422
Ind. Eng. Chem. Res. 1989, 28, 422-427
Modeling of Hydrodemetalation Catalysts? Carmo J. Pereira* and Jean W. Beeckman Research Division, W . R . Grace & C o m p a n y , 7379 Route 32, Columbia, Maryland 21044
A mathematical model that describes t h performance of hydrodemetalation catalysts having a bimodal pore size distribution has been developed. The catalyst pore structure is conceptualized as an aggregate of microporous grains with macroporosity arising from the voids in between these grains. The macropores are assumed to be randomly distributed throughout the volume of the catalyst pellet and have a length characteristic of the pellet radius. Macropores lead to micropores that are assumed to have length of the order of the grain size. Overall demetalation kinetics are described by a first-order kinetic rate law. The virgin catalyst surface and the metal suKde containing deposits that cover the catalyst surface during demetalation each have a characteristic rate constant. The deposits build up on the catalyst surface and cause a spatially varying decrease in pore dimensions within the pellet. The pellet model is extended to predict the performance of a n isothermal fixed bed demetalation reactor which is modeled as a series of mixing cells. In order to test the applicability of the model, experiments were conducted to study the demetalation performance of three catalysts having the shape of a sphere, extrudate, and Minilith using a Boscan vacuum resid feedstock. Model predictions were found to be in good agreement with the experimental observations. The ranges of parameters used in the mathematical model are discussed. During catalytic hydrodemetalation, metal-bearing molecules present in the feed diffuse into the catalyst pellet, react at the active sites on the surface of the catalyst, and deposit as coke and metal sulfides. These deposits, though not as active as the sites originally present on the surface of the catalyst, are also demetalation catalysts. Deposits cover the sites originally present on the surface of the catalyst and continue to further catalyze demetalation. This leads to a buildup of deposits on the surface of the catalyst, and eventually the catalyst pores become filled with deposits. The accumulation of metal sulfide containing deposits within the pores of the catalyst over time is one of the primary causes of irreversible deactivation. Improved demetalation catalysts having higher activity and longer life are being developed by appropriately manipulating the initial surface activity and the pore structure of the catalyst (Hung et al., 1986). Pore structure determines the effective diffusion coefficient for metal-bearing molecules into the catalyst and the capacity of the catalyst for storing deposits. The metals storage capacity of the catalyst often determines catalyst life. As a result of the trend toward decreasing crude quality, larger quantities of atmospheric and vacuum resids with higher metals contents are currently being processed. The metals contained in these heavy feedstocks are found in complex organometallic molecules that are greater than 15 A in diameter (Hensley and Quick, 1980). The sizes of these large metal-bearing molecules are often comparable to the dimensions of the micropores (which generally have diameters in the 75-200-A range), and access of metals into the micropores is restricted due to configurational diffusion limitations. Catalysts containing only micropores when used in the hydrotreatment of resid feeds deactivate rapidly due to pore-mouth plugging by metals deposits. As such, demetalation catalysts used a t the front end of fixed bed demetalation reactors are tailored to have a bimodal pore structure; i.e., in addition to micropores, catalysts also contain macropores (having pore diameters of >lo00 A). These macropores serve as highways for transporting the large metal-bearing molecules into the pellet and continue
* Author to whom correspondence should be addressed.
' A portion of this work was presented at the Division of Petroleum Chemistry, American Chemical Society Meeting, Denver,
May 1987. OSSS-~SS5/S9/262S-0422$01.50/0
to facilitate the transport of metal-bearing molecules into the catalyst even after the micropores near the exterior surfaces of the catalyst are plugged by deposits. The metals deposition problem was first rigorously examined by Rajagopalan and Luss (1979). Recently, Petersen and Smith (19851, Ahn and Smith (19841, Shimura et al. (1986), and Oyekunle and Hughes (1987) have modeled the metals deposition problem. In all of these studies, the pore structure of the catalyst has been described using either a single-pore or the parallel-pore model. Several of the demetalation catalyst modeling papers have been reviewed by Pereira et al. (1987). An excellent review of the progress made toward the design of hydrodemetalation catalysts has been presented by Wei (1987). In this paper, a mathematical model that describes the performance of a bimodal demetalation catalyst pellet has been developed. The model assumes that macropores are randomly distributed throughout the catalyst pellet and that micropores branch out from the macropores. As a result, metal-bearing molecules can access the micropores only through macropores. The model parameters used to predict the performance of catalysts during the hydrotreatment of vacuum resid feedstocks are discussed. To illustrate the applicability of the model, spherical, extrudate, and Minilith-shaped catalysts were experimentally studied using a high metals Boscan vacuum resid feedstock. Model predictions for the demetalation performance of these three catalysts will be compared with experimental data.
Model Equations The bimodal demetalation catalyst contains macropores that are randomly distributed throughout the pellet volume. The macropores are assumed to result from the voids between grains which contain only micropores. The length of the macropores is proportional to the radius of the pellet. Micropore length is assumed to be of the order of the grain size. The model assumes that macropores lead to micropores. A schematic representation of the pore structure model is shown in Figure 1. A similar model has been used to describe the physical structure of a packed bed or a porous solid by Turner (1958). The overall metals deposition rate is taken to be firstorder in total metals concentration. The term for hydrogen partial pressure was included in the rate constant. The 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 423 Pellet External Surface
F
l
m
4 Pel let Centerline
Figure 1. Schematic representation of catalyst pore structure.
effect of the increasing hydrogen sulfide partial pressure along the reactor on the demetalation rate was assumed to be negligible and ignored. The rate constant for metals deposition is given by k = (1- B)kf Bkd for 0 I8 I 1 (1) where k is the overall intrinsic demetalation rate constant, kf is the demetalation rate constant for the fresh or virgin catalyst, kd is the demetalation rate constant for the metals sulfide containing deposits, and 6 is the local fractional coverage of the catalyst surface by metals deposits. 0 is defined as the ratio of the local weight of deposits per unit area of the catalyst surface a t any time (w)to the weight of deposits per unit area of catalyst at monolayer coverage (wJ.For w 1 wg,0 is taken to be unity, and only the metals deposits are active for demetalation. The dimensionless pseudo-steady-state mass conservation equation in a cylindrical micropore is discussed by Rajagopalan and Luss (1979):
where t is time, c is the concentration of metals in the feed, Md is the molecular weight of the deposits, and Pd is the deposit density. The initial condition to eq 4 is f, = 1at t = 0. The micropore is completely plugged when f, = A,. Macropores are assumed to be cylindrical in shape and have an initial diameter of dMoand a length of 1 ~ The . number of macropores and micropores per gram of catalyst are calculated using the measured values of macropore volume ( VM)and micropore volume ( V,). Micropores are distributed uniformly along each macropore throughout the pellet volume. The mass balance equation for metal-bearing molecules in a single macropore accounts for the metals deposition along the walls of the macropore and for the metals deposition within the micropores. The dimensionless pseudo-steady-state mass conservation equation in the macropores is
+
where y is the fractional distance along the macropore from the center of the pellet, n is the shape integer ( n = 0, slab; n = 1, cylinder; n = 2, sphere), and 6 (=V,dMo/VMdmo) is the ratio of the surface area of the micropores to the surface area of the macropores in the pellet. AM, $M, and f M are defined for the macropore in a similar manner as the dimensionless micropore parameters A,, 4,, and f,, respectively. Boundary conditions for eq 5 are dUM(O)/dy = 0 and UM(1)
where z is the fractional distance from the center of the micropore, u, is the dimensionless concentration of the feed metals normalized with respect to the reactor inlet concentration (c), f, (=d,/dmo) is the fraction of the initial micropore diameter (dm0)that is still available for diffusion at any time, and &, (=d,d/dmo) is the ratio of the diameter of the metal-bearing molecule (dmoJto the initial micropore diameter. 4mis the Thiele modulus for the micropore defined as 1,(4k/d,oDmoJ0~5,where 1, is the length of the micropore and Dmolis the bulk diffusion coefficient of the metal-bearing molecule. Dmolis estimated by using the Wilke and Chang correlation (Reid and Sherwood, 1966). When the size of the metal-bearing molecule becomes comparable to the size of the micropore, configurational diffusion becomes important and the fourth-power configurational diffusion relationship of Spry and Sawyer (1975) has been used in eq 2. Boundary conditions for eq 2 are du,(O)/dz = 0 (34 and (3b) where UM is the dimensionless concentration of metals in the macropore just outside the micropore. f, is obtained by solving the metals deposition equation df, ~KcM~u, -=(4) dt dmoPd Um(1)
=
UM
(64
= UR
(6b)
where U R is the dimensionless concentration of metals in the bulk stream just outside the macropore. Should interphase mass transfer become important, eq 6b can be suitably modified to include external mass transfer. The macropore metal deposition equation is similar to eq 4. The pellet demetalation rate is calculated as the flux of metal-bearing molecules entering each macropore times the number of macropores in the pellet. The local weight of deposits per unit area of catalyst surface is given by w = d:(l-
f:)pd/4
for i = m, M
(7)
In addition to metal sulfides, deposits also contain coke, particularly during the initial part of the run. Coke deposition is not explicitly described in the model, and as such, Pd is adjusted to account for the presence of coke in deposits, as discussed later on in the paper. The total weight of deposits in the catalyst pellet is obtained by summing up the deposits in the micropores and macropores of the pellet. The pellet model assumes that the macropores are randomly distributed throughout the pellet volume and that the metal-bearing molecules can enter the micropores only through macropores. This assumption breaks down when the catalyst pellet has a very low macroporosity. In this case, diffusion into the pellet through macropores and micropores located on the external surfaces of the catalyst pellet becomes important. Another situation in which the pellet model may have limited applicability is when the pellet diffusion length, lM,is so small that diffusion through
424 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 Table I. Catalyst Properties catalyst B
A pore structure total pore vol, cm3/g 1.05 0.72 micropore diameter, 8, 114 111 BET surface area, m2/g 204 195 catalyst pellet shape and size shape sphere extrudate diameter, cni 0.180 0.159 length, cm 0.476
Table 11. Properties of API gravity (at 60 O F ) S,wt-% Ni, wppm V wppm
'0°1
C 0.67 122 170
Minilith 0.254 0.254
Boscan Vacuum Resid Feedstock 5.4 Fe, w m m 7.7 5.73 N, wt-% 0.70 130 Conradson C, wt % 18.8 1580 asphaltenes, wt 7'0 25.6
surface micropores contributes significantly to the pellet rate during the life of the catalyst. Such is typically the case in ebullated bed reactors that use 0.079-cm-diameter catalysts. The mean reactor residence time of the catalyst in this application is only 45 days. Real catalysts have pore size distributions of both micropores and macropores. During deactivation, smaller micropores will be completely blocked by metals deposits before micropores having diameters greater than or equal to the average micropore diameter become blocked. Thus, the deactivation rate of real catalysts will differ from the predictions of a model in which average pore dimensions are used. However, the effect of pore size distributions can be easily included in the present model. Real feeds contain a size distribution of metal-bearing molecules and only an average metal-bearing molecule size has been used in the model. Unfortunately, very little information on the size distribution of metal-bearing molecules at reaction conditions is available in the open literature. Pereira et al. (1987) have shown that the maximum in initial metals conversion versus average initial micropore diameter is less sensitive to the average initial micropore diameter if the actual feed molecule size distribution is used in the model instead of an average metal-bearing molecule size. Metal molecule size distribution information, if available, can be included in a general model. The isothermal demetalation reactor is modeled as a series of mixing cells. The dimensionless concentration of metals in the ith stirred tank is given by
where h' is the total number of stirred tanks in series, hd is the hydrodynamic liquid holdup, LSV is the liquid space velocity, t is the reactor void fraction, and pp is the pellet density. h, is obtained from the literature (e.g., Shah (1979)). For upflow reactor operation, hd = 1. The reactor inlet concentration is u R , ~= 1. uR,N is the concentration a t the reactor exit. Metals conversion in the reactor is given by 1 - ~ R f l . Parametric sensitivity calculations for the above model have been discussed by Pereira et al. (1987). Experimental Section Three experimental cobalt-molybdenum catalysts having sphere, cylindrical extrudate, and Minilith shape were
2o 00
5
10
15
20
Figure 2. Vanadium conversion for sphere catalyst (catalyst A) using Boscan vacuum feed; temperature is 644 K for the first 5 days and 672 K for the remainder of the run; pressure is 2000 psi; hydrogen circulation is 4000 scf/bbl. 100
00
60
1 40
20
0
Figure 3. Vanadium conversion for cylinder catalyst (catalyst B) using Boscan vacuum feed; temperature is 644 K for the first 5 days and 672 K for the remainder of the run; pressure is 2000 psi; hydrogen circulation is 4000 scf/bbl.
prepared. The Minilith catalyst, developed in our laboratory, is in the shape of a wheel containing four spokes of outer diameter 2.54 mm. The wall thickness of both the rim and the spokes is 0.4 mm (Pereira et al., 1988). Catalyst properties are shown in Table I. Each catalyst had the same weight percent catalytic metals loading. One hundred cubic centimeters of each catalyst was packed in a reactor having an internal diameter of 2.54 cm. Boscan vacuum resid feedstock was hydroprocessed over a range of liquid hourly space velocities from 0.5 to 2.0 h-' at 2000 psi hydrogen pressure and a t a hydrogen circulation rate of 4000 scf per barrel of feed (scf is standard cubic feet). Feedstock properties are shown in Table 11. The reactor was operated in an upflow mode to ensure complete wetting of the catalyst pellets. The catalysts were aged at a constant temperature and at the base case reactor operating conditions. For the first 5 days, the reactor inlet temperature was maintained a t 644 K. On the sixth day, the inlet reactor temperature was increased to 672 K and held constant at 672 K until the end of the run. The metals concentrations in the hydrotreated product were measured by X-ray fluorescence. Vanadium conversions for the first 21 days on stream for each of the three catalysts are shown in Figures 2-4. A t the conditions of our experiments, thermal (i.e., noncatalytic) removal of vanadium was found to be small. The cylindrical extrudate, catalyst B, was found to deactivate in around 11days, at which time the desulfurization con-
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 425
'" 1
0
5 - "
10 c
15
20
c
Figure 4. Vanadium conversion for Minilith catalyst (catalyst C) using Boscan vacuum feed; temperature is 644 K for the first 5 days and 672 K for the remainder of the run; pressure is 2000 psi; hydrogen circulation is 4000 scf/bbl.
version was only around 25 wt 70. Durability runs for catalysts A and B were terminated due to bed plugging by interparticle metal deposits.
Results and Discussion Solution Algorithm. Pellet eq 1-6 were solved by using a central finite difference scheme. Ten grid points along the micropore direction and 50 grid points along the macropore direction were used in the numerical calculations. The demetalation reactor is modeled as a series of mixing cells. To reduce computational time required to perform the calculations, the number of mixing cells used in the calculations were minimized. Eight mixing cells in series were found to represent the minimum number of cells required to approach plug flow and used to numerically solve the reactor eq 8. An analytical solution for the initial concentration profile of metals in micropores was obtained by solving eq 2 and 3 with f, = 1 and k = k P This solution was used in the macropore eq 5 and 6 with fM = 1 and k = kf to obtain an analytical solution for the initial concentration profile of metals in the macropores. An analytical expression for the flux of metals into the macropores was developed and used together with eq 8 to calculate the initial metals concentration in each mixing cell and the initial metals conversion a t the reactor exit. From the analytical solutions of the initial metals profiles, the metals concentrations were calculated at each of the grid points in the finite difference scheme (i.e., in the micropores and the macropores of pellets located in each of the mixing cells). By use of k = kf, the micropore and macropore metals deposition equations (Le., eq 4 and the analogous metals deposition equation in the macropore, respectively) were used to numerically obtain f, and fM values at each grid point at t = At. It is worthwhile to note that the f, profile will depend on the location of the micropore along the macropore of a pellet. Similarly, the fM profile in a pellet will depend on the axial position of the pellet in the reactor. Once the f, and fM values were determined, eq 7 was used to calculate w at each grid point. Next 0 was determined, and eq 1 was used to obtain k for each grid point a t t = At. By use of these k values and the f, and fM values a t t = At, the metal concentrations in the micropores (u,) and in the macropore (uM) of catalyst pellets located in each of the mixing cells and the reactor axial metals concentration (uR,,) a t t = At were numerically obtained by simultaneously solving eq 2, 3, 5, 6, and 8. Metals conversion a t the exit of the reactor was also calculated. From the metals concentrations, k
values, and f, and fM values at t = At, new f, and fM values for the next time step were numerically obtained, and so on. Model Parameters. Several of the parameters used in the mathematical model were independently determined. Other parameters had to be fitted using the experimental data. This section discusses which parameters were independently determined and the ranges of parameters that were estimated by fitting model predictions to experimental data obtained using vacuum resid feedstocks. The micropore volume (V,), the macropore volume (VM), and volume-averaged macropore diameter (d,") were obtained from Hg porosimetry (Micromeritics Autopore 9200). The catalyst pellet density (p,) was determined standardly from V,, VM,and the skeletal density of the pellet. The average micropore diameter (d,") was calculated by using the relationship d", = 4Vm/s,where s is the BET surface area of the catalyst. The grain size was estimated by electron microscopy. The diffusion length of the micropore (1,) was the ratio of the grain volume to its external surface area. Values of 1, in the range 20-40 pm were used in our calculations. The length of the macropore (1M) is the diffusion length within the demetalation catalyst pellet. Since the macropore eq 5 takes into account the pellet shape, 2M was assumed to be the half-length of the pellet times a tortuosity factor (lMz = ~Rp2).A tortuosity factor (7)of 2 was used in all the calculations. R ,the pellet half-length, is the half-thickness of a flat slab (n = 0), the radius of a cylindrical pellet ( n = l),or the radius of a sphere ( n = 2). There is very little information in the open literature on the size distribution of the metal-bearing molecule a t reaction conditions. Most of the reported size distributions have been based on gel permeation chromatography (GPC) and size-exclusion chromatography together with inductively coupled plasma spectrometry (SEC-ICP) measurements of feeds and products at room temperature. Boscan crude has been characterized in some detail by Takeuchi et al. (1983). Shimura et al. (1986) have reported that the mean diameter for vanadium-bearing molecules in Boscan crude feed is 25 A. Several workers have used kinetic rate laws that are first order, second order, or in between first and second order to describe demetalation kinetics (e.g., Hensley and Quick (1980), T a " et al. (1981), and van Dongen et al. (1980)). The overall demetalation reaction rate in our model was assumed to obey a fiit-order kinetic rate law. The validity of this assumption has been proved for Boscan feed by Shimura et al. (1986) and has been verified by us in independent experiments using a stirred-tank reactor. For purposes of simplicity, only the deposition of vanadium-containing species was considered. This is a reasonable assumption in the case of Boscan vacuum resid because the concentration of nickel is only 7.6 w t % of the total feed metals. Further, the penetration of nickel into the catalyst particle (as determined by electron microprobe analyses of aged catalysts) is shallower than that of vanadium. As such, the catalyst will deactivate due to buildup of deposits containing primarily vanadium sulfides. The rate constant of the fresh catalyst (kf) depends on the intrinsic activity of the catalytic surface. kf (together with the total feed metals concentration (c) and the type of feed processed) influences the initial deactivation rate and the period of initial deactivation (Tamm et al., 1981). kf can be independently determined from fixed-bed or batch autoclave experiments of short duration. A kf value of 5.6 X lo3 cm/s was used for Boscan vacuum feed in our
426 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989
calculations. Recently, Kobayashi et al. (1987) have determined that the initial vanadium removal rate constant cm/s a t 673 K for Khafji feed. is 5.76 x Calculations using the k , values reported above reveal that, although the diffusion into the micropores may be limited due to configurational diffusion, the deposit profiles are fairly uniform within the micropores (i.e., the diffusion-reaction process in the micropores is kinetically controlled). This is because the I , values used in the model result in &, values of less than unity. However, metal concentration profiles along the macropore (i.e., along the pellet coordinate) can be fairly steep depending on the pellet geometry and operating conditions. The Boscan vacuum resid feed used contains a very high metals content of 1710 ppm. As a result, the initial deactivation period in our constant temperature experiments was found to be about 1-2 days. A single set of kd (together with the activation energy for demet,alation by deposits), us,and p d values was determined by fitting the experimental data. These parameters will typically depend on the composition of the feed and on the operating conditions. For example, the vanadium, nickel, and Conradson carbon residue content of the feed, the reactivity of the organometallic molecules present in the feed, and the type of feed (e.g., aromatic content, origin, etc.) will influence the composition of the deposits. Higher hydrogen partial pressure reactor operation will tend to reduce the amount of coke deposition, thereby making the deposits richer in nickel and vanadium sulfides. The monolayer deposition model, as postulated in eq 1, was used to describe the initial deactivation process. In our calculations, we have found the demetalation rate constant for the metal sulfides containing deposits (kd) to be around 50-80’70 of kf. In the hydrotreatment of model nickel porphyrins, Smith and Wei (1985) have used the monolayer deposition model and found that the nickel deposits have 47% of the activity of their fresh catalysts. Of course, the relative demetalation activity of the deposits will depend on the initial surface activity of the catalyst. Activation energies for demetalation by deposits were found to range between 30 and 43 kcal/mol. Activation energies obtained by Hung et al. (1986) and Tamm et al. (1981) using second-order demetalation kinetics were also of similar magnitude. kd = 0.8kfand an activation energy of 43 kcallmol were used in our model to predict the performance of the three catalysts tested using Boscan vacuum resid. Metals deposition values a t monolayer coverage are in the range 0.076-0.176 g of vanadium/cm3 of catalyst, as reported in Tamm et al. (1981). For Boscan vacuum feed, the weight of deposits per unit surface area of the catalyst g of at monolayer coverage, wg,was found to be 4 x vanadium/cm2 of support surface. For the cylindrical catalyst tested (catalyst B), this w,value translates to 0.078 g of vanadium/cm3 of catalyst. Deposit density (p,) values of between 3.5 and 4.5 g/cm3 were used in our calculations. Pure nickel and vanadium sulfides have densities of between 4.7 and 5.8 g/cm3. The density of the mixed metal sulfides resulting from the hydrotreatment of Boscan crude has been estimated at 4.26 g/cm3 (Shimura et al., 1986). The predicted deactivation rate of the catalyst increases as the value of pp is decreased. Comparison of Model with Experiments. In fitting the experimental data, a set of model parameters was determined, as discussed above. These parameters were held constant for each of the three catalyst shapes. Only the pore structure, the size ( R J ,and shape (n)parameters for each catalyst pellet and the reactor void fraction ( e )
were changed during each calculation. The Minilith catalyst was modeled as a flat slab having a half-thickness of 0.02 cm. Comparisons of the experimental data and model predictions for the sphere, cylinder, and Minilith catalysts at 644 and 672 K are shown in Figures 2-4. Model predictions are in good agreement with experimental observations over the first 10 days of reactor operation. After 10 days on stream, the model was found to underpredict the experimentally measured conversions. This deviation could be due to some of the simplifying assumptions used in the model (such as the use of an average reactant molecule diameter and average micropore and macropore diameters instead of the actual feed molecule and pore size distributions) and the buildup of catalytically active interparticle deposits over time. It was speculated that the buildup of external deposits over time was primarily responsible for some of the metals removal activity in the reactor. An indication of the catalytic activity of interparticle deposits is the residual metals conversion activity of the cylindrical catalyst (catalyst B) of about 20 w t %. Also, post-mortem examination of the catalysts revealed the presence of interparticle metals buildup. Catalysts particles at the top of the bed were found to be cemented together by spongy deposits containing vanadium and nickel sulfides as well as some calcium (Pereira et al., 1988). Accordingly, the mathematical model was adjusted to include a buildup of external deposits. Hydrodemetalation activity of the external deposits was assumed to depend on the external surface area per reactor volume of each catalyst and on the total amount of deposits accumulated on the external surfaces of the catalysts a t any time. Model predictions in this case, indicated by the dotted lines in Figures 2-4, are in closer agreement with the experimental data. For the cylindrical catalyst B shown in Figure 4, however, the residual experimentally measured conversion of around 20 wt % is still higher than model predictions. Some of the difference between experiment and theory could possibly be due to thermal metals removal not included in the present model. Conclusions 1. A mathematical model that describes the deactivation performance of demetalation catalysts having a bimodal pore structure has been developed. The model assumes that the macropores are uniformly distributed throughout the pellet volume and that macropores lead to micropores. 2. Three bimodal demetalation catalysts having the shape of a sphere, cylinder, and Minilith were prepared and tested using Boscan vacuum resid feed. Model predictions were found to be in good agreement with experimental data. Model parameters estimated as a result of fitting the model predictions to experimental data were found to have physically realistic values. Acknowledgment The authors thank Dr. L. L. Hegedus for helpful discussions during formulation of the model, Professor V. Hlavacek for developing the numerical algorithm, Dr. W.-C. Cheng for preparing and characterizing the catalysts, and W. Suarez for obtaining the reactor data. Nomenclature c = feed metals concentration, mol/cm3 d = diameter, cm f = open fraction of pore diameter hd = liquid holdup in reactor, cm3/cm3void reactor volume
I n d . E n g . C h e m . Res. 1989, 28, 427-431
k = first-order reaction rate constant, cm/s 1 = diffusion length, cm n = shape factor ( n = 0, flat slab; n = 1, cylinder; n = 2, sphere) s = BET surface area, cm2/g t = time on stream, s u = dimensionless metals concentration w = weight of deposits, g/cm2 surface y = dimensionless distance from center of pellet z = dimensionless distance from center of micropore D = bulk diffusivity of metal bearing molecule, cm2/s LSV = liquid space velocity, l / s M = molecular weight of deposit sulfides, g/mol N = number of stirred tanks in series R = half-length of catalyst pellet, cm V = pore volume of catalyst, cm3/g Greek Letters
4 = dimensionless Thiele parameter, Ij(4k/dj”D,,J0.6 where j=m,M /3 = dimensionless parameter, VmdMo/VMdmo e = reactor void fraction p = density, g/cm3 8 = fraction of surface covered by deposits, w / w , A = ratio of molecule to pore diameter, dmOl/djwhere j = m, M 7 = macropore tortuosity Subscripts
d = deposits f = fresh or virgin i = index m = micropore mol = metal bearing molecule p = pellet s = saturation M = macropore R = reactor Superscript o = initial value Registry No. Co, 7440-48-4; Mo, 7439-98-7; V, 7440-62-2.
Literature Cited Ahn, B.-J.; Smith, J. M. Deactivation of Hydrodesulfurization Catalysts by metals deposition. AZChE J. 1984, 30, 739-746. Hensley, A. L.; Quick, L. M. Effects of catalyst properties and process conditions on the selectivity of resid hydroprocessing.
427
Presented at the AIChE National Meeting, Philadelphia, PA, June 1980. Hung, C.; Howell, R. L.; Johnson, D. R. Hydrodemetallation Catalysts. Chem. Eng. Prog. 1986, 3, 57-61. Kobayashi, S.; Kushiyama, S.; Aizawa, R.; Koinuma, Y.; Inoue, K.; Shimizu, Y.; Egi, K. Kinetic Study on the Hydrotreating of Heavy Oil. 1. Effect of Catalyst Pellet Size in Relation to Pore Size. Znd. Eng. Chem. Res. 1987,26, 2241-2245. Oyekunle, L. 0.; Hughes, R. Catalyst Deactivation during Hydrodemetalization. Znd. Eng. Chem. Res. 1987, 26, 1945-1950. Pereira, C. J.; Donnelly, R. G.; Hegedus, L. L. Design of Hydrodemetallation Catalysts. In Catalyst Deactivation; Petersen, E. E., Bell, A. T., Eds.; Marcel Dekker: New York, 1987. Pereira, C. J.; Cheng, W.-C.; Beeckman, J. W.; Suarez, W. Performance of the Minilith-A Shaped Hydrodemetallation Catalyst. Appl. Catal. 1988, 42, 47-60. Petersen, E. E.; Smith, M. C. Pore Size Distribution Effects on HDS/HDM Catalyst Activity. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1985, 30, 36. Rajagopalan, R.; Luss, D. Influence of Catalyst Pore Size on Demetallation Rate. Znd. Eng. Chem. Process Des. Deu. 1979, 3, 459-465. Reid, R. C.; Sherwood, T. K. The Properties of Gases and Liquids; McGraw Hill; New York, 1966; pp 548-549. Shah, Y. T. Gas-Liquid-Solid Reactor Design; McGraw Hill: New York, 1979. Shimura, M.; Shiroto, Y.; Takeuchi, C. Effect of Catalyst Pore Structure on the Hydrotreating of Heavy Oil. Znd. Eng. Chem. Fundam. 1986,25, 330-337. Smith, B. J.; Wei, J. Catalyst Deactivation during the Hydrodemetallation of Nickel Porphyrin over CoMo/A120a. Presented at the AIChE National Meeting, Chicago, Nov 1985. Spry, J. C.; Sawyer, W. H. Configurational Diffusion Effects in Catalytic Demetallization of Petroleum Feedstocks. Presented a t the AIChE National Meeting, Los Angeles, Nov 1975. Takeuchi, C.; Fukul. Y.; Nakamura, M.; Shiroto, Y. Asphaltene Cracking in Catalytic Hydrotreating of Heavy Oils. 1. Processing of Heavy Oils by Catalytic Hydroprocessing and Solvent Deasphalting. Znd. Eng. Chem. Process Des. Dev. 1983,22,236-257. Tamm, P. W.; Harnsberger, H. F.; Bridge, A. G. Effects of Feed Metals on Catalyst Aging in Hydroprocessing Residuum. Znd. Eng. Chem. Process Des. Deu. 1981,20, 262-273. Turner, G. A. The Flow Structure in Packed Beds. Chem. Eng. Sci. 1958, 7, 156-165. van Dongen, R. H.; Bode, D.; van der Eijk, H.; van Klinken, J. Hydrodemetallization of Heavy Residual Oils in Laboratory Trickle-flow Liquid Recycle Reactors. Znd. Eng. Chem. Process Des. Dev. 1980,19,630-635. Wei, J. Catalyst Design, Hegedus, L. L., Ed.; Wiley: New York, 1987; pp 245-272.
Received f o r reuiew May 11, 1988 Accepted November 28, 1988
Synthesis of Highly Calorific Gaseous Fuel from Syngas on Cobalt-Manganese-Ruthenium Composite Catalysts Tomoyuki Inui,* A k i r a Sakamoto, T a t s u y a Takeguchi, and Yoshiaki Ishigaki Department of Hydrocarbon Chemistry, Faculty of Engineering, K y o t o University, Sakyo-ku, K y o t o 606, J a p a n
To synthesize a highly calorific gaseous fuel from syngas, a composite catalyst system of Co-Mn-Ru supported on alumina calcined at 1060 “C was investigated by a continuous flow reactor under a pressure of 10 kg/cm2. In contrast to the Ni-based composite catalyst, the Co-Mn203-Ru catalyst converts syngas t o a methane-rich gas containing significant concentrations of Cz-C4 paraffins. Consequently, the Co-based catalyst yielded a highly calorific gaseous fuel directly from a coke oven gas. T h e pilot plant test was conducted by using a fluidized bed and confirmed satisfactory performance for more than 3000 h.
A coke oven gas was conventionally used as a city gas. Now in some countries, however, it is steadily being replaced by a natural gas of higher calorific value. To increase the calorific value, more than 10 vol % or about 30 0888-5885/89/2628-0427$01.50/0
carbon mol % C2-C4 hydrocarbons is added to the natural gas, the dominant component of which is methane. In a reaction near atmospheric pressure using CO methanation catalysts such as Ni and Ru, the major hydrocarbon 0 1989 American Chemical Society
