1294
Energy & Fuels 2009, 23, 1294–1298
Assessment of a Thermodynamic Model To Describe Wax Precipitation in Flow Assurance Problems† B. Coto,*,‡ C. Martos,‡ J. J. Espada,‡ M. D. Robustillo,‡ J. L. Pen˜a,§ and S. Go´mez§ Department of Chemical and EnVironmental Technology, ESCET, UniVersidad Rey Juan Carlos, C/Tulipa´n s/n, Mo´stoles, Madrid 28933, Spain, and Alfonso Cortina Technology Centre, REPSOL-YPF, S.A., Mo´stoles, Madrid 28931, Spain ReceiVed July 31, 2008. ReVised Manuscript ReceiVed September 16, 2008
One of the most important problems related to flow assurance is the deposition of paraffinic waxes present in crude oils. These compounds may precipitate when temperature decreases during oil production, transport through pipelines or storage, causing problems in flowlines and equipment that could in the most severe cases stop crude oil production. 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 experimental information, such as detailed crude oil characterization, the experimental WPC, and the wax composition. In this work, experimental WAT and WPC for three crude oils with differences in chemical structure were determined through a multistage fractional precipitation procedure. The trapped crude oil of the precipitated mixtures at each temperature was determined by a 1H nuclear magnetic resonance (NMR) technique, so that the true amount of wax precipitated at each temperature was obtained. Differential scanning calorimetry was also used to determine WAT and WPC. The n-paraffin distribution for the selected crude oils was determined by gas chromatography (GC) techniques, and it was used to predict wax precipitation with a rigorous thermodynamic model available in commercial software. Predictive capabilities of the considered model were tested by comparing the experimental and predicted values of WAT and WPC, obtaining reasonable agreement.
1. Introduction The prediction of the wax appearance temperature (WAT) and the wax precipitation curve (WPC) of crude oils is critical to avoid or minimize flow assurance problems related to wax deposition in flowlines or storage tanks. The availability of these parameters may anticipate and quantify potential wax deposition problems, allowing us to focus on the appropriate solution to mitigate production problems or modifying the design of flowlines and surface facilities in developing projects. The modeling of the precipitation process requires a good knowledge of the liquid-solid equilibrium involved. Different thermodynamic models have been proposed in the literature to describe wax precipitation.1-4 However, the research in this field is still ongoing5-7 because of the impact in the economics of projects for new oil fields in extreme conditions (especially deepwater and arctic production). The main limitation in the † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. Telephone: 34-914887089. Fax: 34-91-4887068. E-mail:
[email protected]. ‡ Universidad Rey Juan Carlos. § Alfonso Cortina Technology Centre. (1) Won, K. W. Fluid Phase Equilib. 1989, 53, 377–396. (2) Lira-Galeana, C.; Firoozabadi, A.; Prausnitz, J. M. AIChE J. 1996, 42, 239–248. (3) Coutinho, J. A. P.; Ruffier-Meray, V. Ind. Eng. Chem. Res. 1997, 36, 4977–4983. (4) Hong-Yan, J.; Bahman, T.; Danesh, A.; Todd, A. Fluid Phase Equilib. 2004, 216, 201–217. (5) Edmonds, B.; Moorwood, T.; Szczepanski, R.; Zhan, X. Energy Fuels 2008, 22, 729–741. (6) Ghanaei, E.; Esmaeilzadeh, F.; Kaljahi, J. F. Fluid Phase Equilib. 2007, 254, 126–137. (7) Dalirsefat, R.; Feyzi, F. Fuel 2007, 86, 1402–1408.
validation of these existing models is the scarcity of reliable experimental data in the literature about crude oil characterization focused in the wax deposition process. Different experimental methods have been reported to precipitate waxes from crude oils,8,9 although many of them present limitations, such as the use of solvents that can modify the liquid-solid equilibrium during the precipitation. Recently, an experimental method based on a multistage precipitation process without dilution of the crude oil has been developed, allowing the WAT and WPC determination.10 The availability of the crude oil composition is critical to obtain reliable predictions for wax deposition. The prediction of WAT is strongly influenced by the wax composition in crude oils. Waxes present in crude oils are mainly formed by linear and branched alkanes (C20-C60) and represent the major risks to produce wax deposition problems because of their low solubility in the oil phase.11 Consequently, the composition of the crude oils in terms of n-paraffin distribution is required to apply commercial thermodynamic models, because most of them use this input information. Different experimental techniques such as gas chromatography (GC) and differential scanning calorimetry (DSC) have been reported to determine n-paraffin (8) Burger, E. D.; Perkins, T. K.; Striegler, J. H. J. Pet. Technol. 1981, 3, 1075–1086. (9) Universal Oil Products (UOP) Method 46-85. Paraffin wax content of petroleum oils and asphalts. UOP Methods, UOP, Inc., Des Plaines, IL, 1985. (10) Coto, B.; Martos, M. C.; Pen˜a, J. L.; Espada, J. J.; Robustillo, M. D. Fuel 2008, 87, 2090–2094. (11) Elsharkawy, A. M.; Al-Sahhaf, T. A.; Fahim, M. A. Fuel 2000, 79, 1047–1055.
10.1021/ef800621x CCC: $40.75 2009 American Chemical Society Published on Web 12/02/2008
Wax Precipitation in Flow Assurance Problems
distribution of crude oils.12,13 Likewise, it can also 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.14 A high amount of trapped oil remains within the precipitated solid as reported elsewhere.15 The determination of this crude oil has a great influence on the correct quantification of the precipitated waxes. Although there are not well-established methods for their quantification, recently, Martos et al.15 determined the trapped crude oil in precipitated mixtures using 1H NMR spectroscopy. Therefore, not only the crude oil composition but also the further characterization of the precipitated solids is needed to accurately predict wax deposition. In this work, a thermodynamic model developed by Coutinho et al.16 included in the commercial software Multiflash was used to predict the wax precipitation. The thermodynamic model can describe wax as a solid solution using Wilson (wax is considered as a single solid solution) or UNIQUAC (modeling the tendency of waxes to split into several separate solid solution phases). This model requires the n-paraffin distribution of the studied petroleum mixture, and it is applicable to both live and dead oils, providing good predictions of WAT and WPC. The aim of this work is to check the predictive capabilities of the model developed by Coutinho et al. with reliable experimental information. 2. Experimental and Computational Section Crude Oils. Three crude oils from Africa with different chemical structure (crude oils B3 and C1 are paraffinic, while crude oil C3 is naphthenic) were used. The crude oils were provided by RepsolYPF. Fractional Precipitation. As previously reported,10 the method is based on a filtration process at controlled temperature. A sample of crude oil is cooled in a cryostat at a slightly higher temperature than its wax appearance temperature (WAT) for 24 h. The crude oil is then filtered using a glass microfiber Whatman filter no. 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 by decreasing the system temperature around 3-5 K each step. The sample of crude oil is not diluted with any solvent to eliminate the effect on the WAT. This procedure allows us to obtain the WAT and the amount of solid precipitated at each temperature, i.e., the wax precipitation curve or WPC. However, as stated before, the results obtained from this method cannot be directly compared to those predicted with the model, but it is necessary to determine the amount of trapped crude oil for each fraction. Total Wax Precipitation. Total wax precipitation was carried out following a modification10 of the method reported by Burger.8 This procedure provides the amount of paraffin waxes precipitated at 253 K, which we use as the total wax content of the crude oil. 1H NMR. A Bruker DRX 500 NMR spectrometer (C/H dual 5 mm probe, frequency of 500 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. This technique was used to determine the aromatic content and degree of branching of the samples following a procedure previously reported,15 which is based on the assumption that wax contains a (12) Coto, B.; Martos, M. C.; Espada, J. J.; Robustillo, M. D.; Pen˜a, J. L. Fluid Phase Equilib. 2008, manuscript submited. (13) Pen˜a, J. L. Repsol YPF. Private comunication. (14) Riazi, M. Ind. Eng. Chem. Res. 1997, 36, 4299–4307. (15) Martos, M. C.; Coto, B.; Espada, J. J.; Robustillo, M. D.; Go´mez, S.; Pen˜a, J. L. Energy Fuels 2008, 22, 708–714. (16) Coutinho, J. A. P.; Edmonds, B.; Moorwood, T.; Szczepanski, R; Zhang, X. Energy Fuels 2006, 20, 1081–1088.
Energy & Fuels, Vol. 23, 2009 1295 negligible amount of aromatic hydrogen atoms; therefore, the Har atoms are only due to the presence of trapped oil. When the amount of aromatic hydrogen atoms in each precipitated fraction is compared to that in the raw crude oil, it is possible to obtain an estimation of the percent of the trapped crude oil or wax porosity. DSC. This technique is usually used to carry out the experimental quantification of wax precipitation in crude oils.2,17,18 In this work, samples were analyzed with a DSC Mettler-Toledo DSC822e. Experimental DSC conditions for crude oil samples were optimized in a previous work.12 First, the sample is heated from 298 to 353 K/min, at 3 K/min. Second, the sample is cooled from 353 to 153 K/min, at 3 K/min, and finally, the sample is heated from 153 to 353 K/min, at 3 K/min. The procedure assumes that the DSC sample behaves as a usual liquid at temperatures above the WAT and as a solid-like sample at temperatures under a very low temperature, under which no further solidifications take place. The heat capacity of the twophase system was obtained by imposing that heat capacity and the first derivative is continuous at both the lowest temperature and the WAT. 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. The cumulative precipitated mass distribution against temperature represents the wax precipitation curve obtained from the integration of the DSC thermogram. The integration process yields the WAT and WPC. GC. The equipment used was an Agilent Technologies 7890-A gas chromatograph equipped with a HP-5MS column, 30 m long and 0.25 mm internal diameter. The detection was carried out by a flame ionization detector (FID). The integration process yields the n-paraffin distribution from the crude oils. Commercial Software. In this work, the commercial software Multiflash, developed by Infochem Computer Services, Ltd., was used to predict wax precipitation. Coutinho’s thermodynamic wax model was used to predict the WAT and amount of wax that comes out of solution. The increasing amount of wax precipitated when the temperature decreases below the WAT was expressed as the WPC. The required information for the calculation process is the n-paraffin distribution and the thermal properties (normal melting point, enthalpy of fusion, etc.). The composition of the wax phase is determined by the known solution behavior in both oil and wax phases. The most common procedure to obtain the n-paraffin distribution is from the PVT analysis (black oil or compositional) combined to the total wax content obtained by the standard Universal Oil Products (UOP) method. In this work, this n-paraffin distribution was obtained by the integration of the GC results.
3. Results and Discussion Wax Precipitation. Total wax content at 253 K, WPC, and WAT for the selected crude oils are shown in Table 1 and plotted in Figure 1. The WPC was obtained using the experimental method previously developed.10 WAT values were determined by linear extrapolation of the WPC values as indicated by Coto et al.10 As can be seen in Figure 1, different results were obtained for the three crude oils regarding their different chemical composition. Thus, for crude oil B-3 (highly paraffinic), the wax content at 253 K was clearly superior to that obtained at the lowest temperature, indicating that a wide range of temperature (between 253 K and WAT) is needed to precipitate all paraffins. Therefore, a wider distribution of n-paraffins could be expected. However, for both C-1 (paraffinic) and C-3 (naphthenic) crude (17) Coutinho, J. A. P.; Ruffier-Me´ray, V. Ind. Eng. Chem. Res. 1997, 36, 4977–4983. (18) Coutinho, J. A. P.; Ruffier-Me´ray, V. Fluid Phase Equilib. 1998, 148, 147–160.
1296 Energy & Fuels, Vol. 23, 2009
Coto et al.
Table 1. Wax Precipitation Results for the Selected Crude Oils fractional precipitation crude oil
wax content at 253 K (wt %)
T (K)
B-3
35.24
333 323 313 308
C-1
23.63
303 298 293 290
C-3
293 278 273 270.5 268 265.5 263
7.57
precipitated solid (wt %)
WAT (K)
2.52 4.47 8.40 21.92
336
3.77 7.26 11.04 23.09
301
0.61 2.51 3.33 4.78 5.21 6.56 7.10
286
oil, narrower distributions of n-paraffins were expected because their WPC showed a narrower range of precipitation temperature.
Figure 1. Results of experimental wax precipitation (total amount of solid against temperature): (4) Crude oil B-3, (0) crude oil C-1, and (O) crude oil C-3. Full black symbols correspond to the total amount of paraffin, and open gray symbols correspond to the linear extrapolated values for the WAT.
In this work, the mixtures precipitated at each temperature were analyzed by means of 1H NMR spectroscopy to obtain the wax porosity15 and determine the real experimental precipitation curve for the three crude oils. 1H NMR Spectroscopy. The content of the different type 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 and their fractions. The CH2/CH3 ratio (Hβ/Hγ) and the calculated trapped crude oil for each precipitated mixture is also listed. Such an amount of occluded crude oil in the precipitated solids was obtained by 1H NMR, following the method previously reported by Martos et al.15 This method assumes that all aromatic hydrogen atoms come from the occluded crude oil trapped in the samples. As it is shown in Table 2, the content of crude oil in the precipitated mixtures was found within the range of 80-99 wt % for the three crude oils. However, trapped crude oil values of the samples precipitated at 253 K were clearly lower. It could indicate a decrease of the amount of trapped crude oil in the precipitated fractions when solvents are used, and the precipitation temperature decreases as reported elsewhere.15 When samples from the different crude oils were compared at 253 K, the naphthenic crude oil (C-3) exhibited a higher content of trapped crude oil than those obtained for the paraffinic crude oils. This can be related to the different structure of the precipitated compounds. These results are in agreement with those obtained for similar crude oils in a previous work.15 The obtained results must be considered with caution as reported by Martos et al.15 because of the relative error in the determination of trapped crude oil by NMR of about (30%. The ratio between Hβ (CH2) and Hγ (CH3) provides information about the degree of branching of the compounds in the different samples. Obtained results were summarized in Table 2. According to these results, it is not possible to determine a relationship between the variation of the branching degree and the precipitation temperature for the different mixtures. This limitation could be related to the presence of trapped crude oil. DSC. Selected crude oils were analyzed by DSC using the experimental procedure previously described. The WAT and WPC were determined for the three crude oils.
Table 2. 1H NMR Results for the Crude Oils and the Precipitated Mixtures sample crude oil B-3 fraction 1 fraction 2 fraction 3 fraction 4 total wax 5 crude oil C-1 fraction 6 fraction 7 fraction 8 fraction 9 total wax 10 crude oil C-3 fraction 11 fraction 12 fraction 13 fraction 14 fraction 15 fraction 16 fraction 17 total wax 18
Hγ (wt %)
Hβ/Hγ
trapped crude oil (wt %)
Crude Oil B-3 65.95 60.76 67.91 66.80 66.88 73.60
26.99 21.11 23.63 24.40 25.01 21.85
2.44 2.88 2.87 2.74 2.67 3.37
90.40 91.25 98.07 90.89 62.13
10.90 10.04 9.98 23.93 9.57 8.00
Crude Oil C-1 56.64 60.72 60.66 54.20 60.68 66.57
28.53 25.36 25.73 18.67 25.94 22.52
1.99 2.39 2.36 2.90 2.34 2.96
98.94 92.36 81.25 96.91 73.81
9.56 9.19 10.93 9.77 14.62 10.73 9.16 10.03 24.22
Crude Oil C-3 51.78 61.74 55.83 60.58 57.91 62.41 60.11 60.97 55.05
34.22 25.13 29.17 25.68 23.61 22.85 26.69 25.18 17.34
1.51 2.46 1.91 2.36 2.45 2.73 2.25 2.42 3.17
88.91 91.79 89.66 87.15 90.46 91.04 86.30 76.41
Har (wt %)
HR (wt %)
333 323 313 308 253
2.66 2.41 2.43 2.61 2.42 1.65
4.40 15.73 6.03 6.19 5.69 2.89
303 298 293 290 253
3.93 3.89 3.63 3.19 3.81 2.90
293 278 273 270.5 268 265.5 263 253
4.43 3.94 4.07 3.98 3.86 4.01 4.04 3.83 3.39
T (K)
Hβ (wt %)
Wax Precipitation in Flow Assurance Problems
Figure 2. DSC thermogram for the three crude oils: B-3 (- - -), C-1 (s), and C-3 ( · · · ).
Figure 3. Wax precipitation curve obtained by DSC for the three crude oils: B-3 (- - -), C-1 (s), and C-3 ( · · · ).
Figure 2 shows the cooling thermograms obtained by DSC for the three crude oils. The WAT value determined by DSC (303.01 K) was very similar to that extrapolated from WPC (301 K) for crude C-1. Likewise, reasonable agreement was found between the WAT obtained by DSC (281.92 K) and that obtained by extrapolation (286 K) for the crude oil C-3 (naphthenic). Nevertheless, a significant difference is appreciated for the crude oil B-3 (highly paraffinic) because a WAT of 322 K was obtained by DSC analysis; meanwhile, the WAT extrapolated was 336 K. This discrepancy could be related to the different structure of the precipitated compounds. DSC analysis also allows for the WPC to be obtained following a method recently developed.12 Figure 3 shows the WPC obtained by DSC for the crude oils considered in this work. DSC-obtained results confirmed those obtained by fractional precipitation, showing that crude oils exhibited different behavior regarding their chemical composition. Thus, the differences found in the slope of the obtained WPC by DSC were in good agreement with those obtained by fractional precipitation. As can be observed, a wider range of temperature was required to precipitate all paraffins for crude oil B-3 than those for crude oils C-1 and C-3, which is in concordance with the WPC obtained by fractional precipitation. n-Paraffin Distribution. As described above, n-paraffin distribution is necessary for models to predict the WPC. Figure
Energy & Fuels, Vol. 23, 2009 1297
Figure 4. n-Paraffin distribution for the three crude oils obtained by GC: (white bars) crude oil B-3, (gray bars) crude oil C-1, and (black bars) crude oil C-3.
4 shows the n-paraffin distribution obtained by GC for the selected crude oils. Crude oil B-3 exhibited a broad n-paraffin distribution and a very high content of n-paraffin mainly in the range between C5 and C22. Crude oil C-1 showed a very similar distribution but with a lower content of n-paraffins. On the opposite end, the naphthenic crude oil C-3 presented an asymmetric distribution, with the maximum content around C6 and a continuous decrease for higher compounds. As a consequence of the distribution shape, n-paraffin distribution for both crude oils B-3 and C-1 was centered at C14 (the point which represents 50% of the integrated distribution), whereas the crude oil C-3 was centered at C10. This is in agreement with the fact that, the heavier the paraffins, the easier it is for them to precipitate. Model Prediction. In this work, Wilson equation was used to predict WAT and WPC. The commercial software allows WPC to be obtained until 273 K. Predicted WAT values for the crude oils C-1 and C-3 (301 and 288 K, respectively) were in agreement with those values obtained by extrapolation of WPC (301 and 286 K for the crude oils C-1 and C-3, respectively). However, higher differences were observed between the WAT values predicted by the model (324 K) and those extrapolated from WPC (336 K) for the crude oil B-3. Nevertheless, the values of the WAT predicted by the model were in good agreement with those yielded by DSC analysis for the paraffinic crude oils (B-3 and C-1). Figure 5 shows the comparison between the corrected WPC obtained by fractional precipitation, the WPC obtained by DSC, and the WPC predicted by the thermodynamic model. The correction of the fractional precipitation WPC was carried out by considering the content of trapped crude oil calculated by 1H NMR. An estimation of the experimental uncertainty is also displayed through the corresponding error bars for the WPC obtained by fractional precipitation. Obtained results were similar for the three different methods and the three crude oils studied. Even when some deviations between experimental corrected values and DSC- and modelcalculated values were observed, the agreement between the obtained results can be considered satisfactory within the experimental uncertainty. Consequently, the available thermodynamic model is accurate enough to predict wax precipitation curves and the correspond-
1298 Energy & Fuels, Vol. 23, 2009
Coto et al.
Figure 5. Wax precipitation curve of the crude oils studied in this work obtained by fractional precipitation (9), DSC (- - -), and thermodynamic model (s). (a) Crude oil B-3, (b) crude oil C-1, and (c) crude oil C-3.
ing WAT values. That agreement was confirmed for three different crude oils using two different experimental methods. 4. Conclusions The WPC of three crude oils was determined by fractional precipitation, and the total wax amount, by precipitation at 253 K. Very different values were found for the WAT values, the WPC, and the total wax amount, showing the different behavior of the three selected crude oils. The trapped crude oil in the precipitated solid was determined from the 1H NMR characterization, and high values were found (around 90% in weight percent) for all the solid fractions. The WPC and the WAT were also determined by DSC for the same crude oils. WPC obtained by precipitation with the correction due to entrapped crude oil is in agreement with that obtained by DSC within the experimental uncertainty. Because both techniques are quite different, the agreement is a consistency test of the obtained results. The wax precipitation for the selected crude oils was simulated using the Multiflash commercial software with the Wilson model, and an n-paraffin distribution was determined
by gas chromatography. Reasonable agreement was found between predicted and experimental values for the WAT and for the WPC in the short range, where results from the three methods are available. Consequently, such a model can be used to predict the wax precipitation behavior of crude oils of different nature from the n-paraffin distribution. Thereafter, the wax precipitation for the selected crude oils was simulated using the Multiflash commercial software, with the n-paraffin distribution previously determined by GC. Reasonable agreement was found between predicted and experimental values for the WAT of paraffinic crude oils. Likewise, experimental and calculated WPC compared favorably within the experimental uncertainty in all cases. Acknowledgment. The authors thank Repsol-YPF for the crude oil supply, the use of experimental facilities, and financial support through the research project “Aseguramiento de flujo de crudos de petro´leo: Estudio de la precipitacio´n de parafinas”. EF800621X
