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Carbonization in the tube bomb leading to needle coke: I. Cocarbonization of a petroleum vacuum residue and a FCC-decant oil into better needle coke
Carbon Vol. 27, No. 3. pp. 359-365. Printed in Great Britain.
1989
oIXIX-h??3~XY $3 (II)+ .lHI SC’19RY Pergamon Press plc
CARBONIZATION IN THE TUBE BOMB LEADING TO NEEDLE COKE: I. COCARBONIZATION OF A PETROLEUM VACUUM RESIDUE AND A FCCDECANT OIL INTO BETTER NEEDLE COKE ISAO MOCHIDA and Yozo KORAI Institute of Advanced Materials Study. Kyushu University 86, Kasuga, Fukuoka 816. Japan TAKASHI OYAMA, YASUHIRO NESUMI, and YOSHIO TODO Marifu Refinery, KOA OIL Co. Ltd., Waki-cho. Yamaguchi 740, Japan (Received
18 March 1988; accepted in revised form 5 October 1988)
Abstract-The
cocarbonization of a Fluidized Catalytic Cracking decant oil (FCCDO) with a petroleum low sulfur vacuum residue (LSVR) was studied at a temperature range of 460 to 480°C by evaluating the qualities of coke lumps produced in a tube bomb in terms of their CTE and anisotropic development. The cocarbonization certainly improved the orientation of flow texture and CTE of the resultant coke, providing the smallest CTE as low as 0.10 x 10~“/“C and 0.36 x 10-b/“C at particular FCCDOiLSVR mixing ratios of 5/5 and 7/3 according to the respective carbonization temperatures of 460 and 480°C. The natures of bulk mesophase and gas evolution at the solidification stage for the axial-rearrangement of mesophase aromatic component, both of which essentially define the coke quality, are strongly influenced by the carbonization reactivities of blended feedstocks and carbonization conditions. FCCDO may moderate the carbonization progress of reactive LSVR and LSVR supply the sufficient gas evolution during the solidification, leading to excellent flow texture of axial arrangement. The formation of mosaic coke in the bottom part of the lump which may deteriolate the quality of the coke produced in a commercial coker appeared to be caused by the phase separation between the paraffin and asphaltene fractions. Such a separation at a early stage of the carbonization was found to be controlled by the blending and the carbonization conditions. Key Words-Needle
coke, tube bomb, petroleum vacuum residue, FCC-decant oil.
1. INTRODUCTION
The cocarbonization of petroleum low sulfur vacuum residues (LSVR) or its thermal tars has been performed with Fluidized Catalytic Cracking decant oil (FCCDO) in a commercial delayed coker to produce better needle cokes[ 11. The cocarbonization has been well documented to modify the anisotropic development[2-61, however, the particular scheme for the production of needle coke is not studied in the cocarbonization. In the present study, cocarbonization of such combination was followed in a tube bomb, which has been proved to provide a coke lump comparable to a coke produced in a delayed coker from same feedstocks in terms of its appearance, optical texture, and CTE value[7,8]. When the cocarbonization in a tube bomb can provide a coke comparable to the commercial one which is certainly produced through some cocarbonization, the optimum carbonization conditions in terms of temperature and pressure as well as the feed composition can be systematically surveyed in an experimental laboratory. The mechanism of cocarbonization and how better coke is produced can be also discussed based on the experimental results. It is realized in the commercial operation of delayed coking that cokes of poor quality are produced at the bottom of the chamber[9-131. The carboni-
zation profile as for the formation can be also studied.
of such a coke
2. EXPERIMENTAL
Some analytical data of a FCCDO and a LSVR are summarized in Table 1. LSVR consisted of 50% saturate, 30% aromatic, 4% resin, and 16% asphaltene according to TLC analysis. While, FCCDO consisted of 28% saturate, 71% aromatic, and 1% resin. There is not asphaltene in FCCDO. ‘H-NMR spectra of the feeds were obtained by 100 MHz NMR and analyzed according to Brown-Ladner method[l4]. The feedstocks were blended at 80°C by stirring. A sample of around 40 g weighed in an aluminium foil tube (diameter 20 mm, height 150 mm) was carbonized in a stainless steel tube bomb that was plunged in a sand bath heated at the prescribed temperature. The heating rate was about 250”Cimin. The heating rate in the preheater of the commercial delayed coker has been reported similar[l5]. The carbonization pressure was adjusted by the initial nitrogen pressure and adequate purging through a control valve during the carbonization. The carbonized product, after a series of the prescribed heating times, was quenched in cold water and its whole lump was recovered in the foil. 359
I. MOCHIDAet al.
360
Table 1. Properties of feedstocks Elemental Analyses (wt%)
LSVR FCCDO
Solubility” (wt%)
TLC-FID (%)
C
H
N
S
0’
C/H
fa2
e3
HS
HI-BS
BI-QS
QI
Satu
Arom
Resin
Asp
86.2 88.9
12.5 9.5
0.4 0.1
0.2 0.4
0.7 1.1
0.6 0.8
0.18 0.54
0.52 0.32
92 100
8 0
0 0
0 0
50 28
30 71
4 1
16 0
‘Difference. Carbon aromaticity according to Brown-Ladner method. ‘Degree of substitution of aromatic nucleus according to Brown-Ladner method. ‘HS, hexane soluble; HI-BS, Hexane insoluble but benzene soluble; BI-QS, benzene insoluble but quinoline soluble; QI, quinoline insoluble.
ented) in the lump coke except for the fine mosaic coke of thin belt at its bottom. More LSVR up to the ratio of 3/7 maintained better uni-axial arrangement although the flow texture units was slightly shortened. The thickness of the bottom mosaic cokes was much the same regardless of the added amount of LSVR. Such anisotropic development in the lump cokes from FCCDO, LSVR, and their mixtures is quantified according to the measurement of anisotropic unit vectors in Table 2. The average length of the anisotropic unit vector (lav) and the averaged axial portion of the unit vectors (fav[l3], an index for uniaxial arrangement), describe quantitatively the anisotropic development of the cokes, although the bottom mosaic texture was omitted from the evaluation. According to fav, the best coke was produced at the ratio of 713, and the ratios of both 515 and 317 allowed higher values than those of cokes from FCCDO and LSVR both alone. Figure 2 illustrates montage photographs of cokes produced under the conditions of 46O”C, 8 kg/cm2. FCCDO gave a similar texture as observed in the coke produced at 480°C. Addition of LSVR slightly increased the length of anisotropic units and improved fav, although the improvement was less than that observed at 480°C. The coke from LSVR alone at 460°C was certainly better than that at 480°C in terms of fav. According to the values of lav and fav, the best coke was produced at the mixing ratio of 515. The ratios of both 713 and 317 provided similar fav values
The lumps were sectioned in parallel to the bomb axis in order to examine the anisotropic texture under reflected polarized light microscope after conventional polishing. A montage photograph of the whole surface was made from 100 micrographs (magnification x 50). Anisotropic units were evaluated by lav (average length of anisotropic unit vector) and fav (average axial portion of the unit vectors), both of which were defined in a previous paper[l3]. The CTE (temperature range, room temperature to 500°C) was also measured after the lump was calcined at 1000°C. Details of anisotropic evaluation and CTE measurement were described in a previous paper[ 131.
3. RESULTS
3.1 Montage photographs of coke lumps produced through the cocarbonization Figure 1 illustrates montage photographs of coke lumps (5 mm height) produced under the conditions of 48O”C, 8 kg/cm* from FCCDO, LSVR, and their mixtures of a series of mixing ratios. FCCDO and LSVR produced coke lumps of flow and flow-mosaic mixed textures, respectively. The former one is an excellent needle coke, while the latter one was much inferior, as reported previously[8]. It should be noted that the latter coke carried a wide area of fine mosaic texture at its bottom part. Addition of LSVR to FCCDO at the ratio of 7/3 (FCCDO/LSVR) certainly improved parallel alignment of flow texture along the bomb axis (abbreviated as uni-axially ori-
Table 2. CTE and anisotropic development
480°C
460°C
lav
lav
(w)
fav
FCCDO
7:FCCD0 3:LSVR
5:FCCD0 5:LSVR
3:FCCD0 7:LSVR
LSVR
19.6
20.0
18.4
17.8
16.2
16.0
17.3
16.8
16.5
15.0
19.0
20.0
19.2
18.2
17.5
16.0
16.0
16.6
16.2
15.4
0.78 Ix 10-6/0c)
of lump cokes
0.90
0.36
0.75
1.20
lmm
to that from FCCDO alone, which was certainly better than that from LSVR alone. The bottom mosaic coke increased certainly at this temperature although such a texture was not still found in the coke from FCCDO alone. The thickness of bottom mosaic belt increased with the increasing amount of LSVR, and LSVR alone gave the thickest one. 3.2 Yield and CTE of Coke Lump Table 3 summarizes the yields of cokes from FCCDO, LSVR, and their mixtures under two conditions. The yields of the mixtures which were much the same under both conditions, coincided approximately to the averaged one of those of FCCDO and LSVR. The CTE values of the calcined lump cokes are summarized in Table 2. The bottom area of mosaic texture was cut off from the sample coke for the CTE measurement. The averages of the values, which were well reproduced in three repeated experiments, distinguished the cokes very definitely. The addition of LSVR to FCCDO certainly improved the value. The best cokes were produced at the mixing ratios of 713 and 515, respectively at 480 and 460°C in terms of CTE and the amount of mosaic coke in bottom part. CTE of the needle coke has been principally related the uni-axial arrangement of graphite layers and the microcracks among the graphite grains[l3]. The degree of uni-axial arrangement is semiquan-
titatively described by the averaged axial component of anisotropic unit vector, fav. in a previous paper[l3]. At present, adequate procedure for the description of microcracks is not proposed yet. The values of fav of the present cokes are correlated to their CTE values in Fig. 3. An excellent linear correlation was established among the cokes produced from FCCDO, LSVR, and their mixtures at different temperatures. Hence, the advantage of cocarbonization in terms of CTE of the resultant cokes can be discussed in connection with the uni-axial arrangement of anisotropic flow texture. 3.3 Bottom Coke of Fine Mosaic Texture As described above, LSVR and its mixtures with FCCDO produced area of mosaic texture at the bottom of the coke lump. Figure 4 shows a montage photographs of whole area of the coke lump produced from mixture (blend ratio: 713 FCCDOi LSVR) at 48o”C, 8 kg/cm’ (A) and microphotographs of the middle and bottom part in larger magnification (B-D). There are various types of flow texture which were axially oriented (C) and not oriented (B) at the middle part of the lump, while the mosaic texture stayed at the bottom part. being clearly distinguished from the flow texture at the middle part. Hence, the thickness of mosaic part at the bottom can be measured easily using the montage photograph[l2]. The thickness of bottom mosaic belts are plotted with the mixing ratio in Fig. 5. where the carboni-
I. MOCHIDAet al.
362 Table 3. Coke yield (wt%)” Feed Carbonization temperature
FCCDO
480°C 460°C
39 40
7:FCCD0 3:LSVR
5:FCCD0 5:LSVR
34 36
LSVR
29 30
23 23
“Carbonization pressure 8 kg/cm’.
zation temperatures were 480°C and 460°C. The thickness were about 1 mm in the cokes produced at 480°C from LSVR alone and its mixtures regardless of the mixing ratio. The thickness was much larger in the cokes produced at 460°C. LSVR gave thickness of 3 mm and the thickness decreased by more FCCDO in the mixture to be zero in the coke from FCCDO alone. Carbonization of mixtures (FCCDOILSVR: 713 and 5/5) was examined under higher pressure (16 kg/cm?) and/or at higher temperature (490°C) to reduce the bottom mosaic cokes. As illustrated in the montage photographs of Fig. 6, the bottom mosaic coke became thinner (height 0.5 mm) when the carbonization temperature and pressure were higher, although it was certainly observable in any cases. The uni-axial arrangement of flow texture was slightly deteriorated by increasing the pressure when cokes produced at the same carbonization temperature were compared. The higher temperature appeared to increase the area of turbulent flow textures as pointed in the figure. 3.4 Carbonization Progress The completion of the carbonization was defined by the appearance and solubility of the coke. The times required for the completion which depended on the carbonization temperature and the feedstock are summarized in Table 4. FCCDO was much more stable than LSVR, requiring 2.5 to 3.0 h for the
0 \
\
t2 0
n
15
0
#p\\ 16 fav ( pm
4.
DISCUSSION
The cocarbonization of FCCDO with LSVR surely improved the coke quality in terms of CTE and uniaxial arrangement of flow texture, producing the best coke at a particular mixing ratio according to the carbonization temperature. Such improvement has been applied in a commercial coker to produce a better coke from more accessible blends of petroleum feedstocks[l]. The present authors have proposed the carbonization mechanism which leads to the needle coke[l6, 17). Two major steps where the bulk mesophase of low viscosity and broad isochromatic areas and its uni-axial rearrangement are achieved compose the whole scheme. In the first step, the aromatic components in the feedstocks are condensed into mesogen molecules of adequate molecular structure and size distribution. Hence, too much reactivity of the component should be controlled by its dissolution in the stable molecules and intermolecular hydrogen transfer from the naphthenic units of the cocarbonization substances[l8]. LSVR carries a considerable amount of asphaltene as shown in Table 1, which is the major origin for the coke and has high reactivity because of its large molecular weight of the aromatic nucleus, many alkyl side chains and mice11 aggromelation. The paraffinic fraction of another major component in LSVR is poor as a solvent for the asphaltene fraction, especially after asphaltene loses the alkyl group by the carbonization. Hence, the mesophase of low viscosity is difficult to be produced from LSVR, smaller aromatic units tending to be introduced. In contrast, rather stable aromatic species in FCCDO
oA k
completion of its carbonization at 480°C while LSVR required 1.5 to 2.0 h. Addition of LSVR shortened the time, depending on the amount of LSVR. The carbonization was very slow at 460°C compared to that at 480°C. FCCDO and LSVR took 12.0 to 12.5 and 5.0 to 5.5 h, respectively, for completion of their carbonization. Again, the addition of LSVR accelerated the carbonization to shorten the time in comparison to that of FCCDO alone. The shorter time for the completion of the carbonization by addition of LSVR to FCCDO suggests that the reactive LSVR accelerates the carbonization of the latter feedstock through some mechanism of radical transfer and propagation.
17
1
Fig. 3. Relation between fav and CTE of lump cokes carbonization temperature. -80°C; 0460°C.
provides bulk mesophase of low viscosity. Its naphthenic structure helps such a mesophase preparation[19-221. Hence, the cocarbonization of FCCDO and LSVR controls the high reactivity and solubility of the asphaltene fraction in LSVR to provide better mesophase from their mixture. In the second stage, appropriate gas evolution is required to rearrange the mesogen molecules uni-
Carbonization
i63
in the tube bomb leading to needle coke. 1. FLOW TEXTURE AXIALLY NOT-ORIENTED
MONTAGE PHOTOGRAPH
FLOW TEXTURE AXIALLY ORIENTED
MOSAIC TEXTURE IN BOTTOM PART
Fig.
4. Montage
photograph 8 kg/cm’
and microphotographs ol’ the coke lump produced from mixture of FCCDOiLSVR (blending ratio 7,!3).
axially along the axis at the solidification process where the viscosity of the mesophase increases. FCCDO certainly carries methyl groups attached to the aromatic ring as a source for such a gas to give an excellent needle coke, as described in a previous paper[l8]. LSVR is plenty of such gas sources be-
u ;i
Ei I 3.0
-
A---_____
5
-4
‘\
E
0 m
0” I3 y
‘\
2.0 -
‘. 0
l.O-
I O
I
0
Fig. 5. Relation height of bottom ture. WWC:
I
I
I,
I
I,,
30 50 70 CONTENT OF FCC DO(%) between the content of FCCDO and mosaic texture. Carbonization tempera046O”C, carbonization pressure.
at 4Xo”C‘ under
cause it carries a number of noncarbonized aromatics, alkyl side chains, and straight paraffins. Hence, the cocarbonization of two feedstocks can control the mesophase formation and provide more gases at its solidification, producing a better coke than those from both feedstocks alone. The mixing ratio and the carbonization temperature influence cooperatively such stage of the carbonization progress through the reactivity, solubility, and hydrogen-donating ability of carbonizing molecules in both feedstocks. Thus, a particular carbonization temperature define a respective particular ratio for the best coke as described above. The carbonization temperatures of 460°C and 480°C exhibited their respective best mixing ratios of S/5 and 713. The higher temperature consumes more donors to moderate the reactive asphaltene and expects less gas evolution from LSVR at the solidification, as suggested from the carbonization profile of FCCDO(lX]. Hence. higher mixing ratio of FCCDO is requested for the best coke at a higher cocarbonization temperature. It is of value to discuss the role and formation mechanism of bottom mosaic coke. It is well recognized that cokes of fine mosaic texture. higher density, and lower quality or even shot cokes are produced at the bottom of the coker chamber(‘)-13). Such cokes are less frequently found in the middle of the chamber. The bottom coke found in the pres-
I.
364
MOCHIDA et al.
i mm
Fig. 6. Montage photographs
of lump cokes produced under variable conditions: A-7/3,
kg/cm2 (FCCDO/LSVR); B-5/.5, 48o”C, 16 kg/cm2 (FCCDO/LSVR); C-7/3, (FCCDO/LSVR);
D-5/5,49O”C,
ent study may originate via the same route. The bottom coke in the chamber is separated to maintain the quality of the whole coke produced in the same chamber. The causes of such cokes may be moderated in the middle chamber, however they may deteriorate the coke quality although influences may not obviously be recognized. The most reactive fraction of the asphaltene in LSVR may be condensed in the early stage of carbonization, losing alkyl side chains and being converted into polycondensed aromatic molecules in the micell[l2]. Such aromatics are hardly soluble in the paraffin-rich matrix to be sedimented to be the bottom, where their carbonization progresses in the highly viscous circumstance with least chances of mesophase growth and less volatile substances, giving fine mosaic cokes of higher density. Such a mechanism which has been proposed in the shot coke production[ll] may also operate in the cocarbonization. Although a large part of the most reactive asphaltene may be dissolved and moderated by the FCCDO to be cocarbonized together with partners, some of most reactive portion still follows the same mechanism to give bottom mosaic coke of reduced amount. The amount of such a portion may strongly depend on the carbonization conditions and the mixing ratio. Higher temperature up to 480°C increase its solubility of the portion in FCCDO and enhances the hydrogen donating ability of FCCDO to decrease the bottom mosaic coke. Such conditions are not necessarily best for the uniaxial arrangement of flow texture in the carbonizaTable 4. Times
for coking
completion
(h)
Feed Carbonization temperature 480°C 460°C “Carbonization
FCCDO
7:FCCD0 3:LSVR
5:FCCD0 5:LSVR
LSVR
2.5-3.0 12.0-12.5
2.0-2.5 11.5-12.0
2.0-2.5 10.5-11.0
1.5-2.0 5.0-5.5
pressure
8 kg/cm*.
16 kg/cm*
48o”C, 16
49O”C, 16 kg/cm2
(FCCDO/LSVR).
tion of major portion as discussed previously[l6]. Further higher temperatures may accelerate the reactivity of the asphaltene to increase the bottom coke. At lower temperatures, the cocarbonization may operate with less portions because of limited solubility and hydrogen transfering ability although the reactivity is reduced, requiring more FCCDO to dissolve or moderate the source of bottom coke. 5. CONCLUSIONS
1. The cocarbonization of a FCCDO with a LSVR was found to improve the orientation of flow texture and CTE of coke lump produced in a tube bomb. 2. Respective optimum blend ratios of the feeds (exist) at the respective carbonization conditions to give the lowest CTE of the coke. 3. The roles of FCCDO and LSVR were discussed in terms of mesophase formation and gas evolution at the solidification stage. 4. The formation of mosaic coke in the bottom part of the coke lump was controlled to a limited extent by blending and the carbonization conditions. REFERENCES 1. C. A. Stocks and V. J. Guercio, Erdol & Kohle Erdgas Petrochemic 38, 31 (1985). 2. H. Marsh and P. L. Walker, Jr., Chemistry and Physics of Carbon, Vol. 15, p. 228. Marcel Dekker, New York (1979). 3. H. Marsh, I. Macefield, and J. Smith, 13th Biennial Conference on Carbon, Extended Abstracts, American Carbon Society, Irvine, CA, p. 21. (1977). 4. I. Mochida, K. Amamoto, K. Maeda, and K. Takeshita, Fuel 56, 49 (1977). 5. I. Mochida, K. Amamoto, K. Maeda, and K. Takeshita, Fuel 57, 225 (1978). 6. Y. Korai and I. Mochida, Fuel 62, 893 (1983). 7. I. Mochida, Y. Korai, Y. Nesumi, and T. Oyama, Ind. Eng. Chem. Prod. Res. Dev. 25, 198 (1986). 8. I. Mochida, Y. Korai, H. Fujitsu, T. Oyama, and Y. Nesumi, Carbon 25, 259 (1987).
Carbonization
in the tube bomb leading to needle coke. I.
9. M. Kakuta, T. Tsuchiya, and M. Kooriki, Tunso 85, 55 (1976). 10. S. Eser, R. G. Jenkins. and F. J. Derbyshire, Carbon 24, 77 (1986).
11. I. Mochida. T. Furuno, Y. Korai, and H. Fujitsu, Oil & Gas Journal 3, 51 (1986). 12. I. Mochida. T. Oyama, Y. 0. Fei, T. Furuno, and Y. Korai. J. Materials Science, 23. 298(1988). 13. I. Mochida, T. Oyama, and Y. Korai. Carbon 25, 273 (1987). 14. J. K. Brown and W. R. Ladner, Fuel 39. 79 (1960). 15. K. Iwamura. J. Japan Petroleum Institute, 20. 360 (1977).
365
16. I. Mochida, T. Oyama. and Y. Korai, Carbon 26. 48 (1988). 17. I. Mochida, T. Oyama, and Y. Korai. Carbon 26, 57 (1988).
18. I. Mochida, T. Oyama, Y. Korai. and Y. Q. Fei. Oil & Gas Journal 86, 73 (1988). 19. Y. D. Park, Y. Korai, and I. Mochida. J. Materials Science 21. 424 (1986). 20. I. Mochida and H. Marsh, Fuel 58, 797 (1979). 21. I. Mochida, T. Ando, K. Maeda, and K. Takeshita. Carbon 14, 123 (1976). 22. I. Mochida, E. Nakamura, K. Maeda, and K. Takeshita. Carbon 14, 123 (1976).
1989
oIXIX-h??3~XY $3 (II)+ .lHI SC’19RY Pergamon Press plc
CARBONIZATION IN THE TUBE BOMB LEADING TO NEEDLE COKE: I. COCARBONIZATION OF A PETROLEUM VACUUM RESIDUE AND A FCCDECANT OIL INTO BETTER NEEDLE COKE ISAO MOCHIDA and Yozo KORAI Institute of Advanced Materials Study. Kyushu University 86, Kasuga, Fukuoka 816. Japan TAKASHI OYAMA, YASUHIRO NESUMI, and YOSHIO TODO Marifu Refinery, KOA OIL Co. Ltd., Waki-cho. Yamaguchi 740, Japan (Received
18 March 1988; accepted in revised form 5 October 1988)
Abstract-The
cocarbonization of a Fluidized Catalytic Cracking decant oil (FCCDO) with a petroleum low sulfur vacuum residue (LSVR) was studied at a temperature range of 460 to 480°C by evaluating the qualities of coke lumps produced in a tube bomb in terms of their CTE and anisotropic development. The cocarbonization certainly improved the orientation of flow texture and CTE of the resultant coke, providing the smallest CTE as low as 0.10 x 10~“/“C and 0.36 x 10-b/“C at particular FCCDOiLSVR mixing ratios of 5/5 and 7/3 according to the respective carbonization temperatures of 460 and 480°C. The natures of bulk mesophase and gas evolution at the solidification stage for the axial-rearrangement of mesophase aromatic component, both of which essentially define the coke quality, are strongly influenced by the carbonization reactivities of blended feedstocks and carbonization conditions. FCCDO may moderate the carbonization progress of reactive LSVR and LSVR supply the sufficient gas evolution during the solidification, leading to excellent flow texture of axial arrangement. The formation of mosaic coke in the bottom part of the lump which may deteriolate the quality of the coke produced in a commercial coker appeared to be caused by the phase separation between the paraffin and asphaltene fractions. Such a separation at a early stage of the carbonization was found to be controlled by the blending and the carbonization conditions. Key Words-Needle
coke, tube bomb, petroleum vacuum residue, FCC-decant oil.
1. INTRODUCTION
The cocarbonization of petroleum low sulfur vacuum residues (LSVR) or its thermal tars has been performed with Fluidized Catalytic Cracking decant oil (FCCDO) in a commercial delayed coker to produce better needle cokes[ 11. The cocarbonization has been well documented to modify the anisotropic development[2-61, however, the particular scheme for the production of needle coke is not studied in the cocarbonization. In the present study, cocarbonization of such combination was followed in a tube bomb, which has been proved to provide a coke lump comparable to a coke produced in a delayed coker from same feedstocks in terms of its appearance, optical texture, and CTE value[7,8]. When the cocarbonization in a tube bomb can provide a coke comparable to the commercial one which is certainly produced through some cocarbonization, the optimum carbonization conditions in terms of temperature and pressure as well as the feed composition can be systematically surveyed in an experimental laboratory. The mechanism of cocarbonization and how better coke is produced can be also discussed based on the experimental results. It is realized in the commercial operation of delayed coking that cokes of poor quality are produced at the bottom of the chamber[9-131. The carboni-
zation profile as for the formation can be also studied.
of such a coke
2. EXPERIMENTAL
Some analytical data of a FCCDO and a LSVR are summarized in Table 1. LSVR consisted of 50% saturate, 30% aromatic, 4% resin, and 16% asphaltene according to TLC analysis. While, FCCDO consisted of 28% saturate, 71% aromatic, and 1% resin. There is not asphaltene in FCCDO. ‘H-NMR spectra of the feeds were obtained by 100 MHz NMR and analyzed according to Brown-Ladner method[l4]. The feedstocks were blended at 80°C by stirring. A sample of around 40 g weighed in an aluminium foil tube (diameter 20 mm, height 150 mm) was carbonized in a stainless steel tube bomb that was plunged in a sand bath heated at the prescribed temperature. The heating rate was about 250”Cimin. The heating rate in the preheater of the commercial delayed coker has been reported similar[l5]. The carbonization pressure was adjusted by the initial nitrogen pressure and adequate purging through a control valve during the carbonization. The carbonized product, after a series of the prescribed heating times, was quenched in cold water and its whole lump was recovered in the foil. 359
I. MOCHIDAet al.
360
Table 1. Properties of feedstocks Elemental Analyses (wt%)
LSVR FCCDO
Solubility” (wt%)
TLC-FID (%)
C
H
N
S
0’
C/H
fa2
e3
HS
HI-BS
BI-QS
QI
Satu
Arom
Resin
Asp
86.2 88.9
12.5 9.5
0.4 0.1
0.2 0.4
0.7 1.1
0.6 0.8
0.18 0.54
0.52 0.32
92 100
8 0
0 0
0 0
50 28
30 71
4 1
16 0
‘Difference. Carbon aromaticity according to Brown-Ladner method. ‘Degree of substitution of aromatic nucleus according to Brown-Ladner method. ‘HS, hexane soluble; HI-BS, Hexane insoluble but benzene soluble; BI-QS, benzene insoluble but quinoline soluble; QI, quinoline insoluble.
ented) in the lump coke except for the fine mosaic coke of thin belt at its bottom. More LSVR up to the ratio of 3/7 maintained better uni-axial arrangement although the flow texture units was slightly shortened. The thickness of the bottom mosaic cokes was much the same regardless of the added amount of LSVR. Such anisotropic development in the lump cokes from FCCDO, LSVR, and their mixtures is quantified according to the measurement of anisotropic unit vectors in Table 2. The average length of the anisotropic unit vector (lav) and the averaged axial portion of the unit vectors (fav[l3], an index for uniaxial arrangement), describe quantitatively the anisotropic development of the cokes, although the bottom mosaic texture was omitted from the evaluation. According to fav, the best coke was produced at the ratio of 713, and the ratios of both 515 and 317 allowed higher values than those of cokes from FCCDO and LSVR both alone. Figure 2 illustrates montage photographs of cokes produced under the conditions of 46O”C, 8 kg/cm2. FCCDO gave a similar texture as observed in the coke produced at 480°C. Addition of LSVR slightly increased the length of anisotropic units and improved fav, although the improvement was less than that observed at 480°C. The coke from LSVR alone at 460°C was certainly better than that at 480°C in terms of fav. According to the values of lav and fav, the best coke was produced at the mixing ratio of 515. The ratios of both 713 and 317 provided similar fav values
The lumps were sectioned in parallel to the bomb axis in order to examine the anisotropic texture under reflected polarized light microscope after conventional polishing. A montage photograph of the whole surface was made from 100 micrographs (magnification x 50). Anisotropic units were evaluated by lav (average length of anisotropic unit vector) and fav (average axial portion of the unit vectors), both of which were defined in a previous paper[l3]. The CTE (temperature range, room temperature to 500°C) was also measured after the lump was calcined at 1000°C. Details of anisotropic evaluation and CTE measurement were described in a previous paper[ 131.
3. RESULTS
3.1 Montage photographs of coke lumps produced through the cocarbonization Figure 1 illustrates montage photographs of coke lumps (5 mm height) produced under the conditions of 48O”C, 8 kg/cm* from FCCDO, LSVR, and their mixtures of a series of mixing ratios. FCCDO and LSVR produced coke lumps of flow and flow-mosaic mixed textures, respectively. The former one is an excellent needle coke, while the latter one was much inferior, as reported previously[8]. It should be noted that the latter coke carried a wide area of fine mosaic texture at its bottom part. Addition of LSVR to FCCDO at the ratio of 7/3 (FCCDO/LSVR) certainly improved parallel alignment of flow texture along the bomb axis (abbreviated as uni-axially ori-
Table 2. CTE and anisotropic development
480°C
460°C
lav
lav
(w)
fav
FCCDO
7:FCCD0 3:LSVR
5:FCCD0 5:LSVR
3:FCCD0 7:LSVR
LSVR
19.6
20.0
18.4
17.8
16.2
16.0
17.3
16.8
16.5
15.0
19.0
20.0
19.2
18.2
17.5
16.0
16.0
16.6
16.2
15.4
0.78 Ix 10-6/0c)
of lump cokes
0.90
0.36
0.75
1.20
lmm
to that from FCCDO alone, which was certainly better than that from LSVR alone. The bottom mosaic coke increased certainly at this temperature although such a texture was not still found in the coke from FCCDO alone. The thickness of bottom mosaic belt increased with the increasing amount of LSVR, and LSVR alone gave the thickest one. 3.2 Yield and CTE of Coke Lump Table 3 summarizes the yields of cokes from FCCDO, LSVR, and their mixtures under two conditions. The yields of the mixtures which were much the same under both conditions, coincided approximately to the averaged one of those of FCCDO and LSVR. The CTE values of the calcined lump cokes are summarized in Table 2. The bottom area of mosaic texture was cut off from the sample coke for the CTE measurement. The averages of the values, which were well reproduced in three repeated experiments, distinguished the cokes very definitely. The addition of LSVR to FCCDO certainly improved the value. The best cokes were produced at the mixing ratios of 713 and 515, respectively at 480 and 460°C in terms of CTE and the amount of mosaic coke in bottom part. CTE of the needle coke has been principally related the uni-axial arrangement of graphite layers and the microcracks among the graphite grains[l3]. The degree of uni-axial arrangement is semiquan-
titatively described by the averaged axial component of anisotropic unit vector, fav. in a previous paper[l3]. At present, adequate procedure for the description of microcracks is not proposed yet. The values of fav of the present cokes are correlated to their CTE values in Fig. 3. An excellent linear correlation was established among the cokes produced from FCCDO, LSVR, and their mixtures at different temperatures. Hence, the advantage of cocarbonization in terms of CTE of the resultant cokes can be discussed in connection with the uni-axial arrangement of anisotropic flow texture. 3.3 Bottom Coke of Fine Mosaic Texture As described above, LSVR and its mixtures with FCCDO produced area of mosaic texture at the bottom of the coke lump. Figure 4 shows a montage photographs of whole area of the coke lump produced from mixture (blend ratio: 713 FCCDOi LSVR) at 48o”C, 8 kg/cm’ (A) and microphotographs of the middle and bottom part in larger magnification (B-D). There are various types of flow texture which were axially oriented (C) and not oriented (B) at the middle part of the lump, while the mosaic texture stayed at the bottom part. being clearly distinguished from the flow texture at the middle part. Hence, the thickness of mosaic part at the bottom can be measured easily using the montage photograph[l2]. The thickness of bottom mosaic belts are plotted with the mixing ratio in Fig. 5. where the carboni-
I. MOCHIDAet al.
362 Table 3. Coke yield (wt%)” Feed Carbonization temperature
FCCDO
480°C 460°C
39 40
7:FCCD0 3:LSVR
5:FCCD0 5:LSVR
34 36
LSVR
29 30
23 23
“Carbonization pressure 8 kg/cm’.
zation temperatures were 480°C and 460°C. The thickness were about 1 mm in the cokes produced at 480°C from LSVR alone and its mixtures regardless of the mixing ratio. The thickness was much larger in the cokes produced at 460°C. LSVR gave thickness of 3 mm and the thickness decreased by more FCCDO in the mixture to be zero in the coke from FCCDO alone. Carbonization of mixtures (FCCDOILSVR: 713 and 5/5) was examined under higher pressure (16 kg/cm?) and/or at higher temperature (490°C) to reduce the bottom mosaic cokes. As illustrated in the montage photographs of Fig. 6, the bottom mosaic coke became thinner (height 0.5 mm) when the carbonization temperature and pressure were higher, although it was certainly observable in any cases. The uni-axial arrangement of flow texture was slightly deteriorated by increasing the pressure when cokes produced at the same carbonization temperature were compared. The higher temperature appeared to increase the area of turbulent flow textures as pointed in the figure. 3.4 Carbonization Progress The completion of the carbonization was defined by the appearance and solubility of the coke. The times required for the completion which depended on the carbonization temperature and the feedstock are summarized in Table 4. FCCDO was much more stable than LSVR, requiring 2.5 to 3.0 h for the
0 \
\
t2 0
n
15
0
#p\\ 16 fav ( pm
4.
DISCUSSION
The cocarbonization of FCCDO with LSVR surely improved the coke quality in terms of CTE and uniaxial arrangement of flow texture, producing the best coke at a particular mixing ratio according to the carbonization temperature. Such improvement has been applied in a commercial coker to produce a better coke from more accessible blends of petroleum feedstocks[l]. The present authors have proposed the carbonization mechanism which leads to the needle coke[l6, 17). Two major steps where the bulk mesophase of low viscosity and broad isochromatic areas and its uni-axial rearrangement are achieved compose the whole scheme. In the first step, the aromatic components in the feedstocks are condensed into mesogen molecules of adequate molecular structure and size distribution. Hence, too much reactivity of the component should be controlled by its dissolution in the stable molecules and intermolecular hydrogen transfer from the naphthenic units of the cocarbonization substances[l8]. LSVR carries a considerable amount of asphaltene as shown in Table 1, which is the major origin for the coke and has high reactivity because of its large molecular weight of the aromatic nucleus, many alkyl side chains and mice11 aggromelation. The paraffinic fraction of another major component in LSVR is poor as a solvent for the asphaltene fraction, especially after asphaltene loses the alkyl group by the carbonization. Hence, the mesophase of low viscosity is difficult to be produced from LSVR, smaller aromatic units tending to be introduced. In contrast, rather stable aromatic species in FCCDO
oA k
completion of its carbonization at 480°C while LSVR required 1.5 to 2.0 h. Addition of LSVR shortened the time, depending on the amount of LSVR. The carbonization was very slow at 460°C compared to that at 480°C. FCCDO and LSVR took 12.0 to 12.5 and 5.0 to 5.5 h, respectively, for completion of their carbonization. Again, the addition of LSVR accelerated the carbonization to shorten the time in comparison to that of FCCDO alone. The shorter time for the completion of the carbonization by addition of LSVR to FCCDO suggests that the reactive LSVR accelerates the carbonization of the latter feedstock through some mechanism of radical transfer and propagation.
17
1
Fig. 3. Relation between fav and CTE of lump cokes carbonization temperature. -80°C; 0460°C.
provides bulk mesophase of low viscosity. Its naphthenic structure helps such a mesophase preparation[19-221. Hence, the cocarbonization of FCCDO and LSVR controls the high reactivity and solubility of the asphaltene fraction in LSVR to provide better mesophase from their mixture. In the second stage, appropriate gas evolution is required to rearrange the mesogen molecules uni-
Carbonization
i63
in the tube bomb leading to needle coke. 1. FLOW TEXTURE AXIALLY NOT-ORIENTED
MONTAGE PHOTOGRAPH
FLOW TEXTURE AXIALLY ORIENTED
MOSAIC TEXTURE IN BOTTOM PART
Fig.
4. Montage
photograph 8 kg/cm’
and microphotographs ol’ the coke lump produced from mixture of FCCDOiLSVR (blending ratio 7,!3).
axially along the axis at the solidification process where the viscosity of the mesophase increases. FCCDO certainly carries methyl groups attached to the aromatic ring as a source for such a gas to give an excellent needle coke, as described in a previous paper[l8]. LSVR is plenty of such gas sources be-
u ;i
Ei I 3.0
-
A---_____
5
-4
‘\
E
0 m
0” I3 y
‘\
2.0 -
‘. 0
l.O-
I O
I
0
Fig. 5. Relation height of bottom ture. WWC:
I
I
I,
I
I,,
30 50 70 CONTENT OF FCC DO(%) between the content of FCCDO and mosaic texture. Carbonization tempera046O”C, carbonization pressure.
at 4Xo”C‘ under
cause it carries a number of noncarbonized aromatics, alkyl side chains, and straight paraffins. Hence, the cocarbonization of two feedstocks can control the mesophase formation and provide more gases at its solidification, producing a better coke than those from both feedstocks alone. The mixing ratio and the carbonization temperature influence cooperatively such stage of the carbonization progress through the reactivity, solubility, and hydrogen-donating ability of carbonizing molecules in both feedstocks. Thus, a particular carbonization temperature define a respective particular ratio for the best coke as described above. The carbonization temperatures of 460°C and 480°C exhibited their respective best mixing ratios of S/5 and 713. The higher temperature consumes more donors to moderate the reactive asphaltene and expects less gas evolution from LSVR at the solidification, as suggested from the carbonization profile of FCCDO(lX]. Hence. higher mixing ratio of FCCDO is requested for the best coke at a higher cocarbonization temperature. It is of value to discuss the role and formation mechanism of bottom mosaic coke. It is well recognized that cokes of fine mosaic texture. higher density, and lower quality or even shot cokes are produced at the bottom of the coker chamber(‘)-13). Such cokes are less frequently found in the middle of the chamber. The bottom coke found in the pres-
I.
364
MOCHIDA et al.
i mm
Fig. 6. Montage photographs
of lump cokes produced under variable conditions: A-7/3,
kg/cm2 (FCCDO/LSVR); B-5/.5, 48o”C, 16 kg/cm2 (FCCDO/LSVR); C-7/3, (FCCDO/LSVR);
D-5/5,49O”C,
ent study may originate via the same route. The bottom coke in the chamber is separated to maintain the quality of the whole coke produced in the same chamber. The causes of such cokes may be moderated in the middle chamber, however they may deteriorate the coke quality although influences may not obviously be recognized. The most reactive fraction of the asphaltene in LSVR may be condensed in the early stage of carbonization, losing alkyl side chains and being converted into polycondensed aromatic molecules in the micell[l2]. Such aromatics are hardly soluble in the paraffin-rich matrix to be sedimented to be the bottom, where their carbonization progresses in the highly viscous circumstance with least chances of mesophase growth and less volatile substances, giving fine mosaic cokes of higher density. Such a mechanism which has been proposed in the shot coke production[ll] may also operate in the cocarbonization. Although a large part of the most reactive asphaltene may be dissolved and moderated by the FCCDO to be cocarbonized together with partners, some of most reactive portion still follows the same mechanism to give bottom mosaic coke of reduced amount. The amount of such a portion may strongly depend on the carbonization conditions and the mixing ratio. Higher temperature up to 480°C increase its solubility of the portion in FCCDO and enhances the hydrogen donating ability of FCCDO to decrease the bottom mosaic coke. Such conditions are not necessarily best for the uniaxial arrangement of flow texture in the carbonizaTable 4. Times
for coking
completion
(h)
Feed Carbonization temperature 480°C 460°C “Carbonization
FCCDO
7:FCCD0 3:LSVR
5:FCCD0 5:LSVR
LSVR
2.5-3.0 12.0-12.5
2.0-2.5 11.5-12.0
2.0-2.5 10.5-11.0
1.5-2.0 5.0-5.5
pressure
8 kg/cm*.
16 kg/cm*
48o”C, 16
49O”C, 16 kg/cm2
(FCCDO/LSVR).
tion of major portion as discussed previously[l6]. Further higher temperatures may accelerate the reactivity of the asphaltene to increase the bottom coke. At lower temperatures, the cocarbonization may operate with less portions because of limited solubility and hydrogen transfering ability although the reactivity is reduced, requiring more FCCDO to dissolve or moderate the source of bottom coke. 5. CONCLUSIONS
1. The cocarbonization of a FCCDO with a LSVR was found to improve the orientation of flow texture and CTE of coke lump produced in a tube bomb. 2. Respective optimum blend ratios of the feeds (exist) at the respective carbonization conditions to give the lowest CTE of the coke. 3. The roles of FCCDO and LSVR were discussed in terms of mesophase formation and gas evolution at the solidification stage. 4. The formation of mosaic coke in the bottom part of the coke lump was controlled to a limited extent by blending and the carbonization conditions. REFERENCES 1. C. A. Stocks and V. J. Guercio, Erdol & Kohle Erdgas Petrochemic 38, 31 (1985). 2. H. Marsh and P. L. Walker, Jr., Chemistry and Physics of Carbon, Vol. 15, p. 228. Marcel Dekker, New York (1979). 3. H. Marsh, I. Macefield, and J. Smith, 13th Biennial Conference on Carbon, Extended Abstracts, American Carbon Society, Irvine, CA, p. 21. (1977). 4. I. Mochida, K. Amamoto, K. Maeda, and K. Takeshita, Fuel 56, 49 (1977). 5. I. Mochida, K. Amamoto, K. Maeda, and K. Takeshita, Fuel 57, 225 (1978). 6. Y. Korai and I. Mochida, Fuel 62, 893 (1983). 7. I. Mochida, Y. Korai, Y. Nesumi, and T. Oyama, Ind. Eng. Chem. Prod. Res. Dev. 25, 198 (1986). 8. I. Mochida, Y. Korai, H. Fujitsu, T. Oyama, and Y. Nesumi, Carbon 25, 259 (1987).
Carbonization
in the tube bomb leading to needle coke. I.
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365
16. I. Mochida, T. Oyama. and Y. Korai, Carbon 26. 48 (1988). 17. I. Mochida, T. Oyama, and Y. Korai. Carbon 26, 57 (1988).
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