Hydrodesulfurization of Fluid Catalytic Cracking Decant Oils in a

This study investigates the hydrodesulfurization (HDS) of fluidized catalytically cracked decant oils used as feedstock for needle coke production. Th...
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Hydrodesulfurization of Fluid Catalytic Cracking Decant Oils in a Laboratory Flow Reactor and Effect of Hydrodesulfurization on Subsequent Coking Ronald T. Wincek, Joseph P. Abrahamson, and Semih Eser* Department of Energy and Mineral Engineering and EMS Energy Institute, The Pennsylvania State University, 114A Hosler Building, University Park, Pennsylvania 16802, United States ABSTRACT: This study investigates the hydrodesulfurization (HDS) of fluidized catalytically cracked decant oils used as feedstock for needle coke production. Three decant oils, representing a high (4.0 wt %), medium (2.5 wt %), and low (0.9 wt %) sulfur content, were hydrotreated in a fixed-bed flow reactor. Removing sulfur from larger ring systems in decant oils is the most effective way of reducing the needle coke sulfur content, because large aromatics are significant contributors to the coke product obtained from delayed coking. Two commercial catalysts with different pore size distributions were tested for their hydrodesulfurization activities, selectivities for specific sulfur-containing species, and hydrogenation of constituent polyaromatic hydrocarbons (PAHs) under different operating conditions. The decant oils and hydrotreated products were analyzed by GC/ MS to determine changes in molecular compositions of the feedstocks. Following HDS, the decant oils and their hydrotreated products were carbonized to produce a semicoke, and the coke was evaluated for mesophase formation and quality. The desirable outcome of decant oil HDS is sulfur removal, particularly from large polyaromatic ring systems, with minimum hydrogen consumption and hydrogenation. The results showed that the desired level of 0.5 wt % of sulfur in both low- and medium-sulfur decant oils could be achieved through HDS over a commercial CoMo catalyst. Furthermore, hydrogenation of the PAH during HDS appeared to slightly improve the mesophase development seen upon subsequent carbonization.

1. INTRODUCTION Highly aromatic decant oil is produced as a bottom product in the fluidized catalytic cracking (FCC) process in a petroleum refinery. Decant oils find use as a desirable feedstock for the production of a premium-grade product, called needle coke, because of its highly anisotropic microstructure.1 The anisotropy of needle coke results from the formation of a liquid crystalline phase called carbonaceous mesophase that is produced by the polymerization and condensation reactions of polyaromatic compounds in a delayed coking process.2−5 Needle coke is the primary filler material used in the production of graphite electrodes for electric-arc furnaces operated for recycling scrap iron and steel. The properties of needle coke that make it suitable for graphite electrode production include graphitizability, high mechanical strength, low coefficient of thermal expansion (CTE), low porosity, and low levels of impurities, such as sulfur, ash, or metals.6 When sulfur is present in the coke above a given threshold, it can result in irreversible thermal expansion during graphitization heat treatment that reaches temperatures as high as 2800 °C. This expansion, commonly termed “puffing,” has been attributed to the release of sulfur in gaseous form, as either CS2 or H2S. The release of these gases can adversely affect the physical properties of the graphitized carbon by decreasing the graphite’s bulk density, strength, and its electrical and thermal conductivity.7−9 The sulfur content in cokes produced by delayed coking is strongly dependent on the nature of the feedstock and its sulfur content. It has been reported that the sulfur content of needle coke increases almost proportionally to the feedstock sulfur contents.10 While there seems to be no consistent correlation © XXXX American Chemical Society

between the occurrence of puffing and the total sulfur content in needle coke, limiting the sulfur content of the decant oil feedstock to 0.5 wt % has proven to prevent puffing during the graphitization heat treatment. Therefore, a sulfur content of 0.5 wt % is commonly targeted when selecting or blending decant oil feedstocks for needle coke production.11 Considering the trends of increasing sulfur content in crude oil and intermediate refinery products, feedstocks slated for premium coke production will require pretreatment to lower the sulfur concentration to acceptable levels. One option for lowering the sulfur content of decant oils is to use a commercially proven hydrotreatment process, i.e., hydrodesulfurization (HDS). Besides reducing the sulfur content, an important consideration for hydrotreatment of decant oils, is to make certain that this pretreatment process does not impair the carbonaceous mesophase development during the subsequent coking for needle coke production. The overall objective of this study is to investigate the effectiveness of catalytic hydrotreatment for sulfur removal from decant oil samples in a laboratory flow reactor and to determine the impact of hydrotreatment on mesophase development by coking the treated decant oil samples in laboratory tubing reactors.

2. EXPERIMENTAL SECTION 2.1. Hydrotreating Flow Reactor. A pilot-scale flow reactor was constructed for evaluating the effects of operating Received: April 10, 2016 Revised: July 3, 2016

A

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Figure 1. Diagram of the hydrotreating flow reactor.

conditions (bed temperature, catalyst type, hydrogen pressure, space velocity, gas-to-liquid ratio, etc.) on the desulfurization of the decant oil feedstocks. The high-pressure and hightemperature fixed-bed flow reactor allows each operating parameter to be independently varied, thus providing a greater amount of flexibility for evaluating the effect of operational parameters on the properties of the hydrotreated products. The hydrotreating flow reactor is shown in Figure 1 with the reactor details provided elsewhere.12 2.2. Decant Oils. Three FCC decant oils (DOs), representing a high (4.00 wt %), medium (2.51 wt %), and low (0.94 wt %) sulfur contents, were selected for this study. These oils represent the wide range of sulfur levels contained in decant oils produced in U.S. refineries.11 The high-sulfur DO represents an extreme case for feedstocks used in the production of needle coke. Therefore, this sample was only used to narrow the type of catalyst selected for further use in this study. The low- and medium-sulfur DOs generally represent the majority of decant oils available for needle coke production within the United States. These samples were used to examine the effect of hydrodesulfurization (HDS) operating conditions on sulfur removal, decant oil hydrogenation, the subsequent mesophase formation, and needle coke quality. Some selected chemical and physical properties of each decant oil are given in Table 1. 2.3. HDS Catalysts. Two commercial catalysts were initially screened for use in the HDS experiments performed on the decant oil feedstocks. The chemical and physical properties of each catalyst are given in Table 2. The first catalyst, designated as catalyst A, had a chemical composition of ∼20.5 wt % MoO3 and 4.3 wt % CoO dispersed on an alumina support. The second, which is a trimetal designated as catalyst B, contained

Table 1. Chemical and Physical Properties of the Decant Oils (DOs) elemental analysis carbon hydrogen nitrogen sulfur C/H ratio API gravity

low-sulfur DO

medium-sulfur DO

high-sulfur DO

89.9 wt % 8.2 wt % 0.1 wt % 0.9 wt % 11.02 1.1

87.8 wt % 7.3 wt % 0.3 wt % 2.5 wt % 12.04 −0.6

86.5 wt % 7.6 wt % 0.2 wt % 4.0 wt % 11.38 −1.0

Table 2. Chemical and Physical Properties of HDS Catalysts chemical composition MoO3 CoO NiO alumina physical properties type form nominal diameter bulk density surface area pore volume

catalyst A

catalyst B

20.5 wt % 4.3 wt % balance

20.0 wt % 3.3 wt % 1.3 wt % balance

extrudate Katform 1.2 mm 48 lb/ft3 214 m2/g 0.55 mL/g

extrudate Quadrilobe 1.6 mm 43 lb/ft3 170 m2/g 0.53 mL/g

similar amounts of MoO3 and CoO with an additional 1.3 wt % of Ni. Catalyst B was selected for evaluation because it contains larger pores, relative to catalyst A. Small-diameter pores in the catalyst can decrease the rate of the HDS reactions by B

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2.6. Total Sulfur Determination of Decant Oils and Desulfurized Products. The total sulfur contained in the parent decant oils and each hydrodesulfurized product was determined using a sulfur−carbon analyzer (Model SC144-DR, LECO Corporation). 2.7. Analysis of Sulfur Compounds in Decant Oils and Desulfurized Products. The GC-amenable sulfur compounds in the parent decant oils and a selected number of HDS products were analyzed using a gas chromotograph (Agilent, Model HP5890) connected to a pulsed flame photometric detector (PFPD) specific to sulfur-containing compounds. 2.8. Coking of Decant Oils. A semicoke was generated from each of the parent decant oils and 10 selected hydrodesulfurized products by coking in tubing reactors. The production of semicokes in tubing reactors allows the evaluation of mesophase development during carbonization. The body of the tubing reactor has an internal diameter of 1/2 in., with an internal volume of ∼15 mL. This small reactor size allows for rapid heating of the liquid sample, followed by rapid quenching of the products after achieving the desired reaction period. Approximately 4 g of decant oil was poured into an open reactor, it was then closed, pressurized with nitrogen, and slowly vented. This process was repeated two additional times, purging all oxygen from the reactor. The reactor valve was closed and the reactor submerged into a preheated sand bath. No agitation was provided with the reactors held stationary within the bath. A reaction temperature of 500 °C was used for a period of 4.5 h to produce semicoke from each decant oil. After the 4.5 h period, the reactors were removed from the sand bath and immediately quenched in cold water. The gaseous products were vented and the resulting solid semicoke cylinder was extruded from the reactor. The cylinder of semicoke, typically measuring ∼1 in. in length and 0.4 in. in diameter, was removed from the foil liner and washed in 20 mL of dichloromethane to remove the liquid products formed during carbonization. The semicoke was then allowed to dry inside a fume hood for 24 h to remove any residual solvent or volatiles. Previous studies showed that, under the selected coking conditions, these batch reactors yield results that are relevant to commercial delayed coking processes.14 2.9. Optical Texture Index Determination. For the purpose of determining the optical texture index of the semicokes, each dried sample was mounted in epoxy and polished with a series of sand papers and alumina polishing slurries. A systematic evaluation of the semicoke surface was performed using polarized reflected light microscopy and an optical microscope (Nikon, Model Microphot-FXAII. A 5× oil objective lens was used with a 0.9 mm × 0.8 mm mask to acquire at least 150 images from the surface of each pellet. The presence of four different optical textures or anisotropic domains described was noted within each image. A statistical analyses of the textures was performed and an overall optical texture (OTI) calculated using the following equation:

restricting the diffusion of reactants and products in and out of the pores. Deposition of metals or coke at the pore opening further restricts the diffusion of reactants and products. The pore size distribution and surface area of each catalyst was measured using Autosorb-1 equipment (Quantachrome Instruments). The Brunauer−Emmett−Teller (BET) equation was applied to the adsorption isotherm to determine surface area, and the Barrett−Joyner−Halenda (BJH) method was used to determine the pore size distribution. The results for each catalyst are summarized in Table 3. The tabulated data indicates Table 3. Pore Size Distributions in the HDS Catalysts total pore volume micropores (%) 300 Å

catalyst A

catalyst B

0.55 mL/g

0.53 mL/g

0

0

9.0% 80.9% 6.7% 2.3% 1.1%

0.3% 47.7% 43.7% 7.1% 1.2%

that the pores for both catalysts can be classified primarily as mesopores with only a small percentage outside the range of 20−300 Å. The average pore diameter was 72 Å for catalyst A and 100 Å for catalyst B. Each catalyst was sulfided prior to use in the HDS experiments with a 1.5 wt % sulfur solution of dimethyl sulfide dissolved in kerosene. The detailed sulfiding procedure has been described elsewhere.12 2.4. Gas Chromatography/Mass Spectrometry Analysis of Decant Oils. To highlight differences between the molecular composition of the starting decant oils, the GCamenable portion of the low- and medium-sulfur DO were analyzed by gas chromatography/mass spectrometry (GC/ MS). The samples were analyzed using a Shimadzu Model GC17A gas chromatograph and a Model QP-5000 mass spectrometer, with reference to EPA Method 8270C.13 Separation of the samples was accomplished by injection into a fused-silica capillary column (30 m long, 0.25 mm inner diameter (ID); Model XTI-5, Restek Technology, PA) with the injector held at 290 °C. The GC oven was programmed to increase in temperature from 25 °C to 140 °C at a rate of 10 °C/min and then increased from 140 °C to 290 °C at a rate of 5 °C/min. An electron ionization of 70 eV was used for analysis and a mass range of 50−550 m/e was detected within the mass spectrometer. To calibrate the GC/MS analysis, a mixture of 30 compounds, including polycyclic aromatic hydrocarbons (PAHs) and normal alkanes (purchased from Supelco, Bellefonte, PA) typically found in the GC-amenable portion of decant oils, was analyzed by the GC/MS system. This allowed for the retention times and responses of known compounds to be determined for the instrument. 2.5. CHN Elemental Analysis of Decant Oils. The parent decant oils and several of the hydrodesulfurized products produced from each decant oil were analyzed for their carbon, hydrogen, and nitrogen content. Theses analyses were performed using a CHN analyzer (Model TrueSpec, LECO Corporation). This analyzer follows a procedure similar to the method described in ASTM Standard D5291.

OTI =

∑ fI

× OTI i

where f i is the numerical fraction of individual texture type i noted during the microscopic analysis, and OTIi is the index assigned to each texture type i. A list of optical texture types, their sizes, and respective OTI indices are provided in Table C

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Energy & Fuels 4.15 This classification was developed to more accurately differentiate the OTIs of needle cokes, which contain greater

HDS activity relative to catalyst B, catalyst A was used in all subsequent HDS testing performed with the low- and mediumsulfur decant oils. Additional hydrodesulfurization experiments were performed to evaluate the effect of operating parameters and the decant oil feedstock on the percent sulfur reduction. During these experiments, the hydrogen pressure, catalyst bed temperature, and LHSV were varied. The initial series of experiments were conducted at hydrogen pressures of 3.4, 6.9, and 10.3 MPa using a LHSV of 1.0 h−1. A decant oil flow rate of 1 mL/min was used to achieve this LHSV. The catalyst bed temperature was varied between 346 °C and 395 °C for each of the hydrogen pressures. This series of experiments examined the effect of varying the hydrogen pressure at a constant catalyst bed temperature and varying the catalyst bed temperature at a constant hydrogen pressure on the sulfur reduction for the lowand medium-sulfur decant oils. Figure 3 plots the percent sulfur reduction at various reaction temperatures and H2 pressures. As the data show, either

Table 4. Nomenclature and Optical Texture Index for Needle Coke Microtextural Descriptiona

a

type of optical texture

size

OTI index

mosaic small domain domain flow domain

60 μm >60 μm long, ≥10 μm wide

1 5 50 100

Data taken from ref 15.

percentages of large anisotropic areas. A significant difference between this classification and that developed for metallurgical coke is the assignment of higher OTI indices to anisotropic areas classified as domain and flow domains.

3. RESULTS AND DISCUSSION 3.1. Catalyst Activity for Sulfur Reduction. The first series of hydrodesulfurization experiments was performed to compare the HDS activity of the two catalysts (A and B) on the high-sulfur decant oil sample. These experiments were performed using catalyst bed temperatures between 331 °C and 415 °C, hydrogen pressures of 3.4, 6.9, and 10.3 MPa, and a liquid hourly space velocity (LHSV) of 0.5 h−1. The percent sulfur reduction, as a function of catalyst bed temperature, for these experiments is plotted in Figure 2. Although catalyst A

Figure 3. Sulfur reduction as a function of catalyst bed temperature and hydrogen pressure for catalyst A (LHSV = 1.0 h−1).

increasing the catalyst bed temperature or the hydrogen pressure results in a greater extent of sulfur reduction in either decant oil. The reactions or catalyst bed temperatures required to achieve the target of 0.5 wt % sulfur in the product are dependent on the H2 pressure and the starting sulfur content in the feedstock. For the medium-sulfur DO, the required temperatures are 380, 370, and 365 °C for the H2 pressures of 3.4, 6.9, and 10.3 MPa, respectively. Feeding the low-sulfur DO, 0.5 wt % sulfur was achievable at the lowest catalyst bed temperature of 345 °C for each H2 pressure. The percent sulfur reduction was found to be dependent on the initial sulfur content in the decant oil feeds. Under the same reaction temperature and H2 pressure, the low-sulfur decant oil (DO-LS) gave a lower sulfur reduction percentage than the mediumsulfur decant oil (DO-M-S), indicating a kinetically controlled conversion of sulfur in the steady-state flow reactor. The plot also illustrates an almost-linear relationship between the catalyst bed temperature and the percent sulfur reduction for each H2 pressure. The wide range of reactivity of the sulfur species in decant oils with less-reactive compounds giving up sulfur at higher temperatures may be responsible for the apparent linear relationship observed between the sulfur reduction and temperature. However, pore diffusion may limit access of the larger polycyclic aromatic sulfur-containing hydrocarbons to active sites on the catalyst surface or restrict the release of the HDS products.

Figure 2. Sulfur reduction, as a function of catalyst bed temperature and hydrogen pressure, for catalysts A and B (LHSV = 0.5 h−1).

was evaluated at fewer temperatures, it is apparent that this traditional CoMo HDS catalyst achieved greater levels of sulfur reduction than catalyst B at the same temperature. Although catalyst B contains larger-diameter pores, it did not produce higher conversions. Therefore, it appears that either the HDS reaction rate for catalyst A may not be limited by internal pore diffusion or the replacement of some Co with Ni reduces the HDS effectiveness of catalyst B. The higher level of sulfur reduction for catalyst A may also be attributed to the greater surface area at similar pore volume (Table 3). While the dispersion of molybdenum, cobalt, or nickel was not measured for either catalyst, the larger surface area for catalyst A may result in higher activity, because of a higher concentration of active sites. Evaluation of catalyst B’s resistance to pore plugging, which is an added benefit of larger pores, is beyond the scope of this work. Because of its greater D

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Energy & Fuels The decant oil flow rate was increased to 2.0 mL/min to achieve a LHSV of 2.0 h−1. An additional series of experiments was then performed using approximately the same H2 pressures and catalyst bed temperatures (catalyst A). These experiments examine the effect of LHSV at various HDS temperatures under a constant H2 pressure and the effect of hydrogen pressure under a constant catalyst bed temperature on the sulfur reduction in two different decant oils. The results of this series of experiments are plotted in Figure 4. As the results show,

The major sulfur compounds contained in the low-sulfur DO are present in the form of benzonaphthothiophenes (BNT), while the medium-sulfur DO contains a significant amount of dibenzothiophenes (DBT), in addition to BNT. The mediumsulfur DO also contains two-ring PASH, benzothiophenes (BT), and their alkylated homologues, which are not present in the low-sulfur DO. The activity for sulfur removal from the low-sulfur and medium-sulfur DOs by catalyst A under different HDS severities is presented in the GC/PFPD chromatograms shown in Figures 5 and 6. The two operating conditions that

Figure 4. Sulfur reduction as a function of catalyst bed temperature and hydrogen pressure for catalyst A (LHSV = 2.0 h−1).

doubling the LHSV reduces the percent sulfur reduction in both the low- and medium-sulfur DOs. A greater reduction in the sulfur was measured in experiments using the mediumsulfur DO versus the low-sulfur DO. Although a rate equation was not derived from these experiments, the difference in the apparent rate of reaction suggests that the reaction is not independent of the sulfur concentration in the decant oil, but is kinetically controlled. The relationship between the catalyst bed temperature and the percent sulfur reduction, which appears linear, is maintained at either LHSV. Comparing the sulfur reduction achieved during testing of the low-sulfur decant oil at a LHSV of 2.0 h−1 illustrates that the goal of 0.5 wt % sulfur in the hydrodesulfurized product can be achieved using the shorter residence time within the reactor. A similar comparison for the medium-sulfur DO shows that 0.5 wt % sulfur in the HDS product was only achieved at high temperatures and pressures. For the low-sulfur DO tested at 2.0 h−1 LHSV, the required temperatures are 370 and 362 °C at H2 pressures of 3.4 and 6.9 MPa, respectively. However, for the medium-sulfur DO, a temperature of 400 °C was sufficient at a H2 pressure of 6.9 and 10.3 MPa to achieve the sulfur target. 3.2. Catalyst Activity for Specific Polycyclic Aromatic Sulfur-Containing Hydrocarbons. To determine the activity of the CoMo catalyst for removing specific sulfur-containing compounds from the decant oils, the sulfur compounds contained within the GC-amenable portion of each decant oil and several of the HDS products were determined using a gas chromatograph equipped with a pulsed flame photometric detector (GC/PFPD). Using the GC/MS and available standards, peaks for 15 of the major sulfur-containing compounds were identified in the medium-sulfur DO chromatogram. Because of the nonlinear response of the GC/PFPD, quantitative results are difficult to obtain. Therefore, these chromatograms provide only a qualitative estimate of the relative concentrations between the two decant oils.16

Figure 5. Sulfur-containing compounds in the low-sulfur DO and HDS products, as shown in the GC/PFPD chromatograms.

were selected for comparison (catalyst bed temperatures of 365 and 395 °C, H2 pressure of 3.4 MPa), represent mild and severe HDS, respectively. Each of these HDS tests was performed at a LHSV of 1.0 h−1. A short-hand notation is used to label each HDS product as follows: (decant feedstock)(catalyst type)(LHSV × 10)-(catalyst bed temperature)(hydrogen pressure). Thus, DO-LA10-365-3.4 represents the HDS product of the low-sulfur decant oil over catalyst A, using a LHSV of 1.0 h−1, a catalyst bed temperature of 365 °C, and a H2 pressure of 3.4 MPa. This format is also used to identify HDS samples that are analyzed in later sections. Although a quantitative analysis of the sulfur compounds in the resulting HDS products is not available, it can be seen in E

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HDS reactions, hydrogenation of PAH, and catalyst deactivation resulting from coke formation. A significant decrease in the total concentration of many PAH was observed with increasing severity of the HDS operating conditions. The reduction of various groups of PAH is illustrated in Figures 7 and 8 for the low- and medium-sulfur

Figure 7. Concentration of PAHs in the low-sulfur decant oil (DO-LS) and HDS products (H2 pressure = 3.4 MPa and LHSV = 1.0 h−1).

Figure 6. Sulfur-containing compounds in the medium-sulfur DO and HDS products, as shown in the GC/PFPD chromatograms. Figure 8. Concentration of PAHs in the medium-sulfur decant oil (DO-M-S) and HDS products (H2 pressure = 3.4 MPa and LHSV = 1.0 h−1).

the chromatograms that the catalyst exhibits good activity toward BT, DBT, and BNT. Under relatively mild reaction conditions (365 °C, 3.4 MPa), most of the BT, DBT, and BNT are removed from both decant oils. The most refractory sulfur species to the CoMo catalyst are the dimethyl-DBT, trimethylDBT, methyl-BNT, dimethyl-BNT, and trimethyl-BNT. These alkylated sulfur compounds are assumed to be those with methyl-induced steric hindrance to the catalyst, such as 4,6DMDBT.16,17 These alkyl DBT and BNT components remain in the HDS products, even under severe hydrotreating conditions (see the bottom GC/PFPD chromatograms in each figure). 3.3. Catalyst Activity for Hydrogenation of PAH. The activity of the HDS catalyst for the hydrogenation of aromatic compounds contained within the decant oils was also evaluated. The molecular composition of the starting decant oils and eight HDS products representing different levels of HDS severity for each decant oil were measured by GC/MS. The HDS operating conditions representing different levels of HDS severity that were chosen for this evaluation included catalyst bed temperatures of 345, 365, 385, and 395 °C, combined with a H2 pressure of 3.4 MPa and a LHSV of 1.0 h−1. When selecting these samples, consideration was given to the effects of increasing catalyst bed temperature and H2 pressures on the

DOs, respectively. This reduction was initially attributed to the hydrogenation of the PAH occurring simultaneously to the HDS reactions. The susceptibility of the major PAH (naphthalene, phenanthrene, pyrene, and chrysene) to hydrogenation under the given reaction conditions varied considerably. As Figures 7 and 8 illustrate, naphthalene and phenanthrene show a steady decrease in their concentration, while pyrene appears relatively resistant to hydrogenation. The chrysenes were found to be most prone to hydrogenation, with ∼60%−70% being converted at a catalyst bed temperature of 395 °C and a H2 pressure of 3.4 MPa. The methylated analogues are included in each of these PAH categories. 3.4. Hydrogen Uptake during HDS Experiments. A second set of HDS products, representing a larger range of HDS conditions, was assembled from the overall list of HDS experiments performed using a LHSV of 1.0 h−1 and catalyst A. This set of samples included several from the previous set and additional samples hydrodesulfurized at H2 pressures of 6.9 and 10.3 MPa. The sulfur reduction for the selected samples ranged from 0 to 89.2% and 0 to 93.5% for the low- and medium-sulfur DO samples, respectively. An elemental analysis for carbon, hydrogen, and nitrogen was performed on each HDS product F

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Table 5. Elemental Analysis and H2 Uptake for the Low- and Medium-Sulfur Decant Oils and Selected HDS Products Elemental Analysis (wt %) sample

sulfur

carbon

hydrogen

C/H ratio

low-sulfur DO DO-LA10-346-3.4 DO-LA10-365-3.4 DO-LA10-385-6.9 DO-LA10-395-10.3 medium-sulfur DO DO-MA10-346-3.4 DO-MA10-365-6.9 DO-MA10-397-3.4 DO-MA10-397-10.3

0.93 0.39 0.31 0.23 0.10 2.51 0.85 0.59 0.34 0.16

89.9 89.9 90.0 90.3 90.4 87.8 89.6 89.5 89.7 87.9

8.16 8.32 8.38 8.68 8.30 7.29 7.97 8.54 8.28 8.85

11.02 10.81 10.74 10.40 10.89 12.04 11.24 10.48 10.83 9.93

H2 uptake (L H2/L DO) 21 28 67 18 88 161 128 201

The coke yield is plotted versus the percent sulfur in the HDS products in Figure 9. The greatest coke yield was

and the parent decant oils. The results of these analyses, in addition to the percent sulfur, are reported in Table 4. The carbon-to-hydrogen ratio (C/H) for each sample was also calculated and listed in the table. As the sulfur is removed from the decant oil through hydrodesulfurization, the concentration of the remaining constituents (carbon, hydrogen, and nitrogen) should increase. The decrease in the C/H ratio with increasing HDS severity can be attributed to the hydrogenation of aromatics during the HDS experiments. Although there are apparent breaks in this trend, these results generally agree with those observed by the GC/MS analyses performed on the previous set of samples hydrodesulfurized at increasing catalyst bed temperatures and a H2 pressure of 3.4 MPa. CoMo catalysts are excellent HDS catalysts but are somewhat less reactive to HDN and the hydrogenation of aromatics. Therefore, CoMo catalysts give rise to relatively low hydrogen consumption.18 Using the elemental compositions provided in Table 5, the hydrogen uptake was calculated for each HDS product. The hydrogen uptake generally increased with increasing severity of the HDS conditions for both the low- and medium-sulfur DOs. This trend also agrees with the observation that greater hydrogenation occurred under more severe conditions. The H2 uptake in the medium-sulfur HDS products is considerably higher than that for the low-sulfur HDS products. This suggests that greater hydrogenation occurred during HDS of the medium-sulfur decant oil. A reduction in the hydrogen uptake values was noted with severe HDS conditions for both the low- and medium-sulfur DOs. This reduction occurred at a H2 pressure of 10.3 MPa for the low-sulfur DO and 3.4 MPa for the medium-sulfur DO. Both tests were performed at a catalyst bed temperature of 395 °C. In 1996, Tanabe et al. also reported the same reduction in H2 uptake when HDS of decant oil occurred at 400 °C.19 3.5. Dependence of Coke Quality on the Extent of HDS. The effect of HDS severity on the quality of mesophase formation and the resulting semicoke was examined for the same subset of HDS products. In general, there is a concern that HDS may negatively affect mesophase development during subsequent coking of the HDS products, because of the possible disruptions of aromatic ring systems during HDS. The samples listed in Table 4 were isothermally carbonized at 500 °C for 4.5 h in tubing reactors. The carbonization of the decant oils produced a solid semicoke bar, in addition to a small volume of liquid and gas products. After the samples were cooled, they were removed from the reactor, the semicoke was rinsed in dichloromethane, and a coke yield (weight percent of decant oil forming semicoke) was determined. The quantity of liquid and gas products were not measured.

Figure 9. Coke yield as a function of sulfur content for the low- and medium-sulfur DOs and selected HDS products.

measured for the parent decant oils, with the yield of the lowsulfur DO being slightly greater than that of the medium-sulfur DO. The coke yield measured for the HDS products of both decants oils decreased as the severity of the HDS operating conditions increased. The target of 0.5 wt % sulfur in the hydrodesulfurized decant oil is also indicated in the figure. There appears to be three different regions in the relationship between the level of sulfur reduction achieved in the decant oils and changes in the coke yield. The first region occurs between the first and second levels of HDS severity or sulfur reduction (0−58.1 wt % for DO-L-S and 0−66.1 wt % for DO-M-S). The coke yield experiences the smallest reduction over this level of HDS. The level of sulfur reduction achieved through HDS appears to have a greater effect on coke yield over the next several tests (58.1−75.3 wt % for low-sulfur DO and 66.1−86.4 wt % for medium-sulfur DO). The coke yield experiences the greatest reduction over this level of HDS. The final region occurs between the highest levels of HDS severity or sulfur reduction (75.3−89.2 wt % for low-sulfur DO and 86.4−93.6 wt %). The reduction of coke yield with increasing level of HDS in this region is similar to that seen in the first region. HDS of the low-sulfur DO to 0.5 wt % sulfur resulted in a 4 wt % decrease in the coke yield from that measured in the parent decant oil. To achieve 0.5 wt % sulfur in the mediumsulfur DO, the coke yield decreased by ∼9 wt %. This greater reduction in the coke yield for the medium-sulfur DO may be attributed to increased hydrogenation during HDS. The coke yields were determined from carbonization performed in sealed reactors that retain the volatiles. Therefore, these results may G

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Figure 10. Polarized-light micrographs of the semicoke from DO-M-S (2.51 wt % sulfur).

Figure 11. Polarized-light micrographs of the semicoke from DO-MA10-397-500 (0.34 wt % sulfur).

low- and medium-sulfur HDS decant oil samples, respectively. The reduction of PAHs, including phenanthrene and chrysene, and the production of their corresponding hydroaromatics during HDS yield a hydrogen-rich feedstock. The presence of the additional hydrogen lowers the rate of reaction allowing additional time for better alignment of mesogens, thus leading to a higher degree of mesophase development, i.e., a higher OTI. Wang and Eser have previously studied the importance of hydroaromatics in mesophase development during the carbonization of decant oils.20,21 Note that the increasing OTI with initial decreases in sulfur content is just coincidental: no relationship is implied between the mesophase development and the sulfur content. At greater levels of sulfur reduction, a decrease in the OTI was measured for the semicokes produced from the HDS products of each decant oil. Although the chemistry of delayed coking is very complex, the primary products from HDS of the decant oils are biphenyls. Because these compounds lack planarity, they can yield semicokes with decreased OTIs. Semicokes produced from the medium-sulfur DO also had a larger increase in OTI with a greater severity of HDS, compared to those produced from the low-sulfur DO. However, the initial OTI of the DO-LS was higher than that determined for DO-M-S. The primary optical texture units observed in the semicokes were domains and flow domains. Only the parent mediumsulfur decant oil contained a significant amount of small domains. A feature not included in the initial classifications of optical texture units, but observed in many of the semicokes produced from the HDS products, was isotropic pitch. This material contained a small number of well-formed mesophase spheres and was not assigned any value in determining the OTI. Although semicokes formed from HDS products of the low-sulfur decant oil contained some isotropic pitch, an increasing amount of isotropic pitch was observed in the semicokes from the medium-sulfur decant oil HDS products with increasing severity of HDS. The presence of the isotropic pitch indicates that the HDS products have a lower reactivity for coking relative to their parent decant oils. It is possible that longer reaction times (greater than 4.5 h) or slightly higher temperatures are required to complete coking of these samples.

not match yields from a full-scale delayed coker venting the volatiles and operating at a significantly lower pressure. To observe the extents of mesophase development, the semicoke produced from each HDS product was mounted in epoxy and polished for examination under a polarized-light microscope. The polarized-light micrographs representing the major optical textures found in the semicokes for the mediumsulfur DO and HDS product containing 0.34 wt % sulfur is shown in Figures 10 and 11. The optical texture was determined for ∼150 points sampled across the surface of the semicoke bar. From these individual optical texture unit counts, an optical texture index (OTI) value was determined for each semicoke. Similar to the coke yield, the OTIs and sulfur contents of the HDS products are plotted in Figure 12. The reproducibility of the OTIs is ±2 units, as indicated by the error bars.

Figure 12. OTI and sulfur content in the HDS products for the lowand medium-sulfur DOs.

The semicoke produced from the low-sulfur DO and its HDS products displayed better mesophase development relative to the medium-sulfur DO and its HDS products. This improvement in coke quality is reflected in the OTIs determined for the two sets of decant oils. The OTIs varied from 88.6 to 91.0 and 78.6 to 86.6 for the low- and mediumsulfur DO samples, respectively. An increase in the OTIs with the level of sulfur reduction was observed in both decant oils, reaching a maximum at ∼0.3 wt % sulfur in the HDS products. This represents a sulfur reduction of ∼68 and 88 wt % for the H

DOI: 10.1021/acs.energyfuels.6b00843 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

(10) Jacob, R. R. Hydrocarbon Process. 1972, 132−136. (11) Guercio, V. J. Oil Gas J. 2010, 108, 96−99. (12) Wincek, R. T. Hydrodesulfurization of Fluid Catalytic Cracking Decant Oils for the Production of Low-Sulfur Needle Coke Feedstocks; M.S. Thesis, The Pennsylvania State University, University Park, PA, 2013. (13) EPA Semivolatile Organic Compounds by Gas Chromatography/Mass Spectrometry. Available via the Internet at: http://www. epa.gov/epaoswer/hazwaste/test/pdfs/8270c.pdf. (14) Mochida, I.; Korai, Y.; Fujitsu, H.; Oyama, T.; Nesumi, Y. Carbon 1987, 25, 259−264. (15) Eser, S. Carbonaceous Mesophase Formation and Molecular Composition of Petroleum Feedstocks. In Supercarbon: Synthesis, Properties and Applications; Yoshimura, S., Chang, R. P. H., Eds.; Springer−Verlag: Berlin, New York, 1998; pp 147−155. (16) Ma, X. L.; Sakanishi, K.; Isoda, T.; Mochida, I. Fuel 1997, 76, 329−339. (17) Shafi, R.; Hutchings, G. J. Catal. Today 2000, 59, 423−442. (18) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating CatalysisScience and Technology, Vol. 11; Springer−Verlag: Berlin, Heidelberg, Germany, 1996. (19) Tanabe, K.; Takada, T.; Newman, B. A.; Satou, M.; Hattori, H. Nippon Enerugi Gakkaishi 1996, 75, 916−924. (20) Wang, G.; Eser, S.; Wiwel, P. Energy Fuels 2007, 21, 3563−3572. (21) Eser, S.; Wang, G. Energy Fuels 2007, 21, 3573−3582.

4. CONCLUSIONS The present work has demonstrated the effect of hydrodesulfurization (HDS) operating parameters on the degree of sulfur reduction in decant oils and the quality and quantity of coke produced by their carbonization. Initial screening tests using a high-sulfur DO (4.00 wt % sulfur) showed that greater sulfur reductions could be achieved with a traditional CoMo catalyst versus a trimetal catalyst containing larger pores. Additional HDS testing with the commercial CoMo catalyst demonstrated that the desired level of 0.5 wt % of sulfur could be achieved in both low- and medium-sulfur DOs. A strong dependence of sulfur reduction on reaction temperature, pressure, and liquid hourly space velocity (LHSV) was demonstrated for the chosen catalyst. Except for sterically hindered alkylated sulfur compounds, strong activity was observed for the removal of most benzothiophenes (BT), dibenzothiophenes (DBT), and benzonaphthothiophenes (BNT), contained in the decant oils. Increasing the severity of HDS resulted in greater hydrogenation of the polycyclic aromatic hydrocarbon (PAH) and reductions in coke yield. It is also clear that hydrogenation of the PAH resulted in initial improvements in mesophase development and coke quality. These improvements can likely be correlated with the availability of additional hydrogen that lowers the rate of coking, allowing for additional time, for a higher degree of mesophase development.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (814) 863-1392. Fax: (814) 865-3248. E-mail: seser@ psu.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Seadrift Coke, L.P. Staff from the EMS Energy Institute are acknowledged for instrument training and assistance with equipment fabrication.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b00843 Energy Fuels XXXX, XXX, XXX−XXX