Influence of Asphaltene Polarity on Crystallization and Gelation of

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Influence of Asphaltene Polarity on Crystallization and Gelation of Waxy Oils Yuzhuo Li, Shanpeng Han, Yingda Lu, and Jinjun Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03553 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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Influence of Asphaltene Polarity on Crystallization and Gelation of Waxy Oils Yuzhuo Li, Shanpeng Han, Yingda Lu*1 and Jinjun Zhang*2

National Engineering Laboratory for Pipeline Safety/MOE Key Laboratory of Petroleum Engineering/Beijing Key Laboratory of Urban Oil & Gas Distribution Technology

China University of Petroleum, Beijing, China *1

Primary corresponding author. Tel: 86-10-8973-2205. E-mail address: [email protected]

*2

Second corresponding author. Tel: 86-10-8973-4627. E-mail address: [email protected]

1 ACS Paragon Plus Environment

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Abstract We report for the first time the results from a systematic investigation of how asphaltenes of different polarity affect crystallization and gelation of waxy oils. The more polar asphaltenes were found to be more aromatic in nature and more highly self-aggregated in the solvent. The presence of less polar asphaltenes in the waxy oil reduced wax appearance temperature and wax precipitation to a greater degree compared to more polar asphaltenes, which was mainly attributed to the difference in the aggregation state of asphaltene of different polarity. Reducing the polarity of asphaltenes present in the oil also resulted in lower gelation temperature, lower storage modulus, and lower yield stress, which was probably because the less polar asphaltenes were more similar to paraffin on the molecular level and thus more readily to interact with wax. Notably, a 99% reduction in yield stress was observed upon the addition of the least polar asphaltenes examined in the present work, in contrast to the 62% yield stress reduction upon the addition of the most polar asphaltenes. This observation may be of industrial significance as they suggest that the crude oil containing less polar asphaltenes may form softer gel or deposit that are more easily to be broken or removed. Microscopic analysis showed that the wax crystals precipitated in the presence of less polar asphaltenes have smaller aspect ratio.

Keywords Waxy oils, Asphaltenes, Polarity, Crystallization, Gelation


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1. Introduction Most crude oils contain wax, also known as n-paraffin, which generally refers to the normal alkanes with carbon number ranging from C17 to C55.1 At relatively high temperatures, wax is dissolved in crude oils in its molecular form. When the oil temperature drops below the wax appearance temperature (WAT), wax will start to precipitate out as wax crystals. This wax precipitation can manifest to a variety of flow assurance issues for the transportation of waxy crude oils, including increased oil viscosity,2 wax deposition,3,4 and oil gelation during temporary operation shut-down.5 Asphaltenes are defined as the fraction of crude oils that is insoluble in light normal alkanes (e.g., n-pentane and n-heptane) but soluble in aromatic solvents (e.g., toluene). 6 Chemically speaking, asphaltenes contain condensed aromatic rings, aliphatic chains, heteroatoms, and in many occasions heavy metals such as Ni, Fe, V.7 A majority of researchers believe that the structure of an asphaltene molecule can be approximated as a poly aromatic condensed core surrounded by a shell of alkyl side chains, the latter of which are believed to provide stability to the molecule. 8 At concentrations higher than Critical Nanoaggregate Concentration (CNAC), asphaltene molecules associate with each other into nanoaggregates and can even further form fractal clusters if conditions are favorable.9 The interactions of asphaltenes and wax have been the subject of many studies on waxy crude oils in the last two decades.10-18 The effect of asphaltenes on the thermodynamic behavior of waxy oils is usually elucidated by examining WAT, the onset temperature at which wax crystallization is microscopically observed. The literature, however, contains conflicting views about the effect. Alcazar-Vara et al. showed that a higher asphaltene concentration generally results in a lower WAT.12 Tinsley et al. demonstrated that the effect of asphaltenes on WAT


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strongly depends on asphaltene concentration and aggregation state, with a low asphaltene concentration decreasing WAT and a high asphaltene concentration increasing WAT.15 Kriz et al. reported that the WAT generally increases with increasing asphaltene concentrations with an unusually high WAT appearing when the asphaltene concentration is around 0.01%. The authors explained this anomaly by stating that 0.01% is around the “critical asphaltene concentration” at which the total surface area of asphaltenes is largest for wax crystallization.13 Regarding the effect of asphaltenes on the gelation of waxy oils, most previous studies have showed that the addition of asphaltenes can reduce gelation temperature and yield stress, but the magnitude of reduction depends on the concentration and chemical nature of asphaltenes.10,12,15 Microscopic examinations showed that when asphaltenes are present, the morphology of wax crystals changes from rod-like to globular in shape and the formed wax crystal network becomes less entangled, leading to a weaker gel strength.10 Though in most cases the gelation temperature and yield stress are found to monotonically decrease with increasing asphaltene contents, a few studies reported an unexpected high gelation temperature and yield stress when the asphaltene content is around 0.01%.13,19 This unusual “high peak” would disappear if the total wax content is lowered or if the wax has a broader carbon number distribution.19 Unfortunately, few convincing explanations have been provided to date to explain these interesting observations. The aggregation state of asphaltenes in the solution is another factor that can affect their effects on the crystallization and gelation of waxy oils. For instance, Lei et al. found that asphaltenes in a more dispersed state tend to interact with wax on a molecular level and hinder wax crystal growth, whereas asphaltenes in a more aggregated state can act as additional junction points bridging different wax crystals.16 In addition to the effect of asphaltene on crystallization and gelation of waxy oils, two recent studies report that adding asphaltenes can reduce wax


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deposition rate and result in the formation of a two-layer deposit morphology, with each layer showing considerably different strength, microstructure, and wax content.17,18 To date, nearly all previous work examined the influence of asphaltenes as a single “pseudocomponent” on the behaviors of waxy oils. The influence of asphaltene with different polarity on crystallization and gelation of waxy oil, however, has received just limited prior attention. To the best of our knowledge, only one previous work reports that more polar asphaltenes reduced the gelation temperature to a larger extent.10 However, this work is limited in that it just studied the gelation temperature, and didn’t systematically assess the effect of asphaltene polarity on other thermodynamic and rheological properties, such as WAT, gelation temperature, and yield stress. These gaps in literature motived our current work. We herein report the results from a systematic and comprehensive study of how asphaltenes of different polarity affect the crystallization and gelation of waxy oils. Synthetic model oil systems were employed to allow for the independent control of asphaltene concentrations. The influences of asphaltene polarity on WAT, wax precipitation curve, gelation temperature and yield stress are identified and reported. We also performed microscopic analysis of the waxy oils containing asphaltenes of different polarity to probe the interactions of wax and asphaltenes on a more microscopic level.

2. Methods 2.A Materials Asphaltenes separated from a Venezuela residue oil were used for this study. This asphaltenes were fractionated into 4 sub-fractions using an adjusted procedure adopted from previous study. 20 , 21

The procedure is outlined in Figure 1. Briefly, the asphaltenes were first dispersed in

dichloromethane at a w/w=1:10 ratio. The asphaltenes of different polarity were obtained by adding different amount of pentane into the mixture followed by centrifuging for 30 min. The


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different fractions of asphaltenes are represented by two subscripts indicating the volumetric ratio of dichloromethane and pentane in the mixture. For instance, the F33/67 asphaltene refers to the asphaltene fraction centrifuged out from the solvent mixture composed of dichloromethane and pentane at v/v=33:67. Based on the “like dissolves like” theory, it is presumable that the F33/67, F25/75, F20/80, F10/90 asphaltenes have a polarity in decreasing order. Compared to the previous procedure, two adjustments were made in the current work. First, the fractionation temperature was maintained at 15 ℃ to prevent the excess evaporation of dichloromethane and pentane. Second, a higher centrifugation speed was employed to separate the less polar asphaltenes to ensure a reliable fractionation effect. Asphaltene

Dichloromethane

100:0 Pentane Pentane

Pentane 33:67 5000 rpm

F33/67 * The

20:80

25:75 5000 rpm

Pentane

7500 rpm

10:90 10000 rpm

F20/80

F25/75

F10/90

ratio on each flask represents the volumetric ratio of Dichloromethane and Pentane in the mixture

Figure 1: Asphaltene fractionation procedure

For the purpose of the present work, synthetic model oil systems were employed to allow for the independent control of asphaltene concentrations. The model oils are composed of wax, asphaltenes, mineral oil, and o-xylene, the last of which provided solubility for the asphaltenes. The mineral oil was obtained from SKLAN Group Company Limited and all other chemicals were obtained from the Sinopharm Chemical Reagent Company Limited. The wax has a melting 6 ACS Paragon Plus Environment

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point of 60-62 ℃ and a carbon number distribution of C20 to C41, as shown in Figure 2. The mineral oil mainly consists of cyclic n-alkanes and it has a density of 0.88 g/cm3 at 25 ℃ and a viscosity of 40.48 mPa·s at 40 ℃. The concentrations of wax and asphaltenes in the mode oil were maintained at 10 wt% and 0.2 wt%, respectively.

Figure 2: Wax carbon number distribution determined by HTGC

The model oils were prepared using the following procedure. The wax was first dissolved in the mineral oil and the mixture was heated at 65 ℃ for 30 min to completely dissolve the wax. Asphaltenes were dispersed in o-xylene by sonicating the mixture at 65 ℃ for 10 min. For the low concentrations of asphaltenes examined in this work (0.2 wt%), a homogeneous asphaltenes/o-xylene mixture can be obtained. These two mixtures were then blended and heated at 80℃ for 2 h with constant stirring. 2.B Asphaltene Characterization The elemental analysis and average molecular weight (MW) of asphaltenes was determined by a Flash EA 1112 Organic Elemental Analyzer and a Waters 1525 Gel Permeation Chromatography, respectively. The asphaltenes were analyzed by 1H-NMR to determine the fractions of different types of hydrogen atoms.12 The results from elemental analysis and 1H7 ACS Paragon Plus Environment

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NMR tests were used to determine the key asphaltene structural parameters including aromatic substitution index δ, number of unit structures per average molecule n, relative length of alkyl side chain L, and aromaticity factor fA via the modified Brown-Ladner method.22 The meanings of each parameter are summarized in Table 1. Table 1: Symbol, unit, and explanation of the key asphaltene structural parameters

Symbol

Unit

Explanation

δ

%

Fraction of substituted peripheral aromatic carbons

n

1

Number of structure units per molecule

L

1

Relative length of alkyl side chain

fA

%

Fraction of aromatic carbons

2.C DSC Measurements Both the WAT and wax precipitation curve of model oils were determined via differential scanning calorimetry (DSC) using a TA Q20 differential scanning calorimeter. The DSC was calibrated using high-purity indium standards. Samples were initially heated to 80 °C and held for 1 min, and then the temperature was decreased to -20 °C at a 5°C/min cooling rate to induce wax precipitation. The test was conducted in a nitrogen environment to prevent the change in oil composition caused by air contacting. The WAT was recorded as the temperature at which the heat flow curve first deviated from the baseline. 2.D Rheological Measurements Gelation temperature. The measurement of gelation temperature was performed by a stresscontrolled HAAKE RS150HIII rheometer equipped with concentric cylinder geometry. The sample was loaded into the rheometer at 50 °C and held for 10 mins. Then it was cooled down at a 0.5 °C/min cooling rate while subjecting to a oscillatory shear stress of 0.02 Pa and 0.5 Hz. The magnitude and frequency of the applied shear stress were pre-determined so that the sample 8 ACS Paragon Plus Environment

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behaves in the linear viscoelasticity region. The gelation temperature was defined as the temperature when the crossover of G’ and G’’ occurs. Yield stress. The yield stress measurement was performed by an Anton PaarRheolab QC rheometer equipped with concentric cylinder geometry. The sample was loaded into the rheometer at 50 °C and held for 10 mins. Then it was cooled to the desired testing temperature at a 0.5 °C/min cooling rate and held for 30 mins. The applied shear stress was then increased at 5 Pa/min and meanwhile the sample’s apparent viscosity was recorded. The yield stress was defined as the stress when a sudden drop in the viscosity was observed. 2.E Microscopy Microscopic examination of the waxy oils was performed using a Nikon OPTIPHOT2-POL polarizing microscope equipped with a Linkam PE60 cooling station and a CCD digital camera. The sample was first heated to 50 ℃ to dissolve all precipitated wax crystals and then cooled down to the desired temperature. The morphology of wax crystals was captured by a CCD digital camera. The aspect ratio and boundary box fractal dimension of wax crystals were obtained by analyzing the microscopic images using Image J software. The particle size distribution of asphaltenes particles were obtained by analyzing the microscopic images using Nano Measure 1.2 software. At each condition, we analyzed 15 images to ensure that the trends obtained are reliable.

3. Results and Discussion This section first provides the results regarding the chemical characterization and the aggregation state of the asphaltenes of different polarity. We then present results of how asphaltene of different polarity affects the thermodynamic properties of waxy oils, that is, the


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WAT and the wax precipitation curve. The section concludes with how the presence of asphaltene of different polarity influences the rheological behavior of waxy oils. 3.A Asphaltene Characterization Table 2: Elemental composition and molecular structural parameters for asphaltene of different polarity

F33/67

F25/75

F20/80

F10/90

C%

83.33

83.22

83.19

83.13

H%

7.61

7.66

7.82

8.10

C/H

0.91

0.90

0.89

0.86

MW

4869

4730

4251

4162

δ

0.50

0.49

0.46

0.48

n

7.77

6.57

5.67

5.99

L

3.42

3.84

4.30

4.16

fA

0.52

0.51

0.50

0.48

Table 2 summarizes the results of asphaltene characterization. The first 4 rows show the results from elemental analysis and molecular weight measurements, respectively, while the last 4 rows summarize the structural parameters determined via modified B-L method. Table 2 shows that the more polar asphaltene generally has a larger C/H ratio, higher molecular weight, higher aromatic substitution index, more structure units, shorter alkyl chain length and higher aromaticty factor. The structure features indicated by these results are that compared to the less polar asphaltene, the more polar asphaltene have bigger and more condensed aromatic cores with shorter and fewer aliphatic chains on the periphery. In other words, the more polar asphaltene is generally more aromatic in nature. These structural differences between asphaltenes of different polarity will lead to differences in the asphaltene’s aggregation behavior and its influences on wax crystallization and gelation, as will be demonstrated in the following sections.


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3.B Aggregation State As shown in previous studies, the aggregation state of asphaltenes may significantly influence their interactions with wax.15,16 Thus it would be necessary to understand the aggregation state of asphaltenes with different polarity under the conditions examined in the current work. Figure 3 compares the size distribution of asphaltenes of different polarity at three o-xylene concentrations. Note that these measurements were conducted at temperatures higher than WAT to avoid the presence of any solid wax crystals. Figure 3(a) shows that in general, the different fractions of asphaltenes exist in crude oil as aggregates with particle size ranging from 1 to 10 µm. The less polar asphaltenes are generally smaller in size and have a narrower size distribution compared to the more polar asphaltenes. These observations suggest that the more polar asphaltenes have higher tendency of self-aggregation,20 which could possibly be attributed to the low hydrogen content and high fa in their molecular structure. As the concentration of o-xylene increases, the difference in the size distribution of asphaltenes with different polarity becomes less pronounced. An aromatic solvent like o-xylene is known to disperse asphaltene by inhibiting aggregation. At 40 wt% o-xylene, the asphaltenes in the solution are well dispersed and the difference in the size distribution of asphaltenes with different polarity almost diminishes, and the asphaltenes with different polarity exist as aggregates with similar size distribution, with the majority of aggregates falling in the size range of 1-3 µm. 0.5 F33/67 F25/75 0.3

F20/80

Polarity decrease

F10/90

0.2 0.1 0.0

0.6

(b) 20wt% O-xylene

Fraction of particles (%)

(a) 10wt% O-xylene 0.4


0.5


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F33/67

0.4

F25/75 0.3

F20/80

Polarity decrease

F10/90

0.2 0.1

1

2 3 4 5 6 7 8 9 Asphaltene particle diameter (µm)

10

F33/67

0.4

F25/75

Polarity F20/80 decrease

0.3

F10/90 0.2 0.1 0.0

0.0 0

(c) 40wt% O-xylene 0.5

0

1

2 3 4 5 6 Asphaltene particle diameter (µm)

7

0

1 2 3 4 5 Asphaltene particle diameter (µm)

6

Figure 3: Comparison of the particle size distribution of asphaltene with different polarity (a) 10 wt% o-xylene; (b) 20 wt% o-xylene; (c) 40 wt% o-xylene


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3.C Wax Appearance Temperature Figure 4 shows the effect of asphaltene polarity on the WAT of oil when the o-xylene content was fixed at 10 wt%. The WAT gradually decreases as the asphaltene polarity decreases. The reduction in WAT is about 3 °C between the oils containing no asphaltene and F10/90 asphaltene, and 1 °C between the oils containing F33/67 asphaltene and F10/90 asphaltene. As mentioned earlier, the effect of asphaltene on WAT strongly depends on the aggregation state of asphaltenes in the solution.15 To further examine this effect, we compared the WAT of oils containing F33/67 asphaltene and F10/90 asphaltene at different o-xylene contents.

Figure 4: Effect of asphaltene polarity on WAT. The o-xylene content was 10 wt%.

Figure 5 shows that at 10% o-xylene, there is a ~2 °C difference in the WAT of the oils containing F33/67 asphaltene and F10/90 asphaltene. When the o-xylene content increased to 40%, the difference becomes almost negligible. These results suggest the aggregation state of asphaltene might be the primary reason responsible for the difference in the WAT of the oils containing asphaltenes of different polarity. Previous investigations suggest that asphaltene aggregates can affect wax crystallization in two competing manners. A smaller asphaltene aggregate can hinder wax crystallization on a molecular level and thus lead to a lower WAT.12


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On the other hand, a larger asphaltene aggregate can serve as the nucleation sites for wax crystallization and result in a higher WAT.13,15 In the present work, the presence of asphaltenes generally causes a lower WAT, indicating the first effect is more dominant. However, as shown in Figure 3, at 10% o-xylene, the oils containing more polar asphaltenes (F33/67 asphaltenes) contain a certain fraction of larger asphaltene aggregates. These large asphaltene aggregates may serve as the nucleation sites for wax crystallization (i.e., the second effect) and thus the WAT is reduced by a less degree. At 40% o-xylene, asphaltenes of different polarity are well dispersed with a similar size distribution and consequently, the difference in WAT diminished.

Figure 5: Effect of asphaltene polarity on WAT at different o-xylene contents

3.D Wax Precipitation Curves Figure 6 compares the wax precipitation curves when asphaltenes of different polarity are present. We only present the portion of the curves in the temperature range of 10-40 °C because this is the temperature range of interest in the present work. Given that all dissolved wax would precipitate out at the lowest temperature (-20 °C) in the DSC measurement, the results in Figure 4 suggest that asphaltenes can generally delay wax precipitation, and the less polar asphaltene produce a stronger delay effect. To understand if these trends are primarily caused by the


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difference in the aggregation state of asphaltenes of different polarity, Figure 7 compares the amount of precipitated wax at 10 °C at different o-xylene contents. As the o-xylene content increases, the difference in the amount of precipitated wax between the oils containing asphaltene of different polarity becomes less pronounced, indicating the asphaltene aggregation state can indeed influence the wax precipitation curve. However, a noticeable difference still exists at 40% o-xylene when asphaltenes of different polarity have similar size distribution, suggesting that the aggregation state is not the only factor affecting wax precipitation and the chemical nature of the asphaltenes of different polarity may also play important roles.

Figure 6: Effect of asphaltene polarity on wax precipitation curve. The o-xylene content was 10 wt%.


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Figure 7: Effect of asphaltene polarity on the wax precipitation at 10 °C at different o-xylene contents.

3.E Rheological Properties The above sections have showed how asphaltene of different polarity affect the thermodynamic properties of waxy oils. In this section we present how asphaltene of different polarity affect the key rheological properties of waxy oils, which are of more significance for industrial practices including efficient pipeline transportation and successful restart operations. 3.E.1. Gelation Temperature Figure 8 compares the oil’s gelation temperature when asphaltene of different polarity is present in the system. Generally, the gelation temperature decreases as the asphaltene polarity decreases. The reduction in gelation temperature is 4 °C for F33/67 asphaltene and as high as 12°C for F10/90 asphaltene. This decreasing trend is in good agreement with the results from Venketesan et al., though the reduction magnitude observed herein (12°C for F10/90) is larger than that reported in their work (5°C for F10/90).10 This larger reduction may be caused by the higher asphaltene concentration (0.2 wt%) used in the present work compared to that (0.1 wt%) used in the work by Venketesan et al.. It may also be caused by the compositional variations of the wax, 15 ACS Paragon Plus Environment

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the asphaltene and the mineral oil used between two studies. Regardless, the results from both investigations suggest that the less polar asphaltene serve as a more effective gelation temperature depressant, probably due to the fact that the less polar asphaltenes are more similar to paraffin on the molecular level.

Figure 8: Effect of asphaltene polarity on gelation temperature. The o-xylene content was 10 wt%.

3.E.2. Storage Modulus Storage modulus G’ is often employed to indicate the strength of waxy gels. 23 Figure 9 shows the variation of G’ in the cooling process of the gelation temperature measurement when asphaltenes of different polarity is present in the system. The G’ remains at a relatively low magnitude when the temperature is above WAT for all cases and starts to increase sharply upon the formation of wax crystals network. To facilitate a more meaningful comparison, in Figure 9 we noted the temperature when G’ reaches 1 Pa for each case. It is clear that when less polar asphaltene is present in the solution, a lower temperature would be required to reach the same gel strength. In other words, the less polar asphaltene can weaken the structure formed by wax crystals. As discussed earlier, this observation is probably attributed to less polar asphaltenes are more easily to interact with wax molecules and introduce deficiencies in the crystal network. 16 ACS Paragon Plus Environment

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Figure 9: Effects of asphaltenes of different polarity on the storage modulus of model oil

3.E.3. Yield Stress In addition to G’, yield stress is a more frequently employed parameter to describe the strength of waxy gels. Figure 10 compares the yield stress of the waxy gels formed at three different temperatures below the gelation temperature when asphaltene of different polarity is present. For all three temperatures examined, the addition of asphaltenes reduces the gel yield stress, which is in agreement with the results from previous investigations.10, 12-15 Figure 10 shows that at a given temperature, the yield stress decreases with decreasing asphaltene polarity and that at a given asphaltene polarity, the yield stress decreases increasing temperatures. For instance, at 10 °C the yield stress is reduced by 62% and 99% upon the addition of F33/67 asphaltene and F10/90 asphaltene, respectively. Notably, the addition of F10/90 asphaltene, the least polar asphaltene, reduces the yield stress at 10°C by almost 99%. To the best of our knowledge, this is the highest reduction in yield stress caused by the addition of asphaltene reported to date. These observations may be of industrial significance as they suggest that the crude oil containing less polar asphaltenes may form softer gel or deposit that are more easily to be broken or removed.


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These results also suggest that blending waxy crude oils with oils containing polar asphaltenes may help to mitigate wax related flow assurance challenges.

Figure 10: Effect of asphaltene polarity on yield stress. The o-xylene content was 10 wt%.

3.E.4. Microscopic Analysis The results of gelation temperature, storage modulus and yield stress all suggest that the waxy oils containing less polar asphaltenes tend to form a weaker gel at lower temperatures. To elucidate the underlying mechanisms for this observation, we performed microscopic analysis of the waxy oils containing asphaltenes of different polarity, as shown in Figure 11. In the absence of asphaltenes, the wax crystals are rod-like whereas when asphaltenes are present, the wax crystals appear to become smaller and transform to more globular in shape. To facilitate a more meaningful comparison, we characterized the aspect ratio and the fractal dimension of wax crystals when asphaltenes of different polarity are present, and these results are summarized in Figure 12. The aspect ratio generally decreases with increasing asphaltene polarity, in agreement with the visual inspection. The fractal dimension remains nearly independent of the asphaltene polarity, suggesting the crystals have similar abilities occupying the space, which is consistent with the results from previous investigations.17,18 18 ACS Paragon Plus Environment

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Figure 11: Microscope images of waxy oils containing 0.2% asphaltenes of different polarity at 25 °C

Figure 12: Effect of asphaltene polarity on the microscopic parameters of wax crystals (a) aspect ratio; (b) fractal dimension

The reduction in the yield stress with decreasing asphaltene polarity is likely caused by a combination of multiple factors. In previous sections, we showed that the precipitated amount of 19 ACS Paragon Plus Environment

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wax at certain temperatures decreases with decreasing asphaltene polarity (Figure 6). Presumably, the gel formed by fewer wax crystals is weaker. Other important factors affecting the gel strength include the morphology of wax crystals composing the gel and the manner these wax crystals interconnect. The results from microscopic analysis show that when less polar asphatlenes are present, the wax crystals the crystals become smaller and have smaller aspect ratio. The network formed by such wax crystals tend to have fewer contacts between crystals and is thus less entangled, both of which may lead to a weaker gel.10 A third factor affecting the gel strength is the aggregation state of asphaltenes. As demonstrated in the studies of and Tisnely el al. and Lei et al., the highly aggregated asphaltenes may serve as the junction points between wax crystals providing “weak points” for the formed gel network.15,16 This mechanism, however, appears to suggest an opposite tendency because the oil containing more polar asphaltenes (more aggregated, as shown in Figure 3) contains more weak points and is more easily to be broken from these “weak points” when it is subjected to external deformation. Thus it is likely that the first two factors are more dominant in determining the gel strength for the particular systems examined in this work.

4. Conclusions Asphaltenes of higher polarity tend to have bigger and more condensed aromatic cores with shorter and fewer aliphatic chains on the periphery compared to less polar asphaltenes. When dispersed in the solution containing 10% o-xylene, the more polar asphaltenes were more highly self-aggregated and displayed a broader particle size distribution. The difference in the particle size distribution between asphaltenes of different polarity diminished when the o-xylene content increased to 40%. The less polar asphaltenes reduced wax appearance temperature and wax precipitation to a larger extent compared to the more polar asphaltenes, which was mainly


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attributed to the difference in the aggregation state of asphaltenes of different polarity. Reducing the polarity of asphaltenes present in the oil also resulted in lower gelation temperature, lower storage modulus and lower yield stress, which was probably because less polar asphaltenes were more similar to paraffin on the molecular level and thus more readily to interact with wax. Interestingly, a 99% reduction in the yield stress at 10 °C was observed with the addition of the least polar asphaltenes examined in the present work, in contrast to the 62% yield stress reduction upon the addition of the most polar asphaltenes. Microscopic analysis showed that the wax crystals precipitated in the presence of less polar asphaltenes have smaller aspect ratio.

5. Acknowledgements The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (No.51534007 and 51134006) and the Science Foundation of China University of Petroleum, Beijing (No. C201602 and No. 2462017YJRC020).

6. References (1) Huang, Z.; Zheng, S.; Fogler, H. S. Wax deposition: experimental characterizations, theoretical modeling, and field practices; CRC Press: Boca Raton, 2015. (2) Ma, C.; Lu, Y.; Chen, C.; Feng, K.; Li, Z.; Wang, X.; Zhang J. Ind. Eng. Chem. Res. 2017, 56, 10920–10928. (3) Lu, Y.; Huang, Z.; Hoffmann, R.; Amundsen, L.; Fogler, H. S.; Sheng, Z. Energy Fuels 2012, 26 (7), 4091–4097. (4) Huang, Z.; Lu, Y.; Hoffmann, R.; Amundsen, L.; Fogler, H. S. Energy Fuels 2011, 25 (11), 5180–5188. (5) Lee, H. S.; Singh, P.; Thomason, W. H.; Fogler, H. S. Energy Fuels 2008, 22 (1), 480−487.


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(6) Speight, J. G. The chemistry and technology of petroleum, 3th ed.; Marcel Dekker: New York, 1999. (7) Sjoblom, J.; Aske, N.; Harald, A. I.; Brandal, O.; Erik Havre, T.; Saether, O.; Westvik, A.; Eng Johnsen, E.; Kallevik, H. Colloid Interface Sci. 2003, 100-102, 399–473. (8) Mullins, O. C. Energy Fuels 2012, 26 (7), 3986–4003. (9) Hoepfner, M. P.; Fogler, H. S. Langmuir 2013, 29 (49), 15423− 15432. (10) Venkatesan, R.; Östlund, J. A.; Chawla, H.; Wattana, P.; Nydén, M.; Fogler, H. S. Energy Fuels 2003, 17 (6), 1630–1640. (11) Xie, K.; Karan, K. Energy Fuels 2005, 19 (4), 1252–1260. (12) Alcazar-Vara, L. A.; Garcia-Martinez, J. A.; Buenrostro-Gonzalez, E. Fuel 2012, 93 (1), 200-212. (13) Kriz, P.; Andersen, S. I. Energy Fuels 2005, 19 (3), 714-745. (14) Oh, K.; Deo, M. Energy Fuels 2009, 23 (3), 1289-1293. (15) Tinsley, J. F.; Jahnke, J. P.; Dettman, H. D.; Prud’home, R. K. Energy Fuels 2009, 23 (4), 2056–2064. (16) Lei, Y.; Han, S.; Zhang, J.; Bao, Y.; Yao, Z.; Xu, Y. Energy Fuels 2014, 28 (4), 23142321. (17) Li, C.; Cai, J.; Yang, F.; Zhang, Y.; Bai, F.; Ma, Y.; Yao, B. J. Pet. Sci. Eng. 2016, 140, 73-84. (18) Yang, F.; Cai, J.; Cheng, L.; Li, C.; Ji, Z.; Yao, B.; Zhao, Y.; Zhang, G. Energy Fuels 2016, 30 (11), 9922-9932. (19) Chen, Y. Cooperative Influence of n-alkanes and asphaltenes on the gelation of waxy Oil; Master dissertation; China University of Petroleum: Beijing, 2007. 22 ACS Paragon Plus Environment

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(20) Nalwaya, V.; Tantayakom, V.; Piumsomboon, P.; Fogler, H. S. Ind. Eng. Chem. Res. 1999, 38(3), 964-972. (21) Wattana, P.; Fogler, H. S.; Yen, A. Energy Fuels 2005, 19 (1), 101-110. (22) Liang, W.; Que, G. H.; Chen, Y. Acta Pet. Sin. 1991, 7, 1-11. (23) Da Silva, J. A. L.; Coutinho, J. A. Rheol. Acta. 2004, 43 (5), 433-441.


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Asphaltene

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Dichloromethane

100:0 Pentane Pentane

Pentane 33:67

5000 rpm

5000 rpm

F33/67 * The

25:75

F25/75

Pentane 20:80

7500 rpm

10:90 10000 rpm

F20/80

F10/90

ratio on each flask represents the volumetric ratio of Dichloromethane and Pentane in the mixture

Figure 1: Asphaltene fractionation procedure ACS Paragon Plus Environment

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Figure 2: Wax carbon number distribution determined by HTGC ACS Paragon Plus Environment

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0.5

(a) 10wt% O-xylene F33/67

0.4

F25/75

0.3

F20/80

Polarity decrease

F10/90

0.2 0.1 0.0

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(b) 20wt% O-xylene


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F33/67

0.4

F25/75 0.3

F20/80

Polarity decrease

F10/90

0.2 0.1

1

2 3 4 5 6 7 8 9 Asphaltene particle diameter (μm)

10

F33/67

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F25/75

Polarity F20/80 decrease

0.3

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0.0 0

(c) 40wt% O-xylene 0.5

0

1

2 3 4 5 6 Asphaltene particle diameter (μm)

7

0

1 2 3 4 5 Asphaltene particle diameter (μm)

Figure 3: Comparison of the particle size distribution of asphaltene with different polarity (a) 10 wt% o-xylene; (b) 20 wt% o-xylene; (c) 40 wt% o-xylene

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Figure 4: Effect of asphaltene polarity on WAT. The o-xylene content was 10 wt%. ACS Paragon Plus Environment

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Figure 5: Effect of asphaltene polarity on WAT at different o-xylene contents

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Figure 6: Effect of asphaltene polarity on wax precipitation curve. The o-xylene content was 10 wt%. ACS Paragon Plus Environment

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Figure 7: Effect of asphaltene polarity on the wax precipitation at 10 °C at different o-xylene contents ACS Paragon Plus Environment

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Figure 8: Effect of asphaltene polarity on gelation temperature. The o-xylene content was 10 wt%. ACS Paragon Plus Environment

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Figure 9: Effects of asphaltenes of different polarity on the storage modulus of model oil ACS Paragon Plus Environment

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Figure 10: Effect of asphaltene polarity on yield stress. The o-xylene content was 10 wt%. ACS Paragon Plus Environment

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Oil containing no asphaltene

Oil containing F33/67 asphaltene




Figure 11: Microscope images of waxy oils containing 0.2% asphaltenes of different polarity at 25 °C ACS Paragon Plus Environment

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Figure 12: Effect of asphaltene polarity on the microscopic parameters of wax crystals (a) aspect ratio; (b) fractal dimension ACS Paragon Plus Environment

Influence of Asphaltene Polarity on Crystallization and Gelation of

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