Figure. (IMAGE)
Caption
Hydrogen gas is a mixture of ortho hydrogen and para hydrogen. Their ratio depends on temperature, and is also greatly affected by the cooling rate. If hydrogen is rapidly cooled without a catalyst, the conversion from ortho hydrogen to para hydrogen is delayed as shown by the red broken line in the figure, and a substantial quantity of energetically unstable ortho hydrogen will remain in the liquid. The residual ortho hydrogen continues to be converted slowly during storage, and releases energy. This gives rise to partial vaporization of liquid hydrogen, and causes boil-off losses during storage. Therefore, it is essential to use a catalyst that completely converts ortho hydrogen to para hydrogen before the liquefaction process (shown by the blue broken line). Such catalyst prevents boil-off losses during storage and increases the hydrogen handling efficiency.
Credit
Hiroshi Mizoguchi (National Institute for Materials Science), Hideki Abe (National Institute for Materials Science), Hideo Hosono (National Institute for Materials Science), and Takeshi Fujita (Kochi University of Technology)
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