Methacrylated Hyperbranched Polyglycerol as a High-Efficiency

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Methacrylated Hyperbranched Polyglycerol as a Novel High-efficient Demulsifier for Oil-in-Water Emulsions Lifeng Zhang, Guijin He, Dengfeng Ye, Ningning Zhan, Yongsheng Guo, and Wenjun Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01631 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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Energy & Fuels

Methacrylated Hyperbranched Polyglycerol as a Novel High-efficient Demulsifier for Oil-in-Water Emulsions

Lifeng Zhang†, Guijin He†, Dengfeng Ye†, Ningning Zhan‡, Yongsheng Guo†, and Wenjun Fang†*

†Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China

‡ CNOOC Energy Technology & Service Limited-Drilling & Production Co., Tianjin 300450, People’s Republic of China

ABSTRACT: To break oil-in-water emulsions with the average size of oil droplets less than 2 μm, a series of hyperbranched polyglycerol (HPG)-based demulsifiers, methacrylated hyperbranched polyglycerol (HPG-MA), are synthesized successfully by controlling the ratio of HPG to glycidyl methacrylate (GMA). Dosage, temperature, settling time, and salinity are taken into account to evaluate the performance of these demulsifiers, respectively. The oil removal ratio with the addition of HPG-MA demulsifier can exceed 86 % within 40 min to reach the equilibrium of demulsification in comparison with ~90 min for previously reported demulsifiers. Because of the specific 1

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branched structure, the demulsifier can multipointly adhere to the oil-water interface, and then shorten the time of adsorption and increase the rupture rate of oil droplets. The oil-water interfacial tensions with the demulsifier in the water phase are further measured to help comprehend the demulsification mechanism. The change of oil droplet size against time, which reflects flocculation and coalescence of oil droplets, is vividly monitored during the process of demulsification. The demulsification performance indicates that the novel HPG-MA demulsifier displays a great promise in petroleum industry. KEYWORDS: oil-in-water (O/W) emulsion; hyperbranched polymer; demulsifier; flocculation; coalescence

1. INTRODUCTION Numerous initially produced crude oil is in the form of oil-in-water (O/W) or water-in-oil (W/O) emulsions.1 Especially in recent years, the booming progress of water-polymer flooding in enhanced oil recovery (EOR) has led to an increasing number of O/W emulsions.2 These emulsions are extremely stable due to the presence of compact or dense films in the oil/water interface with the natural interfacial active materials, such as bitumen, resins, and asphaltenes.3 They further induce a plenty of difficulties in oil transportation and refinery. Hence, the O/W emulsions are always undesirable in petroleum industry.4

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Usually, demulsification technique is applied to resolve the O/W emulsions into bulk phases of oil and water. The traditional demulsification procedure can be defined as a three-stage process including destabilization, aggregation and coalescence, and gravity separation.5-6 Generally, there are several demulsification methods, such as centrifugal method, gravitational method, ultrasonic method, and chemical method. Chemical demulsification is extensively utilized through adding certain amount of chemicals which are usually called demulsifiers. The demulsifiers are amphiphilic compounds with both hydrophobic and hydrophilic groups, and they can reduce the interfacial tension and promote phase separation.7-8 As a result, they enhance the flocculation and coalescence of oils.9 Demulsifiers can be classified into anionic, cationic and non-ionic types. The non-ionic demulsifiers with amphiphilic structures are extensively employed because of low dosage, and good performance. They are not easily affected by electrolytes in comparison with the ionic demulsifiers. Besides, their environmental friendly properties have also attracted much attention.10 For example, ethylcellulose (EC) polymers with different hydroxyl contents and molecular weights have been investigated and the demulsification performance for water-in-diluted bitumen emulsions has been evaluated.11 Branched poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) block co-polymers for the break of W/O emulsions and the inﬂuence of solvent medium on the activities have been discussed.12 The branched PEO-PPO copolymer with hydrophilic segments (EO and OH) shows satisfactory performance. Non-ionic demulsifiers modified 3


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from polyethyleneimine (PEI) with various ethylene oxide (EO) and propylene oxide (PO) have also been designed, and the corresponding demulsification conditions are optimized.13 However, these demulsifiers still have very limited ability to deal with emulsions with the average size of oil droplets around or less than 2 μm.14 Therefore, it is imperative and significant to develop new demulsifiers for the mentioned emulsions. As hyperbranched polymers are three dimensional and topological macromolecules with highly branched structure, they can be easily adhered to oil-water interface and substitute the intrinsic interfacial materials, and their potential applications to the emulsions are attractive.15 A range of dendrimers with the same polyamidoamine (PAMAM) frame but different generations or terminals have been synthesized, and their demulsification performances have been researched.16 It is found that the efficiency of amine-based dendrimers can precede those of the commercial demulsifiers. Hyperbranched polyglycerol (HPG) is a kind of polyether polymer with abundant terminal hydroxyl groups which possess many advantages, such as nontoxicity, convenient storage, simple synthesis and low cost.17 Hence, HPG is apt to be designed as an amphiphilic polymer to serve as an efficient demulsifier.18 In this paper, methacrylated hyperbranched polyglycerol (HPG-MA) is synthesized from hyperbranched polyglycerol (HPG) and glycidyl methacrylate (GMA) with reference to the reported work.18-20 HPG-MA is designed as a novel demulsifier to treat the O/W emulsions with the average size of oil droplets less than 2 μm. The dosage, temperature, settling time, and salinity are taken into account to evaluate the 4


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demulsification performance. Interfacial tensions and the changes of oil droplet size are measured to discuss the demulsification mechanism.

2. EXPERIMENTAL SECTION 2.1. Materials. 1,1,1-Tris (hydroxymethyl) propane (TMP, 98%) and cation-exchange

resin were purchased from Sigma-Aldrich. Glycidyl methacrylate (GMA, 97%), 4-(N,N’-dimethylamino) pyridine (DMAP, 99%), and potassium methylate (95%) were purchased from Aladdin Chemical Reagent Corporation. Methanol (99.5%) and dimethyl sulfoxide (DMSO, 99%) were purchased from Sinpharm Chemical Reagent Corporation. All of the reagents were used as received without further purification. (±)Glycidol (96%) was purified through vacuum distillation before use, which was purchased from Sigma-Aldrich. 2.2. Preparation of HPG. TMP (1.68 g, 12.5 mmol) was partially deprotonated (10%)

by potassium methylate (~1.25 mmol CH3OK) at 65 °C for 15 min. The excess methanol was distilled off from the melt. The purified glycidol (50 mL) was then slowly added into the flask over 24 h with a peristaltic pump at 95 °C under nitrogen atmosphere. After experienced the anionic ring opening polymerization, the product was dissolved in methanol and neutralized over cation-exchange resin. The raw product of HPG was washed twice with acetone. Finally, the sample was dried in vacuum for 24 h at 80 °C, and a transparent and highly viscous product of HPG was obtained.

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2.3. Transesterification of HPG with GMA. Transesterification of HPG with GMA,

shown in Figure 1, with the transesterification fraction controlled to be lower than 0.3 to obtain a water-soluble product, was performed to acquire HPG-MA as the following procedure. Dried HPG (1.0 g) was dissolved into DMSO (9.0 mL) as N2 purging at room temperature (~25 °C). DMAP (2.0 g) and GMA (2.5 mL) were dropwise added and the solution was stirred for 5 h. Subsequently, the crude product was washed twice with ethyl ether and dried at room temperature. The degree of substitution (DS) of the HPG-MA samples is calculated on the basis of 1H NMR determination from the following equation21:

DS=

(Ha +Hb )/2 Hp /6

×100%

(1)

where Ha, and Hb represent the methacryloyl group at the chemical shifts of 5.6, and 6.0 ppm, respectively; Hp denotes the methyl, methylene and methine hydrogens at the chemical shifts between 3.3 and 3.9 ppm.22

HO HO

O HO

HO HO

O

O

O O OH

O

O

O OH

OH

O HO

O O

HO

HO

O

O

O

OH

O

O O HO

O HO

OH

OH OH

OH

CH3 n H 2C C C O CH 3 O

HO

O

O O OH

O

O OH

O O

O

OH

HO

O

OH

O

O

O

O

OH

O OH

O HO

O

OH

O HO

O O

O HO

O

O

O

OH

O

HO

HO

O

O

O

O O

O

OH

O

O

OH

O HO

O

O

OH HO

O

O

O

Figure 1.

Synthesis of HPG-MA through transesterification of HPG with GMA.

2.4. Characterizations of HPG and HPG-MA. The NMR characterizations of the

6


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synthesized HPG and HPG-MA were carried out on a Bruker AVANCE III 500 MHz NMR spectrometer with d6–CH3OD and d6–DMSO as the solvents, respectively. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet iS10 spectrometer (Thermo Fisher Scientific Corp.). Thermal transition behaviors were investigated by a differential scanning calorimeter (DSC, Q2000, TA Instruments) with the scanning rate of 5 °C·min -1 at the range from -80 °C to 80 °C. Gel permeation chromatography ̅̅̅̅n ) and (GPC, Waters) was employed to determine the average molecular weights (M molecular weight distributions of HPG and HPG-MA samples. The system was calibrated through the standard linear polymethylmethacrylate with a narrow molecular weight distribution. N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were used as the eluents under the flow rate of 1 mg·min-1 at 60 °C. The HPG and HPG-MA solutions were prepared to be 3 mg·mL-1 with the existence of crumb LiBr. 2.5. Preparation of oil-in-water emulsions. Diesel (0#, ρ25 °C = 0.823 g· cm-3； η25 °C =

5.43 mPa·s) and deionized water were employed to prepare the O/W emulsions. A mixture composed of 50 g diesel, 0.09 g Span 80, and 0.91 g Tween 80 was added into a 500 mL volumetric flask, and then deionized water was added into the flask reaching to 500mL. Subsequently, the sample was vigorously stirred with a homogenizer (AF-B1 homogenizer, A-FIND) at 20000 r·min -1 for 5 min. The acquired O/W emulsions with the average size of droplets less than 2 μm recorded by dynamic light scattering (DLS, ZEN 3600, Malvern Instruments) were highly stable in the periodic experiments. 2.6. Demulsification tests. The demulsification tests were carried out as follows. The 7


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demulsifier solution (0.5 mL) with a given concentration and the freshly produced emulsion (50 mL) were mixed thoroughly in a 100 mL beaker by machine-vibratory with the frequency of 200-250 min−1 for 90 s.4, 23 A thermostat water bath (Shanghai Jinghong, DKZ-2) was applied to control the mixtures at different temperatures for a given settling time. The bottom phase was collected, and the oil was extracted by n-hexane. UV-vis spectrophotometer (SHIMADZU, UV-2450) was then employed to measure the oil content. Each test was carried out thrice, and the average value of the three repetitions was reported. The corresponding blank tests without adding demulsifiers were performed as the contrasts. The demulsification performance is characterized by the oil removal ratio(R), which is calculated from the following equation:

R=

C0 -C C0

×100%

(2)

where C0 (mg·L−1) and C (mg·L−1) are the initial and final oil contents before and after the addition of demulsifier, respectively.24 The wavelength of characteristic absorption peak of oil, corresponding to C or C0, displays at 335.5 nm. 2.7. Interfacial tension measurement. The interfacial tensions of oil-water interface with

different concentrations of demulsifier in water phase were measured through the pendant-drop method by a digital tensionmeter (DropMeter Standed A-100). Contrastive tests were performed without adding demulsifier or with only adding Tween 80 at a certain concentration in water phase. 2.8. Observations of demulsification process. To understand the demulsification process,

the average sizes of oil droplets and the transmittance values of emulsions were measured. 8


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The time-dependent average size of oil droplets in O/W emulsions was monitored by dynamic light scattering (DLS, ZEN 3600, Malvern Instruments). The transmittance value against the settling time with adding 2000 mg·L-1 of demulsifier into the emulsions was observed by UV-vis spectrophotometer (SHIMADZU, UV-2450).

3. RESULTS AND DISCUSSION 3.1. Characteristic properties of HPG and HPG-MA. The characteristic properties of

̅̅̅̅n ), the degree of hyperbranched polymers, such as the average molecular weight (M branching (DB), and the degree of substitution (DS) can be obtained from the NMR, GPC, DSC and DLS measurements. Typical NMR spectra of HPG and HPG-MA-2 are exhibited in Figures 2 and 3, and the analysis data of HPG and three HPG-MA samples are listed in Table 1.19 In the 1H NMR spectrum of HPG, the broad resonances of methyl, methylene and methine display a signal between 3.3 and 3.9 ppm, while the hydroxyl hydrogens of the solvent produce single peak around 4.9 ppm. In contrast with the HPG sample, the signals of double bond and alkyl group can be clearly observed in 1H NMR spectrum of HPG-MA functionalized with GMA. The characteristic hyperbranched groups emerge as a broad resonance from 3.1 to 4.0 ppm. The signal of newly introduced methyl groups appears at 1.8 ppm. Simultaneously, the chemical shifts of 5.6 and 6.0 ppm indicate the double bond hydrogens. From Equation 1, the DS values of HPG-MA-1, HPG-MA-2, and HPG-MA-3 are calculated to be 12.30 %, 22.53 % and 28.07 %, respectively. 9


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b O

CH3

HPG

O

C

C

HPG-MA-2 HPG

d H

a C

H

c

a b d c

CH,CH2

6

5

CH3

4

3

2

1

0

Chemical shift / ppm

Figure 2. 1H NMR spectra of HPG and HPG-MA-2.

13

C NMR HPG

2L14

L13,L14

L13 D 85

80

2D,2L14

75

70

L13 T

65

60

55

Chemical shift / ppm

Figure 3.

13C

NMR spectrum of HPG. The carbons corresponding to the terminal, dendritic, linear

1,3-uint, and linear 1,4-unit are denoted by T, D, L13, and L14, respectively. Table 1. Characterizations of synthesized HPG and HPG-MA samples. Polymer

VGMA: mHPG

̅̅̅̅ Mn a

̅̅̅̅ Mn b

DBa

DSc / %

Tgd / °C

Tcd / °C

Sizee / nm

HPG

─

2053

2122

0.611

─

-29.87

─

3.12

HPG-MA-1

1:1

─

2281

─

12.30

─

171.12

3.25

HPG-MA-2

2:1

─

2306

─

22.53

─

157.90

3.31

HPG-MA-3

2.5:1

─

2333

─

28.07

─

161.79

3.35

a

Conducted by 13C NMR;b by GPC; c by 1H NMR; d by DSC; e by DLS, Z-average size.

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The Fourier transform infrared (FTIR) spectra of HPG and HPG-MA-2 are shown in Figure 4. A strong absorption band of double bond groups apparently appears at 1562 cm-1 after the transesterification of HPG, and the bands of =C-H and C=O are observed around 3003 cm-1 and 1652 cm-1. The peak ratio of -CH2 group to O-H group apparently diminishes, which represents a successful transesterification of HPG.

=C-H

Transmittance / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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C=C C=O

-OH

-CH2

HPG HPG-MA-2

-OH

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber / cm

Figure 4.

FTIR spectra of HPG and HPG-MA-2.

3.2. Thermal stabilities of HPG and HPG-MA. The typical reversible heat flow

curves from DSC of HPG and HPG-MA are described in Figures 5 and 6. As displayed in Figure 5, the glass-transition temperature (T g ) of HPG is around -29.87 °C. The crosslinking points of HPG-MA-1, HPG-MA-2, and HPG-MA-3 are determined to be 171.12 °C, 157.90 °C and 161.79 °C, respectively; and no glass-transition points are observed. Consequently, HPG-MA would be used in high 11


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temperature reservoirs, which could broaden its application sphere. 4

Heat Flow / Wg

-1

2

0

-2

Tg

-4

-6 -70

-60

-50

-40

-30

-20

-10

Temperature / C

Figure 5.

DSC curve of HPG.

0

-1

-5

Heat Flow / Wg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-10

-15

-20

HPG-MA-1 HPG-MA-2 HPG-MA-3

-25

-30 -50

0

Tc 50

100

150

200

250

Temperature / C

Figure 6.

DSC curves of HPG-MA.

3.3 Demulsification performance of HPG and HPG-MA. The effects of settling time,

temperature, salinity, as well as the demulsifier concentration on the demulsification are systematically carried out. In this work, the demulsifier concentration is separately set as 500 mg·L-1, 1000 mg·L-1, 1500 mg·L-1, and 2000 mg·L-1 in O/W emulsions. The settling 12


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time constantly maintains at 10 min, 20 min, 30 min, or 40 min. The temperature is controlled to be 30 °C, 45 °C, or 60 °C, and the salinity is 2500 mg·L-1, 5000 mg·L-1 , or 10000 mg·L-1. The following parts give their effects on the treatment of emulsions. Effect of different demulsifiers. The changes of oil removal ratio versus demulsifier concentration for different demulsifiers are illustrated in Figure 7. When no demulsifier is added, the oil removal ratio is 7 % owing to the gravity settling alone, which indicates that the emulsion is stable and it can be treated as the blank experiment. When HPG and three HPG-MA samples are added with the concentration of 2000 mg·L -1 , the oil removal ratios reach to be 17 %, 34 %, 86 % and 45 %, respectively. Clearly, the performance of HPG-MA-2 can meet the practical requirements, and the results are ascribed to the ratio between hydrophilic hydroxyl groups and GMA. The appropriate ratio makes HPG-MA have high interfacial activity and replace the intrinsic surfactant to gain the effect of demulsification.

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Energy & Fuels

100

80

Oil removal ratio / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0 500

1000

1500

2000 -1

Demulsifier concentration / mgL

Figure 7. Effect of concentration on oil removal ratio with HPG or HPG-MA as demulsifier at 60 °C after settling 40 min: dashed line, no demulsifier; ●, HPG; ▲, HPG-MA-1; ▼, HPG-MA-2; ◄, HPG-MA-3.

Effect of temperature. The changes of oil removal ratio versus temperature are shown in Figure 8. The oil removal ratio is steadily improved with the increase of temperature. When the temperature varies from 30 °C to 60 °C, the oil removal ratio enhanced 18 % with the demulsifier concentration of 500 mg·L-1 and settling time of 20 min. On the basis of the theory of molecular thermodynamic motion, the movement of demulsifier molecules becomes faster at 60 °C than that at 30 °C, which accelerates the molecules to adhere to oil-water interface, to form loose film and to lessen the intensity. Therefore, a higher temperature can greatly promote the demulsification process.

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100

100

Settling time : 40 min

Settling time : 20 min 80



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60 -1

500 mgL -1 1000 mgL -1 1500 mgL -1 2000 mgL

40

30

45

80

60 -1

500 mgL -1 1000 mgL -1 1500 mgL -1 2000 mgL

40

60

30

Temperature / C

45

60

Temperature / C

Figure 8. Oil removal ratio versus temperature in O/W emulsions with HPG-MA-2 as demulsifier.

Effect of settling time. From Fig. 8, in comparison with the oil removal ratio of 75 % at 20 min, it rises to around 86 % at 40 min under the conditions of 60 °C and 2000 mg·L-1. Thus, it should be identified that the settling time is also a significant factor to affect the demulsification. The changes of oil removal ratio against settling time are detailedly shown in Figure 9. As prolonging the settling time, the oil removal ratio increases. The value of 86 % for settling time of 40 min keeps almost constant with the settling time extended to be 90 min or 120 min. As reported in the literature10, 13, 22, the demulsifiers usually need at least 90 min to reach the equilibrium of demulsification with the maximum value of oil removal ratio. It indicates that the investigated demulsifier can quickly break the emulsion and is conducive to industrial applications. The high efficiency should be ascribed to the branched structure, and the interactions between ether bonds and water molecules. The hyperbranched polymer HPG-MA-2 could be multipointly adhered to the oil-water interface. The large amount of branches make the demulsifier be difficult to access each other and further decrease the thickness of

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interfacial film, which can availably shorten the time of adsorption and is beneficial to demulsification. 100

100

Temperature : 30 C

Temperature : 60 C

80

80

oil removal ratio / %


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40 -1

500 mgL -1 1000 mgL -1 1500 mg/L -1 2000 mg/L

20

60

40 -1

20

0 0

10

20

30

500 mgL -1 1000 mgL -1 1500 mg/L -1 2000 mg/L

40

0 0

10

Time / min

20

30

40 90

100

110

120

Time / min

Figure 9. Oil removal ratio against settling time in O/W emulsions with HPG-MA-2 as demulsifier.

Effect of salinity. On account of the practical techniques of water flooding, polymer flooding, binary system flooding, and alkaline/surfactant/polymer (ASP) flooding, there are many inconveniences to the application of demulsifiers, such as high salinity.25 To simulate the real environment of the applications, mineral salt is added into the deionized water and another array of tests are performed to explore the influence of salinity on the oil removal ratio in O/W emulsions.

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100

80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40 demulsifier only -1 2500 mgL salinity -1 5000 mgL salinity -1 10000 mgL salinity

20

0 0

10

20

30

40

Time / min

Figure 10.

Effect of salinity on oil removal ratio in O/W emulsions with HPG-MA-2 as demulsifier at 60 °C (2000 mg·L-1 demulsifier, m(NaCl):m(CaCl2)=1:1).

The changes of oil removal ratio against settling time with different values of salinity are illustrated in Figure 10. In contrast to the emulsions without salinity, both of the oil removal ratio and the slope of the curves decrease slightly when the salinity increases. The declined scope of the oil removal ratio merely reaches 9.3 % with the salinity of 10000 mg·L-1, and the ratio of demulsification is still approximately 80 %. It is attributed to the hyperbranched polymer demulsifier HPG-MA-2 belonging to non-ionic category, which is hardly affected by the ions. 3.4. Effects on interfacial tension with HPG or HPG-MA as demulsifier. To explore the

demulsification mechanism, interfacial tensions of oil-water interface with HPG or HPG-MA as demulsifier in the water phase are measured. The values of the interfacial tensions are listed in Table 2 and compared with those without adding demulsifier.

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Table 2. Interfacial tensions of the oil-water interface with different demulsifiers in water phase. Concentration (mg·L-1)

Interfacial tension (mN·m-1)

0

37.674

500 1000 1500 2000

33.656 33.625 32.061 31.111

HPG-MA-1

500 1000 1500 2000

21.971 21.480 21.092 20.668

HPG-MA-2

500 1000 1500 2000

15.277 14.936 14.480 14.278

HPG-MA-3

500 1000 1500 2000

18.895 18.548 18.326 18.063

Demulsifier

HPG

It is clearly shown from Table 2 that HPG can not significantly lower the interfacial tension, which could be ascribed to the fact that there are mainly hydrophilic groups but no hydrophobic groups in HPG molecules. Hence, HPG barely possesses the demulsification performance. After HPG is grafted with GMA, the array of HPG-MA has the ability of lowering interfacial intension, and HPG-MA-2 exhibits relatively good performance which thanks to the suitable value of DS. The appropriate DS value endows the hyperbranched polymer to reach the oil-water interface and to attain high interfacial activity. In comparison with the demulsification data, it can be concluded that lowering the interfacial intension is the prerequisite to proceed the demulsification. Furthermore, to gain a better understanding of demulsification process, the interfacial intention with only

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Energy & Fuels

Tween 80 (0.91 g·L-1) or Tween 80/Span 80 (0.09/0.91 g·L-1) in the water phase is measured to be 8.195 mN·m-1 or 8.188 mN·m-1, which is much lower than that with HPG-MA-2 as demulsifier. This difference should be attributed to the structure of hyperbranched molecules. Once increasing the concentration of HPG-MA-2, the molecules can multipointly adhere to the oil-water interface, and compress or substitute the former interfacial substances. Because of the instability of interfacial monolayer, HPG-MA-2 demulsifier achieves the purpose of demulsification. 3.5. Effect of demulsifier on the transmittance. A series of tests were performed through

measuring the transmittance values versus the settling time with the addition of 2000 mg·L-1 HPG-MA-2 in the emulsion. The wavelength of characteristic transmission peak displays at 335.5 nm. The effect of demulsifier on the transmittance is presented in Figure 11. The transmittance value rises from 0.139 % to 46.4 % compared to that of 0.034 % without demulsifier addition, which indicates that HPG-MA-2 is an effective demulsifier. In comparison with the change of the oil removal, the change of the transmittance exhibits a hysteresis phenomenon which results from that the aggregation and coalescence of small oil droplets are quite difficult. With the proceeding of demulsification, the transmittance value is still low due to the dispersion of small oil droplets in the sample. After an induction period, these droplets gradually begin to aggregate and coalesce, and the transmittance value sharply increases. This can be further verified by the observations on the average size of the bottom emulsions. 19


Energy & Fuels

Figure 11. Effect of demulsifier on the transmittance of emulsion at 25°C with the addition of 2000 mg·L-1 HPG-MA-2. The wavelength of characteristic transmission peak is 335.5 nm. 3.6. Effect of demulsifier on the average size. The size distribution and the average size

of the bottom emulsions monitored by DLS over the settling time with the addition of 2000 mg·L-1 HPG-MA-2 are shown in Figure 12.

3000 24

2500

16

Size / nm

2000

12 8

0

1500

1000

4

0

10000

10

500

8000

Ti me /

Figure 12.

(b)

(a)

20

Intensity / %

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6000

20

mi

4000 2000

30

n

40

0

e Siz

0

m

/n

0

10

20

30

40

470

480

Time / min

Effect of demulsifier on the size of emulsion at 25°C with the addition of 2000 mg·L-1 HPG-MA-2: (a) the size distribution; (b) the average size.

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Energy & Fuels

Scheme 1.

Schematic representation of a possible demulsification process.

(a) O/W emulsion; (b) adding HPG-MA as demulsifier; (c) aggregation and coalescence; (d) separation.

With the DLS results that the average size of oil droplets experiences increasing at first, and then decreasing, Scheme 1 is given to elucidate the process of demulsification. The demulsifier can quickly destroy the interfacial film and bring out coalescence of oil droplets with large size. When the settling time extends to 20 min, the largest droplets have almost aggregated and the average size culminates in the emulsion. Subsequently, oil-water separation occurs in the emulsions because of the gravity action. When the settling time is prolonged to 480 min, the average size of oil droplets is around 10 nm, representing there exist only dimer or trimer polymer molecules. Thus, it is concluded that the time-dependent size of the emulsions can increase spontaneously, and then decreases with the boost of demulsification. Together with the transmittance

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Energy & Fuels

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determinations, the observations on the changes of oil droplet can provide a new clue to evaluate the performance of the demulsifiers. CONCLUSIONS A series of methacrylated hyperbranched polyglycerol (HPG-MA) demulsifiers, have been synthesized successfully through transesterification of HPG with GMA, and the performance of demulsifiers can be controlled by adjusting the ratio of HPG to GMA. Under the optimized conditions of demulsification, the oil removal ratio can exceed 86 % with the settling time of 40 min to reach the equilibrium of demulsification, which indicates the potential application of HPG-MA demulsifiers in oilfields. The demulsification process can be observed vividly through measuring the size changes of oil droplets from the bottom phase of oil-in-water emulsions. The DLS and UV-vis observations combined with interfacial intension measurements give a clue to comprehend the demulsification mechanism. This work provides a promising way to use the reflection of microstructure to analysis and understand the process of demulsification. It is beneficial to the development of high-efficient demulsifier for oil-in-water emulsions in petroleum industry.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Tel/Fax: +86-571-88981416. E-mail: [email protected]. 22


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Energy & Fuels

Notes The authors declare no competing financial interest. Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (Nos. 21273201 and J1210042).

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Methacrylated Hyperbranched Polyglycerol as a High-Efficiency

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