2241
Ind. Eng. C h e m . R e s . 1987,26, 2241-2245
Model discrimination criteria confirmed the better fit of the Eley-Rideal model compared to the Mars-Van Krevelen type. Design calculations for technical reactors showed quite reasonable yields for the fiied bed as well as fluidized bed type with the developed catalyst.
Subscript But. = n-butane Registry No. CO, 630-08-0; COz, 124-38-9; C4HI0,106-97-8; maleic anhydride, 108-31-6; vanadium oxide, 11099-11-9; phosphorus oxide, 12640-86-7.
Nomenclature
Arnold, S. C.; Suciu, G. D.; Verde, L.; Neri, A. Hydrocarbon Process. 1985, Sept, 123-126. Berty, J. M. Cat. Rev.-Sci. Eng. 1979, 20(1), 75. Centi, G.: Fornasari, G.: Trifiro, F. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 32-37. Emig, G.; Hoffmann, U.; Hofmann, H. Manual Dechema-course, 1975: Erlanaen, West Germany. Escardino, A.;Sola, C.; Ruiz, F. An. Quim. 1973, 69, 385, 1157. Haber, J. Z . Chem. 1973, 13, 241. Hodnett, B. K. Cat. Rev.-Sci. Eng. 1985,27(3), 373-424. Hofmann, H. Chem.-1ng.-Tech. 1979, 51, 257. Hofmann, H.; Emig, G.; Roder, W. EFCE Public. Ser. No. 37; Pergamon: (ISCRE8) Edinburgh, 1984; pp 419-426. Kazansky, V. B.; Shvets, V. A. J. Catal. 1972,25, 123. Marquardt, D. W. SOC. Znd. Appl. Math. J. 1963,11, 431-441. Mars, P.; Van Krevelen, D. W. Chem. Eng. Sci. 1954,3,(spec. suppl.), 41. Moser, T. P.; Schrader, G. L. J . Catal. 1985, 92, 216-231. Nelder, J. A.; Mead, R. Comput. J . 1965, 7, 308-313. Rideal, E. K. Chem. Ind. 1943,62, 355. Schneider, P. Ph.D. Thesis, University of Erlangen-Nurnberg, 1985. Schneider, P.; Emig, G.; Hofmann, H. Chem.-1ng.-Tech. 1985, 57, 728-736. Sitzmann, W.; Werther, J.; Bock, W.; Emig, G. Ger. Chem. Eng. 1985,8, 107-301. Volta, J. C.; Portefaix, J. L. Appl. Catal. 1985, 18, 1-32. Williams, E. J. Regression Analysis; Wiley: New York, 1959.
dh = particle height, m d, = particle diameter, m d, = tube diameter, m E . = activation energy, kJ/kmol FiD = flame ionization detector G = mass velocity, kg/(m2-s) AHR = reaction enthalpy, kJ/kmol HSY = hourly space yield koj = preexponential factor, kmol/ (kg of catalyst-s-Pa) k . = reaction rate constant, kmol/(kg of catalyst.s.Pa) Ifi = sorption constant, Pa-' 1 = unoccupied site L = length of reactor, m MAN = maleic anhydride p i = partial pressure, Pa Po = total pressure at reactor inlet, Pa r = rate of reaction, kmol/(kg of cata1yst.s) = reactor volume S = selectivity T = temperature, K TCD = thermal conductivity detector x i = mole fraction X = conversion Y = yield
AV
Greek Symbols 6 = surface coverage pbed =
bed density, kg of catalyst/m3 RV
Literature Cited
Received for review June 9, 1986 Revised manuscript received June 29, 1987 Accepted July 29, 1987
Kinetic Study on the Hydrotreating of Heavy Oil. 1. Effect of Catalyst Pellet Size in Relation to Pore Size Satoru Kobayashi,* Satoshi Kushiyama, Reiji Aizawa, Yutaka Koinuma, Keiichi Inoue, Yoshikazu Shimizu, and Kozo Egi National Research Institute for Pollution and Resources, 16-3, Onogawa, Yatabe-mtlchi, Tsukuba, Ibaraki 305, Japan
In order to investigate the relation between hydrodemetalation of heavy oil and the catalyst pellet size, reactions were carried out by a batch reactor using four kinds of commercial HDS catalysts with different pore size distributions. In the vicinity of 1-mm catalyst pellet size, demetalation activity was significantly affected by pellet size. However, as the pellet size decreased, the activity reached a constant level. These results were analyzed by a simple equation including the Thiele modulus, and it was revealed that the equation can be applied even to such complicated reactions as residue hydrodemetalation. By use of the Thiele modulus, it was estimated that the order of the magnitude of the diffusion coefficient (De) was cm2/s for the metal compounds in Khafji residual oil a t 400 "C. Residual oils usually contain a high proportion of sulfur and metal contaminants. These not only contribute to the problems of air pollution but also cause major problems in hydrorefining and catalytic cracking operation. Therefore, many processes to remove sulfur and metals from crude residues have been developed. Various investigators have studied the kinetics of the hydrodesulfurization (HDS) and hydrodemetalation (HDM) of residues. As for HDS, it has been established that the rate of sulfur removal can be expressed as second order with respect to the total sulfur concentration in
residues (Beuther and Schmid, 1963; Ohtsuka et al., 1967; Shimizu et al., 1970; Kosugi and Yoshizawa, 1978). On the other hand, first (Chang and Silvestri, 1974; Riley, 1978) and second (Beuther and Schmid, 1963; Oleck and Sherry, 1977) kinetic orders have been reported for HDM. Many researchers have explained these results by simultaneous first-order kinetic model (Beuther and Schmid, 1963; Kosugi and Yoshizawa, 1978) or reactor performance (Mears, 1974; Paraskos et al., 1975). It is well-known that residue hydrorefining is more or less influenced by intraparticle diffusion limitations, es-
0888-5885/87/2626-2241$01.50/0 0 1987 American Chemical Society
2242 Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 Table I. Properties of Khafji Atmospheric Residue V, ppm 87.0 Ni, ppm 24.4 asphaltene, wt 70(n-pentane insoluble) 11.89 s. wt 7 c 4.03 CCR. wt % 12.05 specific gravity (25 O C / 2 5 "C) 0.9631 kinetic viscosity a t 50 "C, cSt 503 Table 11. Physical and Chemical Properties of Catalysts surface pore mean pellet area, vol, pore density, Mo, Co, name m2/g cm3/g dia., A g/cm3 wt % wt % A-57 240 0.34 57 1.36 5.7 2.8 250 70 B-70 0.44 1.26 6.9 2.7 B-126 175 0.55 126 1.08 6.5 2.5 C-175 160 0.70 175 1.20 11.3 1.8 Pore Diameter, A Figure 1. Pore size distribution of catalysts.
pecially for HDM, and this has been clearly shown from the observation of the intraparticle metal deposition profile of used catalyst by X-ray microanalyzer (Todo et al., 1971; Sato et al., 1971; Tamm et al., 1981). Therefore, in addition to the intrinsic activity of catalysts, the physical properties such as pore size and pellet size must also influence not only the removal of sulfur and metals but also the kinetic expression. There have been reported, however, only a few detailed kinetic studies which take into account the physical properties of catalysts (Shimura et al., 1982). Therefore, in this series of studies, we have carried out experiments changing the physical properties of catalysts for the purpose of considering a kinetic equation which better expresses the result of residue HDS and HDM. By the way, catalyst pellet size is a significant factor for the design of reactors. HDS and HDM conversions are largely influenced by catalyst pellet size based on such effects as reactor performance (in this paper, this term means the influence of liquid holdup, catalyst wetting, and backmixing in trickle-bed reactors) and intraparticle diffusion of catalyst. In the trickle-bed reactor, we cannot investigate solely the influence of intraparticle diffusion without considering the effect of reactor performance. However, in the batch autoclave reactor, we need not consider the effect of reactor performance, so we can obtain properly the relation between catalyst effectiveness factor and pellet size. In this paper, consequently, we will describe the result of the batch autoclave experiments on the influence of pellet size of catalysts. Since the effect of pellet size is supposed to be a function of pore size, experiments were carried out with several catalysts differing in pore size.
catalysts were crushed by a coffee mill and sieved into 12/14 (mean pellet diameter 1.3 mm), 16/20 (0.92), 32/35 (0.46),48/70 (0.25) and 100/200 (0.11) mesh by JIS sieves. This crushing procedure did not alter the main catalyst properties such as pore volume, surface area, pore size distribution, and chemical composition. (3) Analysis. Vanadium and nickel were determined by inductively coupled plasma spectrophotometry. Asphaltenes were precipitated from sample oils with 30 parts rz-pentane to 1part sample. Sulfur was determined by an X-ray fluorescence method.
Experimental Section (1) Experimental Procedure. Reactions were carried
Results and Discussion (1) Catalyst Pellet Size and Activity. The relation-
out by use of a 300-mL autoclave, in which 70 g of Khafji residual oil (see Table I for its properties) and 5 g of catalyst were loaded. Standard reaction conditions included a temperature of 400 "C, initial hydrogen pressure of 100 kg/cr,i2 at 25 OC, and stirring rate of about 800 rpm. The heating and cooling rate were both ca. 7 OC/min. Reaction time, which was 60 min, was defined as the period from the time when the reactor temperature reached 400 "C to the beginning of cooling down. For kinetic study, reactions were carried out for 0 and 60 min at 400 "C, using catalysts C-175 of all pellet sizes. Further, at pellet sizes of 1.3- and 0.11-mm catalyst (2-175, reactions were also carried out for 30 min. (2) Catalysts. Four Co-Mo/A1203 HDS catalysts differing in pore size distribution were used. These were the ones obtained from three commercial sources. Their properties are shown in Figure 1 and Table 11. These
ship between vanadium removal and catalyst pellet size is shown in Figure 2. The figure reveals that the activities decrease with increasing catalyst pellet size in a range of sizes larger than 0.46 mm of catalysts A-57, B-126, and (2-175 and in almost the whole range of sizes of catalyst B-70. This suggests that the reaction of vanadium removal is limited by the intraparticle diffusion; that is, the vanadium compounds contained in large molecular asphaltenes are very difficult to diffuse into pores of catalyst, and the reaction of vanadium removal takes place mostly in the vicinity of the catalyst surface. Catalysts A-57, B-126, and C-175 of less than 0.45-mm pellet size show constant activities. This suggests that, in the range of pellet size, the diffusion rate is large enough compared to the reaction rate so that the whole catalyst is effectively utilized. In the case of catalyst B-70, the pellet size where the activity reaches a constant seems to be much smaller
1 GOT-
C\
-
________
_.
0 2
IO
I
C a t o l y s t Pel l e t Size, cm Figure 2. Relationship between vanadium removal and pellet size.
Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 2243
2.0
/
c
/
40t
30 1
Reaction Time, min.
I
I
lo-'
10-2
Catalyst Pel let Size, cm Figure 3. Relationship between nickel removal and pellet size.
I
C-175
i
?
I
i I
L lo-' C a t a l y s t Pel l e t Size, cm
10-2
Figure 4. hlationship between asphaltene removal and pellet size.
than that of the other catalysts. This may be because catalyst B-70 contains only small-sized pores and no detectable amount of macropores. In contrast, catalyst A-57 has pores of about 25% total pore volume in the range above 200 A diameter, in addition to pores of equal to or even smaller than the size of catalyst B-70 (the pores cannot be seen in Figure 1 since the figure is shown by differentials). This may be the reason why catalyst A-57 is influenced much less by intraparticle diffusion. The relationship between nickel removal and catalyst pellet size is similar to that of vanadium removal, although the activity is lower as shown in Figure 3. Nickel removal is also limited by pore diffusion, because nickel is contained in large asphaltene molecules in the same manner as vanadium. The activity of catalysts A-57 and B-126 of 0.11 mm is a little lower than that of 0.25 mm. These phenomena are also observed in the reaction of vanadium removal. Although the difference is not regarded as experimentally significant, it is conspicuous in the simultaneously proceeding desulfurization reaction. This will be discussed in detail in a separate report. Figure 4 shows the results of the removal of asphaltenes. The figure reveals that the pellet size of catalysts A-57, B-126, and C-175 where the activity becomes constant is larger than that in the reaction of vanadium and nickel removal. This might be because the ratio of the true reaction rate to diffusion rate in the former reaction is smaller than that in the latter reactions. This is incon-
Figure 5. First-order plot of reactions using catalyst C-175:( 0 0 ) DV reaction, (AA)DNi reaction, ( 0 . ) DAs reaction.
sistent, however, with the fact that the demetalation reaction proceeds almost concurrently with the removal of asphaltenes in which vanadium and nickel are concentrated. This matter will be described here. (2) Kinetic Equation. The reaction rate using a heterogeneous catalyst is defined as -(W/dt)(l/ WcS),from which the rate equation of the pseudo-nth order reaction in the batch reactor system is obtained as
In the reaction system used for this study, since the reaction proceeds also during the periods of heating and cooling of the reactor, the reaction during these periods relative to the total reaction cannot be neglected. It is difficult, therefore, to calculate accurately the rate constant at the reaction temperature. However, if the concentration of the reactant at the time when the temperature reaches the predetermined value and that at the time when the temperature begins to decrease are known, the reaction can be investigated based on eq 1. Those concentrations can be determined by use of the results of the 0-min reactioq, in which the rates of heating and cooling are the same as the usual reactions, based on the concept described in the Appendix. Assuming that the change in the rate constant with time is negligibly small, integration of eq 1 in the case of first-order reaction gives
where Co is the concentration of reactant at 0 min at a predetermined temperature and C is the concentration of reactant after 30 or 60 min at a predetermined temperature. Figure 5 shows the plots of f i s t order for the removal of vanadium, nickel, and asphaltenes using catalyst C-175 of sizes 1.3 and 0.11 mm. The figure reveals that all of the reactions can be expressed by first-order equations. (3) Discussion with the Thiele Modulus. Where the intraparticle diffusion affects the overall reaction, the reaction is expressed by the Thiele's equation (eq 3)
?(--A)
Ef= k = 1 ki f tanh f
f
(3)
2244
Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987
Table 111. Intrinsic Rate Constants and Effective Diffusion Coefficients for Demetalation and Asphaltene Removal DV DNi DAs i09ki,cm/s 5.76 4.51 3.93 1 0 ' ~ e ,cm2/s 7.86 7.22 18.03
__I
0 21 001
002
005
01
0 2
Cotalyst P e l l e t S i z e , cm
Figure 8. Relationship between catalyst effectiveness factor and pellet size in asphaltene removal.
c L
W
021-
i 001
002
005
0 ,
02
Catalyst Pellet Size, cm
Figure
6. Relationship between catalyst effectiveness factor and pellet size in vanadium removal. Reaction Time,min I1 1
Reaction Time,min I21
Figure 9. Step of reaction: (1) 0-min reaction, (2) t-min reaction.
asphaltene removal reaction. The reason may be that the contribution of thermal reaction is fairly large in batch reactions, since the residence time of reactants comes to be inevitably long in batch reactions as compared to continuous flow reactions.
Conclusions -_
001
002
005
01
02
Catalyst Pellet S i z e , cm
Figure
7. Relationship between catalyst effectiveness factor and
pellet size in nickel removal.
(Wheeler, 1951) when intrinsic kinetic order is 1. In this section, the applicability of the equation to HDM will be discussed for the case of catalyst C-175. In eq 3, the rate constant (k)for each pellet size can be calculated from eq 2, and the values of S and d, are known. When these values were adapted to eq 3, De and kifor each reaction were numerically calculated by means of a nonlinear least-squares method (Gauss-Newton method). The results are shown in Table 111. In Figures 6-8 are also shown the theoretical relations between Ef and R , which were obtained from the values in Table I11 and eq 3, together with the experimental data points. As for the DV and DNi reactions, the order of De values is cmz/s. This is in good accordance with the value reported by Newson (1970) for the diffusion of vanadium compound in Middle East Crude. Further, the experimental data in Figures 6 and 7 are distributed fairly near the theoretical curves. These results indicate that the simple relation expressed in eq 3 can be applied even to such complicated reactions as residue HDM. On the other hand, De for the reaction of asphaltene removal is very much different from those for the DV and DNi reactions. It is hard to understand considering the fact that most of the vanadium and nickel compounds in the feed are concentrated in asphaltene. This and the large discrepancy of the experimental points in Figure 8 from the theoretical curve suggest that eq 3 is inapplicable to
Hydrodemetalation of residual oil was carried out by a batch autoclave reactor using several kinds of catalysts differing in pore size. The results were analyzed in terms of the relation between the activity and the pellet size of catalysts. The following conclusions were obtained. (1) HDM activity was significantly affected by the change in pellet size, when commercial HDS catalysts of 1-2-mm pellet size are used: (2) The reaction rate of vanadium and nickel removal can be satisfactorily expressed by eq 3, while the equation cannot be applied to the reaction of asphaltene removal. (3) The value of De was estimated by numerical calculation using the apparent reaction rate constants (k)for each pellet size and eq 3. The order of magnitude of lo-' cm2/s was obtained for the metal compounds in Khafji residual oil at 400 "C.
Nomenclature C = concentration of reactant, wt % OF ppm De = effective diffusion coefficient, cm2/s d, = density of oil, g/cm3 d = pellet density of catalyst, g/cm3 I 8= effectivenessfactor, dimensionless f = Thiele modulus, dimensionless k = rate constant for first-order reaction, cm/s ki = intrinsic rate constant for first-order reaction, cm/s MW = molecular weight of reactant, g N = moles of reactant, mol n = reaction order, dimensionless R = particle radius of catalyst, cm S = internal surface area of catalyst, cm2/g t = reaction time, s or min V , = reaction volume, cm3 W , = catalyst weight, g
I n d . Eng. Chem. Res. 1987,26, 2245-2250
2245
atn=1
Appendix The reaction which proceeds during the period from tl to t 2 in Figure 9, plot 2, can be expressed by the conventional equation for pseudo-nth order reaction as
In C4/C, = k(tz - tl) atn # 1 c41-n
- C31-n = k(t,
- tl)
(A7)
Literature Cited As for the reactions during the heating and the cooling steps shown in Figure 9, the following equations can be applied:
Beuther, H.; Schmid, B. K. 6th World Petroleum Congress, Frankfurt, Germany, June 1963, Section 111, Paper 20, p 297. Chang, C. D.; Silvestri,A. J. Ind. Eng. Chem. Process Des. Deu. 1974, 13, 315.
Kosugi, M.; Yoshizawa, T. J. J p n . Pet. Inst. 1978,21, 199. Newson, E. J. Prep.-Am. Chem. Soc., Diu. Pet. Chem. 1970,15(4), A141.
Mears, D. E. In Chemical Reaction Engineering II; Hulburt, H. M., Ed.; ACS Monograph Series 133; American Chemical Society: Washington, DC, 1974; p 218. Ohtsuka, T.; Hasegawa, Y.; Koizumi, M. Bull. J p n . Pet. Inst. 1967,
-Lf"C-. dC = x t 13 k l 3dt
9, 1.
Oleck, S. M.; Sherry, H. S. Ind. Eng. Chem. Process Des. Deu. 1977, 16, 525.
-Lf'C-" dC = xzt4kz4dt Since the rate constants in eq A2-A4 are the functions of temperature alone and the rate of heating is equal to that of cooling, all of the right sides in eq A2-A4 are equal. Then, we can get the concentrations, C1 and Cz,which cannot be measured from experiments, as in the following: atn=l
Paraskos, J. A.; Frayer, J. A.; Shah, Y. T. Ind. Eng. Chem. Process Des. Deu. 1975, 14, 315. Riley, K. L. Prey.-Am. Chem. SOC.,Diu. Pet. Chem. 1978, 23(3), 1104.
Sato, M.; Takayama, N.; Kurita, S.; Kwan, T. Nippon Kagaku Zasshi 1971, 92, 834. Shimizu, Y.; Inoue, K.; Nishikata, H.; Koinuma, Y.; Takemura, Y.; Aizawa, R.: Kobayashi, S.; Egi, K.; Matumoto,K.; Wakao, N. Bull. Jpn. Pet. Inst. 1970, 12, 10: Shimura. M.: Shiroto. Y.: Takeuchi. C. Prem-Am. Chem. Soc.. Diu. Colloid Surf. Chehz.,'Las Vegas Meet., March 1982, 30. Ta", P. W.; Harnsberger, H. F.; Bridge, A. G. Ind. Eng. Chem. Process Des. Deu. 1981, 20, 262. Todo, N.; Kabe, T.; Ogawa, K.; Kurita, M.; Sato, T.; Shimada, K.; Kuriki, Y.; Ohshima, T.; Takematsu, T.; Kodera, Y. Kogyo Kagaku Zasshi 1971, 74(4), 563. Wheeler, A. In Advances i n Catalysis; Academic: New York, 1951; '
C1 = (CoC4)"2
C2 = C3(Co/C4)1'z
(A5)
atn # 1 cll-n
=
(C41-n
C21-. = ( 2 C p +
+ Co1-")/2 c01-n - C*1-")/2
646)
Substituting (A5) and (A6) in (Al) derives
p 249.
Received for review August 14, 1986 Accepted June 26, 1987
Kinetic Study on the Hydrotreating of Heavy Oil. 2. Effect of Catalyst Pore Size Satoru Kobayashi,* Satoshi Kushiyama, Reiji Aizawa, Yutaka Koinuma, Keiichi Inoue, Yoshikazu Shimizu, and Kozo Egi National Research I n s t i t u t e for Pollution and Resources, 16-3, Onogawa, Yatabe-machi, T s u k u b a , Ibaraki 305, Japan
The hydrodemetalation reactions of residual oil were carried out by use of a trickle bed reactor to investigate the relation between catalyst pore diameter and activity. The catalysts used were 3% molybdenum on alumina and silica-alumina. The results showed that the optimum pore diameter was located around 100-150 A at 400 "C. The optimum pore diameter increases with the elevation of reaction temperature, and the curve showing the change of metals removal with pore diameter becomes broader a t higher reaction temperature. The selectivity between two reactions, such as vanadium and nickel removal, passes through a maximum with varying the pore diameter. These phenomena were satisfactorily explained by a simple kinetic equation including the Thiele modulus. In our previous paper (Kobayashi et al., 1986), we investigated the effect of catalyst pellet size on hydrodemetalation (HDM) of residual oil by the use of commercial Co-Mo/A1,03 hydrodesulfurization (HDS) catalysts. The results showed that, above 1-mm pellet sizes for large pore catalysts and even at pellet sizes of less than 0.5 mm for catalysts whose pore diameter was as small as 70 A, HDM activity was significantly affected by pellet size. 0888-5885/87/2626-2245$01.50/0
This indicates that HDM reaction is influenced by intraparticle diffusion in the usual HDS catalysts. This diffusion limitation reduces the effectiveness factor of the catalysts, resulting in the decreased apparent activities of heavy metals removal. It is possible to solve this problem by extending the pore diameter of the catalysts. However, the surface area decreases with increasing pore diameter of the catalysts. This again decreases the apparent activity. 0 1987 American Chemical Society
