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Study of cyclohexane diffusion in Athabasca asphaltenes Abolfazl Noorjahan, Xiaoli Tan, Qi Liu, Murray R Gray, and Phillip Choi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef402312d • Publication Date (Web): 27 Jan 2014 Downloaded from http://pubs.acs.org on January 30, 2014
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Study of cyclohexane diffusion in Athabasca asphaltenes Abolfazl Noorjahan, Xiaoli Tan, Qi Liu, Murray R. Gray and Phillip Choi* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 2V4, Canada * Corresponding author:
[email protected] Abstract The sorption of solvents in asphaltenes, residual bitumen, and organic matter is of interest in oil sands production and extraction. In this study, we investigate the sorption and desorption rates of cyclohexane in a thin film of asphaltene using a gravimetric method. These data were used to model the kinetics and to obtain estimates of the apparent diffusion coefficients. The rate curves for cyclohexane showed biexponential behavior, suggesting the presence of two different transport kinetics. Use of concentrationdependent diffusion coefficients was required to fit the data using Fick’s law. The apparent diffusion 2 coefficients were in the range of 10 to 10 m ⁄s , but the values for sorption were distinctly different from desorption. The results suggest a restructuring of the asphaltene film, with negligible swelling due to the addition and removal of the solvent. Keywords: oil sands, solvent extraction, residual solvent, diffusion
1. Introduction After Saudi Arabia and Venezuela, Alberta’s oil sands deposits represent the third largest proven crude oil reserves in the world1,2. There are currently two methods for bitumen extraction from oil sands.3 For deep deposits, steam assisted gravity drainage (SAGD) technology is used where thermal energy is applied to heat the bitumen in-place and allow it to flow to the well bore and be pumped to the surface. Bitumen recovery by the in-situ technologies is typically less than 60%. In order to enhance the energy efficiency of these processes, injection of a mixture of steam and solvent is under commercial development.4 In this case, the recovery of the solvent from the reservoir is a major concern. Shallow oil sands ores are mined by surface mining methods, which currently contribute 55% of the oil production in Alberta. In this case, the bitumen is recovered by water extraction whereby oil sands are mixed with hot water and caustic and sent through a hydrotransport pipeline to the extraction plant where the bitumen froth is recovered by flotation.
While bitumen extraction using hot water is an economic recovery process, its disadvantage is high fresh water and energy consumption. The volumetric water-to-bitumen ratio in this process is approximately 19. The majority of the process water is recycled, but some of the water is trapped in the wet unsettled tailings, giving net consumption of 2 to 4.5 barrels of fresh water to produce one barrel of oil.3,5 The resulting wet tailings, a mixture of water, fine solids, and residual bitumen, require large tailings ponds which had accumulated to about 650 million by 2006.3 These large volumes of wet tailings are a risk to wildlife and a liability for future remediation.1,5,6 The use of hot water, which has a high heat capacity, contributes to the energy consumption in this process.5 Another input to the extraction process is solvent, which is used to dilute the bitumen froth to enable the removal residual solids and emulsified water.7 The majority of the solvent is recovered, but some is released to the tailings ponds. Depending on the plant, the solvent may be naphtha or a hexane-rich mixture. In the case of the hexane-based process, a portion of the asphaltenes in the oil are precipitated8, so that
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recovery of residual solvent from asphaltenes is a major concern. Solvent extraction of bitumen is an alternative technology that offers high recovery from a range of ores.2,9,10,11 Solvent extraction can greatly decrease or eliminate fresh water demand for the extraction process and eliminate the wet tailings associated with the hot water process. It works at ambient pressure and temperature, so less energy is consumed in the addition of solvent. The solvent-extracted mixture of sand and fine solids, which we call “gangue”, is almost dry and can be returned to the mine area immediately to enable fast reclamation of the landscape. The biggest technical, economic, safety, and environmental challenge of solvent extraction is the recovery of solvents from the gangue.10,11 Several promising solvents have been identified, including aromatics, n-alkanes and cycloalkanes 1,12,13,5 , but a key criterion is the recovery of the solvent by methods such as vacuum or atmospheric drying.14 Cycloalkanes have the advantage of high solvent power, high vapor pressure, and low odor. 1,5 A successful process must remove solvent from the gangue to give residual concentrations lower than circa 250 ppm, better than the current losses of solvents from the froth treatment process. In the gangue, the main capacity for retention of solvent will be in residual bitumen, residual asphaltenes, and insoluble adsorbed organic matter that occurs in oil sands ores.6 For detailed engineering design, we need to predict the diffusion of solvents out of these organic materials, especially at low residual solvent concentrations. Several studies on solvent diffusion in bitumen have shown that the diffusion coefficients are strongly concentration dependent12,15,16,17, with values at low solvent concentrations ranging from 10 to 10 m s at 25 °C. Given the technologies for oil sands production, data on solvent diffusion in asphaltene fractions are important for enhancing solvent recovery from existing froth treatment processes and from future solvent extraction processes. In this study, we examine the rate of desorption or release of cyclohexane solvent from bitumen films by gravimetric measurements, and use these data to
model the concentration-dependent diffusion coefficients.
2. Materials and methods 2.1. Materials The Athabasca asphaltenes used in this study were from the bottom stream of a deasphalting unit processing bitumen from a steam-assisted gravity drainage (SAGD) operation. This material contained 28 wt% pentane-solubles based on the standard method of dissolution then precipitation in 40:1 solvent. Reagent-grade cyclohexane and methylene chloride purchased from Fisher Scientific (Mississauga, Ontario, Canada) were used as received. P-type double-side polished silicon wafers (100) with a 250 µm in thickness and a resistivity of 1−35 Ω cm were purchased from University Wafer (Boston, MA). Nitrogen (99.999%, Praxair) was used as carrier gas for all sorption and desorption experiments.
2.2. Asphaltene film preparation To obtain a regular geometry, a rectangular film of asphaltene was prepared. For this purpose, the silicon wafer was cut into 12 pieces of rectangular shape with known mass and surface area (a total surface area of 35.28 cm ) in order to accommodate these pieces in the sample bucket for following experiments. A 1.0 wt% of Athabasca asphaltene solution in methylene chloride was sprayed on both sides of silicon wafers by using an air brush. The rationale for using the air-brush method was that we needed to coat the silicon wafer on both sides in order to obtain enough mass of asphaltenes in a small sample container for the microbalance in IGA to detect mass change upon solvent absorption/desorption. The current spin coating technique can only yield satisfied films on one side of the substrate. To coat another side of the substrate, spin coating will damage the surface of the side that has already been coated. The airbrush method does not have this problem. The mass of coated asphaltene on these pieces of silicon wafer was pre-calculated to get the desired thickness. For each piece of silicon wafer, the deposition of asphaltene was multistep process where the actual mass of coated
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asphaltenes on silicon wafer was monitored by using analytical balance at each step of spraying following by drying at room temperature in fumehood to evaporate the solvent. After deposition, all asphaltene coated wafers were air dried for 48 hours in a fumehood then weighted to calculate the mean thickness of asphaltene films, which was 7 ± 1.4 µm. Silicon Wafer Asphaltene film Gas phase
,
of vapor generator was fixed at 30 °C to generate the relative pressure of cyclohexane vapor of 0.4 and 0.9 at each sample temperature correspondingly. In each set of experiment, the asphaltene coated silicon wafers were pretreated within the IGA at 70 °C in nitrogen atmosphere with flow rate of 100 ml/min to remove residual solvents and water until the weight of sample leveled off. The total flow rate throughout the experiments was kept at 50 ml/min, whereas the pressure was maintained at a constant value of 105 ± 0.1 kPa and the temperature variation was 10K 10 103 103 5 ∗ 10 > 8.4 ∗ 10K 8.6 ∗ 10 6.3 ∗ 10 2.4 ∗ 10 10 > 10H 10D > 10 10 > 2 ∗ 103 6.8 ∗ 10 4 ∗ 10 103 5 ∗ 10 > 1.5 ∗ 10K 7 ∗ 10 > 2 ∗ 103 4 ∗ 10 > 7.4 ∗ 10K 2 ∗ 10 > 6.4 ∗ 10K
2
(Atmospheric residue) From later stage of naphtha removal and it contains more asphaltenes than bitumen Sorption 4 Desorption 3
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