Changes of Asphaltenes' Structural Phase Characteristics in the

Jan 6, 2016 - Process of Conversion of Heavy Oil in the Hydrothermal Catalytic ... It has been established that in the water vapor media the process o...
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Changes of Asphaltenes’ Structural Phase Characteristics in the Process of Conversion of Heavy Oil in the Hydrothermal Catalytic System G. P. Kayukova,†,‡ A. T. Gubaidullin,† S. M. Petrov,‡ G. V. Romanov,†,‡ N. N. Petrukhina,§ and A. V. Vakhin*,‡ Energy Fuels 2016.30:773-783. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/05/18. For personal use only.

†

A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Kazan 420029, Russia ‡ Kazan (Volga Region) Federal University, Kazan 420000, Russia § Gubkin Russian State University of Oil and Gas, Moscow 117485, Russia ABSTRACT: The composition of heavy crude oil from the Ashal’cha field (Volga-Ural Basin, Republic of Tatarstan) and the peculiarities of the changes of its asphaltenes’ structural phase characteristics in the model hydrothermal−catalytic system have been studied very thoroughly. It has been established that in the water vapor media the process of destruction of the heavy crude oil high-molecular-weight components with the new light fraction formation in the presence of a natural catalyst, namely, hematite, containing iron oxide and at the temperatures of 210, 250, and 300 °C respectively, takes place which has an effect on the changes in its component hydrocarbon, fractional, and structural group composition as well as in the structural parameters of its asphaltenes. As the experiments temperature increases and the water content in the reaction system decreases, the general tendency of growth the asphaltene associates aromaticity factor revealed that in turn it is accompanied by the extension (increase) of the distance between the aromatic layers and polymethylene chain fragments under the reduction of the size of associates and the number of their aromatic layers; this results from the destructive processes course, taking place along/on the most stable asphaltene heteroatomic bonds with/accompanied by further peripheral alkyl fragments breaking off, which has been confirmed by the molecular mass of the fragments above as well as destruction of vanadyl−porphyrin complexes and increase of free radicals concentration. A significant ability of asphaltene associates to immobilize their maltenes has been revealed. Within the process of oxidation cracking at the temperature of 300 °C and under the low water content in the reaction system, the increase in aromaticity and in the degree of association asphaltenes transform to carben−carboids and then to coke and are precipitated out of the oil in the solid form.

1. INTRODUCTION The development of modern oil-producing and -refining industries in the nearest future will be inseparably linked to the development of the unconventional hydrocarbon feedstock resources that include mainly above all highly viscous oils and natural bitumens. The Republic of Tatarstan occupies one of the leading positions among the oil-producing regions of Russia, according to the stocks of heavy crude oils and natural bitumens the share of which significantly exceeds the traditional stocks of light and low-viscosity crude oils.1,2 However, although producing a processing heavy hydrocarbon raw materials, it is inevitable to face some difficulties mainly connected with the high content of high-molecular resinous−asphaltenic compounds in them, the major part of heteroatoms present in the initial stock being concentrated in the compounds above. For example, asphaltenes are capable of clogging and plugging up wells, pipelines, above-ground equipment, and geologic beds’ pores as well as influencing the stability of oil−water emulsions and beds’ wettability,4 which creates problems in the course of oil fields’ development. Moreover, the higher concentration of metals and heteroelements in heavy hydrocarbonic raw materials impacts significantly the technological procedures of their processing,4−6 owing to the coke formation and catalysts © 2016 American Chemical Society

poisoning. All these problems are caused by some special properties of asphaltenes4,7−11 that occur in the oil-disperse systems in the form of supramolecular structures representing associate molecules formation whose sizes vary according to the oil composition and nature as well as to the influence of external factors such as temperature and pressure. At a high degree of association, asphaltenes coagulate and precipitate. At a low degree of association, they are present, as a rule, in a suspended state. Asphaltenes can also be stable without sedimentation in high-viscosity bitumens at exclusively high concentration. In any case, their behavior is ambiguous and unpredictable and defines the main oil-disperse systems properties, such as density, viscosity, and stability in the processes of oil production, transportation, and processing. Therefore, it is important to have information about the changes in the phase-dispersed structure of asphaltenes and their properties in various technological processes. At the moment, the technologies aimed at the production and processing of heavy crude oil stock resulting in the output of so-called “synthetic oil” with lowered viscosity3,12−18 are Received: June 16, 2015 Revised: December 6, 2015 Published: January 6, 2016 773

DOI: 10.1021/acs.energyfuels.5b01328 Energy Fuels 2016, 30, 773−783

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Table 1. General Characteristics and Fractional Analysis of the Ashal’cha Crude Oil and Its Thermal−Catalytic Transformations Productsa fractional analysis (%) asphaltenes experiment no.

density at 200 °C (g/cm3)

HC

BP

ABP

2 3

fraction A

fraction B

Ashal’cha Field Crude (Initial) 57.30 23.60 13.30 36.90 5.80 Experiment 1 Product (T =210 °C, P = 18 MPa, Water/Crude 1:1, Catalyst 2%) 0.9271 77.14 11.48 6.81 18.28 4.57 Experiment 2 Product (T =250 °C, P = 18 MPa, Water/Crude 1:5, Catalyst 2%) 0.9253 78.50 8.23 2.93 11.16 1.13 6,33 Experiment 3 Product (T =300 °C, P = 18 MPa, Water/Crude 1:10, Catalyst 2%) 0.8961 59.61 12.00 1.56 13.56 1.92 0,03 0.9725

1

∑ resins

∑

coke content

5.80 4.57 7.46

2.88

1.95

24.88

a

HC, hydrocarbons; BP, benzene resins; ABP, alcohol−benzene resins; fraction A, fraction of asphaltenes soluble in toluene; fraction B, fraction of asphaltenes insoluble in toluene. temperatures of 210 and 250 °C were followed by a positive thermal effect as evidenced by a short-term temperature increase in comparison with the planned one. Asphaltenes were precipitated from the initial crude oil and the products of experiments by means of the standard technique with the 40-fold surplus of the petroleum ether with the boiling temperature of 40−70 °C produced in Russia in accordance with STPTU COMP 1− 070−08. In the products of the thermo−catalytic experiments, the precipitation of the newly formed oil conversion products, namely, carbens−carboids insoluble in organic solvents, was observed alongside with that of asphaltenes. Deasphaltenizates (malthenes) were analyzed by the method of column liquid adsorptive chromatography on the ASK silica gel with further separation of hydrocarbonic part and two groups of resins, benzene and alcohol−benzene, obtained with the help of alcohol− benzene mix (1:1). The element composition of heavy crude oil conversion products as well as structural composition and characteristics of asphaltenes were investigated with the application of a combination of physical and chemical methods, such as the X-ray powder diffraction, matrix assisted laser desorption/ionization (MALDI), EPR spectroscopy, and elemental and X-ray fluorescent analyses. The element composition of asphaltenes was determined by their burning in the semiautomatic C, H, and N analyzer and with the help of X-ray fluorescent analysis (RFA). The radiographic phase analysis (X-ray diffraction analysis) of asphaltenes powders was carried out at the Department of X-ray Diffraction Studies of the Center of Collaborative Research on the basis of the Laboratory of Diffraction Research Methods at the A. E. Arbuzov Institute of Organic and Physical Chemistry of Kazan Research Center, the Russian Academy of Sciences. Powder diffraction patterns were obtained on the automatic X-ray diffractometer Bruker D8 Advance equipped with the Vario peripheral equipment and Vantec linear coordinate detector. Cu Kα radiation monochromizated by the Johanson’s germanic curved monochromator (λCu Kα1 = 1.5406 Å) with an operating mode of the X-ray tube is 40 kV and 40 mA was used. The experiments were carried out at 23 °C in the Bragg−Brentano geometry with a flat sample. The samples were preliminarily crushed and previously applied on the silicon plate, reducing the background dispersion. The diffraction patterns were registered in the range of dispersion angles of 2θ = 2−70°, the step signal to 0.0081°, and the intensity acquisition time at the point was varied from 0.1 to 0.5 s. There were obtained several diffraction patterns at various experimental modes and with various times of data acquisition for each of the samples. The processing of the data obtained and the structural parameters calculations were executed by means EVA29 and TOPAS V.330 software package. The PDF-2 database of the Joint Committee for Powder Diffraction Standards (JCPDS) was used to identify some crystal phases. MALDI was employed to determine the molecular mass of asphaltenes. The research was carried out by means of a mass

being widely employed. The processes taking place in the water vapors medium and in the presence of rather available black iron oxide catalysts, providing the transformation of heavy crude oil residues into the easily boiling hydrocarbons,19−24 are of great interest. Thus, ferriferous mineral hematite, in particular, is used as a heterogeneous catalyst of heavy crude oil stock conversion20−27 because it accelerates the reactions of hydrogenation of oil stock and the products of its thermal destruction. It is the authors opinion in ref 20 that as a result of the oxidation−reduction reactions in the process of oil residues water pyrolysis with further formation of distillate fractions hematite is converted to magnetite followed by the release of hydrogen which is involved in reactions of hydrogenation and hydrocracking. The attractiveness of this method is connected with low requirements to the quality of oil stock and the possibility to use an inexpensive catalyst in the processes of heavy crude oil conversion. Reference 28 also deals with the fact that the parameters of conditions of the initiated cracking of the Ashal’cha field bitumen in the presence of microspheres, the basis of whose chemical composition is made of iron oxides, results in the formation of liquid products with lower content of resins and higher content of petrol and diesel fractions. In this regard, the purpose of the present work is to investigate the influence of the framework of a natural catalyst, hematite, on the transformations of heavy crude oil composition within the framework of hydrothermal−catalytic processes at various temperatures and with different water content in the reaction system as well as on the changes of structural phase characteristics of crude oil asphaltenes.

2. EXPERIMENTAL SECTION The objects of research are heavy crude oil from the Ashal’cha field (Volga-Ural basin, Republic of Tatarstan) and its conversion products obtained in water vapor in the presence of a ferriferous natural catalyst, hematite. Hematite was used that contains in accordance with its element composition 41.73% iron and 26.54% silicon with admixtures of other elements such as aluminum 12.43%, phosphorus 14.25%, nitrogen 0.67%, and sulfur 2.38%. The experiments on conversion of heavy crude oil were carried out within 2 h in a 1 L autoclave at temperatures of 210, 250, and 300 °C with the ratio of water to oil of 1:1, 1:5, and 1:10, respectively, for the temperatures above and at the initial air pressure of 2 MPa, which was elevated up to 18 MPas during the experiment. At the experiments temperatures of 250 and 300 °C and with the ratio of water to oil of 1:5 and 1:10, the formation of coke with its further precipitation on the reactor walls was observed. It should be noted that the hydrothermal−catalytic processes at 774

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Energy & Fuels Table 2. Element Composition of the Initial Crude Oil Asphaltenes and the Asphaltenes Conversion Products element composition (wt %)

a

target of research

C

H

fraction Aa

80.8

7.8

fraction A

76.2

6.6

fraction Aa fraction Bb coke

78.5 49.1 72.8

6.2 2.5 4.1

fraction Aa fraction Bb coke

80.0 82.6 42.0

5.5 4.1 1.4

N

S

2.9 5.8 Experiment 1 Product 5.7 4.1 Experiment 2 Product 4.4 3.6 5.5 4.6 5.1 2.9 Experiment 3 Product 4.4 3.4 2.2 3.2 3.3 3.2

elements by the method of XFA (rel %) H/C

S

Si

Fe

V

Initial Crude Oil Asphaltenes 1.16 91.4 0.2 0.4 5.3 (T =210 °C, P = 18 MPa, Water/Crude 1:1, Catalyst 1.05 95.4 0.4 0.6 2.7 (T =250 °C, P = 18 MPa, Water/Crude 1:5, Catalyst 0.95 96.6 0.3 2.7 0 0.61 44.7 8.3 36.2 0.8 0.70 72.9 0.2 14.2 0.2 (T = 300 °C, P 18 MPa, Water/Crude 1:10, Catalyst 0.82 97.7 0 1.15 0 0.60 72.4 0.2 13.0 0.4 0.40 56.6 8.1 19.7 1.1

Ni

Ca

Cu

Cr

Zn

1.4 2%) 0.5 2%) 0 5.2 0.5 2%) 0.1 0.7 1.8

0

0

0

0

0.6

0.2

0

0

0 0.5 0.9

0.5 1.4 8.9

0 2.5 1.7

0.2 0 0

0.6 3.4 1.3

0.3 7.4 5.4

0 0.9 4.4

0.1 1.4 0

Fraction A, asphaltenes, soluble in toluene. bFraction B, asphaltenes, insoluble in toluene.

spectrometer Ultraflex III TOF/TOF Bruker. 2,5-Dihydroxybenzoic acid was used as the matrix. The EPR spectra of asphaltenes were taken using the EPR spectrometer SE/X-2544 (Radio PAN, Poland). Two types of signals were registered: a single symmetric signal of R* free stable radicals (g = 2.003) and the multicomponent superthin structure (STS) corresponding to V4+ ions, a part of the vanadyl−porfiryn complexes structure.31 The obtained values of intensity of the corresponding curves in the EPR spectra were normalized in accordance with the sample weight, getting the R* and V4+ content in relative units.

constituent part of cokes and carbon black as well as other high-carbonic compounds and constitute the product of hydrocarbons condensation. There is an intensive formation of coke (24.88%) at essential decrease of the content of asphaltenes and resins at higher temperature (300 °C). There is the new growth of light fractions, but their yield is smaller in comparison with that of experiments 1 and 2. Decrease of quantity of asphaltenes at further deepening of the process of coking is apparently connected with the fact that on the one hand asphaltenes transform into carben−carboids and on the other hand the quantity of products of initial consolidation and resins decreases. 3.2. Composition and Characteristics of Asphaltenes and Other Products of Heavy Crude Oil Conversion. The destructive and hydrogenation processes occurring in the hydrothermal−catalytic system are reflected in the changes in the structure and properties of asphaltenes products (Table 2). 3.2.1. Element Composition. In comparison with the asphaltenes of the initial crude oil, in fraction A asphaltenes there is a decrease in the value of the aromaticity indicator N/ Sat, which gives evidence of the increase in the degree of their structure carbonization under the elevation of experiments temperature and the decrease in the water content in the reaction system. The similar tendency of the aromaticity indicator value to decrease remains unchangeable in the following succession: from asphaltenes (fraction A) to carben−carboids (fraction B) and furthermore to coke. A very low content of carbon in insoluble experiment 2 asphaltenes (fraction B) and experiment 3 coke (49.1 and 42.0%) are of great interest, apparently, because of the particle of rock and catalyst occluded. In the experimental asphaltenes (fraction A), the sulfur content decreases (5.8 → 4.1 → 3.6 → 3.4%), which confirms their destruction along the least stable C−S bonds,32 whereas the nitrogen content increases. It is well-known that nitrogencontaining compounds are the most stable ones under water high-temperature thermolysis.33,34 Therefore, the atoms of nitrogen remain a part of asphaltenes molecules, and owing to the destruction of the alkyl chains, the percentage of nitrogen increases. The similar tendency is noted in the thermal transformations of high-sulfur asphaltite.35 In accordance with the element analysis data, significant changes in the hematite composition take place as well. The content of iron in hematite decreases from 41.73 to 16.48%,

3. RESULTS AND DISCUSSION 3.1. Component Composition of the Heavy Crude Oil Conversion Products. The research undertaken has shown that there are some significant changes in the fractional analysis of heavy crude oil due to the influence of hydrothermal− catalytic factors (Table 1). In all experiments, irrespective of the temperature and amount of water injected into reaction system, crude oil is cracked with the increase in the light and intermediate fractions share at decrease in benzene and alcohol−benzene resins content. At the temperature of 210 °C and with the ratio of water and crude oil of 1:1, the content of benzene and alcohol−benzene resins decreases almost threefold. The decrease in the asphaltenes content at this temperature is not so significant. The content of asphaltenes increases from 5.80 to 7.46% with the temperature increase up to 250 °C and the decrease in the water content in the reaction system. There are two fractions in the asphaltenes structure: a usual one, soluble in aromatic solvents of asphaltenes (fraction A), and the second one, consisting of the substances (fraction B) such as carben− carboids insoluble in toluene. The formation of the coke-like products (2.88%) precipitated on the reactor walls from the liquid phase begins under these conditions. It is important to note that carbens−carboids are highmolecular carbonic compounds17,36,51 that are practically absent in the initial oil because they are obtained as a result of thermal or thermocatalytic and thermooxidizing conversion of oil in accordance with the following pattern: hydrocarbons → resins → asphaltenes → carbens−carboids. The processes of thermal and catalytic conversions of high-molecular oil components are accompanied by the bonds’ rupture with the loss of hydrogen and further development of the conjugation system and with the more thermostable structure formation. Carbens are soluble only in carbon sulfide, whereas carboids are soluble in no solvents/are not soluble at all. Carboids are the main 775

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by the X-ray diffraction method of refs 37−39. The diffractograms of the products under study are presented in Figure 1.

and aluminum and phosphorus decrease from 12.43 to 7.96% and from 14.25 to 9.13%, respectively, after carrying out the experiment at the temperature of 210 °C. At the same time, the content of carbon increases from 0.77 to 2.37%, nitrogen increases from 0.67 to 2.58%, and sulfur increases from 2.38 to 2.57%. The hydrogen content is 1.87%. Everything above confirms that the process of adsorption and coke formation on the hematite surface with the participation of nitrogen- and sulfur-containing fragments does take place. The significant part of the hematite content after the experiment belongs to silicon (58.91 versus 26.54%). 3.2.2. X-ray Fluorescence Analysis. It has been established by the X-ray fluorescence analysis (XFA) method36 that sulfur prevails in the microelement composition of asphaltenes, without taking in to account the basic elements incorporated in their organic part: carbon, hydrogen, and nitrogen. The sulfur content in the initial crude oil asphaltenes is 91.4 rel %, whereas its content increases up to 97.7 rel % in the asphaltenes, which are the experiment products (fraction A). The lowest sulfur content is in the insoluble experiment 2 fraction (fraction B) (44.7 rel %) and in experiment 3 coke (56.6 rel %). The sulfur content in the fractions investigated is in the inverse proportion to the iron content. Thus, there is relatively high content of iron (36.2%) in the insoluble fraction of the experiment 2 asphaltenes and the experiment 3 coke (19.7%). Alongside with the high content of iron, the increased concentrations of vanadium and nickel as well as silicon, nickel, and chrome can be observed in these fractions. There are maximum concentrations of calcium and copper in the composition of the asphaltenes insoluble fraction. The fact that such metals as calcium, copper, chrome, and zinc have not been revealed in the initial asphaltenes is most likely to be connected with their low concentration. In the fraction of insoluble asphaltenes and coke, they are concentrated together with the microcells that are the parts of the catalyst, namely, natural hematite. According to the XFA, hematite contains 42.4% iron, 53.4% silicon, 3.6% aluminum, 0.22% sulfur, 0.20% potassium, 0.15% calcium, and 0.03% copper. In comparison with the initial crude oil asphaltenes, the content of vanadium and nickel in the asphaltenes (fraction A) of the experiment 1 product obtained at the temperature of 210 °C decreases almost twofold, whereas at higher temperatures (250 and 300 °C), the content of these microelements in this fraction drops practically to zero values. This testifies that the destruction of the metal−porphyrin complexes which are parts of asphaltenes takes place. A rather high concentration of vanadium is still retained in carben−carboids (fraction B) and experiment 1 coke (fraction C). There is a rather high concentration of nickel in similar fractions of products of the experiments at temperatures of 250 and 300 °C, especially in experiment 2 coke. The presence of these microelements in the neogenic fractions of carben−carboids and coke indicates that there are not only processes of the organometallic complexes destruction but also processes of metal redistribution with their the subsequent concentration in the structure of the insoluble fractions during the processes of asphaltenes structural phase transformations. 3.2.3. Method of the Powder X-ray Difraction. The structural characteristics of the initial crude oil asphaltenes and various fractions of the asphaltenes, carben−carboids, and coke that were obtained in the course of thermal−catalytic conversion of heavy Ashal’cha crude oil have been investigated

Figure 1. Diffractogram of the Ashal’cha crude oil asphaltenes before its hydrothermal−catalytic transformations: 1, initial asphaltenes; 2, experiment 1 asphaltenes (210 °C); 3, experiment 2 asphaltenes (250 °C); and 4, experiment 3 asphaltenes (300 °C).

According to the data obtained, the diffraction curve of the initial Ashal’cha crude oil asphaltenes (fraction A, curve 1) is typical for a strongly disordered condensed phase of asphaltenes as noted in ref 5 within the framework of the research of heavy crude oil asphaltenes and natural bitumens of Tatarstan. It is characterized by the average intensity reflection with a wide maximum in the area of 2θ = 15−27° and the shoulder, which is quite accurately expressed from the bigger angles in the field of angles 2θ = 25−27°. According to the standard model of asphaltenes structure7−11,40,41 and the analysis of their structure by X-ray diffraction methods,37−39 it is believed that these two most well-observed peaks correspond to the more- or less-ordered aliphatic and aromatic asphaltenes components. The qualitative analysis of the diffraction pictures presented in the figure specifies that the contents and nature of the components distribution are significantly differ in these samples, which depends on the influence of hydrothermal−catalytic factors on crude oil. Although both components are present in the initial asphaltenes (curve 1), the relative increase of the intensity of the reflex “aliphatic” part of is characteristic for the diffractogram of experiment 1 asphaltenes (curve 2). It is perhaps connected with the sedimentation on their surface of the crude oil neogenic aliphatic components, which did not undergo any serious destructive processes even at the temperature of 210 °C. It is known that high-molecular alkanes4,5,7−10,41 are present in the crude oil and crude-containing species and also in the structure of the of asaphltenes units participating in the formation of their supramolecular structures. In accordance with the temperature growth up to 250 and 300 °C in experiments 2 and 3, respectively, the nature of change in the asphaltenes difractograms proves that there is a shift of the main reflection maximum to the aromatic area (Figure 1, curves 3 and 4). The rather high intensity of the curve aromatic shoulder practically in the lack of reflection, which is characteristic for aliphatic structures, gives evidence of the higher degree of carbonization of the asphaltenes obtained at the higher experimental temperatures. 776

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there are multiple interferential peaks proving the presence of additional crystal phases in their composition. With the use of the PDF-2 database, these phases are identified as iron sulfide (FeS) and silicon dioxide (SiO2). The existence of interferential peaks is also marked on a diffractogram of coke in experiment 3 (Figure 2b, curve 3), which points to the presence of the impurities of rock-forming minerals and catalyst that correlate with the data of these fractions’ element analysis as well as previously published data. Thus, it has been established that the associated hydrocarbonic fraction contains not only paraffin crystals but also an organic−mineral fraction insoluble in organic solvents which in turn contains the crystal structures characteristic for minerals occuring within the structure of asphaltenes obtained from the heavy crudes oil and natural bitumens of Tatarstan. It has been assumed as well that the minerals can serve as the germinal phase while also forming the supramolecular structures of the asphaltenes associates in the disperse oil systems. On the basis of this, the role of mineral impurities in the formation and phase stability of the asphaltenes supramolecular structures in the thermal−catalytic processes is quite obvious. The calculation of structural parameters of associates of the asphaltenes and firm high-carbonaceous products of their conversion (Table 3) was carried out on the basis of X-ray

The analysis of the diffactograms of neogenic solid products in experiments 2 and 3 showed (Figure 2) that the intensity of

Table 3. Values of Structural Parameters of the Studied Ashal’cha Crude Oil Conversion Products aromaticity and crystallinity parametersb Lc (Å) target of researcha

fa

dm (Å)

dγ (Å)

I

M II

I

II

Ashal’cha Field Crude (Initial) 0.34 3.5 5.9 18.1 22.0 6.2 7.3 Experiment 1 Product (T = 210 °C, P = 18 MPa, Water/Crude 1:1, Catalyst 2%) fraction A 0.25 3.5 5.7 17.3 20.5 6.0 6.8 Experiment 2 Product (T = 250 °C, P = 18 MPa, Water/Crude 1:5, Catalyst 2%) fraction A 0.56 3.7 6.6 11.2 13.0 4.0 4.5 fraction B 1.00 3.5 26.8 32.2 8.6 10.1 coke 0.51 3.5 5.7 15.7 18.8 5.5 6.4 Experiment 3 Product (T = 300 °C, P = 18 MPa, Water/Crude 1:10, Catalyst 2%) fraction A 0.49 3.7 6.3 11.4 13.8 4.1 4.7 fraction B 0.98 3.7 6.8 5.3 8.7 2.5 3.4 coke 1.00 3.5 24.3 27.6 7.9 8.9

Figure 2. Diffractograms of firm products of hydrothermal−catalyst transformations of Ashal’cha crude: (a) experiment 2 (250 °C); (b) experiment 3 (300 °C). 1, fraction A; 2, fraction B; and 3, fraction C. Green and blue vertical lines correspond to the Bragg’s positions of reflexes for ferrum sulfide (FeS) and silicon dioxide (SiO2). The diffractograms are shifted down from each other for descriptive reasons.

fraction A

the aromatic shoulder on the diffactograms of insoluble asphaltenes (fraction B) and coke (fraction C) is much higher than that of usual asphaltenes in the similar area (fraction A). This fact confirms the high-carbonaceous structures form as a result of separation of the peripheral alkyl substituents from asphaltenes. Hence, they lose their ability to be dissolved in organic solvents5 as happens to usual asphaltenes. It is also important to note the structural differences in the products under study. Thus, the greatest intensity of the aromatic shoulder is characteristic for the diffractograms of the carben−carboids in experiment 2 and the coke in experiment 3. Taking into consideration the high yields of these products and the similarities in the nature of their aromatic parts displayed in the diffractograms, it is possible to assume that having achieved a certain concentration the “insoluble” carben−carboids are precipitated from the dispersion system as coke-like products. This conclusion is quite in line with the commonly known sequence of the hydrogen depletion and the increase in aromaticity along the line of hydrocarbons → resins → asphaltenes of the straight-run residues → asphaltenes of cracking residues → carbens → carboids.41 It should be noted that in the diffractogram of the fraction B insoluble carben−carboids of experiment 2 (Figure 2a, curve 2)

a

Fraction A−asphaltenes, soluble in toluene; fraction B−asphaltenes, not soluble in toluene. bfa, aromaticity; dm, interplanar distance between aromatic layers; dγ, distance between methylene links in saturated structures; Lc, the average height of a pack from aromatic layers; and M, number of aromatic layers in a pack (cluster). I, the sizes determined by a calculation method; II, the sizes calculated by means of the TOPAS program.

diffraction includingthe following: fa, aromaticity factor; dm, interplanar distance between aromatic layers; dγ, distance between methylene links in saturated structures; Lc, the average height of a pack from aromatic layers; and M, number of aromatic layers in a pack. Taking into account the X-ray diffraction data, the calculation was carried out by analogy with the methodological approaches developed in refs 37−39. The structure of asphaltenes (Figure 3) presented in the form of the condensed aromatic layers with 777

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(Table 2) because of some value decrease in fa for the experiment 1 asphaltenes. The increase in aromaticity is a general tendency characteristic for any thermal processes of the oil stocks conversion. Alongside with the increase in temperature of the process, the content of the A1 continent-type highcondensed asphaltenes also goes up, whereas the content of the A2 archipelago-type asphaltenes decreases. The features of the first asphaltenes changing toward the increase in aromaticity and the H/C ratio.43−46 The calculations of structural parameters have also shown that the distance between the aromatic layers (dm) in the experiments 2 and 3 asphaltenes changes from 3.3 to 3.7 Å, and the distance between their methylene links (dγ) in the saturated fragments changes from 5.9 to 6.6 and 6.3 Å (increasing a little). Thus, the values of the parameters characterizing the average height of the multizone formations in the aromatic layers (Lc) and the number of aromatic layers in the pack (M) considerably decrease. These changes are especially characteristic for the experiments 2 and 3 asphaltenes of as a result of the destructive processes. So, the average height of the aromatic layers pack decreases from 18.1 to 11.4 Å, and the number of aromatic layers in the pack decreases from 6.2 to 4.1. The changes in structural parameters of experiment 1 asphaltenes are not so essential. Because asphaltenes are a class of species defined according to their solubility in toluene and insolubility in N-alkanes, the asphaltenes aggregates should have a certain size, and the molecules should have a certain number of aromatic rings46,47 in order to be soluble in toluene. Otherwise, these components will not be soluble in toluene and will be

Figure 3. Cross section of the asphaltenes nanounit37,39

naphthenic and alkyl substituents on the periphery,37,40 in agreement with the standard representations, was used as model. The full profiling analysis42 of the diffraction curves for the products under study, accomplished by means of the TOPAS30 program, made it possible to allocate the interferential peaks used for calculating the dimensional characteristics of the associates and by varying their parameters to minimize the divergence between the precalculated and experimental diffractograms (Table 3 and Figure 4). The results of calculations point to the fact that the general tendency of the values increase in asphaltenes aromaticity (fa; Table 3) is parallel with the increase of experiments’ temperature, even though this tendency is not as unambiguous as the increase in degree of the asphaltenes aromaticity after the experiments according to the data of the elemental analysis

Figure 4. Experimental and precalculated profiles of diffraction used for an assessment of structural parameters of the initial crude oil asphaltenes (top and middle panels, respectively) and the profile of diffraction of the product of its conversion (fraction A; bottom panel). I, experimental curve; II, γ peak; III, graphene peak; IV background line; V, a differential curve between experimental and precalculated data (red continuous line). 1, initial asphaltenes; 2, experiment 1 asphaltenes (210 °C); 3, experiment 2 asphaltenes (250 °C); and 4, experiment 3 asphaltenes (300 °C). 778

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Figure 5. MALDI mass spectra of the experiment products asphaltenes: (a) experiment 1 (210 °C), (b) experiment 2 (250 °C), and (c) experiment 3 (300 °C).

preferable immobilization of high-molecular hydrocarbonic components in its structure. The increase in values of the factor fa for the insoluble carben−carboids and coke points to the destruction of the asphaltenes peripheral alkyl substituents, which leads to phase conversion into the class of insoluble high-carbon substances. This alone is in a good agreement with the results received with the help of other physical and chemical methods. This is confirmed by the changes in their molecular masses (Figure 5) received by the MALDI method, EPR analysis (Table 5) data,31 as well as by the changes in the asphaltenes structural-group composition determined by the IR spectroscopy.51 3.2.4. Matrix-Assisted Laser Desorbtion/Ionization. Apart from the first experiment (210 °C), an essential decrease in the asphaltenes average molecular mass (m/z) from 1710 to 1200 and 950 with the corresponding appropriate increase in the experiments temperature up to 250 and 300 °C, respectively, (Figure 5) has been observed. According to the EPR analysis data (Table 4), the concentration of free radicals (R*) increases in the asphaltenes of the experiments’ products, whereas the concentration of

considered carbenes and carboids. Therefore, after destruction the alkyl substituents with asphaltenes are the only molecules and units whose sizes are not too big to provide solubility in toluene. Carben−carboids of the insoluble fraction (fraction B) of the experiment 2 products as well as the coke from the experiment 3 products are, on the contrary, characterized by the large size of their chain-packed formations. The number of aromatic layers in the pack is also bigger, which is why the fraction B is not soluble in aromatic solvents. Thus, the influence of the conditions of the hydrothermal−catalytic experiments on the change of the asphaltenes structural parameters leads to the alterations in the stability of their associates in the oil disperse systems. A considerable decrease in the content of the insoluble carben−carboids in experiment 3 products in comparison with that of the experiment 2 products (Table 1) allows us to associate this fact with adsorption of carben−carboids in the coke pores, together with the hydrocarbons and resins forming the adsorptive−solvate layers. The smaller values of the coke aromaticity factor (fa) in experiment 2 (Table 3) prove the 779

DOI: 10.1021/acs.energyfuels.5b01328 Energy Fuels 2016, 30, 773−783

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different degree. Moreover, it has been shown that the molecules’ resins penetrate into the microstructure of the asphaltene associates. The authors of ref 10 have put forward the idea that some amount of the dispersion medium, called intermicellar liquid, remains in the interpartial space of asphaltenes, and this circumstance produces a considerable impact on the technological processes connected with oil stock. In ref 50, the composition of aliphatic, polycyclic aromatic hydrocarbons and heteroatomic compounds of the malthenes that are a part of asphaltenes was studied. Associated-withasphaltenes hydrocarbonic fraction has, in contrast to maltenes, a higher concentration of alcohol−benzene resins and crystals of paraffin in the interlayer space of the heavy crude oil asphaltenes and a solid natural bitumen, namely, asphaltite. To study the changes in the composition of the immobilized substances, the asphaltenes were subjected to extractions by heptane within 7−10 days. Extracting the content of benzene and resins makes 88.75% initial asphaltenes (Table 5). As a part of the extract of asphaltenes after thermo-catalytic influence the content of resins decreases twice, the content of hydrocarbons increases. If most of the heptane extract composition of initial asphaltenes is accounted for by benzene and alcohol−benzene resins, the content of which is (88.75%), then after the impact of thermal and catalytic factors on crude oil the content of alcohol−benzene resins in extracts decreases almost twice. The content of hydrocarbons increases as well as in products of experiments. The highest content of hydrocarbons is characteristic of heptane extract from asphaltenes of experiment 2. The content of resins is low in it, especially of alcohol−benzene ones. With an increase in temperature and decrease in the content of a water phase in reaction system, the content of hydrocarbons in heptane extract decreases (experiment 3) as processes of destruction and dehydrogenation proceed more intensively as is shown above. This leads to significant changes of the disperse phase structure of asphaltenes with the formation of fraction of insoluble substances such as carben−carboids, which are dropping out of the system as the coke-like residue. It is possible to conclude from the Tables 3 and 4 that the most aromatic asphaltene units immobilize more malthenes, hydrocarbons in particular. This fact can be evident of the selective immobilization of molecules of high-molecular paraffin in units of slightly soluble high-aromatic asphaltenes noted in ref 47. It is shown that paraffin can get into the “windows” of units of asphaltenes (A1). If units already contain resins, then the phenomenon of a selective immobilization of paraffin is observed to a lesser extent. The total content of resins in the

Table 4. Data of EPR of the Initial Crude Oil Asphaltenes and Products of Its Thermal−Catalytic Transformations target (asphaltenes)

I(R*) for weighted amount

I(V4+) for weighted amount

R*/V4+

Ashal’cha Field Crude (Initial) fraction A 89.22 21.56 4.14 Experiment 1 Product (T = 210 °C, P = 17 MPa, Water/Crude 1:1, Catalyst 2%) fraction A 126.34 11.29 11.19 fraction B 14.63 Experiment 2 Product (T = 250 °C, P = 17 MPa, Water/Crude 1:5, Catalyst 2%) fraction A 10.38 0.09 110.00 fraction B 522.62 0.67 774.71 coke 84.72 0.26 323.33 Experiment 3 Product (T = 300 °C, P = 18 MPa, Water/Crude 1:10, Catalyst 2%) fraction A 16.03 0.49 132.86 fraction B 198.00 0.40 495.0 coke 922.09 0.06 15030

tetravalent vanadium (V+4), the latter being a part of the vanadyl−porfyrin complexes, decreases and the values of the R*/V+4 indicator grow as well.31 This is also consistent with the changes in the values of others above-mentioned indicators, confirming that the destructive processes with the substance effect on the of asphaltenes structure take place. The following peculiarity has been observed: carben−carboids (fraction B) from the experiment 2 products and the experiment 3 coke from are characterized by the biggest content of the paramagnetic centers and, accordingly, by bigger values of the R*/V+4 indicator, which is coherent with the highest degree of their aromaticity. The high content of the paramagnetic centers in the insoluble asphaltenes determines their behavior as centers of association with the supramolecular formation.43 With the increase in the experiments temperature and, as a result the increase in the asphaltenes association degree, the structure above precipitates from the dispersion medium in the form of coke-like products with further sedimentation of other high-molecular components on their surface. 3.3. Composition of the Liquid Products Associated in the Asphaltenes Structure. The results of investigations of numerous authors7−11,43−50 show that in crude the main part of the asphaltenes molecules exist under nature conditions in the form of the associates having the expressed condensed aromatic ring of the characteristic radius (10 Å). The sizes of such crude oil asphaltene associates, according to various methods, vary from 14 to 30−50 Å. It has been noted that the resins of different oil objects stabilize the asphaltene associates to a

Table 5. Fractional Analysis of the Heptane Extracts of the Ashal’cha Crude Oil Asphaltenes before and after Experiments fractional analysis (wt %) experiment number

1 2 3

heptane extract of asphaltenes (HEA)

yield (wt %)

HC

PB

Ashal’cha Field Crude (Initial) HEA 9.07 11.25 42.50 Experiment 1 Product (T = 210 °C, P = 18 MPa, Water/Crude 1:1, Catalyst 2%) HEA, experiment 1 23.94 35.3 41.18 Experiment 2 Product (T = 250 °C, P = 18 MPa, Water/Crude 1:5, Catalyst 2%) HEA, experiment 2 26.79 44.0 31.33 Experiment 3 Product (T = 300 °C, P = 18 MPa, Water/Crude 1:10, Catalyst 2%) HEA, experiment 3 17.64 37.33 45.67 780

PAB

∑ resins

46.25

88.75

23.52

64.70

24.67

56.00

17.00

62.67

DOI: 10.1021/acs.energyfuels.5b01328 Energy Fuels 2016, 30, 773−783

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stable heteroatomic bonds with further separation of peripheral long alkyl substitutes from the resins and asphaltenes followed by the formation of aromatic packs with free radicals high concentration increases. The process above is also accompanied by the destruction of the vanadyl- and nickel-containing porphyrin complexes with the release of metals concentrating in the insoluble experiments products. In the supramolecular structure of asphaltenes, the following occurs:The thickness of their aromatic pack decreases as does the number of layers in the pack. The asphaltenes molecular mass decreases as well. Most intensively, the process of catalytic cracking with the subsequent coke formation takes place at the temperature of 300 °C under the low content of the water phase in the reaction system. There has been revealed a significant ability of the asphaltenes of the crude oil undergoing the hydrothermal reaction to immobilize maltenes, which results from the asphaltenes’ high inclination to the intermolecular interaction with hydrocarbons characterized by the low part and length of alkyl chains not causing sterical difficulties for their molecules to penetrate into the aggregate. The phenomenon of considerable maltenes immobilization, especially hydrocarbons, is of great interest from the technological point of view because it will determine the route of the processes of distillation, deasphaltization, and phase-disperse transformations in the asphaltene associates’ structure in thermal reactions. Thus, the researches of studying the conversion products of heavy crude oil in the presence of the natural oxide catalyst, hematite, revealed the main directions of reactions and transformations of high-molecular-weight components of heavy crude oil in the studied systems that eventually gives the possibility of achieving synthetic oil using this catalyst.

heptane extracts of asphaltenes of the transformed crude oil is lower than that in the asphaltenes of initial oil. High-aromatic asphaltenes have a high content of free radicals and heteroatomic active centers that promote adsorption and immobilization of resins due to interaction of free radicals48 and form hydrogen communications between functional groups of molecules of resins and asphaltenes, for example, carboxyl and nitrogen atoms.49 This is in good agreement with the X-ray structural analysis data (Table 3), showing the growth of the distance between the packs aromatic layers and methylene groups in the saturated fragments of the asphaltenes structure after the experiments. Reference 44 deals with the EPR and IR-spectroscopy methods, by means of which the transportations in the hematite catalyst composition have been registered. Thus, the EPRspectrum of the initial hematite within the range of 0−500 mT definitely demonstrates the resonance band characteristics for ferrum oxide. On the spectrum of hematite extracted from the experiment 1 products (210 °C), the shift of the absorption resonance band directly to the area of the magnetic field higher intensity indicates that the formation of ferrum complexes such as magnetite (III), Fe3O4, and hydroxide (II), Fe(OH)2 or Fe(OH)3, has taken place. The formation of magnetite as a result of interaction between ferrum oxide and water vapor seems to be a very important factor,33 leading to the free hydrogen release, which contributes greatly to the process of hydrogenation of the radicals originating within the process of the catalytic cracking of the heavy crude oil high-molecular components. The experiment 2 (and experiment 3) catalysts’ IR spectra with a wide highly intensive band corresponding to the hydroxyl groups at 3425 cm−1, which was absent in initial catalyst spectrum, also confirms the formation of such ferrum complexes as magnetite (III), Fe3O4, and ferrum hydroxide. However, it could be supposed that the intensive formation of coke in the experiment 3 reaction system will block the active centers of hematite, reducing its activity as a proton donor.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

4. CONCLUSIONS Thus, the result of the undertaken investigation of the products of the heavy crude oil conversion in the presence of a nature oxide catalyst, hematite, allows us to reveal the main reaction routes in the hydrothermal−catalytic system under various temperatures and with different content of water draw some certain conclusions. The asphaltenes structure changes not dramatically at the temperature of 210 °C and with the ratio of water to oil of 1:1 in the reaction system. Under the experimental conditions given, the major hydrocarbon part their structure is not yet destroyed. In the absence of coagulation structures such as carben− carboids, the disperse system remains rather stable. This is supported by the resins adsorptive−solvate films, which define the stability of the asphaltene aggregates, and the presence of water in the system is also possible, which can influence the formation and stabilization of crude oil associates. At temperatures of 250 and 300 °C with a smaller content of water (ratio of 1:5 and 1:10 to crude oil, respectively), some destructive processes start taking place, accompanied by the formation of both light fractions and the products of polymerization and condensation, namely, carben−carboids and coke, that are characterized by the high degree of aromaticity. At the experimental temperature of 250 °C, the hydrocarbons yielded at the expense of destruction of the least-

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. The work was funded by the subsidy allocated to Kazan Federal University for the state assignment in the sphere of scientific activities. We thank A. I. Samigullina for shooting for profiles of diffraction used for an assessment of structural parameters of asphaltenes. The work is performed with support from the grant RFFI No. 15-45-02689 "Povolzhye".

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REFERENCES

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