U.S. patent application number 11/127730 was filed with the patent office on 2006-01-12 for production of substantially free-flowing coke from a deeper cut of vacuum resid in delayed coking.
Invention is credited to Fritz A. Bernatz, Christopher P. Eppig, Theodore Sideropoulos, Michael Siskin.
Application Number | 20060006101 11/127730 |
Document ID | / |
Family ID | 34969659 |
Filed Date | 2006-01-12 |
United States Patent
Application |
20060006101 |
Kind Code |
A1 |
Eppig; Christopher P. ; et
al. |
January 12, 2006 |
Production of substantially free-flowing coke from a deeper cut of
vacuum resid in delayed coking
Abstract
A modified vacuum distillation and delayed coking process for
making substantially free-flowing coke, preferably free-flowing
shot coke. A vacuum resid feedstock is used which contains less
than about 10 wt. % material boiling between 900.degree. F. and
1040.degree. F. as determined by HTSD (High-temperature Simulated
Distillation). The use of such a high boiling resid favors the
formation of shot coke instead of sponge or transition coke. The
distillate recycle reduces coker furnace fouling potential of the
heavier feedstock.
Inventors: |
Eppig; Christopher P.;
(Vienna, VA) ; Siskin; Michael; (Randolph, NJ)
; Bernatz; Fritz A.; (Houston, TX) ; Sideropoulos;
Theodore; (Oakton, VA) |
Correspondence
Address: |
EXXONMOBIL RESEARCH AND ENGINEERING COMPANY
P.O. BOX 900
1545 ROUTE 22 EAST
ANNANDALE
NJ
08801-0900
US
|
Family ID: |
34969659 |
Appl. No.: |
11/127730 |
Filed: |
May 12, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60571346 |
May 14, 2004 |
|
|
|
Current U.S.
Class: |
208/131 |
Current CPC
Class: |
C10B 55/00 20130101;
C10B 57/045 20130101 |
Class at
Publication: |
208/131 |
International
Class: |
C10G 9/14 20060101
C10G009/14 |
Claims
1. A delayed coking process comprising: preparing a vacuum resid
that has less than 10 wt. % 900.degree. F. to 1040.degree. F.
boiling material as measured by HTSD (High-Temperature Simulated
Distillation) and combining with a distillate recycle stream
wherein the distillate recycle stream has boiling range within the
interval of about 450.degree. F. to about 750.degree. F.;
conducting said mixture to a heating zone wherein it is heated to
an effective coking temperature; and conducting said heated mixture
from said heating zone to a coking zone wherein vapor products are
collected overhead and whereby coke with reduced incidence of hot
drums and of a relatively free-flowing nature is formed.
2. The process of claim 1 wherein a substantially free-flowing shot
coke product is produced.
3. The process of claim 1 wherein an additive is introduced into
the feedstock either prior to heating or just prior to it being
introduced in the coker vessel, which additive is an organic
soluble, organic insoluble, or non-organic miscible
metals-containing additive that is effective for the formation of
substantially free-flowing coke.
4. The process of claim 3 wherein the metal of the additive is
selected from the group consisting of potassium, sodium, iron,
nickel, vanadium, tin, molybdenum, manganese, cobalt, calcium,
magnesium, aluminum and mixtures thereof.
5. The delayed coking process of claim 1 wherein the distillate
recycle is in the range of about 1 to 20 volume percent.
6. The delayed coking process of claim 5 wherein the distillate
recycle is in the range of about 0 to 7 volume percent.
7. The delayed coking process of claim 5 wherein the distillation
recycle is in the range of about 0 to 3.5 volume percent.
8. The process of claim 1 which is run with manual or automated
slide valves at the bottom of the drum.
9. The process of claim 5 run with manual or automated slide valves
at the bottom of the coke drum
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims benefit of U.S. Provisional Patent
Application Ser. No. 60/571,346 filed May 14, 2004.
FIELD OF THE INVENTION
[0002] The invention relates to a modified vacuum distillation and
delayed coking process for making substantially free-flowing coke,
preferably free-flowing shot coke. A vacuum resid feedstock is used
which contains less than about 10 wt. % material boiling between
900.degree. F. and 1040.degree. F. as determined by HTSD
(High-temperature Simulated Distillation). The use of such a high
boiling resid favors the formation of shot coke. Use of distillate
recycle in the feed reduces coker furnace fouling potential of the
heavier feedstock.
DESCRIPTION OF RELATED ART
[0003] Delayed coking involves thermal decomposition of
hydrocarbonaceous feedstocks (HFs) including petroleum residua
(resids) and deasphalter bottoms etc. to produce gas, liquid
streams of various boiling ranges, and coke. Delayed coking of HFs
from heavy and heavy sour (high sulfur) crude oils is carried out
primarily as a means of disposing of these low value feedstocks by
converting part of the HFs to more valuable liquid and gas
products.
[0004] In the delayed coking process, a resid feedstock is rapidly
heated in a fired heater or tubular furnace at from about
480.degree. C. to about 520.degree. C. and pressures of about 50 to
550 psig. The heated feedstock is then passed to a coking drum that
is maintained at conditions under which coking occurs, generally at
temperatures above about 800.degree. F. (425.degree. C.), typically
between about 480.degree. C. to about 520.degree. C. (895.degree.
F. to 970.degree. F.), under super-atmospheric pressures of about
15 to 80 psig to allow volatiles that form in the coker drum to be
removed overhead and passed to a fractionator, leaving coke behind.
When the coker drum is full of coke, the heated feed is switched to
another drum and additional hydrocarbon vapors are purged from the
coke drum with steam. The drum is then quenched with water to lower
the temperature to below about 300.degree. F. after which the water
is drained. 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.
[0005] 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 then cut the coke from
the drum. The coke removal step adds considerably to the throughput
time of the overall process. Thus, it would be desirable to be able
to produce a free-flowing coke, in a coker drum, that would not
require the expense and time associated with conventional coke
removal, i.e., it could be drained out of the bottom of the
drum.
[0006] Even though the coking drum may appear to be completely
cooled, some volumes of the bed may have been bypassed by the
cooling water, leaving the bypassed coke very hot (hotter than the
boiling point of water). This phenomenon, sometimes referred to as
"hot spots" or "hot drums", may be the result of a combination of
morphologies of coke being present in the drum, which may contain a
combination of more than one type of solid coke product, i.e.,
sponge coke and shot coke. Since unagglomerated shot coke may cool
faster than other coke morphologies, such as large shot coke masses
or sponge coke, it would be desirable to predominantly produce
free-flowing shot coke in a delayed coker, in order to avoid or
minimize hot drums.
SUMMARY OF THE INVENTION
[0007] In an embodiment, there is provided a delayed coking process
comprising:
[0008] a) preparing a vacuum resid that has less than 10 wt. % 900
to 1040.degree. F. boiling material as measured by HTSD
(High-Temperature Simulated Distillation) and combining with a
distillate recycle stream wherein the distillate recycle stream has
boiling range within the interval of about 450.degree. F. to about
750.degree. F.;
[0009] b) conducting said mixture to a heating zone wherein it is
heated to an effective coking temperature; and
[0010] c) conducting said heated mixture from said heating zone to
a coking zone wherein vapor products are collected overhead and
whereby coke with reduced incidence of hot drums and of a
relatively free-flowing nature is formed.
[0011] In a preferred embodiment, the coking zone is in a delayed
coker drum, and a substantially free-flowing shot coke product is
removed from the coker drum.
[0012] In still another preferred embodiment an additive is
introduced into the feedstock either prior to heating or just prior
to it being introduced in the coker vessel, which additive can be a
metals-containing or metals-free additive. If a metals containing
it is preferably an organic soluble, organic insoluble, or
non-organic miscible metals-containing additive that is effective
for the formation of substantially free-flowing coke.
[0013] In yet another preferred embodiment of the present invention
the metal of the additive is selected from the group consisting of
sodium, potassium, iron, nickel, vanadium, tin, molybdenum,
manganese, aluminum, cobalt, calcium, magnesium, and mixtures
thereof.
[0014] In another embodiment, the metals-containing additive is a
finely ground solid with a high surface area, a natural material of
high surface area, or a fine particle/seed producing additive. Such
high surface area materials include fumed silica and alumina,
catalytic cracker fines, FLEXICOKER cyclone fines, magnesium
sulfate, calcium sulfate, diatomaceous earth, clays, magnesium
silicate, vanadium-containing fly ash and the like. The additives
may be used either alone or in combination.
[0015] In another embodiment substantially metals-free additives
can be used in the practice of the present invention. Non-limiting
examples include elemental sulfur, high surface area substantially
metals-free solids, such as rice hulls, sugars, cellulose, ground
coals, ground auto tires and mineral acids such as sulfuric acid,
phosphoric acid, and their acid anhydrides. It is to be understood
that before or after the resid is treated with the additive, a
caustic species, preferably in aqueous form, may optionally be
added. The caustic can be added before, during, or after the resid
is passed to the coker furnace and heated to coking temperatures.
Spent caustic obtained from hydrocarbon processing can be used.
Such spent caustic can contain dissolved hydrocarbons, and salts of
organic acids, e.g., carboxylic acids, phenols, naphthenic acids
and the like.
[0016] In another embodiment, the process is used in conjunction
with automated coke drum bottom deheading valves, and the product
coke plus cooling water mixture is throttled out the bottom of the
coke drum through the bottom valve.
[0017] If an additive is used, it is desirable to avoid
heterogeneous areas of coke morphology formation. That is, one does
not what locations in the coke drum where the coke is substantially
free flowing and other areas where the coke is substantially
non-free flowing. Dispersing of the additive is accomplished by any
number of ways, preferably by introducing a side stream of the
additive into the feedstream at the desired location. The additive
can be added by solubilization of the additive into the vacuum
resid, or by reducing the viscosity of the vacuum resid prior to
mixing in the additive, e.g., by heating, solvent addition, etc.
High energy mixing or use of static mixing devices may be employed
to assist in dispersal of the additive agent, especially additive
agents that have relatively low solubility in the feedstream.
[0018] Preferably, all or substantially all of the coke formed in
the process is substantially free-flowing coke, more preferably,
free-flowing shot coke. It is also preferred that at least a
portion of volatile species present in the coker drum during and
after coking be separated and conducted away from the process,
preferably overhead of the coker drum.
BRIEF DESCRIPTION OF THE FIGURES
[0019] FIG. 1 is a simplified process flow diagram of one preferred
method of obtaining a deep cut heavy oil stream for use in the
present invention. This figure shows the vacuum distillation system
modified with a steam side stripper, as well as a distillate
recycle stream from the coker main fractionator.
[0020] FIG. 2 is another simplified process flow diagram of another
preferred method for obtaining a deep cut heavy oil stream for use
in the present invention. This figure is similar to that of FIG. 1
hereof except that there is an intermediate resid reheating furnace
for reheating the stream upstream of the stripper.
[0021] FIG. 3 is a cross polarized light optical micrograph showing
coke formed from a transition coke-forming heavy Canadian vacuum
resid containing about 12 wt. % 1000.degree. F. boiling material as
determined by HTSD. The figure shows small domains ranging in size
from about 10 to about 20 micrometers with some coarse mosaic
ranging from about 5 to about 10 micrometers (this microstructure
is associated with bulk coke beds having transition coke
morphology).
[0022] FIG. 4 shows the effect of further distilling the feed so
that it contains only about 2 wt. % 1000.degree. F. boiling
material. The figure is a cross polarized light optical micrograph
showing coke resid formed from the deeper cut resid and shows a
medium/coarse mosaic structure ranging in size from about 2 to
about 10 micrometers (this microstructure is associated with bulk
coke beds having shot coke morphology).
DETAILED DESCRIPTION OF THE INVENTION
[0023] Petroleum vacuum residua ("resid") feedstocks are suitable
for delayed coking. 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 structures that would
inhibit the rate of hydrotreating/hydrocracking and cause catalyst
deactivation; (b) metal contaminants occurring naturally in the
crude or resulting from prior treatment of the crude, which
contaminants would tend to deactivate hydrotreating/hydrocracking
catalysts and interfere with catalyst regeneration; and (c) a
relatively high content of sulfur and nitrogen compounds that give
rise to objectionable quantities of SO.sub.2, SO.sub.3, and
NO.sub.x upon combustion of the petroleum residuum. Nitrogen
compounds present in the resid also have a tendency to deactivate
catalytic cracking catalysts.
[0024] Coke bed morphology is typically described in simplified
terms such as sponge coke, shot coke, transition coke, and needle
coke.
[0025] As previously mentioned, there are generally three different
types of solid delayed coker products that have different values,
appearances and properties, i.e., needle coke, sponge coke, and
shot coke. Needle coke is the highest quality of the three
varieties. Needle coke, upon further thermal treatment, has high
electrical conductivity (and a low coefficient of thermal
expansion) and is used in electric arc steel production. It is
relatively low in sulfur and metals and is frequently produced from
some of the higher quality coker feedstocks that include more
aromatic feedstocks such as slurry and decant oils from catalytic
crackers and thermal cracking tars. Typically, it is not formed by
delayed coking of resid feeds.
[0026] There are additional descriptors of coke too, although
they're less common. For example, a sandy coke is a coke that after
cutting looks to the naked eye much like coarse black beach
sand.
[0027] In an embodiment, resid feedstocks include but are not
limited to 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, coal liquids,
shale oil or combinations of these materials. Atmospheric and
vacuum topped heavy bitumens can also be employed. Feedstocks
typically used in delayed coking are high-boiling hydrocarbonaceous
materials with an API gravity of about 20.degree. or less, and a
Conradson Carbon Residue content of about 0 to 40 weight
percent.
[0028] Vacuum resids are characterized by a number of parameters,
including their boiling point distributions. The boiling point
distribution can be obtained by a physical distillation in a
laboratory, but it is costly to perform this type of analysis.
Another method for determining the boiling point distribution is to
use specialized gas chromatographic techniques that have been
developed for the petroleum industry. One such GC method is
High-temperature Simulated Distillation (HTSD). This method is
described by D. C. Villalanti, et al. In "High-temperature
Simulated Distillation Applications in Petroleum Characterization"
in Encyclopedia of Analytical Chemistry, R. A. Meyers (Ed.), pp.
6726-6741 John Wiley, 2000, and has been found to be effective for
characterizing the boiling point distributions of vacuum residua.
Boiling point distributions are reported as wt. % off versus
atmospheric equivalent boiling point (AEBP) and are report in
increments of 1 wt. %.
[0029] Vacuum distillation is well known in the industry. A number
of variables affect the boiling point distribution of the vacuum
distillation unit bottoms. As refiners tend to try to push ever
more flow through existing units, however, the boiling point
distributions of the vacuum bottoms tend to pick up a higher
percentage of the lowest boiling components.
[0030] It has unexpectedly been found by the inventors hereof that
the components that are contained a virgin resid which boil between
about 900.degree. F. and 1040.degree. F. can have a significant
influence on delayed coker coke morphology is they are present in
an abundance in excess of about 10 wt. % of the entire virgin feed.
Specifically, it has been found that when a resid that otherwise
would make shot coke has the 900.degree. F. to 1040.degree. F.
fraction in excess of about 10 wt. %, it will make a transition
coke, or a bonded shot, and can have appreciable percentage of hot
drums when coked under "typical" delayed coker conditions, e.g.,
DOT=820.degree. F., DOP=15 to 35 psig, and recycle ratio of under
10%, where DOT is drum outlet temperature and DOP is drum outlet
pressure.
[0031] It has been found that by reducing the fraction of
900.degree. F. to 1040.degree. F. AEBP material to under 10 wt. %
pushes coke morphology back to a less bonded and less
self-supporting coke morphology.
[0032] Such deeper cuts of resids can be obtained by any means
available in a petroleum refinery. One means is represented in FIG.
1 hereof wherein atmospheric resid bottoms is conducted via line 10
through a furnace 1 wherein it is heated to a temperature of about
700.degree. F. to about 800.degree. F. then sent via line 20 to
vacuum distillation tower 2 wherein non-condensable material, such
as steam and any small amount of remaining light ends are collected
overhead via line 30, preferably by use of an ejector system (not
shown). A heavy vacuum gas oil cut is removed via line 40. An
intermediate cut is removed via line 50 where it is combined with
vacuum bottoms of line 60 and conducted to outboard stripper 3
where a lighter stream, such as one containing at least a fraction
of any remaining gas oil, is stripped by use of steam injected via
line 70 and sent back to the vacuum distillation tower via line 80.
The stripped vacuum resid bottoms is then conducted via line 90 to
a delayed coker where it is typically introduced near the bottom of
the main fractionator 4, although it can be fed directly to the
coker furnace 5. The bottoms of the main fractionator line 100 are
fed to the coker furnace wherein recycle distillate is introduced
via line 110. Any additives to aid in the desired coking reaction
can be introduced via line 120. The resid stream is heated in coker
furnace 5 to coking temperatures then sent via line 130 to one or
more coker drums (not shown).
[0033] FIG. 2 hereof shows another preferred process scheme for
obtaining a deep cut vacuum resid feed for producing substantially
free-flowing shot coke in a delayed coker. The process scheme is
similar to that shown in FIG. 1 hereof except the intermediate cut
removed from distillation tower 2 is conducted via line 50 and
combined with vacuum distillation bottoms on line 55 and sent
through outboard stripper furnace 6 for reheating to substantially
the same temperature as that of furnace 1. The reheated vacuum
bottoms/intermediate cut stream is conducted via line 60 to
outboard stripper 3.
[0034] The drawback of the deeper cut resids, however, is that they
tend to foul the coker furnace more rapidly than less deeply cut
resids, and this a potential economic debit because this can
increase frequency of furnace cleanout, which in turn reduces
overall throughput of the coker unit. To mitigate the higher
fouling tendency of the deeper cut vacuum resid, a distillate
stream can be added to the coker feed. The boiling point
distribution of the distillate recycle stream is such that it is an
effective mitigator of furnace fouling yet its endpoint is low
enough that it does not affect the coke morphology. An example of
this would be a coker distillate stream with a boiling range of
about 450.degree. F. to 750.degree. F.
[0035] The resid feed pumped to a heater at a pressure of about 50
to 550 psig, where it is heated to a temperature from about
480.degree. F. to about 520.degree. F. It is then discharged into a
coking zone, typically a vertically-oriented, insulated coker drum
through an inlet at the base of the drum. Pressure in the drum is
usually relatively low, such as about 15 to 80 psig to allow
volatiles to be removed overhead. Typical operating temperatures of
the drum will be between about 410.degree. C. and 475.degree. C.
The hot feedstock thermally cracks over a period of time (the
"coking time") in the coker drum, liberating volatiles composed
primarily of hydrocarbon products, that continuously rise through
the coke mass and are collected overhead. The volatile products are
sent to a coker fractionator for distillation and recovery of coker
gases, gasoline, light gas oil, and heavy gas oil. In an
embodiment, a portion of the heavy coker gas oil present in the
product stream introduced into the coker fractionator can be
captured for recycle and combined with the fresh feed (coker feed
component), thereby forming the coker heater or coker furnace
charge. In addition to the volatile products, delayed coking also
forms solid coke product.
[0036] Coke bed morphology is typically described in simplified
terms such as sponge coke, shot coke, transition coke, and needle
coke. Sponge coke, as the name suggests, has a sponge-like
appearance with various sized pores and bubbles "frozen into" a
solid coke matrix. One key attribute of sponge coke produced by
routine coker operating conditions is that the coke is
self-supporting, and typically will not fall out of the bottom of
an un-headed coker drum, which typically has a head diameter of
about 6 feet. The head of the coker drum can be removed either
manually or by use of a throttled slide valve. Needle coke refers
to a specialty coke that has a unique anisotropic structure.
Preparation of coke whose major component is needle coke is known
to those skilled in the art and is not the subject of this
invention.
[0037] Shot coke is a distinctive type of coke. It is comprised of
individual substantially spherical particles that look like BBs.
These individual particles range from substantially spherical to
slightly ellipsoidal with average diameters of about 1 mm to about
10 mm. The particles may be aggregated into larger-sized particles,
e.g., from tennis-ball size to basketball or larger sizes. The shot
coke can sometimes migrate through the coke bed and to the bottom
drain lines of the coke drum and slow, or even block, the quench
water drain process. While shot coke has a lower economic value
that sponge coke, it is the desired product coke for purposes of
this invention because its ease of removal from the coker drum
results in effectively increasing the process capacity which more
than offsets its reduced economic valve.
[0038] At times there appears to be a binder material present
between the individual shot coke particles, and such a coke is
sometimes referred to as "bonded shot" coke. Depending upon the
degree of bonding in the bed of shot coke, the bed may not be
self-supporting, and can flow out of the drum when the drum is
opened. This can be referred to as "fall-out` or "avalanche" and if
unexpected it can be dangerous to operating personnel and it can
also damage equipment.
[0039] The term "transition coke" refers to coke that has
morphology between that of sponge coke and shot coke. For example,
coke that has a mostly sponge-like physical appearance, but with
evidence of small shot spheres that are just beginning to form as
discrete particles in one type of transition coke.
[0040] Coke beds are not necessarily comprised of all of one type
of coke morphology. For example, the bottom of a coke drum can
contain large aggregates of shot, transitioning into a section of
loose shot coke, and finally have a layer of sponge-rich coke at
the top of the bed of coke. There are additional descriptors for
coke, although less common. Such additional descriptors include:
sandy coke which is a coke that after cutting looks to the naked
eye much like coarse black beach sand; and needle coke that refers
to a specialty coke that has a unique anisotropic structure.
Preparation of coke whose major component is needle coke is well
known to those having ordinary skill in the art and is not a
subject of this invention.
[0041] The term "free-flowing" as used herein means that about 500
tons of coke plus its interstitial water in a coker drum can be
drained in less than about 30 minutes through a 60-inch diameter
opening
[0042] It has been discovered that substantially free-flowing shot
coke can be produced by the practice of the present invention by
insuring that the resid feed is one having a substantially higher
initial boiling point than resides conventionally used for delay
coking. As previously mentioned, conventional delayed coking resid
feeds typically have an initial boiling point from about
500.degree. C. to about 526.degree. C., but the resid feeds of the
present invention having an initial boiling point from about
549.degree. C. to about 566.degree. C. will unexpected produce shot
coke over sponge coke.
[0043] Conventional coke processing aids, including an intifoaming
agent, can be employed in the process, for example, delayed coking
wherein a resid feedstock is air-blown to a target softening point
as described in U.S. Pat. No. 3,960,704. 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.
Additives are employed to provide for the formation of the desired,
substantially free-flowing shot coke.
[0044] It is within the scope of this invention to use a suitable
additive to aid in the formation of shot coke, preferably
substantially free-flowing shot coke. In an embodiment, the
additive is an organic soluble metal, such as a metal naphthenate
or a metal acetylacetonate, including mixtures thereof. Preferred
metals are potassium, sodium, iron, nickel, vanadium, tin,
molybdenum, manganese, aluminum, cobalt, calcium, magnesium and
mixtures thereof. Potassium, sodium, iron, aluminum and calcium are
preferred. Additives in the form of species naturally present in
refinery stream can be used. For such additives, the refinery
stream may act as a solvent for the additive, which may assist in
the dispersing the additive in the resid feed. Additives naturally
present in refinery streams include nickel, vanadium, iron, sodium,
and mixtures thereof naturally present in certain resid and resid
fractions (i.e., certain feed streams). The contacting of the
additive and the feed can be accomplished by blending a feed
fraction containing additive species (including feed fractions that
naturally contain such species) into the feed.
[0045] In another embodiment, the metals-containing additive is a
finely ground solid with a high surface area, a natural material of
high surface area, or a fine particle/seed producing additive. Such
high surface area materials include fumed silica and alumina,
catalytic cracker fines, FLEXICOKER cyclone fines, magnesium
sulfate, calcium sulfate, diatomaceous earth, clays, magnesium
silicate, vanadium-containing fly ash and the like. The additives
may be used either alone or in combination.
[0046] It is within the scope of this invention that a metals-free
additive be used. Non-limiting examples of substantially
metals-free additives that can be used in the practice of the
present invention include elemental sulfur, high surface area
substantially metals-free solids, such as rice hulls, sugars,
cellulose, ground coals, ground auto tires. Other additives include
inorganic oxides such as fumed silica and alumina; salts of oxides,
such as ammonium silicate and mineral acids such as sulfuric acid
and phosphoric acid, and their acid anhydrides.
[0047] Preferably, a caustic species is added to the resid coker
feedstock. When used, the caustic species may be added before,
during, or after heating in the coker furnace. Addition of caustic
will reduce the Total Acid Number (TAN) of the resid coker
feedstock and also convert naphthenic acids to metal naphthanates,
e.g., sodium naphthenate.
[0048] Uniform dispersal of the additive into the vacuum resid feed
is desirable to avoid heterogeneous areas of shot coke formation.
Dispersing of the additive is accomplished by any number of ways,
for example, by solubilization of the additive into the vacuum
resid, or by reducing the viscosity of the vacuum resid prior to
mixing in the additive, e.g., by heating, solvent addition, use of
organometallic agents, etc. High energy mixing or use of static
mixing devices may be employed to assist in dispersal of the
additive agent.
[0049] Polarizing light microscopy was used in the examples
(illustrated in FIGS. 1 and 2) for comparing and contrasting
structures of green coke (i.e., non-calcined coke) samples.
[0050] At the macroscopic scale, i.e., at a scale that is readily
evident to the naked eye, petroleum sponge and shot green cokes are
quite different--sponge has a porous sponge-like appearance, and
shot coke has a spherical cluster appearance. However, under
magnification with an optical microscope, or polarized-light
optical microscope, additional differences between different green
coke samples may be seen, and these are dependent upon amount of
magnification.
[0051] For example, utilizing a polarized light microscope, at a
low resolution where 10-micrometer features are discernable, sponge
coke appears highly anisotropic, the center of a typical shot coke
sphere appears much less anisotropic, and the surface of a shot
coke sphere appears fairly anisotropic.
[0052] At higher resolutions, e.g., where 0.5-micrometer features
are discernable (this is near the limit of resolution of optical
microscopy), a green sponge coke sample still appears highly
anisotropic. The center of a shot coke sphere at this resolution is
now revealed to have some anisotropy, but the anisotropy is much
less than that seen in the sponge coke sample.
[0053] It should be noted that the optical anisotropy discussed
herein is not the same as "thermal anisotropy", a term known to
those skilled in the art of coking. Thermal anisotropy refers to
coke bulk thermal properties such as coefficient of thermal
expansion, which is typically measured on cokes which have been
calcined, and fabricated into electrodes.
[0054] It is within the scope of this invention that a metals-free
additive be used to encourage the production of free-flowing coke,
preferably free-flowing shot coke. Non-limiting examples of
metals-free additives include elemental sulfur, high surface area
substantially metals-free solids, such as rice hulls, sugars,
cellulose, ground coals, ground auto tires; inorganic oxides such
as fumed silica and alumina; salts of oxides, such as ammonium
silicate and mineral acids such as sulfuric acid, phosphoric acid,
and acid anhydrides.
[0055] The present invention will be better understood by reference
to the following non-limiting examples that are presented for
illustrative purposes.
EXAMPLE
[0056] A vacuum resid is produced in a refinery and has had the
vacuum overflash reincorporated into it. The refinery is pushing
throughput, and consequently the resid boiling point distribution
is having an increased amount of the lightest portion. The vacuum
resid has an API gravity of 3.7, contains 5.4 wt. % S, and about
10.0 wt. % hydrogen. The boiling point distribution of the front
end as determined by HTSD is as follows in the column labeled "base
case vacuum resid" in the table below. TABLE-US-00001 TABLE Base
Case Resid with second stage Vacuum Resid vacuum distillation HTSD
Wt. % Off AEBP, Deg. F AEBP, Deg. F IBP 554 910 1 698 954 2 813 986
3 858 1003 4 888 1016 5 911 1027 6 929 1036 7 944 1045 8 957 9 969
10 980 11 990 12 999 13 1007 14 1016 15 1024 16 1032 17 1039 Wt. %
1382 - Deg. F 79.9 73.6
[0057] The resid contains about 12 wt. % 900.degree. F. to
1040.degree. F. material. The base case resid is coked in a pilot
plant coker with a drum overhead temperature of 820.degree. F.,
drum overhead pressure of 15 psig and zero recycle. The product
coke has a bonded morphology which appears highly fused throughout
the bed. Microscopic examination of the coke under cross polarized
light reveals mostly small domains (10-20 microns) with coarse
mosaic (5-10 micron). Percentage shot coke by the micrographic
technique is estimate to be about 25%. By a known relationship with
a commercial-scale coker, it is projected that this coke would
yield a bonded shot that would be self-supporting in the commercial
scale coke drum.
[0058] The base case resid then has a second stage vacuum
distillation which removes a portion of the lightest components.
The boiling point distribution of the resid after distillation is
shown in the right column of the table, i.e., after the second
stage vacuum distillation, the resid contains about 7 wt. %
900.degree. F. to 1040.degree. F. material.
[0059] The deeper cut resid is coked in the pilot plant coker with
a drum overhead temperature of 820.degree. F., a drum overhead
pressure of 15 psig, and zero recycle. The product coke is about
80% shot coke. Microscopic examination of the coke under
cross-polarized light reveals mostly medium/coarse mosaic (2-10
microns). Percentage shot coke by the micrographic technique is
estimate to be about 75%. By a known relationship with a
commercial-scale coker, it is projected that this coke would yield
a relatively loose shot that would be non-self supporting in the
commercial scale coke drum.
* * * * *