Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes

Abstract

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. A detailed model of a plate-fin heat exchanger is presented 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. 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%. 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.

Kinetics and Heat Exchanger Design for Catalytic Ortho‐Para Hydrogen Conversion during Liquefaction

“…5 a depicts good accuracy for a high o-H 2 content, or low conversion level. However, as already stated in their original work, the model of Wilhelmsen et al [12] is disadvantageous at higher conversions. Fig.…”

“…Recently, Wilhelmsen et al investigated a catalyst-filled PFHX design in the temperature range from 47.8 K to 29.3 K. They reported thermal gradients and ortho-para-hydrogen conversion to be the two major contributors for exergy losses of the system [12].…”

“…In the recently published PFHX study of Wilhelmsen et al, the authors used the same experimental datasets [14,15] as in this study. Parity plots of the model of Wilhelmsen et al [12] are presented in Fig. 5.…”

“…For large-scale plants the efficiency becomes increasingly important. Recently, a study by Willhemsen et al [12] gave valuable insights to modeling strat-egies for kinetics and heat exchanger design in ortho-para hydrogen conversion as well as on the application to simplified process models. They used a phenomenological kinetic model according to Hutchinson et al [13].…”

“…Until now, available process studies give little or insufficient emphasis on the transition from ortho-to para-hydrogen and the accompanied heat release. Existing studies use staged isothermal conversion [16,17], rely on equilibrium ortho-para conversion without a kinetic model [18], or apply simplified, empirical kinetic models [12,19,20]. In this study, the obtained kinetics are implemented in a one-dimensional pseudo-homogeneous heat exchanger model, which is capable to describe the heat, mass, and momentum transport inside a packed-bed heat exchanger together with the catalytic conversion of orthohydrogen.…”