{'fraction_non_alphanumeric': 0.030862233534839325, 'fraction_numerical': 0.04931594018453707, 'mean_word_length': 3.8819875776397517, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Applications of ortho-para hydrogen catalyst

The underlying theory of hydrogen Ortho-Para conversion has long been known, but the specifics of non-linear heat of conversion from normal to para hydrogen have not been widely disseminated in the cryogenic literature. These factors are reviewed and thermally efficient applications in liquefiers and back conversion cooling systems are illustrated.


Useful Ortho-Para Hydrogen Data
Ortho and para hydrogen is defined by parallel and anti-parallel spins of the atoms as depicted in Figure 1.
Ortho Para Definitive O-P data is shown in Table 1. The first column lists the temperature range of interest. The equilibrium para content for temperatures from 300 down to 20 K are presented in the second column and the related cumulative heats of O-P conversion from normal to para are given in the third column. The cumulative heat of conversion from normal to para hydrogen is plotted in Figure 2 to illustrate the extreme non-linearity of the data. Figure 3 is plotted to focus on the cold end data and optimum location of conversion catalyst in a liquefier.

4.
Analysis As shown in Figure 2, the relation of O-P heat of conversion to temperature is significantly non-linear and impacts the thermal design of hydrogen liquefiers. The heat of conversion from normal to para is 500 kJ/kg in cooling from 300 K to liquid at 20.7 K. From 300 to 200 K the conversion heat removed is 27.27 % of the total, from 200 to 100 K it is 63.63 % and only 9.1 remains to be removed to achieve 100 % para. Actually, any para percentage above 95 % is commercially acceptable. This is equivalent to the equilibrium para level at 90 K. Examination of Figure 3 is particularly revealing. The heat of O-P conversion from 60 down to 20.7 K is only 1.607 kJ/kg or 0.32 % of the total. At a liquid nitrogen bath temperature of 78 K the total O-P conversion heat is 487.938 kJ/kg or 97.59 % of the total. It is clearly advantageous to concentrate the O-P catalyst in a bath cooled with liquid nitrogen.
Carnot work of cryogenic refrigeration is defined by Wc = (Tw -Tc)/Tc. For Tw = 300 K, the Carnot work values are 2.85 W/W at 78 K and 13.8002 W/W at the 20.27 K boiling temperature of parahydrogen for a ratio of 4.8487. This is significant when one considers that 627.9 kJ/kg is required to cool hydrogen gas from 78 to 20.27 K and the latent heat is 445.5 kJ/kg for a total of 1073.4 kJ/kg. O-P conversion in the liquid receiver more than doubles the 20.27 K heat load to 945.5 kJ/kg.

5.
Conclusions 1. O-P conversion is most efficient if done at approximately the 78 K temperature of liquid nitrogen. 2. No catalyst is needed in heat exchangers colder than 78 K but a small amount of catalyst should be installed in the liquid receiver to bring up the para content from 50% to over 97% at the thermal cost of 11.64 kJ/kg or 2.22% of the total. 3. Gas phase conversion in heat exchangers below 200 K can be effective but it is difficult to balance the heat exchangers with the extra heat load. 4. Back conversion cooling with catalyst is not useful below 90 K and is most effective in the range from 100 up to 200 K.