Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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COATING TO REDUCE COKING AND ASSIST WITH DECOKING IN
TRANSFER LINE HEAT EXCHANGER
Field of the Invention
[0001] The invention relates generally to a method of inhibiting coke or
carbon formation and
allowing easier cleaning of metal surfaces of processing equipment during high
temperature
processing of hydrocarbons. More particularly, the invention relates to the
coating of certain
surfaces of a transfer line heat exchanger with a boron-nitride composition to
reduce coke
formation and allow easier cleaning of those surfaces.
Background Art
[0002] In traditional pyrolysis processing using pyrolysis furnaces,
mixtures of hydrocarbons
and steam flow through long coils or tubes which are heated by combustion
gases to produce
olefins, such as ethylene and propylene, as well as other valuable by-
products. Heat is transferred
from the hot combustion gases to the hydrocarbon feedstock passing within the
coils. The
hydrocarbon feedstock is heated within the coils to temperatures typically in
the range of about
7500 to 950 C to form the product stream.
[0003] After passing through the pyrolysis furnace, the product stream is
typically cooled or
"quenched" in a transfer line heat exchanger (TLE) to both stop the reaction,
and to cool for
processing and separation,. A TLE is designed to recover sensible heat from
the hot product
stream leaving the pyrolysis furnace. Heat is transferred from the hot product
mixture to low
pressure steam in the TLE to form high-pressure steam.
[0004] Coke formation is a traditional problem in the TLE as hydrocarbons
are
dehydrogenated, forming a solid residue on the metal and refractory surfaces
of the hot product
side of the TLE. Coke formation and collection in the TLE typically results in
poorer heat
transfer, which in turn results in decreased production of high-pressure
steam. Coke formation in
the TLE also often results in a larger pressure drop across the TLE. This
problem is particularly
acute in the inlet cones of the TLE.
[0005] The typical operating cycle for a TLE is to operate for a period of
time cooling the
product stream from the pyrolysis furnace. During this operation coke forms in
the inlet cone
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plugging up the tubes and tube sheet of the TLE and restricting flow. When the
pressure drop
becomes too high, the TLE will be hot cleaned (decoking cycle) using steam
injected into the
inlet cone to remove coke and open up the tubes of the TLE so more product gas
can flow and
there is less pressure drop. This is only partially effective and after two to
four cycles of
operation and hot cleaning, the TLE inlet cone must be removed so the coke can
be removed
through more aggressive methods. This process of removal of the coke on the
TLE inlet cone is
typically accomplished mechanically, usually entailing hammers and chisels,
which also
damages the inlet cone refractory and the tubesheet of the TLE. Then, the
operation cycle is
started over. Because of the damage done to the inlet cone refractory of the
TLE by the
mechanical removal of coke, the inlet cone refractory must be repaired or
replaced at a
significant cost.
[0006] In addition to the shutdown and startup process of the pyrolysis
furnace, the
mechanical de-coking operation of the TLE itself frequently requires several
days. De-coking
therefore results in increased downtime relative to olefin production time,
frequently amounting
to a several percent loss of olefin production during the course of a year. De-
coking is also
relatively expensive and requires appreciable labor and energy.
[0007] Previous methods have been used to control coke formation. For
instance, coke
inhibitors, i.e., chemical additives, or special coatings of metal surfaces
which suppress coke
formation have been used. Coke inhibitors/surface coating act to passivate
catalytically active
metal sites through chemical bonding interactions, and/or forming a thin layer
to physically
isolate metal sites from coke precursors in the process stream, and/or
interfering with those free
radical reactions leading to coke formation by blocking active sites on
surfaces. Such additives
are expensive and may lead to product gas stream quality issues.
[0008] What is needed is a method of controlling the growth and formation
of coke,
particularly in the inlet cone and tube sheet of the TLE. What is further
needed is a method to
reduce the damage done to the TLE inlet cone and tube sheet during a de-coking
operation.
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Brief Description of the Drawings
[0009] FIG. 1 is a depiction of coke formation on a metal surface of the
TLE.
[0010] FIG. I A is a pictorial depiction of filamentous coke.
[0011] FIG. 1B is a pictorial depiction of filamentous and free radical
coke.
Summary
[0012] The methods described herein relate generally to the field of
reducing and controlling
coke formation in the TLE during the olefin production process.
[0013] In one embodiment of the present disclosure, a method of controlling
coke formation
in pyrolysis furnace process equipment is described, wherein the surface of
the process
equipment is coated with a layer of boron nitride.
[0014] In another embodiment of the present disclosure, a method of
controlling coke
formation in a transfer line heat exchanger is described, wherein a boron
nitride paint is formed
by combining dry boron nitride powder with distilled water. The boron nitride
paint is applied to
a transfer line heat exchanger tube sheet.
[0015] In still another embodiment of the present disclosure, a boron
nitride paint for coating
the surfaces of pyrolysis furnace process equipment is described which
includes providing a dry
boron nitride powder. The dry boron nitride powder contains boron nitride with
a hexagonal
crystalline structure. Distilled water is provided and the boron nitride
powder is mixed with the
distilled water to form the boron nitride paint. The boron nitride paint
comprises between 20 and
45% boron nitride by weight.
Detailed Description
[0016] While not bound by theory, applicants have determined that coke is
classified into two
types: Catalytic coke and Pyrolytic coke. Catalytic coke is formed by
dehydrogenation of
hydrocarbon with catalytic action of metal components on the surface. Metal
components, such
as nickel and iron, may catalyze a hydrocarbon to reduce or eliminate hydrogen
from the olefin,
termed "dehydrogenation." Metal components presenting catalytic activities are
generally in the
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order of Ni>>>>Fe>>Cr, NiO>Ni, Fe0>Fe>Fe203. These metals and oxides catalyze
reaction
to form filament and coil type coke by successive dehydrogenation, as shown in
Fig. 1.
[0017] Catalytic coke tends to be very mechanically hard ("hard coke") and
normally
difficult to remove. Hard coke must often be removed from surfaces by
mechanical means. The
formation of catalytic coke is believed to be most often involved in beginning
the coking process
of the TLE and is believed to act as trap for pyrolytic coke
[0018] Pyrolytic coke is divided into gaseous and condensation coke.
Pyrolytic coke is softer
and generally easier to remove than catalytic coke. Gaseous coke is typically
formed by
dehydrogenation of such light olefinic hydrocarbon as acetylene. Condensation
coke is formed
by condensation, polymerization, and dehydrogenation of heavy aromatic
compounds. Pyroltyic
coke can be classified as globular, black mirror, fluffy or amorphous types
according to
morphology.
[0019] It is believed that the coking process of the TLE begins with the
hydrocarbon reacting
by catalyst action of metal components on metal surfaces and forms filamentous
coke, which
grows and provides deposit sites for various types of coke. Free radical
coking causes coke
filaments to thicken and, as catalytic coke filaments grow, carbon starts to
block metal surfaces.
Tar is formed as condensation collects in the filaments. The filaments formed
by catalytic
coking stop growing when metal particles are covered with carbon and,
afterwards, radical and
condensation coking become dominant.
[0020] FIG. 1 depicts one example of coke formation on typical metal
surface 20 of pyrolysis
furnace TLE 10. Filamentous coke filaments 30 grow outwardly (as shown by
upward arrow 35)
from typical metal surface 20. Free radical coke 40 tends to grow from
filamentous coke
filaments 30, causing filaments 30 to grow and thicken. Filamentous coke
filaments 30 tend to
continue to grow outwardly (as shown by horizontal arrows 45) until active
metal site 50 is
blocked by free radical coke 40, as indicated by blocked metal site 60. FIG.
IA is a pictorial
depiction of filamentous coke during initial formation. FIG. 1B is a pictorial
depiction of
filamentous coke and free radical coke formation.
[0021] In the pyrolysis furnace, the product gas includes the coke
precursors. When the
product gas is quenched to stop the reaction in the TLE before main
fractioner, coking is
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common on gas entry refractory cone wall, the TLE tube (inside) wall and
tubesheet surface.
This coking traditionally causes two problems: pressure drop and spalling,
i.e., the breaking of
the coke into small particles. Either of these problems can result in to
reduced product gas flow
and ultimately a shutdown for decoking.
[0022] In certain embodiments of the present disclosure, a coating is
applied to the TLE
tubesheet and inlet cone refractory and other elements to reduce the amount of
catalytic hard
coke formed during pyrolysis furnace operation. In at least some embodiments
of the present
invention, the coating applied to the TLE tubesheet and inlet cone refractory
is resistant to
temperatures of 1600 F (870 C) or higher, is easy to apply to existing
installations
(retrofitability). In these embodiments, the coating adheres well to metal
and/or refractory
surfaces and is effective as a thin film or layer. In certain embodiments of
the present disclosure,
the coating is water based; in other embodiments the coating is an organic
solvent based
material. In certain embodiments, the coating can be readily sprayed onto
surfaces at room
temperature. It is preferable that the coating be easy to dry and easy to cure
developing good
adhesion or bonding to metal and/or refractory surfaces. Typically, the
coating requires
minimum surface preparation.
[0023] In certain embodiments of the present invention, the coating
material is boron nitride.
Boron nitride has a number of crystalline structures, includes hexagonal,
cubic, and wurtzite. In
certain embodiments of the present disclosure, the TLE coating includes
hexagonal boron nitride.
[0024] The hexagonal crystalline structure of boron nitride is often
lubricious, exhibiting
some of the same properties of solid lubricants as graphite and molybendum
disulfide, with low
shear strength, low abrasivity, a good adherence of solid lubricant film and
superior thermal
stability. Hexagonal boron nitride typically has an oxidation threshold of
approximately 1562 F
(850 C) in an oxidizing atmosphere and up to 1832 F (1000 C) in a reducing
atmosphere. In
certain circumstances, hexagonal boron nitride can be used in inert or vacuum
atmospheres at
temperatures of approximately 3632 F (2000 C). Hexagonal boron nitride tends
to have a high
thermal conductivity with a low thermal expansion. Hexagonal boron nitride
typically has a
relatively high thermal shock resistance compared to other oxide based
refractory compounds.
and tends to be chemically inert to the compounds to which it is exposed in a
pyrolysis furnace.
It therefore tends to have a relatively high resistance to chemical attack
compared to other oxide
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based refractory compounds. and is resistant to chemical corrosion. Boron
nitride tends to have
an excellent parting plane compared to oxide based refractory compounds and
reduces sticking
in glass forming applications.
[0025] As those of skill in the art will appreciate, the thickness of the
coating on the TLE inlet
cone components and tubesheet depends on a number of factors including the
cost of the coating
material and the porosity and/or roughness of the surface to which it is
applied. In certain
embodiments of the present disclosure, the thickness of the coating applied to
the TLE is in the
range of about 25 microns to about 100 microns; in certain other embodiments
of the present
disclosure, the thickness of the coating is between about 40 microns and about
60 microns. In
still other embodiments the thickness of the coating material is about 50
microns. In certain
embodiments of the present disclosure, the thickness of the coating is not
uniform, but varies
across the tubesheet or inlet cone refractory surface, depending on the
characteristics of the
surface and the expected level of coke formation.
[0026] Application of the boron nitride coating may be accomplished by
traditional coating
application methods. In certain embodiments, a water-based boron nitride paint
is made from
dry boron nitride powder and water, such as distilled water. In particular
embodiments, the
water-based boron nitride paint is at 20 to 40% solids concentration by
weight; in other
embodiments, a 20 to 35% concentration is used, although as those of ordinary
skill in the art
will understand with the benefit of this disclosure, a greater or lesser
concentration of solids may
be used.
[0027] In certain embodiments of the present disclosure, the boron nitride
coating is
accomplished by use of a spray gun, such as a DeVilbiss Compact Pressure Spray
Gun, although
this example is non-limiting and any suitable spray gun may be used.
[0028] When applying the boron nitride coating, it is preferable to have
the surface to which
the coating is to be applied to be clean, dry, and as free from grease or oil
as is practical. In
certain embodiments of the present disclosure, the TLE inlet cone and
tubesheet have not been
previously used, i.e., they have not been exposed to process chemicals. In
such embodiments, it
may be necessary to roughen the surface of the area to which the coating is to
be applied in order
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to assist in physical adherence to the surface. Non-limiting examples of
surface roughening
include those specified by SSPC-SP6 and NACE 3-Commercial Blast Cleaning.
[0029] In certain other embodiments, such as when the TLE has been
previously used in a
pyrolysis furnace, the residual coke on metal surfaces such as the TLE
tubesheet inlet surface
and Intrabody Flow Diverter surface are first removed and cleaned by a
hydroblasting procedure,
typically at about 10,000 psi then allowed to air dry. In those embodiments,
typically the
residual coke on the TLE inlet cone refractory is removed and the inlet cone
refractory surface is
dusted to remove residual coke.
[0030] The boron nitride paint may be applied in a single coat or in
multiple coats to achieve
the desired thickness. In particular embodiment of the present disclosure,
when applying
multiple coats of the boron nitride paint, it may be necessary to allow drying
of the boron nitride
paint between coats. The drying may be performed at ambient temperature or at
an elevated
temperature, depending on need. The boron nitride paint may then be allowed to
cure after
completion of the application of all coats. Curing, like drying, may be
accomplished at ambient
temperature or at an elevated temperature depending on need. Typical curing
times are between
60 and 120 minutes, although more or less time may be necessary depending on
such factors as
the thickness of boron nitride paint, the relative humidity, and the ambient
temperature.
[0031] Following the application of the boron nitride paint, the TLE may be
reassembled and
placed in the discharge line of the pyrolysis furnace. It has been determined
by the applicants
that the boron nitride coating often allows for increased run times of the
pyrolysis furnace
between steam decoking. Further, it has been observed by applicants that run
times between
mechanical cleaning of the TLE can be significantly extended as compared to
run times of TLEs
with uncoated tubesheets and inlet cones. While not bound by theory,
applicants believe that the
boron nitride coating acts to inhibit formation of catalytic coke by reducing
or preventing contact
between the hot product stream and the metal catalysts by coating the metal
surfaces. While
pyrolytic coke continues to form, it softer than the hard catalytic coke and
may be more easily
cleaned by steam decoking. It is further believed that the boron nitride
coating provides a
surface with less friction than the uncoated surface, weakening coke adhesion
compared to the
uncoated surface.
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[0032] While this disclosure has focused on the use of the boron nitride
coating of the TLE
tubesheet and inlet cone, those of ordinary skill in the art, with the benefit
of this disclosure, will
recognize that the boron nitride coating may be used with process equipment
having metal
surfaces where coke forms and deposits, and where the temperature does not
exceed 1832
F(1000 C) in oxidation environment and 3632 F (2000 C) in a vacuum or inert
atmosphere.
Such equipment would include, but not be limited to the pyrolysis furnace, the
furnace tubing,
the transfer piping from the furnace to the TLE, the piping between the
primary and secondary
TLEs and the tubes inside the TLEs. This method could also apply to any
similar process where
coke is formed under similar high temperature process conditions.
[0033] This disclosure will now be further illustrated with respect to
certain specific examples
which are not intended to limit the invention, but rather to provide more
specific embodiments as
only a few of many possible embodiments.
Example 1
[0034] A previously used TLE was removed from a pyrolysis furnace process and
disassembled to allow access to the inlet tubesheet surface and TLE cone
internal components.
Coke was removed and the tubesheet and TLE inlet cone were cleaned by
hydroblasting. The TLE components were allowed to dry and remaining dust was
removed.
[0035] A boron nitride paint was made by combining distilled water with a
boron nitride
powder with a hexagonal crystalline structure. Sufficient boron nitride powder
was added to
reach a concentration of 28% concentration by weight.
[0036] A commercial spray gun was pressurized to between 20 and 40 psi using
compressed
air filtered to remove moisture and particulate. The boron nitride paint was
introduced into the
spray gun. A 50 micron thick coating of boron nitride was applied to the TLE
tubesheet and
inside surfaces of the TLE inlet cone. The application was made in two stages
with each stage
applying a 25 micron coating. The boron nitride coating was allowed to dry
approximately 30 to
60 minutes between coats at room temperature, The final boron nitride coating
was cured for 60
to 120 minutes at room temperature.
