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Abstract: A three-compartment BPM-EDI device was assembled by combining bipolar membrane (BPM) and packed bed electrodialysis (EDI) technologies and applied to the electroregeneration of multi-bed ion exchange resins. Experimental results showed that when the regeneration voltage was 60 V and the regeneration time was 60 min, the electroregeneration effect of this device was close to that of chemical regeneration, demonstrating good technical feasibility.
Keywords: Bipolar membrane (BPM); packed bed electrodialysis (EDI); ion exchange resin; electroregeneration
CLC Number: TU991.26
Document Code: C
Article Number: 1000-4602(2001)10-0074-03
The bipolar membrane consists of an anion exchange resin layer (AL), a cation exchange resin layer (CL), and an intermediate hydrophilic layer. Under the action of a DC electric field, it can directly dissociate water into H+ and OH- [1]. A bipolar membrane electrodialysis system, combining bipolar membranes with other anion and cation exchange membranes, can convert and separate salts in aqueous solutions into corresponding acids and bases without introducing components. This principle has shown good technical feasibility in experimental studies of the electroregeneration of mixed-bed ion exchange resins [2]. Now, bipolar membranes and packed-bed electrodialysis technology are combined to assemble a three-compartment BPM-EDI device, which is applied to the electroregeneration of mixed-bed ion exchange resins.
A bipolar membrane three-compartment packed-bed electrodialysis device is formed by arranging cation exchange membranes, bipolar membranes, and anion exchange membranes in a specific order, and filling both sides of the bipolar membrane with anion and cation exchange resins respectively.
Under a certain voltage, the bipolar membrane can directly dissociate water into OH- and H+. In the anion exchange resin chamber, under the influence of an electric field, the anion exchange resin adsorbs and conducts anions in the water, allowing them to pass through the anion membrane into the concentrate chamber. Meanwhile, the OH- ions generated by the bipolar membrane from water dissociation are adsorbed by the anion exchange resin during the desorption of other anions, thus restoring the resin's activity in adsorbing and conducting anions, resulting in regeneration. Similarly, in the cation exchange resin chamber, under the influence of an electric field, cations are adsorbed and transferred through the cation exchange resin into the concentrate chamber via the cation exchange membrane. The H+ ions generated by the bipolar membrane from water dissociation are adsorbed by the cation exchange resin during the desorption of other cations, thus regenerating the resin. When the raw water has a low salinity, under a certain voltage (greater than the operating voltage of the device's limiting current), the bipolar membrane, as well as the interface layers between the anion and cation membranes and the resin particles, undergo varying degrees of polarization. The bipolar membrane will more efficiently dissociate water into H+ and OH-, resulting in better regeneration of the resin in the resin chamber.
2.1 Apparatus
The bipolar membrane was specially manufactured by Shanghai Chemical Plant; the anion and cation exchange membranes used were 3361-BW and 3362-BW produced by Shanghai Chemical Plant; the ion exchange resins used were 001×7 cation resin and 201×7 anion resin produced by Nankai University Chemical Plant; the resin chamber partition was made of rigid polyvinyl chloride board with specifications of 400 mm × 150 mm × 5 mm, processed into a non-circuit, closed-channel inlet and outlet water partition for resin filling; the electrodes were titanium-coated ruthenium (anode) and stainless steel plate (cathode); a 0-100 V thyristor rectifier; a DDS-11A conductivity meter; and a PHS-2C pH meter.
2.2 Method
The anion and cation resins, which had been chemically regenerated, were loaded into the anion and cation resin chambers. Tap water was then passed through to deactivate them. The flow rate was: first through the cation resin chamber, then into the anion resin chamber. The conductivity of the effluent from the anion resin chamber was measured every 5-10 minutes until the resin was deactivated. The experiment used the initial water exchange capacity E0 obtained after the new resin was treated with three acids and three alkalis as the benchmark, while the exchange capacity of the resin after electrostatic regeneration was E. E/E0 represents the degree of regeneration. To simplify the experiment and calculation, the target value for resin regeneration was first determined, and then the failure time T of the resin under the same conditions was compared with the failure time T0 of the new resin.
First, a failure test was conducted on the new resin. The original water conductivity was 550 μs/cm, the water flow rate was 50 L/h, and the water temperature was 16 ℃. T.
Failure was considered when the effluent conductivity was >90 μs/cm, and the treatment time (T0) was 80 min. The following experiments used an effluent conductivity <90 μs/cm as the regeneration target value.
3.1 Relationship between Operating Voltage and Current
The raw water was pretreated by electrodialysis, with a conductivity of 20 μS/cm and a temperature of 16 ℃. The flow rate in each chamber was 5 L/h. The operating voltage was gradually increased from 0 to 100 V in 10V increments, and the current intensity at different voltages was measured.
That when the operating voltage is <30 V, the current increases slowly with increasing voltage, indicating that the voltage is insufficient to cause water dissociation in the bipolar membrane. When the voltage is >30 V, the current increases rapidly with increasing voltage, indicating that water molecules have dissociated into H+ and OH- to act as the current-carrying medium. Simultaneously, the conductivity of the concentrate effluent increases sharply. During operation, with increasing voltage, the pH value of the cation exchange chamber decreased (from 5.27 to 4.01), while the pH value of the anion exchange chamber effluent increased (from 5.98 to 7.81). This indicates that the bipolar membrane's ability to dissociate water is enhanced with increasing operating voltage. The pH values in the concentrate chamber and the electrode chamber did not change significantly, remaining within the range of 4.72–5.69.
3.2 Effect of Voltage on Regeneration
The energizing time was determined to be 50 min. During the experiment, water from the concentrate chamber was directly discharged, while water from other chambers was reused.
That there was almost no regeneration effect when the voltage was <20 V. A significant regeneration effect was observed when the voltage increased to 40 V. The failure time at 60 V was close to that of chemical regeneration. The effect was minimal after 80 V. This indicates that the operating voltage needs to be increased to a certain value to allow sufficient H+ and OH- ions to be generated from water dissociation for thorough resin regeneration. A regeneration voltage of 60 V is recommended for this experimental setup.
3.3 Effect of Energizing Time on Regeneration
The regeneration effect was measured at different energizing times using 60 V as the operating voltage.
That a longer energizing time resulted in a better regeneration effect. The energizing time, between 20 and 60 minutes, showed an almost linear relationship with the failure time. After energizing time > 60 minutes, the failure time trended towards a flattening, indicating that a regeneration time of 60 minutes was optimal.
3.4 Influence of Other Factors on Regeneration Effect
① The flow rate of water in the resin chamber directly affects the contact time between H+ and OH- generated by water dissociation and the resin, thus affecting the regeneration effect. A lower flow rate results in a longer contact time between H+ and OH- generated by water dissociation and the resin, which is more conducive to resin regeneration. However, an excessively low flow rate is not conducive to the migration of salt ions to the concentrate chamber, and a certain degree of turbulence in the chamber is beneficial to resin regeneration. Therefore, there is an optimal flow rate. According to the experimental results of electroregeneration of mixed-bed ion exchange resin [2], 0.5–1.0 cm/s is preferable.
② Better water quality in the regeneration water is more conducive to regeneration. The experiment achieved the expected results using electrodialysis pretreatment of the water. In production, ion exchange resin desalination can meet the requirements for regeneration water. ③ Increasing the influent water temperature reduces water viscosity and increases solution conductivity, which is beneficial for ion migration and resin regeneration. However, excessively high temperatures can damage the ion exchange membrane and resin, and also increase regeneration costs. Therefore, if waste heat is available, the influent water temperature can be appropriately increased within the allowable temperature range of the membrane and resin.
① Combining bipolar membrane (BPM) and packed electrodialysis (EDI) technologies for the electroregeneration of ion exchange resins is technically feasible.
② Regeneration voltage is a critical parameter; regeneration time, water flow rate, water quality, and water temperature also have a certain impact on the regeneration effect.
③ When the regeneration voltage is 60 V and the regeneration time is 60 min, the regeneration effect of this device is close to that of chemical regeneration. References:
[1] Liao Shangzhi, Mo Jianxiong. Development and application of bipolar membranes [J]. Water Treatment Technology, 1995, 21(6): 311-317.
[2] Li Fuqin, Li Qingxue, Wang Dongyun. Experimental study on electroregeneration of mixed-bed ion exchange resin [J]. Journal of Hebei University of Architecture and Technology, 1999, 16(4): 14-16.
