| Airlift reactors are one of the most important classes of modified bubble column reactors in which the liquid circulation flow throughout the reactor is stabilized by means of using a draft-tube or an external tube. They are extensively used in various process industries mainly for the production of several compounds of biological origin, treatment of oil contaminated soils and water effluents, aeration of waste water, and for many catalyzed and un-catalyzed chemical reactions where good mixing conditions are required. This is mainly driven by their simple construction, the ability to maintain sterile conditions, low energy consumption, high mixing rates (without the presence of high local shear rates), and the ability to achieve high inter-phase mass transfer rates. Unfortunately, most of the available experimental data for gas/liquid contacting in ALR were obtained using the air-water system (which is not necessarily representative of industrial situations), and the findings of the few published experimental investigations addressing the influence of system properties are rather scattered and incomplete. Furthermore, only one investigation addressed the use of twin-fluid spargers.;The ability of the twin-fluid sparger to generate fine bubbles in slowly-coalescing electrolyte solutions was found to beneficially impact the performance of the ALR. For example, gas holdup values as high as 23 %, along with liquid circulation velocity of 1.06 m/s, turnover times down to 2.7 s, and mixing times of 16 s could be achieved at superficial velocities as low as 0.03 m/s. The hydrodynamic and mass transfer performance of the ALR was also found to be strongly influenced by changing the bottom clearance height of the reactor. Mass transfer coefficients as high as 0.125 s -1 were obtained at superficial velocities as low as 0.03 m/s which is more than 4-fold higher than those obtained using conventional spargers.;Although these performance indicators are superior to all published results, further improvement may be achieved by adopting a more efficient bubble separator design and/or by increasing the height of the reactor.;The effect of superficial gas velocity, pressure drop across the sparger, salt concentration, and the bottom clearance height on the hydrodynamics (gas holdup, liquid circulation velocity and mixing time, and energy dissipation rates), and mass transfer characteristics (volumetric mass transfer coefficient and the mass transfer coefficient per unit volume of gas present in the reactor) of airlift reactors was investigated using a 0.2 m3 pilot scale unit equipped with a twin-fluid sparger capable of generating fine bubbles. Gas holdup was characterized by using pressure difference method, while the liquid circulation velocity and mixing time were obtained by tracer injection technique. The oxygen transfer rate was determined using the dynamic oxygenation technique. Labview 7.1 software was used to control the experiments, for data acquisition and to analyze the results. |
