Hydrogen liquefaction process with Brayton refrigeration cycle to utilize the cold energy of LNG

A thermodynamic process is investigated and designed for hydrogen liquefaction system to utilize the cold energy of liquefied natural gas (LNG). This study is an initial effort of newly launched five-year governmental project in Korea, aiming at efficient hydrogen liquefiers. Since Korea is a major LNG import country, the cold energy is abundantly available and could be useful in reducing the liquefaction cost. Hydrogen gas at ambient temperature is pre-cooled by LNG and then fed into a closed-cycle Brayton refrigerator. Rigorous thermodynamic analysis is carried out on the process with standard or modified Brayton cycles for the optimal condition to minimize the power consumption. It is revealed that LNG at atmospheric pressure is much more effective in pre-cooling than pressurized LNG (for pipeline distribution), because of the temperature pinch problem in heat exchanger. By taking into consideration the efficiency and other factors such as safety, compactness, and simplicity of operation, 2-stage expansion cycle with LNG pre-cooling is identified as most suitable for the pilot system with a capacity of 0.5 ton/day. Full details of liquefaction process are presented with the optional use of catalysts for ortho-para conversion.