ELECTRODIALYSIS

Electrodialysis is a separation process that offers several features and benefits, some of which are listed below:

• Selective removal or concentration of ions: Electrodialysis can selectively remove or concentrate specific ions from a solution. This makes it useful for a variety of applications, such as desalination of water, recovery of valuable metals from industrial waste streams, and concentration of food and beverage products.
• Scalability: Electrodialysis can be easily scaled up or down to accommodate different flow rates and volumes. This makes it suitable for both small and large-scale applications.
• Low energy consumption: Electrodialysis typically requires less energy than other separation processes, such as reverse osmosis. This can result in lower operating costs and reduced environmental impact.
• High water recovery: Electrodialysis can achieve high water recovery rates, which means less water is wasted during the separation process.
• Continuous operation: Electrodialysis can operate continuously, which makes it suitable for industrial applications that require a constant supply of purified or concentrated products.
• Modular design: Electrodialysis systems can be designed in a modular fashion, allowing for easy maintenance and repair.
• Minimal chemical usage: Electrodialysis requires minimal chemical usage, making it a more environmentally friendly alternative to other separation processes that rely on chemicals.

Overall, electrodialysis is a versatile separation process that offers several advantages over other separation processes. Its scalability, low energy consumption, and ability to selectively remove or concentrate specific ions make it a valuable tool for a variety of industrial applications.

Variations of Electrodialysis (ED)

There are several variations of Electrodialysis (ED), which include:

• Electrodialysis Reversal (EDR): This technique is similar to ED, but with periodic reversal of the direction of the electric field. EDR can help prevent fouling of the membranes and improve the efficiency of ion removal.
• Bipolar Membrane Electrodialysis (BMED): In this technique, a special type of membrane called a bipolar membrane is used, which has both an anion-selective and a cation-selective layer. When an electric field is applied, the bipolar membrane generates a pH gradient, which can be used to separate and concentrate acid and base streams.
• Electrodialysis Metathesis (EDM): This process combines Electrodialysis (ED) with Metathesis reactions. In this technique, an ED stack is used to separate ions from a solution, and then metathesis reactions are used to exchange ions between the separated solutions, resulting in the production of new compounds.
• Reverse Electrodialysis (RED): In RED, a series of alternating cation- and anion-exchange membranes are used to generate a potential difference across the stack. The potential difference can be used to extract energy from solutions of different salinities, such as seawater and fresh water.

Overall, the different variations of ED can be applied to various separation and purification processes, including desalination, wastewater treatment, food and pharmaceutical production, and the extraction of biomolecules.

Electrodialysis Reversal (EDR)

EDR is similar to electrodialysis (ED), but it is a more advanced version that uses a periodic reversal of the electrical potential to maintain performance. The periodic reversal of the electrical potential in EDR helps to prevent fouling of the membranes and improve the efficiency of the process. It also allows for the removal of scaling, which is a common issue with other desalination processes. EDR is capable of removing a wide range of dissolved ions, including sodium, chloride, and sulfate, among others.
EDR is commonly used in industries that require high-purity water, such as the semiconductor, pharmaceutical, and power generation industries. It can also be used for desalination of brackish water and seawater.

The Benefits of Electrodialysis for Desalination as Compared to Reverse Osmosis

Electro Dialysis Reversal (EDR) and Reverse Osmosis (RO) are two commonly used technologies for desalination. Reverse osmosis (RO) and Electro Dialysis Reversal (EDR) are two of the most commonly used membrane desalination technologies. Each technology has its own unique advantages and applications, and they should not be viewed as competing processes. However, the relative economics of each system will depend on various factors, such as water chemistry, process design, and site requirements. Here are some benefits of EDR compared to RO:

• Lower energy consumption: EDR operates at a lower pressure (less than 4 bar) compared to RO (typically 60-70 bar), resulting in lower energy consumption.
• Higher water recovery rate: EDR can achieve a higher water recovery rate than RO, as it can recover up to 95% of the feed water compared to RO, which typically recovers around 70-75%.
• Reduced fouling: EDR has a periodic reversal of the electrical potential, which helps to prevent fouling of the membranes, improving the efficiency and lifespan of the system. In contrast, RO is prone to fouling and scaling, which can reduce its efficiency and lifespan.
• Reduced maintenance: EDR modules are relatively easy to maintain, as they do not require the high-pressure pumps, valves, and piping that are required for RO systems. Additionally, the cleaning process does not affect the water quality or flow rate.
• Minimal waste production: EDR produces less brine and waste compared to RO, making it an environmentally friendly option.
• Greater flexibility: EDR is tunable, meaning it can be adjusted to varying feed water quality, resulting in energy savings. This is not the case for RO, which requires a constant high pressure to deliver consistent quality.

Overall, EDR offers several benefits over RO, including lower energy consumption, reduced fouling, reduced maintenance, and minimal waste production.

Electrodialysis Metathesis (EDM)

Recently, electrodialysis metathesis (EDM) has become a powerful technology for obtaining high-value salt products directly from two feed salts containing cations and anions. EDM is a variant of electrodialysis that uses cation exchange membranes (CEMs) and anion exchange membranes (AEMs) to desalinate two feed streams and concentrate two product streams simultaneously. The cations and anions from the different feed streams can be rearranged into the required substances in the concentrate chambers.
Unlike conventional metathesis reactions, EDM is not an equilibrium process and eliminates the need to separate products from the reaction mixture. The metathesis reactions between any two electrolytes can be accomplished in the EDM cell, even those that cannot occur spontaneously according to the laws of thermodynamics.
EDM is an ultra-high recovery desalination technology that pairs dissolved minerals in feed water with different partners to obtain two concentrated solutions of highly soluble salts (usually one mixed sodium salt and one mixed chloride salt).
EDM has been established to produce chloride-free potash fertilizers (such as K2SO4, KNO3, KH2PO4, etc.) from KCl and other salts containing corresponding anions. Recent studies has shown the use of EDM as a promising possibility for the conversion of lithium resources.

Applications of Electrodialysis Metathesis (EDM)

Electrodialysis metathesis (EDM) has several applications in various industries. Here are some of them:

• Desalination: EDM is an ultra-high recovery desalination technology that can produce two concentrated solutions of highly soluble salts from feedwater. It can effectively remove dissolved minerals from brackish water and seawater, making it suitable for use in water treatment plants.
• Production of high-value salt products: EDM can produce high-value salt products directly from two feed salts containing corresponding cations and anions. This process can be used in the production of various salts such as potassium sulfate, potassium nitrate, and potassium hydrogen phosphate.
• Lithium recovery: EDM has the potential to extract lithium from brines and other sources by rearranging cations and anions from different feed streams. It can convert lithium chloride to other valuable lithium compounds.
• Waste treatment: EDM can treat waste streams containing different types of salts and metals, converting them into valuable products such as fertilizers.
• Food processing: EDM can be used in food processing to extract salt from food products or to concentrate salt solutions.

Overall, EDM has a broad range of applications and can be used in various industries, including water treatment, chemical production, food processing, and waste treatment.

Bipolar Membrane Electrodialysis (BMED)

Bipolar membrane electrodialysis (BMED) is an innovative technique for membrane separation that employs electrodialysis (ED) via a bipolar membrane (BPM). By applying an electric field, H2O can be dissociated into H+ and OH− ions, allowing for the recovery of anions and cations in the solution as acids and bases, respectively, without the need for additional chemical reagents. This feature minimizes both the costs and carbon footprint associated with the process, and enables easy operation and high efficiency. The use of BMED is growing in popularity and showing great potential, making it a subject of considerable research interest.

A common cell configuration used in BMED is the three-compartment cell design, which consists of a bipolar membrane (BPM), an anion exchange membrane (AEM) facing the cathode electrode layer of the BPM, and a cation exchange membrane (CEM) facing the anode electrode layer of the BPM. The advantage of using a three-compartment cell is that it allows for the separation of salt from the acid and base streams produced, thereby enabling the production (or recovery) of two high-purity product streams, one acid and one base.
Another possible configuration for the BMED cell is a two-compartment design, which is particularly useful for handling salts of weak acids or bases. In these cases, the corresponding acid or base solution has poor electrical conductivity, making it impractical to use a three-compartment cell. The two-compartment cell also has the advantage of lower capital and operational costs, requiring less membrane area per repeating unit, and having a less complex process design with only two outlet streams.

How Does Bipolar Membrane Electrodialysis (BMED) Work?

In a typical three-chamber BMED system, a salt solution, referred to as MX, is passed through a salt chamber between the AEM and CEM arrangement, where it is ionized into H+ and X–. In this process, water molecules diffuse through the ion exchange membranes of BPM into the catalytic layer, leading to water dissociation, where H+ ions cross the CEM and react with anions that have migrated from the salt chamber to form the corresponding acid in the acid chamber. Similarly, OH– ions migrate into the alkali chamber with cations from the salt chamber, forming the related alkali. The simultaneous generation of acids and alkalis from salt solutions is the main application of BMED, and the resulting products are usually more valuable compared to salts.

A Comparison Between Bipolar Membrane Electrodialysis (BMED) and Electrolysis Based Membrane Processes

Compared to the membrane electrolysis, BMED is more energy-efficient. The theoretical energy requirement is about 40% of that for water electrolysis with gas evolution, and the stack can be built up from repeating units, reducing unit costs. However, operational costs are increased due to the periodic replacement of the costly BPM membrane. The efficiency of hydroxyl ion and proton generation is limited by BPM performance, and the amount of gaseous byproducts (H2, O2) is significantly reduced since they are not formed in the BPM. The salt leakage through the BPM can lead to contamination of the acid/base product streams with salt ions (estimated at 5%).
Overall, BMED is an effective process for salt splitting and weak acid/base recovery, with economically interesting applications in integrated processes that allow for simultaneous acid/base production.