Numerical modeling of ferrous iron oxidation in a split-rectangular airlift reactor

- CFD is used to simulate of the iron(II) removal from drinking water.
- Coupled hydrodynamics, gas-liquid mass transfer and reaction kinetics are analyzed.
- Global gas hold-up, liquid recirculation velocity and kLa are accurately predicted.
- The single-orifice nozzle induced strong heterogeneities of gas holdup and kLa.
- Iron(II) removal was fairly predicted, showing the effect of these heterogeneities.

In this work, CFD is used to simulate the gas-liquid dispersion coupled to the kinetics of the iron(II) removal from drinking water by aeration process in a split-rectangular airlift reactor. The model aims to describe oxidation of iron(II) into insoluble iron(III) species in water treatment, so that hydrodynamics, oxygen mass transfer, chemical reactions, and pH change can be taken into account. In comparison to the abundant literature using CFD on airlift reactors, this work focuses on a particular reactor in which a highly non-uniform primary gas distribution is ensured by a 3.5 mm diameter single-orifice nozzle located at the bottom center of the riser. This makes convergence more difficult, as local gas velocity is high near the nozzle. A multiphase Euler-Euler model with a unique bubble size and a standard k-ε model of turbulence are used. The 3-D CFD simulations are able to predict the liquid circulation velocity and the average gas holdup both in the riser and the downcomer. The model can also identify the transition from the gas flow regimes without bubbles and with stationary bubbles in the downcomer, while predicted kLa values agree with reoxygenation experiments. Finally, the simulations also predict accurately the oxidation of iron(II) vs. time without or in the presence of iron(III) as a catalyst, and they are also able to detect where local heterogeneities in dissolved oxygen concentration can emerge when pH and/or the amount of catalyst is increased.