Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. 2018, 57, 2068−2074
Optimization of a Pilot Hydrocracking Unit To Improve the Yield and Quality of Jet Fuel Together with Heavy Naphtha and Tail Oil Chong Peng,†,‡ Zhengkai Cao,‡ Yanze Du,‡ Ronghui Zeng,‡ Rong Guo,‡ Xuezhi Duan,*,† and 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, Liaoning Dalian 116045, China
‡
S Supporting Information *
ABSTRACT: Hydrocracking of vacuum gas oil (VGO) is a commonly used process to produce more high-quality transportation fuels and chemicals. In this work, effects of four different operation modes, i.e., SSOTP, FRTHT, FRTHC, and PRTHT, with different recycling modes of tail oil are comparatively studied on the performance of VGO hydrocracking. The FRTHT and FRTHC give rise to a higher yield and/or quality of jet fuel, heavy naphtha, and tail oil compared with that of the other two modes. Moreover, effects of the distillation−cutting scheme are also studied to further increase the yield and quality of jet fuel. It is found that all four of the operation modes show a much higher yield but a slightly higher smoke point for the jet fuel. The insights revealed here could shed new light on the optimization of VGO hydrocracking to produce more targeted products.
1. INTRODUCTION Vacuum gas oil (VGO), one of the atmospheric vacuum distillation cuts of heavy crude oil, has a high boiling point ranging from 370 to 510 °C,1 which is of inherently poor quality because of the high sulfur and nitrogen contents, high molecular weight, and low API gravity.2,3 To satisfy the growing demand of environment-friendly and high-quality transportation fuels and chemicals, especially for the rapid growth of developing Asian markets,4 upgrading is an essential step for the VGO conversion.5 Previously, there are three main upgrading technologies, i.e., hydrocracking, thermal cracking, and catalytic cracking. Among them, the hydrocracking technology is widely used due to the more flexible operation conditions and higher liquid yield.6−9 The hydrocracking process usually consists of two main units, i.e., hydrotreating and hydrocracking reactors, which are used to remove the impurities in the VGO and to produce target products, respectively. Nickel−molybdenum- and nickel−tungsten-based catalysts are widely used in the hydroprocessing.10−12 In particular, the NiMo/γ-Al2O3 catalyst has demonstrated good hydrotreating (HDS and HDN) activity,13 while the NiW/Y-zeolite catalysts have promising hydrocracking activity and targeted product selectivity.14−16 The VGO hydrocracking process has a strong ability to convert the heavy fraction to jet fuel with a high smoke point as well as diesel with a high cetane number,17 in which the proportions of jet fuel and diesel can be varied as needed.6,18 In China, the demand for the jet fuel has been significantly increasing for the prosperity of aviation, and the ratio of diesel to gasoline consumption has been falling continuously.19 In © 2018 American Chemical Society
addition, the heavy naphtha can usually serve as the feedstock for the catalytic reforming units. Therefore, it is necessary to improve the yield and quality of jet fuel and heavy naphtha by developing the advanced technologies for the VGO hydrocracking. In order to achieve higher conversion of VGO and obtain more light products, the tail oil is usually recycled back to the hydrotreating reactor or hydrocracking reactor for further cracking.20 Significant technological developments, such as single-stage once through, single-stage with recycle, and twostage with recycle, are crucial for the process optimization.21 However, very little information is available in the open literature about the influence of different recycling modes of tail oil on the yield and quality of the target products. In this work, two representative commercial non-noble NiMo/Al2O3 and NiW/Y-zeolite catalysts were loaded into the hydrotreating reactor and the hydrocracking reactor, respectively. Effects of four different operation modes, i.e., single stage once through process (SSOTP), full tail oil recycled to hydrotreating reactor (FRTHT), full tail oil recycled to hydrocracking reactor (FRTHC), and partial tail oil recycled to hydrotreating reactor (PRTHT), with different recycling modes of tail oil were comparatively studied on the yield and quality of jet fuel, heavy naphtha, and tail oil. Moreover, effects Received: Revised: Accepted: Published: 2068
November 30, 2017 January 24, 2018 January 25, 2018 January 25, 2018 DOI: 10.1021/acs.iecr.7b04981 Ind. Eng. Chem. Res. 2018, 57, 2068−2074
Article
Industrial & Engineering Chemistry Research
Figure 1. Flow diagrams of a pilot hydrocracking unit with different recycling modes of tail oil: (a) SSOTP, (b) FRTHT, (c) FRTHC, and (d) PRTHT.
The hydrogen consumption = (The volume of hydrogen consumption (L) × 2 (g/mol)/22.4 (L/mol)/the weight of feed H2) × 100%. The potential aromatic content (Ar%) was calculated as follows:
of the distillation−cutting scheme were also studied to further improve the yield and quality of jet fuel.
2. EXPERIMENTAL SECTION Figure 1 schematically illustrates four operation modes of the hydrocracking process, i.e., SSOTP, FRTHT, FRTHC, and PRTHT. All technical parameters were precisely controlled using a distributed control system. Both the top and bottom of the reactors were filled with inert particles to maintain the uniform distribution of the fluids and to prevent catalyst particles from entering into the pipelines. Two representative commercial non-noble catalysts, i.e., catalyst-A and catalyst-B, were loaded into the hydrotreating reactor (i.e., R1) and the hydrocracking reactor (i.e., R2), respectively. The two reactors have the same D/dp > 18 and L/ dp > 350, where the D, L, and dp are the inner diameter of the reactor, bed height, and catalyst size, respectively. Moreover, electrolytic hydrogen obtained through high-pressure deoxygenation and dehydration using silica gel was used with a purity over 99.9 vol%, and its oxygen content was less than 5 μL/L. Compositions of feedstock and products were analyzed by Agilent 5975C using Ar as a carried gas according to ASTM D2425 and SH/T 0606. The nitrogen contents were determined through a nitrogen analyzer (model ANTEK 7000), where the analysis standard was SH/T 0704-2001, the carried gas was Ar, the burning gas was O2, and the burning temperature was 1050 °C. The sulfur content was determined through a sulfur analyzer (ANTEK-9000), where the analysis standard was SH/T0689-2000, the carried gas was Ar, the burning gas was O2, and the burning temperature was 1100 °C. The feedstock and products density at 20 °C were based on the standard ASTM D4052. The distillation range was based on the standard ASTM D86. A 100 mL sample was distilled under suitable conditions. The temperature and volume of the condensate were recorded. The conversion and chemical hydrogen consumption were calculated as follows: The conversion = (1 − the product weight of >370 °C (or >350 °C) fractions/the weight of feedstock) × 100%.
Ar% = a × C6N% + b × C7N% + c × C8N% +
∑ Ar%
a = MB/MC6N = 78/84 = 0.93 b = M T /MC7N = 92/98 = 0.94
c = MX /MC8N = 106/112 = 0.95
where ∑ Ar% is the total aromatic content of the feedstock; a, b, and c are the conversion coefficients; MB, MT, and MX are the molar masses of benzene (B), toluene (T) and xylenes (X); and MCN6, MCN7, and MCN8 are the molar masses of C6 cycloalkane, C7 cycloalkane, and C8 cycloalkane. The BMCI (Bureau of Mines Correlation Index)22 was calculated as follows: BMCI =
48640 15.6 + 473.7 × d15.6 − 456.8 t + 273
where t is the volume average boiling point (°C) and d15.6 15.6 is the specific gravity at 15.6 °C. The index is widely used as an aromatic index for an oil product, assuming that the BMCI values of n-hexane and benzene are 0 and 100, respectively. The BMCI value for the hydrocracking of tail oil should be lower than 20, with the best value normally lower than 12. The deeper the reaction proceeds, the BMCI value is smaller and the yield of ethylene is higher.
3. RESULTS AND DISCUSSION 3.1. Properties of Catalysts. Table 1 shows the compositions and textural properties of the two commercial catalysts, i.e., catalyst-A and catalyst-B, where the surface areas and pore volumes are measured by N2 physisorption using a Micromertics ASAP 2420 instrument. Moreover, the Brönsted and Lewis acid sites are determined by pyridine-adsorbed 2069
DOI: 10.1021/acs.iecr.7b04981 Ind. Eng. Chem. Res. 2018, 57, 2068−2074
Article
Industrial & Engineering Chemistry Research
followed by Scheme 1a and b (Scheme 1c).21 Obviously, for these mechanisms, it is difficult to form C1 and C2 products. Therefore, it can be reasonably concluded that the very low, almost unchaged C1+C2 yields are most likely from the impurities of hydrogen feedstock. According to the C1, C2, C3, i-C4, and n-C4 yields, the C1− C4 gaseous product yields could be calculated, which follow an order of SSOTP < PRTHT < FRTHT < FRTHC (Figure 3a). Based on the mass balance, the C5+liquid product yields are calculated, which follow an order of FRTHC < FRTHT < PRTHT < SSOTP (Figure 3b). Furthermore, for the C5+liquid products, the effects of the operation modes on the targeted jet fuel and heavy naphtha yields are studied, and the results are shown in Figure 3c. It is found that compared to the SSOTP and PRTHT, the FRTHC and FRTHT give rise to higher jet fuel and heavy naphtha yields, which are mainly due to the hydrotreating and hydrocracking of the recycled tail oil. Notably, the FRTHC shows lower H2 consumption significantly affecting the operation cost more than the FRTHT (Figure 3d), which could decrease. Therefore, the FRTHC is suggested as a more appropriate operation mode for the production of jet fuel and heavy naphtha from the VGO hydrocracking. 3.3. Quality of the Jet Fuel, Heavy Naphtha, and Tail Oil. Main properties of the jet fuel obtained from the four modes are summarized in Table S1. It is found that for all the jet fuel obtained from the four operation modes, their densities are 0.786−0.804 (20 °C)/g·cm−3, and their flash point and smoke point are higher than 38 °C and 25 mm, respectively. Moreover, their EBP is lower than 300 °C. These important data could meet the requirements of the ASTMD1655 (US) and Chinese standard of GB 6537-2006 for 3# jet fuel, indicating the high quality of the obtained jet fuel. In addition, the jet fuel obtained from the FRTHT and FRTHC operation modes shows the relatively higher yields (41.20 and 42.70%, respectively), higher flash point (40 °C), higher smoke point (32 mm and 28 mm, respectively), and lower contents of aromatics (3.6 and 4.0%, respectively), indicating that the FRTHT and FRTHC are better operation modes to produce the jet fuel. From the industrial point of view, the heavy naphtha product is used as feedstock for catalytic reforming to produce BTX or gasoline fraction with a high octane number, a low sulfur content, and a low nitrogen content. Main properties of the heavy naphtha obtained from the four operation modes are summarized in Table S2. It is found that they exhibit similar distillation ranges, and they are mainly composed of the paraffins and cyclanes with small contents of the aromatics, where the contents of the paraffins follow an order of SSOTP < PRTHT < FRTHT ≈ FRTHC. Moreover, the aromatic potential content (Ar) is also an important factor for determining the quality of the heavy naphtha. It can be also seen in Table S2 that for the four operation modes, the Ar
Table 1. Main Physico-Chemical Properties of Two Commercial Catalysts catalysts
catalyst-A
compositions, wt%: MoO3 NiO WO3 physical properties: surface area/m2·g−1 pore volume/mL·g−1 shape diameter/mm
23.0−26.0 3.8−4.2
>160 >0.30 sphere 2.1−2.6
catalyst-B
8.5−9.5 22.5−24.5 >250 >0.32 sphere 2.1−2.5
Fourier-transformed infrared (Py-FTIR), and the results are shown in Table 2. It is found that the total Brönsted acidity of the catalyst-B is higher than that of the catalyst-A, supporting that the catalyst-B is more suitable for the VGO hydrocracking. In addition, Figure 2 presents the representative TEM images of the two sulfided catalysts, where typical layered structures of the MoS2 and WS2 phases are obviously observed. 3.2. Effects of Hydrocracking Operation Modes. Catalytic conversion of Iran VGO feedstock in a pilot hydrocracking unit with four different operation modes (i.e., SSOTP, FRTHT, FRTHC, and PRTHT) was carried out at PH2 = 15.7 MPa, LHSV (R1/R2) = 1.0/1.2, and H2/oil ratios (R1/ R2 ) = 900:1/1000:1 (Table 3), where catalyst-A and catalyst-B are used in the R1 and R2, respectively. For the four operation modes, the weighted average bed temperature (WABT) of the R1 was adjusted to control the N content of the hydrotreated Iran VGO lower than 20 μg/g, and that of the R2 to obtain similar one-through conversions (i.e., ∼70%) except for the fourth operation mode (i.e., ∼50%), where the lower oncethrough conversion was obtained to probe the effects of the partial recycle of the tail oil on the yield and quality of the products. Effects of the four hydrocracking operation modes on the product yield and H2 consumption were studied. As shown in Figure 3a, the four operation modes make little difference in the yields of H2S and NH3, possibly due to the highly efficient conversion of the S- and N-containing compounds in the feedstocks after R1 and R2 followed by the almost total separation of the resultant H2S and NH3 in the separator unit. More interestingly, there are no obvious differences in the yields of C1 and C2 but remarkable differences in those of C3, i-C4, and n-C4. Previous studies showed that the cracking of normal paraffins usually proceeds by the double function transformation mechanism (Scheme 1a),23,24 where the new carbocation (R2+) derived from the normal paraffins could not only transfer to paraffins through accepting H− ion but also undergo the β-cracking until C3 and i-C4 were generated; that of cyclanes by means of the paring reaction followed by βscission (Scheme 1b);25 and that of aromatics by means of the hydrogenation saturation to produce polycyclic cyclane
Table 2. Amount of Brönsted and Lewis Acid Sites of Two Commercial Catalysts acidity (mmol/g) 160 °C
250 °C
350 °C
total
catalysts
B
L
B
L
B
L
B
L
catalyst-A catalyst-B
0.039 0.048
0.373 0.225
0.033 0.041
0.029 0.163
0.029 0.024
0.096 0.107
0.101 0.11
0.498 0.495
2070
DOI: 10.1021/acs.iecr.7b04981 Ind. Eng. Chem. Res. 2018, 57, 2068−2074
Article
Industrial & Engineering Chemistry Research
Figure 2. Representative TEM images of the two sulfided catalysts: (a) catalyst-A and (b) catalyst-B.
Table 3. Process Conditions of the Hydrocracking Units Operated by Four Operation Modes operation modes
a
SSOTP
feedstock catalysts PH2/MPa
Iran VGO catalyst-A/catalyst-B 15.7
LHSV (R1/R2)/h−1 H2/oil ratios (R1/R2) WABTa (R1/R2)/°C N contentb/μg·g−1 one-through conversion/vol%
1.0/1.2 900:1/1000:1 379/385 370 °C)
FRTHC (>370 °C)
PRTHT
374/385
379/380
379/376
∼70
∼70
∼50
WABT: weighted average bed temperature. bThe N content of the hydrotreated Iran VGO after the hydrotreating reactor (R1).
Figure 3. Distribution of (a) yield of gaseous product, (b) yield of C5+ liquid, (c) yield of heavy naphtha and jet fuel, and (d) H2 consumption for the four operation modes.
densities are similar, and their sulfur and nitrogen contents are extremely low (
