Numerical and experimental study of the effect of gas evolution in electrolytic pickling

As part of a progressive approach to model the electrolytic pickling process, this paper focuses on the important aspect of hydrogen and oxygen gas evolution on the electrodes and on the steel strip being pickled. The system considered consists of type 316 stainless steel pickled in aqueous sodium sulphate, with lead anodes and stainless steel cathodes. The mathematical model is two-dimensional steady-state, and includes the differential equations describing the effect of migration, giving the potential and current fields, and the Tafel kinetic rate expressions for hydrogen and oxygen gas generation. Experiments were conducted to obtain a better understanding of the process and for model validation. Good agreement between the experimental measurements of the global current efficiency and the model predictions was obtained.     

Hydrodynamic instabilities in electrolytic gas evolution

Both the direct evidence from cine pictures and the analysis of systematic electrochemical investigations support a general explanation of the anode (or cathode) effect, which is based on the conditions of hydrodynamic instability described by Helmholtz and Taylor. Further confirmation of this hydrodynamic interpretation is provided by recent critical current density studies carried out to ultimately establish a method for in situ determination of the alumina content in cryolite-alumina melts.     

基于铜电解槽电流分布估计的烧板故障诊断

铜电解过程中,为了实现阴极棒“烧板”故障的自动诊断和电流分布的实时监测。提出依据红外成像原理,采用红外相机拍摄铜电解槽槽面图像,提取处理后图像阴极棒部位灰度值,结合现场试验建立灰度值与物体表面温度的数学模型,进而求出阴极棒表面温度。其次,应用偏最小二乘法(PLS)分区建立阴极棒表面温度与电流之间的数学模型,整合后得出阴极棒中电流的平方值与阴极棒表面温度、阴极棒坐标点和环境温度为拟线性关系。依据模型导出的阴极棒电流与现场实测电流对比表明:该方法能较准确地测量阴极棒中电流,实现了铜电解过程阴极棒中电流分布的实时监测。此外,能准确自动诊断出发生“烧板”故障的阴极棒,通过阴极棒中电流的监控也能对“烧板”故障进行预测,实现了“烧板”故障的自动诊断。从而降低了阴极棒“烧板”故障的发生,为企业带来了良好的经济效益。     

Mass transfer in electrolytic cells with gas stirring

Mass transfer to wall electrodes was investigated in a circular cell agitated by gas bubbles. Perforated and porous plates were used as gas spargers. Electrodes with varying height and electrolytic solutions having different physical properties were tested. It was found that the enhancing effect of gas bubbles on the mass transfer coefficient is a function of the gas hold-up, irrespective of the velocity of the gas flow and the gas distributor employed. The results were correlated for short mass transfer lengths by the relationship $$Sh = 0.231(ScGa)^{\frac{1}{3}} (L/D_c )^{--0.194 _\varepsilon 0.246}$$ and for fully developed mass transfer by $$Sh_\infty = 0.256(ScGa)^{\frac{1}{3}} \varepsilon ^{0.254}$$      

Effect of gas evolution on current distribution and ohmic resistance in a vertical cell under forced convection conditions

The volume fraction of gas bubbles in a vertical cell with a separator was evaluated on the basis of the Bruggemann equation by taking into account the increase in velocity of the rising gas bubbles when fresh solution without gas bubbles is supplied to the bottom of the cell at constant velocity. This enhancement of the velocity results from an increase in the volume of gases evolving at the working electrode. The following three cases for overpotential at the working electrode were considered: no overpotential, overpotential of the linear type and of the Butler-Volmer type. The volume fraction, _h , at the top of the cell was expressed as a function of the dimensionless height of the cell and kinetic parameters. The total cell resistance can be expressed by {(2/5 _h )[1– _h )^–3/2–1+ _hD;]+µ}₁ d ₁/wh, where ₁ is the resistivity of the solution without gas bubbles,d ₁ the interelectrode distance,w the cell width,h the cell height and the parameter involving overpotential and resistance of the separator. It was found that there is an optimum value of the interelectrode distance. The optimum value is about a quarter of the value for the case of constant gas rise velocity, which corresponds to a closed system.Nomenclature b linear overpotential coefficient - C proportionality constant given by Equation 2 - d ₁ interelectrode distance - d ₂ thickness of the separator - F Faraday constant - h height of the cell - i current density - l total current - t ₀ exchange current density - k parameter given byd ₁(z)^1/2 - n number of electrons transferred - p gas pressure - r dimensionless cell resistance defined by Equation 16 - R gas constant - R _t total cell resistance - T temperature - u auxiliary function defined by Equation 37 - v solution velocity in the cell - v ₀ solution velocity at the bottom of the cell - v _h solution velocity at the top of the cell - V voltage at the working electrode - V _eq voltage at the working electrode when no current flows - w width of the electrode - y axis in the vertical direction from the bottom of the cell - z dimensionless variable fory, defined by Equation 8 - z _h dimensionless variable forh, defined by [C(V– V _eq/(₁ d ₁ v ₀]h - anodic transfer coefficient in the Butler-Volmer equation - volume fraction of gas bubbles in the cell - _h volume fraction of gas bubbles at the top of the cell - dimensionless cell voltage, given by Equation 38 - Butler-Volmer overpotential - Butler-Volmer overpotential when current density,I/wh, flows through the electrode, as described in Equation 42 - µ parameter representing either µ_S, µ_S + µ_L or µ_S + µ_BV - µ_BV ratio defined by Equation 41 - µ_L ratio defined byb/(₁ d ₁) - µ_S ratio defined by ₂ d ₂/₁ d ₁ - ₁ resistivity of the solution phase without gas bubbles - ₁(y) resistivity of the solution phase with gas bubbles at levely - ₂ resistivity of the separator - kinetic parameter in the Butler-Volmer equation, given by Equation 39     

Mass transfer in annular electrolytic cells with gas stirring

Mass transfer towards the inner electrode and the wall electrode was studied in an annular cell stirred with an inert gas bubble flow. Experimental data obtained for the wall electrode follow the relationship found previously for circular cells; namely $$Sh = 0.231(ScGa)^{1/3} (L/D_e )^{ - 0.194_\varepsilon0.246}$$ Study of the influence of gas hold-up on the mass transfer rate towards the inner wall electrode has yielded the following relationship: $$Sh_\infty= 0.315(ScGa)^{1/3_\varepsilon0.231}$$      

On gas bubbles in industrial aluminium cells with prebaked anodes and their influence on the current distribution

The secondary current distribution in industrial aluminium cells with prebaked anodes was calculated, taking into account the gas bubbles. The input data were obtained on the basis of a physical model and data suggested in the literature. The bubbles were modelled in the following manner: (i) as very small bubbles dispersed in a homogeneous layer with higher electrical resistivity than the bulk of the electrolyte, (ii) as large bubbles modelled as discrete slabs with infinite resistivity, and (iii) as a combination of (i) and (ii). The bubble size and the number of bubbles, as well as the resistance of the homogeneous bubble layer, were varied to give an equivalent voltage drop in the range 0.1–0.4 V. Large bubbles (slabs) appeared to have a significant screening effect on the anodic current densities. The anodic current densities between slabs showed local maxima, sometimes reaching twice the value of the working current density (0.75 A cm^–2). The cathodic current densities had local minima underneath the large anodic bubbles, following their position at the anode. Underneath a bubble of 6.1 cm width, the cathodic current density decreased from 0.75 to 0.23 A cm^–2.     

On the distribution of current generation in porous gas electrodes

In the active layer of porous gas electrodes, the spatial distribution of energy generation is determined by several interacting factors, e.g. pore statistics, distribution of active sites, and a set of correlated transport equations. After a short introduction to the problem, it is shown that the transport phenomena can, in this case, be treated in a very simplified manner. In particular, the specific electron resistance can be neglected. Restriction of gas supply can be described by a formalistic gas resistance _g. Thus, the interaction of the different transport parameters can be treated by considering purely electrical models. The relative magnitudes of the different parameters, in the case under study, are of such an order that finally it is only necessary to consider two of them: the specific ionic resistivity of the porous electrode filled partly with liquid electrolyte, and a special parameterp which describes the overvoltage in the region between gaseous phase and electrolyte. As a result, the spatial distribution of current generation can be indicated in the form of analytical expressions and diagrams. One also obtains values of the penetration depth of current generation which do not disagree with practical experience.     

Hydrogen evolution in alkaline solution on electrolytic nickel-cobalt and nickel-iron deposited with different bath compositions and current densities

Nickel-cobalt and nickel-iron electrodeposits were characterized as hydrogen electrodes in alkaline water electrolysis (6 mol/L KOH, 25°C). The nickel-based codeposits were fabricated with different bath compositions and at different current densities. The hydrogen evolution in water electrolysis on the nickel-based codeposits was apparently enhanced as compared with that on nickel. The improvement of the electrocatalytic behaviour of the hydrogen electrodes is attributed to their composition and an increase of their active surface, which are dependent on the electrodeposition conditions.     

Potential and current distribution on the surface of a hydrogen gas diffusion anode

The current and potential distribution for a hydrogen gas-diffusion disc electrode with a relatively high ohmic resistance are investigated. A theoretical model for these distributions is presented. Potential differences between the edge of the electrode and points on the electrode surface have been measured for a hydrogen gas-diffusion electrode loaded with various total currents. From the results it is concluded that the proposed model is very useful to obtain the potential and the current density distribution along a hydrogen-gas diffusion disc electrode. Moreover, the allowable size of cylindrical holes in a perforated plate placed against the rear of the gas diffusion electrode for its current supply, can be calculated to achieve a reasonably uniform current distribution along the gas-diffusion electrode.     

Experimental study on the effect of reactant flow arrangements on the current distribution in proton exchange membrane fuel cells

Current distribution in a proton exchange membrane fuel cell (PEMFC) is significantly influenced by reactant flow configurations. In this study, the current distribution has been measured experimentally using a segmented flow-field plate and printed circuit board (PCB). Local current distributions for a PEMFC with serpentine flow field and three different flow arrangements including co-flow, cross-flow, and counter-flow arrangements for the anode and cathode streams are investigated along with the effect of flow channel orientation. It is shown that the counter-flow arrangement yields most uniform distribution for the current density, whereas the co-flow arrangement results in a considerable variation in the current density from the reactant gas stream inlet to exit. Flow channel orientation can also impact the cell performance and the current distribution appreciably. The limiting hydrogen concentration at the anode side due to the low stoichiometry condition can have a predominant effect on the current distribution and cell performance.     

文留盐构造及其与油气的关系

含盐油气盆地中盐构造控制着油气的生成、运移及成藏。本文提出了文留盐岩深盆浅水成因的观点;沙三中^5沉积期文23盐埋深达300m,盐岩＂软化＂并塑性流动,文东、文西断层开始发育并控制地层沉积,文留盐构造形成;根据盐岩与油气藏关系,该区与盐岩有关的油气藏类型可分为5种。     

Modelling and calculation of the current density distribution evolution at vertical gas-evolving electrodes

During industrial electrolysis, for hydrogen, dichloride or aluminium production, there is bubbles creation at one or two electrodes which imply a great hydrodynamic acceleration but also a quite important electrical field disturbance. This disturbance can lead to the modification of the local current density and to anode effects for example. There is few works concerning the local modelling of coupled electro active species transport and electrochemical processes in a biphasic electrolyte. There are also few local experimental measurements in term of chemical composition, temperature or current density which would allow the numerical calculations validation. Nevertheless, effects like the anode effect, particularly expensive on the point of the process efficiency, should need a better understanding. Nowadays, the respective roles of the local temperature increases, the electro active specie composition or the transport properties modification due to bubbles are not known.The goal of the present work is the modelling and the numerical simulation of the vertical electrode configuration for a biphasic electrolysis process. Bubbles presence is supposed to modify the electrical properties, and then the electro active species diffusive transport and the current density. Bubbles are also motion sources for the electrolysis cell flow, and then hydrodynamic properties are strongly coupled with species transport and electrical field. The present work shows hydrodynamic and electrical properties in a laboratory scale electrolysis cell with a vertical electrode. The numerical algorithm used was the finite volume used in the computational fluid dynamic software Fluent^®.     

阳极进气湿度对质子交换膜水含量及电流密度分布影响

针对质子交换膜中水分布不均匀造成燃料电池性能降低的问题,将膜和催化层中水传递方程进行耦合实现水在膜和催化层之间连续传递,建立了质子交换膜燃料电池三维稳态模型。利用有限元分析软件COMSOL进行模拟计算,研究了阳极气体在不同湿度下膜电流密度分布并组装单电池进行了验证,分析实验模拟结果表明:模拟极化曲线与实验极化曲线吻合良好。湿度对电流密度分布影响很大,低湿度条件下,脊背下方电流密度大于气体流道下方;高湿度条件下,电流密度分布比较均匀;采用Nafion117较厚膜时,高电流密度下,即使阳极加湿,阳极侧也有脱水的可能。     

Pumping effect due to gas evolution in flow-through electrolysers

In electrolysers with recirculation where a gas is evolved, the pumping of electrolyte from a lower to a higher level can be effected by the air-lift effect due to the difference between the densities of the inlet electrolyte and the gaseous dispersion at the outlet. A balance equation for calculation of the rate of flow of the pumped liquid is derived. An equation for the calculation of the mean volume fraction of bubbles in the space between the electrodes is proposed and verified experimentally on a pilot electrolyser. The pumping efficiency of the air-lift effect is determined.Nomenclature a_A,a_C constants of linearized Tafel Equation 7 (V) - b electrode width (m) - b_A,b_C constants of linearized Tafel Equation 7 (V m^–2 A^–1) - c _pE specific heat of electrolyte (J kg^–1 K^–1) - d interelectrode distance (m) - d _E equivalent diameter of interelectrode space (m) - d _T diameter of tubing (m) - E _A,E _C potential of anode and cathode (V) - f correction term, see Equation 11 - F Faraday's constant (96 484 C mol^–1) - g acceleration of gravity (9.81 m s^–2) - H function defined by Equation 16 - I _T total current flowing through electrolyser (A) - l local current density (A m^–2) - j mean current density (A m^–2) - j reduced local current density - K ₁,K _2B criteria defined by Equations 12 and 13 - K ₃ criterion defined by Equation 9 - l pumping height equal tol _E–l _T (m) - l _E electrode height (m) - l _H length of tubing above electrolyser (m) - l _T level height in reservoir (m) - l _v,l _s length of tubing, see Fig. 1 (m) - n _O2,n _H2 number of electrons transferred per molecule of O₂ or H₂ - N _B,N _E pumping power, pumping extrapower, Equations 28, 31 (W) - N _T total power input for electrolysis (W) - p _M, p _p pressure losses in the interelectrode - p _z space, in the inlet tubing and in elbows (N m^–2) - P pressure at the upper edge of the electrode (N m^–2) - R gas constant (J K^–1 mol^–1) - Re, Re _M Reynolds criterion for the electrolyte and for gas dispersion - S _A,S _C thickness of anode and cathode (m) - T temperature (K) - T ₀,T _T temperatures at the inlet and outlet (K) - T temperature difference, T_T–T₀ (K) - U terminal voltage of electrolyser (V) - U increase of the mean voltage drop in the interelectrode space due to presence of bubbles (V) - v _E,v _M velocities of electrolyte and of gas dispersion between electrodes (m s^–1) - v _p velocity of electrolyte in inlet channel (m s^–1) - v _R rising velocity of bubbles (m s^–1) - V_E volume rate of flow of electrolyte (m³ s^–1) - V_G(x) volume rate of flow of gas at heightx (m³ s^–1) - V_GT volume rate of flow of gas at upper electrode edge (m³ s^–1) - x distance from lower electrode edge (m) - (x) volume fraction of bubbles at heightx between electrodes, and its mean value (Equations 5a, 22a) - friction coefficient of electrolyte in a tube - reduced height coordinate,x/l _E - E _pot volume-specific potential energy difference of electrolyte (J m^–3) - E _kin volume-specific kinetic energy difference of electrolyte (J m^–3) - E _dis volume-specific dissipated energy of electrolyte (J m^–3)     

交、直流交替氧化对3005铝合金电解着色的影响

采用交、直流交替氧化的方法,改变3005铝合金在硫酸介质中阳极氧化膜的结构与组成,探讨其对膜层电解着色性能的影响.结果表明,交、直流氧化的顺序及相关电解着色参数对电解着色膜性能产生明显的影响,最佳氧化及着色工艺条件为:直流氧化电流密度1～2 A/dm2,交流氧化电压15～20V,着色电压5～7V,着色温度20～30℃,着色时间2～7 min.由此可获得浅茶色、桃红色、朱红色、紫黑色等一系列具有高装饰性的铝阳极氧化着色膜.     

The effect of metal and solution phase resistances on the current distribution in cylindrical electrochemical reactors

The mathematical modelling of cylindrical electrochemical reactors has been performed taking into account the resistance of the metal phase of the inner electrode and the resistance of the electrolyte. The model assumes that the external electrode is isopotential, the electrochemical reaction on the external electrode has a low polarization resistance (di/dη→∞) and at each axial position in the electrolyte the current is independent of the radial coordinate. The experimental and theoretical current density distributions are compared in order to determine the predictive suitability of the model and a good agreement is observed between them. Furthermore, a comparison is made between this model and a simpler earlier one and an important improvement in the prediction is observed.