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Through the static scale inhibition test, we studied the scale inhibition performance of PBTC and HEDP on calcium carbonate scale under low-temperature conditions (2°C-15°C).

The results show that the scale inhibition performance of PBTC and HEDP on calcium carbonate scale is good at 2°C-15°C. The initial calcium ion + alkalinity has a great influence on the scale inhibition performance of PBTC and HEDP. When the calcium ion + alkalinity increased from (500+600) mg/L to (800+800) mg/L, the scale inhibition rates of PBTC and HEDP decreased from 100% to 65% and 60%, respectively. The presence of PO43-, Zn2+, and turbidity has little effect on the scale inhibition effect of PBTC and HEDP.

Keywords: PBTC, HEDP, low temperature, scale inhibition, calcium carbonate scale.

The air pre-cooling system is an important part of the air separation unit. It is mainly used to reduce the temperature of high temperature compressed air. Air precooling systems typically use circulating cooling water as the heat exchange medium. The process flow of a device is as follows.

The circulating cooling water is divided into two paths. One path is to enter the middle of the air cooling tower to exchange heat with the high-temperature air in the lower section. The air is initially cooled. The circulating water flows back from the bottom of the tower to the circulating cooling water system after heat exchange. The circulating water of the other path first enters the water cooling tower and directly contacts the contaminated nitrogen to exchange heat.

The water temperature drops to 12°C-15 °C. Then exchange heat through the ice machine cooler. The water temperature continues to drop to 5°C-8 °C. Finally, the chilled water flows through the upper part of the air cooling tower and exchanges heat with the air to return to the circulating cooling water system.

In the actual application process, serious fouling was found on the circulating water side of the ice machine heat exchanger. The fouling material is mainly calcium carbonate. In circulating cooling water treatment, the simplest and most effective method is to add scale inhibitor to the circulating cooling water to prevent system fouling.

PBTC and HEDP have an excellent chelating ability for Ca2+, Mg2+, Cu2+, Zn2+ and the like. These two water treatment agents exert a scale inhibition effect by lattice distortion and complex solubilization. In particular, PBTC contains both a phosphonic acid group and a carboxylic acid group. It exhibits excellent scale inhibition performance under high temperature, high hardness, and high pH water conditions.

PBTC and HEDP have scale inhibition and corrosion inhibition functions. The effect of scale inhibition can be achieved by using a small dose. They are the most widely used corrosion and scale inhibitors in circulating cooling water treatment. Whether or not the scale inhibition performance is excellent is particularly important at low temperatures.

In order to find the reason for low temperature fouling on the circulating water side of the ice machine heat exchanger. IRO’s researchers tested the scale inhibition performance of PBTC and HEDP at low temperatures.

1. Experiment part

1.1 Materials and instruments

HEDP (solid content 50%), PBTC (solid content 50%), deionized water, anhydrous calcium chloride, sodium bicarbonate, flask, biochemical incubator, ice machine cooler, measuring cup, beaker.

1.2 Experiment methods

Add a certain amount of anhydrous calcium chloride and sodium bicarbonate to deionized water or tap water. Stir well. The experimental water is obtained. Take 200mL of experimental water and place it in a triangular flask. Add a certain concentration of PBTC or HEDP. Shake well and place it in a biochemical incubator. Keep it at a constant temperature for a certain period of time. filter. The calcium ions in the solution were measured. And calculate the scale inhibition rate.

1.3 Analysis methods

The Ca2+ measurement was performed by EDTA complexometric titration, with reference to GB6986-1986.

2. Results and discussion

2.1 The effect of temperature on scale inhibition

The circulating water passes through the water cooling tower for heat exchange. The temperature drops to 12°C-15°C. After the circulating water is exchanged by the ice machine cooler, the temperature continues to drop to 6°C-8°C.

To this end, the scale inhibition effect of PBTC and HEDP on calcium carbonate at different temperatures was investigated. The researchers used deionized water to dispense water. The initial Ca2+ mass concentration was 496.5 mg/L. The alkalinity is 600 mg/L. The test time was 2h. PBTC and HEDP were added at a concentration of 4 mg/L. The test temperatures were 2°C, 8°C, 15°C, 30°C, 45°C, 60°C. The results are shown in Figure 1.

figure1
We can see from Figure 1. Water samples were added to PBTCA or HEDP to maintain good scale inhibition at 2-60 °C. The Ca2+ mass concentration is close to the initial value. The Ca2+ deposition rate is less than 1%.

Ca2+ in the blank water sample decreased with increasing test temperature. Especially when the test temperature is greater than 30 °C. The drop in Ca2+ is very obvious. This is because the increase in temperature causes Ca(HCO3)2 to be thermally decomposed into CaCO3. The solubility of CaCO3 decreases with increasing temperature. Therefore, more CaCO3 is deposited under high-temperature conditions. This results in a decrease in Ca2+.

In summary, PBTCA and HEDP have good performance in resisting calcium carbonate scale and are less affected by temperature.

2.2 The effect of scale inhibitor mass concentration on scale inhibition

The researchers used deionized water to dispense water. The initial Ca2+ concentration was 494.2 mg/L. The alkalinity is 600 mg/L. The test temperature was 8°C. The test time was 4h. The scale inhibition effect of PBTC and HEDP mass concentration on calcium carbonate was tested. The results are shown in Figure 2.

figure2
We can see from Figure 2. Under the experimental water quality, with the increase of the concentration of the scale inhibitor, the scale inhibition rate of PBTC gradually increased from 93.9% to 100%. The scale inhibition rate of HEDP increased from 93.9% to 100%. This indicates that the mass concentration of PBTCA and HEDP has a certain influence on the scale inhibition effect of calcium carbonate at low temperatures. The increase in the mass concentration of the scale inhibitor is beneficial to increase the scale inhibition rate.

2.3 Effect of test time on scale inhibition

The researchers used deionized water to dispense water. The initial Ca2+ mass concentration was 476.2 mg/L. The alkalinity is 600 mg/L. The test temperature is 8°C. PBTCA and HEDP were added at a concentration of 4 mg/L. Test the effect of test time on the scale inhibition effect.

The results showed that the test time was extended from 2h to 20h. The scale inhibition rate of PBTCA has been maintained at 100%. The scale inhibition rate of HEDP is slightly reduced. But both are greater than 97.0%. It can be seen that the scale inhibition performance of PBTCA and HEDP is better under this test condition. Basically unaffected by the test time.

2.4 Effect of initial calcium ion + alkalinity on scale inhibition

The circulating water enters the water cooling tower and is counter currently contacted with the lower temperature sewage nitrogen. Part of the water evaporates during the gas-liquid contact. Calcium ions and alkalinity are further concentrated.

The researchers used deionized water to dispense water. The test temperature was 8°C. The test time was 4 h. Both PBTCA and HEDP were added at a concentration of 4mg/L. The initial calcium ion + alkalinity was (500 + 600) mg/L, (800 + 800) mg/L, (1 000 + 1 000) mg/L. Investigate the scale inhibition effect of PBTCA and HEDP on calcium carbonate under different calcium ion + alkalinity conditions. The results are shown in Figure 3.

figure3
We can see from Figure 3. When the initial calcium ion + alkalinity is (500 + 600) mg/L, the calcium carbonate scale inhibition rate of both PBTCA and HEDP is 100%. When the initial calcium ion and alkalinity gradually increase, the scale inhibition rate shows a downward trend. When calcium ion + alkalinity was increased to (800+800) mg/L, the scale inhibition rates of PBTCA and HEDP were reduced to 65% and 60%, respectively.

Under normal circumstances, organophosphorus scale inhibitors act by chelation of Ca2+ and lattice distortion of CaCO3. There is also a solubility limit effect. That is, a few milligrams per liter of the drug can stabilize several hundred milligrams per liter of Ca2+. When the calcium ion + alkalinity in the circulating water is high to a certain extent, the ability of PBTC and HEDP to stabilize calcium ions is exceeded. Calcium carbonate crystals are formed. Causes scaling.

2.5 The effect of other fouling factors on scale inhibition

In general, PO43-, Zn2+, and turbidity in circulating water have an effect on calcium carbonate scaling. Therefore, the effects of scale factors such as PO43-, Zn2+, and turbidity on the performance of PBTC and HEDP calcium carbonate scale were investigated.

2.5.1 Effect of PO43- on scale inhibition

Researchers use tap water to dispense water. The initial Ca2+ and alkalinities were 595.2 mg/L and 600 mg/L, respectively. The dosage of PBTCA and HEDP was 4 mg/L. The test time is 12h. The test temperature is 8 °C. Change the PO43- content. Test PO43- Effect on the scale inhibition performance of PBTCA and HEDP. The results are shown in Figure 4.

figure4
We can see from Figure 4. At 8°C, the amount of PO43- is changed, and the corresponding Ca2+ mass concentration is basically unchanged. It shows that the scale inhibition effect of PBTCA and HEDP under low-temperature conditions is basically not affected by PO43-.

2.5.2 Effect of Zn2+ on scale inhibition performance

Researchers use tap water to dispense water. The initial Ca2+ and alkalinities were 595.2 mg/L and 600 mg/L, respectively. The dosage of PBTC and HEDP was 4 mg/L. The test time is 12h. The test temperature is 8°C. Change the Zn2+ content. The effect of Zn2+ on the scale inhibition performance of PBTCA and HEDP was tested. The results are shown in Figure 5.

figure5
We can see from Figure 5. At 8°C, the Zn2+ content was changed, and the corresponding Ca2+ mass concentration was basically unchanged. It indicates that the scale inhibition effect of PBTCA and HEDP under low-temperature conditions is basically not affected by Zn2+.

2.5.3 Effect of turbidity on scale inhibition of chemicals

Researchers use tap water to dispense water. The initial Ca2+ and alkalinities were 595.2 mg/L and 600 mg/L, respectively. The dosage of PBTC and HEDP was 4 mg/L. The test time is 12h. The test temperature was 8°C. The effect of turbidity on the scale inhibition performance of PBTC and HEDP was investigated. Turbidity is controlled by kaolin. The results are shown in Figure 6.

figure6
We can see from Figure 6. At 8°C, the water sample Ca2+ added to the PBTC increased with the increase of turbidity. In the water sample with HEDP, Ca2+ gradually decreased with the increase of turbidity. But the decline is small.

It indicates that the turbidity has little effect on the scale inhibition performance of PBTC under low-temperature conditions, but the scale inhibition performance of HEDP is slightly reduced.

3. Conclusion

(1) PBTC and HEDP have good scale inhibition performance at low-temperature of 2°C-15°C. Temperature and test time have little effect on its scale inhibition performance. The concentration of PBTC and HEDP added has a slight effect on the effect of calcium carbonate scale.

(2) The initial calcium ion + alkalinity has a great influence on the scale inhibition performance of PBTC and HEDP. The scale inhibition rate decreases as the calcium ion + alkalinity increases. When calcium ion + alkalinity is increased from (500+600) mg/L to (800+800) mg/L. The scale inhibition rates of PBTC and HEDP were reduced from 100% to 65% and 60%, respectively.

(3) The presence of PO43- and Zn2+ has little effect on the scale inhibition effect of PBTC and HEDP. Turbidity has little effect on the scale inhibition effect of PBTC. It has a slight influence on the scale inhibition effect of HEDP.

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