I
A. 1. CONN and 1. E. WOLF
\
Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
large Scale Separation of Boron Isotopes As part of the Manhattan Project during World War II, the Standard Oil Co. (Indiana) designed and operated the flrst large scale plant for separating boron ,isotopes. The dimethyl ether complex, selected because of its lower rate of thermal decomposition, was fractionated to obtain a liquid product in which 95% of the boron was present as boron-10. Boron metal was produced from this enriched material by the American Cyanamid Co. (18). The contract for constructing the plant was signed with the Foster-Wheeler Corp. in December 1943. Continuous operation began in November 1944, and when 95% concentration was reached, boron-10 was made continuously for 11 months until the plant was shut down in February 1996. In 1948, it was dismantled, but in 1953, a larger plant was constructed by the Hooker Electrochemical Co. at Model City, N. Y (3). Through a consulting arrangement, experience gained in the plant described here was made available, and the new plant began operation in 1954.
in naturally occurring compounds is composed of two isotopes, one of mass 10 and the other of mass 1 1 . Boron-10 which constitutes 18.8 atom per cent (75, 79) of the total, has an unusually large cross section for capture of low energy neutrons; therefore, in high concentrations, it is particularly valuable as a neutron shielding (23). The nuclear reaction is BORON
on the other two. Providing enough retical plates with minimum thermal decomposition required high-efficiency packing that also had low holdup. Minimizing thermal decomposition made it desirable to operate at low temperatures under vacuum; this in turn introduced problems of water leakage into the system. A wide variety of fractionating columns, commercially available in 1944, were considered (Table I). Of these, Stedman packing was outstanding in having low pressure drop, holdup, and HETP. It was available in circular sizes up to 6-inch diameter and in 6-inch equilateral triangles. Columns of larger cross section could be made by using multiple triangular sections. Gauze Berl saddles (McMahon packing) were also applicable (9, 77), but a t that time they were not available in sufficient quantity. Perforated-plate columns were also considered because of e'ssure drop and low holdup, nch plate spacing required number of columns within reason would have made the operating range too narrow. Thermal decomposition of the comeeds according to the reaction .BF3
(CH30)3,2BF3
Methyl fluoride .is a gas and escapes, complex of methyl borate and trifluoride is a liquid that is more volatile than the ether complex and freezes at 100' F. Therefore it accumulates a t the top of a fractionating column and can plug ensers unless continuously removed. minimize thermal decomposition, steam was selected as an easily controlled heating medium, and holdup
No significant secondary radioactivi;y is produced because the lithium is stable and alpha particles are easily absqrbed. Design Considerations The major problems in designing the fractionation plant were to provide enough theoretical plates without an inordinate number of columns, minimize thermal decomposition, and prevent hydrolysis (4). The solution of each of these problems depends to some extent
e 3CH3F 4-
Packing Type Berl saddles Metal helices Raschigrings Stedman Stedman Bubble trays a
Estimated.
literature Background Subject
Ref.
Separation of B'O investigated ; distillation of BF3 ether com(6, 14, 20) plexes most promising High separation factors of ether complexes of BF3 results from high dissociation in vapor (16) state Distillation of BFa ether complexes resembles other exchange reactions for C, N, and S (IS,27, 88) In large-scale isotope separation, U236 is most difficult to concentrate (10, $4)
of liquid in hot zones was kept as low as possible. Hydrolysis of the complex proceeds according to the reaction: (CH3)zO BF3
+ 2 H 2 0 G BF3.2Hz0 -I(CH3)zO
Methyl ether is also a gas and escapes, but the boron trifluoride dihydrate readily decomposes to give hydrofluoric acid, fluoroboric acid, a series of hydroxyfluoboric acids, and boric acid (22). Some of these acids are highly corrosive to steels. Moreover, salts produced by the corrosion appear to catalyze the thermal decomposition. Hydrolysis can be caused by leakage of not only steam or cooling water, but also air carrying atmospheric moisture. Joints had to be of proper design, and construction had to ensure absolute tightness. Because it was resistant to fluoride corrosion, Monel was used where service was critical or for parts that might be difficult to replace. Because of the small separation factor for boron isotopes, control was a critical design consideration. In a single
Table 1. Comparison of Column Packings Nominal Press. Drop, Packing Diam. Reflux In. H20/ Holdup, Size, of Col., Rate, Theo. Ml./Theo. In. In. Gal./Hr. Plate Plate '/z 3/d' 1/4
X
2.08 6.08 1*/z0
1/4
2 2 2 2 6
6
Outside diameter.
1.3 1.3 1.3 1.3 19 16
0.07" 0.2 1.0 0.045 0.32 0.8
Tray spacing.
41a ... 22Q 13 126 1390
HETP, In. 7.0 2.0 4.7 1.3 1.9 3.0d
Ref (7)
(7) (7) (9) (8)
(1)
Assumed tray efficiency, 50%.
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STEAM EJECTOR
1j
18-INCH COLUMNS
~
I O - I N C H COLUMNS
1
-b - B O T TOMS
6-INCH
COLUMNS
i I
Figure 1. The fractionation plant consisted of nine columns, each of which was maintained at a pressure of 150 mm. of mercury by a single-stage steam ejector
column, with steady control of boil-up and reflux rates to prevent fluctuations in liquid holdup, approximately 1 day was required to reach equilibrium. For a series of columns, a much longer time was required. Therefore, precise control of the two large streams between the columns was needed to maintain the small net production of boron-IO. A small change in either flow could upset concentrations in the columns so that several days would be needed to recover equilibrium.
The Fractionation Plant The fractionation plant was designed to produce 640 grams per day of boron-1 0 in 95% concentration. The equivalent in terms of the liquid complex is 1.56 gallons per day. The boron-10 concentration in the waste taken overhead was arbitrarily set a t 10%; thus, it could be recovered from the waste with only one rectification column and a feed rate of 18 gallons per day. Columns. The 18-inch columns, the largest of the three sizes, were made of 13 triangular sections of Stedman packing in the form of a hexagon having three 12-inch sides and three 6-inch sides (Figures 1 and 2). The 12-inch columns were made of six triangular sections in the form of a regular hexagon. The smallest columns were made with 6-inch circular packing. The separation factor for the boron isotopes was known to be in the neighborhood of 1.015, which required 525 theoretical plates for the plant (Table 11). Each column was expected to have about 90 theoretical plates so
Table II.
Design of Columns
(Av. HETP required, 5.6, inches; expected
HETP, 3.7 inches) Reflux Ratio, Overflow/Bottoms
Col. Diam., In. 18
1232
12
502 270
6
98
that the desired product could be made in six columns, two of each size. T h e third column of each size was included as a spare to replace an inoperable column or to be connected into the system for additional capacity. Alternatively, if only about 60 plates were obtained in each column, all nine columns would be needed to make the desired product, Each column contained 27 feet 4 inches of packing, with a distributor plate at least every 5 feet to give uniform liquid distribution. The use of progressively decreasing column sizes as the boron10 concentration increased was desirable to reduce holdup, bring the unit on stream quicker, and reduce thermal decomposition of the product. These advantages more than offset the penalty resulting from the decreased reflux ratio. The distributor plates had several novel features to ensure efficient operation. The holes were sized to give 3/4 inch of liquid on the plate during normal operation. T o ensure equal flow from all the holes, each was reamed to the exact diameter, and screens were installed over them to prevent plugging. Drip points a t the bottoms of the holes made certain that liquid fell on the packing a t the desired point. A hole was provided for every 5.2 square inches, or less, of packing cross-section (Figure 3). Care in design of the distributor plates is believed to have been essential to the success of the operation. Boron-10 removed as a liquid from the reboiler of the last column, contained much of the corrosion products, scale, and other contaminants washed down through the entire series of fractionating columns. A rerun operation ( I I ) , employing simple batch distillation, was therefore included to clean u p the product. Auxiliaries. Heat exchangers were of standard design, except that the tubes were welded into the tube sheets to prevent leakage. The reboilers were provided with bayonet-type steam tubes, which gave smaller liquid holdup than a U-tube bundle. The condensers con-
INDUSTRIAL AND ENGINEERING CHEMISTRY
tained baffled U-tubes with eight passes of the water through the tubes. The problem of leakage around packing glands used to seal moving equipment is aggravated when the leakage causes corrosion. Packing glands were avoided by use of flexible diaphragms and bellows valves, either of which provides a positive seal, and both of which were commercially available in Monel metal. Diaphragm pumps used to transfer the complex were actuated by reciprocating oil pumps having an adjustable stroke. The hfonel diaphragms were spring-loaded to prevent air from leaking into the oil. Mercury-sealed surge reducers permitted accurate measurement and control of flow. Gravity flow was used wherever possible, so that each column required only one pump. For manual operation, packless globe valves were used almost exclusively; they were of the plug type with a Monel bellows attached to the plug. Valves used in automatic flow control had bellows attached to the \alve stems. Most of the original piping was made up with screwed fittings that were backwelded as a precautionary measure, but later experience indicated that lithargeglycerine cement was satisfactory. .4t points \vhere disassembly of the piping might be necessary and at the tube sheets of exchangers, ring joints mere used. The joints for the column sections were tongue-and-groove flanges put together with copper gaskets. Control. Control of the fractionation plant was principally by flow. Each column had three points of flow control (Figure 4): steam to the reboiler, bottoms stream to the following column, and overhead stream to the preceding column. Steam was supplied from a
Figure 2. The 18-inch columns contained 13 triangular sections of Stedman packing in the form o f a hexagon having three 12-inch sides and three 6-inch sides
BORON ISOTOPES header maintained at constant pressure, and the flow controllers provided steady heat flux to the reboilers. Initially, transfer streams between columns were controlled by setting the bottom liquid rate and automatically resetting the overhead liquid rate by means of a level controller on the reboiler. The time lag with this scheme made it inoperable, and the manual reset of the overhead rate on the basis of indicated reboiler level was finally used. Control of the small difference in rates of the transfer streams was made easier by reducing the flows to 15% of the reflux rates in the columns. At the lower rate of transfer, the product rate became a larger percentage of the transfer streams and the precise difference between them could be maintained more easily. Only a small reduction in separation efficiency resulted. Because the feed, overhead, and bottoms streams were all liquid, they were readily controlled by rottimeters and manually set needle valves. Pressure on the columns was controlled by an air bleed, which in turn was regulated by a differential pressure controller, one leg of which was connected to the manifold and the other to a vaeuum
A. Underside of plate
E. Top C. Packing support D. Plate to hold screen thimbles over distributor holes
pump.
Operation The complex was prepared simply by mixing the dimethyl ether and boron trifluoride gases. Heat is evolved by the reaction, and cooling is needed to obtain the complex in liquid form. Both reactants were supplied from standard pressure cylinders; dimethyl ether has a vapor pressure of about 100 p.s.i. a t 100' F. and was received as a liquid, whereas boron trifluoride was received as a gas a t 2000 p.s.i. The equipment used (26) is shown in Figure 5 ; operation was on a semicontinuous basis. Quantities were controlled so as to give a slight excess of ether in the cooled complex and assure complete reaction of the boron trifluoride. The feed was prepared in batches of 50 gallons, which were degassed and tested for water content before being transferred to storage for charge to the fractionation plant. First attempts to operate the plant resulted in many operating difficulties (6). Leaks in reboilers and condensers, cracking of column sections, failure of transfer pumps, leaks in valves, corrosion and erosion of the steam ejector, and poor efficiency of the 6-inch circular packing prevented continuous operation and production of the desired product. Accordingly, much of the equipment had to be disassembled for repair. Reduction of leaks to a tolerable level was achieved only after consider-
able testing and diligence in reassembly of equipment. Failure of the diaphragms of the transfer pumps was the most troublesome problem, requiring about 3 months of test work and complete redesign of the diaphragm heads and other features (25) before reliable operation was obtained. In the interim, batchwise operation of individual columns for short periods produced 18 gallons of complex containing 53.301, boron-1 0 (30).
Initial operations indicated that the 6-inch circular packing had an HETP of 8 inches or more. Special tests ( 8 ) showed that a disproportionate amount of liquid was running down the column walls and new packing had to be obtained meeting more rigid specifications. As soon as reliable operation of one of the transfer pumps was obtained, two columns were connected in series and continuous operation was begun. Operating conditions were :
TO EJECTOR
b Figure 4. Typical column, To check its operating characteristics, Particularly Pack- STEAM ing efficiency, each could be run independently at total reflux
VOL. 50, NO. 9
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WATER
(CH3120 ROTA METERS
REACTOR
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I I
u
Top of columns Bottom of columns Steam in reboilers
Temp.. F.
2
c
-I
4 P
I
plex and decomposition product. The separation factor was 1.016 instead of the 1.015 used in the design. O n this basis, the eight columns used had 580 theoretical plates instead of 525. The average H E T P was 4.4 inches, although individual columns ranged from 3.5 to 6.3 inches (72). The exchange of enriched boron was demonstrated in laboratory experiments (29) in which the methyl borate-boron fluoride complex, formed by decomposition, rapidly exchanged its boron with the methyl ether-boron fluoride complex. Thus, isotopic equilibrium was rapidly attained between the two complexes under column-operating conditions, and removal of the borate complex did not result in removal of enriched boron from the system. The long period of successful operation was made possible by the extra equipment provided in the design. Any column that showed signs of difficulties was removed from the system, repaired, tested for efficiency, and
194 220 240
At these conditions the average decomposition rate for all the complex in the unit was 1.2%. Additional columns were added to the system, as indicated in Figure 6, A . Ninety-five per cent of boron-10 was first reached after seven columns were operating in series. An eighth column was added before the highest concentration, 97.276 was obtained. The highest production rate, 2.4 gallons per day of complex containing over 95% of boron10, was attained just before the plant was shut down (Figure 6, B). Total production of concentrated complex amounted to 736 gallons. Peak production of 50% above design is accounted for by a slightly better separation factor than expected, use of more plates, and an unanticipated exchange of enriched boron between com-
..x w w
Equipment for preparing the ether complex of boron trifluoride was
BF3
Press. 150mm.Hg 290 mm. Hg 10 p.s.i.g.
.
5
7
2.5 2 .o
2.0-
0 1.5-
I .5
m u I 0
g 8E
I-
1.0-
I .o
0.5-
0.5
C
literature Cited (1) Aston, J. G., Lobo, W. E., Williams, B., ~ N D .ENG.CHEM.39, 718-31 (1947). (2) Bragg, L. B., Zbid., 33, 279 (1941); Foster-Wheeler Corp., Bull. ID-44-2. (3) Chilton, C. H., Chem. Eng. 64, No. 5, 148 (1957). (4) Conn, A. L., Wolf, J. E., U. S. Atomic Energy Commission, Oak Ridge, Tenn , Rept. A-1975, May 24, 1944. (5) Crist, R. H., Kirshenbaum, I. (to U. S. Atomic Energy Commission), U S. Patent 2,796,330 (1957). (6) Eichna, J. R., others, Rept. A-2370, July 23, 1946. (7) Fenske, M. R . , Lawroski, J., Tongberg, C. O., IND ENG.CHEM.30, 297 (1938). (8) Forsythe, M'. L., Wolf, 3. E., Rept. A-2350, June 26,1945. (9) Forsythe, W. L., Stack, T. G., Wolf, J. E., Conn, A . L., Rept. A-2361, Nov. 17, 1945; IND. ENG.CHESI.39, 714-18 (1947). (101 Glasstone. S.. "Sourcebook on Atomic Energy," p.' 202, Van Nostrand, New York. 1950. (ll)-H;ggins, H. B., Stack, T. G., Wolf, J. E., Rept. A-2360, Oct. 30, 1945. (12) Higgins, H. B., Stack, T. G., Wolf, J. E.. Conn, A . L.., Rept. . A-2362. Jan. 28, 1946. (13) Hutchison, C. A., Stewart, D. W., Urey. H.C., J . Chem. Ph3.s. 8, 532-7 (1 940). (14) Kirshenbaum, I., Sabi, N., Schutz, P. W., Rept. A-2120,Sept. 30,1944. (15) Lange, N. A., "Handbook of Chemistry," p. 114, Handbook Publishers, Inc., Sandusky, Ohio, 1956. (16) McCaulay, D. A., Rept. A-2357, Oct. 8, 1945. (17) McMahon, H. O., IND.ENG.CHEM. 39, 712-14 (1947). (18) Murphy, G. M., Kilpatrick, M., Hutchinson, C. A,, Taylor, E. H., Judson, C. M., "National Nuclear Energy Series, Div. 111," vol. 5, p. 20,1952. (19) Ibzd., p. 23. (20) Ibtd., pp. 25-37. (21) Zbzd., p. 70. (22) Zbzd., pp. 122-41. 1231 Rockwell. T.. "Reactor Shielding Design Manua1,';p. 188, Van Nostrandq NPW York. 1956.
-0
0
-
100
IO0
i
70
i s
NUMBER
50-
1234
Acknowledgment Successful completion of this project resulted from the diligent effort of many members of the research and several other departments of the Standard Oil Co. (Indiana). Their work is hereby gratefully acknowledged.
~
0
E a
placed back on stream without greatly affecting the rest of the system.
40
3)4/
5
i4i
8 161
OF COLUMNS 7
OPERATING
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
8
-
60
-
50
(24)-Selak,- P. J., Finke, J. Chem. Eng. Progr. 50, 221 (1954). (25) Stack, T . G., Lukes, J. J., Rcpt. A-2369, July 5, 1946. (26) Stack, T. G., Wolf, J. E., Rept. A-2351. Julv 13.1945. (27) Stewart,'D. W., Cohen, K., J . Chem. Phys. 8, 904 (1940). (28) Thode, H. G., Urey, H. C., Zbzd., 7, 34 (1939). (29) Webb, A. N., Rittschof, Mi. L., Rept. A-2364, Oct. 30, 1945. (30) Wolf, J. E., Higgins, H. B., Mac Fie, K. W.. Lukes. J. J.. Rept. A-2368. June 24,1946. ' RECEIVED for review September 21, 1957 ACCEPTEDApril 4, 1958 Work done by the research department of the Standard Oil Co. (Indiana) under contract W-7418-eng-41 with the Manhattan Engineer District, Corps of Engineers, U. S.Army. I
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