Energy & Fuels 2009, 23, 1249–1252
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Solution Behavior of Naphthenic Acids and Its Effect on the Asphaltenes Precipitation Onset† Carlos A. Carbonezi,‡ Lucilla C. de Almeida,‡ Bruna C. Araujo,‡ Elizabete F. Lucas,§ and Gaspar Gonza´lez*,‡ Petrobras Research Center, AV. Hora´cio Macedo, 950 Cidade UniVersita´ria, Rio de Janeiro, RJ 21941-915, Brazil, and Macromolecules Institute, Federal UniVersity of Rio de Janeiro, CT. Bl. J, Ilha do Funda˜o, Rio de Janeiro, RJ 21945-970, Brazil ReceiVed August 20, 2008. ReVised Manuscript ReceiVed NoVember 6, 2008
Naphthenic acids represent a subfraction of the crude oil resins that contains predominantly carboxylic acids with saturated cyclic structures and corresponds to a complex mixture of compounds. The naphthenic acids can cause deposition problems. These compounds however also play an important role in some phenomena occurring, and then separation from crude oil would improve oil quality during the production. In this contribution, some aspects of the solution behavior of naphthenic acids are presented. It was observed that these compounds form stable liquid mixtures with organic solvents in a wide range of solubility parameters. When added to partially soluble liquid mixtures, such as n-heptane/methanol, they improve the mutual solubility of these liquids, increasing the solubility domain and partition into both liquid phases. The precipitation onset for asphaltenes dissolved in toluene titrated with n-heptane was increased by the addition of naphthenic acids, indicating that these compounds contribute to the solubilization of asphaltenes representing a “good solvent” for this fraction. Applying the polymer solutions to the asphaltenes dissolved in liquid mixtures, treating the naphthenic acids as pure pseudo-components, and assuming that asphaltenes precipitation onset occurs at a particular solvent composition corresponding to a “critical solubility parameter” value from 20 to 23 MPa1/2 were obtained for the naphthenic acids that are in good agreement to those obtained by differential scanning calorimetry (DSC).
1. Introduction Naphthenic acids (NAs) represent a crude oil fraction composed of carboxylic acids with saturated cyclic or polycyclic hydrocarbons with or without branched alkyl chains. Its composition represents a complex mixture of compounds with a broad polydispersity in molecular weight and structure. The increasing interest in NAs in the petroleum industry is due to the metal naphthenate precipitation that causes deposition problems in pipelines and production and treatment facilities.1 These compounds however, also play an important detrimental role in crude oil foaming and emulsification,2,3 scaling,4 corrosion,5 etc. Thus, the separation of these compounds from crude oil or its products or distillate fractions could improve oil quality and would upgrade its market value. A physical-chemical characterization of NAs would be useful to assess the possibility of finding processes and procedures for this NAs separation. † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Petrobras Research Center. § Federal University of Rio de Janeiro. (1) Rousseau, G.; Zhou, H.; Hurtevent, C. International Symposium on Oilfield Scale, Aberdeen, U.K., 2001; Paper SPE 68307. (2) Callaghan, L. C.; McKechnie, A. R.; Ray, J. E.; Wainwright, J. C. SPE J. 1985, 171–175. (3) Skodvin, T.; Sjo¨blom, J.; Saeten, J. O.; Urdahl, O.; Gestblom, B. J. Colloid Interface Sci. 1994, 166, 43–50. (4) Dyer, S. J.; Graham, G. M.; Arnot, C. International Symposium on Oilfield Scale, Aberdeen, U.K., 2003; Paper SPE 80395. ´ lvarez, R. E.; Cano, J. L. Fuel 2004, (5) Laredo, G. C.; Lo´pez, C. R.; A 83, 1689–1695.
The solubility behavior is the main NAs property to develop such kind of processes. However, there are no data available in the literature about NAs solubility parameters or latent heats of vaporization to calculate this parameter. On the other hand, several authors6-12 have studied the asphaltenes precipitation onset and concluded that it depends upon only the solubility parameter. De Boer et al. used this approximation to calculate the solubility parameter of the oil in their model to screen crude oils for precipitation and stability.6 Wiehe studied the solubility of Souedie crude oil in nonpolar solvents of increasing solubility parameter and concluded that the asphaltenes flocculation solubility parameter lies between the values corresponding to the highest nonsolvents (methylcyclohexane, δ ) 15.9 MPa1/2) and the lowest solvent (cyclohexane, δ ) 16.8 MPa1/2).7 On the basis of these results, the author developed, in subsequent publications, a model to predict the compatibility of oils based on the assumption that asphaltenes/resins dispersions present the same flocculation solubility parameter independently of the oil being blended with noncomplexing liquids or other oils8 and successfully applied it to (6) De Boer, R. B.; Leerlooyer, K.; Eigner, M. R. P.; van Bergen, A. R. D. SPE Production and Facilities, Feb 1995; pp 55-61, Paper SPE 24987. (7) Wiehe, I. A. The American Institute of Chemical Engineers (AIChE) Spring National Meeting, Houston, TX, March 14-18, 1999. (8) Wiehe, I. A.; Kennedy, R. J. Energy Fuels 2000, 14, 56–59. (9) Wiehe, I. A.; Kennedy, R. J. Energy Fuels 2000, 14, 60–63. (10) Andersen, S. I. Energy Fuels 1999, 13, 315–322. (11) Gonza´lez, G.; Sousa, M. A.; Lucas, E. F. Energy Fuels 2006, 6, 2544–2551. (12) Souza, M. A.; Oliveira, G. E.; Lucas, E. F.; Gonza´lez, G. Prog. Colloid Polym. Sci. 2004, 128, 283–287.
10.1021/ef800683c CCC: $40.75 2009 American Chemical Society Published on Web 12/12/2008
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Table 1. Refractive Index (RI), Density, Melting Point (mp), Molecular Weight (MW), and Total Acid Number (TAN) for the NAs Used in This Work13 compound
RI
density (g cm-3)
mp (°C)
cyclohexanecarboxylic acid 3-cyclohexanepropionic acid NA
1.461 1.464 1.480
1.030 0.998 0.950
25-28 15-17
a
MW
TAN
128.17 156.23 243.93a
230
Calculated from the TAN value using MW ) 56 105/TAN.
refinery streams.9 Andersen used a similar approximation to interpret some spectroscopic titration data for crude oils presenting different production problems.10 He assumed that asphaltenes precipitation commences at a certain critical solubility parameter (δcr) determined only by the composition of the diluted mixture. More recently, it has been shown that values of, approximately, 16.4-16.6 MPa1/2 may be obtained for the asphaltenes flocculation onset independently of the asphaltenes concentration,11 solubility parameter of the solvent, or origin of the asphaltenes samples.12 These observations may also be used as an alternative method to gather some information on the solubility behavior of NAs when a shift in the asphaltenes precipitation onset is observed because of the presence of NAs. In this contribution, the solubility parameter values for three commercial samples of NAs, obtained by calorimetric studies and asphaltenes precipitation onset, are presented. Additionally, the NA phase behavior in n-heptane/methanol mixtures at 5, 15, and 22 °C is also studied. 2. Experimental Section 2.1. Materials. The three NAs: cyclohexanecarboxylic acid, 3-cyclohexanepropionic acid, and NA were purchased from Fluka and used as received. The first two acids are single compounds with 97 and 98% purity, respectively. The product commercialized by Fluka as NA (hereafter denoted in italics to distinguish it from the crude oil NAs fraction) corresponds to a mixture containing various components. Some physical properties for the three NAs from the supplier catalogue13 are presented in Table 1. Extra pure precipitated salicylic acid (C7H6O3, molecular weight of 138.12) was purchased from E. Merck, Darmstadt, Germany. The asphaltenes used in this work were separated from an asphaltic residue obtained from REDUC Brazilian Refinery following a procedure similar to the standard methods to precipitate and purify this fraction.14 The residue was Soxhlet-extracted with n-heptane (C7) to complete removal of the C7-soluble fraction and dried. The solid was dissolved in toluene, filtered, and dried under vacuum. An infrared spectrum of the powder showed typical peaks of the asphaltenes fraction.11 Methanol and n-heptane were obtained from Vetec Quı´mica Ltd., Rio de Janeiro, Brazil, and were 99.8 and 99.5% pure, respectively. Toluene (99.5%) was purchased from Tedia Brazil. 2.2. Methods. 2.2.1. Calorimetric Measurements. The measurements of enthalpy of vaporization of each compound were performed by the differential scanning calorimetry (DSC) method using a Mettler Model 823. Indium was used for the calibration. The heating rate was 10 °C min-1, and the amount was about 10 mg. The analyses were performed in a helium atmosphere at a rate of 50 mL/min. 2.2.2. Asphaltenes Precipitation Onset. The asphaltenes precipitation onsets were measured by the n-heptane titration method using a Photonics 440 UV-vis spectrometer furnished with a 1 mm microprobe immersed in a 18 mm internal diameter vial. Titrations were carried out by the addition of heptane to 3.0 g of (13) Fluka Riedel-de Hae¨n Catalogue, 1999/2000. (14) Standard test method for determination of asphaltenes (heptanes insoluble) in crude petroleum and petroleum products. Institute of Petroleum, London, U.K., IP 145/01, 2001.
asphaltenes-in-toluene solutions at 0.3 and 0.5 wt %. When NAs were present, 0.5 g of pure NA was added to this solution, completing 3.5 g of solution. The absorbance at 850 nm was measured using a probe with 10 mm path length directly immersed in the solution, and one measurement per minute was normally recorded. The n-heptane flow rate was controlled by a highperformance liquid chromatography (HPLC) pump at 0.200 mL/ min. The asphaltenes precipitation onset was identified as the n-heptane volume corresponding to the minimum in absorbance and expressed in mL of n-heptane/mL of toluene solution. Onset measurements were carried out in duplicates. 2.2.3. Ternary Diagram. Methanol/n-heptane mixtures of different by weight compositions were prepared and left to attain equilibrium under gentle stirring in a thermostatic water bath at 5, 15, and 22 °C. For most compositions, these mixture were turbid. Subsequently, NA was added dropwise from a glass burette, and the composition corresponding to the phase transition was identified as the point in which the turbid mixture suddenly became a single clear phase.
3. Results and Discussion Figure 1 presents the DSC diagrams for the two pure compounds: cyclohexanecarboxylic acid and 3-cyclohexanepropionic acid. The latent heats of vaporization, ∆Hv for the samples were obtained from the peak area integration, following the relation w∆H ) k · area, where w is the sample mass and k is a measurement constant.15 The curves for both samples were similar but presented differences in their volatilization endset values, because of differences in molecular weight and latent heat: cyclohexanecarboxylic acid, MW ) 128.17, endset ) 228.9 °C, and ∆Hv ) 507.63 J g-1; and 3-cyclohexanepropionic acid, MW ) 156.23, endset ) 261.20 °C, and ∆Hv ) 407.43 J g-1. The endset values were used as the boiling point. The solubility parameters for the NAs, δna, may be obtained applying Hildebrand’s definition for this parameter16
( ) ( 1
)
1
∆Ev 2 ∆Hv - RT 2 ) (1) V V where δna is defined as the square root of the cohesive energy density and calculated as the square root of the ratio between the latent energy of vaporization, ∆Ev, and the molar volume, V. Using the previous equation where R is the gas constant and T is the absolute temperature and the data reported in Table 1, the values obtained were 22.9 and 20.2 MPa1/2, respectively, for cyclohexanecarboxylic acid and 3-cyclohexanepropionic acid. The same procedure applied to salicylic acid conduced to a value of 23.08 MPa1/2, which is in a good agreement with the report value17 of 23.72 MPa1/2. Typical curves for asphaltenes solutions titrated with n-heptane are presented in Figure 2. The precipitation onset for asphaltenes dissolved in toluene was around 1.3 mL of n-heptane/mL of solution. As can be inferred by comparing curves a1 and b1, this value does not depend upon the asphaltenes concentration, which is in agreement with previously reported results.12 Furthermore, precipitation onsets determined by UV-vis spectroscopy do not present deviations of linear trend in the plot up to concentrations higher those used in the present work.11 In all of the cases, the addition of NAs resulted in a displacement of the precipitation onset toward higher dilution values. δna )
(15) Wunderlich, B. Thermal Analysis; Academic Press: London, U.K., 1990. (16) Hildebrand, J. H.; Scott, R. L. The Solubility of Nonelectrolytes, 3rd ed.; Dover Publications, Inc.: New York, 1964. (17) Tantishaiyakul, V.; Worakul, N.; Wongpoowarak, W. Int. J. Pharm. 2006, 325, 8–14.
Solution BehaVior of Naphthenic Acids
Energy & Fuels, Vol. 23, 2009 1251
Figure 1. DSC diagrams for cyclohexanecarboxylic acid (left) and 3-cyclohexanepropionic acid (right).
Figure 2. Titration curves for asphaltenes and asphaltenes/NAs solutions with n-heptane: (a) 0.3 wt % asphaltenes/0.5 wt % NA, (b) 0.5 wt % asphaltenes/ 0.5 wt % NA, (c) 0.5 wt % asphaltenes/0.5 wt % cyclohexanecarboxylic acid, and (d) 0.5 wt % asphaltenes/0.5 wt % cyclohexanepropionic acid.
The solubility parameters for the NAs may also be calculated from the shift in the asphaltenes precipitation onset caused by their presence in the solution. The data may be obtained using an equation derived from the polymer solution theory.18 If we consider a mixture containing toluene, n-heptane, NA, and asphaltenes, the solubility parameter at the precipitation onset δonset is given by δonset ) δtφt + δhφh + δnaφna + δaφa
(2)
where δi is the solubility parameter of each component and φi is its corresponding volume fraction. Assuming that NAs corresponds to a single pseudo-component and considering that (18) Barton, A. F. M. Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press: Boca Raton, FL, 1990.
asphaltenes present a fairly high molecular weight compared to the rest of the components and that their volume fraction is rather low, eq 2 reduces to δonset ) δtφt + δhφh + δnaφna
(3)
To obtain the solubility parameter for NAs using eq 3, it is necessary to assume that the asphaltene precipitation onset depends upon only the composition of the solvent medium and occurs at a particular value of the solubility parameter. As described in the Introduction, various authors6-12 have previously used this hypothesis. We used a value of 16.35 MPa1/2 for the δonset in eq 3, which is intermediate between the values reported by Wiehe.7 The values of 21.2, 21.9, and 22.7 were obtained for NA, cyclohexanecarboxylic acid, and 3-cyclohexanepropionic
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Table 2. Solubility Parameters for NAs Calculated from Onset Data and DSC δ (MPa1/2) obtained from
compound
solution/ acid (wt/wt)
onset (v/v)
onset
DSC
NA cyclohexanecarboxylic acid 3-cyclohexanepropionic acid
1:0.3 1:0.3 1:0.3
2.3 2.4 2.6
21.2 21.9 22.7
22.9 20.2
acid, respectively. Table 2 presents the results obtained using the two alternative procedures. The values obtained are comparable to aliphatic alcohols, such as 2-ethyl hexanol or n-octanol (20.8 and 21.1 MPa1/2, respectively),19 and aliphatic acids, such as 2-ethyl hexanoic acid, 22.5 MPa1/2,18 and higher than the values corresponding to aromatic solvents that vary between 18 and 20.3 MPa1/2 18 and are considered good solvents for the asphaltenes. The precipitation onset, i.e., the dilution in mL of C7/mL of asphaltenes solution, increases with the increase in the concentration of NAs in solution. This is illustrated for 3-cyclohexanepropionic acid in Figure 3. Similar results were obtained for cyclohexanecarboxylic acid and NA. The results of linear regression analyses for the plots of the three acids are shown in Table 3. Curves similar to that shown in Figure 3 have been reported for liquids, such as toluene10 and cyclohexane,20 and are characteristic of good solvents, i.e., liquids that enhance the asphaltenes solubilization in the solvent media.18 The value for the onset extrapolated to zero dilution is normally considered the flocculation point or the precipitation onset for the pure undiluted oil or asphaltenes solution.21 The value obtained from these relationships, 1.6-1.7 mL of C7/mL of solution, is somewhat higher but still consistent with the experimentally measured result for the asphaltenes solution in the absence of NAs. Considering that, in all of the cases, NAs are at a relatively high concentration (2.5-25%), it seems reasonable to conclude that NAs modify the onset in a similar way as other good solvents rather than through any specific effects. In fact,
Figure 4. Effect of the temperature and NA content on the phase behavior of methanol/n-heptane mixtures.
chemicals that present specific interactions are effective in increasing the precipitation onset at concentrations of parts per millions, as are, for instance, the cases of nonyl phenol22 or alkyl sulfonic acids.6 In addition, it was observed that NAs dissolve in all proportions in most aliphatic or aromatic hydrocarbons as well as in ethanol and methanol. The phase behavior study of NAs in solution can be useful under several aspects, such as the development of separation methodologies of these compounds present in the crude oil derivatives. Figure 4 shows the miscibility behavior of ternary mixtures of NA, n-heptane, and methanol in three different temperatures. The phase diagram demonstrates the effect of increasing solubility with the increase of temperature. Furthermore, when NAs are added to a mixture of n-heptane/methanol that presents a region of partial miscibility, they improve the mutual solubility of these liquids, increasing the solubility domain and partition into both liquid phases. This last observation may serve as a basis for the development of procedures to separate NAs from crude oil distillate fractions. 4. Conclusions
Figure 3. Precipitation onset for a 0.5 wt % asphaltenes solutions in toluene as a function of the dilutions with 3-cyclohexanepropionic acid. Table 3. Linear Regression Data from the Plots of Flocculation versus NAs/Asphaltenes Solution Ratio compound
intercept
slope
Ra
NA cyclohexanecarboxylic acid 3-cyclohexanepropionic acid
1.7 1.6 1.7
2.4 3.1 3.3
0.9864 0.9777 0.9967
a
R ) correlation coefficient.
Consistent solubility parameter values in the range of 20.2-22.9 MPa1/2 were obtained for three commercial samples of NAs using two alternative procedures. The precipitation onset for asphaltenes dissolved in toluene titrated with n-heptane was increased by the addition of NAs, indicating that these compounds contribute to the solubilization of asphaltenes. The process seems to correspond to a solvent effect rather than to specific interactions. NAs added to partially soluble liquid mixtures, such as n-heptane/methanol, improve the mutual solubility of these liquids, increasing the solubility domain and partition into both liquid phases. This effect may serve as a basis for the development of procedures to separate NAs from crude oil distillates. Acknowledgment. The authors acknowledge Petrobras for authorization of publishing this work. EF800683C (19) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1985. (20) Gharfeh, S.; Yen, A.; Asomaning, S.; Blumer, D. International Conference on Heavy Organic Depositions, Cancun, Mexico, 2000. (21) Heithaus, J. J. J. Inst. Pet. 1962, 48, 45–53. (22) Gonza´lez, G.; Middea, A. Colloids Surf. 1991, 52, 207–217.
