Modeling Frost Formation in Freeze-Out Purification of Gases for Cryogenic Applications

Abstract

Cryogenic refrigeration and liquefaction systems require ultra-high purity refrigerants (helium, argon, hydrogen, etc.) for proper operation. Common contaminants in the refrigerant gases freeze at the operating temperatures of these systems, causing performance degradation of process equipment. Therefore, ultra-high purity refrigerant gas (1.0 ppmv or less contaminants) is often used. However, removal of low levels of moisture (10 ppmv or less) from the refrigerant gas is particularly challenging. Contaminant freeze-out processes in a specifically designed heat exchanger have the potential to achieve effective and efficient purification. Developing an understanding of the contaminant frost formation process is crucial for the proper design of an effective freeze-out heat exchanger. A transient computational model simulating formation and densification of frost on an isothermal cryogenic surface from a contaminated refrigerant gas stream has been developed. The mass and energy conservation equations are discretized and simultaneously solved to obtain the frost layer thickness and frost surface temperature. The model is validated using available experimental data for frost formation from a humid air stream. Several parameters, namely — contaminated gas stream pressure, surface temperature, flow Reynolds number, and carrier (refrigerant) gas affect the interaction between frost formation and densification. The effect of these parameters on the frost formation characteristics has been systematically studied using the developed numerical model. The developed model can be utilized to predict the freeze-out heat exchanger performance degradation and its moisture collection capacity.