Improvement in the desalination performance of membrane capacitive deionization with a bipolar electrode via an energy recovery process

https://doi.org/10.1016/j.cej.2022.135603Get rights and content

Highlights

  • Energy stored in bipolar MCDI is recovered via direct connection manner.

  • Direct connection of two bipolar MCDI induce their partial adsorption and desorption.

  • By using this phenomenon, operation with direct connection reduced energy up to 47%

  • Appropriate time combination can reduce energy with the same desalination performance.

Abstract

In the practical implementation of capacitive deionization (CDI), membrane CDI with a bipolar electrode (bipolar MCDI) is emerging as one of the alternative CDI platforms due to its favorable cell configuration for scale-up and low current originating from the serial connection of electrodes. Nevertheless, one obstacle to practical use is that there are few studies about the energy recovery process for the high energy efficiency of bipolar MCDI, requiring further research. Therefore, in this study, we propose a bipolar MCDI process with energy recovery and assess its potential by analysis of a lab-scale module with a single stack and nine stacks of the bipolar electrode (i.e., 2.4 V and 12 V system, respectively) and a pilot-scale module with 250 stacks (i.e., 300 V system). As a result, the energy consumption of the bipolar MCDI systems was reduced by 43% and 41% in the lab-scale modules with 2.4 V and 12 V systems, respectively, via energy recovery. Furthermore, the energy recovery led to a 40% reduction in the energy consumption of bipolar MCDI, even in the pilot-scale modules of the 300 V system. The results suggest that energy recovery in bipolar MCDI can be one of the essential processes and it has a strong potential for implementation in real industrial and environmental applications.

Introduction

The electrochemical separation method has received large attention as promising water remediation and resource recovery process due to its environmentally benign and energy-efficient operation [1], [2], [3], [4]. In particular, as the water crisis has become a growing global concern, significant research strides have been made in electrochemical desalination technology. Among the electrochemical desalination technology, capacitive deionization (CDI), membrane (MCDI), and electrodialysis has been extensively studied for industrial application [5], [6]. Especially, MCDI is widely studied for treating relatively low salinity water streams such as brackish waters due to its simple implementation [7], [8]. Furthermore, the potential of MCDI as a technology for a specific purpose such as selective removal of ions has been attracted [9], [10], [11]. The ion removal performance of MCDI has been enhanced with the introduction of various electrode materials such as functionalized carbon electrodes [12], [13], [14], [15], intercalating materials [16], [17], [18], [19], and integrating with redox materials [20], [21], [22], [23], [24], [25]. The modification of cell construction [26], [27], [28], [29], and optimization of the operation method [30], [31], [32], [33] have also enhanced the performance of MCDI. In addition, efforts have also been made to develop efficient energy recovery and scale-up methods in MCDI systems that are believed to be necessary for the successful commercial application of this technology [34], [35], [36], [37].

The MCDI modules extend their desalination capacity via stack-up of the electrode as well as the extension of the electrode size [38], [39]. Typically, the stacked electrodes are connected in parallel, which can be defined as a unipolar type. For this unipolar MCDI, all electrode pairs require a terminal to connect with an external power supply, which increases the complexity of module assembly with a larger number of stacked electrodes [39]. In addition, the current is proportionally increased with the number of electrodes in the stack since the total current of the system is distributed at one point to each electrode pair to apply the same voltage (e.g., 1.2 V). Thus, a thick conducting wire is required to control the high current, which can increase the investment cost of the electrical systems and their size [38], [40]. Furthermore, for unipolar MCDI, two types of electrodes sandwiched between cation exchange membranes and anion exchange membranes are alternately stacked between conventional electrodes pairs [41]. These characteristics make the unipolar MCDI module complex, bulky, expensive, as the scale of the module increases.

Recently, the bipolar MCDI module has gained much attention, in which stacked electrodes are connected in series without conducting wiring (Fig. 1a). As compared in Fig. 1b and c, contrary to unipolar MCDI, there is no requirement for terminals for applying electrical energy, since the bipolar electrodes (inner electrode of the module) are evenly polarized via the applied electric field through the outermost electrode pair [42], [43]. Thus, the bipolar MCDI module is operated under a high voltage without increasing current depending on the number of stacked electrodes, which does not require a thick conducting wire [40], [44]. Furthermore, in the bipolar MCDI module, only one type of electrode sandwiched between the cation exchange membrane and the anion exchange membrane is stacked as the inner electrode [40]. Therefore, the bipolar MCDI system can be operated with a low investment cost compared to the unipolar MCDI. In this regard, bipolar MCDI is expected to be a more suitable process for real implementation in industrial and environmental applications. However, for the further success of bipolar MCDI, it is still necessary to improve its energy efficiency during desalination. Nevertheless, to date, there are few studies to improve the energy efficiency of bipolar MCDI significantly limiting its practical application.

Therefore, this study aimed to propose the desalination process of a bipolar MCDI system including direct energy transfer for energy recovery, and assess its potential with a lab-scale and a pilot-scale module. In this study, various bipolar MCDI modules were employed to assess the energy recovery process, such as a lab-scale module with a single stack and nine stacks of the bipolar electrode (i.e., 2.4 V and 12 V system) and a pilot-scale module with 250 stacks (i.e., 300 V system). Because the energy was recovered via a direct energy transfer between two bipolar MCDI modules, this step was denoted as direct P2P (direct energy transfer from MCDI process to MCDI process). The overall process was performed as follows (Fig. 1d). First, as a pretreatment process before energy recovery, the bipolar MCDI modules were operated in an alternating manner with constant cell voltage operation. For example, the first bipolar MCDI module was operated by applying a constant cell voltage of 2.4 V for desalination while the second bipolar MCDI module was operated by applying a reverse voltage of –2.4 V for regeneration. Consequently, the invested energy can be simultaneously charged at both bipolar MCDI modules up to cell voltages of 2.4 V and –2.4 V, respectively, during the desalination and regeneration phase. Second, as shown in Fig. 1d, the energy was transferred between both bipolar MCDI modules by a direct connection leading to energy recovery (first direct P2P). Since the transferred energy is not sufficient to perform a full-cycle operation, this energy partially supports the initial ion adsorption at the first bipolar MCDI module (desalination) and ion desorption at the second bipolar MCDI module (regeneration) by controlling the duration of their electrical connection. Then, additional energy was invested to complete the full cycle operation via an external power source (charging for energy input). For consecutive operations, these processes were alternatively operated between two bipolar MCDI modules via a second direct P2P and reverse charging.

To optimize the direct P2P, the effect of the duration of direct P2P on the energy efficiency and desalination performance of bipolar MCDI operation within 5 min of half-cycle time in a lab-scale module (i.e., 2.4 V and 12 V systems) was examined. Additionally, the potential of direct P2P was assessed with a pilot-scale module (i.e., 300 V system).

Section snippets

Bipolar MCDI module configuration

Fig. 1a shows a schematic illustration of the MCDI module with a bipolar configuration. In the bipolar MCDI module, the bipolar carbon electrodes were coated by cation and anion exchange resins on both sides. The bipolar carbon electrodes were placed between a pair of conventional activated carbon electrodes with an anion exchange membrane and cation exchange membrane. The nonconductive nylon cloth was used as a spacer providing a water stream.

Contrary to conventional MCDI stacked with unipolar

Desalination of bipolar MCDI with direct energy recovery step in 2.4 V system.

Fig. 2 shows the representative effluent concentration and current profile of the first bipolar MCDI module in 2.4 V systems in various durations of first direct P2P, charging (+V), second direct P2P, and reverse charging (–V).

As shown in Fig. 2a, the effluent concentration profile of the first bipolar MCDI module with various direct P2P steps (i.e., P4C1, P3C2, P2C3, and P1C4) showed a bimodal curve for ion adsorption and desorption, while P0C5 had a typical effluent profile of CV/RV operation

Conclusion

In this study, a bipolar MCDI process with energy recovery via direct energy transfer was proposed, and its potential was assessed by analysis of a lab-scale module with a single stack and nine stacks of the bipolar electrode (2.4 V and 12 V system) and a pilot-scale module with approximately 250 stacks (300 V system). As a result of the direct connection of two bipolar MCDI modules having different cell voltages, the energy transfer occurred in both modules with ion adsorption and desorption

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2021R1I1A3040360). The authors are also grateful to Institute of Chemical Processes of Seoul National University.

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