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Energy & Fuels 2005, 19, 1342-1345
Synergies between Asphaltene Stabilizers and Demulsifying Agents Giving Improved Demulsification of Asphaltene-Stabilized Emulsions Joseph L. Stark* and Samuel Asomaning Baker Petrolite, 12645 West Airport Boulevard, Sugar Land, Texas 77478 Received August 6, 2004. Revised Manuscript Received December 6, 2004
Asphaltenes are known to stabilize water-in-oil emulsions, making it more difficult to demulsify and desalt crude oils. Insufficiently demulsified or desalted crude oil can lead to process upsets in the operation of desalters and separator vessels. Asphaltene stability and the methods used in predicting them have been correlated to the ease of demulsification. Two case studies are presented that show the effects of chemical demulsifiers used alone and in combination with asphaltene stabilizers on the efficiency of demulsification operations. The data shows that the asphaltene stabilizer-demulsifier combination does a better job at demulsifying crude oils with asphaltene-stabilized emulsions than when demulsifiers are used alone. While the data shows the action of the asphaltene stabilizer-demulsifier combination is crude specific, a careful choice of such combinations through initial testing can go a long way at solving troublesome asphaltenestabilized emulsion problems in both refineries and oilfield production operations.
1. Introduction One of the first steps in processing crude oil is demulsification, which is the process of breaking unwanted emulsions and separating water from the oil.1 Production operations use separators to break emulsions to increase the value of the oil. Refineries send the crude oil to desalters to remove undesirables from the oil. The desalting process is to mix wash water with the oil in order to remove excess salts, metals, and solids to the effluent water and send the upgraded oil to the distilling section. Asphaltenes are known to stabilize water-inoil emulsions, which leads to increased problems in demulsification.2,3 Asphaltenes can cause upsets in demulsification in a number of ways, including increased oil in the water, water in the oil, precipitation of asphaltenes in the separators or observation of increased solids in the effluent water for desalters, increased current draw for desalters, and increased rag layer. Poor desalting due to asphaltene-stabilized emulsions will lead to increased salts being carried to other areas in the refinery. The salt carryover from the desalter causes problems, such as an increase in tower overhead corrosion, heat exchanger fouling, shortened coker furnace run lengths, and catalyst poisoning. Poor demulsification can lead to water quality problems in * To whom correspondence should be addressed. Phone: (281) 2765827. Fax: 281-276-5491. E-mail:
[email protected]. (1) The Condensed Chemical Dictionary, 8th ed.; Revised by Hawley, G. G.; Van Norstrand Reinhold Co.: New York, 1971. (2) Sjoblom, J.; Saether, O.; Midttun, O.; Ese, M.-H.; Urdal, O.; Fordedal, H. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998. (3) McLean, J. D.; Kilpatrick, P. K. J. Colloid. Interface Sci. 1997, 196, 23.
production separators, resulting in noncompliance with environmental agencies, reduced throughput, and lower valued oil. Asphaltenes are generally considered to be the highest molecular weight component of a crude oil. They are composed of condensed aromatics and heteroatoms such as sulfur, nitrogen, oxygen, nickel, and vanadium. In crude oils asphaltenes are believed to exist as a colloidal suspension made of micelles of an asphaltene core surrounded by resins.4,5 When separated from the oil by the addition of a nonpolar solvent asphaltenes are dark brown to black solids. By definition, asphaltenes are a solubility class; it is the portion of an oil or bitumen that is insoluble in heptane but soluble in benzene or toluene.6,7,8 In an asphaltene-stabilized emulsion the asphaltenes are believed to exist as a rigid, cross-linked network absorbed from the oil to the oil-water interface.9 The high stability of the emulsions is believed to be due to strong interfacial films formed by the asphaltene networks.10,11 It had also been reported that asphaltenes, at their limit of solubility in the oil, tend to readily form (4) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker: New York, 1991. (5) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1978. (6) Mushrush, G. W.; Speight, J. G. Petroleum Products: Instability & Incompatibility; Taylor & Francis, Washington, D.C., 1995. (7) Am. Soc. Test. Mater., Book ASTM Stand, 1975, 24. (8) Speight, J. G.; Moschopedis, S. E. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; Advances in Chemistry 195; American Chemical Society: Washington, D.C., 1981. (9) Kilpatrick, P. K.; Spiecker, M. P. In Encyclopedic Handbook of Emulsion Technology; Sjoblom, J., Ed.; Marcel Dekker: New York, 2001. (10) Acevedo, S.; Escobar, G.; Ranaudo, M. A.; Khazen, J.; Borges, B.; Pereira, J. C.; Me´ndez, B. Energy Fuels 1999, 13, 333. (11) Sams, G. W.; Zaouk, M. Energy Fuels 2000, 14, 31.
10.1021/ef0498014 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/12/2005
Asphaltene Stabilizers and Demulsifying Agents
strong interfacial films, giving the most stable emulsions.9 Stabilization of the asphaltenes in the oil will increase the ease of demulsification.12 Crude oils with stable asphaltenes are desirable, while crude oils with stable emulsions are undesirable. Testing has shown that unstable asphaltenes can cause stabilized emulsions. Efforts to predict asphaltene stability in crude oil have helped to correlate ease of demulsification in desalter operations.13 Given the complexity of crude oils many methods have been developed to predict their stability. The ASITSM (asphaltene stability index test) is one such methods to predict the stability of asphaltenes in crude oil and crude blends.14 (SM-ASITSM is a service mark of Baker Hughes Inc.). In this study a combination of the ASITSM, a laboratory bottle test, and the EDDA (electrostatic desalting/dehydration apparatus test used to simulate the desalting process in refineries) were used to show that improved demulsification is obtained for asphaltene-stabilized emulsions by using an asphaltene stabilizer in combination with demulsifiers. 2. Experimental Section 2.1. Crude Oil Stability Measurements. The ASITSM measures the stability of asphaltenes in crude oils via determination of the onset of the asphaltene flocculation point using a solvent titration method. The onset measurement is done in the asphaltene precipitation detection unit (APDU), which has a solids detection system that uses a near-infrared (NIR) laser to determine the onset of asphaltene flocculation. A sample of oil is heated to a fixed temperature and allowed to come to equilibrium. An asphaltene precipitant is then titrated into the solution at a constant rate and the transmittance of the NIR laser monitored. In the initial stages of titration the transmittance of the laser increases due to the decrease in density of the solution resulting from the addition of nonsolvent. When the asphaltenes begin to flocculate the laser transmittance will decrease, resulting in an inflection in a plot of transmittance versus volume of added nonsolvent. The point of inflection, expressed as an indexsthe asphaltene stability index (ASI)scorresponds to the point of asphaltene precipitation and provides a relative measure of how stable the asphaltenes in the oil are. Oils with high fouling potential have been found to have ASI values between 0 and 100. Oils with low fouling potential have been found to have ASI values of 250 or higher, and marginally fouling oils have been found to have ASI values of between 100 and 250. Fouling potential is related to asphaltene stability. As the asphaltene stability decreases the fouling potential increases. Crude oils with lower asphaltene stability have higher fouling potential. Thus, there appears to be a correlation between the fouling potential of the crude blend, the ASI, and emulsion stability. Since kinetics play an important role in the flocculation and aggregation of asphaltenes, continuous titration was done at a slow rate, and this rate was used in all the experiments carried out for this work. 2.2. Bottle Test. For the bottle test if a sample of emulsified oil is not available one is prepared by adding produced water or some other synthetically prepared brine to oil and mixing with a blender until the oil is well emulsified. Ten milliliters of the well-mixed emulsion is added to a 0.5 oz. prescription bottle. The chemical to be tested is added at a concentration of 500 ppm. A control sample without chemical is also (12) Lissant, K. J.; Demulsification Industrial Applications; Marcel Dekker: New York, 1983. (13) Stark, J. L.; Kremer, L. N.; Nguyen, J. Oil Gas J. 2002, 89. (14) Stark, J. L.; Asomaning, S. Petroleum Sci. Technol. 2003, 21, 569.
Energy & Fuels, Vol. 19, No. 4, 2005 1343 prepared. These samples are mechanically shaken at a low rpm (about 120 cycles/min). After shaking the samples are allowed to stand at room temperature and the water drop over a period of time (usually 5 min) is recorded. The basic sediment and water (BS&W the sum of free water and sediment in a production sample) and the quality of the water are also recorded. 2.3. Desalter Simulation. The desalting process was simulated in the laboratory using the following EDDA procedure. A Waring blender was used to mix the crude oil with 7% wash water in the oil to simulate the mix-valve action ahead of the desalter. The emulsion was then placed in graduated 100 mL glass test cells that contain a special insulated cap having two metal electrodes. The tubes were placed in a heated block, and high-voltage leads were connected to the tube’s electrodes so that an electrical field could be applied to the test cells. The high-voltage power supply is variable and can impress an electrical field equivalent to that found in desalting units. The cells are energized, and the rate at which water is settled in the test cells is noted. Relative chemical demulsifier performance is determined by the speed and amount of water released based upon 100 mL of blended sample and by the sharpness of the water and oil interface. Chemical additives are generally added at a dosage between 5 and 25 ppm. After the test cells have been energized for 2030 min, the current is turned off, the caps removed, and a syringe fitted with a long needle is used to extract a top aliquot of desalted crude. The top cut is centrifuged to determine the degree of dehydration expressed as percent BS&W.
3. Results and Discussion 3.1. Case History 1. In the first example a refinery on the West Cost experienced excessive oil undercarry in the desalter effluent water while running a crude oil charge blended with top pump around. Stabilized emulsions could not be completely resolved in the desalter, and large amounts of oil were going out the bottom with the separated water. The refinery occasionally had to reduce crude oil charge due to the high oil undercarry. It was suspected that unstable asphaltenes were stabilizing the emulsion and precipitating in the desalter. Figure 1 shows the results from the ASITSM of the blended feed, the blended feed with top pump around, and the combined blend with an asphaltene stabilizer. On the basis of the results it was determined that the blended feed was not compatible with the top pump around, causing asphaltene destabilization, which in turn promoted the stable emulsions in the desalter. It can also be seen that the addition of an asphaltene stabilizer greatly increased the stability of the oil. Often asphaltene stabilizers are injected upstream of hot preheat exchangers to mitigate fouling, but in this case the asphaltene stabilizer was added as a pretreatment at the tank farm to promote stability of the asphaltenes, helping to prevent stabilized emulsions and thereby improve desalter efficiency. To examine the impact of the asphaltene stabilizer with the demulsification chemical treatment already used on the desalter, EDDA testing was completed. The original demulsification program included a wetting agent, which is designed to preferentially wet solids so they will not stabilize the emulsion and go into the water phase. Figures 2 and 3 show the results of the EDDA testing of the blended oil with top pump around. Figure 2 shows that the asphaltene stabilizer alone does not appear to increase or inhibit the percent water drop. Figure 3
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Figure 1. Comparisons of the stability of the feed going to the desalter with and without top pump around and then with asphaltene stabilizer added. The least stable blend tested contains the top pump around. The middle curve is of the blended feed alone. The most stable curve is the blended feed with top pump around and asphaltene stabilizer.
Figure 2. Percent water drop from the desalter simulation test. The blank contains no additives to improve demulsification. The test with demulsifier and wetting agent does not contain asphaltene stabilizer and does not give acceptable performance in water drop. Acceptable performance is recovering all of the wash water added. The combination test has demulsifier, wetting agent, and asphaltene stabilizer added and gives acceptable performance.
indicates the addition of asphaltene stabilizer alone significantly reduces the percent BS&W, although still not as effectively as using it with the demulsifier. From the data obtained in Figures 2 and 3 good results were obtained when the asphaltene stabilizer was used in conjunction with the demulsifier program. The combination of asphaltene stabilizer and demulsification additives improves the rate of water drop and reduces the percent BS&W to an acceptable level. The refinery added asphaltene stabilizer to crude oil as it was transferred to storage tanks. The combination of pretreating with asphaltene stabilizer and the normal demulsifier program gave improved performance in the desalter. The rag layer was reduced, and the oil undercarry decreased by about 35%. The refinery also noted a 40% reduction in filterable solids in the oil leaving the desalter.
3.2. Case History 2. In the next example a West Coast offshore production facility was experiencing asphaltene precipitation and deposition in its threephase separators. Because of the sour nature of the oil and associated gas, the asphaltene deposition problem was complicated by the presence of iron sulfide in the deposits. It is known that iron sulfide makes the asphaltene deposits hard and very difficult to remediate. Precipitation of asphaltenes caused additional water quality problems in the production separators resulting in noncompliance with the Mineral Management Service’s (MMS), Department of Transportation’s (DOT), and Environmental Protection Agency’s (EPA) specifications for overboard water. Asphaltene deposition in the separators decreased throughput and resulted in deployment of an additional separator for the condensate stream.
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Figure 3. BS&W remaining in the oil after running the EDDA test. The addition of asphaltene stabilizer significantly reduces the BS&W. The combination contains demulsifier, wetting agent, and asphaltene stabilizer.
Figure 4. Comparison of the stability of the crude oil with and without asphaltene stabilizer.
Laboratory tests performed showed that addition of an asphaltene stabilizer improved the stability of the oil. Figure 4 shows the results of the ASITSM for treated and untreated oil. Bottle tests performed in the field showed that the combination of asphaltene stabilizer with emulsion breaker gave the best performance for reduced asphaltene precipitation. Laboratory testing indicated a 94% reduction in asphaltene deposits with the use of an asphaltene stabilizer. The tests also showed that the oil was dry, less than 3% BS&W. Addition of the asphaltene stabilizer in the field had an immediate and dramatic improvement in preventing asphaltene deposition and a drastic improvement in water quality. The improvement in water quality saved the customer money in the form of fines to governmental bodies. The separator for the condensate was shut down since the water quality specifications were met. Solving
the asphaltene deposition problem in the separators increased throughput. 4. Conclusions The ASITSM was used with other demulsification tests as a predictive tool for identifying asphaltenes as a contributory factor to emulsion stability. When destabilized asphaltenes contribute to stabilized emulsions, using asphaltene stabilizers in combination with demulsifiers shows improved performance in demulsification. A synergistic effect in using asphaltene stabilizers with demulsifiers is evident for both production and refinery operations. Acknowledgment. Our sincere thanks goes to the management of Baker Petrolite for permission to publish this article. EF0498014
