Industrial Extractors

B.  Industrial Extractors

 

Some general trends concerning efforts for improving extraction efficiency and performance can be observed. Development of an extractor implies a compromise between simplicity with respect to the number of moving parts and transporting systems, such as solvent pumps and solid-conveying systems, and efficiency of recirculation pattern for the miscella. Extractors containing compartments, as op- posed to simple conveyer belts, facilitate submersion of the solid, which usually leads to higher extraction efficiencies. Shallow beds that are not divided into separated stages are in danger of forming lakes of solvent freely flowing on the bed surface and mixing miscella of different concentrations. Instead, in deep beds the absolute residence time is often enhanced so that the liquid (miscella or solvent) is sprayed on such a moving bed and dissolving solute on its way, then is drained from the solid in a subsequent section that does not correspond to its solute concentration. For example, full miscella drained from the descend- ing leg of a basket elevator might be collected in the half-miscella tank below the rising leg.

In the following, different types of extractors working with a moving bed are presented. Most of these extractor types have existed for several decades. Technical improvements are mostly confined to some details concerning the position of inlet/outlet streams and recirculation of miscella. The extractors are

 

 

 

 

 

 

 

 

 

 

subdivided into those containing rotating parts and those in which longitudinal movements are carried out. The latter machines make use of conveying elements such as conveyor belts or chains. In general, capital costs are lower than in case of the rotating principle. Moreover, they can be delivered preassembled to a great extent. Since they mostly work with a shallow bed, extraction results may differ from the previously mentioned rotating deep-bed extractors depending on the material being extracted. In general, the range of application of shallow-bed extractors is somewhat confined due to the extractability and permeability of the meal.

The Rotocel extractor is a well-established extraction system developed by Blonox in the early 1960s (5). Then the successor Dravo closed down and fusion of licensees Krupp and Extraktionstechnik in 1990 resulted in production of the Carrousel extractor. The Rotocel consists of up to 18 cells located in a circle, each of them containing perforated bottoms that are periodically opened when the cells pass the discharging section during their rotation. One rotation lasts more than 1 h depending on the extractability of the solid. The rotational energy requirement is fairly low because in spite of large diameters there is little frictional force to be overcome. The Rotocel is a so-called “deep-bed” extractor, with a bed height of 1.8–3 m. The flakes, which may also be introduced as a slurry using miscella, are not moved relative to their neighbors. In this way fragile solid structures can be handled, but there is no additional mixing effect that otherwise could enhance extraction yields. Miscella drawn from the bottom of the cells is collected and used for leaching solid of less bed age. This recircu- lation system consists of up to 10 miscella pumps.

The Carrousel extractor is similar to the Rotocel, except that only the frame of the extraction compartments rotates atop a static sieve tray, the struc- ture of which is shown in Fig. 3. In this way, friction of particles scrapping along the sieve tray bottom needs to be overcome; but on the other hand, parti- cles are kept in movement relative to each other, which enhances mass transfer.

 

 

 

 

Figure 3  Segment of the sieve tray bottom of a carrousel extractor. Original (concen- tric) (a) and actual (b) orientation of sieve gaps.

 

 

 

 

 

 

 

 

 

 

 

One rotation takes about 1 h, which means that at a diameter of 8 m the highest relative velocities of particles with respect to the sieve tray bottom do not exceed

7 mm/s. After being exposed to different extraction stages during one cycle, each compartment arrives at the solid outlet position where the complete solid bed of the respective compartment is discharged through the open bottom. The separation walls of the compartments have a conical cross-section (Fig. 4) that helps in discharging the exhausted meal downward.

Carrousel extractors have diameters of up to 15.3 m. At a bed height of 2 m an extractor of 8 m in diameter has a capacity of 75 m³. Double-deck carrou- sels containing diameters of up to 8 m are also in use (Fig. 4). After passing one cycle in the upper deck, the solid drops onto the lower level for extraction during another rotational period. Such an extractor is capable of extracting oil from 2000 t/d soybeans containing 18% of oil using hexane; the residual oil content amounts to 0.8%. Triple-deck carrousels have also been constructed, but poor accessibility to the middle deck for cleaning and repairing is a disadvan- tage. Figure 5 gives an overview of solid and liquid flow, also indicating the respective extract concentrations in countercurrent extractors like the Carrousel extractor.

The weight of rotating parts can be reduced by using a stationary basket extractor originally fabricated by French Oil Mill Machinery Co. In such an extractor, the compartments filled with solid are stationary and only feed spouts and the positions of the solid discharging facility and draw-off of miscella ro- tate. Usually, 12–20 cells are applied, each of them 1.8–3 m deep. In the mean- time, the French Company switched from the stationary basket principle to car- rousel-type extractors.

Next to rotating extractors also the conveyer belt principle is applied to oil extraction. DeSmet uses sieve tray belts for transporting the meal in their belt extractor. Either fresh solvent or miscella provided by a system of miscella recirculation is sprayed onto the shallow solid bed usually containing heights lower than 0.6 m. The solvent containing the freshly extracted solute drops from the belt into collecting trays. Before leaving the extractor, the solid is treated once more with pure solvent (benzene). A slight vacuum within the extractor is obtained by Venturi nozzles. Belt extractors have the advantage of low appara- tus costs and fairly uncomplicated installation. Extraction results depend on a precise adjustment of transporting velocity, amount of liquid sprayed on the solid, and solid bed permeability. Following efforts to submerge the solid bed to the highest extent possible, lakes may be formed on top of the solid moving bed freely flowing to all sides and mixing half and full miscella with one another.

Similar to rotational extraction systems, horizontal/vertical working ex- tractors offer different ways of moving the solid for passing it through different stages of extraction. In general, recent development is leaning toward stationary screen plates or sieve trays on which the solid is pushed by cells or frames fixed

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4  Interior of a double floor carrousel extractor. (Courtesy of Krupp, Hamburg.)

 

 

to chain conveyors, as in the case of the Crown loop extractor (Fig. 6). The loop extractor is quite compact with one cocurrent and two countercurrent ex- traction zones.

In the sliding cell extractor from Lurgi (Fig. 7), U-shaped cells run on roller tracks pushing the solid material over stationary screen plates. The screen plates consist of rods aligned in a flow direction. The cross-section of the rods

 

 

 

 

 

 

 

 

 

 

 

 

 

_{Figure 5  Concentration profiles of countercurrent extraction, e.g., Carrousel extractor.}

 

 

 

 

Figure 6  Crown loop extractor.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 7  Lurgi sliding cell extractor.

 

 

is V shaped to prevent clogging. The feed material is introduced into the cells of the upper belt by a filling device. After approximately half of the extraction time, the screen plate ends and the feed is dumped through the open cell bottom into the cell, just arriving at the lower screen plate in the feed transfer stage, as indicated in Fig. 7. Before being discharged, the feed is passed through a final drainage zone. The solvent passes through the feed material countercurrently by spraying the collected miscella onto feed at a previous position; it becomes enriched with extract until finally leaving the extractor.

Recently, a sliding bed extractor was developed by Krupp containing a chain-frame conveying system on top of a static sieve tray passing two levels (Fig. 8). Miscella collection is also divided into two levels with a sophisticated recycling system. Full miscella is liberated from fines by integrated hydrocy- clones. Bed height is adjusted at 0.5–1.3 m.

Baskets fixed to a conveyer belt allow either for construction of tall ex- tractors that take little floor space or for horizontal extractors in case one-floor operation is required. A variety of methods concerning collection and recircula- tion of the miscella and solvent-solid feed positions exist, always intending to maximize the number of regions of countercurrent flow while confining the number of pumps required. The basket elevator (Fig. 9), also known as the Bollman extractor (6), contains a rising and a descending leg, each of which contains around 15 baskets that continuously descend and rise undergoing dif- ferent stages of charging/discharging of solid, fresh solvent, and circulated ex-

 

 

 

 

 

 

 

 

Figure 8  Sliding bed extractor. (Courtesy of Krupp, Hamburg.)

 

 

 

 

 

 

 

 

 

 

Figure 9  Basket elevator.

 

 

tract. The solid bed formed in the baskets is rather shallow taking heights be- tween 0.5 and 0.7 m. The solid is fed to the top basket of the descending leg. At this place, half miscella originating from drainage through the more ex- hausted solid beds of the rising leg is introduced draining downward through the descending baskets in a cocurrent fashion. The miscella draining from the bottom basket of the descending leg is collected in a sump and drawn off for further processing. Fresh solvent is sprayed on the top basket of the rising leg draining downward through the rising baskets in a countercurrent fashion, while extract concentration is enhanced resulting in the formerly mentioned half mis- cella, which is collected in a sump at the bottom. After arriving at the top and being contacted to fresh solvent, the exhausted solid is discharged by inverting the basket.

A couple of extractor types make use of the screw transporting principle, such as the vertical screw tower from Buckau Wolf, Germany, the Hildebrandt extractor formerly applied to soybeans, and the horizontal helix from Raffinerie

 

 

 

 

 

 

 

 

 

 

 

Tirlemontoise used for sugar beets, all of which have not been in operation to a great extent (5). Two helicoidal screws transport the solid through the double- screw conveyor called Contex, offered at present by Niro, Denmark. The extrac- tion liquid flows through the solids as a submerged stream by means of gravity due to a slight inclination of the vessel. Complications during operation are caused by disintegration of solid particles and flow conditions that depend on solid compacting.

Multiplate tower extractors like the Bonotto extractor make use of rotating plates or paddles for transporting oilseed flakes until they fall through openings in the plates to the floor below. Solvent and miscella are transported upward in a countercurrent fashion. Such problems as bypassing, fines entrainment, and back mixing of miscella inhibited commercial application.

 

 

C.  Safety Aspects

 

Working with volatile and flammable solvents implies risks. During normal operation, a number of measures, such as the use of explosion-protected equip- ment, working at slight vacuum and continuous control of escaping gases by ignition detectors can for the most part guarantee safe handling. Thus, most accidents involving ignition of solvents or even explosions occur as a result of equipment failure (7). When the plant is shut down and vessels are opened for repair, strict safety guidelines might be missing and residual solvent vapors might come into contact with air, producing flammable mixtures. The U.S. Na- tional Fire Protection Association has formed a committee dedicated to safety of solvent extraction plants, which issued the following statement given in part:

 

NFPA 36 Committee on Solvent Extraction Plants:

Par. 5–8.3: Extractors, Desolventizers, Toasters, Dryers, Spent Flake Conveyers shall be of a design that minimizes the possibility of ignition of product deposits. Such equipment shall be protected by extinguishing systems using inert gas, steam, or a combination of the two, controlled from a safe remote location.

Par. 5–8.1.7: The extractor shall be provided with means to remove solvent vapors so that the concentration of vapors inside the unit in the area where work is required can be maintained at or below 25% of the lower flammable limit, e.g., by a purge fan sized so that it changes the empty air volume of the vessel once every 3 min.

 

 

D.  Conditioning

 

1.  Mechanical Pretreatment

 

Extraction efficiency is highly contingent on preparation of the solid matter that undergoes extraction. Small particles are advantageous to small diffusion

 

 

 

 

 

 

 

 

 

 

resistance within the particles. On the other hand, powders of very small particle size require great efforts of milling or grinding. While preparing the solid bed, reagglomeration may even occur and during operation as a trickle bed extractor channels might be formed, resulting in insufficient extraction yields. If the bed’s permeability decreases pressure drop might increase and lakes may be formed on top of the solid bed. Drainage is retarded, which may result in undesired back mixing in case the solid bed is moved through successive extraction zones within the extractor. Furthermore, the so-called fines of very small particle sizes are at risk of being entrained. In general, the content of particles of diameter below 0.5 mm should not exceed 5–10%.

Oilseed processing preparation methods may be distinguished depending on whether they are combined with mechanical deoiling or are applied for ob- taining solid material of defined size and structure. In the case of oil content above 25 wt%, mechanical deoiling by expression is suitable for economical reasons. Rapeseeds and sunflower seeds both containing more than 40% and corn germs with about 50% oil always undergo previous pressing. Cottonseeds containing about 25% are the limiting case for previous mechanical deoiling. Corn and soy, containing less oil, are usually directly extracted after condition- ing. Mechanical deoiling implies changed characteristics of the feed material for extraction, e.g., moisture and cell structures differ from their natural state due to high pressing temperatures. Even agglomerates may be formed that have to be crushed prior to further processing. Furthermore, different equipment for mechanical pretreatment exist due to the type of force applied. The fluted rolling mill (Fig. 10) mainly cuts larger particles into pieces. A hammer mill (Fig. 11) inserts kinetic energy in particles, which is converted to surface energy by in- creasing surface area during impact with the agitator or the wall. In this case, the fines content is higher than that of the formerly described cutting principle.

A roller mill works by pressing two cylinders against each other. In case there is no differential circumferential speed, the seeds are flaked only by com-

 

 

 

 

Figure 10  Working principle of a fluted breaker rolling mill.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 11  Working principle of a hammer mill.

 

 

 

pressing. A slight differential speed gives rise to shear forces, and seeds may also be ruptured. Extruders being high-shear devices are also capable of ruptur- ing oil cells prior to solvent extraction or mechanical pressing. For processing of high oil content material the extruder may be provided with a drainage cage similar to those used for screw presses described below. In order to obtain parti- cles of defined size, pelletizing or granulating may be applied by pressing, cut- ting, or adding moisture. The adequate pretreatment depends on the individual feed material and must be determined by experience.

As already mentioned, if the content of extractable substances is fairly high, prepressing is carried out prior to extraction. Commonly, the screw presses used easily reduce the oil content in solids to 10% or less. Following solvent extraction requires less solvent because of the reduced amount of extractable substances. In addition, cell structures are destroyed, mechanically liberating enclosed substances. Using a pressure measurement technique described by Eg- gers et al. (8), the screw geometry may be adjusted to obtain an adequate pres- sure profile for achieving an optimal pressing result. Presses of the EP series manufactured by Krupp contain a shaft with a rising outer diameter in flow direction at a constant inner diameter of the strainer cage (Fig. 12) in order to achieve adequate compacting. Recent developments tend to obtain the complete final product only by mechanical treatment (full or final pressing; see section

 

 

 

 

 

 

 

 

 

 

 

 

Figure 12  Screw press. (Courtesy of Krupp, Hamburg.)

 

 

 

below). Therefore, different steps of pressing according to different operating temperatures may be applied for obtaining products of specified quality and quantity. Usually, elevated operating temperatures result from frictional forces. In order to avoid temperature rising, cooling of the press is needed. On the other hand, rising solid temperature before feeding may help performance and en- hance product yield. A press recently developed by Krupp contains different temperature zones within one machine. Here prepressing is carried out at lower temperatures resulting in higher product quality but low yield. Final pressing at elevated temperatures has the objective of enhancing the product yield. Mon- forts has developed a compact double-acting screw press called Komet having a broad range of applicable source material (9) but without any special facility for cooling. Screws of various slopes are used for solids of different hardness, but the shafts always have the same constant outer diameter. Compacting is addi- tionally regulated by use of different nozzles at the cake discharge spout. Very hard solids should be broken or crushed before being fed to the press. If the residual press cake still contains a  considerable amount of extractable sub- stances, solvent extraction follows pressing but the compacted solid cake should pass through a cake breaker first.

Presses manufactured by Anderson International under the name Expeller are in use for prepressing and full pressing of a variety of oil containing material such as cottonseed, peanuts, corn germ, and sesame seed. After conditioning for careful adjustment of its temperature, the feed material enters the downspout where it receives a first pressing by a vertical screw (Fig. 13). Leaving this section, the solid arrives at the horizontal barrel containing a second screw for final pressing. The compacting pressure of the solid cake is regulated by a hy- draulic cone choke at the position of solid discharge. A couple of different

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 13  Pressing by Anderson Expeller. (Courtesy of Anderson International Corp.)

 

 

 

 

 

 

 

 

 

Expeller types are offered to meet specific demands. The Expeller 33 prepress leaves 15% residual oil content in the cake for further solvent extraction work- ing at rates of up to 90 t/d feed material. The 33 duplex press was developed to handle especially hard fibrous and high oil content materials such as copra and palm kernels. The residual solid contains about 7% oil. The high-capacity Model

55 allows processing of more than 25 t/d with residual oil contents of 4–5%. Commonly, Anderson expellers are combined with thermal conditioning like cooking and drying of the feed prior to pressing.

 

2.  Thermal Pretreatment

 

Materials that are destined for oil production and contain a large amount of proteins, such as cottonseeds, soybeans, flaxseeds, sesame seeds, and peanuts, must be cooked prior to pressing in order to coagulate proteins and allow effi- cient recovery of the liberated oil. Therefore, the raw material is maintained for about 20 min at a moisture content of around 10% and temperatures of 90–

95°C. Afterward the material must be dried in a separate step to approximately

3% moisture before entering mechanical pressing so as to stiffen the particles. Materials containing small amounts of proteins only undergo the drying proce- dure. Drying time strongly depends on the physical properties of the solid parti- cles, e.g., size, moisture, and porosity.

Cooking of proteins and starches is combined to adjustment of moisture and porosity by the expander technology. Vapor is introduced into the solid structure during extrusion in a screw press–like device containing steam inlet nozzles at the circumference (Fig. 14). The solid-vapor mixture is expanded at the outlet gap resulting in increased porosity due to explosive-like evaporation from the solid cells. Amandus Kahl offers a ring gap expander disposing of an adjustable ring-formed gap that keeps a defined pressure of the solid moving bed. The resulting material is homogeneous having a high porosity at an in- creased bed density (compacting). Penetration of liquid in posterior solvent ex- traction is facilitated while the resulting homogeneous structure and high density facilitate enhanced solid throughput and safe performance of percolators. The expander-extruder-cooker fabricated by Anderson also allows blending of water into the feed material for cooking at a prescribed moisture level.