Note: Descriptions are shown in the official language in which they were submitted.
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MANUFACT~RE OF_ISOTROPIC CO~E Case No. 8156
Back~round and Sum,mar~v,of,t~e_Invention
Isotroplc coke has a thermal expansion approxim~tely equf~l
along the three ma~or crystalline axes. This thermal expanslon i8
normally expressed as CTE (l.e. coef~icient of thermal expansion) over
5 a glven temperature range such as 30-530C or 30-100C. Isotropic
coke is also indicated by a CTE ratio, whlch ls the ratio of radial
CTE dlvlded by axial CTE measured on a graphiti~ed extruded rod.
Acceptable isotropic coke has a CTE ratio of less than about 1.5 or a
CTE ratio in the range o~ about 1.0-1.5.
Isotropic coke i5 used to produce hexagonal graphite logs
whlch serve as moderators ln hlgh temperatwre gas-cooled nuclear
reactors. Thls type of coke has been produced in the past from
natural products such as gllsonlte. The production of such graphite
logs from gllsonlte and th~ use thereof are descrlbed ln U.S. Pat.
Nos. such as 3,231,521 to Sturges; 3,245,880 to Martln et al; and
3,321,375 to Martin et al. U.S. Pat. No 3,112,181 to Peterson et al
describes the production oE isotropic coke using petroleum
distillates. Contam~nants such as boron, vanadium, and sulfur have
prohibited the use of some materials as the source of isotropic coke
suitable for use in nuclear reactors. Less than about 1.6 weight
percent sulfur is preferred to avoid puffing problems upon
graphitization and abrication of the coke. The supply of isotropic
coke has been limited by availability of source materials, such as
gilsonite and expensive petroleum distillates.
U.S. Patent No 3,960,704 describes a process in which a
residuum, such as bottoms from the fractionation of virgin
feedstocks, is air-blown to increase its softening point. The
air-blo~n resid is then subjected to delayed coking to produce
isotropic coke having a CTE ratio less than l.S.
~esidual oils vary substantially in their sulfur content,
from less than 1.0 wt% to as high as 4.5 wt% or higher. When residual
oils are subjected to coking the amount of sulfur in the resultant
coke i.s from about 1.3 to about 1.5 times as much as the sulfur in the
residual oi]. feedstock. Since it is desirable to obtain an isotropic
coke product containing a minimum amount of sulfur, low-sulfur
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aLr-blown resldual oil~ are preferred a9 coker feedstocks, but these
oils are limited in supply and are more expensive than higher sulfur
feeds.
In accordance with this invention a sulfur-containing
residual oil, which has been contacted with an oxygen-containing gas
at an elevated temperature to increase its softenLtlg point, is
combined with a pyrolysis tar having a lower sulfur content and the
combined material is subjected to delayed coklng to provlde an
isotroplc coke product having a low CTE ratio and reduced sulfur
content.
Prior Art
U.S. Patent No. 3,960,704 to Kegler et al discloses the
production of isotropic coke by air-blowing a petroleum residual oil
and thereafter sub~ecting the air-blown oil to delayed coking. The
coke is subsequently processed to obtain graphite logs for use as
moderators in high temperature, gas-cooled nuclear reactors.
U.S. Patent No. 4,624,775 issued to Eric M. Dickinson
describes a process for making a premium coke by delayed coking of a
mixture of pyrolysis tar and coal tar distillate.
U.S. Patent No. 3,759,822 issued to Hillis 0. Folkins
describes a coking feedstock comprising a mixture of pyrolysis tar and
a heavy cracked oil.
U.S. Patent No. 4,130,475 issued to Daniel F. Cameron et al
describes a process for making premium coke from a feedstock
comprising a mixture oi' atmospheric reduced crude pe~roleum oil and
ethylene tar.
U.S. Patent No.2,922,755 issued to R. C. Hackley describes a
process wherein reduced crude can be mixed with thermal tar to produce
a feedstock mixture for producing premium coke by delayed coking.
U.S. Patent No. 3,112,181 to Peterson et al describes the
production of isotropic coke for use in the manufacture of moderators
employed in nuclear reactors. The coker feedstock used is petroleum
distillate which has been oxygen treated.
U.S. Patent No. 4,111,794 issued to Gerhard Pietzka et al
describes a method for producing pltch coke from a mixture oE coal tar
pitch and pyrolysis oil condensate.
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srieE De~cription_O~ t~e Drawi~
The dr~wing is a schematic diagram of a process unit which
illustrates the process of the invention.
Detailed Desçription of the Invention
The pyrolysis tar used in the process of the inventlon May
be any tar produced by hi~h temperature thermal crac~ing in pyrolysls
furnaces to produce low molecular weight olsfins. In general, olefins
comprising primarily ethylene and lesser amounts of propylene, butene,
and isobutylene are produced by the severe cracking of petroleum
distillates or residues at temperatures from about 1200 to about
1~00F., preferably from about 1300 to about 1600F, at pressures from
atmospheric to about 15 psig and in the presence of a diluent gas.
Typical diluents employed are low boiling hydrocarbons such as
methane, ethane, or propane, although steam is preferred and i~ the
most commonly used diluent. Ethane and propane can also serve as the
cracking stock. Thc product~ of the cracking operation are
predominantly olefinic gases such as ethylene, propylene, and butene.
A heavy pyrolysis tar is obtained from this cracking operation and ls
removed with the effluent and separated by condensation.
Pyrolysis tars obtained in this manner are characterized by
having low sulfur contents, usually from less than 0.1 wt% to about ~
wt% These tars also provide high yields of coke when subjected to
conventional delayed coking.
Residual oils which can be used to produce the isotropic
coke with the process of this invention are those which have not been
sub;ected to extensive ther~al or catalytic cracking; for example
preferred feedstocks are atmospheric or vacuum reduced crudes. Small
amounts of other residual components such as e~tract residuum, thermal
tar, decant oils, and other residua or blends thereof can also be used
in the feedstocks of this invention. The sulfur content of these
residual oils will vary from less than 1.0 wt~ to about 4.5 wt% or
higher, which is substantially higher than the sulfur content of the
typical pyrolysis tars employed in the process. The essential feature
of the feedstocks is thought to be their ability to form cross-linked
molecules under air-blowing conditlons. The preferred feedstocks are
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those which produce substantlal amounts of isotropic coke when
subJected to delayed coking after belng air-blown.
The amount of pyrolysis tar used in this process will vary
depending on the particular residual oil with which it is combined and
the amount of sulfur in such residua]. oil and in the pyrolysls tar.
Any amount o~ pyrolysis tar will provide the desired results, however
the use of larger amounts is more effective in reducing the sulfur
content of the isotropic coke product. Up to 50 wt~ pyrolysis tar or
more may be used in the mixture of p~yrolysis tar and o~ygen-treated
residual oil, however the concentration of pyrolysis tar will usually
constitute between about 15 and about 40 wt% of the mixture,
The mixture of pyrolysis tar and air-blown resid is
converted to isotropic coke by subjecting it to delayed coking. The
manu~acture of coke by delay0d coking refers to the formation of coke
in a coke drum, such as described in U.S. Pat~nt No. 2,922,755 to
Hackley. The delayed coking process typically uses petroleum
feedstock, such as residuum or a mixture of various petroleum
fractions to produce petroleum coke.
Referring now to the drawlng, a residual oil is introduced
through line 2 to air-blowing vessel 4. Wlthin this vessel there is
maintained a body of liquid 8 which is blanketed with in~rt gas,
provided in sufficient quantity to fill the vapor portion 6 of the
air-blowing vessel. The inert gas, which may be steam, nltrogen, or
other gas which i~ not reactive in the process is introduced to vapor
space 6 ~hrough line 12. Air-blown resid is withdrawn from
air-blowing vesssl 4 through line 14 and gases which include the inert
gas, air, and light hydrocarbons are removed overhead from the
air-blowing vessel through line 11.
The air-blowing operation is substantially the same as that
used for producing asphalt and may be a continuous or batch process.
The residual oil charge is heated to a temperature of about 400 to
600F which is slightly below its flash point. Air introduced to
air-blowing vessel 4 through line 10 is bubbled or blown through the
residual oil at a rate of about 20 to about 100 standard cubic feet
per minute per ton of residual oil. The residence time of the
residual oil in air-blowing vessel 4 is controlled to provide a
residual-oil product having a softening point of about 120 to about
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240F and pre~erably from abou~ 1~0F to about 200F. ~hile alr ls
the preEerred blowing agent because of its availability and cost,
other oxygen-containing gases such as o~ygen-enriched air may also be
used if desired. The residence tlme requlred to e~fect the
air-blowing operation will depend on the residual oil whlch is used.
However, the air blowing ordinarily will be completed over a period
from about 2 to about 24 hours of residence time.
The hot air-blown residual oil leaving vessel 4 is coMbi.ned
with hot pyrolysis tar provided through line 16. The mixture of
residual oil and pyrolysis tar is then introduced to fractionator 18
where it is combined with overhead vapors from coke drums 34 and 34a.
Light gases Cl to C3 are removed overhead Erom ~he ~rnctiona~or
through line 20. Heavier materials such as gasoline and light gas oil
are taken from the Eractionator through lines 22 and 24 respectively.
A mixture oE residual oil, pyrolsis tar, and diluent heavy gas oil is
removed from the bottom of fractionator 18 through line 28. The
purpose of the diluent gas oil is to reduce the viscosity of the
mixture and permit easier handling and pu~ping of the mixture to the
delayed coking part of the process. The diluent heavy gas oil which
is part of the gaseous effluent from the coke drums does not
substantially coke and therefore recycles through the system. The
amount of such diluent provided in the residual oil-pyrolysis tar
mixture may be controlled by varying the amount of heavy gas oil
withdrawn from fractionator 18 through line 26.
The mixture of residual oil, pyrolysis tar and heavy gas oil
passes through line 28 and is introduced to coXer furnace 30 wherein
it is heated to temperatures in the range of 875 to about 975F at
pressures of about atmospheric to about 250 psig and is then passed
via line 32 to coke drums 34 and 34a. The coke drums operate on
alternate coking and decoking cycles of about 8 to about 100 hours;
while one drum is being filled with coke the other drum is being
decoked. During the coking cycle each drum operates at a te~perature
between about 830 and about 950F and a pressure from about 15 to
about 200 psig.
The overhead vapor from the coke drums is passed via lines
38 or 38a to fractionator 18 wherein it is separated into various
fractions as previously described. The green coke which is removed
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from the coke drums through outlets 36 and 36a ia further processed
(not shown) to produce hexagonal graphite logs which are used as
moderators in high temperature, gas-cooled nuclear reactors. The
manufacture of such rods involves a series of steps which include
calcination, heating to remove volati:Le hydrocarbons, graphiti~ation
and densifying treatment. These steps, which do not perform a part of
the invention, are described in detail in ~.S. Patent 3,112,181 to
Peterson et al, which patent is incorp~rated herein by reference.
As shown in the drawing the residual oil and pyrolysis tar
lC are fed into a fractionator from which a combined mixture of pyrolysis
tar, residual oil and heavy ~as oil is withdrawn as feed to the
delayed coker. This type o~ operation is typical of a commercial
unit. However, the mixture of pyrolysis tar and residual oil can be
fed directly to a furnace and therea~ter introduced to the coke drums.
In the latter operation the diluent, if used, can be heavy gas oil
obtained from the coking op~ration or another sultable diluent
material.
The air-blowing operation is shown in the figure as a part
of the continuous process. Air-blowing alternatively may be carried
out as a batch operation, in which case the air-blown resld would be
accumulated in a tank or holding vessel from which it could be
introduced continuously to fractionator 18 or to coking furnace 38 as
desired. As another alternativa a plurality of batch air-blowing
vessels could be provided whereby it would be possible to continuously
supply air-blown product for further processing without intermediate
storage.
The isotropic coke produced by the process of the invention
has excellent quality, as indicated by a low CTE ratlo, usually less
than about 1.5, and by low sulfur content, usually not more than about
1.5 percent. The CTE can be measured by any of several standard
methods. For the isotropic coke of this invention, the coke is
crushed and pulverized, dried, and calcined to about 2,400F. This
calcined coke is sized so that about 50 percent passes through a No.
200 U.S, standard sieve. The coke is blended with coal tar pitch
binder, and a small amount of lubricant. The mixture is extruded at
about 1,500 psi lnto electrodes of about three-fourths-inch diameter
and about S inches long. These electrodes are heated slowly up to a
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temperature of about 850~C and heat-soaked for two hours. After A
slow cool-down period (8-10 hours), the baked electrodes are
graphitized at approximately 3000C. Test pieces are machined Erom
the graphitized electrodes. The coefficient of thermal expanslon of
the test specimens is then measured in the axial and radial directions
over the range of about 30-130C heated at a rate o~ about 2C per
minute. The CTE ratio, as used herein, is the ratio oi the radial CT~
to axial CTE of the graphitized elsctrodes.
When sub~ected to coking, t:he pyrolysls tar used in the
process of the invention does not produce an isotropic product yet the
combination of pyrolysis tar and air-blown residual oil when coked
together yields as much as or a higher percentage oi isotropic coke
product than would be obtained from the air-blown residual oil alone.
The process offers a number of advantages over coking only an
air-blown residual oil. ~or example, the isotropic coke product
obtained is more marketable because of its lower sulfur content.
Secondly, less m~terial needs to be alr-blown to make a feedstock for
an equivalent yield o~ coke. Also, less desirable (that is, higher
sulfur? residual oil can be utilized to prepare isotropic coke of the
same sulfur content as would be produced fronl lower sulfur residual
oil without the addition of the pyrolysis tar.
The following example illustrates the results obtained in
carrying out the invention:
Example
An air-blown residual oil and a low-suliur pyrolysis tar
were blended and the mixture was sub~ected to delayed coking. The
air-blown residual oil and pyrolysis tar were also coked separately
under the same conditions. The coking conditions used and the results
obtained from the coking operations are shown in the following table.
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Table
Delayed~k~Conditions:
Temperature, F 840
Pressure, psig 60
Run Time, hr 8
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60wt%/40wt~
Air-Blown Low-sulfur Air Blown resid/
Feedstock ~Q~iPy~olys~is ~ low-sulfuE_pyrotar
Green Coke
Yield, wt~ 28.9 45.8 34 8
Green Coke
Sulfur, wt~ 1.89 0.11 * 1.03
CTE of Graphiti~ed
Rod
Axial, 10 /C48.2 11.2 41.0
Tra~sverse,
10 /C 50.1 33.2 50.5
CTE Ratio,
Transverse/Axial 1.0 3.0 1.2
* Measured on coke calcined at 2400F ~or 2 hours. At such a
low-sulfur content there is little variation in sulfur between green
and calclned coke.
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The data in the table show that the inclusion of coke
derived from pyrolysis tar in the product coke does not greatly affect
the CTEs or CTE ratio despite the fact it amounts to roughly half of
the coke produced. Thus the coke yield and suliur content of the
product coke can be adJusted by appropriate blending of these
feedstock components without substantial deterioration of the isotropy
of the product.
W~ile certain embodiments and details have been shown for
the purpose of illustrating the present invention, it will be
apparent to those skilled in the art that various chan~es and
modifications may be made here1n without departing from the spirit
and/or scope of the invention.
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