Energy & Fuels 1994,8, 1149-1151
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Hydrogenation of Mononuclear Aromatics over a Sulfided NiMo/AlaOa Catalyst Teh C . Ho* Corporate Research Laboratories, Exxon Research and Engineering Co., Annandale, New Jersey 08801 Received April 20, 1994. Revised Manuscript Received June 17,1994@
The kinetics of hydrogenation of m-xylene and tetralin were determined on a commercial sulfided NiMo on y-Al203 catalyst a t 7.0 MPa and over 280-310 "C. The experiments were done in a fured-bed unit under pseudo-first-order kinetic conditions. It is shown that the activation energies for hydrogenation of tetralin and m-xylene are comparable, and the hydrogenation rate of tetralin is about twice that of m-xylene. This approximate 2-fold difference in reactivity is comparable to that observed with unsupported MoSz reported previously under more severe conditions.
Introduction The importance of aromatics hydrogenation perhaps needs no special articulation. Suffice it to say that aromatics hydrogenation is an integral part of commercial hydroprocessing process and that environmental pressures are tightening the aromatic content in distillate fuels. While studies of the catalysis of heteroatom removal have been amply reported, relatively less attention has been paid t o aromatics hydrogenation. It is well-known that hydrogenation of mononuclear aromatics (or monoaromatics for short) is much slower than that of multiring aromatics. For instance, Sapre and Gates1 have shown that in naphthalene hydrogenation, post hydrogenation of tetralins to decalin is some 30 times slower than the hydrogenation of naphthalene to tetralins on a sulfided CoO-MoO3/A1203 catalyst at 325 "C and 7.5 MPa. This is why the liquid products from commercial hydroprocessing very often have a higher monoaromatic content than the feed.2,3In other words, monoaromatics hydrogenation is the bottleneck in the cascade hydrogenation of multiring aromatics. Using high temperature counters thermodynamics and using high pressure counters economics. How to enhance the activity of metal sulfide catalysts for monoaromatics hydrogenation, therefore, is a fertile area of research. In this regard, it should be pointed out that conventional hydrotreating catalysts were developed primarily for heteroatom removal, rather than for aromatics hydrogenation. Accordingly, there is a need to gain a better understanding of the relative reactivities of different monoaromatics. Systematic studies of this topic have been comparatively scarce. Almost all early work4 was conducted in autoclave reactors under rather severe conditions (15-22 MPa and 400-420 "C) on obsolete
* E-mail:
[email protected]. Abstract published in Advance ACS Abstracts, August 1, 1994. (1)Sapre, A. V.;Gates, B. C. Ing. Chem. Process Des. Dev. 1981, 20,68-73. (2)Ho,T.C. Ind. Eng. Res. 1993,32, 1568-1572. (3)Stanislaus, A.; Cooper, A. H. Catal. Rev.-Sci.Eng., 1994,36(1), 75-123. (4)Weisser, 0.; Landa, S. Sulfided Catalysts, Their Properties and Applications; Pergamon Press: London, 1973;p 127. @
catalysts. Recent studies, which were summarized in the reviews by Stanislaus and Cooper3 and Girgis and Gates,5 addressed the effect of substituents on benzene hydrogenation over sulfided NiMo/AlzO3 and NiW/Al203 catalysts. This work was undertaken t o determine the kinetics of individual hydrogenations of m-xylene and tetralin under relatively mild conditions. These compounds were selected as model compounds for alkylbenzenes and hydroaromatics, respectively. Another reason for choosing these compounds is that their reactivities appear to be catalyst dependent. As summarized in Weisser and Landa,4 tetralin is more reactive than m-xylene over unsupported MoS2, but the reverse is true over unsupported WSz. With Ni metal supported on AlzO3, the two have about equal reactivity. In this study, we used a present-day commercial NiMo sulfide on y-Al2O3 catalyst. The experiments were done in a fured-bed unit under pseudo-first-order kinetic conditions. It will be shown that under the conditions used, the activation energies for hydrogenation of tetralin and m-xylene are comparable, and the hydrogenation rate of tetralin is about twice that of m-xylene.
Experimental Section Catalyst. The composition and physical properties of the catalyst are NiO, 3 w t %; MOOS, 16 wt%; surface area, 180 m2/g; and pore volume, 0.5 cm3/g. Prior t o use, the catalyst particles were sized to 20-40-mesh granules and then sulfided with a 10% HzS-in-Hz gas at 360 "Cand atmospheric pressure for 1 h. Procedure and Analysis. The activity tests were conducted in a n automated unit consisting of two independent upflow reactors in a common sand bath. Each reactor was equipped with a calibrated feed buret, pump, gas-liquid separator, and product liquid collector. The reactor was made of a Vs-in. i.d. 316 stainless steel pipe. The reactor pressure, temperature, and hydrogen flow rate were all controlled by a computer. Each reactor was packed with 20 cm3of presulfided catalyst in the central zone and in. alundum in the fore and aft zones. The packing density of the catalyst bed is 0.8 g/cm3. The reactor was essentially isothermal, as indicated by four equally spaced thermocouples across the bed. (5)Girgis, M.J.;Gates, B. C. Ind. Eng. Res. 1991,30,2021-2058.
0887-0624/94/2508-1149$04.50/0 Q 1994 American Chemical Society
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1/LHSV Figure 1. Hydrogenation of tetralin at 7.0 MP,: 280 "C (01, 310 "C (A);C denotes normalized concentration; solid lines are model prediction.
1/LHSV Figure 2. Hydrogenation of m-xylene at 7.0 MP, and 310 "C: concentration of m-xylene (m), concentration of dimethylcyclohexane (A).C denotes normalized concentration; solid lines are model prediction.
All chemicals were purchased from Aldrich Chemical Co. Two different feeds of equal density (0.74 g/cm3 at room temperature) were used, one containing 7.4 wt % tetralin (T) and the other 6.1wt % m-xylene (X).The carrier solvent used was hexadecane. To maintain the catalyst in a sulfide state, both feeds contained 0.05 wt % CS2. Two sets of reaction conditions, both at 7.0 MPa, were used: one at 280 "C and the other 310 "C. The liquid hourly space velocity (LHSV), based on catalyst volume and the inlet conditions, was varied to obtain the concentration profiles for reactants and products. In all runs, a large excess of pure hydrogen was used, corresponding t o a gas-to-liquid ratio of 32 kmol Hdm3 liquid feed. The liquid products were quantified on an HP gas chromatograph. There were no signs of catalyst deactivation throughout the study.
Results Hexadecane was essentially inert under the reaction conditions. The major reaction products were decalin (D), dimethylcyclohexanes (DMCH), and trimethylpentanes (TMCP). These products accounted for more than 98% of the reactants converted. No isomerization of m-xylene was found, as expected from the catalyst and conditions employed. The rate data were modeled by apparent first-order kinetics of the following scheme: kt
T-D
kxl
X
kx2
DMCH
TMCP
(2)
With this simple approach, the relative hydrogenation rate is measured solely by the overall rate constant. The hydrogen concentration term is incorporated into the rate constant, since hydrogen concentration is approximately constant across the reactor. For m-xylene hydrogenation, k,l was determined by fitting the data of m-xylene disappearance. The resulting k,1 was then used to find the rate constant k d for the isomerization reaction. Note that under the conditions used, dehydrogenation was found t o be negligibly slow based both
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1ILHSV Figure 3. Hydrogenation of m-xylene at 7.0 MP, and 280 "C:
concentration of m-xylene (m), concentration of dimethylcyclohexane (A). C denotes normalized concentration; solid lines are model prediction.
on equilibrium considerations3 and data fitting using reversible scheme. Shown in Figures 1-3 are the measured (symbols) and predicted concentrations (solid lines), each as a function of l/LHSV (cm3 cat:h/cm3 of oil). Here the concentrations of reactants and products are normalized with respect to the feed concentration. One can clearly see that tetralin hydrogenates faster than m-xylene. Table 1 summarizes the pseudo-first-order rate constants (in units cm3 of oil/cm3 of cat./h) and the reactivity ratio r defined as
r
kJkXl
(3)
The temperature dependence of r is rather weak, indicating that kt and k , ~have comparable overall apparent activation energies. From Table 1the activation energy for kt was calculated to be 14 kcallmol, compared with 11kcal/mol for k,I. Over the 280-310
Energy & Fuels, Vol. 8, No. 5, 1994 1151
Hydrogenation of Mononuclear Aromatics Table 1. Pseudo-First-Order Rate Constants" 280°C 310°C
kt 2.40 f0.12 4.64 f0.15
kzl
1.38f0.01 2.17f0.10
kzz
0.27 f 0.03 0 . 8 9 3 ~0.14
r 1.74 2.14
Rate constants a r e in units cm3 of oil(cm3 of cat./h)
"C range, r is roughly 2. It is unlikely that pore diffusion limits the reactions. From the value of K t at 310 "C, the catalyst effectiveness factor6 7 can be estimated through trial and error. For an effective cm2/s and an average diffusivity of the order of spherical particle radius of 0.032 cm, 7 is close to unity. The approximate 2-fold difference in reactivity is comparable to that observed with unsupported M o S ~ , ~ (6) Froment, G. F., Bischoff, K. B. Chemical Reactor Analysis and Design; John Wiley: New York, 1990.
suggesting that the addition of Ni primarily increases the number of active sites. It should also be pointed out that with a sulfided NiMo/AlzOs catalyst, Magnabosco7 estimated from the data for a jet fuel that the hydrogenation rate of tetralin is roughly twice that of benzene homologues a t 350 "C and over the 3-10 MPa pressure range.
As alluded to earlier, the hydrogenation reactivities of m-xylene and tetralin are catalyst dependent. This observation cannot be solely explained by Jt-electron density associated with the aromatic ring. Further work is needed. (7)Magnabosco, L.M. In Studies in Surface Science and Catalysis; Trimm, D. L., Akashah, S., Absi-Halabi, M., Bishara, A., Eds.; Elsevier: New York, 1990; Vol. 53, pp 481-496.
