A. A. PALKO Chemistry Division, O a k Ridge National Laboratory, O a k Ridge, Tenn.
Separation of Boron Isotopes in the Bench-Scale Boron Fluoride-Anisole Unit This closed-cycle process, which requires only addition and removal of heat for reflux, offers an improved method for the production of boron isotopes
THE problem of enriching boron isotopes has been studied theoretically (2, 73, 74) and experimentally (4-8, 70-72). As a result a number of enrichment methods have come into use. Gas-liquid chemical exchange seems to be the most promising of these methods for large-scale production of high purity boron isotopes. The boron fluoride-anisole system is a gas-liquid, countercurrent chemical exchange process with a large separation factor, high throughput, and excellent operational stability ( 7 7 ) .
+
CF,H~OCH~.B"F~(~)B'oFa(g) S C6H~OCH3~B'OF(l) B"F,(g) BFa(g)
+
(1)
cool + CEHsOCH?(l)e heat
CeH60CH3,BFS(l) (2) The isotopic fractionation factor, (BIO/ B")1/(B10/B"),, for the exchange reaction 1 is 1.029 a t 25" C.; boron-10 concentrates in the liquid with a half time for the exchange reaction of less than 3 seconds. Reaction 2 provides a convenient and inexpensive mechanism for refluxing the exchange reaction. Cool anisole quantitatively absorbs boron fluoride to form the 1 to 1 boron fluorideanisole complex with the evolution of 12.3 kcal. of heat for each mole of complex formed. This reaction is quantitatively reversible. When heat is supplied, the complex readily dissociates into boron fluoride and anisole. T o utilize these reactions, boron fluoride and the anisole complex were passed countercurrently in the exchange column (Figure 1). Boron-10 concentrated in the liquid phase and boron-11 in the gas phase. The liquid complex leaving the exchange column flowed by gravity into the decomposer system where it was heated. Upon heating, the complex dissociated into boron fluoride and anisole. T h e gas rose upward through the gas cooler into the exchange column and then into the recombiner. T h e anisole meanwhile passed downward through the heat ex-
changer and was pumped to the top of the recombiner. I n the recombiner, boron fluoride and anisole reformed the complex which was then pumped continuously to the top of the exchange column. Thus, a completely closed system was attained. No chemicals were required for reflux; only heat was consumed. Apparatus Columns. The apparatus consisted essentially of four parts-an exchange section, a recombiner, a decomposer, and a solvent purification still-made from standard glass pipe. The exchange column and recombiner, 1-inch outside diameter, were jacketed for temperature control. T h e exchange column, 36 inches long, contained 32 inches of packing. Three types of packing were tested, Helipak, Cannon protruded metal, and glass helices.
Early work was done with a 30-plate, bubble-cap exchange column, but it was replaced with a packed column which had a shorter' equilibrium time, a shorter stage height, and less back pressure. Under normal operating conditions 20 to 30 theoretical stages were attained with the packed column. The recombiner was 22 inches long and contained 20 inches of packing. Temperatures could be measured throughout its length with a movable thermocouple. The decomposer was made from a 30-inch section of 2-irich pipe to which a 1-liter flask was sealed. At the top a cold finger condenser was attached. T h e purification still was of similar construction, except the column was made from 1-inch pipe and the reboiler was a 500-ml. flask. Heat was supplied to these units by means of rheostatcontrolled heating mantles.
Figure 1 . Although this pilot plant was specifically designed to study the boron trifluoride-anisole system, it could be used to study any similar gas-liquid isotopic process which can be refluxed thermally VOL. 51, NO. 2
FEBRUARY 1959
121
Cooling System. A standard, officetype water cooler was modified to supply chilled water to the entire unit. The temperature of the water could be varied from 4 to 28 C. with less than 0.5 O C. fluctuation. An Eastern Model D-11 centrifugal pump, which was used to circulate the water, operated continuously for months without failure. Transfer Lines, Valves, Pumps. All water lines were made from copper tubing, 3 / ~ inch in outside diameter. Transfer lines for process streams were made from either copper, nickel, or stainless steel tubing of 0.25-inch outside diameter. The strongest and most versatile glass-to-metal seals were made with full face, 3/l~-inch metal flanges gasketed with Teflon V or 0 rings against standard glass pipe joints. Holes were bored into the flanges, and transfer lines, valves, and the like were silversoldered directly to the flanges. Changes in the piping could be made easily and quickly. A torque wrench was used to tighten all flange bolts; 20 inch-pounds of torque was sufficient to make the system both vacuum and pressure tight. Because of the corrosive nature of the chemicals handled by the apparatus, all valves were either bellows or diaphragm packless valves of stainless steel or Monel -e.g., Hoke Nos. 411 or 413. These valves operated satisfactorily under vacuum or pressure for long periods of time. Tubing connections were made with Swagelok-type fittings. Packless pumps were used to transfer process streams in the system. Research Appliance Co.’s Model 1000, heavy duty, stainless-steel bellows pump (0 to 3000 ml.) gave satisfactory service for weeks a t a time. O n e set of bellows would operate continuously under process conditions for approximately 40 days. The pulsating stream presented no problem. A calibration tube incorporated directly into the apparatus was used to check the flow rate of the boron fluoride-anisole complex stream. Automation. Automatic operation was achieved with a bellows-operated pressure switch (United Electric Co., Type H-9) which controlled the pump delivering anisole to the recombiner. As boron fluoride and anisole reacted in the recombiner, pressure increased or decreased. An excess of boron fluoride increased pressure causing the switch to close and actuating the anisole pump. As soon as anisole was in excess, the pressure in the system fell, the switch opened, and the pump stopped. By careful adjustment of the pumping rate, pressure fluctuations could be maintained a t f l mm. of mercury, and smooth operation was obtained under these conditions. The switch was adjustable from 0 to 20 inches of water pressure. For normal operation, the system pres-
122
sure was set a t 15 inches of water above atmospheric pressure. This slight positive pressure aided in locating any leaks which developed while the system was operating as boron fluoride fumes strongly upon exposure to air. I t also prevented atmospheric moisture from leaking into the system.
present in the solvent stream could be removed. Nonreactive gases in the system collected a t the top of the recombiner. These gases were always present on startup because tank borqn fluoride contained a small amount of silicon tetrafluoride. Also, residual traces of moisture in the system reacted to form silicon tetrafluoride, methyl fluoride, and hydrogen fluoride. These gases were vented periodically.
Procedure After the apparatus was cleaned. dried, and assembled, it was pressure and vacuum tested; flushed with clean, dry anisole; and evacuated by means of a vacuum pump to remove any air present. A prescribed amount of purified anisole was then admitted to the decomposer and the recombiner. The pumps and decomposer heater were started, and boron fluoride was admitted slowly through a safety trap into the system. Addition continued until the recombiner was approximately half full of the complex, and the pressure of the system was about 850 mm. of mercury. Under these conditions, the system contained approximately 1 gram mole of boron fluoride and 700 ml. of anisole. By the time the required amount of gas had been added to the system, the temperature of the decomposer reboiler was a t the boiling point of anisole. The pumping rate of the liquid complex phase was then set a t the desired level; the system was switched to automatic control and allowed to come to thermal and chemical equilibrium. Samples were taken periodically from all process streams for chemical and isotopic analysis. Temperature readings were recorded continuously from a series of thermocouples arranged strategically throughout the system. The anisole used for these experiments, D u Pont technical grade, was purified by a method described previously ( 7 7 ) . The anisole stream could also be purified continuously while the pilot plant was operating by merely rerouting the anisole from the decomposer through the solvent purification still. Thus, any solids or high boilers
Results Decomposition. Moisture was the determining factor in causing decomposition. Analysis of gas from the top of the recombiner showed a high concentration of methyl fluoride with lesser amounts of hydrogen fluoride and silicon tetrafluoride. Most of the silicon tetrafluoride was introduced as an impurity in the feed gas. Methyl fluoride and hydrogen fluoride were products of hydrolysis by reactions such as the following: BF3
HF
+ 3H20
+
+ C6HbOCH3
H3B03 +
4- 3 HF
CH3F
(3)
+ CeHbOH
(4) Analysis of the effluent from the decomposer showed that both phenol and boric acid were present in sufficient quantities to account for the amount ofmethyl fluoride found in the system. The presence of phenol in the system presented no difficulty as the boron fluoridephenol system has nearly as high a separation factor as does the anisole system ( 5 ) . Because small amounts of cresols are also formed, some rearrangement of the anisole must take place during prolonged contact with boron fluoride, traces of water, and hydrogen fluoride ( 3 ) . Once moisture was eliminated from the system, the apparatus could be operated for weeks with very little decomposition. Fractionation of the solvent a t the end of several runs indicated an average decomposition rate for a dry system of approximately 0,470 of the working inventory per day. T h e solvent
Table I. Analysis of Anisole after Prolonged Use in Boron Fluoride-Anisole System Showed That Metals Did Not Affect Decomposition Rate of the Solvent Length Run No.
R-23b R-2gb
R-30’ R-31d R-32’ R-33,
Solvent
Decomp.
of Run, Recovered, Rate, Days % ’ ’%/Daya 20 19 50 20 37 78
B
Metal in Solution, P.P.M. Fe Cr Ni
128 93.4 80 92 95
78
0.35 0.40 0.40 0.35 0.28
470 169 566 315
3 6 119 2 2.5
2 3
38 5 1
1.2
cu 26 59 54 2.4 3.5
% Working inventory/day. b Nickel packing in exchange column, decomposer, and recombiner. Stainless steel packing. d Same as Run 30 plus addition of copper. e Same as Run 30 plus addition of black iron. f Apparatus contained stainless steel packing, transfer lines, and pumps. Valves were Monel, hence copper is in emuent.
INDUSTRIAL AND ENGINEERINO CHEMISTRY
BORON ISOTOPES l6
i
=
0
, I
I
'
14 0
I
B F 3 - A N I S O L E SYSTEM R U N 15 6 2 m o l e s /hr. c x F L O W R U N 18 4 9 m o l e s / h r . c x F L O W R U N 20 i 3 m o l e s / h r . c x FLOW
1
Y
8
/
RLIN 17-18 RUN 2 2 RUN 2 3
U
c
1
25
r-
5
4C
15
RUN 2 3 A
20
-
~
25
TIME !days:
Figure 2. Boron content of the decomposer stream was well within tolerable limits
was fractionated on a Podbielniak 60plate, concentric tube column having a total holdup of 1.5 ml. ofsolution. A reflux ratio of 30 to 1 was used. Several runs were made to determine the effect of various metals on decomposition rate of the solvent. Even after 20 days of operation with nickel packing (Table I, runs 23 and 23), the solvent remained clean and the amount of decomposition small. R u n 30 showed that stainless steel packing was as satisfactory as nickel. Approximately 10 square feet of bright metal surface in the form of turnings was exposed in various parts of the system with no apparent effect on
Table 11. Reflux Ratio and Number of Stages Needed Depend on Separation Factor and Product Purity Desired" B 10 Product, s VID a0 (Min.)c (Min.)d % 1.05
60.0 95.0 99.9
38.3 90.3 171.5
54.0 100.0 106.2
1.04
60.0 95.0 99.9
47.7 112.4 213.4
67.5 124.8 132.8
1.03
60.0 95.0 99.9
63.2 149.1 283.1
90.0 166.3 177.1
1.02
60.0 95.0 99.9
94.4 222.6 422.7
135.0 249.5 265.6
1.01
60.0 95.0 99.9
187.8 443.0 841.4
270.0 499.0 531.0
1.005
60.0 95.0 99.9
373.9 882.1 1675.1
540.0 998.0 1062.4
a Square cascade-enriching section of boron isotope. Single stage separation factor. e Number of stages. Reflux ratio. V = total upflow, D = product withdrawal (IS).
0
1
2
3
4
5
6
7
8
3
T I M E (hr.)
Figure 3. Total separation increased rapidly from startup, but equilibrium was not attained for several hours
solvent decomposition (Runs 31 and 32). However, there was considerable plating out of copper on the packing in the hot part of the decomposer with an increase in the amount of iron in solution (Run 31). For black iron (Run 32), an extremely high amount of boron was left in the decomposer effluent, though the appearance and the decomposition rate of the solvent were normal. Reflux Efficiency. Because low separation factors are characteristic of isotopic separation processes, reflux efficiency is of great importance. If the desired isotope is lost because of unwanted side reactions or leakage, productivity of the process may be greatly reduced; such losses may make it impossible to achieve desired product purity even with total reflux. For a boron-10 plant, losses a t the product end reflux are not critical, as the natural abundance of boron-10 is high (18.8%) (7). Table I1 shows how the minimum number of stages and minimum reflux ratio vary with the single stage separation factor for a given product concentration. Calculations are for the enriching section of a square cascade. To determine the efficiency of the reflux process, samples were taken from the decomposer effluent while the system was being operated a t steady state. At the end of a run, a large sample was taken from the recombiner. The latter samples were analyzed for total boron while the former were analyzed for boron, copper, iron, chromium, and nickel. The 1 to 1 boron fluorideanisole complex was formed readily in the recombiner even a t 20' C., and boron content of the decomposer stream was well within tolerable limits (Tables I, 111, and IV). Figure 2 shows build-up of boron in effluent with time. '4t the end of run 12 the bubble-cap
exchange column was replaced by a packed column. All copper transfer lines were replaced by nickel and all packing by nickel and later by stainless steel. A marked improvement was noted in the amounts of metals in solution (Tables I and IV, Figure 2). Isotopic Fractionation. Total separation is defined as the ratio of BIO/B1l tops (boron-10 enriched stream) to B1O,/Bl1 bottoms (boron-10 depleted stream). The effects of time, flow rate, column diameter, and packing on the total separation of the boron isotopes in the unit were determined by operating the system a t steady state a t one flow rate for approximately 24 hours before samples were taken for isotopic analysis. Total separation increased rapidly with time from startup, but equilibrium was not attained for several hours (Figure 3). Data obtained from total separation determinations were used to calculate stage heights for the various exchange columns used (Table V). Isotopic fractionation was followed by observing the change in isotopic content of the gas stream at the top and bottom of the exchange column. Samples were taken periodically from the gas stream and analyzed o n a modified G.E. 6inch radius, 60' sector type analytical mass spectrometer. The mass analyt-
Table 111. The Boron Fluoride-Anisole Complex Was Formed Readily in the Recombiner Even a t 20' C. Mole Ratio BFa/Anisole Leaving Temp., Pressure, Run No. Recombiner O C. Mm. H g 16 17 20
1.07 1.06 0.92
6 7 20 ~
VOL. 51, NO. 2
850 950 875 ~
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FEBRUARY 1959
_
_
_
123
_
~
Table IV.
Boron Content of the Decomposer Stream Was Well within Tolerable Limits (Summary of runs 7-12 made with a 3Gplate bubblecap exchange column and copper transfer lines. Decomposer packed with glass beads) Total Reflux Decomposer Effluent, P.P.M. Run No. Time, Hr. Boron Copper Iron
...
...
... ...
... ...
7
4 5
95 167
a
10 14
240 280
9
22
350 40”
1640
60
10
30
480 50“
3000
70
11
38
620 110“
2900
130
12
46
‘
...
...
...
...
3354
...
190a
10“
Duplicate sample-effluent was filtered before analysis. Table V.
Total Separation Decreases and HETP Increases as the Flow Rate Increases
Complex Flow Rate Gal./sq. ft ./hr. Cc./hr.
Exchange Total Temp., Separation C. 1-Inch Exchange Columna 0.05 X 0.1 X 0.1 Inch Helipak packing
6.3 16.2 39.5 65.6 86.9 101.3
2100
34.7 72.3 92.6
720 1500 1920
125
1n.b
25 25 25 25 26 26
1.18 1.20 1.81 2.31 3.18 3.52
24 26 24
3.74 4.87 5.32
25 26 24 26
1.52 2.86 3.40 3.45 4.31
25 25 26 26 26 25 27
2.72 3.46 3.69 3.91 4.07 4.13 4.58
0.24 X 0.24 Inch Cannon Packingd 30 1.345 26 1.318 28 1.281 25 1.245 25 1.228
3.09 3.31 3.70 4. ia 4.46
335 a20 1360
iaoo
2.174 2.137 1.659 1.487 1.333 1.297
HETP
0.16 X 0.16 Inch Cannon packing 1.277 1.207
1 . iaa
1.5-Inch Exchange Column 0.095 X 0.175 X 0.175 Helipak” 8.0 16.1 27.7 37.4 48.3
375 750 1290 1740 2250
7.3 17.5 21.9 27.7 28.4 37.4 46.4
340 815 1020 1290 1320 1740 2160
21.3 21.9 31.2 32.2 41.9
990 1020 1450 1500 1950
1.829 1.377 1.309 1.304 1.237
26
0.16 X 0.16 Inch Cannon Packingd 1.399 1.303 1.282 1.264 1.252 1.248 1.221
,
2-Inch Exchange Column
0.24 X 0.24 Inch Cannon Packinga 10.3 18.2 29.1
a50 1500 2400
1.612 1.446 1.358
27 25 25
Each column was 36 inches long and contained 32 inches of packing. using a = 1.029 and aN = Z(separati0n). Nickel. d 316 stainless steel.
1 24
INDUSTRIAL AND ENGINEERING CHEMISTRY
’
1.91 2.48 2.99
Calculations made
ical technique elsewhere (9).
has
been
discussed
Conclusions
The boron fluoride (gas) with anisoleboron fluoride (liquid) system is suitable for production of high purity boron isotopes. The separation factor, stage height, and decomposition rate are favorable, and operation of the system is relatively simple and economical. Stainless steel and nickel are suitable materials of construction for equipment in contact with the boron fluoridecontaining process streams. With efficient packing, stage heights of 1 to 2 inches may be expected for a 1-inch exchange column and 2 to 3 inches for a 2-inch exchange column at flow rates up to 40 gallons/square foot/hour. ’ Water must be eliminated from the system as it causes hydrolysis of boron fluoride with subsequent decomposition of the anisole. For a dry system, reflux losses a t the boron-10 product end are well within tolerable limits. These preliminary studies indicate that the anisole system is superior to the methyl ether system used today, but a more detailed study is needed for a realistic economic comparison of the two processes. literature Cited (1) Begun, G. M., Palko, A. A., Brown, L. L., J . Phys. Chem. 60,48 (1956). (2) Duncan, J. F., Atomic Energy Re-
search Establ. (Gt. Brit.) Library, Harwell, England C/R., 107, AERE (declassified 1954). (3) Gilman, Henry, “Organic Chemistry,” Vol. 111, p. 70, Wiley, New York, 1953. (4) Green, M., Martin, G. R., Trans. Faraday Sac. 4 8 , 4 1 6 (1952). (5) Healy, R. M., Palko, A. A,, J . Chem. Phys. 28, 211-14 (1958). (6) Holmberg, K. E., Brit. Patent 736,459 (Sept. 7, 1955). (7) Hooker Electrochemical Co., Chem. Eng.64, 148 (1957). (8) Kistemaker, J., ed., “Proceedings of the International Symposium on Isotope Separation,” Amsterdam, North Holland Publishing Co., 1958. (9) Melton, C. E., Gilpatrick, L. O., Baldock, R., Healy, R . M.,Anal. Chem. 28, 1049 (1956). (10) Murphy, G. M., ed., “Separation of Boron Isotopes,” U. S. Atomic Energy Comm., Tech. Information Serv., Oak Ridge, Tenn., NNES 111-5 (1952, declassified May 1957). (11) Palko, A. A,, Healy, R. M., Landau, L., J. Chem. Phys. 28, 214-18 (1958). (12) Panchenkov, G. M., Moiseev, C. D., Markarov, A. V., Doklady Akad. Nauk U.S.S.R. 112, 659-61 (February 1957). (13) Shacter, J., Garrett, G. A., U. S. Atomic Ener y Comm. AECD-1940 (declassified d a y 1948). (14) Urey, H. C., J . Chem. SOG.1947, p. 562. RECEIVED for review June 5, 1958 ACCEPTED September 15, 1958 Based on work performed for the U. S. Atomic Energy Commission by Union Carbide Nuclear Co.
