How Is Liquid Hydrogen Produced?
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The boiling point of hydrogen is '253 degrees C. Liquid hydrogen, denoted LH2, is a common rocket propellant. Liquefaction of hydrogen is used today to reduce the cost of hydrogen transport. Recent consideration of increased use of hydrogen for energy storage (e.g., for use in cars) has prompted the pursuit of more efficient methods of production.
Joule-Thomson Effect
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Instrumental in the liquefaction of gas is the Joule-Thomson effect, which states that a gas, allowed to slowly expand from a high to a low pressure, undergoes a small change in temperature. The direction of this change depends on the initial temperature. Most gases at room temperature cool when expanded. The "inversion temperature" is the point below which expansion produces cooling. So most gases have an inversion temperature higher than room temperature. The critical temperature for hydrogen, however, is 193 degrees K. To liquefy hydrogen, expansion should not begin until 193 degrees K is reached.
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Regenerative Cooling
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In combination with use of its inversion temperature, a method called "regenerative cooling" was first used by James Dewar in 1898 to create liquid hydrogen, though a less stable hydrogen vapor had been produced over a decade before.
The key to regenerative cooling was to use already cooled gas merely as a coolant of more incoming gas, creating a sort of bootstrapping effect.
Dewar used double-walled vessels, with a vacuum between the walls, to prevent introduction of heat by convection. Today, such vessels are called dewars.
A catalyst
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Hydrogen molecules exist in two forms. In orthohydrogen, the two hydrogen protons in the H2 molecule spin in the same direction. In parahydrogen, the two nuclei spin in opposite directions. In liquid hydrogen, orthohydrogen become parahydrogen spontaneously. The conversion releases more heat than is needed to vaporize the liquid. Therefore, liquefied hydrogen evaporates completely on conversion to parahydrogen--even in a perfectly insulated container. To prevent this, liquid hydrogen is catalytically converted to be entirely parahydrogen during the liquefaction process.
Magnetic refrigeration
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With liquid hydrogen use confined to laboratory research and rocket fuel, there has been little motivation to reduce production costs beyond the benefit of economies of scale.
Recent consideration of increased use of hydrogen for energy storage, however, has motivated research into more efficient methods of hydrogen liquefaction, specifically a magnetic liquefier.
Such experimental magnetic refrigerators make use of the magnetocaloric effect, in which some magnetic materials exhaust or absorb heat with the application and removal of an external magnetic field.
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