Reaction Network and - American Chemical Society

Ind. Eng. Chem. Res. 1998, 37, 1235-1242

1235

Hydrodesulfurization of 4-Methyldibenzothiophene and 4,6-Dimethyldibenzothiophene on a CoMo/Al2O3 Catalyst: Reaction Network and Kinetics Vale´ rie Vanrysselberghe, Raphae1 l Le Gall, and Gilbert F. Froment* Laboratorium voor Petrochemische Techniek, Universiteit Gent, Krijgslaan 281, B-9000 Gent, Belgium

The hydrodesulfurization of 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene on a commercial CoMo/Al2O3 catalyst was studied in a multiphase flow reactor in the presence of dibenzothiophene. Hougen-Watson rate equations for the hydrogenolysis of 4-methyldibenzothiophene into 3-methylbiphenyl and H2S and for the hydrogenation of 4-methyldibenzothiophene into partially hydrogenated intermediates were developed. In an identical manner Hougen-Watson kinetics were determined for the hydrogenolysis of 4,6-dimethyldibenzothiophene into 3,3′-dimethylbiphenyl and H2S and for the hydrogenation of 4,6-dimethyldibenzothiophene into dimethyltetra- and dimethylhexahydrodibenzothiophene. Two different types of active sites exist on the catalyst surface: σ sites for hydrogenolysis and τ sites for hydrogenation reactions. The surface reaction between adsorbed reactants and two competitively adsorbed hydrogen atoms was the rate-determining step for both types of reaction. Introduction The structural contributions approach for the modeling of the hydrodesulfurization (HDS) of oil fractions proposed by Froment et al. (1994, 1997) requires the kinetics of the transformation of a number of key components, both “parent” and substituted. Dibenzothiophenes with alkyl substituents in position 4 and/ or 6 such as 4-methyldibenzothiophene (4-MeDBT) and 4,6-dimethyldibenzothiophene (4,6-DiMeDBT) are the most difficult organic sulfur-containing components to remove. Several authors (Houalla et al. (1977, 1980), Kilanowski et al. (1978)) have determined first-order kinetics for the HDS of these components. Kabe et al. (1993) developed Hougen-Watson kinetics for the HDS of dibenzothiophene (DBT), 4-MeDBT, and 4,6-DiMeDBT over a CoMo/Al2O3 catalyst. However, these authors did not determine the retarding effect of H2S and the other desulfurization products on the reaction. A detailed reaction mechanism and rigorous kinetics for the HDS of DBT on a commercial CoMo/Al2O3 catalyst (AKZO Ketjenfine 742) have already been derived in a previous study (Vanrysselberghe and Froment (1996)). It was shown that H2S reduces the hydrogenolysis rate but not the hydrogenation rate of DBT, resulting in an increased selectivity for hydrogenation. The purpose of this paper is to develop HougenWatson rate equations for the HDS of 4-MeDBT and 4,6-DiMeDBT under operating conditions relevant to industrial applications on the AKZO KF 742 CoMo/ Al2O3 catalyst as used in our study on the reaction network and kinetics for the HDS of DBT (Vanrysselberghe and Froment (1996)). Both reactions were studied in the presence of DBT. Hydrodesulfurization of 4-Methyldibenzothiophene Procedure. Details of the experimental equipment, the catalyst properties, and pretreatment procedure were reported in a previous paper (Vanrysselberghe and Froment (1996)). The catalyst was crushed to a size between 710 and 800 µm to avoid any internal diffu-

sional limitations. Experiments were carried out with a solution containing 2 wt % DBT and 0.27 wt % 4-MeDBT in PARAPUR, a mixture of n-paraffins, under the following operating conditions: temperatures, 533593 K; total pressure, 80 bar; molar hydrogen-tohydrocarbon ratios γ, 1.4-4.2. The molar flow rate F0DBT was varied between 1.72 × 10-6 and 3.92 × 10-6 0 kmol/h, and the molar flow rate F4-MeDBT between 2.13 -7 -7 × 10 and 4.86 × 10 kmol/h. The molar hydrogento-methane ratio was 6.4. The total number of experiments amounted to 12. Discussion of the Results. The reaction products of the HDS of DBT were biphenyl (BPH), cyclohexylbenzene (CHB), bicyclohexyl (BCH), and H2S. The reaction products of the HDS of 4-MeDBT were 3-methylbiphenyl (3-MeBPH), cis- and trans-3-methylcyclohexylbenzene (3-MeCHB), 3-cyclohexyltoluene (3-CHT), and H2S. Partially hydrogenated sulfur-containing intermediates such as (methyl)tetra- and (methyl)hexahydrodibenzothiophenes were not detected. The two isomers cis- and trans-3-methylcyclohexylbenzene were lumped. Since the catalyst particles were completely wetted, the fraction of DBT that evaporated, FgDBT, did not participate in the reactions. Consequently, the conversion xDBT of DBT and its conversion xBPH into BPH, xCHB into CHB, and xBCH into BCH are defined as

xDBT )

F0DBT - FgDBT - FlDBT

xBPH ) xCHB ) xBCH )

F0DBT - FgDBT FgBPH + FlBPH F0DBT - FgDBT FgCHB + FlCHB F0DBT - FgDBT FgBCH + FlBCH F0DBT - FgDBT

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1236 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998

The conversion x4-MeDBT of 4-MeDBT and its conversion x3-MeBPH into 3-MeBPH, x3-MeCHB into 3-MeCHB and x3-CHT into 3-CHT are defined as:

x4-MeDBT )

0 g l F4-MeDBT - F4-MeDBT - F4-MeDBT 0 g F4-MeDBT - F4-MeDBT

x3-MeBPH )

x3-MeCHB )

x3-CHT )

g l F3-MeBPH + F3-MeBPH g 0 F4-MeDBT - F4-MeDBT g l F3-MeCHB + F3-MeCHB 0 g F4-MeDBT - F4-MeDBT g l F3-CHT + F3-CHT

0 g F4-MeDBT - F4-MeDBT

The DBT total conversion varied from 42.6 to 85.4% and the 4-MeDBT total conversion from 15.4 to 64.8%, depending on the operating conditions. DBT was mainly desulfurized into BPH and H2S. It also underwent hydrogenation into tetra- and/or hexahydrodibenzothiophene. These products were instantaneously converted into CHB and H2S since the partially hydrogenated intermediates were not detected. BPH was hydrogenated into CHB, which was subsequently hydrogenated into BCH at 573 and 593 K. In contrast, 4-MeDBT was mainly hydrogenated, leading to two types of methyltetra- and methylhexahydrodibenzothiophenes, depending on which benzene moiety in 4-MeDBT underwent hydrogenation. Since these partially hydrogenated intermediates were not detected, these molecules were directly desulfurized into 3-CHT and 3-MeCHB. Hydrogenolysis of 4-MeDBT led to 3-MeBPH and H2S. Hydrogenation of the substituted or the nonsubstituted benzene ring in 3-MeBPH resulted in 3-MeCHB and 3-CHT, respectively. Complete hydrogenation of 3-MeBPH did not occur. Kinetic Analysis. Reaction Scheme. The proposed reaction network for the HDS of 4-MeDBT into 3-MeBPH, 3-MeCHB, 3-CHT, and H2S is shown in Figure 1. The reaction network for the HDS of DBT into BPH, CHB, BCH, and H2S is given in a previous paper. The hydrogenolysis reactions and the hydrogenation reactions were shown to take place on different kinds of active sites, σ and τ (Vanrysselberghe and Froment (1996)). Parameter Estimation. The experiments were performed in a perfectly mixed Robinson-Mahoney flow reactor. The differential method of kinetic analysis was applied (Froment and Bischoff (1990)). The net production rates, Ri, derived from the reaction network, are defined as 1 2 R3-MeBPH ) r4-MeDBT,σ - r3-MeBPH,τ - r3-MeBPH,τ 1 1 R3-CHT ) r4-MeDBT,τ + r3-MeBPH,τ

Figure 1. Reaction network for the HDS of 4-MeDBT into 3-MeBPH, 3-CHT, 3-MeCHB, and H2S.

The net production rates RBPH and RCHB+BCH are defined as

RBPH ) rDBT,σ - rBPH,τ RCHB + RBCH ) rDBT,τ + rBPH,τ The total rate of disappearance of DBT and 4-MeDBT is given by

RDBT ) rDBT,σ + rDBT,τ 1 2 R4-MeDBT ) r4-MeDBT,σ + r4-MeDBT,τ + r4-MeDBT,τ

Experimental values for Ri were directly obtained from the experimental conversions xi:

Ri )

Ri )

xi W/(F0DBT - FgDBT)

i ) DBT, BPH, CHB + BCH

xi

with g - F4-MeDBT ) i ) 4-MeDBT, 3-MeBPH, 3-MeCHB, 3-CHT

0 W/(F4-MeDBT

The reaction mechanism and the Hougen-Watson rate equations derived for the HDS of DBT (Vanrysselberghe and Froment (1996)) were confirmed for the HDS of the components studied in this work. The rate expressions for the different reactions in the simultaneous HDS of DBT and 4-MeDBT are given by

rDBT,σ ) kDBT,σKH,σKDBT,σCDBTCH2/(1 + KDBT,σCDBT +

xKH,σCH

2

2 2 R3-MeCHB ) r4-MeDBT,τ + r3-MeBPH,τ

with

+ KBPH,σCBPH + KH2S,σCH2S +

K4-MeDBT,σC4-MeDBT + K3-MeBPH,σC3-MeBPH)3

Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1237

rDBT,τ ) kDBT,τKH,τKDBT,τCDBTCH2/(1 + KDBT,τCDBT +

xKH,τCH

2

+ KBPH,τCBPH + K4-MeDBT,τC4-MeDBT +

evaluated by means of the F-value, defined by (Froment and Bischoff (1990)) k)vi)n

rBPH,τ ) kBPH,τKH,τKBPH,τCBPHCH2/(1 + KDBT,τCDBT +

Fregr )

k)1i)1

K3-MeBPH,τC3-MeBPH)3

rCHB,τ ) kCHB,τKH,τKCHB,τCCHBCH2/(1 + KDBT,τCDBT + 2

+ KBPH,τCBPH + K4-MeDBT,τC4-MeDBT + K3-MeBPH,τC3-MeBPH)3

r4-MeDBT,σ ) k4-MeDBT,σKH,σK4-MeDBT,σC4-MeDBTCH2/ (1 + KDBT,σCDBT + xKH,σCH2 + KBPH,σCBPH + KH2S,σCH2S + K4-MeDBT,σC4-MeDBT +

In addition, the parameters should satisfy well-established physicochemical laws. Activation energies Ea and adsorption enthalpies (-∆Ha0) should be positive. Boudart et al. (1967) derived constraints on the adsorption entropies (-∆Sa0). For the estimation of the parameters, the following reparametrization of the rate coefficients and the adsorption equilibrium constants was carried out:

[ (

ki,s ) A# exp -

K3-MeBPH,σC3-MeBPH)3 1 r4-MeDBT,τ

)

1 k4-MeDBT,τ KH,τK4-MeDBT,τC4-MeDBTCH2/

(1 + KDBT,τCDBT + xKH,τCH2 + KBPH,τCBPH +

K4-MeDBT,τC4-MeDBT + K3-MeBPH,τC3-MeBPH)3

2 2 r4-MeDBT,τ ) k4-MeDBT,τ KH,τK4-MeDBT,τC4-MeDBTCH2/



1 1 r3-MeBPH,τ ) k3-MeBPH,τ KH,τK3-MeBPH,τC3-MeBPHCH2/




Hydrogen adsorbed competitively and dissociatively on both kinds of active sites, σ and τ. The surface reaction between the adsorbed reactants and two adsorbed hydrogen atoms on both active sites σ and τ was the rate-determining step. It was found that PARAPUR did not adsorb on the σ and τ sites. The parameters related to the HDS of DBT were already determined (Vanrysselberghe and Froment (1996)). The remaining parameters were obtained by minimization of the following multiresponse objective function S(θ) by means of a Marquardt routine: k)v i)n

S(θ) )

(Rik - R ˆ ik)2 f min ∑ ∑ k)1 i)1 θ

In the objective function the detected components DBT, BPH, CHB+BCH, 4-MeDBT, 3-MeBPH, 3-MeCHB, and 3-CHT were included. The significance of the individual parameters was tested by means of their calculated t-values. The significance of the overall regression was

)]

Ea 1 1 Rgas T Tm

[ (

Ki,s ) A# exp -

)]

∆H0a 1 1 Rgas T Tm

Tm is the average temperature of the experiments. The parameter estimates are given as

K4-MeDBT,σ ) 2.346 77 × 101 m3/kmol

[

K4-MeDBT,τ ) 6.036 99 × 10-8 exp k4-MeDBT,σ )

1 ) k4-MeDBT,τ

133 302 kmol/(kgcat h) RgasT

[


[


4.251 12 × 1024 exp 2 ) k4-MeDBT,τ

]

83 802 m3/kmol RgasT

[

1.315 06 × 1011 exp -


2 2 r3-MeBPH,τ ) k3-MeBPH,τ KH,τK3-MeBPH,τC3-MeBPHCH2/

k)vi)n

∑ ∑(Rik - Rˆ ik)2/(nv - p)

xKH,τCH2 + KBPH,τCBPH + K4-MeDBT,τC4-MeDBT +

xKH,τCH

R ˆ ik2/p ∑ ∑ k)1i)1

K3-MeBPH,τC3-MeBPH)3

7.248 09 × 1023 exp -

] ] ]

The parameter estimates, their corresponding 95% confidence intervals, the calculated t-values, and the calculated Fregr-value are shown in Table 1. The regression was found to be significant, and all parameters were statistically significant. The temperature dependence of K4-MeDBT,σ was statistically nonsignificant. The physicochemical rules for the activation energies Ea, for the adsorption enthalpies (-∆Ha0), and for the adsorption entropies (-∆Sa0) were satisfied. The hydrogenation reactions of 3-MeBPH into 3-MeCHB and 3-CHT were negligible in comparison with the hydrogenolysis and hydrogenations of 4-MeDBT, and the corresponding rate coefficients could not be determined. Also the adsorption equilibrium constant of 3-MeBPH on both the σ and τ sites was not significantly different from zero. The comparison between the experimental values Ri and the calculated values R ˆ i is shown in Figure 2. The

1238 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 1. Parameter Estimates, 95% Confidence Intervals, t-Values and FRegr-Value (A# ) Reparametrized Frequency Factor) K4-MeDBT,σ K4-MeDBT,τ k4-MeDBT,σ 1 k4-MeDBT,τ 2 k4-MeDBT,τ

A# ∆H A# Ea A# Ea A# Ea

parameter estimate

lower limit

upper limit

t-value

2.346 77 × 101 4.333 86 -8.3801 80 × 104 4.178 41 × 10-2 1.333 20 × 105 2.334 16 × 10-1 2.69460 × 105 1.40873 × 10-1 2.63603 × 105

1.762 50 × 101 1.030 48 -8.956 76 × 104 3.232 82 × 10-2 1.297 70 × 105 2.191 88 × 10-1 2.60509 × 105 1.32293 × 10-1 2.54574 × 105

2.931 04 × 101 7.637 24 -7.803 59 × 104 5.124 00 × 10-2 1.368 70 × 105 2.476 43 × 10-1 2.78410 × 105 1.49453 × 10-1 2.72633 × 105

8.033 19 2.202 93 -2.906 82 × 101 8.837 69 7.510 65 × 101 3.281 20 × 101 6.02110 × 101 3.28373 × 101 5.83891 × 101

Fregr-value ) 674

4.95 × 10-8 and 4.86 × 10-7 kmol/h. The temperature was varied between 533 and 593 K, the molar hydrogento-hydrocarbon ratio, γ, between 1.1 and 4.0, and the total pressure between 60 and 80 bar. The molar hydrogen-to-methane ratio was 6.4. The total number of experiments amounted to 24. Discussion of the Results. The reaction products of the HDS of 4,6-DiMeDBT were 3,3′-dimethylbiphenyl (3,3′-DiMeBPH), cis- and trans-3-methylcyclohexyltoluene (3-MeCHT), and H2S. Dimethyltetra- or dimethylhexahydrodibenzothiophene were not detected. cis-3Methyl- and trans-3-methylcyclohexyltoluene were lumped. The conversion x4,6-DiMeDBT of 4,6-DiMeDBT and its conversion x3,3′-DiMeBPH into 3,3′-DiMeBPH and x3-MeCHT into 3-MeCHT are defined as follows:

x4,6-DiMeDBT ) 0 g l - F4,6-DiMeDBT - F4,6-DiMeDBT F4,6-DiMeDBT 0 g F4,6-DiMeDBT - F4,6-DiMeDBT

x3,3′-DiMeBPH )

x3-MeCHT )

Figure 2. Parity plots for RDBT, RBPH, RCHB + RBCH, R4-MeDBT, R3-MeBPH, R3-CHT and R3-MeCHB.

agreement between the experimental values and the model predictions is good. Hydrodesulfurization of 4,6-Dimethyldibenzothiophene Procedure. Experiments were performed with a solution containing 2 wt % DBT and 0.067 or 0.286 wt % 4,6-DiMeDBT in PARAPUR. The molar flow rate F0DBT was varied between 1.71 × 10-6 and 3.92 × 10-6 0 kmol/h and the molar flow rate F4,6-DiMeDBT between

g l F3,3′-DiMeBPH + F3,3′-DiMeBPH 0 g F4,6-DiMeDBT - F4,6-DiMeDBT g l F3-MeCHT + F3-MeCHT

0 g F4,6-DiMeDBT - F4,6-DiMeDBT

The DBT total conversion varied from 35.2 to 90.5% and the 4,6-DiMeDBT total conversion from 7.3 to 54.0%. 4,6-DiMeDBT was mainly hydrogenated into partially hydrogenated dimethyldibenzothiophenes prior to sulfur removal. Since the partially hydrogenated intermediates were not detected, these molecules were highly reactive and directly desulfurized into 3-MeCHT and H2S. 4,6-DiMeDBT underwent also a hydrogenolysis reaction, leading to 3,3′-DiMeBPH and H2S. 3,3′DiMeBPH was hydrogenated into 3-MeCHT. Further hydrogenation of 3-MeCHT did not take place. From the experimental results it was seen that 4,6-DiMeDBT was less converted than 4-MeDBT and that 4,6DiMeDBT was more hydrogenated prior to sulfur removal than 4-MeDBT under identical operating conditions. The conversion of the DBTs into the corresponding BPHs decreased in the order DBT > 4-MeDBT > 4,6DiMeDBT, while the conversion of the DBTs into the corresponding CHBs increased in the order DBT < 4-MeDBT < 4,6-DiMeDBT under the same reaction conditions. The adsorption of DBTs with methyl groups in the R position via the sulfur atom is sterically hindered by the substituents, resulting in lower conver-


rBPH,τ ) kBPH,τKH,τKBPH,τCBPHCH2/(1 + KDBT,τCDBT +

xKH,τCH

2

+ KBPH,τCBPH +

K4,6-DiMeDBT,τC4,6-DiMeDBT + K3,3′-DiMeBPH,τC3,3′-DiMeBPH)3 rCHB,τ ) kCHB,τKH,τKCHB,τCCHBCH2/(1 + KDBT,τCDBT +

xKH,τCH

2

+ KBPH,τCBPH +

K4,6-DiMeDBT,τC4,6-DiMeDBT + K3,3′-DiMeBPH,τC3,3′-DiMeBPH)3 r4,6-DiMeDBT,σ ) k4,6-DiMeDBT,σKH,σK4,6-DiMeDBT,σC4,6-DiMeDBTCH2/(1 + Figure 3. Reaction network for the HDS of 4,6-DiMeDBT into 3,3′-DiMeBPH, 3-MeCHT, and H2S.

sions of those sulfur components into the corresponding BPHs than DBT. Kinetic Analysis. Reaction Scheme. The reaction network for the HDS of 4,6-DiMeDBT into 3,3′-DiMeBPH, 3-MeCHT, and H2S deduced from the experimental data is shown in Figure 3. This reaction scheme is identical with that for the HDS of DBT. Parameter Estimation. The parameters were obtained in a manner identical with that of the HDS of 4-MeDBT described above. The net production rates, Ri, derived from the proposed reaction scheme, are defined as

R3,3′-DiMeBPH ) r4,6-DiMeDBT,σ - r3,3′-DiMeBPH,τ

K3,3′-DiMeBPH,σC3,3′-DiMeBPH)3 r4,6-DiMeDBT,τ ) k4,6-DiMeDBT,τKH,τK4,6-DiMeDBT,τC4,6-DiMeDBTCH2/(1 + KDBT,τCDBT + xKH,τCH2 + KBPH,τCBPH + K4,6-DiMeDBT,τC4,6-DiMeDBT +

K3,3′-DiMeBPH,τC3,3′-DiMeBPH)3 r3,3′-DiMeBPH,τ ) k3,3′-DiMeBPH,τKH,τK3,3′-DiMeBPH,τC3,3′-DiMeBPHCH2/(1 + K4,6-DiMeDBT,τC4,6-DiMeDBT +

The total rate of disappearance of 4,6-DiMeDBT is given by

R4,6-DiMeDBT ) r4,6-DiMeDBT,σ + r4,6-DiMeDBT,τ Experimental values for Ri were directly obtained from the experimental conversions xi:

xi 0 W/(F4,6-DiMeDBT

K4,6-DiMeDBT,σC4,6-DiMeDBT +

KDBT,τCDBT + xKH,τCH2 + KBPH,τCBPH +

R3-MeCHT ) r4,6-DiMeDBT,τ + r3,3′-DiMeBPH,τ

Ri )

KDBT,σCDBT + xKH,σCH2 + KBPH,σCBPH +

g - F4,6-DiMeDBT )

For the HDS of 4,6-DiMeDBT reaction mechanism and corresponding Hougen-Watson rate equations identical with those of the HDS of DBT were derived. The rate expressions for the different reactions in the simultaneous HDS of DBT and 4,6-DiMeDBT are given by

K3,3′-DiMeBPH,τC3,3′-DiMeBPH)3 The parameters were obtained by minimization of the following multiresponse objective function S(θ) by means of a Marquardt routine: k)v i)n

S(θ) )

2

K4,6-DiMeDBT,σ ) 1.803 97 × 101 m3/kmol K4,6-DiMeDBT,τ )

k4,6-DiMeDBT,σ )

K4,6-DiMeDBT,σC4,6-DiMeDBT + 3

K3,3′-DiMeBPH,σC3,3′-DiMeBPH) rDBT,τ ) kDBT,τKH,τKDBT,τCDBTCH2/(1 + KDBT,τCDBT + 2

[

1.587 33 × 10-8 exp

+ KBPH,σCBPH + KH2S,σCH2S +

xKH,τCH

θ

The detected components DBT, BPH, CHB + BCH, 4,6DiMeDBT, 3,3′-DiMeBPH, and 3-MeCHT were included in the objective function. The parameter estimates are given as

rDBT,σ ) kDBT,σKH,σKDBT,σCDBTCH2/(1 + KDBT,σCDBT +

xKH,σCH

∑ ∑(Rik - Rˆ ik)2 f min k)1 i)1

+ KBPH,τCBPH +

K4,6-DiMeDBT,τC4,6-DiMeDBT + K3,3′-DiMeBPH,τC3,3′-DiMeBPH)3

[


[


6.445 60 × 107 exp k4,6-DiMeDBT,τ )

]

90 485 m3/kmol RgasT

3.682 08 × 1027 exp -

] ]

The parameter estimates, their corresponding 95% confidence intervals, the calculated t-values, and the

1240 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 2. Parameter Estimates, 95% Confidence Intervals, t-Values and Fregr-Value (A# ) Reparametrized Frequency Factor) K4,6-DiMeDBT,σ K4,6-DiMeDBT,τ k4,6-DiMeDBT,σ k4,6-DiMeDBT,τ

A# ∆H A# Ea A# Ea

parameter estimate

lower limit

upper limit

t-value

1.803 97 × 101 4.17937 -9.048 50 × 104 8.398 65 × 10-3 1.062 23 × 105 5.462 75 × 10-1 2.990 42 × 105

1.398 51 × 101 1.143 75 -9.523 60 × 104 6.610 92 × 10-3 1.022 56 × 105 5.127 06 × 10-1 2.91041 × 105

2.209 43 × 101 7.214 99 -8.573 40 × 104 1.018 64 × 10-2 1.101 90 × 105 5.798 45 × 10-1 3.070 44 × 105

8.898 31 2.422 14 -3.809 07 × 101 9.39586 5.355 33 × 101 3.254 62 × 101 7.474 46 × 101

Fregr-value ) 2538 Table 3. Parameter Values for DBT, 4-MeDBT, and 4,6-DiMeDBT at 573 K ki,τ component

Ki,σ

Ki,τ

ki,σ

1 ki,τ

DBT 4-MeDBT 4,6-DiMeDBT

75.6868 23.4677 18.0397

2.520 21 2.618 87 2.799 14

1.582 51 × 10-1 9.307 44 × 10-2 1.344 72 × 10-2

1.178 34

2 ki,τ

3.083 84 × 10-1 6.867 04 × 10-1 2.05545

calculated Fregr-value are shown in Table 2. The regression was found to be significant, and all parameters were statistically significant. The temperature dependence of K4,6-DiMeDBT,σ was statistically nonsignificant. The activation energies Ea, the adsorption enthalpies (-∆Ha0), and the adsorption entropies (-∆Sa0) satisfied the physicochemical laws. As in the case of 4-MeDBT, the adsorption of 3,3′-DiMeBPH on the σ and τ sites was not significantly different from zero, and the hydrogenation of 3,3′-DiMeBPH was negligible in comparison to the hydrogenolysis and hydrogenation of 4,6-DiMeDBT. The agreement between the experimental values Ri and the model predictions R ˆ i is shown in Figure 4. Comparison of the HDS of DBT, 4-MeDBT, and 4,6-DiMeDBT The values of the various parameters determined for the HDS of DBT, 4-MeDBT, and 4,6-DiMeDBT at 573 K are shown in Table 3. The adsorption equilibrium constant for the DBTs on the σ sites decreased in the order DBT > 4-MeDBT > 4,6-DiMeDBT. The hydrogenolysis reactions involve vertical adsorption of the molecules through the S-atom on the σ sites (Houalla et al. (1978), Kabe et al. (1993)). Methyl substituents in the R position sterically hinder this adsorption. The first methyl group in the R position reduced the adsorption equilibrium constant by a factor of 3.2. The results for 4,6-DiMeDBT indicate that a second methyl group in the R position further reduces the adsorption equilibrium constant only by a factor of 1.3. The adsorption equilibrium constant of the DBTs on the τ sites increased in the order DBT < 4-MeDBT < 4,6-DiMeDBT. This result is in agreement with those of Korre et al. (1994), who observed an increase of the adsorption equilibrium constant of polyaromatics with the number of carbon atoms in the alkyl side chains. The adsorption enthalpy for the DBTs also increased in the order DBT < 4-MeDBT < 4,6-DiMeDBT. The adsorption enthalpy for 4-MeDBT amounted to 84 kJ/ mol. Kabe et al. (1993) determined an adsorption enthalpy for 4-MeDBT of 88 kJ/mol. From the above results it can be concluded that DBTs with alkyl groups in the R position are more strongly adsorbed on the τ sites and less strongly adsorbed on the σ sites than the nonsubstituted DBT. The rate coefficient for the hydrogenolysis of the DBTs decreased in the order DBT > 4-MeDBT > 4,6-

Figure 4. Parity plots for RDBT, RBPH, RCHB + RBCH, R4,6-DiMeDBT, R3,3′-DiMeBPH and R3-MeCHT.

DiMeDBT. The activation energy for the hydrogenolysis of 4-MeDBT was 133 kJ/mol, in agreement with Kabe et al. (1993), who obtained a value of 138 kJ/mol. In our study the activation energy for the hydrogenolysis


Figure 5. Hydrogenolysis rate of DBT, 4-MeDBT, and 4,6DiMeDBT vs concentration H2S in the liquid phase. Conditions: T ) 533 K, pt ) 70 bar, CH2 ) 0.297 mol/L, CBPH ) 0.024 mol/L, CDBT ) 0.039 mol/L, C4-MeDBT ) 0.039 mol/L, and C4,6-DiMeDBT ) 0.039 mol/L.

of the DBTs increased in the order 4,6-DiMeDBT < DBT < 4-MeDBT. The rate coefficient and the activation energy for the different hydrogenation reactions decreased as follows: 1 2 k4,6-DiMeDBT,τ > k4-MeDBT,τ > k4-MeDBT,τ > kDBT,τ 1 2 E4,6-DiMeDBT,τ > E4-MeDBT,τ > E4-MeDBT,τ > EDBT,τ

Figure 5 illustrates one of the possible applications of the derived rate equations, more particularly to reveal the influence of the concentration of H2S in the liquid phase. The effect of the concentration of the latter component on the hydrogenolysis rate of DBT, 4-MeDBT, and 4,6-DiMeDBT calculated from the derived equations is shown. Conclusions Rate equations were derived for the hydrogenolysis and hydrogenation of 4-MeDBT and 4,6-DiMeDBT which were similar to those for the hydrodesulfurization of DBT. The kinetic analysis confirmed the existence of two different types of active sites, σ and τ. On both types of active sites the reactions between adsorbed reactants and two competitively adsorbed hydrogen atoms were rate-determining. Both sulfur components undergo hydrogenolysis with or without prior hydrogenation of the aromatic ring system. The results pointed out that under identical operating conditions the intrinsic rate of hydrogenolysis of the DBTs decreased in the order DBT > 4-MeDBT > 4,6-DiMeDBT and that the intrinsic rate of hydrogenation of the DBTs increased in the order DBT < 4-MeDBT < 4,6-DiMeDBT. Acknowledgment This work was funded by the European Commission under the Joule program Contract No. JOU2-0121. V.V. is also grateful for a contribution from the Center of Excellence Grant awarded to the Laboratorium voor Petrochemische Techniek by the Belgian Ministry of Science and for financial support from Fina Research. R.L.G. was supported by Total. The methyldibenzothiophenes were provided by Total. Nomenclature Ci ) liquid concentration of component i (kmol/mL3) Ea ) activation energy (kJ/kmol)

Fig ) molar gas flow rate of component i (kmol/h) Fil ) molar liquid flow rate of component i (kmol/h) 0 F4,6-DiMeDBT ) molar feed flow rate of 4,6-dimethyldibenzothiophene (kmol/h) 0 F4-MeDBT ) molar feed flow rate of 4-methyldibenzothiophene (kmol/h) F0DBT ) molar feed flow rate of dibenzothiophene (kmol/h) (-∆Ha0) ) heat of adsorption (kJ/kmol) ki,s ) rate coefficient of component i on s sites [kmol/(kgcat h)] Ki,s ) adsorption coefficient of component i on s sites (mL3/ kmol) n ) number of experiments p ) number of parameters pt ) total pressure (bar) rBPH,τ ) rate of hydrogenation of biphenyl into cyclohexylbenzene [kmol/(kgcat h)] rCHB,τ ) rate of hydrogenation of cyclohexylbenzene into bicyclohexyl [kmol/(kgcat h)] rDBT,σ ) rate of hydrogenolysis of dibenzothiophene into biphenyl [kmol/(kgcat h)] rDBT,τ ) rate of hydrogenation of dibenzothiophene into tetra- and/or hexahydrodibenzothiophene [kmol/(kgcat h)] r3,3′-DiMeBPH,τ ) rate of hydrogenation of 3,3′-dimethylbiphenyl into 3-methylcyclohexyltoluene [kmol/(kgcat h)] r4,6-DiMeDBT,σ ) rate of hydrogenolysis of 4,6-dimethyldibenzothiophene into 3,3′-dimethylbiphenyl [kmol/(kgcat h)] r4,6-DiMeDBT,τ ) rate of hydrogenation of 4,6-dimethyldibenzothiophene into dimethyltetra- and/or dimethylhexahydrodibenzothiophene [kmol/(kgcat h)] r3-MeBPH,τ ) rate of hydrogenation of 3-methylbiphenyl into 3-methylcyclohexylbenzene and 3-cyclohexyltoluene [kmol/ (kgcat h)] r4-MeDBT,σ ) rate of hydrogenolysis of 4-methyldibenzothiophene into 3-methylbiphenyl [kmol/(kgcat h)] r4-MeDBT,τ ) rate of hydrogenation of 4-methyldibenzothiophene into methyltetra- and/or methylhexahydrodibenzothiophene [kmol/(kgcat h)] RDBT ) total rate of disappearance of dibenzothiophene [kmol/(kgcat h)]) R4,6-DiMeDBT ) total rate of disappearance of 4,6-dimethyldibenzothiophene [kmol/(kgcat h)] R4-MeDBT ) total rate of disappearance of 4-methyldibenzothiophene [kmol/(kgcat h)] Rgas ) gas constant (kJ/kmol/K) Ri ) net production rate of component i [kmol/(kgcat h)] S(θ) ) objective function (-∆Sa0) ) adsorption entropy (kJ/kmol/K) T ) absolute temperature (K) Tm ) average temperature (K) v ) number of responses xBCH ) conversion of dibenzothiophene into bicyclohexyl xBPH ) conversion of dibenzothiophene into biphenyl xCHB ) conversion of dibenzothiophene into cyclohexylbenzene x3-CHT ) conversion of 4-methyldibenzothiophene into 3-cyclohexyltoluene xDBT ) conversion of dibenzothiophene x3,3′-DiMeBPH ) conversion of 4,6-dimethyldibenzothiophene into 3,3′-dimethylbiphenyl x4,6-DiMeDBT ) conversion of 4,6-dimethyldibenzothiophene x3-MeBPH ) conversion of 4-methyldibenzothiophene into 3-methylbiphenyl x3-MeCHB ) conversion of 4-methyldibenzothiophene into 3-methylcyclohexylbenzene x3-MeCHT ) conversion of 4,6-dimethyldibenzothiophene into 3-methylcyclohexyltoluene x4-MeDBT ) conversion of 4-methyldibenzothiophene W ) total catalyst mass (kgcat)

1242 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Greek Symbols γ ) molar hydrogen-to-hydrocarbon ratio in the feed σ ) hydrogenolysis site τ ) hydrogenation site Subscripts BCH ) bicyclohexyl BPH ) biphenyl CHB ) cyclohexylbenzene 3-CHT ) 3-cyclohexyltoluene DBT ) dibenzothiophene 3,3′-DiMeBPH ) 3,3′-dimethylbiphenyl 4,6-DiMeDBT ) 4,6-dimethyldibenzothiophene H ) atomic hydrogen H2 ) molecular hydrogen H2S ) hydrogen sulfide 3-MeBPH ) 3-methylbiphenyl 3-MeCHB ) 3-methylcyclohexylbenzene 3-MeCHT ) 3-methylcyclohexyltoluene 4-MeDBT ) 4-methyldibenzothiophene σ ) with respect to the hydrogenolysis function τ ) with respect to the hydrogenation function Superscripts 0 ) inlet conditions ∧ ) calculated g ) gas l ) liquid

Literature Cited Boudart, M.; Mears, D. E.; Vannice, M. A. Kinetics of Heterogeneous Catalytic Reactions. Ind. Chim. Belg. 1967, 32, 281. Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design; John Wiley: New York, 1990.

Froment, G. F.; Depauw, G. A.; Vanrysselberghe, V. Kinetic Modeling and Reactor Simulation in Hydrodesulfurization of Oil Fractions. Ind. Eng. Chem. Res. 1994, 33, 2975. Froment, G. F.; Depauw, G. A.; Vanrysselberghe, V. Kinetics of the Catalytic Removal of the Sulphur Components from the Light Cycle Oil of a Catalytic Cracking Unit. Hydrotreatment and Hydrocracking of Oil Fractions; Elsevier Science B. V.: Amsterdam, Netherlands, 1997; p 83. Houalla, M.; Broderick, D. H.; de Beer, V. H. J.; Gates, B. C.; Kwart, H. Hydrodesulfurization of Dibenzothiophene and Related Compounds Catalyzed by Sulfided CoO-MoO3/γ-Al2O3: Effects of Reactant Structure on Reactivity. Prepr.sAm. Chem. Soc., Div. Petrol. Chem. 1977, 22, 941. Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; de Beer, V. H. J.; Gates, B. C.; Kwart, H. Hydrodesulfurization of MethylSubstituted Dibenzothiophenes Catalyzed by Sulfided Co-Mo/ γ-Al2O3. J. Catal. 1980, 61, 523. Kabe, T.; Ishihara, A.; Zhang, Q. Deep Desulfurization of Light Oil. Part 2: Hydrodesulfurization of Dibenzothiophene, 4-Methyldibenzothiophene and 4,6-Dimethyldibenzothiophene. Appl. Catal. 1993, 97, L1. Kilanowski, D. R.; Teeuwen, H.; de Beer, V. H. J.; Gates, B. C.; Schuit, G. C. A.; Kwart, H. Hydrodesulfurization of Thiophene, Benzothiophene, Dibenzothiophene, and Related Compounds Catalyzed by Sulfided CoO-MoO3/γ-Al2O3: Low-Pressure Reactivity Studies. J. Catal. 1978, 55, 129. Korre, S. C.; Neurock, M.; Klein, M. T.; Quann, R. J. Hydrogenation of Polynuclear Aromatic Hydrocarbons. 2. Quantitative Structure/Reactivity Correlations. Chem. Eng. Sci. 1994, 49, 4191. Vanrysselberghe, V.; Froment, G. F. Hydrodesulfurization of Dibenzothiophene on a CoMo/Al2O3 Catalyst: Reaction Network and Kinetics. Ind. Eng. Chem. Res. 1996, 35, 3311.

Received for review July 30, 1997 Revised manuscript received December 2, 1997 Accepted January 22, 1998 IE970533P

Reaction Network and - American Chemical Society

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