Demulsifier performance and dehydration mechanisms in Colombian

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Cite This: Energy Fuels 2017, 31, 10369-10377

Demulsifier Performance and Dehydration Mechanisms in Colombian Heavy Crude Oil Emulsions Diego Pradilla,*,† Jeferson Ramírez,† Fabio Zanetti,‡ and Oscar Á lvarez† †

Grupo de Diseño de Producto y de Proceso (GDPP), Departamento de Ingeniería Química, Universidad de los Andes, Carrera 1 este No. 18A-12, Edificio Mario Laserna, Piso 7, Bogotá, Colombia ‡ The Dow Chemical Company, Diagonal 92 No. 17-42, Bogotá, Colombia ABSTRACT: Important differences arise when chemical demulsification strategies are implemented for heavy crude oils (°API ∼ 10). Traditional methods for screening and selecting an appropriate demulsifier based on bottle tests and lipophilic− hydrophilic parameters (i.e., HLB, RSN, and so on) tend to be less adequate because of the almost negligible density difference between the oil and the water phases. This situation leads to a detriment of the separated water often mixed with undesired dense-packed layers (DPLs) and emulsion layers. In this work, dehydration of heavy crude oil emulsions from a Colombian oilfield was assessed through the use of a wide range of chemical demulsifiers of different functionalities. Through the use of bottle tests and transmission/backscattering measurements, it was shown that the demulsification mechanisms involved in these limiting cases (low density difference) are different. Hence demulsifiers with functional groups that have traditionally performed very well for lighter oils fail when applied to the heavy crude oil cases. Poly(ethylene oxide)/poly(propylene oxide) block copolymer-based products (PEO/PPO) do not seem to have the ability to penetrate the asphaltene network/film at the liquid− liquid interface (separated water, 13) indicates a more water-soluble product while compounds with RSN < 13 are typically insoluble in water. In the range 13 < RSN < 17, the compounds tend to partition at the liquid− liquid interface. Furthermore, it has been reported that when Winsor type-III systems are obtained (i.e., when the surfactant−water and surfactant−oil interactions ratio is close to unity which happens approximately between 11 and 22), water dehydration reaches a maximum.39 Figure 5 shows the

so an alternative analysis needs to be performed as it will be shown later. Emulsion Stability Measurements. Emulsion destabilization can happen in different ways. For example, in some systems coalescence might occur without sedimentation while in others it only occurs once the droplets have settled. In both cases, aggregation needs to be present and these phenomena can be followed by changes in transmission and backscattering of light. Figure 6 shows the transmission and backscattering

Figure 5. Relative solubility number (RSN) of the demulsifiers used in this work as a function of the percentage of separated water.

RSN obtained for each of the demulsifiers used in this work as a function of the separated water. From this figure, it can be seen that there is no direct correlation between the RSN and the amount of separated water for any of the families. The DEMTROL 2000 series which performed best in the bottle tests has a RSN value similar to those of the 1000 and 5000 series, which performed poorly. An explanation for this behavior is that the correlations established in the literature26,40 were developed for lighter oils; thus, the case of heavy crude oils varies mainly because the RSN parameter does not take into account density differences or DPL/residual emulsion formation. As mentioned before, the RSN is only the amount of water needed to reach turbidity in solution; hence, interfacial aspects of a crude oil/water interface are not included. Furthermore, the density difference between the solution of the RSN test and water is different than the density difference between crude oil and water. The interesting aspect of the analysis on the RSN value for different molecules tested is, once again, that the traditional methods for screening and selecting a demulsifier do not fully apply when handling heavy crude oils,

Figure 6. Transmission (a) and backscattering (b) data for heavy crude oil emulsions without addition of demulsifiers at 0 and 24 h after preparation.

data for crude oil emulsions without addition of demulsifiers. The x-axis represents the height of the sample which means that going from left to right will correspond to moving from the bottom to the top of the sample vial. The signals are shown in the y-axis, and the time dependency is represented by the icons in the legend (one at 0 h representing the moment after preparation and one at 24 h). From these plots, it can be seen that that the emulsions are stable and that there is no phase 10373

DOI: 10.1021/acs.energyfuels.7b01021 Energy Fuels 2017, 31, 10369−10377

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Energy & Fuels separation throughout the height of the sample. The changes in transmission and backscattering that are seen at the top of the sample (∼49 mm) correspond to the transition to the oil/air surface. Figure 7 shows the changes in transmission and

Figure 8. Micrographs of the water-in-crude oil emulsions prepared in this work varying the concentration of the dispersed phase.

that even when increasing the concentration of the dispersed phase, the procedure used to manufacture the emulsions did not alter significantly the droplet size, which remained between 1 and 5 μm. The micrographs also show that based on that range of droplet sizes, the freshly prepared emulsions will exhibit high stability, which is consistent with what was observed and the literature.41 The same analysis was performed using all the other demulsifiers listed in Table 2; however, for illustration purposes, Figure 9 only shows the percentage of separated

Figure 9. Percentage of separated water based on the transmission and backscattering signals after 24 h for all the demulsifiers (2500 ppm) used in this work and listed in Table 2.

Figure 7. Transmission (a) and backscattering (b) data for heavy crude oil emulsions destabilized by the blended demulsifier DEMTROL F57 at 2500 ppm at 0 and 24 h after preparation.

water based on the transmission and backscattering signals after 24 h. Once again, it can be seen that demulsifiers that performed best were members of the DEMTROL 2000 series (i.e., 2020 and 2025). Even though one member of the 1000 series (i.e., 1130) showed a good performance, the other members of the series are below 10% which is not indicative of the efficiency of the PEO/PPO chains in preventing the formation of DPL’s/residual emulsion layers. Furthermore, transmission signals and the semiqualitative scale developed to assess the quality of the separated water can be complemented. Figure 10 shows the quality of the separated water according to the demulsifier used. In this plot, a value of 100% in transmission represents pure water. The performance of DEMTROL 2000 series is corroborated in terms of their ability to prevent the formation of DPLs/residual emulsion layers and enhance the separation of clear water. The

backscattering for a crude oil emulsion destabilized by the blended demulsifier F57 at 2500 ppm (see Table 2). First, it can be said that phase separation is taking place after 24 h due to the clear increase in the transmission signal (Figure 7a) evidenced at the bottom of the sample (between 0 and 5 mm). Second, droplet sedimentation is evidenced by the increase of the backscattering signal (Figure 7b) at the bottom of the sample (between 0 and 4 mm) after 24 h. It is important to mention that these peaks are clear which means that the quality of the separated water is good (i.e., 5 according to the grading system presented in the previous section). Finally, the presence of a DPL/emulsion layer is evidenced in the backscattering signal (between 4 and 12 mm). Droplet size measurements (not shown) and the micrographs presented in Figure 8 show 10374


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Figure 10. Quality of the separated water according to the demulsifier used based on the transmission/backscattering data. As a reference, a value of 100% in transmission represents pure water.

of the crude oil which have been shown to be dominant in terms of the interfacial activity of asphaltenes.47 Asphaltenes have been observed to show a pH-dependency47 in terms of interfacial activity at the liquid−liquid interface.25 In fact at neutral pH, the interfacial tension reaches a maximum and this trend correlates to the stability of asphaltene emulsions.34 This behavior has been largely attributed to the ionization of the − COOH groups at the liquid−liquid interface. Evidence for this comes mainly from the fact that such behavior is not observed at the liquid−solid surface.48 Even though the acidity of the crude oil used in this work is relatively low, the asphaltene content is very high (see Table 1). As a consequence there could be several acidic asphaltene molecules per nonacidic molecules which could explain why the polar interactions are so relevant for obtaining a clear water after demulsification and not a formation of a DPL/emulsion layer. It is worth mentioning that a high asphaltene content does not equate to severe emulsification issues. Instead, it is the relationship between the amount of asphaltenes and the acidity of the oil.

DEMTROL 1130 showed a similar performance but for a lower percentage of separated water which means that DPL’s/residual emulsion layers are still being formed. All these results are consistent with observations performed during bottle tests indicating two things: transmission and backscattering measurements are an adequate way for assessing demulsification and the importance of not only considering the percentage of separated water when evaluating the performance of a demulsifier. Functional Group Evaluation. The performance of the different demulsifiers has been assessed based on the percentage of separated water and the quality of the separated water. The latter was judged by using two different criteria, which is something that should be considered when dealing with heavy and extra heavy crude oils. However, a very important aspect of the previous analysis lies on the nature of the functional groups and the type of interactions that will eventually lead to phase separation. It has been previously shown22,42−44 that one of the most effective demulsifiers is the PEO/PPO block copolymer. In fact, many novel demulsifiers present with a similar structure and some of them include PEO/PPO chains. Its efficiency seems to be related to the ability of the chains to penetrate the asphaltene network formed at the oil/water interface and disrupt it by complex formation. Demulsification seems to occur by displacement and subsequent release of demulsifier−asphaltene networks into the oil phase. This behavior varies according to the length of the EO/PO chains45 and the position of the chains along the molecule.40 It occurs at an optimal formulation in which the socalled orogenic displacement is favored.46 However, from the results presented in Figures 2 and 8 it is clear that this is not the case when dealing with a heavy crude oil in which the density difference is very low. In these cases, the action of the EO/PO chains is not enough to strongly interact with the indigenous surfactants in a way that a clear separated water is produced and instead a DPL/emulsion layer is favored. Furthermore, functional groups such as amines (4000 series) and epoxy resins (3000 and 5000 series) do not show an improved performance. It takes a strong polar group, such as the one present in the alkylphenol series (DEMTROL 2000) to favor a clear water content after separation. This might be indicative of the importance of polar interactions enhanced by high acidity

CONCLUSIONS Dehydration of crude oil emulsions made with a Colombian heavy oil (°API ∼ 10) were studied by means of bottle tests and transmission/backscattering measurements. A wide range of chemical demulsifiers of different functionalities (Table 2) were used to assess their efficiency in terms of percentage of the separated water, quality of the separated water, and formation of undesired DPL’s/emulsion layers. It was found that evaluating a demulsifier based only on its ability to separate water (a criterion that works well for lighter oils) can be counterproductive because demulsifiers that traditionally perform well such as PEO-PPO-PEO block copolymers do not prevent the formation of DPLs/emulsion layers generating an additional problem for oil transportation.26,40,49 Evidently, this is not an issue when dealing with lighter oils and/or oils with low acidity/asphaltene content. It is clear that chemical demulsification strategies for heavy and extra-heavy crude oils must be combined with others (i.e., dilution) to make the operation viable and to achieve adequate levels of water separation. In terms of dewatering capabilities and producing a good separation (i.e., minimum formation of DPLs, emulsion layers, clear separated water) the family of demulsifiers that performed best was the DEMTROL 2000 series. This family belongs to the alkoxylated alkylphenol-aldehyde resins, and its efficiency can be related to the importance of strong polar interactions at the liquid−liquid interface. In this case, a possible mechanism for demulsification is the penetration of the demulsifier to the asphaltene network possibly enhanced by the presence of alkyl chains and subsequent strong interactions between the acidic fraction of asphaltenes and the polar group of the demulsifier. A type of orogenic displacement46 followed by surfactant release back to the water phase.50 The performance of other families such as amine and epoxy groups (DEMTROL 3000 and 4000) was very low corroborating the importance of polar interactions for demulsification especially in the case of heavy oils in which the quality of the separated water is highly relevant. Finally, it can be said that transmission and backscattering measurements are a reliable tool for determining the amount of separated water, the formation of DPL’s/emulsion layers and the quality of the separated water. Through a semiquantitative scale (Figures 4 and 9) it was possible to determine these

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parameters and evaluate the performance of different demulsifiers with a new perspective. This analysis will serve as a starting point for demulsifier screening including parameters that are not usually considered in the case of light oils and can be used to design better chemicals of more effective performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+57)-1-3394949 ext. 3095. ORCID

Diego Pradilla: 0000-0001-8810-9526 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge The Dow Chemical Co. for kindly providing the demulsifier samples used in this work.

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REFERENCES

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Demulsifier performance and dehydration mechanisms in Colombian

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