Inhibition of Asphaltene Precipitation by Resins with Various Contents

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Inhibition of Asphaltene Precipitation by Resins with Various Contents of Vanadyl Porphyrins Makhmut Renatovich Yakubov,* Guzalia Rashidovna Abilova, Kirill Olegovich Sinyashin, Dmitry Valerevich Milordov, Elvira Gabidullovna Tazeeva, Svetlana Gabidullinovna Yakubova, Dmitry Nikolaevich Borisov, Pavel Ivanovich Gryaznov, Nikolay Alexandrovich Mironov, and Yulia Yurevna Borisova A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Arbuzov Street 8, Kazan 420088, Russian Federation ABSTRACT: Comparative analysis of the composition and properties of heavy oils from various oil fields has revealed an inverse dependence of the asphaltene/resin ratio upon the vanadium content in resins. This indicates the diminishment of asphaltene precipitation ability with an increase in the content of vanadyl complexes in heavy oil resins. Experiments have shown that the addition of vanadyl porphyrins to resins enhances their inhibition activity toward asphaltene precipitation. For this purpose, vanadyl porphyrins have been concentrated by N,N-dimethylformamide extraction from resins and additionally purified by column chromatography. The obtained vanadyl porphyrin concentrate has been added to resins at the ratio of 1−5 wt %, and its influence on asphaltene stabilization has been further evaluated. Evaluation has been performed by measuring the optical density of deasphalted oil obtained by dilution of the crude oil with 20-fold excess of n-hexane, where resins with various contents of vanadyl porphyrins have been initially dissolved. The change in composition and properties of precipitated asphaltenes has been analyzed, and an increase in their absorbance, aromaticity, and degree of ring fusion with the growth of the vanadyl porphyrin content in resins has been demonstrated. Results have shown the efficiency of vanadyl porphyrins in resins as inhibitors of asphaltene precipitation.

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INTRODUCTION Asphaltenes are the heaviest oil components and possess high molar mass, density, and aromaticity (Ar) and a high content of heteroatomic components and metals. 1 An ability of asphaltenes to aggregate and precipitate in response to environmental conditions leads to the formation of deposits during oil recovery and transportation.2−5 Resins in an oil colloid system act as stabilizers of asphaltene particles and prevent their aggregation.6−8 Different mechanisms of stabilization are suggested: a steric stabilization of resins9 and adsorption of multilayer resins on the surface of asphaltenes.10 Resins improve the stability of asphaltenes and decrease their amount upon precipitation;11,12 also, they can decrease the sizes of asphaltene aggregates.13 The stabilization activity of resins may differ for various crude oils,14 and along with natural amphiphiles in resins, the asphaltene stability may be greatly affected by metalloporphyrins, which are represented in crude oils by vanadyl and nickel complexes. Although there is no clear mechanism of the relationship between metalloporphyrins and asphaltenes, their aggregation ability has shown to be adequate in various references.15,16 Vanadyl and nickel porphyrins, which possess planar aromatic structure with peripheral alkyl substituents, paramagnetism, and size comparable to asphaltene molecules, can, in principle, influence the asphaltene flocculation and, accordingly, their stability. In heavy crude oils with elevated vanadium content, porphyrin metal complexes are mainly represented by vanadyl porphyrins (VPs)17 and their counterparts, whose content in resins may be as high as 1−2 wt %; therefore, the main attention is focused on VPs during the study of their effect on asphaltene precipitation. © 2016 American Chemical Society

To determine the ability of crude oils toward asphaltene precipitation and determine the effect of various compounds on asphaltene stability, conventional methods, such as gravimetry,18 Oliensis spot test,19 and n-heptane titration,20 as well as novel methods based on attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and (NMR) imaging21 are used. Different optical methods are also used for prediction of the onset of asphaltenes. Buckley22 and Wattana et al.23 used refractive index measurements. For these purposes, we employed the method based on the recording of the optical density of deasphalted oil (DAO) during the dilution of crude oil by n-hexane.

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MATERIALS AND METHODS

Recovery of Asphaltenes, Maltenes, and Resins. Asphaltenes were precipitated from petroleum by 20-fold volume excess of nhexane (Figure 1). After 24 h, the obtained precipitate was filtered and washed with boiling n-hexane in a Soxhlet apparatus up to decolorization of flowing solvent to remove as much maltenes as possible. Solvent from maltene solution was removed up to a constant weight using a rotary evaporator. Maltenes were separated into hydrocarbons (HCs) and resins through the adsorption method on silica using n-hexane for HCs and isopropanol/benzene (1:1, v/v) for resins according to the known procedure.24 Asphaltenes were first precipitated by n-hexane, and the hexane-soluble maltenes were separated into HCs and resins. Received: June 20, 2016 Revised: September 30, 2016 Published: October 3, 2016 8997

DOI: 10.1021/acs.energyfuels.6b01503 Energy Fuels 2016, 30, 8997−9002

Article

Energy & Fuels

Figure 1. Separation of nickel and VPs from Ashalchinskoe crude oil resins.

Table 1. Characteristics of Heavy Oils Klaa (cm−1)

content (%)

a

number

oil field, number of well

HC

As

R

A/Rb

oil

DAO

Kla oil/Kla DAO

1 2 3 4 5 6 7 8 9 10 11 12

Zuzeevskoe, 2370 Zuzeevskoe, 956 Zuzeevskoe, 2365 Sbornovskoe, 215 Sbornovskoe, 230 Stepnoozerskoe, 2226 Stepnoozerskoe, 2221 Mayorovskoe, 134 Dachnoe, 3575 Ashalchinskoe, 232 Mordovo-Karmalskoe, 177 Ekatrinovskoe, 6072

72.7 72.6 74.1 64.4 61.3 50.9 53.3 69.7 80.0 72.9 72.0 51.9

6.0 4.9 3.0 2.3 7.1 15.0 18.4 4.4 5.3 5.5 5.9 13.1

21.3 22.5 22.9 33.3 31.6 34.1 28.3 25.9 14.7 21.6 22.1 35

0.28 0.22 0.13 0.08 0.22 0.44 0.65 0.17 0.36 0.25 0.27 0.37

4.4 3.7 2.5 3.1 6.0 10.3 12.0 3.4 4.1 5.2 4.3 10.4

1.68 1.65 1.21 2.37 2.95 2.98 2.88 2.25 1.78 1.98 1.55 3.53

2.64 2.25 2.07 1.32 2.04 3.45 4.17 1.51 2.29 2.62 2.74 2.94

Light absorption coefficient obtained for 0.5 wt % solutions in toluene. bAsphaltene/resin ratio.

Recovery of VP Concentrate from Resins. Extraction of VPs from resins was carried out by N,N-dimethylformamide (DMF, 1:50) by refluxing with a reflux condenser for 20 min. After cooling, the upper layer (extract) was decanted and the solvent was distilled under vacuum. The VP concentrate was recovered from the obtained extract by chromatography. For this purpose, the extract was placed in the column filled with silica and eluted by the solvents with increasing order of polarity: benzene, benzene/chloroform (1:1, v/v), and chloroform. The combined fractions that contained VPs were rotaryevaporated under vacuum. The VP content was determined spectrophotometrically by the absorption band intensity at 575 nm using the following relationship:25

Spectrophotometric Determination of Absorbance and Extinction Coefficient. The optical density (D) of petroleum, DAO, and asphaltenes was determined on a PE-5400 UV photometer at 630 nm. The solutions for measurements were prepared in toluene of “kh.ch.” (chemically pure) brand (for spectroscopy) with the concentration of 0.5 wt % in the case of petroleum and DAO and 0.05 wt % for asphaltenes. All measurements were made in 1 cm thick cells. The light absorption coefficient was calculated according to the following equation:

Kla = D/0.4343cl where Kla is the light absorption coefficient (cm−1), D is the optical density of the object under study, c is the concentration of solution (wt %), and l is the cell thickness (cm). Recording and Processing of Infrared (IR) Spectra. IR spectra were recorded on a JFS-183V IR Fourier spectrometer of Bruker Company in the wavelength range of 4000−400 cm−1. Asphaltene specimens were prepared as KBr pellets. The intensities of intrinsic bands were calculated with baseline correction. Then, the spectral coefficients, Ar, and degree of ring fusion (DRF) were calculated according to the obtained intensity values. Spectral coefficients represent the content of aromatic and condensed structures and could be used to compare the composition of asphaltenes. Ar is defined by the ratio of absorption band intensities of 1600 cm−1 for aromatic CC bonds and 2920 cm−1 for aliphatic C−H bonds. The DRF reflects the fraction of aromatic rings that is included in polycyclic aromatic structures, which is determined by the ratio of

C VP = 0.187hV /(ml) where 0.187 is the scale ratio characterizing the absorption of the medium (mg/mL), h is the height of the maximum absorption of the α band at 575 ± 5 nm (cm) (with the assumption that 0.1D = 1 cm, where D is optical density), m is the weight of the extract (g), V is the adjusted volume of the porphyrin extract (mL), and l is the cell thickness (cm). Determination of the Vanadium Content. The vanadium content was determined by direct flame atomic absorption spectrometry on an AAS-IN spectrophotometer from the calibration curve, using vanadium dibutyldithiocarbamate (II) in the mixture of 80 vol % o-xylene, 10 vol % acetone, and 10 vol % ethanol as a reference solution. 8998

DOI: 10.1021/acs.energyfuels.6b01503 Energy Fuels 2016, 30, 8997−9002

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Energy & Fuels absorption bands at 1600 cm−1 for aromatic CC bonds and 3050 cm−1 for aromatic C−H bonds.

the effect of the amount of toluene in the mixture with hexane on the amount of the formed asphaltene precipitate was evaluated. The amount of toluene in the mixture with hexane varied from 5 to 35% with the step of 5%. After dilution of oils by the toluene/hexane mixture at a 1:20 ratio, the mass of asphaltene precipitate was determined (gravimetric method). Simultaneously, Kla of DAO was determined depending upon the amount of toluene in the mixture with hexane (Figure 4).

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DESIGN OF EXPERIMENTS Evaluation of Heavy Crude Oils by the Ability of Asphaltene Precipitation. The objects of study represented 12 shots of heavy oils from various oil fields in Russian Federation (Table 1). The resins, which act as an asphaltene stabilizer in solution, form solvate shells, which prevent asphaltene precipitation.26 Upon dissolution of solvate shells by light alkanes, asphaltenes precipitate. With an increase in the fraction of resins relative to asphaltenes [asphaltene/resin (A/ R) ratio], the stability of petroleum toward asphaltene precipitation increases.27 The asphaltene content mainly determines the optical density or extinction coefficient (Kla) of petroleum. When colloidal stability of petroleum is disturbed and asphaltenes precipitate, there is a decrease in the light absorption, which enables one to evaluate the amount of asphaltene precipitate according to Kla. The dependence of Kla of the studied heavy oils upon the asphaltene content is given in Figure 2. Resins also affect Kla of oils, although to a

Figure 4. Using Kla for evaluation of the effectiveness of asphaltene precipitation inhibitors.

As exemplified by the oil from the Ashalchinskoe oil field (Figure 5), with the growth of the amount of toluene in the

Figure 2. Dependence of the light absorption coefficient of diluted oils upon the content of asphaltenes in original crude oils.28 Numbers near symbols correspond to numeration of oils in Table 1 (y = 0.6279x + 1.0251; R2 = 0.97).

considerably lower extent, which can be stated by Kla values for DAO, which are nearly 2−3 times lower compared to Kla values for the crude oils under study (Figure 3).

Figure 5. Inhibiting effect of toluene on the precipitation of asphaltenes induced by dilution of the oil from the Ashalchinskoe oil field with 20-fold excess of n-hexane.28

mixture with hexane, Kla of DAO increases. The maximum Kla value of DAO corresponds to that for oil, when the precipitation of asphaltenes is completely inhibited, whereas the minimum Kla for DAO characterizes the complete removal of asphaltenes from oil as in the case of 100% hexane. Therefore, the determination of Kla for DAO fits well in the evaluation of the effectiveness of various substances as the additives in the mixtures with light alkanes for the inhibition of asphaltene precipitation. This method can also be used to determine the minimum content of inhibitor additives in the mixture with light alkanes to slow the precipitation of asphaltenes.

Figure 3. Dependence of the light absorption coefficient of DAO upon the content of resins in original crude oils.28 Numbers near symbols correspond to those in Table 1 (y = 0.0926x − 0.1823; R2 = 0.68).

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DISCUSSION Effect of Resins with Various Contents of VPs on Asphaltene Precipitation. The investigation of heavy oils from various oil fields, where the vanadium content in resins differs by more than 10 times, showed that, with an increase in the vanadium content in resins, the A/R ratio tends to decrease (Figure 6). A series of experiments was performed to evaluate the effect of VPs in resins on asphaltene stability (Table 2). All

To slow asphaltene precipitation, one can use the additives of aromatic VPs, DAO, resins, and various synthetic inhibitors. With the aim to assess the possibility of using Kla for the experimental evaluation of the effectiveness of asphaltene precipitation inhibitors, a series of experiments was carried out. The change of Kla was evaluated along with the amount of asphaltene precipitate (gravimetric method). For all heavy oils, 8999

DOI: 10.1021/acs.energyfuels.6b01503 Energy Fuels 2016, 30, 8997−9002

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Figure 6. Dependence of the A/R ratio upon the vanadium content in resins for studied crude oils. Numbers near symbols correspond to those in Table 1 (y = 0.4727e−5.67x; R2 = 0.65).

Figure 7. Stimulating effect of VP additives to the resins on their inhibition activity toward asphaltene precipitation induced by dilution of the oil from the Ashalchinskoe oil field with 20-fold excess of nhexane.28

studies employed the heavy crude oil of the Ashalchinskoe oil field. When the resin solutions are used as inhibitors of asphaltene precipitation, the optical density of the background is subtracted, which allows for measurement of the absorbance of DAO. Dechaine et al.29 demonstrated the presence of VPs that are tightly bound with asphaltene molecules by covalent or multiple intermolecular interactions and those that are weakly bound to asphaltene molecules by weak intermolecular bonds in oils. The first type of porphyrins has large mass and sizes and is always precipitated with asphaltenes, whereas the second type has a lower mass and preferentially remains in resins. Therefore, the VP concentrate, which was obtained by the DMF extraction from the resins of the oil from the Ashalchinskoe oil field, was used as the additive to resins. To evaluate the effect of VPs on the asphaltene precipitation, the resin specimens with the addition of VP concentrate from 1 to 5 wt % with the step of 1% were compared. The content of VPs in the concentrate was 9.5 wt %. The determination of Kla of DAO for this crude oil showed that the required amount of resins in the mixture with n-hexane is 7.5 wt % for complete inhibition of asphaltene precipitation (Figure 7). However, this amount is by 2.5 wt % less when 5 wt % VP concentrate is added. These experiments allow for the evaluation of the effect of VPs in resins on their activity as the inhibitors of asphaltene precipitation. For this purpose, a series of experiments was carried out followed by the quantity control of the asphaltene precipitate with the dilution of oil by the mixture of n-hexane and 4 wt % resins with the addition of 1−5 wt % VP concentrate. According to the data (Figure 8), there is a proportional decrease for precipitate from 5.1 to 0.9 wt % with the increase in the VP content from 0.084

Figure 8. Yield of asphaltenes precipitated upon dilution of the oil from the Ashalchinskoe oil field with 20-fold excess of n-hexane containing 5 wt % resins with different amounts of VP additives (y = 7.2784e−3.538x; R2 = 0.98).

to 0.560 wt %. The obtained correlation proves that the ability of oils toward asphaltene precipitation decreases with the increase in the content of vanadyl complexes in resins. In this case, the change of the composition of precipitated asphaltenes is also of interest. Therefore, the obtained asphaltenes were compared to each other by Kla, Ar, and DRF. With the decrease in the amount of asphaltene precipitate, there is an increase in Kla, Ar, and DRF (Table 3).

Table 2. List of Experiments experiments

description of experiments

Evaluation of Heavy Crude Oils by the Ability of Asphaltene Precipitation dependence of the light absorption coefficient of diluted oils upon Kla determination of 12 heavy oils the content of asphaltenes in original crude oils dependence of the light absorption coefficient of DAO upon the Kla determination of 12 DAO obtained from heavy oils content of resins in original crude oils inhibiting effect of toluene on precipitation of asphaltenes induced Kla determination of DAO obtained after dilution of the oil from the Ashalchinskoe oil field by dilution of the oil from the Ashalchinskoe oil field with 20-fold with 20-fold excess of n-hexane with toluene; concentration of toluene in hexane of 0, 5, 10, excess of n-hexane 15, and 20 vol % Effect of Resins with Various Contents of VPs on Asphaltene Precipitation recovery of VP concentrate from resins extraction of VP concentrate using DMF from resins obtained from the Ashalchinskoe oil field stimulating effect of VP additives to the resins on their inhibition Kla determination of DAO obtained after dilution of the oil from the Ashalchinskoe oil field activity toward asphaltene precipitation induced by dilution of the with 20-fold excess of n-hexane with VP concentrate; concentration of VP concentrate in oil from the Ashalchinskoe oil field with 20-fold excess of n-hexane resin of 0, 1, 2, 3, 4, and 5 wt % and concentration of resin in hexane of 1, 2, 3, 4, and 5 wt % 9000

DOI: 10.1021/acs.energyfuels.6b01503 Energy Fuels 2016, 30, 8997−9002

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Energy & Fuels Table 3. Yields, Kla Values, and Spectral Coefficients of Asphaltenes Precipitated upon Dilution of the Oil from the Ashalchinskoe Oil Field with 20-Fold Excess of n-Hexane Containing 5 wt % Resins with Different Amounts of VP Additives amount of VP concentrate mixed with resins (wt %)

concentration of VPs in resins (wt %)

yield of asphaltenes (wt %)

0 1 2 3 4 5

0.084 0.180 0.274 0.370 0.465 0.560

5.1 3.8 2.9 2.1 1.5 0.9

Klaa (cm

−1

)

38.8 39.1 41.2 47.8 48.3 48.9

Arb

DRFc

0.20 0.24 0.28 0.32 0.36 0.39

1.03 1.18 1.26 1.40 1.47 1.55

DRF. VPs in crude oil are similar to the structure in the light fraction of asphaltenes according to sizes and molecular mass and act as a dispersant, which reduces the ability of asphaltene nanoaggregates to agglomeration.

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CONCLUSION To evaluate asphaltene precipitation in heavy oils, the Kla ratio of crude oil and DAO was used. In a general experimental procedure of treatment of oil by 20-fold excess of asphaltene precipitant (C5−C7 alkanes), it becomes possible to evaluate aromatic VPs, DAOs, resins, and other substances in the mixture with alkanes as asphaltene precipitation inhibitors. The change of the amount of asphaltene precipitate upon the dilution of oil by the mixture of hexane and resins with various contents of VPs proves their effect on the flocculation and aggregation of asphaltenes. The experiment has shown that the amount of asphaltenes precipitated by the mixture of n-hexane with resins decreases with the increase in the content of VPs in these resins. These results clearly indicate the impact of VPs on inhibiting asphaltene precipitation.

a

Light absorption coefficient obtained for 0.05 wt % solutions in toluene. bAromaticity. cDegree of ring fusion.

The observed changes of the characteristics of asphaltenes are analogous when the composition of the precipitant changes, as exemplified by n-hexane with the addition of toluene (Table 4). For example, in the case of the oil from the Ashalchinskoe

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*E-mail: [email protected].

Table 4. Kla Values and Spectral Coefficients of Asphaltenes Precipitated upon Dilution of the Oil from the Ashalchinskoe Oil Field with 20-Fold Excess of n-Hexane to Which Different Amounts of Toluene Were Added amount of toluene in n-hexane (vol %)

yield of asphaltenes (wt %)

Klaa (cm−1)

Arb

DRFc

0 10 15 20

5.5 3.8 2.8 2.1

37.8 45.5 45.7 48.2

0.16 0.27 0.34 0.45

1.03 1.18 1.33 1.42

AUTHOR INFORMATION

Corresponding Author Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Russian Science Foundation (Project 15-13-00139).

a

Light absorption coefficient obtained for 0.05 wt % solutions in toluene. bAromaticity. cDegree of ring fusion.

oil field, when using the mixture of n-hexane with toluene, there is an increase in Ar and DRF in precipitated asphaltenes with the increase in the amount of toluene. With the variation in the toluene content from 0 to 20% for the mixture with n-hexane, Kla, Ar, and DRF grow about 25−30%. In accordance with the contemporary model of asphaltene aggregation based on molecular thermodynamics,30 polar inhibitors are added to asphaltene nanoaggregates through the active sites and form the environment of a polyaromatic ring. In the case of VPs, which act as solvating agents, the addition to asphaltene aggregates by multiple intermolecular interactions, e.g., through the oxygen atom of vanadyl ion, is possible. Another approach to the addition of VPs to asphaltenes is the axial coordination of vanadium with various bases in asphaltenes.31 Because the least soluble part of asphaltenes with the higher DRF and Ar has a major contribution to the asphaltene flocculation,32 one can suggest the effect of VPs by the increase in the solubility of similar asphaltene structures. A similar scheme was suggested by Leon et al.,33 where after the separation of oil asphaltenes into light and heavy fractions, it was shown that light fractions act as asphaltene precipitation inhibitors. In fact, if we consider the asphaltene aggregate after the addition of resins, nonylphenols, and other solvating agents, the change of the properties of the combined structures should lead to the decrease in Ar and

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NOMENCLATURE VP = vanadyl porphyrin A/R = asphaltene/resin HC = hydrocarbon DAO = deasphalted oil; DAO was obtained after adding 20fold volume excess of n-hepatne with or in the absence of additives to oil; after 24 h, precipitated asphaltenes were filtered and solvent was removed by evaporation Kla = light absorption coefficient Ar = aromaticity, defined by the ratio of absorption band intensities of 1600 cm−1 for aromatic CC bonds and 2920 cm−1 for aliphatic C−H bonds DRF = degree of ring fusion, reflects the fraction of aromatic rings that is included in polycyclic aromatic structures, which is determined by the ratio of absorption bands at 1600 cm−1 for aromatic CC bonds and 3050 cm−1 for aromatic C−H bonds DMF = N,N-dimethylformamide REFERENCES

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