Foam formation in a gas–liquid stirred tank: Impact of impeller type and operating conditions

• Foam height increases over time before reaching a steady-state value. • The equilibrium foam height is reached more slowly at high impeller speeds than at low impeller speeds. • Bubble size and velocity in the liquid affect foam formation and stability. • Foam generation is more influenced by impeller-induced hydrodynamics than gas flow rate. • Strong liquid recirculation at high impeller speed reduces foam height via bubble drawdown. Foam formation occurs in many industrial processes involving gas–liquid contacting and is often undesirable since it can lead to numerous operational and production issues. This study provides new insights into the role of process operating conditions on foam formation in gas–liquid stirred tanks. It explores the impact of gas flow rate, impeller type (radial flow turbine, down-pumping, and up-pumping axial flow impellers), and rotational speed on foam formation for an air-surfactant/water system. An in-depth understanding of the implications of these operating conditions on the quantity of foam formed (via foam height) is analyzed by integrating additional measurements of the gas–liquid flow regime, gas holdup, the size and velocity of bubbles in the liquid, and existing knowledge of the physico-chemical aspects of foam formation. It was found that the gas–liquid flow regime plays an important role in foam formation. Process conditions that produce smaller bubbles lead to larger quantities of foam and higher gas holdup, while larger bubbles result in lower gas holdup and reduced foam formation. Foam quantity typically increases as impeller speed increases and the flow regime transits from flooding to full dispersion. Indeed, an increase in impeller speed leads to smaller bubble size and increased liquid phase recirculation, which in turn results in a longer retention time of bubbles in the liquid phase, allowing more time for surfactant adsorption before reaching the foam layer. This combination of factors increases foam stability and leads to increased foam formation. However, when the impeller speed exceeds a critical value, foam height remains constant or is reduced with a further increase in speed. In such situations, the strong liquid phase flow causes a drawdown and reincorporation of bubbles from the foam layer into the liquid, thereby reducing foam height and increasing gas holdup. At the same tip speed and gas flow rate, the up-pumping axial flow impeller globally produced less foam than the radial disc turbine and the down-pumping axial flow impeller. The up-pumping axial flow impeller is particularly effective in decreasing foam formation by reincorporation of bubbles into the liquid due to its specific liquid flow patterns close to the foam layer.