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Article

The synthesis of a novel dendrimer-based demulsifier and its application inthe 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 ZhaoEnergy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501568b • Publication Date (Web): 19 Aug 2014

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Figure 1a. One cycle of the synthesis procedure of the PAMAM demulsifier. 84x59mm (300 x 300 DPI)

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Figure 1b. Molecular Structure of PAMAM demulsifier. 84x65mm (300 x 300 DPI)

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Figure 2a. 1H NMR spectra of PAMAM demulsifier. 84x59mm (300 x 300 DPI)

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Figure 2b. FTIR spectra of PAMAM demulsifier. 84x59mm (300 x 300 DPI)

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Figure 3. The effect of demulsifier concentration on oil removal rate in O/W emulsions at two certain circumstances.

84x59mm (300 x 300 DPI)

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Figure 4. The effect of demulsifier concentration on oil removal rate in diesel-in-water emulsions 84x59mm (300 x 300 DPI)

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Figure 5. The effect of temperature on oil removal rate in diesel-in-water emulsions. 84x59mm (300 x 300 DPI)

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Figure 6. The effect of settling time on oil removal rate in diesel-in-water emulsions at 30°C. 84x59mm (300 x 300 DPI)

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Figure 7. The effect of settling time on oil removal rate in diesel-in-water emulsions at 50°C. 84x59mm (300 x 300 DPI)

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Figure 8a. Micrographs of typical diesel-in-water emulsions without demulsifier addition after settling for 90 minutes at 70°C

84x87mm (300 x 300 DPI)

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Figure 8b. Micrographs of typical diesel-in-water emulsions with demulsifier addition after settling for 90 minutes at 70°C. (At upper level with oil most occupied)

84x84mm (300 x 300 DPI)

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Figure 8c. Micrographs of typical diesel-in-water emulsions with demulsifier addition after settling for 90 minutes at 70°C. (At lower level with water most occupied)

84x84mm (300 x 300 DPI)

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The synthesis and application of a novel dendrimer-based demulsifier

The synthesis of a novel dendrimer-based 1

demulsifier and its application in the treatment of 2

typical diesel-in-water emulsions with ultrafine oil 3

droplets 4

Xing Yao1, Bin Jiang

1, 2, Luhong Zhang

1*, Yongli Sun

1, Xiaoming Xiao

1, Zhiheng Zhang

2, 5

Zongxian Zhao1 6

1 School of Chemical Engineering and Technology, Tianjin University, Tianjin, P. R. China; 7

2 National Engineering Research Center for Distillation Technology, Tianjin University, 8

Tianjin, P. R. China 9

10

Keywords: Ultrafine oil droplets; Demulsifier; PAMAM; 11

Abstract 12

Waste water resulted from polymer flooding oil recovery generally has a bad impact on 13

subsequent process of enhanced oil recovery. Separating residual oil from O/W emulsion with 14

suitable kinds of demulsifier is one strategy generally adopted by oil companies. Due to the 15

existence of large amounts of ultrafine oil droplets with the average diameter less than 2µm, the 16

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emulsions can be extremely difficult to break up. To solve this problem, an amine-based 1

dendrimer demulsifier PAMAM (polyamidoamine) was synthesized in this study and the 2

efficiency of the demulsifier in dealing with O/W emulsions with ultrafine oil droplets was 3

investigated. Due to its strong interfacial activity and relatively good solubility in water, the 4

dendrimer-based demulsifier can easily attach to emulsified oil droplets in stable diesel-in-water 5

emulsion. The influences of temperature, settling time and concentration of the demulsifier used 6

on the efficiency of the demulsifier were investigated in detail. The optimal operating condition 7

under which more than 90% oil was removed from the original emulsion by the demulsifier was 8

found. In contrast, less than 2% oil was removed from the emulsion without applying the 9

demulsifier under the same conditions. Micrographs showed that the PAMAM demulsifier could 10

lead to the breakup of diesel-in-water emulsions with ultrafine oil droplets by flocculation and 11

coalescence. The surface tension and interfacial tension at diesel-water interface were also 12

measured to give a basic understanding of the demulsification mechanism. Though not perfect in 13

dealing with emulsions with the average oil droplets less than 2µm due to the relatively high 14

demulsifier dosage, its relatively simple synthetic procedure and mild operating condition 15

showed a great promise in industrial application with unique advantages over traditional physical 16

methods. 17

Introduction 18

With the application of polymer flooding technology becoming more widely than ever in 19

enhanced oil recovery, waste water treatment has become a stubborn problem in oil-extraction 20

industry.1 Interfacial active substances resulted from this technology has made the waste water 21

more difficult to handle with traditional methods. Due to the amphiphilic property of certain 22

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molecules aggregating at the interface, the resulting emulsion can be extremely difficult to break 1

for further treatment process.2 Thus it is urgent to remove residual oil from the waste water so 2

that water can be recycled into the reinjection well for second usage. It is obvious that this kind 3

of waste water can be categorized into an oil-in-water (O/W) emulsion, which usually has a high 4

oil content and small oil droplet size. Because certain kinds of interfacial active substances 5

aggregate at the surface of oil droplets, the emulsions can be very stable. Furthermore, with 6

water soluble polymers such as HPAM adsorbing at the oil droplet surface, the aqueous phase 7

become more viscous, which makes the demulsification operation more difficult.3 Without 8

proper treatment, the waste water could do great harm to the environment if directly ejected into 9

rivers and lakes. Many demulsification techniques have been developed, including both physical 10

and chemical methods.4,5

Recently, biodegradable polymers with amphiphilic properties and 11

complex structures have attracted the attention of many scientists due to their environmental 12

friendly properties. Feng et al. (2009) found a nontoxic and biodegradable polymer, 13

ethylcellulose and used it to break up emulsified water from naphtha-diluted bitumen. The 14

ethylcellulose polymer not only showed great dewatering performance and could also assist the 15

removal of fine solids with the water.6 Feng et al. (2011) then investigated the effect of hydroxyl 16

content and molecular weight of the biodegradable ethylcellulose on dewatering rate in 17

water-in-diluted bitumen emulsions. Their results showed that the performance of the demulsifier 18

can also be linked with the molecular structure.7 To enhance the performance of current 19

demulsifiers and develop novel recyclable demulsifiers, scientists have tried to graft amphiphilic 20

polymers onto nanoparticles in order to take advantage of the unique properties of nanoparticles. 21

Peng et al. (2012) developed a novel interfacial active nanoparticles, which can remain highly 22

stable in the organic phase and can attach to the surface of water droplets. Once given a strong 23

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magnetic field, the attached water droplets would respond to the magnetic field accordingly, thus 1

the demulsification process occurred.8 Peng et al. (2012) then investigated the separation 2

efficiency of the demulsifier on heavy naphtha diluted bitumen emulsions. Their investigation 3

showed the recyclability of the demulsifier is amazingly well.9 Li, Shuqiang et al. (2014) also 4

synthesized a novel magnetic demulsifier and investigated its application in the treatment of 5

oil-charged industrial waste water. They then demonstrated the recyclability of the demulsifier 6

through some experiments as well.10

Apart from demulsifiers with traditional structure 7

mentioned above, demulsifiers with novel structures have also been synthesized and tested. The 8

relations between structure and performance are investigated to a great extent as well. Jun Wang 9

et al. (2006) synthesized a series of structurally different dendrimer-based demulsifiers and 10

investigated the performance of these demulsifiers in treating crude oil emulsion. From their 11

research results, they concluded that properly structurally designed dendrimer macromolecules 12

can act as effective demulsifier.11

Jun Wang et al. (2008) synthesized a novel broom molecule 13

and investigated its demulsification performance in treating oil-water emulsion.12

Jun Wang et al. 14

(2010) then reasoned that dendrimers with more branches would demonstrate better 15

demulsification performance. To prove this, they synthesized a series of dendrimer-based 16

demulsifier with the same basic structure and investigated the amount of PEO and PPO within 17

the demulsifier molecular structure on the performance of the demulsifier.13

Zhang et al. (2005) 18

synthesized several kinds of polyether demulsifier with a typical PEO-PPO copolymer as the 19

branch structure. They also concluded that dendrimers with more branches would demonstrate 20

better demulsification performance and the different amount of PPO and PEO affects the 21

interfacial activity and thus has a great influence on the performance of the demulsifier.14

22

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Efficient as they are, these demulsification techniques have their own limitations, which is 1

especially obvious in treating polymer flooding oil-extraction waste water. In most cases, the 2

average diameter of droplets in the emulsion system to be treated is around 5µm and numerous 3

research papers in dealing with emulsions with the average diameter of around 5µm have been 4

published.6-14

However, when dealing with emulsions with much smaller oil droplets, the 5

abilities of most techniques are far from satisfactory from an industry point of view, which is 6

also common in oil recovery industry. Hauke et al. (2002) once used fiber-bed coalescers to deal 7

with emulsions with the average diameter of oil droplets around 2µm and developed a physically 8

founded model describing the coalescence process.15

Apart from that, no research on using 9

demulsifiers to treat emulsions with such small oil droplets has been reported yet. As a result, it 10

is urgent to develop a proper kind of demulsifier, which could result high oil removal rate as well 11

as rapid oil-water separation for emulsions with the average diameter of oil droplets less than 12

2µm. 13

Dendrimers are specially designed macromolecules with certain size, shape and reactivity. 14

Generally, they are branched from a central core, with numerous terminal groups surrounding the 15

core so as to produce an empty interior. This novel kind of dendrimers was developed by 16

Tomalia and Newkome in the 1980s.11

Because specially designed dendrimers with certain 17

interfacial activity can dissolve the original interfacial substances on the surface of the oil 18

droplets rapidly, their potential to break O/W emulsion is extremely strong.11

19

Polyamidoamine (PAMAM) is a kind of dendrimer, which has polar but hydrophobic interior 20

with polar terminal groups on the outer surface. Experimental results showed that the structure of 21

the terminal group contributes most to the demulsification process. Only amine-based dendrimer 22

proved to be effective demulsifier.11

23

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Even though numerous research papers on PAMAM have been published, research on its 1

demulsification ability has not been conducted much. Only Jun Wang et al. investigated the 2

demulsification ability of PAMAM.11, 12

So far, no research using PAMAM to break up 3

diesel-in-water emulsions with the average diameter of oil droplets less than 2µm has been 4

reported yet. 5

In this study, 1, 3-propanediamine were first reacted with methyl acrylate and then with 6

ethanediamine. The resultants were treated with the same synthetic procedure twice (first reacted 7

with methyl acrylate and then with ethanediamine) and then the final products PAMAM were 8

got. The prepared demulsifier was then applied to typical diesel-in-water (O/W) emulsions with 9

the average diameter of oil droplets less than 2µm, which were referred to as the ultrafine oil 10

droplets. The influences of temperature, settling time and demulsifier concentration on the 11

performance of the demulsifier were investigated in detail. This study showed that under certain 12

conditions, the oil removal rate could reach more than 90%, which perfectly meets the industrial 13

requirements. Micrographs of the emulsions with and without treatment of the demulsifier were 14

taken and compared to confirm the flocculation and coalescence process during the 15

demulsification process. Surface tension and interfacial tension of the demulsifier were measured 16

to give a basic understanding of the demulsification mechanism. This is the first report on the 17

synthesis of PAMAM-based demulsifier applied to diesel-in-water emulsions with the average 18

diameter of oil droplets less than 2µm. Though not perfect in dealing with emulsions with 19

ultrafine oil droplets, this study shed light upon a novel chemical method in dealing with this 20

kind of emulsion with obvious advantages over physical methods like fiber-bed coalescers 21

developed by Hauke15

. 22

Experimental Details 23

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Materials. All chemicals were used directly without further purification. Methyl acrylate (AR 1

grade, ≥ 0.98), ethanediamine (AR grade, ≥ 0.98) and methanol (AR grade, ≥ 0.98) were all 2

purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. 1, 3-propanediamine (AR 3

grade, ≥ 0.98) was purchased from Aladdin Reagents. Sodium dodecyl sulfate (SDS) (CR grade) 4

was purchased from Tianjin Guangfu Fine Chemical Research Institute. 5

Synthesis of Amine Dendrimer-Based Demulsifier. 12.5 ml of 1,3-propanediamine was 6

dissolved in 100 ml of methanol, then 130 ml of methyl acrylate was added into the flask. The 7

mixture was stirred for 24 hours at 25°C. The solvent and the unreacted methyl acrylate were 8

removed in a rotatory evaporator and then the resultant was put into a vacuum oven for further 9

purification. The resultant was then dissolved in 100 ml of methanol, then 130ml of 10

ethylenediamine was added into the flask. The mixture was stirred for 24 hours at 25°C. The 11

solvent and unreacted ethylenediamine were removed in a rotatory evaporator and then the 12

resultant was put into a vacuum oven for further purification. The above process was then 13

repeated twice. Then the final product was obtained.11 14

Preparation of diesel-in-water emulsions with ultrafine oil droplets. The emulsions were 15

prepared with deionized water and diesel. 50g of diesel and 1g of SDS were added into a 16

volumetric flask with the volume of one litre. Then deionized water was added into the flask 17

until the volume of the mixture reached one litre. Then the mixture were treated with a 18

homogenizer (Fluke homogenizer, 500W) operated at 10000 rpm for 5 min. The resulting 19

emulsion contained 5% wt% diesel and was referred to as diesel-in-water emulsions. The 20

emulsions obtained as such are very stable within the experimental timeframe and are extremely 21

complex with average drop sizes typically less than 2µm measured by Malvern Mastersizer 22

3000. 23

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Demulsification Test. The ability of the demulsifier was tested by measuring the oil content 1

and the size of oil droplets in diesel-in-water emulsions after the demulsification process 2

finished. In each test, 25 ml of freshly prepared emulsion and 1 ml demulsifier solution with 3

certain concentration were thoroughly mixed in a 25ml colorimeter tube by shaking the mixture 4

200 times by hand.8,9

Then the mixture was put in a water bath (Shanghai Yijing, YQ-120C) 5

under different temperature for different periods of time. Subsequently, the solution at the 6

bottom of the colorimeter tube was taken out and the oil content in it was measured using an 7

ultraviolet spectrophotometer (UNIC, UV-4802). Each sample was repeated 3 times and the oil 8

content reported is the average of the three repetitions. The blank tests were performed for 9

diesel-in-water emulsions without demulsifier addition as a control. The demulsification 10

performance is derived from the oil removal rate, which can be calculated from the equation: 11

� =�� − �

��× 100%

where R (%) is the oil removal rate, Co (mg L-1

) is the initial oil content, and C (mg L-1

) 12

represents the oil content after the demulsifier solution was added.10

13

After settling in the water bath of 70°C for 90 minutes, micrographs of the emulsion sample 14

without any demulsifier addition and that with 2000mg L-1

demulsifier addition were recorded 15

using an optical microscope equipped with a digital video camera linked with a computer. The 16

emulsion sample was put on an object slide and then covered with a cover glass. The image was 17

taken under halogen light. 18

Surface Tension and Interfacial Tension Measurement. The surface tension of the 19

dendrimer-based demulsifier solution with different concentrations was measured using an 20

interfacial tensiometer. The interfacial tension of diesel-water interface with the dendrimer-based 21

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demulsifier with different concentrations in the water phase was also measured using an 1

interfacial tensiometer. The interfacial tension of diesel-water interface without the 2

dendrimer-based demulsifier in the water phase and that with only sodium dodecyl sulfate of 3

certain concentration in the water phase were measured as a control. 4

Results and Discussion 5

Synthesis of PAMAM demulsifier. According to Jun Wang’s research, the demulsification 6

rates increased as the dendrimer generation increased11

. However, when the number of 7

generation increases to some certain extent, the densely-piled surface groups bring great 8

difficulty to the next step of reaction process, which causes insufficient further development of 9

the dendrimer thus making the molecular structure defect.11

Thus the demulsification efficiency 10

of the amine-based dendrimer of the third generation was systematically studied in this paper. 11

Jun Wang’s study on the influence of the ratio of reactants on the yield of the product also 12

showed that when the ratio of methyl acrylate and ethanediamine to the 1,3-propanediamine or 13

the resultants from the previous reaction reaches much more than the molar ratio according to the 14

chemical equation, the yield reaches more than 99.9%.11

Thus during the synthesis procedure, 15

the amount of methyl acrylate and ethanediamine used were much more than the molar ratio 16

required by the chemical equation. 17

The PAMAM demulsifier were synthesized in two-step method with three cycles, and one 18

cycle of the synthesis procedure is shown as Figure 1a. The molecular structure of the final 19

product demulsifier is shown in Figure 1b. 20

To identify the structure, 1H NMR spectra was recorded for the demulsifier. CDCl3 was used 21

as the solvent.11

Experimental results (Figure 2a) indicate that the NMR spectra of the purified 22

product totally matches those reported by reference.16

The Hydrogen-1 chemical shift of the 23

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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 1

(g)2.68 (h)2.59 (i)3.20 (j)2.73 (k)2.68 (l)2.59 (m)3.20 (n)2.73 (o) 1.33. The unmarked chemical 2

shift belongs to the unreacted ethanediamine which is extremely hard to remove from the final 3

product due to the nano-container structure of the molecule.11

The characteristic protons of 4

amino groups appeared at δ1.33 ppm as a broad single peak. The reason why the chemical shift 5

of the amino groups on the surface of the dendrimer molecule is relatively small is that the 6

nitrogen atom linked with the hydrogen atom is not a strong electrophilic atom. The 7

characteristic protons of other groups are also shown in Figure 2a, in which some of the groups 8

in different parts of the molecule shared nearly the same chemical shift. (c, g and k atδ2.68ppm; 9

d, h and l atδ2.59ppm; f, j and n atδ2.73ppm; e, i and m atδ3.20ppm) This can be explained by 10

their highly similar positions inside the molecule as shown in Figure 1b. 11

To further identify the structure of the demulsifier, FTIR spectra was also recorded for the 12

demulsifier. Figure 2b shows the FTIR spectra of PAMAM demulsifier. Typical bands 13

associated with –NH2 vibration are visible at around 3269.60 cm-1

. For –CONH-, the bands were 14

observed at around 1645.80 cm-1

and 1544.38 cm-1

with the former referred to as the stretching 15

of C=O and the latter referred to as the coupling band combined with the bending of N-H and the 16

stretching of C-N. Typical bands associated with C-N vibration at around 1195.50 cm-1

17

confirmed the existence of N-CH2-. No typical bands being observed at around 1740 cm-1

18

showed that hardly any ester-terminated intermediate products existed in the final product. For 19

C-N-CH2-, typical band at 1032.39 cm-1

was observed. For –CH2-, the bands were observed at 20

around 2927.56 cm-1

and 2849.17 cm-1

, which were referred to as the asymmetric and symmetric 21

stretching vibration of –CH2-, respectively. Furthermore, typical bands of –COO-C at 1195.50 22

cm-1

and C=C at 929.98 cm-1

were observed, although the area is extremely small, which 23

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indicated that small amount of ester-terminated intermediate products still existed in the final 1

products. 2

Demulsification of PAMAM demulsifier. Figure 3 shows the relationship between oil 3

removal rates measured by the oil content at the bottom of the emulsion and the demulsifier 4

concentration applied under two certain circumstances. When no demulsifier was added into the 5

emulsion, which can be regarded as a blank test, the oil removal rate under pure gravity was just 6

1.7% after 90 minutes at the temperature of 30 °C that is very near to room temperature. This 7

explains that the emulsions prepared are very stable under normal conditions. To further prove 8

the high stability of the emulsions, another experiment was conducted, in which the settling time 9

was extended to 120 minutes and the temperature was set to 50°C. Again, without any 10

demulsifier addition, the final oil removal rate by natural gravity under the temperature of 50°C 11

after 120 minutes was just 9.1%. The above two blank test shows that the emulsions prepared are 12

extremely stable within a relatively long period of time. 13

From Figure 3, it is clear that when the temperature and settling time increased, the oil 14

removal rate increased as well under the same demulsifier concentration. To further prove the 15

high efficiency of the demulsifier, systematically investigation on the influences of temperature, 16

settling time as well as the demulsifier concentration on the performance of the demulsifier was 17

conducted as follows. 18

The effect of the demulsifier concentration on the oil removal rate at different temperatures 19

was shown in Figure 4. For this purpose, the demulsifier concentration in the diesel-in-water 20

emulsion under study was set as 500mg L-1

, 1000mg L-1

, 1500mg L-1

and 2000mg L-1

in 21

ascending order. Apart from that, the settling time was kept constant at 60 minutes, which is 22

relatively much shorter than the settling time set in another research treating oilfield waste water 23

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with typical O/W emulsions involved.11

Three pairs of experiments with the temperature ranging 1

from 30°C to 70°C were conducted to give a qualitative impression of the demulsifier efficiency. 2

It is obvious that under each temperature, the oil removal rate increased with the increase of 3

demulsifier concentration. When the temperature is 30°C, oil removal rate is very low even if the 4

demulsifier concentration reaches as high as 2000mg L-1

. Less than 50% oil was removed from 5

the emulsion system under this condition, which is undoubtedly far from satisfactory from an 6

industry point of view due to its low efficacy at operating condition near room temperature. To 7

further investigate the factors affecting the performance of the demulsifier, the temperature was 8

raised to 50°C, which is also easy to achieve without much energy in industry, the oil removal 9

rates showed a significant increase, with more than 70% oil being removed when the demulsifier 10

concentration are relatively high. And even when the concentration of the demulsifier is just 11

500mg L-1

, the oil removal rate is near 50%, which has shown much better performance than that 12

in low temperature. From the two experiments shown above, it can be concluded that 13

temperature plays a significant role in improving the performance of the demulsifier. However, 14

even when the temperature reaches 50°C and the demulsifier concentration reaches as high as 15

2000mg L-1

, the oil removal rate is just a little more than 80%, which still cannot meet the 16

requirement standards of industry. Thus experiment with the temperature of 70°C was conducted 17

to further investigate the influence of temperature on the performance of the demulsifier. From 18

the experimental results, it can be seen that at 70°C more than 90% oil was removed from the 19

emulsion system even when the demulsifier concentration is just 500mg L-1

, which perfectly 20

meets the industrial requirements. 21

Satisfactory as it is, the influence of demulsifier concentration on the efficiency of the 22

demulsifier was still unclear, with only a slightly increasing trend being observed. Thus another 23

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four pairs of experiments were conducted to describe this trend in detail. Figure 5 shows the 1

effect of temperature on oil removal rate in diesel-in-water emulsions. The general trend is the 2

same as previously investigated that the oil removal rate increases with the demulsifier 3

concentration. However, it is interesting to see that even though all four curves showed an 4

increasing trend, the difference of demulsifier concentration also plays a part in it. When the 5

demulsifier concentration is as low as 500mg L-1

, the increasing trend of the curve is rather small 6

at low temperatures, with the slope of the curve being just 1.3. But when at high temperatures, 7

the increasing trend of the curve becomes much higher, with the slope of the curve increasing to 8

2.1. This can be explained by the molecular motion of the demulsifier molecules. According to 9

the molecular dynamic theory, when at high temperature, the motion of demulsifier molecules 10

becomes much stronger than at room temperature, which accelerates their transportation towards 11

the surface of oil droplets thus greatly adds to the demulsification process. 12

From Figure 5, it can also be clearly seen that when the demulsifier concentration reaches as 13

high as 2000mg L-1

, the increasing trend of oil removal rate is relatively high at low 14

temperatures, with the slope of the curve being 1.95. But when the temperature ranges are set 15

from 50°C to 70°C, the increasing trend of oil removal rate slows down a lot with the slope of 16

the curve being 0.85. With the oil removal rate increasing to 99%, nearly transparent water phase 17

is got with hardly any oil in it. This could be the contribution of high demulsifier concentration 18

to improving the transportation rate of demulsifier molecules. When the demulsifier 19

concentration is relatively high at high temperature, much more demulsifier molecules are trying 20

to reach the oil droplets in water phase, which greatly adds to the viscosity of water phase. This 21

behavior of demulsifier molecules in turn slows down the rate of their transportation towards the 22

oil droplets.6 In another perspective, when the oil removal rate is already as high as 80%, which 23

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means lots of demulsifier molecules have aggregated onto the surface of oil droplets to replace 1

the original natural surfactants and SDS, the residual positions for other demulsifier molecules 2

are not enough. Thus some of the residual demulsifier molecules would tend to aggregate by 3

themselves due to the high concentration, which further slows down the transportation rate of 4

demulsifier molecules in the water phase thus leading to such experimental results.6 5

Another interesting phenomenon could also be seen in Figure 5. When the demulsifier 6

concentration was varied from 1000mg L-1

to 1500mg L-1

, the oil removal rate only increased 7

slightly at each temperature. Although the variance of increasing trend of oil removal rate with 8

respect to temperature is not clearly observed, it can still be seen that the influence of demulsifier 9

concentration plays a significant part in the demulsification process. An obvious plateau was 10

observed when the demulsifier concentration ranged from 1000mg L-1

to 1500mg L-1

. Thus new 11

pairs of experiments were conducted to investigate the effect of demulsifier concentration on the 12

demulsification performance in detail. 13

Figure 6 shows the effect of settling time on oil removal rate in diesel-in-water emulsions at 14

the temperature of 30°C with each curve indicating a certain demulsifier concentration. From 15

Figure 6, it can be obviously seen that when the settling time is 60 minutes, the oil removal rates 16

of the four experiments at 30°C are all extremely low. Even when the demulsifier concentration 17

reaches more than 2000mg L-1

, the oil removal rate is still slightly more than 40%. However, 18

when the settling time was just extended to 90 minutes, the oil removal rates increased rapidly 19

for all four experiments with different demulsifier concentrations. Especially when the 20

demulsifier concentration was set to as low as 500mg L-1

, the oil removal rate increased from 21

22% to 76% within the period of 30 minutes set previously, which has shown excellent 22

performance of the demulsifier. However, when the settling time was extended to 120 minutes, 23

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the increasing rate is not as high as before. Even when the demulsifier concentration reached as 1

high as 2000mg L-1

, the oil removal rate only increased a little, from 85% to 98%. Although oil 2

removal rate of 98% means that the performance of the demulsifier is extremely excellent, it can 3

be clearly seen that the high demulsifier concentration also made some contribution. Thus 4

experimental results with the demulsifier concentration being set as 1500mg L-1

and 1000mg L-1

5

were compared for the purpose of finding the optimal concentration at 30°C. When the 6

demulsifier concentration was 1500mg L-1

, the oil removal rate at 30°C was 92.6% and when the 7

demulsifier concentration was 1000mg L-1

, the oil removal rate at 30°C was 82.8%. Thus it is 8

clearly that the optimal demulsifier concentration at 30°C was 1500mg L-1

for at this operating 9

condition because industry requires more than 90% oil removal rate. 10

Again, when the demulsifier concentration was 500mg L-1

, only a slightly increase in oil 11

removal rate was observed when the settling time was extended from 90 minutes to 120 minutes. 12

With oil removal rate increasing from 72.5% to 75.6%, it can be concluded that the increase of 13

demulsification efficiency has slowed down a lot at 30°C with the demulsifier concentration of 14

500mg L-1

. According to the increasing trend shown by other three curves of Figure 6, the 15

maximum oil removal rate are obviously higher when oil demulsifier concentration reaches 16

higher level although no clear plateau was observed in Figure 6. 17

In Figure 7, when the temperature was raised to 50°C, obvious plateaus were observed for the 18

two curves with the demulsifier concentration being 2000mg L-1

and 1500mg L-1

. With the 19

demulsifier concentration of 1500mg L-1

, the oil removal rate increased from 91.1% to 92% 20

when the settling time was extended from 90 minutes to 120 minutes, being rather a slightly 21

increase. While the demulsifier concentration reached 2000mg L-1

, the oil removal rate increased 22

from 93.1% to 94.4% when the settling time was extended from 90 minutes to 120 minutes. With 23

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an increase of nearly 1%, it can be regarded that plateau has been reached. When the demulsifier 1

concentration was as low as 1000mg L-1

, the oil removal rate increased from 86.6% to 91% when 2

the settling time was extended from 90 minutes to 120 minutes. Even though with only a slightly 3

increase of nearly 5%, the oil removal rate had reached more than 90%, which perfectly meets 4

the industrial requirements. 5

It is also interesting to see that the curve with the demulsifer concentration of 500mg L-1

6

showed a slightly decrease when the settling time was extended from 90 minutes to 120 minutes. 7

With the oil removal rate decreased from 74% to 70% when the settling time was extended from 8

90 minutes to 120 minutes, it can be assumed that at the concentration of 500mg L-1

, more 9

settling time would not contribute to improving the performance of the demulsifier. This can be 10

explained as follows. At the temperature of 50°C, the molecular motion at the interface, which 11

actually means the surface monolayer of the oil droplets, has reached equilibrium to some extent. 12

With finite amounts of demulsifier molecules in the water phase competing with the original free 13

surfactants such as SDS to get to the interface, the final state has been achieved at the settling 14

time of 90 minutes. As to the slightly decrease when the settling time is extended, it can be 15

assumed that the equilibrium state has not been totally stable, which might result in errors in 16

measurement. 17

From the comparison between Figure 6 and Figure 7, it can be seen that the plateau tends to 18

shift to higher demulsifer concentration band as the temperature rises (From 500mg L-1

to 19

2000mg L-1

). This can be explained as follows. When the temperature is as low as 30°C, with the 20

concentration of demulsifier being only 500mg L-1

, the amount of free demulsifier molecules 21

existing in the water phase is extremely small. Thus increasing temperature to 50°C could not 22

significantly cause more free demulsifier molecules to get to the surface of oil droplets thus lead 23

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to demulsification. It can be clearly seen from the comparison between Figure 6 and Figure 7 1

that when the temperature is raised from 30°C to 50°C, the curve indicating the demulsifier 2

concentration of 500 mg L-1

at the section between 90 minutes and 120 minutes basically 3

remained the same height. However, when higher demulsifier concentration is applied, the 4

amount of free demulsifier molecules existing in the water phase is relatively much larger. Thus 5

when the temperature is raised from 30°C to 50°C, more free demulsifier molecules would try to 6

get to the surface of oil droplets thus lead to demulsification due to stronger molecular motion 7

caused by the increase of temperature. So no obvious plateau is observed for the curve indicating 8

the demulsifier concentration of 1000 mg L-1

. Nevertheless, when the demulsifier concentration 9

is as high as 1500 mg L-1

or 2000 mg L-1

, even within 90 minutes at 50°C, lots of demulsifier 10

molecules have aggregated onto the surface of oil droplets to replace the original natural 11

surfactants and SDS, the residual positions at the surface for other demulsifier molecules are not 12

enough. Thus extending settling time to 120 minutes would not cause more free demulsifier 13

molecules to get to the surface of oil droplets thus leading to demulsification. So obvious plateau 14

is observed for the curve indicating the demulsifier concentration of 1500 mg L-1

or 2000 mg L-1

15

at the section between 90 minutes and 120 minutes. 16

Micrographs of typical diesel-in-water emulsions without and with 2000mg L-1

demulsifier 17

addition after settling for 90 minutes at 70°C are shown separately in Figure 8a, Figure 8b and 18

Figure 8c. As shown in Figure 8a, the oil droplet size in the emulsion system without any 19

demulsifier addition is actually less than 2µm, which is in perfect agreement with the average oil 20

droplet diameter measured by Malvern Mastersizer 3000. In contrast, when treated with the 21

demulsifier with the concentration of 2000mg L-1

at 70°C for 90 minutes, the oil droplet size 22

increased significantly, with the diameter of most oil droplets varied between 20µm and 30µm. 23

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And most of the oil droplets had moved to the upper level of the emulsion (Figure 8b). Hardly 1

any oil droplets remained at the lower level of the emulsion (Figure 8c). It can be obviously seen 2

that the demulsifier greatly improved flocculation and coalescence of emulsified oil droplets in 3

diesel-in-water emulsions. 4

Compared with other similar studies studying the demulsification process with larger droplets 5

(the average diameter being around 5µm) in the emulsions6-14

, it can be clearly seen that the 6

demulsifier dosages they used are relatively lower than this study with other operating conditions 7

more or less the same. This in some sense confirms the great difficulty in treating emulsions with 8

ultrafine droplets (the average diameter being less than 2µm). Though the demulsifier 9

synthesized in this study might not be regarded as the perfect demulsifier in treating emulsions 10

with ultrafine oil droplets for the high demulsifier dosage, it showed strong potentials in dealing 11

with this kind of emulsion, which can be seen from the excellent performance under certain 12

temperature and settling time. Combined with its simple synthetic procedure as well as mild 13

operating conditions, it showed obvious advantages over fiber-bed coalescers developed by 14

Hauke15

from an industrial point of view. 15

Surface tension and interfacial tension study of PAMAM demulsifier. To get a further 16

understanding of the demulsification mechanism, the surface tension of the aqueous solution of 17

PAMAM demulsifier and the interfacial tension of the PAMAM demulsifier at diesel-water 18

interface were measured and compared. Table 1 shows the surface tension of the PAMAM 19

demulsifier aqueous solution and Table 2 shows the interfacial tension of diesel-water interface 20

with the PAMAM demulsifier in the water phase. It can be clearly seen from Table 1 that the 21

PAMAM demulsifier cannot significantly lower the surface tension of pure water no matter what 22

the concentration is. This result perfectly matches Jun Wang’s research that amine terminated 23

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dendrimers cannot reduce the surface tension of pure water and thus do not behave like typical 1

surfactants for air-water interface.11

According to the data shown in Table 2, it can be clearly 2

seen that with certain amount of demulsifier in the water phase, the interfacial tension of 3

diesel-water interface get significantly lowered from 38.12 mN m-1

to less than 8 mN m-1

. Even 4

no clearly interfacial tension variance with the increase of demulsifier concentration was 5

observed, it can still be concluded that lowering the interfacial tension is the precondition for the 6

demulsification process to take place. To further investigate the relationship between interfacial 7

tension and demulsification process, the interfacial tension at diesel-water interface with only 8

SDS of 1 g L-1

in the water phase was also measured. The measured interfacial tension is 3.26 9

mN m-1

, which also confirms the high stability of the emulsion system from the low interfacial 10

tension perspective. Although this value was still a little lower than the interfacial tension at 11

diesel-water interface with demulsifier in the water phase, the demulsification process still 12

occurred. This can be explained by the properties of the interfacial monolayers with demulsifier 13

molecules in it. Once the demulsifier molecules reach the surface of oil droplets, they tend to 14

form nanoaggregates at the interface, which lead to reorientation of the interfacial substances at 15

the interface. This in turn makes the interfacial monolayer more compressible.17

When the 16

amount of demulsifier molecules at the interface increases, the stability of the interfacial 17

monolayer gets weaker because the nanosize aggregates formed at the interface can reduce the 18

mechanical strength of the monolayer, which has been confirmed by AFM images of deposited 19

monolayer.17

From another perspective, the natural surfactants and SDS at the interface could be 20

dissolved into the bulk phases of demulsifier once the demulsifier molecules reached the surface 21

of oil droplets.16

22

Conclusion 23

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According to the study reported above, the following conclusions can be made: 1

(1) Generally, the oil removal rate increases with the increase of temperature, settling time and 2

demulsifier concentration. From the experimental data, it can be seen that with the settling 3

time being 120 minutes, when the temperature was set as 30°C and the demulsifier 4

concentration was set as 1500 mg L-1

, the oil removal rate could reach 92.6%, which 5

perfectly meets the industrial requirements. Increasing the temperature or the demulsifier 6

concentration would not do much more contribution to improving the oil removal rate, which 7

can be seen from the Figure 6 and Figure 7. Micrograph images showed that the PAMAM 8

demulsifier can successfully add to the flocculation and coalescence of oil droplets in the 9

system, which finally leads to the breaking of typical diesel-in-water emulsions. 10

(2) Among the several factors leading to the demulsification process, the most significant factor 11

is temperature, which can be seen from Figure 4 and Figure 5. And the least significant factor 12

is demulsifier concentration, which can be seen from Figure 6 and Figure 7 due to the high 13

similarity of the four curves. The influence of settling time depends on the variance of time 14

period, which can be seen in Figure 6 and Figure 7. 15

(3) Though not perfect in dealing with emulsions with ultrafine oil droplets due to its high 16

demulsifier dosage, this study shed light upon a novel method in dealing with this kind of 17

emulsion. The simple synthetic procedure and mild operating conditions give its unique 18

advantages in dealing with emulsions with ultrafine oil droplets over fiber-bed coalescers 19

developed by Hauke15

. 20

(4) The surface tension and interfacial tension data were given to partially uncover the 21

mechanism of this demulsification process, which showed that the precondition of 22

demulsification process is the ability of the demulsifier to lower the interfacial tension of the 23

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diesel-water interface and that there are other factors contributing to this process such as the 1

properties of the interfacial monolayer changed by the demulsifier molecules, which remains 2

to be studied. 3

Acknowledgements 4

We are grateful for the financial support from the National Natural Science Foundation of 5

China (No. 21336007) 6

References 7

(1) Panthi, K.; Mohanty, K. K., Effect of Alkaline Preflush in an Alkaline-Surfactant-Polymer 8

Flood. Energy Fuels 2013, 27, (2), 764-771. 9

(2) Qi, W.-K.; Yu, Z.-C.; Liu, Y.-Y.; Li, Y.-Y., Removal of emulsion oil from oilfield ASP 10

wastewater by internal circulation flotation and kinetic models. Chem. Eng. Sci. 2013, 91, 11

122-129. 12

(3) Deng, S. B.; Bai, R. B.; Chen, J. P.; Yu, G.; Jiang, Z. P.; Zhou, F. S., Effects of 13

alkaline/surfactant/polymer on stability of oil droplets in produced water from ASP flooding. 14

Colloids Surf., A 2002, 211, (2-3), 275-284. 15

(4) Ma, H. Z.; Wang, B., Electrochemical pilot-scale plant for oil field produced wastewater by 16

M/C/Fe electrodes for injection. J. Hazard. Mater. 2006, 132, (2-3), 237-243. 17

(5) Hafiz, A. A.; El-Din, H. M.; Badawi, A. M., Chemical destabilization of oil-in-water 18

emulsion by novel polymerized diethanolamines. J. Colloid Interface Sci. 2005, 284, (1), 19

167-175. 20

(6) Feng, X.; Xu, Z.; Masliyah, J., Biodegradable Polymer for Demulsification of 21

Water-in-Bitumen Emulsions. Energy Fuels 2009, 23, (1), 451-456. 22

(7) Feng, X.; Wang, S.; Hou, J.; Wang, L.; Cepuch, C.; Masliyah, J.; Xu, Z., Effect of Hydroxyl 23

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Content and Molecular Weight of Biodegradable Ethylcellulose on Demulsification of 1

Water-in-Diluted Bitumen Emulsions. Ind. Eng. Chem. Res. 2011, 50, (10), 6347-6354. 2

(8) Peng, J.; Liu, Q.; Xu, Z.; Masliyah, J., Synthesis of Interfacially Active and Magnetically 3

Responsive Nanoparticles for Multiphase Separation Applications. Adv. Funct. Mater. 2012, 4

22, (8), 1732-1740. 5

(9) Peng, J.; Liu, Q.; Xu, Z.; Masliyah, J., Novel Magnetic Demulsifier for Water Removal from 6

Diluted Bitumen Emulsion. Energy Fuels 2012, 26, (5), 2705-2710. 7

(10) Li, S.; Li, N.; Yang, S.; Liu, F.; Zhou, J., The synthesis of a novel magnetic demulsifier and 8

its application for the demulsification of oil-charged industrial wastewaters. J. Mater. Chem. 9

A 2014, 2, (1), 94-99. 10

(11) Wang, J.; Li, C.-Q.; Li, J.; Yang, J.-Z., Demulsification of crude oil emulsion using 11

polyamidoamine dendrimers. Sep. Sci. Technol. 2007, 42, (9), 2111-2120. 12

(12) Wang, J.; Li, C. Q.; Zhang, S. Y.; Sun, F.; Ge, T. H., Synthesis and characterization of 13

lower generation broom molecules. Chin. Chem. Lett. 2008, 19, (1), 43-46. 14

(13) Wang, J.; Hu, F.-L.; Li, C.-Q.; Li, J.; Yang, Y., Synthesis of dendritic polyether surfactants 15

for demulsification. Sep. Purif. Technol. 2010, 73, (3), 349-354. 16

(14) Zhang, Z. Q.; Xu, G. Y.; Fang, W.; Dong, S. L.; Chen, Y. J., Demulsification by 17

amphiphilic dendrimer copolymers. J. Colloid Interface Sci. 2005, 282, (1), 1-4. 18

(15) Speth, H.; Pfennig, A.; Chatterjee, M.; Franken, H., Coalescence of secondary dispersions in 19

fiber beds. Sep. Purif. Technol. 2002, 29, (2), 113-119. 20

(16) Bhattachar, B. R. New water soluble polymer as well head demulsifiers comprises 21

methacrylate, butyl acrylate, acrylic acid and methacrylic acid units. US5100582-A. 22

(17) Zhang, L. Y.; Xu, Z. H.; Mashyah, J. H., Langmuir and Langmuir-Blodgett films of mixed 23

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asphaltene and a demulsifier. Langmuir 2003, 19, (23), 9730-9741. 1

Figure Captions 2

Figure 1a. One cycle of the synthesis procedure of the PAMAM demulsifier. 3

Figure 1b. Molecular Structure of PAMAM demulsifier. 4

Figure 2a. 1H NMR spectra of PAMAM demulsifier. 5

Figure 2b. FTIR spectra of PAMAM demulsifier. 6

Figure 3. The effect of demulsifier concentration on oil removal rate in O/W emulsions at two certain 7

circumstances. 8

Figure 4. The effect of demulsifier concentration on oil removal rate in diesel-in-water emulsions 9

Figure 5. The effect of temperature on oil removal rate in diesel-in-water emulsions 10

Figure 6. The effect of settling time on oil removal rate in diesel-in-water emulsions at 30°C 11

Figure 7. The effect of settling time on oil removal rate in diesel-in-water emulsions at 50°C 12

Figure 8a. Micrographs with of typical diesel-in-water emulsions without demulsifier addition after settling 13

for 90 minutes at 70°C. 14

Figure 8b. Micrographs of typical diesel-in-water emulsions with demulsifier addition after settling for 90 15

minutes at 70°C. (At upper level with oil most occupied) 16

Figure 8c. Micrographs of typical diesel-in-water emulsions with demulsifier addition after settling for 90 17

minutes at 70°C. (At lower level with water most occupied) 18

19

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Tables 1

Table 1. The surface tension of PAMAM demulsifier aqueous solution 2

Demulsifier

concentration

(mg L-1

)

0.0 1000.0 1500.0 2000.0

Surface

tension (mN

m-1

77.81 76.98 77.38 76.8

3

Table 2. The interfacial tension of diesel-water interface with the PAMAM demulsifier in the 4

water phase 5

Demulsifier

concentration

(mg L-1

)

0.0 1000.0 1500.0 2000.0

Interfacial

tension (mN

m-1

)

38.12 7.89 7.99 7.75

6

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