Hydrodynamics and mass transfer in internal loop airlift contactor with annulus sparger in saline water

Abstract

The investigation of the effect of salinity on the performance of airlift contactor was achieved using the 17L internal loop airlift system with a column height of 1.2 m, and 0.137 m diameter, and a draft tube height of 1 m. The diameters of the draft tubes were altered to vary the ratio between downcomer and riser cross sectional areas (A[subscript d]/A[subscript r]) from 0.061-1.01. The aeration was supplied in the superficial gas velocity (U[subscript sg]) from 0.01-0.07 m/s and the salinity levels were altered from 0, 15, 30, and 45 ppt. The Sauter mean diameter of the bubble appeared to be smaller in saline water than in fresh water due to two main reasons. The first one was the presence of hydrophilic repulsive force which inhibited bubble coalescence, and the second was the high Laplace pressure which promoted the breakup of the bubbles. The range of pressure difference, [delta]P, acting on the bubble which was found to promote bubble coalescence was between 15-20 N/m[superscript 2]. In saline water, the bubble size decreased with superficial gas velocity, u[subscript sg], where a reduction from 6 to 1 mm was observed with an increase in U[subscript sg] from 0.02-0.07 m/s. This was caused by the collision of bubbles at high gas hold-up in the system at higher gas flow rate which occurred at [delta]P greater than 20 N/m[superscript 2] and consequently supported bubble break-up. Axial variation in bubble size was only observed at low u[subscript sg] (less than 0.04 m/s) where bubbles in the bottom section of the airlift was larger than those in the middle and top sections. This was because the conditions in the middle and top sections were turbulent and with high [delta]P (27-65 N/m[superscript 2]), bubble break-up occurred. The effect of downcomer to riser area ratio was quite important and the highest range of [delta]P (approx. 50-97 N/m[superscript 2]) was found at the smallest downcomer area configuration which led to bubble break-up conditions. The overall volumetric mass transfer coefficient appeared lower in the saline water than in the fresh water where the levels of k[subscript L]a could be ordered from high to low as 0 ppt > 30 ppt > 15 ppt > 45 ppt. The specific area was found to be high in the saline water systems, however, the mass transfer coefficient was much higher in the fresh water system than in the saline water. The mass transfer coefficient was controlled by both natural and forced convections which were related significantly to the diameter and slip velocity of the gas bubbles.