1014
Energy & Fuels 2007, 21, 1014-1018
Effect of Water Content and Surfactant Type on Viscosity and Stability of Emulsified Heavy Mukhaizna Crude Oil Naoya Shigemoto,† Rashid S. Al-Maamari,*,‡ Baba Y. Jibril,‡ Akihiko Hirayama,§ and Mark Sueyoshi§ Chemical Technology Department, Shikoku Research Institute, Inc., 2109 Yashima-nishimachi, Takamatsu, Kagawa, 761-0192, Japan, Petroleum and Chemical Engineering Department, College of Engineering, Sultan Qaboos UniVersity, P.O. Box 33, Al-Khoud, PC 123, Sultanate of Oman, and Department of Energy Solutions, Shimizu Corporation, 1-2-3 Shibaura, Minato-ku, Tokyo, 105-8007, Japan ReceiVed June 7, 2006. ReVised Manuscript ReceiVed NoVember 20, 2006
Heavy crude oil from the Mukhaizna oil field in Oman was emulsified with water in an attempt to decrease its viscosity. The oil has a kinematic viscosity of 7160 mm2/s at 30 °C, density of 0.9571 g/cm3 at 15 °C, and asphaltene content of 4.7 wt %. The effects of changes in water content (20-30 wt %), type and concentration of surfactant, addition of a water-soluble polymer, and agitation temperatures (25 and 42 °C) were studied in order to identify the optimum viscosity and stability for transporting the oil. The stability was measured by the water separation rate from the emulsion. For the Mukhaizna heavy crude oil, 21-22 wt % of water content and 0.4 wt % of nonylphenol ether type or higher alcohol alkylene oxide type surfactant were found to be optimum for a stable emulsion with a concomitant significant decrease in viscosity to about 1/3-1/4 that of the original crude oil. This improves the transportability of the heavy crude oil and makes it suitable for use as a power generation fuel.
1. Introduction Heavy crude oil’s mobility in reservoirs and transportation in pipelines remain key challenges to its utilization. In the past, production of such heavy crude oils was disfavored due to discouraging economic feasibility and availability of lighter oils. Recent increasing demands of crude oils have spurred research toward improving the oil flow properties. The viscosity could be reduced by different methods such as heating the oil, blending the heavy with light oil and oil-in-water/surfactant emulsification.1,2 Emulsification has the advantage of lower cost compared with heating and offers the possibility of testing different surfactants or designing new ones based on indigenous constituents of the oil and specific transportation challenges.3,4 One of the early works on this was that of Moa and Marsden5 which showed the reduction of viscosity with the addition of Triton X-114 surfactant. Viscosity was shown to depend on the type of surfactant, among other variables. In addition, the concentration of the Triton X-114 was critical to emulsion stability. High concentration decreased the viscosity, but might have degraded the mechanical shear and the stability of the emulsion.6 A recent successful demonstration of viscosity reduction by emulsification was done using a highly viscous oil (10 000 mPa * To whom correspondence should be addressed. Fax: +968-2414-1354. E-mail:
[email protected]. † Shikoku Research Institute, Inc. ‡ Sultan Qaboos University. § Shimizu Corporation. (1) Crandall, G. R.; Wise, T. H. Can. Pet. 1984, 25, 37-40. (2) Hardy, W. A; Sit, S. P.; Stockwell, A. Oil Gas J. 1989 (March), 39-43. (3) Clark, P. E.; Pilehvari, A. J. Pet. Sci. Eng. 1993, 9, 165-181. (4) Schubert, H.; Armbruster, H. Inst. Chem. Eng. 1992, 32 (1), 14-28. (5) Mao, M. L.; Marsden, S. S. J. Can. Pet. Technol. 1977 (April-June), 54-60. (6) Zakin, J. L.; Pinaire, R.; Borgmeyer, M. E. J. Fluids Eng. 1979, 101, 100-104.
Table 1. Properties of Mukhaizna Heavy Crude Oil item density API gross calorific value net calorific value kinematic viscosity kinematic viscosity kinematic viscosity kinematic viscosity paraffin wax asphaltene water ash
unit
value
item
unit
value
0.9571 16.3 kJ/kg 42 560 kJ/kg 39 970 mm2/s at 30 °C 7160 mm2/s at 50 °C 1360 mm2/s at 75 °C 303 mm2/s at 100 °C 101 wt % 2.1 wt % 4.7 vol % 0.16 wt % 0.033
C H N S salt V Ni Fe Na Mg Ca
wt % wt % wt % wt % ppm ppm ppm ppm ppm ppm ppm
84.4 11.5 0.27 3.46 66 23 37 4 35 1 3
g/cm3 at 15 °C
s at 50 °C) from an oil field in the Orinoco River region of Venezuela. By mixing the oil with 30 wt % water and a small amount of a surfactant, an emulsion called Orimulsion was produced with a drastic decease in viscosity (500-1 000 mPa s at 60 °C).7 Orimulsion has already been commercialized and is being exported to the U.S. and Japan as an emulsion fuel for power generation. On the basis of the foregoing and particularly the success of Orimulsion, we found it of interest to study the effects of different surfactants on the viscosity of Mukhaizna heavy oil available in Oman.8 Emulsification is considered to be one of the effective methods of reducing the viscosity of such oil.7,9 Different surfactants, water contents, agitation temperatures, and polymer addition were explored in attempt to obtain a stable emulsion of reduced kinematic viscosity of 20-1000 mm2/s, a (7) Kennedy, B. A. Am. Soc. Mech. Eng. Fuels Combust. Technol. DiV. 1991, 11, 7-20. (8) Petroleum Development Oman. Mukhaizna Field DeVelopment Plan; 2000; Vol. 1. (9) Pilehvari, A.; Saadevandi, B.; Halvaci, M.; Clark, P. E. Am. Soc. Mech. Eng. Fluids Eng. DiV. 1988, 75, 161-168.
10.1021/ef060259o CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007
Properties of Emulsified HeaVy Mukhaizna Crude Oil
Energy & Fuels, Vol. 21, No. 2, 2007 1015
Table 2. Surfactants Employed for Experiments properties
a
surfactant
type
producer
HLBa
Liponox NC-86 Liponox NC-100 Liponox NC-200 Emulgen 120 Naroacty N-100
poly(oxyethylene)nonylphenol ether poly(oxyethylene)nonylphenol ether poly(oxyethylene)nonylphenol ether poly(oxyethylene)alkylether higher alcohol alkylene oxide
Lion, Japan Lion, Japan Lion, Japan Kao, Japan Sanyo Kasei Company, Japan
12.6 13.3 15.8 15.3 13.3
cloud point (°C) 38.0-41.0 60.0-64.0 78.0-80.0 98.0 63.0
HLB: hydrophile-lipophile balance.
suitable range for pipeline transportation and combustion atomization regardless of temperature.10 1.1. Emulsification Technologies of Viscous Crude Oil. In the emulsification of viscous oil, the nature and concentration of the surfactant determines the emulsion viscosity and stability. The water content is another important factor. It was found to be about 30 wt % for stable Orimulsion.7 This is approximately equal to the theoretical volume percentage of water-in-oil if a closed-packed structure is assumed for the micelle.11,12 Many kinds of surfactants such as anionic, cationic, nonionic, and amphoteric reagents are employed both individually13-17 and in mixture forms.18,19 The use of anionic, cationic, or amphoteric surfactants shows varying degrees of emulsion stability and viscosity reduction.14,16 On the other hand, the stability of the emulsion that employed nonionic reagents exhibits stronger dependence on temperature.14,16 Anionic, cationic, and amphoteric surfactants show the recohesion effect of the oil drops due to electrical charge transfer to the oil surface. The nonionic one also has a similar effect, but in this case, it is through development of a protective barrier around each oil drop.14,16 Furthermore, water-soluble polymers are added in some cases to further stabilize the emulsion.15-17 Generally, the emulsions are formed by mixing different proportions of viscous oil, water (oil:water ) 90-70:10-30), and surfactants (0.01-10 wt %). Sometimes water-soluble polymers (10-4-1 wt %) may be added to improve the stability. The mixture may be agitated at different temperatures. For the surfactants and the water-soluble polymers, a variety of materials with different mixing proportions have been proposed. A recent study has demonstrated the complex effects of the addition of surfactant on the viscosity and other physicochemical properties of the receiving liquid.20 Despite the difficulty in selecting an appropriate surfactant, this approach receives research attention because it could lead to a drastic decrease in viscosity. For instance, viscosity could be decreased from 24 000-40 000 mPa s (60 °C) for a heavy oil to 100 mPa s (60 °C) for the emulsion. (10) Sugasawa, F.; Sugata, K.; Arakawa, Y.; Kuroishi, T.; Ichinose, T.; Hirota, T. Therm. Nucl. Power (Japanese) 2000, 51, 1561-1570. (11) Kaneko, S.; Iwanaga, A.; Hishida, M. Power-Gen. Int. 1996, 1-A: 237-7. (12) Chilenos, M. L.; Taylor, A. S.; Taylor, S. E. Jpn Tokkyo Koho 1995, Hei7-2111. (13) Moriyama, N.; Ogura, T.; Kai, A. Jpn Tokkyo Koho 1994, Hei68422. (14) Moriyama, N.; Ogura, T.; Kai, A. Jpn Tokkyo Koho 1994, Hei68423. (15) Moriyama, N.; Kai, A.; Tokuda, K.; Kitamura, H. Jpn Tokkyo Koho 1993, Hei5-67678. (16) Moriyama, N.; Kai, A.; Tokuda, K.; Kitamura, H. Jpn Tokkyo Koho 1993, Hei5-32439. (17) Moriyama, N.; Ogura, T.; Kai, A. Jpn Tokkyo Koho 1993, Hei531911. (18) Moriyama, N.; Ogura, T.; Kai, A. Jpn Tokkyo Koho 1993, Hei531910. (19) Moriyama, N.; Kai, A., Tokuda, K.; Kitamura, H. Jpn Tokkyo Koho 1993, Hei5-31912. (20) Babay, P. A.; Gettar, R. T.; Silva, M. F.; Thiele, B.; Batistoni, D. A. J. Chromatogr. 2006, 1116, 277-285.
However, selection of optimal surfactants and polymers for different oils remains a key challenge.20,21 2. Experimental Details 2.1. Materials and Characterization. Heavy crude oil, sampled from Mukhaizna oil field, was employed for emulsification experiments. Table 1 shows the oil’s basic properties. The kinematic viscosities of the crude oil are 1360 and 303 mm2/s at 50 and 75 °C, respectively. Nonionic surfactants employed are displayed in Table 2. As a water-soluble polymer, xanthan gum (Dainippon Pharmaceuticals Co. Ltd., Japan, molecular weight ∼ 2 million) was used. Detailed experimental methods for obtaining the basic characteristics of the samples are discussed in an earlier paper.22 2.2. Emulsification Procedure. As an agitation apparatus for the emulsification, a homo mixer (MII-2.5, Tokushu Kika Kogyo, Japan) was employed. Measured amounts of the surfactant and the water-soluble polymer were dissolved in measured amounts of demineralized water. The solution was added to the crude oil in a 1 L agitation vessel. The mixture was maintained at either 25 or 42 °C, corresponding to the low and high ambient temperatures in Oman, respectively. Then, the mixture was vigorously agitated in the mixer at 4000 rpm for 3 min. 2.3. Characterization of the Emulsion. After agitation, the viscosity of the emulsion was measured at 50 °C using a CannonFenske kinematic viscometer. A sample was statically placed at room temperature and the water separation rate was monitored as an indication of the stability of the emulsion. The particle size of the prepared emulsion was measured by a laser diffraction/scatter apparatus (LA920, Horiba Seisakusho, Japan).
3. Results and Discussion 3.1. Effect of Emulsion Preparing Conditions on Viscosity. The emulsions were prepared and their viscosities measured. The effects of water content, surfactant type and concentration, and agitation temperature were investigated. The kinematic viscosities of the emulsions were measured at 50 °C. Figures 1 and 2 show the effect of water content on the kinematic viscosity of the emulsion for different surfactants, agitated at 42 and 25 °C, respectively. 3.1.1. Water Content. A logarithmic plot shows that the relative viscosity [ln (V)] decreases with increase in water content irrespective of the type of surfactant (Figure 1). This relation is in accordance with earlier observations.23,24 The same trend was maintained at a lower agitation temperature as shown for Liponox NC-86 (0.4 wt %) in Figure 2. 3.1.2. Type of Surfactant and its Concentration. In Figure 1, nonylphenol type surfactants with different hydrophile-lipophile balances (HLB) and cloud points were employed at the agitation temperature of 42 °C. At 22% water content, the viscosity of (21) Fingas, M.; Fieldhouse, B. Mar. Pollut. Bull. 2003, 47 (9-12, SepDec), 369-396. (22) Shigemoto, N.; Al-Maamari, R. S.; Jibril, B. Y.; Hirayama, A. Energy Fuels, 2006, 20 (6), 2504-2508. (23) Broughton, G.; Squires, L. J. Phys. Chem. 1938, 42, 253-263. (24) Johnsen, E. E.; Ronningsen, H. P. J. Pet. Sci. Eng. 2003, 38, 2336.
1016 Energy & Fuels, Vol. 21, No. 2, 2007
Figure 1. Effect of water content and surfactant on kinematic viscosity (at 50 °C) of emulsified crude oil: (agitation temperature) 42 °C; (surfactant type/concentration (wt%)) Liponox NC-86/0.4 (O), Liponox NC-100/0.14 (b), Liponox NC-100/0.4 (4), Liponox NC-100/1 (2), Liponox NC-200 /0.4 (0).
Figure 2. Effect of water content and surfactant on kinematic viscosity (at 50 °C) of emulsified crude oil: (agitation temperature) 25 °C; (surfactant type/concentration (wt%)) Liponox NC-86/0.4 (O), Liponox NC-86/1 (b), Emulgen 120/1 (4), Naroacty N-100/1 (0).
the emulsions decreased in the order of Liponox NC-86, Liponox NC-100, and Liponox NC-200. This corresponds to their respective cloud points, or their HLB as shown in Table 2. This observation depends on the water content, as a similar trend could not be obtained at higher water content. Also, the figure shows the viscosity of the emulsion varying with two concentrations of Liponox NC-100 (0.4 and 1 wt %). The viscosity was slightly higher for the lower concentration at the same level of water content. This was further demonstrated when the concentration was decreased to 0.14 wt % of Liponox NC100 at 28 wt % water content. Much higher viscosity was observed as shown in the figure. The high viscosity at low concentration of the surfactant was due to the observed separate phase formation in the emulsion. Nonylphenol type surfactant (Liponox NC-86) and other types, such as poly(oxyethylene) alkylether (Emulgen 120) and higher alcohol alkylene oxide (Naroacty N-100) were employed at the agitation temperature of 25 °C. Liponox NC-86 (0.4 wt %) exhibited a trend similar to that observed at a higher agitation temperature (42 °C). Contrary to the observation of an increase in viscosity with a decrease in Liponox NC-100 concentration, Liponox NC-86 showed (Figure 2) an increase in viscosity with an increase in concentration (0.4-1 wt %). The emulsion viscosities with 21 wt % of water content were lower in the order of Naroacty N-100, Liponox NC-86, and Emulgen 120, where especially Emulgen 120 indicated the lowest viscosity (Figure 2). About 0.4 wt % of the surfactant was sufficient to reduce the viscosity of the emulsion to 67.5 mm2/s.
Shigemoto et al.
Figure 3. Effect of agitation temperature on viscosity of emulsified crude oil: (water content (wt%)/surfactant type/surfactant concentration (wt%)) 22/Liponox NC-86/0.4 (O), 24/Liponox NC-100/0.4 (4).
Figure 4. Effect of polymer addition on kinematic viscosity (at 50 °C) of emulsified crude oil: (agitation temperature) 25 °C; (surfactant) Liponox NC-86 (0.4 wt %); (polymer) xanthan gum.
3.1.3. Agitation Temperature. Figure 3 shows the effect of the agitation temperature on the viscosity of the emulsion. The viscosity increased with an increase in the agitation temperature. Higher viscosities were obtained for Liponox NC-86 (435 mm2/ s) and Liponox NC-100 (633 mm2/s) at 42 and 55 °C. These are near the cloud point of the surfactant employed (Liponox NC-86, 38-41 °C and Liponox NC-100, 60-64 °C). Above the cloud point, the hydrophilic property of the relative amount of surfactant was reduced and the surfactant in micelles of the emulsion decreased thereby promoting separation of water and oil, as observed by higher viscosity. 3.1.4. Addition of Polymer Material. Different amounts of xanthan gum were added to one of the emulsions (22 wt % water and 0.4 wt % Liponox NC-86). The kinematic viscosity of the emulsion with the xanthan gum and the surfactant increased with the increasing concentration of the polymer as shown in Figure 4. For all xanthan gum concentrations tested (0.001-0.01 wt %), the viscosities (116.6-975.5 mm2/s) were higher than that of pure crude oil (1360 mm2/s) at 50 °C. The addition of xanthan gum to the emulsion led to an increase in the viscosity because the aqueous solution of xanthan gum itself was very viscous. However, as discussed below, the addition of xanthan gum improves the stability of the emulsion. 3.2. Effect of Emulsion Preparing Conditions on Stability of Emulsion. The stabilities of emulsions were monitored. Samples were statically placed at room temperature (25 °C) to observe the water separation rate periodically as shown in Figures 5 and 6. 3.2.1. Water Content and Kind of Surfactant. The water separation rates at 25 °C for the emulsions prepared at 42 °C with Liponox NC-86 (O,b) and Liponox NC-100 (0,2)
Properties of Emulsified HeaVy Mukhaizna Crude Oil
Figure 5. Stability of emulsified crude oil over time: (agitation temperature) 42 °C; (surfactant concentration) 0.4 wt %; (water content/ surfactant type) 22/Liponox NC-86 (O), 24/Liponox NC-86 (b), 22/ Liponox NC-100 (4), 24/Liponox NC-100 (2).
Figure 6. Stability of emulsified crude oil over time: (agitation temperature) 25 °C; (surfactant concentration) 0.4 wt %; (surfactant type) Liponox NC-86 (O), Emulgen 120 (4), Naroacty N-100 (0); (water content) 21 wt %.
increased with time and then attained a plateau as shown in Figure 5. The emulsion with lower water content showed the plateau of a lower water separation rate. The emulsion with a water content of 22 wt % and 0.4 wt % of Liponox NC-100 indicated no water separation and, thus, better stability for a long storage period. The same sample showed low viscosity (325 mm2/s at 50 °C) which was about 1/4 of the original heavy crude oil viscosity (1360 mm2/s at 50 °C). The stability may be associated with the cloud point of surfactant employed. A lower storage temperature (25 °C) led to lower water separation, probably due to a higher hydrophilic property which in turn stabilizes the emulsion. As discussed earlier, the viscosity of the emulsion decreases with an increase in the water content. On the other hand, an increase in the water content makes the emulsion more susceptible to separating into two phases, thereby lowering the emulsion stability. Figure 6 also shows the water separation rate for the emulsion prepared at 25 °C. The emulsions with Liponox NC-86 and Naroacty N-100 exhibited similar lower water separation. The emulsion with Emulgen 120 was unstable. Although the emulsion with Emulgen 120 exhibited the lowest viscosity as displayed in Figure 2, it showed a higher water separation rate. From these results, it is observed that the surfactant with the appropriate cloud point of about 60 °C is required to produce a stable emulsion for a storage temperature of about 25 °C. 3.2.2. Agitation Temperature. The water separation rate of the emulsions which were prepared by employing the same surfactant (Liponox NC-86) and the same concentration (0.4 wt %) but different agitation temperatures (42 and 25 °C) are shown in Figures 5 and 6, respectively. Comparing the water
Energy & Fuels, Vol. 21, No. 2, 2007 1017
Figure 7. Stability of emulsified crude oil over time: (agitation temperature) 25 °C; (surfactant) Liponox NC-86 (0.4 wt %); (water content) 22 wt %; (polymer) xanthan gum (wt%) none (b), 0.001 (O), 0.005 (4), 0.01 (0).
Figure 8. Particle size distribution of emulsion: (agitation temperature) 25 °C; (surfactant) Liponox NC-86 (0.4 wt %); (water content) 21 wt %. Table 3. Particle Size Distribution of Emulsion period
median diameter (µm)
range (µm)
2 weeks 1 month 3 months
1.7 1.9 3.0
0.4-7.7 0.4-11.6 0.6-17.4
separation rates for the emulsions prepared at different agitation temperatures, it appears that the agitation temperature has no significant effect on the stability of the emulsion. 3.2.3. Addition of Polymer Material. Figure 7 shows the variation of water separation rates with time for emulsions with and without the xanthan gum. It is apparent that the addition of xanthan gum results in a stabler emulsion but with higher viscosity. It may be observed that the optimum concentration of xanthan gum for a stable emulsion (Figure 7) and low viscosity (Figure 4) is 0.005 wt %. 3.3. Particle Size Distribution of Emulsion and its Stability. Figure 8 shows the particle size distribution of the emulsions (21 wt % water and 0.4 wt % Liponox NC-86). The particles sizes were distributed in the range of 0.4-8 µm with a median diameter of 1.7 µm. The particle sizes had a monomodal distribution. Table 3 indicates change of the particle size range and the median diameter during a long storage period. The particle size increased with time. This is perhaps due to coalescing of small colloids into large ones. This may have decreased the stability of the emulsion. So, probably, the major effect of the type of surfactant and water content is in reducing the coalescing of colloids. The concentration of the surfactants and the amount of water must be chosen to avoid phase separation of the emulsion.
1018 Energy & Fuels, Vol. 21, No. 2, 2007
4. Conclusion The emulsions of Mukhaizna heavy crude oil were prepared by high speed mixing with water in which a small amount of surfactant was added. The kinematic viscosity of the emulsion decreased with an increase in the water content. However, an increase in water content led to less stable emulsion at room temperature. For the Mukhaizna heavy crude oil, about 21-22
Shigemoto et al.
wt % of water content and 0.4 wt % of surfactants were required to have a stable emulsion with viscosity of about 1/4-1/3 that of the original crude oil. The emulsion remained stable for over 100 days. The surfactant with cloud point of about 60 °C, and hydrophile-lipophile balance value of 13.3, was found to produce a stable emulsion with low viscosity. EF060259O
