Electrochemical oxidation of coals: Voltammetry and mass spectrometry

The anodic oxidation of sulphuric acid slurries of lignite, bituminous, and anthracite coals; a high-yield extract of coal; and carbon black, has been investigated voltammetrically and with simultaneous m.s. monitoring of evolved gases. The previously reported evolution of carbon dioxide at very low voltages was shown to be chemically released from minerals. The low rank coals and the asphaltite did show considerable electrochemical activity near to 1.0–1.2 V, mainly from the conversion of leached ferrous ion to the ferric form. Further activity near 1.4–1.5 V arose from the oxidation to CO₂ of leached organic matter. For well-washed low rank coals, the coal extract and the anthracite, CO₂ release was only observed at high (>2.8V) anode voltages, with simultaneous release of oxygen. Electrochemical gasification of coal via anodic oxidation of macromolecules at low voltages did not occur.     

Local mass transport effects in the FM01 laboratory electrolyser

A number of patterns of segmented line electrodes have been manufactured using copper printed circuit board technology. These segmented electrodes have been used to investigate local mass transport effects in ICI's FM01-LC parallel plate electrolyser. It is shown that in the absence of a turbulence promoter the current distribution is uneven. Along the direction of electrolyte flow, a tertiary current distribution is observed. In addition, close to the cell entrance, an uneven current distribution occurs perpendicular to the direction of electrolyte flow; this reflects the design of the electrolyte distributor. With a turbulence promoter the current distribution is more even and the entry effects are much reduced. The turbulence promoter can, however, impose its own pattern on the current distribution perpendicular to the flow.     

Mass transport effects on the electrodeposition of iron-nickel alloys at the presence of additives

Iron-nickel (Fe-Ni) plating bath solution chemistry was studied by determining the Fe-Ni equilibrium concentrations at various pH levels. It was found that the alloy composition is determined by solution equilibria, mass transfer of the electroactive species within the diffusion layer and by the surface coverage of the additives on the electrode. The effect of the rotation speed of the disc electrode and the presence of organic additives on the deposition of Fe-Ni alloys are evaluated. Boric acid increases the absolute iron deposition rate, while it inhibits the rate of nickel reduction. Saccharin and ethylene diamine influence the metal deposition rate but are not as effective as boric acid.     

Polymerization of methacrylamide in the presence of ultrasound and peroxomonosulphate

Polymerization of methacrylamide has been carried out in the presence of ultrasound and peroxomonosulphate for the first time. The rate of polymerization and the rate of disappearance of peroxomonosulphate were followed simultaneously. A first order dependence on monomer and half order dependence with respect to peroxomonosulphate were found towards rate of polymerization. However, a first‐order dependence on peroxomonosulphate was noted with rate of peroxomonosulphate disappearance. A suitable reaction scheme is suggested to fit with the experimental results. The composite rate constants for the polymerization were evaluated and compared for acrylamide and methacrylamide polymerization in the presence of ultrasound. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 524–529, 2000     

Polymerization of methyl acrylate in the presence of ultrasound and peroxodisulfate

A systematic polymerization kinetic study of methyl acrylate with an added initiator, peroxodisulfate, in the presence of low‐power ultrasound was done. The polymerization experiments were conducted at various concentrations of monomer and initiator at different temperatures (303–323 K). The polymerization was found to proceed without an induction period, and the steady state was attained in a fairly short time. The rate of monomer disappearance showed a second‐order dependence on monomer concentration. The chain lengths of the polymer were calculated, and we found that the chain length increased with increasing monomer concentration and decreased with increasing initiator concentration. The reaction scheme proposed is based on the kinetic studies that indicated linear termination by sulfate‐ion‐radical‐incorporating direct reaction between the monomer and the initiator. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Stability of moving surfaces in fluid systems with heat and mass transport — I: Stability in the absence of surface tension effects

A mechanism by which transport processes can produce instability of moving surfaces in fluid systems in considered. Stability of a moving boundary at which fog forms or vaporizes is analyzed using techniques of existing work on stability of moving solid surfaces. It is found that the vaporization case is always stable. But instability is predicted when fog forms at a boundary moving toward an initially supersaturated region. Stability of moving reaction fronts and of moving boundaries in ionic systems is discussed.     

Activating and deactivating mass transport effects in methanol and formic acid oxidation on platinum electrodes

Transient increases in rotation rate at an RDE increase the methanol oxidation current, even though in slower experiments the current decreases with increasing rotation rate as usually reported. Methanol oxidation on smooth polycrystalline platinum rotating disk electrodes in sulfuric acid electrolyte was studied by RDE voltammetry and hydrodynamic impedance spectroscopy combined with cyclic voltammetry. A positive low-frequency real part in the hydrodynamic admittance spectra for the main oxidation peak was used to predict that a transient increase in rotation rate would increase the current, as was observed. In contrast, slow scan rate voltammograms showed a decrease in current with increasing rotation rate. The transient current increase was explained by enhanced production of soluble intermediates, while increased production of adsorbed CO poisoning explained the slower inhibition. Comparative experiments for formic acid oxidation showed increasing current with rotation rate in both hydrodynamic admittance spectra and slow-scan voltammograms.     

The effects of mass transport and chemical reaction on pollutant removal in a three-phase slurry adsorber-reactor

The analysis of the effects of mass transport and chemical reaction on pollutant removal from a gas-phase in a three-phase slurry adsorber-reactor is presented. A mathematical model, consisting of four coupled differential equations, was developed to describe the process and solved numerically. The modeling assumptions included: perfect mixing in the three-phase slurry, Henry's law, a linear adsorption isotherm and a second order, irreversible chemical reaction. The model was found to adequately represent limited existing experimental data and to qualitatively predict the effects of the various transport coefficients on the pollutant removal efficiency of the process. The model was also used for design analysis. Although the model-process achieved significant pollutant removal for representative operating conditions, the percent removal was ultimately limited by mass transport from the gas bubbles to the liquid slurry.     

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.     

Multiphase mass transport with partitioning and inter-phase transport in porous media

We derive the effective mass-transfer coefficient between two fluid phases in a porous medium, one of which is flowing and the other is immobile. A passive tracer is advected by the flowing phase, becomes partitioned at the fluid-fluid interface and diffuses in the immobile phase. We use traditional volume-averaging methods to obtain a unit-cell boundary-value problem for the calculation of the effective mass-transfer coefficient. The problem is controlled by the Peclet number of the flowing phase, by a second dimensionless parameter that captures diffusion and partition in the two phases and by the geometrical properties of the porous medium.We derive asymptotic results for the scaling of the mass-transfer coefficient under various limiting conditions. Then, we use numerical methods that solve for the flow velocity field under Stokes flow conditions, and for the transport problem. The numerical results verify the asymptotic scaling expressions and provide estimates of the coefficient for a number of special cases. In particular, we find that when the immobile phase is wetting the solid (in the form of films), the mass transfer coefficient is larger than in the non-wetting case (where the phase is distributed in the form of blobs). Shape factors for practical applications are also obtained.     

Fluid to particle mass transport in slurries

Equations are derived giving the relationship between the electrochemical potential of the catalyst and mass transport of hydrogen in liquid phase hydrogenation. The result provides a method for measuring local fluid to particle mass transfer coefficients in a slurry reactor. The method is illustrated by evaluating local Sherwood numbers for hydrogen transfer during hydrogenation of nitrobenzoic acid in water medium in the presence of 5% Pd/C.     

Simulation of mass transport in SOFC composite electrodes

The anodes used in SOFCs are composites, formed by a mixture of nickel and YSZ particles. This paper presents a model for this type of electrode, taking mass transport effects into account. The effect of the operating conditions, such as temperature and pressure, is discussed. Also, the effect of the choice of the geometrical parameters, such as electrode thickness and particle radius, on the electrode performance is analysed in detail. In particular, the electrode losses display a minimum for a well-defined radius of the electrode particles, which is related to a trade-off between activation and concentration losses.     

Stability of moving surfaces in fluid systems with heat and mass transport II. Combined effects of transport and density difference between Phases

Stability of interfaces moving as a result of phase transformation or mass transfer is considered. Effects of transport and of the density difference between phases known to significantly influence stability during solidification and combustion respectively are here studied theoretically in fluid systems. The analysis indicates that instability is possible during vaporization of liquids under vacuum and perhaps during growth of cavitation bubbles. But except when interfacial tension is very low, instability is unlikely during mass transfer in liquid-liquid systems because the density difference between phases is small. A density difference can also produce cellular convection at an unstable interface during solidification.     

Scaling transformation for the effects of chemical reaction on free convective heat and mass transfer in the presence of variable stream conditions

Natural convective boundary layer flow and heat and mass transfer of a fluid with temperature-dependent fluid viscosity, chemical reaction and thermal radiation over a vertical stretching surface in the presence of suction is investigated by scaling transformation analysis. Fluid viscosity is assumed to vary as a linear function of temperature. The symmetry groups admitted by the corresponding boundary value problem are obtained by using a special form of Lie-group transformations viz. scaling group of transformations. An exact solution is obtained for translation symmetry and numerical solutions for scaling symmetry. Effects of temperature-dependent fluid viscosity, chemical reaction and thermal radiation on the dimensionless velocity, temperature and concentration profiles are shown graphically. Comparison with previously published work is performed and excellent agreement between the results is obtained. The conclusion is drawn that the flow field and temperature and concentration profiles are significantly influenced by these parameters.     

Enhancement of oxygen mass transfer rate in the presence of nanosized particles

Absorption of gases into a liquid is essentially important for two- or three-phase reactions, because the diffusion of a sparingly soluble gas, like oxygen, across a gas-liquid interface generally limits the reaction rates. Using a third, dispersed phase, the mass transfer rate could be significantly increased. The question arises how the absorption rate can be described in the presence of very fine, nanometer size particles or droplets. Its mathematical model should take into account the specific properties of the nanoparticles, e.g. the Brownian motion of particles, its effect on the diffusion of the bulk phase molecules, the mass transfer rate into the nanoparticles, its dependency on the particle size, etc. The mass transfer rate of oxygen, in the presence of nanometer size, organic droplets, has been investigated both experimentally, using organic submicron n-hexadecane droplets, and theoretically. The effect of the Brownian motion of the nanoparticles as well as its effect on the diffusivity in the nanofluid has been discussed. Accordingly, the enhanced diffusion coefficient, due to the convective motion of the continuous liquid phase induced by the moving particles, has been predicted and its effect on the mass transfer enhancement has been calculated using both homogeneous and heterogeneous mathematical models. The predicted data were compared to the measured ones.     

Investigations in mass transfer enhancement in textiles with ultrasound

A novel application of ultrasound is for the intensification of wet textile treatments, in which mass transfer in the inter- and intra-yarn pores of the textile is the basic physical mechanism. This paper describes a simple methodology for the estimation of mass transfer enhancement in ultrasonic textile treatments. For this study, washing of EMPA 101 fabric, soiled with carbon soot and olive oil, is selected as a model process. In the absence of precise knowledge of the convection velocity resulting due to transient cavitation, a semi-empirical method is used to estimate mass transfer enhancement. The experimental soil removal rate during model process is determined by precise time-controlled ultrasonic treatment of the textile, with the source of cavitation nuclei located close to the textile surface. The mass transfer in the textile during the model process is found to occur in two distinct steps, characterized by two different convective diffusion coefficients. This effect is explained in terms of uneven soil distribution in the inter- and intra-yarn region. The mass transfer enhancement factor, defined as ratio of convective diffusion coefficient to molecular diffusion coefficient of soil particles, is found to be in the range 1000-2000. In addition, it is found that the mass transfer enhancement increases with acoustic pressure amplitude during textile treatment. A qualitative estimate of the convection velocities generated in the vicinity of the bubble is provided using numerical simulations of bubble dynamics equation.     

Tuning the framework polarity in MFI membranes by deboronation: Effect on mass transport

The polarity of a zeolite is an important parameter determining the transport and separation properties in zeolite packed beds and membranes. This study focuses on the systematic variation of the zeolite polarity for membrane applications by varying the amount of silanol groups within the framework of the MFI zeolite by deboronation and consecutive heat treatment from 9 to 4 silanol groups per unit cell. The decrease in the number of silanol groups after heat treatment is confirmed by diffuse reflection infrared Fourier transform (DRIFT) and ²⁹Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The permeation of pure water through the modified silicalite membranes is insensitive to the number of silanol groups, attributed to a decreased adsorption compensated by an increased diffusivity. The ethanol selectivity in water/ethanol mixtures shows an increase with decreasing amount of silanol groups. Residual boron in the zeolite may react with ethanol and decrease its adsorption capacity. A reduction in flux through the debronated MFI membranes was not observed after ethanol/water permeation.     

Magnetic field effects on the mass transport at small electrodes studied by voltammetry and magnetohydrodynamic impedance measurements

The effect of moderate magnetic fields (up to 0.75 T) on the redox reaction of the ferro-ferricyanide couple at small disk electrodes (radii 12.5, 50 and 800 μm) has been investigated using voltammetric techniques and the magnetohydrodynamic (MHD) transfer function analysis. A special feature of the experimental setup is that it works in the time domain and therefore does not need a lock-in amplifier. The magnetic field was applied parallel to the electrode surface and perpendicular to the gravity force. Special focus was given to the influence of the electrode size and the concentrations of the redox species on the MHD effect. In accordance with literature it was found that the MHD effect decreases with decreasing electrode size and concentration of the electroactive species. Furthermore it was found that the characteristic time constant of the MHD effect increases with decreasing concentration of the electroactive species. Combining the information from the voltammetric and the MHD transfer function data we could estimate the magnetically induced flow velocities which were in a reasonable order of magnitude. The implications of our results for MHD stirring in small feature sizes will be discussed shortly.