Ind. Eng. Chem. Res. 1994,33, 2301-2313
2301
Catalytic Hydroprocessing of Simulated Heavy Coal Liquids. 2. Reaction Networks of Aromatic Hydrocarbons and Sulfur and Oxygen Heterocyclic Compounds Michael J. GirgistJ and Bruce C. Gates'J Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and Central Research Laboratory, Mobil Research and Development Corporation, Princeton, New Jersey 08543 The reactivity of a mixture simulating a coal liquid from the SRC-I1 process, with the organonitrogen compounds excluded, was characterized a t 350 "C and 171 atm; the catalyst was presulfided NiMoly-Al203. The reaction products were identified and analyzed quantitatively. Approximate reaction networks were determined for the simultaneously reacting phenanthrene, pyrene, fluoranthene, dibenzothiophene, and dibenzofuran. The reactions in each network are approximated as first order in the organic reactant. The proposed networks allow clarification of pathways that had been proposed earlier with less complete evidence; some previously unidentified pathways have also been identified. The results represent the first determination of quantitative networks of simultaneously reacting compounds in hydroprocessing and help to lay the foundation for modeling of hydroprocessing.
Introduction In the preceding paper (Girgis and Gates, 1994a) of this series characterizing catalytic hydroprocessing reactions, reactivities of each compound in a mixture representing a coal liquid were presented. Here the reaction networks are elucidated for several of the compounds reacting simultaneously in the 'mixture: phenanthrene, pyrene, fluoranthene, dibenzothiophene,and dibenzofuran. The network determinations are based on the product distributions combined with results of investigations of the reactions of some of the intermediates in the networks. Experimental methods and reaction conditions were presented in the preceding paper. The effects of reaction inhibition are to be summarized separately (Girgis and Gates, 199413).
Data Analysis The conversion and product distribution data were obtained as stated in the preceding paper (Girgisand Gates, 1994a). Pseudo-first-order reaction rate constants were estimated by minimizing an objective function equal to the sum of the squares of the differences of measured and predicted concentrations. Predicted concentrations were obtained by numerically solving the rate equations, given a set of rate constants, and then changing values of the rate constant estimates until the objective function was minimized. The computational procedure thus required a function minimization routine coupled to an ordinary differential equation solver. Function minimization was accomplished with the simplex method of Nelder and Mead, and the Gear algorithm was used to solve the differential equations (Press et al., 1986). Initial guesses for the rate constants were obtained with the HimmelblauJones-Bischoff method (Himmelblau et al., 1967). Each reaction in each network was taken to be first order in the organic reactant; this assumption has often been found to
* To whom correspondence should be addressed. Present address: Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. + University of Delaware. t Mobil Research and Development Corp.
be appropriate in catalytic hydroprocessing (Girgis and Gates, 19911, as discussed separately (Girgis and Gates, 1994a).
Results Phenanthrene. Products of the catalytic reaction of phenanthrene with H2 included 9,lO-dihydrophenanthrene, 1,2,3,4-tetrahydrophenanthrene,sym-octahydrophenanthrene, two other octahydrophenanthrenes that are inferred to be asym-octahydrophenanthrenes, and an additional dihydrophenanthrene isomer. The latter was detected only at very low phenanthrene conversions in experimentswith quinoline (a strong inhibitor) in the feed. The chromatographic peaks of the two asym-octahydrophenanthrenes were not entirely resolved, and these two compounds were lumped into one pseudocomponent. The lumping is a justifiable simplification because the components were probably formed from 1,2,3,4-tetrahydrophenanthrene, as discussed below. Concentration profiles of phenanthrene and ita products as a function of conversion are shown in Figure 1for an experiment carried out with the feed composition shown in Table 1, where the run designations are also stated. Selectivities to 9,lO-dihydrophenanthrene and 1,2,3,4tetrahydrophenanthrene,determined by extrapolation to zero conversion, are nonzero, whereas those of the octahydrophenanthrenes are vitrually zero, which implies that the former are primary products and the latter are not (Bhoreet al., 1990). The s u m of the initial selectivities of 9,lO-dihydrophenanthrene and 1,2,3,4-tetrahydrophenanthrene is significantly less thanunity (0.6-0.7);this result suggests that at least one other primary product was formed that was completely converted in all the experiments. Concentration profiles and corresponding selectivity plots obtained from experiments performed at lower phenanthrene conversions resulting from the addition of quinoline to the feed [Table 7 of the preceding paper (Girgisand Gates, 1994a)l suggest that the unknown dihydrophenanthrene is a primary product (Figure 2). The very small initial selectivity for formation of 1,2,3,4tetrahydrophenanthrene (Figure 2) suggests that the unknown dihydrophenanthrene is a 1,2,3,4-tetrahydrophenanthrene precursor, probably having one dihydro-
Q888-5885/94/2633-23Ql$Q4.5Q/Q 0 1994 American Chemical Society
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Figure 1. Concentration profiles and selectivity plots for phenanthrene and its products in experiment B1 [Table 6 of Girgis and Gates (1994a)l.
Figure 2. Concentration profiles and selectivity plota for phenanthrene and ita products in experiment Q3 [Table 7 of Girgis and Gates (1994a)l.
Table 1. Feed Composition in Experiment Using Selected Products of Base Case Reactants:. Experiment D1
These results suggest the reaction network shown in Figure 3, which gives a good fit to the data (Figure 6). A repeat experiment showed that rate constants were reproduced within 10% [Table 7 of the preceding paper (Girgis and Gates, 1994a)1,except for those characterizing the reactions involving the octahydrophenanthrenes;these results are inferred to have been less reproducible because of the large error in determining the low concentrations of octahydrophenanthrenes. The dehydrogenation of 1,2,3,4-tetrahydrophenanthrene to give phenanthrene was assumed to be kinetically significant. The assumption is consistent with the thermodynamics data of Frye (19621, which indicate that at 350 O C the equilibrium constant for the reaction
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reactant naphthalene biphenyl 9,lO-dihydrophenanthrene sym-hexahydropyrene
concn, mmol/L 0.374 0.372 0.373 0.187
a Experimental conditions: 350 O C , 171 atm reactor pressure, 136 atm saturator pressure.
genated outer ring. However, the scatter in the data at low conversions and our inability to determine the structure of the product preclude a more definitive statement of the reaction sequence. Thus, in the phenanthrene network given for the base-case mixture [Figure 3, Girgis and Gates (1994a)l, the unidentified dihydrophenanthrene is not included, and we recognize that treating 1,2,3,44etrahydrophenanthreneas a primary product is a simplification. The maximum observed in the 9,lO-dihydrophenanthrene concentration as a function of inverse space velocity (space time) (Figure 1) suggests that it was either hydrogenated further andlor dehydrogenated to give phenanthrene. The reversibility of phenanthrene hydrogenation is expected (Shabtai et al., 1978; Frye, 1962).To verify the reversibility, an experiment was performed at 350 O C with 9,lO-dihydrophenanthrene as one of the reactants (Table 1). The resulting concentration profiles (Figure 4) indicate clearly that phenanthrene was the primary product of 9,lO-dihydrophenanthene conversion. The results thus demonstrate that, with phenanthrene as the reactant, the 9,lO-dihydrophenanthreneformed is not hydrogenated further, instead being dehydrogenated to give phenanthrene. Thus we conclude that the octahydrophenanthrenes were formed almost exclusively from 1,2,3,4-tetrahydrophenanthrene.Moreover, their constant mole ratio as a function of space velocity (Figure 5) is consistent with their formation in parallel from 1,2,3,4tetrahydrophenanthrene.
phenanthrene
+ 2H2
-
1,2,3,4-tetrahydrophenanthrene(1) (equal to 10-2.71) is not much less than that corresponding to the hydrogenation of phenanthrene to give 9,lOdihydrophenanthrene ( which was shown to be reversible. Including the 1,2,3,4-tetrahydrophenanthrene dehydrogenation in the network also gave a better fit to the data (i-e.,a 3-fold reduction in the objective function value) compared with the network that is the same except for the omission of this reaction. Dehydrogenation reactions of the octahydrophenanthrenes are not included in the network. Under the conditions employed, the rates of these dehydrogenations are inferred to be low because the octahydrophenanthrenes were formed in only low yields (Figure 6). However, Frye's results indicate that the octahydrophenanthreneformation reactions are also reversible. For example, the equilibrium constant for the reaction
-
1,2,3,4-tetrahydrophenanthrene + 2H2 sym-octahydrophenanthrene (2)
is approximately the same as that for reaction 1. Furthermore, octahydrophenanthrene dehydrogenation reac-
Ind. Eng. Chem. Res., Vol. 33, No. 10,1994 2303
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Figure 3. Reaction networks for hydroprocessingof (A) phenanthrene, (B) pyrene, (C)fluoranthene,(D)dibenzofuranand dibenzothiophene, and (E) 5,6,7,&tetrahydro-l-naphtholdetermined in the absence of organonitrogencompounds. The feed compositions and reaction conditions are given in Girgis and Gates (1994a). The numbers over the arrows represent first-order rate constanta in units of L/(g of catalyst-h).
tions would occur to a greater extent at higher phenanthrene conversions. The network shown in Figure 3 is the simplest giving a good representation of the data.
Pyrene. Because pyrene and fluoranthene are isomers, distinguishing pyrene hydrogenation products from hydrogenated hydrofluoranthenes that were converted to
2304 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994 1.5
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Syrn-OHP Concentration, rnmol/L
Figure 5. Evidence suggesting the formation of sym-octahydrophenanthrene and asym-octahydrophenanthrenesin parallel from 1,2,3,4-phenanthrene.
the same extent was impossible on the basis of the mass spectral molecular ion peaks. Furthermore, it was difficult to distinguish the hydrogenated pyrenes from hydrogenated fluoranthenes on the basis of the mass spectra done because of the similarity of the spectra. It was necessary to conduct an experiment with pyrene as the sole reactant with H2 (run P1) to identify its products unambiguously. At 350 "C and a range of space velocities similar to those employed in runs B1 and B2, four pyrene products were found: 4,bdihydropyrene, sym-hexahydropyrene (ie,, 1,2,3,6,7,8-hexahydropyrene), asym-hexahydropyrene (i.e., 1,2,3,3a,4,5,5a-hexahydropyrene),and 4,5,9,10-tetrahydropyrene. All these were also formed in the base-case experiments B1 and B2. sym-Hexahydropyrene was identified readily by co-injection of the pure compound into the gas chromatograph/mass spectrometer; the other compounds were identified from their mass spectra (Girgis, 1988). Decahydropyrenes and perhydropyrenes were not observed; these have been reported as pyrene products in investigations carried out under more severe conditions than those used in this work (Shabtai et al., 1978;Nakatsuji et al., 1978). The data from experiments B1 and B2 were not sufficient to define a unique reaction network because the concentrations of pyrene and its products were virtually
constant at the lower space velocities (Figure 7). These almost constant concentrations are not a consequence of H2 depletion in the reactor. Less than 3 % of the H2 was converted. The almost flat concentration profiles of pyrene and its products (Figure 7) suggest that equilibrium was nearly attained among the reactant and the products shown in the profiles. This inference is consistent with the observation in experiment P1 that an increase in temperature from 350 to 400 "C at an inverse space velocity of 3.4 (g of catalyst.h)/(mmol of pyrene) (one of the larger values) resulted in a decrease in pyrene conversion from 33.4 f 2.8% to 21.5 f 2.6%. The virtual attainment of equilibrium involving pyrene and these products implies that all of the reactions in the pyrene network were reversible under the conditions employed. The rapid approach to a nearly equilibrium mixture made it difficult to determine the reaction pathways. The results of Figure 8A suggest that 4,5-dihydropyrene was a primary product. (The slightly lower selectivity at the lowest conversion is attributed to scatter, on the basis of the comparison with the data from repeat experiment B2.) The remaining selectivities are less conclusive(Figure 8B). Product selectivities based on lower conversion data, obtained when quinoline was present in the feed [Figure 9 and Table 7 of the preceding paper (Girgis and Gates, 1994a)1,slowing the reactions (Girgis and Gates, 1994b), suggest that the extrapolated initial selectivity of 4,5,9,10-
Ind. Eng. Chem. Res., Vol. 33, No. 10,1994 2305 0.8,
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Table 2. Relative Mole Fractions of Pyrene and Hydropyrenes in Products with Pyrene and eym-Hexahydropyrene as Reactants. experiment reactant
B1 B2 D1 pyrene pyrene sym-hexahydropyrene 0.71 0.74 0.78 pyrene 4,bdihydropyrene 0.19 0.18 0.17 4,5,9,10-tetrahydropyrene 0.01 0.02 0.02 sym-hexahydropyrene 0.05 0.04 0.03 asym-hexahydropyrene 0.03 0.02 0.01
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tetrahydropyrene is apparently zero, implying that it is not a primary pyrene hydrogenation product. Thus, the following pathways evidently occur: pyrene
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However, the hexahydropyrene formation sequences are difficult to establish. To provide additional insight into was conducted with these pathways, an experiment (Dl) sym-hexahydropyrene (the only available pyrene product) as a reactant (Table 1). The concentrations of symhexahydropyrene and its products were almost constant at higher values of inverse space velocity, and the relative amounts were similar to those observed in experiments B1 and B2 (Table 2), consistent with the virtualattainment of equilibrium. However sym-hexahydropyrene conversion was too large to permit inference of the pathways (Figure 10). asym-Hexahydropyrene was apparently a primary product. Initial selectivities to the other products were apparently zero, but the extrapolation to zero conversion is not very reliable.
Because these results were not sufficient to define a unique network, an alternative approach was adopted to arrive at approximate reaction pathways. Several different networks were evaluated for consistency with the data from the base-case experiments and experiment D1. All such networks included the pathways of eqs 3 and 4, so that only the hexahydropyrene formation sequences remained to be established. The results of Figure 8 suggest that sym-hexahydropyrene was formed from either pyrene or 4,5-dihydropyrene, since its selectivity was exceeded only by that of 4,5-dihydropyrene. However, all networks in which symhexahydropyrene was assumed to be formed from 4,5dihydropyrene gave predicted 4,5-dihydropyreneprofiles that were inconsistent with the data (Girgis, 1988). The only way to obtain a predicted 4,bdihydropyrene concentration profile that was consistent with the data was to assume that sym-hexahydropyrene was formed exclusively from pyrene and that 4,5-dihydropyrene was not a primary product of sym-hexahydropyrene. asym-Hexahydropyrene formation pathways could not be established on the basis of goodness of fit alone, since the concentration of this product (and thus the contributions of the corresponding residuals to the objective function) were small. It was assumed that asym-hexahydropyrene was formed exclusively from sym-hexahydropyrene; this assumption makes good sense because of (1)the decreasing selectivity for formation of asym-hexahydropyrene in
2306 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994
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experiment D1 (Figure 10) and (2) its lower selectivity relative to that of sym-hexahydropyrene with pyrene as reactant (Figure 8 and 9). On the basis of the foregoing assumptions, the reaction network shown in Figure 3 is proposed for pyrene hydrogenation. This network represents the data well (Figure 11) and also gives a fairly good fit of the data obtained when sym-hexahydropyrene was the reactant (Figure 12). The rate constants estimated from the data obtained in the two base-case experiments were reproduced within 15% [Table 9 of the preceding paper (Girgis and Gates, 1994a)], except for the rate constants involving the formation and dehydrogenation of 4,5,9,10-tetrahydropyrene. The relatively poor reproducibility of these parameters is attributed to the relatively large experimental error in determining the low concentrations of 4,5,9,10-tetrahydropyrene that were formed. The preci-
sion of the rate constants in the pyrene network is less than that of the rate constants in the phenanthrene network [Table 8 of the preceding paper (Girgis and Gates, 1994a)], consistent with the suggestion that the rapid attainment of equilibrium made the estimation of the rate constants difficult. On the other hand, the results of Table 9 of the preceding paper (Girgis and Gates, 1994a) show that inhibition by 5,6,7,8-tetrahydro-l-naphthol was insignificant, consistent with the corresponding phenanthrene results [Table 8 of the preceding paper (Girgisand Gates, 1994a)l. Fluoranthene. The distribution of products formed from fluoranthene was complex, and with few exceptions, fluoranthene products for co-injection in the gas chromatograph to aid in product identification were unavailable, as were the reference mass spectra. Several experiments were thus performed either with fluoranthene alone as the organic reactant with H2 or with selected products of fluoranthene to help identify the products. The results showed that 1,2,3,1Ob-tetrahydrofluoranthenewas the principal product, consistent with earlier results (Lapinas et d.,1987; Nakatauji et al., 1978). Identification of this compound was based on its reported mass spectrum. The other products of fluoranthene conversion formed in appreciable amounts were the following: (1) four compounds having molecular weight 212, designated as Ft5, Ft7, Ft8, and Ft9, (2) two compounds having molecular weight 208, designated as Ft3 and Ft6, and (3) a threepeak cluster designated as Ft4, containing one compound of molecular weight 204, having a peak that was partially merged with those of two other compounds, having molecular weights of 210 and 208. At fluoranthene conversions exceeding 60 % , additional products were formed in small amounts (
