Ind. Eng. Chem. Res. 1997, 36, 2041-2050
2041
Hydrocracking of Polynuclear Aromatic Hydrocarbons. Development of Rate Laws through Inhibition Studies S. C. Korre† and M. T. Klein* Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
R. J. Quann Mobil Research and Development Corporation, Paulsboro, New Jersey 08011
The relationship between molecular structure and reactivity during hydrocracking of model polynuclear aromatic hydrocarbons was examined through detailed kinetic studies. Naphthalene and phenanthrene were reacted over a presulfided NiW/USY zeolite catalyst in an 1-L batch autoclave at PH2 ) 68.1 atm and T ) 350 °C, in a cyclohexane solvent. Pure-component experiments were combined with experiments where hydrocarbons and ammonia were added as inhibitors to aid quantitative network analysis. In all, 21 rate, 18 equilibrium, and 36 adsorption parameters were estimated through fitting of the kinetics data to a dual-site Langmuir-Hinshelwood-Hougen-Watson rate law. Adsorption parameters on both metal and acid sites increased with the number of aromatic rings and the number of saturated carbons; however, the quantitative values were higher for the acid sites. Rate parameters showed that, for a given total number of aromatic rings, hydrogenations at terminal aromatic rings were favored over hydrogenations at internal rings. Isomerizations and ring openings were favored at positions R to an aromatic ring or a tertiary carbon. Equilibrium concentration ratios for all hydrogenation and ring-opening reactions were larger than unity; equilibrium ratios for all isomerizations were less than unity, indicating significant reverse reactions. The hydrocracking networks were organized into the reaction families of hydrogenation, isomerization, ring opening, and dealkylation. The reactivity trends within each reaction family may be used for the development of quantitative structure/reactivity relationships. Introduction The keen interest in catalytic hydrocracking is a direct consequence of its large potential and flexibility as a heavy oil upgrading process. Hydrocracking increases the hydrogen-to-carbon ratio, and thus the fuel value, of an oil, while decreasing its molecular weight. Combined with hydroprocessing, hydrocracking yields a wide range of high-quality products. The combined process is especially appropriate for upgrading streams high in PNAs (polynuclear aromatic hydrocarbons), because of their high hydrogen deficiency. PNAs can account for 40-50% of a heavy feedstock and are also easily formed during other upgrading processes. They are a highly undesirable product, both from processing and environmental points of view. Upgrading streams high in PNAs via hydroprocessing/hydrocracking is generally preferable to their rejection as carbon during coking and is becoming increasingly important in view of the trend toward use of heavier feedstocks. Although paraffin hydrocracking is well understood (Steijns and Froment, 1981; Froment, 1987; Baltanas et al., 1989), detailed quantitative information on PNA hydrocracking is scarce in the literature. The known hydrocracking chemistry of PNAs suggests that the pathways are serial: hydrogenation of an aromatic ring is followed by isomerization of the resulting cyclohexyl moiety to a methylcyclopentyl moiety, prior to ring opening to one or more side chains and eventual dealkylation (Haynes et al., 1983; Lemberton and Guis* Author to whom correspondence should be addressed. † Present address: Exxon Engineering, 180 Park Avenue, Florham Park, NJ 07932-0101. S0888-5885(96)00680-X CCC: $14.00
net, 1984; Lapinas et al., 1987, 1991; Landau et al., 1992; Korre, 1994). In summary of this literature, hydrogenation of an aromatic nucleus can be noted as an essential first step. Hydrogenation reactivity increases with the number of aromatic rings. The effect of naphthenic rings is also positive but less pronounced. Adsorption constants over metal sulfide/alumina catalysts also depend on the structure of the molecule (Korre et al., 1994; Girgis and Gates, 1991). Although hydrogenation of the middle of three or four aromatic rings is kinetically favored over hydrogenation of a terminal aromatic ring, further acid center transformations of the middle ring proceed with slower rates than the terminal rings (Haynes et al., 1983; Lemberton and Guisnet, 1984; Lapinas, 1989). Deep hydrocracking occurs through pathways involving hydrogenation of terminal rings and subsequent acid center transformations. Isomerization of terminal saturated rings to methylsubstituted five-membered rings occurs prior to ring opening to a butyl side chain. This is believed to occur through a protonated cyclopropane (PCP)-mediated isomerization and allows ring opening to a more stable, secondary carbocation (Sullivan et al., 1964; Haynes et al., 1983; Lemberton and Guisnet, 1984). Direct ring opening may also occur to a limited extent in parallel with isomerization, through the formation of a common PCP intermediate (Sie, 1992). Dealkylation of the alkyl-substituted aromatics and hydroaromatics is the final step in the sequence. This occurs with rates proportional to measures of the stability of the leaving alkyl fragment (e.g., carbon number). Strong quantitative correlations have related the dealkylation rate constant to the heat of formation of the leaving alkylcarbenium ion (Mochida and Yoneda, © 1997 American Chemical Society
2042 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
1967; Landau et al., 1992; Neurock and Klein, 1993). Complete dealkylation is favored, and bare-ring aromatics and naphthenaromatics with one ring less than the reactant constitute a large fraction of the final product of the main hydrocracking pathway. These compounds are, in turn, subject to the same reaction sequence, namely, hydrogenation, isomerization, ring opening, and dealkylation. The recursive character of the overall hydrocracking network suggests the existence of governing reaction families for hydrogenation, isomerization, ring opening, and dealkylation. The existence of a reaction family, in turn, suggests the development of quantitative structure/reactivity relationships as a cogent kinetic lumping scheme. This would allow rate, equilibrium, and adsorption constants within each family to be calculated as a function of a reactivity index, pertinent to each reaction, and a small number of adjustable parameters. The implied reduction in adjustable parameters for detailed molecular models is enormous. Reliable quantitative reaction networks are a requisite first step as they provide a representative basis set of compound structures for each candidate reaction family. This approach has already been used in the development of PNA hydrogenation pathways (Korre et al., 1995) and quantitative structure/reactivity relationships for their rate, equilibrium, and adsorption constants (Korre et al., 1994). The object of the present report is to extend this methodology to the acid center transformations during catalytic hydrocracking of PNAs. Experimental Design The experimental plan focused on deducing reliable naphthalene and phenanthrene hydrocracking reaction networks and discerning rate, equilibrium, and adsorption effects. Network deduction was facilitated by thorough product identification. Product rank issues were resolved by complementary experiments using intermediate products as the reactants. Parameter significance was also used as a network deduction tool. Parameter estimation was performed to LangmuirHinshelwood-Hougen-Watson (LHHW) rate expressions, which can accommodate kinetic inhibition. The estimation of the adsorption parameters inherent in the LHHW formalism suggested that experiments with wide variations in the inhibition term should be performed to facilitate the rate/adsorption parameter separation. These variations could be accomplished both by increased reactant loadings (higher concentrations) and by addition of components with stronger adsorption properties (larger adsorption constants than the reactants and products). Since maximum loadings were bound by solubility limitations, the addition of strongly adsorbed inhibitors was the primary tool in this work. These compounds dominated the LHHW denominator and thus allowed decoupling of the intrinsic kinetic and adsorption information in the numerator using the adsorption information in the denominator. Thus, PNAs in size up to phenanthrene and molecules combining small size and significant poisoning power were used as inhibitors. Heterocyclic hydrocarbons, such as pyridine and quinoline, are very potent poisons of the zeolite sites (Fu and Schaffer, 1985), to the extent that the acid function can be completely suppressed (Landau, 1991). Moreover, their hydroprocessing products can be even more strongly adsorbed (tetrahydroquinoline vs quinoline, for example). Previous studies using ammonia with hydroprocessing catalysts at the
temperature interval of interest showed that its proton affinity (and thus the expected adsorption constant) was comparable to that of phenanthrene (LaVopa and Satterfield, 1988; Lee and Satterfield, 1992). Ammonia could thus reversibly inhibit both metal and acid center transformations without completely poisoning the catalyst. Ammonia has the special distinction of being inert under reaction conditions and would thus introduce only one additional adsorption parameter in an experiment. The nature of ammonia inhibition rendered its adsorption reversible (Lee and Satterfield, 1992). Moreover, ammonia is the end product of hydrodenitrogenation of basic nitrogen compounds and is usually present at small concentrations in industrial hydrocracking units. Ammonia thus seemed to be an especially cogent inhibitor for the present hydrocracking studies. In an overview of the experimental program, naphthalene and phenanthrene mixtures were first examined in different initial compositions. The replacement of naphthalene by phenanthrene with the total loading kept constant inhibited naphthalene as well as phenanthrene hydrocracking pathways. Experiments with ammonia as an inhibitor were performed to break the strong statistical correlation between rate and adsorption parameters. The diversity of even these naphthalene and phenanthrene hydrocracking pathways ensured the availability of a variety of probe structures. Close monitoring of reaction products provided the necessary representative basis set of compound structures for each of the candidate reaction families of hydrogenation, isomerization, ring opening, and dealkylation. The naphthalene network provided the prototype of these serial pathways. During phenanthrene hydrocracking, eight more hydrogenations, at least three isomerizations, three ring openings, and three dealkylations were traced. Therefore, a satisfactory sample of the reactivity was obtained using a minimum number of reactants. Experimental Procedure. Hydrocracking experiments were performed in a 1-L batch autoclave. A detailed description is available elsewhere (Korre et al., 1995a; Korre, 1994; Landau, 1991). Reactions took place in the presence of 420 g of cyclohexane as a solvent and 68.1 atm H2 at 350 °C. The total pressure, including cyclohexane vapor pressure at 350 °C, was 191.6 atm. Hydrogen backpressure was kept constant throughout the reaction. The catalyst was a Mobil-treated Zeolyst 753 material provided by Mobil. The catalyst was received in unsulfided form in particles of 0.3 cm. The catalyst (10 g) was placed in a basket mounted on the autoclave stirrer and was presulfided in situ for 135 min at 400 °C in a stream of 10% H2S in H2. A total of 1 mL of CS2 was added with each experiment to keep the catalyst in a sulfided state. The catalyst was equilibrated for 10 h at 350 °C and 68.1 atm H2 during three phenanthrene hydrocracking experiments in cyclohexane (Landau, 1991). Product identification was by gas chromatography/mass spectrometry. Parameter Estimation Strategy. The experimental strategy motivated the exploration of both relative (yield vs conversion data) and absolute (yield vs time data) rates, in turn. Pure-component and mixture experimental data were used simultaneously. Kinetic inhibition was taken into account explicitly by using a dual-site LHHW expression. The dual-site LHHW expression accounted for the bifunctional nature of the catalyst (Froment and Bis-
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2043
choff, 1990). The rate for each product was obtained as a summation of the rates of its transformations on the metal and the acid sites, as in eq 1. In eq 1, Ci, Cj,
∑j kij(Ci - Cj/Kij) ∑l kil(Ci - Cj/Kil)
dCi )
-
+
dt
DH
DA
(1)
and Cl are component concentrations (mol L-1), kji and kli are combined numerator rate parameters (L kgcat-1 s-1) (including intrinsic rate and adsorption constant contributions and hydrogen pressure where applicable), and Kji and Kli are the equilibrium ratios (moli/molj). It follows that the equilibrium ratio is equal to the equilibrium constant (moli/molj/PH2n) multiplied by the hydrogen pressure (PH2) to the hydrogen stoichiometry (n). DH and DA are the adsorption groups for the metal (hydrogenation, H) and zeolite (acid, A) sites, respectively, defined in eq 2. In eq 2, Ci represents the
DH ) 1 +
∑i KiHCi,
DA ) 1 +
∑i KiACi
(2)
component concentration (mol L-1), and KiH and KiA (L mol-1) represent individual component adsorption constants on the metal and acid sites, respectively. Implicit are the assumptions of surface reaction as the ratedetermining step and of a unity adsorption exponent group at all cases. The individual component adsorption constants for both metal (H) and acid (A) sites were estimated by a methodology developed earlier (Korre, 1994; Korre et al., 1994). A linear variation of the logarithm of the adsorption constants with structural increments was imposed. The aromatic ring number (NAR) was used to represent the sp2-hybridized carbons in the aromatic rings, and the saturated carbon number (NSC) was used to represent the sp3-hybridized, saturated carbons (eq 3). The dependence on the aromatic part was included
ln KiH ) aH + bHNAR + cHNSC
(3a)
ln KiA ) aA + bANAR + cANSC
(3b)
to account for the acid/base interaction between catalyst and PNA and the dependence on the saturated part accommodated the number of sites being occupied during adsorption. The parameters aH, bH, cH, aA, bA, and cA in eq 3 were estimated using all the available yield vs time data for naphthalene/phenanthrene hydrocracking. As a result of the dual-site LHHW expression adopted to account for the bifunctional nature of the catalyst, the use of the relative rates (relative to reactant conversion by hydrogenation) resulted in elimination of the LHHW denominators only for the reactions occurring on the same type of site (in this case, metal). Nevertheless, the relative rates still provided a convenient measure of the relative inhibition (eq 4). Since
rij
d(Ci/Ci0)/dt
dyi ) dx
∑j D
j
) dx/dt
∑j
kij(Ci - Cj/Kij) Dj
) r1j
∑j D
H
∑j
k1j(C1 - C1j/K1j) DH
(4)
both reactants naphthalene and phenanthrene were consumed exclusively on the metal sites (with the exception of methylation reactions that never exceeded 1% yield and were thus ignored), the denominator of eq 4 will always contain DH, referring to the metal sites. The parameter Dj in the numerator could refer to either metal or acid sites, depending on the specific chemistry. The formulation in eq 4 provided a convenient manner to study the kinetics over this bifunctional catalyst. For a compound produced and consumed exclusively on metal sites, the yield vs conversion plots are independent of the reactant loading. For a compound produced and consumed on the acid sites, the yield vs conversion plots depend on the ratio of the denominators (eq 5). If
∑j racid ij DH
dyi
(5)
) dx
∑j
r1j
DA
acid sites are more sensitive to poisoning than metal sites, then the yields for a given conversion should decrease with increasing inhibitor loading. Finally, for a compound produced on the metal sites and consumed on the acid sites, the yield for a given conversion should increase with increasing inhibitor concentration (eq 6).
∑j rmetal ∑j racid ij ij DH
dyi ) dx
-
∑j r1j
∑j r1j
DA
(6)
The opposite is true in the event metal sites are more sensitive to poisoning than acid sites. Clearly, if DH and DA vary by the same amount, the y vs x plots should not change, regardless of the absolute values of DH and DA. These arguments will be used in support of a reaction network. Development of Reaction Networks Naphthalene. The hydrocracking reactions of 20 g of naphthalene were studied in the autoclave with 400 g cyclohexane as the solvent and 68.1 atm H2 at 350 °C. Reaction duration was tM ) 1.467 h, resulting in an ultimate conversion of xM ) 0.872. Reaction products included tetralin, n-butylbenzene, 1-, 2-, 3-, and 4-methylindans, indan, trans-decalin, cis-decalin, and decalin isomers, toluene, ethyl- and propylbenzene, methyl-, ethyl- , propyl-, and butylcyclohexanes, 1- and 2-methylnaphthalenes, 1-, 2-, 5-, and 6-methyltetralins, dimethyl- or ethyl- (C2-substituted) naphthalenes, and C2tetralins. Detection of benzene was not possible, as the wide solvent peak overshadowed the expected retention time. All products were identified by co-injections and mass spectral information. Naphthalene conversion increased with reaction time. The conversion rate was higher than that observed over the hydroprocessing catalyst (Korre et al., 1995). No equilibrium limitations were apparent during the reaction. The material balance closure was quantified by the observed products index (OPI), defined as the sum of molar yields of identified products. For naphthalene hydrocracking, OPI included contributions from naphthalene, tetralin, 1- and 2-methylindans, indan, transdecalin, cis- decalin, and decalin isomers, n-butylbenzene, methylnaphthalenes, and methyltetralins. OPI
2044 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 1. Conversion dependence of naphthalene hydrocracking product yields (i) and selectivities (ii): (b) tetralin, (9) 1- and 2-methylindans, (0) n-butylbenzene, (4) indan, (2) cis- and transdecalins. Lines represent parameter estimation results with the network of Figure 2.
decreased with increasing reaction time and naphthalene conversion, as a result of further reactions of the observed products (80% at maximum conversion). The reaction products not included in the OPI contained kinetic information on the dealkylation reactions of n-butylbenzene and further hydrogenations of alkylbenzenes. The deduction of the naphthalene hydrocracking network was facilitated by the yield and selectivity vs conversion Delplots of Figure 1 (Bhore et al., 1990). Tetralin was clearly a primary product, with a projected initial selectivity in excess of 0.8. Methylnaphthalenes were also primary products, while methyltetralins were of secondary product rank. Methylindans were strong secondary products, while decalins were clearly of tertiary rank. Indan and n-butylbenzene were also classified as tertiary products. The information revealed above led to the construction of the naphthalene portion of the network of Figure 2. Naphthalene hydrogenation produced tetralin. Decalins were a product of further tetralin saturation. Methylindans were modeled as resulting from tetralin PCP isomerization over the strongly acidic sites of the USY zeolite, as they were not detected in significant quantities in the hydroprocessing experiments. Along the same lines, zeolite-catalyzed ring opening of methylindan resulted in the production of n-butylbenzene, whereas demethylation led to indan. Products of higher kinetic rank, such as benzene and alkylbenzenes, would evolve from the irreversible dealkylations of n-butylbenzene: the contributions of direct tetralin ring opening and dealkylation were considered negligible. These dealkylation reactions were also assumed to take place on the acid sites and were modeled by overall pseudofirst-order disappearance rate constants.
Phenanthrene. The hydrocracking of 15 g of phenanthrene was studied in the autoclave setup in the presence of 400 g of cyclohexane solvent at 68.1 atm H2 at 350 °C. Two sets of experiments are presented here. For each, the reaction duration (tM) was 3.1 and 4.0 h and maximum conversion (xM) was 0.964 and 0.981, respectively. Phenanthrene conversion increased with reaction time with apparent first-order kinetics. The rate appeared significantly higher than that over the hydroprocessing catalyst (Korre et al., 1995), and no equilibrium limitations were apparent. Phenanthrene hydrocracking resulted in a multitude of products. At 50% conversion, the number of detectable GC peaks (C > 1 ppm) exceeded 150, ranging from light paraffins to alkylphenanthrenes. Identified products included dihydrophenanthrene, tetrahydrophenanthrene, and other isomers of molecular weight 182, symand asym-octahydrophenanthrenes and other isomers of molecular weight 186, methyl-, ethyl-, and butyltetralins, tetralin, naphthalene, methylindans, indan, butylbenzene, and other alkylbenzenes. There was no evidence of dimethyl- or ethylbiphenyls, but small amounts of dimethyl- or ethylcyclohexylbenzenes were detected. The reasoning and methods used for the identification of phenanthrene hydrocracking products are discussed in detail elsewhere (Korre, 1994). In short, the products of phenanthrene hydrocracking with rank up to 3 discussed in this paper are lumped according to the scheme of Appendix A. As was the case for naphthalene, the observed products index, including compounds with retention time higher than that of indan, decreased with time, to a minimum of 20% at maximum conversion (0.964 and 0.981). The development of the network for phenanthrene hydrocracking was facilitated by the yield and selectivity vs conversion Delplots of Figure 3. Clearly, di- and tetrahydrophenanthrenes were the major primary products, as was the case over the hydroprocessing catalyst (Korre et al., 1995). sym- and asym-octahydrophenanthrenes and isotetrahydrophenanthrenes exhibited secondary product behavior, while sym- and asym-isooctahydrophenanthrenes, butylnaphthalenes, butyltetralins, and dimethylcyclohexylbenzenes were clearly of higher rank. Naphthalene and its hydrocracking products were also of rank tertiary or higher. Figure 3b also presents the kinetic behavior of symand asym-octahydrophenanthrenes. Their maximum yields were significantly lower compared with the experiments over the hydroprocessing catalyst (Korre et al., 1995). Further reaction products are well established in the form of sym- and asym-octahydrophenanthrene isomers and butyltetralins (Figure 3c). Further acid center transformations of sym-octahydrophenanthrene appear to be favored over those of asym-octahydrophenanthrene. The ratio of sym-/asym-octahydrophenanthrenes at high phenanthrene conversions clearly decreases in the presence of the acidic catalyst support. Indeed, yields of sym-isoosoctahdrophenanthrenes and 5- and 6-butyltetralins are significantly higher than those of the corresponding asym-octahydrophenanthrene products. The kinetic information presented above combines to suggest the phenanthrene portion of the network of Figure 2. Reversible phenanthrene hydrogenations to di- and tetrahydrophenanthrenes are the primary reactions. Tetrahydrophenanthrene is hydrogenated to symand asym-octahydrophenanthrene, with no further oc-
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2045
Figure 2. Results of parameter estimation to the proposed networks for naphthalene and phenanthrene hydrocracking. (k is in 1.67 × 10-3 L/kgcat/s. Numerator rate parameters are underlined.)
tahydrophenanthrene hydrogenations. Hydrophenanthrenes other than dihydrophenanthrene are subject to PCP isomerizations on the acid sites. Direct opening of 6-carbon-membered rings to butylnaphthalene and butyltetralins was considered negligible. Rather, these products were assumed to derive from ring opening of the PCP-produced isomers. In accordance with tetrahydrophenanthrene hydrogenations, isotetrahydrophenanthrene and n-butylnaphthalene hydrogenations were included. Acid center transformations of butylnaphthalenes and butyltetralins were also taken into account: Irreversible cracking of butylnaphthalenes produced n-alkylnaphthalenes (n < 3) and light gases, while n-alkyltetralins (n < 3), alkylbenzenes, and light gases were produced from irreversible cracking of butyltetralins. n-Alkylnaphthalenes (n < 3), n-alkyltetralins (n < 3), alkylbenzenes, and light gases were not traced throughout the reaction and therefore were not included in the observed products index. The network of Figure 2 summarizes all possible reactions for phenanthrene hydrocracking products up to tertiary rank. Following the structure of the naphthalene network and the literature information, these 17 reactions can be organized into hydrogenations (structures containing two and three aromatic rings), isomerizations (structures containing at least one cyclohexyl ring), ring openings (structures containing at least one methylcyclopentyl ring), and dealkylations (structures containing a butyl side chain). The concept of reaction families is therefore valid in the naphthalene/ phenanthrene hydrocracking network. Experiments with inhibitors will provide the reliable quantitative information needed to translate these qualitative trends into quantitative structure/reactivity relationships.
Inhibition Effects Hydrocarbon Inhibition. The reaction of naphthalene/phenanthrene mixtures was at 68.1 atm H2 at 350 °C in 400 g of cyclohexane solvent. Injection loadings and maximum conversions for each experiment are listed in Table 1. Ultimate conversions varied with loading and reaction duration. The phenanthrene conversion rate was not significantly affected by the initial composition. On the other hand, the naphthalene conversion rate decreased as the phenanthrene content increased. This implies that naphthalene conversion was inhibited by the presence of phenanthrene, but the phenanthrene self-inhibition effect was imperceptible at the total loading range examined. This behavior is consistent with preferential phenanthrene adsorption on the metal sites and at the same time preferential phenanthrene hydrogenation (i.e., KP > KN and kP > kN). The hydrocracking product spectra and reaction networks of the reactant PNAs naphthalene and phenanthrene were qualitatively identical to those of the purecomponent reactions. The concentrations of the products in the network of Figure 2 were traced throughout the reaction. The sum of these concentrations accounted for the observed products index (OPI) for each reactant. The OPI for phenanthrene decreased significantly with conversion, and this behavior appeared to be independent of the initial composition. The OPI for naphthalene also decreased with naphthalene conversion at high naphthalene loadings. At the highest phenanthrene loadings, however, naphthalene OPI increased with conversion, as a result of phenanthrene reactions leading to naphthalene products; naphthalene OPI was virtually invariant at 70% by weight of phenanthrene. The evolution of phenanthrene product yields with
2046 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 2. Conditions for the Hydrocracking of a Naphthalene/Phenanthrene Mixture with Ammonia Inhibition
Figure 3. Conversion dependence of phenanthrene hydrocracking product yields (i) and selectivities (ii): (a) (9,0) dihydrophenanthrene, ([,]) tetrahydrophenanthrene; (b) (9,0) sym-octahydrophenanthrene, ([,]) asym-octahydrophenanthrene; (9,0) isotetrahydrophenanthrene, ([,]) sym-isooctahydrophenanthrene, (b,O) asym-isooctahydrophenanthrene; (d) (9,0) n-butylnaphthalenes, ([,]) 5- and 6-butyltetralins, (b,O) 1- and 2-butyltetralins. Lines represent parameter estimation results. Table 1. Conditions for the Hydrocracking of a Naphthalene/Phenanthrene Mixture (N ) Naphthalene; P ) Phenanthrene; x ) Fractional Conversion) exp. code
N + P (g)
% wt P
% mol P
xN
xP
5% 10% 20% 50% 70% 90%
30.2 20.4 20.2 30.5 25.3 32.3
5.3 10.9 20.1 50.4 70.0 89.9
3.9 8.1 15.3 42.2 62.6 86.5
0.96 0.93 0.88 0.83 0.86 0.66
0.98 0.96 0.91 0.89 0.93 0.87
reactant conversion in the mixture differed only quantitatively from the pure-component results. As was the case in pure phenanthrene hydrocracking, di- and tetrahydrophenanthrene were the major primary products, while isotetrahydrophenanthrene and octahydrophenanthrenes exhibited secondary product behavior. Further products were of tertiary or higher rank. Primary product behavior was consistent with the pure-component hydrocracking results. The maximum tetrahydrophenanthrene yields of approximately 0.14 occurred at phenanthrene conversions of approximately 0.7, and the projected initial selectivity clearly approached 0.25. These results appeared independent of the initial composition. The maximum dihydrophenan-
naph. (g)
phen. (g)
V loader (cc)
P loader (atm)
mol of NH3 (SC)
% NH3
CNH3 (ppm)
9.96 10.00 10.03 10.25 10.02
9.73 10.04 9.99 10.04 9.96
13.5 42.0 42.0 42.0 42.0
1.00 1.00 1.36 1.57 8.03
0.000 55 0.001 71 0.002 32 0.002 67 0.013 71
0.001 00 0.003 12 0.004 24 0.004 88 0.025 03
110 341 465 534 2742
threne yields slightly increased with increasing initial phenanthrene content, from 0.04 for 5% by weight of phenanthrene to 0.07 for 90% by weight of phenanthrene. sym- and asym-octahydrophenanthrene yields increased with increasing initial phenanthrene loading. This behavior is consistent with octahydrophenanthrenes being produced on less inhibitor-sensitive metal sites and consumed mainly on more inhibitor-sensitive acid sites, with similar production and consumption rates (eq 6). The isomerization and ring-opening products of tetrahydrophenanthrene and sym- and asym-octahydrophenanthrene exhibited the opposite behavior. Their yields were slightly decreasing with increasing phenanthrene loading. This is consistent with these compounds being produced and consumed mainly on the acid sites (eq 5). The evolution of naphthalene hydrocracking product yields and selectivities with naphthalene conversion was complicated by the production of naphthalene, tetralin, and methylindans from phenanthrene hydrocracking. All yields and selectivities significantly increased with increasing phenanthrene loading. Therefore, naphthalene hydrocracking could not be studied individually in these mixture experiments, but production from phenanthrene hydrocracking needed to be taken into account as well. Apart from naphthalene production from butylnaphthalenes and tetralin production from butyltetralins, the naphthalene and phenanthrene hydrocracking networks of Figure 2 remained qualitatively unchanged during the mixture experiments. These experimental results were combined with the pure-component information and the ammonia inhibition experiments for rate, equilibrium, and adsorption parameter estimation. Ammonia Inhibition. Reactions with added ammonia as an inhibitor were run in the presence of 400 g of cyclohexane solvent at 68.1 atm H2 at 350 °C. Table 2 summarizes the conditions for these experiments. Gaseous ammonia (Aldrich, 99.9%+) was introduced with slight hydrogen overpressure through the reactant loader into the solvent/catalyst mixture prior to heatup. The amount of ammonia was varied by adjusting the pressure in the loader. To achieve ammonia concentrations lower than 350 ppm, a loader unit of 13.5 cc was used. Injection of a 50% by weight mixture of naphthalene and phenanthrene (dissolved in cyclohexane that included 1 mL of CS2) occurred when the reactor attained the desired 350 °C temperature. Catalyst regeneration through ammonia desorption was performed after each run by increasing the temperature to 420 °C for 3 h under a stream of hydrogen. The activity of the catalyst was verified by phenanthrene hydrocracking. The catalyst life decreased significantly faster in the presence than in the absence of ammonia, requiring catalyst replacement after approximately 15 h on stream. Possible causes for this
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2047
ucts of tetralin acid center transformations (e.g., butylbenzene) did decrease with increasing ammonia loading. Parameter Estimation Results
Figure 4. Evolution of (a) phenanthrene and (b) naphthalene hydrocracking conversion with time in the presence of ammonia. Loading ) 20 g; 50% by weight of phenanthrene in naphthalene. (9) 0 ppm NH3; (0) 110 ppm NH3; ([) 341 ppm NH3; (]) 465 ppm NH3; (2) 534 ppm NH3; (4) 2742 ppm NH3.
behavior include faster coke buildup due to the increased contact time with PNAs resulting from the lower conversion rates. The introduction of ammonia affected metal as well as acid center transformations. Figure 4 demonstrates the decrease in phenanthrene and naphthalene conversion upon introduction of ammonia at the concentrations of Table 2. As was the case for inhibition by phenanthrene, the inhibiting effect was more intense for naphthalene. The sum of the concentrations of the products in the networks of Figure 2 accounted for the observed products index (OPI) for each reactant. The OPI for both reactants was significantly closer to unity when ammonia was used as an inhibitor rather than phenanthrene. The naphthalene OPI in particular was virtually unity for all experiments where ammonia was present. This suggests that reactions leading to products other than those of Figure 2 were virtually eliminated in the presence of ammonia. The phenanthrene OPI was still less than unity at high conversions for all but the highest ammonia loadings. This indicates that further reactions of the compounds of Figure 2 were severely suppressed in the presence of ammonia. The hydrocracking product spectra of the reactant PNAs phenanthrene and naphthalene were similar to the pure compound reactions. Product rank was qualitatively identical to that described in the pure-component experiments. Tetrahydrophenanthrene yields increased slightly in the presence of ammonia. Dihydrophenanthrene yields significantly increased with increasing ammonia concentration, from a maximum of 0.06 in the absence of ammonia to a maximum of 0.2 for the highest ammonia loading, approaching the value of 0.25 observed for reaction with the nonacidic hydroprocessing catalyst (Korre et al., 1995). This suggests that any acid center transformation of dihydrophenanthrene (i.e., coking) was strongly inhibited in the presence of ammonia. symand asym- octahydrophenanthrenes exhibited a similar behavior. This is consistent with di- and octahydrophenanthrenes being produced on the metal sites and consumed mainly on the acid sites. The yields of compounds produced mostly from acid center transformations decreased with increasing inhibitor concentration. This information is also consistent with the acid sites being affected more strongly than the metal sites by the presence of ammonia. The evolution of naphthalene hydrocracking product yields and selectivities with naphthalene conversion was hardly affected by the presence of ammonia, confirming that further tetralin reactions on the acid sites proceed with much lower rates than naphthalene hydrogenation to tetralin. Nevertheless, the small yields of the prod-
The information obtained from the hydrocarbon experiments with and without added ammonia was combined to evaluate rate, equilibrium, and adsorption parameters for the components in the phenanthrene and naphthalene hydrocracking networks and an adsorption parameter for ammonia. More specifically, parameter estimation was performed by matching the results of the experiments of Tables 1 and 2 to the predictions of the dual-site LHHW expression of eq 1. This represented simultaneous correlation of approximately 2300 experimental concentrations for the 17 compounds of the networks in Figure 2. Equation 3a was used for the evaluation of the adsorption parameters on the metal sites and eq 3b for the evaluation of the adsorption parameters on the acid sites. The six parameters of these quantitative structure/reactivity relationships were optimized for the hydrocarbons and the adsorption constant for ammonia along with the rate and equilibrium constants. Adsorption Constants. The best-fit parameters for the hydrocarbon adsorption constant quantitative structure/reactivity relationships corresponding to eqs 3a and 3b are presented in eqs 7 and 8 for the metal (H) and acid (A) sites, respectively. Adsorption constants on
ln KH ) 1.649 + 0.687NAR + 0.0901NSC
(7)
ln KA ) 0.336 + 1.373NAR + 0.132NSC
(8)
both metal and acid sites increased with the number of aromatic rings (NAR) as well as the number of saturated carbons (NSC). This was also the case for the adsorption constants on the metal sites of the hydroprocessing catalyst (Korre et al., 1994). Quantitatively, the coefficients for aromatic rings and saturated carbons on metal sites were similar for both hydroprocessing and hydrocracking catalysts. On the contrary, the coefficients for acid sites (eq 8) were higher than the coefficients for metal sites (eq 7): the aromatic ring coefficient was almost 2 times higher, and the saturated carbon number coefficient was approximately 30% higher. This reflects the experimental observation that the zeolite support is more sensitive to both increased basicity and increased alkyl substitution than the hydroprocessing catalyst. Adsorption constants calculated with the best-fit parameters of eqs 7 (KH, L/mol) and 8 (KA, L/mol) are listed in Table 3. The adsorption constants on the metal sites were of magnitude similar to those evaluated for the hydroprocessing catalyst (Korre et al., 1994). Adsorption constants for 2- and 3-ring aromatic compounds on the acid sites were higher than the corresponding values on the metal sites; the opposite was true for single-ring aromatics. Ammonia was more strongly adsorbed than any hydrocarbon molecule. For example, the adsorption constant for ammonia on the metal sites was almost 1 order of magnitude higher than the adsorption constant for phenanthrene on the metal sites. Moreover, the ammonia adsorption constant for the acid sites was 1-2 orders of magnitude higher than the corresponding phenanthrene adsorption constant (Table 3). The value reported for ammonia in Table 3 is larger than literature information for hydroprocessing catalysts (LaVopa and
2048 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 5. Hydrocarbon and ammonia inhibition on (a) metal and (b) acid catalytic sites. Evolution of denominator-based “activity” terms with time. Loading ) 20 g; 50% by weight of phenanthrene in naphthalene. (9) 5% by weight; (0) 50% by weight; ([) 90% by weight; (]) 465 ppm NH3; (2) 534 ppm NH3; (4) 2742 ppm NH3. Table 3. Best-Fit Adsorption Constants (L/mol) for Hydrocracking Experiments (NAR ) Number of Aromatic Rings; NSC ) Number of Saturated Carbons) compound naphthalene phenanthrene tetralin dihydrophenanthrene tetrahydrophenanthrene i-tetrahydrophenanthrene n-butylonaphthalenes sym-octahydrophenanthrene 5- and 6-butyltetralins asym-octahydrophenanthrene 1- and 2-butyltetralins 1- and 2-methylindans n-butylbenzene decalins dimethylcyclohexylbenzene hexadecylnaphthalene NH3
KH KA NAR NSC (L/mol) (L/mol) ratio 2 3 1 2 2 2 2 1 1 1 1 1 1 0 1 2 0
0 0 4 2 4 4 4 8 8 8 8 4 4 10 8 16 0
19.08 20.83 36.55 80.36 14.57 8.73 23.07 26.48 27.90 33.67 27.90 33.67 27.90 33.67 21.30 14.10 21.30 14.10 21.30 14.10 21.30 14.10 14.57 8.73 14.57 8.73 13.45 4.65 21.30 14.10 87.24 142.09 407 2627
1.09 2.20 0.60 1.15 1.21 1.21 1.21 0.66 0.66 0.66 0.66 0.60 0.60 0.35 0.66 1.63 6.45
Satterfield, 1988) and FCC catalysts at temperatures 500-550 °C (Fu and Schaffer, 1985). The best-fit adsorption constant values of Table 3 allow estimation of the influence of competitive inhibition through the summation of the denominators of eq 2. These summations were determined for the conditions of the experiments of Tables 1 and 2. The values of the denominators were larger than unity, up to a maximum value of 10 for the heaviest feeds. Introduction of ammonia increased the acid sites denominator up to a value of 50 in the most extreme case. This effect can be expressed as an “activity” term multiplying all numerator terms, calculated as the inverse of the denominator for the metal and acid sites separately (1/DH and 1/DA, respectively) and ranging between zero and unity. Figure 5 presents the evolution of these activity terms with time for several experiments of Tables 1 and 2. The highest initial activity was approximately 0.25 for both metal and acid sites, corresponding to the lightest feed (5% by weight of phenanthrene in naphthalene). The activity terms generally increase with reaction duration, as a result of the elimination of multiring aromatics and decrease with the heaviness of the feed and the addition of ammonia. In the highest ammonia concentrations, the increase in activity due to reaction is not enough to counteract the overall activity reduction; as a result, the activity profiles are relatively flat with time. These effects were more intense for the acid sites, as dictated by their increased sensitivity to poisoning expressed in eq 3. Rate and Equilibrium Constants. Parameter estimation to the yield vs time data according to the network of Figure 2 and the LHHW expression of eq 1 resulted in the best-fit rate constants and equilibrium
Figure 6. Parameter estimation results for phenanthrene hydrocracking in the presence of inhibitors. Yield (i) and selectivity (ii) vs conversion plots for (a) dihydrophenanthrene, (b) tetrahydrophenanthrene, and (c) sym-octahydrophenanthrene. Lines represent parameter estimation results with the network of Figure 2. Loading ) 20 g: 50% by weight of phenanthrene in naphthalene. (9) 10% by weight; (0) 20% by weight; ([) 100% by weight; (]) 100% by weight; (2) 341 ppm NH3; (4) 465 ppm NH3; (b) 534 ppm NH3; (O) 2742 ppm NH3.
ratios indicated in Figure 2. The calculated yields are plotted as the curves in Figures 1, 3, and 6. The effects of the increased phenanthrene concentration on the product distribution were very small compared to the effects of increasing ammonia concentration, as demonstrated in Figure 6. This was particularly evident in the results for di- and octahydrophenanthrenes and butylnaphthalenes. The parameters of Figure 2 invite scrutiny of structure/ reactivity trends. Phenanthrene hydrogenation to tetrahydrophenanthrene was favored over that to dihydrophenanthrene. Further hydrogenations of the naphthalenic moieties in tetrahydrophenanthrene, isotetrahydrophenanthrene, and butylnaphthalene proceeded with significant rate constants. In all cases, hydrogenations at the terminal ring, as in tetrahydrophenanthrene to sym-octahydrophenanthrene, isotetrahydrophenanthrene to sym-isooctahydrophenanthrene, and butylnaphthalene to 5- and 6-butyltetralins, were favored over hydrogenations at the internal ring for a constant number of aromatic rings. This is in agreement with the ring saturation structure/reactivity trends over a sulfided CoMo/Al2O3 hydroprocessing catalyst (Korre et al., 1994, 1995). Equilibrium ratios for all hydrogenations were larger than unity, with the exception of phenanthrene hydrogenation at the middle ring, where the equilibrium ratio was less than unity. This implies that dihydrophenanthrene would be less abundant than phenanthrene at equilibrium and confirms the trend for equilibrium ratios less than unity in middle-of-three and middle-of-four aromatic ring hydrogenations discussed in previous publications (Korre et al., 1995a,b). Protonated cyclopropane isomerizations proceeded with substantial rate constants, similar in magnitude to those for ring openings. Isomerization of the cyclo-
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2049
hexyl ring in tetrahydrophenanthrene was the fastest, followed by isomerization of the cyclohexyl ring in symoctahydrophenanthrene, isomerization of the cyclohexyl ring in asym-octahydrophenanthrene, and isomerization of tetralin to methylindan. This ranking is consistent with the ability of the parent structure to stabilize a carbocation, which increases with the addition of tertiary carbons and aromatic rings (Mochida and Yoneda, 1967; Neurock and Klein, 1993). A similar trend is present in ring openings (with the exception of symisooctahydrophenanthrene) and dealkylations (with the exception of butylbenzene). Equilibrium ratios in all isomerizations were less than unity, indicating that the formation of a methylcyclopentyl-substituted molecule is thermodynamically less favored than the formation of a cyclohexyl-substituted moiety. This resulted in low overall reaction rates, indicated, in part, by the low yields of methylcyclopentyl-substituted compounds. On the contrary, ringopening equilibrium ratios were generally larger than unity. Dealkylation equilibrium ratios were much larger than unity; this suggested that the reverse rates were negligible and were thus ignored in the network of Figure 2. Summary and Conclusions The hydrocracking reaction networks of naphthalene and phenanthrene could be organized into the reaction families of hydrogenation, isomerization, ring opening, and dealkylation. This logic, when applied to products of up to tertiary rank, allowed estimation of 21 rate, 18 equilibrium, and 36 adsorption parameters, through fitting to kinetics data. Adsorption parameters on both metal and acid sites increased with the number of aromatic rings and the number of saturated carbons; however, acid sites were more susceptible to both hydrocarbon and ammonia poisoning than metal sites. Rate parameters showed that hydrogenations at terminal aromatic rings were favored over hydrogenations at internal rings. Isomerizations and ring openings were favored at positions R to an aromatic ring or a tertiary carbon. Equilibrium concentration ratios for all hydrogenation and ring-opening reactions were larger than unity; equilibrium ratios in all isomerizations were less than unity, indicating significant reverse reactions. Collectively, this information provides a starting point in the development of quantitative structure/reactivity relationships. Appendix A. Phenanthrene Hydrocracking Product Identification Lumps and Nomenclature
2050 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
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Received for review October 21, 1996 Revised manuscript received March 24, 1997 Accepted March 26, 1997X IE9606808 X Abstract published in Advance ACS Abstracts, May 1, 1997.
