Mass Deposition Rates of Carbon Dioxide onto a Cryogenically Cooled Surface

The rates of CO₂ mass deposition onto cryogenically cooled surfaces are crucial for CO₂ removal processes that rely on cryogenics. A dedicated experimental setup was constructed to measure CO₂ mass deposition rates under controlled conditions. Experiments were carried out with both pure CO₂ and CO₂/N₂ mixtures, growing frost layers up to 8 mm thick. Results demonstrated that heat transfer through the frost layer significantly slows down the mass deposition process. Furthermore, it was found that the addition of N₂ to the gas phase has a considerable influence on mass deposition rates, because it introduces an additional mass transfer resistance toward the frost surface. To describe the experimentally observed behavior, a frost growth model based on mass and energy balances was developed. Expressions for the frost density as a function of the frost temperature and for the effective frost conductivity as a function of the frost density were derived and implemented in the model. When accounting for drift fluxes, the model accurately captures the behavior observed in experiments. The findings of this work highlight the significant impact of heat transfer limitations on processes that accumulate a thick solid CO₂ layer, such as continuously cooled heat exchangers. Conversely, technologies like cryogenically refrigerated packed beds do not develop a thick solid CO₂ layer; calculations showed that a frost layer of 3.24·10⁻⁵ m is formed, resulting in a Biot number well below 0.01, indicating that heat transfer in the frost layer is not limiting.