BACKGROUND OF THE INVENTION
[0001] Electrode grade graphite is manufactured from a commercial grade of coke having an
acicular, anisotropic microstructure called needle coke, see U.S.-A-2,775,549, made
by delayed coking of certain petroleum residues under specific conditions of heat
and pressure. To produce graphite from such coke, it is necessary to heat it to a
temperature in the range of 2000-3000°C, which has the dual function of supplying
energy for the conversion of the carbon in the coke to the graphitic crystalline form
and of volatilizing impurities. When carbon bodies made from such cokes are heated
at temperatures in the vicinity of 1000-2000
0C, various sulfur-containing compounds decompose, attended by a rapid and irreversible
expansion of the carbon body. This phenomenon is termed "puffing". During the production
of graphite articles, particularly high performance graphite electrodes, puffing is
extremely undesirable as it may destroy the structural integrity of the piece and
render it marginal or useless for its intended purpose.
[0002] Puffing of a carbon article made from high sulfur cokes generally starts at about
1500°C, and may result in a volumetric expansion of as much as 25%. It is not simply
an elastic expansion but should be characterized as an inelastic, irreversible expansion.
[0003] The generally accepted explanation of the puffing phenomenon is that in acicular
needle cokes with a relatively large amount of sulfur, sulfur atoms are bonded to
carbon atoms by covalent bonds, either in carbon ring structures or linking rings.
These bonds are less stable at high temperatures than the carbon-to-carbon bonds.
On heating, the carbon-sulfur bonds rupture, the sulfur is freed, then reacts with
hydrogen to form hydrogen sulfide. The simultaneous rupture of these bonds and evolution
of hydrogen sulfide and other sulfur containing materials causes the physical expansion
called puffing.
[0004] Puffing has been avoided in the past by using coke made from petroleum residues low
in sulfur content. This approach is of only limited utility at present since the principal
petroleum crudes currently in use have high sulfur contents, and the cokes made from
their residues will normally exhibit an undesirable degree of puffing.
[0005] Another approach to elimination or alleviation of the puffing problem in manufacture
of graphite articles has been by the use of additives. These additives have usually
been added during the mixing stage when various sizes and grades of coke particles
are mixed, before being wetted with pitch, formed into the desired shape, baked at
an intermediate temperature and graphitized at high temperatures. Additives have included
primarily metal salts and oxides, as disclosed in GB-A-733,073, FR-A-1,491,497, FR-A-2,035,273,
U.S.-A-3,642,962, U.S.-A-3,563,705, U.S.-A-3,842,165, and U.S.-A-3,338,993.
[0006] The patents above disclose the use of iron, sodium, chromium, nickel, cobalt, boron
aluminum, titanium, calcium, zirconium, manganese, magnesium, barium and strontium
compounds as puffing inhibitors. Some compounds of this group are in general usage
and of these a choice is naturally made based upon the effectiveness as a puffing
inhibitor and upon other properties of the graphite article such as electrical resistivity,
tensile strength, modulus of rupture, modulus of elasticity, coefficient of thermal
expansion, and cost.
[0007] Of the above, FR-A-1,491,497 discloses the use of chromium oxide at 0.2-5% in a mixture
with coke and a binder as a catalyst, enabling graphitization to occur at temperatures
in the range of 1200
0-2000
0C.
[0008] FR-A-2,035,273 discloses a low sulfur coke produced by the addition of 0.3-5% of
sodium carbonate to the coking stream mixture and subsequent hydrogenation of the
coke at high temperature.
[0009] GB-A-733,073 discloses the use of oxides of chromium, iron, copper or nickel incorporated
in the grinding stage of coke, mixed with pitch, shaped, baked at 1200°C, and graphitized
at 2500°-2800°C.
[0010] U.S.-A-3,563,705 discloses the use of mixtures of iron or calcium compounds with
small amounts of titanium or zirconium compounds as puffing inhibitors incorporated
into the coke-binder mixture.
[0011] U.S.-A-3,338,993 discloses the use of calcium, magnesium, strontium, and barium fluorides
as puffing inhibitors with raw of calcined coke and binder, mixed, shaped, baked and
graphitized.
[0012] U.S.-A-3,642,962 discloses the use of 1-3% calcium cyanamid or calcium carbide as
desulfurizing agents and puffing inhibitors, mixed wirh raw coke prior to calcining.
[0013] US-A-3 873 427 teaches the combination of a ferruginous material and a metal chloride
as a desulfurizer for coal and coke but is silent on the production of a non-puffing
coke.
[0014] At present, the most common methods of the above are those using iron oxides mixed
dry in the coke-pitch binder blend as puffing inhibitors. These are effective puffing
inhibitors but must be used with caution, as their use tends to increase the coefficient
of thermal expansion or CTE, of the finished product, to an undesirable level.
[0015] CTE is also of vital importance in the production of graphite for certain applications.
Electrodes for electric furnace melting of steel must have a low CTE to avoid excessive
differential expansion at operating temperatures and the resultant spalling,which
in turn causes excessive consumption of the electrode and cost in operation. Other
applications requiring dimensional stability at high temperatures are well-known although
of somewhat less economic importance.
[0016] In general, the addition of any foreign material to a graphitizing carbonaceous mix
will have, in addition to its desired effect, such as puffing inhibition, the effect
of increasing the CTE of the graphite body.
[0017] A needle coke is distinguished by its physical structure when microscopically examined,
showing long needle-like acicular particles. Such cokes, to be suitable for manufacture
of graphite electrodes to be used in ultra-high powered electric steel furnaces, should
have a graphite CTE characteristic of less than 5 x 10'V°C measured over the range
of 0°-50°C. Needle cokes for lower powered electric steel furnaces may have a graphite
CTE characteristic of as much as 7 x 1 C
TV°C over the 0°-50°C range.
[0018] The blends of cokes must be thoroughly mixed to avoid the difficulties present in
making uniform homogeneous blends and in thoroughly coating the particles, which are
often as much as 7 mm. in diameter, with the puffing inhibitor. Both of these difficulties
can lead to non-uniform dispersion of the inhibitor and to puffing, even though there
is sufficient inhibitor present in the total mix to prevent puffing.
[0019] This non-uniformity is particularly troublesome when operating under the newer type
of graphitization processes, which raise the temperature of the carbon bodies (i.e.
electrodes) at a much higher rate than the older processes. The combination of high
sulfur with high rate of temperature rise exacerbates the problem and requires more
stringent treatment to overcome puffing.
[0020] It should be emphasized that overcoming the puffing problem becomes increasingly
more difficult in the larger graphite electrode sizes (above 51 cm (20 in.) diam)
because in such sizes, larger particles of coke are used. Since the puffing inhibitor
only coats the surface of the particles, the coke surface area to inhibitor weight
ratio decreases (for a given weight addition ratio), giving a higher concentration
of inhibitor on the coke particle surfaces for the larger particle blends. Thus a
large amount of the inhibitor is at relatively greater distance from the centers of
the coke particles in the larger coke particle mixes as opposed to the smaller particle
mixes used in smaller electrodes. Migration of the inhibitor into the centers of the
large particles becomes progressively more difficult and less effective as the coke
particles increase in size.
[0021] The puffing problem is further increased with the rate of graphitization of the carbon
bodies. Optimum distribution of the inhibitor throughout the structure of the carbon
body to be graphitized is .-essential as the degree of puffing for any coke particle
blend is highly rate sensitive, being directly related to the rate of temperature
increase during the graphitization cycle. Thus, the figures in certain of the examples
given will show a much higher dynamic puffing at a 14°C/min. temperature rise than
for a 5°C/min. rise.
[0022] The amount of puffing for any given coke-inhibitor blend could be expressed as a
proportionality in the general form:

where DP = dynamic puffing
S = sulfur content of coke
P = mean particle size
AT = rate of temperature increase
I = amount of inhibitor
k = proportionality factor
[0023] Thus it may be seen that increases in sulfur content, particle size, and temperature
rise will increase puffing, while an increase in the inhibitor level will decrease
puffing.
SUMMARY OF THE INVENTION
[0024] A petroleum coker feedstock which would normally produce a puffing coke due to its
high sulfur content is rendered non-puffing by the addition of a small quantity of
puffing inhibitor to the feedstock as a fine particle size powder.
[0025] Puffing inhibitors such as iron oxide and/or calcium fluoride may be pre-dispersed
in a high concentration in a small quantity of the feedstock (fresh feed or furnace
feed), or in compatible material miscible with the feedstock, or dispersed in the
total coker stream and added either batchwise to a batch type coker, or continuously
to the main stream in a delayed coker.
[0026] The use of a fine particle size powder of 100% less than 5 micron diameter, pre-dispersed
in a portion of the feedstock, insures that the final product will be a homogeneous
coke with puffing inhibitor uniformly distributed throughout.
[0027] A current of inert gas or steam bubbled through the batch type coker during the run
aids in keeping the puffing inhibitor in suspension without significantly increasing
the CTE of the finished product during batchwise coking. In a commercial delayed coker
this is not essential. For a description of delayed coking, reference is made to an
article by R. J. Diwoky, Continuous Coking of Residuum by the Delayed Coking Process,
Refiner and Natural Gasoline Manufacturer, Vol. 17, No. 11, Nov. 1938. The present
invention involves the use of this type of coking operation.
[0028] Iron oxide is formed when any of numerous iron bearing materials is calcined, including
organometallic compounds and salts. Minerals such as magnetite (Fe
30
4), limonite (2Fe
lO
3.3H
20); and pyrites (FeS
2) and salts such as ferric sulfate and nitrate when roasted in air are converted to
ferric oxide, and may be used to form the oxide.
[0029] The reactive species may be elemental iron, produced by reduction of the Fe
20
3 by coke during graphitization.
[0030] Calcium fluoride is also highly effective as an inhibitor with slightly superior
performance as compared to iron oxide. Mixtures of the two inhibitors have shown a
synergistic result, being more effective than either of the two when used alone.
[0031] The addition of inhibitor in this manner to the feedstock to the coker produces a
coke which is lower puffing and produces a graphite which has a lower CTE than from
a coke conventionally inhibited by a dry mix.
[0032] The mode of operation of these puffing inhibitors is probably a scavenging reaction,
sulfur reacting with the metallic ions to form the sulfides, then decomposing slowly
at higher temperatures to give a controlled evolution of vaporized elemental sulfur.
[0033] In general the use of any of the additives listed above, when added to a coke particle-pitch
binder mix, will lower the extent of puffing, but at the same time significantly increase
the CTE, of the graphite bodies made from such cokes. I have found that the use of
puffing inhibitor dispersed in the coker feedstock in appropriate amounts prior to
coking gives an unexpected advantage in that it controls puffing of the coke while
increasing the CTE only to a smaller degree (or in some instances not at all), when
compared to the CTE of a graphite body in which the puffing has been eliminated by
adding the same additive to the electrode mix by the conventional dry-mix practice.
[0034] CTE of the graphitized coke was determined by preparing small 1.6 x 12.7 cm (5/8"
x 5") electrodes by the procedure disclosed in U.S.-A-2,775,549 (except for calcination
of the coke to 1250°C), and measuring their elongation over the temperature range
of 0° to 50°C.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] In figure 1 a decant oil, the fractionater tower bottoms from a catalytically cracked
gas oil fraction, also termed slurry oil, or other equivalent hydrocarbon residue,
is conveyed from the fractionater 33 through line 10 and meter 14 to diversion valve
17, where a portion of the feedstock is diverted through valve 13, and meter 15 to
disperser 18. Simultaneously a portion of inhibitor 12 is weighed in scale 16- and
conveyed to disperser 18 where it is dispersed in the feedstock to a specific concentration
by weight. Alternately a compatible liquid and additives from supply 19 are metered
through valve 11 to valve 13 and meter 1 to disperser 18. The inhibitor is dispersed
and discharged through line 22 and meter 23 to mixer 24 where it is mixed with the
main portion of the feedstock coming through line 20 and meter 25, to the exact proportion
desired. The concentrate mixed with the feedstock is pumped by pump 27 through line
26 to furnace 29, then through line 31 to coker drums 28 and 28A, where it is conventionally
delay coked. The overheads are taken off through line 32 and sent to the fractionater
33.
[0036] In the above flowsheet. 18 is the disperser which may be any of several types of
equipment well known in the art, preferably a high shear or colloid mill. Alternately,
a roller or ball mill could be used.
[0037] In practice, a dispersion of approximately 5-50% by wt. of puffing inhibitor in the
feedstock is used as a concentrate.
[0038] The puffing inhibitor dispersion and feedstock are metered into the mixer 24 where
they are mixed in the correct proportions to give a concentration of approximately
0.05-0.5 wt. % puffing inhibitor in the feedstock which is then pumped into the coker
28.
[0039] At the operating temperatures the viscosity of the feedstock is extremely low and
some means is necessary to minimize settling and a concentration of the puffing inhibitor
in the lower portion of the coker during batchwise coker operation. I have found that
by the introduction of a small flow of inert gas bubbled up from the bottom of the
coker drum, I can maintain the puffing inhibitor in a uniform suspension without significantly
raising the CTE of the finished product or lowering the acicular crystal content of
the coke. This is not necessary if the invention is carried out in a commercial delayed
coker.
[0040] The following are examples of specific methods of practicing the invention:
Example 1
[0041] The micronized puffing inhibitors, calcium fluoride and iron oxide (having approximately
the same particle size distribution) were individually mixed with samples of a fresh
feed decant oil coker feedstock, at 0.1 wt. % level in a high speed blender for about
5 minutes. The mixtures were coked under identical conditions in 4 liter resin flasks.
[0042] In an insulated glass resin flask, an inert gas at the rate of 4.5 1/h/kg (0.16 SCFH/kg)
mixture was bubbled up from the bottom of the coking pot to keep the inhibitor uniformly
dispersed in feedstock.

[0043] Dynamic puffing of the cokes was then determined in comparison with uninhibited samples,
and with samples inhibited in the normal manner with dry-mixed iron oxide. The coke
samples had 50% < 0.074 mm. particles and 100% < 0.088 mm. particles.
[0044] Puffing was measured by taking representative samples by the method of ASTM D346-35,
crushing, mixing 100 g coke and 25 g pitch, and molding plugs at 86197.5 k.Pa. (12,500
psi). The plugs were measured by micrometer and placed in a dilatometer. The temperature
was raised to 1200°C over a period of 50 ± 10 min., then the test was run at a temperature
increase of 5° or 12-16°C/min. over the 1200
0-2900°C range, with measurements taken every five minutes. The reported DP is the maximum
degree of elongation (or shrinkage) measured. All of the DP's below were at 14°C/min.
rise except as noted.

[0045] Of the two puffing inhibitors, the calcium fluoride was found to be the more effective.
The addition of either inhibitor to the feedstock significantly decreased puffing
of the resulting coke. The CTE of the resulting cokes was within the range (under
5 x 10
-7/°C considered necessary for a needle coke.
[0046] For less demanding applications, a CTE of the graphite body of as much as 7 x 10
-7/°C may be acceptable, but for ultra high power electrodes for electric steel furnaces
the upper limit is generally 5 x 10
-7/°C.
Example 2
[0047] Another fresh feed sample was tested as in Example 1 using the inhibitors at a higher
level of addition, with the following results:

[0048] Some feedstocks may well need and be beneficially treated with inhibitor additions
of as much as 0.5%, resulting in a 2% ash level in the final coke.
[0049] The examples above are not shown as limitations but merely samples from the wide
variety of petroleum residues currently available.
1. Verfahren zum Erzeugen eines in Graphit von Elektrodenqualität umwandelbaren, nichtblähenden
Nadelkokses mit einem Graphit-Wärmeausdehnungskoeffizienten von höchstens 7 x 10-7/OC im Bereich von 0°C bis 50°C, in dem ein von Erdöl abgeleitetes Ausgangsmaterial,
das in Komponenten des Ausgangsmaterials so viel molekular gebundenen Schwefel enthält,
daß er bei einer Erhitzung auf eine Temperatur von oder über 1400°C zu einer nichtumkehrbaren
volumetrischen Expansion beiträgt, in Anwesenheit eines blähungshemmenden Mittels
einer halbkontinuierlichen Verkoksung unterworfen wird, dadurch gekennzeichnet, daß
in dem Ausgangsmaterial 0,05 bis 0.5 Gew.% des aus Eisen, Calciumfluorid oder Gemischen
derselben bestehenden, blähungshemmenden Mittels dispergiert werden, so daß das blähungshemmende
Mittel während des Verkokens vorhanden ist.
2. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß das blähungshemmende Mittel
zu dem Ausgangsmaterial in Form einer vordispergierten, konzentrierten flüssigen Dispersion
des blähungshemmenden Mittels in einem mit dem Ausgangsmaterial verträglichen, flüssigen
Medium zugesetzt wird.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß das verwendete blähungshemmende
Mittel einen solchen Teilchendurchmesser hat, daß 100% der Teilchen kleiner sind als
5 um und daß 70% kleiner sind als 2 um.
4. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß
das blähungshemmende Mittel in einem Teil des zu verkokenden Ausgangsmaterials dispergiert
wird.