Due to the increasing temperature in water injection system, scales are sediments that are firmly attached on the metal surface in the industrial equipment. These scales, including calcium and ferric oxide precipitations, can cause heat transfer problems. A non-phosphorus water treatment agent APES/AA/AMPS was synthesized by acrylic acid (AA), ammonium allylpolyethoxy sulphate (APES) and 2-acrylamido-2-methyl propanesulfonic acid (AMPS) with the solution of ammonium persulfate as an initiator. The properties of scale inhibition and dispersion of water treatment agent APES/AA/AMPS were studied by using the commonly used standard test methods. The results showed that APES/AA/AMPS had excellent scale inhibition and dispersion performance. The performance of APES/AA/AMPS on strontium sulfate was 93.3% at the dosage of 9 mg·L−1. When the concentration was 4 mg·L−1, the dispersion of iron oxide was optimal according to the solution transmittance of 13.7%. The morphology and crystallinity changes of strontium sulfate in scanning electron microscopy and X-ray powder diffraction showed that the mechanism of the inhibition on strontium sulfate is adsorption.

INTRODUCTION

In recent years, the three biggest water-related production problems involving scale, corrosion and gas hydrates have attracted most interest in research in industrial water injection systems (Doherty et al. 2004; Kelland 2011). The problems of scaling and corrosion associated with the use of natural hard water in cooling towers during recirculation pose great problems from both economical and technical points of view, such as decreased system efficiency and increased frequency of chemical cleaning (Wang et al. 2010; Amjad 2012). With the development of oil and gas fields into the middle and later stages, the enormous changes of pressure, temperature conditions and the thermodynamic instability of water can easily cause fouling. The four commonly occurring scales encountered in the oil industry are calcium carbonate (calcite and aragonite) and sulfate salts of strontium (celestite), barium (barite) and iron oxide (Benbakhti & Bachir-Bey 2010; Jones et al. 2012; Yu et al. 2014). Among them, strontium sulfate is one of the most difficult to remove due to its low solubility (Hamdona et al. 2010; Zang et al. 2011). Meanwhile, the deposition of ferric oxide can cause the decrease of the heat exchange efficiency (Liu et al. 2013). The most effective and commonest method of preventing scale formations is scale inhibitors. These inhibitors are effective because of their inherent performance either to prevent nucleation or to retard the crystal growth of scales.

The development of water treatment agents experienced the process of high-phosphorus, low-phosphorus, and non-phosphorus. Phosphates, 1-hydroxy ethylidine-1-diphosphonic acid (HEDP), ethylenediamine-N,N,N′,N′-tetra(methylenephosphonic acid) (EDTMP), and 1,2-diaminoethanetetrakis-methylene phosphonic acid (Jones et al. 2002; Barouda et al. 2007; Kavitha et al. 2011) are high-phosphorus scale inhibitors, poisoning the crystal growth at concentrations far below stoichiometric amounts of the reactive cations. Chen et al. (2014) studied a low-phosphorus scale inhibitor. The inhibitor has high performance on calcium carbonate, but the inhibitor containing phosphorus can bring water eutrophication and pollution. Thus, developing non-phosphorus water treatment agents is of significant importance. Moreover, the current development trend for water treatment is from single function to multi-function, so that the agent can be used in a wider range of pH and higher calcium tolerance.

In the present study, a non-phosphorus water treatment agent APES/AA/AMPS was prepared by acrylic acid (AA), ammonium allylpolyethoxy sulphate (APES) and 2-acrylamido-2-methyl propanesulfonic acid (AMPS) with water as solvent and ammonium persulfate as initiator. The structural information of the water treatment agent was characterized by Fourier transform infrared (FT-IR) between 4,500 cm−1 and 0 cm−1, which was used to confirm the presence of expected functional groups responsible for the scale inhibition property. The properties of APES/AA/AMPS towards strontium sulfate scale and ferric oxide were studied through static test method. The morphology and crystal structure of strontium sulfate were studied by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). Thus, the water treatment agent APES/AA/AMPS is according to the direction of development of a ‘green’ water treatment agent.

MATERIALS AND METHODS

Materials and instruments

The APES used in the experiment was synthesized according to K. Fu's procedure (Fu et al. 2010). AMPS was supplied by Jiangsu Jianghai Chemical Co., Ltd. AA, SrCl2, Na2SO4, NaCl, FeSO4·2H2O, CaCl2, sodium tetraborate (Na2B4O7·10H2O) and ammonium persulfate were analytically pure grade and were supplied by Zhongdong Chemical Reagent Co., Ltd (Nanjing, China). Deionized water was used throughout the experiments.

Method of preparing APES/AA/AMPS

APES was synthesized from allyloxy polyethoxy ether (APEO) and amido-sulfonic acid at 120 °C in the presence of urea, which is shown in Figure 1. The water treatment agent APES/AA/AMPS was synthesized following the route shown in Figure 2. In brief, the reaction was carried out in a four neck round flask with 5 mL deionized water and 2 g APES, equipped with thermometer and a magnetic stirrer, then it was heated to 80 °C under nitrogen atmosphere. Next, 4 g AA and 1 g AMPS were dissolved into 10 mL deionized water. Meanwhile, 0.5 g ammonium persulfate was dissolved in 15 mL deionized water as initiator. Then the two as-prepared solutions were simultaneously dropwise added into the flask in 1 h. Then, the temperature was increased to 85 °C and continued to react for 2.5 h with stirring. Finally, a pale-yellow liquid with 22.1% solid content was obtained.
Figure 1

Synthesis procedure of APES (n = 9).

Figure 1

Synthesis procedure of APES (n = 9).

Figure 2

Synthesis procedure of APES/AA/AMPS.

Figure 2

Synthesis procedure of APES/AA/AMPS.

Inhibition methods

Strontium sulfate inhibition

The scale effect of water treatment agent APES/AA/AMPS against strontium sulfate was examined according to the oil and gas industry standard of People's Republic of China (SY/T 5673-93) by atomic absorption spectrophotometer. Strontium sulfate precipitation and inhibition were studied in a 7.50 g·L−1 NaCl solution as artificial solvent. Strontium sulfate was prepared by mixing solutions of SrCl2 (8.16 g·L−1) and Na2SO4 (7.31 g·L−1). Strontium sulfate was prepared by mixing SrCl2 (8.16 g·L−1) and Na2SO4 (7.31 g·L−1) in the 50 ml colorimetric tube, and the resulting concentration of Sr2+ was 900 mg·L−1. Then, the required amount of APES/AA/AMPS (0–19 mg·L-1) was added into the colorimetric tube to test its inhibition efficiency. All colorimetric tubes were placed in the water bath at 70 °C for 16 h. The inhibition efficiency of APES/AA/AMPS against strontium sulfate scale was calculated using the following equation: 
formula
where [Sr2+]0 is the concentration of Sr2+ in the absence of both inhibitor and SO42−, [Sr2+]1 is the concentration of Sr2+ in the absence of pristine inhibitor, and [Sr2+]2 is the concentration of Sr2+ in the presence of inhibitor.

Ferric oxide inhibition

First of all, the stock solution of ferric iron was prepared. The aqueous solution (Fe2+, 10 mg·L−1) was configured by dissolving a certain amount of ferrous sulfate into a certain amount of water. By the same method, a solution containing 150 mg·L−1 calcium ion was also obtained. Then the solvent, whose pH was adjusted to 9.0 by sodium tetraborate solution, was added to the as-prepared series of 50 mL colorimetric tubes. The ferric iron stock solution was added into the colorimetric tubes before the addition of water treatment agent APES/AA/AMPS. After that, the colorimetric tubes should be shaken gently. Secondly, 5 mL calcium ion stock solution was added to the solutions. Finally, these colorimetric tubes were placed in the water bath at a temperature of 50°C for 6 h, and the parallel blank test was carried out. The colorimetric tubes were cooled to room temperature after the insulation process. The transmittances of the solution were examined by using a UV spectrophotometer. After the long-term water bath, the lower light transmittance implies ferric oxide particles dispersed in water, which corresponds to better efficacy of APES/AA/AMPS as an iron (III) inhibitor.

Commercial water treatment agents, such as HEDP, diethylenetriamine penta(methylene phosphonic acid) (DTPMP), polyaminopolyether methylenephosphonic acid (PAPEMP), polyepoxysuccinic acid (PESA) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), were also tested to have a comparison of different inhibition efficiencies.

RESULTS AND DISCUSSION

FT-IR analysis of APES/AA/AMPS

The FT-IR spectra of (a) APEO, (b) APES, (c) AMPS and the water treatment agent (d) APES/AA/AMPS are shown in Figure 3. It can be seen from Figure 3(a) and 3(b) that the absorption peaks at 1,350 cm−1 and 950 cm−1 correspond to –S = O asymmetric vibration and N–H bending vibration, while the absorption peak in 1,646 cm−1 of double bond didn't disappear, which showed the molecular structure of APES was confirmed.
Figure 3

The FT-IR spectra of (a) APEO, (b) APES, (c) AMPS, (d) APES/AA/AMPS.

Figure 3

The FT-IR spectra of (a) APEO, (b) APES, (c) AMPS, (d) APES/AA/AMPS.

The peak at 1,648 cm−1 disappears completely in the curve of Figure 3(d) compared with that occurring in Figure 3(b) and 3(c), which demonstrates that the polymerization between AA, APES and AMPS has occurred successfully. It is found that there are characteristic absorption peaks at 1,720 cm−1, 1,630 cm−1, 1,552 cm−1, 1,452 cm−1, 1,174 cm−1, 1,039 cm−1 and 648 cm−1 in Figure 3(d); the peaks that appear at 1,174 cm−1 and 1,039 cm−1 are assigned to aliphatic –S = O symmetry and asymmetry stretching vibrations of sulfonic acid groups respectively, and the S–O stretching vibration peak at 648 cm−1 (Lin et al. 2008; Liu et al. 2012). The residuary weak peak at about 1,630 cm−1 is assigned to the C = O absorption of amide. Correspondingly, the peaks at 1,452 cm−1 and 1,552 cm−1 reveal C–N stretching vibrations and N–H flexural vibrations (Shakkthivel & Vasudevan 2006).

Nuclear magnetic resonance (1H-NMR) analysis

The 1H-NMR spectra of (a) APEO, (b) APES, (c) AMPS, and (d) APES-AA-AMPS are presented in Figure 4. The 1H-NMR data were acquired as follows and the chemical molecular structures were deduced as expected.
Figure 4

The 1H-NMR data of (a) AA, (b) APES, (c) AMPS, (d) APES/AA/AMPS.

Figure 4

The 1H-NMR data of (a) AA, (b) APES, (c) AMPS, (d) APES/AA/AMPS.

APEO (a) ((CD3)2SO, δ ppm): 2.50 (solvent residual peak of (CD3)2SO), 3.32–3.51 (–OCH2CH2–, ether group), 3.94–3.95 (CH2 = CH–CH2–, propenyl protons), 4.55 (–OH, active hydrogen in APEO), 5.12–5.94 (CH2 = CH–CH2–, propenyl protons).

APES (b) ((CD3)2SO, δ ppm): 2.50 (solvent residual peak of (CD3)2SO), 1.03–1.23 (–CH2–, methylene protons), 3.27–3.95(–OCH2CH2–, ether group), 5.11–5.94 (CH2 = CH–CH2–, propenyl protons), 6.91–7.25 (–SO3NH4, proton resonance peak).

AMPS (c) ((CD3)2SO, δ ppm): 2.50–2.51 (solvent residual peak of (CD3)2SO), 1.17–1.43 (–CH3, methyl proton), 2.76 (–CONH–, acylamino), 5.48–6.10 (CH2 = CH–, ethylene protons).

APES/AA/AMPS (d) ((CD3)2SO, δ ppm): 2.50 (solvent residual peak of (CD3)2SO), 1.38 (–CH3, methyl proton), 2.19 (–CONH–, acylamino proton), 3.42–3.62 (–OCH2CH2–, ether group), 4.15 (–CH2–, methylene protons), 6.95–7.29 (–SO3NH4, amino protons).

The disappearing peak at 4.55 ppm (–OH) and appearing peak at 6.91–7.25 (–SO3NH4) in Figure 4(b) indicate that the active hydroxyl group in APEO has reacted with sulfamic acid. Meanwhile, it is obvious that the double bond absorption peaks (δ = 5.12–5.94, 5.11–5.94, 5.18–6.10 ppm) disappear completely compared with Figure 4(d). Conclusively, all of these data further suggest that APES-AA-AMPS was synthesized successfully.

Inhibition efficiency of APES/AA/AMPS on strontium sulfate scale

The water treatment agent APES/AA/AMPS was prepared at different mass ratios of monomers, and the inhibition efficiency to control strontium sulfate scale is shown in Table 1. It can be concluded that the dosage of APES/AA/AMPS and the mass ratios both influence the performance of the water treatment agent on SrSO4. With the increasing dosage of the water treatment agent, the inhibition performance of the water treatment agent on strontium sulfate scale gradually increased. It can be observed obviously that APES/AA/AMPS has the highest efficiency of 93.3% for SrSO4 at the threshold of 9 mg·L−1. Although the different mass ratios of monomers have different performance on SrSO4 and the thresholds are also found changed, the maximum value can be reached above 80% finally. This may be attributed to the active functional groups in the water treatment agent molecular chain which can adsorb Sr2+ effectively, such as sulfonate group (–SO3H), carboxamide (–CONH) group, and carboxyl group (–COOH).

Table 1

The influence of AMPS mass ratios on strontium sulfate scale inhibition

m(APES):m(AA):m Concentration (mg·L−1)/inhibition of SrSO4 (%)
 
(AMPS) 11 13 15 17 
1:4:0.3 63.3 81.9 91.9 87.8 88.4 88.0 88.0 
1:4:0.5 67.3 83.7 93.3 90.7 91.8 90.9 91.7 
1:4:1 50.0 69.4 79.8 82.2 79.0 79.7 79.4 
1:4:1.5 54.4 69.6 77.4 80.1 74.2 73.8 73.3 
m(APES):m(AA):m Concentration (mg·L−1)/inhibition of SrSO4 (%)
 
(AMPS) 11 13 15 17 
1:4:0.3 63.3 81.9 91.9 87.8 88.4 88.0 88.0 
1:4:0.5 67.3 83.7 93.3 90.7 91.8 90.9 91.7 
1:4:1 50.0 69.4 79.8 82.2 79.0 79.7 79.4 
1:4:1.5 54.4 69.6 77.4 80.1 74.2 73.8 73.3 

Comparison of inhibition efficiency on strontium sulfate scale

The comparison of inhibition efficiency on strontium sulfate scale between APES/AA/AMPS (APES: AA: AMPS = 1:4:0.5) and other inhibitors, such as PESA, HEDP, DTPMP (Wang et al. 2006), EDTMP and APES/AA (Amjad & Koutsoukos 2014), was studied. The results are presented in Figure 5 and it can be concluded that APES/AA/AMPS has higher performance than APES/AA, HEDP and DTPMP. In addition, the progressively decreasing order is EDTMP (93.7%) > APES/AA/AMPS (93.3%) > PESA (88.2%) when the dosage is 9 mg·L−1. EDTMP (Barouda et al. 2007) is the most effective inhibitor for strontium sulfate, but it contains a large amount of phosphorus, which can bring pollution and other water problems. Although PESA (Abdel-Aal et al. 2013) is a green natural polymer which has high performance on calcium carbonate, it is commercially higher-cost than other inhibitors and it also has poor inhibition towards other calcium scales. These results suggest that the phosphorus-free terpolymer APES/AA/AMPS has excellent scale inhibition performance, better than most of the major commercial inhibitors except EDTMP. Furthermore, it is environmentally friendly.
Figure 5

Comparison between APES/AA/AMPS and other scale inhibitors on strontium sulfate.

Figure 5

Comparison between APES/AA/AMPS and other scale inhibitors on strontium sulfate.

Inhibition efficiency of APES/AA/AMPS on ferric oxide

The relationship among the dosage of water treatment agent, the monomer mass ratios and the transmittance of solution is shown in Figures 6 and 7. APES/AA/AMPS has the best efficiency on dispersing ferric oxide when the mass ratio of monomers is 4:1:0.3(AA: APES: AMPS), and the minimum transmittance of solution is 13.7% at the dosage of 4 mg·L−1. It also can be seen from Figures 6 and 7 that changing the mass ratio of APES has greater impact on transmittance of solution than that of AMPS in APES/AA/AMPS. As we know, without the water treatment agent, the small Fe2O3 crystal nuclei might assemble together to form larger Fe2O3 scale and precipitate to the bottom of the solution, thus presenting the higher transmittance of solution. When the APES/AA/AMPS is added, the carboxyl groups in polymer molecules can chelate Fe3+, which can forbid small Fe2O3 crystal nuclei to nucleate into larger Fe2O3 scale precipitation, so the ferric oxide can be dispersed.
Figure 6

The influence of changing the ratio of APES in APES/AA/AMPS on the transmittance of solution.

Figure 6

The influence of changing the ratio of APES in APES/AA/AMPS on the transmittance of solution.

Figure 7

The influence of changing the ratio of AMPS in APES/AA/AMPS on the transmittance of solution.

Figure 7

The influence of changing the ratio of AMPS in APES/AA/AMPS on the transmittance of solution.

Comparison of inhibition efficiency on ferric oxide

The inhibition efficiency of different water treatment agent on ferric oxide is compared in Figure 8, such as carboxylic acid (PAA), the environmentally friendly (PESA), the organic phosphorous acid (PAPEMP, PBTC, HEDP) and the as-prepared water treatment agent APES/AA/AMPS. When the dosages of PESA, PAPEMP, PBTC and HEDP are up to 14 mg·L−1, the transmittances of solution are still higher than 75%, implying that they have almost no obvious performance on the dispersion of iron oxide. Although PAA shows certain dispersion ability, the minimum transmittance is above 40%, higher than 13.7% of APES/AA/AMPS. This demonstrates that the synergistic interaction between different functional groups in APES/AA/AMPS may result in a great effect of inhibition efficiency on ferric oxide.
Figure 8

Comparison of inhibition efficiency between APES/AA/AMPS and other scale inhibitors on ferric oxide.

Figure 8

Comparison of inhibition efficiency between APES/AA/AMPS and other scale inhibitors on ferric oxide.

Morphology characterization of strontium sulfate scale

SEM analysis

The precipitated crystals are agglomerated as shown in Figure 9: (a) free inhibitor, (b) 5 mg·L−1, (c) 9 mg·L−1, (d) 13 mg·L−1 of APES/AA/AMPS, using SEM investigation. The morphology of the free inhibitor is the standard celestite and the size of width is about 15 μm, as can be seen from Figure 9(a). The shape and size both change with the concentration change from zero to 13 mg·L−1. When the dosage of inhibitor is 5 mg·L−1, the morphology is brush-shape and most of the strontium sulfate crystals have changed except the edges. Compared with Figure 9(a) and 9(b), the strontium sulfate crystals have all become strips getting together in Figure 9(c). The crystal strips change into a filamentous form whose width is only 1 μm, which can be washed away by flowing water when the dosage is 13 mg·L−1. The result of SEM indicates that the non-phosphorus water treatment agent APES/AA/AMPS can change the morphology and the size of strontium sulfate crystals. Furthermore, the high scale inhibition performance on strontium sulfate can be proved.
Figure 9

SEM of SrSO4: (a) free inhibitor, (b) 5 mg·L−1, (c) 9 mg·L−1, (d) 13 mg·L−1 of APES/AA/AMPS.

Figure 9

SEM of SrSO4: (a) free inhibitor, (b) 5 mg·L−1, (c) 9 mg·L−1, (d) 13 mg·L−1 of APES/AA/AMPS.

XRD analysis

XRD is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The analyzed material is finely ground and homogenized, and average bulk composition is determined. The types and the crystallization intensity of strontium sulfate crystal are shown in Figure 10: (a) free inhibitor, (b) 5 mg·L−1, (c) 9 mg·L−1, (d) 13 mg·L−1 of APES/AA/AMPS. Figure 10(a) displays that the diffraction peaks of the precipitates could be well indexed to the phase of celestite, including the crystal faces of (011), (111), (210), (211), (112), (122), (312) and (323). With increasing concentration of APES/AA/AMPS, the crystallinity becomes smaller than free inhibitor, which can be demonstrated from the clearly increasing peak width at half height. The XRD photographs show that the addition of inhibitor doesn't change the crystal form of strontium sulfate but changes the morphology successfully, which is indicated in Figure 9.
Figure 10

XRD of SrSO4: (a) free inhibitor, (b) 5 mg·L−1, (c) 9 mg·L−1, (d) 13 mg·L−1 of APES/AA/AMPS.

Figure 10

XRD of SrSO4: (a) free inhibitor, (b) 5 mg·L−1, (c) 9 mg·L−1, (d) 13 mg·L−1 of APES/AA/AMPS.

CONCLUSION

The synthesis, characterization and evaluation of a non-phosphorus water treatment agent APES/AA/AMPS have been discussed in this paper. FT-IR and 1H-NMR results identify that the expected structure of APES/AA/AMPS has been obtained successfully. The optimal inhibition on strontium sulfate by adjusting the molar ratios between APES and AMPS was chosen through atomic absorption spectrophotometry. The results of inhibition tests showed that the terpolymer is effective at controlling SrSO4 scale and dispersing ferric oxide. When the mass ratio of APES: AA: AMPS is 1:4:0.5, APES/AA/AMPS exhibited 93.3% inhibition on strontium sulfate at a threshold dosage of 9 mg·L−1. And when the mass ratio of monomers is 4:1:0.3(AA: APES: AMPS), the minimum transmittance of solution is 13.7% at the dosage of 4 mg·L−1, showing excellent performance on dispersing ferric oxide. Then, the efficiency of the as-synthesized APES/AA/AMPS was compared with other commercial inhibitors, and the results proved that the water treatment agent has a similar or better scale inhibition efficacy than commonly used commercial water treatment agents. The results in SEM and XRD indicated that the morphology of strontium sulfate was changed and crystallinity was decreased during the inhibition process. This excellent inhibition efficacy towards strontium sulfate and ferric oxide may be attributed to the synergistic effects of the functional groups like carbonyl, allyloxy, amide, sulfonic acid and ester in as-synthesized APES/AA/AMPS.

ACKNOWLEDGEMENTS

This work was supported by the Prospective Joint Research Project of Jiangsu Province (BY2012196); National Natural Science Foundation of China (51177013); Special funds for Jiangsu Province Scientific and Technological Achievements Projects of China (BA2011186); Program for Training of 333 High-Level Talent, Jiangsu Province of China (BRA2011033); Scientific Innovation Research Foundation of College Graduate in Jiangsu Province (CXLX13-107).

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