Energy & Fuels 2008, 22, 583–586
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Prediction of Asphaltene Self-Precipitation in Dead Crude Oil M. Bayat, M. Sattarin,* and M. Teymouri Petroleum Refining DiVision, Research Institute of Petroleum Industry (RIPI), Tehran, Iran ReceiVed September 8, 2007. ReVised Manuscript ReceiVed NoVember 21, 2007
Asphaltene precipitation in dead crude oil can occur not only by adding saturated solvents such as heptane but also occurs at elevated temperatures. In this work, the relation between asphaltene self-precipitation onset and refractive index (RI) at elevated temperatures is investigated. Experimental measurements of RI for three crude oils are reported at different temperatures. Determination of RI at the onset of precipitation showed that that precipitation occurred at a characteristic RI of 1.42 for each crude oil. The asphaltene content of these samples were in the range 1–11.6 wt %. The sizes of the asphaltene particles formed at elevated temperature were smaller than those formed upon solvents addition.
Introduction Asphaltene is known to flocculate in crude oil and is thought to be responsible for its instability. Crude oil stability depends on composition, pressure, and temperature. The effect of composition and pressure on asphaltene precipitation is generally believed to be stronger than the effect of temperature.1 However, it appears that composition and origin of the oil affect instability of the oil more than the actual amount of asphaltenes.2 Instability and chemical composition incompatibly of crude oils are defined as the colloidal instability index (CII), which is the ratio of the sum of asphaltenes and saturated hydrocarbons to the sum of the resins and aromatics.3 CII ) (asphaltenes + saturates) ⁄ (aromatics + resins) CII of larger than one means unstable crude oil, and precipitation of asphaltene is likely to occur.4 CII can also be used for determination of crude oil stability at room temperature. However, since asphaltene precipitation does not occur just at CII larger than 1, therefore it cannot be used as a quantitative measure to determine asphaltene precipitation onset. The equilibrium of a well-peptized asphaltene system can easily be disturbed by addition of a paraffinic solvent, application of heat, oxidation, or ultraviolet irradiation. In each case, the chemical composition is altered and the aromaticity is decreased, thereby causing a disruption of the equilibrium of colloidal system.3 Addition of n-alkane, as a solvent, to stock-tank oils has often been used for understanding the asphaltene precipitation phenomena. Hirschberg et al.5 and Chang et al.6 carried out their * Corresponding author: e-mail
[email protected]. (1) Leontaritis, K. J.; Mansoori, G. A. Asphaltene Flocculation During oil Prduction and Processing: A Thermodynamic Colloidal Model, SPE Internartional Symposium on Oil Field Chemistry, San Antonio, TX, Feb 1987, SPE 16258. (2) Andersen, S. I. Dissolution of Solid Boscan Asphaltenes in Mixed Solvents. Fuel Sci. Technol. Int. 1994, 12, 51. (3) Mushrush, G. W.; Speight, J. G. Petroleum Products: Instability and Incompatibility; Taylor and Francis: London, 1995; p 298. (4) Barker, K. Understanding Paraffin and Asphaltene Problems in Oil and Gas Wells, PTTC Workshop, July 2003. (5) Hirschberg, A. L.; de Jong, N. J.; Schipper, B. A.; Meyers, J. G. Influence of Temperature and Pressure on Asphaltene Flocculation. Soc. Pet. Eng. J. 1984, 283.
experiments by adding a series of n-alkanes from n-pentane to n-decane to the oil samples and reported precipitation onset composition, amount of material precipitated, and solubility properties of asphaltene. Even though there is no existing standard method to determine oil stability, especially at elevated temperatures, different methods such as spot test,7 P value,8 and turbiscan9 are being used to compare the stability of different oils. However, outcome of a spot test is visual, and hence, the result may be somewhat inaccurate. Even though, determination of P value can show the total amount of n-heptane that can be added to the oil before it becomes unstable, it is time-consuming and is only used for comparing the stability of different oils. Ostlund et al.9 investigated oil stability by utilizing an instrument consisting of an optical scanning device (turbiscan). This method was found to be quick and sensitive, so that the detection of a very small difference in stability is possible. Buckley et al.10 used an improved method for predicting asphaltene precipitation onset according to refractive index (RI). Measurements of RI at the onset of precipitation have shown that for each oil-precipitant combination the onset occurs at a characteristic RI of 1.42-1.44 and is independent of asphaltene content. On the basis of Buckley’s works, asphaltene precipitation onset at characteristic RI between 1.42 and 1.44 and its independence of crude oil type, asphaltene content, and nearly solvent type may be attributed to the transmission of asphaltene from liquid bulk to solid phase. Asphaltene precipitation occurs in crude oil not only by adding n-alkane as a solvent but also happens by increasing temperature, which may be the cause of fouling of crude oil (6) Chang, F.; Sarthi, P.; Jones, R. Modeling of Asphaltene and Wax Precipitation; U.S. Department of Energy, Jan 1991; Topical Report NIPER 498. (7) ASTM D-4740, Standard Test Method for Cleanliness and Compatibility of Residual Fuel by Spot Test, Annual Book of ASTM Standards, 2002; Vol. 5.2. (8) Heithaus, J. J. Measurement and Significance of Asphaltene Peptization. J. Inst. Pet. 1962, 48, 45. (9) Östlund, J. A.; Russel, T.; Walker, S.; Hakansson, R.; Greek, L.; Richards, G. Evalution of a Novel Method to Study Oil Stability (Internet). (10) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J. X.; Gill, B. S. Asphaltene Precipitation and Solvent Properties of Crude Oils. Pet. Sci. Technol. 1998, 16, 251.
10.1021/ef700536z CCC: $40.75 2008 American Chemical Society Published on Web 12/19/2007
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Table 1. Specification of Crude Oil Samples specification
crude oil A
crude oil B
crude oil C
API kinematics viscosity, cSt, at 40 °C asphaltene content, wt % resin content, wt % saturated content, wt % aromatic content, wt % CII
18.9 219.7 11.6 14.4 32 42 0.77
25.8 16.28 5.5 17.9 33.7 37.9 0.84
33.5 6.37 1.0 5.0 54 39.5 1.23
heat exchangers. In this work, the onset temperature of selfprecipitation of asphaltene in relation with characteristic RI has been investigated. Increasing temperature caused self-precipitation of asphaltene to occur at RI of 1.42, which confirmed Buckley’s work.
Figure 1. Refractive index of crude oil A with n-heptane at 20 °C.
Experiment Materials and Equipment. Analytical grade heptane (42 wt % normal heptane and 58 wt % isoheptane, benzene, and toluene free) was used for preparing crude oil-heptane mixtures. A high-pressure reactor, equipped with stirrer (Parr model 4560 mini bench-top reactor), was used for heating of crude oil. A DUR refractometer equipped with a sodium lamp was used for measurement of RI according to the ASTM D-1218 test method.11 A visual analyzer, Quantimet model 570, equipped with an optical microscope was used for taking photographs of the samples. Asphaltene, resin, saturate, and aromatic contents were determined by the SARA method,12 and density was measured according to the ASTM D-5002 test method.13 Procedure. To determine crude oil RI, several mixtures of crude oil and heptane were prepared with ratio of heptane to crude of 10, 20, 35, or 40 by weight. RI of these mixtures was measured at temperatures of 20, 30, 40, and 50 °C. An optical microscope was used for detecting asphaltene precipitation. One drop of each mixture was spotted on a lamella and placed under a microscope for observation. To determine the onset temperature of self-precipitation, 150 mL of each crude oil was placed into the pressure reactor and gradually heated to elevated temperatures while the stirrer speed was set at 300 rpm. Sampling was done through a dipped pipe, after each 5-10 °C temperature increase. Immediately after sampling, a drop of heated crude oil was placed on a lamella for detecting asphaltene precipitates by an optical microscope. In addition by microscopic observation, the rheology of crude oil layer between two glass plates was investigated. Formation of separate smudge was another evidence, for showing asphaltene self-precipitation (Figure 6).
Results and Discussion Table 1 shows some specifications of three different crude oils. Asphaltene content of these crude oils are varied from low (1.0 wt %) to high (11.6 wt %). Also, CII of crude oils A, B, and C are 0.77, 0.84, and 1.23, respectively. This means that crude oil C is in its unstable form. According to Buckley’s results, RI of different ratios of crude oilA/heptane mixtures was measured at 20 °C, and a plot of RI vs wt % of crude oil was drawn. The result is shown in Figure 1. According to Figure 1, by increasing the crude oil content, refractive index of the mixture is increased and reaches 1.42 at 33 wt % of crude oil, (11) ASTM D-1218, Standard Test Method for Refractive Index and Refractive Dispersion of Hydrocarbon Liquids, Annual Book of ASTM Standards, 2002; Vol. 5.1. (12) Wauquier, J. P. Crude Oil Petroleum Products, Process Flowsheets; Institut Francais du Petrole: Paris, France; 1995; p 45. (13) ASTM D-5002, Standard Test Methode for Density and Relative Density of Crude Oils by Digital Density Analayzer, Annual Book of ASTM Standards, 2002; Vol. 5.3.
Figure 2. Optical microscopic photographs of crude oil A-heptane mixture, magnitude 5000: (a) containing 10, 20, 30, and 31.5 wt % crude oil; (b) crude oil A-heptane mixture containing 33 wt % crude oil.
and hence, according to Buckley,10 precipitation of asphaltene is expected to occur. Figure 2 shows optical microscopic photographs of the crude oil A/heptane mixtures with different percentage of the crude oil. These pictures were identical for mixtures containing 10, 20, 30, and 31.5 wt % of crude oil (Figure 2a). These mixtures contain some particle with average size of 2 µm. For the mixture containing 33 wt % crude oil (Figure 2b), large dark aggregates with average size of 50 µm start to appear and the number of aggregates increases for the mixture containing 40 wt % crude oil. According to the above results, the onset of asphaltene precipitation occurs at refractive index of 1.42, which exactly matches Buckley’s results. To detect self-precipitation of asphaltene in crude oil and its relation to RI, measurement of RI at different temperature is required. On the other hand, because of the dark color of the crude oil, measuring its refractive index is almost impossible. To overcome this problem, varying concentrations of crude oil in heptane were prepared and their refractive index was measured. A function of RI (FRI), according to eq 1, was plotted against concentration of the oil and RI of crude oil was obtained by extrapolation. FRI ) (RI2 - 1) ⁄ (2 + RI2)
(1)
Figure 3a-c shows plots of FRI vs concentration (wt %) of crude oils A, B, and C at temperatures of 20, 30, and 50 °C, and the crude oil RI that were obtained by extrapolation is shown in Table 2. Guseva et al.14 found a linear relationship between RI of crude oil and temperature. This relation was found to hold for crude oils A, B, and C at low temperatures and seems to be true at high temperatures, too (Figure 4). (14) Guseva, A. N.; Leifman, I. E. Study of Contraction of Paraffin Waxes Based on Refractometric Data. Chem. Technol. Fuels Oils 1966, 2, 728.
Asphaltene Self-Precipitation in Dead Crude Oil
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Figure 3. (a) FRI of mixtures of crude oil A with n-heptane at 20, 30, and 50 °C. (b) FRI of mixtures of crude oil B with n-heptane at 20, 30, and 50 °C. (c) FRI of mixtures of crude oil C with n-heptane at 20, 30, and 50 °C.
Using the curve-fitting method, eqs 2a, 2b, and 2c were derived respectively for refractive index of crude oils A, B, and C as a function of temperature. RI(crude oil A) ) 1.5331 - 0.0006T (°C)
(2a)
RI(crude oil B) ) 1.4961 - 0.0006T (°C)
(2b)
RI(crude oil C) ) 1.4944 - 0.0006T (°C)
(2c)
These equations show that a refractive index of 1.42 is obtained for crude oils A, B, and C at temperatures of 188.5, 126.8, and 124 °C, respectively. Therefore, it can be expected that selfprecipitation of asphaltene on these crude oils starts at these temperatures. Microscopic inspection of crude oil samples shows
Table 2. Refractive Index of Crude Oils A, B, and C at Temperatures 20, 30, and 50 °C RI temp (°C)
crude oil A
crude oil B
crude oil C
20 30 50
1.5216 1.5148 1.5035
1.4834 1.4766 1.4641
1.4834 1.4767 1.4641
that, when temperature reaches 200, 130, and 115 °C for crude oils A, B, and C, respectively, a small dark spot is observed in the layer between two glass plates (Figure 5). These temperatures are somewhat higher than those predicted by the above equations for crude oils A and B. This difference could be the result of either temperature drop of crude oil flashing through
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Figure 4. RI of crude oils A, B, and C versus temperature.
Figure 5. Image of the crude oil layer between two glass plates at onset temperature of asphaltene precipitation.
sampling or low sensitivity of visual observation to detect onset of self-precipitation or both phenomena. The predicted temperature for crude oil C is a little lower than real one, which may relate to instability of crude oil C. However, the size of these spots is reduced as the temperature drops and disappears completely at temperatures below the ones predicted by the equations. These dark points are asphaltene particles with average size of less than 10 µm, which are much smaller than the particles formed in the presence of heptane, at room temperature. In addition, adsorptions of asphaltene particles on the wall of sampling glass bottle can be observed only at precipitation temperatures (Figure 6). The number of observed asphaltene particles increases when temperatures goes higher than onset temperature for crude oils A and B, which shows continuation of asphaltene precipitation. Crude oil C contains only a little asphaltene content, and nearly all of its asphaltene precipitates at the beginning of the precipitation; therefore, continuation of asphaltene precipitation was not found for this crude oil. Therefore, the above results show that self-precipitation of asphaltene occurs nearly at temperatures at which according to Buckley RI reaches 1.42. These observations suggest that onset temperature of selfprecipitation of asphaltene may be predict by measuring of crude oil refractive index at least at three different temperatures below 50 °C.
Figure 6. Image of the crude oil layer on the wall of sampling bottle: (a) before asphaltene precipitation; (b) after asphaltene precipitation.
Since adsorption of asphaltene on the tube surface of the crude oil preheaters and its subsequent oxidation to cock is the origin of fouling, and it occurs only at temperature above selfprecipitation temperature, therefore determination of this temperature can be helpful in prediction of crude oil heat exchangers fouling. Conclusions Asphaltene precipitation in dead crude oils can occur not only by adding saturated solvents such as heptane but also occurs at elevated temperatures. Self-precipitation of asphaltene in crude oils with different compositions occurs at different temperatures. This phenomenon is only dependent on the composition compatibility of the oil, and it does not depend on the asphaltene content. Because of the composition compatibility, heavy crude oils with high asphaltene content may be more stable than light ones with low asphaltene. The refractive index of crude oil is a suitable parameter for determination of asphaltene precipitation onset. Self-precipitation of asphaltene occurs at characteristic RI of 1.42. Therefore, the onset temperature of self-precipitation can be determined using a linear curve of RI vs temperature. This curve can be obtained by measuring RI at three different temperatures below 50 °C. This is a fast and easy method for determination of onset temperature of self-precipitation of asphaltene. EF700536Z
