Note: Descriptions are shown in the official language in which they were submitted.
21S5~ 33294CA
METHOD FOR PROMOTING THE SPALLING OF
COKE PRODUCED DURING THE THERMAL CRACKING
OF HYDROCARBONS
The present invention relates to processes for the thermal cracking
of hydrocarbons. More specifically, the present invention relates to a method for
promoting the spalling of coke produced during the pyrolytic cracking of
hydrocarbons.
In a process for producing an olefin compound, a fluid stream
containing a saturated hydrocarbon such as ethane, propane, butane, pentane,
naphtha, or lui2~lules of two or more thereof is fed into a thermal (or pyrolytic)
cracking furnace. A diluent fluid such as steam is usually combined with the
hydrocarbon feed m~ter1~1 being introduced into the cracking furnace.
Within the furnace, the saturated hydrocarbon is converted into an
olefinic compound. For example, an ethane stream introduced into the cracking
furnace is converted into ethylene and appreciable amounts of other hydrocarbons.
2155 044 33294CA
A propane stream introduced into the furnace is converted to ethylene and
propylene, and appreciable amounts of other hydrocarbons. Similarly, a ~ e
of saturated hydrocarbons cont~ining ethane, propane, butane, pentane and
naphtha is converted to a mixture of olefinic compounds co~ g ethylene,
5 propylene, butenes, pentenes, and n~phth~lene. Olefinic compounds are an
important class of industrial chemicals. For example, ethylene is a monomer or
comonnmer for m~king polyethylene and other polymers. Other uses of olefinic
compounds are well known to those skilled in the art.
As a result of the thermal cracking of a hydrocarbon, the cracked
10 product stream can also contain appreciable quantities of hydrogen, methane,
acetylene, carbon monoxide, carbon dioxide, and pyrolytic products ot_er than the
olefinic compounds.
During the thermal or pyrolytic cracking of hydrocarbons, a semi-
pure carbon which is termed as "coke" is formed. The coke formed in the cracking
15 process normally deposits upon the surfaces of the cracking tubes of the pyrolytic
cracking furnace of such process. The accumulation of coke upon the surfaces of
the cracking tubes ultimately requires the shut down of the cracking furnace in
order to burn off the coke deposits. The accllm~ tion of coke deposits
necessitates the periodic shutdown of the cracking furnace due to the excessive
20 pressure drop across the cracking furnace tubes and the higher furnace
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temperatures required as a result of the therm~l insulating properties of the
deposited coke.
Compositions known as antifoulants have been used to inhibit ~e
formation and deposition of coke upon the surfaces of cracking furnace tubes and
5 on the metal surfaces of downstream heat exchangers and other process
equipment. In spite of the inhibition in the formation of coke by use of such
antifoulants, the coke buildup during the cracking of hydrocarbons still occurs but
at a slower rate. It can be desirable for there to be minim~l buildup or deposition
of coke upon the cracker tube surfaces. A reduction in coke accllm~ tion on the
10 cracking tubes will increase the run length of a cracking furnace between
shutdowns and thereby improve the cracking operation.
It is an object of this invention to provide an illl~,roved process for
cracking saturated hydrocarbons to produce olefin end-products.
Another object of this inven*on is to provide a method for limi*ng
15 the deposition or accllmlll~*on of coke upon the tube surfaces of a cracking
furnace.
A still further object of this inven*on is to provide a method for
promoting the spalling of coke produced during the pyrolytic cracking of coke
produced during the pyrolytic cracking of hydrocarbons to prevent, limit, or
20 reduce the buildup of coke deposits upon the cracker tube surfaces to thereby
increase cracker furnace run length between shutdowns.
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In accordance with the present invention, spalling of coke produced
during the pyrolytic cracking of a hydrocarbon stream is promoted by passing the
hydrocarbon stream through a tube of a pyrolytic cracking furnace operated under
suitable cracking conditions to thereby produce a cracked product stream. An
5 antifoulant co"~ g tin but having a substantial absence of silicon is added to
the hydrocarbon stream in an amount sufficient to promote the spalling of coke
produced during the pyrolytic cracking of the hydrocarbon stream.
Another embodiment of the invention includes contacting an
antifoulant co,.~ g tin but having a substantial absence of silicon with a tube
10 of a pyrolytic cracking furnace under suitable treatment conditions to thereby
provide a treated tube. A hydrocarbon stream is passed through the treated tube
which is operated under suitable cracking conditions to thereby produce a cracked
product stream having therein spalled coke.
Other objects and advantages of the invention will be apparent from
15 the description of the invention and the appended claims thereof as well as from
the detailed description of the drawing in which:
FIG. 1 is a schem~tic diagram representing the portion of an ethylene
cracking process that includes pyrolytic cracking furnace means and other
elements of the novel process;
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FIG. 2 is a bar chart showing the amount of coke spalled during
ethane cracking from an HK4M alloy tube treated with dimethylsulfide, tin, or a
tin/silicon ~ e; and
FIG. 3 is a bar chart showing the amount of coke spalled during
5 ethane cracking from an HP Modified alloy tube treated with dimethylsulfide, tin,
or tin/silicon.
The process of this invention involves the pyrolytic cracking of
hydrocarbons to produce desirable hydrocarbon end-products. A hydrocarbon
stream is fed or charged to pyrolytic cracking furnace means wherein the
10 hydrocarbon stream is subjected to a severe, high-temperature ellvirol~"~ent to
produce cracked gases. The hydrocarbon stream can comprise any type of
hydrocarbon that is suitable for pyrolytic cracking to olefin compounds.
Preferably, however, the hydrocarbon stream can comprise paraffin hydrocarbons
selected from the group consisting of ethane, propane, butane, pentane, naphtha,
15 and ~ s of any two or more thereof. Naphtha can generally be described as
a complex hydrocarbon mixture having a boiling range of from about 180F to
about 400F as d~l~llllil~ed by the standard testing methods of the American
Society of Testing M~tçri~l~ (ASTM).
As an optional feature of the invention, the hydrocarbon feed being
20 charged to pyrolytic cracking furnace means can be in~im~tely mixed with a
diluent prior to entering pyrolytic cracking furnace means. This diluent can serve
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several positive functions, one of which includes providing desirable reaction
conditions within pyrolytic cracking furnace means for producing the desired
reactant end-products. The diluent does this by providing for a lower partial
pressure of hydrocarbon feed fluid thereby enhancing the cracking reactions
necessary for obtaining the desired olefin products while reducing the amount ofundesirable reaction products such as hydrogen and methane. Also, the lower
partial plC;SSUI~ res~ ing from the ll~lule of the diluent fluid helps in minimi7:ing
the amount of coke deposits that form on the furnace tubes. While any suitable
diluent fluid that provides these benefits can be used, the prefelled diluent fluid
is steam.
The cracking reactions in(luce~ by pyrolytic cracking furnace means
can take place at any suitable temperature that will provide the necessary cracking
to the desirable end-products or to give a desired feed conversion. The actual
cracking temperature utilized will depend upon the composition of the
hydrocarbon feed stream and the desired feed conversion. Generally, the crackingtemperature can range uywaldly to about 2000F or greater depending upon the
amount of cracking or conversion desired and the molecular weight of the
feedstock being cracked. Preferably, however, the cracking temperature will be
in the range of from about 1200F to about l900F. Most preferably, the
cracking temperature can be in the range from 1500F to 1800F.
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The cracked hydrocarbon effluent or cracked hydrocarbons or
cracked product stream from pyrolytic cracking furnace means will generally be
a mixture of hydrocarbons in the gaseous phase. This ~ e of gaseous
hydrocarbons can comprise not only the desirable olefin compounds, such as
5 ethylene, propylene, butylene, and amylene, but also, this cracked hydrocarbon
stream can contain undesirable co~ ting components that include both
oxygenated compounds and acidic compounds and light ends such as hydrogen
and methane.
The cracking furnace means of the i~v~nlive method can be any
10 suitable thenn~l cracking ffirn~ce known in the art. The various cracking furnaces
are well known to those skilled in the art of cracking technology and the choice
of a suitable cracking furnace for use in a cracking process is generally a matter
of plerelel~ce. Such cracking filrn~ces are equipped with at least one cracking tube
to which the hydrocarbon feedstock is charged or fed. The cracking tube provides
15 for and defines a cracking zone contained within the cracking furnace. The
cracking furnace is lltili7e-l to release the heat energy required to provide for the
necessary cracking temperature within the cracking zone in order to induce
cracking reactions therein. Each cracking tube can have any geometry which
suitably defines a volume in which cracking reactions can take place and, thus,
20 will have an inside surface. The term "cracking temperature" as used herein is
defined as being the temperature within the cracking zone defined by a cracking
21SS044 33294CA
tube. The outside wall temperature of the cracking tube can, thus, be higher than
the cracking temperature and possibly substantially higher due to heat transfer
considerations. Typical ples~ s within the cracking zone will generally be in the
range of from about 0 psig to about 100 psig and, preferably from 0 psig to
5 60 psig.
The inventive method provides for or promotes the spalling of coke
produced during the pyrolytic cracking of a hydrocarbon stream. It has been
discovered that the treatment or treating of the tubes of a cracking furnace with a
tin compound only or, more specifically, with a composition having an absence
10 of silicon but comprising tin, coke spalling is promoted. Coke spalling occurs
when the coke formed in the cracking tubes during the cracking of hydrocarbon
either fails to adhere to the tube surfaces thereby forming a layer of coke or when
it is deposited upon the tube surfaces and thereafter chips, flakes or breaks off
such surfaces.
Coke spalling for many cracking operations can be undesirable if the
spalled coke results in damage or plugging of equipment located down stream
from the cracking furnace. However, in situations where the downstream
eqllipm~nt can handle the fr~nent~ of spalled coke or, ~lt~ tively, where means
is provided which can suitably remove the spalled coke contained in a cracked
20 product stream, the promotion of coke spalling can result in increasing the length
of time between decoking of the cracker furnace tubes; because, coke is removed
21~504~
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from the tube surfaces by spalling, or it is pr~v~nled from depositing or adhering
to the tube surfaces. By increasing the length of time between cracker tube
decokings, the furnace production down time is reduced thereby ill,provi~lg
cracking furnace productivity and throughput. Thus, if the eqllir)ment used to
5 process the cracker furnace product stream can handle the spalled coke without
detriment, or, if suitable removal or separation means for removing at least a
portion of the spalled coke contained in the cracked product stream can be
provided, then coke spalling can be desired.
A critical aspect of the instant invention is the use of a composition
10 having an absence of a silicon compound but comprising a tin compound. It has
been discovered that the tre~tment of the tubes of a cracking furnace in accordance
with the methods described herein with a tin compound only, as opposed to a
compound co~ both a tin compound and a silicon compound, unexpectedly
promotes the sp~lling of coke. When a composition cont~inin~ a combination of
15 tin and silicon is used to treat cracker furnace tubes, on the other hand, excessive
spalling of coke is not observed; rather, the formation and deposition of coke
appears to be inhibited. Thus, by combining tin and silicon certain antifoulant
benefits and properties are obtained that are different from those of an antifoulant
col,l;,il~ g a tin compound only or, alternatively, a material having an absence of
20 silicon but comprising tin.
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33294CA
- 10
Any suitable form of tin may be utilized in the antifoulant
composition having an absence of silicon and comprising tin. Element~l tin,
inorganic tin compounds and organic tin compounds as well as ~ ules of any
two or more thereof are suitable sources of tin. The term "tin" generally refers to
5 any one of these tin sources.
Examples of some inorganic tin compounds which can be used
include tin oxides such as stannous oxide and stannic oxide; tin sulfides such as
starmous sulfide and stannic sulfide; tin sulfates such as stannous sulfate and
stannic sulfate; stannic acids such as metastannic acid and thiostannic acid; tin
10 halides such as stannous fluoride, stannous chloride, stannous bromide, stannous
iodide, stannic fluoride, stannic chloride, stannic bromide and stannic iodide; tin
phosphates such as stannic phosphate; tin oxyhalides such as stannous oxychloride
and starmic oxychloride; and the like. Of the inorganic tin compounds those
which do not contain halogen are plerelled as the source of tin.
Examples of some organic tin compounds which can be used incl~lde
tin carboxylates such as stannous formate, stannous acetate, stannous butyrate,
stannous octoate, stannous decanoate, stannous oxalate, stannous benzoate, and
stannous cyclohexanecarboxylate; tin thiocarboxylates such as stannous
thioacetate and stannous dithioacetate; dihydrocarbyltin bis(hydrocarbyl
20 mercaptoaL~anoates) such as dibutyltin bis(isoocylmercaptoacetate) and
dipropyltin bis(butyl mercaptoacetate); tin thiocarbonates such as stannous O-ethyl
215~044
33294CA
11
dithiocarbonate; tin carbonates such as stannous propyl carbonate;
tetrahydrocarbyltin compounds such as tetramethyltin, tetraoctyltin,
tetradodecyltin, and tetraphenyltin; dihydroc~lJyllin oxides such as dipropyltin
oxide; dibutyltin oxide, dioctyltin oxide, and diphenyltin oxide; dihydrocarbyltin
S bis(hydrocarbyl mercaptide)s such as dibutyltin bis(dodecyl mercaptide); tin salts
of phenolic compounds such as stannous thiophenoxide; tin sulfonates such as
stannous ben7~neslllfonate and stannous-p-toluenesulfonate; tin carbamates such
as stannous diethylcarbamate; tin thiocarbamates such as stannous
propylthioc~l,~ and stannous diethyldithiocarbamate; tin phosphites such as
10 stannous diphenyl phosphite; tin phosphates such as stannous dipropyl phosphate;
tin thiophosphates such as stannous O,O-dipropyl thiophosphate, stannous
O,O-dipropyl dithiophosphate and stannic O,O-dipropyl dithiophosphate,
dihydrocarbyltin bis(O,O-dihydrocarbyl thiophosphate)s such as dibutyltin
bis(O,O-dipropyl dithiophosphate); and the like. Organic tin compounds are
15 preferred over inorganic compounds. At present tetrabutyltin is most preferred.
21~S044
33294CA
12
The term "silicon" as used herein refers to silicon sources such as
element~l silicon, inorganic silicon compounds and organic silicon compounds as
well as mixtures of any two or more thereof.
The antifoulant composition described herein is lltili7e~ in the
5 tre~tment of the surfaces of the cracking tubes of a pyrolytic cracking furnace.
The composition is contacted with surfaces of the cracking tubes either by
~eLIeaLillg the cracking tubes with the antifoulant prior to charging the tubes with
a hydrocarbon feed or by adding the antifoulant to the hydrocarbon feed in an
amount effective for treating the cracker tubes.
Any method can be used which suitably treats the tubes of a
cracking furnace by contacting such tubes with the antifoulant under suitable
treatment conditions to thereby provide treated tubes.
The prefel,ed procedure for pretreating the tubes of the cracking
furnace, includes charging to the inlet of the cracking furnace tubes a saturated or
slightly superheated steam having a ~ t~ e in the range of from about 300F
to about 500F. The cracking furnace is fired while charging the tubes with the
steam so as to provide a superheated steam which exits the tubes at a temperature
exceeding that of the steam introduced into the inlet of the tubes. Generally, the
steam effluent will have a temperature upwardly to about 2000F. Thus, the
treating temperature can be in the range of from about 300F to about 2000F,
preferably, from about 400F to about 1800F and, most preferably, from 500F
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to 1 600F. It is desirable for the stearn to be charged to the convection section
of the cracking furnace, therefore, first passing through the convection section
tubes followed by p~sing through the radiant section tubes.
The antifoulant can then be ~(lmixed with the steam being charged
5 to the cracker tubes. The antifoulant can be a~lmixed with the stearn as either a
neat liquid or as a mi~ e of the antifoulant with an inert diluent. In any event,
it is plefe"ed to vaporize or convert into an aerosol either the neat liquid or the
n~lure prior to its introduction into or a~1mixing with the steam. The amount of
antifoulant a~lmixed with the stearn can be such as to provide a concentration of
the antifoulant in the steam in the range of from about 1 ppmm to about 10,000
ppmm, plerelably, from about 10 ppmm to about 1000 ppmw and, most
preferably, from 20 to 200 ppmm.
The ~ ;x~ e of steam and antifoulant is contacted with or charged
to the cracker furnace tubes for a period of time sufficient to provide for treated
15 tubes, which when placed in cracking service, will provide or promote an amount
of coke sp~lling exceeding that which occurs when the antifoulant includes silicon.
Such time period for pl~lleal~lg the cracker tubes is influenced by the specific
geometry of the cracking fumace inchllling its tubes; but, generally, the pretreating
time period can range upwardly to about 12 hours, and longer if required. But,
20 preferably, the period of time for the pretreating can be in the range of from about
0.1 hours to about 12 hours and, most preferably, from 0.5 hours to 10 hours.
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14
In the case where the antifoulant composition is directly a(lmixed
with the hydrocarbon feed to the cracking furnace, it can be added in such an
amount to be effective in promoting the spalling of coke produced during the
pyrolytic cracking of the hydrocarbon feed. Due to the memory effect resulting
5 from the application of the antifoulant the mixing with the hydrocarbon cracker
feed is con-lllcte~ r~ lly as required but, preferably, for periods up to about
12 hours. The concentration of the antifoulant in the hydrocarbon cracker feed
during treating of the cracking furnace tubes can be in the range of from about 1
ppmm to about 10,000 ppmm, preferably, from about 10 ppmm to about 1000
ppmm and, most preferably, from 20 to 200 ppmm.
Now refellmg to FIG. 1, there is illustrated by schematic
representation cracking fumace section 10 of a pyrolytic cracking process system.
Cracking furnace section 10 includes pyrolytic cracking means or cracking furnace
12 for providing heat energy required for inducing the cracking of hydrocarbons.
Cracking furnace 12 defines both convection zone 14 and radiant zone 16.
Respectively within such zones are convection coils as tubes 18 and radiant coils
as tubes 20.
A hydrocarbon feedstock or a mixture of steam and such
hydrocarbon feedstock is conducted to the inlet of convection tubes 18 by way of
20 conduit 22 which is in fluid flow co~ lullication with convection tubes 18.
During the treatment of the tubes of cracking furnace 12, the a(lmixtllre of steam
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and the antifoulant composition can also be conducted to the inlet of convection
tubes 18 through conduit 22. The feed passes through the tubes of cracking
furnace 12 wherein it is heated to a cracking temperature in order to induce
cracking or, in the situation where the tubes are undergoing treatment, to the
5 required tre~tment temperature. The cracked product stream from cracking
furnace 12 passes downstream through conduit 24 to sep~lor 26 which defines
a zone and provides means for removing at least a portion of the spalled coke
contained in the cracked product stream. The spalled coke removed from the
cracked product stream passes from separator 26 by way of conduit 28, and the
10 cracked product stream having at least a portion of the spalled coke contained
therein removed there~iom passes from sep~lor 26 by way of conduit 30.
To provide for the heat energy necessary to operate cracking furnace
12, fuel gas is conveyed through conduit 32 to burners 34 of cracking furnace 12
whereby the fuel gas is burned and heat energy is released.
During the treatment of convection tubes 18 and radiant tubes 20,
the antifoulant composition is conveyed to cracking furnace 12 feed stream
through conduit 36 and ~(lmixed prior to the resulting mixture entering cracking
furnace 12. Interposed in conduit 36 is heat exchanger 38 which provides heat
exchange means for transferring heat energy and to thereby vaporize the feed
20 conversion enhancing composition.
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16
The following example is provided to further illustrate the present
invention.
EXAMPLE
The following two comparisons illustrate the promotion of coke
5 spalling by the addition of a tin compound without the addition of a silicon
compound:
An expenment~l ethane cracker equipped with a HK4M alloy tube
feeding 44 lb~r ethane and 13.2 lb/hr steam was fed 300 ppmw sulfur as dimethyl
sulfide during cracking, a common tre~llnent in current crackers. Conversion was
kept constant at 65% and residence time at 120 milliseconds. Spalled coke
collected after the reactor in a dead leg zone upon completion of the 70 hour run
amounted to 4 grams/day. This same tube was treated with 100 ppmm tetrabutyl
tin, without silicon, for six hours prior to charging ethane. Spalled coke collected
after a 70 hour run amounted to 14.5 grams/day. In a separate run, the HK4M
15 tube was treated with 100 ppmm each of tetrabutyltin and hexamethyldisiloxane
for six hours prior to charging ethane. Spalled coke was collected over a 100 hour
period at a rate of 2.5 grams/day. Data for each of the three experimental runs
with the HK4M tube are pres~nte-l in FIG. 2. As can be observed from an analysis
of the data, the tin only treatment provides for a significantly greater amount of
20 coke spalling than does the tinlsilicon or the dimethyl sulfide compounds.
2155044 33294CA
17
An experiment~l ethane cracker equipped with HP Modified alloy tube
feeding 25.3 lb/hr ethane and 7.6 lb/hr steam was fed 300 ppmw sulfilr as
dimethylsulfide during cracking. Conversion was kept constant at 67% and
residence time was 270 milliseconds. Spalled coke collected after the reactor in
a dead leg zone upon the completion of a 55 hour run amounted to a daily rate of
2 grams/day. This same tube was treated with tetrabutyltin, without silicon, for
six hours prior to charging ethane. Spalled coke collected after a 65 hour run
amounted to 24 grams/day, an increase of twelve times colllpa~ed to the sulfur-
treated run. In a separate run, the HP Modified tube was treated with 100 ppmm
10 each of tetrabutyltin and tetraethylsilane for six hours prior to charging ethane.
Spalled coke was collected over a three day period at a rate of 2 grams/day. Data
for each of the three experimental runs are presented in FIG. 3. As can be
observed from the data, the tin only treatment provides for a significantly greater
amount of coke spalling than does the tin/silicon or the dimethylsulfide
compounds.
Reasonable variations and modifications are possible by those
skilled in the art within the scope of the described invention and the appended
claims.
