Mass transfer at horizontal gas-evolving electrodes

The effect of H₂ and O₂ evolution on the mass transfer coefficient of the reduction of K₃Fe(CN)₆ and the oxidation of K₄Fe(CN)₆ at nickel electrodes was studied up to 105 mA/cm². The relation between the rate gas evolution and the mass transfer coefficient was found to be: log K = a + 0·25 log V for a H₂-evolving horizontal electrode and log K = a + 0·4 log V for an O₂-evolving horizontal electrode.Comparison was made between mass transfer coefficients at horizontal and vertical gas-evolving electrodes. Mass transfer coefficients at horizontal electrodes are much higher than at vertical electrodes.     

Mass transfer at gas-evolving vertical electrodes

Various models have been proposed to describe the mass transfer of indicator ions to gas-evolving electrodes. For verification of the proposed models, the dependence of the mass transfer coefficient of indicator ions,k _j, on the length,L _e, of a gas-evolving electrode may be very useful. Experimental relations betweenk _j andL _e have been determined for oxygen-evolving as well as hydrogen-evolving vertical electrodes in a supporting electrolyte of 1 M KOH. Moreover, a modified hydrodynamic model, where a laminar solution flow is induced by rising bubbles, has been proposed in order to calculatek _j. It has been found that this model is not useful for both types of gas-evolving electrodes. The experimental results support the earlier proposed convection-penetration model for the oxygen-evolving electrode. The solution flow near a vertical electrode, induced by rising bubbles, behaves in a turbulent manner.     

Mass transfer behaviour of gas-evolving particulate-bed electrode

Rates of mass transfer for the cathodic reduction of potassium ferricyanide at a particulate bed of graphite supported on a horizontal nickel disc were studied under H₂-evolving conditions. Variables studied were: H₂ discharge rate, particle size and bed height. The rate of mass transfer was found to increase to a maximum in the presence of the bed which was about 4.5 times compared to that of the supporting disc. The rate of mass transfer was found to increase with H₂ discharge rate, particle size and bed height. Polarization was measured for beds of different particle size and it was found that the presence of the bed increased polarization especially at relatively high current densities, the increase in polarization was independent of particle size of the bed. Comparison with an O₂-evolving particulate electrode was made and possible practical applications were pointed out.Symbols K mass transfer coefficient (cm s^–1) - V H₂ discharge rate (cm³cm^–2s^–1) - I current consumed in reducing potassium ferricyanide (A) - A supporting disc area (cm²) - F Faradays constant (96 500 C mol^–1) - C Potassium ferricyanide concentration (mol cm^–3) - Z number of electrons involved in the reaction     

Mass transfer at packed-bed,gas-evolving electrodes

Mass transfer rates were measured for the cathodic reduction of potassium ferricyanide at a H₂-evolving electrode consisting of a packed bed of spheres. Variables studied were bed height, H₂ discharge rate and ferricyanide concentration. It was found that the mass transfer coefficient (K) is related to the H₂ discharge rate (V) by the equation $$K = aV^{0.325} $$ Bed height and electrolyte concentration were found to have little effect on the mass transfer coefficient. A mathematical model based on the surface renewal theory was formulated to explain the mechanisms of mass transfer at gas-evolving electrodes.     

Mass transfer characteristics of a pilot pulsed baffled reactor

We report experimental measurements on the air-water mass transfer characteristics of a pilot pulsed baffled reactor. The experiments have identified, for the first time, the optimal baffle spacing and the optimal superficial gas velocity for mass transfer in the reactor. The mass transfer characteristics were also examined under various operational conditions and were corrected for the effect of temperature and oxygen probe response. The experimental results indicate that the mass transfer rate depends strongly on the combination of the baffle spacing, oscillation frequency and amplitude. The mass transfer data are also correlated in terms of power density and superficial gas velocity, and compared with those from a stirred tank reactor.     

Mass transfer in an electrochemical reactor with two interacting jets

An electrochemical reactor operated with two identical solution streams injected in opposite directions on the same axis, and leaving it at a normal direction was studied by measuring local and global mass transfer coefficients and visualization of solution flow patterns. This flow configuration was compared to a case where a single stream enters the reactor and leaves it on the same axis. It was found that only the data obtained for the single stream mode can be correlated by the Chilton-Colburn relation, indicating a near laminar boundary layer flow. Global mass transfer coefficients for the single stream mode were found to be slightly higher than those for the interacting jets mode. However, when comparing the two modes by taking into account the dimensionless ratio of the mass transfer coefficient (Sh) to the energy consumption (Eu), it was found that the interacting jets (IJ) mode exhibits a better performance as compared to the single stream mode. The superiority of the IJ mode increases with increasing Reynold's number (Re).Nomenclature A, B adjustable parameters - b half width of channel - C electrolyte ion concentration - d inlet pipe diameter - d microelectrode diameter - D diffusion coefficient - maximum value of mean deviation - E pumping energy - Eu Euler number - F Faraday number - i current to a single microelectrode on an active wall - i current to a single microelectrode in an inert wall - I global diffusion current - k mass transfer coefficient to a single microelectrode in an active wall - k mass transfer coefficient to a single microelectrode in an inert wall - K global mass transfer coefficient - Q volumetric flow rate - Q _T total volumetric flow rate - R radius of the electrochemical reactor - Re Reynolds number - s surface area of a microelectrode - S surface area of the working electrode - Sc Schmidt number - Sh Sherwood number - V _x axial flow velocity alongx-axis - V flow velocity at large distance from the leading edge - V mean flow velocity - x axis tangential to the surface - y axis normal to the surface - z number of electrons transferred in the reaction (z=1 in the present case) Greek letters viscosity - specific gravity - kinematic viscosity (/) - P pressure drop across the reactor - V voltage drop across the reactor Abbreviations ST single stream - IJ interacting jets     

Mass and momentum transfer at the rotating surface of a continuous annular electrochemical reactor

The paper presents and examines the experimental values of local mass transfer coefficients and shear stresses at the surface of the rotating inner cylindrical electrode. Their variations with the hydrodynamical parameters are similar but the test of the Chilton—Colburn analogy is made difficult by the choice of a characteristic flow velocity. An approximative model is proposed which permits the confirmation of the existence of defined vortices with an increased dimension in the flow direction.     

Mass transfer behaviour of a fixed bed electrochemical reactor with a gas evolving upstream counter electrode

Mass transfer rates were determined at a horizontal screen cathode stirred by oxygen bubbles evolved at a horizontal anode placed below the screen by measuring the limiting current of the cathodic reduction of ferricyanide ion from alkaline solution. Variables studied were oxygen discharge rate, ferricyanide concentration and number of closely packed screens forming the cathode. For a single screen cathode the data were correlated by the equation: J = 0.249 (Re Fr)^-0.25 The mass transfer coefficient was found to decrease with increasing the number of screens forming the cathode. Implications of the present work for improving the performance of the flow-through packed bed electrochemical reactor were highlighted.     

电化学反应器流动和传质特性的仿真研究

提出了一种基于双面BDD电极的平行板结构的电化学反应器,它通过阳极和阴极交替排列引导污水多通道蜿蜒流动,实现了三维传质的增强。利用OpenFOAM分析了四阳极结构反应器的流动特性和传质特性,结果表明该结构的反应器在低入口流体雷诺数下传质系数低,分布不均,当入口雷诺数大于500时流体出现不稳定,传质增强。     

Mass transfer characteristics of electrochemical reactors employing gas evolving mesh electrodes

Mass transfer rates were measured at a single screen and a fixed bed of closely packed screens for the simultaneous cathodic reduction of K₃Fe(CN)₆ and anodic oxidation of K₄Fe(CN)₆ in alkaline solution with H₂ and O₂ evolution, respectively. Variables studied were gas discharge rate, number of screens per bed and position of the electrode (vertical and horizontal). For single screen electrodes, the mass transfer coefficient was related to the gas discharge rate by the equations: $$\begin{gathered} K = aV^{0.190} , for H_2 evolving electrodes, \hfill \\ K = aV^{0.469} , for O_2 evolving electrodes \hfill \\ \end{gathered} $$ . Electrode position was found to have no effect on the rate of mass transfer for single and multiscreen electrodes in the case of H₂ and O₂ evolution. Mass transfer coefficients were found to increase with an increasing number of screens per bed in the case of H₂ evolution, while in the case of O₂ evolution the mass transfer coefficient decreased with an increasing number of screens per bed. A mathematical model was formulated to account for the behaviour of the H₂ evolving electrode which, unlike the O₂ evolving electrode, did not obey the penetration model. Power consumption calculations have shown that the beneficial effect of mass transfer enhancement is outweighed by the increase in the voltage drop due to gas evolution in the bed electrode.     

Mass transfer and hydrodynamic characteristics of a high aspect ratio self-ingesting reactor for gas-liquid operations

The mass transfer performance of a gas-liquid self-ingesting stirred reactor is reported both for coalescing and non-coalescing systems. The vessel features are a high aspect ratio and a rather narrow multiple-impeller draft tube, through which the gas phase is ingested and led down to the vessel bottom, where it is finely dispersed into the liquid rising in the annular portion of the vessel. Comparison is made between k_La values determined by several variants of the dynamic method, among which pure oxygen absorption in a previously de-gassed liquid phase. Results show that the gas-liquid mass transfer coefficient values obtained with the last approach are remarkably larger than those measured with all other techniques in which nitrogen is initially dissolved in the liquid phase. Possible reasons behind this discrepancy are discussed.The gas-liquid mass transfer performance of the investigated gas inducing contactor is finally compared with literature data on other self-inducing/ingesting devices. Comparison results encourage further development of the investigated apparatus.     

Effect of mass transfer on the current distribution in monopolar and bipolar electrochemical reactors with a gas-evolving electrode

The current distribution in an electrochemical reactor with vertical parallel-plate electrodes was experimentally determined. The research was performed with monopolar and bipolar electrodes. The reactor has a gas-evolving electrode and at the counter electrode an electrochemical reaction with combined diffusion and charge-transfer kinetic control, takes place. Therefore the kinetics at the counter electrode are influenced by the bubble-induced convection and by the forced convection of the electrolyte. These reactors are found in many electrochemical processes, for example, electrowinning of metals and electrosynthesis. The test reactions were hydrogen evolution at the cathode and the anodic oxidation of sulphite to sulphate from basic solutions. The current distribution shows a minimum at a distance of approximately six times the equivalent diameter of the reactor from the inlet region. This minimum is a consequence of the interaction between forced convection and the bubble-induced convection, which shifts the mass transfer coefficient of the anodic reaction along the reactor. The effects of the total current, the volumetric electrolyte flow rate and the metal phase resistance on the current distribution are also analysed. The experimental current distribution data are compared with theoretical expectations and good agreement is found.     

Application of metallic foams in an electrochemical pulsed flow reactor Part I: Mass transfer performance

This paper describes mass transfer in a porous percolated pulsated electrochemical reactor (E3P reactor), fitted with nickel foam electrodes in an axial configuration. The work is aimed at optimization of the mass transfer conditions in electroorganic reactions such as the oxidative cleavage of diols or the conversion of DAS (diacetone-l-sorbose) into DAG (diacetone-2-keto-l-gulonic acid). The use of nickel foam as an electrode material is of interest for these electrocatalytic reactions due to its high specific surface area (4000 to 11000 m^–1) and its high porosity (over 0.97). The electroreduction of ferricyanide has been chosen as a test reaction in order to correlate the mass transfer coefficient with the overall flow velocity and the amplitude and frequency of the electrolyte pulsation. Four foam grades have been tested.List of symbols a pulsation amplitude (m) - A _ve dynamic specific area of the foam: surface area per volume of material (m^–1) - C ferricyanide concentration in the cell (mol m^–3) - D diffusion coefficient of ferricyanide (m² s^–1 - d _m mean path of a particle in the threedimensional electrode (m) - d _R diameter of the reactor column (m) - d _p mean foam pore diameter of the foam (m) - e thickness of the electrode bed (m) - f pulsation frequency (Hz) - F Faraday number (C mol^–1) - I limiting diffusion current (A) - k _d mass transfer coefficient with pulsation (m s^–1) - k _o mass transfer coefficient without pulsation (m s^–1) - n number of electrons in the electrochemical reaction - Q _v volummetric flow rate through the reactor (m³ s^–1) - Re Reynolds number Re = U _o d _R v ^–1 - Re _pore Reynolds number based on mean pore diameter d _p, Re _pore = U ₀d _p^–1µ^–1 - S active surface area of the electrode (m²) - Sc Schmidt number, Sc = vD ^–1 - Sh Sherwood number, Sh = k _d d _R D ^–1 - Sh _pore Sherwood number based on mean pore diameter d _p, Sh _pore = k _d d _p D ^–1 - Sr Strouhal number, Sr = aU ₀ ^–1 - t _r mean residence time (s) - U ₀ permanent superficial velocity U ₀ = Q _v/(d _R ²/4) (ms^–1) Greek letters porosity of the foam - µ dynamic viscosity (kg m^–1 s^–1) - kinematic viscosity (m² s^–1) - liquid density (kg m^–3) - pulsation, = 2f (rad s^–1) - tortuosity of porous medium     

Mass transfer measurements and modeling in a microchannel photocatalytic reactor

The main objective of this paper was to evaluate the influence of mass transfer on the photocatalytic efficiency at a low flow rate in the order of several mL per hour. Several continuous flow microchannel reactors have been used to study the degradation of salicylic acid (SA) taken as a model pollutant. The photocatalytic degradation of salicylic acid, under UV illumination of 1.5 mW cm⁻², was assessed from the outlet concentration measured by liquid chromatography HPLC. It was shown that the degradation of SA by UV was limited by mass transfer. Numerical simulations have allowed establishing a relationship of the Sherwood number valuable for all the microchannel geometries. Computational fluid dynamics with Comsol Multiphysics is useful for predicting the degradation yield for a given geometry of the microreactor. The best representation of the experimental data is obtained by introducing a kinetic law taking into account mass transfer limitation.     

Mass transport characterization of a novel gas sparged photoelectrochemical reactor

The photoelectrochemical treatment of waste water using immobilised TiO₂ electrodes has been demonstrated to be an attractive alternative to TiO₂ slurry reactors; however, it is generally believed that the diffusion of species to the surface of the catalyst imposes severe mass transfer limitations and hence is a disadvantage of the photoelectrochemical approach. To challenge this view, this paper reports the characterization of the mass transport properties of a novel gas sparged photoelectrochemical reactor. It is shown that passing a constant stream of nitrogen gas through the reactor increased the mass transfer coefficient by an order of magnitude above that in the absence of gas and this was attributed to the turbulent flow regime imposed by rising gas bubbles. It is also demonstrated that the gas–liquid transfer coefficient was greater than that for the rate of liquid diffusion and this has important implications for heterogeneous processes.     

Entrance effect on mass transfer in a parallel plate electrochemical reactor

The influence of different fluid inlet types, slits or tubes, on mass transfer in a rectangular reactor was studied. Measurements of mass transfer coefficients were made using the limiting diffusion current technique based on ferricyanide ion reduction at a large nickel electrode located downstream of abrupt expansions. The overall mass transfer coefficients obtained were 3 to 13 times greater than those obtained in fully developed flows. Overall mass transfer coefficients were correlated for Reynolds numbers ranging from 400 to 3500 by a unique equation by introducing a nondimensional expansion factor defined by the ratio of the fluid inlet cross-section to that of the reactor. The correlation equation obtained was compared with literature data.     

Mass transfer in fluidised bed electrochemical reactors

Electrochemical mass transfer experiments involving the cathodic deposition of copper from aqueous solutions containing H₂SO₄ have been performed for two distinct cases. (a) Determination of mass transfer rates at a plane wall electrode in the presence of a fluidized bed of inert particles (glass beads). In this work bed height, bed size and fluidization conditions have been varied and a correlating equation:

is suggested as applying over the range

Comparison is made with mass transfer data of several other authors, revealing considerable variance. (b) Determination of mass transfer rates between electrolyte and particles within an active bed of fluidized conducting copper particles. Analysis of the data yields a correlating equation:

in the range

This compares very well with another source for electrolytic fluidized bed mass transfer, but is somewhat lower than other equations for particle to fluid non-electrolytic mass transfer.     

定-转子反应器传热特性

采用蒸汽-水体系,在转子转速为400～1400 r·min^-1、水流量为0. 1～0. 5 m³·h^-1、蒸汽充足的实验条件下,研究了定-转子反应器(RSR)的传热特性,初步考察了操作参数对RSR液体分布的影响。实验结果表明：总传热系数随着转速的升高而增大,随着液量的增加而增大但增幅减小。小流量（0.1～0.3 m³·h^-1）与低转速（400 r·min^-1）时,温升不均,表明液体分布不均。最后采用因次分析法推导出定-转子反应器总传热系数关联式,且进行了统计检验,所得关联式与实验数据拟合较好。