OIL AND FAT TECHNOLOGY LECTURES III (Crude Oil Production)

Prof.Dr.Ayta SAYGIN GMKESENYrd.Do.Dr.Fahri YEMOLU

Extraction of Vegetable Oils

Basic approaches :Mechanical Oil Extraction - cold pressing means no heat applied - hot pressing - external heat is appliedSolvent Extraction - organic solvent (hexane, isopropyl alchool) - supercritical solvent (carbondioxide)

Mechanical oil extractionMechanical 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.

Screw presses;In oil industry, screw presses (expellers) are mostly utilized for expression. The main parts of continous-screw press are;Seed feeder,Cone-shaped cageAdjustable cone for press-cake outletWorm (pressure and feed)

Screw pressCooked seed flakesCrude oil Main worm shaftknifecakeAdjustable cone for press-cake outletCone shaped pressure cage

The seeds enter the barrel and falls on the helical pressure worm.During movement in the barell, between worm and cage is gradually reduced and the 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.

Screw Press

Shaft Arrangement-- Screw Press

Cage ArrangementFrench Press

Advantages and disadvantages of the expeller process;

Expellers can be used with almost any kind of oilseeds and nuts. The process is relatively simple and not capital-intensive. While the smallest solvent extraction plant would have a processing capacity of 100-200 tons per day, expellers are available for much smaller capacities, from a few tons per day and up.

The main disadvantage of the screw-press process is its relatively low yield of oil recovery. Even the most powerful presses cannot reduce the level of residual oil in the press-cake below 3 to 5%. In the case of oil-rich seeds such as sesame or peanuts this may still be acceptable. Furthermore, most of the oil left in the cake can be recovered by a stage of solvent extraction. Such two stage processes (pre-press/solvent extraction) are now widely applied . In the case of soybeans, however, a 5% residual oil level in the cake represents an oil loss of about 25%. Solvent extraction of the cake would not be economical, because of the bulk of material which must be processed.

The quality of the meal is therefore a factor of particular importance in the selection of a processing method for soybeans. In this respect, the expeller process has several disadvantages. The first is the poor storage stability of the press-cake, due to its high oil content. Furthermore,the extreme temperatures prevailing in the expeller may impair the nutritive value of the meal protein, mainly by reducing the biological availability of the amino acid lysine. At any rate, expeller press-cake is not suitable for applications requiring a meal with high protein solubility.

Crude oil production (mechanical expression)Cooked flakes

Screw press

Crude oil + seed particles Cake (4-6%oil)

Crude oil seed particles

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

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 mechanical means alone is imposible. The residual oil in cake , therefore, only be removed by a different approach, this being solvent aided extraction.

Single stage leachingSeed flakes +solvents flakes L0Solvents V0 Cake L1 miscella V1 L0 + V0 = L1 + V1

Ideal equilibrium

A + CBB + C A ; B + C A : inert solidB : solventC : oil

Basic principles of solvent extraction: The extraction of oil from oilseeds by means of non-polar solvents is, basically, a process of solid-liquid extraction. The transfer of oil from the solid to the surrounding oil-solvent solution ( miscella ) may be divided into three steps: * diffusion of the solvent into the solid * dissolution of the oil droplets in the solvent * diffusion of the oil from the solid particle to the surrounding liquid

Due to the very high solubility of the oil in the commonly used solvents, the step of dissolution is not a rate limiting factor. The driving force in the diffusional processes is, obviously, the gradient of oil concentration in the direction of diffusion. Due to the relative inertness of the non-oil constituents of the oilseed, equilibrium is reached when the concentration of oil in the miscella within the pores of the solid is equal to the concentration of oil in the free miscella, outside the solid. These considerations lead to a number of practical conclusions:

* 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 the spent 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 through the solid bed or contaminating the miscella with an excessive quantity of fine solid particles.

The effect of flake thickness on the efficiency of solvent extraction

Diffussion extractionSolution extraction

* The rate of extraction can be increased considerably by increasing the temperature in the extractor. Higher temperature 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 have been proposed for increasing the porosity of oilseeds before solvent extraction.

* Although most of the resistance to mass transfer lies within the solid, the rate of extraction can be increased somewhat by providing agitation and free flow in the liquid phase around the solid particles. Too much agitation is to be avoided, in order to prevent extensive disintegration of the flakes.

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 other components 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.

A typical commercial solvent for oil extraction would have a boiling point range (distillation range) of 65 to 70oC and would consist mainly of six-carbon alkanes, hence the name "hexane by which these solvents are commonly used in oil extraction.

The quality parameters which make up the specifications usually include: boiling (distillation) range, maximum non-volatile residue, flash point, maximum sulphur, maximum cyclic hydrocarbons, colour and specific gravity.

Types of ExtractorsSolvent extractors are of two types: batchcontinuousIn 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 until the residual oil content in the batch of flakes is reduced to the desired level.

Batch extractor solventcakemiscella Seed flakes

In continuous extraction, both the oilseeds and the solvent are fed into the 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.

Two different methods can be used to bring the solvent to intimate contact with the oilseed material:

percolation immersion

In the percolation method, the solvent trickles through a thick bed of flakes without filling the void space completely. A film of solvent flows rather rapidly over the surface of the solid particles and efficiently removes the oil which has diffused from the inside to the surface. This mode of contact is preferable whenever the resistance to diffusion inside the flake is relatively low (thin flakes with large surface area, open tissue structure).

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).

Percolation type extractorBelt extractors_(DE SMET extractor); The extractor consists of a horizontal, sealed vessel in which a slowly moving screen belt is installed. Flaked oil seeds are fed on the belt by means of a feeding hopper. A damper attached to the hopper outlet acts as a feed regulating valve and maintains the solids bed on the belt at constant height. This height can be adjusted according to the expected rate of percolation of the miscella through the bed. Difficult percolation is compensated for by lowering bed height. The throughput rate of the extractor is adjusted by changing the belt speed. There are no dividing baffles on the belt and the solid bed is one continuous mass. Yet the extractor is divided to distinct extraction stages by the way in which the miscella stream is advanced.

The solvent is introduced at the spent flake discharge end. It is sprayed on the flakes, percolates through the bed, giving the spent flakes a last wash and removing some oil. The resulting dilute micella is collected in a sectional hopper underneath the belt, from which it is pumped and sprayed again on the flakes at the next section in the direction opposite to belt movement. This process of miscella collection, pumping and spraying at the next section is repeated until the miscella leaves the hopper at the head-end of the extractor, carrying the highest concentration of oil (heavy miscella).

The screen is washed with heavy miscella at the head-end, just before the entrance of fresh flakes, and then again with fresh solvent, right after the discharge of spent flakes.Washing of the screen is essential to prevent clogging. Washing with full miscella at the feed-end provides surface lubrication and prevents adhesion of the flakes to the surface of the screen. The entire extractor vessel is maintained at a slight negative pressure so as to prevent leakage of solvent vapours to the atmosphere.

Belt Extractor (DeSmet) Seed flakes Pure solvent MiscellaCake (0.5% oil)Full miscella 25% oilhigh oil seed flakes

Continuous horizontal extractor

Bollmann extractor

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 for draining. 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.

Sliding cell extractor (Lurgi)

Roto-cell extractor (Reflex extractor-DeSmet)

Rotocell extractor

Hildebrandt extractor(immersion type)

The solid material is extracted according to the immersion method. Screw conveyors are installed in the extractor for transporting the solid material. Again the solvent flows countercurrent to the solid materials through the extractor.

Hildebrandt extractor

Extraction unit (DeSmet)

Post-extraction operations Two streams leave the solvent extraction stage ;an oil-rich fluid extract (full miscella) cake meal (spent flakes) The next operations have the objective of removing and recovering the solvent from each one the two streams.

a.Miscella distillation: Full miscella contains typically 30% oil. Thus, for every ton of crude oil some 2.5 tons of solvent must be removed by distillation. Most manufacturers of solvent extractors also offer miscella distillation systems.

The characteristics of a good miscella distillation system are: good energy economy, minimal heat damage to the crude oil and its components, minimal solvent losses , efficient removal of the last traces of solvent from the oil good operation safety.

The modes of solvent vaporization include flash evaporation, vacuum distillation and steam stripping.

Miscella filtration ; Because of the quality criteria for crude oils, but also to ensure the least possible fluid transport defects (clogging in pumps, 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 longtube type evaporators with a vapor head.

b. Meal desolventizing: The spent flakes carry 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.

The most common type of desolventizer-toaster consists of a vertical cylindrical stack of compartments or "pans". Each compartment is fitted with stirrers or racks attached to a central vertical shaft. Spent flakes are fed at the top of the desolventizer-toaster. The pan floors are equipped with adjustable-speed rotating valve, to permit downward movement of the material , through the pans, at the desirable rate.

Two methods of heating are used: direct steam heating indirect steam heating

For heating with indirect steam, the pans are equipped with double bottoms acting as steam jackets. For direct steam heating, hot live steam is injected into the mass through spargers. The rotating stirrers spread the material and provide the necessary mixing action.

Direct steam is used for three reasons:* The transfer of heat from the heated surface of the pan floor to the oilseed material is slow and difficult, especially after a considerable proportion of the solvent has been removed and no fluid medium is available for heat transfer. In this case, direct contact between the solid material and condensing steam is a more efficient method 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 toasting effect 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.

Desolventizer

Soybean oil production

Sunflower oil productionSunflower 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

Cottonseed oil productionCottoseed

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

Rapeseed oil productionRape seed

Cleaning foreign matter

Flaking

Cooking

Pressing crude oil

Oily cake

Solvent extraction cake l + solvent

Miscella Toaster solvent

Distllation solvent Cake

Crude oil