Article pubs.acs.org/EF
Solvent Extraction of Bitumen from Oil Sands Tong Wang,†,‡ Chao Zhang,†,‡ Ruiyu Zhao,*,†,‡ Chengjun Zhu,†,‡ Chaohe Yang,† and Chenguang Liu†,‡ †
State Key Laboratory of Heavy Oil Processing, and ‡China National Petroleum Corporation (CNPC) Key Laboratory of Catalysis, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China ABSTRACT: In the present study, solvent extraction of bitumen from Xinjiang oil sands was investigated using solvents, such as alkanes, toluene, and chloroform, and mixed solvents, such as n-hexane−ethyl acetate and cyclohexane−methyl ethyl ketone (MEK). The n-heptane asphaltene content in the extracted bitumen was measured by saturate, aromatic, resin, and asphaltene (SARA) analysis, and the average molar mass of asphaltene was tested by vapor pressure osmometry (VPO). The average molar mass of asphaltene was 2850, and the calculated Hansen solubility parameter (HSP) of asphaltene was 20.1 MPa1/2. The operating conditions were well-investigated using cyclohexane as the extraction solvent, and the effect of the volume fractions of the mixed solvents on bitumen recovery was studied. The extraction processes were most appropriate at a stirring velocity of 500 revolutions/min for 30 min at a temperature of 50−60 °C using a ratio of cyclohexane/oil sands of 5:3 (mL/g) based on a high bitumen yield and low extraction cost. Under such conditions, bitumen recovery of about 75% was obtained for cyclohexane extraction, comparable to that of naphtha (78%). Considering the bitumen yield and extraction cost, the appropriate volume fractions of polar solvents were found to be at 0.33 and 0.2 for n-hexane−ethyl acetate and cyclohexane−MEK extractions, respectively. On the basis of the above research and theoretical analysis, a solvent with a high solubility parameter close to that of asphaltene and a parameter distribution close to that of toluene is appropriate for the bitumen extraction of Xinjiang oil sands. The specific parameters were about 18.0−19.0 MPa1/2 for HSP, 65−92 for the dispersion force parameter fraction (fd), 0−15 for the polar force parameter fraction (f p), and 4−22 for the hydrogen-bonding force parameter fraction (f h). The asphaltene content in the extracted bitumen generally has a positive relationship with the bitumen recovery, which is likely due to the mechanism that it is the extracted fuel oil and not the solvent that extracts the asphaltene into the solvent phase.
1. INTRODUCTION As an important unconventional oil resource, oil sands have attracted attention recently. Oil sands have been extracted at scale in Canada since the 1960s.1 In 2005, the bitumen production from oil sands had accounted for 40% of their petroleum production. More than 55% of the bitumen were produced by using water-based extraction processes (WBEPs).2 According to the solids surface wettability, oil sands can be divided into oil-wet, water-wet, and neuter-wet oil sands.3 Water-wet oil sands are usually processed by WBEPs that were developed on the basis of Clark’s pioneering work in the 1920s.4 In this technology, oil sands are mixed with hot alkaline water and then the bitumen froth is collected after flotation and processed further. However, although it brings considerable economic returns, this technology now faces a series of disadvantages. First, the extensive consumption of water is needed in the process. Second, the high greenhouse gas (GHG) and toxic tailing water generated have long-term environmental impacts.5,6 Last, the WBEPs are not suitable for processing oilwet and weathered oil sand ores. Utah tar sands are mostly oilwet or weathered ores and contain a negligible amount of connate water (much less than 1 wt %), and in the absence of connate water, the bitumen is directly bonded to the surface of the sand grains.7 Utah tar sands are not amenable to water extraction because the solids surface is hydrophobic. Thus, the non-aqueous bitumen extraction technologies pyrolysis and solvent extraction were explored, mainly in the United States.8 Considerable research has been performed in an effort to separate bitumen from oil sands using solvent extraction. Hanson et al. investigated a solvent extraction process using © 2014 American Chemical Society
aromatic hydrocarbons, such as toluene, which included extraction, separation, drying, and solvent recovery, and achieved the circulation of solvent with less environmental impact.9 However, the major problems of the process are its complexity and high energy consumption. Funk et al. proposed a process of counter-current extraction−sedimentation−fluidized-bed drying for processing oil sands, in which the bitumen was extracted from minerals using a low boiling point solvent, such as heptane and pentane.10 The solvent recovery can reach 99%; however, the bitumen recovery is not high, and some heavy components (20%) still remain in the sands. Gable et al. proposed a method in which C5−C9 saturated hydrocarbons or the solvent mixed with 2−20% C6−C9 arenes extracted bitumen from oil sands.11 Yang et al. extracted bitumen from oil sands by water-assisted solvent extraction processes (WASEPs).12 The introduction of water can generate a water layer between sands and bitumen solution, which will reduce the fines content in the extracted bitumen solution and make it easy to separate the bitumen solution from the solids. The extraction yield can be compared to that of solvent-direct extraction, and the solvent recovery is greater than the latter. However, it should be noted that, once water is introduced into the extraction process, so are most of the problems associated with the conventional WBEPs. Wu et al. investigated a process for solvent extraction of bitumen from Athabasca oil sands using light hydrocarbon solvents and multiReceived: October 22, 2013 Revised: March 20, 2014 Published: March 21, 2014 2297
dx.doi.org/10.1021/ef402101s | Energy Fuels 2014, 28, 2297−2304
Energy & Fuels
Article
and δ1 and δ2 are the volume fractions of the pure solvent, respectively. In this work, we consider pure and mixed solvents that consist of a light hydrocarbon solvent and a strong polar solvent for the extraction of bitumen from Xinjiang oil sands. The proportions of pure solvents were adjusted to create a range of solubility parameters. The Hansen solubility parameter (HSP) of the asphaltene in the extracted bitumen was calculated. The effect of HSP and its parameter distribution on bitumen recovery were studied to improve the bitumen recovery.
stage extraction.8 The results suggest that, for both high- and low-grade ores, bitumen recovery using the investigated extraction processes is comparable to that of the WBEPs and the quality of extracted bitumen is similar to that of bitumen from WBEPs, followed by naphtha froth treatment. Also, the light hydrocarbon solvents can be easily recovered. Therefore, this method has potential to improve the bitumen yield and solvent recovery. The focus in bitumen extraction has been on the solvents naphtha, reformulated gasoline, benzene, toluene, pyridine, chloroform, methanol, alkane, cycloalkanes, olefin, compressed natural gas (CNG), and coal tar. However, various factors need to be considered for solvent selection, such as the source, cost, recovery, volatility, and toxicity. The bitumen recovery and quality of bitumen are also important. Naphtha is considered a good solvent for separating bitumen from oil sands and has been investigated extensively. Naphtha is widely available and cheap, has a low toxicity and high bitumen recovery, provides a good quality of bitumen, but fails to extract the heavy components, which exist in large amounts in asphaltene. The solubility parameter represents the dissolving capacity of substances in a given solvent, and its value is equal to the square root of the cohesive energy density.13 The solubility parameter is a physical constant that measures the intermiscibility of liquid materials, including rubber. In 1950, Hildebrand and Scott introduced the solubility parameter and theoretically described the relationship between solubility, heat of evaporation, and cohesive energy density.13,14 Hansen divided the solubility parameter into three parts: polar force parameter (δd), dispersion force parameter (δp), and hydrogen-bonding force parameter (δh).15 From the thermodynamical point of view, the process in which solvents extract bitumen from oil sands can be seen as the mixing process of solvents and solute. According to the Hildebrand solution theory (eq 1), if solvent is miscible with solute, which means that the mixing enthalpy is small, they must have the same or almost the same solubility parameters.13,16 If the solubility parameters are similar between macromolecular and low-molecular-weight compounds (D value of
