Preparation of liquid hydrogen

摘要

In the thermal decomposition of methane to produce hydrogen with subsequent liquefaction of the hydrogen, at least part of the cold required for the liquefaction is obtained by the vaporization of liquid methane and the vaporized methane is used to form at least part of the feed to the thermal decomposition step. The methane may be decomposed (a) by pyrolysis in the absence of a catalyst, (b) with steam at 700 DEG -800 DEG C. in the presence of a catalyst, or (c) by partial combustion at 800 DEG -900 DEG C. in the presence of nickel as catalyst. The carbon monoxide produced in method (b) or (c) may be converted to carbon dioxide which may be absorbed in a scrubbing liquid, e.g. water or an aqueous amine or caustic alkali solution. Oxygen for method (c) may be supplied from an air separation plant utilizing the vaporization of liquid methane according to Specification 804,944. Residual impurities in the gaseous hydrogen may be removed by fractional liquefaction also utilizing the vaporization of methane. The vaprized methane in both cases may form part of the feed to the thermal decomposition step. The fractional liquefaction of impurities may be effected through the medium of nitrogen supplied as liquid from an air separation plant. As shown 95-100% pure oxygen at atmospheric pressure or at a pressure of 10-20 atmospheres and preheated gaseous methane at substantially the same pressure are passed from lines 3 and 8 respectively into a partial combustion chamber 4. The reaction product gas, after saturation with water vapour in vessel 10, is passed through a shift converter 15. The resulting gas is passed at a pressure of 10-20 atmospheres through carbon dioxide scrubbers 15 (fed with water or aqueous monoethanolamine solution) and 17 (fed with aqueous sodium hydroxide solution). The carbon dioxide-free hydrogen, if desired together with hydrogen from a gas holder 48, is compressed to 150 atmos. and is passed through a two-stage liquid methane vaporizer comprising heat-exchangers 21 and 21a wherein it is cooled to -160 DEG C., a liquefied impurity separator 26, a heat-exchanger 32 wherein it is further cooled to -208 DEG C., a second liquefied impurity separator 34, low temperature charcoal traps 37, 37a, and a heat exchanger 39. The cold purified gaseous hydrogen is expanded into a vessel 41 at atmospheric pressure and thereby liquefied. Hydrogen gas passes from vessel 41 through heat exchangers 39, 32, 21a and 21 to the gas holder 48. It may alternatively be recompressed and stored in cylinders. The methane to be decomposed is fed from a reservoir 22 and line 25 as a gas into heat-exchanger 21 and from reservoir 22 and line 23 as a liquid into heat-exchangers 21a and 21. Methane from both lines 23 and 25 then flows through line 8 as gas to chamber 4. A nitrogen cycle, comprising heat exchange coil 54, expansion valve 56, heat exchangers 32 and 52, and compressor 50, utilizes the cold generated by vaporization in vessel 22 for cooling the partially purified hydrogen in heat-exchanger 32. The fractionally condensed impurities in vessels 26 and 34 are expanded, passed through heat exchangers 21a and 21, and then passed as gas to waste.