Upgrading of Light Cycle Oil to High-Octane Gasoline through

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Upgrading of Light Cycle Oil to High-Octane Gasoline through Selective Hydrocracking over Non-Noble Metal Bifunctional Catalysts Chong Peng, Zhiming Zhou, Zhen-Min Cheng, and Xiangchen Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04229 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Upgrading of Light Cycle Oil to High-Octane Gasoline through Selective Hydrocracking over Non-Noble Metal Bifunctional Catalysts Chong Peng,†,‡ Zhiming Zhou,*,† Zhenmin Cheng,† Xiangchen Fang*,‡

†

State Key Laboratory of Chemical Engineering, East China University of Science and Technology,

Shanghai 200237, China ‡

Dalian Research Institute of Petroleum and Petrochemicals, SINOPEC, Dalian 116000, China

* Corresponding Authors E-mail: [email protected] (Z. Zhou). E-mail: [email protected] (X. Fang).

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ABSTRACT Three

non-noble

metal

bifunctional

catalysts

(Ni-W/Hβ-Al2O3,

Ni-W/HY-Al2O3

and

Ni-Mo/HY-Al2O3) were prepared and applied to the selective hydrocracking of light cycle oil (LCO) into high-octane gasoline. The structures of catalysts were characterized by various techniques, and the catalytic performances were tested at 400 °C, 8.0 MPa and a liquid hourly space velocity of 0.8 h-1 using a once-through mode at a pilot-scale reactor. Among the three catalysts, Ni-Mo/HY-Al2O3 showed the highest yield of gasoline (43.4 wt% in which aromatics accounted for 31.6%) and the highest research octane number (RON, around 85), which were mainly due to the high total acidity, high surface area and homogeneous distribution of the active phase on the catalyst. In a long-term test (3200 h) with a partial-recycle mode at the pilot reactor, Ni-Mo/HY-Al2O3 exhibited a high and stable gasoline yield (about 53 wt%) with a RON of 92-95, indicating its high performance for the selective hydrocracking of LCO. This catalyst was eventually commercialized in a refinery, which gave rise to a stable gasoline yield of about 40 wt% with a RON of 91 during 250 days on stream. Moreover, the chemical and structural properties of the used catalyst were preserved. Keywords Light cycle oil; High-octane gasoline; Selective hydrocracking; Bifunctional catalyst; Stability

2


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1. INTRODUCTION Light cycle oil (LCO), derived from the fluid catalytic cracking unit and with a boiling point similar to that of diesel, is a poor blending component in diesel pool mainly due to its high aromatic, sulfur and nitrogen contents as well as low API gravity and cetane index.1-4 The aromatic compounds in LCO are composed mainly of polycyclic aromatic hydrocarbons (PAHs), which may account for as high as 90% of LCO.2,5 Upgrading of LCO by complete hydrogenation of aromatics and ring opening of naphthenes is able to yield high cetane number products, but it requires a large quantity of hydrogen, e.g., 7 moles of H2 are needed per mole of naphthalene to produce one mole of 5-methyl-nonane. In contrast, selective hydrocracking of LCO, i.e., partial hydrogenation of PAHs followed by selective ring opening of naphthenic structures, can generate high-octane gasoline with the consumption of a relatively small amount of hydrogen, e.g., 4 moles of H2 are required per mole of naphthalene to yield one mole of benzene. In addition, the demand for gasoline in China increases rapidly in recent years while diesel declines. The ratio of diesel to gasoline has decreased from 2.27:1 in 2005 to 1.64:1 in 2014, which is expected to decline to about 1.1:1 by 2020.6 In this respect, selective hydrocracking of LCO into high-octane gasoline seems to be a more economically viable option for upgrading of LCO in China. Bifunctional catalysts with metal and acid sites are commonly used for selective hydrocracking of PAHs. The former site is mainly responsible for hydrogenation and dehydrogenation reactions, while the latter for isomerization, ring opening and dealkylation reactions.7,8 Catalysts based on noble metals such as Pd/HY,9 Pt/γ-Al2O310 and Pt/AlSBA-15–Al2O311 have been assessed for the selective hydrocracking of PAHs. However, noble metal catalysts are subject to deactivation caused by sulfur and nitrogen poisoning; in addition, they are restricted due to the high cost and limited 3


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availability. These disadvantages have spurred interests in non-noble metal catalysts such as Ni2P/β,12,13 Co-Mo/β,14,15 Ni-W/β,16,17 Ni-W/Hβ-Al2O3,5,18 Ni-Mo/Hβ–HZSM-5,4 Ni-Sn/Hβ,19 Mo2C/γ-Al2O3,10 and W/β.20 Almost all the catalysts mentioned above have been tested using model di- and tri-aromatic compounds, e.g., tetralin,4,9,14,19 naphthalene,9,10,12 methylnaphthalenes,10,11,16,17,20 a mixture of phenanthrene and 1-methylnaphthalene,13 etc. Nevertheless, considering the complex composition of real feedstocks and the possible effect of sulfur- and nitrogen-containing compounds on the activity and stability of catalysts, experiments with real feedstocks are indispensable.2 Upare et al.14,15 synthesized a series of Co-Mo/β catalysts and reported that the best catalyst produced a maximum monoaromatics yield of 54.8% at 99.1% conversion of pyrolysis fuel oil (PFO, a waste product from the naphtha steam cracking process, characteristic of a high content of PAHs such as naphthalene derivatives); moreover, this catalyst exhibited good stability during 140 h of reaction time. However, the above data were obtained with catalyst powder (500 mesh size, about 25 μm) in a laboratory-scale reactor, which are some distance from more meaningful results that could be achieved with catalyst particle (mm-scale) in a pilot-scale reactor; in addition, the global annual output of PFO is much lower than that of LCO. Therefore, for practical purposes, it is of great interest to investigate the performance of shaped catalysts for the selective hydrocracking of LCO at a pilot-scale reactor. In this study, we report how we develop an effective catalyst that can convert LCO to high-octane gasoline through selective hydrocracking. First, three types of non-noble metal bifunctional catalysts were prepared by impregnation method, characterized with various techniques and tested at a pilot-scale reactor using a real LCO feedstock, whereby the structure-activity relationships of 4


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the catalysts were explored. Next, the best catalyst was evaluated for 3200 h on stream at the pilot reactor to assess the long-term stability. Finally, the catalyst was applied at an industry scale, and the structural properties of the used catalyst after 250 days of operation were analyzed and compared with those of the fresh counterpart.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Three non-noble metal catalysts, Ni-W/Hβ-Al2O3, Ni-W/HY-Al2O3 and Ni-Mo/HY-Al2O3, were prepared by impregnation method. The reason why Hβ and HY zeolites are used in this study is that they are most frequently used in commercial hydrocracking catalysts on the one hand,21 and they have been reported effective for the selective hydrocracking of PAHs on the other hand.9,12-20 In addition, the three catalysts have different compositions, textural properties and acidities (see below in Results and Discussion), which are expected to result in different hydrocracking performance. Taking Ni-W/Hβ-Al2O3 as an example, the preparation procedure was as follows: first, Hβ zeolite was mixed with Al2O3 and extruded to form a cylindrical shape (1.4-1.6 mm diameter and 3-8 mm height), which was dried and calcined; next, Ni species (nickel nitrate hexahydrate) and W species (ammonium metatungstate hydrate) were co-impregnated on the Hβ-Al2O3 support; finally, the sample was dried at 120 °C for 2 h and calcined at 500 °C for 4 h. Due to their confidential nature, full details about the catalyst preparation was not provided. The Ni and W (or Mo) loadings in each catalyst, determined by inductively coupled plasma-optical emission spectroscopy, are listed in Table 1.

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Table 1. Composition, Textual Properties and NH3-TPD Results of Fresh Oxidic Catalysts acidity (mmol NH3/g)

Ni

Mo

W

SBET

Vpore

dBJH

(wt%)

(wt%)

(wt%)

(m2/g)

(cm3/g)

(nm)

150-250 °C

250-400 °C

400-500 °C

Sum

Ni-W/Hβ-Al2O3a

4.6

−

17.4

172.1

0.34

6.9

0.27

0.22

0.08

0.57

Ni-W/HY-Al2O3b

5.3

−

19.0

209.3

0.34

8.2

0.22

0.13

0.05

0.40

Ni-Mo/HY-Al2O3b

3.9

10.0

−

396.4

0.28

6.3

0.39

0.33

0.15

0.87

catalyst

a

For Hβ with a SiO2/Al2O3 molar ratio of about 25, the BET surface area, pore volume and BJH pore diameter

determined by the adsorption branch are 542.7 m2/g, 0.53 cm3/g and 5.1 nm, respectively. b

For HY with a SiO2/Al2O3 molar ratio of about 15, the BET surface area, pore volume and BJH pore diameter

determined by the adsorption branch are 685.3 m2/g, 0.47 cm3/g and 3.5 nm, respectively.

2.2. Characterization of Catalysts. The Brunauer-Emmett-Teller (BET) surface area, pore volume and average pore diameter of samples were determined by N2 adsorption-desorption data collected at -196 °C on Micrometrics ASAP 2420. Before analysis the samples were degassed at 133 Pa and 300 °C for 3 h. Powder X-ray diffraction (XRD) on Rigaku D/Max 2550 was conducted to confirm the crystalline structure of sample by using a Cu Kα radiation in the range of 10-80° at a scan rate of 0.02°·s-1. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed on Agilent 725-ES to determine the metal loading of catalyst. High-resolution transmission electron microscopy (HRTEM) was recorded on JEOL JEM-2100 to observe the morphology of catalyst. Temperature-programmed reduction (TPR) of catalyst was carried out on Micromeritics AutoChem 2920 to investigate the metal-support interaction. The sample was reduced from room temperature to 850 °C in 10% H2/Ar at a heating rate of 10 °C/min, and the H2 consumption was monitored by a thermal conductivity detector (TCD). NH3-TPD experiments were also conducted on this instrument. The sample was first heated to 485 °C at a rate of 30 °C/min and hold for 1 h, and then cooled to 150 °C and treated with 5% NH3/He for 1 h. After NH3 adsorption, the sample was flushed with He for 1 h to remove any physically adsorbed NH3. Desorption was monitored by 6


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increasing the temperature from 150 to 500 °C at a rate of 10 °C/min. Raman spectra were recorded from 200 to 1200 cm-1 on Horiba Jobin Yvon Labram HR800 spectrometer using a 532 nm argon ion laser. During the analysis the spectral resolution was 0.65 cm-1 and the laser power was 10 mW. 2.3. LCO Hydrocracking Tests. The pilot-scale hydrocracking installation consisted of oil and hydrogen feed, reaction and separation, and equipment to purify and recycle liquid products and off-gas. A schematic diagram of this installation and detailed description were presented elsewhere.3,18 The reaction unit employed two reactors in series, hydrotreator (HTR) and hydrocracker (HCR), to accomplish hydrotreating in the former and hydrocracking in the latter. In order to guarantee the feed entering the HCR the same, the type of catalyst used in the HTR was unchanged, regardless of which catalyst (Ni-W/Hβ-Al2O3, Ni-W/HY-Al2O3 or Ni-Mo/HY-Al2O3) was used in the HCR. Unless otherwise specified, a once-through process without liquid recycle was used. Before hydrocracking reaction, the catalyst was presulfided using a mixture of 1.5 wt% dimethyl disulfide in a straight jet fuel at 8.0 MPa and 320 °C for 4 h. The liquid hourly space velocity (LHSV) was 1.0 h-1 and the volume flow rate ratio of H2 to oil (FH2/Foil) was 1000:1. After presulfidation, the HCR was adjusted to the desired reaction condition (8.0 MPa, 400 °C, LHSV of 0.8 h-1 and FH2/Foil of 900). The compositions of feedstock and product were analyzed by an Agilent 5975C mass spectrometer according to ASTM D2425 and SH/T 0606. The boiling range distribution was determined by ASTM D86. The sulfur and nitrogen contents were measured using ANTEK-7000 analyzer based on the standards of SH/T 0689-2000 and SH/T 0704-2001, respectively. The liquid density at 20 °C was measured by ASTM D4052. The cetane number (CN) was determined by 7


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ASTM D4737. The research octane number (RON) was measured using the standard of GB/T 5487-1995 in a standard CFR engine operating at 600 rpm, where the sample’s performance was compared to reference fuel blends.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of Catalysts. The N2 adsorption-desorption isotherms and the corresponding pore size distribution (PSD) curves of Ni-W/Hβ-Al2O3, Ni-W/HY-Al2O3 and Ni-Mo/HY-Al2O3 are shown in Figure 1. The specific surface area, pore volume and average pore diameter of each catalyst are listed in Table 1. At high relative pressures between 0.45 and 1.0 on the isotherms, all catalysts exhibit hysteresis loops of type H3 (Figure 1a), suggesting the presence of slit-like mesopores due to the aggregation of plate-like particles.22 Ni-Mo/HY-Al2O3 shows a high N2 adsorption at a relative pressure of 0.4, indicating the contribution of both micropores and mesopores. This is consistent with the PSD curve (Figure 1b) and also in agreement with the highest specific surface area (396.4 m2/g) and the smallest average pore size (6.3 nm) of Ni-Mo/HY-Al2O3 in all the three catalysts. The high surface area of Ni-Mo/HY-Al2O3 gives rise to high dispersion of active species. As displayed in Figure 2, all catalysts after presulfidation show typical layer-like structures assigned to MoS2 in Ni-Mo/HY-Al2O3 and to WS2 in Ni-W/Hβ-Al2O3 and Ni-W/HY-Al2O3, but the distribution of the MoS2 phase appears to be more homogenous on the surface of Ni-Mo/HY-Al2O3.

8


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N2 adsorbed (cm3/g)

250

(a)

Ni-W/H-Al2O3 Ni-W/HY-Al2O3 Ni-Mo/HY-Al2O3

200 150 100 50 0 0.0

0.2

0.4 0.6 Relative pressure

0.8

1.0

0.8 (b) dV/dlog(D) (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 0.2 0.0

1

10 Pore diameter (nm)

100

Figure 1. (a) N2 adsorption-desorption isotherms and (b) pore size distribution curves of fresh oxidic catalysts.

Ni-W/Hβ-Al2O3

20 nm

Ni-W/HY-Al2O3

20 nm

Ni-Mo/HY-Al2O3

Figure 2. HRTEM images of fresh presulfided catalysts.

The total amounts of acid sites of Ni-W/Hβ-Al2O3, Ni-W/HY-Al2O3 and Ni-Mo/HY-Al2O3 as well as the acid site distribution are summarized in Table 1. The amounts of NH3 desorbed in the 9


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temperature ranges of 150-250, 250-400 and 400-500 °C are usually taken as measures of weak, medium and strong strength acid sites, respectively.23 All catalysts exhibit weak, medium and strong acid sites with the main contribution of weak and medium ones, which together account for about 86%, 88% and 83% of the total acidity for Ni-W/Hβ-Al2O3, Ni-W/HY-Al2O3 and Ni-Mo/HY-Al2O3, respectively. As far as the amount of acid sites is concerned, both the total acidity and each type of acidity follow the order Ni-Mo/HY-Al2O3 > Ni-W/Hβ-Al2O3 > Ni-W/HY-Al2O3.

Ni-W/H-Al2O3 Ni-W/HY-Al2O3 Ni-Mo/HY-Al2O3

zeolite Y zeolite 

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-Al2O3

10

20

30

40 2 (o)

50

-Al2O3

60

70

Figure 3. XRD profiles of fresh oxidic catalysts.

Figure 3 displays the XRD patterns of Ni-W/Hβ-Al2O3, Ni-W/HY-Al2O3 and Ni-Mo/HY-Al2O3. For all catalysts, the two diffraction peaks at about 46 and 67° belong to the γ-Al2O3 phase.10,24,25 For Ni-W/HY-Al2O3 and Ni-Mo/HY-Al2O3, the peaks at 10.2, 12.0, 15.8, 18.9, 20.6, 23.9, 27.4 and 31.7° are indexed to the HY zeolite,9,23 but Ni-Mo/HY-Al2O3 exhibits stronger peak intensity than Ni-W/HY-Al2O3, which is attributed to the higher amount of HY zeolite in the former. For Ni-W/Hβ-Al2O3, the peak at 22.4° corresponds to the Hβ zeolite.4,19,23 No diffraction peaks assigned 10


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to crystalline Ni, W and Mo compounds are observed, implying that they are highly dispersed or very fine crystallites are formed, below the detection limit of the XRD technique.14,16,24

358 797

420 532

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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705 559 Ni-W/H-Al2O3 Ni-W/HY-Al2O3 Ni-Mo/HY-Al2O3

100

200

300

400 500 600 Temperature (oC)

700

800

900

Figure 4. H2-TPR profiles of fresh oxidic catalysts.

H2-TPR analysis is used to examine the interaction between metal oxide species and supports. As shown in Figure 4, Ni-Mo/HY-Al2O3 has three reduction peaks centered at 358, 559 and 705 °C, corresponding to the reduction of octahedrally coordinated polymeric Mo species, Ni2+ species and tetrahedral Mo species, respectively.25-27 As for Ni-W/Hβ-Al2O3 and Ni-W/HY-Al2O3, the first reduction peak at 420 or 532 °C is assigned to the reduction of polymeric octahedral W species and most of Ni2+ species, whereas the second peak at 797 or above 850 °C (beyond the upper limit of the TPR apparatus) belongs to the reduction of tetrahedral W species.17,25 The presence of octahedral and tetrahedral species for tungsten oxides (Ni-W/Hβ-Al2O3 and Ni-W/HY-Al2O3) and molybdenum oxides (Ni-Mo/HY-Al2O3) is also evidenced by the Raman analysis (Figure S1 in the Supporting Information). The octahedrally coordinated polymeric Mo and W species are normally considered as the precursors for the active MoS2 and WS2 as they are more readily sulfided than the 11


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tetrahedral species.28-30 The lower reduction temperature for the octahedral Mo species of Ni-Mo/HY-Al2O3 as compared to other two catalysts indicates a lower metal-support interaction and thus easier sulfidation, which is in good agreement with the dispersion of the active phase obtained by HRTEM (Figure 2). 3.2. Pilot-Scale Hydrocracking Test. Figure 5 compares the compositions of gasoline (65-170 °C, left column) and diesel (> 170 °C, right column) fractions of the liquid product over different catalysts using a Qilu LCO feedstock. The important properties of the Qilu LCO are summarized in Table 2. The yield of gasoline, defined as the weight percentage of gasoline in the product relative to the weight of LCO at the start, follows the order Ni-Mo/HY-Al2O3 (43.4 wt%) > Ni-W/HY-Al2O3 (26.7 wt%) > Ni-W/Hβ-Al2O3 (24.0 wt%). The yield of gasoline over Ni-Mo/HY-Al2O3 is much higher than that of either Ni-W/Hβ-Al2O3 or Ni-W/HY-Al2O3, which is mainly ascribed to the high total acidity of Ni-Mo/HY-Al2O3. Indeed, for the selective hydrocracking process, the dependence of the catalyst activity on the total acidity was also reported by other researchers.9,14,16,31. In addition, compared to Ni-W/Hβ-Al2O3 and Ni-W/HY-Al2O3, Ni-Mo/HY-Al2O3 possesses a larger surface area and more homogeneous distribution of the active phase, which together contribute to the increased activity in LCO hydrocracking. The aromatic yield in the gasoline fraction over Ni-Mo/HY-Al2O3 (13.7 wt%, accounting for 31.6% of gasoline) is larger than that over Ni-W/HY-Al2O3 (6.2 wt%, 23.2% of gasoline) or Ni-W/Hβ-Al2O3 (5.7 wt%, 23.8% of gasoline). As a result, the RON of the gasoline fraction over Ni-Mo/HY-Al2O3 (about 85) is higher than that of Ni-W/HY-Al2O3 (71) or Ni-W/Hβ-Al2O3 (72).

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100

90

diesel gasoline

gasoline

40

gasoline

60

85

diesel

80

aromatics

80

75

20

70

0

65 Ni-W/HY-Al2O3 Ni-Mo/HY-Al2O3

Ni-W/H-Al2O3

Research octane number (RON)

naphthene;

diesel

alkane;

Yield of gasoline or diesel (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5. Yields of gasoline and diesel as well as RON over different catalysts at a pilot reactor with a once-through mode (Qilu LCO, 400 °C, 8.0 MPa, LHSV of 0.8 h-1 and FH2/Foil of 900). Table 2. Important Properties of Qilu LCO, Zhenhai LCO and Maoming LCO properties

Qilu LCO

Zhenhai LCO

Maoming LCO

0.921

0.944

0.950

IBP-T10

151-217

136-227

157-224

T30-T50

235-257

252-275

246-273

T70-T90

283-319

303-343

306-350

T95-FBP

332/344

357-371

365-371

C

88.32

89.70

87.54

H

10.54

9.40

10.94

S

1.05

0.81

1.48

N

0.086

0.091

0.040

alkanes

14.8

13.4

23.7

cycloalkanes

9.9

8.3

11.8

aromatics

75.3

78.3

64.5

monoaromatics

29.3

24.0

24.2

diaromatics

41.7

46.9

35.2

triaromatics

4.3

7.4

5.1

17

15

27.5

density (g/cm3) distillation range (°C)

element analysis (wt%)

hydrocarbon distribution (wt%)

cetane number

13


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80 total aromatics monoaromatics diaromatics triaromatics

70 Content of aromatics (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 k l O3 l O3 l O3 stoc Y-A 2 -A 2 Y-A 2 H H H / / / feed o Ni-W Ni-W Ni-M

Figure 6. Aromatic content and distribution in the product over different catalysts.

Figure 6 presents the contents of mono-, di-, tri- and total aromatics in the liquid product over the three catalysts. All catalysts give rise to a decrease in the content of total aromatics as compared to the feedstock, specifically with a decrease in the contents of both di- and tri-aromatics and an increase in the content of monoaromatics. In particular, Ni-Mo/HY-Al2O3 exhibits the highest content of monoaromatics, being around 42 wt%. Figures 5 and 6 indicate that Ni-Mo/HY-Al2O3 can efficiently catalyze the selective hydrocracking of PAHs into monoaromatics. The mechanisms and pathways of selective hydrocracking of PAHs over bifunctional catalysts have been intensively investigated in the literature32-34 and thus are not considered in this study. Worthy of mention is the fact that the aromatic content limitation in China V gasoline standard is 40 vol% (35 vol% in EU V), which is lower than that obtained by Ni-Mo/HY-Al2O3. However, the high aromatic content fuel can be blended with low aromatic content streams to satisfy the clean fuel specification. Indeed, gasoline blending is a key process in most refineries, which combines many streams 14


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420 418 416 414 412 410 95 90 50 40 30 20 10 0

Gasoline yield (wt%) and RON

T (oC)

produced by different units into a mixture in order to meet certain quality specifications.35

Diesel yield (wt%) and CN

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

yield;

40

yield;

RON

CN

30 20 10 0

0

0 40

0 0 0 0 0 0 0 80 120 160 200 240 280 320 Time on stream (h)

Figure 7. Variation of reaction temperature, yields of gasoline and diesel as well as RON and CN with the time-on-stream of Ni-Mo/HY-Al2O3 during 3200 h of pilot-scale operation with a partial-recycle mode (Zhenhai LCO, 413 °C, 8.0 MPa, LHSV of 0.8 h-1 (fresh feed), recycle ratio of 0.45, and FH2/Foil of 1200).

Ni-Mo/HY-Al2O3 is further evaluated for a long time (3200 h) at the pilot reactor using a Zhenhai LCO feedstock with more aromatics (Table 2). Different from the above once-through process, a partial-recycle process (a part of heavy oil is recycled to the HTR inlet with a weight ratio of recycle oil to LCO of 0.45:1) is employed. The operating conditions for the HCR are as follows: inlet temperature, 413 °C; pressure, 8.0 MPa; LHSV, 0.8 h-1 (fresh feed) or 1.16 h-1 (fresh feed + recycle oil); volume flow rate ratio of H2 to oil, 1200. As shown in Figure 7, the yields of gasoline (65-210 °C) and diesel (> 210 °C) during a 3200 h test are stabilized at about 53 wt% and 35 wt%, respectively, despite the fact that the reaction temperature is slightly increased from 413 to 418 °C. Furthermore, the RON is above 90 with a slight increase from about 92 to 95, while the CN is 15


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slightly decreased from around 45 to 44. For the gasoline fraction, the mass fractions of alkane, cycloalkane and aromatics at 413 °C are 22.3, 24.0 and 53.7 wt%, respectively, which are only slightly changed at 418 °C, being 20.5, 25.5 and 54.0 wt%, respectively. In general, the product properties remain stable. The amount of carbon deposits over the catalyst after 3200 h of operation is measured by thermogravimetric analyzer, which is around 5.5 wt% (slightly varying from 5.7 wt% at the inlet of the reactor to 5.3 wt% at the outlet). The long-term pilot-scale experiment demonstrates the excellent performance of Ni-Mo/HY-Al2O3 for selective hydrocracking of LCO. 3.3. Industrial-Scale Hydrocracking Test. Finally, Ni-Mo/HY-Al2O3 is applied to the selective hydrocracking of LCO at an industrial-scale plant of Maoming Petrochemical Company, which consists mainly of a HTR with two beds, a HCR with four beds and a downstream separation system. A partial-recycle process with a weight ratio of recycle oil to LCO of 0.4:1 is used at the industrial plant. The properties of the Maoming LCO are listed in Table 2. The typical operating conditions for the HCR are as follows: inlet temperature, 395 °C; pressure, 9.0 MPa; fresh feed flow rate, 82 t/h; LHSV, 0.7 h-1 (fresh feed) or 0.98 h-1 (fresh feed + recycle oil); volume flow rate ratio of H2 to oil, 800. Figure 8 presents the yields of gasoline (65-210 °C) and diesel (> 210 °C) as well as the RON and CN data during 250 days of operation at the industrial plant. Except for about the first 50 days after startup, the catalyst performance is very stable, with an average yield of 40 wt% for gasoline and 56 wt% for diesel. The remained 4 wt% belongs to gas product, which is composed of CH4 (9%), C2H6 (9%), C3H8 (42%), n-C4H10 (16%) and iso-C4H10 (24%). The RON and CN are stabilized at 91 and 40 on average, respectively. Compared to the feedstock, the CN of the diesel fraction after hydrocracking is raised above 10. All results show that Ni-Mo/HY-Al2O3 can effectively convert LCO to high-octane gasoline on an industrial scale. 16


100 gasoline diesel

80

av. 56%

60 40

av. 40%

20 100 80

RON or CN

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Yield of gasoline or diesel (wt%)

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av. 91 RON CN

60 40

av. 40

20 0

0

50

100

150

200

250

Time on stream (day)

Figure 8. Variation of yields of gasoline and diesel as well as RON and CN with the time-on-stream of Ni-Mo/HY-Al2O3 during 250 days of industrial-scale operation with a partial-recycle mode (Maoming LCO, 395 °C, 9.0 MPa, LHSV of 0.7 h-1 (fresh feed), recycle ratio of 0.4, and FH2/Foil of 800).

The industrial-scale test is ceased after 250 days of operation owing to a scheduled complete plant shutdown for maintenance. 19 used Ni-Mo/HY-Al2O3 samples, located at different positions along the axial direction of the HCR with a constant distance between each other (Figure 9), are withdrawn and analyzed. The sulfur content in different samples is almost the same (around 4.3 ppmw, Figure 9a), indicating that all catalysts are sulfurized completely on the one hand, and no sulfur loss occurs during hydrocracking of LCO on the other hand. However, the nitrogen content in the catalysts gradually decreases with increasing the bed height (from 15.3 to 14.7 ppmw, Figure 9a), which is reasonable because the nitrogen-containing compounds such as NH3 from the HTR preferably adsorb on the acidic sites of catalysts in the upper part of the reactor. The XRD patterns of all used catalysts (Figure 9b) are almost identical and quite similar to that of the fresh catalyst 17


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(Figure 3), implying that the crystalline structure of Ni-Mo/HY-Al2O3 is well preserved during long-term operation, irrespective of the position of catalyst in the reactor.

Figure 9. Variation of chemical and structural properties of used Ni-Mo/HY-Al2O3 (250 days used in an industrial plant) along the axial direction of the hydrocracker: (a) contents of sulfur and nitrogen, (b) XRD patterns, (c) total acidity, and (d) pore size distribution curves.

When it comes to the total acidity of the used catalyst (Figure 9c), it is found that the total acidity first increases rapidly from around 0.4 mmol(NH3)/g at the entrance of the reactor to about 0.5 mmol(NH3)/g at a distance of 1/5 of the reactor height away from the entrance, and then stabilizes at 0.52 mmol(NH3)/g for the last part of the reactor. It should be noted that despite the total acidity of the fresh Ni-Mo/HY-Al2O3 without presulfidation being 0.87 mmol(NH3)/g, it is reduced immediately to 0.53 mmol(NH3)/g after presulfidation of catalyst, as a result of adsorption of nitrogen-containing species contained in the presulfidation solution. It means that after 250 days of industrial operation, most presulfided catalysts (about 80% in the HCR) keep the acidity, indicating the long-term stability of Ni-Mo/HY-Al2O3. The loss of a mall proportion of the total acidity of 18


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Energy & Fuels

catalyst at the inlet section of the reactor is in accordance with the relatively high nitrogen content of catalyst in this zone. We also analyze the variation of the textural properties of used catalysts. As presented in Figure 9d, all catalysts show similar PSD curves with a bimodal pore structure, but the samples placed at the front end of the reactor have smaller surface area and pore volume as compared to those in the downstream part (now shown here), which is mainly due to the deposition of carbonaceous and metallic compounds from the HTR on the surface of Ni-Mo/HY-Al2O3.36,37 Based on the above results on the chemical and structural properties of the 250 day-used catalyst at an industrial-scale reactor as well as on the hydrocracking activity, we draw a conclusion that Ni-Mo/HY-Al2O3 is an excellent catalyst for the selective hydrocracking of LCO into high-octane gasoline.

4. CONCLUSIONS LCO, characteristic of high aromatic content and especially a large proportion of PAHs, is commonly regarded as a poor diesel bending component. However, upgrading of LCO by selective hydrocracking to high-octane gasoline, i.e., partial hydrogenation of PAHs followed by selective ring opening of naphthenic structures, is an economically viable route. In this study, we developed three catalysts, namely, Ni-W/Hβ-Al2O3, Ni-W/HY-Al2O3 and Ni-Mo/HY-Al2O3 for the selective hydrocracking of LCO. The catalysts were characterized by various techniques, which showed that Ni-Mo/HY-Al2O3 possessed larger surface area, higher total acidity, lower metal- support interaction and more homogeneous distribution of the active phase when compared to Ni-W/Hβ-Al2O3 and Ni-W/HY-Al2O3. As a result, Ni-Mo/HY-Al2O3 gave rise to higher gasoline yield (about 43 wt%) and higher RON (85) than Ni-W/Hβ-Al2O3 (24 wt% and 72) and 19


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Ni-W/HY-Al2O3 (27 wt% and 71). In addition, the amount of monoaromatics obtained from Ni-Mo/HY-Al2O3 was larger than that from other catalysts, indicating the higher efficiency of the catalyst in selective conversion of PAHs into monoaromatics. Ni-Mo/HY-Al2O3 showed a good stability, with a stable gasoline yield of about 53 wt% during 3200 h on stream. Finally, Ni-Mo/HY-Al2O3 was successfully commercialized for the selective hydrocracking of LCO. In 250 days of operation in an industrial plant, Ni-Mo/HY-Al2O3 brought about a stable gasoline yield of around 40 wt% and a RON of 91. Furthermore, the catalyst properties such as sulfur and nitrogen contents, crystalline structure, total acidity and pore size distribution were maintained.

 ACKNOWLEDGMENTS The authors thank financial supports from Open Project of State Key Laboratory of Chemical Engineering (SKLChE-18C04) and Dalian Youth Science and Technology Star (2017RQ085).

 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Raman spectra of fresh oxidic catalysts.

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Upgrading of Light Cycle Oil to High-Octane Gasoline through

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