Thermodynamic Design of Hydrogen Liquefaction System with LNG Pre-cooled Brayton Refrigerator

Abstract

A thermodynamic study is carried out for the design of a hydrogen liquefaction systems to employ the helium (He) Brayton refrigerator pre-cooled by Liquefied Natural Gas (LNG). The He Brayton refrigerator is a good option for small-scale liquefaction of hydrogen to take advantage of simple and safe (low-pressure) operation, but typically requires the pre-cooling by liquid nitrogen (LN2) in order to achieve a reasonably high efficiency. It is proposed in this study that LNG could be effectively used as pre-coolant, replacing LN2 in existing systems. This attempt is motivated by the increased usage of LNG for domestic power generation as well as the increased demand of liquid hydrogen. Rigorous thermodynamic cycle analysis is conducted for the hydrogen liquefaction process with He Brayton refrigeration and LNG pre-cooling. The real properties of fluids are incorporated into a process simulator (Aspen HYSYS) to calculate the FOM (Figure of Merit) of liquefaction. The optimal conditions are found to maximize the overall the thermodynamic performance for the purpose of utilizing the cold energy of LNG. Full details of optimized cycle are presented in terms of itemized irreversibility in each component, and the proposed cycle is compared with the existing liquefaction systems.