Agglomeration and Deposition of Coke during Cracking of Petroleum

Energy & Fuels 2007, 21, 1205-1211

1205

Agglomeration and Deposition of Coke during Cracking of Petroleum Vacuum Residue† Weidong Bi, William C. McCaffrey, and Murray R. Gray* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada ReceiVed June 9, 2006. ReVised Manuscript ReceiVed June 29, 2006

Hydrophobic fine solids can reduce the yield of toluene-insoluble coke in thermal cracking of heavy oils at short reaction times. In this work, fine carbon solids were used as additives in the coking of Arab heavyvacuum residue (AHVR) in 1-methylnaphthalene (1-MN) to study the mechanism of this interaction. Both mesophase carbon and spherical graphite particles significantly reduced the coke yield compared with the case of no solid addition. Coke deposited on the surfaces of the two hydrophobic additives. This nucleation of coke deposition on the solid reduced the agglomeration of the coke, giving a better dispersion in the liquid. The highly dispersed coke phase on fine solids was more accessible for reactions with hydrogen-donor compounds in the oil phase, which in turn inhibited the initial rate of coke formation.

Introduction The formation of a toluene-insoluble phase from petroleum fractions during thermal cracking is an important precursor to fouling on heat-transfer surfaces and the formation of coke in processes such as delayed coking. The majority of previous studies have focused on the molecular basis of coke formation, with a particular emphasis on the linkage between the chemical properties of the asphaltenes and the formation of tolueneinsoluble solids,1-4 which we will define as coke for the purpose of this study. While the mechanisms leading to formation of coke deposits on heat-transfer surfaces in coking processes and the formation of solid coke have their roots in the chemistry of the oil components, coking of the heavy fractions of petroleum and bitumen also involves phase change from single-phase liquid to a two-phase or multiphase system.4 Fouling of heat-transfer surfaces involves additional transport processes in the vicinity of the heated metal surface.5 As coke forms from a homogeneous liquid, it has the potential for colloidal interactions between dispersed coke domains, for agglomeration and coalescence of the coke domains, and for wetting of solid surfaces. At longer reaction times, the coke phase can develop ordered domains of mesophase which allow optical analysis of coke structure.6-8 At the early stages of coke formation during the thermal cracking of heavy oils, bitumens, and vacuum residues, the † Presented at the 7th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. E-mail: murray.gray@ ualberta.ca. Telephone: 780-492-7965. Fax: 780-492-2881. (1) Kriz, J. F. Can. J. Chem. Eng. 1994, 72, 85-90. (2) Magaril, R. Z.; Aksenova, E. I. Int. Chem. Eng. 1968, 8, 727-729. (3) Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Fuel Sci. Technol. Int. 1996, 14, 243-260. (4) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32, 2447-2454. (5) Watkinson, A. P.; Wilson, D. I. Exp. Therm. Fluid Sci. 1997, 14, 361-374. (6) Brooks, J. D.; Taylor, G. H. Nature 1965, 206, 697-699. (7) Gentzis T.; Rahimi P.; Malhotra R.; Hirschon, A. S. Fuel Process. Technol. 2001, 69, 191-203. (8) Rahimi, P.; Gentzis, T.; Fairbridge, C. Energy Fuels 1999, 13, 817825.

Table 1. Properties of Arab Heavy-Vacuum Residue property

AHVR

carbon (wt %) hydrogen (wt %) sulfur (wt %) nitrogen (wt %) vanadium (wt ppm) nickel (wt ppm) microcarbon residue (wt %) asphaltenes (wt %)

84.1 9.09 5.38 0.44 199 54.3 22.8 19.8

boiling point distribution (°C) 10% mass 30% mass 50% mass 70% mass

523.0 579.5 635.0 700.0

Table 2. Coke Yields from 25 wt % AHVR in 1-MN with or without Solid Addition at 430 °C and 40 mina carbon additive

coke yield (wt %)

repeat experiments

none mesophase carbon spherical graphite

9.29 ( 0.84 7.15 ( 0.86 6.29 ( 0.23

4 3 3

a Carbonaceous particles were added at 0.06 g/g of AHVR. Error estimates are standard deviations.

initial coke domains in the liquid phase are on the order of a few microns in diameter.9,10 Interactions with hydrophobic clays can give reductions in the yield of coke from liquid-phase cracking of bitumen,11,12 although the mechanism for the reduction in coke is unclear. The advantage of working with fine solids in model systems to understand the mechanisms of initial coke formation is that dispersed fine solids offer a large surface area for interaction with the coke phase, with minimal diffusion resistance or effects from heat transfer and momentum transfer. (9) Wang, S.; Chung, K.; Masliyah, J. H.; Gray, M. R. Fuel 1998, 77, 1647-1653. (10) Rahmani, S.; McCaffrey, W.; Elliott, J. A. W.; Gray, M. R. Ind. Eng. Chem. Res. 2003, 42, 4101-4108. (11) Tanabe, K.; Gray, M. R. Energy Fuels 1997, 11, 1040-1043. (12) Sanaie, N.; Watkinson, A. P.; Bowen, B. D.; Smith, K. J. Fuel, 2001, 80, 1111-1119.

10.1021/ef0602630 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/30/2006

1206 Energy & Fuels, Vol. 21, No. 3, 2007

Bi et al.

Figure 1. SEM micrographs of a mesophase carbon particle after blank experiment: (a) a whole particle and (b) a local area in higher magnification.

Figure 2. SEM micrographs of a spherical graphite particle after blank experiment: (a) a whole particle and (b) a local area in higher magnification.

In this study, we investigated the interactions of tolueneinsoluble solids or coke with powdered carbon materials. Fine particles of mesophase carbon and spherical graphite provided compatible well-defined surfaces for wetting and agglomeration of coke domains. These solids had the added advantage of providing a well-defined surface composition and controlled particle size, in contrast to early work on hydrophobic clays.11,12 By thermally cracking the Arab heavy-vacuum residue (AHVR) in the presence or absence of carbon solids, we were able to study the role of microphase behavior and colloidal interactions on the initial stages of coke formation.

purity, Aldrich, Milwaukee, WI) and 2,3,4-tetrahydronaphthalene (tetralin; laboratory grade, Fisher Scientific, Fair Lawn, NJ). All other solvents were from Fisher Scientific. Mesophase carbon and spherical graphite were used as hydrophobic carbon additives to study the interactions with the coke phase in a liquid suspension. The mesophase carbon was supplied by Kawasaki Steel Corp. (Kobe, Japan) with a size range from 1 to 74 µm and a D50 of 17.7 µm. The spherical graphite was from BTR Energy Materials Co., Ltd. (Shenzhen, China) with a size range from 6 to 43 µm and a D50 of 16.8 µm. Neither material contained detectable sulfur (Antek Model 9000 sulfur and nitrogen analyzer). Coking reactions were carried out in a 15 mL autoclave batch reactor made from stainless steel tubing and fittings (Swagelok, Edmonton, AB). In each experiment, 4.5 g of 1-MN was added to the reactor to help control coalescence of the coke phase during the coking reaction. The other reactants, up to 1.5 g of AHVR and 0.09 g of mesophase carbon or spherical graphite, were added to the reactor without premixing. After the reactants were loaded, the reactor was pressure tested and purged with nitrogen three times at 20 MPa. The reactor was closed at 3 MPa of nitrogen at room

Materials and Methods Arab heavy-vacuum residue (AHVR) was selected as a typical feed material for coking processes. This solids-free material was provided by ExxonMobil Research and Engineering, Clinton, NJ. The properties of AHVR are listed in Table 1, as measured by the Syncrude Canada Analytical Laboratory (Edmonton, AB). Solvents for the reaction of this oil were 1-methylnaphthalene (1-MN, 95%

Agglomeration and Deposition of Coke

Energy & Fuels, Vol. 21, No. 3, 2007 1207

Figure 3. SEM micrographs of a coke particle from 25 wt % AHVR in 1-MN at 430 °C and 40 min: (a) a whole particle and (b) a local area in higher magnification.

Figure 4. SEM micrographs of a particle of mesophase carbon with coke from 25 wt % AHVR in 1-MN with mesophase carbon addition (0.06 g/g AHVR) at 430 °C and 40 min: (a) a whole particle and (b) a local area in higher magnification.

temperature to provide an inert atmosphere. The reactor was immersed in a fluidized sand bath (Model SBS-4, Cole-Parmer, Vernon Hills, IL) that was preheated to the target temperature of 430 °C and agitated at 1 Hz to ensure a uniform temperature during the reaction. After the reaction time of 40 min had elapsed, the reactor was quenched in cold water to terminate the reaction. The gas was then vented, and the liquid and solid products were recovered with 60 mL of toluene and sonicated for 30 min (except as noted) using an Aquasonic Ultrasonic Cleaner (Model 150HT, VWR, West Chester, PA) to ensure extraction of the liquid product from the solid product. The toluene-insoluble (TI) product, either coke or coke with a fine solid additive, was recovered from the mixture by filtration using a 0.22 mm membrane filter (Millipore, Billerica, MA), and then it was washed with toluene until the toluene passed through the membrane without color. To determine the coke yield, the filter cake was weighed after vacuum-drying at 70 °C for 12 h to remove the residual toluene. Solids were prepared for microscopy by dispersion of the wet filter cake in toluene and then drying of a thin film of the suspension on a glass plate to obtain dry particles. Particle size distributions were measured after

the wet filter cake was dispersed in ethanol with a Malvern Mastersizer 2000 (Worcestershire, UK). Samples of the solid particles from coking were prepared for scanning electron microscopy (SEM) by being sputter-coated with chromium. Samples were embedded in either Lucite or green Bakelite and mounted, polished, and sputter-coated to allow examination of cross sections. A JEOL JSM-6301F SEM (Tokyo, Japan) equipped with a Princeton Gamma-Tech energy-dispersive X-ray analyzer (EDX; Rocky Hill, NJ) was used to examine the samples. The surface concentration of sulfur was measured by X-ray photoelectron spectrometry (XPS; Kratos Axis-165 XPS, Manchester, UK). The absorbance of solutions of AHVR or reacted coke material was measured in solution in tetralin using a Shimadzu UV-160 spectrophotometer (Kyoto, Japan) with pure tetralin as a reference.

Results and Discussion Neither the mesophase carbon nor the spherical graphite dissolved under the experimental conditions; therefore, the yield


Bi et al.

Figure 5. SEM micrographs of a particle of spherical graphite with coke from 25 wt % AHVR in 1-MN with spherical graphite addition (0.06 g/g AHVR) at 430 °C and 40 min: (a) a whole particle and (b) a local area in higher magnification.

Figure 6. Cross section micrographs of a particle of mesophase carbon with coke from 25 wt % AHVR in 1-MN with mesophase carbon addition (0.06 g/g AHVR) at 430 °C and 40 min: (a) a whole particle and (b) an edge area in higher magnification.

of coke in the presence of solids was calculated from the total mass of solids after reaction, less the mass of carbon particles added to the reactor. The yield of toluene-insolubles (coke) from thermal cracking of AHVR in a solution of 1-methyl naphthalene at 430 °C for 40 min was reduced by the addition of fine carbon powders (Table 2). This observation was consistent with the prior work of Tanabe and Gray11 and Sanaie et al.,12 which stated that the addition of fine hydrophobic solids would reduce the yield of insoluble coke material under similar reaction conditions. The addition of 6% fine carbon solids in AHVR provided a significant surface area for the deposition of coke material. Figures 1 and 2 illustrate the morphology of the carbon particles before coke deposition, while Figure 3 shows the typical coke material in the absence of carbon particles. The coke solids were clearly formed by agglomeration of smaller domains of insoluble material, ca. 0.3 µm, as illustrated in Figure 3b. This observation was consistent with previous observations by Wang et al.9 and

Rahmani et al.10 Micrographs of particles from coking in the presence of fine carbon solids clearly showed a change in the surface morphology resulting from the deposition of coke. The smooth, glassy surface of the mesophase carbon (Figure 1) was covered by a fused layer of deposited coke that contained embedded spheres of coke material (Figure 4). Similarly, the fused graphite sheets visible in the microstructure of the spherical graphite particles (Figure 2) were largely obscured by a smooth layer of coke material after reaction (Figure 5). Cross sections of the particles confirmed that the coke material agglomerated or deposited on the carbon solids. For example, Figure 6 shows a representative particle of mesophase carbon with adhered coke material. The two morphologies are quite distinct in the cross section, with the porous coke material on top of the layered lamellar structure of the mesophase carbon. This interpretation was further confirmed by EDX analysis. For example, Figure 7 shows a line scan of the sulfur content across a particle of the mesophase carbon after coking with AHVR.



Figure 7. Line scan for sulfur by EDX on the cross section of a particle of mesophase carbon with coke: (a) cross section of the particle and (b) sulfur signal from point 1 to 2.

The mesophase carbon contained negligible sulfur, in contrast to 7.22% sulfur in the coke. The line scan clearly indicates that the sulfur was deposited on the outer edge of the mesophase carbon particle. The micrographs of the solids after reaction suggested that the deposition of coke on the carbon particles was not as a uniform layer but was a combination of agglomeration of small coke domains and deposition on the carbon particles. For example, the uneven coating of porous coke in the cross section in Figure 6 suggests that significant coke formation occurred in the liquid phase, as submicron domains, which then agglomerated on the surface of the mesophase carbon. We would anticipate that the fate of these submicron domains would change with reaction temperature, but this variable was not investigated in this study. Since the effect of fine solids on coke formation was similar for mesophase carbon, spherical graphite (Table 2), and hydrophobic clays,11 molecular patterning of the coke on the particle surfaces was unlikely. Formation of a surface layer could, however, change both the thickness of deposit and its microstructure. This mechanism was investigated by a series of experiments with lower concentrations of AHVR in the reactor to reduce the ratio of coke to carbon solids. At very low concentrations of coke precursors, we would expect the formation of coke directly on the carbon solids without formation of coke domains in the liquid phase. When the concentration of AHVR in 1-methyl naphthalene was reduced from 25 to 0.99 wt %, the carbon solids showed no evidence of the deposition of coke spheres. The mesophase carbon retained its glassy surface morphology (Figure 8a), and the spherical graphite still exhibited irregular plates of carbon

Figure 8. SEM micrographs of particles of mesophase carbon with coke from (a) 0.99 wt % and (b) 9.9 wt % AHVR in 1-MN. Mesophase carbon was added at a ratio of 0.2 g/g AHVR with reaction at 430 °C for 40 min.

(Figure 9a). No sulfur was detected on these particles by EDX, but XPS gave a surface sulfur content of 4.34 wt % for the mesophase carbon with coke material and 2.88 wt % for the spherical graphite with coke material (Table 3). Since both of the two fine solids contained nearly no sulfur, the substantial sulfur content on the surface of each added solid indicated that coke material did deposit on the solid surfaces. At an intermediate concentration of 9.9 wt % AHVR in 1-methyl naphthalene, connected submicron coke spheres and their further coalescence and fusion were observed on the surface of the mesophase carbon (Figure 8b). The morphology of the spherical graphite was obscured by the deposited coke (Figure 9b), but the submicron spheres were not evident. In both cases, the sulfur concentration on the particle surfaces increased as determined by XPS analysis (Table 3). The lack of submicron spheres on the surface of the spherical carbon (Figure 9b) and the small number of such spheres visible at higher concentration of AHVR (Figure 5b) could be caused by either reduced for-


Bi et al.

Figure 10. Comparison of particle size distributions of TI products from 25 wt % AHVR in 1-MN without solid addition and with spherical graphite addition (0.06 g/g AHVR) at 430 °C and 40 min. TI products were suspended in ethanol and shaken for 1 min.

Figure 11. Effect of sonication on particle size distributions of TI products from 25 wt % AHVR in 1-MN without solid addition and with spherical graphite addition (0.06 g/g AHVR) at 430 °C and 40 min. TI products were suspended in ethanol and sonicated for 30 min. Figure 9. SEM micrographs of particles of spherical graphite with coke from (a) 0.99 wt % and (b) 9.9 wt % AHVR in 1-MN with spherical graphite addition (0.2 g/g AHVR) at 430 °C and 40 min. Table 3. Surface Sulfur Content Analysis by XPS solid sample blank mesophase carbon blank spherical graphite mesophase carbon with coke spherical graphite with coke

AHVR concentration in 1-MN (wt %)

surface sulfur content (wt %)

0.99 9.9

0.16 0.45 4.34 6.39

0.99 9.9

2.88 5.57

mation of the spheres in the liquid phase or better wetting and spreading of the spheres on the graphite surface in comparison to the mesophase carbon. The agglomeration of spheres or droplets requires proximity and collision, which decreases as a second-order function as concentration is reduced. Consequently, these observations are also consistent with the kinetics of agglomeration. Particle size analysis confirmed the importance of the formation of submicron spheres and agglomeration at high concentration of AHVR. As illustrated in Figure 10, the particle size

distribution of coke without added carbon was very broad, indicating extensive agglomeration of the coke spheres as illustrated in Figure 3. The narrow distribution of particle sizes of the spherical graphite was still visible in the particles from coking of 25% AHVR with carbon particles (Figure 10), but it was superimposed on a broad distribution. The addition of the spherical carbon did significantly reduce the fraction of the solids in very large particles of over 100 µm diameter. The role of aggregation was examined by sonicating the particles and repeating the analysis. As illustrated in Figure 11, the large agglomerates were completely dispersed into submicron particles. The concentration of the submicron particles was substantially reduced in the presence of the spherical graphite because of the deposition of coke on the graphite surface as illustrated in the micrographs (Figures 5 and 9b). Tanabe and Gray11 proposed that the fine solids reduced the size of the domains of toluene-insoluble material, which enhanced the release of cracked products from the coke material and reduced yield. The volume moment mean diameter (D43) of the coke material represents the volume-averaged distance from the core of coke agglomerates to the bulk liquid phase and provides a measurement of the characteristic length for diffusion within the coke domains. The mean diameter for the


particles in the coke-only sample was 52 µm versus only 38 µm for the sample with spherical graphite because of the reduction in large agglomerates (Figure 10). This D43 value of 38 µm for the coke with spherical graphite includes the graphite particles; therefore, it overstates the diffusion length because the coke deposits in thin layers on the graphite material (Figure 5). Unfortunately, dtermination of the contribution of the graphite particles from the data of Figure 10 was not possible because some agglomeration of the coke-coated graphite particles occurred. Why does the reduction in the size of the coke domains result in a decrease in coke yield? The solubility of fractions, such as the asphaltenes, is sensitive to hydrogen content;4 therefore, hydrogen-donor reactions in the liquid phase can reduce the yield of toluene-insoluble material from asphaltenes. The addition of hydrogen from donors such as tetralin will help maintain solubility of asphaltenes during thermal reactions,13,14 and such compounds occur naturally in petroleum fractions.15 A shorter diffusion path would allow more reaction of material in the coke domains with liquid-phase hydrogen donors, thereby reducing the yield of coke. This hypothesis was tested by reacting the coke solids formed with and without spherical graphite with tetralin. The coke samples were prepared following the same method as the experiments reported in Table 2, but the wet filter cake was dispersed in 4.5 mL of tetralin and reacted again under identical conditions. The product mixture was then filtered to remove unconverted solids, and the absorbance of the resulting tetralin solution was measured to estimate the amount of coke that was redissolved. After 10-fold dilution in tetralin to ensure linear response, the absorbance of product from coke plus (13) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31, 530-536. (14) Rahmani, S.; McCaffrey, W. C.; Gray, M. R. Energy Fuels 2002, 16, 148-154. (15) Gallacher, J.; Snape, C. E.; Dennison, P. R.; Bales, J. R.; Holder, K. A. Fuel 1991, 70, 1266-1270.


spherical graphite was 0.38 ( 0.08, compared to only 0.26 ( 0.05 from coke alone (triplicate experiments). Although the spherical graphite resulted in less coke after the first stage of reaction (Table 2), subsequent reaction with tetralin gave more redissolution. If we assume that the extinction coefficient for the redissolved coke was similar to that of the AHVR at 0.19 mL/mg, then the reaction with tetralin redissolved 90% of the coke from the solids from spherical graphite, compared to only 39% of the coke that formed without addition of spherical graphite. These results suggest that the role of fine solids in suppressing coke yield in the liquid phase was caused by an enhancement of hydrogen-donor reactions between the material in the coke particles and the liquid phase. This study suggests that the interactions between suspended fine solids and the coke phase during its initial stage of formation can include determine where the coke forms, the yield of coke, and the effectiveness of hydrogen-donor reactions between the coke phase and the bulk liquid. Hydrogen-donor compounds are present in a range of petroleum fractions.15 In the case of coking processes, such as delayed coking or fluid coking, these liquid-phase interactions would be highly transient because of the rapid evaporation of the low-boiling liquid components. The wetting of fine particles by coke material could be much more important in fouling processes in fired heaters, where the deposition of coke on fine solids would be highly preferred over deposition on the heat-transfer surfaces. The potential use of fine solids in mitigating such fouling deserves further investigation. Acknowledgment. We thank the Syncrude Canada Limited for analytical support and financial assistance, along with the Natural Sciences and Engineering Research Council of Canada, through the Syncrude/NSERC Chair in Advanced Upgrading of Bitumen. EF0602630

Agglomeration and Deposition of Coke during Cracking of Petroleum

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