Do you ever wonder how hydrogen can be liquefied? Hydrogen is the first element in the Periodic Table, but it has the second lowest boiling point temperature of -423 F. Do you ever wonder who has the lowest boiling point temperature? The answer is Helium, the 2nd element in the Periodic Table. How about the third and the fourth lowest boiling point? They are Neon and Nitrogen. Because of the low boiling temperature, so Helium, Neon and Nitrogen are commonly used to liquefy hydrogen. In today's video, a helium Brayton cycle and liquid nitrogen are used to liquefy hydrogen.
Compressed hydrogen is first introduced to the main cryogenic heat exchanger. Then it is withdrawn to mole sieve adsorbers to remove heavy components that may freeze later. It is further cooled down against helium. and eventually it is liquefied to a state of liquid at atmospheric pressure.
Compressed helium gas enters the main heat exchanger at ambient temperature. Then it expands at an intermediate temperature and further expands at even lower temperature. The 2nd expander achieves the lowest temperature in the system and it is used to liquefy hydrogen. After helium is warmed up in the exchanger, it is sent to the compressor and coolers to form a closed loop.
Liquid nitrogen is used to precool both helium and hydrogen. It is vented to atmosphere after it is warmed up to ambient temperature.
For hydrogen liquefaction, one key parameter to watch out is the temperature approach. On this graph, the X axis is temperature, while the Y axis is the temperature difference between hot and cold fluids. As you can see, there are 4 points that reaches the minimum temperature of about 2.5 F, so it is considered as an optimum design.
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