Energy Fuels 2009, 23, 4575–4583 Published on Web 07/24/2009
: DOI:10.1021/ef900355d
Chemical Destabilization of Crude Oil Emulsions: Effect of Nonionic Surfactants as Emulsion Inhibitors Yanru Fan,* Sebastien Simon, and Johan Sjo¨blom Ugelstad Laboratory, Department of Chemical Engineering Norwegian University of Science and Technology (NTNU) N-7491 Trondheim, Norway Received April 22, 2009. Revised Manuscript Received July 13, 2009
The destabilization of crude oil emulsions by polyoxyethylene nonylphenols as inhibitors has been investigated at a water-to-oil volume ratio of 1:1, as a function of the HLB (hydrophilic-lipophilic balance) and concentration. The results show that the stability of crude oil emulsion begins to level off after a critical surfactant concentration, which seems to correspond to their critical micelle concentration (CMC). A stability minimum was found after the stability plateau region, which corresponds to the inversion of emulsion from W/O to O/W. Furthermore, the inhibitor with HLB = 14.2 has the highest efficiency for demulsification with the highest separation rate and the lowest inversion point, whereas inhibitors with higher HLB are less effective, which may be due to the network formation by their very long oxyethyl headgroups and interactions with indigenous components of the crude oil.
synergism of these surface-active components at the interface that directly relates to their amphiphilicity. In the literature, several concepts have been established correlating the emulsion stability to surfactant hydrophilicity. In 1913, Bancroft7 stated that “a hydrophilic colloid will tend to make water the dispersing phase while a hydrophobic colloid will tend to make water the dispersed phase”. This is the first, but very qualitative, rule correlating the amphiphilicity of emulsifying agent to emulsion type. In 1949, Griffin8 introduced the HLB (hydrophilic-lipophilic balance) concept to describe the amphiphilicity for nonionic surfactants, defining in an empirical and quantitative scale. Surfactants with low HLB tend to give W/O emulsions, whereas high HLB surfactants favor O/W emulsions. Different formulas have been proposed to calculate the HLB value for a given surfactant. For instance, in the case of polyethoxylated nonionic surfactants the HLB was defined as 20 times the weight fraction of the polyoxyethylene part. Griffin’s HLB number was later extended by Davies,9 who introduced a scheme to assign HLB group numbers for different chemical groups composing one surfactant molecule. As an essential feature of the HLB scale, it is assumed that the HLB of a surfactant mixture can be calculated from a linear average mixing rule based on weight composition,
Introduction Demulsification is the process of breaking emulsions in order to separate water from oil, which is also one of the first steps in processing the crude oil. Water is normally present in crude oil reservoirs or is injected as fluid on steam to stimulate oil production. The crude oil and water mixture can form stable W/O emulsions while rising through the well and passing through the valves and pumps.1-3 The quality of the crude oil is highly dependent on the residual contents of water and water-soluble contaminants, which will be problematic for the water treatment part of the processes. The crude oil market demands that water in crudes must be removed to a level of less than 0.5% BSW (bottom, solids, water).1 Therefore, different methods, including both physical and chemical treatment, have been used to separate water from oil.4,5 Chemical demulsification consists of the addition of small amount of demulsifiers (usually 1-1000 ppm) to enhance phase separation, usually using surfactants, polymers, pure solvents, or their mixture.2 Nonionic surfactants have been widely used for demulsification study as model systems, such as fatty esters, alkyl phenol ethers, polyoxypropylene glycol ethers, and fatty amides.1 The previous research has shown that the stability of emulsion is determined by the interfacial layer mixture of both natural surfactants and demulsifiers.6 Therefore, it is very important to fully understand the mechanism of destabilization from studying the interaction or
HLBm ¼
Xi HLBi
ð1Þ
i ¼1
where Xi is the weight fraction of each component.10 This mixing rule has been widely used when selecting emulsifiers for emulsification process.11
*To whom correspondence should be addressed. E-mail: yanru@nt. ntnu.no. (1) Angle, C. W. Encyclopedic Handbook of Emulsion Technology; Sjöblom, J., Ed.; Marcel Dekker: New York, 2001; Ch 24, pp 541-594. (2) Sjo¨blom, J.; Johnsen, E. E.; Westvik, A.; Ese, M. H.; Djuve, J.; Auflem, I. H.; Kallevik, H. Encyclopedic Handbook of Emulsion Technology; Sjöblom, J., Ed.; Marcel Dekker: New York, 2001; Ch 25, pp 595-619. (3) Pe~ na, A. A.; Hirasaki, G. J.; Miller, C. A. Ind. Eng. Chem. Res. 2005, 44, 1139–1149. (4) Djuve, J.; Yang, X.; Fjellanger, I. J.; Sjo¨blom, J.; Pelizzetti, E. Colloid Polym. Sci. 2001, 279, 232–239. (5) Less, S.; Hannisdal, A.; Sjo¨blom, J. J. Dispersion Sci. Technol. 2008, 29, 106–114. (6) Rond on, M.; Bouriat, P.; Lachaise, J.; Salager, J. L. Energy Fuels 2006, 20, 1600–1604. r 2009 American Chemical Society
n X
(7) Bancroft, W. D. J. Phys. Chem. 1913, 17, 501–519. (8) Griffin, W. C. J. Soc. Cosmet. Chem. 1949, 1, 311–326. (9) Davies, J. T. Proceedings 2nd Int. Congress Surface Activity; Butterworth: London, 1957; Vol 1, p 426. (10) Salager, J. L. Pharmaceutical Emulsions and Suspensions, Nielloud, F.; Marti-Mestres, G.; Eds.; Marcel Dekker: New York, 2000, p.41. (11) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons, Ltd: 2002; pp 461-462.
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However, there are some shortcomings with the HLB concept. Indeed, the HLB does not take into account several physiochemical parameters.12 For instance, temperature, electrolyte concentration, oil type and chain length, and cosurfactant concentration can all modify the geometry of the surfactant at an interface, thus changing the curvature of the surfactant layer (the effective HLB at interface) that determines the emulsion properties. Regarding this, Salager et al.13 proposed a concept of HLD (hydrophilic-lipophilic deviation), which takes into account all the above effects. Whenever HLD is positive or negative, the emulsion type is O/ W or W/O, provided that the water-oil ratio is relatively close to unity. At HLD = 0, the emulsion has minimum stability, which corresponds to the three-phase behavior of equilibrated system (also known as Winsor III). HLD is a generalized formulation concept equivalent to Bancroft’s rule and HLB, with the advantage of numerically expressing the contribution of each formulation variable. The HLD concept has been proved to be successful when applied to the emulsion system composed of pure oil phase and certain kinds of surfactants. However, it is still difficult to be used in crude oil emulsion because of the complex variety of components. Therefore, the old HLB concept is still used for the sake of simplicity. In the study for the breaking of crude oil emulsions, some work has been done in the past decades to try to find the relation between the HLB of demulsifiers and their efficiency for demulsify crude emulsions. Aveyard et al.14 studied the effect of HLB of both nonionic and anionic surfactants on the resolution of crude-oil emulsions. They emphasized that changing the total HLB of the system and not the HLB number of the surfactant can promote the demulsification of crude emulsions. Abdel-Azim et al.15 found that the removal of water was favored by an increase in the degree of ethoxylation (HLB from 6 to 14). Goldszal et al.16 studied the demulsification of crude oil by ethoxylated nonylphenols and concluded that the rules established for microemulsion optimization can also be applied to the selection of demulsifiers. Rond on et al.6 studied the breaking of diluted crude oil by nonionic surfactants. They summarized that the optimal demulsification is obtained when the interfacial amphiphile mixture exhibits the same affinity for both phases, and lower concentrations are required to attain this optimum for more hydrophilic demulsifier. Although some conclusions have been obtained, HLB is still not simple to apply to demulsification of crude oil, especially when dealing with different oil fields. More studies are needed in order to get more general rules. In the present work, a systematic study has been performed on the destabilization of Norwegian crude oil emulsion with polyoxyethylene nonylphenol in a relatively wide range of HLB from 10 to 17.8 (EON from 5 to 40), as a function of surfactant concentration. Bottle test and interfacial tension have been used to get complementary information. The
Table 1. Properties of Polyethoxylated Nonylphenol Surfactantsa Solubility surfactant (Igepal) CO-520 CO-610 CO-630 CO-720 CO-850 CO-890
EON
HLB
Mn
5 7 9-10 10.5-12 20-30 40
10 12.2 13 14.2 16 17.8
441 529 617 749 1117 1982
brine (3.5% NaCl) turbid translucent √ √ √ √
toluene √ √ √ √ √ √
a EON refers√to the number of ethylene oxide group. Soluble is represented by .
purpose is to get a basic understanding of the mechanism of destabilization with the application of HLB approach in the demulsification of crude oil. Experimental Section Materials. The commercial nonionic surfactants polyethoxylated nonylphenol (C9H19C6H4(OC2H4)nOH) are Igepal series from Aldrich and Rhodia. They are polydisperse mixture whose degree of ethoxylation is distributed according to Poisson’s law. Their hydrophobic nonyl chains are believed to have a branched structure that results from the polymerization of propylene.17 Their properties are listed in Table 1. The crude oil is from the North Sea. Its properties are summed up in Table 2. In all cases the aqueous phase is composed of 3.5% NaCl and adjusted to pH 7 by NaOH solution. All the demulsifier concentrations mentioned in the following parts are referred to the concentration in aqueous phase. Emulsion Stability Measurement (Bottle Test). Bottle tests are carried out with 15 mL total volume samples with 1:1 water-tooil volume ratio. Demulsifiers with different concentration are added in the water phase before emulsification. The water and oil phase are in contact and are left for 24 h at room temperature, then emulsified using a mixing propeller at 1500 rpm for 5 min. Because surfactant is added in the water phase before emulsification, the function of demulsifier in the present study is as an emulsion-inhibitor. The emulsion stability is evaluated by monitoring the water separation as a function of time. The criterion used here for emulsion stability is the time required for 50% of the total volume to separate. Conductivity. The conductivity of emulsion is measured with a conductivity meter (Inolab Con Level 2P) to determine the emulsion type and inversion point. Interfacial Tension Measurement. A pendant drop method (CAM 200 from KSV instruments) was used to determine the interfacial tension (IFT). All the tests were performed by producing an oil drop into the water phase, except the determination of CMC of demulsifiers (water drop into toluene).
Results Emulsion Stability. For crude oil emulsion stability, the bottle test is a very simple but effective method to select demulsifier for an oil field. In the present work, the bottle tests were performed by measuring the release of water as a function of time. In the absence of demulsifier, the water separation was ca. 10% after 24 h for pure crude oil emulsions. With the addition of demulsifiers in the water phase, there was a variation of water separation as a function of demulsifier concentration and demulsifier type. Some of the results are given in Figure 1 as examples. Since our purpose is just to compare the efficiency of different demulsifiers, the
(12) Binks, B. P. Emulsions - recent advances in understanding. In Modern Aspects of Emulsion Science; Binks, B. P. Ed.; The Royal Society of Chemistry: Cambrige, 1998; p 3. (13) Salager, J. L.; Marquez, N.; Graciaa, A.; Lachaise, J. Langmuir 2000, 16, 5534–5539. (14) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X.; Lu, J. R. Emulsions;A Fundamental and Practical Approach; Sjöblom, J. Ed.; Kluwer Academic publishers: 1992; pp 97-110 (15) Abdel-Azim, A.-A.; Zaki, N. N.; Maysour, N. E. S. Polym. Adv. Technol. 1998, 9, 159. (16) Goldszal, A.; Bourrel, M. Ind. Eng. Chem. Res. 2000, 39, 2746– 2751.
(17) Marquez, N.; Ant on, R. E.; Graciaa, A.; Lachaise, J.; Salager, J. L. Colloids Surf., A 1998, 131, 45–49.
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Table 2. Characterization of Crude Oil Properties saturates (wt %)
aromatics (wt %)
resins (wt %)
asphaltenes (hexane- insoluble) (wt %)
wax (wt %)
water (wt %)
TAN (mg KOH/g)
51.81
37.23
6.31
0.48
4.46
4.05
2.23
Figure 1. The water separation as a function of (A) surfactant concentration for CO-720 and (B) surfactant types at 200 ppm.
Figure 2. The variation of emulsion stability as a function of surfactant concentration. (The filled and empty symbols represent W/O and O/W emulsions, respectively. In some cases, the data points at low concentrations below the stability plateau can not be shown in the graph because the stability is very high.)
bottle test was not performed until 100% water separation. Figure 1A shows that all the curves can be fitted linearly after certain time. The slope of the linear part has been used to derive the separation rate constant of demulsifier in the literature.18 That is, larger slope indicates higher efficiency of emulsion breaking. From Figure 1A, it can be seen that before 100 ppm, the slope increases gradually with concentration. From 100 to 200 ppm, the slope remains almost constant. At 300 ppm, the separation rate is increased to a significantly larger value. Figure 1B illustrates the results obtained at 200 ppm for all the demulsifiers. It seems that the slope of the fitting line for CO-720 is larger than the others, whereas CO-850, CO-610, and CO-630 have similar slope values. In addition, CO-850 can also be considered to be effective since it has the most significant water separation at fixed testing period, although its slope is smaller than CO720. The most hydrophilic CO-890 and the most hydrophobic CO-520 have rather low separation rate. It will be shown later in the discussion part that the order of the demulsification efficiency evaluated by the slopes is consistent with the propensity obtained by other evaluation methods.
In the present study, the time for separation of 50% of the total water was used as a criterion for emulsion stability in the following discussion. The variation of stability as a function of surfactant concentration for demulsifiers with different HLB is illustrated in Figure 2. For most systems, the stability decreases with the increase of concentration until some critical value where the stability levels off in a relatively wide range of concentration within the experimental error. After that, a stability minimum appears and a marked reduction in the emulsion viscosity was observed at the same concentration. In addition, an extremely unstable state was observed around the minimum, where the phase separation occurred after stopping the mixing propeller as if there is no surfactant in the system. This stability minimum corresponds to the inversion of emulsion type, as detected by the conductivity measurement that will be discussed later. Above the inversion point, the stability increases gradually or remains almost constant with the increase of concentration. This is illustrated in Figure 3 with CO-720 as an example. For the most hydrophobic CO520, there is no obvious stability plateau before the inversion
(18) Krawczyk, M. A.; Wasan, D. T.; Shetty, C. S. Ind. Eng. Chem. Res. 1991, 30, 367–375.
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Figure 4. The variations in conductivity of emulsion versus surfactant concentration. (The filled and empty symbols represent W/O and O/W emulsions, respectively).
Figure 3. The illustration of different regions according to the emulsion stability test for CO-720.
point, whereas the stability decreases gradually with the increase of concentration before inversion point. Therefore, CO-520 is not included in the following discussion where stability plateau is discussed. Furthermore, it is later shown in Figure 8D that from CO-610 to CO-850, the inversion concentration is pretty flat, with a reduction for CO-720. Another point that is worth noting is that, after inversion, the emulsion stability of low-HLB surfactant is lower than that of high-HLB surfactant. A possible explanation is that surfactant with a higher HLB value is more hydrophilic, which is a better emulsifier for O/W emulsions. As mentioned above, the type of emulsion has been tested by conductivity measurement. Figure 4 shows the conductivity change of emulsion with the increase of surfactant concentration. At low surfactant concentration, the conductivity value is between 0 and 0.3 μs/cm, which indicates that the external continuous phase is oil. Therefore, the emulsions are W/O emulsions. With the increase of concentration there is a sudden increase in conductivity, which reveals that the external continuous phase changes from oil to water and results in the formation of an O/W emulsion. It should be mentioned that the emulsion is very unstable near the inversion point, so the conductivity was measured immediately after the emulsification in order to get an accurate value. For all the surfactants except CO-720, the conductivity is above 10 ms/cm after transition. However, it is interesting that in the presence of CO-720 intermediate conductivity between 0.15 ms/cm to 5 ms/cm was found at some concentrations just after inversion. Salager et al.19 has suggested that this kind of phenomena in conductivity could be due to the formation of W/O/W multiple emulsion, However, in this study we did not observe the multiple emulsions by microscope. Determination of Critical Micelle Concentration (CMC). According to Figure 2, a stability plateau was found after a critical concentration. This concentration may correspond to the onset of formation of demulsifier micelles in the aqueous phase, which is called CMC. It should be noted that nonionic surfactants may also form some kind of aggregates in nonpolar solvent by intermolecular interaction
Figure 5. The variation of interfacial tension versus surfactant concentration in toluene/brine system.
between EO groups, as studied by Jones et al.20 However, they found that the degree of association is weak and the aggregation number is much smaller compared to their aggregation in water. Therefore, with both an oil and water phase as in the case of emulsion, the aggregation in oil phase can be neglected. The relation between CMC and emulsion stability has also been found by Aveyard et al.21 and Mohammed et al.22 in their demulsification studies. That is, the stability of emulsion levels off or increases after CMC depending on the type of demulsifier. In the present work, we also determined the CMC of nonionic surfactant in toluene/water mixture by measuring interfacial tension (IFT). The IFT is time dependent and was measured as a function of time. The IFT values obtained after 600s are plotted in Figure 5 versus surfactant concentration. As shown in Figure 5, the CMC can be determined from the break point of the two linear parts. For CO-520, there are two transition points, which may be due to its bad solubility in water and the significant transfer of molecules from water
(19) Salager, J. L.; Marquez, L.; Pe~ na, A. A.; Rond on, M.; Silva, F.; Tyrode, E. Ind. Eng. Chem. Res. 2000, 39, 2665–2676. (20) Jones, P.; Wyn-Jones, E.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2735–2749.
(21) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Lu, J. R. J. Colloid Interface Sci. 1990, 139, 128–138. (22) Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Colloids Surf., A 1994, 83, 261–271.
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to toluene phase through interface. Since the stability plateau is also absent for CO-520, it is not included in the further discussion. For comparison, both the change of CMC values and the plateau onset as a function of HLB are summarized in Figure 8A and will be discussed later. The variation of IFT values for different surfactants in Figure 5 should also be noted. It is found that the IFT after CMC decreases with the increasing of HLB, and the lowest IFT is ca. 5.5 mN/m for CO-890. In general, the IFT is determined by the amount and structure of surfactant molecules adsorbed at oil/water interface. In the present study, the difference between different surfactants is only the number of the EO group. Therefore, the major factor affecting the IFT is the adsorbed amount. As mentioned in Table 1, all the nonionic surfactants are soluble in toluene. However, from CO-520 to CO-890, their solubility in water is enhanced with the increase of EO number, which also increases their tendency to stay at the interface and result in lower IFT. Dynamic IFT. The IFT between crude oil and aqueous solution are time-dependent, which may be due to the slow diffusion of some components across the interface and molecular rearrangement at the interface. The main responsible components for the dynamics in IFT are indigenous surfactants from crude oil, including both asphaltenes and resins.23 For the nonionic surfactants used in the present study, they also have time dependence in IFT because they have solubility in both oil and water phase and are polydisperse in the molecular weight. If both indigenous surfactants and nonionic surfactants coexist in one system, the competitive adsorption at the interface will happen, which results in a new kinetics in IFT. The emulsion stability will depend on the result of competition. To investigate the interfacial activity and competitive adsorption of demulsifiers at the crude oil/ water interface, the dynamic IFT was studied, by producing an oil drop in the aqueous solution. For comparison, both pure toluene and crude oil were used as oil drops. These experiments were done at two surfactant concentrations, 50 ppm and 200 ppm, which are before and at the stability plateau, respectively. Since they have the same trend with the change of HLB except that the IFT at 200 ppm is lower than that of 50 ppm, only the results at 50 ppm are presented in Figure 6. Furthermore, a significant discrepancy between panels A and B in Figure 6 is the order of IFT as a function of HLB. In Figure 6B crude oil system, CO-610, CO-630, and CO-850 have very similar equilibrated IFT values, the lowest IFT was observed for CO-720. However, CO-890 has the lowest IFT when the oil phase is toluene, as shown in Figure 6A. This difference is worthy of discussion and will be covered later together with other properties as a function of HLB. To compare the adsorption kinetics, the experimental data in Figure 6 was fitted to a biexponential (four-parameter) decay curve, γ ¼ aeð -btÞ þceð -dtÞ
Figure 6. Dynamic interfacial tension when the oil drop was formed in aqueous solution: (A) toluene drop in 50 ppm demulsifier aqueous solution and (B) crude oil drop in 50 ppm demulsifier aqueous solution. (The inset plot is the curve for crude oil without demulsifier).
constants b and d describe the rate at which the interfacial tension decays, which correspond to the fast decay at initial and the slow decay after that, respectively. The curve fitting was performed using SigmaPlot 10.0 (Systat Software, Inc.) The results are shown in Tables 3 and 4. The decay process is qualitatively explained in the following according to the fitting parameters b and d. Discussion Kinetics of Adsorption at the Interface. The dynamic interfacial tension results can give information on the adsorption kinetics at the oil/water interface, which is helpful to the explanation of demulsification mechanism. In the case of toluene as oil phase, the interface layer is only composed of nonionic surfactants, and the kinetics of surfactant adsorption at the interface can be detected. In the case of crude oil, the interfacial competition between nonionic surfactants and crude indigenous surfactants can be studied. Therefore, the comparison of these two cases can reveal the interaction and replacement of indigenous surfactants by demulsifier. To explain the kinetics, one must try to understand the mechanisms involved when introducing an oil droplet into the water phase. The relaxation process at the liquid-liquid interface can be the consequence of different mechanisms: diffusion, adsorption barriers, structure reorganization,
ð2Þ
where γ is the interfacial tension as a function of time (t), γ0 is the interfacial tension at time 0, and a þ c = γ0. This equation has also been used by Fossen et al.24 for fitting the IFT curves of asphaltene solution in toluene. The decay (23) Buckley, J. S.; Fan, T. Petrophysics 2007, 48, 175–185. (24) Fossen, M.; Kallevik, H.; Knudsen, K. D.; Sjo¨blom, J. Energy Fuels 2007, 21, 1030–1037.
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Table 3. Parameters Obtained from the Fitting of Figure 6A for the Toluene-Water Interface demulsifier CO-520 CO-610 CO-630 CO-720 CO-850 CO-890
a (mN/m)
b (s-1)
c (mN/m)
d (s-1)
R2
6.09 ( 0.16 3.72 ( 0.08 5.50 ( 0.11 4.17 ( 0.10 2.20 ( 0.09 1.21 ( 0.03
(3.90 ( 0.20) 10-3 (5.00 ( 0.20) 10-3 (6.66 ( 0.30) 10-3 (3.90 ( 0.20) 10-3 (5.10 ( 0.40) 10-3 (1.40 ( 0.07) 10-2
23.52 ( 0.18 16.88 ( 0.08 18.76 ( 0.09 16.38 ( 0.11 11.10 ( 0.09 8.61 ( 0.01
(7.73 ( 0.61) 10-5 (9.52 ( 0.42) 10-5 (1.00 ( 0.05) 10-4 (1.00 ( 0.06) 10-4 (1.11 ( 0.07) 10-4 (6.75 ( 0.14) 10-5
0.99 0.98 0.98 0.99 0.97 0.97
Table 4. Parameters Obtained from the Fitting of Figure 6B for the Crude-Water Interface demulsifier No CO-520 CO-610 CO-630 CO-720 CO-850 CO-890
a (mN/m)
b (s-1)
c (mN/m)
d (s-1)
R2
9.68 ( 0.37 4.89 ( 0.09 3.66 ( 0.06 4.75 ( 0.09 1.27 ( 0.06 1.97 ( 0.05 1.51 ( 0.12
(5.00 ( 0.32) 10-4 (1.09 ( 0.04) 10-2 (1.16 ( 0.04) 10-2 (9.70 ( 0.40) 10-3 (9.20 ( 0.80) 10-3 (3.68 ( 0.16) 10-2 (1.21 ( 0.19) 10-1
22.07 ( 0.06 11.07 ( 0.04 5.32 ( 0.03 6.23 ( 0.06 3.35 ( 0.06 5.00 ( 0.01 9.73 ( 0.02
(1.53 ( 0.03) 10-6 (2.00 ( 0.04) 10-4 (3.00 ( 0.06) 10-4 (4.00 ( 0.11) 10-4 (2.00 ( 0.35) 10-4 (3.00 ( 0.04) 10-4 (2.00 ( 0.02) 10-4
0.93 0.98 0.99 0.98 0.98 0.98 0.97
desorption, and mass transfer to the opposite phase.25 The relaxation rate may be different at different equilibrium time, depending on which process is dominating. In the toluene-brine system as shown in Figure 6A and Table 3, the decay in IFT is attributed to the diffusion of nonionic surfactant and their partition into the oil phase. In Table 3, it can be seen that from CO-520 to CO-850, the fast decay constant b has just slightly changed, which may indicate their similarity in structure for the same homologues. However, when the EON increased to 40 for CO-890, a prominent increase in b constant has been found, which is around twice of the other b values. This result may indicate CO-890 has the fastest adsorption to the interface but a low tendency to partition to the oil phase. For pure crude oil-brine system without demulsifier, as shown in the first row of Table 4, the decay constants b and d are much smaller than the other cases in the presence of nonionic surfactants. In this case, the fast decay is usually due to the diffusion and adsorption of resins from bulk phase (oil droplet) to the interface when a fresh W/O interface is formed, since they have lower molecular weight and higher surface activity compared to asphaltenes. The next step represented by the slow decay constant corresponds to the adsorption of higher molecular weight asphaltenes at the interface, which have been proposed to interplay with resins and reorganize into a rigid film with probably multilayer structures.2,24 When demulsifier is coexisting with crude oil, both b and d constants are higher than in Table 3 for the toluene system. This is easy to understand, since both oil phase (indigenous surfactants) and aqueous phase (demulsifier) have surfaceactive agents under this condition, so the interface can be covered by surfactants adsorption from both sides, which leads to a faster decay in IFT. The b constants in Table 4 for CO-850 and CO-890 are increased to a significantly large extent than other surfactants. However, from CO-520 to CO-850, similar slow decay constants d were obtained. On the other hand, in comparison with pure crude oil system, both b and d constants in the presence of demulsifier in Table 4 are much higher. The results may show that when a crude oil droplet is produced in aqueous demulsifier solution, the competition adsorption at the interface happens
between demulsifier and indigenous surfactants. The adsorption of demulsifier from water phase is much faster than crude indigenous surfactants and plays a dominant role in the reduction of IFT. Therefore, the kinetics is actually determined by the properties of demulsifier in the fast decay. However, the coadsorption with indigenous surfactants is unavoidable and also influences the IFT values. Mechanizm of Destabilization. It is of crucial importance to understand the stabilization mechanisms when discussing the efficiency of demulsifiers. It has been well recognized that the stabilization of crude oil W/O emulsion is mainly attributed to the formation of a rigid interfacial film of asphaltenes, where asphaltenes may accumulate as nanosized aggregates at the interface and have interplay with resins as a solubilizing agent. Therefore, the function of emulsion inhibitor is to prevent the formation of this rigid film. Sjo¨blom et al.2 suggested that the addition of chemicals as inhibitors gives substantial enhancement in the efficiency of demulsification, which may be due to either an interfacial competition or a strong bulk interaction between the demulsifier and the crude oil stabilizing components. In the following part, the effect of surfactant inhibitor concentration and HLB on the crude oil emulsion stability will be discussed. Effect of Demulsifier Concentration. An important factor that determines the emulsion stability is the surfactant concentration. Here, we studied the effect of demulsifier concentration in a wide range from W/O until the region of O/W emulsion. At lower concentration before CMC, the emulsion stability decreases significantly compared to pure crude oil. The decrease in stability can be explained in that even a small amount of demulsifier molecules participating at interface can reduce the rigidity of the interfacial film of indigenous surfactants. The interfacial replacement can be traced back to the IFT results. The IFT values at the crude oil/brine interface with 50 ppm demulsifier show marked reduction compared to the case without demulsifier, as shown in Figure 7 with CO-720 as an example. The IFT values are chosen from the end of the kinetic curves, but it is not the real equilibrium condition since the IFT is still decreasing. From Figure 7 we can see a synergism effect of the crude indigenous surfactants and demulsifier in reducing the interfacial tension, which indicates an interaction between these two components. When the surfactant concentration is increased to values above CMC, the participation of extra surfactants
(25) Hannisdal, A. PhD thesis, Norwegian University of Science and Techonogy: Trondheim, 2006.
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Figure 7. The comparison of IFT values for pure crude oil, pure CO720 and their mixture (concentration of CO-720: 50 ppm).
at the interface is unfavorable because the interface is already saturated at CMC and surfactants prefer to form micelles in the water phase. Consequently, the concentration of surfactant at the interface keeps nearly constant. This results in a plateau region vis- a-vis emulsion stability. With further increase in surfactant concentration, the emulsion inversion was found after plateau. This is unexpected since, as mentioned above, the activity of monomeric surfactant at concentration higher than CMC is constant. Here we give some possible explanations for the inversion mechanism. One dominant factor may be related to the interfacial Winsor R-ratio,16 that is, the ratio of the interaction of surface-active agents at the interface with water to their interaction with oil. The situation when R is equal to 1 is the inversion point, which also corresponds to the zero average interface curvature. This situation occurs often in the so-called microemulsions, where a hydrophilic and a hydrophobic surfactant are mixed together. At a certain concentration the system reaches a maximum in interfacial activity with minimum emulsion stability. This region is also called bicontinuous microemulsions. This can be attained when the interfacial proportion of asphaltenes is reduced while the demulsifier proportion is increased to an appropriate value. This has also been discussed by Goldszal et al.,16 who suggested the correlation of demulsification to microemulsion phase behavior. This is related to the amphiphilicity of surfactant, which can be reflected by HLB. In addition, the bulk interaction between the demulsifier and asphaltenes may also contribute to this inversion. This can be solubilization of asphaltene molecules into the hydrophobic region in demulsifier micelles in water phase, or demulsifier can act like resins, which can accumulate around the asphaltenes in oil phase. When the demulsifier concentration is increased to a certain value where the stabilization effect of asphaltenes is completely inhibited, very unstable states are obtained, which indicates the inversion point. Effect of Demulsifier HLB. All the results related to the effect of HLB are summarized in Figure 8, as shown with the variation of different properties as a function of HLB. It can be seen from Figure 8A that the CMC and plateau onset are within the same order and have similar trend versus HLB, which can support the prediction that the occurrence of a stability plateau is due to the beginning of the demulsifier aggregation in bulk phase. The discrepancy between these two parameters can be attributed to the difference in the
Figure 8. The effect of HLB on (A) the concentration of plateau onset and CMC values; (B) equilibrated IFT at crude oil/water and toluene /water interface; (C) the separation rate constant at 200 ppm demulsifier concentration, and the average stability obtained from the plateau region in Figure 2; (D) the demulsifier concentration for inversion of emulsion from W/O to O/W.
polarity of oil phase, which could influence the partition. Figure 8A shows that both CMC and plateau onset are relatively constant from HLB = 12 to HLB = 14.2, and then a decrease was found after that. Generally speaking, CMC is defined as the concentration at which the interface is saturated and the aggregates begin to form in the bulk phase. Two opposite effects may contribute to change the CMC when a oil-water interface is dealt with. On one hand, higher HLB means higher repulsion between headgroups and weaker hydrophobic association, which can lead to the increase of CMC in water phase. This has been demonstrated 4581
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by Hsiao et al., who investigated the CMC in water for this surfactant series by surface tension method. One approximate expression ln(CMC) = 3.87 þ 0.056N has been obtained, where CMC is in micromolar units, and N represents the EO number. On the other hand, the partition between water and oil phase is a crucial factor influencing their CMC values at oil-water interface for this type of nonionic surfactants. M arquez et al.27 studied the partitioning of the ethoxylated nonylphenols between water and n-heptane. Similar equation as that for CMC has been obtained: log K = -4.01 þ 0.45N (K is the partition coefficient defined as the ratio of surfactant concentration in aqueous phase to that in oil phase). In the present study, toluene is used as oil phase, which can give different coefficient of N but the trend is the same as mentioned by M arquez et al.27 Therefore, with the increase of HLB, less surfactant is solubilized in the oil phase whereas more surfactant is available in the aqueous phase for micelle formation. The total concentration that is needed in the system to bring about micellization should decrease, which contributes to the decrease of CMC. When the HLB is lower than 14.2 the variation of EO number is not so significant, these two effects can be counteracted to a large extent, which result in a slight change of CMC. After that, the second effect is predominant and reduction of CMC is observed. Similar explanation can be given for the change of the plateau onset, since it is related to the critical condition when the interface is saturated by both indigenous surfactants and demulsifiers. In Figure 8B, the IFT values at the toluene-water interface are obtained from Figure 6A after a given time for comparison purpose. Please notice that these values are not measured at equilibrium since IFT is still decreasing. For crude oil-water interface, the IFT after 600 s are used for all the surfactants. In the case of the toluene-water system, the IFT decreases with the increase of demulsifier HLB until 17.8, no minimum was observed. However, in the crude oilwater system, the IFT decreases with the HLB until 14.2 and then increases again from 14.2 to 17.8. Meanwhile, the reduction in IFT is also remarkable, when comparing the IFT at toluene/water with those at crude oil/water interface, except for CO-890. This decrease may indicate that the demulsifiers partially displace the indigenous surfactants at the interface and form a mixed interfacial layer. However, for the most hydrophilic CO-890, there is no obvious difference whether toluene or crude oil is used as oil phase. This result may give some suggestion that CO-890 can replace most of the crude indigenous surfactants at the interface. To evaluate the effect of HLB on the demulsification efficiency, the separation rate constant obtained from Figure 1B at 200 ppm demulsifier concentration is also plotted in Figure 8C. As shown, the separation rate has a maximum at HLB 14.2. This is consistent with the IFT change at the crude oil/brine interface in Figure 8B when 200 ppm of demulsifier is present. On the basis of the above discussion, it can also be said that there is a good relation between the change of IFT value and the efficiency of the demulsifier. Therefore, the IFT measurement may be used as a hint to select the demulsifier in the case of the involved
surfactant family. In addition, the average stability of the emulsion within the plateau region in Figure 2 is also given in Figure 8C. Lower stability was observed in the middle range of HLB from 13 to 16. However, the minimum is not obvious for CO-720 in this case. The variation of inversion point is presented in Figure 8D. As can be seen, CO-520 (HLB=10) has the highest inversion concentration, whereas a minimum in the inversion concentration is observed for CO-720 (HLB=14.2, EON = 10.512). The increase of inversion concentration at higher HLB than 14.2 is not anticipated. As aforementioned, the inversion point can be related to the bicontinuous microemulsion where the curvature of interfacial layer is zero. This condition can also be considered that the average molecular packing parameter (MPP) of the interfacial molecules is close to 1 (MPP = v/al, where v and l are the volume and length of the hydrophobic part, respectively; and a is the area of the hydrophilic group).28 For crude oil indigenous surfactants such as asphaltene molecules, their MPP should be larger than 1 since they have very large hydrophobic part and form W/O emulsion. For the nonionic surfactants, their MPP should be lower than 1 since they are usually used as O/W emulsifiers, and the MPP decreases with the increase of HLB. In the emulsion system composed of both crude oil and demulsifier, the MPP of the interfacial layer is determined by both the interfacial proportion and the MPP of the individual components. Demulsifier with MPP < 1 can compensate the large MPP of asphaltenes at the interface, and then the inversion happens when the average MPP is close to 1. Therefore, for demulsifier with smaller MPP, lower interfacial proportion should be needed to get MPP =1. This can only explain the trend with HLB lower than 14.2 but fails to explain the part at higher HLB. For example, the IFT result for CO-890 in Figure 8B indicates that most of the asphaltenes at interface could be replaced by CO-890, so the interfacial proportion of CO-890 should be prominent. This result also suggests that the interfacial activity of CO-890 should be rather high. With the lowest MPP and high interfacial proportion, the inversion concentration should be lower than the other demulsifiers, which is contrary to the experimental result. There should be other explanation. To get more information, we also studied the emulsion stability of the toluene-water (1:1) system only in the presence of CO890. Both 500 and 1000 ppm CO-890 were tested for this kind of experiment, and similar phenomena were observed. After emulsification using the same procedure as for crude oil emulsion, O/W emulsion is produced according to the conductivity measurement. The phase separation happened after several minutes because of creaming, but no obvious coalescence was found between the droplets. The upper layer of compact emulsion droplets preserved the stability for more than one month. This may suggest that the interfacial film has very good viscoelasticity that can resist the coalescence. Considering the structure of CO-890, it is possible that its extremely long EO headgroup can form network structure by intermolecular hydrogen bonds. This kind of structure may also form in the interfacial layer of W/O droplet where a large proportion of CO-890 coexists with asphaltenes, which may impede the inversion of emulsion. This can also explain the higher stability of the emulsion in the present of CO-890 in Figure 2, in comparison with other demulsifiers. A similar
(26) Hsiao, L.; Dunning, H. N.; Lorenz, P. B. J. Phys. Chem. 1956, 60, 657–660. (27) Marquez, N.; Graciaa, A.; Lachaise, J.; Salager, J. L. Langmuir 2002, 18, 6021–6024.
(28) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc. Faraday Trans. II 1976, 72, 1525.
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phenomenon has been observed by Jeribi et al., where the combination of asphaltenes and Triton X-405 (HLB 17.9) provides a better stability to the oil drops than the case without Triton X-405. On the basis of the above discussion it can be concluded that CO-720 with HLB 14.2 and EON 10.5-12 is the most efficient inhibitor for the destabilization of this crude oil emulsion, since it has the highest separation rate constant within the stability plateau and the lowest inversion point. Similar results have been obtained in the literature. Goldszal et al.16 studied the demulsification of Vic-Bilh crude oil by ethoxylated nonylphenol families, and found that the optimal demulsifier has EON from 7 to 12, when salinity changes from 5 to 200 g/L, whereas demulsifier with EON lower or higher than this range gives very low efficiency. Aveyard et al.21 investigated the resolution of Forties crude oil emulsion by ethoxylated octylphenol surfactants. The results also showed that the maximum resolution of water was obtained with EON 9-10. Cooper et al.30 studied the relevance of HLB to demulsification of heavy oil by Pluronic, Span, Tween, and Brij surfactants. They found that the most effective agents for dewatering have HLB values between either 4 and 6 or 13 and 15. It is worth noting that all these results give the similar HLB and EON range for optimal
demulsification, in spite of different operation procedures and different crude oils. Conclusion The influence of inhibitor HLB and concentration on the destabilization of crude oil emulsions has been studied. The IFT results show that there exists significant interaction between crude oil indigenous surfactants and nonionic surfactants. The bottle tests and conductivity results show that the emulsion stability minimum corresponds to the inversion point. The mechanism of inversion can be related to the properties of the interfacial layer composed of both crude oil surfactants and inhibitors. Similar to the bicontinuous microemulsions, the inversion happens when the curvature of interfacial layer is close to zero. In addition, there is a notable stability plateau versus demulsifier concentration after CMC until the inversion concentration. Moreover, the demulsifier with HLB 14.2 (EON = 10.5-12), that is, intermediate HLB, is the most efficient to destabilize the crude oil emulsion. Finally, the IFT between crude oil and demusifier solution can be used as a hint for the evaluation of demulsifier efficiency. Acknowledgment. This work is performed by the FACE centre-a research cooperation between IFE, NTNU, and SINTEF. The centre is funded by The Research Council of Norway and by the following industrial partners: StatoilHydro ASA, Norske ConocoPhillips AS, Vetco Gray Scandinavia AS, Scandpower Petroleum Technology AS, FMC, CD-adapco, ENI Norge AS, and Shell Technology Norway AS.
(29) Jeribi, M.; Almir-Assad, B.; Langevin, D.; Henaut, I.; Argillier, J. F. J. Colloid Interface Sci. 2002, 256, 268–272. (30) Cooper, D. G.; Zajic, J. E.; Cannel, E. J.; Wood, J. W. Can. J. Chem. Eng. 1980, 58, 576–579.
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