Mass production of slush hydrogen.
The major production methods of slush hydrogen are spray, freeze-thaw, and auger [4, 10, 11].
In the spray method as shown in the photo below, a nozzle is used for the adiabatic expansion of liquid hydrogen to produce solid particles. However, the particles produced in this manner tend to melt when mixed into liquid hydrogen, making this method unsuitable for the efficient mass production of slush.
The freeze-thaw method is a relatively straightforward method, and applicable at the experimental level or for small- to medium-scale production.
A vacuum pump is used for pressure reduction of liquid hydrogen at the normal boiling point pressure. As the liquid boils and vaporizes in accordance with the vapor-liquid equilibrium curve, the latent heat is removed and the temperature of the liquid is reduced. When the liquid reaches the triple point pressure, a frozen layer is formed on the liquid surface. When evacuation is stopped, part of the frozen layer melts and solid hydrogen sinks into the liquid. A stirrer is then used to break up the solid into small particles having a diameter of several mm. Slush hydrogen is produced by performing the freeze-thaw cycle repeatedly.
The slush hydrogen and slush nitrogen used for the experimental work are produced using the freeze-thaw method. For slush nitrogen, particle sizes in our experiments are in the range from 0.5 to 2 mm, with an average size of 1.36 mm.
In the auger method, as shown in the above figure, solid hydrogen formed on the heat transfer surface due to heat exchange between liquid hydrogen and lower-temperature gas helium is scraped off by rotating blades known as an auger, to produce fine particles of solid hydrogen. Since the heat exchanger and auger are immersed in the liquid hydrogen, the solid hydrogen particles are mixed into the liquid during the production process.
If cryogenic gas helium and liquid hydrogen are continuously supplied, slush hydrogen can also be continuously produced, making this method suitable for mass production. By adjusting the gap between the auger and heat exchanger surface and/or changing the rotational speed of the auger, the size of solid particles can be controlled.
In our small-scale production experiment as shown in the figure below, at auger speeds from 30 to 80 rpm, solid hydrogen production increases with speed, and the particle size becomes smaller. The maximum solid production rate of 0.062 g/s is obtained at an auger speed of 80 rpm (two blades), equivalent to 5.5 l/h of slush hydrogen with a mass solid fraction of 50 wt.%.
At present, large-scale production experiments have yet to be carried out. A combination of hydrogen liquefier using the helium Brayton cycle and the auger method is already illustrated in the figure of “High-efficiency hydrogen energy system using slush hydrogen”, resulting in the continuous mass production of slush hydrogen.
To achieve practical application of the auger method for high-efficiency mass production, improved performance of the heat exchanger and long-term reliability of the auger mechanism rotating at cryogenic temperatures are necessary.
Production process of slush hydrogen by spray and auger methods.
The spray method is unsuitable for the efficient mass production of slush hydrogen.