V.I. Vasil'eva

Concentration fields of solutions under electrodialysis with ion-exchange membranes

Abstract The laser-interferometric method for measuring concentration profiles in compartments of electrodialyzers with a small intermembrane distance is developed using an optical cell with a moveable membrane. The method of laser interferometry is used for determining of the limiting diffusion current density by the minimal interface concentration of the solution. The method of a local distributive analysis of diffusion boundary layers of two-component solutions by two-frequency laser interferometry is proposed and the distribution of concentrations in the sodium chloride-calcium chloride system is investigated.

Analytical model of laminar flow electrodialysis with ion-exchange membranes

A boundary-value problem for electrodialysis with ion-exchange membranes is posed and its analytical solution obtained. The solution allows one to calculate concentration fields in desalination and brine compartments, the current-density distribution along the flow coordinate and the thickness of diffusion boundary layers. It also makes it possible to estimate the value of local limiting current-density and to obtain the dependence of the process on physico-chemical characteristics of ion-exchange membranes (transport numbers and conductivity). The mathematical model was verified by a local distributive analysis made by means of laser interferometry.

The effect of ion-conducting spacers on mass transfer : numerical analysis and concentration field visualization by means of laser interferometry

Mathematical model is developed for the study of mass transfer processes under the laminar stationary flow in the channels of electromembrane system formed by alternating cation- and anion-exchange membranes at whose surfaces the ion-conducting spacers are fixed. The results of calculations are given of velocity and concentration fields, of current density distribution along the channel length for some values of Reynolds number and the values of the applied voltage. The comparison is made between the calculated mass transfer characteristics and the experimental data obtained by means of laser interferometry.

Competition between diffusion and electroconvection at an ion-selective surface in intensive current regimes.

Considering diffusion near a solid surface and simplifying the shape of concentration profile in diffusion-dominated layer allowed Nernst and Brunner to propose their famous equation for calculating the solute diffusion flux. Intensive (overlimiting) currents generate electroconvection (EC), which is a recently discovered interfacial phenomenon produced by the action of an external electric field on the electric space charge formed near an ion-selective interface. EC microscale vortices effectively mix the depleted solution layer that allows the reduction of diffusion transport limitations. Enhancement of ion transport by EC is important in membrane separation, nano-microfluidics, analytical chemistry, electrode kinetics and some other fields. This paper presents a review of the actual understanding of the transport mechanisms in intensive current regimes, where the role of diffusion declines in the profit of EC. We analyse recent publications devoted to explore the properties of different zones of the diffusion layer. Visualization of concentration profile and fluid current lines are considered as well as mathematical modelling of the overlimiting transfer.

Local Mass Transfer during Electrodialysis with Ion-Exchange Membranes and Spacers

Mass transfer during electrodialysis with ion-conducting spacers in the intermembrane gap is modeled experimentally using a local-distribution analysis of solutions. Due to emergence of back-flow regions near the spacers, local quantities (diffusion layer thicknesses, surface concentrations, Sherwood numbers) are distributed nonuniformly over the channel length. In a smooth channel, due to concentration polarization, local mass transfer rates steadily decrease along the solution supply coordinate, but introducing ion-conducting spacers into the channel intensifies mass transfer because of periodic interruption of diffusion layers and formation of recirculation zones. Effect of geometrical size of spacers on the mass transfer is studied. It is found that the mass transfer rate is maximum for ion-conducting spacers with the ratio 3.5 < l/h< 4.0.

Electrodialysis Kinetics by Laser Interferometry

A new approach to studying electromembrane systems by the laser interferometry method is proposed. Problems in the electrodialysis kinetics that had been solved by this method are reviewed. These include the determination of the size of diffusion boundary layers, the measurement of concentration profiles of solutions at a concentration polarization of ion-selective membranes in solutions of one and two components in isothermal and nonisothermal conditions, the verification of mathematical models for electrodialysis, and the measurement of local Sherwood numbers. A method for measuring concentration profiles in solutions at the interface with bodies of spherical symmetry following the imposition of a gradient of an electric potential on the system is offered. The results of studying the concentration polarization of an ion exchanger granules are analyzed.

Mutual Effect of Concentration Fields in Solutions of Deionization and Concentration Compartments during Electrodialysis with Ion-Exchange Membranes

The conjugated mass transfer processes in the neighboring compartments of an electrodialyzer are studied theoretically and experimentally. Dependences of the concentration fields, thicknesses of diffusion boundary layers, and maximum solution concentration in the flow on the geometric size, initial concentrations, and the electrolyte feed rate are obtained. A laser interferometry method is employed for experimental modeling and theory verification.

Diffusion Boundary Layers in Transport of Aliphatic Acids in Electromembrane Systems

The regularities of the formation of diffusion boundary layers during the transport of aliphatic acids in electromembrane systems are studied and their characteristics obtained experimentally are analyzed. It is shown that their concentration profiles in solutions of the electrodialyzer sections are asymmetric near membranes of different polarity. Experimental values of diffusion layer thicknesses are compared with those following from the known theoretical relations. The equation, which is proposed for the surface concentration, permits the prediction of the operation mode of apparatus during electrodialysis of solutions of aliphatic acids and other weak electrolytes. Specific features characterizing the emergence of a limiting state and the character of the distribution of limiting current densities over the height of the membrane channel are revealed.