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Demulsifier performance and dehydration mechanisms in Colombian heavy crude oil emulsions. Diego Pradilla, Jeferson Ramirez, Fabio Zanetti, and Oscar Alvarez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01021 • Publication Date (Web): 27 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017
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Demulsifier performance and dehydration mechanisms in Colombian heavy crude oil emulsions.
Diego Pradillaa,*, Jeferson Ramíreza, Fabio Zanettib, Oscar Álvareza. a
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. b
The Dow Chemical Company, Diagonal 92 #17-42, Bogotá, Colombia.
*Corresponding Author E-mail:
[email protected] Phone: (+57)-1-3394949 ext. 3095
Conflict of interest disclosure The authors declare no competing financial interest.
KEYWORDS: Chemical demulsification, heavy crude oil emulsions, demulsifier performance, demulsification mechanisms.
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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, etc.) 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 (DPL’s) 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. Polyethylene oxide / Polypropylene 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 < 17%) while the alkylphenolaldehyde resins seem to prevent the formation of DPL’s / emulsion layers possibly through polar interactions, yielding a good quality water phase after separation. INTRODUCTION One of the most complex problems encountered in the petroleum industry is the formation of highly stable water-in-crude oil emulsions and in fewer cases, multiple emulsions1-2. The formation and stabilization of such systems is mainly attributed to the presence of the naturally occurring surface active species such as asphaltenes, resins, naphthenic acids and solid particles such as clays3-5. The mechanisms involved in the long-term stability of these colloidal systems is commonly attributed to the formation of an elastically dominated solidlike interface that hinders coalescence and retards film drainage. Asphaltene crowding, selfassociation, multilayer buildup and resin co-adsorption are some of the key parameters that dominate this phenomenon besides their ability to reduce the interfacial tension6-9. Another particular aspect of the rigid asphaltene layer is that it exhibits a yield stress that is significantly greater than the stress typically prompted by droplet-droplet interactions
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during flocculation. In other words, the resistance to film compression becomes greater than the driving force for coalescence10. To promote oil-water separation by coalescence and hence rupture of the interfacial layer, chemical demulsification strategies can be implemented. Even though it is yet not clear how to predict the interactions at the oil/water interface, a starting idea could be that generally speaking, chemical additives (typically amphiphilic molecules) compete with the indigenous surfactants for the oil/water interface and/or strongly interact at the interface until a complete replacement or displacement is achieved in the case of already dispersed droplets11. Evidently, the nature of the interaction will depend on the type and properties of the demulsifier, the properties of the crude oil and the operating conditions. Consequently, efforts need to be made on designing and selecting adequate systems that perform efficiently at any specific case10. There is an overwhelming amount of chemical structures available as demulsifiers. Some of the widely used chemicals include amphiphilic block copolymers (e.g. polyethylene oxidepolypropylene oxide-polyethylene oxide PEO-PPO-PEO)12, nonylphenols (NP’s)13, alkylphenol polyalkoxylated resins14, polyurethanes, polyalkolxylated amines15, a wide variety of anionic, cationic and nonionic surfactants16, ionic liquids (IL’s)17 and more recently ethyl cellulose (EC)18. The chemical structure of the demulsifiers will determine the mechanisms that will eventually lead to destabilization of emulsions. Thus it will be different in each case and more importantly, the type of interaction at the liquid-liquid interface will vary according to the type of crude oil. The study of demulsifier performance and its interaction with indigenous species at various interfaces/surfaces can be assessed with model solutions (i.e. asphaltene solutions in an aromatic solvent) or directly on crude oils with different properties. When the former is implemented, viscosity and density differences between phases are not considered. When the latter is used, the type of crude oil will influence the assessment of the performance of a given chemical additive. The case of heavy crude oils, in which the density of the crude oil ( ) is similar or close to that of water (°API ~ 10 or ≈ ), or “unconventional oils” as defined by the International Energy Agency19, is of particular interest and highly relevant for countries such as Canada, Norway, Venezuela and Colombia among others. It has been
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estimated that over half of the world oil reserves fall under this category (3.2 trillion barrels approximately)20. In most cases, the clear water content after separation cannot be unequivocally identified and dense packed layer formation (DPL) is enhanced by the action of typical demulsifiers used for dewatering of lighter oils. As a consequence, it can be stated that the mechanisms involved in chemical demulsification seem to be different, and therefore their study is of paramount importance to enhance oil recovery. In a recent work, it was established21-22 that mechanisms that lead to desorption of aged C6asphaltenes from the liquid-liquid interface were initiated by interactions whose strength was strongly related to the type of demulsifier used. This stage was followed by the formation of complex-like structures that disrupted asphaltene multilayers and progressively replaced them. This two-stage process was followed using a high molecular weight amphiphilic PEO-PPO-PEO block copolymer and with a low molecular weight nonionic surfactant. The mechanisms described were always faster for the block copolymer than for the surfactant. In a different study13 using the same crude oil and a series of nonionic surfactants of the nonylphenol family, it was shown that similar mechanisms can be inferred when the stability of the crude oil emulsions was assessed. In this work, the performance of five different families of commercially available and widely used demulsifiers, namely, block copolymers, alkylphenols, epoxy resins alkoxylated and amine-based was studied. The evaluation was performed using a heavy crude oil from a Colombian oilfield and the mechanisms involved during the demulsification process were assessed through emulsion stability (transmission and backscattering) measurements and bottle tests. It was shown that even though the density of the Colombian heavy crude oil is similar to that of the Norwegian continental shelf used in other studies13,
22
, the processes that take place at the liquid-liquid interface when the
demulsifiers interact with all the indigenous components already adsorbed were different. One key feature for this analysis was the quality of the separated water which is known to be influenced by formation and growth of the dense-packed layers (DPL)23 and emulsion layers. The undesirable DPL’s haven been shown to be highly stable and to reduce the separation rates24. Therefore in the present study, the efficiency of the demulsifier was not only assessed through its ability to separate water, but also its ability to prevent the
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formation of a DPL / residual emulsion layer. Other key features were the asphaltene/resin content and the acidity of the crude oil which is represented by the interfacial activity at the liquid-liquid interface enhanced by the presence of –COOH groups25. MATERIALS AND EXPERIMENTAL METHODS Materials. The heavy crude oil used in this work came from a Colombian oil field and its properties are listed in Table 1. The demulsifiers were kindly provided by The Dow Chemical Company and belong to the commercially available series DEMTROL®. Table 2 lists the main functionality, the relative solubility number (RSN) and the nomenclature used throughout the manuscript. As aqueous solution for emulsion preparation, brine (NaCl 1M) was used. Non-ionic surfactants of the Tween® and Span® series were also used and were purchased from Croda®. Table 1. Properties of the heavy crude oil from a Colombian field used in this work. Property 3
Density (Kg/m ) at 25°C* Density (°API) at 15.5°C** pH at 25°C Viscosity (Pas) at 25°C and 1 [1/s]* TAN (mg KOH/g) Water content (% v/v) Saturates (wt %) Aromatics (wt %) Resins (wt %) Asphaltenes (wt %)
Value 1001.8 9.73 7.44 131 0.49 10 23.72 39.41 24.33 12.54
*Measured without the presence of water **60 °F following ASTM D7042-16e2
Relative solubility number (RSN) determination. The RSN of the demulsifiers in this work was experimentally obtained through titration with deionized water. First, a 2.6 %v/v solution of toluene in dimethoxyethane is prepared. Second, demulsifier is added to this solution until a concentration of 0.05M is reached. Third, the titration is started until turbidity is reached according to the parameters established by Wu et al26. The RSN reported corresponds to the amount of water needed (in mL) to reach turbidity. Emulsion preparation. Brine-in-crude oil emulsions were prepared by varying the concentration of the dispersed phase between [10-60] %wt. First, the required heavy crude
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oil quantity was pre-heated to 60°C. Second, brine was added to the heavy crude oil using a peristaltic pump at 5 mL/min. The system is being mixed during the addition at the given tip velocity using a Dispermat LC 55E (VMA-Getzmann GmbH). Third, a homogenization stage of 15 min was given to the emulsions at the same tip velocity. Finally, the emulsification step was carried out in a Rotor-Stator D-15 (Miccra Art) for 15 minutes at the same conditions of the previous step. All the tests done on the emulsions were carried out 30 min after preparation. Bottle tests. This evaluation was performed following the methodology used by Dow Chemical for screening demulsifiers in the oilfields. The first part of the bottle tests were performed by adding a fixed amount (100 mL) of crude oil emulsion in a graduated bottle. Then the bottle was placed in a water bath ensuring a temperature of 60°C. 2500 and 3500 ppm of demulsifier (Figure 1) were added to the crude oil emulsion and then vigorously shaken for 2 min. During the second part of the tests, final water content and residual emulsion in the top layer (also known as thief) were determined. For this, 7.5 mL of the supernatant were diluted in toluene (to 15 mL) and then centrifuged at 3500 rpm for 5 min. To make sure that there was no water in the oil phase, a small amount of emulsion dispersant was added to the mixture and the centrifugation step repeated. The final water content was then registered in mL as well as the residual emulsion. During the final part of the bottle tests, final water content and residual emulsion close to the dense-packed layer (DPL)-oil interface, also known as composite, were determined. For this, a 7.5 mL sample was taken from the vicinities of the DPL / emulsion layer, diluted in toluene and centrifuged at the same conditions of the previous steps. It is important to mention that the reason behind performing the second and third part of the bottle tests was to determine if there was a separation gradient, which is undesirable in terms of reproducibility27. Emulsion stability measurements. Sedimentation, creaming, aggregation, flocculation and coalescence are phenomena that can ultimately give rise to phase separation. These processes can be followed by the changes in transmission signals and backscattering over time. These experiments were performed in a Turbiscan Lab Stability Analyzer (Formulaction SA). For these analysis, an emulsion sample (20mL) was taken after 30 min
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of preparation and analyzed in the instrument. The transmission (0° from the incident light beam) and backscattering (135° from the incident light beam) signals of a pulsed near infrared light source (λ=850 nm) were recorded. The light source moved vertically along the full height of the sample. Both scans consisted of signals collected every 40µm along the height of the sample and taken every hour for 24 hours. All measurements were carried out at 60°C to ensure same conditions as the bottle tests. RESULTS AND DISCUSSION One of the main features of this work is that the chemical demulsification mechanisms that take place at the liquid-liquid interface using a Colombian heavy crude oil are further explored. This included the quality of the separated water, a distinction that is troublesome given the low density difference between water and crude oil, and the extent to which the DPL / emulsion layer is formed. These processes are followed through bottle tests (first part) and stability measurements (second part). To achieve this, a set of chemical demulsifiers of different functionalities and chemical structures (Table 2) was chosen. A comparison with the behavior of a heavy crude oil of similar density from the Norwegian continental shelf reported in the literature is also established.
Table 2. Demulsifiers used in this work, their nomenclature, main functionality and relative solubility number (RSN).
DEMTROL® 1000 Series
Reference
PEO-PPO-PEO Block copolymers
1010 1020 1020E 1030 1040 1114 1130
Relative Solubility Number (RSN) 12.1 10 10 17.8 19 9.7 16.4
2020 2025 2030 2045
12.4 12.8 15.2 15
3000
6
DEMTROL® 2000 Series Alkylphenol Aldehyde resins Alkoxylate DEMTROL® 3000 Series Epoxy resins Alkoxylate
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3001 3010 3020 3027 3030
5 6.3 7.6 10.6 10.5
4017 4026 4110 4115 4120
13 17 10 10 11
5030 5050 5130 5150
13 24.5 11 22.5
F57
N.A
20 80
17.2 12.7
20 80
9.9 6.72
DEMTROL® 4000 Series
Amine-initiated polyol block copolymers
DEMTROL® 5000 Series Modified specialty resins Alkoxylate DEMTROL® F57 Blend of all series TWEEN® Series PEG-Sorbitan monolaurate PEG-Sorbitan monooleate SPAN® Series Sorbitan laurate Sorbitan monooleate
Bottle tests An important aspect for bottle tests is the determination of the optimal dosage for water separation. Figure 1 shows the performance of the demulsifier DEMTROL® F57 (blend of all other series) as a function of time. It is clear from this figure a desired water separation (≈80%) is only achieved after ≈24 h in the best case. At concentrations of demulsifier higher than ≈2500 ppm and based on the error bars, a plateau-like region starts to form which, among other aspects, could be attributed to surfactant overdose7. One possible reason for this performance is that even though interactions between indigenous components of oil and the demulsifiers are taking place at the liquid-liquid interface, the high viscosity of the oil coupled with the almost negligible density difference between oil and water, slow down the actual phase separation process. Taking Jeffreys and Davies28 three-step emulsion rupture description, it can be said that in this case flocculation could dominate (given that < ) and film drainage is delayed. However, as it will be shown
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later, the average droplet size (radius) is between 1-5 µm which falls into the limits of Brownian motion-dominance causing additional problems for flocculation, hence coalescence to happen29. For illustration purposes and given that the viscosity difference is significantly high, a quick analysis of the data presented in Figure 1 was performed. The terminal velocity () was calculated using Stokes’ law (Eq. 1) in which is the gravity, is the particle radius (or mean radius, in this case ̴ 3µm), is the viscosity of the oil and ∆ is the density difference between water and oil. 2 = ∆ 9
Under the conditions of the experiments of this work, the calculated terminal velocity (absolute) is 2.710 / or 2.310 / . Once again, the severe problems when dehydrating heavy and extra-heavy crude oil emulsions are evidenced. With this value of terminal velocity, flocculation is not enhanced, the low resolved water of Figure 1 explained and the mechanisms behind chemical demulsification will vary as it will be seen next. Evidently, all these phenomena go beyond the performance of a chemical as a demulsifier and instead are related to the nature of the separated phases. Figure 2 shows the percentage of separated water for all the demulsifiers listed in Table 2 at 2500 ppm after 24 h. For comparison, Figure 3 shows photographs of the separated water under the same conditions for demulsifiers that exhibited a better performance. Evidently, the best performance was given by the F57 demulsifier which is a blend of all other demulsifiers. It is important to note that this situation is not uncommon: when deciding a chemical demulsification strategy in an oilfield, the service provider performs bottle tests and then chooses those additives that performed best to make blends. It is interesting to see the behavior of the different functional groups of the other series. For example, the DEMTROL® 1000 series exhibits a low performance (≈18% in the best case). This result was unexpected given that PEO-PPO-PEO block copolymers are widely used for their high efficiency in water separation30-31. It can be hypothesized based on Figure 3a that this behavior is due to the ability of this series to form DPL’s / emulsion layers under the conditions of this work. This
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factor is almost irrelevant when the density difference between the oil and water phases is very high, hence this series is traditionally judged as highly efficient and widely used. On the other hand, the performance of the DEMTROL® 2000 series produced the best performance compared to the rest of the demulsifiers used. It can be seen in Figure 3b that even though the separated water is mildly turbid, the DPL / emulsion layer formed is very small. This is an important aspect for addressing demulsifier performance in limiting cases because it means that the residual water is kept at a low value. This series belongs to the Alkylphenol-Aldehyde resin Alkoxylates family and its high performance has been consistently categorized as high for many decades32. It is possible that the action of the phenol group is enhanced by the penetration capabilities of the EO-PO chains that may have disrupted the irreversibly adsorbed asphaltene interfacial film through complexation mechanisms. These complexes could be momentarily more surface active than the indigenous components of crude oil displacing them and hence promoting coalescence22, 33. Figure 2 also illustrates that the demulsification capabilities of the DEMTROL® 3000, 4000 and 5000 series are very low. Even though these series contain basic functionalities, the acid-base interactions (Brønsted–Lowry) do not seem to play an important role in terms of interfacial activity because of the lack of penetration to the asphaltene film
25, 34
. The
non-ionic surfactants of the Tween® and Span® series show some degree of dewatering capabilities. However Figure 3d clearly depicts that the DPL / emulsion layer dominates the separated phase. This could be due to the fact that the Tween® series is water-soluble and so its tendency in these cases would be to promote phase inversion35. Figure 3 can be used to generate a 1 to 5 scale to qualitatively describe the performance of the demulsifiers studied in this work in terms of the type of interface left after separation (DPL / emulsion layer) and the type of separated water (e.g. turbid, presence of solids). Within this scale, 5 is the highest rating representing a positive condition and 1 the lowest representing a negative condition. For example, Figure 3a shows that the separated water is clear and so it was given a value of 5. Regarding the same figure, it is clear that the type of interface formed corresponds to a DPL / emulsion layer and so it was given a value of 1. For figure 3d and following the same criteria, the separated water was given a value of 1 (presence of oil and/or solids and turbidity) and the interface a value of 5 because there is
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no DPL / emulsion layer, only residual oil. Figure 4 shows the results of this evaluation for a selection of the demulsifiers tested and it can be seen that the DEMTROL® 2000 series exhibits the best performance in terms of those two criteria. Once again, the main aspect in this evaluation is not only the amount of separated water but the residual emulsion (or DPL) enhanced by sedimentation. This process is promoted by the viscosity-density difference and the so-called “dynamic surfactant effect” or release of surface active species into the oil phase after coalescence23. For the other series the performance varied according to the length of the EO-PO chains (1000, 3000 series) and the amount of basic groups present (4000, 5000 series) and so no generalization could be made. It is important to mention that similar approaches have been previously used to assess the water resolution and residual emulsions36-37. The relative solubility number (RSN) has been proposed as an alternative to the Hydrophilic-Lipophilic Balance method (HLB) for evaluating the affinity of different surface active agents to a given phase26. In terms of crude oil dehydration, it has been used as a parameter to screen for potential candidates that would perform efficiently based on the acidity of the oil and its salinity38. In simple terms, a high RSN number (> 13) indicates a more water-soluble product while compounds with RSN < 13 are typically insoluble in water. In the range 13 < RSN
