Energy & Fuels 1995,9, 901-905
901
Catalytic Selectivity and H-Transfer in the Hydroconversion of a Petroleum Residue Using Dispersed Catalysts Vicente L. Cebolla,* Luis Membrado, and Jesus Vela Departamento de Procesos Quimicos, Znstituto de Carboquimica, CSZC, Calle Poeta Lucian0 Gracia, 5, 50015 Zaragoza, Spain
Robert Bacaud and Loi'c Rouleau Znstitut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France Received February 2, 1995@
Hydroconversion of a deasphalted vacuum residue of a crude oil has been performed in the presence of various disposable, dispersed catalysts at low concentration (450ppm of metal) under identical conditions: a plasma-prepared nickel-carbon catalyst, an oil-soluble molybdenum naphthenate, and a commercial nickel-molybdenum supported on alumina, in order to obtain some insight into their influence upon the mechanisms of hydrogen transfer, and to evaluate their selectivities toward the production of various hydrocarbon groups. For this last purpose, a quantitative, rapid and accurate method for hydrocarbon group type analysis (saturates f0.5 wt %, alkylaromatics f0.6 wt %, aromatics f1.0 wt %, polars f0.4 wt % and an uneluted asphaltenic group f0.2 wt %) has been used, based on an improved system of thin-layer chromatography with flame ionization detection. The catalysts significantlyaffect the quantitative distribution of hydrocarbon groups without producing new chemical families. The total hydrogen consumption is only slightly increased in the presence of these kind of catalysts. However, a different distribution of the hydrogen is achieved depending on the catalyst. Molybdenum naphthenate exhibits the higher hydrogen incorporation to its derived distillates, which in turn present significantly higher number-average molecular weight and percentage of saturates than those obtained with the other catalysts. For every catalyst studied, the more the incorporation of hydrogen in the distillates, the less the production of coke and gas. Throughout this paper, the agreement between the data obtained from TLC-FID and hydrogen balance is evidenced and explained.
Introduction A deep conversion of petroleum residues is a recognized long-term necessity resulting from the contradictory evolution of supply, consisting of heavier crudes, and an increasing demand for transport fuels and reduced amounts of heating fuels. Residues are characterized by a high average molecular weight, low hydrogen-to-carbon ratio, and high heteroatoms content. The objectives of conversion processes therefore aim to change these basic characteristics in order to meet the specifications of marketable products. Hydroconversion processes, performing hydrogen incorporation, provide a complete valorization of the fossil carbonaceous matter contained in petroleum residues, but their development is hampered by the high level of investment they require. On the other hand, the production cost is directly related to hydrogen consumption. A close control of hydrogen utilization is therefore a key for a better economy of these processes. This control relies upon the use of convenient catalytic systems, working either in ebullated bed reactors or as dispersed-disposable cata1ysts.l @
Abstract published in Advance ACS Abstracts, August 1, 1995.
Hydroconversion involves an extremely complex set of reactions which can roughly be classified according to their impact upon the properties of the resulting products: (1) reactions inducing a reduction of the average molecular weight through C-C or C-hetero atoms bond breaking; ( 2 ) hydrogen transfer reactions, mainly hydrogenation and aromatization; (3) removal of heteroatoms. Evaluating catalytic performance through the characterization of the products generated by hydroconversion reactors for process control is an unrealizable challenge from the analytical point of view. A close control requires a fast determination of multiple analytical parameters and a compromise must be found between information content and fast response of analytical tools. Hydroconversion products are composed of hundreds of molecules and represent a real challenge t o analysts. A first obvious evaluation of catalytic activity consists in a measure of the degree of conversion of residue into light products. Simulated distillation using gas chromatography (e.g., ASTM method D2887I2provides a fast determination of this parameter. However, it only (1)Rouleau, L.; Bacaud, R.; Breysse, M. Prep.-Am. Chem. Soc., Diu. Pet. Chem. 1994, 39 (3), 403-407.
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902 Energy & Fuels, Vol. 9, No. 5, 1995
applies t o the volatile fraction of the products; thus heavy and polar compounds are excluded. Furthermore, it does not give any information concerningthe chemical nature and the quality of the products. Hydrocarbon group type analysis (HGTA) is used to classify the distribution of hydrocarbons in a limited number of classes, (usually saturates, aromatics, resins, and asphaltenes) because the physicochemical properties are determined by the relative content of hydrocarbon types rather than the presence of particular species. The ideal HGTA method should be rapid, adequate for quality control, quantitative, and applicable to the whole samples, without prefractionation. Most extraction, chromatographic, and spectroscopic techniques present important limitations as has been reported for a long time.3 Extraction-based HGTA methods involve time-consuming steps and are strongly dependent upon operational condition^.^ Liquid chromatography techniques (LC) are tedious and time and reactant c~nsuming.~ Furthermore, the ASTM D2007,6 an LC based method, requires removal of asphaltenes prior to sample analysis. Likewise, high performance liquid chromatography (HPLC), although faster, presents problems with regard to the deterioration of columns because of the irreversible adsorption of polar andlor heavy compounds on the column^,^ as well as difficult quantification using conventional HPLC detectors.8 The use of spectroscopic techniques, such as W, generally presents limitations due to the lack of reliable correspondence between the chemical structure of petroleum-related compounds and their spectra as well as its poor resol~tion.~ TLC-FID presents advantages with regard t o others chromatographic techniques, such as the possibilities of analyzing whole samples without previous fractionation, quantifying the heavy/polar uneluted products using FID, as well as the rapidity of the analyses, together with the efficacy of TLC.l0-l2 Use of this technique began some time although some controversy concerning the FID response has been reported.14 In this paper, special emphasis has been given to the application of a rapid quantitative and accurate HGTA method based on an improved TLC-FID system, in order to evaluate the selectivity of the studied dispersed catalysts. The agreement between analytical (2)Drews, A. W. In Manual on Hydrocarbon Analysis, 4th ed.; American Society for Testing of Materials: Philadelphia, 1989 ASTM D2887. (3) Suatoni, J. C.; Garber, H. R. J . Chromatogr. Sci. 1975,13,367371. (4) Mima, M. J.; Schultz, H.; McKinstry, W. E. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.;Academic Press: New York, 1978; Vol. 1, Chapter 19. (5) Cebolla,V. L.; Weber, J. V.; Swistek, M.; Krzton, A.; Wolszczak, J . Fuel 1994, 73, 950- 956. (6)Drews, A. W. In Manual on Hydrocarbon Analysis, 4th ed.; American Society for Testing of Materials: Philadelphia, 1989; ASTM D2007. (7) Zander, M. Fuel Process. Technol. 1988,20, 69-80. (8) Cagniant, D. In Complexation Chromatography; Cagniant, D., Ed.; Cromatographic Science Series, Vol. 57; Marcel Dekker: New York, 1992; p 249. (9) Svehla, G . Analysis of Complex Hydrocarbon Mixtures; Elsevier: Amsterdam, 1981; Vol. 13, Parts A and B. (10)Selucky, M. L. Anal. Chem. 1983,55, 141-143. (11) Ranny, M. Thin-Layer Chromatography with Flame Ionization Detection; D. Reidel Publishing Co.: Dordrecht, The Netherlands, 1987. (12) Ackman, R. G.; McLeod, C. A.; Banerjee, A. J . Planar Chromatogr. 1990,3, 450-490. (13) Ray, J. E.; Oliver, K. M.; Wainwright, J. C. In Petroanalysis 81; Heyden & Son: London, 1981; pp 361-388. (14)Shanta, N. C. J . Chromatogr. 1992, 624, 21-35.
data and hydrogen balance have been analyzed in order to shed light on the role of dispersed catalysts upon hydroconversion of a heavy feed.
Experimental Section Catalysts. The catalysts of this study were used as dispersed, disposable solids. The activity of three different kinds of solids has been evaluated: a nickel-carbon plasmaprepared catalyst obtained by volatilizing nickel electrodes into a hydrocarbon,15 a n oil-soluble molybdenum naphthenate (Shephard Chemicals Co.) and a n alumina-supported Ni-Mo catalyst (Shell 424). Feed. The charge for hydroconversion experiments was a butane-deasphalted oil (DAO)obtained from a 450 OC+ vacuum residue. Its analytical characteristics were WC atomic ratio = 1.64; S = 0.85 wt %; (Ni V) = 5 ppm; viscosity = 60 cSt at 373 K, specific gravity = 1.058 g ~ m - ~ . Hydroconversion Experiments. The feed (100 g), catalyst (450 ppm by weight vs feed, as metal), and hydrogen (140 bar cold) were introduced in a 250 cm3autoclave equipped with a magnetically driven impeller and induction heating. The reaction temperature (440 "C) was maintained for 60 min. The selection of these experimental conditions has been justified elsewhere.16 After a run,the liquid phase was stripped at 40 "C and the totality of gases up t o C4 was recovered, metered, and analyzed by gas chromatography. The material balance of hydrogen was calculated from the metering of feed hydrogen and the analysis of gases. The distribution of hydrogen utilization between the production of gases and incorporation in the liquid products was defined as follows:
+
total H = consumption of gaseous hydrogen (moles of H, for 100 g of DAO) gas H = hydrogen consumption associated with the formation of gases liquid H = total H
- gas H
represents the amount of hydrogen consumed or released by the liquid fraction of the products. The liquid products were submitted to the analytical characterization techniques described in the following sections. TLC-FID System and Operating Parameters. An Iatroscan Mark 5 (Iatron Labs. Inc., Japan) apparatus was used for the chromatographic separation (TLC) and quantification (FID) of DAO and the hydroconversion products. The separations were carried out on chromarods S-I11 (silica gel 5 pm, pore diameter 60 A). In a usual experiment, a set of 10 chromarods are preassembled in a frame, and, after application of the sample and subsequent development with solvents, they are passed at a constant speed through the Hz flame of an FID for quantification of peaks. An improved TLC-FID system has been described, and a study of repeatability of the experiments, choice of standard conditions of analysis, optimization of the separation, and development of quantitative HGTA methods have been performed using DAO." A summary of the procedure is presented here to justify the validity of the results and to support the pertinence of discussion. Given that some doubts as to the acceptability of the quantitative results have been reported using this technique,14 mainly coming from (a) some loss of ionizable material due to the detector configuration,12J4 (b) use of inadequate spot systems,I4and (c) different sensitivity of the electronics in the (15) Rouleau, L.; Bacaud, R.; Breysse, M.; Dufour, J. Appl. Catal. A: General 1999,104, 137-147. (16)Rouleau, L.; Bacaud, R.; Breysse, M.; Dufour, J. Appl. Catal. A: General 1993, 104, 149-159. (17)Vela, J.; Cebolla, V. L.; Membrado, L.; AndrBs, J. M. J. Chromatogr. Sci., in press.
Hydroconversion of a Petroleum Residue data acquisition systems used in the past, the following points of the TLC-FID system have been improved: (1) a new configuration of the detector (Mark 5) in which the ion collector is closer to the chromarods than it was in the former models; (2) the use of an automatic sample spotter (Model 3202DS-02, SES GmbH, Germany); (3) the use of a data acquisition card and the BOREAL software (JMBS Development, France). Samples were solubilized in CHzClz (10 mg mL-'1. Volumes between 0.5 and 2 pL were usually applied on the chromarods. The hydrogen and air flows were 160 and 2100 mL min-l, respectively. The scan duration was 30 s. Other parameters which influence the procedure, such as the temperature of drying of chromarods and their stabilization, were also considered, as explained in the above-mentioned paper.17 After the separation was improved, the development sequence of eluents chosen was (1)n-hexane (38min), (2)toluene (3 min), and (3) CHzClz/methanol, 95/5 v/v (30 5). The peaks separated were attributed, according to FTIR spectra, t o saturates (retention time (rt)= 0.1 min), alkylaromatics (rt= 0.19 min), aromatics (rt = 0.37 min), polars (rt = 0.43 min), and uneluted (rt = 0.47 min). This last peak is generally considered as asphaltenes in petroleum literature when using similar sequences of development.ls Two considerations about the attribution of chemical nature to the separated peaks must be taken into account, according t o their FTIR spectra:17 (i) the fraction referred to as alkylaromatic is composed of saturates and a small concentration of aromatic entities, and (ii) the fraction referred to as polar also presents a strong aromatic character and a small concentration of C-0. Repeatability of TLC-FID Experiments. Retention times were repeatable within f0.01 min. Repeatability of FID response for the separated peaks, expressed as weight percentage, was better than that of ASTM D2007 standard and as follows: saturates f0.5, alkylaromatics f0.6, aromatics f l . O , polars f0.4, uneluted f0.2. Results reported in this work are also given for a confidence interval of 95% and as the average of 10 measurements. Repeatability was not affected by the application volumes or concentration in the studied ranges. TLC-FID under these conditions has shown t o be independent of the operator, overcoming the handicap of classical TLC methods. Quantitative HGTA using TLC-FID. FID response is known to depend on the chemical nature of the separated peaks in TLC-FID. For calibration and quantification purposes, two methods have been tried and compared: (a) method A consists of an absolute calibration using external standards; and (b) method B is a n internal normalization procedure. In method A, FID response is plotted vs the amount of each standard. The content of each fraction of an unknown sample is obtained by interpolation of the corresponding peak area in the calibration curve. In the present study, pure fractions derived from medium pressure liquid chromatography (MPLC) are the most adequate standard. These fractions should correspond exactly to the peaks separated in TLC-FID. For this purpose, TLC-FID has also been a successful off-line system to monitor the MPLC elution and to assess the purity of the fractions. Although method A allows a direct characterization of the MPLC fractions and allows for the possible use of nonlinear calibration relationships, the required step of MPLC fractionation is time and reactive-consuming. If the responses of the different peaks plotted together vs variable amounts @g) of the whole sample (method B) are linear and extrapolate to zero, both methods are theoretically equivalent. However, method B is faster because it does not depend on any prefractionation step and allows a rapid determination of the linear range of the detector (up to 10 pg of whole sample for DAO). Both methods gave comparable quantitative results (Table 1). (18)Fixari, B.; Peureux, S.;Elmouchnino,J.; Le Perchec, P.; Vrinat, M.; Morel, F. Energy Fuels 1994, 8,588-592.
Energy & Fuels, Vol. 9, No. 5, 1995 903 Table 1. Comparison of Methods A and B in the Quantitative Hydrocarbon Group Type (HGT) Analysis of DAO Using TLC-FID method Aa HGT saturates alkylaromatics aromatics polars uneluted
wt%
oc
wt%
36.13 19.90 31.17 11.30 1.50
0.71 0.81 1.47 0.48 0.24
34.90 18.18 32.38 13.20 1.34
method Bb u P 1.20 0.9995 1.36 0.9987 1.82 0.9987 0.83 0.9915 0.37 0.9764
a Absolute calibration. Variety of internal normalization. Standard deviation. d Regression coefficient between 0 and 10 pg.
Table 2. Number-Average Molecular Weight of DAO and Hydroconversion Products, Distillate Yields (370 "C-), and Gas Yield Obtained in the Presence of Various Catalysts run
DAO No catalyst Mo naphthenate Ni plasma Ni-Mo/AlzOa a
mol wta 830 430 570 480 430
distillate yield (wt %) -
55 20.5 35 50
gas yield (wt %) -
4.9 1
1.8 3.5
From VPO (&50).
Other Analytical Techniques. Determinations of numberaverage molecular weight of DAO and its derived catalytic hydroconversion products were carried out using a Knauer vapor pressure osmometer. Calibration was performed using to 1.2 x 10-l m (the benzyl (C14H1002) in the rank 4 x regression coefficient was 0.999). Measurements were performed in CHC13 at 25 "C (six per sample) and extrapolated at infinite dilution. Simulated distillation by gas chromatography was performed using a HT5 capillary column, programmed from 30 to 400 "C at 10 "C min-l (Hewlett-Packard Model 5890). In these conditions, hydrocarbons up to C ~ Zare O eluted. An internal standard (dodecane) was added for calibration when elution was incomplete. DAO was subjected to MPLC in a preparative Quickfit column (10 mm i.d. x 50 cm length) using silica gel (70-230 mesh ASTM, Merck) as stationary phase, and n-hexane, toluene, and CHzClz as eluents, under the conditions described elsewhere." Ramsbottom carbon was evaluated by U V spectroscopy from the absorbance of 0.5%isooctane solutions measured at 340 nm.16
Results and Discussion The first obvious effect of hydroconversion concerns the reduction of the number-average molecular weight determined by vapor pressure osmometry, which is decreased from 850 to about 450 after conversion (Table 2). Concerning the impact of catalysts, the numberaverage molecular weights of the products obtained in the presence of supported Ni-Mo are similar t o those generated during thermal conversion. In contrast, the presence of molybdenum naphthenate leads to an increase of the number-average molecular weight. This change in properties is corroborated by the distribution of distillate (defined as 370 "C-)determined by simulated distillation, and gas production. Thermal, noncatalytic conversion produces higher distillate yield than catalytic hydroconversion, the consequence being more substantial with nickel plasma and molybdenum naphthenate than in the presence of supported Ni-Mo. The same trends are observed concerning gas production which is minimized in the presence of catalysts. Thus,
904 Energy & Fuels, Vol. 9, No. 5, 1995
Cebolla et al.
100
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Figure 1. Quantitative hydrocarbon group type analysis of DAO and its derived hydroconversion products (method A).
0.00
No catalyst
0.50-
-
Retention time Figure 2. TLC-FID chromatograms of DAO (A) and its derived hydroconversion products obtained: in the absence of catalyst (B), using Ni-Mo/AlzOs (C),Ni plasma (D), and Mo naphthenate (E). Retention time (t,) is expressed in minutes.
at first glance, catalysts have a balanced impact upon product distribution in hydroconversion: a positive effect is the reduction of gas formation; but the decrease of distillate yield is apparently a counterproductive effect since the objective of hydroconversion is evidently the production of distillate. However, the properties of the products should also be considered in terms of distribution of hydrocarbon group types and hydrogen content. Results fiom the application of TLC-FID method are shown in Figure 1. As illustrated in Figure 2, the products obtained by hydroconversion of DAO exhibit similar qualitative composition. The apparent differences in retention times mainly reflect the fact of being individual chromarods instead of averages (we must remind that the analysis is performed on the average of 10 such rods). Differences in shape in the alkylaromatic-aromatic zone in this figure belong to a tailing area and can reflect more differences in molecular weight than in chemical nature. FT-IR spectra of the
Figure 3. Hydrogen balance for the hydroconversion runs.
fractions confirmed this hypothesis. This means that chemical families are qualitatively the same as those contained in DAO regardless of the presence of a catalyst. Therefore, any observed variations in quantitative data can unequivocally be attributed to catalytic effects. A clear difference is evidenced between catalytic and noncatalytic experiments. Thermal conversion causes a decrease of saturates and a correlative increase of polar and uneluted fractions. The aromatic fraction is slightly increased, while a reduced contribution of the fraction identified as alkylaromatic is noticed. Given that the polar and the uneluted fractions have also an aromatic character, the total aromaticity of the feed (expressed to as 100%- saturates %) is increased upon thermal conversion. In this context, a decline of alkylaromatic can be interpreted as a reduction of the number and/or length of the alkyl substituent of aromatic entities. When hydroconversion is performed in the presence of any of the dispersed catalysts described in the Experimental Section, a considerable modification of this analytical pattern is observed: polar and uneluted fractions are decreased, aromatic are slightly affected, and an increased contribution of saturates is noticed. The most striking effect is produced by molybdenum naphthenate. The distribution of hydrocarbon group tmes obtained in catalytic hydroconversion and that of DAO are quite similar: If catalysts are classified according to the increase of saturates they induce as compared to thermal conversion, the resulting order (supported Ni-Mo, Ni plasma, Mo naphthenate) is identical to the classification concerning their impact upon reduction in distillate and gas yield. From these alterations of saturates-aromatics distribution and reduction of light products formation, it can be inferred that the impact of catalysts upon the analytical characteristics of hydroconversion products can be analyzed in relation with hydrogen balance. As illustrated by Figure 3 the total hydrogen consumption is only slightly or not affected by the presence of the catalysts used in this study. However, the distribution of hydrogen utilization between gas production and incorporation in the liquid phase is extremely different. As pointed out by the preceding results, thermal conversion produces extensive hydrocracking and generates large amounts of gases and light distillate. These reactions are hydrogen consuming and,
Energy & Fuels, Vol. 9,No. 5, 1995 905
Hydroconversion of a Petroleum Residue 45
s 40
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.-
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25 -300
-200
-100
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...
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Figure 4. Correlation of percentages in saturates determined by TLC-FID with hydrogen incorporation in liquids. ,
t
I
I
distinct pathways: they can condense to form heavier products, or, alternatively, they can be capped and stabilized by hydrogen atoms, provided the rate of production of activated hydrogen is larger than the rate of radicals formation. The presence of active, highly dispersed catalysts such as molybdenum or nickel-based catalysts provides a source of active hydrogen. As a consequence of the fast radical capping, extensive hydrocracking, consecutive to multiple C-C bond ruptures, is avoided, as illustrated by the decrease of gas formation and the reduced production of light distillate. Catalytic systems such as molybdenum naphthenate and plasma Ni possess attractive properties for hydroconversion of residues. The decomposition of molybdenum naphthenate during conversion generates highly dispersed molybdenum sulfide. A distinguishable feature of this system consists in the absence of particulate agglomeration.18 Similarly, plasma-prepared carbonsupported catalysts preserve the dispersion of the active phase.15 This property may influence the hydrogen redistribution and allows these solids to be used at very low catalyst-to-feed ratio and therefore they can be considered excellent candidates as disposable catalysts. It seems possible to evaluate the distributions of hydrogen and hydrocarbon types in order to select adequate catalytic phases, control the addition of catalysts, and, in this manner, modulate hydroconversion.
2-1
-300
-200
-100
0
mmol y 1100 g Feed
100
Conclusions
200
Figure 5. Correlation of Ramsbottom carbon with hydrogen incorporation in liquids.
although the global balance of the reaction indicates a consumption of hydrogen, the quantities of this reactant involved in transformations concerning the liquid products are negative. In contrast, the values of this parameter observed in catalytic conversion are positive, indicating an incorporation of hydrogen in the liquid products. The catalysts were used at identical loading (450 ppm of metal vs feed), thus they can be classified according to their impact upon hydrogen incorporation. The resulting order is supported Ni-Mo, Ni plasma, Mo naphthenate. The preceding results concern analytical characteristics on the one hand and material balance of hydrogen on the other hand. Is there any correlation between hydrogen incorporation and the distribution of the products? A plot of saturates against hydrogen incorporation reveals a linear relationship (Figure 4). In thermal conversion, hydrogen depletion of the liquid products is associated with a correspondingdecrease of saturates, i.e., an increase in total aromaticity. The resulting products are more aromatic than the feed and this increment of aromaticity is illustrated by Figure 5 where Ramsbottom carbon, consisting of heavy polyaromatic entities, is correlated to hydrogen incorporation. A fraction of this hydrogen deficiency of the liquid products is utilized in the production of gases, as evidenced by the material balance in Figure 3. The role of catalysts can be rationalized in the light of these analytical data. The mechanism of hydroconversion involves an initiation step producing radical species through C-C (or C-heteroatoms) bond cleavage. These radicals, once generated, can evolve according to
Comparative experiments of hydroconversion were performed under identical conditions in the presence of various dispersed, disposable catalysts at low concentration. These favor hydrogen incorporation, reduction in the percentage of produced gas, Ramsbottom carbon, and distillate aromaticity. The counterpart is a decrease in distillate production. A strict comparison of activity and selectivity of catalysts, which should be performed at equal conversion level, is out of the scope of our work. However, the comparative experiments' presented here have revealed that the balance of hydrogen transfer is quite different for each studied catalyst. The proposed TLC-FID method allows a sensitive evaluation of the catalytic selectivity through HGTA. A simple test based on the injection of Werent amounts of a whole sample (representative of a set of similar ones) in order to determine the linear range of the detector provides results which are identical to those measured by conventional, time-consuming absolute calibration. Thus, it is a reliable and rapid analytical tool for a quantitative control of hydroconversion of residues. Results obtained using this technique are consistent with hydrogen balance, gas analysis, and Conradson carbon determination and have been useful for a better understanding of hydrogen transfer mechanisms in catalytic hydroconversion.
Acknowledgment. The authors are grateful to Spanish DGICYT (project No. PB93-0100)and European Coal and Steel Community (ECSC, project No. 7220-F/765)for financial support. We also thank CNRS and CSIC for their grant. EF950027H
