Cleavage of Side Chains on Thiophenic Compounds by Supercritical

Sep 22, 2014 - Adam G. Carr , Caleb A. Class , Lawrence Lai , Yuko Kida , Tamba Monrose ... Adam G. Carr , Xingyi Shi , Carlos Domene , Allen K. Leung...
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Cleavage of Side Chains on Thiophenic Compounds by Supercritical Water Treatment of Crude Oil Quantified by Two-Dimensional Gas Chromatography with Sulfur Chemiluminescence Detection Yuko Kida,† Adam G. Carr,‡ and William H. Green* Massachusetts Institute of Technology, Department of Chemical Engineering, 77 Massachusetts Avenue E17-504, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: Two-dimensional gas chromatography with sulfur chemiluminescence detection (GC × GC-SCD) is applied to understand the changes in alkylated thiophenes, benzothiophenes (BTs), and dibenzothiophenes (DBTs) during supercritical water (SCW) upgrading of Arabian Heavy crude oil. It is shown that SCW treatment of heavy crude oil has several important effects: (1) The amount of BTs and DBTs in the distillate range increase, primarily due to cracking of heavier compounds. (2) Most of the long side chains on the thiophenes, BTs, and DBTs crack to form the corresponding thiophenic compounds with shorter side chains. (3) A small amount of the alkylated thiophenes undergo ring closure to form BTs during SCW treatment, and a small amount of the alkylated BTs appear to form DBTs in a similar way. As reported earlier, SCW treatment removes some of the sulfur from the oil phase, presumably as hydrogen sulfide (H2S). Distilling the heavy crude oil into light and heavy fractions and treating these fractions individually with SCW showed these effects more clearly. Model compound studies on hexylthiophenes confirm that SCW cleaves alkyl chains bound to thiophenes.

1. INTRODUCTION Supercritical water (SCW) upgrading of heavy hydrocarbons has been gaining more and more interest in both the scientific and patent literature. SCW treatment has been shown to produce gaseous and liquid fuels from vacuum residue, heavy oil residues, coals, bitumens, and asphaltenes.1−6 It has also been reported to reduce the sulfur level in the liquid phase, presumably by converting some of the fuel sulfur to hydrogen sulfide (H2S).6 Most literature only reports bulk properties and the changes that take place at the molecular level are yet unclear. Details are known about the decomposition of organic sulfides in SCW,7 and several authors have shown that the model compounds thiophene and benzothiophene are essentially inert under normal SCW conditions.8 Here, we analyze SCW treated Arabian Heavy (AH) crude oil with twodimensional (2D) gas chromatography (GC × GC) coupled to a sulfur chemiluminescence detector (SCD) to elucidate the reactions that take place for sulfur compounds during SCW treatment, with a focus on alkylated thiophenes, alkylated benzothiophenes (BTs), and alkylated dibenzothiophenes (DBTs). Two-dimensional (2D) gas chromatography (GC × GC) has become an increasingly popular analytical tool.9 GC × GC is capable of separating compounds in complex mixtures by using two columns with different stationary phases in series. The key component of the 2D GC is the modulator. This component uses two air jets (one cooled by liquid nitrogen and other heated) that work in tandem to trap segments of effluent from the first column, which separates compounds by volatility and then injects the trapped segments into the secondary column, which separates compounds by polarity. There are a number of review papers on GC × GC.9−11 © XXXX American Chemical Society

To our knowledge, gas chromatography with sulfur chemiluminescence detection (GC × GC-SCD) analysis of SCW treated crude oil products has never before been reported in the open literature. Here, we utilize this powerful analytical tool to detect changes in sulfur compounds in AH crude oil during SCW treatment. We also study SCW treatment of fractionated crude oil to clarify the interpretation and the SCW treatment of some model compound alkyl thiophenes to test whether the transformations observed in the crude oil depend on the presence of other components or are due to the intrinsic reactions of the alkyl thiophenes.

2. EXPERIMENTAL METHOD For the crude oil and distillation fraction experiments, 1.0 g of AH crude oil was loaded with 3.5 g of water in a 24 mL 316-stainless steel batch reactor made of Sitec parts described earlier.7 The reactor was purged of air using helium (He), and 20 bar of He was left in the headspace of the reactor upon sealing to prevent water condensation in the cold spots (the tubing leading to the pressure transducer and to the valve) and for ease of gas phase product collection. The sealed reactor was lowered into a 450 °C fluidized sand bath (Techne FB05). Pressure was measured using an Omega pressure transducer, which had a digital readout and recording connected to a recorder and graphical user interface made by National Instruments. In control and model compound experiments, after the reactor was inserted into the sand bath the pressure rose to 320 bar within 10 min and remained constant to within 2%. During the experiments with crude oil, the pressure rose rapidly to about 320 bar but then continued to increase to about 360 bar. The slow increase suggests reactions forming product species which are gases at 450 °C. After 30 min, the reactor Received: July 14, 2014 Revised: September 16, 2014

A

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Figure 1. GC × GC-SCD chromatograms for crude oil before and after SCW treatment. The alkyl benzothiophenes (BTs) and dibenzothiophenes (DBTs) peaks are labeled. was quenched in a water bath. The room temperature liquid product usually came out as an emulsion of oil and water and so the mixture was demulsified by ASTM method D 1796-04. The total sulfur of the oil phase product was measured by X-ray fluorescence (XRF) using a Horiba instrument. For the model compound experiments, mixtures of 3-hexylthiophene, 2-hexylthiophene, ethyl sulfide, and naphthalene were treated in SCW. A total of 5.2 mmol of hexylthiophene was loaded into the reactor with 3.5 g of water. Naphthalene was chosen as an inert standard due to its stability in SCW within the reaction time and temperature range of our experiments.12 A small amount of ethyl sulfide was added in some experiments to assist generation of radicals. The mole ratios of hexylthiophene, naphthalene, and ethyl sulfide were 10:1:1, respectively. Other experiments on 2- and 3-hexylthiophene were conducted without ethyl sulfide using the same ratio of hexylthiophene to naphthalene. These experiments included SCW and “neat” pyrolysis experiments, i.e., without water. The main analytical tool for the crude oil experiments was the GC × GC-SCD system (Leco). The primary column was an RXi-5HT, 30 m length, 250 μm i.d., 0.25 μm film thickness. The secondary column (in a secondary oven held about 15 °C above the temperature of the primary column) was an RXi-17SIL MS, 2 m length, 150 μm i.d., 0.15 μm film thickness. The modulation time was 5 s. An Agilent flame ionization detector (FID) adapter is installed so that the effluent from the GC is analyzed by FID and SCD in series. The injector was held at 300 °C. Known amounts of 3-chlorothiophene were spiked into the oil as a standard for quantification of sulfur compounds. The typical temperature ramp was from 50 to 320 °C in 90 min. GC × GC-SCD chromatograms were analyzed with GC image software (Zoex Corp), which integrates the signal in user chosen regions of the 2D signal trace. In the analysis reported below it was assumed that the integrated SCD response of each peak is exactly proportional to the number of sulfur atoms eluting in that peak. The products of the model compound experiments were analyzed using a GC-FID (Agilent 7890), using an RXi-5HT, 30 m length, 250 μm i.d., 0.25 μm film thickness. A simple vacuum distillation unit was used to separate light and heavy fractions of AH crude oil. A small flow of He was bubbled through the oil to prevent bumping. The liquid distillate was collected in a flask immersed in ice water, and the gases emerging from that flask were passed through a trap immersed in liquid nitrogen. The distillation was halted when the temperature in the boiler reached 320 °C. For more details see the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Crude Oil Analysis. AH crude oil was analyzed by GC × GC-SCD. The chromatogram of crude oil, Figure 1, shows that most of the sulfur compounds eluting through the GC × GC contain BT or larger polycyclic aromatic rings, and the contribution of nonaromatic sulfur compounds and monoaromatics, e.g., alkyl thiophenes, are minor (about 8% of the sulfur seen in the chromatogram). Approximately 43% of the sulfur seen in this chromatogram is contained in 2-ring aromatics, e.g., BTs, and 31% in 3-ring aromatics, e.g., DBTs. About 12% of the sulfur in this chromatogram is in more polar molecules, such as 4+ ring thiophenes. When the total sulfur is added up for this chromatogram, using 3-chlorothiophene as an internal standard, it corresponds to only 1 wt % of the mass of crude oil injected. Since the total sulfur content of the crude oil is 3.0 wt % (measured by XRF, with mineral oil as a solvent), only 1/3 of the sulfur is seen by GC × GC-SCD. This is because only volatile compounds are detected by GC. The heaviest compound detected eluting from the GC × GC columns is C32H66, which has a boiling point of 467 °C. Presumably, the compounds with higher boiling points remain in the GC inlet as liquids. Indeed, when the GC inlet liner is taken out, there are visible spots of crude oil that remained without ever volatilizing and entering the GC column. 3.2. SCW Treatment of Crude Oil. SCW treatment of crude oil at 450 °C for 30 min was effective in cracking hydrocarbons and reducing the amount of sulfur in the oil phase. The total sulfur (measured by XRF) of the product organic liquid phase was 2.3 wt %, a 23% reduction in total sulfur from the original 3.0 wt % in the crude. However, the total sulfur that was detected by GC × GC-SCD was only 1.5 wt %; a 50% increase from the 1.0 wt % sulfur that was seen in the GC × GC-SCD of the original crude oil. This shows that there was some amount of sulfur (1.5 − 1.0 = 0.5 wt %, at least 1 /6th of the total sulfur in the crude) that was originally bound to heavy molecules (>C30) invisible to the GC × GC, but after SCW treatment that sulfur was bound to lighter molecules, B

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Figure 2. Integrated GC × GC-SCD peaks of alkyl BTs and DBTs in AH crude oil, before and after SCW treatment using the groupings shown in Figure 1. Many compounds with large side chains disappear, apparently undergoing side chain cleavage to form lighter BTs and DBTs with short side chains. Some of the light BTs and DBTs formed by SCW treatment come from cleavage of heavy molecules, which are not measured by GC × GCSCD.

which elute through the GC × GC columns. Simultaneously, there appeared to be some cracking of the side chains of the thiophenic compounds contributing to the decrease in higher molecular weight thiophenic compounds and an increase in the lower molecular weight alkyl thiophenes, alkyl BTs, and alkyl DBTs measured by GC × GC-SCD. The BT and DBT peaks can be grouped by molecular weight, as shown in Figure 1.13,14 The individual peaks in these groups are isomers with the same number of carbons. Grouping these compounds is relatively easy for C0−C4 BTs and C0−C2 DBTs (the numbers indicate the number of carbons on the substituents) but as these aromatic rings become more heavily alkylated, the number of isomers increase and it is harder to cleanly separate the groups. The BTs are grouped up to seven carbon side groups and the DBTs up to four carbon side groups. Heavier compounds in each series are lumped together as C8+ BTs and C5+ DBTs. The quantities of each group in Figure 2 show that there was a significant decrease in C8+ BTs but increase in C0−C7 BTs after SCW treatment. This suggests that there was a shift in BTs from heavy to light, due to the reaction in the side chains of the C8+ BTs. However, this did not explain the increase in the total amount of BTs measured by GC × GC, as shown in Table 1. We believe this was due to the cracking of BTs off of

(2) Thiophenic rings themselves are not broken (desulfurized) in the treatment. (3) Cleavage reactions in the “heavy fraction” (the fraction that is beyond the volatility range of the GCs) form smaller thiophenic compounds that fall in the volatility range of the GC. (4) Some alkylated thiophenes are converted to BTs, and some alkylated BTs are converted to DBTs, by ring closing reactions. To provide a clearer test of these hypotheses, distillation of crude oil was performed so that the “light fraction” and the “heavy fraction” can be treated separately. The prediction was that the primary reaction happening in the light fraction would be side chain cleavage. With the heavy fraction, the prediction was that, before treatment, little sulfur would be observed in the GC × GC-SCD volatility range, but after the SCW treatment, the concentration of light thiophenic compounds should increase dramatically. 3.3. Vacuum Distillation of AH Crude Oil. The bottoms (what is left after vacuum distillation) is viscous and black. In total, 65% of the original crude oil mass is left in this fraction. The total sulfur of this fraction was 4.6 wt % measured by XRF. A total of 31 wt % of the original crude oil mass ended up in the distillate fraction and its sulfur content was 0.6 wt %. This fraction is a clear brownish yellow liquid, and 2 wt % of the original crude oil mass passed through the 0 °C distillate collection flask as a gas and was collected in the liquid nitrogen cold trap. This fraction has no sulfur to our detection limit. A total of 98% of the total mass was collected in the system, and the total sulfur adds up to 3.1 wt %, within our 0.1% error bar of the 3.0 wt % sulfur of the original crude oil so both mass balance and sulfur balance is satisfactorily achieved. The GC × GC-SCD chromatogram of the light distillate fraction is shown in Figure 3a. The total sulfur detected in GC × GC-SCD is 0.54 wt %, which was within measurement error of the total sulfur content of this fraction as measured by XRF (0.6 wt % sulfur). Comparing the crude and distillate chromatograms, one can see the distillation partially cut out species with x-axis retention time (RT) >55 min, and no species with RT > 65 min are detectable. The GC × GC-SCD chromatogram of the crude oil heavy (bottoms) fraction is shown in Figure 4a. Note that there is some overlap between the distillate fraction and the bottoms fraction for x-axis RT from 30 to 65 min. With a one stage

Table 1. Group Type Quantification of AH Crude Oil and SCW Treated Product Compounds by GC × GC-SCDa group

crude (%)

treated crude (%)

benzothiophenes dibenzothiophenes thiophene + sulfides 4+ thiophenes total

0.42 0.31 0.08 0.12 0.99

0.63 0.47 0.15 0.27 1.54

a

Numbers reported are total wt % of sulfur in each fraction with 30 or fewer carbon atoms.

the heavy nonvolatile compounds contained in crude oil. Indeed, the total sulfur as measured by GC × GC increased significantly, see Table 1. We propose four hypotheses about the reactions that occur during 450 °C SCW treatment: (1) Side chains on aromatic rings pyrolyze, forming shorter chain aromatics. C

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Figure 3. GC × GC-SCD of (a) crude oil distillate fraction and (b) SCW treated crude oil distillate fraction.

Figure 4. GC × GC-SCD of (a) crude oil bottoms fraction and (b) SCW treated bottoms fraction. The treated material is part b is very different from the starting material shown in part a.

GC × GC-SCD chromatograms do not show much apparent difference (except for in the thiophene + nonaromatic sulfur compound streaks). However, quantification of the groups, Figure 5, shows a clear shift in the molecular weight of all the thiophenic compounds, with a decrease in the C6+ BTs and increases in the lighter BTs. The total amount of BTs remains approximately constant, Table 2, unlike what happened when the whole crude oil underwent SCW treatment. This indicates that there is cracking in the side chains of C6+ BTs, shifting heavy BTs to lower molecular weight BTs. The amount of DBT increased slightly for all molecular weights, meaning there was net generation of DBTs, probably some cyclization from some of the long chain alkyl BTs and alkyl thiophenes. Below it is shown that BTs are generated from alkyl thiophenes under these conditions. The most dramatic change in the distillate caused by SCW treatment was that about half of the nonpolar organosulfur

vacuum distillation, a sharp cut could not be obtained. The total sulfur detected by GC × GC-SCD in the heavy fraction was 1.3 wt %. The total sulfur of this fraction measured by XRF is 4.6 wt % so most of the sulfur in this fraction is in molecules too heavy to pass through the GC × GC. 3.4. SCW Treatment of Distillate Fraction. The 450 °C treatment of the distillate fraction in SCW had a small apparent effect shown in Figure 3. The total sulfur detected by GC × GC-SCD after treatment is 0.52 wt %, i.e., only slightly less than the untreated distillate. The chromatograms of the treated distillate look similar to the original distillate fraction; however, careful analysis and quantification of the peak volumes shows that there are shifts in the molecular weights within the alkyl thiophene, alkyl BT, and alkyl DBT streaks. The BT and DBT peaks are grouped in a similar manner for distillates as for crude oil. The DBTs are only grouped up to C4 DBT due to the rapidly fading peaks after the C3 DBTs. The D

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Figure 5. BTs and DBTs in crude oil distillate before and after SCW treatment.

original bottoms fraction (Figure 4a). The BTs and DBTs are concentrated in the short alkyl range and there is an emergence of light nonpolar organosulfur compounds. The total sulfur that is detected by GC × GC-SCD is 2.6 wt %, which is double that measured by the GC × GC-SCD of the original bottoms fraction. The total sulfur measured by XRF, on the other hand, decreased from 4.6 wt % to 3.8 wt %. The change in the sulfur compound distribution due to SCW treatment, Figure 6, is more obvious for the bottoms fraction than for whole crude. There is a decrease in C8+ BTs while there is a dramatic increase in C2−C7 BTs. The increase in these compounds is much larger than the decrease in C8+ BTs, and the total amount of BTs increased by 70% confirming the fact that there are BTs appearing in the volatile range (volatile enough to elute in GCs) from cracking of heavy fractions in the bottoms mixture, as shown in Table 3. There is a net increase in total DBTs, particularly for the light DBTs, which are presumably being formed by cracking of the heavy molecules not detectable by GC × GC. There are also large increases in the measured amount of C4+ thiophenes and a significant amount of very light nonpolar organosulfur species (thiophenes and sulfides) are formed. Presumably, these are also formed by the cracking of molecules that are too heavy to pass through the GC × GC.

Table 2. Group Type Quantification of Crude Distillates before and after SCW Treatment by GC × GC-SCDa group

distillate (%)

treated distillate (%)

benzothiophenes dibenzothiophenes thiophene + sulfides 4+ thiophenes total

0.33 0.06 0.16 0.00 0.55

0.35 0.08 0.09 0.00 0.52

a

The numbers are total mass of sulfur in each group of molecules divided by the total mass of the distillate fraction. Only molecules with 30 or fewer carbon atoms are detected by GC × GC-SCD.

species were converted. The numbers in Table 2 suggest that about half of the nonpolar organosulfur species that reacted formed BTs and DBTs, and the other half formed sulfur species which left the organic and liquid phase, e.g., H2S. This would be expected, if about half of the nonpolar organosulfur species are reactive sulfides and disulfides and the other half contains stable thiophene rings; unfortunately, with our current GC × GC columns and methods we cannot separate these two types of nonpolar organosulfur compounds. 3.5. SCW Treatment of Bottoms Fraction. The 450 °C treatment of the bottoms fraction gave very interesting results. The GC × GC-SCD chromatogram, shown in Figure 4b, shows emergence of light compounds that were not observed in the

Figure 6. BTs and DBTs in crude oil bottoms before and after SCW treatment. Many light BTs and DBTs appear in the chromatogram after SCW treatment; most of these are reaction products formed from sulfur-containing species too heavy to pass through the GC. E

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Table 3. Group Type Quantification of Bottoms Fraction before and after SCW Treatment by GC × GC-SCDa group

bottoms (%)

treated bottoms (%)

benzothiophenes dibenzothiophenes thiophene + sulfides 4+ thiophenes total

0.48 0.43 0.08 0.25 1.28

0.82 0.70 0.21 0.77 2.53

Table 5. Molar Yields of Sulfur Containing Degradation Products from 2-Hexylthiophene Decomposition at 450 °C in SCW and Pyrolysis without Watera 2-hexylthiophene

a

The SCW treatment converts heavy sulfur-containing molecules into lighter molecules detected by the GC × GC-SCD.

3.6. SCW Treatment of 2- and 3-Hexylthiophene. The results reported earlier suggest that during SCW treatment, alkyl side chains cleave off aromatic sulfur cores, e.g., thiophene and BT. To test our hypothesis, two hexylthiophene isomers were treated in SCW. We note that Smith and Savage15−21 studied the pyrolysis of alkyl benzenes and similar aromatics in the 1990s, and they observed cleavage of the alkyl chains leading primarily to methyl and ethyl arenes. However, to our knowledge, no one has reported doing that type of experiment using SCW. Furthermore, no one has tested if alkylthiophenes behave similarly to alkylaromatic rings that do not contain sulfur under these conditions. In some of our experiments, a small amount of ethyl sulfide was added to simulate the presence of aliphatic sulfur compounds in crude oil, which served as free radical initiators, and so might be expected to accelerate the reactions. To test the effect of SCW on the pyrolysis of alkylthiophenes, the mixture was also thermally cracked at 450 °C without water. The results of the 3-hexylthiophene conversion are shown in Table 4, and the 2-hexylthiophene conversion in Table 5. The

3-hexylthiophene

species

SCW

neat

SCW

neat

3-methylthiophene 3-ethylthiophene 3-vinylthiophene 3-propylthiophene 3-isopropylthiophene 3-hexylthiophene ethylbenzothiophene sum

0.29 0.1

0.3 0.14

0.02 0.01 0.26 0.00 0.68

0.25 0.00 0.69

0.14 0.06 0.05 0.02 0.01 0.32 0.00 0.61

0.16 0.08 0.03 0.02 0.01 0.31 0.04 0.64

SCW

neat

0.04 0.20 0.17 0.03 0.03 0.02 0.09 0.08 0.65

0.05 0.21 0.11 0.03 0.03 0.02 0.03 0.02 0.49

a

The lower sulfur recovery from neat pyrolysis (only 49% of the sulfur is detected by GC) is likely the result of the formation of larger aromatic structures which cannot pass through the GC.

the ethylsulfide case, suggesting the sulfide also modestly affects the selectivity. The major products of the 2-hexylthiophene decomposition were similar to the 3-hexylthiophene case, with the major products observed at the 2-position rather than the 3-position on the thiophene ring. Similar relative concentrations were observed, although the overall conversion was a little higher. Furthermore, a higher concentration of aromatic compounds was observed, including the ring closure products BT and ethylbenzothiophene. In the neat pyrolysis of 2-hexylthiophene, a lower proportion of the thiophenic rings were observed passing through the GC, about 10% less than in the 3hexylthiophene studies based on molar yield. For all cases studied, between 49% and 70% of the original number of thiophenic rings were measured in the products by GC. However, the sulfur recovery in each experiment conducted was greater than 95% as measured by XRF, i.e., the sulfur atoms are remaining in the oil phase, but after the SCW treatment or pyrolysis, many of them are no longer detectable by GC-FID. It is likely that the balance of the thiophenic rings in both the 2- and 3-hexylthiophene reactions ended up within compounds that are too heavy to be detected by the GC. The presence of these heavy compounds is evident from the dark color of the product oil. The recovery of a higher concentration of benzothiophenic and branched benzothiophenic compounds in the 2-hexylthiophene neat pyrolysis case, with a lower total thiophene yield may be further evidence of this, as shown in Table 5. Although little difference is observed between the SCW and neat pyrolysis processed samples in the GC-FID, the neat pyrolysis products (no SCW) have a darker color than the SCW treated product (see photographs in the Supporting Information ). This suggests a possible coke suppression effect in the presence of SCW. Vinylarenes are known to polymerize, and the missing 30−50% of the rings could be ending up as macromolecules, some of them with strong color. The possibility of a thiophene ring opening leading to desulfurization cannot be completely eliminated; however, we did not observe any of the C6+ hydrocarbon coproducts that might be expected from conventional desulfurization processes, and the XRF data show that the sulfur atoms remained in the oil phase.

Table 4. Molar Yields of Sulfur-Containing Degradation Products from 3-Hexylthiophene Decomposition at 450 °C in SCW and after Pyrolysis without Watera 3-hexylthiophene + ethylsulfide

species thiophene 2-methylthiophene 2-ethylthiophene 2-isopropylthiophene 2-propylthiophene BT 2-hexylthiophene ethylbenzothiophene sum

a

The corresponding expected alkane and alkene co-products were also observed, e.g., pentene is a major product with a yield similar to methylthiophene.

major products of the 3-hexylthiophene decomposition were methylthiophene and ethylthiophene for both SCW treatment and neat pyrolysis. Between 60% and 70% by molar yield of the thiophenic rings were recovered and identified by GC-FID. Similar products were observed both with and without ethyl sulfide. The overall conversion of 3-hexylthiophene was slightly higher when ethylsulfide was present. This is presumably the result of the larger radical pool present in the ethylsulfide study. The relative yield of methyl- and ethylthiophene was higher in F

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(4) Morimoto, M.; Sugimoto, Y.; Saotome, Y.; Sato, S.; Takanohashi, T. Effect of Supercritical Water on Upgrading Reaction of Oil Sand Bitumen. J. Supercrit. Fluids 2010, 55 (1), 223. (5) Zhao, L.-Q.; Cheng, Z.-M.; Ding, Y.; Yuan, P.-Q.; Lu, S.-X.; Yuan, W.-K. Experimental Study on Vacuum Residuum Upgrading through Pyrolysis in Supercritical Water. Energy Fuels 2006, 20 (5), 2067. (6) Kozhevnikov, I. V.; Nuzhdin, A. L.; Martyanov, O. N. Transformation of Petroleum Asphaltenes in Supercritical Water. J. Supercrit. Fluids 2010, 55 (1), 217. (7) Kida, Y.; Class, C. A.; Concepcion, A. J.; Timko, M. T.; Green, W. H. Combining Experiment and Theory to Elucidate the Role of Supercritical Water in Sulfide Decomposition. Phys. Chem. Chem. Phys. 2014, 16, 9220−9228. (8) Vogelaar, B. M.; Makkee, M.; Moulijn, J. A. Applicability of Supercritical Water as a Reaction Medium for Desulfurization and Demetallization of Gasoil. Fuel Process. Technol. 1999, 61 (3), 265− 277. (9) Meinert, C.; Meierhenrich, U. J. A New Dimension in Separation Science: Comprehensive Two-Dimensional Gas Chromatography. Angew. Chem., Int. Ed. 2012, 51 (42), 10460. (10) Seeley, J. V.; Seeley, S. K. Multidimensional Gas Chromatography: Fundamental Advances and New Applications. Anal. Chem. 2014, 85 (2), 557. (11) Murray, J. A. Qualitative and Quantitative Approaches in Comprehensive Two-Dimensional Gas Chromatography. J. Chromatogr. A 2012, 1261, 58. (12) Savage, P. E. Organic Chemical Reactions in Supercritical Water. Chem. Rev. 1999, 99, 603−621. (13) Schoenmakers, P. J.; Oomen, J. L. M. M.; Blomberg, J.; Genuit, W.; van Velzen, G. Comparison of Comprehensive Two-Dimensional Gas Chromatography and Gas Chromatography Mass Spectrometry for the Characterization of Complex Hydrocarbon Mixtures. J. Chromatogr. A 2000, 892 (12), 29. (14) Vendeuvre, C.; Ruiz-Guerrero, R.; Bertoncini, F.; Duval, L.; Thibaut, D.; Hennion, M.-C. Characterization of Middle Distillates by Comprehensive Two-Dimensional Gas Chromatography (GC×GC): A Powerful Alternative for Performing Various Standard Analysis of Middle Distillates. J. Chromatogr. A 2005, 1086, 21. (15) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics: Structure and Reactivity. AIChE J. 1991, 37 (11), 1613. (16) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics. 7. Hydrogenolysis in Binary Mixtures. Energy Fuels 1994, 8 (3), 545. (17) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics. VI. Detailed Chemical Kinetic Modeling. Chem. Eng. Sci. 1994, 49 (2), 259. (18) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics. 5. Pyrolysis of Methylanthracenes. AIChE J. 1993, 39 (8), 1355. (19) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics. 4. Hydrogenolysis Mechanisms in 1-alkylpyrene Pyrolysis. Energy Fuels 1992, 6 (2), 195. (20) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics. 1. Pathways, Kinetics, and Mechanisms for 1dodecylpyrene Pyrolysis. Ind. Eng. Chem. Res. 1991, 30 (2), 331. (21) Smith, C. M.; Savage, P. E. Reactions of Polycyclic Alkylaromatics. 2. Pyrolysis of 1,3-diarylpropanes. Energy Fuels 1991, 5 (1), 146.

4. CONCLUSIONS GC × GC-SCD analysis of SCW treated AH crude oil and its fractions provides important information on the product sulfur compound distribution. SCW treatment at 450 °C has several important effects: (1) under SCW treatment, the heaviest fractions of crude oil (beyond the volatility range of the GCs) react to form lighter compounds containing thiophenic rings. These reactions significantly reduce the sulfur level in the heavy fraction and double the sulfur level in the light fraction, increasing the amount of lower molecular weight thiophenic compounds; (2) SCW treatment cracks long side chains on sulfur containing aromatic rings (thiophenes, BTs and DBTs), reducing their molecular weight; (3) SCW treatment partially desulfurizes the oil, particularly the heavy fraction, both by cracking off thiophenic compounds and also by converting sulfur atoms bound in sulfide linkages and thiols into H2S7. Model compound experiments confirmed cleavage of side chains on alkylthiophenes. SCW treatment was shown to be ineffective at breaking thiophenic rings that would result in total desulfurization; instead there was a small amount of ring formation (e.g., alkylthiophenes becoming BTs). The side chain cracking reactions proceed at about the same rate in several different environments (with and without water). Analysis with GC × GC-SCD gave insight into the effect of SCW treatment at a molecular level.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the experimental apparatus. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses †

Y.K.: Dow Chemical Company, Freeport, TX. A.G.C.: Aerodyne Research Inc., Billerica, MA 01821.

‡

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from Saudi Aramco under Contract 6600023444. We thank Professor Michael T. Timko at Worcester Polytechnic Institute and Dr. Pushkaraj Patwardhan for early efforts in setting up the GC × GC-SCD. We thank Marko Djokic, Thomas Dijkmans, and Professor Kevin Van Geem from the Marin Group at Ghent University for assistance with the GC × GC-SCD data analysis. We thank Lawrence Lai for assistance with some of the experiments. Helpful discussions with Caleb A. Class are gratefully acknowledged.

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REFERENCES

(1) Cheng, Z.-M.; Ding, Y.; Zhao, L.-Q.; Yuan, P.-Q.; Yuan, W.-K. Effects of Supercritical Water in Vacuum Residue Upgrading. Energy Fuels 2009, 23 (6), 3178. (2) Choi, K.-H. Supercritical Water Process to Upgrade Petroleum. World Patent Application WO2013066852 A1, May 10, 2013. (3) Kayukawa, T.; Fujimoto, T.; Inoue, N.; Teratani, S.; Nagamatsu, S. Method for Producing Upgraded Oil from Heavy Oil by Utilizing Supercritical Water and Apparatus for Producing Upgraded Oil. Patent Application CA2772095 A1, October 19, 2012. G

dx.doi.org/10.1021/ef5015956 | Energy Fuels XXXX, XXX, XXX−XXX