Electrochemical mass transfer studies in open cavities

Experimental measurements on free convection mass transfer in open cavities are described. The electrochemical deposition of copper at the inner surface of a cathodically polarized copper cylinder, open at one end and immersed in acidified copper sulphate was used to make the measurements. The effects on the rate of mass transfer of the concentration of the copper sulphate, the viscosity of the solution, the angle of orientation, and the dimensions of the cylinder were investigated. The data are presented as an empirical relation between the Sherwood number, the Rayleigh number, the Schmidt number, the angle of orientation and the ratio of the diameter to the depth of the cylinder. Comparison of the results with the available heat transfer data was not entirely satisfactory for a number of reasons that are discussed in the paper.Nomenclature C _b bulk concentration of Cu⁺⁺ (mol cm^–3) - C _b bulk concentration of H₂SO₄ (mol cm^–3) - C _o concentration of Cu⁺⁺ at cathode (mol cm^–3) - C _o concentration of H₂SO₄ at cathode (mol cm^–3) - D cavity diameter (cm) - D diffusivity of CuSO₄ (cm² s^–1) - D diffusivity of H₂SO₄ (cm² s^–1) - Gr Grashof number [dimensionless] (=Ra/Sc) - g acceleration due to gravity (=981 cm s^–2) - H cavity depth (cm) - h coefficient of heat transfer (Wm ^–2 K^–1) - i _L limiting current density (mA cm^–2) - K mass transfer coefficient (cm s^–1) - K ₁,K ₂ parameters in Equation 1 depending on the angle of orientation () of the cavity (see Table 3 for values) [dimensionless] - k thermal conductivity (W m^–1 K^–1) - L ^* characteristic dimension of the system (=D for cylindrical cavity) (cm) - m exponent on the Rayleigh number in Equation 1 (see Table 3 for values) [dimensionless] - Nu Nusselt number (=hL ^* k^–1) [dimensionless] - n exponent on the Schmidt number in Equation 1 (see Table 3 for values) [dimensionless] - Pr Prandtl number (=v/k) [dimensionless] - Ra Rayleigh number (defined in Equation 2) [dimensionless] - Sc Schmidt number (=v/D) [dimensionless] - Sh Sherwood number (=KD/D) [dimensionless] - ^t H⁺ transference number for H⁺ [dimensionless] - ^t Cu⁺⁺ transference number for Cu⁺⁺ [dimensionless] - specific densification coefficient for CuSO₄ [(1/)/C] (cm³ mol^–1) - specific densification coefficient for H₂SO₄ [(1/)/C] (cm³ mol^–1) - k thermal diffusivity (cm² s^–1) - dynamic viscosity of the electrolyte (g cm^–1 s^–1) - kinematic viscosity of the electrolyte (= /)(cm² s^–1) - density of the electrolyte (g cm^–3) - angle of orientation of the cavity measured between the axis of the cavity and gravitational vector (see Fig. 1) [degrees] - parameter of Hasegawaet al. [4] (=(2H/D))5/4 Pr^{– 1/2}) [dimensionless]     

Impedance spectroscopy study of electrochromism in sputtered iridium oxide films

The a.c. impedance response of sputtered iridium oxide films (SIROFs) was studied at room temperature in 1M H₂SO₄ between 1mHz and 50mHz. The spectra were recorded as a function of applied potential in the range of electrochromic properties from 0.0 to 1.0V vs SCE and before and after an electrochemical treatment consisting of alternatively colouring and bleaching the electrode. The spectra were analysed with help of an equivalent circuit. Between 0.4 and 1.0V, the spectra can be interpreted as due to electrochemical proton insertion in a single phased compound. From the data, hydrogen chemical diffusion coefficients with values ranging from 2 × 10^–8 to 1.1 × 10^–7cm²s^–1 are found. It is shown that this parameter increases fourfold after the cycling treatment and significantly decreases with the amount of inserted hydrogen. Below 0.4V spectrum changes are observed over the intermediate frequency range studied, indicating some changes of the interfacial reactivity which remain to be clarified.     

Experimental investigation of the primary and secondary current distribution in a rotating cylinder Hull cell

A rotating cylinder cell having a nonuniform current distribution similar to the traditional Hull cell is presented. The rotating cylinder Hull (RCH) cell consists of an inner cylinder electrode coaxial with a stationary outer insulating tube. Due to its well-defined, uniform mass-transfer distribution, whose magnitude can be easily varied, this cell can be used to study processes involving current distribution and mass-transfer effects simultaneously. Primary and secondary current distributions along the rotating electrode have been calculated and experimentally verified by depositing copper.List of symbols c distance between the cathode and the insulating tube (cm) - F Faraday's constant (96 484.6 C mol^–1) - h cathode length (cm) - i local current density (A cm^–2) - i _L limiting current density (A cm^–2) - i _ave average current density along the cathode (A cm^–2) - i ₀ exchange current density (A cm^–2) - I total current (A) - M atomic weight of copper (63.54 g mol^–1) - n valence - r _p polarization resistance () - t deposition time (s) - V _c cathode potential (V) - Wa _T Wagner number for a Tafel kinetic approximation - x/h dimensionless distance along the cathode surface - z atomic number Greek symbols _a anodic Tafel constant (V) - _c cathodic Tafel constant (V) - solution potential (V) - overpotential at the cathode surface (V) - density of copper (8.86 g cm^–3) - electrolyte conductivity ( cm^–1) - deposit thickness (cm) - _ave average deposit thickness (cm) - surface normal (cm)     

Long service life IrO2/Ta2O5 electrodes for electroflotation

Ti/IrO₂-Ta₂O₅ electrodes prepared by thermal decomposition of the respective chlorides were successfully employed as oxygen evolving electrodes for electroflotation of waste water contaminated with dispersed peptides and oils. Service lives and rates of dissolution of the Ti/IrO₂-Ta₂O₅ electrodes were measured by means of accelerated life tests, e.g. electrolysis in 0.5M H₂SO₄ at 25°C and j = 2 A cm^–2. The steady-state rate of dissolution of the IrO₂ active layer was reached after 600–700 h (0.095 g Ir h^–1 cm^–2) which is 200–300 times lower than the initial dissolution rate. The steady-state rate of dissolution of iridium was found to be proportional to the applied current density (in the range 0.5–3 A cm^–2 ). The oxygen overpotential increased slightly during electrolysis (59–82 mV for j = 0.1 A cm^–2 ) and the increase was higher for the lower content of iridium in an active surface layer. The service life of Ti/IrO₂ (65 mol%)-Ta₂O₅ (35 mol%) in industrial conditions of electrochemical devices was estimated to be greater than five years.List of symbols a constant in Tafel equation (mV) - b slope in Tafel equation (mV dec^–1) - E potential (V) - f mole fraction of iridium in the active layer - j current density (A cm^–2) - l number of layers - m _Ir content of iridium in the active layer (mg cm^–2) - r dissolution rate of the IrO₂ active layer (g Ir h^–1 cm^–2) - T _c calcination temperature (°C) - _{O 2} oxygen overpotential (mV) - _{O 2} difference in oxygen overpotential (mV) - _A service life in accelerated service life tests (h) - _S service life in accelerated service life tests related to 0.1 mg Ir cm^–2 (h) - _p polarization time in accelerated service life tests (h)     

β-AgVO3· 4AgI and AgNbO3· 4AgI: two new solid silver electrolytes at room temperature

The specific conductivity of compounds in the-AgVO₃-AgI system above 50 mol% AgI has been measured. A maximum value of the conductivity was observed at 80 mol% AgI. The temperature dependence of the specific conductivity of both the compounds-AgVO₃ · 4AgI and AgNbO₃ · 4AgI was also measured. Activation energies are 3.7 kcal mol^–1 and 3.8 kcal mol^–1 respectively. The specific conductivity of the-AgVO₃ based compound is much higher than that of the AgNbO₃ analogue. The silver transport number in both compounds has been determined by Tubandt's method. In both cases a value of 1.00 ± 0.02 was found. Galvanic cells of the type Ag(s)/electrolyte (s)/C(I₂) were constructed. Electrochemical measurements on these cells indicate a better performance of those cells with a-AgVO₃ · 4AgI electrolyte with respect to the ones with an AgNbO₃ · 4AgI electrolyte.     

Occurrence and Correlations of Monoterpene Hydrocarbon Enantiomers in Pinus sylvestris and Picea abies

The relative amounts and enantiomeric compositions of monoterpene hydrocarbons in branch and trunk xylem, in needles, and in resin from apical buds in 18 Pinus sylvestris trees have been determined and compared with the terpene content in branch xylem and needles of Picea abies. Besides the high amount of (+)-3-carene, an excess of (+)--pinene has been found in P. sylvestris, whereas in P. abies (–)--pinene dominates over (+)--pinene. In P. sylvestris, clear positive correlations were found between (+)--pinene and (+)-camphene in the four tissues analyzed. Good positive correlations were also observed between (–)--pinene and (–)-camphene in the two types of xylem, between (+)--pinene and (+)--pinene in the resin, and between tricyclene and (–)-camphene in resin and needles. In P. abies, positive correlations were found between (+)--pinene and (+)-camphene in the branch xylem and between tricyclene and (–)-camphene as well as between (–)--pinene and (–)-camphene in the needles. Complex relationships between (–)--pinene and (–)--pinene were found both in the P. abies and in the P. sylvestris tissues. The importance of the enantiomeric composition of -pinene for the host selection of Ips typographus, Tomicus piniperda, and Hylobius abietis is discussed.     

Studies concerning charged nickel hydroxide electrodes I. Measurement of reversible potentials

Reversible potentials (E _R) have been measured for nickel hydroxide/oxyhydroxide couples over a range of KOH concentrations from 0·01–10 M. It is shown that the couples derived from the parent- and-Ni(OH)₂ systems can be distinguished by the relative change in KOH level on oxidation and reduction. In the case of couples derived from the-class of materials a dependence of 0·470 moles of KOH per 2e change is found compared with 0·102 moles of KOH per 2e change for the-class of materials. Couples derived from the- and-Ni(OH)₂ systems can be encountered in a series of activated and de-activated forms having a range of formal potentialsE ₀ . Activated. and de-activated-Ni(OH)₂/-NiOOH couples are found to lie in the range 0·443–0·470 V whilst-Ni(OH)₂/-NiOOH couples lie in the range 0·392–0·440 V w.r.t. Hg/HgO/KOH. It is demonstrated for de-activated,-Ni(OH)₂/-NiOOH couples thatE _R is independent of the degree of oxidation of the nickel cation between states of charge of 25% and 70%. SimilarlyE _R is constant for states of charge between 12% and 60% for activated-Ni(OH)₂/-NiOOH couples. The constant potential regions are considered to be derived from heterogeneous equilibria between pairs of co-existing phases both containing nickel in upper and lower states of oxidation. Differences inE ₀ between the activated and de-activated couples are considered to be related to the degree of order/disorder in the crystal lattice.     

Natural convection mass transfer at horizontal screens

The free convection mass transfer behaviour of horizontal screens has been investigated experimentally using an electrochemical technique involving the measurement of the limiting currents for the cathodic deposition of copper from acidified copper sulphate solutions. Screen diameter and copper sulphate concentration have been varied to provide a range ofSc.Gr from 22×10⁸ to 26×10¹⁰. Under these conditions, the data for a single screen are correlated by the equation:Sh=0.375(Sc.Gr)^0.305 Results have been compared with previous work on free convection at horizontal solid surfaces where mass transfer coefficients are somewhat lower.Mass transfer coefficients have been measured also for arrays of closely spaced parallel horizontal screens. The mass transfer coefficient was found to decrease with the number of screens forming the array.Symbols and units A area of mass transfer surface, cm² - C _b bulk concentration of ionic species, mol cm^–3 - D diffusivity, cm²s^–1 - F Faraday number, 96494 C g [equiv^–1] - Z number of electrons involved in the reaction - I _L limiting current, A - K mass transfer coefficient, cm s^–1 - Sh Sherwood number, dK/D - Sc Schmidt number,/D or/D - Gr Grashof numbergd ³/ ² _s - solution dynamic viscosity, g cm s^–1 - solution kinematic viscosity, cm² s^–1 - solution density, g cm^–3 - density difference between bulk solution and electrode/solution interface, g cm^–3 - _s solution density at electrode/solution interface, g cm^–3 - d screen diameter, cm - g gravitational acceleration, cm s^–2 On leave of absence, Chemical Engineering Department, Alexandria University, Alexandria, Egypt.     

Kinetics of copper electrodeposition in citrate electrolytes

The kinetics of copper electrocrystallization in citrate electrolytes (0.5M CuSO₄, 0.01 to 2M sodium citrate) and citrate ammonia electrolytes (up to pH 10.5) were investigated. The addition of citrate strongly inhibits the copper reduction. For citrate concentrations ranging from 0.6 to 0.8 M, the impedance plots exhibit two separate capacitive features. The low frequency loop has a characteristic frequency which depends mainly on the electrode rotation speed. Its size increases with increasing current density or citrate concentration and decreases with increasing electrode rotation speed. A reaction path is proposed to account for the main features of the reduction kinetics (polarization curves, current dependence of the current efficiency and impedance plots) observed in the range 0.5 to 0.8 M citrate concentrations. This involves the reduction of cupric complex species into a compound that can be either included as a whole into the deposit or decomplexed to produce the metal deposit. The resulting excess free complexing ions at the interface would adsorb and inhibit the reduction of complexed species. With a charge transfer reaction occurring in two steps coupled by the soluble Cu(I) intermediate which is able to diffuse into the solution, this model can also account for the low current efficiencies observed in citrate ammonia electrolytes and their dependencies upon the current density and electrode rotation speed.Nomenclature b, b ₁, b ₁ ^* Tafel coefficients (V^–1) - bulk concentration of complexed species (mol cm^–3) - (si*) concentration of intermediate C^* atx=0 (mol cm^–3) - C concentration of (Cu Cit H)^2– atx=0 (mol cm^–3) - C C variation due to E - C concentration of complexing agent (Cit)^3- at the distancex (mol cm^–3) - C _o concentrationC atx=0 (mol cm^–3) - C _o C _o variation due to E - Cv _s bulk concentrationC (mol cm^–3) - (Cit H), (Cu), (Compl) molecular weights (g) - C _dl double layer capacitance (F cm^–2) - D diffusion coefficient of (Cit)^3- (cm²s^–1) - D ₁ diffusion coefficient of C^* (cm²s^–1) - E electrode potential (V) - f ₁ frequency in Equation 25 (s^–1) - F Faraday's constant (96 500 A smol^–1) - i, i ₁, i ₁ ^* current densities (A cm^–2) - i i variation due to E - Im(Z) imaginary part ofZ - j - k ₁, k ₁ ^* , K₁, K ₁ ^* , K₂, K rate constants (cms^–1) - K rate constant (s^–1) - K ₃ rate constant (cm³ A^–1s^–1) - R _t transfer resistance (cm²) - R _p polarization resistance (cm²) - Re(Z) real part ofZ - t time (s) - x distance from the electrode (cm) - Z _f faradaic impedance (cm²) - Z electrode impedance (cm²) Greek symbols maximal surface concentration of complexing species (molcm^–2) - thickness of Nernst diffusion layer (cm) - , ₁, ₂ current efficiencies - angular frequency (rads^–1) - electrode rotation speed (revmin^–1) - =K ^–1(s) - _d diffusion time constant (s) - electrode coverage by adsorbed complexing species - (in0) electrode coverage due toC _s - variation due to E     

Diffusion-controlled current distributions near cell entries and corners

This paper reports experimental work undertaken to explore diffusion-controlled current distributions immediately downstream of sudden changes in flow cross-sectional area such as may occur at the entry to electrochemical flow cells. Nozzle flows expanding into an axisymmetric circular duct and into a square duct have been investigated using the reduction of ferricyanide ions on nickel micro-electrodes as the electrode process. The spanwise distribution of current has also been studied for the case of the square cell where secondary corner flows are significant.Nomenclature A electrode area (cm²) - c bulk concentration of transferring ions (mol dm^–3) - D cell diameter (cm) - D Diffusion coefficient (cm₂s^–1) - F Faraday number (96 486 C mol^–1) - I limiting electrolysis current (A) - k mass transfer coefficient (cm s^–1) - N nozzle diameter (cm) - u mean fluid velocity (cm s^–1) - x distance downstream from point of entry to cell (cm) - z number of electrons exchanged - electrolyte viscosity (g s^–1 cm^–1) - electrolyte density (g cm^–3) - (Re)_D duct Reynolds number,Du/ - (Re)_N nozzle Reynolds number,Nu/ - (Sc) Schmidt number,/D) - (Sh) Sherwood number,kD/D)     

Investigations in platinum metal group electrochemistry: II The Pd(II)-Pd° reduction

The Norbide boron carbide electrode has been satisfactorily applied to polarographic studies of Pd(II)–Pd° and some other systems involving deposition of metal. By its means the following thermodynamic and kinetic data have been established: standard oxidation-reduction potentials, Pd²⁺–Pd°, 0.91 V; Ag⁺–Ag°, 0.805 V; stability constants, PdCl ₄ ^2– , log ₄, 9·38; logK ₄, 1·44; Pd(SO₄) ₂ ^2– , log ₂, 3·16; activation energies, Pd²⁺–Pd°:Q _D, 18·6; Q°, 188 kJ mole^–1. Analytical applications have been briefly examined.List of symbols A Area of the working electrode - (A°) Apparent frequency factor of the Arrhenius relationship - n Nominally the product of the transfer coefficient, , and the number of electrons,n, involved in an electrochemical process. In practice it is the value obtained from the slopeRT/anF of the lineE v. ln(i ₁–i)/i orv. ln(i ₁–i) - _j Product of dissociation constants of successive complexes:K ₁×K ₂×...×K _j - C ₀ Bulk concentration in the aqueous phase of species undergoing electrochemical reduction or oxidation - D ₀ Diffusivity of that species in the aqueous phase immediately adjacent to the electrode surface - Thickness of a diffusion layer - E _1/2 Half-wave potential, at whichi=i ₁/2 in a polarographic wave of the formE=E _1/2+RT/anF ln(i ₁–i)/i - E _mid Potential at whichi=i ₁/2 in a wave of the formE=E _mid+RT/anF ln(i ₁–i)/i - E _1/2 Displacement of half-wave potential caused by complexing of reducing species - _1/2 Overpotential at the half-wave potentialE _1/2 - _mid Overpotential atE _mid - f Activity coefficient, e.g.f _Pd ^2+(x=0) the activity coefficient of Pd²⁺ species in the aqueous phase at the electrode surface - i ₁ Limiting current - i Current at any stage of the electro-chemical processes governed byE v. ln(i ₁–i)/i relationships - j Number of complexing ligands associated with a cation—e.g. for PdCl ₃ ^– =3 - Q Arrhenius activation energy of the electrochemical process of a reduction at a working electrode [8] - Q _D Arrhenius activation energy of the diffusion stage of an electrochmical reduction [8]     

Creep and long-term strength of magnesia ceramics

Conclusions A study was made of the creep and long-term strength of magnesium oxide ceramics with crystal sizes of 12 and 25 at 1400–1550°C and 200–600 kg/cm². Ceramics with smaller crystal sizes are characterized, in identical conditions, by a higher creep rate, but lower long-term strength. The rate of the steady creep and the time prior to destruction are related for magnesia by the ratio _p=A()^–2. Magnesium oxide ceramics are characterized by the mechanism of destruction during creep.Translated from Ogneupory, No.6, pp.38–43, June, 1972.     

Ethyl m-Digallate from Red Maple, Acer rubrum L., as the Major Resistance Factor to Forest Tent Caterpillar, Malacosoma disstria Hbn.

An ethanolic extract of red maple (Acer rubrum L.) leaves (RME) applied to trembling aspen (Populus tremuloides Michx.) leaves reduced feeding in choice test assays with forest tent caterpillar larvae (Malacosoma disstria Hbn.) (FTC), whereas a trembling aspen foliage extract, similarly applied, stimulated feeding. Compounds isolated from the RME were gallic acid, methyl gallate, ethyl gallate, m-digallate, ethyl m-digallate, 1-O-galloyl--D-glucose, 1-O-galloyl--L-rhamnose, kaempferol 3-O--D-glucoside, kaempferol 3-O--D-galactoside, kaempferol 3-O--L-rhamnoside, kaempferol-3-O-rhamnoglucoside, quercetin 3-O--D-glucoside, quercetin 3-O--L-rhamnoside and quercetin 3-O-rhamnoglucoside, (–)-epicatechin, (+)-catechin and ellagic acid. All of the gallates, (–)-epicatechin, and kaempferol 3-O--L-rhamnoside deterred feeding on trembling aspen leaf disks when applied at 0.28 mg/cm². The two digallates deterred feeding by 90% and were the most effective. HPLC analysis indicated that ethyl m-digallate is present in amounts 10–100 × higher in RME (2.5–250 mg/g) than any other compound. Thus, ethyl m-digallate appears to be the major compound protecting red maple from feeding by FTC, with a minor contribution from other gallates.     

Characteristics of Heterophase Radical Graft Polymerization of Methyl Methacrylate on Polymer Fibres

Graft copolymers with graft PMMA were synthesized using the Cu²⁺—H₂O₂ redox system. The conditions for obtaining graft PCA—PMMA and HC-PMMA of 40–60 composite composition with grafting efficiency at the 85% level are found. The effect of the polymer matrix on the kinetic and thermodynamic properties of graft polymerization of MMA is revealed. Possible schemes of the reactions of graft polymerization of MMA to polycaproamide and hydrated cellulose are proposed based on quantum chemical calculations of the enthalpy of formation of the graft copolymers.The research was conducted under MNTPP Project No. 203 Chemical Engineering (Section 2 General Chemical Engineering) in the scientific-industrial program Higher-Education Research on Priority Directions in Science and Engineering.__________Translated from Khimicheskie Volokna, No. 1) pp. 19–23, January–February, 2005.     

Application of porous flow through electrodes: IV. Hydrogen evolution on packed bed electrodes of iron spheres in flowing alkaline solutions

Packed bed electrodes of small iron spheres have been used for the electrolytic production of hydrogen from alkaline solutions at different temperatures under conditions of electrolyte flow. The effects of temperature, electrolyte type, concentration and flow rate on the polarization behavior of the electrode were evaluated and analyzed. It was shown that increases in the conductivity of the electrolyte or the operating temperature decreases the potential required to support the reaction. The generated gas bubbles disperse in the pore electrolyte, resulting in an increase in its resistivity and, subsequently, an increase in the potential. It was shown that some gas bubbles are trapped within the porous electrode. The implications of the trapped gas bubbles on the behaviour of the electrode are discussed.Nomenclature A geometrical cross-sectional area (cm²) - a empirical constant (cm³ C^–1) - b RT/F in volt, withR the gas constant,T the absolute temperature - E ₀ electrode potential at the entry face (V) - E _L electrode potential at the exit face (V) - F Faradays's constant - i ₀ exchange current density of the electrode reaction (A cm^–2) - i _L experimentally measured current density at the exist face (A cm^–2) - L bed thickness (cm) - q tortuosity - Q electrolyte volume flow rate (cm³ s^–1) - V electrolyte flow rate,V=Q/A (cm s^–1) - S specific surface area of the bed (cm^–1) - x position in the electrode - transfer coefficient - gas void fraction - ₀ polarization at the entry face (V) - _L polarization at the exit face (V) - porosity - pore electrolyte resistivity ( cm) - ₀ resistivity of the bubble-free pore electrolyte ( cm) - ₀ ^b resistivity of the bulk electrolyte ( cm)     

Optimal design of packed bed cells for high conversion

In connection with the electrochemical purification of metal containing waste waters, the realization of a high concentration decrease per pass is one of the goals of design optimization. For a packed bed cell with crossed current and electrolyte flow directions high conversion in conjunction with a large space time yield requires limiting current conditions for the whole electrode. For establishing the concentration profiles in the direction of flow a plug flow model is used. These considerations result in a new packed bed electrode geometry for which an analytical bed depth function is derived. The basic engineering equations of such packed bed electrodes are given, and design equations for different arrangements are developed. The reliability of this scaling-up method is shown by comparison of theoretically predicted and experimental performance data of two cells. Engineering aspects such as easy matching of cells to waste water properties and parametric sensitivity are discussed. Some technical applications are reported.Nomenclature and constants used in the calculations A _s specific electrode surface (cm^–1) - b(y) width of the packed bed (cm) - c(y) metal concentration (mol cm^–3) - C _e ^t total equivalent concentration of electroactive species (mol cm^–3) - D diffusion coefficient (cm² s^–1) - D _c conversion degree (1) - d _p(y) diameter of packed bed particles (cm) - F Faraday number (96.487 As mol^–1) - h(y) bed depth parallel to current flow direction (cm) - i() current density (A cm^–2) - i _b bed current density (A cm^–2) - i _g[c(y)] diffusion limited current density (A cm^–2) - mean current density of metal deposition (A cm^–2) - k(y) mass transfer coefficient (cm s^–1) - k 0.8121×10^–3 cms^–1/2 - U cell voltage (V) - u(y) flow velocity (cm s^–1) - v voidage (0.56) - v _A volume of anode compartement (cm³) - V _B volume of packed bed electrode (cm³) - v _D volume flow rate (cm³ s^–1) - W water parameter (mol cm^–2 A^–1) - x coordinate parallel to current flow (cm) - y coordinate parallel to electrolyte flow (cm) - y _ST ^E space time yield of the electrode (s^–1 or m³h^–1l^–1) - y _ST ^C space time yield of the cell (s^–1 or m³h^–1l^–1) - z coordinate normal to current and electrolyte flow (cm) - z _i charge number (1) - current efficiency (1) - ₁ overpotential near the feeder electrode (V) - ₂ overpotential near the membrane (V) - ₂- ₁ (V) - (x, y) overpotential at point (x, y) (V) - _s particle potential (V) - _s electrolyte potential (V) - X electrolyte conductivity (S cm^–1) - X _p particle conductivity (S cm^–1) - _s electrolyte conductivity (S cm^–1) - v kinematic viscosity (cm² s^–1) - slope of the feeder electrode (1)     

Effects of gas bubbles on the current and potential profiles within porous flow-through electrodes

This paper presents a mathematical model to calculate the distributions of currenti(x), potentialE(x), gas void fraction (x) and pore electrolyte resistivity (x) within porous flow-through electrodes producing hydrogen. It takes into consideration the following effects: (i) the kinetics of the interfacial charge transfer step, (ii) the effect of the non-uniformly generated gas bubbles on the resistivity of the gas-electrolyte dispersion within the pores of the electrode (x) and (iii) the convective transport of the electrolyte through the pores. These effects appear in the form of three dimensional groups i.e.K=i _o L where i_o is the exchange current density, is the specific surface area of the electrode andL its thickness.= ⁰ L where ⁰ is the pore electrolyte resistivity and =/Q where is a constant, =tortuosity/porosity of the porous electrode andQ is the superficial electrolyte volume flow rate within it. Two more dimensionless groups appear: i.e. the parameter of the ohmic effect =K/b and the kinetic-transport parameterI=K. The model equations were solved fori(x),E(x), (x) and (x) for various values of the above groups.Nomenclature specific surface area of the bed, area per unit volume (cm^–1) - b RT/F in volts, whereR is the gas constant,T is the absolute temperature (K) - B =[1–(I ² Z/4)], Equation 9a - C =(1–B ²), Equation 9b - E(L) potential at the exit face (V) - E(0) potential at the entry face (V) - E(x) potential at distancex within the electrode (V) - E _rev reversible potential of the electrochemical reaction (V) - F Faraday's constant, 96500 C eq^–1 - i _o exchange current density of the electrode reaction (A cm^–2 of true surface area) - i(L) current density at the exit face (A cm^–2 of geometrical cross-sectional area of the packed bed) - I K =i _oL(/Q) (dimensionless group), Equation 7d - K =i _oL, effective exchange current density of the packed bed (A cm^–2) Equation 7a - L bed thickness (cm) - q tortuosity factor (dimensionless) - Q superficial electrolyte volume flow rate (cm³ s^–1) - x =position in the electrode (cm) - Z =exp [(0)], Equation 7f - transfer coefficient, =0.5 - =K/b=(i ₀ L ⁰ L)/b (dimensionless group) Equation 7e - (x) gas void fraction atx (dimensionless) - = ⁰ L, effective resistivity of the bubble-free pore electrolyte for the entire thickness of the electrode ( cm²) - (0) polarization at the entry face (V) - (L) polarization at the exit face (V) - =q/, labyrinth factor - constant (cm³ C^–1), Equation 3a - =/Q (A ^–1) conversion factor, Equation 3b - porosity of the bed - (x) effective resistivity of the gas-electrolyte dispersion within the pores ( cm) - ⁰ effective resistivity of the bubble-free pore electrolyte ( cm)     

Detonation Properties and Electrical Conductivity of Explosive–Metal Additive Mixtures

Detonation properties of mixtures of condensed high explosives with metal additives are studied. A scheme of measurement of high electrical conductivity of detonation products ( > 10 ^–1 · cm^–1) with a time resolution of 10 nsec is developed. It is shown that the properties of detonation products depend significantly on the content of the additive in the HE and on dispersion and density of the mixture. The electrical conductivity of detonation products of the compositions examined reaches 5 · 10^{3 –1} · cm^–1, which is more than three orders higher than the electrical conductivity of the HE without the additive. Significant variation of electrical conductivity of detonation products over the conducting region thickness has been found. The main conductivity corresponds to a sector 1 mm long near the detonation front. The overdriven state of the detonation wave has a strong effect on electrical conductivity and conducting region thickness. It is assumed that the behavior of electrical conductivity with time is caused by successive processes of shock compression of the HE, excitation of the chemical reaction (including the reaction of the additive with detonation products), and expansion of detonation products. The measurement technique used is highly informative due to the possibility of studying detonation in various regimes.     

First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

In situ Raman spectroscopy at temperatures up to 500°C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO₂ oxidation. Vanadia/silica catalysts impregnated with Cs₂SO₄ were exposed to various SO₂/O₂/SO₃ atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported model V₂O₅–Cs₂SO₄ and V₂O₅–Cs₂S₂O₇ molten salts. The data indicate that (1) the V^V complex V^VO₂(SO₄)₂ ^3– (with characteristic bands at 1034 cm^–1 due to (V=O) and 940 cm^–1 due to sulfate) and Cs₂SO₄ dominate the catalyst surface after calcination; (2) upon admission of SO₃/O₂ the excess sulfate is converted to pyrosulfate and the V^V dimer (V^VO)₂O(SO₄)₄ ^4– (with characteristic bands at 1046 cm^–1 due to (V=O), 830 cm^–1 due to bridging S–O along S–O–V and 770 cm^–1 due to V–O–V) is formed and (3) admission of SO₂ causes reduction of V^V to V^IV (with the (V=O) shifting to 1024 cm^–1) and to V^IV precipitation below 420°C.     

Modelling of gas-evolving electrolysis cells. III. TheiR drop at gas-evolving electrodes

Based on a potentiostatic interrupter technique theiR drop of the bubble layer in front of gas-evolving electrodes of various shapes has been investigated. At small plane electrodes the dependency ofiR drop on electrode inclination has been studied for hydrogen, oxygen and chlorine evolution. In all systems a slightly up-faced orientation results in a gas bubble layer structure of minimumiR drop. Also for expanded metal electrodes of different shapes theiR drop across the electrode diaphragm gap has been studied. The fractional open cross-section and the inclination angle of the electrode blades have been identified as important parameters with respect to the gas diverting effect. These tendencies have also been confirmed for a pilot cell of 1 m height.Nomenclature b' Tafel slope (V) - c ₀ double layer capacity (F cm^–2) - d thickness (cm) - E electrode potential (V) - F Faraday number (96487 As mol^–1) - i current density (A cm^–2) - R area resistance ( cm²) - R gas constant (8.3144 Ws deg^–1 mol^–1) - T temperature (K) - t time (s) - u _g ⁰ superficial gas velocity (cm s^–1) - u _sw swarm velocity (cm s^–1) - U voltage (V) Greek symbols inclination angle (^o) - symmetry factor (1) - _g gas voidage (1) - _m maximum gas voidage. (1) - overvolgate (V) - electrolyte conductivity (S cm^–1) - _g number of electrons (1) 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.