Inhibition of asphaltene precipitation in Brazilian crude oils using new oil soluble amphiphiles

Inhibition of asphaltene precipitation in Brazilian crude oils using new oil soluble amphiphiles

Journal of Petroleum Science and Engineering 51 (2006) 26 – 36 www.elsevier.com/locate/petrol Inhibition of asphaltene precipitation in Brazilian cru...

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Journal of Petroleum Science and Engineering 51 (2006) 26 – 36 www.elsevier.com/locate/petrol

Inhibition of asphaltene precipitation in Brazilian crude oils using new oil soluble amphiphiles Luiz Carlos Rocha Junior, Maira Silva Ferreira, Antonio Carlos da Silva Ramos * Departamento de Tecnologia Quı´mica — Universidade Federal do Maranha˜o, Sa˜o Luis — MA, Brazil Accepted 4 November 2005 Dedicated to Prof. Rahoma Sadeg Mohamed

Abstract Asphaltene and resins are heavy fractions of petroleum responsible for serious problems during petroleum production. These problems include the formation of organic deposits in oil reservoirs; wells, transport pipelines and equipment, and can significantly increase the production operational costs. The nature and behavior of asphaltene in crude oils is complex and changes in temperature, pressure and composition of crude oils during production can result in precipitation of asphaltene components. Potential solutions for these problems include physical removal of deposits, solvent washes and treatment with dispersant agents. The use of soluble amphiphile oils provides the most practical and economical solution for deposits treatment. In this work, the inhibitory capacity of a number of new chemical additives to asphaltene precipitation was examined in three types of Brazilian crude oils. Low molar mass ethoxylated nonylphenols, vegetable oils (coconut essential oil, sweet almond, andiroba and sandalwood oil) and organic acids (linoleic, caprylic and palmytic) displayed highest capacity to inhibit asphaltene deposition. The dissolution capacity of some additives was evaluated for two asphaltenic deposits in aliphatic solvents. The remarkable solubilization effect displayed by dodecylbenzenesulfonic acid, confirmed the importance of acid–base interactions in this process. The results also revealed distinct mechanisms for asphaltene solubilization/dispersion in aliphatic solvents and inhibition of asphaltene precipitation in crude oils. D 2006 Elsevier B.V. All rights reserved. Keywords: Asphaltene; Deposit; Precipitation; Polymeric dispersants; Amphiphiles

1. Introduction Organic deposits formation is a problem that seriously affects the petroleum industry worldwide with drastic economical implications which depend on the extension of the phenomenon. In the literature there are many accounts relating the problems generated by the deposition of heavy fractions in the various steps of the production process (Islam, 1994; Leontaritis and Man-

* Corresponding author. E-mail address: [email protected] (A.C. da Silva Ramos). 0920-4105/$ - see front matter D 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2005.11.006

soori, 1988, Ramos, 2001). Deposits formation during petroleum production causes several operational troubles such as total or partial blocking of pipelines and reservoirs, changes in wettability and equipment damages. Changes in the balancing of the crude oil phases during production and processing may lead to the formation of solid phases through the precipitation of heavy fractions, such as asphaltenes, resins and paraffin. Due to their aggregative nature (Ramos et al., 2001; Ramos, 2001; Mohamed et al., 1999a; Loh et al., 1999), asphaltene and resins are important deposit forming crude oils heavy fractions. These fractions are consti-

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tuted by molecules with polycondensated aromatic rings and lateral aliphatic chains, acid–base functional groups and complexed metals (Leo´n, 1998; Al-Sahhaf et al., 2002), varying in size as well as in their aggregation tendency. The IP143/84 (1989) procedure, defines asphaltene as a precipitated of crude oil fluid produced by the addition of an exceeding portion of nheptane, thus forming amorphous particles soluble in toluene or benzene. Measures for solving asphaltene precipitation and deposition problems are typically three fold: (a) the development of theoretical models (Vasquez and Mansoori, 2000; Pacheco-Sanchez and Mansoori, 1998; Victorov and Firoozabadi, 1996; Islam, 1994; (b) the study of asphaltene interfacial and colloidal behavior in crude oils and model systems (Ramos, 2001; Ramos et al., 2001; Mohamed et al., 1999a; Carbognani, 2001) and; (c) the estimation of chemical additives for the inhibition of asphaltene precipitation (Chang and Fogler, 1994a,b; Mohamed et al., 1999b; Gonza´lez and Middea, 1991; Rogel and Leo´n, 2001). With respect to the estimation of chemical additives for the inhibition of asphaltene precipitation, several procedures are used to remove or prevent deposits formation throughout the production, transportation and refining operations, among which we can cite: mechanical removal, use of ultrasonic techniques, cleaning with solvents, removal with hot fluids or water steam and the addition of inhibitors and dispersants (Ramos, 2001). Solvent treatment comes to be an excellent alternative; however the most applicable such as toluene, xylene, benzene and chlorate solvents, are flammable, carcinogenic, dangerous for handling and harmful for the environment. In addition, many of those techniques may cause pauses in the production. Using of substances which effectively stabilize or solubilize asphaltene in crude oils is either a preventive or remedial measure which, in addition, saves costs and ease its application (Stephenson, 1990). Market products with amphiphile agents are used as asphaltene precipitation inhibitors and the Pfeiffer micelle (Pffeifer and Saal, 1940), one of the first attempts to explain crude oils colloidal behavior, is still used as to justify the additives effect on the asphaltene stabilization. In several researches it is assumed that asphaltene particles are kept dispersed in crude oils due to interactions between natural surfactant polar groups (resins) and asphaltene superficial charges. The analysis of changes in crude oils composition by low molecular mass n-alkane titration is a common technique to identify the additive’s affectivity over asphaltene and, in this case, such affectivity depends

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on the higher additive’s capacity to maintain the asphaltene stabilized in the oil phase (Ramos et al., 2001; Mohamed et al., 1999b; Subodhsen et al., 1999). Commercial additives are mixtures difficult to characterize once they are produced with no scientific criteria and, in general, they are effective only with crude oils already tested (Boer et al., 1992). A few studies have presented alternatives to identify and develop more effective substances. Nevertheless, due to the complex nature of the crude oils it is necessary to estimate these substances with as many types of crude oils as possible in order to obtain more sustainable results. Usually, the studies have addressed the use of nonionic amphiphiles, such as the ethoxilated nonylphenols for asphaltene stabilization process (Gonza´lez and Middea, 1991; Loh et al., 1999; Mohamed et al., 1999b; Ramos et al., 2001, Ibrahim and Idem, 2004a), and the importance of acid–base interactions for asphaltene dissolution process in aliphatic solvents (Chang and Fogler, 1993, 1994a,b; Ramos et al., 2001). In the search for new inhibitors, the low solubility of certain substances in crude oils has become a barrier for the development of more effective mixtures as well as a restriction for those products practical use. Ramos et al. (2001), selected diverse polymers with functional groups similar to the ones usually found in the research concerning asphaltene stabilization in crude oils and aliphatic solvents, being most of them total or partially insoluble in the estimated crude oils. In the literature, few studies have evaluated the use of vegetable oils for preventing asphaltene precipitation. Vegetable oils are mixtures that, due to their chemical nature, are more soluble in crude oils which, in addition, contain some substances with characteristics similar to those found in amphiphiles already estimated. Cashew-nut shell liquid, for example, almost totally formed by phenolic compounds with linear unsaturated alkyl chains (fifteen carbon atoms) meta-substituted in aromatic rings, performed well in the peptization and inhibition of asphaltene precipitation (Moreira et al., 1999). It was also observed that changes in pressure and temperature can cause an increase in the asphaltene stability, thus reducing the inhibition rate necessary for preventing its deposition (Aquino-Olivos et al., 2001; Boer et al., 1992). The use of vegetable oils as inhibitors of asphaltene precipitation is a plausible economical measure because of its low production cost, cheaper than the cost of the majority of the commercial highly elaborated products used as dispersants, its easy handling and operational application which does not cause environmental threats. Yet, essences and vegetable oils present in their chem-

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ical composition substances with functional groups already estimated in the literature as effective in the inhibition of asphaltene precipitation (Moreira et al., 1999), besides the fact that these substances are chemically compatible with crude oils, and thus usable during the production process without affecting the crude oil original quality. In this work, vegetable oils and derivatives were estimated against those of certain classical amphiphiles, in situations involving both, the asphaltene precipitation and the asphaltene solubilization capacity in aliphatic solvents. 2. Experimental section The A, B, C crude oils used in this work came from important Brazilian petroleum fields, and the asphaltenes were separated from the crude oils according to IP 143/84 procedure (1989), Ramos et al., 2001. To obtain the asphaltenes, n-heptane, n-pentane and toluene were used (all three purchased from Carlos Erba, with purity higher than 99%). The obtained asphaltenes were labeled as C7I-insoluble in n-heptane or as C5I-insoluble in n-pentane. The following substances were evaluated as asphaltene precipitation inhibitors: (i) Ionic surfactants — dodecylbenzenesulfonic acid (DBSA) purchased from Hoechst; sebacic acid, from BHD; caprylic acid, also from BHD; linoleic acid, from Acros Organics; palmytic acid, from Vetec; salicylic acid, from Isofar; 8-hydroxyquinoline acid, from Merck; alizarin, also from Merck; m-hydroxybenzoic acid, from Sigma; cetyltrimethylammonium bromide (CTAB) from Aldrich; ethylparaben and methylparaben from Sigma; sodium dodecyl sulfate (SDS) from Sigma; poly (ethylene-co-acrylic acid) from Aldrich; (ii) Nonionic surfactants — (nonylphenols ethoxylated — Renex), Renex 18, Renex 40, Renex 100 and Renex 1000, from Oxiteno; (iii) Vegetable oils — soy oil, buriti oil, copaı´ba oil, pequi oil, babac¸u oil, sweet almond oil, resin oil, andiroba oil, arruda sabina oil, Brazil nut oil, grape seed’s oil, essential eucalyptus oil, pepper Jamaican essential oil, pinewood essential oil, sandalwood essential oil, coconut essential oil, lemon essential oil. All the oils were commercial samples, purchased in Sa˜o Luis, State of Maranha˜o, Brazil, and were used as received; (iv) Vegetable oil derivatives — a-terpinene, g-terpinene, linalyl acetate, linalool and eugenol (all

from Dierberg S.A. oil essentials industry— Brazil). The asphaltene precipitation onset was determined by n-heptane titration of the crude oil, and the particle formation was viewed with the help of an Olympus BX51/BX52 optical microscope. The additive effectiveness to inhibit precipitation was estimated by comparing the precipitation onset either, with or without the use of additives. The additive’s capacity to solubilize asphaltene was estimated in aliphatic solvents (n-heptane and n-pentane), considering the ones that better affected the asphaltene precipitation. Asphaltene concentration in the supernatant was determined by means of a Cary50 type spectrophotometer from Varian, in the visible region (400 nm), and the increase in the asphaltene concentration was considered either, as a marker of the additive’s inhibitory effectiveness or ineffectiveness in the asphaltene precipitation. 3. Results and discussion 3.1. Asphaltene precipitation onset in crude oils The evaluated crude oils presented different characteristics, as shown in Table 1 for C7I and C5I asphaltene content and for the precipitation onset. Yet, crude oil C presents a great amount of suspended paraffin. Generically, crude oils A and B are classified as asphaltenic, but the latter derives from a mixture of several crude oils, and C is a paraffinic crude oil. A, B, and C crude oils individual precipitation onset is shown in Table 1 and represents the smallest nheptane amount to produce precipitation, as determined by optical microscopy. Experiments were carried out at 28 F 1 8C, in duplicate, and the results were used as reference to estimate the capacity of the additives to inhibit asphaltene precipitation. On the other hand, in this work, the precipitation onset was used just as reference to estimate the additive’s effectiveness to hold the asphaltene within the oil phase. Recent works (Ibrahim and Idem, 2004b,c) have shown that asphaltene precipitation behavior is a function either, of Table 1 Onset of precipitation at a temperature of 28 F 1 8C and asphaltene content for different Brazilian crude oils Petroleum

Onset precipitation (mL n-heptane/g oil)

C5I (wt.%)

C7I (wt.%)

A B C

3.1 F 0.1 2.2 F 0.1 2.8 F 0.1

7.7 F 0.3 3.3 F 0.5 7.0 F 0.3

4.2 F 0.3 1.2 F 0.4 2.8 F 0.4

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the crude oil nature or the structural characteristics of the asphaltene molecules; yet, they also show that the asphaltene stabilization mechanism is equally a function of the additive’s nature. In some cases, the effectiveness can be related with a significant decrease in the amount of precipitated asphaltene, not necessarily occurring onset shifting (Ibrahim and Idem, 2004b,d). Yet, it can be verified that the conditions along the processing and production of crude oils are different from those found in the laboratory and, in this case, our results furnish only an indication that supposedly favors the practical use of these substances that, for sure, need further accurate estimation under field conditions. For the three crude oils, the C5I asphaltene content was always bigger than C7I asphaltene, which is a characteristic found in crude oils from different sources (Fotland et al., 1993). The fact that the B crude oil presents lower precipitation onset compared with the A and C crude oils, added with the A crude oil higher C5I and C7I asphaltene content against the A and C crude oils, is not related with the asphaltenes tendency toward precipitation. In Brazil, asphaltene precipitation is usually found in light crude oils with lower asphaltene content. Nevertheless, problems related to asphaltenes deposition are frequently observed in light crude oils and derivatives in the diverse production stages, irrespective of its origins. 3.2. Inhibitors effect on asphaltene precipitation in crude oils Ramos et al. (2001) reported troubles when solubilizing some additives already tested in other studies. Either the insolubility or partial solubility can lead to

Fig. 1. Inhibition capacities of chemical additives: Renex 18, 40 and 100 in oil-A.

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Fig. 2. Inhibition capacities of chemical additives: Renex 18, 40 and 100 in oil-B.

mistaken experimental interpretations and turn unviable the practical use of those substances. In this work, it was carefully observed whether the additives, throughout the estimated concentration range (about 8% in weight), were or not soluble in the crude oils, either by the lack of solids or emulsions. That amount represents a far higher percent compared with the one used in loco (about a few parts per million); however it was necessary in order to establish a relationship between the structural parameters of the additive molecules and their effectiveness to inhibit asphaltenes precipitation. In this work the graphs show only the soluble additives that acted better as inhibitors of asphaltenes precipitation in the evaluated concentrations. All probes concerning the study of asphaltenes precipitation were carried out under the same experimental conditions, with uncertainty of (0.1 mL/g for each measurement). 3.2.1. Renex effect over the precipitation onset Renex ethoxylated chain increase was followed by a higher insolubility in the crude oils, reason for which the Renex 1000 was found insoluble in the three oils, as confirmed by the presence of particles viewed by optical microscopy. The remaining three Renex were soluble within the estimated concentration range. Figs. 1 2 and 3 show the Renex effect on the asphaltene precipitation in A, B and C crude oils. The dotted line shows the precipitation onset without additives and the experimental points over this line show the additive’s efficiency to inhibit the asphaltene precipitation as induced by the addition of n-heptane. Three Renex (18, 40 and 100) were effective in the inhibition of asphaltenes precipitation, disregarding the crude oils nature. These results reinforce the capacity attributed to ethoxilated nonylphenols to inhibit crude

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oils from different sources (Ramos, 2001; Leo´n et al., 2001; Gonza´lez and Middea, 1991; Loh et al., 1999). The higher the Renex concentration the higher its inhibition effect in the crude oils, being Renex 18 and Renex 40 the ones which yielded better results, especially with the A and B crude oils. With crude oil C, the three Renex acted similarly in the inhibition of asphaltenes precipitation, provably due to the Renex smaller solubility in this crude oil and, consequently, their higher affinity with the asphaltene surface. One should notice that (Fig. 4) the Renex 18 effect over the three oils is very alike indicating, in principle, that the asphaltene stabilization mechanism is identical for the estimated crude oils. 3.2.2. Organic acids effect on the precipitation onset As to evaluate the effects of the hydrocarbon chain length, the presence of functional groups and the occurrence of double bounds, the following organic acids were tested: palmytic acid, linoleic acid, caprylic acid, sebacic acid and salicylic acid (see Fig. 5, for their chemical structures). The acids were tested with crude oils A and C and the best results were found in the following order: palmytic acid, linoleic acid and caprylic acid, as shown in Figs. 6 and 7. Within the estimated range of concentration, an increase in the additive’s concentration is proportional to the asphaltenes precipitation inhibition effectiveness. The sebacic acid, with the same number of carbon atoms as the caprylic acid and with two functional groups in the extremity, did not show any inhibitory effect in the asphaltenes precipitation, reason for which was not included in Figs. 6 and 7. Similar results as the ones presented by the sebacic acid were also observed in two different crude oils, sug-

Fig. 3. Inhibition capacities of chemical additives: Renex 18, 40 and 100 in oil-C.

Fig. 4. Effect of Renex 18 on the asphaltene precipitation onset in crude oils A, B and C.

gesting that the stabilization mechanism probably occurs due to the adsorption of just one functional group, reinforcing former proposals related with asphaltenes stabilization produced by the additives surfactant activity (Ramos et al., 2001). In the same context, we suggest that the carboxyl functional group from the sebacic acid interacts with species in the asphaltene particle surface, exposing the hydrocarbon chain to the external medium. Palmytic acid best effect, compared with the caprylic acid, can be correlated with the increase in the alkyl chain length, consequently increasing its surfactant character. This result, also obtained for the two crude oils, reinforces the former proposal for the asphaltene stabilization. The linoleic acid, which has two double bonds with larger hydrocarbon chains than the palmytic acid, because of its character prominently polar, was less efficient in the asphaltene stabilization process. The salicylic acid, which chemical structure presents functional groups similar to those of organic acids and Renex, produced a strong destabilization in the asphaltene reducing the A crude oil precipitation to 2.9 mL of n-heptane/g oil and the B crude oil to 1.0 mL n-heptane/g oil, both accounting for 3% wt/wt of the additive. In general, results concerning the organic acids study suggest that the balance between the additive lyophilic and lyophobic portions rule the asphaltenes stabilization process. 3.2.3. Vegetable oils’ effect on the precipitation onset The choice of vegetable oils as potential asphaltene inhibitors is backed by the following factors: first because of the importance of finding new substances with higher solubility; second because of the good results

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Fig. 5. Organic acids chemical structure: (a) palmytic acid (hexadecanoic acid); (b) linoleic (9,12-octadecadyenoic acid); (c) caprylic acid (octanoic acid); (d) sebacic acid (decanodyoic acid) and salicylic acid.

obtained in the present tests with organic acids, taking into account the fact that vegetable oils are mixtures rich in free organic acids or forming glycerides; third because they are easy to obtain and are cheaper than most polymeric dispersants commercially used and, finally, once Maranha˜o State is plenty in such substances, their use could lead to positive socio-economical consequences. Essential vegetable oils were randomly chosen according to their availability in Sa˜o Luis city; nevertheless we tried to estimate products of different nature, especially the more abundant ones. Tests were conducted in crude oils A and C. As vegetable oils sold in Sa˜o Luis market lack quality control varying in their composition according to the way they are processed (raw matter, mixing with adul-

terants, etc.) (Vasconcelos, 2000), special care was taken as to estimate at least three different samples of vegetable oils for each type of crude oil. In Fig. 8, the asphaltene onset represents the average value of at least two vegetable oil samples that presented the closer values. As viewed in Fig. 8, the essential coconut oil performed excellently in the inhibition of A crude oil asphaltene precipitation, as compared with other additives, fact that, in principle, recommends its application in field tests. Good results in the inhibition of asphaltene precipitation were also obtained for andiroba oil, sandalwood essential oil, pinewood essential oil, Brazil nut oil, grape seed oil, pepper Jamaican essential oil and sweet almond oil and equivalent results were observed for the same additives in C crude oil.

Fig. 6. Inhibition capacities of chemical additives: organic acids (palmytic, linoleic and caprylic) in oil A.

Fig. 7. Inhibition capacities of chemical additives: organic acids (palmytic, linoleic and caprylic) in oil C.

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Fig. 8. Asphaltene precipitation onset in crude oil A as a function of the additives amount.

For pepper Jamaican essential oil, for example, the inhibitory effect on the asphaltene precipitation in crude oils A and C could, in principle, be attributed to the presence of eugenol, a substance which presents a phenolic group and is highly concentrated in this oil (Mouchrek Filho, 2000). Experiments using an eugenol standard, however, showed a poor effect on the asphaltene stabilization process (in the A crude oil, 3.14% wt/ wt eugenol shifted the precipitation onset from 3.1 to 3.3 mL n-heptane/g oil). Eugenol molecular structure is similar to that of some nonylphenolic resins, which also had little effect in the asphaltene precipitation inhibition, compared with the nonylphenols (Renex) or native resins (Leo´n et al., 2001). Probably the bigger effect of the pepper Jamaican essential oil found in this work can be attributed to the presence of other substances in this oil. This result, together with the good effects found when using these vegetable oils in the inhibition of the A and C crude oils asphaltene precipitation, suggest that they can be fractioned as to allow the identification of substances with greater capacity to inhibit asphaltene precipitation, as reported by Moreira et al. (1999), during the application of cashew-nut oil shell liquid, where it was proved that, beside the oil, two substances containing phenolic groups in their chemical structure, cardanol and polycardanol, obtained from their fractionation, were also active. Up to this date, however, there are no systematic studies that establish a relationship between the additive efficiency in inhibiting asphaltene precipitation induced by the addition of flocculants and its efficiency in the field. As a rule, all substances pointed out in this article must be further tested in the field during production, where asphaltene precipitation conditions can substantially differ from experimental laboratory conditions.

3.3. Chemical additives evaluation in asphaltene solubilization Initially, as we are dealing with a polydisperse sample (crude oil fraction), it was necessary to estimate the asphaltene behavior in toluene as to relate it with the spectrophotometer response. The aim of this study is to determine a wavelength that allows the monitoring of asphaltene concentration. Fig. 9 shows the UV–vis; spectra-taken for different C5I and C7I asphaltene concentration in crude oils A and C. Although the profile of the curve shows a lack of absorption peaks, this is a typical behavior of polydisperse systems (characteristic of crude oil fractions). 400 nm was the selected wavelength to investigate the asphaltene concentration because that is a wave range that allows the application of the Lambert–Beer law (considering up to 1.0 unit absorbance readings as maximum) and also due to literature references (Bru¨ggemann and Freitas, 1995), that recommend the check-

Fig. 9. Absorbance spectra obtained from a solution of C5I and C7I in toluene at 28 F 1 8C.

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Fig. 10. Absorbance as a function of C7I and C5I asphaltene concentration from A and C crude oils, in toluene.

ing of polyaromatic hydrocarbon concentration in that wavelength range. Asphaltene C7I and C5I calibration curves (in A and C crude oils) in toluene (T = 28 F 1 8C room temperature) presented a linear behavior within the evaluated concentration range, as shown in Fig. 10 for the C7I-A asphaltene, and they were further used to study the asphaltene solubilization in aliphatic solvents as to determine their concentration in the supernatant. In order to determine the kinetics of the solubilization/dispersion, preliminary tests were undertaken with some of the additives, as shown in Fig. 11 for the octanoic acid (caprylic acid) and for the andiroba vegetable oil, in which an initial increase of the absorbance can be observed, followed by a plateau region or saturation, as well as two distinct behaviors: on the one hand it occurs the absorbance increasing in function of time, on the other the absorbance remains unchanged. The stabilization time for asphaltene absorbance was about 70 h for the andiroba oil and 90 h for the octanoic acid. After that, only small oscillations in the readings were observed, with no more than 3% variation with respect to the absorbance value. To be sure, all experiments on the additives efficiency in aliphatic solvents were carried out during at least 168 h (seven days).

In the asphaltene solubility study, only the additives listed in Figs. 12 and 13, soluble in n-heptane and n-pentane within the determined concentration range, were tested. Model solvents such as the n-heptane and n-pentane are more representative in this case, once they are crude oil derivatives and simulate the oil low dielectric constant. Fig. 12 shows the results of asphaltene C7I solubility, from oil A, in n-heptane. Dodecylbenzenessulfonic

Fig. 11. Kinetic study of C7I and C5I asphaltene solubilization from crude oil A in n-pentane and n-heptane, respectively, as a function of the octanoic acid and andiroba vegetable oil combined action.

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Fig. 12. C7I asphaltene from oil A — solubilization in n-heptane in presence of additives.

acid (DBSA) had an excellent performance as well as the Renex 100. Similar behavior was observed for asphaltene C5I in n-pentane (Fig. 13). Proof of the dodecylbezenesulfonic acid (DBSA) effectiveness in dissolving C7I and C5I deposits, corroborates former reports by Boer et al. (1992), Chang and Fogler (1993), Loh et al. (1999), and Ramos et al. (2001), revealing the importance of acid–base interactions in the asphaltene dissolution process. Weak acids, however, such as linoleic and octanoic ones, did not present significant results. Nevertheless, DBSA presented less expressive results on the asphaltene precipitation inhibition. This result points out that the asphaltene stabilization mechanism in crude oils is different from the asphaltene stabilization mechanism in crude aliphatic solvents, reinforcing the idea that, in crude oils, the asphaltene can be dispersed by additives due to hydrophobic interactions with asphaltene particles, while solubilization is the process that controls the additive activity on asphaltene in aliphatic solvents.

In practice, vegetable oils did not have any effect in the solubilization tests with a model solvent, with the best result furnished by coconut essential oil and Brazil nut oil. Results with C7I and C5I asphaltene of C crude oil match the ones previously obtained for crude oil A; yet, they prove that DBSA and Renex efficiency does not depend on the asphaltene source used in this experiment. 4. Conclusions In this work several substances were screened and selected as potential asphaltene precipitation inhibitors and/or as potential asphaltene deposit solubilizers. Once operational variables in the field may be very distinct from laboratory experiments, the necessity of evaluating these substances bin locoQ is emphasized. Low molecular mass ethoxilated nonylphenols performed well in the inhibition of asphaltene in Brazilian crude oils, confirming the efficiency of these substances in diverse sources of crude oils. Nevertheless, one observed restriction was

Fig. 13. C5I asphaltene from oil A — solubilization in n-pentane in presence of additives.

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Renex insolubility due to an increase in the ethoxilated chain. The stabilization mechanism for Renex 18 and Renex 40 (in the three tested crude oils) does not depend on the crude oil origin. It was also confirmed that vegetable oils could be used as inhibitors, while revealing products rather soluble in crude oils compared with various insoluble polymeric dispersants, turning these results practical and economically valuable. Vegetable oils that performed better were sweet almond, andiroba, coconut essential oil and sandalwood essential oil. Organic acids such as palmytic, linoleic and caprylic acid showed fairly efficient in the asphaltene precipitation inhibition, addressing towards an asphaltene stabilization mechanism as a function of its surfactant capacity. These results also indicate that vegetable oils can further be fractioned while searching for substances with higher inhibition potential. With respect to the studies on asphaltene solubility in aliphatic solvents, even when the solvents simulate the crude oils apolar medium they may not be representative of crude oils. In this case, it is also noted the scarcity of studies that establish a relationship between the additives effectiveness and their capacity in the field, as for example when one attempts to solubilize deposits in the transport pipeline. Yet, our study proves that DBSA and Renex efficiency does not depend on the asphaltene nature. Renex 18 and Renex 40 presented less significant results and Renex 100 performed slightly better. These results were practically opposite to the ones found for the asphaltene precipitation inhibition and, in principle, they point out that both the asphaltene solubilization in aliphatic solvents and the precipitation inhibition mechanisms in crude oils are distinct with respect to the interactions between asphaltenes and additives. Acknowledgments The authors thank the Brazilian agencies CAPES, for financial support, and CNPq, for the research grants, and address especial thanks to Prof. Watson Loh, for some useful discussions. References Al-Sahhaf, T.A., Fahim, M.A., Elkilani, A.S., 2002. Retardation of precipitation by addition of toluene, resins, deasphalted oil and surfactants. Fluid Phase Equilib. 194–197, 1045 – 1057. Aquino-Olivos, M.A., Buenorostro-Gonzalez, E., Andersen, S.I., Lira-Galeana, C., 2001. Investigations of inhibition of asphaltene precipitation at high pressure using bottomhole samples. Energy Fuels 15, 236 – 240.

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