Isotope separation by multiphoton dissociation of methylamine with an infrared laser

摘要

Methylamine (CH.sub.3 NH.sub.2) is dissociated into CH.sub.3 · and NH.sub.2 · radicals when irradiated with the output from an infrared laser. Ammonia (NH.sub.3) is then formed inherently through radical and molecular collisions. By tuning the laser to the appropriate frequency, the dissociation process is isotopically selective. As a consequence, ammonia, or other nitrogen containing compounds, are obtained which are selectively enriched in .sup.14 N or .sup.15 N. Carbon isotopes may be separated with essentially the same technique. .sup. PPBACKGROUND OF THE INVENTIONPP1. Field of the InventionPPThe present invention relates to enrichment and separation of isotopes, in particular .sup.14 N and .sup. 15 N, and .sup.12 C and .sup.13 C by multiphoton dissociation of methylamine.PP 2. Description of the Prior ArtP PThe projected use of .sup.15 N in the nuclear industry is expected to reach 500,000 kg/year. Major use is in fuel pellets and cladding in uranium and plutonium nuclear reactors. Also, .sup.15 N and . sup.13 C are presently used as tracers in the fields of medicine and research. Low cost schemes for separating these isotopes are accordingly required.PPAmbartsumyan et al. in Vol. 17, Journal of Experimental and Theoretical Physics Letters, pp. 63-65 (1973) disclose isotope separation of .sup.14 N and .sup.15 N by a two-step photodissociation of .sup.14 NH.sub.3 and .sup.15 NH.sub.3, in which monochromatic radiation of a frequency &ngr;.sub.1 selectively excites a vibrational transition of molecules of only one isotopic composition. The molecules are simultaneously illuminated with light of frequency &ngr;. sub. 2, the quantum energy of which is sufficient for photodissociation of only the vibrationally excited molecules. However, this is a costly process and provides a possibility of isotopic scrambling due to four different intermediate chemical reactions.PP Robinson et al. in U.S. Pat. No. 4,049,515, issued Sept. 20, 1977, disclose laser isotope separation schemes by multiple photon absorption. Briefly, the schemes involve irradiating a molecular species having at least two isotopes of an element with infrared laser light of a frequency which selectively excites to a vibrational level only those molecules of the molecular species containing a particular isotope. Use of multiple photon absorption produces a sufficiently energetic vibrational state such that the molecules containing the particular isotope undergo a chemical reaction, such as dissociation or reaction with a second molecular species. The patent discloses two examples for which laser induced enrichment was obtained; namely the enrichment of .sup.34 S in natural SF. sub.6 and .sup.11 B in natural BCl.sub.3. However, there is no teaching therein of a method for selecting a candidate molecular species from the essentially infinite number of species which exist for any given element for which the process of Robinson et al. is applicable. When a molecule is subjected to infrared radiation in the manner taught by Robinson et al. , the isotopic shift is generally masked by other vibrational modes or, if unmasked, has a magnitude lower than that required for isotope separation. In addition, it has not been possible to precisely predict the manner in which a molecule will dissociate or react with other species when subjected to high intensity infrared radiation. Hence, no general method can be given for devising a laser-chemical reaction system which will effect removal of the desired isotopic species. As a result, it has heretofore not been possible to predict which molecules lend themselves to laser isotope separation by multiple photon absorption. PPAlthough present usage of .sup.15 N is small, projected use in core elements of liquid-metal fast breeder reactors is considerable, as mentioned above. Current separation of .sup.15 N is accomplished by NO distillation, or by chemical exchange between NO and HNO.sub.3. The latter process has an enrichment factor of about 1.055. The only proposed separation of nitrogen isotopes with a laser discussed above (Ambartsumyan et al.) suggests an isotopic enrichment factor of about 4. However, the considerable potential for isotopic scrambling renders the proposed process unsuitable on a commercial scale.PP Carbon isotopes are presently separated by low temperature distillation of carbon monoxide and by gas phase thermal diffusion of methane. The carbon monoxide process for separating .sup.13 C is based on a vapor pressure differential that yields an enrichment factor of about 1.011. PPSUMMARY OF THE INVENTIONPPThe present invention provides a process for obtaining nitrogen containing compounds which are enriched in a particular isotope of nitrogen. The process comprises exposing gaseous methylamine containing two or more isotopes of nitrogen to substantially monochromatic infrared laser radiation. The frequency of the infrared radiation is coincident with a vibration absorption band of the methylamine, producing dissociation of the methylamine into CH.sub.3 · and NH.sub.2 · radicals. Through molecular and radical collisions, the NH.sub.2 · radicals abstract a hydrogen atom, forming ammonia. The addition of other gaseous materials, such as oxygen, can be added to the methylamine to scavenge the NH.sub.2 · radicals to form other nitrogen containing compounds. The process is accomplished isotopically selectively by employing a vibrational band which exhibits an isotope shift. Specifically, .sup.15 NH.sub.3 was obtained with an enrichment factor of 1.65.PPThe process provided herein may be directly applied to the enrichment of carbon isotopes by slightly modifying the wavelength of the laser radiation employed.