Abstract
ABSTRACTA modified airlift reactor (ALR) with a cross-shaped internal was developed to achieve beneficial flow conditions for an advantageous biological process. To study the effect of introduced internal on the reactor's hydrodynamics, computational fluid dynamics simulation method was applied to predict the flow characteristics verified with experimental data. The internal introduction into a conventional ALR resulted in increased liquid velocities in the riser and the downcomer for 12% and 16%, respectively, maintaining the superficial gas velocity of 3 cm/s. The internal allowed the total gas holdup slightly decreased for 4.8%, whereas the turbulent kinetic energy in the riser was significantly reduced for 13%. The cross-shaped internal modified the flow structure in the ALR's bottom zone due to, presumably, substitution of non-elastic collisions of liquid micro-lumps moving in counter-current directions with elastic collisions of micro-lumps with the wall of the internal, reducing the energy dissipation at the U-turn of the flow. The internal unified the direction of liquid velocity vectors in the bottom zone bringing an order to the liquid momenta in the flows close to the parallel ones and thus, saving energy in the ALR's operation.
Highlights
Airlift reactors (ALRs) are considered as promising ones in various industrial applications including process industries, biological fermentation and wastewater treatment [1,2]
The ALR, modified with a cross-shaped internal, used in the experiments was composed of a conventional ALR and a cross-shaped internal mounted beneath the riser
One can see that the decreasing grid size leads to small variances for the gas holdup and liquid velocities: when the mesh sizes were of 2 and 3 mm, the differences of simulation results for the gas holdup and the liquid velocity in the riser, and the liquid velocity in the downcomer were within 8.7%, 4.5% and 2.5%, respectively
Summary
Airlift reactors (ALRs) are considered as promising ones in various industrial applications including process industries, biological fermentation and wastewater treatment [1,2]. The ALRs’ advantages include their simple construction, absence of moving parts, large throughput on a continuous basis, low shear stress, efficient mixing and mass transfer with low energy requirement [3,4]. It has been shown that these internals enhance the performance of the reactors or gain special destination in the flow pattern. Airlift reactors are applied massively as biological reactors in large-scale wastewater treatment plants (WWTPs) [9]. The unstable flow pattern in the ALRs used in wastewater treatment results in increased turbulence shear stress and energy consumption making the efficiency of mixing and mass transfer decreased
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