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Original Paper
Hydrotreating of FCC decant oil as a needle coke feedstock
KatsumoriTANABE*l,4, TomonoriTAKADA*1, Bruce A .NEWMAN*2,
MasaakiSATOU*3 andHideshiHATTORI*3
*1 Mizushima Refinery, Japan Energy Corporation , 2-1 Ushio-dori, Kurashiki, Okayama, 712 Japan
*2 Conoco Inc., P. O. Box 1267, Ponca City, OK, 74603, USA
*3 Center for Advanced Research of Energy Technology ,
Hokkaido University,
N-13 W-8, Kita-ku, Sapporo, 060 Japan
(Received February 13, 1996)
Two levels of hydro-desulfurizing experiments using FCC decant oil were performed with a
conventional petroleum hydrorefining catalyst in order to obtain detailed information on
hydro-desulfurizing of feed oil and coke derived from it. Hydro-desulfurizing products and the
feed decant oil were analyzed by the HPLC-MS method, and the carbonization was carried out in a
small batch reactor after which properties of coke produced were measured.
There is no difference with sulfur condensation ratios in cokes formed from hydro- desulfu-
rized and unhydro-desulfurized feedstocks. Low sulfur coke clearly can be produced from low
sulfur feedstocks which have beenhydro-desulfurized. Coke coefficient of thermal expansion
(CTE) decreases by feedstock hydro-desulfurizing, but coke CTE from the more severely hydro-desulfurized decant oil is higher than that from the mildly hydro-desulfurized decant oil.
Under the severehydro-desulfurizing conditions, hydrogenation of aromatic rings occurs, as does
naphthenic ring opening and dealkylation. Severehydro-desulfurizing also causes a decrease in
hydrogen donor ability and an increase in the carbon number of alkyl side chain for one of com-
pound classes.
Key Words
Decant oil, Needle coke, Hydro-desulfurizing, Coefficient of thermal expansion
1. Introduction
Needle coke, which generally is produced from
petroleum heavy residua and coal tar pitch using
commercial delayed coking, is a raw material for
graphite electrodes in the steel industry. The most important properties of needle coke are low
puffing and low coefficient of thermal expansion
(CTE)1). The puffing phenomenon is believed to occur through the evolution of contaminants from*4 To whom correspondence should be addressed
Hydrotreating of FCC decant oil as a needle coke feedstock (TANABE 他) 917
softened carbon in the graphitizing process 2)3)
The larger the puffing, the weaker the electrode
strength becomes. In addition to being suscepti-
ble to puffing, graphite electrodes are used at a
high temperature, which promotes both large
temperature gradients and strong thermal shock.
Therefore, high performance electrodes can be
manufactured only from low CTE cokes.
It is believed that hydrodesulfurization of coke
feed oils reduces coke puffing because sulfur3) is
one of the contaminants which causes puffing.
Additionally, coke CTE is expected to change be-
cause the chemical structure of the hydrodesulfu-
rized oil is altered. It is well known that prop-
erties of needle coke depend strongly upon feed-
stock properties4)•`7). Detailed information on
feedstock and coke structural change associated
with variation in hydrodesulfurizing severity,
however, has been extremely limited.
This work was pursued to obtain detailed in-
formation on hydrodesulfurizing of feed oil and
the consequent impact upon coke properties.
Two levels of hydrodesulfurizing experiments us-
ing FCC decant oil, which has been commercially
recognized as the best petroleum feedstock for the
needle coke, were performed with a conventional
petroleum hydrorefining catalyst. Hydrodesulfu-
rized products and the feed decant oil were analy-
zed by the HPLC-MS method") to measure
change of aromatic and hydroaromatic components
with change in reaction severity. Carbonization
of both hydrodesulfurized and unhydrodesulfu-
rized feeds was carried out in a laboratory-scale
batch reactor.
2. Experimental
2.1 Hydrodesulfurizing for FCC decant oil
An FCC decant oil derived from Middle East
crude was used in the present study. It contains
0.73 wt% sulfur, and the 10% and 90% boiling
points are 342 and 486 •Ž, respectively, under
atmospheric pressure. General properties of the
decant oil (FD-DO) are shown in Table 1.
A commercially available hydrodesulfurizing
catalyst, consisting of cobalt and molybdenum
Table 1 Properties of a Feed Decant Oil and
Hydrodesulfurized Decant Oils
supported on alumina, was used. Prior to use,
the catalyst was sulfided in-situ using a gas oil
spiked with 1.0 wt% carbon disulfide at tempera-
tures increasing from 150 to 300 C.
A conventional high-pressure bench unit, equip-
ped with a reactor of 25 mm inner diameter, 1200 mm length, and 100 ml catalyst loading capacity,
was used in the hydrodesulfurizing experiments.
The reaction temperature was the average of five
temperatures measured along the catalyst bed
height at regular intervals. A 7 -alumina sup-
port was placed in the preheater section. Two levels of reaction severity, using temperatures of
340 and 400 t as the mild and severe conditions,
were employed in these experiments. Other reac-
tion conditions were the same in both cases.
2.2 Analysis of oils
Analyses of the hydrodesulfurized decant oils
and their feed decant oil were carried out using
the following apparatus: API gravity with a
Antompar model DMA-45 according to JIS K
2249; Conradson Carbon Residue with a Tanaka
Chemical model ACR-5 according to JIS K 2270;
sulfur content by X-ray fluorescence analysis
with a Tanaka Chemical model RX 500 SA accord-
ing to JIS K 2541; carbon and hydrogen contents
by a CHN-analyzer with a Perkin-Elmer model
2400; and boiling points with a Hewlett-Packard
model HP 5880 A according to JIS K 2254.
918 ― 「日本 エ ネル ギ ー学 会 誌 」 第75巻 第10号(1996) ―
The oil samples also were analyzed with an
HPLC-MS procedure which is described else-
where 8)9). The samples were separated into
seven hydrocarbon fractions, called'compound
classes' by HPLC. The liquid chromatograph
used was a Jasco model BIP-I and 880-PU equip-
ped with a Yamamura Chemical Laboratories mod-
el SH-643-5 S-5 120 A NH2 column. A Jasco
model UNIDEC-100-IV and a Showa Denko model
Shodex SE-11 detectors were used. A low vol-
tage ionization (10 eV) EI-MS technique was
used to analyze the aromatic hydrocarbon com-
pound classes. A sample was directly introduced
into the heat chamber. The spectra were obtained
with a Hitachi model M-52 MS system.
2.3 Carbonization
A sample oil of 20 g was charged in a small
stainless tube reactor similar to the one10) which
has been reported to produce cokes with their
properties comparable to commercial needle cokes.
The reactor was equipped with a cracked oil re-
servoir and a relief valve. Carbonization was
carried out in a fluidized sand bath. The carbo-
nization temperature was kept at 475 •Ž, and the
carbonization pressure was adjusted by the initial
nitrogen pressure and manual control of the valve
to be 5 Kg/cm2G. The holding time was 20
hours. After the reaction, the tube reactor was
cooled at room temperature to allow recovery of
coke and oil produced.
2.4 Analysis of coke
The cokes obtained were heat-treated at 1,400
•Ž for 15 min in a flow of argon with an electric
furnace. Sulfur content of calcined coke was me-
asured by burning the sample in an oxygen flow
in a ceramic tube according to JIS K 2541.
CTE of the calcined coke was measured accord-
ing to a conventional method. After coke pulver-
ization, a test specimen (5 mm x 5 mm x 20 mm)
was made by molding the coke powder and binder
pitch. Baking of the test specimens was carried
out at 1,000 •Ž for 4 hours using an electric fur-
nace. The CTE perpendicular to the loading
direction was measured between room tempera-
ture and 300 •Ž with a Mac Science model
TD-5010 dilatometer.
3. Results and Discussion
3.1 Physical and chemical properties of oils
It generally is reported7)11) that decant oils con-
sist of 60 to 95% of aromatics, which carry some
alkyl substitutions, saturate and resin fractions.
They also carry significant sulfur atoms. One of
main objectives of commercial hydrodesulfurizing
of decant oil is to reduce the sulfur content.
General properties of the feed decant oil and
the mild and severe hydrodesulfurized decant oil
(MH-DO, SH-DO) are summarized in Table 1.
The API gravity increases from 7.23 to 9.31 in
the MH-DO, and the SH-DO has the same API
gravity as the MH-DO. The CCR decreases from
2.38 to 0.98 wt% under the mild condition.
However, the CCR of the SH-DO decreases only to
1.49 wt%. The sulfur contents of MH-DO and
SH-DO were 0.26 and 0.06 wt% which yield de-
sulfurization ratios of 64% and 92% under the
mild and severe conditions, respectively. Sul-
fur2)3) in coke is thought to be one of the impurity
materials which causes puffing. Therefore, it
should be beneficial to reduce the sulfur content
of the coker feedstock to 0.06 wt% from the view-
point of coke puffing. Table 1 also gives the
elemental composition of the sample oils. The
atomic H/C ratios of the sample oils are calculated
as 1.22, 1.31 and 1.27 for FD-DO, MH-DO and
SH-DO, respectively. It is clear that some hyd-
rogen was added to the decant oils by hydrodesul-
furizing. Hydrogen uptake values were calcu-
lated using these elemental compositions to be 92
and 59 liter-H2/liter-product oil in MH-DO and
SH-DO, respectively. It should be noted that the
hydrogen uptake of the SH-DO is significantly
lower than that of the MH-DO.
Ten, 50 and 90% boiling point also are given in
Table 1. These values decrease from 342 , 407
and 486 •Ž for the unhydrodesulfurized feedstock
to 317, 396 and 474 •Ž , respectively, under the se-
vere condition. It is apparent, therefore, that
Hydrotreating of FCC decant oil as a needle coke feedstock (TANABE 他) 919
some light oil fraction is produced under the se-
vere condition. Comparing SH-DO with MH-DO
from the viewpoint of their chemical composition
and general properties, it also appears that a de-
crease of saturate, an increase of aromatics, and a
decrease of alkyl side chains occur under the se-
vere condition. These effects would be expected
to cause a decrease of API gravity. On the other
hand, SH-DO shifted to lighter boiling points.
Therefore, the actual API gravity of SH-DO is the
same as that of MH-DO. In other words, it
appears that there should be a difference in the
chemical compositions and/or chemical structures
between MH-DO and SH-DO because API gravity
of both are the same and their distillation prop-
erties, CCR, and H/C ratio vary.
Hydrodesulfurized decant oils and the feed de-
cant oil were separated into seven compound class
fractions by an HPLC equipped with an amine col-
umn. The compound class contents of sample oils
determined by gravimetry are shown in Fig.1. P,
M, D1, D2, T1, T2 and PP represent paraffin,
monoaromatics, naphthalene type diaromatics,
biphenyl type diaromatics, triaromatics, tet-
raaromatics and poly/polar compounds, respec-
tively. There are significant variations in these
compound class contents.
P of MH-DO is the same as the feed and P of
SH-DO is slightly larger than that of the feed.
M, D1 and D2 increase under the mild condition,
and slightly decrease under the severe condition.
Conversely, T1, T2 and PP decrease under the
mild condition and increase under the severe con-
Fig. 1 HPLC Separation of Feed and
Hydrodesulfurized Decant Oils
dition. For MH-DO, it is assumed that decom-
position of polar compounds and hydrogenation of
polyaromatic rings cause both the decrease of T1,
T2 and PP fractions and the equivalent increase
of M, D1 and D2 fractions. Under the reaction
conditions in this study, a high degree of hyd-
rogenation of aromatic rings apparently did not
occur because the P fraction did not significantly
increase.
Tanabe et al 9) reported that for hydrorefining
of an SRC-II heavy distillate, naphthenic ring
opening and dealkylation occur at 420•Ž and
higher temperatures, producing measurable
amounts of light oil. In this study, unfortunately,
a material balance of the hydrodesulfurizing ex-
periments was not completed, however, a signifi-
cant amount of lighter oil should be generated. It
also is suggested that some of the naphthenic
rings produced from aromatic rings by hydrogena-
tion decomposed. Therefore, under the severe
condition, M, D1, and D2 (mainly hydroaromatics)
decrease because a part of these fractions ran
away out of the product oil by naphthenic ring
opening and dealkylation. As the result, T1, T2
and PP contents increase relatively. Relative
contents of T2 and PP to T1 (T1=1.00) are
0.44 and 0.96, 0.39 and 0.89 under the mild and
severe conditions, respectively. Therefore, it is
noted that under the severe condition T2 and PP
compounds still decrease both by deheteroatom
reactions and by hydrogenation of the aromatic
ring.
Compound type analyses of the aromatic com-
pound classes (M, D1, D2, T1 T2) separated
by HPLC were carried out by EI-MS. Molecular
ions without fragmentation were detected by the
LV (10eV)-EI. method 8) 9). Consequently, the
m/z peaks are the molecular weights. The
molecular weight distribution measured included
odd mass numbers because of the presence of iso-
topes. In this study, the odd mass numbers were
intentionally neglected to simplify the mass spec-
trum. The structural assignment of components
in the oils was carried out on the basis of a com-
bination of HPLC separation characteristics and
920 ― 「日本 エ ネル ギ ー 学 会誌 」 第75巻 第10号 (1996) ―
compound type analyses by MS. 8) 9).
In this study, possible compound types are
assumed to be benzene (Z=-6), naphthalene (Z=-12) , anthracene (Z=-18), pyrene (Z=-22), ben-zopyrene (Z=-28), dibenzopyrene (Z=-34) and
coronene (Z=-36) and their hydrogenated com-
pounds. Shown in Fig. 2 are the major compo-nents of the decant oil and hydrodesulfurized de-
cant oils. The aromatic fraction of the FD-DO
contained mainly anthracenes, pyrenes, and benzo-
pyrenes, but little naphthalenes. The anthracenes included dihydro-, tetrahydro-, octahydro-and
Fig. 2 Distributions of Compound Types
anthracene, and the pyrenes included dihydro-,
tetrahydro-, octahydro-, decahydro-and pyrene.
It was found that mild hydrodesulfurizing caused
anthracene, pyrene and dihydro-pyrene to de-
crease, and conversely, tetrahydro-, octahydro-
anthracene and tetrahydro-, hexahydro-and
decahydro-pyrene to increase with the severe
hydrodesulfurizing, anthracene, dihydropyrene
and pyrene contents were higher than those from
the mild hydrodesulfurizing.
Benjamin12) reported that tetralin mainly was
converted to naphthalene by dehydrogenation dur-
ing pyrolysis. We can't deny completely a possi-
bility of dehydrogenation of naphthenic ring com-
pounds under the severe condition because a mass
valance on the hydrodesulfurizing experiment
wasn't enough. The boiling point of the hyd-
rodesulfurized oil under the severe condition be-
come low and shown in Fig. 2, benzopyrenes de-
creased, and pyrenes and anthracenes increased.
If dehydrogenation had been dominant under the
severe condition, the amounts of those compounds
would not be changed. From this result it again
appears that hydrogenation of polyaromatic hyd-
rocarbon occurs under the mild condition, and de-
composition of naphthenic ring components occurs
under the severe condition.
3.2 Properties of coke
The hydrogenated decant oils and the parent
decant oil were carbonized using a pressurized
small batch reactor. The green coke was calcined
at 1, 400•Ž. Coke yields, sulfur contents, and
CTE of the calcined coke are shown in Table 2.
The coke yield of FD-DO was 53.1wt% and
those of MH-DO and SH-DO were 45.4 and 47.5
wt%, respectively. It is well known that a good
Table 2 Coke Yield and Sulfur and CTE of
Calcined Coke
Hydrotreating of FCC decant oil as a needle coke feedstock(TANABE他) 921
relationship exists between coke yield and CCR of
the feed oil 13). A similar relation was found in
this study. The coke yield decreased with an in-
crease of H/C ratio and a decrease of CCR of feed
oils.
The sulfur content decreases from 0.77 to 0.30
and 0.07wt% in the MH-DO and SH-DO, respec-
tively. The primary object of hydrodesulfurizing
of the feed decant oil, therefore, was fully accom-
plished. A sulfur condensation ratio (describing the
fraction of sulfur in the feed incorporated into
coke) can be calculated as follows:
(1)
where: Cs (%)=sulfur condensation ratio in
coke; Sc=wt% of sulfur content of coke; Yc=coke
yield; So=wt% of sulfur content of feed oil. Cs of FD-DO was 56% and those of MH-DO
and SH-DO were 52 and 55%, respectively.
Therefore, we see that slightly over half of the
feed oil sulfur was concentrated in the coke and
that the sulfur condensation ratio didn't depend
upon the sulfur content of the coker feedstock. It
should be noted that the removal of heteroatom by
hydrodesulfurizing often leads to the degradation
of aromatic rings, resulting, in the extreme, in no
coke formation from the molecules after the re-
moval of heteroatoms. In such case, the content
of heteroatoms in the feed is certainly reduced;
however, less reduction of heteroatom contents in
the coke is achieved 14). Further, if sulfur re-
maining in decant oil after hydrodesulfurizing is
easier to condense in coke than sulfur removed by
hydrodesulfurizing, the sulfur condensation ratio
should increase with hydrodesulfurizing severity.
However, because sulfur condensation ratio didn't
change significantly (52 and 55%) with an in-
crease of desulfurization ratio (64 and 92%), sul-
fur remaining in hydrogenated oil is roughly the
same as sulfur removed from the viewpoint of
condensation into coke. It is for this reason that
hydrodesulfurizing of decant oil is very effective
in reducing the sulfur content of coke.
Low CTE is one of the most important prop-
erties which are required in needle coke. Coke
CTEs also are shown in Table 2. CTE of
FD-DO was 2.20•~10-6/•Ž and those of MH-DO
and SH-DO were 1.96 and 2.15•~10-6/•Ž, re-
spectively.
There have been many studies of relationships
between feed oil properties and coke properties.
Mochida et al11) reported that development of ani-
sotropic texture in mesophase correlates well with
coke CTE. Marsh et al 15) reported that hyd-
rogenation of coal-extract solutions evidently
facilitates the physical and chemical requirements
for growth and coalescence of mesophase. It has
been reported also that the size of optical texture
of coke is, in general, directly proportional to the
degree of aromaticity of the feedstocks 6) 7) 16) and
that heteroatoms have an negative effect on coke
properties 17) 18)
In this study, the MH-DO has a lower sulfur
content and a higher aromatic fraction yield
(Fig. 1) than FD-DO. For the above reasons,
MH-DO shows lower CTE than FD-DO. On the
other hand, it is not clear why CTE of SH-DO is
higher than that of MH-DO. It is necessary to
discuss the chemical composition of SH-DO.
3.3 Feedstock property and coke CTE
Carbonization schemes leading to needle coke
are generally understood to consists of next steps
: destructive distillation, mesophase sphere forma-
tion, growth and coalescence, bulk mesophase
laying down parallel to the bottom, growth of bulk
mesophase in the whole region, and rearrangement
of mesophase planar molecules into uniaxial
orientation and, finally, solidification. Thus,
three major factors essentially determined the
property of needle coke: anisotropic development,
viscosity of the bulk mesophase, and gas
evolution 19). Therefore, some properties should
be required for feedstock of needle coke 20). A lot
of amount of paraffin in the decant oil cause the
formation of bottom mosaic coke which has less
property, because such paraffin-rich matrix hard-
ly dissolve polyaromatic hydrocarbons which is
produced at the very earliest stages of the carbo-
922 ― 「日本 エ ネ ル ギ ー学 会 誌 」 第75巻 第10号 (1996)―
nization. Partially hydrogenated aromatic hydro-
carbons are believed to be excellent hydrogen-
donating species which moderate the carbonization
reaction in the mesophase development and gas
evolution for uniaxial rearrangement. Heter-
oatoms in the feedstock are believed to disturb the
anisotropic development because they act as reac-
tive sites for condensation which cause three
dimensional bridge bond, and withdraw hydrogen
from donor species which have a roll to keep the
viscosity of bulk mesophase low.
Table 3 shows contents of aromatics and hyd-
roaromatics of the coker feed oils calculated from
the data of MS analyses. Both compounds are
carbonized into coke while losing their aliphatic
carbons and hydrogens 7). As predicted before,
hydroaromatic content is shown in Table 3 to in-
crease during hydrodesulfurizing from 33.9 to
52.3wt% under the mild condition and to de-
crease to 44.3wt% under the severe condition. Yokono et al 21) reported that residues forming
cokes with good optical texture both have a high
ability as a hydrogen donor and show a low spin
concentration in the early stage of carbonization.
It is thought 22) that donatable hydrogen stabilize
radical components produced during thermal
cracking, therefore causing lower viscosity 11) of
the reaction system and higher development of
mesophase texture. Proton Donor Quality Index
(PDQI) 23) 24) was proposed in order to evaluate hydrogen donor ability for coal liquefaction sol-
vents by means of mass spectroscopic data.
PDQI reflects actually the maximum donatable
hydrogen (mg) of naphthenic rings in 1g of the
sample. PDQI calculated by this method is shown
in Table 4. PDQI of FD-DO was 6.9 and those
of MH-DO and SH-DO were 14.0 and 10.2, respec-
tively. It is found that the higher PDQI value
does, indeed, correlate with the lower coke CTE,
Table 3 Aroamics and Hydroaromatics Contents
demonstrating the importance of hydrogen donor
ability.
Mochida et al 25) reported that heat-treatment of
FCC-DO removes paraffins and alkyl groups of
longer chains on aromatic rings, thus producing a
more highly aromatized FCC-DO which is capable
of dissolving viscous mesophase that causes bot-
tom mosaic structure. It also is reported that
feedstocks which carry short alkyl chains 6) 16)
have slow carbonization reaction rate, thereby
promoting more highly developed mesophase tex-tures. Table 5 shows average alkyl chain carbon
numbers calculated from MS data of each com-
pound class. Compared to the FD-DO, both MH-DO and SH-DO numbers for M, D1, and D2
decrease. However, M for SH-DO is much larger
than that for MH-DO. The change of average
alkyl carbon number qualitatively agrees with
coke CTE order (coke CTE increases with an in-
crease of average alkyl carbon number of feed-
stock oil) in this study. Therefore, in this study,
the carbon number of alkyl side chains may be
one of the possible factors affecting coke CTE.
There are a few studies investigating directly
the relationships between chemical composition of
feed oils and coke CTE. Liu et al 26) reported a
multi-regression analysis between coke optical
Table 4 Proton Donor Quality Index
Table 5 Average Alkyl Side Chain Carbon Number
Hydrotreating of FCC decant oil as a needle coke feedstock(TANABE他) 923
anisotropic texture index (OTI) and decant oils
compounds as measured by GC/MS. He found
the following equation:
(2)
where: FL=fluorene content; NAP=naph-
thalene content; PY=pyrene content; CR=
chrycene content; PH=phenanthrene content; BI=
biphenyl content; Alk=alkane content.
The equation suggests that fluorene, naph-
thalene, pyrene, chrysene and phenanthrene have
a positive effect upon optical texture and biphenyl
and alkane have a negative effect.
OTI of FD-DO, MH-DO and SH-DO are calcu-
lated using the above equation. In this calcula-
tion, chrysene and phenanthrene contents are sub-
stituted by benzopyrene and anthracene, and the
fluorene term is neglected because fluorene could
not be distinguished in this study. OTI calcu-
lated in such way for FD-DO, MH-DO and
SH-DO are 20.6, 26.5 and 24.8, respectively.
Calculated OTI and coke CTE appear, therefore,
to have a good relationship. It also is clear that
coke CTEs depend strongly upon feedstock che-
mical composition. However, at this time there is
little reliability for this equation both because we
have no idea why fluorenes should have the larger
positive coefficient and because heavy components
of decant oils weren't analyzed.
As mentioned above, we suggest there are three
reasons why SH-DO has a higher CTE than
MH-DO, despite the sulfur content of SH-DO is
lower than that of MH-DO. First is a decrease of
hydrogen donor ability caused by a decrease of
hydroaromatic compounds. The decrease of hyd-
rogen donor ability should cause the increase the
viscosity of bulk mesophsse and to deteriorate the
development of anisotropic texture. Second is an
large increase of the alkyl carbon number in the
mono-aromatic compound class. A coking reac-
tion rate becomes faster because of it. The high-
er coking rate disturbs the growth and coalesc-
ence of mesophase sphere. Third is an increase
of the paraffin fraction content. It decreases the
solubility of the matrix phase. In other words,
this study demonstrates that it is possible to re-
duce coke sulfur by feedstock hydrodesulfurizing,
and also to reduce the coke CTE. Under a severe
hydrodesulfurizing condition (higher temperature),
however, chemical structure of the hydrodesulfu-
rized oil is dramatically changed, reducing the
positive effect of hydrodesulfurizing upon coke CTE. It should be effective on both coke CTE
and puffing to reduce a specific velocity instead of
increasing temperature for hydrodesulfurizing. It
should be examined from the view point of econo-
mics of commercial plants.
It is thought that there is an optimum carbo-
nization condition 11) for each feedstock. In this
study, the chemical composition and the structure
of the severely hydrodesulfurized oil were
changed significantly, but the carbonization condi-
tion was constant for all feedstocks. Therefore,
it may have been possible to obtain coke of re-
latively lower CTE using a different carbonization
condition for the SH-DO.
4. Conclusions
This study on carbonization of hydrodesulfu-
rized decant oils at different severities of reaction
led us to the following conclusions:
(1) There is no difference in sulfur condensa-
tion ratios in cokes formed from hydrodesulfu-
rized and unhydrodesulfurized feedstocks. In
other words, low sulfur coke clearly can be pro-
duced from low sulfur feedstocks which have been
hydrodesulfurized.
(2) Coke CTE decreases by feedstock hyd-rodesulfurizing, but coke CTE from the more sev-
erely hydrodesulfurized decant oil is higher than
CTE from the mildly hydrodesulfurized decant oil.
(3) Under severe hydrodesulfurizing condi-tions, hydrogenation of aromatic rings occurs, as
does naphthenic ring opening and dealkylation.
Severe hydrodesulfurizing also causes a decrease
of hydrogen donor ability and an increase of the
alkyl side chain length of the coker feedstock.
924 ― 「日本エ ネルギー学会誌」 第75巻 第10号(1996)―
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