Unified study of flow regimes and gas holdup in the presence of positive and negative surfactants in a non-uniformly aerated bubble column

In this paper, a study on the global gas holdup and hydrodynamic flow regimes developed in a partially aerated bubble column at variable air superficial velocities (UG) in the presence of positive and negative surfactants is presented. According to the results obtained, despite the different liquid phase properties variation caused by the presence of positive (alcohols) and negative (electrolytes) surfactants, both reduce coalescence and the effect in the gas holdup (εG) is equivalent: it increases with the surfactant concentration (C) but only when the (C/Ct) ratio is clearly above 1, being Ct the transition concentration. Contrary to the results obtained for totally aerated bubble columns, for lower values of the (C/Ct) ratio, the holdup remains practically invariable. Considering the crucial role that C and Ct play in the resulting εG, a new prediction equation for εG accounting for the ratio (C/Ct) and UG is presented and its performance for both types of surfactants validated. Additionally, visual and wall pressure fluctuations studies reveal that the vortical flow (VF), characterized by an oscillating bubble plume, prevails in ultrapure water (UPW) but results destabilized in the presence of surfactants. This destabilization results in an evolution to a pseudo-steady flow regime, the double cell turbulent flow regime (DCTF), characterized by a quasi-static bubble jet, located at the column centerline that determines the appearance of two static symmetrical vortices

► Unified study of flow regimes and εG in a partially aerated bubble column containing surfactants.
► εG increases with the surfactant concentration but only when the (C/Ct) ratio is above 1.
► A new prediction equation for εG depending on (C/Ct) and UG and its validation is presented.
► The vortical flow prevails in UPW but is destabilized in the presence of surfactants.
► The evolution of the vortical flow is the double cell turbulent flow regime.