Extraction of Soybeans
I
M e c h a n i s m with V a r i o u s Solvents
DONALD F. OTHMER and WALTER A. JAATINEN Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
Commercial extractors for soybeans can be designed by using simple, bench-scale determinations to give constants for rate equations which are dependent on the Hagen-Poiseuille relations for viscous fluid flow
OF
THE 2], billion pounds of soybean oil made in the United States each year, oil by solvent extraction is increasing brcause of higher yields and superior product. Extraction is basically a problem of fluid dynamics of the solvent in the intricate capillary system of the cells in the crushed flakes ( 7 ) ; extraction rate is defined by the Hagen-Poiseuille law covering viscous flow in capillaries and is independent of molecular diffusion, and almost unaffected by countercurrent action. usually held important in extractor design and operation. A technique based on simple laboratory determinations was developed to give immediately the necessary data for designing plant extractors. T h e logarithmic plot of residual oil remaining after laboratory extraction us. time is a straight line. This gives immediately both the requisite time for extraction and the volume of a commercial extractor. Only one solvent, hexane, was used. T h e present study investigated the applicability with other solvents of the postulated fluid dynamics and mechanism, methods and technique for determining data for design, and nonessentiality of the usual countercurrent use of solvent.
Technically pure solvents were chosen for their physical properties. B.P., O c. 56.1 101.6 46.3 76.8 61.2 82.4 101.5 110.6 87.2
Soybean flakes (Spencer Kellogg Co.) from standard cracking and flaking roll operations, had average thicknesses of 0.007.0.0125, and 0.023 inch by statistical measurements. T h e 0.0125-inch flakes were from a different batch of beans; variables Tvere minimized by conditioning all flakes to approximately the same equilibrium moisture content. Laboratory methods of extraction, analysis, and other techniques were as described ( 7 ) . A slightly different shaking apparatus was used; but the degree of agitation is relatively unimportant (2). “Bound oil” (that remaining in flakes
\\ere also made for one Meek without continuous agitation; it was assumed that equilibrium \vas reached. Ihere Mas practically no change in C (residual oil) bet\\een 500 m i n u t e and 1 u e r k . Discussion of Results -4 logarithmic plot of C zs. t gave straight lines, conforming to the equation previous1)- used (Figure 1 and Table I). C = mt--b (1 1 T h e slope of the line is b; ~n is a constant depending on flake thickness. The line breaks abruptly; extraction stops at different values of residual oil, depending on solvent and oil concentration. Different Solvents. The genrral character of the extraction lines on the logarithmic plot is similar for all solvents.
Table 1. Summary of All Extraction Equations Initial Extracting Miscella Concn., K t .
Solvent
56
h
m
hIin. Applicable Limit of C Detn. by Equilibrium
Oil
Average Flake Thickness, 0.0125 Inch Acetone Carbon tetrachloride
Experimental Procedure
Acetone Butyl bromide Carbon disulfide Carbon tetrachloride Chloroform Isopropyl alcohol (987,) Nitromethane Toluene Trichloroethylene
Mithout loss in weight after repeated extractions with fresh solvent for one week), total extractables, and moisture were determined a t 25” C. Series of runs Ivere made \\ith different amounts of oil in solution in the different solvents and different flake thicknesses. Flasks containing samples of the oil-bearing flakes and five times the Lveight of solvent were shaken; the oil extracted was determined by evaporating a sample of the solvent withdrawn from each flask 1, 2, 4, 7 , 12, 20. 35, 60, 120, and 500 minutes after starting each run. T h e per cent concentration of oil i n the miscella remaining after each extraction time \vas the difference between the original amount and the amount extracted, calculated b>- a material balance. Extractions
0-20 31.06 45.15 0
9.23 24.05
Chloroform Trichloroethylene Carbon disulfide Butyl bromide Toluene Isopropyl alcohol
0
10.026 25.137 0-10 24.672 0
20.321 34.123 0-10 29.43 0-10 31.236 0
16.5 17 18 17.5 18 31 12.5 13 20 12 15.0 7.4 8.2 11.5 15.5 20 14 18.5 39
-0.46 -0.422 -0.319 -0.635 -0.526 -0.282 -0.55 - 0.444 -0.355 -0.645 -0.372 -0.782 -0.69 -0.36 -0.671 -0.554 -0.35 -0.168 -0.44
0.034 0.0343 0.0673 0.0145 0.0266 0.1289 0.0161 0.0235 0.0598 0.0129 0.0397 0.0061 0.0093 0.0254 0.0124 0.0241 0.0238 0.0926
...
Average Flake Thickness, 0.023 Inch Acetone Carbon tetrachloride
0-20 2.5.279 35.12 0
14.248
29 33 30 34 40
-0.385 -0.368 -0.255 -0.477 -0.435
0.0449 0.0485 0.0757 0.0336 0.0362
Average Flake Thickness, 0.007 Inch Acetone Carbon tetrachloride
0-20 31.002 40.672 0
9.987
8.2 9.0 9.6 10 10.5
-0.332 -0,287 -0.203 -0.617 -0.535
VOL. 51, NO. 4
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:t
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I 1 1 1 1 1
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EXTRACTION TIME I N MINUTES
Figure 1.
Residual oil after extraction gives a straight line a logarithmic plot, down to a value where extraction ceases
vs. time on
Extraction of 0.01 25-inch soybean flakes with 1, nitromethane. ?, isopropyl alcohol. 4, toluene. 5, carbon tetrachloride. 6, chloroform. 7, butyl bromide. 8, trichloroethylene. 9, carbon disulfide. For acetone, 3, an average i s given using misceiia containing O to 2070 oil at start
However. nitromethane is unsuitable for extraction because of immiscibility with soybean oil (Figure 1). The small amounts of material extracted may represent matter other than oil, which is probably also present in other extractions. Isopropyl alcohol gave a relatively loiv initial extraction rate, probably because of IOW solubility of the oil at the temperature used (commercial extractions use a higher temperature). Leaching of moisture frorr. the flakes mav account for the break in the line. Miscella Concentration. Previously. Concentration of oil as high as 207, in hexane solvent (miscella) had no effect upon extraction rate ( 7 ) . In this study with ever\ solvent except acetone. extraction rates decrease with lO7c or more oil in the miscella. Figure 2 represents a typical plot for miscella based on carbon tetrachloride. Effects of other solvent concentrations are indicated in Table I. Acetone gave no discernible decrease in rate, until miscella concentrations above 20% were used. 0 07
~
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10
100 EXTRACTION TIME I N MINUTES
I
loo0
Figure 2. Carbon tetrachloride line with 0" oil shows faster and more complete extraction of 0.01 25-inch soybean flakes than synthetic miscella containing 10% oil or rniscella with 25y0 oil
Values a t 1 minute (log 1 = 0) are plotted in Figure 5. Specific surface, 5':for each flake thickness, calculated from the specific volume for 0.007-, 0.0125, and 0.023-inch-thick flakes, was 25.8, 14.4, and 7.8 square inches per gram inerts. respectively. Figure 5 shows that
As extraction proceeds, residual oil always decreases to a minimum where extraction stops. This value of "equilibrium" oil represents bound oil plus oil extractable Lvith fresh solvent, but now contained in the extracting miscella in the voids or in equilibrium. The equilibrium oil is higher with higher miscella concentrations, (Table I and Figure 3 for residual oil after 500 minutes' extraction). Flake Thickness a n d Specific Surface. Flakes 0.023 and 0.007 inch thick Ivere extracted \vith acetone and carbon tetrachloride, representing physical properties seemingly at extremes. The constants in Table I and the lines of Figure 4 varied for carbon tetrachloride. The batch from which the 0.0125-inch flakes came affected the extractability and hence slope of the line in Figure 4. Flake thickness controls the specific surface, S, available for extraction. The extraction rate dropped considerably with thicker flakes, as before ( 7 ) ; and the same correlating equation held. m depends on thickness (Figure 4).
m = 2.7 S-1
This, combined gives:
cs
=
Xvith 2.7t-t,
Equation
(2) 1. (3)
Similarly for the acetone extractions: CS = 2 . 3 t - h (41 The actual value of slope b for each flake thickness should be used unless the family of curves determined for each flake thickness (Figure 4) are parallel. .As the equilibrium residual oil for each flake thickness of C is approached, the experimental data vary from these equations. Extraction Rate (weight of oil removed per unit weight of inert solids per unit time). This is the negative of the value of the rate of change of residual oil content: C'. Equations for different
I
0
2 006-
E 1 1I
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I-
w Lt
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5 005-
>
n
5 goo2
2
g 001 2 1
W 0
0
/!
20
1, chloroform. 2, trichloroethylene. 3, acetone. 4, carbon disulfide, 5, toluene. 6, carbon tetrachloride. 7, butyl bromide INDUSTRIAL AND ENGINEERING CHEMISTRY
A 0
-
10 100 E X T R A C T I O N TIME IN M I N U T E S
50
Figure 3. Various solvents extracting 0.01 25-inch soybean flakes show increase of equilibrium residual oil with increasing miscella concentration
544
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30 40 WEIGHT PERCENT MISCELLA CONCENTRATION 10
0
Figure 4.
Carbon tetrachloride shows more complete o.ol 25- or 0.007-inch flakes extractions of 0.023-inch soybean flakes than 0.
Averages of several runs
SOYBEAN EXTRACTION flake thicknesses are obtained by differentiating the equation for C and substituting values of the constants for the flake thickness (Equation 2 and Table I ) . Thickness, Inch
Equation
I
a Extraction w i t h Carbon Tetrachloride w 0.007 -dC/dt = 0.1235 C2.62SL.62
0.0125 0.023
-dC/dt -dC/dt
The rate of extraction with acetone did not decrease until the miscella concentration exceeded 20'30; with carbon tetrachloride miscella it decreased a t approximately l0y0 oil. As the oil concentration u p to approximately IO'% had no effect on extraction rate, it can be inferred that extraction is not irnproved by countercurrent operations a t low miscella concentrations. If the oil in a miscella is initially high and its physical properties are greatly different from a miscella of a lower oil content, the rate of extraction will be lower. Capillary Hydraulics. T h e rate of viscous flow through a system of capillary passages is defined ( 7 ) by a modified ,Hagen-Poiseuille equation :
3-
0.1334C2.6i5S'.5i6 I0.0587 C3.09S2.09 z
-
-
--
-
Extractions w i t h Acetone
0.007
0.0125 0.023
-dC/dt 0.0273 C4.0'S3.0' -dC/dt = 0.0755 C3.17S2.1i -dC/dt 0.0442 C3.60S2.60 0.1 -
Logarithmic plots in Figure 6 are straight lines, showing the tremendous decrease of rate for a decrease in residual oil; for carbon tetrachloride extractions on a 0,007-inch flake, a tenfold decrease in residual oil decreases extraction rate 440-fold. Similarly, the rate depends on the surface area or thickness of the flake, and varies as an exponent, a power of S from 1.575 to 2.09, depending on flake thickness. Hence, carbon tetrachloride is superior to acetone, which compares with hexane ( 7 ) . Bound or Unextractable Oil. Oil concentration in the miscella affects extraction. The maximum possible extraction \yas determined by long-time extraction with fresh solvent until no further oil was removed. This eliminated an effect of oil in the solvent on the equilibrium values. The unextractable remaining oil is the bound oil: for 0.007-inch flakes, 0.00656 gram of oil per gram of dry inert solids; for 0.0125inch flakes, 0.00754; for 0.023-inch flakes, 0.0151. Increase of flake thickness thus increases bound oil and these values are expressed by C = 0.28L-0.778. Disruption of bean cells in flaking prior to extraction is evidently greater for thinner flakes, to give less bound oil.
IO
I SPECIFIC
Figure
SURFACE
40 IN SO. IN.
5. Logarithmic plot
A.
Intercepts m of lines in Figure 4 at 1 minute (log I = 0)give straight-line function on logarithmic paper vs. specific surface, S, o f flakes;
,, = 2.7
-dC/dt
=
K3 ( ? ) f ( C )
s-1
where dC/dt is the change with time of residual oil during extraction, K is a constant, y is surface tension, p is density, and p is viscosity of the miscella. C defines other aspects of the residual oil and of the system as a whole which are constant within the experiment. The rate equations derived from experimental results are also a function of C. Thus, the extraction rate increases with increase of surface tension arid density, and decreases with increase of viscosity (Table 11). y p / p always decreases with increasing miscella concentration (Figure 7), contrary to the values for hexane below 20% oil in miscella, which are almost constant, a fortuitous chance of previous studies (7). All deviations from the original extraction curves using pure solvent occur when y p / p decreases below approximately 30. T h e rate of extraction for the solvents a t the same time of extraction decreases in practically the sarne
8. Intercepts m, from acetone extractions give function similar t o Figure 4; m = 2.3s-'
Mechanismof Extraction Initial contact of solvent dissolves oil from the flake surface and the outer layers of broken cells. T h e solvent then penetrates into the voids and capillaries to dissolve more oil. Concentration differences occur in the maze of capillaries; and capillary flow moves the oil nearer the bulk miscella, where further solution occurs. As oil concentration in the flake decreases, the concentration gradients decrease, and oil concentration increases in the extracting miscella. T h e potential for flow, therefore, also decreases; and the rate decreases rapidly down to the equilibrium oil condition, where further extraction ceases. Changes in some physical properties of the miscella as oil concentration increases control the rate of oil extraction.
80
-
--
decreases tremendously as residual oil decreases and flake thickness increases from 0.007 to 0.0125 to 0.023
70
&
6o
8 A. B.
Carbon tetrachloride Acetone x 40 2
trichloroethylene. 3, acetone. carbon disulflde. 5, toluene. G H S O I L / G M INERT
SOLIDS
OMS OIL/GM INERT SOLIDS
previous work ( I )
4, 6, W E I G H T PERCENT OIL IN S O L U T I O N
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order when determined a t the miscella concentration for the same time. Methods for Extractor Design
Commercial extractors may be designed from data secured as above, with modifications of earlier technique which correlated laboratory data with Only simple laboratory plant data (7). bench measurements are required; a flask in a simple mechanical shaker is used as a contactor. Operational requirements should first be established, including the retention ratio and the maximum tolerable values of residual oil in the extracted solids for discharge. The bound oil is determined by extended extractions with fresh solvent for at least two flake thicknesses. A logarithmic plot of bound oil against flake thickness gives a straight line which determines the retained oil in the solids under the best extraction conditions-viz., pure solvent contact. Maximum flake thickness is thus fixed. Laboratory extractions are made for the selected flake thickness and one other, at two or more extraction times, to determine the residual oil, C. C is plotted on logarithmic paper
against the time of extraction, t. The straight lines obtained have the equation C = mt-b, in which the applicable limit of C is determined by the amount of bound oil in fresh solvent extractions, or by the equilibrium oil. The contact time necessary to obtain the desired C may be determined from these lines by inspection. If the desired valve is not approached with the original flake size, a thinner flake is specified. The effects of low miscella concentrations on the final value of C are shown. Extractions should also be made with higher initial oil-solvent concentrations to determine the maximum allo~vable miscella concentration for extraction without affecting the rate of extraction or the final amount of residual oil. The maximum allowable miscella concentration allows calculation of the minimum volume of solvent required. The extractor volume (the sum of the volume of all sections) is that required to give the necessary holdup or extracting time. The amounts of solids in contact with the solvent required to extract the oil and give the tolerable final miscella concentration are specified. The Xvashing of miscella from extracted solids should be considered a separate operation, althouqh the fresh solvent after such use $vi11 be used for extraction.
Table II. Variation of Density, Surface Tension, Viscosity, and Ratio of Surface Tension X Density/Viscosity with Various Solvents in Soybean Oil Surface Oil in Density, Viscosity, Tension, Soh., Dynes/Cm. G./Cc., CPS., Solvent ?P/,U Y YP % P @ 26.7 39.5 0 1.48 0.563 70.2 Chloroform
Carbon tetrachloride
Carbon disulfide
Toluene
Acetone
Butyl bromide Trichloroethylene
546
5.72 14.07 25.12 35.12 45.21 0 4.42 6.92 15.85 25.5 35.13 48.3 0 7.65 15.95 25.32 36.20 49.33 0 10.49 26.7 36.71 46.21 0 4.88 9.43 19.76 25.0 31.5 38.0 55.0 0 15.95 25.8 0 8.816 27.56 50.85
1.423 1.354 1.212 1.095 1.141 1.584 1.534 1.501 1.417 1.308 1.225 1.141 1.26 1.184 1.218 1.135 1.081 1.032 0.862 0.849 0.869 0.879 0.880 0.789 0.791 0.794 0.816 0.822 0.827 0.831 0.868 1.236 1.176 1.138 1.44 1.378 1.244 1.119
0.782 1.248 2.02 5.21 7.71 0.968 1.225 1.34 2.31 3.38 5.2 8.7 0.368 0.508 0.80 1.24 2.15 4.10 0.59 0.74 1.103 1.66 2.53 0.329 0.356 0.415 0.625 0.760 1.042 1.23 3.43 0.595 1.085 1.21 0.537 0.875 3.64 5.43
INDUSTRIAL AND ENGINEERING CHEMISTRY
26.7 26.4 26.6 26.8 27.1 26.8 27.8 25.3 26.9 26.9 27.4 28.0 33.5 33.4 33.4 33.6 33.6 33.6 28.4 27.4 28.0 28.5 28.7 23.7 22.8 23.6 24.4 24.9 25.4 25.7 27.7 22.9 23.16 23.7 27.6 28.1 28.9 30.3
38.0 35.8 32.3 29.4 31.0 42.5 42.6 38.0 38.1 35.2 33.6 32.0 42.3 39.6 40.7 38.2 36.4 34.7 24.4 23.2 24.3 25.0 25.2 18.7 18.0 18.75 19.9 20.5 21.0 21.4 24.0 28.3 27.3 25.2 39.8 38.8 35.9 33.9
48.4 28.4 16.15 5.88 3.86 44.0 34.8 28.2 16.5 10.42 6.45 3.67 114.6 70.8 50.7 30.7 16.9 8.46 41.7 31.4 22.1 15.1 9.96 56.8 50.5 45.2 31.8 27.0 20.1 17.4 6.95 47.5 25.1 22.2 74.2 44.3 11.4 6.24
Conclusions
The residual oil and rate of extraction are exponential functions of the time of extraction and are continuous functions, down to the equilibrium oil content, which defines maximum extraction and is a function of miscella concentration and flake thickness. The rate of extraction increases greatly with decrease in flake thickness and increase in residual oil and is an exponential function. The extraction action of different solvents depends on relative physical properties. Rates of extraction are increased by low, viscosities, high densities. and high surface tensions. If the physical properties of the oilsolvent solution change markedly with concentration, increase of oil lowers the extraction rate and the maximum extraction obtainable or increases the bound oil. Solvents less dense than soybean oil, whose solutions \vith soybean oil change very little with respect to surface tension and viscosity (if initially lo^), affect extraction rate Lvith increasing miscella concentrations less than solvents of greater density than the oil, which have solutions with soybean oil relatively constant in surface tension and viscosity. Acetone solutions exhibited constant extraction rates from 0 to ZOyc by weight of oil, as did hexane ( 7 ) ; both have effectively constant values for this range ofdkl.
The Hagen-Poiseuille equation for viscous flow in capillaries expresses the rate of extraction as a function of the residual oil, and shows the potential driving force for extraction to be a ratio of the physical properties, y p / p . Experimental results of the extraction rate decrease approximately in the same order as for miscella Concentration. For any thickness of flake, and any solvent, rate of extraction does not decrease a t y p ; p above 30. The amount of bound oil increases with flame thickness. Extractions with acetone correlatcad \vel1 with those with hexane ( I ) . Other solvent systems exhibited several variances, especially hvhen the oil in the miscella affected its physical properties. Laboratory techniques allow the ready and confident design of commercial extractors, by methods nolv corroborated and extended with some modifications. Acknowledgment
The authors thank the Spencer Kellogg Co. for supplying sol bean flakes. Literature Cited (1) Othmer, D. F., Aqarwal, J. C., Chem. Eng. Proqr. 51, 372 (1955). (2) Ruth,'B. F., Ibtd., 44, -1 (1948).
RECFIVED for review April 10, 1958 A C C E P T E D October 23, 1958
