Seawater has complex water quality with high hardness, high sulfate content, and high chloride ion content. During operation, seawater frequently contacts the heat exchange tubes in the evaporator, continuously concentrating. The total hardness and sulfate concentration of the concentrated brine in the bottom area of the shell are higher than those of the raw seawater sprayed from the upper part, leading to increased ion concentration and a supersaturated state of salts, which intensifies the scaling tendency. This easily causes the deposition of calcium sulfate scale, carbonate scale, and other inorganic scales on the heat exchange tubes. Long-term accumulation of these scales reduces heat transfer efficiency and water production capacity.
Therefore, it is necessary to add antiscalant and dispersant to control scaling during actual operation. Antiscalant can effectively inhibit the deposition of inorganic salts on the surface of heat exchangers, thereby improving the operational efficiency of the device and reducing operational costs. According to the operating characteristics of thermal seawater desalination plants, we synthesized a terpolymer antiscalant containing carboxyl and sulfonic acid groups, and tested its temperature resistance. In accordance with relevant standards, the scale inhibition performance of the agent was evaluated using Bohai Bay seawater, and the effect of the copolymer on the crystal morphology of scale samples was analyzed by scanning electron microscopy (SEM).
Sodium hydroxide, molecular weight regulator, initiator, etc., were all industrial grade. Calcium chloride, sodium bicarbonate, sodium sulfate, sodium tetraborate, potassium hydroxide, disodium edetate, ferrous sulfate, hydrochloric acid, etc., were all analytical grade reagents. The instruments used included a W201 constant temperature water bath (Shanghai Sanshen Medical Equipment Co., Ltd.), a TU-1810 UV-Vis spectrophotometer (Beijing Puxi General Instrument Co., Ltd.), a variable speed stirrer (Shanghai Sanshen Medical Equipment Co., Ltd.), a pH meter SG78 (Mettler-Toledo), a potentiometric titrator DL53 (Mettler-Toledo), a scanning electron microscope (model JSM-6390LV), and a thermogravimetric analyzer (NETZSCH).
A static experiment was conducted. A certain volume of water with specific pH value and hardness was prepared, and a quantitative amount of copolymer was added. The mixture was incubated at a certain temperature for a specific period, then cooled and filtered. The content of the corresponding component in the filtrate was determined to calculate the antiscalant rate.
Test method:
Refer to GB/T 16632-2008.
Test water quality conditions:
[Ca²⁺] = 250 mg/L,
[HCO₃⁻] = 250 mg/L,
Temperature: 80℃/60℃,
Time: 10 h.
The content of Ca²⁺ was determined by EDTA complexometric titration (METTLER-DL53 potentiometric titrator).
The scale inhibition rate was calculated using the following formula (1):
Determination of calcium sulfate antiscalant performance:
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Raw water: Coastal seawater from the southern Bohai Bay in Tangshan City, Hebei Province.
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Process: Concentrated to approximately 1.5 times using the distillation process (calcium ions and sulfate ions were additionally supplemented to appropriately increase their concentrations), followed by pre-clarification and filtration.
Experimental water quality conditions:
[Ca²⁺] = 2900 mg/L, [Mg²⁺] = 2000 mg/L, [SO₄²⁻] = 8000 mg/L, [Cl⁻] = 48000 mg/L,
Temperature: 70℃,
Time: 10 h.
The content of Ca²⁺ in the filtered filtrate was determined (METTLER-DL53 potentiometric titrator).
Figure 1 shows the thermogravimetric analysis of the self-synthesized copolymer. It can be seen that the copolymer has excellent temperature resistance. A weight loss peak appears around 85℃, which may be attributed to small molecule water. Another weight loss peak occurs around 250℃, indicating the start of polymer decomposition. These results demonstrate that the synthesized polymer has good thermal stability, fully meeting the operating conditions of thermal seawater desalination distillation devices (such as MSF, MED, etc.).
Figure 1 TG-DTG Analysis of Copolymer
The results are shown in Figure 2.
Figure 2 Effect of Dosage on Antiscalant of Calcium Carbonate
As can be seen from Figure 2:
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When the copolymer dosage is 2-4 mg/L, the scale inhibition rate changes significantly with the increase in dosage; when the dosage is 5-6 mg/L, the change in antiscalant rate tends to be gentle.
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From the perspective of test temperatures, at the same dosage, the scale inhibition rate at 60℃ is higher than that at 80℃. Higher temperatures lead to a more severe scaling tendency of calcium carbonate.
The antiscalant performance of the copolymer against calcium sulfate scale was determined under the following experimental conditions:
Experimental water quality conditions:
[Ca²⁺] = 2900 mg/L,
[Mg²⁺] = 2000 mg/L,
[SO₄²⁻] = 8000 mg/L,
[Cl⁻] = 48000 mg/L,
Temperature: 70℃,
Time: 10 h.
The specific results are shown in Figure 3. Similar to the calcium carbonate antiscalant rate, the same trend is observed: when the scale inhibitor dosage is 2-4 mg/L, the antiscalant rate changes significantly with the increase in dosage; when the dosage is 5-6 mg/L, the change tends to be gentle. This indicates that the copolymer has a low-dose effect, meaning it can achieve high antiscalant efficiency even at low dosages.
Figure 3 Effect of Dosage on Antiscalant of Calcium Sulfate
To further investigate the mechanism of the copolymer's antiscalant on calcium sulfate scale, scanning electron microscopy analysis was performed on the precipitates collected from the calcium sulfate antiscalant test, as shown in Figure 4 below.
Figure 4(a) Formation of Calcium Sulfate Crystal without Antiscalant
Figure 4(b) Formation of Calcium Sulfate Crystal with Antiscalant
It can be seen from Figure 4 that the copolymer causes a fundamental change in the micro-morphology of calcium sulfate scale. Without the addition of the antiscalant, calcium sulfate crystals have a highly regular structure, mostly in strip (columnar) shapes. With the addition of the antiscalant, the calcium sulfate crystals become very irregular, significantly fragmented, with more surface pores and loose texture. The introduction of the antiscalant alters the geometric shape of the crystals and causes lattice distortion. The polar groups occupy certain active growth sites on the crystal growth surface, thereby hindering the normal combination of sulfate ions and calcium ions. This results in disordered and irregular lattice arrangement, making the scale loose and porous, which is easily washed away and not prone to deposition.
Scaling is the most significant hazard for thermal seawater desalination distillation devices and a relatively complex problem in thermal seawater desalination technology. The multi-component copolymer synthesized by free radical polymerization not only has excellent thermal stability but also exhibits good antiscalant performance when tested under the actual water quality conditions of concentrated seawater. The synthesized copolymer meets the operational requirements of the distillation seawater desalination process and has promising application prospects.
