Article pubs.acs.org/EF
The Influence of Metal Deposits on Residue Hydrodemetallization Catalysts in the Absence and Presence of Coke Yanzi Jia,* Qinghe Yang, Shuling Sun, Hong Nie, and Dadong Li Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China ABSTRACT: Spent hydrodemetallization (HDM) catalysts were collected from pilot-scale reactors with different time-onstream, and they were treated with an extraction or regeneration method for further investigation. The fresh, spent, and regenerated samples were characterized systematically. Their catalytic performances were assessed using model molecules and Kuwait atmospheric residue as the reactants. Compared with conventional NiMo/Al2O3 catalysts, deposited metal sulfides had a notable increase in their HDM activity but an obvious decrease in their naphathelene hydrogenation activity. With short contact time, metal deposits accelerated residue HDM activity, and they were also involved in the loss of hydrodesulfurization (HDS) activity. At the initial stage of operation, HDM activity was reduced significantly due to the formation of coke. However, coke and metal deposits together contributed to the loss of HDS activity and the rapid fall of long period on-stream HDM activity. Meanwhile, batch scale reactors or fixed-bed microreactors were employed with a short contact time (less than 80 h) to quantitatively identify the cause of deactivation by coke and metal deposits.8,9 Usually, metals are progressively deposited throughout the hydrotreating process, which means catalysts with a short operation time cannot reflect the effect of coke and metal deposits on the whole process. Accordingly, the investigation of samples withdrawn from an industrial or a pilot reactor can provide information of great value. The objective of this study was to determine the influence of metal deposits on commercial HDM catalysts under pilot plant conditions with or without coke. Spent HDM catalysts with different time-on-stream (TOS) were collected from several pilot units and then treated with the extraction or regeneration method for further investigation. These catalysts represent typical phases of catalyst deactivation in commercial practice. The samples were characterized systematically using advanced techniques. At the same time, their catalytic performances were assessed using model molecules and Kuwait AR as the reactant. The composite influence of coke and metal deposits was studied by a comparison between the fresh and spent catalysts. The effect of metal deposits on the performance of HDM catalysts was also elucidated by comparing the fresh catalysts with the regenerated ones. Finally, the impact of coke on the autoactivity of metal deposits was identified according to the relationship between the spent and regenerated HDM samples.
1. INTRODUCTION To satisfy the specifications imposed by regulations and the increased demand for middle distillates, the main objectives of residue upgrading processes include hydrodemetallization (HDM), hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and Conradson carbon reduction (HDCCR). Unlike HDS and HDN, HDM generates deposits of metal sulfides (mainly nickel and vanadium sulfides) on the catalysts, leading to an irreversible deactivation and recycling of the HDM catalysts as a knotty problem. Besides metal deposits, coke also contributes to the deactivation of HDM catalysts. All of these result in a short cycle length of the HDM catalysts (usually for one year). It is widely accepted that the metal deposits deactivate HDM catalyst activity due to the covering of the active sites or the pore plugging of the catalyst. However, some researchers claimed that these deposited sulfides can also act as catalysts for hydrotreating reactions.1,2 Metal deposits are of significant importance in catalytic performance because the catalysts can accumulate large amounts of vanadium and nickel during residue hydroprocessing. Consequently, a better understanding of metal deposits, the coke, and their composite effect could define options to make the most of the metal deposits’ autoactivity, extend HDM catalyst life, and thus bring enormous economic benefit. Ideally, catalysts should be studied under industrially relevant conditions. However, it is difficult to mimic these conditions in laboratory experiments because the operation of HDM catalysts takes a very long time. Because of limited access to these industrial HDM catalysts, researchers3−7 frequently employ model compounds, such as nickel-tetraphenylporphin or vanadyl-tetraphenylporphin, to simulate the metal deposited course. The problems of the model compound lie in two aspects: (1) ruling out the influence of coke, and (2) ignoring the impact of vanadium’s U-shape distribution along the radius on the diffusion gradient of the residue molecules. Because of its higher reaction rate, the vanadium tends to concentrate at the surface of the catalyst, whereas the nickel distributes more evenly throughout the catalyst pellet. © XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Pretreatment of the Samples. The HDM catalysts A (MoO3 = 7.5 wt %, NiO = 1.3 wt %) and B (MoO3 = 5.0 wt %, NiO = 1.0 wt %) were selected as the reference catalysts. Both catalysts A and B were specially developed HDM catalysts for pretreatment of residual oils. Received: July 9, 2015 Revised: February 19, 2016
A
DOI: 10.1021/acs.energyfuels.5b01553 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
V3+, V4+, and V5+ belong to the V 2p3/2 peak area of the corresponding valence. On the basis of the fitting results of Ni 2p3/2 XPS spectra, the INiMoS/INiS was defined as
The atmospheric and vacuum residues (AR and VR, respectively) from Iran were processed in a continuous hydroprocessing unit provided with a trickle-bed reactor, respectively. Table 1 lists all of the
Table 1. Origins of Differently Aged NiMo/Al2O3 HDM Catalysts
INiMoS/INiS = NiMoS/NiS
catalyst
A
A
B
TOS/h temperature/°C pressure/MPa LHSV/h−1 H2/oil/(NL/L) feedstock Ni in feedstock/μg·g−1 V in feedstock/μg·g−1
100 380 14 0.2 700 Iran AR 29.4 92.0
4000 380 14 0.2 700 Iran AR 29.4 92.0
800 380 14 0.2 700 Iran VR 59.6 142.0
The transmission electron microscopy (TEM) images of the aforementioned sulfided catalysts were recorded on a TECNAI G2 F20 S-TWIN microscope, FEI. At least 20 micrographs were taken for each sample, and the average slab length and stacking number were determined by manually measuring all of the slabs per sample. The average slab length (Laverage) and stacking layer number (Naverage) of (Ni)MoS2 were calculated according to eqs 5 and 6.
∑ li/n
(5)
Naverage =
∑ niNi/n
(6)
X naphathelene = (w0 − we)/w0
(7)
Yi = wi /w0
(8)
where w0 and we denote the mass fractions of naphthalene in the feedstock and products, respectively, and wi is the mass fraction of component i in the products. Residue hydrotreating experiments were carried out in a 500 mL batch reactor operating under the fixed-bed reaction conditions. The feed used was a Kuwait AR with sulfur content of 5.0 wt %, Ni content of 26.5 μg/g, and V content of 80.0 μg/g. The experiments were carried out at a hydrogen pressure of 8.0 MPa, a reaction temperature of 380 °C, and a reaction time of 2 h under a stirring rate of 200 r/ min. A catalyst weight of 16.0 g was used to treat 250.0 g of feed. The catalyst was presulfided ex situ for 10 h at a pressure of 3.2 MPa and a temperature of 320 °C with the sulfiding feed composed of 5 wt % CS2 in kerosene solvent. The sulfided catalyst was transferred into the reactor in nitrogen atmosphere very quickly, preventing the contact of catalyst with air for a long period. Sulfur contents of the feed and effluents were measured by X-ray fluorescence analysis, and metals (Ni, V) were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). The removal rates of sulfur, nickel, and
(1)
Ni sulfidation = (NiS + NiMoS)/(NiS + NiMoS + NiO + Ni satellites)
Laverage =
where li is the slab length, Ni is the stacking layer number of a MoS2 nano slab, ni is the number of slabs in Ni layers, and n is the total number of slabs in the statistical regions. For the elemental composition of the selected area to be determined further, the TEM bright field was transferred to the high-angular annular dark field (HAADF) mode on a TECNAI G2 F20 S-TWIN microscope, FEI. The scanning transmission electron microscopy (STEM) images of the spent catalysts were recorded, and semiquantitative estimation was investigated by energy dispersive X-ray (EDX) analysis. 2.3. Catalytic Activity Assessment. The naphthalene HYD activity assessment was carried out in a continuous flow fixed-bed microreactor. The tests were operated at 4.0 MPa and 300 °C with the catalyst loading equating to 1.0 g. A decane solution containing 1.0 wt % naphthalene was used as the model feed, and the experiments were conducted at a feed flow rate of 0.2 mL/min and a H2 flow rate of 200 mL/min. For each run, the weighed catalyst was diluted with quartz powder, and the solid mixture was placed in a stainless steel reactor (8.0 mm in inner diameter). Prior to catalytic reaction tests, the catalysts were sulfided in situ with a mixture consisting of carbon disulfide (CS2, 5 wt %) and cyclohexane (95 wt %). The sulfidation was conducted at a total pressure of 4.0 MPa and a temperature of 360 °C for 3 h. Under the experimental conditions, naphthalene was hydrogenated to trans- and cis-decaline via the partially hydrogenated intermediate tetralin.10 The liquid products of the HYD reaction were analyzed by a gas chromatograph using an HP-1 column heated from 50 to 130 °C and a flame ionization detector at 350 °C. The naphathelene conversion Xnaphathelene, as well as yields of tetralin and decaline Yi, can be calculated according to the definitions
experimental conditions for catalysts A and B. The spent catalysts maintained at 100, 800, and 4000 h TOS withdrawn from the pilotscale reactors contained some residual oil from the process. They were toluene extracted in Soxhlet for 12 h to remove the residual oil (with the prefix S-). Each spent catalyst was subgrouped into two parts: one was directly used for characterization and activity assessment, and the other was regenerated at 430 °C for 4 h (with the prefix R-). To be clear, S-A-t represents oil-free spent catalyst A, R-A-t stands for regeneration of the spent catalyst A, and t is the TOS of the catalysts. 2.2. Characterizations. The BET specific surface area, pore volume, and pore size distribution of fresh, spent, and regenerated catalysts were measured by nitrogen adsorption at −196 °C (using the Quantachrome AUTOSORB-6B surface area and pore size analyzer). The cross-sectional profile of metal deposits in the spent catalysts was measured by a scanning electron microscope (SEM, FEI Quanta 200F). The XRD patterns were collected on a Philips X’Pert diffractometer equipped with a secondary graphite monochromator operating at 40 kV and 30 mA and employing nickel-filtered Cu Kα radiation (λ = 0.154 nm). Carbon and sulfur contents of the spent catalysts were determined using an IR detector attached to a CS-600 LECO analyzer. The thermogravimetric differential thermal analysis (TG-DTA) system TA-SDT Q600 (TA, USA) was adjusted to operate at the following conditions: dynamic atmosphere at 100 mL/min and heating rate of 10 °C/min from 50 to 800 °C. The metal contents of the spent catalysts were determined on a 3271E X-ray fluorescence (XRF) analyzer (Rigaku Industrial Corporation). The X-ray photoelectron spectroscopy (XPS) spectra of the sulfided catalysts were recorded on a Thermo Fischer-VG ESCALAB 250 spectrometer. The C 1s contamination peak at 284.8 eV was used for binding energy calibration. For the sulfided catalyst, the sample preparation was carried out under a controlled atmosphere (Ar with O2 and H2O content being less than 15 μg/g). The above-used sulfide catalysts were freshly prepared according to the same ex situ sulfidation procedures as used by the residue hydrotreating test in the following section. The XPS spectra of Mo, Ni, and V species were fitted. According to the deconvolution results, the sulfidation degree of Mo, Ni, and V species in catalysts, denoted as Mosulfidation, Nisulfidation, and Vsulfidation, were defined by eqs 1−3, respectively. Mosulfidation = Mo 4 + /(Mo 4 + + Mo5 + + Mo6 +)
(4)
(2)
Vsulfidation = (V2 + + V3 + + V 4 +)/(V2 + + V3 + + V 4 + + V 5 +) (3) where Mo4+, Mo5+, and Mo6+ represent the Mo 3d peak area of Mo4+ (MoS2), Mo5+ (MoOxSy), and Mo6+ (MoO3), respectively. NiS, NiMoS, NiO, and Ni satellites stand for the Ni 2p3/2 peak area of Ni2+ (NiS), Ni2+ (NiMoS), Ni2+ (NiO), and Ni satellites, respectively. V2+, B
DOI: 10.1021/acs.energyfuels.5b01553 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels vanadium, which can reflect the HDS and HDM activities of catalyst samples, were given by removal rates of sulfur = (R S,F − R S,P)/R S,F
(9)
removal rates of nickel = (RNi,F − RNi,P)/RNi,F
(10)
removal rates of vanadium = (R V,F − R V,P)/R V,F
(11)
where RS,F, RNi,F, and RV,F are the mass fraction of sulfur, nickel, and vanadium in the feeds, respectively, and RS,P, RNi,P and RV,P denote the mass fraction of sulfur, nickel, and vanadium in the products, respectively.
3. RESULTS 3.1. Textural Properties of Fresh, Spent, and Regenerated Catalysts. Table 2 presents the physical
Figure 2. Pore size distribution of fresh, spent, and regenerated catalyst B.
is a major contributor to deactivation during the initial phase of residue hydroprocessing.9 For the samples exposed to 800 and 4000 h on-stream, after regeneration, their pore volumes and average pore diameters were still much lower than those of the fresh catalysts, indicating pore plugging introduced by metal buildup for long periods on-stream. In Table 2, for long period on-stream samples (800 and 4000 h), pore volumes increased accordingly upon regeneration, whereas surface areas were still lower than the spent catalysts. Figures 1 and 2 show that for S-A-4000h and S-B-800h, their pore volumes with diameters below 7 nm fell rapidly after regeneration. When coke was eliminated by burning, the disappearance of this surface resulted in the loss of surface area. Therefore, such pores could be ascribed to the void accumulated by deposited metal species bound or isolated with coke during the operation.12 3.2. Deposition of C, S, and Metals on the Spent Catalysts. Table 3 displays the contents of carbon, sulfur, and metals, referring to 100 g of fresh catalyst for the samples maintained at different TOS.
Table 2. Pore Structures of the Catalysts sample
SBET/m2 g−1
Vg/mL g−1
average pore diameter/nm
A S-A-100h R-A-100h S-A-4000h R-A-4000h B S-B-800h R-B-800h
136 123 125 84 58 150 124 103
0.63 0.37 0.57 0.24 0.27 0.90 0.46 0.51
18.4 12.1 18.2 11.3 18.6 24.1 14.9 19.8
properties of the fresh, spent, and regenerated catalysts. The specific surface area and pore volume of catalyst B was larger than that of A. It clearly shows that the specific surface area and pore volume of the spent catalysts was reduced with respect to the corresponding fresh catalyst. Meanwhile, the loss of the physical properties increased for long periods on-stream. Figures 1 and 2 show the pore size distributions of the samples. In Figure 1, the centroid of the pore size distributions
Table 3. C, S, Ni, and V Content for Samples at Different TOS sample
C/wt %
S/wt %
Ni/wt %
V/wt %
S-A-100h S-A-4000h S-B-800h
10.03 10.71 14.80
4.39 15.42 6.26
1.74 7.71 4.11
2.13 30.38 14.94
Increase of sulfur (S) content (in Table 3) with the TOS and with the metal (Ni and V) content indicates that the metals had been deposited on the catalyst in sulfide form because their equilibrium constants were favored under the typical hydrotreating conditions.13 The TG analyses for S-A-100h, S-A-4000h, and S-B-800h are shown in Figure 3. In TG analysis, two types of coke were defined when reactivity toward oxidation was used as criteria for classification: reactive, reversible or soft and refractory, or irreversible or hard.14−17 The temperature range between 350 and 425 °C was related to the combustion of the carbonaceous compounds that can be transformed at relatively low temperature (soft coke), and the zone located at 570 and 665 °C contained the oxidation of hard coke.18 As illustrated from the TG curve in Figure 3, upon increasing TOS, the ratio between these two types of coke shifted toward refractory coke.19
Figure 1. Pore size distribution of fresh, spent, and regenerated catalyst A.
moved from 15.7 nm for fresh catalyst A to around 10.2 nm for spent sample S-A-4000h. In Figure 2, the same tendency was also observed for fresh catalyst B and spent sample S-B-800h. This indicates pore blocking mainly due to coke and metal deposits.11 Figure 1 also shows that the pore size distribution of R-A100h almost overlapped with fresh sample A, which suggests that the pore structure was regained upon regeneration of catalyst A at 100 h on-stream. These results confirm that the change of pore structure is mainly due to coke formation, which C
DOI: 10.1021/acs.energyfuels.5b01553 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 4. XRD patterns of the spent samples: △, NiV2S4; ○, V3S4; □, VS2; and ★, V2S3.
(JCPDS), the above-mentioned crystalline phases can be attributed to vanadium sulfides that incorporated nickel into their structure with a stoichiometry close to Ni(1−x)V(2+x)S4(0 ≤ x ≤ 1). Some representative examples could be NiV2S4 (peaks at 2θ = 17.4, 35, 45, 46, and 54.8 deg; JCPDS ID: 36-1132), V3S4 (peaks at 2θ = 17, 34.7, 34.8, 44.4, and 45 deg; JCPDS ID: 83-0864), VS2 (peaks at 2θ = 15.4, 35.8, 45.2, 57.1, and 58.3 deg; JCPDS ID: 36-1139), and V2S3 (peaks at 2θ = 15.4, 34.5, 34.9, 44.3, and 44.9 deg; JCPDS ID: 19-1407). Similar findings were also reported in the literature.2,11 In Figure 4, Ni3S2 phase (JCPDS ID: 02-0772) reported by Rana et al.20 was not observed, which may be associated with the small crystalline size (
