Hydrotreating Reactivities of Atmospheric Residues and Correlation

May 21, 2018 - Petroleum Research Center, Kuwait Institute for Scientific Research, Safat 13109, ... previous study in hydrotreating of AR from Kuwait...
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Article Cite This: Energy Fuels 2018, 32, 6726−6736

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Hydrotreating Reactivities of Atmospheric Residues and Correlation with Their Composition and Properties Qingyan Cui,† Xiaoliang Ma,*,‡ Koji Nakano,§ Koji Nakabayashi,† Jin Miyawaki,† Adel Al-Mutairi,‡ Abdulazim MJ Marafi,‡ Joo-Il Park,‡,⊥ Seong-Ho Yoon,† and Isao Mochida*,∥ †

Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan Petroleum Research Center, Kuwait Institute for Scientific Research, Safat 13109, Kuwait § JGC Catalysts and Chemicals Ltd, Fukuoka 212-0013, Japan ∥ Kyushu Environmental Evaluation Association, Fukuoka, Japan ⊥ College of Engineering, Hanbat National University, Daejeon 305-719, Republic of Korea

Energy Fuels 2018.32:6726-6736. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 01/12/19. For personal use only.

‡

ABSTRACT: In order to better understand the effects of composition and properties of atmospheric residues (AR) on their reactivities for hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodevanadium (HDV), hydrodenickel (HDNi), hydrodeasphaltene (HDAsp), and hydrodeconradson-carbon-residue (HDCCR) in the hydrotreating process, analysis and characterization of three ARs (AA-AR, AB-AR, and AM-AR) from Arabia crudes were conducted, and their hydrotreating reactivities were evaluated in a pilot unit over a catalyst system at 370 °C under a H2 pressure of 13.5 MPa by comparing the conversions of the various species and their rate constants on each catalyst. The overall reactivity of various species decreases in the order of vanadium species > sulfur species ≈ asphaltenes > nickel species > Conradson carbon residue precursor > nitrogen species, regardless of the sources of the ARs. Reactivities of the three ARs in HDS, HDV, and HDAsp increase in the order of AB-AR < AA-AR < AM-AR, while reactivities of the three ARs in HDNi, HDCCR, and HDN are similar. The higher nitrogen and asphaltenes concentrations and larger density of AR have strong and negative effects on the HDS, HDV, and HDAsp reactivities but no significant effect on the HDN, HDNi, and HDCCR reactivities. The B parameter obtained from electron spin resonance analysis can be a good index to predict the HDV reactivity of AR.

1. INTRODUCTION In recent years, the crude oil becomes heavier and poorer, containing more heteroatoms of sulfur (S), nitrogen (N) and metals, whereas the global demand for fuels constantly increases, and the environmental regulations to limit the S concentration in them become more stringent. Thus, refining of the heavier and poorer crude oil to produce cleaner fuels is urgently needed to satisfy the market demand. Hydrotreating of atmospheric residue (AR) is one of the normal and practical processes in current petroleum refineries to treat petroleum heavy fractions for producing clean fuels. As well-known, in addition of the catalyst activity1−4 and operating conditions, the composition and properties of AR have strong effects on the performance of the hydrotreating process. Consequently, a fundamental understanding of the reactivities of different ARs for hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodevanadium (HDV), hydrodenickel (HDNi), hydrodeasphaltenes (HDAsp), and hydrodeconradson-carbonresidue (HDCCR) is crucial for improving the hydrotreating performance through the catalyst development, optimization of the operating conditions, feedstock blending, and better processing design. Many studies in HDS, HDN, hydrodemetallization (HDM), HDCCR, and HDAsp reactivities of heavy oils have been reported in the literature.5−9 It has been found that the refractory S species of the dibenzothiophenes with alkyl groups at 4- and/or 6-position, such as 4,6-dimethyldibenzothiophene (4,6-DMDBT), are hard to be removed in the conventional © 2018 American Chemical Society

hydrotreating process, due to the steric hindrance of the alkyl groups to the approach of the sulfur atom to the active sites,10−12 and HDS of the individual S compounds follows the pseudo-first-order kinetics.12,13 Recently, in the study of the HDS and HDM of vacuum residue, Ferreira et al.14 found that the reactivities for HDS and HDM seemed to be linked to the molecular structure and size and the heteroatoms concentrations, highlighting the importance of the molecular size. The essential reactions in the HDN process were the hydrogenation of the N-ring followed by hydrogenolysis of the C−N bond, and the HDN reactivity depended on the molecular structure of the N compounds.15 HDN chemistry of heavy oil indicates that some intrinsic properties of the feedstock, such as three and larger ring aromatic compounds, profoundly influence the nitrogen removal.7 Vanadium (V) and nickel (Ni) porphyrins are the common metal compounds in petroleum, associating with their surrounding molecules through noncovalent, van der Waals’ force, aromatic π−π stacking, and/or hydrogen bond.17,18 A previous study in hydrotreating of AR from Kuwait Export Crude showed that the V removal depended on the catalyst porosity, and the catalyst with larger pores promoted the V removal, while the Ni removal depended on the NiMo loading.19 In the study of hydrotreating Maya residues, Callejas Received: April 2, 2018 Revised: May 20, 2018 Published: May 21, 2018 6726

DOI: 10.1021/acs.energyfuels.8b01150 Energy Fuels 2018, 32, 6726−6736

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Energy & Fuels

HDN, HDV, HDNi, HDAsp, and HDCCR were evaluated in a pilot unit with a series of reactors over a hydrotreating catalyst system on the basis of the conversions and the rate constants for various species. The correlations of the various reactivities of the three ARs with their compositions and properties were conducted. The objective of this study is to get a better understanding of the effects of AR compositions and properties on the various reactivities in the hydrotreating process, which benefits to the development of better catalyst, optimization of the operating conditions, design of the catalyst beds, and blending of feedstocks for improving AR hydrotreating performance.

et al. found that the conversion of the various species decreased in the order of V species > Ni species > S species > N species.20 Asphaltenes are presented in the petroleum fraction in a delicate balance, which can be easily upset, resulting in precipitation and coke formation in various processes. The presence of asphaltenes reduced the efficiency of the hydrotreating and the conversion of the hydrocracking due to the coke formation causing the deactivation of the catalysts.21−23 In recent years, many studies in the petroleum asphaltenes for a better understanding of their changes in the hydrotreating process have been reported in the literature.24−26 High reaction temperature and low space velocity were found to be beneficial to the asphaltenes removal in the hydrotreating of Maya residue, and the asphaltenes removal was shown to be parallel and linear to the metal removal.27 Marafi et al. reported that the activity of the HDM catalyst decreased in the order of HDS > HDV > HDNi > HDCCR > HDAsp > HDN in the hydrotreating of AR, while the activity of the HDS catalyst was in the order of HDS > HDV > HDAsp > HDCCR ≈ HDNi > HDN.28 In the study of the kinetics of S, N, V, and Ni removals from Maya residue, Callejas and Martinez found that the S, N, V, and Ni removal reactions followed the second-, half-, half-, and firstorder kinetics, respectively,20 and the asphaltene conversion fitted the 0.5-order kinetics.27 Ferdous et al. reported that both HDN and HDS of heavy gas oil can be described by the firstorder kinetics.16 Choudhary et al. studied the HDN chemistry of eight heavy oils, in which the nitrogen removal reaction was modeled by an apparent first-order reaction.7 In the catalytic hydrotreating of heavy oil, Shimura et al. found that the reaction kinetics were first-order for vanadium removal and second-order for asphaltene removal, and the catalyst with the optimum pore diameter favored the removal of vanadium and asphaltenes.29 Alvarez30 investigated the residue hydroprocessing in a multifixed-bed reactor system and found the HDS, HDN, HDNi, HDV, and HDAsp followed as 1.17, 2.00, 0.55, 1.56, and 0.75 order reaction. In study of the CCR conversion in the hydrotreating of a Maya crude, Trasobares et al. reported that the CCR conversion fitted the 0.5 order reaction.31 However, Marafi et al. found that the HDCCR kinetics in hydrotreating a Kuwaiti atmospheric residue followed as the second-order reaction.32 It looks like that the kinetics order reported by different research for HDS, HDN, HDNi, HDV, HDAsp, and HDCCR are quite different, resulting in difficulty for comparing the reactivities by directly using the obtained rate constants. In general, ARs from various sources present different reactivities in the hydrotreating process on the basis of their physicochemical properties and compositions.14,19 However, how the composition and properties of AR influence the HDS, HDN, HDV, HDNi, HDAsp, and HDCCR reactivities over various catalysts is still unclear, although this information is important for understanding the reactivities of various ARs and for improving the hydrotreating performance of ARs, especially those from heavy crudes. On the other hand, many previous studies used the different kinetics orders to describe the reactions of the different species,16,20 which makes it difficult to compare their reactivities directly using their rate constants. In the present study, three ARs (AM-AR, AA-AR, and ABAR) were analyzed and characterized in detail by element analysis, SARA analysis, gel permeation chromatography (GPC), X-ray diffractometer (XRD), electron spin resonance (ESR), and so on. The reactivities of the three ARs for HDS,

2. EXPERIMENTAL SECTION 2.1. Atmospheric Residues and Catalysts. Three atmospheric residues (ARs), namely AA-AR from an Arabia crude A, AB-AR from a heavy Arabia crude B, and AM-AR from an Arabia Medium crude, were used in this study, which were provided by Kuwait Oil Company (KOC), Kuwait National Petroleum Company (KNPC), and JGC Catalysts and Chemicals Ltd. (JGC&CC), respectively. A series of commercial catalysts (NiMo/Al2O3) from JGC&CC were used in this study, which were named as HDM1, HDM2, HDM3, and HDS1. The composition and properties of these catalysts are shown in Table 1. The pore diameter and pore volume decrease in the order of HDM1 > HDM2 > HDM3 > HDS1, while the MoO3 and NiO contents increases in the order of HDM1 < HDM2 < HDM3 < HDS1.

Table 1. Composition and Properties of the NiMo/Al2O3 Catalysts catalysts

surface area (m2 g−1)

pore volume (cm3 g−1)

pore diameter (nm)

MoO3 content (wt %)

NiO content (wt %)

HDM1 HDM2 HDM3 HDS1

170 160 140 175

0.73 0.69 0.65 0.64

30 30 25 13

3.3 8.5 10.8 11.8

0.3 2.3 2.7 2.9

2.2. Pilot Hydrotreating Test. The hydrotreating of the three ARs was conducted in a reaction system with five fixed-bed reactors in series. The internal diameter of the reactors was 20 mm. The catalyst loading in the reactors is shown in Figure 1. The five reactors were used for easy sampling after each catalyst bed. The reactors were loaded in turn by 40 mL of HDM1, 40 mL of HDM2, 20 mL of HDM3 plus 20 mL of HDS1, 40 mL of HDS1, and 40 mL of HDS1, respectively. Prior to the hydrotreating testing, the catalysts were presulfurized using a mixture of gas oil with 2 wt % of carbon disulfide

Figure 1. Catalyst loading amount and sampling outlets in the reactor system. 6727

DOI: 10.1021/acs.energyfuels.8b01150 Energy Fuels 2018, 32, 6726−6736

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Energy & Fuels at 240 °C for 5 h and then at 290 °C for 10 h. The hydrotreatment of ARs was conducted at 370 °C under a H2 pressure of 13.5 MPa with a feed flow rate of 60 mL/h and a H2-to-oil ratio of 800 (v/v). The liquid hourly space velocity (LHSV) for total catalyst beds was 0.3 h−1. When the pilot unit was stable at the designed conditions for a period of 48 h, the product samples were collected first from the outlet of the fifth reactor and then the third, second, and first reactors in order to avoid the disturbance to the pilot unit from the sampling. The mass balances for all hydrotreating three ARs were higher than 97.0%. 2.3. Analysis of Feedstocks and Products. In order to characterize further the composition and properties of the saturates, aromatics, resins, and asphaltenes (SARA) fractions in the feedstocks, the SARA separation and analysis were conducted, as described as follows: the AR sample was dissolved in n-heptane at a ratio of 1/50 (w/w) under stirring at 60 °C for 5 h and then at 20 °C setting overnight. The solution was filtered, and the solid cake was washed with n-heptane until there was no color in the filtrate. The insoluble portion was extracted with toluene in a Soxhlet apparatus, and then the solvent in the solution was removed by rotary evaporating. The received sample was dried at 110 °C for 5 h and weighted to determine the asphaltenes content. The solvent in the filtrate was removed in a rotary evaporator at 30 °C under a pressure of 0.01 MPa for 1 h to obtain maltenes, which was further fractionated into saturates, aromatics, and resins through a glass column packed with activated neutral alumina. The maltenes with a small amount of nheptane was added on the top of the packed column, and then the column was sequentially eluted with n-heptane, toluene, and a mixture of toluene and methanol (9/1, v/v) at a solvent-to-maltene ratio of 250 mL to 1 g. The solvent in the eluate was removed in a rotary evaporator at 30 °C under a pressure of 0.01 MPa for 1 h to obtain saturates, aromatics, and resins. The asphaltenes of three ARs were named as AB-As, AA-As, and AM-As, respectively, and their resins were named as AB-R, AA-R, and AM-R, respectively. The element analysis of H, C, and S contents was measured by the X-ray fluorescence analysis according to the method of ASTM D2622. The N content was analyzed using chemical luminescence based on the method of ASTM D4629. The V and Ni contents were measured using the inductively coupled plasma (ICP). The content of asphaltenes as the n-C7 insoluble substance was measured according to the method of ASTM D6560. The content of asphaltenes measured by this method was slightly different from that of the SARA separation for three ARs, but the trend of asphaltenes content in the different ARs was the same as the two methods. The amount of Conradson carbon residue (CCR) was measured by the method of ASTM D4530. Density of ARs and products was measured using a vibration type density meter according to the method of ASTM D1298. Gel permeation chromatography (1200 Infinity Series, Agilent Technologies, Santa Clara, CA) with an ultraviolet diode array detector (DAD at 261 nm) (GPC-UV) was used to measure the molecular size distribution of the ARs and their fractions. X-ray diffraction (XRD) analysis of the sample was performed by an AXS D8 ADVANCE diffractometer (Bruker, Billerica MA) with the Cu Kα radiation (40 kV, 30 mA). The intensity was recorded from 5 to 80° in steps of 0.02° s−1. Electron spin resonance (ESR) spectra were obtained on a JESFA200 ESR spectrometer with an X-band Bridge (JEOL Ltd., Tokyo, Japan) under a standard 100 kHz field modulation. The detail measurement method and the calculation of B parameter value can be found in our previous paper.33 Here, B parameter is an indicator for the rotational mobility of V = O, as it is a sensitive parameter for the tetragonal distortion of V = O.

Table 2. Composition and Properties of Three ARs

3. RESULTS AND DISCUSSION 3.1. Analysis and Characterization of Three ARs. The concentrations of C, H, S, N, V, and Ni in the three ARs were measured, and the results are listed in Table 2. It is clear that the concentrations of all heteroatoms in the ARs increase in the order of AM-AR < AA-AR < AB-AR, and the H/C atomic ratio decreases in the order of AM-AR > AA-AR > AB-AR, although

In order to get insight into the molecular size distributions of the three ARs, the GPC-UV chromatograms of the three ARs and their resins and asphaltenes were obtained, and the results are shown in Figure 3. The molecular size distributions of ARs, resins, and asphaltenes are in a range corresponding to an elution time from 17.0 to 26.0 min, 18.0 to 26.0 min, and 16.0 to 26.0 min, respectively, indicating that the average molecular

density (g/mL) C (wt %) H (wt %) H/C (atomic ratio) S (wt %) N (ppmw) V (ppmw) Ni (ppmw) asphaltenes (wt %) CCR (wt %)

AM-AR

AA-AR

AB-AR

0.975 84.28 11.15 1.59 4.12 2205 63.6 20.8 4.9 10.9

0.977 84.05 11.03 1.57 4.26 2410 70.9 21.9 4.5 11.6

1.025 82.86 10.14 1.47 5.45 2850 155.0 43.2 7.9 16.1

their concentrations in AA-AR are closer to those in AM-AR. Moreover, the density of the ARs increases in the order of AMAR < AA-AR ≪ AB-AR. The results indicate that the AB-AR is much heavier and dirtier than AM-AR and AA-AR. The high-temperature simulation distillation curves of the three ARs are shown in Figure 2. Comparison of these curves

Figure 2. High-temperature simulation distillation curves of three ARs.

indicates that the boiling points of the compounds and distributions in the three ARs are similar, except in the light fraction (less than 370 °C), where the boiling points of the compounds in AM-AR are lower in comparison with those in AA-AR and AB-AR. The SARA separation and analysis were conducted for the three ARs. The measured concentrations of saturates, aromatics, resins, and asphaltenes are shown in Table 3. ABAR gives much higher concentrations of aromatics, resins, and asphaltenes and obviously lower concentration of saturates in comparison with those in AA-AR and AM-AR. Table 3. SARA Compositions of AM-AR, AA-AR, and AB-AR

6728

fractions

AB-AR

AA-AR

AM-AR

saturates (wt %) aromatics (wt %) resins (wt %) asphaltenes (wt %)

19.07 52.43 17.01 11.49

31.20 47.85 14.30 6.65

28.93 48.69 15.30 7.04

DOI: 10.1021/acs.energyfuels.8b01150 Energy Fuels 2018, 32, 6726−6736

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Figure 3. GPC-UV chromatograms of (a) ARs and their (b) resins and (c) asphaltenes.

shown in Table 4. The B parameter, calculated from the ESR anisotropic spectra, has been proposed to be used as an index to reflect the molecular entanglement between the V = O complexes and their matrixes in the ARs and their asphaltenes.33−35 The obtained B value decreases in the order of AB (AR: 4.78, As: 4.51) > AA (AR: 4.66, As: 4.43) > AM (AR: 4.51, As: 4.32) for both the ARs and their asphaltenes, indicating a stronger constraint of the matrixes on the V = O mobility by the surrounding large aromatic molecules in AB-AR and its asphaltenes than in the other two ARs and their asphaltenes. 3.2. Reactivity of Three ARs on the Basis of Conversion over Whole Catalysts. The hydrotreatment of AM-AR, AA-AR, and AB-AR at 370 °C under a H2 pressure of 13.5 MPa over a series of HDM and HDS catalysts in the reactor system was conducted. The concentrations of S, N, V, Ni, asphaltenes, and CCR in products from the outlet of reactors 1, 2, 3, and 5 were analyzed, and the results are listed in Table 5. The overall conversions of S, N, V, Ni, asphaltenes, and CCR for the three ARs in the whole hydrotreating process are shown in Figure 6. The overall conversions for the different species are quite different, which decrease in the order of V species > S species > asphaltenes > Ni species > CCR precursors > N species, regardless of the sources of ARs, except for the conversions of asphaltenes for AM-AR and AB-AR, indicating that the reactivities of the different species in ARs decrease in the same order. It is clear that this catalyst system is effective for removing the S and V species with a conversion more than or close to 80 wt %. However, the conversion of the N compounds is the lowest, being about 40 wt %, indicating that the N compounds are the most difficult to be removed among all these species in the hydrotreating process. For the metals species, the conversion of V is obviously higher than that of Ni, which is consistent with the previous findings.19,36−40 It is possible that the oxygen atom connected to V makes it more active, and thus promotes the V removal.41 The significantly different conversions for V and Ni species imply that these reactions may not occur synchronously.

size of the asphaltenes is larger than those of AR and resins, regardless of the sources of ARs. No obvious difference of the molecular size distribution in the resins among the three ARs is found. The molecular size distribution of asphaltenes can be divided into two parts, the larger molecule size part corresponding to an elution time range from 16.0 to 19.0 min, and the smaller molecule size part corresponding to an elution time range from 19.0 to 23.0 min. AM-As shows slightly larger ratio of the smaller molecule size part to the larger molecule size part, indicating that the average asphaltenes molecular size in AM-As is smaller than those in AA-As and AB-As. The XRD patterns of the three asphaltenes are shown in Figure 4. The diffraction peaks are at about 10 to 30°, which

Figure 4. XRD patterns of the asphaltenes.

can be attributed to the paraffinic and aromatic aggregation. The sharper shoulder peak at 26° increases in the order of AMAs < AA-As < AB-As, indicating that the aromatic π−π stacking in AB-As is stronger than those in AM-As and AA-As. Figure 5 shows ESR spectra of V = O of the AR and asphaltenes dissolved in 1-methylnaphthalene at 50 °C. The ESR spectra are anisotropic for all samples, and it is difficult to find the difference by direct comparison of their spectra. ESR parameters values of V = O of the ARs and their asphaltenes are 6729

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Figure 5. ESR spectra of V = O in (a) ARs and (b) asphaltenes dissolved in 1-methylnaphthalene at 50 °C.

Table 4. ESR Parameters of V = O for ARs and Asphaltenes Dissolved in 1-Methylnaphthalene at 50 °C g/

g//

B

1.9617 1.9618 1.9619 1.9613 1.9615 1.9617

1.9938 1.9936 1.9935 1.9932 1.9931 1.9929

4.78 4.66 4.60 4.51 4.43 4.32

samples ARs

asphaltenes

AB-AR AA-AR AM-AR AB-As AA-As AM-As

However, the finding of the reactivity order of various species in this study is slightly different from that (V species > Ni species > S species > N species) reported by Callejas and Martinez.20 The different reactivity orders for S species and Ni species indicate that the chemical structure of the S species and Ni species in the two residues may be different, or the relative activities of the catalysts used in the two systems for removal of the S and Ni species may be different. Asphaltenes made up by the large polyaromatic compounds with high concentrations of S, N, V, and Ni heteroatoms are usually considered as precursors of the coke in AR hydrotreating. CCR is often used to predict the tendency of the feedstock to form carbon deposits at high temperature in an inert atmosphere, which is also related to coke formation in the refining process. The observed conversions (>70%) of asphaltenes in the three ARs are significantly higher than those ( AA-AR > AB-AR, while for the HDN, HDNi, and HDCCR reactions, the reactivities are close to each other, except for the HDV and HDNi reactions over HDM1 catalyst. This result is in agreement with the findings derived from the total conversions (see section 3.2). The lower reactivities of AM-AR for HDV and HDNi over HDM1 catalyst are probably due to the different molecular structures of the V and Ni species and/or the inhibition effect of the coexisting compounds on the HDV and HDNi over HDM1catalyst. 3.4. Effect of the Composition and Properties of ARs on Reactivities. In general, the reactivities of the V, Ni, S, N, asphaltenes, and CCR species in AR depend on both the inherent reactivity of the species, which is defined by the reactants’ chemical structures and their distributions, and the inhibition caused by the coexisting compounds in the feedstock. The present study shows that the reactivities of the three ARs for HDS, HDV, and HDAsp reactions decrease obviously in the order of AM-AR > AA-AR > AB-AR. No significant difference in the boiling point and the molecular size distribution is found among AB-AR, AA-AR, and AM-AR, as shown in Figures 2 and 3), indicating that the molecular size may be not one of the major factors that determine their reactivities for HDS, HDV, and HDAsp in the present study. However, the heteroatoms in the asphaltenes with smaller molecular size, as shown in Figure 3c, may show the weaker constraint and more easily escape from the surrounding molecules to approach the active sites of the catalyst, which may explain the higher reactivity of AM-AR in the hydrotreating process. The detail characterization of the molecular structures of S, V, and asphaltenes species as well as their distributions is needed to confirm whether there are differences in their molecular structures and distributions. On the other hand, the N and asphaltenes concentrations as well as the density, as listed in Table 2, decrease in the order of AB-AR > AA-AR > AM-AR, indicating that the coexisting N, asphaltenes, and aromatic species may affect their reactivities for HDS and HDV, and the coexisting N and aromatic species may affect their HDAsp reactivity. The HDAsp reactivity in the order of AM-As > AA-As > AB-As may be ascribed to their different aromatic π−π stacking in asphaltenes, as shown in Figure 4, possibly due to the stronger aromatic stacking, which inhibits the hydrogenation and hydrocracking reactions. In addition, the stronger aromatic stacking may more strongly wrap the heteroatoms with the larger aromatic species in asphaltenes, resulting in difficult approaching of the heteroatoms to the active sites on the catalyst surface. The detail correlations of the HDS rate constants with the N and asphaltene content as well as the density over various catalysts were conducted, and the results are shown in Figure 8. It is clear that the HDS rate constants decrease with the increasing N and asphaltene contents as well as the density, regardless of the catalysts, with a R2 value higher than 0.85 (except for the effects of the asphaltenes and density over HDM1 catalyst, R2 = 0.51−0.83). The result indicates that the presence of a larger number of N and asphaltene species as well as higher density of feedstock has significantly negative effects on the HDS. The negative effect of N compounds on HDS of

the N species are much more difficult to be removed than the S species in all catalyst beds. For the HDV, the rate constant decreases significantly in the catalyst order of HDM1 > HDM2 > (HDM3+HDS1) > HDS1, regardless of the sources of ARs, implying that the HDM catalysts are more effective for HDV, and the reactivity of the V species in the beds decreases in the same order. The rate constant for HDNi decreases slightly in the catalyst order of HDM1 > HDM2 > (HDM3+HDS1) > HDS1, except that HDM1 catalyst gives the smallest rate constant for HDNi of AM-AR. In comparison of the rate constants for HDNi and HDV, the rate constants for HDV over different catalyst beds are about 1.2−2.4 times larger than those for the corresponding HDNi, regardless of the feedstocks, indicating that the V species in ARs have higher reactivity than the coexisting Ni species in the hydrotreating process, regardless of the catalysts, which has also been discussed in section 3.2. Interestingly, it is found further that the ratio of the rate constant for HDV to the rate constant HDNi (kHDV/kHDNi) decreases in the catalyst order of HDM1 > HDM2 > (HDM3+HDS1) > HDS1. It implies that the HDM catalysts with larger pore size are more favorable to HDV, while the catalyst with higher hydrogenation activity (higher NiMo loading) may be more beneficial to HDNi. This is consistent with the previous finding by Alfadhli et al.19 The V porphyrin molecule contains an oxygen atom protruding from the molecule’s macrocycle, which may promote the reactivity of the V porphyrin. Consequently, the V porphyrin is easier to be converted in comparison with the coexisting Ni porphyrin.48,49 The Ni atom in petroleum is bound with aromatic compounds, so that the catalyst with high hydrogenation activity may be necessary to weaken the Ni compound polarity by saturation of the double bonds and rearrangement the Ni compound charge.50 The rate constants for HDAsp show almost a decreasing tendency with the catalyst bed order of HDM1 > HDM2 > (HDM3+HDS1) > HDS1. In addition of the more reactive asphaltenes molecules in the feed for the forwarder catalyst beds, the catalysts with large pores may be beneficial to the conversion of asphaltenes, which agrees with the reports in the literature.51,52 However, the rate constants for HDCCR over various catalysts are close to each other. Considering that the major CCR species (precursors) remained in the subsequent catalyst beds might be more refractory, the catalyst with higher hydrogenation activity, such as HDS1, shows to be more favorable to remove the CCR species. The different change trends of the rate constants between HDAsp and HDCCR indicate that the two reactions may be less dependent. 3.3.3. Reactivities of Various Species over Different Catalysts. The reactivities of various species in ARs are quite different. On the basis of their pseudo-first-order rate constants, as shown in Figure 7, the following reactivity tendencies of the species over different catalyst beds can be found: (1) V species > asphaltenes ≈ S species > Ni species > CCR species > N species in the HDM1 catalyst bed; (2) V species ≈ S species > asphaltenes > Ni species > CCR species > N species in the HDM2 catalyst bed; (3) V species ≈ S species > asphaltenes > Ni species > CCR species > N species in the HDM3+HDS1 catalyst bed; and (4) S species > V species ≈ asphaltenes > Ni species > CCR species ≈ N species in the HDS1 catalyst bed. In general, regardless of the catalyst types, V and S species are the two species with the highest reactivity, while the CCR and N species are the ones with the lowest reactivity among the six examined species. In order to improve the performance of the 6732

DOI: 10.1021/acs.energyfuels.8b01150 Energy Fuels 2018, 32, 6726−6736

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Figure 9. Correlations of HDV rate constants with (a) asphaltene content and (b) density of feedstock over various catalysts.

density (with the R2 values higher than 0.82, except for the HDM1 catalyst), indicating that the higher asphaltene content and larger density have negative effects on the V removal. Our previous study has reported that the B parameter calculated on the basis of the ESR spectrum is a good index to reflect the molecular entanglement between the V = O complexes and their matrixes in AR.34 Consequently, the B parameter values of ARs should have a good relationship with their HDV reactivities, as they reflect the extent of constraint on the V = O mobility, and thus affect the HDV reactivity. A correlation of the overall HDV conversion with the B values of the three feedstocks, which are derived from the ESR spectra (Figure 5) of V = O, is shown in Figure 10. An excellent linear relationship between the HDV conversions and the B values of ARs is found, confirming that the B value can be a good index to estimate the HDV reactivity of AR. The results from the present study show that AM-AR, AAAR, and AB-AR have similar reactivities for HDN, HDCCR, and HDNi according to their total conversions and the rate constants on each catalyst bed (except for the HDNi on HDM1 catalyst for AM-AR), which implies that the molecular structures and distributions of the N, Ni, and CCR species in the three ARs may be similar, and the effects of the different concentrations of the coexisting compounds (such as S species, asphaltene species, and the density) in the feedstocks on HDN, HDNi, and HDCCR may also be ignored. More attention in future study should be paid to how to identify the refractory N, Ni, and CCR species and to improve the hydrotreating performance for removing them.

Figure 8. Correlation of HDS rate constants with (a) N content, (b) asphaltene content, and (c) density of feedstock over various catalysts.

the middle distillates has been well-reported in the literature.53,54 The effect of asphaltenes on the HDS can be explained that the presence of asphaltenes in the feedstock blocks the way for the S species to approach the active sites on the catalyst surface. The feedstock with higher concentration of aromatics, indicated by the higher density and the stronger aromatic π−π stacking, as shown in Figure 4, can be another reason for the decrease of the HDS reactivity in the order of AM-AR > AA-AR > AB-AR. The correlations between the HDV rate constant with asphaltene content and density over various catalysts are shown in Figure 9. The result shows a clear tendency that the HDV rate constants decrease with increasing asphaltene content and 6733

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Energy & Fuels Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Japan Cooperation Center Petroleum (JCCP), the Kuwait Oil Company (KOC), and the Kuwait Institution for Scientific Research (KISR) for their support and collaboration on this joint project. Acknowledgement is also extended to Kuwait National Petroleum Company (KNPC) for the in-kind contribution and technical support.

■ Figure 10. Correlation of overall HDV conversions with the B values derived from ESR characterization.

4. CONCLUSIONS Composition and properties of the three ARs were analyzed and characterized. Their hydrotreating reactivities for HDS, HDN, HDV, HDNi, HDAsp, and HDCCR over a hydrotreating catalyst system were evaluated and correlated with their composition and properties. On the basis of this study, some conclusions can be summarized as follows: (1) various species in AR feedstocks show quite different reactivities, decreasing almost in the order of V species > S species ≈ asphaltenes > Ni species > CCR precursor > N species, regardless of the sources of ARs. (2) The hydrodemetallization catalysts with larger pore size, such as HDM1 and HDM2, are more efficient for removing V and asphaltene species, while the catalyst with higher hydrogenation activity, such as HDS1, is required for removing the refractory S, N, Ni, and CCR species in AR. (3) Reactivities of the three ARs for HDS, HDAsp, and HDV increase in the order of AB-AR < AA-AR < AM-AR, while their reactivities for HDNi, HDCCR, and HDN are similar, according to the total conversions and the rate constants over each catalyst. (4) The higher N and asphaltenes concentrations and the larger density of AR have strong and negative effects on the HDS, HDV, and HDAsp reactivities, but such factors have no significant effect on the HDN, HDNi, and HDCCR reactivities. (5) The B parameter value calculated on the basis of the ESR spectrum can be a good index to predict the HDV reactivity of AR. It should be highlighted that the effects of the composition and properties of AR on their HDS and HDM reactivities are not limited to only concentrations of the coexisting N, asphaltene, and aromatic species, but also depend on the molecular structures and distributions of the S, metal, and asphaltene species in the feedstock. More advanced technologies for characterizing these species at a molecular level are required for building a better correlation between the composition and properties of ARs and their hydrotreating reactivities.

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ABBREVIATIONS AR = atmospheric residue As = asphaltenes R = resins AB = heavy Arabia crude B AA = Arabia crude A AM = Arabia Medium crude S = sulfur N = nitrogen V = vanadium Ni = nickel CCR = Conradson carbon residue HDS = hydrodesulfurization HDN = hydrodenitrogenation HDV = hydrodevanadium HDNi = hydrodenickel HDM = hydrodemetallization HDAsp = hydrodeasphaltene HDCCR = hydrodeconradson-carbon-residue GPC = gel permeation chromatography XRD = X-ray diffractometer ESR = electron spin resonance SARA = saturates-aromatics-resins-asphalenes LHSV = liquid hourly space velocity REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (096)5 2495 6920. *E-mail: [email protected]. Tel: (081)0926620410. ORCID

Qingyan Cui: 0000-0001-7367-5082 Xiaoliang Ma: 0000-0003-0450-0662 6734

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