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channels avoiding differential pressures is needed (this is more important in large stacks). Submerged cylinders perpendicular to the flow have been tested at lab scale and simulated in two dimensions [189, 195, 234]. Interestingly, if cations and anions have different diffusivities in solution, as in the case of NaCl (DCl- > DNa+), floating promoters of mixing optimize mass transfer when placed slightly closer to the AEM than to the CEM, due to the obtained asymmetric distribution of fluid velocity inside the channel [189]. Dimensional stability can be maintained also by the novel design proposed by Koutsou et al. [271] (Figure 11 (m)), where the partially floating spacer comprises spherical nodes in contact with the membranes and symmetrically connected with a mesh of smaller cylindrical filaments. 4.4.2 Conductive spacers and profiled membranes The most significant disadvantage of net polymeric spacers is that they are commonly made by non- conductive materials, thus increasing the compartment’s electrical resistance; instead, the use of conductive materials spacers may be a good option for lowering the energy consumption, thus improving the efficiency of ED units. In the mid-1970s, Kedem [46, 47] prepared spacers made by ion exchange material. The multiple benefits deriving from the use of conductive spacers were evident: mass transfer enhancement (reduction of polarization and increase of ilim) due to the increase of the active area, reduction of Ohmic (shadow effect) and non-Ohmic resistances, high current efficiency at high current density, stable pH (reduction of water splitting), higher effectiveness at low concentrations of diluate and achievement of very low concentrations. After more than 20 years, commercial spacers were modified into conductive spacers by chemical reaction [285] or coating [286], confirming the previous results. In addition, it was pointed out that (i) the consequences (water splitting, scaling and fouling) of uneven distributions of feed solutions among and within the channels were significantly mitigated by the lower resistance provided by conducting spacers [285]; (ii) a larger effectiveness was achieved in electrodialyzers provided with heterogeneous IEMs which are, by their nature, more polarizing, and with conductive spacers with higher ion exchange capacity [286]. The effectiveness of chemically modified spacers was also proven by a subsequent work [144]. Conductive spacers can be useful also for special applications, e.g. for the selective separation of nitrates from drinking water [287] by using a cation conductive woven spacer with granules of nitrate- selective anion exchange resin and a modified AEM. Local effects of a conducting spacer were analysed by Shaposhnik et al. [185] by experiments (laser interferometry) and simulations. Fragments of ion exchange material were cut and pasted on the membranes (with glue only on the non-working surface of membrane). The membrane/channel configuration obtained in this way is very similar to what can be obtained by profiling the membranes. 38PDF Image | Electrodialysis for water desalination
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