Session: 11-08-01: Fundamentals of Phase-Change Including Micro/Nanoscale Effects-Boiling, Evaporation, Freezing and Condensation

Paper Number: 94985

94985 - Modeling Frost Formation in Freeze-Out Purification of Gases for Cryogenic Applications 

Cryogenic refrigeration and liquefaction systems require ultra-high purity refrigerants (typically, gases such as helium, argon, hydrogen, etc.) for their proper operation. Common contaminants (such as constituents of air, residual oil from process equipment, etc.) in the refrigerant gases have a melting point greater than the operating temperature of the cryogenic system, resulting in all contaminants freezing at the refrigerator. The frozen contaminants have the potential to cause performance degradation of the process heat exchangers, and even failure of rotating equipment (e.g. turbo-expanders). To mitigate this issue, ultra-high purity refrigerant gas (99.9999% or better) is often used in these cryogenic systems. However, removal of low levels of moisture (10 ppm_v or less) from the refrigerant gas is particularly challenging. Contaminant freeze-out processes carried out in a properly designed heat exchanger have the potential to achieve effective and efficient (with less utility consumption) purification of refrigerant gases. Developing an understanding of the contaminant frost formation process, especially the impact of frost on flow area and associated transport processes is crucial for the proper design of an effective freeze-out heat exchanger. A transient computational model simulating the formation and densification of frost on an isothermal cryogenic surface (flat plate) from a contaminated refrigerant gas stream has been developed. The mass and energy conservation equations for frost and the refrigerant gas are discretized and simultaneously solved to obtain the frost layer thickness and frost surface temperature. Correlations available from the literature are used to obtain frost thermo-physical properties. 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, and flow Reynolds number affect the interaction between frost formation and densification, as well as the associated transport phenomena. The effect of these parameters on the frost formation characteristics has been systematically studied using the developed numerical model. The carrier (refrigerant) gas has a significant impact on the frost saturation concentration, frost formation temperature, associated transport phenomena, and thereby the frost formation physics. The effect of carrier (refrigerant) gas on the frost formation characteristics has been studied using the developed model. Especially three common cryogenic refrigerant gases, namely – helium, hydrogen, and argon has been chosen for this study, and their effects on frost formation and densification are compared to that from humid air. The developed model can be utilized to predict the freeze-out heat exchanger performance degradation, as well as the moisture collection capacity of such a heat exchanger.

Presenting Author: Duncan Kroll Michigan State University / Facility for Rare Isotope Beams

Presenting Author Biography: Duncan Kroll is a Ph.D. candidate in the Department of Mechanical Engineering at Michigan State University (MSU). He is working as a graduate research assistant at the Facility for Rare Isotope Beams. His main research interests are in cryogenics, including helium purification, freeze-out heat exchangers, and optimization of thermal systems. Presently he is studying the mechanics of frost formation in a freeze-out heat exchanger for gas purification. He received his Bachelor’s Degree in Chemical Engineering and his Master’s Degree in Mechanical Engineering in 2018 and 2020 respectively - both from Michigan State University.

Authors:

Duncan Kroll Michigan State University / Facility for Rare Isotope Beams
Nusair Hasan Michigan State University / Facility for Rare Isotope Beams