Liquid hydrogen.html
 
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IUPAC Liquid hydrogen
Other names Hydrogen (cryogenic liquid); Hydrogen, refrigrated liquid; LH2, Para Hydrogen
Formula LH2
UN number 1966, Class 2
CASNo 1333-74-0
MeltingPt K=14.01, C=−259.14, F=−434.45
BoilingPt K=20.28, C=−252.87, F=−423.17
Triple point K=13.8033, kPa=7.042
RTECS MW8900000
PEL-OSHA Simple asphyxiant
ACGIH TLV-TWA Simple asphyxiant

Appearance Colorless, odorless
LEL 4 %
UEL 75 %
Auto ignition C=500, F=932

Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. It is a common liquid rocket fuel for rocket applications. Hydrogen is found naturally in the molecular H2 form.

To exist as a liquid, H2 must be pressurized and cooled to a very low temperature, 20.27 K (−423.17 °F/−252.87°C).1 One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of hydrogen storage. As in any gas, storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. Once liquified it can be maintained as a liquid in pressurized and thermally insulated containers.

Liquid hydrogen consists of 99.79% parahydrogen, 0.21% orthohydrogen.2

Contents

History

Further information: Timeline of low-temperature technology

1756 - The first documented public demonstration of artificial refrigeration by William Cullen3, Gaspard Monge liquefied the first gas producing liquid sulfur dioxide in 1784. Michael Faraday liquified ammonia to cause cooling, Oliver Evans designed the first closed circuit refrigeration machine in 1805, Jacob Perkins patented the first refrigerating machine in 1834 and John Gorrie patented his mechanical refrigeration machine in 1851 in the US to make ice to cool the air45, Siemens introduced the Regenerative cooling concept in 1857, Carl von Linde patented equipment to liquefy air using tile Joule Thomson expansion process and regenerative cooling6 in 1876, in 1885 Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K; critical pressure, 13.3 atmospheres; and boiling point, 23 K. Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929.

Spin isomers of hydrogen

Room temperature hydrogen consists mostly of the orthohydrogen form. After production, liquid hydrogen is in a metastable state and must be converted into the parahydrogen isomer form to avoid the exothermic reaction that occur when it changes at low temperatures, this is usually performed using a catalyst like ferric oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromic oxide, or some nickel compounds7.

Uses

NFPA 704 placard for stationary hydrogen facilities
Liquid hydrogen tank in the BMW Hydrogen 7.

In liquid hydrogen fuelled rocket engines, liquid hydrogen is used as a coolant to cool the engine nozzle (Regenerative cooling (rocket)) and other parts before being mixed with the oxidizer (often liquid oxygen (LOX)) and burned. The resulting exhaust of such LH2 - LOX engines is very clean water with traces of ozone and hydrogen peroxide.

Liquified hydrogen can be used as a fuel in an internal combustion engine or fuel cell. Various submarines (Type 212 submarine, Type 214 submarine) and concept hydrogen vehicles have been built using this form of hydrogen (see DeepC, BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for LNG. However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless LH2 is injected instead of gas, hydrogen-fueled piston engines usually require larger fuel systems. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.

Liquid hydrogen is also used to cool neutrons to be used in neutron scattering, since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision).

Advantages

Hydrogen has one of the highest gravimetric energy densities of all available fuels, which means it has very high energy content per unit mass making it one of the lightest fuels available (143 MJ/kg, 40 percent more than other rocket fuels).[1].

Producing “zero emissions”, the byproducts of its combustion with oxygen alone are mainly water vapor.

Drawbacks

One liter of hydrogen weighs only 0.07 kg. That is a density of 70.99 g/L (at 20 K).Liquid hydrogen requires cryogenic storage technology such as the special thermally insulated containers and requires special handling common to all cryogenic fuels. This is similar to, but more severe than Liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away. (Typically it will evaporate at a rate of 1% per day.[2]) It also shares many of the same safety issues as other forms of hydrogen, as well as being cold enough to liquefy atmospheric oxygen which can be an explosion hazard.

Hydrogen burns with a very high flame temperature but which emits light which is nearly invisible to the naked eye and is a safety hazard. Typical piston engines burning hydrogen in ambient air (not simply oxygen) produce high amounts of NOx pollution.

See also

Tank for liquid hydrogen of Linde, Museum Autovision, Altlußheim, Germany

References

  1. ^ Chemical elements data references
  2. ^ Liquid Air/LH2
  3. ^ William Cullen, Of the Cold Produced by Evaporating Fluids and of Some Other Means of Producing Cold, in Essays and Observations Physical and Literary Read Before a Society in Edinburgh and Published by Them, II, (Edinburgh 1756)
  4. ^ 1851 John Gorrie
  5. ^ 1851 Patent 8080
  6. ^ Hydrogen through the Nineteenth Century
  7. ^ Ortho-Para conversion. Pag. 13
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