Cryogenic 11CO₂ capture system design for RIB project

A cryogenic radioactive ¹¹CO₂ trap is under development for the Radioactive Ion Beam project in VECC. The trap uses the process of de-sublimation (frosting) and sublimation for separating radioactive ¹¹CO₂ from the (¹¹CO₂/N₂) mixture. Radioactive ¹¹CO₂ produced in the accelerator will be mixed with dry N₂, before feeding to the trap for separation. The volumetric concentration of ¹¹CO₂ in the mixture will be around 5%. The mixture will be passed through the coiled stainless steel tube submerged in liquid N₂. A pump maintains the pressure of the coil below 1 atmosphere, preventing the condensation of N₂ gas, but allowing trapping of gases like ¹¹CO₂, NO₂, etc. Completing the entire separation process well within the half-life of the ¹¹C is a challenge in the design of the trap. This paper presents the overall scheme of the trap as well as its numerical modelling of frosting of ¹¹CO₂ inside the tube. A parametric study is performed to study the effect of channel diameter, flow rate, and ¹¹CO₂ concentration on the frost mass, frost layer thickness and transfer coefficient and obtain optimum values of the parameters. (author)