Article pubs.acs.org/EF
Exploratory Investigation for the Coking Behavior during Slurry-bed Hydrocracking of Coal Tar Atmospheric Residue Wenan Deng,*,† Juntao Du,*,† Chuan Li,† Lele Wu,‡ Zailong Zhang,† and Ruilong Guo† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Huadong), Qingdao, Shandong 266580, P.R.C. CNOOC EnerTech-Safety & Environmental Protection Co., Tianjin 300456, China
‡
ABSTRACT: The coking behavior of coal tar atmospheric residue (CTAR) during slurry-bed hydrocracking was studied. The CTAR hydrocracking was performed in the autoclave at 430 °C with H2 pressure of 13 MPa, comparing with that of the Merey atmospheric residue (MRAR). The properties of C7-asphaltene and toluene-insoluble in feedstocks and products were analyzed respectively to describe the CTAR coking behavior during slurry-bed hydrocracking. The experimental results showed that CTAR with higher asphaltene content (30 wt%) produced less amount of coke than MRAR with asphaltene content (9.29 wt%), and little coke on the inner surface of reactor was found. The toluene-insoluble in the feedstock contained the larger carbonaceous particles (about 7 μm) and the inorganic fine particles (about 1 μm). Both sulfurized catalyst particles and inorganic fine particles in CTAR provided independent condensation nuclear or growth nuclear for macromolecular radicals during slurry-bed hydrocracking. These particles promoted the dispersion of spherical coke precursor and inhibited the coalescence of coke precursor. The larger carbonaceous particles in CTAR carried spherical coke particles as coke-carrier.
1. INTRODUCTION
dispersion of nucleation center and the inhibition of mesophase precursor coalescence.21−23 It was generally believed that the slurry-bed hydrocracking reactions followed the free radical mechanism. The macromolecular radicals (maltene and asphaltene) could carry out condensation and self-aggregation, which were the immediate cause of coke precursor formation.24−26 Meanwhile, the sulfided catalysts particles were highly dispersed into the oil, showing good hydrogenation activity in promoting hydrogenation. The catalyst particles were accumulated at the interface between the oil phase and the coke precursor, then the coalescence of the coke precursors were inhibited. Finally, the coke precursors encapsulated the catalysts to form catalystcoke mixtures, the mixtures were dispersed in the oil phase.1,27−30 Most of research focused on petroleum residual oil in slurrybed hydrocracking, whereas few studies involved the coke formation of CTAR. Considering the obvious difference of properties between coal tar and petroleum residual oil, these studies investigated the coking features of CTAR during slurrybed hydrocracking, and revealed the coking behavior by analyzing the properties of C7-asphaltene (CTAR-asp) and toluene-insoluble in feedstocks and products.
Hydroconversion has been the efficient path for converting inferior heavy oil to high-value distillates oil, but the coke formation inside the reactor has been proved to be an inevitable problem, which led to equipment fouling, catalyst deactivation, and operation burden.1,2 So far, as to the mechanism of coke formation, the researchers have not reached a consensus. Low/middle temperature coal tar was an inferior oil produced by coal pyrolysis and was an alternative fuel.3−7 Especially, the coal tar atmospheric residue (>350 °C, CTAR) accounted for about 50 wt% in coal tar and produced serious operational troubles in conventional hydrotreating, due to the worst properties, such as high metal content, high asphaltene content, high ash content etc.3,5 But, in hydroconversion processes, slurry-bed hydrocracking could achieve high conversion for heavy oil, and it had fewer limitations in feedstock.8−11 Therefore, the slurry-bed hydrocracking was a good choice to deal with CTAR. But the operation cycle was restricted by coke formation in reactor,1,12 coke could be classified into the coke in the liquid phase (cokeliq) and the coke on the reactor surface (cokesur), and the cokesur formation was a key factor in slurry-bed hydrocracking.7,13,14 The researchers had a common understanding that heavy oil was a kind of relatively stable colloidal system, which resulted from the relative balance of all sorts of macromoleculars. When the balance was changed, the process of coke formation could be described as a scheme: maltene → asphaltene → sediment → coke.15,16 It was worth mentioning that the phase separation was about Wiehe model for coke formation, which was caused by the aggregation and flocculation of asphaltenes because of incompatibility.17,18 Asphaltene played a key role in the initial stage of sediment formation, which was caused by asphaltene incompatibility in the heavy oils.19,20 In addition, fine particles inhibited coke formation in the thermal cracking because of the © XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. The medium/low temperature coal tar atmospheric residue (CTAR, > 350 °C) was obtained from the North of Shaanxi Province, and the petroleum-based crude oil Merey atmospheric residue (MRAR) was used as comparison. Their properties are shown in Table 1. Received: July 21, 2016 Revised: September 21, 2016
A
DOI: 10.1021/acs.energyfuels.6b01707 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
2.4. Analysis of Asphaltene and Coke. Elemental analyses were performed in a VARIO EL (Elementar, Ltd., Germany). The relative molecular mass was determined by vapor pressure osmometry (VPO) Knauer tester. The 1H NMR spectra of samples were obtained on a Bruker AVANCE 2500 spectrometer with a spectral width of 500 MHz, CDCl3 as a solvent, and tetramethy silane (TMS) as internal standard. Crystal structures of samples were detected by XRD, with a D8-ADVANCE X-ray diffraction (Bruker in Germany), using Cu Kα radiation, scanning range of 5° to 75°, and the slope angle was 0.05°. The functional group structures of samples were collected with a NEXUS-FT IR spectrometer. The configurations of samples were observed by SEM, with a JSM-7500F scanning electron microscope.
Table 1. Properties of CTAR and MRAR
a
parameters
CTAR
MRAR
density(20 °C)/g·mL−1 condensation point/°C TI/wt% w(carbon residue)/wt% w(ash content)/wt% Ni/μg·g−1 V/μg·g−1 Fe/μg·g−1 Ca/μg·g−1 C/wt% H/wt% S/wt% N/wt % O/wt% H/C saturate/wt% aromatics/wt% resin/wt% C7−asphaltene/wt%
1.021 > 50 2.84 17.96 0.14 27.7 28.2 393.1 226.4 84.67 7.84 0.22 0.87 6.40 1.11 14.57 35.62 20.35 29.46
0.9976 22 −a 15.33 0.10 65.2 443.0 21.2 − 84.82 10.87 2.89 0.63 0.79 1.54 31.49 39.44 12.06 9.29
3. RESULTS AND DISCUSSION 3.1. Hydrocracking of CTAR. On the basis of results presented in Table 1, compared with petroleum-based MRAR, the H/C atomic ratio of CTAR was lower, and the contents of carbon residue, ash, C7-asphaltene (CTAR-asp), and tolueneinsoluble (CTAR-TI) in CTAR were considerably higher, especially the content of CTAR-asp was up to 30 wt%. It might be speculated that CTAR was richer in coke precursor than in MRAR. The distribution of products of CTAR or MRAR during slurry-bed hydrocracking were shown in Table 2. As the catalyst content increased, the yield of total amounts of coke decreased, because sulfurized catalyst significantly inhibited coke formation. Although the contents of CTAR-asp and CTAR-TI in CTAR were considerably higher than that of MRAR, the VR conversion and the yield of middle distillate production in CTAR hydrocracking were better than those of MRAR, and the total amounts of coke in CTAR and in MRAR were approximate after hydrocracking. Moreover, the content of CTAR-cokesur was only 0.01−0.5 wt% and was far less than that of the content of MRAR-cokesur (0.12−3.01 wt%). The content of CTAR-cokeliq was far more than that of CTAR-cokesur, which was close to the total amount coke in CTAR slurry-bed reaction. The total amounts of coke in CTAR products were less than the content of CTAR-TI in the feedstock. It could be assumed that the behavior of coking during CTAR slurry-bed hydrocracking might be associated with the composition and structure of CTAR-asp and CTAR-TI, which must be different from the petroleum-based MRAR. 3.2. Characterization of CTAR-asp. The element composition of asphaltenes were shown in Table 3. Compared with MRAR asphaltene (MRAR-asp), the content of O and N in CTAR-asp was higher, and the H/C atom ratio (0.86) of CTAR-asp was lower.33,34 The results showed that CTAR-asp had more polycyclic aromatic hydrocarbon than petroleumbase MRAR-asp, which might be the potential coke precursor in hydrocracking.35 The CTAR-asp and the MRAR-asp were analyzed by the 1H NMR, the structural parameters were calculated by the improved Brown-Ladner method, as shown in Table 4. Both the contents of HA and Hα in CTAR-asp accounted for 80% in the total hydrogen, which were more than that of MRAR-asp, but the content of Hβ, Hγ was lower. The relative molecular mass of CTAR-asp was 462, which was far less than that of petroleum asphaltene.36 And the average molecular formulas of CTAR-asp and MRAR-asp were C31.40H26.80S0.05N0.51O3.06 and C133.35H144.70S2.99N0.01O3.97, respectively. It could be speculated that the number of carbon atoms in CTAR-asp alkyl side chain was generally within three and its aromatic sheet unit was smaller than the petroleum asphaltene. Compared with MRARasp, the fA of CTAR-asp was higher, f N/f P, σ, and RN was lower.
“−” means “not detected”.
2.2. Catalyst. The mixture of molybdenum naphthenate and nickel naphthenate was used as catalyst precursor during slurry-bed hydrocracking, as the specific metal mass ratio of Mo to Ni.31,32 2.3. Hydrocracking Reaction. Figure 1 shows the operation procedure of hydrocracking experiments. The slurry-bed hydro-
Figure 1. Scheme of the operation procedure of hydrocracking experiments. cracking reactions were carried out in the 500 mL autoclave with a mixer at 420−440 °C, under H2 initial pressure of 9 MPa (equivalently reaction 13 MPa) for 1 h. The autoclave was loaded with 150 g CTAR or MRAR, with 0−400 μg·g−1 catalysts expressed as metal mass and a certain amount of sulfur, and was mixed at 750 rad/min for 1 h before the reaction. The reactor was cooled by cold water rapidly after reaction. The coke on the inner surface of autoclave was washed with toluene and collected to get cokesur. The naphtha (500 °C) were separated from liquid products by atmospheric and vacuum distillation. The tolueneinsoluble of VR was defined as cokeliq. And the toluene-insoluble was obtained from the feedstocks by toluene extraction, then the asphaltene was obtained from the toluene-soluble of feedstocks by nheptane extraction, Finally, three parallel hydrocracking experiments were conducted and the average results are given in Table 2. B
DOI: 10.1021/acs.energyfuels.6b01707 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 2. Product Distribution of CTAR and MRAR Hydrocracking in the Same Reaction Conditions product distribution/wt% feed
temperature/°C
catalyst/μg·g−1
gas
naphtha
diesel
VGO
VR
cokesur/wt%
cokeliq/wt%
total coke/wt%
CTAR
430
MRAR
420 440 430
0 100 200 300 200 200 0 100 200 300 200 200
9.12 9.65 7.20 7.31 5.50 7.69 8.09 9.42 5.22 5.56 4.28 10.25
22.03 6.40 8.42 11.18 7.30 10.94 17.59 15.88 11.27 11.37 7.91 16.80
27.89 36.03 38.17 34.39 33.27 37.34 30.27 25.17 31.15 32.18 29.61 31.57
26.93 35.04 35.75 36.57 43.78 34.86 20.65 25.11 28.6 27.65 30.31 20.11
8.92 10.68 9.15 9.31 10.15 9.16 16.19 21.61 22.66 22.41 27.20 17.72
1.35 0.50 0.01 0.02 0 0.02 5.78 2.56 0.46 0.12 0.12 3.01
3.76 1.70 1.30 1.22 1.01 1.37 1.43 0.25 0.64 0.71 0.57 0.55
5.11 2.20 1.31 1.24 1.01 1.39 7.21 2.81 1.10 0.83 0.69 3.55
420 440
CTAR-asp was less than MRAR-asp in the slurry bed hydrocracking. Therefore, it concluded that the macromolecular radicals of CTAR-asp were not easily aggregated to form coke, this was the reason why the higher asphaltenes content of CTAR produced less coke than that of MRAR in the hydrocracking processing. 3.3. Properties Analysis of CTAR-TI and Cokeliq. The element composition of CTAR-TI in feedstock is shown in Table 5. Compared with CTAR-asp, the content of O and N of CTAR-TI was higher, the H/C atom ratio (0.78) of CTAR-TI was lower, and the ash and metal content was higher. The element composition of CTAR-TI was almost equal to the element composition of coal in the reported literature.37 The results showed that CTAR-TI in feedstock contained the carbonaceous particles and inorganic material, and CTAR-TI was likely to be the fragments of coal particles. The calculated parameters of XRD pattern for CTAR-asp, CTAR-TI, CTAR-cokeliq, MRAR-asp, and MRAR-coke are listed in Table 6.38 The crystalline parameters of CTAR-TI in
Table 3. Elemental Analysis of CTAR-asp and MRAR-asp elemental
CTAR-asp
MRAR-asp
C/wt% H/wt% S/wt% N/wt% O (by difference)/wt% H/C (atomic ratio)
81.67 5.85 0.33 1.54 10.61 0.86
82.13 7.48 4.90 2.23 3.26 1.09
Table 4. Average Molecular Structural Parameters of CTARasp and MRAR-asp parameter
CTAR-asp
MRAR-asp
HA (atomic %) Hα (atomic %) Hβ (atomic %) Hγ (atomic %) M (relative molecular mass VPO) fA (aromatic carbon weight ratio) f N (naphthenic carbon weight ratio) f P (alkyl carbon weight ratio) RT (total rings per average molecule) RA (aromatic rings per average molecule) RN (naphthenic rings per average molecule) CA (aromatic carbons per average molecule) CN (naphthenic carbons per average molecule) CP (alkyl carbons per average molecule) CS (saturated carbons per average molecule) HA/CA (aromatic rings condensation degree) σ (aromatic rings substitution degree)
39.5 40.1 16.8 3.6 462 0.74 0.17 0.08 7.31 5.35 1.96 23.39 5.38 2.63 8.01 0.69 0.44
8.6 22.6 49.3 19.5 1950 0.53 0.17 0.33 28.63 20.87 7.76 66.62 23.28 43.34 66.62 0.42 0.59
Table 6. Crystalline Parameters Derived from XRD Patterna parameters
dm/Å
dγ/Å
La/Å
Lc/Å
M
NOar
fa
CTAR-asp CTAR-TI CTAR-cokeliq MRAR-asp MRAR-cokeliq
3.7 3.6 3.5 3.6 3.5
4.6 4.4 4.3 4.5 4.4
2.7 3.8 3.2 6.4 3.5
3.2 3.6 5.3 4.5 6.6
1.9 2.0 2.5 2.4 2.9
1.1 1.4 1.2 2.5 1.3
0.72 0.88 0.83 0.55 0.79
a
dm, the average interaromatic layer distance. dγ, the distance between the aliphatic chains and naphthenic sheets. La, the average diameter of the aromatic sheet. Lc, the average height of the stack of the aromatic sheet. M, the number of aromatic sheets per stacked cluster. NOar, the number of aromatic rings in per aromatic sheet. fa, the aromaticity of molecules.
These data showed that the CTAR-asp aromatic ring ratio was higher and alkyl substituent scale was less, aromatic condensation degree was lower and resulted in the intermolecular association which was weaker than petroleumbase MRAR-asp. Moreover, the C−C bond of short aromatic side chain in CTAR-asp was not easily broken compared to the long alkyl side chain in MRAR-asp. These meant that the possibility of polycyclic aromatic free radical generated from
Table 6 were similar to coal in the reported literature.39,40 It further illustrated that CTAR-TI contained carbonaceous particles (polycyclic aromatic hydrocarbons with a small amount of alkane side) and inorganic minerals. The La and Lc of CTAR-cokeliq were less than MRAR-cokeliq, this suggested
Table 5. Properties Analysis of CTAR-TI in Feedstock element analysis/wt%
metal analysis/μg·g−1
ash/wt%
C
H
S
N
O
H/C
77.29
5.04
0.44
1.63
15.60
0.78 C
15.72
Fe
Ca
Na
Mg
410.3
832.5
526.3
449.9
DOI: 10.1021/acs.energyfuels.6b01707 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels CTAR-cokeliq had smaller aromatic sheet size and stack height than those of MRAR-cokeliq. The analysis of IR spectra were used to further determine the relation between CTAR-cokeliq and CTAR-TI during slurry-bed hydrocracking, as shown in Figure 2.41 It found that the
Figure 4. SEM photos of CTAR-cokeliq (reaction conditions: 430 °C, 200 μg·g−1, 9 MPa, 1h) (a) magnified 5000 times; (b) magnified 35 000 times.
kind was larger particle (about 7 μm) in Figure 3a including carbonaceous particles, minerals particles and coal particles, and the other kind was smaller particle (about 1 μm) in Figure 3b including organic particles, inorganic or carbonaceous microcrystalline particles.42,43 Figure 4 showed the dispersion of cokeliq in the product system. Compared with the loose distribution of larger particles in Figure 3a, there were the dispersed and dense distribution of the smaller and homogeneous particles in Figure 4a. In addition, the magnified 35 000 times SEM photos for cokeliq surface in Figure 4b showed that a large number of small spherical particles accumulated on the surface of these big particles. The results could be speculated that the larger particles decomposed again, and at the same time sulfurized catalyst inhibited these larger particles getting together. In addition, the macromolecular free radicals or coke precursor were absorbed on the surface of microcrystalline particles or catalyst particles, generating small spherical coke (in Figure 4b). These particles inhibited the growth of coke, because of the dispersion of nucleation center and the inhibition of mesophase precursor coalescence.23,44 Figure 5 showed the SEM photos of cokeliq from the slurrybed hydrocracking or thermal hydrocracking of CTAR and MRAR under the same reaction conditions. As shown in Figure 5a, it was clear that the stacking structure of CTAR-cokeliq in catalytic hydrocracking was accumulated by about
