Ortho-para-hydrogen conversion in the liquefier
Ortho-para-hydrogen conversion in the liquefier
We saw that hydrogen can exist in two different forms-parahydrogen and ortho-hydrogen. The ortho-para concentration in equilibrium hydrogen depends upon the temperature of the hydrogen. Near room temperature, the composition is practically 75 percent ortho-hydrogen and 25 percent para-hydrogen, whereas at the normal boiling point of hydrogen, the equilibrium composition is almost all para-hydrogen. When hydrogen gas is passed through a liquefaction system, the gas does not remain in the heat exchangers long enough for the equilibrium composition to be established at a particular temperature. The result is that the fresh liquid has practically the room-temperature ortho-para composition and will, if left alone in the liquid receiver, undergo the exothermic reaction there. The changeover from ortho- to para-hydrogen involves a heat of conversion that is greater than the heat of vaporization of para-hydrogen; therefore, serious boil-off losses will result unless measures are taken to prevent it. This is a problem peculiar to hydrogen-liquefaction systems that must be solved in any efficient system.
A catalyst may be used to speed up the conversion process, while the heat of conversion is absorbed in the liquefaction system before the liquid is stored in the liquid receiver. Because the heat of conversion results in an increase in liquid evaporated, it is advantageous to carry out as much of the conversion in the liquid-nitrogen bath as possible. The nitrogen is much less costly to produce than the liquid hydrogen. Note from Table 2.7 that the equilibrium composition at temperature near 70 K (126°R), corresponding to liquid nitrogen boiling under vacuum, is approximately 55 to 60 percent para-hydrogen. Thus if the conversion is complete at this temperature, the energy released in the• liquid receiver is reduced by almost one-half.
Two possible arrangements for ortho-para conversion are shown in Fig. 3.28. In the first arrangement, the hydrogen' is passed through the catalyst in the liquid-nitrogen bath, expanded through the expansion valve into the liquid receiver, and drawn through a catalyst bed before passing into a storage vessel. The hydrogen that is evaporated due to the heat of conversion flows back through the heat exchanger and furnishes additional refrigeration to the incoming stream. The second arrangement is similar to the first one, except that the high-pressure stream is divided into two parts before the expansion valve. One part is expanded through an expansion valve and flows through a catalyst bed immersed in a liquid-hydrogen bath; the converted hydrogen is passed to a storage vessel
The other part of the high-pressure stream is expanded through another expansion valve into the liquid receiver to furnish refrigeration for the catalyst bed; the vapor is passed back through the heat exchanger to cool down the incoming gas. The second arrangement allows approximately 20 percent higher liquid-hydrogen yields compared with the first arrangement.
Some of the catalysts that have proved effective are (1) hydrous ferric oxide, (2) chromic oxide on alumina particles, (3) charcoal and silica gel, and (4) nickel-based catalyst. Of these, hydrous ferric oxide is the most active; that is, a relatively small volume of catalyst is required to produce practically complete conversion to the equilibrium composition. The conversion process is speeded up for any of the catalysts if they are ground into fine pellets, which offer a larger surface area per unit volume than do large chunks of material.
Certain impurities will "poison" the catalysts or severely reduce their effectiveness (Scott et al. 1964). Methane, carbon monoxide, and ethylene act as temporary poisons, whereas chlorine, hydrogen chloride, and hydrogen sulfide permanently decrease the catalyst activity. It is important to remove these materials from the hydrogen feed stream before they enter the liquefier.
References
- Cryogenic Systems - Barron R. F
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