Slush hydrogen is a combination of
liquid hydrogen and
solid hydrogen at the
triple point with a lower temperature and a higher density than
liquid hydrogen. It is commonly formed by repeating a freeze-thaw process.
This is most easily done by bringing liquid hydrogen near its boiling point and then reducing pressure using a vacuum pump. The decrease in pressure causes the liquid hydrogen to vaporize/boil - which removes latent heat, and ultimately decreases the temperature of the liquid hydrogen. Solid hydrogen is formed on the surface of the boiling liquid (between the gas/liquid interface) as the liquid is cooled and reaches its triple point. The vacuum pump is stopped, causing an increase of pressure, the solid hydrogen formed on the surface partially melts and begins to sink. The solid hydrogen is agitated in the liquid and the process is repeated. The resulting hydrogen slush has an increased density of 16–20% when compared to liquid hydrogen.
[Christopher P. McKeehan, Terry L. Hardy, Margaret V. Whalen, Maureen T. Kudlac, Matthew E. Moran, Thomas M. Tomsik and Mark S. Haberbusch (April 1995). A summary of Slush hydrogen. NASA] It is proposed as a
rocket fuel in place of liquid hydrogen in order to use smaller fuel tanks and thus reduce the
dry weight of the vehicle.
[ Density . Astronautix.com. Retrieved on 2012-12-29.]
Production
The continuous freeze technique used for slush hydrogen involves pulling a continuous vacuum
over triple point liquid and using a solid hydrogen mechanical ice-breaker to disrupt the surface of the freezing
hydrogen.
[Mark S. Haberbusch and Nancy B. McNelis (1996). Comparison of the continuous freeze slush hydrogen production. NASA Technical Memorandum 107324. Retrieved on 2012-12-29.][R. O. Voth (February 1978). Producing Liquid-Solid Mixtures of Hydrogen Using an Auger. Cryogenics Division. Institute for Basic Standards National Bureau of Standards, Boulder, Colorado (report for NASA). Retrieved on 2012-12-29.][A.S. Rapial and D.E. Daney (May 1969). 1966 – Preparation and characterization of slush hydrogen and nitrogen gels. Cryogenics Division. Institute for Basic Standards National Bureau of Standards, Boulder, Colorado (report for NASA). Retrieved on 2012-12-29.]
See also
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Compressed hydrogen
-
Hydrogen safety
-
Metallic hydrogen
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Timeline of hydrogen technologies
-
Liquefaction of gases
