Energy Fuels 2010, 24, 2221–2226 Published on Web 02/17/2010
: DOI:10.1021/ef900784w
Characterization of Brazilian Crude Oil Samples To Improve the Prediction of Wax Precipitation in Flow Assurance Problems† C. Martos,*,‡ B. Coto,‡ J. J. Espada,‡ M. D. Robustillo,‡ J. L. Pe~ na,§ and D. Merino-Garcia§ ‡ Department of Chemical and Energy Technology, Escuela Superior de Ciencias Experimentales y Tecnologı´a (ESCET), Universidad Rey Juan Carlos, C/Tulip an s/n, 28933 M ostoles, Madrid, Spain, and §Repsol Technology Centre, 28931 M ostoles, Madrid, Spain
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Received July 24, 2009. Revised Manuscript Received October 23, 2009
The deposition of paraffinic waxes in offshore Brazilian crude oils is a high potential problem in terms of flow assurance. These compounds precipitate when the temperature decreases, causing damage in equipment and operation. Several models have been proposed to determine the wax appearance temperature (WAT) and the wax precipitation curve (WPC) to predict potential wax deposition problems and their magnitude. However, their application is limited because of the scarcity of reliable input data. In this work, WAT and WPC were obtained by a multi-stage fractional precipitation for three Brazilian crude oils. The wax content at -20 °C was obtained using a modification of Burger’s method. Precipitated solid at each temperature was characterized by means of proton nuclear magnetic resonance (1H NMR) to determine the amount of entrapped crude oil. The n-paraffin distribution for the selected crude oils was determined by high-temperature gas chromatography (HTGC), and it was used to predict wax precipitation with a rigorous thermodynamic model. Discrepancies between experimental and predicted results were found, probably related to the presence of non-paraffinic compounds in the studied crude oils. DSC results of some precipitated fractions showed signals that may support this hypothesis. Gas chromatography-mass spectrometry (GC-MS) analysis was used to determine qualitatively the nature of such compounds, but no conclusive results were obtained. literature to describe wax precipitation.5-10 However, the application of these models is limited because of the scarcity of reliable experimental data on wax precipitation. The main variables in wax precipitation are the wax appearance temperature (WAT) and the wax precipitation curve (WPC). Different experimental methods have been reported to precipitate waxes from crude oils,11-15 although many of them present limitations, such as the use of solvents that can modify the liquid-solid equilibrium during the precipitation. To overcome this limitation, an experimental method based on a multi-stage precipitation process without dilution of the crude oil allowing for both WAT and WPC determination has been previously reported.16 The crude oil composition is crucial to obtain reliable predictions for wax deposition because the WAT is strongly influenced by the paraffin distribution in crude oils. Waxes present in crude oils are mainly formed by linear and branched alkanes (C20þ) and represent the major risk to produce wax
1. Introduction The increasing demand of fossil fuel resources has led to the Brazilian government and many oil companies from all over the world to invest a lot of money for new offshore exploration. This leads to a greater impact of paraffin deposition problems because of the low water temperature that the crude oil has to face during transportation. The accumulation of these solids on the walls can cause problems in pipelines and equipment and even stop production.1-4 Such problems are well-known within the petroleum industry, and a big research effort is being made to develop procedures to anticipate wax deposition problems. Precipitation modeling requires a detailed characterization of the raw crude oil and the compounds present in the solid phase but also a good knowledge of the liquid-solid equilibrium and the fluid dynamics involved in the deposition process. Different thermodynamic models have been proposed in the
(8) Hong-Yan, J.; Bahman, T.; Danesh, A.; Todd, A. Fluid Phase Equilib. 2004, 216, 201–217. (9) Edmonds, B.; Moorwood, T.; Szczepanski, R.; Zhan, X. Energy Fuels 2008, 22, 729–741. (10) Ghanaei, E.; Esmaeilzadeh, F.; Kaljahi, J. F. Fluid Phase Equilib. 2007, 254, 126–137. (11) Burger, E. D.; Perkins, T. K.; Striegler, J. H. J. Pet. Technol. 1981, 3, 1075–1086. (12) UOP Method 46-85, Paraffin wax content of petroleum oils and asphalts. UOP Methods, UOP, Inc., Des Plaines, IL, 1985. (13) Musser, B. J.; Kilpatrick, P. K. Energy Fuels 1997, 12, 715–725. (14) Handoo, J.; Gupta, A. K.; Agrawal, K. M. Pet. Sci. Technol. 1997, 15, 347–356. (15) Nermen, H. M.; Magdy, T. Z. Pet. Sci. Technol. 2004, 11-12, 15553–15569. (16) Coto, B.; Martos, M. C.; Pe~ na, J. L.; Espada, J. J.; Robustillo, M. D. Fuel 2008, 87, 2090–2094.
†
Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed. Telephone: 34-914888123. Fax: 34-91-4887068. E-mail:
[email protected]. (1) Coutinho, J. A. P. Ind. Eng. Chem. Res. 1998, 37, 4870–4875. (2) Ronninngsen, H. P.; Bjorndal, B.; Hansen, A. B; Pedersen, W. B. Energy Fuels 1991, 5, 895–908. (3) Pauly, J. P; Daridon, J. L.; Coutinho, J. A. P. Fluid Phase Equilib. 1998, 149, 191–207. (4) Pauly, J. P; Daridon, J. L.; Coutinho, J. A. P. Fluid Phase Equilib. 2004, 224, 237–244. (5) Won, K. W. Fluid Phase Equilib. 1989, 53, 377–396. (6) Lira-Galeana, C.; Firoozabadi, A.; Prausnitz, J. M. AIChE J. 1996, 42, 239–248. (7) Coutinho, J. A. P.; Ruffier-Meray, V. Ind. Eng. Chem. Res. 1997, 36, 4977–4983. r 2010 American Chemical Society
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: DOI:10.1021/ef900784w
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by decreasing the system temperature around 3-5 °C for each step. The sample of crude oil is not diluted during filtration with any solvent to eliminate its effect on the WAT. This procedure allows obtaining the WAT and the amount of solid precipitated at each temperature (i.e., the WPC). However, as stated before, the results obtained by this method cannot be directly compared to those predicted with the model, because it is necessary to determine the amount of trapped crude oil in each fraction. Wax Content. The amount of paraffin waxes precipitated at -20 °C was determined following a modification16 of the method reported by Burger et al.11 The crude oil is dissolved in n-pentane and stirred during 30 min. Acetone (acetone/n-pentane ratio of 3:1) is added to the mixture and cooled to 253 K for 24 h. The solid phase present in the oil is separated by filtration in a Buchner funnel using a glass microfibre Whatman Filter No. 934. The solid phase is redissolved in n-hexane to remove asphaltenes. After solvent removal, the final product is weighted. 1 H NMR Spectroscopy. A Varian Mercury Plus NMR spectrometer (C/H dual 5 mm probe, frequency of 400 MHz) was used to quantify the different types of hydrogen atoms. Samples were dissolved in deuterated chloroform in 5 mm sample tubes. The number of scans was 64, with a 30° pulse and a 1 s delay time between scans. The trapped crude oil of the samples was also determined following a procedure previously reported,22 which is based on the supposition that wax contains a negligible amount of aromatic hydrogen atoms (Har); therefore, they are only due to the presence of entrapped oil. When the amount of aromatic hydrogen atoms in each precipitated fraction are compared to that in the raw crude oil, it is possible to obtain an estimation of the percent of the entrapped crude oil or wax porosity, considering the paraffin solid previously separated at each temperature during the fractional precipitation process. Besides, this technique was used to determine the degree of branching of the mixtures. DSC. Samples were analyzed with a DSC Mettler-Toledo DSC822e. First, the sample is heated from 20 to 80 °C at 3 °C/ min. Then, the sample is cooled from 80 to -120 °C at 3 °C/min, and finally, the sample is heated from -120 to 80 °C at the same rate. This technique is usually used to experimentally quantify the amount of wax precipitation in crude oils.24,25 The difference between the DSC curve and the baseline is a direct measurement of the total heat involved in the phase change, which is converted into the corresponding mass using a calibration from pure n-paraffins. The baseline is determined by assuming that the sample behaves as a usual liquid at temperatures above the WAT and as a solid-like at very low temperatures and that the two-phase system behavior can be obtained by imposing the continuity of the heat capacity and first derivative. DSC measured heat is converted into mass through the following equation:17
deposition problems because of their low solubility in the oil phase.17 For that reason, the composition of the crude oil in terms of n-paraffin distribution is required as input for modeling, although some evidence suggest that other compounds can precipitate simultaneously. Experimental gas chromatographic (GC) techniques are commonly used to determine n-paraffin distribution of crude oils.18-20 However, the results can be inaccurate when very heavy n-paraffins are present in the crude oil. Likewise, the n-paraffin distribution can be obtained from the true boiling point (TBP) curve of the crude oil using correlations and distribution functions, such as that proposed by Riazi et al.21 As reported in the literature, a high amount of trapped oil remains within the precipitated solid.22 Consequently, the knowledge of this variable is also required to correctly quantify the amount of precipitated solids. Martos et al.22 developed a method to determine the trapped crude oil in precipitated mixtures based on proton nuclear magnetic resonance (1H NMR) spectroscopy. In this work, the WAT and the WPC were obtained by a multi-stage fractional precipitation procedure for three Brazilian crude oils. The wax content at -20 °C was obtained using a modification of Burger’s method.11 The trapped crude oil of the precipitated mixtures at each temperature was determined by the 1H NMR technique. Precipitated solid at each temperature was characterized by means of DSC and 1H NMR. Modeling of wax precipitation was performed using a model developed by Coutinho et al.,23 included in the commercial software Multiflash. The n-paraffin distribution of the crude oils was required as input information and determined by high-temperature gas chromatography (HTGC). The comparison between experimental and calculated values showed deviations and may be related to the presence of non-paraffinic compounds in the precipitated solids. DSC analysis for some precipitated fractions suggests the presence of non-paraffinic compounds in some of the studied crude oils. These fractions were analyzed by gas chromatographymass spectrometry (GC-MS) to qualitatively determine the presence of these compounds, although the obtained results cannot definitively confirm this hypothesis. 2. Experimental and Computational Section Crude Oils. Three naphthenic water-free dead crude oils from Brazil provided by Repsol (BR-1, BR-2, and BR-3) were used in this work. Fractional Precipitation. The experimental method is based on a filtration process at controlled temperature and has been previously reported in the literature.16 A sample of crude oil is cooled in a cryostat at a slightly higher temperature than its WAT (previously determined by DSC analysis) for 24 h. The sample is then filtered using a glass microfiber Whatman filter 934 for at least 2 h. The solid phase is washed down with acetone to reduce the trapped crude oil and then is recovered by solution in dichloromethane. This procedure can be repeated 4 or 5 times
Δm Hi ¼ cMi Tm, i
ð1Þ
where, for a given n-alkane i, ΔmHi is the molar melting heat (in J mol-1), Mi is the molecular weight (in g mol-1), and Tm,i is the melting temperature (in K). The value for parameter c (c = 0.6111) was taken from the literature.17 Equation 1 yields a precipitated mass distribution against temperature, whose cumulative sum represents the WPC obtained from the integration of the DSC thermogram. HTGC. A Varian CP-3800 gas chromatograph, equipped with a Chrompack-WCOT Ultimetal column (10 m long,
(17) Elsharkawy, A. M.; Al-Sahhaf, T. A.; Fahim, M. A. Fuel 2000, 79, 1047–1055. (18) Nermen, H. M.; Magdy, T. Z. Pet. Sci. Technol. 2005, 23, 483– 493. (19) Jayalakshmi, V.; Selvavathi, V.; Sekar, M. S.; Sairam, B. Pet. Sci. Technol. 1999, 17, 843–856. (20) Khan, H. U.; Sharma, R. L.; Nautiyal, S. P.; Agrawal, K. M.; Schmidth, P. Pet. Sci. Technol. 2000, 18, 889–899. (21) Riazi, M. Ind. Eng. Chem. Res. 1997, 36, 4299–4307. (22) Martos, M. C.; Coto, B.; Espada, J. J.; Robustillo, M. D.; G omez, S.; Pe~ na, J. L. Energy Fuels 2008, 22, 708–714. (23) Coutinho, J. A. P.; Edmonds, B.; Moorwood, T.; Szczepanski, R.; Zhang, X. Energy Fuels 2006, 20, 1081–1088.
(24) Coutinho, J. A. P.; Ruffier-Meray, V. Ind. Eng. Chem. Res. 1997, 36, 4977–4983. (25) Coutinho, J. A. P.; Ruffier-Meray, V. Fluid Phase Equilib. 1998, 148, 147–160.
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Martos et al. Table 2. 1H NMR Results for the Crude Oils and the Precipitated Mixtures
Table 1. Wax Precipitation Results for the Selected Crude Oils
Figure 1. Results of experimental wax precipitation (Total amount of solid against temperature) for the three crude oils. Δ BR-1; 0 BR-2; O BR-3. Full black symbols correspond to the wax content at -20°C and full gray symbols correspond to the linear extrapolated values for the WAT.
0.53 mm internal diameter, and 0.17 μm thick stationary phase) was used in this work. The detection was carried out by a flame ionization detector (FID). The injector and the column oven were heated from 45 to 425 °C at 16 °C/min and maintained at 425 °C for 20 min. The detector was set at 430 °C during all of the analysis. The calibration of the equipment was carried out using an n-paraffin mixture within the range C5-C60. The integration of the chromatogram yields the n-paraffin distribution from the crude oils. GC-MS. A Varian CP-3800 gas chromatograph equipped with a factorFour VF-1 ms capillary column (30 m long, 0.25 mm internal diameter, and stationary phase of 1% diphenyl/ 95% dimethylpolysiloxane co-polymer, with a film thickness of 0.25 μm). The detection was carried out by a Varian 1200 series mass detector using electronic impact at 70 eV. The injection port was set at a temperature of 350 °C. The GC oven temperature is set at 40 °C for 5 min, and then the temperature is increased at 5 °C/min up to 300 °C and finally held for 5 min. The helium carrier gas flow was 1 mL/min. Commercial Software. The Multiflash commercial software developed by Infochem Computer Services, Ltd., which includes Coutinho’s thermodynamic wax model, was used to predict wax precipitation (WAT and WPC). The required input information for the calculation tool was the n-paraffin distribution and the thermal properties
Figure 2. DSC cooling thermogram for the three crude oils: (- - -) BR-1, (;) BR-2, and ( 3 3 3 ) BR-3.
Figure 3. WPC obtained by DSC for the three crude oils: (- - -) BR-1, (;) BR-2, and ( 3 3 3 ) BR-3.
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the WPC values, as indicated by Coto et al. The absolute average deviations of fractional precipitation measurements were (0.3 wt %. As can be seen, in Figure 1, the wax content at -20 °C was in all cases clearly larger than that obtained at the lowest temperature by fractional precipitation, thus indicating that a wide range of temperatures (between -20 °C and WAT) is needed to precipitate all paraffins. Therefore, a wide distribution of n-paraffins could be expected for the studied crude oils. In order to obtain the true WPC for the three crude oils the trapped crude oil must be calculated. 1 H NMR Spectroscopy. The content of the different types of hydrogen atoms was obtained by 1H NMR spectroscopy. Table 2 shows the amount of Har, HR, Hβ, and Hγ in weight percent for the crude oils studied, their precipitated fractions, and the wax precipitated at -20 °C. The CH2/CH3 ratio (Hβ/Hγ) and the calculated entrapped crude oil for each precipitated mixture are also listed. The amount of occluded crude oil in the precipitated solids was obtained by 1H NMR, following the method previously reported by Martos et al.,22 which assumes that all aromatic hydrogen atoms come from the crude oil entrapped in the samples. As shown in Table 2, the content of crude oil in the precipitated mixtures was found within the range of 80-99 wt % of total precipitated mixture in all cases. However, entrapped crude oil values of the samples precipitated at -20 °C were clearly lower, probably because of the use of solvents during the precipitation process, as reported
of pure n-paraffins (normal melting point, enthalpy of fusion, etc.)
3. Results and Discussion Wax Precipitation. The wax content at -20 °C, WPC (expressed as wt % of the raw crude oil), and WAT for the selected crude oils are shown in Table 1 and plotted in Figure 1. WAT values were determined by linear extrapolation from
Figure 4. n-Paraffin distribution obtained by HTGC for the three crude oils: (black bar) BR-1, (light gray bar) BR-2, and (gray bar) BR-3.
Figure 5. WPC of the crude oils studied in this work obtained by (9) fractional precipitation, (- - -) DSC, and (;) thermodynamic model for (a) crude oil BR-1, (b) crude oil BR-2, and (c) crude oil BR-3.
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elsewhere. When samples precipitated at -20 °C from different crude oils are compared, BR-3 showed the lowest content of trapped crude oil. This can be related to the different structure of the precipitated compounds, although the obtained results must be considered with caution because the relative error in the determination of trapped crude oil by NMR is about (30%, as reported by Martos et al.22 The ratio between Hβ (CH2) and Hγ (CH3) provides information about the degree of branching of the compounds in the different samples. Obtained results are summarized in Table 2. According to these results, it is not possible to establish a relationship between the branching degree and the WAT for the different mixtures. This can be due to the high amount of entrapped crude oil that makes the 1 H NMR technique insensitive to check these differences. DSC. The crude oils considered in this work were analyzed by DSC using the experimental procedure previously described. The WAT and the WPC were determined for the three crude oils. Figure 2 shows the cooling thermograms obtained by DSC for the three crude oils. Reasonable agreement was found between WAT determined by DSC, 18.9 °C for crude oil BR-2 and 17.4 °C for crude oil BR-3, and that extrapolated from experimental WPC, 24.0 °C for crude oil BR-2 and 21.8 °C for crude BR-3. Nevertheless, a significant difference was observed for crude oil BR-1; the DSC WAT was 18.6 °C against 26.7 °C obtained from the experimental WPC. This discrepancy could be related to the different structure of the precipitated compounds and the very low content of paraffin in this crude oil. WAT values extrapolated from WPC should be considered with caution because WPC is not strictly linear; thus, DSC values were more accurate. Figure 3 shows the WPC obtained by DSC for the crude oils considered in this work. DSC-obtained results for crude oils BR-1 and BR-3 were very similar, but crude oil BR-2 showed a higher content of precipitated solid. This result is in good agreement with that obtained for the WAT. n-Paraffin Distribution. Figure 4 shows the n-paraffin distribution (in wt % for each carbon number) obtained by HTGC for the selected crude oils. All crude oils exhibited a broad n-paraffin distribution in the range between C10 and C60. The distributions are similar, showing the maximum content around C17 in all cases. However, crude oil BR-2 showed the highest content of n-paraffin, which is in good agreement with the results obtained for WPC and WAT from DSC analysis. However, because the HTGC in the high carbon atom region yielded extremely low values, the n-paraffin distribution was completed by extrapolation in this region. To apply the model, n-paraffin distribution must be corrected to match up the total amount of paraffin obtained by DSC. Model Prediction. In this work, Multiflash commercial software was used to predict WPC for the selected crude oils from the n-paraffin distribution previously determined. Figure 5 shows the comparison between the experimental WPC obtained by fractional precipitation corrected by 1H NMR to discount the trapped crude oil, the WPC obtained by DSC, and the WPC predicted by the thermodynamic model. Reasonable agreement between the experimental and calculated WPC was obtained for crude oil BR-2. However, discrepancies were found for crude oils BR-1 and BR-3. Two possibilities were considered to explain these deviations: the uncertainties in the n-paraffin distribution and/or the extrapolation procedure used for a high number of carbon atoms 22
Figure 6. DSC cooling thermograms for crude oil BR-3 and their precipitated fractions.
Figure 7. GC-MS spectra for the fraction precipitated at 20 °C from crude oil BR-3: (a) paraffins (m/z 85), (b) hopanes (m/z 191), and (c) esteranes (m/z 217).
and the presence of non-paraffinic compounds in the precipitated solids. To check these possibilities, further characterization of the precipitated fractions obtained from crude oil BR-3 was carried out. Figure 6 shows the thermograms of crude oil BR-3 and all of their precipitated fractions. As can be seen, the fraction precipitated at the highest temperature (20 °C) shows a double peak, which is not present in the rest of the fractions. This signal can be attributed to the presence of high-molecularweight paraffins or non-paraffinic compounds (hopanes and esteranes) that can precipitate when the temperature is decreased. Despite the low content of these compounds in most crude oils, sometimes a significant amount can be 2225
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found regarding the origin of the crude. These compounds show a complex molecular structure, but they can be detected by GC-MS analysis, because they show characteristic signals.26 In this work, GC-MS spectra at m/z ratios of 85 (paraffins), 191 (hopanes), and 217 (esteranes) for the fraction precipitated at 20 °C were acquired and shown in Figure 7. The obtained results showed a very low intensity for hopanes and esteranes. Consequently, GC-MS results are not conclusive because the amount of these compounds detected by GC-MS was not high enough to support the remarkable double peak obtained by DSC analysis. Therefore, a more likely explanation should be the presence of very heavy paraffins in the precipitated solids. However, such a conclusion cannot considered definitive because it is very difficult to check that point experimentally.
observed, showing similar behavior for all crude oils. The trapped crude oil in the precipitated fractions of all of the crude oils was determined by 1H NMR spectroscopy, obtaining high values in all cases (>80 wt %). DSC analysis was used to determine the WAT and the WPC. The studied crude oils show a very low content of paraffins, as determined by this experimental technique. For that, despite high WAT values, the amount of precipitated paraffin when decreasing the temperature is low. Discrepancies between WPC obtained by DSC analysis and that experimentally determined were found probably because of the very low content of paraffins in these crude oils. WPC was also obtained using the Multiflash commercial software, with the n-paraffin distribution determined by HTGC analysis. Differences between the predicted WPC and that obtained experimentally were found. These differences could be explained by the presence of non-paraffinic compounds or, more likely, according to the obtained results, by the presence of high-molecular-weight paraffins, very difficult to be determined by the available experimental techniques.
4. Conclusions The WPC of three Brazilian crude oils was determined by fractional precipitation and the total amount of wax by precipitation at -20 °C. No remarkable differences were
Acknowledgment. The authors thank Repsol for the crude oil supply and the financial support through the research project “Aseguramiento de flujo de crudos de petr oleo: Estudio de la precipitaci on de parafinas”.
(26) Wei, Z.; Moldowan, J. M.; Peters, K. E.; Wang, Y.; Xiang, W. Org. Geochem. 2007, 38, 1910–1926.
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