Ortho-para Hydrogen Conversion Research Articles

Comparative energy and exergy analysis of ortho-para hydrogen and non-ortho-para hydrogen conversion in hydrogen liquefaction

This study reports the comparative energy and exergy analysis of ortho-para hydrogen and non-ortho-para hydrogen conversion in hydrogen liquefaction process. Two cases were simulated, case A – hydrogen liquefaction with ortho-parahydrogen conversion and case B – hydrogen liquefaction without ortho-parahydrogen conversion. This is the first study that presents a comparative energy and exergy analysis between such two cases. In this research, a hydrogen liquefaction process was designed adopting cascaded five-stage Brayton refrigeration cycle. The process was simulated in Aspen PLUS. The process used a mixed refrigerant (of liquefied natural gas) refrigeration cycle to precool the gaseous hydrogen feed from 26 °C temperature to −192 °C temperature, and mixed refrigerant (of nelium) was subsequently used to further deep-cool the the hydrogen stream from −192 °C temperature to −245.99 °C temperature in the cryogenic section of the process. Liquefaction was achieved by expanding the hydrogen through Joule-Thomson valve at −248.37 °C and 1 bar. The simulated results of the two cases showed the specific energy consumption of case A to be 8.45 kWhr/kgLH, and that of case B to be 15.65 kWhr/kgLH respectively. The results also indicated a total exergy efficiency of 92.42% in case A and 87.18% in case B. The research results showed that the hydrogen liquefaction designed with configuration of ortho-parahydrogen conversion has better performance indicators than the liquefaction without ortho-parahydrogen conversion. Therefore, hydrogen liquefaction with ortho-parahydrogen conversion can be considered in the design and development of new hydrogen liquefaction plants. Process optimization is recommended to further enhance the specific energy consumption and exergy efficiency of both processes.

Study on the flow and heat transfer characteristics of gaseous hydrogen in heat exchange tubes coupled with ortho-para hydrogen conversion

Hydrogen as a clean energy to deal with carbon emissions has been favored by many scholars, while the problems of low efficiency and high energy consumption in the process of liquefaction storage and transportation have become one of the important factors restricting the development of it. The combination of heat exchanger and ortho-para (O–P) hydrogen converter has attracted considerable attention as a high efficiency and low energy consumption technology. In this paper, based on the catalyst-filled spiral wound heat exchanger (CSWHE) and coupled with ortho-para hydrogen conversion, the flow and heat transfer characteristics of gaseous hydrogen are studied. Firstly, low-temperature O–P hydrogen catalytic conversion experiments and flow characteristics experiments were carried out by filling catalyst in the heat exchange tube. and an equation for calculating pressure drop in the section filled catalyst was proposed. Secondly, a new numerical model was established validated by the experiments. The influence of catalyst on heat transfer and the effects of velocity on the heat transfer performance and O–P hydrogen conversion efficiency were studied by numerical simulation. Finally, this paper analyzed the significant factor affecting the para hydrogen content at outlet: reaction time. And an equation for calculating para hydrogen content at outlet with reaction time was proposed. This study is expected to provide a reference for the design of CSWHE.

Development of a research cryostat for direct thermoacoustic cooling and conversion of hydrogen

Hydrogen thermoacoustics is a relatively unexplored but promising topic for development of new cryocoolers and hydrogen liquefaction. This paper describes a new research cryostat designed to investigate hydrogen as a working fluid in thermoacoustic systems. The cryostat is a modular, turbo-molecular pump driven, vacuum chamber with a 4.2 K pulse-tube cryocooler capable of 45 W of cooling at 50 K. Using this cryostat, a feasibility study of direct thermoacoustic cooling with ortho-parahydrogen conversion is initiated. An acoustic resonator system is designed to have the fundamental acoustic mode excited via imposed temperature gradient in a porous stack between the ambient environment and cold bar connected to the cryocooler. The generated acoustic waves transfer heat inside another porous stack to produce a refrigeration effect at cryogenic temperatures. In addition, the regenerator solid matrix will be catalyzed for ortho-parahydrogen conversion to demonstrate a novel combination of cooling and conversion of cryogenic hydrogen in a single device. Design, construction, and performance of the cryostat and modelling results for one thermoacoustic system are reported.

Ortho-parahydrogen catalyst test facility

With the growing interest in hydrogen as one pillar of the future energy economy, the relevance of hydrogen liquefaction for storage and transportation is increasing rapidly. However, the liquefaction process still offers much potential for improvement in terms of cost and efficiency. One problem faced with achieving more cost- and energy-efficient liquefiers is the uncertainty associated with the dimensioning of the ortho-para converters. Literature data on the existing ortho-para catalysts stem mainly from the 1950s and 1960s and are partly inconsistent. Therefore, a new facility for the comprehensive investigation of catalytic ortho-parahydrogen conversion was set up at the Dresden University of Technology (TU Dresden) as part of the government-funded HyCat (“Improvement of the Ortho-Para-Catalysts for Hydrogen Liquefaction”) project. It enables the testing of catalysts in the whole operational range of ortho-para converters in modern large-scale liquefaction plants by allowing the independent variation of temperature, pressure, flow rate, inlet ortho-para ratio, and sample reactor internal geometry. The setup has been tested and validated preliminarily. It can soon be used to produce new high-quality data sets on the performance of the commercial catalyst Ionex (hydrous ferric oxide), for the qualification of alternative catalysts, and for gaining a deeper understanding of the reaction kinetics involved.

Analysis of standing-wave stack for direct cooling and ortho-parahydrogen conversion of cryogenic hydrogen

Direct cooling of cryogenic hydrogen can be achieved by flow through a stack or regenerator of a thermoacoustic refrigerator. In addition, using a catalytic stack, the ortho-parahydrogen transformation can be conveniently realized in the same device. Analysis of this system element is carried out by integrating one-dimensional thermoacoustic equations with addition of empirical ortho-parahydrogen conversion reaction. Calculations in a standing-wave catalyzed setup demonstrate a selection process for optimal acoustic impedance and stack pore dimensions, showing significant advantage over non-catalyzed hydrogen- and helium-based stacks of similar kind. Results are presented for hydrogen flow characteristics and distributed heat load due to ortho-parahydrogen conversion inside a stack. The stack performance is also quantified at variable flow rate of hydrogen, stack length, mean pressure, and supplied acoustic power.

Experimental and numerical study on the offshore adaptability of new FLH2 floating hydrogen liquefaction production storage and offloading unit

With the development of hydrogen production technology from offshore wind power and natural gas, the deep-sea energy interconnection system using hydrogen energy as the medium is constantly improving. The floating hydrogen liquefaction production storage and offloading unit (LH2-FPSO, FLH2) is an effective method for large-scale utilization of offshore hydrogen energy. By filling the heat exchanger with ortho-parahydrogen conversion catalyst, a new porous medium heat exchange channel is formed. Integrating the spiral wound heat exchanger with the ortho-parahydrogen catalytic conversion reactor can reduce the deck area of FLH2 process system and lower the investment cost of floating hydrogen liquefaction device. The performance of FLH2 can be influenced by the offshore wave and typhoon. In this paper, the methods of floating cryogenic visualization experiment, theoretical analysis and numerical simulation are conducted to study the coupled flow and heat transfer mechanism of working fluid applied in FLH2 under offshore conditions. The results show that the sea state has little influence on the ortho-parahydrogen conversion and cooling process of hydrogen in the porous heat exchange tube. The complex flow and heat transfer challenges brought by the integrated process of new floating hydrogen liquefaction and ortho-parahydrogen conversion have been overcome. With the effect of co-current shear of gas-phase mixed refrigerant, a large dry area will not appear on the outer wall surface of porous tube under offshore conditions. The FLH2 process system shows good adaptability at sea.

The thermodynamic analysis on the catalytical ortho-para hydrogen conversion during the hydrogen liquefaction process

The catalytic conversion of ortho-para hydrogen is an important part in the hydrogen liquefaction process. Isothermal conversion, adiabatic conversion and continuous conversion are three main methods of the catalytic conversion in the hydrogen liquefaction process. However, there are few studies on the analysis and comparison of the three methods. In this paper, a theoretical model of the hydrogen liquefaction process is developed and calculated by MATLAB. The influences of the conversion methods, the number of adiabatic conversion stages and the parahydrogen concentration requirements in products are discussed and analyzed. The results show that the consumption work of the hydrogen liquefaction process with continuous conversion is the lowest. The consumption work of the isothermal conversion is 29.42 % higher than that of the continuous conversion, the consumption work of the adiabatic conversion with different stage numbers is 3.00 %–15.67 % higher than that of the continuous conversion when the parahydrogen percentage requirement in the product is 95 %.

Roles of catalysts’ porous media feature and catalytic conversion effect on the performance of plate-fin heat exchangers in hydrogen liquefaction

Ortho-para hydrogen conversion is essential for hydrogen liquefaction and filling catalysts inside channels of plate-fin heat exchangers (PFHXs) is the most efficient method of conversion. However, the roles of the catalysts’ porous media feature and catalytic conversion effect on the performance of PFHXs filled with ortho-para hydrogen conversion catalysts are barely discussed. Three cases, i.e., Case A (normal PFHX), Case B (PFHX filled with non-catalytic particles acting as porous media), and Case C (PFHX filled with catalysts), are numerically studied and compared to illustrate the influence of filled ortho-para hydrogen conversion catalysts on the performance of the PFHX. The thermal-hydraulic performance coefficient (ηTHP) is defined to illustrate the combined performance of thermal (j factor) and hydraulic performance (f factor), and the conversion efficiency (ηCON) is defined to evaluate the conversion performance of the heat exchanger. The comparison between Case A and Case B shows that the particles acting as porous media restrain the hydraulic performance of normal PFHX while enhance the thermal performance. In comparison with Case B, as the ortho-para hydrogen conversion effect is considered in Case C, j factor is reduced by 1.8 % and the flow resistance is increased by 8.9 %, denoting that the conversion heat leads to deterioration of heat transfer and hydraulic performance. Parametric analyses show that, when the Reynolds number rises from 214.0 to 1569.1, ηTHP and ηCON in Case C increase by 304.9 % and 10.2 % respectively, implying that the Reynolds number is quite influential to the performance. Beside, the proportion of conversion heat in the total heat flux rises along the flow direction and tends to level off, reaching about 45 %.

Thermodynamic analysis of the effect of initial ortho-hydrogen concentration on thermal behaviors for liquid hydrogen tanks

Due to the extra evaporation loss caused by the ortho-para hydrogen conversion (OPHC) during the storage and transportation of liquid hydrogen (LH2), the initial concentration of ortho-hydrogen (o-H2) plays an important role in the large-scale safe operation and utilization of LH2. However, there are very limited studies focusing on clarifying the impact of the OPHC on thermal behaviors of LH2 tanks. In light of this, this paper establishes and develops an extended thermal multi-zone model that couples the OPHC process with the mass and energy conservation equations of the ullage and liquid for an LH2 tank. With the help of the model, a thermodynamic analysis of the pressurization rate is performed to obtain the dominant parameter that evaluates the effect of the initial o-H2 concentration on the pressure build-up process. The results indicate that the interfacial mass transfer rate caused by the OPHC is the dominant parameter accounting for more than 90% of the effect of all parameters on the pressurization rate during a 10-day process with an initial liquid fill ratio of 90% and a heat leakage of 40 W. Additionally, the effect of the initial o-H2 concentration on the optimal liquid fill ratio is discussed in this paper. The results indicate that when the initial o-H2 concentration changes from 0.2% to 10%, the optimal liquid fill ratio decreases from 82.5% to 74%. The present work provides theoretical support and guidance for controlling the o-H2 concentration to realize the long-term and high-efficiency storage of LH2.

Structural improvement of catalyst filled channel of continuous ortho-para hydrogen conversion technology

The continuous ortho-para hydrogen conversion (OPHC) technology is a novel and effective method for solving the OPHC challenge in hydrogen liquefaction, whose key application is the catalyst filled plate-fin heat exchanger (CFPFHE). However, the incoordination of the OPHC and heat transfer greatly limits the overall performances of the CFPFHE. To solve it, this paper attempts various catalyst filled channels to improve the overall performances of the CFPFHE by numerical simulation. Firstly, for the perforated fin channel, the special perforated structure makes its overall performances slightly better than the plain fin channel. Secondly, considering the excellent heat transfer of the catalyst filled channel and the demand increasing the catalyst filled amount, the plate channel is proposed by removing the fins between the plates. The results indicate that the surface heat transfer coefficient αh of the plate channel matches well with the serrated fin channel of the cold side. Compared with the plain fin channel, the pressure drop Δp of the plate channel is decreased by 25.12% ∼ 27.89%, and the OPHC index YYpH2 is increased by 2.62% ∼ 4.50%. From the perspective of YYpH2 ≥ 95%, the allowable working condition of the plate channel is more than 3 times that of the plain fin channel. Meanwhile, the plate channel with a large plate spacing dh sacrifices a part of αh, but it greatly improves the flow performance and OPHC performance. Further, the performance correlations of the plate channel are obtained to facilitate the design of the plate channel in the CFPFHE.

Study on multi-stream plate fin heat exchanger coupled with ortho-para hydrogen conversion based on distributed parameter model

The distributed parameter model is used to simulate full domain of multi-stream CFPFHE (catalyst filled plate fin heat exchanger). The heat transfer due to the extended catalyst surface area, the ortho-para hydrogen conversion heat and the flow resistance in porous catalyst are integrated in the model. The results show that increasing the mass flow of fluid channel filled with catalyst not only decreases HEPUP (heat exchange amount per unit pump power), but also weakens efficiency of o-p conversion. For example, when the mass flow of fluid A (in which catalyst is filled) increases from 0.06 kg/s to 0.084 kg/s, the pump power increases by 34.6%, and the HEPUP and the average outlet para-hydrogen decreases by 30.9% and 7.0% respectively. In addition, A deviation parameter ϕ is defined to qualitatively describe the degree of inlet maldistribution. Although no catalyst filled in fluid C channel, the effect of ϕC on the outlet para-hydrogen mass fraction is obvious. With ϕC increasing from 0 to 1.5, the outlet para-hydrogen mass fraction in the layer numbered by 42 (middle layer of fluid A) increases by 6.8%, decreases by 11.9% and 11.9% respectively, in the layer numbered by 6 (upmost layer of fluid A) and 76 (downmost layer of fluid A). The increase of inlet maldistribution of fluid C prevents the potential of catalyst filled in up and down layers of fluid A from being fully utilized.

Numerical model improvement of a plate-fin heat exchanger filled with catalyst for hydrogen liquefaction

The continuous ortho-para hydrogen conversion technology represented by the plate-fin heat exchanger filled with catalyst (PFHEFC) is one of the key technologies for the breakthrough of large-scale hydrogen liquefaction system. An appropriate numerical model of the PFHEFC can provide reference and guidance for its design. The thermal-hydraulic behaviors of the catalyst filled layer (CFL) are essential for the model development of the PFHEFC. In this paper, the thermal-hydraulic correlations of the high-pressure and low-temperature hydrogen in the CFL are investigated by the numerical simulation and then used to improve the existing numerical models of the PFHEFC. The results show that the simulation results of the face centered cubic (FCC) packing structure are close to the results of some classical correlations of the CFL with most relative deviations within ±10% and can be used to approximate the actual CFL. And taking 40 K as the dividing point, the new thermal-hydraulic correlations of the high-pressure and low-temperature hydrogen in the CFL are obtained with the relative deviations within ±10%. Further, the New model of the PFHEFC is developed with the new thermal-hydraulic correlations. The New model has some margin in the heat transfer performance, considers the special thermal-hydraulic performances of the high-pressure and low-temperature hydrogen in the CFL, and is consistent with the existing numerical models in the ortho-para hydrogen conversion performance. It shows the New model is more suitable for the PFHEFC and can provide more accurate results for the design of the PFHEFC than the existing numerical models.

Performances comparison of adsorption hydrogen storage tanks at a wide temperature and pressure zone

Large-scale application of hydrogen requires safe, reliable and efficient storage technologies. Among the existing hydrogen storage technologies, cryo-compressed hydrogen (CcH2) storage has the advantages of high hydrogen storage density, low energy consumption and no ortho-para hydrogen conversion. But it still needs higher hydrogen storage pressure when reaching higher hydrogen storage density. In order to reduce hydrogen storage pressure and improve storage density, solid adsorption technology is introduced in CcH2. Activated carbon and metal-organic framework materials (MOFs) are employed as adsorbents in this paper. The gravimetric/volumetric hydrogen storage capacities of different adsorption tanks are studied and compared with the hydrogen storage conditions of 1–55 MPa at 77–298 K. The results show that the hydrogen storage density of CcH2 combined with adsorption is higher than that of pure adsorption hydrogen storage, and the storage pressure is lower than that of pure CcH2 under the same hydrogen storage capacity. And the combination of two hydrogen storage technologies can achieve a high hydrogen storage capacity equivalent to that of liquid hydrogen at a lower pressure.

Study of Polythermal Ortho-Para Hydrogen Conversion Process and its Industrial Implementation

Study of Polythermal Ortho-Para Hydrogen Conversion Process and its Industrial Implementation

Comparative study on thermodynamic performance of hydrogen liquefaction processes with various ortho-para hydrogen conversion methods

Catalytic conversion of ortho-para hydrogen is essential for hydrogen liquefaction. Among the three ortho-para hydrogen conversion methods, i.e., isothermal, adiabatic and continuous conversion, it has already been accepted that the continuous conversion is the most energy-efficient. However, studies on quantitative analyses and the underling mechanisms are still lacking. In this paper, the three conversion methods are evaluated based on a reference 5 t/d-scale hydrogen liquefaction process with a self-developed numerical model. Optimizations are conducted to identify the lowest specific energy consumption (SEC) of the hydrogen liquefaction process with different conversion methods. The three conversion methods are compared in terms of the temperature distribution along the process, the conversion-heat-associated exergy and SEC. The optimization results show that the continuous-conversion based process reaches the lowest SEC, i.e., 11.38 kWh/kgLH2, which is 21.8% and 28.7% lower than the adiabatic and isothermal ones. Exergy analyses indicate that the exergy consumption associated with dissipating the conversion heat is the lowest for the continuous conversion (139.54 kW), followed by the adiabatic conversion and then the isothermal conversion, i.e., 174.94 kW and 273.90 kW, respectively. This work demonstrates the differences quantitively and reveals its mechanism thermodynamically, which would be helpful for understanding, and designing hydrogen liquefaction processes.

A novel integrated hydrogen and natural gas liquefaction process utilizing a modified double mixed refrigerant process pre-cooling system

The hydrogen liquefaction process is energy-intensive due to the lower liquefaction temperature. To improve the energy efficiency of the hydrogen liquefaction process, a novel integrated liquefaction process employing a modified double mixed refrigerant process pre-cooling is proposed and analyzed. The three-stage adiabatic ortho–para hydrogen conversion makes the process compact. Furthermore, the particle swarm optimization algorithm is utilized to find the optimal decision variables. The result reveals that the total specific energy consumption (SEC) of the process is 6.5921 kWh/kgLH2-LNG, which is 15.83% lower than the base case. The SECs for the production of LNG and LH2 are 0.2445 kWh/kgLNG and 6.3476 kWh/kgLH2. Moreover, the exergy analysis and thermodynamic evaluation reveal that the exergy efficiency, coefficient of performance, and figure of merit of the process are 49.26%, 0.2306, and 0.49, exhibiting high thermodynamic performance. Finally, the sensitivity analysis indicates that the evaporation pressure of the sub-cooling refrigerant has the greatest impact on the performance of the process. The proposed process has an important reference value in the design of the combined production of LH2 and LNG from industrial by-products and the H2 liquefaction process.

Numerical study on comprehensive performance of flow and heat transfer coupled with ortho-para hydrogen conversion

The energy consumption of hydrogen liquefaction can be effectively reduced by filling the plate-fin heat exchanger with the catalyst to achieve continuous conversion of ortho-para hydrogen while cooling. In this study, a three-dimensional numerical model of heat transfer and flow coupled with ortho-para hydrogen conversion is established, and the simulation results are verified by the experimental data. Both the temperature field and the outlet parahydrogen fraction demonstrate better accuracy of the local thermal non-equilibrium model results compared to the local thermal equilibrium model results, and the thermal non-equilibrium effect is most significant at the inlet of the porous medium, where the temperature difference between the fluid and the solid exceeds 2 K. Furthermore, it is indicated that two typical stages exist in the hot channel: the heat transfer dominates in the inlet section and the ortho-para hydrogen conversion efficiency decreases significantly, even once falling below 30%. In the thermal fully developed section, the catalytic performance is effectively improved along with the decay of velocity, with a 10% improvement compared to the lowest point of performance. Finally, an integrated comprehensive performance evaluation criterion is summarized regarding the overall performance of heat transfer, flow and catalysis, with excellent comprehensive performance being achieved at Reynolds numbers of 170 to 300. This research can provide a reference for the comprehensive performance study of catalyst-filled plate-fin heat exchanger for hydrogen.

Significance of ortho-para hydrogen conversion in the performance of hydrogen liquefaction process

Hydrogen is an energy vector and is produced just like electricity. In order to overcome the shortcomings associated with its low molecular weight and energy density per unit volume, hydrogen is liquefied for storage and transportation purposes. The liquefaction of hydrogen differs from that of other substances as it involves the reactive transformation of its isomeric states. At 25 °C, molecular hydrogen consists of 75% orthohydrogen and 25% of parahydrogen. As the normal boiling point, hydrogen essentially exists in the para-state, which is preferred because of its lower boil-off gas rate. However, the conversion of ortho-to-para hydrogen is an exothermic reaction, and this enthalpy of conversion enhances the total reversible work by about 15%. Little work has been done regarding ortho-to-para hydrogen conversion from the process systems point of view. Therefore, parametric analysis of this vital conversion reaction was studied with potential impact on the performance of cryogenic heat exchangers, reactors configuration and mode of operation, and probable impact on the energy efficiency of the liquefaction process. An alternate approach to simulate the reaction is also proposed. The results show that the current approaches to process design need to be changed. The study opens avenues for more in-depth analysis and optimization approaches to present a holistic framework for future integrated energy systems.

Performance study on plain fin filled with catalyst in a plate fin heat exchanger for hydrogen liquefaction

In order to develop the correlations of the fin performances of the catalyst filled plate fin heat exchanger (CFPFHE) for hydrogen liquefaction, a numerical model of the plain fin filled with catalyst at 30–80 K is established. The effects of temperature, the structural parameters of the fin and catalyst layer and the operating condition on the fin performances of the fin channel are analyzed and discussed. The results show that the temperature distribution patterns and fin performances at 30–40 K and above 40 K have obvious differences. The results above 40 K can be characterized by that at 70–80 K. The sensitivity analysis shows that the distributions of sensitivity at 70–80 K and 30–40 K are similar. The heat transfer and ortho-para hydrogen conversion performances are mainly affected by the structural parameters of the fin and the operating condition, while the flow performance is mainly affected by the structural parameters of the catalyst layer. The correlations of the flow and heat transfer performances at 70–80 K and 30–40 K are obtained by fitting the data points on the response surface with more than 97% of the fitting degrees and within ±15% of the deviations. Meanwhile, the correlations of the ortho-para hydrogen conversion performance show that the mass space velocities at 70–80 K and 30–40 K should be lower than 1.25 and 7.50 kg m−3 s−1 respectively to reach the standard of the ortho-para hydrogen conversion. The correlations of the fin performances can be used as the basis of optimization design of the CFPFHE.

Experimental and numerical study on tube-side flow and heat transfer characteristics of superheated vapor flow in catalyst-filled LH2 spiral wound heat exchanger

High energy consumption is considered to be one of the most persistent problems in liquid hydrogen (LH2) plants. The combination of heat exchanger and ortho-para (O–P) hydrogen conversion has attracted considerable attention as a cutting-edge technology to reduce energy consumption. The flow and heat transfer characteristics of O–P hydrogen conversion catalyst-filled spiral wound heat exchanger (SWHE) were investigated in this study in two steps. In the first step, pressure-drop experiments were performed in a tube filled with porous media. The results indicated that the pressure drop was overestimated when using Ergun's equation. Therefore, a new empirical pressure-drop correlation for a channel filled with O–P catalyst was formulated. Subsequently, a novel heat transfer model was established based on this correlation for further numerical simulations. The distributions of the temperature, pressure, and para hydrogen content in a catalyst-filled tube were determined. In addition, the influence of the flow rate on the heat exchange coefficient and outlet para hydrogen was clarified; it was found that, with an increase in the flow rate, the heat exchange coefficient increased, whereas the outlet para hydrogen content decreased. At a flow rate of 0.5 m3/h, the para hydrogen content increased by 44% after hydrogen flowed through the channel filled with the O–P catalyst. Furthermore, a prediction model for the para hydrogen content with a flow rate range of 0–1.5 m3/h was derived. This study provides promising theoretical evidence for the engineering application of SWHEs filled with O–P catalysts in large-scale hydrogen liquefaction units.