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Monodispersed nickel and cobalt nanoparticles in desulfurization of thiophene for in-situ upgrading of heavy crude oil
Fuel 211 (2018) 697–703
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Full Length Article
Monodispersed nickel and cobalt nanoparticles in desulfurization of thiophene for in-situ upgrading of heavy crude oil Kun Guoa,b, Vidar Folke Hansenc, Hailong Lid, Zhixin Yua,b,
MARK
⁎
a
Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway The National IOR Centre of Norway, University of Stavanger, 4036 Stavanger, Norway c Department of Mechanical and Structural Engineering and Materials Science, University of Stavanger, 4036 Stavanger, Norway d Department of Energy, Building and Environment, Mälardalen University, 72123 Västerås, Sweden b
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoparticle Nickel Cobalt In-situ upgrading Hydrodesulfurization Thiophene
Monodispersed nickel (Ni) and cobalt (Co) nanoparticles (NPs) with different sizes are synthesized via the thermal decomposition of organometallic precursors by controlling the reaction temperature and surfactant amount. X-ray diffraction analysis of the as-prepared NP samples shows the formation of cubic Ni metal phases with good crystallinity, while the cubic Co metal samples are semi-amorphous. Transmission electron microscopy characterization further confirms that two Ni NP samples with average sizes of 9 and 27 nm, and Co NPs with an average size of 6 nm are successfully prepared with a narrow size distribution. Furthermore, catalytic performance of these monodispersed NPs towards the hydrodesulfurization (HDS) reaction, which plays a pivotal role in the upgrading of heavy crude oil, is evaluated under reservoir-relevant conditions using thiophene as a sulfur-containing model compound. Different parameters including particle size, catalyst dosage, hydrogen donor ratio, temperature, and reaction duration are systematically studied to optimize the catalytic HDS performance. The morphology and size of the spent NP catalysts after the reaction are also analyzed. The results show that the 9 nm Ni NPs exhibit the best HDS activity and stability compared with other catalysts, which suggests that such well-dispersed Ni NPs are promising candidates for the in-situ upgrading and recovery of heavy crude oil from underground reservoirs.
1. Introduction
needs additional water separation and recycling facilities. It is therefore necessary to improve this technique to reduce the environmental footprint. Since the discovery of aquathermolysis reactions between injected steam and crude oil in 1980s [16], the concept of in-situ catalytic upgrading and recovery of heavy crude oil has been proposed and developed [3,7]. Extra catalysts are introduced into the reservoirs to facilitate the aquathermolysis reactions, which include a series of hydrocracking, hydrodesulfurization (HDS), hydrodenitrogenation, hydrodeoxygenation and hydrodemetallization reactions. Due to these reactions, large hydrocarbon molecules are cracked into small derivatives and the viscosity and oil quality are thus improved [17]. The reservoir is turned into an underground refinery to ease the extraction and production of heavy crude oil. With regard to catalysts in aquathermolysis reactions, metallic nanoparticle (NP) has emerged to be a competitive alternative compared to other water-soluble, oil-soluble, amphiphilic, minerals and zeolites, and solid superacids catalysts [1,3,7]. Owing to the inherent catalytic activity, high specific surface area and accessibility of active sites, metallic NPs are reported to give rise to high viscosity reduction of
To address the challenges of ever-increasing energy demand and declining crude oil reserves, research in both academia and industry is devoting more attention to unconventional oil, which includes heavy and extra-heavy oils. Compared with conventional oil, the exploration and production of heavy crude oil is a challenging task due to its special physicochemical properties. More specifically, heavy oil is characterized by high viscosity or poor mobility, high fraction of asphaltenes and resins with large molecules, and considerable amounts of heteroatoms, including sulfur, nitrogen, oxygen and metals [1–9]. So far, numerous technologies for the exploration and production of heavy oil have been reported, such as thermal injection [10], chemical injection [11–14], and biological degradation [15]. Among them, thermal injection is regarded as the most effective one and has been widely implemented. This technique is, however, energy-intensive because external energy needs to be supplied by combustion of natural gas to generate steam with high temperature and pressure. In addition, this process causes the emission of large amounts of greenhouse gases and
⁎
Corresponding author at: Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway. E-mail address: [email protected] (Z. Yu).
http://dx.doi.org/10.1016/j.fuel.2017.09.097 Received 14 February 2017; Received in revised form 14 September 2017; Accepted 26 September 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. TEM images and corresponding particle size distribution of the as-prepared Co NPs (a, b) and Ni NPs (c, d and e, f).
∼90% at temperature of ∼300 °C and pressure of > 3 MPa. Furthermore, the propagation of such NPs through porous rocks is demonstrated with little particle retention [18–21]. Nevertheless, the underlying mechanism behind the reactions remains largely unknown and further efforts are necessary to optimize the NP parameters in terms of metal type, particle size, suspension stability and structure design. Previous work mainly uses commercially available metallic NPs or submicron particles, which have wide size distributions and can aggregate severely. Thus, it is difficult to fundamentally study the influence of individual property of the metal NPs on their catalytic activity. To this end, the preparation of monodispersed NP with narrow size distribution remains desirable. Besides, the size effect on catalytic
activity is a frontier research topic in catalysis science. Despite smaller particle size entails higher metallic surface area-to-volume ratio and more active sites, the high surface energy of nanosized particles makes them tend to aggregate and thus lose the stability and activity. By precisely controlling the particle size in the synthesis process, numerous studies have reported the influence of particle size on the catalytic activity for different reactions [22–24]. To the best of our knowledge, the size-activity correlation for the aquathermolysis of heavy crude oil has never been investigated. Furthermore, current industrial catalysts for hydrotreating are prepared in the form of mixed metallic oxides, which need to be reduced to metallic states in the pretreatment process. Direct synthesis of metallic NPs can avoid the pre-reduction process, 698
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Fig. 4. Thiophene conversion using Co-6 NPs with thiophene/tetralin ratios of 10:10, 16:4 and 18:2 at temperature of 160 °C and reaction duration of 24 h.
Fig. 2. XRD patterns of the as-prepared Co NPs and two commercial Co samples.
Fig. 5. Thiophene conversion using Co-6 NPs with catalyst amounts of 20, 50, 100 and 200 mg at temperature of 160 °C, reaction duration of 24 h, and thiophene/tetralin ratio of 18:2.
stainless steel autoclave reactor to simulate an underground reservoir by controlling the reaction conditions at lower temperatures and pressures, similar to the conditions in an oil reservoir. We intend to explore two types of metal NPs (nickel (Ni) and cobalt (Co)) as catalysts in the HDS reaction using thiophene as a sulfur-containing model compound. Ni and Co NPs are in house prepared via the thermal decomposition method. These NPs, together with some commercial Ni and Co particles, are studied in the HDS reaction to investigate the sizedependent activity of metallic catalysts. In addition, ratio between thiophene and hydrogen donor, catalyst dosage, temperature and reaction duration are optimized. To understand the catalytic behavior of NPs, the morphology and size of NP catalysts after the HDS reaction are also characterized. The results presented in this work are important to gain fundamental knowledge for the application of metallic NPs as catalysts in the in-situ upgrading and recovery of heavy crude oil.
Fig. 3. XRD patterns of the as-prepared Ni NPs and two commercial Ni samples.
which is of practical significance if the catalysts are to be applied in the subsurface reservoirs. Given that the actual aquathermolysis contains a series of complex reactions, it is preferable to investigate the various reactions separately using model compounds to represent the crude oil molecules. Among all the existing chemical bonds in hydrocarbon molecules, CeS bond is found to have the smallest bond-dissociation energy. The breaking of CeS bond contributes greatly to the viscosity reduction and crude oil quality upgrading [1,3,25]. Furthermore, desulfurization of crude oil in the oil refineries is becoming an increasingly important task due to the stringent environmental and clean-fuel legislations. Currently, alumina supported NiMo or CoMo sulfide catalysts are applied in industrial HDS processes [26–30]. The HDS reaction is normally conducted in a fixedbed reactor at high temperatures of 300–400 °C and pressures of 3–13 MPa. The deep desulfurization requirements lead to large capital investment and big operational cost of the HDS facilities. Therefore, it is desirable to reduce the sulfur content of crude oil even before it reaches the refinery. In this work, the HDS reaction is performed inside a Teflon-lined
2. Experimental 2.1. Chemicals All chemicals were purchased from Sigma-Aldrich unless otherwise indicated and were used as received without further treatment. These chemicals included nickel(II) acetylacetonate (Ni(acac)2, 95%), 699
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Fig. 6. Thiophene conversion of all Co (a) and Ni (b) samples at different temperatures of 120, 160 and 180 °C.
room temperature overnight. 2.3. Preparation of Ni NPs Ni NPs were synthesized via the modified heat up method [32–34]. For the synthesis of 9 nm Ni NPs, briefly, a 250 mL round bottom threeneck flask containing 10 mL of OAm, 6 mL of TOP and 0.3023 g of Ni (acac)2 was heated up to 100 °C. The flask was connected with a nitrogen inlet, a reflux condenser attached with a bubbler. After purging the flask with nitrogen for at least 30 min, the flask was transferred to another oil bath, which was preheated to 230 °C. In a short time, the solution color changed from green to black, indicating that colloidal NPs were formed. The reaction was held for 20 min and then the flask was taken out from the oil bath and cooled down naturally. Both the reaction medium and the oil bath were stirred with magnetic stir bars at 1000 rpm. Afterwards, a mixture of hexane and ethanol was added to extract the black NPs and centrifugation was conducted to separate them. This process was repeated at least 3 times to remove residual surfactants and impurities. No size selective precipitation procedure was further performed in this preparation. The final powder was then collected by drying the sample in nitrogen at room temperature overnight. For the synthesis of 27 nm Ni NPs, the same procedure was applied except that 3 mL of TOP and 0.4578 g of Ni(acac)2 were added.
Fig. 7. Thiophene conversion of four Ni catalysts with different reaction durations of 12, 24 and 48 h at temperature of 160 °C.
dicobalt octacarbonyl moistened with hexane (1–10%) as a stabilizer (Co2(CO)8, ≥90%), Ni nanopowder (< 100 nm), Ni powder (< 1 µm), Co nanopowder (Alfa Aesar, 25–30 nm), Co powder (∼2 µm), oleylamine (OAm, technical grade, 70%), trioctylphosphine (TOP, 97%), hexane (≥99%), toluene (≥99.5%), 1,2-dichlorobenzene (DCB, 99%, anhydrous), oleic acid (OA, ≥99%), trioctylphosphine oxide (TOPO, 99%), thiophene (≥99%), and 1,2,3,4-tetrahydronaphthalene (tetralin, 99%).
2.4. Materials characterization X-ray powder diffraction (XRD) was conducted to characterize the crystal phase of the samples. The diffraction patterns were recorded on a Bruker-AXS Microdiffractometer (D8 ADVANCE) using Cu Kα radiation source (λ=1.54 Å). Scanning angles for all samples were set in the 2θ range of 10–90° with a step interval of 2.25 °/min. Peaks were indexed according to the database established by Joint Committee on Powder Diffraction Standards (JCPDS). The morphology and structure of the NP catalysts were also characterized by transmission electron microscopy (TEM, JEOL JEM-2100F) with an accelerating voltage of 200 kV. In the specimen preparation, one droplet of the Co or Ni NP suspension was dropped onto a copper grid coated with carbon film (400 mesh, TAAB). The grid was dried at room temperature and protected under a continuous flow of nitrogen to avoid oxidation.
2.2. Preparation of Co NPs Co NPs were synthesized via the modified hot injection method [31]. Briefly, a 250 mL round bottom three-neck flask containing 12 mL of DCB, 0.1 g of TOPO and 0.2 mL of OA was heated up to reflux. The flask was connected with a nitrogen inlet, a reflux condenser attached with a bubbler. After purging the flask with nitrogen for at least 30 min, 0.54 g of Co2(CO)8 dissolved in 3 mL of DCB was rapidly injected into the flask. Upon injection, the solution turned dark black immediately, indicating the decomposition of Co2(CO)8 and the formation of colloidal Co NPs. The reaction was held for 20 min and then the flask was taken out from the oil bath and cooled down naturally. Both the reaction medium and the oil bath were stirred with magnetic stir bars at 1000 rpm. Afterwards, excessive ethanol was added to extract the black NPs and centrifugation at a speed of 6000 rpm for 10 min was conducted to separate them. This process was repeated at least 3 times to remove residual surfactants and impurities. No size selective precipitation procedure was further performed in this preparation. The final powder was then collected by drying the sample in nitrogen at
2.5. HDS of thiophene HDS reaction of thiophene was performed in a 50 mL Teflon-lined stainless steel autoclave reactor. Initially, thiophene as the sulfur-containing model compound and tetralin as the hydrogen donor in a total volume of 20 mL together with the NP catalysts were added into the reactor. No diluting solvent was added to ensure thiophene was excessive in the reactions. The Teflon container was then sonicated for 700
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evaluate the HDS activity of different catalysts, which was carried out with an Agilent 7820 Gas Chromatograph equipped with a J & W HP-5 column (length 30 m, diameter 0.25 mm, thickness 1.0 µm) and a FID detector. In each analysis, 80 µL liquid from the reactor was mixed with 1.6 mL of toluene solvent, and this mixture was utilized to measure thiophene concentration. Each sample test was repeated three times and the average value was taken as the thiophene concentration. The thiophene conversion was expressed as the equation below, where C0 was the initial thiophene content and C1 was the thiophene content after the reaction.
Thiophene conversion =
C0−C1 × 100% C0
3. Results and discussion To determine the morphology and particle size distribution (PSD) of the products, TEM images are recorded for each sample together with the corresponding PSD from statistic data, as shown in Fig. 1. Fig.1a shows a typical TEM image of the as-prepared Co NPs. These spherical NPs are monodispersed and no apparent agglomeration is observed, which is beneficial to have largely exposed metal active sites. Based on a statistic count of particle size (Gauss fit, over 1000 counts) in Fig. 1b, an average particle size of 6 nm is calculated and a narrow size distribution is confirmed. Similarly, monodispersed spherical Ni NPs are also observed in Fig. 1c and e, and statistic counts of particle sizes in Fig. 1d and f confirm an average particle size of 9 and 27 nm, respectively. It should be pointed out that thermal decomposition of organometallic precursors has been widely studied for the controllable synthesis of transition metal NPs. By adjusting the precursor and surfactant amounts, the size of the resulted Ni NPs can be precisely controlled [35]. Furthermore, TEM images of the commercial Co (25–30 nm) and Ni (< 100 nm) NPs are also shown in Figs. S1 and S2, respectively. It can be observed that both samples aggregate into much larger particle sizes and present poor dispersity. The Co samples with different sizes are designated as Co-6, Co-30, and Co-2000 in order of increasing size, whereas Ni samples are similarly designated as Ni-9, Ni27, Ni-100 and Ni-1000. The crystal phase and crystallinity of the as-prepared metal NPs are further analyzed by XRD. Figs. 2 and 3 show the diffraction patterns of the Co and Ni samples, respectively. According to the three characteristic peaks at 44.2°, 51.5° and 75.9° in Fig. 2, both Co-6 and Co-30 NPs can be indexed to cubic α-Co phase, corresponding to the JCPDS card No. 89-7093. The high intensity indicates a relatively good crystallinity of Co-30 NP sample, while the poor peak intensity of the Co-6 NPs indicates either its small particle size as observed from TEM images or its semi-amorphous nature. In addition, the Co-2000 sample presents a hexagonal phase since the peaks at 41.6°, 44.5°, 47.4°, 62.5°, 75.8°, and 84.1° match well with JCPDS card No. 89-7094. With respect to the Ni samples in Fig. 3, three characteristic peaks at 44.5°, 51.8° and 76.4° are observed and they are all indexed to cubic Ni phase, conforming to JCPDS card No. 04-0850. The exceptionally sharp peaks of the Ni-100 and Ni-1000 samples suggest that both samples are well crystalline and the particle sizes are large. Comparatively, the peaks of the Ni-9 and Ni27 NPs are broader and weaker, implying relatively small particle sizes, which are in accordance with the results from TEM characterization. Thiophene is a heterocyclic compound with the formula of C4H4S. This five-membered ring is very commonly found in the asphaltene component of heavy crude oil [36–38]. Herein, we intend to study the catalytic activities of metallic particles towards the HDS reaction as part of the aquathermolysis reaction. Thiophene as the sulfur-containing compound is in liquid phase and tetralin is used as a hydrogen donor. Reaction temperature is controlled in the range of 120–180 °C and reaction pressure is below 3 MPa. These conditions are relatively close to an actual reservoir [36], which is of high importance for the implementation of in-situ upgrading technology underground.
Fig. 8. TEM images and corresponding particle size distribution (inlets) of the as-prepared Co-6 NPs (a), Ni-9 NPs (b) and Ni-27 NPs (c) after the HDS reaction.
10 min to disperse the NPs. The mixture was sealed tightly and the reactor was placed inside an electronic oven preheated to 120–180 °C for a duration of 12–48 h. After the reaction, the reactor was cooled down naturally. Thiophene content after the reaction was analyzed to 701
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To explain the enhanced HDS activity of Ni NPs, we further conduct TEM analysis of the Co-6, Ni-9 and Ni-27 catalysts after HDS reaction at 160 °C for 24 h. Fig. 8 shows the TEM images of these three catalysts and the corresponding PSD (inlets in Fig. 8b and c). After the HDS reaction, Co-6 NPs agglomerate into much larger particle sizes, which can explain the activity degradation of Co NPs. On the contrary, well-dispersed NPs can be observed in Fig. 8b and c for the Ni-9 and Ni-27 catalysts, respectively, although slight aggregation still occurs. PSD curves confirm the average particle sizes of 12 and 31 nm for Ni-9 and Ni-27 NPs, respectively, indicating that Ni catalysts are relatively stable in the HDS reaction. These results suggest that the as-prepared Ni NPs display better stability than the Co NPs. The high stability of the Ni NPs at reaction temperature ensures their high HDS activity at reservoir relevant conditions.
Co samples are selected to examine the influence of catalyst dosage and thiophene/tetralin ratio on the catalytic activity. Fig. 4 presents the thiophene conversion over Co-6 NPs as catalysts at temperature of 160 °C and reaction duration of 24 h. Thiophene/tetralin ratio varies from 10:10, 16:4 to 18:2 in a total volume of 20 mL. Co NP dosage is maintained constant at 20 mg. The highest thiophene conversion of 4.7% is achieved when the ratio is 18:2, compared to 2.8% and 3.1% at ratios of 10:10 and 16:4, respectively. This could be explained by the adsorption of tetralin molecule on the active metal sites when their concentration is high, in accordance with the findings from the Liu’s group [36]. For similar reactions they proposed that hydrogen donors adsorb on the catalyst surface and block the catalytic sites at reaction conditions, which is reasonable given that the boiling point of tetralin is 204 °C, higher than that of thiophene. In the consecutive HDS activity tests, the volume ratio is constantly maintained at 18:2. Fig. 5 shows the thiophene conversion using Co-6 NPs with four different catalyst dosages of 20, 50, 100 and 200 mg and without catalyst as blank test. Reaction temperature of 160 °C, reaction duration of 24 h and feedstock ratio of 18:2 remain the same. In the blank experiment, thiophene is hardly converted, implying the thermal stability of this aromatic compound. However, with the increase of catalyst amount from 20 mg to 100 mg, the conversion rate increases gradually from 4.7% to 8.6%. This can be explained by that the number of available catalytically active sites increases with catalyst amount and thus more thiophene is converted in the HDS reaction. However, further increasing the amount from 100 to 200 mg results in nearly the same conversion. Temperature as a key reaction parameter is investigated with both Co and Ni catalysts. Fig. 6a shows the conversion of three Co catalysts with the same reaction duration of 24 h. Co-6 NPs achieve the highest conversion at all temperatures of 120, 160 and 180 °C, compared with the other two Co catalysts. Meanwhile, the conversion increases substantially as the temperature is raised from 160 to 180 °C, indicating that the HDS activity of Co metal is significantly enhanced at elevated temperatures. In Fig. 6b, the same trend is observed that the HDS activity is improved with reduced Ni particle size as well as elevated temperature. Specifically, Ni-9 NPs present high thiophene conversion rates of 10.5% and 11.3% at 160 and 180 °C, respectively, which are almost twice as high as 5.6% and 6.5% of Co-6 NPs. This suggests that Ni metal displays better activity in the activation of C–S bond of thiophene than Co, which might be ascribed to the more crystalline nature of Ni NPs or their higher intrinsic activity. More interestingly, the conversion is almost stable when further increasing the temperature from 160 to 180 °C, suggesting that for Ni catalysts, further increase of temperature does not result in large activity enhancement. Since Co and Ni catalysts exhibit different temperature-activity trends, it could be speculated that the catalytic reactions for the two metallic NPs might follow different reaction pathways. Furthermore, HDS reaction with four Ni catalysts of different sizes are performed for different reaction periods of 12, 24 and 48 h to explore the time-dependent activity. As shown in Fig. 7, apparent increase of conversion can be observed for all Ni catalysts when extending the reaction duration from 12 to 24 h. Afterwards, the conversions remain unchanged except that conversion over Ni-9 NPs is slightly improved to 11.9% when prolonging the reaction time to 48 h. This result reveals that the reaction time can be optimized to reach the desirable HDS activity of NP catalysts. For a fair comparison, the thiophene conversion at temperature of 160 °C and reaction time of 24 h is used to calculate the average reaction rate, as shown in Figs. S3 and S4, which directly illustrates the size-dependent activity of Co and Ni NPs in the HDS reaction. It demonstrates that for both Ni and Co, the catalytic activity decreases significantly when the particle sizes increase to a certain range, while further increase of the sizes has little influence on the catalytic activity. The same trend has been reported for Co catalysts for Fischer–Tropsch synthesis by de Jong et al., [24] even though supported Co catalysts have been applied.
4. Conclusions In the aquathermolysis of heavy crude oil, HDS is an important reaction involved in the upgrading process. To investigate the size-dependent activity of metal NPs, Co and Ni catalysts of various particle sizes are synthesized with narrow particle size distribution. Meanwhile, parameters of thiophene/tetralin ratio, catalyst dosage, temperature, and reaction duration are systematically optimized. The size dependent activity has been demonstrated for both Ni and Co NPs. It turns out that Ni metallic NPs exhibit better activity in the activation of CeS bond than the Co NPs, which could be attributed to the inherent activity and high thermal stability in the reaction process. This study demonstrates the opportunity for the application of well-dispersed metallic NPs in the in-situ upgrading and recovery of heavy crude oil under reservoir relevant conditions, which contributes to improved oil quality as well as less demanding downstream HDS operation in refineries. Acknowledgements This work was financially supported by the National IOR Centre of Norway at University of Stavanger and the industrial partners of the Centre. The authors appreciate the assistance of Priscille Cuvillier in performing the TEM tests and Jorunn Hamre Vrålstad for the GC analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2017.09.097. References [1] Muraza O, Galadima A. Aquathermolysis of heavy oil: a review and perspective on catalyst development. Fuel 2015;157:219–31. [2] Muraza O. Hydrous pyrolysis of heavy oil using solid acid minerals for viscosity reduction. J Anal Appl Pyrol 2015;114:1–10. [3] Hashemi R, Nassar NN, Almao PP. Nanoparticle technology for heavy oil in-situ upgrading and recovery enhancement: opportunities and challenges. Appl Energy 2014;133:374–87. [4] Mai A, Bryan J, Goodarzi N, Kantzas A. Insights into non-thermal recovery of heavy oil. J Can Petrol Technol 2013;48(03):27–35. [5] Almao PP. In situ upgrading of bitumen and heavy oils via nanocatalysis. Can J Chem Eng 2012;90(2):320–9. [6] Shah A, Fishwick R, Wood J, Leeke G, Rigby S, Greaves M. A review of novel techniques for heavy oil and bitumen extraction and upgrading. Energy Environ Sci 2010;3(6):700–14. [7] Maity SK, Ancheyta J, Marroquín G. Catalytic aquathermolysis used for viscosity reduction of heavy crude oils: a review. Energy Fuels 2010;24(5):2809–16. [8] Guo K, Li H, Yu Z. In-situ heavy and extra-heavy oil recovery: a review. Fuel 2016;185:886–902. [9] Guo K, Li H, Yu Z. Metallic nanoparticles for enhanced heavy oil recovery: promises and challenges. Energy Procedia 2015;75:2068–73. [10] Zhao DW, Wang J, Gates ID. Thermal recovery strategies for thin heavy oil reservoirs. Fuel 2014;117:431–41. [11] Pei H, Zhang G, Ge J, Zhang L, Ma M. Effect of the addition of low molecular weight alcohols on heavy oil recovery during alkaline flooding. Ind Eng Chem Res
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Monodispersed nickel and cobalt nanoparticles in desulfurization of thiophene for in-situ upgrading of heavy crude oil Kun Guoa,b, Vidar Folke Hansenc, Hailong Lid, Zhixin Yua,b,
MARK
⁎
a
Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway The National IOR Centre of Norway, University of Stavanger, 4036 Stavanger, Norway c Department of Mechanical and Structural Engineering and Materials Science, University of Stavanger, 4036 Stavanger, Norway d Department of Energy, Building and Environment, Mälardalen University, 72123 Västerås, Sweden b
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoparticle Nickel Cobalt In-situ upgrading Hydrodesulfurization Thiophene
Monodispersed nickel (Ni) and cobalt (Co) nanoparticles (NPs) with different sizes are synthesized via the thermal decomposition of organometallic precursors by controlling the reaction temperature and surfactant amount. X-ray diffraction analysis of the as-prepared NP samples shows the formation of cubic Ni metal phases with good crystallinity, while the cubic Co metal samples are semi-amorphous. Transmission electron microscopy characterization further confirms that two Ni NP samples with average sizes of 9 and 27 nm, and Co NPs with an average size of 6 nm are successfully prepared with a narrow size distribution. Furthermore, catalytic performance of these monodispersed NPs towards the hydrodesulfurization (HDS) reaction, which plays a pivotal role in the upgrading of heavy crude oil, is evaluated under reservoir-relevant conditions using thiophene as a sulfur-containing model compound. Different parameters including particle size, catalyst dosage, hydrogen donor ratio, temperature, and reaction duration are systematically studied to optimize the catalytic HDS performance. The morphology and size of the spent NP catalysts after the reaction are also analyzed. The results show that the 9 nm Ni NPs exhibit the best HDS activity and stability compared with other catalysts, which suggests that such well-dispersed Ni NPs are promising candidates for the in-situ upgrading and recovery of heavy crude oil from underground reservoirs.
1. Introduction
needs additional water separation and recycling facilities. It is therefore necessary to improve this technique to reduce the environmental footprint. Since the discovery of aquathermolysis reactions between injected steam and crude oil in 1980s [16], the concept of in-situ catalytic upgrading and recovery of heavy crude oil has been proposed and developed [3,7]. Extra catalysts are introduced into the reservoirs to facilitate the aquathermolysis reactions, which include a series of hydrocracking, hydrodesulfurization (HDS), hydrodenitrogenation, hydrodeoxygenation and hydrodemetallization reactions. Due to these reactions, large hydrocarbon molecules are cracked into small derivatives and the viscosity and oil quality are thus improved [17]. The reservoir is turned into an underground refinery to ease the extraction and production of heavy crude oil. With regard to catalysts in aquathermolysis reactions, metallic nanoparticle (NP) has emerged to be a competitive alternative compared to other water-soluble, oil-soluble, amphiphilic, minerals and zeolites, and solid superacids catalysts [1,3,7]. Owing to the inherent catalytic activity, high specific surface area and accessibility of active sites, metallic NPs are reported to give rise to high viscosity reduction of
To address the challenges of ever-increasing energy demand and declining crude oil reserves, research in both academia and industry is devoting more attention to unconventional oil, which includes heavy and extra-heavy oils. Compared with conventional oil, the exploration and production of heavy crude oil is a challenging task due to its special physicochemical properties. More specifically, heavy oil is characterized by high viscosity or poor mobility, high fraction of asphaltenes and resins with large molecules, and considerable amounts of heteroatoms, including sulfur, nitrogen, oxygen and metals [1–9]. So far, numerous technologies for the exploration and production of heavy oil have been reported, such as thermal injection [10], chemical injection [11–14], and biological degradation [15]. Among them, thermal injection is regarded as the most effective one and has been widely implemented. This technique is, however, energy-intensive because external energy needs to be supplied by combustion of natural gas to generate steam with high temperature and pressure. In addition, this process causes the emission of large amounts of greenhouse gases and
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Corresponding author at: Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway. E-mail address: [email protected] (Z. Yu).
http://dx.doi.org/10.1016/j.fuel.2017.09.097 Received 14 February 2017; Received in revised form 14 September 2017; Accepted 26 September 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. TEM images and corresponding particle size distribution of the as-prepared Co NPs (a, b) and Ni NPs (c, d and e, f).
∼90% at temperature of ∼300 °C and pressure of > 3 MPa. Furthermore, the propagation of such NPs through porous rocks is demonstrated with little particle retention [18–21]. Nevertheless, the underlying mechanism behind the reactions remains largely unknown and further efforts are necessary to optimize the NP parameters in terms of metal type, particle size, suspension stability and structure design. Previous work mainly uses commercially available metallic NPs or submicron particles, which have wide size distributions and can aggregate severely. Thus, it is difficult to fundamentally study the influence of individual property of the metal NPs on their catalytic activity. To this end, the preparation of monodispersed NP with narrow size distribution remains desirable. Besides, the size effect on catalytic
activity is a frontier research topic in catalysis science. Despite smaller particle size entails higher metallic surface area-to-volume ratio and more active sites, the high surface energy of nanosized particles makes them tend to aggregate and thus lose the stability and activity. By precisely controlling the particle size in the synthesis process, numerous studies have reported the influence of particle size on the catalytic activity for different reactions [22–24]. To the best of our knowledge, the size-activity correlation for the aquathermolysis of heavy crude oil has never been investigated. Furthermore, current industrial catalysts for hydrotreating are prepared in the form of mixed metallic oxides, which need to be reduced to metallic states in the pretreatment process. Direct synthesis of metallic NPs can avoid the pre-reduction process, 698
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Fig. 4. Thiophene conversion using Co-6 NPs with thiophene/tetralin ratios of 10:10, 16:4 and 18:2 at temperature of 160 °C and reaction duration of 24 h.
Fig. 2. XRD patterns of the as-prepared Co NPs and two commercial Co samples.
Fig. 5. Thiophene conversion using Co-6 NPs with catalyst amounts of 20, 50, 100 and 200 mg at temperature of 160 °C, reaction duration of 24 h, and thiophene/tetralin ratio of 18:2.
stainless steel autoclave reactor to simulate an underground reservoir by controlling the reaction conditions at lower temperatures and pressures, similar to the conditions in an oil reservoir. We intend to explore two types of metal NPs (nickel (Ni) and cobalt (Co)) as catalysts in the HDS reaction using thiophene as a sulfur-containing model compound. Ni and Co NPs are in house prepared via the thermal decomposition method. These NPs, together with some commercial Ni and Co particles, are studied in the HDS reaction to investigate the sizedependent activity of metallic catalysts. In addition, ratio between thiophene and hydrogen donor, catalyst dosage, temperature and reaction duration are optimized. To understand the catalytic behavior of NPs, the morphology and size of NP catalysts after the HDS reaction are also characterized. The results presented in this work are important to gain fundamental knowledge for the application of metallic NPs as catalysts in the in-situ upgrading and recovery of heavy crude oil.
Fig. 3. XRD patterns of the as-prepared Ni NPs and two commercial Ni samples.
which is of practical significance if the catalysts are to be applied in the subsurface reservoirs. Given that the actual aquathermolysis contains a series of complex reactions, it is preferable to investigate the various reactions separately using model compounds to represent the crude oil molecules. Among all the existing chemical bonds in hydrocarbon molecules, CeS bond is found to have the smallest bond-dissociation energy. The breaking of CeS bond contributes greatly to the viscosity reduction and crude oil quality upgrading [1,3,25]. Furthermore, desulfurization of crude oil in the oil refineries is becoming an increasingly important task due to the stringent environmental and clean-fuel legislations. Currently, alumina supported NiMo or CoMo sulfide catalysts are applied in industrial HDS processes [26–30]. The HDS reaction is normally conducted in a fixedbed reactor at high temperatures of 300–400 °C and pressures of 3–13 MPa. The deep desulfurization requirements lead to large capital investment and big operational cost of the HDS facilities. Therefore, it is desirable to reduce the sulfur content of crude oil even before it reaches the refinery. In this work, the HDS reaction is performed inside a Teflon-lined
2. Experimental 2.1. Chemicals All chemicals were purchased from Sigma-Aldrich unless otherwise indicated and were used as received without further treatment. These chemicals included nickel(II) acetylacetonate (Ni(acac)2, 95%), 699
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Fig. 6. Thiophene conversion of all Co (a) and Ni (b) samples at different temperatures of 120, 160 and 180 °C.
room temperature overnight. 2.3. Preparation of Ni NPs Ni NPs were synthesized via the modified heat up method [32–34]. For the synthesis of 9 nm Ni NPs, briefly, a 250 mL round bottom threeneck flask containing 10 mL of OAm, 6 mL of TOP and 0.3023 g of Ni (acac)2 was heated up to 100 °C. The flask was connected with a nitrogen inlet, a reflux condenser attached with a bubbler. After purging the flask with nitrogen for at least 30 min, the flask was transferred to another oil bath, which was preheated to 230 °C. In a short time, the solution color changed from green to black, indicating that colloidal NPs were formed. The reaction was held for 20 min and then the flask was taken out from the oil bath and cooled down naturally. Both the reaction medium and the oil bath were stirred with magnetic stir bars at 1000 rpm. Afterwards, a mixture of hexane and ethanol was added to extract the black NPs and centrifugation was conducted to separate them. This process was repeated at least 3 times to remove residual surfactants and impurities. No size selective precipitation procedure was further performed in this preparation. The final powder was then collected by drying the sample in nitrogen at room temperature overnight. For the synthesis of 27 nm Ni NPs, the same procedure was applied except that 3 mL of TOP and 0.4578 g of Ni(acac)2 were added.
Fig. 7. Thiophene conversion of four Ni catalysts with different reaction durations of 12, 24 and 48 h at temperature of 160 °C.
dicobalt octacarbonyl moistened with hexane (1–10%) as a stabilizer (Co2(CO)8, ≥90%), Ni nanopowder (< 100 nm), Ni powder (< 1 µm), Co nanopowder (Alfa Aesar, 25–30 nm), Co powder (∼2 µm), oleylamine (OAm, technical grade, 70%), trioctylphosphine (TOP, 97%), hexane (≥99%), toluene (≥99.5%), 1,2-dichlorobenzene (DCB, 99%, anhydrous), oleic acid (OA, ≥99%), trioctylphosphine oxide (TOPO, 99%), thiophene (≥99%), and 1,2,3,4-tetrahydronaphthalene (tetralin, 99%).
2.4. Materials characterization X-ray powder diffraction (XRD) was conducted to characterize the crystal phase of the samples. The diffraction patterns were recorded on a Bruker-AXS Microdiffractometer (D8 ADVANCE) using Cu Kα radiation source (λ=1.54 Å). Scanning angles for all samples were set in the 2θ range of 10–90° with a step interval of 2.25 °/min. Peaks were indexed according to the database established by Joint Committee on Powder Diffraction Standards (JCPDS). The morphology and structure of the NP catalysts were also characterized by transmission electron microscopy (TEM, JEOL JEM-2100F) with an accelerating voltage of 200 kV. In the specimen preparation, one droplet of the Co or Ni NP suspension was dropped onto a copper grid coated with carbon film (400 mesh, TAAB). The grid was dried at room temperature and protected under a continuous flow of nitrogen to avoid oxidation.
2.2. Preparation of Co NPs Co NPs were synthesized via the modified hot injection method [31]. Briefly, a 250 mL round bottom three-neck flask containing 12 mL of DCB, 0.1 g of TOPO and 0.2 mL of OA was heated up to reflux. The flask was connected with a nitrogen inlet, a reflux condenser attached with a bubbler. After purging the flask with nitrogen for at least 30 min, 0.54 g of Co2(CO)8 dissolved in 3 mL of DCB was rapidly injected into the flask. Upon injection, the solution turned dark black immediately, indicating the decomposition of Co2(CO)8 and the formation of colloidal Co NPs. The reaction was held for 20 min and then the flask was taken out from the oil bath and cooled down naturally. Both the reaction medium and the oil bath were stirred with magnetic stir bars at 1000 rpm. Afterwards, excessive ethanol was added to extract the black NPs and centrifugation at a speed of 6000 rpm for 10 min was conducted to separate them. This process was repeated at least 3 times to remove residual surfactants and impurities. No size selective precipitation procedure was further performed in this preparation. The final powder was then collected by drying the sample in nitrogen at
2.5. HDS of thiophene HDS reaction of thiophene was performed in a 50 mL Teflon-lined stainless steel autoclave reactor. Initially, thiophene as the sulfur-containing model compound and tetralin as the hydrogen donor in a total volume of 20 mL together with the NP catalysts were added into the reactor. No diluting solvent was added to ensure thiophene was excessive in the reactions. The Teflon container was then sonicated for 700
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evaluate the HDS activity of different catalysts, which was carried out with an Agilent 7820 Gas Chromatograph equipped with a J & W HP-5 column (length 30 m, diameter 0.25 mm, thickness 1.0 µm) and a FID detector. In each analysis, 80 µL liquid from the reactor was mixed with 1.6 mL of toluene solvent, and this mixture was utilized to measure thiophene concentration. Each sample test was repeated three times and the average value was taken as the thiophene concentration. The thiophene conversion was expressed as the equation below, where C0 was the initial thiophene content and C1 was the thiophene content after the reaction.
Thiophene conversion =
C0−C1 × 100% C0
3. Results and discussion To determine the morphology and particle size distribution (PSD) of the products, TEM images are recorded for each sample together with the corresponding PSD from statistic data, as shown in Fig. 1. Fig.1a shows a typical TEM image of the as-prepared Co NPs. These spherical NPs are monodispersed and no apparent agglomeration is observed, which is beneficial to have largely exposed metal active sites. Based on a statistic count of particle size (Gauss fit, over 1000 counts) in Fig. 1b, an average particle size of 6 nm is calculated and a narrow size distribution is confirmed. Similarly, monodispersed spherical Ni NPs are also observed in Fig. 1c and e, and statistic counts of particle sizes in Fig. 1d and f confirm an average particle size of 9 and 27 nm, respectively. It should be pointed out that thermal decomposition of organometallic precursors has been widely studied for the controllable synthesis of transition metal NPs. By adjusting the precursor and surfactant amounts, the size of the resulted Ni NPs can be precisely controlled [35]. Furthermore, TEM images of the commercial Co (25–30 nm) and Ni (< 100 nm) NPs are also shown in Figs. S1 and S2, respectively. It can be observed that both samples aggregate into much larger particle sizes and present poor dispersity. The Co samples with different sizes are designated as Co-6, Co-30, and Co-2000 in order of increasing size, whereas Ni samples are similarly designated as Ni-9, Ni27, Ni-100 and Ni-1000. The crystal phase and crystallinity of the as-prepared metal NPs are further analyzed by XRD. Figs. 2 and 3 show the diffraction patterns of the Co and Ni samples, respectively. According to the three characteristic peaks at 44.2°, 51.5° and 75.9° in Fig. 2, both Co-6 and Co-30 NPs can be indexed to cubic α-Co phase, corresponding to the JCPDS card No. 89-7093. The high intensity indicates a relatively good crystallinity of Co-30 NP sample, while the poor peak intensity of the Co-6 NPs indicates either its small particle size as observed from TEM images or its semi-amorphous nature. In addition, the Co-2000 sample presents a hexagonal phase since the peaks at 41.6°, 44.5°, 47.4°, 62.5°, 75.8°, and 84.1° match well with JCPDS card No. 89-7094. With respect to the Ni samples in Fig. 3, three characteristic peaks at 44.5°, 51.8° and 76.4° are observed and they are all indexed to cubic Ni phase, conforming to JCPDS card No. 04-0850. The exceptionally sharp peaks of the Ni-100 and Ni-1000 samples suggest that both samples are well crystalline and the particle sizes are large. Comparatively, the peaks of the Ni-9 and Ni27 NPs are broader and weaker, implying relatively small particle sizes, which are in accordance with the results from TEM characterization. Thiophene is a heterocyclic compound with the formula of C4H4S. This five-membered ring is very commonly found in the asphaltene component of heavy crude oil [36–38]. Herein, we intend to study the catalytic activities of metallic particles towards the HDS reaction as part of the aquathermolysis reaction. Thiophene as the sulfur-containing compound is in liquid phase and tetralin is used as a hydrogen donor. Reaction temperature is controlled in the range of 120–180 °C and reaction pressure is below 3 MPa. These conditions are relatively close to an actual reservoir [36], which is of high importance for the implementation of in-situ upgrading technology underground.
Fig. 8. TEM images and corresponding particle size distribution (inlets) of the as-prepared Co-6 NPs (a), Ni-9 NPs (b) and Ni-27 NPs (c) after the HDS reaction.
10 min to disperse the NPs. The mixture was sealed tightly and the reactor was placed inside an electronic oven preheated to 120–180 °C for a duration of 12–48 h. After the reaction, the reactor was cooled down naturally. Thiophene content after the reaction was analyzed to 701
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To explain the enhanced HDS activity of Ni NPs, we further conduct TEM analysis of the Co-6, Ni-9 and Ni-27 catalysts after HDS reaction at 160 °C for 24 h. Fig. 8 shows the TEM images of these three catalysts and the corresponding PSD (inlets in Fig. 8b and c). After the HDS reaction, Co-6 NPs agglomerate into much larger particle sizes, which can explain the activity degradation of Co NPs. On the contrary, well-dispersed NPs can be observed in Fig. 8b and c for the Ni-9 and Ni-27 catalysts, respectively, although slight aggregation still occurs. PSD curves confirm the average particle sizes of 12 and 31 nm for Ni-9 and Ni-27 NPs, respectively, indicating that Ni catalysts are relatively stable in the HDS reaction. These results suggest that the as-prepared Ni NPs display better stability than the Co NPs. The high stability of the Ni NPs at reaction temperature ensures their high HDS activity at reservoir relevant conditions.
Co samples are selected to examine the influence of catalyst dosage and thiophene/tetralin ratio on the catalytic activity. Fig. 4 presents the thiophene conversion over Co-6 NPs as catalysts at temperature of 160 °C and reaction duration of 24 h. Thiophene/tetralin ratio varies from 10:10, 16:4 to 18:2 in a total volume of 20 mL. Co NP dosage is maintained constant at 20 mg. The highest thiophene conversion of 4.7% is achieved when the ratio is 18:2, compared to 2.8% and 3.1% at ratios of 10:10 and 16:4, respectively. This could be explained by the adsorption of tetralin molecule on the active metal sites when their concentration is high, in accordance with the findings from the Liu’s group [36]. For similar reactions they proposed that hydrogen donors adsorb on the catalyst surface and block the catalytic sites at reaction conditions, which is reasonable given that the boiling point of tetralin is 204 °C, higher than that of thiophene. In the consecutive HDS activity tests, the volume ratio is constantly maintained at 18:2. Fig. 5 shows the thiophene conversion using Co-6 NPs with four different catalyst dosages of 20, 50, 100 and 200 mg and without catalyst as blank test. Reaction temperature of 160 °C, reaction duration of 24 h and feedstock ratio of 18:2 remain the same. In the blank experiment, thiophene is hardly converted, implying the thermal stability of this aromatic compound. However, with the increase of catalyst amount from 20 mg to 100 mg, the conversion rate increases gradually from 4.7% to 8.6%. This can be explained by that the number of available catalytically active sites increases with catalyst amount and thus more thiophene is converted in the HDS reaction. However, further increasing the amount from 100 to 200 mg results in nearly the same conversion. Temperature as a key reaction parameter is investigated with both Co and Ni catalysts. Fig. 6a shows the conversion of three Co catalysts with the same reaction duration of 24 h. Co-6 NPs achieve the highest conversion at all temperatures of 120, 160 and 180 °C, compared with the other two Co catalysts. Meanwhile, the conversion increases substantially as the temperature is raised from 160 to 180 °C, indicating that the HDS activity of Co metal is significantly enhanced at elevated temperatures. In Fig. 6b, the same trend is observed that the HDS activity is improved with reduced Ni particle size as well as elevated temperature. Specifically, Ni-9 NPs present high thiophene conversion rates of 10.5% and 11.3% at 160 and 180 °C, respectively, which are almost twice as high as 5.6% and 6.5% of Co-6 NPs. This suggests that Ni metal displays better activity in the activation of C–S bond of thiophene than Co, which might be ascribed to the more crystalline nature of Ni NPs or their higher intrinsic activity. More interestingly, the conversion is almost stable when further increasing the temperature from 160 to 180 °C, suggesting that for Ni catalysts, further increase of temperature does not result in large activity enhancement. Since Co and Ni catalysts exhibit different temperature-activity trends, it could be speculated that the catalytic reactions for the two metallic NPs might follow different reaction pathways. Furthermore, HDS reaction with four Ni catalysts of different sizes are performed for different reaction periods of 12, 24 and 48 h to explore the time-dependent activity. As shown in Fig. 7, apparent increase of conversion can be observed for all Ni catalysts when extending the reaction duration from 12 to 24 h. Afterwards, the conversions remain unchanged except that conversion over Ni-9 NPs is slightly improved to 11.9% when prolonging the reaction time to 48 h. This result reveals that the reaction time can be optimized to reach the desirable HDS activity of NP catalysts. For a fair comparison, the thiophene conversion at temperature of 160 °C and reaction time of 24 h is used to calculate the average reaction rate, as shown in Figs. S3 and S4, which directly illustrates the size-dependent activity of Co and Ni NPs in the HDS reaction. It demonstrates that for both Ni and Co, the catalytic activity decreases significantly when the particle sizes increase to a certain range, while further increase of the sizes has little influence on the catalytic activity. The same trend has been reported for Co catalysts for Fischer–Tropsch synthesis by de Jong et al., [24] even though supported Co catalysts have been applied.
4. Conclusions In the aquathermolysis of heavy crude oil, HDS is an important reaction involved in the upgrading process. To investigate the size-dependent activity of metal NPs, Co and Ni catalysts of various particle sizes are synthesized with narrow particle size distribution. Meanwhile, parameters of thiophene/tetralin ratio, catalyst dosage, temperature, and reaction duration are systematically optimized. The size dependent activity has been demonstrated for both Ni and Co NPs. It turns out that Ni metallic NPs exhibit better activity in the activation of CeS bond than the Co NPs, which could be attributed to the inherent activity and high thermal stability in the reaction process. This study demonstrates the opportunity for the application of well-dispersed metallic NPs in the in-situ upgrading and recovery of heavy crude oil under reservoir relevant conditions, which contributes to improved oil quality as well as less demanding downstream HDS operation in refineries. Acknowledgements This work was financially supported by the National IOR Centre of Norway at University of Stavanger and the industrial partners of the Centre. The authors appreciate the assistance of Priscille Cuvillier in performing the TEM tests and Jorunn Hamre Vrålstad for the GC analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2017.09.097. References [1] Muraza O, Galadima A. Aquathermolysis of heavy oil: a review and perspective on catalyst development. Fuel 2015;157:219–31. [2] Muraza O. Hydrous pyrolysis of heavy oil using solid acid minerals for viscosity reduction. J Anal Appl Pyrol 2015;114:1–10. [3] Hashemi R, Nassar NN, Almao PP. Nanoparticle technology for heavy oil in-situ upgrading and recovery enhancement: opportunities and challenges. Appl Energy 2014;133:374–87. [4] Mai A, Bryan J, Goodarzi N, Kantzas A. Insights into non-thermal recovery of heavy oil. J Can Petrol Technol 2013;48(03):27–35. [5] Almao PP. In situ upgrading of bitumen and heavy oils via nanocatalysis. Can J Chem Eng 2012;90(2):320–9. [6] Shah A, Fishwick R, Wood J, Leeke G, Rigby S, Greaves M. A review of novel techniques for heavy oil and bitumen extraction and upgrading. Energy Environ Sci 2010;3(6):700–14. [7] Maity SK, Ancheyta J, Marroquín G. Catalytic aquathermolysis used for viscosity reduction of heavy crude oils: a review. Energy Fuels 2010;24(5):2809–16. [8] Guo K, Li H, Yu Z. In-situ heavy and extra-heavy oil recovery: a review. Fuel 2016;185:886–902. [9] Guo K, Li H, Yu Z. Metallic nanoparticles for enhanced heavy oil recovery: promises and challenges. Energy Procedia 2015;75:2068–73. [10] Zhao DW, Wang J, Gates ID. Thermal recovery strategies for thin heavy oil reservoirs. Fuel 2014;117:431–41. [11] Pei H, Zhang G, Ge J, Zhang L, Ma M. Effect of the addition of low molecular weight alcohols on heavy oil recovery during alkaline flooding. Ind Eng Chem Res
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