H2 Liquefier Optimization

The control and operation of H2 liquefiers is an old and well-established technology, which will not be discussed in detail. Instead, the emphasis will be placed on its optimization through the recovery of compressor energy in expander turbines and through optimized compressor and temperature controls. 

The evaporation of liquid nitrogen (LN2) in HE-1 brings down the H2 temperature below its inversion point. In the process of liquefaction, one must consider this inversion point temperature (-5°C, or -95°F), because the behavior of H2 changes (reverses) at this point. Below the inversion temperature, when the pressure is reduced, the temperature of H2 will drop. Above that temperature the opposite occurs a drop in pressure causes a rise in temperature. 

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After the H2 has been cooled to below its inversion temperature, it enters HE-2 for further cooling. HE-2 serves to condense some of the total H2 feed (the F23 portion of the total F25). F23 is shown by the heavy line. The cooling required to condense F23 is provided by evaporating some of the LH2 (F27) in the evaporator pipe in HE-2. 

If the energy of compression is not recovered in the heat pumps, the liquefaction efficiency will be low (35-60%). If the letdown valves are replaced by turboexpanders (ETG-2 and ETG-3), which will recover some of the compression energy during pressure letdown, and if helium or neon refrigerants are used, the liquefaction efficiency can theoretically reach 80% (see Table 1.46). 

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