Addition of Nonendogenous Paraffins in Brazilian Crude Oils and their

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Addition of Nonendogenous Paraffins in Brazilian Crude Oils and their Effects in Emulsions Stability and Interfacial Properties José Francisco Romero Yanes, Filipe Feitosa, Frederico Ribeiro do Carmo, and Hosiberto Batista de Sant'Ana Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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Addition of Nonendogenous Paraffins in Brazilian Crude Oils and their Effects in Emulsions Stability and Interfacial Properties

José Francisco Romero Yanes1, Filipe Xavier Feitosa1, Frederico Ribeiro do Carmo2, Hosiberto Batista de Sant’Ana1* 1 Grupo

de Pesquisa em Termofluidodinâmica Aplicada, Chemical Engineering Department, Universidade Federal do Ceará, Fortaleza, CE, Brazil

2

Grupo de Estudos em Termodinâmica Química, Engineering and Technology Department, Universidade Federal Rural do Semi-Árido, Mossoró, RN, Brazil *email: [email protected] Telephone: +55 85 3366-9610

Abstract

Emulsion’s formation is recognized to be a challenging problem during crude oil production and processing. Emulsion stability and interfacial properties were investigated in this work for paraffinic modified Brazilian crude oils. Crude oil modifications were made by the addition of two different nonendogenous paraffins: n-hexadecane and a commercial paraffinic pool (melting point range 53-57 °C) to a heavy crude oil, here named P1, based on asphaltenes destabilization results previously published [Fluid Phase Equilib. 474 116-125; 2018]. Paraffinic modified oils were emulsified with synthetic brine (pH 7 and 60 g·L-1 of NaCl) to identify emulsion’s phases formed when varying water content from 10 to 90 v/v %, and temperature from 30 to 80 °C. From the emulsion’s phase diagrams, it was determined that all paraffinic modified oils allow the inclusion of at least 10% v/v more water as stable emulsified phase, also with a notable decrease in required energy to promote emulsification.

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It means that, less stir velocity was needed in the emulsification process for paraffinic modified crude oils with remarkable effect for paraffinic pool modified sample. Interfacial tension measurements between brine and modified crude oils was evaluated, showing a reduction when compared to the unmodified oil in all the range of temperature tested. These results could be related to the asphaltenes solubility variation in modified crude oils. Additionally, wax crystals formation, detected by wax appearance temperature (WAT) measurements and polarized light optical microscopy, also contribute to emulsion stability for crude oil modified with paraffinic pool.

Keywords: asphaltenes; paraffins; emulsions; interfacial tension; flow assurance.


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1. Introduction Crude oil production and processing are often affected by emulsion formation, normally associated to pumping problems due to the increasing in emulsion viscosity; to corrosion effects by the presence of salt in emulsified brine; and, the need of additional treatments process to remove water 1. Water is commonly associated to petroleum reservoir as brines or during waterflooding process 2. For this reason, its presence could form dispersed emulsified systems such as water in oil emulsions (WOE) due to turbulence, mixing, and agitation that may occurs on downhole wellbore facilities (valves, pumps, and pipes) 3. It is well known that emulsified water is normally stabilized by natural surfactants present in crude oil, such as heavy and polar species like asphaltenes and resins

4,5.

Asphaltenes are

defined as the heaviest, aromatic and polar compounds of the crude oils, normally soluble in aromatic solvents as toluene and insoluble in light paraffins as heptane 6,7. Some asphaltenes fractions act as surfactants stabilizing emulsion when the crude oil is mixed with water after proper agitation. Asphaltenes normally form a thin film around the water droplets avoiding coalescence process 4. In the last decades many efforts have been made in order to better-understand the influence of different crude oil compounds in emulsion stability 8–14. Due to the crude oil compositional complexity, it has been a common practice to use synthetic model solutions to evaluate crude oil fractions influence in emulsion behavior

8,11,13.

Thorough this procedure, it has been

demonstrated that asphaltene composition and aggregation are closely related to crude oil emulsion stability 9,11,12. Moreover, it was reported that the dispersion of emulsifier agents 13


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(asphaltenes, resins and saturates) influence the emulsion stability acting as interfacial stabilizers. Asphaltenes presence in a minimal quantity is fundamental for emulsion formation

13,

and emulsions become more stable when asphaltenes aggregation reach

precipitation onset conditions

11.

One of the constrains of these approaches is the

simplification of the crude oil’s composition by considering the model solutions, avoiding the natural solvency properties of crude oil constituents, and consequently their influence in the emulsification mechanism. Although it has been demonstrated that resins and saturates do not act as emulsion stabilizers by themselves

8,13,15,

these fractions could modify the aggregation of asphaltenes and then

the emulsion behavior. Solubility of asphaltenes has been related to emulsions stability in model systems by varying the paraffinic/aromatic ratio in synthetic crude oils, or by inducing the asphaltenes flocculation in asphaltenes model solutions

8,11,13.

Similar effects in

asphaltenes solvency has been reported for crude oil modifications with paraffins, being a more realistic approach to the asphaltenes behavior in the natural crude oil blend

16.

This

change in asphaltene solubility could affect the resulting asphaltenes availability as emulsion’s stabilizers. Besides modifying asphaltenes solvency in crude oils, paraffinic compounds have been related to WOE stability, especially for high molecular weight paraffins

8,11,17,18.

Paraffinic

compounds in crude oils could affect the emulsion’s behavior by two different mechanisms. Firstly, relying on viscosity changes 11 that might affect the coalescence of emulsion droplets. Secondly, based on paraffinic solid precipitation that could act as emulsion stabilizer at temperatures below the wax appearance temperature (WAT)

17,18.

Even though paraffinic

crystals were recognized as hydrophobic specie, it could present an appropriate size (around


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1 µm) that could be responsible for the emulsion stabilization

18.

Additionally, paraffinic

crystals could equally act as support to adsorb amphiphilic species on their surfaces 19. In this work, nonendogenous paraffins were included on a dead crude oil to evaluate how they affect emulsion’s stability. This approach attempts to approximate to a more realistic crude oil system instead of using an oversimplified asphaltenes model solutions; preserving natural oil fractions and their interactions. Emulsion stability, interfacial tension, and polarized light microscopy data were evaluated for a Brazilian crude oil with the following modifications: the addition of 4, 6, and 10 wt % of n-hexadecane and a commercial paraffinic pool (melting point range 53-57 °C). These modifications were taken from previous results relating the effect of the addition of two nonendogenous paraffins in Brazilian crude oils on the asphaltenes solvency variation 16.

2. Materials and Methods Six different paraffinic modifications of a Brazilian dead crude oil were tested in terms of emulsion stability and interfacial properties. The procedure for crude oil characterization, paraffin incorporation, emulsification and interfacial tests are described below.

2.1 Crude oil samples and characterization


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Brazilian crude oil supplied by Petrobras (Petróleo Brasileiro S.A.) was used in this work, here named P1. Crude oil thermal treatment and its characterization are briefly described as follows. Crude oil thermal history was erased by heating the samples until 60 °C for 4 h, attempting to dissolute any wax crystals and to evaporate light ends 9. API gravity and viscosity were analyzed by using an Anton-Paar SVM 3000 viscodensimeter at 20 and 60 °C. This equipment was equipped with a U-tube principle density cell. Viscosity measurement were performed following the modified Couette principle. A small volume cell (2.5 mL) composed by two rotating tubes, one of them rotating at constant speed around the sample and another one that rotates slower. This apparatus has been calibrated by using a standard oil (CN-6773) supplied by Anton-Paar at a temperature range from 273.15 to 393.15 K. Viscosity analyses were also performed in a range of 0 °C to 80 °C to determine wax appearance temperature (WAT) of the crude oil. The WAT was found as the temperature that intercepts two different linear regions on a semi-logarithm drawing of viscosity against the inverse of the absolute temperature 20,21. Standard uncertainties (u) are reported as: u(T) = 0.01 K, u(P) = 0.1 kPa, u(η) = 0.02·η mPa∙s, and u(ρ) = 0.0015 g∙cm-3, where T is temperature, P is pressure, η is viscosity and ρ is density. Crude oil asphaltene content was measured following a single stage n-heptane addition, as described by Alboudwarej et al. 22. A 3 g sample of pretreated crude oil was mixed with 120 mL of n-heptane and sonicated for 45 min at room temperature. After 24 h, the solution was filtered under vacuum using a Millipore 0.22 µm mesh filter. Heptane soluble or maltenes were used for saturates, aromatics and resins (SAR) analysis. Filtered asphaltenes were Soxhlet-washed with hot n-heptane (~75 °C) until the solvent in the upper section of the extractor became colorless. Washed asphaltenes were recovered in a previously weighted flask via Soxhlet using hot toluene. Asphaltenes mass was determined by weighting the flask


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content after drying at 60 °C under vacuum. It is important to mention that toluene residues were undetected after Soxhlet extraction. Crude oils SAR content was performed using liquid chromatography fractionation (ASTM D2007M) 23. Analytical grade (> 99.8 %) solvents (nheptane, toluene, dichloromethane, and methanol) were supplied by Sigma-Aldrich. No further purification process was performed. Total amount of waxes was also determined by using acetone precipitation technique

24,25

following the modified UOP 46-64 method 26. Water content was determined by Karl-Fisher titration and salt content of the crude oil was determined by the IP 77 method 27,28. For this method, ultrapure Milli-Q water (conductivity of 18.2  0.2 mcm, at 298.15 K) was used to extract crude oil salts. Sodium chloride (NaCl) content was determined by Mohr titration (in mg/crude oil-dm3)

28.

Table 1 summarizes crude oil characterization analyses.

Experimental uncertainties for SARA and WAT were determined by four independent repeat measurements, given: saturates and aromatics content ± 2, wt%; resins content ± 3, wt%; asphaltenes content ± 0.05, wt%; and WAT ± 1, °C.


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Table 1. Characterization and properties of unmodified crude oil P1. Properties ° API

16.5

viscosity (20 °C), mPa·s

2200

viscosity (60 °C), mPa·s

107

saturates content ± 2, wt%

32

aromatics content ± 2, wt%

27

resins content ± 3, wt%

35

asphaltenes content ± 0.05, wt%

6.34

WAT ± 1, °C

24

wax content, wt%

3.4

salt content, mg NaCl/dm3

277

water content, wt%

0.57

2.2 Crude oils modifications with paraffins Previous results showed a decrease in asphaltene’s precipitation onset of 5 wt% of n-heptane, when 10 wt% of paraffin (higher than C16) was included in the oil; also, no variation in asphaltene onset was detected when 4 wt% of paraffin was added


16.

For this reason, two

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different commercial paraffins were used to modify the crude oil in a proportion of 4, 6 and 10 wt%. n-hexadecane (C16, Sigma-Aldrich 99 %, melting point 18 °C) and a commercial paraffin pool ASTM-D87 (melting temperature range of 53 to 57 °C supplied by Sigma Aldrich) were tested. Paraffins carbon number also agree with reported distribution of native paraffins from typical paraffinic crude oils 29,30. All paraffins were used without any further purification. For crude oil modification, 120 g of oil and the calculated mass of each paraffin were heated to 60 °C. Then the paraffin was added slowly to the oil sample and homogenized by using a spatula for at least 30 min, to avoid local concentration of the paraffins

31.

The modified

crude oils were cooled and equilibrated at room temperature for 24 hours. Modified crude oils were named indicating the pure petroleum name P1; the paraffin mass fraction (4, 6 or 10); and, the paraffin addition (C16 or Pool), e.g., P1-10C16. Modified crude oils were characterized in terms of API gravity, viscosity, and WAT by the same experimental procedure described in Section 2.1.

2.3 Emulsions preparation and analysis Unmodified and paraffinic modified crude oils were emulsified with synthetic brine to identify formed phases. Brine consists in a solution of sodium chloride at 60 g·L-1 (Dinâmica Ind. Br., 99.8 % w/w) in deionized water. Brine pH was adjusted to 7 by adding a solution of hydrochloric acid and sodium hydroxide. Oils emulsification was made with a Digital UltraTurrax IKA T25® where a total amount of 15 mL of brine and oil at the required water proportion were mixed for 2 min. Due to viscosity


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variations of modified paraffinic oils, the emulsification stir velocity had to be adjusted for each different sample to obtain an emulsion with a comparable mean droplet diameter (MDD). An MDD of 10 µm was selected for all crude oils systems when emulsified at a proportion of 50 % v/v with brine, the procedure is described as follows. It was prepared 50 % v/v water oil systems and emulsified at different stirring velocities ranging between 2,300 and 10,000 rpm. A sample of the emulsified phase was taken and analyzed by optical microscopy in order to determine the MDD of the emulsion. The stir velocity for the specific oil system was selected when the MDD satisfy a value of 10 ± 0.5 µm, and maintained constant for the water content variation. A schematic procedure for the determination of the stirring velocity for each crude oil tested is described in Figure 1. Specifications of optical treatments for MDD determination are given in the following paragraphs.

Figure 1. Procedure for stirring velocity determination and emulsion phase diagram evaluation for the different crude oils tested.


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All emulsions were prepared in 50 mL Falcon tubes at room temperature and transferred for bottle test flasks, i.e., graduated conic glass flasks. Those bottles were submerged in a thermal bath LAUDA-ALPHA®, with temperature control LAUDA-ET15® at 25 °C. After 1 hour of stabilization, phase identification was performed for the detection of water (W), oil (O), oilwater emulsion (OWE) and water-oil emulsion (WOE) phases. Visual color identification was performed, being blackish for oil phases and brownish for WOE phases, exemplified in Figure 2.

Figure 2. Visual phase identification of emulsion phases formed: water (W), oil (O), oilwater emulsion (OWE), and water-oil emulsion (WOE), with P1 crude oil at different water content, a) total WOE with 30% v/v of water, and b) all emulsions phases (W, O, OWE, and WOE) in 50 %v/v water content system. W phase is identified as clear phase in the bottom of the flask, and O phase normally appear as a black fine layer in the top surface of the system.


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Also, micrographs were taken to corroborate existing phases. A sample was taken from the emulsified phases with a spatula to determine the emulsion MDD using an optical ALLTION® microscopy with a magnification of 10.0 x 4.5 x 1.5 coupled with a digital camera. Images were registered and analyzed using the open-source software ImageJ®, which allows to perform droplet count and area distribution analysis. Diameter di (µm) was calculated from the area of each droplet considering circular droplets. Droplet diameter histograms were done and the counts of each class counti were calculated. Emulsion’s MDD was calculated with the relation of the droplets volume, droplets area, and droplet count known as the Sauter mean diameter expression 32:

𝑛

𝑑32 =

∑𝑖 = 1𝑑3𝑖 𝑐𝑜𝑛𝑡𝑖

(1)

𝑛

∑𝑖 = 1𝑑2𝑖 𝑐𝑜𝑛𝑡𝑖

Systems of 50% v/v crude oil-brine were analyzed in order to adjust the emulsification velocity for each crude oil to obtain a d32 diameter around 10 ± 0.5 µm. Three independent measurements were performed for error calculations. Then, water proportion of the systems was varied from 10 to 90% v/v and emulsified at the selected velocity for each oil. The systems were heated in a thermal bath from 25 °C to 80°C with 20 min equilibration steps, and phase identification was performed following the procedure detailed previously. Phase diagrams were constructed indicating the water proportion and temperature where a new phase was detected.


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2.4 Interfacial analysis crude oil-brine Crude oil and crude oil-brine emulsions were analyzed under polarized light microscopy to identify possible waxy crystals. It is well known that waxy crystals show a notable bright under polarized light 20. An Olympus BX51® with 90° polarized light filters coupled with a digital camera was employed. Samples of unmodified petroleum, paraffinic modified crude oils and emulsions of those crude oils were analyzed by polarized light microscopy at room temperature. Additionally, interfacial tension was measured between crudes oils and brine. It was used a digital Kruss K20® tensiometer, using the Du Noüy ring method. A mass-known platinumiridium alloy ring was located in the interphase of the crude oil-water system and the force required to move out the ring from the interface was measured and related to the interfacial tension of the system. Interfacial tensions measurements were reported at equilibrium, i.e., no interfacial tension varied with time. Six independent measurements were performed to report the standard deviation. The interfacial tension measurements were made for 25, 50, and 70 °C, with an appropriate tensiometer thermostat jacket for accurate temperature-controlled measurements. Pt-100 temperature sensor placed direct on samples indicated temperatures with 0.1 °C uncertainty.

3. Results and Discussion

3.1 Emulsion behavior of modified crude oils


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Table 2 depicts the properties of unmodified and paraffinic modified crude oils and the results of the emulsions preparation conditions, i.e., the obtained velocity for the target emulsion’s MDD, and the resulting MDD for each system. One can note that for all paraffinic modified crude oils, the stirring velocity for emulsification process decreases when compared to the unmodified oil P1. For crude oils modified with n-hexadecane, stir velocity and viscosity decrease by increasing paraffin content were observed. These results could be related to the low viscosity of the lighter paraffin tested, that could be directly related to less required energy to emulsification, decreasing stir velocity.

Table 2. Crude oils viscosity, WAT, emulsification stirring velocity and MDD for 50% v/v crudes oil-brine emulsions. viscosity

stirring velocity,

crude oil

WAT ± 1°C (20° C), mPa.s

MDD + 0,5 μm (rpm)

P1

2,200

24

10,000

9.4

P1-10C16

476

15

6,000

9.5

P1-6C16

790

20

7,000

9.7

P1-4C16

1,020

23

8,000

9.7

P1-10Pool

28,600

34

-

-

P1-6Pool

4,120

33

2,300

9.6

P1-4Pool

1,820

32

6,000

9.5


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Although the paraffin pool was heavier than the n-hexadecane, its influence in the emulsification stir velocity was higher. Viscosity of paraffin pool modified samples increased with the increase in paraffin content, and WAT measurements suggests the presence of waxy crystals at ambient temperature. It can be noted that with 4 wt % of paraffin pool, the stirring velocity decreased from 10,000 rpm for the P1 crude oil to 6,000 rpm. For the crude oil modified with 6 wt % the stirring velocity decreased to the limit of the stirring at 2,300 rpm to achieve the desired MDD. With 10 wt % of paraffin pool addition, it was not possible to prepare an emulsion with MDD around 10 µm. Even at the lowest stirring velocity allowed by the apparatus, the resulting MDD was lower than the target (10 µm). These results suggest that not only the crude oil viscosity affects the emulsion behavior, but also the conditions of the chemical species in the petroleum sample. Emulsification stir velocity for each crude oil is also listed in Table 2. From these values, emulsions were prepared varying the brine/oil ratio from 10 to 90% v/v. Variations of MDD for changes in water proportion were analyzed and presented in Figures 3 and Figure 4. In Figure 3 it can be noticed that lower MDD was obtained for P1 emulsified system with lower water content. Although this variation, differences in the MDD were the same for the different modified crude oils, as can be seen in Figure 4, allowing the comparison of the different systems. Micrographs were also compared in order to better illustrate these observations.


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10 v/v % 20 v/v % 30 v/v % 40 v/v % 50 v/v % 60 v/v % 70 v/v % 80 v/v % 90 v/v %

0.024 0.022 0.020

fv, volumetric frequency

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

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0.018 0.016 0.014 0.012

100µm

P1 50% v/v water

0.010 100µm

0.008 0.006

P1 70% v/v water

0.004 0.002 0.000 1

10

droplet diameter, m

100µm

Figure 3. Adjusted log-normal droplet size distribution for volumetric frequency of P1 emulsions at different water/oil ratio at 25 °C, and comparative micrographs for P1 30, 50 and 70% v/v water emulsion systems.


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17 P1-4C16 60% v/v water

14 13

mean droplet diameter (MDD), m

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

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11 P1 60% v/v water

10 9 8

P1 P1 10C16 P1 6C16 P1 4C16 P1 6Pool P1 4Pool

7 6 5 4 3

100µm

P1-4Pool 60% v/v water

2 0

10

20

30

40

50

60

70

80

90

water proportion, v/v %

100 100µm

Figure 4. Emulsion’s MDD for each crude oil as function of the water proportion at 25 °C, and comparative micrographs for emulsions of P1, P1-4C16 and P1-4Pool at 60% v/v water proportion.

All identified phases of emulsified systems with P1 crude oil are presented in Figure 5, as function of water proportion and temperature. For systems with 10 and 20% v/v water it was obtained only a WOE phase, and with the increase of temperature a free water W phase appears. Between 30 and 90% v/v of water and temperatures lower than 70 °C, all phases were observed. In this range, temperature increase contributes to the disappearance of the OWE phase.


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80

70

Temperature, °C

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

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O W WOE

WOE W

60

50

O W WOE OWE

WOE

40

30

0

10

20

30

40

50

60

70

80

90

water proportion, v/v %

Figure 5. Phase diagrams for emulsified systems using P1 crude oil, as function of the water proportion and temperature.

Phase diagram for P1 crude oil samples modified with n-hexadecane at different proportion is presented in Figure 6. When compared with unmodified P1 behavior, it can be noticed that for all modified crude oils the total WOE region increases from 20 to 30% v/v of water. This implies that the modification of crude oils with hexadecane allows the inclusion of 10 v/v% more water as emulsion. It is important to point out that this phenomenon was achieved even with less energy (by the reduced stir velocity), despite the viscosity decrease that can promote water droplets coalescence. A reduction in OWE region was noted as the paraffin content increase, probably associated with the reduction of the stirring velocity.


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19 a ) Emulsion diagram for P1-10C16 systems 80

Temperature, C

70

O W WOE

60

WOE 50

40

O W WOE OWE

30

0

10

20

30

40

50

60

70

80

90

water proportion, % v/v

b) Emulsion diagram for P1-6C16 systems 80

O W WOE

Temperature, °C

70

60

WOE 50

O W WOE OWE

40

30

0

10

20

30

40

50

60

70

80

90


c) Emulsion diagram for P1-4C16 systems 80

O W WOE

70

Temperature, C

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

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WOE 50

O W WOE OWE

40

30

0

10

20

30

40

50

60

70

80

90


Figure 6. Phase diagrams for emulsified systems using modified P1 with n-hexadecane at a) 10 wt %, b) 6 wt %, c) 4 wt %, as function of water proportion and temperature.


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Similar results were observed for the emulsified systems with paraffin pool, as shown on Figure 7. P1-4Pool systems allowed the increase in total WOE region from 20 to 30% v/v. When paraffin pool addition was increased to 6 wt%, the total WOE region increased to 40% v/v of water; i.e., when the paraffin content increased more water was included as a stable emulsion. The OWE region was totally reduced by the notably decrease in stir velocity. a)

Emulsion diagram for P1-6Pool systems 80

Temperature, °C

70

O W WOE

60

WOE

50

40

30

WOE/OWE 0

10

20

30

40

50

60

70

80

90


b) Emulsion diagram for P1-4Pool systems 80

70

Temperature, °C

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

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O W WOE

60

WOE 50

40

30

WOE/OWE 0

10

20

30

40

50

60

70

80

90


Figure 7. Phase diagrams for emulsified systems using modified P1 with paraffin pool at a) 6 wt %, b) 4 wt %, as function of water proportion and temperature.


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It is important to remark that all the modified crude oils systems were emulsified with a lower stirring velocity, and also it was achieved an increase in the total WOE region. It has been reported that paraffins has not interfacial contribution due to its low polarity they can affect resins and asphaltenes solubility

11,15.

8,13,

however

Also, for the paraffin pool modified

crude oils, the presence of waxy crystals was expected by the low WAT detected and that could contribute to the emulsion formation and water stabilization. Decrease of stirring velocity could also be associated to the presence of waxy crystals on paraffin pool modified samples. It has been reported that microcrystals of paraffins could act as emulsion stabilizers 17,18

and the results of polarized light microscopy will be analyzed in the following sections.

3.2 Interfacial properties for modified paraffinic oils In order to study the possible factors that influences the increase of emulsion’s stability in paraffinic modified crude oils, interfacial analysis measurements were performed. Interfacial tension measurements between different crude oils samples and the brine are presented in Figure 8. As expected, it was observed a reduction on the interfacial tension with the temperature increase for all the samples. These results could be associated to a thermodynamic condition that favor the migration of active species to the interface 33.


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13,5

P1 P1 10C16 P1 6C16 P1 4C16 P1 6Pool P1 4Pool

13,0 12,5

interfacial tension, mN/m

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12,0 11,5 11,0 10,5 10,0 9,5 9,0 20

30

40

50

60

70

temperature, °C

Figure 8. Interfacial tension crude oil-brine for P1 and paraffinic modified crude oils as function of temperature.

When compared the paraffinic modified crude oils systems with the base P1 crude oil, it was noticed that the interfacial tension was reduced in all cases when the paraffin was included in the crude oil. Interfacial tension reduction for paraffinic modified crude oils systems can be related to the enhanced water stabilization under emulsification. With a lower interfacial tension, the emulsified system is less unstable and has a reduced potential to separate 33. Diminution on interfacial tension could be related to the availability of surface-active species. Our recent paper 16 demonstrates how inclusion of n-hexadecane and paraffin pool affects the asphaltene precipitation behavior in the tested petroleum P1, both paraffins reduced the precipitation onset at the same extension when included in crude oil. These changes in


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asphaltenes aggregation and solubility of paraffinic modified crude oils could favor the migration of asphaltenes to the interface as surface active species, reducing then the interfacial tension. Interfacial tension reduction for paraffinic modified oils could also be associated to the decrease in stirring energy resulted when the crude oils were emulsified. In order to explain the notable diminution in emulsification stirring velocity and the emulsion’s behavior of modified crude oils with paraffin pool, the crude oils and the emulsified system were analyzed by polarized light microscopy. Figure 9 present the polarized light micrographs of base P1 crude oil and modified crude oils with n-hexadecane and paraffin pool. It can be noted that only for crude oils samples of P1 modified with paraffin pool, waxy crystals were detected, and identified with a notable bright. It is also worth to note that this observation coincides with the WAT determination by viscosimetry analysis.


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24 P1

100µm

P1-4%Pool

P1-10%Pool

P1-6%Pool

100µm

P1-4%C16

P1-10%C16

P1-6%C16

100µm

100µm

100µm

100µm

100µm

Figure 9. Polarized light micrographs for P1 and modified crude oils with paraffins at ambient temperature.

Wax crystals were also detected for emulsified systems with 6 and 4 wt % of paraffin pool, as one can note in Figure 10. Paraffin crystals were detected on the water-oil interface and, in some cases, between independent droplets avoiding coalescence. Paraffin crystals appears as solids barriers that prevent water droplets to approach and coalesce. This stabilization mechanism by waxy solids has been reported before 17, and it could explain the lower stirring energy required to emulsify the modified crude oils with paraffin pool.


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P1-4%Pool 50 % v/v water

100µm

P1-6%Pool 50 % v/v water

100µm

Figure 10. Polarized light micrograph for 50% v/v water emulsified system with a) P1-4Pool, and b) P1-6Pool, at ambient temperature and pressure.

Changes in asphaltenes aggregation and solubilization can be related to the emulsion’s behavior of paraffinic modified oils. Similar results were obtained for emulsified model solution of heptol (n-heptane + toluene), asphaltenes and water; when asphaltenes were near the flocculation onset, the most stable emulsions were obtained 11. For the systems without any waxy solid crystals, such as modified crude oils with nhexadecane, variations in solvency of asphaltenes by the paraffin addition can be related to the observed emulsion’s behavior. It permits the stable inclusion of more 10% v/v emulsified


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26

water with only 4 wt% of n-hexadecane in the crude oil; and also shows a diminution on interfacial tension when compared with the unmodified P1 crude oil. Solid stabilization mechanism could also be identified for systems with paraffinic pool modified crude oils. Waxy solids act as emulsion stabilizers, which can be related to the considerable reduction of energy required for emulsification when only included 4 wt % of paraffin pool in the crude oil, obtaining lower stirring velocities with the increase of the paraffin content.

4. Conclusions Modified crude oils with nonendogenous paraffins were analyzed in order to evaluate the effects on emulsion behavior and stability. It was demonstrated that the aggregation of the chemical species in the crude oil is a fundamental factor in emulsion stability. For the systems tested in this work, all paraffinic crude oils allowed the increase of 10% v/v of water in the WOE region. Also, notable less energy or stirring velocity was required to achieve the same average droplet diameter than the emulsion with the unmodified crude oil. These phenomena coincide with the reduction in interfacial tension for modified oils when compared to the unmodified crude oil. Affectation on asphaltenes solubility in paraffinic modified crude oils can be related to the enhanced emulsification stability observed, as well as the reduction in the interfacial tension. Asphaltenes solubility data shows that precipitation onset is reduced in the same extend when same inclusions of the paraffins (n-hexadecane or paraffin pool) were tested. Also, for paraffinic pool modified crude oils it was detected contributions of waxy solids in the resulted


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emulsions. Waxy crystals detected by WAT measurements and polarized light microscopy were also observed in the water-oil interface for crude oil modified with the paraffin pool.

5. Acknowledgments The authors thanks to the Organization of American States OAS and Brazilian Council of Scientific and Technological Development CNPq for financial supports and Petrobras for crude oils samples supplying.

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Kilpatrick, P. K. Water-in-Crude Oil Emulsion Stabilization: Review and Unanswered Questions. Energy Fuels 2012, 26 (7), 4017–4026.

(2)

Aguilera, B. M.; Delgado, J. G.; Cárdenas, A. L. Water-in-Oil Emulsions Stabilized by Asphaltenes Obtained from Venezuelan Crude Oils. J. Dispers. Sci. Technol. 2010, 31 (3), 359–363.

(3)

Wong, S. F.; Lim, J. S.; Dol, S. S. Crude Oil Emulsion: A Review on Formation, Classification and Stability of Water-in-Oil Emulsions. J. Pet. Sci. Eng. 2015, 135, 498–504.

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Tchoukov, P.; Yang, F.; Xu, Z.; Dabros, T.; Czarnecki, J.; Sjöblom, J. Role of Asphaltenes in Stabilizing Thin Liquid Emulsion Films. Langmuir 2014, 30 (11), 3024–3033.


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Speight, J. G. Petroleum Asphaltenes - Part 1: Asphaltenes, Resins and the Structure of Petroleum. Oil Gas Sci. Technol. 2004, 59 (5), 467–477.

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Chilingarian, G. V.; Yen, T. F. Asphaltenes and Asphalts, 1; Elsevier, 1994.

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Zhang, J.; Tian, D.; Lin, M.; Yang, Z.; Dong, Z. Effect of Resins, Waxes and Asphaltenes on Water-Oil Interfacial Properties and Emulsion Stability. Colloids Surf. Physicochem. Eng. Asp. 2016, 507, 1–6.

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Addition of Nonendogenous Paraffins in Brazilian Crude Oils and their

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