Electric mass transport of sodium chloride through cation-exchange membrane MK-40 in dilute sodium chloride solutions: A rotating membrane disk study

The polarization properties of an electromembrane system consisting of an MK-40 membrane and a dilute sodium chloride solution are investigated with an experimental apparatus, which includes a rotating membrane disk with a horizontally positioned membrane. For the electrochemical systems of MK-40/0.01 M NaCl and MK-40/0.001 M NaCl, effective ion transport numbers and partial current-voltage curves are determined for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. The space-charge distribution in the diffusion layer and in the membrane is calculated for various current densities and rotation rates of the membrane. It is shown that when electric-current densities are greater than the limiting value, ion fluxes of the salt increase as a result of a decrease in the effective thickness of the diffusion layer. This decrease is caused by the development of space charge, electroconvection, water dissociation, and the exaltation effect in the region near the membrane. It has been established that in dilute solutions the limiting current is not purely electrodiffusive in nature.     

Influence of the nature of membrane ionogenic groups on water dissociation and electrolyte ion transport: A rotating membrane disk study

Polarization properties of electromembrane systems (EMS) consisting of a heterogeneous membrane, either the MK-41 phosphonic acid membrane or the MK-40 sulfonic acid membrane, and dilute sodium chloride solutions are investigated with the rotating membrane disk method. For the MK-41/0.01 M NaCl and MK-41/0.001 M NaCl EMS, effective ion transport numbers and partial current-voltage curves (CVC) are measured for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. With the theory of the overlimiting state of EMS, internal parameters of the systems under investigation—the diffusion-layer thickness, the space-charge distribution, and electric-field strengths in the diffusion layer and in the membrane—are calculated from experimentally obtained CVC and the dependence of effective transport numbers on current density. The catalytic influence of ionogenic groups on the dissociation rate of water is analyzed quantitatively. Partial CVC for H⁺ ions are calculated for the space-charge region in MK-40 and MK-41 membranes. Analogous CVC for bipolar membranes containing sulfonic acid and phosphonic acid groups are compared. The dissociation mechanism of water is the same in all EMS and is independent of the membrane type and the nature of the functional groups.     

Modeling of mass and charge transfer in an inverted rotating disk electrode (IRDE) reactor

In this work, the validation of a newly constructed inverted rotating disk electrode (IRDE) reactor is reported. Compared to the rotating disk electrode (RDE) reactor, the working electrode is changed in position from the top to the bottom of the electrochemical cell. The IRDE reactor is designed to facilitate the actual study of gas evolution reactions. It is studied whether the first-order analytical expression for the velocity field in an RDE reactor is also acceptable for an IRDE configuration. To that purpose, the kinetic parameters of the well-known ferri/ferro cyanide redox system are determined in both configurations and compared. This is done qualitatively by comparing the polarization curves obtained in the inverted and the conventional RDE configuration. Additionally, a statistically founded fitting algorithm is used to quantitatively determine the model parameters of the oxidation and reduction reaction. Not only the diffusion coefficients of Fe²⁺ and Fe³⁺ are calculated, but also the rate constants (k_ox and k_red) and the transfer coefficients (α_ox and α_red) are quantified and compared together with their respective standard deviation. It is found that the parameters of mass and charge transfer in both configurations agree well. So it is concluded that the same analytical equations of mass and charge transfer can be used in both the RDE and the IRDE reactor.     

固体粒子对板式膜吸收过程的传质强化

分别采用纯CO2-去离子水和不同浓度的NaOH溶液为实验体系,在板式膜器中研究了第三相固体粒子对膜吸收过程传质效果的影响．分别考察了在不同粒子种类、搅拌转速、传质体系、化学反应强度、膜孔隙率等因素下固体粒子对传质强化的影响．结果表明,随着粒子固含率的增大,传质系数和增强因子均有所提高,当粒子固含率增大到一定范围后,传质系数和增强因子的变化趋于平缓．在固含率一定的条件下,不同种类的固体粒子对膜吸收过程的强化效果随着固体粒子密度的增加而减小．传质系数随着搅拌转速的增大而增大,但高搅拌转速下固体粒子的强化作用减弱．膜吸收过程的传质系数和增强因子随着化学反应强度的增强而增加．随着粒子固含率的增大,不同膜孔隙率对传质效果的差异减小,且孔隙率越小,固体粒子对膜吸收传质过程的强化效果越好．其中,对于纯CO2-去离子水体系,当孔隙率为20％,粒子固含率为1．5gL^（-1）时,固体粒子的加入可使传质系数提高1．45倍,增强因子可达2．45．     

Determination of the mass transfer coefficients in direct immersion solid‐phase microextraction

Diffusion of the analytes across the diffusion boundary layers and subsequently through the fiber coatings determines the extraction kinetics of solid‐phase microextraction in aqueous matrices. Besides, the matrix effects can distort the behaviors of the analytes transferring across the diffusion boundary layers. However, these processes were always studied via certain simplification, which often left the mass transfer through the fiber coatings unconsidered and the matrix effects partially investigated. Herein, a comprehensive study on the mass transfer processes in direct immersion solid‐phase microextraction was presented. Under different agitation speeds, it was determined that the mass transfer coefficients across the diffusion boundary layers were three to six orders larger than those through the fiber coatings. However, the mass transfer across the diffusion boundary layers was generally the major rate‐limiting step. In addition, the shuttle effect and the barrier effect, which were responsible for accelerating and retarding the extraction kinetics, respectively, were found to be the dominant matrix effect alternately under different agitation speeds. This study comprehensively illustrated the major rate‐limiting step and the dominant matrix effects through recording the mass transfer coefficients.     

Nonsteady-state processes of water transfer in solid-polymer membrane: a mathematical model

The regularities of electroosmotic and diffusion water transport are shown in the case of hydrogen ionization in the porous anode layer applied on the proton-exchange membrane surface. The possibility of critical phenomena due to drying of a membrane region adjacent to the anode is shown.     

The role of migration mass transfer in the electrodeposition of nickel from sulfate-chloride and chloride solutions containing succinic acid

The electrodeposition of nickel from 0.5 M sulfate-succinate-chloride (I) and 0.3 and 0.5 M chloride-cuccinate (II) solutions at a temperature of 50°C is studied. It is found that, depending on the pH₀ of solution and the concentration of succinic acid and nickel, the maximum cathodic current density for producing high-quality nickel deposits is to 35 A/dm² in electrolytes I and to 60 A/dm² in electrolytes II. This corresponds to the metal deposition rates to 23 and 48 A/dm², respectively. It is shown experimentally that high buffer properties of solutions (succinate buffer) are one of the reasons for high permissible current densities of nickel electrodeposition. The computer-calculated data showed that the intensification of nickel electrodeposition is caused also by the acceleration of mass transfer due to the presence of a considerable fraction of nickel in the form of positively charged complexes and a high concentration of components stabilizing pH_S in the near-cathode layer.     

A protocol for the measurement of all the parameters of the mass transfer kinetics in columns used in liquid chromatography

Band broadening in chromatography results from the combination of the dispersive effects that are associated with the different steps involved in the migration of compound bands along the column. These steps include longitudinal diffusion, trans-particle mass transfer, external film mass transfer, overall eddy diffusion, including trans-column, short-range inter-channel, trans-channel eddy diffusion, and the possible, additional mass transfer contributions arising from heat friction and the thermal heterogeneity of the column. We describe a series of experiments that provide the data needed to determine the coefficients of the contributions to band broadening of each one of these individual mass transfer steps. This specifically designed protocol can provide key information regarding the kinetic performance of columns used in liquid chromatography and explain why different columns behave so differently. The limitations, accuracy and precision of these methods are discussed. Further avenues of research that could improve the characterization of the mass transfer mechanisms in chromatographic columns, possibly contributing to the development of better columns, are suggested.     

A study of heat and mass transfer of Non-Newtonian fluid with surface chemical reaction

Considering the significance of non-Newtonian fluid usage in manufacturing such as molten plastics, polymeric materials, pulps, and so on, significant efforts have been made to investigate the phenomenon of non-Newtonian fluids. In this article the influences of heat and mass transfer on non-Newtonian Walter's B fluid flow over uppermost catalytic surface of a paraboloid is encountered. An elasticity of the fluid layer is considered in the freestream together with heat source/sink and has the tendency to cause heat flow in the fluid saturated domain. The flow problem of two-dimensional Walter's B fluid is represented using Law of conservation of mass, momentum, heat, and concentration along with thermal and solutal chemical reactive boundary conditions. The governing equations are non-linear partial differential equation and are non-dimensionalized by employing stream function and similarity transformation. The final dimensionless equations yielded are coupled non-linear ordinary differential equations. Furthermore, shooting technique along with RK-4th order method is used to get the numerical results. Graphs and tables are modeled by using MATLAB software to check the effects of Walter's B parameter, Chemical reaction parameter and Thickness parameter on temperature, velocity, and concentration profiles. Tabular analysis shows the results of some physical parameters like skin friction coefficient, Nusselt number and Sherwood number due to the variation of Walter's B parameter, thickness parameter and chemical reactive parameter.     

Cathode of a fuel cell with a solid polymer electrolyte: Calculating overall currents in the presence of a gas-diffusion layer

Mathematical apparatus, which makes it possible to perform calculations of the current-voltage characteristics of cathodes of fuel cells with a solid polymer electrolyte in conditions where there are present extraneous diffusion restrictions is proposed. In so doing, the partial pressure of oxygen and the absolute pressure of gas in the gas chamber may assume any values. First of all presented are the results of calculations of the current-voltage characteristics intrinsic to active layers of the air and oxygen cathodes, which are performed under the assumption that the extraneous diffusion restrictions are absent altogether. Thereafter, in the same conditions (at the same parameters that characterize the active layer of a cathode), obtained are results of a calculation of the current-voltage characteristics inherent in the air and oxygen cathodes in the presence of extraneous diffusion restrictions. Afterward there is performed an analysis of the way a gas-diffusion layer restricts the process of generation of current in a cathode and of what measures should be taken in order for the extraneous diffusion restrictions to become less significant.     

Hydrogen diffusion through a steel membrane in ethylene glycol solutions of hydrochloric acid containing sodium hydroarsenate: the effect of anodic polarization

The kinetics of the hydrogen evolution reaction on iron and the hydrogen diffusion rate (i _H) through a membrane of steel 3 are studied. The study is carried out in conditions of anodic polarization (E _a), in ethylene glycol and water–ethylene glycol solutions of HCl with a constant ionic strength, which contain As(V) predominantly in the form of H₃AsO₄. It is shown that the dependence of i _H on E _a passes through a maximum in media with different concentrations of water and HCl, different solvation form of the proton, and different nature of species that solvate the working surface of the membrane. The effect of E_a on i _H disappears in a certain range of anodic potentials. The presence of the maximum is shown to be due to the formation of two forms of adsorbed hydrogen (H^r,H^s). That i _H is independent of E _a is connected with chemical dissolution of the membranes working side.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 181–189.Original Russian Text Copyright © 2005 by Vigdorovich, Tsygankova, Kopylova.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Multi-location peak parking method: an important new tool for the study of mass transfer kinetics in liquid chromatography

The peak parking (PP) method probes the longitudinal diffusion coefficient of a compound at a single location along the chromatographic column. We extended to a so-called multi-location peak parking (MLPP) method, in which a large number of axial locations along the column are selected in order to check the validity of the conventional PP method and to reveal possible defaults in the structure of the packed bed or pitfalls of the PP and the MLPP methods. MLPP was applied to a series of HILIC columns, including a 5.0 μm Venusil, a 3.0 μm Luna-diol, three 2.7 μm Halo, and a 1.7 μm Kinetex columns. The results demonstrate that the MLPP method may reveal local heterogeneities in the axial diffusion of small retained low molecular weight compounds along the column. Most importantly, experiments show that the sample zone should not be parked in the entrance of the column (i.e., at <1/10 th of the column length). The abrupt drop in the flow rate considerably affects the peak shape and prevents scientists from using the conventional PP method. Practical solutions to cope with that problem are proposed and their success/failure are discussed.     

A lubricated stagnation point flow of nanofluid with heat and mass transfer phenomenon: Significance to hydraulic systems

The improved thermal association of heat transfer is considerably observed due to interaction of nanoparticles in recent days. The lubrication phenomenon with heat and mass transfer effects plays a key role in the hydraulic systems. In current research, the thermal impact of nanofluid over a lubricated stretching surfaces near a stagnation point analytical has been studied. A thin layer of lubricating fluid with a variable thickness provides lubrication. The inspection of thermophoresis and Brownian motion phenomenon is illustrated via Boungrino model. The analytical finding of refurbished boundary layer ordinary differential equations is obtained by a reliable and proficient technique namely variational iteration method (VIM). The Lagrange Multiplier is a potent tool in proposed technique to reduce the computational work. In addition, a numerical comparison is presented to show the effectiveness of this study. The range of flow parameters is based on theoretical flow assumptions. Physical inspection of involved parameters on velocities, temperatures, concentrations, and other quantities of interest when lubrication is presented. The current results present applications in polymer process, manufacturing systems, heat transfer and hydraulic systems.     

Coupled mass transfer and reaction in water of an oil soluble chemical: A CT scan study

Chemical compounds of the family of alkoxysilanes, such as tetramethyl orthosilicate (TMOS), are soluble in oil but can be dissolved in water where they undergo a hydrolysis and a condensation reaction to form a stable silica gel. We have investigated in detail the coupled mass transfer–reaction process that takes place when oil with dissolved TMOS is brought in contact with water in a well-defined geometry. The main physical parameters (initial TMOS concentration in the oil phase, initial water to oil volume ratio, and water pH and salinity) were varied during the experiment. X-ray CT was used to visualize and quantify concentration changes in the oil and the water phases. We found that varying the initial concentration does not affect mass transfer rates out of the oil phase, but increasing initial concentrations do speed up gelation. The water to oil volume ratio has a larger impact on mass transfer, with a transfer rate that increases with increasing water to oil volume ratio. Low pH of the water phase promotes mass transfer, whereas high pH and addition of salt has no significant effect. Gelation is however more profoundly affected by both increasing pH and addition of salts.     

Charge injection and transfer tuning of a series of Pt complexes through oligothiophenes: A theoretical study

The geometries, frontier molecular orbitals, charge injection and transfer, as well as spectroscopic properties of a series of novel bis(8-hydroxyquinolinato) (Ptq₂) derivatives with oligothiophenes moiety were investigated theoretically. The calculated results indicated that incorporating of oligothiophene units into the Ptq₂ had major role in the tailoring the optical properties. Furthermore, from the analyses of ionization potentials (IP), electron affinity (EA), reorganization energy (λ), it was found that these Pt derivatives have excellent hole and electron injection and transfer ability by introducing the oligothiophenes at the 7-positions of the quinoline ligand. These results are favorable to establish the structure-photophysical property relationship for enhancing the carrier transfer abilities of Ptq₂.     

Concentration changes in near-electrode layers during electrolysis of copper(II) sulfate and potassium dicyanoaurate: A spectrophotometric study

A spectrophotometric method is suggested for measuring concentration changes in near-electrode layers. The method is based on the interaction between a monochromatic light beam, traversing the electrochemical reaction zone, and metal ions. Concentration changes in the near-cathode layer during electrolysis of copper(II) in sulfate electrolytes and potassium dicyanoaurate in phosphoric electrolytes are studied.     

A numerical and experimental study of mass transfer in spacer-filled channels: Effects of spacer geometrical characteristics and Schmidt number

A systematic study is presented on the effect of both Reynolds (Re) and Schmidt (Sc) numbers on Sherwood (Sh) number, for narrow channels with spacers, closely simulating conditions of feed-side channels in spiral-wound modules. Results of direct numerical simulations performed for the three-dimensional geometries resulting from non-woven cylindrical filament (net-type) spacers show the significant influence of spacer geometry on local Sh distributions. These distributions exhibit a tendency to be displaced towards lower values for more sparse spacer geometries, and towards higher values as the angle between crossing filaments is increased. Additionally, the distribution of the local time-averaged mass transfer coefficients is generally similar to the corresponding distribution of shear stresses at the channel walls, with a tendency to exhibit closer similarity with increasing Sc, as one would expect. To validate the results of numerical simulation, a significant amount of mass transfer data is reported for nine different prototype spacer geometries, plus a common commercial spacer, and three Sc numbers in each case. Correlations of average Sh are obtained for each geometry, in terms of Re and Sc numbers; the exponent of Sh dependence on Sc is near 0.4, as is also obtained from the numerical simulations. Moreover, in agreement with the numerical results, the experimental data reveal a similar trend in the dependence of the average Sh number on spacer geometrical characteristics, namely decreasing with increasing ratio L/D (of cell side L over filament diameter D) and increasing with the filaments crossing angle β. A mass transfer correlation is also proposed for the square-cell commercial spacer commonly used in RO/NF modules.