Systematic thermodynamic approach for designing mixed refrigerants used in hydrogen precooling process

Abstract

Designing an optimal mixed-refrigerant (MR) composition for any cryogenic refrigeration process plays a key role in the process overall performance. However, the existing approaches of developing mixed-refrigerants for these processes are difficult, time-consuming, and semi-random trial and error approaches. Therefore, for the first time, this study presents a systematic thermodynamic approach to optimally design mixed-refrigerants for any cryogenic process. In particular, the developed approach is conducted for the hydrogen precooling process (HPP) with a precooling temperature less than −193 °C. This approach consists of four criteria and seven major steps needed for the development of an energy-efficient HPP. Using this approach, 15 new mixed-refrigerants are created, analyzed, and compared based on the specific energy consumption (SEC) and coefficient of performance of the HPP. Further sensitivity analyses for the major operating conditions of the HPP are conducted for the best five cases. A minimum SEC of 1.03 kWh/khH2Feed at wet-cooling conditions and of 1.32 kWh/khH2Feed at dry-cooling conditions are achieved, which is remarkably 78.81% lower than the SEC of the existing HPP in commercial plants. This study also found that eight-component MR is the best option to create optimal mixtures for cryogenic cooling processes with perfect match between the composite curves of all heat exchangers of the HPP. While methane, nitrogen, and hydrogen components are essential for the MR to achieve energy-efficient HPP, the use of ethylene instead of ethane, H2 instead of neon, and R14 instead of NH3 is recommended to optimize the SEC.