S. Rode

Hydrodynamics of liquid flow in packed beds: an experimental study using electrochemical shear rate sensors

A literature survey on the electrochemical sensor technique shows that the simple relation between the average measured diffusional current and the local shear rate of the liquid flow holds even for extremely non-homogeneous flow conditions with high-amplitude fluctuations. Even though the frequency response of electrochemical probes is unsatisfactory, their transfer function has been recently determined and low-amplitude—high-frequency fluctuations of the instantaneous local shear rate can be investigated. Small circular probes have been successfully used to study hydrodynamics of liquid flow in packed-bed reactors. The local flow is extremely non-homogeneous in space and, for high Reynolds numbers, quite fluctuating in time. The onset of chaotic, time-dependent flow occurs at superficial Reynolds numbers of about 110–150. The analysis of the characteristic times of the velocity gradient fluctuations in the chaotic flow regime indicates the existence of liquid agregates having the characteristic dimensions of the porous media.

Complexation Chemistry in Copper Plating from Citrate Baths

An experimental and theoretical study of the influence of solution chemistry on the electrodeposition of copper from complexing citrate baths is proposed and discussed. The behavior of the system is described in terms of the relative distribution of various copper-citrate complexes, combined with a model mechanism for electrodeposition kinetics involving an adsorbed blocking intermediate. Studies of partial-current polarization curves for copper deposition over a wide range of solution pH and free citrate concentration substantiate the mechanism and offer convincing evidence for the significant role of solution chemistry in the electroreduction process. In addition to the copper system, the mechanism proposed offers a framework that may be useful for the study of other metals and alloys electrodeposited from complexing baths containing citrate or citrate-like molecules.

Hydrodynamics and liquid—solid mass transfer mechanisms in packed beds operating in cocurrent gas—liquid downflow: An experimental study using electrochemical shear rate sensors

Overall and local hydrodynamics and liquid—solid mass transfer mechanisms were investigated in a laboratory scale packed bed reactor operating in gas—liquid downflow. The mean liquid saturation and the liquid—solid mass transfer rate were determined using classical electrochemical techniques and the local instantaneous hydrodynamics were analyzed using electrochemical shear rate sensors. The experimental results as well as theoretical considerations enabled us to contribute to the elucidation of gas—liquid flow mechanisms, especially in high-interaction regimes. In pulse flow at low liquid flow rates (L < 10kg m−2 s−1) the wall is, on average, not entirely wetted, this might explain hot spot occurence in industrial fixed-bed reactors. In dispersed bubble flow and in the liquid rich slugs of pulse flow, the local instantaneous liquid-wall shear rate is characterized by high-amplitude—high-frequency fluctuations. The comparison of the space averaged shear rate measurements with the overall mass transfer rate indicates that the liquid—solid mass transfer mechanism is laminar in nature and may be modelized by a succession of developing laminar boundary layers. An overall mechanical force balance on the liquid shows that the average drag of the liquid by the gas is very small, compared to the total energy dissipated by the gas in the reactor. All the experimental results obtained in this work as well as several literature data can be explained by a flow mechanism in dispersed bubble flow, where the liquid flow is dominated by viscous forces whereas the gas bubbles pass through the packed bed by pressure pulses.

Prediction of pressure drop and liquid saturation in trickle-bed reactors operated in high interaction regimes

Simple models, based on liquid-solid and gas-liquid interaction are proposed in order to model a gas-liquid flow in trickle-bed reactors operating in high interaction regimes. Good results are obtained, when the liquid-solid and the gas-liquid interactions are modeled separately. Besides the classical Ergun equation, a simple model of the liquid-solid interactions, based on a boundary layer flow is proposed. This approach results in a relation which is very close to the well-known liquid saturation correlation proposed by Specchia and Baldi (1977, Chem. Engng Sci.32, 515–523), and might be its theoretical justification. The gas-liquid interaction is modeled using a drift-flux approach. The different models are tested against the data bank of about 1500 saturation and pressure drop measurements, established mainly by the Nancy research group. The importance of the geometrical characteristics of the porous media and their incomplete description are emphasized. Finally simple predictive correlations based on the models are proposed, tested against the data bank and compared to literature correlations.

The use of microelectrodes for the determination of flow regimes in a trickle-bed reactor

The utilization of microelectrodes in a non-conducting wall with subsequent signal analysis allowed the determination of flow regime transitions: trickling/pulsing, trickling/dispersed bubble and dispersed bubble/pulsing in a trickle-bed reactor by the analysis of the rate of fluctuation of the liquid—solid mass transfer coefficient (velocity gradient) variations as a function of liquid and gas flow rates.

Complexation Chemistry in Nickel and Copper-Nickel Alloy Plating from Citrate Baths

Following a previous study of the influence of solution chemistry on the electrodeposition kinetics for copper deposition from complexing citrate baths, the present work examines the corresponding influence for nickel andcopper-nickel alloys. The distributions of various metal-citrate chelates, computed as a function of bath composition and pH, are combined with model mechanisms for electrodeposition kinetics involving adsorbed blocking intermediates. The methodology permits not only interpretation of the partial-current curves measured in nickel-citrate baths, but also of the partial-current curves determined for copper and for nickel in codeposition citrate baths. For the case of codeposition, the copper behavior is similar to that observed without nickel, whereas the nickel deposition is catalyzed by the copper. When correctly accounting for solution chemistry, a simple model is sufficient to describe the essential features of the single nickel and the codeposition kinetics.

Bubble flow mechanisms in trickle beds—an experimental study using image processing

Abstract The mechanisms of bubble motion in concurrent gas–liquid down flow through trickle beds are investigated. The laboratory reactor is a structured quasi-two-dimensional porous medium with an average pore diameter close to the values encountered in trickle beds. The accuracy of the reactor design is demonstrated by hydrodynamic investigations on the reactor scale where it is shown that the flow regimes encountered and the experimental pressure drop are comparable to those observed in trickle beds. The investigations on the pore scale are focused on the dispersed bubble flow regime where the liquid flow is continuous and the gas is divided into elongated bubbles. The bubble motion is recorded with the aid of a high-speed video camera and the images are processed and analysed in a quantitative manner. The investigations clearly show that in dispersed bubble flow, the bubbles are frequently pulsing on the pore scale. The mechanism of this flow pattern is discussed.

Segmented thin-gap flow cells for process intensification in electrosynthesis

Design calculations are presented for a single-pass high-conversion electrochemical reactor suitable for process intensification in electroorganic synthesis. The key feature of the design is the use of a segmented working electrode, combined with a small anode—cathode gap. Each working electrode segment is operated at an optimal local current density, defined with respect to the local diffusion—limited current density of the reacting species. Two reactor configurations are considered:(i) an adiabatic reactor, and (ii) an isothermal reactor with integrated heat exchange. Calculated results for the devices in a classical electroorganic synthesis system, the methoxylation of 4-methoxy-toluene, are presented and the general features and performance characteristics of the cell are compared with those of a more conventional capillary-gap cell, currently used industrially. For an electrode gap of 0.1 mm, the average current density attainable in the novel design is of the order of 2700 A m−2 in the adiabatic reactor and of the order of 7100 A m−2 in the isothermal reactor, respectively, 5 and 14 times higher than the current densities applied in the current industrial process. In addition to process intensification, other advantages of the proposed technology are the absence of reactant recycle, short residence times and plug flow of the reagents, all of which contribute to improved process selectivity.

Thin-Gap Single-Pass High-Conversion Reactor for Organic Electrosynthesis II. Application to the Anodic Methoxylation of 4-Methoxytoluene

The four-electron anodic methoxylation of 4-methoxytoluene has been performed in a thin-gap, single-pass, high-conversion reactor at ambient temperature and pressure, with an inlet reagent concentration of 0.1 M, methanol solvent and coreactant, 0.01 M KF supporting electrolyte, and current densities up to 240 A m -2 . Product selectivities greater than 90% have been obtained for reagent conversions greater than 95%, corresponding to very high product yields in a simple single-pass technology. The generated volumetric hydrogen flux is 3-7 times higher than the volumetric liquid flux, resulting in a substantial decrease of the electrical conductivity in the 100 μm wide interelectrode gap. Nevertheless, the experimental results compare very well to reactor simulations performed with the electrochemical cell model developed in Part I of this study [J. Electrochem. Soc. 155, E193 (2008).] Adjustment of a single parameter, the value of the average mass-transfer coefficient, is sufficient to fit the simulations to the experimental results, and the results suggest that the electrochemical reactor model developed should be of use for application to other electrochemical reaction systems also.

Thin-Gap Single-Pass High-Conversion Reactor for Organic Electrosynthesis I. Model Development

Design calculations are presented for a thin-gap, single-pass, high-conversion electrochemical cell suitable for process intensification in electro-organic synthesis. The interest of a microstructured design in electro-organic synthesis is highlighted, and it is shown that the specific productivity of the electrochemical cell is inversely proportional to the interelectrode gap. A dimensionless reactor model is developed for the case of an electro-organic synthesis characterized by three consecutive oxidation steps, coupled with possible mass-transfer limitations. For a given range of kinetic parameters, the reactor performance depends on only three independent dimensionless parameters: a Wagner-like number, a number of transfer units, and a dimensionless current. Model simulations are performed for the case of methoxylation of 4-methoxyanisole and provide identification of optimal operating conditions for process performance: a Wagner number higher than 0.5, a number of transfer units between 6 and 12, and a dimensionless current close to unity. In practice, the number of transfer units and the dimensionless current are easily adjusted, whereas it is much more difficult to adjust the Wagner number. For the thin-gap cell geometry, low Wagner numbers lead to uniform current distributions which favor undesired reactions at the reactor outlet, where the reagent concentrations are low. For industrial application, high-pressure operation can be advantageous. If the system is operated at a sufficiently high pressure (P > 10 bar), the drawback of hydrogen evolution at the counter electrode can be drastically reduced and the single-pass, high-conversion cell is feasible even at high levels of reagent concentrations.