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Energy & Fuels 2003, 17, 62-68
The Influence of Alumina on the Performance of FCC Catalysts during Hydrotreated VGO Catalytic Cracking S. Al-Khattaf† Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran, 31261 Saudi Arabia Received March 13, 2002
Two categories of FCC catalysts were prepared. The first one is an FCC catalyst containing Y-zeolite and amorphous matrix. The second category is amorphous catalyst without Y-zeolite. Alumina was systematically added to both catalyst categories. The addition of alumina to the FCC catalyst containing Y-zeolite forms an active matrix. The inactive matrix of the catalyst is composed of kaolin filler and silica sol binder. The experimental data clearly show that alumina addition to both catalyst categories substantially increased both catalyst surface area and acidity. The addition of alumina to the amorphous catalyst resulted in a significant increase in hydrotreated VGO conversion. However, similar alumina addition to Y-zeolite containing catalysts did not appear to have a significant effect on the hydrotreated VGO conversion.
1. Introduction Commercial FCC catalysts are manufactured using 1-2-µm zeolites dispersed on an amorphous matrix forming 60-µm particles.1,2 Zeolites are extremely active and provide catalyst activity, selectivity, and stability.3 Their regular pore dimensions make them selective as to which molecules are adsorbed or converted. Hence, Y-zeolite is the active and the most important component in FCC catalysts. Y-zeolite provides the major part of the surface area and the active sites.1,2 Thus it is the key component, which controls catalyst activity and selectivity.4-9 The catalytic activity of Y-zeolite is mainly controlled by its unit cell size (UCS) and to a lesser extent by its crystal size. Recently, Al-Khattaf and de Lasa have studied the effect of Y-zeolite crystal size on the activity and selectivity of FCC catalysts.10-12 Most commercial FCC catalysts contain between 15 and 40 wt % zeolite.2 The activity and selectivity of FCC catalysts are mainly controlled via three processing strategies:13 † E-mail:
[email protected]. Phone: 9663-8601429. Fax: 96638604234. (1) Scherzer, J. In Fluid Catalytic Cracking: Science and Technology; Magee, J. S., Mitchell, M. M., Eds.; Elsevier: Amsterdam, 1993. (2) Humphries, D.; Harris, D.; O’Connor, P. In Fluid Catalytic Cracking: Science and Technology; Magee, J. S., Mitchell, M. M., Eds.; Elsevier: Amsterdam, 1993; pp 41. (3) Humphries, A.; Wilcox, J. R. Oil Gas J. Feb 6, 1989. (4) Pine, L. A.; Maher, F. K.; Wachter, W. A. J. Catal. 1984, 85, 466. (5) Arribas, J.; Corma, A.; Fornes, V.; Melo, F. J. Catal. 1987, 108, 135. (6) Ino, T.; Al-Khattaf, S. Appl. Catal. 1996, 142, 5. (7) Al-Khattaf, S. Appl. Catal. 2002, 231, 293. (8) Rajagopalan, K.; Peters, A. W. J. Catal. 1987, 106, 410. (9) Ritter, R. E.;Creighton, J. E.; Roberies, T. G.; Chin, D. S.; Wear, C. C. NPRA Annual Meeting, Los Angeles, March 1986. (10) Al-Khattaf, S.; de Lasa, H. I. Ind. Eng. Chem. Res. 1999, 38, 1350. (11) Al-Khattaf, S.; de Lasa, H. I. Can. J. Chem Eng. 2001, 3, 79, 341. (12) Al-Khattaf, S.; de Lasa, H. I. Appl. Catal. 2002, 226, 139.
zeolite characteristics such as silica/alumina ratio which is directly related to unit cell size, the presence of additives such as ZSM-5, and the type of matrix in the the FCC catalysts as the active matrix can contribute to the cracking reactions. The matrix comprises 60-85 wt % of the commercial FCC catalyst and usually contains synthetic and natural components. Clay is the natural component and amorphous silica or silica-alumina is the synthetic portion.14 Matrix composition can influence catalyst performance but to a lesser extent than the zeolite component.14 An inactive matrix does not contain enough strong acid sites that can influence the catalyst performance. On the other hand, an active matrix contains acid sites associated with aluminum atoms (e.g., alumina). However, only those sites that have sufficient strength are able to contribute to the cracking reactions.15 Among the different alumina available commercially, boehmite is the most important one that is used in catalyst formulations. The high acidity of boehmite is the major reason behind its catalytic activity. Its addition to the catalyst can improve the catalyst performance, improves attrition resistance and serves as a metal trap.1 Active matrix is believed to improve bottoms upgrading and LCO quality by increasing the cetane number.1,15 The large pore of the matrix >500 Å facilitates large heavy oil molecule (10-100 Å) diffusion and may cause cracking if the matrix contains acid sites with certain strength. The active matrix is characterized by high surface area and acidity.1,15 Usually active matrix enhances bottom conversion and gasoline octane.1,15-18 However, it also produces (13) Magee, J. S.; Letzsch, W. S. In Fluid Catalytic Cracking III; Materials and Processes; Occelli, M. L., O’Connor, P., Eds.; American Chemical Society: Washington, DC, 1994. (14) Scherzer, J. In Fluid Catalytic Cracking III; Materials and Processes; Occelli, M. L., and O’Connor, P., Eds.; American Chemical Society: Washington, DC, 1994. (15) Scherzer, J. Catal. Rev.-Sci. Eng. 1989, 31, 3, 215.
10.1021/ef020066a CCC: $25.00 © 2003 American Chemical Society Published on Web 12/05/2002
Performance of FCC Catalysts
more coke, dry gas, and less gasoline than inactive matrix.16-18 The effect of alumina addition to FCC catalyst matrix on processing different kinds of VGO feedstock was studied by Otterstedt et al.17 They observed that alumina addition to the catalyst matrix had increased the conversion of the heavier feed oil but at the same time increased coke selectivity and decreased gasoline selectivity. However, alumina addition did not have a significant effect during lighter feed oil processing. Coke and gasoline selectivities were found to have similar behavior to those of heavier feed oil. It has been reported that the olefin/paraffin ratio decreases by increasing zeolitic content and decreasing matrix activity.19 Moreover, Scherzer15 claimed that increasing matrix activity (by adding alumina) would increase both olefin/paraffin ratio and gasoline octane. This was attributed to lower hydrogen transfer reaction rate versus cracking reaction rate over amorphous matrix component. Thus, it is clearly known that alumina addition to a FCC catalyst matrix can influence the catalyst performance by increasing catalyst acidity and surface area. Consequently, the VGO catalytic cracking might be enhanced by alumina addition to the matrix without changing the USY-zeolite structure. In the present study, a thorough investigation for the role of active matrix on catalytic cracking of hydrotreated VGO has been conducted. Systematic addition of alumina to both matrix samples and FCC catalysts containing USY-zeolite has been studied. Testing both catalyst systems under standard MAT conditions has been carried out. The activity of matrix with alumina and catalyst containing alumina is determined. Furthermore, the effect of alumina addition on product selectivity is highlighted. 2. Experimental Section 2.1. Catalyst Preparation. USY-zeolite was mixed with kaolin clay and stabilized silica sol (SI-550) supplied by Catalyst and Chemicals Industries Co. The resulting slurry was heated over a sand bath with continuous stirring. The dried product was crushed and sieved to the proper particle size (520-710 µm). The base catalyst, the one with inactive matrix (CAT0), contained 30% zeolite, 50% kaolin, and 20% silica sol binder. The prepared catalysts were calcined in air at 600 °C for 2 h. Finally the catalysts were treated with 100% steam at 810 °C for 6 h. The series of catalysts containing the active matrices were prepared in the same manner as the one containing inactive matrix with the exception that part of the kaolin was replaced by boehmite. The amount of boehmite used in these preparations corresponded to 5 wt % (CAT5), 10 wt % (CAT10), and 20 wt % (CAT20). The three catalysts CAT5, CAT10 and CAT20 all contained 30 wt % USY-zeolite and 20 wt % silica sol. The other component is the kaolin as shown Table 1. The series of matrix samples are prepared as follows; inactive matrix contains only kaolin (M0). Active matrices were prepared in the same manner as the FCC catalysts with active matrix except that in these series no zeolite is present. Part (16) Van de Gander, P.; Penslay, R. M.; Chuang, K. C.; Cormier, W. E.; Walterman, G. M.; Boldingh, E. P. Advanced Fluid Catalytic Cracking Technology; AIChE Symposium Series vol. 88, no. 291; AIChE: New York; pp 21-29. (17) Otterstedt, J. E.; Zhu, Y.; Sterte, J. Appl. Catal. 1988, 38, 143. (18) Sterte, J.; Otterstedt, J. E. Appl. Catal. 1988, 38, 131. (19) de Jong, J. I. Ketjen Catalysis. Symposium 1986, Scheveningen, The Netherlands, Paper F-2.
Energy & Fuels, Vol. 17, No. 1, 2003 63 Table 1. Catalyst Composition Content catalyst
Y-zeolite (wt %)
silica sol (wt %)
kaolin (wt %)
alumina (wt %)
CAT0 CAT5 CAT10 CAT20 M0 M5 M10 M20
30 30 30 30 0 0 0 0
20 20 20 20 0 20 20 20
50 45 40 30 100 75 70 60
0 5 10 20 0 5 10 20
Table 2. Treatment and Characterization of Catalysts
catalyst
treatment
unit cell size (Å)
CAT0 CAT5 CAT10 CAT20 M0 M5 M10 M20
steaming, 810 °C steaming, 810 °C steaming, 810 °C steaming, 810 °C steaming, 810 °C steaming, 810°C steaming, 810 °C steaming, 810 °C
24.206 24.206 24.206 24.206 -
surface Al acidity area (m2/g) (wt %) (mmol/g) 123 177 180 196 22 25 31.7 60
0 5 10 20 0 5 10 20
0.0190 0.0296 0.0311 0.0377 0.006 0.0088 0.0114 0.0227
of kaolin was substituted by boehmite. The amount of boehmite used in these preparations corresponded to 5 wt % (M5), 10 wt % (M10), and 20 wt % (M20) see Table 1. All matrices were calcined in air at 600 °C for 2 h. Then they were treated with 100% steam at 810 °C for 6 h (see Table 2). 2.2. Catalyst Characterization. The unit cell size was determined by X-ray diffraction following ASTM D-3942-80. High purity silicon powder (99.999%) was used as the calibration standard. Surface area of the sample catalyst were measured by nitrogen adsorption at -196 °C. The zeolite used has 75.9 m2/g surface area (this surface area is for the zeolite which is steamed at 810 °C for 8 h) and negligible Na content. Moreover, the unit cell size is 24.206 Å. Hence its silica/ alumina ratio can be found from this unit cell size. The acid property of the catalyst was characterized by pyridine temperature programmed desorption (TPD). In all experiments, 50 mg of sample was charged in a tubular cell. Prior to obtaining TPD spectra, the sample was outgassed at 400 °C for 30 min in flowing helium and then cooled to 150 °C. At that temperature, pyridine was adsorbed on the sample by injecting pulses of pyridine (2ul/ pulse). The injection of pyridine was repeated until the amount of pyridine detected was the same for the last two injections. After the adsorption of pyridine was saturated, the sample was flushed at 150 °C for 1 h with helium to remove excess pyridine and then temperature programmed at 30 °C/min to 1000 °C in flowing helium at 30 mL/min. An FID detector was used to monitor the desorbed pyridine. The properties of the catalysts used in the present study are presented in Table 2. 2.3. Catalyst Evaluation. Catalytic experiments were carried out in a microactivity test (MAT) unit (fixed bed), which had been designed according to the ASTM D-3907 method with minor modifications. The reactor was operated at atmospheric pressure and 520 °C. For a given catalyst, the conversion was varied by varying the catalyst-to-oil ratio (C/O) in the range of 0.5-3.0. The C/O ratio is defined as the amount of catalyst divided by the total amount of oil fed in a given time on stream and was varied by changing the weight of the catalyst while the total amount of oil fed and the time were kept constant, i.e., 1 g of the oil was charged to the reactor in 30 s (see Table 3). Thermal effects and changes in the bed volume were minimized by diluting the catalyst with kaolin particles (having the same size as the catalyst particle), and the total weight of the catalyst bed was kept at 3 g. The distribution of gaseous products was analyzed by gas chromatographies. The boiling point range of the liquid products was determined by
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Al-Khattaf
Table 3. MAT Operating Conditions temperature feed rate amount of catalyst feed type
520 °C 1 g/30 s 0.5-3.0 g VGO
Table 4. MAT Feed Oil Properties specific gravity (15/4 °C) sulfur (wt %) Conradson carbon (wt %) refractive index (15 °C) bromine number Ni (ppm) V (ppm)
0.8821 0.18 0.09 1.4719 3.2
