The research on electrodialysis technology began in Germany. In 1903, Morse and Pierce placed the two electrodes inside the dialysis bag and the external solution, and found that the charged impurities could be quickly removed from the gel. In 1924, Pauli used a chemical design Principle, improved Morse's experimental device, trying to reduce polarization and increase mass transfer rate. But it wasn't until Juda's first trial production of highly selective ion-exchange membranes in 1950 that the electrodialysis technology entered a practical stage, which experienced three major innovations: the application of selective ion-exchange membranes; the design of multi-compartment electricity Dialysis component; adopts frequent inverted operation mode. With the continuous innovation and improvement of the performance of various aspects of the ion exchange membrane and the structure of the electrodialysis device, the electrodialysis technology has entered a new stage of development, and its application prospects are also broader.
Electrodialysis (eletrodialysis, abbreviated as ED) technology is a combination of electrochemical process and dialysis diffusion process. Driven by an external DC electric field, the potential difference is used as the driving force, and the electrolyte is separated from the solution using the selective permeability of the ion exchange membrane When it comes out, the anions and cations move to the anode and the cathode, respectively, so as to achieve the purposes of concentration, desalination, refining and purification of the solution.
The electrodialysis device mainly includes an electrodialyzer body and auxiliary equipment. The electrodialyzer body is composed of a membrane stack, a pole region and a clamping device. Auxiliary equipment refers to various material-liquid tanks, water pumps, DC power supplies, and water pretreatment equipment.
Ion exchange membranes in electrodialysis can be divided into two types: cation exchange membranes (cationic membranes) and anion exchange membranes (anion membranes). The selective permeability of the ion exchange membrane is mainly manifested in that the cation membrane allows cations to pass through and repels blocking anions, and the anion membrane allows anions to pass through and repels blocking cations. The ion exchange membrane does not need to be regenerated.
According to the type of ion selective membrane used and the mode of operation, the electrically driven membrane separation process can be divided into ordinary electrodialysis (CED), bipolar membrane electrodialysis (BMED), electrolytic electrodialysis (EED), and selectivity. Electrodialysis (SED), packed bed electrodialysis (EDI), etc. Among them, ordinary electrodialysis and bipolar membrane electrodialysis are two common membrane separation processes in the electric drive process. The process that can achieve the desalination and concentration of the target solution is called CED; the process that uses bipolar membranes and ordinary anion / cation exchange membranes to achieve the target material liquid acid and alkali production is called BMED; the use of a multivalent ion The ability of selective ions to selectively pass through the membrane to achieve separation between ions with different charge numbers in the solution is called SED; an anion / cation exchange membrane with acid and alkali resistance is used, which can produce acid and alkali through electrode reaction The process is called EED; the process of using ordinary anion / cation exchange membranes and ion exchange resins that assist in ion migration to achieve solution desalination to obtain ultrapure water is called EDI.
Ordinary electrodialysis uses ordinary anion / cation exchange membranes. Under the action of an electric field, ions in the solution migrate. Ordinary electrodialysis membrane stacks are generally composed of ordinary anion and cation exchange membranes, electrodes at both ends, flow channel grids and sealing gaskets. After an electric field is applied, the cations in the desalination chamber move toward the cathode through the cation exchange membrane under the action of the positive electrode, and are blocked by the anion exchange membrane in the adjacent compartment; the anions in the desalination chamber pass through the action of the negative electrode. The anion exchange membrane moves towards the positive electrode, while being blocked by the cation exchange membrane of the adjacent compartment, and the aggregation occurs in the concentration chamber, respectively, so that the desalting and concentration of the solution are realized.
The operating modes of electrodialysis can be constant current, constant voltage and pulsed current. The constant current operation can maintain the stable rate of ions in the solution for mass transfer, but it is easy to reach the limit current density during the experiment, and the hydrolytic separation occurs on the surface of the membrane. When the constant voltage operation is adopted, the current at both ends of the membrane stack follows the membrane stack. The resistance increases and decreases, so it is not easy to reach the limit current density during operation, but the mass transfer rate is slower during the process, and the operation time is longer compared to the constant current process; the pulsed electrodialysis operation is mainly used to reduce the electrodialysis process Membrane fouling, especially when handling organic feed-liquid systems that tend to cause membrane fouling.
Bipolar membrane electrodialysis is different from ordinary electrodialysis. It uses a new type of ion exchange membrane-bipolar membrane that can catalyze hydrolysis and ionization online. Bipolar membranes are generally composed of cation exchange membrane layer, intermediate hydrolysis ionization catalyst layer and anion exchange. The membrane is superimposed. Under the action of the electric field, the bipolar membrane can hydrolyze in the hydrolytic ionization catalyst layer, so that the water molecules are converted online to H + and OH-, and are driven through the cation exchange membrane layer by the electric field. And anion exchange membrane layer, and migrated to the solution on both sides in contact with the bipolar membrane.
The theoretical energy consumption of bipolar membrane hydrolysis is much lower than that of the electrolytic process, and its rate is 5 × 107 times. Therefore, the bipolar membrane has been widely used, especially in the fields of biological engineering, food engineering and environmental protection. There are many theoretical models of bipolar membrane hydrolysis, among which the second Wien effect, and the chemical reaction model of protonation and deprotonation between weak acids and weak base groups are reported and cited. Among them, the second Wien effect emphasizes that the hydrolysis of bipolar membranes is promoted by the electric field. The rate of hydrolysis of bipolar membranes is directly related to the applied electric field. The chemical reaction model emphasizes the existence of weak acid and weak base groups. The importance of reversible protonation and deprotonation reactions for hydrolysis.
Packed bed electrodialysis (also known as electro-deionization technology, EDI for short) is a membrane separation technology combining electrodialysis and ion exchange. Usually, the anion and cation exchange resins are filled in the fresh room partition of the electrodialyser and combined with ion exchange resin. And ion exchange membrane, a new water treatment technology to realize the deionization process under the action of DC electric field. This technology combines the advantages of conventional electrodialysis with continuous operation and ion exchange deep desalination, concentrating the advantages of electrodialysis and ion exchange methods, while overcoming the prone to concentration polarization in the conventional electrodialysis process, and the need for regenerated resin and intermittent ion exchange. Disadvantages of operation. At present, this technology is mainly used in high-purity water (preparation of ultrapure water). With the further development of packed bed electrodialysis technology, it will help promote the application of this technology in a wider range of fields.
Packed bed electrodialysis equipment generally consists of ion exchange membranes, separators, electrodes, clamping devices, solution flow channels and pipelines. The membrane stack is composed of alternating thin and thick chambers, which is different from conventional electrodialysis. It is an exchange medium filled with anions and cations, such as ion exchange resins, ion exchange fibers, and inorganic ion exchangers. Its working principle includes ion migration and resin regeneration, that is, under the action of a DC electric field, positive and negative ions in the fresh water move along the channels formed by the resin bed and the ion exchange membrane to the negative and positive directions, respectively, and the positive and negative ions pass through the positive and negative ions, respectively. The anion exchange membrane enters the adjacent thick chamber, thereby generating fresh water and concentrated water. The filling of the exchange resin particles increases the conductivity of the electrodialysis lightening chamber, thereby reducing the polarization phenomenon and improving the deionization ability, and the quality of the effluent water is improved. When the limit current is exceeded, the interface layer near the membrane and the resin is polarized to ionize water into H + and OH-, which can be adsorbed by mixed anion and cation exchange resins, respectively, and replace the electrolyte ions adsorbed by the resin, so that the resin is obtained. regeneration.
Low energy consumption: no phase change in the electrodialysis process, low operating costs, low power consumption, and significant economic benefits; operation at room temperature, especially suitable for heat-sensitive systems, product quality is more stable; regeneration also consumes electricity, no acid or alkali Save material costs.
Long service life: The device has simple pretreatment process and durable equipment; the dedicated membrane and electrode can be used for more than 5 years, and the separator can be used for more than 10 years; and it is easy to operate and maintain.
Strong anti-pollution ability: Because electrodialysis is not a filtering type, it has a strong anti-pollution ability, and the quality of raw water is relatively low.
Flexible device design: The desalination rate and raw water recovery rate of the system device can be flexibly designed according to requirements. The desalination rate can reach 30% to 99% and the recovery rate can reach 40% to 90%. The entire system is simple to operate and easy to realize mechanization and automation. Control; small space, omitted mixing bed and regeneration device; simple operation, low noise. The water production is continuous and stable, and the effluent quality is high.
Low environmental pollution: During operation, the process is clean and does not require frequent regeneration with acids, alkalis or other agents; no high-pressure operation is required, which avoids the impact of noise on the environment; significant environmental protection benefits and high operational safety.
Due to its excellent desalination and concentration properties, it is widely used in the fields of seawater desalination, sewage treatment, environmental protection and industrial production. Pressure-driven reverse osmosis, electrically driven electrodialysis, and thermally driven pervaporation are all effective methods. The electrodialysis desalination process using electricity has been widely used due to its advantages such as low energy consumption, high water recovery, long membrane life, and low membrane pollution. The cost of electrodialysis seawater desalination can be as low as 1.2 yuan / ton, which has a strong competitive advantage.
Another feature of electrodialysis is that the effective components in wastewater or materials can be separated and purified, which is mainly reflected in the application of bipolar membrane electrodialysis / ordinary electrodialysis to organic acid production. The traditional organic acid production and purification process has a long route, which requires a large amount of acid and alkali to be consumed, and a large amount of waste residue is also generated in the process. This not only has high operating costs, but also causes serious environmental pollution. Therefore, by utilizing the characteristics of bipolar membrane electrodialysis that can produce acid and alkali on-line, the fermentation broth is passed to the bipolar membrane electrodialysis membrane reactor, and the organic acid salt can be converted online into by-products of organic acid and alkali. It is reused until the pH of the alcohol solution is adjusted. At the same time, the unfermented components such as bacteria, protein and glucose in the fermentation broth do not move in the electric field, so they are retained in the fermentation broth. High, the entire process does not require the introduction of additional acids and alkalis to efficiently extract organic acids. On the other hand, because the bipolar membrane electrodialysis can realize the on-line removal of organic acids in the fermentation broth, the inhibitory effect of the accumulation of organic acids on the fermentation process is avoided.