Energy & Fuels 1997, 11, 1127-1136
1127
Nonconventional Residuum Upgrading by Solvent Deasphalting and Fluid Catalytic Cracking† Siauw H. Ng National Centre for Upgrading Technology, 1 Oil Patch Drive, Suite A202, Devon, Alberta, Canada T9G 1A8 Received January 17, 1997X
This work presents the deasphalting characteristics of nonconventional vacuum tower bottoms (VTB) from Athabasca oil sands bitumen and Lloydminster heavy oil. These materials were deasphalted using four solvents (propane, n-butane, n-pentane, and n-heptane) over a range of solvent/VTB ratios, temperatures, and pressures. The objective was to provide an experimental data base for assessing the applicability of deasphalting and catalytic cracking technology to nonconventional residuum upgrading. Sufficient data were collected from deasphalting experiments to develop ternary phase envelopes for the eight solvent-residuum systems and to establish the effects of solvent type and operating conditions on deasphalted oil (DAO) yield and quality. Deasphalting experiments were carried out in a single equilibrium cell and solvent-rich and lean phases were analyzed for oils, resins, and asphaltenes. The system was represented as a pseudoternary system where the resins could be combined with the oils or with the asphaltenes as asphalt. The deasphalting results showed that as the molecular weight of solvent decreased from n-heptane to propane, the DAO yield decreased while its quality improved: Conradson carbon residue (CCR), sulfur, nitrogen, and metals, as well as the density, decreased, and the oil content of DAO increased. Upon catalytic cracking at constant severity, product yields could be correlated with either oil or asphalt content of DAO. For a given composition of DAO, it was found that roughly 50% of oils and 20% of resins in DAO were converted to gasoline.
Introduction As western Canadian reserves of conventional oil continue to decline, Canadian refiners have to depend increasingly on nonconventional feedstocks such as those derived from oil sands bitumen, heavy oils, and resids to produce transportation fuels. The known reserves of oil sands bitumen and heavy oils in Canada (mainly in the province of Alberta) are over 200 billion m3,1 more than 5 times larger than Saudi Arabia’s oil reserves. If these nonconventional resources can be fully recovered, Canada’s oil supply can be secured for several centuries. Currently, production from the four existing upgraders in western Canada is about 325 000 barrels per day.2 Oil sands bitumen and heavy oils contain 40-50 vol % 525 °C+ vacuum residuum, compared with about 10 vol % in Alberta light crude. This makes the whole crude high in molecular weight and viscosity and low in API gravity (9-16 °API, compared with 36 °API for Alberta light crude). Further, the nonconventional raw crudes, especially the heavy fractions, are extremely rich in impurities (sulfur, nitrogen, oxygen, and metals) and polynuclear aromatics (PNA). Current processes to upgrade these raw crudes in western Canada include coking and hydrocracking followed by hydrotreating of † A different version of this paper on kinetic modeling using Lloydminster DAO cracking data was published in Fuel (Pope and Ng, 1990). X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Alberta’s Reserves of Crude Oil, Oil Sands, Gas, Natural Gas Liquids and Sulphur; Energy Resources Conservation Board (ERCB): Calgary, Alberta, 1994 (December); p 3-2. (2) Oil Patch Mag. 1995, 14(1) (February/March), 6; in CANMET Supports Commercialization of Upgrading Technology; Master Publications: Edmonton, Alberta, Canada.
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the distillates. These processes are relatively capitaland energy-intensive. It was thought that solvent extraction might provide a cheaper means of upgrading for smaller-scale operations. It is well-known that suitable solvents at certain conditions can effectively precipitate asphaltenes, the complex PNA molecules containing high levels of heteroatoms. Asphaltenes are known to be detrimental to refinery processes and are responsible for catalyst deactivation and coke laydown.3 After separation and evaporation of the solvent, the oil recovered, termed deasphalted oil or DAO, can be used as a feedstock for fluid catalytic cracking (FCC) or production of lubricating oils or specialty products. A number of asphaltene-related and solvent deasphalting studies involving Canadian nonconventional feedstocks have been reported in the literature. Mitchell and Speight4 established a correlation between the weight of asphaltenes precipitated from Athabasca bitumen and the solubility parameter of the hydrocarbon solvents used. The polynuclear aromatic systems in asphaltenes isolated from Athabasca bitumen were also reported by Speight.5 Kokal et al.6 measured precipitation of asphaltenes in Canadian heavy oils by injecting light hydrocarbon gases, e.g., methane, propane, ethane/propane mixtures, and carbon dioxide, at various temperatures and pressures. In their study, a thermodynamic model proposed by Hirshberg et al.7 (3) Speight, J. G. In Catalysis on the Energy Scene; Kaliaguine, S., Mahay, A., Eds.; Elsevier: Amsterdam, 1984; p 515. (4) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149-152. (5) Speight, J. G. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1986, 31(4), 818. (6) Kokal, S. L.; Najman, J.; Sayegh, S. G.; George, A. E. J. Can. Pet. Technol. 1992, 31(4), 24-30.
Published 1997 by the American Chemical Society
1128 Energy & Fuels, Vol. 11, No. 6, 1997
based on Flory-Huggins theory8 was used to correlate the experimental data and predict precipitate formation. Recently, Brons and Yu9 reported the solvent deasphalting effects on Cold Lake bitumen, with respect to the property changes of DAOs and asphaltenes. The solvents used were propane, n-butane, isobutane, and n-pentane. With respect to process study using general feedstocks, Bousquet and Laboural10 conducted pilot plant deasphalting on vacuum residua from light and heavy Arabian crude oils using propane, butane, and pentane. The recovered DAOs were cracked using the ARCO FCC pilot unit. The results showed good economics for this upgrading option: Cash flow could be significantly increased by adding deasphalting to a base refinery having facilities for atmospheric and vacuum distillation plus FCC. Pope and Ng11 assessed the catalytic performances of several DAOs using a riser kinetic model. Several extraction-based commercial refining processes such as Demex,12 ROSE13,14 (residuum oil supercritical extraction), and Mellon15 were reported in the literature. The objective of the present study was to evaluate the viability and processability of DAOs from Canadian nonconventional residua as feedstocks to produce transportation fuels. This paper describes and compares the deasphalting characteristics of two vacuum tower bottoms (VTBs) derived from Athabasca oil sands bitumen and Lloydminster (Lloyd) heavy oil, and the cracking performance of the resulting DAOs. Results for the Athabasca samples are emphasized since some of the catalytic cracking data of Lloyd DAO have been reported previously.11 Experimental Section Materials. Athabasca VTB was obtained by distillation (ASTM D1160) of a coker feed (305 °C+) from a commercial plant which produces synthetic crude from oil sands bitumen. The distillation produced 12.4 wt % coker feed as heavy distillate (IBP-360 °C), 24.1 wt % as vacuum gas oil (360544 °C), and 63.5 wt % as VTB (544 °C+). Lloydminster VTB was obtained from a refinery in eastern Canada that processes pipeline-quality Lloydminster crude oil. A metals-tolerant cracking catalyst, Davison DA-440, was obtained from a commercial FCC unit. Characterization of Oil and Catalyst Samples. Characterization of feedstocks and extraction products was performed using ASTM and other supplementary methods. The results are shown in Tables 1 and 2. Note that in this study the term “asphaltenes” is defined as n-heptane-insolubles determined by ASTM D3279. In extraction experiments, the DAO recovered from the solvent-rich phase and the residue recovered from the solvent-lean phase were analyzed for oils, resins, and asphaltenes. The first two were n-heptane-solubles (maltenes) and were determined using an Attapulgus clay column. Here, 33 g of clay was used to separate 1 g of maltenes dissolved in 10 mL of n-heptane. The oil was eluted (7) Hirschberg, A.; deJong, L. N. J.; Schipper, B. A.; Meijer, J. G. Soc. Pet. Eng. J. 1984, 34(3), 283-293. (8) Prausnitz, J. M. Molecular Thermodynamics of Fluid-Phase Equilibria; Prentice-Hall Inc.: Englewood Cliffs, NJ, 1969. (9) Brons, G.; Yu, J. M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40(4), 785-793. (10) Bousquet, J.; Laboural, T. Oil Gas J. 1987, 85(16), 62-68. (11) Pope, A. E.; Ng, S. H. Fuel 1990, 69, 539-546. (12) Penning, R. T.; Vickers, A. G.; Shah, B. R. Hydrocarbon Process., 1982, May, 145-150. (13) Nelson, S. E.; Roodman, R. G. Chem. Eng. Prog. 1985, May, 63-68. (14) Low, J. Y.; Hood, R. L.; Lynch, K. Z. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40(4), 780-784. (15) Anon. Oil Gas J. 1994, 92(22), 87-88.
Ng Table 1. Feedstock Characterization Athabasca VTB
Lloydminster VTB
-0.7 66 11 2002 959 24.3 6390 1820 5.93 130 310 470 0.8 64 46 36.7 17.3
6.3 36 192 205 113 18.3 4964 1352 4.53 84 189 0.51 0.1
