Article pubs.acs.org/EF
Characterization of Hydrotreated Decant Oils. Effect of Different Severities of Hydrotreating on Decant Oil Chemical Composition Maria M. Escallon,* Dania A. Fonseca, and Harold H. Schobert EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania, 16802, United States ABSTRACT: Different severities of hydrotreatment were performed on a raw decant oil and one of its subsequent hydrotreated decant oils. Six hydrotreated decant oils with different chemical composition were obtained during this process. The products obtained from the raw decant oil showed difficulty in sulfur removal. Higher desulfurization was obtained after repassing the first drum of product (EI-132) at a reduced feed rate (LHSV 0.5h−1), higher temperature (734 °F and 750 °F), and higher reactor pressure (8.3 MPa). Several analyses were performed to characterize the decant oils: elemental analysis, 1H and 13C NMR, asphaltene content, average boiling point, API gravity, GC/MS, and viscosity. Two structural parameters derived from the 1H NMR spectra and elemental analyses were calculated: aromaticity (fa) and the fraction of aromatic edge carbons carrying substituents (σ). All the information gathered from the different techniques was helpful to understand the chemical transformations taking place during hydrotreatment of decant oils. It was observed that hydrogenation of aromatic rings and hydrodealkylation were the dominant reactions in the severely hydrotreated decant oils (EI-137 and EI-138) while the formation of alkyl side chains was observed in the low and intermediate hydrotreated decant oils, (EI-133 and EI-134) and (EI-135 and EI136), respectively.
1. INTRODUCTION Decant oil (DO), a heavy product obtained from the fluid catalytic cracking (FCC) process, is considered as the preferred raw material in the production of needle coke for manufacturing graphite electrodes. Due to their chemical complexity, the characterization of decant oils, and petroleum resids in general, is a challenge that has to be addressed in order to determine whether the DO has the proper chemical and physical characteristics to be used in the production of a premium coke (needle coke). High aromatic content, low sulfur levels, and low asphaltene content are the preferred characteristics that a starting material should have in order to obtain a premium coke.1 Some of the desirable characteristics listed by Goval et al.1 are API gravity values between 0 and 10.0; sulfur and nitrogen content from 0.0 to 0.7 wt %; and aromatic content between 50.0 and 80.0 wt %. However, the scarcity of starting materials fitting those requirements has led to efforts to modify the chemical composition, for example, by subjecting the feedstock to hydrotreatment to decrease the sulfur levels.2 Mochida et al.2 proposed hydrogenation, hydrocracking, or pyrolytic treatments as ways to modify the chemical structure of starting materials. Some examples include hydrogenation of coal tar,3 hydrotreating of solvent refined coal,4 and hydrotreating or hydrogenation of fluid catalytically cracked decant oil (FCC-DO).5 A hydrotreatment process was carried out for the purpose of modifying the chemical structure of the decant oils is used in this study. Very little information is reported in the literature related to the use of multiple analytical techniques to establish correlations of structural parameters and physical properties in decant oils. One work combined nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, and elemental analysis results to determine structural parameters of a decant oil sample.6 Linear correlations between the aromatic indexes obtained from NMR and FTIR as well as between this © 2012 American Chemical Society
parameter with the C/H ratios obtained from elemental analysis were found.6 In this study, an untreated decant oil and six hydrotreated samples were characterized by elemental analysis, 1H and 13C NMR, asphaltene content, average boiling point, API gravity, gas chromatography/mass spectrometry (GC/MS), and viscosity. Two structural parameters derived from 1H NMR and elemental analysis were calculated: aromaticity, fa, and fraction of aromatic edge carbons carrying substituents, σ. Correlations between the results from the different techniques are discussed. These correlations provide insight as to the possible chemical reactions that the decant oils experience when exposed to different levels of hydrotreating.
2. MATERIALS AND METHODS 2.1. Hydrotreatment Experiments. The fresh decant oil was provided by United Refining Co., Warren, PA. This decant oil went through two stages of settling, where the settling agent aided with the solids settling. The decant oil was shipped from United Refinery to Intertek-PARC, Harmaville, PA, for hydrotreating. The reactor used for the experiment was PARC’s adiabatic hydrotreatment pilot unit, P67. The configuration used for this study, P67, consisted of two down flow reactors operated in series. The no. 1 reactor had multiple catalyst beds separated by glass wool and quartz chips, while the no. 2 reactor, which was a single bed, had glass wool and quartz at the inlet and outlet. Hydrogen was recycled after amine scrubbing to remove H2S, and the purity was maintained at 95 to 98%.7 The catalyst used for the hydrotreatment of the decant oils was Criterion NiMo Syncat-137 preimpregnated with a sulfur compound. A commercial diesel oil containing 0.25% sulfur as dimethyl disulfide Received: August 2, 2012 Revised: November 9, 2012 Published: November 15, 2012 478
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in addition to the naturally occurring sulfur in the base diesel oil (about 300 ppm) was used as the catalyst activation feedstock.7 A summary of the hydrotreating operation conditions used for each run is shown in Table 1. The run feed was set at 5500 g/h (about 1
LHSV), and the inlet hydrogen rate at 2400 scf/bbl. The liquid hourly space velocity (LHSV) was obtained by using the feed weight gathered per hour and converting it to volume using the feed density in g/mL. The feed density and the catalyst weights and volumes were entered into the computer at the beginning of the each run. The decant oil was processed for 10 days.7 The raw decant oil was processed at four different conditions, and then, the first drum of product was processed at three different conditions, representing a total of seven levels of severity. A summary of the different conditions of hydrotreatment is shown in Scheme 1. The reactor temperature and feed rate (LHSV) were varied in the first four runs (EI-132 to EI-135); however, they did not meet the desulfurization target of 95%. Therefore, the first drum of product (EI-132 or Drum 1) was repassed using different conditions. The decant oil EI-136 was obtained by reducing the feed rate to 0.5 h−1, but the desulfurization level was still 88%. Then, for decant oils EI-137 and EI-138, the reactor pressure and temperature were increased while keeping the feed rate. Finally, decant oils EI-137 and EI-138 reached the desulfurization level of 99%. A lower reactor temperature was used in EI-138, compared to EI-137, in order to attempt to produce a product with a lower desulfurization level (less than 88% as obtained for EI-136). However, EI-138 maintained 99% desulfurization in the limited time remaining. 2.2. Decant Oil Analyses. A LECO 600 CHN analyzer was used to measure total carbon, hydrogen, and nitrogen contents in the decant oil samples. The total sulfur content was obtained using a LECO SC 132 analyzer. The asphaltene content in the decant oils was
Table 1. Hydrotreating Operating Conditions Summary Catalyst: Criterion Syncat-37 charge reactor 1 reactor 2 total R1+R2 feed ID EI-107 EI-107 EI-107 EI-107 EI-132 EI-132 EI-132
mL g 2148 2474 2656 3060 4804 5534 run or product drum feed g/h, inlet H2 rate drum ID no. 8 h avg (SCFB) EI-132 1 5490 2500 EI-133 2 5274 2600 EI-134 3 4009 3450 EI-135 4 3956 3400 Started Second Pass of Drum No. 1 EI-136 5 2691 4600 EI-137 6 2745 3000 EI-138 7 2745 3100
H2 consum. (SCFB) 750 908 1100 1150 1167 1268 1300
Scheme 1. Summary of the Different Conditions of Hydrotreatment Performed on the Raw Decant Oil and on Its First Drum of Producta
a
*HDS level; **HDN level 479
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Table 2. Elemental Composition and Characteristics of the Decant Oil Samplesa derived from EI-107
a
derived from EI-132
analysis
EI-107
EI-133
EI-134
EI-135
EI-136
EI-137
EI-138
API gravity asphaltene content, wt % viscosity, centipoises Ultimate Analysis, wt % C ± 0.27 H ± 0.13 N ± 0.08 S ± 0.01 H/C (mol/mol)
−4.1 0.21 904.8
0.3 0.07 300.5
1.4 0.08 161.1
1.8 0.15 151.7
2.8 0.16 101.3
6.1 0.23 42.0
6.4 0.26 40.9
89.59 7.32 0.22 2.99 0.98
90.09 8.40 0.18 1.39 1.12
89.93 8.98 0.24 0.94 1.20
90.8 8.71 0.17 0.44 1.15
90.23 8.98 0.50 0.33 1.19
90.02 10.00 0.10 0.03 1.33
90.59 9.24 0.12 0.02 1.22
± is the reproducibility of a measurement. Calculations are reported elsewhere28.
quantified following the ASTM designation D 3279-07.8 The calculations were made based on the heptane-insoluble fraction. The API gravity was determined using eq 1.15 The density (ρ) was calculated following the ASTM designation D 1217-93,9 and the specific gravity (δ) was corrected from the observed temperature (room temperature) to 60 °F using standard tabulated corrections.10
API =
141.5 − 131.5 δ60 ° F
were injected into a GC-17A Shimadzu gas chromatograph coupled with a QP-5000 mass spectrometer. A XTi-5 column from Restek (30 m × 0.25 mm × 0.25 μm) was used for separation of the different compounds. The oven was set at 40 °C, held for 4 min, and then heated to 180 °C at a rate of 20 °C/min. It was then heated to 310 °C at a rate of 4 °C/min and kept at this temperature for 10 min. The injector and detector temperatures were set at 290 °C, the split ratio was 15:1, and the column flow was kept at 1 mL/min using helium as carrier gas. Select ion chromatograms (SIC) were generated following the procedure outlined in ASTM D2425.14 The peaks in the chromatogram were identified by comparing the spectra with the NIST 107 mass spectral library, and the area percentage was calculated. The liquid-state NMR analyses were carried out with a Bruker AMX 360 NMR operating at 9.4 T and 70° tip angle. Deuterated chloroform, CDCl3, was used as a solvent. Samples were dissolved in a 1/1 volume ratio in CDCl3 containing 1 vol % of tetramethylsilane (TMS) as an internal standard. The different parameters used for 1H NMR were TD, time domain 16 384; NS, number of acquisitions 256; D1, recycle delay 5 s, and P1, pulse width 5 μs. The different proton types in the 1H NMR spectrum were defined by the chemical shift ranges (relative to TMS) reported in Table 5.15,16 The distribution of protons was determined by 1H NMR by dividing the integrated signal corresponding to the specific group (Hx) by the total integrated signal (H, 0.7−9.2 ppm), as shown in eq 5.
(1)
where δ60°F is the specific gravity at 60 °F corresponding to each different decant oil. The viscosity was determined following the ASTM designation D 88-07.11 The results are expressed in seconds; therefore, they have to be converted to absolute viscosity (centipoises) using eq 2.
⎡ 149.7 ⎤ μ = δT⎢0.219t − ⎥ ⎣ t ⎦
(2)
where μ = liquid viscosity, centipoises; δT = specific gravity of liquid at temperature of liquid; and t = time, seconds. An estimate of δT can be made using eq 3,
δ T = δ T0[1 + α(T0 − T )]
(3)
where δT0 = specific gravity of liquid measured at T0 used as a reference point and α = coefficient of thermal expansion of petroleum products with respect to T0. For crude oil, α = 0.0009/°C (if T0 and T are in °C). T0 is the temperature of measurement or room temperature, and T is 30 °C. The simulated distillation gas chromatography (SimDist GC) technique was used to determine the boiling point distributions of the decant oils, following the ASTM designation D 2887-06a.12 A Hewlett-Packard HP 5890 GC Series II model fitted with a flame ionization detector (FID) and a carborane siloxane polymer column (10 m × 0.53 mm) from Restek was used for these analyses. Approximately 0.03−0.04 g of decant oil was diluted with 1.2−1.5 g of carbon disulfide. The samples were then placed in the HewlettPackard automatic injection tray, and 1.0 μL was injected into the oncolumn injector. The oven temperature was kept for 4 min at 40 °C; then, the temperature was raised to 325 °C at a rate of 10 °C/min and held at that temperature for 15 min. The SimDis Expert 6.3 software was used to process the data. It provided the yield by weight percentage of any given fraction of decant oils. The average boiling point (ABP) was calculated according to eq 4, which is based in a modified procedure reported by Kegler.13
T + T2 + T3 + T4 + T5 ABP = 1 5
Hx * =
Hx H
(5)
where x = AR, α, β, γ Aromaticity, fa, was calculated using the Brown and Ladner method,17 which has been frequently used for petroleum oils. This method is stated as follows:16
⎡ ⎛ C ⎞ ⎛ H * ⎞ ⎛ Hβ* ⎞ ⎛ H γ * ⎞⎤ ⎛ C ⎞ ⎟−⎜ ⎟⎥/⎜ ⎟ fa = ⎢⎜ ⎟ − ⎜ α ⎟ − ⎜⎜ ⎢⎣⎝ H ⎠ ⎝ x ⎠ ⎝ yi ⎟⎠ ⎜⎝ yii ⎟⎠⎥⎦ ⎝ H ⎠
(6)
where x = 2; yi = 2; yii = 3. These values have been reported elsewhere.16 The degree of substitution of aromatic rings (σ) is defined as that fraction of the aromatic edge carbons that is substituted. Then,16 Hα * x
( )+( ) σ= ( )+( )+H Hα * x
O H
O H
AR *
(7)
13
The parameters used for C NMR were the following: TD 65,536; NS 102.4; D1 2 s; and P1 8 μs. The carbon peak assignments shown in Table 6 followed those reported by Rodriguez et al.18
(4)
where: T1, T2, T3, T4, and T5 represent the temperatures where 10, 30, 50, 70, and 90 wt % of the sample was distilled, respectively. For GC/MS analysis, no pretreatment was necessary because the asphaltene content was very low, so the GC/MS column was not in jeopardy. Around 0.02−0.03 g of each sample was diluted in approximately 1 g of dichloromethane, and 1.0 μL of these solutions
3. RESULTS 3.1. Elemental Analysis, API Gravity, Asphaltene Content, and Viscosity. Table 2 shows the elemental analysis 480
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results and H/C ratios for the original decant oil and its hydrotreated versions. The decreasing sulfur content in the hydrotreated decant oils indicates the effectiveness of the hydrotreatment and the increasing hydrogen content reveals that substitution of sulfur atoms by hydrogen atoms and possible hydrogenation of aromatic compounds were taking place, as will be shown later in the 13C NMR and GC/MS sections. The results of the API gravity are also reported in Table 2. EI-107 has the lowest API gravity whereas EI-138 the highest. As a rule of thumb, API gravity is inversely related to sulfur content;19 this expected trend was observed in our set of samples. API gravity is also directly related to H/C ratio. A decrease in the viscosity of the different samples with the increase in the degree of hydrotreatment was also observed in Table 2. The asphaltene content is an important parameter since the preferred feedstock to obtain a premium coke should have less than 8% in asphaltenes.1 As observed in Table 2, all the decant oils studied in this work have a very low percentage of asphaltenes (below 0.26 wt %). 3.2. Simulated Distillation Analysis. The average boiling point was determined by using a modified simulated distillation procedure.13 To calculate the average boiling point, we used
Also, it was observed that the lower the average boiling point, the lower the viscosity, which shows agreement with Altgelt and
Figure 2. Relationship between average boiling point (°C) and viscosity.
Boduszynski.20 Figure 2 shows a logarithmic relationship between average boiling point and viscosity with r2 = 0.99. 3.3. GC/MS Analysis. The chemical composition of the decant oils was determined by GC/MS. The area percentage of each molecular type is reported in Table 4. EI-107 has the highest amount of polycyclic aromatic compounds (≥3 rings) plus heteroatoms (PAC+het) while hydroaromatic compounds were barely detected. The amount of polycyclic compounds (≥3 rings) plus heteroatoms decreased with the increase in hydrotreatment severity, while the amount of cycloalkanes increased. A much higher increase of cycloalkanes was observed for the severe hydrotreated decant oils (EI-137 and EI-138). These results are consistent with the fact that the increase in hydrogen percentage observed by elemental analysis is due to hydrogenation of the aromatic compounds. It is also observed in Table 4 that the amount of alkylbenzenes was increasing from the untreated decant oil to oils with low and intermediate levels of hydrotreatment (EI-133 to EI-136). This suggests that the ring saturation and opening (possibly accompanied by heteroatom loss) is a dominant reaction at this level of hydrotreating, as observed in the GC/ MS analyses. This behavior breaks down for EI-137 and EI-138, likely because the hydrotreating is so severe that multiple reaction pathways (e.g., hydrodealkylation) were taking place. The percentage of paraffins for all the decant oils was very low. A small increase of paraffins was observed for the severe hydrotreated decant oils (EI-137 and EI-138), which could also indicate that hydrodealkylation reactions were taking place. A correlation between the API gravity and the polycyclic aromatic content plus heteroatoms is shown in Figure 3. As expected, hydrogenation processes saturate aromatic rings to hydroaromatics and/or cycloalkanes, thereby increasing the API gravity (i.e., reducing the density).21 According to Korre et al.,22 hydrogenation reactivity increased with the number of aromatic rings, and according to Stanislaus and Cooper,23 condensed multiring aromatic compounds are hydrogenated more easily than corresponding monoaromatics. Findings made by Korre et al.22 and Stanislaus and Cooper23 would explain the low formation of naphthalenes (two fused rings), as observed in Table 4, except for sample EI137. Since EI-137 was subjected to the highest hydrotreatment level among the hydrotreated decant oils, EI-137 likely experienced, to some extent, hydrogenation of three- to fourfused aromatic ring compounds, followed by cracking and dealkylation, and then, hydrogenation of the resulting two-
Table 3. Result Obtained by SimDis GC sample ID
IBPa
FBPb
BRc
ABPd ± 1.1 °C
EI-107 EI-133 EI-134 EI-135 EI-136 EI-137 EI-138
234.1 229.7 202.7 212.0 154.0 122.0 110.8
518.8 556.8 510.2 512.9 562.6 506.6 512.8
284.7 327.1 307.7 300.9 408.6 384.6 402.0
414.6 400.7 392.9 391.4 388.0 370.1 371.0
a IBP: initial boiling point. bFBP: final boiling point. cBR: boiling range = FBP − IBP. dABP: average boiling point.
temperatures corresponding to 10, 30, 50, 70, and 90 wt % distillation of the sample. The results are reported in Table 3 along with the initial boiling point, final boiling point, and boiling range data. As the degree of hydrotreatment increases, the initial boiling point decreases while the boiling range increases. As a result of the hydrogenation, lighter components are formed. An inverse correlation between the average boiling point and H, wt % is observed in this study (Figure 1) and corresponds with results previously reported in the literature.20
Figure 1. Relationship between average boiling point (°C) and hydrogen content. 481
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Table 4. Composition of Decant Oils Determined by GC/MS (Area %) derived from EI-107
a
derived from Drum 1
compound group
EI-107
EI-133
EI-134
EI-135
EI-136
EI-137
EI-138
Paraffins Cycloalkanes saturated cyclicsa decalins total Hydroaromatics indanes tetralins total alkyl benzenes Naphthalenes PAC+het
0.43
0.00
1.07
0.42
0.00
3.68
2.05
0.99 0.00 0.99
1.28 0.00 1.28
1.98 0.00 1.98
3.73 0.00 3.73
3.89 0.00 3.89
6.73 6.27 13.00
14.40 6.69 21.09
0.10 0.00 0.10 7.00 3.24 88.24
0.11 4.47 4.58 21.04 4.03 69.08
2.77 6.53 9.30 24.85 5.61 57.19
6.92 2.98 9.90 17.02 2.95 65.99
5.59 1.57 7.16 24.62 2.60 61.63
2.32 6.15 8.47 13.32 8.92 52.61
4.19 6.81 11.00 11.42 4.16 50.28
Other than decalins.
differences in the percentage of hydrogen in the different environments were observed among the different samples. The fraction of the various types of hydrogen was calculated by introducing the total hydrogen percent (% H, wt %) obtained by elemental analysis as follows: Hx actual =
⎛ %Hx ⎞ ⎜ ⎟(%H, wt%) ⎝ 100 ⎠
(8)
A linear relationship between the extent of hydrogenation, defined as the H/C ratio experimentally calculated on the decant oils, and the fraction of the various types of hydrogen is shown in Figure 4. Figure 4a shows a linear relationship between the fraction of Hα and the extent of hydrogenation. As hydrogenation increases, the amounts of benzylic α-CH2 and α-CH3 also increased. Although Hα covers α-CH3 benzylic, the hydroaromatics and cycloalkanes observed by GC/MS suggest that most of Hα corresponds to benzylic α-CH2. HAR also increases with the increase in hydrogenation (Figure 4b). The HAR might be formed as the result of hydrogenation of substituted aromatic carbons present in some molecules, for instance by hydrodealkylation. The decant oils EI-137 and EI-138 are cases in which hydrodealkylation is likely important, since their alkylbenzene contents (by GC/MS) are the lowest, as shown in Table 4. This suggests that hydrodealkylation may occur at severe hydrotreatment reaction conditions. As the hydrogenation increases, the amounts of Hβ also increased (Figure 4c). In addition to the reaction proposed in Figure 1a, another reaction that can account for the production of Hβ is the ring-opening reaction of hydroaromatics between carbons in positions 2 and 3. The decant oils EI-133 to EI-136
Figure 3. Relationship between PAC+het obtained by GC/MS and API gravity.
fused ring compounds. It is important to mention that, for aromatic hydrocarbons containing more than one ring, hydrogenation proceeds via successive reversible steps.23 It is also observed in Table 4 that EI-137 and EI-138, which are the severely hydrotreated decant oils, show the lowest content of PAC+het, which suggests that higher pressures favor aromatic conversion. This is confirmed by Stanislaus and Cooper,23 who observed that high pressures favor low equilibrium concentration of aromatics (high conversions). Stanislaus and Cooper23 reported that the aromatic content decreases with increasing reaction temperature, but increases as the temperature is further raised. This behavior was found in the samples EI-137 and EI-138, where both were derived from the same feedstock and obtained at the same pressure and LHSV. EI-137 displays higher PAC+het than EI-138, and it was hydrotreated at higher reaction temperature. 3.4. NMR Analysis. Table 5 shows the range of chemical shifts and the assignment of proton signals.15 Very few Table 5. Distribution of Hydrogen (% H) Obtained by 1H NMR
derived from EI-107 hydrogen assignments single ring and more than one aromatic ring α-CH2, α-CH3 benzylic β-CH2 tetralins, β-CH2 indans, β-CH3, remote CH2, β-CH2 alicyclics remote CH3 total aliphatic region
derived from EI-132
chemical shift region (ppm)
EI-107
EI-133
EI-134
EI-135
EI-136
EI-137
EI-138
HAR Hα Hβ
9.2−6.2 4.4−1.7 1.7−1.0
38.74 43.66 12.43
37.69 44.53 12.34
38.51 43.41 12.70
38.54 43.10 12.73
37.59 43.90 12.64
37.60 43.98 12.60
38.06 44.03 12.64
Hγ T-Ali-H
1.0−0.7 4.4−0.7
5.17 61.26
5.44 62.31
5.38 61.49
5.63 61.46
5.87 62.41
5.82 62.40
5.27 61.94
symbol
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Figure 4. Actual fraction of the various types of hydrogen as a function of the extent of hydrogenation (H/C) and possible reactions that may occur as result of hydrotreatment.
Table 6. Distribution of Carbon (% C) Obtained by 13C NMR derived from EI-107 carbon assignments methyl methylene N-aliphatica O-aliphatica olefinic protonated aromatic internal aromatic naphthenic substituted aromatics heteroatom (N,O,S)b total aliphatic fraction total aromatic fraction a
derived from EI-132
symbol
chemical shift region (ppm)
EI-107
EI-133
EI-134
EI-135
EI-136
EI-137
EI-138
CH3 CH2 >CH NCal OCal HCCH HARC >CAR >CH2 N,O,S−CAR T-Ali-C T-Aro-C
22.5−11.0 37.0−22.5 60.0−37.0 65.0−60.0 75.0−65.0 118−108 128−118 135−128 138−135 160−138 22.5−75.0 108−160
14.59 9.10 3.94 0.13 0.08 1.49 44.54 17.36 4.08 4.69 27.84 72.16
13.09 14.92 2.75 0 0 1.44 43.98 17.67 4.06 2.75 30.76 69.24
14.17 23.23 6.35 0 0 1.55 29.12 17.44 3.88 4.28 43.75 56.25
14.61 20.74 5.53 0 0 1.52 34.83 15.70 3.20 3.88 40.88 59.12
13.12 23.28 5.72 0 0 0.54 31.45 18.03 3.75 4.11 42.12 57.88
14.07 25.93 7.53 0 0 1.22 30.54 14.13 3.04 3.54 47.53 52.47
12.54 28.14 9.59 0 0 1.38 30.55 12.69 2.55 2.57 50.27 49.73
Aliphatic heteroatom. bAromatic heteroatom.
provide examples of this reaction. The alkylbenzene contents of these samples are higher among the decant oils (Table 4). The final correlation (Figure 4d) shows that as the hydrogenation increases, the Hγ increased. One reaction that could lead to an increase of Hγ is the ring-opening reaction of hydroaromatics between carbons in position 4 and 5. This reaction also accounts for the production of Hα, Hβ, and HAR. However, the plot of Hγ (actual) versus H/C has the lowest slope, which suggests that the probability of the formation of
Hγ (actual), if it happens, is the lowest among the different types of protons. From the slopes of Figure 4, the order of hydrogen-type formation is Hα > HAR > Hβ > Hγ. The band assignments for the liquid-state 13C NMR analysis were selected based on the breakdowns reported by Rodriguez et al.18 The percent of each functional group in the different decant oils is reported in Table 6. An increase in the percentage of −CH2− (in the aliphatic region) with hydrogenation was observed. This corresponds to an increase of methylene 483
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were changed. Two pairs of decant oils, (1) EI-132 and EI-133 and (2) EI-134 and EI-135, were obtained at relatively similar LHSV while the temperature was progressively increasing. The results indicated that an increase in temperature favors the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), as expected and in agreement with Boahene et al.,24 Owusu-Boakye et al.,25,26 and Mapiour et al.27 The decant oils EI-137 and EI-138 were obtained using the same LHSV and pressure but changing the temperature. In this case, the decant oil EI-138, which was obtained at a lower temperature when compared to EI-137, reached comparable HDS level than EI-137 but a higher HDN. This is in agreement with the results of Owusu-Boakye et al.,26 who studied the sulfur and nitrogen conversion at the optimum pressure of 11.0 MPa and LHSV of 6 h−1. Owusu-Boakye et al.26 showed that HDS and HDN increased when the temperature was changed from 644 °F (340°) to 716 °F (380 °C); however, higher temperatures may decrease the HDN effectiveness while the HDS remains almost constant. In Scheme 1, it is observed that an increase in pressure and a decrease in space velocity lead to higher HDS and HDN levels. Boaehne et al.24 have reported that by decreasing the LHSV the extent of sulfur and nitrogen conversions is increased. Optimum conditions of hydrotreatment can be achieved by varying one parameter at a time (P, T, and LHSV). In this work, slight changes in more than one parameter were sometimes observed because the runs were performed in a pilot plant. However, it was observed that severe operating conditions such as high temperatures (734 °F), low space velocities (0.51 h−1), and high pressures (8.3 MPa) were found to be necessary to achieve the desulfurization target. The increase in the hydrogen percentage and decrease in sulfur content with an increase in the hydrotreatment severity conditions was an indication of the effectiveness of the process. Also, the decrease in viscosity and average boiling point with hydrotreatment indicates the formation of lighter compounds, which could result from hydrodealkylation, hydrocracking, and aromatic ring saturation reactions. From the GC/MS and NMR analysis, it can be deduced that the possible reactions that occurred as result of hydrotreatment were (1) saturation of one or more aromatic rings with the formation of hydroaromatics and cycloalkanes, (2) hydrodealkylation, and (3) formation of alkyl side chains due to ringopening reactions of hydroaromatic compounds. It was observed that the amount of cycloalkanes and hydroaromatics increased as the severity of hydrotreatment increased. Formation of alkyl side chains occurred in the decant oils derived from the raw decant oil (see Scheme 1). In the case of the samples obtained from Drum 1, formation of alkyl side chains was the most probable reaction for EI-136, while dealkylation was more favorable for EI-137 and EI-138. Despite experiencing dealkylation, an increase in aromaticity in EI-137 and EI-138 is not observed, due to the extensive formation of cycloalkanes; indeed, the saturation of aromatic rings was a dominant reaction. Decant oils EI-133 and EI-134, obtained from EI-107, display formation of alkyl side chains as the most probable reaction. An increase in the amount of alkylbenzenes was observed by GC/ MS. On the other hand, hydrodealkylation reactions were more favored for EI-135, since a decrease in the amount of alkylbenzenes was observed when compared to EI-133 and EI-134. It is important to recall that EI-135 experienced the
carbons in the alkyl side chains and hydroaromatic rings. The percentage of aliphatic carbon connected to one hydrogen (>CH−) also increased with hydrogenation. This is correlated with an increase in the amount of substituted cycloalkanes. On the other hand, a decrease was observed in the percentage of protonated, internal, and naphthenic-substituted aromatic carbons with the increase in the severity of hydrotreatment. This indicates that hydrogenation of aromatic compounds and ring-opening process was taking place and corroborates the information from GC/MS analysis. Table 6 shows that the percentage of −CH2− was increasing with hydrotreatment and this amount was higher than the percentage of −CH3, indicating that the substitution of aromatic rings was mainly with naphthenic or alkylbenzene compounds. Table 7. Aromaticity (fa) and degree of Substitution of Aromatic Rings (σ) for the Different Decant Oils derived from EI-107
derived from EI-132
structural param.
EI107
EI133
EI134
EI135
EI136
EI137
EI138
fa σ
0.74 0.36
0.70 0.37
0.68 0.36
0.70 0.36
0.69 0.37
0.65 0.37
0.68 0.37
Table 7 shows the aromaticity, fa, and σ results obtained using eqs 6 and 7, respectively. The API gravity (Table 2) is inversely related to aromaticity, fa. These parameters show a decrease in aromaticity when comparing EI-107 with its hydrotreated decant oils, while the degree of substitution of aromatic rings is almost constant, meaning that if any hydrocracking of side chains occurred, it was at carbons β- or further from the ring. A correlation between the total aromatic carbon obtained from liquid state 13C NMR and the aromaticity with H/C (elemental analysis) is observed in Figure 5. In both cases,
Figure 5. Relation between H/C as a function of aromatic carbon TAro-C (13C NMR) and fa.
aromaticity, fa, or aromaticity (T-Aro-C) decreases as the extent of hydrogenation increased.
4. DISCUSSION The desulfurization and denitrogenation levels were determined by varying the hydrotreatment conditions. As observed in Scheme 1, for the first four runs (EI-132 to EI-135), the pressure was kept constant while the LHSV and temperature 484
dx.doi.org/10.1021/ef301296t | Energy Fuels 2013, 27, 478−486
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Scheme 2. Potential Reactions Correlated with Hydrotreatment Operating Conditions
5. CONCLUSION
most severe hydrotreatment conditions among the decant oils derived from EI-107. A slightly higher amount of alkylbenzenes and double the amount of hydroaromatics were observed in EI-134 compared to EI-133. From 13C NMR, EI-134 displays 1.5 times the amount of −CH2− and 2.3 times of >CH− as compared to EI133. This suggests that the alkylbenzenes present in EI-134 have longer, and likely more branched, alkane chains than in EI133. EI-135 and EI-136 show similar cycloalkane contents. EI-135 displays lower alkylbenzene content, and hence, higher aromaticity when compared to EI-136. Decant oils EI-137 and EI-138 were obtained at higher pressures. As observed by GC/MS, the chemical composition changed dramatically between EI-136 and EI-137 (due to increase in pressure and temperature), suggesting that hydrodealkylation occurred; it is important to note that alkylbenzenes are the lowest among all decant oils and cycloalkanes are the highest. The chemical compositions of EI-137 and EI-138 display similarities except for the higher formation of cycloalkanes for EI-138, which is nearly 1.5 times higher. Since the only difference to obtain EI-137 and EI-138 is the reaction temperature, this shows that a lower temperature, 734 °F (390 °C) favors aromatic conversion into cycloalkanes. In addition to undertaking this work to obtain insights into the reaction processes in the hydrotreatment of decant oil, we also wished to produce a family of such hydrotreated oils to use as feedstocks in the delayed coking of decant oil−coal blends. The results of that work will be published in subsequent papers.
Hydrodesulfurization, hydrodenitrogenation, and saturation of aromatic compounds saturation occurs in successive steps. It is out of the scope of the current paper provide mechanisms and step-by-step reactions due to the complexity of the decant oils. This paper provides the most probable reaction(s), based on the analytical techniques performed on the final product (hydrotreated decant oils), which ultimately will tell how the chemical composition influences the needle coke formation when decant oil is used as a feedstock. This topic will be addressed in a subsequent paper. According to Scheme 2, the samples were divided into three groups: low hydrotreated, intermediate hydrotreated, and severely hydrotreated decant oils. During the hydrotreatment process, the PCA+het decreases, as is shown in Table 3. The PCA+het compounds saturate and give origin to different reactions: fused ring saturation, dehydroalkylation, and/or alkylation. It was observed that the most probable reactions for the decant oils subjected under severe hydrotreatment conditions is fused ring saturation and hydrodealkylation. The most probable reaction for decant oils subjected to low and intermediate hydrotreatment conditions is the formation of alkyl side chain.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: (814) 876-0071. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 485
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(13) Kegler, W. H. Control of feedstock for delayed coking. U.S. Patent No. 4,043,898, Aug. 23, 1977. (14) ASTM Standard D2425, Standard Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry; ASTM International: West Conshohocken, PA, 2007; DOI:10.1520/C0033-03R06,. (15) Zander, M. Chemistry and properties of coal-tar and petroleum pitch. In Sciences of Carbon Materials; Marsh, H., Rodriguez-Reinoso, F., Eds.; Publicaciones Universidad de Alicante: Alicante, Spain, 2000. (16) Collin, P. J.; Tyler, R. J.; Wilson, M. A. Fuel 1980, 59, 479−486. (17) Brown, J. K.; Ladner, W. R. Fuel 1960, 39, 87−96. (18) Rodriguez, J.; Tierney, J. W.; Wender, I. Fuel 1994, 73, 1870− 1875. (19) Schobert, H. H. The Chemistry of Hydrocarbon Fuels; Butterworths: Boston, MA, 1990; Chapter 5. (20) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker: New York, 1994. (21) McVicker, G. B.; Schorfheide, J. J.; Baird Jr. W. C.; Touvelle, M. S.; Daage, M.; Klein, D. P.; Ellis, E. S.; Vaughan, D. E. W.; Chen, J.; Hantzer, S. S. Desulfurization and ring opening of petroleum streams. U.S. Patent No. 6,103,106, Aug. 15, 2000. (22) Korre, S. C.; Klein, M. T.; Quann, R. J. Ind. Eng. Chem. Res. 1995, 34, 101−117. (23) Stanislaus, A.; Cooper, B. Catal. Rev 1994, 36, 75−123. (24) Boahene, P.E. Hydroprocessing of heavy gas oils using FeW/ SBA-15 catalyst: Experimentals, optimization of metals loading, and kinetics study. Catal. Today 2012, DOI: 10.1016/j.cattod.2012.04.064. (25) Owusu-Boakye, A.; D., A. K.; Ferdous, D.; Adjaye, J. Ind. Eng. Chem. Res. 2005, 7935−7944. (26) Owusu-Boakye, A.; D., A. K.; Ferdous, D. Energy Fuels 2005, 19, 1763−1773. (27) Mapiour, M.; Sundaramurthy, V.; Dalai, A. K.; Adjaye, J. Fuel 2010, 89, 2536−2543. (28) Escallon, M. M. Petroleum and petroleum blends as feedstocks in laboratory-scale and pilot-scale cokers to obtain carbons of potentially high value. PhD Thesis, The Pennsylvania State University, University Park, PA, Dec. 2008.
ACKNOWLEDGMENTS This work was funded by the United States Department of Energy (DOE). The authors thank Dr. Ö mer Gül, now of GrafTech Inc., Parma, OH, and the late Dr. David Clifford, for the training, guidance, and help in the decant oil characterization. Thanks to Dr. Luis Ayala for the discussions on viscosity and Dr. Geoffrey Wilson, now of Wilson & Associates Inc., for the discussions on hydrotreatment. The authors also express gratitude to Dr. Leslie Rudnick, now of Ultrachem Inc., Wilmington, DE, and to Dr. Caroline Burgess Clifford for their helpful advice and comments during this research.
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DEFINITIONS, ABBREVIATIONS, AND ACRONYMS extent of hydrogenation = H/C ratio experimentally calculated on the decant oils hydrodealkylation = the process of removing methyl or alkyl groups from a molecule, as in the conversion of toluene to benzene. A hydrogen atom replaced the alkyl groups untreated decant oil = fresh decant oil or EI-107 DO = decant oil FCC-DO = fluid catalytically cracked decant oil HDS = hydrodesulfurization HDN = hydrodenitrogenation PCA + het = polycyclic aromatic compounds plus heteroatoms T-Aro-C = total aromatic fraction (13C NMR)
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REFERENCES
(1) Goval, S. K.; Kolstad, J. J.; Hauschildt, F. W.; Venardos, D. G; Joval, C. L. M. Process for producing needle coke. U.S. Patent No. 5,286,371, Feb. 15, 1994. (2) Mochida, I.; Oyama, T.; Korai, Y.; Qing, F. Y. Fuel 1988, 67, 1171−1181. (3) Murakami, T.; Nakaniwa, M.; Nakayama, Y.; Masuo, M. Hydrogenation catalyst for coal tar, a method of hydrogenation of coal tar with use of such catalyst and a method of producing super needle coke from the hydrogenation product of coal tar. U.S. Patent No. 4,855,037, Aug. 8, 1989. (4) Hoover, D. Process for production of premium grade needle coke from a hydrotreated SRC material. U.S. Patent No. 4,737,261, Aug. 8 1988. (5) Kelley, A.; Block, M. J.; Skripek, M. Method for producing needle coke. U.S. Patent No. 4,521,278, June 4, 1985 (6) Castro, A. T. J. Braz. Chem. Soc. 2006, 17, 1181−1185. (7) Rudnick, L.; Boehman, A.; Song, C.; Miller, B.; Mitchell, G. D. Refinery integration of by-products from coal-derived jet fuels. SemiAnnual Progress Report to the Department of Energy on Grant DE-FC2603NT41828; The Pennsylvania State University, University Park, PA, Nov. 17, 2005; pp 40−45. E-print archive http://www.osti.gov/ bridge/purl.cover.jsp?purl=/840426/ (accessed Oct 30, 2012). (8) ASTM Standard D3279, Standard Test Method for Heptane Insolubles; ASTM International: West Conshohocken, PA, 2007; DOI:10.1520/D3279-07. (9) ASTM Standard D1217, Standard Test Method for Density and Relative Density (Specific Gravity) of Liquids by Bingham Pycnometer; ASTM International: West Conshohocken, PA, 2007; DOI:10.1520/ D1217-93R07. (10) Petroleum Measurement Tables; ASTM: Philadelphia, PA, 1980; Vol. 1, Table 5A. (11) ASTM Standard D88, Standard Test Method for Saybolt Viscosity; ASTM International: West Conshohocken, PA, 2007; DOI: 10.1520/D0088-07. (12) ASTM Standard D2887, Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography; ASTM International: West Conshohocken, PA, 2007, DOI:10.1520/D288706A, www.astm.org 486
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