Individual ionic activities and mass transfer in anodic chlorate formation

The individual activity of hypochlorite ion in concentrated neutral salt (NaCl-NaClO₃) solutions has been estimated kinetically by measurement of the Faradaic efficiency in a chlorate producing system comprised of a cell and external reactor, with provision for brine re-circulation. By use of this activity in the diffusion relationships developed by Ibl and Landolt [1–3], reasonable agreement has been achieved between the experimental chlorate yield and the yield expected based on the combined hydrolytic and diffusional flux of active chlorine. In this way it has been shown as expected from basic theory that the gradient of chemical potential rather than the concentration difference represents the driving force of diffusion. A formal activity coefficient for active chlorine species is suggested as being a useful term when considering the chlorate cell system.     

工业次氯酸钠溶液中氯酸钠含量的测定

利用次氯酸钠和氯酸钠的氧化能力不同,以双氧水消解次氯酸钠的影响,以硫酸中和次氯酸钠溶液中的氢氧化钠,以硫酸亚铁作还原剂,将试样中的氯酸钠还原,再用重铬酸钾标准溶液氧化过量的硫酸亚铁,从而测定工业次氯酸钠溶液中氯酸钠的准确浓度.方法简单可靠,测得的回收率高,检测极限（质量浓度）可达0.05g/L以下.     

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.     

Mechanism of anodic oxidation of chlorate to perchlorate on platinum electrodes

The kinetics and mechanism of the anodic oxidation of chlorate to perchlorate on platinum electrodes have been investigated. The current efficiency for perchlorate formation and the electrode potential have been determined as a function of current density for various solution compositions, flow rates and pHs at 50°C. The results have been compared with theoretical relations between the ratio of the current efficiencies for perchlorate and oxygen formation, the electrode potential, the concentration of chlorate at the electrode surface and the current density for various possible mechanisms. It is concluded that the formation of an adsorbed hydroxyl radical is the first step in the overall electrode reaction. The mechanism proposed for the C10₄ perchlorate and oxygen formation is:

Probing anodic reaction kinetics and interfacial mass transport of a direct formic acid fuel cell using a nanostructured palladium-gold alloy microelectrode

The anodic reaction kinetics and interfacial mass transport of a direct polymer electrolyte membrane formic acid fuel cell have been investigated in an all solid-state electrochemical cell using a highly active nanostructured palladium-gold alloy microelectrode as an in situ probe. Well-defined “S-shaped” steady-state cyclic voltammograms exhibiting current-rising region at lower overpotentials and limiting current region at higher overpotentials have been first obtained for the electrochemical oxidation of formic acid at varying temperature. The “S-shaped” steady state polarization curves and chronoamperometric curves enable convenient measurements of the anodic reaction kinetics and interfacial mass transport of formic acid under real polymer electrolyte membrane conditions. It is encouragingly found that formic acid can be directly oxidized to CO₂ with the first electron transfer being the likely rate-determining step and the formation of surface poison can be neglected. The exchange current density for the electrooxidation of formic acid is on the order of magnitude of 10⁻⁷ A cm⁻² in the temperature range of 20-60 °C. The permeability and diffusion coefficient of formic acid through a Nafion^® 117 membrane are of the order of magnitude of 10⁻⁹ mol cm⁻¹ s⁻¹ and 10⁻⁶ cm² s⁻¹, respectively. The combination of a nanostructured microelectrode and an all solid-state electrochemical cell offers a versatile approach to evaluate potential electrocatalysts for fuel cells and electrochemical sensors employing polymer electrolyte membranes.     

Electrode current distribution in a hypochlorite cell

Electrochemical production of gases, e.g. Cl₂, H₂ and O₂, is generally carried out in vertical electrolysers with a narrow electrode gap. The evolution of gas bubbles, on one hand, speeds up the mass transport; on the other it increases the solution resistance and also the cell potential. The gas void fraction in the cell increases with increasing height and, consequently, the current density is expected to decrease with increasing height. Insight into the effects of various parameters on the current distribution and the ohmic resistance in the cell is of the utmost importance in understanding the electrochemical processes at gas-evolving electrodes. An example of the described phenomena is the on-site production of hypochlorite by means of a vertical cell. Experiments were carried out with a working electrode consisting of 20 equal segments and an undivided counter electrode. It has been found that the current distribution over the anode is affected by various electrolysis parameters. The current density,j, decreased linearly with increasing distance,h, from the leading edge of the electode. The absolute value of the slope of theI/h straight line increased with increasing average current density and temperature, and with decreasing velocity of the solution, NaCl concentration and interelectrode gap.Nomenclature a ₁ constant - b _a anodic Tafel slope (V) - b _c cathodic Tafel slope (V) - B current distribution factor - B ₀ current distribution factor att _e=0 - c _NaCl sodium chloride concentration (kmol m^–3) - d_wt interelectrode gap (mm) - h distance from the leading edge of the segmented electrode (m) - H total height of the segmented electrode (m) - I current (A) - I _s current through a segment (A) - j ₀ exchange current density (kA m^–2) - j _av mean current density (kA m^–2) - j _t current density at the top of the segmented electrode (h=H) (kA m^–2) - j _b current density at the bottom of the segmented electrode (h=0) (kA m^–2) - n _s number of a segment of the segmented electrode from its leading edge - R _s unit surface resistance of solution ( m²) - R _{s, b} unit surface resistance of solution at the bottom of the segmented electrode ( m²) - R _{s, t} unit surface resistance of solution at the top of the segmented electrode ( m²) - t _e time of electrolysis (h) - T temperature (K) - U _c cell voltage (V) - U ₀ reversible cell voltage (V) - v ₀ solution flow rate of the bulk solution in the cell at the level of the leading edge of the electrode (m s^–1) - resistivity of the solution ( m) - _a anodic overpotential (V) - _c cathodic overpotential (V) - gas void fraction - _b gas void fraction ath=0 - _t gas void fraction ath=H Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

旋转式微通道内液液萃取的流型及传质

利用高速摄影仪观测了2-乙基己基磷酸单2-乙基己基酯（油相）与硝酸铬水溶液（水相）在旋转式微通道（RME）内部的液-液两相流流型,主要观察到了滴状流型（上油下水片状流、滴状片状流、滴状带状流）、带状流型（带状流、紊乱流）、上水下油流型（上水下油滴状流、上水下油片状环状流、上水下油带状流、上水下油丝状流）,共3类,9种。考察了微通道设备的尺寸、内筒转速以及进料流速对流型的影响。发现了随着流速和转速的增加,RME内部的流型从滴状流型到带状流型再到上水下油流型的一个转变。进一步利用水油两相的韦伯数将微通道内部的惯性力和黏性力相关联,绘制了流型图,发现了RME内部惯性、黏性力的线性变化关系。最后在9种不同的流型下进行铬的萃取实验,找到了可实现较高萃取级效率的流型。该旋转式微通道内部流型的研究可以为后期的实验和设备的应用奠定基础。     

Hydrodynamic and mass transport phenomena in a multiple-electrode magnetoelectrolytic cell

The effect of imposing a uniform magnetic field on an electrolytic cell with multiple parallel-plate electrodes is analysed in terms of hydrodynamic and mass transport phenomena with particular regard to cell design in the laminar flow regime.List of Symbols a electrode length - B magnetic flux density - b electrode separation distance - d electrode height - d _e equivalent diameter =4bd/(b+2d) - F MHD body force density =JB - f _c liquid head loss factor in curvature - g acceleration due to gravity - h _c liquid head loss due to curvature - h _f liquid head loss due to friction - h _M liquid head generated by the MHD body force - J magnitude of current density - (Pe) Peclet number = (Re)(Sc) - r _c curvature of the path of the floating particle on the electrolyte surface - (Ra) Rayleigh number = (Sc)(Gr) [(Gr): Grashof number] - (Re) Reynolds number - S slope of an open-flow channel - (Sc) Schmidt number - (Sh) Sherwood number, based on the electrode height as characteristic length - T _p time required for the completion of a vortex motion about the electrode for a floating particle - ¯V average electrolyte velocity - ¯V _s average surface velocity in electrolyte - aspect ratio =b/d - dynamic viscosity of electrolyte - v kinematic viscosity of electrolyte - density of electrolyte Abbreviations CVD cell voltage drop - MHD magnetohydrodynamic     

The effect of pH on graphite wear in a chlorate cell process

An apparatus and a method for graphite wear investigation at constant pH values in a chlorate cell process have been developed and presented. The same set-up could also be used in a chlorine cell process. Experimental and compilative data show the existence of an optimal pH region providing minimal graphite consumption in electrolytic chlorate production. The latter also coincides with the same pH range which provides maximal rates of active chlorine chemical conversion into chlorate [1], and thereby maximal current yields as well. Therefore, by adjusting the pH, one simultaneously optimizes both the graphite wear and the whole electrolytic process.Paper partly presented at 22nd Meeting of the International Society of Electrochemistry (I.S.E., formerly C.I.T.C.E.), Dubrovnik, September 1971.     

The production of chlorate with a bipolar pump cell

Chlorate has been produced in a system consisting of an electrochemical pump cell (space-time ～ 150 ms) and a holding volume. The overall current efficiency has been determined as a function of ratio of tank:cell volume, electrolyte concentration and applied voltage, and the interaction of the electrochemical and chemical reactions investigated. At low applied voltages the reduction of dispersed and dissolved oxygen provides an alternative counter reaction to the more normal evolution of hydrogen, although only at low current densities on graphite. Space-time yields range from 0.52 to 8.76 mol h^?1 dm^?3 at energy yields of 2.65 to 7.59 kW h kg^?1.     

Coupled transport of heat and mass in laminar tube flow

An analysis is performed of the mutually dependent heat and mass transfer for gas flow in a circular tube. Sublimation occurs at the inner surface of the wall, while the outer surface is thermally insulated from the external environment. The flow is assumed to be laminar and hydrodynamically fully developed. Mathematical expressions are derived and numerical results are given for bulk temperature and bulk mass fraction, wall temperature and wall mass fraction, wall heat and mass fluxes, local Nusselt number, temperature and mass fraction profiles, and length of the development region. Comparisons are made of the results based on a parabolic velocity profile with those based on a slug flow profile.     

Influence of intraparticle total pressure change on pore mass transport

Influence of intraparticle total pressure change on pore mass transport is investigated in physical gas phase adsorption of a single component from an inert carrier medium. The Dusty Gas Model is applied to quantify pore mass transport, assuming local equilibrium between pore fluid and adsorbed phase. Calculated results for single pellet adsorption kinetics of cyclohexane on activated carbon and CO₂ on molecular sieve 5 Å are compared with experimental data. It is found that the total pressure drop in the pore system may be as much as 1% of the ambient total pressure. This results in a maximum viscous flow contribution of 13% for the cases studied. Since this contribution is obtained only under conditions of low overall transport rates of the adsorbed component, the assumption of isobaric conditions within the pore system of a porous adsorbent appears justified for most cases of practical calculations.     

Kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on lead dioxide electrodes

The kinetics and mechanism of anodic oxidation of chlorate ion to perchlorate ion on titanium-substrate lead dioxide electrodes have been investigated experimentally and theoretically. It has been demonstrated that the ionic strength of the solution has a marked effect on the rate of perchlorate formation, whereas the pH of the solution does not influence the reaction rate. Experimental data have also been obtained on the dependence of the reaction rate on the concentration of chlorate ion in the solution at constant ionic strength. With these data, diagnostic kinetic criteria have been deduced and compared with corresponding quantities predicted for various possible mechanisms including double layer effects on electrode kinetics. It has thus been shown that the most probable mechanisms for anodic chlorate oxidation on lead dioxide anodes involve the discharge of a water molecule in a one-electron transfer step to give an adsorbed hydroxyl radical as the rate-determining step for the overall reaction.Nomenclature anodic energy transfer coefficient - ₂ potential of outer Helmholtz plane with respect to solution - _M potential of metal with respect to solution - dielectric constant of solution - ₂ permittivity of free space - faradaic efficiency for anodic chlorate oxidation - A adsorbed intermediate in Reaction 2 - B bulk species in Reaction 2 - c _A concentration of A at outer Helmholtz plane - c _B concentration of B at outer Helmholtz plane - c _B ⁰ concentration of B in bulk - c _ClO3 ^– /0 concentration of ClO ₃ ^– in bulk - c _ClO4 ^– /0 concentration of ClO ₄ ^– in bulk - E electrode potential corrected for ohmic drop - E _a electrode potential as measured against reference electrode - E _s ⁰ standard electrode potential of Reaction 2 - E _z potential of zero charge of the anode in test solution - F Faraday constant - f F/(RT) - I _t current at anode - I _OER current used for oxygen evolution reaction at anode - I current used for chlorate oxidation (=I _t–I _OER) at anode - i _t I _t/anode area - i _OER I _OER/anode area - i I/anode area - J total concentration of (uni-univalent) electrolytes in solution - K ₂ integral capacitance of compact part of double layer - K _s standard rate constant for Reaction 2, corrected for double layer effects - n _s number of electrons involved in Reaction 2 - p ln(–i)/lnc _ClO3 ^– /0 - q _M charge density on metal surface - Q ₁ quantity of electricity passed in given time interval - Q _OER quantity of electricity required for oxygen evolution reaction in given time interval - R ohmic resistance between anode and Luggin tip - R gas constant - r ln(–i)/lnJ - s ln(–i)/ pH - T absolute temperature - t ln(–i)/E - u (₂ RT/2)^1/2 - V volume of gases evolved in given time interval - V _H volume of hydrogen evolved in given time interval - Z _B charge on species B     

Influence of the mass flow rate of secondary air on the gas/particle flow characteristics in the near-burner region of a double swirl flow burner

The influence of the mass flow rate of secondary air on the gas/particle flow characteristics of a double swirl flow burner, in the near-burner region, was measured by a three-component particle-dynamics anemometer, in conjunction with a gas/particle two-phase test facility. Velocities, particle volume flux profiles, and normalized particle number concentrations were obtained. The relationship between the gas/particle flows and the combustion characteristics of the burners was discussed. For different mass flow rates of secondary air, annular recirculation zones formed only in the region of r/d=0.3–0.6 at x/d=0.1–0.3. With an increasing mass flow rate of secondary air, the peaks of the root mean square (RMS) axial fluctuating velocities, radial mean velocities, RMS radial fluctuating velocities, and tangential velocities all increased, while the recirculation increased slightly. There was a low particle volume flux in the central zone of the burner. At x/d=0.1–0.7, the profiles of particle volume flux had two peaks in the secondary air flow zone near the wall. With an increasing mass flow rate of secondary air, the peak of particle volume flux in the secondary air flow zone decreased, but the peak of particle volume flux near the wall increased. In section x/d=0.1–0.5, the particle diameter in the central zone of the burner was always less than the particle diameter at other locations.