100% saturated liquid hydrogen production: Mixed-refrigerant cascaded process with two-stage ortho-to-para hydrogen conversion

Abstract

To reduce CO2 emissions and address climate change concerns, most futuristic studies investigating 100% renewable energy sources and subsequent power-to-gas/fuel/liquid/X technological developments have been based on hydrogen (H-2). The long-term storage and transportation of H-2 over long distances restrict its feasibility as an energy vector, mainly due to its low energy density. Liquefaction is a promising approach for overcoming these issues. However, it requires a large amount of energy, and if H-2 itself is used to provide this energy, then 25% to 35% of the initial quantity of H-2 is consumed. The existing H-2 liquefaction plants have specific energy consumption values in the range of 10-12 kWh/kg(LH2) and exergy efficiencies in the range of 20%-30% with complicated configurations. Therefore, a thermodynamically efficient and compact design is required to facilitate a roadmap to H-2 economy. This paper proposes a simple, energy-efficient, and cost-effective process for H-2 liquefaction. Three refrigeration cycles with optimal mixed-refrigerant compositions are used, which makes the proposed process energy-efficient. Additionally, two-stage ortho-to-para conversion makes the process compact. The proposed process is unique in terms of its configuration and mixed-refrigerant combination. The modified coordinate descent approach was adopted to identify the optimal design variables for the proposed H-2 liquefaction process. The proposed process consumes an energy of 6.45 kWh/kg(LH2), which is 36.5% and 16.1% lower than that consumed by the base design of the proposed process and a published base case, respectively. Additionally, the exergy efficiency of the proposed process is 47.2%. This study will help process engineers achieve a sustainable green economy by improving the competitiveness of H-2 storage and transportation over long distances.