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Heterogeneous Distribution of Adsorbed Bitumen on Fine Solids from Solvent-based Extraction of Oil Sands Probed by AFM Jing Liu, Jingyi Wang, Jun Huang, Xin Cui, Xiaoli Tan, Qi Liu, and Hongbo Zeng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00396 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 6, 2017
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Heterogeneous Distribution of Adsorbed Bitumen on Fine Solids from Solvent-based Extraction of Oil Sands Probed by AFM Jing Liu, Jingyi Wang, Jun Huang, Xin Cui, Xiaoli Tan, Qi Liu, Hongbo Zeng* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada Email:
[email protected]; Phone: +1-780-492-1044
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Abstract Mineral particles encountered in oil production and many chemical processes are generally adsorbed with organics (e.g. bitumen), playing an important role in determining their wettability and interaction behaviors. In this work, the surface properties of fine solids, collected from solvent-based extraction of Athabasca oil sands using cyclohexane as the extraction solvent, have been systematically characterized by several complementary techniques. The fine solids were shown to be mainly composed of silica and aluminosilicate clays, analyzed using X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The particle wettability was determined using the Washburn method. The mineral particle surfaces were further characterized by atomic force microscope (AFM) techniques. Various regimes on the particle surfaces with and without adsorbed bitumen were identified and distinguished using PeakForce quantitative nanomechanics AFM imaging of surface topography, adhesion, modulus and deformation simultaneously in air. The results demonstrated that the adsorbed bitumen was heterogeneously distributed on the particle surfaces. Such surface heterogeneity was further confirmed by AFM force mapping using a hydrophobized AFM tip on fine solids in water. The force-separation profiles obtained on relatively hydrophilic mineral regimes could be well fitted by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) model, while additional hydrophobic interaction should be included in the modified DLVO model for the interactions on more hydrophobic domains (with adsorbed bitumen). This work provides a facile and useful methodology for characterizing the surface properties of fine solids in oil production, with implications for an improved understanding of the interaction mechanisms of particles suspended in bitumen products in oil production. The methodology can be readily extended to characterizing the surface properties of many other particles and substrates in a wide range of chemical processes and engineering applications.
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1. Introduction The oil sands deposits in Alberta is the world’s third-largest oil reserve.1 Extraction of bitumen from oil sands ores is of vital importance in the oil production process. The current economical water-based extraction of oil sands, however, requires excessive fresh water usage, almost all of which ends up in toxic extraction tailings.2-3 Extraction of oil sands using organic solvents has been considered as a potential alternative to the water-based extraction because it is capable of dramatically decreasing the need for fresh water and eliminating the challenging tailings issue resulted from the water extraction process.4-9 Nevertheless, solvent-based extraction process has not been commercialized despite the research efforts over the past two decades. One major problem impeding its industrial application is that considerable amounts of fine solids, several µm or less in size, can stably suspend in the bitumen-solvent mixture and migrate into the extracted bitumen products.7 The presence of these fine solids renders the bitumen product unsuitable for pipeline transport and as refinery feed.10-14 Adsorbed organics on fine solids are believed to play an important role in the stability of fine solids in the complex organic media.15 Previous studies show that bitumen components can strongly adsorb onto rock reservoirs and clays mostly due to physical adsorption (though chemical binding might be also present for some specific components), making the mineral solids more oil-wet.16-18 The wettability of fine solids largely depends on the surface coverage of adsorbed bitumen, consequently affecting the interactions of these fine particles and the degree of migration of fine solids into bitumen products during the solvent-based extraction process.19 It
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has been hypothesized in previous reports that the distribution of adsorbed bitumen on mineral particles is discontinuous.10, 20 Thus, characterizing the surface properties of fine mineral solids and the distribution of adsorbed organics on surfaces of these solid particles has attracted consideration attention. The average wettability of these mineral particles was characterized by contact angle measurement.19 Other analytic techniques such as time-of-flight secondary ion mass spectroscopy (ToF-SIMS)21-22 and X-ray photoelectron spectroscopy (XPS)23-24 have been also used to characterize the surface properties of the mineral solid particles. Nevertheless, characterizations of the particle surfaces and verification of heterogeneous distribution of adsorbed bitumen on fine solids still remain incomplete for the development of solvent-based extraction process for the oil sands industry. Atomic force microscope (AFM) is a useful technique for characterizing the distinct structures,25-26 patterns27-28 or functional sites29-31 along with their distributions on various material surfaces in a wide range of research fields. AFM force mapping method was applied to characterize the distribution of specific molecular recognition sites on living cells by probing ligand-receptor interaction forces through a functionalized tip.32-33 The AFM nanomechanical mapping in the so-called PeakForce quantitative nanomechanics (QNM) mode enables measurements of mechanical properties of materials at nano-scale, which can help distinguish the distribution of different material components on the surfaces. Lohse et al.34 and Cui et al. 35 have used the PeakForce QNM technique to characterize the distribution of nanobubbles or nanodroplets generated on hydrophobic substrates by distinguishing soft structures from the rigid
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substrates in terms of nano-mechanical properties. These previous studies suggest the applicability of AFM in characterizing the distribution of adsorbed bitumen on the surface of mineral solids in the solvent-based extraction process of oil sands. In this work, the fine mineral solids were collected from the solvent-based extraction process of Althabasca oil sands. Cyclohexane was used as the solvent for the extraction experiment due to its good performance with high bitumen recovery, high evaporation rate from extracted sands and low solids content in the recovered bitumen, , as reported previously.7 The chemical compositions of solid surfaces were characterized using XPS and energy dispersive X-ray spectroscopy (EDX), and the wettability of solids was determined by contact angle measurements using Washburn method. The particle surfaces and distribution of adsorbed organics on fine solid particles were studied using PeakForce QNM imaging and AFM force mapping by characterizing the nano-mechanical properties and hydrophobicity across the mineral surfaces. This work provides useful insights into the surface properties and the interaction mechanism of the fine solids suspended in bitumen from the solvent-based extraction process of oil sands, which is of both fundamental and practical importance for the oil industry.
2. Materials and Experimental Methods 2.1 Materials. The oil sands ore samples were rich-grade Athabasca oil sands ore provided by Syncrude Canada Ltd. The ore has 13.5 wt% bitumen, 3.0 wt% water, 11.2 wt% fine solids, and almost 70 wt% coarse sands. ACS grade cyclohexane (Fisher Scientific, Canada) was used as the solvent in the solvent-based extraction process of oil sands. UV curing adhesive (Norland 5 ACS Paragon Plus Environment
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optical adhesive 81, Norland Products, Inc., NJ) was used for fixing centrifuged fine solids on the glass slide. Athabasca bitumen provided by Syncrude Canada Ltd was used to prepare bitumen coating as the model sample. Silicon wafer with 0.5 µm thermal oxide (silica) (nanoFAB, University of Alberta) was used as the substrate of model samples. Octadecyltrichlorosilane (OTS) (C18H37SiCl3, 95%, ACROS Organics) was used to prepare the hydrophobized AFM tip. 2.2 Collection of Fine Mineral Solids. The protocol used in this work for the solvent extraction of bitumen from oil sands ores follows a reported procedure by Nikakhtari et al.7 Details on collecting fine mineral solids from solvent-extracted bitumen can be found elsewhere.9, 19 A brief schematic of the protocol is shown in Figure 1, summarized as follows: (1) 150 g Athabasca oil sands and 100 g cyclohexane were mixed in a Teflon bottle and rotated end-to-end for 10 min in a rotary mixer, (2) the mixture was allowed to settle in a graduated cylinder for 30 min, (3) the remaining slurry in the bottom portion of the cylinder was mixed with 50 g cyclohexane, agitated for 10 min, (4) the supernatant products as the
