Petroleum coke structure: Influence of feedstock composition

Carbon,Vol. 31, No. 2, pp. 383-390,1993 Printedin Great Britain.

OLWi-6223193 $6.00 + .oO Cowight Q 1993 PergamonPressLtd.

PETROLEUM COKE STRU~URE: IN~UENCE OF FEEDSTOCK COMPOSITION MIRA LEGIN-KOLAR and DUBRAVKA UGARKOVIC University of Zagreb, Faculty of Metallurgy Sisak Aleja narodnih heroja 3,440OO Sisak, Croatia (Received 21 March 1992; accepted in revisedform

27 August 1992)

Abstract-The influence of a coking crude on the structure of calcined petroleum coke and the subsequent structural changes in coke induced by hid-tem~mture treatment up to 2400@ were investigated. The surface morphology, crystallite height (LJ, and interlayer spacing (C&Q)were determined as a function of temperature and heating time. High-temperature treatment had a positive effect on the coke structure so that with increasing temperature and heating time, the crystallite size increased while the interlayer spacing decreased. The results showed that not only temperature but also coking crude were major factors influencing the structure of petroleum coke. Key Words-Coking

feedstock, crystallite height, interlayer spacing, petroleum coke, surface morphol-

1. INTR~DU~ION

Premium grade petroleum coke is manufactured from the residues of primary and secondary petro-’ leum rehning with the atomic H:C ratio from I .O to 1.4.

The coking crude should have a high content of polycyclic aromatic hydrocarbons and both low asphaltenes (<39/o) content and detrimental metallic and nonmetallic constituents[ 1f. During the coking process, the crude passes through various phases of structural transformation. Around 43o”C, large planar mesophase molecules are generated that, by parallel alignment, combine to form lamellalike structural elements. Those make the basic structure of “graphite carbons.” The mode of arrangement of basic mesophase units will predetermine the final coke microstructure[2,3]. Up to 700°C almost all gases and volatile compounds, generated by polymerization and condensation of the feedstock components, will be separated. Substantial structural changes occur between 700 and 13OO”C,when coke density increases, coke volume and sulphur content decrease, and intensive growth of particles takes place, that is, crystallites begin to appear in the coke structure. Major changes in coke porosity will take place only above 1ooo”C and will depend to a great extent on the presence of thermally stable sulphur, whose separation from the coke structure occurs above 14OO”C[4,5]. During the carbonization process, in the temperature range between 1300” and 18Oo”C, intensive structural arrangement and crystallite growth begin, as does transformation or separation of metallic and nonmetallic micr~onstituents, caused by thermal decomposition of organic molecules and condensation of aromatic rings[6]. Above 1800°C netlike hexagonal layers begin to assemble into “stacks,” and the tw~dimensionai structure starts transforming into a graphitelike three-dimensional one[7-91. But not all carbons are graphitizable; only “graphite carbons” have that ca383

pacity. To those belongs petroleum coke, whose graphitizability can be determined from crystallographic and optical structural parameters[ 10,111. The proportion of characteristic textural units (planar, cylindrical, and wrinkled lamellae; finegrained, medium-grained and coarse-grained mosaics) will determine the shape, size and distribution of anisotropic domains[ 12,131. This article deals with the results of investigation of crystallographic and optical parameters of three petroleum cokes, manufactured from different coking crudes, before and after temperature treatment.

2. EXPERIMENTAL

Three petroleum cokes were treated in a temperature range of 1300”-24Oo”C, with different heating rates and heating times. Changes in crystallographic parameters (L, and d,,) before and after high-temperature treatment were measured by the method of X-ray diffraction. Optical texture was determined using the methods of optical microscopy with reflected polarized light and scanning electron microscopy (SEM analysis).

The samples of cokes calcined at 1250°C and used for analysis were manufactured in the Sisak Petroleum Refinery and atmospheric residue (AR) and from a mixture of atmospheric residue, gasoline pyrolysis residue (GPR), and FCC decant oil (FCC Do) in different ratios. Some analytical data of feedstocks are summarized in Table 1. The basic characteristics of the examined coke samples are shown in Table 2.

2.2 High-temperature treatment Coke samples (200 pm grain size) were placed in graphite crucibles and heated in a hi-tem~mture “Astro” furnace type 1000-3060 FP in an inert argon atmosphere at 1300”, 1500”, 1800”, 2000”, 2200”, and 2400°C. The heating rates were 10”/min and

384

M. LEGIN-KOLARand D.

UGARKOVIC

Table 1. Properties of feedstock Coking feedstock Atmospheric residue (AR)

Characteristics Real density 15/15” C (g.cmm3) Carbon (%) Sulphur (%) Aromatics (%) Asphaltenes (%) Ash (%) Quinoline insoluble (%) Metal content (rg.g-‘) V Ni Fe Na

Gasoline pyrolysis residue (GPR)

FCC decant oil (FCC DO)

1.048 89.2 0.22 51.6 6.9 0.015 CO.1

1.000 87.1 1.05 63.1 0.3 0.018 CO.1

0.941 85.2 0.81 29.5 t&8 <0:1 0.5 40.0 6.2 3.8

60”/min, respectively; the soaking times were 10 minutes, 1 hour, and 5 hours.

2.3 Determination of crystallographic parameters Crystallite height (L,) and interlayer spacing (doo2) were determined from the 002 X-ray diffraction line (Cok,) using natural graphite with dwz= 0.336 nm as an internal standard. A “Philips” Norelco X-ray apparatus was used. The degree ofgraphitization (g) was calculated according to Maire and MCring[ 141.

2.4 Determination of optical structure The polished coke samples (15-mm grain size) were examined by the method of optical microscopy in a “Carl Zeiss” polarized light optical microscope type Nf-Pol, equipped with a camera. For examination of surface morphology by means of SEM, coke samples were first etched in chromic acid at 150°C for 4 hours. Then they were washed in distilled water and dried. The preparations were coated with a thin copper layer in a vacuum evaporator and photographed in a beam of secondary electrons using a “Jeol” JXA50A microscope.

2.2 15.8 25.7 18.2

5.0 28.2 7.8 25.2

3. RESULTS AND DISCUSSION

The investigation of the effect of temperature on crystallite height, at constant heating rate and heating time, shows that between 1700’ and 2400°C all samples exhibited a linear rise in L, values. In the temperature range from 1300” to 17Oo”C, because of the separation of sulphur and metallic microconstituents, crystallite growth was slowed down or inhibited, and structure impairment brought about. Even reduction in L, values could be noted (Fig. 1). The results of investigation on the effect of heating rate and heating time, at constant temperature, show that the highest L, values ( 12- 15 nm) were obtained for all samples when they were heated at 2400°C for 5 hours. In that case the effect of heating rate was almost negligible. However, at temperatures below 2ooo”C even heating time did not produce an appreciable increase in L,. values because the crystalline structure, which is known to be highly sensitive to temperature, only began to take shape over that temperature range. Differences in crystallite size growth were found to depend on the type of coke and its feedstock.

Table 2. The basic properties of calcined cokes Coke A

Coke B

Coke C

70% Atmospheric residue (AR) 30% gasoline pyrolysis residue (GPR)

60% AR 40% FCC decant oil (FCC DO)

50% AR 25% GPR 25% (FCC DO) 2.13 0.79 0.13

Characteristics Coking feedstock Bulk density* (gem-‘) Sulphur (%) Ash (96) Metal content (*)I g-‘) V Ni Fe K Ca

2.11 0.76 0.17

2.1 I 0.95 0.18

4.60 127.30 18.40 78.00

5.20 130.00 32.00 80.10 128.50

115.00

*Measured by the pycnometer method with n-butyl alcohol.

2.70 111.40 10.40 22.00 123.80

Petroleum coke structure: influence of feedstock composition

385

COKE A-A O-B iOT -j

--c

I

V-iO’C/min t=Wmin

9-

/I 7

/-

8I

: /

0,34LO -

/

37 -

I x300

COKE

-/

1503

lax

2m

2200

1

24w

Temper&xc

36-

-

-c V: IO’Clmin t ZWrniri

&i

Fig. I. Influence of temperature on crystallite height (L,,). maximum L‘~values were always obtained for the coke samples manufactured from the feedstock with a higher content of pyrolysis residue. Namely, according to literature data[8,15], during the mesophase transfo~ation of pyrolysis residue and decant oil, big planar molecules are generated wherefrom, by polymerization ofthe aromatics, lamellae are formed as basic structural elements of “graphite carbons.” Above 1XKYC, the structure turns into a three-dimensional one, triggering graphitization of the carbon substance. The transition is feasible only if the crystallite size at this stage is over 10 nm, regardless of the type of carbon. Differences in L, values will point out differences in a carbon’s susceptibility to graphitization. As in this investigation, the L,. values for all the three coke samples were comparable, corresponding at 2400°C to the values for “graphite carbons,” structural differencescould not be determined on the basis of this parameter alone. Investigation of the effect of temperature on interlayer spacing (G&J demonstrated irregular changes of this parameter at lower temperatures. A decrease in values was noted only above 2000°C (Fig. 2). Apparently, the temperature range investigated was not high enough for this type of coke to cause a major decrease in doo2values. According to some authors[ 161 with certain carbonized substances, a doozvalue of 0.3430 nm was reached at only 18oo”C, whereas others1 17,181 claim this value to be feasible only above 2000°C provided a metal catalyst was added. The coke samples examined demonstrated a slight increase in interlayer spacing between 1300” and 18OVC, which can be attributed to structural impairment induced by the separation of volatile components, sulphur, and metals. With a further rise in temperature, doozreached the lowest values in all samples at 2400°C. This was confirmed with values for the de-

Fig. 2. Influence of temperature on interlayer spacing (dwZ).

gree of ~phiti~tion, which was for sample C, 6.9%, twice as high as that for sample A, 3.5%. For sample B, the degree of graphitization was 0%. Differences in crystallographic parameters of the coke samples examined also became manifest through their optical textures. Because studies of crystallite height and interlayer spacing showed largest structural changes to have taken place at highest temperatures, we performed an investigation of surface morphology on samples that were submitted to temperature treatment at 2400°C. Microphotographs of the calcined coke surfaces examined under reflected polarized light before and after heat treatment are given in Fig. 3. The structures of coke samples A and C revealed a lamellar texture with parallel alignment of small, not quite homogeneous, flakes. Equally anisotropic domains were rather small, particularly in sample B. The mosaic texture, which was present in all the three samples, was composed of extended, variously orientated, medium-grained structure. After high-temperature tr~tment, no major changes in the coke optical texture were observed. After etching of sample surface with chromic acid SEM microphotographs taken in the beam of secondary electrons displayed new textural com~nents. In sample A after treatment at 24Oo”C, the newly developed components had the form of flat (Fig. 4[e]), wrinkled and cylindrical lamellae (Fig. 4[b,c,e]), or a “fish-bone” configuration where, at large magnilication, layers of small lamellae were also discernible (Fig. 4[d]). Figure 5 shows a rather poorly developed structure of coke B. Iamellar areas appear to be small (Fig. 5[c,d]) and intersected with irregular variously ori-

386

M. LEGIN-KOLARand D. UGARKOVIC

Fig. 3. Polarized light micrographs of calcined coke (magn. X 110):(A-C) before HTT, (a-c) after HTT at 2400°C.

entated structures (Fig. 5[b,e]). In coke C the multilayered lamellar structure (flat and cylindrical lamellae) was present even before HTT (Fig. 6[a]). After heating to 2400°C almost all components characteristic of needle coke became visible: the lamellar (Fig. 6[b,d]) and threadlike (Fig. 6[c]) ones as well as rolled lamellae assembled into a stack with prismatic edges and basic planes. The latter structure was especially well developed (Fig. 6[e]). It was a sign of the coke’s good susceptibility to graphitization. The results showed that the method of qualitative microanalysis in a beam of secondary electrons (the SEM method) was more suitable for the determination of textural components because it revaled the struc-

tures that were not visible under reflected polarized light. The SEM analysis proved to be a very convenient, fast, and direct method for textural characterization of cokes. It was found to provide data on coke behaviour in the process of temperature treatment and to help predict its graphitizability. Investigations showed that in addition to heating temperature and heating time, the composition of coking crude had a major impact on the coke structure. The coke manufactured from a feedstock with a large proportion of light atmospheric residue, which contained small concentrations of aromatic constituents (Table 1) and almost all inorganic microconsti-

Petroleum coke structure: influence of feedstock composition

Fig. 4. SEM micrographs of coke A surface: (a) before and (b-e) after b-X3000,c-X1000,d-X5000,e-X500).

tuents that had accumulated during petroleum refining, is said to have a medium grade structure. Such were cokes A and 3 in this work. The medium-grade structure enabled grapbitization of coke at higher temperatures or in the presence of a catalyst, although the use of a graphitization catalyst may make the final product useless because it can bring in to the graphite product unwant~ impurities.

387

HIT at 2400°C (a-X 500,

However, if pyrolysis residue and decanted oil were added to the coking crude, the content of aromatic components increased, and the H:C ratio decreased. This in turn facilitated the formation of textural components (lamellar, needlelike, threadlike) that made graphitization feasible at temperatures below 3000°C. Our previous investigations[ 19-Z I J with the same coking fedstocks, based on Croatian Moslavina

M. LEGIN-KOLARand D. LJGARKOVIC

388

Fig. 5.

SEM micrographs of coke B surface: (a) before and (b-e) after HTT at 2400°C (a-X b-X 3000, c-X 1000, d-X 5000, e-X 3000).

basin crude oil, showed that individual components, as well as their mutual interaction, play a major role in the production of good quality coke. Because decanted oil has less aliphatic character than pyrolysis residues[22], the coke C manufactured from atmospheric residue, pyrolysis residue, and decant oil in the ratio 0.5:0.25:0.25 after high-temperature treatment showed the density value 2.182 g cm-‘, and longitudinal CTE values from 1.5 to 1.8 X 10-6/“C. According to the structural designations[ 121, the coke C could be ranked as super-regular with smaller

100,

amounts of premium structure, and coke B and C as regular (primary).

4. CONCLUSION Our investigations showed that changes in crystallite height (LJ and interlayer spacing (&J depended on heating temperature and heating time. Among the coke samples examined, the highest L,value was that of coke C ( 14.9 nm) at 24Oo’C, heating time 5 hours,

389

Petroleum coke structure: influence of feedstock composition

Fig, 6. SEM micrographs of coke C surface: (a) before and (b-e) after HTT at 2400°C (a-X b-X 1000, c--X3000, d-X 1500, e-X3000).

and heating rate iO“C/min. Under those conditions, the doozfor that sample was 0.3434 nm, and the de-

gree of graphitization was 6.9%. The study of the mo~hology of coke surface showed that all the three samples contained famelfar and mosaic structural components, but in different percentage. Temperature treatment at 2400°C brought about the development of different textural units. According to those units, provided crystallographic parameters and feedstock composition were known, coke samples could be classified.

500,

Acknowledgement-This work was performed with the sup port of the Scientific Research Fund of R. Croatia.

REFERENCES I. 0. H. Stormont, Oil uns Gus .I. 11, 1969 (1969).

2. 3. 4. 5.

J. D. Brooks and G. M. Taylor, Curbon3, 185 (1965). Y. D. Park and I. Mochida. Carbon 27,925 ( 1989). S. R. Brandtzaegand H. A. Oye, Carboi 26,163 (1988). K. I. Fuiimoto. I. Mochida. Y. Todo. T. Ovama. R. Yamash& and H. Marsh, C&bon 27, $09 (i984): 6. T. Noda, Carbon 18,3 (1980).

390

M. LEGIN-KOLARand D. UGARKOVIC

7. E. A. Heintz, Fuel 64, I 192 (1985). 8. L. S. Singer, Fuel 60,839 (I 98 I). 9. S. Keshun, W. Chengfong, and L. Jifu, Fuel 64, 174 (1985). 10. A. Oberlin, M. Oberlin, and M. Monthioux, 3rdIntw wt. Carbon, Conf: Baden-Baden, p. 453 ( 1980). I 1. H. Marsh, J. G. Piasecha, and A. Grint, Fuel 59, 343 ( 1980). 12. R. W. Pysz, S. L. Hoff, and E. A. Heintz, Carbon 27, 935 (1989). 13. H. Marsh, M. Forrest, and L. A. Pacheco, Fuel@ 423 (1981). 14. J. Maire and J. Meting, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 6, p. 125. Marcel Dekker, New York ( 1970).

15. L. S. Singer and J. C. Lewis, Carban 16,417 (1978). Iti. J. M. AbizgiIjdin, R. M. Usmanov, and R. H. Sadljikov, Neftepererab. Neftehim. 6,9 (1975). 17. A. S. Schwartz and J. C. Bokros, Carbon 5, 325 ( 1967). 1X. H. N. Murty, D, L. Biederman, and E. A. Heintz, Fuel 57,442 ( 1978). 19. D. UgarkoviE and M. Legin-Kolar, Carbon 24, 195 ( 1986). 20. D. UgarkoviC and D. Premerl, Fue166, 143 l(l987). 2 1. M. Legin-Kolar and D. Ugarkovie, Fuel 71(1992) (in press).

22. 1. Mochida, T. Oyama, Y. Korai, and Y. Fei Quing, Fuel67, 1171 (1988).