Hydrogen is one of the most important raw materials in the chemical industry. It is expected that in the foreseeable future hydrogen consumption will increase drastically due to its use as an ecologically clean fuel.
Among existing hydrogen generation methods the purest gas is obtained by means of water electrolysis. This process is not widely used in industry due to high energy requirements. Less costly hydrogen generation methods are based on natural gas or oil conversion by steam or carbon monoxide. The most serious disadvantage of such methods is low hydrogen purity due to the presence of CO, CO.sub.2, CH.sub.4 and H.sub.2 O. Large quantities of hydrogen are also formed during hydrocracking and catalytic reforming of oil fractions. The impurities present in these cases are mainly the products of such processes.
The development of an effective hydrogen separation and purification method is therefore of critical importance. As a rule, existing hydrogen purification processes include several stages based on methods such as absorption, adsorption, rectification of liquidized gas and fractionation condensation. Many stage processes drastically increase the cost of hydrogen.
Membrane separation is regarded nowadays as a most preferred method for production of purified hydrogen and, according to comparative evaluation presented in U.S. Pat. No. 4,265,745 is increasingly cost effective. The availability of membrane materials, which are either selectively permeable for hydrogen, or, alternatively allow for penetration of various gases other than hydrogen, makes it possible to develop a one-stage hydrogen purification process.
Some of the best known hydrogen separation membranes, which have also found some limited industrial applications, are thin Pd or Pd-alloy films. The separation process in this case is based on the ability of hydrogen to dissolve in palladium and diffuse through it. The fact that no other gasses are soluble in palladium manifests itself in extremely high selectivity (&gt;1000), which is suitable for one stage hydrogen purification. The drawback of palladium membranes results from the mechanism of hydrogen permeation through them. It is based on the diffusion through a non-porous solid material, where the diffusion rate is determined by the equation: EQU D=k P.sup.a, 0.5&lt;a&lt;1.0
where k is a constant and P is pressure.
Under those conditions any considerable hydrogen permeability can be obtained at high pressures only. Due to low values of the diffusion constant the maximum permeability is limited to about 2000 barrers. Another serious disadvantage of palladium membranes is their extremely high cost. Various palladium alloy membranes with higher diffusion constant and lower cost have been introduced in recent years. Y. Sakamoto, F. L. Chen, M. Furukawa, N. Noguchi, J. Alloy and Comp., 185 (1992) 191.; Y. Sakamoto, F. L. Chen, Y. Kinari, J. Alloy and Comp., 205 (1994) 205!.
Other successful hydrogen separation membranes are based on polymeric materials W. J. Koros, G. K. Fleming, J. Membr.Sci., 83 (1993) 1.!, porous ceramics Inorganic membranes, R. R. Bhave ed., Van Nostrand Reinolds, N.Y., 1991, p.155! and R. J. R. Uhlhorn, K.Keiser, A. J. Burgraff, J. Membr. Sci., 66 (1992) 259.!, zeolite molecular sieves PCT Patent WO 90/092231! and Meng-Dong Jia, K. V. Reinemann, R. D. Behling, J. Membrane Sci., 82 (1993) 15.! and molecular sieving carbons W. J. Koros, G. K. Fleming, J. Membr.Sci., 83 (1993) 1.!.
A different and very promising class of hydrogen separation membranes is based upon the use of proton-conductors. At present two main types of proton conductors are known. The first type includes comparatively wide spectrum of materials of oxide and sulfide nature H. Iwakawa, H. Uchida, N. Naeda, Solid State Ionics, 11 (1992) 109.! and K. G. Frase, G. C. Farrington, J. O. Thomas, Ann. Rev. Mater Sci., 14 (1984) 279.!. As a rule such materials do not contain protons and are capable of proton inclusion and transport due to existence of cation exchange sites in their structures F. M. Ingberger, J. Non-Crystal. Solids, (1980) 39.!. The formation of such sites and therefore any considerable proton conductivity in such materials occurs at elevated temperatures only.
Low temperature proton transport occurs in the proton conductors of a second type. It comprises most of solid inorganic poly-acids, i.e. proton containing compounds. The proton conductive properties of such materials have been studied in great detail. It has been established that only poly-antimonic acids and acidic phosphates of polyvalent metals exhibit proton transport of considerable value. In poly-antimonic acids protons are located in the channels of the crystalline structure. Inacidic polyphosphates of zirconium and titanium protons, which are a part of phosphates groups, form layers between metal atoms. In both cases the concentration of protons in specific directions and comparatively low distances between protons result.
It is an object of the invention to suggest a novel ceramic based membrane for use in hydrogen separation.