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
1887
50, r:
f’” c
Figure 1. Laboratory glass flash vaporization still
An elegant alternative is to cool the two-phase distillate in t h e original receiver in a refrigerator a t -10 to -20 “C. T h e water will solidify and, in this process, reject the dissolved hydrocarbons from the crystal matrix into the liquid phase. This is the basis of the purification principle in zone melting and in normal freezing ( 2 ) . In the absence of a refrigerator, one can also slowly cool the lower portion of the distillate container in dry ice t o induce crystallization of the aqueous solution from the bottom up. After the water is frozen, the hydrocarbon layer can be recovered by decanting. E. Flash Vaporization. Emulsified mixtures of heavy (and not so heavy) residual stocks and water are obtained from, e.g., the cleanup of oil spills, API separator skims, etc. Even the water from crude oil desalters may form fairly
iaa8
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
persistent emulsions. These products appear homogeneous and may contain as much as 80% water. An economical approach to process fairly large quantities of this material is to use flash vaporization. Figure 1shows a schematic diagram of the laboratory unit which we use. A more detailed drawing has been published earlier (3). Vaporization is usually carried out a t atmospheric pressure a t temperatures as high as 400 O F , depending on the composition of the feedstock. The product is drawn from the reservoir and pumped into the heating coil by a Zenith gear pump with a variable speed controller. T h e products are preheated to the desired temperature in the glass, double-helically wound coils and then flashed in a 15-inch by 1-inch flash chamber, where t h e separation between vapors and liquid takes place. The overhead products are condensed and collected in the distillate receiver. The bottom product goes through a gooseneck trap in the line (which acts as a liquid seal) t o the bottom receiver. Heating of the coils and the flash chamber is carried out with a heating fluid, the boiling temperature of which can be adjusted by the choice of the fluid used and by the pressure in the system. The overhead products consist of an immiscible two-phase mixture of water and light hydrocarbons which can be separated by the freezing technique described in Section D. The amount of water in the bottom product depends on t h e severity of the separation but can be easily adjusted to levels of 0.1% or less.
LITERATURE CITED (1) ASTM D 2892-73, 1975 Annual Book of ASTM Standards, p 823. (2) T. H. Gouw, in “Progress in Separation and Purification”, Vol. I , E. S. Perry, Ed., Interscience, New York, N.Y., p 968. (3) J. A. Lodtwccd, R. L. LeTourneau, R. Matteson, and F. Sipos. AM/. Chem., 23, 1398 (1951).
RECEIVED for review June 6, 1977. Accepted June 29, 1977.
