Energy Fuels 2010, 24, 4430–4438 Published on Web 07/28/2010
: DOI:10.1021/ef1002809
Advances in Quantitative Analysis of Heavy Petroleum Fractions by Liquid Chromatography-High-Temperature Comprehensive Two-Dimensional Gas Chromatography: Breakthrough for Conversion Processes Thomas Dutriez,† Marion Courtiade,*,† Didier Thiebaut,‡ Hugues Dulot,† Julie Borras,† Fabrice Bertoncini,† and Marie-Claire Hennion‡ †
IFP, BP3, 69360 Solaize, France, and ‡ESPCI, PECSA UMR CNRS 7195, 10 rue Vauquelin, 75231 Paris Cedex 05, France Received March 10, 2010. Revised Manuscript Received June 10, 2010
The implementation of high-temperature two-dimensional gas chromatography (HT-2D-GC) represents an important advance for heavy petroleum fraction analysis. Offline liquid chromatography (LC) separation before HT-2D-GC was applied to different vacuum gas oils (VGOs); results were in agreement with those obtained from reference methods. These conditions allowed for an extended quantification of heavy saturated compounds, including iso-paraffins and naphthenes. Weight distributions by carbon atoms number and chemical structures could be built for VGO cuts thanks to this innovative separation. These breakthrough progresses for the characterization of heavy petroleum fractions represent utterly new data to study conversion processes (including coking and hydrotreatment/hydrocracking) and to improve kinetic models, which could be based on actual chemical composition.
silica-alumina or a zeolithic structure in an alumina matrix and a metal (Pt, Mo/W, and Ni). The coking process also deals with heavy cuts, allowing for the rejection of metal in coke. It produces high-quality coke from vacuum residues but also low-quality liquid products with high contents of olefins, aromatics, and heteroelements. VGO cuts represent about 10-30 wt % of coking products and then need to be purified by a HT process. A kinetic modeling of the reactions involved in the processes is often used to study, simulate, and optimize conversion processes, such as HT2-4 or HCK.5,6 In the past, more or less detailed models were implemented:7 lumped kinetic models, continuum lumping, or structure-oriented lumping. The disadvantage of these models is that they depend upon kinetic parameters to describe the feedstock composition. More recently, the single event concept8 was developed. The “single event” concept uses carbenium ion chemistry to describe the elementary reactions at a molecular level. In this case, parameters are not dependent upon feed properties, but applicability remains difficult for real feedstocks. A molecular analytical characterization of process feedstocks and products is necessary for model implementation. Lighter cuts can be analyzed by standard analytical methods (gasoline cut) or recent two-dimensional gas chromatography (2D-GC) ones for atmospheric gas oils.9-11 However,
1. Introduction The proportion of heavy fractions varies from 20 wt % for an Arabian light crude oil to more than 50 wt % for a nonconventional Canadian or Venezuelan crude oil. Therefore, their purification and conversion into valuable fuels are extremely interesting for the petroleum industry. Among the most common processes implemented to treat vacuum gas oils (VGOs), hydrotreatment (HT) and hydrocracking (HCK) become references for the production of jet fuels or diesels from VGOs. These two processes are operated under high hydrogen pressure to purify (for HT) and convert (for HCK). They produce high-quality fuels containing a low proportion of aromatic components, sulfur, and nitrogen. Until now, different types of process modifications have been developed, each including different efficiencies: single stage once through, single stage with liquid recycle, and two stage.1 As a first step, HT is essential to eliminate impurities by hydrogenating aromatic rings and removing sulfur and nitrogen. The HT process is carried out with adapted metal-based catalysts on sulfided NiMo, CoMo, or NiW on alumina, under high hydrogen pressure (more than 100 bar) and at high temperature (approximately 400 °C). The aim of this process is also to preserve acid catalysts of the HCK process from poisoning effects of impurities. The HCK process consists of the conversion of heavy molecules into lighter ones, produced using bifunctional catalysts under high pressure. Typical HCK catalysts consist of a combination of an acid function (which induces isomerization and cracking via carbenium ions) and a metallic function (which allows for the (de)-hydrogenation HCK reactions). Catalysts are composed by the association of
(2) Jimenez, F.; Kafarov, V.; Nunez, M. Chem. Eng. J. 2007, 134, 200– 208. (3) Remesat, D.; Young, B.; Svrcek, W. Y. Chem. Eng. Res. Des. 2009, 87, 153–165. (4) Charon-Revellin, N.; Dulot, H.; L opez-Garcı´ a, C.; Jose, J. Oil. Gas. Sci. Technol. 2010, in press. (5) Balasubramanian, P.; Pushpavanam, S. Fuel 2008, 87, 1660–1672. (6) Kumar, H.; Froment, G. F. Ind. Eng. Chem. Res. 2007, 46, 5881– 5897. (7) Ancheyta, J.; Rana, M. S.; Furimsky, E. Catal. Today 2005, 109, 76–85. (8) Froment, G. F. Catal. Rev. 2005, 47, 83–124. (9) Ruiz-Guerrero, R.; Vendeuvre, C.; Thiebaut, D.; Bertoncini, F.; Espinat, D. J. Chromatogr. Sci. 2006, 44, 566–574.
*To whom correspondence should be addressed. Telephone: þ33-0478-02-20-76. Fax: þ33-04-78-02-27-45. E-mail: marion.courtiade@ ifp.fr. (1) Wauquier, J. P. P etrole Brut, Produits P etroliers, Sch emas de Fabrication; Technip: Paris, France, 1994. r 2010 American Chemical Society
4430
pubs.acs.org/EF
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
no method is currently optimized to provide an accurate molecular quantitative description of heavier fractions, including the 350þ product after HT and the unconverted product after HCK. Nevertheless, VGO cuts represent key matrices, obviously for feedstocks but also after HT (60-80 wt %) or after HCK (10-30 wt %), because impurities and unconverted compounds are concentrated in these fractions. Because no single technique can currently bring sufficiently detailed results for heavy fractions,12 a combination of analytical data is often used in mathematic algorithms (i.e., molecular reconstructions13) to build a representative mixture for heavy petroleum samples. Even if these mathematical models have been used to study different conversion processes, including HT and HCK,14 the lack of analytical data combining chemical structures and molecular weight induced biases.15 This limits the understanding of HT and HCK steps, especially to identify and quantify inhibitors and refractory compounds. Considering process requirements and limitations of molecular reconstructions, new analytical methods dedicated to heavy fractions are necessary. Although high-resolution mass spectrometry with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS)16 recently provided unsurpassed identification of VGO compounds, it cannot provide quantitative data and, hence, cannot be used for kinetic modeling. New analytical methods based on high-temperature twodimensional gas chromatography (HT-2D-GC) techniques17,18 hyphenated with liquid chromatography (LC) fractionation19 have recently been considered as good alternatives. Indeed, when two independent chemical dimensions20 (volatility and polarity) are combined, a VGO is analyzed as a function of chemical structures and carbon atoms number. LC preseparation provides more space (peak capacity) for 2D separation. After HT-2D-GC conditions were adapted19 and the second-dimension duration for each different fraction was tuned, it was possible to obtain almost molecular details on saturated and aromatic fractions. Therefore, extended quantification of saturated compounds with new quantitative results on iso-paraffins and naphthenes was recently obtained.19 Conventional 2D-GC was previously considered as a new way to study kinetic modeling,21 and it seems17-19 that HT-2DGC may also become one. In this study, LC-(HT-2D-GC) was applied to different VGOs to check the applicability of this new method to a wide
range of samples and to confirm its potential to quantitatively analyze heavy petroleum samples. In the second part of the study, special attention is paid to conversion processes, including narrow VGO cuts after the coking process. A comparison of feedstock, HT, and HCK unconverted VGOs is also presented. The last part of this study is devoted to the evaluation of its advantages to study conversion processes and involved reactions to provide kinetic models with more detailed input data. 2. Experimental Section 2.1. Standard Method Analysis. The high-temperature simulated distillation [adapted from American Society for Testing and Materials (ASTM) D2887] was performed at a constant helium flow (10 mL/min) using a HP 6890 gas chromatograph (Agilent Technologies, Massy, France) equipped with a flame ionization detector (FID) (400 °C) and a cool on-column injector. A metallic capillary column MXT-1 (Restek, France) Silcosteeltreated stainless steel (10 m 0.53 mm 0.5 μm) was heated from 35 to 390 °C at 10 °C/min. A simulated distillation curve was obtained using ChromDis software (Gecil Process, France). Mass spectrometric analyses were derived from the Fisher method, using a mass spectrometer (Autoconcept, MSI, U.K.) equipped with a magnetic sector.22 After the resin fraction was removed by LC, the samples were ionized by electronic ionization at 70 eV. A mass spectrometric resolution of 10 000 allowed for the separation of multiplets and identical mass rating. The quantification of each hydrocarbon family was obtained using a correlation matrix based on an average carbon atom number determined by high-temperature simulated distillation. 2.2. Petroleum Samples and Conversion Processes. Highgrade solvents were purchased from Sigma-Aldrich (St. Quentin Fallavier, France). Petroleum samples (VGOs) (different geographic origins, separation type, or catalytic processes) were supplied by IFP-Lyon (Table 1). For LC fractionation, samples were diluted in n-heptane. After LC separation, fractions were evaporated. Saturated and aromatic fractions were heated for 10 min at 80 °C, then diluted in CS2 (1/5) (to guarantee complete dissolution), and finally, analyzed in corresponding HT-2D-GC configurations. The coking process was performed on a feedstock vacuum residue; coke of different qualities (delayed or fluid coking) and also lighter cuts were produced. The resulting liquid products are often highly olefinic and contain a high proportion of heteroelements. The VGO cut (frequently corresponding to approximately 10-30 wt % of products) was vacuum-distilled according to ASTM D2892, and three narrows cuts were obtained: 350420 °C (VGO H), 420-500 °C (VGO I), and 500þ °C (VGO J). HT and HCK processes are used for the production of highquality gas oil or jet fuel from VGOs. Both steps were applied on the feedstock VGO D. Processes were carried out with adapted catalysts under high hydrogen pressure and at high temperature. HT consisted of the purification of heavy cuts and HCK of the conversion. The proportion of the VGO cut is high and represents an essential fraction in products, especially after HT (generally about 60-80 wt %) but also after HCK (generally about 10-30 wt %). Therefore, VGO cuts after HT (VGO K) as well as those unconverted after HCK (VGO L) were used for this study. 2.3. LC-(HT-2D-GC) Experiments. 2.3.1. LC Separation. Group-type separation by LC of VGO samples was performed applying a procedure derived from the ASTM D2007 method. Preparative adsorption LC was carried out using a column filled with silica and alumina (Merck, Darmstadt, Germany). A pump (model 510, Waters, Guyancourt, France) was used to deliver the mobile phase. Three fractions were collected by increasing
(10) Adam, F.; Bertoncini, F.; Brodusch, N.; Durand, E.; Thiebaut, D.; Espinat, D.; Hennion, M. C. J. Chromatogr. A 2007, 1148, 55–64. (11) Adam, F.; Bertoncini, F.; Thiebaut, D.; Courtiade, M.; Hennion, M. C. J. Chromatogr. A 2010, 1217, 1386–1394. (12) Merdrignac, I.; Espinat, D. Oil. Gas. Sci. Technol. 2007, 62, 7–32. (13) Hudebine, D. Reconstruction moleculaire de coupes petrolieres. Ph.D. Thesis, Ecole Normale Superieure de Lyon, Lyon, France, 2003. (14) Verstraete, J. J.; Revellin, N.; Dulot, H.; Hudebine, D. Abstr. Pap.;Am. Chem. Soc. 2004, 227, No. U1070. (15) Revellin, N. Modelisation cinetique de l’hydrotraitement des distillats sous vide. Ph.D. Thesis, Ecole Normale Superieure de Lyon, Lyon, France, 2006. (16) Al-Hajji, A. A.; Muller, H.; Koseoglu, O. R. Oil Gas Sci. Technol. 2008, 63, 115–128. (17) Dutriez, T.; Courtiade, M.; Thiebaut, D.; Dulot, H.; Bertoncini, F.; Vial, J.; Hennion, M. C. J. Chromatogr. A 2009, 1216, 2905–2912. (18) Dutriez, T.; Courtiade, M.; Thiebaut, D.; Dulot, H.; Hennion, M.-C. Fuel 2010, 89, 2338–2345. (19) Dutriez, T.; Courtiade, M.; Thiebaut, D.; Dulot, H.; Bertoncini, F.; Hennion, M.-C. J. Sep. Sci. 2010, 33, 1787-1796. (20) Giddings, J. C. J. High Resolut. Chromatogr. 1987, 10, 319–323. (21) Van Geem, K. M.; Reyniers, M. F.; Marin, G. B. Oil Gas Sci. Technol. 2008, 63, 79–94.
(22) Fafet, A.; Bonnard, J.; Prigent, F. Oil Gas Sci. Technol. 1999, 54, 453–462.
4431
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
Table 1. Origin, Physical Characteristics, and Elementary Composition of VGOs Studied by LC-(HT-2D-GC) samples
process
geographic origin
density at 15 °C (g/cm3)a
total sulfur (% S)b
total nitrogen (ppm N)c
boiling point interval (°C)d
VGO A VGO B VGO C VGO D VGO E VGO F VGO G VGO H VGO I VGO J VGO K VGO L
straight-run distillation straight-run distillation straight-run distillation mixture of straight run and VGOs straight-run distillation straight-run distillation vacuum residue HCK vacuum residue coking vacuum residue coking vacuum residue coking VGOs HT VGOs HCK
type I type II type II type II type III Venezuela Middle East Europe Europe Europe Middle East Middle East
0.8808 0.9282 0.9414 0.9237 0.9429 0.9811 0.9531 0.9360 0.9490 0.9886 0.8646 0.8327
0.13 2.03 2.92 2.41 0.42 3.44 1.31
830 1527 1357 836 1655 2667 1717
ε ε
ε ε
380-566 376-554 399-582 366-533 358-550 375-545 403-547 345-435 382-490 464-600 375-521 376-513
a Determined by NF ISO12185. b Determined by NF ISO14596 or ISO20884. c Determined by NF 07058 or ASTM D4629. d Determined by HT SimDist (ASTM D2887) (5-95% weight).
Table 2. Quantification Results after LC Fractionation of VGOsa
the mobile-phase eluting power (n-heptane, dichloromethane, and methanol): saturated compounds, aromatic compounds, and resin fraction. Then, LC fractions were evaporated and weighted to allow for a weight repartition by group type. 2.3.2. HT-2D-GC. Experiments were carried out using a 2D chromatograph (trace GC, Thermo, Italy) equipped with a CO2 dual-jet modulator, a split injector at 370 °C (1/100) (0.2 μL), and a FID system (370 °C, 100 Hz). H2, air, and He (makeup) flows were 35, 400, and 35 mL/min, respectively. Analytical gases were provided by Air Liquid (Feyzin, France) at a minimum purity of 99.99%. Helium was used as a carrier gas at a constant flow rate. In this study, column sets consisted of the combination of a nonpolar first column (DB1-HT, dimethylpolysiloxane, J&W Scientific) and a midpolar second column [BPX-50, (50% phenyl)polysilphenylene-siloxane, SGE] placed in the same oven. For all VGOs and aromatic fraction analyses, the same column set was used: DB1-HT (10 m 0.32 mm, 0.1 μm) and BPX-50 (0.5 m 0.1 mm, 0.1 μm) considering the previous study.17 For saturated fractions, a column set composed of DB1-HT (10 m 0.32 mm, 0.1 μm) and BPX-50 (1.5 m 0.1 mm, 0.1 μm) was used.17 The GC system was operated under temperature-programmed conditions, from 100 °C to the maximum temperature of the column set (BPX-50, 370 °C) at 2 °C/min. The system was operated under a constant flow, close to the optimum rate of the first column:23 1.2 mL/min in both sets. Two-dimensional modulation, provided by dual-stage carbon dioxide jets, was carried out at the beginning of the second column. The temperature of cryogenic jets was set at -40 °C (room-temperature conditions), adjusting the inner diameter of restrictors used for CO2 expansion. The modulation period was set to 20 s. 2.4. Data Treatment. Raw data of FID signals were acquired using Polycard software (Thermo, Italy) and exported as a CSV file for further data processing. GC GC contour plotting, retention time measurement, blob fitting, and peak integration were performed using 2DChrom (Thermo, Italy). Intensities were visualized with contrasting colors, ranging from pale blue to dark blue for minor and major peaks, respectively.
samples
saturates (%, w/w)
aromatics (%, w/w)
resins (%, w/w)
VGO A VGO B VGO C VGO D VGO E VGO F VGO G VGO H VGO I VGO J VGO K
71.8 48.2 37.5 47.3 50 26.6 25.7 47.2 43 27 87.8
22.5 43.7 52.9 46.9 40.5 60.9 65.2 44.6 42.1 42.7 11.3
5.7 8.1 9.6 5.8 9.5 12.5 9.1 8.2 14.9 30.3 0.9
a LC method repeatability. Confidence interval (CI) = 0.011C (concentration found).
Section; the petroleum matrices were separated in three fractions of increasing polarities: saturates, aromatics, and resins. Quantification results after solvent evaporation are in Table 2. The proportion of each chemical class changes as a function of geographic origins or conversion processes. Because resins are composed of the most aromatic and polar compounds, they were not taken into account in this study, even if their proportions were not negligible. This is also the case in most of the MS methods, owing to distortions of the quantitative results.22 3.1.2. Qualitative Remarks. After the offline LC separation of VGOs, saturated and aromatic fractions were analyzed by HT-2D-GC under the conditions described in section 2.3. Two-dimensional contour plots of four saturated fractions are in Figure 1. Actually, 2D contour plots represent a bidimensional view of the studied samples. In the first dimension, a separation by the carbon atoms number was obtained using an apolar column. The second dimension was a midpolar separation, including phenyl groups; separation by rings number was obtained in this dimension. On the basis of the previous identifications performed by time of flight/mass spectrometry (TOF/MS),19 2D elution zones were distinguished: n-paraffins, iso-paraffins, and naphthenes by rings number. A visual comparison of the 2D contour plots clearly provided evidence that different compositions correspond to different blue intensities. For those four saturated fractions, intensities of biomarker compounds (acyclic isoprenoid alkanes, hopanes, and steranes) are very different and related to the type of VGO. For three straight-run VGOs (A, B, and E), the presence of
3. Results and Discussion 3.1. Application to Various VGOs. In the first part of this study, LC-(HT-2D-GC) was applied to a wide range of VGOs, to set the method for group-type quantification in comparison to reference methods. For this purpose, eight VGOs (A-G and K) with different origins were chosen as a first working basis. 3.1.1. LC Separation. Group-type separation by LC was performed on VGOs A-K, as described in the Experimental (23) Beens, J.; Janssen, H. G.; Adahchour, M.; Brinkman, U. A. T. J. Chromatogr. A 2005, 1086, 141–150.
4432
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
Figure 1. Two-dimensional contour plots of four saturated fractions from four VGOs. See conditions in section 2.3 (2L = 1.5 m). H, hopanes; S, steranes; 1N, mononaphthenes; 2N, dinaphthenes; 3N, trinaphthenes.
Figure 2. Two-dimensional contour plots of two aromatic fractions from two VGOs. See conditions in section 2.3 (2L = 0.5 m). 1A, monoaromatics; 2A, diaromatics; 3A, triaromatics; 4Aþ, tetra-aromatics and more.
each biomarker group is confirmed. A high proportion of biomarkers is supposed in VGO E, corresponding to resistance to thermal degradation and different maturities. Concerning the VGO G, obtained from a conversion process, only hopanoic biomarkers were evidenced, while acyclic isoprenoids have disappeared. Moreover, the high number of compounds between 1t = 80 and 100 min, on the 2D contour plot, induced high coelutions between saturates and mononaphthenes. Aromatic fractions of each VGO were analyzed under the conditions described in section 2.3. Two-dimensional contour plots of two aromatic fractions are in Figure 2. For the identification of aromatic 2D elution zones, the results of a previous study19 based on retention times of test mixtures were considered. Actually, the selectivity by the aromatic ring is clearly not completely suitable, because the distinction of each group type is not obvious. It is even less straightforward for some aromatic fractions, such as VGO E, for which almost no distinction is possible. Nevertheless, the same integration mask was applied for all aromatic fractions. The orange dotted line shows the separation between
saturated and monoaromatic solutes, which was taken into account for quantitative analysis by HT-2D-GC without any previous LC fractionation. 3.1.3. Quantitative Results. The quantification procedure consisted of (i) determining a group-type quantification by a 100% normalization of each fraction and (ii) taking into account the saturates/aromatics ratio determined by LC fractionation. For each fraction, a discrimination factor was implemented along the first dimension as determined in ref 19. Quantification results for saturated and aromatic compounds in eight VGOs were compiled in Figure 3. Actually, the proportion of saturates is different for each VGO. In the case of straight-run VGOs, the polynaphthene content is supposed to be due to biomarkers (hopanes and steranes). For VGO H, obtained from HT, a high proportion of naphthene is observed because they are formed by hydrogenation of aromatic rings during the HT process. We point out that new information is available for all VGOs: the iso-paraffin content. Concerning aromatic compounds, the proportion of each class depends upon VGO samples 4433
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
Figure 3. Quantitative results for (left) saturated and (right) aromatic fractions by LC-(HT-2D-GC) analysis of eight VGOs.
Figure 4. Comparisons of quantitative results for (A) saturates and (B) aromatics by LC-(HT-2D-GC) and MS Fischer.
and the process involved. The low proportion of polyaromatic compounds in the hydrotreated product (VGO H) is confirmed, while a high content of polyaromatics is found in the HCK product (VGO G). 3.1.3.1. Comparisons to the MS Method. To compare our results to those obtained using the MS Fischer reference method, parity diagrams were built. LC-(HT-2D-GC) results for saturates and aromatics compared to those of the MS Fischer method are displayed in panels A and B of Figure 4, respectively. Samples showing the highest differences (VGOs C and G) were indicated by an arrow, with the corresponding label. While HT-2D-GC only allowed for the construction of parity diagrams for aromatics18 with quantitative biases, LC(HT-2D-GC) allows for a better comparison between the two techniques thanks to the improved group-type resolution between saturates and monoaromatics. Therefore, for all aromatic group types, a better correlation is observed (Figure 4B). Concerning saturated compounds (Figure 4A), the parity diagram takes into account the fact that the polynaphthenic group was composed of naphthenes having more than one saturated ring. For saturates, it can be noticed that a good overall correlation exists between the MS Fischer method and LC-(HT-2D-GC), for all samples. 3.1.3.2. Quantification by the Carbon Atoms Number. Thanks to the use of bidimensional techniques, weight distributions by the carbon atoms number and chemical structure could be obtained for each VGO. As explained in the literature,19 their plot was obvious for the saturated fraction, although for the aromatic fraction, the distinction by carbon atoms number is hopeless. However, in using group contribu-
Figure 5. Weight distributions of hopanes by carbon atoms number for six straight-run VGOs obtained by LC-(HT-2D-GC). See conditions in section 2.3.
tion methods19 via a discretization step, quantitative distribution of aromatics by the carbon atoms number can be obtained. In comparison to the usual method (MS Fischer method), these data represent a breakthrough progress for the analytical characterization of heavy petroleum fractions. It is now possible to quantitatively compare samples by carbon atom, which opens new possibilities for their study. For example, LC-(HT-2D-GC) can be used for a geochemical study of VGOs comparing the biomarker content. As shown in Figure 5, distributions of hopanes depend upon the geographic origin of crude oils and could be related to geologic type or maturity.24 3.2. Interest for Understanding of Processes. As demonstrated above, an offline LC-(HT-2D-GC) analysis allows for access to innovative descriptions of heavy cuts (350þ). (24) Peters, K. E. The Biomarker Guide; Cambridge University Press: New York, 2005; Vol. 2.
4434
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
Figure 6. Two-dimensional contour plots of (top) saturated and (bottom) aromatic fractions of a 350þ product from a coking process on a feedstock VGO: VGOs H-J. See conditions in section 2.3 (2L = 0.5 m for aromatics, A; 2L = 1.5 m for saturates, S). P, paraffins; 1N, mononaphthenes; 2Nþ, dinaphthenes and more; 1A, monoaromatics; 2A, diaromatics; 3A, triaromatics; 4Aþ, tetra-aromatics and more.
Figure 7. Comparison of chemical group contents for narrow VGO cuts (VGOs H-J).
To illustrate the analytical potential of this technique for process studies, analytical schemes were prepared for two conversion processes of heavy cuts: coking and HT/HCK of VGO cuts. 3.2.1. Coking Process. VGO cuts frequently represent an important fraction after the coking process. The aim of this part was to determine a fine chemical description of a VGO coking product on different ebullition ranges. Therefore, the VGO cut was first separated in narrow cuts: 350-420 °C (VGO H), 420-500 °C (VGO I), and 500þ °C (VGO J) before initiation of LC-(HT-2D-GC) analysis. The determination of 2D elution zones and the quantification procedure were carried out as described previously. Two-dimensional contour plots of each fraction are in Figure 6. As expected, compounds were translated along the apolar first dimension, following the ebullition range of VGO cuts. Because no distinction of olefins was possible on coking cuts, the same 2D elution zones were kept. For aromatic fractions,
the decrease of the second-dimension selectivity at high temperatures can clearly be deduced from the loss of resolution by the aromatic rings number. It has to be pointed out that a low content of heavy alkanes is also found in the heavier aromatic fraction, because of their low solubility in solvents used for LC fractionation. A group-type quantification was completed for each cut, as explained in previous sections, and results are compiled in Figure 7. These results confirm that n-paraffin content decreases according to the ebullition range, whereas the aromatic and isomerization content increases. For a more accurate description of the chemical evolution on ebullition range, weight distributions of three cuts was also drawn (Figure 8). Therefore, it is possible to follow the distribution of chemical families for each cut and also determine the profile for each group. These results confirm that LC-(HT-2D-GC) can describe the chemical evolution along the ebullition range for coking products, which allows for a deeper study. However, 4435
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
Figure 8. Weight distributions by the carbon atoms number and chemical structure of VGO H (350-420 °C), VGO I (420-500 °C), and VGO J (500þ °C) obtained by LC-(HT-2D-GC). See conditions in section 2.3.
Figure 9. Two-dimensional contour plots of saturated fractions for (left) feedstock VGO D and (right) HT VGO. See conditions in section 2.3. 1N, mononaphthenes; 2&3N, di- and trinaphthenes; 2Nþ, dinaphthenes and more; 4N, tetranaphthenes; 5N, pentanaphthenes.
it still lacks information on heavy olefins, which is also true for reference analytical methods. To proceed to an accurate study of upgrading processes, feedstock and product distributions should be compared. 3.2.2. HT/HCK Processes. To understand the HT and HCK processes even more, developed conditions were implemented for feedstock and process product (350þ) VGOs. Therefore, VGO D and HT products (VGO K) were analyzed under LC-(HT-2D-GC) conditions. On the contrary, the HCK unconverted product (VGO L) was only analyzed under HT-2DGC conditions for saturates, because the aromatic content was very low (less than 2% as determined by the MS method). Contour plots of feedstock and HT VGOs are displayed in Figure 9 (saturated fractions) and Figure 10 (aromatic fractions).
Concerning saturated fractions, a visual examination confirms that polynaphthenic compounds are less important after HT, because of the dilution effect with other saturated compounds or the cracking of these structures at high temperatures. With regard to the aromatic fraction of HT VGO, polyaromatic structures have almost disappeared compared to those of the feedstock. Consequently, most aromatic compounds after HT are concentrated in mono- or diaromatic elution zones. The loss of group-type resolution must be due to the increase of the hydrogenated chemical structures number, including naphtheno-aromatic coumpounds. For the quantification step, the same 2D integration masks were taken into account for both samples. Concerning the HCK VGO (VGO L), the 2D contour plot is shown in Figure 11. 4436
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
Figure 10. Two-dimensional contour plots of aromatic fractions for (left) feedstock VGO D and (right) HT VGO. See conditions in section 2.3. 1A, monoaromatics; 2A, diaromatics; 3A, triaromatics; 4Aþ, tetra-aromatics and more.
Figure 11. Two-dimensional contour plot of VGO L. See conditions in section 2.3 (2L = 1.5 m). 1N, mononaphthenes; 2Nþ, dinaphthenes and more.
A qualitative analysis shows that the 2D elution zone of mononaphthenes is less intense, while the intensity of iso-paraffins is greatly increased. These first remarks are consistent with HCK reactions of hydrogenation and hydroisomerization involved in the preparation of this VGO. A quantification step was performed as explained earlier. Results for three VGOs are compiled in Figure 12, according to the number of rings (aromatic or naphthenic). The straight-run VGO D contains a lot of aromatic compounds, while the proportion of naphthenic compounds is much more important in the HT VGO. During the HT step, aromatics are converted by hydrogenation reactions of one or more of the aromatic rings in the same compound. Thus, a direct comparison of the content variation between compounds having the same rings number must be taken carefully. Concerning poly-ring structures (more than four rings), HT did not cause any significant increase of polynaphthene content but tetra-aromatic content decreased. This could be explained mostly by partial hydrogenation of polyaromatic compounds or conversion by cracking in lighter compounds in the gas oil range. With regard to other compounds with naphthenic or aromatic rings (one, two, or three), the content of aromatics has drastically decreased, after HT, while the naphthenes content has increased. Besides, a low rise of the paraffin content was observed. For the HCK VGO, results were allowed to confirm quantitatively the improvement of aromatic ring hydrogenation and the opening of naphthenic rings. The proportion of iso-paraffins became very high, which is related to the HCK mechanism. LC-(HT-2D-GC) conditions have brought a better description for the heavy cut evolution because its quantitative results are consistent with the reactions occurring in HT and
Figure 12. Comparison of group-type content determined by LC(HT-2D-GC) before HT (VGO D), after HT (VGO K), and after HCK (VGO L).
HCK processes. To go further in a better understanding of reactions involved in each step, weight distributions were built for the three VGOs (Figure 13). Thus, an easier visualization of transformations, occurring during each step along the carbon atoms number range, is illustrated by smoothed curves. Therefore, the previous bimodal distribution of VGO D cannot be seen after the HT process (VGO K) because of the hydrogenation and the light cracking effect of the catalyst involved in the HT step. For HCK VGO, cracking is confirmed by the high proportion of compounds in the low carbon atoms number range. These distributions allow for the access to molecular content by the carbon atoms number, which is key data to study HT and HCK processes by kinetic modeling. These results can be directly applied to kinetic modeling after converting weight fractions into molar fractions. The conversion of these results into a new analytical constraint during the molecular reconstruction step can be another way to exploit these new data. For a thorough study of HT/HCK processes, the same distributions should be built for lighter cuts, to make a global balance between feedstock VGO and products. For this purpose, current analytical methods should be enough because, as mentioned in the Introduction, one-dimensional GC for gasoline cuts and recent 2D-GC methods for gas oil cuts11 are already available. 4437
Energy Fuels 2010, 24, 4430–4438
: DOI:10.1021/ef1002809
Dutriez et al.
Figure 13. Weight distributions by carbon atoms number and chemical structures smoothed in curves for feedstock (VGO D), HT (VGO K), and HCK (VGO L) VGOs.
In the last part of this study, the application of LC-(HT2D-GC) to two conversion process effluents (coking and hydroconversion) provided a true molecular description of the feedstock or products (350þ), which is, up to now, a key point to understand the involved reactions. Despite the fact that the LC-(HT-2D-GC) technique provides new results for VGO cuts, future improvements are possible because there is still a lack of information on olefinic compounds, especially after upgrading processes, such as coking. Moreover, no quantitative data on the resin fraction are available; however, resins represent between 5 and 10 wt % of a VGO cut and are considered as composed of most refractory compounds for HT.
4. Conclusion 19
Previously developed LC-(HT-2D-GC) conditions were applied to several VGOs coming from various origins and processes. Using an offline LC pre-separation, reported coelutions between polynaphthenes and monoaromatics were suppressed, which allowed for better correlations with the MS Fischer method for almost all VGOs. In addition, an innovative description and quantification of iso-paraffin and polynaphthene contents in VGOs were possible. Besides, the lack of stationary-phase selectivity at high temperatures prevents suitable group-type resolutions between aromatic classes. However, this analytical method represents a breakthrough competitive technique for an estimation of VGO group-type quantification. When weight distributions were assessed according to the chemical family and carbon atom number, a deeper and more accurate description or comparison of VGOs can be obtained, which appears to be necessary for geochemistry or process studies.
Acknowledgment. The team of Jeremy Ponthus, Agnes Fonverne, and Badaoui Omais are warmly acknowledged, respectively, for the analysis of VGO samples by the MS Fischer method, the LC fractionation of heavy samples, and the preparation of this paper.
4438
