Article pubs.acs.org/EF
Selective Asphaltene Precipitation from Hydroconverted Bottoms Jeramie J. Adams,*,† John F. Schabron,† Joseph F. Rovani, Jr.,† Justin Boysen,† Frans G. A. van den Berg,‡ Carl Mesters,§ and Nihar Phalak§ †
Western Research Institute, 3474 North 3rd Street, Laramie, Wyoming 82072, United States Shell Global Solutions International, B. V. Grasweg 31, 1031 HW Amsterdam, The Netherlands § Shell International Exploration & Production, Inc., 3333 Highway 6 South, Houston, Texas 77082, United States ‡
ABSTRACT: A novel method is described that selectively precipitates the least soluble asphaltenic material from hydroconverted bottoms (HCB) using a dual immiscible solvent approach. This approach separates asphaltenes from the deasphalted (DAO)/precipitant layer by adding acetonitrile (CH3CN) as a second immiscible phase. The three phase system precipitates asphaltenes to the bottom which becomes separated from the top DAO/precipitant layer by a middle CH3CN layer. Due to some CH3CN dissolving in the DAO/precipitant layer, the solvent parameter (and possibly other chromatographic or specific interactions) of the precipitating medium is increased causing less overall asphaltenes to be precipitated, but more of the least soluble precoke-like asphaltenes are selectively precipitated. This treatment produces DAO that is more thermally stable with regard to coke formation compared to DAO generated using only the aliphatic precipitant which precipitates out more asphaltenes. The immiscible CH3CN layer reaches a steady state saturation level of HCB components and can be reused multiple times without additional treatment. HCB components that become dissolved into the CH3CN are primarily smaller molecular weight aromatics with very little contamination from Ni and V complexes.
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INTRODUCTION
structures, and some molecules were shown to contain larger fused aromatic cores than previously recognized.5 The molecular forces which are responsible for holding together asphaltene aggregates, and the attractive forces between resulting aggregates, are the same types of forces responsible for adsorption phenomena at solid surface interfaces, emulsion stabilization (through adsorption at the oil/water interfaces), and various fouling phenomena that may or may not be heat induced. It has been determined that at very low concentrations asphaltenes exist as monomers and at slightly higher concentrations dimers form through primarily π−π stacking interactions.6 As the size of the aggregate grows, the combination of sterically blocking aliphatic side chains weakens further π−π stacking. With increasing concentration nanoaggregates appear consisting of less than 10 molecules, and with further increased concentration clusters of nanoaggregates appear which are also formed from groups of about 10 nanoaggregates or less.2,3 Alternatively, other forces such as dipole−dipole, acid−base, hydrogen bonding, hydrophobic pockets, and electrostatic interactions can dominate to form supramolecular structures.7 The combination of these forces, and others including π−π interactions, is responsible for knitting together asphaltene aggregates. These forces are relatively weak and are not rigidly fixed for any given aggregate but are dependent upon the conditions that surround the aggregates (temperature, concentration, amount, and type of other molecules in the mixture, etc.). As conditions change surrounding the asphaltenes, the forces which hold them together rearrange to find new thermodynamic local minimums
Asphaltenes are the heaviest, densest, most aromatic, polar, and viscous building components within oil. For practical reasons they are defined by a solubility classsoluble in aromatic solvents, such as toluene, but not in paraffinic solvents, such as heptane or pentane. As as result they are not restricted to any particular type of molecule but rather consist of a continuum of molecules. Molecular identification is further complicated by the fact that asphaltene molecules self-associate, even at very low concentrations in good asphaltene solvents such as toluene (50−100 mg/L),1−3 and they can trap other types of molecules within the aggregated structures. In addition, waxes can also coprecipitate with the mixture. Asphaltenes also include metal porphyrins and other chelating metal structures of primarily iron, nickel, and vanadium. Nevertheless, the majority of asphaltene molecules generally consist of a central fused polyaromatic (between 5 and 10 aromatic rings) core structure which is decorated with a few percent of N, O, and S heteroatoms and aliphatic side chains, cyclics, and bridges to other aliphatic, aromatic, or functional group moieties. These molecules also adsorb visible light causing them to appear brown in color. Asphaltene molecules consist of a broad distribution of molecular weights which are dependent upon the source of the asphaltenes but are most often reported to average around 700−750 Da.2,4 Recently, scanning tunneling microscopy coupled with atomic force microscopy has been used to elucidate molecular structures for several individual petroleum and coal derived asphaltene molecules. The results showed that most petroleum asphaltenes do consist of fused polyaromatic cores with peripheral aliphatic side chains; however, some structures did show smaller polymaromatic cores linked by single bonds indicating some “archipelago” type © XXXX American Chemical Society
Received: August 10, 2016 Revised: September 26, 2016
A
DOI: 10.1021/acs.energyfuels.6b02008 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
ing process which uses an aliphatic solvent and polar solvent to improve the asphaltene stability of hydroconverted bottoms. This process selectively precipitates more methylene chloridesoluble asphaltenes than cyclohexane-soluble asphaltenes when compared to deasphalting using only the aliphatic solvent.
in energyin accord with the guiding principle of supramolecular chemistry via self-assembly.7Attractions between the nanoaggregates to form clusters of nanoaggregates are generally believed to occur through combinations of less specific van der Waals forces.8 Asphaltenes are known to cause a wide range of problems in the petroleum industry for both upstream and downstream operations. To mitigate upstream problems, asphaltenes are often treated with additives9,10 or are removed through various processes.11 One common method for removing asphaltenes is through solvent deasphalting.11−13 The deasphalted oil (DAO) has made it possible to efficiently upgrade portions of heavy oils and residua by catalytic processes by preserving catalyst activity and lifetime.11,14 Hydrocracking is a process developed to efficiently produce more valuable light and middle distillates from heavier, usually more aromatic, feedstocks with minimum coke formation. The process utilizes catalysts under high pressures of hydrogen and high temperatures to accelerate bond breaking and hydrogenation.11,15 Reaction conditions are tuned so that dealkylation and hydrogenation are balanced and favored as opposed to dehydrogenation, aromatization, and radical formation which lead to condensation reactions to form coke material. Heavy ends, or bottoms, from the hydrocracking process contain a combination of a highly aliphatic materials and highly aromatic materials with smaller more aromatic asphaltenes and very little resinous material.14 Hydroconversion more specifically deals with hydroprocessing residua which is upgraded under more intense conditions. Whole oils or residua can be separated into heptane-soluble material (maltenes) and various asphaltene subfractions by precipitation and re-dissolution onto an inert stationary phase with various solvents and mixtures of increasing strength. This process sequentially and selectively dissolves portions of the whole asphaltenes that have higher polarity as the solvent strength is increased.16,17 Automated and preparatory scale methodsknown as the Asphaltene Determinatorhave been optimized to separate asphaltenes into cyclohexane-soluble, toluene-soluble, and methylene chloride-soluble asphaltenes.17 At cracking temperatures ≥ 400 °C the cyclohexane-soluble asphaltenes decrease dramatically and methylene chloridesoluble asphaltenes increase rapidly.16,17 The ratio of these two asphaltene subfractions can be used as a coke index providing a metric for determining the proximity of a cracking/pyrolysis process to the onset of coke formation. The cyclohexanesoluble portion of the asphaltenes is the least aromatic and polar while the methylene chloride-soluble portion of the asphaltenes is the most aromatic and polar due to lack of aliphatic side chains due to dealkylation.17 Since asphaltenes also span a continuum of molecules, and thus a continuum in solubility, the toluene-soluble portion of asphaltenes contains material which is less aromatic at one extreme and material which is more aromatic at the other extreme with most of the material falling somewhere in the middle. Therefore, to improve already upgraded materials for use as other refining feedstockswhich already contain very little cyclohexane-soluble asphaltenes and significantly more methylene chloride-soluble asphaltenes18it is intuitive that the cyclohexane-soluble asphaltenes should be retained while the methylene chloride-soluble asphaltenes be rejected. By improving upgraded feedstocks in this way, subsequent refining products will produce less coke within heat exchangers and furnaces. This work describes a binary/dual solvent deasphalt-
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EXPERIMENTAL SECTION
Hydroconverted residue (HCB) from a commercial LC-Finer were provided by Shell. All solvents and inert gases were reagent or technical grade, respectively. Solvents and inert gases were purchased from commercial sources and used as received. Analytical Asphaltene Determinator Separation. The oncolumn asphaltene precipitation and re-dissolution experiments Asphaltene Determinator (AD)were conducted on custom ground PTFE packed high performance liquid chromatography (HPLC) columns as described in detail by Schabron et al.17 The analytical setup uses a Waters 717plus auto sampler, a Waters 60F pump with a model 600 controller, a Waters 2489 ultraviolet/visible absorbance detector, and a Waters 2424 evaporative light scattering detector (ELSD). Solutions of the sample oils were prepared as 10% (w:v) solutions in chlorobenzene, and solutions of asphaltenes were prepared at 2% (w:v) solutions. The solutions were filtered through 0.45 μm syringe filters into autosampler vials. Portions of 20 μL were injected for the analytical scale AD separation. All separation profiles were electronically blank subtracted prior to peak integration. Analysis of the asphaltene content and asphaltene solubility profile by the AD provides rapid information about amount of asphaltene removed upon precipitation and the selectivity of asphaltene subfractions removed. In addition to quick turnaround times and high repeatability compared to gravimetric asphaltene separations, the method requires 1 g or less of sample allowing for smaller scale evaluations while using significantly less solvent than traditional methods. Saturates, Aromatics, Resins−Asphaltene Determinator Analysis. The fully automated saturates, aromatics, resins−asphaltene determinator (SAR-AD) analysis was performed according to published procedures.19,20 The method couples the chromatographic SAR separation of the maltenes with the AD solubility separation of the asphaltenes. Dual Solvent Deasphalting. Asphaltene precipitation experiments using pentane or heptane with MeOH or CH3CN were initially conducted in 1 oz glass vials fitted with PTFE lined lids. An amount of HCB was weighed into the vials; the solvents were added and the vials capped. The vials were placed on a vortex mixer and mixed at 550 rpm. In most cases the vials were agitated for 30 min. After mixing, the vials were allowed to stand at ambient temperature. For experiments which contained lower concentrations of HCB (≤1:2 HCB:precipitant) the vials were allowed to settle for 1 h before separating the solvents and asphaltenes; for experiments with larger HCB concentrations they were allowed to sit for 24 h prior to separating. When the HCB concentration was ≤1:2 HCB:pentane, the CH3CN layer was less dense than the asphaltenes, causing the asphaltenes to precipitate to the bottom and the pentane/DAO to be located on top of the CH3CN layer. Conversely, when the HCB concentration was >1:2 HCB:pentane, the asphaltenes were still located at the bottom but the pentane/DAO layer was in the middle and the CH3CN layer was at the top. The solvent layers were separated from each other and from the asphaltenes using a pipet to transfer the precipitant/DAO layer and CH3CN layer into different vials while leaving the asphaltenes behind in the original vial. For the sealed centrifuge tube experiments, HCB was weighed into a 100 mL centrifuge tube and the appropriate solvents were added by pipet. A new neoprene stopper was used to seal the tube using three to four rubber bands and wire. The sealed centrifuge tube was placed on its side in an ultrasonic bath (Bransonic 220) filled with distilled water so that at least half, but no more than three-fourths, of the centrifuge tube was covered by water. The sealed centrifuge tubes were sonicated for 30 min and removed from the ultrasonic bath and centrifuged at 1450 RCF for 15 min prior to separating the solvent layers from the B
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Energy & Fuels asphaltenes. By using ultrasonic mixing, weak emulsions were formed necessitating centrifuging to more rapidly separate the layers. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) tests were conducted using a TA Instruments Q5000 TGA (TA Instruments, New Castle, DE, USA). No more than 20 mg of material was placed in platinum TGA pans and heated at 10 °C/min under nitrogen to 600 °C. The sweep gas was switched to air, and the samples were held at 600 °C for 15 min. Ni and V Analysis. Ni and V analyses were performed by Huffman Laboratories in Golden, CO, USA. The oils were digested by heating about 0.5 g of oil and concentrated sulfuric acid on a hot plate to dryness followed by ashing in a muffle furnace at 520 °C. The ash was dissolved in aqueous nitric acid, and the nitric acid solutions were sent to Huffman Laboratories. The samples were analyzed by inductively coupled plasma atomic emission spectroscopy capable of analyzing 30 elements. Two blanks and a QC sampleof a well characterized Unitar round-robin Athabasca bitumen with a known metals contentwere digested and analyzed to provide limits of detection and quantification, and as a quality control check. Gravimetric Isolation of Asphaltenes. HCB was weighed and mixed with an excess of heptane (40:1 (v:w)) with magnetic stirring. The mixture was heated to 70 °C, allowed to cool to ambient temperature, and stirred overnight to allow for full precipitation of asphaltenes. The mixture was allowed to sit undisturbed for 30 min and subsequently vacuum filtered using a 10 μm medium frit sintered glass filter with repeated heptane rinsing. The filter cake was usually rinsed with at least another 40 volumes of heptane until the filtrate was colorless. The filtrate was subjected to a second filtration using a 0.45 μm polytetrafluoroethylene (PTFE) filter, and this filter cake was also rinsed with about another 40 volumes of heptane. Residual solvent was removed from the asphaltenes by placing the samples in a vacuum oven under full dynamic vacuum at 110 °C for at least 1 h. Partial Solubility Test. A 15 mL centrifuge tube was filled with 3 g of HCB deasphalted oil prepared from HCB using 40 volumes of heptane (C7-DAO). CH3CN (3 mL) and pentane (6 mL) were added to the centrifuge tube to give a final ratio of 1:1:2 C7DAO:CH3CN:pentane. The centrifuge tube was sealed and agitated using an ultrasonic bath for 30 min and allowed to separate under gravity. There was about 2.0 mL of an orange CH3CN layer indicating that about 1 mL of CH3CN had dissolved into the C7-DAO/pentane layer.
solvents were chosen so that the density of the polar solvent was low enough (less than about 0.9 g/mL) to allow efficient transport of the asphaltenes (about 1.2 g/mL) to the bottom under gravity, but high enough so that it efficiently separated from a less dense DAO/precipitant layer causing the DAO/ precipitant layer to be located on the top of the polar solvent. This method of precipitating and separating asphaltenes is advantageous over other methods because it allows for a convenient separation at ambient temperature and pressure while taking advantage of gravity and solvent immiscibility to drive the separation of asphaltenes from DAO. Additionally, this method does not require filtration to remove the asphaltenes from the DAO/precipitant, and the polar solvent can be recycled without additional treatment. Another factor considered was the density of the precipitant and amount of oil, since if the DAO + precipitant density becomes greater than the density of the polar solvent, then a phase inversion of the solvent layers occurs causing the polar solvent to be on the top of the DAO/precipitant layer and the DAO/precipitant layer comes into contact with the precipitated asphaltenes. The last consideration is that the polar solvent and precipitant should be as immiscible as possible at the process conditions to minimize partial cosolubility of solvents so that distinct layers are formed. The initial biphasic dual (or binary) solvent systems investigated were methanol (MeOH) or acetonitrile (CH3CN) with pentane or heptane. The oil to be deasphalted was hydroconverted bottoms (HCB) which contains a much higher amount of the least soluble precoke-like methylene chloride-soluble asphaltenes compared to more aliphatic cyclohexane-soluble asphaltenes. For dual solvent asphaltene removal, the use of MeOH with C6 or above hydrocarbons has been previously disclosed in U.S. Patent 4,493,765 but the use of acetonitrile has not been discussed to our knowledge.27 An example separation using n-heptane and MeOH with HCB is given in Figure 1. Other dual solvent methods have been
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RESULTS AND DISCUSSION Biphasic Dual Solvent Asphaltene Precipitation. A new approach has been reported that selectively precipitates asphaltenes in a three phase system with the DAO/precipitant layer at the top, water in the middle, and asphaltenes at the bottom.19−21 In this process, an emulsion is initially formed and a precipitant is added to condition the emulsion causing asphaltenes floccules to accumulate and grow at the emulsion interfaces. Then pressure, heat, and additional diluent are applied to rapidly rupture the emulsion causing the asphaltenes to settle to the bottom leaving a DAO/precipitant layer above the water and asphaltenes. During this process asphaltenes overcome their hydrophobic repulsion to water and penetrate the water layer and are physically separated from the DAO/ precipitant. The asphaltenes are then collected by filtration followed by diluent washing to recover additional oil or by depressurization.21−23 Surfactants have also been reported to improve the process.24 In this current work, a novel nonaqueous approach was developed to facilitate selective separation of asphaltenes by using a high polarity solvent that was immiscible in aliphatic precipitating solvents. The solvent system was designed so that the asphaltenes would precipitate to the bottom and become physically separated from the top DAO/precipitant layer by a middle immiscible polar solvent barrier. To achieve this the
Figure 1. Separation using 2 g of HCB, 17 mL of heptane, and 15 mL of MeOH. The dark top layer is DAO + heptane, the red middle layer is MeOH, and the bottom dark material is the asphaltenes.
reported, but they generally separate oil into an asphaltene poor fraction dissolved in a weak solvent and an immiscible asphaltene rich fraction dissolved in a strong solvent and there is no barrier between the two phases.25−27 The quality of the DAO was gauged from the amount of asphaltenes remaining in the DAO, the gravimetric amount of asphaltenes precipitated, and the coke index of the DAO assayed by the on-column precipitation and re-dissolution method known as Asphaltene Determinator. The AD separates whole oils into heptane-soluble material (maltenes), cycloC
DOI: 10.1021/acs.energyfuels.6b02008 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels hexane-soluble, toluene-soluble, and methylene chloride-soluble asphaltenes. For the asphaltene subfractions, as the polarity of the extracting solvents increases, the polarity and aromaticity of the asphaltenes which report to that fraction also increase. The AD coke index is a metric showing the pyrolysis severity of an oil; since cracking cleaves aliphatic side chains and naphthenic cyclics, this reduces the amount of more alkyl containing cyclohexane-soluble asphaltenes and increases the most aromatic methylene chloride-soluble asphaltenes.17 Since the cyclohexane-soluble fraction contains a significant amount of aliphatic groups and the methylene chloride-soluble fraction contains precoke-like material, the DAO from HCB can be improved by selectively precipitating a minimum amount of asphaltenic material enriched in methylene chloride-soluble asphaltenes without removing the cyclohexane-soluble asphaltenes. From previous cracking/pyrolysis studies, in the absence of hydrogen, it was shown that a 500 nm AD coke index of about 2 or less is the cutoff for which precoke material begins to form. This means that residua, oils, and DAO with a coke index above 2 are relatively more stable with regards to formation of precoke material which ultimately leads to coke formation.17 It should be noted that this cutoff was determined from pyrolysis reactions in the absence of hydrogen so the absolute value of 2 may not be appropriate for HCB which was catalytically processed under sever conditions in the presence of hydrogen. Nevertheless, an increase in this valuerelative to HCB which has a 500 nm AD coke index of around 0.5will indicate an improvement in the quality of the oil. From the HCB separation using MeOH and heptane that is shown in Figure 1, the DAO/heptane, MeOH layers, and asphaltenes were separated from each other. The solvents were evaporated under a stream of N2, and the residues were dried under vacuum at 100 °C. AD data showed that the DAO had 75% less asphaltenes and the 500 nm AD coke index was greatly improved going from 0.5 to 1.6. HCB is the bottoms stream processed up to the verge of stability, so the 67% increase in the coke index demonstrates a substantial improvement in the thermal stability of the resulting DAO. An experiment was conducted to visually illustrate how gravity and immiscible organic solvents provide the driving forces to separate the asphaltenes from the DAO/precipitant mixture. An HCB/heptane slurry was prepared, and MeOH was added into the bottom of the same vial. In this experiment the HCB/heptane slurry raised to the top of the more dense MeOH layer. Over time gravity caused the precipitated asphaltenes to settle to the bottom of the DAO/precipitant layer, and after about 2 h the asphaltenes broke through the interfacial tension barrier between the two immiscible solvent layers and fell as a plug to the bottom of the vial, as shown in Figure 2. From this experiment, the DAO/heptane, MeOH layer, and asphaltenes were separated and dried. AD analysis of the fractions showed that the DAO was similar to the previous experiment, and the orange/red material which reported to the MeOH layer had less than 0.5% asphaltenes. The asphaltenes, on the other hand, were shown to be contaminated by some DAO. AD data of the MeOH/heptane asphaltenes are compared to asphaltenes precipitated with 40 volumes of heptane in Figure 3. In this figure it can be seen that the MeOH/heptane asphaltenes contain 37 wt % more heptanesoluble material. It is not surprising that the MeOH/heptane asphaltenes from the experiment shown in Figure 2 contained
Figure 2. Photographs showing settling of precipitated asphaltenes. A 2 g amount of HCB and 17 mL of heptane were mixed with a vortex mixer at 550 rpm for 30 min, and immediately 15 mL of MeOH was syringed into the bottom of the vial, raising the HCB/heptane slurry. Over the course of 2 h the asphaltenes had settled to the bottom of the vial.
Figure 3. AD asphaltene solubility profile for HCB asphaltenes precipitated with 40 volumes of heptane (C7) and asphaltenes precipitated from MeOH/heptane as explained in the preceding Figure 2.
more oil components since the large plug that passed through the MeOH/heptane interface was encased with DAO/heptane. A similar experiment was conducted using HCB, heptane, and CH3CN. In this experiment a plug of asphaltenes also broke through the CH3CN barrier and settled at the bottom of the vial. But in this experiment the asphaltenes settled more quickly taking less than 1 h as opposed to 2 h when MeOH was used. This is likely since CH3CN contains less hydrogen bonding character than MeOH and is thus less hydrophilic. AD data from this experiment were similar to those for the MeOH experiment except that material which dissolved in the CH3CN material contained about twice as much asphaltenes and the DAO from this experiment had a higher coke index of 1.4 compared to 1.1 in the case of MeOH. Since the amount of asphaltenes in the DAO from the MeOH and CH3CN experiments were nearly identical, but the coke index was higher for the CH3CN separation, it was concluded that the CH3CN separation was more selective. Since CH3CN was more selective at precipitating the least soluble asphaltenes, and it had a greater rate of asphaltene settling than MeOH, it was investigated further. Asphaltene precipitation experiments were conducted using a vortex mixer to mix HCB, precipitant, and CH3CN. For these experiments both heptane and pentane were used to determine if the D
DOI: 10.1021/acs.energyfuels.6b02008 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels precipitant densities and immiscibility characteristics had a significant impact on the separation. In fact a significant difference in the asphaltene settling behavior was observed between the two precipitants. After mixing HCB, CH3CN, and heptane (1:6.5:7.5, w:v:v), the asphaltenes settled out as a webbed network of asphaltenes after 5−10 min. However, when conducting the same experiment with pentane, the asphaltenes settled to the bottom of the vial in a discrete mass within a few minutes. The difference in the asphaltene settling behavior between heptane and pentane precipitants in the presence of CH3CN is shown in Figure 4.
Table 2. Select AD Data for the Separation of 2 g of HCB with 13 mL of CH3CN and 15 mL of Heptane or Pentane
Table 1. Gravimetric Mass Balance Data for 2 g of HCB Separated with 13 mL of CH3CN and 15 mL of Heptane or Pentane pentane/CH3CN separation
grams
grams
2.0722
initial HCB
2.0004
heptane DAO CH3CN-soluble asphaltenes
1.6475 0.1187 0.2501
pentane DAO CH3CN-soluble asphaltenes
1.4404 0.1721 0.3968
total difference
2.0163 0.0559
total difference
2.0093 −0.0089
HCB sample control
0.62
3.29
DAO from heptane/CH3CN separation CH3CN solubles from heptane separation
1.50
0.39
1.48
1.24
DAO from pentane/CH3CN separation CH3CN solubles from pentane separation
1.81
0.30
1.48
0.94
Table 3. Gravimetric Mass Balance Data for the Capacity Study Using 2−8 g of HCB Separated with 8 mL of CH3CN and 16 mL of Pentane
The material balance for the DAO, CH3CN-soluble material, and asphaltenes for this set of experiments is given in Table 1
initial HCB
% AD asphaltenes
was in the middle, and the DAO/precipitant was at the top. Experiments were conducted with 2−8 g of HCB in 8 mL of CH3CN and 16 mL of pentane using a vortex mixer at 550 rpm. This study showed that the amount of recovered DAO and precipitated asphaltenes increased linearly with respect to the amount of HCB used. The amount of material dissolved in CH3CN seemed to increase slightly, but it did not increase with a good linear correlation (later experiments showed that the amount of material which becomes dissolved in CH3CN stays about constant). Table 3 shows the gravimetric mass balance data for the initial capacity study, and Figure 5 shows a plot of the gravimetric data.
Figure 4. Two pictures from the same experiment. The picture on the left was taken without the camera flash and the picture on the right was taken with the flash so that the webbed network of asphaltenes can be clearly seen in the case of heptane. The vials contained 2 g of HCB, 13 mL of CH3CN, and 15 mL of pentane (left) or heptane (right). The vials were mixed using a vortex mixer at 550 rpm and were left undisturbed for about 10 min.
heptane/CH3CN separation
500 nm coke index
sample
HCB (g)
DAO
CH3CN-soluble
asphaltenes
2.0002 3.0004 4.0000 5.0003 6.0002 7.0000 8.0000
1.4147 2.1415 2.8734 3.6451 4.3202 5.2018 6.0152
0.1428 0.1731 0.1980 0.2375 0.2490 0.3272 0.4494
0.4189 0.6587 0.8643 1.0840 1.3181 1.4678 1.5017
The DAO from the capacity study was analyzed by the AD to determine the amount of AD asphaltenes present and the 500 nm coke index. These data showed that as the amount of HCB increased, while holding the solvent volumes constant, the
and select AD data are presented in Table 2. The AD data shows that the DAO from the pentane separation contained slightly less asphaltenes than the DAO from the heptane separation. The asphaltene content in the CH3CN-soluble layer is also less for the pentane separation, despite more material becoming soluble in the CH3CN layer. As expected, as more asphaltenes were precipitated when pentane was used the coke index also improved going from 1.50 for the heptane separation to 1.81 for the pentane separation. HCB Capacity Study for CH3CN/Pentane Asphaltene Precipitation. A capacity study was carried out to determine which HCB to solvent ratios precipitated the least amount of asphaltenes while maintaining a maximum 500 nm AD coke index. The solvent ratios were selected in a range so that the separated asphaltenes went to the bottom, the CH3CN layer
Figure 5. Plot of the mass balance data from Table 3 for 2−8 g of HCB separated with 8 mL of CH3CN and 16 mL of pentane. E
DOI: 10.1021/acs.energyfuels.6b02008 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels amount of AD asphaltenes in the DAO also increased. However, the 500 nm AD coke index continued to improve until around a 1:2 HCB to pentane ratio. The initial AD coke index for HCB was 0.58, whereas the DAO separated from 7 g of HCB had a dramatically improved coke index of 2.61. A plot of select AD data is given in Figure 6 along with tabulated
Table 4. Mass Balance and AD Data from Two Experiments Where the Order of Solvent Addition Was Different but the Amount of HCB (8 g), CH3CN (8 mL), and Pentane (16 mL) and Mixing Conditions Were Held Constanta pentane first
CH3CN first
initial HCB (g) asphaltenes (g) DAO (g) CH3CN-soluble (g)
8.0002 1.6610 6.0694 0.2612
8.0004 1.6990 6.0273 0.2723
total (g)
7.9916
7.9986
0.72 0.91
0.70 1.19
DAO ELSD AD asphaltenes 500 nm AD Coke Index
a In one experiment pentane was added first and then CH3CN, and in the other experiment CH3CN was added first and then pentane.
Figure 6. 500 nm WRI coke index and ELSD AD asphaltenes data from the DAO plotted against the amount of HCB separated with 8 mL of CH3CN and 16 mL of pentane.
amount of pentane and HCB in the presence of CH3CN, only 12 wt % asphaltenes were precipitated. Likewise, AD data of the DAO showed that separations using CH3CN had twice as many asphaltenes but that the coke index was slightly better: 2.5 for pentane/CH3CN separation vs 2.4 for pentane only separation. These results confirm that the addition of CH3CN drastically enhances the selectivity of the separation by precipitating much less asphaltenes that are more enriched in the worst precoke type asphaltenes. Table 5 shows the mass balance results for the sealed centrifuge tube experiments, and Figure 7 shows select AD data. Repeats from two to four experiments showed that the gravimetric results from the centrifuge tube experiments were consistent when using only pentane to precipitate the asphaltenes (430 °C) compared to whole HCB which has about 25 wt % material in the pyrolysis region. For the CH3CN-soluble material, if a Gaussian distribution is assumed for the volatile material then very little of the material in the pyrolysis region is actually pyrolytic carbon which is represented by the small higher temperature shoulder around 490−500 °C. It is reasonable to expect that CH3CN may selectively extract some metal porphyrins or other metal chelating species due to the orange/red color of the CH3CN layer, and also since the literature shows that CH3CN can be used to extract petroleum porphyrins.28−33 However, metals analysis of the DAO, CH3CN-soluble material, and asphaltenes showed that the concentrations of Ni and V are the lowest in the CH3CNsoluble material. This is consistent with other literature findings that showed that even though porphyrins may be somewhat selectively extracted by CH3CN the overall metals content is low in this fraction.30 The extremely low concentration in the case of HCB is also consistent with other reports that show as metal porphyrins and other metal species lose their aliphatic side chains during the hydrocracking process they become more concentrated in the asphaltenes compared to unprocessed oils.34 Table 14 compares the metals distribution for the DAO and asphaltenes that were separated using only pentane to the fractions separated with CH3CN and pentane. The data show that the DAO from the pentane only separation and the pentane/CH3CN separation contain about the same amount of Ni and V. This shows that the CH3CN separation does not cause significantly more metals to report to the DAO even though it removes much less asphaltenes. Therefore, separations carried out with CH3CN also have the added
the CH3CN-soluble material was extremely deficient in saturates and asphaltenes but contained slightly more resins. The distribution of HCB components that were soluble in CH3CN changed slightly depending upon the ratios of solvents and the method used for the separation. For large scale sealed centrifuge tube experiments, using a 1:2 pentane to HCB ratio and an ultrasonic bath for mixing, the SAR-AD data showed that the CH3CN material still contained a significant majority of aromatics (87%), but it was also slightly more concentrated in saturates (6%) and resins (6%) while the overall amount of dissolved asphaltenes stayed about the same (Table 12). Thermogravimetric analysis (TGA) was performed on the CH3CN-soluble material extracted from HCB. The data were compared to the TGA curve from the whole HCB which showed that the CH3CN-soluble material contains relatively smaller volatile molecules, as shown in Figure 14. The figure
Figure 14. TGA curves for HCB and HCB components that were extracted in CH3CN.
clearly shows that there is a maximum in the molecular weight corresponding to a boiling point of around 500 °C for the material which can be dissolved in CH3CN (the five condensed ring molecule benzo[a]pyrene has a boiling point around 495 °C). The peak shape also indicates that there is a relatively tighter distribution of molecular weights that are extracted by CH3CN. Since SAR-AD data showed that only 0.2−6% of J
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Table 13. Gravimetric TGA Values for HCB and CH3CN-Soluble Material Extracted from HCB during the Separation of 25 g of HCBa volatiles
pyroysis
carbon residue
total
sample
initial wt (mg)
%
mg
%
mg
%
mg
mg
HOSB control CH3CN material
5.514 5.585
65.56 82.89
3.615 4.630
25.14 9.31
1.386 0.520
9.16 7.62
0.505 0.426
5.506 5.576
The cutoff between the volatiles and pyrolysis regions were determined to be 430 °C based upon an onset of pyrolysis calculated from the large upper temperature peak from the derivative curve of the feed bitumen. Carbon residue was determined from the burn off of material at 600 °C when the nitrogen purge was switch to air.
a
Table 14. Ni and V Analysis of Whole HCB; DAO, and Asphaltenes from HCB Separated using Only Pentane; and DAO, CH3CN-Soluble Material, and Asphaltenes from HCB Separated using Both CH3CN and Pentane HCB pentane/CH3CN separation DAO CH3CN asphaltenes total HCB pentane only separation DAO asphaltenes
wt %
V (ppm)
Ni (ppm)
72.7 7.1 20.1
7 10 229
9 28 383
76.4 23.6
4 188
total whole HCB
5 319
V (ppmw)
Ni (ppmw)
5 1 46
6 2 77
52
85
3 44
4 75
47 50.5
79 81.4
Figure 15. Data from the partial solubility test of CH3CN and pentane. An initial amount of CH3CN (CH3CNi) and pentane (pentanei) were measured into graduated 15 mL centrifuge tubes which were sealed and then shaken twice by hand and then allowed to stand undisturbed for several minutes before recording the level of the new meniscus (Meniscusf).
benefit of selectively precipitating metal species with the asphaltenes. Mechanistic Considerations for Selective Asphaltene Precipitation with Pentane in the Presence of CH3CN. Preliminary steps were taken to understand the mechanism by which CH3CN promotes selective asphaltene precipitation from HCB. Some proposed mechanistic aspects include the following: (1) Partial solubility of CH3CN into the pentane increases the solubility parameter of the pentane phase. (2) Partial solubility of pentane into the CH3CN phase reduces the amount of pentane available for precipitation. (3) The chromatographic strength of pentane/CH3CN is greater than the resulting solubility parameter of pentane and CH3CN.35
(4) Specific interactions between CH3CN with aromatic molecules or other oil components through its lone pair of electrons or CN π-electrons disrupt asphaltene aggregation.36 (5) CH3CN selectively removes a specific class of low solubility parameter compounds from HCB which causes the HCB/pentane mixture to be a stronger solvent helping to dissolve asphaltenes. The first two mechanisms deal with the partial cosolubilities of the solvent system: the amount of CH3CN that dissolves in pentane or the amount of pentane that dissolves in CH3CN. These two aspects were investigated by simply mixing different amounts of CH3CN with pentane (total volume of solvents equaled 15 mL) in sealed graduated centrifuge tubes and observing the new level of the resulting meniscus that formed between the immiscible phases. From this simple test it was observed that when less than 2.0 mL of CH3CN was used the K
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which can overlap better with aromatic π-electron systems of asphaltenes and resins to disrupt their associations. The fourth mechanistic consideration is closely related to the third; a recent review has been published concerning biphasic solvent systems and how they work to selectively dissolve polyaromatics.36 In this review the authors explain that the mechanism by which mixtures of alkanes with dimethylformamide (DMF) enhance the solubility of polyaromatics is through specific interactions between the electron lone pairs of DMF with the π-electrons of polyaromatic molecules. This may also be somewhat true for CH3CN since it also has a lone pair of electrons. The last possible major mechanism is that CH3CN dissolves a specific class of low solubility parameter HCB compounds so that the resulting HCB/pentane phase has a greater solubility parameter and thus a greater ability to dissolve asphaltenes. For example, if the CH3CN selectively extracted saturates (strong asphaltene destabilizing fraction), removing them from the HCB/pentane layer, then the resulting HCB/pentane layer would become a better solvent for the asphaltenes. However, this mechanism is not supported because mostly aromatics and some resins are extracted by CH3CN, with very little saturates. Extracting aromatics from HCB would make the resulting HCB/pentane layer less able to dissolve asphaltenes which should cause even more asphaltenes to be precipitatednot less precipitated asphaltenes as observed. Conclusions for Dual Solvent Separation of HCB. A concept was developed involving immiscibleor partially solubledual solvents as a way to selectively precipitate asphaltenes from HCB. By using CH3CN with pentane, up to 53% less asphaltenes can be precipitated relative to using only pentane as the precipitant. Although significantly less asphaltenes are precipitated when using CH3CN, the 500 nm AD coke index is about the same compared to using only pentane. This shows that the quality of the DAO from the two precipitation methods are about the same with respect to the formation of precoke and coke products under pyrolysis conditions. The addition of CH3CN modifies the separation so that more of the least soluble and most aromatic methylene chloride-soluble asphaltenes are precipitated relative to the more soluble and less aromatic cyclohexane-soluble asphaltenes. By using a diluent instead of pentane, at the same HCB:precipitant ratios, even less asphaltenes can be precipitated. Even when significantly less asphaltenes were precipitated using a diluent in the presence of CH3CN, the 500 nm coke index was still dramatically improved. This approach shows how by modulating the solvent power of the precipitant and combining it with CH3CN, the most aromatic and polar asphaltenic fraction can be selectively precipitated from HCB. Ni and V metal species were preferentially concentrated in the asphaltenes to the same extent for separations with or without CH3CN. This demonstrates that although the CH3CN separation precipitated significantly less asphaltenes, the metal containing materials are still selectively precipitated. It was also demonstrated that the CH3CN could be recycled without affecting the quality of subsequent separations. Mechanistic studies showed that a small amount of the strong solvent CH3CN becomes dissolved in the pentane/DAO layer boosting the solubility parameter of the resulting precipitating mixture. Other possible mechanisms which were not ruled out include the chromatographic effects of the mixture and specific interactions between CH3CN molecules and asphaltenes (Lewis acid or base interactions, coordination,
CH3CN became increasingly soluble in pentane; and that when more than 2.5 mL of the CH3CN was used the pentane became increasingly soluble in CH3CN (Figure 15). The partial solubility test showed that when the ratio of CH3CN to pentane was about 1:5−6 there was very little cosolubility of the two solvents. At the 1:2 ratio of CH3CN to pentane, as was used for several of the separations, the partial cosolubility test shows that there is slightly more pentane (11 vol %) soluble in the CH3CN. However, this small loss of pentane into the CH3CN layer does not explain why significantly less asphaltenes are precipitated when CH3CN is present. For the pentane only separations when the amount of pentane is reduced by half (going from 1:2 HCB to pentane ratio to a 1:1 HCB to pentane ratio), the amount of asphaltenes precipitated was reduced by only 24 wt %, but when CH3CN was present at a 1:2:1 HCB:pentane:CH3CN ratio, there was a 44 wt % drop in asphaltenes precipitated relative to the 1:2 HCB:pentane separation. Clearly a loss of 11 vol % of pentane is not enough to make such a large difference in the amount of asphaltenes precipitated when CH3CN is present. The results from the simple partial cosolubility test using CH3CN and pentane do not explain why significantly less asphaltenes were precipitated. This is because the model does not take into consideration how HCB modifies the solubility parameter of the solvents and thus their resulting partial cosolubility. A second partial cosolubility test was conducted with HCB DAO that was prepared using 40 volumes of heptane to precipitate the asphaltenes (C7-DAO), pentane, and CH3CN. The results were significantly different from the partial cosolubility test without C7-DAO. In the absence of DAO there was 11 vol % pentane dissolved in CH3CN at the 1:2 ratio of CH3CN:pentane. However, in the presence of C7-DAO, at a 1:2:1 C7-DAO:pentane:CH3CN ratio, there was about 14 vol % CH3CN dissolved in the pentane layer. The presence of C7DAO significantly shifts the partial cosolubilities of pentane and CH3CN. A mechanism dominated by the partial solubility of CH3CN in pentane is expected to be largely governed by the solubility parameter of the resulting pentane/CH3CN mixture. By neglecting the solubility parameter of the DAO, the solubility parameter of CH3CN dissolved in pentane can be calculated since solubility parameters are usually assumed to be additive. CH3CN has a Hildebrand solubility parameter of 24.4 and pentane is 14.5 MPa1/2. By using the results from the partial solubility experiment for 1:2:1 C7-DAO:pentane:CH3CN, this showed that 1 mL of CH3CN dissolved into 6 mL of the pentane. The resultant 1:6 CH3CN:pentane solution would have a solubility parameter of 16.1 MPa1/2. It is clear that since some CH3CN becomes dissolved in the HCB/pentane layer, the solubility parameter of the precipitating mixture is increased which would cause less asphaltenes to be precipitated. This however may not be the only driving factor for the enhanced selectivity of precipitation. With respect to the third mechanism, it has been demonstrated that solvents with the same solubility parameter can actually dissolve/ precipitate significantly different amounts of asphaltenes. For example, it was shown that cyclohexane precipitates 20−50 wt % more asphaltenes than a 1:1 toluene:heptane mixture although both the solvent and mixture have the same solubility parameter (16.8 MPa1/2).35 This is due to the stronger chromatographic effect of the aromatic π-electrons of toluene L
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Energy & Fuels or π-interactions). Characterization of the material which dissolved in CH3CN showed that it contained mostly smaller volatile aromatic compounds with trace amounts of Ni and V.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Shell for providing funding and allowing publication of this work. REFERENCES
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