Evolution of Asphaltene Structure during Hydroconversion Conditions

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Energy & Fuels 2006, 20, 2028-2036

Evolution of Asphaltene Structure during Hydroconversion Conditions Isabelle Merdrignac, Anne-Agathe Quoineaud, and Thierry Gauthier* IFP, CEDI Rene Nanarre, BP3, Vernaison 69390, France ReceiVed February 6, 2006. ReVised Manuscript ReceiVed June 8, 2006

The evolution of asphaltenes has been studied under hydroconversion conditions with an ebullated bed on a Buzurgan (Middle Eastern) feedstock. A bench unit was used to produce effluents in residue conversion conditions in the range of 55-85 wt %. Under those conditions, asphaltene conversion was observed in the range of 62-89 wt %. Asphaltenes from the feedstock and unconverted asphaltenes were then characterized using size exclusion chromatography (to evaluate asphaltene size) and 13C nuclear magnetic resonance (to evaluate the evolution of the average molecular structure parameters of asphaltenes). The work clearly shows that (i) the asphaltene unit size decreases when conversion increases and (ii) the aromaticity of asphaltenes increases due to dealkylation.

Introduction Residue conversion processes are becoming increasingly important in the world today, because of several market and economic factors. The residual fuel oil demand continues to decline, while, at the same time, there is an increasing demand for motor fuels. At the same time, the crude oil market production of light conventional crude oils is declining and projected to decline further in the future. This light oil production is being replaced by heavier, nonconventional crude oils. The residue of heavy crude oils can be upgraded through various existing processes.1,2 One of the challenges to face while upgrading residues is to handle asphaltene that is contained in the feed properly. Asphaltenes consist of a heterogeneous mixture of highly polydispersed molecules, in terms of size and chemical composition, with a high content of heteroatoms (sulfur, nitrogen, metals such as nickel and vanadium, ...).3 Those heavy molecules are dissolved in the entire residue structure but can precipitate very easily because of changes in operating conditions such as temperature or changes in composition of the residue. Furthermore, asphaltenes are known to be coke precursors in acid catalysis and catalyst inhibitors. Asphaltenes are defined by their insolubility in a paraffinic solvent. They are composed of various chemical species, including polyaromatic and polyheteroaromatic units bearing alicyclic sites that are substituted and connected to aliphatic chains with or without heteroatoms. Metals may also be included in this complex structure, through various chemical structures, such as porphyrines.4 In solution, they exhibit self-assembly and colloidal behavior, depending on the operating conditions. To * Author to whom correspondence should be addressed. Tel.: 33 4 78 02 20 97. Fax: 33 4 78 02 70 12. E-mail: [email protected]. (1) Lepage, J. F.; Chatila, S. G.; Davidson, M. In Residue and HeaVy Oil Processing; Editions Technip: Paris, 1992. (2) Gray, M. R. Upgrading Petroleum Residues and HeaVy Oils; Marcel Dekker: New York, 1994. (3) Sheu, E. Y. Energy Fuels 2002, 16, 74-82. (4) Strausz, O. P.; Lown, E. M. In The Chemistry of Alberta Oil Sands, Bitumens and HeaVy Oils; Alberta Energy Research Institute (AERI): Calgary, Alberta, Canada, 2003.

characterize such complex structures, several models have been proposed in the literature.5,6 The continental configuration defines asphaltenes with a large central aromatic region, whereas the archipelago model would describe molecules with a smaller aromatic region linked by bridging alkanes.7-9 Molecular associations are initiated by aromatic region staking (by four or five) to form the elementary particles, which further selfassociate to form colloidal aggregates. Hydrogen and π-π bonds are the main interactions involved in such association mechanisms. However, with the archipelago type, the aggregate formation is more complex, because intramolecular associations are possible.10 Numerous studies regarding asphaltene analysis have been achieved. Chemical and colloidal characterization must be thoroughly distinguished.11 Several methods can be used, such as elemental analyses, 13C nuclear magnetic resonance (NMR), size exclusion chromatography (SEC), or different coupling such as SEC-mass spectrometry (SEC-MS). Different available strategies are available to handle the upgrading of heavy residue feedstocks that contain asphaltenes: asphaltenes can either be removed from the residue feedstock or converted. Asphaltene removal is industrially feasible through solvent deasphalting (liquid-liquid extraction), producing asphalts on one side and deasphalted oil on the other side, or through a coking process (thermal cracking where asphaltene molecules are converted to coke). However, it is also possible to consider asphaltene conversion into valuable hydrocarbons. This requires severe operating conditions at high temperature and hydrogen partial pressure in the presence of an hydrogenation catalyst with low acidic support to limit coke (5) Pfeiffer, J. P.; Saal, R. N. J. Phys. Chem. 1940, 44, 139-149. (6) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33, 15871594. (7) Murgich, J.; Rodriguez, J. M.; Aray, Y. Energy Fuels 1996, 10, 6876. (8) Andersen, S. I.; Del Rio-Garcia, J. M.; Khvostitchenko, D.; Shakir, S.; Lira-Galeana, C. Langmuir 2001, 17, 307-313. (9) Yarranton, H. W.; Beck, L.; Alboudwarej, H.; Svrrcek, W. Y. Prepr.s Am. Chem. Soc., DiV. Pet. Chem. 2002, 47 (4), 336. (10) Murgich, J. Mol. Simul. 2003, 29, 451. (11) Merdrignac, I.; Espinat, D. Oil Gas Sci. Technol., to be published.

10.1021/ef060048j CCC: $33.50 © 2006 American Chemical Society Published on Web 07/19/2006

Asphaltene Structure during HydroconVersion

Figure 1. H-Oil reactor scheme.

formation. High asphaltene hydroconversion levels are then possible under these conditions and the yield of liquid produced by such processes is much higher than that with asphaltene removal processes. The high metal content of feeds that contain asphaltenes, as well as the control of reaction exothermicity, make it necessary to operate residue hydroconversion processes with specific technologies, such as ebullated-bed technology. Indeed, metals such as nickel and vanadium are concentrated in residue feeds and will progressively deposit and concentrate on the hydroconversion catalyst. Catalyst addition is then required to maintain catalyst activity during operation. Furthermore, high conversion level in the presence of hydrogen generates significant heat of reaction that must be controlled via adequate mixing to avoid large thermal gradients along the reactor. The ebullated bed reactor that is used in the H-Oil process to convert and upgrade petroleum residue, heavy oils, and bitumen12,13 is shown in Figure 1. Similar to a fixed-bed reactor, the ebullated-bed reactor (Figure 1) is used to contact hydrocarbon feedstock, hydrogen, and a bed of hydrogenation catalyst. However, the innovation of the ebullated-bed reactor was to maintain individual catalyst particles in a state of constant motion or fluidized state, by recycling an internal liquid stream. The internal liquid recycle stream is obtained from an internal vapor/liquid separation device (recycle cup), which provides suction to the recycle or ebullating pump. Inherent advantages of the ebullated bed reactor are excellent temperature control as well as low and constant pressure drop over several years of continuous operation, because bed plugging and channelling are eliminated. Very importantly, fresh catalyst can be added and spent catalyst withdrawn to control the level of catalyst activity in the reactor. There are seven H-Oil Plants in operation throughout the world, operating with various feedstocks and with demonstrated vacuum residue conversion levels of 45%85%. This operating experience includes feedstocks with more than 700 wt ppm metals and specific experience in heavy crude upgrading for the production of synthetic crude oil. The objective of this work is to characterize asphaltene structure in a residue feedstock and follow the evolution of asphaltenes as a function of hydroconversion conditions representative of industrial operation. Such information is important to better understand the reaction mechanisms that occur under hydroconversion conditions. There are a few published studies in the literature to characterize the evolution of asphaltenes under hydrotreatment (12) Colyar, J. J.; Wisdom, L. I. The H-Oil Process: A Worldwide Leader in Vacuum Residue Processing. Presented at the National Petroleum Refiners Association Annual Meeting, San Antonio, TX, 1997. (13) Duddy, J. E.; Wisdom, L. I.; Kressmann, S.; Gauthier, T. Presented at the 3rd Bottom of the Barrel Technology Conference and Exhibition, Antwerp, Belgium, October 20-21, 2004.

Energy & Fuels, Vol. 20, No. 5, 2006 2029

conditions and, therefore, essentially at moderate residue conversion. Buch et al.14 studied the evolution of asphaltene structure using fluorescence depolarization techniques and NMR analysis at moderate temperature (in the range of 350-390 °C) on a Middle Eastern crude. Under their conditions, residue conversion remained low. Ancheyta et al.15 also studied the evolution of asphaltenes using vapor-phase osmometry and NMR analysis on Maya and Isthmus feedstocks with low asphaltene conversion (540 °C in R1 feed, R2 feed, and R2 atmospheric residue effluent, respectively. As discussed previously, asphaltenes are defined by their insolubility in paraffinic solvents. Asphaltene recovery in R1 AsC7 effluent (xAsC7 R2F ) or in the second reactor effluent (xAR ) is performed through a method derived from the norm NF T60-115 with n-heptane at 80 °C with an oil/heptane ratio of 1/50. C7 asphaltene conversion is then determined as follows in each stage by comparison with xAsC7 R1F : XAsC7 R1 (wt % ) ) 100 ×

AsC7 MR1FxAsC7 R1F - MR2FxR2F

AsC7 XR1+R2 (wt % ) ) 100 ×

MR1FxAsC7 R1F AsC7 MR1FxR1F - MARxAsC7 AR

MR1FxAsC7 1R1F

On the bench unit, catalyst deactivation is not compensated by catalyst addition. Therefore, the activity of catalyst decreases as a function of time. Catalyst deactivation is a function of catalyst age, which can be expressed as a function of the residue volumetric flow rate (Qff) and the mass of catalyst (Mc):

∫Q t

Age (t) )

0

ff

dt

Mc

Size Exclusion Chromatography (SEC). SEC was performed on a Waters 150CV+ system, using a refractive index detector. The system was controlled using a Millenium chromatography manager. Calibration was performed using 10 monodisperse polystyrene standards with masses in the range of 162-120 000 g/mol (Polymer Laboratories). Samples were injected at a concentration of 5 g/L in tetrahydrofuran (THF) with a volume of 50 µL. The temperature was fixed at 40 °C and the flow rate was fixed to 0.7 mL/min. Three columns that were packed with polystyrene-divinylbenzene supports (PS-DVB, Polymer Laboratories) were chosen; the corresponding porosities are 100, 1000, and 10000 Å, and the column characteristics are as follows: packing particle size, dp ) 5 µm;



Figure 2. Scheme of the ebullated-bed bench unit.

column length, L ) 300 mm; and internal diameter, 8 mm. The SEC data enable one to describe the weight distributions according to weight averages, calculated as follows:

∑ NM i

Mn )

∑N i

Mw )

catalyst age liquid hourly space velocity, LHSV (h-1) reactor temperature (°C) reactor pressure (bar) X540°+ (wt %) XAsC7 (wt %)

i

2 i

∑ NM i

i

∑ NM

3

∑ NM

2

i

Mz )

(m3/kg)

i

∑ NM

i

i

i

∑ NM i

Mz+1 )

∑ NM i

Table 2. Main Operating Conditions and Performances

4 i 3

i

where Ni represents the number of molecules with a molecular weight of Mi, and the molecular weight Mi is given in units of g/mol. Basically, low weights are represented by Mn, whereas higher weights are represented by higher orders of the distributions (Mw, Mz, Mz+1). The choice of these operating conditions has been described elsewhere.17 Size Exclusion Chromatography-Mass Spectrometry Coupling (SEC-MS). On-line SEC-MS coupling was achieved with the mass spectrometer instrument QStar-Pulsar (Applied Biosystems) without splitting. The heater temperature is 350-400 °C. Analyses were performed in positive-ion mode. The solvent chosen (17) Merdrignac, I.; Truchy, C.; Robert, E.; Guibard, I.; Kressmann, S. Pet. Sci. Technol. 2004, 22 (7&8), 1003-1022.

Condition 1

Condition 2

R1

R1

R2

0.19 0.30 427.5 427.1 159.9 156 55.8 74.1 62.7 85.8

R2

0.36 0.20 427.5 426.8 159.9 156.1 67.2 85.4 75.1 89.0

here is dichloromethane (DCM), and only one PS-DVB column (with a porosity of 1000 Å) is used. 13C Nuclear Magnetic Resonance (13C NMR). NMR experiments were performed on an Advanced 300 MHz Bruker spectrometer, using a 10 mm BBO 1H/X/D NMR probe. The chemical shifts were referenced using CDCl3 as the solvent. Samples were prepared by mixing 100 mg of asphaltenes or resins in 3 mL of CDCl3 to obtain a homogeneous solution. 13C NMR direct acquisition spectra were realized with a 60° flip angle at a radio frequency pulse of 20 kHz, which provided the quantity of saturated and unsaturated C atoms. In addition, two 13C NMR experiments, based on the scalar coupling between proton and carbons, were conducted to obtain data about paraffinic, naphtenic, and aromatic carbon species. The spin-echo experiment allowed the aromatic and aliphatic carbon species to be quantified separately, and the Attached Proton Test (APT) series was applied to identify and quantify the proportion of C atoms, as a function of the number of proton in their neighborhood.

Results and Discussion In Table 2, we have enclosed the main operating conditions achieved during the two bench-unit tests. Operation under the conditions of LHSV ) 0.3 h-1 and 427 °C led to a residue conversion of 55.8 wt % at the outlet of the first reactor, and to a total conversion of 74.1 wt %, considering the two reactors. By decreasing the feed flow rate, we could then adjust the LHSV value to 0.2 h-1. The longer residence time at the same temperature resulted in a higher residue conversion of 67.2 wt

2032 Energy & Fuels, Vol. 20, No. 5, 2006

Figure 3. Asphaltene conversion as a function of residue conversion.

% at the first reactor outlet and 85.4 wt %, considering the two reactors. Under the last condition, attaining a residue conversion of 85.4 wt % means that 85.4 wt % of the feed boiling above 540 °C is converted to lighter and more-valuable products. Depending on the conversion level, the specific gravity of C5+ effluent decreases, because of the increasing light fractions that result from cracking. At a low residue conversion level (55.8 wt %), the C5+ specific gravity is ∼0.94 but decreases to 0.87 at high conversion (85.4 wt %). Stable operation was achieved during those two periods, and no operation problem was reported on the bench unit. The sediment content in the atmospheric residue that was produced was evaluated using the I.P.375 standard method. Under the conditions of the test, the sediment value remained 80 wt % is achievable, even at the higher LHSV studied. To characterize the asphaltenes, samples of the feedstock and bench-unit effluents were analyzed. Asphaltene Size Estimates Using SEC and SEC-MS Coupling. One of the important parameters generally used to represent the colloidal state is the molecular weight (MW). However, the characterization of asphaltenes, in term of molecular masses, is complex, because of their high polydispersity, heterogeneity, and propensity to form aggregates. The association state of these molecules is known to be very sensitive to experimental conditions such as concentration, temperature, and solvent.18,19 Because of these properties, interpretation of analytical results may be tedious. It has been shown that the MW of asphaltenes measured with different technologies can vary by several orders of magnitude, because the asphaltenic structures are not measured in the same state.20 In this study, we chose the SEC analysis, because it can be routinely used to estimate the apparent MW of oil fractions. In the case of asphaltene measurements, however, the results may be biased, because of association and nonsize effects.17,21 In Figure 4, SEC chromatograms that correspond to asphaltenes in the feedstock and in effluents are plotted at different conversion levels. Clearly, the chromatogram of the feedstock (18) Tissot, B. ReV. Inst. Fr. Petrol. 1981, 36 (4), 429-446. (19) Szewczyk, V.; Behar, F.; Behar, E.; Scarsella, M. ReV. Inst. Fr. Petrol. 1996, 51 (4), 575-590. (20) Speight, J. G.; Wernick, D. L.; Gould, K. A.; Overfield, R. E.; Rao, B. M. L.; Savage, D. W. ReV. Inst. Fr. Petrol. 1985, 40, 1, 51-61.

Merdrignac et al.

Figure 4. Size exclusion chromatography (SEC) chromatograms obtained for the feed and effluents asphaltenes at different conversion levels.

exhibits a bimodal profile, typical of the asphaltene distribution between the associated (high-molecular-weight) and lessassociated states (low-molecular-weight) of the aggregates. The low-molecular-weight tailing of the chromatogram trace may originate from the nonsize effects, which are basically due to the adsorption of species on the stationary phase of the column. The comparison between apparent molecular weight distributions of the feed and effluents shows a significant evolution with conversion. After the first reactor, the mass profile of the hydrotreated asphaltenes is shifted toward lower apparent masses. The distribution at low conversion (LHSV ) 0.3 h-1; XAsC7 ) 62.7 wt %) is still bimodal, but the lower apparent masses remain predominant. When conversion increases, this tendency becomes more pronounced and the molecular weight distribution becomes quasi-monomodal. The results can be explained by the combination of two phenomena: (1) Asphaltene aggregates of high apparent molecular weight are converted to smaller species or form structures that present better dissociative properties than the initial species, or (2) Asphaltene aggregates of high apparent molecular masses are converted to structures that develop specific adsorption properties with the stationary phase of the column, such as highly polycondensed aromatic compounds. In regard to the comparison between the two effluents from the bench unit (R1 + R2), the results show that the MW distribution continues to be shifted toward lower apparent MW values to ultimately reach a Gaussian form. In conclusion, there is a general decrease in the size of the asphaltene aggregates when the severity of conversion increases. The initial asphaltenes seem to be converted either in smaller aggregates or in species that form smaller aggregates than those in the initial feed. To further investigate the impact of conversion on asphaltene size, we compared our results with others results obtained with the same feed but treated in different conditions in a fixed-bed process at much lower temperature; the residue conversion level was kept lower, to keep fixed-bed operation viable, but a deeper feed hydrotreatment was achieved.16 For the results presented here, the residue conversion X540°+ in the fixed bed was in the range of 14%-48%, whereas the asphaltene conversion was in the range of 41%-82%. In the fixed-bed process, the thermal contribution is less pronounced than in the ebullated bed reactor: indeed, temperature is much lower and conversion is essentially due to catalytic contribution. Chromatograms obtained for asphaltenes in the feed and in the effluents, from either the fixed bed (FB) or the ebullated bed (EB), are illustrated in Figure 5. The results show that there (21) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809822.


Figure 5. SEC chromatograms of the feed and effluents asphaltenes in the ebullated bed (EB), compared to effluents in the fixed bed (FB) at lower conversion.

Figure 6. Qualitative evolution of the molecular weight (Mp) of asphaltene aggregates as a function of the residue conversion.

is a continuous evolution between feed, effluents from the fixed bed, and then effluents from the ebullated beds. Residue conversion, which, to some extent, resumes the severity of operation, seems to be a relevant parameter to follow this evolution. Under the fixed-bed conditions (lower conversion), chromatographic traces show a decrease of the apparent MW; however, large aggregates are still remaining. It seems that asphaltene dissociation (or conversion into smaller aggregates) is less pronounced in that case, although asphaltene conversion can be quite high and similar to some of the asphaltene conversion encountered under ebullated-bed conditions. According to the results, SEC enhances the change of asphaltene aggregate size, as a function of conversion. It seems that, during the conversion process, asphaltenes first dissociate into smaller aggregates before being converted. Under ebullatedbed conditions, most of the asphaltenes that remain in unconverted effluents seem to be dissociated, which is not the case of unconverted asphaltenes downstream from the fixed-bed reactor operating at a lower severity. To further illustrate the evolution of asphaltenes as a function of conversion, in Figure 6, we plotted the average molecular weight (Mp) of asphaltene aggregates as a function of the residue conversion. We defined Mp as the molecular weight observed at the top of the main SEC chromatogram peak. Here, molecular weights are only indicative values, because molecular weights determined by SEC are only relative (equivalent polystyrenes) but not absolute.22 Such value represents qualitatively only the evolution of asphaltene aggregates, because of the response factor, which is directly correlated to the molecule nature. (22) Domin, M.; Herod, A.; Kandiyoti, R.; Larsen, J. W.; Lazaro, M. J.; Li, S.; Rahimi, P. Energy Fuels 1999, 13, 552-557.


Concerning the feed and effluents from a fixed bed at low residue conversion, results show the predominance of high apparent molecular weights, representative of large aggregates (see Figure 6). When residue conversion varies in the range of 30-50 wt %, a dramatic change in asphaltene aggregates size is noticed. At higher residue conversions (>50%), the plot shows a predominance of low masses, which is characteristic of dissociated, smaller, and/or more-aromatic species. It is clear (see Figure 6) that their seems to be a continuity with the two processes in the evolution of asphaltene size, as a function of conversion. However, further work including H-Oil operation at moderate conversion shall be done to confirm that residue conversion is a relevant parameter to understand asphaltene aggregates dissociation. Other parameters such as temperature may also have to be considered. As already discussed, SEC is essentially qualitative and does not allow accurate estimation of the absolute average molecular weights of asphaltene aggregates. Therefore, we attempted to use SEC-MS coupling. With this technology, estimation of the MW is less affected by the side effects that can occur in chromatography, where aggregation should not be promoted. In a previous study, three different types of coupling were performed to check the validity of the measures:23(i) off-line coupling of SEC with laser desorption ionization-time-of-flight mass spectrometry (LDI-TOF), (ii) on-line coupling of SEC with atmospheric pressure chemical ionization (APCI)-quadripole/time-of-flight (Q/TOF) mass spectrometry, and (iii) online coupling of SEC with atmospheric pressure photoionization (APPI)-quadripole/time-of-flight (Q/TOF) mass spectrometry. Results showed that analyses performed in SEC-MS coupling give similar results, regardless of the ionization source used in MS. APCI is a soft ionization technique that allows the ionization of medium- and low-polarity compounds. Generally, singly charged ions are obtained, in positive-ion mode, without fragmentation. APCI sources are compatible with usual flow rates of the mobile phase (up to 1.5 mL/min) and are easily compatible with on-line high-performance liquid chromatography (HPLC) coupling.24 Based on these elements, SEC-APCIQ/TOF on-line coupling is the technique that has been selected in this study. Analysis with the SEC-MS technique was conducted on asphaltenes from the feed and from the effluent obtained at the highest conversion level. For technical reasons, the solvent used for the chromatography was dichloromethane (DCM) instead of THF. As a consequence, the resultant SEC profile became monomodal rather than bimodal, as previously observed for the feed. As discussed previously, the solvent impacts the associative state of asphaltene units, which is known to be very sensitive to experimental conditions.17 The aggregation state of the asphaltene and adsorption on the support differ, relative to the solvent that is used. This point confirms how the asphaltene aggregation state in solution is related to the experimental conditions. As a result, the mass spectra corresponding to each SEC fraction showed that mass is a function of the retention time. The average MW of asphaltene units in the feed was in the range of 300-1300 g/mol, with an average value of 580 g/mol. In the effluent that results from hydroconversion, it decreased to 440 g/mol. A simplification of mass spectra is also significant. (23) Merdrignac, I.; Desmazie`res, B.; Terrier, P.; Delobel, A.; Laprevote, O. Presented at the Heavy Organics Deposition International Conference, Los Cabos, Mexico, November 14-19, 2004. (24) Desmazie`res, B.; Merdrignac, I.; Lapre´vote, O.; Terrier, P. Presented at the 5th International Conference on Petroleum Phase Behavior and Fouling, Characterization of Petroleum Macromolecules Session, Banff, Alberta, Canada, June 13-17, 2004.


This first attempt to characterize asphaltenes, using SEC analyses, suggests that the asphaltene species progressively dissociate during the hydroconversion process. This dissociation probably corresponds to the conversion of asphaltenes from particles to more individual units. SEC-MS provides additional information and evidence that the average molecular weight of the asphaltene unit may change during hydroconversion. To understand how the remaining asphaltenes in effluents are different from feed asphaltenes, a detailed investigation of the chemical structure of asphaltenes is needed. Evolution of Asphaltene Structure as a Function of Conversion Using 13C Nuclear Magnetic Resonance. 13C NMR is a spectroscopic technique that enables one to characterize the various carbon types found in asphaltenes quantitatively. Therefore, we attempted to use this technique to analyze asphaltenes from the feedstock and from the effluents of the ebullated-bed bench unit to evaluate chemical structure changes as a function of conversion. Classically, the determination of the relative percentages of aromatic, naphthenic, and paraffinic carbon in an hydrocarbon sample can be determined using the measurement of refractive index, density, and molecular weight, as well as the ASTM D3238-80 correlation25 to obtain the relative carbon percentages. Patt et al.26 and Jakobsen et al27 proposed an alternative technique: the Attached Proton Test (APT) NMR method. The APT NMR experiment is based on the scalar coupling between carbon and a proton. It allows one to distinguish carbon close to an even (quaternary carbons and methylene groups) or an odd (methyne and methyl groups) proton number. The characterization of average molecular parameters has been performed using a classical 13C NMR experiment (the socalled “direct 13C” method) and a series of APT NMR analyses reported by Shoolery and co-workers26 and spin-echoes. The direct 13C method is used to obtain the total amount of aromatic (Caro) and aliphatic (Cali) carbon species by integration of the 160-100 ppm and 70-0 ppm chemical shift areas, respectively. The quantity of CH (CHaro) and quaternary carbon (Cq,aro) present in the aromatic portion is calculated using the integration of the 160-100 ppm region of the 13C spin-echo and 13C APT, corresponding to 1/2JCH with JCH ) 160 Hz, which is characteristic of protonated aromatic carbons. The percentages of CH3, CH2, CH, and quaternary carbon (Cq) present in the aliphatic component are measured using integration of the 70-0 ppm region of the 13C spin-echo and 13C APT NMR, corresponding to 1/2JCH and 1/JCH with JCH ) 125 Hz, which is typical of sp3 carbons. Using the amount of total aromatic carbon and the detailed percentages of CH and Cq, it is possible to calculate the substitution index, which is defined as the ratio between the substituted and the substitutable aromatic carbons and the condensation index, which corresponds to the average number of condensed aromatic rings. The carbon repartition in asphaltenes, between aliphatic carbon (Cali) and aromatic carbons (Caro), is represented in Figure 7, as a function of conversion. When conversion increases, 13C NMR analysis clearly shows that the asphaltene structure becomes more aromatic. Initially, in the feedstock, carbon repartition is almost even. As a function of conversion, the remaining carbon in asphaltene becomes more and more (25) ASTM Standard D3238-80, 1982 Annual Book of ASTM Standards Part 25; American Society for Testing and Materials: Philadelphia, PA, 1982. (26) Patt, S. L.; Ruben, D.; Shoolery, J. N. J. Magn. Reson. 1982, 46, 535-539. (27) Jakobsen, H. J.; Sorensen, O. W.; Brey, W. S.; Kanyha, P. J. J. Magn. Reson. 1982, 48, 328-335.

Merdrignac et al.

Figure 7. Evolution of aromatic (Caro) and aliphatic (Cali) carbon in asphaltenes, as a function of residue conversion.

Figure 8. Evolution of CH/Cq repartition in asphaltene aromatic carbon, as a function of residue conversion.

aromatic. The amount of aromatic molecules increases in the asphaltene effluent as the conversion level increases. Because of asphaltene conversion, the overall asphaltene content decreases. However, the trend observed in Figure 7 shows that conversion impacts the repartition between aromatic carbon and aliphatic carbon. The higher aromatic carbon content can only be explained by a more rapid conversion of the aliphatic components of asphaltenes. Indeed, other mechanisms such as polycondensation into aromatic cycles and dehydrogenation of naphthenes are not possible here, because of the operating conditions. The high hydrogen partial pressure and hydrogenation catalyst indeed promote hydrogenation and stabilize intermediate species that are initiated during cracking, to avoid condensation of the molecules. The evolution of CH/Cq repartition in asphaltene aromatic carbon is shown in Figure 8. CH aromatic carbon corresponds to unsubstituted carbons, whereas Cq represents the proportion of quaternary carbons, either condensed or substituted. Clearly, there is a slight increase in the proportion of unsubstituted carbon when conversion increases, which can be explained by dealkylation (mechanism a), cracking of adjacent aromatic structures (mechanism b), or conversion of the naphthenic portion of naphtheno-aromatic molecules (mechanism c), as represented in Figure 9. Hydroconversion conditions in ebullated bed are rather severe and characterized by high temperature; therefore, thermal cracking mechanisms with formation of intermediate radicals and β-scission of aliphatic chains should be favored. Therefore, mechanism b or c is more likely to occur in the conversion of asphaltenes. To investigate the aromatic carbon evolution



Figure 9. Some cracking reaction mechanisms, explaining the evolution of CH/Cq in asphaltene molecular evolution.

Figure 10. Schematization of (a) peri-condensated rings and (b) catacondensated rings. Figure 12. Evolution of CH3 and CH2 in asphaltenes (in terms of weight percentage), as a function of the residue conversion.

Figure 11. Substitution index as a function of residue conversion as a function of residue conversion.

further, we calculated the substitution index as a function of residue conversion. Indeed, the evolution of aromatic species in asphaltene is usually described using the substitution index (IS) and the peri- and cata-condensated indexes (Icp and Icc, respectively). These indexes (Icp and Icc) allow the average number of aromatic cycles that compose the structure to be evaluated. The substitution index IS represents the ratio between the substituted and the potentially substitutable aromatic carbons; Icp and Icc are determined empirically and calculated as the ratio between aromatic substituted carbons and the total aromatic carbons quantity, as represented by the following equations:

IS ) Icp )

Cq,sub Caro - Cq,cond

1 + 2(Cq,cond/Caro) 1 - [3Cq,cond/(2Caro)]

Icc )

1 + 2(Cq,cond/Caro) 1 - 2(Cq,cond/Caro)

where Caro corresponds to the total amount of aromatic carbons, Cq,sub corresponds to the amount of quaternary aromatic carbons substituted by an aliphatic chain, and Cq,cond corresponds to the amount of quaternary condensed aromatic carbons. Peri-condensated and cata-condensated rings are model molecules, such as pyrene and naphthalene, as shown in Figure 10a and b, respectively. Figure 11 shows that IS decreases when the process severity is increased. 13C NMR experiments show that the proportion of substituted C atoms decreases. This observation is consistent with the mechanisms previously proposed, considering the

cracking of adjacent aromatic structures and naphtheno-aromatic structures and also a decrease of aliphatic chain bounds. In regard to Icc and Icp, it was observed that they do not vary significantly as a function of residue conversion. Icc and Icp remain constant and have values of Icc ) 15 and Icp ) 5. Assuming that aromatic rings are comprised of an average of peri- and cata-condensated forms, the number of aromatic rings in an asphaltene entity is therefore included between the values of 5 (pure peri-condensated form) and 15 (pure cata-condensated form), depending on the reconstruction hypothesis, but it does seem to be rather constant. The stability of Iperi and Icata probably means that the conversion of asphaltenes does not modify the aromatic ring arrangement in the remaining asphaltenes: there is probably no significant condensation of aromatic rings. 13C NMR also gives the possibility to characterize the repartition of carbon aliphatic species between C, CH, CH2 and CH3. C and CH species remain very low both in the feedstock and the effluents and are not discussed here. In Figure 12, we have represented the evolution CH3 and CH2 aliphatic carbons in asphaltenes as a function of residue conversion. The increasing severity of the process conditions leads to a reduction of the length of carbon chains. Indeed, the ratio CH2/CH3 decreases. This trend is in good agreement with the dealkylation mechanism of aromatic rings induced by β-scission, as discussed previously. Dealkylation generates an increase in the number of aromatic carbons that remain in the asphaltenes, whereas cracked aliphatic carbons enter into lighter fractions and disappear from the asphaltenes. Because of β-scission, only CH3 remains as an alkyl on aromatic structures, and, therefore, the ratio CH3/CH2 increases progressively. To summarize, NMR asphaltene characterization clearly shows that the unconverted asphaltene structure changes during hydroconversion. Progressively, asphaltenes become more aromatic and this is probably mainly due to dealkylation mechanisms, as suggested by detailed characterization. Furthermore, apparently, no aromatic condensation occurs, despite the increase in process condition severity. It is interesting to combine NMR information together with SEC-MS results. According to SEC-MS, the MW of the asphaltene can be estimated to be ∼580 g/mol for the feedstock and ∼440 g/mol in effluents, as discussed previously for dissociated elementary structures. This is rather consistent with the number of aromatic rings evaluated through NMR. If we assume that the MW difference is linked to dealkylation, and if we furthermore suppose that, at high conversion, dealkylation of asphaltenes is complete, then, based on SEC estimates, we can deduce that the elementary structure of asphaltenes in


Buzurgan feedstock may be, on average, the combination of ∼7-9 aromatic rings, together with ∼10-12 branched aliphatic carbons (on average). However, the analysis proposed in this paper remains simple. It does not take heteroelements into account, which must be considered for more-detailed explanations. Conclusion Hydroconversion in an ebullated bed is a very efficient way to convert asphaltenes. Asphaltene conversions as high as 89% could be achieved. This work clearly shows that asphaltene conversion is faster than overall residue conversion. As a consequence, the asphaltene content in the remaining residue portions decreases. A detailed investigation of the asphaltene structure, as a function of conversion, shows that unconverted or remaining asphaltenes change as a function of the conversion.

Merdrignac et al.

Size exclusion chromatography (SEC) measurements clearly suggest that the remaining asphaltenes are dissociated in smaller aggregates when the conversion level attains a value of ∼50%. This is important for catalytic aspects, because such species are smaller and will migrate more easily into a catalyst than larger asphaltene particles. Apart from asphaltene dissociation, the work clearly shows that the remaining asphaltenes are more aromatic than asphaltenes in the feedstock, which is due mainly to dealkylation. Acknowledgment. The authors would like to acknowledge Cyril Collado, Isabelle Guibard, Stephane Kressmann, Alain Quignard, Alain Ranc, and Jan Verstraete for their contributions, help, and advice regarding this work. EF060048J

Evolution of Asphaltene Structure during Hydroconversion Conditions

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