RICHARD L. KENYOS Associate Editor
1x1
collaboration with
S . F. KRUSE iiND S.
P. C L l R K
Central Soya Company, Decatur. Ind.
T
countercurrent to the miscella flow, has also been reported to be very successful (9). The continuous screw press method employs a barrel, constructed of bars to allon the oil to escape, t,hrough which t h e heated, dried beans are forced by means of a screv. This screw is designed t o increase the pressure progressively up to 10 to 20 tons per square inch. -4t the same time the temperature rises t o 300" I?. and above ( 7 ) . Solvent extraction, also a continuous process, is effected by rolling the cracked beans into flakes of carefully regdated thickness, treating them with hot solvent (by any of a number of systems) to remove the oil, and then evaporating the solvent from both oil and spent flakes to leave the finished products. The solvent extraction method is used almost exclusively in Europe, \There both batch and continuous systems are einployed (3,6). I n the United States, as previously mentioned, it is second t o the screw press process but gron4ng in favor. The relative importance is indicated by Table I. The outstanding advantage of solvent extrastion is the high percentage of oil which is removed from the seed. The content of residual oil in the meal is usually beIoiv I and often as low as 0.5y0,and constitutes 95 t o 98y0 removal of the oil. This is marlredly superior t o screw pressing ;vhich removes only
HE processing of soybeans for oil has a long history, particularly in the Orient, but is coniparatiTely new in the United States, where its development has taken place almost entirely within the last thirty years. The three t,ypes of proccessing generally used in this operation are hydraulic pressing, expeller or rotary screw pressing, and solvent extraction. Of these three, hydraulic pressing is the oldest and its use is declining. Screw pressing has been distinctly the most popular method, and remains so but is being replaced t o some extent by solvent extraction. Solvent extraction is more complicated and has required more developmental work and technical direction. B y this time, however, many of the obstacles to the success of this process have been overcome, and it is gaining in favor as it is especially advantageous for use with soybeans. All hydraulic pressing is executed batchx+se. In the most commonly used hydraulic process, the beans are flaked by repeated passage between steel rolls, cooked in a series of steamheated pans, formed into cakes which are wrapped in hair mats, and submit,ted t o pressures of 1500 t o 1600 pounds per square inch by a hydraulic ram. The oil is forced out by pressure from the direct force of the ram on t,he cake ( 8 ) . The extraction system in which the oil bearing material is propelled, by perforated paddles, through a series of semicircular pockets, in the direction 186
February 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
about SO%, and to the hydraulic method which recovers about 75%. Physical characteristics have hindered the adoption of solvent extraction for other seeds but not for the more adaptable soybean. For efficient and rapid extraction the seed must be rolled into a thin flake, Where there is a considerably higher oil content than in the soybean, as is often true, it is difficult to form a firm, rugged flake which will not disintegrate during the process Consequently, reduction of oil content by pressing is often necessary prior t o extraction (3). This is not true with the soybean. Cost is also a matter of prime significancein a comparison of the processes. The original outlay for a solvent extraction plant is higher than for a screw press plant b ~ 3 3 ' /to~ 50% depending on capacity. In spite of the original cost, plus a 15 to 25% higher operating cost, the solvent extraction plant will yield a greater return on the investment because of the increased percentage of oil obtained, its finer quality, and its very high value as compared to the meal. Safety is another factor of particular importance because of the handling of large quantities of flammable solvent. The design of a solvent extraction plant must conform to special and rigorous regulations to obtain a favorable insurance rate. At best, this rate is still comparatively high. Many other special precautions, and more skilled labor are thus required. However, this has its compensation in dividends often reaped from more skilled operators through more efficient production. SOLVENT EXTRACTION EQUIPMENT
Goss, in his reviews of solvent extraction (f2, S), describes a number of systems for the process. He indicates that we are indebted to the German industry for the basis of much of our present technology, and discussesseveral units developed there as well as some American outgrowths. One of the systems of German origin most widely used in the United States is the Bollman system. The bean flakes, in sieve baskets attached to an endless chain, constantly moving in bucket elevator fashion, are soaked with solvent which drains in both concurrent and countercurrent directions. The Hildebrandt system employs a U-tube through which the solids are moved by screw conveyers while the solvent is circulated in a countercurrent direction. A type of extractor made in the United States ( I ) , and widely employed here, consists of a vertical column containing slotted horizontal plates. The flakes fall vertically downward from one plate t o another after being carried around the circumference of each plate by a scraper arm. Solvent, introduced at the bottom moves countercurrently, and the miscella, or solution of extracted oil, flows out at the top. A number of other systems are described in the aforementioned articles, as well as in the authoritetive treatise by Markley and Goss (8).
187
screw press method, has expanded through adoption of solvent extraction, and is continuing to develop its solvent techniques in further expansion. The method originally used by Central Soya Company and still employed is screw press processing. A flow sheet of this installation is shown in Figure 1. In this system the beans are taken through cracking rolls, then dried in steam-tube dryers to reduce the moisture to about 3% and elevate the temperature to 260" F. The cracked beans pass through steam-jacketed, tempering conveyers where they are heated to 275' F., and then into the expellers. In the screw presses the bean particles are forced by means of a vertical feed m e w hopper into a horizontal barrel equipped with a screw which progressively subjects the beans to increased pressures in passing through the chamber. This barrel is constructed of bars, spaced 0.010 to 0.020 inch apart. Here a pressure of about 20 tons per square inch and temperature of 350' F. are developed. The oil, along with some fines, drains between the bars and into a settling tank. A portion of the oil is passed through heat exchangers where its temperature is reduced t o about 150" F. This oil is then flooded over the barrels to wash off the foots, or meal, and to cool the outside of the barrel. This enhances the shearingeffect on the thermoplastic mass of beans within, thus effecting a greater expulsion of oil. The oil returns to the settling tank where it is drawn from the top, double-filtered in plate and frame filter presses, and collected as finished product. The foots are drawn off and returned to the presses dong with fresh cracked beans. The cake is forced from the presses and cooled by water spray which also returns its moisture content back to the normal range of about 12%, then further cooled by air.
.
PROBLEMS IN SOLVENT EXTRACTION
In the development and initial operation of the solvent extraction process, with unit operations which varied radically from the screw press method, the Central Soya Company dealt with a number of challenging chemical engineering problems.
SCREW PRESS EQUIPMENT
An excellent opportunity to observe the solvent and screw press processes in operation together is provided by the Central Soya Company, Decatur, Ind. This company began processing soybeans by the
Top of Five-Story Extractor Unit Top section of extractor shows hopper for incoming flakes, lines carrying half miscella ( r i g h t ) , and sight glasses for observing internal operation.
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Figure 1. Flow Sheet of Screw Press Process Xloisture conditions and seasonal changes in the beans, and the amount of the oil can be readily extracted, whereas the remainder regulation of equipment in adaptation t o variations in these concan be removed only with greater difficulty. ditions, constitute a problem of utmost importance. Thus, The shape, size, and bulk density of the flakes also require tempering, storing of beans of like condition together, and the attention. A bed of flakes is needed vihich will give enough void for the free passage of solvent through the entire mass of flakes careful and uniform drying of wet beans to avoid burning or case hardening, must be given detailed attention. and at the same time give sufficient retardation t o create a SoxhThe efficiency of extraction depends heavily on the proper let effect in each basket. 9 correlation of all these 1.cquir.ecracking and flaking of the beans. Moisture and temperature ments demands a detailed study. of the beans, type, number, design, and arrangement of cracking In handling a high-protein organic mat,erial under conditions rolls, as well as their speed, differential speed, and set,ting, enter of temperature and humidity above the normal, a problem into the cracking of the beans t o the proper size wit,hout mashing arises in conveying. Dust, accumulates and, with the attendant or development of excessive fines. The condition of the cracked heat and moisture condensation, begins to deteriorate, forming pieces is a major determinant in the formation of thin tough soggy masses along the conveying lines, particularly a t corners, flakes on rolling. These flakes must allow optimum penetration crevices, or dumping points. This not only results in fouling of of the solvent and diffusion of the oil, and retain their structures without breaking down into flour and dust, throughout the TABLE I. SOLVENT EXTRACTIOX O F OIL FROM SOYBEANS" ext,raction and solvent removal process. Tot,al The importance of the structure of the cropYear Screw Press Solvent Hydraulic Extd., % of Total Tons of Total Tons % of Total Tons 0ct.-Oct. Tons flake has been shown in studies of Lhe 1936-37 422,800 68 81,604 13 114,136 19 618,540 application of diffusion theory t o the 37-38 637,532 70 155,591 17 114,167 13 909,290 38-39 967,173 72 214,iei 16 158,093 1,330,427 soybean (4,IO),wherein it was found that 39-40 1,274,016 75 364,055 20 92,081 5 1,712,152 simple diffusion theory for uniform por40-41 1,425,501 74 443,685 23 52,486 3 1,921,672 2 2,313,938 74 567,986 24 41,257 41-42 1,714,695 ous solids does not correlate the ex42-43 2,531,035 63 653,797 16 818,775 21 4,003,607 ie 784,158 4,269,192 43-44 2,785,866 66 699,168 traction data for soybean flakes; the 10 l8 4,602,062 70 918,956 20 437,332 44-45 3,245,775 28 363,325 8 4,783,804 structure of the flakes is considered to 45-46 3,073,262 e4 1,347,217 be the cause. It was shown, however, a Estimates through 1940 based on data compiled by the Bureau of the Census in cooperation that data from a number of experiments with the United States Regional Industrial Soybean Products Laboratory, January 1941 (6); reproduced through the courtesy of the Chemical Publishing Company. Estimates 1941-45 from d a t a could be significantly correlated. For a compiled b y the Bureau of the Census and published through t h e courtesy of t h a t bureau. particular structure of flake a certain
Tebruarp 1948
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minimize maintenance cost. Extensive study has led Central Soya equipment and the mixing of decaying material with the clean to the conclusion that for corroeion resistance in the solvent exproduct, but also causes damaging corrosion. I n addition t o traction process, Type 316 stainless steel and the aluminum aonveying equipment designed to avoid such accumulations, alloy 2Sl/zH are the most satisfactory. The former should be removal of dust and moisture-laden air through an effective used where abrasion resistance and high strength are required. aspiration system has minimized this problem. A number of othel factors which experience has shown to be In the actual extraction, a principal concern is procurement of important in the design of a plant have been discussed by Tray the highest possible oil content in the miscella to minimize and Bilbe (11). steam costs in the distillation and solvent recovery systems. To achieve high oil content in the miscella and still obtain satisfacSOLVENT EXTRACTION PLANT tory extraction, consideration must be given to the points, The Bollman system of solvent extraction is employed by the method, and rate of introduction of the solvent onto the p r o p Central Soya Company. This system was selected and ordered erly prepared flakes. in January 1938, after a study of plants in Germany by N. F. Evaporation and recovery of solvent from the oil to yield a Kruse and H. C. Offutt as representatives of the company. At high grade oil and suffer a minimum loss of solvent require careful this time the Hansa-Muhle Company, primarily a processor of study, planning, design of equipment, and operating techniques. oil seeds, was just going into the design of plants for sale. The Correlation of temperature and pressure a t an optimum point extractor, evaporation and desolventizing equipment, and other are, of course, particularly important. major units for a 275-ton-per-day plant (larger than any preIn the evaporationand recovery of solvent from extracted flakes, viously sold by that company) were shipped t o the Decatur, entrainment of dust by the vapor passing t o the condensers may Ind., plant and installed under the supervision of three German foul the condensers very rapidly. Entrainment may be reduced engineers who remained in Decatur through the f i s t few months by control of vapor velocities, by admitting live steam to mainof operation. Operation was begun in November 1937. tain a maximum practical level of moisture in the flakes, and by Constant development work since that time has increased drawing the vapor off in sections where the Aakes are wettest the production from the designed rate of 275 tons to the present with solvent instead of in sections where they are dry and dusty. 405 tons per day. It can also be controlled by adjustment of the speeds an$ paddle, One other plant of this type was installed in the United States pitches of the conveyers in the system t o provide sufficient by Hansa-Muhle. Several American companies now manuagitation for good heat transfer without undue increase of dust. facture plants of similar design. Dust settling chambers and vapor scrubbers can be used to reduce t h e dust in the vapors. Wide tube spacing in condensers hanSOLVENT EXTRACTION PROCESS dling dirty vapors is also essential. The solvent extraction process may be considered as consisting As has been indicated, safety factors are important. It is of three parts: preparation of beans, extraction of oil from the necessary to keep this in mind in both the design and operation of beans, and reclamation of solvent from the oil and meal. A flow sheet of the plant is shown in Figure 2, and a materials balance of a plant. Ample ventilation must be provided for a housed and data on oil concentration at various process steps are given plant by large windows, free circulation of air, and avoidance of in Table 11. dead areas, This is aided in the present plant by the use of STORAGE AND PREPARATION OF BEANS. A t bean harvest steel grill floors designed so that a t least 40% of the floor area is time in October and November, a large volume of beans is reopen i o allow circulation of air. Air from the fan ventilation system has been vented at a high level. This system is being changed in existing and new plants, in accordance with insurance compbny recommendations, through the use of a high capacity ventilating fan which forces the air down and out at the lowest floor level; this provides rapid removal of the heavier-than-air solvent vapors t o the outside air and dilution below danger level. To prevent the carrying of solvent vapors into the sewer by waste water from the solvent redamation system, water from the separation tank (where water is separated from recovered solvent) is passed through an evaporator from which it goes t o the sewer. Solvent vapors are stripped off, condensed, and returned to the separation tank. All motors, switches, conduits, and other electrical equipment are of the explosionproof type approved by the Underwriters' Laboratories for use in the presence of explosive vapors. All hammers, prybars, and other routine tools are made of rubber, plastic, or brass, t o avoid sparks. Oil Distillation System By no means least in importance is Left, section of extractor shows drive and cam, which aperate basket loading device, and solvent and half mimcella valves; foreground dotor and reducer fop conveyer from extractor to the selection of materials of construcSchneckenst right, evaporators; background, conliemate tanks and vapor absorbers; flooring, tion t o prevent loss of production and steel grill.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
ceived by truck and rail for storage. However, purchase and receipt of beans continue throughout the year. The facilities for handling and storing beans are of the standard type used throughout the grain industry. Storage is in concrete silos which will hold about 45,000 bushels each, Before they are ready for processing, the beans are cleaned, dried (if above about 13% moisture), and blended by transfer from one bin to another to temper them to like conditions. Beans are brought into the mill by belt conveyer and bucket elevator, collected in storage bins, measured through automatic scales, passed over a magnetic belt to remove stray metal, then fed through a surge hopper into the cracking rolls. Each cracking roll unit consists of three pairs of corrugated rolls in vertical series turning toward each otlier at different speeds with sharpto-sharp contact of the corrugations. This .gives cutting and cracking, rather than mashing, of the beans. The beans are cracked to a size to allow about 12% to pass a perforated screen with '/*pinch openings and to leave about 8% on a 7/s4-inch perforated screen. The cracked beans are passed over a '/*-inch perforated dehulling shaker screen where fines drop out and hulls rise to the top to be removed by fan aspiration. The dehulled cracked beans are then carried by a screw conveyer to the conditioner. The moisture content of the beans coming from the elevator is found to range from 9 to 13.5%. For optimum operation of the solvent extraction process, 9.5 to 10% moisture is desirable, and preparatory flaking is accomplished most satisfactorily a t 165" t o 175" F. The desired conditioning of the cracked beans is achieved by means of a lrotary steam-tube dryer, 30 feet long and 6 feet in diameter, equipped with water sprays so that moisture may be either added or removed. Control is maintained through frequent, regular sampling, and corresponding manual adjustment. The beans are then carried by a horizontal Redler conveyer to the four flaking machines. Each machine contains two sets of rolls operating in parallel flow. Each set consists of a pair of smooth-surface rolls regulated to produce flakes about 0.008 inch thick in one pass. After passing the flaking rolls, the flakes are conveyed to a Redler conveyer for movement to the extractor by a paddle conveyer which breaks them up somewhat and contributes to adjustment of their bulk density to the desired value. The Bollman system of extraction gives two passages of solvent through the beans, fresh solvent in the countercurrent and half miscella in the concurrent direction. The halfmiscella is the product of the extraction by the fresh solvent s rayed on the partially extracted flakes as they rise through tge up side of the elevator system of the extractor. The half miscella, sprayed on the fresh flakes as they enter the cxtractor, flows through on the down side to yield the full miscella, from which the oil is obtained by evaporation of the solvent. The Bollman type extractor used by Central Soya carries thirty-eight baskets, each 6 feet 6 inches long, 2 feet 4 inches wide, and 21 inches deep. At the loading, or No. 1 position, are set spray pipes in position to wet the flakes with half miscella as they enter the basket. Beside and slightly above positions 4 and 5 on the down side are perforated pipes (in fixed positions parallel to the long axis of the baskets) from which half miscella
Cracking Rolls Where Beans are Cracked to Specification before Passing into Conditioner and Flaking Rolls
191
TABLE11. MATERIALSBALANCEFOR SOLVENTEXTRACTION PROCESS Actual Material Contained in 1000 Lb. of Flaked Beans and 960 Lb. of Hexane Solvent@, n--.
Pointb
Material
,"o;fd
Oil
Water
Hexane
Total
Into Extractor
A
Flakes Hexane
B
721
... 721
D
Flakes Fullmiscella
E
721 721
D A T A ON
Point
B C E F G
H
J a
b
'
184
... 184
...
95 -..
960 960
L
95
Out of Extractor 5 95 %1 .. -
184
461
499 960
95
1000
960 1960 1282
678 1960
M I S C ~ L LAA0 IT P A S S E S TAROUQH S Y S T E M , BASEDO N ABOVX MATERIALB BALANCE Wt. Solution, Stage Freshsolvent Half miscella Full misoella Out of first three evauorators Out of falling film evaporator Out of first vacuum column Out of second vacuum column
Oil, %
iKi7 25-28 55-60 89-91 99.5-99.7 99.85-99.95
Lb.
960 1200 678 312 199
180 179
uantities shown are for representative operating conditions. %own in flow sheet (Figure 2 ) .
can be sprayed. Similar spray units for fresh solvent are placed in positions 5 and 6 from the top on the rising side. The entire extraction unit is housed in a vapor-tight, fabricated steel tank, approximately 45 feet high and 15 feet by 8 feet in cross section. Flakes are fed in through a vapor-tight double hopper on the top of the tower. The hopper gates are operated by cams on the main shaft of the extraction tower drive. When the first hopper is fdled with a set load and dumps to the second hopper, the upper gates immediately close and the lower hopper dump8 into the extractor basket just as it starts down the tower. Directly beneath the zenith position in the basket train is a bin for receiving wet flakes as they are dumped from the top basket after completion of the extraction cycle. Paddle conveyers run from this bin out opposite sides of the extractor tower to steamjacketed Schnecken conveyers where solvent is removed. The speed of the paddle conveyers from the wet flake bin is manually coordinated with that of the extractor so that the level in the bin provides a seal between the extractor and the Schneckens. The solvent is drawn from a hi h tank in which it is maintained at 136" F. by steam coils. Skefiysolve B, containing a mixture of hexanea, having a specific gravity of 0.685 a t 60" F. and a boiling range of 146"to 156"F. is used as a solvent. The sequence of the solvent cycle beginning with the introduction of fresh solvent onto the partially extracted flakes is as follows: (a) A total of 96 gallon6 of fresh solvent a t 136" F. is sprayed from the automatically actuated system onto the two adjacent baskets of partially extracted flakes in positions 5 and 6 on the up side each time the. continuously moving elevator advances one position, at 58- to 60-second intervals; thus each basket of wet flakes is sprayed twice with fresh solvent. The solvent drips through the baskets below to the floor pan, where it is collected as half miscella. (There k i n g very little additional holdup by the already wet flakes, and some drainage from the baskets passing over from the down side, the total half miscella collected per 96 gallons of fresh solvent is about 120 gallons, containing. about 15% oil.) The half miscella is pumped through a settling chamber to remove large flake particles which might clog the spray pipes, and then to a high tank where it is reheated to 136" F., and sprayed onto the down baskets. (b) As a basket is loaded with flakes it is sprayed with 30 to 35 gallons of half miscella, which is sufficient to wet the load thoroughly. As the hopper closes, the main spray valve opens and the sprays at positions 4 and 5 on the down side empty the remainder of the 120 gallons of half miscella on their respective baskets. (This entire procedure is repeated each time the train advances one position.) The resulting full miscella is collected in the floor pan. (Here on the down side, starting with fresh flakes, there is considerable holdup and carryover, so that the amount of full miscella collected is about 65 gallons per 96 gallons of original fresh solvent.) (c) The full miscella, contaiping 25 to 28% oil, is through leaf filters (covered with 8.53 ounces of single-#?i!: twill, to remove solids), then through a small surge tank to a heat
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Leaf Filters for Reiiioi a1 of Solids from Full Rliscella
exchanger, heated by the vapors of reclaimed solvent distilled from a falling film evaporator, and thence to the first evaporator. which it enters a t about 145 O F. SOLVENT EVAPORATIOX ASD RECLAMATION FROY MISCELLA. The first part of the evaporation system consists of three atmospheric pressure evaporators in series, the first larger than the last two, containing inlets and outlets on approximately the same level so that the solution flows through by gravity. These are heated by horizontal bundles of steam coils to maintain a vigorous evaporation of solvent. The miscella ent.ers the first, of t,hese a t t,he rat,e a t u-hich it comes from the extractor. The vapor from these evaporators passes through condensers and directly to the work tank. From this tank a small part of the loxer layer, which contains some moisture, is pumped to the separation t,ank,whereas the major port,ion is pumped to the high tank supplying the extractor. From these evaporators the solution passes, by gravit>y,to a falling film evaporator. This unit consists of a vertical bundle of 212 tubes, each 8 feet inches in length and 1 inch in external diameter, encased in a steel jacket. I t is heated by steam a t 15 to 20 pounds per square inch in the jacket. The oil solution runs down the inside of these tubes. By this process the concentration of oil in t.he solution is increased from 55-60YG to about 90%. The vapors pass t,hrough a heat exchanger which is used to heat the full miscella, thence t,o a condenser, and into the separation tank. The next unit is a vacuum column, which is a steel cylinder 2 feet 8 inches in diameter and 44 feet high, cont,aininga30-foot section of 1'/2-inch ceramic Berl saddle packing which extends nearly to the top, The oil is pumped into the top of this column, which operat,es a t 21 to 22 inches of mercury vacuum, and flows down over the saddles and is collected in the unpacked space a t the bottom. Live steam is introduced into the column a t the bottom and also just above the oil level, so that it. bubbles up through the oil and passes upward through the column in contact with the downward current of oil to give steam distillation or scrubbing. The steam and solvent pass out the top of the column through a condenser and into the separation tank. The oil residue, now containing 99.5 to 99.7% oil, is pumped through a double-pipe, steam heat exchanger to bring its temperature to about 245" F., and into the top of a second vacuum column which serves as a final stripper to ensure complete solvent and moisture removal. This is an unpacked steel column, 4 feet in diameter and 30 feet high. I t operates a t 26 to 28 inches of mercury vacuum. The oil is sprayed in a t t,he top of the column while live steam is injected into the bottom. The vapors are pulled out the top and through a barometric condenser and steam jet ejector to the sewer. The residue, which is the final oil product and is now 99.85 to 99.95% oil, is pumped out the bottom of the column to the storage tank. The average yield of oil per 540-pound basket of beans is 96 to 100 pounds, constituting 97 to 987, extraction as shown later. FLAKES AND MEAL. The flakes entering the extractor contain 17 to 19% oil and 9.5 to 10% moisture. These are loaded
about 540 pounds per basket. After completing the solvent cycle, as described, they are drained approximately 5 minutes between the last spray position and the dump position in the extractor tower. At this point the basket load weighs about 690 pounds and contains about 35% solvent, 7 to 8% water, and. 0.4% 011. l ' ~ e s ewet flakes are carried into the meal desolventizing system made up of one unit on each of two sides of the extractor. Each unit consists of two, six-high tiers of Schnecken conveyers connected in series. The two tiers operate in parallel flow and empty into the same deodorizing drum a t the bottom. The Bchneckens are steam-jacketed paddle conveyers 16 feet 8 inches long by 19314 inches inside diameter. The meal passes from these units into the drum a t a t'emperature of about 150" F., and still cont'ains about' 10% solvent. The deodorizing drum is, a st'eam-jacketed 4-foOt diameter unit cont'aining a steam coil mounted around the center shaft of a ribbon-type screw conveyer equipped with lifting blades to agitate the flakes. Live steam is b l o m into this drum to remove the last traces of solvent from the flakes before they are discharged a t about 200" to. 220" F. About 20 minutes are required for flakes to pass through the system from the extractor to the drum discharge. The flakes from the drums on both of the units discharge intD the same screw conveyer, where some moisture is removed by flash evaporation, and then int,o a Redler conveyer for rnoveinent to the meal manufacturing building. The average yield per 540-pound basket is about 430 pounds of meal with a moisture content of about 8% and oil content of 0.6 to 0.7yG. From this meal 97 to 98% of the oil has bccn extract,ed: (540 X 0.184) - (430 X 0.0066) X 100 540 X 0.184
=
07.27,
The vapors from the Schneckens and deodorizers pass through dust settling chambers and into condensers. The eondensate flows into the separation tank, where the water is separated from the solvent. Throughout the process, beginning with the extractor, solvent vapois are present. iill of the units involved are vapor-tight but each is vented directly to a surface condenser or, in the case of evaporators, has its corresponding condenser vented to the vapor recovery system. The condensate is carried to the separation tank. The final vent from the recovery system is connected through a pair of activated carbon adsorption towers from which solvent-free air is discharged to the atmosphere. Uncondensed vapors are adsorbed by the activated carbon towers which are alternately regenerated with steam and the effluent condensed and carried to the separation tank. The solvent loss during the entire process averages approximately two gallons per ton of beans processed. ECOIVOMICS
The following cos6 and materials relations are based on experiencp in a plant % here both the solvent and screw press methods are used to process 130,000 to 150,000 tons of beans annually by each method: Yield, % of Total Screw Solvent press extraction method method 14.0
83.4
17.4 79.0
2.6
3.6
.4ssuming, in consideration of prices current during the snmmer of 1947 that oil is worth $0.20 per pound, meal $60.00 per ton, and the cost of beans is $3.00 per bushel, the following margin will be produced on the basis of the above yields: Screw
Income per bushel Oil Meal Total oil and meal Less cost of beans per bushel Materials margin
Press Method $1 68 1 50 $3.18 $3 00 ___ 80.18
-
Solvent Extraction Method $2 08 1 42 __
S3 50
s3 00 __ $0 50
The materials margin difference amounts to 80.32 per bushel or $10.70 per ton. Solvent cost, peculiar to the extraction process, does not modify this relation appreciably, amounting to only about $0.30 per ton
INDUSTRIAL AND ENGINEERING CHEMISTRY
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193
or less than $0.01 per bushel, Subtraction of this amount still leaves a difference of $10.40 per ton, and a highly favorable ratio for the solvent process:
materials margin ratio
solvent
screw press
::::
_-
=
2.71
Operating cost comparisons, excluding supervision, depreciation, insurance, etc., per unit of product for plants’of equal capacitv are: Processing Cost Ratio, SolventScrew Press
Percentage of Total Cost Bolvent extraction Screw press method method
e
FUTURE DEVELOPMENTS
Thus the development of a successful solvent extraction plant is described and its operation shown. The amount of chemical engineering thought and the ingenuity required to reach this stage have been greater than can be readily discerned from a brief description. However, the process cannot be considered t o be perfected as yet. In the development described and in studies incorporated by Central Soya into a new plant now under construction, other possibilities for improvement have been seen as highly desirable. Continuous sampling and moisture recording and control throughout the bean cracking, bean conditioning, and flaking operations, as well as in the entire meal desolventizing and deodorizing stages, would contribute greatly, through uniformity of adjustment to conditions, to a more thoroughly efficient operation and better product. I n connection with this operation, as well as others, instrumentation offers promise. This is particularly true for improved temperature control and regulation by which detailed operator attention could be reduced and variations eliminated. I n the same way, further application of instrumentation may be made to regulate and coordinate the flow of matetials between unit operations. The quality of final products, as well as refining losses, is greatly affected by the presence of even small quantities of foreign insoluble substances in the oil. Thus any increases in efficiency of
Flaking Rolls Where Flakes about 0.008 Inch Thick Are Produced front Conditioned Beans
filtration are valuable. Leaf filters used in filtering the miscella, while removing the major portion of solids, do not produce the highest possible clarity. Studies of this problem are expected t o yield valuable results both in quality of products and economical operation. In handling high-protein materials, heat always offers a threat of decomposition or change, and the same is true in the case of the oil. Thus improvement in removal of solvent from either -the meal or oil, a t lower temperatures to preserve the original quality of the material yet, give solvent-free products, would constitute desirable progress. Some of the improved types of evaporation equipment, such as rising film evaporators of natural or forced circulation type, may have merit in application to this problem. In the operation of a solvent extraction plant there are, of course, occasions for shutdowns for cleaning, repair, or adjustment. I n such cases the clearing of solvent vapors is necessary and offers a problem in large units such as the extractor or evaporators. This problem is being solved in new plants by the installation of an inert gas purging system which will minimize time in shutdowns and improve the safety considerations. The machine operates by the combustion of fuel oil with air, giving an oxygen-free gas which is blown through the system t o remove all solvent vapors. In view of the present growing concern over underground water supply, cooling water problems demand consideration and would apply to the solvent extraction plant which uses large quantities of cooling water. This indicates the importance of studies of condenser design, heat transfer coefficients, tube construction, vapor and cooling water velocities, use of recycled water from cooling towers, and related problems. Extensive experience with solvent extraction has indicated the necessity for particular attention t o mechanical details which have bearing on repair and maintenance cost. I n the designing of future plants special consideration is being given to more exacting specifications and greater factors of safety on such items as bearings, size and composition of shafts, intricate control mechanisniP, Lower Section of Extractor Unit and other precision machinery. Extractor and control board for Schneckens; steam-jacketed drum (right) where live Another primary factor in the succebs steam is injected to deodorize meal and remove laat maces of molvent; cone-bottom of any plant is the exacting training of superdust collector between Schneckenm and condennera in vapor receiving system.
194
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ins tallation of Equipment in Extraction Building of New Central Soya Company Plant at Gibson City, Ill. visors and operators in the unit operations of the plant. Not only should exacting conditions be known and understood but the requirements should be observed. Increasing observation of this practical principle is always desirable in any process and is being taken into consideration by Central Soya in the operation of its new plant. Supervisors are trained in the existing plants, and new units are startrd under a corps composed of these men and experienced veteran operators from existing plants. Using experience gained from development and operation of present installations, these supervisors will rigorously train new operators in the operation of the new plant a t the most highly developed levels of the technique.
Vol. 40, No. 2
in these years if profits can be made, some beans will be procured and processed in screw presses by operators without solvent plants. The processors of soybeans may be divided into two groupsnamely, the large processors and the small processors. The large processors usually operate both screw press and solvent plants whereas the small processors usually operate onlv screw press plants. If the margin between the cost of beans and the prices of oil and meal decreases, the operation of a screw press plant may become unprofitable. However, ti?o factors will contribute to the continued operation of some screw presses w e n when operation is unprofitable for others. I n the first place a company Lvhich operates both screw press and solvent plants has service departments t o maintain, whether the solvent and screi-i press plants or only the solvent plants are operating. This overhead expense is not diminished in proportion to the decreased production if the screw presses ale shut down. A spreading of bhe overhead expense to as much production as possible is desirable because the unit cost of the overhead is thus loivered. If beans are available, the screw presses should operate as long as receipts from their finished products are as great or greater than the actual proceasing costs. Secondly, there are many small screw press plants mhivh operate with lower costs for labor, bean handling and storage, supervision, service department, and other expenses than the large processors. These may continue to operate profitably on a smaller bean cost-oil-meal price differential than the large processors. Some screw prebs mills are equipped l o mill oil seeds other than soybeans. This enables them to obtain a lower unit operating expense by purchasing whatever beans they can process at a profit, and completing the year’s operation with other oil seeds. Until their present equipment (much of it installed during the war) is worn out or requires excessive repairs, these small screw press processors ivill continue to operate to some extent. Therefore, even though the solvent process is more profitable than screw pressing, it is apparent that there will not be a complete replacement of the screw press units in the next few years. There is the fact that solvent plant operators can afford to pay more for soybeans, which in turn may affect the screw press processor by placing his raw material price out of the range for profitable operation. Hence, the increasing ratio of solvent extraction capacity to screw press capacity could exert some influence in raising thr price of soybeans.
SUMMARY AND FUTURE OUTLOOK
This article has described a plant for the solvent extraction of oil from soybeans and has briefly compared its cost of operation and monetary return with that of a plant for extraction of oil by the screw press process. The solvent process was shown to be considerably more profitable ; this naturally raises the question of how soon the solvent process will replace the screw press process to the extent that these two have replaced the hydraulic process. Of course, the answer to this question is largely dependent on future economic conditions. Hoivever, some natural trends in the soybean processing industry may be predicted. As shown in Table I, the trend in new processing plants is strongly in favor of thc solvent process. Since the end of the Tar this trend has increased considerably. However, even a t the present rate of increase, a number of years will pass before the solvent processing capacity is great enough to process the entire bean crop a t its present level. Some screw presses will be run until that time. In a year of small bean crops, when a company operating both processes cannot purchase enough beans t o operate both a full capacity, the screw press naturally will take what beans are left after the solvent process has been applied to its capacity. Even
ACKNOWLEDGMENT
The authors wish to acknodedge, with appreciation, the valuable technical assistance of R. L. Thomas of the Central Soya Company in the preparation of this article. LITERATURE CITED
Allis-Chalmers Mfg. Co., ‘Continuous Solvent Extraction of Cottonseed Oil” (1947). Goss, W. H., Oil M i l l Gazetteer, 51, 11 (1947). Goss, W.H.,Oi2 & Soap, 23, 241 (1946). King, C. O.,Katz, D. L., and Brier, J. C., Trans. Am. I n s t . Chem. Engrs., 40,533-66 (1944). Markley, K. S., and Goss, TV. H., “Soybean Chemistry and Technology,” p. 147, Brooklyn, Chemical Publishing Co., 1944. Ibid., p. 148. Ibid., p. 150. Ibid., p. 199.
McKinney, R. S., Rose, W. G., and Kennedy, A. B.. IND. END.CHEM.,36, 138, (1944). Osburn, J. C., and Kats, D. L., Trans. Am. Inst. Chem. Engrs., 40,511-31 (1944).
Tray, S. E., and Bilbe, C. W., Chem. Eng., 54, 139 (1947). REcrnIvl~November 4,1947.
