ARTICLE pubs.acs.org/EF
Destabilization of Petroleum Emulsions: Evaluation of the Influence of the Solvent on Additives Viviane F. Pacheco, Luciana Spinelli, Elizabete F. Lucas, and Claudia R. E. Mansur* Institute of Macromolecules (IMA), Federal University of Rio de Janeiro (UFRJ), Avenida Horacio Macedo, 2030, Ilha do Fund~ao, 21941-598 Rio de Janeiro, Rio de Janeiro (RJ), Brazil ABSTRACT: This paper evaluates the influence of the type of solvent medium on the activity of two linear and two branched poly(ethylene oxide)poly(propylene oxide) (PEOPPO) block co-polymers used to destabilize water-in-oil emulsions. These co-polymers were dissolved in xylene/ethanol (75:25) and toluene, which are commonly employed in commercial formulations, and water and solbrax/butanol (65:35) as a proposition of this work. The results show that the branched PEOPPO co-polymer with hydrophilic segments (EO and OH) adjacent to each other performed best in breaking down the emulsions studied. This efficiency was better when the used solvent media was xylene/ethanol (75:25) or solbrax/butanol (65:35). The former solvent mixture improved the diffusion of the molecules of the additive in the oil phase of the emulsion and acted as a co-additive; the later only presented action as a co-additive.
1. INTRODUCTION In oilfields, emulsions can be found in almost all areas: in the reservoirs, producing wells, intermediate production equipment, pipelines, primary processing installations, and flowlines. The occurrence of emulsions is a common phenomenon and is particularly caused when formation water is co-produced with crude oil. Suitable methods have been developed to obtain the petroleum presenting reduced water content, but as the oil is extracted from a field, the percentage of produced water can rise, reaching about 70%, because of the application of recovery techniques involving water injection.1 Emulsifiers have surface activity. They concentrate at the interface of the water in crude oil emulsion and form a mechanically resistant viscoelastic film around the droplets. This film acts as a protective layer that restricts the coalescence process, thus reducing the emulsion destabilization rate.2 Many authors25 indicate asphaltenes as one of the main factors of the stabilization of water-in-oil emulsions. Because of the amphiphilic nature of asphaltenes, some researchers stated that asphaltene molecules would aggregate like surfactants and form micelles above a certain concentration [critical micelle concentration (cmc) or as other researches refer it as critical aggregation concentration (cac)].68 Asphaltenes and their molecular aggregates function as surfactants and tend to migrate to the wateroil (WO) interface, making emulsions more stable. These emulsions can be destabilized by, among other processes, the addition of chemical demulsifier compounds (normally in concentrations ranging from 10 to 1000 ppm), to improve the separation rate of the emulsion. They cause the interfacial film to become thinner, allowing the droplets to come closer and coalesce, so that the phases separate.9 Polymers are often used as demulsifiers, as well as in many other oil production operations.10 Those used for this purpose are composed of non-ionic molecules, with polar mass around 5000 g/mol, and contain hydrophilic and hydrophobic parts.11 The hydrophilic part includes oxyethylene, hydroxyl, carboxyl, or r 2011 American Chemical Society
amine groups, and the hydrophobic part is composed of alkyl, alkylphenol, or oxypropylene groups.12 Among the commercial demulsifiers used are ethoxylated phenolformaldehyde resins and poly(ethylene oxide)poly(propylene oxide) (PEOPPO) block co-polymers. Among the properties sought in demulsifiers are rapid adsorption at the WO interface, the capacity to displace the natural emulsifiers that stabilized emulsions, and activity to make the films and the WO interface thinner and more fragile.9 Commercial demulsifiers are normally formulated from a mixture of two or more bases, dispersed in organic solvents (usually aromatic solvents and alcohols), which act as coadditives and/or make the formulation less viscous and thus easier to pump. This paper reports the results of experiments to evaluate the influence of the type of solvent medium on the activity of two linear and two branched PEOPPO block co-polymers used to destabilize water-in-oil emulsions.
2. EXPERIMENTAL SECTION 2.1. Materials. The linear and branched PEOPPO block copolymers were donated by Dow Química Ltda., S~ao Paulo, Brazil. A total of four co-polymers were studied, two linear and two branched copolymers, named L1, L2, R1, and R2, respectively. The PEOPPO block co-polymers used in this work were of four types, two monofunctional linear co-polymers (called L1 and L2) and two branched co-polymers (called R1 and R2). Co-polymers R1 and L1 have adjacent hydrophilic segments (that is, the PEO segment and OH group are next to each other), while in R2 and L2, these segments are alternated (that is, the PEO segment is separated from the OH group by the hydrophobic PPO segment).13 The specification sheets supplied by the manufacturer indicate that these co-polymers are synthesized Received: December 30, 2010 Revised: March 14, 2011 Published: March 14, 2011 1659
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Table 1. Characterization of PEOPPO Block Co-polymers
a
By SEC. b By 1H NMR. c From refs 14 and 15.
industrially via an anionic mechanism, which produces molecules with relatively narrow polydispersion (Mw/Mn). The structures of the co-polymers were determined by hydrogen nuclear magnetic resonance spectroscopy (1H NMR) and size-exclusion chromatography (SEC).14,15 Table 1 shows the molar masses of the co-polymers and the values obtained for the ratio Mw/M n, confirming their narrow polydispersity, mainly for the branched co-polymers (R). It also shows the structures of the co-polymers, identified by the number of repeated ethylene oxide (EO) and propylene oxide (PO) units. These numerical values were used to calculate the EO/PO ratio, which indicates the degree of affinity of the molecules for the aqueous or organic media; the higher the EO/ PO ratio, the more hydrophilic the surfactant. The EO/PO ratios obtained for R1, R2, L1, and L2 were 0.19, 0.47, 0.51, and 0.84, respectively, providing evidence of the greater affinity of these molecules for the organic medium. As expected, R1 has much greater affinity for the organic medium than L2. The solvent media for the additives used in this work were toluene, xylene/ethanol (75:25), solbrax/butanol (65:35), and water. Commercial toluene, n-butyl alcohol, ethyl alcohol [pro analysis (pa)], and xylene (pa) were purchased from Vetec Química Fina Ltda., Brazil. Before use, the toluene was distilled and dried under metallic sodium. The others were used as received. The solvent solbrax was donated by Petrobras Distribuidora, Brazil, and was used as received. The main characteristics of the solvents of the solbrax eco series are their low concentration of aromatic compounds, olefins and sulfur. This makes them less toxic and reduces their odor. They are produced by catalytic hydrogenation of petroleum distillates at high pressure and have a molecular distribution between 9 and 26 carbon atoms, with a distillation range between 140 and 310 °C. The solbrax solvent used in this work has a distillation range between 255 and 285 °C, for which reason it is called solbrax eco 255/285. Its main characteristics are its high flash point and nearly imperceptible odor. The crude oil sample came from a Brazilian oil field. The chemical and physicochemical characteristics of this sample, with respect to the level of saturates, aromatics, resins, and asphaltenes (SARA), water content, density [degrees American Petroleum Institute (API)], and levels of nitrogen, sulfur, nickel, and vanadium, were determined according to the procedures described in a previous paper.16 The results are shown in Table 2. The water content of the crude oil (0.59 wt %) was relatively low, characteristic of a dehydrated sample. The amount of water present in
Table 2. Chemical and Physicochemical Characteristics of Petroleum properties water content (wt %)
petroleum 0.59
API (deg)
29.90
saturates (wt %)
58.40
aromatics (wt %)
26.20
resins (wt %)
14.61
asphaltenes (wt %)
0.79
resins/asphaltenes nitrogen (wt %)
18.50 0.30
sulfur (wt %)
0.32
nickel content (mg/kg)
4.0
vanadium content (mg/kg)
6.0
this sample can be considered negligible; therefore, it did not interfere in the results obtained from measuring the volumes of water incorporated in the oil phase during the experimental procedures carried out. The API density was 29.9°. According to some authors, oils with API density values between 22° and 30° are classified as medium and those below 20° are considered heavy. Therefore, this sample can be considered non-heavy.17 Besides the density and water content, the other relevant parameters to characterize a crude oil sample, shown in Table 2, are the content of the fractions extracted from the oil, the ratio between resins and asphaltenes, and the concentration of metals. The low concentration of asphaltenes (0.79 wt %) found in this oil indicated it was suitable for our purposes, because the asphaltenes present in an oil sample have a significant influence on the stability of water-in-oil emulsions and our intent was not to work with highly stable (hard to separate) emulsions. Synthetic emulsions prepared from samples of this type of oil must show clear differences in the capacity for demulsification when comparing different demulsifier systems. 2.2. Methods. 2.2.1. Preparation of PEOPPO Block Co-polymer Solutions and Water-in-Oil Emulsions. The PEOPPO block copolymers were dissolved in different solvents [toluene, xylene, ethanol, xylene/ethanol (75:25), solbrax, solbrax/propanol, and solbrax/ butanol]. To evaluate the efficiency of co-polymers as demulsifiers, 1660
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solutions of 40% (w/v) of each additive were prepared and, from these, removed the volume calculated as necessary to conduct each test. To prepare the saltwater-in-oil emulsions, the saltwater solution, composed of 55 000 ppm of the salts NaCl/CaCl (10:1), was prepared as described in a previous paper.18 Then, the performance of copolymers solutions was evaluated in the laboratory from these synthetic emulsions prepared using Ultra Turrax T 50 agitator at 6000 rpm for 3 min. The saltwater was added to the crude oil, and the water/oil ratio in the emulsion was 50:50. 2.2.2. Physicochemical Characterization of the PEOPPO Block Copolymers. Techniques to determine solubility and surface/interface tension were used to evaluate the behavior of the co-polymers in different solvents. 2.2.2.1. Evaluation of the Solubility of the PEOPPO Block Copolymers. Initially, the solubility tests were performed in different solvent media for co-polymer R1, at a concentration of 40% (w/v), at room temperature, with toluene, xylene, ethanol, xylene/ethanol (75:25), solbrax, solbrax/propanol, and solbrax/butanol. These two later solvent mixtures were tested in several proportions. Then, all four co-polymers were evaluated at concentrations of 40, 400, 800, and 1200 ppm, using toluene, xylene/ethanol (75:25), solbrax/butanol (65:35), and water as the solvent systems. The solubility tests of the co-polymer samples were performed in a test tube immersed in a beaker containing water, placed over an agitation and heating plate. The solutions containing each additive were heated and then slowly cooled. The temperatures were determined by a thermometer placed in the test tube. The temperature range analyzed was 1075 °C. Two duplicates were prepared of each solution, and there were two cloud point temperature readings for each solution, determined by the average between the temperature at which the first indication of clouding appeared and the temperature at which the clouding disappeared. All of these tests were performed by visual inspection. 2.2.2.2. Analysis of Surface/Interface Tension of the PEOPPO Block Co-polymer Solutions. The surface tension measurements were determined by the du No€uy ring method using a Kr€uss model K10 tensiometer at 25 °C. All of the measurements were made in triplicate, and only the values with variation less than 1 mN/m were considered. For each co-polymer, a graph was plotted of the average tension value (in mN/m) as a function of the logarithm of the concentration (in %, w/v). Then, this graph was used in each case to determine the cmc. The saltwateroil interface tension measurements were obtained by the pendant drop method using a DataPhysics model OCA-20 automatic contact angle meter (goniometer), to observe the interfacial behavior of the co-polymer as a function of time. All of these measurements were made at 25 °C. When dissolved in water, the co-polymers were dispersed in the aqueous phase (saltwater), and when dissolved in toluene, xylene/ ethanol (75:25), and solbrax/butanol (65:35), the co-polymers were dispersed in the oil phase (crude oil). All of the analyses were carried out at a concentration of 160 ppm of the co-polymer. In pendant drop tests, a droplet of oil is formed at the end of a straight needle (d = 0.89 mm) connected to a syringe with electronic control of the droplet, within an optical glass cuvette containing a fixed volume of the co-polymer solution. The droplet is subject to interfacial tension and gravitational forces and is filmed by a camera. The measurements were taken at 1 min intervals, and the interface tension (γ) was determined by digitalizing the images and analyzing the profile of the droplets by applying the YoungLaplace equation (eq 1)19 ΔP ¼ ðFo Fa Þgh γð1=r1 Þ þ ð1=r2 Þ
ð1Þ
where ΔP is the pressure difference across the interface (inside and outside the droplet), Fo and Fa are the densities of the oil and aqueous
phases, respectively, g is gravity, h is the height of the liquid column of the droplet, and r1 and r2 are the principal radii of curvature. The errors of these measurements are indicated in the equipment manual as being 5%. 2.2.3. Tests of the Gravitational Separation Efficiency of the PEOPPO Co-polymers. The performance of the co-polymer solutions prepared in different solvents (water, xylene/ethanol, solbrax/butanol, and toluene), as described previously, was evaluated using WO gravitational separation tests or bottle tests to test the as-prepared emulsion. The test and the emulsion preparation were described in a previous publication.18 The efficiency of each formulation used in these tests was calculated by applying eq 215,18 EFWO ¼ ðVWS =VWT Þ 100
ð2Þ
where EFWO is the efficiency of gravitational separation of water and oil, in % by volume, VWS is the volume of water separated during the test, in mL, and VWT is the volume of total water inside the test tube, in mL. All measurements were taken in triplicate. The values presented are the means accompanied by the respective standard deviations.
3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterization of the PEOPPO Block Co-polymers. 3.1.1. Solubility Testing of the PEOPPO Block Co-polymers. First, solubility tests of the co-polymers at
room temperature were carried out in different solvent media. These were toluene, xylene, ethanol, and xylene/ethanol (75:25). Among these, toluene and xylene/ethanol (75:25) are the dispersants normally used for formulations containing demulsifying additives.13,1720 Water and solbrax were also used as solvents. The latter, as mentioned before, has low concentrations of aromatics and olefinic compounds. Because solbrax is not a good solvent for the PEOPPO block co-polymers studied, mixtures of the former with other alcoholbased solvents were prepared to enhance the ability to dissolve these co-polymers. The solbrax/solvent ratios were varied to obtain mixtures at which these solvents were miscible and the copolymers were dissolved. The mixtures of solbrax (15.1 MPa1/2)21 with ethanol (26.0 MPa1/2)22 were immiscible. For this reason, mixtures of this solvent with alcohols were formulated having lower solubility parameters (propanol, 23.5 MPa1/2; butanol, 23.3 MPa1/2),22 in the respective proportions of 75:25 for solbrax/propanol and solbrax/butanol and 65:35 for solbrax/butanol. According to the solubility tests, the minimum butanol concentration miscible with the solbrax and also sufficient to dissolve the co-polymers was 35%. The co-polymers were insoluble in the other mixtures. Thus, the results indicate that solbrax 255/285 can replace aromatic compounds (such as xylene or toluene) to dissolve additives used as demulsifiers, as long as it is mixed with solvents, such as propanol or butanol, whose addition permits adjusting the solubility parameter of the mixture to the solubility of the additive of interest. The solubility tests of the co-polymers in water, toluene, xylene/ethanol (75:25), and solbrax/butanol (65:35) were performed while varying the temperature, with the co-polymers added in different concentrations (40, 400, and 1200 ppm). All of the co-polymers (R1, R2, L1, and L2) were soluble in toluene, xylene/ethanol (75:25), and solbrax/butanol (65:35), throughout the temperature range analyzed (2565 °C). There 1661
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Table 3. Evaluation of the Solubility of the Co-polymers in Water
Table 4. CMC Results of PEOPPO Co-polymers in Aqueous Solution surface tension at
cloud point ((1 °C) concentrations (ppm)
L1
area occupied per
co-polymer
cmc (mN m1)
59
R1 R2
36.6 a
0.01 a
cmc (%, w/v) molecule (nm2/molecule)
R1
R2
L2
40
36
55
400
27
41
56
47
L1
36.0
0.70
0.30
1200
23
35
47
42
L2
a
a
8.09
0.30 0.55
a
was no loss of solubility of the co-polymers, even at the highest concentrations. As expected, the analysis of the solubility of the co-polymers in water showed that they lost solubility as the temperature increased. The reason is that their solubility is related to the hydration of the polar groups present in the molecules (the oxyethylene groups in the co-polymers studied).13 The hydration of the oxygen atoms of ether by hydrogen bonds results from the interaction of at least two water molecules per EO unit, and this hydration increases with an increasing length of the PEO chain.23 As the temperature of these systems rises, the hydrogen bonds are broken and the non-ionic surfactants lose their solubility in water. The cloud point is the temperature below which there is a single phase of a molecular solution. Above the cloud point, a non-ionic surfactant loses its solubility in water, causing the formation of two phases and giving the mixture a cloudy aspect.24 To investigate the effect of the temperature on the solubility of the co-polymers in aqueous medium (up to 65 °C), the cloud points were measured at concentrations of 40, 400, and 1200 ppm. The results are shown in Table 3. It can be seen that, as expected, increasing the concentration hampered solubility; i.e., as the co-polymer concentration increased, the cloud point temperatures fell. Of the linear co-polymers, L1 was more soluble in water than L2, although it has a lower EO/PO ratio (Table 1). This means that the difference in the EO/PO ratio was not enough to overcome the influence of the positioning of the hydrophilic (PEO and OH) and lipophilic (CH3 and PPO) segments in the molecule, as shown in previous studies.13,15,18 Of the branched copolymers, R2 was more soluble in the temperature range studied. This behavior can be explained by the higher EO/PO ratio in the chain of co-polymer R2, in which case the influence of the position of the hydrophilic/lipophilic segments does not overcome the difference between the EO/PO ratios. Co-polymer R1 has an extremely low EO/PO ratio (0.19; Table 1), which otherwise would make it totally insoluble in water. However, its adjacent structure partially counteracts this, increasing its solubility.13,15,18 3.1.2. Determination of the Surface Tension of the Aqueous Solutions of the PEOPPO Block Co-polymers. To study the activity of the co-polymers at the waterair interface, the surface tension in the function of the co-polymer concentration in aqueous solution was measured, using the du No€uy ring method. The variation of the surface tension as a function of the concentration leads to determination of the mass of the surfactant adsorbed at the surface, through the Gibbs adsorption isotherm (eq 3)19 Γa ¼ ð1=RTÞðDγ=DlnCÞT
ð3Þ
where Γa is the surface concentration (adsorption), γ is the surface tension, C is the concentration of the solution, T is
It was not possible to determine these values using surface tension as a function of the log concentration of the co-polymer.24
the temperature at which the measurement is made, and R is the universal gas constant. The area (A) occupied by a surfactant molecule at the water air interface can be calculated by eq 419 A ¼ 1=Γa NA
ð4Þ
where NA is Avogadro’s number. The surface tension measurements of the aqueous co-polymer solutions were used in this work to help understand the adsorption of co-polymer molecules at the interfaces. For this purpose, the areas occupied by a co-polymer molecule at the waterair interfaces by eq 4 were determined. The cmc values were calculated in a previous work.25 For the co-polymers with adjacent structures (R1 and L1), these areas were determined near the cmc. In the case of co-polymers R2 and L2, which do not have a well-defined cmc, the areas were calculated in the concentration interval from 0.5 to 5% (w/v) (final range of the experiment, that is, the concentration range where the chains of these co-polymers are most tightly packed). The results show that the co-polymers with adjacent structures (R1 and L1) take up the smallest areas at the waterair interface (Table 4). This behavior can be attributed to tighter packing of the molecules of these co-polymers at the interface.25 From the values presented in Table 4, it can be seen that the area occupied by the molecule of co-polymer R1 (0.30 nm2/ molecule) is similar to the molecule of co-polymer L1 (0.30 nm2/molecule), even though the molar mass of R1 is 3 times that of L1 (Table 1). The authors present as a suggestion the following contribution for this behavior. Surfactant molecules tend to be adsorbed at the interface with the apolar segment turned away from the aqueous solution, while the polar segment is inside the solution. In the case of adjacent co-polymer R1, the position of the molecule at the interface makes each of its three branches behave like an individual molecule.26 The area taken up by a molecule of co-polymer L2 at the surface of the aqueous solution was much greater than the areas found for the other co-polymers. Besides the fact that this copolymer has an alternate structure, which already works to increase this value, it was observed that the surface of the aqueous solution was still not completely saturated.26 3.1.3. Determination of the WO Interfacial Tension of the PEOPPO Block Co-polymers. The measurements of the WO interfacial tension were performed using the pendant drop method. This method allows us to follow the variation of the WO interfacial tension values as a function of time. This, in turn, enables us to obtain these values after the system has reached equilibrium, that is, after the processes of diffusion, 1662
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Figure 1. Effect of the structure and time in measures of WO interfacial tension with co-polymers solutions, at 160 ppm, in xylene/ ethanol disperses in oil. Temperature = 25 °C.
Figure 2. Effect of the solvent bulk of R1 co-polymer, at 160 ppm, in measures of WO interfacial tension. Temperature = 25 °C.
adsorption, reorganization at the interface, desorption, and transfer of the molecular mass to the other phase have occurred. 3.1.3.1. Influence of the Structure of the Co-polymers. The interfacial tension values as a function of time can be correlated with the efficiency of forming the WO interfacial film in the emulsion. A previous study27 showed that, the faster the minimum interfacial tension values are reached, the quicker the diffusion of the molecules to the interface, leading to the formation of thinner and less elastic films that rupture more readily. To observe the influence of the structure of the co-polymers, they were dissolved in a mixture of xylene/ethanol (75:25) and then added to the oil phase at a concentration of 160 ppm. The results in Figure 1 show that all of the PEOPPO copolymers reduced the WO interfacial tension. This agrees with the results shown in Table 4. At first, the interfacial tension values decline with time, indicating that the molecules of the copolymers gradually migrate to the interface, adsorbing as monolayers according to the Gibbs isotherm. Co-polymer R1 was most effective at reducing the interfacial tension; the droplet ruptured after 8 min of analysis. This behavior is related to the higher content of PO units in its chains (Table 1), which favors the process of diffusion in the oil. Indeed, there appears to be an inverse relation between the EO/PO ratio of the co-polymers and the capacity to reduce the interfacial tension. The EO/PO ratio increases in the following order: R1 < R2 < L1 < L2, while the ability to reduce the tension varies in the reverse order. As already discussed, this molecular architecture has a significant influence in this type of test as well; R2 has a slightly lower EO/PO ratio than L1, but its ability to reduce the tension is much better. 3.1.3.2. Influence of the Solvent Medium in Which the Copolymer Was Dissolved. To assess the influence of the solvent medium in which the co-polymer was dissolved on its ability to reduce the WO interfacial tension, co-polymer R1 and the solvent media toluene, water, and solbrax/butanol (65:35) were chosen, besides xylene/ethanol (75:25), whose test results are reported previously. These analyses were also performed using
the pendant drop method. In the case of the organic solvents, the co-polymer was dissolved in the solvent and added to the oil, forming the interface with the water. In the case of the water used as the solvent for the co-polymer, this solution was added to the water that forms the interface with the oil. The results obtained are shown in Figure 2. It can be observed that when the aqueous co-polymer solution was added to the water phase, there was a greater reduction in interfacial tension after 18 min. This result reflects the high hydrophobicity of this co-polymer (EO/PO ratio = 0.19), which tends to migrate quickly to the interface to minimize its contact with the water. Among the solvent media in which co-polymer R1 was dispersed in the oil phase, the xylene/ethanol (75:25) mixture provided the fastest reduction of the interfacial tension, reaching a value of ∼5.5 mN/m after 8 min. For the rest of these systems, the tension decreased more gradually, and even after the longest measurement time (18 min), the tension still appeared to be declining slightly. The WO interfacial tension values obtained at the end of the test can be interpreted as representing the influence of the solvent medium of the co-polymer on its ability to reduce interfacial tension. The effectiveness of these three co-polymer media was, in decreasing order: xylene/ethanol (75:25) > toluene > solbrax/butanol (65:35). This behavior appears to be directly related to the solubility parameter (δ) of the dispersion media of 20.0 MPa1/2 > 18.2 MPa1/2 > 17.9 MPa1/2, respectively. The solubility parameter values were obtained from the literature21,22 and, in the case of the solvent mixtures, calculated from the volume fraction of the solubility parameters of the pure solvents, also obtained from the literature. These results indicate that the more hydrophilic solvent media facilitated the dispersion of the co-polymer in the crude oil. 3.2. Evaluation of the Performance of the PEOPPO Block Co-polymers in the Bottle Tests. The performance of the copolymers in destabilizing the WO emulsions was evaluated by the bottle test.16 To evaluate the stability of the synthetic WO emulsions used in this work, emulsions with saltwater as the aqueous phase and the crude oil sample as the oil phase were prepared, without any 1663
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Figure 3. Efficiency of gravitational separation of synthetic emulsion WO of the co-polymer R1 solubilized in xylene/ethanol (75:25) and added to the emulsion in different concentrations. Initial concentration of the co-polymer solution = 40% (w/v). Oily phase of the emulsion = P1 petroleum. Temperature = 45 °C.
Figure 4. Efficiency of gravitational separation of synthetic emulsion WO of the co-polymer R2 solubilized in xylene/ethanol (75:25) and added to the emulsion in different concentrations. Initial concentration of the co-polymer solution = 40% (w/v). Oily phase of the emulsion = P1 petroleum. Temperature = 45 °C.
additive. These emulsions were then placed in a heated bath at the temperature for carrying out the gravitational separation tests (45 °C). The WO emulsions remained stable after 70 min, a sufficiently long period for the addition of any of the selected demulsifiers to cause them to destabilize in shorter periods than this. 3.2.1. Influence of Adding the Solvent on the Stability of the WO Emulsions. To investigate the role of the solvent used to dissolve the co-polymers on the stability of the WO emulsions, tests were first performed only adding the solvent [water, toluene, xylene/ethanol (75:25), and solbrax/butanol (65:35)] to the emulsion in sufficient quantities to obtain a final solvent concentration of 160 ppm. The water and toluene did not cause any significant change in the emulsion that could be observed with the naked eye. However, in the presence of solbrax/butanol (65:35) and xylene/ ethanol (75:25), the emulsions changed color (becoming clearer), mainly when observed at the bottom of the tube used
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Figure 5. Efficiency of gravitational separation of synthetic emulsion WO of the co-polymer L1 solubilized in xylene/ethanol (75:25) and added to the emulsion in different concentrations. Initial concentration of the co-polymer solution = 40% (w/v). Oily phase of the emulsion = P1 petroleum. Temperature = 45 °C.
in the assays. This behavior appears to indicate the start of the droplet coalescence process, showing that these solvent media, especially solbrax/butanol (65:35), might be acting as co-additives in the demulsification process. This behavior suggests that alcohols can also be acting as a co-surfactant. 3.2.2. Influence of the Type and Concentration of the Additive on the WO Emulsions. To assess the influence of the concentration of each co-polymer on breaking down the emulsions formed, they were dissolved in xylene/ethanol (75:25), at a concentration of 40% (w/v). Aliquots of this solution were then added to the emulsions, in quantities sufficient to obtain final additive concentrations of 40, 80, 160, and 400 ppm, after which the gravitational separation was observed as a function of time. In all cases, an increase in the concentration of the additive improved its performance in destabilizing the emulsion. Co-polymer R1 was the most efficient additive in breaking down the emulsion (Figure 3). After 15 min at a concentration of 80 ppm, approximately 90% of the water had separated out. Complete phase separation was achieved at a concentration of 160 ppm after 15 min. The phase separation was much less efficient for the other copolymers (R2, L1, and L2). Figures 46 show that the separation efficiencies of these co-polymers, at a concentration of 400 ppm after 15 min, were about 78, 78, and 65% for R2, L1, and L2, respectively. At the lowest concentrations (80 and 40 ppm), co-polymer L1 was more efficient than co-polymer R2. Figure 7 shows the graphs of all of the co-polymers, at the concentration of 80 ppm. In general, the efficiency of the gravitational separation for the samples tested increased in the following order: L2 < R2 < L1 < R1. Although R1 was the most efficient in reducing the interfacial tension and L2 was the least efficient (Figure 1), there was no direct relationship between the efficiencies of the additives and this property. The results showed that there is some relation between the gravitational separation and the solubility preference of the additive in the oil phase (Table 3). R1 may have performed best because it had the highest bulk phase concentration. If so, for these polymer additives, those that are poorly soluble in the aqueous phase perform better than those that are soluble in both phases. 1664
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Figure 6. Efficiency of gravitational separation of synthetic emulsion WO of the co-polymer L2 solubilized in xylene/ethanol (75:25) and added to the emulsion in different concentrations. Initial concentration of the co-polymer solution = 40% (w/v). Oily phase of the emulsion = P1 petroleum. Temperature = 45 °C.
Figure 7. Efficiency of gravitational separation of synthetic emulsion WO of the co-polymers solubilized in xylene/ethanol (75:25) and added to the emulsion at a concentration of 80 ppm. Initial concentration of the co-polymer solution = 40% (w/v). Oily phase of the emulsion = P1 petroleum. Temperature = 45 °C.
However, there was a relationship among the efficiency of the gravitational separation, the molecular architecture, and the area occupied at the interface. The gravitational separation efficiency increased as the area occupied per molecule at the WO interface declined (Table 4). The smallest areas were attained by the molecules with adjacent architecture. The literature6,16 shows that the molecular aggregates formed by asphaltenes helps stabilize WO emulsions, forming a barrier at the interfaces. Within a determined limit,16 the greater the aggregate size, the more difficult it will be for a demulsifying surfactant to reach the interface, because of the smaller number of voids at the interface. The molecules of the co-polymers that take up smaller areas at the interfaces more easily fill the spaces left by the asphaltenes at the WO interface, allowing for more efficient gravitational separation in less time.
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Figure 8. Efficiency of gravitational separation of synthetic emulsion WO of the co-polymer R1 solubilized in different solvent bulks. Temperature = 45 °C.
The better efficiency of R1 in relation to L1 can be related to the tighter packing of the molecules of the former. 3.2.3. Influence of the Dispersant Solvent of Co-polymer R1. To evaluate the influence of the solvent medium in which the additive is dispersed on the efficiency of gravitational separation of the emulsion, formulations of R1 were prepared (the most efficient co-polymer in this process) in all of the solvent media evaluated in this work, water, toluene, solbrax/butanol (65:35), and xylene/ethanol (75:25), all at a co-polymer concentration of 40% (w/v). The final co-polymer concentration in the emulsions was 80 ppm. The results are shown in Figure 8. The gravitational separation efficiency of co-polymer R1 was highest when it was dissolved in xylene/ethanol or solbrax/ butanol, and it was least efficient when dissolved in water. The results of reduction of WO interfacial tension as a function of the solvent medium (Figure 2) showed that the xylene/ethanol (75:25) provided a faster reduction of the interfacial tension values (6 mN/m in 4 s), indicating that this solvent medium favors the diffusion of the molecules of the co-polymer in the oil phase. The best efficiency of this mixture (∼35%) occurred after 5 min. The equal efficiency of the solbrax/butanol mixture, although it does not favor the diffusion of the molecules of the additive as much, can be associated with its action as a co-additive in separating the WO emulsion, as was the case of the xylene/ ethanol mixture. Visual observation of the aqueous phase after the bottle tests showed that, when the solvent medium used was water, a large amount of oil was trapped on the walls of the glass tube, indicating that this medium does not favor diffusion of the molecules of the additive in the oil phase. This behavior was also observed when toluene was used as the solvent medium.
4. CONCLUSIONS Among the PEOPPO block co-polymers tested in this work, the most efficient in promoting gravitational separation of waterin-oil emulsions was the branched co-polymer with adjacent polar and apolar segments (R1). 1665
dx.doi.org/10.1021/ef101769e |Energy Fuels 2011, 25, 1659–1666
Energy & Fuels Co-polymer R1 performed best when dissolved in xylene/ ethanol (75:25) or solbrax/butanol (65:35). This result is due to the facility of the molecules of these co-polymers to diffuse in the oil phase of the emulsion and the action of the solvent medium used as a co-additive. Demulsifier formulations traditionally use aromatic solvents, such as xylene and toluene, which are highly toxic. The results presented here, for light oil and with low asphaltene content, besides shedding light on the aspects that influence the WO gravitational separation process, clearly show that it is possible to substitute aromatic solvents by solbrax 255/285, as long as it is mixed with another solvent, such as isobutyl alcohol, in an adequate proportion, to promote the solubility of the PEO PPO block co-polymer.
’ AUTHOR INFORMATION
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’ ACKNOWLEDGMENT We thank the Brazilian National Research Council (CNPq), Carlos Chagas Filho Foundation for Research Support (FAPERJ), and Petrobras for financial support and the Dow Chemical and Petrobras companies for providing the samples. We also thank Dr. J. B. Ramalho for lending the goniometer. ’ REFERENCES (1) Sztukowski, D. M.; Yarranton, H. W. J. Colloid Interface Sci. 2005, 285, 821–833. (2) Khadim, M. A.; Sarbar, M. A. J. Pet. Sci. Technol. 1999, 23, 213–221. (3) Madge, D. N.; Garner, W. N. Miner. Eng. 2007, 20, 387–394. (4) Yarranton, H. W.; Sztukowski, D. M.; Urrutia, P. J. Colloid Interface Sci. 2007, 310, 246–252. (5) Yarranton, H. W.; Urrutia, P.; Sztukowski, D. M. J. Colloid Interface Sci. 2007, 310, 253–259. (6) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14, 38–42. (7) Andreatta, G.; Goncalves, C. C.; Buffin, G.; Bostrom, N.; Quintella, C. M.; Arteaga-Larios, F.; Perez, E.; Mullins, O. C. Energy Fuels 2005, 19, 1282–1289. (8) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11. (9) Kim, Y. H.; Wasan, D. T. Ind. Eng. Chem. Res. 1996, 35, 1141–1149. (10) Lucas, E. F.; Mansur, C. R. E.; Spinelli, L.; Queiros, Y. G. C. Pure Appl. Chem. 2009, 81 (3), 473–494. (11) Pe~na, A. A.; Hirasaki, G. J.; Miller, C. A. Ind. Eng. Chem. Res. 2005, 44, 1139–1149. (12) Wu, J.; Xu, Y.; Dabros, T.; Hamza, H. Energy Fuels 2003, 17, 1554–1559. (13) Mansur, C. R. E.; Oliveira, C. M. F.; Gonzalez, G.; Lucas, E. F. J. Appl. Polym. Sci. 1997, 66, 1767–1772. (14) Oliveira, C. M. F.; Lucas, E. F. Polym. Bull. 1990, 24, 363–370. (15) Mansur, C. R. E.; Lechuga, F. C.; Mauro, A. C.; Gonzalez, G.; Lucas, E. F. J. Appl. Polym. Sci. 2007, 106 (5), 2947–2954. (16) Ramalho, J. B. V. S.; Lechuga, F. C.; Lucas, E. F. Quim. Nova 2010, 33 (8), 1664–1670. (17) Arnold, K. E.; Smith, H. V. Crude oil emulsions. In Petroleum Engineering Handbook, 3rd ed.; Bradley, H. B., Ed.; Society of Petroleum Engineers: Richardson, TX, 1992; Chapter 19. (18) Mansur, C. R. E.; Barboza, S. P.; Gonzalez, G.; Lucas, E. F. J. Colloid Interface Sci. 2004, 271 (1), 232–238. 1666
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