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Yield behavior of gelled waxy oil in water-in-oil emulsion at temperatures below ice formation
Fuel 90 (2011) 2113–2117
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Yield behavior of gelled waxy oil in water-in-oil emulsion at temperatures below ice formation Kyeongseok Oh ⇑, Milind D. Deo ⇑ Chemical Engineering Department, University of Utah, Salt Lake City, UT 84112, United States
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 2 January 2011 Accepted 20 February 2011 Available online 5 March 2011 Keywords: Yield stress Gelled waxy oil Emulsion Viscosity
a b s t r a c t Paraffinic waxes precipitate from bulk oil when oil temperatures are lower than the oil wax appearance temperature. The oil can form a gel if the temperature goes below the pour point, especially under quiescent conditions. The strength of the gelled waxy oil increases as temperature decreases further. Application of a mechanical shear deforms and fractures the gel. It is shown that this strength reduction in the gel is irreversible under isothermal conditions. In subsequent cooling, the prior fractured gel even showed much less yield stress than the gel from the shear-free condition at measured temperature. This study explored the gel strength behavior in water-in-oil (w/o) emulsion state. Three different model oils, water-free oil, 10 wt.% w/o and 30 wt.% w/o, were used to determine the yield stress using vane method. Both emulsified oils showed less yield stress values at temperatures between the pour points and ice temperature. Compared to water-free oil at temperatures below ice formation, the higher yield stresses were observed in 10 wt.% w/o oil; however, the lower yield stresses in 30 wt.% w/o oil. Subsequent cooling option after prior gel breakage was also examined. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Highly paraffinic crude oils can form gels in pipelines leading to serious flow blockages. The waxes are soluble in the oil above the wax appearance temperature (WAT), and the oil is Newtonian. The oil exhibits shear thinning behavior below the wax appearance temperature as the wax precipitates [1–4]. It has been shown previously that the emulsified waxy oils also show shear thinning [5– 7]. Water–oil emulsions may form during oil production since water is also produced along with oil in most oil production operations. Natural surfactants in the crude oil, mainly asphaltenes and resins, are known to stabilize the water-in-oil (w/o) emulsions because they carry both the hydrophobic and the hydrophilic functional groups. Hydrophobic wax particles can adsorb on the surface of emulsified water droplets by interacting with natural surfactants at temperatures below WAT in the w/o emulsion. During cooling, newly generated wax particles can cover the surface of water droplets and build the gel network in the oil phase [5] impeding the free movement of water droplets. Emulsified water droplets act like dispersed wax particles and increase the bulk viscosity of the oil as well as yield stress [5,6].
⇑ Corresponding authors. Current address: SK Innovation Global Technology, Taejon 305-712, Republic of Korea (K. Oh). E-mail addresses: [email protected] (K. Oh), [email protected] (M.D. Deo). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.02.030
Three-dimensional gel networks are formed in pipelines on rapid cooling under quiescent conditions. Once the gel develops and the flow stops, certain level of upstream pressure needs to be applied to overcome the yield stress of the gel along the pipeline for restart [8]. A force balance on the gelled length of the pipeline gives the pressure requirement as shown in following equation [9,10].
DP ¼
4sL D
ð1Þ
Here, DP, s, L, and D represent the applied pressure drop, yield stress of the gel, pipeline length, and diameter, respectively. The strength of wax gel network depends on its temperature and shear history. Variations in rheological properties have been studied as function of temperature, cooling rate, and shear rate [4,11–14]. The yield stress is higher at lower temperatures and under quiescent slow rate of cooling [11]. When mechanical shear is applied at temperatures below the pour point, and the gel subsequently cooled, gel formation can be suppressed, even when the magnitude of the shear stress is small [4]. It should be noted that the pour point is not the sole indicator of complete gel network creation and flow blockage. The pour point (ASTM D97 method [15]) is determined when about two weight percent of the soluble waxes come out of solution causing the oil to not flow ‘‘freely’’. The oil with low pour point may demonstrate a high yield stress after a brief cooling period. It is important to know how the yield strength develops at temperatures below the pour point to help determine restart pressure requirement.
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Table 1 Compositions of water-free model oil and 10% and 30% emulsions. Emulsion
Water (wt.%)
Water-free 10% water 30% water
0 10 30
Oil Phase Mineral oil (wt.%)
Wax (wt.%)
Span 80 (wt.%)
87 78.3 61.11
3 2.7 1.89
10 9 7
Rheological studies of w/o emulsion have shown that the oil viscosity increases at higher water content to about 70% watercut, as does the yield stress of the emulsified oils [6]. The yield behavior of emulsified oils at temperatures below the pour point and further cooling to ice formation is of significant interest. We can hypothesize two different types of yield behavior below the ice point. One possibility is an increase in yield stress with water droplets acting as dispersed wax particles. Additional gel strength is expected to be generated as wax film entraps ice, even though majority of the gel strength is usually attributed to the wax gel network. This scenario could be envisioned if there is adequate wax to account for all of the water/ice in the system. In the second scenario, the water droplets may hinder the continuity of the wax network causing a reduction in yield stress, essentially acting as a wax inhibitor. Wax inhibitors and asphaltenes have been shown to decrease the gel strength [12,16,17]. In this study, we compared the yield strength of emulsified oils to water-free oil at temperatures below the pour point of the oil. Temperatures below ice formation were also explored. The water-free and emulsified oils were prepared with excess surfactant to ensure formation and sustenance of emulsion. The yield stress was measured after breaking the gel and cooling the samples further to study the evolution of yield stress under secondary cooling.
Volume fraction of water (/)
Pour point (°C)
0 0.09 0.27
14 13 13
Laborlux 12 Pol, Olympus DP 11 camera attached). Images of waxfree emulsions (magnified X100) are shown in Fig. 2. These particular images were chosen due to the clear visibility of the water droplets.
2.2. Viscosity and yield stress measurements Brookfield RVDV-II + was used to measure the rheological properties. Shear stress and viscosity measurements were performed using the cone-and-plate geometry. The torque reading for the yield stress determination was obtained using the vane fixture. An external cooling bath was connected to control temperatures in both the cone-and-plate and vane geometries. The maximum allowable torque in the instrument was 0.7187 mN.m. All the measurements were begun at 50 °C. In the vane method, the cooling rate was fixed at 0.8 °C/min to the designated temperature and
2. Experiments 2.1. Emulsion preparation Oil phase was prepared by mixing a food-grade wax, white mineral oil (Superla-7) and Span-80 (HLB = 4.3). Deionized water was added to the oil to prepare 10 wt.% w/o and 30 wt.% w/o emulsions at 50 °C. The emulsion constituents are shown in Table 1. The wax composition was measured using high temperature gas chromatography, and the carbon number distributions of wax are shown in Fig. 1. Magnetic stirring was used to make the emulsion. The increase in mixture opaqueness was significant. Formation of the emulsified water droplets was confirmed using a microscope (Leitz
Fig. 1. Carbon number distribution of wax added to the model oil as determined by SIMDIS.
Fig. 2. Microscopic images of 10% and 30% w/o emulsion samples. The images were taken from the sample prepared without wax components to show the emulsion state clearly.
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the torque reading was recorded after aging the gelled oil for 90min duration at each temperature interval. The vane spindle had a diameter of 8.026 mm (0.316 in.) and a length of 16.053 mm (0.632 in.). Thus, the ratio of length to diameter was two. Inner diameter of the jacketed cylinder into which the sample was loaded was 33 mm, and this may have prevented slip during vane rotation. Oil level placed in vane cell was about 60 mm. A thermocouple was placed to measure the temperature of the gel in the cell right after vane rotation. The temperature difference between the core of the gel and the outside was less than 0.3 °C. Freeze–thaw demulsification can lead to phase separation [18]; hence, fresh emulsion samples were used for the new cooling schedule. The procedure is also described elsewhere [19].
3. Results and discussion 3.1. Viscosity of w/o emulsions Fig. 3 shows the viscosities of three samples: water-free oil, 10 wt.% w/o emulsion, and 30 wt.% w/o emulsion. The viscosities are plotted as function of shear rates and temperatures. At temperatures higher than 25 °C, the viscosity values of the three samples were constant regardless of shear rates employed. At temperatures below 25 °C, significant shear thinning is observed for all the three samples. The viscosity values are higher for both the 10 wt.% and 30 wt.% emulsions across the board with higher measured values for the 30 wt.% w/o emulsion. The viscosity values more than doubled when 30% water was added to the oil in comparison with the water-free samples. This is consistent with increase in viscosity
Fig. 4. Shear stresses of water-free, 10% w/o emulsion, and 30% w/o emulsion samples.
trends reported in the literature [5,6]. Visintin et al. [5] suggested that the wax layers on the surface of emulsified water droplets generate additional dispersed phase resulting in the increase in viscosity as well as storage modulus. Water may act like wax particles in its emulsion state. Shear stress versus shear rate is presented in Fig. 4. The shear stress was also dependent on the shear rate at temperatures below 25 °C.
Fig. 3. Viscosities of water-free, 10% w/o emulsion, and 30% w/o emulsion samples.
Fig. 5. Yield stress values of water-free, 10% w/o emulsion, and 30% w/o emulsion samples.
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Fig. 6. Yield stress values with subsequent cooling option after prior gel breakage.
3.2. Yield stress of w/o emulsions Shown in Eq. (2) is the correlation derived by Nguyen and Boger [20,21] between the maximum torque reading obtained from the rheometer and the yield stress for use with vane rheometry.
sy ¼ T max
p 2
3
d
1 H 1 þ d 3
ð2Þ
Here, H and d represent the length (or height) of vane blade and diameter of vane rotation, respectively. Tmax represents the maximum torque reading. Based on the maximum torque rating of the viscometer, the maximum yield stress that could be measured with this arrangement was 380 Pa.
Fig. 5 shows the static yield stress of three samples. Linear increase in yield stress is clearly observed in the temperature range of –7.8 to 7.3 °C for the water-free sample. This linear increase for model waxy oils was shown in our previous work [12]. In general, lower yield stresses were observed in the 10 wt.% w/o and 30 wt.% w/o emulsions at temperatures between the pour point and ice temperature. In the case of 10 wt.% w/o emulsion, the yield stress crossover was observed near the ice temperature, 0 °C. For this sample, the yield stress increased steeply as the sample was cooled below the ice temperature, but the slope became more or less parallel to the pour point line of water-free oil as the cooling continued. The higher yield values at temperatures below ice temperature does match the first hypothesis suggested in the introduction. The ice particles entrapped by wax film increase the yield stress of this sample beyond the values for the water-free oil. The lower yield stress values between the pour point temperature and the ice point for this sample was surprising and does not fit that postulation. This may be due to the limited amount of wax in our samples. The amount of wax in the water-free samples was 3%. This may have been inadequate to entrap all of the water droplets. The trend observed here is different from that reported in the literature [5,6], which was an increase in yield stress with water content at temperatures above ice temperature. The amount of wax in the samples in those experiments was significantly higher than the amount in our experiments. Visintin et al. [5] used a waxy oil containing 9.25 wt.% wax and Paso et al. [6] used a waxy oil containing 24 wt.% of C19 + n-alkanes. When wax amounts are lower, incomplete wax films may allow emulsified water to move freely during the gelling process and not act like wax particles. The yield stresses were uniformly lower than those of water-free oil in the case of the 30% water–oil emulsion. The yield stresses did increase as the temperature decreased, but did not cross over, as in the 10% emulsion case. The wax networks may be disrupted by
Fig. 7. Yield stress values with subsequent cooling option: (a) water-free, (b) 10% w/o emulsion, and (c) 30% w/o emulsion.
K. Oh, M.D. Deo / Fuel 90 (2011) 2113–2117
the amount of water present in this system, leading to uniform reduction in yield stress. 3.3. Yield stress behavior after gel breakage and subsequent cooling Conceptual diagram of yield stress with subsequent cooling option was presented in the previous paper [19] and is shown in Fig. 6. Maximum torque reading in the viscometer is plotted as the yield stress proxy versus temperature. Static yield stress was determined by identifying the maximum torque during the vane rotation inside gelled oil. Once the gel was broken, the second yield values (sa,2, sb,2, sc,2 in Fig. 6) were determined at the same temperature after some time. The values were much less than the first measurement because the gel network was broken. However, it should be noted that a non-zero yield stress value was measured even though the value was much lower than the value in the first measurement. Broken gels may exhibit slurry flow during restart. In order to examine whether the yield strength is regained after gel breakage, the gel was cooled again and the yield stress measured. The third-time yield values (sa,3, sb,3, sc,3 in Fig. 6), after subsequent cooling, showed that yield stress increased from the second-time yield values, but never did reach the initial levels. New wax generated during subsequent cooling may contribute to this gel strength increase but is still smaller than the value of the undisturbed gel at the same temperature (sd,1 sa,3 in Fig. 6). The results of water-free and emulsion yield stress measurements after gel breakage and subsequent cooling are shown in Fig. 7. The trend shown in Fig. 6 is also observed in w/o emulsion samples. The cooling rate was 0.8 °C. Once the first measurement was determined, the second measurement of yield stress was performed in 5 min under isothermal condition. The yield stresses were initially measured at 2 °C. Once the gel was broken, the yield stress measurements were repeated at the same temperature. The values from the second measurement were much less for all three samples as expected. As the samples were cooled further, yield stresses of the samples increased, but did not reach the initial (un-broken) yield stress values at the same temperature. It also appears that the water-free samples regained more of their yield stress compared to the emulsions. 4. Conclusions The viscosities and the yield stress values of emulsified oils were compared with a water-free waxy model oil at temperatures below pour point, with some measurements well below the ice point. In viscosity measurements, water-free and emulsified oils showed the shear thinning behavior at temperatures below their WAT, as shown in previous literature. In water-free oil, the yield stresses increased more or less linearly as the temperature decreased. Lower yield stress values compared to the water-free oil were measured for the 10 wt.% w/o and 30 wt.% w/o at temperatures between pour point and ice point. This trend is opposite to the one reported in the literature, most likely due to the much lower amount of wax in the model oil The yield stress values of the 10% w/o emulsion crossed over to values higher than those of the water-free oil below the ice point, while those of the 30% w/o emulsion continued to increase but stay below the values for the water-free oil. It is possible that the small
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amount of water in the 10% w/o emulsion strengthens the gels at temperatures below the ice point while the larger amount of water in the 30% w/o emulsion disrupts networks, effectively acting as an inhibitor. Yield stress of the gel once broken and subsequently cooled never did reach the initial yield stress values for the same samples at the same temperature. This study shows that the yield behavior of water oil emulsions depends on a number of factors including but not limited to wax and water content, and temperature and shear history. Acknowledgement The authors acknowledge the assistance of Professor Zhigang Fang in Metallurgical Engineering Department. References [1] Rønningsen HP. Rheological behaviour of gelled, waxy North Sea crude oils. J Pet Sci Eng 1992;7:177–213. [2] Visintin RFG, Lapasin R, Vignati E, D’Antona P, Lockhart TP. Rheological behavior and structural interpretation of waxy crude oil gels. Langmuir 2005;21:6240–9. [3] Ahn S, Wang KS, Shuler PJ, Creek JL. Paraffin crystal and deposition control by emulsification, SPE 93357 presented at the 2005 SPE international symposium on oilfield chemistry held in Houston, TX; 2–4 February, 2005. [4] Singh P, Fogler HS, Nagarajan N. Prediction of the wax content of the incipient wax-oil gel in a pipeline: an application of the controlled-stress rheometer. J Rheol 1999;43:1437–59. [5] Visintin RFG, Lockhart TP, Lapasin R, Antona PD. Structure of waxy crude oil emulsion gels. J Non-Newton Fluid Mech 2008;149:34–9. [6] Paso K, Silset A, Sørland G, Gonçalves M de AL, Sjöblom J. Characterization of the formation flowability and resolution of Brazilian crude oil emulsion. Energy Fuels 2009;23:471–80. [7] Farah MA, Oliveira RC, Caldas JN, Rajagopal K. Viscosity of water-in-oil emulsions: variation with temperature and water volume fraction. J Pet Sci Eng 2005;48:169–84. [8] Golczynski TS, Kempton EC. Understanding wax problems leads to deepwater flow assurance solutions. World Oil 2006;227:D7–D10. [9] Uhde A, Kopp G. Pipeline problems resulting from the handling of waxy crudes. J Inst Petrol 1971;57:63–72. [10] Davenport TC, Somper RSH. The yield value and breakdown of crude oil gels. J Inst Petrol 1971;57:86–105. [11] Venkatesan R, Nagarajan NR, Paso K, Yi Y-B, Sastry AM, Fogler HS. The strength of paraffin gels formed under static and flow conditions. Chem Eng Sci 2005;60:3587–98. [12] Oh K, Deo MD. Characteristics of wax gel formation in the presence of asphaltenes. Energy Fuels 2009;23:1289–93. [13] Wardhaugh LT, Boger DV. The measurement and description of the yielding behavior of waxy crude oil. J Rheol 1991;35:1121–56. [14] Chang C, Boger DV, Nguyen QD. The yielding of waxy crude oils. Ind Eng Chem Res 1998;37:1551–9. [15] Annual Book of ASTM-Standards. Petroleum products, lubrications. West Conshohocken, Pennsylvania: American Society for Testing and Materials; 1999 [Section 5]. [16] Guo X, Pethica BA, Huang JS, Prud’homme RK. Crystallization of long-chain nparaffins from solutions and melts as observed by differential scanning calorimetry. Macromolecules 2004;37:5638–45. [17] Pedersen KS, Rønningsen HP. Influence of wax inhibitors on wax appearance temperature, pour point, and viscosity of waxy crude oils. Energy Fuels 2003;17:321–8. [18] Lin C, He G, Dong C, Liu H, Xiao G, Liu Y. Effect of oil phase transition on freeze/ thaw-induced demulsification of water-in-oil emulsions. Langmuir 2008;24:5291–8. [19] Oh K, Gandhi K, Magda JJ, Deo MD. Yield stress of wax gel using vane method. Pet Sci Technol 2009;27:2063–73. [20] Nguyen QD, Boger DV. Yield stress measurement for concentrated suspensions. J Rheol 1983;27:321–49. [21] Nguyen QD, Boger DV. Direct yield stress measurement with the vane method. J Rheol 1985;29:335–47.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Yield behavior of gelled waxy oil in water-in-oil emulsion at temperatures below ice formation Kyeongseok Oh ⇑, Milind D. Deo ⇑ Chemical Engineering Department, University of Utah, Salt Lake City, UT 84112, United States
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 2 January 2011 Accepted 20 February 2011 Available online 5 March 2011 Keywords: Yield stress Gelled waxy oil Emulsion Viscosity
a b s t r a c t Paraffinic waxes precipitate from bulk oil when oil temperatures are lower than the oil wax appearance temperature. The oil can form a gel if the temperature goes below the pour point, especially under quiescent conditions. The strength of the gelled waxy oil increases as temperature decreases further. Application of a mechanical shear deforms and fractures the gel. It is shown that this strength reduction in the gel is irreversible under isothermal conditions. In subsequent cooling, the prior fractured gel even showed much less yield stress than the gel from the shear-free condition at measured temperature. This study explored the gel strength behavior in water-in-oil (w/o) emulsion state. Three different model oils, water-free oil, 10 wt.% w/o and 30 wt.% w/o, were used to determine the yield stress using vane method. Both emulsified oils showed less yield stress values at temperatures between the pour points and ice temperature. Compared to water-free oil at temperatures below ice formation, the higher yield stresses were observed in 10 wt.% w/o oil; however, the lower yield stresses in 30 wt.% w/o oil. Subsequent cooling option after prior gel breakage was also examined. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Highly paraffinic crude oils can form gels in pipelines leading to serious flow blockages. The waxes are soluble in the oil above the wax appearance temperature (WAT), and the oil is Newtonian. The oil exhibits shear thinning behavior below the wax appearance temperature as the wax precipitates [1–4]. It has been shown previously that the emulsified waxy oils also show shear thinning [5– 7]. Water–oil emulsions may form during oil production since water is also produced along with oil in most oil production operations. Natural surfactants in the crude oil, mainly asphaltenes and resins, are known to stabilize the water-in-oil (w/o) emulsions because they carry both the hydrophobic and the hydrophilic functional groups. Hydrophobic wax particles can adsorb on the surface of emulsified water droplets by interacting with natural surfactants at temperatures below WAT in the w/o emulsion. During cooling, newly generated wax particles can cover the surface of water droplets and build the gel network in the oil phase [5] impeding the free movement of water droplets. Emulsified water droplets act like dispersed wax particles and increase the bulk viscosity of the oil as well as yield stress [5,6].
⇑ Corresponding authors. Current address: SK Innovation Global Technology, Taejon 305-712, Republic of Korea (K. Oh). E-mail addresses: [email protected] (K. Oh), [email protected] (M.D. Deo). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.02.030
Three-dimensional gel networks are formed in pipelines on rapid cooling under quiescent conditions. Once the gel develops and the flow stops, certain level of upstream pressure needs to be applied to overcome the yield stress of the gel along the pipeline for restart [8]. A force balance on the gelled length of the pipeline gives the pressure requirement as shown in following equation [9,10].
DP ¼
4sL D
ð1Þ
Here, DP, s, L, and D represent the applied pressure drop, yield stress of the gel, pipeline length, and diameter, respectively. The strength of wax gel network depends on its temperature and shear history. Variations in rheological properties have been studied as function of temperature, cooling rate, and shear rate [4,11–14]. The yield stress is higher at lower temperatures and under quiescent slow rate of cooling [11]. When mechanical shear is applied at temperatures below the pour point, and the gel subsequently cooled, gel formation can be suppressed, even when the magnitude of the shear stress is small [4]. It should be noted that the pour point is not the sole indicator of complete gel network creation and flow blockage. The pour point (ASTM D97 method [15]) is determined when about two weight percent of the soluble waxes come out of solution causing the oil to not flow ‘‘freely’’. The oil with low pour point may demonstrate a high yield stress after a brief cooling period. It is important to know how the yield strength develops at temperatures below the pour point to help determine restart pressure requirement.
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Table 1 Compositions of water-free model oil and 10% and 30% emulsions. Emulsion
Water (wt.%)
Water-free 10% water 30% water
0 10 30
Oil Phase Mineral oil (wt.%)
Wax (wt.%)
Span 80 (wt.%)
87 78.3 61.11
3 2.7 1.89
10 9 7
Rheological studies of w/o emulsion have shown that the oil viscosity increases at higher water content to about 70% watercut, as does the yield stress of the emulsified oils [6]. The yield behavior of emulsified oils at temperatures below the pour point and further cooling to ice formation is of significant interest. We can hypothesize two different types of yield behavior below the ice point. One possibility is an increase in yield stress with water droplets acting as dispersed wax particles. Additional gel strength is expected to be generated as wax film entraps ice, even though majority of the gel strength is usually attributed to the wax gel network. This scenario could be envisioned if there is adequate wax to account for all of the water/ice in the system. In the second scenario, the water droplets may hinder the continuity of the wax network causing a reduction in yield stress, essentially acting as a wax inhibitor. Wax inhibitors and asphaltenes have been shown to decrease the gel strength [12,16,17]. In this study, we compared the yield strength of emulsified oils to water-free oil at temperatures below the pour point of the oil. Temperatures below ice formation were also explored. The water-free and emulsified oils were prepared with excess surfactant to ensure formation and sustenance of emulsion. The yield stress was measured after breaking the gel and cooling the samples further to study the evolution of yield stress under secondary cooling.
Volume fraction of water (/)
Pour point (°C)
0 0.09 0.27
14 13 13
Laborlux 12 Pol, Olympus DP 11 camera attached). Images of waxfree emulsions (magnified X100) are shown in Fig. 2. These particular images were chosen due to the clear visibility of the water droplets.
2.2. Viscosity and yield stress measurements Brookfield RVDV-II + was used to measure the rheological properties. Shear stress and viscosity measurements were performed using the cone-and-plate geometry. The torque reading for the yield stress determination was obtained using the vane fixture. An external cooling bath was connected to control temperatures in both the cone-and-plate and vane geometries. The maximum allowable torque in the instrument was 0.7187 mN.m. All the measurements were begun at 50 °C. In the vane method, the cooling rate was fixed at 0.8 °C/min to the designated temperature and
2. Experiments 2.1. Emulsion preparation Oil phase was prepared by mixing a food-grade wax, white mineral oil (Superla-7) and Span-80 (HLB = 4.3). Deionized water was added to the oil to prepare 10 wt.% w/o and 30 wt.% w/o emulsions at 50 °C. The emulsion constituents are shown in Table 1. The wax composition was measured using high temperature gas chromatography, and the carbon number distributions of wax are shown in Fig. 1. Magnetic stirring was used to make the emulsion. The increase in mixture opaqueness was significant. Formation of the emulsified water droplets was confirmed using a microscope (Leitz
Fig. 1. Carbon number distribution of wax added to the model oil as determined by SIMDIS.
Fig. 2. Microscopic images of 10% and 30% w/o emulsion samples. The images were taken from the sample prepared without wax components to show the emulsion state clearly.
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the torque reading was recorded after aging the gelled oil for 90min duration at each temperature interval. The vane spindle had a diameter of 8.026 mm (0.316 in.) and a length of 16.053 mm (0.632 in.). Thus, the ratio of length to diameter was two. Inner diameter of the jacketed cylinder into which the sample was loaded was 33 mm, and this may have prevented slip during vane rotation. Oil level placed in vane cell was about 60 mm. A thermocouple was placed to measure the temperature of the gel in the cell right after vane rotation. The temperature difference between the core of the gel and the outside was less than 0.3 °C. Freeze–thaw demulsification can lead to phase separation [18]; hence, fresh emulsion samples were used for the new cooling schedule. The procedure is also described elsewhere [19].
3. Results and discussion 3.1. Viscosity of w/o emulsions Fig. 3 shows the viscosities of three samples: water-free oil, 10 wt.% w/o emulsion, and 30 wt.% w/o emulsion. The viscosities are plotted as function of shear rates and temperatures. At temperatures higher than 25 °C, the viscosity values of the three samples were constant regardless of shear rates employed. At temperatures below 25 °C, significant shear thinning is observed for all the three samples. The viscosity values are higher for both the 10 wt.% and 30 wt.% emulsions across the board with higher measured values for the 30 wt.% w/o emulsion. The viscosity values more than doubled when 30% water was added to the oil in comparison with the water-free samples. This is consistent with increase in viscosity
Fig. 4. Shear stresses of water-free, 10% w/o emulsion, and 30% w/o emulsion samples.
trends reported in the literature [5,6]. Visintin et al. [5] suggested that the wax layers on the surface of emulsified water droplets generate additional dispersed phase resulting in the increase in viscosity as well as storage modulus. Water may act like wax particles in its emulsion state. Shear stress versus shear rate is presented in Fig. 4. The shear stress was also dependent on the shear rate at temperatures below 25 °C.
Fig. 3. Viscosities of water-free, 10% w/o emulsion, and 30% w/o emulsion samples.
Fig. 5. Yield stress values of water-free, 10% w/o emulsion, and 30% w/o emulsion samples.
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Fig. 6. Yield stress values with subsequent cooling option after prior gel breakage.
3.2. Yield stress of w/o emulsions Shown in Eq. (2) is the correlation derived by Nguyen and Boger [20,21] between the maximum torque reading obtained from the rheometer and the yield stress for use with vane rheometry.
sy ¼ T max
p 2
3
d
1 H 1 þ d 3
ð2Þ
Here, H and d represent the length (or height) of vane blade and diameter of vane rotation, respectively. Tmax represents the maximum torque reading. Based on the maximum torque rating of the viscometer, the maximum yield stress that could be measured with this arrangement was 380 Pa.
Fig. 5 shows the static yield stress of three samples. Linear increase in yield stress is clearly observed in the temperature range of –7.8 to 7.3 °C for the water-free sample. This linear increase for model waxy oils was shown in our previous work [12]. In general, lower yield stresses were observed in the 10 wt.% w/o and 30 wt.% w/o emulsions at temperatures between the pour point and ice temperature. In the case of 10 wt.% w/o emulsion, the yield stress crossover was observed near the ice temperature, 0 °C. For this sample, the yield stress increased steeply as the sample was cooled below the ice temperature, but the slope became more or less parallel to the pour point line of water-free oil as the cooling continued. The higher yield values at temperatures below ice temperature does match the first hypothesis suggested in the introduction. The ice particles entrapped by wax film increase the yield stress of this sample beyond the values for the water-free oil. The lower yield stress values between the pour point temperature and the ice point for this sample was surprising and does not fit that postulation. This may be due to the limited amount of wax in our samples. The amount of wax in the water-free samples was 3%. This may have been inadequate to entrap all of the water droplets. The trend observed here is different from that reported in the literature [5,6], which was an increase in yield stress with water content at temperatures above ice temperature. The amount of wax in the samples in those experiments was significantly higher than the amount in our experiments. Visintin et al. [5] used a waxy oil containing 9.25 wt.% wax and Paso et al. [6] used a waxy oil containing 24 wt.% of C19 + n-alkanes. When wax amounts are lower, incomplete wax films may allow emulsified water to move freely during the gelling process and not act like wax particles. The yield stresses were uniformly lower than those of water-free oil in the case of the 30% water–oil emulsion. The yield stresses did increase as the temperature decreased, but did not cross over, as in the 10% emulsion case. The wax networks may be disrupted by
Fig. 7. Yield stress values with subsequent cooling option: (a) water-free, (b) 10% w/o emulsion, and (c) 30% w/o emulsion.
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the amount of water present in this system, leading to uniform reduction in yield stress. 3.3. Yield stress behavior after gel breakage and subsequent cooling Conceptual diagram of yield stress with subsequent cooling option was presented in the previous paper [19] and is shown in Fig. 6. Maximum torque reading in the viscometer is plotted as the yield stress proxy versus temperature. Static yield stress was determined by identifying the maximum torque during the vane rotation inside gelled oil. Once the gel was broken, the second yield values (sa,2, sb,2, sc,2 in Fig. 6) were determined at the same temperature after some time. The values were much less than the first measurement because the gel network was broken. However, it should be noted that a non-zero yield stress value was measured even though the value was much lower than the value in the first measurement. Broken gels may exhibit slurry flow during restart. In order to examine whether the yield strength is regained after gel breakage, the gel was cooled again and the yield stress measured. The third-time yield values (sa,3, sb,3, sc,3 in Fig. 6), after subsequent cooling, showed that yield stress increased from the second-time yield values, but never did reach the initial levels. New wax generated during subsequent cooling may contribute to this gel strength increase but is still smaller than the value of the undisturbed gel at the same temperature (sd,1 sa,3 in Fig. 6). The results of water-free and emulsion yield stress measurements after gel breakage and subsequent cooling are shown in Fig. 7. The trend shown in Fig. 6 is also observed in w/o emulsion samples. The cooling rate was 0.8 °C. Once the first measurement was determined, the second measurement of yield stress was performed in 5 min under isothermal condition. The yield stresses were initially measured at 2 °C. Once the gel was broken, the yield stress measurements were repeated at the same temperature. The values from the second measurement were much less for all three samples as expected. As the samples were cooled further, yield stresses of the samples increased, but did not reach the initial (un-broken) yield stress values at the same temperature. It also appears that the water-free samples regained more of their yield stress compared to the emulsions. 4. Conclusions The viscosities and the yield stress values of emulsified oils were compared with a water-free waxy model oil at temperatures below pour point, with some measurements well below the ice point. In viscosity measurements, water-free and emulsified oils showed the shear thinning behavior at temperatures below their WAT, as shown in previous literature. In water-free oil, the yield stresses increased more or less linearly as the temperature decreased. Lower yield stress values compared to the water-free oil were measured for the 10 wt.% w/o and 30 wt.% w/o at temperatures between pour point and ice point. This trend is opposite to the one reported in the literature, most likely due to the much lower amount of wax in the model oil The yield stress values of the 10% w/o emulsion crossed over to values higher than those of the water-free oil below the ice point, while those of the 30% w/o emulsion continued to increase but stay below the values for the water-free oil. It is possible that the small
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amount of water in the 10% w/o emulsion strengthens the gels at temperatures below the ice point while the larger amount of water in the 30% w/o emulsion disrupts networks, effectively acting as an inhibitor. Yield stress of the gel once broken and subsequently cooled never did reach the initial yield stress values for the same samples at the same temperature. This study shows that the yield behavior of water oil emulsions depends on a number of factors including but not limited to wax and water content, and temperature and shear history. Acknowledgement The authors acknowledge the assistance of Professor Zhigang Fang in Metallurgical Engineering Department. References [1] Rønningsen HP. Rheological behaviour of gelled, waxy North Sea crude oils. J Pet Sci Eng 1992;7:177–213. [2] Visintin RFG, Lapasin R, Vignati E, D’Antona P, Lockhart TP. Rheological behavior and structural interpretation of waxy crude oil gels. Langmuir 2005;21:6240–9. [3] Ahn S, Wang KS, Shuler PJ, Creek JL. Paraffin crystal and deposition control by emulsification, SPE 93357 presented at the 2005 SPE international symposium on oilfield chemistry held in Houston, TX; 2–4 February, 2005. [4] Singh P, Fogler HS, Nagarajan N. Prediction of the wax content of the incipient wax-oil gel in a pipeline: an application of the controlled-stress rheometer. J Rheol 1999;43:1437–59. [5] Visintin RFG, Lockhart TP, Lapasin R, Antona PD. Structure of waxy crude oil emulsion gels. J Non-Newton Fluid Mech 2008;149:34–9. [6] Paso K, Silset A, Sørland G, Gonçalves M de AL, Sjöblom J. Characterization of the formation flowability and resolution of Brazilian crude oil emulsion. Energy Fuels 2009;23:471–80. [7] Farah MA, Oliveira RC, Caldas JN, Rajagopal K. Viscosity of water-in-oil emulsions: variation with temperature and water volume fraction. J Pet Sci Eng 2005;48:169–84. [8] Golczynski TS, Kempton EC. Understanding wax problems leads to deepwater flow assurance solutions. World Oil 2006;227:D7–D10. [9] Uhde A, Kopp G. Pipeline problems resulting from the handling of waxy crudes. J Inst Petrol 1971;57:63–72. [10] Davenport TC, Somper RSH. The yield value and breakdown of crude oil gels. J Inst Petrol 1971;57:86–105. [11] Venkatesan R, Nagarajan NR, Paso K, Yi Y-B, Sastry AM, Fogler HS. The strength of paraffin gels formed under static and flow conditions. Chem Eng Sci 2005;60:3587–98. [12] Oh K, Deo MD. Characteristics of wax gel formation in the presence of asphaltenes. Energy Fuels 2009;23:1289–93. [13] Wardhaugh LT, Boger DV. The measurement and description of the yielding behavior of waxy crude oil. J Rheol 1991;35:1121–56. [14] Chang C, Boger DV, Nguyen QD. The yielding of waxy crude oils. Ind Eng Chem Res 1998;37:1551–9. [15] Annual Book of ASTM-Standards. Petroleum products, lubrications. West Conshohocken, Pennsylvania: American Society for Testing and Materials; 1999 [Section 5]. [16] Guo X, Pethica BA, Huang JS, Prud’homme RK. Crystallization of long-chain nparaffins from solutions and melts as observed by differential scanning calorimetry. Macromolecules 2004;37:5638–45. [17] Pedersen KS, Rønningsen HP. Influence of wax inhibitors on wax appearance temperature, pour point, and viscosity of waxy crude oils. Energy Fuels 2003;17:321–8. [18] Lin C, He G, Dong C, Liu H, Xiao G, Liu Y. Effect of oil phase transition on freeze/ thaw-induced demulsification of water-in-oil emulsions. Langmuir 2008;24:5291–8. [19] Oh K, Gandhi K, Magda JJ, Deo MD. Yield stress of wax gel using vane method. Pet Sci Technol 2009;27:2063–73. [20] Nguyen QD, Boger DV. Yield stress measurement for concentrated suspensions. J Rheol 1983;27:321–49. [21] Nguyen QD, Boger DV. Direct yield stress measurement with the vane method. J Rheol 1985;29:335–47.