Hydrogen liquefaction is a significantly energy-intensive process. Compared to established liquefied energy commodities, such as LNG, the process requires deeper cryogenic temperature and, uniquely, is subject to additional heat load associated with exothermic ortho-to-para hydrogen isomer conversion. The latter challenge is currently the subject of substantial fundamental and applied research activity, as isomer conversion in the presence of materials (catalysts) that accelerate the process is not well understood.
The Objective
This project aimed to contribute to the understanding of cryogenic hydrogen isomer conversion in the context of industrial hydrogen liquefaction, as a foundation for higher-fidelity simulation and accurate process design, through a sequence of literature review, simulation and modelling, and experimental characterisation of the cryogenic hydrogen ortho-para (OP) isomer conversion.
Project Findings
The project began with reviewing the current state-of-knowledge, through a detailed literature review of ortho-para hydrogen conversion studies, compilation of a comprehensive database, and assessment of the suitability of kinetic models for predicting catalyst performance. The review encompassed data for a range of catalyst materials, with an emphasis on hydrous ferric oxide (HFO) materials, and assessed the quality of experimental data from each source. This highlighted that the available data is limited in coverage and identified a lack of consistency in the measurement methodology and reporting of data. Of the kinetic models assessed, it was found that a first-order kinetic model was sufficient, although the Langmuir-Hinshelwood model would be preferred if sufficient reference-quality data becomes available.
A series of experimental kinetic measurements were conducted of OP conversion for a commercial HFO catalyst. For this, a dedicated experimental apparatus was developed incorporating cryogenic hydrogen flow control for pressures up to 4 MPa and temperatures down to 40 K, with Raman spectroscopy measurement of OP isomer composition. A wide-ranging data set was acquired over conditions that would be expected for hydrogen liquefaction heat exchanger operation. Additionally, the role of catalyst activation conditions and trace impurity specification on catalyst performance were experimentally investigated over time, to establish requirements for process specification.
Commercial process simulation software packages are not able to account for the effect of temperature-dependent hydrogen OP conversion. To address this, a custom 2D model was developed to quantify the effect of OP conversion on the performance of a catalyst-lined heat exchanger. Utilising this model, mathematical functions were established for key parameters related to hydrogen liquefaction process flow conditions. When implemented within commercial process simulation software, this enables the thermodynamic contribution of hydrogen OP isomer conversion to be included in the evaluation of hydrogen liquefaction cycle design.
Next Steps
A key finding from this project is the importance of managing the condition of the catalyst employed for hydrogen OP conversion, beginning with appropriate activation procedure and continuing with monitoring and control of feed-gas impurities. Even at trace concentrations, the low temperatures of hydrogen liquefaction will result in accumulation of impurities within cryogenic process units and deactivation of catalysts. A suggested area of future research is development of low-cost sensors for inline monitoring of cryogenic hydrogen isomer composition and real-time process control.
Project Researchers
- Dr. Paul L. Stanwix
- Dr. Einar O. Fridjonsson
- Dr. Gongkui Xiao
- Ms. Guinevere M. Sellner
Project Status
Complete
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