Numerical analysis of cryogenic LOX-LN2 mass transfer characteristics under different textured surfaces

Abstract

Cryogenic distillation efficiency is strongly influenced by structured packings where the separation process intensely occurs in an air separation unit. Packings with surface treatments are usually employed for mass transfer enhancement in current studies mostly conducting on room-temperature fluids. However, researches on cryogenic fluids like liquid oxygen and nitrogen are relatively limited, to what degree indeed the textures could enhance the cryogenic mass transfer process remains unknown. Besides, the exact region of the textures where the mass transfer is enhanced the most, which might help to better optimize the textures’ dimensions, is not revealed. In this work, a 2-D CFD model was developed to investigate cryogenic oxygen-nitrogen mass transfer in structured packing elements at a small scale. Two corrugated packing surfaces were employed and compared with the plane plate. Flow characteristics including flow intensity, vortices, and liquid fluctuations were analyzed. The results showed that the thickness of the liquid film on corrugated surfaces (0.3–0.4 mm) was larger than that on plane plates (0.2–0.3 mm). Nonetheless, the mass transfer on corrugated surfaces was better than that on plane plates, mainly resulting from the liquid disturbances which changed the flow characteristics. Due to larger flow intensity, larger vortices at the trough region, and more intense flow fluctuations, sinusoidal textures could enhance the cryogenic mass transfer process by 50%, better than triangular ones with 25%. It was in the textures trough where the mass transfer was promoted the most, on the account of the generated vortices there facilitating the liquid-gas mixture. Furthermore, currently used texture dimensions that are mainly designed based on room temperature fluids (water, etc.) may not be the fit under cryogenic conditions according to the investigation, more exploration is in need.