Ultrasonic lnsonation Effect on liquid-Solid Extraction DUDLEY THOMPSON AND D. G. SUTHERLAND‘ Department o f Chemical engineering, Virginia Polyfechnic Insfifufe, Blacksburg, Va.
As a
limited and preliminary step in preparing for an investigation of the effect of ultrasonic energy on mass transfer in the unit operation of liquid-liquid extraction, the effect of 400-kilocycle insonation on liquid-solid extraction was studied utilizing the system n-hexane and peanuts. Employing as a basis of calculation the crude oil obtained in a Soxhlet extractor during an 8-hour period, extraction efficiencies were calculated. At a maximum intensity of 63.3 volt-amperes per square centimeter (calculated as intensity of radio frequency energy produced at the power tubes) the extraction efficiency in 6 minutes was 90.2%, 2.76 times the efficiency of the control sample maintained at ambient room temperature. Increased extraction was postulated to be the consequence of extension of phase boundaries by dispersion of adhered particles and partial disruption of oil cells, agitation selectively at phase boundaries where there is a mismatch of acoustical impedance and gross stirring of solvent and suspended solids, and thermal effect.
A
S A limited and preliminary step in preparing for an investigation of the effect of ultrasonic energy on mass transfer In the unit operation of liquid-liquid extraction, the effect of 400-kllocycle insonation on liquid-solid extraction was studied utilizing the system n-hexane and peanuts. Interest shown by a number of Investigators in this work, which is admittedly incomplete, has prompted its submission for publication a t this time, primarily for the reason that there has been a remarkable absence of published accounts dealing directly with the unit operations relatlng to mass transfer. There is reason to believe that some work has been done in this area even though it has not been published. However, the physical and chemical effects of ultrasonic energy have been widely reported ( f - f g , 16, 19,do). Solvent Extraction of Protein from Brewer’s Yeast Employing Audible Insonation. Solvent extraction of protein from brewer’s yeast with a 5’% sodium chloride solution in the presence of an acoustical field was reported ( 4 ) . A magnetostriction generator was employed with frequency in the audible range. Data show that insonetion produced slightly higher extraction values than were obtained in noninsonated control tests. Increased extraction was accredited to the dissolved gases in the water diffusing through the yeast cell membranes, making possible explosion of the yeast cells during insonation. Extraction of Oil from Fish Materials Employing Insonation. A patent wa8 granted (13) for a process whereby audible and inaudible lnsonation was applied to the extraction of oil from fish material. Frequency of insonation was stated not to be critical, when high power level was maintained, although frequencies low in the audible range were employed. Fish fragments were coarsely ground to produce a pulpy mass which was subjected to compressional vibrations at high power levels. Inteneity was suEcient to produce cavitation within the pulpy mass, causing rupture of cellular fish materials. Fish oils were released from the oil cells and could be removed mechanically. Unit Extraction Operation. Previous investigators a t the Virginia Polytechnic Institute have pointed out the chemical engineering aspects (14)of the applications of ultrasonic energy and emphasized the unit operations ( 16, 17‘) and unit processes Present address, U. 8 Army.
June 1955
(18). As an extension of the investigations of the application of ultrasonic energy to the unit operations, the effect of this form of energy on the unit operation liquid-solid extraction was studied. I n particular, peanuts were selected as the solid from which the oil was to be extracted with the solvent n-hexane. There was no particular reason for the specific selection of the system, except that peanuts were readily available and data had been collected on the extraction of peanut oil with n-hexane in the absence of ultrasonic energy.
Experimental The purpose of this investigation was to determine the effects of intensity and time of 400-kilocycle insonation on solvent extraction of oil from peanuts. Plan of Investigation. Oil was extracted from crushed peanut meat with n-hexane in the presence of a 400-kilocycle ultrasonic field. Employing as a basis the mass of crude oil extracted with n-hexane in a Soxhlet extractor during an 8-hour period, extraction efficiencies were computed for 6-minute periods of insonation a t various intensities of ultrasonic energy above the cavitation level. I n order that a concept might be obtained of the equivalent mechanical stirring required to bracket the efficiencies produced by insonation, tests were performed t o ascertain the values. During insonation the thermal paths were observed. These thermal paths were repeated in the absence of insonation to demonstrate the fact that the extraction obtained was not solely due to thermal energy. Ultrasonic Energy. The source of ultrasonic energy was the Crystalab Ultra-Sonorator, model number SL 520. Figure 1 shows the electromechanical transducer which converts high frequency electrical energy to high frequency mechanical energy. The four stable crystal-controlled frequencies available were 400, 700, 1000, and 1500 kilocycles per second. Only the 400kilocycle frequency was employed in these tests. Intensities. Under the conditions of this investigation, less than half of the energy produced a t the power tubes of the generator appears as acoustical energy within the reaction vessel in conducting ultrasonic tests ( 17). Estimates by other investigators employing different generators and transducers indicate that the ratio of acoustical energy realized to high frequency
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ascertain the rate of agitation, in revolutions per minute, that would produce on the same time basis an extraction efficiency less than the lowest intensity insonation employed and likewise the rate of agitation that would produce an extraction efficiency greater than the highest intensity insonation employed. Comparison of Insonation with Thermal Effects. Thermal-time curves were plotted for each path of the four intensities of insonation. I n the absence of ultrasonic energy the dynamic thermaltime paths were repeated on similar test samples. The purpose of these tests was to demonstrate the effect that thermal energy alone had on the extraction efficiency. Data. The effects of intensity and time of insonation on the %hexane extraction of oil from peanuts are given in Figure 2. The effects of mechanical agitation on the n-hexane extraction of oil from peanuts are given in Figure 3. Data for extraction of peanut oil with n-hexane under similar thermal conditions encountered in insonation are given in Figure 4.
Discussion
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Extraction efficiency was increased (90.2/32.6 = 2.76) by 400-kilocycle insonation. Maximum increase in extraction efficiency (61.4/32.6 i= 1.88)following the thermal path of maximum insonation could not account for the increase in extraction efficiency of insonation observed and would indicate that the phenomenological mechanism of the effects of insonation are complex. Agitation supplied to the suspension by the rotor of the magnetic stirrer was essentially a gross movement of solvent, solute, and inert organic matter. iipproximately 1200
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electPica1 energy produced may be less than 1 to 10. Instrumentation was not available for measuring directly acoustical intensities within the reaction vessel. Intensities were expressed as the volt-amperes produced a t the power tubes (assuming a power factor of 1) divided by the internal cross-sectional area of the cylindrical test vessel, 6.16 square centimeters. The internal cross-sectional area of the reaction vessel and the upper effective area of the flat quartz transducing crystal were essentially the same. The units of intensity employed were voltamperes per square centimeter. Test Vessel. A simple test vessel was employed for this investigation. It was constructed of a borosilicate glass cylinder having an outside diameter of 32 millimeters and an inside diameter of 28 millimeters. I t s height was approximately 25 centimeters, A disk of 0.001-insh nickel was soldered to the lower end of the cylinder, which had been platinized. Peanuts. Peanuts, obtained from the Tidewater Field Station, Holland, Va., were shelled by hand, After the shelled peanuts were soaked in distilled water, the skins were -removed. The kernels were atmospherically dried for 24 hours and ground in a mortar. Particles passing a number 16 screen and retained on a number 20 screen were dried a t 110" C. for 3 hours and were employed as the feed, which was stored in a desiccator. Approximately 5 grams were employed for each test. Solvent-feed ratios were studied in the range 1: 1 to 5 : 1. Suspension volume was held to a minimum t o ensure reasonable uniformity of intensity in the reaction mass yet was limited by the fact that sufficient quantity had to be employed to permit accurate analyses. An optimum ratio of 3: 1 was selected based primarily on analytical methods, although concentration of mixture and insonation techniques were considered. Gravimetric determinations were employed in analyses. Comparison of Insonation with Mechanical Agitation. After the insonation tests were accomplished similar tests were condwted employing mechanical agitation only. A laboratory magnetic stirrer was utilized. The purposes of these tests were to 1168
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Effect of extraction efficiency of 400-kilocycle insonation
revolutions per minute were required t o produce the extraction efficiency indicated by maximum insonation. Probably, there are at least three principal separate mechanisms taking place in the suspension simultaneously under the influence OP ultrasonic energy: 1. Extension of phase boundaries by dispersion of adhered particles and partial disruption of oil cells. 2. Agitation selectively a t phase boundaries where there is a mismatch of acoustical impedance and gross stirring of solvent and suspended solids. 3. Thermal effect. This approach to and analysis of the problem is empirical and leaves much t o be desired. However, this is precisely the essential stepwise path that has been followed by investigators entering many new areas of study. Instrumentation, precise measurements, understanding, and explanation of the successive and simultaneous mechanisms involved must follow.
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1. Increasing the conductance by decreasing interfacial resistance to mass transfer. 2. Increasing the interfacial area by reducing the size and increasing the number of particles of the dispersed phase. 3. Increasing the concentration gradient across the interface by decreasing the concentration gradient within a phase.
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Figure 4. Effect on extraction efficiency of dynamic thermal paths equivalent to insonation intensities Postulation
Since this investigation was taken as a preliminary step in the study of the effect of insonation on the related unit operation of liquid-liquid extraction and since the essential elements of mass transfer across a phase boundary are involved in both instances, there may be justification for some postulation with respect to mass transfer, extrapolating from liquid-solid to’ liquid-liquid extraction. It is postulated that the selective microagitation a t phase boundaries serves to reduce interfacial resistances t o mass transfer and that both the microagitation as well as the gross stirring re-
June 1955
Acknowledgment
This investigation was made possible through the support of the Virginia Engineering Experiment Station.
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Figure 3.
duces the concentration gradient within a single liquid phase. Observations made during this investigation suggest further study of mass transfer in the unit operation of liquid-liquid extraction. I n fact, such an investigation has been initiated and is under way. The possibility exists that ultrasonic (and sonic) energy may affect t,he mass transfer rate in a positive manner by at least three separate and simultaneous mechanisms :
literature Cited (1) Beranek, L. L., “Acoustic Measurements,” pp. 1-36, 184, 40739, Wiley, New York, 1949. (2) Bergmann, L., “Der Ultraschall und seine Anwendung in Wissenschaft und Technik,” 6th ed., pp. 762-849, 864-908, S. Hirzel Verlag, Zurich, 1964. (3) Cady, W. G., “Piezoelectricity,” pp. 667-98, AlcGraw-Hill Book Co., New York, 1946. (4) Grove, H. D., Jr., M. Sc. thesis, University of Iowa, 1948. (5) Inskeep, G. C., IND. ENG.CHEM.,46, 1 3 A (December 1954). (6) Kinsler, L. E., and Frey, A. R., “Fundamentals of Acoustics,” pp. 467-98, Wiley, New York, 1950. (7) Mason, W. P., “Piezoelectric Crystals and Their Application t o Ultrasonics,” pp. 234-309, 325-89, Van Nostrand, New York, 1950. (8) Rlassa, Frank, “Acoustic Design Charts,” pp. 4-5, 213-14, Blakiston, Philadelphia, 1942, (9) Morse, P. M., “Vibration and Sound,” pp. 133-78, McGrawHill Book Co., New York, 1936. (10) Olson, H. F., “Dynamical Snalogies,” pp. 148-52, 165-70, Van Nostrand. New York. 1943. (11) Rayleigh, Lord, “Theoryof Sound,” 2nded., Vol. 11,pp. 312-42, Dover Publications, New York, 1945. (12) Richardson, E. G., “Ultrasonic Physics,” pp. 158-208, 252--76, Elsevier, New York, 1952. (13) Shropshire, R. F., U. S. Patent 2,473,453 (June 14, 1949). (14) Thompson, D., Ckem. E n g . Progr., 46, 3-6 (1950). (15) Thompson, D., Chem. Eng. Progr. Symp. Ser., No. 1, Vol. 47, 1951. (16) Thompson, D., “Ultrasonic Coagulation of Phosphate Tailing,” Virginia Polytechnic Institute, Eng. Exp. Sta., Bull. 75, July 1950. (17) Thompson, D., and Vilbrandt, F. C., IND.ENG.C H m f . , 46, 1172-80 (1954). (18) Thompson, D., Vilbrandt, F. C., and Gray, W. C., Jr., < J .Acoust. Soc. Anher., 25, 485-90 11953). (19) Vigoureux, P., “Ultrasonics,” pp. 99-142, Wiley, New York, 1951. (20) Wood, A. B., “Textbook of Sound,” pp. 148, 152, 158, 558, G. Bell and Sons, Ltd., London, 1946. RECEIVED f o r review January 28, 1965.
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