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Modeling of Frost Growth and Evaporation of Refrigeration Mixtures in a Fin-and-Tube Heat Exchanger

Biogas upgrading process consists in removing undesirable components such as hydrogen sulfide, water vapor, carbon dioxide and other pollutants in order to produce bio methane. One of the dehydration processes consists in frosting the water vapor contained in the biogas. Ice crystals can be then defrosted either by melting or by sublimation. Many models have been developed to simulate a frost layer on a cold plate using empirical correlations, valid for limited temperatures range. In this study, the frosting process has been modelled using a numerical approach of heat and mass transfer during its formation on the external side of a fin-and-tube heat exchanger. This model is valid for temperatures from 0°C to -50°C and lower. Frost density, thermal conductivity and diffusion equations are calculated via empirical correlations. The evaporation of several refrigeration mixtures is also modelled. The model takes into account the whole heat exchanger path. Simulation results show that the frost mass growth on fins and rows is followed by a decreased heat transfer rate. The porous structure of the frost layer is made of solid crystals and air pores. On one hand, the thermal resistance of the frost layer is relatively high, leading to a decrease in the heat exchanger performance. On the other hand, the thickness of the frost layer reduces the external flow passage surface, leading to higher pressure drop on the external side. This model has been validated by comparing numerical and experimental results for the heat transfer rate.