Al2

Ind. Eng. Chem. Res. 1996, 35, 3311-3318

3311

Hydrodesulfurization of Dibenzothiophene on a CoMo/Al2O3 Catalyst: Reaction Network and Kinetics Vale´ rie Vanrysselberghe and Gilbert F. Froment*

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Laboratorium voor Petrochemische Techniek, Rijksuniversiteit Gent, Krijgslaan 281, B-9000 Gent, Belgium

The hydrodesulfurization of dibenzothiophene on a commercial CoMo/Al2O3 catalyst was studied in a multiphase reactor. The operating conditions were varied over the following range: temperatures, 513-573 K; total pressures, 50-80 bar; molar hydrogen to hydrocarbon ratios, 1.1-4.1. Hougen-Watson rate equations for the hydrogenolysis of dibenzothiophene into biphenyl and H2S, for the hydrogenation of dibenzothiophene into tetra- and hexahydrodibenzothiophene, for the hydrogenation of biphenyl into cyclohexylbenzene, and for the subsequent hydrogenation of cyclohexylbenzene into bicyclohexyl were developed. Two different types of active sites were considered: σ sites for hydrogenolysis and τ sites for hydrogenation. The surface reaction between adsorbed reactants and two competitively adsorbed hydrogen atoms was found to be the rate-determining step for both types of reaction. Introduction Thiophenic components are known to be the most refractory organic sulfur-containing components. Rigorous kinetics for the hydrodesulfurization (HDS) of thiophene and benzothiophene have already been derived (Van Parys and Froment, 1986; Van Parys et al., 1986). For dibenzothiophene, hydrodesulfurization rate equations have been reported by Broderick and Gates (1981) and recently by Edvinsson and Irandoust (1993). Broderick and Gates (1981) neglected the hydrogenation of biphenyl into cyclohexylbenzene, while Edvinsson and Irandoust (1993) did not determine the influence of the H2S concentration on the reaction rates. Furthermore, cyclohexylbenzene and bicyclohexyl were lumped, so that no rate equation was developed for the hydrogenation of cyclohexylbenzene into bicyclohexyl. The present study is part of a wider effort aiming at a more rigorous kinetic modeling of the hydrodesulfurization of oil fractions introduced by Froment et al. (1994) and in which the kinetics of HDS of substituted thiophenes, benzothiophenes, and dibenzothiophenes are related to those of the unsubstituted components through electronic and steric factors. The purpose of this paper is to develop rate equations for all reactions in the network for the hydrodesulfurization of dibenzothiophene on a commercial CoMo/Al2O3 catalyst under operating conditions relevant to industrial applications. Experimental Setup Figure 1. Experimental setup.

A flow scheme of the experimental setup is shown in Figure 1. The liquid hydrocarbon was fed into the reactor with a high-pressure pump (Spectra-Physics P1000) (1). The hydrocarbon feed was metered with a Sarto¨rius electrobalance type BA 4100 (2). The hydrogen, the hydrogen sulfide, the nitrogen, and the methane feed were controlled and measured with a set of electronic mass flow controllers of the type Brooks 5850 TR (3). Hydrogen sulfide and nitrogen were used in the pretreatment of the catalyst. Methane was used as internal standard for the on-line analysis of the reactor effluent. The gases and the liquid feed were preheated (4) and mixed (5) before entering the reactor (6). The effluent section was also heated to avoid condensation. The reaction was carried out in a multiphase RobinsonMahoney reactor. Complete mixing in this reactor was verified by residence time distribution studies. Visual S0888-5885(96)00099-1 CCC: $12.00

observations in a glass vessel showed the gas to be perfectly distributed over the liquid phase (Yu and Froment, unpublished). The reactor is made of stainless steel and can operate at pressures up to 140 bar and at temperatures up to 350 °C. The temperature was measured by means of thermocouples and controlled by a PID temperature controller. The pressure was controlled by a back pressure regulator (7). The effluent of the reactor consisted of gas and liquid phases at high pressure and high temperature. Both phases were separated by means of a cyclone (8). The liquid was collected in the liquid holder (9). The gas product stream entered a mist eliminator (10) to recuperate entrained liquid droplets that were added to the already recuperated liquid phase. The cyclone, the liquid holder, and the demister were kept at the same pressure and © 1996 American Chemical Society

3312 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996

temperature as in the reactor to avoid changes in composition of both phases. A small fraction of the gas phase was taken for on-line gas chromatographic (GC) (11) analysis. The remaining gas phase was cooled (12), so as to condense heavy fractions and then was scrubbed by means of a sodium hydroxide solution (13) to remove hydrogen sulfide. The liquid product was cooled (14) and flashed (15) under ambient conditions. The light gases, dissolved in the liquid phase, were partially desorbed and collected in a gas buret (16). The flow rates and compositions of the desorbed gas mixture and the liquid product were measured and analyzed off-line by means of the GC. The GC contained a TCD detector, a Hayesep D packed column, and a backflush column OV-101 to analyze light gases such as hydrogen, hydrogen sulfide, and methane. The GC also contained a FID detector and a capillary column HP-5 for the analysis of the hydrocarbons. Procedure Experiments were performed using a solution of 2 wt % dibenzothiophene (DBT) (0.35 wt % sulfur) dissolved in Parapur (FINA). Parapur is a paraffinic mixture containing 5.23 wt % n-decane, 48.42 wt % n-undecane, 33.64 wt % n-dodecane, 12.52 wt % n-tridecane, and 0.19 wt % n-tetradecane. Experiments were carried out at total pressures between 50 and 80 bar and at temperatures between 513 and 573 K. The molar flow rate F0DBT was varied between 1.74 × 10-6 and 4.04 × 10-6 kmol/h and the molar hydrogen to hydrocarbon ratio, γ, between 1.1 and 4.1. The molar hydrogen to methane ratio was kept constant at 6.4 for all experiments, except for some experiments at 70 bar where the molar hydrogen to methane ratio was kept constant at 7.0. The total number of experiments amounted to 98. The catalyst used was the commercial HDS catalyst AKZO Ketjenfine 742 containing 5-30 wt % MoO3, 1-10 wt % CoO, 0-6 wt % SiO2, and 0-10 wt % P2O5 on an alumina support and with a surface area of 264 mcat2/gcat and a pore volume of 0.52 cm3/gcat. It was crushed to a size between 710 and 800 µm to avoid diffusional limitations, and 2.53 gcat was diluted with nonporous inert alumina. Diffusional limitations were calculated using the Weisz-Prater criterium. In addition, it was shown experimentally that no diffusional limitations exist with 0.71-0.80 mm particles. Experiments with 1.2 mm particles as well as with 0.63-0.90 mm particles gave identical values for the reaction rates for given conditions. The catalyst was dried at 423 K in the reactor by means of nitrogen and was then heated to 623 K, still under nitrogen. It was pretreated by a 15 vol % mixture of H2S in H2 during 6 h at 1 bar and 623 K. The gas flow rate was 10 L(STP)/h. The catalyst was cooled under nitrogen. It was then stabilized with the liquid feed at T ) 533 K, pt ) 50 bar, F0DBT ) 1.97 × 10-6 kmol/h, γ ) 4.1, and H2/CH4 ) 6.4. The stability of the catalyst activity and selectivity was frequently checked by performing experiments at reference operating conditions. Catalyst deactivation and hydrocracking of the solvent Parapur did not occur under the conditions mentioned above. Discussion of the Results The reaction products of the HDS of DBT were biphenyl (BPH), cyclohexylbenzene (CHB), bicyclohexyl (BCH), and H2S. Tetra- (THDBT) and hexahydrodibenzothiophene (HHDBT) were not detected.

0 g Figure 2. Conversions as a function of W/(FDBT - FDBT ): (b) total conversion of DBT, (9) conversion of DBT into BPH, (2) conversion of DBT into CHB. Experimental conditions: T ) 553 K, pt ) 60 bar, H2/CH4 ) 6.39, and H2/HC ) 1.10.

The catalytic reactions only take place in the completely wetted catalyst. Consequently, the fraction of DBT that evaporated, FgDBT, is excluded from the contact with the catalyst surface and does not participate in the reactions. For a given molar feed rate, F0DBT, the molar flow rate of DBT in contact with the active sites is F0DBT - FgDBT. Consequently, the space time is defined as W/(F0DBT - FgDBT) and the conversion xDBT of DBT as well as its conversion xBPH into BPH, xCHB into CHB, and xBCH into BCH are defined as:

xDBT )

0 g l - FDBT - FDBT FDBT

xBPH )

xCHB )

xBCH )

0 g FDBT - FDBT g l + FBPH FBPH 0 g FDBT - FDBT

FgCHB + FlCHB 0 g FDBT - FDBT

FgBCH + FlBCH 0 g FDBT - FDBT

The DBT total conversion varied from 12.3 to 87.0%, depending on the operating conditions. As expected, the conversions xDBT, xBPH, xCHB, and xBCH increased with temperature. DBT was mainly desulfurized into BPH and H2S. It was also hydrogenated into THDBT and/ or HHDBT. Since these partially hydrogenated DBTs were not detected, they are highly reactive intermediates that are instantaneously converted into CHB and H2S. BPH was further hydrogenated into CHB. Complete hydrogenation of BPH only occurred at 573 K and led to very small amounts of BCH. Typical sets of conversion vs spacetime plots are shown in Figures 2 and 3. The dependence of the conversions xDBT, xBPH, and xCHB on γ is given in Figure 4 for a given value of W/ (F0DBT - FgDBT), T, pt, and H2/CH4. The dependence of the selectivity for hydrogenation defined by the ratio xCHB/xDBT and of the H2S liquid concentration CH2S on γ

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3313

0 Figure 3. Total conversion of DBT as a function of W/(FDBT g FDBT ) at various temperatures: (b) 513, (9) 533, (2) 553, and (O) 573 K. Experimental conditions: pt ) 80 bar, H2/CH4 ) 6.38, and H2/HC ) 1.33.

Figure 6. Selectivity for hydrogenation, xCHB/xDBT, and conversions as a function of total pressure, pt,: (b) total conversion of DBT, (9) conversion of DBT into BPH, (2) conversion of DBT into 0 g CHB. Experimental conditions: T ) 553 K, W/(FDBT - FDBT )) 1275 kgcat h kmol-1, H2/CH4 ) 6.39, and H2/HC ) 4.13.

upon hydrogenation and hydrogenolysis (Van Parys and Froment, 1986; Van Parys et al., 1986; Edvinsson and Irandoust, 1993). The dependence of xDBT, xBPH, and xCHB and of the selectivity for hydrogenation xCHB/xDBT on the total pressure, pt, is shown in Figure 6 for a given value of W/(F0DBT - FgDBT), T, γ, and H2/CH4. Within the range of 50-80 bar, xDBT, xBPH, and xCHB increased with the total pressure. The selectivity for hydrogenation reactions was favored by higher total pressures. Kinetic Analysis Reaction Scheme. The following reaction network for the HDS of DBT into BPH, CHB, BCH, and H2S is deduced from the experimental program:

S

Figure 4. Conversions as a function of H2/HC: (b) total conversion of DBT, (9) conversion of DBT into BPH, (2) conversion of DBT into CHB. Experimental conditions: T ) 533 K, pt ) 70 bar, 0 g W/(FDBT - FDBT ) ) 1029 kgcat h kmol-1, and H2/CH4 ) 7.02.

rDBT,τ

rDBT,σ

+ H2S S

S rBPH,τ

+ H2S rCHB,τ

Figure 5. Selectivity for hydrogenation, xCHB/xDBT, and H2S liquid concentration, CH2S, as a function of H2/HC. Experimental condi0 g tions: T ) 533 K, pt ) 70 bar, W/(FDBT - FDBT ) ) 1029 kgcat h -1 kmol , and H2/CH4 ) 7.02.

is shown in Figure 5. A higher γ resulted in higher conversions. Furthermore, H2S increased the selectivity for hydrogenation, indicating a different effect of H2S

The hydrogenolysis reactions and the hydrogenation reactions are considered to take place on different kinds of active sites, σ and τ. The existence of two different types of active sites has already been pointed out by Delmon (1979), Gates et al. (1979), Broderick and Gates (1981), Vrinat (1983), Van Parys and Froment (1986), Van Parys et al. (1986), and Edvinsson and Irandoust (1993). Parameter Estimation. The experiments were performed in a perfectly mixed flow reactor. The differential method of kinetic analysis was applied (Froment and Bischoff, 1990). The net production rates,

3314 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996

Ri, derived from the above reaction scheme, are defined as

RBPH ) rDBT,σ - rBPH,τ RCHB ) rDBT,τ + rBPH,τ - rCHB,τ RBCH ) rCHB,τ

When the calculated value Fadeq exceeds the tabulated R-percentage point of the F-distribution with nv - p ∑(ns - 1)v and ∑(ns - 1)v degrees of freedom, there is a probability of 1 - R that the model is inadequate and the model is rejected. The significance of the individual parameters is tested by means of their calculated t-values. The significance of the overall regression is evaluated by means of the F-value, defined by Froment and Bischoff (1990)

The total rate of disappearance of DBT is given by

RDBT ) rDBT,σ + rDBT,τ

Fregr )

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

Ri )

xi 0 W/(FDBT

g - FDBT )

Several plausible reaction mechanisms and corresponding Hougen-Watson rate equations were derived. The various reaction mechanisms only differ by the way of adsorption of hydrogen: atomically (A) or molecularly (M); competitively in hydrogenolysis (σc) and hydrogenation (τc); noncompetitively on a third type of active sites (σnc, τnc); noncompetitively on the active sites for hydrogenation (στ) or hydrogenolysis (τσ). For the adsorption of atomic hydrogen, the addition of the first H atom, the addition of the second H atom, and the simultaneous addition of two hydrogen atoms were considered. Reaction with H2 directly from the liquid phase was also considered (σER, τER). The sulfur atom of DBT, THDBT, or HHDBT remains on the catalyst surface after reaction. Its removal occurs via reaction with H2 directly from the liquid phase (SER) or via a mechanism corresponding with the mechanism of hydrogenolysis on the σ sites (Sσ, Sτ, Snc). Either the adsorption of the reactants or the surface reaction between the adsorbed species or the desorption of the reaction products on both active sites σ and τ could be the rate-determining step (rds). The discrimination involved 174 rival models. The parameters were obtained by minimization of the following multiresponse objective function S(θ) by means of a Marquardt routine: h)v k)v

S(θ) )

h)v k)v

i)n

R ˆ ihR ˆ ik/p ∑ ∑ σhk∑ h)1 k)1 i)1 h)v k)v

i)n

(Rih - R ˆ ih)(Rik - R ˆ ik)/(nv - p) ∑∑ ∑ i)1 σhk

h)1 k)1

When the calculated Fregr value exceeds the tabulated R percentage point of the F distribution with degrees of freedom (p, nv - p), the regression is considered to be meaningful. Among the rival models, the one with the highest Fregr value is considered the best. However, the parameters have to satisfy physicochemical laws. The adsorption equilibrium constants have to obey the van’t Hoff temperature dependence; the rate coefficients have to obey the Arrhenius temperature dependence. Constraints on the adsorption entropies were derived by Boudart et al. (1967). 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 -

)]

Ea 1 1 Rgas T Tm

[ (

)]

∆H0a 1 1 Ki,s ) A exp Rgas T Tm #

Tm is the average temperature of the experiments, here 540 K. The discrimination between the rival models was based on statistical tests and physicochemical criteria. The discrimination and parameter estimation led to the reaction mechanism AσcτcSσ: (a) hydrogenolysis of DBT into BPH and H2S on the σ sites

DBT + σ h DBT.σ

i)n

σhk∑(Rih - R ˆ ih)(Rik - R ˆ ik) f min ∑ ∑ h)1 k)1 i)1 θ

The experimental program contained a number of replicated experiments so that the “pure error” sum of squares could be calculated and the model adequacy could be tested by means of the following F-test (Froment and Bischoff, 1990):

H2 + 2σ h 2H.σ

DBT.σ + 2H.σ f BPH.σ + S.σ + σ

(rds)

S.σ + 2H.σ h H2S.σ + 2σ lack of fit sum of squares s)r

nv - p Fadeq )

∑(ns - 1)v

BPH.σ h BPH + σ

s)1

pure error sum of squares

H2S.σ h H2S + σ

s)r

∑(ns - 1)v

s)1

(b) hydrogenation of DBT into THDBT and HHDBT on the τ sites, followed by hydrogenolysis into CHB and

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3315

H2S on the σ sites

rBPH,τ )

DBT + τ h DBT.τ H2 + 2τ h 2H.τ DBT.τ + 2H.τ h DHDBT.τ + 2τ

(rds)

DHDBT.τ + 2H.τ h THDBT.τ + 2τ

rCHB,τ )

kBPH,τKH,τKBPH,τCBPHCH2 (1 + KDBT,τCDBT + xKH,τCH2 + KBPH,τCBPH)3 kCHB,τKH,τKCHB,τCCHBCH2 (1 + KDBT,τCDBT + xKH,τCH2 + KBPH,τCBPH)3

with

THDBT.τ h THDBT + τ THDBT.τ + 2H.τ h HHDBT.τ + 2τ

KDBT,σ ) 7.56868 × 101

(m3/kmol)

[

HHDBT.τ h HHDBT + τ

113232 RgasT

]

(m3/kmol)

[

]

(m3/kmol)

[

]

KH,σ ) 3.36312 × 10-11 exp

THDBT + σ h THDBT.σ HHDBT + σ h HHDBT.σ

KBPH,σ ) 3.84984 × 10-4 exp

H2 + 2σ h 2H.σ THDBT.σ + 2H.σ f PHCH.σ + S.σ + σ PHCH.σ + 2H.σ h CHB.σ + 2σ

KH2S,σ ) 1.47118 × 10-8 exp

48214 RgasT

105670 RgasT

(m3/kmol)

HHDBT.σ + 2H.σ f CHB.σ + S.σ + σ

[

S.σ + 2H.σ h H2S.σ + 2σ

kDBT,σ ) 2.44336 × 1010 exp -

CHB.σ h CHB + σ

(kmol kgcat-1 h-1)

H2S.σ h H2S + σ (c) hydrogenation of BPH into CHB on the τ sites

]

(m3/kmol)

142693 RgasT

]

(m3/kmol)

[

]

(m3/kmol)

KDBT,τ ) 2.50395 × 10-7 exp

BPH + τ h BPH.τ H2 + 2τ h 2H.τ BPH.τ + 2H.τ f PHCHD.τ + 2τ

KH,τ ) 1.40255 × 10-15 exp (rds)

PHCHD.τ + 2H.τ h PHCH.τ + 2τ PHCH.τ + 2H.τ h CHB.τ + 2τ CHB.τ h CHB + τ (d) hydrogenation of CHB into BCH on the τ sites

[

76840 RgasT

[

KBPH,τ ) 4.96685 × 10-4 exp

37899 RgasT

[

kDBT,τ ) 2.86757 × 1016 exp -

CHB + τ h CHB.τ

]

186190 RgasT

(kmol kgcat-1 h-1)

H2 + 2τ h 2H.τ CHB.τ + 2H.τ f CHCHD.τ + 2τ

]

122770 RgasT

(rds)

CHCHD.τ + 2H.τ h CHCH.τ + 2τ CHCH.τ + 2H.τ h BCH.τ + 2τ BCH.τ h BCH + τ

[

kBPH,τ ) 3.41120 × 1023 exp -

]

255714 RgasT

(kmol kgcat-1 h-1) kCHB,τKCHB,τ (573 K) ) 3.38631 × 10-1 (m3 kgcat-1 h-1)

The corresponding rate expressions are given by rDBT,σ ) kDBT,σKH,σKDBT,σCDBTCH2 (1 + KDBT,σCDBT + xKH,σCH2 + KBPH,σCBPH + KH2S,σCH2S)3

rDBT,τ )

kDBT,τKH,τKDBT,τCDBTCH2 (1 + KDBT,τCDBT + xKH,τCH2 + KBPH,τCBPH)3

The parameter estimates, their corresponding 95% confidence intervals, the calculated t-values, the calculated Fregr value, and Fadeq value are shown in Table 1. The selected model was found to be statistically adequate. The regression was found to be significant and all parameters were statistically significant. The temperature dependence of KDBT,σ was statistically nonsignificant. The physicochemical rules for the activation energies, Ea, for the adsorption enthalpies

3316 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 1. Parameter Estimates, 95% Confidence Intervals, t Values, Fregr Value, and Fadeq Valuea KDBT,σ KBPH,σ

parameter estimate

lower limit

upper limit

t value

7.56868 × 101 1.79037 × 101 -4.82140 × 104 3.07994 -1.13232 × 105 2.49703 × 102 -1.05670 × 105 3.18252 × 10-2 1.22770 × 105 6.87701 -7.68399 × 104 2.31738 -3.78986 × 104 9.13800 × 10-2 -1.42693 × 105 2.70832 × 10-2 1.86190 × 105 5.99185 × 10-2 2.55714 × 105 3.38631 × 10-1

7.35331 × 101 1.39478 × 101 -5.55333 × 104 1.68108 -1.22443 × 105 2.32196 × 102 -1.10430 × 105 1.97179 × 10-2 1.15508 × 105 6.87182 -8.87463 × 104 2.31659 -4.60500 × 104 8.95236 × 10-2 -1.56505 × 105 2.48567 × 10-2 1.76425 × 105 1.12721 × 10-2 2.48667 × 105 3.33916 × 10-1

7.78405 × 101 2.18595 × 101 -4.08947 × 104 4.47880 -1.04020 × 105 2.67210 × 102 -1.00910 × 105 4.39326 × 10-2 1.30032 × 105 6.88220 -6.49335 × 104 2.31816 -2.97473 × 104 9.32364 × 10-2 -1.28881 × 105 2.93098 × 10-2 1.95955 × 105 1.08565 × 10-1 2.62761 × 105 3.43345 × 10-1

7.02849 × 101 9.05173 -1.31744 × 101 4.40349 -2.45847 × 101 2.85260 × 101 -4.44026 × 101 5.25719 3.38130 × 101 2.64876 × 103 -1.29074 × 101 5.89640 × 103 -9.29873 9.84469 × 101 -2.06625 × 101 2.43274 × 101 3.81358 × 101 2.46343 7.25727 × 101 1.43647 × 102

A# ∆H A# ∆H A# ∆H A# Ea A# ∆H A# ∆H A# ∆H A# Ea A# Ea

KH,σ KH2S,σ kDBT,σ KDBT,τ KBPH,τ KH,τ kDBT,τ kBPH,τ kCHB,τKCHB,τ (573 K) a

Fregr value ) 20366; Fadeq value ) 1.30.

Table 2. Adsorption Equilibrium Constants and Rate Coefficients at 573 K KDBT,σ ) 7.56868 × 101 KH,σ ) 7.01679 × 10-1 KH2S,σ ) 6.27912 × 101 KBPH,σ ) 9.53728 kDBT,σ ) 1.58251 × 10-1 KDBT,τ ) 2.52021 KH,τ ) 1.41658 × 10-2 KBPH,τ ) 1.41256 kDBT,τ ) 3.08384 × 10-1 kBPH,τ ) 1.69206 kCHB,τKCHB,τ ) 3.38631 × 10-1

m3/kmol m3/kmol m3/kmol m3/kmol kmol/(kgcat h) m3/kmol m3/kmol m3/kmol kmol/(kgcat h) kmol/(kgcat h) m3/(kgcat h)

(-∆H0a), and for the adsorption entropies (-∆S0a) mentioned in Froment and Bischoff (1990) were satisfied:

Ea > 0 (-∆H0a) > 0 0 < (-∆S0a) < S0g 48.14 < (-∆S0a) < 51.04 + 0.0014(-∆H0a) The values for the adsorption equilibrium constants and for the rate coefficients at 573 K are given in Table 2. The adsorption of DBT, BPH, and hydrogen is stronger on the σ sites than on the τ sites. The significant differences between KDBT,σ and KDBT,τ, between KBPH,σ and KBPH,τ, and between KH,σ and KH,τ are a posteriori evidence for the distinction between sites for hydrogenolysis and hydrogenation. The adsorption entropy (-∆S0a) for BPH, H2, and H2S on the σ sites was calculated to be 65.4, 100.3, and 150.0 kJ kmol-1 K-1, respectively; the adsorption entropy (-∆S0a) for DBT, BPH, and H2 on the τ sites was 126.4, 63.3, and 142.2 kJ kmol-1 K-1, respectively. Activation energies for DBT hydrodesulfurization presented in the literature are between 60 and 163 kJ/mol (Broderick and Gates 1981; Edvinsson and Irandoust, 1993; O’Brien et al., 1986; Singhal et al., 1981; Vrinat, 1983). The heat of adsorption for DBT and H2 reported in the literature range from 18.8 to 51.8 kJ/mol and from 25.1 to 138.5 kJ/mol (Broderick and Gates, 1981; Edvinsson and Irandoust, 1993; O’Brien et al., 1986; Singhal et al., 1981; Vrinat and de Mourgues, 1981).

Figure 7. Parity plots for RDBT (a) and RBPH (b).

Toyoshima and Somorjai (1979) found a value of 146.5 kJ/mol as the heat of adsorption for H2. The values presented in this paper are in good agreement with the values reported in the literature.

Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3317

Acknowledgment This work was funded by the European Commission under the Joule Program Contract 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. Nomenclature

Figure 8. Parity plots for RCHB (a) and RBCH (b).

The comparison between the experimental values Ri and the calculated values R ˆ i are shown in Figures 7 and 8. As can be seen from these figures, there is a very good agreement between the experimental values and the model predictions.

Ci ) liquid concentration of component i, kmol/mL3 Ea ) activation energy, kJ/kmol Fgi ) molar gas flow rate of component i, kmol/h Fli ) molar liquid flow rate of component i, kmol/h 0 ) molar feed flow rate of dibenzothiophene, kmol/h FDBT (-∆H0a) ) 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 ns ) number of replicated 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) RDBT ) total rate of disappearance of dibenzothiophene, kmol/(kgcat h) Rgas ) gas constant, kJ kmol-1 K-1 Ri ) net production rate of component i, kmol/(kgcat h) S(θ) ) objective function (-∆S0a) ) adsorption entropy, kJ kmol-1 K-1 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 xDBT ) conversion of dibenzothiophene W ) total catalyst mass, kgcat Greek Symbols

Conclusions The kinetic study of the hydrodesulfurization of dibenzothiophene pointed out that hydrogenolysis and hydrogenation reactions occur on two different types of active sites. On both types of active sites the surface reactions between adsorbed reactants and two competitively adsorbed hydrogen atoms were rate determining. Dibenzothiophene undergoes hydrogenolysis with or without prior hydrogenation of the aromatic ring system. Similar rate equations were derived by Van Parys and Froment (1986) for the gas phase HDS of thiophene and by Van Parys et al. (1986) for the gas phase HDS of benzothiophene. Under identical operating conditions, the intrinsic rate of thiophene hydrogenolysis was found to be higher than that of benzothiophene hydrogenolysis. The rate of dibenzothiophene hydrogenolysis as determined from the multiphase experimentation of this work was lower than that of thiophene and benzothiophene.

γ ) molar hydrogen to hydrocarbon ratio in the feed σ ) hydrogenolysis site σhk ) (h,k) element of the inverse of the covariance matrix of the experimental errors on R τ ) hydrogenation site Subscripts BCH ) bicyclohexyl BPH ) biphenyl CHB ) cyclohexylbenzene DBT ) dibenzothiophene H ) atomic hydrogen H2 ) molecular hydrogen H2S ) hydrogen sulfide σ ) with respect to the hydrogenolysis function τ ) with respect to the hydrogenation function Superscripts ˆ ) calculated g ) gas l ) liquid

3318 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996

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Toyoshima, I.; Somorjai, G. A. Heats of Chemisorption of O2, H2, CO, CO2, and N2, on Polycristalline and Single Crystal Transition Metal Surfaces. Catal. Rev. Sci. Eng. 1979, 19, 105. Van Parys, I. A.; Froment, G. F. Kinetics of Hydrodesulfurization on a CoMo/γ-Al2O3 Catalyst. 1. Kinetics of the Hydrogenolysis of Thiophene. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 431. Van Parys, I. A.; Hosten, L. H.; Froment, G. F. Kinetics of Hydrodesulfurization on a CoMo/γ-Al2O3 Catalyst. 2. Kinetics of the Hydrogenolysis of Benzothiophene. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 437. Vrinat, M. L. The Kinetics of the Hydrodesulfurization Process: A Review. Appl. Catal. 1983, 6, 137. Vrinat, M. L.; de Mourgues, L. Etude Cine´tique sous Pression en Pre´sence de Catalyseurs Sulfure´s Co-Mo/Al2O3, Mo/Al2O3 et Co/Al2O3: Hydrode´sulfuration du Dibenzothiophe`ne. J. Chim. Phys. 1982, 79, 45.

Received for review February 9, 1996 Revised manuscript received May 14, 1996 Accepted June 12, 1996X IE960099B X Abstract published in Advance ACS Abstracts, September 1, 1996.