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Trapping of Aromatic Compounds during Coking of Athabasca Vacuum Residue Murray R. Gray,*,† William C. McCaffrey,† Naras Srinivasan,† and Keng Chung‡ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6, and Syncrude Canada Ltd., Edmonton Research Centre, 9421 17th Ave, Edmonton, Alberta T6N 1H4 Received July 19, 2002 Introduction The formation of coke solids from liquid petroleum fractions involves a complex series of chemical reactions and physical processes. Reactions such as cracking and polymerization can cause incompatibility in the liquid phase, leading to the formation of a separate phase. During its early stage of formation, this material is insoluble in toluene, but can be dissolved in more polar solvents such as pyridine or quinoline. Subsequent reactions and phase separation within the insoluble phase can lead to the formation of anisotropic lamellar mesophase, which is visible under polarized light and is insoluble in quinoline.1 The fluid or plastic behavior of toluene-insoluble material gives a distinct interface,2 and this fluid behavior can persist for an extended period depending on the rate of cross linking.1 The initial separation of the toluene-insoluble or coke phase has been attributed to the incompatibility of the newly forming phase with the bulk of the oil phase.3 This approach is valuable for analyzing the kinetics of coke formation,3,4 but direct evidence for the role of phase equilibrium is lacking. This fundamental question has direct relevance to the quest to increase liquid product yields from coking processes. Phase separation due to aromaticity of asphaltenic fragments would be expected to result in partitioning of aromatic species between the two phases. Subsequent cross-linking of the coke material could trap products in the coke by covalent bonding or by a cage effect, analogous to the retention of a solvent in a polymer gel. When tracers, such as 14C- or 13C-labeled compounds are added to vacuum residue, then subjected to thermal cracking, a portion of the isotopic label is associated with the coke product.5,6 This retention of the tracer in the coke could be due to either covalent bonding to the coke, or to physical trapping as in a gel. * Author for correspondence: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6. Phone: 780-492-7965. Fax: 780-492-2881. E-mail:
[email protected]. † Department of Chemical and Materials Engineering. ‡ Syncrude Canada Ltd. (1) White, J. L. In Petroleum Derived Carbons; Deviney, M. L., O’Grady, T. M. Eds.; ACS Symposium Series 21; American Chemical Society: Washington, DC, 1976. (2) Wang, S.; Chung, K.; Masliyah, J. H.; Gray, M. R. Fuel 1998, 77, 1647-1653. (3) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32, 2447-2456. (4) Rahmani, S.; McCaffrey, W.; Gray, M. R. Energy Fuels 2002, 16, 148-154. (5) Fixari, B.; Perchec, P. L.; Kopinke, F.-D.; Zimmermann, G.; Decroocq, D.; Thomas, M. Fuel 1994, 73, 505-509. (6) Dutta, R. P.; McCaffrey, W. C.; Gray, M. R.; Muehlenbachs, K. Energy Fuels 2001, 15, 1087-1093.
Figure 1.
This study examines the trapping of aromatic tracer compounds in coke in order to determine the role of physical trapping processes in the formation of coke. Anthracene, chrysene and perylene were added to Athabasca vacuum residue and subjected to coking conditions. The toluene-insoluble coke was recovered and dissolved in quinoline in order to liberate unreacted, trapped tracer compounds for GC analysis. To determine the possible role of toluene in extracting components from the coke, the experiments were repeated using a pentane-toluene mixture to recover insoluble material after reaction. Experimental Section Vacuum residue from Athabasca bitumen was used as the reactant (Syncrude Canada Ltd., 524 °C+ fraction, 1086.8 kg/ m3, 5.8% sulfur, 0.7% nitrogen, 24.7 wt % pentane asphaltenes and 27.8% microcarbon residue, and 1.8% toluene insolubles and 1.25% ash). A series of polycyclic aromatic hydrocarbons were selected as aromatic tracers. Anthracene (3 ring), chrysene (4 ring), and perylene (5 ring, Figure 1) were obtained from Aldrich (Mississauga, Ontario), and used as received. Solvents were from Fisher Scientific (Toronto, Ontario). Experiments were carried out in a 15 mL batch microreactor made from Swagelock fittings and tubing. The tracer compounds were added to vacuum residue at a ratio of approximately 1:10. The tracers were normally blended with warmed vacuum residue feed at ca. 100 °C. In two experiments, the tracer was dissolved in toluene, mixed with vacuum residue and evaporated to dryness. The reactor was loaded with 3 g of a mixture of vacuum residue and tracer compound and then pressure tested with nitrogen at 4 MPa. The gas was then vented, and the reactor was pressurized twice more with nitrogen and vented to purge residual oxygen. Finally, the reactor was pressurized with nitrogen to an initial pressure of 3.5 MPa. The reactor was then heated in a fluidized sand bath and agitated at ca. 1 Hz for 35 min. The contents of the reactor reached the final temperature within 5 min. All of the reactions were carried out at 430 °C under a nitrogen environ-
10.1021/ef020154v CCC: $25.00 © 2003 American Chemical Society Published on Web 01/30/2003
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ment. The reaction was terminated by quenching the reactor in cold water. Liquid product and insoluble solids were washed out of the reactor with 40 parts or more of toluene, then kept overnight at 70 °C to ensure the separation of liquid products from the solid toluene insoluble (TI) fraction. In some experiments, a mixture of toluene and pentane was used as a more paraffinic solvent. Insoluble material was then removed from the wash solution by filtration on a 0.22 µm Millipore filter. The yield of insoluble material was determined by weighing the filter after drying in a vacuum at 70 °C for 12 h. Samples of the insoluble solids were digested with quinoline by sonication at 80 °C for 1 h, then filtered using 0.22 µm filters to remove the quinoline insoluble (QI) fraction. Yields of insolubles were calculated on an ash-free basis as follows:
TI )
QI )
wTI - wVRfa wVR(1 - fa)
(wQI/fTI) - wVRfa wVR(1 - fa)
(1)
(2)
where wTI, wVR, and wQI are the masses of vacuum residue, TI and QI respectively; fa is the weight fraction of ash in the vacuum residue; and fTI is the fraction of the TI material that was samples for the determination of QI. The quinoline-soluble filtrate was washed with quinoline and toluene. Toluene was evaporated first using a water aspirator, then the quinoline was evaporated almost to dryness using a vacuum pump and a hot oil bath. The resulting solution was quantitatively transferred to a volumetric flask with quinoline as the solvent, then diluted with toluene to appropriate concentrations and analyzed by gas chromatography (GC). The concentrations of anthracene, chrysene and perylene in the reaction product were analyzed by a Hewlett-Packard 5890 GC using a HP-1 cross-linked methyl silicone gum column (25 m, 0.32 mm ID, and 0.17 µm) equipped with FID detector and a computer for storing the chromatograms. The temperature program for the GC was as follows: initial oven temperature of 50 °C, then heating to 300 °C at a rate of 5 °C/minute and then held at constant temperature for 15 min. Standard solutions were prepared to calibrate the GC response. The GC peaks from the tracers in the product mixture were identified by spiking products with the authentic compounds. A method adapted from Green et al.7 and Liotta et al.8 was used to measure swelling of the insoluble materials. Samples of TI were packed into glass tubes (diameter of 1 or 3 cm), then centrifuged at 2500g for 5 min, and the height of the packed column of TI was measured. Excess of the desired solvent (pentane or toluene or quinoline) was added to the column to immerse all the TI and left to stand for 24 h. The capillary tubes were then centrifuged again at 2500g for 5 min, and the height of the TI column was measured. The ratio of the change in height due to the solvent to the initial dry TI was taken as a measure of swelling.
Results and Discussion In the absence of vacuum residue, control experiments showed that anthracene, chrysene and perylene were stable at the reaction conditions of 430 °C for 35 min, with negligible conversion as measured by GC. The yields of TI and QI were not affected by the presence of anthracene or perylene relative to vacuum residue alone (Table 1). Gas yields were not determined. In the first (7) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935-938. (8) Liotta, R.; Brons, G.; Issacs, J. Fuel 1983, 62, 781-791.
Table 1. Yield of Toluene and Quinoline Insolubles from Athabasca Vacuum Residuea
tracer
solvent
solvent insoluble wt % of feed
noneb anthraceneb chryseneb perylene anthracenec chrysene perylene
toluene toluene toluene toluene pentold pentol pentol
11.6 ( 0.6 10.9 ( 0.7 16.6 ( 3.2 12.2 19.0 18.3 19.0
quinoline insoluble wt % of feed 2.7 ( 0.2 1.4 ( 1.2 5.3 ( 2.1 2.8 1.1 1.1 2.0
a Yields on an ash-free basis, all reactions at 430 °C for 35 min, 3.5 MPa cold nitrogen b Data from 3 repeat experiments; error estimates are standard deviations. c Mean of two experiments. d Pentane + toluene, 50:50 volume ratio.
Table 2. Tracer Concentrations in Product Fractions
tracer
solvent
tracer in solvent solubles, %a
anthracene chrysene perylene anthracene chrysene perylene
toluene toluene toluene pentol pentol pentol
55.2 ( 7.4 95.6 ( 0.4 67.9 56.1 ( 1.8 92.7 ( 3.8 51.7 ( 5.0
tracer in quinoline solubles, %a
partition ratiob
0.06 ( 0.04 0.09 ( 0.01 0.03 0.03 (0.00 0.09 8.35
0.0011 0.0009 0.0004 0.0005 0.0010 0.16
a Standard deviations for experiments with toluene are for analysis of replicate reaction experiments. Standard deviations for experiments with pentol are from repeat GC analysis. b Partition ratio ) [concentration of tracer in solvent insoluble - QS fraction]/[unreacted. tracer in solvent].
two experiments with anthracene, the feed was prepared by dissolving the feed and tracer in toluene and evaporating the solvent to dryness, while in the third the warmed residue was blended with the tracer. The yields were equivalent to the blank; therefore, the warm blending method was used for all subsequent experiments. A blank extraction experiment was carried out by mixing TI with quinoline and a known mass of anthracene, then digesting and removing the quinoline solvent and analyzing the extract for anthracene. The recovery of the anthracene from the quinoline solution was 82%. Losses were likely due to evaporation of anthracene with the quinoline solvent. Only chrysene had an effect on the yield of TI, giving an increase from 11.6% in the blank to 16.6% in the presence of chrysene (Table 1). All of the experiments with pentane-toluene mixture as a solvent gave more insoluble material, in the range 18-19%, as would be expected for a solvent with lower ability to dissolve asphaltenic material. In every case, a large majority of the TI or pentol-insoluble material was soluble in quinoline, which enabled the analysis of the majority of the insoluble material (TI or pentol-insoluble) for the presence of the tracer compounds. The concentrations of tracers in the toluene-soluble (TS) and the toluene-insoluble-quinoline-soluble (TIQS) fractions are listed in Table 2. Very small amounts of anthracene, chrysene, or perylene were recovered from the TI-QS fraction. Some anthracene and perylene were lost due to reaction with the vacuum residue, whereas most of the chrysene was recovered from the solvents. The loss of tracers to the QI fraction cannot be ruled out, but given the lack of trapping in the TIQS fraction, trapping in the QI seems unlikely. Other experimental techniques would be required to check
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Table 3. Solvent Swelling of TI Material sample 1 sample 2
solvent
% swelling
pentane toluene pentane toluene
5.1 16.1 8.5 16.1
partitioning of tracers into the QI material. The ratio of the concentrations of the tracer in the TI-QS and the TS fractions is given as the partition ratio in Table 2. The small value of this ratio emphasizes the low recovery of tracer from the coke. Note that this conclusion is not affected by the loss of some anthracene during sample preparation, while evaporation losses of chrysene and perylene would be even lower. These data clearly show that aromatic tracer compounds were not physically trapped in the TI fraction formed from vacuum residue. After reaction, the tracers were either in the oil phase, or had reacted with components in the oil or the TI. The low recovery of tracer from the TI suggested either that the aromatic compounds did not partition with the TI phase in the reactor or that the experimental method was not able to detect or probe such phase behavior. After the reaction was quenched, toluene was used to extract the reactor contents. If toluene were too strong a solvent, then it might extract the TI material so well that the tracer compounds were completely removed from the coke matrix. Swelling tests on TI samples were used to determine whether the toluene could swell the coke matrix, and thereby release the tracer compounds. The data of Table 3 show representative results, comparing swelling in toluene to swelling in n-pentane as a control. Toluene swelled the TI more than pentane, therefore, trapped compounds in the TI material could potentially be extracted. To prevent such extraction of the TI, but to allow washing of the reactor and filtration of solids, we selected a less aromatic solvent. A toluene-n-pentane mixture (50:50 vol/vol) or “pentol” is equivalent in solvent properties to the unreacted bitumen.9 This solvent mixture was used in experiments with anthracene, chrysene and perylene (Tables 1 and 2). The use of a weaker solvent did not significantly change the recovery of anthracene or chrysene from the quinoline soluble fraction. Perylene gave very low solubility in the pentol solvent, based on dissolution of pure crystals at room temperature. Consequently, most of the perylene tracer was not extracted and passed into the quinoline solution. As a result, the high apparent concentration of perylene in the experiment with pentol (Table 2) is not representative of the composition of the TI. The observations in this study suggested that trapping of low-molecular-weight components in the coke, measured as TI, was not significant. When toluene was used as the solvent, the partitioning of the tracers decreased with increasing number of aromatic rings, which is opposite to the trend expected due to aromatic partitioning. These observations are consistent with the (9) Yarranton, H. W. “Asphaltene Solubility and Asphaltene Stabilized Water-in-Oil Emulsions”, Ph.D. Thesis, University of Alberta, Dept of Chemical and Materials Engineering, Edmonton, Alberta, 1997.
work of Fixari et al.5 who detected insignificant amounts of 14C label in coke when labeled polynuclear aromatic compounds were added to vacuum residues prior to cracking and coking. When 14C-chrysene was blended with Safaniya vacuum residue and reacted at 430 °C, the yield of TI was 15 wt %, but only 1.4% of the tracer 14C appeared in the TI fraction. This result reinforces the present study, because it shows that the total incorporation of chrysene in the solids (both TI-QS and QI) by all possible mechanisms was very low. Four explanations are possible to explain the lack of tracer compounds in insoluble material from vacuum residues, both from this study and Fixari et al.:5 1. The insoluble solids that form after quenching the reactor do not represent the material that separates from the oil at reaction temperatures. Direct sampling of the TI from a reactor will always give some toluenesoluble phase, because the TI likely forms as a dispersion of fine particles or droplets in the oil phase, rather than a discrete phase in the bottom of the reactor with a definite phase boundary (Wang et al.2) 2. The tracer compounds in this study and in Fixari et al.5 were too small to partition into the new aromaticrich phase. This explanation might apply to the threering aromatic anthracene, but it is less plausible for four ring (chrysene) and five-ring (perylene) polycyclic aromatic hydrocarbons. 3. The tracers in the separated coke phase formed covalent bonds before the reaction was quenched. This explanation is unlikely, given the data of Fixari et al.5 and the solubility of a large fraction of the TI in quinoline, showing a low degree of cross-linking. 4. The separation of the insoluble coke is driven by some process other than incompatibility of aromatic compounds. The best candidate mechanism is polymerization, partly because coking is normally thought to involve polymerization reactions. In this case, the precipitation of TI material would follow polymerization of aromatic and polar species in the liquid phase. The lack of reactive groups on chrysene would account for its low concentration in the TI in the 14C-labeled experiments of Fixari et al.5 In contrast, aromatics with reactive side chains, such as alkyl-pyrenes, can form insoluble products even in model reactions.10,11 The present study did not confirm the hypothesis of phase separation during coking due to incompatibility of aromatic species in the liquid phase. The data presented here do not disprove the hypothesis, but they do suggest that further examination of the fundamentals of coking is required to verify the mechanism by which the coke phase separates from the liquid. Acknowledgment. The authors gratefully acknowledge the helpful comments of Vince Nowlan, Syncrude Canada, experimental assistance from Tuyet Le and financial support from the Syncrude/NSERC Industrial Research Chair in Advanced Upgrading of Bitumen. EF020154V (10) Savage, P. E.; Jacobs, G. E.; Javanmardian, M. Ind. Eng. Chem. Res. 1989, 28, 645-653. (11) Freund, H.; Matturro, M. G.; Olmstead, W. N.; Reynolds, R. P.; Upton, T. H. Energy Fuels 1991, 5, 840-846.
