Our paper entitled: Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes has been published in the International Journal of Hydrogen Energy.
A present key barrier for implementing large-scale hydrogen liquefaction plants is their high power consumption. The cryogenic heat exchangers are responsible for a significant part of the exergy destruction in these plants and we evaluate in this work strategies to increase their efficiency.
In the work, we present a detailed mathematical model of a plate-fin heat exchanger that incorporates the geometry of the heat exchanger, nonequilibrium ortho-para conversion and correlations to account for the pressure drop and heat transfer coefficients due to possible boiling/condensation of the refrigerant at the lowest temperatures. Plate fin heat exchangers have fins that increase the heat transfer area. Catalyst is placed in the layers where there is conversion from ortho-hydrogen to para-hydrogen as illustrated in the figure below.
Based on available experimental data, a correlation for the ortho-para conversion kinetics is developed, which reproduces available experimental data with an average deviation of 2.2%. The correlation (solid lines) is compared to some of the available experimental data in the figure below.
In a plate-fin heat exchanger that is used to cool the hydrogen from 47.8 K to 29.3 K with hydrogen as refrigerant, we find that the two main sources of exergy destruction are thermal gradients and ortho-para hydrogen conversion, being responsible for 69% and 29% of the exergy destruction respectively. A route to reduce the exergy destruction from the ortho-para hydrogen conversion is to use a more efficient catalyst, where we find that a doubling of the catalytic activity in comparison to ferric-oxide, as demonstrated by nickel oxide-silica catalyst, reduces the exergy destruction by 9%. A possible route to reduce the exergy destruction from thermal gradients is to employ an evaporating mixture of helium and neon at the cold-side of the heat exchanger, which reduces the exergy destruction by 7%. We find that a combination of hydrogen and helium-neon as refrigerants at high and low temperatures respectively, enables a reduction of the exergy destruction by 35%. A combination of both improved catalyst and the use of hydrogen and helium-neon as refrigerants gives the possibility to reduce the exergy destruction in the cryogenic heat exchangers by 43%. The limited efficiency of the ortho-para catalyst represents a barrier for further improvement of the efficiency.
The work describes new routes to follow in order to improve the efficiency and reduce the power requirement to liquefy hydrogen and was carried out in in collaboration with D. Berstad, Ailo Aasen, Petter Nekså and Geir Skaugen from SINTEF Energy Research and the Norwegian university of science and technology.