Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DELAYED COKING PROCESS FOR
PRODUCING ANISOTROPIC FREE-FLOWING SHOT COKE
FIELD OF THE INVENTION
[0001] The present invention relates to a delayed coking process wherein
substantially all of the coke produced is free-flowing anisoWopic shot coke. A
cokes feedstock, such as a vacuum residuum, is heated with an oxidizing agent,
such as air, to increase the level of one or more of asphaltenes, polars, and
organically bound oxygen groups. The oxidized feedstock is then heated to
coking temperatures and passed to a cokes drum for an effective amount of time
to allow volatiles to evolve and to produce a substantially free-flowing
anisoti-opic shot coke.
DESCRIPTION OF RELATED ART
[0002] Delayed coking has been practiced for many years. The process
broadly involves thermal decomposition of peh~oleum residua (resids) to
produce
gas, liquid streams of various boiling ranges, and coke. Delayed coking of
resids
fi-om heavy, and heavy sour (high sulfur) crude oils is cazTied out primarily
as a
means of disposing of these low value feedstocks by converting paa of the
resids
to more valuable liquid and gas products. Although the resulting coke is
generally thought of as a low value by-product, it does have some value as a
fuel
(fuel grade), electrodes for aluminum manufacture (anode grade), etc.
[0003] In the delayed coking process, the feedstock is rapidly heated in a
fired heater or tubular furnace. It is then passed to a coking drum that is
maintained at conditions under which coking occurs, generally at temperatures
above about 400°C under super-atmospheric pressures. The heated
residuum
feed fuuther decomposes in the cokes drum to form volatile components that are
removed overhead and passed to a fractionator leaving coke behind. When the
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coker chum is full of coke the heated feed is switched to another chum and
hydrocarbon vapors are purged from the coke drum with steam. The drum is
then quenched with water to lower the temperaW re to about 200-300°F
after
which the water is dr aired. When the cooling step is complete, the drum is
opened and the coke is removed after drilling and/or cutting using high
velocity
water jets.
[0004] For example, a high speed, high impact water jet is used to cut the
coke from the drum. A hole is typically bored in the coke from water jet
nozzles
located on a boring tool. Nozzles oriented horizontally on the head of a
cutting
tool cut the coke from the drum. The coke removal process adds considerably
to the throughput time of the process. That is, since it takes approximately 1
to 6
hours, typically about 3 hours to drill-out and remove the resulting coke
mass,
the cokes drum turn-around time and process costs are increased. Thus, it
would
be desirable to produce a free-flowing coke in the cokes duum that would not
requ>le the expense and time associated with conventional agglomerated coke
mass removal.
[0005] Further, even though the coking drum may appear to be completely
cooled, occasionally, a problem arises which is refeiTed to in the ant as a
"hot
drum." This problem occurs when areas of the dmm do not completely cool.
This may be the result of a combination of morphologies of coke in the chum
resulting in a non-uniform drum. The drum may contain a combination of more
than one type of solid coke product, i.e., needle coke, sponge coke and shot .
coke. BB-sized shot coke may cool faster than another coke, such as large shot
coke masses or sponge coke. Avoiding "hot drums" is another reason for
pr oducing predominantly shot coke in a delayed cokes.
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[0006] Attempts have been made to produce predominantly, or substantially
all of a single type of coke during delayed coking. For example, U.S. Patent
No.
5,258,115, which is incorporated herein by reference, teaches a delayed coking
process wherein spent caustic is introduced into a delayed cokes feed, or into
the
cokes drum itself, to produce shot coke to help alleviate the hot drum
problem.
It also reduces cooling time.
[0007] Further, U.S. Patent No. 3,960,704, which is also incorporated herein
by reference, teaches a delayed coking process wherein isotropic coke is the
product. Isotropic coke is coke that has thermal expansion approximately equal
along the three crystalline axes. This is achieved by air blowing a petroleum
resid feedstock to a certain softening point and conning the coking process at
relatively high recycle ratios and preferably with a diluent oil.
[0008] Although delayed coking lias been in commercial use for many years,
there still remains a need in the art for improvements that can shorten the
coke
removal time.
SUMMARY ~F THE INVENTION
(0009] In accordance with the present invention there is provided a delayed
coking process wherein substantially all of the coke produced is substantially
free flowing anisotTOpic shot coke, which process comprises:
a) contacting a vacuum resid feed with an oxidizing agent at a temperature
from about 150°C to about 375°C for an effective amount of time
to
significantly increase the amount of asphaltenes and organically bound oxygen
in the resid;
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b) heating said oxidized resid feed to a temperature effective for cokiilg
said feed;
c) charging said heated oxidized resid to a delayed cokes duum at a
presswe from about 15 to 50 psig for an effective amount of time to produce
volatiles and anisoti~opic substantially fi~ee-flowing shot coke;
d) removing at least a poution of said volatiles overhead; and
e) removing the anisoti~opic substantially free-flowing shot coke product
fi~om the cokes drum.
[0010] Also in accordance with the present invention there is provided a
delayed coking process comprising:
a) contacting a vacuum resid with an oxidizing agent at a temperature
fi~om about 150°C to about 375°C for an effective amount of time
to
significantly increase the amount of asphaltenes and/or polars and other
organically bound oxygen groups in the resid;
b) heating said oxidized resid to a temperature effective for coking said
feed;
c) charging said heated oxidized resid to a delayed cokes dl-um at a
pressure from about 15 to 50 psig for an effective amount of time to produce
volatiles and a substantially free-flowing anisoh-opic shot coke;
d) removing at least a portion of the volatiles overhead;
e) quenching the remaining hot coke bed with water;
f) removing the resulting anisotl-opic substantially free-flowing shot coke
product fi-om the cokes drum.
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[0011] In one prefeiTed embodiment of the present invention, the oxidizing
agent is air.
[0012] In another prefewed embodiment of the present invention a caustic can
be added to the oxidized resid cokes feedstock before,. during, or after
heating in
the cokes fiunace.
BRIEF DESCRIPTION OF THE FIGURE
[0013] Figure 1 hereof is a cross polarized light photomicrograph of coke
resulting from a San Joaquin Valley vacuum residuum that was not treated with
an oxidizing agent prior to coking. The area of view is 170 microns by 136
lnicr ons.
[0014] Figure 2 hereof is a photomicrograph of coke resulting from a San
Joaquin Valley vacuum residuum that was treated with air for 3 hours at a
temperature from 185°C to 225°C prior to coking. The area of
view is 170
microns by 136 microns.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Feedstocks suitable for the delayed coking process of the present
invention are petroleum vacuum residua. Such petroleum residua are frequently
obtained after removal of distillates from crude feedstocks under vacuum and
are
characterized as being comprised of components of large molecular size and
weight, generally containing: (a) asphaltenes and other high molecular weight
aromatic stx-uctures that would inhibit the rate of hydroti-
eating/hydrocracking
and cause catalyst deactivation; (b) metal contaminants occmTing naturally in
the
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crude or resulting from prior treatment of the crude, which contaminants would
tend to deactivate hydroti-eating/hydrocracking catalysts and intenere with
catalyst regeneration; and (c) a relatively high content of sulfur and
nitrogen
compounds that give rise to objectionable quantities of S02, S03, and NO,;
upon
combustion of the petroleum residuum. Nitrogen compounds also have a
tendency to deactivate catalytic cracking catalysts. Typical examples of coker
peholeum feedstocks which are contemplated for use in the present invention,
include residues from the atmospheric and vacuum distillation of petroleum
crudes or the atmospheric or vacuum distillation of heavy oils, visbroken
resids,
tars from deasphalting units or combinations of these materials. Atmospheric
and vacuum topped heavy bitumens can also be employed. Typically, these
feedstocks are high-boiling hych~ocarbonaceous materials having a nominal
initial boiling point of about 538°C or higher, an API gravity of about
20° or
less, and a Com~adson Carbon Residue content of about 0 to 40 weight percent.
[0016] The coking process of the present invention is delayed coking, which
is well known in the ant. Generally, in the delayed coking process, a bottoms
fiaction, such as a petroleum residuum chargestock is pumped to a heater at a
pressure of about 50 to 550 psig, where it is heated to a temperahue from
about
480°C to about 520°C. It is then discharged into a veuically
oriented insulated
cokes drum tluough an inlet at the base of the drum. Pressure in the drum is
usually relatively low, such as about 15 to 50 psig to allow volatiles to be
removed overhead. Typical operating temperaW res of the drum will be between
about 410°C and 475°C. The hot feedstock thermally cracks over a
period of
time in the cokes drum, liberating volatiles composed primarily of hydrocarbon
products, that continuously rise tln-ough the coke mass and are collected over-
head. The volatile products are sent to a cokes fractionator for distillation
and
recovery of cokes gases, gasoline, light gas oil, and heavy ga.s oil. At least
a
portion of the heavy cokes gas oil present in the product stream introduced
into
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the coker fractionator is captured for recycle and combined with the fresh
feed
(cokes feed component), thereby forming the cokes heater or cokes furnace
charge.
(0017] There are generally three different types of solid delayed cokes
products that have different values, appearances and properties. These are
needle coke, sponge coke and shot coke. Needle coke is the highest quality of
the three varieties. Needle coke, upon further themnal treatment, has high
conductivity and is used in elech~ic anc steel production. It is relatively
low in
sulfur and metals and is produced from some of the higher quality cokes
feedstocks that include more aromatic feedstocks such as slurry and decant
oils
fiom catalytic crackers and thermal cracking tars as opposed to the
asphaltenes
and resins.
[0018] Sponge coke, a lower quality coke, sometimes called "regular coke",
is most often formed in refineries. Low quality refinery cokes feedstocks
having
significant amounts of asphaltenes, heteroatoms and metals produce this lower
quality coke. If the sulfur and metals content is low enough, sponge coke can
be
used for the manufacture of electrodes for the aluminum industry. If the
sulfiu
and metals content is too high, then the coke can be used as fuel. The name
"sponge coke" comes from its porous, sponge-like appearance. Conventional
delayed coking processes, using the prefewed vacuum resid feedstock of the
present invention, will typically produce sponge coke, which is produced as an
agglomerated mass that needs an extensive removal process including drilling
and water-jet technology. This adds considerable time and costs to the
process.
[0019] Shot coke has been considered the lowest quality coke because it has
the highest sulfur and metals content, the lowest electrical conductivity and
is
the most difficult to grind. The term "shot coke" comes from its shape which
is
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similar to that of BB sized (about 1/1G inch to 3/8 inch) balls. Shot coke,
lilee the
other types of coke, has a tendency to agglomerate, especially in admixture
with
sponge coke, into larger masses, sometimes larger than a foot in diameter.
This
can cause refinery equipment and processing problems. Shot coke is usually
made from the lowest quality high resin-asphaltene feeds and makes a good high
sulfur fuel source, pal-ticularly for use in cement kilns and steel
manufacture.
The inventors hereof have unexpectedly found that substantially free-flowing
anisotropic shot coke can be produced by first treating the residuum feedstock
with an oxidizing agent to substantially increase the contents of its
asphaltene,
and/or polars fractions, such as those containing organically bound oxygen
like
ketones, carboxylic acids, etc. The residuum feed is subjected. to the
oxidizing
agent, preferably air, at effective temperahires, i.e., at temperatures that
will
encourage the formation of asphaltenes and organically bound oxygen groups to
form. Such temperatures will typically be from about 150°C to about
325°C,
preferably from about 185°C to about 280°C, more preferably from
about 185°C
to about 250°C. The oxidizing agent can be in any suitable form
including gas,
liquid or solid. Non-limiting examples of oxidizing agents that can be used in
the practice of the present invention include air, oxygen, ozone, hydrogen
peroxide, organic peroxides, hydroperoxides, lnorgan lc peracids, inorganic
oxides and peroxides and salts of oxides, sulfuric acid, and nitric acid.
PrefelTed
is air. It is to be understood that after the resid is t<~eated with the
oxidizing
agent, a caustic, preferably a spent caustic, may optionally be added. The
spent
caustic can also be added before, during, or after the oxidized resid is
passed to
the cokes furnace and heated to coking temperatures. The caustic will be an .
alkali-metal material preferably a spent caustic soda and/or potash sizeam
that is
typically used in various refinery processes. Such spent caustic streams
typically
contain one or more of sodium and potassium, sulfur, and other wastes,
including organic contaminants that vary depending on the hydrocarbon source
but can be organic acids, dissolved hydrocarbons, phenols, naphthenic acids,
and
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salts of organic acids. The spent caustic stream will usually have a
relatively
high water content, typically about 50 wt% to 95 wt% water, more typically
from about 65 wt% to about 80 wt%.
[0020] The precise conditions at which the residuum feedstock is treated with
the oxidizing agent is feed dependent. That is, the conditions at which the
feed
is treated with the oxidizing agent is dependent on the composition and
properties of the feed to be coked. These conditions can be determined by one
having ordinary skill in the art without undue experimentation. Several mns
are
made with a particular feed at different oxidizing times and temperatures
followed by coking. The resulting coke is then analyzed by use of a micro-.
carbon test procedure and microscopy as set forth in the examples hereto. The
desired coke morphology that will produce substantially free-flowing coke is a
coke microstructure of discrete micro-domains having an average size of about
1
to 10 ~.m, preferably from about 1 to 5 ~.m, somewhat like a mosaic (Figure 2
hereof). Coke microstructure that represents coke that is not free-flowing
anisoti-opic shot coke is the microstructure represented in Figure 1 hereof
that
show a coke mice osh-ucture that is composed substantially of non-discrete, or
substantially large flow domains up to about 60 E.un or greater in size,
typically
fi~om about 10 to 60p.m.
[0021] U.S. Patent No. 3,960,704 which is incorporated herein by reference,
teaches delayed coking wherein a resid feedstock is air blown to a target
softening point. The air blown feed is then passed to delayed coking process
that is operated at conditions that will favor the formation of isori~opic
coke.
That is, coke particles having substantially equal thermal expansion
properties
along the three major crystalline axes. This '704 patent requires relatively
high
recycle ratios and an additional amount of oil as a diluent to produce a
pellet-
type isoh~opic coke. For example, the recycle ratio of this '704 patent is
from
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about 1 to 5. This coiTelates to 100% to 500% recycle based on fresh feed.
Although up to about 15% recycle can be used in the practice of the present
invention it is prefeiTed that no recycle be used. The presently claimed
delayed
coking process does not produce isotropic pellet-type coke - it produces
substantially free-flowing anisoh~opic shot coke. Also, the shot coke that
results
from the practice of the present invention can be easily removed from the
coker
drum without drilling or the use of water-jet cutting technology. While shot
coke has been produced by conventional methods it is typically agglomerated to
such a degree that water jet technology is needed for its removal.
[0022] It is important to the practice of the present invention that the resid
feedstock be first treated with an oxidizing agent to substantially increase
its
level of asphaltenes, polars, and organically bound oxygen groups that
encourages the formation of anisoti~opic substantially free-flowing shot coke.
It
is also important to the practice of the present invention that the cokes drum
be
kept at relatively low pressures in order to allow as much of the evolving
volatiles to be collected overhead. This helps prevent agglomeration of the
resulting shot coke. The recycle ratio, that is the volumetric ratio of
furnace
charge (vacuum resid plus recycle oil) to fresh feed to the continuous delayed
cokes operation should also be kept as low as possible. The use of recycle
ratio
for delayed coking is taught in more detail in LJ.S. Patent No. 3,116,231
which is
incorporated herein by reference.
[0023] The present invention will be better understood by reference to the
following examples that are presented for illustrative proposes only and are
not
to be taken as limiting the invention in any way.
Example s
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(0024] General Procedure: Approximately 180 g each of five different
petroleum residua were added to a 500 cc round bottom flask equipped with a
Therm-O-Watch conh~oller, a mechanical blade stiiTer, and a condenser attached
to a Dean-Stark trap to recover any light ends and water generated during the
reaction. The residuum was heated to 180°C at which time air was
introduced
into the hot residuum feed under its surface by means of a sparger tube. The
temperature was raised said controlled to between 220°C to 230°C
and the flow
rate of air was controlled at 0.675 ft~llw for tlwee hours or as required
depending
on the desired degree of oxidation. The sparger tube was removed after the
desired time and the flask was allowed to cool to room temperature.
(0025] Deasphalting Procedure: A mixture of fresh or oxidized cokes feed
and n-heptane were added to a 250 cc round bottom flask in a ratio of 1 part
feed
to 8 pans n-heptane and allowed to stir for 1G hours at room temperature. The
mixture was then filtered through a coarse Buchner funnel to separate the
precipitated asphaltenes. The solids were dried in a vacuum oven at
100°C
overnight. The heptane was evaporated from the oil/heptane mixture to recover
the deasphalted oil. The amount of asphaltenes produced from the oxidized feed
was compared to the amount generated from the starting residuum under the
same deasphalting procedure. The results are presented in the following table:
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M
cd 'b
N
c~
U
a\
x
N O
cd O
C",
O
O
d'
O
O o
~,U
O
Cla N
N
~.0
3
v~ O
a
a
o .~ ~
, ~ o
~
a1, ~
o
.o~
0
w
0
~_ N o0
N A ~ ~ c~
O
O
3
w ~ ~~ M
w
a
N
N ~ N
H 3 '~
. O
b
a\
N
N
N
_+~
C~
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[0026] Microcarbon residue tests were performed on the above feeds to generate
cokes to be evaluated by microscopy. The following is the procedure used for
the
microcarbon tests:
Heatin~Profile Time min N2 Flow (cc/min)
Heat from room tem erature 10 66
to 100C
Heat from 100C to 300C then 30 66/19.5
to
500C
Hold at 500C 15 19.5
Cool to room temperature ~ 40 ~ 19.5
[0027] Figures l and 2 are cross polarized light photomicrographs showing the
microstxwcture of the resulting coke from a San Joaquin Valley residuum for
both
the untreated residuum and the residuum treated with air in accordance with
the
above procedure. The viewing area for both is 170 microns by 136 microns. The
untx-eated residuum resulted in a coke with a microstructure that was not
discrete
fme domains. The domains were relatively large (10-30 ~.m) flow domains. This
indicates that a mixture of shot coke and sponge coke will be produced in the
cokes
chum of a delayed cokes. The microsti-ucW re (Figure 2) of the resulting coke
from
the residuum sample that was first air oxidized shows relatively fine (2-5 pm)
discrete fine domains indicating that fi~ee-flowing shot coke will be produced
in the
cokes drum of a delayed cokes. Following the same procedure, the following
changes in flow domain sizes were observed: a Midwest Vacuum Resid (10-50 ~xn
to 2-3 p,m), a Louisiana Sweet Vacuum Resid (20-60 p.m to 2 to 5 p,m) in six
hours,
a Maya Vacuum Resid (2-10 l.~m - no change), and a Heavy Ca~ladian Vacuum
Resid ( 10-20 ~.un to 2-10 p.m).
