Mass transfer in an annular electrodialysis cell in pulsating flow

The effect of pulsating flow on the mass transfer in an annular electrodialysis cell has been studied in terms of the limiting current. The results indicate that the limiting current is influenced by the fluid velocity, the pulsation amplitude and the pulsation frequency, giving an increase of 400% with respect to the steady state. For a given amplitude, the dimensionless velocity, 0 (0 = a/v), can be taken as a representative parameter of the pulsation effect on the mass transfer. The fractional increase in the Sherwood number in pulsating flow with respect to the steady state has been correlated in terms of the dimensionless velocity, 0, and the Stokes number, 1 (1 = Deq (/)1/2), giving the correlation:     

The oxidative dehydrogenation of ethane over molybdenum-vanadium-niobium oxide catalysts: the role of catalyst composition

The oxidative dehydrogenation of ethane has been studied at atmospheric pressure using molybdenum-vanadium-niobium oxide catalysts in the temperature range of 350–450 °C. The presence of all three oxides together is necessary in order to have active and selective catalysts. The best results have been obtained using a mixture having a Mo V Nb ratio of 19 5 1. Our studies of the variation of oxide composition suggest that the active phase is based on molybdenum and vanadium. Niobium enhances the intrinsic activity of the molybdenum-vanadium combination and improves the selectivity by inhibiting the total oxidation of ethane to carbon dioxide. The apparent activation energies for the conversion of ethane to ethylene, carbon monoxide and carbon dioxide were 18, 27 and 17 kcal/mol, respectively. The addition of water vapor to the gas stream does not affect the product distribution on this catalyst.     

Voidage of bulk solution in a tall vertical gasevolving cell

Vertical electrolysers with parallel-plate electrodes and with a narrow interelectrode gap or with narrow gaps between the membrane and each of the electrodes are used industrially to produce gases. Conductivity measurements were carried out with a small conductivity cell built within one compartment of the cell and just above the top of the working electrode. KOH solutions were used as electrolytes and gas bubbles were evolved at a 20-segment electrode. The effect of various parameters, including current passed through the 20-segments electrode, flow rate of liquid, temperature and nature of gas evolved, was studied. From the normalized conductivity measured, the gas voidage was obtained. It was found that for a bubbly flow the gas voidage just above the gas-evolving electrode is given by the relation:/(max – ) = 1.4 Qg/Qlwhere max is the maximum voidage for gas bubbles in a solution; max=0.69 for oxygen as well as hydrogen in a concentrated and a dilute KOH solution at different liquid flow rates. Qg is the volumetric flow rate of gas saturated with water vapour and Ql is the volumetric flow rate of liquid. This relation can be used to calculate the voidage distribution in the bulk solution of the cell compartment with a gas-evolving electrode.     

A parametric study of copper deposition in a fluidized bed electrode

The behaviour of a fluidized bed electrode of copper particles in an electrolyte of deoxygenated 5×10^–1 mol dm^–3Na₂SO₄–10^–3mol dm^–3H₂SO₄ containing low levels of Cu(II), is described as a function of applied potential, bed depth, flow rate, particle size range, Cu(II) concentration and temperature. The observed (cross sectional) current densities were more than two orders of magnitude greater than in the absence of the bed, and current efficiencies for copper deposition were typically 99%.No wholly mass transport limited currents were obtained, due to the range of overpotentials within the bed. The dependence of the cell current on the experimental variables (excluding temperature) was determined by regression analysis. The values of exponents for some of the variables are close to those expected, while others (for concentration and flow rate) reveal interactions between the experimental parameters. Nevertheless the values of the correlation coefficient matrix are low (except for the term relating expansion and flow rate), so that cross terms may be neglected in modelling the system at the first level of approximation.Nomenclature d mean particle diameter (mm) - E electrode potential, ( _m– _s)_r+(x) (V vs ref) wherer denotes the value of ( _m- _s) at the reversible potential - I (membrane) current density (A m^–2) - L static bed depth (mm) - M concentration of electroactive species (mol dm^–3) - T catholyte temperature (K) - u catholyte flow rate (mm s^–1) - x distance in the bed from the feeder electrode atx=0 - XL expanded bed depth (mm) - bed expansion (fraction of static bed depth) - _m metal phase potential (V) - _s solution phase potential (V) - _m metal phase resistivity (ohm m) - _s solution phase effective resistivity (ohm m) - overpotential (V)     

Electrowinning magnesium from its oxide in a melt containing neodymium chloride

A new process for the electrolytic production of magnesium from its oxide is described, based on the reaction MgO + NdCl3MgCl2 + NdOCl. This is followed by electrolysis of the resulting MgCl2/NdCl3/NdOCl melt using a carbon anode, the proposed electrolysis reaction being 2MgCl2+C+ 2NdOCl2Mg + CO2 + 2NdCl3. XRD studies confirm the formation of NdOCl when MgO is fused with NdCl3. Electrolysis of the melt, using a molten tin cathode and a graphite anode, produced a gas containing chlorine, suggesting some direct electrolysis of chlorides. At 700°C only chlorine was found, but at 750°C and above chlorine and carbon dioxide were evolved, the chlorine:carbon dioxide ratio decreasing markedly as the current density was decreased from 3600 to 890Am–2. This was consistent with the measured cell voltages, which at 750°C and above fell below the calculated decomposition voltage of MgCl2 at current densities of less than 1000 A m–2. There is extensive co-deposition of neodymium with the magnesium with the molten tin cathode, but when magnesium metal was equilibrated with the MgCl2/NdCl3/NdOCl melt only 0.5wt% of neodymium entered the metal.     

Electrochemical and gas evolution characteristics of direct methanol fuel cells with stainless steel mesh flow beds

Carbon dioxide gas management and bipolar plate/(flow bed) design are important to the development of direct methanol fuel cell (DMFC) systems. If the gas produced at the anode is not removed rapidly and efficiently a gradual deterioration in electrical performance can occur. This paper examines the feasibility of using stainless steel mesh materials as flow beds for the DMFC. A flow visualization study, using a high-speed video camera and appropriate computer software, of the anode side, carbon dioxide gas evolution and flow behaviour with flow beds based on stainless steel mesh is reported. The electrochemical behaviour of the direct methanol fuel cell with stainless steel flow beds is also reported. A number of the flow bed designs, based on stainless steel mesh, showed promising behaviour in terms of gas removal characteristics and electrical performance.     

Selective Oxidation of 1- and 2-Propanol with Molecular Oxygen by Noble Metal Catalysis in “Supercritical” Carbon Dioxide

The selective oxidation of 1- and 2-propanol by molecular oxygen over supported platinum catalysts was investigated in supercritical carbon dioxide as an environmentally benign and safe reaction medium. The reaction occurs exclusively to acetone or propionic aldehyde and propionic acid in a single-phase region at 100–190 bar and at a mild temperature (40 °C). Compared to conversions in aqueous solution, catalyst stability is significantly enhanced in supercritical carbon dioxide and depends on the oxygen concentration in the reaction medium. Thus, at least a fourfold higher substrate/catalyst ratio than with water as a solvent can be used. Platinum catalysts with nanoporous silica (MCM-41, silicalite-1) as a support are also active for the oxidation of 2-propanol in supercritical carbon dioxide.     

Low temperature electrochemical oxidation of sulfur dioxide

The low temperature electrochemical oxidation of sulfur dioxide on carbon gas diffusion electrodes modified with cobalt phthalocyanine (CoPc) was studied. It was found that when operating with mixtures of air containing up to 20% by volume SO₂ a basic problem of the electrooxidation of SO₂, namely its permeation into the electrolyte, can be resolved. It is demonstrated that sulfur dioxide can be oxidized in the presence of air on active carbon, modified with cobalt phthalocyanine without external current at room temperature. The electrocatalytic character of the process is established and the rate dependence on the concentrations of H₂SO₄ and SO₂ up to 0.30 vol % in air is displayed. An original method for the cold oxidation of SO₂ to sulfuric acid is proposed.     

Wacker type and π-allyl type oxidations of propylene controlled by fuel cell system in the gas phase

Oxidation of propylene applying the (C₃H₆, Pd/H₃PO₄/Pt, O₂) fuel cell system in the gas phase produced acrolein and acrylic acid, the -allyl type oxidation products. However, addition of H₂PdCl₄ or HCl to the electrolyte reduced the -allyl oxidation, but enhanced Wacker type oxidation producing acetone. Direction to either Wacker type or -allyl type oxidation can easily be controlled electrochemically by applying the fuel cell system in the gas phase.     

Electrochemical carboxylation of organic substrates. Synthesis of carboxylic derivatives of acenaphthylene

The electrochemical carboxylation of acenaphthylene totrans-acenaphthene-1,2-dicarboxylic acid and acenaphthene-1-carboxylic acid has been investigated. The yields of the synthesis are influenced by the cathode material by the current density and, in the case of acenaphthene-1-carboxylic acid, by the pressure of carbon dioxide. In preparative experiments, performed in a gas lift electrochemical cell, up to 100 g oftrans-acenaphthene-1,2-dicarboxylic acid were obtained at cathodic current densities of 40 mA cm^–2 and with current yields greater than or equal to 80%.