- Email: [email protected]
Evaluation of a novel CaSO4 scale inhibitor for a reverse osmosis system
Desalination 214 (2007) 193–199
Evaluation of a novel CaSO4 scale inhibitor for a reverse osmosis system Hesheng Li, Wei Liu*, Xijuan Qi College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China Tel. +86 (10) 6445 3979; Fax: +86 (10) 6443 4773; email: [email protected] Received 6 April 2006; Accepted 13 October 2006
Abstract The aim of this study is to certify the performances of a new type high-effective scale inhibitor we have developed for calcium sulfate scale inhibition. The inhibitor is a non-phosphorous copolymer with multi-functional groups, and its molecular weight is controlled to assure its high efficiency of scale inhibition. Compared with the known scale inhibitors of Flocon-135 and Flocon-100 by means of beaker experiments, it is proved that the performances of the new type inhibitor are of low dose, high tolerance for calcium ions, steady inhibition efficiency at the wide range of pH and iron concentration in the feed water, and little scale in a longer period at a certain dosage of inhibitor. Keywords: Scale inhibitor; Reverse osmosis; Calcium sulfate scale; Non-phosphorous
1. Introduction Potable water is lacking in many places of the world, so seawater desalination and sewage water recycling are widely used [1,2]. Reverse osmosis technique is one of the important membrane separation techniques to obtain high quality water. In the process of reverse osmosis, membrane fouling restricts its wide application because of the decrease of operating efficiency and longevity. Membrane fouling is the blockage of aperture *Corresponding author.
resulting from adsorption and deposition of granules, colloid particles, scale and microorganisms on the membrane [3]. It is well known that pretreatment of the feed water to prevent the membrane fouling is the pivotal part in reverse osmosis systems. One of the most common applied pretreatment methods to prevent or control the scaling of different salts in feed water is to add scale inhibitors. At first, acids and sodium hexa-metaphosphate (SHMP) were widely used to inhibit calcium sulfate and calcium carbonate scales [4].
0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.10.023
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H. Li et al. / Desalination 214 (2007) 193–199
However, these two chemicals cause some serious problems. For example, when over-dosed, they cause serious piping materials corrosion and the corrosion products tend to plugging the membrane. On other occasions, SHMP acts as the food for bacteria that can thrive in the phosphate environment and plug the membrane. The other problem is that acids and SHMP solution have to be prepared everyday, which is an inconvenient task for the operators. Furthermore, in an aqueous solution, SHMP tend to be hydrolyzed to form orthophosphate which will combine with calcium ion to form an insoluble sludge of calcium orthophosphate. As far as we know, most of reverse osmosis commercial inhibitors now applied are organic polymer-based inhibitors of which there are numerous products formulated from various monomers, copolymers and terpolymers, with predominantly carboxylic and sulfonic acid, etc. [5–10]. But both the phosphorous and nonphosphorous inhibitors have some drawbacks in application. For example, the widely used inhibitor of Flocon-135 has very good compatibility with the presence of iron in feed water, but it is a phosphenocarboxylic acid polymer so it has the troubles in the conditions of demanding nonphosphorous water system. Being polyacrylic acid polymer, Flocon-100 has relatively good performances of the non-phosphorous inhibitors on CaCO3 and CaSO4 scale under the condition of certain range pH, but its efficiency of inhibition decrease in feed water with low pH and high iron concentration. Therefore, it is necessary to develop a new type non-phosphorous scale inhibitor with high scale inhibition efficiency under the conditions of lower dosage of inhibitor, wider range of pH, higher supersaturation and well compatibility with the presence of iron in feed water simultaneously. The efficiency of an inhibitor is influenced by many factors such as its chemical formulation, its concentration, supersaturation degree of the insoluble salts, iron ion concentration, and the pH
value of the feed water. To optimize the industry application condition, such as the proper dosage, the efficiency of an inhibitor has to be evaluated first on laboratory scale. In industrial application of reverse osmosis water, the calcium sulfate scale is one of the main causes of membrane fouling. In this paper, the efficiency of a new type of non-phosphorous inhibitor was examined against calcium sulfate scale. Most of performances of the studied inhibitor were compared with inhibitors Flocon-135 and Flocon-100 through beaker experiment.
2. Experimental The supersaturated solutions of calcium sulfate were prepared with anhydrous CaCl2, anhydrous Na2SO4 and distilled water. A new type or compared inhibitor was prepared and added into the solution. The adding inhibitor dosage is determined according to the same the concentrations of active ingredients of different inhibitors in each test water sample though the concentrations of active ingredients of each commercial inhibitor are different. The water solutions containing various dosages were adjusted to a pH value of 5.7. The beaker experiments were performed at a temperature of 70EC for 10 hours. An aliquot of the supernatant liquid was withdrawn for analysis to determine the concentration of soluble calcium ions using a standard solution of EDTA titration. The efficiency of scale inhibition was calculated based on the concentration of calcium ions remaining in the supernatant liquid from the flask containing no inhibitor and the flask containing a known amount of the inhibitor, respectively. The efficiency of scale inhibition was calculated as the following:
⎡ Ca 2+ ⎤ ⎣ ⎦1 × 100 Efficiency of scale inhibition = 2 + ⎡ Ca ⎤ ⎣ ⎦ 2
H. Li et al. / Desalination 214 (2007) 193–199
195
where [Ca2+]1 is the concentration of soluble calcium ion, mg/L and [Ca2+]2 is the concentration of total calcium ion, mg/L.
3. Results and discussion 3.1. Effect of the molecular weight on efficiency of scale inhibition The aqueous solution of the new type scale inhibitor whose molecular weight varied with the synthesis condition was obtained through polyreaction. The effect of the copolymer molecular weight on the efficiency of scale inhibition was measured with beaker experiments under the pH value of 5.7, the supersaturation ratio r of calcium sulfate of 7.92×104 and the inhibitor dosage of 8 mg/L. Here the supersaturation ratio r of calcium sulfate is defined as the ratio of the ionic product [Ca2+]×[SO2! 4] and the solubility product KSP :
r=
Fig. 1. Efficiency of scale inhibition vs. the inhibitor molecular weight.
⎤ [Ca 2+ ] × ⎡⎣SO24 ⎦ K SP
The test results are shown in Fig. 1. When the inhibitor molecular weight (MW) is 4000 to 6000, the efficiency of scale inhibition of inhibitor decreased obviously with the increasing of MW. When the MW is larger than 6000, it has no obvious influence on the efficiency. It can be expect that the efficiency will reach higher when the inhibitor MW is lower than 4000, but the inhibitor with that low MW is still to be synthesized. 3.2. Effect of the dosage on the efficiency of scale inhibition The effect of the dosage on the efficiency of scale inhibition was tested with several repeating runs conducted in terms of various inhibitor dosages in the range from 2 to 16 mg/L and
Fig. 2. Efficiency of scale inhibition vs. dosage.
various concentrations of calcium ions. Fig. 2 shows the results. It can be seen that the efficiency of scale inhibition increases evidently with the increasing inhibitor dosage when the dosage is low. However, when the dosage reaches a threshold, the efficiency of scale inhibition increases little with increasing inhibitor dosage. And the threshold decreases with decreasing calcium ions concentration. The efficiency of scale inhibition of the studied inhibitor was compared with that of Flocon-135 and Flocon-100 under various
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Fig. 3. Efficiency of scale inhibition vs. dosage.
dosages. The results are shown in Fig. 3. It can be seen that the studied inhibitor can reach the same efficiency of scale inhibition as that of both Flocon-135 and Flocon-100 at a much lower dosage. The threshold value of the efficiency of the studied inhibitor is also much smaller than that of both Flocon-135 and Flocon-100, so the studied inhibitor can be effectively used at a much lower dosage so as to save the cost. 3.3. Effect of the pH value on the efficiency of scale inhibition The relationship between the efficiency of scale inhibition and the pH value was examined under a dosage level of 8 mg/L and a supersaturation of 7.92×104. Fig. 4 shows the results. It can be seen that the pH value has little effect on the efficiency of scale inhibition, so the calcium sulfate scale formation is insensitive to pH value of the water sample when the studied inhibitor exists. Although pH influences the CaSO4 solubility, our experiment results verify that in the range of the tested supersaturation and the pH of the feed water, the influence can be neglected when the inhibitor is added in the feed water. Therefore the inhibitor can be applied well under wide range of pH value. The reason may be
Fig. 4. Efficiency of scale inhibition vs. pH value of the water sample.
attributed to the structure of the copolymer inhibitor. The copolymer inhibitor belongs to polyelectrolyte, and it has both anion and cation functional groups [11]. The efficiency of scale inhibition of Flocon-135 inhibitor is also insensitive to the pH value of the water sample, but the efficiency of scale inhibition of the studied inhibitor was higher than that of Flocon-135. Although to some extent the Flocon-100 is sensitive to the pH value of the water sample, its inhibition efficiency is higher than that of Flocon135 almost at all the pH range tested. But the inhibition efficiency of Flocon-100 is also lower than that of the studied inhibitor at all the pH range tested. Therefore the studied inhibitor is the best inhibitor for various pH ranges. 3.4. Effect of supersaturation ratio r of calcium sulfate on the efficiency of scale inhibition The increase of the supersaturation ratio r of calcium sulfate raises the tendency for calcium sulfate precipitation. Under the condition of the same inhibitor dosage and supersaturation ratio r, the higher efficiency of scale inhibition is, the more outstanding the scale inhibitor is. Under the pH value of 5.7 and the inhibitor dosage of
H. Li et al. / Desalination 214 (2007) 193–199
Fig. 5. Efficiency of scale inhibition vs. the supersaturation ratio r of calcium sulfate.
8 mg/L, the relationship between the efficiency of scale inhibition and supersaturation ratio r of calcium sulfate was tested through changing the calcium ions concentration or sulfate ions. The results are shown in Fig. 5. It can be seen that the efficiency of scale inhibition decreases with increasing supersaturation ratio r or calcium ions concentration. During the process of the reverse osmosis, the supersaturation ratio r of calcium sulfate becomes higher and higher because the calcium ions in the aqueous solution accumulate at the side of the membrane. Therefore, the scale inhibitor that has high tolerance for calcium ions or the supersaturation ratio r of calcium sulfate is required in the reverse osmosis system. Fig. 5 shows that the efficiency of scale inhibition of the studied inhibitor is only slightly fluctuant at all the test range of the supersaturation ratio r whereas the efficiency of both Flocon-135 and Flocon-100 decreased observably at the same range. Therefore, the studied inhibitor has high tolerance for calcium ions and could prevent membrane fouling with calcium sulfate scales more effectively. 3.5. Effect of the iron ion concentration in feed water on efficiency of scale inhibition On normal condition, there is a little iron ion
197
Fig. 6. Efficiency of scale inhibition vs. iron ion concentration in water.
in the feed water due to the slight pipe corrosion, so the effect of the iron ion concentration on the efficiency of scale inhibition has to be considered. The effect was examined under the inhibitor dosage of 8mg/L, the pH value of 5.7 and the supersaturation ratio r of calcium sulfate of 7.92×104. The results are shown in Fig. 6. It can be seen that the efficiencies of scale inhibition of all the three inhibitors decrease with increasing iron ions concentration. But the efficiency of scale inhibition of the studied inhibitor is higher than that of both Flocon-135 and Flocon-100 at all the iron ions concentration range. 3.6. Effect of the time on the scale deposition For a practical system, the occasional accident such as the inhibitor feeding pump or watercirculating pump failure cannot be avoided, so the inhibitor is expected to keep the high efficiency of scale inhibition in a relative long period in order to prevent the membrane fouling in the static liquid. The efficiency of scale inhibition for a long period was tested under a certain inhibitor dosage and various other conditions. Fig. 7 shows the test results of scale deposition under various dosages of scale inhibitor for
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Fig. 7. Scale deposition vs. time at 70EC of various dosages.
Fig. 8 Scale deposition vs. time at 25EC of various dosages.
various periods at a temperature of 70EC. When the inhibitor is not added, scale grows rapidly with time during the initial 28 h. Define the scale deposition amount corresponding to the calculated efficiency of scale inhibition being greater than 95% as little scale deposition, then it can be considered that the scale deposit little during the first 12 h when the dosage of the inhibitor is 10 mg/L. And the scale deposition increases larger and larger with time after the initial period of time, but it gradually reaches a constant amount. When the dosage of the inhibitor is 30 mg/L, the scale has the same increase tendency as that of the dosage of the inhibitor being 10 mg/L, but the period with little scale deposition is prolonged and the final constant scale amount is decreased. When the dosage is 60 mg/L, the scale amount keeps little during all the test period. Fig. 8 shows the scale deposition vs. time under the test condition of 25EC and various inhibitor dosages. The little scale deposition periods are respectively 72 h, 72 h and 32 h when the dosages are 24 mg/L, 16 mg/L and 10 mg/L, respectively. Compare Fig. 7 with Fig. 8. It can also be seen that the little scale period is prolonged when the test temperature is decreased at the inhibitor dosage of 10mg/L.
Fig. 9. Scale deposition vs. time at various supersaturations.
Fig. 9 shows the scale deposition vs. time under the same temperature, inhibitor dosage, and various supersaturations ratio r. The little scale deposition periods are respectively 34 h, 22 h, and 16 h when the r is 150, 250, and 350, respectively. It can also be seen that the little scale periods are prolonged with supersaturation decreasing. Take Figs. 7–9 into account. It can be seen that even the supersaturations ratio r is as high as 350 and the temperature is as high as 70EC, the
H. Li et al. / Desalination 214 (2007) 193–199
membrane fouling resulting from calcium sulfate scale can also be effectively prevented for a long period at the dosage of the inhibitor of 60 mg/L. When the supersaturations ratio r or the temperature is decreased, the actual application dosage can be decreased correspondingly. Furthermore, if the actual application dosage is lower than that of preventing membrane fouling effec-tively, additional inhibitor can be added into the liquid when the accidents do happen. The above test results can be a reference of the dosage needed.
4. Conclusions The results of this study clearly show that the performances of scale inhibition of the new type inhibitor are outstanding relative to known polymer inhibitors. Its characteristics are as following: C It is low cost and non-phosphorous. C Its efficiency is strongly dependent upon its molecular weight (MW) and its inhibition performance was high at the MW of about 4000. C The results of beaker experiments show that the new inhibitor is more excellent than both
199
Flocon-135 and Flocon-100 against calcium sulfate scale in the aspects of smaller dosage, wider range of pH value, higher tolerance for calcium ions or higher supersaturation and being effected less by iron ion. And the new type inhibitor performs very well under inclement application conditions such as interruption of dosing pump or pH value excursion.
References [1] H. Zidouri, Desalination, 131 (2000) 137–145. [2] M. Al-Shammiri and M. Al-Dawas, Desalination, 110 (1997) 37–48. [3] G. Zhao, M. Luo and G. Liu, Industry Water Treatment (in Chinese), 20 (2000) 25–27. [4] R.Y. Ning and J.P. Netwig, Desalination, 143 (2002) 29–34. [5] H.A. El Dahan and H.S. Hegazy, Desalination, 127 (2000) 111–118. [6] R.J. Lipinki, US Patent No. 4, 138, 353, 1979. [7] R. Christensen, US Patent No.4, 406, 811, 1983. [8] Nalco Chemical Co., J.P. 58 199 877, 1983. [9] D. Hasson, D. Bramson, B. Limoni-Relis and R. Semiat, Desalination, 108 (1996) 67–79. [10] M. Al-Shammiri, M. Safar and M. Al-Dawas, Desalination, 128 (2000) 1–16. [11] W. Liu, H. Li, X. Qi and T. Wang, Ch. Patent No. 10,056,952, 2006.
Evaluation of a novel CaSO4 scale inhibitor for a reverse osmosis system Hesheng Li, Wei Liu*, Xijuan Qi College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China Tel. +86 (10) 6445 3979; Fax: +86 (10) 6443 4773; email: [email protected] Received 6 April 2006; Accepted 13 October 2006
Abstract The aim of this study is to certify the performances of a new type high-effective scale inhibitor we have developed for calcium sulfate scale inhibition. The inhibitor is a non-phosphorous copolymer with multi-functional groups, and its molecular weight is controlled to assure its high efficiency of scale inhibition. Compared with the known scale inhibitors of Flocon-135 and Flocon-100 by means of beaker experiments, it is proved that the performances of the new type inhibitor are of low dose, high tolerance for calcium ions, steady inhibition efficiency at the wide range of pH and iron concentration in the feed water, and little scale in a longer period at a certain dosage of inhibitor. Keywords: Scale inhibitor; Reverse osmosis; Calcium sulfate scale; Non-phosphorous
1. Introduction Potable water is lacking in many places of the world, so seawater desalination and sewage water recycling are widely used [1,2]. Reverse osmosis technique is one of the important membrane separation techniques to obtain high quality water. In the process of reverse osmosis, membrane fouling restricts its wide application because of the decrease of operating efficiency and longevity. Membrane fouling is the blockage of aperture *Corresponding author.
resulting from adsorption and deposition of granules, colloid particles, scale and microorganisms on the membrane [3]. It is well known that pretreatment of the feed water to prevent the membrane fouling is the pivotal part in reverse osmosis systems. One of the most common applied pretreatment methods to prevent or control the scaling of different salts in feed water is to add scale inhibitors. At first, acids and sodium hexa-metaphosphate (SHMP) were widely used to inhibit calcium sulfate and calcium carbonate scales [4].
0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.10.023
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H. Li et al. / Desalination 214 (2007) 193–199
However, these two chemicals cause some serious problems. For example, when over-dosed, they cause serious piping materials corrosion and the corrosion products tend to plugging the membrane. On other occasions, SHMP acts as the food for bacteria that can thrive in the phosphate environment and plug the membrane. The other problem is that acids and SHMP solution have to be prepared everyday, which is an inconvenient task for the operators. Furthermore, in an aqueous solution, SHMP tend to be hydrolyzed to form orthophosphate which will combine with calcium ion to form an insoluble sludge of calcium orthophosphate. As far as we know, most of reverse osmosis commercial inhibitors now applied are organic polymer-based inhibitors of which there are numerous products formulated from various monomers, copolymers and terpolymers, with predominantly carboxylic and sulfonic acid, etc. [5–10]. But both the phosphorous and nonphosphorous inhibitors have some drawbacks in application. For example, the widely used inhibitor of Flocon-135 has very good compatibility with the presence of iron in feed water, but it is a phosphenocarboxylic acid polymer so it has the troubles in the conditions of demanding nonphosphorous water system. Being polyacrylic acid polymer, Flocon-100 has relatively good performances of the non-phosphorous inhibitors on CaCO3 and CaSO4 scale under the condition of certain range pH, but its efficiency of inhibition decrease in feed water with low pH and high iron concentration. Therefore, it is necessary to develop a new type non-phosphorous scale inhibitor with high scale inhibition efficiency under the conditions of lower dosage of inhibitor, wider range of pH, higher supersaturation and well compatibility with the presence of iron in feed water simultaneously. The efficiency of an inhibitor is influenced by many factors such as its chemical formulation, its concentration, supersaturation degree of the insoluble salts, iron ion concentration, and the pH
value of the feed water. To optimize the industry application condition, such as the proper dosage, the efficiency of an inhibitor has to be evaluated first on laboratory scale. In industrial application of reverse osmosis water, the calcium sulfate scale is one of the main causes of membrane fouling. In this paper, the efficiency of a new type of non-phosphorous inhibitor was examined against calcium sulfate scale. Most of performances of the studied inhibitor were compared with inhibitors Flocon-135 and Flocon-100 through beaker experiment.
2. Experimental The supersaturated solutions of calcium sulfate were prepared with anhydrous CaCl2, anhydrous Na2SO4 and distilled water. A new type or compared inhibitor was prepared and added into the solution. The adding inhibitor dosage is determined according to the same the concentrations of active ingredients of different inhibitors in each test water sample though the concentrations of active ingredients of each commercial inhibitor are different. The water solutions containing various dosages were adjusted to a pH value of 5.7. The beaker experiments were performed at a temperature of 70EC for 10 hours. An aliquot of the supernatant liquid was withdrawn for analysis to determine the concentration of soluble calcium ions using a standard solution of EDTA titration. The efficiency of scale inhibition was calculated based on the concentration of calcium ions remaining in the supernatant liquid from the flask containing no inhibitor and the flask containing a known amount of the inhibitor, respectively. The efficiency of scale inhibition was calculated as the following:
⎡ Ca 2+ ⎤ ⎣ ⎦1 × 100 Efficiency of scale inhibition = 2 + ⎡ Ca ⎤ ⎣ ⎦ 2
H. Li et al. / Desalination 214 (2007) 193–199
195
where [Ca2+]1 is the concentration of soluble calcium ion, mg/L and [Ca2+]2 is the concentration of total calcium ion, mg/L.
3. Results and discussion 3.1. Effect of the molecular weight on efficiency of scale inhibition The aqueous solution of the new type scale inhibitor whose molecular weight varied with the synthesis condition was obtained through polyreaction. The effect of the copolymer molecular weight on the efficiency of scale inhibition was measured with beaker experiments under the pH value of 5.7, the supersaturation ratio r of calcium sulfate of 7.92×104 and the inhibitor dosage of 8 mg/L. Here the supersaturation ratio r of calcium sulfate is defined as the ratio of the ionic product [Ca2+]×[SO2! 4] and the solubility product KSP :
r=
Fig. 1. Efficiency of scale inhibition vs. the inhibitor molecular weight.
⎤ [Ca 2+ ] × ⎡⎣SO24 ⎦ K SP
The test results are shown in Fig. 1. When the inhibitor molecular weight (MW) is 4000 to 6000, the efficiency of scale inhibition of inhibitor decreased obviously with the increasing of MW. When the MW is larger than 6000, it has no obvious influence on the efficiency. It can be expect that the efficiency will reach higher when the inhibitor MW is lower than 4000, but the inhibitor with that low MW is still to be synthesized. 3.2. Effect of the dosage on the efficiency of scale inhibition The effect of the dosage on the efficiency of scale inhibition was tested with several repeating runs conducted in terms of various inhibitor dosages in the range from 2 to 16 mg/L and
Fig. 2. Efficiency of scale inhibition vs. dosage.
various concentrations of calcium ions. Fig. 2 shows the results. It can be seen that the efficiency of scale inhibition increases evidently with the increasing inhibitor dosage when the dosage is low. However, when the dosage reaches a threshold, the efficiency of scale inhibition increases little with increasing inhibitor dosage. And the threshold decreases with decreasing calcium ions concentration. The efficiency of scale inhibition of the studied inhibitor was compared with that of Flocon-135 and Flocon-100 under various
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H. Li et al. / Desalination 214 (2007) 193–199
Fig. 3. Efficiency of scale inhibition vs. dosage.
dosages. The results are shown in Fig. 3. It can be seen that the studied inhibitor can reach the same efficiency of scale inhibition as that of both Flocon-135 and Flocon-100 at a much lower dosage. The threshold value of the efficiency of the studied inhibitor is also much smaller than that of both Flocon-135 and Flocon-100, so the studied inhibitor can be effectively used at a much lower dosage so as to save the cost. 3.3. Effect of the pH value on the efficiency of scale inhibition The relationship between the efficiency of scale inhibition and the pH value was examined under a dosage level of 8 mg/L and a supersaturation of 7.92×104. Fig. 4 shows the results. It can be seen that the pH value has little effect on the efficiency of scale inhibition, so the calcium sulfate scale formation is insensitive to pH value of the water sample when the studied inhibitor exists. Although pH influences the CaSO4 solubility, our experiment results verify that in the range of the tested supersaturation and the pH of the feed water, the influence can be neglected when the inhibitor is added in the feed water. Therefore the inhibitor can be applied well under wide range of pH value. The reason may be
Fig. 4. Efficiency of scale inhibition vs. pH value of the water sample.
attributed to the structure of the copolymer inhibitor. The copolymer inhibitor belongs to polyelectrolyte, and it has both anion and cation functional groups [11]. The efficiency of scale inhibition of Flocon-135 inhibitor is also insensitive to the pH value of the water sample, but the efficiency of scale inhibition of the studied inhibitor was higher than that of Flocon-135. Although to some extent the Flocon-100 is sensitive to the pH value of the water sample, its inhibition efficiency is higher than that of Flocon135 almost at all the pH range tested. But the inhibition efficiency of Flocon-100 is also lower than that of the studied inhibitor at all the pH range tested. Therefore the studied inhibitor is the best inhibitor for various pH ranges. 3.4. Effect of supersaturation ratio r of calcium sulfate on the efficiency of scale inhibition The increase of the supersaturation ratio r of calcium sulfate raises the tendency for calcium sulfate precipitation. Under the condition of the same inhibitor dosage and supersaturation ratio r, the higher efficiency of scale inhibition is, the more outstanding the scale inhibitor is. Under the pH value of 5.7 and the inhibitor dosage of
H. Li et al. / Desalination 214 (2007) 193–199
Fig. 5. Efficiency of scale inhibition vs. the supersaturation ratio r of calcium sulfate.
8 mg/L, the relationship between the efficiency of scale inhibition and supersaturation ratio r of calcium sulfate was tested through changing the calcium ions concentration or sulfate ions. The results are shown in Fig. 5. It can be seen that the efficiency of scale inhibition decreases with increasing supersaturation ratio r or calcium ions concentration. During the process of the reverse osmosis, the supersaturation ratio r of calcium sulfate becomes higher and higher because the calcium ions in the aqueous solution accumulate at the side of the membrane. Therefore, the scale inhibitor that has high tolerance for calcium ions or the supersaturation ratio r of calcium sulfate is required in the reverse osmosis system. Fig. 5 shows that the efficiency of scale inhibition of the studied inhibitor is only slightly fluctuant at all the test range of the supersaturation ratio r whereas the efficiency of both Flocon-135 and Flocon-100 decreased observably at the same range. Therefore, the studied inhibitor has high tolerance for calcium ions and could prevent membrane fouling with calcium sulfate scales more effectively. 3.5. Effect of the iron ion concentration in feed water on efficiency of scale inhibition On normal condition, there is a little iron ion
197
Fig. 6. Efficiency of scale inhibition vs. iron ion concentration in water.
in the feed water due to the slight pipe corrosion, so the effect of the iron ion concentration on the efficiency of scale inhibition has to be considered. The effect was examined under the inhibitor dosage of 8mg/L, the pH value of 5.7 and the supersaturation ratio r of calcium sulfate of 7.92×104. The results are shown in Fig. 6. It can be seen that the efficiencies of scale inhibition of all the three inhibitors decrease with increasing iron ions concentration. But the efficiency of scale inhibition of the studied inhibitor is higher than that of both Flocon-135 and Flocon-100 at all the iron ions concentration range. 3.6. Effect of the time on the scale deposition For a practical system, the occasional accident such as the inhibitor feeding pump or watercirculating pump failure cannot be avoided, so the inhibitor is expected to keep the high efficiency of scale inhibition in a relative long period in order to prevent the membrane fouling in the static liquid. The efficiency of scale inhibition for a long period was tested under a certain inhibitor dosage and various other conditions. Fig. 7 shows the test results of scale deposition under various dosages of scale inhibitor for
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H. Li et al. / Desalination 214 (2007) 193–199
Fig. 7. Scale deposition vs. time at 70EC of various dosages.
Fig. 8 Scale deposition vs. time at 25EC of various dosages.
various periods at a temperature of 70EC. When the inhibitor is not added, scale grows rapidly with time during the initial 28 h. Define the scale deposition amount corresponding to the calculated efficiency of scale inhibition being greater than 95% as little scale deposition, then it can be considered that the scale deposit little during the first 12 h when the dosage of the inhibitor is 10 mg/L. And the scale deposition increases larger and larger with time after the initial period of time, but it gradually reaches a constant amount. When the dosage of the inhibitor is 30 mg/L, the scale has the same increase tendency as that of the dosage of the inhibitor being 10 mg/L, but the period with little scale deposition is prolonged and the final constant scale amount is decreased. When the dosage is 60 mg/L, the scale amount keeps little during all the test period. Fig. 8 shows the scale deposition vs. time under the test condition of 25EC and various inhibitor dosages. The little scale deposition periods are respectively 72 h, 72 h and 32 h when the dosages are 24 mg/L, 16 mg/L and 10 mg/L, respectively. Compare Fig. 7 with Fig. 8. It can also be seen that the little scale period is prolonged when the test temperature is decreased at the inhibitor dosage of 10mg/L.
Fig. 9. Scale deposition vs. time at various supersaturations.
Fig. 9 shows the scale deposition vs. time under the same temperature, inhibitor dosage, and various supersaturations ratio r. The little scale deposition periods are respectively 34 h, 22 h, and 16 h when the r is 150, 250, and 350, respectively. It can also be seen that the little scale periods are prolonged with supersaturation decreasing. Take Figs. 7–9 into account. It can be seen that even the supersaturations ratio r is as high as 350 and the temperature is as high as 70EC, the
H. Li et al. / Desalination 214 (2007) 193–199
membrane fouling resulting from calcium sulfate scale can also be effectively prevented for a long period at the dosage of the inhibitor of 60 mg/L. When the supersaturations ratio r or the temperature is decreased, the actual application dosage can be decreased correspondingly. Furthermore, if the actual application dosage is lower than that of preventing membrane fouling effec-tively, additional inhibitor can be added into the liquid when the accidents do happen. The above test results can be a reference of the dosage needed.
4. Conclusions The results of this study clearly show that the performances of scale inhibition of the new type inhibitor are outstanding relative to known polymer inhibitors. Its characteristics are as following: C It is low cost and non-phosphorous. C Its efficiency is strongly dependent upon its molecular weight (MW) and its inhibition performance was high at the MW of about 4000. C The results of beaker experiments show that the new inhibitor is more excellent than both
199
Flocon-135 and Flocon-100 against calcium sulfate scale in the aspects of smaller dosage, wider range of pH value, higher tolerance for calcium ions or higher supersaturation and being effected less by iron ion. And the new type inhibitor performs very well under inclement application conditions such as interruption of dosing pump or pH value excursion.
References [1] H. Zidouri, Desalination, 131 (2000) 137–145. [2] M. Al-Shammiri and M. Al-Dawas, Desalination, 110 (1997) 37–48. [3] G. Zhao, M. Luo and G. Liu, Industry Water Treatment (in Chinese), 20 (2000) 25–27. [4] R.Y. Ning and J.P. Netwig, Desalination, 143 (2002) 29–34. [5] H.A. El Dahan and H.S. Hegazy, Desalination, 127 (2000) 111–118. [6] R.J. Lipinki, US Patent No. 4, 138, 353, 1979. [7] R. Christensen, US Patent No.4, 406, 811, 1983. [8] Nalco Chemical Co., J.P. 58 199 877, 1983. [9] D. Hasson, D. Bramson, B. Limoni-Relis and R. Semiat, Desalination, 108 (1996) 67–79. [10] M. Al-Shammiri, M. Safar and M. Al-Dawas, Desalination, 128 (2000) 1–16. [11] W. Liu, H. Li, X. Qi and T. Wang, Ch. Patent No. 10,056,952, 2006.