1140
Znd. Eng. C h e m . Res. 1995,34, 1140-1148
Kinetic Study of Naphthalene Hydrogenation over Pt/Al203 Catalyst Ting-Chia Huang* and Ben-Chang Kang Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China 701
The hydrogenation of the biaromatic compound naphthalene was studied in a trickle bed reactor at 473-533 K, 1.7-8.7 MPa, and 1.5-8.0 liquid hourly space velocity (LHSV) over WAl203 catalyst. The hydrogenation activity and selectivity were investigated by using a reaction network including isomerization of decalin and a power-law kinetic model. The rates of tetralin hydrogenation and cis-decalin isomerization could be described by pseudo-first-order kinetics in excess hydrogen. The apparent activation energies for hydrogenation of tetralin to cis-decalin and trans-decalin and isomerization of cis-decalin are 9.88,7.25, and 14.75 kcdmol, respectively. These values match those reported in the literature. The reaction orders of hydrogen for the hydrogenation of tetralin to cis-decalin and truns-decalin are 1.88 and 1.06, respectively. For the isomerization of cis-decalin, the reaction order of hydrogen decreases with increasing pressure and approaches 0 a t pressure higher than 5.17 MPa.
Introduction High aromatics content in diesel fuel lowers fuel quality and contributes significantly to the formation of undesired emissions in exhaust gases (Barry et al., 1985; Lindsay et al., 1992; Ullman, 1989). The stipulated maximum levels of aromatics in fuel are different in different areas. The California Air Resources Board, anticipating the future U.S.specification, has proposed a state reference fuel in which an aromatics limit is specified (10 or 20 vol % depending on the refinery throughput) (van der Berg, 1993). In Sweden, there are three diesel specifications: a limit of 5 wt % aromatics in class 1, 20 wt % in class 2, and 25 wt % in class 3 (Cooper et al., 1993). There are many approaches to solve the aromatics reduction problem (Sakaniski et al., 1991; Suchanek and Hamilton, 1991; Suchanek, 1990). One of the most famous approaches is conventional hydrotreating catalysts in first-stage operation and Pt catalysts in secondstage operation. The hydrotreating of polyaromatics down to monoaromatics is rather easy. However, the saturation of the final ring is difficult because of resonance stabilization of the monoaromatic ring (Asim et al., 1991). Deep aromatic saturation is a reversible reaction favored at low temperature (Girgis and Gates, 1991). Hence, the NiMo or NiW/alumina in secondstage operation is only preferred for moderate levels of saturation. For deep aromatic saturation a noble-metal catalyst in the second stage is preferred. There has been much research on the hydrogenation of aromatic compounds catalyzed by sulfided CoMolyA 1 2 0 3 (Broderick et al., 1987; Pazter et al., 1979; Sapre and Gates, 1982, 1981) or NiWly-Al203 (Wilson et al., 1985; Wilson and Kriz, 1984). Also, numerous studies have been carried out for the hydrogenation of benzene (or toluene) over Pt catalysts (Basset et al., 1975; Ceckiewicz and Delmon, 1987; Orozco and Webb, 1983; Gutierrez-Ortiz et al., 1993). However, there is only a little information about the reactions of biaromatics (or polyaromatics) over supported noble metal catalyst under high pressure (Koussathana et al., 1991; Koussathana et al., 1992; Sakanishi et al., 1989). The reaction kinetics of the hydrogenation of aromatics with metal oxide and metal sulfide catalysts, under
* Author to whom correspondence should be addressed. 0888-5885/95/2634-1140$09.00/0
high hydrogen pressure, followed a first-order dependence on the hydrocarbon and zero-order dependence on hydrogen, which was in excess. Koussathana et al. (1991) obtained the same result with supported noble metal catalysts. All of those results were focused on the aromatics hydrogenation in the hydroprocessing of coal-derived liquids or the coal liquefaction technology where tetralin was used as a hydrogen donor. In this work, the biaromatic compound naphthalene dissolved in n-hexadecane was used to simulate the aromatic compounds in diesel fuel and to study the kinetics of hydrogenation reaction. For deep biaromatic saturation in this study, a reaction model has been suggested, and the selectivity of cis-decalin in the products has been studied.
Experimental Section Catalyst Preparation and Characterization. United Catalysts Inc. Cs-331-4 alumina particle, 2545 mesh (355-710 pm), was used as the catalyst support. To ensure the textural characteristics of the alumina did not change during the catalyst preparation, the alumina support was calcined at 773 K for 4 h. The method of incipient wetness impregnation of the support was employed; i.e., y-AlzO3 was impregnated with appropriate amounts of aqueous solution of chloroplatinic acid (Merck Products). Impregnated support was kept in air overnight and calcined at 383 K for 4 h, followed by 673 K for 4 h. The metal content of the catalysts was 0.6 wt %. The surface area (BET) and pore volume of the support and catalyst were determined from the nitrogen adsorption-desorption isotherms at 77 K, using a Micromeritics 2400 apparatus. "he measurements were performed on the oxidized form of the catalyst samples. Catalysts were characterized in terms of exposed metallic surface area and metal dispersion by selective chemisorptionwith carbon monoxide. Carbon monoxide uptake was determined in a constant-volume, highvacuum apparatus with Micromeritics Chemisorb 2800. The CO to platinum surface atom stoichiometric ratio was supposed to be 1:l (Wells, 1985). The volume of CO adsorbed for monolayer coverage was obtained by the difference between total adsorption and physical adsorption (Delannay, 1984). The Pt concentration of catalyst was determined by 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1141
9
LIQUID PRODUCT
Figure 1. Simplified flow diagram of continuous hydrogenation test unit. (1)feed tank, (2) balance; (3) feed pump; (4) reactor; (5) forward pressure regulator; (6)mass flowmeter and controller; (7) check valve; (8)safety relief valve; (9) gas-liquid separator; (10)wettest meter; (11)cooler; (12) back-pressure regulator.
inductively coupled plasma-atomic emission spectrometry (Jarrell-Ash, Model 1100). Reactants Used. The biaromatic model compound naphthalene (Merck Products, purity '99%) dissolved in n-hexadecane (Merck Products, purity >99%) was used to study the kinetics of hydrogenation reaction. Hydrogen used in the reaction system was supplied by Sanfu Co. (in Taiwan), with a purity of 99.99%. Reaction System. The bench-scale cocurrent downflow trickle bed reactor system shown in Figure 1was used in this study. The reactor was heated and controlled by means of three electric resistances and three thermocouples. The reaction temperature was monitored with three thermocouples. One of the thermocouples was set in the center of the reaction zone; the other two were set outside the tube reactor a t the top and bottom of the catalyst bed, respectively. The temperature difference between these thermocouples was less than 1.5 "C at steady state. The radial temperature difference was neglected due to small reactor diameter. Hence we assumed the catalyst bed was isothermal. Hydrogen gas flow rate was controlled and metered to the reactor by means of a Brooks mass flowmeter and controller. The liquid and gas products from the reactor passed through a back-pressure regulator and were cooled in a double pipe heat exchanger to ambient temperature before they entered the gasliquid separation system. Liquid sampling parts were provided immediately after the cooler and the separator. The liquid product from the separator was emptied before sampling. The gases were passcd through a wettest meter before venting to the atmosphere. The evaluation of kinetic data from the catalytic fured bed reactor was usually based on the assumption of plug flow. However, in the small-scale reactor, deviations from plug flow might be caused by several factors such as axial dispersion, wall effect, and channeling. Several criteria (Montagna and Shah, 1975; Satterfield, 1975) have been reported to evaluate the minimum reactor length and diameter necessary to avoid a significant dispersion effect. In general, a radial aspect ratio (ratio of bed diameter to the catalyst particle diameter) greater than 10 and axial aspect ratio (ratio of the catalyst bed length to the catalyst particle diameter) greater than
20 are required for an isothermal system. However, for a nonisothermal system, the radial aspect ratio and axial aspect ratio should be greater than 30 and 60, respectively (Tsai et al., 1991). The equivalent spherical diameter of the catalyst used in this study was smaller than 710 pm (25-45 mesh). The stainless steel tube reactor had internal diameter 11 mm, outer diameter 20 mm, and length 430 mm. According to the results of Mears (1971) for plug flow
In this study, the conversion of the hydrogenation of naphthalene to decalin was high. The catalyst bed was diluted with 45-75 mesh (0.354-0.195 mm) ceramic powder to avoid the dispersion effect and to obtain a homogeneous thermal distribution in the reactor (Klinken and Dongen, 1980). The volume ratio of inert ceramic powder to catalyst was about 2, and catalyst layer and ceramic layer were loaded one after another. In this work, the length of the catalyst bed Z was 140 mm, the average particle in the bed was about 0.3 mm, and the value of Zld, was about 467. The Peclet number for this typical bench-scale trickle bed reactor was estimated to be about 0.2. In most of this study, the conversion of tetralin hydrogenation was lower than 0.99. According t o eq 1,we could assume that the flow in reactor was plug flow. In addition, the remaining spaces of catalyst bed at the top and the bottom of reactor were filled with 25-45 mesh ceramic powder to avoid the entrance and exit effects. Catalysts were reduced in situ with flowing hydrogen a t 673 K for 4 h before the feed was introduced. After reducing of the catalyst, the reactor was cooled to the desired temperature; then the reactants were introduced. During a run with a given catalyst, the temperature and pressure were kept constant, but the flow rates were changed randomly. At each temperature and each pressure, at least four flow rates were chosen to obtain the conversion, and at least one flow rate at each temperature and pressure was duplicated. The hydrogenation reactions were studied under steady-state operation at pressure 1.72, 3.45, 5.17, 6.89, and 8.62
1142 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 Table 1. Characteristics of the Support and the Prepared Catalyst support
BET surface area, m2/g pore volume, cmVg average pore radius, nm medium pore radius, nm actual t'F content, w t B metallic dispersion
198.9 0.658 6.58 6.0 0.0
catalyst 195.0 0.668 6.62 6.20 0.63 0.924
foo.oo 80.00
i t 1
c
Table 2. Results of Duplicated Experiments at 513 K, 5.17 MPa, and 2.7 h-l LHSV
time,min 95 150 230 311 400 490
mole fraction of product composition x 100% tetralin cis-decalin trans-decalin run1 run2 runl run2 runl run2 0.00 0.68 0.99 1.53 1.68 1.78
0.00 0.22 0.38 1.04 1.45 1.61
17.81 33.07 44.49 44.82 44.80 44.72
13.85 29.41 43.60 45.09 46.08 45.90
82.09 66.25 54.52 53.64 53.53 53.50
86.15 70.37 56.02 53.87 52.47 52.51
MPa, temperature, 473, 493, 513, 533, and 553 K, hydrogedoil ratio 1000 mLN/mL, and LHSV 1.54-8.00 h-l. The liquid samples were withdrawn from the separator with a sampling valve and analyzed by a gas chromatograph (Carle Erba, Model DP700). A 10 m by 0.5 mm fused-silicawide-bore column, type WCOT, with liquid phase CP-SIL-5CB, was used to separate reactants and products. The chromatographic analysis was conducted under temperature programming conditions.
Figure 2. Effect of space time on hydrogenation of naphthalene at 513 K, 3.45 MPa.
mow TETRALIN O~ a0M A 0 CIS-DECALIN TRANS-DECALIN 0 80
0"CrNAPHTHALENE cco33 TOTAL DECALIN
Results and Discussion Catalyst Characterization. The characteristics of the support and the prepared catalyst are shown in Table 1. The textural characteristics of the alumina support did not change during the catalyst preparation. Highly dispersed platinum catalyst was obtained in this work. Its actual content of Pt was 0.63 wt %. Activity Test. Blank experiments were performed by using pure y-Al2O3 as catalyst. There was no observable conversion even under conditions more severe (607 K) than this work in the presence of Pt catalyst. These results confirmed that all the observed reactions were catalyzed by W y - A l 2 0 3 , consistent with the result of Sapre and Gates (1981). In all the catalysis experiments, mass balance obtained from weights of input and output liquid and output gas composition agreed well, i.e., greater than 99%based on weight and time average at steady state. The same batch-prepared catalyst was used for all tests in this work. In each run a new mass of the fresh catalyst was loaded into the reactor; the temperature, pressure, and Hdfeed ratio were kept the same, but the feed flow rates were changed randomly. The first experimental condition was repeated at the end of the series of experiments with the same loading of the catalyst. No significant catalyst deactivation was detected. The reproducibility of experiment was checked by different loading of fresh catalysts. The results are shown in Table 2. The reaction approached steady state after 400 min, and the reproducibility was very good at steady state. The conversion of naphthalene produced three hydrogenated products for all operation variables studied. The conversion and the quantitative products relied on the reaction conditions. The reaction was sequential;
i
'I
2 00.00
0.20 L HS V '(hr) 0.40
0.60
Figure 3. Effect of space time on hydrogenation of naphthalene a t 513 K, 5.17 MPa.
i.e., naphthalene was hydrogenated to tetralin, followed by subsequent hydrogenation t o cis- or trans-decalin (Girgis and Gates, 1991). Figures 2 and 3 show that tetralin could be converted to trans- and cis-decalin separately at low conversion, as reported by Sapre and Gates (1981). However, the interaction between cis- and trans-decalin could not be neglected at higher decalin concentration. A new reaction mechanism and model were proposed to explain the reactions and the selectivity of cis-decalin which contained higher heat combustion energy than the trans form. Reaction Kinetics. The corrections for heat transfer limitations (Mears, 1971) and mass transfer limitations (Satterfield, 1970; Patzer et al., 1979) showed that no significant heat and mass transfer existed within the reactor when the particle size of catalysts was small enough. To avoid the mass transfer limitation, the particle size of catalyst in the reaction system was controlled between 355 and 710 pm. The influence of
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1143 mass transfer limitations would be calculated and checked experimentally as described in the following. According t o this study and the reaction mechanism suggested by Weitkamp (1968),a modified reaction model was proposed in the following:
From eqs 5,8,9,and 10,
transldecalin (TD)
The reaction model was based on the following assumptions: (1)To a good approximation, the reactor was a piston-flow reactor. (2)The reactor operated nearly isothermally. (3)Intraparticle and interparticle transport resistances were negligible. (4)Hydrogen in the reactor was in great excess and constant, so the reversible reactions of hydrogenation were neglected. (5) The change of volume in liquid phase was neglected. Generally, the rate expression of Langmuir-Hinshelwood type should include absorption terms for naphthalene, tetralin, decalin, hydrogen, and solvent. For WAl2O3 catalyst, there are two reaction paths for aromatics hydrogenation. One is that aromatics absorb on the metal surface and react with hydrogen, and the other is that aromatics absorb on the acid sites of the metal-oxide interfacial region and then react with hydrogen spillover from the Pt metal surface (Lin and Vannice, 1993). However, the Langmuir-Hinshelwood model has assumed the active surface is uniform in the reaction. Without the support of spectroscopic and tracer experiments, we did not consider that the reaction mechanism of naphthalene hydrogenation followed the Langmuir-Hinshelwood model. The experimental data in this work were fitted reasonably by the simple powerlaw model. On the basis of the previous assumptions and reaction scheme, the rate equations could be expressed as the following:
where
Parameters k~’, k2’, k3’, k4’, and k-i could be obtained from eqs 9-12, but those were very complicated. In this study, kl’ was much larger than k2’ and k3’ (Sapre and Gates, 1981; Wilson et al., 1985) and k4’ was much larger than k-4’ (Weitkamp, 1968). Let kl’ be infinite and k-4’ be zero. The above equations could be simplified as the following:
and
(3) From eq 14
where
+
where
The sum of rate constants (k2’ k3’) was determined from the slope Of& vs t plot on a semilogarithmic scale. Experimental results at various temperatures and pressures are shown in Figures 4 and 5. The sums of rate constants (k2’ k3’) at four different temperatures (473, 493,513,and 533 K) and five levels of reactor pressure (1.72,3.45,5.17,6.89, and 8.62MPa) were determined. The results showed that there was a well correlated pseudo-first-order reaction profile for the hydrogenation of tetralin and the hydrogenation activity increased with the increase of temperature and pressure. Using the optimal method (Gill et al., 19811,constants B and k4’ could be obtained from z and reactor outlet compound concentrations. The least-square criterion
+
From eqs 4 and 8,
1144 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995
-7.00 O ' O
-2 00
0
Table 3. Temperature Dependency of ki' Values at 5.17 MPa temp, K kz', h-' k3', h-' k4', h-'
h \ 1
\
473 493 513 533
4
-
4-3 00 -
1.031 1.982 3.130 6.650
0.105 0.441 1.600 3.314
Table 4. Pressure Dependency of ki' Values at 513 K press., MPa kz', h-' k3', h-' k4', h-'
\
-2 % Fi
L
2.459 4.078 11.080 24.060
1.72 3.45 5.17 6.89 8.62
0.895 2.204 2.820 3.226 5.890
0.949 5.416 11.080 13.274 23.110
0.414 1.123 1.600 1.542 1.453
4.00
\
I
3.00
1
1
0.20
0.40
0.60
LHS Ir'(hr)
2.00
Figure 4. Pseudo-first-order kinetics for tetralin hydrogenation over WAlzO3 at 5.17 MPa and different temperatures. 0.00
100
7 I
I
I
I
I
'2
L
000
4 -1.00
-
-2.00
-
-1.00
\"\
-2 00
*
-2
-3.00
1.80
L
F: 4-3.00 -
1
2.00
i___\_\_
2.20
1/T X 1000 Figure 6. Arrhenius plot for the rate constants ki'. The lines represent the least squares fit at 5.17 MPa.
-4.00 -
'
1
I
-5.00 0.00
0.20
0.40
I
constants 122' and k~' were calculated from eq 16, and are listed in Tables 3 arid 4. Arrhenius plots for the temperature dependence of the first-order rate constant
0.60
LHS if ' (hr) Figure 5. Pseudo-first-order kinetics for tetralin hydrogenation over PffAlzO3 at 513 K and different pressures.
was used in the objection function L
F = c(BXcD(j)+ exp(-k,'z(j)) - XT(j))2 (17) j=l
The gradients of function 17 are shown by the following:
where j is the number of experimental runs. The parameters kq' and B were estimated by the above equations and the steepest descent method. The
kit = Aiexp(-E,,,/RT)
(20)
are presented in Figure 6, where R is the gas constant. In Figure 6, E , i was calculated from the slope. The apparent activation energies of tetralin hydrogenation to cis- and trans-decalin were 9.88 and 7.25 kcalfmol, respectively. The apparent activation energy of isomerization from cis-decalin to trans-decalin was 14.75 kcaV mol. The activation energy of hydrogen diffusion in liquids has been reported between 3.0 and 4.0 kcalfmol (Odenbrand and Lundin, 1980). Hence the diffusion resistance of hydrogen in liquid film could be neglected in this work. If the activation energy is larger than 6 kcal/mol, it may imply that the operation is in a reaction-controlled region (Bond, 1987). The data further confirmed that the reaction was kinetically controlled. Activation energies of some aromatic compounds on different catalysts have been reported and listed in Table 5, but the data about tetralin have not been found in the literature. The apparent activation energies for the hydrogenation of benzene on supported platinum catalysts reported in the literature range from 7 to 27 kcaVmol (Parmaliana et al., 1984). Bassel et al. (1975)
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1146 Table 5. Activation Energy for Hydrogenation of Aromatic Hydrocarbons over Supported Catalysts catalyst
4.00
activation energy, kcal/mol 16.9 14.3
aromatic type benzene aromatic carbon 13C NMR (Syncrude B) benzocycloparaffms biphenyl benzene benzene benzene benzene benzene benzene benzene I
Wilson et al. (1985) Sapra and Gates (1982) Basset et al. (1975) Gutierrez-Ortiz et al. (1993) Nakano et al. (1982) Lin and Vannice (1993) Lin and Vannice (1993) Chou and Vannice (1987) Chou and Vannice (1987)
10.5 33.8 9.7 17.3 21.6 11.4 11.4 12.6 12.2
1.88 and 1.06,respectively. However, the effect of pressure on isomerization of cis-decalin to trans-decalin is complicated. The reaction order of hydrogen decreased and then approached 0 with increasing pressure. Increasing of pressure increased the concentration of hydrogen in the liquid phase and the amount of hydrogen adsorbed on the catalyst. When the active sites for ,the isomerization of cis-decalin were all occupied by hydrogen, the isomerization reaction rate remained constant with increasing hydrogen pressure. The Langmuir model was used to correlate k3' with pressure:
I
3.00
2.00
;;c
reference Voorhoeve and Stuiver (1971) Wilson and Kriz (1984)
T
1.00
L
Ft
4
0.00
0.1002~~~
k3' = 1
-1.00
-2.00
5.
Figure 7. Effect of hydrogen pressure on the rate constants k/ a t 513 K.
found that the apparent activation energy was 9.5 & 0.5 kcaVmol independent of the preparation methods of the Pt/A1203 catalysts. The data are in good agreement with our experimental results. Miyazawa and Pitzer (1958)have reported that the standard heats of formation for cis-decalin and transdecalin are -40.38 and -43.57 kcdmol, respectively. trans-Decalin is more stable than cis-decalin. The cistype partial hydrogenated intermediate complex of tetralin would be less stable than the trans-type. Therefore, it was reasonable that the activation energy for tetralin hydrogenation to cis-decalin was larger than that to trans-decalin. Conflicting and contradictory reaction orders have been reported. Reaction orders for hydrogen ranging from 0 to 0.5 and even up to 3 have been reported (Parmalian et al., 1984). For a great number of catalytic hydrogenations in the liquid phase over a broad range of hydrogen pressure, the reaction order of hydrogen changed from 1 to 0 with increasing pressure (Gut et al., 1986;Gutierrez-Ortiz et al., 1993). By applying an empirical power law to eq 7, let
(21) where an is the empirical exponent of hydrogen. The exponents were determined from the slope of In k,' vs In P. Figure 7 shows that the reaction orders of hydrogen for tetralin to cis-decalin and trans-decalin are
+ 0.065PH3
(22)
The result predicted from the Langmuir model is also shown in Figure 7. Tetralin contains an aromatic ring which could strongly adsorb on catalyst metal active sites or different active sites and might be a different reaction mechanism. The effect of hydrogen on tetralin hydrogenation was simpler than on decalin isomerization. The different empirical exponents of hydrogen between cis-decalin and trans-decalin was explained by the reaction steps suggested by Weitkamp (1968),as shown in Figure 8. Evidence from the deuteration of olefins was strongly against the trans-addition hydrogen mechanism. Some evidence showed that olefins played an important rule in the reaction scheme. Tetralin reacted directly with hydrogen to form cis-decalin at high pressure, or through the intermediate octalins. But tetralin could not convert directly into trans-decalin; it needed to convert to A1.9-octalin and then to transdecalin. Therefore, the influence of hydrogen partial pressure for the formation of trans-decalin (reaction order of hydrogen is 1.06)would be less than for cisdecalin (reaction order of hydrogen is 1.88). Instead of naphthalene, cis-decalin was used as a reactant in the same reaction system and same reaction condition to check the isomerization of cis-decalin. From experimental results, it was evident that cis-decalin easily isomerized t o trans-decalin in the same reaction condition and catalyst. The rate constant of isomerization was determined from the slope O f X C D vs z plot on a semilogarithmic scale. Figure 9 shows the results of cis-decalin isomerization a t 5.17 MPa and two different temperatures, 493 and 513 K. The rate constants were 5.4 and 11.77 h-l for 493 and 513 K, respectively. According to the Arrhenius law, the activation energy was 19.8 kcavmol, which matched the above experimental result. The rate constants obtained in this experiment were about 1 order larger than the above
1146 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
Figure 8. Road map for the hydrogenation of naphthalene to decalin. 0.00
\
I
I
I
1
T=5 13K v q p e o T=493K
QDQQQ
-I
50
nn
-3.00
It
1
- 4 . ‘8o o
0.40
0.60
0.20
L HS Ir (hr)
Figure 9. Pseudo-first-order kinetics for cis-decalin isomerization over PffAl~O3at 5.17 MPa and different temperatures.
Table 6. Comparison of Experimental kj’ with Predicted ki’ at 503 K and 4.31 MPa k.’ I , h-1 exptl predicted 6.043 7.121 k z‘ k3’ k4’
2.296 0.741
2.601 0.921
results. Because the naphthalene and tetralin were more easily adsorbed on catalyst than tetralin, they acted as poisons to the isomerization of decalin. Therefore, pure cis-decalin had a higher rate constant of isomerization than in the naphthalene hydrogenated system. The curves in Figures 2 and 3 are the results calculated from the proposed reaction model, and the experimental data are also indicated. Another experiment at 503 K, 4.31 MPa, and 1000 cm3~/cm3 Hz/oil ratio was done to check the reaction model. The experimental rate constants ki’ were compared with the values predicted from eqs 20-22, and are shown in Table 6. As can be seen from Table 6, the error was about 15%. Hence the proposed reaction model is reasonable. Mass transfer resistance was checked experimentally with a diagnostic procedure suggested by Koros and Nowak (1967) by varying the active metal content of the catalyst pellets, all the other parameters being constant. If the reaction rate varied linearly with the metal content (active size), no internal and external mass transfer limitations were expected (Gianetto and Spec-
-10 I 0.0
I
I
1.0
2.0
Pt CONTENT OF CATALYST ( W t W ) Figure 10. Relation of the rate constant of tetralin hydrogenation and Pt content of catalyst.
chia, 1992). The relation of the hydrogenation rate constant of tetralin and Pt content from experimental results in the same reaction system is shown in Figure 10. At Pt metal content lower than 1.0 wt %, the relation of rate constant and metal content is nearly linear. When the Pt metal content was higher than 1.0 wt %, the reaction rate constant decreased due to the decreasing of metal dispersion. In this study, the Pt content was 0.6 wt %; therefore its diffusion resistance could be neglected. The experimental Thiele modulus was also checked by the following equation (23)
At the highest reaction temperature 533 Kin this study, the sum of rate constants (Kz’ + k3’) was 30.71 h-l, the density of catalyst particle gp was 0.72 g/cm3, and the effective diffusivity of liquid reactant Del was 6.91 x c d s (Prasher et al., 1978). Therefore 9 I 1 and the intraparticle diffusion could be neglected. External liquid film mass transfer was not a significant resistance unless the following inequality held (Satterfield, 1975):
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1147 Table 7. Effect of Temperature and LHSV-l on Selectivity of cie-Decalinto hm-Decalin in Products of Naphthalene Hydrogenation at 5.17 MPa
LHSV-' (h) 0.611 0.364 0.199 0.630 0.376 0.210 0.627 0.371
temp, K 553 553 553 533 533 533 513 513
mole fraction of total decalin (XCD + XTD) 0.985 0.985 0.985 1.000 1.000 1.000 1.000 0.989
XcdXm 0.144 0.191 0.382 0.171 0.366 0.695 0.530 0.860
Table 8. Effect of Pressure and LHf4V-l on Selectivity of cie-Decalin for Naphthalene Hydrogenation at 513 K mole fraction of total decalin press., MPa 3.45 5.17 5.17 6.89 6.89 8.62 8.62
LHSV-', h 0.647 0.371 0.627 0.369 0.636 0.374 0.637
+ XTD)
(XCD
0.985 0.989 1.000 1.000 1.000 1.000 1.000
XcdXm 0.57 0.86 0.53 0.97 0.62 0.97 0.67
The mass transfer coefficient kl, through the liquid film may be approximated by the following equation:
kls = 2De1ah From eq 25,kl, was estimated to be about 3.86 x c d s . At the highest temperature of this study, the lefthand side of eq 24 was equal to 4.17 x cds. Therefore the external mass transfer could be neglected in this work. According to eq 13 and eq 15, the selectivity of cisdecalin in the nearly saturated naphthalene could be obtained from the following equation:
XTD
-1
+ (exp(-(lz,' + k,')t) -
Therefore, when XT was close t o zero, the selectivity of cis-decalin decreased with increasing space timehemperature. The results are shown in Table 7. To get the maximum heat combustion energy value of hydrogenated naphthalene products, the space time would approach 4/(kz' kg'). The effect of pressure on the selectivity of cis-decalin in this reaction was complicated as shown in Table 8 and eq 26. At high pressure, more than 5.17 MPa, the calculated selectivity, of cis-decalin from eq 26 increased by increasing pressure. The same trend is shown in Table 8.
+
Conclusion To improve the knowledge of the hydrogenation reaction of aromatic compounds in diesel fuel, a model diaromatic compound (naphthalene) dissolved in inert solvent (n-hexadecane) was studied over WAl2O3 catalyst in a trickle bed reactor. The influence of temperature and pressure on hydrogenation activity has been described by a simple reaction network which agreed well with the experimental data. According to the
proposed reaction model, the hydrogenation rate constants and isomerization rate constant have been calculated. The apparent activation energies for tetralin hydrogenation to cis-decalin and trans-decalin were found to be 9.88 and 7.25 kcdmol, respectively. These values are consistent with those reported in the literature. For isomerization of cis-decalin to trans-decalin, the activation energy calculated from the proposed model was 14.75kcal/mol, which matched the value obtained from the experiment (19.8kcal/mol) of pure cis-decalin isomerization in the same condition. The reaction orders of hydrogen for the hydrogenation of tetralin to cis- and trans-decalin were 1.88and 1.06, respectively. For the isomerization of cis-decalin to trans-decalin, the reaction order of hydrogen decreased with increasing pressure and then approached 0 at pressure higher than 5.17 MPa. By application of the reaction model, the highest combustion energy of hydrogenated naphthalene can be obtained without increasing the operation cost and hydrogen consumption.
Acknowledgment The authors wish to express their gratitude for the support of this work by the Refining and Manufacturing Research Center of Chinese Petroleum Coorporation.
Nomenclature a = external surface area of catalyst per volume of catalyst bed, cm2/cm3 A, = preexponential factor C = concentration of liquid reactant, mol/cm3 D = metal dispersion D. = axial dispersion, cm2/s Del = effective diffusivity of liquid reactant, cm2/s d, = particle diameter, cm E , , = activation energy, kcal/mol fn(PH) = relation of hydrogen pressure with reaction rate j = the number of experiment run kc = kc'/fn(PH) 12,' = rate constant, h-' 121, = mass transfer coefficient for overall transfer through the liquid, c d s n = reaction order P = pressure, MPa P H = partial pressure of hydrogen, MPa R = gas constant, kcal4mol.K) r = reaction rate, moY(s.cm3) T = temperature, K V = superficial velocity, c d s Xi = mole fraction of component i in products 2 = total bed length, cm E = porosity of the bed Qp = catalyst particle density, g/cm3 z = (LHSV1-l = space time, h = Thiele modulus C#J
Abbreviations CD = cis-decalin LHSV = liquid hourly space velocity N = naphthalene T = tetralin TD = trans-decalin
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