IMPREGNATED CHARCOAL FOR REMOVING RADIOACTIVE MOLECULES FROM GASES

Abstract: Impregnated adsorbent for removing radioactive molecules from gas wherein charcoal is impregnated by spraying or other simple techniques with a substituted 1,4-diazabicy clo[2.2.2]octane which is a liquid at atmospheric pressure and a range of normal operating temperatures.

Description: IMPREGNATED CHARCOAL FOR REMOVING RADIOACTIVE MOLECULES FROM GASES Description Technical Field This invention is in the field of adsorbent mate¬ rials for removing radioactive molecules, such as methyl iodide, from gases. Background Art It is often necessary to trap or remove ' radio- active molecules from gases. This is particularly true regarding nuclear energy plants, where off-gas streams often contain radioactive molecules, including alkyl halides such as methyl iodide. Such radioactivity is usually caused by isotopes, such as iodine 131. A common method employed for removing radioactive molecules from such gas streams involves the use of gas filtration systems containing beds of adsorbent materials, such as charcoal. When used in this capacity, charcoal is frequently treated to increase its surface area; such treated charcoal is frequently called activated charcoal, and is intended to be included within the term charcoal as used herein. Such adsorbent beds are normally operated at room temperatures . although they must be capable of with- standing higher temperatures under emergency condi¬ tions. To increase the adsorption efficiency of such beds, the adsorbent may be impregnated with substances O].:PI /,.„ vι:o . that increase its tendency to adsorb certain molecules such as alkyl halides. Examples of impregnants pre¬ viously used with activated charcoal for this purpose are set forth in U.S. Patent No. 3,453,807, issued to R. Taylor in 1969. In the Taylor patent, activated charcoal is im¬ pregnated with a water-soluble secondary or tertiary amine. One such tertiary a ine (referred to in the claims as a secondary amine) is stated to be triethyl- enediamine (TEDA) , which might also be named 1,4- diazabicyclo[2.2.2]octane. This substance can be represented by the following chemical formula: While 1,4-diazabicyclo[2.2.2]octane has proven to be somewhat suitable for trapping radioactive alkyl halides and other molecules, it suffers from several disadvantages. One significant disadvantage is that this substance is a crystalline solid at atmospheric pressure and normal operating temperatures. Therefore, it must be dissolved in a solvent, such as water, prior to application to the charcoal. This has proven to be a complex and troublesome operation in the production of impregnated charcoal. In addition, this substance tends to be relatively volatile. Therefore, it tends to escape from impregnated charcoal through the process of sublimation. 1, -diazabicyclo[2.2.2]octane can be modified to render it a liquid at atmospheric pressure over a substantial range of temperatures. This can be ac- complished by substituting any of several groups for one of the hydrogen atoms on the 1,4-diazabicyclo[2.2.2]- o - octane. Such appended groups disrupt the symmetrical shape of the unsubstitued molecules, reducing its tendency to crystallize. Such substituted compounds can be represented by the following structural ; formula: wherein the combination of R substituents, one or more of which are not hydrogen, cause the compound to be a liquid at atmospheric pressure. Suitable R substituents can comprise lower alkyl groups, i.e., C--C β alkyl. Examples of such compounds include 2-methyl-l,4-diazabicyclo[2.2.2] octane; 2,5- dimethyl-l,4-diazabicyclo[2,2,2] octane; 2,6-dimethyl- 1,4-diazabicyclo[2.2.2]octane, -2-ethyl-l,4- diazabicyclo[2.2.2]octane; 2,5,7-trimethyl-l,4- diazabicyclo[2.2.2]octane; and 2-propyl-l,4-diazabicyclo [2.2.2]octane. These alkyl-substituted compounds are described in the patent literature in U.S. Patent Nos. 3,297,701 (Brader et al, 1967) and 3,325,547 (Cour et al, 1967); British Patent No. 1,045,091 (Farkas et al, 1966); and German Offen. No. 2,215,474, the teach¬ ings of which, in regard to the syntheses and properties of these compounds, are hereby incorporated by reference. Alternately, charcoal may be impregnated with an aqueous mixture containing a tertiary amine (such as diazabicyclo [2.2.2]octane) and an inorganic salt of • iodine or bromine, as described in U.S. Patent No. 4,040,802 (Dietz et al, 1977). German Offen. 2,629,302 (Dabby, 1977) reveals the use of impregnant compounds substituted with lower alkyl groups, such as 2-methyl-l,4-diazabicyclo [2.2.2]octane, applied to the adsorbent material alumina. However, the process of applying the impregnant to the alumina is complex and wasteful. In addition, that document does not disclose information about the kinetics of the adsorption reaction, e.g., the rate at which gas is diffused through the material and the rate at which the alkyl halides are adsorbed on the impregnated alumina. Rapid adsorption rates are very important in the use of adsorbents to remove radioactive molecules from nuclear reactor off-gases. Disclosure of the Invention The invention described herein arises out of the discovery that several surprising and very useful re- suits occur when 1,4-diazabicyclo[2.2.2]octane, which has been substituted to make it a liquid at room temperature and atmospheric pressure, is applied to charcoal that is used as an adsorbent for radioactive gases. These advances are substantially superior to the prior art regarding both unsubstituted 1,4- diazabicyclo[2.2.2]octane, used as an impregnate on charcoal, as well as substituted 1,4-diazabicyclo- [2.2.2]octane used as an impregnate on alumina. The first surprising advantage relates to the process of applying the impregnate to the adsorbent. When applying substituted 1,4-diazabicyclo [2.2.2]~ octane, German Offen. 2,629,302 required the adsorbent to be pre-heated under a vacuum, cooled, treated with excess impregnate, and allowed to stand for an hour. The excess impregnate was then removed, by an undisclosed process. The sorbent was then dried for several hours in a vacuum oven; pre¬ sumably, impregnate that evaporated during that period was either recycled or disposed of. Unexpectedly, it has been discovered that the process of applying substituted 1,4-diazabicyclo- [2.2.2]octane to charcoal is a great deal simpler than the process of applying this same material to alumina. This apparently results from very sub¬ stantial differences between the pore structure within and diffusion rates through the two adsorbents. The inventors have discovered that the application of sub¬ stituted 1,4-diazabicyclo[2.2.2]octane does not re- quire pretreatment of the adsorbent with heat and vacuum, nor the use of excess impregnate, nor the removal of excess, impregnate. Instead, substituted 1,4-diazabicyclo[2.2.2]octane can be applied to char¬ coal through a single operation, such as conventional spraying.. If the impregnate is simply sprayed over charcoal absorbent, it will tend to diffuse by capil¬ lary action, forming a relatively uniform thin film on very large areas of the charcoal. This is in marked and unexpected contrast to the diffusion of the same compound when applied to alumina. Several other fluid application techniques can also be used to apply substituted 1,4-diazabicyclo- [2.2.2]octane to charcoal. For example, the impreg¬ nate could be vaporized and allowed to condense upon the adsorbent. Alternately, the charcoal could be immersed fully or partially in the impregnate, which would coat the charcoal by capillary action. The second major advantage of using charcoal rather than alumina results from the fact that charcoal has a much greater surface area than the same quantity of alumina. A gram of high-quality charcoal has a • total surface area of approximately 1,000 to 1,500 square meters, while a gram of adsorbent alumina has a surface area of approximately 100 to 350 square meters. Therefore, charcoal will tend to retain more impregnate and more radioactive molecules than will alumina. _oι:FI Another benefit which can be obtained by employing charcoal instead of alumina derives from the fact that charcoal is relatively more hydrophobic than alumina. Therefore, both impregnate molecules and alkyl halides will tend to adsorb more readily to charcoal than to alumina in the presence of substantial humidity, a condition which normally exists in the nuclear reactor uses contemplated by this invention. Impregnate mole¬ cules and alkyl halides will also tend to bond more tightly to charcoal than to alumina, thereby being retained more efficiently in the adsorbent bed. A still further surprising result obtained by using charcoal rather than alumina as adsorbent material is the rate at which radioactive molecules cling to impregnated charcoal. As described in the examples below, a gas stream containing radioactive methyl iodide was passed through an impregnated char¬ coal bed with a contact time of only about one-fourth of a second. However, the impregnated charcoal removed substantially more than 99% of the input methyl iodide. Best Mode of Carrying Out the Invention This invention employs charcoal as an adsorbent material to remove radioactive molecules from gas. The appropriate particle size, bed configuration and dimensions, pretreatment techniques, and other suchparameters relating to the use of charcoal in this capacity can be chosen for each application. In gener¬ al, suchparameters will be known to those skilled in the art, or can be determined using no more than routine experimentation. The charcoal is impregnated with a substituted 1,4-diazabicyclo [2.2.2]octane, wherein the substituent(s) causes the substituted compound to be a liquid at atmospheric pressure and room temperatures. One preferred impregnant is 2-methyl-l,4- diazabicyclo[2.2.2]octane, which also can be called methyl-triethylene diamine. This substance can be represented by the following formula: The impregnate is applied to charcoal by any of several conventional fluid handling techniques. A preferred way to apply impregnate to charcoal is by spraying the impregnate with a nebulizer using air pressure. Impregnation can be accomplished before the charcoal is loaded into an adsorbent bed, or after unimpregnated charcoal is loaded into an ad¬ sorbent bed with suitable dimensions and configuration. Other agents can be added to the specific liquid impregnant chosen, prior to its application to the charcoal. Similarly, two or more diffirent liquid impregnants according to this invention could be mixed together. It is also possible to dissolve unsub- stituted crystalline 1,4-diazabicyclo [2.2.2]octane in one or more of the liquid impregnants described herein, such as 2-methyl-l,4-diazabicyclo[2.2.2]octane. All such possibilities are included within the scope of this invention. The amount of impregnate employed for any given application can vary. In general, amounts between •about one percent and about ten percent by weight, based upon the weight of unimpregnated adsorbent, .are considered suitable. Once the impregnated charcoal is loaded into an adsorbent bed, the bed is installed within a gas handling system using conventional technology. A preferred way to install an adsorbent bed in a nuclear reactor is to install the bed by means of piping and valves between a source of radioactive gas and an outlet to the atmosphere. Any gas that is released by the reactor to the atmosphere must pass through the adsorbent bed, which removes " radioactive molecules from the gas before it is released. Example 1 Charcoal derived from a coconut base material, 8/16 mesh, steam activated, and : having a surface area of about 1,000 square meters per gram was employed. Before the test began, the charcoal was pre-equilibrated with air at 25°C, 95% relative humidity, in order to obtain reproducible results. About 4.5%,- by weight, 2-methyl-l,4-diazabicyclo- [2.2.2] octane was then sprayed upon the charcoal with a nebulizer at about 60 pounds of air pressure. The additional test conditions were: Temperature - 25°C Bed Depth - 2 inches Air Velocity - 7.84 inches/second Contact time - 0.254 seconds Relative humidity - 95% Pressure - Atmospheric Concentration of radioactively labelled 3 Methyl Iodide - 2 mg/meter Feed Duration - 2 hours Elution Period - 2 hours. At the end of the four hour test period, 99.92% of the methyl iodide had been retained within the test bed. 'As much as 3.0% of the methyl iodide could have passed through the charcoal and still met a currently existing commercial standard. Example 2 Charcoal impregnated with 2-methyl-l,4-diazabicylo- [2.2.2]octane, prepared as described in Example 1, was tested at a relatively high temperature. The test conditions were: Temperature - 130°C Relative Humidity - 95% Bed Depth - 2 inches Air Velocity - 8 inches/second Contact time - 0.25 seconds Pressure - 42.5 psia Concentration of radioactively labelled Methyl Iodide - 1.75 mg/ Feed Duration - 2 hours Elution Period - 2 hours Under these conditions, the impregnated charcoal retained 99.75% of the input methyl iodide. Industrial Applicability The invention described herein has industrial applicability in nuclear power plants and other such applications wherein it is desirable and/or necessary to remove radioactive molecules, such as methyl iodide, from a volume of gas. Equivalents Those skilled in the art will recognize, or be able to ascertain using no more than routine experi¬ mentation, many equivalents to the embodiments specifically described herein. Such equivalents are intended to be covered by the following claims.

Claims: Claims ' 1. In the use of charcoal for removing radioactive molecules from gas: The improvement wherein said charcoal is impregnated with " a substituted 1,4-diazabicyclo[2.2.2]octane which is a liquid at room temperature and atmospheric pressure. 2. An adsorbent comprising charcoal impregnated with ■a compound represented by the structural formula: wherein ents which cause the compound to be a liquid at room temperature. 3. An improvement of Claim 2 wherein at least one of the R substituents comprises a lower alkyl group. 4. An improvement of Claims 2 or 3 wherein one of the R substituents comprises a methyl group and the other R substituents are hydrogen. 5. A charcoal adsorbent impregnated with 2-methyl, 1,4-diazabicyclo [2.2.2]octane. 6. A method for trapping radioactive gaseous products in a gas stream by passing said gas stream through charcoal impregnated with a substituted 1,4- diazabicyclo[2.2.2]octane which is a liquid at room termperature and atmospheric pressure. ■ ΪE 1. A method of Claim 6 wherein said substituted 1,4-diazabicyclo[2.2.2]octane is a compound represented by the structural formula: wherein R. through R. 2 are individually selected from hydrogen and lower alkyl groups. 8. A method of applying an impregnate comprising 1,4-diazabicylo[2.2.2]octane to an adsorbent comprising the following steps: a. substituting for one or more of the hydrogen atoms on said 1,4-diaza- bicyclo[2.2.2loctane a substituent group selected so that the resulting compound is a liquid at room tempera¬ ture and atmospheric pressure; and b. applying said substituted impregnate to said adsorbent by a technique selected from the following group of techniques: spraying, condensation, partial immersion, and total immersion. 9. An improvement of Claim 8 wherein at least one said substituent group comprises a lower alkyl group.

Inventors: LASKIE J, UNDERHILL D
Applicants: LASKIE J, UNDERHILL D
Application year: 1981
Main classification: B01D53
IPC codes: B01D53, B01J20, G21F9