Residence time distribution measurements in an external-loop airlift reactor: Study of the hydrodynamics of the liquid circulation induced by the hydrogen bubbles

A detailed study of the residence time distribution (RTD) analysis of liquid phase has been performed in an external-loop airlift reactor of 20 L nominal volume, regarded as a global unit and discriminating its different sections (riser, gas–liquid separator and downcomer) using the tracer response technique. The reactor was used as an electrochemical reactor in order to carry out the electrocoagulation/electroflotation (EC/EF). The gas phase created in the riser is the hydrogen produced by water electrolysis.In order to use this reactor for a continuous EC/EF, hydrodynamic studies were carried out to control the operating conditions and to help modelling the electrocoagulation. Current density, position of the electrodes in the riser and the volumetric liquid flow (inlet flow) are the key parameters for the hydrodynamics. The experimental results revealed that both in the downcomer and the riser–separator zones, the flow model is axial dispersion. Interesting results were obtained: –The superficial liquid velocity (ULd) at the downcomer, decreased when the volume inlet flow increased (0<QL<2 L/min). –The Peclet number obtained in the downcomer was correlated to the current density and the electrodes position. –In the riser–separator zone the Peclet number decreased with the superficial liquid velocity in the riser indicating that the dispersion increased with an increase of turbulence created in the separator by an increase of liquid velocity. –The percentage of flow that quits the reactor without reacting increased when the main flow increased and the current intensity decreased. The global RTD can be reconstituted by the signal resulting from the junction and that from riser–separator and downcomer zone by using the convolution technique. The experimental results confirm this reconstitution. The experiments confirm also that the liquid crosses the reactor without achieving loops in the case of the continuous flow

Defluoridation of Morocco drinking water by electrocoagulation/electroflottation in an electrochemical external-loop airlift reactor

An innovative application of external-loop airlift reactors as electrocoagulation/electroflotation cells with Al electrodes for defluoridation of drinking water was developed. Liquid overall recirculation and mixing were induced only by hydrogen microbubbles electrochemically generated from the cathode. This application was carried out in a 20 L external-loop air liftreactor both under semi-batch and continuous flow conditions. Results showed that liquid recirculation could be correlated to current density and gas–liquid dispersion height in the separator. Experimental data obtained at optimum conditions that favored simultaneously mixing and flotation confirmed that concentrations lower than 1.5 mg/L could be achieved when initial concentrations were between 10 and 20 mg/L. The effects of conductivity and pH agreed with the literature. Conversely, the low electrode surface vs. reactor volume ratio merged the formation of fluoroaluminum microflocs near the electrodes to fluoride adsorption on these particles in the riser and the separator sections, which differed from conventional EC cells. Consequently, defluoridation could be achieved at lower energy and electrode consumptions than in the literature. An optimum current density was defined at j = 6 mA/cm2 for pH 5, accounting simultaneously for mixing, reaction time, yield and operating costs. A promising attempt of transposition from batch to continuous process was also reported in this work, as flotation avoids the need for a downstream settling unit

Defluoridation of drinking water by electrocoagulation/electroflotation in a stirred tank reactor with a comparative performance to an external-loop airlift reactor

Defluoridation using batch electrocoagulation/electroflotation (EC/EF) was carried out in two reactors for comparison purpose: a stirred tank reactor (STR) close to a conventional EC cell and an external-loop airlift reactor (ELAR) that was recently described as an innovative reactor for EC. The respective influences of current density, initial concentration and initial pH on the efficiency of defluoridation were investigated. The same trends were observed in both reactors, but the efficiency was higher in the STR at the beginning of the electrolysis, whereas similar values were usually achieved after 15 min operation. The influence of the initial pH was explained using the analyses of sludge composition and residual soluble aluminum species in the effluents, and it was related to the prevailing mechanisms of defluoridation. Fluoride removal and sludge reduction were both favored by an initial pH around 4, but this value required an additional pre-treatment for pH adjustment. Finally, electric energy consumption was similar in both reactors when current density was lower than 12 mA/cm2, but mixing and complete flotation of the pollutants were achieved without additional mechanical power in the ELAR, using only the overall liquid recirculation induced by H2 microbubbles generated by water electrolysis, which makes subsequent treatments easier to carry out

Kinetic study of defluoridation of drinking water by electrocoagulation/electroflotation in a stirred tank reactor and in an external-loop airlift reactor

A kinetic study of defluoridation of drinking water was carried out using the electrocoagulation/electroflotation process in two batch reactors of identical volume (20 L): a stirred tank reactor (STR) and an external-loop airlift reactor (ELALR). When the evolution of fluoride content was independent of stirring speed, experimental results showed that the kinetics of fluoride removal could be modelled using a variable-order-kinetic (VOK) approach coupled with a Langmuir–Freundlich adsorption model in the STR. Conversely, when mixing was less efficient, which is the case in the ELALR, experimental data could be fitted adequately only using a pseudo-first-order model. This constitutes however only an empirical approach based on a lumped parameter that accounts simultaneously for mass transfer, adsorption and electrochemical steps. In this case, only regression analysis could be used to establish a quantitative relationship between the kinetic constant and the operating conditions, such as current density and initial fluoride concentration