Effect of Petroleum Feedstock and Reaction Conditions on the

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Effect of Petroleum Feedstock and Reaction Conditions on the Structure of Coal-Petroleum Co-Cokes and Heat-Treated Products Mhlwazi S. Nyathi,* Caroline Burgess Clifford, and Harold H. Schobert EMS Energy Institute and Department of Energy and Mineral Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 ABSTRACT: Bituminous coal/petroleum co-cokes were produced by coking 4:1 blends of vacuum resid (VR)/coal and decant oil (DO)/coal at temperatures of 465 and 500 °C for reaction times of 12 and 18 h, under autogenous pressure in microautoclave reactors. Co-cokes were calcined at 1420 °C and graphitized at 3000 °C for 24 h. Optical microscopy, surface area measurements, X-ray diffraction, temperature-programmed oxidation, and Raman spectroscopy were used to characterize the products. Product yield distribution analysis suggested an increase in co-coke yield as reaction severity index increases, although the increase yield is small at higher index values. It was found that higher reaction temperature (500 °C) or shorter reaction time (12 h) leads to an increase in the amount of mosaic carbon at the expense of textural components necessary for the formation of anisotropic structure, namely, domains and flow domains in the co-coke texture. Characterization of graphitized cocokes showed that the quality, as expressed by the degree of graphitization, crystallite dimensions, Raman disorder parameter, and oxidation reactivity temperature of the final product, is dependent on the nature of the precursor co-coke, with products obtained from co-cokes produced at 500 °C showing a higher structural disorder than the corresponding products produced at 465 °C. The products obtained from DO/coal blend also displayed better structural order than products derived from VR/coal.

1. INTRODUCTION For more than a decade, our research group at the EMS Energy Institute at The Pennsylvania State University has been investigating the concept of co-coking highly fluid bituminous coals with various petroleum-derived materials,1−6 such as fluid catalytic cracking decant oil (DO) or vacuum resid (VR), under typical delayed coking conditions, such as 465 °C and residence times of up to 18 h. Although this process was developed for production of coal-based liquids that would ultimately be converted into clean middle distillate fuels, it also yields a carbon product referred to as co-coke. In addition to studying the nature of the liquid products,4,6 the EMS Energy Institute has dedicated part of the research to studying the structure of the co-coke because of its potentially high market value. Previous studies have shown that, depending on the nature of the feedstock and operating conditions, co-coke yield and morphology vary.1−3,5 Being able to vary the co-coke morphology by changing the nature of feedstock and reaction conditions offers the potential of a variety of commercial uses for the co-coke or its heat-treated derivatives. The co-coke has been and is being evaluated for applications in production of graphite for arc furnace electrodes,5 production of activated carbon, and aluminum-smelting anodes. Fickinger and colleagues studied the effect of the nature of feedstock on the structure of the co-coke by using different bituminous coals in a blend with the vacuum resid.1,3 The effect of changing coal with the same petroleum feedstock was found to be minimal, since the coals used were similar. Escallón studied both the effect of the nature of feedstock and reaction pressure on the co-coke structure by hydrotreating the decant oil prior to blending with coal and by varying the pressure of the reaction using different reactor designs.5 A correlation between hydrogen transferability and co-coke quality was found when operating at higher © 2012 American Chemical Society

pressure. In this study, we sought to investigate the effect of the nature of petroleum feed in a blend (by using vacuum resid and decant oil), reaction temperature, and time on the co-coke structure, and ultimately on the structure of the final product obtained when the co-coke is subjected to graphitization conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. The coal used in this study was supplied by the Penn State Coal Sample Bank. The sample was prepared by cleaning a coal that was obtained from the James Cell Effluent in A. T. Massey’s Marfork Cleaning Plant in Raleigh County, WV. After cleaning, the coal sample (−60 mesh) was sealed in foil laminate bag under argon. Two petroleum feedstocks used in this study were vacuum resid (VR) obtained from Valero and decant oil (DO) from BP. The properties of the feedstocks are summarized in Table 1. 2.2. Reaction Procedure. The reactions were performed in vertical microautoclave reactors of 25 mL capacity, constructed of type 316 stainless steel. The feedstock (5 g) was loaded into a reactor. The feedstock was a VR/coal or DO/coal blend, at a 4:1 (solvent:coal) ratio, mixed prior to loading into the reactor. The reactor was then purged three times with nitrogen to remove air and to ensure that there was no leakage, the purging pressure being 7 MPa. The reactor was then sealed at atmospheric pressure. The reactor was immersed in a sand bath that had been preheated to 465 or 500 °C. The reaction times were 12 and 18 h. The pressure was autogenous, reaching 6.9− 10.3 MPa. At the end of the reaction time, the reactor was quenched in cold water, and the solid product was Soxhlet-extracted, first with pentane and then with tetrahydrofuran. Soxhlet-extraction was done for the purpose of separating and recovering the liquid products from the solid product, where pentane and tetrahydrofuran recover lower and higher boiling liquids, respectively. The solid product from Received: March 17, 2012 Revised: May 15, 2012 Published: June 1, 2012 4413

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77 K and maximum manifold pressure of 0.123 kPa. The adsorption measurements were translated to surface area based on the Brunauer− Emmett−Teller (BET) equation. Porosity distribution was calculated using the original density functional theory (DFT) considering slit pores.7 Co-cokes were also analyzed using optical microscopy. Samples were ground to powder and then embedded in a cold-setting epoxy resin. After the epoxy hardened, the blocks were polished and then analyzed using a Zeiss Universal research microscope. The micrographs were collected at a magnification of 625× in oil immersion objective using a cross-polarized white-light illumination. The pointcount analysis was performed by traversing the sample based upon a 0.4 × 0.4 mm grid and using a textural classification scheme modified from Oya et al.8 by Eser9 for petroleum-derived semicoke and ASTM D5061 for metallurgical coke derived from coal.10 Samples were ground to fine powder using mortar and pestle and then sprinkled on the surface of the quartz zero-background sample holder for X-ray diffraction (XRD) analysis. The analysis was carried out using a PANalytical X’Pert Pro powder diffractometer with X’celerator detector. Ni-filtered Cu Kα radiation produced at 45 kV and 40 mA was used for the analysis. A continuous scan with a step size of 0.01° 2θ and scan step time of 100 s, running from 5 to 90 degrees 2θ, was acquired. Data acquisition was done using MDI Jade 9 software. The peak broadening due to instrumental factors was corrected with the use of an external standard, where silicon was used for that purpose. The peak intensity and full width at half-maximum (fwhm) were recorded. The interlayer spacing (d002) was calculated from the (002) peak using the Bragg equation. The mean crystallite height (Lc) and crystallite width (La) were calculated from the broadening of the Gaussian profiles for the (002) and (110) peak using the Scherrer equation.11 Raman spectra were obtained from XY Raman spectrometer WITec CRM200 that utilizes an argon laser of 488 nm wavelength and equipped with a 1024 pixel charge-coupled device (CCD) camera. The spectra were recorded in a backscattering configuration under the confocal WITec microscope attached to the instrument, using a numerical objective lens of 40× objective to focus the laser beam at a power of 25 mW. Extended scans were made over a range from 82 to 4460 cm−1 to obtain both first- and second-order Raman bands, at 5 s integration with two repetitions. The peak intensity, full width at halfmaximum (fwhm), and frequency of bands were measured using a Gaussian curve-fitting module of the Origin software. Temperature-programmed oxidation (TPO) was performed using a LECO RC 612 multiphase carbon analyzer. Samples were oxidized by reaction with ultrahigh purity O2 in a furnace over a CuO catalyst bed. The sample was heated 30 °C/min in O2 flowing at a rate of 750 mL/ min to a maximum temperature of 900 °C, with a hold period of 6 min at the 900 °C. The resulting carbon dioxide was quantitatively measured as a function of furnace temperature using a calibrated infrared (IR) detector.

Table 1. Properties of the Feedstock coal

DO

Proximate Analysis (dry) fixed carbon, wt % 66.5 6.26 volatile matter, wt % 32.6 93.6 ash, wt % 0.9 0.05 Ultimate Analysis (dry ash free basis) carbon, wt % 90.0 89.4 hydrogen, wt % 5.5 7.72 nitrogen, wt % 1.7 0.03 sulfur, wt % 0.8 0.8 oxygen, wt % (diff) 1.9 0.17 API gravity (60 °F) n.d.a 1.7 1 H NMR total aromatics, wt % n.d. 30.7 total aliphatics, wt % n.d. 69.3 13 C NMR total aromatics, wt % n.d. 59.7 total aliphatics, wt % n.d. 40.3 a

VR 24.01 75.74 0.25 88.1 9.8 0.20 1.0 0.19 −4.8 7.9 92.1 18.3 81.7

n.d. denotes not determined.

Soxhlet-extraction was then dried and stored for characterization and further treatment. Throughout the discussion in this work, co-coke refers to a Soxhlet-extracted solid product. Additional co-coking reactions were carried out whereby reaction temperatures of 450, 465, 480, and 500 °C, and times of 6, 12, 18, and 24 h were used. This set of experiments was solely meant for determination of reaction severity index, which accounts for a coupled effect of reaction temperature and time on product yield. These reactions were carried out using the described reaction procedure. Calcination and graphitization of samples were carried out using an induction furnace at GrafTech, Parma, Ohio. About 5 g of the sample were placed into a graphite capsule and then inserted into a tube furnace that was purged continuously with argon. Calcination was carried out by heat-treating the samples at 1420 °C at a heating rate of 4 °C/min and holding at the final temperature for 1 h. The calcined samples were then heat-treated at 3000 °C for 24 h and then cooled to ambient temperature. 2.3. Analytical Procedures. Proximate analyses were conducted using a LECO 400 Proximate Analyzer following the ASTM method D4625-86, while ultimate analyses were carried out using a LECO CHN 600 Analyzer following the ASTM method D5142-04. A LECO SC 132 was used to determine the total sulfur content following the ASTM method D5016-08. The decant oil and vacuum resid were analyzed using a Bruker AMX 360 NMR operating with a magnetic field of 9.4 T and 70° tip angle. Samples were dissolved in a 1:1 volume ratio with deuterated chloroform (CDCl3) containing 1 vol % of tetramethylsilane (TMS) as an internal standard and charged to a 5 mm tube for analysis. For 1H NMR, a time domain of 16 384 s was used with a total of 256 acquisitions. The recycle delay and pulse width used were 5 s and 5 μs, respectively. For 13C NMR, a time domain of 65 536 s and number of acquisition equal to 1024 were used. The recycle delay and pulse width used were 2 s and 8 μs, respectively. The simulated distillation−gas chromatography (SimDis-GC) measurements of decant oil and vacuum resid were made according to ASTM 2887 method by using an HP 5890 GC-FID fitted with MXT-500 simulated distillation column (10 m, 0−53 mm inner diameter, and 2.56 μm Restek). Carrier gas flow rate was adjusted to 13 mL/min. The sample was injected at 40 °C and held at this temperature for 4 min and then ramped to 350 °C at rate of 15 °C/ min. The temperature was held at 350 °C for an additional 10 min. SimDis Expert 6.3 software was used to calculate the percentage of fractions. Surface area and porosity analyses were carried out using a Micromeritics ASAP 2020. The samples were degassed for 24 h in a vacuum prior to measurement. Nitrogen gas was used for analysis, at

3. RESULTS AND DISCUSSION 3.1. Product Yield Distribution and Co-coke Structure. The assessment of product yield distribution reveals the behavior of feedstocks when reacted alone or when blended, that is, the interaction of feedstocks in a blend when reacted at different reaction conditions.1,2 Figure 1 shows the effect of reaction temperature on the product yield distribution for cocoking DO/coal and VR/coal blends at different reaction times. Both plots show a decrease in the total liquids yield as the reaction temperature is increased from 465 to 500 °C. The decrease in the liquids yield is accompanied by an increase in gas and solid product yields. This is observed at both 12- and 18-h reaction times, indicating that the effect of reaction temperature occurs even at shorter reaction times, such as 12 h. This is attributed to relatively greater cracking reactions at 500 °C, which convert primary liquid products into gas with a concomitant increase in solid formation. This behavior has 4414

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rich in alkyl substituents tend to be highly reactive, as defined by the extent of radical initiation and termination reactions.16 The vacuum resid used in this study has an aliphatic carbon content of 81.7 wt %, while decant oil has 40 wt %, as determined using nuclear magnetic resonance spectrometry (Table 1). A reaction severity index (RSI) was used to compare product yield distributions on common basis, and it serves to couple the effect of reaction temperature and time into a single variable to examine the effect on co-coke yield.17,18 The RSI was calculated using the following expression: RSI = 2 × reaction time (h) + (temperature (°C) − 450 °C). This is the approach that was proposed and used by Petrakis et al.17 Figure 2 shows the

Figure 2. Variation of co-coke yield with reaction severity index.

Figure 1. Product yield distribution for co-coking at various temperatures and reaction times: (A) VR/coal and (B) DO/coal blend.

variation in the co-coke yield as a function of RSI. The line is not a regression fit of the data but is for visualization purposes only. Both blends show a positive slope, with an obvious increase in co-coke yield at low RSI values, and slight increase in co-coke yield at higher RSI values. The observed change in co-cokes yield with increase in RSI is analogous to the “coke jump” that was reported by Fickinger and colleagues when assessing the coke yield as a function of temperature.3 For instance, an increase of severity index from 12 to 24, VR/coal reaction experienced 20% increase in co-coke yield; however, the co-coke yield increased by 2% for an increase of the severity index from 86 and 98. Therefore, the co-coke yield can be increased by increasing the reaction severity, but one approaches a limit of doing so as RSI values become higher and higher. Figure 2 also shows that the DO/coal blend provides a higher solid product yield than VR/coal blend. To account for this, the coking inhibition index (CII) was determined. CII is an empirical method for estimating the relative coking propensity of petroleum streams.19 The CII is a function of specific gravity, sulfur content, and aromaticity, as expressed by the US Bureau of Mines Correlation Index (BMCI): BMCI = (48640/VABP) + 473.7SG − 456.8, where VABP is volume average boiling point, in Kelvin, and SG is specific gravity. The VABP was calculated from simulated distillation gas chromatography (SimDis-GC) data as the average of cut point temperatures for 10, 30, 50, 70, and 90% loss of petroleum feed. The CII is then calculated using the following equation: CII = 153.4 − [5.1 × (BMCI1/2 × (1 + S/1.3) + 2API)], where S is

been reported in previous studies3 and is attributed to an increase in the rate of polymerization and condensation reactions, leading to an increase in the viscosity of the liquid system.12−14 The consequence of increased viscosity is resolidification of the liquid medium and an increase in the solid product yield. Running reactions at different times revealed a decrease in the total liquids yield accompanied by an increase in the gas and solid product yields as the reaction time was increased from 12 to 18 h. This trend was observed whether the reaction is carried out at 465 or 500 °C. These product yield variations with reaction time are due to prolonged thermal exposure of the liquids, which promotes cracking of primary liquids, leading to an increase in gas and solid product yields. Also, a longer reaction time gives the reactants enough time to interact, and the consequence of that is an increase in the solid product yield.1,2 The interaction of the reactants is manifested by nonlinear combinations of the products from the feedstocks reacted individually and by optical textural distribution as viewed in optical microscopy. Since the product of interest in this work is the co-coke, as opposed to liquid or gas products, longer reaction time is favorable because it improves the cocoke yield. However, these changes in product yields with change in reaction temperature and time are not as drastic in the DO/coal reaction as they are in the VR/coal reaction. This is attributed to a generally high aliphatics concentration in vacuum resids.15,16 Mochida reported that feedstocks that are 4415

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Table 2. Simulated Distillation GC Result for BP Decant Oil and Valero Vacuum Resid DO VR

wt % off

initial bp

10

30

50

70

90

final bp

BMCI

CII

temp., °C temp., °C

132 238

342 479

369 582

388 622

405 673

420 798

519 1105

119 118

72 77

weight percentage of sulfur and API is equal to (141.5/SG − 131.5). In predicting the tendency to deposit coke, a high CII implies less tendency to form coke than a feedstock that has a low CII. Table 2 shows the SimDis-GC data, BMCI and CII for both DO and VR. The CII for the decant oil used in this work is 72, whereas for the vacuum resid is 77. The literature suggests that higher CII corresponds to lower coke yield;19 therefore, the observed CII values for VR and DO are in agreement with the product yield distribution presented above, whereby VR/coal blends tend to have lower co-coke yields than DO/coal blends. To determine whether an interaction between the reactants had occurred during co-coking, expected yields were calculated and checked against experimental yields. To determine the expected yields, control reactions were carried out by coking each feedstock by itself. The yield distributions observed in control reactions were then used to calculate the expected yields from blends, assuming that the expected yield would be the weighted linear average of the yields observed from each feedstock reacted by itself. The linear (expected) yield represents the distribution that would be obtained if the reactants in a blend were not interacting.20 Table 3 presents the

to be the case for a DO/coal blend, as shown by an increase in the difference between expected and experimental yield when the reaction is carried out at 500 °C. A change in the optical textural distribution in co-coke structure with a change in the reaction conditions elucidates the effect of conditions on critical stages of the coking process: mesophase sphere growth, coalescence of mesophase spheres, and resolidification. Figure 3 shows examples of various textures that were observed. The top and the right-hand side of Figure 3(A) show domains derived from vacuum resid, whereas the bottom left corner of the micrograph shows a texture originating from the interaction of vacuum resid with coal, showing the presence of regions that display good interaction between coal and vacuum resid. Figure 3(B) shows areas of interaction and areas of poor interaction (label X) between the vacuum resid and coal. The micrograph also shows the presence of pores (label Y). Although vacuoles were observed in all co-cokes, co-cokes obtained at 500 °C tended to have more pores than those produced at 465 °C. This is indicative of gases being released over a larger volume of the reacting mixture when operating at the higher temperature, due to the generation of gases over a larger area and a reaction progress that does not favor an orderly release of gases. Table 4 shows the amounts of petroleum-derived textural components in co-cokes obtained from coking DO/coal and VR/coal blends at various reaction conditions. An increase in the reaction temperature leads to a decrease in the amount of domains (>60 μm) and flow domains (L > 60 μm, W < 10 μm). Contrary to the effects of temperature increase, an increase in reaction time results in an increase in the amount of domains and flow domains. The inverse relationship between the amount of large domains (domains plus flow domains) and mosaic carbon ( 60 μm, W < 10 μm

VR/coal-465 °C, 12 h VR/coal-500 °C, 12 h DO/coal-465 °C, 12 h DO/coal-500 °C, 12 h VR/coal-465 °C, 18 h VR/coal-500 °C, 18 h DO/coal-465 °C, 18 h DO/coal-500 °C, 18 h

1.2 1.4 0.9 1.3 0.5 1.1 0.5 0.9

21.4 27.6 15.6 26.2 17.5 28.1 11.5 21.2

62.9 57.4 64.7 56.4 61.1 55.0 62.3 56.7

1.8 0.6 3.6 1.8 5.2 1.8 8.8 4.9

1.1 0.5 1.6 0.9 1.7 1.2 2.4 1.6

Table 5. Surface Area Measurement and H/C for Co-cokes sample

surface area, m2/g

total pore vol., cm3/g

pore size range, nm

avg pore size, nm

H/C

VR/coal-465 °C, 18 h VR/coal-500 °C, 18 h DO/coal-465 °C, 18 h DO/coal-500 °C, 18 h

1.68 2.45 1.15 2.08

0.0022 0.0029 0.0030 0.0026

1.7−185.2 1.6−192.8 1.8−183.5 1.6−196.2

13.0 8.6 16.9 11.9

0.474 0.381 0.408 0.380

465 to 500 °C or decreasing the reaction time from 18 to 12 h decreases the amount of textural components necessary for the formation of anisotropic structure in a co-coke. A decrease in the amount of these components is attributed to fast reaction rates attained when operating at higher reaction temperature, which promote retrogressive reactions that limit the growth of mesophase spheres, whereas a decrease in the amount of large domains when operating at a shorter reaction time is ascribed to early termination of the reaction, which limits the growth of spheres.13,16,24 Surface area measurements were performed for studying the pore structure of co-cokes. Table 5 presents surface area measurements of co-cokes from 18-h reactions. The data in Table 5 show that both blends experience an increase in the cocoke surface area and pore sizes at the higher reaction temperature. However, with respect to the total pore volume, VR/coal shows an increase while DO/coal shows a decrease. Also, co-cokes obtained at 500 °C have a wider range of pore sizes than those obtained at 465 °C. Despite the wider pore size range, co-cokes obtained at 500 °C have a smaller average pore size than the corresponding co-cokes obtained at 465 °C. Both wider pore size range and smaller average pore size in co-cokes obtained at 500 °C are indicative of a greater volume of gases being released throughout the reacting mixture due to the severe reaction conditions. The importance of the nature of the petroleum feed in a blend is shown by a larger average pore size for co-cokes obtained from DO/coal than for co-cokes obtained from VR/coal. This possibly reflects the different ability to generate gases between these two petroleum feeds, whereby the VR generates more gas than DO. The authors base this suggestion on the results of the studies conducted by Mochida and colleagues, whereby gas evolution was controlled by varying the pressure of the system.25,26 Their findings showed that excess gas evolution leads to a flaky and excessively porous coke. Furthermore, this suggestion is consistent with the observation made in the discussion of optical micrographs where it was noted that, by comparison, micrographs of cocokes produced at 500 °C are more porous due to high gas generation when the reaction is carried out at 500 °C. Figure 4 depicts a relationship between average pore size and the amount of domains and flow domains. The line is not a regression fit of the data but is for visualization purposes only.

Figure 4. Average pore size as a function of the sum of domains and flow domains (D + FD) for co-cokes obtained at 18 h.

There is an increase in average pore size as the amount of large domains (D + FD) increases. The observed relationship shows the consistency of data and also suggests that gas generation and coalescence of mesophase spheres are a function of reaction temperature and feedstock. Table 5 shows the H/C of the co-cokes. Co-cokes produced at 500 °C have slightly lower H/C ratio than those obtained at 465 °C. This difference implies that, when operating at 500 °C, the amount of material lost to form gaseous products increases, and more hydrogen is also lost with those fractions, hence a decrease in the H/C of the co-coke. For example, gas yields for a VR/coal reactions carried out at 465 and 500 °C are 17.3 and 26.6 wt %, respectively. The overall effect of an increase gas formation is an increase in the total pore volume. 3.2. Graphitized Co-coke Structure. Table 6 presents the X-ray diffraction parameters of graphitized co-cokes. The standard deviation is given in brackets. The interlayer spacing (d002) values were used to calculate the degree of graphitization, DOG, using the following expression: DOG = (0.3440 − d002)/ (0.3440 − 0.3354), where 0.3440 nm is the interlayer spacing of carbon with no graphitic order and 0.3354 nm being the interlayer spacing of graphite.27 The probability of random 4417

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Table 6. X-ray Diffraction Parameters for Graphitized Co-cokes sample VR/coal-465 °C, 18 h VR/coal-500 °C, 18 h DO/coal-465 °C, 18 h DO/coal-500 °C, 18 h

d002, nm 0.3368 0.3374 0.3360 0.3371

(0.0003) (0.0002) (0.0004) (0.0003)

Lc, nm 22.1 20.8 31.5 28.9

La, nm

(1.1) (0.8) (1.5) (0.9)

orientation between any two neighboring layers, p, is given by d002 = 0.3354 + 0.0086p, with the assumption that the interlayer spacing of nongraphitic carbons is 0.3440 nm, and 0.0086 nm is the incurred difference between the interlayer spacing values of graphitic and nongraphitic carbons.28 The use of p provides an alternative method of accounting for the disorder in the graphitic structure, and normally, DOG and p should sum to 1. The average number of aromatic layer planes in each stack, N, was determined using the expression N = Lc/d002.29 Although graphitized co-cokes show characteristics of a graphitic structure, such as the appearance of bands characteristic of structural three-dimensionality, namely (110), (112), and (006) located at approximately 77, 83, and 87°, respectively,30,31 their interlayer spacing values are larger than interlayer spacing of graphite (0.3354 nm). Table 6 shows that, for both blends, the effect of the increasing coking temperature is to increase d002, hence a decrease in the degree of graphitization and increase in the probability of disorientation between carbon layer planes of the resulting product upon graphitization of co-cokes. These changes indicate that the products synthesized by graphitizing the co-cokes obtained at 500 °C are structurally less developed than those of 465 °C origin. Consistent with changes in the degree of graphitization, the crystallite parameters (Lc and La) of the final product decrease with an increase in coking temperature. Optical microscopy showed that an increase in coking temperature decreased the amount of large domains (domains plus flow domains) and increased the amount of mosaic carbon by promoting early resolidification. Early resolidification limits the coalescence process and deprives the lamellae enough time to stack. Such effects are inherited by graphitization products; as manifested by a decrease in crystallite parameters (Lc and La) and N of products obtained from co-cokes produced under these conditions. Moreover, these results show the dependence of the graphitization product quality on the nature of the petroleum feed, with DO-based products being structurally superior to their VR counterparts, as shown by a lower degree of graphitization and smaller crystallite parameters for VR/coal products than those for the corresponding DO/coal products. Raman spectra of graphitized co-cokes showed three peaks in the first-order region (1000−2000 cm−1): a D-peak (defectinduced) at ∼1360 cm−1, a prominent G-peak (ordered carboninduced) at ∼1580 cm−1, and a less intense D′-peak at ∼1615 cm−1. The spectra showed characteristics of graphitic structure since the narrowing of the G-band relates to narrow phonon distribution that occurs in materials with highly oriented surfaces.32,33 The splitting of the second-order region (2000− 3000 cm−1) peak, the G′-peak located at ∼2700 cm−1, into G′1 at ∼2695 cm−1 and G′2 at ∼2735 cm−1 also signaled the presence of three-dimensional ordering of graphene layers in graphitized co-cokes.34−36 The use of the ratio of relative intensities of D-bands (ID) and G-bands (IG) for evaluating the in-plane structural order and accounting for structural differences in highly ordered carbon materials is well-known.32−38 The average bandwidths

31.0 27.6 42.0 37.0

(1.5) (1.3) (1.7) (1.9)

DOG

p

N

0.8365 0.7559 0.9298 0.8017

0.1634 0.2440 0.0701 0.1982

65.61 61.62 93.74 85.73

(full width at half-maximum, fwhm) and disorder parameters (ID/(ID + IG) are presented in Table 7. The standard deviation Table 7. Raman Spectroscopy Parameters and Oxidation Reactivity Temperatures for Graphitized Co-cokes fwhm (cm−1)

sample

Dband

Gband

ID/(ID + IG), %

oxidation reactivity temp., °C

VR/coal-465 °C, 18 h VR/coal-500 °C, 18 h DO/coal-465 °C, 18 h DO/coal-500 °C, 18 h

51.96 53.69 48.64 50.21

25.54 28.06 20.54 22.94

11.48 (0.12) 13.71 (0.15) 3.88 (0.11) 4.97 (0.14)

832 786 883 832

is given in brackets. Both bandwidths and disorder parameters suggest the dependence of the structure of the final product on the structure of its precursor co-coke. An increase in coking temperature from 465 to 500 °C leads to an increase in bandwidths and disorder parameter. A decrease in the disorder parameter is indicative of an improvement of the crystallite orientation in a graphitic material.32,34,36 The values also indicate the dependence of the final product structure on the nature of the petroleum feed, as shown by larger bandwidths and disorder parameters for products obtained from VR/coal than those obtained from DO/coal. Since the disorder parameter is an indicator of the bidimensional crystallite order, it is inversely proportional to crystallite width (La).32,35,39,40 A similar trend is observed in this work, with samples with lower disorder parameter exhibiting larger La values. Temperature-programmed oxidation has been used to study the structure of carbon materials.36,41,42 Since the reaction of carbons with oxygen progresses at particular active sites including structural defects and labile atoms,43 carbon materials with high structural ordering react at higher temperatures than those with poor structural organization. Table 7 lists the oxidation reactivity temperatures for graphitized co-cokes. The oxidation temperatures are comparable to temperatures reported by González et al.; synthetic and natural graphite reacted with oxygen at ∼830 and 1000 °C, respectively.36 In agreement with X-ray diffraction and Raman spectroscopy, the data in Table 7 show that an increase in the coking temperature serves to increase the structural disorder of the final product. There is also a clear dependence of oxidation reactivity temperature on the nature of the petroleum feed. Products obtained from VR/coal react at lower temperatures than the corresponding DO/coal products. Since oxidation proceeds through active sites and La directly relates to the ratio of the number of edge sites to the number of interior basal plane sites, a relationship between La and temperature oxidation peak has been investigated in prior studies.36 Consistent with the literature, the data presented here show that samples with larger La tend to oxidize at higher temperatures. 4418

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4. CONCLUSIONS The effects of co-coking reaction conditions (temperature and time) and the nature of the petroleum feed on the structure of the co-coke and its graphitization product were investigated by characterizing the microstructure of the resulting co-coke before and after graphitization. The overall analysis of product yield distribution shows that the co-coke yield increases with an increase in reaction severity index, although at high index values there is not much increase in co-coke yield. An increase in reaction temperature or a decrease in reaction time was found to increase the amount of mosaic carbon at the expense of textural components necessary for the formation of anisotropic structure in a co-coke, namely, domains and flow domains. Lower atomic H/C ratios and higher pore volumes in co-cokes obtained at higher reaction temperature indicated an increase in the amount of gases generated and were associated with faster reaction rates. Consistent with the literature, shorter reaction time deprives the system enough time to form large and elongated domains. The dependence of the graphitization product on the nature of the precursor co-coke was shown by a change in the degree of graphitization and crystallite parameters, as measured using X-ray diffractometry. In agreement with X-ray diffraction, Raman spectroscopy and temperature programmed oxidation showed that an increase in co-coking temperature from 465 to 500 °C increases the structural disorder in the final product, as shown by an increase in the disorder parameter and a decrease in the oxidation reactivity temperature. Due to the higher aromaticity of DO, products obtained from DO/coal showed higher structural order than those from VR/coal.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Air Force Office of Scientific Research and the Consortium for Premium Carbon Products from Coal for financial support of this project. The authors also thank Gary Mitchell, Nichole Wonderling, and Joe Stit for help with data analysis. The authors also thank Peter Stansberry, of GrafTech, Parma, Ohio, for his help with calcination and graphitization of samples.

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REFERENCES

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dx.doi.org/10.1021/ef300467k | Energy Fuels 2012, 26, 4413−4419