Note: Descriptions are shown in the official language in which they were submitted.
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HYDROCARBON THERMAL CRACKING USING ATMOSPHERIC RESIDUUM
BACKGROUND OF INVENTION
FIELD OF INVENTION
This invention relates to the thermal cracking of hydrocarbons using a
vaporization unit in combination with a pyrolysis furnace wherein at least a
preponderance of the liquid hydrocarbonaceous feed to be cracked in the
furnace is
first vaporized in the vaporization unit. More particularly, this invention
relates to the
use of atmospheric residuum as a significant liquid hydrocarbonaceous feed
component for the vaporization unit and furnace.
DESCRIPTION OF THE PRIOR ART
Thermal (pyrolysis) cracking of hydrocarbons is a non-catalytic petrochemical
process that is widely used to produce olefins such as ethylene, propylene,
butenes,
butadiene, and aromatics such as benzene, toluene, and xylenes.
Basically, a hydrocarbon containing feedstock is mixed with steam which
serves as a diluent to keep the hydrocarbon molecules separated. The
steam/hydrocarbon mixture is preheated in the convection zone of the furnace
to
from about 900 to about 1,000 degrees Fahrenheit ( F or F), and then enters
the
reaction (radiant) zone where it is very quickly heated to a severe
hydrocarbon
thermal cracking temperature in the range of from about 1,400 to about 1,550F.
Thermal cracking is accomplished without the aid of any catalyst.
This process is carried out in a pyrolysis furnace (steam cracker) at
pressures
in the reaction zone ranging from about 10 to about 30 psig. Pyrolysis
furnaces have
internally thereof a convection section (zone) and a separate radiant section
(zone).
Preheating functions are primarily accomplished in the convection section,
while
severe cracking mostly occurs in the radiant section.
After thermal cracking, depending on the nature of the primary feed to the
pyrolysis furnace, the effluent from that furnace can contain gaseous
hydrocarbons
of great variety, e.g., from one to thirty-five carbon atoms per molecule.
These
gaseous hydrocarbons can be saturated, monounsaturated, and polyunsaturated,
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and can be aliphatic, alicyclics, and/or aromatic. The cracked gas can also
contain
significant amounts of molecular hydrogen (hydrogen).
The cracked product is then further processed in the olefin production plant
to
produce, as products of the plant, various separate individual streams of high
purity
such as hydrogen, ethylene, propylene, mixed hydrocarbons having four carbon
atoms per molecule, fuel oil, and pyrolysis gasoline. Each separate individual
stream aforesaid is a valuable commercial product in its own right. Thus, an
olefin
production plant currently takes a part (fraction) of a whole crude stream or
condensate, and generates therefrom a plurality of separate, valuable
products.
to
Thermal cracking first came into use in 1913 and was first applied to gaseous
ethane as the primary feed to the cracking furnace for the purpose of making
ethylene. Since that time, the industry has evolved to using heavier and more
complex hydrocarbonaceous gaseous and/or liquid feeds as the primary feed for
the
cracking furnace. Such feeds can now employ a fraction of whole crude or
condensate which is essentially totally vaporized while thermally cracking
same.
The cracked product can contain, for example, about 1 weight percent (wt.%)
hydrogen, about 10 wt.% methane, about 25 wt.% ethylene, and about 17 wt.%
propylene, all wt.% being based on the total weight of that product, with the
remainder consisting mostly of other hydrocarbon molecules having from 4 to 35
carbon atoms per molecule.
Natural gas and whole crude oil(s) were formed naturally in a number of
subterranean geologic formations (formations) of widely varying porosities.
Many of
these formations were capped by impervious layers of rock. Natural gas and
whole
crude oil (crude oil) also accumulated in various stratigraphic traps below
the earth's
surface. Vast amounts of both natural gas and/or crude oil were thus collected
to
form hydrocarbon bearing formations at varying depths below the earth's
surface.
Much of this natural gas was in close physical contact with crude oil, and,
therefore,
absorbed a number of lighter molecules from the crude oil.
When a well bore is drilled into the earth and pierces one or more of such
hydrocarbon bearing formations, natural gas and/or crude oil can be recovered
through that well bore to the earth's surface.
The terms "whole crude oil" and "crude oil" as used herein means liquid (at
normally prevailing conditions of temperature and pressure at the earth's
surface)
crude oil as it issues from a wellhead separate from any natural gas that may
be
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present, and excepting any treatment such crude oil may receive to render it
acceptable for transport to a crude oil refinery and/or conventional
distillation in such
a refinery. This treatment would include such steps as desalting. Thus, it is
crude oil
that is suitable for distillation or other fractionation in a refinery, but
which has not
undergone any such distillation or fractionation. It could include, but does
not
necessarily always include, non-boiling entities such as asphaltenes or tar.
As such,
it is difficult if not impossible to provide a boiling range for whole crude
oil.
Accordingly, whole crude oil could be one or more crude oils straight from an
oil field
pipeline and/or conventional crude oil storage facility, as availability
dictates, without
any prior fractionation thereof.
Natural gas, like crude oil, can vary widely in its composition as produced to
the earth's surface, but generally contains a significant amount, most often a
major
amount, i.e., greater than about 50 weight percent (wt. /0), methane. Natural
gas
often also carries minor amounts (less than about 50 wt. %), often less than
about 20
wt. %, of one or more of ethane, propane, butane, nitrogen, carbon dioxide,
hydrogen sulfide, and the like. Many, but not all, natural gas streams as
produced
from the earth can contain minor amounts (less than about 50 wt. %), often
less than
about 20 wt. %, of hydrocarbons having from 5 to 12, inclusive, carbon atoms
per
molecule (C5 to C12) that are not normally gaseous at generally prevailing
ambient
atmospheric conditions of temperature and pressure at the earth's surface, and
that
can condense out of the natural gas once it is produced to the earth's
surface. All
wt.% are based on the total weight of the natural gas stream in question.
When various natural gas streams are produced to the earth's surface, a
hydrocarbon composition often naturally condenses out of the thus produced
natural
gas stream under the then prevailing conditions of temperature and pressure at
the
earth's surface where that stream is collected. There is thus produced a
normally
liquid hydrocarbonaceous condensate separate from the normally gaseous natural
gas under the same prevailing conditions. The normally gaseous natural gas can
contain methane, ethane, propane, and butane. The normally liquid hydrocarbon
fraction that condenses from the produced natural gas stream is generally
referred to
as "condensate," and generally contains molecules heavier than butane (C5 to
about
C20 or slightly higher). After separation from the produced natural gas, this
liquid
condensate fraction is processed separately from the remaining gaseous
fraction
that is normally referred to as natural gas.
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Thus, condensate recovered from a natural gas stream as first produced to
the earth's surface is not the exact same material, composition wise, as
natural gas
(primarily methane). Neither is it the same material, composition wise, as
crude oil.
Condensate occupies a niche between normally gaseous natural gas and normally
liquid whole crude oil. Condensate contains hydrocarbons heavier than normally
gaseous natural gas, and a range of hydrocarbons that are at the lightest end
of
whole crude oil.
Condensate, unlike crude oil, can be characterized by way of its boiling point
range. Condensates normally boil in the range of from about 100 to about 650F.
With this boiling range, condensates contain a wide variety of
hydrocarbonaceous
materials. These materials can include compounds that make up fractions that
are
commonly referred to as naphtha, kerosene, diesel fuel(s), and gas oil (fuel
oil,
furnace oil, heating oil, and the like).
Naphtha and associated lighter boiling materials (naphtha) are in the C5 to
C10, inclusive, range, and are the lightest boiling range fractions in
condensate,
boiling in the range of from about 100 to about 400F.
Petroleum middle distillates (kerosene, diesel, atmospheric gas oil) are
generally in the C10 to about C20 or slightly higher range, and generally
boil, in their
majority, in the range of from about 350 to about 650F. They are, individually
and
collectively, referred to herein as "distillate," "distillates," or "middle
distillates."
Distillate compositions can have a boiling point lower than 350F and/or higher
than
650F, and such distillates are included in the 350-650F range aforesaid, and
in this
invention.
Atmospheric residuum (resid or residua) typically boils at a temperature
of from about 650F up to its end boiling point where only non-boiling entities
such as
asphaltenes and tar are left. Atmospheric resid is formed by processing crude
oil/condensate in an atmospheric thermal distillation tower. Atmospheric resid
is not
the same as vacuum residuum which is 'formed in a vacuum assisted thermal
distillation tower, and has a boiling range of from about 1,000F up to its end
boiling
point where only non-boiling entities remain.
The olefin production industry is now progressing beyond the use of fractions
of crude oil or condensate (gaseous and/or liquid) as the primary feed for a
cracking
furnace to the use of whole crude oil and/or condensate itself.
Recently, U.S. Patent Number 6,743,961 (hereafter "USP '961") issued to
Donald H. Powers. This patent relates to cracking whole crude oil by employing
a
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vaporization/mild cracking zone that contains packing. This zone is operated
in a
manner such that the liquid phase of the whole crude that has not already been
vaporized is held in that zone until cracking/vaporization of the more
tenacious
hydrocarbon liquid components is maximized. This allows only a minimum of
solid
residue formation which residue remains behind as a deposit on the packing.
This
residue is later burned off the packing by conventional steam air decoking,
ideally
during the normal furnace decoking cycle, see column 7, lines 50-58 of that
patent.
Thus, the second zone 9 of that patent serves as a trap for components,
including
hydrocarbonaceous materials, of the crude oil feed that cannot be cracked or
I() vaporized under the conditions employed in the process, see column 8,
lines 60-64
of that patent.
Still more recently, U.S. Patent 7,019,187 issued to Donald H. Powers. This
patent is directed to the process disclosed in USP '961, but employs a mildly
acidic
cracking catalyst to drive the overall function of the vaporization/mild
cracking unit
more toward the mild cracking end of the vaporization (without prior mild
cracking) ¨
mild cracking (followed by vaporization) spectrum.
U.S. Patent Number 6,979,757 to Donald H. Powers is directed to the process
disclosed in USP '961, but that invention removes at least part of the liquid
hydrocarbons remaining in the vaporization/mild cracking unit that are not yet
vaporized or mildly cracked. These liquid hydrocarbon components of the crude
oil
feed are drawn from near the bottom of that unit and passed to a separate
controlled
cavitation device to provide additional cracking energy for those tenacious
hydrocarbon components that have previously resisted vaporization and mild
cracking. Thus, that invention also seeks to drive the overall process in the
vaporization/mild cracking unit more toward the mild cracking end of the
vaporization-mild cracking spectrum aforesaid.
U.S. Patent Number 7,374,664, filed September 2, 2005,
having common inventorship and assignee with USP '961, is directed to the
process
of using whole crude oil as the feedstock for an olefin plant to produce a
mixture of
hydrocarbon vapor and liquid. The vaporous hydrocarbon is separated from the
remaining liquid and the vapor passed to a severe cracking operation. The
remaining liquid is vaporized using a quench oil to minimize coke forming
reactions.
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U.S. Patent Number 7,396,449, filed March 1, 2006,
having common inventorship and assignee with USP '961, is directed to the use
of
condensate as the dominant liquid hydrocarbonaceous feed for the vaporization
unit
and furnace.
U.S. Patent Number 7,550,642, filed October 20, 2006,
having common inventorship and assignee with USP '961, is directed to the
integration of the vaporization unit/furnace combination with a crude oil
refinery.
During periods of increased gasoline demand the gasoline supply (pool) can
be increased by subjecting various crude oil fractions, including distillates,
to various
to refinery catalytic cracking processes such as fluid catalytic cracking.
Thus, the
quantity of gasolineinaptha produced from a barrel of crude oil can be
increased if
desired. This is not so with distillates as defined above. The amount of
distillate
recovered from a barrel of crude oil is fixed and cannot be increased as it
can with
gasoline. The only way to increase distillate production to increase the
distillate
supply for the distillate pool is by refining additional barrels of crude oil.
By this invention valuable distillates that are in short supply are saved for
the
distillate pool, and not consumed in the cracking process.
SUMMARY OF THE INVENTION
In accordance with this invention, the aforesaid combination of a
vaporization.
unit/cracking furnace is operated with its feed containing a significant
amount of
residuum from at least one atmospheric thermal distillation column, i.e.,
atmospheric
resid.
DESCRIPTION OF THE DRAWING
Figure 1 shows a simplified flow sheet for the whole crude oil/condensate
cracking process described hereinabove.
DETAILED DESCRIPTION OF THE INVENTION
The terms "hydrocarbon," "hydrocarbons," and "hydrocarbonaceous" as used
herein do not mean materials strictly or only containing hydrogen atoms and
carbon
atoms. Such terms include materials that are hydrocarbonaceous in nature in
that
they primarily or essentially are composed of hydrogen and carbon atoms, but
can
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contain other elements such as oxygen, sulfur, nitrogen, metals, inorganic
salts, and
the like, even in significant amounts.
The term "gaseous" as used in this invention means one or more gases in an
essentially vaporous state, for example, steam alone, a mixture of steam and
hydrocarbon vapor, and the like.
Coke, as used herein, means a high molecular weight carbonaceous solid,
and includes compounds formed from the condensation of polynuclear aromatics.
An olefin producing plant useful with this invention would include a pyrolysis
(thermal cracking) furnace for initially receiving and thermally cracking the
feed.
Pyrolysis furnaces for steam cracking of hydrocarbons heat by means of
convection
and radiation, and comprise a series of preheating, circulation, and cracking
tubes,
usually bundles of such tubes, for preheating, transporting, and cracking the
hydrocarbon feed. The high cracking heat is supplied by burners disposed in
the
radiant section (sometimes called "radiation section") of the furnace. The
waste gas
from these burners is circulated through the convection section of the furnace
to
provide the heat necessary for preheating the incoming hydrocarbon feed. The
convection and radiant sections of the furnace are joined at the "cross-over,"
and the
tubes referred to hereinabove carry the hydrocarbon feed from the interior of
one
section to the interior of the next.
In a typical furnace, the convection section can contain multiple sub-zones.
For example, the feed can be initially preheated in a first upper sub-zone,
boiler feed
water heated in a second sub-zone, mixed feed and steam heated in a third sub-
zone, steam superheated in a fourth sub-zone, and the final feed/steam mixture
split
into multiple sub-streams and preheated in a lower (bottom) or fifth sub-zone.
The
number of sub-zones and their functions can vary considerably. Each sub-zone
can
carry a plurality of conduits carrying furnace feed there through, many of
which are
sinusoidal in configuration. The convection section operates at much less
severe
operating conditions than the radiant section.
Cracking furnaces are designed for rapid heating in the radiant section
starting at the radiant tube (coil) inlet where reaction velocity constants
are low
because of low temperature. Most of the heat transferred simply raises the
hydrocarbons from the inlet temperature to the reaction temperature. In the
middle of
the coil, the rate of temperature rise is lower but the cracking rates are
appreciable.
At the coil outlet, the rate of temperature rise increases somewhat but not as
rapidly
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as at the inlet. The rate of disappearance of the reactant is the product of
its
reaction velocity constant times its localized concentration. At the end of
the coil,
reactant concentration is low and additional cracking can be obtained by
increasing
the process gas temperature.
Steam dilution of the feed hydrocarbon lowers the hydrocarbon partial
pressure, enhances olefin formation, and reduces any tendency toward coke
formation in the radiant tubes.
Cracking furnaces typically have rectangular fireboxes with upright tubes
centrally located between radiant refractory walls. The tubes are supported
from
m their top.
Firing of the radiant section is accomplished with wall or floor mounted
burners or a combination of both using gaseous or combined gaseous/liquid
fuels.
Fireboxes are typically under slight negative pressure, most often with upward
flow
of flue gas. Flue gas flow into the convection section is established by at
least one
is of natural draft or induced draft fans.
Radiant coils are usually hung in a single plane down the center of the fire
box.
They can be nested in a single plane or placed parallel in a staggered, double-
row
tube arrangement. Heat transfer from the burners to the radiant tubes occurs
largely
by radiation, hence the thermo "radiant section," where the hydrocarbons are
heated
20 to from about 1,400 F to about 1,550 F and thereby subjected to severe
cracking,
and coke formation.
The initially empty radiant coil is, therefore, a fired tubular chemical
reactor.
Hydrocarbon feed to the furnace is preheated to from about 900 F to about
1,000 F
in the convection section by convectional heating from the flue gas from the
radiant
25 section, steam dilution of the feed in the convection section, or the
like. After
preheating, in a conventional commercial furnace, the feed is ready for entry
into the
radiant section.
The cracked gaseous hydrocarbons leaving the radiant section are rapidly
reduced in temperature to prevent destruction of the cracking pattern. Cooling
of the
30 cracked gases before further processing of same downstream in the olefin
production plant recovers a large amount of energy as high pressure steam for
re-
use in the furnace and/or olefin plant. This is often accomplished with the
use of
transfer-line exchangers that are well known in the art.
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With a liquid hydrocarbon feedstock downstream processing, although it can
vary from plant to plant, typically employs an oil quench of the furnace
effluent after
heat exchange of same in, for example, the transfer-line exchanger aforesaid.
Thereafter, the cracked hydrocarbon stream is subjected to primary
fractionation to
remove heavy liquids, followed by compression of uncondensed hydrocarbons, and
acid gas and water removal therefrom. Various desired products are then
individually separated, e.g., ethylene, propylene, a mixture of hydrocarbons
having
four carbon atoms per molecule, fuel oil, pyrolysis gasoline, and a high
purity
=
hydrogen stream.
io Figure
1 shows one embodiment of a cracking process that uses whole crude
oil and/or condensate as the dominant (primary) furnace feed. Figure 1 is very
diagrammatic for sake of simplicity and brevity since, as discussed above,
actual
furnaces are complex structures.
Figure 1 shows a liquid cracking furnace 1 wherein a crude oil/condensate
primary feed 2 is passed in to an upper feed preheat sub-zone 3 in the upper,
cooler
reaches of the convection section of furnace 1. Steam 6 is also superheated in
an
upper level of the convection section of the furnace.
The pre-heated cracking feed stream is then passed by way of pipe (line) 10
to a vaporization unit 11 (see USP '961), which unit is separated into an
upper
vaporization zone 12 and a lower vaporization zone 13. This unit 11 achieves
primarily (predominately) vaporization of at least a significant portion of
the naphtha
and gasoline boiling range and lighter materials that remain in the liquid
state after
the pre-heating step. Gaseous materials that are associated with the preheated
feed
as received by unit 11, and additional gaseous materials formed in zone 12,
are
removed from zone 12 by way of line 14. Thus, line 14 carries away essentially
all
the lighter hydrocarbon vapors, e.g., naphtha and gasoline boiling range and
lighter,
that are present in zone 12. Liquid distillate present in zone 12, with or
without some
liquid gasoline and/or naphtha, is removed there from via line 15 and passed
into the
upper interior of lower zone 13. Zones 12 and 13, in this embodiment, are
separated
from fluid communication with one another by an impermeable wall 16, which can
be
a solid tray. Line 15 represents external fluid down flow communication
between
zones 12 and 13. In lieu thereof, or in addition thereto, zones 12 and 13 can
have
internal fluid communication there between by modifying wall 16 to be at least
in part
liquid permeable by use of one or more trays designed to allow liquid to pass
down
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into the interior of zone 13 and vapor up into the interior of zone 12. For
example,
instead of an impermeable wall 16, a chimney tray could be used in which case
liquid within unit 11 would flow internally down into section 13 instead of
externally of
unit 11 via line 15. In this internal down flow case, distributor 18 becomes
optional.
By whatever way liquid is removed from zone 12 to zone 13, that liquid moves
downwardly into zone 13, and thus can encounter at least one liquid
distribution
device 18. Device 18 evenly distributes liquid across the transverse cross
section of
unit 11 so that the liquid will flow uniformly across the width of the tower
into contact
with packing 19.
Dilution steam 6 passes through superheat zone sub-20, and then, via line 21
in to a lower portion 22 of zone 13 below packing 19. In packing 19 liquid and
steam
from line 21 intimately mix with one another thus vaporizing some of liquid
15. This
newly formed vapor, along with dilution steam 21, is removed from zone 13 via
line
17 and added to the vapor in line 14 to form a combined hydrocarbon vapor
product
in line 25. Stream 25 can contain essentially hydrocarbon vapor from feed 2,
e.g.,
gasoline and naphtha, and steam.
Stream 17 thus represents a part of feed stream 2 plus dilution steam 21 less
liquid distillate(s) and heavier from feed 2 that are present in bottoms
stream 26.
Stream 25 is passed through a header (not shown) whereby stream 25 is split
into
multiple sub-streams and passed through multiple conduits (not shown) into
convection section pre-heat sub-zone 27 of furnace 1. Section 27 is in a
lower, and
therefore hotter, section of furnace 1. Section 27 is used for preheating
stream 25 to
a temperature, aforesaid, suitable for cracking in radiant zone 29.
After substantial heating in section 27, stream 25 passes by way of line 28
into radiant section 29. Again, the multiple, individual streams that normally
pass
from sub-zone 27 to and through zone 29 are represented as a single flow
stream 28
for sake of brevity.
In radiant firebox 29 of furnace 1, feed from line 28, which contains numerous
varying hydrocarbon components, is subjected to severe thermal cracking
conditions
as aforesaid.
The cracked product leaves radiant firebox 29 by way of line 30 for further
processing in the remainder of the olefin plant downstream of furnace 1 as
described
hereinabove and shown in USP '961.
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When using crude oil and/or condensate as the significant component(s) of
feed 2, substantial amounts of distillates are ultimately passed into furnace
1 and
cracked thereby converting such distillates into lighter components.
Accordingly,
these distillates are lost as a source of distillate supply for jet fuel,
diesel fuels and
home heating oils.
Pursuant to this invention, feed 2 contains, as a significant component
thereof,
residuum obtained from the atmospheric thermal distillation of at least one
crude oil
and not the crude oil/condensate feed of the prior art. Although one or more
atmospheric residua can be essentially the sole component of feed material 2,
this
invention is not so limited so long as atmospheric residuum is a significant
constituent in feed 2 and line 10.
Note that this invention does not apply to vacuum resid or catalytic cracking.
For purposes of this invention, feed 2 can enter furnace 1 at a temperature of
from about ambient up to about 300F at a pressure from slightly above
atmospheric
up to about 100 psig (hereafter "atmospheric to 100 psig"). Feed 2 can enter
zone
12 via line 10 at a temperature of from about ambient to about 700F at a
pressure of
from atmospheric to 100 psig.
Stream 14 can be essentially all hydrocarbon vapor formed from feed 2 and is
at a temperature of from about ambient to about 700F at a pressure of from
atmospheric to 100 psig.
Stream 15 can be essentially all the remaining liquid from feed 2 less that
which was vaporized in pre-heater 3 and is at a temperature of from about
ambient
to about 700F at a pressure of from slightly above atmospheric up to about 100
psig
(hereafter "atmospheric to 100 psig").
The combination of streams 14 and 17, as represented by stream 25, can be
at a temperature of from about 600 to about 800 F at a pressure of from
atmospheric
to 100 psig, and contain, for example, an overall steam/hydrocarbon ratio of
from
about 0.1 to about 2, preferably from about 0.1 to about 1, pounds of steam
per
pound of hydrocarbon.
In zone 13, dilution ratios (hot gas/liquid droplets) will vary widely because
the
compositions of atmospheric resid and condensate vary widely. Generally, the
hot
gas, e.g., steam and hydrocarbon at the top of zone 13 can be present in a
ratio of
steam to hydrocarbon of from about 0.1/1 to about 5/1.
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Steam is an example of a suitable hot gas introduced by way of line 21. Other
materials can be present in the steam employed. Stream 6 can be that type of
steam normally used in a conventional cracking plant. Such gases are
preferably at
a temperature sufficient to volatilize a substantial fraction of the liquid
hydrocarbon
15 that enters zone 13. Generally, the gas entering zone 13 from conduit 21
will be
at least about 650F, preferably from about 900 to about 1,200F at from
atmospheric
to 100 psig. Such gases will, for sake of simplicity, hereafter be referred to
in terms
of steam alone.
Stream 17 can be a mixture of steam and hydrocarbon vapor that has a
boiling point lower than about 1,200F. Stream 17 can be at a temperature of
from
about 600 to about 800F at a pressure of from atmospheric to 100 psig.
Steam from line 21 does not serve just as a diluent for partial pressure
purposes as is the normal case in a cracking operation. Rather, steam from
line 21
provides not only a diluting function, but also additional vaporizing and mild
cracking
is energy
for the hydrocarbons that remain in the liquid state. This is accomplished
with just sufficient energy to achieve vaporization and/or mild cracking of
heavier
hydrocarbon components in the atmospheric resid and by controlling the energy
input. For example, by using steam in line 21, substantial vaporization/mild
cracking
of feed 2 liquid is achieved. The very high steam dilution ratio and the
highest
temperature steam are thereby provided where they are needed most as liquid
hydrocarbon droplets move progressively lower in zone 13.
The atmospheric resid feed employed in this invention can be from a single
or multiple sources, and, therefore, can be a single resid or a mixture of two
or more
residua. Atmospheric resid useful in this invention can have a wide boiling
range,
particularly when mixtures of residua are employed, but will generally be in a
boiling
range of from about 600F to the boiling end point where only non-boiling
entities
remain.
Pursuant to this invention, light materials such as light gasoline, naphtha,
and
gas oils boiling lighter (lower) than about 1,000F, all as defined
hereinabove,
remaining in the atmospheric residuum feed will be vaporized in unit 11 and
removed
by way of either line 14 or 17 or both and fed to furnace 1 as described
hereinabove.
In addition, hydrocarbonaceous entities heavier than the lighter entities
mentioned
above in this paragraph can, at least in part, be mildly cracked or otherwise
broken
down in unit 11 to lighter hydrocarbonaceous entities such as those mentioned
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above, and those just formed lighter entities removed by way of line 17 as
feed for
=
furnace 1. The remainder of the residuum feed, if any, is removed by way of
line 26
for disposition elsewhere.
Atmospheric resid bottoms from an atmospheric thermal distillation tower
(atmospheric tower) are primarily composed of a gas oil component boiling in
the
range of from about 600 to about 1,000F and a heavier fraction boiling in a
temperature range of from about 1,000F up to its end boiling point where only
non-
boiling entities remain. A vacuum assisted thermal distillation tower (vacuum
tower)
typically separates this gas oil component from its associated heavier
fraction
aforesaid, thus freeing the gas oil fraction for separate recovery and
beneficial use
elsewhere.
By the process of this invention, the need for a vacuum tower to recover the
gas oil component contained in atmospheric resid is eliminated, without
eliminating
the function thereof. This is accomplished in this invention by using
atmospheric
resid as a significant portion of the feed 2 (and 10) to stripper 11 of Figure
1. In
section 13 of stripper 11 the counter current flow of downward moving
hydrocarbon
liquid and upward flowing steam 21 enables the heaviest (highest boiling
point)
liquids to be contacted at the highest steam to oil ratio and with the highest
temperature steam at the same time. This creates an efficient operation for
the
vaporization and mild cracking of the atmospheric resid thereby forming
additional
lighter materials from that resid, and freeing the gas oil component of that
resid, all
for recovery by way of line 17 as additional cracking feed for furnace 1. It
can be
seen that this invention utilizes for breaking down atmospheric resid, energy
in
stripper 11 that is normally utilized in the operation of furnace 1, while
eliminating the
energy cost of operating a separate vacuum tower. Similarly the capital and
other
costs for a vacuum tower are eliminated without loss of the function of that
vacuum
tower in breaking down atmospheric resid and separating the useful gas oil
component thereof.
The amount of atmospheric resid employed in feed 10 pursuant to this
invention can be a significant component of the overall feed 2. The
atmospheric
resid component can be at least about 20 wt.% of the total weight of the feed,
but it
is not necessarily strictly within this range.
Depending on the specific physical and chemical characteristics of the
atmospheric resid added to feed 2, other materials can be added to that feed.
Such
13
CA 02683943 2014-09-02
additional materials can include light gasoline, naphtha, natural gasoline,
and/or
condensate. Naphtha can be employed in the form of full range naphtha, light
naphtha, medium naphtha, heavy naphtha, or mixtures of two or more thereof.
The
light gasoline can have a boiling range of from that of pentane (C5) to about
158F..
Full range naphtha, which includes light, medium, and heavy naphtha fractions,
can
have a boiling range of from about 158 to about 350F. The boiling ranges for
the
light, medium, and heavy naphtha fractions can be, respectively, from about
158 to
about 212F, from about 212 to about 302F, and from about 302 to about 350F.
If light materials, as aforesaid, are added to the atmospheric resid in feed
2, it
can be preferable, again depending on the specific characteristics of this
resid feed
in line 2, that lighter fractions such as light gasoline and light naphtha be
added to
that resid feed, thereby leaving medium and/or heavy naphtha fractions for
addition
to the gasoline pool.
The amount of light material(s) thus deliberately added to the atmospheric
resid in feed 2 can vary widely depending on the desires of the operator, but
the
resid in feed 2 will remain a significant component of the feed in line 10 to
vaporization unit 11.
The deliberate re-mixing of already separated light materials with atmospheric
resid is counter intuitive and not done in the art. However, the addition of
one or
more of these light materials to the atmospheric resid is highly useful in
this invention
because they help lift the gas oil from the atmospheric resid in stripper 11.
Depending on the characteristics of the resid in line 2, the amount of residua
added to line 2, and present in line 10, after addition of one or more light
materials
thereto as aforesaid, can be less than 20 wt. % and still be within the scope
of the
invention.
It can be seen, that, with this invention, the distillates that are removed by
the
atmospheric tower are preserved for separate use such as for diesel fuel and
kerosene, and a novel combination of atmospheric resid and lighter material in
stripper 11 is used to form additional cracking feedstock from atmospheric
resid that
would otherwise be lost for this purpose.
EXAMPLE
An atmospheric residuum characterized as Skikda atmospheric resid from
Algeria is mixed in equal parts by weight with natural gasoline, and fed
directly into
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CA 02683943 2009-10-14
WO 2008/143744
PCT/US2008/004875
the preheat section 3 of the convection section of pyrolysis furnace 1. This
feed is at
260F and 80 psig. In this convection section this residuum feed is preheated
to
about 690F at about 60 psig, and then passed into vaporization unit 11 wherein
a
mixture of gasoline, naphtha and gas oil gases at about 690F and 60 psig are
separated in zone 12 of that unit.
The separated gases are removed from zone 12 for transfer to the convection
preheat sub-zone 27 of the same furnace. The preheated stream is then passed
to
radiant section 29.
The hydrocarbon liquid remaining from residuum feed 2, after separation from
io accompanying hydrocarbon gases aforesaid, is transferred to lower
section 13 and
allowed to fall downwardly in that section toward the bottom thereof.
Preheated
steam 21 at about 1,050F is introduced near the bottom of zone 13 to give a
steam
to hydrocarbon ratio in section 13 of about 1. The falling liquid droplets are
in
counter current flow with the steam that is rising from the bottom of zone 13
toward
the top thereof. With respect to the liquid falling downwardly in zone 13, the
steam
to liquid hydrocarbon ratio increases from the top to bottom of section 19.
A mixture of steam and vapor 17 which contains the gas oil fraction at about
760F is withdrawn from near the top of zone 13 and mixed with the gases
earlier
removed from zone 12 via line 14 to form a composite steam/hydrocarbon vapor
stream 25 containing about 0.4 pounds of steam per pound of hydrocarbon
present.
This composite stream is preheated in sub-zone 27 to about 1,000F at less than
about 50 psig, and then passed into radiant firebox 29 for cracking at a
temperature
in the range of 1,400 F to 1,550 F.
Bottoms product 26 of unit 11 is removed at a temperature of about 900 F,
and pressure of about 60 psig, and passed to the downstream processing
equipment
to produce fuel oil therefrom.
