Application of electrochemical oxidation for treatment of industrial wastewater — the influence of reactor hydrodynamics on direct and mediated processes

The influence of kinetic and hydrodynamic factors in electrochemical reactors used in the removal of pollutants from industrial wastewater is shown, distinguishing between the two main types of reactions, namely direct and mediated electro‐oxidation. The effect of stirring during treatment of four different types of wastewater is reported. Whilst for direct electro‐oxidation of pollutants, the influence of agitation on the performance of the reactor can be easily predicted from a mass transfer correlation, its effect during electro‐oxidation mediated in the homogeneous phase by a redox couple is not straightforward. The Hatta number can be a useful criterion to apply to electrochemical reactors performing mediated oxidation of compounds (in analogy to gas–liquid reactions), so as to define whether the reaction occurs in the bulk of the reactor or near the electrode, and thus can be affected differently by stirring. The hydrodynamic conditions in the reactor for treatment of industrial wastewater can affect the differential selectivity of the removal of pollutants and this can be used for optimising the performance of the reactor with respect to a target pollutant. Copyright © 2006 Society of Chemical Industry     

Electrochemical recovery of nickel from nickel sulfamate plating effluents

Nickel sulfamate solutions are widely used for industrial nickel plating, when electrodeposits with low stress are required. Partial decomposition of sulfamate with decreasing pH below ca. 2.5 degrades the properties of nickel electrodeposits, decreases the charge yield and results in spent solutions, from which nickel must be recovered before they could be discharged to sewers. Results are reported of charge yields for nickel recovery from an industrial sulfamate effluent, using an electrochemical reactor operated at constant current in batch-recycle mode and incorporating a nickel mesh cathode, a Ti/Ta₂O₅–IrO₂ mesh anode and a cation-permeable membrane to prevent anodic oxidation of sulfamate. A micro-kinetic model was developed, treating the processes of nickel(II) and proton reduction in sulfamate solutions as two multi-step reactions involving adsorbed intermediates, Ni_ads^I and H_ads, respectively. The unknown kinetic parameters were obtained using gPROMS software by iterative fitting of the model to experimental data obtained over a range of nickel(II) concentrations and bulk solution pH, enabling evaluation of nickel(II) reduction charge yields as a function of nickel(II) concentration, bulk pH and electrode potential. A model combining the micro-kinetic equations with mass and charge balances on the reactor was used to determine the control parameters for electrochemical recovery of elemental metal from nickel(II) in batch-recycle mode. It was determined experimentally that a decrease in catholyte pH to values below ca. 2.5 resulted in a decrease in nickel(II) reduction charge yields to values below 0.9. The decrease in catholyte pH, caused by the flux of protons from the anolyte where they were generated via anodic oxygen evolution, was obviated by continuous addition of NaOH at a rate determined by the model, permitting nickel(II) recovery with an average charge yield of 0.94.     

Oxygen storage in automobile exhaust catalyst

The charge transfer reactions between a Pt catalyst and yttria-stabilized zirconia (YSZ) support have been studied electrochemically during CO oxidation over a range of gas phase compositions and temperatures typical of automotive exhaust. From a combination of electrochemical and reaction-rate measurements, a model for the participation of the support in the oxidation reactions is derived. It is estimated from the model that the contribution of the charge transfer reactions occurring at the three-phase interface between Pt, YSZ and the gas phase, to the total rate of CO oxidation is negligible under steady operating conditions (gas phase composition and temperature). However, under cyclic conditions typical of those encountered in automotive exhaust, these charge transfer reactions can lead to an enhancement (≈ 10% at 828 K) in the rate of CO oxidation.     

Optimal design and operation of a natural gas tri-reforming reactor for DME synthesis

Korea Gas Corporation (KOGAS) is developing a new di-methyl-ether (DME) plant. Syngas is provided by natural gas tri-reforming, in a reactor consisting of a homogenous part where oxidation leads to a temperature increase required for the reforming reactions and a catalytic part where the reforming reactions take place. A first principle model for the tri-reforming reactor is developed. A kinetic mechanism is proposed combining homogeneous gas-phase reactions and heterogeneous catalytic reactions. The proposed model is systematically calibrated and validated with global sensitivity analysis followed by global parameter estimation against concentration measurements of a lab-scale prototype reactor and comparisons of the sensitivity of the outlet as a function of inlet composition and design parameters with experimental results. The validated model is finally used for the optimization of design variables such as length ratio of homogeneous and heterogeneous section and operational variables such as the feed composition.     

An approximate solution to the model of a sparged packed bed electrode reactor

An approximate analytical solution to the mathematical model of a sparged packed bed electrochemical reactor (SPBER) in which a gaseous reactant undergoes direct anodic oxidation is presented. The model is based on two strongly nonlinear partial differential equations and is solved by the inverse operator method (IOM). The solution is used to study the direct anodic oxidation of propylene. The approximate IOM solution is useful in that it enables the direct investigation of the effect of model parameters on the operation of the reactor. Current density against electrode potential, polarization data, for the anodic oxidation of propylene in the SPBER is presented. Nonlinear parameter estimate methods are used to fit the model to experimental data to obtain physically meaningful values of kinetic parameters.     

邻二甲苯液相氧化动力学的研究

在搅拌釜（φ107×350mm）中进行邻二甲苯液相氧化动力学的研究,其结果表明,在本试验条件下（搅拌釜转速为1200rpm空釜线速≥1.5cm/s,氧化温度在130℃以下）可以消除传质影响。 本研究建立了邻二甲苯（OX）氧化的动力学方程式（在 OX的转化率<50％时适用）,为气液反应器放大提供计算氧化速度的动力学 模型。 本工作还对邻二甲苯氧化七组分的主副反应动力学进行研究,测定了不同温度下各反应的速率常数和活化能。其数值与采用鼓泡塔氧化的结果相一致,再次表明邻二甲苯氧化为串、并联一级反应。     

Interactions of CO, HCl, and SOx in pulverised coal flames

The effect of HCl and SO₂ on CO oxidation in pulverised coal flames was investigated experimentally and kinetically in an entrained flow combustion reactor. Two bituminous coals (German ‘Goettelborn’ and a Polish coal) were used as fuels with a feeding rate of 1 or 1.5 kg/h. HCl or SO₂ is introduced into the reactor premixed with the primary air. Experimental results indicate that HCl addition may inhibit CO oxidation in coal flames and increases CO emission. Reducing temperature in the reactor will enhance the inhibitory effect of HCl on CO oxidation. The measured CO profiles along the reactor height clearly show that the addition of HCl may inhibit CO oxidation. In the experimental range of SO₂ addition, the inhibiting effect of SO₂ on CO oxidation is less significant than HCl. A detailed kinetic mechanism is used to model the reactions, and the controlling reactions are analysed.     

微反应器内甲苯气固催化氧化反应动力学

微通道反应器具有优良的传热、传质性能，能有效避免催化剂床层内热点的形成，为研究强放热反应动力学提供有利条件。开展了微反应器内的V2O5/TiO2催化剂上的甲苯气相选择氧化动力学研究，在简化反应网络的基础上建立了动力学模型，并给出动力学参数。该模型能较好地反映和预测较宽的反应条件范围内的甲苯气固相催化氧化反应转化率及产物分布，为优化操作条件提供依据。     

液相氧化反应失控过程的动态流程模拟

随着化工工业现代化、绿色化概念的普及,氧化工艺特别是新型氧化工艺在当前工艺中所占比重日益增加,但氧化反应多为强放热反应,反应所涉及的物料往往也具有不稳定性. 为防止反应安全事故的发生,有必要进行系统的反应安全性研究. 针对液相氧化反应,本工作利用流程模拟手段,建立了带控制条件的半间歇动态反应器模型. 介绍了反应器模型的结构、传热设置和工艺控制设置方案. 选择丙烯环氧化反应开展了模拟研究,设置了相关的动力学参数和反应器参数. 通过模拟正常反应过程对设计的反应器参数进行核算,证明在正常工艺条件下可保证反应平稳进行. 利用所建立的模型模拟了密闭绝热、冷却失效和冷却水调节阀故障条件下的多场景反应危险性,得到了各场景下反应器温度、压力和反应器内物料组成的变化曲线,为后续工作中制定合理的安全控制措施提供数据支持.     

Effect of mass transfer on the current distribution in monopolar and bipolar electrochemical reactors with a gas-evolving electrode

The current distribution in an electrochemical reactor with vertical parallel-plate electrodes was experimentally determined. The research was performed with monopolar and bipolar electrodes. The reactor has a gas-evolving electrode and at the counter electrode an electrochemical reaction with combined diffusion and charge-transfer kinetic control, takes place. Therefore the kinetics at the counter electrode are influenced by the bubble-induced convection and by the forced convection of the electrolyte. These reactors are found in many electrochemical processes, for example, electrowinning of metals and electrosynthesis. The test reactions were hydrogen evolution at the cathode and the anodic oxidation of sulphite to sulphate from basic solutions. The current distribution shows a minimum at a distance of approximately six times the equivalent diameter of the reactor from the inlet region. This minimum is a consequence of the interaction between forced convection and the bubble-induced convection, which shifts the mass transfer coefficient of the anodic reaction along the reactor. The effects of the total current, the volumetric electrolyte flow rate and the metal phase resistance on the current distribution are also analysed. The experimental current distribution data are compared with theoretical expectations and good agreement is found.     

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.     

Deposition of tin oxide,iridium and iridium oxide films by metal-organic chemical vapor deposition for electrochemical wastewater treatment

In this research, the specific electrodes were prepared by metal-organic chemical vapor deposition (MOCVD) in a hot-wall CVD reactor with the presence of O₂ under reduced pressure. The Ir protective layer was deposited by using (Methylcyclopentadienyl) (1,5-cyclooctadiene) iridium (I), (MeCp)Ir(COD), as precursor. Tetraethyltin (TET) was used as precursor for the deposition of SnO₂ active layer. The optimum condition for Ir film deposition was at 300 °C, 125 of O₂/(MeCp)Ir(COD) molar ratio and 12 Torr of total pressure. While that of SnO₂ active layer was at 380 °C, 1200 of O₂/TET molar ratio and 15 Torr of total pressure. The prepared SnO₂/Ir/Ti electrodes were tested for anodic oxidation of organic pollutant in a simple three-electrode electrochemical reactor using oxalic acid as model solution. The electrochemical experiments indicate that more than 80% of organic pollutant was removed after 2.1 Ah/L of charge has been applied. The kinetic investigation gives a two-step process for organic pollutant degradation, the kinetic was zero-order and first-order with respect to TOC of model solution for high and low TOC concentrations, respectively.     

Reduction of metal ions in dilute solutions using a GBC-reactor: Part I: Experimental study on the laboratory scale reduction of ferric ions

To reduce metal ions in dilute solutions a new type of electrochemical reactor has been developed: the GBC-reactor. This reactor consists of a gas diffusion electrode coupled with a packed bed electrode. The working principle of the reactor is based upon two main reactions: the catalytic oxidation of hydrogen gas in the gas diffusion electrode and the simultaneous reduction of metal ions on the packed bed electrode. This process occurs spontaneously without the need for an external power supply when the Gibbs free energy of the total reaction is negative. To study the behaviour of the reactor the reduction of ferric ions was used as a model system. The experimental results, obtained from varying a number of key process parameters, could be described using a very simple macroscopic rate equation. It is concluded that the reduction of ferric ions in a GBC-reactor is controlled by both mass transfer and electrochemical kinetics. To carry out scale-up and optimization studies a reactor model incorporating the potential distribution in the packed bed electrode is, however, necessary.     

On the modelling of multidisciplinary electrochemical systems with application on the electrochemical conversion of CO2 to formate/formic acid

This paper presents a model-based approach on the analysis of complex multidisciplinary electrochemical processes, with implementation on a reactor for the electrochemical conversion of CO₂ to formate/formic acid. The process is regarded as a system of interacting physical and electrochemical mechanisms. A process model is developed by combining individual mathematical sub-models of the mechanisms, organised at groups of compartments following the physical process structure. This approach results in a generic reconfigurable model that can be used as a part of integrated systems, and to test design modifications. The approach is demonstrated on an electrochemical cell, where CO₂ is converted to formate/formic acid. The model captures the molar transportation under electric field, the two-phase flow effects, and the key electrochemical reactions. The model is calibrated and validated against experimental data obtained from a continuous flow cell. The key parameters affecting the process performance are discussed through scale-up analysis.     

Reduction of metal ions in dilute solutions using a GBC-reactor: Part I: Experimental study on the laboratory scale reduction of ferric ions

To reduce metal ions in dilute solutions a new type of electrochemical reactor has been developed: the GBC-reactor. This reactor consists of a gas diffusion electrode coupled with a packed bed electrode. The working principle of the reactor is based upon two main reactions: the catalytic oxidation of hydrogen gas in the gas diffusion electrode and the simultaneous reduction of metal ions on the packed bed electrode. This process occurs spontaneously without the need for an external power supply when the Gibbs free energy of the total reaction is negative. To study the behaviour of the reactor the reduction of ferric ions was used as a model system. The experimental results, obtained from varying a number of key process parameters, could be described using a very simple macroscopic rate equation. It is concluded that the reduction of ferric ions in a GBC-reactor is controlled by both mass transfer and electrochemical kinetics. To carry out scale-up and optimization studies a reactor model incorporating the potential distribution in the packed bed electrode is, however, necessary.     

Electrochemical cell current requirements for toxic organic waste destruction in Ce(IV)-mediated electrochemical oxidation process

The electrochemical cell for cerium oxidation and reactor for organic destruction are the most important operation units for the successful working mediated electrochemical oxidation (MEO) process. In this study, electrochemical cells with DSA electrodes of two types, single stack and double stack connected in series, were used. The performances towards the electrochemical generation of Ce(IV) in nitric acid media at 80 °C were studied. The current-voltage curves and cerium electrolysis kinetics showed the dependence on number of cell stacks needed to be connected in series for the destruction of a given quantity of organic pollutant. The presence of an optimum region for Ce(III) oxidation with a contribution of oxygen evolution, especially at low Ce(III) concentration (high conversion ratios), was found. The cells were applied for the Ce(IV) regeneration during the organic destruction. The cell and reactor processes were fitted in a simple model proposed and used to calculate the current needed in terms of Ce(III) oxidation rate and the number of cell stacks required for maintaining Ce(IV)/Ce(III) ratio at the same level during the organic destruction. This consideration was based on the kinetic model previously developed by us for the organic destruction in the MEO process.     

Theoretical and experimental studies of the effect of side reactions in copper deposition from dilute solutions on packed-bed electrodes

This paper analyses the effect of side reactions, such as the reduction of oxygen or Fe(III), on copper deposition from dilute acid sulfate solutions. Fundamental studies with a rotating disc electrode were performed to determine the potential regions where the reactions take place and also to calculate kinetic parameters under the conditions of the experiments. Applied studies carried out in a through-flow electrochemical reactor with a packed bed cathode formed by a stack of nets are reported. The experimental data are compared with theoretical results, according to the continuous model, to corroborate its reliability for the case of simultaneous electrochemical reactions. It was found that the agreement between the mathematical treatment and the experimental data is similar to the case of a single reaction in the electrode. Taking into account the side reactions, criteria for the selection of bed thickness parallel to the current flow are discussed.     

EXPERIMENTAL STUDY OF A COMPLEX CATALYTIC REACTION NETWORK IN A TRICKLE-BED REACTOR

In the present report a relatively complex reaction system including several side reactions to the desired main reaction has been studied in a trickle-bed reactor. In addition the experimental part—which showed very good reproducibility of the results—an attempt to a reasonable modelling of the reactor was tried. The starting point was the differential equation system describing the kinetics of the reactions as well as the corresponding kinetic parameters, which had been established earlier in a slurry-reactor. It was possible to demonstrate that the behaviour of a trickle-bed reactor can be simulated by a relatively simple approach containing the basic kinetic equations, the wetting efficiency of the catalyst particles and a one-dimensional, pseudohomogeneous reactor model, the cell model. The so calculated concentration curves of the different reactants and products fitted the experimental values very closely. For those reasons, and especially for the reasonable effort needed for very satisfactory modeling, the results and proceeding steps outlined here might be of particular interest for further applications of trickle-bed reactors.