Influence of salinity on bubble size distribution and gas–liquid mass transfer in airlift contactors

The investigation of the effect of salinity on the performance of airlift contactor was achieved using the 17 l internal loop airlift with height of 1.2 m, and 0.137 m diameter. Various draft tubes with different diameter sizes were provided to vary the ratio between downcomer and riser cross-sectional areas (Ad/Ar) from 0.061 to 1.01. The superficial gas velocity (usg) was supplied in a range from 0.01 to 0.07 m/s and the salinity was adjusted from 0 to 45 ppt. The Sauter mean diameter of the bubble (dBs) appeared to be smaller in saline water than in fresh water. Bubble size was regulated by two factors. The first one was the hydrophilic repulsive force which inhibited bubble coalescence whereas the second was the Laplace pressure which controlled the coalescence and breakup of bubbles. The range of pressure difference, ΔP, acting on the bubble that promoted bubble coalescence was between 15 and 20 N/m2 below which bubble coalescence was inhibited and above which bubble breakage prevailed. In saline water, dBs decreased with usg. This was caused by the collision and breakup of bubbles at high gas holdup which occurred at ΔP greater than 20 N/m2. Axial variation in dBs was only observed at low usg (less than 0.04 m/s) where bubbles in the bottom section of the airlift were larger than those in the middle and top sections. It was anticipated that the middle and top sections exhibited higher turbulent conditions than the bottom section at this low aeration rate. The effect of draft tube size was quite important where the smallest draft tube (smallest downcomer area) best promoted the breakup of the bubbles with a relatively high ΔP of approximately 50–97 N/m2. The effect of salinity on the overall volumetric mass transfer coefficient (kLa) was only apparent at high aeration rate where the fresh water provided a higher kLa than the saline water. In fact, the specific area (a) was high in the saline water systems, however, the mass transfer coefficient (kL) was higher in the fresh water system than saline water. Finally, a general correlation for the estimation of kL in the airlift system was proposed.