Application of metallic foams in electrochemical reactors of filter-press type Part I: Flow characterization

The aim of the present paper is to investigate the application of nickel foam electrodes in classical filter-press type electrochemical cells. For this purpose the ElectroSynCell®, commercialized by Electrocell AB, has been used. The interest of using metallic foams linked to a classical plane plate is to improve the performance of the electrochemical cell by increasing the electrode surface area. Moreover, for some industrial applications it is possible to use the cell without a membrane. In the proposed configuration, the working electrodes consist of metallic foam linked to a plane plate and the auxiliary electrode is simply a plane plate. The imbalance between the surface areas of working and auxiliary electrodes allows operation with a single hydraulic circuit. This paper, focuses on the study of residence time distribution and pressure drop in order to compare the flow behaviour with that in a classical configuration. A model for the reactive zone of the cell is also given.Notation D _ax axial dispersion coefficient (m2 s^–1) - ER root mean square error - F(s) transfer function in Laplace domain - I number of continuous stirred tank reactors in the first cascade (model of the two cascades) - J number of continuous stirred tank reactors in the second cascade (model of the two cascades) - L length of the path in the reactor (m) - P pressure drop (Pa) - Pe _r Peclet number of reactor - Q _v volumic flow rate (m³ s^–1) - _s mean residence time (s) - u interstitial velocity (m s^–1) - U _o mean superficial velocity (m s^–1) - X(t) experimental signal at the inlet of the reactor - Y(t) experimental signal at the outlet of the reactor - Y _ca1(t) calculated signal at the outlet of the reactor Greek letters porosity of the duct in the reactor - standard deviation - ₁ mean residence time in the first cascade (s) - ₂ mean residence time in the second cascade (s)     

几何结构对滤压式电解槽内部传质的影响

Effect of configuration （structure of electrode, interelectrode gap, positions of inlet and outlet, volume of the cell and additional nets） on mass transfer characteristic of a filter-press type electrochemical cell has been studied. The mass transfer coefficients on the electrodes were obtained by using the well-known technique based on the determination of limiting diffusion current. It is found that mass transfer coefficients with mesh electrode are greater than that of with plate electrode. Mass transfer coefficient is decreased with interelectrode gap. While interelectrode gap achieved a certain value （7 ram）, mass transfer coefficient is steady, no more declining. Mass transfer characteristic for different positions of inlet and outlet are different and dimensionless number groups correlated equations are obtained by experiment. Mass transfer characteristic is the best when inlet located on the top and outlet on the bottom of the cell respectively. While magnified the volume of the cell to eight times, mass transfer characteristic changes little. Mass transfer characteristic without nets is lower than that of with additional nets in the exit region, but higher than that of with additional nets in the entry region.     

Characterization of the reaction environment in a filter-press redox flow reactor

The characteristics of a divided, industrial scale electrochemical reactor with five bipolar electrodes (each having a projected area of 0.72 m²) were examined in terms of mass transport, pressure drop and flow dispersion. Global mass transport data were obtained by monitoring the (first order) concentration decay of dissolved bromine (which was generated in situ by constant current electrolysis of a 1 mol dm⁻³ NaBr_(aq)). The global mass transport properties have been compared with those reported in the literature for other electrochemical reactors. The pressure drop over the reactor was calculated as a function of the mean electrolyte flow velocity and flow dispersion experiments showed the existence of slow and fast phases, two-phase flow being observed at lower velocities.     

Problems in modelling and the study of electrochemical reactor dynamics

Proper understanding of the unsteady state behavior (start-up, shut-down, process perturbation effects, etc. …) of electro-chemical reactors requires realistic mathematical modelling. Inexact current knowledge of all thermodynamic, hydrodynamic and kinetic aspects of electrochemical systems, in addition to inherent mathematical complexities requires approximations to allow the characterization of such systems via control engineering principles. The usefulness of the process dynamics approach is illustrated by practical examples.     

Application of metallic foams in electrochemical reactors of the filter-press type: Part II Mass transfer performance

This paper focuses on mass transfer characteristics of classical filter-press electrochemical reactors without membranes. In the tested configuration, the working electrode consists of a lane plate with a sheet of foam and the counter-electrode consists of a plane plate with a turbulence promoter. The global mass transfer coefficients of the two electrodes have the same order of magnitude. Moreover, a comparison with literature data shows that their values remain in the range of those previously presented. Due to the high specific surface area of the foam used (A _ve, = 6400 m^–1), the ratio of the surface area of the working electrode to that of the counter electrode is 15. The electroreduction of ferricyanide has been carried out to test the performance of this configuration. The value of the final conversion has been compared to that calculated from mass transfer coefficients and surface areas of the electrodes.List of symbols A _ve dynamic specific surface area of the foam: surface area per volume of material (m^–1) - A_ve dynamic specific surface area of the electrode consisting of a plate and a sheet of foam: surface area per volume of electrode (m^–1) - A _vs static specific surface area (m^–1) - C _in ferricyanide concentration at the inlet of the cell (mol m^–3) - C _out ferricyanide concentration at the outlet of the cell (molm^–3) - D diffusion coefficient (m² s^–1) - d _h equivalent hydraulic diameter, dh = 2lh (l + h)^–1 (m) - F Faraday number (C mol^–1) - h channel thickness (m) - I limiting diffusion current (A) - I _{c a} final limiting diffusion current intensity at the anode (A) - I _cf final limiting diffusion current intensity at the cathode (A) - k _a mass transfer coefficient at the anode (m s^–1) - k _c mass transfer coefficient at the cathode (ms^–1) - k _d mass transfer coefficient (m s^–1) - l channel width (m) - n number of electrons in the electrochemical reaction - Q _v volumetric flow rate in the channel (m³ s^–1) - Re Reynolds number, Re = U ₀ d _h v ^–1 - S active surface area of the electrode (m²) - S _a surface area of the anode (m²) - S _c surface area of the cathode (m ²) - S _c Schmidt number, Sc = v D ^–1 - Sh Sherwood number, Sh = k _d D _h/D - U ₀ superficial velocity (m s^–1) - V volume offered to fluid flow in the volumic electrode (m³) - V volume of one tank reactor in the cascade (m³) - X conversion - X _f final conversion Greek letters porosity - v kinematic viscosity (m² s^–1) - density (kg s^–1) - residence time in a continuous stirred tank reactor = /Q _v (s)     

The modelling of concentration—time relationships in recirculating electrochemical reactor systems

Approximate and rigorous models are presented and compared for the prediction of concentration—time and current—time relationships for electrochemical reactor systems operating with continuous recirculation of the electrolyte. The reservoir is considered as a well mixed tank while the reactor is considered both as a plug and as a perfectly back mixed system. Application to the depletion in concentration of Cu²⁺ containing electrolytes using fluidized bed reactors is discussed.     

Mathematical reactor modelling for coupled chemical and electrochemical processes: the ECE system

A procedure for modelling electrochemical reactions and reactors which involve heterogeneous reaction, homogeneous fast chemical reaction and diffusional mass transport is described. The procedure can be applied to any combination of first order reaction processes utilising numerical routines for the solution of initial value differential equations. By the use of collocation it can be extended to higher order processes. The reactor types considered are batch, plug flow and dynamic continuous stirred tanks and reactors with recycle. Operation with either potentiostatic, galvanostatic or constant cell voltage control is described and illustrated using the ECE reaction mechanism, involving successive electrochemical, chemical and electrochemical reaction.     

Prediction of mass transport profiles in a laboratory filter-press electrolyser by computational fluid dynamics modelling

A commercial computational fluid dynamics code (Fluent) has been used to analyze the performance of a unit cell laboratory; the filter-press reactor (FM01-LC) operating with characteristic linear flow velocities between 0.024 m s⁻¹ and 0.110 m s⁻¹. The electrolyte flow through the reactor channel was numerically simulated using a finite volume approach to the solution of the Navier-Stokes equations. The flow patterns in the reactor were obtained and the mean linear electrolyte velocity was evaluated and substituted into a general mass transport correlation to calculate the mass transport coefficients. In the region of 150 < Re < 550, mass transport coefficients were obtained with a relative error between 5% and 29% respect to the experimental k_m values. The differences between theoretical and experimental values are discussed.     

Performance of a multipurpose research electrochemical reactor

This paper reports on a multipurpose research electrochemical reactor with an innovative design feature, which is based on a filter press arrangement with inclined segmented electrodes and under a modular assembly. Under bipolar connection, the fraction of leakage current is lower than 4%, depending on the bipolar Wagner number, and the current distribution is closely uniform. When a turbulence promoter is used, the local mass-transfer coefficient shows a variation of ±10% with respect to its mean value. The fluidodynamics of the reactor responds to the dispersion model with a Peclet number higher than 10. It is concluded that this reactor is convenient for laboratory research.     

Global modelling of a gas-liquid-solid airlift reactor

This paper presents a global model of three phase flow (gas-liquid-solid) in an internal airlift reactor. The airlift is composed of four zones: a riser (on the aerated side on the internal wall), a downcomer (on the opposite side) and two turning zones above and below the internal wall. Tap water is the liquid continuous phase and the dispersed phases are air bubbles and polyethylene particles. The global modelling of the airlift involves mass and momentum equations for the three phases. The model enables phase velocities and phase volume fractions to be estimated, which can be compared to experimental data. Closure relations for the gas and solid drift velocities are based on the model proposed by Zuber and Findlay. The drift flux coefficients are derived from CFD numerical simulations of the airlift. Gas bubble and solid particle averaged slip velocities are deduced from momentum balances, including drag coefficient correlations. The link between Zuber and Findlay model and the two-fluid model is established. In the experiment as well as in the model, the gas flow rate is fixed. However, the liquid and solid flow rates are unknown. Two closure relations are needed to predict these flow rates: the first closure relation expresses that the volume of solid injected into the airlift remains constant; the second closure relation expresses a global balance between the difference of column height in the riser and the downcomer and the total pressure drop in the airlift. The main parameters of a three phase airlift reactor, like gas and solid volume fractions, are well predicted by the global model. With increasing solid filling rate (40%), the model starts to depart from the experimental values as soon as coalescence of bubbles appears.     

The electrochemical recovery of iron in a two-compartment batch electrochemical reactor

A model has been developed for a two-compartment batch electrochemical reactor in which simultaneous iron deposition and hydrogen evolution occur at the cathode, the anodic reaction being the production of hydrogen ions. The model incorporates ionic diffusivities through the diaphragm and a composite kinetic parameter for the cathodic reactions. In order to apply the model to data from a small batch reactor, the diffusivities have been obtained under conditions in which hydrogen alone is evolved at the cathode. The value of the kinetic parameter has been chosen to provide the best agreement between the model predictions and the experimental results. Satisfactory predictions of the current efficiency for iron deposition and the final concentrations in the reactor has been obtained, the largest discrepancies between predicted and actual concentrations being at high anolyte hydrogen ion concentrations. Reasons for these discrepancies are discussed.     

Electrooxidation of benzyl alcohol and benzaldehyde on a nickel oxy-hydroxide electrode in a filter-press type cell

The electrooxidation of benzyl alcohol and benzaldehyde in alkaline medium was carried out in a filter-press type cell on a nickel oxy-hydroxide electrode under different experimental conditions. An overpotential occurs in the presence of organic molecules in the solution shifting oxygen evolution towards higher potentials. The results obtained were conclusive that benzyl alcohol and benzaldehyde electrooxidation on NiOOH layers yielded benzoic acid as the main final reaction product. Chromatographic analysis of the bulk solution showed that the electrocatalytic oxidation of harmful molecules was carried out until the formation of acid compounds (benzoic acid) as an ultimate stage, suggesting that a Ni anode can be used successfully for waste remediation.     

The application of flow dispersion models to the FM01-LC laboratory filter-press reactor

The flow distribution in the rectangular channel of a laboratory filter-press electrochemical reactor was evaluated using three flow models namely: (a) axial dispersion, (b) sum of two phases and (c) fast and stagnant zones. In the case of the axial-dispersion model, several methods have been used to calculate the Peclet number; the moment method, the non-linear least-squares and the Laplace transform technique. Several boundary conditions, involving different physical and experimental assumptions of the flow were used to solve the partial differential equation that describes the flow behaviour. A total of nine expressions to examine flow dispersion has been used. The comparison of experimental and predicted response signals was made by evaluating the root mean squared error. A data fit in real time has been found to be a better choice as solutions based on the evaluation of moments are prone to error due the overweight of the signal at long times. Data fitting in the Laplace plane is very accurate but it does not guarantee a good fit in real time. Models based on the sum of a fast and a slow or stagnant phase resulted in solutions having very low values of the extension of the slow and stagnant phases, the assumption of a single phase with some degree of dispersion was considered more appropriate.     

Dynamic simulation of a parallel-plate electrochemical fluorination reactor

A time-dependent mathematical model of a parallel-plate reactor was developed to study the electrochemical fluorination of organic compounds dissolved in anhydrous hydrogen fluoride. The model incorporates two-phase flow with differential material, energy and pressure balances. Dynamic results are presented that show the effect of disturbances to the whole reactor or one of the cells of the multicell reactor. The effect of disturbances in the cell current and inlet electrolyte flow rate on the cell voltage, molar flow rates, and current efficiency is studied. Also, the effect of a blockage in inlet flow to one cell in the cell pack is studied for cases when heat transfer is either present and absent between the adjoining cells.     

Dynamic modelling of a three-phase catalytic slurry reactor

Two dynamic models for a three-phase catalytic slurry reactor with appropriate solution procedures were developed in this work. The models consist of mass and heat balance equations for the catalyst particles, for the gas and liquid bulk phases as well as for the heat exchange through the jacket of the reactor. The models of the tubular reactor were applied to describe the dynamic behaviour of the reactor during the hydrogenation of o-cresol on Ni/SiO₂ catalyst. These models differ in solid phase modelling, which allows to evaluate the reactor dynamic behaviour prediction capacity. The models successfully reproduce the main characteristics of the reactor dynamic behaviour.     

Mass transfer characteristics of a novel gas-evolving electrochemical reactor

Mass transfer coefficients were measured for the deposition of copper from acidified copper sulphate solution at a vertical-plate cathode stirred by the oxygen evolved at a coplanar, vertical-plate lead anode placed upstream from the cathode. The variables studied were: oxygen discharge rate, electrolyte concentration and height of the cathode. The cathodic mass transfer coefficient was increased by a factor of the order of 2 to 5 over the natural convection value depending on the rate of oxygen discharge at the lead anode. The relationship between the mass transfer coefficient and the oxygen discharge rate was found to be logK=a+0.296 logV.The mass transfer coefficient was found to decrease initially with increasing electrode height, but then reached a constant value independent of further change in electrode height. A new, modified parallelplate reactor stirred by the counter electrode gases is described. The advantage of the design is that it offers more efficient stirring with no added power consumption. Details of the design are discussed.Nomenclature C concentration of CuS0₄ (mol cm^–3) - F Faraday's constant - h electrode height (cm) - K mass transfer coefficient (cm s^–1) - l _L limiting current density (A cm^–2) - V gas discharge rate (cm s^–1) - z number of electrons involved in the reaction On leave from the Department of Chemical Engineering, Faculty of Engineering, Alexandria University, Alexandria Egypt.     

Steady state and local stability in a plug-flow electrochemical reactor

Approximate analytical methods are presented for the estimation of steady state electrolytic concentration and temperature profiles, and for the determination of stable behaviour under small perturbations in their inlet values.     

The continuous stirred tank electrochemical reactor. An overview of dynamic and steady state analysis for design and modelling

The continuous stirred tank reactor is frequently adopted as a model for electrochemical reactors. This article brings together the various important aspects of the model: dynamics, thermal characteristics, residence time distributions and steady state characteristics and gives an overview of the design procedures.     

An experimental study of a pilot-scale undivided and quasi-undivided electrochemical reactor

The performance of a pilot-scale (200 cm²) electrochemical reactor of the undivided parallel plate type, is reported, using the Ce⁴⁺ ?Ce³⁺ redox couple as test reaction in a closed loop. The effect of electrode surface texture, anode—cathode area ratio and other parameters are evaluated. Results are also reported for the quasi-divided operational mode, where one electrode is covered with an inert cloth. Such cells are shown to have minimal back-reactions and hence to offer an effective alternative to the traditional divided cell. The results are compared with a simple model in which mass transport is assumed to be due solely to co-evolution of gas.