Numerical investigation on the flow characteristics of liquid oxygen and water in the structured packing

The distillation efficiency of the cryogenic air separation unit is significantly affected by the inside flow performance of the cryogenic oxygen and nitrogen mixture in the structured packing. Owing to the difficulties in cryogenic experiments, the development of structured packings in cryogenic distillation often relies on the hydraulic experiments at room temperatures, while the real flow characteristics of the cryogenic fluids in the structured packing are hardly known. In this work, with the help of CFD simulation, a 3-D model combining VOF method was built to study the flow characteristics of liquid oxygen (LOX) in the structured packing, and comparisons with water were also carried out. Effects of liquid load and equivalent contact angle on the flow patterns, the liquid hold-up, the interfacial area and the average film thickness were analyzed. Results show that the flow of water is mainly rivulets, while LOX can form film flow on the packing surface in a large fluid load range due to its small contact angle and surface tension. Because the interfacial area of LOX isn't strongly affected by liquid load, its liquid film thickness could be the main consideration for heat transfer, mass transfer and load selection. Since the variation of contact angle has slight effect on the flow parameters of LOX, the optimization of structured packings for cryogenic distillation may not be expected by the surface micro texture improvements.