Note: Descriptions are shown in the official language in which they were submitted.
REDUCED FOULING FROM THE CONVECTION SECTION OF A CRACKER
FIELD OF THE INVENTION
The present invention relates to an initial heat treatment of hydrocarbons at
temperatures up to about 560 C in for example the convection section of a
heater.
Typically the hydrocarbon is then fed to the radiant section of a furnace
where it is
further cracked at temperatures typically between 800 and 950 C. Downstream
of
the radiant section is a transfer line leading to a quench section. To extend
the
operation of the process it is desirable to minimize coke build up from the
convection
section.
Initially, when the tubes in a radiant section were not treated or there was
no
careful control over the cracking in the radiant section, the time between off
line
operation to "burn" out the accumulated coke, may have ranged from about 30 to
about 65 days. Now, the run time for the radiant section of the furnaces can
be
extended to 100 days or more. To achieve these longer run times, more effort
is
being focused on the reducing coke formation from the convection section.
BACKGROUND OF THE INVENTION
U.S. patent 4986222 issued Jan., 22 1991 to Pickell et al., assigned to Amoco
Corporation. The patent relates to furnaces for oil refiners or petrochemical
plants.
The patent discloses thermal treatment for high sulphur naphtha and hydrogen
to
produce gasoline and aromatic feedstocks, not olefins. At claim 8 and Col. 5
line 65
to Col. 6 line 2 the patent teaches stainless steel heat treated coils in the
convection
section. The arrangement is "wrong" as the convection section seems to convey
the
feed from the radiant section to the desulphurization unit. However, the
patent
teaches the concept of using stainless steel tubing in the convection section.
The
patent does not differentiate between the various types of stainless steel.
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U.S. 7402237 issued July 22, 2008 to McCoy assigned to ExxonMobil
Chemicals Patents Inc. teaches vanadium pentoxide would also destroy the
protective oxide layer on 304 stainless steel tubes to be used in the lower
part of the
convection section" (Col. 6 line 40 - 45). The reference clearly teaches using
304
stainless on the lower part of the convection section. The tubes in the
convection do
not appear to be coated. Again the reference does not differentiate between
the
various types of stainless steel. This patent is addressing feedstocks that
contain
salts and or particulates.
U.S. patent application 20110014372 published January 20, 2011 in the name
of Webber et al. The patent did not issue. It was filed by Lyondell. Paragraph
39
and 47 of the disclosure teach that conduit 5 may be stainless steel. From the
figure
conduit 5 passes through the convection section of the furnace. Also
paragraphs 35
and 42 teach the line through the convection section and the radiant section
may
have the same internal coating. The published application teaches it is
necessary
that phosphorus is coated on the internal surface of the tubes or passes.
Phosphorus can have several disadvantages. It is an environmental concern and
needs continued replenishment. If it enters the grain structure it lowers the
grain
boundary melting point weakening the metal. The passes or tubes used in the
convection sections of the furnace of the present disclosure do not have
internal
coatings.
The present invention seeks to provide for the use of a high Cr /Ni alloy
having less than about 70% iron, to have a significantly lower propensity to
coke
over conventional carbon steels and higher iron content stainless steels (e.g.
304).
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SUMMARY OF THE INVENTION
In one embodiment, the present invention provides a convection section of a
furnace to treat hydrocarbons, wherein not less than 50% of the coil length
upstream
from the outlet of the convection section, comprises less than 66 wt% of Fe
and the
balance a mixture of 23-26 wt % of Cr; 19-22 wt % of Ni; and one or more
components selected from the group consisting of C, Mn, Si, P, and S.
A further embodiment provides a convection section of a furnace wherein not
less than 75 % of the coil length upstream from the outlet of the convection
section
have the composition as above.
A further embodiment provides the convection section of a furnace to treat
hydrocarbons as above, wherein the coils further comprise 0.08 to 0.2 wt% C;
1.5 to
2.5 wt % Mn; 1.5 to 3 wt % of Si, 0.04 to 0.05 wt % P; and 0.25 to 0.35 wt% of
S.
A further embodiment provides the convection section of a furnace as above,
wherein the hydrocarbons comprise one or more from C2-4 paraffins.
A further embodiment provides a method to reduce fouling from the
convection section of a furnace to treat hydrocarbons wherein not less than
50% of
the coil length upstream from the outlet of the convection section comprise
less than
66 wt% of Fe and the balance a mixture of 23-26 wt % of Cr; 19-22 wt (:)/0 of
Ni; and
one or more components selected from the group consisting of C, Mn, Si, P, and
S.
A further embodiment provides an above method wherein not less than 75 %
of the coil length upstream from the outlet of the convection section have the
above
composition.
A further embodiment provides an above method wherein the coils further
comprise 0.08 to 0.2 wt% C; 1.5 to 2.5 wt % Mn; 1.5 to 3 wt % of Si, 0.04 to
0.05 wt
% P; and 0.25 to 0.35 wt% of S.
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A further embodiment provides an above method wherein the hydrocarbons
comprise one or more C2-4 paraffins.
A further embodiment provides an above method wherein the hydrocarbon is
ethane.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of an ethylene furnace.
Figure 2 is a plot of flow coefficients for an ethylene cracker in which the
carbon steel tubes in the convection section were replaced with stainless
steel 310
(SS 310).
DETAILED DESCRIPTION
Numbers Ranges
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
invention 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.
Notwithstanding that the numerical ranges and parameters setting forth the
broad scope of the invention are approximations, the numerical values set
forth in
the specific examples are reported as precisely as possible. Any numerical
values,
however, inherently contain certain errors necessarily resulting from the
standard
deviation found in their respective testing measurements.
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Also, it should be understood that any numerical range recited herein is
intended to include all sub-ranges subsumed therein. For example, a range of
"1 to
10" is intended to include all sub-ranges between and including the recited
minimum
value of 1 and the recited maximum value of 10; that is, having a minimum
value
equal to or greater than 1 and a maximum value of equal to or less than 10.
Because the disclosed numerical ranges are continuous, they include every
value
between the minimum and maximum values. Unless expressly indicated otherwise,
the various numerical ranges specified in this application are approximations.
All compositional ranges expressed herein are limited in total to and do not
exceed 100 percent (volume percent or weight percent or mole percent) in
practice.
Where multiple components can be present in a composition, the sum of the
maximum amounts of each component can exceed 100 percent, with the
understanding that, and as those skilled in the art readily understand, the
amounts of
the components actually used will conform to the maximum of 100 percent.
Figure 1 is a schematic diagram of a furnace (cracker) which may be used in
any conventional application. One particularly useful application is in the
cracking of
chemicals feedstocks, preferably ethane, but the furnace could also be used
with a
naphtha feed or mixed feeds.
In a cracker 1, such as an ethylene cracker, the feed stock 2 enters a coil 3
typically passing through the exhaust area 4, typically referred to as the
convection
section. The feed is preheated in the convection section to a sub-cracking
temperature. Typically in a cracker, steam is fed to the convection section 4
through
a parallel set of coils 6 to preheat it and then blended with the feedstock
stream. At
the back end of the cracker is a quench unit 7 which cools the cracked gas and
heats water in a heat exchanger 8 to generate steam. Steam from the heat
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exchanger 8 is fed through a separate set of coils 9 in the convection section
4 to
further heat the steam for plant use.
The feed exits the convection section typically travels through the furnace
radiant section 5. In the furnace radiant section the coil may also be
serpentine in
configuration. There are a number of furnace configurations such as a single
radiant
section (fire box per the figure), parallel radiant sections (fire boxes), or
it may
comprise two radiant sections (fire boxes) in series, one cooler (cold box)
and one
hotter (hot box). However, both radiant sections typically share a common
exhaust
or convection section 4.
The feed is further heated in the furnace radiant sections by number of
burners 10, fed with a hydrocarbon fuel. Preferably the fuel is a fluid, most
preferably a gas such as natural gas or natural gas mixture with other
combustible
gases, such as hydrogen. Low pressure combustion air is provided to burners 10
from a fan 11 through a duct system 12 or naturally aspirated through burner
10
registers. Each burner 10 has an associated variable air or oxygen flow
controller 13
such as a damper or valve.
The control system for the furnace comprises a number of sensors or probes.
In the arch 4 there is an oxygen probe 14 connected by electrical or optical
cable 15
to a microprocessor 16. The microprocessor 16 is connected by an electrical or
optical cable 17 to the fan 11. Increasing or decreasing the air fan speed or
adjusting a set of fan louvers is used to control the amount of air supplied
to the
burners and, thus, dry oxygen in the exhaust gases.
Microprocessor 16 is connected by one or more electrical or optical cable(s)
18 to each air flow controller 13, and it can process the signals from the
following
probes:
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i) an air pressure probe 19 which reads the pressure drop across the air
or oxygen flow controller;
ii) a temperature probe 20, attached to the outlet piping of the radiant
section and measuring the cracked gas temperature;
iii) a temperature probe 21, installed on the furnace wall proximate,
typically within 1.5 meter about 5 feet from, the burner (e.g. the radiant
section of the
furnace wall);
iv) temperature probes 22 on sections of the coil or furnace tubes 3;
v) one or more (e.g. an array) remote sensor(s) 23 may be mounted on or
adjacent a furnace wall, in a further embodiment.
While a significant amount of metallurgy has been developed to reduce coking
in the radiant sections of the furnace, generally in the convection section
the coils
are made from carbon steel. As the furnace run length increases, coking from
the
tubes or passes in the convection section of the furnace can become a problem.
The issue of coking from the coils in the convection section of a furnace to
treat hydrocarbons may be reduced by using a coil comprising less than 66 wt%
of
Fe and the balance a mixture of 23-26 wt % of Cr; 19-22 wt % of Ni; and one or
more
components selected from the group consisting of C, Mn, Si, P, and S such as
for
example a steel selected from the group consisting of stainless steel (SS)
310, 310S
and 314 (SAE designation). In some embodiments the steel may further comprise
0.08 to 0.2 wt% C; 1.5 to 2.5 wt % Mn; 1.5 to 3 wt % of Si, 0.04 to 0.05 wt %
P; and
0.25 to 0.35 wt% of S. The steel may further comprise a total of trace
elements such
as titanium, niobium, in a total amount of up to 2 wt %.
The coil in the convection section may be made entirely of one of the above
.. stainless steels. Composite coils could also be used comprising sections of
the coil
made using the above steel in the hotter areas of the convection section
(closer to
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the outlet of the convection section) in combination with sections of the coil
made
from steels having a higher iron content such as carbon steel and low nickel
(typically less than 12 wt% Ni) austenitic stainless steels in the cooler
areas of the
convection section (farther from the feed outlet of the convection section).
Typically
wherein not less than 50%, in some embodiments not less than 75%, of the coil
length (immediately) upstream from the feed outlet of the convection section
comprise less than 66 wt% of Fe and the balance a mixture of 23-26 wt % of Cr;
19-
22 wt % of Ni; and one or more components selected from the group consisting
of C,
Mn, Si, P, and S.
The coils or passes of the present invention do not have an internal coating
of
phosphorus.
The present invention also provides a method to reduce fouling from the
convection section of a furnace to treat hydrocarbons wherein not less than
50% of
the coil length from the outlet of the convection section comprising less than
66 wt%
of Fe and the balance a mixture of 23-26 wt % of Cr; 19-22 wt % of Ni; and one
or
more components selected from the group consisting of C, Mn, Si, P, and S.
The convection section feed typically operates at temperatures from about
350 C to about 800 C, typically 400 C to about 800 C. The feed may be
naphtha
feed or derived from natural gas. Typically the feed will be a C2_4 paraffin.
In some
embodiments the feed predominantly comprises, more than about 85 vol. %
ethane.
In some instances coil treatment agents such as dimethyl disulphide may be
added to the feed in amounts up to about 0.005 wt %.
In one embodiment the present invention provides a convection section of a
furnace to treat hydrocarbons wherein not less than 50% of the coil length
upstream
.. from the feed outlet of the convection section, comprise less than 66 wt%
of Fe and
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the balance a mixture of 23-26 wt A of Cr; 19-22 wt % of Ni; and one or more
components selected from the group consisting of C, Mn, Si, P, and S.
In a further embodiment there is provided a convection section of a furnace
wherein not less than 75 % of the coil length upstream from the feed outlet of
the
convection section have the composition of any other embodiment.
in a further embodiment there is provided the convection section of a furnace
to treat hydrocarbons according to one or more other embodiments wherein the
coils
further comprise 0.08 to 0.2 wt% C; 1.5 to 2.5 wt % Mn; 1.5 to 3 wt A of Si,
0.04 to
0.05 wt % P; and 0.25 to 0.35 wt% of S.
In a further embodiment there is provided the convection section of a furnace
according one or more other embodiments, wherein the hydrocarbons comprise one
or more from C2-4 paraffins.
In a further embodiment there is provided a method to reduce fouling in the
convection section of a furnace to treat hydrocarbons wherein not less than
50% of
the coil length from the feed outlet of the convection section comprising less
than 66
wt% of Fe and the balance a mixture of 23-26 wt % of Cr; 19-22 wt % of Ni; and
one
or more components selected from the group consisting of C, Mn, Si, P, and S.
In a further embodiment there is provided a method, wherein not less than 75
% of the coil length from the feed outlet of the convection section have the
composition of one or more other embodiments.
In a further embodiment there is provided a method according to one or more
other embodiments, wherein the coils further comprise 0.08 to 0.2 wt% C; 1.5
to 2.5
wt % Mn; 1.5 to 3 wt % of Si, 0.04 to 0.05 wt % P; and 0.25 to 0.35 wt% of S.
In a further embodiment there is provided a method according to one or more
other embodiments, wherein the hydrocarbons comprise one or more from C2-4
paraffins.
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In a further embodiment there is provided a method according to one or more
embodiments, wherein the hydrocarbon is ethane.
The present invention is illustrated by the following non limiting examples.
In a commercial ethylene cracker using ethane as a feed having a high
nickel/chrome stainless steel in the furnace tubes in the radiant section and
carbon
steel in the convection section, the better run times between decoking ranges
from
134 days to 238 days. The convection section of the furnace was rebuilt and
the
carbon steel tubes were replaced with SS310 tubes. The run time was extended
to
406 days. The furnace run was terminated due to a full plant outage and not
because the furnace required an outage. What is important to note is that
after the
rebuild the flow coefficient for the best run was significantly extended and
also the
profile for the flow coefficient was very flat compared to the prior runs with
carbon
steel.
This is shown in figure 2.
This shows using stainless steel 310 significantly reduces coking from the
convection section of a cracker.
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