Note: Descriptions are shown in the official language in which they were submitted.
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HIGH GAS VELOCITY START-UP OF AN ETHYLENE CRACKING FURNACE
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No.
62/964,243,
filed January 22, 2020, which is hereby incorporated by reference in its
entirety.
TECHNICAL FIELD
The present disclosure relates to the field of hydrocarbon cracking furnaces.
In
particular, the present disclosure relates to start-up procedures for
hydrocarbon cracking
furnaces.
BACKGROUND ART
In an industrial steam cracker, a feed is passed through several reactors or
"furnaces", each furnace including a radiant section with tubular metal coils,
before exiting
the furnace at an elevated temperature, typically above 750 C. At these
temperatures the
steam and feed, typically an alkane, usually a lower molecular weight alkane
such as ethane,
propane, butane and mixtures thereof, or heavier feed stock including naphtha,
heavy
aromatic concentrate (HAC) and heavy aromatic gas oil (HAGO) or any of the
vacuum gas
oils, undergoes a rearrangement yielding alkenes, including but not limited to
ethylene,
propylene, and butene, as well as hydrogen and other co-products. Over time,
heavy
hydrocarbons and coke build up on the internal surface of the radiant tubes
increasing the
pressure drop across the tube and reducing the thermal and cracking efficiency
process in
that tube or coil. The furnace is taken off-line (hydrocarbon feed is no
longer passing
through the coil) and the coil(s) is decoked after which the furnace is
returned to operation.
Decokes can be costly as they represent time periods where costs are incurred
without production of commercially valuable products. Reducing the time to
perform a
decoke can save money, but options for doing so are limited. Increasing the
time between
decokes by increasing the run length is another option for reducing the
financial effect of
the costly downtime. Provided herein is a start-up method for a hydrocarbon
cracking
furnace that can extend run length.
SUMMARY OF INVENTION
The present disclosure seeks to provide a start-up procedure to extend a run
length
of a hydrocarbon cracking furnace, the hydrocarbon cracking furnace including
at least one
furnace tube, wherein the at least one furnace tube includes at least one
inlet, at least one
outlet, and a point of incipient cracking in between the inlet and outlet, the
start-up
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procedure including: introducing a fluid including hydrocarbon and dilution
steam to the at
least one inlet; establishing and maintaining the fluid at the inlet to a
temperature of
between 25 C and 225 C; determining a number average molecular weight of the
fluid
proximate the inlet; determining a number average molecular weight of the
fluid proximate
the outlet; calculating an average of the number average molecular weights of
the fluids at
the inlet and the outlet; measuring a pressure drop of the fluid from the
inlet to the outlet;
calculating a fluid velocity at the point of incipient cracking; controlling
the fluid velocity at
the point of incipient cracking to a range between at least 90 to at most 115
m/s for at least 5
days by varying the flow rate and/or one or more of the fluid properties of
the fluid at the
inlet; wherein the start-up procedure extends the run length by at least 15%
relative to a
start-up procedure conducted in the same manner but wherein the fluid velocity
at the point
of incipient cracking is not monitored and/or controlled.
The present disclosure also seeks to provide a start-up procedure to extend a
run
length of a hydrocarbon cracking furnace, the hydrocarbon cracking furnace
including at
least one furnace tube, wherein the at least one furnace tube includes at
least one inlet, at
least one outlet, and a point of incipient cracking in between the inlet and
outlet, the start-up
procedure including: introducing a fluid including hydrocarbon dilution steam
to the at least
one inlet; establishing and maintaining the fluid at the inlet to a
temperature of between
C and 225 C; determining a number average molecular weight of the fluid
proximate the
20 inlet; determining a number average molecular weight of the fluid
proximate the outlet;
calculating an average of the number average molecular weights of the fluids
at the inlet and
the outlet; measuring a pressure drop of the fluid from the inlet to the
outlet; calculating a
fluid velocity at the point of incipient cracking; controlling the fluid
velocity at the point of
incipient cracking to a range between at least 90 to at most 115 m/s for at
least 5 days by
25 varying the flow rate and/or one or more of the fluid properties of the
fluid at the inlet;
wherein the start-up procedure extends the run length by at least 8 days
relative to a start-up
procedure conducted in the same manner but wherein the fluid velocity at the
point of
incipient cracking is not monitored and/or controlled.
BRIEF DESCRIPTION OF DRAWINGS
To easily identify the discussion of any particular element or act, the most
significant digit or digits in a reference number refer to the figure number
in which that
element is first introduced.
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Figure 1 illustrates a steam hydrocarbon cracking furnace 100 layout in a
schematic
process flow diagram in accordance with one embodiment.
Figure 2 illustrates a steam hydrocarbon cracking furnace 200 layout in a
schematic
process flow diagram in accordance with one embodiment.
Figure 3 illustrates a steam hydrocarbon cracking furnace 300 layout in a
schematic
process flow diagram in accordance with one embodiment.
Figure 4 illustrates a steam hydrocarbon cracking furnace 400 layout in a
schematic
process flow diagram in accordance with one embodiment.
Figure 5 illustrates a graph of the gas velocity in an industrial steam
hydrocarbon
cracking furnace over a 500 day period which includes 3 separate runs.
DESCRIPTION OF EMBODIMENTS
Reference will now be made in detail to certain embodiments of the disclosed
subject matter. While the disclosed subject matter will be described in
conjunction with the
enumerated claims, it will be understood that the exemplified subject matter
is not intended
to limit the claims to the disclosed subject matter.
Other than in the operating examples or where otherwise indicated, all numbers
or
expressions referring to quantities of ingredients, reaction conditions, etc.
used in the
specification and claims are to be understood as modified in all instances by
the term
"about". Accordingly, unless indicated to the contrary, the numerical
parameters set forth in
the following specification and attached claims are approximations that can
vary depending
upon the properties that the present disclosure desires to obtain. At the very
least, and not as
an attempt to limit the application of the doctrine of equivalents to the
scope of the claims,
each numerical parameter should at least be construed in light of the number
of reported
significant digits and by applying ordinary rounding techniques.
Definitions
"Aromatic hydrocarbon" refers to a hydrocarbon with sigma bonds and
delocalized
pi electrons between carbon atoms forming a circle.
"Cl ¨ C4 alkane" refers to one or more of methane, ethane, propane, and
butane.
"Dilution steam" refers to steam added to the hydrocarbon to be cracked in a
hydrocarbon cracking furnace.
"Fluid properties" refers to properties of the fluid, including, but not
limited to,
density, viscosity, temperature, pressure, specific volume, specific weight,
specific gravity,
and number average molecular weight.
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"Furnace tube" refers to a conduit, often described as a pass and where
multiple
passes are linked together in what is typically referred to as a coil, that
can be used in a
furnace, through which the fluid to be heated flows.
"Heavy aromatic distillate" refers to a combination of hydrocarbons obtained
from
distillation of aromatic streams. It consists predominantly of aromatic
hydrocarbons having
carbon numbers predominantly in the range of C9 through C16 and boiling in the
range of
approximately 165 C to 290 C. It can be a co-product from ethylene production.
(also
known as HAD)
"Hydrocarbon" refers to an organic compound consisting entirely of hydrogen
and
carbon.
"Hydrocarbon cracking furnace" refers to a furnace designed to break down or
crack
hydrocarbons, typically alkanes into alkenes.
"Ideal Gas Law" refers to the equation of state of a hypothetical ideal gas;
it is a
good approximation of the behavior of many gases under many conditions and may
be used
in place of measuring or calculating the actual properties of a gas. The Ideal
Gas Law is
often written as: PV=nRT, where P, V and T are the pressure, volume and
temperature, n is
the number of moles of gas, and R is the ideal gas constant.
"Naphtha" refers to a flammable liquid hydrocarbon mixture. Mixtures labelled
naphtha have been produced from natural gas condensates, petroleum
distillates, and the
distillation of coal tar and peat. In different industries and regions naphtha
may also be
crude oil or refined products such as kerosene.
"Number average molecular weight" refers to the total weight of the sample
divided
by the number of molecules in the sample.
"Point of incipient cracking" refers to the location in the hydrocarbon
cracking
furnace where the hydrocarbon starts to crack, forming radicals. This location
is often
approximated, and is based on many factors, including the pressure in the
hydrocarbon
cracking furnace, feed composition, coke formation, desired products, etc. The
temperature
at the point of incipient cracking can range as low as 370 C to as high as 850
C in industrial
furnaces, typically 500-550 C. For this disclosure, the location where the
temperature first
reaches 525 C was chosen as it is typical temperature for a furnace cracking a
predominately ethane feed. The pressure at the point of incipient cracking was
determined
using SPYRO Suite 7 software from the Pyrotec Division of Technip Benelux
B.V.
"Pressure drop" refers to the difference in total pressure between two points
of a
fluid carrying network.
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"Run length" refers to the length of time that a furnace tube is in
hydrocarbon
cracking operation.
"Start-up procedure" refers to the steps followed to start a hydrocarbon
cracking
furnace until it is producing the desired alkenes.
One method of producing alkenes such as ethylene is by steam cracking, in
which
hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons, by
thermally
cracking with the use of dilution steam in a bank of pyrolysis furnaces. The
starting
hydrocarbons can be Cl ¨ C4 alkanes, naphtha, or aromatic hydrocarbons such as
heavy
aromatic distillate (HAD).
In an industrial hydrocarbon cracking furnace, there are typically several
reactors or
coils. Within the radiant section of each furnace are tubular metal coils or
furnace tubes
with one or more zones with one or more passes per furnace tube which proceed
through a
furnace where the fluid within exits the furnace tube at an elevated
temperature typically
above 750 C, usually in the range of 800 C to 900 C, the feed passing through
the furnace
tube in the radiant section of a cracker for a period of time from
milliseconds to several
seconds. The one or more zones can be included of a convection section or pre-
heat section
and a radiant cracking section. After the fluid exits the radiant cracking
section, it can be
sent to a quench section to lower the fluid temperature. The temperature of
the fluid
entering the pre-heat section is typically between 25 C and 225 C, whereas the
temperature
of the fluid entering the radiant section is typically between 600 C and 750
C. The
temperature at the point of incipient cracking can range as low as 370 C to as
high as 850 C
in industrial furnaces, typically 500-550 C at typical pressures and
compositions, with
525 C being used in this disclosure in all calculations. At these temperatures
the dilution
steam and hydrocarbon feed, typically an alkane, typically a lower molecular
weight alkane
such as ethane, propane, butane and mixtures thereof, or heavier feed stock
including
naphtha, heavy aromatic distillate (HAD) and heavy aromatic gas oil (HAGO) or
any of the
vacuum gas oils, undergoes a rearrangement yielding alkenes, including but not
limited to
ethylene, propylene and butene as well as hydrogen and other coproducts.
Over time, heavy hydrocarbons and a carbonaceous deposit, or coke-like
material,
build up on the internal surface of the furnace tube increasing the pressure
drop across the
furnace tube and reducing the thermal and cracking efficiency process in that
furnace tube.
Coking is an unwanted side reaction from hydrocarbon cracking. It is a major
operational
problem in the radiant section of hydrocarbon cracking furnaces and transfer
line
exchangers. Dilution steam lowers the hydrocarbon partial pressure of the
cracked
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compounds and reduces the tendency of coke deposition on the furnace tubes.
Dilution
steam increases the distance between fouling components and can be used to
increase the
velocity of the gas by adding its volume to the flow.
Coke is an undesired but inevitable side product of the pyrolysis. Surface
catalyzed
reactions lead to the formation of filamentous coke. In many cases, the coke
formation is
caused by nickel and iron on the alloy surface. Amorphous coke is formed in
the gas phase.
Increased pressure drop, impaired heat transfer and higher fuel consumption
due to the coke
cause high production losses. The external furnace tube skin temperature can
also rise,
which influences the process selectivity and leads to even more rapid coke
formation. The
formed coke must be removed by controlled reaction with steam and air. It is a
non-
productive downtime of the steam cracker furnace. Decoking cycles lead to
shorter coil life
of the steam cracker furnaces. During a decoking cycle, the furnace is taken
off-line (i.e.,
hydrocarbon feed is no longer passing through the furnace tube) and the
furnace tube is
decoked, after which the furnace is returned to operation.
The present disclosure relates to field of hydrocarbon cracking furnaces. In
particular, the present disclosure relates to start-up procedures for
hydrocarbon cracking
furnaces. The start-up procedures can be implemented for the first start-up of
the
hydrocarbon cracking furnace, after a decoking procedure, or any other time
the
hydrocarbon cracking furnace is starting up. The hydrocarbon cracking furnaces
are
typically taken off-line or may require being shut down on a periodic basis to
remove coke
accumulated on the internal surfaces of the furnace tubes. The present
disclosure is suitable
for any cracking process where dilution steam with hydrocarbon molecules are
converted to
other hydrocarbon molecules at elevated temperatures where coke is a byproduct
on the
furnace tubes or reactors, such as a fluid catalyst cracker or a steam
cracker, used to produce
alkenes from corresponding alkanes at elevated temperatures.
The present disclosure seeks to provide a method for managing initial coke
deposition on a hydrocarbon cracking furnace's tube wall inner surface by
maximizing
boundary layer turbulence of the hydrocarbon and any diluent present. The
greater the
velocity at the temperature in the furnace tube where cracking begins, the
greater the
scrubbing action of the turbulent flow, which minimizes the amount of coke
laydown onto
the furnace tube of a newly built, rebuilt, or de-coked hydrocarbon cracking
furnace. The
high gas velocity minimizes the thickness of the laminar flow of the boundary
layer at the
furnace tube wall providing a scouring effect upon the newly formed and
typically
deposited coke.
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Figure 1 shows a schematic drawing of a hydrocarbon cracking furnace 100. In
this
steam cracking furnace version, a dilution steam 102 is combined with a
hydrocarbon feed
104 to be cracked. The combined fluid enters a pre-heat section 106. The fluid
continues
through the apparatus into a radiant section 108. The fluid now contains
dilution steam,
some of the initial hydrocarbon feed and newly made cracked gas. Then the
fluid enters a
quench section 110.
Figure 2 shows a schematic drawing of a hydrocarbon cracking furnace 200. In
this
steam cracking furnace version, a dilution steam 202 is combined with a
hydrocarbon feed
204 to be cracked. The dilution steam 202 is preheated in a dilution steam pre-
heat section
212. The combined fluid enters a pre-heat section combined stream pre-heat
section 214.
The dilution steam pre-heat section 212 and the combined stream pre-heat
section 214 make
up a pre-heat section 206. The fluid continues through the apparatus into a
radiant section
208. The fluid now contains dilution steam, some of the initial hydrocarbon
feed and newly
made cracked gas. Then the fluid enters a quench section 210.
Figure 3 shows a schematic drawing of a hydrocarbon cracking furnace 300. In
this
steam cracking furnace version, dilution steam 302 is combined with
hydrocarbon feed 304
to be cracked. The dilution steam 302 is preheated in a dilution steam pre-
heat section 312
and hydrocarbon feed 304 is preheated in a hydrocarbon feed pre-heat section
314. The
combined fluid enters a combined stream pre-heat section 316. The dilution
steam pre-heat
section 312, the hydrocarbon feed pre-heat section 314 and the combined stream
pre-heat
section 316 make up a pre-heat section 306. The fluid continues through the
apparatus into a
radiant section 308. The fluid now contains dilution steam, some of the
initial hydrocarbon
feed and newly made cracked gas. The fluid enters a quench section 310.
Figure 4 shows a schematic drawing of a hydrocarbon cracking furnace 400. In
this
steam cracking furnace version, dilution steam 402 is combined with
hydrocarbon feed 404
to be cracked. The dilution steam 402 is preheated in a dilution steam pre-
heat section 412.
The hydrocarbon feed 404 is preheated in a hydrocarbon feed pre-heat section
414. The
hydrocarbon feed 404 to be cracked joins the dilution steam 402 on the outside
of
hydrocarbon cracking furnace 400. The combined fluid re-enters the combined
stream pre-
heat section 416. The dilution steam pre-heat section 412, the hydrocarbon
feed pre-heat
section 414 and the combined stream pre-heat section 416 make up a pre-heat
section 406.
The fluid continues through the apparatus into a radiant section 408. The
fluid now contains
dilution steam, some of the initial hydrocarbon feed and newly made cracked
gas. The fluid
enters a quench section 410.
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Decoking
In decoking a steam cracker furnace several different methods are available.
In one
method, the coke is physically scoured from the internal furnace tube walls.
Typically, a
relatively high velocity stream of air, steam or a mixture thereof passes
through the furnace
tube resulting in small particulate materials being included in the effluent
stream. As the
particulates pass through the furnace tube, the coke on the internal wall is
scoured or eroded
from the wall. One issue with this type of treatment is the erosion of the
internal surface of
the furnace tube, fittings and downstream equipment. An additional concern
with this type
of treatment is downstream plugging with coke particulates scoured from the
furnace tube
walls.
An alternate treatment to decoke the furnace tube is to react or "burn" the
carbon
accumulation from the furnace tube wall. When the furnace is taken off-line,
air and steam
are passed through the furnace tube at an elevated temperature to cause the
coke to react or
burn. The progress of the decoking process may be measured in several
different ways
including measuring the carbon dioxide and carbon monoxide content in the
effluent gasses
leaving the furnace, measuring the furnace tube metal temperature, measuring
changes in
the furnace tube outlet pressure or changes in the furnace tube fouling
factors. A mixture of
steam and air is passed through the coil while it is maintained at an elevated
temperature
from about 750 C to about 900 C, in some embodiments from 780 C to 850 C in
some
embodiments from 800 C to 830 C. The amount of air fed to the tube or coil
depends on the
furnace and the coil design. In some instances, the air may be fed to the coil
at a rate from
about 10 kg/hr to about 1000 kg/hour. Steam is fed to the reactor to provide
an initial weight
ratio of steam to air from about 200:1 to about 170:3. The decoke is completed
when the
amount of gasified carbon (CO2 and CO) in the effluent stream from the tube or
coil is
below about 2,000 ppm of CO2. In some embodiments of the procedure, the rate
of air feed
to the coil may be increased up to about 1000 kg/hr/coil as a post burn and/or
as a surface
polishing step.
During the decoke procedure the temperature in the combustion side of the
cracker
(sometimes called the radiant box or zone) may range from about 760 C to about
1100 C.
The rate of decoking needs to be controlled to minimize or limit spalling of
coke
from the inner surface of the coil as this may interfere with downstream
operation. Also
during decoking, the external temperature of the tube should be maintained as
uniform as
possible to prevent damage to the tube.
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The decoking may be finished with a steam scour at a steam feed rate of not
less
than 3500 kg/hr/reactor for a time from 0.5 to 10 hours, in some embodiments
from about 6
to 9 hours under the same temperature conditions as the decoke burn-out
process.
In some embodiments, an anti-coking agent may also be included in the steam
feed
for the polish treatment or subsequent to the polish treatment. Many anti-
coking agents are
known to those skilled in the art. In some embodiments the anti-coking agent
may be
chosen from compounds of the formula RSA' with n being the mean sulphur number
ranging from 1 to 12 and R and R' chosen from H and a linear or branched C1-C6
alkyl,
cycloalkyl or aryl radicals. The anti-coking agent is added to the polish feed
or a steam feed
if the treatment is subsequent to the polish in an amount from 15 ppm to 2,500
ppm, for a
period of time from 0.5 to 24, hours, preferably from about 1 to 6 hours at
which time
decreasing the dosing rate may begin.
Materials of the Furnace Tubes
The furnace tubes in hydrocarbon cracking furnaces are typically made of
steel. The
present disclosure is applicable to steels typically including at least 12 wt%
Cr, preferably at
least 16 wt% of Cr. The steel may be chosen from 304 stainless steel, 310
stainless steel,
315 stainless steel, 316 stainless steel, austenitic stainless steel and HP,
HT, HU, HK, HW
and HX stainless steel. In one embodiment the stainless steel, preferably heat
resistant
stainless steel typically includes from 13 to 50, preferably 20 to 50, most
preferably from 20
to 38 wt% of Cr. The stainless steel may further include from 20 to 50,
preferably from 25
to 50 most preferably from 25 to 48, desirably from about 30 to 45 wt% of Ni.
The balance
of the stainless steel is substantially iron.
The present disclosure may also be used with nickel and/or cobalt based
extreme
austenitic high temperature alloys (HTAs). Typically, the HTAs include a major
amount of
nickel or cobalt. Typically, the high temperature nickel-based alloys include
from about 50
to 70, preferably from about 55 to 65 wt% of Ni; from about 20 to 10 wt% of
Cr; from
about 20 to 10 wt% of Co; and from about 5 to 9 wt% of Fe and the balance one
or more of
the trace elements noted below to bring the composition up to 100 wt%.
Typically, the high
temperature cobalt based alloys include from 40 to 65 wt% of Co; from 15 to 20
wt% of Cr;
from 20 to 13 wt% of Ni; less than 4 wt% of Fe and the balance one or more of
the trace
elements noted below to bring the composition up to 100 wt%.
In some embodiments of the disclosure the substrate may further include at
least 0.2
wt%, up to 3 wt% typically 1.0 wt%, up to 2.5 wt% preferably not more than 2
wt% of
manganese from 0.3 to 2, preferably 0.8 to 1.6 typically less than 1.9 wt% of
Si; less than 3,
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typically less than 2 wt% of titanium, niobium (typically less than 2.0,
preferably less than
1.5 wt% of niobium) and all other trace metals; and carbon in an amount of
less than 2.0
wt%.
The present disclosure may also be used with 35 wt% nickel and 45 wt% chromium
based alloys with an amount of aluminum of up to 4% with a propensity to form
an
aluminum oxide layer or an alumina layer on the inner surface of a reactor or
pass.
Furnace Tube Boundary Layer
The process of the present disclosure uses the practice of managing initial
coke
deposition in a cracking furnace's radiant coil internal surface or tube wall
inner surface by
maximizing the boundary layer turbulence of the furnace tube with the
characteristics of the
fluid being cracked, i.e. hydrocarbon plus diluent.
As the gas mix enters the furnace inlet tube at a high pressure, and as it
increases in
temperature along the length of the furnace tubes, the volumetric change in
the gas results in
a velocity increase of the gas as it travels to the outlet end of the furnace.
The greater the
velocity at the point of incipient cracking, the greater the surface
turbulence minimizing the
amount coke laydown onto the internal surface of a furnace tube of a newly
built, rebuilt or
de-coked hydrocarbon cracking furnace. The high gas velocity minimizes the
thickness of
the laminar flow of the boundary at the furnace tube wall providing a scouring
effect upon
the newly formed and deposited coke.
Using the Ideal Gas Law, desired gas velocity is determined by including the
mass
flow rate, molecular weight of the fluid, temperature of the fluid, the
pressure, and area of
the tube, as calculated thusly:
r-rti mg)
2;1 RI _________________________________________________ 22 la
MIV2
V 3 13"ab8 3600Pabs
V' ____________________________________________________________________
A
wherein v is gas velocity (m/s), V is gas volume (m3/s), A is pipe area (m2),
rn is mass flow
rate of a gas (g/hr), MW is a molecular weight (g/mol) of a gas, z is the
compressibility
factor of a gas, R is the ideal gas constant (8.314 J=mo1l=K-1), T is the
temperature (K), P
- abs
is the absolute pressure (Pa), and r is the pipe radius (m), and the
subscripts 1 and 2 refer to
gas 1 and gas 2, respectively. The ellipsis indicates there could be more than
two
components in the gas.
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A feed gas chromatograph (GC) can provide the number average molecular weight
of the hydrocarbon portion at the inlet of the furnace tube. For example, a
hydrocarbon
cracking furnace might have a front-end "sweetening" system, wherein about 65%
of the
total hydrocarbon feed to the hydrocarbon cracking furnace has gone through an
amine
contactor and has become saturated with water. This hydrocarbon can be blended
with dry,
recycled hydrocarbon so the water content's influence on the molecular weight
of the
hydrocarbon is not considered. The molecular weight of the dilution steam can
be estimated
as 18.015. The number average molecular weight of the fluid entering the
furnace can then
calculated thusly:
MW
enter
lithc / (MW he
from v(.. rnsIgam / (18,015)
where rnhc is the total mass flow of hydrocarbon, and thsteam is the total
mass flow of the
dilution stream. An analogous method can be used to calculate the number
average
molecular weight of the gas at the outlet of the furnace tube.
The number average molecular weights can be varied by changing the composition
of the fluid, such as by varying the steam to hydrocarbon ratio, or changing
the types of
hydrocarbons in the feed.
Start-Up Procedure
The start-up procedure after commissioning a furnace tube, or after a decoking
has
taken place, or for any other reason that the hydrocarbon cracking furnace is
starting up can
include a number of steps that are well known in the art.
The fluid velocity at the point of incipient cracking is not typically
measured or
controlled. If not measured, the fluid velocity at the point of incipient
cracking can be
estimated by measuring or calculating the number average molecular weight of
the fluid at
the inlet of the furnace tube, measuring or calculating the number average
molecular weight
of the fluid at the outlet of the furnace tube, calculating an average of the
number average
molecular weights of the fluids at the inlet and the outlet to estimate the
number average
molecular weight at the point of incipient cracking, measuring or calculating
the fluid
velocities and the fluid pressures at the inlet and outlet and a pressure drop
of the fluid from
the inlet to the outlet. Using the Ideal Gas Law as above, the fluid velocity
at the point of
incipient cracking can be calculated.
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The fluid velocity at the point of incipient cracking is surprisingly a key
variable in
the operation of an industrial stream hydrocarbon cracking furnace, including
the run length
of the furnace before it needs to be shut down to perform a decoke. A
hydrocarbon cracking
furnace start-up in which the fluid velocity at the point of incipient
cracking is maintained at
a sufficient velocity from the start-up shall reduce the coke buildup on the
inner walls of the
furnace tube. The fluid velocity at the point of incipient cracking should be
at least about 80
m/s to about 120 m/s, preferably at least about 85 m/s to about 115 m/s,
preferably at least
about 90 m/s to about 115 m/s, preferably at least about 90 m/s to about 110
m/s, preferably
at least about 85 m/s to about 105 m/s, preferably at least about 90 m/s to
about 105 m/s,
preferably at least about 95 m/s to about 105 m/s. The fluid velocity at the
point of incipient
cracking, chosen to be 525 C, in this disclosure, should be maintained for at
least five (5)
24-hour days, preferably 10 days, preferably 20 days.
The fluid velocity entering the radiant section can be greater than 295 ft/sec
or 90
m/s, preferably greater than 311 ft/sec or 95 m/s, preferably greater than 340
ft/sec or 103.6
m/s. This velocity can be attained within five hours of introducing feed into
the furnace
tube, preferably with three hours, most preferably within less than 60
minutes.
EXAMPLES
The following example is presented for the purpose of illustrating selected
embodiments of this disclosure; it being understood that the example presented
does not
limit the claims.
Figure 5 shows the change in gas velocity (fluid velocity) over a 500 hundred
day
period, during which three consecutive runs of an industrial steam hydrocarbon
cracking
furnace were conducted. The three bars on the figure are plotted versus the x-
axis showing
82 days (run 1), 39 days (run 2), and 343 days (run 3) for each of the three
runs. The days
indicates the number of cracking days, which is the number of days online,
from
hydrocarbon feed-in to hydrocarbon feed-out. The furnace was decoked after
each run.
The left y-axis on Figure 5 indicates the calculated value of the fluid
velocity at the
point of incipient cracking, the values calculated using the procedure
described above.
An example calculation of the gas velocity at the 525 C point of the coil for
Figure
5, run 1, is shown as follows:
= Hydrocarbon feed composition: >98 mol% ethane
= Hydrocarbon feed MW: 29.96 g/mol
= z for ethane = 0.9915
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CA 03166744 2022-07-04
WO 2021/148942
PCT/IB2021/050384
= Mass of hydrocarbon feed: 5,000,000 g/hr
= Diluent composition: >99 mol% gaseous water
= Diluent MW: 18.015 g/mol
= z for diluent: 0.9613
= Mass of diluent: 1,500,000 g/hr
= Incipient cracking temperature: 525 C + 273.15 = 698.15 K
= Pressure at incipient cracking: 300,000 Pa + 92,000 Pa (ambient pressure
at
900 m above sea level)
= R = 8.31441 J=mo1-1=K-1
= Time t = 3600 s/hr
= Pi = 3.141593
Therefore velocity= 91.14 m/s.
For run 1, the gas velocity at the point of incipient cracking was only
allowed to
exceed 90 m/s for three days, resulting in a run length of 82 days before the
hydrocarbon
cracking furnace required decoking. For run 2, the gas velocity at the point
of incipient
cracking was only allowed to exceed 90 m/s for less than 1.5 days, resulting
in a run length
of 39 days before the hydrocarbon cracking furnace required decoking. Finally,
for run 3,
the gas velocity at the point of incipient cracking was allowed to exceed 90
m/s for greater
than 20 days, resulting a run length of 343 days before the hydrocarbon
cracking furnace
required decoking.
These results demonstrate that run length of a hydrocarbon cracking furnace
can be
improved by controlling the fluid velocity at the point of incipient cracking,
including
maintaining a fluid velocity at the point of incipient cracking of at least 90
m/s, for at least
the first 5 days after start-up.
INDUSTIAL APPLICABILITY
The disclosure is related to operation of hydrocarbon cracking furnaces.
Specifically, a start-up procedure is disclosed which allows longer run times
before
decoking is required.
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