U.S. patent application number 11/803905 was filed with the patent office on 2008-11-20 for hydrocarbon thermal cracking using atmospheric residuum.
Invention is credited to Donald H. Powers.
Application Number | 20080283445 11/803905 |
Document ID | / |
Family ID | 39938364 |
Filed Date | 2008-11-20 |
United States Patent
Application |
20080283445 |
Kind Code |
A1 |
Powers; Donald H. |
November 20, 2008 |
Hydrocarbon thermal cracking using atmospheric residuum
Abstract
A method for thermally cracking a hydrocarbonaceous feed wherein
the feed is first gasified in a vaporization unit. A significant
component of the feed is residua from the atmospheric thermal
distillation of crude oil.
Inventors: |
Powers; Donald H.; (Houston,
TX) |
Correspondence
Address: |
LyondellBasell Industries
3801 WEST CHESTER PIKE
NEWTOWN SQUARE
PA
19073
US
|
Family ID: |
39938364 |
Appl. No.: |
11/803905 |
Filed: |
May 16, 2007 |
Current U.S.
Class: |
208/85 ;
208/106 |
Current CPC
Class: |
C10G 9/02 20130101 |
Class at
Publication: |
208/85 ;
208/106 |
International
Class: |
C10G 1/00 20060101
C10G001/00 |
Claims
1. In a method for thermally cracking a hydrocarbonaceous feed in
at least one cracking furnace wherein said feed is first subjected
to a vaporization step in which at least a preponderance of said
feed is vaporized and the vapors thus formed are passed to said at
least one cracking furnace as the feed for said furnace, the
improvement comprising providing cracking feed composed in
significant amount of at least one residuum formed from the
atmospheric thermal distillation of crude oil, and subjecting said
atmospheric residuum feed to said vaporization step.
2. The method of claim 1 wherein said vaporization step employs at
least first and second vaporization zones, said first vaporization
zone receives said residuum feed and separates gaseous materials
associated with said residuum feed as received and any additional
gaseous materials formed in said first vaporization zone, said
separated gaseous materials are passed from said first vaporization
zone to said at least one cracking furnace as feed therefore, said
second vaporization zone receives from said first vaporization zone
liquid residuum feed that was not vaporized in said first
vaporization zone and subjects said liquid residuum feed to at
least one of heating and mild cracking in said second vaporization
zone until a significant amount of said liquid residuum feed in
said second zone is vaporized, and the gaseous materials formed in
said second zone are removed therefrom and passed to said at least
one cracking furnace as feed therefore.
3. The method of claim 2 wherein said gaseous materials from said
first and second vaporization zones are passed to the same cracking
furnace.
4. The method of claim 1 wherein said atmospheric residuum feed is
composed of a mixture of at least two differing residua.
5. The method of claim 1 wherein said atmospheric residuum feed has
a boiling range of from about 600 F to its boiling end point.
6. The method of claim 1 wherein there is added to said residuum
feed to said vaporization step at least one of light gasoline, at
least one naphtha fraction, natural gasoline, and condensate.
7. The method of claim 6 wherein said light gasoline boils in the
range of from about the boiling point of pentane to about 158 F,
said naphtha fraction boils in the range of from about 158 to about
350 F, and said condensate boils in a range of from about 100 to
about 650 F.
8. The method of claim 6 wherein said naphtha fraction is composed
of at least one of light naphtha, medium naphtha, and heavy
naphtha.
9. The method of claim 8 wherein said light naphtha boils in the
range of from about 158 to about 212 F, said medium naphtha boils
in the range of from about 212 to about 302 F, and said heavy
naphtha boils in the range of from about 302 to about 350 F.
10. The method of claim 1 wherein said cracking feed contains at
least about 20 wt. % atmospheric residuum based on the total weight
of said feed.
Description
BACKGROUND OF INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Prior Art
[0004] 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.
[0005] 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 (.degree. 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 1,550 F. Thermal cracking is accomplished without
the aid of any catalyst.
[0006] 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.
[0007] 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, and can be aliphatic, alicyclics, and/or aromatic.
The cracked gas can also contain significant amounts of molecular
hydrogen (hydrogen).
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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 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.
[0013] 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. %), 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.
[0014] 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.
[0015] 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.
[0016] 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 650 F. 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).
[0017] 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 400 F.
[0018] 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 650 F. They are, individually and collectively,
referred to herein as "distillate," "distillates," or "middle
distillates." Distillate compositions can have a boiling point
lower than 350 F and/or higher than 650 F, and such distillates are
included in the 350-650 F range aforesaid, and in this invention.
Atmospheric residuum (resid or residua) typically boils at a
temperature of from about 650 F 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
1,000 F up to its end boiling point where only non-boiling entities
remain.
[0019] 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.
[0020] Recently, U.S. Pat. No. 6,743,961 (hereafter "USP '961")
issued to Donald H. Powers. This patent relates to cracking whole
crude oil by employing a 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 vaporized under the
conditions employed in the process, see column 8, lines 60-64 of
that patent.
[0021] Still more recently, U.S. Pat. No. 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.
[0022] U.S. Pat. No. 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.
[0023] The disclosures of the foregoing patents, in their entirety,
are incorporated herein by reference.
[0024] U.S. patent application Ser. No. 11/219,166, filed Sep. 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.
[0025] U.S. patent application Ser. No. 11/365,212, filed Mar. 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.
[0026] U.S. patent application Ser. No. 11/584,722, filed Oct. 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.
[0027] During periods of increased gasoline demand the gasoline
supply (pool) can be increased by subjecting various crude oil
fractions, including distillates, to various refinery catalytic
cracking processes such as fluid catalytic cracking. Thus, the
quantity of gasoline/naptha 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.
[0028] 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
[0029] 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
[0030] FIG. 1 shows a simplified flow sheet for the whole crude
oil/condensate cracking process described hereinabove.
DETAILED DESCRIPTION OF THE INVENTION
[0031] 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 contain other elements such as oxygen, sulfur, nitrogen,
metals, inorganic salts, and the like, even in significant
amounts.
[0032] 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.
[0033] Coke, as used herein, means a high molecular weight
carbonaceous solid, and includes compounds formed from the
condensation of polynuclear aromatics.
[0034] 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.
[0035] 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.
[0036] 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 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.
[0037] 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.
[0038] Cracking furnaces typically have rectangular fireboxes with
upright tubes centrally located between radiant refractory walls.
The tubes are supported from their top.
[0039] 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
of natural draft or induced draft fans.
[0040] 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 to from about 1,400.degree. F. to about
1,550.degree. F. and thereby subjected to severe cracking, and coke
formation.
[0041] The initially empty radiant coil is, therefore, a fired
tubular chemical reactor. Hydrocarbon feed to the furnace is
preheated to from about 900.degree. F. to about 1,000.degree. F. in
the convection section by convectional heating from the flue gas
from the radiant 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.
[0042] The cracked gaseous hydrocarbons leaving the radiant section
are rapidly reduced in temperature to prevent destruction of the
cracking pattern. Cooling of the 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.
[0043] 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.
[0044] FIG. 1 shows one embodiment of a cracking process that uses
whole crude oil and/or condensate as the dominant (primary) furnace
feed. FIG. 1 is very diagrammatic for sake of simplicity and
brevity since, as discussed above, actual furnaces are complex
structures.
[0045] FIG. 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.
[0046] 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 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] Note that this invention does not apply to vacuum resid or
catalytic cracking.
[0056] For purposes of this invention, feed 2 can enter furnace 1
at a temperature of from about ambient up to about 300 F 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 700 F at a
pressure of from atmospheric to 100 psig.
[0057] Stream 14 can be essentially all hydrocarbon vapor formed
from feed 2 and is at a temperature of from about ambient to about
700 F at a pressure of from atmospheric to 100 psig.
[0058] 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 700 F at a pressure of
from slightly above atmospheric up to about 100 psig (hereafter
"atmospheric to 100 psig").
[0059] 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.
[0060] 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.
[0061] 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 650 F, preferably
from about 900 to about 1,200 F at from atmospheric to 100 psig.
Such gases will, for sake of simplicity, hereafter be referred to
in terms of steam alone.
[0062] Stream 17 can be a mixture of steam and hydrocarbon vapor
that has a boiling point lower than about 1,200 F. Stream 17 can be
at a temperature of from about 600 to about 800 F at a pressure of
from atmospheric to 100 psig.
[0063] 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 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.
[0064] 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 600 F to the boiling end point where
only non-boiling entities remain.
[0065] Pursuant to this invention, light materials such as light
gasoline, naphtha, and gas oils boiling lighter (lower) than about
1,000 F, 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
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.
[0066] 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
1,000 F and a heavier fraction boiling in a temperature range of
from about 1,000 F 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.
[0067] 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 FIG.
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.
[0068] 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.
[0069] 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 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 158 F. Full range naphtha,
which includes light, medium, and heavy naphtha fractions, can have
a boiling range of from about 158 to about 350 F. The boiling
ranges for the light, medium, and heavy naphtha fractions can be,
respectively, from about 158 to about 212 F, from about 212 to
about 302 F, and from about 302 to about 350 F.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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 spirit of this
invention.
[0074] 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
[0075] An atmospheric residuum characterized as Skikda atmospheric
resid from Algeria is mixed in equal parts by weight with natural
gasoline, and fed directly into the preheat section 3 of the
convection section of pyrolysis furnace 1. This feed is at 260 F
and 80 psig. In this convection section this residuum feed is
preheated to about 690 F at about 60 psig, and then passed into
vaporization unit 11 wherein a mixture of gasoline, naphtha and gas
oil gases at about 690 F and 60 psig are separated in zone 12 of
that unit.
[0076] 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.
[0077] The hydrocarbon liquid remaining from residuum feed 2, after
separation from 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
1,050 F 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.
[0078] A mixture of steam and vapor 17 which contains the gas oil
fraction at about 760 F 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 1,000 F at
less than about 50 psig, and then passed into radiant firebox 29
for cracking at a temperature in the range of 1,400.degree. F. to
1,550.degree. F.
[0079] 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.
* * * * *