Ind. Eng. Chem. Res. 1996, 35, 1141-1149
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Effect of Demulsifier Partitioning on the Destabilization of Water-in-Oil Emulsions Young H. Kim and Darsh T. Wasan* Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616
The factors affecting the demulsification and interfacial behavior of water-in-oil emulsions in the presence of oil-soluble demulsifiers were investigated. Using both model water-in-oil and water-in-crude oil emulsion systems with demulsifiers with different chemical structures, the effects of demulsifier partitioning on the interfacial and film rheological properties were studied. The experimental results were compared and related with the demulsifier performance. There is a one-to-one correlation between the performance of demulsifier and the interfacial activity of the partitioned demulsifier; the partitioned demulsifier components exhibit an increase in static and dynamic interfacial activity, low dynamic interfacial and film tension, and a low film dilational modulus with a high adsorption rate-low interfacial tension gradient (MarangoniGibbs stabilizing effect) and have excellent demulsification performance. Introduction Demulsification is a process which involves flocculation, coalescence, and separation of two immiscible liquids. According to Bancroft (1913), the stability of any emulsion is largely affected by the nature of the adsorbed layer and the stability of the film is strongly dependent upon the surfactant adsorption and interfacial rheological properties such as elasticity, interfacial tension gradients, and interfacial viscosity. Zapryanov et al. (1983) have developed a model to describe the rate of drainage of axisymmetric plane parallel emulsion films in relation to droplet-droplet coalescence phenomena. In their analysis, they determined that the drainage time of a surfactant-stabilized emulsion film is dependent not only on the interfacial viscosity but also on the value of the tension gradient, Kσ ) dσ/dC, which represents the activity of the surfactant at the interface. The drainage velocity of the thinning film is dependent upon the forces acting at the interface of the film (Figure 1a). As two emulsion drops approach each other, liquid flows out of the film and thinning occurs. However, the flowing liquid film carries with it the adsorbed surfactant molecules due to the convective flux and thus disturbs the equilibrium, creating a concentration gradient at the interface (Figure 1b). This concentration gradient produces a variation in the local value of the interfacial tension (tension gradient) which generates a force opposing the flow of liquid out of the film. The sum of the interfacial stress and the tangential bulk stress on the liquid in the droplet must counterbalance the tangential bulk stress at the liquid film interface, which results in a reduction in the mobility of the film surface and a decrease in the rate of film thinning. Chemical demulsification is a process in which the film thinning rate is enhanced and stability of the film is reduced by a chemical demulsifier. It has been established that the role of the demulsifier is to change the interfacial rheological properties and to destabilize the surfactant-stabilized emulsion films (Eley et al., 1987; Mohammed et al., 1993, 1994). The principal role of the chemical demulsifier is to enhance film drainage by suppressing the tension gradient (Mohammed et al., * Author to whom correspondence should be addressed.
0888-5885/96/2635-1141$12.00/0
Figure 1. Film drainage process: (a) drop approach and emulsion film formation between drops; (b) tangential stress balance at the film interface during film drainage.
1993, 1994). It can be shown that not only should the interfacial viscosity be low in order to enhance the film thinning but also the interfacial concentration and the activity (Kσ) of the demulsifier must be sufficiently high to suppress the tension gradient. Most commercial demulsifiers that are used to break up water-in-oil (W/O) emulsions are oil-soluble. The interfacial activity of these oil-soluble demulsifier molecules is controlled by the rate of the bulk diffusion process from the bulk phase to the interface and the adsorption barrier at the W/O interface. It has been found that the efficiency of the demulsifier may relate to hydrophilic-lipophilic balance (HLB) behavior of the demulsification system (Cooper et al., 1980; Averyard et al., 1992). In certain cases, some components of the oil-soluble demulsifier can be dissolved (partitioned) into the water drop phase (dispersed phase); this process is known as partitioning. When the demulsifier components exist in the dispersed (droplet) phase, the parti© 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
tioned demulsifier components in the droplet phase can strongly affect the W/O dynamic interfacial properties, such as the interfacial tension gradient or the Marangoni-Gibbs stabilizing effect. Ivanov (1980) studied the thinning of emulsion films when the surfactant is soluble in the dispersed phase. His analysis showed that due to the circulation of the fluids in the droplet, as well as the geometry of the system, the surfactant molecules inside the droplet are easily transported to the film surfaces. Thus, a surfactant which is soluble in the droplet phase can easily suppress the surface concentration gradients (which have arisen from the convective flux of the draining film liquid) and suppress the tension gradients. This process results in rapid film draining and films with short lifetimes. Malhotra and Wasan (1987) showed that this result holds true for partially miscible surfactants. Krawczyk et al. (1991) identified the partitioning of the demulsifier as an important parameter in the mechanism of demulsification. He measured the partition coefficient (Kp) of demulsifiers, which is defined as the ratio of demulsifier concentration in the water droplet phase to the concentration in the oil phase, at equilibrium. He concluded from experimental evidence that a demulsifier with a partition coefficient close to unity has a better performance. Bhardwaj and Hartland (1993, 1994a,b) found that the partitioned demulsifier effectively reduces interfacial tension and increases demulsification performance by rapid adsorption of the demulsifier on the O/W interface. It is generally believed that the most effective demulsifier is one which partitions equally into the water and oil phases. This balance would lead to a maximum in the surface adsorption of demulsifier and a minimum in the interfacial tension gradient (Berger et al., 1987). Shetty et al. (1992) have found that a water-soluble demulsifier can effectively destabilize the water-in-oil emulsions. They studied the effects of demulsifiers with varying HLB’s and molecular weights on the destabilization of water-in-oil emulsions; they concluded that a demulsifier can have a very good performance when the demulsifier contains a high percentage of the hydrophilic group (high HLB number) and a low molecular weight. Cooper et al. (1980) and Averyard et al. (1992) found that the HLB of a system is an important parameter for effective demulsification. Usually, commercial demulsifiers are blended mixtures of several components which have various chemical structures and polymeric materials, as well as a wide molecular weight distribution. In this case, each component of the demulsifier possesses a different partitioning ability and a different interfacial activity due to various chemical structures or types. In order for the partitioned components to effectively suppress the tension gradients, they should possess sufficiently high interfacial activity. Thus, the interfacial activity of partitioned demulsifier molecules should be considered as well as the partition coefficient (amount of partitioned demulsifier into the droplet phase). In certain cases, the partitioned demulsifier may not enhance the demulsification process with a high partition coefficient if the partitioned components are not interfacially active. In this paper, the effect of the demulsifier partitioning on the W/O emulsion stability involving the partition coefficient, interfacial activity of the partitioned demulsifier, and W/O interfacial and emulsion film (W/O/W, oil film between water droplets phase) rheological properties have been considered. The results are cor-
Table 1. System A Demulsifiers demulsifier
chemical group
RP-4011 F-17 RP-2327 R-77 RP-0484
polyamines, glycols alkylaryl sulfonates alkylaryl sulfonate, phenolic resin, polyamines phenolic resin, glycols, aryl sulfonates alkylaryl sulfonate, polyamines, phenolic resins
Table 2. System B Demulsifiers demulsifier
chemical group
mol wt
RE-1748 RE-1753 RE-1868 RE-1756
EO/PO EO/PO, diepoxide phenolic resins phenolic resins
8000 8000 3000 3700
related to the demulsifier performance and water-inoil emulsion stability. Experimental Section Materials. To generalize and ensure the accuracy of experimental results and the theoretical consideration of the partitioning effect of demulsifier on the emulsion stability, two different types of water-in-oil emulsions and demulsifier sets (system A and system B) were investigated. Emulsions. Model Oil Emulsion. The model oil consisted of 1.0 of asphaltenes (derived from a California crude oil (Anderson and Birdi, 1990))/L of an alkanearomatic solution of 70% heptane and 30% toluene. The solution (model oil) in which the asphaltenes were dissolved shall henceforth be called “heptol”. The emulsions were generated with the model oil [(70 vol %) which possesses 1.0 g of asphaltene in heptol and pure water (30 vol %)] by vigorous agitation in a graduated cylinder (average emulsion drop size was 30 µm). The emulsion preparation methods used in this investigation were identical to ensure that the mechanical energy input to the system remained constant. Pure water was used for the droplet (dispersed) phase. Crude Oil Emulsion. The crude oil used was California Heavy Crude Oil (viscosity ) 2000 cP at 25 °C, density ) 0.93 g/mL, asphaltene content ) 3.2 wt %, resin content ) 2.0 wt %). Before using the crude oil, the water and solid particles which are naturally present in the crude oil were removed by centrifugation for 30 min with a 15 000 rpm rotational speed at 25 °C. The emulsions were prepared with California crude oil and a 1500 ppm brine solution. In a 1000 mL beaker, the crude oil was stirred at 25 °C by a stirring motor (2000 rpm), while the water phase was dropped onto the crude oil from a buret. The final average drop size of the W/O emulsions was 5.4 µm, and the water content was 10 vol %. For the water phase (droplet phase), a brine solution (1500 ppm NaCl) was used. Demulsifiers. System A. The demulsifiers used for system A were dispersed mixtures of alkylaryl sulfonate, phenolic resins, and polyamines obtained from Petrolite Corp. (Table 1). System B. The demulsifiers used were ethylene oxide (EO)/propylene oxide (PO) copolymers and phenolic resin from Baker Performance Chemicals Inc. (shown in Table 2). Emulsion Stability and Demulsifier Effectiveness. Batch emulsion stability tests for the water-inoil (W/O) emulsion were conducted on the crude oil and model systems. System A Demulsifiers. Conventional bottle tests were used to measure the demulsifier performance for
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1143 Table 3. Emulsion System and Stability Measurement system A emulsion system water oil demulsifiers emulsion stability test a
Figure 2. Apparatus for the measurement of dynamic interfacial tension (drop volume method).
system A demulsifiers. The model oil emulsions prepared were transferred into graduated cylinders, and a known amount of each demulsifier was injected into the emulsions using a microsyringe. The amount of demulsifier injected was 45 ppm concentration relative to the emulsion oil phase. After adding the demulsifier, the volume of water settled was noted against time at 25 °C. System B Demulsifiers. The stirred tank test (Bhardwaj and Hartland, 1994a,b) was used for system B demulsifiers for the measurement of emulsion stability and demulsifier performance. A known amount (20 g) of the crude oil emulsions prepared was transferred into a jacketed beaker, which was maintained at 80 °C. The beaker was connected to a constant-temperature bath and a circulation pump (Figure 2). After thermal equilibrium, a known amount of demulsifier (45 ppm in oil phase) was added into the beaker using a syringe and the emulsion drop size change was measured as a function of time. During the experiment, the emulsions were stirred with a magnetic stirrer (60 rpm) to regulate the emulsion density and exclude the gravitational effect. The stirring velocity (60 rpm) ensured that the stirring did not restabilize the emulsions coalesced in the range of emulsion size. In the measurement of emulsion size during the experiment, a photomicrogrraphic technique was employed. A sample of the emulsion was placed on a Haward cell and photographed through a Nikon microscope with magnifications of 40× and 10×. The first photographs were taken before the addition of the demulsifier, and additional photographs were taken at different times throughout the experiment. The photographs were converted into slides for the purpose of determining the size distribution using a digital droplet size analyzer. For the calculation of the number of drops, the volumetric mean droplet size was used and the volumetric number of drops (N) per unit volume was calculated by following equation:
N)
w (4/3)πr*3
(1)
where w is water content (vol %) and r* is the volumetric mean drop size (radius). With the measured data, N, at various time intervals, the flocculation (K) and coalescence rate (a) constants were calculated by fitting the data to the following kinetic equation (Borwankar et al., 1992):
N N0
[
) e-Gθ eG + G ln θ +
]
∞
Gn+1(θn - 1)
n)1
n × n!
∑
(2)
pure water 1 g of asphaltene/1 L of heptol alkyl sulfonate, resin, polyamines bottle test (water separation test)
system B 1500 ppm NaCl solution crude oil EO/PO,a resin stirred-tank test (coalescence rate)
Ethylene oxide/propylene oxide copolymer.
where G ) K/aN0 and θ ) 1 + aN0t; N0 is the initial number of droplets in unit volume, and t is time. The types of emulsions and emulsion stability tests for the two demulsifier systems (system A and system B) are summarized in Table 3. Partition Coefficient. The partition coefficient (Kp) is defined as the equilibrium ratio of the demulsifier concentration in the water phase (Cw) to the demulsifier concentration in the oil phase (Co) (Averyard et al., 1992). Thus,
Kp ) Cw/Co
(3)
The partition coefficient for asphaltenes/resins (which are stabilizing surfactants of the W/O emulsions and virtually insoluble in deionized water) is zero. Partition coefficients greater than unity are indicative of preferentially water-soluble compounds. Preferentially oilsoluble compounds are characterized by partition coefficients less than unity. Due to the complex chemical mixture of the demulsifier and the difference of the partitioning ability of each component of the demulsifier, measurement of the exact partition coefficient would be nearly impossible. The approximate partition coefficients were measured as described below (Krawczyk et al., 1991; Bhardwaj and Hartland, 1993, 1994a,b). Equilibrated systems were prepared by contacting the water and oil phases in a graduated cylinder under vigorous agitation. The volumes of the water phase and the oil phase were 50 and 100 mL, respectively. The initial concentration of each demulsifier in the oil phase was 45 ppm, and the asphaltene concentration was 1.0 g/L of heptol. After equilibrium for 3 or 4 days, the contents of the graduated cylinder was centrifuged at 5000 rpm for 2 h. The two immiscible phases were separated using a separatory funnel, and the separated oil phase was used to determine the partition coefficient. The separated water phase was stored for the subsequent experiment of the interfacial activity measurement. To determine an approximate value of the partition coefficient, a calibration plot of the interfacial tension of the oil phase containing known quantities of the demulsifier against the pure water has to be ascertained. The interfacial tension of the equilibrated and separated oil phases against pure water was measured by the Wilhelmy plate method (Kruss Model K8). The concentration of the partitioned demulsifier in the separated oil phase was determined by comparing the equilibrium interfacial tension measured against the calibration plot. The resulting mass balance allows the calculation of the equilibrium demulsifier concentration in the oil and water phases and the determination of the partition coefficient.
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Interfacial Activity of a Partitioned Demulsifier. To reduce the interfacial tension gradient or Marangoni-Gibbs stabilizing effect on the W/O interface and destabilize the W/O emulsions, the partitioned demulsifier should have high interfacial activity. The measurements of the interfacial activity of a partitioned demulsifier were conducted using the following procedure: The separated water phase from the previous experiment containing partitioned components of the demulsifier was diluted as the ratio di. The dilution ratio (di) is given by eq 4:
di )
separated water vol. separated water vol. + added fresh water vol. (4)
To evaluate the interfacial activity of a partitioned demulsifier, the change in the interfacial tension of the separated water phase (against the model oil phase without any demulsifier) was measured as a function of the dilution ratio. The interfacial activity of a partitioned demulsifier (δi) is obtained from the shape of the interfacial tension isotherm. It is given by
(
δi ) -
)
∂σ ∂ log di
(5)
The term δi is a measure of the effectiveness of the partitioned demulsifier molecules to lower the tension gradients in the film surfaces. The term is relative because the exact concentrations of the partitioned demulsifier components are unknown. Dynamic Interfacial Tension and Dynamic Interfacial Activity. Since the film drainage during the coalescence process results in dynamic interfacial properties, it is essential to know the dynamic interfacial tension and the activity. The dynamic interfacial tensions were measured for the best and the worst performance demulsifiers of the system A and system B demulsifiers using model oil and water phases. To study the partitioning effect of the demulsifier on the dynamic properties, measurements were made of the dynamic interfacial tension for the system of 45 ppm demulsifier in oil/fresh water (before-partition system) and preequilibrated water/oil like the previous separated phases (after-partition system). The drop volume method (Jho and Burke, 1983) was used to measure the dynamic interfacial tension (σ) with the relationship:
σ)
∆FgV0 F(V0/rc3) rc
(6)
where rc is the capillary radius, V0 is the volume of a detached drop, ∆F is the density difference between water and oil phases, g is the gravimetric constant, and F(V0/rc3) is the correction factor. The experimental apparatus (Figure 2) used to measure the dynamic interfacial tension included a syringe pump (SAGE Instrument Model 220), syringes (Hamilton 5 and 2.5 mL), a Teflon line connection, and a Teflon capillary with radius (rc) ) 1.2 mm (Figure 2). The water phase was pumped at a constant flow rate with the syringe pump to form and detach the water drops at the tip of the capillary. The drop frequency (the number of drops formed per unit of time) was calculated
Figure 3. Schematic diagram of the film experimental apparatus.
from the measured dropping time. With a given water flow rate, the drop volume (V) was also calculated and the dynamic interfacial tension was obtained against the drop frequency. In addition, dynamic activity was defined by the slope of the dynamic interfacial tension curve as a function of the drop frequency. Interfacial Shear Viscosity. Hydrodynamically, the mobile W/O interface produces unstable emulsions. The mobile film surface will produce a low hinderance of the film flow during the film thinning and thus yield a higher film thinning rate and lower film stability. For the interfacial mobility measurement, interfacial shear viscosity of the water/oil (W/O) interface between the water phase and the oil phase in the presence of various demulsifiers was measured using a viscous traction deep-channel viscometer (Wasan et al., 1971). The centerline velocity at the interface was measured to calculate the interfacial shear viscosity with the known velocity of the deep channel viscometer dish. By solving momentum balance equations, the interfacial shear viscosity can be calculated (Wasan et al., 1971). Emulsion Film Properties. To understand the role of the demulsifier on the destabilization mechanism in the emulsion film, emulsion film rheological properties as well as the single interfacial rheological properties should be considered (Kim et al., 1995). Using the device and technique shown in Figure 3 (Kim et al., 1995), W/O emulsion film (W/O/W type) rheological properties were measured in the presence of a demulsifier. In film rheological property measurement, a thin oil film in the water phase was formed at the tip of a glass capillary. To understand the role of the partitioned demulsifier in the drop phase during the film thinning, dynamic film tension and film stress relaxation measurements were performed for the beforepartitioned and after-partitioned systems described in the previous section. In the measurement of the dynamic film tension, the film tension was measured while the film was expanded. In the film stressrelaxation experiment, the film was expanded suddenly at time ) 0 and the decreasing film tension was measured as a function of time. Results and Discussion The measured demulsifier performances which were obtained by the bottle test for the system A demulsifiers are shown in Figure 4. The measured partition coefficients of system A demulsifiers are presented in Table 4. In system A (Figure 4), the order of demulsifier performance is RP-4011 > R-77 > RP-2327 . F-17 g
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1145
Figure 4. Water drainage performances of system A demulsifiers.
Figure 6. Normalized number of drops vs time in the tank test for system B demulsifiers. A steeper curve represents higher performance of the demulsifier.
Figure 5. Static interfacial tension vs dilution ratio of partitioned demulsifiers of system A. A higher slope indicates higher interfacial activity. Table 4. Partition Coefficients of System A Demulsifiers in the Oil/Water Emulsion System demulsifier
RP-4011
F-17
RP-2327
R-77
RP-0484
Kp ( 0.002
0.40
0.21
0.19
0.19
0.16
Table 5. Interfacial Activities of Partitioned Demulsifiers of System A in the Oil and Water System no demulsifier RP-4011 R-77 RP-2327 RP-0484 F-17 demulsifier δi ( 0.005
2.11
1.03
0.65
0.55
0.35
0.05
RP-484. RP-4011 separates the water phase in the quickest time (highest performance), and it has the highest partition coefficient in Table 4. However, note that the RP-2327 and R-77 demulsifiers have very different performances with the same partition coefficients and F-17 does not have better performance with a higher partition coefficient. It should be emphasized that the partition coefficient alone cannot be a definitive parameter to reveal the effects of a partitioned demulsifier on the demulsification enhancement. In other words, a demulsifier having a high partition coefficient may not suppress the interfacial tension gradient of the emulsion film if the components of the partitioned demulsifier in the drop phase are not interfacially active. When we consider the interfacial activity of the partitioned demulsifier for system A (Figure 5 and Table 5), the best performing demulsifier still has the highest interfacial activity of the partitioned demulsifiers. In fact, there is a one-to-one correlation between the interfacial activity of the partitioned demulsifier and the demulsifier performance. The demulsifier performances for system B demulsifiers measured by the tank tests (Bhardwaj and Hartland, 1994a,b) are shown in Figure 6. The graph shows the comparison of a kinetic model (Borwankar et al., 1992) with the experimental data for the number of
Figure 7. Static interfacial tensions vs dilution ratio of partitioned demulsifiers of system B. A higher slope indicates higher interfacial activity. Table 6. Partition Coefficients and Interfacial Activities of System B Demulsifiers Partitioned in the Oil and Water System demulsifier
RE-1748
RE-1753
RE-1868
RE-1756
Kp ( 0.002 δi ( 0.005
1.97 0.75
1.05 0.56
0.63 0.32
0.15 0.01
drops as a function of time. As time progresses, the rate-controlling process of demulsification changes from coalescence controlled to flocculation controlled. The calculated coalescence rate constants represent the performances of the demulsifiers for the film-thinning process in the demulsification. The better demulsifier has the higher coalescence rate constant. With the measured interfacial activity data of the partitioned demulsifier as shown in Figure 7 and summarized in Table 6, the same correlation was observed between the demulsifier performance and the interfacial activities of the partitioned demulsifier components in system B. The dynamic interfacial tensions of the partitioned demulsifier against drop frequency of system A for the worst (F-17) and the best (RP-4011) demulsifiers are shown in Figure 8. The best demulsifier has the lowest dynamic interfacial tension and the highest interfacial activity (which was calculated by the initial slope of the interfacial tension curve against frequency in Figure 8). The same trend was obtained for system B (Figure 9). These results show that, in order to displace the adsorbed asphaltenes effectively, a good demulsifier needs to possess partitionable components having low dynamic interfacial tension and high dynamic interfacial activity.
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Figure 8. Dynamic interfacial tension vs drop frequency for the best (RP-4011) and the worst (F-17) performance demulsifiers of system A demulsifiers.
Figure 11. Effect of partition on the dynamic interfacial tension for the best (RE-1748) and the worst (RE-1756) demulsifiers of system B demulsifiers. Table 7. Interfacial Shear Viscosity of the Model Oil and Water System for the Best and Worst Demulsifiers in System A
Figure 9. Dynamic interfacial tension vs drop frequency for the best (RE-1748) and the worst (RE-1756) performance demulsifiers of system B demulsifiers.
Figure 10. Effect of partition on the dynamic interfacial tension for the best (RP-4011) and the worst (F-17) demulsifiers of system A demulsifiers.
Considering the effect of a partitioned demulsifier on dynamic interfacial tension, the dynamic interfacial tension measurements were performed for the beforeand after-partition systems using the best and worst performance demulsifiers of system A and system B (Figures 10 and 11). In the high-performance demulsifiers (RP-4011 and RE-1748), the dynamic interfacial tension is decreased and the dynamic interfacial activity is enhanced when the demulsifier is partitioned (after-
demulsifier
no demulsifier
interfacial viscosity (sP)
3.5 × 10-2
RP-0484
RP-4011
2.4 × 10-2 5.8 × 10-4
partition of the demulsifier). However, in case of the low-performance demulsifiers, no change of the dynamic interfacial tension was observed because the demulsifier does not partition into the droplet phase. As a result, it can be said that the partitioned demulsifier suppresses dynamic interfacial tension and increases dynamic interfacial activity. The suppressed interfacial tension and enhanced dynamic interfacial activity are caused by the enhancement of a mass-transfer rate to effectively adsorb the demulsifier on the film surfaces (Zapryanov et al., 1983; Ivanov, 1980; Bhardwaj and Hartland, 1993, 1994a,b). The lowering of the dynamic interfacial tension when the demulsifier is partitioned into the water droplet phase proves that the demulsifier mass-transfer rate is enhanced by the partitioned demulsifier, probably due to convective mass transfer in the droplet phase (Ivanov, 1980) and/or the effective mass transfer from both water and oil phases (Bhardwaj and Hartland, 1993, 1994a,b). The interfacial shear viscosity measured for system A is shown in Table 7. Without the demulsifier, the asphaltene adsorbed on the oil/water interface gives a very high interfacial shear viscosity. The worst demulsifier (RP-484) does not displace the asphaltenes effectively, and interfacial shear viscosity is still high. The best demulsifier (RP-4011), however, reduces the interfacial shear viscosity by an order of 2. Kinetically, lowering the interfacial shear viscosity is necessary for an effective demulsification process. Referring to the result of the interfacial shear viscosity measurement as shown in Table 7, the demulsifier reduces the interfacial shear viscosity and increases the interfacial mobility, destabilizing the water-in-oil emulsions. In the results of the dynamic interfacial measurement for before- and after-partitioned systems, the lowered dynamic interfacial tension with the higher slope suggests that the adsorption rate of the demulsifier is improved by the partitioned demulsifier (which exists in the water droplet phase). In the case of an afterpartitioned system, the demulsifier is present in the oil and water droplet phases; the demulsifier adsorbs from both phases. The rate of demulsifier adsorption on the
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1147
Figure 12. Effect of partition on film stress relaxation of the water-in-crude oil emulsion film in the presence of 45 ppm RE1756 demulsifiers at 40 °C.
Figure 13. Effect of partition on film stress relaxation of the water-in-crude oil emulsion film in the presence of 45 ppm RE1748 demulsifiers at 40 °C.
W/O interface is improved and results in a higher efficiency of demulsification (Kim et al., 1995). Usually, the demulsifier adsorption is diffusion-controlled, and the diffusivity of the demulsifier can be described as a mean value of diffusivities in the oil and water phases when the demulsifier is partitioned (after-partitioned system). If the demulsifier is soluble in both oil and water phases (if initially both phases were equilibrium), then the diffusivity (D) can be shown as (Hunsel and Joos, 1987):
D=
(x
Co D + Co + Cw o
x
)
Cw D Co + Cw w
2
(7)
where Co and Cw are the demulsifier concentrations in oil and water phases and Do and Dw are the diffusion coefficients in oil and water phases, respectively.
D)
1 (D + KpDw + xKpDoDw) 1 + Kp o 1 (D + KpDw) D≈ 1 + Kp o
(8) (9)
where Kp is the partition coefficient ()Cw/Co). If a demulsifier is insoluble in the water phase, the coefficient Kp is small and the demulsifier adsorption rate will depend on the oil phase diffusivity (Do) in the diffusion-controlled adsorption process. If a demulsifier, however, is soluble in both oil and water phases (i.e., partitioned), the demulsifier can be transferred onto the interface from both the water and oil phases. An increase of the adsorption rate of the demulsifier may result (Bhardwaj and Hartland, 1993, 1994a,b). In Figures 12 and 13, the W/O emulsion film stress relaxations are shown in the presence of worst (RE1756) and best (RE-1748) performance demulsifiers of system B at 40 °C, respectively. Note that, in this case, the oil phase is a heavy crude oil which has a relatively high (approximately 1000-1500 cP) bulk viscosity. The worst performance demulsifier, RE-1756, does not effectively reduce the dynamic film tension, which indicates the demulsifier has a very low surface activity (Figure 12). However, the high-performance demulsifier, RE-1748, effectively decreases the film tension after the film is expanded. Note that the film tension does not effectively decrease without the partitioned demulsifier RE-1748. Without the partitioning of the demulsifier (RE-1748), the film tension cannot be effectively
Figure 14. Dynamic film tension vs ln(A/A0) in the presence of 45 ppm RE-1756 demulsifier at 40 °C. The rate of film expansion was 1.9 × 10-4 cm2/s. The slope of the curve represents the film dilational modulus.
reduced (Figure 13). No effect of before- and afterpartitioned systems was observed in the RE-1756 demulsifier due to the very low partition coefficient (Figure 12). It is interesting that the demulsifier can adsorb to a greater extent on the film surfaces when the demulsifier is present in both the water drop phase and the oil film phase than just the oil film phase only. When the demulsifier molecules are dispersed in a liquid medium, the diffusivity (D) can be described as:
D ) kT/3πηd
(10)
where d is the diameter of a molecule. Equation 10 shows that, at a constant-temperature condition, demulsifier diffusivity can be very low when the bulk viscosity of the medium is extremely high. In the case of water-in-crude oil emulsions, the viscosity of the continuous phase is relatively high so that the diffusivity in the oil phase (Do) is very low. In that condition, the demulsifier diffusivity (D) will be mainly dependent upon the diffusivity in the water phase (Dw). In the crude oil case, during the film thinning, the demulsifier cannot be adsorbed on the W/O interface without partitioning because the demulsifier diffusivity in the oil phase (Do) can be considerably low when the bulk viscosity of the oil phase is extremely high. In Figures 14 and 15, the dynamic film tensions measured are shown for the RE-1756 and RE-1748 demulsifiers, respectively. As in Figure 15, in the presence of effective demulsifier RE-1748, the dynamic
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Figure 15. Dynamic film tension vs ln(A/A0) in the presence of 45 ppm RE-1748 demulsifier at 40 °C. The rate of film expansion was 1.9 × 10-4 cm2/s. The slope of the curve represents the film dilational modulus.
film tension is relatively low and the film dilational modulus is very low when the demulsifier is partitioned; this proves that the Marangoni-Gibbs stabilizing effect can be diminished by the partitioned effective demulsifier RE-1748. A lower film dilational modulus caused a higher adsorption of RE-1748 partitioned demulsifier, which implies a lower surface tension gradient of the film during film expansion (film thinning). The film dilational modulus (Eγ) is calculated from the slope of the dynamic film tension vs logarithmic value of normalized film area expanded as:
Eγ )
∂γ ∂ ln(At/A0)
(11)
where γ is the dynamic film tension (mN/m) and A0 and At are the initial film area and the expanded film area at any time, respectively. Conclusions Using both a model water-oil system and a watercrude oil system with demulsifiers with various chemical structures, the effect of demulsifier partitioning on the demulsification in water-in-oil emulsions was established. Furthermore, the measurements of interfacial and emulsion film rheological properties allowed the understanding of the demulsification mechanisms with a chemical demulsifier. There is a one-to-one correlation between the performance of the demulsifier and the degree to which it partitions (partition coefficient); the partitioned demulsifier components exhibited an increased dynamic interfacial activity. The partitioned demulsifier components exhibit a high degree of static and dynamic interfacial activity, low interfacial shear viscosity, a low film dilational modulus with a high adsorption rate, and excellent demulsification performance. It is believed that hydrophilic groups in the demulsifier and lower molecular weight favor partitioning into the water droplet phase. When the emulsion film thins, the effectiveness of the demulsifier in reducing the interfacial tension gradient (Marangoni-Gibbs effect) is strongly affected by the degree of partitioning. At this point, a high interfacial activity of the partitioned demulsifier component is required to effectively reduce the interfacial tension gradient.
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Received for review June 20, 1995 Revised manuscript received January 18, 1996 Accepted February 6, 1996X IE950372U
X Abstract published in Advance ACS Abstracts, March 15, 1996.
