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[Abstract] This paper briefly describes the significance of improving the operating cycle of a desalination plant, how to improve the operating cycle, and several aspects that need attention when improving the desalination cycle.
[Keywords] Ion exchange resin; Exchange capacity; Regeneration quality
Jiang Aimei
Abstract: In this paper, we recite the meaning of saving water and how to improve the performance period of a desalination plant, for that we should pay attention in several areas.
Key words: ion exchange resin; exchange capacity; quality of reproduction
Water is a precious and non-renewable resource for humankind; a good environment is the foundation of human survival. As a preliminary device for producing boiler water, a desalination plant inevitably consumes a large amount of water for regeneration and inevitably generates a large amount of wastewater, which inevitably has a certain impact on the environment. Therefore, improving the operating cycle of the desalination plant to reduce its self-consumption of water not only has high economic benefits but also has important social significance.
The pretreated Yangtze River water, after reaching the desalination unit, first passes through a weak cation exchanger bed (D9208) to remove carbonate hardness, then through a strong cation exchanger bed (D9207) to remove all cations, followed by decarbonation in a decarbonator (D9206), and finally to a dual-layer bed of weak and strong anion exchangers (D9205) to remove anions before proceeding to the next process.
The process flow diagram is as follows:
Note: D9208 is a weak cation exchanger.
D9207 is a strong cation exchanger.
D9206 is a decarbonator.
D9205 is a combination of weak and strong anion exchangers.
Yellow represents a blower, white represents a water pump.
1. Improve Equipment Reliability
1.1 The water distribution and drainage pipes of the desalination unit must be unobstructed, without any flow deviation. Uneven water distribution and poor drainage will lead to flow deviation, causing premature breakdown of the protective resin layer even before the resin has deteriorated, thus shortening the production cycle.
1.2 Valves and monitoring systems must be sensitive and easy to use. Malfunctioning valves and monitoring systems will cause considerable trouble for normal operation and regeneration, and may even shorten the production cycle.
1.3 The decarbonizer in the desalination unit may seem like the least important piece of equipment, but this is not the case. Although it doesn't have many valves and doesn't require regeneration, its importance becomes apparent with a thorough analysis of the raw water. HCO3- constitutes the vast majority of anions. High decarbonization efficiency of the decarbonizer significantly reduces the load on the anion bed, thereby increasing the operating cycle of the desalination unit. This effect may not be very noticeable in summer, as water quality is better and the water temperature is suitable for the optimal operating temperature of the anion resin. However, it becomes particularly significant in winter. Although the raw water can be heated to around 30℃, the desalination unit's operating cycle decreases due to the dry season and deteriorating water quality. Moreover, in most cases, SiO2 prematurely degrades. This indicates that winter water quality significantly impacts the working exchange capacity of anion exchange resins than cation exchange resins, necessitating a decarbonizer with high decarbonization efficiency. After decarbonization, the CO2 content of the water should be less than 5 mg/L. At our plant, the CO2 content after decarbonization is around 18 mg/L. In summer, the operating cycle can reach about 30 hours, but in winter it drops to only about 24 hours, often falling below 20 hours, primarily due to premature SiO2 degrades.
1.4. Equipment, including water distribution pipes, drainage pipes, and decarbonizer packing, must be regularly inspected. During normal operation, any malfunctions must be addressed promptly; operation with defects is prohibited.
2. Select the appropriate resin type and replace it promptly based on its usage.
To ensure the normal cycle water production of the desalination unit, the ion exchange resin should be selected rationally based on the raw water quality (HCO3- 85.4ppm, Cl- 17.8ppm, SO42- 30%, NO3- 0.3ppm, NO2- 0.02ppm, SiO2 5.0ppm, Ca2+ 35.3ppm, Mg2+ 2.9ppm, Na+ 1.9ppm, total iron 1.3ppm, NH4+ 0.3ppm), the required effluent quality (conductivity less than 5.0μs, SiO2 less than 100ppb), the basic performance of the ion exchange resin, and the performance of the ion exchange equipment, through a technical and economic comparison. The performance indicators of the resin should be analyzed regularly, and resins with a significant decrease in exchange capacity or mechanical strength should be replaced immediately.
The water used in the desalination unit is pretreated in a clarification tank. The turbidity, Cl- content, and Fe3+ content of the influent all significantly affect the desalination operation cycle. Since the coagulation agent is a FeCl3 solution, the FeCl3 solution must contain minimal impurities. With influent turbidity less than 5 ppm, the dosage (Cl- content controlled between 15 ppm and 25 ppm) should be kept at the lower limit. Fe3+ and Fe2+ can contaminate the resin, causing resin poisoning, thereby reducing the resin's exchange capacity and deteriorating its performance; Cl- will occupy the anion exchange capacity, thus shortening the desalination operation cycle.
After desalination production is completed, the resin needs to be regenerated. The quality of regeneration directly determines the desalination operation cycle. Besides strictly ensuring the quality of the regeneration agents, human factors play a significant role. To improve regeneration quality, the following points should be noted:
4.1 If the resin fails prematurely, the operator must be able to determine whether the premature failure is in the cation exchange bed or the anion exchange bed based on the analysis data. This allows for targeted regeneration, with appropriate addition of acid or alkali to facilitate the next operation.
4.2 During the regeneration process, the operator must carefully check all valves, pumps, and interlocking actions according to the regeneration steps to prevent valve malfunctions or misoperations that could affect regeneration quality or even cause resin runaway.
4.3 We use countercurrent regeneration because the regenerated solution always contacts the protective resin layer with the lowest failure rate, ensuring the most thorough regeneration of this layer and resulting in good effluent quality. Furthermore, the "hook-out effect" of convection regeneration shortens the affinity difference between exchange ions, making regeneration easier and reducing regenerator consumption, typically 1.2 to 1.5 times. The key to countercurrent regeneration is preventing resin layer disorder. Therefore, the top pressure must be well controlled to ensure proper resin layering. Operators must constantly monitor and adjust the top pressure to ensure proper layering. Layering disruption will significantly shorten the regeneration cycle.
4.4. Control the regenerator temperature between 30 and 40°C. Temperature greatly affects the regeneration of ion exchange resins, especially strongly basic anion exchange resins.
4.5. Control the regenerator concentration and flow rate. Since concentrated sulfuric acid is used as the regenerator, controlling the regenerator concentration is crucial. Sulfuric acid reacts with the Ca2+ ions exchanged from the resin to form CaSO4·2H2O, which has a low solubility product. This CaSO4·2H2O can clog resin micropores, reducing the resin's exchange capacity. Therefore, our plant uses stepwise regeneration. Experience shows that when regenerating with sulfuric acid, first use 0.8% dilute sulfuric acid for 30% of the total volume, then gradually increase the acid concentration to 2%. This not only prevents CaSO4·2H2O precipitation but also improves the regeneration effect. During regeneration, it is crucial to avoid dehydrating D9208 and prevent calcium buildup in the cation exchange resin. Regularly monitor the color of the regeneration wastewater to determine if the regenerated solution concentration is appropriate. To ensure a complete exchange reaction during regeneration, the contact time between the regenerated solution and the exchange resin should generally be at least 30 minutes.
4.6. During regeneration, carefully check the drainage for obstruction and whether the regenerated solution is flowing excessively. Obstructed drainage may indicate calcium buildup in the resin, requiring a halt to regeneration and backwashing, or even scrubbing, to remove the deposited CaSO4. Excessive regenerated solution flow may indicate resin caking. Prompt backwashing is necessary to loosen the caking; otherwise, the regeneration effect will be poor.
4.7. In the later stages of regeneration, when cleaning D9208, measure the Ca2+ elution rate. If Ca2+ < 0.8 mmol/L, cleaning can be stopped early to conserve water and avoid depleting the resin's exchange capacity. If the Ca2+ content remains high at the end of the cleaning process, the cleaning time can be appropriately extended, but not excessively. Prolonged cleaning wastes water and depletes the resin's exchange capacity.
4.8. Resin should be regenerated promptly after it becomes ineffective. Our plant uses a gel-type uniformly porous strong-base anion resin with very small pores. When the strong-base anion resin becomes ineffective, it adsorbs a large amount of SiO2-silica compounds. If left for an extended period, these silica compounds associate with the exchange groups and dehydrate, forming high-molecular-weight polymers. These polymers deposit on the resin surface or in the pores, making elution during regeneration difficult, thus reducing the number of active exchange groups and lowering the resin's exchange capacity.
4.9. After a certain number of cycles, a thorough manual backwash is necessary, reaching a certain expansion height. This washes away broken resin to prevent increased flow resistance, reduced unit output, and loosening of the resin layer to prevent flow deviation. After a period of time, an appropriate amount of resin should be added to replenish the broken resin lost during the backwash. 4.10. A reasonable operating cycle is necessary. This cycle should be determined based on the water quality and the actual conditions of the equipment. Otherwise, if the cycle is too short, the equipment may reach the specified operating cycle, but it will be underutilized, wasting the resin's exchange capacity and self-consumed water, thus causing waste. However, it should not be set too high, otherwise it will be overloaded and unable to reach the normal operating cycle.
To improve the operating cycle of the desalination unit, we should focus on improving equipment reliability, selecting good resin, controlling the influent water quality, and ensuring regeneration quality.
Currently, the overall operating status of our plant's desalination unit is good. However, there are still shortcomings in decarbonization effect and timely resin replacement, which need continuous optimization to achieve safe, stable, long-term, full-capacity, and high-quality operation.
[References]
[1] Zhou Bensheng, Industrial Water Treatment Technology, Second Edition, Chemical Industry Press, 2005
[2] 300 Questions on Water Treatment Technology, Luzhou Tianhua Plant Research and Design Institute
