Article Cite This: Energy Fuels 2018, 32, 1087−1095
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Flocculation of Asphaltenes by Polymers: Influence of Polymer Solubility Conditions Claudia P. P. Mazzeo,† Flaviane A. Stedille,‡ Claudia R. E. Mansur,†,§ Antônio C. S. Ramos,‡ and Elizabete F. Lucas*,†,§
Energy Fuels 2018.32:1087-1095. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/07/19. For personal use only.
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Universidade Federal do Rio de Janeiro, Instituto de Macromoléculas Professora Eloisa Mano, LMCP-Av. Horácio Macedo, 2030, Bloco J, Ilha do Fundão, Rio de Janeiro, RJ 21941-598, Brazil ‡ Centro de Engenharias (CEng), Universidade Federal de Pelotas, Praça Domingues Rodrigues, 2, Pelotas, Rio Grande do Sul 96010-440, Brazil § Universidade Federal do Rio de Janeiro, COPPE/PEMM, LADPOL, Av. Horácio Macedo, 2030, bloco F, Ilha do Fundão, Rio de Janeiro, Rio de Janeiro 21941-972, Brazil ABSTRACT: Sulfonated polystyrene has shown flexible action as an asphaltene dispersant/flocculant as a function of the degree of sulfonation and concentration used. In this work, samples of sulfonated polystyrene with different sulfonation degrees were assessed in precipitation assays in model asphaltene systems, with variation of the asphaltene fractions (asphaltenes extracted by n-pentane and n-heptane, C5I and C7I, respectively), asphaltene concentration, polymer concentration, and medium used to dissolve the polymer and asphaltenes. The precipitation tests were carried out with an ultraviolet−visible spectrometer, and the absorbance values were converted into asphaltene concentration values in solution by using calibration curves. The results showed that the concentration of sulfonic groups at which the polymer performs best as an asphaltene flocculant is 10 mol %. The dependence of the polymer’s effect as a flocculant or stabilizer of asphaltenes in function of its hydrophilicity and concentration was confirmed. Moreover, the results indicate there is a strong relationship between the polymer’s solubility in the medium and its flocculant action, which is significantly more effective when the polymer does not have strong affinity for the medium.
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INTRODUCTION Asphaltenes are the most polar and heaviest compounds in crude oil. They are formed by polycondensed aromatic rings linked to aliphatic chains or rings, generally containing acid and base functional groups, with elements such as sulfur, oxygen, nitrogen, vanadium, and nickel. It is these elements (S, O, N, and heavy metals) that form the variety of polar groups in asphaltenes, such as aldehydes, carbolic acids, and starches. Asphaltenes precipitate from petroleum by the addition of excess n-heptane or n-pentane, and are totally miscible in toluene or benzene at room temperature.1−7 However, other techniques have been applied to separate them.8 One of the main problems found in the petroleum industry is the destabilization of asphaltenes in crude oil.6,9−12 Due to their strong tendency to associate, asphaltenes form deposits, both in the reservoirs and during transport and processing operations, drastically increasing costs.13−15 Variations in temperature, pressure, and composition of the oil during the various production and processing steps can destabilize the asphaltenes, causing them to precipitate.16−22 It has been shown that the aggregation potential is related to asphaltenes composition.23 In general, the problems related to asphaltenes can be classified as due to aggregation (formation of fine particles in suspension) and deposition (precipitation followed by formation of larger particles). This deposition reduces the capacity of storage tanks, fouls pipes, alters the wettability of the reservoir rock formation, and forms emulsions and foam.6,18,19,24,25 © 2018 American Chemical Society
Resins form another fraction of crude oil and are often studied along with asphaltenes. The main distinction between these fractions is their solubility in normal paraffinic solvents, in which asphaltenes are insoluble and resins are soluble.6,26−28 Among many explanations, it is partially accepted that asphaltenes are found in petroleum as colloidal dispersions,24 peptized by resins, which are relatively polar substances. These interactions between asphaltenes and resins lead to the formation of micelles, the presence of which can be demonstrated by studies of the molar mass and observations of aggregation of asphaltenes in diluted solutions, indicating this can occur at extremely low concentrations.16 On the basis of these studies, it can be assumed that asphaltenes form a nonaqueous colloidal system.1,6,8,25−28 Some authors have investigated the mechanism by which these macromolecules are stabilized. Knowledge of the interaction mechanism sheds light on the exact position and nature of the sites involved in the interaction, the relations among the molecules involved (including solvent molecules), the system’s energy at each stage of the process, and how fast all the changes occur.29 Evidently, it is very difficult to clarify all these points, especially when dealing with high complexity involving molecules of various natures and sizes, such as petroleum.30,31 Received: August 31, 2017 Revised: January 17, 2018 Published: January 18, 2018 1087
DOI: 10.1021/acs.energyfuels.7b02577 Energy Fuels 2018, 32, 1087−1095
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Table 1. Qualitative Solubility of PS and PSS in Mixtures of Toluene and Isopropyl Alcohol in Different Hildebrand Solubility Parameter, at 25 °C
In some cases, the deposition of asphaltenes can be prevented by changing the processing conditions, such as the temperature and pressure, so that the crude oil will not be subjected to conditions that favor aggregation. However, usually these conditions cannot be varied easily, mainly during production, so it is necessary to add asphaltene aggregation inhibitors and dispersants.32−34 Little attention has been paid to the flocculation of asphaltenes by polymers, because the main interest of the oil industry is to stabilize this fraction.35−38 However, such investigation can be useful to obtain an understanding of the mechanisms of precipitation and dispersion of asphaltenes.39 Moreover, asphaltene flocculants can be applied in deasphalting processes, which consist of removing the asphaltenic material from the residues left after producing certain derivatives, such as paraffins or lubricating oils. Another application of these flocculants is to improve heavy crude by reducing the asphaltenic fraction, to raise the market price of the oil.40−42 In a previous study by our group,43 samples of sulfonated polystyrene were synthesized to assess their use as asphaltene stabilizers/flocculants. Because of the more hydrophilic character of these substances (in relation to nonsulfonated polystyrene), the samples tested were insoluble in toluene but soluble in toluene/acetone mixtures. For this reason, this mixture (in a 60/40 ratio) was used in the asphaltene precipitation tests carried out. However, the solvent acetone is an asphaltene destabilization agent, and its presence caused precipitation of around 30% of the asphaltenes present in the solution. This paper reports the results of tests to study the asphaltene stabilization or flocculation mechanisms promoted by the addition of sulfonated polystyrene with varied degrees of sulfonation. For this, another solvent mixture (toluene/ isopropyl alcohol) and other testing methods were used than those reported previously.43
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(M̅ w ) average molar mass obtained for the polystyrene (PS) used in the sulfonation reaction were, respectively, 87 300 g/mol and 234 200 g/mol. The sulfonation degrees obtained for the polymers were 7 and 10 mol % (Table 1). Figure 1 shows chemical structures of the PS and PSS samples.
Figure 1. Chemical structures of polystyrene (PS) and sulfonated polystyrene (PSS).
Obtaining the Asphaltene Fractions. The asphaltene fractions were extracted by the solubility difference from asphaltic residue ASPR, according to the procedure described previously.43 The extraction was induced by the addition of an excess of nonsolvent (n-pentane or n-heptane) to the residue in a Soxhlet extractor and the names were assigned according to the nonsolvent employed to obtain, respectively, the asphaltene fractions A-C5I and A-C7I. Solubility of the Chemical Additives. The solubility tests were carried out in a test tube immersed in a beaker containing water heated over a hot plate. The solutions, containing 1 wt %/v of additive, were heated and then cooled. The temperatures were measured by placing a thermometer inside the test tube. Each solution was tested in duplicate and for all solutions two cloud point measurements were made. The cloud point was determined as the average between the temperature when the first indication of clouding occurred and the temperature when it disappeared. The temperature range used in this test was 5 to 60 °C. The numerical values of the solubility parameters of the solvents used (toluene and isopropyl alcohol) were taken from the literature.44 The solubility parameter of the solvent mixtures was calculated using the weighted average between the δ value of the solvent and its fraction by volume in the mixture, according to eq 1.
EXPERIMENTAL SECTION
Materials. Isopropyl alcohol (P.A. purity grade 99.0%), n-heptane (P.A. 99.5%), and n-pentane (P.A. 99.5%) were acquired from Vetec ́ Quimica Fina (Rio de Janeiro, Brazil), while toluene (HPLC grade) was supplied by Tedia Brasil (Rio de Janeiro, Brazil). The asphaltic petroleum residue (ASPR) sample was donated by the Petrobras Research Center (CENPES). The sulfonated polystyrene samples (PSS) were prepared as part of our previous study.43 The polymer sulfonation was carried out by reacting polystyrene with acetylsulfate at 40 °C, during different reaction times: 10 and 15 min, producing samples named PSS5 and PSS6, respectively. Indexes “a” and “b” were used for two different batches at the same reaction conditions. The number (M̅ n ) and weight
δμιξ = (δ1 × ϕ1) + (δ2 × ϕ2)
(1)
where δ denotes the solubility parameter and ϕ is the fraction by volume of each solvent in the mixture. Stabilization of the Asphaltenes. The destabilization/stabilization of the asphaltenes was evaluated by the precipitation test, in a Varian Cary 50 ultraviolet spectrometer with a cuvette having a 2 mm optical path. The absorption values of the asphaltenes in solution were measured at 850 nm, the wavelength at which there is absorption by the asphaltenes but not the additives used. 1088
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Initially we performed control tests, i.e., without the addition of an additive in the asphaltene solutions, carried out with mixtures of heptane/toluene and isopropyl alcohol/toluene (0, 10, 20, 30, 40, 50, 60, 70, 80, and 90% heptane or isopropyl alcohol, respectively, in relation to total volume). The concentration of the asphaltene samples (A-C7 and A-C5) was 0.5 wt %/v. In another test, 1 mL of the stock solution of asphaltenes in toluene was placed in each of a series of 10 mL centrifuge tubes, after which the toluene/isopropyl alcohol mixture (50/40) in varied volumes and the solvent n-heptane were added, so that the final mixtures contained 0.5 wt %/v of asphaltenes, 60/40 toluene/isopropyl alcohol, and varied amounts of n-heptane. In all the tests, the samples were prepared by magnetic stirring. Since asphaltenes undergo a kinetic process of precipitation and flocculation45,46 and it was not the aim of this work to study such phenomena, all samples were left at rest for the same time: 24 h. After resting, the samples were centrifuged at 3000 rpm for 30 min. The absorbance values of the supernatants were read in an ultraviolet spectrometer at 850 nm and converted into the percentage of stabilized asphaltenes from the response curves plotted. Although the apparent absorption of visible light has been demonstrated to be a scattering mechanism,47 it was possible to monitor the precipitation phenomena in this work. Assays of Asphaltene Precipitation in the Presence of Additives. More than one system was utilized to test the asphaltene precipitation in the presence of additives. In these tests, different solvents were used to prepare the asphaltene dispersions as well as the additive solutions. Precipitation Tests of Asphaltenes in Toluene in the Presence of Additives in Toluene/Isopropyl Alcohol (60/40). The precipitation test was used to evaluate the destabilization/stabilization of the asphaltenes obtained by extraction with n-heptane (A-C7I) and npentane (A-C5I). The asphaltene stock solutions were prepared in concentrations of 1%, 2.5%, and 5 wt %/v, then left at rest for 24 h, after which they were placed in an ultrasound bath for 15 min to ensure complete solubilization of the asphaltenes. The volume of solution in the round-bottom beaker was adjusted before the precipitation tests. The stock solution containing the additive was prepared at a concentration of 40 wt %/v using the mixture of toluene/isopropyl alcohol (60/40 ratio) as solvent. This solution was also left at rest for 24 h to allow complete solubilization of the additive. A series of ∼10 mL centrifuge tubes was filled with 1 mL of the stock solution of asphaltenes in toluene and 9 mL of the same solvent to obtain the desired final concentrations (0.1%, 0.25%, and 0.5 wt %/v). Then aliquots of the additive solution (40 wt %/v) were added in the tubes containing the asphaltene solutions; for each tube, the volume of additive added varied between 2.5 and 200 μL, according to the desired final concentration (0.01% to 0.8 wt %/v, respectively). Although there was variation in the final volume of the asphaltene solution according to the additive concentration, this volume was disregarded in the final calculations because of the small quantity of additive solution added. Precipitation Tests of Asphaltenes Carried out in the Presence of Additive Both Dissolved in the Toluene/Isopropyl Alcohol Mixture. In this case, both the stock solutions of asphaltenes A-C5I and A-C7I and the additive stock solution (40 wt %/v) were prepared in mixtures of toluene/isopropyl alcohol (60/40). A series of ∼10 mL centrifuge tubes was filled with 1 mL of the asphaltene solution in toluene and 9 mL of a mixture of toluene/ isopropyl alcohol (ratio of 50/40). The final proportion in 10 mL was 60/40 of toluene/isopropyl alcohol considering the 1 mL of asphaltene solution added at the start. The aliquots of the additive solution (40 wt %/v) were added in the tubes containing the asphaltene solution, and the same procedure described previously was carried out. The asphaltene concentration employed was 0.5 wt %/v.
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RESULTS AND DISCUSSION
Selection of the Initial Solubility Conditions between the Polymers and Solvent Mixture. In studies of asphaltenes in model systems, toluene has been the solvent most widely used. In our previous work43 on the stabilization of asphaltenes with sulfonated polymers (PPS), we used a mixture of toluene and acetone. Acetone, with a higher Hildebrand solubility parameter than toluene, was selected to ensure the solubility of the PPS in the medium. The complete dissolution of the substances involved is a necessary initial condition for subsequent studies of the interaction between polymers and asphaltenes. It is known, however, that acetone molecules form dimers in dipole−dipole interactions, shielding the ketone group by the methyl group, which impedes the solubilization of the phenyl group of polystyrene.48 This fact can result in distortions of the solubility of polymers in the presence of acetone, and since our interest here was to interpret the behavior in function of the solubility parameter of the mixture, we chose isopropyl alcohol instead of acetone. We initially evaluated the mutual solubility between toluene and isopropyl alcohol in different volumetric proportions. Under the conditions evaluated, the solvents were mutually soluble. The Hildebrand solubility parameter of each mixture was calculated by eq 1 and found to vary between 18.2 MPa1/2 for toluene and 23.5 MPa1/2 for isopropyl alcohol. This range of solubility parameter values defined the conditions under which the interaction between asphaltenes and polymers was studied (Table 1). The qualitative solubility of the polymers PS and PSS at the concentration of 1% in toluene was characterized by the formation of a clean and transparent solution (absence of turbidity), as judged visually. The working temperature was defined as the range between 5 and 60 °C. The data reported in Table 1 were obtained at 25 °C. In these conditions, the PS totally dissolved in pure toluene while the PPS samples were insoluble. This result agrees with what was observed in the previous study43 for the toluene and acetone. For the PS, under the conditions evaluated, the insolubility started in proportions of isopropyl alcohol greater than 20 v/v %, that is, with a solubility parameter greater than 19.8 MPa1/2, while the PPS samples were insoluble in pure toluene, justifying the use of a substance with higher solubility parameter to guarantee dissolution of the PPS in the medium. For the mixture of toluene and isopropyl alcohol, we found a solubility range of the sulfonated polymers between 10 v/v% and 50 v/v % of isopropyl alcohol in toluene for the PPS samples with sulfonation degree of 7 mol %, a range that increased to 60 v/v % of isopropyl alcohol when the sulfonation degree of PSS was increased to 10 mol %. The mixtures were prepared to find conditions that favor the complete solubility between the components, not to represent their quantitative solubility. For example, we did not test whether only a small amount of isopropyl alcohol would be sufficient to dissolve the PSS, so the parameter value of 18.7 MPa1/2 does not imply a lower solubility limit of these systems. The results of qualitative solubility shown in Table 1 indicate the less hydrophilic character of PS in relation to the modified samples, since the solubility was restricted to solvents with solubility parameters lower than 19.3 MPa1/2. Consequently, the increase of the sulfonation degree contributed to reduce the polymer’s hydrophobicity, as expected for this modification.49,50 1089
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its components are δD = 18.2 MPa1/2, δP = 6.3 MPa1/2, and δH = 6.1 MPa1/2.53,54 The asphaltenes are precipitated by the addition of a nonsolvent with higher solubility parameter than toluene, so for the A-C7I fraction the value of 18.7 MPa1/2 represents the condition in which the molecules with lower solubility parameter leave the solution, such that this is the upper parameter limit for the destabilization of asphaltenes C7I.37,55−59 After studying the influence of isopropyl alcohol on the precipitation of asphaltenes, we also evaluated the precipitation with a nonsolvent having a lower solubility parameter, nheptane, with 15.3 MPa1/2. Figure 3 panels a and b show,
It can also be observed that the sample PSS6, for being more hydrophilic, was soluble in a parameter range up to ∼21.4 MPa1/2. All the PSS samples were soluble in the interval from 18.7 to 20.9 MPa1/2, indicating this was the best interval to select the suitable conditions for the objectives of this study. Behavior of the Asphaltenes in the Model Systems. We first investigated the phase behavior of the asphaltenes in toluene using isopropyl alcohol as nonsolvent, with two asphaltene fractions: A-C7I (insoluble in heptane) and A-C5I (insoluble in pentane), both obtained from an asphaltic residue. The destabilization of the asphaltenes was observed at isopropyl alcohol concentrations lower than 10 v/v% for A-C5I and starting at just above 10 v/v% for A-C7I, as shown in Figure 2. Therefore, the increase of the medium’s solubility
Figure 2. Precipitation test of asphaltenes A-C5I (●) and A-C7I (■) dispersed in toluene, using isopropyl alcohol as nonsolvent.
parameter by adding isopropyl alcohol resulted in partial insolubility of the components of the asphaltene fractions. This result indicates that fraction C5I has molecules with lower solubility in isopropyl alcohol than does fraction C7I, which can be explained by the higher content of resin molecules in A-C5I. The content of resins in A-C5I has also been used explain the greater tendency for precipitation of C7I in previous works,17,18,37,51,52 in which the precipitation occurred in function of the addition of a flocculant with low Hildebrand solubility parameter (e.g., n-heptane). For the quantity of 40% isopropyl alcohol (δ = 20.3 MPa1/2), the initial concentration of asphaltenes declined by about 10%. Therefore, we considered that for this proportion, 10% of the asphaltenes in the medium had already precipitated. It is important to highlight that the asphaltenes solubility in a solvent depends better on the three different components of the solubility parameter, that are dispersion forces (δD), permanent dipole-permanent dipole forces (δP), and hydrogen bondings (δH).53 Isopropyl alcohol is a nonsolvent for asphaltenes since its δP and δH components are relatively high (9.1 and 16.4 MPa1/2, respectively) and the δD component is relatively low (15.8 MPa1/2), if they are compared with the components of toluene (δD = 18.0 MPa1/2, δP = 1.4 MPa1/2, and δH = 2.0 MPa1/2). A mixture of toluene/isopropyl alcohol 60/ 40 (δ = 20.3 MPa1/2) does not completely dissolve asphaltenes; its components would be δD = 17.1 MPa1/2, δP = 4.5 MPa1/2, and δH = 7.8 MPa1/2. Other solvents presenting a solubility parameter close to this mixture are able to solubilize asphaltenes since their δH component is lower and δD is higher. For example, methylene chloride can solubilize asphaltenes; its solubility parameter is 20.2 MPa1/2; however,
Figure 3. Precipitation test of asphaltenes dispersed in toluene (●) and in toluene/isopropyl alcohol (60/40) (■), using n-heptane as nonsolvent: (a) A-C5I and (b) A-C7I.
respectively, the results obtained for asphaltene fractions C5I and C7I in two distinct solvents: pure toluene and toluene/ isopropyl alcohol 60/40 (the latter being the condition in which the asphaltenes are partially soluble). As shown in Figure 3, the precipitation onset of the asphaltenes occurred in higher proportions of n-heptane for the systems with A-C5I than for A-C7I. This result reveals the presence of more soluble components in heptane in the composition of A-C5I, in agreement with results reported previously in the literature18,48,59−61 and with the results obtained in the assay with destabilization induced by isopropyl alcohol (Figure 2). Assuming that the asphaltene fraction C7I started to precipitate in 50 v/v% of n-heptane, this fraction started to precipitate at a solubility parameter of ∼16.5 MPa1/2. Therefore, for A-C7I it was possible to determine the solubility range in terms of Hildebrand parameters as being between 16.7 and 18.7 MPa1/2, which is near the interval already reported before,59 determined by the intensity of 1090
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an influence on the final solubility parameter due to the excess of n-heptane, which tends to shift the solubility parameter to a value near 15.3 MPa1/2 and maintain it there. These preliminary observations provide support for a better interpretation of the interaction of polymers and asphaltenes in the presentation of the results that follow, particularly whether this interaction results in solubilization or stabilization of particles. Evaluation of Precipitation/Stabilization of Asphaltenes in the Presence of Additives. The additives PSS5a, PSS5b, and PSS6a were selected for this study based on previous findings, which indicated the influence of these types of polymers on the phase behavior of asphaltenes.43 The precipitation tests of asphaltene fractions A-C5I and AC7I were performed while varying the concentration of asphaltenes and additives. Figure 4 panels a and b show the
interactions using UV−vis spectrometry: 17.2 to 19.0 MPa for C7I asphaltene extracted from an asphaltic residue. The literature also reports a value of 16.2 MPa1/2 as a lower solubility parameter threshold for precipitation of asphaltenes by the addition of n-heptane. The value of 16.2 MPa1/2 is usually applied in models of crude oil compatibility in which it is assumed that the asphaltenes start precipitating at the same destabilization parameter independent of the nature of the crude oil.57,58 These results are also in agreement with those found here, since the asphaltenes in petroleum tend to be more stable than the A-C7I fraction in a model system due to the presence of molecules similar to those of C7I, but with lower polarities, helping to stabilize the more polar asphaltenes present in the oil. More stable systems precipitate with higher n-heptane concentrations, causing the solubility parameter at the precipitation onset to be lower than that observed in model system containing A-C7I. Although not our direct objective, we can infer that the solubility parameter range of the A-C7I fraction (extracted in this study) in the model system is from 16.7 to 18.7 MPa1/2. As a consequence, a good solvent for A-C7I should have a solubility parameter of 17.7 ± 1.3 MPa1/2 (solubility radius), which justifies the use of toluene as asphaltene solvent (solubility parameter of 18.2 MPa1/2), considering for this analysis only the total solubility parameter, that is, Hildebrand Solubility Parameter. Regardless of the nature of the asphaltenes, the solutions with pure toluene were more soluble than those with toluene and isopropyl alcohol: (1) By using isopropyl alcohol, the quantity of asphaltenes that precipitated was always greater, up to slightly above the precipitation onset, since isopropyl alcohol in the proportion studied (60/40) precipitated about 10% of the initial concentration of asphaltenes (Figure 2). (2) The precipitation onset was lower for the toluene/ isopropyl alcohol solvent than for the toluene. In fact, asphaltenes are a class of polydispersed molecules in relation to polarity, so the addition of isopropyl alcohol to the medium causes precipitation of the less polar molecules, which helps the stabilization of the more polar molecules when an asphaltenes nonsolvent with low δ (in this case n-heptane) is added. The absence of these molecules dispersed thus facilitates the precipitation of the more polar molecules and a lower volume of nheptane is required to attain precipitation onset, as observed in Figure 3. As n-heptane was added, we would expect the solubility of the less polar asphaltene molecules when the system achieved the range of solubility parameters in which these molecules are soluble, followed by the precipitation from the liquid phase of the more polar asphaltene molecules when the system achieved the precipitation solubility parameter of these molecules. Although the asphaltene precipitation phenomenon is reversible, many authors have reported the presence of hysteresis, which disfavors the resolubilization of the asphaltenic particles.62 This can explain the fact that the solubilization did not occur at the beginning of n-heptane addition. We also observed, just beyond the asphaltenes precipitation condition, a larger quantity of asphaltenes precipitated in the toluene/isopropyl alcohol in comparison with the pure toluene. This difference gradually declined at higher n-heptane content, because the concentration of the initial mixture ceased having
Figure 4. Concentration of soluble asphaltenes A-C5I, at initial concentrations of 0.10 wt/v% (●), 0.25 wt/v% (■), and 0.50 wt/v% (▲), in function of polymer concentration: (a) PSS5b and (b) PSS6a. Asphaltenes were previously dissolved in toluene.
results of concentration of A-C5I in the medium in function of the concentration of PSS5b and PSS6a, respectively. The effect of the initial concentration of asphaltenes was also investigated. In this test, the additive was dissolved in the toluene/isopropyl alcohol mixture (60/40) and a small volume of this solution, varying from 25 to 200 μL, was added to 10 mL of the solution of asphaltenes in toluene to reach additive concentrations between 0.01 and 0.8 wt/v%, respectively. Since this additive is insoluble in toluene, it precipitates a condition that is different from that addressed in our previous work.43 Figure 4 panels a and b reveal that the concentration of asphaltenes in solution diminished with rising concentration of additives, indicating the polymer induced the flocculation of the asphaltenes. This reduction was more significant at concen1091
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Energy & Fuels trations below 0.2 (wt/v%) and for the systems with higher concentrations of asphaltenes. The influence of increasing the quantity of isopropyl alcohol as a function of the increase in the polymer concentration was not considered, since the level of this solvent only varied from 0.01 to 0.8 v/v% in relation to toluene, so its influence can be considered negligible, as can be seen in Figure 2. Figure 4b shows that in the polymer concentration range of 0.2 to 0.4 (wt/v%), half of the asphaltenes precipitated with initial asphaltene concentrations of 0.25 and 0.5 (wt/v%). With a polymer concentration higher than 0.4 wt/v%, the concentration of asphaltenes in the solution remained virtually constant. This result suggests that a determined fraction of asphaltenes is flocculated by the polymer, probably that with greater polarity due to the better interaction with the chain of the polymer, which is also polar. The behavior of PSS5b and PSS6a was similar, although PSS6a exhibited a more pronounced flocculant action than PSS5b: for initial asphaltene concentration of 0.25 wt/v% and additive concentration of 0.3 wt/v%, the concentration of soluble asphaltenes in the medium declined from 0.33 to 0.22 wt/v% with PSS5b (7 mol % sulfonation) and from 0.27 to 0.11 wt/v% with PSS6a (10 mol % sulfonation). In our previous work,43 comparing samples of polystyrene with sulfonation degrees of 4, 5, 7, 12, and 13 mol %, we observed that 7 mol % was the optimal degree for good flocculant action when the additive remained soluble in the solution. The results of the present study show that when the polymer was insoluble in the medium, the sulfonation degree of 10 mol % was more effective in removing the asphaltenes from the solution. Similar behavior was found for asphaltene fraction A-C7I, as shown in Figure 5 panels a and b. The additive PSS6a, at a concentration of 0.25 wt/v%, caused a larger decline in the concentration of dispersed asphaltenes (from 0.35 to 0.10 wt/v %) than PSS5b (from 0.27 to 0.15 wt/v%), confirming the more effective flocculant action of the polymer with sulfonation of 10 mol %. The percentage of fraction A-C7I removed from the solution was significantly higher (∼70%) than that of A-C5I. This is coherent with the fact that fraction A-C5I is more stable than C7I due to the wider polarity distribution of its molecules,16,37,42,63,64 and also because the polymer, with polar character, interacts preferentially with the more polar asphaltene molecules, indicating that the most likely action mechanism was adsorption of the asphaltenes on the polymers. To check the influence of solubilization of the polymer on its action on the phase behavior of the asphaltenes, we performed new experiments, keeping the solvent at 60/40 toluene/ isopropyl alcohol to guarantee solubility of the polymer in the medium. Figures 6 and 7 show, respectively, the effect of concentration of PSS6 and PSS5 on the quantity of asphaltene fraction A-C5I that remained soluble in the medium. For PSS6a (Figure 6), which has sulfonation of 10 mol %, the optimal polymer concentration to achieve maximum flocculant action was ∼0.5 wt/v%. In turn, the optimal concentration for maximum flocculant action with use of PSS5a and PSS5b (both with sulfonation of 7 mol %) is shown in Figure 7. In this particular experiment, it was possible to assess the influence of random distribution of the sulfonic groups with a single concentration of asphaltene fraction A-C5I. The two polymer samples showed very similar behavior, with the maximum flocculation point at the same concentration (0.3 wt/v%). Thus, it can be suggested that the positioning of the sulfonic
Figure 5. Concentration of soluble asphaltenes A-C7I, at initial concentrations of 0.10 wt/v% (●), 0.25 wt/v% (■), and 0.50 wt/v% (▲), as a function of polymer concentration: (a) PSS5b and (b) PSS6a. Asphaltenes were previously dissolved in toluene.
Figure 6. Concentration of soluble asphaltenes A-C5I as a function of PSS6a concentration. Asphaltenes were previously dissolved in toluene/isopropyl alcohol (60/40).
groups in the polymer chain does not significantly alter the additive’s activity, so it is only necessary to consider the sulfonation degree and concentration of polymer utilized. This type of behavior was observed in our previous study43 with the toluene/acetone, with another sample of asphaltenes and polystyrene samples having sulfonation degrees other than 10 mol %. Figure 6 of that study43 presented a proposed mechanism for stabilization of asphaltenes by PSS, according to which the increase in polymer concentration and/or sulfonation degree improves the process of stabilizing asphaltenes. Despite use of the expression “stabilization of asphaltenes”, the probable mechanism is molecular and reflects the interaction of asphaltene monomers along the polymer chain. 1092
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Energy & Fuels
only an approximation. For PSS5b it is 7 asphaltene molecules for each 10 functional groups. These results indicate that the asphaltenes remain in solution through the interaction of the functional groups in the chemical structure of the asphaltenes with the sulfonic groups, indicating the molecular nature of this mechanism and confirming the proposed mechanism presented earlier.43
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CONCLUSIONS The flocculant action of amphiphilic polymers depends on their degree of hydrophobicity and concentration. For sulfonated polystyrene, the optimal sulfonation degree for flocculant action is ∼10 mol %, and the position of the sulfonic groups in the polymer’s chain does not appear to change its activity. The existence of a concentration of hydrophilic groups at which inversion of the polymer’s behavior occurs (flocculant ↔ dispersant) was confirmed. The affinity/solubility of the polymer in the dispersion medium has a significant influence on its activity: if there is affinity with the medium, the polymer’s action as a flocculant or dispersant is related to its concentration in the medium, but if there is no affinity, the polymer behaves only as a flocculant. Moreover, the percentage of asphaltenes flocculated by the addition of an n-alkane (e.g., n-heptane) is directly related to the procedure followed in adding the polymer: if the polymer becomes immediately insoluble in the medium, a higher quantity of asphaltenes is precipitated after the addition of the nonsolvent, while a smaller quantity of asphaltene precipitates when the polymer remains soluble after being added to the system.
Figure 7. Concentration of soluble asphaltenes A-C5I as a function of PSS5a (■) and PSS5b (●) concentration. Asphaltenes were previously dissolved in toluene/isopropyl alcohol (60/40).
Comparison of this type of test with that performed with a solvent in which the polymer did not remain soluble in the medium shows that the polymer’s activity significantly depends on its affinity/solubility in the dispersion medium. If affinity exists, the polymer shows maximum flocculant activity at a certain concentration, which depends on its sulfonation degree. However, when no affinity exists between the polymer and medium, the flocculant activity is directly related to the increase of its concentration in the medium, reaching a plateau of maximum flocculation in which the quantity of asphaltenes removed from the solution is a function of the polymer’s sulfonation. Figures 6 and 7 show the two behaviors as a function of polymer concentration. In the first, the concentration of asphaltenes in the solution decreases and in the second there is a proportional increase between the additive’s concentration and the asphaltenes’ concentration in the solution. The second pattern suggests a constant relation between the number of polymer molecules necessary to keep the asphaltenes in solution. In this case, we assumed the behavior is first-order linear whose derivative provides a relation between the mass of asphaltenes and mass of polymers, which corresponded to 0.36 for PSS5a and 0.25 for PSS5b. Then, with the value of 500 g/ mol for molar mass of the asphaltenes19 and 87 300 g/mol for the polymer, it was possible to calculate the ratio between the number of asphaltenes and polymer molecules, which was 62 for PSS5a and 43 for PSS5b. These results reflect the polydispersion degree of each polymer and also indicate that the polymer molecules interact with much larger number of asphaltene molecules. To assess the relation between the sulfonation degree and number of asphaltene molecules in the solution, we calculated the number of mers of the PSS by dividing the polymer’s molar mass (87 300 g/mol) by that of the monomer (99 g/mol), resulting in an average value of 882 mers per PSS chain. Considering that 7% of the mers of PSS5a and PSS5b were sulfonated, we calculated a total of 62 functional groups. This number coincides with the number of asphaltene molecules that interact with PSS5a in the 1:1 proportion between asphaltenes and the sulfonic functional group. We stress, however, that because of the characteristics of the asphaltene and polymer molecules, and due to the high degree of polydispersion, or also because of the distribution of the functional groups in the molecular structure, the value found is
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Claudia R. E. Mansur: 0000-0001-9624-7056 Elizabete F. Lucas: 0000-0002-9454-9517 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank CNPq (307193/2016-0), CAPES, and FAPERJ (E-26/201.233/2014) for financial support and Petrobras for the asphaltic petroleum residue (ASPR).
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