trode system, though the cell certainly is sensitive to temperature changes. The temperature in the laboratory was sufficiently constant that simple protection against drafts yielded stable operation. It seems certain that compensation can be accomplished, probably by means of a suitable network of thermistors. Study of this problem will be continued. The application of the system to practical atmospheric and laboratory problems is also being investigated.
ment Printing Office, Wwhington, D . C., 1959.
ACKNOWLEDGMENT
The p H meter for this study was loanedbY Beckman Instruments, Inc. LITERATURE CITED
(1) Baez, A. P., Ing. quim. ( M e r . ) 4, 22-6
(1959).
u.s. Public Health Service, Proceedings, National Conference on Air Pollution, P.H.S. Publication Yo. 654, pp. 106-9. Govern-
(2) Hewson, E. W., in
(3) 23, 13-27 (1959).
Ana"
Keeling, C. D., Tellus 12, 200-3 (1960). (5) Toren, P. E., Heinrich, B. J., ANAL. CHEM. 29, 1854-6 (1957). (4)
RECEIVED for review December 1, 1961. Accepted March 5, 1962. Work performed at the Laboratory of Engineering and Physical Sciences, Division of 4ir Pollution, Public Health Service, U. S. Department of Health, Education, and Welfare.
An Efficient Still for Milligram Samples of High-Boiling Materials MAX BLUMER1 Shell Development Co.,
A Division o f Shell Oil Co., Houston, rex.
b A new still has been designed for the efficient rectification of milligram quantities of materials in the 200 to 850 molecular weight range. Separation is achieved b y recycling of the volatiles in an evacuated tube moving through a temperature gradient. Remixing of liquid distillates in the tube is prevented b y a silicone film of reduced wettability. A distillation is completed in less than an hour, sample recovery is quantitative, and the thermal hazard per theoretical plate is of the same order as that for a conventional falling-film still.
T
of high boiling materials from natural sources has in the past, been severely hampered by the lack of an efficient technique for molecular size separation on the milligram level. Distillation, which would be ideally suited for obtaining approximate molecular weight distribution data rapidly and inexpensively, is not easily applied to milligram samples. Conventional reflux distillation on this sample level is reported by Morton and Xahoney (8). Microsublimation techniques have been described by Von Elbe and Scott (IO, 11), Behrens and Fischer ( 2 ) , Behrens, Melchior, and Thalacker (S), Flaschentrkger, AbdelWahab, and Habib Labib (6),Bates ( I ) , and Thomas (9). -4microstill is under development a t the Battelle Institute ( 6 ) . The design by Von Elbe and Scott appears to offer the greatest plate efficiency. We have modified their design to operate at much higher temperatures, HE ANALYSIS
Present address, Woods Hole Oceanographic Institution, Woods Hole, Mass. 704
ANALYTICAL CHEMISTRY
to permit true distillations of liquids, and to increase the charge capacity by a factor of 2.5. DESCRIPTION OF STILL
A still has been constructed for the separation of milligram samples of highboiling materials, utilizing the principle of rectification in a temperature gradient (Figure 1). Such a still has several advantages for the microanalysis of volatile materials. I single straight piece of glass tubing serves the same purpose as still pot, rectification column, take-off head, and fraction collector in conventional stills. The still has essentially no holdup; all the recycling material represents product. Furthermore, there is no gas flow out of the tube after its initial evacuation, and consequently, there are no material losses. Power to the furnace and motor unit is supplied from the 117-volt line and stabilized by a 1-kva electronic voltage regulator. The power input to the two furnace sections is controlled b y means of the two powerstats. The distilling tube is driven b y a horsepower motor. Its speed is reduced by a Zeromax variable speed transmission h-hich is controlled from the 15turn Helipot dial. A 100 to 1 worm reduction gear further reduces the speed. With a clutch, controlled from the front panel, the chain driving the carriage and distillation tube can be disengaged from the power unit. An adjustable stop and a microswitch permit the termination of the distillation a t a preselected time. The on and off periods of the heaters are controlled by a 7-day time switch. Figure 2 shows the inside construction of the furnace unit. Two aluminum furnaces are used. The constant temperature section (unit a t left) is heated throughout its length and is insulated toward the outside and the gradient furnace. The latter is heated by four
short high-wattage heating eleiiieiits and cooled with tap water a t the opposite end. It is also heavily insulated to establish a nearly linear temperature gradient between the hot (up to 360' C.) and the water cooled (approximately 30" C.) cold end. The gradient furnace is anodized to prevent corrosion of its tapered cold end and of the cooling fins The vacuum source, including a manostat with adjustable leak, holds the system pressure constant to i 5 microns. The distilling tube and the dry ice trap, which is slipped over the distilling tube to prevent the loss of very volatile materials, are shown in Figure 3. If the condensate were permitted to form a continuous liquid film on the wall of the tube, diffusion, convection, and gravity flow would lead to an apparent loss of efficiency. For this reason, the stills of Von Elbe and of Behrens perform best in sublimations where the distillate collects as a solid phase and remixing is reduced to a minimum. T o prevent remixing of the product, the inside of the distilling tube is treated with a silicone (5% solution of methyltrichlorosilane or dimethyldichlorosilane in isooctane) which reacts with the glass to form a stable, nonvolatile coating. This changes the interfacial tension between distillate and wall. The condensate now only partially wets the glass and collects in small wellseparated droplets, instead of in a continuous film. OPERATION OF STILL
Solid and liquid samples are best introduced into the still in a small disposable aluminum boat. Static charges on the tube, persistent because of the hydrophobic silicone film, are dissipated nTith a polonium brush. The charge can also be introduced in a lowboiling solvent-e.g., isopentane-which is boiled off before the start of the distillation. To begin a distillation, the
,,,--FURNACE
[BLACK
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TOP
A
-DISTILLING
TUBE
MICROSWITCH
/-
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Figure 1.
tube, containing a sample, is evacuated and introduced into the cold end of the gradient furnace. The mechanical drive moves the tube slowly and at a constant rate through the temperature gradient. Volatile materials present in the charge evaporate as soon as they reach a sufficient temperature and condense on a cooler section of the wall. As the tube continues its movement toward higher temperatures, re-evaporation and recondensation take place, and a steady state develops with the escape rate of the vapor balanced against the movement of the condensate with the wall of the tube. The position a t which a condensate recycles in the temperature gradient can be related to its boiling point. Equilibrium is never established during the distillation; as a matter of fact, no separation Fould be obtained a t equilibrium, since the entire distillate would collect a t the coldest point of the tube. If the distillation were stopped when the sample compartment reached the hottest part of the gradient, the charge mould be resolved very unevenly. Lowboiling fractions which escape soon after the start of the distillation would have recycled a sufficient number of times, b u t the highest-boiling fractions would just have escaped from the residue. We therefore continue the recycling process by moving the tube into the constant temperature furnace, in which the temperature remains a t the highest value of the gradient. No more distillate is produced in this furnace, b u t the material collected in the gradient continues to recycle for additional rectification. For recovery of the product, the tube is broken after scoring it deeply with a dry diamond saw. This results in a clean fracture and minimizes the loss of materials with the glass fragments. The distillates arc washed out with a
REV.
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v
TURNS
Exploded view of the EPR ultramicromolecular still
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upon the sample. The probability for the transition of a molecule from the vapor phase to the liquid phase depends upon the wall condition and area; it is increased in the presence of distillate droplets. Thus, it was found that the zone of 2 mg. of chrysene was displaced '/4 inch toward the hot end of the gradient in the presence of 50 mg. of a paraffin was boiling in the same range. This shift corresponds to an apparent change of the atmospheric boiling point of approximately 6' to 8' C. This charge effect is suificiently
low-boiling solvent and filtered through a small cotton plug in a funnel. The products are weighed after solvent removal. CALIBRATION OF STILL
The still is empirically calibrated by distilling a test material and analyzing the products for their molecular weights or boiling points. The calibrating material should resemble the actual charge in size and type, since the location, a t which the distillate recycles, depends
*--A0
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Distilling tube and cold trap VOL. 34,
NO. 6, MAY 1962
705
small to be neglected, except in calibrations and in the most dcnianding distillations. OPERATING VARIABLES
Size of Charge. The product is accumulated in the form of small separate droplets on the silicone treated wall of the tube. The maximum permissible wall coverage, beyond which efficiency is lost due to remixing of the distillate, depends upon the interfacial tension between n-all and product. It varies from sample to sample. As a rule, normal paraffin and aromatic hydrocarbon distillates h a w less tendency to rcconibine bhan isoparaffins and naphthenic hydrocarbons. The maximum perniissible charge depends upon the maximum wall coverage and also upon t,he boilingpoint distribution of thc material. Obviously, it' must be smaller for narrow than for wide boiling-range material, especially if it yields a large residue. The maximum charge for mixtures with fairly even distillate covcragci over 10 to 14 inches of t'he tubc is gencrally approximately i o to 90 mg. for nornial paraffins and aromatics, and 50 to 60 mg. for isoparaffins and naphthenes. The minilnuin charge of the still is debermined by the technique for processing the distillates rather than by the design of the still. The vapor content of the tube during distjillat8ionis comparable to the holdup of a conventional still. It amounts to about 2 fig. in the case of a wide range mixture. Thus, the sample to holdup ratio rcmains high even for submilligram rliargcs Samples of 5 nig. h a w bccn distilled with quantitative recowry n-ithia t'hc weighing errors of a microh:hicc. For smaller samples, processing of thr products becomes increasingly difficult, but' in special cases (scparntioii of radio:tctivcly tagged conipountls 01' of biochemically active matcrinls), tho still should prove useful e v ~ nin thc microgram range. Use of Packed Columns. It should be possible to incre:rw the charge cayacity and plntc efficiency of the I1 by packing the distilling tube. me experiments with pure polyaroni:itic hydrocarbons in a tube packed witli silicone c-oatctl quartz grains int1ic:lted :I definite dccwase i n zone 1engt)h and thms an inrrease in Hotvcver. thc poor tj- of t,hc 1)arking (a liiglil>- conducting 1ia(tradccane (254" C. boiling point) is lost from t,hc system, unless it is trapped in a dry ice trap slipptd over the distilling tube. .it a pressure of 1000 microns, tet,radecanc is also completely retained within the gradient. Separation of Ion-er-boiling materials is possible by cooling the furnacc below room trmprrature. To r c . d u c ~ ~the load on the rrfrigcration system, a twin cooling mantle might be used with tap water followd by a refrigerant. The thermal instability of highboiling materials determines t,he upper limit of the still. d maximum operating temperature of 270°-2i5' C. is recommended for general use of t,he still. The highest-boiling distillatrs recovered a t this t(mperaturc, a t a pressure of 70 microns, and with f mm. per niinlite travel, hay(> an equivalent atniospheric boiling point of 570" C. (corresponding to a ClS normal paraffin). Cracking of hydrocarbon materials has never been observed under these conditions. Pet.roleum waxes have been distilled a t higher temperatures 11-ithout cracking (310" C. and 360" C. corresponding to atmospheric boiling points of 650" C. and 700' C.),but sensitive high-boiling mat,erials from nat,ural sources have occasionally been destroyed. A reduction of the thermal hazard is possible and might permit utilization of tlic still even at a 360" C. maximum trmprraturr. Thermal Hazard. T h e thermal hazard of :I still is defined as a quantit y de 1) r nd Pn t u 1) on the t e niper a t ure to n-hich a charge is exposed and upon the duration of the rsposure. Hickman ant1 E i n b r c ~( 7 ) cmplo~-cda hazard indcx t o rornl)nrr the rxposure of material in diffcrcnt stills. However, their indcx is h s c d on thtl equilibrium concept, and it is not direc~t~ly applicable to our stently-st,at,est,ill. TT'ith a ratr of t r a n ~ lof 14 mm. per minute, a complctc tlistillation in tht, microstill rcquirrs 50 minutes. .llt,liough this srcms long, :i considerablc part of tlie snniplc, 1111 t'o 90%, never rrnclics thc liottcst part' of thc furnnc~c. The dist'illntc rccyclw in tlic gradicnt furnace a t :i tcmpxaturc a t Ivliich its vapor prcssurc a \ - ~ r a g r snhout 4 microns. Sornial paraffins of 20 and 30 carbon atonia, foy instance, cyclc a t teniperatuva of 80" C. and 1-10" C. High-boiling tlistillntcP and the rcsirlur are exlmctl to higher trnipCrnturrs, hut, for a rcducc~lduration. T h e tliwnial exposure of the rcaitlur can be rcduccd froiii 25 to 2-5 minutcs by introducing tlic tubc a t once into
the constant-temperature furnace and topping it for a few minutes. The residue is separated from the unresolved condensate by cutting the tube or sealing it' off betwren the two products. The dist'illate is recharged to a new tube or into the sealed end of the original one and rrdistilled. The average vapor pressure of a distilland is 4 microns: this is allproximately the same as in conventional molecular stills. The exposure to temperatures above 100" C. is approximately 30 minutrs and can be decrcased to about 15 minutes by reducing the length of travel from 24 to 18 inches. The corrrsponding exposure time in a molecular-pot' still is about 1 hour; in a falling-film still about 1 to 2 minutes; and in a wntrifugal still, it is approximately 1 second. The ratio of thermal hazard t'o the number of thcoretical plates ( 7 ) is about the same for the falling-film still (an exposure of 1 to 2 minutes and 1 plate) and the prcsent still (an exposure of 15 minutes and an estimated 6 to 10 plates). During the distillation, air is present in the distilling tube in addition to the vapor of t'he distillate. Very sensitive materials should be distilled under nit'rogen with nitrogen bleeding into the manostat. This precaution is not necessary for reasonably stable materials such as most, petroleum hydrocarbons. Relative Separating Efficiency. The present steady state still fundamentally differs from the conventional equilibrium-type stills; the plate concept is not applicable. As a measure of its relative efficiency, we have compared the resolution of some samples n-ith t h a t obtained in conventional stills of known efficiency. Thus, a mixture of et'hyl-hexylphthalate and rthyl-1icxyl-sc.hacate was seliaratcd. Th(x charge was held small (10 to 15 mg.) to avoid excessive n.all covcragc~and consrcubir-e rrcombination of thc distillates. In principlc, one should analyze an infinitely small fraction of thc highrst boiling distillatc; because of the small charge, this \\-asnot Iiossiblc. TTitli a maxinium furnacc tcniprraturr of 310" C., a pressure of 100 microns and a rat'e of travel of i nim. prr minute, we obtained R total zone length of 23 ' 4 inchcs. \I'e analyzed 1 '1 inch a t the high and 1 inch a t the lonend by rcfractomctry. The rcsolutioii of the c1i:u.g~was found to be equivalent to that obtainablc in a six plate Yigreux still. This valuc may he biased toivard thc loiv side because of the rather excessive frat-tion of distillatc taken for rr conil~arison was niade by distilling a s:iml)le of ,z conimcrcial normal paraffin \\-ax under optimum conditions in the 1)' 250 microns, 14 nnn. pcr niinutc t'ravcl). A relatirely large charge had to be used
to provide suficicnt, s:iniple for subscquent mass-spectral an:tlysis. T h r carbon number distribution of thc ccntcr fraction was thcm cwmp~rtdto t,hnt of a dist'illatc of itl(,iitical yit4tl and boiling rang(' oljtainetl fmii thc s:tin~ \\.ax in a Vigrciux column oprratcrl at an c#icicncy of six to cight 1jlntc.s. H c ~ rblie cffic * i t w \ - of the l)rcwint still was slightly infcrior to the Yigrcus rolumri. It thus :ippwirs that t h r 1)r~sent~ still is capablc of n resoliit'ioii quivalent to a t least that obtained in a conrcntional sixplate still. Efficiency of Still as Function of Pressure, Temperature, and Rate of Travel of Distilling Tube. T h e posit'ion of a condensate in the distilling tube and the resolution of a mixture are influenced by the temperature gradient, the system pressure, a n d t h e rate of movement in the tube. These effects were investigated in a series of distillations of a 11-ide-boiling range normal par:tffin mixture. Sarron- sections, rcwxercd from 51/, t,o 53/4 inches (Figure 4) and from 9'/? to 93,'r inches froin the cold end of the furnace, were :inalyzed mass spectromctrically for their carbon number distribution. Lowering the maximum tcmpcrature of thc furnace and thus rcducing the sttlepness of the tcnipcrat'ure gradient grmtly improve the resolution (curve 1 and 2). The major nornial paraffin increases in concentration from 28 to 49%, whilr t,he averagr carbon number clcrrcascs from 28 t,o 24. -in increase in the ,system pressure from 7 to 250
Table I. Distillation of Hydrocarbon Fractions Ilistillation conditions: maximum temperature, 27%' C. ; pressure, 60 microns; rate of travel, 14 mm. per minute Sample I, 37.8 Mg. Sample 11, 6.03 Mg. Isoparaffins-Saphthenes Sornial Paraffins Estimate \T-eight, Weight Boiling Fraction mg. Per cent mg. Per cent Range, C. Cold trap 4.2 11.1 ... ... Below 280 1+2 7.1 18.8 3.01 49.9 280-365 3 7.2 19.0 1.57 26.0 365420 4 14.4 38.1 0.66 10.9 420-470 5 2.6 6.9 0.31 5.1 470-515 6 0.8 2.1 0.07 1.2 515-570 Residue 1.4 3.7 __ 0.29 4.8 Above 570 37.7 99.7 5.91 97.9 ~
~
microns (curves 3 and 4) reduces the average carbon number of the cut from 28 to 25 and simultaneously improves the resolution. -4s Bon-man and Tipson (4) pointed out for similar stills, the diffusirity of the vapor a t lo\\ pressures is too large to support a temperature gradient, and the plate efficiency is reduced until, in the extreme case, only a tn o-plate separation can be achieved. d n increase in the rate of travel of the tube leads to recycling a t a higher temperature and vapor pressure. Thus, the average carbon number a t identical positions in the tube is lonered from 27 to between 26 and 25 (curves 5 and 6). Faster travel improves the plate efficiency of the still. With a stationary tube, no resolution would result a t equilibrium. The increase in resolution lvith faster travel appears to
be due to a transition from a n equilibrium-state to a steady-state mechanisin. The effects a t higher carbon numbers (Figure 5 ) are identical except for the wider peaks and l o n w maxima due to the reduced vapor-pressure differences between adjacent normal paraffins. Curves 7 and 8 show the improvement in resolution with a lonering of the temperature gradient, curves 7 and 9 demonstrate hoiv much an increase in the systcm pressure improves the efficiency of the distillation, and curves 10, 7 , and 11, in that order, show the effect of increasing the rate of travel from 3.5 to 7 and 14 mm. pcr minute. -4 compromise betn een high plate efficiency, wide distillation range, and minimum thermal hazard must be made each time the distillation conditions for
2
50 -
>
rn.n/m,n
Figure 4. Influence of temperature, pressure, and rate of travel on resolution-average
C-number 24 to 28
VOL. 34, NO. 6, M A Y 1962
707
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ER
ANALYTICAL CHEMISTRY
a steeper gradient and a t lower pressure and thus increase the distillation limit and lower the thermal hazard. For routine analyses of hydrocarbon materials, we operate the still with a maximum temperature of 270” C., a system pressure of 60 microns, and a rate of travel of 14 mm. per minute. The tube is cut a t 2-inch intervals. This permits the distillation of materials boiling up to approximately 570” C. APPLICATIONS
T n o stills have been used for the distillation of many hundreds of samples, most of which were high-boiling hydrocarbon concentrates. Typical distillation results on a sample of normal size (38 mg.) and on a very small sample (6 mg.) are shom-n in Table I. Besides hydrocarbon mixtures, many other materials such as resins, the lower molecular weight fraction of asphaltenes, and various organic chemicals have been separated. The still is used for examination of the purity of commercial products and for the purification of small samples. It \yould appear to be useful also to reduce sample complexity previous to mass spectral or gas chromatographic analysis.
44
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Influence of temperature, pressure, and rate of travel on resolution-average
a certain charge are selected. Reducing the maximum temperature of the still improves the resolution but lowers the distillation limit. Increasing the system pressure improves the plate efficiency but lowers the distillation limit and may lead to oxidation and cracking in the vapor phase. At very high pressures, according to Bowman and Tipson (4, the vapor flow in such columns becomes laminar, and inadequate mixing takes place with a resulting loss in efficiency. This was confirmed by a drop in efficiency upon increase of the system pressure from 250 to 730 microns. ;iccordingly, a system pressure of about 250 microns is recommended for optimum plate efficiency. A lower pressure would be advantageous for the distillation of labile materials. A high rate of travel is desirable, because i t improves the plate efficiency of the still and reduces the thermal hazard. On the other hand, i t lowers the distillation limit, and a t very fast travel, some efficiency may be lost if the diffusion in the liquid phase is too slow to replenish the surface with volatile materials. Often some plate efficiency can be sacrificed. If the distillation tube is to be cut into 1- to 2-inch sections instead of 1/4-inch sections, one may operate at
708
36
2-NdMBEQ 270
C-number 34 to 40
ACKNOWLEDGMENT
The author thanks the management of Shell Development Co. for permission to publish this paper. The author also thanks E. C. Reinhardt and J. Richardson who have contributed materially to the physical design of the still. LITERATURE CITED
(1) Bates, T. H., Chem. & Ind. 1958,
1319.
( 2 ) Behrens, M., Fischer, A., -Vaturwzssenschujkn 41, 13 (1954). (3) Behrens, M., Melchior, H., Thalacker, R., Ibzd., 44,490 (1957).
(4) ,Bovman, J. R., Tipson, R. S., “Technique of Organic Chemistry,” Vol. IV, p. 473, Interscience, New Pork, 1951. (5) Chem. Eng. News 39, No. 10, 50 (1961). (6) Flaschentrager, B., Abdel-Wahab, S. hf., Habib Labib, G., Mzkrochim. Acta 46,390 (1957). (7) Hickman, K. C. D., Embree, N. D., Ind. Eng. Chem. 40, 135 (1948). (8) Morton, A. A., Mahoney, J. F., IND.ENG. CHEM.,ANAL.ED. 13, 494 (1941). (9) Thomas, J. F., Sanborn, E. N., Mukai, Tebbens, B. D., ANAL.CHEM. 30,1954 (1958). (10) Von Elbe, G., Scott, B. B., IND. ESG. CHEM.,ANAL.ED. 10, 284 (1938). (11) Von Elbe, G., Scott, B. B., U. S. Patent 2,198,848 (April 30, 1940). RECEIVED for review November 16, 1961. Accepted March 16, 1961.
