Energy & Fuels 2000, 14, 11-13
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Mitigation of Fouling in Bitumen Furnaces by Pigging Richard J. Parker* National Centre for Upgrading Technology, Devon, Alberta, Canada, T9G 1A8
Richard A. McFarlane Alberta Research Council, Edmonton, Alberta, Canada, T6N 1E4 Received May 27, 1999. Revised Manuscript Received September 17, 1999
Processing of Athabasca oil sands bitumen requires preheating of the feed to temperatures in the range of 350-500 °C. At these temperatures significant amounts of solid foulant can be deposited on the walls of the furnaces. Ultimately, buildup of the deposit causes premature shutdown of the process, either because of excessively high tube skin temperatures or because of high pressure drop across the furnace. Steam/air de-coking, which has been practiced in the past, is unable to eliminate all the deposit since the foulant contains significant quantities of noncarbonaceous material. Currently, the preferred method to address this problem is pigging assisted by high-pressure water. The feed to the furnaces typically contains close to 0.5 wt % inorganic solids originating from the mineral matrix from which the bitumen is extracted. Elemental analysis of the feed identifies 300-450 ppm iron, much of which can be attributed to the components of the inorganic matrix, such as pyrite, iron carbonate, and iron-bearing clays. However, it is expected that soluble organic iron species, such as naphthenates and carboxylates, are more likely to decompose at the temperatures in the furnaces and to convert to iron sulfide and deposit within the furnace. Samples of foulant were collected from two furnace pigging operations, one at the Suncor Energy Inc. coker furnace and one at the Syncrude Canada Ltd. LC Finer furnace. The samples were taken at timed intervals that allowed a profile of the foulant within the cross-section of the furnace tube to be completed. After washing and drying, the foulant samples were analyzed for carbon, sulfur, and a variety of metals and inspected by optical petrography. Coker furnace foulants exhibited high concentrations of both carbonaceous material and iron sulfide. The iron sulfide concentration was highest at the wall and declined as the distance from the wall increased. The carbon content showed the reverse trend, suggesting that coke formation became the more important factor as the deposition process proceeded. This reverse trend coincided with the increase in tube skin temperature. Fluid and skin temperatures are much lower in the LC Finer furnace than in the coker furnace; therefore, the tendency to coke formation is reduced. As a result, the composition of the foulant from the LC Finer furnace was more constant and consisted almost entirely of iron sulfide. It was estimated that the amount of iron deposited as foulant corresponded to about 0.05 wt % of the total iron in the feed that passed through these furnaces.
Introduction Fouling of furnaces has been shown to be a common occurrence in a variety of refinery processes.1,2 For the most part, the foulants have been shown to be carbonaceous and often derived from asphaltenes present in the oil resid.3,4 High concentrations of vanadium and nickel are often associated with the asphaltenes and are also contained in the foulants. A “dryout” mechanism can explain the deposition5 of solid particles from liquid * Corresponding author. (1) Lemke, H. K.; Stephenson, W. K. Proceedings of the A. I. Ch. E. 1997 Spring National Meeting; Houston, Texas, March 1997. (2) Bannayan, M. A.; Lemke, H. K.; Stephenson, W. K. Catalysts in Petroleum Refining and Petrochemical Industries, 1995; Absi-Halibi, M., et al., Eds.; Elsevier: New York, 1996; pp 273-281. (3) Wiehe, I. A. A Phase-Separation Kinetic Model for Coke Formation. Ind. Eng. Chem. Res. 1993, 32, pp 2447-2454. (4) Storm, D. A.; Decanio, S. J.; Edwards, J. C.; Sheu, E. Y. Sediment Formation during Heavy Oil Upgrading. Pet. Sci. Tech. 1997, 15 (1&2), 77-102.
streams. A major component in some foulants is iron sulfide, yet iron is present at much lower concentration in the feed than either vanadium or nickel. Recently, Panchal6 proposed a mechanism to account for iron sulfide formation that offers oil soluble iron species, such as naphthenates, as the source of iron. These oil soluble salts decompose at furnace temperatures and subsequently react with sulfur compounds either present in or derived from the resid. Athabasca bitumen is recovered from a sand matrix by mining, followed by extraction with hot water. These processes leave considerable quantities, typically 0.5 wt %, of the inorganic constituents from the mineral matrix in the extracted bitumen. Subsequently, the bitumen (5) Perera, W. G.; Rafique, K. Coking in a Fixed Heater. The Chemical Engineer, Feb. 1976, 107-111. (6) Panchal, C. B.; Halpern, Y.; Kuru, W. C.; Miller, G. Understanding Heat Exchanger Fouling and Its Mitigation. Proceedings of the Engineering Foundation Conference; Lucca, Italy, May 1997.
10.1021/ef990105r CCC: $19.00 © 2000 American Chemical Society Published on Web 11/24/1999
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Energy & Fuels, Vol. 14, No. 1, 2000
is converted into a synthetic crude oil by delayed coking, fluid coking, or LC Fining. Both the delayed coking operation at Suncor Energy Inc. and the LC Fining plant at Syncrude Canada Ltd. have experienced shutdowns because of the deposition of foulants within the furnaces leading to the respective processes. Temperatures are raised to almost 500 °C in the coking furnaces, and the feed is at or above coking conditions for some of this time. Over a period of several weeks, foulant builds up in the furnaces and forms an insulating layer. Consequently, the furnace skin temperature must be increased to ensure that the exit feed temperature is maintained. This process continues until the temperature limit of the furnace tube metal is reached. Temperatures in the LC Finer furnace are more moderate. Currently, the feed exit temperature is held below 350 °C, which is less than the coking temperature of the bitumen, and does not need to be raised before a shutdown for routine maintenance. There have been shutdowns in the past as the result of an increase in ∆P across the furnace. This problem was more severe when feed exit temperatures were closer to 370 °C. Both Suncor and Syncrude employ contractors to remove foulants from their furnaces by a pigging procedure. The following paper describes the collection of foulant samples during two such operations. The foulants were subjected to chemical and optical characterization. The nature of the foulants provided insights into the physical and chemical processes by which deposition occurred. Experimental Section Solid foulants were separated from aqueous effluent streams during pigging of two coker furnaces. The pigging operation cleaned out the two cells of the furnace simultaneously but independently. Samples were collected hourly until the effluent water was free of solids larger than approximately 0.5 mm. Solids were washed with water and stored in plastic bags. Samples for analysis were dried at 50 °C. Particle size of the solids from the LC Finer furnace was much smaller than from the coker furnace. Samples of the water stream were collected in a 5-L bucket. The solids were concentrated by decantation into a small plastic bottle. After further settling and decantation, the solids were washed with tetrahydrofuran to remove water and oil, and then dried. Both sets of dried samples were then subjected to elemental analysis. Sub-samples were ashed and the residue dissolved in acid prior to analysis for metals by ion-coupled plasma. Petrographic examination performed on these samples has been reported elsewhere.7
Results and Discussion Typical properties of the Athabasca bitumen fed to Suncor and Syncrude furnaces are given in Table 1. The bitumen has low API gravity, extremely high viscosity, a low hydrogen-to-carbon ratio, and a high content of heteroatoms and metals. The asphaltenes are a likely source of coke during processing. The ash and metals are derived from two sources: (a) soluble vanadium and (7) Gentzis, T.; Parker, R. J.; McFarlane, R. A. Proceedings of the 216th ACS National Meeting; Boston, Massachusetts, August 1998.
Parker and McFarlane Table 1. Properties of Athabasca Bitumen API gravity
7.7
° API
viscosity @25 °C carbon hydrogen nitrogen sulfur asphaltenes MCR resid 524+
166000 83.0 10.2 0.47 5.05 18.8 13.4 53.0
cps wt % wt % wt % wt % wt % wt % wt %
tolueneinsoluble ash iron vanadium nickel silicon aluminum calcium
0.28
wt %
0.47 336 230 84 567 530 77
wt % ppm ppm ppm ppm ppm ppm
Table 2. Properties of Foulant from Coker Furnace time C H S Fe V Ni Ca Al Si (h) wt % wt % wt % wt % wt % wt % wt % wt % wt % 0.00 0.42 1.00 1.58 4.25 5.25 10.45 11.45 15.45
58.4 53.8 57.2 53.7 52.1 53.9 37.0 51.6 37.3
2.3 2.7 1.8 1.7 1.4 1.4 1.3 1.4 1.1
12.9 13.1 14.5 15.4 15.8 14.4 21.0 16.2 19.7
12.2 15.8 16.8 19.3 20.3 19.3 29.2 20.1 26.4
0.15 0.15 0.17 0.15 0.16 0.15 0.09 0.14 0.09
0.17 0.15 0.14 0.13 0.15 0.13 0.23 0.19 0.38
0.24 0.22 0.21 0.20 0.22 0.18 0.20 0.22 0.16
1.56 1.33 0.99 0.88 0.90 0.71 0.54 0.80 0.56
2.29 2.00 1.54 1.37 1.42 1.15 0.94 1.31 0.93
Figure 1. Coker furnace foulant: major elements.
Figure 2. Coker furnace foulant: minor elements.
nickel porphyrins inherent to the oil, and (b) clays that remained in the feed after extraction of the oil sand. Complete analysis was performed on selected foulant samples from the south side tubes of Suncor furnace 5F4 (Table 2). Several trends can be discerned from the data. First, the major components of the foulant are carbon, sulfur, and iron, and second, during pigging there is an increase in the iron and sulfur which coincides with a decrease in the carbon (Figure 1). The minor components, defined as hydrogen, silicon, aluminum, and calcium, follow the declining trend observed for carbon (Figure 2). The data are plotted as a function of the time that the sample was collected which means that the last samples to be collected were removed from close to or at the wall of the furnace tubes. Conversely, the early samples were dislodged from large foulant nodules that projected into the center of the tube. The
Mitigation of Fouling in Bitumen Furnaces by Pigging
presence of these nodules could be discerned at certain points in the furnace by changes in ∆P during passage of the pigs through the tubes. The highest localized concentration of foulant was indicated by pressure drop to be at approximately tubes 8-10 (there are a total of 29 tubes in the radiant section of the furnace). Also, during the initial pigging operation, a small pig was employed which would only remove the thicker foulant deposits. As the foulant layer became progressively thinner, larger pigs were needed to ensure complete removal of all of the deposits. The foulants at the tube surface contained the highest concentrations of iron and sulfur. If we assume that the sulfur content of the coke component of the foulant is 6 wt %, then the Fe:S ratio is essentially a constant at 1:1 which corresponds to pyrrhotite. The carbon or coke content of the foulant was at a maximum farthest from the tube wall, that is, closest to the bulk fluid flow. The ratio of carbon to hydrogen increased in deposits closer to the wall. This increase is consistent both with a higher skin temperature at the initial point of formation and a loss of hydrogen since the coke at the wall has spent a longer period at an elevated temperature. Petrographic examination confirmed both these scenarios. Coke at the wall was compacted and more structured, while near the bulk it was looser and amorphous. Calcium, aluminum, and silicon levels confirmed the presence of clay minerals in the foulant. All these elements increased toward the center of the tube and at a relatively constant ratio. As described earlier, minerals are initially present in the feed stream at about 0.5 wt % or 0.2 vol %. Together, the clay mineral content of the feed exceeds that of the iron by severalfold. If the mechanism for the deposition of the foulant was one that relied on adhesion of entrained solid at dry spots at the tube surface, a higher clay mineral content would be expected. Certainly, flow conditions in the furnace tubes would be predicted to promote the dry spot mechanism since flashing of part of the feed will occur as the feed temperature increases from 250 to 500 °C. Petrographic examination of the coke showed that it had an open structure near the bulk fluid and was more porous. Clays were trapped within this structure. The metals derived from the organic components of the feed showed differing trends in their relative proportions of the foulants. The vanadium was relatively constant at about 1500 ppm. This value is close to the one expected if all the vanadium is concentrated in the asphaltenic (coke forming) components of the bitumen feed. The nickel content of the feed was about one-fourth of the vanadium content, yet the Ni:V ratio generally exceeded one. If the nickel porphyrins are more reactive than the vanadium porphyrins, nickel could concentrate in the foulant. Alternatively, nickel is a component of both the furnace tube steel and other vessels, and there is a possibility that the nickel may have originated from these sources. In summary, we might envision the following sequence of events for the deposition of the foulant. Initially, iron sulfide is produced in the bulk liquid bitumen, part of which deposits on the tube wall. Mesophase or coke precursors attach onto this surface,
Energy & Fuels, Vol. 14, No. 1, 2000 13 Table 3. Properties of Foulant from LC Finer Furnace C H S Fe V Ni Ca Al Si sample wt % wt % wt % wt % wt % wt % wt % wt % wt % SF-1 SF-4 SF-8 SF-12 SF-16 SF-19 SF-22
8.3 6.3 1.0 1.8 3.4 2.6 3.8
0.60 0.50 0 0 0 0 0
29.3 32.0 36.5 31.1 26.2 24.3 23.3
47.1 54.4 59.9 57.0 52.7 53.8 50.2
0.03 0.02 0.02 0.02 0.02 0.02 0.02
0.06 0.04 0.06 0.05 0.08 0.07 0.07
0.11 0.06 0.47 0.05 0.09 0.11 0.12
0.82 0.50 0.35 0.35 0.38 0.34 0.35
1.14 0.69 0.56 0.57 0.68 0.74 0.73
and at very high skin temperatures (well in excess of 600 °C) coke is formed from these species. As the coke and iron sulfide deposit builds, the temperature at the interface between the oil and the deposit drops. To ensure that the feed temperature meets its target, the skin temperature must be raised further by burning more fuel, thus accelerating coke formation. Farthest from the wall, the coke is deposited later, and this tendency is confirmed by the lower reflectance in oil which is indicative of a lower temperature of formation. Solids are therefore more easily entrapped in the resulting porous structure. The appearance and nature of the foulant recovered from the pigging of the LC Finer were quite different compared with the coker furnace (Table 3). The most abundant elements, by far, were iron and sulfur. In contrast to the coker furnace, carbon was virtually nonexistent in the deposit. The concentration of iron sulfide was greatest at the wall. The iron-to-sulfur ratio is also close to the 1:1 ratio found for the coker furnace. Clay elements were much less than iron despite their greater abundance in the feed. Again the Ni:V ratio is well above that found in the feed and reinforce the argument that nickel porphyrins are more reactive under the process conditions. Conditions in the LC Finer furnace were milder than those found in the coker furnace. The sequence of events leading to the deposition of iron sulfide remained the same albeit at a slower rate because of lower bulk and skin temperatures. Petrographic examination indicated that there was some coke which had granular or mosaic anisotropy and was apparently cemented to the iron sulfide particles. The total amount of solid mineral matter and soluble iron compounds which passed through both the coker and LC Finer furnaces was enormous compared with the quantity removed during pigging. An estimate for the LC Finer furnace suggests that only 0.2 ppm iron from the feed was deposited, corresponding to less than 0.05 wt % of the total iron passed. Athabasca bitumen recovered by nonmining methods has little mineral content and less than 10 ppm iron. Yet, this amount would still be sufficient to form iron sulfide deposits in the quantities found in this study. Acknowledgment. The authors acknowledge the financial support received from Suncor Energy Inc., Syncrude Canada Ltd., and the National Centre for Upgrading Technology, and thank them for permission to publish these findings. EF990105R
