O. V. Grigorchuk

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.

Diffusion boundary layers during electrodialysis

A laser interferometry method for direct measurements of thicknesses of diffusion boundary layers is described. The method is employed to explore electromembrane processes. Thicknesses of diffusion boundary layers at below and above limiting diffusion current densities are measured and compared with theoretical values. It is shown that the diffusion boundary layers in narrow channels of electrodialyzer sections overlap one another at high current densities. This radically alters traditional notions concerning the diffusion boundary layer. An interpretation for variations in the thickness of a diffusion boundary layer at current densities in excess of the limiting diffusion current density is given.

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.

Mathematical model of electrodialysis with ion-exchange membranes and inert spacers

A mathematical model describing the two-dimensional concentration field of an electrodialysis device with inert spacers is proposed. The boundary-value problem includes the Navier-Stokes, continuity, and steady-state convective diffusion equations and well-defined conditions and is solved by the control-volume numerical method. Results are expressed in the form of functional relationships of generalized variables. It is shown that when channels of the electrodialysis device are filled with spacers that do not conduct electric current, mass transport increases by several times in comparison to devices with open channels. The possibility is discussed for replacing the inert spacers with ones that conduct ion, not only in the complete demineralization of natural waters, but also in the desalination of brackish ground waters.

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.