Article pubs.acs.org/EF
Robust Aqueous−Nonaqueous Hybrid Process for Bitumen Extraction from Mineable Athabasca Oil Sands Sanjay Kumar Harjai,† Chris Flury,† Jacob Masliyah,† Jaroslaw Drelich,‡ and Zhenghe Xu*,† †
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2G6, Canada Department of Materials Science and Engineering, Michigan Technological University, Houghton, Michigan 49931, United States
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ABSTRACT: Mining and processing of Athabasca oil sands in Alberta, Canada, is a great success story of government−industry collaboration, fulfilling increased domestic and worldwide demands for oil. However, economic and environmental incentives still exist in the oil sands industry to enhance oil recovery and reduce energy consumption and water use, while minimizing greenhouse gas emissions and tailings ponds. The current industrial bitumen extraction processes, after surface mining, are exclusively water-based and operate at elevated temperatures, typically between 45 and 50 °C. Robust low-temperature processes, with reduced in-take of feedwater, that are less sensitive to ore characteristics are in a strong demand from both environmental and economical point of views. In response to this demand, we propose a robust aqueous−nonaqueous hybrid bitumen extraction process, in which diluent such as kerosene and naphtha is added to the oil sands prior to oil sands slurry preparation to decrease bitumen viscosity and enhance bitumen liberation. With the proposed hybrid bitumen extraction process, the oil sand processing temperature can be reduced to ambient temperature. To prove this concept, bitumen recovery tests were carried out on four Athabasca oil sand ores of good to poor processability, using a Denver flotation cell operated at ambient temperature. Adding kerosene or naphtha to oil sands at 4−11 wt % of the bitumen content was found to significantly enhance flotation recovery of bitumen and bitumen froth quality, especially for poor processing ores. It was found that kerosene addition not only increased bitumen liberation kinetics determined using our novel in situ bitumen liberation visualization flow cell (BLVFC) but also improved bitumen aeration measured by induction time apparatus.
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INTRODUCTION With great efforts to unlock the vast oil resources in Athabasca oil sands, hot water-based processes pioneered by Clark have been the principal technology for bitumen extraction from mineable oil sands.1 Although oil production from these oil sands resources is providing a security for fossil fuel needs into a foreseeable future, the industry is facing great challenges of water shortage and green-house gas emissions related to bitumen production from mineable oil sands. To alleviate the challenges associated with water shortage, a great deal of effort has been made on developing either effective water-recycle technologies2,3 or nonaqueous-based extraction processes.4−6 The concepts of using supercritical CO2,7,8 ionic liquids,4 or solvent9 for bitumen extraction have been proposed and tested at laboratory scales. Although promising, the technical and economical challenges inherited in these concepts set the roadblocks for them to move forward to pilot scale tests. With regard to the reduction in green house gas emissions, effort has been made in developing lower temperature extraction processes. The use of a hydrotransport slurry pipeline for oil sands conditioning to reduce the temperature of bitumen extraction from the original 80−90 °C to current limit of 45 °C has been a successful story of oil sands industry in reducing energy intensity and green house gas emissions of oil production from oil sands.10,11 However, a further reduction in operating temperature with current water-based extraction technology is considered impractical. Furthermore, the performance of processing at lower temperatures has been found to be more sensitive to ore characteristics and water chemistry. There is a clear need to develop a robust, low © 2012 American Chemical Society
temperature process for bitumen production from mineable oil sands, which is the subject of this paper. In dealing with difficult ores such as Utah oil (tar) sands, Hupka et al.12−15 developed a diluent-assisted bitumen extraction process at the University of Utah. In this process, about 5 to 10 wt % of kerosene on the basis of bitumen content was added to Utah oil sands and water-based bitumen extraction was conducted at 50−55 °C and mild alkalinity (pH 8−9) controlled by sodium carbonate addition. By applying the knowledge of solid hydrocarbon (coal) flotation using kerosene and methyl isobutyl carbinol (MIBC) as process aids, OSLO (other six lease owners) developed two distinct extraction processes known as OSLO cold water extraction at 5−30 °C (OCWE) and OSLO hot water extraction at 30−90 °C (OHWE), collectively known as OSLO processes.16−18 In OSLO processes, kerosene (as collector) and MIBC (as frother) at a 2:1 mass ratio and 0.3 wt % of bitumen content were used as process aids to improve bitumen recovery. By eliminating the use of caustic as process aids, a unique feature of these two OSLO processes is the improved settling of extraction tailings.18 Based on the promising results of OSLO, Syncrude extended the OCWE process to low energy extraction (LEE) process operated at temperatures between 5 and 35 °C. In the LEE process, MIBC and kerosene at a 1:2 mass ratio were used as the process aids. The amount of MIBC and kerosene required to achieve an acceptable process Received: February 14, 2012 Revised: April 19, 2012 Published: April 19, 2012 2920
dx.doi.org/10.1021/ef300270j | Energy Fuels 2012, 26, 2920−2927
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Figure 1. Schematics of current water-based bitumen extraction process (solid connections) and a hypothetical aqueous−nonaqueous hybrid bitumen extraction process (dotted connections). A portion of organic solvent currently used in froth cleaning is added up front in oil sands ores prior to slurry preparation to reduce bitumen viscosity and hence bitumen extraction processing temperature.
recovery from the Athabasca oil sands. More recent analysis performed by Long et al.11 suggests that the viscosity threshold value for the Athabasca oil sands is about 7 Pa·s. Although the viscosity reduction by diluent addition is a key element in improving bitumen recovery, the alternation of interfacial properties by solvent addition should not be overlooked as the bitumen liberation is largely controlled by interfacial properties.24
performance was found to vary, depending on the ore grade. Low-grade oil sands need more MIBC/kerosene to attain desired levels of bitumen recovery.19 Further development of LEE process by Sury et al.16 led to a patented process of low temperature bitumen extraction. In this patented process, the oil sand slurry was vigorously mixed with a conditioning and frothing agent for a sufficient period to release bitumen from oil sands at temperatures above the freezing point to 35 °C, preferably in the range of 2 to 15 °C. The conditioning agent in this case was kerosene, diesel, or kerosene/diesel blend at a concentration of 100−800 ppm. MIBC at 50−400 ppm was used as frother. Khraisha et al.20 investigated the water-based extraction of oil sand from Wadi Isal in Jordan. In their study, bitumen extraction was found to be extremely poor, even at temperatures as high as 80 °C. With kerosene addition, bitumen recovery increased up to seven times as compared to hot water extraction without NaOH addition. Several investigators concluded that there is a close correlation between bitumen recovery and bitumen viscosity.12−15,21,22 Hupka et al.13−15 demonstrated that a reduction in bitumen viscosity allows the processing temperature to be lowered by at least 40 degrees as compared to the original Clark’s process developed for bitumen extraction with boiling water. Kerosene or other solvent was typically added to the oil sands during their pretreatment step to reduce bitumen viscosity.23 The amount of kerosene added depends on the original bitumen viscosity, oil sand grade, and intended processing temperature. The kerosene addition levels can be predicted by using the bitumen viscosity curves developed as a part of the study. The study shows that to achieve a satisfactory coefficient of separation, the bitumen viscosity must be reduced to below 2 Pa·s at the processing temperature for a predetermined conditioning time (penetration time necessary for the diluent to mix with oil sands prior to digestion in order to uniformly reduce the bitumen viscosity). Schramm et al.21 also obtained a bitumen viscosity threshold value of similar magnitude, about 3 Pa·s, to achieve a satisfactory bitumen
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CONCEPT OF AQUEOUS−NONAQUEOUS HYBRID EXTRACTION PROCESS With the proven improvement in bitumen recovery by solvent addition and difficulties encountered in nonaqueous-based extraction processes,6 it is natural to look into a water−diluent hybrid process that is robust in response to the variability of oil sands ores and water chemistry, and effective at low (ambient) operating temperatures. This is particularly attractive if one closely examines the current water-based bitumen production process, as shown by solid-line connected flowchart of Figure 1. In a water based extraction process, mined oil sands ores are mixed with hot water and chemical additives, mainly caustics, to make a slurry. The resultant slurry is conditioned through slurry hydrotransport pipelines, in which bitumen becomes liberated and aerated, an invention of the oil sands industry. The conditioned slurry is then fed to a separation vessel where the aerated bitumen floats to the top of the vessel as bitumen froth, typically containing 60 wt % bitumen, 30 wt % water, and 10 wt % solids. To meet the requirement of upgrading by hydrocracking and/or hydrotreating, the bitumen froth needs to be cleaned by adding a large amount of solvent, typically at 1:2 of naphtha/bitumen volume ratio or 2.5:1 of hexane/ bitumen volume ratio. After separating the solids and water by gravity separation, the solvent-diluted bitumen is sent to a solvent recovery unit where the clean bitumen of less than 2 wt % water and 0.5 wt % solids is produced, while the recovered solvent is recycled back to the process. Since an ample amount of solvent is used anyway in the current bitumen production process, it would be feasible to distribute the solvent addition 2921
dx.doi.org/10.1021/ef300270j | Energy Fuels 2012, 26, 2920−2927
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0.3 and 68 ± 2 mN/m, respectively. The process water contained 14− 20 ppm K+, 503−743 ppm Na+, 19−20 ppm Mg2+, 48−56 ppm Ca2+, 431 ppm Cl−, 63 ppm SO42−, 647 ppm HCO3−, and
