A. Storck

The use of micro-electrodes in the study of the flow regimes in a packed bed reactor with single phase liquid flow

Abstract The utilization of micro-electrodes in a non-conducting wall with subsequent signal analysis allowed the determination and the study of the flow regimes in a packed bed reactor with single-phase liquid flow. In effect, the analysis of the rate of fluctuation of the liquid—solid mass transfer coefficient (velocity gradient) as a function of liquid flow rate clearly showed the transition between inertial flow regime (nonlinear steady-state laminar) and turbulent flow regime. This transition, located between Reynolds number values of 110 and 370, corresponds to an unsteady-state laminar flow regime. Examination of the power spectral densities of these fluctuations led to a better understanding of liquid flow behaviour during this transition.

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.

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.

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.

Hydrodynamics and gas-liquid-solid interfacial parameters of co-current downward two-phase flow in trickle-bed reactors

Abstract Design of trickle-fixed bed reactors requires knowledge of the hydrodynamics of two-phase co-current downward flow through fixed porous catalytic media and interfacial parameters. Unfortunately, most of the published papers deal exclusively with the hydrodynamics of an air-water system and the determination of gas-liquid-solid interfacial parameters in highly ionic solutions. In this paper, we present some experimental results on the hydrodynamics, pressure drop, liquid holdup, different flow patterns, gas-liquid interfacial areas and liquid-side mass transfer coefficients for organic non-viscous and viscous liquids and liquid-solid mass transfer coefficients with different packings: glass beads (dp = 1.16 × 10−3 m and 4 × 10−3 m), spherical catalyst (dp = 2.4 × 10−3 m) and glass Raschig rings (dp = 6.48 × 10−3 m). Comparison between our values and correlations in the literature will be discussed.

Electrochemical oxidation of sulphite ions at graphite electrodes

The electrochemical oxidation of sodium sulphite has been studied in aqueous sodium sulphate solution at two different graphite electrodes, one being of natural graphite (EC) and the other impregnated with phenol (ECK). The objective of the present work was to obtain some insight into the direct oxidation as well as the indirect oxidation, via produced oxygen radical species, of sulphite on non-metal electrodes. For this reason a study of the oxidation of sulphite in the concentration range between 0–0.10m in aqueous sodium sulphate using a batch electrochemical reactor, operating potentiostatically, was undertaken. The potential range was chosen between 1.0 to 2.5 V/SCE, and the concentration of the supporting electrolyte, sodium sulphate, was kept constant at 0.5m. A kinetic Tafel type law, considering irreversible behaviour for the direct sulphite oxidation and the mass transfer performance in regards to the experimental conditions were applied to predict the time variations of the sulphite conversion.

Liquid—solid mass transfer in a packed bed with downward cocurrent gas—liquid flow: An organic liquid phase with high Schmidt number

Abstract An electrochemical technique was used to measure the overall liquid—solid mass transfer coefficients at high Schmidt number in downward cocurrent gas

Electrochemical scrubbing of SO2-containing gas: Coupling absorption to electrochemical reaction

Abstract This paper deals with a fundamental approach of electrochemical scrubbing of SO 2 -containing waste gas. The study has been conducted using a laboratory device, which allows a comprehensive investigation of the coupling phenomena between the main process involved: absorption into a sulfuric solution, transfer to the electrode and anode reaction. This work is based on both theoretical and experimental aspects of previous investigations: gas solubility, electrode kinetics and transfer phenomena in multiphase reactors.

Coupling between electrolysis and liquid-liquid extraction in an undivided electrochemical reactor applied to the oxidation of Ce3+ to Ce4+ in an emulsion Part II. Cell modelling

The effect ofin situ simultaneous extraction of tetravalent cerium by an organic phase (di-2 ethylhexylphosphoric acid in kerosene) on the performance of an undivided batch electrochemical reactor using the Ce3+/Ce4+ system was investigated. The influence of the most important parameters (initial concentration of Ce3+, composition and volume percentage of the organic phase) were studied experimentally under potentiostatic control of the anode potential. A conversion factor of Ce3+ and extraction factor larger than 90% are obtained for the best operating conditions, but the presence of the organic phase and the necessity of avoiding oxygen gas evolution considerably reduce the operating anodic current densities. The transient cell behaviour and the final “equilibrium” state conditions are found to be in very good qualitative agreement with the conclusions of a preliminary study of the electrochemical kinetics of the Ce3+/Ce4+ system and extraction mechanisms of Ce4+ by the organic phase.

Hydrodynamic study of a trickle-bed reactor by means of microelectrodes: analysis of the probability densities

The technique of microelectrodes in a non-conducting wall was used to determine the flow regimes in a trickle-bed reactor and to characterize the wall wetting by the analysis of the Probability Density Functions (PDF). The identification of flow regime transitions (trickling/pulsing, trickling/dispersed and dispersed/pulsing) and the study of the wall wetting are carried out respectively by the analysis of the variations of the fluctuation rate of the local liquid/solid mass transfer coefficient and the wetting rate as functions of the gas and liquid flow rates.