Experimental Study of the Electro-Deionization Process 1

Abstract: This study investigated the effects of different operating conditions on the quality of EDI permeate through an experimental investigation of the post-reverse osmosis (RO) electrodeionization (EDI) process. The optimal operating parameters for EDI deionization were explored, and the stability of the EDI permeate process was also examined. The experiments showed that the EDI process can operate continuously for extended periods and produce high-quality pure water. Furthermore, it was found that increasing the operating current of the EDI membrane stack yields high-quality pure water; lower conductivity of the raw water entering the EDI membrane stack results in better permeate quality; appropriately increasing the water recovery rate of the EDI membrane stack leads to higher purity permeate; and appropriately increasing the temperature of the EDI raw water is beneficial for maximizing the desalination effect of EDI and obtaining high-quality pure water. Keywords: Electrodeionized water; Pure water; Electrodialysis; Ion exchange membrane

CLC Number: TU991.26+3

Document Code: A

Article Number: 1005-829X(2000)09-0011-03

Author Biography: Liu Hongbin (1968-), graduated from the Institute of Chemical Engineering, Tianjin University in 1997, PhD, Assistant Researcher.

Received Date: 2000-04-06

Electrodeionization (EDI) is a novel membrane separation technology that organically combines electrodialysis and ion exchange. It retains the advantages of continuous desalination in electrodialysis and deep desalination in ion exchange resins, while overcoming the limitations of deep desalination in electrodialysis, the adverse effects of concentration polarization, and the inconvenience and environmental pollution caused by the need for acid and alkali regeneration of ion exchange resins. EDI (Extracorporeal Dioxide) allows for continuous long-term operation of the deionization process and produces high-quality pure water, thus having broad application prospects in high-purity water preparation.

This paper investigates the effects of different operating conditions on the quality of EDI product water through experimental research on the EDI process, explores the optimal operating parameters for deionization, and examines the stability of the EDI product water process.

Experimental Section

Experimental Components

The EDI membrane stack was self-made, using all domestically produced raw materials.

Experimental Procedure

Tap water pre-filtered with activated carbon reverse osmosis EDI functional microporous filter membrane product water.

Data Acquisition

The voltage and current of the EDI membrane stack were measured using a multimeter. The flow rates of the EDI concentrate and deionized water were measured using a graduated cylinder and stopwatch, with Vc and Vp representing the flow rates of concentrate and deionized water, respectively. The conductivity of the raw water and concentrate was measured using a DDS-307 conductivity meter with temperature compensation. The conductivity of the freshwater was measured online using an RM220 resistivity meter and then converted to conductivity.

Experimental Methods

Reverse osmosis water was used as the feed water for EDI. A mixed-bed ion exchange resin was filled into the EDI desalination chamber partition, with anion and cation resins mixed at a specific volume ratio. Under different operating conditions, the current, voltage, and permeate conductivity of the EDI membrane stack were recorded to investigate the influencing factors on the permeate conductivity during the EDI process.

Results and Discussion

Voltage-Current Characteristics of the EDI Membrane Stack

The membrane stack current increases with increasing voltage. When the voltage increases to a certain level, the current increase becomes significantly larger. However, in electrodialysis, when the voltage increases to a certain level, the current increase becomes significantly smaller.

The reason for the above phenomenon is that when the membrane stack voltage gradually increases, the resin in the dilute chamber and the water in the concentration diffusion layer on the membrane surface undergo water dissociation under the action of the potential gradient, producing H+ and OH-. These not only carry part of the current but also displace salt ions from the mixed-bed resin filled in the dilute chamber, electroregenerating the resin into H- and OH- forms, allowing more ions to participate in the load current. As the voltage increases, the membrane stack current increases. When the membrane stack voltage rises to a certain level, water dissociation intensifies, producing excess H+ and OH- in the dilute chamber, further improving the conductivity of the dilute chamber, thus causing a larger increase in the membrane stack current, resulting in an inflection point on the voltage-current relationship curve.

Effect of Operating Current on Product Water Conductivity

Different operating voltages can lead to different membrane stack currents, and different membrane stack currents will result in different operating states of the EDI process, which will inevitably affect the conductivity of the EDI product water. Therefore, the membrane stack current is one of the most important factors affecting the conductivity of EDI product water. Under certain concentrate and desalination flow rates and a raw water conductivity of 12 μS/cm, the operating current of the membrane stack was varied, and the changes in product water conductivity were recorded. As the current increased, the EDI product water conductivity decreased rapidly.

Influence of Raw Water Quality on Product Water Conductivity

When the raw water conductivity is low, the EDI product water conductivity is also low, indicating good product water quality. This is because low raw water conductivity means a lower ion content, directly leading to improved product water quality. Simultaneously, the low ion concentration results in a larger potential gradient on the resin and membrane surfaces in the desalination chamber, leading to enhanced water dissociation and a greater quantity of H+ and OH- ions. This improves the regeneration effect on the anion and cation exchange resins packed in the desalination chamber, thus resulting in low product water conductivity.

Effect of EDI Membrane Stack Water Recovery Rate on Product Water Conductivity

The formula for calculating the EDI membrane stack water recovery rate is: Recovery Rate = Vp + Vc × 100%

The EDI membrane stack water recovery rate has a significant impact on the EDI product water conductivity.

The product water conductivity decreases rapidly with increasing EDI water recovery rate, and the change tends to level off when the water recovery rate is above 67%. This is because an increased water recovery rate leads to a larger desalination flow rate, which improves the hydraulic state in the desalination chamber, reduces the thickness of the stagnant layer on the resin particle surface, decreases the resistance of the desalination chamber, increases the membrane stack current, and promotes ion diffusion and migration; conversely, a smaller concentrate flow rate increases the ion concentration in the concentrate chamber, which in turn increases the membrane stack current. However, as the water recovery rate further increases, the difference in ion concentration between the concentrate and desalination chambers becomes larger, and the reverse migration of ions (i.e., the migration of ions from the concentrate chamber to the desalination chamber) becomes greater. Therefore, the change in product water conductivity tends to level off when the water recovery rate is above 67%. 2.5 Effect of Raw Water Temperature on Product Water Conductivity

As the EDI raw water temperature increases, the product water conductivity decreases. This is because higher raw water temperature accelerates the migration and diffusion of ions in the resin and membrane, which is beneficial for the deionization process.

Stability of the EDI Product Water Process

We operated the EDI unit for an extended period under different operating conditions, totaling 600 hours. Throughout the operation, the EDI unit remained stable and reliable, and the product water quality varied according to changes in operating conditions.

Over time, the product water conductivity gradually decreased, eventually stabilizing at 0.065 μS/cm, while the pH value remained around 6.2. 3. Conclusions

(1) Increasing the operating current of the EDI membrane stack can yield high-quality pure water, but from the perspective of improving the current efficiency of the membrane stack, the operating current should not be too high; (2) The better the pre-desalination effect, that is, the lower the conductivity of the raw water entering the EDI membrane stack, the better the quality of the EDI product water; (3) Appropriately increasing the water recovery rate of the EDI membrane stack can yield product water with higher purity. For small EDI devices, it is generally advisable to control it at 75%, and the concentrate can be recycled; (4) Appropriately increasing the temperature of the EDI raw water is beneficial to exerting the desalination effect of EDI and obtaining high-quality pure water.

Liu Hongbin, Gong Chengyuan, Su Jianyong, Zhu Mengfu (Institute of Health Equipment, Academy of Military Medical Sciences, Tianjin 300161, China)