Research on the Process for Removing Heat-Stable Salts from Organic Amine Absorbents via Electrodialysis

Abstract

The use of organic amine absorbents in CO₂ capture technologies is highly significant. The widespread application of this technique is limited by the heat-stable salts (HSSs) produced during the cyclic absorption–desorption process. This research focused on the HSS removal process using electrodialysis technology and systematically examined the effects of operating voltage, initial concentration, pH, current density, the ratio of liquid volume in the enriched chamber to that in the diluting chamber, and the type of ion-exchange membrane on desalination efficiency, energy consumption, and amine loss. An increase in both voltage and initial concentration significantly enhances the rate of water migration. The rate of ion migration is observed to follow the order of Cl⁻ > ${\mathrm{S}\mathrm{O}}_4^{2-}$> F⁻ in a homogeneous membrane, while in a heterogeneous membrane, the order is ${\mathrm{SO}}_4^{2-}$ > Cl⁻ > F⁻. The optimal operating voltage is 10 V, with a pH level of 8 resulting in the highest ${\mathrm{SO}}_4^{2-}$ removal efficiency. An industrial scenario validated the optimized process conditions, which balanced energy consumption with desalination efficiency. This methodology is essential not only for providing a viable solution for the industrial purification of organic amines but also for promoting the environmentally sustainable development of carbon capture technologies.

1. Introduction

One of the hottest topics in carbon emission reduction technology is CO₂ capture using organic amine absorbents. Among the challenges faced are the formation of heat-stable salts (HSSs) and the high energy consumption associated with the capture process, both of which hinder the widespread use of organic amine CO₂ capture technology. The organic amine loses its active sites for absorbing CO₂ as a direct result of HSS production, which reduces the effectiveness of the decarburization process. The cost of CO₂ capture is further increased by issues like increased equipment corrosion and material foaming in the absorption and regeneration towers [1]. Therefore, as the main factor affecting the absorption effect of the organic amine solution [2], its removal is a key link in the carbon capture process using the organic amine method. Several HSS purification technologies for the organic amine CO₂ capture process have been created both domestically and internationally, such as distillation recovery [3], adsorption [4], ion exchange, and electrodialysis [5].

The distillation recovery method regenerates the active amine in the lean amine solution by heating and distillation. Tavan Y et al. conducted an investigation into the elimination of HSS from the amine solution via vacuum distillation, and they juxtaposed the empirical industrial data with simulation outcomes derived from Aspen Hysys [6]. The results were basically consistent, indicating the effectiveness and applicability of the vacuum distillation technology in amine purification. Cai P. et al. investigated the application of vacuum distillation technology within the context of membrane distillation, successfully regenerating a non-functional methyl diethanolamine (MDEA) solution under conditions of 10.0 kPa and 65 °C [7]. The study reported a recovery rate of 97% for the MDEA solution. However, the energy consumption of distillation technology is generally high. Especially when recovering amine liquids under vacuum conditions, the high temperature and the corrosiveness of the distillation effluent will damage the equipment. Even if corrosion-resistant materials such as stainless steel are used, it is difficult to completely avoid it. Moreover, the process impurities generated are often harmful to the environment, which will increase the difficulty and cost of subsequent treatment.

The adsorption method is based on the selective adsorption of impurity molecules by adsorption materials such as activated carbon, thereby realizing the purification and separation of amine solutions. Edathil et al. successfully synthesized a magnetic alginate gel material (Ca-Fe₂O₃) and applied it to remove organic acid anions in the MDEA solution, showing applicability in specific application scenarios [8]. Yan et al. developed a synthetic material for the fixed-bed adsorption process [9]. This synthetic material can deeply purify and remove impurities in various foaming pollutants, especially the sulfate impurities abundantly present in the amine solution [10]. Compared with traditional activated carbon, these new adsorbents with unique surface structures and chemical properties have shown significant advantages in key indicators such as purification efficiency and economic cost. However, there is still a lack of detailed research and reports on the regeneration ability of adsorption materials. Furthermore, the strength of the adsorbent’s adsorption of H₂O can also cause a multiple-fold change in energy consumption. Therefore, its high energy consumption has become a significant challenge hindering its commercialization [11].

Ion-exchange technology is a technology developed based on the purification of HSSs, which can perform reversible ion exchange between the solid phase and the liquid phase. Research on the utilization of anion-exchange resins as adsorbents for the removal of HSSs from lean amine solvents remains limited. Morgan et al. described the first commercial application of ion-exchange resins in amine recovery [12]. The recycled amine solution contaminated with sodium chloride flows downward through the bed of strong basic anion-exchange resins, where chloride ions are replaced by hydroxide ions, and the resin is regenerated with the solution of 1 mol·L⁻¹. Jameh used an anion-exchange resin to remove HSSs from the solution, but did not mention the properties or composition of the anion-exchange resin used [13]. It is important to note that during the ion-exchange process, the resin may become contaminated over prolonged usage, leading to a reduction in its operational efficacy. Consequently, regular performance assessments and timely regeneration treatments are essential [14]. Nevertheless, the financial investment and scale required for the ion-regeneration process and the treatment of high salt concentrations are relatively substantial [15]. Furthermore, the diverse origins and types of HSS encountered in the CO₂ capture process significantly impair the absorption efficiency of organic amines, indicating that the existing advanced purification technologies for HSSs in organic amine solutions remain inadequate. Therefore, it is necessary to develop and propose in-depth purification technologies for HSSs to better cope with the current environmental protection and resource conservation challenges in China and promote green and sustainable development.

Electrodialysis is a continuous and efficient separation technique that leverages the osmotic selectivity of ion-exchange membranes, specifically anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) to facilitate the migration of charged ions from the raw diluting chamber to the enriched chamber when subjected to a direct current electric field. This process effectively retains uncharged components within the freshwater chamber, thereby achieving the separation of charged and uncharged species. The application of this technology is extensive, encompassing the desalination of brackish and alkaline waters, the desalination of organic products, and the treatment of saline waste liquids. Furthermore, it is currently recognized as a prominent method within the field of purification. Hong M et al. used a small electrodialysis device to remove HSSs from the aqueous alkanolamine solution when exploring the purification method of the aqueous alkanolamine solution, which not only improved the purity of the aqueous alkanolamine solution but also helped to optimize the entire production process [16]. Zhou Z et al. proposed a novel bipolar membrane electrodialysis (BMED) process to tackle the issues of low efficiency in removing heat-stable salts (HSSs) from lean amine solutions via the traditional resin exchange method, as well as the requirement for substantial amounts of alkali for regeneration. This offers guidance for the industrial application of HSS separation from desulfurization wastewater and other relevant sources [17].

Grushevenko et al. further advanced the utilization of electrodialysis technology by implementing a two-step electrodialysis procedure aimed at selectively removing sulfates from MEA solutions post CO₂ absorption [18]. This method is characterized by its systematic approach and has yielded significant results. Volkov et al. conducted a more comprehensive study, utilizing electrodialysis to eliminate HSSs from MEA with varying CO₂ loadings [19]. Their findings indicated that up to 70% of HSSs could be removed within a 30-min timeframe. This outcome not only underscored the efficacy of electrodialysis in HSS removal but also offered innovative strategies for addressing the HSS challenges associated with CO₂ absorption in alkanolamine solutions.

Table 1. Characteristics of various heat-stable salt purification technologies.

Project	Electrodialysis	Ion Exchange	Distillation Recovery	Adsorption Method
Technical principle	Removal of charged ions driven by voltage difference	Ion-exchange resin adsorbs ionic impurities	Thermal recovery of alkanolamine and water from amine solution degradation products and heat—stable salts	Adsorbent materials purify insoluble impurities
Feed requirements	Cold lean amine solution	Cold lean amine solution with solid particles and hydrocarbon impurities removed	Neutralized HSS	Pretreated cold lean amine solution
Required chemicals	An equivalent amount of ${\mathrm{R}}_3{\mathrm{N}\mathrm{H}}^{+}$$\mathrm{and} \mathrm{NaOH}$	$\mathrm{NaOH}$$\mathrm{and} {\mathrm{H}}_2{\mathrm{SO}}_4$ for resin regeneration	An equivalent amount of alkali to ${\mathrm{R}}_3{\mathrm{NH}}^{+}$	Agents for adsorbent regeneration
Types of impurities that can be purified	HSS	HSS impurities and some amides, amino acids, etc.	All non—volatile impurities	Insoluble impurities; HSS and surfactants
Amine solution recovery rate	Nearly 98%	Nearly 99%	85~95%	Nearly 99%
Cost	Moderate	Moderate	High	Relatively low
Technical advantages	No limit on the salt concentration in the raw material; low chemical consumption	Suitable for the treatment of low-concentration HSSs; low energy consumption	Capable of purifying a wide variety of pollutants with a small total amount after pollutant concentration	Simple operation, low cost, and a wide range of pollution purification
References	[5]	[12]	[3]	[4]

also summarizes the characteristics of various HSS purification technologies.

Distillation technology is characterized by high energy consumption and the adsorption method is constrained by the limited regeneration capacity of adsorbent materials. In ion-exchange processes, the regeneration of ion-exchange resins, coupled with the associated equipment costs and challenges related to handling high salt concentrations, remains relatively expensive. Consequently, these purification techniques are currently inadequate for achieving thorough removal of heat-stable salts (HSSs) in organic amine solutions. In contrast, electrodialysis, a continuous and efficient separation technique, selectively separates ionic species in aqueous solutions by applying a voltage across semi-permeable ion-exchange membranes. This paper primarily investigates the separation of HSSs (anions) from a mixed solution utilizing electrodialysis. The research explores the impact of various operating conditions, facilitated by a specially designed membrane stack, on the performance of electrodialysis separation and energy consumption. Key factors examined include operating voltage, initial concentration, initial value, current density, the volume ratio of the enriched to diluting chamber, and the characteristics of the membranes employed.

2. Materials and Methods

2.1. Electrodialysis Device

The electrodialyzer is the focal point of the electrodialysis apparatus, and Figure 1 illustrates the electrodialysis principle. Spacers divide the essential part, the ion-exchange membrane, which is alternately positioned between the positive and negative electrodes. Each membrane pair consists of one spacer, one AEM, and one CEM. The voltage for each pair of membranes is between 0.5 and 1 V. Zhejiang Lanran Technology Co., Ltd. (Hangzhou, China). supplied the desktop electrodialyzer utilized in this experiment, which was put together using the one-pole two-stage assembly method. The three pumps have a maximum flow rate of 1.8 min⁻¹ and operate under constant voltage (maximum 18 V) or constant current (maximum 3 A). The three solution tanks (diluting chamber, enriched chamber, and electrode water chamber) have a combined maximum storage capacity of 1 L. These tanks are utilized for the storage of raw solution, concentrated solution, and electrode solution, respectively. The experimental setup employed a membrane stack consisting of ten cation-exchange membranes and ten anion-exchange membranes, with electrodes measuring 47 mm × 107 mm, constructed from titanium-coated ruthenium. The homogeneous and heterogeneous membranes utilized in this study were produced by Zhejiang Lanran Technology Co., Ltd. (Hangzhou, China). Upon the application of voltage, anions migrate from the freshwater chamber to the anode through the anion-exchange membrane, where they are subsequently captured by the cation-exchange membrane located in the enriched chamber. The experiment was conducted at ambient temperature, and samples were systematically collected from both the diluting chamber and the enriched chamber for analytical testing.

2.2. Ion-Exchange Membrane

The core component of electrodialysis technology is the ion-exchange membrane, which is fabricated from a specialized polymer material containing active ion-exchange groups. The membrane is divided by specially engineered partitions and is alternately positioned between the anode and cathode end plates. Anion-exchange membranes generally contain quaternary ammonium functional groups, with R representing the base membrane structure. In contrast, cation-exchange membranes typically incorporate sulfonic acid functional groups (R-SO₃H), where R denotes the base membrane, -SO³⁻ signifies the fixed ion, and H⁺ corresponds to the dissociable ion. The characteristic functional group configurations of ion-exchange membranes are depicted in Figure 2. When an electric field is applied, only ions with specific charges are permitted to traverse the ion-exchange membrane, while other ions are effectively obstructed. This selective permeability enables the membrane to efficiently separate ions by accurately screening and allowing the passage of target ions.

Based on the macroscopic architecture of ion-exchange membranes, they can be categorized into homogeneous and heterogeneous types. Homogeneous membranes are fabricated by incorporating active functional groups into an inert matrix, resulting in a chemical structure characterized by small pores. In contrast, heterogeneous membranes are produced by blending powdered ion-exchange resin with a binder, leading to a chemically non-uniform structure. The ion-exchange channels of both homogeneous and heterogeneous membranes employed in the electrodialysis process are illustrated in Figure 3. To ensure sustained performance stability over extended periods, the ion-exchange membrane must exhibit high total diffusion resistance and robust chemical stability, thereby preventing degradation or damage from chemical exposure. The primary performance parameters of the ion-exchange membrane utilized in this study are summarized in

Table 2. Ion-exchange membrane performance parameters.

Index Name	Homogeneous Membrane		Heterogeneous Membrane
	Cation-Exchange Membrane	Anion-Exchange Membrane	Cation-Exchange Membrane	Anion-Exchange Membrane
$\mathrm{Resistance} \left(\mathsf{\Omega }\cdot \mathrm{c}{\mathrm{m}}^{-2}\right)$	3.0	2.4	$2\sim 3$	$2\sim 3$
Bursting Strength (MPa)	≥0.40	≥0.25	0.6	0.6
Temperature Resistance(°C)	5~40	5~40	15~50	15~50
$\mathrm{Resistance} \mathrm{to} \mathrm{p}\mathrm{H}$ Value	1~11	1~11	1~14	1~14
Diaphragm Thickness (mm)	0.17	0.14	$0.2\pm 0.05$	$0.2\pm 0.05$
Diaphragm Size (mm)	$550\times 1100$	$550\times 1100$	$550\times 1100$	$550\times 1100$

The performance parameters of the ion-exchange membrane were provided by Zhejiang Lanran Technology Co., Ltd.

.

Given that the performance of the ion-exchange membrane in the electrodialysis process is intricately linked to its structural characteristics, Fourier transform infrared (FTIR) spectroscopy and microscopic morphological analyses of both the cross-section and surface were conducted. These analyses aimed to assess the conditions of the membrane matrix morphology and the distribution of ion-exchange sites, specifically the resin particles. The ion-exchange membrane utilized in this study is a polymerizable composition created by impregnating an ion-exchange resin into a substrate made of polyolefin woven fabric. The FITR-infrared spectrogram is illustrated in Figure 4: the upper red line indicates the infrared peak corresponding to the AEM, while the lower black line represents the infrared peak associated with the CEM. The presence of residual hygroscopic water molecules within the unmodified resin component is evidenced by the O-H stretching vibrations and O-H in-plane scissor bending vibrations observed at the peaks of 3400 cm⁻¹ and 1600 cm⁻¹, respectively.

An SEM analysis of the ion-exchange membrane was conducted. The cross-sectional morphology of the CEM is illustrated in Figure 5, where resin particles that enhance the mechanical strength of the membrane are observed on the surface of the thick support layer, characterized by a loose porous structure, as well as within the membrane matrix. The functional groups present on the resin particles may provide conductive regions within the ion-exchange membrane, facilitating the formation of flow channels for ion transport during the electrodialysis process. The surface morphology of the AEM is depicted in Figure 6a,b. The rough surface structure of the AEM is advantageous as it can enhance water flux by increasing the likelihood of interaction between water molecules and various ions. Although Figure 6c,d do not reveal any prominent large pores, the discontinuity of the adhesive and the aggregation of resin particles contribute to the presence of numerous small pores and cracks. To provide a clearer representation of the distribution of functional groups on the membrane, a mapping test was conducted. As illustrated in Figure 7a,b, it was determined that the sulfur (S) element constituted 55% of the ion-exchange groups in the CEM, while the nitrogen (N) element accounted for 63% in the AEM.

2.3. Experimental Methods

2.3.1. Operation Method

The electrodialysis experiment was conducted utilizing a constant-voltage mode, with applied voltages of 5 V and 15 V, respectively. Upon the application of an electric field, cations and anions migrated across the ion-exchange membrane into the enriched water chamber. The electrode water chamber was filled with a solution of Na₂SO₄ at a specified concentration (1~3%), which helped to create a stable electric field and successfully stopped the electrodes from corroding and becoming contaminated. Amine solutions and salt solutions of varying concentrations were introduced into the diluting chamber and the enriched chamber, respectively, with volumes ranging from 500 to 1500 mL. The circulation flow rate was maintained at approximately 70 L h⁻¹. Each solution was circulated for a duration of 30 to 80 min, during which the changes in conductivity of both two chambers were recorded synchronously. The experiment concluded when the conductivity fell below 1 mS·cm⁻¹. All experimental procedures were conducted at ambient temperature [20].

2.3.2. Analysis and Calculation Methods

An ultraviolet spectrophotometer was employed to quantify various organic amines, specifically monoethanolamine (MEA) and methyldiethanolamine (MDEA). Anions were identified through ion chromatography. The concentration of sodium hydroxide was assessed using phenolphthalein as an indicator. Conductivity measurements were obtained with a conductivity meter. The pH of the solution was determined using a combined meter. Fluoride concentration was measured utilizing an ion-selective electrode (fluoride ion electrode) in conjunction with a standard reference electrode connected to a Metrohm ion meter. To adjust the total ionic strength, a total ionic strength adjustment buffer (TISAB) solution was prepared, consisting of 58.5 g of trisodium citrate and 85 g of sodium nitrate, with nitric acid added to achieve a pH range of 5 to 6. The chloride concentration was quantified through potentiometric titration using an automatic titrator, while the sulfate concentration was determined via gravimetric analysis.

The average flux J (mmol·(m²·s)⁻¹) was used to represent the average quantity of salt and water transferred through the ion-exchange membrane per unit time to evaluate the performance of the ion-exchange membrane [21].

$$J_{\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4}={\displaystyle \frac{C_{\mathrm{t}}{V}_{\mathrm{t}}-C_0{V}_0}{N{\mathrm{A}}_{\mathrm{t}}}}$$

(1)

$$J_{H_{2}O}={\displaystyle \frac{V_{\mathrm{t}}-V_0}{18N{A}_{\mathrm{t}}}}$$

(2)

Among the terms, V_t and V₀ refer to the volume of the solution in the diluting chamber at time t and 0; t (s) refers to the operating time; C_t and C₀ (mmol·L⁻¹) refer to the concentrations at time t and 0; A_t (0.055 m²) refers to the effective membrane area; N (N = 10) refers to the number of repeating units.

The desalination rate is one of the key indicators for measuring the performance of electrodialysis and the separation effect of ion-exchange membranes. In the electrodialysis process, it refers to the ratio of the reduced salt amount in the freshwater chamber to the salt content of the raw material, which directly reflects the desalination capacity of the electrodialysis system [22]. Its calculation formula is

$$f={\displaystyle \frac{C_{\mathrm{d}0}{V}_{\mathrm{d}0}-C_{\mathrm{d}2}{V}_{\mathrm{d}2}}{C_{\mathrm{d}0}{V}_{\mathrm{d}0}}}\times 100\mathrm{\%}$$

(3)

Among the terms, V_d0, V_d2 (L) refers to the initial and final volumes of diluting, f(%) refers to the desalination rate; C_d0, C_d2 (mol·L⁻¹) refers to the initial and final salt concentrations in the diluting chamber.

As an important parameter for evaluating the economic efficiency of the electrodialysis process, the value of power consumption directly reflects the energy consumption of the electrodialysis system during operation [23]. Its calculation formula is

$$W={\displaystyle \frac{{\int }_0^{t}UIdt}{n}}$$

(4)

Among the terms,$W$(Wh·L⁻¹) refers to the amount of energy consumed by an electrodialysis device in one hour of operation; U (V) refers to the voltage drop across the membrane in the electrodialyzer; t (h) refers to the time required for desalination; I (A) refers to the current; n (mol) refers to the molar amount of the regenerated amine solution.

The current efficiency (η, %) is calculated using the following formula [24]:

$$\eta ={\displaystyle \frac{Z\left(C_{\mathrm{t}}^{\prime }-C_0^{\prime }\right)V\mathrm{F}}{\mathrm{N}{\int }_0^{t}Idt}}\times 100\%$$

(5)

Among the terms, Z refers to the charge number of the ion; $C_0^{\prime }$ and $C_{\mathrm{t}}^{\prime }$ (mol·L⁻¹) refer to the molar concentrations at time 0 and t; F (96,485 C·mol⁻¹) is the Faraday constant; V (L) refers to the solution volume; I (A) refers to the current; N (N = 10) refers to the number of repeating units in the membrane stack.

The loss rate is calculated by the following formula [25]:

$$\mathrm{loss} \mathrm{late}={\displaystyle \frac{{\omega }_0-{\omega }_{\mathrm{t}}}{{\omega }_0}}\times 100\mathrm{\%}$$

(6)

Among the terms, ω₀ (%) refers to the initial mass fraction of the amine solution in the diluting chamber; ω_t (%) refers to the mass fraction of the amine solution in the diluting chamber at time t.

3. Results

Following the regeneration process in the desorption tower, the amine solution is found to contain a significant concentration of HSSs, which are comprised of various anions. Each anion is capable of binding to a single amine molecule, resulting in a scenario where the salt is unable to release the associated organic amine molecules during the regeneration process that involves heating. Consequently, electrodialysis technology was employed to perform desalination experiments on a simulated HSS. The initial solutions in the diluting chamber included NaCl, NaF, and Na₂SO₄. The study aimed to investigate the migration behaviors of these HSS anions during the electrodialysis experiments. Additionally, the research examined the influence of several factors on the electrodialysis separation performance and energy consumption, including operating voltage, initial concentration, initial values, current density, the volume ratio of enriched to diluting chambers, and the types of membranes used. Furthermore, the migration behavior of anions in an actual amine solution sample was validated.

3.1. Effect of Operating Voltage

In this experiment, the electrodialysis process is conducted in a constant-voltage mode, with the operating voltage set within the range of 5 to 15 volts. To optimize energy efficiency in the electrode chamber, the number of membrane stacks is increased. Figure 8 illustrates the conductivity of the freshwater chamber at various operating voltages. The data presented in the figure indicate a temporal decrease in the conductivity of the diluting chamber across different operating voltages, signifying the effective removal of salt from the freshwater solution. As cations and anions migrate towards the enriched chamber via the CEM and AEM, respectively, the concentration of salt ions in the diluting chamber diminishes, leading to a corresponding reduction in conductivity. Notably, the slope of the conductivity curve exhibits a decreasing trend over time, with a rapid decline in conductivity observed during the initial phase of the experiment, followed by a deceleration in the ion migration process in the later stages. At the same time, it can be found from the figure that the time required for the desalination experiment decreases with the increase in the operating voltage because the driving force of electrodialysis is the current field. The greater the current, the greater the driving force.

Different concentrations of ${\mathrm{SO}}_4^{2-}$ (16.9 g·L⁻¹), Cl⁻ (6.07 g·L⁻¹), and F⁻ (2.21 g·L⁻¹) were simulated and prepared, and the change laws under different voltages were discussed.

1.

Effect of operating voltage on ${\mathrm{SO}}_4^{2-}$.

Figure 9 illustrates the relationship between operating voltage and the reduction in salt concentration in the diluting chamber. As depicted in Figure 9a, an incremental increase in operating voltage correlates with a substantial decrease in salt concentration, which diminishes from an initial value of 16.9 g·L⁻¹ to below 0.5 g·L⁻¹. Furthermore, the duration required for this desalination process decreases with increasing voltage, thereby demonstrating the beneficial impact of voltage on the desalination rate. An analysis of voltage and current variations during the initial 180 min allowed for the calculation of energy consumption and current efficiency at different voltages, as presented in Figure 9b. Notably, as the applied voltage escalates from 5 V to 15 V, the energy consumption necessary for the treatment of a unit volume of ${\mathrm{SO}}_4^{2-}$ solution consistently rises, reaching a significant level of 11.2 Wh·L⁻¹ at 15 V. Additionally, the current efficiency exhibits a trend of initial increase followed by a decrease with rising operating voltage, achieving a peak current efficiency of 68.12% at 10 V.

2.

Influence of operating voltage on Cl⁻.

Figure 10 shows that as the voltage gradually increases, the time required for the chloride salt content in the diluting chamber to decrease from 6.07 g·L⁻¹ to below 0.5 g·L⁻¹ gradually shortens. At a voltage of 15 V, the energy consumption significantly increases to 7.51 Wh·L⁻¹. The current efficiency first increases and then decreases as the operating voltage increases, reaching a maximum current efficiency of 65.2% at 10 V.

3.

Influence of operating voltage on F⁻.

Figure 11 shows that as the operating voltage gradually increases, the time required for the fluoride salt content in the diluting chamber decrease from 2.21 g·L⁻¹ to below 0.5 g·L⁻¹ gradually shortens. At a voltage of 15 V, the energy consumption significantly increases to 5.2 Wh·L⁻¹. The current efficiency first increases and then decreases as the operating voltage increases, reaching a maximum current efficiency of 55.4% at 10 V.

The impact of electrodialysis on the removal of individual anions is significant. At lower voltages, the rate of desalination is comparatively slow, and energy consumption is minimal. However, as the operating voltage is elevated, the strength of the electric field increases, thereby enhancing the driving force for ion migration and mass transfer, which results in a greater number of ions traversing the ion-exchange membrane per unit time. Nonetheless, higher operating voltages necessitate increased energy to overcome resistance, leading to a corresponding rise in energy consumption. Additionally, with the increase in operating voltage, issues such as water decomposition, co-ion migration, and membrane surface scaling become more pronounced, ultimately diminishing current efficiency. Consequently, an operating voltage of 10 V has been identified as optimal.

3.2. Influence of Initial Concentration

The alteration in conductivity was investigated under an operating voltage of 10 V, with initial concentrations of ${\mathrm{SO}}_4^{2-}$, Cl⁻, and F⁻ in the diluting chamber solution set at 5 g·L⁻¹. The migration behaviors of the three ions within the combined solution were analyzed. In this phase of the experiment, heterogeneous ion-exchange membranes were utilized in the electrodialysis apparatus, which exhibit a pronounced steric hindrance effect on specific ion types due to their highly cross-linked structural characteristics. As illustrated in Figure 12a, the patterns of conductivity reduction during the migration of the three ions in the pure solution were largely consistent. However, when the ions coexisted in the same solution, Figure 12b reveals that ${\mathrm{SO}}_4^{2-}$ and Cl⁻ migrated to the enriched chambers more rapidly, while the migration of F⁻ significantly increased following the majority of ${\mathrm{SO}}_4^{2-}$ and Cl⁻ migrating to the enriched chamber. The migration of ions is driven by the electric field and concentration gradients, with divalent ions such as ${\mathrm{SO}}_4^{2-}$ occupying a greater number of ion-exchange sites compared to the monovalent Cl⁻. Consequently, the migration of ${\mathrm{SO}}_4^{2-}$ surpasses that of Cl⁻. The monovalent F⁻ ions demonstrate relatively lower removal efficiency during electrodialysis, attributed to their smaller ionic radius and higher solubility. Thus, the order of anion migration in electrodialysis utilizing heterogeneous membranes is established as ${\mathrm{SO}}_4^{2-}$ > Cl⁻ > F⁻.

3.3. Influence of the Initial pH Value

The charge, degree of ionization, and the extent of bound water associated with ions are all affected by the pH of the solution. Figure 13 illustrates the influence of pH on the migration of ${\mathrm{SO}}_4^{2-}$, Cl⁻, and F⁻. It is apparent that pH significantly impacts the migration of ${\mathrm{SO}}_4^{2-}$ and F⁻, while it does not affect the migration of Cl⁻. The removal rate of F⁻ under acidic conditions increases with rising pH, whereas the opposite trend is observed in alkaline conditions. In alkaline environments, the migration of F⁻ is markedly diminished due to competition from OH⁻ for mass transfer, which leads to the conversion of charged F⁻ into HF molecules. The maximum removal rate of ${\mathrm{SO}}_4^{2-}$ occurs at a pH of 8.

3.4. Influence of Current Density

A clearance rate of 95.7% can be attained on an industrial scale by reducing the ion concentration to 1g·L⁻¹ under four current densities, as seen in Figure 14a, which is influenced by the current effect and concentration diffusion. At a fixed operating current density, the current efficiency falls as the current density drops in the same solution as seen in Figure 14b. The current efficiency will also drop because ion migration will transport some water molecules over the ion-exchange membrane, lowering the acidity of the solution in the concentrated water chamber. The current efficiency is in opposition to the energy consumption. Energy consumption increases as current efficiency decreases.

3.5. Influence of the Volume Ratio of the Enriched/Diluting Chambers

In the electrodialysis concentration experiment, the objective is to maximize the recovery of the original solution within the diluting chamber. This is achieved by varying the proportions of high-salt water introduced into both the diluting chamber and the enriched chamber, thereby facilitating a higher concentration of the solution. It is observed that, prior to reaching the maximum concentration limit of electrodialysis, an increased volume of solution in the diluting chamber correlates with a greater recovery of the original solution. By manipulating the volume of the diluting chamber, researchers can alter the volume ratio of the solutions in both the concentrated and diluting chambers, thereby investigating the concentration effects of electrodialysis across different volume ratios. Figure 15 shows the change in concentration with time under different volume ratios of the enriched/diluting chambers. In the circulation mode, the initial concentrations of the ${\mathrm{SO}}_4^{2-}$ solutions in the concentrated and dilute chambers are both 10.16 g·L⁻¹. As the initial volume of the Na₂SO₄ solution in the dilute chamber increases continuously, the concentration change in the dilute chamber is relatively large, while that in the concentrated chamber is relatively small. The reason is that the concentration difference between the two sides of the membrane gradually increases, and the high concentration gradient leads to the reverse diffusion of ions from the enriched chamber to the diluting chamber.

3.6. Influence of the Membrane

Heterogeneous and homogeneous membranes were selected for the experiment. The migration of Cl⁻, F⁻, and ${\mathrm{SO}}_4^{2-}$ is shown in Figure 16. In the heterogeneous membrane, ${\mathrm{SO}}_4^{2-}$ has the fastest migration rate due to its high concentration, followed by Cl⁻. In addition, because the fixed groups and active ions in the heterogeneous membrane have a strong affinity and selective permeability for ${\mathrm{SO}}_4^{2-}$, it is more conducive to the migration of ${\mathrm{SO}}_4^{2-}$.

Compared with the heterogeneous membrane, in the homogeneous membrane experiment, the migration rate of ${\mathrm{SO}}_4^{2-}$ slows down, while that of Cl⁻ speeds up, indicating that the homogeneous membrane has a higher selectivity for Cl⁻. For F⁻, regardless of the type of membrane, its migration rate is the lowest. The main reason is that F⁻ has the highest hydration free energy, which hinders the migration of F⁻. Therefore, when using a heterogeneous membrane, the order of ion migration rates is ${\mathrm{SO}}_4^{2-}$ > Cl⁻ > F⁻; when using a homogeneous membrane, the order is Cl⁻ > ${\mathrm{SO}}_4^{2-}$ >F⁻.

3.7. Influence on Water Migration

Initial research on the water migration phenomenon during the electrodialysis desalination process has indicated that a portion of the water traverses the ion-exchange membrane [26,27,28]. This migration primarily occurs in the forms of osmotic water and electro-osmotic water. The predominant factor influencing the separation process in electrodialysis is electro-osmotic water [29]. This type of water movement involves the transport of certain water molecules in a hydrated state alongside ions, driven by the application of an electric field. Consequently, the effects of voltage and initial concentration on the water migration process were systematically examined. Solutions containing 25 g·L⁻¹ ${\mathrm{SO}}_4^{2-}$ were prepared in the diluting chamber, and the apparatus was operated in a cyclical manner for a duration of 120 min.

1.

Effect of voltage on water migration

As illustrated in Figure 17, the data presents the volume of water migration from the diluting chamber to the enriched chamber across various operating voltages. The findings indicate a progressive increase in water migration correlating with elevated voltage levels. Notably, a greater voltage differential results in a more pronounced increase in water migration. This phenomenon can be attributed to the fact that an increase in voltage enhances the driving force for ion migration, thereby accelerating the formation of bound water with ions and subsequently augmenting the overall volume of water migration.

2.

The influence of initial concentration on water migration

As shown in Figure 18, the water migration amount increases with the increase in the initial concentration in the diluting chamber. When the initial concentration is above 10 g·L⁻¹, the increase in the water migration amount is obvious.

3.8. Effect on Amines

To investigate the impact of HSS concentration on the depletion of amine solution, MEA solutions with HSS mass fractions of 1%, 2%, and 3% were prepared. A 30 wt.% MEA solution was simulated for comparison. The process employed constant-pressure desalination, with a voltage maintained at 10 V. The solution was circulated through the electrodialysis apparatus at a flow rate of 1.16 L·min⁻¹ for a duration of 180 min. Samples from both the chambers were collected at regular intervals for analysis.

The removal rates of anions and the concentration of amine were subsequently calculated, with amine loss determined based on the amine content in the diluting chamber. As indicated in

Table 3. Mobility and MEA amine loss for different salt concentrations.

Salt Concentration (%)	Salt Concentration at the End (%)	Removal Rate (%)	Amine Concentration at the End (%)	Amine Loss (%)
1	0.19	81.0	30.25	16.70
2	0.43	78.5	30.47	17.13
3	0.77	74.3	31.04	19.25

, a significant increase in amine loss was observed with rising salt concentrations, suggesting that the HSS impede the purification process of the amine solution and adversely affect its recyclability.

The depletion of MDEA is also influenced by both electro-osmosis and concentration-gradient osmosis. Given that the initial amine originates from a lean amine solution, the removal of HSSs from the solution during the electrodialysis process inevitably leads to the diffusion of amine molecules from the diluting chamber to the enriched chamber. Consequently, this study primarily focused on the impact of the MDEA amine concentration gradient on the electrodialysis process. The findings indicate that the migration effect is optimized at a concentration of 30 wt.%, resulting in a reduced loss of amine. This observation is corroborated by the data presented in

Table 4. Ion mobility and amine loss at different MDEA amine concentrations.

MDEA Concentration (%)	Cl− Mobility (%)	${\mathbf{S}\mathbf{O}}_4^{2-}$ Mobility (%)	Amine Concentration at the End (%)	Amine Loss (%)
30	93.84	85.15	21.34	20.15
40	90.15	82.47	40.12	26.60
50	88.27	79.27	50.27	26.40

; it is consistent with the concentration of acidic gas removed by MDEA.

4. Industrial Case Analysis

The electrodialysis technique was employed to investigate and assess a genuine sample of the lean amine solution sourced from the sodium nitrite production facility of Jinchang Company. The results of the experiments are presented in

Table 5. Comparison of electrodialysis results of lean amine solution.

Inspection Items	Before Desalination	After Desalination	Removal Rate (%)	Inspection Standards
Amine concentration (%)	30	32.48	-	GB/T31589-2015 [30]
Amine loss (%)	-	18.7	-	-
Thermally stable salt content (%) (standard content < 1%)	6.24	0.65	89.6%	GB/T9725-2007 [31]
pH	3.68	3.63	-	GB/T6920-2019 [32]
Conductivity (mS·cm−1)	53.3	1.32	-	GB/T11007-2008 [33]
Fluoride (mg·L−1) (standard content < 10)	18.4	1.54	91.6%	SY/T7001-2014 [34]
Chloride (mg·L−1) (standard content < 500)	196.7	46.47	76.4%	SY/T7001-2014 [34]
Sulfate (mg·L−1) (standard content < 500)	92,680	75.82	99%	SY/T7001-2014 [34]
Sodium ion (mg·L−1) (empirical requirement < 500)	15,175	448	97%	SY/T7001-2014 [34]
Appearance	Yellowish-green	Pale yellow	Obvious	Visual inspection

. Prior to the purification process utilizing the electrodialysis apparatus, the effective loading of the amine solution was relatively low, while its HSS content was notably high. During the purification process, there was a significant reduction in the HSS content of the amine solution, indicating a considerable increase in effective loading. The purification effect likewise attained the level of a new absorbent and was readily apparent from appearance.

5. Conclusions

An experimental investigation was carried out to examine the process of electrodialysis desalination in conjunction with lean amine solutions. The study involved the simulation of salt-laden solutions as well as the analysis of actual lean amine solution samples. Various parameters were assessed, including operating voltage, concentration of raw materials, value, current density, the volume ratio of concentrated to dilute chambers, and the characteristics of the membranes, all of which influence the electrodialysis regeneration process. The findings indicated the following:

1.

An operating voltage of 10 V was identified as the optimal condition in terms of energy consumption and current efficiency. Under equivalent concentration conditions, Cl⁻ and ${\mathrm{SO}}_4^{2-}$ exhibited preferential migration. The migration of Cl⁻ was found to be independent of the value, while the migration of F⁻ peaked at a value range of 4 to 6. The migration of ${\mathrm{SO}}_4^{2-}$ reached its maximum at a value of 8. Current efficiencies for ${\mathrm{SO}}_4^{2-}$, Cl⁻, and F⁻ under high current density conditions were recorded at 68.12%, 65.2%, and 55.4%, respectively. Correspondingly, the energy consumptions at the limiting voltage were measured at 11.2 Wh·L⁻¹, 7.51 Wh·L⁻¹, and 5.2 Wh·L⁻¹.

2.

Water permeation often leads to a decrease in current efficiency. The water migration amount gradually increases with the increase in voltage and the initial concentration of the freshwater chamber. The concentration of the diluting chamber varies greatly under different volume ratios of enriched/diluting chambers, and the ion removal rate in the freshwater chamber is greater than 98%. The order of ion migration rates for homogeneous membranes is Cl⁻ > ${\mathrm{SO}}_4^{2-}$ >F⁻, and the order of ion migration rates for heterogeneous membranes is ${\mathrm{SO}}_4^{2-}$ > Cl⁻ > F⁻.

3.

Thermally stable salts play a role in deactivating and contributing to the losses of organic amine solutions during the carbon dioxide capture process. Research indicates that the electrodialysis method leads to an amine loss exceeding 15%. Among the various formulations, a 30 wt.% MEA lean amine solution exhibits the lowest amine loss at 16.7%, and its regeneration concentration is the most closely aligned with the concentration of the removed acid gas.

As a result, electrodialysis technology can efficiently extract sulfates from organic amines in addition to purifying and regenerating the lean amine solution. However, more tests are required to confirm the economic costs and operating impacts.