US007977524B2

(12) United States Patent (10) Patent No.: US 7,977,524 B2

McCoy et al. (45) Date of Patent: Jul. 12, 2011
(54) PROCESS FOR DECOKING A FURNACE FOR (56) References Cited
CRACKINGA HYDROCARBON FEED
U.S. PATENT DOCUMENTS
(75) Inventors: James N. McCoy, Houston, TX (US); 5.536.390 A 7/1996 Herrmann
Richard Charles Stell, Houston, TX 2005/0261532 A1 11/2005 Stell et al.
(US) 2005/0277797 A1 12/2005 Maeda et al.
2006/0014992 A1 1/2006 Stell et al. ..................... 585,648
(73) Assignee: ExxonMobil Chemical Patents Inc., 2006/0249428 A1 11, 2006 Stell et al.
Houston, TX (US) FOREIGN PATENT DOCUMENTS
(*) Notice: Subject to any disclaimer, the term of this GB 1306962 12/1973
patent is extended or adjusted under 35 * cited by examiner
U.S.C. 154(b) by 563 days.
(21) Appl. No.: 11/953,307
y x- - - 9
Primary Examiner — Prem C Singh
(22) Filed: Dec. 10, 2007 (57) ABSTRACT
O O A process for decoking a convection section of a furnace for
(65) Prior Publication Data cracking a hydrocarbon feed, the furnace comprising a radi
US 2008/O2O7974 A1 Aug. 28, 2008 ant section having burners that generate radiant heat and hot
O O flue gas, and the convection section having at least one heat
Related U.S. Application Data exchange tube for conveying the hydrocarbon feed. The pro
(60) Provisional application No. 60/902,769, filed on Feb. cess includes the step of establishing a flue gas temperature
22, 2007. within the convection section of the furnace immediately
adjacent the at least one convection section heat exchange
(51) Int. Cl. tube so as to effect a film surface temperature of less than
CD7C 4/00 (2006.01) about 540°C. (about 1000°F.) within at least one convection
(52) U.S. Cl. ........ 585/652; 585/648; 422/105:422/188: section heat exchange tube, wherein said flue gas temperature
422/194; 208/48 Q: 208/85 establishing step is effective to decoke the at least one con
(58) Field of Classification Search .................. 585/648, vection section heat exchange tube. A process for cracking
585/652: 422/105, 188, 194: 208/48 Q. hydrocarbon feed in a furnace is also provided.
208/85
See application file for complete search history. 22 Claims, 1 Drawing Sheet

18 as Exhaust

Radiant
Section net
U.S. Patent Jul. 12, 2011 US 7977,524 B2

Fue (as Exhaust

of 2
3. to Radiant D
Section net.
Radiant
Section
 US 7,977,524 B2
1. 2
PROCESS FOR DECOKINGA FURNACE FOR appreciated, when the price of natural gas is high relative to
CRACKINGA HYDROCARBON FEED crude, gas cracking tends to be disadvantaged when com
pared with the cracking of Virgin crudes and/or condensates,
PRIORITY CLAIM or the distilled liquid products from those feeds. (e.g., naph
tha, kerosene, field natural gasoline, etc). In such an economic
This application claims priority to and the benefit of U.S. environment, it would be desirable to extend the range of
Ser. No. 60/902,769, filed Feb. 22, 2007. useful feedstocks to include liquid feedstocks that yield
higher levels of tar.
FIELD OF THE INVENTION Advantaged steam cracking feeds frequently contain
10 asphaltenes, which lay down as coke in the convection section
The present invention relates to the field of thermal crack of conventional pyrolysis furnaces. Contaminated conden
ing of hydrocarbons for the production of olefins, particularly sates and full range virgin gas oils (FRVGO) with up to 400
low molecular weight olefins such as ethylene. More particu ppm asphaltenes are typical of such advantaged feeds. How
larly this invention relates to the removal of coke deposits that ever, feeds with greater than 100 ppm asphaltenes cause the
form during such a thermal cracking process. 15 thickness of the coke layer to increase rapidly in part because
the coke produced by the asphaltenes typically is found
BACKGROUND OF THE INVENTION within a few rows of the heat exchange tubes of the convec
tion section. Since pressure drop is a strong function of tubing
Steam cracking, also referred to as pyrolysis, has long been diameter, a fast growing coke layer causes the convection
used to crack various hydrocarbon feedstocks into olefins, section pressure drop to increase rapidly. For example, a
preferably light olefins such as ethylene, propylene, and one-half inch layer of coke in a five inch diameter tube triples
butenes. Conventional steam cracking utilizes a pyrolysis the pressure drop across the tube, while the same one-half
furnace that has two main sections: a convection section and inch layer of coke in a three inch diameter tube increases the
a radiant section. The hydrocarbon feedstock typically enters pressure drop by nine times. As such, it would be desirable to
the convection section of the furnace as a liquid (except for 25 have an improved process for decoking a furnace for cracking
light feedstocks which enteras a vapor) wherein it is typically a hydrocarbon feed to facilitate the use of advantaged steam
heated and vaporized by indirect contact with hot flue gas cracking feeds.
from the radiant section and by direct contact with steam. The G.B. Patent No. 1,306,962 proposes a process for ther
vaporized feedstock and steam mixture is then introduced mally cracking hydrocarbons wherein on-stream decoking is
into the radiant section where the cracking takes place. The 30 employed. It is asserted that an advantage of more frequent
resulting products comprising olefins leave the pyrolysis fur decoking is that decoking will be accomplished more readily
nace for further downstream processing, including quench since the coke will not have had a chance to calcine over a
ing. long period. It is believed that the process proposed relates to
Olefin gas cracker Systems are normally designed to crack decoking a radiant section of a furnace, rather than dealing
ethane, propane and on occasion butane, but typically lack the 35 with the unique issues of convection section decoking.
flexibility to crack heavier liquid feedstocks, particularly U.S. Pat. No. 5,536,390 proposes the thermal decoking of
those that produce tarinamounts greater than one percent. As cracking gas coolers that operate with low gas pressure. This
gas feeds tend to produce little tar, primary, secondary, and is said to be accomplished by controlling the temperature of a
even tertiary transfer line exchangers (TLEs) are utilized to cleaning gas delivered to the cooler. The temperature control
recover energy through the generation of high pressure and 40 is said to be achieved by mixing a cleaning gas, which has
medium pressure steam, as the furnace effluent cools from the been heated in a cracking oven, with a stream of relatively
furnace outlet to the quench tower inlet. The process gas is cool cleaning gas upstream of the cooler.
normally then fed to a quench tower wherein the process gas U.S. Patent Publication No. 2006/0249428 proposes a pro
is further cooled by direct contacting with quench water. cess for steam cracking heavy hydrocarbon feedstocks con
Conventional steam cracking systems have been effective 45 taining non-volatile hydrocarbons. The process includes the
for cracking high-quality feedstocks which contain a large steps of heating the heavy hydrocarbon feedstock, mixing the
fraction of light Volatile hydrocarbons, such as gas oil and heavy hydrocarbon feedstock with a fluid and/or a primary
naphtha. However, Steam cracking economics sometimes dilution steam stream to form a mixture, flashing the mixture
favor cracking lower cost feedstocks containing resids Such to form a vapor phase and a liquid phase, and varying the
as, by way of non-limiting examples, atmospheric residue, 50 amount of the fluid and/or the primary dilution steam stream
e.g., atmospheric pipe still bottoms, and crude oil. Crude oil mixed with the heavy hydrocarbon feedstock in accordance
and atmospheric residue often contain high molecular with at least one selected operating parameter of the process,
weight, non-volatile components with boiling points in such as the temperature of the flash stream before entering the
excess of 595°C. (1100°F). The non-volatile components of flash drum.
these feedstocks lay down as coke in the convection section of 55 Despite these advances in the art, there is a need for an
conventional pyrolysis furnaces. Only very low levels of non improved process for decoking a furnace for cracking a
Volatile components can be tolerated in the convection sec hydrocarbon feed.
tion downstream of the point where the lighter components
have fully vaporized. Cracking heavier feeds. Such as kero SUMMARY OF THE INVENTION
senes and gas oils, produces large amounts oftar, which leads 60
to rapid coking in the radiant section of the furnace, often In one aspect, provided is a process for decoking a convec
requiring costly shutdowns for cleaning. tion section of a furnace for cracking a hydrocarbon feed, the
Steam crackers designed to operate on gaseous feedstocks, furnace comprising a radiant section having burners that gen
while limited in feedstock flexibility, require significantly erate radiant heat and hot flue gas, and the convection section
lower investment when compared to liquid feed crackers 65 having at least one heat exchange tube for conveying the
designed for naphtha and/or heavy feedstocks that produce hydrocarbon feed. The process includes the step of establish
higher amounts of tar and byproducts. However, as may be ing a flue gas temperature within the convection section of the
 US 7,977,524 B2
3 4
furnace immediately adjacent the at least one convection BRIEF DESCRIPTION OF THE DRAWING
section heat exchange tube so as to effect a film Surface
temperature of less than about 540° C. (1000 F) within at The invention is further explained in the description that
least one convection section heat exchange tube, wherein the follows with reference to the drawing illustrating, by way of
flue gas temperature establishing step is effective to decoke 5 non-limiting examples, various embodiments of the invention
the at least one convection section heat exchange tube. wherein:
In another aspect, the process further includes the steps of The FIGURE illustrates a schematic flow diagram of a
interrupting the flow of hydrocarbon feed to the at least one process as disclosed herein employed with a pyrolysis fur
convection section heat exchange tube; passing an air/steam nace, with particular emphasis on the convection section of
decoking feed mixture having a first air/steam ratio through 10 the furnace.
the at least one convection section heat exchange tube; and DETAILED DESCRIPTION OF PREFERRED
increasing the air/steam decoking feed mixture temperature EMBODIMENTS
entering the convection section to decoke an upper portion of
the convection section. 15 Various aspects will now be described with reference to
In yet another aspect, the process further includes the steps specific embodiments selected for purposes of illustration. It
of reducing the air/steam ratio of the air/steam decoking feed will be appreciated that the spirit and scope of the process and
mixture to a second air/steam ratio prior to the step of increas system disclosed herein is not limited to the selected embodi
ing the air/steam decoking feed mixture temperature entering ments. Moreover, it is to be noted that the FIGURE provided
the convection section. herein is not drawn to any particular proportion or scale, and
In still yet another aspect, the process is conducted at that many variations can be made to the illustrated embodi
intervals sufficient to prevent extensive crosslinking of the ments. Reference is now made to the figures, wherein like
coke. numerals are used to designate like parts throughout.
In a further aspect, the furnace further comprises a steam When an amount, concentration, or other value or param
Superheater capable of being Supplied with a stream of desu 25 eters is given as a list of upper preferable values and lower
perheater water, and the step of increasing the air/steam preferable values, this is to be understood as specifically
decoking feed mixture temperature entering the convection disclosing all ranges formed from any pair of an upper pre
section includes the step of reducing the Supply of desuper ferred value and a lower preferred value, regardless whether
heater water to the steam superheater ranges are separately disclosed.
In a yet further aspect, provided is a process for decoking a 30 Feedstocks that may be employed herein may be any feed
furnace for cracking a hydrocarbon feed, the furnace com stock adapted for cracking insofar as they may be cracked into
prising a radiant section having burners that generate radiant various olefins, and may contain heavy fractions such as
high-boiling fractions and evaporation residuum fractions.
heat and hot flue gas, and a convection section at least one Such feedstocks also include condensates and full range Vir
having heat exchange tube for conveying the hydrocarbon 35 gin gas oils (FRVGO). The liquid feedstocks that may be
feed, the convection section having upper, middle and lower employed herein include, not only those heavy fraction-con
portions thereof. The process includes the steps of taking the taining feedstocks adapted for cracking such as condensate,
at least one heat exchange tube off stream by halting the flow but also those having an appropriate proportion of high-qual
of hydrocarbon feed thereto; passing an air/steam decoking ity feed stocks such as naphtha blended thereto.
feed mixture having a first air/steam ratio through the at least 40 Referring now to the FIGURE, a pyrolysis furnace 10
one heat exchange tube, and increasing the air/steam decok includes a lower radiant section 12, an intermediate convec
ing feed mixture temperature entering the convection section tion section 14 and an upper flue gas exhaust section 16. In the
to decoke the upper portion of the convection section, radiant section 12, radiant burners (not shown) provide radi
wherein the process is conducted for a period of time effective antheat to a hydrocarbon feed to produce the desired products
for decoking the at least one heat exchange tube. 45 by thermal cracking of the feed. The burners generate hot gas
In a further aspect, provided is a process for cracking that flows upwardly through convection section 14 and then
hydrocarbon feed in a furnace, the furnace comprising a radi out of the furnace 10 through flue gas exhaust section 16.
ant section having burners that generate radiant heat and hot As shown in the FIGURE, hydrocarbon feed 18 enters an
flue gas, and a convection section having at least one heat upper portion of the convection section 14 where it is pre
exchange tube for conveying the hydrocarbon feed, the con 50 heated. The preheating of the hydrocarbon feed can take any
vection section having upper, middle and lower portions form known by those of ordinary skill in the art. Generally, the
thereof. The process includes the steps of preheating the heating includes indirect contact of the feed 18 in the upper
hydrocarbon feed in the heat exchange tubes in the convection convection section 14 of the furnace 10 with hot flue gases
from the radiant section 12 of the furnace 10. This can be
section by indirect heat exchange with the hot flue gas from 55 accomplished, by way of non-limiting example, by passing
the radiant section to provide preheated feed, heating the feed the feed 18through heat exchange tubes 20 located within the
mixture in the at least one heat exchange tube in the convec convection section 14 of the furnace 10. The preheated feed
tion section by indirect heat transfer with hot flue gas from the 22 has a temperature between about 95° C. to about 315° C.
radiant section to form a heated feed mixture, taking at least (about 200° F to about 600°F) or between about 150° C. to
one heat exchange tube off stream by halting the flow of 60 about 260° C. (about 300° F. to about 500 F) or between
hydrocarbon feed thereto, passing an air/steam decoking feed about 175° C. to about 260° C. (about 350° F to about 500°
mixture having a first air/steam ratio through the at least one F.).
heat exchange tube, and increasing the air/steam decoking After the preheated hydrocarbon feed 18 exits the convec
feed mixture temperature entering the convection section to tion section 14 at 22, water 24 and dilution steam 26 are added
decoke the upper portion of the convection section. 65 thereto to form a mixture. Water 24 is added to the preheated
These and other features will be apparent from the detailed feed 18 in an amount of from at least about 1% to 100% based
description taken with reference to accompanying drawings. on the total amount of water 24 and dilution steam 26 added
 US 7,977,524 B2
5 6
by weight oran amount of at least about 3% (i.e., about 3% to mixture reaches its dry point. Contaminated condensates and
about 100% water) based on water 24 and dilution steam 26 full range VGOs (FRVGO) with up to 400 ppm asphaltenes
by weight or at least about 10% or at least about 30%, based are typical advantaged feeds. However, feeds with greater
on water 24 and dilution steam 26 by weight. It is understood than 100 ppm asphaltenes cause the thickness of the coke
that, in accordance with one form, 100% water could be layer to increase rapidly in part because the asphaltenes lay
added to the hydrocarbon feed 18 such that no dilution steam down in only five convection rows of heat exchange tubes 20.
is added. The sum of the water flow and dilution steam flow For light feedstocks, this occurs in the upper portion of con
provides the total desired reaction Zone HO required to vection section 14.
achieve the desired hydrocarbon partial pressure. The rate of coking varies with the type of feed employed
As shown, water 24 may be added to the preheated feed 22 10
but nevertheless is continuous and, therefore, the coke builds
prior to addition of dilution steam 26. It is believed that this up and reduces the effective cross-sectional area of the tube,
order of addition may reduce undesirable pressure fluctua thereby necessitating higher pressures to maintain a constant
tions in the process stream originating from mixing the throughput. For example, a one-half inch layer of coke in a
hydrocarbon feed 22, water 24 and dilution steam 26. As may
be appreciated by those skilled in the art, such fluctuations are 15 five inch diameter tube triples the pressure drop across the
commonly referred to as a water-hammer or Steam-hammer. tube, while the same one-halfinch layer of coke in a three inch
While the addition of water 24 and dilution steam 26 to the diameter tube increases the pressure drop by nine times.
preheated hydrocarbon feed 22 could be accomplished using Since coke is an effective insulator, its formation on tube
any known mixing device, it is preferred to use a sparger walls must be accompanied by a sharp increase in furnace
assembly 28. Water 24 is preferably added in a first sparger tube temperature in order to maintain cracking efficiency.
30. As shown, first sparger 30 comprises an inner perforated High operating temperatures, however, result in a decrease in
conduit 32 surrounded by an outer conduit 34 so as to forman tube life, which limits the practical temperature that can be
annular flow space 36 between the inner and outer conduits 32 employed, as well as the ultimate conversion and yield.
and 34, respectively. The preheated hydrocarbon feed 22 During the Subsequent decoke, the steam/air mixture in the
flows through an annular flow space. Also preferably, water 25 upper convection section 14 may be too cold to burn coke. In
24 flows through the inner perforated conduit 32 and is this case, coke can only be removed by a cold shutdown of
injected into the preheated hydrocarbon feed 22 through the furnace 10, cutting off the convection return bends, hydro
openings (perforations) shown in inner conduit 32. blasting the coke, re-welding the return bends and finally
Dilution steam 26 may be added to the preheated hydro re-starting furnace 10. As may be appreciated by those skilled
carbon feed 22 in a second sparger 38. As shown, second 30 in the art, this is an expensive and time-consuming process.
sparger 38 includes an inner perforated conduit 40 sur To address these issues, in one form, a process for decoking
rounded by an outer conduit 42 so as to form an annular flow a convection section 14 of a furnace 10 for cracking a hydro
space 44 between the inner and outer conduits 40 and 42. carbon feed is provided, the furnace 10 including a radiant
respectively. The preheated hydrocarbon feed 22 to which the section 12 having burners (not shown) that generate radiant
water 24 has been added flows through the annular flow space 35 heat and hot flue gas, and convection section 14 having at least
44. Thereafter, dilution steam 26 flows through the inner one heat exchange tube 20 for conveying the hydrocarbon
perforated conduit 40 and is injected into the preheated feed. The process includes the step of establishing a flue gas
hydrocarbon feed 22 through the openings (perforations) temperature within convection section 14 of the furnace 10,
shown in inner conduit 40. immediately adjacent the at least one convection section heat
In another form, the first and second spargers 30 and 38. 40 exchange tube 20 so as to effect a film surface temperature of
respectively, are part of a sparger assembly 28, as shown, in less than about 540° C. (about 1000 F) within at least one
which the first and second spargers 30 and 38, respectively, convection section heat exchange tube 20, wherein said flue
are connected in fluid flow communication in series. The first gas temperature establishing step is effective to decoke the at
and second spargers 30 and 38 are interconnected in fluid flow least one convection section heat exchange tube 20. The flue
communication in series by fluid flow interconnector 46. 45 gas temperature so established within the convection section
As further illustrated, upon exiting the sparger assembly may be at least about 540° C. (about 1000°F).
28, the mixture 48 of hydrocarbon feed 22, water 24 and In practice, one or more tubes 20 are taken off stream (with
dilution steam 26 flows back into furnace 10 wherein the or without shutting down the furnace 10) by cutting out the
mixture 48 is further heated in a lower portion of convection normal feed thereto and passing a decoking feed through the
section 14. The further heating of the hydrocarbon feed can 50 tube or tubes 20 in sufficient amount to remove the coke from
take any form known by those of ordinary skill in the art. The the interior of the tubes. After decoking, the tube or tubes 20
further heating may include indirect contact of the feed in the are returned to normal flow by cutting out the decoking feed
lower convection section 14 of the furnace 10 with hot flue and returning the decoked tube or tubes to normal service.
gases from the radiant section 12 of the furnace. This can be In another form, the process further includes the steps of
accomplished, by way of non-limiting example, by passing 55 interrupting the flow of hydrocarbon feed to the at least one
the feed through heat exchange tubes 50 located within the convection section heat exchange tube, passing an air/steam
convection section 14 of the furnace 10. Following the addi decoking feed mixture having a first air/steam ratio through
tional heating of the mixture at 50, the resulting heated mix the at least one convection section heat exchange tube and
ture exits the convection section at 52 and then flows to the increasing the air/steam decoking feed mixture temperature
radiant section of the furnace for thermal cracking of the 60 entering the convection section to decoke an upper portion of
hydrocarbon. The heated feed to the radiant section may have the convection section.
a temperature between about 425°C. to about 760° C. (about Convection section coke can be burned off at as low as
800° F. to about 1400°F) or about 560° C. to about 730° C. about 850 F. film temperature. Burning coke in the upper
(about 1050 F. to about 1350°F). Zones of convection section 14 is accomplished by adjusting
As mentioned above, advantaged Steam cracking feeds 65 the decoke flue gas temperature and burning the coke in tubes
frequently contain asphaltenes, which will lay down as coke 20 located above the high pressure steam superheater bank
in the convection section 14 of furnace 10 as feed/steam 82. To yield a 455° C. (850 F) film temperature during
 US 7,977,524 B2
7 8
decoking a flue gas temperature of about 540° C. (about comprising a radiant section 12 having burners (not shown)
1000°F) to about 595°C. (about 1100°F) is required. that generate radiant heat and hot flue gas, and a convection
In yet another form, the process further includes the steps section 14 having at least one heat exchange tube 20 for
of reducing the air/steam ratio of the air/steam decoking feed conveying the hydrocarbon feed, the convection section 14
mixture to a second air/steam ratio prior to the step of increas 5 having upper, middle and lower portions thereof. The process
ing the air/steam decoking feed mixture temperature entering includes the steps of taking the at least one heat exchange tube
the convection section. Although a low air/steam ratio 20 off stream by halting the flow of hydrocarbon feed thereto;
reduces combustion kinetics, it also prevents temperature passing an air/steam decoking feed mixture having a first
runaways. Yet Sufficiently high combustion rates are still pos air/steam ratio through the at least one heat exchange tube 20,
sible at a higher temperature when employing a low air/steam 10
and increasing the air/steam decoking feed mixture tempera
ratio. ture entering the convection section 10 to decoke the upper
In anotherform, the step of increasing the air/steam decok portion of the convection section 14, wherein the process is
ing feed mixture temperature entering the convection section conducted for a period of time effective for decoking the at
least one heat exchange tube 20.
includes the step of increasing flue gas oxygen content. As In a still yet further aspect, provided is a process for crack
may be appreciated, significantly higher crossover tempera 15
ing hydrocarbon feed in a furnace 10, the furnace 10 com
tures and film temperatures may be achieved by increasing prising a radiant section 12 having burners (not shown) that
the flue gas O content and firing. For example, during decok generate radiant heat and hot flue gas, and a convection sec
ing, increasing the flue gas O content from 11.8 to 12.2 vol. tion 14 having at least one heat exchange tube 20 for convey
%, wet, increases firing by only 10%, but increases the cross ing the hydrocarbon feed, the convection section 14 having
over temperature by about 10° C. (about 50°F) and the flue upper, middle and lower portions thereof. The process
gas and film temperature in the upper convection section 14 includes the steps of preheating the hydrocarbon feed in the at
by about 25°C. (about 75°F). In the use of this form, flue gas least one heat exchange tube 20 in the convection section 14
oxygen measuring instrumentation 88 can be employed. Flue by indirect heat exchange with the hot flue gas from the
gas oxygen measuring instrumentation 88 should cover the radiant section 12 to provide preheated feed, heating the feed
entire range of 0 to 15% O2, rather than the more common 25 mixture in heat exchange tubes 20 in the convection section
range of 0 to 10% O. In this form, it may be beneficial to 14 by indirect heat transfer with hot flue gas from the radiant
employ a low alloy steel in the fabrication of the convection section 12 to form a heated feed mixture, taking at least one
section heat exchange tubes 20 rather than carbon steel, to heat exchange tube 20 off stream by halting the flow of
accommodate the hotter flue gas. hydrocarbon feed thereto, passing an air/steam decoking feed
In still yet another form, the decoking scheduling is modi 30 mixture having a first air/steam ratio through the at least one
fied to allow burning of the coke to assure that the coke is heat exchange tube 20, and increasing the air/steam decoking
hydrogen-rich and to prevent extensive crosslinking of the feed mixture temperature entering the convection section 12
coke. Although this form may reduce furnace run length, as to decoke the upper portion of the convection section 14.
may be appreciated by those skilled in the art, this is not Additionally, the FIGURE further illustrates a control sys
necessary. For example, furnace 10 can crack a relatively 35 tem having utility in the processes disclosed herein. The
clean feed for the majority of a run; then be switched to a light process temperature provides an input to a controller 54
feedstock contaminated with resid. which controls the flow rate of water via a flow meter 56 and
In a further form, the operating procedure is modified to a control valve 58. The water then enters the sparger assembly
include contaminated feeds that lay down coke in the convec 28. When the process temperature is too high, controller 54
tion section where the film temperature during decoking is 40 increases the flow of water 24.
about 455° C. (850° F.) rather than about 540° C. (about Controller 54 also sends the flow rate signal to a computer
1000 F). For a typical quench header furnace, this means control application schematically shown at 60, which deter
that the coke will lay down about two rows higher in the mine the dilution steam flow rate as detailed below. A pre-set
convection section 14. flow rate of the hydrocarbon feed 18 is measured by flow
In a still furtherform, the furnace further comprises a steam 45 meter 62, which is an input to controller 64, which in turn
superheater 82 capable of being supplied with a stream of sends a signal to feed control valve 66. Controller 64 also
desuperheater water 84, and the step of increasing the air/ sends the feed rate signal to a computer control application
steam decoking feed mixture temperature entering the con 68, which determines the total HO to the radiant section 12
vection section 12 includes the step of reducing the Supply of by multiplying the feed rate by a pre-set total HO to feed rate
desuperheater water to the steam superheater 82. As may be 50 ratio. The total H2O rate signal is the second input to com
appreciated, the operation of the high pressure Steam Super puter application 60. Computer application 60 subtracts the
heater 82 may be modified, the design of the steam super water flow rate from the total HO rate; the difference is the
heater 82 may be modified, or both may be modified, to yield set point for the dilution steam controller 70. Flow meter 72
a hotter flue gas above the steam superheater 82. Hotter flue measures the dilution steam rate, which is also an input to the
gas temperature requires reducing the heat (Q) absorbed by 55 controller 70. When water flow rate increases, as discussed
the steam Superheater 82. Since Q=UAAT, with O.A. above, the set point inputted to the dilution steam controller
being nearly constant, reducing AT is the only way to 70 decreases. Controller 70 then instructs control valve 74 to
reduce the heat absorbed. Reducing AT requires increasing reduce the dilution the steam rate 76 to the new set point.
the high pressure steam temperature throughout the steam When the process temperature 78 is too low the control
superheater 82. 60 scheme instructs control valve 58 to reduce water rate and
In one form, AT is reduced by turning off the desuper instructs control valve 74 to increase the steam rate while
heater water 84 at valve 86 and/or allowing a portion of the maintaining constant total H2O rate.
saturated high pressure steam supplied by line 80 to bypass
the steam superheater 82 through line 90, through the use of Example
valves 90 and 94. 65
In a yet further form, provided is a process for decoking a In this example AT is reduced by turning off the desu
furnace 10 for cracking a hydrocarbon feed, the furnace 10 perheater water and/or allowing a portion of the Saturated
 US 7,977,524 B2
10
high pressure steam to bypass the steam Superheater (SSH). establishing a flue gas temperature within the convection
The Table below summarizes the performance of these two section of the furnace immediately adjacent the at least
options for a furnace during decoking. one convection section heat exchange tube so as to effect
TABLE
Normal SSH No desuperheater No desuperheater water
operations Water 50% HP steam bypass
SSH duty, MBtuhr 16 14 8.5
SSH duty, Mkg-m/hr 1721 1505 914
SSH outlettemp. 504°C. (940°F) 599° C. (1110°F) 677° C. (1250°F)
Flue temp. above SSH 404°C. (760°F) 435° C. (815°F) 538°C. (1000°F)
Film temp. above SSH 379° C. (715 F) 407 C. (765°F) 493° C. (920°F)

As shown above, simply turning off the desuperheater 15 a film surface temperature of less than about 540° C.
water does not reduce the heat absorbed by the steam super within at least one convection section heat exchange
heater sufficiently to burn coke, since the film temperature is tube, wherein said flue gas temperature establishing step
only 407°C. (765°F) in the row above the steam superheater. is effective to decoke the at least one convection section
However, turning off the desuperheater water and bypassing heat exchange tube;
half of the high pressure steam does reduce the heat absorbed and wherein the furnace further comprises a steam Super
by the steam superheater sufficiently to burn coke. This heater disposed below the at least one convection section
occurs because the steam Superheater absorbs only about heat exchange tube in the furnace and capable of being
one-half the heat absorbed during normal operations. The Supplied with a stream of desuperheater water, and
film temperature above the steam superheater is 493°C. (920° 25 wherein said step of increasing the air/steam decoking
F.), which is more than adequate to burn coke. feed mixture temperature entering the convection sec
It would be expected that the bypass option will require tion includes the step of reducing the Supply of desuper
steam Superheater tubes that are thicker and possess better heater water to the steam Superheater during the decok
metallurgy and may also require alloy steel in the process ing of the upper convection section of the furnace.
rows above the steam Superheater. In addition, controlling the 30 2. The process of claim 1, wherein the flue gas temperature
steam Superheater outlet temperature is more difficult during so established within the convection section of the furnace
bypass operations. However, bypassing only at the end of a immediately adjacent the at least one convection section heat
decoke process mitigates the effect that this control issue has exchange tube is at least about 540°C.
on the plant high pressure steam system. 3. The process of claim 1, wherein the film surface tem
All patents, test procedures, and other documents cited 35 perature of the at least one convection section heat exchange
herein, including priority documents, are fully incorporated tube is at least about 455° C.
by reference to the extent such disclosure is not inconsistent 4. The process of claim 1, wherein the steps of passing an
with this invention and for all jurisdictions in which such air/steam decoking feed mixture and increasing the air/steam
incorporation is permitted. decoking feed mixture temperature are conducted for a period
While the illustrative embodiments of the invention have 40 of time effective for decoking the at least one convection
been described with particularity, it will be understood that section heat exchange tube.
various other modifications will be apparent to and can be 5. The process of claim 4, further comprising the step of
readily made by those skilled in the art without departing reducing the air/steam ratio of the air/steam decoking feed
from the spirit and scope of the invention. Accordingly, it is mixture to a second air/steam ratio prior to the step of increas
not intended that the scope of the claims appended hereto be 45 ing the air/steam decoking feed mixture temperature entering
limited to the examples and descriptions set forth herein but the convection section.
rather that the claims be construed as encompassing all the 6. The process of claim 1, wherein the heat exchange tubes
features of patentable novelty which reside in the invention, of the convection section are formed of low alloy steel.
including all features which would be treated as equivalents 7. The process of claim 1, wherein the process is conducted
thereof by those skilled in the art to which the invention 50 at intervals sufficient to prevent extensive crosslinking of the
pertains. coke.
8. The process of claim 1, wherein the flue gas temperature
What is claimed is: is between about 540° C. and about 595 C.
1. A process for decoking a convection section of a furnace 9. A process for decoking a furnace for cracking a hydro
for cracking a hydrocarbon feed, the furnace comprising a 55 carbon feed, the furnace comprising a radiant section having
radiant section having burners that generate radiant heat and burners that generate radiant heat and hot flue gas, and a
hot flue gas, and the convection section having at least one convection section having at least one heat exchange tube for
heat exchange tube for conveying the hydrocarbon feed, said conveying the hydrocarbon feed, the convection section hav
process comprising the step of ing upper, middle and lower portions thereof, the process
interrupting the flow of hydrocarbon feed to the at least one 60 comprising the steps of
convection section heat exchange tube; (a) taking the at least one heat exchange tube disposed in
passing an air/steam decoking feed mixture having a first the middle or upper sections of the convection section
air/steam ratio through the at least one convection sec off stream by halting the flow of hydrocarbon feed
tion heat exchange tube; and thereto;
increasing the air/steam decoking feed mixture tempera 65 (b) passing an air/steam decoking feed mixture having a
ture within the tube entering the convection section to first air/steam ratio through the at least one heat
decoke; exchange tube; and
 US 7,977,524 B2
11 12
(c) increasing the air/steam decoking feed mixture tem (a) preheating the hydrocarbon feed in the heat exchange
perature entering the convection section to decoke the tubes in the convection section by indirect heat exchange
upper portion of the convection section, with the hot flue gas from the radiant section to provide
wherein steps (b) and (c) are conducted for a period of time preheated feed;
effective for decoking the at least one heat exchange (b) heating the feed mixture in the at least one heat
tube; and exchange tube in the convection section by indirect heat
wherein the furnace further comprises a steam Superheater transfer with hot flue gas from the radiant section to form
capable of being Supplied with a stream of desuperheater a heated feed mixture;
water, and wherein said step of increasing the air/steam (c) taking the at least one heat exchange tube disposed in
decoking feed mixture temperature entering the upper 10 the middle or upper sections of the convection section
convection section includes the step of reducing the off stream by halting the flow of hydrocarbon feed
Supply of desuperheater water to the Steam Superheater. thereto;
10. The process of claim 9, wherein said step of increasing (d) passing an air/steam decoking feed mixture having a
the air/steam decoking feed mixture temperature entering the first air/steam ratio through the at least one heat
convection section includes the step of increasing flue gas
15 exchange tube; and
OXygen content. (e) increasing the air/steam decoking feed mixture tem
11. The process of claim 9, wherein said step of increasing perature entering the convection section to decoke the
the air/steam decoking feed mixture temperature entering the upper portion of the convection section,
convection section includes the step of increasing burner wherein steps (d) and (e) are conducted for a period of time
firing rate for the radiant section burners. effective for decoking the at least one heat exchange tube
12. The process of claim 9, wherein the upper and middle and wherein the furnace further comprises a steam
convection section heat exchange tubes are formed of low Superheater capable of being Supplied with a stream of
alloy steel. desuperheater water, and wherein said step of increasing
13. The process of claim 9, wherein said step of increasing the air/steam decoking feed mixture temperature enter
the air/steam decoking feed mixture temperature entering the 25 ing the convection section includes the step of reducing
convection section includes the step of eliminating the Supply the Supply of desuperheater water to the steam Super
of desuperheater water to the steam superheater. heater during the decoking of the upper convection sec
tion of the furnace.
14. The process of claim 13, wherein said step of increasing 18. The process of claim 17, further comprising the step of
the air/steam decoking feed mixture temperature entering the 30 reducing the air/steam ratio of the air/steam decoking feed
convection section also includes the step of bypassing a por mixture to a second air/steam ratio prior to said step of
tion of steam supplied to the steam Superheater. increasing the air/steam decoking feed mixture temperature
15. The process of claim 13, wherein the flue gas tempera entering the convection section.
ture is between about 540° C. and about 595 C. and is
effective to decoke the at least one convection section heat 19. The process of claim 17, wherein the upper and middle
exchange tube. 35 convection section heat exchange tubes are formed of low
16. The process of claim 15, wherein the at least one alloy steel.
convection section heat exchange tube has a film Surface 20. The process of claim 17, wherein the flue gas tempera
ture is between about 540° C. and about 595 C.
temperature of at least about 455° C. 21. The process of claim 20, wherein the flue gas tempera
17. A process for cracking hydrocarbon feed in a furnace, 40 ture is effective to decoke the at least one convection section
the furnace comprising a radiant section having burners that heat exchange tube.
generate radiant heat and hot flue gas, and a convection sec 22. The process of claim 20, wherein the at least one
tion having at least one heat exchange tube for conveying the convection section heat exchange tube has a film Surface
hydrocarbon feed, the convection section having upper, temperature of at least about 455° C.
middle and lower portions thereof, the process comprising the
steps of: k k k k k