Hydrogen liquefaction ortho-para conversion

The theoretical minimum work for hydrogen liquefaction depends on the pressure of the hydrogen feed, the rate of ortho-para conversion and the temperature difference between ambient temperature and the temperature of the liquid hydrogen. The following formula is valid for ambient input and output pressures ... 

The conversion from the ortho form of hydrogen to the para form, an approximately 75 percent conversion, takes place in either the liquefaction process or in the storage tank. Since the conversion is exothermic, a number of catalyst beds at successively lower temperatures are incorporated into the design of the liquefaction plant, so that very little conversion occurs in the storage tank. This minimizes the loss of liquid hydrogen from the storage tank. 

If ortho-hydrogen remains after liquefaction, the heat of transformation described previously will be released slowly as the conversion proceeds. Long-term storage of hydrogen requires that the hydrogen be converted from its ortho form to its para form to minimize boil-off losses. This can be accomplished using a number of catalysts including activated carbon, platinized asbestos, ferric... 

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