Liquid hydrogen
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| Names | |||
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| IUPAC name
Hydrogen
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| Systematic IUPAC name
Liquid hydrogen | |||
| Other names
Hydrogen (cryogenic liquid), Refrigerated hydrogen; LH2,para-hydrogen
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| Identifiers | |||
3D model (JSmol)
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| ChEBI | |||
| ChemSpider | |||
| KEGG | |||
PubChemCID
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| RTECS number |
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| UNII | |||
| UN number | 1966 | ||
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| Properties | |||
| H2(l) | |||
| Molar mass | 2.016g·mol−1 | ||
| Appearance | Colorless liquid | ||
| Density | 0.07085 g/cm3(4.423 lb/cu ft)[1] | ||
| Melting point | −259.14 °C (−434.45 °F; 14.01 K)[2] | ||
| Boiling point | −252.87 °C (−423.17 °F; 20.28 K)[2] | ||
| Hazards | |||
| GHSlabelling:[3] | |||
 
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| Danger | |||
| H220,H280 | |||
| P210,P377,P381,P403 | |||
| NFPA 704(fire diamond) | |||
| 571 °C (1,060 °F; 844 K)[2] | |||
| Explosive limits | LEL 4.0%; UEL 74.2% (in air)[2] | ||
Except where otherwise noted, data are given for materials in theirstandard state(at 25 °C [77 °F], 100 kPa).
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Liquid hydrogen(H2(l)) is theliquid stateof the elementhydrogen.Hydrogen is found naturally in themolecularH2form.[4]
To exist as a liquid, H2must be cooled below itscritical pointof 33K.However, for it to be in a fully liquid state atatmospheric pressure,H2needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F).[5]A common method of obtaining liquid hydrogen involves acompressorresembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form ofhydrogen storage.Storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid for some time in thermally insulated containers.[6]
There are twospin isomers of hydrogen;whereas room temperature hydrogen is mostly orthohydrogen, liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.[5]
Hydrogen requires a theoretical minimum of 3.3 kWh/kg (12 MJ/kg) to liquefy, and 3.9 kWh/kg (14 MJ/kg) including converting the hydrogen to the para isomer, but practically generally takes 10–13 kWh/kg (36–47 MJ/kg) compared to a 33 kWh/kg (119 MJ/kg) heating value of hydrogen.[7]
History
[edit]
In 1885,Zygmunt Florenty Wróblewskipublished hydrogen's critical temperature as 33 K (−240.2 °C; −400.3 °F); critical pressure, 13.3 standard atmospheres (195 psi); and boiling point, 23 K (−250.2 °C; −418.3 °F).
Hydrogenwas liquefied byJames Dewarin 1898 by usingregenerative coolingand his invention, thevacuum flask.The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen, was achieved byPaul HarteckandKarl Friedrich Bonhoefferin 1929.
Spin isomers of hydrogen
[edit]The two nuclei in a dihydrogen molecule can have two differentspinstates. Parahydrogen, in which the twonuclear spinsare antiparallel, is more stable than orthohydrogen, in which the two are parallel. At room temperature, gaseous hydrogen is mostly in the ortho isomeric form due to thermal energy, but an ortho-enriched mixture is onlymetastablewhen liquified at low temperature. It slowly undergoes anexothermic reactionto become the para isomer, with enough energy released as heat to cause some of the liquid to boil.[8]To prevent loss of the liquid during long-term storage, it is therefore intentionally converted to the para isomer as part of the production process, typically using acatalystsuch asiron(III) oxide,activated carbon,platinized asbestos, rare earth metals, uranium compounds,chromium(III) oxide,or some nickel compounds.[8]
Uses
[edit]Liquid hydrogen is a commonliquidrocket fuelforrocketryapplication and is used byNASAand theU.S. Air Force,which operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons).[9]
In mostrocket enginesfueled by liquid hydrogen, it firstcoolsthe nozzle and other parts before being mixed with the oxidizer, usuallyliquid oxygen,and burned to produce water with traces ofozoneandhydrogen peroxide.Practical H2–O2rocket 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, which can increasespecific impulse,despite the incomplete combustion.
Liquid hydrogen can be used as the fuel for aninternal combustion engineorfuel cell.Various submarines, including theType 212 submarine,Type 214 submarine,and others, and concepthydrogen vehicleshave been built using this form of hydrogen, such as theDeepC,BMW H2R,and others. Due to its similarity, builders can sometimes modify and share equipment with systems designed forliquefied natural gas(LNG). Liquid hydrogen is being investigated as azero carbon fuelforaircraft.Because of the lowervolumetric energy,the hydrogen volumes needed for combustion are large. Unlessdirect injectionis 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 inneutron scattering.Since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision). Finally, superheated liquid hydrogen was used in manybubble chamberexperiments.
The firstthermonuclear bomb,Ivy Mike,used liquiddeuterium,also known as hydrogen-2, for nuclear fusion.
Properties
[edit]The product of hydrogen combustion in a pure oxygen environment is solely water vapor. However, the high combustion temperatures and present atmospheric nitrogen can result in the breaking of N≡N bonds, forming toxic NOx if no exhaust scrubbing is done.[10]Since water is often considered harmless to the environment, an engine burning it can be considered "zero emissions". In aviation, however, water vapor emitted in the atmosphere contributes toglobal warming(to a lesser extent than CO2).[11]Liquid hydrogen also has a much higherspecific energythan gasoline, natural gas, or diesel.[12]
The density of liquid hydrogen is only 70.85 kg/m3(at 20K), arelative densityof just 0.07. Although the specific energy is more than twice that of other fuels, this gives it a remarkably low volumetricenergy density,many fold lower.
Liquid hydrogen requirescryogenicstorage technology such as special thermally insulated containers and requires special handling common to allcryogenic fuels.This is similar to, but more severe thanliquid oxygen.Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically at a rate of 1% per day[12]). It also shares many of the samesafety issuesas other forms of hydrogen, as well as being cold enough to liquefy, or even solidify atmospheric oxygen, which can be an explosion hazard.
Thetriple pointof hydrogen is at 13.81 K[5]and 7.042 kPa.[13]
Safety
[edit]Due to its cold temperatures, liquid hydrogen is a hazard forcold burns.Hydrogen itself is biologically inert and its only human health hazard as a vapor is displacement of oxygen, resulting in asphyxiation, and its very high flammability and ability to detonate when mixed with air. Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended. Unlike ambient-temperature gaseous hydrogen, which is lighter than air, hydrogen recently vaporized from liquid is so cold that it is heavier than air and can form flammable heavier-than-air air–hydrogen mixtures.
See also
[edit]References
[edit]- ^Thermophysical Properties of Hydrogen,nist.gov, accessed 2012-09-14
- ^abcdInformation specific to liquid hydrogenArchived2009-07-17 at theWayback Machine,harvard.edu, accessed 2009-06-12
- ^GHS:GESTIS 007010
- ^"We've Got (Rocket) Chemistry, Part 1".NASA Blog.15 April 2016.Retrieved3 October2021.
- ^abcIPTS-1968,iupac.org, accessed 2020-01-01
- ^"Liquid Hydrogen Delivery".Energy.gov.Retrieved2022-07-30.
- ^Gardiner, Monterey (2009-10-26).DOE Hydrogen and Fuel Cells Program Record: Energy requirements for hydrogen gas compression and liquefaction as related to vehicle storage needs(PDF)(Report). United States Department of Energy.
- ^ab"Liquefaction of" Permanent "Gases"(PDF of lecture notes).2011.Retrieved2017-10-16.
- ^Flynn, Thomas (2004).Cryogenic Engineering, Second Edition, Revised and Expanded.CRC Press. p. 401.ISBN978-0-203-02699-1.
- ^Lewis, Alastair C. (2021-07-22)."Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NOx emissions".Environmental Science: Atmospheres.1(5): 201–207.doi:10.1039/D1EA00037C.ISSN2634-3606.S2CID236732702.
- ^Nojoumi, H. (2008-11-10). "Greenhouse gas emissions assessment of hydrogen and kerosene-fueled aircraft propulsion".International Journal of Hydrogen Energy.34(3): 1363–1369.doi:10.1016/j.ijhydene.2008.11.017.
- ^abHydrogen As an Alternative FuelArchived2008-08-08 at theWayback Machine.Almc.army.mil. Retrieved on 2011-08-28.
- ^Cengel, Yunus A. and Turner, Robert H. (2004).Fundamentals of thermal-fluid sciences,McGraw-Hill, p. 78,ISBN0-07-297675-6