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.
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
Strontium sulfate inhibition
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 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
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).
|m(APES):m(AA):m .||Concentration (mg·L−1)/inhibition of SrSO4 (%)|
|(AMPS) .||5 .||7 .||9 .||11 .||13 .||15 .||17 .|
|m(APES):m(AA):m .||Concentration (mg·L−1)/inhibition of SrSO4 (%)|
|(AMPS) .||5 .||7 .||9 .||11 .||13 .||15 .||17 .|
Comparison of inhibition efficiency on strontium sulfate scale
Inhibition efficiency of APES/AA/AMPS on ferric oxide
Comparison of inhibition efficiency on ferric oxide
Morphology characterization of strontium sulfate scale
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.
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).