Influence of Crystal Size of USY Zeolite on Performance of Hydro-upgrading Catalysts

Transcription

1 Catalyst Research China Petroleum Processing and Petrochemical Technology 2012,Vol. 14, No. 1, pp March 30, 2012 Influence of Crystal Size of USY Zeolite on Performance of Hydro-upgrading Catalysts Wang Fucun 1, 2 ; Zhu Jinling 2 ; Yan Zifeng 1 (1. State Key Laboratory of Heavy Oil Processing, CNPC Key Laboratory of Catalysis, China University of Petroleum, Qingdao ; 2. Daqing Petrochemical Research Center of PetroChina, Daqing ) Abstract: The effect of crystal size of USY zeolite on the performance of hydro-upgrading catalysts for treating catalytically cracked (FCC) LCO (light cycle oil) was studied. Three W-Ni catalysts supported on USY zeolites with different crystal sizes and Al 2 O 3 were prepared by impregnation method. The catalysts were characterized by XRD and BET methods, and evaluated in a micro-reactor using tetralin as the model compound and in an 100-mL hydrogenation test unit using FCC LCO as the feedstock. By contrast, catalyst made from smaller crystal-size USY zeolite had higher external surface area and shorter pore length, having more hydrogenation activity sites and short contact time of reactant molecules with acidity sites. The evaluation results showed that the catalyst prepared on the basis of small crystal-size USY zeolite had higher tetralin conversion and better hydro-upgrading performance for treating FCC LCO. Key words: crystal size, USY, hydro-upgrading catalyst 1 Introduction Diesel is extensively used as a liquid fuel both in highway transportation vehicles and off-highway transportation systems since diesel engines are by 25% 40% more fuel-efficient than comparable gasoline engines [1]. Therefore, low-grade streams such as light cycle oil (LCO) delivered from FCC units will need to be processed at refineries to supply an additional volume of high quality diesel. However, high aromatic content in FCC LCO lowers diesel fuel quality, especially in terms of cetane number, and brings about the formation of undesirable exhaust gases emissions from diesel engines. As a result of the gradual transition to more stringent environmental regulations, much attention has been paid to the deep reduction of aromatics contained in diesel. Under such circumstances, there is a considerable interest in the development of new catalysts for hydro-upgrading of FCC LCO [2-6]. It has been proved that the addition of acidic components into the traditional alumina-supported hydrotreating catalysts can greatly enhance their hydro-upgrading performance. Among the acidic components, USY zeolite is the most extensively studied and widely applied in commercial scale catalyst [7-9]. The purpose of our research is to improve the quality of the catalytically cracked diesel components (LCO), mainly in order to decrease the aromatic content and increase the cetane number to realize a premium diesel production using different USY zeolites as acidic component of hydro-upgrading catalysts. So far, the comparison between hydro-upgrading catalysts using USY zeolites with different crystal sizes is still not well-documented. Accordingly, the purpose of this article is to examine the effect of different crystal sizes of the USY zeolite on the hydro-upgrading catalyst performance. 2 Experimental 2.1 Catalyst preparation Three Ni-W type catalysts, NWY-1, NWY-2 and NWY- 3, were prepared with the same metal loading of WO 3 and NiO. The NWY-1, NWY-2 and NWY-3 catalyst samples were prepared through co-impregnating the extrudates of gamma-alumina together with each one of USY1, USY2 and USY3 samples (which were provided by the Nankai University) used as the carrier with a mixed solution of Ni(NO 3 ) 2 6H 2 O and (NH 4 ) 6 H 2 W 12 O 40 xh 2 O, followed by Corrresponding Author: Dr. Wang Fucun, address: wfc459@petrochina.com.cn. 32

2 Wang Fucun, et al. Influence of Crystal Size of USY Zeolite on Performance of Hydro-upgrading Catalysts drying at 393 K for 4 h and calcinating at 783 K for 4 h. All the catalyst samples had a fixed amount of every component, denoting that the mass fraction of Y type zeolite and gamma-alumina was 20% and 50%, respectively. 2.2 Characterization of catalysts X-Ray diffractometry (XRD) The relative crystallinity and unit cell parameters of zeolites were calculated from XRD patterns that were collected on a Shimadzu XRD6000 difractometer using CuKα radiation. The relative crystallinity was estimated by comparing the eight peak intensities of modified sample with a standard NaY zeolite sample. The framework Si/Al ratio was obtained from the calculated unit cell parameters by using the Breck-Flanigen equation [10]. The crystal size of the (111) crystal plane of USY zeolite samples was measured by XRD line-broadening [11], while correlating the analysis of crystallite size with the broadening of diffraction peak using the Scherrer equation BET (N 2 adsorption/desorption technique) Nitrogen adsorption/desorption measurements were performed at 77 K in liquid nitrogen on a Tristar 3000 type surface area and pore structure instrument. Surface area of catalyst sample was calculated using the multipoint BET equation. Pore volume of catalyst sample was calculated from the maximum adsorption amount of nitrogen at p/p 0 = Catalytic activity evaluation Hydrogenation of tetralin Since monoaromatics are predominant among the possible aromatic compounds in the FCC LCO, tetralin has been used as a probe molecule which is able to diffuse into the supercages of the zeolite USY [12-13]. So tetralin was chosen as the model compound for hydro-upgrading catalyst activity evaluation. The catalytic experiments were performed in a micro-reactor. The system consisted of the liquid and gas feeding sections, a high-pressure reactor, a heater with a temperature controller for precisely controlling the temperature of the catalyst bed, and a high-pressure gas-liquid separator. The reaction was carried out at temperatures of 573 K and 613 K in H 2 flow under a total pressure of 5.0 MPa. Before the catalytic hydrogenation run, the catalyst was sulfided with 8v% of CS 2 dissolved in n-decane at 583 K for 4 h. The reactor was then brought to the reaction temperature and pressurized to 5.0 MPa. The liquid hourly space velocity (LHSV) and H 2 to hydrocarbon ratio were maintained at 1.5 h -1 and 500, respectively. The reaction time was 10 hours. The tetralin feed was diluted with n-decane at a weight ratio of 15:85. The liquid products were collected and were identified by GC-MS and quantified by an Agilent 7890A gas chromatograph. The GC was equipped with a DM-5 capillary column (30 m 0.25 mm 0.32 μm). The conversion of tetralin was calculated as follows: Tetralin conversion=[(t feed -T product )/ T feed ] 100% where T feed and T product denote the tetralin content in the feed and product, respectively Hydro-upgrading of FCC LCO In contrast with FCC LCO, tetralin is free of sulfur and nitrogen impurities, which are considered to be poisons for metal catalysts and can influence the activity of hydrotreating catalysts. The evaluation results for tetralin may not be persuasive, therefore hydro-upgrading of Daqing FCC LCO (with a cetane number (CN) of 30) over the NWY-1, NWY-2 and NWY-3 catalysts, respectively, was also conducted on the 100-mL continuous flow fixedbed hydrogenation test units operating in compliance with a single-stage and once-through process. Cetane number of the feedstocks and diesel products was determined according to the national standard method GB/T , and product distillation range was measured according to the ASTM D86 method. The hydro-upgrading catalysts were sulfided in situ before evaluation, and the sulfidation oil was a kind of straight-run kerosene containing 2 m% of CS 2. ΔCN of three catalysts was expressed as follows: ΔCN=CN product -CN feed where CN product and CN feed denote the cetane number of the product and feed, respectively. The diesel yield was expressed as follows: Yield=Weight diesel in product /Weight feed 3 Results and discussion 3.1 XRD The framework Si/Al ratio, relative crystallinity and crys- 33

3 China Petroleum Processing and Petrochemical Technology 2012,14(1):32-36 tal size of three USY zeolite samples are listed in Table 1. Table 1 Properties of USY zeolite samples measured by XRD Sample USY1 USY2 USY3 Crystal size, nm SiO 2 /Al 2 O 3 molar ratio Relative crystallinity, % Table 1 depicts that three USY zeolite samples had no significant essential differences in their Si/Al ratio and relative crystallinity, but the crystal size decreased gradually in the following order: USY1>USY2 >USY BET surface area measurements According to N 2 adsorption/desorption analysis of USY1, USY2 and USY3 samples presented in Table 2, USY1, USY2 and USY3 samples had the same BET surface area and total volume. The lower the crystal size, the higher the external surface area would be, which is beneficial to an increased catalytic activity of hydro-upgrading catalysts [14]. Table 2 Pore structures of USY zeolite samples Sample BET surface area, m²/g External surface area, m²/g Total volume, ml/g USY USY USY Comparison of hydroupgrading performance of catalysts The tetralin conversion performance of three catalyst samples is shown in Figure 1. Figure 1 demonstrates that the crucial factor for hydro-upgrading performance was not related with the reaction temperature, but was associated with the hydro-upgrading nature of the catalyst itself. The catalyst NWY-3 had the highest tetralin conversion and the catalyst NWY-1 had the lowest tetralin conversion among three catalyst samples at a reaction temperature of either 573 K or 613 K. Based on the afore-mentioned results, it can be concluded that the higher conversion of zeolite-containing catalyst was associated primarily with different USY zeolite samples, and the catalyst composed of USY zeolite with smaller crystal size had higher external surface area, which could enhance the hydro-upgrading activity of the catalyst. Figure 1 Tetralin conversion on catalysts NWY-1, NWY-2 and NWY-3 at 573K and 613K, respectively NWY-1; NWY-2; NWY Comparison on performance of three catalysts for hydro-upgrading of FCC LCO The hydro-upgrading performance evaluation results of catalysts NWY-1, NWY-2 and NWY-3 using Daqing FCC LCO as the feedstock are depicted in Table 3. The evaluation results exhibited that the NWY-3 catalyst showed better ΔCN (cetane number improvement) and higher diesel yield than NWY-1 and NWY-2 at a reaction pressure of either 6.0 MPa or 8.0 MPa. Higher cetane number improvement was associated primarily with the higher external surface area of the USY3 zeolite, so there are more active sites on NWY-3 catalyst compared to those of NWY-1 catalyst and NWY-2 catalyst, which can increase the hydrogenation activity of hydro-upgrading catalyst. With respect to the product properties, we can see that the end boiling temperature of products obtained during hydrogenation reaction on three catalysts increased in the following order: NWY-1<NWY-2<NWY-3. This outcome was attributed to the larger crystal size of USY zeolite, because the longer pore length of USY zeolite could result in slow diffusion of reactant molecules from the acidity sites, leading to a longer time in contact with the catalyst, so that the diesel paraffin molecules obtained from dealkylation of aromatics and saturates may be over-cracked to gasoline molecules resulting in reduced diesel yield [15-16]. The test results revealed that the USY zeolite with smaller crystal size was more suitable for preparation of hydro-upgrading catalysts compared with the larger crystal size USY zeolite. The evaluation results revealed that there is a comparable catalytic performance of three catalysts in hydrogenation of model compound 34

4 Wang Fucun, et al. Influence of Crystal Size of USY Zeolite on Performance of Hydro-upgrading Catalysts Table 3 The evaluation results of NWY-1, NWY-2 and NWY-3 catalysts Items Feedstock NWY-1 NWY-2 NWY-3 NWY-1 NWY-2 NWY-3 Operating conditions Ⅰ Ⅱ Pressure, MPa Temperature, K H 2 /oil volume ratio 600:1 600:1 LHSV, h Oil properties: Density (20 ), g/cm Distillation, K IBP 50% EBP Yield, m% Cetane number ΔCN Aromatic content, v% and hydro-upgrading of FCC LCO feedstock. 4 Conclusions In the present study it was found that the crystal size of USY zeolite had an important influence on the hydroupgrading catalyst performance. Reduction of crystal sizes can lead to higher external surface area of the USY zeolite. The decrease in crystal size can shorten the diffusional path, and undesirable diffusional limitations can be reduced or eliminated. The catalyst evaluation results revealed that the hydro-upgrading catalyst prepared on the basis of USY zeolite with smaller crystal size had higher tetralin conversion and better hydro-upgrading performance for treating FCC LCO along with higher diesel yield, therefore the USY zeolite sample with smaller crystal size is more suitable for preparation of hydro-upgrading catalysts compared with the USY zeolite sample with larger crystal size. Acknowledgment: Financial support from the China National Petroleum Corporation (CNPC) (2011B ) is gratefully acknowledged. References [1] Antony S, Abdulazeem M, Mohan S R. Recent advances in the science and technology of ultra- low sulfur diesel (ULSD) production[j]. Catal Today, 2010, 153(1/2): 1-68 [2] Song C S. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel[j]. Catal Today, 2003, 86(1/4): [3] Huang L, Huang Q L, Xiao H N, et al. Al-MCM-48 as a potential hydrotreating catalyst support: I-Synthesis and adsorption study[j]. Microporous and Mesoporous Mater, 2008, 111(1): [4] Fujikawa T, Idei K, Ebihara T, Mizuguchi H, et al. Aromatic hydrogenation of distillates over SiO 2 -Al 2 O 3 -supported noble metal catalysts[j]. Appl Catal A, 2000, 192(2): [5] Yin Hailiang, Zhou Tongna, Liu Yunqi, et al. Synthesis of nanocrystal zeolite Y and its application in the catalyst for hydrodesulfurization and hydrodenitrogenation of diesel fuel[j]. Petroleum Processing and Petrochemicals, 2011, 42(7): (in Chinese) [6] Guo Jintao, Tian Ran, Zhang Zhihua, et al. Research of a new bulk catalyst for gas oil hydrotreating[j]. Petroleum Processing and Petrochemicals, 2010, 41(6): (in Chinese) [7] Ding L H, Zheng Y, Zhang Z S, et al. Hydrotreating of light cycle oil using WNi catalysts containing hydrothermally and chemically treated zeolite Y [J]. Catal Today, 2007, 125(3/4): [8] Bataille F, Lemberton J L, Pérot G, et al. Sulfided Mo and CoMo supported on zeolite as hydrodesulfurization cata- 35

5 China Petroleum Processing and Petrochemical Technology 2012,14(1):32-36 lysts: Transformation of dibenzothiophene and 4,6-dimethyldibenzothiophene[J]. Appl Catal A, 2001, 220(1/2): [9] Naoyuki K, Ki-Hyouk C, Yozo K, et al. Optimum coating of USY as a support component of NiMoS on alumina for deep HDS of gas oil[j]. Appl Catal A, 2004, 276(1/2): [10] Breck D W, Flanigen E M. Molecular Sieves[M]. London Society of Chemical Industry, 1968: 47 [11] Michael T, Mark L, Tatsuya O. Characterization of zeolite L nanoclusters[j]. Chem Mater, 1995, 7(9): [12] Santikunaporn M, Herrera J E, Jongpatiwut S, et al. Ring opening of decalin and tetralin on HY and Pt/HY zeolite catalysts[j]. J Catal, 2004, 228(1): [13] Liu H R, Meng X C, Zhao D S, et al. The effect of sulfur compound on the hydrogenation of tetralin over a Pd-Pt/ HDAY catalyst[j]. Chem Eng J, 2008, 140(1/3): [14] Yamamura M, Chaki K, Wakatsuki T, et al. Synthesis of ZSM-5 zeolite with small crystal size and its catalytic performance for ethylene oligomerization[j]. Zeolites, 1994, 14(6): [15] Hu L Y, Xie S J, Wang Q X, et al. Advances in synthesis and application of superfine zeolites[j]. Petrochemical Technology, 2007, 36(12): (in Chinese) [16] Wu J, Qin Y N, Ma Z, et al. synthesis and application of NaY zeolite from coal-bearing kaolin[j]. Chemical Industry and Engineering, 2006, 23(1): (in Chinese) Commercial Application of Catalytic Pyrolysis Technology for Maximization of Olefins Yield Has Been Launched Following the successful commercial test of catalytic pyrolysis technology aimed at cracking of resid in a twostage riser reactor for maximization of propylene yield (TMP) at PetroChina s Daqing Refining and Chemical Branch Company, other two TMP units are to come online. Another 1.0 Mt/a TMP unit owned by Sinochem s Hongrun Petrochemical Company in Weifang is in the phase of detailed engineering design. Hence this technology is experiencing a prelude of commercial application ready to be unveiled. The TMP technology, which is developed by China Petroleum University (East China) and the engineering design of which is undertaken by PetroChina s East China Design Branch Company, operates on a two-stage riser catalytic cracking platform, which upon retaining the original advantages such as reaction in various stages, catalyst transfer, short reaction time and large catalyst/ oil ratio, has also realized a combined feed inlet, a low reaction temperature and an appropriate reaction duration. By adopting high density of fluidized catalyst, this technique has solved the contradictions between maximization of propylene output and minimization of dry gas with high H 2 content to guarantee a maximized propylene yield along with high-quality light oil. The results of commercial tests of the TMP technology have revealed that this technology by using atmospheric heavy oil as the feedstock can increase the propylene yield coupled with harvesting good yields and quality of other products. For example upon processing the Daqing AR this technology can achieve an over 20% propylene yield, less than 5.5% yield of dry gas (containing about 50% ethylene), and the olefin content in gasoline is less than 35% with an octane rating of close to 97 RON. Compared to other RFCC technologies, the TMP technology has an obvious advantage in terms of propylene yield, product quality and dry gas yield. In addition, the TMP technology requires mild reaction conditions, and its reaction temperature is comparable with that of conventional catalytic cracking process. According to experts of oil refining industry, commercialization of the TMP technology not only can provide a new route for utilization of heavy oil to manufacture propylene, but also can solve the problem for reducing olefin content in FCC naphtha, apart from increasing the diesel/gasoline ratio in finished oil products pool and improving the product slate to apparently enhance economic benefits of petrochemical enterprises. 36