Spontaneous desublimation of carbon dioxide in turbo-expander applied for cryogenic carbon capture

Desublimation-based cryogenic carbon capture has drawn particular attention to mitigate the climate issue of global warming. Turbo-expander based refrigeration system is a potential option to provide cooling at 150 K for CO₂ desublimation, while the CO₂ spontaneous desublimation in turbo-expander complicates the expansion process and leads to wetness losses. In this paper, the non-equilibrium desublimation process is included in the one-dimensional mean streamline model of a cryogenic turbo-expander, and the expansion process with CO₂ desublimation in air is investigated. The distribution of CO₂ desublimation parameters in the flow passage of turbo-expander is obtained, and the two-phase expansion processes are classified into two types including desublimation expansion and subcooled expansion. A parametric study shows that a decrease in inlet temperature, an increase in pressure ratio and inlet CO₂ concentration lead to a larger wetness loss, where inlet temperature is the most important parameter, followed by pressure ratio and inlet CO₂ concentration. For a turbo-expander with inlet pressure of 300 kPa and CO₂ concentration of 0.64%, the significant desublimation occurs at a subcooling degree of 23.7 K, leading to a wetness loss of 1.2%. To avoid desublimation as well as wetness loss in passage of a turbo-expander, the designed maximum subcooling degree is supposed to be smaller than 15 K for the turbo-expander of a cryogenic carbon capture system.