Influence of Resins on Crystallization and Gelation of Waxy Oils

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Influence of Resins on Crystallization and Gelation of Waxy Oils Jialin Dai, Jinjun Zhang, and Chaohui Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03488 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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Influence of Resins on Crystallization and Gelation of Waxy Oils Jialin Dai, Jinjun Zhang*, Chaohui Chen National Engineering Laboratory for Pipeline Safety, Ministry of Education (MOE) Key Laboratory of Petroleum Engineering, Beijing Key Laboratory of Urban Oil & Gas Distribution Technology, China University of Petroleum (Beijing), Beijing 102249, China

ABSTRACT: In the present work, we employed model oil systems to examine the effects of resins upon the gelation and crystallization of waxy oils. Two types of waxes were explored, namely n-tetracosane (Wax A) and a commercial wax with a melting temperature of 52-54 °C (Wax B). The resins were extracted from a deoiled asphalt from Venezuelan residue by SARA fractionation method. The results from negative-ion ESI FT-ICR MS suggest that resins consist of 1-4 fused benzenes rings or 1-2 fused naphthalene rings constructed by N1, N1O1, N1S1, N1O1S1, N1O2, O1, O1S1 and O2 class species containing stacking aromatic rings. For both types of waxes examined, adding resins to the waxy oils suppresses wax precipitation and modifies the morphology of wax crystals, which collectively lead to lower gelation temperature and lower yield stress. Up to 7 °C of reduction in gelation temperature, 60% of reduction in yield stress,

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and 6 °C of reduction in wax appearance temperature were achieved when the waxy oils contain 7 wt% resins. While for the Wax A model oil, the gelation temperature decreases gradually with increasing resin content, for the Wax B model oil the resin effect is pronounced only at the resin concentration below 0.2 wt%. The reduction in yield stress caused by adding resins is also greater for the Wax A model oil than Wax B model oil. The different resin effects for the Wax A model oil and Wax B model oil might be caused by the compositional variations between Wax A and Wax B.

1. INTRODUCTION Wax contained in crude oils is generally the normal alkanes, which have carbon number ranging from C17 to C55 [1]. When the oil temperature is above WAT (wax appearance temperature), wax is dissolved in waxy oils. When the oil temperature is below the WAT, dissolved wax starts to precipitate out and form wax crystals. This wax precipitation can cause a variety of issues for the safe transportation of waxy oils [2-7]. Asphaltenes are heavy fractions of crude oils, which are insoluble in n-pentane or nheptane but soluble in benzene or methylbenzene

[8].

Asphaltenes are complex

molecules that contain condensed aromatic rings, naphthenic rings, alkyl chains and heteroatoms [9]. It has been shown that the aromatic core of an asphaltene molecule is surrounded by a number of naphthenic rings, and both of aromatic cores and naphthenic


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rings contain alkyl chains [10]. Asphaltene molecules can associate into aggregates, or even clusters if conditions are favorable [11]. Resins are another type of heavy fraction of crude oils, which are soluble in n-pentane or n-heptane. Resins are chemically similar to asphaltenes and contain condensed aromatic rings, naphthenic rings, alkyl chains and heteroatoms as well. N1, N2, N1O1, N1O2, N2O1, N1S1, S1, S2, S3, O1S1, O1S2, O2S1, O1 and O2 class species have been identified by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS)

[12-14],

which consist of fused benzenes

rings or naphthalene rings. The π−π interactions between the resin molecules can drive resin molecules aggregate into particles, whose sizes enlarge when resin concentrations are increased [15]. The interactions between resin molecules result in the crystal structure of resins. Results from XRD showed that three peaks centered at 2θ =20°~60° can be observed for resins [16]. Both asphaltenes and resins have been shown to affect the rheological properties of waxy oil [15, 17-22]. The effects of asphaltenes on gelation temperature, yield stress, WAT, and structure of wax crystals of waxy oil have been studied for many years, but the literature still contains conflicting views [17-20]. Many studies have reported that adding asphaltenes can decrease the gelation temperature, yield stress and WAT of waxy oil [17-19].

Nevertheless, higher asphaltenes concentration can also increase the WAT [19].

Moreover, an unexpected “high peak” of gelation temperature, yield stress and WAT were observed around asphaltene concentrations of 0.01%

[20].

In addition to

concentration, asphaltene aggregation state is also a factor that can affect the gelation


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and crystallization of waxy oil

[19].

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Dispersed asphaltenes could inhibit wax crystal

growth, and aggregated asphaltenes could bridge wax crystals

[21],

which is in good

agreement with the low concentration asphaltenes decreasing WAT and high concentration asphaltenes increasing WAT

[19].

Asphaltenes also could make wax

crystals become globular from rod-like and smaller in shape and lead to the formation of a weaker gel [22]. The effects of resins on viscosity of waxy oils have been studied. The viscosity of waxy oils has been found to increase with the increasing resin content [15, 23-25]. Besides, the resins can affect the performances of the magnetic treatment

[26] [27]

and chemical

treatment [28] [29] of waxy oil. Almost all previous studies focused on the effects of resins on viscosity, whereas the impacts on the gelation and crystallization of waxy oils have received just limited prior attention. Our current work is intended to fill these gaps. We examined the influence of resins upon the gelation and crystallization of waxy oils. Two types of model oil systems were employed. The effects of resins on the gelation temperature and yield stress of model oils were first presented. Then the influences of resins on WAT, precipitated wax concentration, structure of wax crystals were studied to reveal how resins affect the gelation temperature and yield stress of model oils. 2. Materials and methods 2.1. Materials


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The resins were extracted from a deoiled asphalt from Venezuelan residue by SARA fractionation method. The procedure is outlined in Figure 1

[30].

The

asphaltenes of a deoiled asphalt from Venezuelan residue was n-heptane insoluble. The SARA mass fractions are listed in Table 1.

Figure 1. Separation procedure of saturates, aromatics, resins and asphaltenes. Table 1. Quantitative assays of saturates, aromatics, resins and asphaltenes in Venezuelan deoiled asphalt fractions

yields (wt%)

Saturates

1.51

Aromatics

8.01

Resins

30.18

Asphaltenes

56.36

In order to independently control the resin concentration, two types of model oil systems were employed, and the synthetic procedure has been reported [31].


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Both two types of model oil systems were composed of waxes, resins, 97% pure n-decane, and 97% pure o-xylene, which provided solubility for the resins. The concentrations of waxes in the mode oils were maintained at 15 wt %, the concentrations of o-xylene in the mode oils were maintained at 30 wt % and resins concentrations in the model oils were varied from 0 wt % to 7 wt %, respectively. The n-decane and o-xylene were obtained from Aladdin Group Company, Limited. Two types of waxes were explored, namely Wax A (the analytically pure n-tetracosane) obtained from Aladdin Group Company, Limited and Wax B (a commercial wax with a melting temperature of 52-54 °C) obtained from Sinopharm Chemical Reagent Company, Limited. Wax A only has the n-C24 and Wax B has a continuous carbon-number distribution ranging from C14 to C37, which contains 84% n-alkanes and 16% branched- and cycloalkanes as shown in Figure 2.


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Figure 2. Composition of Wax B as determined by high temperature gas chromatography (HTGC). 2.2. Resin fractionation and characterization Resins were firstly fractioned into subfractions of different polarity, namely Resins 1 (R1), Resins 2 (R2) and Resins 3 (R3), and then each subfraction was further fractioned into acidic, alkaline and neutral subfractions. FT-ICR was employed to analyze the resins molecules

[12-14]:

Acidic resins molecules can

be analyzed by Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance-Mass Spectrometry (Negative-Ion ESI FT-ICR), alkaline resins molecules can be analyzed by Positive-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance-Mass Spectrometry (Positive-Ion ESI FT-ICR), and neutral resins molecules can be analysis by both negativeion ESI FT-ICR and positive-ion ESI FT-ICR. 1g resins were firstly fractioned into Resins 1 (R1), Resins 2 (R2) and Resins 3 (R3) using the solvent systems in sequence: (1) n-heptane/benzene (85:15, v/v, 100 mL), (2) n-heptane/benzene (1:1, v/v, 100 mL), (3) benzene/ethanol (1:1, v/v, 40 mL), benzene (40 mL), ethanol(40 mL). The procedure is outlined in Figure 3 [32]. The mass fractions of Resins 1, Resins 2 and Resins 3 are listed in Table 2.


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Figure 3. Separation procedure of Resins1, Resins2, Resins3. Table 2. Quantitative assays of Resins1, Resins2 and Resins3 in parent resins. subfractions

yields (wt%)

Resins1

37.36

Resins2

27.57

Resins3

33.53

Residue amount

1.54

1g Resins X (X was 1, 2 and 3) were fractioned into Acidic Resins X, Neutral Resins X and Alkaline Resins X. Resins X (X was 1, 2 and 3) were fractionated into Acidic Resins X and non - Acidic Resins X though an alkaline alumina (about 40g, 100-200 mesh, with 9 wt% H2O) using the solvent systems in sequence: (1) benzene (100 mL), (2) benzene/ethanol (1:1, v/v, 40 mL), benzene (40 mL), ethanol (40 mL). Non - Acidic Resins X were fractionated into Neutral Resins X and Alkaline Resins X though an acidic alumina (about 40g, 100-200 mesh, with 4 wt% H2O) column using the solvent systems in sequence: (1) cyclohexane/ethyl acetate (4:1, v/v, 100 mL), (2) benzene/ethanol (1:1, v/v, 40 mL), benzene (40 mL), ethanol (40 mL). The procedure is outlined in Figure


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4

[12].

The Acidic Resins X, Neutral Resins X and Alkaline Resins X mass

fractions are listed in Table 3.

Figure 4. Separation procedure of Acidic Resins X, Neutral Resins X and Alkaline Resins X, X is 1, 2 and 3. Table 3. Quantitative assays of Acidic Resins X, Neutral Resins X and Alkaline Resins X in Resins X. subfractions

yields (wt%)

Acidic Resins 1

4.02

Neutral Resins 1

94.11

Alkaline Resins 1

0

Acidic Resins 2

10.15

Neutral Resins 2

87.58

Alkaline Resins 2

0

Acidic Resins 3

38.29


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Neutral Resins 3

58.63

Alkaline Resins 3

0

Acidic Resins X and Neutral Resins X (X was 1, 2 and 3) were recovered, both of which can be analyze by negative-ion ESI FT-ICR. Bruker Apex Ultra FT-ICR MS equipped with a 9.4 T superconducting magnet and ESI in negativeion model

[12]

was employed to analyze the molecular composition of R1, R2

and R3. The sample preparation for negative-ion ESI FT-ICR analysis and the equipment operating conditions and data processing have been reported [12, 3335].

2.3. Differential Scanning Calorimetry Both the WAT and the precipitation curve of the model oils were determined via a TA Q20 differential scanning calorimeter. The sample was placed into sealed aluminum crucible to prevent the evaporation of solvent. A sample was initially heated to 80 °C fast, held at 80 °C for 1 min, and cooled to −20 °C with a rate of 5 °C/min to induce wax precipitation [36] [37]. 2.4. Rheological Measurements. 2.4.1. Gelation Temperature. The gelation temperature of the model oils was determined via a stress-controlled HAAKE RS150HIII rheometer, which was equipped with concentric cylinder


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geometry [31]. A sample was heated to the 50 °C, held at 50 °C for 10 min, and cooled with a rate of 0.5 °C/min while subjecting to oscillatory shear stress of 0.02 Pa and 0.5 Hz at rheometer. The magnitude and frequency of the imposed stress have been pre-determined to ensure that the sample behaves in the linear viscoelasticity region. The gelation temperature was defined as the temperature at which the crossover of G′ and G″ takes place. 2.4.2. Yield Stress. The yield stress of the model oils was determined via an Anton PaarRheolab QC rheometer, which was equipped with concentric vane geometry

[31].

A

sample was heated to the 50 °C, held at 50 °C for 10 min, and cooled to the desired testing temperature with a rate of 0.5 °C/min and held for 30 min at rheometer. The shear stress was increased with a rate of 50 Pa/min, and the shear strain of the sample was recorded simultaneously. The yield stress was defined as the stress at which a sudden increase in the shear strain was observed. 2.5. Microscopy The morphology of resins and wax crystals was determined via a Nikon OPTIPHOT2-POL polarizing microscope, which was equipped with a Linkam PE60 cooling station, and were captured via a CCD digital camera [31]. A sample was heated to the 50 °C, held at 50 °C for 10 min, and cooled to the desired


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testing temperature with a rate of 0.5 °C/min. The particle size distribution of resins particles and the fractal dimension of wax crystals were determined via Image J software. The 12 images were analyzed to ensure the reliability of results at each condition. 2.6. X-ray diffraction The crystal structure of resins and wax crystals were determined via X-ray diffraction at the temperature of 20 °C. The high-speed centrifuge was used to separate wax crystals from the model oils. The separation procedure is as follows [36]: (1) The model oil was heated to the 50 °C, and then placed in a centrifuge tube. (2) The centrifuge tube was loaded into the centrifuge cell and held at 50 °C for 10 min. (3) The model oil was then cooled to 8 °C at a rate of 0.5 °C/min and held at 8 °C for 30 min. (4) At 8 °C, the model oil was centrifuged for 45 min at a speed of 12,000 rpm. (5) The wax crystals were collected and used for the subsequent X-ray diffraction experiments. The wavelength of the X-rays was 0.154 nm. A sample was placed in the sample holder and scanned in the Bragg angle ranging from 1.0 to 35° with a rate of 8°/min. The diffraction intensities were recorded at a step size of 0.02° in 2θ [36]. 3. Results and discussion The results of resins characterization including molecular structure and aggregation state have been firstly reported. Then the effects of resins on the


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gelation temperature and yield stress are presented. The influence of resins on crystallization containing WAT, precipitated wax concentration, morphology and crystal structure of wax crystals are discussed in the end. 3.1. Resins Characterization In this section, the molecular structure of resins is firstly discussed. Then the aggregation state is presented. 3.1.1. Molecular structure The broad-band negative-ion ESI FT-ICR MS spectra of subfractions R1−R3 is displayed in Figure 5. The data shows that the molecular weight of resins ranges from 200 to 600Da. The relative abundance of the dominant class species in resins is displayed on Figure 6. N1, N1O1, N1S1, N1O2, N1O1S1, O1, O1S1, and O2 species are observed in the resins.

6. 吸收峰


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Figure 5. Broad-band negative-ion ESI FT-ICRMS spectrum of the subfractions R1−R3.

Figure 6. Relative abundances of the dominant class species in subfractions R1−R3. The isoabundance maps of DBE as a function of the carbon number for N1, N1O1, and N1S1 class species can be seen in Figure 7 and those for O1, O1S1, O2 and N1O2 class species can be seen in Figure 8. The DBE distribution center for N1O1 and N1S1 species is at 14, and the DBE distribution center for N1 species is at 12 in Resins 1. An enhancement of 2 shows that oxygen could be in furanic and sulfur atoms could be in thiophenic rings [12]. N1, N1O1 and N1S1 species have a higher DBE and higher carbon number distribution center in Resins 2 and Resins 3, which show that molecular structures of Resins 2 and Resins 3 may contain more condensed aromatic rings. The O1 and O1S1 species could be phenolic compounds and sulfur - containing phenolic


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compounds

[12].

The O2 class species could be carboxylic acids, whose DBE

carbon number distribution center is at 2 (no rings), naphthenic acids, whose DBE carbon number distribution center is ranging from 3 to 6 (2−5 naphthenic rings) and aromatic acids whose DBE carbon number distribution center is more than 5 [12]. The N1O2 class species could be nitrogen-containing carboxylic acids, whose DBE carbon number distribution center is ranging from 11 to 14 [12]. [12]

The molecular structures of N1, N1O1, N1S1, O1, O1S1, O2 and N1O2 species

can be seen in the insets of Figure 7 and Figure 8, which consist of 1-4 fused

benzenes rings or 1-2 fused naphthalene rings. Almost all resins molecules are amphoteric compounds containing two basic moieties: a polar moiety and a nonpolar moiety, which is similar to polymeric wax inhibitors and pour point depressants

[22].

Polar moieties could be pyrrole

rings, phenol rings, furanic rings, thiophenic rings, sulfur-containing phenol rings, whose benzenes rings varying from 1 to 4 or naphthalene rings varying from 1 to 2. Nonpolar moieties could be long alkyl chains. Resins may interact with wax crystals via nucleation, adsorption or co-crystallization or solubilization [22].


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Figure 7. Isoabundance maps of DBE as a function of the carbon number for N1, N1O1 and N1S1 class species.


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Figure 8. Isoabundance maps of DBE as a function of the carbon number for O1, O1S1, O2 and N1O2 class species. 3.1.2. Aggregation state The aggregation state of asphaltenes has been reported to significantly affect their interactions with wax

[19] [21].

aggregation state of resins

[15]

Thus, it is necessary to examine the under the conditions examined. The

concentrations of waxes and o-xylene in the mode oils were maintained at 15 wt % and 30 wt %, respectively. The particle size distribution of resins at concentrations of 1 wt%, 3 wt% and 7 wt% is shown in Figure 9. These measurements were conducted at 40 °C which is higher than the WAT to avoid the presence of wax crystals. Figure 9 demonstrates the particle size distribution of resins at different concentrations. When the resin content is below 3 wt%, the resin particles are smaller in size and have a narrower size distribution compared to those observed at 7 wt%. As resin concentration reaches 7 wt%, the fraction of resins ranging from 1 to 2 μm dramatically decreases and those above 10 μm dramatically increases. These results suggest that high concentration resins could promote the self-aggregation of resin particles, which is in good agreement with the findings from previous research

[15],

and this trend may be attributed to the fact that increasing resin


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concentration diminishes the distances between resin molecules thus facilitating resin assembling.

Figure 9. Particle size distribution of resins. The concentrations of waxes and o-xylene in the mode oils were maintained at 15 wt % and 30 wt %, respectively. 3.2. Effect of resins on rheological properties of waxy oils In this section, the effects of resins on gelation temperature and yield stress of waxy oils are discussed, which are crucial for safety of waxy oil transportations and successful restart operations. 3.2.1. Gelation temperature The effect of the resins on the gelation temperature is displayed on Figure 10. For both waxy oil systems, an increase in resin concentration results in a decrease in the gelation temperature. Up to 7 °C of reduction in the gelation temperature is achieved when the waxy oil contains 7 wt% resins. The


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decreasing trend of gelation temperature is in good agreement with the effects of some asphaltenes

[17-19]

and pour point depressants

[38].

The molecular

structures of resins, as one of heavy fractions of crude oil, are similar to asphaltenes, indicating that interaction between resins and wax is similar to asphaltenes. It may also be caused by the fact that the molecular structures of resins are similar to pour point depressants, co-crystallizing with wax. Regardless, the result suggests that the resins can act as a natural gelation temperature depressant. The efficiency of the resins on decreasing the gelation temperature depends on the polydispersity of the wax. For the Wax A model oil, the gelation temperature gradually decreases from 17.1 °C to 12.5 °C with increasing resin content, and for the Wax B model oil, the resins effect is pronounced only at the resin concentration below 0.2 wt%, and the gelation temperature remarkably decreases from 24.5 °C to 18.5 °C at the concentration of 0.2 wt%. The stronger effect of resins at low concentration for Wax B waxy oil may be caused by the compositional variations between Wax A and Wax B. Wax A only has n-C24 and Wax B has n-alkanes, branched- alkanes and cyclo-alkanes ranging from C14 to C37. The performance may be caused by wax polydispersity or may be caused by branched- alkanes and cyclo- alkanes. The presence of the resin may have a percolation threshold for the Wax B model oil, which is similar to the percolation effect of micro-crystalline wax in altering the aspect ratio


[39].

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The resins are active as gelation suppressers at concentrations of 0.2 wt%, while the micro-crystalline wax is active at concentrations above 1 wt%. The resins may have higher interfacial activity than micro-crystalline wax.

Figure 10. Effect of resins on gelation temperature of model oils. 3.2.2. Yield stress The effect of the resins on the yield stress at 8 °C is displayed on Figure 11. For both waxy oil systems, an increase in the resin concentration results in a decrease in the yield stress. Up to 60% of reduction in the yield stress is achieved when the waxy oil contains 7 wt% resins. The decreasing trend in the yield stress is also in good agreement with the effects of some asphaltenes [1719]

and pour point depressants [38]. The aggregation state of resins is similar to

asphaltenes, indicating that interaction between resins and wax crystals may cause waxy oils to form softer gels. It may also be caused by the fact that the


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molecular structures of resins are similar to those of pour point depressants, thus interrupting wax crystal growth [22]. The efficiency of the resins on decreasing the yield stress depends on the polydispersity of the wax. The reduction in yield stress caused by adding resins is greater for the waxy oil containing Wax A than Wax B. For the Wax A model oil, the resin effect is pronounced at the resin concentration of 1 wt%, where the yield stress remarkably decreases from 4200 Pa to 2400 Pa. For the Wax B model oil, the yield stress gradually decreases from 1900 Pa to 600 Pa with increasing resin concentration. Stronger effect of resins at low concentration for Wax A model oil may be caused by the compositional variations between Wax A and Wax B. Wax A only has n-C24, which can crystalize into triclinic crystals [36]

and Wax B has n-alkanes, branched-alkanes and cyclo-alkanes ranging

from C14 to C37, which can crystalize into orthorhombic crystals [36]. The triclinic crystals may be affected by resins easier than orthorhombic crystals. The effects of resin on the behaviors of Wax A model oil is similar to the influences of micro-crystalline wax upon the behaviors of waxy oils containing macrocrystalline wax [39].


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Figure 11. Effect of resins on yield stress of model oils. 3.3. Effect of resins on wax crystallization Wax crystallization causes the formation of waxy gels. In this section, the effects of resins on wax crystallization are first discussed, which include WAT, precipitated wax concentration at both gelation temperature and 8°C, morphology and crystal structure of wax crystals. Then the results are used to analyze the effects of resins on gelation temperature and yield stress. 3.3.1. WAT The effect of the resins on the WAT is displayed on Figure 12. For both waxy oil systems, an increase in resin concentration results in a decrease in WAT. Up to 6 °C of reduction in WAT is achieved when the waxy oil contains 7 wt% resins. The decrease in WAT demonstrate that resins suppress wax precipitation and model oil needs to be decreased ΔT1 further to precipitate wax (ΔT1 is defined as the difference between the WAT of virgin model oil and


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the WAT of resins-contained model oil, ΔT1A for Wax A model oil and ΔT1B for Wax B model oil). Adding resins decreases the thermodynamic phase transition temperature of waxy oils, and increases the solubility of waxes in waxy oils. Contradictory trends are observed regarding the influence of asphaltenes

[40]

and pour point depressants [41] on the wax appearance temperature. Hence the decrease in WAT is in good agreement with the effects of some asphaltenes [18]

and is contrary to with the effects of other asphaltenes

[20].The

decrease in

WAT is in good agreement with the effects of the comb-type pour point depressants

[41]

and is contrary to the effects of co-polymerized ethylene pour

point depressants [41]. The efficiency of resins on decreasing the WAT depends on the polydispersity of the wax. While for the Wax A model oil, the WAT gradually decreases from 18.4 °C to 14.3 °C with increasing resin content, and for the Wax B model oil, the resins effect is pronounced only at the resin concentration below 0.2 wt%, with the WAT remarkably decreasing from 25.4 °C to 21.1 °C when the resin content increases from 0 to 0.2 wt%. At low resin concentrations, the effect of resins on Wax B model oil is higher than Wax A model oil, which could be caused by enhancement activity of the high concentration of cyclo+branched alkanes [22] of Wax B.


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Figure 12. Effect of resins on WAT of model oils, ΔT1A for Wax A model oil and ΔT1B for Wax B model oil. 3.3.2. Precipitated wax concentration at gelation temperature The effect of resins on the precipitated wax concentration at gelation temperature is displayed on Figure 13. For both waxy oil systems, adding resins results in an increase in precipitated wax concentration at gelation temperature, which shows that more precipitated wax is needed to cause the formation of waxy gels for resins-contained waxy oils and shows that adding resins may lead the formation of weak gels. In addition, adding resins results in precipitated wax concentrations having the variation tendency of M type, indicating a variety of interactions between resins and wax at different resin contents. The increase in precipitated wax concentration at gelation temperature shows that resins–contained model oils need to be decreased ΔT2 further to precipitate enough wax concentration to cause the formation of waxy gels (ΔT2


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is the gelation temperature of resins-contained model oil subtracted the temperature when the precipitated wax concentration of resins-contained model oil is equal to the precipitated wax concentration of virgin model oil at gelation temperature, ΔT2A for Wax A model oil and ΔT2B for Wax B model oil). The effect of resins on precipitated wax concentration at gelation temperature depends on the polydispersity of the wax, which may be caused by fact that the compositional variations between Wax A and Wax B. According to percolation theory, resins may reduce the aspect ratio of both Wax A crystals and Wax B crystals, which leads to more precipitated wax at the gelation temperature [42].

Figure 13. Effect of resins on precipitated wax concentration of Wax A model oil (Left) and Wax B model oil (Right) at gelation temperature, ΔT2A for Wax A model oil and ΔT2B for Wax B model oil. Both the suppressed wax precipitation (increasing the solubility of waxes in waxy oil) and modified wax crystals (increasing the precipitated wax concentration at gelation temperature) contribute to the decrease in the gelation


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temperature of waxy oils to some extent. At temperatures below the WAT, adding resins cause waxy oils to be decreased ΔT2 further to precipitate enough wax concentration to format waxy gels. The ΔT2 is displayed on Figure 14. ΔT2A is less than 1 °C, which is less than the decrease in gelation temperature of Wax A model oils. ΔT2B is less than 3 °C, which is less than the decrease in gelation temperature of Wax B model oils. Besides changing wax crystals, there could be others factors leading to the decrease in the gelation temperature of model oils. At temperatures above the WAT, adding resins leads model oils to be decreased ΔT1 further to precipitate wax. The ΔT1 is also displayed on Figure 14. ΔT1A is up to 4 °C and ΔT1B is up to 6 °C, which could be another factor leading to the decrease in the gelation temperature of model oils. In addition, the noticeable decrease in gelation temperature of Wax B model oils at low resin concentrations is in good agreement with the noticeable decrease in WAT caused by cyclo+branched alkanes at the low resin concentration.


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Figure 14. ΔT1 and ΔT2 of Wax A model oil (Left) and Wax B model oil (Right), A is resin concentration of Wax A model oil, B is resin concentration of Wax B model oil. 3.3.3. Precipitated wax concentration at 8 °C The effect of resins on the precipitated wax concentration at 8 °C is displayed on Figure 15. For both waxy oil systems, an increase in resin concentration results in a decrease in precipitated wax concentration at 8 °C, which shows that resins suppress wax precipitation. The effect of resins suppressing wax precipitation is in good agreement with the effect of resins decreasing the wax appearance temperature. The efficiency of the resins on decreasing the precipitated wax concentration at 8 °C depends on the polydispersity of the wax. The decrease in precipitated wax concentration at 8 °C caused by adding resins is higher for Wax A model oils than Wax B model oils.


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Figure 15. Effect of resins on precipitated wax concentration of model oils at 8 °C. 3.3.4. Microscopic analysis of wax crystals The microscopic analysis of wax crystals at 8 °C is displayed on Figure 16 and Figure 17. Wax A crystals are branch–like, which is in good agreement with the effects of pour point depressants on the dotriacontane model oil [43]. In order to confirm these crystals are indeed wax crystals, the system was heated to 40 °C (above WAT) and held for 10 min. The disappearance of crystals at higher temperatures confirmed our speculation. The process can be seen in Figure 18. Bright spots have been observed in Figure 18 (D). Note that there is no wax in this model oil, and hence the bright spots are not wax crystals. Due to the crystal structure of resins

[16],

those bright spots could be the resins particles caused

by the stacking aromatic rings[15] of N1, N1O1, N1S1, O1, O1S1, O2 and N1O2 species. For both waxy oil systems, resins observably changed the structure of wax crystals at 8 °C, especially the Wax A crystals. Moreover, the influence of the resins on structure depends on the polydispersity of the wax. The Wax A crystals are slice-like in the virgin model oil, and branch-like in the resinscontained model oil. Resin particles may provide 1-D restricted geometry to wax crystal, which may cause the 1-D confined crystallization


[44].

Resins may

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also act as natural pour point depressants, whose nonpolar groups may cocrystalize with wax and incorporate into wax crystals. The Wax B crystals are rod- like in the virgin model oil, and smaller, needle-like in the resins-contained model oil. The fractal dimension

[31]

of Wax A crystals and Wax B crystals is

displayed in Figure 19. The increase in resin concentration results in the decrease in fractal dimension.

Figure 16. Microscope images of Wax A model oil at 8 °C, A: 0% resins, B: 1% resins, C: 3% resins, D: 7% resins.


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Figure 17. Microscope images of Wax B model oil at 8 °C, A: 0% resins, B: 1% resins, C: 3% resins, D: 7% resins.


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Figure 18. Microscope images of branch-like crystals is heated, A : 8 °C, B : 18 °C, C : 20 °C, D : 40 °C and held for 10min.

Figure 19. Area box fractal dimension of Wax A crystals and Wax B crystals at 8 °C. 3.3.5 Crystal structure of wax crystals We first performed X-ray diffraction of resins, and the result is shown in Figure 20. One peak is observed at 2θ ≈ 20°, indicating that resin particles contain crystal structure. This observation is also confirmed by previous research [16].


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Figure 20. Powder diffraction patterns (λ = 0.154 nm) of resins, 2θ =5°~30°. The X-ray diffraction of wax crystals at 8 °C is displayed on Figure 21. For both model oil systems, adding resins considerably affect the structure of wax crystals, especially those in Wax A model oil. Moreover, the influence of resins depends on the polydispersity of the wax. The disappearance of (001) series in Wax A crystals and (002) series in Wax B crystals demonstrates that resin particles may provide 2-D restricted geometry to wax crystals, which may cause the 2-D confined crystallization and a randomization of the z-coordinates for both Wax A and Wax B crystals [45]. For Wax A crystals, the change of the peaks at 2θ≈20°~25°may be caused by the co-crystallization between resins and wax [22].

For Wax B crystals, adding resins do not change the peaks at 2θ≈20°~25°,

suggesting that wax crystals may not be modified by resins, which may be caused by the co-crystallization of alkanes molecules of Wax B.


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Figure 21. Powder diffraction patterns (λ = 0.154 nm) of Wax A crystals and Wax B crystals at 8 °C, A : Wax A crystals, 2θ=1°~10°, B: Wax A crystals, 2θ=10°~35°, C: Wax B crystals, 2θ=1°~10°, D : Wax B crystals, 2θ=10°~35°. Resins suppressing wax precipitation lead to the decrease in precipitated wax concentration at 8 °C. Resins changing wax crystals lead to the change of both morphology and crystal structure of wax crystals. For both model oil systems, there is little decrease in precipitated wax concentration at the resin concentration of 1 wt%, but noticeable decrease in yield stress. Hence the resins changing wax crystals could be the major factor leading to the decrease in yield stress at the resin concentration of 1 wt%, which could be morphology or crystal structure or both. There is almost nothing changed in both of morphology and crystal structure at the resin concentration varied from 1 wt% to 7 wt%, and the resins suppressing wax precipitation could be the major factor leading to the decrease in yield stress. 4. Conclusion Resins consist of 1-4 fused benzenes rings or 1-2 fused naphthalene rings constructed by N1, N1O1, N1S1, N1O1S1, N1O2, O1, O1S1 and O2 class species. The interaction of resin molecules results in larger resin particles at higher resin contents and cause resin particles to have crystal structure.


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The effects of resins upon the gelation and crystallization of waxy model oils were studied. Both monodisperse n-tetracosane (Wax A) and polydisperse commercial wax (Wax B) were examined. For both types of waxes, adding resins to the waxy oils suppresses wax precipitation and leads to the formation of weaker gels. Wax appearance temperature has been found to decrease with increasing resin content. More precipitated wax is required to induce the gelation of resins-contained waxy oils. The precipitated wax concentration at the gelation temperature varies with the resin contents in an irregular “M” manner, indicating a variety of interactions between resins and wax at different conditions. Up to 7 °C of reduction in gelation temperature and 6 °C of reduction in wax appearance temperature were achieved when the waxy oils contain 7 wt% resins. The effects of resins on gelation temperature, yield stress, crystallization depend on wax composition. While for the Wax A model oil, the gelation temperature decreases gradually with increasing resin content, for the Wax B model oil the resin effect is pronounced only at the resin concentration below 0.2 wt%. The reduction in yield stress caused by adding resins is also greater for the Wax A model oil than Wax B model oil, especially at low resin concentration. The resins cause the formation of branch-like crystals for the Wax A model oil, and smaller, needle-like crystals for Wax B model oil.


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AUTHOR INFORMATION Corresponding Authors *Telephone: 86-10-8973-4627. E-mail: [email protected]. ACKNOWLEDGMENT The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (51534007). The authors thank Yingda Lu at the China University of Petroleum (Beijing) for his intensive editions. References (1) Huang, Z.Y.; Zheng, S.; Fogler, H. S. Wax Deposition: Experimental Characterizations, Theoretical Modeling, and Field Practices; CRC Press: Boca Raton, FL, 2015. (2) Ma, C. B.; Lu, Y. D.; Chen, C. H.; Feng, K.; Li, Z. X.; Wang, X. Y.; Zhang, J. J. Electrical treatment of waxy crude oil to improve its cold flowability. Industrial & Engineering Chemistry Research 2017, 56, 10920−10928. (3) Lu, Y. D.; Huang, Z. Y.; Hoffmann, R.; Amundsen, L.; Fogler, H. S. Counterintuitive effects of the oil flow rate on wax deposition. Energy Fuels 2012, 26 (7), 4091−4097.


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Influence of Resins on Crystallization and Gelation of Waxy Oils

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