Abstract
Crude oils containing large quantities of waxes, and in situations where sudden temperature drops occur, a crystalline network can be formed, generating flow difficulties in production, transfer and offloading lines. Despite the scientific and economic importance of this phenomenon, correlations between the behavior of petroleum in relation to its pour point are scarce in the literature. In this work, nine crude oil samples were characterized regarding density, water content, wax fraction (one- and two-dimensional chromatography), pour point, yield stress and wax appearance temperature. The results showed that the storage conditions and oil characteristics had a strong influence on the pour point. Yield stress was influenced by initial temperature of the sample, cooling rate and shear rate during cooling. Finally, the content of aromatic compounds in oil with low concentration of n-alkanes can contribute to increase the pour point.
Highlights
In the reservoir, crude oil behaves as a Newtonian fluid, since the waxes with high molar mass are in solution, causing the oil to act as a monophasic liquid, i.e., in laminar flow, in which the viscosity is only a function of temperature, independent of the shear rate.[1,2] As soon as the oil leaves the reservoir and flows through the line at a lower temperature, the temperature gradient between the cold wall of the pipe and the hot oil causes the waxes to precipitate
This is a factor proposed by Universal Oil Products (UOP) to classify petroleum according to its nature, as paraffinic (KUOP ≥ 12.5), naphthenic (KUOP < 11.5), intermediate (11.5 ≤ KUOP ≤ 12.1) or aromatic (KUOP ≤ 10)
The results obtained by one-dimensional chromatography along with the data obtained by two‐dimensional chromatography (Figure 4) revealed that crude oil AP07 had the highest content of n-alkanes present in the C15-C44 range (GC-FID) (13.95% m/m), a level much higher than the other crude oils analyzed (4.03, 3.93 and 3.75% m/m for samples AP09, AP02 and AP04, respectively), in line with its very high pour point (36 °C) and greater yield stress (> 10,000 Pa)
Summary
Crude oil behaves as a Newtonian fluid, since the waxes with high molar mass are in solution, causing the oil to act as a monophasic liquid, i.e., in laminar flow, in which the viscosity is only a function of temperature, independent of the shear rate.[1,2] As soon as the oil leaves the reservoir and flows through the line at a lower temperature, the temperature gradient between the cold wall of the pipe and the hot oil causes the waxes to precipitate. The nine oil samples were characterized regarding density, water content, pour point, wax appearance temperature (WAT), yield stress, mass of saturates, content of n-alkanes, isoalkanes + cycloalkanes (UCM) by one‐dimensional gas chromatography (GC) in the C15-C44 range; contents of branched alkanes, cycloalkanes and some aromatic compounds in relevant abundance, by twodimensional gas chromatography (GC×GC-TOFMS) in the C6-C40 range.
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