A study of the carbonization of ethylene tar pitch and needle coke formation

A study of the carbonization of ethylene tar pitch and needle coke formation lsao Mochida,

You Cling Fei and Yozo Korai

Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 876, Japan (Received 70 August 7989; revised 26 December 7989)

Carbonization of ethylene tar pitch (ETP) and its hydrogenated products was studied in a tube bomb to find the appropriate carbonization conditions for producing a good needle coke. At the relatively low temperature of 460°C under a pressure of 784 kPa for 10 h, ETP produced lump coke of low CTE and of excellent flow texture without any mosaic coke being formed at the bottom of the reactor (bottom mosaic coke). A higher carbonization temperature (480°C) gave a mosaic texture coke of large CTE, although the carbonization was complete in 6 h. A lower carbonization temperature (440°C) or pressures above 784 kPa resulted in bottom mosaic coke. The catalytic hydrogenation of ETP improved the anisotropic texture of coke carbonization at 48O”C, but could not eliminate the mosaic coke forming at the reactor bottom. N.m.r. and g.p.c. analysis revealed that ETP contained some oletins and high molecular weight substances in the major 2-3 ring aromatic hvdrocarbons. The roles of such minor components in the formation of

needle and mosaic cokes are discussed. (Keywords:

coke; tar; pitch)

Ethylene tar pitch (ETP) is now produced from ethylene crackers on a large scale (world-wide, over 10 million tonnes per year) as the residual oil. Most of this ETP,

which comes from naphtha, is soluble in benzene, highly aromatic and essentially free from heteroatoms and inorganic ash. Its application as the feedstock for needle coke or pitch-based carbon fibre, however, is very limited. This is mainly because of its high carbonization reactivity, which interferes with the growth of mesophase under ordinary carbonization conditions. Some coking feeds are known’-3 to produce mosaic textured coke at the reactor bottom because of the reactive components, and this reduces the homogeneity of the needle coke. The production of needle coke from ETP has been reported4, probably after some preliminary heattreatment. Otherwise, poor quality is inevitable under commercial conditions. Aluminium chloride has been found5*6 to be very effective in modifying ETP to give a suitable precursor for carbon fibre production and a co-carbonization additive. Sanada et al.‘,’ reported that thermal and hydrogenation treatment could modify ETP to facilitate the development of large anisotropic texture mesophase. Nevertheless, the practical evaluation of ETP as a feedstock for needle coke, and the cause of its poor performance as a feedstock, have been examined to only a small extent. In this study, the carbonization of ETP was studied at different temperatures and pressures in a tube bomb. Tube bombs have been shown to provide lump needle coke from some feedstocks under the appropriate carbonization conditions’-’ ’ and also to reproduce bottom mosaic coke from some feedslT2. The lump cokes produced were evaluated as needle coke in terms of their optical texture and CTE value. ETP was also catalytically hydrogenated to modify its carbonization properties. Such a study may reveal the practical precedure required to produce a ‘puffring-free’ needle coke from ETP, because of the extremely small amount of heteroatoms and 001&2361/90/060667~5 0 1990 Butterworth-Heinemann

Ltd.

inorganic ash present in the coke. The carbonization chemistry of ETP can be discussed at the same time, to find reasons why ETP exhibits such properties. EXPERIMENTAL Table 1 shows some properties of the ethylene tar pitch (b.p. 2OWWO”C) used in the study. Its carbonization was carried out in a tube bomb (55 ml) under O-3.1 MPa pressure in a sand bath at 440-500°C. The coking time for completion was determined microscopically as that required for the elimination of mesophase spheres in the carbonized product. The resultant lump coke was evaluated in terms of anisotropic texture, CTE value, and nitrogen and ash contents after calcination at 1000°C. Details of the carbonization and evaluation procedures have been described previously’-12. ETP was hydrogenated in an autoclave (200 ml), with a commercial Ni-Mo catalyst (Shell 324: NiO, 3.4 wt%; MOO,, 20 wt%) sulphurized with 5 ~01% H,S/H, at 360°C for 6 h. The hydrogenation conditions were 200°C for 1 h at 2.9 MPa (initial hydrogen pressure) and 380°C for 1 h at 9.8 MPa, for mild and severe conditions respectively. ETP and its hydrogenated products were analysed by ‘H n.m.r. and gel-permeation chromatography (g.p.c.) in solvents CDCl, and tetrahydrofuran, respectively. The

Table 1

Properties

of feed

Carbon (wt%) Hydrogen (wt%) Nitrogen (wt%) Ash (wt%) Difference (wt%) H/C (atomic ratio) Carbon aromaticity BS (wt%)

92.40 7.30 0.04 0.03 0.23 0.95 0.77 100

FUEL,

1990,

Vol 69, June

667

Carbonization

Figure

1

Figure 2

of ethylene

Montage

photographs

Higher magnification

tar pitch and needle

of lump cokes prepared

micrographs

coke formation:

from ETP under 784 kPa pressure

of lump cokes at bottom

regions (carbonization

molecular weight scale on the g.p.c. was determined by commercially available aromatic hydrocarbons in combination with synthetic aromatic pitches characterized by field desorption (f.d.) m.s.r3. RESULTS Injluence Figure

of carbonization

temperature

I shows

montage micrographs of lump cokes produced from ethylene tar pitch (ETP) under a carbonization pressure of 784 kPa at various temperatures. Each montage shows nearly the whole section of coke lump parallel to the reactor tube axis, and consists of about 30 low magnification photographs (x 50). Carbonization temperatures of both 500°C and 480°C gave cokes of mosaic anisotropy with a number of large spherical pores. In contrast, large flow texture aligned uniaxially was produced at 460°C. Orientation of flow texture became rather random at the lower temperature of 440°C. The carbonization time for coking completion increased from 2 h to 20 h as the carbonization temperature decreased from 500°C to 44O”C, whereas the

668

FUEL, 1990, Vol 69, June

I. Mochida

Table 2

at: a, 440°C; b, 460°C; c, 480°C; d, 500°C

pressure 784 kPa): a, 440°C; b, 460°C; c, 480°C; d, 500°C

Properties

Temperatureb

et al.

(“C)

CTE (x 10m6/“C) N content (wt%) Ash content (wt%) Bottom-mosaic Coke yield (wt%) Coking time (h)

of cokes” 440

460

480

500

1.0 _ _ some 28 20

0.7 0.04 0.03 little 31 10

1.5 0.03 0.13 much 33 4

1.7 0.09 0.09 much 31 2

“Calcined at 1000°C for 1 h after carbonization b In combination with carbonization pressure of 784 kPa

fairly high coke yield of around 30 wt% was almost independent of the carbonization temperature, as shown in Table 2. Figure 2 illustrates representative micrographs (magnification x 50) of bottom areas of coke lumps shown in Figure I. Although this bottom area was rather thin in the whole section, a different texture could be observed for different carbonization temperatures. For example, at both 500°C and 480°C the mosaic texture was the same as that in upper areas, whereas the flow texture prevailed to the bottom in the coke produced at 460°C.

Carbonization of ethylene tar pitch and needle

Figure 3 Higher magnification 1.57 MPa; d, 3.14 MPa

micrographs

of lump

cokes

at bottom

regions

(carbonization

coke formation: I. Mochida

temperature

460°C):

a, 392 kPa;

et al.

b, 784 kPa;

c,

Figure 4 Montage photographs of lump cokes prepared at 480°C under 784 kPa pressure from: a, hydrogenated ETP at 200°C under 2.94 MPa: b, hydrogenated ETP at 380°C under 9.81 MPa

micrographs of lump cokes at bottom Figure 5 Higher magnification regions (480°C 784 kPa): a, hydrogenated ETP at 200°C under 2.94 MPa; b, hydrogenated ETP at 380°C under 9.81 MPa

In marked contrast, a thin layer of finer mosaic texture was observed at the bottom for the coke prepared at 440°C.

always showed excellent flow type anisotropy regardless of the pressure, the texture at the bottom became finer as the pressure increased. Mosaic textures barely appeared under pressures lower than 784 kPa, whereas they became a layer at pressures above this, and the thickness increased distinctly as the carbonization pressure increased to 3.1 MPa.

Some properties qf the cokes,from ETP Table 2 summarizes some properties of lump cokes produced from ETP at various temperatures. After calcination, the lump coke produced at 460°C exhibited the lowest CTE value of 0.7 x 1O-6/“C (Z 5OO”C), in agreement with the excellent flow texture of uniaxial orientation. Both higher and lower temperatures gave cokes of larger CTE value, reflecting their mosaic texture and poor orientation of flow texture, respectively, even if the bottom portion was removed before the CTE measurement. These cokes had nitrogen and ash levels below and around 0.1 wt%, respectively. Zrzfluence of carbonization pressure Figure 3 shows micrographs (magnification x 50) of the bottom areas of cokes produced at 460°C under pressures up to 3.1 MPa. Although the anisotropic texture predominant in the upper areas of coke lumps

Carbonization of hydrogenated ETP Figure 4 shows montage micrographs of lump cokes from two hydrogenated ETPs carbonized under 784 kPa pressure at 480°C for 6 h. The mild hydrogenation enlarged the optical texture in the carbonized coke, compared with that from original ETP under the same carbonization conditions. More flow textures could be observed, although mosaic textures still occupied the majority of the surface. The severe hydrogenation provided much larger flow textures orientated randomly and decreased the coke yield to 19 wt%. Figure 5 shows micrographs of the bottom portions of the two cokes. Much fine mosaic texture still remained. Some clusters of mosaic texture surrounded with flow

FUEL, 1990, Vol 69, June

669

Carbonization

of ethylene

tar pitch and needle

coke formation:

I. Mochida

et al.

group on the nucleus. In addition, some characteristic peaks appeared in the 5-7 ppm range. These were assigned to olefinic hydrogens, although their intensity was not strong. Mild hydrogenation hardly changed the spectra except for the disappearance of olefinic peaks. Severe hydrogenation increased aliphatic hydrogens to the extent that they became the predominant species. In particular, the B-hydrogen peaks at 1.5-2.0 ppm increased remarkably and the a-hydrogen peaks shifted to the low magnetic field, indicating that a considerable amount of naphthenic rings was produced during the hydrogenation. Figure 7 shows the gel-permeation chromatographs of the original and hydrogenated ETP. Although the major peaks of the original and mildly hydrogenated materials were around 300 molecular weight, a significant number of molecules with molecular weight ranging from 5OG4000 also existed. They were probably a kind of asphaltene consisting of a number of nuclei of small aromatic rings, since large fused aromatic rings could not be recognized in the low field of the ‘H n.m.r. spectra. Severe hydrogenation decreased the amount of the larger molecular weight component and increased that around 350 molecular weight to form another peak. Nevertheless, a significant number of molecules of 50&1500 molecular weight still remained. DISCUSSION Carbonization in a tube bomb revealed that excellent needle coke of very low nitrogen and ash contents could be produced from ethylene tar pitch (ETP) without bottom mosaic coke at a relatively low temperature of 46O”C, although the coking time of 10 h required for completion may be too long for a commercial process. It can provide an excellent filler coke for graphite electrode without the puffing attributed to heteroatoms and inorganic ash’&16, suggesting a route to utilize the increasing amount of residual oil for the production of high-value carbon materials. Only mosaic texture coke of large CTE was produced

Figure 6 ‘H n.m.r. spectraof ETP feed and its hydrogenated products: a, original ETP; b, hydrogenated ETP at 200°C under 2.94 MPa; c, hydrogenated ETP at 380°C under 9.81 MPa

texture were found hydrogenated ETP.

in the coke from the severely

N.m.r. and g.p.c. analyses of ETP products

and its hydrogenated

Figure 6 shows the ‘H n.m.r. spectra of the original and hydrogenated ETP. The majority of the hydrogen in the original ETP was in aromatic situations, with peaks in the range higher than 8.4 ppm, indicating structural units of a 2-3 ring aromatic nucleus. A considerable amount of a-hydrogen and a certain amount of b-hydrogen was observed, suggesting one methyl or ethyl

670

FUEL, 1990, Vol 69, June

12

10

8 Retention

Figure 7 Gel-permeation chromatogram in THF: -, ---, hydrogenated ETP at 200°C under 2.94 MPa; -. ETP at 380°C under 9.81 MPa

I

6 trme(min)

original

ETP;

-, hydrogenated

Carbonization

of ethylene

from ETP at temperatures higher than 480°C at which temperature decant oils and coal tar pitches give excellent indicating a high carbonization needle cokes’,“, reactivity in spite of its high aromaticity and very small amount of heteroatoms. The minor amounts of some olelins and high molecular weight substances in the predominantly 2-3 ring aromatic hydrocarbons of the ETP are thought to influence the carbonization reactions. The range of the appropriate (in terms of needle coke formation) carbonization temperatures and pressures was very narrow for ETP. Temperatures of 440°C and 480°C or pressures above 784 kPa failed to develop the excellent flow texture of uniaxial orientation or to provide lump coke free from bottom mosaic texture. Such carbonization properties can be discussed from the viewpoint of the composition, structure and reactivity of ETP. Mild hydrogenation eliminated the olefins and enlarged units of anisotropic texture in the coke produced at 480°C. Oletins can inhibit anisotropic development at this temperature. However, there are other factors leading to poor quality coke, because mosaic texture coke was formed at the reactor bottom even from the hydrogenated ETP. The fraction of high molecular weight remained after even severe hydrogenation, and probably resulted in bottom mosaic coke. A considerable amount of naphthenic hydrogen introduced in severe hydrogenation failed to eliminate the bottom mosaic coke completely. The carbonization tempeature and pressure can strongly influence the reactivities and solubilities of the fractions in ETP, and affect their smooth cocarbonization1,2,9. At too low a temperature, the carbonization of all substances is so slow that gas evolution at the bulk mesophase solidification is too small for uniaxial alignment, although the anisotropic texture can grow. The solubility of the high molecular weight fraction in the carbonization matrix may be limited. Hence, it tends to precipitate to the reactor bottom where the carbonization at high viscosity leads to a mosaic reactive texture layer’,*. At too high a temperature, olefins as well as high molecular weight substances may initiate the polycondensation reactions and rapidly increase the viscosity of the mesophase to complete carbonization in a short time, producing a rather uniform

tar pitch and needle

coke formation:

I. Mochida

et al.

mosaic texture in the whole area. Hydrogenation suppresses the reactivity of oletins to improve the high molanisotropic texture 6,8. However, unmodified ecular weight substances can cause mosaic texture formation at the reactor bottom because of high reactivity and low solubility of their mesophase derivatives. Thus, a very narrow range of temperature is defined for the development and uniaxial orientation of mesophase without precipitation of mosaic texture. Bottom mosaic cokes presumably result from rapid carbonization reactions of reactive substances. They may form less soluble mesogens, which suffer phase separation from a matrix of inadequate dissolving ability, leading to the bottom mosaic coke. Higher carbonization pressure can hold more volatile substances as a poor solvent/partner in the carbonization system and enhance the phase separation even at the appropriate temperature for mesophase development in the main upper areas of the reactor, giving more bottom mosaic coke. REFERENCES 1 2 3 4 5 6 I 8 9 10 11 12 13 14 15

16

Nesumi, Y., Todo, Y., Oyama, T. et al. Carbon 1989,3, 359 Mochida, I., Korai, Y., Oyama, T. et al. Curbon 1989, 3, 367 Eser, S., Jenkins, R. G. and Derbyshire, F. J. Carbon 1986,24,77 Alberta, B. ‘Proceedings 5th International Carbon and Graphite Conference’, London, UK, 1979, p. 365 Mochida, I., Sone, Y., Korai, Y. and Fujitsu, H. J. Petr. Inst. Jpn. 1983, 3, 487 Mochida, I., Sone, Y. and Korai, Y. J. Materials Science Letter 1985, 4, 1237 Honda, H., Kimura, H., Sanada, Y. er a[. Carbon 1970,8,18 1 Obara, T., Yokono, T., Miyazawa, K. and Sanada, Y. Carbon 1981, 19, 263 Mochida, I., Oyama, T., Korai, Y. and Fei, Y. Q. Fuel 1988, 67, 1171 Mochida, I., Fei, Y. Q., Oyama, T. et al. J. Mat. Sci. 1987, 22, 3989 Mochida, I., Korai, Y., Nesumi, Y. and Oyama, T. Znd. Eng. Chem. Prod. Res. Dev. 1986, 2, 189 Mochida, I., Fei, Y. Q., Fujimoto, K. et al. Carbon 1989,3,375 Mochida, I., Shimizu, K., Korai, Y. et al. Carbon submitted for publication Wagner, M. H. and Wilhelmi, G. ‘Extended Abstracts, 17th Biennial Conference on Carbon’, Worcester, USA, 1987, p. 113 Fujimoto, K., Yamada, M., Yamashita, R. and Nagino, H. ‘Proceedings 4th International Carbon Conference’, BadenBaden, 1986, p. 116 Jakzer, L., Jasienko, S. and Lukoszek, J. ‘Proceedings 4th International Carbon Conference’, Baden-Baden, 1986, p. 166

FUEL, 1990, Vol 69, June

671