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Membranes 2020, 10, 221 27 of 72 For the energy consumption, no general trend can be drawn, and opposite trends are even observed. The optimum PEF conditions, in terms of energy consumption, seem to be dictated by the composition of the solution as well as the potential fouling that PEF can mitigated. The impact of PEF on fouling mitigation cannot be dissociated from the global process performance where energy consumption is one of the main components. For Mass Transfer PEF was also tested in the context of mass transfer for improving demineralization or/and organic acid recovery as well as divalent cation selectivity. Concerning demineralization, it was reported on polymer-flooding produced water that PEF improved the demineralization percentage and according to the authors, the shorter the pulse duration, the higher the demineralization rate [84] (Table 1). However, the only exception was for the runs with 0.1 s pulses, which rendered low demineralization percentages, possibly due to the high frequency that could cause a closest packing of the foulant: partially hydrolyzed polyacrylamide (HPAM). For regimes with the same pulse and different pause period, longer pauses yielded lower demineralization of the stream. For a feed stream consisting of brackish water + HPAM, the best demineralization rate was obtained employing the regime 1 s/1 s [84]. This is confirmed on Figure 15 representing the demineralization rate as a function of frequency (on a logarithmic scale) and where the condition 1 s/1 s corresponds to a frequency of 0.5 Hz, and is the optimum. On the opposite, on sweet whey demineralization, the highest demineralization rate was obtained for PEF combinations 0.1 s/0.1 s with a demineralization rate 81% superior to CC mode for the same number of charges transported (Figure 15). It is important to mention that PEF application of 1 s/1 s condition was the second-best condition, however with a demineralization rate only 10.5% higher than CC [64] (Table 1). This was explained by the emergence of electroconvective vortices (ECVs) at the beginning of each pulse due to the appearance of a voltage spike. Since their lifetime are around 0.5 s, these ECV did not have time to fade off during the whole process in the case of high-frequency PEFs increasing consequently the mass transfer [65] (for more explanation see Section 4.2.1). On acid whey demineralization and lactic acid recovery by ED, it was observed that the combination of pulse durations between 5–50 s and, pause durations between 5–35 s led to a similar global demineralization rate around 67% [83,113] (Table 1). Following the results of simulation compared with experimental data obtained on a laboratory-scale electrodialysis stack, with a 0.1 M NaCl solution, Sistat et al. showed that mass transfer under PEF of a sufficiently high frequency (>1 Hz) was higher than the one in conventional steady state direct current (DC) mode, if a same average voltage is applied [121]. The advantage increases with frequency and reaches a plateau at about 100 Hz (Figure 15). The authors explained these results by the fact that when applying a pulse after a pause, there are low ohmic resistance and low diffusion potential drop caused by partial concentration restoration in close vicinity of the membrane beneficial to the mass transfer. This allows passage of an instantaneous current of a high density, which can essentially exceed the limiting current density in steady state DC conditions. However, at low frequencies this gain is rapidly vanished by increasing concentration polarization during the pulse; thereby the mass transfer in PEF mode is lower than that in DC mode. Although these experiments were carried-out with a model solution containing only 0.1 M NaCl and a duration of each experimental run of 10 min, a similar conclusion was obtained by Lemay et al. [64] on sweet whey (Figure 15). Indeed, the authors reported on this complex food solution that the mass transfer efficiency increased with the frequency of the pulse/pause combination; 5 Hz (0.1 s/0.1 s) being the best condition (Figure 15) (for more information see Section 4.2.1). However, recently, Dufton et al. [83] on demineralization and recovery of lactic acid from acid whey observed that an overall enhancement of the ED process was visible through the increasing of frequency, even for frequencies comprised between 0.014–0.1 Hz. So, the beneficial impacts of frequency were not only visible for high-frequency PEFs (higher than 1 Hz) in regard to mass transfer improvements.PDF Image | Electrodialytic Processes
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