Fluid Catalytic Cracking of Hydrogenated Light Cycle Oil for Maximum

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Fluid Catalytic Cracking of Hydrogenated Light Cycle Oil for Maximum Gasoline Production: Effect of Catalyst Composition Haina Zhang, Xiaolin Zhu,* Xiaocheng Chen, Peipei Miao, Chunchao Yang, and Chunyi Li* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China ABSTRACT: Selective hydrogenation and subsequent catalytic cracking of light cycle oil (LCO) from a fluid catalytic cracking unit is expected to produce more high-octane-number gasoline. In this process, the multi-ring aromatics are selectively hydrogenated and transformed to naphthenic aromatics, which are further converted into the gasoline fraction through cracking reaction. This work has systematically studied the effect of catalyst composition on the cracking performance of hydrogenated LCO (hydro-LCO). The results indicate that, the cracking activity of LCO was substantially improved after hydrogenation. In comparison to the ZSM-5-zeolite-based catalyst, both an efficient conversion of hydro-LCO to gasoline and a greatly enhanced hydrogen transfer reaction were obtained over the Y-zeolite-based catalyst, further resulting in a higher hydrogen utilization efficiency. In addition, the active acid sites for hydro-LCO cracking were inferred, and a possible reaction network was proposed.

1. INTRODUCTION In recent years, worldwide crude oil has become much heavier and inferior. However, the demand for heavy fuel oil has decreased, and the demand for light oils has increased considerably, especially for gasoline and diesel.1−3 The fluid catalytic cracking (FCC) process plays an important role in converting heavy crude oil into light oil products. However, in some extreme cases, the light cycle oil (LCO) in the FCC unit, usually used as a blending component of diesel oil, is greatly deteriorated in quality and can hardly be improved. Therefore, how to convert these inferior LCO has become an urgent issue. Moreover, considering the increasing gasoline demand in the domestic market, the method to efficiently transform the inferior LCO into high-octane-number gasoline has become the focus of this work. In composition, the most abundant constituent in LCO is aromatic compounds, especially the multi-ring aromatics,4 directly inducing a low cetane number of diesel oil.5 However, the aromatic molecules usually deliver a high octane number for gasoline. Considering this, the selective conversion of multiring aromatics into single-ring aromatics would be a possible strategy to upgrade inferior LCO and produce high-octanenumber gasoline simultaneously. Under typical FCC conditions, the multi-ring aromatics are more preferentially condensed than cracked into small molecules. When subjected to selective hydrogenation, these multi-ring aromatics can be partially saturated to naphthenic aromatics, which can be further cracked into single-ring aromatic molecules constituting the high-octane-number gasoline.6 In general, the inferior LCO rich in multi-ring aromatics is a potential feedstock for producing high-octane-number gasoline through the FCC and hydrogenation coupling process. Given the above discussions, the FCC and hydrogenation coupling process, of which the processing scheme is depicted in Figure 1, has been proposed for efficient conversion of the inferior LCO to high-quality gasoline. In this coupling process, the heavy feed oil is treated in the FCC unit under typical operating conditions. Afterward, the oil gas yielded from the © XXXX American Chemical Society

Figure 1. Schematic diagram of the FCC and hydrogenation coupling process.

FCC unit is distillated into gas products, gasoline, LCO, heavy cycle oil (HCO), and slurry through a fractionator. Ultimately, the LCO rich in multi-ring aromatics is selectively hydrogenated into naphthenic aromatics, and then this stream (hydro-LCO) is recycled into the FCC unit for further cracking reactions to produce high-octane-number gasoline. Besides the feedstock property7−10 and operating conditions,1,11 the catalyst composition also affects the product distribution remarkably. Although a series of studies have been conducted to investigate the influence of the catalyst composition on the products in conventional FCC processes,12−16 the study on the catalyst effect in this FCC and hydrogenation coupling process is still lacking. Consequently, it is really necessary to probe into the effect of catalyst composition and determine which type of catalyst (Y or ZSM-5 zeolite based) is more likely to be used in such a coupling process. Received: January 17, 2017 Revised: February 28, 2017 Published: March 2, 2017 A

DOI: 10.1021/acs.energyfuels.7b00185 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels In this work, the FCC and hydrogenation coupling process for efficient conversion of LCO to gasoline fraction has been systematically studied. The cracking performance of LCO and hydro-LCO is primarily compared. Afterward, the effect of the catalyst composition on the conversion of hydro-LCO was investigated, and the catalyst structure−activity relationship was further probed. Ultimately, on the basis of the changing composition of the LCO fraction, the reaction pathway of the multi-ring aromatics in this selective hydrogenation and FCC coupled process was deduced.

2. EXPERIMENTAL SECTION 2.1. Feedstock and Catalyst. The LCO feedstock used in this work was obtained from an industrial FCC unit of PetroChina Co., Ltd. LCO was mildly hydrogenated in a fixed-bed reactor filled with a commercial hydrogenation catalyst at the temperature of 350 °C, pressure of 8.7 MPa, hydrogen/oil ratio of 1000, and weight hourly space time (WHSV) of 1.2 h−1. The as-yielded stream is denoted as hydro-LCO, and the detailed properties of LCO and hydro-LCO are both listed in Table 1. Moreover, the hydrocarbon compositions of

Figure 2. Schematic diagram of the fixed-bed microreactor unit. The products yielded from the microreactor were separated into gas and liquid phases through the ice bath. The gas products were analyzed using a Bruker 450 gas chromatograph equipped with a flame ionization detector and a thermal conductivity detector. The liquid products were weighed and then analyzed by simulated distillation (ASTM D2887) on another Bruker 450 gas chromatograph to determine the yields of gasoline [from initial boiling point (IBP) to 204 °C], LCO (from 205 to 350 °C), and HCO (above 350 °C). The amount of coke deposition on the spent catalyst was estimated by thermogravimetry. Herein, the LCO conversion is defined as the overall yields of dry gas, liquefied petroleum gas (LPG), gasoline, and coke. 2.3. Catalyst Characterization. Nitrogen adsorption−desorption isotherms of the catalyst samples were performed using the Quantachrome Autosorb-1 equipment at 77 K. The surface area was obtained on the basis of the Brunauer−Emmett−Teller (BET) method, and the pore sizes were calculated according to the Barrett−Joyner−Halenda (BJH) method applied to the adsorption branch of the isotherm. Moreover, the particle size distribution of the catalyst was determined by the Malvern particle size analyzer (Mastersizer 2000, U.K.). The density of acid sites was characterized by temperatureprogrammed desorption of ammonia (NH3-TPD). Herein, the acid type was determined by pyridine adsorption infrared (IR) spectra. The amount of different types of acid sites could be obtained from the detection of pyridine absorbed on the catalyst by IR transmittance spectroscopy.18 Pyridine adsorption was carried out by exposing the catalyst at the pyridine atmosphere for 1 h and further evacuated for 1 h at room temperature. Afterward, the spectra were tested and recorded by a Nicolet 6700 spectrometer at 6 cm−1 resolution.

Table 1. Properties of the LCO and Hydro-LCO Feedstock item density at 20 °C (kg m−3) carbon residue (wt %) elemental composition (wt %) C H viscosity (mm2 s−1) 20 °C 50 °C condensation point (°C) flash point (°C)

LCO

hydro-LCO

934.0 0.34

902.1 120 μm

Y-cat

Z5-cat

1.052 140 0.17

0.908 158 0.19

36.65 32.34 18.64 12.37

24.25 39.12 22.32 14.31

3. RESULTS AND DISCUSSION 3.1. Properties and Compositions of LCO and HydroLCO. After hydrogenation treatment, the hydrogen content of LCO was increased by 1.2 wt %, consequently giving rise to a significantly improved LCO property (see Table 1). In comparison to LCO, the density of hydro-LCO was decreased from 934.0 to 902.1 kg m−3, the carbon residue was reduced from 0.34 to 0.02 wt %, and the viscosity was also lowered to some extent. The improvement is primarily attributed to a lower concentration of aromatics, probably resulting from their saturation to naphthenes and paraffins during hydrogenation. It is also worth noting that the majority of two- and threering aromatics in LCO were partially saturated to one-ring aromatics and naphthenes after hydrogenation, and the onering aromatics even account for more than 50 wt % of the

2.2. Experimental Apparatus. The activity tests were conducted in a fixed-bed microreactor unit (Figure 2) to investigate the effect of the catalyst composition on the cracking performance of LCO and hydro-LCO. During each experiment, 5 g of catalyst and abundant quartz sand were loaded into the stainless-steel microreactor (12 mm inner diameter), and the reaction was carried out at 510 °C with 5 g/g catalyst-to-oil (CTO) ratio and 9.6 h−1 WHSV. B


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Although the catalyst composition exhibits a similar influence on the cracking of LCO and hydro-LCO, their product distributions deviated from each other significantly. In comparison to LCO, the cracking performance of hydro-LCO was obviously improved. After hydrogenation treatment, a greatly enhanced conversion of LCO to gasoline was obtained for hydro-LCO and the condensation reaction of LCO to HCO and the coking reaction were both effectively inhibited. Meanwhile, the yield of LPG deviated slightly between LCO and hydro-LCO, thereby leading to a promoted production of liquid product (including LPG, gasoline, and LCO) after hydrogenation. The improved catalytic cracking performance of hydro-LCO over LCO is primarily attributed to their difference in composition. LCO without hydrogenation is rich in two- and three-ring aromatics (see Figure 3), which are difficult to be cracked under FCC conditions. In contrast, these multi-ring aromatics are selectively converted to one-ring aromatics and naphthenes through hydrogenation treatment, thereby giving rise to a higher cracking activity to the gasoline fraction and a lower tendency to condensation reactions. The compositions of dry gas yielded from hydro-LCO cracking over the four different catalyst samples are given in Figure 5. With the increasing percentage of Y-cat, the content of paraffins in dry gas, including CH4 and C2H6, increased, while that of C2H4 decreased. Meanwhile, the hydrogen content also shows a slight decreasing trend over Y-cat, which probably resulted from the enhanced hydrogen transfer reaction. During the FCC process, the naphthenes and aromatics suffer from dehydrogenation and condensation and the as-yielded hydrogen are likely to saturate the olefins into paraffins.19−21 The decreasing content of hydrogen in dry gas also indicates a higher utilization efficiency of hydrogen over Ycat. Figure 6 illustrates the LPG composition of hydro-LCO cracking over different catalyst samples. As reflected by the decreasing content of olefins (C3H6 and C4H8) and their transformation to paraffins (C3H8 and C4H10), the intensified hydrogen transfer reaction over Y-cat was further evidenced, which is consistent with the previous report.22−25 Similarly, the content of paraffins in both LPG and dry gas obtained from LCO cracking increased obviously with the increasing content

hydro-LCO fraction (see Figure 3). Specifically, the content of multi-ring aromatics was reduced by 43.92 wt %, and the

Figure 3. Hydrocarbon compositions of LCO before and after hydrogenation.

content of one-ring aromatics and naphthenes/paraffins was increased by 36.73 and 7.19 wt %, respectively, indicating that the multi-ring aromatics were selectively hydrogenated and partially saturated. These one-ring aromatics and naphthenes are more likely to be cracked under typical FCC conditions and enter the gasoline boiling range and, thereby, can be considered as important constituents for gasoline production. 3.2. Catalytic Cracking Performance of Different Zeolite-Based Catalysts. In a fixed-bed microreactor, the cracking performance of LCO and hydro-LCO were both evaluated over a series of catalysts with different compositions. The catalyst samples included Y-cat, Z5-cat, and their mixtures of different mass ratios [denoted as Y/Z5-cat (1:1) and Y/Z5cat (2:3)]. According to the product distributions shown in Figure 4, both the conversions of LCO and hydro-LCO to gasoline were increased by the increasing portion of Y-cat but the coking rate increased inevitably, while the condensation of LCO and hydro-LCO to HCO was enhanced over the Z5-cat and the yield of LPG was also elevated to some extent.

Figure 4. Product distributions of (a) LCO and (b) hydro-LCO cracking over the four different catalysts at the temperature of 510 °C, CTO of 5 g/ g, and WHSV of 9.60 h−1. C


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Energy & Fuels

Figure 7. HTI of LCO and hydro-LCO cracking over the four different catalysts.

Figure 5. Dry gas compositions of hydro-LCO cracking over the four different catalysts.

coking reaction is inevitable. In the case that the light olefins are desired products, the ZSM-5-zeolite-based catalyst effectively inhibiting the hydrogen transfer reaction is preferred. 3.3. Relationship between Catalyst Property and Catalytic Performance. The acidic property of the catalyst influences its catalytic performance significantly. The NH3-TPD measurement was conducted to characterize the catalyst acidity. In general, the TPD profiles of Y-cat and Z5-cat can be divided into two regions (see Figure 8a): a low-temperature region from 50 to 275 °C representing the weak acid and a hightemperature region from 275 to 500 °C standing for the strong acid. Through integrating the area of these two regions, the relative amount of weak and strong acids is estimated. In comparison to Z5-cat, the weak acid of Y-cat decreased obviously, while the strong acid increased by about 18%. Interestingly, this value is close to the difference in hydro-LCO cracking ability (see Figure 4b), suggesting a close relationship between the strong acid amount and catalyst activity. To take a deeper insight into the effect of acid type, pyridine Fourier transform infrared spectroscopy (FTIR) of the four different catalyst samples were taken, and the spectra are illustrated in Figure 8b. The peak around 1440 cm−1 is attributed to C−H bending of pyridine adsorbed on the Lewis acid sites,26 and an increase of the Lewis acid strength would lead to a shift of this bond toward higher frequencies as a result of more electron cloud of the C−H bond attracted to the Lewis sites.27 The peak at 1540 cm−1 is assigned to pyridine adsorbed on the Brønsted acid sites, and the peak around 1490 cm−1 is associated with both Brønsted acid and Lewis acid sites.28 In comparison to Z5-cat, the Lewis acid sites of Y-cat were greatly reduced but the concentration of Brønsted acid sites was close to each other. Considering the higher LCO conversion to gasoline over Y-cat than that over Z5-cat, the strong Brønsted acid sites are deduced to play an important role in the catalytic cracking of partially saturated naphthenic aromatics. The structure of the zeolite used to fabricate the catalyst also generates considerable influence on the cracking reaction.29 Besides the micropore channel structures and directions, the difference between ZSM-5 and Y zeolites mainly lies in their micropore sizes. The micropore size of the ZSM-5 zeolite is in the range of 0.51−0.55 nm, while that of the Y zeolite is greatly enlarged up to 0.74 nm.30 The main components of LCO and

Figure 6. LPG compositions of hydro-LCO cracking over the four different catalysts.

of Y-cat in the mixed catalyst, and an greatly enhanced hydrogen transfer reaction was also observed. To further probe into the hydrogen transfer reaction, the hydrogen transfer index (HTI) defined as the weight ratio of (C3° + C4°)/(C3= + C4=) is introduced,3,21 with a larger value representing a higher reaction intensity. Consistent with the above results, a higher hydrogen transfer intensity was obtained over Y-cat (see Figure 7), while Z5-cat exhibited outstanding shape selectivity and suppressed the hydrogen transfer reaction remarkably, consequently resulting in high propylene and butene contents in LPG. Notably, a higher hydrogen transfer intensity was obtained for hydro-LCO than LCO without hydrogenation. This result indicates that the FCC and hydrogenation coupling process can not only efficiently convert the inferior LCO to high-octane-number gasoline but also guarantee a low olefin content of the gasoline fraction. In summary, the catalyst composed of Y zeolite can efficiently convert the hydrogenated LCO into high-quality gasoline but an increasing intensity of hydrogen transfer and D


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Energy & Fuels

Figure 8. (a) NH3-TPD profiles and (b) pyridine IR spectra of different catalyst samples.

hydro-LCO are two-ring aromatics and naphthenic aromatics, respectively. When tetralin is taken as a typical example, its kinetic diameter is around 0.62 nm;31 therefore, it can only be cracked on the external surface of the ZSM-5 zeolite as a result of diffusional limitation but can be converted on both the external surface and internal micropores of the Y zeolite. Consequently, a higher conversion of both LCO and hydroLCO has been achieved over the Y-zeolite-based catalyst. Moreover, the Y zeolite also provides enough space for the hydrogen transfer reaction,32 thereby resulting in a higher coking rate and a lower concentration of light olefins. 3.4. Reaction Pathway in the Selective Hydrogenation and FCC Coupled Process. Given that the inferior LCO rich in multi-ring aromatics is hardly cracked under typical FCC conditions, the FCC and hydrogenation coupling process has been proposed to selectively saturate these aromatic molecules to naphthenic aromatics and further crack them to produce high-octane-number gasoline. On the basis of the composition of LCO before and after hydrogenation as well as the product distribution of subsequent catalytic cracking, the reaction pathway of the multi-ring aromatics is inferred in Figure 9.

Through selective hydrogenation, the tetralin molecules are partially saturated to C10 naphthenic aromatics, which are further cracked to gasoline-range one-ring aromatics and light olefins under the catalysis of strong Brønsted acid sites. Theoretically, the as-yielded gasoline fraction enriching in aromatic components possesses a high octane number. In contrast, the phenanthrene molecules are much harder to be hydrogenated, and the yielded two-ring aromatics by the cracking reaction are still in the diesel fraction. Meanwhile, the hydrogen transfer between the naphthenic aromatics and light olefins occurs intensively over the Y zeolite, thereby resulting in a high coking rate and generating abundant light alkanes. The ZSM-5-zeolite-based catalyst is more preferred to suppress the hydrogen transfer reaction, but the conversion of LCO to gasoline decreases inevitably.

4. CONCLUSION In this work, the FCC and hydrogenation coupling process for efficient conversion of inferior LCO to high-quality gasoline is presented and the effect of catalyst composition on the cracking performance of hydro-LCO has been systematically studied. The results indicate that the hydrogenation treatment improved the cracking activity of LCO remarkably. In comparison to Z5cat, both a higher LCO conversion and a greatly intensified hydrogen transfer reaction were obtained over Y-cat, thereby yielding a higher hydrogen utilization efficiency but a much faster coking rate. Moreover, the strong Brønsted acid sites were speculated to play an important role in the hydro-LCO cracking reaction. Ultimately, a possible reaction network was proposed on the basis of the composition of LCO before and after hydrogenation as well as the FCC product distributions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaolin Zhu: 0000-0002-6527-100X Chunyi Li: 0000-0002-2595-4690 Notes

Figure 9. Reaction network of multi-ring aromatics in the FCC and hydrogenation coupling process.

The authors declare no competing financial interest. E


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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21606257 and U1362201) and the Fundamental Research Funds for the Central Universities (17CX02015A and 16CX06013A).

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Fluid Catalytic Cracking of Hydrogenated Light Cycle Oil for Maximum

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