Dimensionless performance mapping of cryogenic plate-fin heat exchangers with ortho-para hydrogen continuous conversion for hydrogen liquefaction

Continuous conversion heat exchangers are recognized as the next-generation technology for large-scale and energy-efficient hydrogen liquefaction plants. However, their structural design and operational optimization are impeded by the absence of generalised evaluation methodology, as well as unclear coupling mechanisms of convection and conversion. This study develops a novel dimensionless model incorporating the number of heat transfer units and the Damköhler number, to describe the convective transport and catalytic conversion process for continuous conversion heat exchangers. Their comprehensive performance is evaluated by defining the cooling, conversion, and total effectiveness, based on which performance maps are subsequently proposed for three typical temperature zones. The scaling results demonstrate that the conversion-convection characteristics along the heat exchanger train exhibit multiple complex stages, heavily contingent upon the space velocity and the ratio of heat capacity rates between hot and cold fluids. To achieve the effectiveness of over 0.90, these two operational parameters should be below about 1000 h−1 and 0.50, respectively. The characteristic variables are recommended to meet Da0 ≥ 8 and NTU0 ≥ 16 as a minimum requirement for various heat exchangers. These findings offer a theoretical foundation for high-efficient design and operation of continuous conversion heat exchangers in future hydrogen liquefaction plants.