Effect of Comb-type Copolymers with Various Pendants on Flow Ability

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Effect of Comb-type Copolymers with Various Pendants on Flow Ability of Heavy Crude Oil Jun Xu, Hejian Jiang, Tao Li, Xiaoming Wei, Tongshuai Wang, Jing Huang, Weina Wang, Anthony L. Smith, Jie Wang, Rui Zhang, Yisheng Xu, Li Li, Robert K Prud'homme, and Xuhong Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00674 • Publication Date (Web): 23 Apr 2015 Downloaded from http://pubs.acs.org on April 26, 2015

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Industrial & Engineering Chemistry Research

1

Effect of Comb-type Copolymers with Various Pendants on Flow

2

Ability of Heavy Crude Oil

3

1

4

Jun Xu1*, Hejian Jiang1, Tao Li1, Xiaoming Wei2, Tongshuai Wang1, Jing Huang , Weina

5

Wang1, Anthony L. Smith1, Jie Wang1, Rui Zhang1, Yisheng Xu1, Li Li1, Robert K.

6

Prud’homme3, Xuhong Guo1,4*

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1

9

Technology, Shanghai 200237, China

State Key Laboratory of Chemical Engineering, East China University of Science and

10

2

Petrochina Liaohe Oilfield Company, Panjin 124010, China

11

3

Department of Chemical Engineering and Princeton Materials Institute, Princeton University,

12

Princeton, New Jersey 08544

13

4

14

Xinjiang Production and Construction Corps for Materials Chemical Engineering, Shihezi

15

University, Xinjiang 832000, PR China

Key Laboratory of Xinjiang Uygur Autonomous Region and Engineering Research Center of

16 17 18 19

*To whom correspondence should be addressed. E-mail: [email protected] (Jun Xu) or

20

[email protected] (Xuhong Guo). Tel number: +86-021-64252362 1

ACS Paragon Plus Environment


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Abstract

1 2

Modification of the paraffin crystallization and flow ability of waxy crude oil is of vital

3

importance during transportation and restart processes at low temperature. To investigate the

4

influence of pendants in comb-type copolymers on the cold flow ability of crude oil, maleic

5

anhydride-α-octadecene copolymer and its derivatives with octadecyl (MAC), phenyl (AMAC)

6

or naphthalene (NMAC) pendants were synthesized. These derivatives, when added to waxy

7

crude oil, change the size and quantity of the paraffin crystals observed by Polarizing Light

8

Microscopy (PLM), improve the flow ability of waxy oils by reducing the viscosity and yield

9

stress revealed by rheometer, and decrease the paraffin crystallization temperature and

10

quantity of wax precipitation determined by Differential Scanning Calorimetry (DSC).

11

AMAC had the greatest effect followed by MAC and NMAC respectively. It seems that small

12

aromatic pendants improve the flow ability of waxy oils by adsorbing on the surface of

13

asphaltenes, while large aromatic pendants impair the assembly of copolymers with

14

asphaltenes by a higher steric hindrance.

15 16

Keywords: waxy crude oil, comb-type copolymer, wax crystal inhibitor, rheology.

17 18 19

1. Introduction

20

Crude oil is a complex mixture of saturated hydrocarbons, aromatics, paraffins,

21

asphaltenes and resins [1]. Crude oil, having a high content of paraffins, often exhibits a high 2


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wax apparent temperature (WAT). Below this temperature paraffins begin to precipitate,

2

crystallize, and form a “house-of-card” network, by the overlapping and interlocking of

3

orthorhombic paraffin crystals [2]. For this reason, oil with a high content of paraffins often

4

shows high viscosity, high yield stress, and follows non-Newtonian fluid behavior. It is widely

5

accepted that the deposition of paraffins and the aggregation of asphaltenes are the main causes

6

of flow difficulties with crude oil [3]. Deposited paraffin crystals, at low temperature, causes

7

severe problems in oil production, storage, and transportation [4]. Enormous amounts of

8

energy are wasted to heat and pump the crude oil from reservoir to refinery. To solve these

9

problems, thermal, mechanical and chemical methods are employed [5]. Among them, the

10

most recognized and economical approach is to add polymeric wax crystal inhibitors or Pour

11

Point Depressant (PPD) additives to the crude oil [6].

12

Asphaltenes and resins also influence the flow behavior of crude oil [7-10]. It has been

13

found that asphaltenes, in the model oil, along with paraffins, and aromatic solvent improve

14

the performance of PPD [1,3,7,10,11]. Asphaltenes can suppress the deposition of paraffins

15

just like a natural PPD [8,12-13]. PPD molecules, asphaltenes and resins often produce new

16

agglomerate, providing extra nucleation sites during paraffin crystallization. The formation

17

of large paraffin crystals is suppressed and the pour point of oils is decreased [8].Yi and

18

Zhang [9] found that the flow ability of waxy crude oil is heavily depended on its asphaltene

19

content.

20

Due to the important role of wax crystal inhibitors in oil production and transportation,

21

various wax crystal inhibitors have been synthesized, such as ethylene and vinyl acetate (EVA)

22

copolymer [14-15], ethylene-butene copolymers (PE-PEB) [16-18], alkyl esters of

23

styrene-maleic anhydride derivatives [11,19], and maleic anhydride/acrylic acid-α-olefins

24

copolymers grafted with long-chain alkyl alcohols or amines [20-21]. These copolymers

25

usually contain polar and nonpolar segments in their polymer chains. The polar segments 3



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always bear –COOH, –COO–, –CONH2, or –CONH– functional groups, while the nonpolar

2

segments are grafted by long alkyl side chains. The structure of copolymers, especially the

3

proportion of polar and nonpolar segments, plays an essential role in affecting the rheological

4

properties of crude oil [22, 23].

5

In this work, in order to study the effect of aromatic pendants on the performance of

6

comb-type copolymers by improving the cold flow of Liaohe high waxy oil, poly(maleic

7

anhydride-α -olefin) copolymer derivatives were synthesized by introducing alkyl, aromatic

8

and naphthalene pendants into the copolymer’s backbone. Rheological methods, Polarizing

9

Light Microscope (PLM) and Differential Scanning Calorimetry (DSC) were exploited to

10

study the effect of copolymers on the rheological behavior and crystallization of model oil

11

and crude oil.

12

2. Experimental

13

2.1 Materials

14

Decane (anhydrous, 99%), maleic anhydride (MA) (99%), α-octadecene (95%), benzoyl

15

peroxide

(99%)

and

o-xylene

(98%),

α-octadecylamine

16

1-naphthylamine(99%) were purchased from Alfa company and used as obtained.

17

1-methylnaphthalene (MN), and the two kinds of paraffin (No. p100928 and No. p100934)

18

were purchased from Aladdin company, whose melting points are in the range 52~54°C (No.

19

p100928) and 60~62 °C (No. p100934) respectively. The crude oil samples studied in this

20

work were obtained from Petroleum Liaohe Oilfield Company. The content of paraffin and

21

asphaltenes in crude oil is 38.5% and 15.6%, measured according to the standard

22

SY/T7550-2004. The density of crude oil is 0.8676 g/cm3, the freezing point is 54°C, and the

4


(97%),

aniline

(97%),

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viscosity at 70°C is 7.2 mPa.s, tested following the standard GB/T 1884-2000,

2

SY/T0541-2009 and SY/T0520-2008, respectively.

3

2.2 Preparation of Model Oil

4

To mimic the Liaohe crude oil, the model oils were prepared by mixing wax (30%)

5

dissolved in decane and asphaltene (20%) dissolved in 1-methylnaphthalene, as well as 0.1 ~

6

0.5% of copolymers [22].The wax is a blend of two paraffin mixtures, each of which contains

7

50%. The distribution of alkanes ranging from C16-C37 of the blend was analyzed and

8

compared to the wax extracted from the Liaohe crude oil, as shown in Figure 1. The

9

asphaltenes were extracted from the Liaohe crude oil [23-24]. In this article, crude oil and

10

model oil are abbreviated as CO and MO, respectively.

11 12

Fig. 1. Alkane distribution of purchased wax blend and wax extracted from Liaohe crude oil

13

analyzed by GC-MS.

14 15 16

2.3 Synthesis of maleic anhydride copolymers derivatives Comb-type copolymers were synthesized by radical polymerization. First, poly(α-olefin 5



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1

-alt- maleic anhydride) was polymerized by a-olefin and maleic anhydride (molar ratio =

2

1.1/1) at 120 °C for 1.5 hour, with 0.1% of benzoyl peroxide as the initiator, and o-xylene as

3

the solvent. The reaction was protected by a nitrogen atmosphere. Then, octadecylamine,

4

aniline or 1-naphthylamine with various molar ratio to maleic anhydride were fed into the

5

maleic anhydride copolymers and allowed to react at 105-110 °C for 15 hours. The crude

6

products were purified by precipitation with an excess of methanol, filtered, washed by hot

7

water, and dried by vacuum drying. The obtained copolymer derivatives are named MAC,

8

AMAC and NMAC, respectively (Figure 2).

9 10

Fig. 2. Chemical structure of comb-type copolymers. (a) MAC, (b) AMAC, (c) NMAC.

11

2.4 1H NMR spectra and SEC

12

1

H NMR spectra were recorded on a BRUKER AVANCE 500 spectrometer operating at

13

500 MHz with deuterated chloroform as the solvent. The molecular weight and molecular

14

weight distribution of the MACs were measured by Waters 1525 SEC equipped with UV-vis

15

detector, using THF as the mobile phase and polystyrene samples as standards.

16

MACs, NMACs and AMACs were obtained by varying the feeding ratios of maleic 6


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anhydride/amine (1:0.5, 1:1, and 1:2), which are named MAC0.5, MAC1.0, MAC2.0,

2

NMAC0.5, NMAC1.0, NMAC2.0, AMAC0.5, AMAC1.0 and AMAC2.0, respectively. The

3

amidation ratio (f), which means the number of amine groups introduced onto the copolymer

4

and should be between 0 and 2, was calculated based on the 1H NMR spectra (Figure 3) and

5

Table 1 [22].

6

In order to compare the effect of various pendants on the flow ability of crude oil,

7

MAC1.0, AMAC2.0 and NMAC2.0, which has a close amidation ratio, were chosen, briefly

8

marked as MAC, AMAC and NMAC.

9 10

Fig. 3. 1H NMR spectra of comb-type copolymers with various pendants. (a) MAC, (b)

11

AMAC, (c) NMAC.

12 7



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1 2

Table 1

3

Feeding ratio of monomer and amidation ratio (f) of MAC, NMAC and AMAC copolymers

4

calculated from the integration of 1H NMR spectra. Copolymer

Feeding Ratio of maleic

Amidation Ratio (f)

anhydride/amine MAC0.5

1:0.5

0.45

MAC1.0

1:1

0.89

NMAC0.5

1:0.5

0.35

NMAC1.0

1:1

0.60

NMAC2.0

1:2

0.87

AMAC0.5

1:0.5

0.31

AMAC1.0

1:1

0.58

AMAC2.0

1:2

0.83

5

Feeding ratio of monomer and amidation ratio (f) of MAC, NMAC and AMAC copolymers

6

calculated from the integration of 1H NMR spectra

7 8 9 10

The molecular weight and polydispersity of poly(octadecylene - maleic anhydride) copolymers are 7300 and 1.43, respectively. 2.5 Rheological measurements

8


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All of the rheological measurements were done using the MCR501 rheometer from

2

Anton-Paar Physical Company (Austria). Before being measured, model oil samples were

3

heated to 70°C and kept for 0.5 hour, and the crude oil samples were heated to 80°C and kept

4

for 0.5 hour in order to remove any thermal and shearing history.

5

Yield Stress. The yield stress (τy) is defined as the stress below which no flow occurs. An

6

operational definition of τy is the stress at the transition between the creep and liquid-like

7

viscosity regimes. As the stress increases, the gel creeps for a brief period of time and then

8

catastrophically fails. The stress at failure is recorded as the yield stress [22]. The yield stress

9

of model oil and crude oil was measured at 0 and 20 °C, respectively.

10

Viscosity. The viscosity of model oil upon temperature changing from 55 to 30 °C was

11

measured at a constant shear rate of 10 s-1. Model oil exhibits as a waxy gel, since

12

precipitated small wax crystals dispersed in decane in this temperature range. When the

13

temperature is above 55°C, all of the wax dissolved in decane and model oil shows

14

Newtonian fluid behavior. Under 30°C, both wax and asphaltenes precipitate out of solution.

15

For the same reason, crude oil samples are measured at a constant shear rate of 10 s-1 from 70

16

to 45 °C.

17

Pour Point. The pour points of oil samples, in the absence and presence of copolymers,

18

were determined from viscosity-temperature curves, which are defined as the temperature at

19

which viscosity increases dramatically as the temperature decreases. This reveals the lowest

20

temperature at which oil samples can freely flow.

9



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Thixotropic and Recovery Properties. The thixotropy property reflects the structural

2

strength of waxy gel. This value can be evaluated by measuring the hysteresis loop. The

3

smaller the area of hysteresis loop, the weaker is the structural strength, which implies a better

4

flow ability of fluids.

5

Crude oil samples were heated to 80 °C, loaded in the coaxial cylinder sensor system, and

6

then cooled to 50 °C at the rate of 1 °C/min. The shear rate was linearly changed in the

7

following pattern, as shown in Equation 1:

 R × t;  R × (2t1 − t );

γ& = 

8

9 10

Where

0 ≤ t ≤ t1 t1 ≤ t ≤ 2t1

(1)

is the shear rate (s-1), R is the changing rate of shear rate (2 s-1/s), t is the

shearing time, and t1 is the increased time of shearing which was equal to 10 min.

11

The recovery properties of crude oil were measured using the small amplitude

12

oscillatory pattern. Oil samples were heated to 80 °C, loaded in the coaxial cylinder sensor

13

system, cooled to 50 °C at the rate of 1 °C/min, and then subjected to a shearing at 10 s-1 for

14

10 min. Small amplitude oscillatory for 60 min was applied to measure the static recovery

15

property of the oil samples.

16

2.6 Polarized Light Microscope

17

Morphology of paraffin crystals was observed using a LEICA DM2500P Polarized Light

18

Microscope (PLM) with a Linkam THMS 600 cold/hot stage. Images were captured using a

19

CCD camera connected to a PC via a WT-1000GM imaging board. A tiny quantity of model oil

20

or crude oil sample was transferred on a glass slide for observation at 20°C. Images of model 10


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1

oil and crude oil were both taken at 20°C.

2

2.7 Differential Scanning Calorimetry

3

Crystallization of paraffins in waxy gels was researched by TA2000/MDSC2910

4

Differential Scanning Calorimetry (DSC) apparatus from TA Instruments. All samples were

5

heated to 100 °C and kept at this temperature for 30 min to remove any thermal history. The

6

cooling rate was set to 10 °C/min, from 80 to -20°C for model oil and from 80 to 0 °C for crude

7

oil. The enthalpy and onset temperature of transitions were analyzed using the TA Universal

8

Analysis software.

9 10

3. Results and Discussion

11

3.1 Rheology

12

Yield Stress. The yield stress as a function of the copolymer concentration in both model

13

oil and Liaohe crude oil is shown in Figure 4. From Figure 4a, it is found that all three

14

copolymers have a positive effect on the reduction of yield stress for model oil. The reduction

15

of yield stress increases with the increase of copolymer concentration. The same trend is also

16

found for the crude oil, as shown in Figure 4b. Among the three copolymers, the largest

17

reduction of yield stress was realized by the dosage of AMAC for both oils, and the smallest

18

reduction was obtained by the dosage of NMAC. It seems that the phenyl pendants “match”

19

with the asphaltenes better than the naphthalene pendants. That is to say, AMAC assembles

20

with asphaltenes easily more than NMAC. In fact, large aromatic pendants with a short

21

spacer have a weak assembly with asphaltenes because of high steric hindrance. Only small 11



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1

benzyl pendants with a short spacer assemble strongly with asphaltenes, inhibit the

2

aggregation of asphaltenes, and thus decrease the yield stress of both model oil and crude oil.

3 4

Fig. 4. Yield stress of oils as a function of copolymer concentration. (a) Model oil at 0 °C, (b)

5

Crude oil at 20 °C.

6 7

Viscosity. Viscosity of model oil and crude oil under various temperatures and

8

concentrations of copolymers are compared in Figure 5a and Figure 5b. When the temperature

9

decreases but is above the WAT, both kinds of oil are a Newtonin fluid. In Figure 5a, it is

10

found that there is only a slight increase of the viscosity with the decrease of temperature for

11

model oil. However, the increase of viscosity for the crude oils is much higher, as shown in

12

Figure 5b. Despite this, the viscosity of both kinds of oil nearly linearly increases with the

13

decrease of temperature. As the temperature drops below WAT, different oils exhibit different

14

behavior. With the formation of the waxy gels by rapid precipitation of wax crystals in model

15

oil, comes the dramatic increase in viscosity of model oil which converts the model oil from

16

Newtonian to a non-Newtonian fluid. Though a transition can be found for the crude oil, there 12


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1

is no significant increase of the viscosity around the WAT. Similar to the results of yield stress

2

measurements, the viscosity is reduced most by AMAC for both kinds of oil among three

3

copolymers. The reduction of viscosity is smallest by NMAC.

4 5

Fig. 5. Viscosity of oils as a function of temperature with 0.5% of NMAC, MAC and AMAC.

6

(a) Model oil, (b) Crude oil.

7 8 9

The relationship between the viscosity of oils and temperature above the WAT can be stated as Equation 2 [25].

10

µ = A × eE / RT

11

Where, A is a constant depending on the flow entropy of activation, Ea is the fluid

12

activation energy reflecting the internal friction between molecules, and R is the universal gas

13

constant which is 8.314 J/mol·K.

a

(2)

14

The curves of viscosity as a function of temperature above the WAT for model oil was

15

fitted by Equation 2 and the corresponding parameters are shown in Table 2. From Table 2, it

16

is found that all of the Ea for model oil in the presence of copolymer is lower than that of 13



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1

model oil without any copolymer. This means that copolymers impair the influence of

2

temperature on the viscosity of model oil. Among the three model oils in the presence of

3

copolymers, model oil in presence of AMAC exhibits the lowest E a , while that in presence of

4

NMAC shows the highest Ea, verifying the inference from yield stress measurements.

5 6

Table 2

7

Parameters in Arrhenius equation for model oil in the absence and presence of different

8

copolymers. Samples

A

Ea

Correlation Index

Model Oil

5.56 × 10-6

1.89 × 104

99.7%

+0.5% NMAC

8.72 × 10-6

1.72 × 104

99.7%

+0.5% MAC

9.73 × 10-6

1.60 × 104

99.6%

+0.5% AMAC

10.1 × 10-6

1.57 × 104

99.5%

9

Thixotropic Property. Thixotropic property reflects both the recover ability and

10

structural strength of waxy gels. The restarting ability of crude oil in pipelines also has been

11

studied by this means [26-27]. The hysteresis loop and small-amplitude oscillatory shear are

12

introduced to study the structural recovery of crude oil.

13

The recoverability and structural strength of oil samples were investigated by measuring

14

the hysteresis loop. As shown in Figure 6, in the upward curves, the crude oil experienced a

15

creep which results in a rapid increase of shear stress at the beginning of shear. When the

16

shear strain exceeds the yield point of waxy gels, the wax crystals are degraded and the gel 14


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1

begins to flow. The yield point, corresponding to the peak of shear stress, is called the yield

2

stress [28]. The yield stress obtained from the hysteresis loop is consistent with the previous

3

yield stress measurements. With the increase of shear rate, the shear stress drops and then

4

tends to be stable.

5 6

Fig. 6. The hysteresis loops of Liaohe crude oil in absence and presence of copolymers at the

7

same concentration of 0.5%.

8 9

From Figure 6, it is found that the area of hysteresis loops for crude oil in the presence

10

of copolymers is smaller than that of crude oil in absence of any copolymer. By integrating

11

the area of hysteresis loops of crude oil in absence of any copolymer, in presence of NMAC,

12

MAC and AMAC, the integration is calculated out and the values are 235.4, 157.9, 126.7 and

13

115.8 Pa/(s·ml), respectively. The sequence of integration of hysteresis loop area is consistent

14

with the yield stress and viscosity measurements, which also proves the previous inference.

15



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1

In addition, the formation process of waxy gels was studied by researching the

2

isothermal structure recovery [29-30]. The storage modulus of Liaohe waxy crude oils under

3

small-amplitude oscillation pattern [31-34] was measured during the isothermal process. The

4

storage modulus curves were then fitted by Equation 3 [35-36].

5

ln Gt′ = ln G∞′ − (ln G∞′ − ln G0′ ) × e− ct

m

(3)

6

Where G’t is the storage modulus at time t, G’0 is the initial modulus value at time t0,

7

G’∞ is the equilibrium storage modulus which reflects the structural strength of waxy gels.

8

Both c and m are structural constants, in which c is the recovery rate of the storage modulus,

9

and m is the recovery time till reaching the equilibrium.

10 11

Fig. 7. The fitting curves and storage modulus as a function of time for Liaohe crude oil in

12

absence and presence of copolymers.

13

16


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1

The fitting curves and storage modulus as a function of time for Liaohe crude oil are

2

shown in Figure 7. From this Figure, it is found that the presence of copolymers decreases the

3

storage modulus of waxy crude oil. Among the three copolymers, the sequence of reduction

4

of the storage modulus for crude oils is AMAC > MAC > NMAC, which is also consistent

5

with the previous rheological measurements.

6

The correlation coefficients of the fitting equations are shown in Table 3. From this

7

Table, it is found that the equation fits well the storage modulus data. Among the three

8

copolymers, G’0 and G’∞ is reduced by AMAC > MAC > NMAC, indicating that small

9

phenyl pendants in copolymers impair the structural strength and recover ability of waxy

10

gels.

11

Table 3

12

The correlation coefficients of the storage modulus fitting equation for Liaohe crude oil in the

13

absence and presence of copolymers. Oil samples

G0′

G∞′

c

m

R

Crude Oil

338

1014

0.087

0.899

99.4%

+0.5%NMAC

233

698

0.099

0.798

99.5%

+0.5%MAC

148

557

0.200

0.676

99.5%

+0.5%AMAC

48

286

0.201

0.662

99.4%

14 15

The same sequence is also found for the value of c, indicating the recovery rate of crude

16

oil is AMAC > MAC > NMAC. The identical sequence of reduction of m is also found, 17



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

1

which implies that it takes a longer time to recover the strain of waxy gels after the dosage of

2

copolymers, and the recovery time is AMAC > MAC > NMAC.

3

In a word, the correlation coefficients prove that the copolymers impair the strain

4

recoverability of waxy crude oil in varying degrees by changing the isothermal and quiescent

5

behavior of wax crystals.

6

3.2 Microscope

7

Morphology of wax crystals in the absence and presence of MAC, AMAC and NMAC

8

was observed by polarizing light microscope at 20°C. Morphology of wax in model oil is

9

shown in Figure 8a, c, and e and that in crude oil is shown in Figure 8b, d, and f. As shown in

10

Figure 8a, wax crystals appear as an irregular shape and are closely packed in the absence of

11

copolymer. In the presence of copolymers, the shape of wax crystals has no obvious change,

12

but the quantity is reduced (Figure 8c, e, and g). In the presence of copolymers, the quantity

13

of wax crystals were reduced by AMAC > MAC > NMAC. This is consistent with the

14

rheological measurements for model oil. Similar changes are also found in crude oil (Figure

15

8d, f, and h).

16

Consistent with the previous observations in microscopy, phenyl pendant in AMAC is

17

more helpful on the hindrance of wax crystallization than the naphthalene pendant and long

18

alkyl side-chains.

19

18


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1 2

Fig.8. Polarizing light micrographs of model and crude oil in the absence and presence of

19



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

1

copolymers. (a) MO, (c) MO+ 0.5%NMAC, (e) MO+ 0.5%MAC, (g) MO+ 0.5%AMAC; (b)

2

CO, (d) CO+ 0.5%NMAC, (f) CO+ 0.5%MAC, (h) CO+ 0.5%AMAC.

3 4

As shown in Figure 9b, all the cooling curves of crude oil exhibit two distinct transitions

5

[7-8]. Similar to the model oil, in presence of copolymers, the WAT is decreased and the

6

enthalpy of transition is reduced, though the decrease of WAT is lower than that of model oil.

7

Identically, in presence of copolymers, the WAT and enthalpy of transitions of crude oil are

8

decreased by AMAC > MAC > NMAC, similar to the measurements of model oils.

9 10

Fig. 9. DSC thermogram of model oil and Liaohe crude oil in the absence and presence of

11

MAC, AMAC, NMAC.

12 13

According to the crystallization theory, only when the radius of the crystal nucleus

14

exceeds the critical radius ( rk ), can a stable crystal nucleus exist and grow, as shown of

15

Equation 4.

20


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1


rk =

2σ l − s MTe ρ∆H p ∆T

(4)

2

Where ∆H p is the enthalpy of wax crystallization, σ l − s is the interfacial tension

3

between the wax crystals and the light hydrocarbon. ρ is the density of wax crystal and M

4

is molecular weight of paraffin. ∆H p and σ l − s are decreased by the dosage of copolymers,

5

and ∆H p are decreased more than σ l − s , which results in the increase of rk [37]. With the

6

increase of rk , the precipitation of wax from the light hydrocarbon changes more and more

7

difficult. The solubility of wax in hydrocarbon increases and the WAT of oils decreases.

8

The thermal effect, during the wax precipitation in a specified temperature range (Q),

9

can be calculated by integrating the enthalpy of cold transitions ranging from the WAT to the

10

specified temperature. The concentration of wax precipitated from oil (Cw) is deduced by

11

Equation 5, where m pre is the weight of precipitated wax, mtotal is the total weight of wax in

12

the oil, and Q is suggested to be 210 J/g [9]

13 14

Fig. 10. The weight percentage of wax precipitated from the model oil (a) and crude oil (b) in

15


16 21



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

′ m pre Q Cw = = mtotal Q

1

Page 22 of 32

(5)

2

The weight percentage of wax precipitated from model oil and crude oil changing with

3

temperature is shown in Figure 10. From Figure 10a, it is found that the weight percentage of

4

wax precipitated from the model oils in the presence of copolymers is smaller than that in

5

absence of copolymers. With the temperature further decreased, the gap between the weight

6

percentage of wax precipitated from the model oils in the absence and presence of

7

copolymers becomes smaller [38]. Similarly, the weight percentage of wax precipitated from

8

the crude oil with copolymer is much smaller than that without copolymer (Figure 10b). But

9

the difference of wax precipitation weight percentage between the crude oils in the absence

10

and presence of copolymers becomes larger upon reducing temperature, which seems

11

opposite to the result of model oil.

12

The mechanism of comb-type copolymers improving the flow ability of crude oils by

13

assembling with the paraffins and asphaltenes in waxy crude oil is proposed, as shown in

14

Figure 11. The nonpolar long-chain alkyl pendants of copolymer inhibit the regular

15

arrangement of paraffin crystals by inserting themselves into the paraffin molecules, and

16

inhibit the formation of a wax network, which has been proposed in our previous paper [22].

17

In addition, the aromatic pendants in a copolymer disperse or partially damage the

18

aggregation of asphaltenes by the adsorption onto the surface of asphaltenes (Figure 11). By

19

means of copolymer assembly with paraffin and asphaltenes, the cold flow ability of crude oil

20

is dramatically improved. Among the three copolymers, the flow ability of model oil and

22


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1

crude oil is improved in the order of AMAC > MAC > NMAC. It seems that AMAC

2

copolymer with phenyl pendants is more easily adsorbed on the surface of asphaltene

3

molecules and prevents them from aggregating. NMAC copolymer, in which naphthalene

4

pendants are supposed to be appropriate for assembling with asphaltenes, show a poor ability

5

to improve the flow of oils probably due to the high steric hindrance, whose performance is

6

even worse than MAC copolymer.

7 8

Fig. 11. The possible mechanism of comb-type aromatic copolymers for improving the cold

9

flow ability of waxy crude oil by assembling with paraffins and asphaltenes.

10 11

23



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

1

Conclusions

2

Maleic anhydride-α-octadecene copolymer derivatives with different pendants (MAC

3

with alkyl group, AMAC with phenyl one and NMAC with naphthyl one) were synthesized.

4

Their chemical structure were characterized and confirmed by 1H NMR spectra. Rheological

5

methods, Polarizing Light Microscope and Differential Scanning Calorimetry were exploited

6

to explore the effect of copolymers with various pendants on improving the flow ability of

7

heavy crude oils. The presence of copolymers reduces the yield stress of model oil and crude

8

oil, improves their thixotropy properties, decreases the crystal size, and decreases both the

9

crystallization temperature and enthalpies of wax crystals as observed by rheology, PLM and

10

DSC measurements. Various pendants in copolymers have a different effect on the

11

performance of oil flow improvement. Among the three copolymers, AMAC shows the best

12

effect on improving the cold flow ability of heavy crude oils by decreasing the yield stress

13

and viscosity, reducing the quantity of wax crystals, and forming a weak structure of wax

14

crystal after shearing. NMAC shows the worst performance. It seems the higher steric

15

hindrance of naphthyl pendants impair the assembly of NMAC with asphaltenes. The

16

sequence of copolymers improving the flow ability of waxy oils is AMAC > MAC > NMAC,

17

which was also confirmed by molecular simulation. The possible mechanism of comb-type

18

copolymers for improving the cold flow ability of waxy crude oil by assembling with

19

paraffins and asphaltenes has been proposed.

20 21 24


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1


Acknowledgement

2

Financial support by National Natural Science Foundation of China (51003028,

3

21004021, 21306049 and 51273063), the Fundamental Research Funds for the Central

4

Universities, the Higher School Specialized Research Fund for the Doctoral Program

5

(20110074110003), and 111 Project Grant (B08021) is gratefully acknowledged. The authors

6

also thank Petrochina Liaohe Oilfield Company for affording oil samples and technological

7

supports.

8 9 10 11 12 13 14 15 16 17 18 19 20 21

References (1) Venkatesan, R.; Ostlund, J. A.; Chawla, H.; Wattana, P.; Nyden, M.; Fogler, H. S. The Effect of Asphaltenes on the Gelatin of Waxy Oils. Energy Fuels 2003, 17, 1630. (2) Soni, H. P.; Bharambe, D. P.; Nagar, A. Synthesis of Chemical Additives and Their Effect on Akholjuni Crude Oil. Indian J. Chem. Technol. 2005, 12, 55. (3) Kriz, P.; Andersen, S. I. Effect of Asphaltenes on Crude Oil Wax Crystallization. Energy Fuels 2005, 19, 948. (4) Castro, L. V.; Vazquez, F. Copolymers as Flow Improvers for Mexican Crude Oils. Energy Fuels 2008, 22, 4006. (5) Beiny, D. H.; Mullin, J. W. Solubilities of Higher Normal Alkanes in m-Xylene. J. Chem. Eng. Data 1987, 32, 9. (6) Victor, V. L.; Roman M.B.; Rustem, Z. S. Calculation of Dipole Moment of Fractal Asphaltene Cluster. J. Disper. Sci. Technol. 2011, 32, 1502. 25



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(7) Tinsley, J. F; Jahnke, J. P; Adamson, D. H; Guo, X. H; Kriegel, R; Saini, R; Dettman, H. D;

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Prud'home, R. K. Waxy Gels with Asphaltenes 2: Use of Wax Control Polymers. Energy

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Fuels 2009, 23, 2065.

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(8) Fang, L.; Zhang, X, D.; Ma, J. H.; Zhang, B. T. Investigation into a Pour Point Depressant for Shengli Crude Oil. Ind. Eng. Chem. Res. 2012, 51, 11605.

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(9) Yi, S. Z.; Zhang, J. J. Relationship between Waxy Crude oil Composition and Change in

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the Morphology and Structure of Wax Crystals Induced by Pour-point-depressant

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Beneficiation. Energy Fuels 2011, 25, 1686.

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(10) Oliveira, G. E.; Mansur, C. R. E.; Lucas, E. F. The Effect of Asphaltenes, Naphthenic

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Acids, and Polymeric Inhibitors on the Pour Point of Paraffins Solutions. J. Disper. Sci.

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Technol. 2007, 28, 349.

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(11) Xu, J.; Qian, H Q.; Xing, S. L.; Li, L.; Guo, X. H.; Synthesis of Poly(maleic acid

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alkylamide-co-a-olefin-co-styrene) Co-polymers and Their Effect on the Yield Stress and

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Morphology of Waxy Gels with Asphaltenes. Energy Fuels 2011, 25,573.

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(12) Chanda, D.; Sarmah, A.; Borthakur, A.; Rao, K. V.; Subrahmanyam B.; Das, H. C.

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Combined Effect of Asphaltenes and Flow Improvers on the Rheological Behaviour of

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Indian Waxy Crude Oil. Fuels 1998, 77, 1163.

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(13) Luis, A.; Eduardo, B. Experimental Study of the Influence of Solvent and Asphaltenes

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on Liquid-Solid Phase Behavior of Paraffinic Model Systems by Using DSC and FT-IR

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Techniques. J. Therm. Anal. Calorim. 2012, 107, 1321.

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(14) Gilby, G. W. The Use of Ethylene-Vinyl Acetate Copolymers as Flow Improvers and

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Wax Deposition Inhibitors in Waxy Crude Oil. Special Publication-RSC 1983, 45, 108.

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(15) Ashbaugh, H. S.; Guo, X. H.; Schwahn, D.; Prud’homme, R. K.; Richter, D.; Fetters, L. J.

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Interaction of Paraffin Wax Gels with Ethylene/vinyl Acetate Co-polymers. Energy Fuels

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2005, 19, 138.

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(16) Guo, X. H.; Pethica, B. A.; Huang, J.; Prud’homme R. K.; Adamson DH; Fetters L. J.

6

Crystallization of Mixed Paraffin from Model Waxy Oil and the Influence of

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Micro-crystallize Poly(ethylene-butene) Random Copolymers. Energy Fuels 2004, 18,

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930.

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(17) Guo, X. H.; Pethice, B. A.; Huang, J. S.; Prud’homme, R. K. Crystallization of

10

Long-chain n-Paraffins from Solutions and Melts as Observed by Differential Scanning

11

Calorimetry. Macromolecules 2004, 37, 5638.

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(18) Li, L.; Guo, X. H.; Adamson, D. H.; Pethica, B. A.; Huang, J. S.; Prud’homme, R. K.

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Flow Improvement of Waxy Oils by Modulating Long-chain Paraffin Crystallization

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with Comb Polymers: An Observation by X-ray Diffraction. Ind. Eng. Chem. Res. 2011,

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50, 316.

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(19) Al-Sabagh, A. M.; Noor, E. D.; Morsi, R. E.; Elsabee, M. Z. Styrene-maleic Anhydride

17

Copolymer Esters as Flow Improvers of Waxy Crude Oil. J. Petrol. Sci. Eng. 2009, 65,

18

139.

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(20) Wu, Y. M.; Ni, G. D.; Yang, F.; Li, C. X.; Dong, G. L. Modified Maleic Anhydride

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Co-polymers as Pour Point Depressants and Their Effects on Waxy Crude Oil Rheology.

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(21) Soni, H. P.; Kiranbala, K. S.; Agrawal, A. N.; Bharambe, D. P. Designing Maleic

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Anhydride-α-Olefin Copolymers Combs as Wax Crystal Growth Nucleators. Fuel

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Process. Technol. 2010, 91, 997.

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(22) Xu, J.; Xing, S. L.; Qian, H. Q.; Chen, S.; Wei, X. M.; Zhang, R.; Li, L.; Guo, X. H.

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Effect of Polar/Nonpolar Groups in Comb-Type Copolymers on Cold Flow Ability and

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Paraffin Crystallization of Waxy Oils. Fuel 2013, 103, 600.

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Crystallization and Dissolution Temperature, and Newtonian and Non-newtonian Flow

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Properties. Energy Fuels 1991, 5, 895.

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Paraffin Crystals in Waxy Crude Oils Cooled in Quiescent Conditions and under Flow.

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Fuel 2003, 82, 127.

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Waxy Crude Oil after Static and Dynamic Cooling. J. Petrol. Sci. Eng. 2011, 77, 351.

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(35) Barnes, H. A. Thixotropy - A Review. J. Non-Newtonian Fluid Mech. 1997, 70, 1.

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(36) Mewis, J.; Wagner, N. J. Thixotropy. Adv. Colloid Interface Sci. 2009, 147, 214.

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(37) Hennessy, A. J.; Neville, A.; Roberts, K. J. An Examination of Additive-mediated Wax

16 17 18

Nucleation in Oil Pipeline Environments. J. Cryst. Growth 1999, 198, 830. (38) Chen, W. H.; Zhao, Z. C.; Yin, C. Y. The Interaction of Wax with Pour Point Depressants. Fuel 2010, 89, 1127.

19

29



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1

Figure Captions

2

Fig. 1. Alkane distribution of purchased wax blend and wax extracted from Liaohe crude oil

3

analyzed by GC-MS.

4

Fig. 2. Chemical structure of comb-type copolymers. (a) MAC, (b) AMAC, (c) NMAC.

5

Fig. 3. 1H NMR spectra of comb-type copolymers with various pendants. (a) MAC, (b)

6

AMAC, (c) NMAC.

7

Fig. 4. Yield stress of oils as a function of copolymer concentration. (a) Model oil at 0 °C, (b)

8

Crude oil at 20 °C.

9

Fig. 5. Viscosity of oils as a function of temperature with 0.5% of NMAC, MAC and AMAC.

10

(a) Model oil, (b) Crude oil.

11

Fig. 6. The hysteresis loops of Liaohe crude oil in absence and presence of copolymers at the

12

same concentration of 0.5%.

13

Fig. 7. The fitting curves and storage modulus as a function of time for Liaohe crude oil in

14


15

Fig.8. Polarizing light micrographs of model and crude oil in the absence and presence of

16

copolymers. (a) MO, (c) MO+ 0.5%NMAC, (e) MO+ 0.5%MAC, (g) MO+ 0.5%AMAC; (b)

17

CO, (d) CO+ 0.5%NMAC, (f) CO+ 0.5%MAC, (h) CO+ 0.5%AMAC.

18

Fig. 9. DSC thermogram of model oil and Liaohe crude oil in the absence and presence of

19

MAC, AMAC, NMAC.

20

Fig. 10. The weight percentage of wax precipitated from the model oil (a) and crude oil (b) in

21

absence and presence of copolymers. 30


Page 30 of 32

Page 31 of 32

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1

Fig. 11. The possible mechanism of comb-type aromatic copolymers for improving the cold

2

flow ability of waxy crude oil by assembling with paraffins and asphaltenes.

3

31



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

1

Graphics for the Table of Content

2

32


Page 32 of 32

Effect of Comb-type Copolymers with Various Pendants on Flow Ability

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