Energy Fuels 2010, 24, 1146–1155 Published on Web 12/30/2009
: DOI:10.1021/ef901065c
Effect of Emulsion Characteristics on Wax Deposition from Water-in-Waxy Crude Oil Emulsions under Static Cooling Conditions Yu Zhang, Jing Gong,* Yongfei Ren, and Pengyu Wang Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum, 102249 Beijing, China Received September 21, 2009. Revised Manuscript Received December 11, 2009
Water-in-waxy crude oil emulsions with different water cuts were prepared with three different stirring speeds. The droplet size and distribution of the dispersed phase were determined by measuring the droplet diameter with trinocular biomicroscopy. The effects of the droplet size and distribution of the dispersed phase, water cut, and coldfinger temperature on wax deposition for water-in-waxy crude oil emulsions were studied experimentally using the coldfinger apparatus. The results show that the three influencial factors, including the coldfinger temperature, water cut, and droplet size and distribution, have a significant effect on wax deposition for the twophase oil-water system. In addition, 24 test results referring to different test conditions have been analyzed statistically. The significance analyses of influencial factors have been accomplished by using the F-test method from the data obtained from variance analysis. The results show that, for water-in-waxy crude oil emulsion, the influence degree on wax deposition in order is the coldfinger temperature, water cut, and the droplet size and distribution. At the end of the experiments, the samples of wax deposits collected from the coldfinger experiments were analyzed using high-temperature gas chromatography (HTGC) to determine the percentage of iso-alkanes, n-alkanes, and total wax in the deposits. The results of this study provide a reference and an insight for the further study on the two-phase oil-water wax deposition in flow loop and actual pipelines and should be of significant interest to develop a reliable prediction model in the future.
the negligible gravity settling and Brownian diffusion. On the basis of either the wax deposition mechanisms or the heattransfer approach with combining the data obtained from the coldfinger apparatus and flow loops, various models1-19 used to predict the wax deposition rate in the single flow have been
Introduction With the exploration of the deepwater oil fields and the construction of the subsea pipelines, a new problem, wax deposition occurring in the multiphase flow pipelines, has to be faced and significantly requires corresponding solutions for multiphase transportation technology. Wax molecules in the waxy crude oil can crystallize out, and solid wax particles can deposit on the cold pipe wall, when the waxy crude oil is transported in a cold environment that is not only below the oil temperature but also below its wax appearance temperature (WAT) or cloud point. Owing to the occurrence of wax deposition, transfer pressure will increase and the transportation capacity of the pipeline will be reduced with a decreasing area open to flow. To make matters worse, it may lead to the blockage of flow lines with as little as 2% of precipitated wax solids in the bulk oil, which can cause enormous financial losses to the petroleum industry. Wax deposition has become a focus studied by the global petroleum industry with the construction of the offshore pipelines. Hence, the study on wax deposition in multiphase flow will be of great benefit to the flow assurance of offshore pipelines. Wax deposition in the multiphase flow is quite a complex process influenced together by components of the oil, water cut, oil temperature, temperature difference between the bulk oil and the ambient, velocity, flow pattern, pipe material, deposition time, etc. For the study on wax deposition in singlephase flow, the mechanisms of wax deposition have been generally considered as molecular diffusion, shear dispersion, gravity settling and Brownian diffusion and the mechanism of molecular diffusion has been recognized to be dominant with
(1) Bern, P. A.; Withers, V. R.; Cairns, R. J. R. Wax deposition in crude pipelines. Presented at European Offshore Petroleum Conference and Exhibition, London, U.K., Oct 21-24, 1980; Paper EUR 206. (2) Burger, E. D.; Perkins, T. K.; Striegler, J. H. Studies of wax deposition in the trans Alaska pipeline. J. Pet. Technol. 1981, 33, 1075. (3) Weingarten, J. S.; Euchner, J. A. Methods for predicting wax precipitation and deposition. SPE Prod. Eng. 1988, 2, 121–126. (4) Majeed, A.; Bringedal, B.; Overa, S. Model calculates wax deposition for North Sea oils. Oil Gas J. 1990, 18, 52–59. (5) Hamouda, A. A.; Ravnoy, J. M. Prediction of wax deposition in pipelines and field experience on the influence of wax on drag-reducer performance. Presented at the 24th Annual Offshore Technology Conference (OTC), Houston, TX, 1992; OTC 7060. (6) Hamouda, A. A.; Viken, B. K. Wax deposition mechanism under high-pressure and in presence of light hydrocarbons. Presented at the Society of Petroleum Engineers (SPE) International Symposium on Oilfield Chemistry, New Orleans, LA, 1993; SPE 25189. (7) Hamouda, A. A.; Davidsen, S. An approach for simulation of paraffin deposition in pipelines as a function of flow characteristics with a reference to Teesside oil pipeline. Presented at the Society of Petroleum Engineers (SPE) International Symposium on Oilfield Chemistry, San Antonio, TX, 1995; SPE 28966. (8) Hsu, J. J. C.; Brubaker, J. P. Wax deposition scale-up modeling for waxy crude production lines. Presented at the 27th Annual Offshore Technology Conference (OTC), Houston, TX, May 1-4, 1995; OTC 7778. (9) Matzain, A. Single phase liquid paraffin deposition modeling. M. S. Thesis, University of Tulsa, Tulsa, OK, 1996. (10) Ribeiro, F. S.; Mendes, P. R. S.; Braga, S. L. Obstruction of pipelines due to paraffin deposition during the fow of crude oils. Int. J. Heat Mass Transfer 1997, 18, 40–48. (11) Hsu, J. J. C.; Lian, S. J.; Liu, M.; Bi, H. X.; Guo, C. Z. Validation of wax deposition model by a field test. Presented at the Society of Petroleum Engineers (SPE) International Conference and Exhibition, Beijing, China, 1998; SPE 48867.
*To whom correspondence should be addressed. Telephone: þ86-1089733804. E-mail:
[email protected]. r 2009 American Chemical Society
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proposed. The concept of aging was proposed and incorporated in the modeling while taking the effect of shear stripping into account. In addition, the Ostwald Ripening was confirmed to be another aging mechanism of wax deposits.25 On the basis of the crystallization classic theory26 and boundary layer theory27 and data obtained from coldfinger tests, mathematical models for wax deposition were proposed. From a comparison to wax deposition in single-phase flow, the study on wax deposition in multiphase flow is still in a preliminary stage. For the two-phase gas-oil system in which the asymmetric thickness distribution of wax deposits exists under various flow patterns, an online measurement to the thickness of wax deposits for the two-phase gas-oil flow, called liquid displacement-level detection (LD-LD), was proposed,28 which was conducted without depressurization, restarting the test system, imposing any influence on the in situ and overall heat transfer, and frequently removing measuring sections during the measurement. Furthermore, considering the effect of the flow pattern, wax deposition kinetic models of oil-gas two-phase flow were proposed with the combination of wax deposition mechanisms for single-phase flow and data
obtained from the tests for wax deposition of the two-phase gas-oil flow.29-31 However, the published literature sources that specifically address the prediction of wax deposition in the two-phase oil-water flowlines or wellbores are very limited. Thus far, for the two-phase oil-water system, a few explorative experiments have been carried out using the coldfinger apparatus32 and flow loop facility.33-37 Unlike the single oil situation for the two-phase oil-water system, emulsion preparation patterns including the stirring speed, stirring temperature, and addition method of the water phase have an extremely significant effect on emulsion characteristics.38 During the course of emulsion preparations, different droplet sizes and distributions generated by varying the mixing speed of the stirrer or varying the water cut may have a significant effect on the wax deposition; however, few literature sources have reported this aspect. In this study, the influencial factors including the droplet size and distribution of the dispersed phase, water cut, and cold ambient temperature on wax deposition for water-inwaxy crude oil emulsions were studied experimentally using the coldfinger apparatus and the wax content in the deposits were analyzed using high-temperature gas chromatography (HTGC). The results provide a reference and an insight for the future study on a reliable prediction model for the two-phase oil-water wax deposition in flow loop.
(12) Lund, H. J. Investigation of paraffin deposition during singlephase liquid flow in pipelines. M.S. Thesis, University of Tulsa, Tulsa, OK, 1998. (13) Bidmus, H. O.; Mehrotra, A. K. Heat-transfer analogy for wax deposition from paraffinic mixtures. Ind. Eng. Chem. Res. 2004, 43, 791. (14) Parthasarathi, P.; Mehrotra, A. K. Solid deposition from multicomponent wax-solvent mixtures in a benchscale flow-loop apparatus with heat transfer. Energy Fuels 2005, 19, 1387. (15) Jennings, D. W.; Weispfenning, K. Effect of shear and temperature on wax deposition: Coldfinger investigation with a Gulf of Mexico crude oil. Energy Fuels 2005, 19, 1376. (16) Fong, N.; Mehrotra, A. K. Deposition under turbulent flow of wax-solvent mixtures in a bench-scale flow-loop apparatus with heat transfer. Energy Fuels 2007, 21, 1263. (17) Mehrotra, A. K.; Bhat, N. V. Modeling the effect of shear stress on deposition from “waxy” mixtures under laminar flow with heat transfer. Energy Fuels 2007, 21, 1277. (18) Merino-Garcia, D.; Margarone, M.; Correra, S. Kinetics of waxy gel formation from batch experiments. Energy Fuels 2007, 21, 1287. (19) Bidmus, H. O.; Mehrotra, A. K. Measurement of the liquiddeposit interface temperature during solids deposition from wax-solvent mixtures under static cooling conditions. Energy Fuels 2008, 22, 1174. (20) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. R. Formation and aging of incipient thin film wax-oil gels. AIChE J. 2000, 46, 1059–1073. (21) Hernandez, O. C. Investigation of single-phase paraffin deposition characteristics. M.S. Thesis, University of Tulsa, Tulsa, OK, 2002. (22) Hernandez, O. C.; Hensly, H.; Sarica, C. Improvements in singlephase paraffin deposition modeling. SPE Prod. Facil. 2004, 11, 237–244. (23) Venkatesan, R.; Nagarajan, N. R.; Paso, K.; Yi, Y. B. The strength of paraffin gels formed under static and flow conditions. Chem. Eng. Sci. 2005, 60, 3587–3598. (24) Venkatesan, R. The deposition and rheology of organic gels. Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 2004. (25) Coutinho, J. A. P.; Lopes da Silva, J. A.; Ferreira, A.; Soares, M. R.; Daridon, J. L. Evidence for the aging of wax deposits in crude oils by Ostwald Ripening. Pet. Sci. Technol. 2003, 21, 381–391. (26) dos Santos, J. S. T.; Fernandes, A. C.; Giulietti, M. Study of the paraffin deposit formation using the cold finger methodology for Brazilian crude oils. J. Pet. Sci. Eng. 2004, 45, 47–60. (27) Correra, S.; Fasano, A.; Fusi, L.; Primicerio, M. Modelling wax diffusion in crude oils: The cold finger device. Appl. Math. Modell. 2007, 31, 2286–2298. (28) Chen, X. T.; Butler, T.; Brill, J. P. Techniques for measuring wax thickness during single and multiphase flow. Presented at the 1997 Society of Petroleum Engineers (SPE) Annual Technical Conference and Exhibition, San Antonio, TX, Oct 5-8, 1997; SPE 38773. (29) Matzain, A.; Zhang, H. Q. Investigation of paraffin deposition during multiphase flow in pipelines and wellbores;Part 1: Experiments. Proceedings of ETCE/OMAE 2000 Joint Conference on Energy for the New Millenium, New Orleans, LA, 2000; pp 753-759. (30) Matzain, A.; Mandar, A. Investigation of paraffin deposition during multiphase flow in pipelines and wellbores;Part 2: Modeling. ASME J. Energy Resour. Technol. 2001, 123, 150–157.
Experimental Section Waxy Crude Oil Sample. The experimental oil sample used in the study is a typical waxy crude oil coming from the actual transportation pipeline, Qinhuangdao-Beijing pipeline, which is characterized by transporting the crude oil of high wax content, which can lead to severe wax deposition and even gelling problems when the pipeline is shut down. The physical properties of the waxy crude oil sample used in the experiments are as follows: the density at the temperature of 20 °C is 870.8 kg/m3; the wax content is 18.72% by weight; and the WAT and gel point temperature are 47 and 34 °C, respectively. Note that both the wax content and the WAT were determined using a differential scanning calorimeter (DSC) under cooling conditions with the cooling rate of 5 °C/min. Thermal spectra, which are curves of heat influx versus temperature, are shown in Figure 1. The temperature, at which the curve deviated (31) (a) Matzain, A. Multiphase flow wax deposition modeling. Ph.D. Thesis, University of Tulsa, Tulsa, OK, 1999. (b) Kilincer, N. Multiphase paraffin deposition behavior of a Garden Banks condensate. M.S. Thesis, University of Tulsa, Tulsa, OK, 2003. (32) Couto, G. H.; Chen, H.; Dellecase, E. An investigation of twophase oil/water paraffin deposition. Presented at the 2006 Offshore Technology Conference (OTC), Houston, TX, May 1-4, 2006; OTC 17963. (33) Hsu, J. J. C.; Santamaria, M. M.; Brubaker, J. P. Wax deposition of waxy live crudes under turbulent flow conditions. Presented at the 69th Annual Technical Conference and Exhibition, New Orleans, LA, Sept 25-28, 1994; SPE 28480. (34) Hsu, J. J. C.; Elphingstone, G. M.; Greenhill, K. L. Modeling of multiphase wax deposition. ASME J. Energy Resour. Technol. 1999, 121, 81–85. (35) Gao, C. Investigation of long term paraffin deposition behavior for South Pelto oil. M.S. Thesis, University of Tulsa, Tulsa, OK, 2003. (36) Bordalo, S. N. Experimental study of oil/water flow with paraffin precipitation in subsea pipelines. Presented at the 2007 Society of Petroleum Engineers (SPE) Annual Technical Conference and Exhibition, Anaheim, CA, Nov 11-14, 2007; SPE 110810. (37) Bruno, A.; Sarica, C.; Chen, H.; Volk, M. Paraffin deposition during the flow of water-in-oil and oil-in-water dispersions in pipes. Presented at the 2008 Society of Petroleum Engineers (SPE) Annual Technical Conference and Exhibition, Denver, CO, Sept 21-24, 2008; SPE 114747. (38) Dou, D.; Gong, J. Experimental investigation on apparent viscosity of heavy oil-water emulsions. Chem. Eng. 2006, 34, 39–42.
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Figure 1. Thermal spectra curve of the tested oil sample during cooling.
Figure 3. Effect of the residence time of 48 h on the droplet size and distribution for emulsions with the water cut of 10 and 40% prepared with the stirring speed of 400 rpm.
speeds applied by the stirrer can generate distinct emulsion characteristics. To study the effect of different droplet sizes and distributions on wax deposition in water-in-waxy crude oil emulsions, consequently, emulsions for testing conditions were prepared at the temperature of 55 °C with the gradual addition of water at 55 °C and, meanwhile, with the mixing speed of 400, 800, and 1200 rpm. During the course of preparations, the stirring period was kept for 10 min for each stirring speed. The water cuts of emulsions are 10, 20, 30, and 40%, respectively. The type of stirrer used during emulsion preparations is KIKARW20DZM n with the range from 0 to 2400 rpm. The type of heat bath is HWY-10, with (0.1 °C of temperature-controlled accuracy. The objective of the experiments is to study the wax deposition characteristics on the premise that the type of emulsions is water-in-oil. However, the emulsions will become less stable with the increasing water cut. In other words, waterin-oil emulsion will not be stable any longer and the dispersed phase water droplets will agglomerate, leading to settling because of gravity. If unstable emulsions are used to study the wax deposition experimentally, the “effective” water cuts in the unstable emulsions will decrease with the increasing free water, consequently unable to reflect the real wax deposition situation. On the basis of this fact, emulsions with different water cuts were placed stationarily in the heat bath for 48 h to observe the stability after preparations. The results showed that during that period no free water was observed. It is known that the water droplets may flocculate and settle in the bottom with the time, if serious, which could cause a change in the gradient of colors inside the sample (from black at the top to brown at the bottom). However, the change in the gradient of colors inside the testing sample was not observed. In addition, the effect of the residence time of 48 h on the droplet size and distribution for emulsions is shown in Figure 3. In Figure 3, the two emulsions with the water cut of 10 and 40%, which were prepared with the stirring speed of 400 rpm, are selected as the representations. The emulsion samples were collected at the same vertical locations (at the bottom) to measure the droplet size and distribution. The results show that the curves of droplet size and distribution only slightly shift to the right after 48 h; i.e., the amount of small droplets decreases, and the amount of relatively larger droplets increases. Hence, the experimental residence time could be set for 48 h, which not only assured that the type of emulsions for wax deposition tests was the water-in-oil but also assured that the emulsions were stable all of the time during the course of wax deposition tests. It is noted that the droplet size and distribution of the dispersed phase in actual pipelines mainly depend upon the velocity, flow pattern, oil physical properties, and water cut. Although different droplet sizes and distributions can be generated by varying the water cut and the mixing speed of the
Figure 2. Sketch of the coldfinger apparatus.
from the baseline, was the WAT. The curve was able to be used to calculate the average latent heat of wax crystallization by integrating the peak corresponding to the phase change. The wax content of the sample could be obtained with the average heat of wax crystallization.39 The type of DSC is TA2000/ MDSC2910 with 0.1 μW of heat flux accuracy and 0.1 °C of temperature-controlled accuracy. Experimental Apparatus. The coldfinger apparatus consists of a heat-bath temperature controller, a cold-bath temperature controller, coldfingers, and stainless-steel vessels. The temperatures of emulsion and coldfinger surface are adjusted and maintained by circulating hot and cold water in heat- and cold-bath temperature controllers, respectively. A stainless-steel vessel containing the emulsion is placed inside the heat-bath temperature controller to keep the emulsion bulk temperature constant for testing conditions. A coldfinger offers the surface for wax deposition when immersed into the emulsion. The sketch of the coldfinger apparatus is shown in Figure 2. The coldfinger apparatus was designed to conduct up to six tests simultaneously, so that it was more convenient to study the effect of different particle size distributions on wax deposition in water-in-waxy crude oil emulsions. Furthermore, owing to the more simple structure, the more easy operation and control and much less required amount of oil using the coldfinger apparatus can study the wax deposition qualitatively and quantitatively much more conveniently than using the test flow loop. Emulsion Preparations. It has been considered that emulsion preparation patterns including stirring speed, stirring temperature, and addition method of water have an extremely significant effect on emulsion characteristics. During the course of preparations for emulsion, various sizes and distributions of dispersed phase water droplets generated by different stirring (39) Huang, Q. Y.; Wang, J. F.; Zhang, J. J. Physical properties of wax deposits on the walls of crude pipelines. Pet. Sci. 2009, 6, 64–68.
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Figure 4. Images at the water cut of 40% for emulsions with the stirring speeds of (a) 400 rpm, (b) 800 rpm, and (c) 1200 rpm.
stirring rate (i.e., mixing speed), stirring temperature, and addition method of the water phase have an extremely significant effect on emulsion characteristics. During the course of emulsion preparations, different droplet sizes and distributions can be generated by varying the mixing speed of the stirrer even under the same water cut conditions. For the purpose of studying the effect of different droplet sizes and distributions on wax deposition, emulsions at each water cut were created with the mixing speed of 400, 800, and 1200 rpm. The trinocular biomicroscopy of CN15 type equipped with the Anymicro DSS digital camera was used to observe and shoot the dispersed phase droplets, and in combination with the relevant processing software (as shown in Figure 4), the diameters of dispersed phase droplets can be determined and the droplet size distribution can be obtained. Droplet sizes and distributions under the conditions of different mixing speed and water cuts are shown in Figure 5. Note that the droplet size distribution is in number and not in volume. It can be seen from the results in Figure 5 that the droplet diameter and distribution of the dispersed phase of emulsion, which is prepared at a constant water cut with a fixed stirring speed, are not uniform but concentrate within a certain diameter range (i.e., peaks exist in the curves of droplet size and distribution). In addition, the curves of droplet size and distribution shift to the left with the increasing stirring speeds during the course of the preparations for emulsions; i.e., the amount of smaller droplets increases, and the amount of relatively large droplets decreases. The reason for this phenomenon is that a higher stirring speed providing
stirrer, they may not be completely the same as the droplet size and distribution of the dispersed phase in actual pipelines because of different flow regimes. Experimental Procedure and Method. Emulsions with various particle size distributions and different water cuts were poured into six stainless-steel vessels. These six stainless-steel vessels having contained emulsions were weighed with the mass named M1-M6 before being placed inside ther heat-bath temperature controller, which had been set at the temperature of 55 °C in advance. After that, six coldfingers also having been maintained at 55 °C were immersed into the emulsions, respectively. After 30 min, the temperatures of the cold and hot baths were regulated quickly from 55 °C to the desired testing temperatures. As soon as the testing temperature arrived, the time was recorded and considered as the beginning of the tests. The tests ended after 48 h. Coldfingers were pulled from vessels. The deposits were removed from coldfingers and sampled for HTGC analyses. Then, the six stainless-steel vessels containing emulsions that had already caused wax deposition were weighed again with the mass named m1-m6. It is obvious that the difference between M1 and m1 is the mass of deposits on coldfinger 1. By analogy, we can obtain the rest.
Results and Discussion In the study, emulsions in the stainless-steel vessels were kept static without the stirring speed during the course of wax deposition; i.e., the wax deposition occurred under the static cooling conditions. Effect of the Droplet Size and Distribution. As mentioned previously, emulsion preparation patterns, including the 1149
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Figure 5. Droplet size and distribution curve for emulsions with the water cuts of (a) 10%, (b) 20%, (c) 30%, and (d) 40%.
Figure 6. Deposition rate versus stirring speed with the emulsion temperature of 45 °C and the coldfinger temperatures of (a) 35 °C and (b) 32.5 °C under different water cut conditions.
the two-phase oil-water system with more energy can generate a more drastic shear strength, as a consequence, leading to the water phase being dispersed as smaller droplets. The effect of the droplet size and distribution of the dispersed phase on wax deposition under static cooling conditions is shown in Figure 6. Note that the tests in Figure 6 were conducted with the emulsion temperature of 45 °C and the coldfinger temperatures of 35 and 32.5 °C. The results show that the wax deposition rate decreases as the stirring
speed at which the emulsions were prepared increases. In other words, the wax deposition rate decreases with the decreasing droplet diameters of the dispersed phase and the resulting increasing amount of smaller droplets. The reasonable explanation for the phenomenon may be as follows: Wax molecules existing in water-in-waxy crude oil emulsion are recognized to diffuse toward the surface of the coldfinger only through the continuous phase (i.e., oil phase) because of their negligible solubility in the dispersed phase (i.e., water 1150
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Figure 7. Apparent viscosity versus the temperature at different stirring speeds for emulsions with the water cuts of (a) 10%, (b) 20%, (c) 30%, and (d) 40%.
phase); however, a higher stirring speed will generate smaller and more water droplets under the same water cut conditions, which has potential to form more severe obstacles hindering the wax molecules from diffusing toward the coldfinger surface and results in the increasing apparent viscosity of emulsions (as shown in Figure 7), consequently diminishing the diffusion coefficient of wax molecules according to the classical Fick’s mass diffusion law. In addition, it can be seen that the wax deposition rate decreases sharply when the stirring speed varies from 400 to 800 rpm and the wax deposition rate decreases slightly when the stirring speed varies from 800 to 1200 rpm. This is because the distinction of the droplet size and distribution in the emulsions prepared with the stirring speeds of 800 and 1200 rpm is relatively smaller than that with the stirring speeds of 400 and 800 rpm. Effect of the Water Cut and the Coldfinger Temperature. Figure 8 shows the droplet size and distribution and cumulative frequency curves with the stirring speed of 400, 800, and 1200 rpm. From the results in Figure 8, it can be seen that the amount of relatively smaller droplets in the dispersed phase decreases and the amount of larger droplets increases as the volume fraction of dispersed phase (i.e., water cut) increases under the same stirring speed conditions. The cumulative frequency for droplet size in Figure 8 is defined as the ratio of the amount of droplets less than a certain diameter to the total droplets count, from which it can be seen more clearly that the amount of smaller droplets
decreases and the amount of relatively large droplets increases with the increasing water cut. This is because the energy provided by a constant stirring speed is the same for the emulsions with different water cuts, which is able to disperse the water phase into lots of smaller droplets under the condition of the relatively low water cuts. Accordingly, more energy will be required to disperse the water phase with the increasing water cut. However, because of the constant energy provided by the constant stirring speed, it is reasonable that the dispersion degree at high water cut is lower than that at a low water cut. The wax deposition rate as a function of the water cut with the coldfinger temperatures of 35 and 32.5 °C is shown in Figure 9. The results show that, under the same water cut conditions, the wax deposition rate increases with the decrease in the temperature of the coldfinger surface (i.e., the increase in the temperature difference between the emulsion and the coldfinger surface), obviously, which is in agreement with the variation trend of wax deposition in a single phase (i.e., oil) obtained by previous researchers. The reason is that both the radial temperature gradient and the concentration gradient with respect to the temperature increase as the temperature of the coldfinger surface decreases under the constant emulsion temperature conditions, which will result in a higher wax deposition rate according to the classical Fick’s mass diffusion law. In addition, the results in Figure 9 also show another important issue that the wax deposition rate in W/O emulsions 1151
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Figure 8. Droplet size and distribution and cumulative frequency curves for emulsions with the stirring speeds of (a) 400 rpm, (b) 800 rpm, and (c) 1200 rpm.
decreases almost linearly with the increase in the water cut. From the combination of the results in Figures 8 and 9, in other words, the fact that under the same stirring speed conditions the amount of relatively smaller droplets in the dispersed phase decreases and the amount of larger droplets increases as the volume fraction of the dispersed phase increases will lead to the decrease in the wax deposition rate. The explanation for this phenomenon may be as follows: (1) The diffusion coefficient of wax molecules will be diminished with the increasing apparent viscosity of emulsions caused by the increasing water cut (as shown in Figure10),
consequently, which leads to the decrease in the wax deposition rate. (2) A larger volume fraction of the water phase will generate more water droplets, consequently slowering the time that wax moves and deposits onto the coldfinger surface by distorting the diffusive path of wax molecules toward the coldfinger surface, which ultimately lead to a lower wax deposition rate. (3) For all tests, the total volume of each emulsion was maintained constant (300 mL). Hence, the volume of the oil phase will decrease with the increasing water cut, which will lead to a lower amount of wax molecules available for deposition (i.e., the wax deposition 1152
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Figure 9. Comparison of the deposition rate versus the water cut with the coldfinger temperatures of 35 and 32.5 °C at the stirring speeds of (a) 400 rpm, (b) 800 rpm, and (c) 1200 rpm.
Figure 10. Apparent viscosity versus the temperature at a shear rate of 500 s-1 for emulsions prepared with the stirring speed of 800 rpm.
Figure 11. Comparison of the wax deposition rate between methods A and B.
rate will decrease). A further study on this aspect has been shown in the Effect of the Experimental Method section. It is noted that, in this study, the emulsion presence is considered to have a negligible effect on the solid-liquid behavior of waxes. A reasonable interpretation is as follows: Because of the negligible solubility of wax molecules in the dispersed water phase, wax molecules existing in water-inwaxy crude oil emulsion are recognized to diffuse toward the surface of the coldfinger only through the continuous phase (i.e., oil phase). For the water-in-oil emulsion under the static conditions, the liquid phase that is in contact with the
liquid-solid interface is practically the oil phase because of the characteristic of the continuous phase. Hence, the solidliquid behavior of waxes may not be affected for the waterin-oil emulsion. Effect of the Experimental Method. All tests mentioned above were conducted on the premise of the constant volume of emulsions (300 mL), the experimental method of which is named method A here. Method A drew the conclusion that the mass of deposits decreased with an increasing water content for the same temperature difference by means of the coldfinger experiments. However, method A was con1153
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ducted with emulsions of constant total volume. Obviously, the volume of the oil phase available for the tests was reduced as the water phase increased because of the constant total volume of emulsion, causing less amount of wax available for deposition. Therefore, it is difficult to determine whether the increasing water content was responsible for the decreasing amount of deposits for the same temperature difference. On the basis of the problem mentioned above, method B was designed instead of method A to further study the effect of the water content on wax deposition. The tests in method B were conducted with emulsions, in which the oil phase maintained a constant volume of 200 mL, regardless of the water content (i.e., the total volume of emulsion will increase with the increasing water cut). In addition, the temperature
condition in method B was selected as the same as that in method A with the heat bath temperature at 45 °C and the cold bath temperature at 35 °C. Because of the constant volume of the oil phase in method B, the total volume of emulsion increases with the increasing water content, leading to the increasing liquid height in contact with the coldfinger in the stainless-steel vessel (i.e., a larger surface area for deposition is offered), which has a positive effect on the wax deposition possibly leading to more deposits. To compare method B with method A, the wax deposition rate in method B had to be calculated on the basis of the mass of deposits and deposition area. Figure 11 shows the comparison of the wax deposition rate between methods A and B, from which it can be seen that there is almost no distinction in the wax deposition rate between the two experimental methods, which indicates that the decrease in the oil volume in method A because of the increasing water cut is not responsible for the decrease in the wax deposition rate mentioned in the Effect of the Water Cut and the Coldfinger Temperature section, but the effect of the water cut accounts for the decreasing wax deposition rate. In addition, a problem should be noted that, because the volume of emulsion in method B is not the same as that in method A, the distribution of water droplets in emulsion may be affected when the same stirring speed is applied under the same water cut conditions. However, it can be seen from
Table 1. Significance Analyses of Influencial Factors in W/O Emulsion influence factor
F value
F critical value (a = 0.01)
significancea
coldfinger temperature (°C) water cut (%) stirring speed (rpm)
1581.97
8.4
//
5.18 6.11
// //
93.44 82.71
a Note that / represents significance and // represents high significance.
Figure 12. Cumulative mass fraction (C17-C45) obtained by HTGC analyses as a function of the stirring speed with the water cuts of (a) 10%, (b) 20%, (c) 30%, and (d) 40% under the conditions of emulsion temperature of 45 °C and coldfinger temperature of 32.5 °C.
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: DOI:10.1021/ef901065c
Zhang et al.
the results in Figure 11 that, under the same stirring speed conditions, there is almost no distinction in the wax deposition rate between the two experimental methods even though the distribution of water droplets is not completely identical because of the different emulsion volumes. The possible explanation may be as follows: The tests in method B were conducted with emulsions in which the oil phase was maintained at a constant volume of 200 mL, regardless of the water content; therefore, under the condition of the water cuts of 10, 20, 30, and 40%, the total volume of emulsion in method B will be 222, 250, 286, and 333 mL, respectively. In comparison to the constant emulsion volume of 300 mL in method A, the variation degree of emulsion volume in method B may not be large enough to cause a significant change in the size and distribution of water droplets under the same stirring speed conditions. Therefore, the wax deposition rate in method B is almost identical to that in method A under the same stirring speed and water cut conditions. Statistical Data Analysis. Wax deposition in W/O emulsion is a complex process, especially affected by the cold environmental temperature (i.e., the coldfinger temperature), water cut, and droplet size and distribution of the dispersed phase, which is subject to the stirring speed during the course of emulsion preparations. However, the influence degree of each influencial factor is not the same for the wax deposition rate in W/O emulsion. In this section, 24 test results referring to different test conditions have been analyzed statistically. The significance analyses of influencial factors including the coldfinger temperature, water cut, and stirring speed have been accomplished using the F-test method from the data obtained from variance analysis. The results in Table 1 show that the three influencial factors studied above are all deemed highly significant by comparing each F value with a corresponding F critical value. However, it can be seen that the influence degree of the coldfinger temperature on wax deposition is bigger than that of the other two influencial factors because of the larger difference value between its F value and corresponding F critical value. By analogy, the influence degree on wax deposition in order is the coldfinger temperature, water cut, and the droplet size and distribution, which is subject to the stirring speed during the course of emulsion preparations. Wax Content in Deposits. Wax constituents existing in the oil used in the experiments mainly consist of n- and iso-alkanes within the range of carbon numbers from 17 to 45. The samples of wax deposits collected from the coldfinger experiments were analyzed using HTGC. The analyses provide information on the percentage of wax in the deposits, which is crucial to predict the amount of deposit by developing a reliable model. The results in Figure 12 show the cumulative mass fraction (C17-C45) obtained by HTGC analyses as a function of the stirring speed with the water cut of 10, 20, 30, and 40%, respectively, under the condition of the emulsion temperature of 45 °C and coldfinger temperature of 32.5 °C. Note that the contents of iso-alkanes, n-alkanes, and total wax in the deposit are represented in the legend as IC%, NC%, and C%, respectively. It is found that the contents of iso-alkanes, n-alkanes, and total wax in the deposits are independent of the stirring speed at the same water cut. Likewise, the results in Figure 12 also show that the water cut at the same stirring speed has no effect on the contents of iso-alkanes, n-alkanes, and total wax in the deposits either. In other words, the droplet diameter and distribution of dispersed phase of W/O emulsion have a negligible effect on the wax content in the deposits, which should be of significant interest to be incorporated into a future prediction model.
Conclusion During the course of emulsion preparations, different droplet sizes and distributions can be generated by varying the mixing speed of the stirrer even under the same water cut conditions. The results show that the wax deposition rate will decrease as the stirring speed increases under the same water cut conditions; i.e., wax deposition rate decreases with the decreasing droplet diameters of the dispersed phase and the resulting increasing amount of smaller droplets under the same water cut conditions. Under the same stirring speed conditions, the amount of relatively smaller droplets in the dispersed phase will decrease and the amount of larger droplets increases as the volume fraction of the dispersed phase (i.e., water cut) increases. The results show that the wax deposition rate in W/O emulsions decreases almost linearly with the increase in the water cut. In other words, the fact that under the same stirring speed conditions the amount of relatively smaller droplets in the dispersed phase decreases and the amount of larger droplets increases as the volume fraction of dispersed phase increases will lead to the decrease in the wax deposition rate. Furthermore, the wax deposition rate increases with the decrease in the temperature of coldfinger surface, which is obviously in agreement with the variation trend of wax deposition in single oil obtained by previous researchers. Two methods of coldfinger experiments were carried out and compared to further study the effect of the water content on wax deposition. The result indicates that the decrease in the oil volume in method A because of the increasing water cut is not responsible for the decrease in the wax deposition rate but the single effect of the water cut accounts for the decreasing wax deposition rate. Significance analyses of influencial factors have been accomplished using the F-test method from the data obtained from variance analysis of the 24 test results, which show that the influence degree on wax deposition in order for water-inwaxy crude oil emulsion is the coldfinger temperature, water cut, and the droplet size and distribution. The samples of wax deposits collected from the coldfinger experiments were analyzed using HTGC. The results show that the droplet diameter and distribution of the dispersed phase of W/O emulsion have a negligible effect on the contents of iso-alkanes, n-alkanes, and total wax in the deposits, which should be of significant interest to be incorporated into a reliable prediction model to be developed in the future. However, a problem should also be noted that there still exists the limitation that the coldfinger experiments may not represent the actual flow conditions because the coldfinger experiments were carried out just under the static conditions, where the effects of flow regime and shear stripping are not reflected. To apply the results of coldfinger to the field through simulations, the effects of the flow regime and shear stripping will have to be considered by conducting the flow loop experiment. Even so, the results of this study provide a reference and an insight for the further study on the two-phase oil-water wax deposition in flow loop and actual pipelines and should be of significant interest to develop a reliable prediction model in the future. Acknowledgment. The authors thank the financial support from the National Hi-Tech Research and Development Program (863 Program, Grant 2006AA09Z357) and the Key National Science and Technology Specific Project (2008ZX05000-026004). 1155
