Article pubs.acs.org/EF
Production of Gasoline and Diesel from Coal Tar via Its Catalytic Hydrogenation in Serial Fixed Beds Tao Kan, Xiaoyan Sun, Hongyan Wang, Chunshan Li,* and Usman Muhammad Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Clean liquid fuel was produced from the catalytic hydrogenation of coal tar using two serial fixed beds. Hydrofining catalyst of MoNi/γ-Al2O3 and hydrocracking catalyst of WNiP/γ-Al2O3-USY were filled in the first and second fixed beds, respectively. In the initial catalyst screening tests, the typical fixed experimental conditions were as follows: hydrogen pressure of 8 MPa, liquid hourly space velocity of 0.8 h−1, hydrogen-to-tar volume ratio of 1600, temperature in first fixed bed at 360 °C, and temperature in second fixed bed at 380 °C. Gasoline (≤180 °C) and diesel (180−360 °C) fractions were then separated from the effluent oil. Their fuel indexes were determined to assess the hydrogenation performance. The effect of pressure (6−10 MPa) on the hydrogenation performance was also investigated by keeping other experimental conditions constant. The catalysts showed good stability in activity in the test of catalyst life. The analysis results of the products indicated that raw coal tar could be promisingly upgraded by catalytic hydrogenation in the serial fixed beds. Nitrogen and sulfur contents were greatly reduced from 1.69 and 0.96 wt % in the feed to less than 10 and 50 ppm, respectively, in the products.
1. INTRODUCTION Concern on the petroleum depletion crisis and rising fuel prices is increasing. Hence, major efforts are being dedicated to the development of various usable energy sources. Abundant coal tar is produced every year by coal carbonization and gasification. 1 Coal tar is a complex dark brown mixture consisting of aliphatic, alicyclic, aromatic, and heterocyclic compounds, especially some extremely complicated PAH (polycyclic aromatic hydrocarbon) compounds with molecular weight up to several thousands.2 Around 500 kinds of organic compounds have been identified in coal tar.3 There are some obvious defects in the physical and chemical properties of coal tar, including high viscosity, thermal instability, corrosiveness, etc., which limit its application as an energy fuel for combustion. Therefore, an upgrading process for coal tar is required before its application.1 By hydrogenation, coal tar can be used as an alternative source of transportation fuel such as gasoline and diesel. Meanwhile, liquid fuel production is currently subject to stricter environmental standards than before.4 Thus, environmental and economic benefits will be definitely linked to the production of clean transportation fuel with ultralow heteroatom (such as S) content from hydrogenation of coal tar. However, the complexity of coal tar has driven researchers to mainly focus on a pure model compound such as phenanthrene,5 thiophene,6 and dibenzothiophene,7,8 rather than on a real fraction. Results about the conversion efficiency of the model compounds were extensively reported. The kinetics of hydrodesulfurization (HDS) was also well studied,9−11 and Bej et al. reviewed the recent studies aiming at searching for an efficient 4,6-DMDBT hydrodesulfurization catalyst.12 In the previous study,13 MoCo/γ-Al2O3 and WNi/γ-Al2O3 catalysts were used to process the coal tar material. Promising results including high sulfur and nitrogen removals were achieved. © 2012 American Chemical Society
However, it was found that some fuel indexes of products were still not so satisfactory. For example, the cetane number of the diesel product obtained under the hydrogen pressure of 8 MPa was at a low level of 42.4, which indicated that many polycyclic aromatic molecules still remained in the diesel product. This was also confirmed by the GC−MS analysis. In the present work, hydrofining catalyst of MoNi/γ-Al2O3 and hydrocracking catalyst of WNiP/γ-Al2O3−USY were installed in the first and second fixed beds, respectively, for the sake of enhancing the hydrocracking of the feedstock and obtaining high-quality fuel products.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characteration. Commercial γ−Al2O3 granules (20−40 mesh) were used as the support of hydrofining catalyst MoNi/γ-Al2O3. The support of hydrocracking catalyst WNiP/ γ-Al2O3−USY was first molded by a catalyst extruding machine using a mixture of USY molecular sieve, pseudoboehmite powder, sesbania cannabina powder, polyvinyl alcohol, HNO3 solution, etc., as the raw material. The extruded mixture was then calcined at 500 °C for 4 h, after which the support of hydrocracking catalyst was obtained. Both hydrofining and hydrocracking catalysts were prepared following the same procedures as described in a previous study,13 including the pretreatment of the catalyst support, ultrasonic-assisted incipient wetness impregnation, and temperature-programmed calcination. Ammonium molybdate [(NH 4 ) 6 Mo 7 O 24 ·4H 2 O], ammonium metatungstate hydrate [H40N10O41W12·xH2O], nickel nitrate [Ni(NO3)2·6H2O], and phosphoric acid were used as the precursors of active components. For the series of MoNi/γ-Al2O3 catalysts (named as HF-1−HF-4), the MoO3 content was designed to range from 10% to 25% with an interval of 5%, and the NiO content was fixed at 4%. As to the series of WNiP/γ-Al2O3−USY Received: March 12, 2012 Revised: May 24, 2012 Published: May 25, 2012 3604
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catalysts (named as HC-1−HC-4), the WO3 loadings were 13%, 18%, 23%, and 28%, and the NiO and P loadings were fixed at 4% and 3%, respectively. Before each run, fresh hydrofining catalyst (e.g., HF-1) and hydrocracking catalyst (e.g., HC-1) were filled in the first and second fixed beds, respectively. The amounts of various active species presenting on the catalysts were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, model: IRIS Intrepid II XSP, ThermoFisher Co., Ltd.). The Brunauer−Emmett−Teller (BET) surface areas and pore volumes of the catalysts were measured by an adsorption equipment (Micromeritics) using N2 gas. X-ray diffraction (XRD) was performed using a diffractometer (model: X’Pert PRO MPD, PANalytical Co., Ltd.) with Cu Kα radiation filtered by a graphic monochromator at 40 kV and 40 mA. High-resolution transmission electron microscopy (TEM, model: JEM-2100, JEOL Co., Ltd.) was also utilized to investigate the catalyst structure. 2.2. Reaction System and Product Analysis. The distillate (under 360 °C) of the coal tar was used as feedstock, the properties of which were listed in Table 1. The hydrogenation of the feedstock was
unit was composed of a preheater, a hydrofining reactor, and a hydrocracking reactor. The middle section of each fixed bed reactor was filled with 30 mL of catalyst. The product separation and collection unit included a water cooler, a gas−liquid separator, a lye washer, and so on. Before each run of coal tar hydrogenation, the catalysts were presulfurized using 2 wt % dimethyl disulfide in aviation kerosene. The liquid product was distilled into gasoline (360 °C) fractions. The gasoline and diesel yields were calculated from their respective weight divided by the weight of coal tar fed into the system. The gasoline and diesel fractions were then subjected to the following analyses: (i) determination of the distillation range by the Engler distillation method (standard: ASTM D86); (ii) density on DMA 5000 (Anton Paar, Austria); (iii) C and H elemental analyses using an Elementar VARIO ELIII (Germany), N and S analyses using a KY-3000SN (Jiangsu Jiangyan KEYUAN Electronic Instrument Co. Ltd., standards: ASTM D5453 and D4629); (iv) research octane number (RON) and antiknock index (AKI) determinations for gasoline; (v) cetane value and solidifying point for diesel; and (vi) detailed composition determination by capillary column gas chromatography− mass spectroscopy (GC−MS) analysis (model: Agilent 6890N with a 30 m × 0.25 mm × 0.25 μm HP-5MS capillary column).
Table 1. Properties of Coal Tar Fraction properties elemental analysis/wt % C H N S H/C molar ratio distillation range/°C IBP 10% 50% 90% density (20 °C)/(g mL−1)
value
3. RESULTS AND DISCUSSION 3.1. Catalyst Screening Tests. In this work, the dark brown raw material was successfully converted into transparent liquid fuel after the hydrogenation treatment. As compared to the raw material, the density of the gasoline and diesel was decreased from more than 1 g·mL−1 into about 0.8−0.9 g·mL−1, which indicated the upgrading of the feedstock. There are complex performance indicators for gasoline and diesel products, such as yields, S and N contents, distillation range, RON for gasoline, and cetane value for diesel. In this study, the main objective is to produce diesel as much as possible on the premise that the key indexes of diesel (such as S and N contents and cetane value) meet the requirements. Catalyst screening tests were performed under typical experimental conditions, that is, hydrogen pressure (PH2) of 8 MPa, liquid hourly space velocity (LHSV) of 0.8 h−1, hydrogen-to-coal tar volume ratio (H2/oil ratio) of 1600,
84.86 8.39 1.69 0.96 1.19 118 196 261 306 1.0078
carried out in a continuous two fixed-beds system. As shown in Figure 1, the entire reaction system was mainly made up of three units, that is, the reactant feeding unit, the hydrogenation unit, as well as the product separation and collection unit. The reactant feeding unit consisted of a tar supply line and a high-pressure hydrogen supply line. The hydrogenation
Figure 1. Flow diagram of coal tar hydrogenation process. 3605
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Table 2. Results of the Catalyst Sieving Step Ia hydrocracking catalyst in the second stage HC-1
HC-2
HC-3
HC-4
product properties
gasoline
diesel
gasoline
diesel
gasoline
diesel
gasoline
diesel
yield/wt % S and N analysis S (ppm) N (ppm) distillation range/°C IBP 10% 50% 90% RON AKI cetane value solidifying point/°C
26.9
64.1
28.2
62.3
29.0
61.4
29.5
61.4
157 177
162 124
94 56
81 39
54 40
47 21
44 18
49 21
77 103 133 217 93.6 90.0
167 200 255 323
82 110 133 215 92.4 87.6
168 213 265 335
75 100 130 210 93.0 88.8
170 208 260 317
75 105 127 198 91.8 87.2
175 205 257 310
52.4 5.1
54.6 8.5
57.7 5.1
58.9 2.6
Experimental conditions: Thf = 360 °C, Thc = 380 °C, PH2 = 8 MPa, LHSV = 0.8 h−1, and H2/oil ratio = 1600. Fresh HF-4 catalyst was constantly filled in the first hydrofining fixed-bed in each run.
a
Table 3. Results of the Catalyst Sieving Step IIa hydrofining catalyst in the first stage HF-1
HF-2
HF-3
HF-4
product properties
gasoline
diesel
gasoline
diesel
gasoline
diesel
gasoline
diesel
yield/wt % S and N analysis S (ppm) N (ppm) distillation range/°C IBP 10% 50% 90% RON AKI cetane value solidifying point/°C
15.1
69.8
23.5
64.5
26
65
29.5
61.4
70 56
50 42
52 31
44 18
49 21
169 204 269 328
86 100 122 201 92.7 88.1
170 211 270 332
75 105 127 198 91.8 87.2
175 205 257 310
126 179
142 183
102 54
71 95 133 218 96.4 91.6
170 207 275 346
83 103 125 214 92.4 87.8
52.4 12.5
58.0 5.1
57.9 5.1
58.9 2.6
a Experimental conditions: Thf = 360 °C, Thc = 380 °C, PH2 = 8 MPa, LHSV = 0.8 h−1, H2/oil ratio = 1600. Fresh HC-4 catalyst was constantly filled in the second hydrofining fixed-bed in each run.
hydrofining temperature in the first fixed bed (Thf) of 360 °C, and hydrocracking temperature in the second fixed bed (Thc) of 380 °C. The process of catalyst screening included the following two steps: (I) Hydrofining catalyst of HF-4 was always filled in the first fixed bed, and the hydrocracking catalyst in the second fixed bed was varied from HC-1 to HC-4 to determine the best hydrocracking catalyst in the second stage. (II) Similarly, the selected best hydrocracking catalyst was kept constantly in the second bed, and different hdyrofining catalysts in the first stage were tested. The results of the above steps are listed in Tables 2 and 3, respectively. As can be seen from Table 2, the gasoline yield gradually increased from 26.9 to 29.5 wt %, while the diesel yield showed the opposite trend when the hydrocraking catalyst was varied from HC-1 to HC-4. The total light oil (i.e., gasoline and diesel) yield was calculated to be around 90 wt %, which should be attributed to the facts that some atoms such as oxygen and sulfur in the feedstock were converted into H2O, H2S, NH3, etc., and a small fraction of residual heavy oil was not successfully hydrogenated into light oil.
Sulfur content is one of the most significant fuel indexes. In the current study, the sulfur contents in the gasoline and diesel products were greatly reduced to 157 and 162 ppm, respectively, as compared to the sulfur value of 0.96 wt % in the feedstock. Meanwhile, increasing the active metal content of the hydrocraking catalyst, more sulfur atoms could be removed from the oil products. Low sulfur contents of 44 and 49 ppm in gasoline and diesel, respectively, were achieved by applying HC-4 catalyst. On the other hand, the nitrogen contents underwent an even sharper drop to 18 and 21 ppm in gasoline and diesel, respectively, from 1.69 wt % in the feed by using HC-4 catalyst, which indicated that the reaction rate constant for hydrodenitrogenation (HDN) is obviously higher than the value for hydrodesulfurization. The recent investigations into the nature of the inhibition effects by means of scanning tunnelling microscopy (STM) and density functional theory (DFT) have provided new atomic-scale details of HDN and HDS reactions, revealing that the metallic-like brim sites located adjacent to the edges were involved in the reactions.14−16 3606
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Table 4. Effect of Pressure on Properties of Gasoline and Diesel Productsa effect of pressure on gasoline
a
effect of pressure on diesel
product properties
6 MPa
7 MPa
8 MPa
9 MPa
10 MPa
6 MPa
7 MPa
8 MPa
9 MPa
10 MPa
yield/wt % S and N analysis S (ppm) N (ppm) distillation range/oC IBP 10% 50% 90% RON AKI cetane value solidifying point/oC
22.3
24.8
29.5
29.6
30.8
66.0
64.8
61.4
61.3
60.8
92 66
44 18
40 12
39 13
113 146
85 79
49 21
49 16
43 9
78 112 177 220 93.0 89.1
75 105 127 198 91.8 87.2
76 105 134 191 92.1 87.3
75 104 130 185 90.9 86.8
178 195 270 340
175 201 263 332
175 205 257 310
171 207 255 312
162 200 271 324
115 89 80 115 198 235 93.3 89.8
51.5 11.6
56.2 8.5
58.9 2.6
59.4 2.6
58.6 3.5
Other experimental conditions: Thf = 360 °C, Thc = 380 °C, LHSV = 0.8 h−1, and H2/oil ratio = 1600.
from the feedstock as much as possible and simultaneously have the moderate ability of hydrocracking. 3.2. Effect of Pressure on Product Properties. The comprehensive reaction performance estimated by the yields and properties of products is greatly influenced by the significant reaction parameters including temperature, pressure, H2/oil ratio, LHSV, etc. Operation at very high temperatures is undesirable, and the catalyst should be operated at temperatures below 400 °C.19 In the following serial tests, the effect of hydrogen pressure (6−10 MPa at an interval of 1 MPa) on the reaction performance was investigated, while other experimental conditions such as Thf of 360 °C, Thc of 380 °C, H2/oil ratio of 1600, and LHSV of 0.8 h−1 were kept constant. The results with regard to different pressures are listed in Table 4. With increasing the pressure from 6 to 10 MPa, the gasoline yield was remarkably upgraded from 22.3 to 30.8 wt %, accompanied by the obvious decrease in diesel yield from 66.0 to 60.8 wt %. What is noticeable to mention is that the sulfur and nitrogen contents of the gasoline product were stepwise but obviously reduced from 115 and 89 ppm to 44 and 18 ppm, respectively, when increasing the pressure from 6 to 8 MPa, whereas the corresponding values underwent a slight decrease with raising the pressure from 8 to 10 MPa. A similar phenomenon was also observed for the diesel product. This indicated that the removals of sulfur and nitrogen atoms were not so sensitive to the pressure higher than 8 MPa, which might be ascribed to that the S and N atoms could be easily eliminated through hydrogenation in the early phase, but the unconverted heterocyclic compounds with complex structures were difficult to hydrogenate to get rid of S and N atoms in the later phase due to the steric hindrance.20 Besides, the contact probability of the heterocyclic compounds with the catalysts was probably reduced due to their lower concentrations, which may also account for the above phenomenon. Thus, very high hydrogen pressure is not so preferable for obtaining products with much better quality. Additionally, higher pressure will also result in higher operating cost and some security problems. The HDS reaction network proceeds by a mechanism of direct hydrogenolysis or the aromatic ring hydrogenation prior to sulfur removal, and the preferred reaction pathway for HDN reaction initially involves the saturation of the
Other essential fuel indexes such as distillation range, RON and AKI for gasoline, and cetane value and solidifying point for diesel were also presented in Table 2. For the gasoline product, the lowest RON value of 91.8 was obtained, while the second fixed bed was filled with HC-4. It could be explained as the fact that the lowest content of aromatic hydrocarbons was produced through HC-4 catalyst with the highest WO3 loading among HC-x (x = 1, 2, 3, and 4) catalysts, resulting in the lowest RON value. As to the cetane value index for diesel, n-hexadecane is assigned a cetane value of 100 for its short delay period during ignition, while α-methyl naphthalene is assigned 0.17 The cetane value order of hydrocarbons with the same value of carbon atoms is alkane > monocyclic alkane > polycyclic alkane > monocyclic aromatic > polycyclic aromatic.18 Thus, a gradual increase in cetane value could be observed as the catalyst was varied from HC-1 to HC-4. After the screening step I, the HC-4 catalyst was finally selected and hereafter always filled in the second fixed bed to perform the downstream reactions including hydrocracking, hydrogenation, etc., in the following runs. In step II, hydrofining catalysts ranging from HF-1 to HF-4 in the first fixed bed were employed to evaluate the effect of the active Mo content on the product properties, the results of which were depicted in Table 3. Being similar to step I, higher gasoline yield accompanied by the inverse trend of diesel yield was also observed as increasing the Mo content of the hydrofining catalyst. Meanwhile, the diminishing changing tendency of sulfur and nitrogen elements in gasoline and diesel products was also detected. An overall decay of RON and AKI indexes for gasoline product as well as solidifying point for diesel occurred. On the basis of the above results, it was obviously indicated that the products with higher quality were favored by the higher amounts of active metals for both hydrocracking and hydrofining catalysts in our investigation range. After the twosteps screening tests, the hydrofining catalyst (MoNi/γ-Al2O3) of HF-4 and the hydrocracking catalyst (WNiP/γ-Al2O3−USY) of HC-4 showed better performance than the other combinations of catalysts did. Thus, they were finally employed in the following experiment. However, the increase in metal loadings resulted in the transfer of diesel yield into gasoline yield. Therefore, further work is required to exploit more suitable catalysts that are able to remove S and N elements 3607
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Table 5. GC−MS Analysis of Gasoline Producta
aromatic ring carrying the heteroatom before C−N bond scission takes place.19 The removal of nitrogen compounds contained in coal-derived naphtha was also well investigated by Liaw et al.21 As can be seen from Table 4, other indexes such as RON and AKI for the gasoline product as well as cetane value and solidifying point for the diesel product were also profoundly influenced by the hydrogen pressure as expected. At the same time, the H/C molar ratio for gasoline and diesel products increased with hydrogen pressure, as shown in Figure 2.
molar tR content (min) (%) formula
Figure 2. Effects of hydrogen pressure on H/C molar ratios of gasoline and diesel products. Other experimental conditions: Thf = 360 °C, Thc = 380 °C, LHSV = 0.8 h−1, and H2/oil ratio = 1600.
The H/C molar ratio rose from 1.90 at 6 MPa to 1.92 at 8 MPa, which was followed by an increase to 1.93 when raising the hydrogen pressure from 8 to 10 MPa. The values for the diesel were 1.61, 1.72, and 1.74 at the three pressures, respectively. For both gasoline and diesel, the H/C molar ratio was upgraded when the pressure was varied from 6 to 8 MPa, the increasing tendency of which, however, slowed when further increasing the pressure. 3.3. Components in Gasoline and Diesel Products. The detailed compositions of the gasoline and diesel products were finally analyzed by GC−MS. For the gasoline product, about 200 peaks were successfully detected, a majority of which were composed of substituted cyclohexanes and aromatic organics such as varieties of naphthalenes. The main components of the gasoline product are listed in Table 5. Substituted cyclohexanes were the most abundant materials, such as methyl-, ethyl-, cis-1-ethyl-3-methyl-, and propyl-cyclohexanes, with respective molar contents of 3.066%, 5.397%, 3.820%, and 3.421%. Among these materials, ethyl-cyclohexane was the most dominant one. trans-2-Methyl-1,1′-bicyclohexyl and 1,2,3,4-tetrahydronaphthalene with high fractions of 3.197% and 3.382% were also the main products in the gasoline. These substituted cyclohexanes may originate from the corresponding benzenes with different branched chains, and the naphthalenes may result from the thermal decomposition and hydrocracking of polycyclic aromatics. For the diesel oil, more than 250 peaks corresponding to a large number of components with carbon numbers ranging from 7 to 24 appear in the GC−MS graph. Substituted and partially saturated naphthalenes such as 1,2,3,4-tetrahydro2,6-dimethyl-naphthalene as well as saturated ring-containing hydrocarbons such as tetradecahydro-anthracene mainly constituted the diesel oil. 1,2,3,4-Tetrahydro-6-methyl-naphthalene was the most abundant component in the diesel product with a
1.172 1.557 2.134 2.396 2.534 2.957 3.111 3.758 3.996 4.62 4.735 5.328 5.812 6.42 7.521 8.236 8.875 9.544 9.991 11.914 12.692 12.815
0.75 3.066 2.987 1.152 2.499 1.07 5.397 2.261 2.602 3.820 0.781 2.814 0.824 3.421 0.813 2.118 0.784 3.197 0.89 2.414 1.744 1.135
C6H12 C7H14 C8H16 C8H16 C8H16 C8H16 C8H16 C8H10 C8H10 C9H18 C8H10 C9H18 C9H18 C9H18 C9H16 C9H12 C9H16 C13H24 C9H12 C9H10 C10H18 C10H18
13.884 14.115 14.2 14.946 16.832 18.563 20.094 20.856 21.626
0.995 1.154 0.836 0.935 3.382 0.725 1.595 0.943 0.737
C10H12 C10H12 C11H20 C11H20 C10H12 C11H14 C11H14 C11H14 C12H16
molecular weight
compound name cyclohexane cyclohexane, methylcyclohexane, 1,3-dimethyl-, ciscyclohexane, 1,2-dimethyl-, transcyclohexane, 1,4-dimethyl-, transcyclohexane, 1,2-dimethyl-, ciscyclohexane, ethylEethylbenzene p-xylene cis-1-ethyl-3-methyl-cyclohexane o-xylene cyclohexane, 1-ethyl-4-methyl-, transcyclohexane, (1-methylethyl)cyclohexane, propyl1H-indene, octahydro-, transbenzene, 1-ethyl-3-methyl1-methylcyclooctene 1,1′-bicyclohexyl, 2-methyl-, transbenzene, 1,2,4-trimethylindane naphthalene, decahydrocyclohexane, 1-methyl-3-(1methylethenyl)-, cis2,4-dimethylstyrene benzene, 1-ethenyl-4-ethyltrans-decalin, 2-methylnaphthalene, decahydro-2-methylnaphthalene, 1,2,3,4-tetrahydrobenzene, cyclopentylnaphthalene, 1,2,3,4-tetrahydro-6-methylnaphthalene, 1,2,3,4-tetrahydro-5-methyl naphthalene, 1,2,3,4-tetrahydro-2,7dimethyl-
84 98 112 112 112 112 112 106 106 126 106 126 126 126 124 120 124 180 120 118 138 138 132 132 152 152 132 146 146 146 160
Experimental conditions: Thf = 360 °C, Thc = 380 °C, PH2 = 10 MPa, LHSV = 0.8 h−1, and H2/oil ratio = 1600. a
molar content of 2.401%. Besides, large amounts of branched and straight-chain paraffins, for example, 2,6,10-trimethyl-tetradecane (C17H36), heptadecane (C17H36), nonadecane (C19H40), and heneicosane (C21H44), were also identified. 3.4. Characteristics of Catalysts and Process Mass Balance. The characteristics of the preferential HF-4 and HC-4 catalysts in the first and second fixed beds, respectively, were investigated. As presented in Table 6, both the HF-4 and the Table 6. Catalyst Characterization composition (wt %) catalyst
Mo
HF-4 HC-4
16.0
W
Ni
P
BET area (m2/g)
pore vol (cm3/g)
20.9
3.1 3.0
2.8
232 248
0.57 0.48
HC-4 catalysts possessed a BET surface area of more than 230 m2/g and a pore volume of around 0.5 cm3/g, which were the indications of potential high hydrogenation activity. In the XRD analyses, for both HF-4 and HC-4 catalysts, only broad XRD lines of the supports (not shown here) were exhibited, and no obvious special peaks other than the supports were detected, indicating 3608
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Figure 3. TEM micrographs of catalysts: (A) fresh HF-4 and (B) fresh HC-4.
Table 7. Mass Balances (per day) over the Reaction Systema effluents (g) materials fed (g)
a
liquid
solid
gas
summary
tar
H2
oil
water
coke in reactor
sulfur in reactor
H2
hydrocarbons
CO
CO2
others
580.5
82.8
553.8
40.1
2.9
2.3
52.6
13.4
1.7
1.2
11.6
materials fed (g) 663.3 error: 2.4%
effluents (g) 679.6
Experimental conditions: Thf = 360 °C, Thc = 380 °C, PH2 = 10 MPa, LHSV = 0.8 h−1, and H2/oil ratio = 1600.
were well homogenized on the respective supports for HF-4 and HC-4 catalysts, which was consistent with the above conclusion from the XRD results. Catalyst lifetime is a significant factor for the industrialization of coal tar hydrogenation. In this work, a catalyst duration time of 150 h was tested, during which the product yields as well as the sulfur and nitrogen contents of gasoline and diesel products were selected as the indexes. As shown in Figure 4A and B, after a runtime of 150 h, the values of the indexes did not show an obvious decrease. Mass balance (per day) over the reaction system during the run at 10 MPa is also exhibited in Table 7. A part of fed hydrogen was consumed by reacting with coal tar, and several gas species such as hydrocarbons, CO, CO2, NH3, H2S, etc., were produced at the same time. According to the calculation, there is a small acceptable error of 2.4% between the masses of the materials fed and the effluents. 3.5. Comparison between This Work and Previous Research. Table 8 lists some previous researches relating to the hydrogenation of coal tar or its fractions, including feedstock, reaction conditions, catalysts used, main results, and the data source. Very limited works were performed on the production transportation fuels via hydrotreating of coal tar. Creosote oil as a coal tar fraction has been utilized to produce gasoline and diesel. However, very high hydrogen pressure (usually >10 MPa, even as high as 25.5 MPa) was imposed on the feedstock to achieve better hydrogenation performance. Some fuel indexes such as RON and cetane value still did not meet the expectation. In this work, the distillate (under 360 °C) of the coal tar used as feedstock differed from the materials listed above, which makes this work distinct from other previous works. Meanwhile, a relatively low hydrogen pressure of 6 MPa was attempted, and good hydrogen performance was successfully achieved.
Figure 4. Analysis of catalyst life: (A) gasoline indexes with time; (B) diesel indexes with time. Experimental conditions: Thf = 360 °C, Thc = 380 °C, PH2 = 8 MPa, LHSV = 0.8 h−1, and H2/oil ratio = 1600.
that for the HF-4 and HC-4 catalysts, the metal oxides were highly dispersed over the supports. The above results are well accordant with those of a previous study.22 The TEM micrographs of the fresh HF-4 and HC-4 catalysts were described in Figure 3A and B, respectively. It could be observed from the images that the active metals and promoters 3609
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coal-derived products mesophase coal tar pitch
■
AUTHOR INFORMATION
Corresponding Author
*Tel./fax: +86-10-82547800. E-mail:
[email protected].
Calculated from the data in the literature. bMesophase coal tar pitch was hydrogenated through electrochemical technology.
ref 28 ref 29b product distribution (toluene, p-cresol, xylenes, etc.) was analyzed hydrogen content and H/C ratio were increased 7.6
10 wt % in feedstock 15.2
380 (first stage) and 420 (second stage) 275−450 coal liquid heavy distillate
4. CONCLUSION Hydrofining (MoNi/γ-Al2O3) and hydrocracking (WNiP/γAl2O3−USY) catalysts with different metal loadings were prepared, which were filled in the first and second fixed beds, respectively, where hydrogenation of coal tar distillate was performed. After preliminary catalyst screening tests, optimum combination of catalysts was finally determined. The effect of pressure on the hydrogenation performance was also investigated. XRD and TEM characterizations as well as the lifetime test of the catalysts showed promising results. Gasoline fraction with low sulfur and nitrogen contents of 39 and 13 ppm and diesel fraction with 43 and 9 ppm were obtained, as compared to the respective values of 0.96 and 1.69 wt % in the feedstock. GC−MS analyses revealed that the gasoline and diesel products mainly comprised substituted cyclohexanes, substituted and partially saturated naphthalenes, and saturated paraffins. As compared to other previous studies, a relatively lower hydrogen pressure of 6 MPa was successfully applied to obtain good-quality products.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful to the National Natural Science Foundation of China (No. 21006113) and the National Basic Research Program of China (973 Program No. 2009CB219900).
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a
ref 27
NiMo/Al2O3 (first stage) and Co−Mo/Al2O3 (second stage) NiMo/Al2O3) or CoMo/Al2O3
ref 25 ref 26 tungsten sulfide NiMo/Al2O3 2000 0.59 1.5 wt % in feedstock 25.5 13 420 340 creosote oil 20 wt % of anthracene oil in toluene
ref 19
naphtha yield (200 °C) = 60 wt %, CN = 37 gasoline yield = 120 vol %, moter octane number = 68 stable 9,l0-dihydroanthracene and 1,2,3,4tetrahydroanthracene concentrations at 30 and 45 mmol/L, respectively denitrogenation of 96% and good paraffin cracking of 61% CoMo/γ-Al2O3 2000 15 430
0.9
NiMo/γ-Al2O3 1500 0.5 17.5
13.8 450
380
creosote S 6400 ppm, N 15 400 ppm medium creosote
0.8−2.1a 10.3 350−500
coal tar S 0.8456 wt %, N 0.6438 wt % yallourn brown coal tar
ref 19
ref 24
oil yield, 83 wt %;a boiling range, 98% at 450 °C; molecular weight, 227−237; oxygen content, 0.3−2.3 wt % diesel CN = 48, sulfur in diesel, 27 ppm; nitrogen in diesel, 64 ppm about 500−1250a
NiMo/Al2O3
ref 23
Article
gasoline yield, about 25−70 vol %; diesel yield, about 10−55 vol %; sulfur removal, >65%; nitrogen removal, >30% NiW/SiO2−Al2O3
H2/oil ratio LHSV pressure/ MPa T/oC material
reaction conditions
Table 8. Previous Research Relating to Hydrogenation of Coal Tar or Its Fractions
catalysts used
main results
data source
Energy & Fuels
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