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NOBLE GAS ABSORPTION PROCESS 2,793,507 5/1957 Hnilicka 55/66 X 3,192,687 7/1965 Silva Et Al 55/33 [75] Inventor: Jess W

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NOBLE GAS ABSORPTION PROCESS 2,793,507 5/1957 Hnilicka 55/66 X 3,192,687 7/1965 Silva Et Al 55/33 [75] Inventor: Jess W 3 890 United States Patent MI BEST AVAILABLE COPY [11] > >l2l Thomas [45] June 17, 1975 [54] NOBLE GAS ABSORPTION PROCESS 2,793,507 5/1957 Hnilicka 55/66 X 3,192,687 7/1965 Silva et al 55/33 [75] Inventor: Jess W. Thomas, Chatham, N.J. 3,542,525 1 1/1970 Pigford et al 55/62 X [73] Assignee: The United States of America as 3,713,272 1/1973 Barrere, Jr. et al..... 55/62 X represented by the United States Energy Research and Development Primary Examiner—Charles N. Hart Administration, Washington, D.C. Assistant Examiner—Ethel R. Cross Attorney, Agent, or Firm—John A. Horan; Leonard [22] Filed: Feb. 19, 1974 Belkin [21] Appl. No.: 443,262 [57] ABSTRACT [52] U.S. CI 55/62; 55/66 The method of removing a noble gas from air compris- [51] Int. CI BOld 53/02 ing the use of activated carbon filters in stages in [58] Field of Search 55/66, 74, 62, 58 which absorption and desorption steps in succession are conducted in order to increase the capacity of the [56] References Cited filters. UNITED STATES PATENTS 1,906,917 1/1931 Peters 55/66 X 6 Claims, 8 Drawing Figures PATENTED JUN17 1975 3,890,121 UIILCUCTLT 1 1 Fig. 1 Fig. 2 WET METER Fig. 4 PATENTED JON 17 1975 CM-TCT n oin.;.' HUMIDIFYING TtfSES Fig. 3 PATENTED JUN17 i9?s SHEET 3 Fig. 5 Fig. 7 1.4 DESORBING CONDITIONS DESORBING CONDITIONS AIR BATH TEMPERATURE 120 °C AIR BATH TEMPERATURE I50°C 1.2 AIR VOLUME 248 L. AIR VOLUME 100 L. TEST SERIES MT I TEST SERIES MT 3 o 1.0 DESORBING CONDITIONS DESORBING CONDITIONS AIR BATH TEMPERATURE 20 °C AIR BATH TEMPERATURE 150 °C AIR VOLUME 101 L. AIR VOLUME 56 L. TEST SERIES MT 2 TEST SERIES MT 4 0 10 20 30 40 ADSORPTION TIME, MINUTES Fig. 6 Fig. 8 3,888,733 NOBLE GAS ABSORPTION PROCESS and purified in the second filter. As an example, if the first filter cleans a given volume of air and one tenth BACKGROUND OF THE INVENTION that volume is employed to redeposit the noble gas in The invention described herein was made in course the second filter, there is an amplifying factor of ten, so of employment with the U.S. Atomic Energy Commis- that the second filter is capable of adsorbing ten times sion. the noble gas of the first filter. Extending this process It is known that one of the principal internal radiation to a third charcoal bed with the same amplifying factor, hazards in the mining of uranium is the presence of the bed would have the capacity of cleaning 100 times radon due to its immediate degradation products as de- the given volume cleaned by the first filter. scribed in U.S. Pat. No. 3,555,278 issued Jan. 12, 1971. 10 It is therefore a principal object of this invention to However, although radon daughters are the principal provide an efficient process for the removal of a noble inhalation hazard, rather than the radon itself, removal gas such as krytop, xenon, or radon from air. of the daughters alone is unsatisfactory since unaccept- Other objects and advantages of this invention will able concentration levels of the daughters are rapidly hereinafter be obvious from the following description produced from relatively small amounts of radon. 15 of preferred embodiments of this invention. At present, the radon daughter hazard is controlled by high air ventilation rates. This can be quite expen- BRIEF DESCRIPTION OF THE DRAWING sive, particularly for deep mines, since long ventilating FIGS. 1 and 2 show schematically two embodiments, shafts and large exhaust blowers are required. respectively, of this invention. The use of carbon to remove radon in mines has been 20 FIG. 3 illustrates schematically radon adsorption ap- previously considered, but previous studies concluded paratus employed in one example of this invention. generally that the quantity of carbon required to ac- FIG. 4 illustrates schematically desorption apparatus complish this function would be excessive, and hence employed in the same example. impractical. FIGS. 5-8 show transmission curves for various test 25 SUMMARY OF THE PRESENT INVENTION It is known, as pointed out in "Measurement of Dy- DESCRIPTION OF THE PREFERRED namic Adsorption Coefficients For Noble bases on Ac- EMBODIMENTS tivated Carbon" by Siegiwarth, et al., appearing in the As shown in FIG. 1, air containing radon to be re- Proceedings of the 12th AEC Air Cleaning Conference 30 moved is passed (as shown by the solid lines) through (1972) that the rate of penetration, or breakthrough, an activated charcoal filter bed 10 until the latter starts of krypton and xenon through a carbon bed is indepen- permitting radon to pass. Then bed 10 is heated to a dent of the noble gas concentration when the latter is suitable temperature while simultaneously a second present in low concentration. This suggests also that a flow of air (as shown by the broken lines) is passed given quantity of carbon will purify a given quantity of through bed 10 to carry away the radon released by air, regardless of the radon concentrations in the enter- heating. The second flow of air which is at a rate which ing air. is substantially below that of the first flow, is cooled The present invention utilizes the principle described and passed through a second activated filter bed 12, above to provide a practical method of employing car- which will remove all of the radon. If desired, addi- bon to remove a noble gas such as krypton, xenon or 40 tional stages, such as a third filter bed 14 operating at radon from air when present in relatively small a further reduction in the rate of air flow may be em- amounts. ployed. If the volume of air depositing radon in filter 10 In accordance with the principles of the invention, is Vi, and the volume of air depositing radon in filter there is provided a method for the removal of the noble 12 is V2, then the radon adsorbed by filter 12 is Vi/V2 45 gas such as radon from air comprising the steps of pass- times the amount of radon adsorbed by filter 10. The ing a finite volume of the air through a charcoal filter third stage filter 14 provides an additional multiplica- to adsorb the radon, then passing a substantially re- tion or amplification stage. duced volume of air through said filter after heating the In the operation of this process, there appears to be latter to a temperature sufficient to release the radon no critical or [imitating conditions as to temperature or 50 so that latter is carried away in the reduced volume of pressure. However, adsorption is more effective as the air, followed by cooling and passing the reduced vol- temperature is lowered and the higher the desorption ume of air containing the radon through a second char- temperature the higher the radon concentration which coal filter to adsorb all the radon present. The first fil- can be obtained in the effluent. For the purposes of this ter may be utilized again to adsorb radon from a second invention, adsorption is preferably conducted with the 55 finite volume of air followed by the previously men- filter at a temperature less than about 100° C whereas tioned steps to add this radon removed to the radon ad- during desorption the filter temperature should be at sorbed in the second filter. The process is fully applica- least 20° C higher than the sorption temperature. It is ble to the other noble gases kryton and xenon. the elevation of temperature from adsorption to de- sorption which produces the amplification, and the As the rate of penetration, or breakthrough, of the 60 noble gas through a carbon bed is independent of the greater the elevation, the greater will be the amplifica- noble gas concentration provided it is present in low tion. Increasing the desorption temperature increases the rapidity of desorption and this increases the con- concentration, so that a given quantity of carbon will centration which can be obtained. With regard to pres- purify a given quantity of air, it is seen that the process described has an "amplifying factor." That is, the sec- 65 sure, both adsorption and desorption are conducted at ond filter will accept a number of charges equal to the about atmospheric pressure, with only enough pressure ratio of the finite volume of air being purified in the i.e. of the order of 2 inches of water, to provide flow. first filter to the volume of air removing the noble gas Increase in system pressure will cause a larger quantity 3,890 ,121 3 4 of radon to be adsorbed per unit volume of carbon, and through a radium chloride solution containing several decrease of pressure the opposite. millicuries of Ra-226. The rate of production of radon With regard to concentration of the radon, in a ura- was found to be about 900,000 pCi/min. An "aerosol nium mine without ventilation, the radon is typically canister" containing two layers of high efficiency filter present in 50,000 pCi/liter which is equal to about 3.5 5 paper was used downstream of the solution to remove X 10~n vol %. This process as herein described will op- any entrained aerosol from the radium solution. The erate successfully up to concentrations of about 1015 main air supply for the apparatus entered through flow- pCi/liter so that for the particular application described meter F-l at 69 1/min and was humidified to about 9.0 e.g., removal of radon from air found in uranium mines, mg/1 by means of the humidifying tubes shown.
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