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Flow loop apparatus to study the effect of solvent, temperature and additives on asphaltene precipitation
Journal of Petroleum Science and Engineering 23 Ž1999. 133–143 www.elsevier.comrlocaterjpetscieng
Flow loop apparatus to study the effect of solvent, temperature and additives on asphaltene precipitation Subodhsen Peramanu ) , Patrick F. Clarke, Barry B. Pruden Department of Chemical and Petroleum Engineering, UniÕersity of Calgary, Calgary, Alberta, Canada T2N 1N4 Received 29 December 1998; accepted 17 May 1999
Abstract A detection technique was developed for the onset of asphaltene precipitation based on the measurement of pressure drop across an in-line filter. An in-line filter of a nominal size of 60 mm was used to capture the precipitate and thus detect the onset, which was indicated by a dramatic increase in the pressure drop across the filter. Experimental runs required only 4 h to complete and the apparatus is capable of testing opaque samples at high temperatures and pressures. The effects of solvent type and temperature on the onset of precipitation were investigated for Athabasca and Cold Lake bitumens. For both bitumens, the qualitative behavior of the onsets with solvent type Žcarbon number. and temperature were similar, displaying an increase in the onsets at lower carbon number and temperature values and decrease at the higher values. To identify the effectiveness of resin type compounds to dissolve asphaltenes, 20 g of various aromatic, hydrogen donor, heteroatom and surfactant compounds were added to 100 g samples of Athabasca and Cold Lake bitumens. It was found that aromatic and hydrogen donor compounds increased the onset ratio by a slight to moderate degree, heteroatom compounds increased the onset ratio by a moderate degree, whereas surfactants increased the onset ratio by the highest degree. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Asphaltene precipitation; Precipitation onset; Precipitation detection; Precipitation inhibition; Thermal effects; Concentration effects
1. Introduction Asphaltene flocculation and precipitation is a common problem in both upstream and downstream petroleum operations. Asphaltenes are high molecular weight brown to black solids consisting of a complex mixture of heavy crude constituents. Asphaltenes are usually defined as that fraction which is soluble in toluene and insoluble in n-pentane or )
Corresponding author. Tel.: q1-403-250-4740; fax: q1-403250-0633; E-mail: [email protected]
n-heptane at a dilution ratio of 40 volumes of solvent per volume of petroleum sample ŽSpeight et al., 1984.. In production, refining, and upgrading processes there are instances where a black solid deposit of asphaltenes forms due to changes in temperature, pressure, andror composition. Asphaltene precipitation is unwanted during production, transportation, and upgrading processes involving hydrogen addition such as hydrocracking and hydrotreating. Asphaltene precipitation however, is desired in upgrading processes involving carbon rejection such as coking and solvent deasphalting. For both, the situa-
0920-4105r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 4 1 0 5 Ž 9 9 . 0 0 0 1 2 - 1
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tions were flocculation is avoided and sought, the point at which the asphaltenes flocculate and precipitate is valuable information. Recent studies ŽAndersen, 1992; Clarke and Pruden, 1996. have shown that the process of asphaltene flocculationrprecipitation is not completely reversible. This indicates that asphaltenes in petroleum are not solely in the dissolved state. In addition, an extensively wide range of asphaltene size distributions suggests that asphaltenes may be partly dissolved and partly in the colloidal state ŽKawanaka et al., 1989; Leontaritis and Mansoori, 1987. as shown in Fig. 1. Asphaltene molecules in the dissolved state do not have surrounding resin molecules, whereas the asphaltene molecules in the colloidal state are surrounded by resin molecules as shown. Pfeiffer and Saal Ž1940. first indicated that asphaltenes are colloidally suspended in oils or bitumens. Since this first report, many authors reported on their findings of asphaltene colloid size and shape. Small-angle neutron ŽSANS. and small-angle X-ray scattering ŽSAXS. are two popular techniques that have been used to demonstrate that the colloids could be various possible shapes Žspherical, disk, and cylindrical. with a radius of gyration in the size ˚ A small sample of range of approximately 5 to 70 A. the papers that report asphaltene dimensions using SAXS andror SANS are: Sheu et al. Ž1992., Carnahan et al. Ž1993. and Sahimi et al. Ž1997.. Galtsev et al. Ž1995. used electron-nuclear double resonance ŽENDOR. spectroscopy to provide additional evidence that asphaltene micelles form condensed aro-
Fig. 1. Asphaltenes in dissolved and colloidal forms in diluted bitumen.
matic sheets with a characteristic radius of up to 50 ˚ It is evident from the SANS and SAXS studies A. that asphaltenes in an undisturbed state are less than ˚ and once they start to flocculate they do so 100 A very quickly and grow to a size of approximately 100 mm or more. Thus, the flocculation is accompanied by an increase in size by four orders of magnitude. Numerous methods are available for determining the point at which asphaltene flocculationrprecipitation starts. These methods include filtration followed by gravimetric analysis, visual observation by microscopy, the absorbance of an electromagnetic beam, particle size analysis, and the measurement of properties such as viscosity and electrical conductivity. By determining the weight percent of asphaltenes collected on filter paper at different normal alkane to bitumen ratios, many authors have generated a plot of weight percent precipitated vs. the solvent to bitumen ratio. The data are then extrapolated to the dilution ratio for when no solids would be collected, which is the onset. This technique has been used by numerous authors such as Kokal et al. Ž1992., Leontaritis et al. Ž1994. and Rassamdana et al. Ž1996.. Leontaritis et al. Ž1994. used 0.1, 0.45, and 1.0 mm membrane filters. This method is difficult to perform at low solvent to bitumen ratios since heavy oils and bitumens are very viscous, and therefore it is difficult to obtain an accurate weight measurement for the precipitate. Unfortunately, the low solvent to bitumen ratio conditions are where such accurate measurements are needed the most. Visual observations of asphaltene size using microscopes are also used to obtain onsets. Heithaus Ž1962. used a microscope of 400 times magnification and Hirschberg et al. Ž1984. used a magnification of 200 times. Methods based on the absorbance andror scattering of electromagnetic radiation are also widely used as detailed by de Boer et al. Ž1995. and Jamaluddin et al. Ž1996.. Using a He–Ne laser, Thomas et al. Ž1992. determined the onset from a plot of photodiode voltage vs. mole fraction of alkane solvent added. Ferworn et al. Ž1993. used a laser particle size analysis method to determine the size of flocculated asphaltenes. Unfortunately, the minimum solvent to bitumen ratio of a specimen that could be analyzed was five, thus, determination of the onset of flocculation was not possible. However, they
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demonstrated that the asphaltene flocs for Cold Lake bitumen formed using n-heptane as solvent were 196, 241 and 286 mm in diameter for solvent to bitumen ratios of 5, 20, and 40 to 1, respectively. They also found that the asphaltene micelles formed micron size flocs within 1 min, thus, demonstrating that the flocculation process occurs in the order of seconds. Recently, an asphaltene flocculationrprecipitation detection technique using viscosity measurements was presented by Escobedo and Mansoori Ž1995.. Plots of viscosity vs. weight fraction of solvent added were generated for both asphaltene precipitating solvents Ž n-pentane, n-heptane, and n-nonane. and dissolving solvents such as toluene. The plots for both dissolving and precipitating solvent were plotted on the same chart, and a small but abrupt deviation in the viscosity curve was evident at the point of asphaltene flocculation. An asphaltene precipitation detection technique based on the electrical conductivity of a crude oil and precipitating solvent sample has been presented by Fotland et al. Ž1993.. Results were interpreted to give the onset of precipitation and the weight fraction of asphaltenes precipitated. From the above discussion, it is evident that there are various techniques that can potentially be used to determine the onset of asphaltene flocculationrprecipitation. However, with the exception of a few, these techniques are difficult to implement when the sample is bitumen either due to its opacity or due to high viscosity. In addition, some methods require that numerous individual samples be made, that are time consuming to conduct. Clarke and Pruden Ž1996, 1997, 1998. developed a technique using heat transfer analysis in which asphaltene precipitation was identified by monitoring temperature profiles at the bottom of a vessel where the heat transfer resistance increased due to the formation of a precipitate layer. Most runs were performed using Cold Lake bitumen and a few for Athabasca bitumen. By testing various chemical additives they showed that phenanthrene, a tri-aromatic compound, and nonylphenol, a surfactant compound, were most effective in dissolving asphaltenes. Although the method of Clarke and Pruden was successfully applied for asphaltene precipitation from bitumens, it had shortcomings. The method required that a cycle be repeated over and over, and for each
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cycle an injection was performed and the contents stirred thoroughly followed by at least 3 h for the solids to settle. The experimental runs were therefore very long, up to 40 h, and for some runs the lower layer temperature fluctuated and the onset determination was difficult. In the present paper, a method for detecting the onset of asphaltene flocculation is described that utilizes the fact that the flocculation process occurs very quickly and that the flocs quickly grow to a size of 100 mm or more. The technique is a flow loop method that uses an in-line filter to capture asphaltene flocs as precipitant is slowly and continuously added to bitumen. An increase in pressure drop across a filter was used to determine the point of asphaltene precipitation. Each experimental run took less than 4 h to complete and the onset was easily identified.
2. Experimental 2.1. Apparatus The experimental apparatus consisted of a 750-ml vessel with provisions for stirring and heating as shown in Fig. 2. Stirring was accomplished using a magnetic drive mixer ŽDyna-Mag, Pressure Products Industries, Warminster, PA. and heating using a clamp heater wired to a temperature controller ŽOmron, Kyoto, Japan.. The vessel contents were circulated through a sintered stainless steel in-line filter ŽSwagelok, Solon, OH. of nominal pore size 60 mm using a metering pump ŽBran-Luebbe, Buffalo Grove, IL.. The 60-mm filter had pore size distribution ranging from 50 to 75 mm. Other smaller size filters tested caused high pressure drop and blockage of the filter due to the high viscosity of bitumens. Pressure drop across the filter was measured by a differential pressure transmitter ŽRosemount, Eden Prairie, MN.. A bypass valve was installed across the in-line filter to aid in the smooth start-up of experiments. The bitumen sample was blanketed with nitrogen and a pressure relief valve was installed on the vessel for safety. The vessel and tubing were insulated to minimize heat loss. Thermocouples were located in the vessel and near the in-line filter to ensure isothermal operation. A small injection pump
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Fig. 2. Schematic diagram of the flow loop apparatus.
ŽMilton Roy, Ivyland, PA. continuously dispensed the solvent from either reservoir into the vessel. The vessel temperature, in-line filter temperature, vessel pressure, and pressure drop signal across the in-line filter were recorded using a data acquisition board ŽKeithley, Taunton, MA. and stored in a personal computer. 2.2. Procedure For each experiment, 100 g of bitumen was thoroughly mixed with 25 ml of solvent and placed in the vessel. Next, the contents were blanketed with nitrogen and then heated to the desired temperature. Once the bitumen sample reached the desired temperature, a valve underneath the vessel was opened, the circulating pump was turned on and the sample was circulated at approximately 200 mlrmin. Ini-
tially, the flow was through a fully opened bypass valve mounted across the in-line filter. The bypass valve was then closed slowly until all of the sample was passing through the in-line filter. The operation was fully automated once the bypass valve was closed. It was essential to use a new in-line filter for each run since it was impossible to clean the used filters thoroughly. After the bitumen was circulating through the flow loop for about 15 min, solvent was injected into the vessel continuously at a rate of 2 mlrmin. As the solvent was added, the viscosity of the sample decreased and the pressure drop across the filter dropped slowly. When the addition reached a certain amount, the pressure drop across the in-line filter started increasing as the asphaltenes were captured in the filter. The solvent to bitumen ratio at which the pressure drop just started to increase indicated the onset of asphaltene precipitation.
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Each run took about 4 h to complete and ultimately had about 5 volumes of solvent added to the original bitumen. After each run, the liquid contents were drained from the apparatus using a nitrogen purge and the in-line filter was removed. Solids stuck to the vessel were scraped and removed. The apparatus was then cleaned by flowing toluene through the loop for about 1 h.
Table 2 Properties of solvents and additives used for experiments
SolÕents n-Heptane n-Octane n-Decane n-Dodecane
100.205 114.232 142.286 170.340
684 703 730 748
15.1 15.6 15.8 16.2
3. Materials
AdditiÕes Aromatics Benzene Toluene m-Xylene 1,2,4-TMB Naphthalene Žsolid. Phenantherene Žsolid.
78.114 92.141 106.168 120.195 128.174 178.234
885 867 864 880 – –
18.8 18.2 18.0 17.6 20.3 20.1
Hydrogen donors Tetralin Decalin
132.206 138.254
973 896
19.4 18.0
Heteroatoms Pyridine Quinoline Indole Žsolid. Benzothiophene Žsolid. Creosol
79.102 129.162 117.152 134.202 138.168
983 1095 – – 1092
21.9 22.1 23.2 21.7 22.8
Surfactants Nonylphenol DDBSA
220.357 326.504
937 1060
19.4 22.6
3.1. Bitumen samples Athabasca and Cold Lake bitumen samples were tested; the properties of which are given in Table 1. Athabasca bitumen was denser than Cold Lake bitumen due to the slightly higher percentage of resins and asphaltenes, and lower percentage of saturates. Peramanu et al. Ž1999. reported the molecular weight and specific gravity distributions for these bitumens and their SARA ŽSaturates, Aromatics, Resins and Asphaltenes. fractions which are essential for bitumen characterization, computation of thermodynamic properties and studies of phase equilibria. 3.2. SolÕents and additiÕes The solvents n-heptane, n-octane, n-decane and n-dodecane were tested to identify the effect of Table 1 Properties of Athabasca and Cold Lake bitumens
API gravity Viscosity ŽPa s. at 248C Saturates Žwt.%. Aromatics Žwt.%. Resins Žwt.%. Asphaltenes Žwt.%. Resinsrasphaltenes Žwt.rwt.. Colloidal Index ŽCI. Carbon Žwt.%. Hydrogen Žwt.%. Sulfur Žwt.%. Oxygen Žwt.%. Nitrogen Žwt.%. Other Žwt.%. Total heteroatoms Žwt.%.
Athabasca
Cold Lake
8.05 323 17.27 39.70 25.75 17.28 1.49 1.89 83.34 10.26 4.64 1.08 0.53 0.15 6.4
10.71 65 20.74 39.20 24.81 15.25 1.63 1.78 83.62 10.50 4.56 0.86 0.45 0.01 5.9
Molecular weight Žkgrkmol.
Liquid density Žkgrm3 .
Solubility parameter ŽJrcm3 . 0.5
carbon number, and thereby the effect of solvent solubility parameter, on the onset values. The properties of these solvents listed in Table 2 show that the solubility parameter increases with increasing solvent carbon number. Knowledge of the chemical characteristics of the asphaltene and resin fractions is an essential factor in the choice of chemical additives. Asphaltenes contain the highest molecular weight species, the most aromatic and typically has the largest weight percentage of heteroatoms. The oil fraction contains the lowest molecular weight species, is of low aromaticity and has very few heteroatoms. Resins act as a transition between the asphaltenes and oil and have intermediate molecular weight, aromaticity, and heteroatom content. Due to the molecular complexity of the resins and asphaltenes, and difficulties in their
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isolation, determination of their individual chemical species is not possible at present. Instead, detection of the key functional groups is used to understand their important chemical features. Identification of N, S and O heteroatom functional groups were also used to select possible additives. Four types of additives were considered for their ability to shift the asphaltene onset to higher solvent to bitumen ratios. The types of additives tested were: aromatics, hydrogen donors, heteroatom compounds, and surfactants. The additives given in Table 2 are similar to those used by Clarke and Pruden Ž1997, 1998. but include more aromatic compounds. Aromatic compounds such as benzene, toluene, 1,2,4-trimethylbenzene, m-xylene, naphthalene and phenanthrene were used as additives since it is well documented ŽSpeight, 1991. that aromatic compounds dissolve asphaltenes. For compounds containing heteroatoms, the nitrogen compounds pyridine, quinoline and indole were tested, since infrared spectroscopic data ŽSpeight, 1991. has indicated that nitrogen can be found in resins in the form of pyrroles or indoles. A sulfur containing compound, benzothiophene, was tested because it has been identified ŽSpeight, 1982. that Athabasca asphaltenes contain this form of sulfur. This identification was carried out from observations of the rate of desulfurization of S-containing polymers with Raney nickel. Acetylation of the resins, combined with detailed infrared spectroscopic examination ŽSpeight, 1991. has established the presence of oxygen as ester functions, acid functions as well as carbonyl Žketone or quinone. functions. Therefore, the additive, 2-methoxy-4-methylphenol was chosen since it contains oxygen in both a phenolic hydroxyl group and a methoxy group. The hydrogen donor solvents tetralin and decalin were also tested, since it is well known that these solvents provide hydrogen to suppress coke formation in residue upgrading and coal liquefaction ŽCarlson et al., 1958.. In the formation of coke, asphaltenes polymerize, possibly after flocculation, to form mesophase and then coke. It was demonstrated that ŽClarke and Pruden, 1997. these donor solvents inhibit asphaltene flocculation by increasing the oil’s ability to solubilize asphaltenes. Asphaltenes are the most polar fraction in a bitumen or crude, and flocculated asphaltenes possess a
positive charge ŽLichaa and Herrera, 1975.. The resin fraction on the other hand is less polar than the asphaltenes. Solely based on the difference in the polarity of these two fractions, and the importance of the resins in dispersing the asphaltenes in the oil fraction, one would expect that resins have surface active properties ŽMcLean and Kilpatrick, 1997.. Therefore, it follows that surfactants should be able to inhibit asphaltene precipitation. Chang and Fogler Ž1993. demonstrated that the nonionic surfactant nonylphenol and anionic surfactant n-dodecylbenzene sulfonic acid ŽDDBSA. were effective in dissolving asphaltenes. Clarke and Pruden Ž1998. using their heat transfer apparatus confirmed that nonylphenol was effective in dissolving asphaltenes. However, contrary to Chang and Fogler’s findings, Clarke and Pruden Ž1998. observed that dodecylbenzene sulfonic acid was a poor asphaltene inhibitor. Therefore, in this work, both nonylphenol and dodecylbenzene sulfonic acid were tested for their effectiveness in dissolving asphaltenes.
4. Results and discussion When an asphaltene dissolving solvent Žtoluene. is added to bitumen the viscosity of the mixture decreases. This results in a decrease in the pressure drop across the in-line filter for a constant circulation rate as shown in Fig. 3. If an asphaltene precipitating solvent Ž n-heptane. is added, the pressure drop decreases at the beginning since the mixture viscosity decreases. When asphaltenes start to precipitate, the solids accumulate on the in-line filter and thereby increase the pressure drop as shown in Fig. 4. Therefore, the onset of precipitation is indicated by the minima of the pressure drop profile, the point where the pressure drop just starts to increase. Since it was difficult to identify the minima directly from the pressure drop profile, the following procedure was adopted. First, the data was confined to a range that bracketed the minimum point. A fourth order curve was then fitted through the data, and the minimum was determined by differentiating the fourth order equation and equating it to zero. For the runs at high temperatures, it was important to keep the vessel pressures well above the bubble point pressure of the solvent since the objec-
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asphaltene precipitation. Above the bubble point pressure, the increase in pressure decreases the asphaltene precipitation but the effect is minor. The decreased asphaltene precipitation with pressure above the bubble point is due to the increased solubility parameter of the solvent favoring asphaltene dissolution. The result of these effects is that asphaltene deposition is more severe at the bubble point pressure. Therefore, to avoid the system reaching the bubble point, higher starting pressures were used for the high temperature runs. Since the vessel pressure was not controlled, initially the vessel pressure increased due to heating from room temperature to a desired temperature and later, the pressure increased exponentially due to solvent addition. For a run starting at 180 kPa, the final pressure reached as high as 520 kPa. A typical pressure profile is given by Fig. 5. Fig. 3. Typical experimental pressure drop curve for asphaltene dissolving solvent Žtoluene..
tive was to maintain the system in the liquid phase. For a system below its bubble point pressure, the increase in pressure increases the dissolution of the solvent vapor in the sample resulting into increased
Fig. 4. Typical experimental pressure drop curve for asphaltene precipitating solvent Ž n-heptane..
4.1. SolÕent runs Asphaltene onsets were measured for both Athabasca and Cold Lake bitumens with n-heptane, n-octane, n-decane, and n-dodecane solvents at 808C. Fig. 6 gives the measured onsets solvent carbon
Fig. 5. Typical curve for vessel pressure variation with solvent to bitumen ratio for an experimental run.
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Fig. 6. Effect of solvent type on the precipitation onset for Athabasca and Cold Lake bitumens at 808C.
number for Athabasca and Cold Lake bitumens. The onsets for Athabasca bitumen were in the range of 1.35–1.61 g solventrg bitumen and the onsets for Cold Lake bitumen ranged from 1.17 to 1.46 g solventrg bitumen. The higher values of onsets for Athabasca bitumen than Cold Lake bitumen indicate that asphaltene stability cannot be predicted from the weight ratio of resins to asphaltenes. If the stability of the asphaltenes is indicated by resin to asphaltenes ratio ŽTable 1., then Athabasca bitumen would be less stable since the ratio for Athabasca bitumen is 1.49 and for Cold Lake bitumen it is 1.63. Athabasca bitumen’s lower resins to asphaltene ratio would suggest that it has a slightly greater tendency for precipitation, and it would be expected that less solvent would be needed to initiate the asphaltene precipitation. However, the present results suggest that stability is rather governed by the colloidal index ŽLoeber et al., 1998. and weight percent of heteroatoms. The colloidal index is defined as the ratio of dispersed constituents Žaromaticsq resins. to the flocculated constituents Žsaturatesq asphaltenes.. A higher colloidal index means that the asphaltenes are more peptized by the resins in the oil base medium, and higher heteroatom content means the
asphaltenes are more stable since heteroatoms are largely responsible for the strong bond between resins and asphaltenes. Due to the higher colloidal index and higher amount of heteroatoms in Athabasca bitumen ŽTable 1., it is more stable and therefore more solvent is required to cause asphaltene precipitation. The solvent to bitumen ratio at onset increases slightly as the carbon number increases ŽFig. 6.. The onset reaches a maximum value at approximately n-C 10 and then decreases with an increase in carbon number. One possible reason for this maximum in the profile is that solution phenomenon plays a major role at low carbon numbers and colloidal phenomenon plays a major role at high carbon numbers. According to solution theory, when a paraffin solvent is added to bitumen the thermodynamic equilibrium shifts due to differences in solubility parameters of the asphaltenes and that of the remaining solution. Since the solubility parameter increases with an increase in carbon number, the asphaltenes will be more soluble at higher carbon numbers and therefore result into higher onset ratios. Although resins are more soluble than asphaltenes because of their solubility parameters being closer to solvent solubility parameter, this does not apply to resins surrounding the asphaltene molecules ŽFig. 1. due to strong bonding between resin and asphaltene molecules Žcolloidal state.. However, as the carbon number increases, the solubility parameter of the solvent approaches resin’s solubility parameter and the attractive forces between solvent and resin molecules are strong enough to disturb the adsorption equilibrium between resin and asphaltenes. The asphaltene particles become free and flocculate irreversibly resulting in decreased solubility, and hence lower onset ratios. 4.2. Temperature runs Temperature runs were performed with n-heptane solvent from 60 to 1208C. The results are given in Fig. 7 for Athabasca and Cold Lake bitumens. Although pure n-heptane boils at 98.458C at 1 atm, boiling did not occur at the experimental conditions since n-heptane was mixed with bitumen and the vessel pressures were always higher than the bubble point pressures. Fig. 7 indicates that the onset goes through a maximum and this behavior is similar to
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the onset variation with carbon numbers as presented in Fig. 6. Again, this behavior is expected when solution phenomenon is dominant at low temperatures and colloidal phenomenon is dominant at higher temperatures. At low temperatures, according to solution theory, when the temperature increases asphaltenes would be more soluble, and hence the precipitation onset increases. However, according to the colloidal theory, dissolution seems to occur between resin and asphaltenes ŽFig. 1. at high temperatures and this counteracts the increased solubility. 4.3. AdditiÕe runs Runs were performed with the additive amount of 20 g in 100 g of bitumen Ž16.7 wt.%. with n-heptane solvent at 808C. The results are plotted against additive solubility parameter in Figs. 8 and 9 for Athabasca and Cold Lake bitumens, respectively. Most of the solubility parameters were obtained from Hansen and Beerbower Ž1971. and Grulke Ž1989., however, for the four compounds indole, creosol, benzothiophene and DDBSA, the solubility parameters were calculated using the group molar attraction constants of Small Ž1953. and Hoy Ž1970.. The broken horizontal line in the figures indicates the
Fig. 7. Effect of temperature on the precipitation onset for Athabasca and Cold Lake bitumens with n-heptane solvent.
Fig. 8. Effect of additive on the precipitation onset for Athabasca bitumen with n-heptane solvent at 808C ŽAdditive amounts 20 gr100 g bitumen..
onset when no additive was used. For Athabasca bitumen with no additive, the onset was 1.35 g solventrg bitumen and for Cold Lake bitumen the onset was 1.17 g solventrg bitumen. All the aromatics, hydrogen donors, heteroatoms and surfactants dissolve asphaltenes and therefore result in a higher solvent to bitumen ratio for the precipitation onset. The random placement of onset points in the figures indicates that there is no direct correlation for the onset and the additive’s solubility parameter. For Athabasca bitumen, the increase in the onset ratio by adding single ring aromatic compounds and hydrogen donors was less than for Cold Lake bitumen. However, the increase in the onset ratio caused by adding double and multiple ring aromatics Žnaphthalene and phenanthrene., heteroatom compounds, and surfactants, were almost the same for both bitumens. In general, for both bitumens, aromatic and hydrogen donor compounds caused a slight to moderate increase in the onset ratio, heteroatom compounds caused a moderate increase in the onset ratio, whereas surfactants caused the greatest increase in
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through a maximum giving a dome shaped curve. This is due to solution phenomenon playing a major role at low carbon numbers and temperatures, and colloidal phenomenon playing a major role at high carbon numbers and temperatures. Aromatic and hydrogen donor additives increased the onset ratio by a slight to moderate degree, heteroatoms increased the onset ratio by a moderate degree, and surfactants increased the onset ratio significantly. Among the surfactants, the anionic surfactant, DDBSA, was more effective than the nonionic surfactant, nonylphenol.
Acknowledgements
Fig. 9. Effect of additive on the precipitation onset for Cold Lake bitumen with n-heptane solvent at 808C ŽAdditive amounts 20 gr100 g bitumen..
the onset ratio. The surfactants were most effective since they posses an alkyl tail which makes this portion of the molecule lyophilic Žoil ‘‘loving’’., and an aromatic core with a head group which makes this portion compatible with the polar and highly aromatic asphaltenes. DDBSA was more effective than nonylphenol because of the anionic head group which has a stronger affinity towards asphaltene molecules than the hydroxyl head group.
5. Conclusions Asphaltene precipitation detection using a pressure drop technique in a flow loop apparatus was fast, reasonably accurate, and the method could handle opaque solvents at high temperatures and pressures. At 808C, the onset of precipitation of asphaltenes from Athabasca and Cold Lake bitumens occurred at 1.35 and 1.17 n-heptane to bitumen weight ratio, respectively. When plotted against carbon number and temperature, the onset ratios went
The authors would like to acknowledge the support of all the agencies and industrial sponsors of the University of Calgary’s Industrial Hydrogen Chair. Also, the authors are grateful to Mr. Dan Wong and Mr. Chandresh Singh for performing the experimental work, and Dr. Harvey Yarranton for his useful comments.
References Andersen, S.I., 1992. Hysteresis in precipitation and dissolution of petroleum asphaltenes. Fuel Sci. Technol. Int. 10, 1743–1749. Carlson, C.S., Langer, A.W., Stewart, J., Hill, R.M., 1958. Thermal hydrogenation: transfer of hydrogen from tetralin to cracked residua. Ind. Eng. Chem. 50, 1067–1070. Carnahan, N.F., Quintero, L., Pfund, D.M., Fulton, J.L., Smith, R.D., Capel, M., Leontaritis, K., 1993. A small-angle X-ray scattering study of the effect of pressure on the aggregation of asphaltene fractions in petroleum fluids under near-critical solvent conditions. Langmuir 9 Ž8., 2035–2044. Chang, C.L., Fogler, H.S., 1993. Asphaltene stabilization in alkyl solvents using oil-soluble amphiphiles. SPE Int. Symp. Oilfield Chem., SPE 25185, 339–349. Clarke, P.F., Pruden, B.B., 1996. Heat transfer analysis for detection of asphaltene precipitation and resuspension. 47th Annual Technical Meeting of The Petroleum Society, Calgary, June 10–12, Paper 96-112. Clarke, P.F., Pruden, B.B., 1997. Asphaltene precipitation: detection using heat transfer analysis, and inhibition using chemical additives. Fuel 76 Ž7., 607–614. Clarke, P.F., Pruden, B.B., 1998. Asphaltene precipitation from Cold Lake and Athabasca bitumens. Pet. Sci. Technol. 16 Ž3–4., 287–305.
S. Peramanu et al.r Journal of Petroleum Science and Engineering 23 (1999) 133–143 de Boer, R.B., Leerlooyer, K., Eigner, M.R.P., van Bergen, A.R.D., 1995. Screening of crude oils for asphalt precipitation: theory, practice, and the selection of inhibitors. SPE Prod. Facil. ŽFebruary. 55–61. Escobedo, J., Mansoori, G.A., 1995. Viscosity determination of the onset of asphaltene flocculation: a novel method. SPE Prod. Facil. ŽMay. 115–118. Ferworn, K.A., Mehrotra, A.K., Svrcek, W.Y., 1993. Measurement of asphaltene agglomeration from Cold Lake bitumen diluted with n-alkanes. Can. J. Chem. Eng. 71, 699–703. Fotland, P., Anfindsen, H., Fadnes, F.H., 1993. Detection of asphaltene precipitation and amounts precipitated by measurement of electrical conductivity. Fluid Phase Equilib. 82, 157– 164. Galtsev, V.E., Ametov, M., Grinberg, O.Y., 1995. Asphaltene association in crude oil as studied by ENDOR. Fuel 74 Ž5., 670–673. Grulke, E.A., 1989. Solubility Parameter Values. Polymer Handbook, 3rd edn. Wiley, New York, pp. VII519–VII559. Hansen, C.M., Beerbower, A., 1971. Solubility Parameters. Encyclopedia of Chemical Technology, Suppl. Vol., 2nd edn. Wiley, New York. Heithaus, J.J., 1962. Measurement and significance of asphaltene peptization. J. Inst. Pet. 48, 45–53, February. Hirschberg, A., deJong, L.N.J., Schipper, B.A., Meijer, J.G., 1984. Influence of temperature and pressure on asphaltene flocculation. SPE J., June, 283–289. Hoy, K.L., 1970. New values of the solubility parameters from vapor pressure data. J. Paint Technol. 42, 76–118. Jamaluddin, A.K.M., Nazarko, T.W., Sills, S., Fuhr, B.J., 1996. Deasphalted oil: a natural asphaltene solvent. SPE Prod. Facil. ŽAugust. 161–165. Kawanaka, S., Leontaritis, K.J., Park, S.J., Mansoori, G.A., 1989. Thermodynamic and colloidal models of asphaltene flocculation. Oil Field Chemistry. ACS Symposium Series No. 396, Chap. 24. American Chemical Society, Washington, DC, pp. 443–458. Kokal, S.L., Najman, J., Sayegh, S.G., George, A.E., 1992. Measurement and correlation of asphaltene precipitation from heavy oils by gas injection. J. Can. Pet. Technol. 31 Ž4., 24–30. Leontaritis, K.J., Mansoori, G.A., 1987. Asphaltene flocculation during oil production and processing: a thermodynamic col-
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loidal model. SPE Int. Symp. Oilfield Chem., San Antonio, TX, SPE 16258. Leontaritis, K.J., Amaefule, J.O., Charles, R.E., 1994. A systematic approach for the prevention and treatment of formation damage caused by asphaltene deposition. SPE Prod. Facil. ŽAugust. 157–164. Lichaa, P.M., Herrera, L., 1975. Electrical and other effects related to the formation and prevention of asphaltenes deposition. SPE 5304, 107–125. Loeber, L., Muller, G., Morel, J., Sutton, O., 1998. Bitumen in colloid science: a chemical, structural and rheological approach. Fuel 77 Ž13., 1443–1450. McLean, J.D., Kilpatrick, P.K., 1997. Effects of asphaltene solvency on stability of water-in-crude-oil emulsions. J. Colloid Interface Sci. 189, 242–253. Peramanu, S., Pruden, B.B., Rahimi, P., 1999. Molecular weight and specific gravity distributions for Athabasca and Cold Lake bitumens and their SARA fractions. Accepted for Publication in Ind. Eng. Chem. Res. Pfeiffer, J.P., Saal, R.N.J., 1940. Asphaltic bitumen as colloid system. J. Phys. Chem. 44, 139–149. Rassamdana, H., Dabir, B., Nematy, M., Farhani, M., Sahimi, M., 1996. Asphalt flocculation and deposition: the onset of precipitation. AIChE J. 42 Ž1., 10–22. Sahimi, M., Rassamdana, H., Dabir, B., 1997. Asphalt formation and precipitation: experimental studies and theoretical modeling. SPE J. 2 Ž2., 157–169. Sheu, E., Liang, K.S., Sinha, S.K., Overfield, R.E., 1992. Polydispersity analysis of asphaltene solutions in toluene. J. Colloid Interface Sci. 153 Ž2., 399–410. Small, P.A., 1953. Some factors affecting the solubility of polymers. J. Appl. Chem. 3, 71–80. Speight, J.G., 1982. The structure of petroleum asphaltenes — current concepts. Alberta Res. Counc., Inf. Ser. 81. Speight, J.G., 1991. The Chemistry and Technology of Petroleum, 2nd edn. Marcel Dekker, New York. Speight, J.G., Long, R.B., Trowbridge, T.D., 1984. Factors influencing the separation of asphaltenes from heavy petroleum feedstocks. Fuel 63, 616–620. Thomas, F.B., Bennion, D.B., Bennion, D.W., Hunter, B.E., 1992. Experimental and theoretical studies of solids precipitation from reservoir fluids. J. Can. Pet. Technol. 31 Ž1., 22–31.
Flow loop apparatus to study the effect of solvent, temperature and additives on asphaltene precipitation Subodhsen Peramanu ) , Patrick F. Clarke, Barry B. Pruden Department of Chemical and Petroleum Engineering, UniÕersity of Calgary, Calgary, Alberta, Canada T2N 1N4 Received 29 December 1998; accepted 17 May 1999
Abstract A detection technique was developed for the onset of asphaltene precipitation based on the measurement of pressure drop across an in-line filter. An in-line filter of a nominal size of 60 mm was used to capture the precipitate and thus detect the onset, which was indicated by a dramatic increase in the pressure drop across the filter. Experimental runs required only 4 h to complete and the apparatus is capable of testing opaque samples at high temperatures and pressures. The effects of solvent type and temperature on the onset of precipitation were investigated for Athabasca and Cold Lake bitumens. For both bitumens, the qualitative behavior of the onsets with solvent type Žcarbon number. and temperature were similar, displaying an increase in the onsets at lower carbon number and temperature values and decrease at the higher values. To identify the effectiveness of resin type compounds to dissolve asphaltenes, 20 g of various aromatic, hydrogen donor, heteroatom and surfactant compounds were added to 100 g samples of Athabasca and Cold Lake bitumens. It was found that aromatic and hydrogen donor compounds increased the onset ratio by a slight to moderate degree, heteroatom compounds increased the onset ratio by a moderate degree, whereas surfactants increased the onset ratio by the highest degree. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Asphaltene precipitation; Precipitation onset; Precipitation detection; Precipitation inhibition; Thermal effects; Concentration effects
1. Introduction Asphaltene flocculation and precipitation is a common problem in both upstream and downstream petroleum operations. Asphaltenes are high molecular weight brown to black solids consisting of a complex mixture of heavy crude constituents. Asphaltenes are usually defined as that fraction which is soluble in toluene and insoluble in n-pentane or )
Corresponding author. Tel.: q1-403-250-4740; fax: q1-403250-0633; E-mail: [email protected]
n-heptane at a dilution ratio of 40 volumes of solvent per volume of petroleum sample ŽSpeight et al., 1984.. In production, refining, and upgrading processes there are instances where a black solid deposit of asphaltenes forms due to changes in temperature, pressure, andror composition. Asphaltene precipitation is unwanted during production, transportation, and upgrading processes involving hydrogen addition such as hydrocracking and hydrotreating. Asphaltene precipitation however, is desired in upgrading processes involving carbon rejection such as coking and solvent deasphalting. For both, the situa-
0920-4105r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 4 1 0 5 Ž 9 9 . 0 0 0 1 2 - 1
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tions were flocculation is avoided and sought, the point at which the asphaltenes flocculate and precipitate is valuable information. Recent studies ŽAndersen, 1992; Clarke and Pruden, 1996. have shown that the process of asphaltene flocculationrprecipitation is not completely reversible. This indicates that asphaltenes in petroleum are not solely in the dissolved state. In addition, an extensively wide range of asphaltene size distributions suggests that asphaltenes may be partly dissolved and partly in the colloidal state ŽKawanaka et al., 1989; Leontaritis and Mansoori, 1987. as shown in Fig. 1. Asphaltene molecules in the dissolved state do not have surrounding resin molecules, whereas the asphaltene molecules in the colloidal state are surrounded by resin molecules as shown. Pfeiffer and Saal Ž1940. first indicated that asphaltenes are colloidally suspended in oils or bitumens. Since this first report, many authors reported on their findings of asphaltene colloid size and shape. Small-angle neutron ŽSANS. and small-angle X-ray scattering ŽSAXS. are two popular techniques that have been used to demonstrate that the colloids could be various possible shapes Žspherical, disk, and cylindrical. with a radius of gyration in the size ˚ A small sample of range of approximately 5 to 70 A. the papers that report asphaltene dimensions using SAXS andror SANS are: Sheu et al. Ž1992., Carnahan et al. Ž1993. and Sahimi et al. Ž1997.. Galtsev et al. Ž1995. used electron-nuclear double resonance ŽENDOR. spectroscopy to provide additional evidence that asphaltene micelles form condensed aro-
Fig. 1. Asphaltenes in dissolved and colloidal forms in diluted bitumen.
matic sheets with a characteristic radius of up to 50 ˚ It is evident from the SANS and SAXS studies A. that asphaltenes in an undisturbed state are less than ˚ and once they start to flocculate they do so 100 A very quickly and grow to a size of approximately 100 mm or more. Thus, the flocculation is accompanied by an increase in size by four orders of magnitude. Numerous methods are available for determining the point at which asphaltene flocculationrprecipitation starts. These methods include filtration followed by gravimetric analysis, visual observation by microscopy, the absorbance of an electromagnetic beam, particle size analysis, and the measurement of properties such as viscosity and electrical conductivity. By determining the weight percent of asphaltenes collected on filter paper at different normal alkane to bitumen ratios, many authors have generated a plot of weight percent precipitated vs. the solvent to bitumen ratio. The data are then extrapolated to the dilution ratio for when no solids would be collected, which is the onset. This technique has been used by numerous authors such as Kokal et al. Ž1992., Leontaritis et al. Ž1994. and Rassamdana et al. Ž1996.. Leontaritis et al. Ž1994. used 0.1, 0.45, and 1.0 mm membrane filters. This method is difficult to perform at low solvent to bitumen ratios since heavy oils and bitumens are very viscous, and therefore it is difficult to obtain an accurate weight measurement for the precipitate. Unfortunately, the low solvent to bitumen ratio conditions are where such accurate measurements are needed the most. Visual observations of asphaltene size using microscopes are also used to obtain onsets. Heithaus Ž1962. used a microscope of 400 times magnification and Hirschberg et al. Ž1984. used a magnification of 200 times. Methods based on the absorbance andror scattering of electromagnetic radiation are also widely used as detailed by de Boer et al. Ž1995. and Jamaluddin et al. Ž1996.. Using a He–Ne laser, Thomas et al. Ž1992. determined the onset from a plot of photodiode voltage vs. mole fraction of alkane solvent added. Ferworn et al. Ž1993. used a laser particle size analysis method to determine the size of flocculated asphaltenes. Unfortunately, the minimum solvent to bitumen ratio of a specimen that could be analyzed was five, thus, determination of the onset of flocculation was not possible. However, they
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demonstrated that the asphaltene flocs for Cold Lake bitumen formed using n-heptane as solvent were 196, 241 and 286 mm in diameter for solvent to bitumen ratios of 5, 20, and 40 to 1, respectively. They also found that the asphaltene micelles formed micron size flocs within 1 min, thus, demonstrating that the flocculation process occurs in the order of seconds. Recently, an asphaltene flocculationrprecipitation detection technique using viscosity measurements was presented by Escobedo and Mansoori Ž1995.. Plots of viscosity vs. weight fraction of solvent added were generated for both asphaltene precipitating solvents Ž n-pentane, n-heptane, and n-nonane. and dissolving solvents such as toluene. The plots for both dissolving and precipitating solvent were plotted on the same chart, and a small but abrupt deviation in the viscosity curve was evident at the point of asphaltene flocculation. An asphaltene precipitation detection technique based on the electrical conductivity of a crude oil and precipitating solvent sample has been presented by Fotland et al. Ž1993.. Results were interpreted to give the onset of precipitation and the weight fraction of asphaltenes precipitated. From the above discussion, it is evident that there are various techniques that can potentially be used to determine the onset of asphaltene flocculationrprecipitation. However, with the exception of a few, these techniques are difficult to implement when the sample is bitumen either due to its opacity or due to high viscosity. In addition, some methods require that numerous individual samples be made, that are time consuming to conduct. Clarke and Pruden Ž1996, 1997, 1998. developed a technique using heat transfer analysis in which asphaltene precipitation was identified by monitoring temperature profiles at the bottom of a vessel where the heat transfer resistance increased due to the formation of a precipitate layer. Most runs were performed using Cold Lake bitumen and a few for Athabasca bitumen. By testing various chemical additives they showed that phenanthrene, a tri-aromatic compound, and nonylphenol, a surfactant compound, were most effective in dissolving asphaltenes. Although the method of Clarke and Pruden was successfully applied for asphaltene precipitation from bitumens, it had shortcomings. The method required that a cycle be repeated over and over, and for each
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cycle an injection was performed and the contents stirred thoroughly followed by at least 3 h for the solids to settle. The experimental runs were therefore very long, up to 40 h, and for some runs the lower layer temperature fluctuated and the onset determination was difficult. In the present paper, a method for detecting the onset of asphaltene flocculation is described that utilizes the fact that the flocculation process occurs very quickly and that the flocs quickly grow to a size of 100 mm or more. The technique is a flow loop method that uses an in-line filter to capture asphaltene flocs as precipitant is slowly and continuously added to bitumen. An increase in pressure drop across a filter was used to determine the point of asphaltene precipitation. Each experimental run took less than 4 h to complete and the onset was easily identified.
2. Experimental 2.1. Apparatus The experimental apparatus consisted of a 750-ml vessel with provisions for stirring and heating as shown in Fig. 2. Stirring was accomplished using a magnetic drive mixer ŽDyna-Mag, Pressure Products Industries, Warminster, PA. and heating using a clamp heater wired to a temperature controller ŽOmron, Kyoto, Japan.. The vessel contents were circulated through a sintered stainless steel in-line filter ŽSwagelok, Solon, OH. of nominal pore size 60 mm using a metering pump ŽBran-Luebbe, Buffalo Grove, IL.. The 60-mm filter had pore size distribution ranging from 50 to 75 mm. Other smaller size filters tested caused high pressure drop and blockage of the filter due to the high viscosity of bitumens. Pressure drop across the filter was measured by a differential pressure transmitter ŽRosemount, Eden Prairie, MN.. A bypass valve was installed across the in-line filter to aid in the smooth start-up of experiments. The bitumen sample was blanketed with nitrogen and a pressure relief valve was installed on the vessel for safety. The vessel and tubing were insulated to minimize heat loss. Thermocouples were located in the vessel and near the in-line filter to ensure isothermal operation. A small injection pump
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Fig. 2. Schematic diagram of the flow loop apparatus.
ŽMilton Roy, Ivyland, PA. continuously dispensed the solvent from either reservoir into the vessel. The vessel temperature, in-line filter temperature, vessel pressure, and pressure drop signal across the in-line filter were recorded using a data acquisition board ŽKeithley, Taunton, MA. and stored in a personal computer. 2.2. Procedure For each experiment, 100 g of bitumen was thoroughly mixed with 25 ml of solvent and placed in the vessel. Next, the contents were blanketed with nitrogen and then heated to the desired temperature. Once the bitumen sample reached the desired temperature, a valve underneath the vessel was opened, the circulating pump was turned on and the sample was circulated at approximately 200 mlrmin. Ini-
tially, the flow was through a fully opened bypass valve mounted across the in-line filter. The bypass valve was then closed slowly until all of the sample was passing through the in-line filter. The operation was fully automated once the bypass valve was closed. It was essential to use a new in-line filter for each run since it was impossible to clean the used filters thoroughly. After the bitumen was circulating through the flow loop for about 15 min, solvent was injected into the vessel continuously at a rate of 2 mlrmin. As the solvent was added, the viscosity of the sample decreased and the pressure drop across the filter dropped slowly. When the addition reached a certain amount, the pressure drop across the in-line filter started increasing as the asphaltenes were captured in the filter. The solvent to bitumen ratio at which the pressure drop just started to increase indicated the onset of asphaltene precipitation.
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Each run took about 4 h to complete and ultimately had about 5 volumes of solvent added to the original bitumen. After each run, the liquid contents were drained from the apparatus using a nitrogen purge and the in-line filter was removed. Solids stuck to the vessel were scraped and removed. The apparatus was then cleaned by flowing toluene through the loop for about 1 h.
Table 2 Properties of solvents and additives used for experiments
SolÕents n-Heptane n-Octane n-Decane n-Dodecane
100.205 114.232 142.286 170.340
684 703 730 748
15.1 15.6 15.8 16.2
3. Materials
AdditiÕes Aromatics Benzene Toluene m-Xylene 1,2,4-TMB Naphthalene Žsolid. Phenantherene Žsolid.
78.114 92.141 106.168 120.195 128.174 178.234
885 867 864 880 – –
18.8 18.2 18.0 17.6 20.3 20.1
Hydrogen donors Tetralin Decalin
132.206 138.254
973 896
19.4 18.0
Heteroatoms Pyridine Quinoline Indole Žsolid. Benzothiophene Žsolid. Creosol
79.102 129.162 117.152 134.202 138.168
983 1095 – – 1092
21.9 22.1 23.2 21.7 22.8
Surfactants Nonylphenol DDBSA
220.357 326.504
937 1060
19.4 22.6
3.1. Bitumen samples Athabasca and Cold Lake bitumen samples were tested; the properties of which are given in Table 1. Athabasca bitumen was denser than Cold Lake bitumen due to the slightly higher percentage of resins and asphaltenes, and lower percentage of saturates. Peramanu et al. Ž1999. reported the molecular weight and specific gravity distributions for these bitumens and their SARA ŽSaturates, Aromatics, Resins and Asphaltenes. fractions which are essential for bitumen characterization, computation of thermodynamic properties and studies of phase equilibria. 3.2. SolÕents and additiÕes The solvents n-heptane, n-octane, n-decane and n-dodecane were tested to identify the effect of Table 1 Properties of Athabasca and Cold Lake bitumens
API gravity Viscosity ŽPa s. at 248C Saturates Žwt.%. Aromatics Žwt.%. Resins Žwt.%. Asphaltenes Žwt.%. Resinsrasphaltenes Žwt.rwt.. Colloidal Index ŽCI. Carbon Žwt.%. Hydrogen Žwt.%. Sulfur Žwt.%. Oxygen Žwt.%. Nitrogen Žwt.%. Other Žwt.%. Total heteroatoms Žwt.%.
Athabasca
Cold Lake
8.05 323 17.27 39.70 25.75 17.28 1.49 1.89 83.34 10.26 4.64 1.08 0.53 0.15 6.4
10.71 65 20.74 39.20 24.81 15.25 1.63 1.78 83.62 10.50 4.56 0.86 0.45 0.01 5.9
Molecular weight Žkgrkmol.
Liquid density Žkgrm3 .
Solubility parameter ŽJrcm3 . 0.5
carbon number, and thereby the effect of solvent solubility parameter, on the onset values. The properties of these solvents listed in Table 2 show that the solubility parameter increases with increasing solvent carbon number. Knowledge of the chemical characteristics of the asphaltene and resin fractions is an essential factor in the choice of chemical additives. Asphaltenes contain the highest molecular weight species, the most aromatic and typically has the largest weight percentage of heteroatoms. The oil fraction contains the lowest molecular weight species, is of low aromaticity and has very few heteroatoms. Resins act as a transition between the asphaltenes and oil and have intermediate molecular weight, aromaticity, and heteroatom content. Due to the molecular complexity of the resins and asphaltenes, and difficulties in their
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isolation, determination of their individual chemical species is not possible at present. Instead, detection of the key functional groups is used to understand their important chemical features. Identification of N, S and O heteroatom functional groups were also used to select possible additives. Four types of additives were considered for their ability to shift the asphaltene onset to higher solvent to bitumen ratios. The types of additives tested were: aromatics, hydrogen donors, heteroatom compounds, and surfactants. The additives given in Table 2 are similar to those used by Clarke and Pruden Ž1997, 1998. but include more aromatic compounds. Aromatic compounds such as benzene, toluene, 1,2,4-trimethylbenzene, m-xylene, naphthalene and phenanthrene were used as additives since it is well documented ŽSpeight, 1991. that aromatic compounds dissolve asphaltenes. For compounds containing heteroatoms, the nitrogen compounds pyridine, quinoline and indole were tested, since infrared spectroscopic data ŽSpeight, 1991. has indicated that nitrogen can be found in resins in the form of pyrroles or indoles. A sulfur containing compound, benzothiophene, was tested because it has been identified ŽSpeight, 1982. that Athabasca asphaltenes contain this form of sulfur. This identification was carried out from observations of the rate of desulfurization of S-containing polymers with Raney nickel. Acetylation of the resins, combined with detailed infrared spectroscopic examination ŽSpeight, 1991. has established the presence of oxygen as ester functions, acid functions as well as carbonyl Žketone or quinone. functions. Therefore, the additive, 2-methoxy-4-methylphenol was chosen since it contains oxygen in both a phenolic hydroxyl group and a methoxy group. The hydrogen donor solvents tetralin and decalin were also tested, since it is well known that these solvents provide hydrogen to suppress coke formation in residue upgrading and coal liquefaction ŽCarlson et al., 1958.. In the formation of coke, asphaltenes polymerize, possibly after flocculation, to form mesophase and then coke. It was demonstrated that ŽClarke and Pruden, 1997. these donor solvents inhibit asphaltene flocculation by increasing the oil’s ability to solubilize asphaltenes. Asphaltenes are the most polar fraction in a bitumen or crude, and flocculated asphaltenes possess a
positive charge ŽLichaa and Herrera, 1975.. The resin fraction on the other hand is less polar than the asphaltenes. Solely based on the difference in the polarity of these two fractions, and the importance of the resins in dispersing the asphaltenes in the oil fraction, one would expect that resins have surface active properties ŽMcLean and Kilpatrick, 1997.. Therefore, it follows that surfactants should be able to inhibit asphaltene precipitation. Chang and Fogler Ž1993. demonstrated that the nonionic surfactant nonylphenol and anionic surfactant n-dodecylbenzene sulfonic acid ŽDDBSA. were effective in dissolving asphaltenes. Clarke and Pruden Ž1998. using their heat transfer apparatus confirmed that nonylphenol was effective in dissolving asphaltenes. However, contrary to Chang and Fogler’s findings, Clarke and Pruden Ž1998. observed that dodecylbenzene sulfonic acid was a poor asphaltene inhibitor. Therefore, in this work, both nonylphenol and dodecylbenzene sulfonic acid were tested for their effectiveness in dissolving asphaltenes.
4. Results and discussion When an asphaltene dissolving solvent Žtoluene. is added to bitumen the viscosity of the mixture decreases. This results in a decrease in the pressure drop across the in-line filter for a constant circulation rate as shown in Fig. 3. If an asphaltene precipitating solvent Ž n-heptane. is added, the pressure drop decreases at the beginning since the mixture viscosity decreases. When asphaltenes start to precipitate, the solids accumulate on the in-line filter and thereby increase the pressure drop as shown in Fig. 4. Therefore, the onset of precipitation is indicated by the minima of the pressure drop profile, the point where the pressure drop just starts to increase. Since it was difficult to identify the minima directly from the pressure drop profile, the following procedure was adopted. First, the data was confined to a range that bracketed the minimum point. A fourth order curve was then fitted through the data, and the minimum was determined by differentiating the fourth order equation and equating it to zero. For the runs at high temperatures, it was important to keep the vessel pressures well above the bubble point pressure of the solvent since the objec-
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asphaltene precipitation. Above the bubble point pressure, the increase in pressure decreases the asphaltene precipitation but the effect is minor. The decreased asphaltene precipitation with pressure above the bubble point is due to the increased solubility parameter of the solvent favoring asphaltene dissolution. The result of these effects is that asphaltene deposition is more severe at the bubble point pressure. Therefore, to avoid the system reaching the bubble point, higher starting pressures were used for the high temperature runs. Since the vessel pressure was not controlled, initially the vessel pressure increased due to heating from room temperature to a desired temperature and later, the pressure increased exponentially due to solvent addition. For a run starting at 180 kPa, the final pressure reached as high as 520 kPa. A typical pressure profile is given by Fig. 5. Fig. 3. Typical experimental pressure drop curve for asphaltene dissolving solvent Žtoluene..
tive was to maintain the system in the liquid phase. For a system below its bubble point pressure, the increase in pressure increases the dissolution of the solvent vapor in the sample resulting into increased
Fig. 4. Typical experimental pressure drop curve for asphaltene precipitating solvent Ž n-heptane..
4.1. SolÕent runs Asphaltene onsets were measured for both Athabasca and Cold Lake bitumens with n-heptane, n-octane, n-decane, and n-dodecane solvents at 808C. Fig. 6 gives the measured onsets solvent carbon
Fig. 5. Typical curve for vessel pressure variation with solvent to bitumen ratio for an experimental run.
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Fig. 6. Effect of solvent type on the precipitation onset for Athabasca and Cold Lake bitumens at 808C.
number for Athabasca and Cold Lake bitumens. The onsets for Athabasca bitumen were in the range of 1.35–1.61 g solventrg bitumen and the onsets for Cold Lake bitumen ranged from 1.17 to 1.46 g solventrg bitumen. The higher values of onsets for Athabasca bitumen than Cold Lake bitumen indicate that asphaltene stability cannot be predicted from the weight ratio of resins to asphaltenes. If the stability of the asphaltenes is indicated by resin to asphaltenes ratio ŽTable 1., then Athabasca bitumen would be less stable since the ratio for Athabasca bitumen is 1.49 and for Cold Lake bitumen it is 1.63. Athabasca bitumen’s lower resins to asphaltene ratio would suggest that it has a slightly greater tendency for precipitation, and it would be expected that less solvent would be needed to initiate the asphaltene precipitation. However, the present results suggest that stability is rather governed by the colloidal index ŽLoeber et al., 1998. and weight percent of heteroatoms. The colloidal index is defined as the ratio of dispersed constituents Žaromaticsq resins. to the flocculated constituents Žsaturatesq asphaltenes.. A higher colloidal index means that the asphaltenes are more peptized by the resins in the oil base medium, and higher heteroatom content means the
asphaltenes are more stable since heteroatoms are largely responsible for the strong bond between resins and asphaltenes. Due to the higher colloidal index and higher amount of heteroatoms in Athabasca bitumen ŽTable 1., it is more stable and therefore more solvent is required to cause asphaltene precipitation. The solvent to bitumen ratio at onset increases slightly as the carbon number increases ŽFig. 6.. The onset reaches a maximum value at approximately n-C 10 and then decreases with an increase in carbon number. One possible reason for this maximum in the profile is that solution phenomenon plays a major role at low carbon numbers and colloidal phenomenon plays a major role at high carbon numbers. According to solution theory, when a paraffin solvent is added to bitumen the thermodynamic equilibrium shifts due to differences in solubility parameters of the asphaltenes and that of the remaining solution. Since the solubility parameter increases with an increase in carbon number, the asphaltenes will be more soluble at higher carbon numbers and therefore result into higher onset ratios. Although resins are more soluble than asphaltenes because of their solubility parameters being closer to solvent solubility parameter, this does not apply to resins surrounding the asphaltene molecules ŽFig. 1. due to strong bonding between resin and asphaltene molecules Žcolloidal state.. However, as the carbon number increases, the solubility parameter of the solvent approaches resin’s solubility parameter and the attractive forces between solvent and resin molecules are strong enough to disturb the adsorption equilibrium between resin and asphaltenes. The asphaltene particles become free and flocculate irreversibly resulting in decreased solubility, and hence lower onset ratios. 4.2. Temperature runs Temperature runs were performed with n-heptane solvent from 60 to 1208C. The results are given in Fig. 7 for Athabasca and Cold Lake bitumens. Although pure n-heptane boils at 98.458C at 1 atm, boiling did not occur at the experimental conditions since n-heptane was mixed with bitumen and the vessel pressures were always higher than the bubble point pressures. Fig. 7 indicates that the onset goes through a maximum and this behavior is similar to
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the onset variation with carbon numbers as presented in Fig. 6. Again, this behavior is expected when solution phenomenon is dominant at low temperatures and colloidal phenomenon is dominant at higher temperatures. At low temperatures, according to solution theory, when the temperature increases asphaltenes would be more soluble, and hence the precipitation onset increases. However, according to the colloidal theory, dissolution seems to occur between resin and asphaltenes ŽFig. 1. at high temperatures and this counteracts the increased solubility. 4.3. AdditiÕe runs Runs were performed with the additive amount of 20 g in 100 g of bitumen Ž16.7 wt.%. with n-heptane solvent at 808C. The results are plotted against additive solubility parameter in Figs. 8 and 9 for Athabasca and Cold Lake bitumens, respectively. Most of the solubility parameters were obtained from Hansen and Beerbower Ž1971. and Grulke Ž1989., however, for the four compounds indole, creosol, benzothiophene and DDBSA, the solubility parameters were calculated using the group molar attraction constants of Small Ž1953. and Hoy Ž1970.. The broken horizontal line in the figures indicates the
Fig. 7. Effect of temperature on the precipitation onset for Athabasca and Cold Lake bitumens with n-heptane solvent.
Fig. 8. Effect of additive on the precipitation onset for Athabasca bitumen with n-heptane solvent at 808C ŽAdditive amounts 20 gr100 g bitumen..
onset when no additive was used. For Athabasca bitumen with no additive, the onset was 1.35 g solventrg bitumen and for Cold Lake bitumen the onset was 1.17 g solventrg bitumen. All the aromatics, hydrogen donors, heteroatoms and surfactants dissolve asphaltenes and therefore result in a higher solvent to bitumen ratio for the precipitation onset. The random placement of onset points in the figures indicates that there is no direct correlation for the onset and the additive’s solubility parameter. For Athabasca bitumen, the increase in the onset ratio by adding single ring aromatic compounds and hydrogen donors was less than for Cold Lake bitumen. However, the increase in the onset ratio caused by adding double and multiple ring aromatics Žnaphthalene and phenanthrene., heteroatom compounds, and surfactants, were almost the same for both bitumens. In general, for both bitumens, aromatic and hydrogen donor compounds caused a slight to moderate increase in the onset ratio, heteroatom compounds caused a moderate increase in the onset ratio, whereas surfactants caused the greatest increase in
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through a maximum giving a dome shaped curve. This is due to solution phenomenon playing a major role at low carbon numbers and temperatures, and colloidal phenomenon playing a major role at high carbon numbers and temperatures. Aromatic and hydrogen donor additives increased the onset ratio by a slight to moderate degree, heteroatoms increased the onset ratio by a moderate degree, and surfactants increased the onset ratio significantly. Among the surfactants, the anionic surfactant, DDBSA, was more effective than the nonionic surfactant, nonylphenol.
Acknowledgements
Fig. 9. Effect of additive on the precipitation onset for Cold Lake bitumen with n-heptane solvent at 808C ŽAdditive amounts 20 gr100 g bitumen..
the onset ratio. The surfactants were most effective since they posses an alkyl tail which makes this portion of the molecule lyophilic Žoil ‘‘loving’’., and an aromatic core with a head group which makes this portion compatible with the polar and highly aromatic asphaltenes. DDBSA was more effective than nonylphenol because of the anionic head group which has a stronger affinity towards asphaltene molecules than the hydroxyl head group.
5. Conclusions Asphaltene precipitation detection using a pressure drop technique in a flow loop apparatus was fast, reasonably accurate, and the method could handle opaque solvents at high temperatures and pressures. At 808C, the onset of precipitation of asphaltenes from Athabasca and Cold Lake bitumens occurred at 1.35 and 1.17 n-heptane to bitumen weight ratio, respectively. When plotted against carbon number and temperature, the onset ratios went
The authors would like to acknowledge the support of all the agencies and industrial sponsors of the University of Calgary’s Industrial Hydrogen Chair. Also, the authors are grateful to Mr. Dan Wong and Mr. Chandresh Singh for performing the experimental work, and Dr. Harvey Yarranton for his useful comments.
References Andersen, S.I., 1992. Hysteresis in precipitation and dissolution of petroleum asphaltenes. Fuel Sci. Technol. Int. 10, 1743–1749. Carlson, C.S., Langer, A.W., Stewart, J., Hill, R.M., 1958. Thermal hydrogenation: transfer of hydrogen from tetralin to cracked residua. Ind. Eng. Chem. 50, 1067–1070. Carnahan, N.F., Quintero, L., Pfund, D.M., Fulton, J.L., Smith, R.D., Capel, M., Leontaritis, K., 1993. A small-angle X-ray scattering study of the effect of pressure on the aggregation of asphaltene fractions in petroleum fluids under near-critical solvent conditions. Langmuir 9 Ž8., 2035–2044. Chang, C.L., Fogler, H.S., 1993. Asphaltene stabilization in alkyl solvents using oil-soluble amphiphiles. SPE Int. Symp. Oilfield Chem., SPE 25185, 339–349. Clarke, P.F., Pruden, B.B., 1996. Heat transfer analysis for detection of asphaltene precipitation and resuspension. 47th Annual Technical Meeting of The Petroleum Society, Calgary, June 10–12, Paper 96-112. Clarke, P.F., Pruden, B.B., 1997. Asphaltene precipitation: detection using heat transfer analysis, and inhibition using chemical additives. Fuel 76 Ž7., 607–614. Clarke, P.F., Pruden, B.B., 1998. Asphaltene precipitation from Cold Lake and Athabasca bitumens. Pet. Sci. Technol. 16 Ž3–4., 287–305.
S. Peramanu et al.r Journal of Petroleum Science and Engineering 23 (1999) 133–143 de Boer, R.B., Leerlooyer, K., Eigner, M.R.P., van Bergen, A.R.D., 1995. Screening of crude oils for asphalt precipitation: theory, practice, and the selection of inhibitors. SPE Prod. Facil. ŽFebruary. 55–61. Escobedo, J., Mansoori, G.A., 1995. Viscosity determination of the onset of asphaltene flocculation: a novel method. SPE Prod. Facil. ŽMay. 115–118. Ferworn, K.A., Mehrotra, A.K., Svrcek, W.Y., 1993. Measurement of asphaltene agglomeration from Cold Lake bitumen diluted with n-alkanes. Can. J. Chem. Eng. 71, 699–703. Fotland, P., Anfindsen, H., Fadnes, F.H., 1993. Detection of asphaltene precipitation and amounts precipitated by measurement of electrical conductivity. Fluid Phase Equilib. 82, 157– 164. Galtsev, V.E., Ametov, M., Grinberg, O.Y., 1995. Asphaltene association in crude oil as studied by ENDOR. Fuel 74 Ž5., 670–673. Grulke, E.A., 1989. Solubility Parameter Values. Polymer Handbook, 3rd edn. Wiley, New York, pp. VII519–VII559. Hansen, C.M., Beerbower, A., 1971. Solubility Parameters. Encyclopedia of Chemical Technology, Suppl. Vol., 2nd edn. Wiley, New York. Heithaus, J.J., 1962. Measurement and significance of asphaltene peptization. J. Inst. Pet. 48, 45–53, February. Hirschberg, A., deJong, L.N.J., Schipper, B.A., Meijer, J.G., 1984. Influence of temperature and pressure on asphaltene flocculation. SPE J., June, 283–289. Hoy, K.L., 1970. New values of the solubility parameters from vapor pressure data. J. Paint Technol. 42, 76–118. Jamaluddin, A.K.M., Nazarko, T.W., Sills, S., Fuhr, B.J., 1996. Deasphalted oil: a natural asphaltene solvent. SPE Prod. Facil. ŽAugust. 161–165. Kawanaka, S., Leontaritis, K.J., Park, S.J., Mansoori, G.A., 1989. Thermodynamic and colloidal models of asphaltene flocculation. Oil Field Chemistry. ACS Symposium Series No. 396, Chap. 24. American Chemical Society, Washington, DC, pp. 443–458. Kokal, S.L., Najman, J., Sayegh, S.G., George, A.E., 1992. Measurement and correlation of asphaltene precipitation from heavy oils by gas injection. J. Can. Pet. Technol. 31 Ž4., 24–30. Leontaritis, K.J., Mansoori, G.A., 1987. Asphaltene flocculation during oil production and processing: a thermodynamic col-
143
loidal model. SPE Int. Symp. Oilfield Chem., San Antonio, TX, SPE 16258. Leontaritis, K.J., Amaefule, J.O., Charles, R.E., 1994. A systematic approach for the prevention and treatment of formation damage caused by asphaltene deposition. SPE Prod. Facil. ŽAugust. 157–164. Lichaa, P.M., Herrera, L., 1975. Electrical and other effects related to the formation and prevention of asphaltenes deposition. SPE 5304, 107–125. Loeber, L., Muller, G., Morel, J., Sutton, O., 1998. Bitumen in colloid science: a chemical, structural and rheological approach. Fuel 77 Ž13., 1443–1450. McLean, J.D., Kilpatrick, P.K., 1997. Effects of asphaltene solvency on stability of water-in-crude-oil emulsions. J. Colloid Interface Sci. 189, 242–253. Peramanu, S., Pruden, B.B., Rahimi, P., 1999. Molecular weight and specific gravity distributions for Athabasca and Cold Lake bitumens and their SARA fractions. Accepted for Publication in Ind. Eng. Chem. Res. Pfeiffer, J.P., Saal, R.N.J., 1940. Asphaltic bitumen as colloid system. J. Phys. Chem. 44, 139–149. Rassamdana, H., Dabir, B., Nematy, M., Farhani, M., Sahimi, M., 1996. Asphalt flocculation and deposition: the onset of precipitation. AIChE J. 42 Ž1., 10–22. Sahimi, M., Rassamdana, H., Dabir, B., 1997. Asphalt formation and precipitation: experimental studies and theoretical modeling. SPE J. 2 Ž2., 157–169. Sheu, E., Liang, K.S., Sinha, S.K., Overfield, R.E., 1992. Polydispersity analysis of asphaltene solutions in toluene. J. Colloid Interface Sci. 153 Ž2., 399–410. Small, P.A., 1953. Some factors affecting the solubility of polymers. J. Appl. Chem. 3, 71–80. Speight, J.G., 1982. The structure of petroleum asphaltenes — current concepts. Alberta Res. Counc., Inf. Ser. 81. Speight, J.G., 1991. The Chemistry and Technology of Petroleum, 2nd edn. Marcel Dekker, New York. Speight, J.G., Long, R.B., Trowbridge, T.D., 1984. Factors influencing the separation of asphaltenes from heavy petroleum feedstocks. Fuel 63, 616–620. Thomas, F.B., Bennion, D.B., Bennion, D.W., Hunter, B.E., 1992. Experimental and theoretical studies of solids precipitation from reservoir fluids. J. Can. Pet. Technol. 31 Ž1., 22–31.