Cryogenic carbon dioxide capture by the desublimation method has the advantage of being contamination free and energy efficient under high concentration. Due to the difficulty of collecting solid CO2 after desublimation, this method has not been applied. In this paper, a visual experimental setup for carbon dioxide desublimation and sublimation on a low temperature surface is introduced in detail. The core part of the experimental set-up is a visual tube-in-tube counterflow heat exchanger consisting of a Pyrex glass tube with a larger diameter and a stainless-steel tube with a smaller diameter. The crystal growth and dissipation occur on the precooled outer surface of the inner tube, which is recorded by a camera. When carbon dioxide desublimates under different working conditions, such as temperature and supersaturation, the growth shape and growth rate of the solid are different. The thermal properties of the solid such as porosity, thermal conductivity and density are also different, leading to variations in the sublimation process and speed. Three distinct sublimation processes are discussed in this paper to provide guidance on the actual carbon capture process.

Heat exchangers with helical tubes are applied widely in various industrial fields and cryogenic applications. Currently, the most common ones are the spiral wound heat exchangers, which consists of many layers of spiral tubes with different coil diameters in a closed shell. Most of the fluid in the shell-side passes directly through the gap between two adjacent layers, which weakens the turbulence and the associated heat transfer coefficient. A novel twisted helical bundle geometry is proposed in this paper of which the thermal performance is explored using CFD simulation. The heat exchanger uses helium gas as the working fluid and operates between 300 K and 30 K. The curve of each tube in the bundle is generated by different 3D sinusoidal equations, and the tubes at adjacent layers are twisted together thereby producing the same end-to-end length for each tube. By simulating a spiral wound heat exchanger using different turbulent equations and comparing the results with experimental data from literature, the k-omega SST model is found to be the most suitable one to simulate this kind of geometry. Nusselt numbers and friction factors are obtained under different Re values and compared with those of standard spiral wound heat exchangers. The creative twisted geometry can improve the Nu number although also increasing the friction factor. Geometry parameters such as pitch length are being further explored to obtain an optimal thermal performance.