N. Yu. Eliseev

Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy: I. Structure of the absorbance spectrum

Abstract The aim of this paper is to provide new data on the properties of molecular aggregation in toluene solutions of crude oils and of solid asphaltenes. The shape of the optical absorption spectrum was found to be sensitive to the details of asphaltene aggregation processes. In dilute solutions, these processes are apparently determined by the net concentrations of asphaltenes; other oil constituents play a secondary role. Our experimental data indicate that molecular solutions of asphaltenes are possible only for concentrations below 1 mg/l. With increasing asphaltene content, more and more complex molecular aggregates are formed. In particular, asphaltene dimers evidently are the predominant species in the range of 5–15 mg/l, while stable “nanocrystallites” (dimer pairs) are predominant at concentrations ≈90 mg/l. Aggregates at higher concentrations may be viewed as assemblies of such “crystallites”. The observed gradual aggregation process is distinct from conventional micellisation phenomena with step-like changes at critical micelle concentrations (CMCs).

Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy: II. Concentration dependencies of absorptivities

Abstract The properties of molecular aggregation in toluene solutions of a crude oil and solid asphaltenes are determined almost solely by the concentration of asphaltenes, as shown by absorptivity measurements at 315–750 nm. From non-monotonic concentration dependencies of absorptivities, it is concluded that asphaltene monomers are abundant in solutions with asphaltene concentrations below 1–5 mg/l, while molecular aggregates are effectively formed above 20–25 mg/l. The most stable oligomers are a dimer and a dimer pair (Yens “nanocrystallite” [NC]). Nanocrystallites act as building blocks for more complex aggregates at asphaltene concentrations exceeding 90–100 mg/l. These optical absorption results are supported by studies of Rayleigh scattering in asphaltene solutions.

Rheological evidence of structural phase transitions in asphaltene-containing petroleum fluids

Abstract The aim of this paper is to provide data on the rheological/structural properties of “synthetic oils,” composed of light hydrocarbons (toluene) and a heavy fraction, containing asphaltenes (vacuum residue, VR). Samples with asphaltene concentrations 20–85 g/l have been studied at temperatures 0–60 °C and shear rates up to 1500 s−1. The non-Newtonian flow curves were approximated by the Bingham and the Herschel–Bulkley models to determine the apparent yield stress and the shear-rate exponent as functions of the asphaltene concentration and the temperature. Sharp variations of these parameters were attributed to formation/destruction of extended ordered structures in asphaltene colloid suspensions. Structural changes were observed in the temperature range 20–30 °C, particularly important for industrial processes of reservoir development and pipeline transportation. A molecular model of the observed macroscopic effects takes into account possible first-order structural phase transitions in the nanometer-size resin/asphaltene colloid microparticles.

Characteristics of viscous flow of liquid media with resins and asphaltenes

The extensive utilization of deposits of highly viscous crude oils is stimulating a wider investigation of the physicochemical processes involving macromolecular resins and asphaltenes (RA). The reliability of operation of turbolines in pumping crude oils with a high solid point is determined to a significant degree by the possibility of controlling the flow of liquid colloidal systems whose disperse phase is primarily represented by asphaltenes. However, there is no sufficiently complete information on cross-linking mechanisms involving molecules of RA and the degree of the effect of the structures formed on the rheology of the liquids for such control. We used viscometry to investigate the characteristics of association and dispersion of RA in relatively simple model liquid media. The effect of the concentration composition of liquid media were previously primarily investigated in such studies [1 – 3]. The step for changing the temperature was too high: 10°C and higher, to determine any thermal characteristics of viscous flow.

Some Features of the Supramolecular Structures in Petroleum Media

The structure of the disperse phase of crude oils and liquid petroleum products is determined by the characteristics of formation of molecular aggregates of asphaltenes. The characteristics of these processes in dilute solutions of crude oil in toluene were investigated by measuring the proton spin‐spin relaxation time as a function of the concentration of asphaltenes. These measurements allowed explaining the conditions of formation of dimers and more complex aggregates of asphaltene molecules. The data obtained by the NMR‐relaxation method are confirmed by the results of measurements of the viscosity and optical absorption of the petroleum solutions investigated.

Negative Viscosity Anomaly of Liquid Petroleum Products after Heat Treatment

Heat treatment is a common method of improving the properties of heavy crude oils and petroleum products. Measurements showed for the first time that after heat treatment at 28—40°C, the rheological parameters of petroleum systems become much worse. The results obtained are important for improving shipping and storage technologies in conditions of periodic temperature drops. A theoretical explanation of these effects is given with consideration of the phase diagrams of petroleum systems and thermally induced changes in their colloidal structure.

ELECTROPHYSICAL PROPERTIES OF LIQUID HYDROCARBON MEDIA

The electrophysical properties of crude oils and liquid petroleum products play an important role in the study of their disperse structure, primarily determined by the content of high-molecular-weight resin-asphaltene components (RAC). The great interest in these properties is also related to the industrial use of electric fields for both intensifying technological processes (dehydration, desalting, dewaxing) and for quality control of crude oils and petroleum products.

Effect of Asphaltenes on the Thermal Properties of Petroleum and Bitumen Emulsions

The thermal properties and characteristics of the molecular structure of petroleum and bitumen emulsions were investigated by viscometer and Rayleigh light scattering. The viscosity, structural properties, and character of the intermolecular interactions in the emulsions are determined by the temperature of formation of these media. Formation at “critical” temperatures close to 36 – 38°C initiates a structural phase transition that alters the size and activity of molecular aggregates of asphaltenes. These aggregates serve as binder material in three—dimensional supramolecular structures containing wax microcrystals. The long (up to several months) “memory” of the emulsions concerning the conditions of formation is due to the strength of the three—dimensional structures that contain not only hydrogen but also covalent bonds.

Selection of the conditions for the optimum effect of metal-containing additives in oils

The antifriction properties of lubricating oils can be significantly improved by additives containing organic substances and metal salts [I-4]. The elevated efficiency of an additive prepared by dissolving copper sulfate in glycerin was demonstrated in [3]; good results were also obtained with additives based on tin, silver, gold, and some other metal salts soluble in ethers and polyhydric alcohols. The friction coefficient decreased by 10-16 times and wear decreased by 8-10 times when these additives were used [2, 3].