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Chemical Characterization of GPC Fractions of Athabasca Bitumen Asphaltenes Isolated before and after Thermal Treatment† Heather Dettman,* Adrienne Inman, and Sara Salmon National Centre for Upgrading Technology, 1 Oil Patch Drive, Suite A202, Devon, Alberta, Canada T9G 1A8
Kerry Scott and Bryan Fuhr Alberta Research Council, 250 Karl Clark Road, Edmonton, Alberta, Canada T6N 1E4 Received October 22, 2004. Revised Manuscript Received April 10, 2005
The object of this work was to determine whether new information could be obtained by using gel permeation chromatography (GPC) to fractionate asphaltene samples prior to analysis. In particular, GPC elution profiles, elemental analyses, molecular weights by vapor pressure osmometry (VPO), and boiling point distributions of the asphaltenes isolated from the original Athabasca bitumen feed (Feed) and from its total liquid product (TLP) after visbreaking were compared. The analyses showed that for GPC run using chloroform, fractionation was based on size where elemental analyses and boiling point distributions indicated that the earlier eluting fractions were not aggregates of later eluting fractions. The largest TLP asphaltene species were slightly smaller in size (by GPC and VPO) to those in the Feed asphaltenes; the smallest TLP asphaltene species were smaller than those isolated from the Feed asphaltenes and contained material with an initial boiling point of 340 °C despite both vacuum distillation (524 °C cutpoint) and pentane extraction being used during asphaltene preparation. For comparable molecular sizes by GPC and VPO, the TLP asphaltene fractions had lower H/C ratios and so were more aromatic and consisted of higher boiling material than the Feed asphaltene fractions. VPO results and elemental analysis trends confirmed that pentane extraction leaves behind molecules (asphaltenes) on the basis of some combination of size, aromatic content, and polarity. The significance of the various fractions of asphaltene species isolated remains to be evaluated in terms of their contributions to bitumen and heavy oil behavior during both production and thermal processing.
Introduction There continues to be considerable discussion over the “structure” and “size” of asphaltene molecules.1-5 As a fraction of petroleum that is insoluble in alkane solvents such as pentane or heptane, asphaltenes are the species that tend to cause the most problems during petroleum transportation and refining. Yet, of all petroleum fractions, their molecular structures are least understood. The physical separation of petroleum molecular species before analysis is a standard approach for characterization of distillate fractions (bp < 524 °C [975 °F]). For example, the combination of gas chromatography and mass spectrometry (GC-MS) is a powerful tool for † Presented at the 5th International Conference on Petroleum Phase Behavior and Fouling. * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Strausz, O. P.; Mojelsky, T. W.; Faraji, F.; Lown, E. M.; Peng, P. Energy Fuels 1999, 13, 207-227. (2) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 1123711245. (3) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677-684. (4) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809822. (5) Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. Pet. Sci. Technol. 2003, 21, 461-489.
determining distillate hydrocarbon compositions. The use of GPC columns for fractionation of heavy hydrocarbons such as asphaltenes has been studied.6,7 However, there has been little work published on the chemical characterization of the fractions obtained.8 This paper describes the use of gel permeation chromatography (GPC) to generate fractions of asphaltenes prior to chemical characterization. In particular, the pentane-asphaltenes derived from the Underground Test Facility (UTF) Athabasca bitumen before and after thermal treatment (visbreaking) were prepared and analyzed. The results of vapor pressure osmometry (VPO), high-temperature simulated distillation (HTSD), and elemental analyses of the GPC fractions are compared. Experimental Section Visbreaking was performed on UTF Athabasca bitumen (Feed) by thermally treating 400-g batches of bitumen in a 1-L autoclave, under nitrogen. Temperature was increased to (6) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandyoti, R. Energy Fuels 2004, 18, 778-788. (7) Carbognani, L. Pet. Sci. Technol. 2003, 21, 1685-1703. (8) Carbognani, L.; Espidel, J. Pet. Sci. Technol. 2003, 21, 17051720.
10.1021/ef0497356 CCC: $30.25 Published 2005 by the American Chemical Society Published on Web 05/14/2005
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Table 1. Elution Volumes for Feed and TLP Asphaltene GPC Fractions volume (mL) fraction feed 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
156 167 178 188 199 209 220 230 241 251 261 272 282 292 302 312 322 332
volume (mL)
TLP fraction feed
171 183 194 206 217 229 240 252 264 275 287 299 310 322 334
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
342 352 362 372 382 392 402 411 421 431 440 450 459 469 478 487 497 506
volume (mL)
TLP fraction feed 345 357 369 381 392 404 416 428 440 452 464 475 487 499 511
52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67
516 525 535 544 553 563 572 582 591 600 610 619 629 638 647 657
TLP 523 535 547 559 571 583 595 607 619 630 642 653
405 °C at a rate of 2.3 °C/min and was held at this temperature for 1 h. These conditions were chosen as they were just below the conditions where coke was visible in the vessel. The process resulted in 34.5 wt % conversion of vacuum residue to lighter products including a gas make of 2.6 wt % feed. The toluene insoluble content of the total liquid product was 2.1 wt % after 31 days. Asphaltenes were precipitated from the D1160 vacuum residues [boiling point (bp) +524 °C] of Feed and total liquid product (TLP) with pentane, using a single treatment of the procedure outlined in Peramanu et al.9 This method includes adding 40-volumes of pentane, sonicating in a bath sonicator for 45 min, leaving the mixture to rest overnight at room temperature, then sonicating for an additional 30 min before filtering, and washing with pentane until the eluent is colorless. This method gave 36.2 and 49.2 wt % asphaltenes from the Feed and TLP residues, respectively. GPC was run on the asphaltenes using Bio-beads S-X1 purchased from Bio-Rad. These beads are reported to have a molecular weight separation range from 600 to 14 000 g/mol. Two 4-ft columns (volume of ∼580 mL each) were prepared using beads suspended in tetrahydrofuran. The columns were connected in series and were washed with three bed volumes of chloroform (void volume was ∼190 mL). The pump flow rate was set to 0.7 mL/min for all runs with pressures of 3 and 9 psi measured for the Feed and TLP asphaltene runs, respectively (SX-1 beads can withstand pressures up to 100 psi). It was found that the elution rate was not constant for all samples and so fraction volumes were measured at regular intervals to be able to standardize elution profiles by volume rather than by time. The volumes for the fractions analyzed are given in Table 1. Fraction numbers for the Feed asphaltenes were used to refer to both Feed and TLP asphaltene samples, corresponding to the elution volumes listed. For each run, approximately 2 g of asphaltene sample was dissolved in 5 mL of chloroform and was sonicated in a bath sonicator for at least 1 h to homogenize. Fractions were collected in 20-mL test tubes using an LKB fraction collector, taking 30 h to complete. Fractions were dried under nitrogen in a TurboVap evaporator with water bath temperature at 45 °C. Fractions were then put in a vacuum oven at 45 °C overnight before final weights were measured. A total of four runs were performed for each of the Feed and TLP asphaltenes. Equivalent fractions from each run were combined using chloroform and then were redried before analyses. Two additional GPC runs were prepared and combined for the Feed asphaltenes for elemental analyses; fractions were further combined as indicated below. One additional GPC run was performed using 950 mg of the Feed fractions 18 and 19 to test fraction elution reproducibility. (9) Peramanu, S.; Pruden, B. B.; Rahimi, P. Ind. Eng. Chem. Res. 1999, 38, 3121-3130.
Figure 1. Gel permeation chromatography (GPC) elution profiles for the asphaltenes isolated from UTF Athabasca bitumen (Feed) (upper panel) and from its total liquid product (TLP) after visbreaking (lower panel). The error bars indicate the range of variation found between elution profiles from repeated chromatography runs, normalized to 2 g of asphaltenes per run. Molecular weights were measured by vapor pressure osmometry (VPO) using o-dichlorobenzene at 120 °C, as described in Peramanu et al.9 High-temperature simulated distillation (HTSD) was performed on an Agilent gas chromatograph using Analytical Control software (American Society for Testing and Materials [ASTM] 6352-02). Analyses for carbon, hydrogen, and nitrogen contents were performed using a LECO CHN analyzer (ASTM D5373).10 Sulfur content was measured using ASTM D4239 on a LECO sulfur analyzer (Model SC432). The following Feed asphaltene GPC fractions were pooled to have enough material: 24 + 25 (referred to as F25); 28 + 29 (F29); 32 + 33 + 34 (F33); 36 + 37 + 38 (F37); and 40 + 41 + 42 (F41).
Results and Discussion The object of this work was to determine whether new information could be obtained by using gel permeation chromatography to fractionate asphaltene samples prior to analysis. In particular, gel permeation chromatography (GPC) elution profiles, elemental analyses, molecular weights by VPO, and boiling point distributions of the asphaltenes isolated from the original Athabasca bitumen feed (Feed) and from its total liquid product (TLP) after visbreaking were compared. The GPC elution profiles for Athabasca Feed and TLP asphaltenes are shown in Figure 1. An error bar for each fraction indicates the repeatability of the elution profile (10) See, for example: Annual Book of ASTM Standards; 2001; Section 5, Petroleum Products, Lubricants, and Fossil Fuels, Volume 05.06; American Society for Testing and Materials: West Conshohocken, PA, 2001.
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Figure 2. Carbon, hydrogen, nitrogen, and sulfur contents of the Feed (left panels) and TLP (right panels) asphaltenes and their selected GPC fractions. The Feed fractions that are composites of more than one fraction include F25 (24 + 25), F29 (28 + 29), F33 (32 + 33 + 34), F37 (36 + 37 + 38) and F41 (40 + 41 + 42).
for the Feed and TLP asphaltene samples. The flow rate for the TLP asphaltenes was slightly lower than that of the Feed, presumably because of toluene insoluble material (below). Consequently, the volumes of fractions were monitored to be able to compare elution profiles. The Feed asphaltenes started to elute and were off the column sooner than those of the TLP asphaltenes. The TLP asphaltenes consisted of a lower content of the largest molecular species (T17-T26) (40 wt %) compared to that of the Feed asphaltenes (F16-F23) (55 wt %). The TLP contained 2.1 wt % toluene insolubles (TI). Assuming that these species were concentrated in the asphaltenes fraction, the TLP asphaltenes would have a TI of 12.4 wt %. We have found that chloroform can dissolve some types of TI material, and so we decided to run the columns without removing the TI material to obtain the fullest information of the types of species generated during visbreaking. Black material was deposited in the upper portion of the first GPC column; however, the lowest recovery from the TLP GPC runs was 95 wt %; Feed GPC runs obtained better than 97 wt % recovery. The most “cokelike” material was found in the earliest eluting TLP fraction, that is, T20. Figure 2 shows the results of elemental analyses for the Feed, TLP, and selected GPC fractions. It was expected that the original Feed and TLP elemental contents would be between the values of those obtained for their GPC fractions. However, repeat analyses of the original samples did not, for example, reduce the carbon content of the original Feed asphaltenes. This may reflect either incomplete combustion of the sample
during the analysis or inhomogenegity within the batch of asphaltenes. The carbon contents of the original TLP asphaltenes and its GPC fractions were greater, and the hydrogen contents lesser, than those of the Feed asphaltenes. The TLP asphaltenes and its GPC fractions had slightly lower sulfur contents and similar nitrogen contents compared to those of the Feed asphaltenes. The carbon contents of the Feed GPC fractions appeared to increase slightly in the later eluting fractions. The carbon contents of the TLP GPC fractions were quite similar. Interestingly, for similar elution volumes (F17-F41 compared to T19-T40), similar trends were followed for hydrogen, nitrogen, and sulfur contents: hydrogen contents and H/C ratios increased, and nitrogen and sulfur contents decreased. For the TLP asphaltenes, the trends for the later eluting fractions (T44-T57) changed: hydrogen contents and H/C ratios decreased, nitrogen contents were similar, and sulfur contents increased. The variations found between the GPC fractions for both the Feed and TLP asphaltenes suggest that the earliest eluting fractions were not aggregates of the later eluting fractions. The separation of species by GPC may not only be based on the sizes of the species but also on their shapes. Two molecules of the same molecular weight will move through the column packing material at different rates depending on whether the molecule is a spherical shape (faster) or a rod shape (slower). Also, polar molecules may tend to adsorb to the material, slowing their rate of elution. In Figure 1, it was seen that Feed asphaltene species eluted slightly earlier than TLP asphaltene species. This suggests that earliest eluting Feed as-
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Figure 3. Vapor pressure osmometry (VPO) results for the original Feed and TLP asphaltenes and their selected GPC fractions. Error bars indicate the 10% repeatability error found for the molecular weight standards.
phaltenes were larger or more spherical or were less polar than the earliest eluting TLP asphaltenes. Figure 3 shows the average molecular weights of the asphaltenes from the original Feed and TLP and their GPC fractions measured by VPO. The results for the original Feed and TLP asphaltenes were 2630 g/mol and 1230 g/mol, respectively. The ranges of molecular weights of the GPC fractions for the Feed and TLP asphaltenes were from 660 to 6300 g/mol. The molecular weights correlated well between the fractions relative to their elution from the GPC column. The last species to elute from the column were the smallest molecular weight species in the TLP asphaltenes; the largest were the Feed asphaltene species that eluted earliest from the column. That the VPO results were quite superimposable throughout the Feed and TLP elution profiles suggests that shape and adsorption effects were not major influences and that the elution profile resulted from size differences of the components. Pentane extraction leaves behind molecules (asphaltenes) on the basis of size and aromatic content and polarity. Figure 2 showed that the latest eluting GPC fractions, for the TLP asphaltenes, had decreasing H/C ratios and increasing sulfur contents. Adding the results from Figure 3, asphaltene species with average molecular weights less that 1000 g/mol became more aromatic and had increasing sulfur contents, as average size decreased. The results illustrated that the smallest molecules were asphaltenes because of their aromatic carbon and heteroatom contents. Whether the true size range of the asphaltene components was from 660 to 6300 g/mol remains to be confirmed. Aside from the possibility of aggregation during the VPO measurement (below), the choice of calibration standards was not ideal. Usual analytical practice for calibrating a test response versus values of a chosen characteristic includes selection of calibration standards that are structurally similar to the compounds to be measured and that have values of the characteristic that cover the range of responses expected. The standards used to calibrate VPO, benzil and sucrose-octaacetate, contained relatively high contents of oxygen compared to petroleum hydrocarbons, including asphaltenes. Also, they had relatively low molecular weights (210.23 and 678.61 g/mol, respectively). Consequently, small differences between the calibration
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Figure 4. GPC elution profile of the asphaltenes isolated from the Feed and that of two of its fractions combined and rerun, F18 and F19.
standards and their equivalent-sized hydrocarbons would become significant differences when extrapolated to higher molecular weights. The suggestion that adsorption to the Bio-beads was not significant during elution with chloroform was further supported by both the asphaltene recovery from the column (>97 wt % for the Feed asphaltenes and 95 wt % for the TLP asphaltenes) and the nitrogen content results (Figure 2). The nitrogen contents of the Feed and TLP asphaltenes were similar and had the same trend in that the first eluting species had the highest nitrogen contents and the last eluting species had the lowest nitrogen contents. If nitrogen species were adsorbing to the column material, it would be expected that the nitrogen contents would be the greatest for later eluting species. There have been discussions of the aggregation state of asphaltenes in solvents other than chloroform.1-5 To see directly whether the earlier eluting species were actually aggregates of the later eluting species, equal quantities of two Feed fractions, F18 and F19, were pooled and rerun on the GPC column (Figure 4). The expectation was that after sonication and re-elution from the column, aggregated components would form a new equilibrium mixture and the elution profile would be similar to that of the Feed asphaltenes (Figure 1). Figure 4 shows that the combined Fractions F18 and F19 began to elute with similar volume of eluent as they did in the Feed asphaltenes. The profile was slightly broader than that in the Feed but there was negligible tail in the profile for these fractions; the profile returned to baseline by 330 mL. The observation that the elution profile for Fractions 18 and 19 was broader than their original elution profile in the Feed was probably due to the amount of sample used. Approximately twice as much material was used for the GPC run for Fractions 18 and 19 combined compared to the quantity of those samples present in the original Feed asphaltenes. Consequently, there would be more competition for the same pore sizes in the column, effectively “overloading” the column, and so the elution profile would be broader. A second possible explanation could be that the sonication and re-elution of the material may actually have redistributed aggregates that contain “very sticky” monomers. However, HTSD results in Figure 5 (below) indicate that 60 wt % of the material boils over 750 °C, suggesting that if there were aggregates in the chloroform, they consisted of relatively large molecular species.
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Figure 5. Contents of asphaltene components with boiling points of >750 °C for the Feed and TLP asphaltenes and their selected GPC fractions. Error bars were calculated using the repeatability ranges given for ASTM method D 6352-02.
Figure 6. Contents of asphaltene components with boiling points of
