Hydrotreating Reactivities of Atmospheric Residues and Correlation

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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 AlMutairi, Abdulazim MJ Marafi, Joo-Il park, Seong-Ho Yoon, and Isao Mochida Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01150 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Hydrotreating Reactivities of Atmospheric Residues and Correlation with Their Composition and Properties Qingyan Cui a, Xiaoliang Ma *,b Koji Nakano c, Koji Nakabayashi a, Jin Miyawaki a, Adel Al-Mutairi b, Abdulazim MJ Marafi b, Joo-Il Park b, e, Seong-Ho Yoon a, Isao Mochida *,d a

Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka, Japan

b

Petroleum Research Center, Kuwait Institute for Scientific Research, Safat, Kuwait

c

JGC Catalysts and Chemicals Ltd, Fukuoka, Japan

d

Kyushu Environmental Evaluation Association, Fukuoka, Japan

e

Colloge of Engineering, Hanbat National University, Republic of Korea

ABSTRACT In order to better understand the effects of compositions 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, 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
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 3 ACS Paragon Plus Environment

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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 first order 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 first order 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 was the first-order for vanadium removal and the second-order for asphaltene removal, and the catalyst with the optimum pore diameters favored to the removal of vanadium and asphaltenes.

29

Alvarez

30

investigated the

residue hydroprocessing in a multi-fixed-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. 4 ACS Paragon Plus Environment

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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 researchs 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 by direct using their rate constants. In the present study, three ARs (AM-AR, AA-AR and AB-AR) were analyzed and characterized in detail by element analysis, SARA analysis, gel permeation chromatography (GPC), X-ray diffractometer (XRD) and electron spin resonance (ESR) and so on. The reactivities of the three ARs for HDS, 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 5 ACS Paragon Plus Environment

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of the operating conditions, design of the catalyst beds and blending of feedstocks for improving AR hydrotreating performance. 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 increased in the order of HDM1 < HDM2 < HDM3 < HDS1. Table 1 Composition and properties of the NiMo/Al2O3 catalysts. Catalysts HDM1

Surface area (m2 g-1) 170

Pore volume (cm3 g-1) 0.73

Pore diameter (nm) 30

MoO3 content (wt %) 3.3

NiO content (wt %) 0.3

HDM2

160

0.69

30

8.5

2.3

HDM3

140

0.65

25

10.8

2.7

HDS1

175

0.64

13

11.8

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 6 ACS Paragon Plus Environment

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ml of HDS1, respectively. Prior to the hydrotreating testing, the catalysts were pre-sulfurized using a mixture of gas oil with 2 wt % of carbon disulfide at 240 oC for 5 h, and then at 290 oC for 10 h. The hydrotreatment of ARs was conducted at 370 oC 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 firstly from the outlet of the 5th reactor, and then the 3rd, 2nd and 1st reactors in the 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 %.

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

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 oC setting overnight. The solution was filtered, and the solid cake was washed with n-heptane until no color in the filtrate. The insoluble portion was extracted with

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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 oC 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 n-heptane 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 oC 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 contents in the different ARs was same by 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 8 ACS Paragon Plus Environment

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Clara, CA, USA) 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, USA) with the Cu Kα radiation (40 kV, 30 mA). The intensity was recorded from 5 to 80 o in steps of 0.02 o 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. 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 their concentrations in AA-AR are more close to those in AM-AR. Moreover, the density of the ARs increases in the order of AM-AR < AA-AR 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.

AB-As AA-As AM-As

AB-AR AA-AR AM-AR Intensity /a.u.

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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a

b

250

300

mT

350

400

250

300

mT

350

400

Figure 5. ESR spectra of V=O in (a) ARs and (b) asphaltenes dissolved in 1-methylnaphthalene at 50 oC. Table 4 ESR parameters of V=O for ARs and asphaltenes dissolved in 1-methylnaphthalene at 50 oC. Samples g// g┴ B ARs

Asphaltenes

AB-AR

1.9617

1.9938

4.78

AA-AR

1.9618

1.9936

4.66

AM-AR

1.9619

1.9935

4.60

AB-As

1.9613

1.9932

4.51

AA-As

1.9615

1.9931

4.43

AM-As

1.9617

1.9929

4.32

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.


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Table 5 Composition and Property of the hydrotreated products. Samples

AA-AR

AB-AR

AM-AR

P-1

P-2

P-3

P-5

P-1

P-2

P-3

P-5

P-1

P-2

P-3

P-5

Density, g/ml

0.964

0.949

0.940

0.926

1.001

0.982

0.970

0.951

0.961

0.947

0.938

0.925

S, wt %

3.35

2.20

1.54

0.73

4.60

3.33

2.47

1.25

2.86

1.80

1.22

0.56

N, ppm

2320

2200

1985

1520

2720

2545

2310

1775

2115

1965

1635

1255

CCR,wt %

10.3

8.7

7.2

5.2

13.4

11.2

9.3

6.9

9.3

7.5

6.4

4.8

Asp.,wt %

3.2

2.4

1.8

1.0

6.2

4.5

3.6

2.4

3.0

1.8

1.1

0.5

Ni, ppm

16.1

12.4

9.5

6.1

30.2

22.5

18.2

12.2

17.2

12.7

9.5

5.5

V, ppm

42.6

27.2

18.4

10.0

93.8

63.3

45.6

27.8

39.7

24.7

15.8

7.8

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. 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 14 ACS Paragon Plus Environment

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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.

AM-AR AA-AR AB-AR

100 90 Conversion / wt %

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80 70 60 50 40 30

HDS

HDN

HDAsp HDCCR

HDV

HDNi

Figure 6. Total conversions of three ARs for HDV HDNi, HDS, HDN, HDAsp and HDCCR reactions.

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 (< 56 %) of CCR, indicating that asphaltenes are easier to be removed than the CCR precursors in the hydrotreating process. When comparing the reactivities of the three ARs for various reactions, it is found that the reactivities for HDS, HDAsp and HDV increase in the order of AB-AR < AA-AR < AM-AR according to the conversions shown in Figure 6. It indicates that the 15 ACS Paragon Plus Environment

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S, V and asphaltene species in AB-AR are the most difficult to be removed among the three ARs, while the S, V and asphaltene species in AM-AR are the highest reactivity. Such results may come from two reasons: 1) the intrinsic reactivities of the S, V and asphaltene species in AB-AR are lower due to their molecular structure, and/or 2) the co-existing compounds and matrixes in AB-AR inhibit more significantly the HDS, HDAsp and HDV reactions. Interestingly, on the other hand, it is found that the reactivities of the three ARs for HDNi and HDCCR are very close, indicating that the molecular structures of the Ni species and the CCR precursors are similar, and the co-existing compounds in the AR have a similar effect on the HDNi and HDCCR reactions. 3.3. Reactivities Based on the Rate Constants over Different Catalyst Beds.

3.3.1. Rate Constants of Various Species in Three ARs over Different Catalyst Beds. Understanding of the reaction rates of the ARs for HDS, HDN, HDV, HDNi, HDAsp and HDCCR over different catalyst beds is important for design of the catalyst and the catalyst bed. In many cases, the reaction rates for HDN, HDV, HDNi, HDAsp and HDCCR can be descripted by the first order kinetics, as indicated in the literature,7,13,29,42 although other order kinetics have also been reported,28 depending on the conversion and distribution of the reactant species with different reactivities.30, 41-45 The apparent HDS kinetics behavior can be described approximately by the first order reaction, when liquid hourly space velocity (LHSV) is higher, or the conversion is lower.16,46 In principle, an individual compound in hydrotreating obeys mostly the first order kinetics.

47

In the real feed cases, the empirical reaction order for various

compounds, such as S, N, V, Ni and asphaltenes compounds, is usually estimated by the regression analysis to fit the experimental data. Consequently, the empirical reaction 16 ACS Paragon Plus Environment

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order does not only depend on the reaction mechanism, but also depend on the distribution of the species with different reactivities and the extent of the conversions. Since using different reaction orders for HDS, HDN, HDV, HDNi, HDAsp and HDCCR makes it difficult to compare directly the reactivities of the different species on the basis of their rate constants, in the present study, the pseudo-first-order kinetics was used to estimate the rate constants for all six reactions in each catalyst bed. It should be highlighted that the rate constants determined by the pseudo-first-order kinetics model in this study for the various species is to be used as the indexes for comparing the reactivities, but not for a kinetics study. The pseudo-first-order rate constant was calculated by using equation:

k = LHSV⋅ ln (Co/ Ct) where Co and Ct is the concentration of the species entering and out of the catalyst bed,

LHSV is the liquit hourly space velocity for each corresponding catalyst bed, k is the rate constant. The rate constants calculated on the basis of the hydrotreating results are shown in Figure 7 for the different ARs over each catalyst bed.

3.3.2. Activities of Different Catalysts. In general, the rate constant value reflects both the reactivity of the reactant and the activity of the catalyst. The rate constant values measured for HDS over different catalyst beds are close to each other, as shown in Figure 7(a), except for the smaller rate constant value of HDM1 catalyst, indicating that the HDM1 is lower active for HDS than others. As well known, AR contains various S compounds with different HDS reactivities. The major S compounds remained in the latter catalyst beds are ones with lower reactivity. Considering the smaller rate constant for HDM1 catalyst and the lower reactivities of the S compounds in the subsequent HDM2, HDM3+HDS1 and HDS1 17 ACS Paragon Plus Environment

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catalysts beds, it is clear that the HDS activity of the catalysts increases in the order of HDM1 < HDM2 < (HDM3+HDS1) < HDS1. Figure 7(b) shows that the rate constant of HDN increases gradually in the order of HDM1 < HDM2 < (HDM3+HDS1) < HDS1, regardless of the feedstocks, definitely indicating the significant increase in the HDN activity of the catalysts in the same order. These changes in the catalyst activity for HDS and HDN can be attributed to the increase of the active metal (MoO3 and NiO) loading in the catalysts, as shown in Table 1. In comparison of the rate constants for HDS and HDN, it is found that the rate constant for HDS is about 2.4-8.8 times larger than those for the corresponding HDN, suggesting that 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 co-existing Ni species in the hydrotreating process, regardless of the catalysts, which has also been discussed in 3.2 section. 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 18 ACS Paragon Plus Environment

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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 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.


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(a) HDS

(b) HDN

HDM1

HDM1

HDM2

HDM2

HDM3+ HDS1

HDM3+ HDS1

AM-AR AA-AR AB-AR

HDS1

0.0

0.2

0.4 0.6 -1 Rate constant (k) / h

0.8

AM-AR AA-AR AB-AR

HDS1

0.0

1.0

0.2


HDM1

HDM1

HDM2

HDM2

HDM3+ HDS1

HDM3+ HDS1

0.0

AM-AR AA-AR AB-AR 0.2


0.8

AM-AR AA-AR AB-AR

HDS1

1.0

0.0

0.2


(e) HDAsp HDM1

HDM2

HDM2

HDM3+ HDS1

HDM3+ HDS1

AM-AR AA-AR AB-AR

HDS1

0.2


0.8

1.0

(f) HDCCR

HDM1

0.0

1.0

(d) HDNi

(c) HDV

HDS1

0.8

0.8

AM-AR AA-AR AB-AR

HDS1

1.0

0.0

0.2


0.8

1.0

Figure 7. Rate constants estimated on the basis of hydrotreating results for (a) HDS, (b) HDN, (c) HDV, (d) HDNi, (e) HDAsp and (f) HDCCR of three ARs over each catalyst bed.

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 20 ACS Paragon Plus Environment

Page 21 of 34 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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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 catalyst system for HDN, HDS and HDCCR, the catalyst with higher hydrogenation activity may be required.

3.3.4. Reactivities of AR for Various Reactions over Different Catalysts. In comparison of the reactivities of the three ARs for various reactions over different catalyst beds, it is found that for the HDS, HDV and HDAsp reactions, the reactivities of the three feedstocks decrease in the order of AM-AR > 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 co-existing 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 co-existing 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 > 21 ACS Paragon Plus Environment

Energy & Fuels 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

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(a), 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 3(c), 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 co-existing N, asphaltenes and aromatic species may affect their reactivities for HDS and HDV, and the co-existing 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.


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0.8

HDS rate constant /h

-1

HDM1 HDM2 HDM3+HDS1 HDS1

0.6

2

2

R =0.932

R =0.960

2

R =0.866

0.4 2

R =0.829

a

0.2 1200

1600

2000 2400 N content /ppm

0.8

HDS rate constant / h

-1

R =0.991 0.6

3200

2

R =0.993 2

R =0.985 0.4 2

R =0.510 0.2

b 0

2

4 6 8 Asphaltene content / wt %

0.8 2

R =0.984 -1

2800


2

HDS rate constant /h

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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2

R =0.909

0.6

10


2

R =0.963 0.4 2

R =0.655

c

0.2 0.90

0.95 1.00 Density /gm/ml

1.05

Figure 8. Correlation of HDS rate constants with (a) N content, (b) asphaltene content and (c) density of feedstock over various catalysts. 23 ACS Paragon Plus Environment

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The detail correlations of the HDS rate constants with the N and asphaltenes contents 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 asphaltenes 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 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 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.


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HDV rate constant /h

-1

0.8

0.6

2

R =0.997 2

R =1.000

0.4 2

R =0.945

0.2

a

0

2

4 6 8 Asphaltene content /wt %

10


-1

0.8 HDV rate constant /h

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

2

R =0.998 2

R =0.835

0.4

0.2 0.90

2

R =0.829

b

0.95 1.00 Density /gm/ml

1.05

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

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, 25 ACS Paragon Plus Environment

Energy & Fuels

confirming that the B value can be a good index to estimate the HDV reactivity of AR.

90 88 HDV Conversion /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

Page 26 of 34

y=-31.667x+233.467 2 R =1.000

86 84 82 80 4.55

4.60

4.65 4.70 B value

4.75

4.80

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

The results from the present study show that AM-AR, AA-AR 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 co-existing 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. 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: 26 ACS Paragon Plus Environment

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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 co-existing 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. ABBREVIATIONS AR: atmospheric residue 27 ACS Paragon Plus Environment

Energy & Fuels 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

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 AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (I. Mochida), [email protected] (X. Ma); Tel: (081)0926620410 (I. Mochida) , (096)5 2495 6920(X. Ma.) 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 28 ACS Paragon Plus Environment

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Hydrotreating Reactivities of Atmospheric Residues and Correlation

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