EDMOND S. PERRY Distillation Products Industries, Division of Eastman Kodak Co., Rochester, N. Y
The Brush Still Distillations of High Molecular Weight Organics
The brush still offers a successful method for the fractional distillation of essential oils and other high molecular weight organic materials. Its high separatory power, low pressure drop, and general utility over a wide pressure range places the brush still in a unique position for the distillation of high-boiling and/or heat-labile substances.
THE
brush still u i t h a n evaluation of its performance at micron pressures is described in the accompanying paper ( J ) . This still was shoivn to have a relativeh high degree of fractionating
power under high vacuum and a reasonably low thermal hazard due to the small pressure drop imposed by the sparsely bristled brush. Thus. the brush still should be useful for processing high molecular Lveight substances of either synthetic or natural origin. Several substances falling Lvithin this category have been examined in the brush stills. One such product is tall oil which consists mostly of fatty and rosin acids. both present in about equal quantities and together comprise 80 to 90% of the oil. Other natural occurring mixtures subjected to brush distillations are citral, ionone mixtures. cedarwood oil, and peppermint oil. Polyethylene glycol is an example of a synthetic product also treated by this type ofdistillation. These substances form an interesting group of materials because the average vapor pressures of each differ sufficiently that the distillations involved extend over a pressure range from 1 p to 6 mm. of mercury. These results shoiv. then. that the brush stills can be useful in the moderate pressure range as \vel1 as at high vacuum. The purpose of this article is to present a n account of the vacuum fracrionation of these various substances in the brush still. Experimental
Figure 1 .
Brush still
The brush still used for the present distillations differs from the test still already cited by having a n external pot heater of 500-watt capacity and an aircooled alembic-type product condenser of the kind used on “boiling point” stills (6). The brush shaft was also air cooled. A photograph of a recent laboratory model brush still is shown in Figure 1. I n operation, the pot heater was always maintained a t the lowest wattage which. in conjunction with the column heater. gave the best qualities in the condensate film on the column wall in accordance with the test results given in the preceding paper ( 4 ) . Distillations were conducted in batchwise manner and tvithout
the use of head reflux. Fractions were cut on a weight basis and a large number were usually taken. Complete analysis of fractions was not always possible owing to the unavailability of the special methods or techniques required. In these cases the data are presented mostly by plots involving temperature or refractibe index versus per cent distilled. These, supported by infrared spectrophotometric analysis of selected fractions. served to identify some of the various constituents of the mixtures.
Tall Oil T h e tall oil used for this work \vas a so-called “steamed” grade commercial product which had a dark color and contained a crystalline deposit at room temperature. The original oil and distillate fractions were analyzed for rosin acid and fatty acid contents by the method of Hastings and Pollak ( 2 ) . The fractions Lvere easy to analyze but the dark color of the original oil made it more difficult to detect the colorimetric end points during titration. Per cent rosin acid and fatty acid were calculated by the relationships used by Hastings and Pollak
(4. The experimental data and results for the distillation are given in Figure 2 in lvhich the fraction composition is plotted as a function of the per cent distilled. Two such curves are indicated-one based on total fraction weight and the other on a nonsaponifiable-free basis. In addition the rate of elimination of the fatty acids is shown as the distillation progresses. For completeness the rosin acid composition of each fraction is also plotted in the same figure. T h e curves of Figure 2 show that the fatty acids are separated in high purity from the rosin acids of tall oil by the brush still. -4volatile nonacid impurity was present in this oil as is evident from the lmver purity of the first two fractions. With the exception of these t\vo fractions the nonacid content of the fatty acid VOL. 48, NO. 9
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rosin acid constituents. At least 95% of the input fatty acids can be removed as a light-colored product free of the characteristic tall oil odor and color. Its purity would be in excess of 90% fatty acids.
Cedarwood Oil
Figure 2 yield fractions is quite low. The distillation curves indicate that the fatty acid distillates are contaminated with a persistent 2 to 3% rosin acids. This is believed to be due to the limitations of the analytical method and that the fatty acid fractions are purer than indicated in the plotted data. Herrlinger and Compeau ( 3 ) have shown that the method of Hastings and Pollak gives high results for rosin acids in fatty acid mixtures of low rosin acid content. They have found that the latter method indicates rosin acid values of 1.5 to 3.67, when no rosin acids are present a t all. and in samples actually containing 17c rosin acids, the results obtained by various analysts were 3.6 to 4.1%. The elimination or yield of the fatty acids during the course of the distillation follows a linear relationship up to the point where the rosin acids begin to appear in the distillates. The curve is a
straight line until 90y0 of the original fatty acids have distilled out, and a t this point only 43Y0 of the charge is taken overhead. i\t the SO?', distilled mark, 97% of the fatty acids are in the distillate. Because the changes in visual properties for the series of fractions \yere quite striking, a brief mention of them seems worth while. The first t\vo fractions were soft, dark-colored solids, from which colorless crystals separated. These fractions contained the strong characteristic odor of the original tall oil. The next two fractions retained only a slight reminiscence of the pinelike odor but odor was absent in all the others. Fractions 15 to 18 (rosin acids) were amber-colored, glass) solids quite characteristic of rosin acids. From a consideration of the foregoing results. it can be concluded that tall oil can be successfully processed in the brush still to separate the fatty acids from the
1.5140--170~
1.5100--150
2 2
Y)
l.5060--1302
e
-Uq
C
1.5020--110
a) Q E
P 1.4980--90
1.4940--70
1.4900-
50
Weight % distilled Figure 3
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The cedarwood oil available for this distillation was a sample of American red cedarwood oil ( 7 ) . This lightbodied. amber-colored liquid as received was distilled in the brush still a t 5 m m .of mercury pressure. Fractions were taken so that each comprised less than 5% of the charge. The resulting 27 fractions amounted to more than 92% of the original charge. Fractions 1 through 16, which total 5670 of the charge, were colorless liquids. Beginning with 17 and through 26, the fractions were white solids a t room temperature. The last fraction was a light-yellow liquid. The experimental results are plotted in Figure
3. The cuives in Figure 3 are plots of distilling temperatures and refractive indices versus the per cent distilled. The temperature curve shows that the oil contains at least two major constituents with se\,eral others present in lesser amounts. The refractive index curve covers a limited range because above the %%-distilled mark (fractions 17 through 26) the distillates were solids a t 25' C. Fraction 27, however, was a liquid of high refractive index-namely, 1.5 141. By spectrophotometric analyses in the infrared of several of the fractions the identity of some of the constituents was established. Fractioiis 2> 5, 8, and 12 were mostly hydrocarbon in character, indicating that in this range the distillate was principally composed of the cedrenes. The spectra for fraction 2 revealed the presence of very weak bands for hydroxyl and carbonyl groupings. Fractions 15 and 16 retained the hydrocarbon characteristics but the appearance of a new band a t 1515 cm.? suggested the probability that these ma!. be aromatic hydrocarbons. There was no evidence of either hydroxyl or carbonyl groupings in these fractions. T h e identity of this substance is not knoivn. The spectra of fractions 17, 18, 32, and 25 is entirely different from that for 15 and 16. The band a t 1515 cm.-' is gone. and there is a strong hydroxyl band but no carbonyl. The spectrum is similar to that for pure crystalline cedrol except that the hydroxyl group is displaced slightly. This might be due to an isomeric form. Fraction 26 appears to be intermediate between the crystalline cedrol and the cedrenol. I t has a strong hydroxyl band with evidence of weak carbonyl absorption. I n fraction 27 there is strong hydroxyl band, and the spectrum in general is similar to that of cedrenol.
The relative positions of these constituents are indicated in Figure 3. This graphical representation of the infrared results permits a reasonable estimation of the quantities of these constituents when used in conjunction with the curves of the figure. Thus, the following values represent probable percentages of the indentified constituents: Per Cent Cedrene 48-52 Cedrol 28-30 Cedrenol 10-12 Peppermint Oil The peppermint oil subjected to this brush-still fractionation was a "natural" American oil ( 7 ) which had received no prior treatment. It possessed the usual properties accorded this type of oil such as a pale-yellow color, refractive index, 1.4608 a t 25' C.. specific gravity. 0.899. and optical rotation, about -21 '. The brush distillation was performed a t 6 mm. of mercury pressure. A series of 15 fractions was taken M hich comprised 91.17, of the charge. The first ten fractions, or 55.3y0 of the input, were clear colorless liquids; the next four, 11 through 14, solidified on cooling; and fraction 15 was liquid. Fractions 1 and 2 possessed a surprisingly disagreeable odor which is perhaps accountable by the many volatile constituents which are known to occur in small quantities in peppermint oil. The familiar sweet odor reminiscent of the peppermint flavors accompanied fractions 4 through 8. The odor of these fractions was much more pleasing than that of the original sample. The solid fractions possessed a distinctive menthol odor. Fraction 15 had a special aroma of its oivn and did not resemble either the menthol or menthone. The distillation data are plotted in Figure 4. Distillation temperatures and refractive indices are both plotted as functions of the per cent distilled. The three distinct regions or plateaus in the curves indicate the presence of the three major constituents of peppermint oil. The analyses by infrared spectrophotometry of selected fractions locate and verify these major constituents. These are also included graphically in Figure 4. The fractions of the first plateau consist mostly of hydrocarbon constituents such as menthenes but probably also contain other volatiles such as low molecular weight fatty acids. The menthone yield occurs along the center region. and finally the last fractions contain the menthol. An estimate of the probable magnitudes of these various constituents are indicated on the figure.
Peppermint Oil { 6 m m pressure )
0
80 I
Weight % d i s t i l l e d Figure 4
only by the position of a double bond in the ring structure. Their boiling points lie closely together, and therefore they are difficult to separate by fractional distillation. The ionones are very sensitive to heat and must be treated with a minimum of thermal hazard. The distillation of ionone mixtures in the brush still having five or six theoretical plates has not in itself demonstrated any remarkable separatory power for the still. By comparison, however. to a type of packed column known to be efficient a t moderate vacuum, the performance of this brush still appears more striking as can be shown by the distillation of ionone mixtures in both stills. Instead of the usual distillation curves, the data for ionone distillations are plotted in terms of yield of p-ionone for 1 C
0 'u 100-
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I
any desired level of concentration. This was done in the following manner: Since the p-ionone constituent is the high boiler. the greatest concentration should be in the last fractions of the series. Starting with the last fractions, therefore: and blending with it the preceding fractions one at a time, a series of composites result from which the $-ionone composition and the yield in any particular blend can be determined. Such values are plotted in Figure 5 for typical distillations of ionone mixtures whose B-ionone concentration was 80%. The ordinate represents the p-ionone concentration in the composite fractions and the abscissas the p-ionone yield. The first point for the brush still, for example, represents a composite of the last distillate fraction in which 17.291, of the I
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1
lonone ( 2 mm pressure) d
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0)
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c
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0)
C
0
C
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a lonone Commercial p-ionone usually contains alpha-ionone as an impurity. These substances are isomeric and differ
Accumulative percent Ionone of input Ionone Figure 5 VOL. 48, NO. 9
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c
7 -
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I 60
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70
80
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Crude citral Z O O p pressure
1.4880
1
i
I. 4 8 0 0 h /
1 8 1.4780
0
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IO
20
30
I
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4G
50
90
W e i g h t p e r c e n t distilled Figure
original $-ionone is contained at 95.7% concentration. The second point is for another composite in which 60.4% of the original p-ionone is prrsent in 95% concentration. And so forth for the entirr curve. T h e curves for each still in Figure 5 are not greatly dissimilar. T h e packed column \vas a spiral screen column of the kind described by Stallcup, Fuguitt, and Hawkins (.5), T h e efficiency of the present column was estimated from distillations of terpene mixtures to be about 20 theoretical plates at 20 mm. of mercury pressure. T h e ionone distillations were done at 100 grams ‘hour throughputs. 40: 1 and 20: 1 reflux ratios, and 2 mm. of mercury pressure. Since the test and ionone distillations were done a t different pressures, it is difficult to attribute ho\c much effect, if any. the decreased pressure has on the efficiency of the still. However, Hawkins and Brent (5) have
60 -
-
1
I
6
shown recently that reduced pressures have no pronounced effect on the efficiencies of these columns. By comparing the two curvcs of Figure 5: therefore, it appears that the performance of the brush still used for these distillations is equal to, or perhaps rven bettcr than. that of the 20-platc. fractionating column. In addition. the lower thermal hazard of the brush still is evident from the higher level of its curve for the last fractions (first points on curve). This difference in the yield of p-ionone is due to thermal decomposition because of the higher temperatures required in the spiral screen still over thosc in the brush still for distillations at comparable head pressures. Citral
Citral is difficult to purifv bv distillation becausr it decomposes reddily at
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Polyethylene glycol 200
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1
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temperatures above 60’ C. Attempts to purify the crude product by molecular distillation resulted in little or no separation of the undesirable constituents. Crude citral was found to successfully sustain treatment in the brush still by maintaining the boiler temperature a t 50@ C. Under this condition citral distilled easily and satisfactorily at 200niicron pressure. T h e plot of refractive index vcrsus per cent distilled given in Figure 6 is not greatly illuminating, but in the absence of better analytical data i t shows the range over which the citral composition is uniform. Infrared anal>-ses of several fractions in this region established a high degree of citral purity in these fractions. From these results it is deduced that 55 to 607, of the distillate is high quality citral. Another indication of the success oi‘ this distillation is that these fractions have a desirable taste. This is verification that the citral was not thermally altered during distillation. PoIyet hy Ie ne
GIy c 01
T h e brush distillation results of pol>-ethylene glycol 200 are graphed in Figure 7 as refractive index versus per cent distilled. T h e various plateaus in the curvc indicate the presence of several polymeric species. This glycol distilled to the extent of 927,. and even as the last fraction was being removed there was no evidence of decomposition. Other polyethylene glycols have been subjected to processing in the brush still. ”Nona-Polyethylene” glycol was found to yield a 20y6 low boiling fraction and thereafter a product of uniform composition comprising about 60% of thc remaining material. The resulting higher boiling residue showed indication of decomposition at temperatures above
200@c.
Literature Cited (1) Guenther, E., “The Essential Oils,”
vol. 2, Van Yostrand. New York.
1949. ( 2 ) Hastings. R.. Pollak. .A,. 021 3 .Soup 16, 101 (1939). ( 3 ) Herrlinger, R.. Compeau. G., A m . O i l Chemists‘ Soc. 29, 342 (1952). ( 4 ) Perry, E. S., Cox, .D., IND.ENG.CmM. 48, 1473 (1956); Perry, E. S.: Mansing. J.. U. S. Patent 2,539,699
4 201
(Jan. 30: 1951);
Hickman, K..
Zbtd., 2,609,335(Sept. 2, 1952).
( 5 ) Stallcup, D.. Fuguitt, R. E., Hawkins. J. E., IND.ENG. CHEM.,ANAL.ED. 14, 503 (1942); Hawkins. J. E.. Brent, J. A., IND. ENG.CHEM.43, 2611 (1951). (6) Techniques of Organic Chemistry (A. Weissberger, editor), vol. 4, chapt. 6, p. 512, Interscience. New York. 1951.
Weight % distilled Figure
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INDUSTRIAL AND ENGINEERING CHEMISTRY
RECEIVED for review November 28, 1955 .ACCEPTED March 14, 1956 Communication No. 228 from the Laboratories of Distillation Products Industries.
