Production of concentrated hydrogen from dilute, lean reducing gas by use of the steam-oxide process (part 1). Behavior of iron oxide medium in the reduction-oxidation cycles

Concentrated hydrogen was produced from dilute hydrogen gas by use of the periodic pulse technique coupled with the use of iron oxide medium. The amount of concentrated H₂ thus produced was almost proportional to the width of the dilute H₂ pulse and to the concentration of H₂ in the pulse, but it was independent of the width of the H₂O pulse. The yield of concentrated H₂ from dilute H₂ gas was as high as 50-60% at 700°C. However, the yield decreased markedly with decreasing reaction temperature, and concentrated H₂ was scarcely produced at 400°C. Detailed kinetics of the reaction of dilute H₂ with the preoxidized iron oxide medium and the reaction of H₂O with the prereduced medium were investigated by use of high speed gaschromatograph. It was shown that the reaction of dilute H₂ with the preoxidized medium proceed edreadily at 700°C and the conversion of H₂ in this process was close to the equilibrium conversion. At lower temperatures such as 300 and 500°C, the reaction of the medium with dilute H₂ was decreased by decreased equilibrium conversion and by decreased rate of the surface reaction and/or diffusion of the oxygen or iron ion in the bulk of the medium. The oxidation of the prereduced medium with H₂O proceeded rapidly at 700°C, and the conversion of H₂O was close to equilibrium conversion. At lower temperatures, the rate of oxidation of the medium with H₂O was decreased significantly by decreased rate of surface reaction and/or by the migration of the oxygen or iron ion in the bulk of the medium. From these results and discussion, the characteristics of the iron oxide medium were clarified, and the necessity for developing new kinds of media was emphasized.