Study of the Mechanism of Structure-Forming Processes in Crude Oils
Abstract Generated abstract
This study examines structure formation in paraffin-containing crude oils from Western Siberia, focusing on viscosity-temperature behavior, heat treatment, holding time, and the kinetics of structural strengthening. Dynamic viscosity measurements and limiting shear stress measurements using a tangentially displaced plate show that heat treatment at about 60 to 70°C has the strongest effect at negative temperatures and that this effect persists for several days. The results indicate that the strength of the paraffin structural lattice depends on temperature, formation conditions, and the chemical nature of the crude oil, with lower temperatures producing stronger structures through changes in particle-contact bonding.
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STUDY OF THE MECHANISM OF STRUCTURE-FORMING PROCESSES IN CRUDE OILS
N. V. Yudina, G. G. Krinitsyn
(Presented by the scientific-methodological seminar of the organic departments of the Faculty of Chemical Technology)
The study of the structural-mechanical properties of crude oils has recently been of great practical and theoretical interest in connection with discussion of the mechanism of structure formation during the emergence and development of the solid phase of paraffin. Numerous experimental studies related to the solution of this question have been carried out [1, 2]. However, in such complex systems, which are mixtures of hydrocarbons and other organic compounds of different molecular weights, it is difficult to reveal the mechanism by which a crystalline structure is formed. In connection with the study of the process of formation of the solid phase in paraffin-containing systems, the influence of heat treatment on the structural-mechanical properties of crude oils is of great importance. We investigated the viscosity-temperature properties of crude oils from Western Siberia and the effect on them of heat treatment and holding time. The results obtained are given in Table 1.
It follows from this that the crude oils studied obey the previously established relationships, namely: the optimum heat-treatment temperature is 60–70°C; the effect of heat treatment is most strongly manifested in the region of negative temperatures, persisting for 2–3 days.
The study of the processes of development of spatial structures in crude oils was based on the concepts of physicomechanical mechanics: the processes of emergence and development of structures proceed in time and are governed mainly by kinetic regularities.
Fig. 1. Kinetics of the increase in structural strength with time for Varyogan crude oil (numbers at the curves are temperatures, °C).
The kinetics of structure formation was characterized by us by the increase in the strength of structures—the limiting shear stress
Table 1
Effect of heat treatment and holding time on the dynamic viscosity of oils
| Oil | Holding time, min, days | Heating temperature, °C | −50 | −40 | −30 | −20 | −10 | 0 | 20 | 30 | 40 | 50 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Varieganskaya | without holding | not therm. | 65,00 | 16,00 | 6,45 | 2,93 | 1,87 | 1,25 | — | 0,415 | 0,373 | 0,348 |
| Varieganskaya | 50 | 50 | 13,39 | 9,72 | 4,07 | 2,16 | 1,56 | — | 0,382 | 0,364 | 0,364 | 0,338 |
| Varieganskaya | 70 | 70 | 8,44 | 6,14 | 3,20 | 1,75 | 1,01 | — | — | — | — | — |
| Varieganskaya | 2 | 50 | 13,38 | 8,95 | 5,01 | 2,82 | 1,79 | — | 0,382 | 0,361 | 0,350 | 0,338 |
| Varieganskaya | 3 | 50 | 43,10 | 14,20 | 6,20 | 2,08 | 1,21 | — | 0,179 | 0,382 | 0,364 | 0,369 |
| Varieganskaya | 3 | 70 | 16,50 | 6,85 | 3,92 | 1,66 | 1,04 | 0,6310 | 0,376 | 0,361 | 0,351 | 0,339 |
| Klyuchevskaya | without holding | not therm. | 88,00 | 29,40 | 17,40 | 3,98 | 3,46 | 1,300 | 0,790 | 0,679 | 0,577 | 0,500 |
| Klyuchevskaya | without holding | 50 | 29,80 | 10,70 | 8,15 | 2,46 | 1,29 | 0,865 | 0,760 | 0,665 | 0,545 | 0,485 |
| Klyuchevskaya | without holding | 70 | 16,40 | 8,77 | 3,74 | 2,18 | 1,16 | 0,901 | — | — | 0,477 | 0,295 |
| Klyuchevskaya | 2 | 50 | 34,90 | 29,40 | 5,95 | 2,60 | 1,58 | 0,890 | 0,760 | 0,665 | 0,545 | 0,485 |
| Klyuchevskaya | 2 | 70 | 17,10 | 9,45 | 3,43 | 1,92 | 1,56 | — | — | — | 0,410 | 0,369 |
| Klyuchevskaya | 3 | 70 | 25,70 | 11,50 | 4,89 | 2,37 | 1,32 | 1,19 | 0,709 | 0,616 | 0,512 | — |
| Klyuchevskaya | 5 | 50 | 88,00 | 28,80 | 17,90 | 3,98 | 2,20 | 1,26 | 0,790 | 0,679 | 0,314 | 0,500 |
| Klyuchevskaya | 5 | 70 | 52,50 | 20,90 | 8,21 | 1,65 | 1,18 | 1,05 | 0,776 | 0,706 | 0,585 | — |
at different time intervals by the method of a tangentially displaced plate. The magnitude of the limiting shear stress is proportional to rigidity over a wide temperature range. Measurements of the quantity under study were in all cases carried out at a constant deformation rate. The first measurement was made when the experimental temperature had been established in the system.
The kinetic curves shown in Fig. 1 make it possible to study the mechanism of structure-forming processes in crude oils. In the initial stage of the structure-formation process, over the course of 1 hour, a slight increase in the limiting shear stress is observed. At the same time, a decrease in temperature leads to a sharp increase in the strength of the structure. This indicates a change in the number or nature of the bonds arising at contacts between particles as the temperature is lowered [3].
Analysis of the experiments performed shows that the structural lattices formed in crude oils have different strengths. The general pattern is that the strength of the structural lattice depends on the conditions of its formation and on the chemical nature of the crude oil. The duration of structure formation in paraffinic crude oils differs: in low-resin crude oils the structure-formation process ends quickly, after which the strength of the structure decreases, apparently as a result of enlargement of the crystals.
Conclusions
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It has been established that the strength of the structural lattice of paraffin depends on the temperature and the nature of the crude oil.
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The increase in the strength of the structure when the temperature is lowered indicates a change in the bonds arising at contacts between particles.
References
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A. E. Bobrova, V. N. Izmailova, P. A. Rehbinder. Study of the limiting shear stress and assessment of contacts in lyophilic disperse systems—gelatin gels. Kolloidnyi zhurnal, 1972, vol. 36, pp. 6–9.