Research articleCatalytic cracking of vacuum gasoil over -SVR, ITH, and MFI zeolites as FCC catalyst additives
Graphical abstract
Introduction
The primary objective of the fluidized catalytic cracking (FCC) process is to increase the production of gasoline by processing vacuum gasoil (VGO) and other low-value feeds such as atmospheric residues [1], [2]. FCC gasoline continues to be a major component in the total gasoline pool produced in an integrated refinery. As the market conditions evolve, FCC units are being optimized to produce light olefins along with transportation fuels [1], [3]. Conventional FCC units produce approximately 4–6 wt% propylene. Revamping operating conditions and catalyst system enabled over 20–25 wt% propylene production [1], [2]. High propylene yield from FCC units is a result of high VGO conversion and selective cracking which maximizes monomolecular reactions and minimizes secondary reactions [3], [4]. The most cost effective route to enhance light olefins yield from FCC units is the use of medium-pore zeolite additives [5], [6], [7]. MFI additive cracks reactive species in gasoline fraction to LPG olefins. Other zeolite structures such as SSZ-74, SSZ-33, MFI, TNU-9, IM-5 MCM-36, MCM-22 and ferrierite were evaluated to increase the yield of light olefins in VGO catalytic cracking [5]. Maximum yield of propylene of 11 wt% was reported over MCM-36 and ferrierite compared to 7 wt% over the base catalyst [5].
Adewuyi et al. [8] investigated blending of various amounts of MFI additive to USY FCC catalyst in VGO cracking. Maximum enhancement in propylene yield of 11.8 wt% compared to 5.4 wt% over the base catalyst was found at 25 wt% additive level. After further increase in MFI addition, reactive species in gasoline fraction were completely converted and significant decrease in the conversion was observed due to the dilution effect [8]. The reason is a poor ability of bulkier molecules of gas oil to diffuse through the small pore of MFI to reach the active sites. Li et al. compared the catalytic conversion of VGO using MFI and USY based catalysts [9]. The authors reported that at temperature of 560 °C and high C/O ratio equal to 8 g/g the conversion over MFI as base catalyst can reach comparable levels to USY catalyst. Catalytic cracking processes such as deep catalytic cracking (DCC) utilized MFI based catalyst under more severe conditions to enhance propylene yield [1], [4].
The successful zeolite additive in enhancing propylene yield must be active to for cracking of gasoline fraction selectively to light olefins. Zeolites activity and selectivity are greatly influenced by their acidic properties such as, acid sites type (Brønsted or Lewis), concentration, strength and location [10], [11], [12]. Several studies investigated the effect of the number of acid sites on MFI catalytic behaviour in hydrocarbons cracking [13], [14]. Direct correlation exists between the number of acid sites and product yields. Framework tetrahedral aluminum is the source of zeolite acidity. Therefore, zeolite Si/Al molar ratio has major impact on its performance as catalysts [14]. Lu et al. investigated the effect of MFI Si/Al molar ratio as FCC catalyst additive in the cracking of C4 alkanes [10]. They found that high Si/Al molar ratio (> 80) was more effective in producing light olefins. The hydrogen transfer reactions were minimized due to a lower number of acid sites. Other studies described elevated hydrothermal stability correlated to high Si/Al molar ratio [15], [16]. Low aluminum content reduces the level of Al extraction from the framework. This increases framework stability and improves the efficiency in light olefins production [16].
High silica –SVR zeolite is characterized by 3-dimensional 10-ring channel system with pore openings at 5.5 × 5.7, 5.2 × 5.9 and 5.2 × 5.6 Å [17]. Zeolite ITH possesses interconnecting 9 − (4.0 × 4.9 Å), 10 − (4.8 × 5.7 Å) and 10 − (4.7 × 5.1 Å) ring channels. It has been prepared by Corma and coworkers in germanosilicate reaction mixture [18]. The incorporation of Al into ITH framework, accompanied with a formation of strong acid sites, was achieved either by applying direct template-assisted hydrothermal synthesis (Si/Al = 30–65) [19] or by post-synthesis alumination (Si/Al = 34–70) [20]. It was shown, that small pore dimensions and strong acidity in directly synthesized Al-containing ITH zeolite (Si/Al = 30) increased propylene production in VGO cracking compared with MFI [19].
In this work, three medium-pore aluminosilicate zeolites of -SVR (10 × 10 × 10-ring channels), MFI (10 × 10 × 10-ring channels) and ITH (10 × 10 × 9-ring channels) topology with varying Si/Al molar ratio were investigated for their catalytic performance as FCC additives in the cracking of hydrotreated VGO derived from Arabian Light crude oil. Zeolite additives were compared by VGO conversion and products yields. In addition, effect of crystal size using MFI zeolites, catalytic parameters such as C3/gasoline ratio, C3 olefinicity and hydrogen transfer coefficient were also investigated.
Section snippets
Materials
The E-Cat was a commercial equilibrium zeolite catalyst acquired from a local refinery. It has a Si/Al molar ratio of 3.3, BET area of 158 m2/g and pore volume of 0.14 cm3/g. The E-Cat was calcined for 5 h at 550 °C in air to have the sample free of coke according to ASTM test method D3907 [21]. The zeolites MFI, -SVR and ITH additives were activated for 5 h at 550 °C to get the H-form. The zeolites were pelletized, crushed and 80–90 μ size of the material was sieved out. The E-Cat containing additive
Catalyst characterization
XRD patterns of –SVR, ITH, and MFI samples are shown in Fig. S-1. The diffraction patterns are consistent with those published for standard samples in IZA [27]. Aluminated ITH-34 and ITH-48 zeolites display sharp diffraction lines at the characteristic 2-theta positions (Fig. 1), matching well with those reported in the literature [28]. No diffraction lines of Al(NO3)3 or other crystalline admixtures were observed in XRD patterns of ITH zeolites after alumination procedures.
Fig. S-2 shows SEM
Reaction scheme
To explore the impact of Si/Al molar ratio on apparent activation energies for VGO catalytic cracking, kinetic study was performed over E-Cat and the additives MFI-30, MFI-280 and MFI-2000. A 4-lump kinetic model is used to represent cracking reactions as proposed in Scheme 1.The model considers the cracking of VGO to the main products gasoline, gases and coke. It also accounts for the cracking of gasoline to gases where the experimental data showed correlation between gases yields (including
Conclusions
Three medium-pore zeolites -SVR, ITH and MFI with different Si/Al ratios were investigated as FCC catalyst additives for VGO cracking. All E-Cat/additives showed an increase in LPG yield at the expense of gasoline. Higher propylene and light olefin yields result from two factors: high gasoline over-cracking and low hydrogen transfer reactions. Increasing Si/Al ratio minimized hydrogen transfer reactions due to lower density of acid sites; however, it decreased gasoline over-cracking. In wide
Acknowledgement
The authors acknowledge the support from the Saudi Ministry of Education in establishing of the Center of Research Excellence in Petroleum Refining & Petrochemicals (CoRE-PRP) at KFUPM. J. Č. thanks the Czech Science Foundation for support (P106/12/G015), M. S. acknowledges the Czech Science Foundation for funding (14-30898P).
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