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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 Ω ¯ is able to characterise the mass transfer across the falling film gas-liquid interface. [ABSTRACT FROM AUTHOR]

• Twin-liquid film flow has been observed on a spoked rotating disk reactor. • Interplay between the confined free film and wall-bounded film leads to mass transfer intensification. • The circumferential length of the opening window should be accommodated to fluid viscosity. Spoked rotating disk reactor is a common gas–liquid mass transfer equipment for highly viscous fluids. The hydrodynamics of the twin-liquid film on the disk is numerically investigated. The simulated film thickness, which agrees well with the experimental results, shows that film flow patterns shift at different rotational speeds. The alternation between the windows and spokes brings diverse film flow pattern, i.e. , twin-liquid film, where a confined free film and a wall-bounded film coexist and interplay. The twin-liquid film results in 70% thinning of the film thickness and 30% increase in the surface renewal frequency compared to the wall-bounded film. Based on the trajectory of the tracking particles, intense velocity fluctuations within the twin-liquid film can be observed. Given the pressure gradient within the opening window increasing exponentially with fluid viscosity, we suggest that the circumferential length of the open window be proportional to fluid viscosity. The results demonstrate good adaptability of the optimization strategy, which can guide the scaling-up of the spoked rotating disk reactors for highly viscous fluids. [ABSTRACT FROM AUTHOR]