【Daily Question】Chemical Engineering Principles 305: Ion Exchanger Sequence (January 25)

If the raw water first passes through a strong-base anion exchanger, precipitates such as calcium carbonate, magnesium hydroxide, and iron hydroxide adhere to the surface of the resin, making it difficult to elute them； If it is placed after a strong acid cation exchanger, then the only cations that reach the anion exchanger are H+, making the solution acidic. This reduces the effect of counterions and allows the reaction to proceed more thoroughly. Therefore, cation exchangers are generally installed in front of anion exchangers.

If the raw water first passes through a strong-base anion exchanger, precipitates such as calcium carbonate, magnesium hydroxide, and iron hydroxide adhere to the surface of the resin, making it difficult to elute them； If it is placed after a strong acid cation exchanger, then the only cations that reach the anion exchanger are H+, making the solution acidic. This reduces the effect of counterions and allows the reaction to proceed more thoroughly. Therefore, cation exchangers are generally placed in front of anion exchangers.

Why is the cation exchanger placed in front of the anion exchanger? Answer: 1) When anionic exchange resin becomes ineffective and needs to be regenerated, NaOH is used for this purpose. If the anion bed is located at the front, the OH- ions in the regenerating agent get adsorbed onto the anion resin during regeneration. During operation, these ions react with cations such as Ca2+, Mg2+, and Fe3+ present in water, resulting in the formation of precipitates like Ca(OH)2, Mg(OH)2, Fe(OH)3, and Ca(HSiO3)2. These precipitates adhere to the surface of the anion resin, blocking it and contaminating it, thereby preventing further ion exchange. Moreover, they are difficult to remove. 2) The exchange capacity of anion exchange resins is much lower than that of cation exchange resins, and they are also highly susceptible to contamination by organic substances. Therefore, if the anion bed is placed before the cation bed, it will be more prone to organic contamination, resulting in an even lower exchange capacity, which is detrimental to desalinated water treatment. 3) One of the most difficult aspects in desalinated water treatment is the removal of silicate ions HSiO3- from the water, which is accomplished using strong alkaline anion exchange resins. However, the silicate ion HSiO3- exists as the salt form NaHSiO3 in alkaline water, whereas in acidic water it exists in the form of silicic acid (H2SiO3). Strong-base anion exchange resins have a much greater capacity to exchange silicic acid than silicates; therefore, exchange is best carried out in acidic water. The effluent from cation exchange columns is acidic, so placing the anion bed after the cation bed is highly beneficial for removing silicate ions from the water. 4) The exchange reaction of ion exchange resins is reversible. This is an anti-ion effect; therefore, a strong exchange potential is required for ion exchange to proceed smoothly. Strong acid cation resins with high exchange capacity are placed in the first stage; the H+ ions exchanged out rapidly react with the anions in water to form inorganic acids. Subsequently, the OH- ions exchanged out by the anion resins cause H+ and OH- to combine to form water, thereby eliminating the influence of counterions, which is highly beneficial for the anion exchange reaction. 5) The acidic effluent from the cation exchanger can neutralize the alkalinity (HCO3-) in the water; the resulting H2CO3 can be removed through a decarburizer. Therefore, the cation exchanger can reduce the load on the anion exchanger in advance.

Why is the cation exchanger placed in front of the anion exchanger? Answer: 1) When anionic exchange resin becomes ineffective and needs to be regenerated, NaOH is used for this purpose. If the anion bed is located at the front, the OH- ions in the regenerating agent get adsorbed onto the anion resin during regeneration. During operation, these ions react with cations such as Ca2+, Mg2+, and Fe3+ present in water, resulting in the formation of precipitates like Ca(OH)2, Mg(OH)2, Fe(OH)3, and Ca(HSiO3)2. These precipitates adhere to the surface of the anion resin, blocking it and contaminating it, thereby preventing further ion exchange. Moreover, they are difficult to remove. 2) The exchange capacity of anion exchange resins is much lower than that of cation exchange resins, and they are also highly susceptible to contamination by organic substances. Therefore, if the anion bed is placed before the cation bed, it will be more prone to organic contamination, resulting in an even lower exchange capacity, which is detrimental to desalinated water treatment. 3) One of the most difficult aspects in desalinated water treatment is the removal of silicate ions HSiO3- from the water, which is accomplished using strong alkaline anion exchange resins. However, the silicate ion HSiO3- exists as the salt form NaHSiO3 in alkaline water, whereas in acidic water it exists in the form of silicic acid (H2SiO3). Strong-base anion exchange resins have a much greater capacity to exchange silicic acid than silicates; therefore, exchange is best carried out in acidic water. The effluent from cation exchange columns is acidic, so placing the anion bed after the cation bed is highly beneficial for removing silicate ions from the water. 4) The exchange reaction of ion exchange resins is reversible. This is an anti-ion effect; therefore, a strong exchange potential is required for ion exchange to proceed smoothly. Strong acid cation resins with high exchange capacity are placed in the first stage; the H+ ions exchanged out rapidly react with the anions in water to form inorganic acids. Subsequently, the OH- ions exchanged out by the anion resins cause H+ and OH- to combine to form water, thereby eliminating the influence of counterions, which is highly beneficial for the anion exchange reaction. 5) The acidic effluent from the cation exchanger can neutralize the alkalinity (HCO3-) in the water; the resulting H2CO3 can be removed through a decarburizer. Therefore, the cation exchanger can reduce the load on the anion exchanger in advance.

If the raw water first passes through a strong-base anion exchanger, precipitates such as calcium carbonate, magnesium hydroxide, and iron hydroxide adhere to the surface of the resin, making it difficult to elute them； If it is placed after a strong acid cation exchanger, then the only cations that reach the anion exchanger are H+, making the solution acidic. This reduces the effect of counterions and allows the reaction to proceed more thoroughly. Therefore, cation exchangers are generally placed in front of anion exchangers.

If the raw water first passes through a strong-base anion exchanger, precipitates such as calcium carbonate, magnesium hydroxide, and iron hydroxide adhere to the surface of the resin, making it difficult to elute them； If it is placed after a strong acid cation exchanger, the cations that enter the anion exchanger are essentially only H+, making the solution acidic. This reduces the effect of counterions and allows the reaction to proceed more thoroughly. That’s why cation exchangers are generally placed in front of anion exchangers.

If the anion exchanger is placed first, the cations in water (such as Ca2+, Mg2+, etc.) will combine with OH- ions to form precipitates, which can clog the equipment.

1) When anion exchange resin becomes ineffective and needs to be regenerated, NaOH is used for this purpose. If the anion bed is located at the front, the OH- ions in the regenerating agent get adsorbed onto the anion resin during regeneration. During operation, these ions react with cations such as Ca2+, Mg2+, and Fe3+ present in water, resulting in the formation of precipitates such as Ca(OH)2, Mg(OH)2, Fe(OH)3, and Ca(HSiO3)2. These precipitates adhere to the surface of the anion resin, blocking it and contaminating it, thereby preventing further ion exchange. Moreover, they are difficult to remove. 2) The exchange capacity of anion exchange resins is much lower than that of cation exchange resins, and they are also highly susceptible to contamination by organic substances. Therefore, if the anion bed is placed before the cation bed, it will be more prone to organic contamination, resulting in an even lower exchange capacity, which is detrimental to the treatment of deionized water. 3) One of the most difficult aspects in desalinated water treatment is the removal of silicate ions HSiO3- from the water, which is accomplished using strong alkaline anion exchange resins. However, the silicate ion HSiO3- exists as the salt form NaHSiO3 in alkaline water, whereas in acidic water it exists in the form of silicic acid (H2SiO3). Strong-base anion exchange resins have a much greater capacity to exchange silicic acid than silicates; therefore, exchange is best carried out in acidic water. The effluent from cation exchange columns is acidic, so placing the anion bed after the cation bed is highly advantageous for removing silicate ions from the water. 4) The exchange reaction of ion exchange resins is reversible. This is an anti-ion effect; therefore, a strong exchange potential is required for ion exchange to proceed smoothly. Strong acid cation resins with high exchange capacity are placed in the first stage; the H+ ions exchanged out quickly react with the anions in water to form inorganic acids. Subsequently, the OH- ions exchanged out by the anion resins cause H+ and OH- to combine to form water, thereby eliminating the influence of counterions, which is highly beneficial for the anion exchange reaction. 5) The acidic effluent from the cation exchanger can neutralize the alkalinity (HCO3-) in the water; the resulting H2CO3 can be removed using a decarburizer. Therefore, the cation exchanger can reduce the load on the anion exchanger in advance.

If the raw water first passes through a strong-base anion exchanger, precipitates such as calcium carbonate, magnesium hydroxide, and iron hydroxide adhere to the surface of the resin, making it difficult to elute them； If it is placed after a strong acid cation exchanger, then the only cations that reach the anion exchanger are H+, making the solution acidic. This reduces the effect of counterions and allows the reaction to proceed more thoroughly. Therefore, cation exchangers are generally placed in front of anion exchangers

If the raw water first passes through a strong-base anion exchanger, precipitates such as calcium carbonate, magnesium hydroxide, and iron hydroxide adhere to the surface of the resin, making it difficult to elute them； If it is placed after a strong acid cation exchanger, then the only cations that reach the anion exchanger are H+, making the solution acidic. This reduces the effect of counterions and allows the reaction to proceed more thoroughly. Therefore, cation exchangers are generally placed in front of anion exchangers.