Production of slush hydrogen

Most slush hydrogen has been produced so far by means of the freeze thaw method [22]. In this technique the gaseous phase above the liquid hydrogen is pumped to pull a vacuum until the liquid is cooled down below the freezing point (13.8 K) and a film of solid hydrogen forms on the surface. The system pressure is then stopped and the solid 

The time of the freezing period can be extended by spraying triple point liquid hydrogen onto the surface, the spray / freeze method, which could be a step towards a continuous process with no further need of a thawing period [40]. 

A Small Scale Slush Hydrogen Facility has been constructed by NASA to study ways of optimizing the SLH2 production process and provide a test bed for advanced instrumentation. The tank with a total volume of 0.76 is vacuum jacketed and wrapped in several inches multilayer insulation plus a liquid nitrogen shield in the upper tank part. A 1 kW heater is used to simulate a heat leak or a warming up. The tank also offers the option to install a liquid helium slush auger for research on this method of production. In a 17 m long and 0.05 m diameter vacuum jacketed transfer line, studies on flow characteristics and instrumentation can be conducted [40]. 

A smaller slush hydrogen production system has been constructed in Japan with a volume of 60 liters. Both the freeze-thaw and the auger production method have been investigated revealing the creation of finer fractions with a higher fluidity with the latter method [61]. 

Hydrogen Production, Tutorial during the 10th World Hydrogen Energy Conf., Cocoa Beach, USA, June 19, 1994. 

---