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Energy & Fuels 1999, 13, 346-350
Solids Associated with the Asphaltene Fraction of Oil Sands Bitumen L. S. Kotlyar,* B. D. Sparks, and J. R. Woods National Research Council of Canada, Institute for Chemical Process and Environmental Technology, Montreal Road Campus, Ottawa, Ontario, Canada K1A 0R6
K. H. Chung Syncrude Canada Ltd., Edmonton Research Centre, 9421 17th Avenue, Edmonton, Alberta, Canada T6N 1H4 Received September 29, 1998
Bitumen separated from Athabasca oil sands by the hot water extraction process (HWEP) contains residual salty water and inorganic solids. Because of their strong interaction with bitumen, the solids fraction has been designated bitumen associated solids, abbreviated as BS. The major constituent of BS is ultrafine, aluminosilicate, clay crystallites. There is a minor contribution from sulfur- and titanium-bearing minerals. The surfaces of BS particles are rendered asphaltene-like owing to the adsorption of polar, aromatic, toluene-insoluble organics. Most of these solids are removed when asphaltenes are precipitated during treatment of bitumen with solvents less polar in nature than the naphtha in current use. Owing to their bi-wettable surface characteristics, the BS are likely to occur in association with water droplets in the bitumen phase. These droplets, or clusters, have an asphaltene-like exterior and exist as a stable colloidal dispersion in the maltene component of bitumen. These factors explain the intractable nature of the water and solids remaining with bitumen after conventional froth treatment by dilution with aromatic naphtha followed by centrifugation. During fluid, or delayed coking of bitumen, most of the BS are removed with the coke. However, deposition of the carbon-rich solids may also contribute to fouling in reactor systems and catalyst deactivation in catalytic hydroprocessing. Also, ultrafine BS and salt particles may themselves become entrained in the volatile overhead liquids and cause corrosion and fouling in downstream process units. Dilution of bitumen froth with a less polar solvent than naphtha reduces the overall stability of asphaltene micelles in the maltene component of bitumen. Asphaltene then co-precipitates with the inorganic particles and water to form a “rag layer”. This process yields bitumen of excellent quality, in terms of solids and water content. However, in some cases, the product losses are unacceptable relative to the conventional froth treatment approach using centrifugation. It is expected that bitumen recovery can be improved by identifying a solvent, or solvent blend, capable of selective flocculation and precipitation of the clay-water clusters while precipitating only a minor amount of asphaltene. Preliminary results show that this approach is a realistic proposition.
Introduction The oil sands deposits of Northern Alberta contain about 1.3 trillion barrels of crude oil equivalent. The Athabasca is the largest of the four major deposits with nearly three-quarters of Canada’s oil sands reserves. About one-tenth of the Athabasca formation lies within 50 m of the surface; bitumen from this zone is economically recoverable by conventional surface mining techniques.1,2 Commercial oil sands mining plants currently use a modified hot water extraction process, based on principles developed in the 1920’s by Dr. K. A. Clark. In this process oil sands is conditioned with hot water, steam, (1) Hocking, M. B. The chemistry of oil recovery from bituminous sands. J. Chem. Educ. 1977, 54, 725. (2) Berkowitz, N.; Speight, J. G. The oil sands of Alberta. Fuel 1975, 54, 138.
and caustic to release the bitumen from the sand and other solids. Gravimetric separation and air floatation then produce bitumen froth, middlings, and coarse tailings. The froth contains significant amounts of water and solids that must be removed before bitumen upgrading. The conventional froth treatment procedure includes dilution with a partly aromatic naphtha followed by two stages of centrifugation. At this stage, the treated bitumen still contains ultrafine solids, BS, as well as water with dissolved salts. The BS are primarily ultrafine, aluminosilicate, clay crystallites. There is a minor contribution from sulfur- and titanium-bearing minerals. The surfaces of the particles are highly active and readily adsorb polar, aromatic, toluene-insoluble, organic components, similar to asphaltenes. Before bitumen upgrading, the naphtha diluent and other volatile components are removed by atmospheric topping at elevated temperature. The mineral solids and
10.1021/ef980204p CCC: $18.00 © 1999 American Chemical Society Published on Web 02/12/1999
Asphaltene Fraction of Oil Sands Bitumen
salt particles remain with the topped bitumen. During upgrading, some of these solids are entrained with volatile overheads and are carried over to other process units. The high chloride content of the salt residues causes corrosion and fouling problems in downstream upgrading units. Also, the BS component is a potential contributor to coke formation both in reactor vessels and on catalyst surfaces.3 Also, entrainment of BS is known to cause fouling in packed bed hydrotreaters, leading to excessive pressure build-up and premature shutdown of hydrotreater units.4 From a process point of view, it is desirable to remove these intractable components from the bitumen before it enters the upgrading plant. One potential approach is to substitute a less polar solvent for the naphtha diluent used in conventional froth treatment.5 This approach produces a clean bitumen layer and a so-called “rag layer” in which the rejected components are concentrated. A significant problem with this technique is that, depending on operating conditions, bitumen recovery, in a once through process, is only 85% compared to values of 95-98% achieved in the conventional naphtha dilution-centrifugation process. In order to overcome such process-related problems, it is critical to develop a better understanding of the nature of the BS component of bitumen. It is the purpose of this work to summarize and discuss the information available on the properties of BS and their role in bitumen quality and processing. Experimental Section The following samples were supplied by Syncrude Canada Ltd., Edmonton Research Centre: (1) A coker feed bitumen (bitumen A), obtained using conventional froth treatment with naphtha as a diluent. This material has been used as a standard in many round-robin bitumen characterization studies. (2) Samples of estuarine and marine transition oil sands with bitumen concentrations in the range 8.9-12.7 w/w % and fines contents falling between 10.3 and 22 w/w % of dry bitumen-free oil sands.6 (3) Four rag layer samples from bitumen froth treatment with less polar solvent.7,8 (4) An insoluble end-cut fraction from bitumen pitch separated by supercritical fluid extraction with pentane.9 Separation Schemes. A batch extraction unit (BEU)10 was used to separate bitumen from the three ores as primary (PFB) (3) Chan, E. W.; Chung, K. H.; Veljkovic, M.; Liu, J. K. Hydrodynamics and fines capture in packed bed hydrotreaters. Paper presented at the International Petroleum and Petrochemical Technology Symposium, Beijing, September 15-17, 1994. (4) Chung, K.; Xu, C.; Gray, M.; Zhao, Y.; Kotlyar, L.; Sparks, B. The chemistry, reactivity, and processability of Athabasca bitumen pitch. Rev. Process Chem. Eng. 1998, 1, 41-79. (5) Shelfantook, W. E.; Tipman, R. N. Effect of alternative diluents in froth treatment. Research paper presented at the Syncrude R&D Seminar on alternate diluents, Edmonton, November 1995. (6) O’Carroll, J.; Kotlyar, L. S.; Sparks, B. D.; Toll, F. N.; Hardacre, K.; Cuddy, G. Factors affecting bitumen recovery from oil sands. Part 3: Effect of lithological facies on bitumen froth quality; ICPET Internal Report No. PET-1429-98S, 1998. (7) Kotlyar, L. S.; Sparks, B. D.; Woods, J. R.; Raymond, S. Athabasca oil sands: Solids associated with toluene insoluble organic matter and rag layer formation; ICPET Internal Report No. PET-140997S, 1997. (8) Kotlyar, L. S.; Woods, J. R.; Sparks, B. D.; LePage, Y.; L’Hostis, D. L. Characterization of the rag layer formed during addition of paraffinic solvents to bitumen froth from the hot water extraction process; ICPET Internal Report No. PET-1370-96S, 1996. (9) Chung, K.; Xu, C.; Hu, Y.; Wang, R. Supercritical fluid extraction reveals resid properties. Oil Gas J. 1997. (10) Syncrude Analytical Methods of Oil Sand and Bitumen Processing; Syncrude Canada Ltd.: Edmonton, Alberta, 1979; p 23.
Energy & Fuels, Vol. 13, No. 2, 1999 347 and secondary (SFB) froths. Process water for the BEU tests was synthesized by adding 500 ppm Na+, 250 ppm Cl-, and 915 ppm HCO-3 to distilled water; the pH was 8.8. One part by weight of each primary froth was heated and mixed with 0.45 parts by weight of hot toluene on a high intensity Spex shaker for 5 min. Centrifugation of the bitumen solutions for 10 min at a relative centrifugal force (RCF) of 1500 gravities produced toluene solutions with water and solids levels similar to those found in diluted bitumen from the conventional HWEP. Bitumen was separated from the rag layers by the soxhlet extraction-Dean and Stark method with toluene as the solvent.11 The bitumen solutions were concentrated using a RapidVap evaporation system. Samples were loaded into 600 mL glass tubes and desolventized at 80 °C for 2.5 h under a vacuum of 80 kPa and with a vortex speed of 28 cycles per min. Separation of asphaltenes was accomplished by dissolving each sample in 1.5 volumes of warm toluene followed by dilution with approximately 40 volumes of n-pentane.12 In addition, bitumen A was treated with a pentane-toluene blend with a molar ratio of 0.9:1.5. This treatment precipitated a much smaller amount of asphaltene-like material. In all cases, the BS were completely removed by entrainment in the precipitated asphaltene. The BS were recovered from the precipitates by centrifuging 5 w/w % toluene solutions of each asphaltene sample at an RCF of 366 000 gravities for 1 h. Prior to characterization, the recovered solids were washed free of bitumen by repeated treatment with a mixture of toluene and water until the toluene phase was free of color.13 Analysis. Carbon, hydrogen, and nitrogen contents were measured with a Perkin-Elmer, model 240, CHN analyzer. Sulfur was determined by the titrimetric, oxygen flask, combustion method, using a Scho¨niger-type combustion apparatus. DC arc emission spectroscopy (DCAES) determined metals. Gel permeation chromatography (GPC) was used to measure the weight average molecular weight (MW) for each asphaltene sample using monodisperse polystyrene standards for calibration. Standards, with weight average molecular weights in the range 162-43 000, were supplied by Phenomenex (Torrance, CA) and Polymer Labs (Church Stretton, U.K.). Surface spectra, by X-ray photoelectron spectroscopy (XPS), were recorded using a PHI 5500 Instrument (PHI Electronics, Eden Prairie, MN) with an Al KR source of X-rays. The pressure inside the analysis chamber was always below 8 × 10-9 Torr during measurements. An electron flood gun was used to neutralize any charge developing on sample surfaces while the spectra were recorded. High-resolution spectra were obtained at a pass energy of 29.6 eV, while survey spectra were recorded at a pass energy of 156 eV. Several repetitions were made to ensure data reproducibility. The inorganic samples were also examined by transmission electron microscopy (TEM), using a Philips CM20 200 kV electron microscope equipped with an Oxford Instruments energy dispersive X-ray diffraction (LINK EDX) detector and a charge-coupled device (CCD) camera. The CCD camera produces electronically enhanced image contrast, allowing extremely thin objects to be viewed at high resolution. The EDX detector allows detection and quantitation of elements heavier than nitrogen. Use of the TEM in diffraction mode produces standard electron diffraction patterns that, in conjunction with EDX analysis, allow mineral identification. (11) Syncrude Analytical Methods of Oil Sand and Bitumen Processing; Syncrude Canada Ltd.: Edmonton, Alberta, 1979; p 58. (12) Mitchell, D. L.; Speight, J. G. The solubility of asphaltenes in hydrocarbon solvents. Fuel 1973, 52, 149. (13) Kotlyar, L. S.; Sparks, B. D.; Woods, J. R.; Raymond, S.; Le Page, Y.; Shelfantook, W. Distribution and types of solids associated with bitumen Fuel Sci. Technol. 1998, 16, 1.
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Table 1. Yield of Asphaltenes (n-pentane insolubles) and Bitumen-Associated Solids (BS) asphaltenes
a
BS (w/w %)
sample
yield (w/w % bitumen)
MW (GPC)
concn in asphaltenes
calculated concn in parent bitumen
bitumen Aa bitumen Ab primary froth bitumen rag layer bitumen end-cut
16.4 2.3 16.7-17.6 21.8-51.8 94.0
6600 9000 6100-6400 6700-7003 6606
5.2 34.8 1.8-3.4 4.6-6.4 11.0
0.9 0.8 0.3-0.6 1.4-2.5 10.3
n-Pentane insoluble. b Blend (toluene/n-pentane with molar ratio of 0.9:1.5) insolubles.
Results Asphaltenes. Table 1 summarizes the yield and elemental analysis of asphaltenes, from different sources, on a BS-free basis. The asphaltene content of each sample showed the following trend: end-cut pitch sample (94 w/w %), rag layers (21.8-51.8 w/w %), bitumen A and PFBs (16.4-17.6 w/w %). The weight average molecular weights (MW) of the rag layer asphaltenes were somewhat greater than those derived from the whole bitumen samples. The solvent-blend insolubles from bitumen A gave a significantly lower yield of higher molecular weight asphaltene, compared to that from the pentane treatment of the same sample. Solids. The amounts of BS material separated from the various asphaltene samples are also summarized in Table 1. Concentrations of solids followed the same trend as the asphaltene yields: end-cut pitch sample (11 w/w %) > bitumen A and rag layers (5-6 w/w %) > PFBs (2-3 w/w %). The solvent blend insolubles from bitumen A had the highest solids content at 35 w/w %. Back calculation from asphaltene yields and solids content of the pentane and solvent blend insolubles from bitumen A gave virtually the same solids content for the parent bitumen, i.e., 0.9 and 0.8 w/w % respectively. These results indicate that solids in the parent bitumen can be removed by precipitation of only a small fraction of the asphaltene originally present. Figure 1 presents TEM, EDX, and electron diffraction patterns for a typical BS sample. Aluminosilicate, ultrafine clays, i.e., crystallites with a lateral extension of less than 100 nm and thickness of
