Crystallization of Petroleum Paraffins
Abstract Generated abstract
This study examines the relationship between the onset of petroleum paraffin crystallization and the solidification of paraffin-containing oils from Western Siberia. Solidification temperatures were determined by loss of fluidity, while mass paraffin crystallization was followed through cooling curves and simultaneous microscopic observation, with additional tests of heat-treatment temperature and holding time. The results indicate that temperature arrests on cooling curves correspond to sequential precipitation of paraffin crystals and occur above the pour point, whereas oil solidification results from coagulation of the crystalline paraffin mass into a spatial network that immobilizes the liquid phase. Heat treatment can lower the solidification temperature, apparently by altering crystal formation and favoring structures less able to form such a network.
Full Text
Crystallization of Petroleum Paraffins
N. V. Yudina, N. V. Polyaeva
(Presented by the Scientific-Methodological Seminar of the Organic Departments of the Chemical-Technological Faculty)
The process of crystallization in paraffin-containing systems has been studied by many researchers [1, 2]. Nevertheless, the relationship between the onset of crystallization and solidification in one and the same system has been little studied. This is explained by the complex nature of petroleum, which is a mixture of hydrocarbons and other organic compounds of varying molecular weight.
We present the results of such studies for oils of Western Siberia in Table 1 and in Fig. 2.
The solidification temperature was estimated from the loss of fluidity [3], when a dispersed structure is formed that immobilizes
Fig. 1. Schematic of the apparatus for studying the crystallization of petroleum paraffins.
1 — air blower; 2 — clamp; 3 — rheometer; 4 — drying bottle; 5 — Dewar vessel; 6 — cuvette; 7 — thermocouple; 8 — galvanometer; 9 — microscope.
the entire volume of the liquid phase. The temperature of mass separation of paraffin crystals was determined from the cooling curves with simultaneous observation under a microscope. The schematic of the apparatus for studying paraffin crystallization is shown in Fig. 1.
Table 1
Effect of heat treatment and holding time on the pour point of oils
| Oil | Pour point at heat-treatment temperature, °C: 20 | Pour point at heat-treatment temperature, °C: 30 | Pour point at heat-treatment temperature, °C: 40 | Pour point at heat-treatment temperature, °C: 50 | Pour point at heat-treatment temperature, °C: 60 | Pour point at heat-treatment temperature, °C: 70 | Pour point at heat-treatment temperature, °C: 80 | Pour point at heat-treatment temperature, °C: 90 | Pour point upon holding the oil after heat treatment at +70°C, days: 1 | Pour point upon holding the oil after heat treatment at +70°C, days: 2 | Pour point upon holding the oil after heat treatment at +70°C, days: 3 | Pour point upon holding the oil after heat treatment at +70°C, days: 4 | Pour point upon holding the oil after heat treatment at +70°C, days: 5 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Nikolskaya | −11 | −13 | −13 | −15 | −24 | −30 | −15 | −18 | −29 | −27 | −12 | −11 | −10 |
| Pervomaiskaya | −25 | −27 | −26 | −34 | −56 | −53 | −31 | −30 | −49 | −42 | −28 | −28 | −24 |
| Ozernaya | −14 | −12 | −14 | −16 | −24 | −54 | −38 | −30 | −29 | −23 | −20 | −19 | −15 |
| Shkainskaya | −30 | −31 | −32 | −35 | −65 | −68 | −54 | −45 | −68 | −68 | −68 | −65 | −65 |
| Varieganskaya | −4 | −9 | −11 | −15 | −27 | −45 | −38 | −29 | −45 | −45 | −45 | −45 | −43 |
When oil is cooled, paraffins separate out at definite temperatures depending on molecular weight and structure. In this process, hydrocarbons having the highest melting point separate first. A crystal lattice is formed at the moment the oils solidify. The stability of this temperature depends on the content of a given amount of hydrocarbon and on the structure of their crystals [4].
The cooling curves shown in Fig. 2 have temperature arrests. They appear as a result of the precipitation of solid hydrocarbons having similar melting points and molecular weights. It should be noted that the temperature arrests lie somewhat higher than the solidification temperatures of the oils. Apparently, the temperature arrests are characterized by the serial precipitation of paraffin crystals, which is confirmed by direct observation of them under a microscope. The solidification of oils, however, is due to the formation of a coagulation structure, which encompasses the entire mass of the oil and deprives it of mobility.
Fig. 2. Cooling curves of oils. Ozernaia: 1 — untreated; 2 — heat treatment at +50°C; 3 — heat treatment at +70°C. Permian: 1 — untreated; 2 — heat treatment at +50°C.
Of no small importance in the study of the crystallization of petroleum paraffins is the determination of the effect of heat treatment on the solidification temperature. It is believed that the solidification temperature of oil decreases owing to an increase in the activity of asphaltene–resin substances upon heating, or that the cause is a decrease in the solvate shell of the crystals. If, as a result of heat treatment of the oil, it is possible to obtain a coarse-crystalline structure incapable of forming a spatial network, then it solidifies at a lower temperature [5]. The cooling curves in this case indicate a decrease in the temperature at which crystals separate out.
Conclusions
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It has been shown that the solidification temperature of oils is determined by coagulation of the crystalline mass of paraffin into a structural network.
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A mechanism of oil solidification has been established, with the processes of paraffin-crystal separation and coagulation-network formation separated in time.
Literature
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E. A. Aleksandrova, A. P. Grishin, A. A. Petrova. Study of the temperatures of the onset of crystallization and structural solidification of paraffin solutions. Izv. vyzov SSSR, Neft i gaz, 1971, No. 6, pp. 47–51.
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N. I. Chernozhukov, V. V. Vainshtein, B. I. Kartinin. On the crystallization of solid paraffins of oils. KhTTiM, 1969, No. 6, pp. 15–18.
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B. M. Rybak. Analysis of Oil and Petroleum Products. Moscow: Gostoptekhizdat, 1962.
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B. U. Urazgaliev, G. A. Bedik, Zh. E. Koshebekov. Rheological properties of commercial oils of Emba and Mangyshlak. Proceedings of the Institute of Petroleum Chemistry and Natural Salts, Academy of Sciences of the Kazakh SSR, vol. 1, 1970, pp. 3–9.
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O. M. Alchikova, V. P. Konovalov. On the problem of lowering the solidification temperature of high-paraffin oils. Proceedings of the Institute of Petroleum Chemistry and Natural Salts, Academy of Sciences of the Kazakh SSR, vol. 3, 1971, pp. 96–99.