916

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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