I
MEYER STEINBERG and BERNARD MANOWITZ Brookhaven National Laboratory, Upton, N. Y.
Recovery of Fission Product Noble Gases Kerosine-base solvents are suitable for use in a continuous absorptionstripping process for the 'separation and recovery of xenon and krypton
A
MAJOR FRACTION (2, 79) of the products of thermal fission of uranium-235 is gaseous materials and, in particular, the noble gases, xenon and krypton. Reactor operation requires their periodic removal (78). Because of expanding nuclear operations, the danger of polluting the atmosphere by accumulation of fission xenon and krypton is constantly increasing. This alone could dictate their removal from fission product offgases (7) ; however, new uses recently developed for xenon (77, 20) and krypton-85 (9, 73), considered in conjunction with the relatively low supply of naturally occurring xenon and krypton in the atmosphere ( 7 7 , 75)) provide additional incentive for their recovery. Fission product gases can emanate directly from a nuclear reactor plant, as in the case of homogeneous fluid fueled reactors, or from plants where solid fuels are processed after intervals of burning in the reactor (76). In the former case xenon andArypton may be diluted with an inert carrier gas; in the latter, off-gases from fuel dissolution vessels usually contain nitric oxide, nitrous oxide, water, hydrogen, oxygen, and nitrogen ( 7 , 22, 25).
Solubility Measurements The extent of solubility of a gas in a liquid when no interaction takes place can be estimated from Hildebrand's theory of the solubility of nonelectrolytes (72): The solubility tends to be maxi-
mum when the cohesive energy density, solubility parameter, (CED)'12 or 6, of the solute approaches that of the solvent. For xenon and krypton the (CED)1/2 values a t boiling points are calculated to be 8.4 and 7.9. With this as a basis, solvents having (CED)'12 values close to the above were mainly considered for investigation. Procedures. Experimental procedures for measuring equilibrium values of solubility included the use of a modified McDaniel method (70), a static tracer technique, a dynamic tracer technique, and the Van Slyke method (23, 24). Care was taken to outgas the solvents before transfer to the measuring tubes. Technically pure xenon and krypton in cylinders under pressure were supplied by the Matheson Co. and the solvents were supplied from various sources as technically or chemically pure. A dynamic tracer method was developed for measurement of solubility of xenon and krypton gas a t low partial pressure in a carrier stream such as nitrogen. The noble gas containing tracer and carrier stream was bubbled through a known volume of solvent. T h e effluent gas from the saturation tube was analyzed with the use of a n in-line continuously recording count-rate meter. Continuous temperature recording of the solution aided in measuring dynamic equilibrium a t low temperatures. When the outlet concentration reached the value of the inlet concentration, the
tracer-laden stream was turned off and a nitrogen purge and sparge carried the noble gas out of the solvent and through the continuous analyzer. From the measured flow rate of the sparge stream and the time trace of the activity streaming through the analyzer, it was possible to determine the quantity of noble gas absorbed by the solvent and thus the Henry's law constant. A fourth procedure, the Van Slyke method (23, 24), was used mainly for measuring solubilities of the less soluble helium, argon, nitrogen, and oxygen. Results. Measurements made by the four methods agreed to within 5 to 10%. A compilation of the solubilities measured in this study, with a few recent literature values, are given in Table I. Analysis of the data indicates that the solubility of xenon and krypton increases as the solvent becomes more nonpolar. Comparing olive oil with a 50% emulsion of olive oil in water shows that the solubility decreases about 6 times. There ip no increasing solubility effect due to increasing surface activity on account of emulsification. The less viscous silicone oil had a 63% higher xenon solubility than the more viscous 10-centistoke oil, and indicates an effect of viscosity and probably surface tension. An attempt to correlate the data of Table I in terms of the cohesive energy density is shown in Figure 1. The solubility data tend to go through a maximum in the vicinity of a (CED)l/Z
An absorption-stripping process was developed for concentration of xenon and krypton from a fuel-processing plant VOL. 51, NO. 1
JANUARY 1959
47
value of 8.0, confirming Hildebrand's theory. However, in the case of xenon, in the longer chain aliphatic hydrocarbons, such as n-heptane and kerosine (molecular weight 175, which largely consists of C12H26), there is a positive deviation from Raoult's law, indicating an interaction between solute gas and solvent liquid. For the case of krypton in kerosine positive deviation from Raoult's law is less. A review of the solubility data in Table I indicates that the kerosine-base solvents --i.e.> kerosine, Amsco 123-15 (American Mineral Spirits Solvent Co. No. 140), and Ultrasene (Atlantic Refining Co. solvent)-come closest to meeting the criteria for a suitable process solvent. A further study of the solubility of the noble gases in Amsco 123-15 over a range of temperatures and concentrations was undertaken. Table I1 indicates that the solubility decreases 4.5 times when the temperature is increased from 24' to 150'; there is still an appreciable solubility of xenon in Amsco at 150" C. ; within experimental error, Henry's law holds over the entire temperature range studied. A plot of log concentration us. l , / T gave essentially a straight line. The heat of solution AH,, calculated from the slope of this line, gave a value of -3040 cal. per gram-mole, which is approximately equal to the heat of vaporization of xenon? AHv = -3021 cal. per gram-mole. This indicates that the attractive forces between the solute xenon molecules and the solvent Amsco molecules are about the same as between the molecules of xenon done. Similar data for krypton show that solubility decreases 2.95 times when the temperature increases from 24' to 150' C., and increases 2.68 times when the temperature decreases from 24' to -55' C. Again within experimental error (about 10%) Henry's law is applicable over a wide range of gas phase concentration and temperature conditions. The heat of solution, AH, = -1700 cal., which is 460 cal. less than the heat of vaporization of krypton, indicates a lesser attractive force between the krypton and Amsco molecules than the krypton molecules themselves. Attractive forces between the solvent molecules are greater than the attractive forces between the solvent and solute. This confirms the conclusion reached earlier, that there is less of a positive deviation from Raoult's law for krypton than for xenon.
Rates of Absorption The rate of absorption of xenon and krypton in Amsco was measured in a countercurrent absorption column using auxiliary flow and analysis devices as shown in Figure 2. Tagged noble gas was fed into an empty gas cylinder and then pressurized with nitrogen to a predeter-
48
INDUSTRIAL AND ENGINEERING CHEMISTRY
mined value. The gas after passing through a regulator was metered and pdssed to the bottom of a glass column 1.5 inches in diameter, packed with '/4inch ceramic Raschig rings. Liquid Amsco, pumped to the top of the column, was distributed by means of a screen over the top of the packing, and flowed dou n the column, countercurrent to the rising gas stream. The gas cylinder concentration was measured prior and subsequent to making a run and the effluent gas stream was analyzed manually and b) means of the continuous inline gamma scintillation counter. The maximum error in the mass balance across the column was approximately 187, and in many cases did not exceed loyo. The results obtained were analyzed in terms of the number of trans-
Table I.
fer units and the over-all height of a transfer unit, HOG, based on the gas stream. According to Colburn ( 6 ) , the following equations apply \$,hen the effluent solution is dilute, Henry's law holds, constant flon. conditions exist, and pure solvent is fed to the column.
T'alues of -YoG and HOGderived from experimental runs !+it11xenon and Amsco
Solubility of Xenon and Krypton Increases as the Solvent Bocornes More Nonpolar
Solvent Water Aniline Acetic acid Xylene (tech. grade) p-Xylene Toluene Mesitylene ?+Hexane tt-Heptane n-Dodecane Kerosine (Amend Drug CO.) Amsco 123-15 (59.6wt. % paraffin, 27.2 wt. 7cnaphthene, 13.2 wt. yo aromatics) Ultrasene (80 wt. % paraffin, 20 wt. % naphthene) Dow Corning silicone oil 200 (dimethyl siloxane, 1 cs.) 200 (dimethyl siloxane, 10 cs.) 702 (contains diphenyl groups) Halocarbon 437 (trichloroheptafluo robutane) Olive oil Pine oil Terphenyl Dowtherm A (diphenyl-diphenylene oxide) 40 vol. % toluene, 60 vol. yopine oil Oil in water emulsion (50 vol. yo olive oil) Dow Corning Anti-Foam A (60 wt. yosilicone oil in water) Koppers Emulsion K-900, (50 wt. % styrenebutadiene in water) 83 wt. yomebhanol, 17 wt. water (eutectic)
Xenon Solubibv Temp., c c. / C C . 15-
L
'2 1.01 I
,
4
0
2
4
Figure 1 , ,
,
,
I
6 8 ( C E O I"':
I
I
IO 12 I?, (CAL / c c )
I
-,--7--7--l--&-
16
18
20
22
24
Correlation of xenon solubilities
when feeding gas streams containing 432 to 969 p.p.m. (by volume) of xenon in nitrogen, indicated transfer unit heights in the order of 0.33 to 0.51 foot. When H O G values are plotted as a function of the liquid flow rate, there is an indication of an increase of the H O G with the 0.13 power of L. A similar set of runs was performed with krypton and Amsco, using a gas feed containing 41.6 to 5300 p.p.m. in nitrogen. For krypton, the HOGv a l u s range u p to 0.85 foot and Hog increases as the 0.70 power of L. There does not seem to be a trend with gas flow rate, although the data are somewhat scattered. This would indicate that the liquid film dominates the rate of mass transfer of the noble gas into the liquid. Further rate measurements with columns of larger diameter would be required to determine more precisely the controlling features of the mass transfer rate. The ratio of the rates of absorption of xenon to krypton expressed in terms of the liquid flow rate is: ~
II.
Liauio
SUPPLY MNTAINER
Kerosine-base solvents come closest to meeting criteria for process solvent
Tabla
26
At a liquid flow rate of 15,000 lb./hr./ sq. ft. the rate of absorption of xenon is about twice that of krypton. Of the absorption systems investigated and used industrially, probably chlorine-water most closely resembles the xenon-Amsco system in solubility and rate data. T h e system carbon dioxide-water has characteristics similar to the krypton-Amsco system.
Process Design Based on the foregoing, a system of noble gas removal and recovery was developed to process off-gas effluent from a 1-metric-ton-per-day fuel processing plant. Because the higher oxides of nitrogen could react with Amsco, and the nitrous oxide, if present, would dissolve (measurement of its solubility in Amsco gave 2.95 cc. per cc.), it is necessary to remave these constituents either by a caustic wash and catalytic hydrogen reduction, or by the following procedure. T h e oxides of nitrogen may be burned directly with hydrogen in a gas burner and exit gases cooled with a water spray. The gases, mainly nitrogen with a smaller
24 60 110 150
PHOTO MULTIPLIER
amount of hydrogen and containing the noble gases, pass u p through a countercurrent packed absorption column. T h e effluent Amsco stream containing the noble gases is then steam-stripped and the noble gases are concentrated in the overhead condenser. It is assumed that it would be desirable to remove and recover 99% of the krypton present in the off-gas stream. As nitrogen is soluble in Amsco, the maximum increase in concentration of krypton in a nitrogen stream for one cycle of absorption and stripping, neglecting differences in rate of absorption, ,is the ratio of their respective solubilities. This value given in Table I11 is 4.5. T h e rate of absorption of nitrogen in Amsco is probably much less than that of krypton. However, because of the much
Table I l l . Relative Solubilities and Volatilities of Gases in Amsco 123-1 5 Xenon is more easily separated from other gases than krypton, but the krypton separation can be improved by lowering the temperature
'
31.0 67.0 90.0 139.0
662 655 659 1670
1.50 0.56 0.44 0.34 0.19
70 190 238 313 544
41.6 48 92 92 92
Temp., O C . 24
- 55
Solubility of Xenon and Krypton in Amsco 123-1 5
3.42 1.60 1.18 0.76
CONTINUOUS COUNTING METER
100
Solubilities, Cc./Cc.
33.4 63.0 97.5 157.0
KRYPTON
- 55
CRYSTAL DETECTOR
Figure 2. Xenon and krypton absorption rate was measured in a countercurrent column with auxiliary flow and analysis devices
Within experimental error Henry's law holds over a wide range of gas-phase concentration and temperature Low Concentration of Xenon 100% Xenon or Krypton Concentration in Gas ( P = 1 Atm.) or Krypton in Gas Phase Conen.," p.p.m. Henry's Solubility, cc. Henry's by volume in eon st.. Temp., gas/ce. const., H = p.p./x c. liquid H = p.p./x NZa t start XENON 24 60 110 150
LIQUID
RECEIVER
71 173 251 290 497
a Concentration of xenon or krypton gas a t start of equilibration; approximately equal volumes of gas and liquid used for measurement. p.p. = partial pressure in atmospheres.
Nz 0 2
He Ar Kr Xe
0.125
...
0.066 0.42 1.50
...
0.126 0.159 0.043 0.29 0.56 3.30
0.136
... ...
... ...
0.30
Reiative Solubilities Kr/Nz Xe/N2 Kr/Oz Kr/He Kr/A Xe/Kr
12.0
4.5 26.2 3.5 13.0 1.9 5.9
...
...
22.7 3.6
...
2.2
... ... ... ... ...
Relative Volatilities (Ratio of Vapor Pressures)' 23.1 pNn/pKr 14.7 7.2 pA/pKr 6.0 3.2 pKr/pXe 6.0 Extrapolated above critical. ~~~~
VOL. 51, NO. 1
... ...
...
~~
JANUARY 1959
49
larger Concentration, the Amsco would become quickly saturated with nitrogen. T o limit the height of the absorber a n absorption factor, L,/kG,, greater than 1.O is required and for the present design 1.1 was used. This means that an additional 10% of the nitrogen will enter the Amsco and be given off in the stripper. A concentration factor of about 4.0 can then be expected. The process flowsheet (page 47) includes flow quantities. The flow rate of total gas was estimated for 1 metric ton of a n enriched 15% uranium-aluminum fuel element (7). T h e generalized pressure drop correlations for dumped packing (74) were used to estimate the pressure drop so as to be below the load point, and the correlation of Dodge and Dwyer ( 8 ) was used to convert the H O G values from the experimental data on ‘/4 inch packing to 1 inch Raschig ring packing. Pertinent Design Data for Absorber
Basis Noble gas concn.
A’OG
Packing HOG Packed height Tower diameter L G
Total pressure drop Concentration factor
9970 removal of Kr and 10070 removal of Xe 300 p.p.m.
24 1-inch Raschig rings 1.0 foot 24 feet 30 inches 66.5 gal./min. or 5300 lb. /hr./sq. ft. 5.0 cu. ft./min. or 4.5 lb./ hr./sq. it. 2.0 p.s.i.g. 4.0
How to improve the operation. 1. By reducing the recovery of krypton to 90% and allowing a larger liquid a larger pressure droploading-i.e., both height and diameter of the column could be materially reduced. For example. assuming a loading of 15,000 lb./hr./sq. ft. and 90y0recovery, SO,= 6. HOG= 2. the height of the column is reduced to 12 feet and the diameter to 18 inches. A sieve or plate column, because of the lower pressure drop per unit height, may have some advantage. 2. Operating the absorber under higher pressure u70uld appreciably decrease both the size of the column and the volume flow rate of Amsco. For example, operating at 10-atm. pressure the quantity of liquid would decrease by a factor of 10 and the diameter. by a factor of about 3. The height would decrease correspondingly. The stripper would be reduced essentially to a flash evaporator by operating at atmospheric pressure. This would also reduce the heat exchange load considerably. 3. Operating at a lower temperature would increase the concentration factor, reduce the liquid handling problem, and decrease tower dimensions. 4. Instead of using a steam strip, operating the stripper under vacuum or stripping with carbon dioxide and sub-
50
INDUSTRIAL AND ENGINEERING CHEMISTRY
sequently washing the enriched gas with caustic would eliminate the heat exchange equipment. 5 . For improving the over-all concentration of noble gas, additional absorption-stripping stages could be added. T o increase the concentration of noble gas from 300 p.p.m. to 7.775, four absorption-stripping stages would be required. 6. With the use of a n absorberfractionator column, the xenon and krypton in the Amsco could be greatly enriched before stripping. A highly concentrated noble gas product could thus be obtained in a two-column unit and would be the most economical way of operating the process. Conclusions
An advantage of using an absorptionstripping system over charcoal (3)adsorption is that the noble gas can be removed almost completely in a continuous operation a t ambient temperatures. A difficulty with the Amsco absorptionstripping method is that relatively large volumes of organic have to be handled when operating at atmospheric pressure. The absorption-stripping system could be advantageously used where an inexpensive “head end” or primary bulk separation of gaseous activity is required, either for storage until safe disposal to the atmosphere is allowable, or for further processing for complete enrichment or purification of xenon and krypton. Other possibilities include the processing of homogeneous reactor purge gas streams, where a suitable carrier gas, such as helium, can be chosen to allow a n increased single-stage concentration factor. This could also be used in processing gas streams from pyrometallurgical processing plants, where there is freedom in the choice of a vehicle gas in purging the gaseous fission products. Nomenclature
concentration in liquid phase, mole fraction, m o1e/ m o1e v.p. = vapor pressure of solute gas, atm. cy = Ostwald solubility coefficient, cc. gas/cc. solvent a t total pressure of 1 atm.; STP is a t 15” C. and 1 atm.: ~r = 23,700d . ’ 23,700d v.p. x M L ’ =ax ‘WL ICED)’/’ = solubility parameter, cohesive energy density, latent heat of vaporization (or 6) per unit volume to 1/2 power d = density of liquid, grams per cc. = molecular weight of liquid L MC = molecular weight of gas Y = concentration Y n gas ehase, mole fraction-mole/mole NOG = number of transfer units based on gas stream HOG = over-all height of transfer unit, feet
x
=
I
*
equilibrium constant k = y / x ; when Henry’s law holds, k = H a t 1 atm. mass flow rate of gas = lb./hr./sq. ft. of column cross section Ib. moles/hr./sq. ft. mass transfer rate coefficient based on gas phase, lb. moles/hr./cu. ft. atm. mass flow rate of liquid, lb./hr./sq. ft. lb. moles/hr./sq. ft. height of column, feet Henry’s law constant Literature Cited
(1) Blanco, R. E., “Symposium on the Reprocessing of Irradiated Fuels,” U. S. +:y,r_nri_c>EnergyComm., TID-7534, Book (2) Blomeke, J. O., Todd, M. F., Oak Ridge Natl. Lab., ORNL 2127 (TID 4500, 13th ed.), Part I, vol. 1 and 2, 10G7 . , - I .
(3) Bruce, F. R., Fletcher, J. M., Hyman, H. H.. Katz. J. J.. “Process Chemistrv.” Progriss in ’Nucle‘ar Energy Series f11, Pergamon Press, London, 1956. (4) Clever, H. L., J . Phys. Chem. 61, 10823 (1957). (5) Clever, H. L., others, Ibid.,61, 1078-82 11957). (6)’ Colburn, A. P., IND.ENG.CHEM.33, 459 11941). (7) Cdler, F. L., Oak Ridge Natl. Lab., ORNL CF 57-3-114 (1957). (8) Dodge, B. F., Dwyer, 0. E., I X J .ENG. CHEM.-33, 485 (194.1): (9) Fastovski!, V. G., Kislorod 4, 5-14 (1947). (10) Furman, E. H., “Scott’s Standard Methods of Chemical Analysis,” vol. 11, 5th ed., Van Nostrand, New York, 1939. (11) Gluekauf, E., Kitt, G. P., Proc. Roy. Sac. (London) A 195, 557-65 (1956). (12) Hildebrand, J. H., Scott, R. L., “Solubility of Nonelectrolytes,” Reinhold, New York, 1950. (1 3) Industrial Laboratories, news release (November 1957). (14) Leva, Max, “Tower Packings and Packed Tower Design,” U. S. Stoneware Co., Akron, Ohio, 1953. (15) Linde Air Products Co., Bull. F-1002A 119531. (1G) Manowitz, B., Advances zn Chem. Eng. 2, Chap. 3 (1958). (17) Northrop, J. A., Nobles, R., Xudeonzcs 14, No. 4, 36-7 (1954). 118) Remolds. M. R.. A’uclcar Sci. and Eng. 1, 374-82 (1956). (19) Robinson, M. T., Krause, J. F., Ibid., 1 , 216-21 (1956). (20) Sayres, A., Wu, C. S., Rev. Sct. Inslr. 28, 758-64 (1957). (21) Seidell, A,, “Solubilities of Inorganic and Metal Organic Compounds,” Van Nostrand, New York, 1940. (22) U. S. Atomic Energy Comm., Selected Reference Material. “Chemical Processing and Equipment,” TID-5276 (1955). (23)O’CTanSlyke, D. D., J . Rid. Chern. 130, 545--54 (1939). (24)’-clan Slyke, D. D., Neill, J. M., Ibid., 61, 523-73 (1924). (25) Weeren, H. O., Oak Ridge Natl. Lab.. ORNL. CF 51-7-120, TID-1110 (1951). RECEIVED for review April 7, 1958 ACCEPTED October 24, 1958 Division of Industrial and Engineering Chemistry, Nuclear Technology Subdivision, 133rd Meeting, ACS, San Francisco, Calif., April 1958. ~
