Numerical simulation of carbon dioxide (CO₂) absorption and interfacial mass transfer across vertically wavy falling film

The process of CO₂ absorption in many currently used chemical devices can be typically characterised and analysed by mass transfer across a thin falling film gas-liquid interface. There is still a lack of general predictability of the transfer quantity based on the basic hydrodynamic parameters involved in such mass transfer processes, and certain important phenomena associated with the transfer still remain unexplained. A novel concept that utilises the correlation between the vorticity neighbouring the falling film gas-liquid interface and the gas concentration to characterise CO₂ absorption mass transfer by the falling film is proposed in this work. Numerical simulations using a volume of fluid (VOF) approach were performed for a vertical falling film arrangement. The wave hydrodynamics and the associated mass transfer are discussed, and the numerical results are compared with the existing experimental empirical relationships. In particular, the mass transfer across the falling film interface is interpreted as the passive scalar entrapment and entrainment by the interfacial vortices. The numerical simulation clearly indicates that two types of vortices exist: wall-bounded vortices associated with primary shear in the falling film and interfacial vortices associated with weaker shear in the vicinity of the interface. The numerical simulation reveals that mass transfer across the falling film is highly correlated with the interfacial vorticity Ω_i, thus indicating the use of cΩi- is able to characterise the mass transfer across the falling film gas-liquid interface.