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In this section, the processes for the separation of the protein (meal) fraction from the oil will be reviewed. At present, there are only two competitive oil-milling processes for soybeans: the screw-press (expeller) process and extraction by solvents.
3-1 The expeller (screw press) process)
3-1-1 Operation principles
Continuous pressing by means of expellers (also known as screw presses) is a widely applied process for the extraction of oil from oilseeds and nuts . It replaces the historical method for the batchwise extraction of oil by mechanical or hydraulic pressing. The expeller (Fig. 7) consists of a screw (or worm), rotating inside a cylindrical cage (barrel). The material to be pressed is fed between the screw and the barrel and propelled by the rotating screw in a direction parallel to the axis. The configuration of the screw and its shaft is such that the material is progressively compressed as it moves on, towards the discharge end of the cylinder. The compression effect can be achieved, for example, by decreasing the clearance between the screw shaft and the cage ( progressive or step-wise increase of the shaft diameter ) or by reducing the length of the screw flight in the direction of the axial movement The gradually increasing pressure releases the oil which flows out of the press through the slots provided on the periphery of the barrel, while the press-cake continues to move in the direction of the shaft, towards a discharge gate installed at the other extremity of the machine.
Before entering the expeller, the oilseeds must be cleaned, dehulled (optional), flaked, cooked and dried. Flaking facilitates oil release in the press by decreasing the distance that the oil will have to travel to reach the particle surface. Cooking in the presence of moisture is essential for the denaturation of the proteins and, to some degree, for the coalescence of the oil droplets. Cooking plasticizes the flakes, renders them less brittle and thus reduces the extent of flake disintegration as a result of shear in the press. Extensive flake disintegration would reduce oil yield and produce a crude oil with a high content of fine solid particles (foots). After cooking, excess moisture is removed in order to avoid the formation of muddy emulsions in the press. Cooking is usually achieved by mixing the flakes with live steam. Additional heat may be provided by indirect steam, while thoroughly mixing the mass.3-1-2 Advantages and disadvantages of the expeller process
Expellers can be used with almost any kind of oilseeds and nuts. Therefore, in a multi-purpose plant built to process different types of raw materials and not only soybeans, the expeller process may prove advantageous. 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. Pre-press/solvent extraction processes are, therefore, not applied to soybeans.
It will be recalled from Chapter 1, that in the case of soybeans, the commercial value of the meal is usually higher than the income from sales of the corresponding quantity of oil. 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.3-1-3 Equipment
Unlike solvent extraction equipment which is supplied by a relatively small number of manufacturers, screw presses with a widely varying degree of sophistication are available from a multitude of sources. Yet, considerable technical improvement and advanced features can be found in the models offered by the leading manufacturers. Such features include: multi-stage pressing to increase oil yield, better feed rate control, water cooled barrel and shaft, ease of maintenance and repair, improvements in the drive and transmission, sanitary construction, safety features etc. Most press manufacturers also supply cooker-dryer units, designed to operate with the press. Cooker-dryers may be horizontal (jacketed screw conveyor type), but the most common types consist of vertical stacks of round chambers (rings) equipped with paddle stirrers. (Fig. 7)
Fig 7: Oil expeller (screw press) with cooker-dryer
This design is indeed very common in operations for heat treating oilseed material and will be encountered in flake conditioners, desolventizers, meal dryers and coolers.
3-2 The solvent extraction process3-2-1 Operation principles
A flow-diagram describing the solvent extraction process for soybeans is given in Fig.8. The process consists of the following stages:
a- Receiving and storage of soybeans.
b- Preparation of the raw material for extraction.
c- Solvent extraction.
d- Recovery of the solvent from the extract (micella).
e- Desolventizing/toasting of the meal.
Figure 8: Solvent extraction of soybean oil; flow diagram
3-2-2 Receiving and storage of soybeans
Nowadays, soybeans are received at the factory, almost exclusively in bulk, by truck or rail. They are weighed, unloaded and conveyed to the main storage silos. The size of the silos depend on the frequency of reception and the availability of other storage facilities in the region. Normally the main storage volume should correspond to the raw materials needed for a few months of operation at full capacity.
Pneumatic conveying is used in large installations while mechanical conveyors and elevators are more common in smaller plants. It is extremely important to maintain good sanitary conditions on and around the receiving areas and especially, to protect the seeds from contact with moisture. The receiving area, which consists of outdoors installations with a fair amount of movement of people and vehicles, tends to be one of the most critical parts of the factory, from the sanitation point of view.
As soybeans are purchased by grade, it is necessary to draw representative samples for quality evaluation from each lot at the point of reception. The samples are analyzed for moisture, foreign materials, colour, broken beans etc. in order to determine the compliance of the lot with the specified grade criteria. It is also advisable to determine oil and protein content, free fatty acids and other quality factors for the sake of proper bookkeeping, even if these criteria are not part of the standard grading and pricing system.
The typical storage facility in soybean oil plants is the vertical cylindrical silo. In recent years the conventional concrete silo is being replaced by steel silos of different types. A recent innovation in this area is a silo construction method based on the use of a steel strip wound in the form of a continuous spiral, each winding being fastened to the next one by crimping. The steel strip is supplied as compact coils, thus reducing the cost of transportation of bulky pre-fabricated constructions. One of the advantages of the metal silos is the speed of erection.3-2-3 Preparation for extraction
This stage comprises drying, tempering, cleaning, classification (optional), cracking, dehulling (optional), conditioning and flaking. A flow diagram for the conventional preparation of soybeans prior to solvent extraction is given in Fig.9.
Figure 9: Conventinal Preparation
System for Soybeans
a- Drying: If the soybeans are to be dehulled before extraction, they must be dried to a moisture content below 10% in order to facilitate separation of the hulls. This is achieved in vertical gas or oil fired forced circulation driers. If the natural moisture content of the beans is 10% or less, or if dehulling is not practised, drying as a preparation step can be omitted.
b- Tempering: After cooling, the dried soybeans are stored in bins for 2 to 5 days, in order to allow for moisture equilibration by diffusion. This is called tempering. The tempering bins, which are usually outdoors silos of the vertical type, also serve as working bins (day bins), to secure uninterrupted feeding of the plant. As all the subsequent steps of processing are continuous, it is necessary to monitor the flow of soybeans from the working bins to the processing plant, in accordance with the planned processing capacity. This is done by means of automatic balances installed at the feed-end of the line.
c: Cleaning: The soybeans are subjected to a number of cleaning operations throughout the process. Tramp iron is removed by magnetic separators. In moderate capacity installations these can be magnets attached to conveyors or chutes carrying a stream of beans. For larger plants, revolving drum type magnets which permit continuous removal of tramp iron from magnet surface are used.Both permanent magnets and electromagnets can be used. Permanent magnets have the advantage of being practically maintenance-free. Furthermore, they do not consume electrical power. Since the beans may become re-contaminated with stray iron (loose nuts and bolts, nails etc.) as they pass through the machinery, magnetic cleaning is not a one-time operation but must be repeated several times along the line. It is therefore advisable to install magnetic separators at the entrance of each machine where the presence of metal particles may cause serious damage (cracking mills, flaking machines etc.)
Stones, sand, dust and other foreign materials are usually removed by conventional seed cleaners. Typically, the seed cleaner consists of a two-deck vibrating screen.(Fig. 9). The upper screen retains the stones and other coarse materials but allows whole soybeans to fall through. The lower screen retains the soybeans but lets finer particles such as sand to pass through. Light trash, free hull particles and dust are removed by aspiration and trapped in cyclones.
d: Classification: The purpose of this operation is to separate split beans from whole beans. This step is optional and it is applied only if the meal is to be processed for human consumption. Classification is carried-out by a simple sifting operation.
e: Cracking: The purpose of this operation is to break the seeds into smaller particles in preparation for flaking. If the beans have been dried to 10% moisture and tempered as described above, cracking also loosens the hulls and permits their separation by aspiration. Ideally, the seeds should be broken to 4 to 6 pieces of fairly uniform size. Production of fines should be minimized. Cracking machines consist of pairs of counter-rotating, corrugated rolls. One roll in each pair rotates faster than the other, to provide the shearing effect necessary to break the seed. Roll diameter is in the order of 25 cm. Roll length depends on the capacity. Two or three pairs of rolls are provided, mounted one on top of the other. A vibrating conveyor secures feeding of the mill at a uniform rate. The corrugations on the upper pair of rolls are coarser and deeper than those on the lower pairs.
A vibrating screen is provided at the exit from the mill. This is where the stream of broken particles is separated into hulls (removed by aspiration for further processing), oversize particles (returned to the cracking mill), meats of the correct size (sent to conditioning and flaking) and fines (usually mixed with the meats for conditioning).
The surface of cracking rolls is subject to considerable wear. After a certain service period, it may be necessary to renew the corrugations (refluting). Good quality rolls may be refluted several times before it becomes necessary to replace them.
Figure 10: Seed cleaner with multiaspirator and cyclone
f: Conditioning: The purpose of this operation is to increase the plasticity of the meats, in preparation for flaking. The conditioner is similar to the cooker described in connection with expellers. It can be a horizontal screw conveyor type heated reactor or a vertical stacked cooker. Heat can be provided by indirect steam or by direct steam injection, the latter being used to increase the moisture content when necessary. The meats are heated to 65-70oC and the moisture content is brought to 10.5-11%. At this point the plasticity of the meats is such that they can be flattened by pressure in the flaker, without breaking.
g: Flaking: Flaking machines consist of a pair of horizontal counter-rotating smooth steel rolls. Typical roll sizes are in the range of 60-80 cm. in diameter. The rolls are pressed one against the other by means of heavy springs or by controlled hydraulic systems. Conditioned soybean cotyledon particles are fed between the rolls and they are flattened as the rolls rotate one against the other. The roll-to-roll pressure can be regulated and it determines the average thickness of the flakes. The main purpose of flaking is to increase the contact surface between the oilseed tissues and the solvent, and to reduce the distance that the solvent and the extract will have to travel in the process of extraction. It is also believed that flaking disrupts the oilseed cells to some degree and thus makes the oil droplets more available for solvent extraction. Typical values for flake thickness are in the range of 0.2 to 0.35 millimetres.
Flaking rolls require maintenance as they wear considerably. To maintain the smoothness of the surfaces and to secure good contact between the rolls at every point, the rolls are reground from time to time. This requires expertise and accurate machines. In order to compensate for uneven thermal expansion, the rolls are manufactured not as perfect cylinders but with a slightly curved profile, thinner at both ends and thicker in the middle. Furthermore the wear is usually not uniformly distributed and tends to be more extensive at the middle. Some manufacturers supply grinding devices which allow the roll ends to be reground without removal of the rolls.
h: Alternative processes: The processes described above are conventional oil-mill operations. Recently, improved processes have been suggested for individual steps or for the whole seed preparation line.
The " Hot Dehulling (Popping) System ", offered by Buhler-Miag Ltd. makes use of a "shock treatment" to loosen the hulls.
Soybeans with a moisture content of about 13% are preheated to 60oC, then contacted with a stream of hot air in a fluidized bed unit. This treatment causes popping of the hull. Now the seeds are split in half by impact and the hulls are separated by air. The dehulled split beans are further cracked and flaked. The main advantage of the process is its lower energy consumption, since the multiple heating and cooling, drying and humidification steps of conventional dehulling are obviated. The short duration of the heat treatment step prevents extensive protein denaturation. The reduction in NSI (Nitrogen Solubility Index) is claimed to be essentially the same as in conventional dehulled flake preparation. A process flow-diagram for the Hot Dehulling System is given in Fig.11.
Figure 11: The BUHLER Hot Dehulling (Popping) System
The "Alcon Process" (Fig. 12) offered by Lurgi GmbH, consists of a series of operations installed between the conventional preparation line (right after the flaking mill) and the extractor. The flakes are humidified and heated in a conditioner, maintained at the desired moisture content and temperature for 15-20 minutes (tempering), then dried and cooled before being led to the extractor. This is, essentially, an agglomeration process, whereby the flakes are fused into more compact, porous granules. The following benefits are claimed:
a: The bulk density of the modified granules is by 50% higher than that of the original flakes (550 against 360 kg/m3). This results in a corresponding increase in extractor capacity.
b: The rate of percolation of micella or solvent through the granules is tripled. This results in improved extractor efficiency (see below).
c: Solvent retention in the spent granules is about 25%, while conventional spent flakes may retain as much as 35% solvent. As a result, desolventizer capacity is increased, oil yield is improved and energy is saved.
d: During preparation and extraction, certain enzymes reduce the hydratability of the phospholipids. The thermal treatment associated with the Alcon process inactivates these enzymes and improves the efficiency and yield of the oil degumming process.
e: Due to the thermal treatment mentioned above, meal toasting requirements are less severe.
Figure 12: LURGI's "Alcon" Process ef pre-extraction preparation
In the Pellet method suggested by the FRENCH Oil Mill Machinery Company, the crushed material is extruded as pellets. The extruder which is called "the Enhanser Press", is equipped with special ports for the injection of steam or water into the barrel. The mass is pressed through the holes on a die plate, expands as a result of the sudden evaporation of water and yields firm pellets with sufficient internal porosity but a bulk density higher than that of flakes. The advantages claimed are essentially the same as those of the other agglomeration processes.
A drawing showing the FRENCH Enhancer Press is given in Fig. 13.
Figure 13: The FRENCH "Enhancer" Press
a- 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 ( micella ) 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 micella within the pores of the solid is equal to the concentration of oil in the free micella, 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 micella from the spent solid extremely difficult. Instead, the oilseeds are rolled into thin flakes, as described in the previous paragraph, thus reducing one dimension to facilitate diffusion, without impairing too much the flow of solvent through the solid bed or contaminating the micella with an excessive quantity of fine solid particles. The effect of flake thickness on the efficiency of solvent extraction is demonstrated in Fig.14.
* 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 micella viscosity. In fact, it is customary to heat the solvent and the intermediate micella to the highest temperature which would still provide an acceptable level of safety.
* 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 (See para. 3-2-3-h ).
* 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.
Figure 14: Effect of Flake Thickness on Extraction Efficiency
* Since the concentration gradient is the factor responsible for moving the oil out of the solid, it is important to keep this gradient high, at each point within the extractor. This effect is obtained most economically by the principle of counter-current multistage extraction. The process is divided to a number of contact stages . Each stage comprises means for mixing the solid and the solvent phases and for separating the two streams after extraction has been achieved. In going from one stage to the next, the flakes and the solvent move in opposite directions. Thus, flakes with the lowest oil content are contacted with the leanest solvent, resulting in high oil yield and high driving force throughout the extractor. The principle of counter-current extraction is shown in Fig.15.
Figure 15 : Principle of Counter-current Extraction Applied to the "Carrousel Extractor"
A detailed discussion of the theoretical basis for the design of multistage solid-liquid extraction processes is beyond the scope of the present work. We shall outline here only its principal practical consequences, as far as they provide useful criteria for the selection and operation of an extractor.
Two different methods can be used to bring the solvent to intimate contact with the oilseed material: percolation and flooding. 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 flooding 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).
The number of contact stages necessary to perform a given extraction operation depend on the following variables:
* Flakes/solvent ratio: If the quantity of solvent used to extract oil from one ton of flakes is increased, a smaller number of contact stages will be needed to achieve a given extraction job. However, the full micella resulting from the process would be less concentrated in oil, meaning that we would have to evaporate larger quantities of solvent for each ton of product, and hence, spend more on energy.
* Oil yield: If the number of stages is increased while all other variables are kept unchanged, the proportion of oil left in the spent flakes will be lower and therefore,the oil yield will be higher. The relationship between the number of stages and residual oil in the meal is shown in Fig. 16.
Figure 16: Effect of Number of Stages on Residual Oil
* Percolation: The quantity of solvent or micella retained within the capillaries and pores of the solid after drainage is called "bound extract" or "bound solvent". This quantity depends on the properties of the flakes and solvent as well as the drainage conditions. Easy percolation of the solvent through the solid bed leaves less extract in the capillaries after drainage and results therefore, in a reduction of the number of contact stages needed. Proper preparation and handling of the flakes are important to ensure high percolation rate.
b- Choice of solvents:
An ideal solvent for the extraction of oil from soybeans 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 micella 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 ant the micella warm.
* The solvent should be chemically inert to oil and other components of the soybean.
* Absolute absence of toxicity and carcinogenicity, for the solvent and its residues.
* Non-inflammable, non-explosive.
* Commercial availability in large quantities and low cost.
Unfortunately, the ideal solvent possessing all these properties does not exist. Most of the requirements, with the notable exception of flammability and explosiveness, are met by low-boiling hydrocarbon fractions obtained from petroleum. 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 known in the U.S.A.. "Hexane " solvents for the extraction of edible oil must comply with strict quality specifications. 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.
The main shortcoming of light hydrocarbon solvents is their flammability and the explosiveness of mixtures of their vapours and air. Safety considerations gave led to the enforcement of special standards for buildings and installations in solvent extraction plants. All the electrical installations have to be explosion-proof. The discharge end of all vents have to be equipped with refrigerated condensers to minimize escape of solvent vapours to the atmosphere. Very strict safety measures are taken to prevent the hazard of sparks in and around the plant. All these add to the high cost of erection and operation of solvent extraction plants.Even so, accidents are not uncommon.
The continuous search for alternative solvents is, therefore understandable. One such solvent, trichloroethylene, was in commercial use for a short period in the early 1940's, but had to be abandoned when it was discovered that the meal prepared in this way was toxic to animals. Another alternative approach makes use of "supercritical extraction" with liquid carbon dioxide under high pressure. Although technically feasible, supercritical extraction of soybean oil is not commercially viable at present, due to the high cost of the equipment and the relatively poor oil dissolving capacity of carbon dioxide near its critical point. Alcohols constitute yet another class of potential solvents for oil extraction. Water-free (absolute) low aliphatic alcohols such as ethanol and isopropanol are fairly good solvents for oils at high temperature but the solubility of oils in these solvents decreases drastically as the temperature is lowered. This high dependence of solubility on temperature is precisely the principle on which alcohol extraction processes are based. Extraction takes place at high temperature. The micella is then cooled. Saturation occurs and excess oil separates as a distinct phase which can be recovered by centrifugation. The solvent is reheated and sent back to the extractor. These alcohols are less flammable then hexane, but precautions are still necessary. Despite considerable research efforts to develop alternative solvent systems, extraction with light hydrocarbons continues to be, practically, the only commercial solvent extraction process for soybean oil.
c- Types of extractors:
Solvent extractors are of three types: batch, semi-continuous and continuous.
In batch processes, a certain quantity of flakes is contacted with a certain volume of fresh solvent. The micella 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 extractors as industrial units are now obsolete. Laboratory and pilot plant size extractors are still used for experimentation and instruction purposes.
Semi-continuous systems consist of several batch extractors connected in series. The solvent or micella flows from one extractor to the next one in the series. The material in the first extractor is the most exhausted, since it has been treated with fresh solvent. After a while, the second extractor is made "head" of the series and connected to the fresh solvent line. The spent flakes are discharged from the first extractor, which is then filled with a batch of fresh flakes and is connected to the system as the "tail" unit, and so on.
Semi-continuous systems of the type described above are seldom used for the solvent extraction of soybeans. However, the same principle is applied in one of the widely known solvent extraction systems for other oilseeds: the FRENCH Stationary Basket Extractor.
The FRENCH extractor (Fig. 17)is essentially a vertical cylindrical vessel, divided into a number of tall vertical sections or "baskets" by radial walls. The baskets are stationary. Solvent or micella is fed at the top of the basket and percolates through the deep bed of solids. Using a system of moving micella showers, the oilseed material is contacted with micella at decreasing oil content, and finally with fresh solvent, thus achieving countercurrent extraction, without moving the solid bed. In its recent version, the FRENCH stationary basket extractor is equipped with a rotating basket bottom, to achieve automatic discharge of the baskets at the correct time and to render the extractor nearly continuous. The capacities of units supplied since 1975 for soybean oil extraction, range from 100 to 3000 tons per day.
Figure 17: The FRENCH Stationary Basket Extractor
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. The most prominent types will be described in the next paragraphs.
* Belt extractors_ the DE SMET extractor: This extractor, offered by the Belgian De Smet Company and its subsidiaries in many countries, was developed in 1946 by J.A. De Smet at the "Nouvelles Huileries Anversoises" oil mill in Belgium. According to the company, since then over 450 plants using the DE SMET process have been built in various parts of the world.
A drawing describing the DE SMET Extractor is given in Fig.18. The extractor consists of a horizontal, sealed vessel in which a slowly moving screen belt is installed. Flaked soybeans 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 micella through the bed. Difficult percolation is compensated for by lowering bed height. For properly flaked soybeans, the height of the flake bed at the head end of the extractor is normally 6 to 8 feet (180 to 240 cm.). 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 micella stream is advanced. The solvent is introduced at the spent flake discharge end ( i.e. at the end opposite to the flake feeding side of the extractor ). 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 micella collection, pumping and spraying at the next section is repeated until the micella leaves the hopper at the head-end of the extractor, carrying the highest concentration of oil (heavy micella). The screen is washed with heavy micella 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 micella 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.
Figure 18: The DE SMET Extractor
According to the manufacturers, DE SMET extraction plants have been built for capacities ranging from 25 to 3000 tons of raw material per day. Solvent losses are 0.07% to 0.3% and the residual oil content of the extracted material is 0.25% to 0.6%.
* Moving basket extractors: 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 (carrousel 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.
In the horizontal moving basket extractors manufactured by the LURGI Company, the "basket" or "cell" is formed by an endless bucket belt and a separate perforated bottom. The bottom can be fixed perforated plates on which the bucket separations slide (sliding cell design) or screen belt conveyors moving with the buckets. Both types are shown in Fig.19.
Figure 19: Two Types of Basket (Cell) Extractors
Another type of horizontal basket extractor, featuring tilting baskets or trays, is manufactured by the HLS Company Ltd. The operation principle of the T.O.M. (Turning Over of Material) HLS extractor is shown in Fig.20. Each basket in the extractor can be flooded, permitting immersion and percolation in the same extractor. In order to overcome the problem of the formation of a dense surface layer of compressed fines, the trays or baskets are inverted at the end of the conveying chain. The material falls to the basket or tray below. The impermeable surface layer is broken and the oilseed material undergoes mixing in the process of its transfer from one level to the other. Extraction continues as the material moves, in reversed direction, on the lower (return) side of the conveyor. Thus, unlike most horizontal extractors, in the HLS Extractor the inlet for fresh raw material and the outlet for the spent flakes are on the same end of the shell.
Figure 20: The HLS Basket Extractor
* Carrousel extractors somewhat resemble the cylindrical FRENCH extractor described above, but here, the "baskets" rotate around the axis of the cylinder while the solvent/micella circuitry is fixed. The construction principle of the Carrousel Extractor, manufactured by EXTRACTIONSTECHNIK GmbH, is shown in Fig.21. The following description of the extractor and its operation is from an article by Dr. Ing. Wolfgang Kehse:
" The extractor consists of a single-part rotor with an inner and outer cylindrical wall. The ring-shape interspace is divided by radially arranged conical partition walls into a number of chambers (10 to 20) . It is slowly rotated usually by chain drive, the larger gear rim of which is placed round the rotor. Smaller extractors may be directly driven by a central shaft. These rotation speeds vary from one rotation in 20 minutes up to one rotation in 4 to 5 hours, and are adjustable. The rotor rotates above a slitted bottom with only a few millimetres' gap. This slitted bottom is constructed of profiled rods with a trapezoidal cross-section. This profile causes the slits which are at their surface about 0.8 mm wide to become wider further down. The specific advantage of this slitted bottom, however, is that the slits are exactly concentric with the rotor shaft. The raw material is filled into the chambers and thus form a compact layer which can reach a height of from 0.5 to 2.5 meters, depending on the material to be extracted. The height of the rotor corresponds to this. Therefore, a free space of about 200 mm above the layer remains, which is filled with liquid solvent during the time that the chamber is being sprayed with solvent.
Depending on the required time for extraction, the material
is moved at a speed of 1-10 mm/sec., over the concentric slits in the bottom.
Because the slits are arranged parallel to the direction of movement of the
material, no mechanical forces apart from the sliding resistance are exerted
on the extraction material and subsequently no plugging of the slits can occur.
While moving over the slitted bottom, the bed of material is percolated by micella
of different concentrations, beginning with the end-micella having the highest
concentration immediately after feeding of the solid material up to the pure
solvent at the end of its passage. The micella passes through the bed of material
and the slitted bottom and is the collected in chambers separated by weirs in
the lower part of the extractor. From there it is pumped back onto the bed of
material. The discharge of the extracted solid material is effected through
the slitted bottom by a hole as wide as a rotor chamber and allowing the contents
to drop down into a discharge chute where it is moved on for further processing
by a screw conveyor.
The partition walls of the chambers are conically widening downward so that any sticking of the chamber contents is impossible."
According to the manufacturer, Carrousel Extractors are available in capacities from 20 up to 4000 tons per 24 hours. The largest extractor (4000 tpd.) has a nominal diameter of 15 m.
Fig. 21: The "CARROUSEL" Extractor
3-2-5 Post-extraction operations
Two streams leave the solvent extraction stage: an oil-rich fluid extract (full micella) and solvent-laden spent flakes. The next operations have the objective of removing and recovering the solvent from each one the two streams.
a: Micella distillation: Full micella 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 micella distillation systems. The characteristics of a good micella 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 and, of course, good operation safety. The modes of solvent vaporization include flash evaporation, vacuum distillation and steam stripping.
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.
In desolventizing-toasting (DT) applied in the production of soybean oil meal for animal feeding, the time-temperature-moisture profile of the process permits, in addition to solvent removal, a heat treatment sufficient to inactivate the undesirable enzymes and inhibitors and to improve the palatability of the meal to animals (toasting). 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 and 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. 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.
The various models of vertical stack type DT's differ in the sequence of direct/indirect heating zones and several other features. In the FRENCH DT shown in Fig. 22, the top pans are indirect steam heated. They constitute the pre-desolventizing zone. The bottom pans are direct steam heated and they serve as the toasting/stripping zone. The meal coming out of this DT has about 18% moisture and a temperature of about 105oC. It has to be dried and cooled. A separate dryer/cooler (DC) is used for this purpose (see below).
Figure 22: The FRENCH Desolventizer-toaster (DT)
The DE SMET DT shown in Fig. 23 has 4 to 10 pans with steam heated bottoms. The apparatus is maintained at a slight negative pressure.
The meal dryer-cooler (DC) is similar to the DT in construction, but much shorter. Ambient air is used to dry and cool the meal before storage or bagging. The construction of a self-standing DC unit, offered by FRENCH, is shown in Fig. 24.
The DT and DC units can also be combined into one piece of equipment. Most manufacturers of desolventizing equipment also offer combined DTDC units. The operating principle of such a system, sold by LURGI is shown in Fig. 25.
A photograph of a 1200 ton per day desolventizer-toaster-dryer is given in Fig. 26.
Figure 23: The DE SMET Desolventizer-Toaster (DT)
Figure 24: Self-standing dryer-cooler (DC)
Figure 25: Combined Desolventizer-toaster-dryer cooler
Figure 26: Desolventizer-toaster. Nominal capacity
While desolventizing-toasting is the standard method for the manufacture of soybean oil meal for animal feeding, this process is not suitable for the production of "white flakes", i.e. meal with minimum protein denaturation. As it can be seen in Fig.27, protein denaturation ( expressed as the reduction in Nitrogen Solubility Index, NSI) by treatment with live steam is very rapid. White flakes, which are the starting material for the production of soybean protein isolates, most concentrates and texturized products, must have a high NSI value.
Figure 27: Effect of Heat Treatment on Nitrogen Solubility of Soybean Oilmeal (Source: Weber, 1973)
The best method of desolventizing for the production of white flakes is flash desolventizing (FD). In this process, the solvent laden spent flakes coming out from the extractor are fluidized in a stream of superheated solvent vapours. The superheat of the vapour provides the energy for the evaporation of solvent from the flakes. The turbulent nature of the flake-vapour flow permits extremely rap[id heat and mass transfer. Protein denaturation is minimized, mainly because of the short heating time. A short stripping stage may be necessary to complete solvent removal and rapid cooling is a must for preventing undue reduction of NDI. The flow-diagram of a flash desolventizing system is shown in Fig.28.
Figure 28: Flash Desolventizing System
DE SMET s.a. (1990)
Commercial Communication Extraction De Smet s.a., Edegem, Belgium
EXTRACTIONTECHNIK G.m.b.H. (1990)
Commercial Communication Deutsche Babcock Group, Hamburg, Germany
FRENCH U.S.A. (1990)
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Commercial Communication H.L.S. Industrial Engineering Company, Petah-Tikva Israel
Commercial Communication Lurgi G.m.b.H., Frankfurt am Main, Germany
Moore, N.H. (1983)
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