Znd. Eng. Chem. Res. 1991,30, 1801-1810 Venkatesan, V. M. Fluidized Bed Thermal Recovery of Synthetic Crude from Bituminous Sands of Utah. Ph.D. Dissertation, University of Utah,Salt Lake City, 1980. Wang, J. The Production of Hydrocarbon Liquids from a Bitumen Impregnated Sandstone in a Fluidized Bed Pyrolysis Reactor.
1801
M.S.Thesis, University of Utah, Salt Lake City, 1983. Received for review July 17,1990 Revised manuscript received January 23,1991 Accepted March 22,1991
Hydrodesulfurization and Hydrodemetalation Reactions of Residue Oils over CoMo/Aluminum Borate Catalysts in a Trickle Bed Reactor Ming-Chang Tsai and Yu-Wen Chen* Department of Chemical Engineering, National Central University, Chung-Li, 32054 Taiwan
Ben-Chang Kang, J u n g - C h u n g Wu, a n d Li-Jen Leu Refining & Manufacturing Research Center, Chinese Petroleum Corporation, Chia- Yi, 60036 Taiwan
A series of aluminum borates (AB) with various Al/B mole ratios was prepared by the precipitation method. These samples have been characterized with respect to surface areas, pore volumes, pore size distributions, thermal stabilities, and mechanical strength. The results indicated that the exhibited properties are dependent on the Al/B ratio of the material. The monodisperse pore size distributions of these samples imply that it is a true microcomposite structure rather than a mixture of the individual materials. Hydrodesulfurization (HDS) and hydrodemetalation (HDM) of heavy Kuwait atmospheric residuum over CoMo/AB catalysts were carried out in a bench-scale trickle bed reactor at 663 K and 7582 kPa. The weight hourly space velocity of residue oils was 1.5, and the hydrogen flow rate was kept constant a t 300 mL/min(STP). The results showed that these catalysts are much more active than the conventional CoMo/A1203 catalyst in HDS and HDM reactions. The results of desulfurization activity are mainly interpreted on the basis of difference in dispersion and the interaction of Mo species with the support. The demetalation activity was strongly influenced by the intraparticle diffusion of metal porphyrins. A general correlation could be established between the HDS activity and the reducibility of oxidic precursor. Larger surface area and high reducibility of active metal may produce more active sites and result in high HDS activity. In addition, larger pore size can reduce the pore diffusion resistance and may produce more opportunity for HDM reaction. Introduction With sweet crudes in short supply and with increasing oil prices, hydroprocessing of heavy residual feedstocks becomes more attractive, resulting in even more stringent demands on the catalysts employed in the process (Topsoe et al., 1986). Petroleum residua, such as those from atmospheric and vacuum distillation of crude oil, that contain a high proportion of sulfur, metal, nitrogen, and asphaltene contaminants not only contribute to the problem of air pollution but also cause major problems in most refining processes such as hydrotreating and fluid catalytic cracking operation. Since asphaltenes are large molecules that consist of highly heterocyclic and aromatic rings to which sulfur, metals, and nitrogen are bonded, residuum hydrodesulfurization treatment is extremely difficult. Therefore, the removal of high concentrations of metals and sulfur from residuum for its effective use as fuel oil or as the charge stocks for cracking and hydrocracking operations is an important subject. Several authors (Ohtsuka, 1977; Stanislaus et al., 1988) have reported that hydrotreating of heavy oils is generally carried out over alumina-supported catalysts containing combinations of cobalt and molybdenum salts or nickel and molybdenum salts where cobalt and nickel play an important role in promoting the active sites of molybdenum-based sulfide catalysts in HDS. Previous reports (Tanabe, 1970) have shown that alumina-boria catalysts
* To whom correspondence should be addressed.
can be used as cracking catalysts. Pine (1976) employed boria-alumina compositions in the hydrocarbon conversion process and proved that they are particularly useful in the hydrocracking of petroleum feedstocks over the boriaalumina supported on various combinations of zeolite, nickel oxide, and molybdenum oxide. Sat0 et al. (1987) also reported that alumina-supported boria exhibited high catalytic efficiency for the vapor-phase Beckmann rearrangement of cyclohexanone oxime. Peil et al. (1989) reported that aluminum borate mixed oxides have unique pore structures and high surface acidities. In the present study, a precipitation technique at constant pH value was used to prepare a series of aluminum borates. Desulfurization and demetalation of heavy residue oils over CoMo/aluminum borate catalysts were tested in a trickle-bed reactor. The chemical and physical properties of these catalysts were characterized to obtain the correlation with their catalytic properties. Experimental Section Support Preparation. The aluminum borate support was prepared from common solutions of aluminum nitrate (A1(N03)3-9H,0)and boric acid (H3B03)using an ammonium hydroxide solution as a precipitant. A well-stirred container was charged with distilled water as buffer. The previously described two solutions were slowly added to this distilled water with the rate of addition controlled in order to maintain the pH of the solution at a constant value of 8.0 f 0.1. The resulting precipitate was filtered, washed with distilled water, and dried overnight at 373 K,
0888-5885/91/2630-1801$02.50/00 1991 American Chemical Society
1802 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
is nearly proportional to the rate of reduction. followed by calcination at 773 K in a muffle furnace for 5 h. By altering the relative amount of aluminum nitrate Bench-ScaleTrickle Bed Reactor. The bench-scale and boric acid, a desired aluminum borate composite could cocurrent downflow trickle bed reactor was used in this be obtained. study by modifying a commercial-scale one from the petroleum industry for HDS and HDM reactions. It is very Catalyst Preparation. The oven-dried uncalcined important to ensure the data are free from the influence aluminum borate material was grinded to powder, and then the solutions of (NH4)6M070u-4H20 and C O ( N O ~ ) ~ . ~ H ~ of O unwanted transport effects as much as possible, no matter whether in the small-scale or commercial-scale (both from Merck) in appropriate distilled water were reactor (Anderson, 1968; Rakesh and James, 1984). impregnated successively, where Moo3 content was 1 2 wt % and Co/Mo mole ratio was 0.6. The mixture was exThe evaluation of kinetic data from fixed bed catalytic truded by adding an appropriate amount of water. The reactor is usually based on the assumption of plug flow. extrusion diameter was about 1/16 in. (1 in. = 2.54 cm). However, in the small-scale reactor, deviations from plug The wet extrusion was dried at atmospheric pressure ovflow may be caused by several effects such as axial disernight and calcined at 373 K for 12 h, followed by calpersion, wall effect, and channeling. Several authors cination at 773 K for 5 h. This calcination temperature (Montagna and Shah, 1975; Satterfield, 1975) have preshas been reported to give optimal HDS activity (Stanislaus ented many criteria to evaluate the minimum reactor et al., 1988). For convenience, we denoted CoMo/alumilength and diameter necessary to avoid a significant disnum borate as CoMo/ABx, where x represents the Al/B persion effect. In general, a radial aspect ratio (ratio of mole ratio. bed diameter to the catalyst particle diameter) greater than 10 and axial aspect ratio (ratio of the catalyst bed length Characterization. Surface areas and pore volumes of to the catalyst particle diameter) greater than 20 seem to catalyst samples were determined by nitrogen adsorption suffice for an isothermal system. However, for a noniat 77 K (BET method) using a Micromeritics 2600 surface area analyzer. Pore size distributions for all supports and sothermal system, the radial aspect ratio and axial aspect catalysts were measured by the nitrogen adsorption meratio should be greater than 30 and 60, respectively. thod with a Micromeritics Digisorb 2500 instrument. This A cylindrical catalyst with diameter of 1.5 mm and enables us to measure pore size distributions in the radius length of 4 mm (Le., the equivalent spherical diameter is range 10-300 A. The measurements were performed on 2.4 mm) was used in this study. A stainless steel tube the oxidic form of the catalyst samples. Sample weights reactor of internal diameter 14.3 mm, outer diameter 25.4 of about 500 mg were used. Since the average pore sizes mm, and length 430 mm was applied. From the above were not large, the mercury penetration method was not criteria, radial dispersion seems to exist. The policy is that valid for such catalysts in the present study. the catalyst bed was diluted with 50-70-mesh sand (Merck) in order to reduce the dispersion effect and to create a In order to investigate the thermal properties of alumore homogeneous thermal distribution in the reactor. In minum borate materials, differential thermal analysis a typical run, the reactor was packed with 20-g CoMo/AB (DTA) and thermal gravimetric analysis (TGA) were carried out on Perkin-Elmer 1700 and TGS-2 analyzers, catalyst extrudates. Each of the catalyst extrudates was respectively. The metal contents of aluminum borate carefully hand-picked. The length of catalyst used was supports and CoMo/aluminum borate catalysts were controlled within the range 3-5 mm. Sand (4 g, 4 g, 4 g, characterized by means of inductively coupled plasma 4 g, 4 g) and catalyst (3 g, 5 g, 6 g, 6 g) were loaded one atom emission spectroscopy (Jarrell Ash Model 1100). To after another. The inert to catalyst weight ratio is about 2. The height of catalyst bed was about 350 mm in all investigate the crystallographic properties of the catalysts, cases. In addition, on the basis of the considerations of X-ray powder diffraction (XRD) analysis was performed avoiding entrance effects and exit effects, as a distributor, by using a Siemens D-500 X-ray diffractometer operated and as packing to support the catalyst bed, the remaining at 30 kV and 20 mA with Cu K a radiation. The mespaces at the top and bottom of the reactor were filled with chanical strength of catalysts was studied by a Toyama 40-60-mesh ceramics (Merck). Since a packed-bed reactor Sangyo TH-203CP tablet hardness tester. is usually operated as an integral reactor, Le., with a subTemperature-Programmed Reduction. The temstantial change of conversion from entrance to exit and perature programmed reduction (TPR) technique has been results in the reaction more severe at the top region in the extensively used to chemically characterize the supportreactor, the arrangement of catalyst extrudate was ined-metal catalysts. In this study, TPR was used to discreased gradually from inlet to outlet. The reactor was tinguish different reducibilities on various C00-M003/AB heated and controlled by means of three electric resistCatalysts. The catalyst was loaded in a U-type quartz tube ances, and the temperature over the reaction zone was kept reactor, and the catalyst bed was held between two quartz constant. Careful and skillful control of temperature was wool plugs. In order to remove the adsorbed water of needed. The reaction temperature was monitored with catalyst before proceeding with TPR, the sample (100 mg) three thermocouples. One of the thermocouples was set was flushed with nitrogen at a flow rate of 100 mL/min in the center of tube reactor; the other two were located at 393 K for 4 h. A 10% H2/Ar mixture gas stream at a outside the tube reactor along the length of the reactor. flow rate of 50 mL/min was then passed through the reactor to reduce the catalyst. The temperature of the rePresulfiding the Catalysts. In order to promote the actor was programmed to increase linearly from room activity of catalyst, presulfiding treatment was necessary. temperature to 1143 K at a heating rate of 10 K/min and The diesel oil feedstock, which was doped to 1wt % sulfur then retained isothermally for 1 h. The water formed by addition of dimethyl disulfide, was passed through the either by reduction or from dehydration process was reactor with the following temperature program: heated from room temperature to 448 K and kept for 2 h; then trapped in a 4A molecular sieve. The temperature of the catalyst was recorded by a two-pen recorder, and apparent increased to 523 K and held for 4 h; after that, increased hydrogen consumption data were monitored continuously to 598 K and retained until sulfiding completely. The with a thermal conductivity detector (TCD); both result weight hourly space velocity (WHSV) of diesel oil was 2.4. in a TPR spectrum. By use of a high flow rate to minimize The hydrogen flow rate was 300 mL/min(STP) and the pressure was maintained at 2757 kPa. diffusion resistance and thermal effects, the TCD response
Ind. Eng. Chem. Res., Vol. 30, No. 8,1991 1803
vent
I
11
I:) 82
Figure 1. Flow diagram of HDS and HDM teat unit: (1) ATB tank; (2) stop valve; (3) feed pump; (4) check valve; (5) reactor; (6) oven; (7) receiver; (8) separator; (9) back-pressure regulator; (10) condenser; (11) absorber; (12) compressor; (13) three-way valve; (14) needle valve; (15) mass flow controller. Table I. Physical and Chemical Properties of Kuwait Residue Oil OAPI 16.8 sulfur, wt % 3.72 nitrogen, w t % 0.21 Conradson carbon residue, wt % 10.1 nickel, ppm 14.0 vanadium, ppm 53.0 H/C ratio 0.146 ASTM distillation 5% 554 K 584 K 10% 633 K 20% 30 % 673 K 40 % 704 K 50 % 743 K 60% 794 K 65 % 811 K
HDS and HDM Activity Test. HDS and HDM of heavy residue oils over CoMo/AB catalyst were carried out in a bench-scale trickle bed reactor at 663 K and 7582 Wa. The continuous downflow trickle bed reactor system is shown in Figure 1. After presulfiding of the catalyst, the Kuwait atmospheric tower bottom (ATB) residue was passed through the reactor with a WHSV of 1.5. The hydrogen flow rate was the same as that in the presulfiding step. The characteristics of Kuwait ATB residue feedstock are shown in Table I. The ATB residue feedstock was supplied by a Lewa proportioning pump (type FCM1) and passed the reactor (0.5mL/min) at 663 K and 7582 kPa. Pure hydrogen was supplied by a high-pressure compressor (Novaswiss compressor). The hydrogen flow rate (300 mL/min at STP) was set by using a Brooks mass flow controller (Model 5872 A). The ATB liquid feed and hydrogen gas were passed over the fixed bed of catalyst in a cocurrent downflow mode. The liquid product was collected in high-pressure accumulators. Gas from the high-pressure accumulators then left through a backpressure regulator (Mity-Mite Model 90 W) and passed through a wet test meter. Every effort was made to maintain the reactor isothermally. Temperature profiles with 2-5 K variations were typically observed. A t an appropriate time, the hydrotreated residue oil samples were withdrawn to measure sulfur and metal (nickel, vanadium) contents by X-ray fluorescence spectrometry (Oxford, LAB-X 2000) and ICP-AES (Jarrell Ash Model 1100), respectively. The flow pattern diagram reported by Charpentier and Favier (1975)was used to check the flow type. The analysis indicated that the reactor in this work was indeed operated in the trickle flow mode.
Table 11. Physical Properties of Aluminum Borate supports surf. area, pore v01, av pore SUDDOrt m2/g mL/a diam, A AB1 92 0.40 174 AB2 210 0.46 88 AB3.5 270 0.36 53 AB5 229 0.35 61 AB8 242 0.36 60 AB10 0.40 229 70 AB12 0.39 239 65 0.44 AB20 294 60 Table 111. Physical Characteristics of CoMo/Aluminum Borate Catalysts surf. area, pore vol, av pore catalyst m2/g mL/g diam,A CoMo/ABl 81 0.31 153 CoMo/AB2 174 0.30 69 CoMoIAB3.5 231 0.27 47 CoMo/AB5 211 0.29 55 CoMo/AB8 209 0.28 54 CoMo/AB10 214 0.30 56 CoMo/AB12 207 0.30 58 CoMo/AB20 268 0.32 48 190
300 -250 m
150
L I
m W
200
_.
m
s150
110
5
90
5
L
L
wl 2
100
70
m
50
-50 2
1 6
1
6 lb 15 l i 16 18 20 AL/B
Figure 2. Surface area and average pore diameter versus A1 to B mole ratio.
Results and Discussion Textural Properties. Surface areas and pore volumes of aluminum borate supports and CoMo/AB catalysts are given in Tables I1 and 111, respectively. Examination of Tables I1 and I11 shows that the incorporation of a little boron into the alumina structure had little effect on the specific surface area, whereas, when the amount of incorporated boron was large, the specific surface area was strongly influenced by the boron content. However, no matter whether the AI to B atomic ratio was stoichiometric or nonstoichiometric, there was hardly any effect on pore volume. These results are in agreement with those in the literature (Peil et al., 1989). The results presented in Table I11 demonstrate that both the surface areas and pore volumes of aluminum borates were decreased as the cobalt and molybdenum were impregnated, possibly due to the plugging of small pores by impregnation. This phenomenon is probably due to the redistribution of active material via the vapor phase as M o O ~ ( O Hand ) ~ COO(OH)~ (Sonnemans and Mars, 1973;Thomas et al., 1983). Figure 2 shows the BET surface areas and the average pore diameters versus A1 to B mole ratios of aluminum borates. The average pore diameter shown here was introduced by the common definition of 4V,/S, for cylindrical pore, where V, and S, represent the pore volume and internal
1804 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 Table IV. Pore Volume Dietribution Data of Aluminum Borate Supports pore radius range, - A support 10-20 20-30 30-40 40-50 50-75 0.008 AB1 0.010 0.009 0.019 0.098 AB2 0.017 0.167 0.202 0.039 0.021 AB3.5 0.062 0.194 0.018 0.014 0.072 0.008 AB5 0.049 0.206 0.069 0.018 0.057 0.239 0.048 0.011 0.003 AB8 AB10 0.008 0.076 0.293 0.024 0.003 AB12 0.044 0.278 0.048 0.010 0.009 AB20 0.147 0.250 0.023 0.007 0.006
75-100 0.109 0.005 0.003 0 0 0
0.004 0.004
Table V. Pore Volume Distribution Data of CoMo/Aluminum Borate Catalysts Dore radius range, A catalyst 10-20 20-30 30-40 40-50 50-75 75-100 100-150 CM/AB1° 0.006 0.008 0.008 0.020 0.086 0.085 0.093 0.076 0.133 0.008 0.012 0.010 0.010 CM/AB2 0.025 CMlAB3.5 0.123 0.100 0.008 0.004 0.002 0.002 0.004 0.054 CM/AB5 0.147 0.011 0.005 0.002 0.008 0.006 CM/AB8 0.157 0.066 0.005 0.003 0.003 0.002 0.003 CM/AB10 0.178 0.093 0.005 0.002 0.003 0.002 0.005 0.185 0.043 0.006 0.004 0.004 0.003 CM/AB12 0.004 CMIABPO 0.207 0.041 0.006 0.004 0.004 0.003 0.011
100-150 0.132 0.006 0 0 0 0 0
15e200 0.012 0.003 0 0 0 0 0 0
0.004
150-200 0.005 0.005 0.005 0.019 0.004 0.008 0.006
0.014
200-300 0 0.006 0.008 0.034 0.010 0.014 0.021 0.014
>300 0 0 0.012 0.019 0.024 0.006 0.021 0.016
"CM/ABl denotes CoMo/ABl.
surface area of catalyst,respectively. The effect of addition of boron on the characteristics of pore structure is, therefore, clearer. The results of pore size distribution measurements of aluminum borates and CoMo/AB catalysts are shown in Tables IV and V, respectively. The pore size distributions of supports have the same trend as the surface areas and pore volumes, Le., shifted toward smaller pores after impregnation of Co and Mo. From examination of the pore size distributions of CoMo/aluminum borate cataly~ts,one could reach the following general conclusions. The medium pore diameter, i.e., the pore size for dV,/dr, maximum in the pore size distribution, has a value between 35 and 40 A for all but one of the catalysts. The exception is the CoMo/ABl catalyst, which has a very broad pore size distribution curve in the range of about 50-120 A, and its mean pore diameter is, then, difficult to determine. The other important information that can be extracted from the pore size distributions is the lack of any bimodal pore distribution as would be expected for materials consisting of simply a mixture of two oxides. This result indicates that these materials are not just mixtures of alumina and diboron trioxide. To be more specific, the monodisperse pore size distribution of aluminum borate implies that it is a true microcomposite structure material rather than a mixture of the individual materials. Mechanical Strength. The mechanical strength of a catalyst generally includes attrition resistance, hardness, and compressive strength. In a packed bed reactor, the resistance to crushing of catalyst is especially important, but there are as yet no accepted standard tests analogous to the common use of the BET method for determining total surface area. This is in part because many factors may come out during the preparation of catalyst, such as resulting from chemical attack or thermal shock, that are highly specific to the process and result in much reduction in mechanical strength. However, various methods have been developed by ASTM and have been used to assess catalyst strength, including the side crushing strength, the bulk crushing strength, and the attrition resistance. Crush tests may be made in a variety of ways on individual pellets or on beds of catalyst. In order to investigate the effect of boron content on mechanical strength, the side crushing strength of the
B/Al
Figure 3. Mechanical strength versus B to A1 mole ratio.
catalysts was measured. In the traditional measurement of side crushing strength, a catalyst particle is placed between metal plates to which an increasing pressure is a p plied until the particle breaks. Such a method has been described in ASTM D4179/82 for tablets and spheres though not for extrudates. Shell Company developed a similar method to measure the crushing strength per unit length of extrudak, this modified method was used in this study. Figure 3 demonstrates the mechanical strength versus boron to aluminum mole ratio. The results show an exponential correlation. The higher the boron content in CoMo/AB catalyst, the weaker is the strength. This implies that the incorporation of boron into the alumina framework may reduce the interactions between particles. Since no other binder or lubricant additives except distilled water were used during the course of extruding process, it is, therefore, hypothesized that the main bonding types between particles are AI-0-B, A1-0-AI, and B-0-B. (These chemical bonds between particles may be formed by means of calcination.) This indicates that the probabilities of forming the A l U B and B U B chemical bonds will rise as the boron content increases and finally result in a weaker mechanical strength of the catalyst. Differential Thermal Analysis. The thermal stabilities of various aluminum borate materials were examined by DTA, TGA, and XRD. The DTA curves are plotted in Figure 4, and the results gave us information on dehydration and phase transformation for aluminum borates. The DTA curves for all samples demonstrate
Ind. Eng. Chem. Res., Vol. 30,No. 8, 1991 1805 a : AB( 1 1
b : AB( 2 1 c :ABl3.5)
II
OTA
I
h
TGA
I
+ .C
3
> m * L
.-
e 4
150
330
510
690
870
Temperature ('C) I
70
290
510 730 950 1170 Temperature ( Y 1
Figure 4. Differential thermal analysis of aluminum borate materials.
unregularly at low temperature; these phenomena may be due to the instability of the instruments. However, the dehydration phenomena can still substantially be observed by DTA curve. As the temperature increases, two exotherms were observed by DTA for all the aluminum borates which occurred in two temperature ranges. The first exotherm took place between 280 and 800 O C and the second between 800 and 1200 "C. The first exotherm can be attributed to the initial formation of the oxide and the combustion of the impurity. Since the instability of boron compounds was identified as described previously, it is, therefore, the probabilities of forming the impurities such as nitrate bonded with a dramatically broad temperature range. One interesting feature is that the trend of broad exotherms for all samples is the same. In contrast, the second exotherm occurred a t higher temperatures and concentrated in a relative narrow temperature range for each sample. Unlike the first exotherm, the temperature of the second exotherm was from 800 "C shifted toward about 1200 O C and the exotherm signal was gradually decreased as the A1 to B mole ratio varied from 1 to 20. The second exotherm is ascribed to the phase transformation, Le., the formation of a crystalline phase characteristic of aluminum borates. Thermal Gravimetric Analysis. The TGA curves are presented in Figure 5. Examination of TGA curves showed that they exhibit a sharp decrease at lower temperatures followed by a gradual decrease a t higher temperatures for all samples. The sharp decrease in the weight of aluminum borates at low temperature can be attributed to the release of the majority of the moisture, and the gradual decrease at high temperature can be ascribed to the release of the surface and structural water and the burning off of impurities. Unfortunately, the weight loss that results from the phase transformation and surface hydroxyl condensation could not be observed in this study, since the detected temperature was not high enough. However, the results of TGA are in part consistent with the results of DTA. X-ray Powder Diffraction. The XRD results showed that all the aluminum borates demonstrate an amorphous structure after calcination at 500 "C, even the aluminum borate material with a ratio of B/A1 of zero (i.e., pure alumina). However, if conventional alumina is calcined
Figure 5. Thermal gravimetric analysis of aluminum borate materials.
at this temperature, the y-AlzO3 phase is formed (Marcelin et al., 1984). This may be due in part to the preparation method and medicine source employed in this study. The characteristics of alumina prepared in this study are different from those of traditional alumina obtained by hydrolysis of aluminum isopropoxide. Gielisse and Foster (1962) presented a tentative phase diagram for the A1203-B203system, showing that the dominant phase between temperatures of 300 and 2000 OC was expected to be 9Al2O3*2BzO3 for the materials consisting of less than 10% Bz03 and 2Al2O3-BzO3for the materials consisting of more than 25% Bz03between the temperatures of 470 and 1035 "C. However, all these phases were not detected for samples calcined at 1000 "C in this study. Different XRD results can be due in part to the alternative synthesis method, since their aluminum borates (or so-called alumina-boric oxide mixtures) were prepared from high-purity y-alumina and crystalline boric oxide in the literature (Gieliesse and Foster, 1962). It is interesting to note that the alumina, prepared in this study, appears as XRD signals of a-Al2O3phase after calcination at 1000 O C , even though its intensity is not very high. This indicates that the thermal stability of alumina prepared in this study is higher than the conventional one. In addition, the 9A1203.2Bz03phase is more stable than the 2A1203-B203phase. The crystallization of 2AlzO3*B2O3 from an amorphous phase and the decomposition of 2AlZO3.B2O3 into 9Al2O3-2BzO3 have been previously reported (Gielisse and Foster, 1962; Peil et al., 1989). The XRD patterns of CoMo/ABl and CoMo/AB2 demonstrate a Moo3 crystalline phase although the intensity is not strong. It is hypothesized that the crystalline bulk oxide and surface compounds are formed. However, those supported on other aluminum borate supports did not show a crystalline phase. It is hypothesized that microcrystals are formed. However, the crystals are too small to be detected by XRD. The results can be interpreted in terms of the differences in metal dispersion and surface acid amount of supports. AB1 and AB2 have relatively smaller surface areas than others and thus possess relatively high metal contents per unit surface area (so-called lower dispersions). This would result in the presence of bulk compounds. The presence of bulk compounds is also influenced by the other factor, i.e., the pH value of impregnation environment. In an acid impregnation solution, some surface hydroxyl of the support was ionized to possess positive
1806 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 Table VI. Characteristics of CoMo/Aluminum Borate and Commercial Catalysts CoMotABl CoMotAB2 CoMOtAB3.5 CoMotAB10 BET surf. area, m2/g 81 174 231 214 0.30 0.27 pore vol, mL/g 0.31 0.30 8.41 7.79 Mo, wt % 8.64 7.53 2.94 co, wt % 3.16 3.09 2.82 0 0 Ni, wt % 0 0 0 Ti, w t % 0 0 0 42.1 60.2 72.5 66.8 sulfup removal, w t % 53.2 52.1 58.2 53.5 metalb removal, w t %
GC-106 129 0.51 8.82 1.25 0.44 0.09 51.5 44.7
GC-107 127 0.46 8.16 0.54 3.03 5.80 55.0
OPbAt steady state.
.-C
A
3
0
a
i,
8
12
16
20
Ai/B
m c L
Figure 7. TPR T,,
.-
n
versus A1 to B mole ratio.
L
G
200
400
600
Temperature
800
('C 1
Figure 6. TPR patterns of active metals Co and Mo supported on various supports.
charge and preferred to adsorb the oxomolybdenum anions. This phenomenon can easily be identified from Table VI, since the same metal (Co and Mo) amounts were used for each carrier during impregnation. However, the exact impregnated metal content on the aluminum borate support was decreased as Al/B mole ratio of the support increased. The equilibrium relationship between molybdate species may be written as follows (Wang and Hall, 1980; Duchet et al., 1983): 7[M004]~-+ 8H+ [Mo,024Je- + 4Hz0 (1)
+.
Duchet et ai. (1983) proposed that the polymeric (octahedral) molybdate species are more easily formed than the monomeric (tetrahedral) ones in the presence of acid. It is known that the AB1 and AB2 materials have higher acid concentrations, and polymeric Mo oxides should be easily formed on these supports. In contrast, the monomeric molybdate species are easily formed on the aluminum borate supports with high Al/B ratios, which have lower acid concentrations. On the basis of the results of DTA, TGA, and XRD, it is clearly concluded that the aluminum borate materials possess a thermal stability superior to that of alumina, even if only a little boron was incorporated into the alumina structure. Temperature-Programmed Reduction. The TPR patterns of active metals Co and Mo supported on various aluminum borate supports are shown in Figure 6, which demonstrates a two-peak profile for all samples. The low-temperature-reductionpeak has been attributed to the reduction of octahedrally surrounded Mo6+multilayer, and the high-temperature-reduction peak can be attributed to
the reduction of tetrahedrally and octahedrally surrounded Mo6+monolayer (Thomas et al., 1982a,b; Arnoldy et al., 1985a,b). Figure 7 shows the low-temperature-reductionpeak T,, versus AI to B mole ratio of the catalyst. The T,, sharply decreased at first and then maintained a nearly constant value as boron content increased. This indicates that, for high boron content materials, the lower the A1 to B mole ratio, the higher the T,, is. However, the boron content had little effect on T,, for Al/B greater than 3.5. It has been reported (Arnoldy et al., 1985a,b)that in the oxidic catalyst the Mo-0-A1 links polarize the Mo-0 bonds, making them more difficult to reduce. In this study, A13+ in the BO3" environment has a stronger ability to polarize the Mo-0 bonds than in pure A1203. The results indicated that the higher the Al/B mole ratio of AB, the smaller is the acid amount, the lower is the T,, in the TPR spectrum, and the greater is the reducibility of MOO,. HDS Activity. Since all the catalysts tested were advanced HDS catalysts, under normally recommended operation conditions all samples gave greater than 90% conversion of the sulfur compounds to hydrogen sulfide. In order to magnify any difference in activity, the tests were carried out under conditions that were less severe for the catalysts than those used in commercial practice. Under the conditions used in this study, the different catalysts showed differences of conversions varying over the range 30-909'0. Figure 8 shows the comparisons between the activity and stability of CoMo/AB catalysts and those of commercial catalystssuch as Chevron GC-106 and GC-107. The results showed that the Al to B mole ratio has little effect on either the stability or the activity except CoMo/ABl, whose activity and stability are poor, Comparing with the commercial catalysts (GC-106 and GC-1071, it was found that the CoMo/AB catalysts are much more active, at least in the fmt 72 h. The coke formation in HDS catalysts results in a rapid decline in activity in the initial period and the metal deposition results in a slow decline in activity. In order to get rid of the influence of the coke deposition, the initial reaction data were chosen for comparison in the
Ind. Eng. Chem. Res., Vol. 30, No. 8,1991 1807 KUWAIT RESlOUE
0.8
I
I !
.-
20
IO
3b
&
sb
60
70
Time on stream I h r 1 Figure 8. ATB HDS activity versus time on stream.
I
.
I Pore Volume (30-70A diameter 0) pores) ~ ~ ( 3 0 - 7 cm'/g
--
60-
Figure 11. HDS activity versus effective pore volume.
c
L
3
vl 3
50-
LO
0'
. V GC-107
340
,bo
Iio
i o
A
Surface Area ( m2/g)
Figure 12. HDS activity versus specific surface area.
Relationship between HDS Activity and Sulfidability. It has been widely accepted that Mo is an essential part of hydrodesulfurization catalysts CoMo/A1203and NiMo/A120B. Duchet et al. (1983) proposed that Mo supported on carbon carrier still has a high activity even in the absence of promoter such as Co or Ni. It is, therefore, of importance to investigate which characteristics of Mo catalysts determine the HDS activity. The prepared catalysts were sulfided to obtain their active form. During the course of sulfidation, reduction (e.g., by hydrogen) and sulfidation (e.g., by dimethyl disulfide) may take place in competition with each other. There are three paths that refer to sulfidation (Hallie, 1982), where the best sulfided path is when reduction and sulfidation proceed simultaneously. At present, at least four models have been reported in accordance with the structure of active catalytic sites,which include the monolayer model (Lipsch and Schuit, 1969; Schuit and Gates, 19731, the intercalation model (de Beer and Schuit, 1976), the contact synergism model (Delmon, 1977), and the Co-Mo-S model (Topsoe et al., 1986). Different methods of preparation and measurements result in the differences of interpretation among models. Among various models, the Co-Mo-S model was the best accepted. In order to investigate the effect of Mo on HDS activity, the surface coverage of catalyst was used in this study. The
1808 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 2.0 I
1.5-
&
1.0 -
c
a
\ &
1 0 co"o/ABlll c o w w a i
cono/&n(i 11
\. L
A
A
cono/~llol
A
oc-107
W
0
0.5-
A
O\
CO"O/AQlii'
40 400
500 Tmax
600
('C)
Figure 14. HDS activity versus TPR 7'-.
where X, is the sulfur removal (wt %), Co is the sulfur concentration in the feed (mol/L), k, is the HDS secondorder reaction rate constant (L/(mol*h)),and the WHSV is the weight hourly space velocity (l/h). The QTOF represents how many volumes of ATB were treated per mole of active metal per second. Since Co was recognized as a promoter, the quantity of Co was not counted for active metal, i.e., the calculated surface coverage only included Mo. Figure 13 shows the QTOF values of the catalyst versus surface coverage of Mo atoms. The QTOF was strongly influenced by the surface coverage. The QTOF (or the HDS activity) rapidly decreased to about 2.5 Mo atoms/nm2, and then leveled off. One interesting feature of Figure 13 is that the phenomenon of QTOF as a function of the surface coverage is more similar to Mo/Si02 than to Mo/A1203 (Thomas et al., 1983; Scheffer et al., 1988). This can be interpreted on the basis of support effect: probably the incorporated boron, whether the amount of boron was much or little, transferred the characteristics of alumina and resulted in the characteristics of aluminum borates rather similar to silica ones. The sulfidability of catalyst especially for Moog was generally used to explain the HDS activity. It is known that sflidability depends on dispersion and on interaction with the support. In this study, the Moo3 bulk compounds are present on CoMo/ABl and CoMo/AB2, indicating a lower dispersion, and the rate of sulfiding is, therefore, determined to a large extent by mass-transfer limitations. Thus, a sulfide layer is formed around an oxidic core of MOO* Even if the sulfiding approaches completely as the time on stream increases, i.e., the oxidic core also be sulfided, only the front MoSz is a precursor and, therefore,
contributes to the HDS activity. In contrast, the masstransfer limitation is absent in highly disperse samples such as COMO/AB3.5 and CoMo/AB10, and the sulfiding rate is influenced by the interaction with the support. It has been widely accepted that, in the CoMo supported catalyst TPR profile, the first peak at low temperature is the octahedrally surrounded Mo6+"multilayer" reduction peak, and the second peak a t high temperature is the octahedrally and tetrahedrally surrounded Mo6+ "monolayer" reduction peak (Arnoldy et al., 1985a,b). However, the monolayer species has little contribution to HDS activity, and only the multilayer MOO, is the precursor. Chen et al. (1990) studied the reducibility of the M o / M and CoMo/AAP catalysts and reported that the position of the low-temperature-reduction peak is not affected by adding the second metal Co at the Co/Mo mole ratio ca. 0.6. The same results for CoMo/y-A1203have also been reported (Arnoldy and Moulijn, 1985; Arnoldy et al., 1985a,b). These imply that the effect of Co on TPR pattern of CoMo-supported catalyst may be negligible especially for the low-temperature reduction peak. Therefore, it is meaningful to find a possible correlation between T,, (the temperature of the first reduction peak maximum) and HDS activity. The HDS activity versus TPR T,, is represented in Figure 14. The resulta showed that the HDS activity is strongly influenced by the TPR 2'". The higher the T- is, the lower the HDS activity is. From the TPR patterns, CoMo/ABl and CoMo/AB2 both exhibit strong interactions with the supports. A strong interaction decreases the covalent character of the Mo-S bond and results in the reduction of anion vacancy. The anion vacancies on Mo are the active sites for HDS reaction; in other words, the strong support interaction hinders the formation of active sulfide phase and results in poor HDS activity. A similar explanation can be found in the literature (Duchet et al., 1983). In addition, the stronger support interaction (the higher T-) yielded the majority of surface active sites with a higher activation energy. It, therefore, results in a poor HDS activity. CoMo/ABl and CoMo/AB2 both possess lower dispersion and stronger support interaction and result in poor HDS activity. However, the dispersion effect and the interaction effect cannot be interpreted very well from the differences of the HDS activity for CoMo/AB3.5 and CoMo/AB10 samples, since their surface coverages and support interactions are nearly the same. A tentative explanation for the HDS activities over CoMo/B3.5 and CoMo/AB10 is that the acid properties of the catalysts predominate over the HDS activity at higher Al to B mole ratio. HDM Activity. Since the asphaltene sheets in the residue oils are composed of nickel and vanadium por-
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1809
1
I ? I
04 0
30
15
I
I
45
GO
75
Time On Stream ( h )
Figure 17. Nickel removal veruss time on stream. a
0
10
20
30
LO
I
I
50
60
95 I
!
70
Time On Stream [ h r
Figure 16. ATB HDM activity versus time on stream.
-
/O
I
/
'I
I
0
15
30
45
GO
7
Time On S t r e a m ( h )
Figure 18. Vanadium removal versus time on stream. 50
0
40
80
120
160
200
Average Pore Diameter(i\)
Figure 16. Metal removal versus average pore diameter.
phyrins, such asphaltene sheets are stacked one above the other bounded by 7~ bonds of the polynuclear aromatic centers and aliphatic bridges, which results in the molecular sizes of so-called asphaltenes being larger than sulfur compounds. Therefore, the mechanisms of HDS and HDM are quite different. Figure 15 shows the comparison between the activities of CoMo/AB catalysts and that of the commercial demetalation catalyst Chevron GC-106. The results showed that all the CoMo/AB catalysts possess stabilities superior to that of GC-106. In addition, as far as metal removal is concerned, all the CoMo/AB catalysta are also more active than the commercial one. Unlike the situation of desulfurization activity, the CoMo/ABl catalyst gives the highest demetalation activity. It is meaningful and interesting to find a possible correlation between the average pore diameter of catalyst and the HDM activity. The results are represented in Figure 16. The HDM activity was strongly influenced by the pore size of catalyst. Obviously, the extent of effect of pore diffusion on HDM reaction is far larger than on HDS reaction. In addition, it is worth mentioning that although the larger average pore diameter can give the higher percentage of metal removal, the extent of catalyst decay for the larger pore catalyst is also greater than that for the smaller pore one, simulheously. This cross-linked problem can be attributed to the pore diffusion effect and the coke deactivation behavior for the former and the latter, respectively. In regard to the coke phenomena, it is well-known that small surface area and high surface acid concentration may result in serious coke formation which deposits on the surface of the catalyst and makes the activity rapidly decay. This coking characteristic is valid for both HDS and HDM reactions in the initial period and
can be perfectly used to interpret the deactivation of HDS and HDM in this study. The CoMo/AB3.5 has a small pore relative to others, i.e., its pore diffusion resistance is greater than others. One would expect that the percent of metal removal on this catalyst should be not high. In contrast, it poeseases a very high HDM activity. It should be noted that the sulfur contents are in wt % , but the metal contents (V+ Ni) are in ppm. In addition, it must be emphasized that the CoMo/AB3.5 catalyst has a high surface area and a very good metal dispersion. This would result in a very high activity for HDS and HDM. The high HDS activity for CoMo/AB3.5 has been confirmed previously. At this stage, although the intraparticle diffusion for CoMo/AB3.5 is slower than others, once metal porphyrin diffuses to the surface of active sites, it will react immediately. Moreover, the metal content is less than the sulfur content in residuum. This would result in the high HDM activity of CoMo/AB3.5. In order to further investigate the demetalation activity, the denickelation and devanadization were included for discussion. Figures 17 and 18 show the denickelation and devanadization versus time on stream, respectively. It is clearly seen that the devanadization activity is higher than the denickelation activity for all catalysts. This is because vanadium is more active than nickel; Le., the ability of vanadium to be adsorbed on active site of catalyst is stronger than nickel. Hung and James (1980) considered the stereochemistry of nickel- and vanadium-bearing structures and expected the rate of vanadium removal to be greater than the rate of nickel removal. In addition, they also claimed that the presence of vanadyl compounds will suppress the nickel removal reaction, and the reverse suppression is less significant. Conclusion Incorporation of a little boron into the alumina framework had little effect on the specific surface area. In
1810 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
contrast, when the amount of incorporated boron is large, the specific surface area is strongly influenced by the boron content. However, no matter whether the aluminum borate material is stoichiometric or nonstoichiometric, there is hardly any effect on pore volume. The instability of boron compound can easily be viewed from the elemental analysis results. One interesting feature is that no matter how the stoichiometry varied, only approximately half of the boron was incorporated into the alumina structure. In addition, increasing boron content reduced the interaction between interparticles, which resulted in a weaker attrition strength of the catalyst. The monodispersed pore size distributions for all aluminum borate samples indicated that it is a true microcomposite rather than a mixture of alumina and diboron trioxide. These materials also possess a better thermal stability than alumina. The results showed that the CoMo/AB catalysts are more active than the conventional CoMo/A1203 catalyst in HDS and HDM reactions. From the activity viewpoint, CoMo/AB catalyst may be useful and can be used as an HDS and HDM catalyst. A correlation exists between the reducibility of the oxidic precursor of CoMo/aluminum borate and the HDS activity of the sulfided samples. The higher the reducibility, the higher is the HDS activity. The HDS activities of catalysts on different supports can be rationalized on the basis of the nature of species in the oxidic catalysts. This strongly suggests that during sulfiding the dispersion and at least a part of the itneraction with the support of the oxidic species are preserved. Larger surface area and high reducibility of active metal may produce more active sites and result in a high HDS activity. In addition, large pore size can reduce the pore diffusion and produces more opportunity for HDM reaction.
Acknowledgment Financial support from the Chinese Petroleum Corporation of the Republic of China is gratefully acknowledged.
Nomenclature co = feed sulfur concentration, wt 90 k, = apparent rate constant for second-order rate equation, L/(mo1.h) = pore radius of catalyst, 8, = internal surface area of catalyst, m*/g = pore volume of catalyst, cm3/g dHSV = weight hourly space velocity, l / h X, = desulfurization conversion, wt %
3
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Received for review July 16, 1990 Revised manuscript received April 5, 1991 Accepted April 22, 1991
