【Daily Question】Chemical Engineering Principles 306: Exchanger Sequence (January 26)
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This post was last edited by “Back to Cambridge” on February 5, 2016, at 17:08. Starting today, the Chemical Engineering Theory section is launching the “Question of the Day” activity, aimed at helping everyone reinforce their basic knowledge in chemical engineering. Subsequently, series such as “Fundamentals of Chemical Engineering”, “Mass Transfer and Separation”, “Chemical Thermodynamics”, and “Chemical Process Technology” will be introduced. We hope for your active participation! Wishing you all a Merry Christmas~~ For replies to the “Question of the Day” activity, answers can be viewed directly; however, the thread will be closed after one day! ! To encourage continued participation from everyone this year! You get 3 wealth rewards just for participating, and an additional 4 rewards for correct answers~~~ Short answer question: Why is the cation exchanger placed before the anion exchanger? Answer: 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 only cations that reach the anion exchanger are H+, resulting in an acidic solution. This reduces the effect of counterions and enables the reaction to proceed more thoroughly. That’s why cation exchangers are generally placed in front of anion exchangers.
If passed through a strong alkali anion exchanger first, precipitates such as calcium carbonate, magnesium hydroxide, and iron hydroxide adhere to the surface of the resin and are difficult to elute； If it is placed after a strong acid cation exchanger, the only cations that reach the anion exchanger are H+, resulting in an acidic solution. This reduces the effect of counterions and enables the reaction to proceed more thoroughly
Why is the cation exchanger placed in front of the anion exchanger? Answer: 1. The function of a carbon remover is to remove carbon dioxide. 2. When the raw water passes through the cation resin, the cations in the water are adsorbed, and the H+ ions carried by the resin are released into the water, making it acidic; when the pH
Why is the cation exchanger placed in front of the anion exchanger? Answer: When raw water passes through the cation resin, the cations in the water are adsorbed, and the H+ ions carried by the resin are released into the water, making it acidic; when the pH
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 anionic 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 anionic 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 alkali 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; thus, exchange is most effectively 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.
The theoretical setup principle for the equipment in the chemical desalination system is raw water → cation exchanger → carbon remover → anion exchanger → deionized water. Due to the low exchange capacity of strongly basic anion exchange resins, HCO3- in water can be effectively removed in the form of CO2, thereby reducing the load on the strongly basic anion exchangers and the amount of alkali required, extending their operational life, and creating favorable conditions for the anion resins to adsorb silicate ions.
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 anionic 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 anionic 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 alkali 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 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.
1. The function of a carbon remover is to remove carbon dioxide. 2. When the raw water passes through the cation resin, the cations in the water are adsorbed, and the H+ ions carried by the resin are released into the water, making it acidic; when the pH
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+, resulting in an acidic solution. This reduces the effect of counterions and enables the reaction to proceed more thoroughly. Therefore, cation exchangers are generally placed before anion exchangers
Answer: 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 only cations that reach the anion exchanger are H+, resulting in an acidic solution. This reduces the effect of counterions and enables the reaction to proceed more thoroughly. That’s why cation exchangers are generally placed in front of anion exchangers.
Why is the cation exchanger placed in front of the anion exchanger? 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+, resulting in an acidic solution. This reduces the effect of counterions and enables the reaction to proceed more thoroughly. Therefore, cation exchangers are generally placed before anion exchangers