Abstract
Electrodialysis (ED) has been demonstrated as an effective membrane method for desalination, concentration, and separation. Electroconvection (EC) is a phenomenon which can essentially increase the mass transfer rate and reduce the undesirable water splitting effect. Efforts by a number of researchers are ongoing to create conditions for developing EC, in particular, through the formation of electrical heterogeneity on the membrane surface. We attempt, for the first time, to optimize the parameters of surface electrical heterogeneity for ion-exchange membranes used in a laboratory ED cell. Thirteen different patterns on the surface of two Neosepta anion-exchange membranes, AMX and AMX-Sb, were tested. Low-conductive fluoropolymer spots were formed on the membrane surface using the electrospinning technique. Spots in the form of squares, rectangles, and circles with different sizes and distances between them were applied. We found that the spots’ shape did not have a visible effect. The best effect, i.e., the maximum mass transfer rate and the minimum water splitting rate, was found when the spots’ size was close to that of the diffusion layer thickness, δ (about 250 μm in the experimental conditions), and the distance between the spots was slightly larger than δ, such that the fraction of the screened surface was about 20%.
Highlights
Today electrodialysis (ED) is a well-developed membrane technique which has many applications, such as desalination and concentration of solutions, separation of ions, acid and alkali production, energy generation, and others [1,2,3,4].In the course of electrodialysis of dilute solutions, the surface properties of ion-exchange membranes (IEM) significantly affect the performance of the separation process
It has been shown that the deposition of relatively hydrophobic poorly conducting fluoropolymer spots on the surface of a homogeneous membrane allows increasing mass transfer rate by up to 1.5 times and reducing water splitting rate near the membrane surface
It is possible to turn the factor of EC intensification into dominant over the factor of increasing concentration polarization (CP)
Summary
Today electrodialysis (ED) is a well-developed membrane technique which has many applications, such as desalination and concentration of solutions, separation of ions, acid and alkali production, energy generation, and others [1,2,3,4].In the course of electrodialysis of dilute solutions, the surface properties of ion-exchange membranes (IEM) significantly affect the performance of the separation process. In contrast to the effect of H+ and OH– ion generation, EC is considered as an extremely desirable effect which causes a significant increase in mass transfer [8,10,19,20], and leads to a decrease in the water splitting rate [17,18,21,22] This effect is explained by the fact that an increase in EC mixing of the near-membrane solution layer leads to an increase in the concentration of salt ions above the critical value at which the generation of H+ and OH– ions begins
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