The present invention relates to a gas separator for separating a specific gas from a mixed gas by diffusion.
Heretofore, as a technique for obtaining a specific gas from a mixed gas, there is known a separation method by the use of an organic or an inorganic gas separating film. Among the separating films, examples of a hydrogen separating film for use in a film separation method include organic polymeric films of polyimide, polysulfone and the like, and inorganic compound films of palladium, palladium alloys and the like, and examples of an oxygen separating film include films of silver and silver alloys. The palladium film and the palladium alloy films have heat resistance and can obtain extremely high-purity hydrogen.
Palladium and the palladium alloys have characteristics which allow hydrogen to be dissolved therein and which allow hydrogen to permeate therethrough, and by the utilization of the characteristics, a thin film comprising palladium or the palladium alloy has been widely used as a gas separator for separating hydrogen from a mixed gas containing hydrogen. However, the thin film comprising palladium itself is weak in mechanical strength, and so, in Japanese Patent Application Laid-open No. 62-273030, palladium or the palladium alloy is deposited on the surface of an inorganic porous support of a porous glass, porous ceramics, a porous aluminum oxide or the like to increase the mechanical strength of the thin film comprising palladium or the palladium alloy.
Japanese Patent Application Laid-open No. 3-146122 discloses a method for preparing a hydrogen separator which comprises first forming a palladium thin film on the surface of a heat-resistant porous substrate by a chemical plating process, and further forming a silver thin film on the palladium thin film by the chemical plating process, followed by a heat treatment. According to this disclosed method, a hydrogen separator having the porous substrate and a palladium alloy thin film covering it can be obtained. In this palladium alloy thin film, palladium and silver are uniformly distributed by the above-mentioned heat treatment.
In addition, U.S. Pat. No. 3,359,705 discloses a silver thin film for separating oxygen.
However, these gas separators have a drawback that a material gas to be subjected to the gas separation leaks into a purified gas through holes (hereinafter referred to as "throughhole-defects") which extend through the gas separating film comprising the metal for separating the gas. Therefore, the concentration of hydrogen in the purified gas deteriorates as much as the leaked material gas.
In order to remove these throughhole-defects, there is a method of thickening the gas separating film comprised of the metal for separating the gas, but this method has a problem that a gas permeability of the gas separating film deteriorates and hence a gas separation efficiency also deteriorates. However, this method prevents a material gas from getting mixed in a refined gas, and therefore, a hydrogen gas with high purity can be obtained.
Further, Japanese Patent Application Laid-Open No. 6-277472 discloses that pores are closed by filling a metal having a gas separating ability inside the pores opened on the surface of a porous substrate. According to this method, a leakage of a material gas into a refined gas can be avoided without deteriorating an efficiency of separating a gas.
However, when a thickness of a gas separating film is made thick, the method has a problem that adhesive properties between the gas separating film and a substrate such as a porous film are weak, and when the hydrogen separator obtained by the method is actually used in a gas separation process, the gas separating film peels in a short period of time. In consequence, such a hydrogen separator cannot be used continuously for a long term in order to carry out the gas separation.
When a hydrogen separating film disclosed in Japanese Patent Application Laid-Open No. 6-277472 is used, it is desired that a frequency of an exfoliation is decreased though a continuous gas separation for a long term in comparison with a separator having a thickened gas separation film.
The present invention is made in light of the background of the problems of the above-mentioned conventional techniques, and an object of the present invention is to provide a gas separator which can prevent a material gas to be subjected to a gas separation from leaking into a purified gas and which hardly has an exfoliation of a gas separating film and which has excellent durability in comparison with a conventional gas separator.