Falling film hydrodynamics of black liquor under evaporative conditions

Bubbles were observed in a thin, evaporating, falling film of black liquor (a fluid mixture generated during the pulp production) on the exterior wall of a research evaporator. Because the presence of bubbles could not be explained by nucleate boiling, a combination of turbulent vapour entrainment and effects due to surface-active compounds – surfactants – is proposed. Black liquor contains numerous surfactants, which are likely to enhance bubble formation and stabilization in the fluid and on the film interface. One observed important effect of bubble formation was fluid loss due to bubble-bursting aerosolization (sputtering). Also, bubbles and bubble processes probably alter the film velocity-profile and heat transfer resistance, thereby affecting heat transfer across the film and hence evaporator efficiency.