GB833837A - Improvements in and relating to the concentrating of hydrogen - Google Patents

Abstract

<PICT:0833837/III/1> A gas stream of increased hydrogen concentration is produced by passing a mixture of gases comprising hydrogen at elevated temperature and pressure through a diffusion zone separated into an upstream portion and a downstream portion by a membrane (as defined below) comprising a hydrogen-permeable Group VIII metal supported against the upstream pressure by a porous, rigid, compressed, sintered steel matrix and collecting a hydrogen-enriched gas downstream of said matrix and a hydrogen-reduced gas upstream of said membrane. The membrane may be a continuous foil comprising said metal or may consist of particles comprising said metal distributed in the matrix, thus providing membrane elements therein. The invention also comprises an apparatus in which a matrix membrane combination is located in a housing so as to divide it into two compartments, one of which is provided with an inlet and an outlet for the hydrogen-containing gas mixture and the hydrogen-reduced gas stream respectively and the other with an outlet for the hydrogen-enriched stream. Fig. 3 illustrates an apparatus of this type wherein two membranes of the continuous foil type, 31 and 41, are employed, said membranes being supported by matrices 12 and 13, respectively, which together with the housing define a chamber 14 into which hydrogen diffuses and is removed by outlet 15. To chambers 8 and 9 having inlets 10 and 11 is supplied the hydrogen-containing gas and the mixture, after loss of hydrogen through the membranes, is taken off by outlets 16 and 17. Two or more such cells may be connected in series with or without intermediate compressors to increase the purity of the hydrogen-rich product or to recover more hydrogen from the hydrogen-reduced product. The invention may be operated at pressures above 0.65 ats., preferably 1.4-100 ats.; atmospheric pressure may be used on the downstream side, though there is exemplified the use of a downstream pressure of 20.4 ats. Temperatures of 390-540 DEG C. are referred to. The membrane may consist of a Group VIII metal; Series 4 and 6 are preferred and palladium, iron, nickel and platinum are referred to. The metals may be used alone or as alloys, e.g. with copper, silver, gold, molybdenum or boron. The matrix may be prepared by sintering under pressure iron or stainless steel powder, having a particle size between 0.1 and 800 microns; heating may be performed by passing an electric current through the mass of particles. The following methods of supporting the membrane on or in the matrix are referred to: (1) a relatively thick (0.5-20 microns) foil may simply be placed in contact with the upstream surface of the matrix (as in Fig. 3, and in Fig. 2, not shown); (2) a thin foil may be placed upon the matrix and subjected to fluid pressure as great as that desired in the separation and the purity of hydrogen diffusing therethrough tested; if the hydrogen is not of sufficient purity further foils may be added and the procedure repeated as many times as necessary to give the desired result; (3) as (2) but the foil may be pressed into the matrix by burnishing after each application (the pore diameter of the matrix should not be greater than 100 microns in this case); (4) powdered metal may be placed on the surface of the matrix and burnished or polished; (5) a layer (e.g. of platinum-silver alloy) may be electroplated on the surface of the matrix; (6) vapours of the desired metal (e.g. produced in an electric are or, particularly for palladium or palladiumsilver alloys, under vacuum conditions) may be condensed on the matrix; (7) molten metal may be sprayed on to the matrix; this is particularly useful where the membrane metal has a low melting-point; (8) the matrix may be impregnated with a decomposable salt of the hydrogenpermeable metal (e.g. palladium nitrate) or a mixture of salts of the metal components of the alloy, and the salt(s) then reduced in situ, e.g. with hydrogen, or by precipitation with hydrogen sulphide followed by heating in air or oxygen; the permeability of the membrane may then be tested and, if desired, further quantities of metals introduced in the same way; and (9) the powdered matrix material may be mixed, compacted and sintered, together with the membrane metal; this method is particularly suitable with stainless steel matrices. Mixtures of gases referred to are hydrogen and sulphur trioxide, hydrogen and low molecular weight hydrocarbons, e.g. methane, ethane and ethylene, which may be obtained in cracking oils and which may contain only a fractional percentage of hydrogen, and hydrogen and nitrogen which may be formed in the autothermic reaction of a methane-steam-air mixture. The latter two mixtures are exemplified. The invention may also be used to increase the proportion of hydrogen in a recycle stream of gas in a reforming reaction, which may contain methane and hydrogen sulphide.ALSO:A porous, rigid, compressed sintered steel matrix may be coated p with a layer of a Group VIII metal or an alloy thereof by (1) placing a continuous foil of the metal on the matrix and pressing the foil into the matrix by burnishing: this may be repeated a number of times; (2) placing the powdered metal on the matrix and burnishing or polishing; (3) permitting vapours of the metal to impinge upon the matrix, e.g. by supporting the matrix above an electric arc in which the metal is heated, or by vaporizing the metal at e.g. 0.01-10 mm. Hg. pressure; or (4) spraying the molten metal upon the matrix maintained at a temperature below the melting point of the metal: this process may also be repeated a number of times. The matrix may be prepared by sintering under pressure iron or stainless steel powder. The preferred Group VIII metals are those of series 4 and 6, palladium, platinum, iron and nickel being specified. Alloys with copper, silver, gold, molybdenum and boron are also mentioned.