Ind. Eng. Chem. Res. 1987,26, 1945-1950
1945
Catalyst Deactivation during Hydrodemetallization Layioye 0. Oyekunle* Department of Chemical Engineering, University of Lagos, Lagos, Nigeria
Ronald Hughes Department of Chemical Engineering, University of Salford, Salford M5 4WT,England
A pore-plugging model has been used to analyze catalyst deactivation by metal deposition, which prevents diffusion of large reactant molecules into the pores. A uniform pore structure was assumed and catalyst lifetimes have been predicted at limiting values of activity for a single, isothermal catalyst particle undergoing a second-order reaction. Drop in activity correlates well with the quantity of metals deposited within the catalyst pores, and the effectiveness factor changes significantly with catalyst age. Excellent correspondence was observed between the results of numerical calculations and those reported in the literature. Hydrodemetallization (HDM) is an established process for the removal of metal impurities in residuums used as catalytic cracker or coker feed and in direct residual conversion processes. Heavy crudes and residual oils are now being upgraded to lighter and more valuable products through hydroconversion processes. The high metal and sulfur contents of these heavy feeds present processing difficulties and can give rise to serious environmental problems. The metals present in the asphaltene molecules deposit on the catalyst surfaces, and this leads to deactivation, especially poremouth poisoning (Newson, 1975; Ohtsuka, 1977; Oyekunle and Hughes, 1984; Ahn and Smith, 1984). Coking is a reversible deactivation which also occurs during HDM. However, metals have greater tendency than coke to form deposits where relative pore size is large, and coke deposition is suppressed by increasing hydrogen partial pressure, thus increasing the selectivity of demetallization (Ohtsuka, 1977; Prasher et al., 1978). Consequently, deposition of metals is the principal cause of reduced activity, and coke is readily burnt off during catalyst regeneration. Demetallization is carried out in a trickle bed reactor filled with a proprietary catalyst in the presence of hydrogen. Hydroconversion processes produce a considerable amount of deactivated catalyst for which regeneration procedures are currently under development. Research efforts are directed at developing long-life catalysts. Research into catalyst deactivation has resulted in better catalysts and reactor designs. Two-stage reactors, first for HDM and second for catalytic cracking or hydrocracking, have been developed. A further improvement has resulted in different catalyst bed layers with an upper section of wide pore radii and a lower section of small pore radii (Schuetze and Hofman, 1984). Different reactor types are possible, and in some flow reactors fresh catalysts are added periodically. Periodic catalyst regeneration during HDM process is also practiced. This allows higher conversion (5040%) at higher temperature than most residual hydroconversion processes. Inspite of increasing reactor temperature, catalyst lifetime is limited to 1 year maximum (Schuetze and Hofman, 1984).
Previous Studies Many studies have been carried out in recent years, and various models have been developed to predict catalyst life. Newson (1975) has provided a pore-pluggingmodel based on Wheeler’s (1951) intrapore diffusion model and estimated catalyst life from the effectiveness factor of the * T o whom all correspondence should be addressed. 0888-5885/87/2626-1945$01.50/0
demetallization reaction determined for various feeds. A model developed by Rajagopalan and Luss (1979) employed the rate of demetallization for predicting catalyst life. Hughes and Mann (1978) presented a model for computing the activity of plugged catalysts assuming a wedge-shaped metal deposit. Ahn and Smith (1984) formulated equations for catalyst activity as a function of time for demetallization and evaluated effectiveness factor as a function of reduced time for a single, isothermal catalyst particle. In all these models, a first-order rate expression has been employed. Oyekunle and Hughes (1984) postulated a simple pore-pluggingmodel to describe catalyst deactivation due to metal deposition for a second-order kinetics which is more consistent with the actual observations for demetallization (Scott and Bridge, 1971; Newson, 1975; Ohtsuka, 1977; Ahn and Smith, 1984). The model was used to predict catalyst life based on limiting demetallization rate for catalyst with uniform pore sizes. In the present study, catalyst lifetime has been estimated from (i) effectiveness factor of the demetallization reaction, (ii) metal deposition thickness within the catalyst pores, and (iii) the rate of demetallization. The results are compared with the predictions of other prior models.
Pore-Plugging Model Diffusion in a Poisoned Pore Mouth. During hydrodemetallization, the feed reactant contains significant concentrations of organometallic compounds, mainly of vanadium and nickel, which react to give inorganic products. These compounds have high molecular weights ranging from several hundreds to 100000. The diameters of the molecules have a size distribution from 2 X to m with peaks at 7 X 10-9-10-8 m (Ohtsuka, 1977; Rajagopalan and Luss, 1979). These dimensions approach or exceed catalyst pore size, and diffusion into the small pores of the catalyst is greatly retarded. A model describing catalyst deactivation due to metal deposition has already been presented by Oyekunle and Hughes (1984). During the demetallization reaction, the metal sulfide products formed plug the pores of the catalyst, and the deactivation process occurs in an irreversible manner. It is assumed that the outer pore mouth becomes plugged uniformly, while the inner recesses are completely clean (Wheeler, 1951). Scheme I shows the plugging of a single catalyst pore. The rate of metal deposition is controlled by intraparticle diffusion, and deposition occurs radially on the deposited metal surface at varying thicknesses. The metal deposition thickness increases with time, and this thickness will be 0 1987 American Chemical Society
1946 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 Scheme I. Diffusion and Reaction with a Poisoned Pore Mouth
d 4
* z greatest at the pore mouth while the inner recess of the catalyst pore is still fresh. The reactant concentration at the pore mouth is Co,and after an initial metal deposition on the fresh catalyst pore, there is restricted diffusion of the large organometallic molecules into the pore. The restricted diffusivity is given by D, = D f ( N (1) where D is the bulk diffusivity and
:
0.6
ou
0.2
0 0
0.2
Q6 08 1.0 DISTANCE ALONG THE PORE, J
0.4
Figure 1. Concentration profile along the pores.
where rm is the radius of the reactant molecule and rp is the instantaneous pore radius. Spry and Sawyer (1975) proposed an empirical relationship which gives f(A) = (1 -
X)4
sistance to the reactant molecules within the catalyst pore. Equation 6 now becomes
(3)
where the new dimensionless parameters for the restricted diffusion are
It then follows that D, = D ( l (4) Deactivation Reaction. In an idealized pore structure model, the catalyst pellet is assumed to consist of an assemblage of open cylinders of uniform radius. The pellet is also assumed to be isothermal in the reactor, and the reaction is second order with respect to the feed and metal concentrations. The equation for diffusion and reaction in a fresh catalyst pore is given by d2C 2 dC D - = -K,C" + (5) dt dX2 ro where K, is the intrinsic rate constant per unit surface area of pore. If we introduce dimensionless parameters a = c/c, (54
p =x/L
T)
(5b)
ho = L( 2K,COn-l ' I 2
tD L2
7 = -
Equation 5 becomes
with boundary conditions a = l
da _ -0
at p = O at / 3 = 1
dT
a = l
at
7 = O
The original pore diameter is progressively reduced by metal deposition which leads to increasing diffusional re-
tD, L2 The effective diffusivity parameter, h,, will vary with time because rp varies with time and with the radial distance. However, rp has been used to normalize the radial distance and a transient term has been included in eq 10. Also, the time constants for catalyst life are on the order of months. The concentration profiles (cy) along the pores at different time intervals were evaluated by the use of orthogonal collocation method with four internal collocation points and the Runge-Kutta polynomial approximation to solve eq 10. All the data used in the model calculations were obtained from literature sources, and they can be considered representative of commercial and experimental HDM catalysts (Newson, 1975; Ohtsuka, 1977; Rajagopalan and Luss, 1979; Chantong and Massoth, 1983; van Vuuren et al., 1984). Figures 1 and 2 show how the concentration ( a )varies with the distance along the pore (p) for two different pore sizes. The two figures illustrate the large decrease in the concentration of the reactant molecules within the pore, and pore area is only partly available for reaction. At low values of T , (10.02), the reactant molecules are spread along the whole length of the pore. When T , is greater than 0.02, the concentration ( a )reduces to zero at various values of 0, signifying some resistance to the flow of the reactant molecules. Comparison of Figures 1 and 2 at T , = 0.005 shows that at /3 = 1.0, the concentration a = 0.21 for ro = 4 nm and a = 0.28 for ro = 12 nm. This confirms the fact that the larger pore allows more reactant molecules into inner re7,
=-
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 1947 y = l at t = O
d
i
0.8
0 a
06
0.4
0.2
n -
0
0.2
0.4 0.6 04 1.0 DlSlANCE ALONG THE PORE,B
Figure 2. Concentration profile along the pores.
cesses of the pore. At higher values of 7,above 0.02, there is no large difference between the values of p in both Figures 1 and 2, showing that the pore mouth becomes ultimately plugged whatever the size of the pore. The values of CY consequently approaches zero discontinuously. At rr = 0.08, the reactant molecules can only get to within 35% of the pore length, confirming pore mouth plugging. Deposition of Metals with t h e Pore. The organometallic constituents of the HDM feed, primarily vanadium and nickel, react out of the feed and combine with the hydrogen sulfide to produce solid deposits of metal sulfides. The pore-plugging rate is the sum of all the metal sulfide deposition rates. The rate of metal deposition per unit length of pore is dm - ~ 2 n r & C ~ M , ~ _ dt where e = number of metal sulfide molecules per molecule of reactant and M,, = molecular weight of metal sulfide deposit. With metal deposition thickness, 6, m = pmsr(ro2- rp2) = pmsa(rO2 - (rowhere pma= density of metal sulfide deposit. With y = rp/rO, m = pmsnro2(l- y2)
(12)
Therefore,
and dy
-=-
eKsCOnMmsa"
dt
Pms
Hence,
where Pms
with the boundary condition
(14)
The deposited metal sulfides can be assumed to have the formula V2S3 and NiS (e = 2), while the density of metal sulfide (pms) is assigned a value of 3000 kg m-3 (Newson, 1975). These assumptions are supported by calculations based on the analyses of used catalysts. The metal deposition thickness, 6, is related to y by 6 = ro(l - y) (16) Solution of eq 14 when n = 2 yields values for varying concentration profiles (a)obtained for different pore sizes. The metal deposition thickness, 6, is then computed through eq 16. Figure 3 shows that the metal deposition thickness increases with the number of days the catalyst is on stream. However, the metal deposition decreases with increasing pore size for catalysts of the same age. It can also be seen that pore sizes of 4 nm or below will be completely plugged within about 100 days, while for larger pore sizes (10 nm and above) complete pore blockage will not occur until after 1 year. Effectiveness Factor and Demetallization Rate. Recognition that intraparticle limitation of mass transport could occur in catalyst pellets undergoing reaction led to the concept of an effectiveness factor. For isothermal situations and normal kinetics, the effectiveness factor can be given by an analytical expression through solution of the equations modeling diffusion and reaction inside the catalyst pellet. The instantaneous reaction rate in a single pore is given by
R, =
$,=,
x=L
2nrd(,Cn dx = 2nr&KsCo"~
(17)
where the instantaneous effectiveness factor is
and a and y are functions of p and n = 2. Therefore, 1
R, = 2nr&KSCo21 0 a2y dp
(19)
The effectiveness factor, q , accounts for the concentration gradients and the diffusional resistance due to the plugging of the pores by metal deposition. Equation 19 shows that the rate of demetallization can be conveniently represented by the effectiveness factor, 7. The idealized pore structure model due to Wheeler (1951) can be assumed in evaluating the rate of demetallization in a catalyst pellet. The pellet is believed to be an assemblage of open cylinders of uniform and identical radius. Equation 19 then becomes e 2 1 R, = R, = -8KSCo2$ a2y dp (20) nro2L21J2 ro 0 where R, is the reaction rate in the pellet. Equation 18 was solved by orthogonal collocation method, and values of the effectiveness factor were determined. The reaction rates for single catalyst pellets of different average pore sizes were evaluated through eq 20. The values of effectiveness factor (7)for different pore sizes are plotted as a function of reduced time (7)in Figure 4. When the catalyst is fresh and without any metal deposition, the effectiveness of the internal surface increases with time because there is little or no resistance to diffusion and the entire internal surface is available for
1948 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 0.7
0.6 E
0,5 m W
2 L
u
0.L
2
0
0. 3 v)
0
n W
a
0.2
2
a
c W
I
0. 1
0
0
200
100
CATALYST
300 AGE,
DAYS ,
Figure 3. Metal deposition thickness as a function of catalyst age.
0'40
F
1
I
1
u
! 0.30 U
i m
p
0.20
W
2
t W
::
0.10
W
: I
0
tL-.--
I
0
I
I
0.05
I
0.10
I
I
REDUCED TIME,
Figure 4. Variation of effectiveness factor
( 7 ) with
I
0.15
0.20
T
reduced time.
reaction. The effectiveness factor reaches a maximum when r is about 0.16, and it then starts to drop progressively as r increases and the pores become plugged. Due to the presence of large-sized organometallic compounds, high catalytic effectiveness is not achieved, but it is improved by increasing the mean pore size of the catalyst. Figure 5 shows the variation of effectiveness factor (7) as a function of catalyst age for different pore sizes. The value of 7 decreases uniformly during catalyst aging and changes more significantly with catalysts having smaller pore sizes. The plots also indicate the advantage of catalyst with larger pores having a greater value of effectiveness factor because they provide easy access for the large reactant molecules. The dependence of demetallization rate on the pore size at various catalyst ages is shown in Figure 6. The initial reaction rate for the fresh catalyst decreases with in-
creasing pore size, indicating that the reaction is occurring at or near the surface of the pellet. Catalyst with smaller pores has larger surface area and therefore exhibits greater initial activity. For all pore sizes, the activity decreases with time on stream due to metal deposition and increasing diffusional resistance. The high initial demetallization rate of catalyst with pores of smaller size decreases more rapidly than that of pores of larger size. the plots also indicate a total loss of activity for catalysts of 6-nm pore radius or less. Catalyst activity is totally lost after 140,173, 235, and 405 days for pore sizes of 2, 3, 4,and 6 nm, respectively. Model Predictions In commercial operating practices, catalysts are withdrawn when the activity has deteriorated to a relatively low value, and fresh catalysts are added on stream. Since
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 1949 0.5
-
c 0.4 U
2
?
0.3
m
m z
W
-
0.2
r
U W
1L LL
w 0. 1
0 0
100
2 00
3 00 CATALYST
AGE
4 00
DAYS
Figure 5. Variation of effectiveness factor with catalyst age. Table I. Summary of Lifetime Predictions Obtained in This Study catalyst lifetime, days effective50% of pore size, deposition ness demetallizainitial" nm thickness factor tion rate activity 2 130 3 145 4 64 95 173 110 6 122 200 280 220 8 190 305 405 330 250 445 475 455 10 12 340 450 475 480
" Values from previous study (Oyekunle and Hughes, 1984). total loss of activity occurs when substantial amount of metals are deposited on the catalyst, the plots of metal deposition thickness with catalyst age (Figure 3) can be used to predict catalyst life. Ozaki et al. (1975) showed that catalyst activity was lost by 30 g of vanadium deposition on 100 g of catalyst. Ohtsuka (1977) reported a 50-65 wt % metals deposits on new catalysts as an appropriate tolerance level. A limiting 50% metal deposition thickness (Figure 3) yields 64,122,190,250, and 340 days for catalysts of 4, 6, 8, 10, and 12 nm, respectively. The effectiveness of a catalyst is closely related to the resistance to diffusion within the pores. Newson (1970, 1972) determined the effectiveness factor of the demetallization reaction for various feeds and estimated the catalyst life. An effectiveness factor of 0.2 is reported reasonable for demetallization reaction for a 0.16-cm-diameter spherical pellet. Observation of the plots in Figure 5 shows that an effectiveness factor of 0.2 can be used to obtain the lifetimes of 95,200,305, and 445 days for catalysts of 4-, 6-, 8-, and 10-nm pore radius, respectively. Figure 6 can also be used to evaluate catalyst life, and as shown in a previous study (Oyekunle and Hughes, 1984), Table 11. Some Catalyst Lifetimes Reported in the Literature catalyst life, days method 145.8 model prediction 187.5-208.3 commercial plant (Gulf) 155 commercial plant kinetic studies (85% efficiency) 145 commercial plant kinetic studies (80% efficiency) 41.7 model prediction 125 same 41.7 same 291.7 same
a limiting rate of 2 X m4 kmol-' s-l seems adequate. The plots (Figure 6) yield lifetimes of 173, 280, 405, and 475 days for pore sizes of 4, 6, 8, and 10 nm, respectively. These are in agreement with the previously determined data. Figure 6 also predicts catalyst life for smaller pore sizes: 130 and 145 days for pore radii of 2 and 3 nm, respectively. As can be seen from the plots, catalysts with average pore sizes of 7 nm and above will have a lifetime of 1year or more. Catalysts with a smaller pore radius will have to be replaced before the end of 1 year. Table I compares catalyst lifetimes obtained through different parameters. The lowest values were obtained through metal deposition thickness because the pores are assumed to be 50% plugged. In commercial practice, reactant molecules can still diffuse through the pores especially with catalysts having large pore sizes. Catalyst life prediction through effectiveness factor has the same magnitude with that obtained from a 50% reduction of initial activity (Oyekunle and Hughes, 1984). The very large values obtained through limiting demetallization rate show that a high limiting rate has been assumed. The data indicated in Table I compare favorably with catalyst lifetimes reported in the literature (Table 11). Newson's (1970, 1972, 1975) model predictions reported catalyst life varying from 41.7 to 291.7 days. Gulf commercial plants provided values ranging from 187.5 to 208.3 days (Newson, 1975), while the kinetic studies of commercial plants by Ozaki et al. (1975) yielded 145 and 155 days. Conclusions Catalyst deactivation by pore mouth plugging during hydrodemetallization has been studied. Lifetimes of catalysts have been predicted at 50% pore blockage and at limiting values of both effectiveness factor and demesource Newson, 1970, 1972, 1975 same Ozaki et al., 1975 same Ohtsuka, 1977 same same same
feedstock
Kuwait, 340 OC same Khafji Kuwait Gach Saran (Iranian Heavy) Arabian Light
1950 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
kmol m-3 Co = initial concentration of reactant, 2 X D = bulk diffusivity, 7 X m2 s-l D, = restricted diffusivity, m2 s-l ho = Thiele modulus (eq 5c) h, = Thiele modulus with restricted diffusivity (eq loa) K , = deposition velocity defined by eq 15 K, = reaction rate constant per unit surface area, 5 X m4 kmol-' s-l L = half pore length, m
*g
+ W
4
a
-1
d W
I
w
cl
0
2
c
6
PORE
8
a
12
RADIUS, n m .
Figure 6. Dependence of demetallizationrate on pore size and time on stream.
tallization rate. A uniform pore structure was assumed, and catalyst with small pore sizes has a lifetime on the order of days. Large pore-sized catalyst can last for more than a year before it needs to be replaced. These results indicate that an appropriate pore structure is required for achieving the desirable goals in HDM processes. Residual feedstock hydroprocessing is still confronted with the problem of achieving high catalytic effectiveness due to the presence of organometallic contaminants in the feed. The details of metals accumulation within the catalyst pellets are complex, and the geometry of the pore structure of common catalyst is still poorly defined. Also, the kinetics of HDM reactions has not been fully understood. More knowledge on catalyst poisoning supported by experimental data will open up the way to better utilization of the presently used catalysts. Acknowledgment This work was supported by the Department of Chemical Engineering, University of Salford. Nomenclature C = instantaneous concentration of reactant, kmol m-3
L, = maximum pore length equivalent to pellet diameter, m m = mass of metal deposit per unit length of pore M,, = molecular weight of metal sulfide deposit n = order of reaction, 2 R, = instantaneous reaction rate in a pore R, = reaction rate in a single pellet r p = instantaneous pore radius, nm m rm = radius of reactant molecule, 6.25 X ro = initial pore radius, nm t = time element, s x = distance coordinate along the length of a pore Greek Symbols a = dimensionless concentration, C/Co /3 = dimensionless distance, x / L y = ratio of instantaneous pore radius to the initial pore radius, r /ro 6 = metal Jeposition thickness, nm t = number of metal sulfide molecules per molecule of reactant, 2 9 = instantaneous effectiveness factor 0 = porosity, 0.5 A = ratio of molar to pore radius, rm/rp pms = density of metal sulfide, 3000 kg m-3 T = dimensionless time (eq 5d) r, = dimensionless time for restricted diffusivity (eq lob) Literature Cited
Ahn, B.; Smith, J. M. AIChE J. 1984, 30, 739. Chantong, A.; Massoth, F. E. AIChE J. 1983, 29, 725. Hughes, C. C.; Mann, R. ACS Symp. Ser. 1978,65, 201. Newson, E. J. Prepr. Pap.-Am. Chem. SOC., Diu. Pet. Chem., 1970, A-141. Newson, E. J. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1972, 17, 49. Newson, E. J. Ind. Eng. Chem. Process Des. Dev. 1975, 14, 27. Ohtsuka, T. Catal. Reu. Sci. Eng. 1977, 16, 291. Oyekunle, L. 0.; Hughes, R. Chem. Eng. Res. Des. 1984, 62, 339. Ozaki, H.; Satomi, Y.; Hisamitsu,T. 9th World Petroleum Congress, Tokyo, 1975, PD 18, p 4. Prasher, B. D.; Gabriel, G. A.; Ma, Y. H. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 266. Rajagopalan,K.; Luss, D. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 459. Schuetze, B.; Hofman, H. Hydrocarbon Process. 1984,63, 75. Scott, J. W.; Bridge, A. G. Adu. Chem. Ser. 1971, 103, 1. Spry, J. C.; Sawyer, W. H. 68th Annual AIChE Meeting, Los Angeles, Nov 1975, Prepr. 30C. van Vuuren, D. S.; Stander, C. M.; Glasser, D. AIChE J. 1984, 30, 593. Wheeler, A. Adu. Catal. 1951, 3, 249.
Received for reuiew October 23, 1985 Revised manuscript received April 28, 1986 Accepted May 11, 1987
