SEPARATION PROCESSES
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I
Separation of Isotopes by Fractional Distillation of Water B
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lower pot Tubular heater Steam supply line Heavy water concentrate tank
MERLE RANDALL AND WELLS ALAN WEBB University of California, Berkeley, Calif.
I J K L
L‘
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0 P $ m y condenser acuum regulator Q
R Vacuum receiver S T
Vacuum pump Elector for auxiliary vacuum Main cool in^ water supply V Auxiliary water supply W flooding indicator X Botiom water level gauge Y Concentrate tank level gauge z Vent a Vacuum line cooler
U
Heavy concentrate -liquid Heavy concentrate -vapor Common water -liquid Common water -vapor--
Scale
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In the rush to obtain a small quantity of a newly discovered element the pure scientist is often forced into near factoryscaleoperation. The ratio of the vapor pressures of ordinary and heavy water is 0.949 at 100’ C. and 0.90 at 50’ C. This paper describes the fractionating column by which the concentration of deuterium oxide in ordinary water was increased to such a concentration that the pure deuterium oxide could be easily obtained by the electrolytic method.
I” = 6 ’ - 0
FIGURE 1. COLUMN1
stallation, and preliminary operation, was completed within 20 days), it was impossible to make everything as might have been desired or even to keep complete notes of all events, No apology is offered, for the column adequately fulfilled its primary purpose-namely, the furnishing of “heavy concentrate” at a time when no other primary source of deuterium was locally available.
HI8 paper describes the preliminaryresults in the concentration of the heavy isotopes of hydrogen and oxygen in the fractionating column, 12 inches (30.5 cm.) in diameter and 72 feet (22 meters ) high, constructed in June, 1933, and referred to by Lewis (6,s)in August, 1933. Nearly all the deuterium oxide made by Lewis and Macdonald was prepared by electrolysis from a bottom fraction of this still or from a bottom fraction which had been further concentrated by fractionation in a second column, 2 inches X 72 feet (5 cm. x 22 meters). The sample of “heavy concentrate,’’ examined by Latimer and Young ( I ) , which showed indications of H3 by the magneto-optic method of Allison, was originally a concentrate from column 2. This brief description of the columns and a discussion of some of their operating characteristics are given, in order to complete the record of the above pioneer experiments. Because of the necessity of furnishing a maximum quantity of “heavy concentrate” as rapidly as possible, and the consequent need of speed in the assembly of equipment (the entire column 1, including purchase of materials, c~nstruct~ion, in-
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Column 1 A diagram of column 1 is given in Figure 1. Washburn, Smith, and Frandsen (8), and Washburn and Smith (7) showed the possibility of fractionation of the isotopic forms of water by distillation. The experiments of Lewis and
Cornish (4) had indicated an increase in the density of the bottom fraction, and a decrease in the density of the top fraction when water was distilled a t atmospheric pressure in an efficient total reflux packed column. The experiments of Lewis and Macdonald (5, 6) had shown an increase in the ratio of the vapor pressure of water t o that of deuteriumtoxide as the temperature is lowered. 221
INDUSTRIAL AND ENGINEERING CHEMISTRY
228
The authors' column was therefore designed to operate under less than atmospheric pressure, not only to obtain a gain in fractionating effect but also to avoid a possible loss of concentrate by leakage outward. The column is operated as a semicontinuous total reflux column with the feed on the top plate; the feed also serves in place of the reflux, for it is obviously richer in heavy constituents than any fraction condensed from the vapor. The feed was the condensate from the heaters; it was ordinary distilled water and was greater in amount than the vapor from the top plate.
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1200 I100
100 080
I000 900 800
070 060
700 600
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2 050 03C
500 400 300
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200 I00 0
2 040 I
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8 I0 I,? I4 16 18 202224 26 2830 2 4 6 8 /0 I2 14 16 I8 June 1933 July 1933
FIQURE
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< 3 2 9
FEED RATEAND INCREASE I N DENSITY IN INITI.4L RUN O F COLUMN 1
About 54 cubic feet (1,500 liters) of packing material were needed to fill the column. A brief survey of available materials and preliminary measurements showed that aluminum turnings and borings from the manufacture of gas engine pistons were available; they gave a theoretical equivalent plate height of about 5 inches (12 cm.) in small short test columns, and the flooding capacity of well-selected borings was of the order of 7 ml. per sq. cm. per minute, or 5 liters per minute for the 12-inch (30.5-cm.) diameter column. As was to be expected, the borings were not all like the sample submitted, and although only the material remaining on a 0.19-inch (0.47-em.) mesh screen was used, the flooding rate proved to be about 2 liters per minute; owing to the corrosion of the borings in portions of the column, the flooding rate decreased during the 8 months the column was in operation to less than 500 ml. per minute. The flooding rate is determined by the rate of the least satisfactory portion. There was always a slow action of the hot water upon the borings, as evidenced by the presence of hydrogen in the gases from the condensed vapor. The evolution of hydrogen as well as the presence of air in the injected cooling water limited the feed rate possible when using an ejector for vacuum to approximately 1 liter per minute. The later operations a t higher feed rates were made using the automatically controlled vacuum pump to remove the gases from the condenser. The main column was of 12-inch standard steel pipe which had served as an exhaust line from the turbines in theuniversity power plant. T$e dirt and scale were partly removed by means of a revolving wire brush. When the packing was later removed, the surface of the iron was clean, and there appeared to be no more corrosion of the borings near the iron than in the center of the column. The borings were supported by means of an inverted ordinary aluminum collander with many additional 0.19-inch (0.47-cm.) holes; the rim was supported by a brass rifi'g resting upon crossed 0.5-inch (1.27-em.) steel rods inserted into recesses near the bottom flange. The vapors in such a long packed column tend to
VOL. 31, NO. 2
channel through the middle, the liquid reflux running down the sides. There was no time t o provide for redistribution of the liquid reflux, and the number of theoretical plates in the column is much less than is indicated by the experiments with small short columns. At the outset, since the authors were pioneering, all accessory equipment was either installed in duplicate or provision was made for the installation of a second unit to be used while the original unit was being repaired or altered. Thus the heaters, the condensers, the feed regulators, etc., were all in duplicate or of alternate design. The authors were thus able to operate the column continuously for periods as long as 90 days. In order to keep the bottom volume small, the bottom of the column was designed as a combined centrifugal steam separator and bottom reservoir. Later it was found desirable to insert a 25-gallon (94.5-liter) tank in one of the bottom circulating lines. This tank served to settle aluminum hydroxide which was carried down from the packing and lengthened the interval between necessary cleaning8 of the tubes in the heaters. To begin operations, enough distilled water was put in the bottom of the colunin to give circulation through the heaters. The vacuum was brought t o 25 inches (62.5 em.) of mercury and the column was slowly heated. Water was added a t the bottom as required to maintain a constant level. When vapor began t o flow t o the condenser, the feed of distilled water was started and the heat supply to the jacket of the operating heater was increased to such a point that all the liquid flowing down through the packing was evaporated as fast as it reached the bottom. There was some fluctuation in the rate of evaporation in the heaters and also in the rate of reflux, and these surges served to decrease the yield of heavy isotopes. The decrease in yield caused by surges seemed to be greater in the case of the oxygen isotope. In this type of column the total yield of heavy isotope under steady conditions is determined by the efficiency of the top plate and the ratio of the volatilities of the light and heavy isotopes. If the quantity of liquid on the top plate is small, then a relatively small excess of ascending vapor over feed may for a time cause a net loss of heavy isotopes in the column. If the quantity of liquid on the top plate is made large, then surges in the rate of vapor or reflux will be of less consequence and such losses will be decreased. In the early experiments the authors used a 50-foot (15.2-meter) atmospheric U-tube as a barometric seal and reducing valve for the feed. Although the temperature of the feed was kept a few degrees below that of the top of the column, occasionally bubbles of air and vapor became entrained in this tube, giving an air-lift effect in the vacuum leg of the seal, which resulted in breaking the vacuum. Later, reliable control of the feed rate was obtained by means of a device consisting of an auxiliary feed tank with a constant-level float; a calibrated orifice delivered the feed into a second small auxiliary feed tank with a large flat float operating a globe valve in which a copper bellows had been substituted for the usual packing gland and which acted as a reducing valve to deliver to the column just the quantity of water fed to the second auxiliary tank. The condensate from the heaters as well as from the other stills was collected in a 10-gallon (37.8-liter) tank and delivered to the feed tank a t the top of the column by a l-inch (2.5-cm.) Crane tilt trap and later by a steam pump. The steam used as the lift kept the water in the feed tank hot. For a time an auxiliary feed storage tank was used. An automatic recorder and time interval indicator on the lift trap served as a measure of the regularity of the operation of the heaters and of the operation of the still itself. All piping was insulated with 85 per cent magnesia asbestos insulation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
FEBRUARY, 1939
Performance of Column 1
The experimental arrangement is shown in Figure 3. All the liquid overflowing the pot into the heater is immediately returned to the column as vapor, so the operation of the column is controlled entirely by the rate of feed which is slightly below the flooding rate. The column was filled a t the outset with the same aluminum borings as column 1. When a charge of heavy concentrate with an increased density of about 180 parts per million was fed from the top tank, an increase in density of At = 7.5" (1390 parts per million) was obtained in the lower part of the column in about a week. The "increase in density" of the water in the top tank decreased by about half every 5 days. The authors' original plan was t o allow the concentrate produced by column 2 t o accumulate until they had 10 gallons (37.8 liters) of water (about 1400 parts per million density increase). With this it was hoped to charge the top tank again through a second run with column 2 and obtain a heavy water of high concentration from the bottom of the column. However, the demand for concentrate was so great that this could not be done. All concentrates made in column 2 were added to fractions of corresponding density processed in Lewis and Macdonald's electrolytic plant for the concentration of deuterium oxide. The aluminum packing soon blackened and corroded and the throughput of the column became only a few milliliters per minute. The column was then cleaned and refilled with Nos. 12 and 14 iron single jack chain in the lower part and with No. 14 brass jack chain in the upper part. Column 2 was operated many times and required very little attention.
A summary of the performance of column 1 during the first 5 weeks of its operation (June 8 to July 18, 1933) is shown in Figure 2. The solid curve is the increase in density of the bottom fraction measured in terms of At, used by Lewis and Macdonald (6), and in terms of the increase in density in parts per million (1" At = 170 parts per million increase in density). The dotted curve gives the rate of feed in milliliters per minute. The density increased almost linearly for the first 20 days, or until June 28 when the column appears to have reached a steady state between upper and lower concentrations. The increase in density a t this time was about 180 parts per million. The operation of the column during the period from July 2 to 5 was very unsteady, and the lower pot became flooded. This necessitated the reinoval of 72 liters of water with an average concentration of At = 0.74 (126 parts per million increase in density), The column again resumed a steady increase in At somewhat more rapid than in the earlier run (influenced no doubt by the greater rate of feed), and it again became steady a t approximately 180 parts per million increase in density. The vacuum during this period was being maintained by the ejector and was not steady. An effort was made to maintain the vacuum a t 25 inches (62.5 em.) of mercury, under which conditions the top temperature was about 130" F. (54.4" C.) and the bottom temperature from 150" to 170" F. (65.8" to 76.7" c.), depending upon the holdup in the column. The increase in density in the bottom fraction was due partly t o hydrogen isotopes and partly to oxygen isotopes. In a separate communication Lewis (3) reported his isotopic analysis of the bottom fraction. He showed that when the increase in density was 182 parts per million over ordinary water, 97 parts mere due to accumulated deuterium and 73 parts t o the heavy oxygen kotopes. That the check was not exact they attributed to experimental error. Column 2 The theory of operation of the second column' was to have a large constant volume of about 11 gallons (42 liters) a t the top and a small volume (about 1 liter) in a pot a t the bottom; the operation of the column served continually to lower the density of the top and to increase the density of the lower fraction. The condensate a t the top of the column was returned continually into the bottom of the top tank while the feed came out of the top of this tank. If there is no mixing in the tank (which is never fully realized), every time an amount of water equal to the entire content of the top tank is fed into the top of the column the water remaining would be depleted in heavy isotopes by the enrichment factor determined by the temperature of the top plate. This process will continue with smaller and smaller efficiency until the ratio of the concentration of each isotope in the top and bottom containers reaches a steady-state composition characteristic of the isotope, which is given by the equation
A Column-2 galv iron pipe B Bottom thermomeler
C
D
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0
logA where A
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=
log a d n
(1)
limiting enrichment at steady state
av. enrichment on each individual theoretical plate = number of the theoretical plate
1 The junior author did not assist in the construction and operation of column 2.
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W Vacuum auge X Water mB,, trap Vacuum line Z Mercury level contactor a Sobnoid-operated valve b Relav C Vaciium pump
FIGURE 3. COLUMN 2
INDUSTRIAL, AND ENGINEERING CHEMISTRY
230
Present* State of Distillation Project The tubular heaters of column 1 have shown a tendency to develop ‘(leaks” which allow steam to enter the bottom fraction (thereby reducing the possible enrichment of the bottom fraction). The heaters have been redesigned so as to avoid this trouble completely. The aluminum packing has been removed from column 1, and it is being filled with shoe eyelets known as United Shoe Machinery Corporation No. 2G brass eyelets. Suitable deflectors are being installed in order to counteract channeling. Two additional i’z-foot (21.9-meter) columns, one 2 inches (5 cm.) and the other 1inch (2.5 cm.) in diameter, are also in use. Further theoretical treatment of results is deferred to a later paper.
Summary Preliminary results have shown that the 12-inch (30-cm.) column will accumulate better than 5 grams of deuterium oxide per day in the form of dilute aqueous solution and that pure Hz018 and possibly H20” should be obtained by fractional distillation within a reasonable time. 9
VOL. 31, NO. 2
Acknowledgment The authors wish to thank G. N. Lewis, Ronald Macdonald, and Robert E. Cornish for their interest in the project, J. C. Potts for analysis of the heavy concentrates, and also E. J. Caldwell, Wilbur H. Lear, Albert W. Deutschman, George F. Johnson, and Michael Arnado who assisted in the construction and operation of these columns, and to the Works Progress Administration for clerical and mechanical assistance (OP 465-03-3-147). They also wish to acknowledge the kindness of the United Shoe Machinery Corporation which furnished a large quantity of No. 2G brass eyelets a t a price reasonable enough for them t o be used for packing.
Literature Cited (1) Latimer and Young, Phys. Rev., [2]44,690 (1933). (2) Lewis, G.N., J. Am. Chem. Soc., 55, 3502 (1933). (3) Lewis, IX Congr. intern. g u h . pura aplicada, 2, 5-32 (1,934) (4) Lewis and Cornish, J . Am. Chem. Soc., 55,2616 (1933). (5) Lewis and Maodonald, Ibid., 55, 305 (1933). (6) Lewis and Macdonald, J . Chem. Phys., 1 , 3 4 1 (1933). (7) Washburn and Smith, Ibid., 1,426 (1933). (8) Washburn, Smith, and Frandsen, Ibid., 1,288 (1933). RBCEIWDJuly 20. 1938.
May 1, 1935.
Effect of Pressure on Lubricating Greases BRUCE B. FARRINGTON AND ROBERT L. HUMPHREYS Standard Oil Company of California, San Francisco, Calif.
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REVIOUS reports in the literature on the effect of pressure on lubricating greases have dealt with the effect of Pressure on the flow characteristics of this type of material. Several types of pressure viscometers have been described, all being designed to measure yield point and viscositv a t various flow rates. TCe present report, however, describes results obtained with an interesting grease press designed by Herschel (2). This apparatus confines the grease under test while pressure is placed upon it, and immediately absorbs any oil squeezed through the filter-paper membranes. The factors governing the retention of lubricating oil by the soap fibers of the grease can thus be studied. A photograph of the disassembled grease press is shown in Figure 1, most of the parts being inverted to show details. A is the receptacle, B the clamping disk, C the latch, D the plunger, E extra 5-pound weight, F grease sample on filter paper, G blotter disk, and H a grease molding washer which the authors have found very convenient. The washer is made of brass, outside diameter 2 inches and inside diameter 0.94 inch, tapered to 1 inch, and molds ap roximately 2 grams (31 grains) of grease. By the use of this mofd the same sized pellet of grease is obtained every test run, an essential factor in producing concordant results. The grease sample is placed between two filter papers enclosed in two blotting-paper disks, and the whole clamped in the press. Pressure is applied by inserting the plunger. The grease and filter papers are weighed before pressing and after selected time intervals and the oil loss is determined by difference.
In Figure 2 is shown the grease press assembled, the parts being lettered as before. A wooden base on which the apparatus is supported can be seen in the photograph, as well as a ressed sample of grease with accompanying filter papers and &tern. The plunger shown weighs 5 pounds but additional weights can be used if desired. The area t o which pressure is applied is 2.76 square inches.
Effect of Grease Consistency on Oil Loss In Table I are shown the various greases tested, the code numbers used in the following figures, and various physical constants of the greases. TABLE I. CHARACTERISTICS OF GREASES TESTED Grease
Type of Soap
A. S. T. M. Worked Penetration Oil
% 1
Calcium
3
Calcium Calcium Calcium Sodium Sodium Aluminum Aluminum Snow-white petrolatum
i
9
10
343 220 170 364 331 302 220 355 210 220
85.5
77.5 67.3 87.8 87.6 88.0
76.0 88.0 80.0 *.
Oil Visooaity at
Soap 1OOOF. % S. U.S. 14.5 300 22.5 300 32.7 300 12.4 110 12.4 520 12.0 180 25.0 1so 12.0 275 275 20.0
..
In Table I1 (Figure 3) are shown the results of tests on three calcium soap greases differing in mineral oil content
