1040
Energy & Fuels 1997, 11, 1040-1043
Role of Fine Solids in the Coking of Vacuum Residues Katsumori Tanabe† and Murray R. Gray*,‡ Mizushima Refinery, Japan Energy Corp., 2-1 Ushi-dori, Kurashiki, Okayama, 712, Japan, and Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2G6, Canada Received March 24, 1997X
The yield of coke as a function of time was measured for vacuum residue from Athabasca in a batch reactor at 430 °C. The presence of fine solids in the residue inhibited coke formation, delaying the onset of coking and reducing the yield of coke by as much as 7-9 yield % after 20-30 min. Similar trends were observed in coke formation from an asphaltene fraction of Athabasca bitumen. Experiments with an asphaltene fraction showed that coke yields at short reaction times were very sensitive to subtle changes in the reaction mixture. These results suggest that fine solids may help to disperse a coke-precursor phase at reactor conditions, preventing coalescence and thereby altering the rate of formation of solid coke.
Introduction Coking of vacuum residues from petroleum and bitumen is of interest in a number of process technologies, from delayed coking to hydrocracking. In coking processes, the objective is to maximize the yield of cracked products while also producing a coke material of the desired quality. In hydrocracking processes, on the other hand, the objective is to avoid the formation of coke deposits. An understanding of the mechanisms and kinetics of coke formation is important in both cases. Coke is defined operationally in petroleum refining as a hydrocarbon material that is insoluble in an aromatic solvent such as benzene or toluene. Therefore, the kinetics of coke formation necessarily consider the formation of a complex mixture of components. The chemical species in the coke product will depend on the composition of the starting vacuum residue and the process history of the coke. These factors are well understood for mesophase coke, that is, for the formation of a structured anisotropic phase within the coke material. Mesophase formation is desirable for the production of graphitic carbon products.1 Its formation is aided by the use of highly aromatic, low-sulfur feeds such as FCC decant oil and by careful control of process temperatures.2-4 The formation of coke is usually attributed to condensation and polymerization of aromatic components, eventually giving a carbon-rich material. Unfortunately, the terminology for heavy hydrocarbon pitches * To whom correspondence should be addressed. Telephone: (403) 492-7965. Fax: (403) 492-2881. E-mail:
[email protected]. † Japan Energy Corp. ‡ University of Alberta. X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Marsh, H.; Cornford, C. In Petroleum-Derived Carbons; Devinney, M. L.; O’Grady, T. M., Eds.; American Chemical Society: Washington, DC, 1976; pp 266-281. (2) Marsh, H.; Latham, C. S. In Petroleum-Derived Carbons; Bacha, J. D., Newman, J. W., White, J. L., Eds.; American Chemical Society: Washington, DC, 1986; pp 1-28. (3) Mochida, I.; Oyama, T.; Korai, Y.; Fei, Y.-Q. Fuel 1988, 67, 11711181. (4) Tanabe, K.; Takada, T.; Newman, B. A.; Satou, M.; Hattori, H. J. Jpn. Inst. Energy 1996, 75, 916-924.
S0887-0624(97)00045-5 CCC: $14.00
and cokes is a confusing mix of definitions that depend on operational definitions such a solubility in a solvent. Further hampering our understanding is the difference in phase behavior between the reactor conditions in situ and the conditions in the laboratory where samples are analyzed. At the temperatures typical of coking and residue hydrocracking processes (in excess of 420 °C), coke materials can exhibit fluid properties such as coalescence and deformation by shear.3 Petroleum coke materials, therefore, should be considered as potential thermoplastics at reactor conditions. The viscosity of the coke material at reactor conditions will depend on the extent of cross-linking and devolatilization reactions. Depending on the composition of the liquid phase at reactor conditions, separation of a dense aromatic liquid phase may precede actual coke formation.3,5,6 This isotropic pitch liquid will be rich in components that are insoluble in pentane (asphaltenes) and toluene (coke), but at short reaction times it will be soluble in stronger solvents such as quinoline and trichlorobenzene. With time, mesophase may form as spheres that can coalesce depending on the hydrodynamics of the isotropic liquid and the mesophase itself.3 Micrographs of these coke materials provide compelling evidence of their liquid behavior at reactor temperatures. Alternatively, rapid cross-linking may prevent formation of mesophase by increasing the viscosity of the coke phase and may eventually eliminate thermoplastic behavior. When a significant quantity of pentane-soluble material is present in the reactor, as in hydrocracking, then the thermoplastic coke phase will form in contact with a less viscous oil phase. An alternative mechanistic view of coke formation has been suggested by Storm et al.7 They suggest that coke formation proceeds via the formation of micelles of asphaltene molecules, analogous to flocculation of a dispersed polymer in a liquid phase. Based on data for the size of dispersed asphaltenes and rheological mea(5) Dukhedin-Lalla, L.; Yushun, S.; Shaw, J. M. Fluid Phase Equilib. 1989, 53, 415-422. (6) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32, 2447-2454. (7) Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Energy Fuels 1995, 9, 168-176.
© 1997 American Chemical Society
Solids in Coking of Residues
surements, they suggest that the dispersed asphaltene components flocculate as temperature increases over 150 °C to give microstructural changes in the fluid. These flocs would in turn give coke formation at reactor temperatures. An alternative explanation for the changes in rheology observed by Storm et al.7 would be the formation of dispersed droplets of liquid phase. Rheological measurements during mesophase formation have been used to follow the accumulation of the new dispersed phase; viscosity tends to increase as the volume fraction of mesophase increases.8 Despite the importance of coking phenomena, and the possible importance of phase behavior, very few reports have appeared on the kinetics of coke formation. One of the most comprehensive studies was that of Banerjee et al.,9 who investigated the kinetics of coke formation from fractions of vacuum bottoms from petroleum, heavy oil, and bitumen. They found that coke was formed most rapidly from the asphaltene fraction of the oil and that increased aromaticity correlated with a higher rate of coke formation. An induction period is sometimes observed in coking, where no coke is formed for a duration after the mixture reaches reaction temperature and then coke formation proceeds at a high rate.6 The coke product concentrates some of the components in the feed, such as nitrogen, vanadium, and mineral solids.10 When metals such as molybdenum are added to heavy oils as hydroprocessing catalysts, the resulting metal sulfides are concentrated in the coke fraction.11 Although mineral materials such as iron sulfides and clay minerals are ubiquitous in vacuum residues, their potential for altering the kinetics of coke formation has not been considered. Beuther et al.12 found that fine catalyst solids (γ-alumina and η-alumina) inhibited the formation of large domains of mesophase. Instead, only very small domains were observed. They attributed this inhibition to the formation of coke on the surface of the alumina, but this hypothesis was not tested by any direct experiments nor were the details of the experimental conditions reported. An alternative explanation is that the addition of fine solids interfered with the coalescence of the mesophase, as observed in coal tar by Dubois et al.13 and Bradford et al.14 Coke formation involves the formation of a new phase either via a liquid-phase separation3,5,6 or by flocculation of asphaltene micelles.7 In either case we would expect the developing coke material to interact with mineral solids via mechanisms such as nucleation of the coke phase or flocculation with coke solids. Fine clays can stabilize oil-water emulsions.15 Therefore, by analogy, we may expect fine solids with suitable surface proper(8) Rand, B. In Petroleum-Derived Carbons; Bacha, J. D., Newman, J. W., White, J. L., Eds.; American Chemical Society: Washington, DC, 1986; pp 45-61. (9) Banerjee, D. K.; Laidler, K. J.; Nandi, B.; Patmore, D. J. Fuel 1986, 65, 480-484. (10) Gray, M. R. Upgrading Petroleum Residues and Heavy Oils; Marcel Dekker: New York, 1994. (11) Peureux, S.; Bonnamy, S.; Fixari, B.; Lambert, F.; Le Perchec, P.; Pepin-Donat, B.; Vrinat, M. Bull. Soc. Chim. Belg. 1995, 104, 359366. (12) Beuther, H.; Larson, O. A.; Perotta, A. J. In Catalyst Deactivation; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1980; pp 271-282. (13) Dubois, J.; Ahache, C.; White, J. L. Metallography 1970, 3, 337368. (14) Bradford, D. J.; Greenhalgh, E.; Kingshott, R.; Senior, A.; Bailey, P. A. Third Conference on Industrial Carbons and Graphite; Society of Chemical Industry: London, 1971; pp 520-527. (15) Yan, N.; Masliyah, J. Colloids Surf. A 1995, 96, 229-242.
Energy & Fuels, Vol. 11, No. 5, 1997 1041 Table 1. Composition of Athabasca Vacuum Residue and Supercritical Fluid Extraction (SFE) Residue property
vacuum residue
SFE residue
toluene insolubles, wt % S, wt % N, wt % V, wppm aromatic C, % asphaltenes, wt % MCR, wt % average molecular weight
1.8 3.13 3,500 291 36 29.3 13.1 1690
4.9 6.51 10,500 877 49 84.7 48.9 4662
ties to stabilize an emulsion of coke precursors in oil. The objective of this study was to study the role of mineral solids in the kinetics of coke formation to understand how alteration of the concentration of mineral solids could alter the subsequent processability of the oil. Materials and Methods The experiments used two residue materials: Athabasca bitumen vacuum residue (524 °C+) and residue from extraction of Athabasca vacuum residue with supercritical pentane.16 The properties of the feed materials are listed in Table 1. Solids were removed from the vacuum residue by digesting with 40 parts toluene for 12 h, then filtering through a 0.25 µm membrane filter (Millipore). The filtered solids were vacuum-dried at 90 °C for 2 h. The solids are mainly kaolinite (ca. 75%) with some illite and montmorillonite.17 Centrifugation of the bitumen during the extraction process removed particles larger than 50 µm. Solvent was removed from the filtered oil by rotary evaporation followed by heating on a hot plate for 12 h at 120 °C then vacuum-drying for 12 h at 120 °C. The solids were removed from the supercritical extraction residue by diluting the oil 4:1 in methylene chloride and then centrifuging at 3100 rpm for 1 h. The liquid was decanted, and the methylene chloride was evaporated by rotary evaporation and then by vacuum-drying for 12 h at 70 °C. The oils were stored at 5 °C until they were loaded into the reactor. The feeds were reacted in a batch microreactor made from Swagelock fittings and tubing to give an internal volume of ca. 15 mL. The reactor was filled with liquid and then pressure tested with nitrogen at 20 MPa. The gas was then vented, and the reactor was pressurized twice more with nitrogen and vented to purge residual oxygen. The reactor was immersed in a fluidized sand bath and agitated at ca. 1 Hz for the duration of the reaction interval, then cooled in water. The contents of the reactor reached the final reaction temperature within 5 min. Total reaction times, measured from immersion in the sand bath, ranged from 10 to 90 min. The standard reaction conditions were a temperature of 430 °C and a cold pressure of 0.1 MPa nitrogen, giving ca. 0.24 MPa at reaction temperature. The reactor was loaded with 3 g of feed. Gas yield was determined by weighing the reactor before and after venting the gas. Liquid product and coke were washed out of the reactor with 40 parts toluene, then heated to 70 °C for 12 h to ensure extraction of liquid products from the solid coke. Solids were removed from the toluene solution by filtration on a 0.25 µm Millipore filter. The solids were then washed in toluene and vacuum-dried at 90 °C for 2 h. Toluene was removed from the filtrate by rotary evaporation, then oil was blended with 120 mL of n-pentane and mixed overnight. The solid asphaltene precipitate was removed by filtration, then dried at 90 °C for 2 h. The pentane was removed from the pentane-soluble fraction by rotary evaporation and then by vacuum-drying at 50 °C to constant weight. Yields were calculated as follows: (16) Chung, K. H.; Xu, C.; Hu, Y.; Wang, R. Oil Gas J. 1997, 95 (3), 66-69. (17) Kasperski, K. L. AOSTRA J. Res. 1992, 8, 11-53.
1042 Energy & Fuels, Vol. 11, No. 5, 1997 coke yield )
mTI - msolids mfeed - msolids
asphaltene yield )
mPI-TS mfeed - msolids
Tanabe and R Gray (1)
(2)
oil yield )
mPS mfeed - msolids
(3)
gas yield )
mgas mfeed - msolids
(4)
Coke was defined as the toluene-insoluble fraction corrected for the solids in the original feed oil, which were predominantly clay minerals. In selected experiments, solids were mixed back into the solids-free vacuum residue at a concentration of 1.8% by weight using two different methods. In the sonication method, residue was diluted with 10 parts methylene chloride. The solids were added to the solution, then the mixture was sonicated for 60 min. Solvent was removed by drying on a hot plate for 12 h and then by vacuum-drying at 50 °C for 12 h. In the heating plus agitation method the residue and the solids were added to a reactor, which was then heated to 200 °C for 60 min to disperse the solids.
Results and Discussion Coke Formation from Vacuum Residue. The yields of asphaltenes and coke from Athabasca vacuum residue and from solids-free vacuum residue as a function of time are illustrated in Figure 1. Repeat experiments showed that the reproducibility of the measurements was (1 wt %. The vacuum residue gave an induction time of 10-15 min before appreciable amounts of coke were formed. Removal of the solids eliminated this induction time so that the solids-free vacuum residue gave more coke at every time interval. After 90 min of reaction, the two feeds gave similar coke yields, although the yield from the solids-free feed was slightly higher. The trend for the asphaltenes was reversed in that the solids-free residue gave lower asphaltene yields between 20 and 60 min of reaction. Yields of oils (pentane-soluble liquids) and gases were comparable with both feeds. Oil yield decreased in both cases from 68 to 72% after 10 min to 45% after 90 min (Table 2). Gas yield increased monotonically from 1.8% at 10 min to 13.0-13.8% after 90 min with both feeds. The material balance was in the range of 94-103% with a mean value of 97.7%. The differences in coking could be due to two factors: chemical alteration of the residue due to dilution with toluene, filtration, and subsequent drying; interactions of the solids with the developing coke phase. To rule out the former cause, a control sample of vacuum residue was prepared by diluting with toluene and then by following the same drying procedure as in preparing the solids-free feed. Only filtration was omitted so that the sample contained the original solids. The results from this control sample are presented in Table 3, and the data clearly show that the yields were unchanged from that of the original VR, within experimental variablity. The difference in coke and asphaltene yields between the two feeds, therefore, must be attributed to interactions between the fine suspended solids and the developing coke phase. If the fine solids inhibit the transformation of asphaltenes into coke, as indicated by the data of Figure
Figure 1. Yield of asphaltenes and coke from reaction of Athabasca vacuum residue and solids-free Athabasca vacuum residue. Reactions were at 430 °C under a nitrogen atmosphere. Experiments at times from 10 to 60 min were repeated two or three times. Error bars for experiments conducted in triplicate show the standard deviation. Table 2. Mean Yields of Oil and Gases from Replicate Coking Experiments at 430 °C VRa
VR, solids-freea
time, min
oil
gas
oil
gas
0 10 20 30 40 60 90
70.1 72.2 63.7 66.7 60.1 55.1 45.3
0 1.8 3.8 5.3 6.4 9.4 13.8
66.3 68.4 63.9 61.8 56.5 53.4 45.3
0 1.7 3.0 4.9 8.8 10.4 13.0
a
VR ) vacuum residue.
Table 3. Coke and Asphaltene Yields from Athabasca Residues at 430 °C solids addition time min
VRa
VR solids-freea
control VR
0 30 40 60
0 5.8 10.2 18.5
0 13.3 16.2 21.2
0 30 40 60
29.8 23.3 19.9 13.9
Asphaltene Yield, wt % 32.6 31.5 34.0 16.6 19.7 16.4 14.5 17.0 14.8 11.6 15.1 13.5
sonication
Coke Yield, wt % 0 7.7 10.8 18.8
0 12.8 15.0 18.8
heating + agitation 0 17.2
13.8
a VR ) vacuum residue. Mean values are given for replicate experiments.
1, then the addition of solids to the solids-free material should result in behavior similar to the original vacuum residue. Two methods were used to mix 1.8 wt % of dried filtered solids back into the solids-free vacuum residue: sonication and heating plus agitation. The data of Table 3 show that both samples gave the similar yields of coke and asphaltene to the solids-free vacuum
Solids in Coking of Residues
Energy & Fuels, Vol. 11, No. 5, 1997 1043
Table 4. Yields of Coke and Asphaltenes for SFE Residue at 430 °C SFE residue
SFE residue, solids-free
time, min
coke
asphaltene
coke
asphaltene
0 30 40 60
0.0 31.8 39.6 49.5
89.1 30.6 19.1 11.1
0.0 42.8 49.9 53.1
89.4 16.0 14.6 8.5
residue. This result was likely due to poor redispersion of the fine solids in the vacuum residue, after being collected as a dried cake from filtration. Coke Formation from SFE Residue. The same trends due to the presence of fine solids was observed with SFE residue as with vacuum residue (Table 4). The presence of the fine solids clearly inhibited the transformation of asphaltene components into coke, even at much higher initial concentrations of asphaltenes. These results confirm the observations from the vacuum residue feeds. The fractional conversion of asphaltenes was higher in the SFE residue, ranging from 0.74 at 30 min to 0.89 at 90 min compared to 0.36 and 0.59, respectively, in the vacuum residue. Any formation of asphaltene components from the oils in the vacuum residue would tend to give an apparent low asphaltene conversion. In contrast, such reactions would be less important in the SFE residue, which contained a much lower fraction of oil. Mechanism of Solids-Coke Interaction. The presence of finely dispersed solids inhibited the rate of coke formation, both in the vacuum residue and in the SFE residue. The inhibition was most pronounced at short reaction times, when the solids gave rise to an induction period with very low coke formation. After 90 min, the yield of coke was reduced slightly due to fine solids. From these observations, we conclude that fine solids inhibit the initial development of coke. If fine solids caused either nucleation of the coke phase, or flocculation/coalescence of coke material, then fine solids would enhance the rate of coking. These mechanisms, therefore, were not significant. Fine solids could inhibit free radical chain reactions by providing a surface for termination of radicals. Finely divided clay minerals would provide more surface area than the walls of the reactor and could reduce the overall concentration of radicals by increasing the rate of heterogeneous termination reactions. The formation of gases during coking is due to cracking of aliphatic groups in the residue. Any significant suppression of free radical reactions due to the presence of solids would necessarily suppress the formation of gases at each time interval. The data of Table 2 show that the presence of solids had no effect on the formation of gas. Therefore, the fine solids did not suppress free radical reactions. The most plausible remaining mechanism is stabilization of highly dispersed coke precursors by solids, analogous to the stabilization of oil-in-water and waterin-oil emulsions by fine solids. If the solids accumulate at the interface between the oil and the coke (see Figure 2 for schematic diagram of interfaces), then the coalescence of the coke into larger droplets would be inhibited. This mechanism would also account for the effective collection of mineral solids in coke products.11 The role of the solids in the oil/coke mixture would be equivalent to the observations of Beuther et al.12 and Dubois et al.13 wherein fine solids inhibited the coalescence of me-
Figure 2. Schematic diagram of clays at interfaces of thermoplastic coke dispersed in oil. Table 5. Yields of Coke from Diluted SFE Residue at 430 °C, 40 min Reaction
molecular weight
diluent aromatic N C, % wppm
1210 1120 950 820
43 43 36 40
7900 7530 7370 6810
resin wt %
wt % diluent
mole fraction SFE residue
59.5 54.3 46.3 34.8
77.8 76.9 74.0 69.9
0.076 0.074 0.074 0.078
coke wt % 15.0 12.9 8.5 7.7
sophase in a mixture of isotropic pitch and mesophase. A highly dispersed coke phase would have more contact with hydrogen donor compounds in the oil phase, which would in turn inhibit the rate of dehydrogenation and polymerization. To systematically alter the composition of the oil phase and observe its effect on coking, SFE residue was reacted in mixtures with successively lighter fractions from supercritical pentane extraction of the 524 °C+ fraction of Athabasca bitumen.16 Weights were adjusted to maintain a constant molar concentration of SFE residue in the diluting material. The results listed in Table 5 clearly show that addition of a succession of lower molecular weight diluents with less aromatic carbon gave a progressive reduction in coke yield. In this series of experiments, the concentration of resins in the reacting mixture was successively reduced, which would normally be expected to destabilize the asphaltenes and enhance coking. Instead, the more dissimilar mixtures gave lower coke yield. These experiments do not prove that solids act by dispersing coke liquids in the oil phase, but they do suggest that the kinetics of coke formation are sensitive to subtle changes in the chemistry of the liquid phase at constant time, temperature, and concentration. Given such a degree of sensitivity, it is reasonable to suppose that macroscopic phase behavior of coke materials at reactor conditions will have a profound influence on the apparent rates of reaction. Conclusions The presence of naturally occurring fine mineral solids in the residue fraction of Athabasca bitumen suppressed the rate of formation of coke relative to solids-free feed. The same trend was observed for a residue of supercritical fluid extraction with pentane. Suppression of coking gave a concomitant increase in the concentration of asphaltenes at a given reaction time. The observed changes in reaction rate were consistent with macroscopic interactions of fine solids with a dispersed phase of thermoplastic coke. EF970045U
