
the oil present in the cake by mechanicalmeans alone is imposible. The residual oil
in cake , therefore, only be removed by a
different approach, this being solvent aided
extraction.
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Single stage leaching
Seed flakes +
solvents
flakes L0
Solvents V0
Cake L1
miscella V1
L0 + V0 = L1 + V1
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Ideal equilibrium
A + C
B B + C
A ; B + C
A : inert solid
B : solvent
C : oil
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Basic principles of solvent extraction:The
extraction of oil from oilseeds by means of
non-polar solvents is, basically, a processof solid-liquid extraction. The transfer of oil
from the solid to the surrounding oil-
solvent solution ( miscella ) may be dividedinto three steps:
*diffusion of the solvent into the solid
*dissolution of the oil droplets in thesolvent
*diffusion of the oil from the solid particle
to the surrounding liquid
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Due to the very high solubility of the oil in
the commonly used solvents, the step ofdissolution is not a rate limiting factor. Thedriving force in the diffusional processesis, obviously, the gradient of oilconcentration in the direction of diffusion.Due to the relative inertness of the non-oilconstituents of the oilseed, equilibrium isreached when the concentration of oil inthe miscella within the pores of the solid isequal to the concentration of oil in the freemiscella, outside the solid. Theseconsiderations lead to a number ofpractical conclusions:
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* Since the rate-limiting process is diffusion, much
can be gained by reducing the size of the solid
particle. Yet, the raw material cannot be ground
to a fine powder, because this would impair the
flow of solvent around the particles and would
make the separation of the miscella from thespent solid extremely difficult. The oilseeds are
rolled into thin flakes, thus reducing one
dimension to facilitate diffusion, without
impairing too much the flow of solvent throughthe solid bed or contaminating the miscella with
an excessive quantity of fine solid particles.
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The effect of flake thickness on the efficiency of
solvent extraction
Diffussion extraction
Solution extraction
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*The rate of extraction can be increased
considerably by increasing the
temperature in the extractor. Highertemperature means higher solubility of the
oil, higher diffusion coefficients and lower
miscella viscosity.
*An open, porous structure of the solid
material is preferable, because such a
structure facilitates diffusion as well as
percolation. A number of processes havebeen proposed for increasing the porosity
of oilseeds before solvent extraction.
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*Although most of the resistance to
mass transfer lies within the solid, therate of extraction can be increased
somewhat by providing agitation and
free flow in the liquid phase aroundthe solid particles. Too much agitation
is to be avoided, in order to prevent
extensive disintegration of the flakes.
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Choice of solvents:An ideal solvent for the extraction of oil from oil seeds should possess
the following properties:
* Good solubility of the oil.
* Poor solubility of non-oil components.* High volatility (i.e. low boiling point), so that complete removal of
the solvent from the miscella and the meal by evaporation is
feasible and easy.* Yet, the boiling point should not be too low, so that extraction can
be carried out at a somewhat high temperature to facilitate mass
transfer.* Low viscosity.* Low latent heat of evaporation, so that less energy is needed for
solvent recovery.* Low specific heat, so that less energy is needed for keeping the
solvent and the miscella warm.
* The solvent should be chemically inert to oil and othercomponents of the seed flakes.
* Absolute absence of toxicity and carcinogenicity, for the solvent
and its residues.* Non-inflammable, non-explosive.* Non-corrosive
* Commercial availability in large quantities and low cost.
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Types of Extractors
Solvent extractors are of two types:
batch
continuous
In batch processes, a certain quantity of
flakes is contacted with a certain volume
of fresh solvent. The miscella is drained
off, distilled and the solvent is
recirculated through the extractor untilthe residual oil content in the batch of
flakes is reduced to the desired level.
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Batch extractor
solvent
cake
miscella
Seed flakes
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In continuous extraction, both the
oilseeds and the solvent are fed intothe extractor continuously. The
different available types are
characterized by their geometrical
configuration and the method by which
solids and solvents are moved one in
relation to the other, in counter-current
fashion.
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Two different methods can be used to
bring the solvent to intimate contact with
the oilseed material:
percolation
immersion
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In the percolation method, the solventtrickles through a thick bed of flakes
without filling the void space completely.A film of solvent flows rather rapidlyover the surface of the solid particlesand efficiently removes the oil which has
diffused from the inside to the surface.This mode of contact is preferablewhenever the resistance to diffusion
inside the flake is relatively low (thinflakes with large surface area, opentissue structure).
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In the immersion mode,the solid particles
are totally immersed in a slowly moving,
continuous phase of solvent. Immersion
works better with materials offering a
greater internal resistance to oil transfer(thick particles, dense tissue structure).
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Percolation type extractor
Belt extractors_(DE SMET extractor);The extractorconsists of a horizontal, sealed vessel in which a
slowly moving screen belt is installed. Flaked oilseeds are fed on the belt by means of a feedinghopper. A damper attached to the hopper outletacts as a feed regulating valve and maintains thesolids bed on the belt at constant height. This
height can be adjusted according to the expectedrate of percolation of the miscella through the bed.Difficult percolation is compensated for bylowering bed height. The throughput rate of theextractor is adjusted by changing the belt speed.There are no dividing baffles on the belt andthe solid bed is one continuous mass. Yetthe extractor is divided to distinct extractionstages by the way in which the miscellastream is advanced.
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The solvent is introduced at the spent flakedischarge end. It is sprayed on the flakes,percolates through the bed, giving the spentflakes a last wash and removing some oil.The resulting dilute micella is collected in asectional hopper underneath the belt, from
which it is pumped and sprayed again onthe flakes at the next section in the directionopposite to belt movement. This process ofmiscella collection, pumping and spraying
at the next section is repeated until themiscella leaves the hopper at the head-endof the extractor, carrying the highestconcentration of oil (heavy miscella).
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Belt Extractor DeSmet)
Seed flakesPure solvent
Miscella
Cake (0.5% oil)Full miscella 25% oil
high oil
seed
flakes
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Continuous horizontal extractor
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Basket type- Sliding cell extractor (Lurgi)
In this class of extractors, the flakes do not constitute a continuous mass but are
filled into separate, delimited elements (baskets) with perforated bottoms fordraining. The baskets can be moved vertically (bucket elevator extractors),
horizontally ( frame belt and sliding cell extractors), or can be rotated around a
vertical axis (roto-cell extractors). Vertical bucket-chain extractors are among the
first industrial solvent extractors constructed for continuous operation. Many are
still in operation but they are less frequently found in more recent installations.
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Sliding cell extractor (Lurgi)
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Roto-cell extractor
Reflex extractor-DeSmet)
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Hildebrandt extractor
immersion type)
The solid material is extracted according to
the immersion method. Screw conveyors
are installed in the extractor fortransporting the solid material. Again the
solvent flows countercurrent to the solid
materials through the extractor.
Hildebrandt extractor
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Hildebrandt extractor
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Extraction unit (DeSmet)
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Post-extraction operations
Two streams leave the solvent extraction stage ;
an oil-rich fluid extract(full miscella)
cakemeal(spent flakes)The next operations have the objective ofremoving and recovering the solvent from each
one the two streams.
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a.Miscella distillation:Full miscella contains typically30% oil. Thus, for every ton of crude oil some 2.5 tons ofsolvent must be removed by distillation. Most
manufacturers of solvent extractors also offer miscelladistillation systems.
The characteristics of a good miscella distillationsystem are:
good energy economy,
minimal heat damage to the crude oil and itscomponents,
minimal solvent losses ,
efficient removal of the last traces of solvent from
the oil good operation safety.
The modes of solvent vaporization include flashevaporation, vacuum distillationand steam stripping.
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Miscella filtration ;Because of the quality criteria
for crude oils, but also to ensure the least
possible fluid transport defects (clogging inpumps, pipes etc.) and heat transfer
resistances, the miscella must be freed of
solide meal particles with special closed filter
presses before proceeding to distillation.Miscella distillation ; Distillation is the most
energy consuming part of the total extraction
process.In general the evaporation is carried
out in two or three stages, mostly in longtubetype evaporators with a vapor head.
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b. Meal desolventizing:The spent flakescarry with them about 35% solvent. The
removal and recovery of this portion of
the solvent is also one of the most
critical operations in oil mill practice,since it determines, to a large extent, the
quality of the meal and its derivatives.
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The most common type of desolventizer-
toaster consists of a vertical cylindrical
stack of compartments or "pans". Eachcompartment is fitted with stirrers or racks
attached to a central vertical shaft. Spent
flakes are fed at the top of thedesolventizer-toaster. The pan floors are
equipped with adjustable-speed rotating
valve, to permit downward movement ofthe material , through the pans, at the
desirable rate.
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Direct steam is used for three reasons:
*The transfer of heat from the heated surface of the panfloor to the oilseed material is slow and difficult,
especially after a considerable proportion of the solventhas been removed and no fluid medium is available forheat transfer. In this case, direct contact between thesolid material and condensing steam is a more efficientmethod of heating. Condensation of the steam adds
moisture to the flakes.* The added moisture facilitates the protein denaturation
reactions leading to the inactivation of trypsin inhibitor(for soybean cake). It is also believed that the toastingeffect accomplished by the combined action of heat and
moisture enhances the palatability of the meal to animals.*The steam distillation effect is necessary in order to
remove last traces of solvent from the meal.
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Desolventizer
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Soybean oil production
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Sunflower oil production
Sunflower seed
Cleaning foreign matter
Dehulling hulls
Flaking
Cooking
Pressing crude oil
Oily cake
Solvent extraction cake l + solvent
Miscella Toaster solvent
Distllation solvent Cake
Crude oil
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Cottonseed oil production
Cottoseed
Cleaning foreign matter
Delinting lints
Dehulling hulls
Flaking
Cooking
Pressing crude oil
Oily cake
Solvent extraction cake l + solvent
Miscella Toaster solvent
Distllation solvent Cake
Crude oil
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Rapeseed oil production
Rape seed
Cleaning foreign matter
Flaking
Cooking
Pressing crude oil
Oily cake
Solvent extraction cake l + solvent
Miscella Toaster solvent
Distllation solvent Cake
Crude oil