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Emulsification of heavy crude oil in water for pipeline transportation
Journal of Petroleum Science and Engineering 71 (2010) 205–211
Contents lists available at ScienceDirect
Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l
Research paper
Emulsification of heavy crude oil in water for pipeline transportation S.N. Ashrafizadeh ⁎, M. Kamran Research Lab for Advanced Separation Processes, Department of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran
a r t i c l e
i n f o
Article history: Received 4 December 2009 Accepted 16 February 2010 Keywords: crude oil pipeline emulsion viscosity stability Triton X-100 surfactant
a b s t r a c t The stability and viscosity of W/O emulsions and their application for heavy oil pipeline transportation were investigated using two Iranian crude oil samples. An Iranian heavy crude oil sample named West Paydar and a blend of diesel and bitumen were used to produce heavy crude oil emulsions in water. The diverse factors affecting the properties and stability of emulsions were investigated. There was a restricted limit of 60 vol.% for crude oil content in the emulsions, beyond that limit the emulsions were inverted to water-in-oil emulsions. Different crude oil-in-water emulsions were prepared through addition of Triton X-100 surfactant. According to performed investigations, emulsification reduces the viscosity of the crude oil samples. However the viscosity of the emulsions increased by increasing the oil content of the emulsion, surfactant concentration, speed and time of mixing, salt concentration, and pH of the aqueous phase, while temperature of homogenization process substantially reduced the viscosity of the prepared emulsion. The stability of crude oil-in-water emulsions decreased by increasing the oil content while increasing the surfactant concentration, time and speed of mixing, pH of the aqueous phase and temperature enhanced the emulsion stability. The stability of crude oil emulsions was also increased by increasing the salt concentration. The main applicable observation of this research is that heavy crude oil-in-water emulsions can be highly stabilized simply by increasing the pH of the aqueous phase to basic values. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrocarbon resources are very important regarding the fact that they include about 65% of the world's overall energy resources (Langevin et al., 2004). Nowadays crude oil is the most important hydrocarbon resource of the world and heavy crudes account for a large fraction of the world's potentially recoverable oil reserves (Chilingar and Yen, 1980; Langevin et al., 2004). However, the heavy crude oils have a little portion in the world's oil production due to their high viscosities which cause problems in their pipeline transportation (Plegue et al., 1989). Production of heavy crudes is expected to increase significantly in the near future as low viscosity crudes are depleted (Plegue et al., 1989). Several alternative transportation methods for heavy crudes have been proposed and employed, including preheating of the crude oil with subsequent heating of the pipeline (Layrisse, 1998; Saniere et al., 2004), dilution with lighter crude oils (Iona, 1978), partial upgrading (MacWilliams and Eadie, 1993), and injection of a water sheath around the viscous crude. All the above-mentioned methods experience logistic, technical, or economic disadvantages, however. One of the newest pipeline techniques is the transport of viscous crudes as oil-in-water (O/W) emulsions (Lappin and Saur, 1989; Gregoli
⁎ Corresponding author. Tel.: + 98 21 77240402; fax: +98 21 77240309. E-mail address: ashrafi@iust.ac.ir (S.N. Ashrafizadeh). 0920-4105/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2010.02.005
et al., 2006). In this method, by the aid of suitable surfactants, the oil phase becomes dispersed in the water phase and stable oil-in-water emulsions are formed. The result causes a significant reduction in the oil viscosity, i.e. the produced emulsion has a viscosity in a range about 50– 200 cP, and therefore in the transportation costs and problems. This method can be very effective in the transportation of crude oils with viscosities higher than 1000 cP especially in cold regions. Besides, since water is the continuous phase, crude oil has no contact with the pipe wall and this reduces the pipe corrosion (e.g. in the crudes with high sulfur content) and prevents forming of sediments in pipes (e.g. in the crudes with high asphaltene content) (Poynter and Tigrina, 1970). To transport the crude oil using emulsion systems three steps are performed, including producing the oil-in-water emulsions, transportation of produced emulsions to the desired destination, and finally separation of oil and water phases (Poynter and Tigrina, 1970). To form the emulsions, the common method of homogenization is used, however, the new methods of emulsification by membranes and ultrasonic waves are being studied recently (Lidietta et al., 2003; Lin and Chen 2006). After transformation of crude oil is accomplished several different methods can be applied to separate the oil and water phases. Some of the important methods are thermal demulsification, electrodemulsification, chemical demulsification, freeze–thaw method, and demulsification by membranes (Yan and Masliyah, 1998; Srijaroonrat et al., 1999). Different oil-in-water emulsions have been made by different crude oil samples. The use of surfactants and water to form stable O/W
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emulsions with crude oils is the subject of a series of patents (Titus, 1973; Ahmed et al., 1999). The technical viability of this method has been demonstrated in an Indonesian pipeline in 1963 and in a 13 mile long, 8 inch diameter, pipeline in California (Ahmed et al., 1999). The purpose of the current research is to investigate the various factors affecting the preparation of a stable crude O/W emulsion for two Iranian oil samples, namely, West Paydar crude oil, and a blend of diesel and bitumen. The study investigates the influence of oil content of the emulsion, type and concentration of the surfactant, speed and duration of mixing, salt concentration and pH of the aqueous phase, and the temperature of homogenization on the stability and viscosity of the emulsion. 2. Materials and methods 2.1. Materials All chemicals used were analytical grade reagents from Merck. Aqueous solutions were made using tap water. Triton X-100 (polyethylene glycol octylphenol ether) with chemical formulation of C33H60O10 was used as surfactant. This surfactant is a nonionic hydrophilic surfactant which is suitable to form O/W emulsions. Hydrochloric acid and sodium hydroxide were employed to adjust the pH of the aqueous phase. Sodium chloride was used to adjust the water phase salinity. The homogenization method was used to perform the emulsification process and a laboratory scale Polytron hemogenizer, PT-1200C model; with maximum rotation speed of 22,000 rpm from Kinematica AG (Switzerland) was employed. An electric adjustable heater was used whenever heating was required. A Precisa pH900 pH meter was used to adjust and measure the pH of aqueous solution. A polyvisc viscometer, Viscostar model from Kinematica (Switzerland) was employed for viscosity measurements. 2.2. Specifications of Iranian oil samples As mentioned above, an Iranian oil sample of West Paydar crude oil and a blend of diesel and bitumen were employed to perform the experiments. The characteristics of these samples are given in this section. 2.2.1. West Paydar crude oil West Paydar crude oil was employed to perform the first series of experiments. This sample was supplied by the National Iranian Oil Company. The specification of this sample is illustrated in Table 1. According to the observed characteristics, West Paydar crude oil belongs to the heavy crude oils with high tenacity category. The viscosity of this sample equals 198 cP at 25 °C (77 °F). The West Paydar crude oil pipeline is closed at the cold winters because of high viscosity and adhesiveness of it (NIOC-RIPI PVT department report, 2005). 2.2.2. The blend of diesel and bitumen The blend of diesel and bitumen was used to perform the second series of experiments. To simulate an extra-heavy oil sample, diesel and bitumen were blended to produce a synthetic crude oil with a viscosity equal to those of extra-heavy crude oils. The specifications of
Table 1 Specifications of West Paydar crude oil (NIOC-RIPI PVT department report, 2005). Characteristics
Unit
Amount
Experimental method
Saturated Aromatics Resins Asphaltenes Wax appearance temperature Wax
wt.% wt.% wt.% wt.% °F wt.%
34.22 38.82 19.96 6.58 122 3.56
SARA SARA SARA SARA Viscosity BP-237
the 85/100 bitumen employed for this purpose are shown in Table 2 (Iranian Institute of Standards and Industrial Research, 2008). Diesel was mechanically mixed with the bitumen at the temperature of 50 °C until a homogeneous solution was formed. The viscosity of the obtained solution was adjusted by adding various amounts of diesel to bitumen. The effect of diesel quantity on the obtained viscosity of the blend is shown in Table 3. 2.3. Experimental procedure Two series of experiments were performed using two different samples of crude oils. In both series, the influence of operating parameters including oil content of the emulsion (40–80 wt.%), surfactant concentration (0.5–4 wt.%), speed of mixing (1000–15,000 rpm), duration of mixing (5–40 min) salt concentration, pH of the aqueous phase (5–9), and temperature of homogenization (25–65 °C) on the stability and viscosity of the emulsion was investigated. In each series of experiments, oil-in-water emulsions were prepared using various amounts of particular oil samples while other parameters were fixed at desirable values. Therefore, the maximum limit of oil content for each sample was revealed. Beyond that limit, phase inversion would occur. The produced emulsions were classified in two parts, one part for the viscosity and another part for the stability measurements. After finding the maximum limit of oil content for each sample, further investigations were carried out at the obtained maximum limit. 2.4. Stability of the prepared emulsions The emulsion stability was measured based on the amount of separated water from the prepared emulsions after 24 h. O/W emulsions prepared at different conditions were tested for their stability by transferring the emulsions into laboratory graduated tubes; the latter were left at room temperature to rest for a while. The volume of separated water was recorded after 24 h since the time homogenization was performed. By dividing the amount of water separated from the emulsion to the initial amount of water in the emulsion, the percentage of separated water from the prepared emulsions was achieved. 3. Results and discussion 3.1. Effect of oil content To prepare the O/W emulsions, the concentration of Triton X-100 surfactant in water was kept constant, namely 2 wt.% at the temperature of 25 °C and pH of 7, while speed and duration of mixing were 6000 rpm and 20 min, respectively. For each particular type of the crude oils, the oil content of the emulsion was varied from 40 to 80 vol.% with respect to the total volume of the emulsion. Fig. 1 demonstrates the effect of oil content on the viscosity and stability of the emulsions. By increasing the oil content up to 60 vol.%, the viscosity of the emulsions slightly decreases. However, beyond this limit the viscosity increases significantly due to the occurrence of phase inversion. The Table 2 Specification of bitumen used in blend production (Iranian Institute of Standards and Industrial Research, 2008). 85/100 bitumen specification
Amount
Experimental method
Density at 25 °C (g/cm3) Penetration at 25 °C Softening point (°C) Weight reduction caused by heating (wt.%) Penetration reduction (%) Fire point (°C) Solubility in carbon sulfide (wt.%) Spot test
1–1.05 85/100 45–52 0.05 20 225 99.5 Negative
D-70 D-5 D-36 D-6 D-6, D-5 D-92 D-4 A.A.S.H.O.T.102
S.N. Ashrafizadeh, M. Kamran / Journal of Petroleum Science and Engineering 71 (2010) 205–211 Table 3 Viscosity of obtained blends with various diesel contents. The quantity of diesel in the blend (%)
The final viscosity of obtained blend (cP)
30 40 50 60
1970 285 89 35
maximum oil content limit plays a very important role in designing the emulsion transport system. Obviously, it is desirable to reduce the water content of the emulsions as much less as possible to enhance the efficiency of the transportation system; less pipe space will be occupied by water. On the other hand, beyond a certain limit, increasing the oil content of the emulsion would result in a significant enhancement in its viscosity due to the occurrence of the phase inversion. The stability of crude oil-in-water emulsion slightly decreases with increasing the oil content of the emulsion up to 60 vol.%. Beyond this critical value, the stability of the emulsion experiences a significant decrease; afterwards the stability continues to increase with increasing the oil content. The oil content of 70 vol.% is the point of phase inversion and has the lowest stability. At this point the emulsion is converted to W/O emulsion. Therefore, the maximum oil content of 60 vol.% was considered as an optimum value to run the rest of experiments. The raise in the emulsion viscosity due to increase in the oil volume fraction up to the phase inversion point is understandable, since by increasing the oil volume fraction which possess a higher viscosity the viscosity of the mixture it is expected to increase. However, the jump in the viscosity at the phase inversion point can be attributed to the fact that viscosity of the emulsions is a strong function of the continuous phase viscosity; and a weak function of that in the dispersed phase. As such, it is well expected that in the phase inversion point at which the oil phase becomes the predominant phase, viscosity of the emulsion exhibits a high jump. The latter phenomena would bring the worst conditions in the pipeline transportation and would oblige the lowest stable conditions to the system. Therefore, it seems necessary for the pipeline transportation systems to keep a proper distance from the phase inversion threshold (Martinez et al., 1998). At the same time, stability of the emulsion would decrease at higher volume fractions of the oil dispersed phase due to increase in the coalescence of emulsion droplets which occurs as a consequence of increase in the effective entropy of droplet collisions (Ahmed et al., 1999).
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3.2. Effect of emulsifier concentration The influence of Triton X-100 emulsifier on the viscosity and stability of the emulsion was investigated. To prepare the O/W emulsions, the oil content of the emulsion was kept constant at its optimum value, i.e. 60 vol.%, while the other conditions were temperature of 25 °C, pH of 7, and speed and duration of mixing were 6000 rpm and 20 min, respectively. The concentration of Triton X-100 surfactant in water was varied from 0.5 to 4 wt.%. Fig. 2 demonstrates the effect of surfactant concentration on the viscosity and stability of the emulsions, respectively. As it can be observed, increasing the concentration of surfactant has resulted in a slight increase in the viscosity of the emulsion, while the stability has significantly increased. Increasing the surfactant concentration causes an increase in the amount of barriers between the two phases and provides a better distribution of dispersed droplets in the continuous phase. The latter would result in the formation of emulsions with higher stabilities. It is notable that Triton X-100 is a viscous liquid. Thus increasing its concentration in the emulsion increases the viscosity of the emulsion (Eirong and Lempe, 2006). At the same time, increasing the surfactant concentration would lower the interfacial tension which would facilitate the breakage of droplets into smaller ones. The latter would result in a more stable emulsion of higher viscosity (Sakka, 2002). Eventually, decision about the surfactant concentration should be made upon the surfactant cost and the economy of the process. Further investigations on the characteristics of the O/W emulsions were performed with surfactant concentration of 2 wt.%, due to the appropriate stability of the emulsions which was obtained at this concentration. 3.3. Effect of speed and duration of mixing To investigate the influence of mixing speed on the viscosity and stability of the emulsions, the behavior of either West Paydar and/or diesel–bitumen blend crude oil-in-water emulsions was studied at different mixing speeds of 1000, 3000, 6000, 10,000 and 15,000 rpm. The other operating conditions were: temperature of 25 °C, pH of 7, mixing duration of 20 min, surfactant concentration of 2 wt.%, and oil content of 60 vol.%. Results are shown in Fig. 3. Similarly, to investigate the influence of mixing time on the viscosity and stability of the emulsions, the West Paydar and/or diesel–bitumen crude oil-inwater emulsions were homogenized for either of 5, 10, 20, 30, and 40 min. The other conditions were: temperature of 25 °C, pH of 7, surfactant concentration of 2 wt.%, oil content of 60 vol.%, and mixing speed of 6000 rpm. Results of these experiments are shown in Fig. 4.
Fig. 1. Effect of oil content on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
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Fig. 2. Effect of emulsifier concentration on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
Fig. 3. Effect of speed of mixing on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
Increasing either speed and time of mixing has a similar effect on the emulsions quality. Their increase has a slight increasing effect on the viscosity of the emulsions while increases the stability of the emulsions up to a desirable level. On the other hand, reducing the mixing speed to less than 3000 rpm and mixing time to less than 10 min, would significantly reduce the quality of the emulsions. For a crude oil-in-water emulsion with a specified volume fraction of crude oil and surfactant concentration, increasing the speed and
time of mixing results in the production of droplets with smaller sizes, which causes an increase in the interfacial area and particle to particle interaction, which would finally increases the stability of the emulsion. At the same time, decreasing the size of the oil droplets, i.e. dispersed phase, results in a slight increase in the viscosity of the emulsions (Stachurski and MichaŁek, 1996; Zaki, 1997). The obtained observations on the increase in the emulsion viscosity due to increase
Fig. 4. Effect of mixing duration on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
S.N. Ashrafizadeh, M. Kamran / Journal of Petroleum Science and Engineering 71 (2010) 205–211
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Fig. 5. Effect of salt concentration on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
in the agitation speed, which produces droplets of smaller size, are in agreement with the findings of Pal et al. (1992) as well as those of Briceno et al. (1997).
droplets of dispersed phase would also form which would represent higher stabilities. 3.5. Effect of pH
3.4. Effect of salt concentration The influence of salt concentration in the water phase of the emulsion, on the viscosity and stability of the emulsions was investigated. The O/W emulsions were prepared using the oil content of 60 vol.%, surfactant concentration of 2 wt.%, mixing speed of 6000 rpm, temperature of 25 °C, pH of 7, and mixing time of 20 min. The concentration of salt in the water phase was varied from 0 to 40,000 ppm; the salt concentration in the tap water was assumed to be negligible. Fig. 5 illustrates the effect of salt concentration on the viscosity and stability of the emulsions. As it can be observed, increasing the salt concentration has increased the viscosity of the emulsions. Increasing the salt concentration has reduced the amount of separated water as well. Similar observation has been noticed from the various aqueous surfactant systems through which the viscosity has been increased by the addition of salt. At the same time, the ions would act as barriers among the oil droplets and the water phase. Thus increasing the salinity of the aqueous phase, results in the enhancement of the emulsions stability. Ahmed et al. (1999) reported the same observation, i.e. increase in the viscosity of the emulsion due to increase in the aqueous phase salinity, and attributed this behavior to the lower interfacial tension of the oil and aqueous phases at higher aqueous salinities. It is obvious that at lower interfacial tensions smaller
In the last stage of this series of experiments the influence of pH of the aqueous phase on the viscosity and stability of the prepared emulsions was investigated. The O/W emulsions were prepared using the oil content of 60 vol.%, surfactant concentration of 2 wt.%, mixing speed of 6000 rpm, temperature of 25 °C, and mixing duration of 20 min. The pH of the aqueous phase was adjusted at either of 5, 6, 7, 8, and 9. Fig. 6 illustrates the effect of pH on the viscosity and stability of the emulsions. As it can be observed, increasing the pH of the solution has resulted in a negligible enhancement in the viscosity of the emulsions while that has increased the emulsions stability significantly. Increasing the pH of the continuous phase of the emulsions from 6 to 9 causes an increase in the absolute value of zeta potential of the droplets which results in the formation of emulsions with higher stabilities (Sakka, 2002). Yang et al. (2007) observed that oil-in-water emulsions became more stable at higher pH values and attributed this phenomenon to the higher affinity of surfactant molecules towards aggregation at higher pH values. 3.6. Effect of temperature of homogenization Regarding the fact that the diesel–bitumen oil sample resembles synthetic extra-heavy crude, temperature of the homogenization process
Fig. 6. Effect of pH on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
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Fig. 7. Effect of temperature of homogenization on the viscosity and stability of diesel–bitumen blend emulsions.
can be very effective in its emulsion making performance. To investigate the effect of this parameter on the emulsification behavior of diesel– bitumen blend, emulsions were prepared using the fixed oil content of 60 vol.%, surfactant concentration of 2 wt.%, mixing speed of 10,000 rpm, pH of 7, and mixing duration of 20 min. The temperature was adjusted at either values of 25, 40, and 50 °C. Fig. 7 illustrates the results. As it can be seen, raising the temperature of homogenization has resulted in emulsions with lower viscosities and higher stabilities. It is notable that increasing the temperature of mixing would result in softening the bituminous particles and therefore, a monotonous dispersion of oil droplets in the water phase would occur.
crude oil. For high viscosity samples such as synthesis crude oil, heating the homogenization vessel resulted in the production of more stable emulsions with lower viscosities.
4. Conclusions
References
The oil-in-water emulsions were successfully prepared using Iranian oil samples of West Paydar crude oil and a blend of diesel– bitumen synthetic crude with water in the presence of Triton X-100 surfactant. Using water as the continuous phase has several benefits in pipeline oil transportation including: decreased amount of viscosity which leads to a reduced energy consumption and easier pumping in oil transportation, and lack of contact between the crude and pipe walls which results in less erosion and precipitation inside pipes. The viscosity of the emulsion was found to increase as the oil content of the emulsion increased up to 60% for both samples. Increasing the oil content beyond this value causes a sudden increase in the emulsion viscosity. This is the point in which phase inversion occurs and therefore, emulsions made with oil contents higher than this value cannot be suitable for this purpose. A similar trend of increasing of viscosity was observed in emulsions prepared from both samples by increasing the surfactant and salt concentration in water, the time and speed of mixing, and pH of the aqueous phase. Increasing the temperature of homogenization process reduced the viscosity of emulsions produced by the particular synthetic crude sample. The stability of O/W emulsions of both samples was found to decrease as the oil content of the emulsion increases up to the phase inversion point. After this point the emulsion converts to W/O emulsion and the stability of emulsion starts to increase with increasing the oil content. For both samples the oil content of 70 vol.% found to be the phase inversion point. The stability of O/W emulsions of both samples was found to increase as the surfactant and salt concentration, the speed and time of mixing of the emulsion, and the pH of aqueous phase and temperature of homogenization increase. The lowest emulsifier concentration required for producing a stable emulsion (the maximum water separation after 24 h ∼ 5%) was 2 wt.%. The minimum duration for the time of mixing for producing stable emulsion was 20 min for both samples. The minimum speed of mixing for producing a stable emulsion was 6000 rpm for west Paydar and 10,000 rpm for synthesis
Ahmed, N.S., Nassar, A.M., Zaki, N.N., Gharieb, H.Kh., 1999. Formation of fluid heavy oilin-water emulsions for pipeline transportation. J. Fuel 78, 593–600. Briceno, M., Isabel, R.M., Bullón, J., Salager, J.L., 1997. Customizing drop size distribution to change emulsion viscosity, 2nd world congress on emulsion - 2ème Congrès Mondial de l'Emulsion CME2, Bordeaux, France, paper 2-1-094. Chilingar, G.V., Yen, T.F., 1980. Enhanced recovery of residual and heavy oils, In: Schumacher, M.M. (Ed.), 2nd ed. Noyes Data Corporation, Park Ridge, NJ, pp. 403–418. Eirong, J.I., Lempe, D.A., 2006. Calculation of viscosities of liquid mixtures using Eyring's theory in combination with cubic equations of state. Chin. J. Chem. Eng. 14 (6), 770–779. Gregoli, A.A., Hamshar, J.A., Olah, A.M., Riley, C.J., Rimmer, D.P., 2006. Preparation of stable crude oil transport emulsions, Us Patent No 4,725,287. Iona, M., 1978. Process for producing low-density low sulfur crude oil, US Patent No 4,092,238. Iranian Institute of Standards & Industrial Research, 2008. Iranian national standards of mineral and constructional materials, Report No 126, Serial No 00203663. Langevin, D., Poteau, S., Henaut, I., Argillier, J.F., 2004. Crude oil emulsion properties and their application to heavy oil transportation. Oil Gas Sci. Tech. Rev. IFP 59 (5), 511–521. Lappin, G.R., Saur, J.D., 1989. Alpha Olefins Applications Handbook. CRC Press, New York. Layrisse,R., 1998. Viscous hydrocarbon-in-water emulsions, US Patent No 4,795,478. Lidietta, G., Li, N., Drioli, E., 2003. Reparation of oil-in-water emulsions using polyamide 10 kDa hollow fiber membrane. J. Membr. Sci. 217, 173–180. Lin, C.Y., Chen, L.W., 2006. Emulsification characteristics of three and two-phase emulsions prepared by the ultrasonic emulsification method. J. Fuel Process. Tech. 87, 309–317. MacWilliams, M.A., Eadie, W., 1993. Process and apparatus for partial upgrading, Canadian Patent No 1313639. Martinez, A.E., Intevep, S.A., Arirachakaran, S., Shoham, O., Brill, J.P., 1998. Prediction of dispersion viscosity of oil/water mixture flow in horizontal pipes. SPE Annual Technical Conference and Exhibition, Texas, pp. 427–438. NIOC-RIPI PVT department report, 2005. West Paydar reservoir, reservoir well #4, report No 45460156. Pal, R., Yan, Y., Masliyah, J.H., 1992. In: Schramm, L.L. (Ed.), Emulsions Fundamentals and Applications in the Petroleum Industry. American Chemical Society, Washington DC, pp. 141–145. Plegue, T.H., Frank, S.G., Zakin, J.L., 1989. Studies of water-continuous emulsions of heavy crude oils prepared by alkali treatment. SPE Prod. Eng. 4 (2), 181–183. Poynter, G., Tigrina, S., 1970. Pipelining O/W mixtures, US Patent No 3,519,006. Sakka, S., 2002. Sol–gel science and technology topics in fundamental research and applications. Sol–Gel Prepared Ferroelectrics and Related Materials, vol. 1. Kluwer academic publisher, New York, pp. 33–35. Saniere, A., Hénaut, I., Argillier, J.F., 2004. Pipeline transportation of heavy oils, a strategic, economic and technological challenge. Oil Gas Sci. Tech. Rev. IFP 59 (5), 455–466.
Acknowledgement The National Iranian Oil Engineering & Construction Co. (NIOEC) is highly acknowledged for its financial support during the course of this project.
S.N. Ashrafizadeh, M. Kamran / Journal of Petroleum Science and Engineering 71 (2010) 205–211 Srijaroonrat, P., Julien, E., Aurelle, Y., 1999. Unstable secondary oil/water emulsion treatment using ultrafiltration, fouling control by back flushing. J. Membr. Sci. 159, 11–20. Stachurski, J., MichaŁek, M., 1996. The effect of the ζ potential on the stability of a non-polar oil-in-water emulsion. J. Colloid Interface Sci. 184 (2), 433–436. Titus, P.E., 1973. Pipeline transportation of waxy products, US Patent No 3,776,248. Yan, N., Masliyah, J.H., 1998. Demulsification of solids-stabilized oil-in-water emulsions. J. Colloids Surf., A Physicochem. Eng. Asp. 11, 15–20.
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Contents lists available at ScienceDirect
Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l
Research paper
Emulsification of heavy crude oil in water for pipeline transportation S.N. Ashrafizadeh ⁎, M. Kamran Research Lab for Advanced Separation Processes, Department of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran
a r t i c l e
i n f o
Article history: Received 4 December 2009 Accepted 16 February 2010 Keywords: crude oil pipeline emulsion viscosity stability Triton X-100 surfactant
a b s t r a c t The stability and viscosity of W/O emulsions and their application for heavy oil pipeline transportation were investigated using two Iranian crude oil samples. An Iranian heavy crude oil sample named West Paydar and a blend of diesel and bitumen were used to produce heavy crude oil emulsions in water. The diverse factors affecting the properties and stability of emulsions were investigated. There was a restricted limit of 60 vol.% for crude oil content in the emulsions, beyond that limit the emulsions were inverted to water-in-oil emulsions. Different crude oil-in-water emulsions were prepared through addition of Triton X-100 surfactant. According to performed investigations, emulsification reduces the viscosity of the crude oil samples. However the viscosity of the emulsions increased by increasing the oil content of the emulsion, surfactant concentration, speed and time of mixing, salt concentration, and pH of the aqueous phase, while temperature of homogenization process substantially reduced the viscosity of the prepared emulsion. The stability of crude oil-in-water emulsions decreased by increasing the oil content while increasing the surfactant concentration, time and speed of mixing, pH of the aqueous phase and temperature enhanced the emulsion stability. The stability of crude oil emulsions was also increased by increasing the salt concentration. The main applicable observation of this research is that heavy crude oil-in-water emulsions can be highly stabilized simply by increasing the pH of the aqueous phase to basic values. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrocarbon resources are very important regarding the fact that they include about 65% of the world's overall energy resources (Langevin et al., 2004). Nowadays crude oil is the most important hydrocarbon resource of the world and heavy crudes account for a large fraction of the world's potentially recoverable oil reserves (Chilingar and Yen, 1980; Langevin et al., 2004). However, the heavy crude oils have a little portion in the world's oil production due to their high viscosities which cause problems in their pipeline transportation (Plegue et al., 1989). Production of heavy crudes is expected to increase significantly in the near future as low viscosity crudes are depleted (Plegue et al., 1989). Several alternative transportation methods for heavy crudes have been proposed and employed, including preheating of the crude oil with subsequent heating of the pipeline (Layrisse, 1998; Saniere et al., 2004), dilution with lighter crude oils (Iona, 1978), partial upgrading (MacWilliams and Eadie, 1993), and injection of a water sheath around the viscous crude. All the above-mentioned methods experience logistic, technical, or economic disadvantages, however. One of the newest pipeline techniques is the transport of viscous crudes as oil-in-water (O/W) emulsions (Lappin and Saur, 1989; Gregoli
⁎ Corresponding author. Tel.: + 98 21 77240402; fax: +98 21 77240309. E-mail address: ashrafi@iust.ac.ir (S.N. Ashrafizadeh). 0920-4105/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2010.02.005
et al., 2006). In this method, by the aid of suitable surfactants, the oil phase becomes dispersed in the water phase and stable oil-in-water emulsions are formed. The result causes a significant reduction in the oil viscosity, i.e. the produced emulsion has a viscosity in a range about 50– 200 cP, and therefore in the transportation costs and problems. This method can be very effective in the transportation of crude oils with viscosities higher than 1000 cP especially in cold regions. Besides, since water is the continuous phase, crude oil has no contact with the pipe wall and this reduces the pipe corrosion (e.g. in the crudes with high sulfur content) and prevents forming of sediments in pipes (e.g. in the crudes with high asphaltene content) (Poynter and Tigrina, 1970). To transport the crude oil using emulsion systems three steps are performed, including producing the oil-in-water emulsions, transportation of produced emulsions to the desired destination, and finally separation of oil and water phases (Poynter and Tigrina, 1970). To form the emulsions, the common method of homogenization is used, however, the new methods of emulsification by membranes and ultrasonic waves are being studied recently (Lidietta et al., 2003; Lin and Chen 2006). After transformation of crude oil is accomplished several different methods can be applied to separate the oil and water phases. Some of the important methods are thermal demulsification, electrodemulsification, chemical demulsification, freeze–thaw method, and demulsification by membranes (Yan and Masliyah, 1998; Srijaroonrat et al., 1999). Different oil-in-water emulsions have been made by different crude oil samples. The use of surfactants and water to form stable O/W
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emulsions with crude oils is the subject of a series of patents (Titus, 1973; Ahmed et al., 1999). The technical viability of this method has been demonstrated in an Indonesian pipeline in 1963 and in a 13 mile long, 8 inch diameter, pipeline in California (Ahmed et al., 1999). The purpose of the current research is to investigate the various factors affecting the preparation of a stable crude O/W emulsion for two Iranian oil samples, namely, West Paydar crude oil, and a blend of diesel and bitumen. The study investigates the influence of oil content of the emulsion, type and concentration of the surfactant, speed and duration of mixing, salt concentration and pH of the aqueous phase, and the temperature of homogenization on the stability and viscosity of the emulsion. 2. Materials and methods 2.1. Materials All chemicals used were analytical grade reagents from Merck. Aqueous solutions were made using tap water. Triton X-100 (polyethylene glycol octylphenol ether) with chemical formulation of C33H60O10 was used as surfactant. This surfactant is a nonionic hydrophilic surfactant which is suitable to form O/W emulsions. Hydrochloric acid and sodium hydroxide were employed to adjust the pH of the aqueous phase. Sodium chloride was used to adjust the water phase salinity. The homogenization method was used to perform the emulsification process and a laboratory scale Polytron hemogenizer, PT-1200C model; with maximum rotation speed of 22,000 rpm from Kinematica AG (Switzerland) was employed. An electric adjustable heater was used whenever heating was required. A Precisa pH900 pH meter was used to adjust and measure the pH of aqueous solution. A polyvisc viscometer, Viscostar model from Kinematica (Switzerland) was employed for viscosity measurements. 2.2. Specifications of Iranian oil samples As mentioned above, an Iranian oil sample of West Paydar crude oil and a blend of diesel and bitumen were employed to perform the experiments. The characteristics of these samples are given in this section. 2.2.1. West Paydar crude oil West Paydar crude oil was employed to perform the first series of experiments. This sample was supplied by the National Iranian Oil Company. The specification of this sample is illustrated in Table 1. According to the observed characteristics, West Paydar crude oil belongs to the heavy crude oils with high tenacity category. The viscosity of this sample equals 198 cP at 25 °C (77 °F). The West Paydar crude oil pipeline is closed at the cold winters because of high viscosity and adhesiveness of it (NIOC-RIPI PVT department report, 2005). 2.2.2. The blend of diesel and bitumen The blend of diesel and bitumen was used to perform the second series of experiments. To simulate an extra-heavy oil sample, diesel and bitumen were blended to produce a synthetic crude oil with a viscosity equal to those of extra-heavy crude oils. The specifications of
Table 1 Specifications of West Paydar crude oil (NIOC-RIPI PVT department report, 2005). Characteristics
Unit
Amount
Experimental method
Saturated Aromatics Resins Asphaltenes Wax appearance temperature Wax
wt.% wt.% wt.% wt.% °F wt.%
34.22 38.82 19.96 6.58 122 3.56
SARA SARA SARA SARA Viscosity BP-237
the 85/100 bitumen employed for this purpose are shown in Table 2 (Iranian Institute of Standards and Industrial Research, 2008). Diesel was mechanically mixed with the bitumen at the temperature of 50 °C until a homogeneous solution was formed. The viscosity of the obtained solution was adjusted by adding various amounts of diesel to bitumen. The effect of diesel quantity on the obtained viscosity of the blend is shown in Table 3. 2.3. Experimental procedure Two series of experiments were performed using two different samples of crude oils. In both series, the influence of operating parameters including oil content of the emulsion (40–80 wt.%), surfactant concentration (0.5–4 wt.%), speed of mixing (1000–15,000 rpm), duration of mixing (5–40 min) salt concentration, pH of the aqueous phase (5–9), and temperature of homogenization (25–65 °C) on the stability and viscosity of the emulsion was investigated. In each series of experiments, oil-in-water emulsions were prepared using various amounts of particular oil samples while other parameters were fixed at desirable values. Therefore, the maximum limit of oil content for each sample was revealed. Beyond that limit, phase inversion would occur. The produced emulsions were classified in two parts, one part for the viscosity and another part for the stability measurements. After finding the maximum limit of oil content for each sample, further investigations were carried out at the obtained maximum limit. 2.4. Stability of the prepared emulsions The emulsion stability was measured based on the amount of separated water from the prepared emulsions after 24 h. O/W emulsions prepared at different conditions were tested for their stability by transferring the emulsions into laboratory graduated tubes; the latter were left at room temperature to rest for a while. The volume of separated water was recorded after 24 h since the time homogenization was performed. By dividing the amount of water separated from the emulsion to the initial amount of water in the emulsion, the percentage of separated water from the prepared emulsions was achieved. 3. Results and discussion 3.1. Effect of oil content To prepare the O/W emulsions, the concentration of Triton X-100 surfactant in water was kept constant, namely 2 wt.% at the temperature of 25 °C and pH of 7, while speed and duration of mixing were 6000 rpm and 20 min, respectively. For each particular type of the crude oils, the oil content of the emulsion was varied from 40 to 80 vol.% with respect to the total volume of the emulsion. Fig. 1 demonstrates the effect of oil content on the viscosity and stability of the emulsions. By increasing the oil content up to 60 vol.%, the viscosity of the emulsions slightly decreases. However, beyond this limit the viscosity increases significantly due to the occurrence of phase inversion. The Table 2 Specification of bitumen used in blend production (Iranian Institute of Standards and Industrial Research, 2008). 85/100 bitumen specification
Amount
Experimental method
Density at 25 °C (g/cm3) Penetration at 25 °C Softening point (°C) Weight reduction caused by heating (wt.%) Penetration reduction (%) Fire point (°C) Solubility in carbon sulfide (wt.%) Spot test
1–1.05 85/100 45–52 0.05 20 225 99.5 Negative
D-70 D-5 D-36 D-6 D-6, D-5 D-92 D-4 A.A.S.H.O.T.102
S.N. Ashrafizadeh, M. Kamran / Journal of Petroleum Science and Engineering 71 (2010) 205–211 Table 3 Viscosity of obtained blends with various diesel contents. The quantity of diesel in the blend (%)
The final viscosity of obtained blend (cP)
30 40 50 60
1970 285 89 35
maximum oil content limit plays a very important role in designing the emulsion transport system. Obviously, it is desirable to reduce the water content of the emulsions as much less as possible to enhance the efficiency of the transportation system; less pipe space will be occupied by water. On the other hand, beyond a certain limit, increasing the oil content of the emulsion would result in a significant enhancement in its viscosity due to the occurrence of the phase inversion. The stability of crude oil-in-water emulsion slightly decreases with increasing the oil content of the emulsion up to 60 vol.%. Beyond this critical value, the stability of the emulsion experiences a significant decrease; afterwards the stability continues to increase with increasing the oil content. The oil content of 70 vol.% is the point of phase inversion and has the lowest stability. At this point the emulsion is converted to W/O emulsion. Therefore, the maximum oil content of 60 vol.% was considered as an optimum value to run the rest of experiments. The raise in the emulsion viscosity due to increase in the oil volume fraction up to the phase inversion point is understandable, since by increasing the oil volume fraction which possess a higher viscosity the viscosity of the mixture it is expected to increase. However, the jump in the viscosity at the phase inversion point can be attributed to the fact that viscosity of the emulsions is a strong function of the continuous phase viscosity; and a weak function of that in the dispersed phase. As such, it is well expected that in the phase inversion point at which the oil phase becomes the predominant phase, viscosity of the emulsion exhibits a high jump. The latter phenomena would bring the worst conditions in the pipeline transportation and would oblige the lowest stable conditions to the system. Therefore, it seems necessary for the pipeline transportation systems to keep a proper distance from the phase inversion threshold (Martinez et al., 1998). At the same time, stability of the emulsion would decrease at higher volume fractions of the oil dispersed phase due to increase in the coalescence of emulsion droplets which occurs as a consequence of increase in the effective entropy of droplet collisions (Ahmed et al., 1999).
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3.2. Effect of emulsifier concentration The influence of Triton X-100 emulsifier on the viscosity and stability of the emulsion was investigated. To prepare the O/W emulsions, the oil content of the emulsion was kept constant at its optimum value, i.e. 60 vol.%, while the other conditions were temperature of 25 °C, pH of 7, and speed and duration of mixing were 6000 rpm and 20 min, respectively. The concentration of Triton X-100 surfactant in water was varied from 0.5 to 4 wt.%. Fig. 2 demonstrates the effect of surfactant concentration on the viscosity and stability of the emulsions, respectively. As it can be observed, increasing the concentration of surfactant has resulted in a slight increase in the viscosity of the emulsion, while the stability has significantly increased. Increasing the surfactant concentration causes an increase in the amount of barriers between the two phases and provides a better distribution of dispersed droplets in the continuous phase. The latter would result in the formation of emulsions with higher stabilities. It is notable that Triton X-100 is a viscous liquid. Thus increasing its concentration in the emulsion increases the viscosity of the emulsion (Eirong and Lempe, 2006). At the same time, increasing the surfactant concentration would lower the interfacial tension which would facilitate the breakage of droplets into smaller ones. The latter would result in a more stable emulsion of higher viscosity (Sakka, 2002). Eventually, decision about the surfactant concentration should be made upon the surfactant cost and the economy of the process. Further investigations on the characteristics of the O/W emulsions were performed with surfactant concentration of 2 wt.%, due to the appropriate stability of the emulsions which was obtained at this concentration. 3.3. Effect of speed and duration of mixing To investigate the influence of mixing speed on the viscosity and stability of the emulsions, the behavior of either West Paydar and/or diesel–bitumen blend crude oil-in-water emulsions was studied at different mixing speeds of 1000, 3000, 6000, 10,000 and 15,000 rpm. The other operating conditions were: temperature of 25 °C, pH of 7, mixing duration of 20 min, surfactant concentration of 2 wt.%, and oil content of 60 vol.%. Results are shown in Fig. 3. Similarly, to investigate the influence of mixing time on the viscosity and stability of the emulsions, the West Paydar and/or diesel–bitumen crude oil-inwater emulsions were homogenized for either of 5, 10, 20, 30, and 40 min. The other conditions were: temperature of 25 °C, pH of 7, surfactant concentration of 2 wt.%, oil content of 60 vol.%, and mixing speed of 6000 rpm. Results of these experiments are shown in Fig. 4.
Fig. 1. Effect of oil content on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
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Fig. 2. Effect of emulsifier concentration on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
Fig. 3. Effect of speed of mixing on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
Increasing either speed and time of mixing has a similar effect on the emulsions quality. Their increase has a slight increasing effect on the viscosity of the emulsions while increases the stability of the emulsions up to a desirable level. On the other hand, reducing the mixing speed to less than 3000 rpm and mixing time to less than 10 min, would significantly reduce the quality of the emulsions. For a crude oil-in-water emulsion with a specified volume fraction of crude oil and surfactant concentration, increasing the speed and
time of mixing results in the production of droplets with smaller sizes, which causes an increase in the interfacial area and particle to particle interaction, which would finally increases the stability of the emulsion. At the same time, decreasing the size of the oil droplets, i.e. dispersed phase, results in a slight increase in the viscosity of the emulsions (Stachurski and MichaŁek, 1996; Zaki, 1997). The obtained observations on the increase in the emulsion viscosity due to increase
Fig. 4. Effect of mixing duration on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
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Fig. 5. Effect of salt concentration on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
in the agitation speed, which produces droplets of smaller size, are in agreement with the findings of Pal et al. (1992) as well as those of Briceno et al. (1997).
droplets of dispersed phase would also form which would represent higher stabilities. 3.5. Effect of pH
3.4. Effect of salt concentration The influence of salt concentration in the water phase of the emulsion, on the viscosity and stability of the emulsions was investigated. The O/W emulsions were prepared using the oil content of 60 vol.%, surfactant concentration of 2 wt.%, mixing speed of 6000 rpm, temperature of 25 °C, pH of 7, and mixing time of 20 min. The concentration of salt in the water phase was varied from 0 to 40,000 ppm; the salt concentration in the tap water was assumed to be negligible. Fig. 5 illustrates the effect of salt concentration on the viscosity and stability of the emulsions. As it can be observed, increasing the salt concentration has increased the viscosity of the emulsions. Increasing the salt concentration has reduced the amount of separated water as well. Similar observation has been noticed from the various aqueous surfactant systems through which the viscosity has been increased by the addition of salt. At the same time, the ions would act as barriers among the oil droplets and the water phase. Thus increasing the salinity of the aqueous phase, results in the enhancement of the emulsions stability. Ahmed et al. (1999) reported the same observation, i.e. increase in the viscosity of the emulsion due to increase in the aqueous phase salinity, and attributed this behavior to the lower interfacial tension of the oil and aqueous phases at higher aqueous salinities. It is obvious that at lower interfacial tensions smaller
In the last stage of this series of experiments the influence of pH of the aqueous phase on the viscosity and stability of the prepared emulsions was investigated. The O/W emulsions were prepared using the oil content of 60 vol.%, surfactant concentration of 2 wt.%, mixing speed of 6000 rpm, temperature of 25 °C, and mixing duration of 20 min. The pH of the aqueous phase was adjusted at either of 5, 6, 7, 8, and 9. Fig. 6 illustrates the effect of pH on the viscosity and stability of the emulsions. As it can be observed, increasing the pH of the solution has resulted in a negligible enhancement in the viscosity of the emulsions while that has increased the emulsions stability significantly. Increasing the pH of the continuous phase of the emulsions from 6 to 9 causes an increase in the absolute value of zeta potential of the droplets which results in the formation of emulsions with higher stabilities (Sakka, 2002). Yang et al. (2007) observed that oil-in-water emulsions became more stable at higher pH values and attributed this phenomenon to the higher affinity of surfactant molecules towards aggregation at higher pH values. 3.6. Effect of temperature of homogenization Regarding the fact that the diesel–bitumen oil sample resembles synthetic extra-heavy crude, temperature of the homogenization process
Fig. 6. Effect of pH on the viscosity and stability of Iranian oil sample emulsions (dashed lines present viscosity).
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Fig. 7. Effect of temperature of homogenization on the viscosity and stability of diesel–bitumen blend emulsions.
can be very effective in its emulsion making performance. To investigate the effect of this parameter on the emulsification behavior of diesel– bitumen blend, emulsions were prepared using the fixed oil content of 60 vol.%, surfactant concentration of 2 wt.%, mixing speed of 10,000 rpm, pH of 7, and mixing duration of 20 min. The temperature was adjusted at either values of 25, 40, and 50 °C. Fig. 7 illustrates the results. As it can be seen, raising the temperature of homogenization has resulted in emulsions with lower viscosities and higher stabilities. It is notable that increasing the temperature of mixing would result in softening the bituminous particles and therefore, a monotonous dispersion of oil droplets in the water phase would occur.
crude oil. For high viscosity samples such as synthesis crude oil, heating the homogenization vessel resulted in the production of more stable emulsions with lower viscosities.
4. Conclusions
References
The oil-in-water emulsions were successfully prepared using Iranian oil samples of West Paydar crude oil and a blend of diesel– bitumen synthetic crude with water in the presence of Triton X-100 surfactant. Using water as the continuous phase has several benefits in pipeline oil transportation including: decreased amount of viscosity which leads to a reduced energy consumption and easier pumping in oil transportation, and lack of contact between the crude and pipe walls which results in less erosion and precipitation inside pipes. The viscosity of the emulsion was found to increase as the oil content of the emulsion increased up to 60% for both samples. Increasing the oil content beyond this value causes a sudden increase in the emulsion viscosity. This is the point in which phase inversion occurs and therefore, emulsions made with oil contents higher than this value cannot be suitable for this purpose. A similar trend of increasing of viscosity was observed in emulsions prepared from both samples by increasing the surfactant and salt concentration in water, the time and speed of mixing, and pH of the aqueous phase. Increasing the temperature of homogenization process reduced the viscosity of emulsions produced by the particular synthetic crude sample. The stability of O/W emulsions of both samples was found to decrease as the oil content of the emulsion increases up to the phase inversion point. After this point the emulsion converts to W/O emulsion and the stability of emulsion starts to increase with increasing the oil content. For both samples the oil content of 70 vol.% found to be the phase inversion point. The stability of O/W emulsions of both samples was found to increase as the surfactant and salt concentration, the speed and time of mixing of the emulsion, and the pH of aqueous phase and temperature of homogenization increase. The lowest emulsifier concentration required for producing a stable emulsion (the maximum water separation after 24 h ∼ 5%) was 2 wt.%. The minimum duration for the time of mixing for producing stable emulsion was 20 min for both samples. The minimum speed of mixing for producing a stable emulsion was 6000 rpm for west Paydar and 10,000 rpm for synthesis
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Acknowledgement The National Iranian Oil Engineering & Construction Co. (NIOEC) is highly acknowledged for its financial support during the course of this project.
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