ARTICLE pubs.acs.org/EF
Ionic Liquid Enhanced Solvent Extraction for Bitumen Recovery from Oil Sands Xingang Li,†,‡,§ Wenjun Sun,†,§ Guozhong Wu,† Lin He,† Hong Li,†,‡ and Hong Sui*,†,‡ † ‡
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China National Engineering Research Centre for Distillation Technology, Tianjin 300072, China ABSTRACT: An ionic liquid (IL) with low viscosity, 1-ethyl-3-methyl imidazolium tetrafluoroborate ([Emim][BF4]), was used to enhance bitumen recovery from oil sands by solvent extraction using a composite solvent of n-heptane and acetone. Results demonstrated that [Emim][BF4] increased the bitumen recovery by up to 95% at room temperature. Much less clay fines in the recovered bitumen than those using solvent extraction without IL, and organic residue was not observed in the spent sands. This technology circumvented the issue of tailing water because only a small amount of water was used to recycle IL, and organic solvent could be readily regenerated by distillation. The low viscosity of [Emim][BF4] made it outperform other alternative ILs for application in the oil sand industry. Results also highlighted the role of acetone in this technology (i.e., decrease IL viscosity and reduce IL consumption). The optimal solvent extraction conditions were found to be stirring for 10 min at 450 rpm at 25 °C. The optimized ratio of acetone to nheptane in the composite solvent and the ratio of organic solvents to oil sands was determined as 2:6 (v/v) and 4:1 (v/w), respectively.
1. INTRODUCTION A dramatically increasing interest in the Canadian oil sand, a complex mixture of sand, clays, water, and bitumen, concerns the recovery of nonrenewable resource as an alternative to relieve the issue of global energy shortage. As a promising “energy carrier”, Canada’s oil sands contain about 900 billion barrels of crude oil equivalent and have the potential to yield 250 billion barrels of marketable “synthetic crude oil”.1,2 Because of the increased awareness of the considerably renewable energy resource in oil sands and the benefits of sustainable development, numerous efforts have been devoted to develop technologies to separate bitumen from oil sands. One of the most popular approaches is the hot water extraction process (HWEP) which has been successfully applied in the oil sands industry.3,4 In this technology, hot (or warm) water and processing aids (e.g., NaOH) are mixed with oil sands to liberate bitumen from the minerals by shearing the oil sand particles. A bitumen-rich froth is formed by aeration and flotation. The organic solvent (e.g., naphtha or hexane) is then used to dilute the froth, decrease the viscosity of bitumen, and facilitate the removal of minerals and water droplets.4 8 One of the issues of water-based extraction is the enormous consumption of water which results in 4 m3 of fluid tailings for treating 1 m3 of oil sands.9,10 Although about 70% of the water is recyclable, the oil sands processing is still an ever-growing inventory of processaffected waters and tailing pond materials. The fine particles suspended in tailing water settle very slowly.11 After years of settling, a stable suspension in water (known as mature fine tailings) is formed by the fine particles, high concentration of naphthenic acids, sodium sulfate, and unrecovered bitumen, which is banned to discharge in accordance with the Canadian policy because it is toxic to a variety of organisms once released into the environment.10 13 Moreover, the naphthenic acids in the tailing water are highly corrosive to equipment during the water reusing process.12 The solid fines and salty water in the r 2011 American Chemical Society
extracted bitumen will deactivate catalysts and also cause corrosion and fouling in downstream process units. Therefore, there is a demand for the development of alternative technologies capable of avoiding the above drawbacks. Alternatively, organic solvent extraction technology was developed to recover bitumen from oil sands. The aromatic solvents (e.g., benzene and toluene) are widely used because they have the similar solubility parameter with bitumen, thereby removing bitumen effectively.14 17 However, it is difficult to extend this technology to the oil sands industry because these organic solvents are volatile, flammable, toxic, and difficult to recycle from the spent sands. Therefore, efforts were devoted to develop environmentally friendly solvents with high efficiency. The ionic liquids (ILs), green solvents having unique properties (e.g., thermal and chemical stability) and widely used for synthesis and separation,18 21 were applied to enhance solvent extraction of bitumen from oil sands. A recent study22,23 demonstrated that the [Bmmim][BF4], a typical IL, improved the bitumen recovery to 90% in the solvent extraction using toluene at room temperature. With the assistance of IL, bitumen was essentially removed from mineral fines leaving the relatively clean sands. The issues associated with tailing water were addressed as only a slight amount of water was used to remove IL from the residual sands and clays. Additionally, it is easy to separate water and IL by evaporation due to the IL’s negligible vapor pressure.22 24 The high performance of IL in the solvent extraction was mainly because it engaged in electrostatic interactions with the surfaces of sands and their associated ions. This decreased the adhesive force between bitumen and silica by 1 order of magnitude, which facilitated desorption and dissolution of Received: July 26, 2011 Revised: October 8, 2011 Published: October 13, 2011 5224
dx.doi.org/10.1021/ef2010942 | Energy Fuels 2011, 25, 5224–5231
Energy & Fuels Table 1. Structure and Properties of the Ionic Liquids Used in This Studya
ARTICLE
Table 2. Solvents Used for Bitumen Recovery from Oil Sands (2.0 ( 0.1 g) scenario
solvent toluene
1 2
(8 mL) toluene
a
[Bmmim][BF4]
b
[Emim][BF4]
c
mixture of [Bmmim][BF4] and
d
mixture of [Emim][BF4]
(3.0 ( 0.1 g)
(8 mL) and
(3.0 ( 0.1 g) NaOH (
