Note: Descriptions are shown in the official language in which they were submitted.
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PULSED AIR DECOKING
FIELD OF THE INVENTION
The present invention relates to the field of
hydrocarbon cracking and more specifically to the
efficient maintenance of hydrocarbon reaction systems
which require periodic decoking.
BACKGROUND OF THE INVENTION
Cracking hydrocarbons from long chain molecules
to gasoline and other useful short chain hydrocarbons
results in ~he production of significant secondary by-
products of the reaction, most notably carbonaceous
deposits or coke. The effects of the coke produced in
the reaction are considerable, especially in fired
tubular furnace reactors and the like utilizing process
reactor tubes in which the cracking reaction occurs.
During the reaction, the coke by-products form
deposits, inter alia, as a layer on the inside of the
process tubes of the reactor. The coke depos`its increase
the pressure drop and inhibit heat transfer across the
tubes thereby penalizing the process. The coke must,
therefore, be removed pPriodically to restore the
pressure drop and heat transfer to normal levels.
The coke removal process, generally called
decoking, is commonly carried out by combustion (also
known as steam-air decoking), by steam reaction or by
mechanical removal. Steam-air decoking is performed by
introducing a small quantity of air in a steam matrix,
usually starting at about 1-2% and increasing in stages
to about 20% by weight of air relative to steam, into the
fouled process tubes to burn-off the coke from the in~ide
of the tubes. In contrast, steam reaction decoking
requires the introduction of steam to the tubes at high
temperatures to react with the coke. An example of the
steam reaction decoking is found in United States Patent
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No. 4,3~6,694. Finally, mechanical decoking makes u~e of
physical means for breaking loose and scouring the coke
- from the inside of the process tubes, generally a high
pressure water jet, i.e. at a pressure of from about 700
5 to 1000 bar.
of these methods, steam-air decoking can be the
most efficient, however, to be efficient requires close
monitoring of the process to maintain a reasonable
burning rate. If the burning rate is too low the
decoking operation will require a longer period of time,
resulting in the processing operation remaining off
stream for a longer period. On the other hand, if the
burning rate is too high the excessive temperature will
damage or even burn through the process tubes.
In conventional steam-air decoking the burning
rate is monitored by measuring the concentration of
carbon dioxide from time to time in the decoking effluent
gas. In conventional decoking of the fired tube reactor,
overheating of the process tubes is safeguarded again~t
by visual inspection of the tubes in the firebox and/or
observing tube temperatures by means of a pyrometer.
It is therefore an object of the present
invention to provide a method of decoking which can be
performed over a short period of time, thereby reducing
reactor downtime.
It is another ob~ect of the invention to
provide a method of decoking which can be automatically
monitored to maximize combustion while avoiding damage to
the reactor, thus limiting operator involvement.
S W MARY OF ~HE INVENTION
These and other ob~ects are achieved by the
present invention which i9 directed to a method of
decoking a coke fouled hydrocarbon reaction pathway ln a
hydrocarbon reaction system comprising the steps of:
(a) interrupting the hydrocarbon feed;
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(b) injecting an air pulse into a steam matrix
into the reaction pathway for combustion of the coke,
said air pulse being of sufficient concentration for
vigorous combustion of the coke for a period of time
limited in duration so as not to raise the temperature of
the reactor components above their design temperature,
thereby producing an effluent gas; ~-
(c) interrupting the injection of the air into
the pathway for a period of time to subdue combustion and
allow the temperature of the reactor components heated
during combustion to decrease;
(d) sequentially repeating steps (b) and (c)
until the effluent gas has a C2 content of less than
about 0.2 volume %; and
(e) resuming the hydrocarbon feed. The
additional qtep of continuously monitoring the C02 content
of the effluent gas produced during decoking by using
sensing means such as a gravitometer, infra-red analyzer
or other means is also contemplated.
The above process allows the usè of higher
weight concentrations of air in steam, i.e. in the range
of from about 20% to about 50% by weight. The increased
concentration of air results in vigorous burning that
might seriously overheat the tubes but for the
interruption of the air pulse, allowing the combustion
and heat generation to subside or extinguish. Another
air pulse is then in~ected into the steam matrix after a
short time interval to re~uvenate or reinitiate the coke
! ' combustion, with the on/off sequence repeated until
decoking is complete.
The frequency of the air pulse and the
concentration of the air in steam durlng the pulse in a
partlcular envlronment varles with both the geometry of
the reactor pathway and the characteristlcs of the
feedstock being processed. As the decoking progresses,
the concentration of air in the steam would normally be
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increased to ensure rapid and thorough combustion of the
coke. The frequency of the air pulses would be regulated
to prevent large, damaging temperature swings in the tube
metal.
Further, the process of alternately heating and
cooling the coke and the tube metal by the pulsed burning
will create a tendency for some of the coke to spall from
the tube walls, thus further accelerating the decoking
process.
The rate of burning is established by a C2
sensing means such as a RANAREX gravitometer or a direct
reading C2 analyzer such as an infra-red analyzer. The
gravitometer measures the specific gravity of the
effluent gas relative to air and thus senses the
concentration of carbon dioxide in the effluent, as the
molecular weight of C2 i9 high relative to that of air.
Slmilarly, the infra-red CO2 analyzer measures the CO2
content of a gas stream such as the effluent ga~
directly. The CO2 content and the rate of air in~ection
establishes the rate of coke burning.
The CO2 sensing means is closely linked to the
decoking effluent by means of a sampling line. The CO2
sensing means is adapted to generate an electrical signal
which is used to terminate the pulsing once the COz level
has dropped below the desired point. Automatic means to
control either the total volume of air being injected
during a pulse or the frequency of the air pulses, as
appropriate, is contemplated. The choice of air volume
or frequency controls depends on the mechanical means
chosen to regulate the air flow.
For example, decoking of a fired tubular
furnace for cracking hydrocarbons to ethylene and other
oleflnic compound~ under the present invention i~
achieved by in~ecting a pulse of air at a weight
concentration relative to the steam flow for a period of
time to initiate combustion and burn the coke. At the
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end of this period the air would be interrupted either
totally or partially, subduing or extinguishing the
combustion, to avoid raising the temperature of the tubes
beyond their design temperature. After the end of the
air pulse interruption, a subsequent air pulse i5
injected into the steam matrix with the on-off cycle
repeated many times until the CO2 content of the effluent
gas during the air pulse is less than about 0.2 volume %
indicating that the effluent is substantially free of CO2.
The period of time that the air flow is on and
the period it is interrupted, as well as the air
concentration during the process are established by the
CO2 sensing means and generally change along the decoking
process. An air flow is then left on for a period of
time to "air polish" the pathway.
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BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings are included to better
illustrate the present invention without limiting the
invention in any manner whatsoever.
FIGURE 1 is a schematic view of a reactor
furnace for use with the present invention.
FIGURE 2 is a graph of the specific gravity of
the effluent during decoking under the present process.
FIGURE 3 is a graph of the specific gravity of ;
the effluent during decoking under the present process
from a downstream gravitometer.
FIGURE 4 is a plot of temperature at various
points along the length of a process tube during decoking
by the present method.
FIGURE 5 is a graph of temperature over time
which follows the air pulses and the movement of coke
burn toward and past a thermocouple.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to the figures, and specifically
FIGURE 1, the present invention is shown in the
environment of a fired tubular furnace (1) as used for
cracking hydrocarbons to ethylene and other olefinic
compounds. It is understood that the invention can be
used with any suitable cracking furnace or hydrocarbon
reaction system which needs periodic decoking.
The fired tubular furnace (1) has a convection
section (2) and a cracking section (3). The furnace (1)
contains one or more process tubes (4) through which
hydrocarbons fed through a hydroca~bon feed line (6) are
cracked to produce product gases upon the application of
heat, with carbonaceous deposits referred to herein as
coke produced as a by-product. Steam from steam conduit
22 i8 used as a diluent during the reaction process.
Heat is supplied by a heating medium introduced to the
exterior of the process tube~ (4) in the cracking section
(3) of the furnace (1) through heating medium inlet (8),
and exiting through an exhaust (10). As the product
gases move through the process tubes (4) and out the
product gas exit (12) the product gases are then passed
through one`or more quench/heat exchangers (14) and (16).
The product gases are then directed along line (18) to
downstream processing equipment such as a quench tower
and separation means (not shown).
To implement the present invention, a valve
(20) is placed in the hydrocarbon feed line (6) to
interrupt the hydrocarbon feed. After interruption of
the hydrocarbon feed flow, an air pulse from an air
conduit (24) is in~ected into the flow of a steam matrix
along a steam conduit (22). The relative volume and
duratlon of air flow through the air conduit (24) and
into the 8team condult (22) 19 controlled by a valve (26)
placed in the alr flow condult (24). An alternatlve
embodiment includes a nitrogen conduit (not shown) to
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deliver a nitrogen flow to the steam matrix in place of
air when the air flow is interrupted.
During decoking downstream valving (34)
controls direction of the effluent gas and coke spall
- 5 away from the product separation system (not shown).
When the hydrocarbon feed valve (20) is closed the valve
(34) to the downstream processing system (not shown) is
also closed and a decoking effluent valve (36) is opened
to direct the decoking effluent along an effluent line
(38) to the cracking section (3) of the furnace (1)
exterior to the process tubes (4) for coke spall
combustion, or optionally to a decoking drum (40) with
gas ~ents (42). Since air cannot be introduced to the
separation system, the valve (34) to the processlng
system is closed and the effluent valve (36) is opened to
direct the effluent away from the separation system upon
interruption of the hydrocarbon feed flow and
introduction of air into the steam matrix for decoking.
Automatic activation of the feed, steam, air, processing
and effluent valves (20), (21), (26), (34) and (36) is
contemplated to provide an automatic decoking system. -
Additionally, C2 sensing means (28), such as a
gravitometer with a range of from about 0.6 to about 1.2,
or any CO2 analyzer with a continuous read out, is closely
linked to the decoking effluent by means of a sampling
line (30). The sampling line (30) is preferably linked
to the effluent line (38) downstream of the effluent
valve (36) so that sampling takes place only during
decoking. The CO2 sensing means (28) preferably measures
the content of C2 in the effluent gas continuously. The
CO2 sensing means (28) is adapted to generate an
electrical signal which is used to determine the end of
the pulsing or decoklng cycle and possibly control the
volume of air in~ected into the steam matxix during the
air pulse and/or the frequency of air pulses, as
appropriate. When the CO2 content in the effluent gas is
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measured at less than 0.2 volume % it signlfies that no
more burning is taking place, i.e. that no coke is left
~ to be burned, and the cracking process can resume.
j Alternatively, the sampling line (30a) for the
CO2 sensing means (28a) can be taken from product line
(18) or from a point closer to the furnace (1) such as at
product gas/outlet (12) or between the heat exchangers
(14) and (16). Although sensing means closer to the
furnace (1) tend to be more responsive, the downstream
sampling at lower temperatures is a simpler installation.
The decoking process of the present invention,
utilizing the above structure, is ~ut into effect after
the cracking operation has been run for a period of time
resulting in a coke buildup on the walls of the metal
tubes (4) of the furnace (1), resulting in an increase in
pre~sure drop and decreased heat transfe~ across the tube
walls. The present process for removal of the coke, or
decoking, is employed without the need to shut down or
cool the furnace.
The present process is initiated by
interrupting the hydrocarbon feed at valve (20), closing
the product gas valve (34) and opening the decoking valve
(36). A pulse of air is introduced into the steam
matrix, in conduit (22) from air flow conduit (24) by the
opening and closing of the air valve (26). The steam
(and air pulse) is then heated in the convection section
to about 900F, up to about 1400F. In the most
preferred embodiment, the relative volume of air (i.e.
concentration in the steam matrix) and duration of the
pulse is automatically fixed by the mechanical means
used. The signal from the CO2 sensing means (28) is used
to stop the pulsing process and initiate an "air polish",
as later described, if desired.
The concentration of air in the steam matrix
during the air pulse is contemplated to be in the range
of from about 20 to about 50% by weight and varies
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loosely inversely with the duration of the pulse, i.e.
the greater concentration of air in the steam matrix the
shorter the duration. Generally, the duration of the
pulse is long enough to initiate vigorous combustion of
the coke but not so long as to raise the temperature
above the predetermined design temperature of the metal
process tubes (4) or other reactor components. The
preferred duration is estimated to be in the range of
about 10 to about 50 seconds when a concentration of
about 40 weight ~ air in steam is used.
Additionally, the interruption of the air flow
at valve (26) is for a period of time to subdue or even
extinguish combustion and heat generation and allow the
process tube temperature to decrease. The duration of
the air flow interruption is generally less than the
duration of the air pulse, i.e. in the range of from
about 5 to about 30 seconds, however, the duration i9
controlled to ensure that damage to the tubes is avoided
and the process is efficiently run.
The amount and duration of the air flow
described above is contemplated from results achieved on
a bench scale unit. The optimal values are determinable
by experimentation on a full scale furnace by one skilled
in the art.
EXAMPLE
Pulsed air decoking as described in the present
disclosure has been tried three times in decoking a bench
! scale pyrolysis unit (BSU) following propane Gracking
operations of up to 5 hour duration.
A RANAREX gravitometer with a 0.3 to 1.3 range
was close-coupled to the cracking tube outlet (12). As
seen ln FIGURE 2, rapid swings in effluent specific
gravlty are clearly visible, indicating that the swings
in CO2 are instantaneous upon the injection and
interruption of an air pulse, virtually like turning on
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and off a switch. Similarly, the first set of data
recorded on the usual RANAREX gravitometer (28) located
much further downstream, shown in FIGURE 3, shows the
same back and forth response of effluent specific
gravity, or CO2 concentration, as the air was pulsed on
and off alternatively with N2. -
The air pulse duration used was essentially 45
seconds on and 45 seconds off, arbitrarily chosen for the
present because the swings on both the upstream and
downstream RANAREX gravitometers and on temperature
indicators on the exterior of the tubes (4) suggested
that duration for maximizing the a~mount of burn on each
air pulse cycle. A constant air concentration to steam
of 40 weight % throughout the burn was used. Although
automatic operation is contemplated, the pulsing was done
manually, switching instantaneously from air to nitrogen
in the steam matrix thus inducing a swing in effluent
specific gravity from 1.08 at maximum CO2 (21%) to the
nitrogen level of 0.966. ~he signal drop slightly below
the nitrogen level was due to some steam-carb;on reaction
which continued when the air was cut off.
Each time the pulsed air technique was used,
the progression of the burn through the coil was observed
by means of the skin thermocouples (TI's) spaced along
the tube length. As seen in FIGURE 4, the TI at the
mldpoint of the tube coil (4) where coke buildup usually
begins, shows the first temperature blip and the other
TI's further along the tube coil (4) show temperature
increases (50-150F) sequentially as the burn progresses
toward the outlet.
Because the tube (4) has the usual rising
temperature profile, the rate of burnins speeds up
significantly toward the coil outlet (12). Therefore,
the burn time or aLr pulse may be shortened as the
decoking progresses down the tube coil (4).
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The TI's located inside the process stream at
the outlet (12) show virtually no temperature change at
the start of the decoking process when the burn is in the
middle of the tube (4) (where the coke is first laid
down). As the burn progresses the tube outlet
temperature (TOT) is increasingly affected by the
approaching burning front and ends up swinging more than
the tube metal temperatures (TMTs). A potentiometer was
attached to the key TOT measured on the process side,
enabling the process effluent temperature swings to be
followed very rapidly (see FIGURE 5). The combination of
the steam flow (heat sink), the slow response of the 24
point strip chart recorder, and the automatic cut~ing
back by the controllers of the electric heat input ~s
enough to mask the process-side temperature swing (TOT)
when the burn is well back in the tube.
The temperature swings induced on the tube (4)
by the pulsed burn need not be too wide, i.e. about
200F. At the roughly 1800F operating temperature of
the coil no metallurgical problems occur.
Some "peristaltic" effect of the temperature
swings are inevitable, possibly inducing more coke
spalling than durinq a normal decoking. If the coke
deposit were unusually heavy, and the spalling were great
enough to cause a tube blockage, this could be
detrimental. In the usual case, however, the additional
spalling, if it occurs, would speed up the decoking
process and be a benefit. Coke spall was observed during
this process example, as seen in virtually all decokings,
both steam and air, but no measure of any difference in
the amount of spall was noted.
When the decoking was complete the RANAREX
9ignal settled at the level for air. At thls point the
pulsinq process was stopped and the air left on for a
period of about 10 minutes to "air polish" the tube coil
(4). The chart of FIGURE 2 marked RANAREX No . 2 shows
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this. On this chart the decoking after 5 hours of
cracking propane was finished in about 20 minutes. The
coking rate in this 40 foot by ~ and ~ inch I.D. swaged
coil is relatively higher than in the commercial coil it
simulates, probably proportional to the difference in
surface to throughput ratio (about l9). Using the pulsed
air system, less time was spent than normally necessary
to complete decoking.
While the invention has been described in
detail and with specific reference to embodiments
thereof, various changes and modifications will be
apparent to one skilled in the art without departing from
the spirit and scope thereof.
