Abstract
Ion-selective membranes are an important component of electrodialysis stacks for desalination. Manufacturing imperfections or slight inhomogeneity of the material can lead to minute membrane surface imperfections. Two-dimensional solutions of the coupled Poisson–Nernst–Planck and Navier–Stokes equations were sought for a perfectly smooth membrane and for membranes with well-defined small-amplitude harmonic surface roughness. The simulations were carried out with the validated rheoEFoam solver by Pimenta and Alves. In the overlimiting regime, the electric field is strong enough for an electrokinetic instability to occur. The instability leads to disturbance growth and the formation of electro-convection cells, which strongly increase the current density. The present simulations show that with an increasing ion concentration and applied voltage, the instability becomes stronger and the overlimiting regime is reached earlier. The limiting current density shows a noticeable dependence on the wavelength of the surface roughness. When the wavelength of the surface roughness is incommensurate with the wavelength of the naturally occurring instability, the limiting current density is increased. Since production membranes will always have some degree of surface roughness, this suggests that membrane surface treatments which favor certain wavelengths may have an effect on the overall membrane performance.
Highlights
Desalination of alternative waters, such as brackish water, seawater, municipal, and industrial wastewater, has become a critical strategy to expand traditional water supplies and to alleviate water shortages [1]
An ED stack consists of pairs of anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) arranged alternatingly between an anode and a cathode (Figure 1)
The structures are caused by an electrokinetic instability (EKI)
Summary
Desalination of alternative waters, such as brackish water, seawater, municipal, and industrial wastewater, has become a critical strategy to expand traditional water supplies and to alleviate water shortages [1]. The opposite occurs when the negatively charged anions migrate toward the anode This results in an alternatively increasing ion concentration in one compartment (concentrate) and decreasing salt concentration in the other (diluate). Electrodialysis reversal (EDR) is a modified ED process, where the electrical polarity of the electrodes is reversed periodically This results in a reversal in the direction of the ion transport. Electrodialysis and EDR have been utilized for decades to desalinate brackish water, treat and industrial wastewater, and produce NaCl from seawater [1]. Any improvement of the effectiveness of the ED process based on a reduction the methods that overcome inherent operating limitations have strong implications for environmental energy losses (e.g., Chehayeb and Lienhard [3]) and methods that overcome inherent operating sustainability and strong the water, food, and chemical industries. The paper concludes with a brief discussion of the results and their relevance for ED
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