Experimental Study of NaCl Electrodialysis Using DESALT Pilot Equipment

Ghafoor Ahmad, Jack Ma, Muhammad Mujahid

YASA ET (Shanghai) Co., Ltd. 2388, Xiupu Road, Pudong New Area, Shanghai, China Phone: +86 136 3643 1077

E-mail: info@yasa.ltd

Abstract

This study presents a comprehensive performance evaluation of the electrodialysis (ED) process for desalinating a 10 g·L⁻¹ sodium chloride (NaCl) solution using a pilot-scale YASA ET DESALT unit. The system was configured with a YSF ED stack comprising ten cell pairs, operated under a constant applied voltage of 14 V. Real-time monitoring of key electrochemical parameters, including conductivity, current density, temperature, and cumulative energy consumption, was conducted via an integrated PLC-Modbus interface. The results demonstrate effective and selective ion migration, evidenced by a systematic decrease in dilute conductivity (from an initial 15.8 mS/cm to 5.2 mS/cm over 120 minutes) and a proportional increase in concentrate conductivity. A characteristic decay in current density from 45 mA/cm² to 18 mA/cm² was observed, directly correlated with increasing ohmic resistance due to ion depletion in the dilute stream. Energy analysis revealed a linear increase in cumulative energy consumption, underscoring the escalating work required to maintain ion transport against a growing concentration gradient and polarization effects. The system exhibited notable operational stability with minimal fluctuations, confirming the robustness of the pilot unit for detailed process optimization and scale-up studies relevant to industrial desalination and resource recovery applications.

1.    Introduction

The global challenge of water scarcity necessitates advanced separation technologies for the desalination of brackish and saline water sources, as well as for the treatment of complex industrial effluents. Among these technologies, electrodialysis (ED) has emerged as a mature and versatile electromembrane process. Its operational principle involves the selective transport of ions through alternating cation- and anion-exchange membranes under an applied electric field, thereby achieving desalination or concentration in respective compartments. Compared to pressure-driven processes like reverse osmosis, ED offers distinct advantages for specific applications, including high recovery rates, exceptional tolerance for feedwater with higher salinity or fouling potential, and the capability for selective ion separation.

The scalability of ED from laboratory prototypes to full-scale industrial plants, however, presents significant engineering challenges. While the fundamental electrochemical principles are well-understood, the practical performance is governed by a complex interplay of operational parameters, membrane properties, and fluid dynamics. Key factors such as current density, flow rate, and feed concentration directly influence critical outcomes, including ion removal kinetics, energy consumption, and the onset of limiting current density due to concentration polarization. These phenomena can only be accurately characterized at a pilot scale, where the hydraulics, membrane area, and stack design more closely mimic industrial systems.

Pilot-scale studies are therefore indispensable, serving as a critical bridge between foundational research and commercial implementation. They provide a robust platform for validating long-term membrane stability, optimizing energy efficiency, and generating reliable techno-economic data. A thorough investigation of the intrinsic relationships between voltage, current, and resistance under realistic conditions is paramount for predicting performance and controlling costs in larger-scale applications.

This study presents a systematic performance analysis of NaCl desalination using a DESALT Pilot ED unit. The primary objective is to elucidate the fundamental correlations between ion transport, evidenced by conductivity changes, and the resulting electrical and energy profiles under constant voltage operation. By monitoring the temporal evolution of current density and cumulative energy consumption, this work aims to quantitatively link the progressive ion depletion in the dilute stream to the increasing system resistance and energy demand. The findings provide critical insights into the operational dynamics of pilot-scale ED, contributing valuable data for the design and optimization of energy-efficient desalination and resource recovery processes.

2.    Materials and Methods

2.1. Electrodialysis System and Configuration

The experiments were conducted using a DESALT ED(BM) Pilot System (YASA ET), an advanced modular unit designed for both electrodialysis (ED) and bipolar membrane electrodialysis (EDBM). A comprehensive view of the pilot system, including the front operational panel and the rear plumbing and stack assembly, is presented in Figure 1. The core of the system is a YSF electrodialysis stack, configured with ten cell pairs. Each cell pair consists of an alternating sequence of cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs), separated by polypropylene spacers that define the flow channels. The effective membrane area for each sheet was 270 mm × 110 mm.

Figure 1 DESALT ED(BM) Pilot System a) front view of system; b) back view of system.

The ion-exchange membranes used were characterized by the following properties, as provided by the manufacturer: an ion-exchange capacity of 0.90-1.10 mmol/g, a wet thickness of 70-80 µm, and a water uptake of 20-25 wt.% at 25˚C, as shown in Table 1. The area resistance ranged between 4.5-5.5 Ω·cm², and the transport number exceeded 0.97, confirming high permselectivity. The membranes were stable across a broad pH range of 0-14 and operational temperatures of 15-40˚C.

Table 1. Characteristics of the ion-exchange membranes used in the ED stack.

Property	Value
Ion-exchange capacity	0.90 - 1.10 mmol/g
Wet thickness	70 - 80 µm
Water uptake	20 - 25 wt.%
Area resistance	4.5 - 5.5 Ω·cm²
Transport number	> 0.97
pH range	0 - 14
Temperature range	15 - 40 °C

2.2.         Feed Solutions and Chemical Reagents

Aqueous sodium chloride (NaCl, ACS reagent grade) solutions were prepared using deionized water. To establish a stable initial condition and minimize osmotic water transfer, the dilute (feed) and concentrate streams were both initiated with identical 10 g·L⁻¹ NaCl solutions. The electrode rinse solution (electrolyte) was prepared at a higher concentration of 20 g·L⁻¹ NaCl to ensure sufficient ionic conductivity between the anode and cathode, thereby stabilizing the applied electric field and minimizing parasitic voltage drops.

2.3.         Operational Procedure and Data Acquisition

The system was operated in a constant voltage mode, with a direct current power supply maintaining a potential of 14 V across the entire membrane stack. The volumetric flow rates for the dilute and concentrate loops were precisely controlled at 70 L·h⁻¹ each using integrated metering pumps, while the electrolyte loop was maintained at 40 L·h⁻¹. Each hydraulic circuit contained approximately 1 L of solution, which was recirculated in a batch mode for the duration of the experiment.

Before each experimental run, the entire system was rinsed with tap water for 15-20 minutes to remove any residual ions, followed by a deionized water rinse. The prepared NaCl solutions were then introduced into their respective compartments. A programmable logic controller (PLC) integrated with a Modbus communication protocol was employed for automated system control and data acquisition. Sensors continuously monitored the pH, temperature, and conductivity in all streams. The applied voltage and resultant current were recorded simultaneously. All parameters were logged at five-minute intervals throughout the 120-minute experimental duration to ensure a high-resolution dataset for subsequent analysis.

2.4.         Electrodialysis Mechanism and Governing Reactions

The principle of electrodialysis relies on the selective electromigration of ions under an applied electric field. In a stack with alternating CEMs and AEMs, cations (e.g., Na⁺) migrate toward the cathode. They pass through CEMs but are blocked by AEMs. Conversely, anions (e.g., Cl⁻) migrate toward the anode, passing through AEMs but being blocked by CEMs. This selective transport results in the depletion of ions in alternate compartments (dilute) and the concentration of ions in the others (concentrate).

The overall process is driven by the electrochemical reactions at the electrodes. In a sodium chloride electrolyte, the primary reactions are:

At the Anode (Oxidation):

2Cl⁻→ Cl₂​(g) + 2e⁻

This reaction can be accompanied by the secondary reaction of water oxidation, especially at lower chloride concentrations:

2H₂​O → O₂​(g) + 4H⁺ + 4e⁻

At the Cathode (Reduction):

2H₂​O + 2e⁻ → H₂​(g) + 2OH⁻

The protons (H⁺) generated at the anode and hydroxide ions (OH⁻) generated at the cathode are typically neutralized by the high-concentration electrolyte rinse stream, which prevents extreme pH shifts from damaging the stack components.

3. Results and Discussion

3.1. Ion Transport and Conductivity Dynamics

The dynamic changes in electrical conductivity within the dilute and concentrated compartments provide direct evidence of ion transport during the electrodialysis process. As depicted in Figure 2, the conductivity in the dilute stream decreased progressively from an initial value of 15.8 mS/cm to 5.2 mS/cm over 120 minutes. This systematic reduction confirms the effective electromigration of Na⁺ and Cl⁻ ions out of the dilute compartment through the respective ion-exchange membranes.

Conversely, the concentrate stream exhibited a complementary and proportional increase in conductivity, rising from 15.8 mS/cm to 25.4 mS/cm. This symmetrical behavior validates the fundamental principle of ED, wherein ions are selectively extracted from one stream and accumulated in another under an applied electric field. The initial period was characterized by a high rate of conductivity change, indicative of a high ion flux driven by a strong concentration gradient. As desalination proceeded, this rate gradually stabilized, signaling the approach toward a new ionic equilibrium where the driving force for electromigration is balanced by the increasing tendency for back-diffusion from the concentrate.

Figure 2 Electrical Conductivity (mS/cm) vs. Time (min).

3.2. Thermal Buildup from Joule Heating and Ionic Friction

A gradual temperature increase was observed in both the dilute and concentrated circulating loops, as shown in Figure 3. The temperature rose steadily from an ambient 22.5 °C to 28.5 °C. This thermal effect is primarily attributed to Joule heating, a phenomenon where electrical energy is dissipated as heat due to the ohmic resistance (I²R losses) of the ionic solutions and the membrane stack.

Additional contributions to the temperature rise include the frictional resistance encountered by ions migrating through the membrane matrix and the exothermic energy changes associated with ion hydration and dehydration at the membrane-solution interfaces. In a closed recirculation system with limited heat dissipation, this generated heat accumulates, leading to the observed steady thermal buildup. This temperature profile is a typical and inherent characteristic of a functioning electrodialysis system.

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Figure 3 Temperature (°C) vs. Time (min).

3.3. Current Decay as an Indicator of Increasing System Resistance

The current-time profile, presented in Figure 4, exhibits a characteristic decay from an initial high of approximately 45 mA/cm² to a final steady value near 18 mA/cm². This trend is a direct consequence of the changing electrical resistance of the system under constant applied voltage.

Initially, the high ionic concentration in the dilute stream provides high conductivity, resulting in low overall resistance and high current. As electrodialysis progresses, the successful removal of ions from the dilute compartment depletes the number of charge carriers, thereby increasing the solution's electrical resistance.

According to Ohm's Law (V=IR), for a constant voltage (V=14 V), an increase in resistance (R) necessitates a decrease in current (I). The eventual stabilization of the current at a lower level indicates that the system has reached a state where ion removal rates have slowed significantly due to high resistance and pronounced concentration polarization at the membrane surfaces.

Figure 4 Current vs. Time (min).

3.4. Temporal Evolution of Energy Demand

The cumulative energy consumption, calculated from the real-time voltage and current data, demonstrated a linear increase over the operational period (Figure 5). This linear trend represents the continuous power input required to maintain ion transport as the concentration gradient between the compartments widens.

At the start of the process, energy consumption per unit of salt removed was relatively low, reflecting efficient ion migration under conditions of high conductivity. However, as concentration polarization developed and the resistance of the dilute compartment increased, a greater amount of electrical energy was required to maintain the same potential difference and force ions across the membranes. The linear rise in cumulative energy underscores the growing energy demand to overcome these increasingly unfavorable conditions, highlighting a key efficiency challenge in batch ED operations.

Figure 5 Energy Consumption vs. Time (min).

3.5. Interplay Between Operating Current and Energy Efficiency

Figure 6 illustrates the critical correlation between operating current and the system's energy efficiency. At high initial current levels, the specific energy consumption (energy per ion removed) was low. This is because the energy input was utilized efficiently for ion transport in a low-resistance environment.

As the process advanced and the current declined due to increasing resistance, the specific energy consumption rose significantly. This inverse relationship is fundamental to electrodialysis: as ionic depletion occurs, the system requires more energy to move each subsequent ion. The data clearly shows that operating at lower currents, which correspond to higher levels of desalination, incurs a penalty in energy efficiency. This insight is crucial for optimizing ED processes, suggesting that for applications requiring high purity, the energy cost increases non-linearly, and strategies to mitigate polarization are essential.

Figure 6  Energy Consumption vs. Current.

4. Conclusion

This study conclusively demonstrates the efficient desalination of a NaCl solution using a pilot-scale electrodialysis (ED) system. The process was quantitatively validated through key electrochemical metrics: a significant decrease in dilute conductivity confirmed effective ion migration. At the same time, a corresponding current decay under constant voltage directly reflected the increasing system resistance due to ion depletion. The linear rise in cumulative energy consumption highlighted the growing work required to overcome this resistance and concentration polarization, revealing a key efficiency trade-off at higher desalination levels.

The DESALT pilot system exhibited excellent operational stability and reproducibility throughout the experiments. These findings confirm the system's reliability for precise evaluation of membrane performance and process optimization studies. The elucidated relationships between ion transport, electrical resistance, and energy demand provide a critical empirical foundation for scaling up ED technology for industrial applications in brackish water desalination and resource recovery.