Theoretical and Experimental Studies of the Dynamic Behaviour of Plug Flow Electrochemical Reactors for a Step Change in Flow Rate

This work presents an analysis of the dynamic behaviour of a plug flow electrochemical reactor for a step change in flow rate. The mathematical model takes into account the temporal variation of the mass transfer coefficient of the electrochemical reaction by means of an empirical expression with one parameter. To simplify the numerical solution of the general differential equation different assumptions are made. Thus, analytical equations of the transient current response are obtained. The theoretical treatments are compared with experimental results, obtained from copper deposition in a parallel-plate electrochemical reactor, in order to determine the reliability and the correlation capability of the mathematical models. The influence of the empirical parameter values on the performance of the reactor is also analysed.     

Macrokinetic analysis of polarisation characteristics of gas-diffusion electrodes in contact with liquid electrolytes, Part II: Oxygen reduction as example for a higher order reaction

Classical partial oxidation processes often suffer from low selectivities. Promising alternatives are electrochemical processes where the oxidation takes place at a packed-bed anode while an oxygen-consuming gas–liquid membrane is used as cathode. As a basis for the reliable design of such a process, the performance of oxygen-consuming gas-diffusion electrodes (GDE) is investigated experimentally and is analysed based on a rigorous model accounting for the reaction microkinetics and all relevant mass and charge transport phenomena. The results indicate that oxygen is transported in the gas-filled pores by Knudsen diffusion and that the cathodic oxygen reduction follows a parallel reaction scheme forming hydrogen peroxide at the carbon black support and water at the applied platinum catalyst particles.     

Modeling of a Tubular‐SOFC: The Effect of the Thermal Radiation of Fuel Components and CO Participating in the Electrochemical Process

A mathematical model based on first principles is developed to study the effect of heat and electrochemical phenomena on a tubul solid oxide fuel cell (SOFC). The model accounts fordiffusion, inherent impedance, transport (momentum, heat and mass transfer) processes, internal reforming/shifting reaction, electrochemical processes, and potential losses (activation, concentration, and ohmic losses). Thermal radiation of fuel gaseous components is considered in detail in this work in contrast to other reported work in the literature. The effect of thermal radiation on SOFC performance is shown by comparing with a model without this factor. Simulation results indicate that at higher inlet fuel flow pressures and also larger SOFC lengths the effect of thermal radiation on SOFC temperature becomes more significant. In this study, the H₂ and CO oxidation is also studied and the effect of CO oxidation on SOFC performance is reported. The results show that the model which accounts for the electrochemical reaction ofCO results in better SOFC performance than other reported models. This work also reveals that at low inlet fuel flow pressures the CO and H₂ electrochemical reactions are competitive and significantly dependent on the CO/H₂ ratio inside the triple phase boundary.     

Mass transfer modeling and simulation at a rotating cylinder electrode (RCE) reactor under turbulent flow for copper recovery

This work presents the numerical simulation of a laboratory reactor with rotating cylinder electrode (RCE) and a six-plate counter electrode that is used in studies on heavy metal recovery. The rate of electrode rotation and the potential applied are of such magnitude that the electrochemical reactor works in conditions of mass transport control under turbulent flow to obtain high recovery rates and formation of dendritic metal deposits. For hydrodynamics, the Reynolds averaged Navier–Stokes (RANS) equations were solved using the standard k–ε turbulence model, as well as wall functions based on the universal velocity distribution in the near-wall region. Results of 3-D simulations of the velocity field show clearly the formation of the turbulence Taylor vortex flow. For mass transfer, convection–diffusion equation was solved using the Kays–Crawford model for turbulent Schmidt number and Launder–Spalding wall functions adapted for mass transfer. Kinetics of copper recovery from aqueous solutions containing 0.019 M CuSO₄ and 1 M H₂SO₄, in the range of rotation speed of 400–1100 rpm, was adequately fit (error <8%) during the electrolysis time to achieve a final recovery of 85% for potentiostatic and 60% for galvanostatic experiments. The fitting parameter of the concentration wall function used in all experiments was A=2.9.     

Internal stress and magnetic properties of electrodeposited amorphous Fe–P alloys

The aim of this work is to investigate the use of a new type of reactor for electroorganic synthesis. The concept of the reactor is based on the principle of the porous percolated pulsed electrode (E3P) which was primarily developed at commercial scale for metal recovery in waste waters. The reactor is fitted with a three-dimensional electrode, of axial configuration, consisting of ordered stacks of discs of expanded metal. It can be supplied either by a homogeneous electrolyte or by an emulsion generated by an external ultrasonic system. The pulsation of the electrolyte represents a very effective means of improving mass transfer rates at the electrode. Under two phase conditions, the role of the pulsation is also to ensure the hydraulic transport of the emulsion and to increase the three phase contacts between the aqueous phase, the organic phase and the electrode. The efficiency of the reactor was tested using both homogeneous and two phase liquid–liquid electrolytes in the direct reduction process of an aromatic ketone. This study reports the effects of the pulsation on the mass transfer rate of acetophenone at the electrode. A comparative study of the behaviour of the E3P reactor towards different media is accomplished. Particular attention is paid to the chemical and faradaic yields, as well as to the selectivity of the reaction.     

Electrochemical machining under pulsed current conditions

The possibility of using pulsed current in electrochemical machining at low electrolyte flow rate has been investigated. Theoretical aspects of predicting electrolyte heating and limiting rate of mass transport are discussed in terms of simplified models. High rate dissolution of nickel in sodium chloride solutions under pulsed current conditions was investigated in a flow channel cell by studying the influence of different pulse parameters on anode potential, surface microtexture, surface roughness and current efficiency of metal dissolution. Obtained results indicate that anode potential and surface finish are controlled by mass transport in agreement with steady state behavior. Maximum current density applicable under pulsed current conditions is limited by the occurrence of sparking.     

Computer modelling and numerical analysis of hydrodynamics and heat transfer in non-porous catalytic reactor for the decomposition of ammonia

Chemical reactors exhibit very complex behaviours such as multiple steady states, oscillations, etc. resulting from complex linkage between the transport processes and the non-linear chemical reaction kinetics. Ammonia is a potential hydrogen source for a number of fuel cell applications for small scale power generation useful for portable equipments. In the present work, we analyse the fluid dynamics and heat transfer in catalytic microreactor systems for the decomposition of ammonia over a monolayer Ni non-porous catalyst. The overall model for this convective-diffusive-reactive system consists of a flow model, a mass transport model, an energy conservation model and a reaction kinetics model for ammonia decomposition. The flow model is described by the Stokes equation for a creeping flow regime. The mass transport and energy conservation models are based on convective-diffusion equations. The rate of ammonia decomposition can be measured as a function of the catalyst activity and ammonia concentration. A standard Galerkin finite element technique has been applied for the solution of the flow equations. A slightly perturbed form of the mass continuity equation is used to satisfy the Ladyzhenskaya-Babuška-Brezzi stability criterion. For the solution of convection-diffusion equations, a streamline inconsistent upwind finite element scheme has been chosen to avoid any spurious oscillations. C⁰-continuous 9-noded Lagrangian biquadratic isoparametric finite elements are used for the approximation of the field variables. A second-order Taylor-Galerkin time-stepping scheme has been chosen for the temporal discretisation of the flow equations whilst an implicit theta method has been used for convection-diffusion equations. The results are presented in the form of velocity vectors and concentration, temperature contours and are examined for stability, convergence and theoretical consistency.     

The effect of catalyst film thickness on the electrochemical promotion of ethylene oxidation on Pt

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion of catalysis (EPOC or NEMCA effect) was investigated for the model catalytic reaction of C₂H₄ oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement ρ is up to 400 for thinner (0.2 μm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 μm) films. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O²⁻ species which causes electrochemical promotion.     

The influence of mass transport on the effectiveness of particulate bed electrodes

Approximate methods of analysis are presented for the estimation of effectiveness of particulate bed electrodes under activation control. The approximate analytical expressions based on a Taylor series and an asymptotic form give reasonable agreement to exact solutions. The effect of the various system parameters are characterized by two dimensionless group ν² and δ. The influence of mass transport on the rate of electrochemical reaction is treated in an analogous manner and is characterized by an additional dimensionless parameter γ representing the ratio of the maximum (diffusion limited) and equilibrium rates of reaction.     

Reactor engineering models of complex electrochemical reaction schemes—I. Potentiostatic operation of parallel and series reactions in ideal reactors

Mathematical models of complex electrochemical reaction schemes are developed in which mass transport of reactant and products at the electrode surface plays an important part in describing overall reaction rates. Parallel and series reactions are treated in two ideal reactor configurations: batch and plug flow. Cell operation is restricted to potentiostatic operation which allows the model solutions to be obtained in analytical form. The model is used to simulate the behaviour of a potentially useful organic synthesis; the electroreduction of oxalic acid to glyoxylic acid.     

In situ controlled electrochemical promotion of catalyst surfaces: Pd-catalysed ethylene oxidation

The catalytic activity of polycrystalline Pd films deposited on 8 mol% Y₂O₃-stabilized–ZrO₂ (YSZ), an O^2–-conductor, can be altered reversibly by varying the potential of the Pd catalyst film via the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 290–360 °C and atmospheric total pressure. The rate of C₂H₄ oxidation can be reversibly enhanced by up to 45% by supplying O^2– to the catalyst via positive current application. The steady-state rate change is typically 10³–10⁴ times larger than the steady-state rate I/2F of electrochemical supply or removal of promoting oxide ions. The observed behaviour is discussed on the basis of previous NEMCA studies and the mechanism of the reaction.     

CFD evaluation of internal manifold effects on mass transport distribution in a laboratory filter-press flow cell

The internal manifold geometry strongly influences the flow distribution inside an electrochemical reactor. The mass transport coefficient is a function of the flow pattern and is a key parameter in successful electrochemical reactor design and scale-up. In this work, a commercial computational flow dynamics (CFD) package was used to describe the flow pattern in the FM01-LC reactor at controlled volumetric flow rates (corresponding to mean linear flow velocities past the electrode surface between 0.024 and 0.11 m s^?1). Numerical Re numbers were obtained for each local flow velocity at different positions in the reactor channel. From a known mass transport correlation (based on dimensionless groups, i.e. Sh, Re, Sc), numerical k _m values were obtained (in the range 200 < Re < 1,000) at different positions in the reactor channel. Computed k _m numbers are compared against experimental values. This computational approach could be useful in reactor design or selection since it facilitates a fast, preliminary reactor flow and mass transport characterisation without experimental electrochemical measurements.     

模拟循环冷却系统黄铜管的腐蚀电化学

用研制的黄铜管腐蚀监测传感器对模拟循环冷却系统的黄铜管进行了电化学阻抗谱和电化学噪声测试.电化学阻抗谱测试结果表明,缓蚀剂能够在金属表面上形成保护性膜层抑制腐蚀过程.流速提高时溶液中氧的扩散速度提高,使缓蚀剂在HAl77-2管表面上的成膜作用加强,腐蚀速度降低.电化学噪声测试结果表明,提高流速对HSn70-1管局部腐蚀敏感性影响不大,而HAl77-2管在较高流速下局部腐蚀敏感性提高,表明电化学噪声能用于循环冷却系统黄铜管的腐蚀监测.流速对逆水流方向传感器的影响大于顺水流方向,因此实际应用时传感器应顺水流方向放置,以减小流速的影响.研究结果表明,所研制的传感器适用于动态条件下黄铜管的电化学测试.     

Steady-state mass transport at stationary discs under divergent laminar radial flow conditions

This paper deals with global and local wall-to-liquid mass transfer under divergent laminar radial flow conditions between two parallel stationary discs. Approximate theoretical expressions for local and overall transport rates in piston flow and Poiseuille flow, and empirical correlations from the literature, are compared to experimental observations utilizing the electrochemical method of rate measurement. The experimental results support the theoretical approach for this laminar flow regime, the latter expanding the scope of the applicability of electrochemical methods presented in the literature.     

环丙沙星、诺氟沙星、氧氟沙星在1 mol·L~(-1) HCl中对碳钢的缓蚀性能及机理研究(英文)

The inhibiting effect of ciprofloxacin,norfloxacin and ofloxacin on the corrosion of mild steel in 1 mol·L-1 HCl and the mechanism were studied at different temperatures using mass loss measurement,electrochemical method,and X-ray photoelectron spectroscopy(XPS) .Effective inhibition was shown by mass loss,potentiodynamic polarization and impedance spectroscopy measurement.The corrosion rate of the metal in the mass loss measurement,and the corrosion reaction on cathode and anode in the electrochemical measurement were accelerated when temperature was increased.XPS results showed that the inhibitors adsorbed effectively on the metal surface.     

Removal of triclosan by conductive‐diamond electrolysis and sonoelectrolysis

BACKGROUND: Removal of triclosan by conductive‐diamond electrochemical oxidation (CDEO) and sonoelectrochemical oxidation (CDSEO), either in methanol/water or methanol solutions is studied in the range of concentration 0.1 to 100 mg dm^?3. Effects of current density and of electrolyte on the process efficiency are assessed. RESULTS: Concentration of triclosan can be removed below the detection limit of HPLC. In methanol/water solutions, there are two stages: a rapid decrease in the concentration and a less efficient process in which the rate is slower. In methanol solutions, experimental results clearly fit first‐order kinetics. Mediated oxidation plays a significant role. Electrolyses in sulphate media are more efficient than in chloride media and the second low‐rate stage in methanol/water solution appears for higher triclosan removal. The main intermediates are catechol, chlorohydroquinone, 4‐chlorocatechol, acetic acid and dichloroacetic acid. They were only detected during the electrolysis of highly concentrated solution, suggesting a very energetic oxidation. CONCLUSIONS: CDEO and CDSEO are very efficient for the removal of triclosan. CDSEO improves mass‐transfer processes and gives a more efficient removal at lower concentrations. The removal of methanol is improved significantly, in spite of its concentration being well over the limiting value for mass transport control. The reaction sequence for triclosan mineralization is consistent with literature reports. © 2012 Society of Chemical Industry     

Cyclone flow cell for the investigation of gas-diffusion electrodes

Electrochemical gas–liquid reactions can be efficiently carried out at porous gas diffusion electrodes (GDE). These electrodes are simultaneously in contact with a gas phase and a liquid phase. For the design and scale-up of electrochemical reactors based on these GDE their macrokinetic behaviour (i.e., the interaction of reaction and internal mass transport phenomena) must be investigated under well defined external mass transfer conditions and controlled wetting conditions. To meet these requirements, a novel cyclone cell has been designed in which two vortex flow fields are realised on either side of a horizontally positioned GDE. The external mass transfer coefficients in this cell are determined from limiting current measurements for the oxidation of Fe(CN₆)^4–.     

On diffusion, dispersion and reaction in porous media

The upscaling process of mass transport with chemical reaction in porous media is carried out using the method of volume averaging under diffusive and dispersive conditions. We study cases in which the (first-order) reaction takes place in the fluid that saturates the porous medium or when the reaction occurs at the solid–fluid interface. The upscaling process leads to average transport equations, which are expressed in terms of effective medium coefficients for (diffusive or dispersive) mass transport and reaction that are computed by solving the associated closure problems. Our analysis shows that these effective coefficients depend, in general, upon the nature and magnitude of the microscopic reaction rate as well as of the essential geometrical structure of the solid matrix and the flow rate. This study also shows that if the reaction rate at the microscale is arbitrarily large, the capabilities of the upscaled models are hindered, which is in agreement with the breakdown of the physical sense of the microscale formulation.     

Electrochemical behavior of high-pressure synthetic boron doped diamond powder electrodes

Boron doped diamond (BDD) was synthesized under high pressure and high temperature using B-doped graphite intercalation compositions (GICs) as carbon sources. The electrochemical characteristics of high-pressure synthetic BDD powder electrodes were investigated by measuring the cyclic voltammetry curves and AC impedance spectrum. For the [Fe(CN)₆]^3−/4− redox couple, the electrode reaction process is reversible or quasi-reversible at the scan rates of 0.01-1.0 V/s. At the low scan rate the linear relation between peak current and square root of scan rate indicates that the electrode process was a diffusion-controlled mass transport process. The electrochemical behavior is similar to a planar electrode. With the increasing of the scan rate the electrode process is controlled by the mass transport plus kinetic process. AC impedance spectra exhibit the porous structure characteristic of BDD powder electrode.