Journal of the Japan Petroleum Institute, 55, (5), 319-325 (2012)
319
[Regular Paper]
Effect of Acid Properties of Catalysts on Fluid Catalytic Cracking of Residual Oil Yoshifumi HIRAMATSU†1)＊, Youhei AITA†1), and Takashi UMEKI†2) †1)
†2)
Technology & Engineering Center, Idemitsu Kosan Co., Ltd., 2-1 Anesakikaigan, Ichihara, Chiba 299-0194, JAPAN Advanced Technology Research Laboratories, Idemitsu Kosan Co., Ltd., 1280 Kami-izumi, Sodegaura, Chiba 299-0293, JAPAN (Received February 29, 2012)
This article describes the relationship between the different acidic properties of catalytic active sites and their catalytic cracking ability using desulfurized atmospheric residue (DSAR) as a feed. Experiments were carried out in a fixed fluid bed reactor. The acid strength was determined by the temperature of NH3 desorption peak and the number of acid was defined by the amounts of NH3. The catalysts from Y-type zeolites with different unit cell sizes were prepared and evaluated. The lowest number of acid site catalyst shows the lowest conversion and the highest gasoline yield, while the highest one shows the highest conversion and the lowest gasoline yield. Furthermore, the catalysts using wider range of acid site number are evaluated. It was found that the gasoline yield reaches a maximum at a certain number of acid sites for the catalytic cracking of DSAR, but not for that of desulfurized vacuum gas oil (DSVGO). It is important for cracking from DSAR to gasoline to control the number of acid site. Based on these results, an attempt was made to identify the intermediate materials produced during the catalytic cracking of DSAR. Keywords Fluid catalytic cracking, Zeolite catalyst, Acid property, Gasoline yield, Residual oil
1.
Introduction
Fluid catalytic cracking (FCC) has been the most important and flexible conversion process in petroleum refineries. It is economically beneficial that low-value residues having high molecular weights can be cracked into valuable constituents, such as gasoline, diesel, and petrochemicals. Therefore, an understanding of the reaction mechanism of FCC of heavy molecules is critical for increasing the selectivity of valuable products. In order to improve FCC performance, therefore, identifying the major factors that influence the FCC reaction, such as the catalyst structure, feed properties, and operating conditions, is crucial. This article describes the investigation of active sites of FCC catalysts, particularly for optimization of zeolite acidity, because acidic sites are recognized to be the major active sites for catalytic cracking. Commercial FCC catalysts are made from 1-2 μm Y-type zeolites and silica and/or alumina sols or gels to form 75 μm particles with kaolin clay as a filler1). Zeolites are extremely active, particularly in hydrocarbon conversion reactions, and their regular pore structures make them selective for certain molecules, ＊ ＊
To whom correspondence should be addressed. E-mail:
[email protected]
which are adsorbed or converted. Among them, Y-type zeolites are the active, and thus the most important, component in FCC catalysts. They provide the major portion of the surface area and active sites2), and control both the catalyst activity and selectivity3). The formation of coke also influences the catalyst activity as the formed coke can cover some of the active sites and/or block the zeolite channels, which can make the inner active sites inaccessible for the reactant molecules3),4). In FCC reactions, coke formation takes place rapidly (in less than a second), causing a decline in the catalyst activity. Consequently, catalyst deactivation is a significant issue in catalytic cracking, and numerous studies have been carried out on this reaction1),3),5). The catalytic activity of Y-type zeolites is mainly determined by the unit cell size (UCS), which can be controlled through steaming and/or acid treatment. Al-Khattaf reported that the product distribution of vacuum gas oil (VGO) cracking could be controlled by using FCC catalysts6) prepared from Y-type zeolites with different UCS. Sahoo et al.3) studied the effect of steam dealumination on the acidity and strength distribution of ZSM-5. They reported that the density of acid sites decreased with an increase in the steaming severity, and that coke formation was mainly a function of the acid site density rather than the acid strength dis-
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tribution. They considered that strong acid sites catalyze not only cracking, but also hydrogen transfer reactions, which convert paraffin to aromatics, and even coke2),3),5). Although the effect of the UCS upon VGO conversion has been reported by many researchers, the effect of the UCS on the FCC of residual oil is scarcely reported in the open literature. This article describes the relationship between the acid properties of catalytic active sites of FCC catalysts and their catalytic cracking ability using desulfurized atmospheric residue (DSAR) and desulfurized vacuum gas oil (DSVGO) feeds. 2.
Experimental
2. 1. Catalyst Preparation Typically, HSZ-H330 (UCS＝2.456 nm) from Tosoh Corp. was used as the parent Y-type zeolite. Alumina, silica, and silica_alumina matrixes and silica sol binder were purchased from JGC Catalysts and Chemicals Ltd. Kaolin clay (ASP-170, Tsuchiya Kogyo Co., Ltd.) was used as the filler. These materials were mixed in a slur-
Fig. 1●Simplified Manufacturing Process for Calcined FCC Catalysts Table 1
ry, spray dried, and carried through the series of procedures shown in Fig. 1 to produce the FCC catalysts. Each catalyst contained 10-30 wt% of the Y-type zeolite, 40-60 wt% of kaolin clay, 0-20 wt% of alumina, 0-20 wt% of silica_alumina, and 0-20 wt% of the silica sol binder. The prepared catalysts were spray dried and then washed three times with 1 wt% ammonium nitrate in water to exchange the sodium (Na) ions and then with ion-exchanged water to remove the ammonium nitrate. The washed catalysts were dried at 393 K for 3 h and then calcined in air at 773 K for 2 h. Typical catalysts examined in this study are named CAT1-CAT4, all of which contained the same number of Y-type zeolite and the original Y-type zeolite, HSZ-H330, was treated in different steaming conditions in order to give systematic decreases in UCS in the order of CAT1 to CAT4 as shown in Table 1. To extend the number of acid sites wider, supplementary catalysts were prepared and listed in Table 2, where CatA to CatF contained different Y-type zeolites in origin. 2. 2. Catalyst Characterization X-ray diffraction (XRD) patterns were recorded with an Ultima III diffractometer (Rigaku Corp.) using CuKα radiation (λ＝1.5418 Å, 1 Å＝10-10 m). The unit cell size of the Y-type zeolite component was determined from the average 2θ values of the (642) and (555) peaks, which were corrected for the 2θ of Si (111) as 24.443° using Si (NIST 640a) as an internal standard. Nitrogen adsorption measurements were carried out with a Quantachrome Autosorb-6 at 77 K. The total surface areas were determined by the multi-point B.E.T. method from 0.01 to 0.1 P/P 0. The surface areas of the micropores and the (meso＋macro) pores were determined using the T-plot method. The acid properties of the catalyst was determined by ammonia temperature programmed desorption (NH3TPD)7). A typical NH3-TPD profile of a Y-type zeolite consists of a low temperature peak (LP) at ～ 473 K and a high temperature peak (HP) at ～ 523 K. The LP is attributed to the desorption of physically adsorbed NH3, while the HP is due to the desorption of NH3 strongly adsorbed on the acid sites. For the NH3-TPD measurements, a meshed catalyst sample (16-32 mesh, ～ 100 mg) was charged into a tubular quartz cell. The sample was maintained at
Different Unit Cell Sizes (UCS) of Four Types of Catalysts Prepared Using Different Steaming Conditions
Catalyst name
UCS [nm]
CAT1 CAT2 CAT3 CAT4
2.456 2.451 2.446 2.439
Number of acid sites
Acid stength
Surface area
case A [mmol/g]
case B [mmol/g]
case B [kJ/mol]
total [m2/g]
micro [m2/g]
meso＋macro [m2/g]
0.29 0.28 0.24 0.16
0.23 0.20 0.18 0.12
124 124 125 127
245 244 236 226
181 182 177 163
64 62 59 63
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321 Table 2
Catalyst name CatA
CatB
CatC
CatD CatE CatF
Different Unit Cell Sizes of Types of Catalysts Surface area
Steaming time [h]
Number of acid sites case B [mmol/g]
Acid stength case B [kJ/mol]
total [m2/g]
micro [m2/g]
meso＋macro [m2/g]
0 2 4 6 0 2 4 6 0 2 4 6 0 0 0
0.15 0.05 0.04 0.04 0.15 0.10 0.05 0.04 0.15 0.04 0.04 0.03 0.21 0.19 0.19
131 131 130 130 132 138 132 131 132 132 131 132 129 129 130
227 194 157 150 223 162 159 157 223 172 167 166 242 238 236
164 101 85 78 165 82 84 85 157 94 91 87 179 179 173
64 94 72 72 58 80 75 72 66 78 76 79 64 59 63
773 K for 1 h in flowing helium and then cooled down to 373 K. Next, NH3 was adsorbed onto the sample by flowing a 0.5 % NH3/He mixed gas at 100 mL/min for 45 min. After NH3 adsorption, two types of pretreatments described below were applied. For case A, the sample was flushed at 373 K for 1 h in a 50 mL/min He flow to remove the excess adsorbed NH3. For case B, after removal of the excess adsorbed NH3 as in case A, the sample was exposed to saturated water vapor at room temperature in a 50 mL/min He flow at 373 K for 4 h, and then subsequently flushed at 373 K for 1 h in 50 mL/min He flow. The exposure to water vapor (case B) has been reported to selectively eliminate the LP7). After the above pretreatments, each sample was heated at a rate of 20 K/min up to 983 K and flushed in a 50 mL/min He flow. The amount of NH3 was determined using a mass spectrometer. 2. 3. Catalyst Evaluation Evaluation of catalyst activity was carried out in a fixed fluid bed reactor known as the Advanced Catalyst Evaluation Micro Activity Testing (ACE-MAT, Kayser Technology Inc.) reactor. It is a fixed fluid bed reactor that uses nitrogen flow and is operated at atmospheric pressure and 803 K. Conversions are adjusted by varying the catalyst to oil weight ratio (C/O), in the range of 3.75-6.0, where C/O is the amount of catalyst (9 g) divided by the total amount of oil feed in a given time on stream. In this paper, catalytic activities were expressed at C/O of 5.0. DSAR and DSVGO, as shown in Table 3, were used at the feedstocks. The distribution of gaseous products was analyzed using a gas chromatography. The boiling point range of the liquid products was determined using simulated distillation gas. The products from the ACE-MAT unit were classified based on the boiling point range as follows: (1) Gases, C1-C4
Table 3
Feed Properties of DSAR and DSVGO
Property Density at 15 ℃ Sulfur Viscosity at 50 ℃ CCRa) Ni V
[g/mL] [wt%] [mm2/s] [wt%] [wtppm] [wtppm]
DSAR
DSVGO
0.9233 0.27 155 4.2 3.1 3.2
0.8857 0.23 15 0.02 ＜1 ＜1
a) Conradson carbon residue (ASTM D189).
(2) Gasoline, C5-463 K (3) Light cycle oil (LCO), 463-633 K (4) Slurry oil (SLO) or clarified slurry oil (CLO), over 633 K. The carbon deposited on the spent catalyst (the coke) was burned out at 948 K in air. The combustion flue gas was converted completely to carbon dioxide by a copper catalyst, and the amount of coke was determined using an infrared gas analyzer. The product material balances were calculated, and only runs with balances ＞95 w t % w e r e a c c e p t e d f o r d a t a a n a l y s i s . Conversion is defined as 100 wt%−(LCO＋SLO). 3.
Results and Discussion
3. 1. Catalysts Prepared with Different UCSs Four types of catalysts can be distinguished by their characteristic UCSs. Compared to CAT1 as a base catalyst, CAT2, CAT3, and CAT4 have lower UCSs— 0.005, 0.010, and 0.015 nm, respectively. The influences of steaming conditions of zeolite on surface areas (Table 1) and on pore size distributions (Fig. 2) of the catalysts were also investigated. The (meso＋macro) pore surface area, which represents the external surface area, was nearly the same for each catalyst. This result is confirmed by the fact that all of
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Fig. 2●Pore Size Distributions of Four Types of Catalysts as Determined by the N2 Adsorption Method Fig. 4●NH3-TPD Spectra after the Case B Treatment of Four Types of Catalysts
Fig. 5●DSAR Product Yield Distributions by ACE-MAT of Four Types of Catalysts Fig. 3●NH3-TPD Spectra after the Case A Treatment of Four Types of Catalysts
the catalysts show equivalent pore size distributions. On the other hand, the micro surface area decreases with decrease in the UCS. Next, information on the number of acid sites and acid strength was obtained using NH3-TPD. The number of acid sites was determined as the equivalent area of the LP＋HP obtained for the catalyst prepared using the method in case A, as shown in Fig. 3 and listed in Table 1. The number of acid sites decreased with a decrease in the UCS (CAT1＞CAT2＞CAT3＞CAT4). In addition, the number of acid sites as defined by the equivalent area of the HP alone was also determined, and as can be seen in Fig. 4, follows the same trend observed for the number derived from the LP＋HP. It is noteworthy that the peak top temperature of the HP is nearly the same for the four catalysts, indicating that the acid strength calculated by the method of Sawa et al.8) is same for all four catalysts. The retention of a similar pore size distribution and external surface area for all four catalysts suggests that the steaming conditions were not severe enough to disintegrate the crystalline structure of the original Y-type zeolite. Furthermore, the steaming varied only the number of acid sites while keeping the acid strength constant. Based on these two results, it can be concluded that the steaming conditions may have only induced dealumination of the catalysts.
3. 2. Catalyst Evaluation Using ACE-MAT Figure 5 shows the product yield distributions for the FCC of DSAR in the ACE-MAT reactor using the four catalysts. Conversion is defined as 100 wt%− (LCO＋SLO). CAT4 shows the lowest conversion, the lowest coke yield and the highest gasoline yield, while CAT1 shows the highest conversion, the highest coke yield and the lowest gasoline yield. It has been the general consensus that a higher activity (conversion) correlates fairly well with a higher UCS, which is an approximate measure of the aluminum content of the zeolite framework and thus of the number of acid sites9). The ACE-MAT evaluation results are consistent with the consensus. In contrast to the conversion trend, however, the gasoline yield is highest for CAT4. Because CAT4 contains the lowest number of acid sites, one might speculate that there is an optimum level of acidity for enhanced gasoline yield, especially for residual oil catalytic cracking. In order to confirm this speculation, another set of catalysts was prepared and evaluated in the ACE-MAT reactor using both DSAR and DSVGO to consider the molecular size effect. The catalysts studied in these experiments contained CAT1-4, CatA-F and steamed CatA-C (showed in Table 2). Figure 6 depicts the relationship between the number of acid sites (as determined by NH3-TPD using the case B method), and the conversion and product yields obtained in the ACE-
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Conversion (wt%)＝100−LCO−SLO. Fig. 6
Relationship between the Number of Acid Sites and the Conversion/Yield
MAT reactor. When DSAR was used as the feed, the gasoline yield reached a maximum when the number of acid sites equaled ca. 0.10 mmol/g, while both the conversion and the coke yield increased with an increase in the number of acid sites. On the other hand, using DSVGO as the feed, the gasoline yield increased with an increase in the number of acid sites in a manner similar to that of both the conversion and the coke yield. The higher conversion for DSVGO compared with that for DSAR can be accounted for by the apparent difference in the average molecular sizes of the compounds in the DSVGO and DSAR feeds. Figure 7 suggested by Masuda10) shows a schematic image for catalytic cracking, where the DSVGO or DSAR feed compounds are sequentially converted to smaller molecules inside the different sized pore structures that are present in typical FCC catalysts. Masuda and Sato11) considered the diffusion-limited to approach of zeolite acid sites in the FCC catalysts on DSAR cracking case. Because VGO molecules are able to access the zeolite micropores more readily than AR molecules, the conversion of DSVGO is generally higher than that of DSAR. The existence of a maximum for the gasoline yield only in the DSAR cracking may be interpreted as follows. It can be assumed that the main precursors for the cracked gasoline, which are shown as the precracked oils in Fig. 7, have a similar size as the LCO molecules and are smaller than those of VGO. The
population of the pre-cracked oil inside the catalyst pores for the DSAR cracking will therefore be much smaller than that for the DSVGO cracking. Sato11) suggested that it is important to have a pore diameter which is 3-6 times of a molecule size which is wanted to crack. Masuda10) also presented that Y-type zeolites are suitable for the higher gasoline selectivity compared with silica _ alumina and mordenite in terms of acid strength distribution. But he did not discuss about the number of acid sites. In such lean conditions for the pre-cracked oil reaction, the higher number of acid sites would result in over-cracking of the gasoline to gases and LPG. These results suggest that to obtain a higher gasoline yield for DSAR cracking, catalysts should be prepared with a suitable number of acid sites or a larger pore size, which would create a wider reaction area. 4.
Conclusions
A series of FCC catalysts from Y-type zeolites with different UCSs were prepared by changing the steaming conditions. A decrease in the number of acid sites due to a decrease in the UCS of the Y-type zeolite was observed, while the pore size distribution did not change. The catalyst with the lowest number of acid sites exhibited the highest gasoline yield and the lowest conversion for DSAR cracking reactions. In order to elucidate these phenomena, another set of
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Fig. 7●Simplified Reaction Scheme for Catalytic Cracking Showing the Typical Molecules and Molecular Sizes (modified the Fig. 3 of Masuda, Shokubai, 45, (1), 31 (2003)11).)
catalysts with a wider range of acid site number were prepared and evaluated. When DSAR was used as the feed, the gasoline yield reached a maximum when the number of acid sites was ca. 0.10 mmol/g, while both the conversion and the coke yield increased with an increase in the number of acid sites. On the other hand, using DSVGO as the feed, the gasoline yield increased with increasing in the number of acid sites, as did the conversion and the coke yield. The existence of a maximum in the gasoline yield only for the DSAR cracking suggests that the population of pre-cracked oil molecules inside the catalyst pores determines the gasoline yield in connection with the number of acid sites. Acknowledgments This work was supported by the Japan Petroleum Energy Center (JPEC) as a technological development project supported financially by the Japan’s Ministry of Economy, Trade and Industry. We gratefully acknowl-
edge Dr. K. Inamura (Idemitsu Kosan Co., Ltd.) for helpful discussions. References 1) Sadeghbeigi, R., “Fluid Catalytic Cracking Handbook,” Gulf Publishing Company, Houston, TX (2000), p.94. 2) Sadeghbeigi, R., “Fluid Catalytic Cracking Handbook,” Gulf Publishing Company, Houston, TX (2000), p.90. 3) Sahoo, S. K., Viswanadham, N., Ray, N., Gupya, J. K., Singh, I. D., Appl. Catal., 205, 1 (2001). 4) Reyniers, M. F., Tang, Y., Marin, G. B., Appl. Catal., 202, 65 (2000). 5) Ino, T., Al-Khattaf, S., Appl. Catal., 142, 5 (1996). 6) Al-Khattaf, S., Appl. Catal. A: General, 231, 293 (2002). 7) Katada, N., Niwa, M., Zeolites, 21, 45 (2004). 8) Sawa, M., Niwa, M., Murakami, Y., Zeolites, 10, 307 (1990). 9) Rigutto, M. S., Veen, R., Huve, L., Cejka, J., Stud. Surf. Sci. Catal., 168, 855 (2007). 10) Masuda, T., Shokubai, 45, (1), 29 (2003). 11) Sato, S., Morimoto, Y., Takatsuka, T., Hashimoto, H., Sekiyu Gakkaishi (J. Jpn. Petrol. Inst.), 29, (1), 60 (1986).
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要 旨 流動接触分解触媒の酸性質が重質重油分解へ与える影響 平松 義文†1)，會田 洋兵†1)，梅木 孝†2) †1)
出光興産（株）生産技術センター，299-0194 千葉県市原市姉崎海岸2-1
†2)
出光興産（株）先進技術研究所 解析技術センター，299-0293 千葉県袖ケ浦市上泉1280
流動接触分解触媒の有する分解活性点である酸性質が重質重
せた触媒を調製し，評価した結果，ある特定の酸量においてガ
油の分解へ与える影響を流動床タイプの反応器を用いて検討し
ソリン収率は最大点を示すことが明らかとなった。減圧軽油留
た。酸強度はアンモニア TPD のピーク位置から決定し，アン
分を原料油と用いた場合と異なり，重質重油からガソリンを得
モニア吸着量から酸量を定義した。ゼオライト含有量が一定
るには最適な酸量を有していることが重要と考える。ガソリン
で，格子定数が異なる触媒を調製し，反応評価を行った。その
は重質重油が分解し，ガスへと転換するところでの中間物質に
結果，酸量が高くなるにつれ転化率も高くなったが，最も酸量
あたるため，ガソリン収率向上のためには，適度な酸量に調整
が少ない触媒が最もガソリン収率が高かった。酸量の影響をよ
することに加え，重質重油に適した反応場を創出することが重
り幅広く確かめるため，ゼオライト含有量，格子定数を変化さ
要である。
J. Jpn. Petrol. Inst., Vol. 55,
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