INVESTIGATION OF RESIN–ASPHALTENE SUBSTANCES IN NIZHNEVARTOVSK CRUDE OILS

N. M. Smolyaninova, L. D. Kriger, L. A. Igumnova

(Presented by the scientific-methodological seminar of the organic departments of the Faculty of Chemical Technology)

Knowledge of the chemical structure and nature of resin–asphaltene substances, quite apart from the great importance of this question for solving purely practical problems, is of exceptional scientific value for the geochemistry of organic compounds.

The present work is devoted to the study of the chemical nature of resins and asphaltenes from the crude oils of two of the largest fields in Western Siberia—the Sovetskoe and Samotlor fields. A brief characterization of the oils is as follows: well No. 23 and No. 14, respectively; density \(\rho^{20}_{4}\)—0.8629 and 0.8525; molecular weight—228 and 213; paraffin content—4.39 and 3.64%; amount of silica-gel resins—9.88 and 8.53%; asphaltenes—0.79 and 0.71%; yield of fractions up to \(200^\circ\text{C}\)—30.8 and 28.8%; up to \(300^\circ\text{C}\)—53.0 and 49.0 wt. %, respectively (the first values are for the Sovetskoe oil, the second for the Samotlor oil) [1, 2].

The indicated substances were isolated from the oil by the VNII NP method [3, 4], which excludes the action of high temperatures and chemically active substances, making it possible to obtain resins and asphaltenes in an unchanged state.

The investigation was carried out using elemental, functional, and X-ray structural analyses, as well as methods of selective dissolution, infrared spectroscopy, and derivatography.

The carbon and hydrogen contents were determined by the micromethod, sulfur by the accelerated method according to GOST 1437-56, nitrogen by Kjeldahl, and molecular weight by cryoscopy with benzene. Functional analysis was used to evaluate the amount of oxygen-containing groups (CO, OH, COOH). The hydroxyl number was determined by the oximation reaction, and carboxyl groups by acetylation followed by potentiometric titration of the products with an aqueous alkali solution [4]. X-ray diffraction patterns were recorded on a URS-50 IM instrument; IR spectra were obtained on a UR-10 spectrophotometer. Interpretation of the spectra was carried out on the basis of spectral atlases and literature data.

The characteristics of the resins and asphaltenes according to elemental and functional analyses are given in Table 1. These data show that, in terms of molecular weight, resins and asphaltenes are the highest-molecular-weight compounds in the oil, and moreover heteroorganic compounds, as indicated by the results of elemental analysis. Asphaltenes contain less hydrogen and more carbon than resins,

Table 1

Characteristics of resins and asphaltenes

which, given the greater molecular mass of the former, makes it possible to suppose a higher degree of condensation of their molecules.

Table 2

Interplanar spacings and band intensities for asphaltenes

Various oxygen-containing functional groups—hydroxyl, carbonyl, and carboxyl—were found in small amounts in the resins and asphaltenes. The presence of such groups is also confirmed by infrared spectra recorded in the region 700–3400 cm\(^{-1}\).

In all the spectra absorption of the carbonyl group is observed at 1710–1730 cm\(^{-1}\), of the ether group at 1060 and 1230 cm\(^{-1}\), and of the carboxyl group at 1725 cm\(^{-1}\). The presence of absorption bands at 2896, 2925, 1460, and 1380 cm\(^{-1}\) indicates the presence of side aliphatic chains, while maxima at 760–825, 1010, 1260, and 1576–1600 cm\(^{-1}\) attest to the presence of aromatic substituted structures in the molecules of resin–asphaltene substances.

On the basis of the X-ray patterns obtained (Table 2), it may be concluded that asphaltenes are amorphous substances with some indication of structural ordering, as evidenced by two large peaks in the X-ray patterns. For them, maxima 1010 and 0002, characteristic of graphite, were found. These maxima correspond to interplanar spacings of 3.5 Å and 4.5 Å. The first, as Sergienko [1] indicates, corresponds to the distance in a hexamethyl-substituted benzene ring, while the second is found in aliphatic hydrocarbons in the liquid state. The presence of this spacing serves as another confirmation of the presence of alkyl radicals in asphaltene molecules.

Resins are not individual compounds; to simplify their composition, selective dissolution was carried out using a consecutive series of solvents of different polarity (\(\mathrm{CCl}_4\), acetone, benzene, alcohol–benzene) [1], followed by spectral study of the narrow resin fractions. The yields of extracts are given in Table 3.

Table 3

Yields of resin fractions soluble in various solvents

The crude oils of the Sovetskoye and Samotlor fields are naphthenic–paraffinic; therefore, the greater part of the resin (52 and 57%, respectively) is extracted by \(\mathrm{CCl}_4\).

The IR spectra of the narrow resin fractions indicate that fractions of the same name from the resins of both crude oils are similar and differ only,

degree of oxidation. In general, the molecules contain aromatic rings (1600, 810, 880, 765–750 cm\(^{-1}\)), —CH\(_2\)— groups belonging to aliphatic (720, 1380 cm\(^{-1}\)) and naphthenic (970, 1460 cm\(^{-1}\)) hydrocarbons, as well as carbonyl, carboxyl, and hydroxyl groups (1710–1725 cm\(^{-1}\); 3200–3400 cm\(^{-1}\)), and —CH\(_3\)— (1380, 2870, 2920 cm\(^{-1}\)) groups.

Derivatographic analysis*) was carried out on an instrument manufactured by “Metrimpex,” supplemented with a system for sampling and gas-chromatographic analysis of the gases of thermal destruction. The rate of temperature rise was 5°C/min, the analysis time 200 min, and the rate of suction of the decomposition products 25 ml/min.

The results of derivatographic analysis of the resin-asphaltene substances of oil make it possible to note a number of general regularities in their behavior during thermal destruction. The curves of the rate of change in weight (Figs. 1, 2, 3, 4) and of the formation of gaseous products indicate that the main changes in the structure and chemical

Fig. 1. Derivatogram of asphaltenes of Samotlor crude oil

Fig. 2. Derivatogram of asphaltenes of Soviet crude oil

composition occur in the temperature range up to 600°C. These transformations are associated with the formation of volatile products of thermal decomposition, among which the major share belongs to liquid resinous substances and water. The similar character of the process of decomposition of resins and asphaltenes is also reflected by the temperature ranges in which the corresponding gaseous

*) The derivatography was performed by V. M. Ikrin.

of various products ($\mathrm{CO_2}$, $\mathrm{H_2}$, $\mathrm{CO}$, $\mathrm{CH_4}$), the maxima of formation of which coincide in most cases. This is probably associated with processes of destruction of substances similar in structural respect. And since the least thermally stable bonds are $\mathrm{O{-}C}$, $\mathrm{C_{ar.}{-}C_{aliph.}}$, and $\mathrm{C_{aliph.}{-}C_{aliph.}}$, it may be assumed that the principal changes occur in the peripheral part of the structural units of resins and asphaltenes. The condensed aromatic part undergoes almost no changes in the temperature interval up to $600^\circ\mathrm{C}$. Consequently, at the initial stage of thermolysis there is cleavage of the functional groups —$\mathrm{COOH}$, —$\mathrm{CO}$, —$\mathrm{OH}$ and of aliphatic chains.

Fig. 3. Derivatogram of resins from Samotlor crude oil

Fig. 4. Derivatogram of resins from Soviet crude oil

Above $600^\circ\mathrm{C}$ the pyrolysis products are the gases $\mathrm{CO}$, $\mathrm{H_2}$, $\mathrm{CH_4}$, the formation of which is associated not only with bond-breaking processes but also with the formation of new, stronger bonds, as may be indicated by the insignificant changes in weight of the TG sample.

In addition to the general regularities, there are also differences in the behavior of resins and asphaltenes. This is expressed in the quantity of products formed at the corresponding stages of pyrolysis of the samples.

The results of elemental, functional, X-ray structural, and spectral analyses provide grounds for considering asphaltenes to be more condensed substances than resins. Therefore, the yield of liquid and gaseous thermolysis products from resinous substances should pre-

exceed that for asphaltenes, as is evident from the curves for product formation and from the values of the rates of their evolution.

With respect to the aliphatic groupings of the structural units of resin–asphaltene substances, the following may be said: the amount of methane formed during decomposition of asphaltenes and exceeding that for resins may be, to some extent, a measure of the branching and length of the aliphatic chains. The greater the chain length, the more hydrocarbons with a number of carbon atoms greater than 1 are formed. This also indicates a greater diversity of the peripheral part of the structural units of resinous substances.

Above 600°C the amount of gaseous products is small; the main share of them consists of CO and H₂. In this case the asphaltenes give somewhat more CO than the resins, which is probably associated with the higher content in the latter of heterocyclic oxygen, whose thermal stability is higher than that of other oxygen-containing groups; this does not contradict the elemental-composition data.

The kinetic analysis carried out for the thermal decomposition of the samples gave activation-energy values that correspond to the rupture of the corresponding chemical bonds and do not contradict the literature data.

Table 4

Activation energy of the overall process of thermal decomposition and formation of gas components

The values given in Table 4 for the activation energy of the overall process of destruction and of the formation of individual gas components for resins and asphaltenes from the same crude oil vary within small ranges. This indicates that, in the same temperature intervals, similar structural units of the macromolecule are cleaved. As the thermolysis temperature increases, the activation energy increases, which indicates the destruction of increasingly stronger bonds. The anomalously high value of the activation energy for hydrogen formation in asphaltenes may be explained by its release as a result of condensation processes.

If the resin and asphaltene samples isolated from different crude oils are compared, a difference may be observed in their behavior during thermal decomposition.

The thermal decomposition of resin–asphaltene substances from Samotlor crude oil is more pronounced, which is manifested in a narrower temperature interval of decomposition and in a smaller number of maxima appearing on the curves of weight loss and formation of gaseous products. This may be evidence of a more homogeneous structure of the substances of Samotlor crude oil; that is, the number and composition of structural groupings in the resin–asphaltene substances of the Soviet crude oil are more varied than in the Samotlor oil, while the higher activation-energy values for samples of Samotlor crude oil indicate that they are characterized by the presence of more stable bonds, which are broken at the corresponding temperatures.

Conclusions

1. Resin–asphaltene substances were isolated; their elemental and functional compositions were determined; IR spectra were recorded; and X-ray diffraction patterns of asphaltenes were obtained.

2. It was shown that the resins and asphaltenes of the crude oils studied are high-molecular heteroorganic compounds.

3. Resinous substances are distinguished by a smaller amount of condensed structures in the composition of the macromolecules.

4. Selective extraction of resins was carried out with $\mathrm{CCl_4}$, $\mathrm{C_6H_6}$, acetone, and alcohol–benzene. IR spectra were recorded for the narrow resin fractions obtained.

5. It was established that the resinous substances of Samotlor crude oil contain more aromatic structures, whereas those of the Soviet crude oil contain more naphthenic structures.

6. Derivatographic analysis of resins and asphaltenes was carried out, with study of the products of thermal destruction by gas–liquid chromatography.

7. Curves were constructed for thermal decomposition and for the dynamics of gas evolution: $\mathrm{CO_2}$, $\mathrm{CO}$, $\mathrm{CH_4}$, $\mathrm{H_2}$.

8. The activation energy was calculated for the overall process of thermal destruction and for the formation of individual gas components.

References

1. S. I. Smolyaninov, N. M. Smolyaninova, K. K. Stramkovskaya. Problems of Chemistry. Issue 26. Tomsk, Tomsk State University Publishing House, 1969, p. 22.

2. N. M. Smolyaninova, S. I. Smolyaninov, L. E. Aleksandrova, V. K. Zhurba, V. B. Orlovskii. Izvestiya TPI, vol. 253. Tomsk, Tomsk State University Publishing House, 1976, p. 36.

3. S. R. Sergienko. High-Molecular Compounds of Petroleum. Moscow, “Khimiya,” 1964.

4. Manual for the Analysis of Petroleum. Moscow, “Nedra,” 1966.