It is well known that in nuclear fuel reprocessing plants a certain amount of radioactive krypton gas is released during chemical processes which are used to dissolve spent nuclear fuel rods. Rather than allow such radioactive gas to contaminate the atmosphere, environmental standards generally require that such krypton be recovered and stored for a period of about 100 years. As the half-life of krypton is approximately 10 years, storage for 100 years will reduce the radioactivity of the krypton to virtually insignificant levels, at which time it will be comparatively safe to use or release the gas to atmosphere. Storage of a large quantity of gas for a period of 100 years obviously poses substantial problems and cost with respect to such storage, even if only in terms of the space required. Therefor, it has been an objective of the art to develop an efficient process for separation of these radioactive gases so that the least amount possible of the particular gas need be stored. Hence, there is and has been a need for an efficient method of separating radioactive krypton from other gases.
In one common type of nuclear fuel reprocessing plant the krypton arrives at the separation point mixed with argon, xenon and nitrogen; other gaseous products of the fuel dissolution process such as oxygen and hydrocarbons having been removed by catalytic combustion or adsorption methods. However, it is difficult to remove xenon from nitrogen by distillation methods as the pressures and temperatures typically used are such that any xenon present tends to freeze and clog columns, etc. By comparison, Kr and Xe can be separated from oxygen with relative ease, although there are some freezing problems with this process as well. Hence, the art has recognized that a need exists for a highly efficient and inexpensive process for separation of the gases krypton and xenon from nitrogen. In particular, a method is needed for the efficient separation of xenon, as the krypton can then be distilled from the nitrogen.
Another approach to the problem of nuclear fuel reprocessing plants involves a similar process to that already described except that the krypton and xenon come mixed together with helium gas which, of course, is also inert. A process for the separation of the krypton and xenon from helium is described in U.S. Pat. No. 4,080,429. The described process is carried out in sealed containers and the retorts and reaction vessels utilized are continually flushed with helium. The krypton and xenon are separated from the helium by passing the mixture into a large container having surfaces cooled by liquid nitrogen. As the freezing point of the krypton and xenon are above the liquefaction temperature of nitrogen, while that of helium is below such nitrogen boiling point, the helium remains gaseous while the krypton and xenon freeze out on to the walls of the container while the helium flows therethrough. However, the fact that the krypton has a significant partial pressure--1.7 mm Hg at liquid nitrogen temperatures means that some of the krypton will be entrained in and necessarily escape with the helium gas streams; therefore, the helium must be further processed, in order to ensure that virtually all of the krypton has been removed therefrom. Moreover, the xenon and krypton remaining in the container are subsequently separated from each other in order to avoid storing xenon for unnecessarily long periods of time. This approach, while not unworkable, is not especially efficient and, in particular, is not well suited to the separation of nitrogen from krypton and xenon because much more nitrogen is used in the nitrogen-based approach than is helium in the other method discussed above. Thus, even more krypton and xenon will tend to become entrained with the nitrogen than with the helium. Consequently, it is an object of the invention to provide an efficient, simple and workable method for separating one or more gases from a multi-component feed gas stream.
It is a further object of the invention to provide a method whereby large quantities of nitrogen can be thoroughly separated from a gas mixture containing minor quantities of krypton and xenon.
It is a further object of the invention to provide a general method for separation of gases which have large differences in partial pressure at a given temperature from one another.
Other objects of the present invention will become apparent from the detailed description of an exemplary embodiment thereof which follows and the novel features of the invention will be particularly pointed out in conjunction with the claims appended hereto.