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Ind. Eng. Chem. Res. 1990, 29, 1830-1840
Hydrodesulfurization Reactions of Residual Oils over CoMo/Alumina-Aluminum Phosphate Catalysts in a Trickle Bed Reactor Yu-Wen Chen,* Wen-Chang Hsu, and Chiang-Shiang Lin Department of Chemical Engineering, National Central University, Chung-Li, 32054 Taiwan, ROC
Ben-Chang Kang, Shwu-Tzy Wu, Li-Jen Leu, and Jung-Chung Wu Refining & Manufacturing Research Center, Chinese Petroleum Corporation, Chia-Yi, 60036 Taiwan, ROC an important role in the conversion of sulfur-laden residual oils. Cobalt molybdate catalysts are commonly used for the process. In the present study, a series of alumina-aluminum phosphates (AAP) was prepared by a precipitation technique. These materials were used as a support of CoMo HDS catalysts. The samples were characterized by X-ray diffraction, BET surface area, and mercury-penetration pore volume measurements. The temperature-programmed reduction method was used to monitor the reducibility of Mo02. HDS of Kuwait atmospheric residuum over these catalysts was carried out in a cocurrent down-flow trickle bed reactor at 663 K and 7582 kPa. The reaction data can be fitted very well with a pseudo-second-order rate equation. The activation energy is 29 kcal/mol. A correlation exists between the reducibility of the oxidic precursor and the HDS activity of the sulfided samples. The higher the reducibility of the oxidic samples, the higher the HDS activity of the sulfided samples. Larger surface area, smaller acid amount, and weaker interaction of AAP supports make the metal more highly disperse, produce more active sites, and result in a high initial HDS activity.
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Catalytic hydrodesulfurization (HDS) plays
Introduction
variable pore size distribution and surface area properties depending on its stoichiometry. It has a monodispersed pore distribution. It is resistant to surface area loss when contacted with water and has been used successfully as a catalyst support for a liquid-phase hydrogenation reaction (Campelo et al., 1982; Pine, 1975; Stirton, 1948). Furthermore, it has a characteristic of large pore without losing much surface area (Chen et al., 1987). The present study aims to shield more light on this catalyst system by evaluating the RHDS properties of AAP-supported Co and Mo catalysts. A series of CoMoAAP catalysts with various Al/P ratios and nearly constant metal contents was prepared and tested for their HDS activity. A commercial catalyst GC-106 from Gulf was included for comparison. Kinetic studies of HDS reactions were also carried out in a trickle bed reactor. In addition, temperature-programmed reduction (TPR) and X-ray powder diffraction techniques were used to characterize the catalysts.
Residual oils usually contain high contents of sulfur. These not only lead to the problems of air pollution but also cause major problems in hydrotreating and fluid catalytic cracking operations. Therefore, it is important to remove sulfur from residues. Many hydrodesulfurization (HDS) process have been developed (Topsoe et al., 1986). Hydrodesulfurization of heavy oils is generally carried out over alumina-supported catalysts containing combinations of cobalt and molybdenum salts or nickel and molybdenum salts (Ohtsuka, 1977; Stanislaus et al., 1988). The catalysts are prepared in the oxidic form, but their actual active states are obtained by sulfiding before use. It is found that there is no need for the exclusive use of alumina support in HDS catalysts. Mo-based carbonsupported catalysts have much higher HDS activity than the corresponding Al203-supported catalysts (Schmitt et al., 1976; Duchet et al., 1983). Silica-supported and Y zeolite supported molybdenum oxide and tungsten oxide have also been used for HDS reaction (Thomas et al., 1983; Cid et al., 1987). It is well-known that the residue hydrodesulfurization (RHDS) reaction is influenced by pore diffusion in the usual HDS catalysts (Johnson et al., 1986). This diffusion limitation would reduce the effectiveness factor of the catalysts, resulting in the decrease of the apparent activities of sulfur removal. However, the surface area of the catalyst is increased with decreasing the pore diameter of the catalyst. In order to investigate the relationship between catalyst pore structure and RHDS activity, it would be ideal for all the support materials to have the following characteristics: (1) the chemical properties are the same, (2) the pore size distribution is monodispersed with a narrow range of sizes, (3) the total pore volumes are the same, and (4) the pellet sizes are the same (Kobayashi et al., 1987). To our best knowledge, alumina-aluminum phosphate (AAP) is the most suitable material to meet the above conditions. AAP is easily prepared and exhibits *
Experimental Section (1) AAP Support Preparation. AAP supports were
prepared by the coprecipitation method as reported in previous works (Chen et al., 1988; Marcelin et al., 1983). Aluminum nitrate 1( 03)3·9 20 (Merck) was completely dissolved into doubly distilled water to form a 0.5 M solution; phosphoric acid H3P04 (Wako, 85%) was then added slowly in a suitable amount. After the addition, the acidic solution was well stirred for at least 10 min. A basic ammonium hydroxide aqueous solution was prepared by mixing the NH4OH (Merck) and distilled water at volume ratio 1:1. Both the acidic solution and the basic solution were slowly added to a well-stirred vessel containing 1 L of distilled water as a stirring medium. The pH value was maintained at 8.00 ± 0.02 throughout the precipitation process. The titration was continued until the acidic solution was consumed. The resulting precipitates were filtered, washed with distilled water, and dried at 393 K overnight, followed by 12 h of calcination in air at 773 K in the muffle furnace. Controlling the stoichiometry of the Al/P atomic ratio allowed a desired AAP composite to be obtained. The
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1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No.
9, 1990
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L--------------0 Scheme of TPR apparatus. (1) TCD detector. (2) U type reactor. (3) Tube furnace. (4) Molecular sieve trap. (5) Thermocouple. (6) Temperature controller. (7) Recorder.
Figure
1.
areas and pore volumes of the samples were determined by nitrogen adsorption (BET method) using a Quantasorb adsorption unit manufactured by Quantachrome Corp. The pore size distributions were measured by the mercury penetration method on an Autopore II 9220. The sample powder was first pressed into a disk in a die and then crushed into granules for penetration method. The Al to P ratios were obtained by elementary analysis using a Kontron NS-35 ICP atomic emission spectrophotometer. Materials were first dissolved by aqua regia and then diluted to a concentration of about 100 ppm for measurement. We denote the AAP sample, its Al/P atomic ratio = x, as AAPx. For example, AAP8 indicates that the Al/P atomic ratio is 8. After calcination, dry clump AAP and 3 wt % methylcellulose as lubricant were well grinded, and then an appropriate amount of distilled water was added. The mixture was then extruded. The wet extrusion was dried in air at room temperature, dried at 383 K for 4 h, and calcined at 773 K for 4 h. The extrusion diameter was about 1/16 in. (1 in. = 2.54 cm). (2) C0O-M0O3/AAP Preparation. C0O-M0O3/AAP catalysts were prepared by incipient wetness impregnation of supports in the form of extrusion with solutions of ( 4) 7024·4 20 and Co(N03)2-6H20, successively in distilled water. Mo was added first, and catalyst was dried and calcined before addition of Co. The first impregnation (Mo) was dried at atmospheric pressure overnight and calcined at 383 K for 4 h, followed by 773 K for 4 h. The second impregnation (Co) was dried and calcined in the same way. This calcination temperature has been reported to give optimal HDS activity (Stanislaus et al., 1988). The catalysts prepared in this study contain nearly constant amounts of Mo and Co. The metal contents of the catalysts were determined by means of ICP-AES (Jarrell-ash 1100). All catalysts prepared in this study were characterized by X-ray diffraction to investigate their crystallographic properties. X-ray diffraction patterns were obtained by using a Siemens D-500 operated at 30 kV and 20 mA with Cu Ka radiation.
surface
(3) Temperature-Programmed Reduction (TPR). TPR was used to distinguish different reducibility of M0O3 on various supports. A schematic diagram of the TPR apparatus is shown in Figure 1. C0M0/AAP catalyst was set in a U-type quartz reactor tube. The catalyst bed was held between two quartz wool plugs. The sample (0.2 g) was heated in a N2 flow at 393 K for 4 h to desorb water
and cooled to ambient temperature. Then the sample was heated in a H2/Ar reducing gas mixture (H2 = 5 mL/min, Ar = 50 mL/min) from room temperature to 1123 K by a heating rate of 5 K/min and retained at 1123 K for 1 h.
Water, formed by reduction or dehydration, was trapped in a 4A molecular sieve column. After the gas mixture had passed through the molecular sieves, its composition was monitored continuously with a thermal conductivity detector (TCD). By use of a high flow rate to minimize diffusion resistance and thermal effects, the TCD response
(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.
Figure 2. Simplified flow diagram of HDS test unit.
4
g
sand g
catalyst
5 g
catalyst
6 g
catalyst
6
catalyst
3
4
4
4
4
Figure
3.
g
g
g
g
sand
sand
sand
g
sand
Catalyst bed arrangement.
of reduction. Time lag and concentration gradient could be neglected in this study, as shown in Appendix 1. (4) RHDS Activity Measurement. The RHDS reaction was performed in a stainless steel tube reactor. In a typical run, the reactor was packed with 20 g of catalyst extrusion. Each of the catalyst extrusions was carefully hand-picked. The length of catalyst used was controlled within the range 3-5 mm. The catalyst bed was diluted with sand (50-70 mesh). Sand (4 g, 4 g, 4 g, 4 g, 4 g) and catalyst (3 g, 5 g, 6 g, 6 g) were loaded one after another as shown in Figure 3. The inert to catalyst weight ratio is about 2. The continuous cocurrent down-flow trickle bed reactor system is shown in Figure 2. The reactor was heated and controlled by means of three electric resistances, and the temperature over the reaction zone was kept constant. Careful and skillful control of temperature is needed. The reaction temperature was monitored with three thermocouples. One of the thermocouples was set in the center of tube reactor; the other two were located outside the tube reactor along the length of the reactor. The catalyst bed height was ca. 35 cm in all cases. In order is nearly proportional to the rate
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Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
Table I. Physical and Chemical Properties of Kuwait Residue Oil API 16.8 sulfur, wt % nitrogen, wt %
500
1--—--r600 ‘
J
1
3.72 0.21 10.1 14.0 53.0 0.146
Conradson carbon residue, wt % Ni, ppm V, ppm
H/C ratio ASTM distillation 5% 10% 20% 30% 40% 50% 60% 65%
554 584 633 673 704 743 794 811
K K K K K K K K
Figure 4. Surface area and average pore diameter versus Al/P mole
to avoid entrance and exit effects, the remaining space at the top and bottom of the reactor was filled with 40-60mesh ceramics. The design of a laboratory reactor is to ensure that the data are free from the influence of unwanted transport effect (Anderson, 1968; Doraiswamy and Tajbl, 1974; Rakesh and James, 1984). In order to ensure that channeling and heat-transfer effects at the reactor wall are not limiting, the radial aspect ratio (ratio of the bed diameter to the catalyst particle diameter) should be greater than 4 (Doraiswamy and Tajbl, 1974). The evaluation of kinetic data from a fixed bed catalytic reactor is usually based on the assumption of plug flow. However, marked deviations from plug flow can occur in an experimental reactor due to axial eddy dispersion. Doraiswamy and Tajbl (1974) also suggested that if the axial aspect ratio is greater than 30, axial dispersion and axial heat condition effects can be neglected. For the cylindrical catalyst of diameter 1.5 mm and length 4 mm, the equivalent spherical diameter is 2.669 mm (Mears, 1971). In order to satisfy the above criteria, the internal diameter of the reactor should be greater than 10.68 mm and the length of the catalyst bed should be greater than 80.07 mm. In this study, a strainless steel tube reactor of internal diameter 14.27 mm, outer diameter 25.4 mm, and length 430 mm was used. Therefore, channeling, wall heat-transfer effects, axial dispersion, and axial heat condition effects are negligible in this study. Presulfiding Treatment. Undoubtedly it is accepted that presulfiding of the catalyst results in a higher catalytic activity. In this study, a spiked feedstock method was used for the presulfiding of the catalysts, where dimethyl disulfide (DMDS) was used as a spiking agent. Before sulfiding, a N2 flow purge was flowing through the reactor at 393 K overnight to desorb water. A hydrogen stream (7582 kPa and 300 mL/min) was then switched in. The diesel oil feedstock doped to a level of 1 wt % sulfur by the addition of dimethyl disulfide (DMDS) was passed through the reactor (1 mL/min) with a temperature program. The following temperature program was applied: heat from room temperature to 448 K and retain at 448 K for 2 h, and then increased to 523 K and hold for 4 h; after that, increase to 598 K for 4 h. HDS Activity Measurement. The characteristics of the atmosphere tower bottom (ATB) residue feedstock are shown in Table I. The ATB residue feedstock was sup-
ratio.
plied by a Lewa proportioning pump (type FCM1) and passed the reactor (0.5 mL/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 5872A). The ATB liquid feed and hydrogen gas were passed over the fixed bed of catalyst in a cocurrent down-flow mode. The liquid product was collected in high-pressure accumulators. Gas from the high-pressure accumulators then left through a back pressure regulator (Mity-Mite Model 90W) 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. At an appropriate time, the liquid samples were withdrawn from the separator with a sampling valve to measure sulfur contents by X-ray fluorescence (Oxford, LAB-X2000). In a single run, the total number of hours on stream was 72 h. The flow pattern diagram reported by Charpentier and Favier (1975) was used to check the flow type. The analysis indicated that the flow pattern of the reactor in this work is indeed in the trickling flow mode (Appendix 2).
Results and Discussion (1) Catalyst Characterization. The physical properties of AAP are listed in Table II and plotted in Figure 4. It is shown that the surface area increases and average
pore diameter decreases as the Al/P atomic ratio increases. A desired surface area and pore diameter could be obtained easily by varying the Al/P atomic ratio (Chen et al., 1986). The results are in good agreement with those reported in the literature (Vogel et al., 1983; Marcelin et al., 1983; Marcelin et al., 1984). The results presented in Table III demonstrate that after impregnation of Co and Mo on the carriers the surface area significantly decreases, possibly due to the plugging of small pores by impregnation. Yet the trend of high surface area at high Al/P atomic ratio still exists. The results of temperature-programmed desorption (TPD) of ammonia reported in our previous work (Wang, 1987) indicate that AAP has only one type of acid site with medium acid strength. The acid concentration of AAP
Table II. Characteristics of AAP Carriers surface area, m2/g pore volume, mL/g avg pore diameter, Á
AAP1
AAP2
AAP3.5
AAP5
AAP6
AAP8
71
127 1.16
190 1.29 271.4
254 1.33 210.0
260 1.28 196.6
313 0.64 81.7
0.94 532.5
365.6
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
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Table III. Characteristics of CoMo/AAP Catalysts
CMAAPl” catalyst BET surface area, m2/g Mo03, wt % CoO, wt %
37
11.75 3.78 0.62 1.277 45.98 0.425 0.156
Co/Mo
apparent ATP RHDS rate constant, mL/(g-h) initial sulfur removal, % ATB RHDS QTOF, mL/(mol-s) deactivation rate constant
“CMAAP1 denotes CoMo supported
Al/P
Figure
5.
Acid amounts
decreases as the
versus
mole
on
AAP1.
6
CMAAP3.5
CMAAP6
100 12.49 4.01 0.62 3.135 67.64 1.045 0.103
147 12.68
CMAAP8
129 13.23 1.59 0.23
157
12.48 4.20 0.65 4.291 74.10 1.430 0.047
4.03 0.61 3.665 70.96 1.221 0.079
GC-1066
3.549 70.29 1.072 0.056
Asymmetrical quadruloble form.
ratio
Al/P mole ratio of AAP.
Al/P atomic ratio increases, as
shown in
5.
Figure The X-ray diffraction patterns of CoMo/AAPl and CoMo/AAP2 (12 wt % Mo03, Co/Mo = 0.6) demonstrate a crystalline phase is formed, as shown in Figure 6. The positions of the main lines are in partial agreement with those of the Mo03 phase, indicating that the structure of the crystallite on the carrier surface differs from those of pure Mo03 and CoO. This indicates that a mixture of crystalline bulk oxide and surface compounds is possibly formed on these two catalysts and can be attributed to the metal oxide-support interaction. A similar result for W03 supported on Si02 has been reported (Cid et al., 1987). However, those supported on AAP3.5, AAP6, and AAP16 (Figure 6) gave an amorphous phase, instead of a crystalline phase. AAP1 and AAP2 have small surface areas relative to others; therefore, bulk metal oxide crystalline is easily formed on these catalysts. At other AAPs with large surface areas, microcrystalline metal oxide (
