Experimental and Kinetic Studies of Aromatic Hydrogenation

Ind. Eng. Chem. Res. 2005, 44, 7935-7944

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Experimental and Kinetic Studies of Aromatic Hydrogenation, Hydrodesulfurization, and Hydrodenitrogenation of Light Gas Oils Derived from Athabasca Bitumen Abena Owusu-Boakye,† Ajay K. Dalai,*,† Deena Ferdous,† and John Adjaye‡ Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan, 57 Campus Drive Saskatoon, Saskatchewan, S7N 5A9, Canada, and Syncrude Edmonton Research Centre, Edmonton, 9421 17th Avenue, Edmonton, Alberta, T6N 1H4 Canada

In this work, a systematic experimental and kinetic study of hydroprocessing of light gas oils (LGOs) such as vacuum LGO (VLGO), atmospheric LGO (ALGO), and hydrotreated LGO (HLGO) using NiW/Al2O3 and commercial NiMo/Al2O3 catalysts has been conducted. Experiments were performed by varying temperature from 340 to 390 °C, at a constant pressure and liquid hourly space velocity of 11.0 MPa and 0.6 h-1, respectively. H2/feed ratio was maintained at 550 mL/ mL throughout the experiments. Appreciable hydrogenation of aromatics (AHYD) was achieved by the NiW/Al2O3 catalyst at low temperatures and at high severities of hydrotreating. However, the hydrogenation activity of NiMo/Al2O3 was superior to that of the NiW/Al2O3 catalyst. For hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities, higher conversions of 95-98.8 and 96-99 wt %, respectively, were attained for the commercial NiMo/Al2O3 catalyst throughout the temperature range studied. Simulated distillation of the feed showed that VLGO contained the most complex and heaviest compounds followed by HLGO and ALGO. Diesel selectivity in both ALGO and HLGO increased with hydrotreating temperature, but in the case of VLGO, it decreased with temperature. Kinetics studies showed that dearomatization of the HLGO feed was the most difficult, followed by ALGO and then the VLGO. Kinetics of ALGO and VLGO were best described by a pseudo-first-order reaction mechanism while the 1.3 power law kinetics worked well with HLGO. Introduction Hydrogenation of aromatics (AHYD) in middle distillates is well-known in the petroleum refining industry as a result of the surging environmental measures to improve the quality of diesel fuel. More strict aromatics and cetane number specifications along with maximum sulfur content limitations in diesel fuels have been introduced round the world. Effective October 1993, the California Air Resource Board (CARB) passed regulations to limit aromatics and sulfur contents of diesel to 10 vol % and 0.05 wt %, respectively.1,2 Generally, AHYD is more difficult than hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) due to the type and amounts of aromatic species present in the oil as well as the complexity of the reactions.3 Hence, to be able to reduce aromatics present in middle distillates which serve as feedstock for the production of diesel fuel, a thorough understanding of the process variables, catalyst type, and interaction of these variables on the chemistry and thermodynamics is necessary. A number of studies4-7 have been conducted to investigate the effects of process variables on AHYD as well as HDS and HDN. In our previous study8 experiments were performed between 340 and 390 °C, at pressures of 6.9-12.4 MPa and liquid hourly space velocity (LHSV) values of 0.5-2.0 h-1, to investigate the effects of process variables and their interaction on * Corresponding author. Tel.: (306) 966-4771. Fax: (306) 966-4777. E-mail: [email protected]. † University of Saskatchewan. ‡ Syncrude Edmonton Research Centre.

hydroprocessing of bitumen-derived light gas oil. Results showed that the interaction between temperature and pressure was the most significant factor affecting AHYD while HDS and HDN were greatly influenced by the interaction between the LHSV and temperature. Considering all the factors affecting AHYD, hydrogen partial pressure is the most significant factor affecting AHYD.9 Another important factor affecting the extent of hydrotreating (AHYD) is the catalyst type used for hydrotreating. Molybdenum sulfide catalysts with Ni and Co promoters have been used extensively for upgrading of synthetic crude. However, it has been observed that using these catalysts for refining shows less efficient hydrogenation activity and a shorter lifetime.10 This phenomenon was explained to be mainly due to differences in properties of the starting feed material.11 Depending on the origin of petroleum crude, the amount and type of compounds found in each feed are different. Generally, distillates from synthetic sources are particularly high in aromatics, sulfur, and nitrogen contents compared to distillates from conventional petroleum crude.12 Hence, the drive to improve the product quality of fuel has resulted in a keen interest in the catalysis industry to develop a more selective catalyst for AHYD as well as HDS and HDN. There are differing opinions on the hydrogenation activity of NiW/Al2O3, but generally, among the different Co(Ni)-Mo(W) combinations, NiW/Al2O3 sulfide catalysts are known to display the best performance for hydrogenation of aromatic compounds.3 Superior hydrogenation of a laboratory-prepared NiW/ Al2O3 catalyst compared to commercial NiMo/Al2O3 is

10.1021/ie0500826 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/17/2005

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Table 1. Physical Properties of Light Gas Oil Feeds temp [°C] at various distillations 13C aniline density N [wppm] BN [wppm S [wppm] C [wt %] H [wt %] NMR point [g/cm3] 5 wt % 25 wt % 50 wt % 75 wt % 95 wt % HLGO VLGO ALGO BLGO

1773 634 290 461

1211 285 153 247

7149 26780 15020 17420

86.5 85.4 85.7 85.5

12.89 11.92 12.66 12.06

reported for upgrading of a coal-derived kerosene and gas oil feed.11 In another study,12 aromatic compounds contained in middle distillate fractions from synthetic crude were hydrogenated with presulfided NiW/Al2O3 catalysts at a temperature of 380 °C, LHSV of 0.75 h-1, and pressure of 17.0 MPa. At these conditions, almost complete conversion (97%) of aromatics was achieved in the synthetic crude oil. However, higher NiMo hydrogenation activity compared to NiW (all on alumina support) is reported in the literature7 for aromatic hydrogenation in Kuwait heavy gas oil in a single-stage hydrotreater at the following conditions: temperature of 300-400 °C, pressure of 4.5-12.5 MPa, and LHSV of 1.0-2.5 h-1. The primary objective of this work is to investigate the effectiveness of sulfided NiW/Al2O3 for hydrogenation of aromatic compounds and removal of heteroatoms in bitumen-derived gas oils produced from Athabasca oil sands. Secondly, since feed composition is one of the essential factors affecting product quality, yield, and selectivity, differences in hydrotreating reactivities of three other light gas oil feeds namely, vacuum light gas oil (VLGO), atmospheric light gas oil (ALGO), and hydrocrack light gas oil (HLGO), over NiW on alumina catalyst will be investigated. Furthermore, the effects of temperature on product distribution and diesel selectivity will be studied. Kinetics of hydrotreating will also be studied in the different LGO feedstocks. Experimental Section Catalyst Preparation. The NiW/Al2O3 catalyst was prepared by the incipient wetness impregnation method with extruded γ-alumina. In this method, a solution containing 3.0 wt % Ni in nickel nitrate, 15 wt % tungsten in ammonium metatungstate, and 2.5 wt % phosphorus in phosphoric acid in water was impregnated onto the alumina support. Prior to impregnation, the support, γ-Al2O3 (obtained from Sud Chemical India Ltd., New Delhi) was dried at 120 °C overnight to remove any moisture. Following impregnation, the catalyst was dried again at a temperature of 120 °C for 12 h and then calcined at 500 °C for another 4 h. Catalyst Characterization. BET surface area, pore volume, and size of the fresh and spent catalysts were determined using an automated gas (N2) adsorption analyzer ASAP 2000 (Micrometrics) with pure nitrogen gas (99.9% pure). About 0.05 g of sample was used and before each analysis; the catalyst sample was evacuated at 200 °C for 4 h in a vacuum to remove all adsorbed moisture from the catalyst surface. Prior to surface area/ porosity determinations, all spent catalysts were thoroughly washed with hexane solution to remove volatiles as well as gas oil present on the surface and in the pores of the catalysts. Cleaned catalysts were then dried in an oven at 120 °C for 12 h. High-resolution transmission electron microscopic (TEM) analyses of sulfided NiMo/Al2O3 and NiW/Al2O3 catalysts were carried out with a Philips CM20 electron microscope with an LaB6 filament as a source of

24.0 23.9 15.5 17.1

51.6 43.6 51.9

0.9718 0.9409 0.8922 0.9007

178.5 288 180.0 190.5

236.0 324.5 251.5 262.0

290.5 352.5 294.5 310.0

343 382.5 338.5 355.0

410.0 435.5 409.0 420.0

electrons and operated at 200 kV. For TEM analysis each cleaned sample was powdered. A small amount of the powdered sample was then sprinkled on a piece of Parafilm and a droplet of water was added to it. A 400 mesh carbon coated grid was floated on the droplet of water and then picked. The retained droplet of material was allowed to air-dry on the grid and then mounted on a specimen holder. Catalyst Performance Test. Details of the experimental procedure are described elsewhere.8 The experiment consisted of two phases: the first part was the catalyst performance test where the hydrogenation activity of NiW/Al2O3 was tested using a blend of light gas oil fractions from Athabasca oil sands. Commercial NiMo/Al2O3 was used as a reference catalyst. The reaction conditions used were LHSV of 0.6 h-1, total pressure of 11.0 MPa, reaction temperature of 340-390 °C, and H2/oil ratio of 550 mL/mL. The second part of the experiment, focused on determining the differences in reactivity of total aromatics, sulfur, and nitrogen present in light gas oils from different production processes, was done using vacuum (VLGO), atmospheric (ALGO), and hydrocrack (HLGO) light gas oils. The physical and chemical properties of the feedstocks used are shown in Table 1. At each set of experimental conditions, experiments were carried out for a period of 3 days and product samples were collected after every 24 h. The first product sample collected after 24 h was discarded, and steady state was observed from analyses of samples collected after the second and third days. The data given in Figures 2 and 3 are the averages of the ones collected after days 2 and 3, when steady state prevailed. Experiments were continued for 55 days for both catalysts. To ensure reproducibility of the results, the activity of each catalyst was tested after every 15 days at the temperature, pressure, and LHSV of 375 °C, 9.0 MPa, and 1.0 h-1, respectively. Aromatics, sulfur, and nitrogen conversions at this condition were varied only in the small range of 2-4 wt %, indicating that the catalyst had not deactivated over the entire period of experiments. Feed and sample products were also analyzed for total aromatics contents and total sulfur and nitrogen contents using a 500 MHz Avance NMR spectrometer and ANTEK 9000 sulfur and nitrogen analyzer, respectively. Boiling point distribution of the samples was also determined by GC-simulated distillation according to ASTM D2887 HT procedure. Results and Discussion Catalyst Characterization. BET surface area, pore volume, and pore diameter of fresh NiW/Al2O3 and commercial catalysts are given in Table 2. The surface area of laboratory-prepared NiW/Al2O3 catalyst is slightly higher than that of commercial catalyst. The pore volume and pore diameter of this catalyst were also higher than those of commercial catalyst. TEM analysis of sulfided NiMo/Al2O3 and NiW/Al2O3 catalysts was performed to determine the dispersion of

Ind. Eng. Chem. Res., Vol. 44, No. 21, 2005 7937

study.8 Aromatics, sulfur, and nitrogen conversions were defined as

conversion )

Figure 1. TEM micrographs of sulfided (a) NiMo and (b) NiW catalyst on Al2O3 support. Table 2. Characteristics of NiW and Commercial NiMo Catalysts on Al2O3 Support NiW BET surface area [m2/g] pore volume [cm3/g] pore size [Å]

NiMo

fresh

spent

fresh

spent

174 0.495 114.2

75 0.161 86.2

169 0.412 97.8

114 0.237 83.2

MoS2 and WS2, which are the active phases of hydrotreating catalysts in their working states.13,14 Parts a and b, respectively, of Figure 1 show the TEM micrographs of the NiMo/Al2O3 and NiW/Al2O3 catalysts. The lattice images of the MoS2 and WS2 slabs are indicated by the solid black lines in both parts. The sulfided NiMo/Al2O3 catalyst showed higher dispersion of MoS2 slab compared to that of WS2 in sulfided NiW/ Al2O3 catalyst. Catalyst Performance Test. The performance tests were carried out to determine the effectiveness of NiW/ Al2O3 catalyst for AHYD, HDS, and HDN activities. A blend of light gas oils was used for the performance study, and the experiments were performed by varying the reaction temperature between 340 and 380 °C at a pressure of 11.0 MPa and LHSV of 0.6 h-1. The selection of the pressure and space velocity were based on optimum operating conditions obtained for hydrogenation of aromatics in middle distillates in a previous

[feed] - [products] × 100% [feed]

(1)

where [feed] and [products] are feed and product concentrations, respectively. Figure 2 shows the overall hydrogenation activity of NiW/Al2O3 and NiMo/Al2O3 as measured by 13C NMR. It is observed in both cases that the rate of hydrogenation increases with temperature throughout the temperature range. This is an indication of negligible effect of thermodynamic equilibrium on the reaction; hence the reaction can be said to be kinetically controlled. This is because, generally, aromatic hydrogenation is kinetically and thermodynamically controlled. In the kinetically controlled region, an increase in temperature increases the rate of forward reaction, resulting in a higher conversion of aromatics. However, at higher temperatures, thermodynamic effects dominate, thus leading to a shift in equilibrium in favor of the reverse reaction (dehydrogenation), which results in more aromatics being produced in the hydrotreated products and AHYD passing through a minimum. From Figure 2, it is also observed that the hydrogenation activity of NiW/Al2O3 is higher than that of NiMo/ Al2O3 at lower temperatures, i.e., 340-350 °C, but as the temperature is increased, between 365 and 380 °C, the opposite is observed. Generally, NiW is observed to have a higher hydrogenation activity than NiMo, but it can be inferred from the BET surface area measurements that the low performance of the NiW/Al2O3 may be a result of the decrease in surface area and pore volume of the catalyst during hydrotreating. High severities of hydrotreating may have triggered catalyst coking, thus reducing the stability and activity of the catalyst. The low performance of NiW/Al2O3 could also be attributed to the lower dispersion of the WS2 species on the alumina support compared with the MoS2 species as observed from the TEM results (see Figure 1). In the HDS and HDN activities shown in parts a and b, respectively, of Figure 3, the commercial NiMo maintained higher performance throughout with increasing temperatures. Relatively high conversions of nitrogen (96-99 wt %) were observed for the commercial catalyst. This is in agreement with the well-known efficacy of the catalyst for HDN and HDS.15 At higher temperature 380 °C similar HDS activities of approximately 100% conversion were observed for both NiW/Al2O3 and NiMo/Al2O3. Although it has been established from this study that NiMo/Al2O3 has a superior performance over NiW/Al2O3, it is difficult to identify the real factors responsible for the somewhat low performance of the NiW/Al2O3 since both catalysts are from different sources and the composition of the commercial NiMo is unknown, thus introducing many unquantified variations in the system. Hydrotreating of Light Gas Oils Using NiW/ Al2O3 Catalyst. Following the activity tests, three other LGO feeds were hydrotreated with NiW/Al2O3 to investigate the differences in the reactivity of these oils which are produced under different process conditions. The different feeds used were vacuum light gas oil (VLGO), atmospheric light gas oil (ALGO), and hydrocrack light gas oil (HLGO). ALGO, with a boiling range of 160393 °C, is produced under atmospheric pressure conditions. Bottoms from the atmospheric distillation unit are

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Figure 2. Comparison of AHYD activity of NiW and NiMo on γ-alumina support at a pressure of 11.0 MPa, LHSV of 0.6 h-1, and H2/oil ratio of 550 mL/mL.

sent to the vacuum distillation plant where VLGO (271-482 °C) is produced under vacuum conditions to lower the boiling temperature of the material, thereby allowing distillation without excessive decomposition. HLGO (163-404 °C), on the other hand, is produced under high-pressure conditions via catalytic hydrogenation of hydrogen. It has a very high hydrogen/carbon ratio (H/C) and is dominated by monoaromatics. Hydrotreating experiments were performed by varying the reaction temperature from 340 to 390 °C, at a pressure of 11.0 MPa, and LHSV of 0.6 h-1. Figure 4 shows the extent of conversion of aromatics when the LGO feeds are hydrotreated. Each point on the graph represents a fraction of aromatic carbons converted over total aromatic species originally present in the feed samples. Unlike VLGO, hydrogenation of the aromatic carbons in ALGO and HLGO are not affected by thermodynamic equilibrium effects. Thus AHYD in the latter feeds are said to be kinetically controlled, whereas AHYD in VLGO is affected by both kinetics and thermodynamic equilibrium effects.12 When a reaction is kinetically controlled, increasing temperature increases the rate of reaction, thus increasing conversion. However, with the onset of thermodynamic effects at higher reaction temperatures, there is a decrease in conversion owing to the shift in equilibrium. This effect is observed in Figure 4 for VLGO, where AHYD passes through a maximum of 83.0% at 365 °C. For the other two LGO feeds, maximum conversions of 71 and 67% were achieved at 390 °C in ALGO and HLGO, respectively. The order of conversion of total aromatic compounds in all feeds is found to be VLGO >ALGO > HLGO (Figure 4). This trend in conversion is unusual since VLGO is a heavier fraction, as it would contain more fused multi-ring aromatic compounds, which should be more difficult to hydrogenate. However, the observed trend is a result of the ease of hydrogenation of the higher order aromatic compounds which are more thermodynamically favored than hydrogenation of monoaromatics.3 Pretreatment of the hydrotreated light

gas oil (HLGO) fraction hydrogenates most of the higher order aromatic compounds to monoaromatics; hence the dominant aromatic group present in the feed is monoaromatics. These groups of aromatic compounds have a very stable resonance structure, making them the most difficult groups of aromatic hydrocarbons to hydrogenate (rate-limiting step).3,16 Consequently, severe hydrotreating conditions are required to completely saturate the aromatic rings. The HDS and HDN activities of NiW/Al2O3 in the three feeds are shown in parts a and b, respectively, of Figure 5. It is observed that increasing the temperature enhances both the HDS and HDN processes. Sulfur conversion ranges from as low as 86.9 wt % in ALGO to a high of 97.6 wt % in VLGO. Both the lowest and highest conversions of nitrogen-containing species (7299 wt %) were observed in VLGO. Comparing the HDS and HDN activities, the latter appeared to be higher in all cases. Simulated Distillation Studies. Since feed composition is one of the factors affecting product quality, selectivity, and yield,4 simulated distillation of the various LGO feeds were performed to determine the distribution of their boiling points in relation to the types and amounts of sulfur, nitrogen, and aromatic compounds contained in each feed. This analysis helps in the characterization of the feed and in further understanding their differences in hydrotreating reactivity. From the simulated distillation curves of the three feeds shown in Figure 6, it is obvious that VLGO contains the most complex and less reactive species followed by HLGO and ALGO. Differences in the distribution of the boiling temperature are mainly due to the sulfur content in the feed. Above 50 wt %, the distillation curves for both ALGO and HLGO are the same, which may be a result of the presence of similar sulfur species contained in both feeds.4,5 Effect of Temperature on Diesel Selectivity. The feed and products were grouped into three main cuts based on their boiling point distributions: gasoline, G (40-205 °C); diesel, D (206-343 °C); and the heavy


Figure 3. Comparison of NiW and NiMo activity for (a) HDS and (b) HDN at a pressure of 11.0 MPa, LHSV of 0.6 h-1, and H2/oil ratio of 550 mL/mL.

ends, H (343+ °C). Earlier studies5 have shown that temperature is the key operating variable influencing product distribution and selectivity. Thus, increasing temperature increases the conversion of heavier petroleum fractions to light fractions via hydrotreating and mild hydrocracking. Mild hydrocracking occurs at higher severities of hydrotreating. The combination of desulfurized, denitrogenated lighter products of lower overall aromaticity makes hydrocracking an ideal process for producing diesel fuel meeting more stringent fuel specifications.17 Since our focus is to reduce the aromatic contents in middle distillates (149-371 °C) suitable for diesel production, this section of the study looks at the effects of temperature on diesel selectivity in ALGO, HLGO, and VLGO. Diesel selectivity was defined as

selectivity )

desired fraction [wt %] × 100% undesired fractions [wt %] (2)

From Figure 7, which shows the effects of temperature on diesel selectivity, it is observed that selectivity

in both ALGO and HLGO increase with temperature. This means that, as temperature increases, more diesel fractions are produced. In the case of VLGO feed, which has a cut point of 288-436 °C (see Figure 5), diesel selectivity decreased with temperature. Diesel selectivity in VLGO is about 7 times higher than those of ALGO and HLGO at 340 °C, which then decreases to ∼1.0% at 390 °C. This is because at low severities of hydrotreating (340-350 °C) diesel is predominantly produced from the heavier fractions with very low conversion to the lighter fractions. As the temperature is increased, i.e., from 365 to 390 °C, more gasoline and naphtha fractions are produced from both the diesel and the heavier ends with a net decrease in the production of diesel fuel. Hence, low severities of HT of VLGO feed will result in the production of large quantities of diesel fuel compared to the other light gas oil feeds. Kinetic Studies. (a) Kinetics of AHYD. Knowledge of kinetics is necessary for selecting the appropriate catalyst for hydrotreating and optimizing process variables.3 Kinetic parameters for AHYD were derived using

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Figure 4. Extent of AHYD in ALGO, HLGO, and VLGO over NiW/γ-AL2O3 at a pressure of 11.0 MPa, LHSV, 0.6 h-1, and H2/oil ratio of 550 mL/mL. Table 3. Kinetic Parameters for AHYD, HDS, and HDN activation energy, E [kJ/mol]

feedstock

ko

ALGO HLGO VLGO (forward) VLGO (reverse)

58.6 90.6 52.8 80.0

AHYD 4 × 104 4.4 × 106 1.4 × 104 7.3 × 106

ALGO HLGO VLGO

53.4 29.3 44.2

HDS 7.3 × 104 7.3 × 10 5.4 × 102

ALGO HLGO VLGO

44.6 54.1 112.0

HDN 6.63 × 103 2.4 × 104 7.2 × 108

reaction order, n

k1 [h-1] R2

1.0 1.3 1.0 1.0

0.9951 0.9994 0.9996 0.9999

1.4 1.2 1.3

0.9931 0.9983 0.9978

1.0 1.0 1.0

Table 4. Intrinsic Overall Kinetic Parameters of Product Distribution in ALGO, HLGO, and VLGO Feedsa

0.9924 0.9903 0.9985

the power law reaction rate model and the representative reaction

A + nH2 / AH

(3)

Based on the concentration patterns discussed in the previous section, ALGO and HLGO concentrations were best described by the irreversible power law model in eq 4 while that of VLGO was obtained by assuming a pseudo-first-order reversible power law reaction rate model in eq 5.18

dCA ) kiCin dt

(4)

dCA ) kfPmCAn - kr(1 - CA) dt

(5)

-

Table 3 summarizes the kinetic parameters for AHYD in the three feeds. Data for the temperature dependence of AHYD determined from Arrhenius plots are also shown in Table 4. The energy requirements per mole of aromatic hydrocarbons, expressed in terms of the activation energy, varied between 53 and 125 kJ/mol. The highest activation energy occurred in HLGO. This is because of the presence of highly stable aromatic species such as monoaromatics owing to the pretreatment of the

a

k2 [h-1]

k3 [h-1]

340 350 365 380 390 E [kJ/mol] ln ko r2

Atmospheric Light Gas Oil (ALGO) 0.1199 0.0680 0.1390 0.0800 0.1900 0.1050 0.2700 0.1302 0.3501 0.1437 72.9 51.8 12.1 7.5 0.9900 0.9951

0.0023 0.0039 0.0080 0.0134 0.0179 139.1 21.3 0.9921

340 350 365 380 390 E [kJ/mol] ln ko r2

Hydrocrack Light Gas Oil (HLGO) 0.0794 0.0475 0.0813 0.0510 0.0865 0.0592 0.0942 0.0647 0.0933 0.0707 11.3 26.8 -0.33 2.21 0.9942 0.9953

0.0116 0.0117 0.0120 0.0121 0.0122 3.93 -3.69 0.9719

340 350 365 380 390 E [kJ/mol] ln ko r2

Vacuum Light Gas Oil (VLGO) 0.1922 0.0151 0.2861 0.0220 0.4016 0.0403 0.5855 0.0711 0.7965 0.1207 92.1 138.1 16.4 22.9 0.9939 0.9951

0.0704 0.0805 0.1255 0.1861 0.2446 86.7 14.3 0.9903

Pressure, 11.0 MPa; LHSV, 0.6 h-1; H2/oil ratio, 550 mL/mL.

feed. Hydrogenation of monoaromatics is a critical step in AHYD.3 Monoaromatics have a very stable resonance structure which makes them the most difficult group of aromatic hydrocarbons (HC) to hydrogenate (ratedetermining step) compared to higher order aromatic groups such as the di-, tri-, and polyaromatics.16 It can be inferred from the activation energy values that a greater proportion of the ALGO feed is made up of polyaromatics. Hence less energy is required to saturate the aromatic rings to cycloalkanes and straight-chain alkanes. (b) Kinetic Studies of HDS and HDN. The increasing demand on nitrogen and sulfur contents in diesel fuel has also forced the development of more active hydrotreating catalysts. HDS and HDN are among the


Figure 5. Effect of temperature on (a) HDS and (b) HDN at a pressure of 11.0 MPa, LHSV of 0.6 h-1, and H2/oil ratio of 550 mL/mL.

largest processes in petroleum refining to produce clean transportation fuels. Under typical hydrotreating conditions these reactions occur simultaneously and may influence each other by competitive adsorption.19 Thus, it is important to know more about the mechanism and kinetics of HDS and HDN processes and how these reactions influence each other. The intrinsic kinetic parameters shown in Table 3 were determined by fitting the experimental data to eq 3 and the Arrhenius plots. The reaction orders for HDS ranged from 1.2 to 1.4, while the kinetics of HDN was best described by a pseudo-first-order reaction mechanism. The lower reaction orders observed for the nitrogen compounds compared to that of the sulfur species clearly indicate that nitrogen adsorbs much stronger than sulfur, thus inhibiting HDS reactions. Several authors5,16,20 have noted that basic nitrogen-containing components are among the strongest inhibitors of HDS reactions. It has also been found that nitrogen com-

pounds inhibit the hydrogenation but not the desulfurization of sulfur compounds. However, presently there is no agreement about the active sites for the C-S and C-N bond cleavage reactions. It is even debated whether the catalytic sites facilitating C-S bond breaking are the same as those for C-N bond breaking.20 Activation energy for HDS ranged between 29 and 53 kJ/mol, contrary to the theoretical calculations by Neurock and Van Santen21 which predict the activation energy for sulfur removal to be 73 kJ/mol. It is also observed from Table 3 that the lowest activation energy and reaction order of 29.3 kJ/mol and 1.2, respectively, were realized for the HLGO feedstock. This is because of the pretreatment of the feed which removes any complex sulfur species which are less reactive. The activation energies for HDN ranged from 44 to 112 kJ/ mol, which are relatively higher than that of HDS. This is because nitrogen removal is observed to undergo two main stages: hydrogenation followed by cleavage of the

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Figure 6. Simulated distillation curves of VLGO, HLGO, and ALGO.

Figure 7. Diesel selectivity in VLGO, HLGO, and ALGO at a pressure of 11.0 MPa, LHSV of 0.6 h-1, and H2/oil ratio of 550 mL/mL.

C-N bond (hydrogenolysis).16 Hence more energy is required to actively remove the nitrogen species from the feed. From the analysis of the kinetics parameters, it is more difficult to hydrogenate aromatic species in the virgin light gas oil (ALGO) than HLGO due to the pretreatment of the latter feed. VLGO hydrogenation follows a reversible reaction owing to the presence of more complex aromatic species in the feed. The ease of sulfur and nitrogen removal follows the order HLGO > VLGO > ALGO and ALGO > HLGO > VLGO, respectively. (c) Overall Kinetics. Analysis of the overall kinetics of conversion of heavy species to lighter boiling fractions was based on the reaction pathway presented in Scheme 1.

Evaluation of the kinetics parameters for the product distribution was derived from nonlinear least squares approximation using pseudo-first-order reaction models (see eqs 6-14).

for species H: dCH ) -(k1 + k2)CH dt

(6)

CH ) CHOe-(k1+k2)t

(7)

dCD ) k1CH - k3CD dt

(8)

for species D:

CD )

Scheme 1. Reaction Pathway for Conversion between 340 and 390 °C

-k1CHOe-(k1+k2)t - Xk3 + Xk1 + Xk2 -k3 + k2 + k1

(9)

where

X)

e-k3t(k1CHO - CDOk3 + CDOk1 + CDOk2) -k3 + k2 + k1

(10)


for species G:

(

dCG ) k2CH + k3CD dt

(11)

CG ) k1k2CH - k3k1CH + k1YCHO + k2YCHO + k1YCDO + 2k2YCDO - k22CH + k2k3CH - k3YCDO k22 k 2 k3 YCDO + YCDO + Zk3k1 + Zk3k2 - Zk12 k1 k1

)

2Zk1k2 - Zk22 (-k3k1 - k3k2 + k12 + 2k1k2 + k22)-1 (12)

shaped nature of the concentration pattern. Kinetics of ALGO and VLGO were best described by pseudo-firstorder irreversible and reversible power law reaction mechanisms, respectively. Kinetic data of AHYD in HLGO were best described by a 1.3 power law reaction model. Simulated distillation curves of the LGO feeds showed that VLGO contains the complex and heaviest compounds followed by HLGO and ALGO. Overall kinetics of product in the LGO feeds was modeled using a pseudo-first-order power law reaction mechanism. Diesel selectivity in both ALGO and HLGO increased with temperature, but the opposite was observed for the VLGO feed. Selectivity in VLGO was about 7 times higher than that of the other feedstocks at 340 °C, which then decreased to ∼1.0 wt % at 390 °C.

where

Nomenclature

Z) k1CHO + CDOk1 + CDOk2 - k2CHO - CGOk1 - CGOk2 k1 + k 2 (13)

CD ) concentration of diesel [wt %] CG ) concentration of gasoline [wt %] CH ) concentration of heavy fractions [wt %] Ci ) fractional concentration of aromatics, sulfur, or nitrogen species H2S ) hydrogen sulfide gas kf ) forward rate constant ki ) reaction rate constant kr ) reverse rate constant m, n ) reaction order with respect to hydrogen pressure and aromatics content r2 ) regression coefficient t ) time [h]

and

Y ) k1e-k3t

(14)

Analysis of the variation of the rate constants with temperature in Table 4 shows the following: 1. The rate constants from VLGO follow the order k1 > k3 > k2. The relatively low k2 value is an indication that step II is the rate-limiting step. Hence the heavier cuts are preferentially converted to diesel with a major portion of gasoline being produced from the diesel fractions. 2. In the case of ALGO, variation of the rate constant was observed to occur in the order k1 > k2 > k3, with step III being the limiting reaction. Conversion of diesel to gasoline is only significant at higher severities of hydrotreating, i.e., 380-390 °C. 3. The rate of formation of diesel from the H-species in the HLGO feed is higher than the rate of production of gasoline. This is observed by the trend k1 > k2 > k3. 4. Considering the rate constant values for diesel production (k1,VLGO > k1,ALGO > k1,HLGO), VLGO feed gave the highest amount of diesel produced followed by ALGO and finally HLGO. Conclusions In summary, at lower severities of hydotreating, NiW on alumina activity for AHYD was observed to be higher than the commercial NiMo, which changed as the reaction temperature was raised. This was attributed to specific interactions of the Ni-W components of the catalyst with certain components of the feedstock at high temperatures, thus causing premature deactivation of the catalyst. NiMo activities for both HDS and HDN were higher throughout the temperature range studied. Comparing AHYD activity in the three feeds, lower rates of reaction (low conversion) were observed for HLGO due to the high proportion of monoaromatics present in the feed. The concentration pattern in AHYD for the various feeds showed an irreversible increase in reaction rate for both ALGO and HLGO. However, AHYD in VLGO was limited by thermodynamic equilibrium constraints, and this is shown by the dome-

Abbreviations AHYD ) aromatic hydrogenation ALGO ) atmospheric light gas oil D ) diesel fraction [wt %] G ) gasoline fraction [wt %] H ) heavy petroleum fractions [wt %] HDN ) hydrodenitrogenation HDS ) hydrodesulfurization HLGO ) hydrocrack light gas oil HT ) hydrotreated samples LGO ) light gas oil LGOB ) light gas oil blend LHSV ) liquid hourly space velocity VLGO ) vacuum light gas oil

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Received for review January 20, 2005 Revised manuscript received August 4, 2005 Accepted August 16, 2005 IE0500826

Experimental and Kinetic Studies of Aromatic Hydrogenation

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