Article pubs.acs.org/EF
Synthesis of a Novel Dendrimer-Based Demulsifier and Its Application in the Treatment of Typical Diesel-in-Water Emulsions with Ultrafine Oil Droplets Xing Yao,† Bin Jiang,†,‡ Luhong Zhang,*,† Yongli Sun,† Xiaoming Xiao,† Zhiheng Zhang,‡ and Zongxian Zhao† †
School of Chemical Engineering and Technology and ‡National Engineering Research Center for Distillation Technology, Tianjin University, Tianjin 300072, P. R. China ABSTRACT: Waste water resulted from polymer flooding oil recovery generally has a bad impact on the subsequent process of enhanced oil recovery. Separating residual oil from oil/water (O/W) emulsion with suitable kinds of demulsifier is one strategy generally adopted by oil companies. Because of the existence of large amounts of ultrafine oil droplets with the average diameter less than 2 μm, the emulsions can be extremely difficult to break up. To solve this problem, an amine-based dendrimer demulsifier PAMAM (polyamidoamine) was synthesized in this study, and the efficiency of the demulsifier in dealing with O/W emulsions with ultrafine oil droplets was investigated. Because of its strong interfacial activity and relatively good solubility in water, the dendrimer-based demulsifier can easily attach to emulsified oil droplets in a stable diesel-in-water emulsion. The influences of temperature, settling time, and concentration of the demulsifier used on the efficiency of the demulsifier were investigated in detail. The optimal operating condition under which more than 90% oil was removed from the original emulsion by the demulsifier was found. In contrast, less than 2% oil was removed from the emulsion without applying the demulsifier under the same conditions. Micrographs showed that the PAMAM demulsifier could lead to the breakup of diesel-in-water emulsions with ultrafine oil droplets by flocculation and coalescence. The surface tension and interfacial tension at the diesel− water interface were also measured to give a basic understanding of the demulsification mechanism. Though not perfect in dealing with emulsions with the average oil droplets less than 2 μm due to the relatively high demulsifier dosage, its relatively simple synthetic procedure and mild operating conditions showed a great promise in industrial applications with unique advantages over traditional physical methods.
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INTRODUCTION
nontoxic and biodegradable polymer, ethylcellulose, and used it to break up emulsified water from naphtha-diluted bitumen. The ethylcellulose polymer not only showed great dewatering performance and could also assist the removal of fine solids with the water.6 Feng et al. (2011) then investigated the effect of hydroxyl content and molecular weight of the biodegradable ethylcellulose on dewatering rate in water-in-diluted bitumen emulsions. Their results showed that the performance of the demulsifier can also be linked with the molecular structure.7 To enhance the performance of current demulsifiers and develop novel recyclable demulsifiers, scientists have tried to graft amphiphilic polymers onto nanoparticles in order to take advantage of the unique properties of nanoparticles. Peng et al. (2012) developed a novel interfacial active nanoparticles, which can remain highly stable in the organic phase and can attach to the surface of water droplets. Once given a strong magnetic field, the attached water droplets would respond to the magnetic field accordingly; thus the demulsification process occurred.8 Peng et al. (2012) then investigated the separation efficiency of the demulsifier on heavy naphtha diluted bitumen emulsions. Their investigation showed the recyclability of the demulsifier is amazingly well.9 Li et al. (2014) also synthesized a novel magnetic demulsifier and investigated its application in
With the application of polymer flooding technology becoming more widely than ever in enhanced oil recovery, wastewater treatment has become a stubborn problem in the oil-extraction industry.1 Interfacial active substances resulted from this technology has made the wastewater more difficult to handle with traditional methods. Because of the amphiphilic property of certain molecules aggregating at the interface, the resulting emulsion can be extremely difficult to break for further treatment process.2 Thus, it is urgent to remove residual oil from the wastewater so that water can be recycled into the reinjection well for second usage. It is obvious that this kind of wastewater can be categorized into an oil-in-water (O/W) emulsion, which usually has a high oil content and small oil droplet size. Because certain kinds of interfacial active substances aggregate at the surface of oil droplets, the emulsions can be very stable. Furthermore, with water-soluble polymers such as HPAM adsorbing at the oil droplet surface, the aqueous phase become more viscous, which makes the demulsification operation more difficult.3 Without proper treatment, the wastewater could do great harm to the environment if directly ejected into rivers and lakes. Many demulsification techniques have been developed, including both physical and chemical methods.4,5 Recently, biodegradable polymers with amphiphilic properties and complex structures have attracted the attention of many scientists due to their environmental friendly properties. Feng et al. (2009) found a © 2014 American Chemical Society
Received: July 12, 2014 Revised: August 10, 2014 Published: August 19, 2014 5998
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the treatment of oil-charged industrial wastewater. They then demonstrated the recyclability of the demulsifier through some experiments as well.10 Apart from demulsifiers with traditional structure mentioned above, demulsifiers with novel structures have also been synthesized and tested. The relations between structure and performance are investigated to a great extent as well. Wang et al. (2006) synthesized a series of structurally different dendrimer-based demulsifiers and investigated the performance of these demulsifiers in treating crude oil emulsion. From their research results, they concluded that properly structurally designed dendrimer macromolecules can act as an effective demulsifier.11 Wang et al. (2008) synthesized a novel broom molecule and investigated its demulsification performance in treating the oil−water emulsion.12 Wang et al. (2010) then reasoned that dendrimers with more branches would demonstrate a better demulsification performance. To prove this, they synthesized a series of dendrimer-based demulsifier with the same basic structure and investigated the amount of PEO and PPO within the demulsifier molecular structure on the performance of the demulsifier.13 Zhang et al. (2005) synthesized several kinds of polyether demulsifier with a typical PEO−PPO copolymer as the branch structure. They also concluded that dendrimers with more branches would demonstrate better demulsification performance and the different amounts of PPO and PEO affect the interfacial activity and thus has a great influence on the performance of the demulsifier.14 Efficient as they are, these demulsification techniques have their own limitations, which is especially obvious in treating polymer flooding oil-extraction wastewater. In most cases, the average diameter of droplets in the emulsion system to be treated is around 5 μm, and numerous research papers in dealing with emulsions with the average diameter of around 5 μm have been published.6−14 However, when dealing with emulsions with much smaller oil droplets, the abilities of most techniques are far from satisfactory from an industry point of view, which is also common in the oil recovery industry. Speth et al. (2002) once used fiber-bed coalescers to deal with emulsions with the average diameter of oil droplets around 2 μm and developed a physically founded model describing the coalescence process.15 Apart from that, no research on using demulsifiers to treat emulsions with such small oil droplets has been reported yet. As a result, it is urgent to develop a proper kind of demulsifier, which could result in a high oil removal rate as well as rapid oil−water separation for emulsions with the average diameter of oil droplets less than 2 μm. Dendrimers are specially designed macromolecules with a certain size, shape, and reactivity. Generally, they are branched from a central core, with numerous terminal groups surrounding the core so as to produce an empty interior. This novel kind of dendrimers was developed by Tomalia and Newkome in the 1980s. 11 Because specially designed dendrimers with certain interfacial activity can dissolve the original interfacial substances on the surface of the oil droplets rapidly, their potential to break the O/W emulsion is extremely strong.11 Polyamidoamine (PAMAM) is a kind of dendrimer, which has a polar but hydrophobic interior with polar terminal groups on the outer surface. Experimental results showed that the structure of the terminal group contributes most to the demulsification process. Only the amine-based dendrimer proved to be an effective demulsifier.11
Even though numerous research papers on PAMAM have been published, research on its demulsification ability has not been conducted much. Only Wang et al. investigated the demulsification ability of PAMAM.11,12 So far, no research using PAMAM to break up diesel-in-water emulsions with the average diameter of oil droplets less than 2 μm has been reported yet. In this study, 1,3-propanediamine were first reacted with methyl acrylate and then with ethanediamine. The resultants were treated with the same synthetic procedure twice (first reacted with methyl acrylate and then with ethanediamine), and then the final products PAMAM were obtained. The prepared demulsifier was then applied to typical diesel-in-water (O/W) emulsions with the average diameter of oil droplets less than 2 μm, which were referred to as the ultrafine oil droplets. The influences of temperature, settling time, and demulsifier concentration on the performance of the demulsifier were investigated in detail. This study showed that, under certain conditions, the oil removal rate could reach more than 90%, which perfectly meets the industrial requirements. Micrographs of the emulsions with and without treatment of the demulsifier were taken and compared to confirm the flocculation and coalescence process during the demulsification process. Surface tension and interfacial tension of the demulsifier were measured to give a basic understanding of the demulsification mechanism. This is the first report on the synthesis of PAMAM-based demulsifier applied to diesel-in-water emulsions with the average diameter of oil droplets less than 2 μm. Though not perfect in dealing with emulsions with ultrafine oil droplets, this study shed light upon a novel chemical method in dealing with this kind of emulsion with obvious advantages over physical methods like fiber-bed coalescers developed by Speth.15
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EXPERIMENTAL DETAILS
Materials. All chemicals were used directly without further purification. Methyl acrylate (AR grade, ≥ 0.98), ethanediamine (AR grade, ≥ 0.98), and methanol (AR grade, ≥ 0.98) were all purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. 1,3-Propanediamine (AR grade, ≥ 0.98) was purchased from Aladdin Reagents. Sodium dodecyl sulfate (SDS) (CR grade) was purchased from Tianjin Guangfu Fine Chemical Research Institute. Synthesis of an Amine Dendrimer-Based Demulsifier. A portion of 12.5 mL of 1,3-propanediamine was dissolved in 100 mL of methanol; then 130 mL of methyl acrylate was added into the flask. The mixture was stirred for 24 h at 25 °C. The solvent and the unreacted methyl acrylate were removed in a rotatory evaporator, and then the resultant was put into a vacuum oven for further purification. The resultant was then dissolved in 100 mL of methanol; then 130 mL of ethylenediamine was added into the flask. The mixture was stirred for 24 h at 25 °C. The solvent and unreacted ethylenediamine were removed in a rotatory evaporator, and then the resultant was put into a vacuum oven for further purification. The above process was then repeated twice. Then the final product was obtained.11 Preparation of Diesel-in-Water Emulsions with Ultrafine Oil Droplets. The emulsions were prepared with deionized water and diesel. A sample of 50 g of diesel and 1 g of SDS were added into a volumetric flask with the volume of 1 L. Then deionized water was added into the flask until the volume of the mixture reached one liter. Then the mixture were treated with a homogenizer (Fluke homogenizer, 500W) operated at 10 000 rpm for 5 min. The resulting emulsion contained 5 wt %% diesel and was referred to as diesel-inwater emulsions. The emulsions obtained as such are very stable within the experimental time frame and are extremely complex with average drop sizes typically less than 2 μm measured by Malvern Mastersizer 3000. 5999
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Demulsification Test. The ability of the demulsifier was tested by measuring the oil content and the size of oil droplets in diesel-in-water emulsions after the demulsification process finished. In each test, 25 mL of freshly prepared emulsion and 1 mL of demulsifier solution with a certain concentration were thoroughly mixed in a 25 mL colorimeter tube by shaking the mixture 200 times by hand.8,9 Then the mixture was put in a water bath (Shanghai Yijing, YQ-120C) under different temperatures for different periods of time. Subsequently, the solution at the bottom of the colorimeter tube was taken out, and the oil content in it was measured using an ultraviolet spectrophotometer (UNIC, UV-4802). Each sample was repeated three times, and the oil content reported is the average of the three repetitions. The blank tests were performed for diesel-in-water emulsions without demulsifier addition as a control. The demulsification performance is derived from the oil removal rate, which can be calculated from the equation:
R=
C0 − C × 100% C0
where R (%) is the oil removal rate, C0 (mg L−1) is the initial oil content, and C (mg L−1) represents the oil content after the demulsifier solution was added.10 After settling in the water bath of 70 °C for 90 min, micrographs of the emulsion sample without any demulsifier addition and that with 2000 mg L−1 demulsifier addition were recorded using an optical microscope equipped with a digital video camera linked with a computer. The emulsion sample was put on an object slide and then covered with a cover glass. The image was taken under halogen light. Surface Tension and Interfacial Tension Measurement. The surface tension of the dendrimer-based demulsifier solution with different concentrations was measured using an interfacial tensiometer. The interfacial tension of diesel−water interface with the dendrimerbased demulsifier with different concentrations in the water phase was also measured using an interfacial tensiometer. The interfacial tension of diesel−water interface without the dendrimer-based demulsifier in the water phase and that with only sodium dodecyl sulfate of certain concentration in the water phase were measured as a control.
Figure 1. (a) One cycle of the synthesis procedure of the PAMAM demulsifier. (b) Molecular structure of PAMAM demulsifier.
RESULTS AND DISCUSSION Synthesis of PAMAM Demulsifier. According to Wang et al.’s research, the demulsification rates increased as the dendrimer generation increased.11 However, when the number of generation increases to some certain extent, the densely piled surface groups bring great difficulty to the next step of reaction process, which causes insufficient further development of the dendrimer, thus making the molecular structure defect.11 Thus, the demulsification efficiency of the amine-based dendrimer of the third generation was systematically studied in this paper. Wang’s study on the influence of the ratio of reactants on the yield of the product also showed that, when the ratio of methyl acrylate and ethanediamine to the 1,3-propanediamine or the resultants from the previous reaction reaches much more than the molar ratio according to the chemical equation, the yield reaches more than 99.9%.11 Thus, during the synthesis procedure, the amounts of methyl acrylate and ethanediamine used were much more than the molar ratio required by the chemical equation. The PAMAM demulsifier were synthesized in two-step method with three cycles, and one cycle of the synthesis procedure is shown as Figure 1a. The molecular structure of the final product demulsifier is shown in Figure 1b. To identify the structure, 1H NMR spectra were recorded for the demulsifier. CDCl3 was used as the solvent.11 Experimental results (Figure 2a) indicate that the NMR spectra of the purified product totally matches those reported by ref 16. The hydrogen-1 chemical shift of the demulsifier showed in Figure 1 is as follows: (a) 2.39, (b) 2.46, (c) 2.68, (d) 2.59, (e) 3.20, (f)
2.73, (g) 2.68, (h) 2.59, (i) 3.20, (j) 2.73, (k) 2.68, (l) 2.59, (m) 3.20, (n) 2.73, and (o) 1.33. The unmarked chemical shift belongs to the unreacted ethanediamine which is extremely hard to remove from the final product due to the nanocontainer structure of the molecule.11 The characteristic protons of amino groups appeared at δ 1.33 ppm as a broad single peak. The reason why the chemical shift of the amino groups on the surface of the dendrimer molecule is relatively small is that the nitrogen atom linked with the hydrogen atom is not a strong electrophilic atom. The characteristic protons of other groups are also shown in Figure 2a, in which some of the groups in different parts of the molecule shared nearly the same chemical shift (c, g, and k at δ 2.68 ppm; d, h, and l at δ 2.59 ppm; f, j, and n at δ 2.73 ppm; e, i, and m at δ 3.20 ppm). This can be explained by their highly similar positions inside the molecule as shown in Figure 1b. To further identify the structure of the demulsifier, FTIR spectra were also recorded for the demulsifier. Figure 2 shows the FTIR spectra of PAMAM demulsifier. Typical bands associated with −NH2 vibration are visible at around 3269.60 cm−1. For −CONH−, the bands were observed at around 1645.80 and 1544.38 cm−1 with the former referred to as the stretching of CO and the latter referred to as the coupling band combined with the bending of N−H and the stretching of C−N. Typical bands associated with C−N vibration at around 1195.50 cm−1 confirmed the existence of N−CH2−. No typical bands being observed at around 1740 cm−1 showed that hardly any ester-terminated intermediate products existed in the final product. For C−N−CH2−, the typical band at 1032.39 cm−1
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Figure 3. Effect of demulsifier concentration on the oil removal rate in O/W emulsions at two certain circumstances.
emulsions prepared are extremely stable within a relatively long period of time. From Figure 3, it is clear that when the temperature and settling time increased, the oil removal rate increased as well under the same demulsifier concentration. To further prove the high efficiency of the demulsifier, systematically investigation on the influences of temperature, settling time as well as the demulsifier concentration on the performance of the demulsifier was conducted as follows. The effect of the demulsifier concentration on the oil removal rate at different temperatures is shown in Figure 4. For
Figure 2. (a) 1H NMR spectra of PAMAM demulsifier. (b) FTIR spectra of PAMAM demulsifier.
was observed. For −CH2−, the bands were observed at around 2927.56 and 2849.17 cm−1, which were referred to as the asymmetric and symmetric stretching vibration of −CH2−, respectively. Furthermore, typical bands of −COO-C at 1195.50 cm−1 and CC at 929.98 cm−1 were observed, although the area is extremely small, which indicated that small amount of ester-terminated intermediate products still existed in the final products. Demulsification of PAMAM Demulsifier. Figure 3 shows the relationship between oil removal rates measured by the oil content at the bottom of the emulsion and the demulsifier concentration applied under two certain circumstances. When no demulsifier was added into the emulsion, which can be regarded as a blank test, the oil removal rate under pure gravity was just 1.7% after 90 min at the temperature of 30 °C that is very near to room temperature. This explains that the emulsions prepared are very stable under normal conditions. To further prove the high stability of the emulsions, another experiment was conducted, in which the settling time was extended to 120 min and the temperature was set to 50 °C. Again, without any demulsifier addition, the final oil removal rate by natural gravity under the temperature of 50 °C after 120 min was just 9.1%. The above two blank test shows that the
Figure 4. Effect of demulsifier concentration on the oil removal rate in diesel-in-water emulsions.
this purpose, the demulsifier concentration in the diesel-inwater emulsion under study was set as 500 mg L−1, 1000 mg L−1, 1500 mg L−1, and 2000 mg L−1 in ascending order. Apart from that, the settling time was kept constant at 60 min, which is relatively much shorter than the settling time set in another research treating oilfield wastewater with typical O/W emulsions involved.11 Three pairs of experiments with the temperature ranging from 30 to 70 °C were conducted to give a qualitative impression of the demulsifier efficiency. It is obvious that, under each temperature, the oil removal rate increased 6001
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becomes much higher, with the slope of the curve increasing to 2.1. This can be explained by the molecular motion of the demulsifier molecules. According to the molecular dynamic theory, when at high temperature, the motion of demulsifier molecules becomes much stronger than at room temperature, which accelerates their transportation toward the surface of oil droplets thus greatly adds to the demulsification process. From Figure 5, it can also be clearly seen that when the demulsifier concentration reaches as high as 2000 mg L−1, the increasing trend of oil removal rate is relatively high at low temperatures, with the slope of the curve being 1.95. But when the temperature ranges are set from 50 to 70 °C, the increasing trend of oil removal rate slows down a lot with the slope of the curve being 0.85. With the oil removal rate increasing to 99%, nearly transparent water phase is got with hardly any oil in it. This could be the contribution of high demulsifier concentration to improving the transportation rate of demulsifier molecules. When the demulsifier concentration is relatively high at high temperature, much more demulsifier molecules are trying to reach the oil droplets in the water phase, which greatly adds to the viscosity of the water phase. This behavior of demulsifier molecules in turn slows down the rate of their transportation toward the oil droplets.6 In another perspective, when the oil removal rate is already as high as 80%, which means lots of demulsifier molecules have aggregated onto the surface of oil droplets to replace the original natural surfactants and SDS, the residual positions for other demulsifier molecules are not enough. Thus, some of the residual demulsifier molecules would tend to aggregate by themselves due to the high concentration, which further slows down the transportation rate of demulsifier molecules in the water phase, thus leading to such experimental results.6 Another interesting phenomenon could also be seen in Figure 5. When the demulsifier concentration was varied from 1000 mg L−1 to 1500 mg L−1, the oil removal rate only increased slightly at each temperature. Although the variance of increasing trend of oil removal rate with respect to temperature is not clearly observed, it can still be seen that the influence of demulsifier concentration plays a significant part in the demulsification process. An obvious plateau was observed when the demulsifier concentration ranged from 1000 to 1500 mg L−1. Thus, new pairs of experiments were conducted to investigate the effect of demulsifier concentration on the demulsification performance in detail. Figure 6 shows the effect of settling time on oil removal rate in diesel-in-water emulsions at the temperature of 30 °C with each curve indicating a certain demulsifier concentration. From Figure 6, it can be obviously seen that, when the settling time is 60 min, the oil removal rates of the four experiments at 30 °C are all extremely low. Even when the demulsifier concentration reaches more than 2000 mg L−1, the oil removal rate is still slightly more than 40%. However, when the settling time was just extended to 90 min, the oil removal rates increased rapidly for all four experiments with different demulsifier concentrations. Especially when the demulsifier concentration was set to as low as 500 mg L−1, the oil removal rate increased from 22% to 76% within the period of 30 min set previously, which has shown excellent performance of the demulsifier. However, when the settling time was extended to 120 min, the increasing rate is not as high as before. Even when the demulsifier concentration reached as high as 2000 mg L−1, the oil removal rate only increased a little, from 85% to 98%. Although oil removal rate of 98% means that the performance of the
with the increase of demulsifier concentration. When the temperature is 30 °C, the oil removal rate is very low even if the demulsifier concentration reaches as high as 2000 mg L−1. Less than 50% oil was removed from the emulsion system under this condition, which is undoubtedly far from satisfactory from an industry point of view due to its low efficacy at operating condition near room temperature. To further investigate the factors affecting the performance of the demulsifier, the temperature was raised to 50 °C, which is also easy to achieve without much energy in industry, the oil removal rates showed a significant increase, with more than 70% oil being removed when the demulsifier concentration are relatively high. And even when the concentration of the demulsifier is just 500 mg L−1, the oil removal rate is near 50%, which has shown much better performance than that in low temperature. From the two experiments shown above, it can be concluded that temperature plays a significant role in improving the performance of the demulsifier. However, even when the temperature reaches 50 °C and the demulsifier concentration reaches as high as 2000 mg L−1, the oil removal rate is just a little more than 80%, which still cannot meet the requirement standards of industry. Thus, experiment with the temperature of 70 °C was conducted to further investigate the influence of temperature on the performance of the demulsifier. From the experimental results, it can be seen that at 70 °C more than 90% oil was removed from the emulsion system even when the demulsifier concentration is just 500 mg L−1, which perfectly meets the industrial requirements. Satisfactory as it is, the influence of demulsifier concentration on the efficiency of the demulsifier was still unclear, with only a slightly increasing trend being observed. Thus, another four pairs of experiments were conducted to describe this trend in detail. Figure 5 shows the effect of temperature on oil removal
Figure 5. Effect of temperature on the oil removal rate in diesel-inwater emulsions.
rate in diesel-in-water emulsions. The general trend is the same as previously investigated that the oil removal rate increases with the demulsifier concentration. However, it is interesting to see that, even though all four curves showed an increasing trend, the difference of demulsifier concentration also plays a part in it. When the demulsifier concentration is as low as 500 mg L−1, the increasing trend of the curve is rather small at low temperatures, with the slope of the curve being just 1.3. But when at high temperatures, the increasing trend of the curve 6002
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Figure 6. Effect of settling time on the oil removal rate in diesel-inwater emulsions at 30 °C.
Figure 7. Effect of settling time on the oil removal rate in diesel-inwater emulsions at 50 °C.
demulsifier is extremely excellent, it can be clearly seen that the high demulsifier concentration also made some contribution. Thus, experimental results with the demulsifier concentration being set as 1500 mg L−1 and 1000 mg L−1 were compared for the purpose of finding the optimal concentration at 30 °C. When the demulsifier concentration was 1500 mg L−1, the oil removal rate at 30 °C was 92.6%, and when the demulsifier concentration was 1000 mg L−1, the oil removal rate at 30 °C was 82.8%. Thus, it is clearly that the optimal demulsifier concentration at 30 °C was 1500 mg L−1 for at this operating condition because industry requires more than 90% oil removal rate. Again, when the demulsifier concentration was 500 mg L−1, only a slightly increase in oil removal rate was observed when the settling time was extended from 90 to 120 min. With oil removal rate increasing from 72.5% to 75.6%, it can be concluded that the increase of demulsification efficiency has slowed down a lot at 30 °C with the demulsifier concentration of 500 mg L−1. According to the increasing trend shown by other three curves of Figure 6, the maximum oil removal rate are obviously higher when oil demulsifier concentration reaches a higher level although no clear plateau was observed in Figure 6. In Figure 7, when the temperature was raised to 50 °C, obvious plateaus were observed for the two curves with the demulsifier concentration being 2000 mg L−1 and 1500 mg L−1. With the demulsifier concentration of 1500 mg L−1, the oil removal rate increased from 91.1% to 92% when the settling time was extended from 90 to 120 min, being rather a slightly increase. While the demulsifier concentration reached 2000 mg L−1, the oil removal rate increased from 93.1% to 94.4% when the settling time was extended from 90 to 120 min. With an increase of nearly 1%, it can be regarded that plateau has been reached. When the demulsifier concentration was as low as 1000 mg L−1, the oil removal rate increased from 86.6% to 91% when the settling time was extended from 90 to 120 min. Even though with only a slightly increase of nearly 5%, the oil removal rate had reached more than 90%, which perfectly meets the industrial requirements. It is also interesting to see that the curve with the demulsifer concentration of 500 mg L−1 showed a slightly decrease when the settling time was extended from 90 to 120 min. With the oil
removal rate decreased from 74% to 70% when the settling time was extended from 90 to 120 min, it can be assumed that at the concentration of 500 mg L−1, more settling time would not contribute to improving the performance of the demulsifier. This can be explained as follows. At the temperature of 50 °C, the molecular motion at the interface, which actually means the surface monolayer of the oil droplets, has reached equilibrium to some extent. With finite amounts of demulsifier molecules in the water phase competing with the original free surfactants such as SDS to get to the interface, the final state has been achieved at the settling time of 90 min. As to the slightly decrease when the settling time is extended, it can be assumed that the equilibrium state has not been totally stable, which might result in errors in measurement. From the comparison between Figure 6 and Figure 7, it can be seen that the plateau tends to shift to higher demulsifer concentration band as the temperature rises (from 500 mg L−1 to 2000 mg L−1). This can be explained as follows. When the temperature is as low as 30 °C, with the concentration of demulsifier being only 500 mg L−1, the amount of free demulsifier molecules existing in the water phase is extremely small. Thus, increasing temperature to 50 °C could not significantly cause more free demulsifier molecules to get to the surface of oil droplets thus leading to demulsification. It can be clearly seen from the comparison between Figure 6 and Figure 7 that, when the temperature is raised from 30 to 50 °C, the curve indicating the demulsifier concentration of 500 mg L−1 at the section between 90 and 120 min basically remained the same height. However, when a higher demulsifier concentration is applied, the amount of free demulsifier molecules existing in the water phase is relatively much larger. Thus, when the temperature is raised from 30 to 50 °C, more free demulsifier molecules would try to get to the surface of oil droplets thus leading to demulsification due to stronger molecular motion caused by the increase of temperature. So no obvious plateau is observed for the curve indicating the demulsifier concentration of 1000 mg L −1 . Nevertheless, when the demulsifier concentration is as high as 1500 mg L−1 or 2000 mg L−1, even within 90 min at 50 °C, lots of demulsifier molecules have aggregated onto the surface of oil droplets to replace the original natural surfactants and SDS, the residual positions at the surface for other demulsifier molecules are not enough. 6003
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any oil droplets remained at the lower level of the emulsion (Figure 8c). It can be obviously seen that the demulsifier greatly improved flocculation and coalescence of emulsified oil droplets in diesel-in-water emulsions. Compared with other similar studies studying the demulsification process with larger droplets (the average diameter being around 5 μm) in the emulsions,6−14 it can be clearly seen that the demulsifier dosages they used are relatively lower than this study with other operating conditions more or less the same. This in some sense confirms the great difficulty in treating emulsions with ultrafine droplets (the average diameter being less than 2 μm). Though the demulsifier synthesized in this study might not be regarded as the perfect demulsifier in treating emulsions with ultrafine oil droplets for the high demulsifier dosage, it showed strong potentials in dealing with this kind of emulsion, which can be seen from the excellent performance under a certain temperature and settling time. Combined with its simple synthetic procedure as well as mild operating conditions, it showed obvious advantages over fiberbed coalescers developed by Speth15 from an industrial point of view. Surface Tension and Interfacial Tension Study of PAMAM Demulsifier. To get a further understanding of the demulsification mechanism, the surface tension of the aqueous solution of PAMAM demulsifier and the interfacial tension of the PAMAM demulsifier at the diesel−water interface were measured and compared. Table 1 shows the surface tension of
Thus, extending settling time to 120 min would not cause more free demulsifier molecules to get to the surface of oil droplets thus leading to demulsification. So obvious plateau is observed for the curve indicating the demulsifier concentration of 1500 mg L−1 or 2000 mg L−1 at the section between 90 and 120 min. Micrographs of typical diesel-in-water emulsions without and with 2000 mg L−1 demulsifier addition after settling for 90 min at 70 °C are shown separately in Figure 8a, b, and c. As shown
Table 1. Surface Tension of PAMAM Demulsifier Aqueous Solution demulsifier concentration (mg L−1) surface tension k(mN m−1)
0.0
1000.0
1500.0
2000.0
77.81
76.98
77.38
76.8
Table 2. Interfacial Tension of the Diesel−Water Interface with the PAMAM Demulsifier in the Water Phase demulsifier concentration (mg L−1) interfacial tension (mN m−1)
0.0
1000.0
1500.0
2000.0
38.12
7.89
7.99
7.75
the PAMAM demulsifier aqueous solution, and Table 2 shows the interfacial tension of the diesel−water interface with the PAMAM demulsifier in the water phase. It can be clearly seen from Table 1 that the PAMAM demulsifier cannot significantly lower the surface tension of pure water no matter what the concentration is. This result perfectly matches Wang’s research that amine terminated dendrimers cannot reduce the surface tension of pure water and thus do not behave like typical surfactants for air−water interface.11 According to the data shown in Table 2, it can be clearly seen that, with a certain amount of demulsifier in the water phase, the interfacial tension of the diesel−water interface get significantly lowered from 38.12 mN m−1 to less than 8 mN m−1. Even no clearly interfacial tension variance with the increase of demulsifier concentration was observed, it can still be concluded that lowering the interfacial tension is the precondition for the demulsification process to take place. To further investigate the relationship between interfacial tension and demulsification process, the interfacial tension at diesel-water interface with only SDS of 1 g L−1 in the water phase was also measured. The
Figure 8. Micrographs with of typical diesel-in-water emulsions without demulsifier addition after settling for 90 min at 70 °C (a), at a upper level with oil most occupied (b), and at a lower level with water most occupied (c).
in Figure 8a, the oil droplet size in the emulsion system without any demulsifier addition is actually less than 2 μm, which is in perfect agreement with the average oil droplet diameter measured by Malvern Mastersizer 3000. In contrast, when treated with the demulsifier with the concentration of 2000 mg L−1 at 70 °C for 90 min, the oil droplet size increased significantly, with the diameter of most oil droplets varied between 20 and 30 μm. And most of the oil droplets had moved to the upper level of the emulsion (Figure 8b). Hardly 6004
dx.doi.org/10.1021/ef501568b | Energy Fuels 2014, 28, 5998−6005
Energy & Fuels
Article
measured interfacial tension is 3.26 mN m−1, which also confirms the high stability of the emulsion system from the low interfacial tension perspective. Although this value was still a little lower than the interfacial tension at diesel-water interface with demulsifier in the water phase, the demulsification process still occurred. This can be explained by the properties of the interfacial monolayers with demulsifier molecules in it. Once the demulsifier molecules reach the surface of oil droplets, they tend to form nanoaggregates at the interface, which lead to reorientation of the interfacial substances at the interface. This in turn makes the interfacial monolayer more compressible.17 When the amount of demulsifier molecules at the interface increases, the stability of the interfacial monolayer gets weaker because the nanosize aggregates formed at the interface can reduce the mechanical strength of the monolayer, which has been confirmed by AFM images of deposited monolayer.17 From another perspective, the natural surfactants and SDS at the interface could be dissolved into the bulk phases of demulsifier once the demulsifier molecules reached the surface of oil droplets.16
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (No. 21336007).
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CONCLUSION According to the study reported above, the following conclusions can be made: (1) Generally, the oil removal rate increases with the increase of temperature, settling time, and demulsifier concentration. From the experimental data, it can be seen that with the settling time being 120 min, when the temperature was set as 30 °C and the demulsifier concentration was set as 1500 mg L−1, the oil removal rate could reach 92.6%, which perfectly meets the industrial requirements. Increasing the temperature or the demulsifier concentration would not do much more contribution to improving the oil removal rate, which can be seen from Figure 6 and Figure 7. Micrograph images showed that the PAMAM demulsifier can successfully add to the flocculation and coalescence of oil droplets in the system, which finally leads to the breaking of typical diesel-in-water emulsions. (2) Among the several factors leading to the demulsification process, the most significant factor is temperature, which can be seen from Figure 4 and Figure 5. The least significant factor is demulsifier concentration, which can be seen from Figure 6 and Figure 7 due to the high similarity of the four curves. The influence of settling time depends on the variance of time period, which can be seen in Figure 6 and Figure 7. (3) Though not perfect in dealing with emulsions with ultrafine oil droplets due to its high demulsifier dosage, this study shed light upon a novel method in dealing with this kind of emulsion. The simple synthetic procedure and mild operating conditions give its unique advantages in dealing with emulsions with ultrafine oil droplets over fiber-bed coalescers developed by Speth.15 (4) The surface tension and interfacial tension data were given to partially uncover the mechanism of this demulsification process, which showed that the precondition of demulsification process is the ability of the demulsifier to lower the interfacial tension of the diesel−water interface and that there are other factors contributing to this process such as the properties of the interfacial monolayer changed by the demulsifier molecules, which remains to be studied.
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