Upgrading Heavy Crude Oils and Extra Heavy Fractions in

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Upgrading heavy crude oils and extra heavy fractions in supercritical methanol Muhammad Kashif Khan, Winarto Kwek, and Jaehoon Kim Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02524 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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Energy & Fuels

Upgrading heavy crude oils and extra heavy fractions in supercritical methanol

Muhammad Kashif Khana, Winarto Kweka, Jaehoon Kim*,a,b

a

School of Mechanical Engineering, Sungkyunkwan University

2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea b

SKKU Advanced Institute of Nano Technology (SAINT),

2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea

*

Corresponding author. E-mail: [email protected]; tel.:+82-31-299-4843; fax: +82-31-290-

5889

1

ACS Paragon Plus Environment

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Abstract Herein, we report a method of upgrading unconventional crude oils and extra heavy fractions using supercritical methanol (scMeOH) and compare it to supercritical water (scH2O)-based and pyrolytic upgrading. The yields and properties of upgraded oil are explored as functions of operating parameters (temperature, pressure, concentration) and feedstocks for high-acid crudes (Laguna, Bachaquero-13), a heavy crude (Rubiales), and a vacuum tower bottom (VTB). As a result, scMeOH upgrading of unconventional crude oils at 400 °C and 30 MPa effectively reduced their asphaltene content to ~0 wt% and increased that of naphtha-to-diesel fractions to 30–40 wt%. Conversely, a considerable amount of asphaltenes (8.8–10.0 wt%) was present in oil upgraded using scH2O and pyrolysis. Additionally, scMeOH upgrading resulted in a more effective reduction of the total acid number (TAN) of high-acid crudes (< 0.5 mg-KOH/g-oil) compared to values achieved by scH2O and pyrolysis methods. Finally, scMeOH treatment significantly reduced the metal (Ni, V, Fe) content of the upgraded oil. The effective asphaltene content and TAN reduction realized in scMeOH was attributed to its hydrogen donation and esterification ability, with plausible mechanisms of scMeOH, scH2O, and pyrolytic upgrading presented and discussed in detail. Keywords: supercritical methanol, supercritical water, upgrading, unconventional crude oils, asphaltenes

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1. Introduction The rapid economic growth and technological improvements of the last few decades together with the fast depletion of conventional crude oils steadily increase the importance of unconventional crude oils (high-acid crudes, heavy crudes, bitumen, and oil sand), which account for approximately 10% of the global oil production.1,2 The fundamental characteristics of unconventional crude oils making them different from conventional crudes are high viscosity and density, chemical complexity, high acidity, increased level of metal and heteroatom impurities, high asphaltene content, low API gravity, and a low H/C ratio.3 Although these unfavorable characteristics make them difficult to process in currently existing refineries, unconventional crudes are considered highly promising and practical alternatives for replacing conventional crudes in the near future. To date, the large-scale processing of unconventional crude is achieved using upgrading techniques developed for treating heavy residue fractions such as vacuum residue and vacuum tower bottom (VTB). These residue upgrading techniques can be classified into two major groups, namely hydrogen addition (e.g., hydrotreating and hydrocracking) and carbon rejection (e.g., solvent deasphalting, delayed coking, visbreaking, and thermal cracking).4,5 Although these techniques allow the residue to be converted into light fractions, some of their limitations make them unsuitable for unconventional crude oils. For example, thermal cracking requires very high operating temperatures in the range of 500–1000 °C, resulting in extensive coke formation,4 while solvent deasphalting requires large amounts of expensive solvents for separating highmolecular-weight components, with the yields of recovered deasphalted oil typically equaling 30–50%.6-8 Finally, catalytic deactivation caused by coking and inherently present metals (e.g., Ni, V, Ca, Fe) and high consumption of expensive hydrogen are major drawbacks of the 3


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hydrogen addition method.9,10 Supercritical water (scH2O, Pc = 22.1 MPa, Tc = 374 °C) has recently been widely utilized for upgrading heavy fractions (e.g., vacuum residue, asphaltenes, and coal tar) and heavy crude oils (e.g., bitumen, oil shale, and oil sand) due to exhibiting unique properties such as a high propensity to solubilize organic substances and pressure- and temperature-controlled tunability of physicochemical properties.5,11-13 Heavy crude components are well soluble in supercritical water, which increases reaction rates and suppresses coke formation.12 Despite the clear advantages of scH2O upgrading such as moderate coke formation and affordable process cost, as compared to carbon rejection and hydrogen addition processes,11 it still exhibits several limitations, such as high operating temperatures, high asphaltene content and low H/C ratio of the produced oil (reduction efficiency < 40%), low conversion efficiency of heavy fractions, and low light oil yield.14 For example, scH2O bitumen upgrading at 450 °C for 90 min moderately reduced asphaltene content from 16.5 to 9.0 wt%, achieving a reduction efficiency of 45% and being accompanied by a significant degree of coking (13 wt%).15 In addition, the efficiency of metal (e.g., V, Ni, Fe, Ca) and heteroatom (N and S) impurity removal from petroleum feedstocks during scH2O upgrading is somewhat limited, e.g., upgrading at 420 °C and 25 MPa for 60 min reduced the levels of S, N, Ni, and V in vacuum residue by 32, 15, 83, and 85%, respectively.16,17 Moreover, reduction of the total acid number (TAN) in scH2O requires severe reaction conditions. For instance, upgrading at 490 °C and 45 MPa for 90 min was required to convert 83.0% of the neat naphthenic acid mixture.18 Therefore, more effective, economically viable, and simpler methods need to be developed for enhanced utilization of unconventional crude oils. Herein, we demonstrate for the first time that supercritical methanol (scMeOH, Tc = 239 °C, 4


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Pc = 8.08 MPa) is a very effective medium for upgrading unconventional crudes to produce asphaltene-free oil. Previously, scMeOH has been utilized for upgrading heavy crude fractions (de-oiled asphaltenes) to achieve catalyst- and external-hydrogen-free impurity removal (S, N, Ni, V) and reduce asphaltene and non-distillate fraction contents.14 The unique in situ hydrogen donation ability (donation of α-hydrogen as hydride and the Meerwein-Ponndorf reduction),19,20 low dielectric constant, low degree of hydrogen bonding,21,22 high diffusivity, high average chemical potential energy,23 and unique reactivity (e.g., esterification)24 of scMeOH make it a suitable medium for upgrading unconventional crude oils without utilizing external catalysts and molecular hydrogen. This study aimed to utilize scMeOH for reducing the asphaltene content, TAN, non-distillate fraction content, and metal (Ni, V, Fe, Ca) and heteroatom (S, N, O) impurity levels in unconventional crude oil, investigating the effect of operating parameters (temperature, pressure, crude concentration) and crude oil type (Laguna, Bachaquero-13, and Rubiales). ScMeOH upgrading was compared with pyrolytic, scH2O, and (scH2O + scMeOH) upgrading to gain insights into the role of scMeOH in treating unconventional crude oils. In addition, the feedstock dependency of scMeOH upgrading was investigated by subjecting VTB (produced by vacuum residue hydrocracking followed by vacuum distillation) to the above treatment.

2. Experimental 2.1. Chemicals and feedstocks Three types of unconventional crudes (Laguna, Venezuela; Bachaquero-13, Venezuela; Rubiales, Columbia) and conventional crude oil (Kuwait) were provided by SK Innovation (South Korea). VTB was provided by GS-Caltex (South Korea). HPLC grade isopropyl alcohol, dichloromethane, and methanol were obtained from Honeywell Burdick & Jackson® (USA). Deionized water, hexane (99.5%), and toluene (99.5%) were purchased from Sigma Aldrich 5


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(USA). High-purity nitrogen (99.99%) was purchased from the JC Gas Company (South Korea). 2.2. Reaction protocol in scMeOH A custom-built batch reactor system, comprising stainless steel (SUS-316) with an inner volume of 140 mL, was used for upgrading unconventional crude oils and VTB in scMeOH, with details of the reaction protocol and reactor assembly provided elsewhere.25 The yields of upgraded oil, coke, and gases were calculated as follows. Upgraded oil yield wt% =

!

"!"#"" ! $%&

!, !

× 100%

Coke yield wt% = "!"#"" ! $%& × 100% / !

Gas yield wt% = "!"#"" ! $%& × 100%

(1)

(2)

(3)

The yields of gas and upgraded oil were based on dry ash free (daf) heavy oils and the coke yield was calculated on dry basis. The ash content in all the feedstocks, measured using TGA, were less than 0.1 wt% (see Figure 1d). The overall average mass balance based on the daf heavy crude oil was in the range of 97.0–99.0% in case of scH2O reaction, and 95.5–97.7% in case of scMeOH reaction, and 98.5–101.1% in case of scH2O+scMeOH reaction. The total mass balance of the scH2O+scMeOH reaction was slightly higher than 100% in some cases because enhanced self-decomposition of supercritical alcohols at a high temperature (400 °C) in the presence of water may produce low-molecular-weight fractions,26 which could contribute to increase the upgraded oil yield.

6


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2.3. Product analysis The inorganic content of feedstocks was measured using thermogravimetric analysis (TGA; Q50, TA Instruments, USA) in a temperature range of 30 to 800 °C at a heating rate of 10 °C min–1 and an air flow rate of 60 mL min–1. TANs of feedstocks and upgraded oils were determined by a potentiometric method (ASTM D664-11a) using a Metrohm 877 Titrino plus instrument (Metrohm Ltd., Switzerland). The error range was within ± 5% for all the TAN measurements. A mixture of toluene (50 vol%), 2-propanol (49.5 vol%), and deionized water (0.5 vol%) was used as a solvent, and TAN and TAN reduction were calculated as follows. TAN 3

4 567

8 =

9:4 ; 567 × 535 °C, > C44).31,32 The investigated unconventional crude oils were very similar to each other in terms of the obtained fractions, with the VGO and non-distillable residue fractions accounting for ~40 and 31–37 wt%, respectively. Conversely, VTB exhibited the highest non-distillable fraction content of 82 wt%. When compared to conventional crude oil from Kuwait, the obtained boiling point distribution shows that all unconventional crude oils and VTB mainly comprised high-boiling fractions, in good agreement with the results of SARA analysis. Figure 1d shows TGA profiles of feedstocks collected under airflow conditions, revealing that unconventional crude oils and VTB contained around 0.07–0.1 wt% ash, which was typically composed of SiO2, Al2O3, ZnO, K2O, and Fe2O3 (Table S1).

The high content of resins and asphaltenes in VTB compared to that of

unconventional crude oils was confirmed by TGA: at 450 °C, VTB showed the least weight loss 9


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of 35 wt%, whereas that of unconventional crude oils equaled 60–68 wt% at the same temperature. 3.2. Comparison of upgrading techniques Pyrolytic, scH2O, scMeOH, and (scH2O + scMeOH) upgrading techniques were compared at 400 °C (1-h reaction) using Laguna as a feedstock (Figure 2). Pyrolysis in an atmosphere of N2 resulted in a low oil yield (74.5 wt%) and a high coke yield (22.7 wt%). During pyrolysis, most high-molecular-weight compounds (asphaltenes and resins) underwent coking under hydrogendeficient conditions33 and the asphaltene content of pyrolytically produced oil marginally decreased from 27.5 to 20.7 area% (or 12.9 to 10.1 wt%). ScH2O upgrading of Laguna at 30 MPa and 9.1 wt% of Laguna increased the oil yield to 89.2 wt%, effectively suppressing the formation of coke (4.9 wt%). However, as in the case of pyrolysis, the use of scH2O was not effective in reducing the asphaltene content of upgraded oil, which was still high (19.0 area%, 8.8 wt%). Conversely, the utilization of scMeOH achieved almost complete removal of asphaltene and a substantial increase of saturates + aromatics content from 53.6 (Laguna) to 75.5 area% (upgraded oil), clearly indicated the superiority of scMeOH for reducing asphaltene content. The yield of coke produced in scMeOH was higher (20 wt%) than that in scH2O (4.9 wt%), suggesting that the effective removal of asphaltene in the former case was related to the formation of coke as a byproduct. Therefore, scMeOH could effectively stabilize the aliphatic hydrocarbon radicals produced by C-C bond scission of polyaromatic centers. On the other hand, the highly reactive aromatic cores of asphlatenes fused together to form coke. The enhanced polymerization of asphaltenes core radicals might be the reason for selective accumulation of heteroatoms (S and N) and metallic impurities (Ni, V, Fe and Ca) into the coke phase.34,35 Since the dielectric constant (ε) of methanol decreases with increasing pressure and temperature 10


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(e.g., εscMeOH = 2.4 at 320 °C and 20.8 MPa36), low-molecular-weight non-polar fractions of Laguna are well-soluble in scMeOH, while the high-molecular-weight fractions can be well dispersed in the same. However, the effect of solubility and dispersion may not be the only factors responsible for the effective asphaltene content reduction, since its efficiency in scH2O was not as effective as that in scMeOH, although crude oil fractions also exhibit good solubility and dispersion in scH2O at 400 °C and 30 MPa.37 Therefore, the above phenomenon can be attributed to the unique reactivity of scMeOH, which can donate its α-hydrogen in hydride form20 or take part in Meerwein-Ponndorf reduction,19 accelerating the conversion of highmolecular-weight asphaltenes into saturates and aromatics. On the other hand, scH2O itself is not capable of donating hydrogen during upgrading reactions.38 Gas phase analysis results (Figure S2) evidenced the significantly enhanced production of hydrogen during the reaction in scMeOH, as compared to pyrolytic and scH2O reactions. Although the yield of pyrolytically upgraded oil was low, its content of naphtha-to-diesel fractions significantly increased (16 to 36 wt%), and that of non-distillable residue fraction decreased from 39 to 18 wt% (Figure 2c). The residue fraction contents of oils upgraded using scH2O and scMeOH were similar, with the naphtha-to-diesel fractions being slightly less abundant (25–29 wt%) compared to those of pyrolytically produced oil. As shown in Figure 2d, scMeOH was highly efficient in reducing the TAN of unconventional crude oil from 5.1 to 0.44 mg-KOH/g-oil (reduction efficiency of 91.4%), meeting crude distillation unit requirements (< 0.5 mg-KOH/g-oil), whereas oil upgraded in scH2O exhibited a much higher TAN (3.47 mg-KOH/g-oil). Pyrolysis of Laguna reduced its TAN to 0.94 mg-KOH/g-oil, corresponding to a reduction efficiency of 81.6%. The main deacidification

mechanism

of

pyrolytic

process

and

11


scH2O

upgrading

is

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decarboxylation/decarbonylation by thermal cracking,18,39 while esterification is considered to be the dominant pathway of NA removal in the presence of scMeOH.40 A mixture of scH2O and scMeOH (1:1 w/w) was tested to investigate the feasibility of increasing upgraded oil yield while reducing its asphaltene content. The yield of oil produced in the mixed medium (89.2 wt%) exceeded that produced in scMeOH, while a non-negligible amount of asphaltenes was still present in the upgraded oil (5.8 area%, 2.6 wt%). Moreover, the scH2O + scMeOH mixture effectively reduced the TAN to 0.68 mg-KOH/g-oil. Plausible mechanisms of asphaltene conversion during pyrolytic and scH2O-/scMeOH-based upgrading are shown in Figure 3. The average molecular weight of asphaltenes is ~750 g mol–1, with the molecular weight range equaling 400–1000 g mol–1.41 Asphaltenes feature polycyclic aromatic hydrocarbon cores connected with five- or six-membered rings containing heteroatoms (e.g., pyrrole and pyridine).42 The unique structure and self-aggregation tendency of asphaltene molecules can lead to the formation of their nanoaggregates and clusters. For example, at a critical nanoaggregation concentration of 50–150 mgL-1, 4–10 asphaltene molecules form nanoaggregates with a size of ~2 nm. As the asphaltene concentration increases to 2–5 gL-1, approximately eight nanoaggregates combine to form clusters with a size of ~6 nm.43 In scMeOH, the extent of asphaltene clustering could be reduced due to the high diffusivity of this solvent, which is good for dissolving crude oil components. The thermal breakage of aliphatic chains (C–C, 359.0 ± 4 kJ mol–1)44 and carbon-heteroatom bonds (C–O, 348.5 ± 4 kJ mol–1; C– N, 343.9 ± 4 kJ mol–1; C–S, 323.0 ± 8 kJ mol–1)44 connected to aromatic cores can produce hydrocarbon radicals.45 Thermal scission of aliphatic asphaltene chains (e.g., H-abstraction, βscission, C–C bond cleavage) produces aliphatic hydrocarbons, and the co-formed aromatic radicals can be converted into coke or aromatic species by reacting with hydrogen produced from 12


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scMeOH/crude oil.38 As in the case of scMeOH, asphaltene cluster breakdown and good dispersion of asphaltene nanoaggregates and/or individual asphaltene molecules are expected to be observed in scH2O.37 However, scH2O is not capable of producing hydrogen for effective radical stabilization, and thus the reduction of asphaltenes in this case is much less significant, as reflected by decreased coke formation. In contrast, during pyrolysis, asphaltene clusters are dispersed in the maltene fraction, which is a less effective dispersion medium than scMeOH or scH2O and can additionally participate in coke formation during pyrolysis.46-48 Therefore, it can be concluded that the solubilizing power and hydrogen donating ability of scMeOH are important for the reduction of asphaltene content and suppression of coke formation. 3.3 Effect of temperature The effect of temperature on the yields and properties of upgraded oil was explored in scMeOH using Laguna as a feedstock (30 MPa, 16.7 wt% of Laguna, 1 h), with the obtained results (Figure 4) indicating a trade-off between the coke yield and asphaltene content of upgraded oil (Figures 4a–b). As the reaction temperature increased from 300 to 400 °C, the coke yield increased from 0.6 to 20.0 wt%, whereas the asphaltene content of upgraded oil decreased from 26.2 to 0 area% (12.3 to 0.0 wt%). In addition, the non-distillable residue content gradually decreased with increasing temperature (Figure 4c). At high temperatures, cracking in scMeOH enhanced the yield of low-boiling fractions produced from resins and asphaltenes, as discussed in the previous section. The observed TAN reduction (Figure 4d) indicates that high-temperature operation (above 350 °C) is necessary for the effective removal of NAs from unconventional crude oil. In the presence of scMeOH, the dominant deacidification pathway of naphthenic acid is considered to be esterification.49 The extent of proton liberation from scMeOH can be enhanced when the reaction temperature increased from 350 to 400 °C, which could increase the 13


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rate of acid-catalyzed esterification, resulting in high TAN reduction.25 The reduction of hydrogen bonding between methanol molecules as temperature increases from 350 (0.23) to 400 °C (0.17) at constant pressure of 30 MPa could support the enhanced acid-catalyzed esterification reaction.50 In addition, thermal decarboxylation of alkyl-CO2 and aryl-CO2 bonds could increase at the high reaction temperature, which plays a role in the enhanced deacidification of naphthenic acids at the elevated temperature of 400 °C.

3.4. Effect of pressure The effect of pressure on the yields and properties of upgraded oil was explored in scMeOH using Laguna as a feedstock (400 °C, 16.7 wt%, 1 h), with the results shown in Figure 5. The pressure inside the reactor was controlled by adjusting the amount of methanol and Laguna at a fixed concentration. As the pressure increased from 15 to 30 MPa, the upgraded oil yield was not significantly changed, whereas the coke yield decreased and the gas yield increased (Figure 5a). The above pressure increment at 400 °C increased the density of scMeOH from 0.21 to 0.32 g cm–3. In addition, the diffusivity of scMeOH also increases with increasing pressure (e.g., from 5.2 ×10–8 to 6.1 ×10–8 m2 s–1 for a pressure increase of 15 to 30 MPa at 240 °C51), possibly resulting in effective penetration of the enriched heavy crude phase and suppression of coke formation by stabilizing reactive aromatic radicals.21 Literature reports indicate that scH2O upgrading of vacuum residue and low-rank coal showed a similar trend, i.e., an increase of water density at constant temperature increased the conversion and liquid/gas yields by mitigating coke formation.52,53

Similarly, the increase of scH2O pressure from 30 to 45 MPa at 490 °C

suppressed the formation of coke from the NA mixture.18 The reduction of asphaltene content in oils upgraded at 15 and 30 MPa was very similar 14


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(Figure 5b). Low-pressure upgrading at 15 MPa effectively reduced the content of the nondistillate/residue fraction by 87.2%, significantly increasing that of the naphtha-to-diesel fraction from 16 (Laguna) to 44 wt% (Figure 5c). The enhanced cracking activity at low pressure is held responsible for the higher amount of produced light fractions, as compared to that obtained at high pressure. The increased amount of light fractions obtained at 15 MPa might also be ascribed to the enhanced formation of coke from high-boiling fractions (asphaltenes and resins) during upgrading, resulting in a smaller amount of these fractions remaining in upgraded oil. On the other hand, acidity reduction was more pronounced in the case of high-pressure upgrading (Figure 5d), indicating enhanced esterification due to the increased local concentration of scMeOH molecules in the reaction medium. As the pressure of scMeOH increases, the density of scMeOH increases; for example, at 400 °C, when the pressure increased from 15 to 30 MPa, the density of the scMeOH increased from ~ 0.21 to 0.32 g cm-3.21 This suggests that the local concentration of the methanol molecules increases with increasing pressure. This increase in local clustering of methanol molecules at high pressures was observed by molecular dynamic simulation and nuclear magnetic resonance.54 The increased local clustering of methanol molecules could increase the interaction of naphthenic acids and methanol molecules, which could result in enhanced esterification. A similar trend was observed during the synthesis of fatty acid methyl esters (FAME); the yield of FAME increased from 43.0 to 75.0% when the pressure of scMeOH increased from 10 to 30 MPa with soybean oil-to-methanol ratio of 1:40 at temperature of 300 °C.55 3.5. Effect of feedstocks Various types of unconventional crude oils and VTB were upgraded in scMeOH (400 °C, 30 MPa, 16.7 wt%, 1 h) to verify the effectiveness of this solvent for upgrading diverse feedstocks. 15


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Figure 6a shows that the yields of upgraded oils and coke obtained from unconventional crude oils were similar, exhibiting an error range of ±4–5%. VTB showed an upgraded oil yield similar to that of Laguna and Bachaquero-13, whereas its coke yield was much higher (34.2 wt%). In view of the fact that resins and asphaltenes tend to condensate into coke at elevated temperatures, the above finding was attributed to the higher total content of resins and asphaltenes (58 wt%) in VTB compared to that of unconventional crude oils (Figure 1a). The much lower yield of gases observed for VTB (0.5 wt%) in comparison to that of unconventional crude oils (8.1–11.2 wt%) was rationalized by the lower content of easily cleavable low-molecular-weight saturates and aromatics in the former. As shown in Figure 6b, scMeOH upgrading was highly effective in reducing asphaltene contents in various types of unconventional crude oils, achieving a reduction efficiency above 96% and producing upgraded oils with asphaltene contents below 0.5 wt%. For VTB, the above efficiency was slightly lower (87.3%). Compared to their corresponding feedstocks, oils upgraded in scMeOH were enriched in naphtha-to-diesel fractions (24–32 wt%) and depleted in non-distillable fractions (18–21 wt%). Oil obtained by upgrading VTB contained a smaller amount of naphtha-to-diesel fractions than upgraded unconventional crude oils due to the initially high non-distillable fraction content of the former. Although the conversion of VGO and non-distillate fraction into the naphtha-todiesel fraction is extremely difficult to understand because of the highly complex nature of unconventional crude oils, which contain a multitude of chemical species, it can be speculated that in the presence of scMeOH, the residue fraction is first converted into VGO and subsequently transformed into low-boiling fractions. The residue fraction is considered to be enriched in asphaltenes and high-molecular-weight resins, while the VGO fraction contains 16


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mono- and di-aromatic hydrocarbons connected by alkyl side chains.56 Due to the combinatorial effect of thermal cracking and hydrogen donation from scMeOH under the examined reaction conditions, the above alkyl chains could be converted into short-chain aliphatic hydrocarbons and alkylated monoaromatic derivatives. The abundance of saturates and aromatics in upgraded oils (Figure 6b) was in good agreement with the simulated distillation results, confirming our speculation. The TGA profiles of Laguna feedstock and upgraded oil/coke formed during scMeOH upgrading are shown in Figure S3. The contents of high-boiling and high-molecularweight fractions in upgraded oil were significantly reduced, in good agreement with the results of simulated distillation. As shown in Figure 6d, scMeOH is a very effective medium for reducing TANs of various feedstocks. As in the case of Bachaquero-13, the TAN decreased from 4.2 to 0.25 mg-KOH/g-oil after scMeOH upgrading. Again, deacidification of crude feedstocks was attributed to the esterification of NAs with scMeOH to produce the corresponding methyl esters.25 3.6 Removal of impurities The nitrogen and sulfur contents of unconventional crudes and VTB before and after upgrading in scMeOH are shown in Table 1. The above upgrading effectively reduced the nitrogen content in the produced oil to values below the elemental analysis detection limit (≤ 0.01 wt%) for Rubiales, Bachaquero-13, and VTB, while Laguna showed a 96.5% decrease from 0.2 to 0.07 wt%. The sulfur content reduction of upgraded oil was in the range of 20–25%. In addition, scMeOH upgrading was highly effective for removing metal impurities, achieving reduction efficiencies above 90% for Ca, V, and Ni present in unconventional crude oils. As shown in Figure S3, the ash content of coke (~ 1.0 wt%) was higher than that of Laguna and upgraded oil, suggesting that metal impurities selectively migrated into the coke phase 17


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during scMeOH upgrading. In addition, Table S2 reveals that coke showed an increased heteroatom content compared to Laguna and upgraded oil. The accumulation of metal impurities and heteroatoms in coke confirmed the role of asphaltenes and resins as coke precursors, as discussed in the previous section. Heavy metals (Ni, V, Fe) are typically embedded in asphaltene molecules in the form of oil-soluble porphyrin structures,57 with the increased concentration of nitrogen and sulfur in coke explained in a similar way, since almost 75–90% of these heteroatoms are present in resins and asphaltenes as amide, organic sulfoxide, thiophene, and benzothiophene linkages.58,59 Previously, demetallization was performed employing several physical and chemical methods including catalytic hydroprocessing and the use of metal passivators to avoid catalyst poisoning and ash formation, which is highly detrimental to automobile engines and boiler reactor walls.57 Therefore, the efficient metal impurity removal in scMeOH benefits the wide utilization of unconventional crude oils. Finally, as in the case of Fe, V, and Ni, a significant reduction of Ca content in upgraded oils was observed. Since Ca is typically present as calcium naphthenate in unconventional crude oils, it is expected to be efficiently removed by the formation of esters in scMeOH.

4. Conclusion Three types of unconventional crude oil and vacuum tower bottom were upgraded in supercritical methanol (scMeOH) at various temperatures (300–400 °C) and pressures (15–30 MPa) for 1 h. scMeOH upgrading at 400 °C achieved almost complete asphaltene removal, while oils produced by supercritical water-based upgrading or pyrolysis exhibited asphaltene contents of 8.8–10.1 wt%. In addition, scMeOH upgrading effectively reduced the total acid number (TAN) from 4.2–5.1 to below 0.3 mg-KOH/g-oil, while the TANs of oils upgraded in scH2O were still high (3.5 mg-KOH/g-oil). The nitrogen content of upgraded oils was reduced to values 18


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below the detection limit of elemental analysis (0.01 wt%), and the content of metal impurities (Ca, V, Fe, and Ni) was decreased by one or two orders of magnitude. The described approach can benefit the effective utilization of various types of unconventional crude oils in existing refineries without requiring major operational and technological amendments.

Acknowledgements This work was supported by the Energy Efficiency & Resources Core Technology Program (No. 20152010103120) and the New & Renewable Energy Core Technology Program (No. 20143030090940) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy, Republic of Korea. Additional support was provided by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (grant number: NRF-2016R1A2B3008800).

Supporting information The asphaltenes and resins calibration curves, XRF analysis of the vacuum tower bottom, composition of the gases produced during Laguna heavy oil upgrading, TGA and elemental analysis (CHNSO) of heavy oil (Laguna), upgraded oil and coke. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure Captions Figure 1. Characterization of unconventional crude oils and VTB: (a) SARA analysis, (b) TAN, (c) boiling point distribution, and (d) TGA. Figure 2. Comparison of upgrading techniques using Laguna as a feedstock (400 °C, 30 MPa, 9.1 wt%, 1 h): (a) product yield, (b) SARA analysis, (c) boiling point distribution, and (d) TAN. Figure 3. Plausible mechanism of liquid oil and coke formation from asphaltenes during scMeOH- and scH2O-based upgrading and pyrolysis. Figure 4. Effect of temperature on Laguna upgrading in scMeOH (30 MPa, 16.7 wt%, 1 h): (a) product yield, (b) SARA analysis, (c) boiling point distribution, and (d) TAN. Figure 5. Effect of pressure on Laguna upgrading in scMeOH (400 oC, 16.7 wt%, 1 h): (a) product yield, (b) SARA analysis, (c) boiling point distribution, and (d) TAN. Figure 6. Effect of various feedstocks on upgrading in scMeOH (400 oC, 30 MPa, 16.7 wt%, 1 h): (a) product yield, (b) SARA analysis, (c) boiling point distribution, and (d) TAN.

24


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Figure 1

25


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AGO Naphtha

Diesel 100 60

(c)

60 30 40 20 20

10

TAN Reduction (%)

40

5.0 4.5

80

50

80

5.5

(d)

4.0 3.5

60

3.0 2.5 40

2.0 1.5

20

1.0 0.5

0 0

0 Laguna

Pyrolysis

scH2O

scH2O + scMeOH

0.0 Laguna

Pyrolysis

scMeOH

scH2O

scH2O + scMeOH scMeOH

Figure 2

26


TAN (mg-KOH/g-oil)

Residue VGO Jet Fuel / Kerosene

Residue reduction (%)

100

Fraction recovery (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 3

27


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4

28


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Figure 5

29


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Figure 6

30


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Table 1. Characteristics of unconventional crude oils and VTB, and the properties of the corresponding upgraded oils obtained in scMeOH (400 °C, 30 MPa, 16.7 wt%, 1 h).

Property Saturates + aromatics (wt%) Resins (wt%)

Rubiales

Upgraded Upgraded Laguna Bach-13 oil oil

Upgraded VTB oil

Upgraded CDUa oil Requirements

70.4

87.1

60.2

78.7

63.6

75.3

41.7

75.9

14.6

12.4

26.9

23.3

24.2

24.4

50.0

23.1

0.5

12.9

0

12.2

0.3

8.3

1.0

1.6

Asphaltenes (wt%) 15.0 TAN (mg-KOH/g0.55 oil) C (wt%) 87.4

0.05

5.1

0.32

4.2

0.25

0.81

0.07

< 0.5

86.7

85.2

82.6

85.5

78.7

86.9

80.2

-

H (wt%)

10.6

10.7

10.3

10.3

10.3

9.7

8.8

8.9

-

O (wt%)

0.78

0.4

1.58

1.49

1.37

0.87

0.37

0.17

-

S (wt%)

1.1

0.93

2.8

2.22

2.7

2.07

3.27

2.4

2.28

N (wt%)

0.04

N.D.b

0.2

0.07

0.1

N.D.

0.04

N.D.

0.12

H/C ratio

1.46

1.48

1.45

1.50

1.45

1.48

1.22

1.33

-

Ca (ppmw)

318

16

315

11

131

9

14

3

1

Fe (ppmw)

43

11

76

3

24

3

25

3

5

V (ppmw)

130

8

395

31

402

29

68

2

3

32

1

Ni (ppmw) 34 3 50 3 48 a CDU: Specification of a crude distillation unit in a South Korea refinery. b Not detectable. 31


≤ 90

Upgrading Heavy Crude Oils and Extra Heavy Fractions in

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