Diffusion Phenomena in Solvent Deresination of Guayule Rubber d
THOMAS F. BANIGAN, JR. U.S . Natural Rubber Research Station, Salinas, Calij. use, or of investigating some promising future process, or perhaps of isolating some of the by-product resin. Unfortunately, these specific objectives have regularly been reached without obtaining basic rate data. It therefore seemed desirable to give some attention t o guayule rubber deresination as a diffusion phenomenon. I n this way some fundamental rate data can be obtained which should prove useful in the design and operation of large scale extraction systems.
ATURAL rubber of excellent quality has been obtained by acetone deresination of the resinous worms from the pebble milling of freshly harvested comminuted guayule shrub (4). The removal of the resin fraction is so rapid and complete in this instance that it is difficult to study factors which influence the rate and extent of deresination. Since the intelligent design of a n efficient extraction process, requires a thorough knowledge of these factors, it seemed desirable t o prepare the material in a form more adaptable to fundamental study. The present paper reports a n investigation of diffusion phenomena occurring during the solvent extraction of resin from thin sheets of resinous guayule rubber impressed on stainless steel screens.
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THEORY
REVIEW OF PREVIOUS WORK
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Hauser and Le Beau carried out Soxhlet type extractions with thin sheets of resinous guayule rubber for the purpose of comparing the effectiveness of various resin solvents (6). The solvents that they investigated can be arranged in the following order of decreasing efficiency for resin removal at their boiling points: furfural, acetone, nitropropane, ethyl alcohol, and methanol. They noted, however, that furfural was inferior to acetone a t lower temperatures. Nishimura et aE.(9) preferred glacial acetic acid for the deresination of resinous guayule in the form of oven-dried spongy sheets, This solvent was apparently even more effective than acetone for removing the deteriorative substances which they found in guayule rubber obtained in the above fashion. Recently Wood and Fanning (14) carried out a series of diffusion type deresinations with boiling acetone on sheets of resinous guayule milled “as thin as possible and then cut into small pieces.” These smaller sheets were supported horizontally on properly spaced pieces of galvanized window screen, in a Soxhlet type extractor. Extractions were conducted for periods of 1,4, 8, and 12 hours, respectively. I n 12 hours the resin content had dropped from 19.4 to 2.7%. Since this corresponded to a slower rate than that obtained by them in another series of experiments with mastication, it appears likely that the extraction rate was being limited by diffusion. Their data have now been plotted (see Figure l), and a diffusion limitation to deresination is indeed evidenced by the close conformity of the experimental extraction curve t o the theoretical curve based on Fiok’s diffusion law (a). The semilogarithmic plot of the fraction of resin remaining (q/qo) versus time is a straight line after t = 1 hour. Since the sheet thickness is not known, the diffusion coefficient cannot be calculated, and therefore the accompanying theoretical curve is based merely on the experimentally determined extraction rate. Clark et al. ( 4 )have reported on batch countercurrent deresinations. Warm acetone was employed in the preparation of 4000 pounds of low-resin content, high viscosity guayule rubber suitable for truck carcass stock compound. This work revealed various significant advantages for the deresination of guayule in the form of moist worms. The small size and open structure of these spongelike particles were shown to favor rapid deresination. Much of this foregoing work has been approached either from the standpoint of obtaining superior quality rubber for further
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Despite the suitability of guayule worms as a n ideal physical form for rapid large scale deresination, they were not well suited to the present work for several reasons. First, the average worm size is so small that deresination proceeds too rapidly for the procurement of accurate rate data. Secondly, the factors responsible for controlling worm size in the milling operation are imperfectly understood, and consequently considerable size variations can exist (0.50 mm. f 0.25 mm. diameter) ( l a ) . Since rate varies inversely with the square of the slab thickness in the diffusion equation, such uncertainty would impair the value of any calculations, Lastly, at the time this work was initiated no means of storage were known by which wet worms could be preserved in a desirable unconglomerated state. The physical form that appeared best suited, and was finally adopted, was the thin film obtained by impression of dry resinous rubber into the interstices of stainless steel screens. Since the slab thicknesses of these thin films could be accurately determined and reproduced, a treatment of the extraction data based on the theory of diffusion became feasible. The theory of diffusion which applies t o the process of solvent extraction of oils, or in this case resin, from porous solids has been proposed by Barrer ( 3 ) as the best method of correlating extraction data. Modified forms of the equation may be applied t o many nonuniform solids (6, 7 ) , especially those where structure is a variable (IO). The general equation which applies to nonstationary mass transfer by diffusion is Fick’s law:
where c = concentration of solute at point z distance from the origin; t = time; D = diffusion coefficient (also called diffusion constant), the amount of material which passes a plane of unit area in unit time when under unit concentration gradient. I n order to integrate this equation t o usable form, i t is necessary t o specify the conditions under which the diffusion is taking place. According t o Fick’s law certain conditions must be met: 1. The diffusion coefficient, D, must be a constant independent of solute concentration, c. 2. The structure of the slab must be homogeneous and isotropic. 3. The initial distribution of resin in the slab must be uniform. 4. The thickness of the slab must be small compared t o the surface dimensions and diffusion through the edge must be negligible. 5 . The temperature must be constant.
INDUSTRIAL AND ENGINEERING CHEMISTRY
978
6. T h e thickness of the slab must be uniform and the same for all slabs extracted a t the same time. The integration of the general diffusion equation for these conditions is given by Boucher and coworkers ( 3 ) . Solution of the equation leads t o the infinite series
where q qo
2L
D n
= quantity of resin in a unit volume or weight of rubber a t t seconds after diffusion has started = initial quantity of resin in rubber, at t = ta = thickness of rubber slab = diffusion constant = term in the series
The series of Equation 2 may be expanded to give
.. (3)
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Figure 1. Extraction Curve Based on Data of Wood and Fanning (IS) Theoretical curye, shown for comparison, is derived from diffusion law using same extraction rate (slope)
The series converges so rapidly that a few terms are sufficient for most practical calculations. I n fact, except for very small values of t , all but the first term of the series may be neglected. Equation 3 then reduces t o
Vol. 45, No. 3
Parthenium argentaturn Gray (variety 593), grown under dryland conditions near Salinas, Calif. After scrub milling, the worms were dried over a &hour period in a circulating air oven operated at 40" C., passed through a 12inch wash mill three times during the drying period, then wrapped in Holland cloth, and refrigerated until needed. No changes were observed in the extraction characteristics of this rubber during the 3 months' duration of the investigation. Analysis of the dry rubber showed 26.09% acetone solubles and 4.770 of acetone and benzene insoluhles (1). The diffusion samples were prepared by impressing 4.0-gram portions of the dry rubber into the interstices of 35-mesh stainless steel screens measuring 12 cm. square. The pressings were made in a small hydraulic press, operated a t 5000 pounds per square inch over a 10-minute period with plat'en temperatures of 45' to 55' C. The viscosity of the resinous rubber was systematically reduced by cold milling to a Mooney viscosity of 20 or less, to facilitate smooth flow into the screen surface within the desired temperature range. Thus, diffusion samples were routinely prepared in which the rubber was spread into circular uniformly thick films, free of bubbles and wrinkles. Without this physical breakdown, a platen temperature of about 70" t o 75' C. was required for comparable flow. It vas suspected t h a t the higher temperature might produce a change in some of the resin components, consequently altering their extractability. This possibility was not rigorously tested, although several experiments designed t o reveal such effects failed t o show any significant differences due to polymer breakdown or the higher platen temperature. Apparently, minor changes in the conditions outlined above for sample preparation do not have much effect on the extraction data. Sheets of greater thickness were conveniently prepared by pressing either 8 or 12 grams of rubber into stacks of tw-o or three screens, respectively. Pressing and subsequent handling of the pressed samples were facilitated by the use of properly cut sheets of polyethylene film, placed above and below the sample screens in the press. This film performed elegantly as a press release agent and yet adhered sufficiently t o the rubber-impregnated screens t o cover t>hemuniformly while thickness measurements were taken. These measurements were made on a pressor foot thickness gage conforming t o American Society for Testing Materials requirements (1). It was regular practice to stack the covered screens in a desiccator and store in a refrigerator until needed (within 3 days). The protective film was removed shortly before use. Screens both with and without rubber were weighed directly on an analytical balance t o 0.1 mg. Extraction was conducted in a constant temperature bath which consisted of a heavy-walled borosilicate glass battery jar, measuring 12 inches in height and 12 inches in diameter and recessed into the accompanying base which contained the thermostat controls. Constant t.emperature (h0.15" C.) was maintained at desired settings by a sealed Beckman type mercury thermoregulator which actuated a sealed immersion heater placed directly in the solvent contained in the glass jar. An efficient air stirrer provided the necessary agitation. The extraction samples were placed in a rectangular cont'ainer, made of 16mesh copper plated screen, measuring 13 cm. square by 20 c,m.
Loot,
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and
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ACETONE AT 25'--
0 60
(5) 0 40
This is the equation of a straight line when log,^ q / y o is plotted against t. The slope of this line can be used t o evaluate the diffusion constant. It is beyond the scope of this paper to take into account the interference of the solid on the true diffusion coefficient (10). Equation 5 as presented will be used in subsequent sections of this paper, both to find the apparent diffusion coefficient from data and to calculate the extraction curve when the coefficient is known.
o ao 0 PO
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0.10
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EXPERIMENTAL PROCEDURE
The rubber used in this work was sampled from a typical 20pound batch produced in the pilot plant of this Station by pebble milling of freshly harvested guayule shrub ( 4 , 11). The 7-yearold shrub from which the resinous worms were prepared was
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Figure 2. Extraction Curves for Experiments 1 to 3 Showing Effect of Sample Thickness on Extraction Rate
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1953
TABLEI. DIFFUSION COEFFICIENTSFOR DERESINATION OF GUAYULE RUBBER FILMS WITH VARIOUS SOLVENTS Experiment No. 1 2 3
Thickness Cm.' 0.185 0.137 0.069 0.066
Temperature O C: 25 25 25 e5
' Solvent D , Sq. Cm./Min. Acetone 1.23 X 10-6 Acetone 1.16 5 10-6 Acetone 4 Ethyl alcohol 8.06 X 1 0 1 5 Methyl ethyl ketone, water saturated 0.066 25 6 Methylethyl ketone 0.066 25 3 . 0 X 10-aa 7 Ethyl alcohol 0.067 37.5 1.69 X 10-0 8 Acetone 0.130 37.5 1.44 x io-( 'E Method for measuring D becomes less accurate a t very high or very low extraction rates.
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high and open at the top; wire rods served as removable separators, positioning the screens in the bath. Extraction took place over about a 10-hour period with samples withdrawn a t approximately logarithmic intervals. Since volatile solvents were used, a 24-hour period of air drying in a darkened room proved sufficient t o attain constant wei hts. T h e wei ht loss on each screen was related t o its time, t, in &e bath. '?he solvents investigated included acetone (d:g 0.7865); 95% ethyl alcohol (dq: 0.808); methyl ethyl ketone (dZo 0.805); methyl ethyl ketone saturated with water (ca. 11%water) at 20' C. (dZo 0.836). For each extraction 14 liters of fresh solvent were used. With rubber of 26.09% resin content (40 = 0.353 gram per gram of resin-free dry rubber) this volume provided a solvent-torubber ratio of a t least 500 t o 1 in all cases. Any influence exerted by the 0.2% or less of resin on the extraction behavior of these solvents did not appear t o be significant in the present calculations.
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with acetone at 25" C. using samples 0.066 cm. thick. The first of these extractions was carried out with essentially unmilled resinous rubber with a Mooney viscosity of 45, pressed into samples at 70' to 75" C. instead of the usual more heavily milled rubber pressed at 50" C. This was done to determine whether the amount of milling routinely given to the other samples altered the diffusion coefficients. The second series was merely a duplicate of experiment 3. The extraction curves from these two additional experiments (not plotted here) bore a close resemblance t o the plot of experiment 3. An earlier experiment with acetone at 30" C. had been carried out with a less refined thermoregulating device. However, since the calculated diffusion coefficient seemed in agreement with later data it was utilized in verifying the temperature relationship shown in Figure 6. This expression is based on the assumption that the data can be represented as a straight line over the region shown. Guayule resin, like many plant extracts, is a mixture of compounds which possesses different relative solubilities in the usual resin solvents (IS). The completeness with which these compounds are removed by any particular solvent is no less important than the rate of their removal, because the fraction of resin remaining has a definite bearing on the quality of the rubber produced. Therefore, the solvents used in this rate study were also evaluated in terms of the extent of resin removal. Samples of resinous rubber were analyzed by a n ASTM procedure (I), modified in that determinations were made with each of the above solvents as well as with acetone. The results are summarized in Table I1 along with some comparative values derived from q/po a t 10 hours taken from the rate plots, the straight-line portions of which are shown in Figures 2 and 3.
RESULTS
Eleven series of experimental extractions were made to determine q/qo as a function of time. The curves for eight of these are shown in Figures 2 to 4. A series of theoretical extraction curves calculated from Equation 5, using experimentally determined extraction rates [ D / ( 2 L ) 2 ]is, presented in Figure 5. The relationship for the diffusion coefficient of acetone versus temperature is illustrated in Figure 6. Most of the extractions gave rate curves containing long straight-line portions. I n such cases it is valid to calculate the diffusion coefficient from the straight-line portion of the semilogarithmic plots shown in the figures. These calculated diffusion coefficients, together with other pertinent data, are summarized in Table I. In Figure 5 the theoretical curves corresponding to experiments 1 to 3 were based on a n average D ~for s acetone of 1.20 X I n addition t o these experiments, two other runs were made
TABLE 11. COMPARISON OF RESINSOLVENTS BASEDON EXTENT OF DERESINATION OF DRYRESINOUS RUBBER Resin Removed, % Diffusion ASTM q/qo a t Solvent 10 Hr. experiments analyses Ethyl alcohol 0.20 22.0 25.3 Acetone 0.055 25.0 26.1 Methyl ethyl ketone azeotrope= 0.064 24.8 27.2 Methyl ethyl ketone 0.00 26.1 27.9 Composition of azeotrope approximates that of the water-saturate a t 25' C.
The ASTM values should be considered as the truer measure of the total extractables in the rubber with respect to the several solvents, since they are based on a n exhaustive extraction method employing refluxing solvents. The diffusion data probably correlate more closely with large scale deresination experience. De-
ACETONE 0 IO
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Figure 3. Extraction Curves for Experiments 4 to 6 Using Sheets 0.066 Cm. Thick a t 25' C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 3
5 . Methyl ethyl ketone, water saturate (20" C.), produced an extraction curve remarkably like the acetone curve. This is illustrated in experiments 3 and 5 and suggests similar diffusion coefficients.
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5
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Figure 5. Theoretical Extraction Curves Based on Measured Sheet Thicknesses and Diffusion
Coefficients Determined Experimentally at 25" C.
The current work also reveals that the simple diffusion theory does not properly describe the acetone deresination of rubber films thinner than about 1 mm. This is in line with an earlier finding-namely, that the acetone deresination of moist worms proceeds a t a constant rate only after the fraction of the resin initially present has decreased to lOy0 or less depending on temperature (4). These two cases display- a graphical rescmblance when the logarithm of 4 / 4 0 is plotted against the logarithm of the extraction time. In Figure 7 the upper portions of these extraction curves-the region over which most of the resin is removedapproach linearity. However, since Fick's diffusion law does not apply here, no attempt will be made to consider these more complicated systems rigorously. Qualitatively, it may be said that for very thin slabs or in the extreme case, worms, the extraction rate differs from that predicted from simple theory in a way which is probably consistent with the physical nature of the material and with the observations of other workers.
Curve numbers refer to experiment numbers listed in Table 1
1.00
spite the procedural differences, a reasonable agreement of order exists between these two methods for comparing the efficiency of resin solvents with respect t o the extent of deresination.
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DISCUSSION
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The foregoing data reveal that extraction of resin under controlled conditions from properly prepared samples of guayule rubber conforms fairly well with Fick's law of diffusion. From these findings a number of interesting observations may be drawn.
0 os
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1. Acetone a t 25" C. has a diffusion coefficient of 1.20 X 10-5 square centimeters per minute. At slab thickness in excess of about 1 mm., this constant appears independent of thickness in keeping with the diffusion law. At lower thicknesses a deviation from theory exists which will be treated in some detail later.
1.1
1.0 25
35
30
40
TEHPERITURE. .C.
Figure 6. Effect of Extraction Temperature o n Diffusion Coefficient of Acetone Interoept, 5.8 X 10-6; slope, 2.26 X 10-1
2. The coefficient for acetone is shown to have a rate of change with temperature expressible as follows for values of T near room temperature: D 2.26 X X T 5.8 X 3. The diffusion coefficients for ethyl alcohol, acetone, and methyl ethyl ketone are in the following approximate ratios, respectively: 0.07, 1.0, and 2.5. 4. The coefficient for ethyl alcohol appears to have a rate of change with temperature of 7.1 X lo-* square centimeters per minute per degree centigrade near room temperature.
+
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0.S
1.0
1 . 0 3,O
SO
IO
noufls
Figure 7. Coinparison of Experimental Extraction Curves for Deresination of Guayule Rubber In form of "worms"
US.
thin sheets
Osburn and Katz (IO)have shown, for example, that the structure of the slab has an important bearing on the extraction rate. I n their work with soybean flakes, curved extraction rates were found which were shoa-n t o be similar to theoretical curves for extraction with two different diffusion coefficients. A method was offered for finding the two coefficients from the extraction curve when certain assumptions could reasonably be made. The curved extraction rates (semilogarithmic plot) for guayule worm deresination ( 4 ) may likewise result from two different diffusion coefficients. This effect could be brought about by factors such as the following: ( a ) the presence of water in the worms; ( b ) nonuniform resin dibtribution; ( c ) solubility differences for various resin components; and ( d ) thickness variations. Factors ( a ) and ( b ) probably exert less influence than factors ( e ) and ( d ) in the light of the current work on thin dry sheets. The first two factors were absent in experiment 3, yet the extraction curve for this experiment (0.066 em. sheet with acetone a t 25" C.) bears a closer resemblance t o a worm deresination curve ( 4 ) than to any theoretical curve, plotted from Fick's law calculationi. Factor ( e )-solubility differences for various resin components-might be partly responsible for the nonconformity of thin slabs since great solubility differences are known to exist ( 8 )among various resin fractions. Such differences, accentuated by a high ratio of surface area to weight, would favor rapid dissolution of the more soluble components, thereby leaving less soluble substances to diffuse out a t a n over-all slower rate. Factor (d)-thickness variations-undoubtedly play a major role
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1953
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with thin slabs. A standard deviation from the mean of 0.005 cm. (typical for these experiments) for a n individual thickness measurement, corresponds to only 2.7% of a 0.185-cm. slab, but 7.670 of a 0.066-cm. slab. The diffusion coefficients reported here ,were determined with thin sheets of dry rubber and, consequently, should be considered most pertinent to guayule in that form. Present evidence indicates that the findings enumerated above have a t least limited application t o resin extraction from rubber in other physical forms, such as worms. For example, it has been found that the solvents used in this work behave in worm deresination in a manner consistent with their diffusion rates reported here. Also, temperature changes have the same rather minor effect on the rate of worm extractions as on thin sheet extractions (a rise of nearly 50’ C. being required to double the rate with acetone). Temperature can have a decided effect on the completeness of the deresination, depending on the choice of solvent. A fuller knowledge of the role which the phenomenon of diffusion exerts in the mechanism of guayule deresination must await further detailed investigations.
ance in preparation of the figures; and to Ruth V. Crook of this laboratory for the resin analyses.
ACKNOWLEDGMENTS
18, 1945. (13) Walter, E. D., J. Am. Chem. SOC..66, 419 (1944). (14) Wood, J. W., and Fanning, R. J., Rubber A g e , 68, 195 (1950). RECEIVED for review June 30, 1952. ACCEPTED October 8, 1952.
The author is grateful to Eleanor C. Taylor, of the Bureau of Plant Industry, Soils, and Agricultural Engineering, for assist-
LITERATURE CITED
Am. Soc. Testing Materials, “Standards on Rubber Products,” Philadelphia, 1948. (2) Barrer, R. M., “Diffusion in and Through Solids,” Cambridge University Press, 1952. (3) Boucher, D. F., Brier, J. C., and Osburn, J. O., Trans. Am. Inst.
(1)
Chem. Engrs., 38,967 (1942).
(4)Clark, F. E., Banigan, T. F., Jr., Meeks, J. W., and Feustel, I. C., IND. ENQ.CKEM.,45,572 (1953). (5) Fan, H. P., Morris, J. C., and Wakeham, H., Ibid., 40, 195 (1948).
( 6 ) Hauser, E. A., and Le Beau, D. S., India Rubber World, 108, 37 (1943).
(7)
King, $3. O., Katz, D. L., and Brier, J. C., Trans. Am. Inst.
Chem. Engrs., 40, 533 (1944). ( 8 ) Meeks, J. W., Banigan, T. F., Jr., and Planck, R. W., U. S. Patent 2,572,046 (Oct. 23, 1951).
Nishimura, M. S., Hirosawa, F. N., and Emerson, R., IND. ENQ. CHEM.,39,1477 (1947). ( I O ) Osburn, J. O., and Katz, D. L., Trans. Am. Inst. Chem. Enars.. 40, 511 (1944). (11) Taylor, K. W., Econ. Botanu, 5, 255 (1951). (12) Tint, H., Salinas Tech. File, Emergency Rubber Proiect. Jan. ~. (9)
Adsorption of Ammonia on Fuller’s Earth and Gas-Adsorbent Carbon G. L. BRIDGER AND R. D. SINNER1 Department of Chemical and Mining Engineering, Iowa State College, Ames, Iowa
T
H E adsorption of ammonia by various solids has been the subject of numerous invest&ations (1, 2). However, a literature search failed t o reveal data on the adsorption of ammonia a t constant pressure at temperatures just above its condensation point. The purpose of this paper is to present such data of sufficient accuracy for engineering design for the adsorp tion of ammonia on two adsorbents at atmospheric pressure and
a t temperatures near the condensation temperature of ammonia, -33.4”
c.
I n preliminary experiments, activated alumina, bauxite, bentonite, bone char, china clay, silica gel, zeolite, fuller’s earth, and gas-adsorbent carbon were tested for ammonia adsorption capacity at -5” and -30” C. The fuller’s earth and gasadsorbent carbon had the greatest adsorptive capacity and consequently were used in the more detailed studies. ADSORBENTS
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The fuller’s earth used was Attaclay SF produced by the Attapulgus Clay Co., Philadelphia, Pa. The clay, originally in powder form, was compressed into tablets l / 4 inch in diameter and 1/16 inch in thickness. The bulk density of the tablets in a S/4-inch column was approximately 0.56 gram per cc. The gas-adsorbent carbon used was Columbia Grade 4SXW, 6 to 8 mesh, produced by Carbide and Carbon Chemicals Corp., New York, N. Y. In both cases the adsorbents were dried a t 120’ C. for 18 hours before using. APPARATUS AND PROCEDURE I 0.001 40
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Figure 1. Adsorption Isobars for Ammonia on GasAdsorbent Carbon and Fuller’s Earth at Atmospheric Pressure
The procedure used in determining adsorptive capacities was chosen t o give results of sufficient accuracy for design of commercial adsorbers. A drying tube of Drierite was used to remove water vapor from the inlet ammonia stream. An aluminum coil placed in a 1
Present address, The Ethyl C o w , Baton Rouge, La.
