Energy Fuels 2010, 24, 6054–6060 Published on Web 10/08/2010
: DOI:10.1021/ef1010176
Study of the Aqueous Chemical Interactions between a Synthetic Tetra-acid and Divalent Cations as a Model for the Formation of Metal Naphthenate Deposits Ola Sundman,*,†,‡ Sebastien Simon,§ Erland L. Nordgard,§ and Johan Sj€ oblom§ †
Department of Chemistry, Umea University, SE 901 87, Umea, Sweden, and §Department of Chemical Engineering, Ugelstad Laboratory, NO 7491, Trondheim, Norway. ‡Present address: Department of Forest Products Technology, Aalto University, P.O. Box 16300, 00076 Aalto, Finland. Received August 4, 2010. Revised Manuscript Received September 20, 2010
The previously presented synthetic tetra-acid model compound BP10 was used to investigate the chemistry behind the formation of metal naphthenate deposits. The interactions between BP10 and the cations Ba2þ, Ca2þ, Hþ, Mg2þ, and Sr2þ were investigated using potentiometric titrations, metal ion depletion by inductively coupled plasma-atomic emission spectrometry (ICP-AES), pH measurements, and elemental analysis of precipitates, in 20-600 mM NaCl ionic medium. The interactions of BP10 with the monovalent Naþ are discussed on the basis of a previous study. The data given indicate that Ca2þ shows the strongest affinity toward BP10 and Ba2þ, and Sr2þ form approximately equally stable solid phases with BP10, while Mg2þ is less tightly bound to the tetra-acid. Hþ interacts more strongly than the Me2þ ions, and Naþ shows a rather small affinity for BP10. No soluble complexes could be detected, and all products in the chemical reactions are therefore believed to be solid materials. We suggest that BP10 show the following preference of cations: Hþ . Ca2þ > Ba2þ ≈ Sr2þ > Mg2þ . Naþ. This order could be due to the hydration state and size of the cations. In comparison to typical concentrations found of each in saline water, it is proposed that the dominance of Ca2þ in naphthenate deposits is dependent upon both availability and selectivity.
for Arn, with Me2þ.1,7,8 In these studies, interesting differences in physiochemical behavior between the different counterions could be spotted. Nonetheless, a question yet to answer is whether the dominance of calcium in the deposits is due to selectivity or whether the availability of calcium in the environment is the predominant factor; i.e., it would be interesting to investigate if indigenous naphthenic C80 tetra-acids (Arn) show selectivity toward Ca2þ. However, the experimental problems when working with Arn have increased the interest of synthetic model compounds instead. 7,8 In particular, the model compound BP10 (cf. Figure 1) has been shown to have similar behavior to Arn acids.7,8 Moreover, the acid/base properties of BP10 and several other synthetic tetra-acids have been reported.9 Because of its similarities to Arn, BP10 is perhaps the most interesting synthetic tetra-acid from an industrial perspective, and therefore, this particular compound was used in the present study. The study has been aimed at obtaining a better understanding of the relative strength of the bulk interactions between BP10 and several cations and evaluating whether BP10 shows selectivity toward Ca2þ. The indications from previous studies show that the results gained from model compounds, such as BP10, are relevant for Arn, and thus, using BP10 also here will increase the knowledge of the chemistry involved in naphthenate scaling. A long-term goal, beyond the scope of this particular investigation, is to fully understand and model the driving forces and kinetics involved in the formation of calcium naphthenate deposits.
Introduction The industrial problems in crude oil production, especially offshore, related to the contact between saline aqueous and petroleum phases, with the latter containing naphthenic acids, are well-documented.1,2 Especially, the naphthenic tetraacids, Arn, present at parts per million (ppm) level in the oil, are known to cause scaling problems and deposits together with Ca2þ present in saline water.3-5 Although it is clear from the literature that the dominating cation in these deposits is Ca2þ, it is not fully known why.6 Previous studies have focused on physiochemical properties of Arn and interfacial interactions of naphthenic mono-acids and model compounds *To whom correspondence should be addressed: Department of Forest Products Technology, Aalto University, P.O. Box 16300, 00076 Aalto, Finland. E-mail:
[email protected]. (1) Brandal, O.; Sjoblom, J.; Oye, G. J. Dispersion Sci. Technol. 2004, 25, 367–374. (2) Hurtevent, C.; Rousseau, G.; Bourrel, M.; Brocart, B. In Emulsion and Emulsion Stability, 2nd ed.; Sj€oblom, J., Ed.; CRC Press: Boca Raton, FL, 2006; pp 477-516. (3) Brandal, Ø.; Hanneseth, A.-M. D.; Hemmingsen, P. V.; Sj€ oblom, J.; Kim, S.; Rodgers, R. P.; Marshall, A. G. J. Dispersion Sci. Technol. 2006, 27, 295–305. (4) Baugh, T. D.; Grande, K. V.; Mediaas, H.; Vinstad, J. E.; Wolf, N. O. The discovery of high molecular weight naphthenic acids (ARN acids) responsible for calcium naphthenate deposits. Proceedings of the Society of Petroleum Engineers (SPE) 7th International Symposium on Oilfield Scale; Aberdeen, U.K., 2004. (5) Brocart, B.; Bourrel, M.; Hurtevent, C.; Volle, J.-L.; Escoffier, B. J. Dispersion Sci. Technol. 2007, 28, 331–337. (6) Shepherd, A. G.; Thomson, G.; Westacott, R.; Sorbie, K. S.; Turner, M.; Smith, P. C. Analysis of organic field deposits: New types of calcium naphthenate scale or the effect of chemical treatment?. Proceedings of the Society of Petroleum Engineers (SPE) 7th International Symposium on Oilfield Scale; Aberdeen, U.K., 2006. (7) Nordgard, E. L.; Sj€ oblom, J. J. Dispersion Sci. Technol. 2008, 29, 1114–1122. r 2010 American Chemical Society
(8) Nordgard, E. L.; Magnusson, H.; Hanneseth, A.-M. D.; Sj€ oblom, J. Colloids Surf., A 2009, 340, 99–108. (9) Sundman, O.; Nordgard, E. L.; Grimes, B.; Sjoblom, J. Langmuir 2010, 26, 1619–1629.
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Energy Fuels 2010, 24, 6054–6060
: DOI:10.1021/ef1010176
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Figure 1. Molecular structures of the used tetra-acid BP10 and one of the most dominating indigenous naphthenic C80 tetra-acids (Arn).19
Experimental Section
from CO2 contamination using inert N2 gas. The cell used for the determination of [Hþ] was as follows:
Titration and metal ion depletion experiments have been performed at the Department of Chemistry, Umea University, Umea, Sweden, in rooms thermostatted at 25 ( 1 °C. During the titrations, all measurement cells were protected from airborne CO2 g using a flow of washed [bubbled through 10% H2SO4 aq and 10% NaOHaq] N2 g. Synthesis of BP10 and preparation of Me2þ-BP10 precipitates were carried out at Ugelstad Laboratory at Norwegian University of Science and Technology (NTNU), Trondheim, Norway. Chemicals. A synthetic tetracarboxylic acid, BP10 (>95%), was prepared in solid form at the Ugelstad Laboratory, NTNU, Trondheim, Norway,7 and used as such. For all experiments, except the preparation of precipitates, all aqueous solutions were prepared from deionized (Milli-Q 185 Plus) and boiled (to remove all gas) water. Dried (180 °C overnight) NaCls (AnalR Normapur) was used to prepare all ionic media. The HCl solutions were prepared from dilute HCl (AnalR Normapur), calibrated using dried (80 °C overnight) Trizma Base (Sigma). The dilute NaOH solutions were prepared from a concentrated NaOH solution, filtered through a glass funnel to remove a contamination of solid carbonates, and calibrated via titration with dilute HClaq of known concentration. Solutions of the divalent metal chlorides were prepared from their salts, all used as received. For the preparation of precipitate experiments, Milli-Q water (Millipore simplicity), NaCls (g99.5%, Merck), and NaOHs (99%, VWR) were used. In contrast to previous experiments, NaOH solutions were not filtered. Potentiometric Titrations. All titrations were preformed on the high-precision potentiometric titration equipment, developed at location, and used in several previous papers.9-18 All titrations were performed in an oil bath thermostatted at 25 ( 0.1 °C. During the titrations, all solutions were protected
- Ag/AgCl | ionic media (NaCl) || solution/suspension in NaClaq | glass electrode þ The potential (E, in mV) of this cell is given by eq 1 E ¼ E0 þ
RT ln 10 log½Hþ þ Ej F
ð1Þ
where E0 is an apparatus constant. R in eq 1 is the gas constant, T is the absolute temperature, and F is the Faraday constant. Ej is a function of [Hþ], the two junction potentials jac and jalk, and the ionic product of water, Kw, according to eq 2. Ej ¼ jac ½Hþ þ jalk Kw ½Hþ - 1
ð2Þ
In eq 2, relevant values for Kw, jac, and jalk were adopted from Sj€ oberg et al.10 The equilibrium condition was set to a drift in E less than 0.2 mV/h. The titrations in the alkaline direction were prepared by mixing ionic media, dilute HCl, and a small amount of the Me2þ solution and then adding a known amount of BP10 in solid form. The E0 values for these titrations were internally calibrated in the initial part of each titration by reacting the excess of Hþ, originating from the HCl solution, with OH-, in 4-10 steps, until the acid was nearly neutralized. The titrations in the opposite direction were externally calibrated using a one-point calibration in HCl of known concentration and applying eqs 1 and 2. The samples were prepared by mixing ionic media, Me2þ solution, and the 10 mM BP10 solution previously described. Dilute HClaq solutions were used for titrations in the acidic direction, and dilute NaOHaq solutions were used in the alkaline direction. Z Plots. The data from the potentiometric titrations are illustrated in the form of Z plots, where Z is defined as the average number of protons adsorbed to each molecule of BP10. This definition is shown in eq 3
(10) Sj€ oberg, S.; H€agglund, Y.; Nordin, A.; Ingri, N. Mar. Chem. 1983, 13, 35–44. € (11) Ohman, L.-O.; Sj€ oberg, S. Acta Chem. Scand. 1981, 35, 201. (12) Laine, J.; L€ ovgren, L.; Stenius, P.; Sj€ oberg, S. Colloids Surf., A 1994, 88, 277–287. € (13) Sundman, O.; Persson, P.; Ohman, L.-O. J. Colloid Interface Sci. 2008, 328, 248. (14) Leone, L.; Ferri, D.; Manfredi, C.; Persson, P.; Shchukarev, A.; Sj€ oberg, S. Environ. Sci. Technol. 2007, 41, 6465–6471. € (15) Athley, K.; Ulmgren, P.; Ohman, L. O. Nord. Pulp Pap. Res. J. 2001, 16, 195–203. € (16) Laiti, E.; Ohman, L. O. J. Colloid Interface Sci. 1996, 183, 441– 452. (17) Hofslagare, O.; Sj€ oberg, S.; Samuelsson, G. Chem. Speciation Bioavailability 1994, 6, 95–102. (18) Sj€ oberg, S.; L€ ovgren, L. Aquat. Sci. 1993, 55, 324–335.
Z ¼ -
½COO- H - h þ Kw h- 1 ¼ ½BP10 B
ð3Þ
where H and B are the total concentration (mol/dm3) of protons and tetra-acid, respectively, h is the concentration (mol/dm3) of free protons, [Hþ], and Kw is the autoionization product of water at 600 mM Na(Cl). Metal Ion Depletion Experiments. Stock BP10 solutions (10 mM) were prepared by dissolving BP10 in a just above stoichiometric amount of 0.6 M NaOHaq and then diluting this 6055
Energy Fuels 2010, 24, 6054–6060
: DOI:10.1021/ef1010176
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in a volumetric flask using 0.6 M NaClaq. Using this stock solution, batches with BP10, HClaq, ionic medium, and Me2þ were prepared in 15 mL plastic test tubes. The samples were then left on an end-over-end rotation test tube holder for >48 h. After equilibration, the pH was carefully measured using a combination pH electrode (Orion Ross 8103SC, Thermo Electron Corporation), filled with 0.6 M NaCl, and calibrated using a solution of known [Hþ] and using eq 1. After this, the solid material was removed by centrifugation at 5000 rpm for 10 min, and the remaining clear solutions were filtered through 0.22 μm filters before analysis. The Me2þ content in the filtrates could then be measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin-Elmer Optima 2000 DV). Preparation and Characterization of Me2þ/BP10 Precipitates. Precipitates were prepared by adding dropwise 10 mL of BP10 solution to 65 mL of MeCl2 solution (with Me2þ being Sr2þ, Ba2þ, Ca2þ, and/or Mg2þ) under stirring and at different Me2þ/ BP10 ratios. The final concentrations are given in the text. Both solutions were previously adjusted to pH 9.5 and contained either 20 or 600 mM NaCl. The pH was readjusted to 9.5 1 h later. After 1 day of stirring, the solutions were filtered under vacuum through a 0.45 μm high-volume, low-pressure (HVLP) filter (Millipore), and the recovered precipitates were washed with Milli-Q water and dried at 60 °C under a N2 stream. The composition of the precipitates was determined by the Laboratory SGS Multilab (Evry, France) by thermal conductivity measurements for C and H, potentiometry for Cl, and ICP-AES measurements after mineralization in bomb Milestone by microwave for metal content. The uncertainties are the following (in relative amounts): calcium, (20%; magnesium, (13%; and sodium, (9%.
Figure 2. Titration data from potentiometric titrations of BP10 in 600 mM NaCl in the alkaline direction. The titration curves illustrate data from titrations with 3:1 Me2þ/BP10 with Ca2þ, Ba2þ, Mg2þ, and Sr2þ. Data from titrations performed without any Me2þ are shown for comparison. [BP10]0 = 0.3-0.4 mM in all titrations.
In this study, low critical micelle concentration (cmc) values (10-4-10-6 M) were found for BP10 under basic conditions and the cmc was lowered upon increased salinity. A forthcoming paper is showing that the micelles of deprotonated BP10 is relatively small (2-3 nm) with the occurrence of varying amounts of vesicles, depending upon the conditions. During the previous titration experiments, the water-insoluble fully protonated tetra-acid was in equilibrium with a micellar phase composed of deprotonated tetra-acid molecules, and because of the negatively charged micelle surface, this equilibrium shifted the titration curve to higher pH values. Thus, contrary to simple monoprotic fatty acids, which, in the monomeric state, show a typical pKa value of 4.9 for chain lengths >5, the apparent pKa value of BP10 ranged from 6.8 to 8, depending upon the salinity of the system. However, when a divalent cation is present in solution, any deprotonated tetra-acid will immediately react with the cation before a micellar solution can be formed. As a result of the absence or a decreased micellization tendency, the pH in which deprotonation takes place is shifted to lower pH values compared to when there are no divalent cations present. The curves seem to flatten out at Z=-1, probably because of the formation of a more complex solution containing solid tetra-acid and different complexes of cations and partially deprotonated species, which may cause some degree of aggregation, again affecting the deprotonation to higher pH values. After Z = -2, the deviation from non-metal-containing solution increases, because more divalent cations can bind to the complexes formed, decreasing the complex polarity and, thereby, formation of micelles in solution. In panels a and b of Figure 3, the titration data in the acidic direction is shown, comparing the four divalent cations to the parent titration data for BP10 only. The results are shown at an excess of BP10 in Figure 3a and at an excess of Me2þ in Figure 3b. When the cation concentration is lower than the concentration of BP10 (Figure 3a), the titration curves with all four cations show that Me2þ competes with Hþ regarding binding to BP10. For these data, with an excess of BP10, any distinction between the four Me2þ ions is difficult to make. However, when the Me2þ is in excess (>2:1), it is clear
Results Potentiometric Titrations. As a first approach to study the binding of divalent cations to a tetracarboxylic acid in an aqueous solution, potentiometric pH titrations were carried out correspondingly to previous studies without the presence of any divalent cation. Because the Me2þ ions compete with Hþ for affinity to BP10, a shift in the potentiometric titration curve in the acidic direction is expected as a result of the presence of such ions. This phenomenon can be studied by comparing the titration curves from potentiometric titrations recorded with Me2þ ions present with the respective curves without Me2þ ions. Furthermore, indications of selectivity can be seen by comparing the titration curves originating from titrations with different Me2þ. In Figure 2, the titration data from titrations in the alkaline direction is shown. It is obvious that the presence of an excess of all four Me2þ ions give some shift of the titration curve in the acidic direction, showing that Hþ ions are released from BP10 at lower -log[Hþ] when the Me2þ ions are present. For three of the Me2þ ions, Ba2þ, Ca2þ, and Sr2þ, this shift of the titration behavior is seen throughout most of the curve, while for Mg2þ, this phenomenon is only seen at Z values between 0 and -1.5. Actually, the data with Mg2þ present are overlapping with the native data, i.e., data with no Me2þ present, at Z < -1.5. This is evidence of a quite weak bonding behavior between BP10 and Mg2þ. The strong influence at initial deprotonation of the tetra-acid can be understood from a micellization point of view. As was claimed in previous titration studies,9 the unusual high apparent pKa values obtained when titrating BP10 originate from the occurrence of micelles when the deprotonated tetra-acid was formed. (19) Lutnaes, B. F.; Brandal, Ø.; Sj€ oblom, J.; Krane, J. Org. Biomol. Chem. 2006, 4, 616.
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Figure 3. Titration data from potentiometric titrations of BP10 in 600 mM NaCl in the acidic direction with (a) [Me2þ]/[BP10] = 0.5 and (b) [Me2þ]/[BP10] g 2. [BP10]0 = 0.37 mM in all titrations.
that no abrupt increase of the [Me2þ] can be seen at higher [Me2þ]tot. At increasing cation concentrations, the initially formed 1:1 Me2þ/BP10 stoichiometry in the formed solid materials is not constant but gradually approaches a 2:1 stoichiometry, which is strange and indicates complicated reaction mechanisms. To further unravel the differences in affinity between BP10 and the different Me2þ ions, a series of six batches was prepared, where all Me2þ ions were present simultaneously. It was considered likely that the different Me2þ ions would result in different aqueous equilibrium concentrations, even at Z=-4, if they truly are bound differently. The data, presented in Figure 5b, show that the difference becomes noticeable only at an excess of [Me2þ]tot added. Again, Mg2þ shows the least affinity toward precipitating with BP10 tetra-acid when all four Me2þ ions are present in the solution. Analysis of Me2þ-BP10 Precipitates. Measuring the depletion of [Me2þ] before and after precipitating with BP10 is an indirect method to gain information about the stoichiometry in the reaction products. In addition, this method does not take into account co-precipitation of other species, e.g., NaCl, which, at the high ionic strength (600 mM) that simulates saline seawater, may influence the precipitation products. It was thus decided to precipitate BP10 and Me2þ at both 20 and 600 mM NaCl and to analyze the elemental composition and cation content directly. This could also indicate whether NaCl co-precipitated with BP10/Me2þ and verify the results with the [Me2þ] depletion experiments. In Figure 6, the data of elemental analysis of the formed precipitates are shown. Only Ca2þ and Mg2þ containing single divalent cation systems were studied. At 20 mM NaCl and equimolar conditions of BP10 and the divalent cations, Ca2þ gives a near 2:1 solid composition with little incorporation of sodium but significantly more sodium is incorporated when the divalent cation is Mg2þ. Obtaining a ∼2:1 solid with Ca2þ with initially equimolar conditions dictates that the system tends to consume the divalent cation to form the 2:1 solid with half of the available BP10, instead of forming a 1:1 complex with all available BP10 molecules. Thus, the stability of this complex is rather low and highly prone to react with another divalent cation to precipitate out of the solution, possibly as a result of cross-linking. When the sodium content was increased to 600 mM NaCl, the results (Figure 6b) show a higher incorporation of Naþ in
Figure 4. Effect on pH from the addition of different concentrations of Ca2þ, Ba2þ, Mg2þ, and Sr2þ in 600 mM NaCl. The concentration of BP10 was 3.33 mM in the experiments. The pH in the reference solutions were 7.95 (Ca2þ and Sr2þ) and 7.82 (Ba2þ and Mg2þ) (Z = -2).
from -4 Ba2þ Sr2þ > Mg2þ . Naþ which is similar to the pattern reported by Brandal et al. for interfacial reactions of naphthenic mono-acids1 and that by Nordgard et al. for interfacial reactions of BP10.8 This ranking can partly be explained by considering the number of coordinated H2O molecules around the divalent cations and their respective radius, thus considering the electrostatic shielding density from the coordinated H2O molecules. Indeed, small Mg2þ with a hydration state of 6 would experience the highest shielding density compared to the other alkaline earth cations. This shielding could explain why Mg2þ has the lowest affinity for BP10. There may be two reasons why the reactivity of Me2þ follows the observed pattern. Smaller cations would experience more shielding from the same number of coordinated H2O molecules and would, thus, experience less complexation tendencies with the carboxylates in BP10. Another explanation could come from deviations in the binding enthalpy upon the reaction between Me2þ and BP10 and, thereby, different Gibbs free energy. To highlight if the reactivity observed in this paper is due to binding enthalpy differences, experiments using isothermal titration calorimetry (ITC) are currently ongoing in our and collaborating laboratories and will be presented in a future paper. The differences of affinity among the three other cations (Ca2þ, Sr2þ, and Ba2þ) could result from a difference of size, and perhaps an optimal cation size relative to the hydration state is obtained with Ca2þ. Here, also ITC is expected to bring more information about these differences.
Acknowledgment. The authors thank the JIP consortium, consisting of AkzoNobel, Baker Petrolite, BP, Champion Technologies, Chevron, Clariant Oil Services, ConcoPhillips, Shell Global Solutions, Statoil ASA, Talisman Energy, and Total, for financial support of the present work. Supporting Information Available: Additional experimental data for elemental composition, mass and total charge balances for precipitates of BP10 and a single cation, and mixtures of two different divalent cations. This material is available free of charge via the Internet at http://pubs.acs.org. (20) Stumm, W.; Morgan, J. J. Aquatic Chemistry;Chemical Equilibria and Rates in Natural Waters, 3rd ed.; Wiley: New York, 1996.
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