Mass transfer characteristics of a novel gas-evolving electrochemical reactor

Mass transfer coefficients were measured for the deposition of copper from acidified copper sulphate solution at a vertical-plate cathode stirred by the oxygen evolved at a coplanar, vertical-plate lead anode placed upstream from the cathode. The variables studied were: oxygen discharge rate, electrolyte concentration and height of the cathode. The cathodic mass transfer coefficient was increased by a factor of the order of 2 to 5 over the natural convection value depending on the rate of oxygen discharge at the lead anode. The relationship between the mass transfer coefficient and the oxygen discharge rate was found to be logK=a+0.296 logV.The mass transfer coefficient was found to decrease initially with increasing electrode height, but then reached a constant value independent of further change in electrode height. A new, modified parallelplate reactor stirred by the counter electrode gases is described. The advantage of the design is that it offers more efficient stirring with no added power consumption. Details of the design are discussed.Nomenclature C concentration of CuS0₄ (mol cm^–3) - F Faraday's constant - h electrode height (cm) - K mass transfer coefficient (cm s^–1) - l _L limiting current density (A cm^–2) - V gas discharge rate (cm s^–1) - z number of electrons involved in the reaction On leave from the Department of Chemical Engineering, Faculty of Engineering, Alexandria University, Alexandria Egypt.     

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     

Effect of gas evolution on mass transfer in an electrochemical reactor

Experiments were conducted to study the effect of gas bubbles generated at platinum microelectrodes, on mass transfer at a series of copper strip segmented electrodes strategically located on both sides of microelectrodes in a vertical parallel-plate reactor. Mass transfer was measured in the absence and presence of gas bubbles, without and with superimposed liquid flow. Mass transfer results were compared, wherever possible, with available correlations for similar conditions, and found to be in good agreement. Mass transfer was observed to depend on whether one or all copper strip electrodes were switched on, due to dissipation of the concentration boundary layer in the interelectrode gaps. Experimental data show that mass transfer was significantly enhanced in the vicinity of gas generating microelectrodes, when there was forced flow of electrolyte. The increase in mass transfer coefficient was as much as fivefold. Since similar enhancement did not occur with quiescent liquid, the enhanced mass transfer was probably caused by a complex interplay of gas bubbles and forced flow.List of symbols A electrode area (cm²) - a constant in the correlation (k = aRe ^m , cm s^–1) - C _{R, bulk} concentration of the reactant in the bulk (mol^–1 dm^–3) - D diffusion coefficient (cm² s^–1) - d _h hydraulic diameter of the reactor (cm) - F Faraday constant - Gr Grashof number =gL ³/² (dimensionless) - g gravitational acceleration (cm s^–2) - i _g gas current density (A cm^–2) - i _L mass transfer limiting current density (A cm^–2) - k mass transfer coefficient (cm s^–1) - L characteristic length (cm) - m exponent in correlations - n number of electrons involved in overall electrode reaction, dimensionless - Re Reynolds number =Ud _h^–1 (dimensionless) - Sc Schmidt number = D ^–1 (dimensionless) - Sh Sherwood number =kLD ^–1 (dimensionless) - U mean bulk velocity (cm s^–1) - x distance (cm) - _N equivalent Nernst diffusion layer thickness (cm) - kinematic viscosity (cm² s^–1) - density difference = (_L – ), (g cm^–3) - _L density of the liquid (g cm^–3) - average density of the two-phase mixture (g cm^–3) - void fraction (volumetric gas flow/gas and liquid flow)     

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.     

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 in gas–liquid stirred reactor with various triple-impeller combinations

The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30 m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT + 2CBYN, PDT + 2CBYW, PDT + 2WHD, HEDT + 2WHD and HEDT + 2WHU, respectively. The results show that the relative power demand of HEDT+2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT+2WHD shows the best mass transfer performance and HEDT+2WHU shows the worst mass trans-fer performance under al operating conditions. At high uG and a given power input, the impel er combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design.     

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.     

Mass transfer in the spinning catalyst basket reactor

A mathematical model is derived for the evaluation of the relative velocity between the gas and the pellets in two configurations of the spinning catalyst basket reactor. This slip velocity is then used to calculate the particle Reynolds number and hence mass transfer coefficients from well known correlations for mass transfer in packed beds. Experimentally determined mass transfer coefficients were found to be in good agreement with the model. The effect on mass transfer rates of the position of the pellet and rotor dimensions for the cruciform spinning catalyst basket reactor is seen to be well predicted from the model. The performance of a typical spinning catalyst basket reactor is calculated when a catalytic oxidation reaction takes place at various jacket temperatures and rotational speeds.     

Mass transfer in laminar liquid jets: Measurement of diffusion coefficients

Laminar liquid jets used in mass transfer studies have been generally discharged from convergent nozzles or orifices. The use of long straight capillary tubes to discharge the liquid jet is studied in this work and numerical values of the jet diameter and surface velocity are obtained. These computed values are applied to predict the actual absorption rates in the laminar jet for different values of N_J= N_Re/N_Fr, N_we and the axial coordinate. The effect of N_we is slight and can be disregarded in analysing experimental data. Experimental values of absorption of CO₂ in water and water-glycerol solutions are obtained using a laminar liquid jet discharged from capillary tube and good agreement is found between the computed values and the measured absorption rates. The results are applied to the determination of diffusion coefficient for the CO₂ in water glycerol solutions.     

Mass transfer behaviour of a flow-by fixed bed electrochemical reactor composed of a vertical stack of screens under single and upward two phase flow

Rates of mass transfer were studied at a vertical array of closely packed screens under single and two phase (gas–liquid) flow by measuring the limiting current for the cathodic reduction of ferricyanide ions. Variables studied were screen characteristics (mesh number and wire diameter), physical properties of the solution, solution flow rate, gas flow rate and the effect of surface active agents. The single phase data were correlated by the equation:J = 0.52 Re_L^-0.55while the two phase data were correlated by the equations:Sh=0.87 Sc^0.33 Re_L^0.35 Re_g^0.12for the conditions 10 < Re < 125 and 1.4 < Re_g < 77; andSh=0.62 Sc^0.33Re_L^0.11Re_g^0.25for the conditions 1.1 < Re_L < 22 and 1.4 < Re_g < 77. The presence of surfactant was found to reduce the rate of mass transfer in both single phase and two phase flow, the percentage reduction being higher in the case of single phase flow.     

Mass transfer and pressure distributions in electrochemical pump cells

Macroscopic and microscopic measurements of mass transfer rate have been made in a pump cell operating at up 6000 rpm. The dependence of the average Sherwood number is given by

where G = h/r_o is the gap ratio, Re is the channel Reynolds number and R_ø is the rotational Reynolds number. The mass transfer rate is so high, and the diffusion layer thickness consequently so small (～2 × 10^?2mm), that the rotor behaves essentially as a free disc even when the gap is less than 0.1 mm. Local measurements using microelectrodes confirm this behaviour, and show that the current density increases monotonically towards exit (in contrast to the behaviour of a non-rotational, capillary gap cell).Agreement between measured and theoretically derived pressure distributions is fair, and both show that pressures below atmospheric are easily attained at entry. Under almost all operating conditions there is then a net rise in pressure across the disc so the cell may be used as its own pump; an estimate of pumping power is derived.     

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.     

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 limitations on fixed-bed reactor for Fischer-Tropsch synthesis

Mass transfer limitations on fixed-bed for Fischer-Tropsch synthesis were investigated by changing synthesis gas superficial velocity, catalyst pellet size, and catalyst amount. To study external mass transfer limitation, synthesis gas superficial velocity was changed from 8.47 × 10^− 4 m s^− 1 to 3.39 × 10^− 3 m s^− 1. As a result, the synthesis gas superficial velocity of 3.39 × 10^− 3 m s^− 1 was most suitable for hydrocarbon chain growth resulting to liquid hydrocarbon formation. In case of internal mass transfer limitations, the effects of catalyst pellet size and catalyst amount (W_cat/F) were discussed. The large catalyst pellet showed higher C₅₊ selectivity and a lower α value compared to the small pellet because of more severe internal mass transfer limitations of α-olefin and long-chained hydrocarbons in the large pellet, respectively. Catalyst amount (W_cat/F) was inversely proportional to the internal mass transfer limitation because increased catalyst amount gave more time for liquid hydrocarbon products to diffuse from the catalyst pellet and, therefore, the catalyst amount of 4.5 g (W_cat/F = 45 g_cat min L^− 1) was most appropriate for liquid hydrocarbon formation.     

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 aerated fermentation broths in a stirred tank reactor

The volumetric mass transfer coefficients, k_La, measured in a pilot plant (0.1 m³) and an industrial (67.5 m³) fermentor during an actual fermentation process are presented. Problems related to the estimation of the phsyical properties as well as to the correlation of experimental data and to scale up procedures are discussed. Although the scale up factor was rather high, both sets of data could be represented by single correlation. Comparison of the experimental data with several available correlations demonstrated the need for pilot plant experiments and scale up procedures, since it is almost impossible to take into account all relevant system properties.     

Mass transfer and reaction of trimethylamine in a three-phase biological reactor

A model system, consisting of a mixed culture of microorganisms, was used to study the performance of a bench-scale packed bed bubble column (PBC) bioreactor treating trimethylamine. Three mesophilic bacterial strains were selected, mixed and acclimatized to grow with trimethylamine as the sole carbon source; degradation kinetic constants were determined for a bacterial suspension system. Gas transfer to the liquid phase was modelled in order to characterize solute transport and availability into the biofilm. The overall performance of the system is discussed based on mass transfer considerations.