Liquid hydrogen cargo transfer assisted by make-up boil-off gas compressor

Abstract

This paper proposes a liquid hydrogen (LH2) unloading system assisted by make-up boil-off gas (BOG) compressor and conducts a feasibility study. The cryogenic submerged pumps used in liquefied gas unloading have low efficiency and technical readiness, which transfer a significant amount of heat to cryogenic cargo, causing BOG. To address these issues, make-up BOG compressor-assisted unloading system is proposed. The unloading process modeling was based on mass and energy balance and feasibility study was conducted comparing pump-assisted unloading with compressor-assisted unloading. Furthermore, a sensitivity study was performed to identify factors affecting the unloading process. To evaluate the feasibility of the compressor-assisted unloading system, it was analyzed in terms of pressure, unloading mass and vaporization rate, and energy consumption of the pump and compressor. As a result of the compressor-assisted unloading, the carrier tank unloaded LH2 within the maximum allowable pressure of the tank, 400 kPa. The transferred LH2 mass and vaporization rate were comparable to pump-assisted unloading, and the power consumption was 16 – 33% of pump-assisted unloading. In addition, as a result of the sensitivity study, a substantial amount of make-up BOG reduced the vaporized gas in the carrier tank by 50%, and maintaining thermal stratification in the carrier tank showed the lowest vaporization rate. When the unloading time was reduced, the pressure drop in the pipeline increased, and the pressure of the carrier tank increased. In the case of high temperature of make-up BOG, the system was operated at a less amount of make-up BOG, but the vaporized gas in the carrier tank increased. As a result of LNG unloading, a pressure drop was 10 times higher than the LH2 unloading system, requiring the carrier tank to pressurize 370 kPa above. The vaporization rate was 1.4 times and compressor power consumption was 12 times higher than LH2 unloading, respectively.