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 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

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

Numerical modelling of iron (II) oxidation by aeration in an airlift reactor

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

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Euler-Euler large eddy simulations of the gas–liquid flow in a cylindrical bubble column

International audienceIn this work Euler-Euler Large Eddy Simulations (LES) of dispersed turbulent gas–liquid flows in a cylindrical bubble column are presented. Besides, predictions are compared with experimental data from Vial et al. 2000 using laser Doppler velocimetry (LDV). Two test cases are considered where vortical-spiral and turbulent flow regimes occur. The sub-grid scale (SGS) modelling is based on the Smagorinsky kernel with model constant and the one-equation model for SGS kinetic energy. The emphasis of this work is to analyse the performance of the one-equation SGS model for the prediction of bubbly flow in a three-dimensional high aspect ratio bubble column () of 20 and investigate the influence of the superficial gas velocity using the OpenFOAM package. The model is compared with the Smagorinsky SGS model and the mixture model in terms of the axial liquid velocity, the gas hold-up and liquid velocity fluctuations. The bubble induced turbulence and various interfacial forces including the drag, virtual mass and turbulent dispersion where incorporated in the current model. Overall, the predictions of the liquid velocities are in good agreement with experimental measurement using the one-equation SGS model and the Smagorinsky model which improve the mixture model in the core and near-wall regions. However, small discrepancies in the gas hold-up are observed in the bubble plume region and the mixture model performs much better. The numerical simulations confirm that the energy spectra of the resolved liquid velocities in churn-turbulent regime follows the classical −5/3 law for low frequency regions and are close to −3 for high frequencies. More details of the instantaneous local flow structure have been obtained by the Euler-Euler LES model including large-scale structures and vortices developed in the bubble plume edge

Hybrid Electrocoagulation/Electroflotation/Electrodisinfection (EC-EF-ED) process as a pretreatment for seawater desalination

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Electrochemical Oxidation as Treatment for Contaminated Wastewaters by Carbamazepine: Process Optimization Through Response Surface Methodology.

The electrochemical oxidation (ECO) of carbamazepine (CBZ), an antiepileptic drug, has been carried out in this study. A response surface methodology approach (RSM) was used in order to optimize the treatment process for CBZ removal on synthetic effluent. Four different operating parameters (current intensity, treatment time, recycling flow rate, and anode type) were chosen as key factors while a single response (CBZ removal) was considered. In the first part of the study, a factorial design (FD) methodology was carried out in order to evaluate the effects and interactions between the selected factors. Results showed that anode type is the most important parameters affecting CBZ degradation (with 67% of the overall effect) followed by the treatment time, the current intensity, and then the recirculation flow rate. Subsequently, a central composite design (CCD) was conducted in order to optimize the overall process taking into account efficiency (CBZ removal) and energy consumption. The contribution of direct and indirect effects of CBZ electro-oxidation was also investigated. As expected, direct oxidation was the most dominant mechanism during ECO with approximately 66% whereas indirect oxidation contributed with only 12%. Finally, the determined optimal conditions were applied on real pharmaceutical wastewater. Despite the effect matrix, 84% of CBZ was obtained after only 100 min of treatment with 23% of mineralization. Finally, CBZ by-products such as salicylic acid, catechol, and anthranilic have been detected during the oxidation process

Global sensitivity analysis to identify influential model input on thermal risk parameters: To cottonseed oil epoxidation

International audienceThermal runaway is still an important cause of accident in chemical industry. To evaluate the risk of such events, thermal risk assessment, which is a part of process safety, must be done. This assessment determines the safe operating conditions of a process by evaluating the thermal risk of an exothermic system. Nevertheless, based on thermal risk assessment, it is not possible to know which model inputs have the most influence on the thermal risk parameters. The knowledge of the most influential model inputs on thermal risk parameters is important to establish adequate safety barriers. Global sensitivity analysis was used to evaluate the influence of model inputs and their interaction on thermal risk parameters. It was performed on the exothermic system: epoxidation of cottonseed oil by performic acid in semibatch mode under isoperibolic conditions. The maximum reaction temperature and the time to reach this maximum reaction temperature were chosen as thermal risk parameters. We have also studied the influence of model inputs on the temperature rise. In the operating conditions of this study, it appeared that two parameters have the most influence on maximum reaction temperature and the temperature rise: the initial concentration of hydrogen peroxide and the jacket temperature, and one parameter for the time to reach this temperature: the jacket temperature