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Using hydrogen as a working fluid in cryocoolers can potentially benefit cryocooling technologies and hydrogen liquefaction. Moreover, in flow-through thermoacoustic systems, hydrogen can be efficiently cooled and undergo ortho-parahydrogen isomeric conversion, which is important for the efficient storage of cryogenic hydrogen. A traveling-wave regenerator is analyzed in this study, using the thermoacoustic theory with a superimposed mean flow and an empirical correlation for hydrogen isomer conversion. A regenerator with hydrogen fluid is shown to achieve higher performance in comparison with helium as the working fluid. However, the hydrogen system performance degrades at supercritical pressures and subcritical temperatures in compressed liquid states. In regenerators with mean flow, using hydrogen as the working fluid leads to higher cooling powers and efficiencies, but helium systems are able to achieve colder temperatures.
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Temperature lowering of liquid nitrogen via injection of helium gas bubbles improves the generation of parahydrogen‐enriched gas
Abstract The para spin isomer of hydrogen gas possesses high nuclear spin order that can enhance the NMR signals of a variety of molecular species. Hydrogen is routinely enriched in the para spin state by lowering the gas temperature while flowing through a catalyst. Although parahydrogen enrichments approaching 100% are achievable near the H2liquefaction temperature of 20 K, many experimentalists operate at liquid nitrogen temperatures (77 K) due to the lower associated costs and overall simplicity of the parahydrogen generator. Parahydrogen that is generated at 77 K provides an enrichment value of ~51% of the para spin isomer; while useful, there are many applications that can benefit from low‐cost access to higher parahydrogen enrichments. Here, we introduce a method of improving parahydrogen enrichment values using a liquid nitrogen‐cooled generator that operates at temperatures less than 77 K. The boiling temperature of liquid nitrogen is lowered through internal evaporation into helium gas bubbles that are injected into the liquid. Changes to liquid nitrogen temperatures and parahydrogen enrichment values were monitored as a function of helium gas flow rate. The injected helium bubbles lowered the liquid nitrogen temperature to ~65.5 K, and parahydrogen enrichments of up to ~59% were achieved; this represents an ~16% improvement compared with the expected parahydrogen fraction at 77 K. This technique is simple to implement in standard liquid nitrogen‐cooled parahydrogen generators and may be of interest to a wide range of scientists that require a cost‐effective approach to improving parahydrogen enrichment values.
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- Award ID(s):
- 2238852
- PAR ID:
- 10484305
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Magnetic Resonance in Chemistry
- Volume:
- 62
- Issue:
- 2
- ISSN:
- 0749-1581
- Format(s):
- Medium: X Size: p. 94-100
- Size(s):
- p. 94-100
- Sponsoring Org:
- National Science Foundation
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