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  • 1
    Electronic Resource
    Electronic Resource
    Optimal time-varying potential control (1993)
    Springer
    Journal of applied electrochemistry 23 (1993), S. 715-727 
    Details
    ISSN: 1572-8838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Notes: Abstract As an illustration of the potential utility of optimal-control theory, we determine the time-varying electrode potential which maximizes the desired product produced from a coupled, chemical-electrochemical reaction sequence occurring in a well-mixed batch reactor for a specified reaction time. The reactant is electrochemically reduced to a stable intermediate which is itself a reactant for two competing parallel reactions: a homogeneous chemical decomposition to the desired product, or a further electrochemical reduction to an undesired product. If the transfer coefficient of the first reaction is greater than that of the second, then chattering control, in which the potential switches at an infinite frequency between two limits, is optimal. If the transfer coefficients have the opposite relationship, then a continuous, time-varying potential is optimal. We compare the results of applying the optimal, chattering-potential control with those resulting from the best continuous and steady controls. Improved selectivity results from a chattering control and may be effected even in the presence of significant mass-transfer resistance. Since an infinite-frequency control cannot be actually implemented, we discuss how a high-frequency, rectangular waveform can be determined which results in essentially the same product distribution as a chattering control. A qualitative, simple-to-apply method to determine whether selectivity enhancement is attainable using chattering controls is also illustrated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00243341
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  • 2
    Electronic Resource
    Electronic Resource
    Optimal time-varying cell-voltage control of a parallel-plate reactor (1994)
    Springer
    Journal of applied electrochemistry 24 (1994), S. 1116-1123 
    Details
    ISSN: 1572-8838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Notes: Abstract As a means to illustrate the calculation of an optimal cell-voltage control for a parallel-plate reactor, we determine the time-varying cell voltage that maximizes p-aminophenol produced from the electroreduction of nitrobenzene in a differential-conversion reactor operated in a batch mode; that is, the electrolyte is continuously recirculated from a batch holding tank through the reactor in which a low conversion per pass occurs. A rationale is given for restricting the search for the optimal control for this particular reaction network to a chattering-cell voltage that switches between a priori chosen minimum and maximum values. The optimal, time-varying duty cycle is computed using a gradient-search technique. The predicted concentrations are dependent upon the reaction time; for the conditions examined here, an improvement of twenty-five percent in the production and nine-hundred percent in the selectivity of p-aminophenol may be achieved by using the optimal, time-varying voltage in comparison to the best steady value. Since a chattering control is a mathematical construct, we illustrate that a rectangular, high-frequency waveform may be applied to yield results which are indistinguishable from those effected by a chattering cell voltage. The period of the waveform must be short enough so that surface concentrations are time invariant over it and yet, simultaneously, must be long enough so that double-layer charging does not account for a significant passage of coulombs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00241309
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  • 3
    Electronic Resource
    Electronic Resource
    Kinetic study of the electroreduction of nitrobenzene (1990)
    Springer
    Journal of applied electrochemistry 20 (1990), S. 370-376 
    Details
    ISSN: 1572-8838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A reaction kinetic study has been performed for the reduction of nitrobenzene on a Cu electrode in 1m H2SO4 in a 50∶50 (Vol%) mixture of water and 1-propanol at 27°C. The study was carried out on a rotating disc electrode for which the current-potential data were supplemented with product-concentration measurements. The resulting rate expressions represent a reaction mechanism for the reduction of nitrobenzene to aniline and p-aminophenol through the common intermediate phenylhydroxylamine, and incorporate the dependence on reactant concentration and potential for the three predominant reaction pathways. The three major reaction steps were studied independently by performing experiments in which phenylhydroxylamine only was used as the reactant to complement those experiments in which nitrobenzene was used. The kinetic expressions found from measuring the rates of the individual reactions were consistent with the results of experiments in which all the reactions were carried out simultaneously. The expressions obtained are suitable for use in reactor design, modelling and control, and of equal importance, the methodology outlined to extract kinetic parameters from the current and concentration data serves as a model for application to other reaction systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01076043
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  • 4
    Electronic Resource
    Electronic Resource
    A model for Kolbe electrolysis in a parallel plate reactor (1998)
    Springer
    Journal of applied electrochemistry 28 (1998), S. 1157-1166 
    Details
    ISSN: 1572-8838
    Keywords: Kolbe electrolysis ; parallel plate reactor ; model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A parallel-plate reactor model is developed for the Kolbe electrolysis of acetate to ethane and carbon dioxide with hydrogen evolution as the counterelectrode reaction. The parallel-plate reactor is considered to consist of three zones: a turbulent bulk region in which streamwise convection is the dominant mass-transport mechanism (plug-flow model) and a thin diffusion layer at each electrode where diffusion and migration mass transport are dominant (Nernst diffusion-layer model). The acetic acid solution is supported with sodium hydroxide, and the reactor is under steady cell-potential control. Gaseous products are tracked by a hypothetical gas layer which increases in thickness in the streamwise direction. The gas phase is assumed to be an ideal, three-component mixture of hydrogen, carbon dioxide and ethane; the liquid phase consists of acetate, proton, acetic acid, and sodium and hydroxyl ions. The model predicts streamwise profiles of concentration, current density, gas-void fraction, and gas and liquid velocities in addition to reactant conversion, and cell-polarization characteristics. The average current density exhibits a maximum at a base-to-acid ratio of 0.96 due to the weak-acid/strong-base chemistry and a broad maximum at an interelectrode spacing of 0.37 cm resulting from minimized ohmic losses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1003495828226
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  • 5
    Electronic Resource
    Electronic Resource
    Transient analysis of a flow-through porous electrode at the limiting current (1981)
    Springer
    Journal of applied electrochemistry 11 (1981), S. 145-152 
    Details
    ISSN: 1572-8838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Notes: Abstract The equations governing the transients in the concentration, current and potential response of a porous flow-through electrode at the limiting current for a single reactant in a well-supported electrolyte have been solved. It was assumed that a potentiostatic step was applied to an electrode with a uniform feed concentration. The dispersive flux of reactant was assumed to be negligible but double-layer charging effects were taken into consideration. If the double-layer time constant is much less than the fluid residence time (νC/ɛϰ ≪ 1), it is quantitatively shown that the capacitive current may be neglected in interpreting the current-time response of the electrode when examined in the fluid residence time frame. If the entire electrode is to operate at the limiting current, it is quantitatively shown that the solution phase ohmic drop can become significant early in the transient such that secondary reactions may become important. The ability to interpretI versust data in terms of the limiting current species mass transfer coefficient is removed under these conditions. The results support the qualitative arguments made by Newman and Tiedemann in their comprehensive review article on porous flow-through electrodes. Finally, it is shown that ln(Faradaic current) versust can be approximately linear in a limited time span, although no useful information can be obtained from such a plot.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00610974
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  • 6
    Electronic Resource
    Electronic Resource
    A model for the Kolbe reaction of acetate in a parallel-plate reactor (1996)
    Springer
    Journal of applied electrochemistry 26 (1996), S. 175-185 
    Details
    ISSN: 1572-8838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Notes: Abstract A mathematical model is developed for the study of the Kolbe oxidative dimerization of acetate to ethane and carbon dioxide in a parallel-plate reactor operating at a fixed cell potential, with hydrogen evolution being the cathode reaction. The volume of gas evolved into the interelectrode gap is tracked by constructing a hypothetical gas layer which increases in thickness with the streamwise direction in a manner determined by solution to the model equations; concurrently, the liquid flows in a hypothetical layer which decreases in thickness. The three-component gas phase is assumed to be ideal, and the liquid phase is an aqueous mixture of five species: acetate, proton, sodium and hydroxyl ions and acetic acid. The model predicts the concentration profiles and the streamwise variation of the gas-void fraction, reaction current density and liquid and gas velocities. Gas evolution causes a decreasing current density in the streamwise direction and an increasing gas and liquid velocity. The concentrations of acetic acid and proton decrease in the streamwise direction, while hydroxyl concentration increases; the decrease in acetate concentration, however, is not significant until the local base-to-acid ratio is near unity because of the buffering effect from undissociated acetic acid. The average current density increases with inlet solution velocity and cell potential and asymptotically approaches the secondary current limit. There exists an optimal interelectrode separation where the cell resistance is minimum. The average current density exhibits a shallow maximum with the baseto-acid ratio of the feed, but decreases precipitously when the ratio is near unity due to the rapid decrease in the proton concentration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00364068
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  • 7
    Electronic Resource
    Electronic Resource
    Selectivity modification in electroorganic reactions by periodic current control: Electroreduction of nitrobenzene (1985)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 31 (1985), S. 1578-1580 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The electroreduction of nitrobenzene in deoxygenated acid solution generates predominantly either p-aminophenol or aniline via the common electrochemically generated intermediate, phenylhydroxylamine. Square wave current control increases the reaction selectivity for p-aminophenol, which is produced in a chemical pathway from the intermediate over that obtained at the average current of the oscillation. Data collected under electrokinetic-controlled reaction conditions on a planar copper electrode in a water-propanol-sulfuric acid medium show an order of magnitude increase in selectivity at 100 Hz over that measured at the average direct current. The electrode is depolarized under periodic current control which results in lower rates of undesired electrochemical reactions with a consequential increase in chemical product selectivity. Implications and limitations of periodic electrochemical reaction rate control are discussed.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690310925
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    Paper (German National Licenses)
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