Liquid hydrogen.html


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. 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

1 History
2 Spin isomers of hydrogen
3 Uses
4 Advantages
5 Drawbacks
6 See also
7 References

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

It is a common liquid rocket fuel for rocket applications. In most rocket engines fueled by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the oxidizer (usually liquid oxygen (LOX)) and burned to produce water with traces of ozone and hydrogen peroxide. Practical H2/O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust that can actually increase specific impulse despite the incomplete combustion.

Liquid hydrogen can be used as the fuel storage for 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.

NFPA 704 placard for stationary hydrogen facilities

Liquid hydrogen tank in the BMW Hydrogen 7.

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, however see "drawbacks" below.

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.

References

^ Chemical elements data references
^ Liquid Air/LH2
^ 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)
^ 1851 John Gorrie
^ 1851 Patent 8080
^ Hydrogen through the Nineteenth Century
^ Ortho-Para conversion. Pag. 13