Hydrogen is considered a key energy source that is becoming increasingly popular in today’s energy-conscious world. It must be in liquid form for both transport and use.
At 298 K, molecular hydrogen comprises 75 % orthohydrogen and 25 % parahydrogen. Hydrogen must be converted from the ortho to the para form for energy efficiency and to prevent boiloff. This conversion is typically carried out at the boiling point of hydrogen, 20 K, using an iron oxide catalyst.
At the standard boiling point, hydrogen exists in its thermodynamically preferable para-state. Raman spectroscopy is the ideal analytical technique for monitoring this low-temperature conversion process. Raman analyzers are becoming increasingly popular in the analysis of industrial processes such as hydrogen production due to their rapid measurement capability and the devices’ stability. In addition, it’s advantageous that data analysis can be streamlined to peak integration routines for accurate compositional monitoring such as liquid hydrogen production.
Introduction
Hydrogen is considered a key player in a move toward a decarbonized energy future. Reliable transportation and storage of liquid hydrogen are pivotal components for its widespread use. Ensuring safety and efficiency in its liquefaction and storage is vital to making it a viable energy solution.
Importantly, being able to determine the best liquefaction methods and transport/storage technologies involves analyzing multiple factors, including energy use, material choices, and the handling of liquid hydrogen’s unique properties, such as its temperature-influenced ortho-para hydrogen conversion and its potential to create boil-off gas.1
Hydrogen exists as two isomers, ortho and para, based on proton nuclear spin. A temperature-dependent equilibrium dictates their ratio, with a 75% ortho isomer makeup at room temperature shifting to nearly 100% para at liquid hydrogen temperatures (20 K).
This transition releases a considerable amount of energy, influencing liquefaction and storage processes. Due to quantum effects, the ortho-to-parahydrogen transition needs an external catalyst. Otherwise, it is slow, taking days or weeks if cooled without one. This exothermic conversion releases more heat (525 kJ/kg) than hydrogen’s vaporization enthalpy at its boiling point (448 kJ/kg), which affects storage strategies.1,2
Ensuring the efficiency of ortho-to-parahydrogen conversion is critical for hydrogen liquefaction and storage on an industrial scale. Rapid liquefaction that does not allow for adequate conversion times can result in excessive boil-off gas and potential over-pressurization of storage tanks, presenting safety risks. Moreover, the slow conversion process presents a considerable barrier to long-term storage, as the heat generated can evaporate over 70 % of the stored liquid hydrogen.1
Conversely, boil-off can be reduced by reversing the reaction where parahydrogen converts back to orthohydrogen as this endothermic process can further cool the liquid hydrogen. This conversion plays a key role in maintaining the cryogenic temperature of stored hydrogen, improving stability and efficiency during storage.2
To speed up the conversion process, catalyst materials are incorporated into the design of industrial hydrogen liquefaction systems; this makes it possible to convert from one state to another within minutes. However, there is an urgent need for detailed data on the reaction rates unique to these catalysts to improve the accuracy of kinetic models used in reactor design and to determine the most effective reactor configurations.3
In recent years, Raman spectroscopy has seen significant advancements and become a powerful analytical technique in various industrial applications. Advancements in lasers, solid-state hardware, and fiber optics have played a key role in facilitating these developments.
Raman spectroscopy, specifically in process analytical technology (PAT) applications, has seen a considerable increase in use. Its ease of interface, non-destructive sampling, and quick cycle times have enabled successful integration into diverse application areas such as biopharma, oil and gas, petrochemical, and fermentation sectors.
The expanded use of Raman spectroscopy in industrial applications comes from combining it with chemometric methods, which has contributed to its versatility and effectiveness in various fields. As an additional benefit, minimal adjustments are required when employing Raman spectroscopy in workflows for in-line measurements.
This study demonstrates the use of a process Raman analyzer conducting in-line compositional analysis to determine the ortho/ para ratio and the rate of conversion between the hydrogen isomers under different experimental conditions.
Common Ortho-Para Measurement Techniques
Raman spectroscopy, NMR spectroscopy, and thermal conductivity measurements are key techniques used for detecting ortho and para hydrogen. Raman spectroscopy stands out for this purpose due to its capability to deliver relative measurements of the isomer ratios without the need for calibration, along with its online measurement capabilities.2
However, questions about its long-term performance in cryogenic conditions and sensitivity to different pressures remain. NMR spectroscopy can directly detect orthohydrogen (parahydrogen is NMR invisible) but requires calibration and can face challenges with signal-to-noise ratios at lower temperatures. However, certain sophisticated techniques can improve parahydrogen signals.4
Thermal conductivity measurements take advantage of the significant thermal conductivity differences between the isomers but are limited by pressure and temperature conditions, and are unsuitable for liquid hydrogen.5 The rapid analysis times and easy-to-use sample interface (utilizing a high-pressure at low-temperature flow cell) make Raman spectroscopy ideal for monitoring the conversion of ortho to para hydrogen.
The Raman spectrum of hydrogen includes specific peaks that relate to the ortho and para isomers of the diatom. Because Raman peaks scale linearly with concentration, it is possible to track the peak areas of the specific ortho and para peaks and use them to calculate the compositional ratio. A relatively straightforward background correction and baseline correction boost the accuracy of the determined peak areas and subsequent compositional ratios.
Experimental
All spectra were acquired using a Thermo Scientific™ MarqMetrix™ All-In-One Process Raman Analyzer. The data-collection parameters were set to collect a new dark-subtracted spectrum every 10 seconds. The spectra were obtained using a Thermo Scientific™ MarqMetrix™ FlowCell Sampling Optic (Figure 1) rated to 2500 psi and a temperature range from cryogenic temperatures to 350 K.
Once the data was collected, Solo 9.2.1 (Eigenvector Research, Inc., Manson, WA) was employed for processing. Background removal was conducted using a spectrum of helium, followed by trend analysis that accounts for baseline variation. Lastly, normalization was carried out to account for variation in pressure and flow. To generate real-time determinations of the ortho and para content of the liquid hydrogen, the resultant model files were transferred to the Raman instrument and were deployed using Solo_Predictor 4.3 (Eigenvector Research, Inc., Manson, WA) (Figure 2).
Figure 1. Integrated high pressure low temperature flow cell. Image Credit: Thermo Fisher Scientific
Figure 2. Progression of data analysis. Image Credit: Thermo Fisher Scientific
In this study, hydrogen gas was initially cooled in a heat exchanger immersed in liquid nitrogen and then fed through a packed-bed reactor containing a commercially available iron-based catalyst. This process enabled the conversion of orthohydrogen to parahydrogen, altering its equilibrium from 75 % ortho at room temperature to 50 %, effectively balancing the ortho-para ratio. Temperature, pressure, and flow rate were regulated and monitored.
A Raman probe was deployed downstream of the reactor to measure the ortho- and parahydrogen concentrations in real-time, delivering immediate insights into the efficiency of the conversion process.
Results and Discussion
Raman spectroscopy was effectively used to measure the catalytic conversion of orthohydrogen to parahydrogen. The success of the outcome was due in part to the integrated flow cell sampling optic used to take the measurements. The flow cell employed was particularly compact as it was designed for in-line utilization, and connected via compression fittings. This configuration enabled a simple “plug-and-play” installation. It also ensured a leak-free operation, eliminating the need for the user to adjust or address potential complications related to the laser’s focal points or other adjustments.
The experiments performed with the flow cell in this study were conducted at an ambient temperature. Nevertheless, it is possible to submerge the flow cell in liquid nitrogen (77 K) or operate it in conjunction with a cryocooler to reach temperatures as low as 20 K, presuming the probe stem is adequately long enough and thermally regulated to prevent damage to the electronic components.
The spectra acquired using the flow cell displayed clear transitions of ortho- to parahydrogen. The signal-to-noise ratio was outstanding, facilitating clear resolution and discrimination of the relevant liquid hydrogen peaks (Figure 3). The corresponding rotational states (J) and zero vibrational transition states (S) for a measured hydrogen spectrum are shown in Figure 3.
The quantified Raman shifts, in relation to the parahydrogen peaks, are recorded at 354 and 814 cm-1, whereas the orthohydrogen peaks can be seen at 587 and 1034 cm-1, respectively. These findings remain consistent with the hydrogen Raman spectra delineated in the literature2, confirming that the measured shifts are accurate and reliable within the context of established spectroscopic data.
The instrument sensitivity facilitated a high degree of resolution of subsequent samples and precise monitoring of the catalytic conversions. Utilizing these signals, several experiments were conducted to monitor various factors including conversion efficiency and kinetic rates. The applied chemometric methods did not necessitate any additional calibration other than the acknowledgment that at room temperature, the ratio of ortho to parahydrogen is around 3:1.
Figure 3. Spectrum of normal hydrogen taken at room temperature and 1000 psi pressure after background removal. Image Credit: Thermo Fisher Scientific
Conclusion
This study demonstrates that Raman spectroscopy is an ideal spectroscopic technique for monitoring the ortho-to-parahydrogen conversion at cryogenic temperatures. Rapid data acquisition rates, excellent signal-to-noise performance, and resolution make it possible to acquire relevant information on conversion rates, catalyst efficiency, and performance. By eliminating background interferants, especially the 578 cm-1 peak, very small changes in peak height and area can be confidently monitored, resulting in extremely accurate conversion rates.
The cryogenic flow cell facilitates measurement in liquid hydrogen at 20 K, further boosting precision, accuracy, and analysis speed due to the higher density of the liquid phase compared to the gas phase. It is anticipated that analysis times will be reduced from 10 seconds to 200 milliseconds, ushering in an era of catalysis research using extremely fast-acting materials. This will allow significant volumes of hydrogen to undergo 100 % conversion to the parahydrogen form in a short amount time, ensuring efficient storage and transportation of the liquid form with minimal losses due to boil-off.
The success of this study further paves the way for other arenas that require the accurate determination of hydrogen and its isotopes, such as reducing carbon emissions by blending considerable levels of hydrogen into natural gas fuel for power generation or monitoring hydrogen/deuterium feed to a future fusion reactor for power generation.
References and Further Reading
- Al Ghafri, S.Z. et al., Hydrogen liquefaction: a review of the fundamental physics, engineering practice and future opportunities. Energy Environ. Sci., 2022, 15, 2690-2731.
- Bunge, C., A High-Fidelity, Continuous Ortho-Parahydrogen Measurement and Conversion System. PhD Thesis (2021), Washington State University.
- Ilisca, E. and F. Ghiglieno, Nuclear conversion theory: molecular hydrogen in non-magnetic insulators. Royal Society Open Science, 2016. 3(9): p. 160042.
- Krasae-in, S. et al., Development of large-scale hydrogen liquefaction processes from 1898 to 2009. International Journal of Hydrogen Energy, 2010. 35(10): p. 4524-4533.
- Zhou, D. et al., Determination of the Ortho-Para Ratio in Gaseous Hydrogen Mixtures. Journal of Low Temperature Physics, 2004. 134(1): p. 401-406.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Handheld Elemental & Radiation Detection.
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