U.S. patent application number 10/383960 was filed with the patent office on 2004-09-09 for coker operation without recycle.
This patent application is currently assigned to Chevron U.S.A. Inc.. Invention is credited to Bell, Robert V., Klasnich, Steve.
Application Number | 20040173504 10/383960 |
Document ID | / |
Family ID | 32927168 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040173504 |
Kind Code |
A1 |
Klasnich, Steve ; et
al. |
September 9, 2004 |
Coker operation without recycle
Abstract
A process for coking a heavy oil feedstock with elimination of
recycle is disclosed. In a preferred embodiment, heavy hydrocarbon
feed is directly passed to the coking vessels, coker overhead
vapors are combined and passed directly to a fractionator and
fractionator bottoms are recovered as product for further
processing in other refining systems. Distillate coker product is
not used to reduce the heavy hydrocarbon feed viscosity or to
manage coke fouling in the coker furnace.
Inventors: |
Klasnich, Steve; (Martinez,
CA) ; Bell, Robert V.; (Mobile, AL) |
Correspondence
Address: |
CHEVRON TEXACO CORPORATION
P.O. BOX 6006
SAN RAMON
CA
94583-0806
US
|
Assignee: |
Chevron U.S.A. Inc.
|
Family ID: |
32927168 |
Appl. No.: |
10/383960 |
Filed: |
March 7, 2003 |
Current U.S.
Class: |
208/131 |
Current CPC
Class: |
C10B 55/00 20130101;
C10G 9/005 20130101 |
Class at
Publication: |
208/131 |
International
Class: |
C10G 009/14 |
Claims
What is claimed:
1. A method for coking a heavy hydrocarbon feed comprising: (a)
directly feeding the heavy hydrocarbon feed through at least one
pre-heater to a coking furnace; (b) heating the feed in the coking
furnace to coking temperatures; (c) alternately feeding the heated
feed directly to a first coke vessel and a second coke vessel to
form a first hydrocarbon vapor product and a first coke
accumulation in the first vessel and a second coke accumulation in
the second vessel.
2. The method of claim 1 further comprising, alternately steam
stripping the first and the second coke accumulations to form a
second hydrocarbon vapor product and feeding the second hydrocarbon
vapor product to a blowdown system to form a first condensate.
3. The method of claim 2 further comprising, alternately emptying
the first coke accumulation from the first coke vessel and the
second coke accumulation from the second vessel and alternately
feeding the second hydrocarbon vapor through the empty vessel to a
condensate drum to form a second condensate.
4. The method of claim 3 further comprising, combining the first
condensate, the second condensate and the first hydrocarbon vapor
product to form a quenched combination and feeding the quenched
combination to the fractionator to form one or more hydrocarbon
products.
5. The method of claim 1 wherein the heavy hydrocarbon feed is
heated to temperatures in the range of about 850.degree. F. to
about 1100.degree. F.
6. The method of claim 5 wherein the heavy hydrocarbon feed is
heated to temperatures in the range of about 900.degree. F. to
about 975.degree. F.
7. The method of claim 4 wherein the hydrocarbon products are
selected from the group consisting of coker fractionator bottoms,
heavy coker gas oil, light coker gas oil, a jet fuel cut, light
naphtha, heavy naphtha and process gas.
8. The method of claim 7 wherein the heavy coker gas oil product
has a boiling range between about 650.degree. F. and 1150.degree.
F.;
9. The method of claim 7 wherein the light coker gas oil product
has a boiling range between about 350.degree. F. and 750.degree.
F.;
10. The method of claim 7 wherein the jet fuel product has a
boiling range between about 250.degree. F. and 570.degree. F.;
11. The method of claim 7 wherein the heavy naphtha product has a
boiling range between about 180.degree. F. and 400.degree. F.;
12. The method of claim 7 wherein the light naphtha product has a
boiling range between about 50.degree. F. and 250.degree. F.;
13. The method of claim 7 wherein the process gas product has a
boiling range less than about 100.degree. F.;
14. The method of claim 7 wherein the coker fractionator bottoms
product has a boiling range above about 650.degree. F.
15. The method of claim 1 wherein step (a) further comprises
feeding the heavy hydrocarbon feed through a surge drum before
feeding to the coker furnace.
16. The method of claim 7 further comprising feeding the coker
fractionator bottoms product to other refining processes.
17. The method of claim 7 further comprising feeding at least a
portion of the heavy coker gas oil to the fractionator.
18. The method of claim 18 further comprising feeding the entire
stream of heavy coker gas oil to the fractionator.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of heavy
hydrocarbon refining by the delayed coking process.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an improved method for delayed
coking and involves elimination of all recycle, in particular,
heavy coker gas oil and fractionator bottoms recycle to improve
heavy hydrocarbon feed throughput. Delayed coking is a well-known
oil refining process that is used to convert very heavy hydrocarbon
feed stocks into useful liquid fuel products. In this process the
heavy hydrocarbon feed is heated rapidly to cracking temperatures
at elevated pressure and fed into a coke drum. The heated feed
drops in pressure as it enters the coke drum causing lower boiling
range components to vaporize. The larger molecules in the heated
feed rapidly crack into lower boiling volatile components, leaving
behind a solid carbonaceous material known in the art as petroleum
coke or, simply, coke.
[0003] The volatile component, a vaporous hydrocarbon mixture, is
fed overhead to a fractionator vessel which serves the dual purpose
of separating (fractioning) the volatile component into coker
products and serving as a coker feed surge drum. Coker system
fractionator products typically include process gas, light and
heavy naphtha, light and heavy gas oil, and fractionator bottoms.
Heavier components of the vapor mixture that condense before
reaching the fractionator trays fall to the bottom of the
fractionator where they are mixed with incoming fresh feed. These
heavier components that condense into the fresh feed in the
fractionator are known as "natural recycle". Lighter components
condense on the fractionator trays, are collected and fed to other
refining systems for further processing. As this process continues,
coke accumulates in the active drum until it is filled to a safe
level, at which time the heated feed is diverted to a "sister" coke
drum to continue the above process. The full drum is isolated from
the coking system and the accumulated coke is removed to prepare
the drum for repetition of the above described cycle.
[0004] A conventional coking process is illustrated in FIG. 1. This
conventional process utilizes two coke drums for coking a heavy
feedstock, and a fractionator for separating a vapor product from
the coke drums and for recovering one or more liquid distillate
products. Fresh coker feedstock from line 10 passes through heat
exchangers 12 and 14, where it is preheated. The fresh coker
feedstock may be unstable to heating, and, as such, may cause
deposits to form on the heat exchange surfaces during heating. To
minimize this coke deposition, a portion of coker distillate
product 86 is combined with the fresh feed through line 88 prior to
preheating in heat exchanger 12. Amounts of distillate are
generally from about 2 to about 50 parts by volume of distillate
per 100 parts of fresh feed, and preferably about 5 to about 30
parts for most cases. The preheated feed is then introduced through
line 16 to the bottom of coker fractionator 22, where it is
combined with the bottoms from the fractionator. Alternatively, a
portion of the heavy coker gas oil 18 may be combined with the
fresh feed 10 through line 80.
[0005] Fractionator bottoms 26 are heated in furnace 28 and passed
from line 30 via valve 90 and into either coke drum 36 through
conduit 32 or into coke drum 38 through conduit 34. Feed to the
coke drum 36 is a mixture of fresh feed 10, heavy gas oil recycle
product 20, distillate product recycle 88, and bottoms product from
fractionator 22. The heavy coker gas oil recycle 20 may be combined
with the fractionator bottoms product either internal to the
fractionator 22 (as shown), or externally into line 26. The heavy
gas oil comes from several sources. As shown in FIG. 1, heavy coker
gas oil is withdrawn from fractionator 22 via line 18, and a
portion of the heavy gas oil is returned to the fractionator via
line 20 where it is utilized to knock down entrained material and
condense the heavier components of the vapor in fractionator 22.
Heavy gas oil 18 withdrawn from the fractionator 22 is also used to
quench the vapor overhead product 50 through line 24 and to
condense the heavier boiling material in the overhead product
50.
[0006] The feed for coking is thus passed from the fractionator
through furnace 28 for heating the feedstock to coking temperatures
and from there, alternatively, to the coke drums. The mixture 26 is
heated in furnace 28 to temperatures normally in the range of about
850.degree. F. to 1100.degree. F., and preferably in the range of
900.degree. F. to 975.degree. F. A furnace that heats the mixture
rapidly to such temperatures, such as a pipe still, is normally
used. The mixture exits the furnace through line 30 at
substantially the above-indicated temperatures and is introduced
into the bottom of coke drum 36. The mixture is charged to the coke
drum at pressures usually ranging between about 20 to 200 psig,
though higher pressures may be used if desired. The coke drum is
insulated and may also be heated, such as by introduction of hot
gas and vapor from a sister vessel into the drum, so as to maintain
the drum's contents at a temperature in the range of about
800.degree. F. to about 1200.degree. F., more usually 750.degree.
F. to 950.degree. F. Inside the drum the heavy hydrocarbon in the
mixture thermally cracks to form cracked vapors and coke.
[0007] The vapors are continuously removed overhead from the drum
through line 40. Coke accumulates in the drum until it reaches a
predetermined safe level at which time the feed to the drum is shut
off and switched to the alternate coke drum 38. The operation of
drum 38 is identical to that of drum 36. This switching permits
drum 36 to be taken out of service, opened, and the accumulated
coke removed therefrom using conventional techniques. The
hydrocarbon vapors that are taken overhead from the coke drum(s)
are carried by line 50 to a fractionator 22. Even though the coker
vessels 36 and 38 are operated alternately the overhead hydrocarbon
vapor products flow continuously via line 50 to the fractionator
22.
[0008] After the Coke drum 36 is filled with coke, the feed 30 is
redirected to the alternate coke drum 38, steam 92 is immediately
introduced to drum 36 to strip out any remaining hydrocarbon
liquid. The drum is stripped with steam and the resulting vapor is
fed to the fractionator through line 50 for a period of time, then
the vapors are redirected to the blowdown system 48 via line 44.
Heavier oils stripped out of the drum to the blowdown system are
condensed. This condensed material is then pumped into the feed
stream 16 through line 58 on the way to the fractionator 22 and,
thus, represents a source of recycle.
[0009] Coke in drum 36 is removed by a drilling operation. This
process is well known, and does not require detailed explanation
here. After drum 36 is drilled and is empty of coke, the drum is
preheated, using product vapor from drum 38. This may be
accomplished by diverting a portion of the vapor from drum 38
through drum 36 in a reverse direction through line 96. The
resulting flow of condensed liquid and uncondensed vapor exiting
through the bottom of the drum through line 52 is routed to a coke
condensate drum 54. Vapor leaves the coke condensate drum 54
through a balance line 56 with the fractionator 22. Liquid 98 is
routed from the coke condensate drum 54 into the blowdown system
48, and from there via line 58 into the feed stream 16. This
represents another source of recycle.
[0010] Likewise, after the Coke drum 38 is filled with coke, the
feed 30 is redirected to the first coke drum 36. Steam 94 is
immediately introduced to drum 38 in order to strip out any
remaining hydrocarbon liquid. The drum is stripped with steam to
the fractionator through line 50 via line 42 for a period of time
and then the drum vapors are redirected to the blowdown system 48
via line 46. Preheating drum 38 using product vapor from drum 36,
with condensed liquid and uncondensed vapor out of the drum bottom
of vessel 38 being routed to the coke condensate drum 54 is
accomplished in the way described above for drum 36. However, the
product vapor stream (corresponding to line 96) and the condensed
liquid and uncondensed vapor stream (corresponding to line 52) are
not illustrated in FIG. 1.
[0011] Products recovered from fractionator 22 include heavy
Naphtha 86, light naphtha 76 and a process gas overhead product 70.
In FIG. 1, a distillate stream in line 78 is recovered from
fractionation, and stripped using stripper 82. The bottoms from the
stripper are cooled in exchanger 12 and recovered as distillate
product 86. A portion of the distillate product is combined with
fresh feed via line 88, using the method previously described. The
overhead from stripper 82 is returned as reflux to fractionator 22
via line 84. A fractionator overhead product 62 is cooled in
exchanger 64 and passed, via line 66 to separation zone 68. A
portion of separation zone liquid 72 is returned as reflux to the
fractionator 22 via line 74, and at least a portion of the
remainder recovered as light naphtha through line 76. A process gas
overhead product 70 is also recovered from separation zone 68.
[0012] As is evident from the above description, conventional
coking systems use substantial amounts of "recycle" in the coker
feed, principally to reduce viscosity of the feed and minimize
fouling of the coker furnace. However, recycle in the feed
effectively reduces coking capacity and the production of valued
light hydrocarbons; thus, there is a need in the industry to
improve the coking process to enhance recovery of valued, light
hydrocarbons. In this regard, the process taught in U.S. Pat. No.
4,394,250 includes adding a catalyst and hydrogen to the coker feed
to facilitate production of high amounts of useful light products.
However, '250 also includes recycle of the bottoms from the
fractionator to the coker feed.
[0013] Similarly, U.S. Pat. No. 4,455,219 describes a conventional
coking process in which feed to the coker includes fresh feed,
heavy fractionator bottoms recycle and coker gas oil added as a
diluent to minimize recycle of heavier fractions. As noted above,
the heavy coker gas oil is a cracked product from the coking
reactions in the coke drum. This gas oil can be processed elsewhere
in the refinery for the production of useful liquid products; it
does not require additional reaction in the coker. Thus, the
addition of the heavy coker gas oil in the coker feed consumes coke
drum capacity which is more economically utilized for the raw,
heavy feed which must be coked.
[0014] Whereas, the '219 patent does not resolve the problem of
heavy coker gas oil in the coker feed U.S. Pat. No. 4,518,487
directly addresses the problem and purports to eliminate
fractionator bottom and heavy coker gas oil recycle. In '487, feed
to the coker furnace is not combined with fractionator bottoms;
instead a higher boiling range distillate is combined with the
fresh feed in amounts necessary to prevent fouling of the furnace
tubes. This higher boiling range distillate is drawn off the coker
product stream and, thus represents recycle, although not of heavy
fractionator bottoms.
[0015] The present invention further improves upon '487 and
provides for complete elimination of all coker product recycle and
thereby increases throughput of feed to the delayed coker process
resulting in greater and mores economic recovery of valued light
hydrocarbon products. Coker furnace tube fouling is minimized and
managed by non-recycle methods, such as by on-line steam
spalling.
SUMMARY OF THE INVENTION
[0016] The present invention is directed to a process for coking a
heavy oil feedstock with elimination of all of recycle.
Conventional coking processes require recycle of lighter distillate
product streams to reduce the viscosity of feed streams, to reduce
carbon deposition in heating zones, and to facilitate recovery of
liquids from high temperature streams exiting the coking vessel or
within fractionation zones. However, adding recycle streams to the
coking vessel effectively reduces the amount of heavy oil feed
stocks that may be processed for coke production.
[0017] The process of the present invention provides for a greater
throughput of heavy oil feedstock in a coking process. Among other
factors, the present invention is based on the discovery of methods
for increasing recovery of products from a coking process rather
than recycling the products back to the process, as in the prior
art. In one preferred embodiment, the entire bottoms product from
the coking fractionator is recovered as a product stream rather
than being recycled to the coking process for additional coking. In
another preferred embodiment, fresh feed is first passed through a
surge drum then directly to the coking drums without passing
through the coker fractionator. In another preferred embodiment,
liquid products from the coking process are passed directly to the
fractionator rather than being blended with the fresh coker feed.
Thus, the most important and unique aspects of the present
invention include: (1) directly passing heavy hydrocarbon feed to
coking vessels; (2) combining coker overhead vapors; (3) passing
the overhead vapor combination directly to a fractionator and (4)
recovering fractionator bottoms as product for further processing
in other refining systems.
[0018] Accordingly, the present invention provides a method for
coking a heavy hydrocarbon feed comprising:
[0019] (a) directly feeding the heavy hydrocarbon feed through at
least one pre-heater to a coking furnace;
[0020] (b) heating the feed in the coking furnace to coking
temperatures;
[0021] (c) alternately feeding the heated feed directly to a first
coke vessel and a second coke vessel to form a first hydrocarbon
vapor product and a first coke accumulation in the first vessel and
a second coke accumulation in the second vessel.
[0022] Another aspect of the invention comprises combining coker
overhead vapor and liquid products from a blowdown system and
condenser in a quench step and feeding the combination to the
fractionator for separation into coker products. In a preferred
embodiment, the product stream recovered from the coker
fractionator includes coker fractionator bottoms, heavy coker gas
oil, light coker gas oil, a jet fuel cut, light naphtha, heavy
naphtha, and process gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 illustrates the prior art coking process and depicts
various points in the prior art process where product recycle takes
place.
[0024] FIG. 2 illustrates a specific embodiment of the present
invention and depicts the elimination of coker product recycle.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Suitable hydrocarbon feed stocks for delayed coking are
described in the art. The feedstock may be derived from petroleum,
shale, coal, tar and/or other hydrocarbon sources. It is typically
heavy, low-grade oil such as heavy virgin crude, reduced crude,
topped crude, residua from refining processes such as thermal or
catalytic cracking processes or blends of such stocks. These feed
stocks may be hydrotreated, if desired, before being fed to the
coking process to remove sulfur, metals, and other
contaminants.
[0026] One embodiment of the present invention is illustrated in
FIG. 2. In the present invention, a surge drum or tank 104 is for
temporary in-line storage of the fresh coker feed 110 and serves
the purpose of absorbing or minimizing sudden changes in the
pressure or flow rate of feed into the coking system. Surge drum
104 has a capacity of between 0.1 and 100 minutes of feed
throughput, based on the design feedrate of the coking process. The
fresh feed 110 is fed to at least one heat exchanger 114. Then, in
contrast to the conventional process, the preheated feed is passed
through the surge drum 104 and line 126 to furnace 128 and heated
therein to coking temperatures. From the furnace 128, the heated
feed is passed via line 130, through valve 190 and into either coke
drum 136 through conduit 132 or into coke drum 138 through conduit
134. In the preferred process of the present invention, the fresh
feed is not passed to the fractionation column 122, nor does it
contain bottoms product from the fractionator 122. More preferably,
the heated coker feedstock 130 does not contain any fractionator
bottoms product, heavy coker gas oil or any coker product
recycle.
[0027] Furnace 128, typically a pipe still type furnace, heats the
feedstock mixture to coking temperatures, normally in the range of
about 850.degree. F. to 1100.degree. F., and preferably in the
range of 900.degree. F. to 975.degree. F. The mixture exits the
furnace 128 through line 130 and is alternately introduced into the
bottom of either coke drum 136 or 138, at substantially the
above-indicated temperatures and at pressures usually ranging
between about 20 to 200 psig, though higher pressures may be used
if desired. The coke drum is insulated and may also be pre-heated,
such as by introduction of hot gas and vapor from the sister vessel
into the drum, so as to maintain the drum's contents at a
temperature in the range of about 800.degree. F. to about
1200.degree. F., more usually 750.degree. F. to 950.degree. F.
Inside the drum, the heavy hydrocarbon in the mixture thermally
cracks to form cracked hydrocarbon vapors and coke.
[0028] The vapors are continuously removed overhead from the active
drum through either line 140 or 142. The vapors that are taken
overhead from the coke drum(s) are carried by line 150 to a
fractionator 122. Coke accumulates in the active drum until it
reaches a predetermined level at which time the feed to the drum is
shut off and switched to the second, sister coke drum 138. The
operation of drum 138 is identical to that of drum 136. This
switching permits drum 136 to be taken out of service, opened, and
the accumulated coke removed therefrom using conventional
techniques.
[0029] After the coke drum 136 is filled with coke, and the feed
130 is redirected to the second coke drum 138, steam is immediately
introduced through 192 to remove any remaining hydrocarbon liquid.
The steam-stripped liquid is passed to fractionator 122 through
line 150 for a period of time and then the drum vapors are
redirected to the blowdown system 148 via line 144. Heavier oils
stripped out of the drum to the blowdown system are condensed.
While the prior art processes use coker gas oil recovered from the
fractionator for quenching the vapors, the present process provides
that the oil accumulation in the blowdown system 148 is injected
into the coke drum overhead vapor 150 as quench, thus delivering
this oil to the fractionator 122 for further distillation and
product recovery. This arrangement provides the refiner the
capability of increasing the quench 158 of the coke drum overhead
vapor 150 and reducing the flash zone temperature in the
fractionator 122 without increasing recycle to the coking zones.
Increased quench of the overhead vapor will further reduce coking
of the products streams in the fractionator, thus extending
fractionator run time between turnarounds.
[0030] After drum 136 is drilled and is empty of coke, the drum is
preheated, using product vapor from drum 138. This may be
accomplished by diverting a portion of the vapor from drum 138
through line 196 into drum 136. The resulting flow of condensed
liquid and uncondensed vapor out the drum bottom through line 152
is routed to coke condensate drum 154. Vapor leaves the coke
condensate drum 154 through a balance line 156 and into
fractionator 122. Liquid 198 is routed from the coke condensate
drum 154 into the blowdown system 148, from where it is combined
with other liquid products, e.g. stream 144, for quenching the
vapors in stream 150 through stream 158.
[0031] Likewise, after the Coke drum 138 is filled with coke, the
feed 130 is redirected to the first coke drum 136. Steam 194 is
immediately introduced to drum 138. The liquid produced during the
steam stripping operation is passed to fractionator 122 through
lines 142 and 150 for a period of time and then the drum vapors are
redirected to the blowdown system 148 through line 146. Preheating
drum 138 using product vapor from drum 136, with condensed liquid
and uncondensed vapor out of the drum bottom of vessel 138 being
routed to the coke condensate drum 154 is accomplished in the way
described above for drum 136.
[0032] Products recovered from fractionator 122 may include a
bottoms product 108, a heavy coker gas oil product 118, a light gas
oil product 119, a jet fuel distillate 120 heavy naphtha 121, a
light naphtha 176 and a process gas overhead product 170. The heavy
coker gas oil is typically returned to the fractionator 122 via
line 116 to cool the fractionator bottoms product and reduce coke
formation in the fractionator 122. Boiling ranges of the various
product fractions are broadly defined in Table I, below.
1 Product Boiling range Bottoms Product 108 >650.degree. F.
Heavy Coker Gas Oil 118 650.degree. F.-1150.degree. F. Light Coker
Gas Oil 119 350.degree. F.-750.degree. F. Jet Fuel Distillate 120
250.degree. F.-570.degree. F. Heavy Naphtha Product 121 180.degree.
F.-400.degree. F. Light Naphtha Product 176 50.degree.
F.-250.degree. F. Process Gas 170 <100.degree. F.
[0033] The above description of preferred embodiments of the
invention is intended to be descriptive and not limiting as to the
scope of the invention, which is defined by the following
claims.
* * * * *