LITERATURE CITED
153 (1959). (4) “Indicators”, E. Bishop, Ed., Pergamon Press, Oxford, 1972.
(1) F. A. Cooper and J . C. Qualey, Analyst, (London). 91, 363 (1966). (2) G. Marinenko and J. K. Taylor, Anal. Chern., 40, 1645 (1968). (3) J. K. Taylor and S. W. Smith, J . Res. NaN. Bur. Stand., Sect. A , 63,
RECEIVED for review May 23, 1977. Accepted July 18, 1977.
Purification of Polyphenyl Ethers by Vacuum Distillation Thomas A. Elwood Department of Chemistry, University of Utah, Salt Lake City, Utah 84 7 72
Polyphenyl ether is a commonly used diffusion pump fluid in mass spectrometers and other vacuum apparatus (1-4). (The ethers present are mostly meta-substituted and contain three t o seven phenyl groups.) Because of either routine maintenance or pumping accidents, the oil must be changed occasionally. Major users of the oil can have five-gallon quantities redistilled by Consolidated Vacuum Corporation, Rochester, N.Y ,, or one-gallon quantities redistilled by Molecular Distillation, Inc., Titusville, Pa. Small laboratories, however, will find difficulty even approaching a gallon of used oil over a several year period. Our laboratory, for instance, collected only two liters of used oil over a 10-year span. The purchase of several moderate size diffusion pumps and the recent price increases for polyphenyl ether brought us to the point where redistillation of the used oil was practical.
EXPERIMENTAL Apparatus. The all glass/Teflon system is shown in Figure 1. The flash is 500 mL (Kimex);all joints are 24/40 Standard Taper with Teflon sleeves (Nalgene or Kontes). The upper half of the flask was wrapped with ‘/4-in.wet asbestos and then dried. The center arm of the flask holds a Teflon adapter (Kontes) which in turn holds a 21-cm length of “marine barometer tubing”, approximately 0.05 bore glass capillary (DME, 2-5 s or 6-12 s, Sargent-Welch). An identical Teflon adapter is used to hold the thermometer at the top of the distillation head. Moderate vacuum (0.1-0.3 Torr) is provided by a mechanical pump (Duo Seal, Sargent-Welch) attached t o a simple manifold comprised of a monometer and a variable leak. Heating is via a heating mantle (Glas-Col) and a variable transformer (Powerstat, Superior Electric). Procedure. The flask is filled with 300 mL of used oil with the capillary and adapter removed. A slight vacuum is applied to the apparatus (-100 Torr), using the adjustable air leak in the vacuum line, then the capillary and adapter are inserted into center neck of the flask. The capillary should begin to produce small bubbles in the fluid immediately; also, the liquid will begin to foam as it degases. The air leak is closed slowly until full vacuum is attained and all foaming has ceased. The variable transformer is turned to full power. Distillation begins at about 230 “C. This first fraction (usually 10-30 mL) is collected until a constant distillation temperature is reached and the distillate is a clear, pale yellow. The distillation is stopped by cutting the power and removing the heating mantle. After the flask is cool, the vacuum is reduced by opening the air leak, then vented by pulling the capillary and adapter. The bottom of the capillary should be wiped immediately and rinsed with a solvent such as dichlorornethane. The collection vessel is replaced and the distillation is reinitiated as above. The purified oil boils at about 250 OC with a decent vacuum and is collected over about a 10” range. (One batch was run with a McLeod gauge present in the line; the product boiled at 263-270 “C/0.25 Torr.) Typically, all but 10-30 mL distills over. After the apparatus is cooled and vented, the residue is rinsed out with dichloromethane, and the apparatus is ready for another batch. If the dirty polyphenyl ether contains solvents, the distillation is initiated at atmospheric pressure with a stopper in place of the capillary and ice bath around the first collection vessel. When distillation ceases, the power is turned off, and the collection vessel 1886
A N A L Y T I C A L CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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A ‘F-
u0 ..
Flgure 1. Apparatus for the vacuum distillation of polyphenyl ether
is changed. The vacuum distillation is then begun as described above. Diffusion pump fluid contaminated with roughing pump fluid requires additional steps. The two fluids are only partially miscible which allows the lighter roughing fluid t o be removed by decantation and the use of a suction pipet. Then, for ease in handling, half a volume of dichloromethane is added to a given volume of the dirty oil. In a separatory funnel, the oil is extracted with a succession of 3% NaOH in water, concentrated HC1 acid, and three portions of distilled water; the oil is always the lower phase. An appropriate volume is poured into the distillation flask, and the procedure as given above for the distillation of oil plus solvent is undertaken.
DISCUSSION The major impurities in used polyphenyl ether result from pyrolysis and air oxidation of the oil a t elevated temperatures when the vacuum system is inadvertently vented or partially vented. The simple vacuum distillation, shown in Figure 1, is capable of removing these impurities in 300-mL batches. Teflon sleeves and capillary/ thermometer holders were used to avoid silicone or other stopcock grease contamination. The use of a capillary is a common method for causing ebulation in a phlegmatic liquid heated under vacuum. Neither Carborundum boiling chips nor a rapidly spinning magnetic stirrer worked for the polyphenyl ether. From about 2000 mL of used oil, we reclaimed approximately 1500 mL. Under visual inspection, the fluid showed the same clarity and color as the original product. Bell jar tests on this oil revealed no detectable differences from new. The use of nitrogen or argon vs. air into the capillary was
found to be of little value for our uses. However, if the fluid is to be used in a low gas flow, ultra high vacuum system, the use of a gas may be worthwhile. T h e extraction procedures outlined in the Experimental section for polyphenyl ether contaminated with mechanical pump oil were deemed necessary because of the possible presence of surfactants.
ACKNOWLEDGMENT The comments of David A. Herold and the artistry of Herbert E. Brant during manuscript preparation are greatly appreciated.
LITERATURE CITED (1) K . C. 0.Hickrnan, Nature(London),187 405 (1960). (2) K . C. D Hickrnan (to Rochester Institide of Technology), U.S. Patent 3,034,700 (filed March 11, 1960). (3) S. Aftergut, R . J. Blackington, and G.P. Brown, Chem. Ind. (London) 1090 (1959). (4) K . J. Sax. W. S. Saari. C. L. Mahoney, and J. M. Gordon, J , Org. Chem., 25, 1590 (1960).
RECEIVED for review May 31, 1977. Accepted July 13, 1977. Support from the National Center for Toxicological Research is gratefully acknowledged.
Removal of Water in the Distillation of Hydrocarbon Mixtures T.
H. Gouw
Chevron Research Company, Richmond, California 94802
T h e presence of water in hydrocarbon systems almost always leads to trouble if this mixture is to be distilled in a regular packed or equivalent distillation column. Water has a very high heat of vaporization, necessitating more than the usual amount of heat to the reboiler. The very high surface tension and its immiscibility with the average hydrocarbon constituent in the mixture result in a lower column efficiency. T h e steam formed during the distillation will, in most cases, act as a carrier gas; and substantially larger amounts of high boiling components may be found in the distillate than what would be expected from the distillation cut point. In addition, water is easily superheated, leading to excessive "bumping" in the reboiler during the distillation process. In one case, we even experienced a collapse of a glass distillation column. If the water is present as a separate phase, it can obviously be removed by mechanical means such as by decanting or by centrifugation. In some cases, however, this water is emulsified with the hydrocarbon stock. There is also some water dissolved in the hydrocarbon matrix. Centrifugation is not always applicable to solve this problem. Of the several approaches to process these samples, we have found the following the most useful in practice. A. Heating. In a surprisingly large number of cases, a separate water phase can be induced by storing the sample a t elevated temperatures for extended periods of time. T o prevent the loss of light material, a reflux condenser may have to be mounted on the vessel. A better approach is to heat the mixture in a vessel which can be pressurized. For safety, a relief valve should be installed. The system can then be maintained a t 180-200 O F for several hours. If a separate water phase is formed, the system should first be cooled prior to removal of the hydrocarbon phase. Freezing the water a t this point (see Section D) is also an excellent method to separate the two phases. B. Addition of Chemical Reagents. The ASTM D 1160 method suggests as a convenient approach for dehydration, the addition of 8-12 mesh fused calcium chloride to the sample preheated to 180 O F . This is followed by vigorous stirring, cooling, and decanting of the supernatant oil phase. Although applicable to many samples of low viscosity with fairly high initial boiling points, several drawbacks are obvious. Precautions have to be taken to avoid losses of light ends. In relation to other hydrocarbon types, a proportionately larger amount of polar material may be removed in the solid phase. Some salt may dissolve in the oil which would affect the properties of the residual product in a high temperature end-point distillation.
C. Azeotrope Formation. The addition of excess of an azeotroping agent which is moderately to completely miscible with water results in a smooth distillation and a complete removal of the water from the feedstock. If the additive is not miscible with water, the distillat ion may not proceed as smoothly. As azeotroping agents, we have successfully used isopropanol or n-butanol, but other crganic compounds can obviously also be employed. About !i-10 times or 3-5 times, respectively, of the amount of these alcohols as the estimated amount of water present is added to the sample prior to the distillation. This is a very elegant approach, but there are drawbacks. The additive constitutes an additional expense. In some cases, the introduction of a foreign substance may be objectionable because the distillation products in that particular boiling range will be contaminated by this material. Another, and more important, objection is the loss of the boiling curve information in this region. A prime example is in the distillation of crude oils. D. Water Removal by a Preliminary Distillation Step. The determination of the true boiling point ( T B P ) curve of a crude oil as, e.g.. specified by the ASTM Method D 2892 ( I ) , is fraught with erratic data for the lower temperature range of the distillation curve if more than ,a few-tenths percent of water is present in the feedstock. (This standard is currently being revised.) Removal of this moisture by azeotropic distillation is obviously impractical. The standard approach is to carry out a preliminary distillation with total takeoff in a low efficiency column. This distillation is carried out until all water is removed which, in a crude oil distillation, may correspond to a vapor temperature of about 275-300 O F . The distillate will consist of two immiscible phases. No emulsification is present in this mixture because of the fairly high water to hydrocarbon ratio and because of the absence of heavier hydrocarbon components and traces of surface active components present in the original oil. T h e water is now removed, and the hydrocarbon phase is reblended into the reboiler prior to subjecting the water-free mixture to the standard distillation procedure. The obvious method to remove water from the distillate is by the use of a separatory funnel or by decanting. However, in this approach, it is very difficult to prevent some loss of the lower molecular weight hydrocarbons in the mixture. Evaporation losses occur during the water withdrawal step. Some hydrocarbons are also lost because of a small but finite solubility in water. ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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