Antifoulants for thermal cracking processes

The formation of carbon on metals exposed to hydrocarbons in a thermal cracking process is reduced by contacting such metals with an antifoulant selected from the group consisting of tin, a combination of tin and antimony, a combination of germanium and antimony, a combination of tin and germanium and a combination of tin, antimony and germanium.

This invention relates to processes for the thermal cracking of a gaseous 
stream containing hydrocarbons. In one aspect this invention relates to a 
method for reducing the formation of carbon on the cracking tubes in 
furnaces used for the thermal cracking of a gaseous stream containing 
hydrocarbons and in any heat exchangers used to cool the effluent flowing 
from the furnaces. In another aspect this invention relates to particular 
antifoulants which are useful for reducing the rate of formation of carbon 
on the walls of such cracking tubes and in such heat exchangers. 
The cracking furnace forms the heart of many chemical manufacturing 
processes. Often, the performance of the cracking furnace will carry the 
burden of the major profit potential of the entire manufacturing process. 
Thus, it is extremely desirable to maximize the performance of the 
cracking furnace. 
In a manufacturing process such as the manufacture of ethylene, feed gas 
such as ethane and/or propane and/or naphtha is fed into the cracking 
furnace. A diluent fluid such as steam is usually combined with the feed 
material being provided to the cracking furnace. Within the furnace, the 
feed stream which has been combined with the diluent fluid is converted to 
a gaseous mixture which primarily contains hydrogen, methane, ethylene, 
propylene, butadiene, and small amounts of heavier gases. At the furnace 
exit this mixture is cooled, which allows removal of most of the heavier 
gases, and compressed. 
The compressed mixture is routed through various distillation columns where 
the individual components such as ethylene are purified and separated. The 
separated products, of which ethylene is the major product, then leave the 
ethylene plant to be used in numerous other processes for the manufacture 
of a wide variety of secondary products. 
The primary function of the cracking furnace is to convert the feed stream 
to ethylene and/or propylene. A semi-pure carbon which is termed "coke" is 
formed in the cracking furnace as a result of the furnace cracking 
operation. Coke is also formed in the heat exchangers used to cool the 
gaseous mixture flowing from the cracking furnace. Coke formation 
generally results from a combination of a homogeneous thermal reaction in 
the gas phase (thermal coking) and a heterogeneous catalytic reaction 
between the hydrocarbon in the gas phase and the metals in the walls of 
the cracking tubes or heat exchangers (catalytic coking). 
Coke is generally referred to as forming on the metal surfaces of the 
cracking tubes which are contacted with the feed stream and on the metal 
surfaces of the heat exchangers which are contacted with the gaseous 
effluent from the cracking furnace. However, it should be recognized that 
coke may form on connecting conduits and other metal surfaces which are 
exposed to hydrocarbons at high temperatures. Thus, the term "Metals" will 
be used hereinafter to refer to all metal surfaces in a cracking process 
which are exposed to hydrocarbons and which are subject to coke 
deposition. 
A normal operating procedure for a cracking furnace is to periodically shut 
down the furnace in order to burn out the deposits of coke. This downtime 
results in a substantial loss of production. In addition, coke is an 
excellent thermal insulator. Thus, as coke is deposited, higher furnace 
temperatures are required to maintain the gas temperature in the cracking 
zone at a desired level. Such higher temperatures increase fuel 
consumption and will eventually result in shorter tube life. 
Another problem associated with carbon formation is erosion of the Metals, 
which occurs in two fashions. First, it is well known that in the 
formation of catalytic coke the metal catalyst particle is removed or 
displaced from the surface and entrained within the coke. This phenomenon 
results in extremely rapid metal loss and, ultimately, Metals failure. A 
second type of erosion is caused by carbon particles that are dislodged 
from the tube walls and enter the gas stream. The abrasive action of these 
particles can be particularly severe on the return bends in the furnace 
tube. 
Yet another and more subtle effect of coke formation occurs when coke 
enters the furnace tube alloy in the form of a solid solution. The carbon 
then reacts with the chromium in the alloy and chromium carbide 
precipitates. This phenomena, known as carburization, causes the alloy to 
lose its original oxidation resistance, thereby becoming susceptible to 
chemical attack. The mechanical properties of the tube are also adversely 
affected. Carburization may also occur with respect to iron and nickel in 
the alloys. 
It is thus an object of this invention to provide a method for reducing the 
formation of coke on the Metals. It is another object of this invention to 
provide particular antifoulants which are useful for reducing the 
formation of carbon on the Metals. 
In accordance with the present invention, an antifoulant selected from the 
group consisting of tin, a combination of tin and antimony, a combination 
of germanium and antimony, a combination of tin and germanium and a 
combination of tin, antimony and germanium is contacted with the Metals 
either by pretreating the Metals with the antifoulant, adding the 
antifoulant to the hydrocarbon feedstock flowing to the cracking furnace 
or both. The use of the antifoulant substantially reduces the formation of 
coke on the Metals which substantially reduces the adverse consequences 
which attend such coke formation.

The invention is described in terms of a cracking furnace used in a process 
for the manufacture of ethylene. However, the applicability of the 
invention described herein extends to other processes wherein a cracking 
furnace is utilized to crack a feed material into some desired components 
and the formation of coke on the walls of the cracking tubes in the 
cracking furnace or other metal surfaces associated with the cracking 
process is a problem. 
Any suitable form of germanium may be utilized in the combination of 
germanium and antimony antifoulant, in the combination of tin and 
germanium antifoulant or in the combination of tin, antimony and germanium 
antifoulant. Elemental germanium, inorganic compounds and organic 
germanium compounds as well as mixtures of any two or more thereof are 
suitable sources of germanium. The term "germanium" generally refers to 
any one of these germanium sources. 
Examples of some inorganic germanium compounds that can be used include the 
halides, nitrides, hydrides, oxides, sulfides, imides, sulfates, and 
phosphates. Of the inorganic germanium compounds, those which do not 
contain halogen are preferred. 
Examples of organic germanium compounds that can be used include compounds 
of the formula 
##STR1## 
wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are selected independently 
from the group consisting of hydrogen, halogen, hydrocarbyl, and 
oxyhydrocarbyl. The hydrocarbyl and oxyhydrocarbyl radicals can have from 
1-20 carbon atoms which may be substituted with halogen, nitrogen, 
phosphorus, or sulfur. Exemplary hydrocarbyl radicals are alkyl, alkenyl, 
cycloalkyl, aryl, and combinations thereof, such as alkylaryl or 
alkylcycloalkyl. Germanium compounds such as tetrabutylgermanium, 
germanium tetraethoxide, tetraphenylgermanium, germanium tetraphenoxide, 
and diphenyldibromogermanium can be employed. At present germanium 
tetraethoxide is preferred. 
Any suitable form of antimony may be utilized in the combination of tin and 
antimony antifoulant, in the combination of germanium and antimony 
antifoulant or in the combination of tin, antimony and germanium 
antifoulant. Elemental antimony, inorganic antimony compounds and organic 
antimony compounds as well as mixtures of any two or more thereof are 
suitable sources of antimony. The term "antimony" generally refers to any 
one of these antimony sources. 
Examples of some inorganic antimony compounds which can be used include 
antimony oxides such as antimony trioxide, antimony tetroxide, and 
antimony pentoxide; antimony sulfides such as antimony trisulfide and 
antimony pentasulfide; antimony sulfates such as antimony trisulfate; 
antimonic acids such as metaantimonic acid, orthoantimonic acid and 
pyroantimonic acid; antimony halides such as antimony trifluoride, 
antimony trichloride, antimony tribromide, antimony triiodide, antimony 
pentafluoride and antimony pentachloride; antimonyl halides such as 
antimonyl chloride and antimonyl trichloride. Of the inorganic antimony 
compounds, those which do not contain halogen are preferred. 
Examples of some organic antimony compounds which can be used include 
antimony carboxylates such as antimony triformate, antimony trioctoate, 
antimony triacetate, antimony tridodecanoate, antimony trioctadecanoate, 
antimony tribenzoate, and antimony tris(cyclohexenecarboxylate); antimony 
thiocarboxylates such as antimony tris(thioacetate), antimony 
tris(dithioacetate) and antimony tris(dithiopentanoate); antimony 
thiocarbonates such as antimony tris(O-propyl dithiocarbonate); antimony 
carbonates such as antimony tris(ethyl carbonates); trihydrocarbylantimony 
compounds such as triphenylantimony; trihydrocarbylantimony oxides such as 
triphenylantimony oxide; antimony salts of phenolic compounds such as 
antimony triphenoxide; antimony salts of thiophenolic compounds such as 
antimony tris(-thiophenoxide); antimony sulfonates such as antimony 
tris(benzenesulfonate) and antimony tris(p-toluenesulfonate); antimony 
carbamates such as antimony tris(diethylcarbamate); antimony 
thiocarbamates such as antimony tris(dipropyldithiocarbamate), antimony 
tris(-phenyldithiocarbamate) and antimony tris(butylthiocarbamate); 
antimony phosphites such as antimony tris(-diphenyl phosphite); antimony 
phosphates such as antimony tris(dipropyl)phosphate; antimony 
thiophosphates such as antimony tris(O,O-dipropyl thiophosphate) and 
antimony tris(O,O-dipropyl dithiophosphate) and the like. At present 
antimony 2-ethylhexanoate is preferred. 
Any suitable form of tin may be utilized as the tin antifoulant, in the 
combination of tin and antimony antifoulant, in the combination of tin and 
germanium antifoulant or in the combination of tin, antimony and germanium 
antifoulant. Elemental tin, inorganic tin compounds, and organic tin 
compounds as well as mixtures of any two or more thereof are suitable 
sources of tin. The term "tin" generally refers to 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 
stannous sulfide and stannic sulfide; tin sulfates such as stannous 
sulfate and stannic sulfate; stannic acids such as metastannic acid and 
thiostannic acid; tin 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 stannic 
oxychloride; and the like. Of the inorganic tin compounds those which do 
not contain halogen are preferred as the source of tin. 
Examples of some organic tin compounds which can be used include 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 mercaptoalkanoates) such as dibutyltin bis(isooctyl 
mercaptoacetate) and dipropyltin bis(butyl mercaptoacetate); tin 
thiocarbonates such as stannous O-ethyl dithiocarbonate; tin carbonates 
such as stannous propyl carbonate; tetrahydrocarbyltin compounds such as 
tetrabutyltin, tetraoctyltin, tetradodecyltin, and tetraphenyltin; 
dihydrocarbyltin oxides such as dipropyltin oxide, dibutyltin oxide, 
dioctyltin oxide, and diphenyltin oxide; dihydrocarbyltin bis(hydrocarbyl 
mercaptide)s such as dibutyltin bis(dodecyl mercaptide); tin salts of 
phenolic compounds such as stannous thiophenoxide; tin sulfonates such as 
stannous benzenesulfonate and stannous-p-toluenesulfonate; tin carbamates 
such as stannous diethylcarbamate; tin thiocarbamates such as stannous 
propylthiocarbamate and stannous diethyldithiocarbamate; tin phosphites 
such as 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. At present stannous 2-ethylhexanoate is preferred. 
Any of the listed sources of tin may be combined with any of the listed 
sources of antimony or germanium to form the combination of tin and 
antimony antifoulant, the combination of tin and germanium antifoulant or 
the combination of tin, antimony and germanium antifoulant. In like 
manner, any of the listed sources of germanium may be combined with any of 
the listed sources of antimony to form the combination of germanium and 
antimony antifoulant. 
Any suitable concentration of antimony in the combination of tin and 
antimony antifoulant may be utilized. A concentration of antimony in the 
range of about 10 mole percent to about 75 mole percent is presently 
preferred because the effect of the combination of tin and antimony 
antifoulant is reduced outside of this range. In like manner, any suitable 
concentration of antimony may be utilized in the combination of germanium 
and antimony antifoulant. A concentration of antimony in the range of 
about 10 mole percent to about 75 mole percent is presently preferred 
because the effect of the combination of germanium and antimony 
antifoulant is reduced outside of this range. 
Any suitable concentration of germanium may be utilized in the combination 
of tin and germanium antifoulant. A concentration of germanium in the 
range of about 10 mole percent to about 75 mole percent is presently 
preferred because it is believed that the effect of the combination of tin 
and germanium antifoulant would be reduced outside this range. 
Any suitable concentration of antimony in the combination of tin, antimony 
and germanium may be utilized. A concentration in the range of about 10 
mole percent to about 65 mole percent is presently preferred. In like 
manner, a concentration of germanium in the range of about 10 mole percent 
to about 65 mole percent is presently preferred. 
In general, the combination antifoulants of the present invention are 
effective to reduce the buildup of coke on any of the high temperature 
steels. The tin antifoulant is considered to be effective to reduce the 
buildup of coke on any of the high temperature steels other than steels 
having an iron content of about 98 weight percent or higher. Commonly used 
steels in cracking tubes are Incoloy 800, Inconel 600, HK40, 11/4 
chromium-1/2 molybdenum steel, and Type 304 Stainless Steel. The 
composition of these steels in weight percent is as follows: 
__________________________________________________________________________ 
STEEL Ni Cu 
C Fe S Cr Mo P Mn Si 
__________________________________________________________________________ 
Inconel 600 
72 .5 
.15 8.0 15.5 
Incoloy 800 
32.5 .75 
.10 45.6 21.0 0.04 max 
HK-40 19.0-22.0 
0.35-0.45 
balance 
0.40 max 
23.0-27.0 1.5 max 
1.75 max 
.congruent. 50 
11/4Cr--1/2Mo balance 
0.40 max 
0.99-1.46 
0.40-0.65 
0.035 max 
0.36-0.69 
0.13-0.32 
.congruent. 98 
304SS 9.0 .08 72 19 
__________________________________________________________________________ 
The antifoulants of the present invention may be contacted with the Metals 
either by pretreating the Metals with the antifoulant, adding the 
antifoulant to the hydrocarbon containing feedstock or preferably both. 
If the Metals are to be pretreated, a preferred pretreatment method is to 
contact the Metals with a solution of the antifoulant. The cracking tubes 
are preferably flooded with the antifoulant. The antifoulant is allowed to 
remain in contact with the surface of the cracking tubes for any suitable 
length of time. A time of at least about one minute is preferred to insure 
that all of the surface of the cracking tube has been treated. The contact 
time would typically be about ten minutes or longer in a commercial 
operation. However, it is not believed that the longer times are of any 
substantial benefit other than to fully assure an operator that the 
cracking tube has been treated. 
It is typically necessary to spray or brush the antifoulant solution on the 
Metals to be treated other than the cracking tubes but flooding can be 
used if the equipment can be subjected to flooding. 
Any suitable solvent may be utilized to prepare the solution of 
antifoulant. Suitable solvents include water, oxygen-containing organic 
liquids such as alcohols, ketones and esters and aliphatic and aromatic 
hydrocarbons and their derivatives. The presently preferred solvents are 
normal hexane and toluene although kerosene would be a typically used 
solvent in a commercial operation. 
Any suitable concentration of the antifoulant in the solution may be 
utilized. It is desirable to use a concentration of at least 0.1 molar and 
concentrations may be 1 molar or higher with the strength of the 
concentrations being limited by metallurgical and economic considerations. 
The presently preferred concentration of antifoulant in the solution is in 
the range of about 0.2 molar to about 0.5 molar. 
Solutions of antifoulants can also be applied to the surfaces of the 
cracking tube by spraying or brushing when the surfaces are accessible but 
application in this manner has been found to provide less protection 
against coke deposition than immersion. The cracking tubes can also be 
treated with finely divided powders of the antifoulants but, again, this 
method is not considered to be particularly effective. 
In addition to pretreating of the Metals with the antifoulant or as an 
alternate method of contacting the Metals with the antifoulant, any 
suitable concentration of the antifoulant may be added to the feed stream 
flowing through the cracking tube. A concentration of antifoulant in the 
feed stream of at least ten parts per million by weight of the metal(s) 
contained in the antifoulant based on the weight of the hydrocarbon 
portion of the feed stream should be used. Presently preferred 
concentrations of antifoulant metals in the feed stream are in the range 
of about 20 parts per million to about 100 parts per million based on the 
weight of the hydrocarbon portion of the feed stream. Higher 
concentrations of the antifoulant may be added to the feed stream but the 
effectiveness of the antifoulant does not substantially increase and 
economic considerations generally preclude the use of higher 
concentrations. 
The antifoulant may be added to the feed stream in any suitable manner. 
Preferably, the addition of the antifoulant is made under conditions 
whereby the antifoulant becomes highly dispersed. Preferably, the 
antifoulant is injected in solution through an orifice under pressure to 
atomize the solution. The solvents previously discussed may be utilized to 
form the solutions. The concentration of the antifoulant in the solution 
should be such as to provide the desired concentration of antifoulant in 
the feed stream. 
Steam is generally utilized as a diluent for the hydrocarbon containing 
feedstock flowing to the cracking furnace. The steam/hydrocarbon molar 
ratio should not be allowed to exceed 2:1 when the tin antifoulant of the 
present invention is being used since the effectiveness of the tin 
antifoulant is substantially reduced at steam/hydrocarbon molar ratios 
above 2:1. The preferred steam/hydrocarbon molar ratio is in the range of 
about 0.25:1 to about 0.75:1 to enhance the effectiveness of the tin 
antifoulant. 
The steam/hydrocarbon molar ratio is considered to have very little effect 
on the use of the combination of tin and antimony antifoulant, the 
combination of germanium and antimony antifoulant, the combination of 
germanium and tin antifoulant or the combination of tin, antimony and 
germanium antifoulant. It is believed that the steam/hydrocarbon molar 
ratio is critical for tin alone because the tin antifoulant is volatile at 
high steam/hydrocarbon molar ratios. The combination antifoulants do not 
seem to exhibit this same volatility. 
The cracking furnace may be operated at any suitable temperature and 
pressure. In the process of steam cracking of light hydrocarbons to 
ethylene, the temperature of the fluid flowing through the cracking tubes 
increases during its transit through the tubes and will attain a maximum 
temperature at the exit of the cracking furnace of about 850.degree. C. 
The wall temperature of the cracking tubes will be higher and may be 
substantially higher as an insulating layer of coke accumulates within the 
tubes. Furnace temperatures of nearly 2000.degree. C. may be employed. 
Typical pressures for a cracking operation will generally be in the range 
of about 10 to about 20 psig at the outlet of the cracking tube. 
Before referring specifically to the examples which will be utilized to 
further illustrate the present invention, the laboratory apparatus will be 
described by referring to FIG. 1 in which a 9 millimeter quartz reactor 11 
is illustrated. A part of the quartz reactor 11 is located inside the 
electric furnace 12. A metal coupon 13 is supported inside the reactor 11 
on a two millimeter quartz rod 14 so as to provide only a minimal 
restriction to the flow of gases through the reactor 11. A hydrocarbon 
feed stream (ethylene) is provided to the reactor 11 through the 
combination of conduit means 16 and 17. Air is provided to the reactor 11 
through the combination of conduit means 18 and 17. 
Nitrogen flowing through conduit means 21 is passed through a heated 
saturator 22 and is provided through conduit means 24 to the reactor 11. 
Water is provided to the saturator 22 from the tank 26 through conduit 
means 27. Conduit means 28 is utilized for pressure equalization. 
Steam is generated by saturating the nitrogen carrier gas flowing through 
the saturator 22. The steam/nitrogen ratio is varied by adjusting the 
temperature of the electrically heated saturator 22. 
The reaction effluent is withdrawn from the reactor 11 through conduit 
means 31. Provision is made for diverting the reaction effluent to a gas 
chromatograph as desired for analysis. 
In determining the rate of coke deposition on the metal coupon, the 
quantity of carbon monoxide produced during the cracking process was 
considered to be proportional to the quantity of coke deposited on the 
metal coupon. The rationale for this method of evaluating the 
effectiveness of the antifoulants was the assumption that carbon monoxide 
was produced from deposited coke by the carbon-steam reaction. Metal 
coupons examined at the conclusion of cracking runs bore essentially no 
free carbon which supports the assumption that the coke had been gasified 
with steam. 
The selectivity of the converted ethylene to carbon monoxide was calculated 
according to equation 1 in which nitrogen was used as an internal 
standard. 
##EQU1## 
The conversion was calculated according to equation 2. 
##EQU2## 
The CO level for the entire cycle was calculated as a weighted average of 
all the analyses taken during a cycle according to equation 3. 
##EQU3## 
The percent selectivity is directly related to the quantity of carbon 
monoxide in the effluent flowing from the reactor. 
EXAMPLE 1 
Incoloy 800 coupons, 1".times.1/4".times.1/16", were employed in this 
example. Prior to the application of a coating, each Incoloy 800 coupon 
was thoroughly cleaned with acetone. Each antifoulant was then applied by 
immersing the coupon in a minimum of 4 mL of the antifoulant/solvent 
solution for 1 minute. A new coupon was used for each antifoulant. The 
coating was then followed by heat treatment in air at 700.degree. C. for 1 
minute to decompose the antifoulant to its oxide and to remove any 
residual solvent. A blank coupon, used for comparisons, was prepared by 
washing the coupon in acetone and heat treating in air at 700.degree. C. 
for 1 minute without any coating. The preparation of the various coatings 
are given below. 
0.5 M Sb: 2.76 g of Sb(C.sub.8 H.sub.15 O.sub.2).sub.3 was mixed with 
enough pure n-hexane to make 10.0 mL of solution referred to hereinafter 
as solution A. 
0.5 M Ge: 1.26 g Ge(OC.sub.2 H.sub.5).sub.4 was dissolved in enough 
absolute ethanol to make 10.0 mL of solution referred to hereinafter as 
solution B. 
0.5 M Sn: 2.02 g of Sn(C.sub.8 H.sub.15 O.sub.2).sub.2 was dissolved in 
enough pure n-hexane to make 10.0 mL of solution referred to hereinafter 
as solution C. 
0.5 M Sn-Sb: 0.81 g of Sn(C.sub.8 H.sub.15 O.sub.2).sub.2 was dissolved in 
enough pure grade n-hexane to make 4.0 mL of solution. The antimony 
solution was prepared by dissolving 1.10 g of Sb(C.sub.8 H.sub.15 
O.sub.2).sub.3 in enough pure n-hexane to make 4 mL. The two solutions 
were combined and mixed thoroughly and the resulting mixture is referred 
to hereinafter as solution D. 
0.5 M Ge-Sb: 5.0 g of Ge(OC.sub.2 H.sub.5).sub.4 was dissolved in enough 
absolute ethanol to make 40.0 mL. Then 1.1 g of Sb(C.sub.8 H.sub.15 
O.sub.2).sub.3 was dissolved in enough pure n-hexane to make 4.0 mL. The 
1:1 Ge-Sb solution was prepared by mixing 4.0 mL of each solution together 
and is referred to hereinafter as solution E. 
0.5 M Ge-Sn: 1.26 g Ge(OC.sub.2 H.sub.5).sub.4 was dissolved in absolute 
ethanol and diluted with alcohol to exactly 10 mL. 2.7 g Sn(C.sub.8 
H.sub.15 O.sub.2).sub.3, was dissolved in pure n-hexane and diluted with 
n-hexane to exactly 10 mL. The two solutions were combined and mixed and 
are referred to hereinafter as solution F. 
0.1 M Sn-Sb: A 2.0 mL aliquot of solution D was added to a graduated 
cylinder and enough toluene was added to make 10.0 mL. The resulting 
solution is referred to hereinafter as solution G. 
0.1 M Sn-Sb-Ge: 0.68 g of Sn(C.sub.8 H.sub.15 O.sub.2).sub.2, 0.92 g of 
Sb(C.sub.8 H.sub.15 O.sub.2).sub.3 and 0.42 g of Ge(OC.sub.2 
H.sub.5).sub.4 were dissolved in enough toluene to make 10.0 mL. A 2.0 mL 
aliquot of this solution was added to a graduated cylinder and enough 
toluene was added to make 10.0 mL. The resulting solution is referred to 
hereinafter as solution H. 
The temperature of the quartz reactor was maintained so that the hottest 
zone was 900.+-.5.degree. C. A coupon was placed in the reactor while the 
reactor was at reaction temperature. 
A typical run consisted of three 20 hour coking cycles (ethylene, nitrogen 
and steam), each of which was followed by a 5 minute nitrogen purge and a 
50 minute decoking cycle (nitrogen, steam and air). During a coking cycle, 
a gas mixture consisting of 73 mL per minute ethylene, 145 mL per minute 
nitrogen and 73 mL per minute steam passed downflow through the reactor. 
Periodically, snap samples of the reactor effluent were analyzed in a gas 
chromatograph. The steam/hydrocarbon molar ratio was 1:1. 
Table I summarizes results of cyclic runs (with either 2 or 3 cycles) made 
with Incoloy 800 coupons that had been immersed in the test solutions A-H 
previously described. 
TABLE I 
______________________________________ 
Time Weighted Selectivity to CO 
Run Solution Cycle 1 Cycle 2 
Cycle 3 
______________________________________ 
1 None (Control) 
19.9 21.5 24.2 
2 A 15.6 18.3 -- 
3 B 18.5 30.4 -- 
4 C 5.6 8.8 21.6 
5 D 0.74 2.2 4.9 
6 E 1.5 5.0 -- 
7 F 10.9 19.1 20.5 
8 F 4.2 8.9 15.1 
9 F 3.1 9.1 18.7 
10 G 5.8 9.9 16.2 
11 H 2.9 8.7 15.8 
12 H 2.8 5.0 -- 
______________________________________ 
Results of runs 2, 3, and 4 in which tin, antimony and germanium were used 
separately, show that only tin was effective in substantially reducing the 
rate of carbon deposition on Incoloy 800 under conditions simulating those 
in an ethane cracking process. However, binary combinations of these 
elements used in runs 5, 6, 7, 8 and 9 show some very surprising effects. 
Run 5, in which tin and antimony were combined, and run 6, in which 
germanium and antimony were combined, show that these combinations are 
unexpectedly much more effective than results of runs in which they were 
used separately would lead one to expect. 
Runs 8 and 9 show an improvement over tin alone that is unexpected in view 
of the effect of germanium alone. However, the combination of tin and 
germanium antifoulant does not show the dramatic improvement exhibited by 
the combination of tin and antimony antifoulant and the combination of 
germanium and antimony antifoulant and thus this antifoulant is not the 
preferred antifoulant among the combination antifoulants. 
It is not known why run 7 was less effective than tin alone. However, runs 
8 and 9 are considered to be more exemplary of the effect of the 
combination of tin and germanium antifoulant and it is believed that this 
antifoulant is more effective than tin alone. 
In runs 10, 11 and 12, 0.1 M solutions were used in order to show the 
improvement provided by the trinary combination. Higher concentrations 
such as 0.5 M have a tendency to mask the improvement. A comparison of 
runs 10, 11 or 12 shows that the combination of tin, antimony and 
germanium antifoulant is significantly more effective than the best binary 
combination (Sn-Sb). 
EXAMPLE 2 
Using the process conditions of Example 1, a plurality of three cycle runs 
were made using antifoulants which contained different ratios of tin and 
antimony and different ratios of germanium and antimony. Each run employed 
a new Incoloy 800 coupon which had been cleaned and treated as described 
in Example 1. The antifoulant solutions were prepared as described in 
Example 1 with the exception that the ratio of the elements was varied. 
The results of these tests are illustrated in FIGS. 2 and 3. 
Referring to FIG. 2, it can be seen that the combination of tin and 
antimony was particularly effective when the concentration of antimony 
ranged from about 10 mole percent to about 75 mole percent. Outside of 
this range, the effectiveness of the combination of tin and antimony was 
reduced particularly in the second and third cycles. 
Referring now to FIG. 3, it can again be seen that the combination of 
germanium and antimony was effective when the concentration of antimony 
was in the range of about 10 mole percent to about 75 mole percent. Again, 
the effectiveness of the combination of germanium and antimony is reduced 
outside of this range. Also, it can be noted that the effectiveness of the 
combination of germanium and antimony is reduced more each cycle than was 
seen in the combination of tin and antimony. 
It is believed that the combination of tin and germanium antifoulant would 
act essentially the same as the combination of tin and antimony 
antifoulant with respect to effectiveness as a function of concentration 
and thus a concentration of germanium in the combination of tin and 
germanium antifoulant is preferably in the range of about 10 mole percent 
to about 75 mole percent. 
EXAMPLE 3 
Coupons of 11/4 chromium-1/2molybdenum steel alloy, which is the alloy 
commonly used in transfer line heat exchangers in commercial ethylene 
cracking units, were cleaned in the manner described in Example 1. 
Separate coupons were then treated with solutions A, C or D of Example 1. 
Each coupon, including a control coupon, was then subjected to cyclic runs 
under the conditions set forth in Example 1. The results are summarized in 
Table II. Because of experimental difficulties associated with analysis of 
effluent gases, the reported observations are at different times but are 
nevertheless considered to provide a comparison of the effectiveness of 
the antifoulants. 
TABLE II 
______________________________________ 
Treating Agent 
Time, Hr. C.sub.2 H.sub.4 Conv., % 
Sel. to CO, % 
______________________________________ 
None* 2 13.8 43.6 
4 19.1 70.8 
6 23.3 83.5 
8 24.2 86.5 
Sn 24 31.2 82.6 
30 25.6 82.1 
36 23.6 82.4 
53 45.5 97.1 
Sb 5 42.0 86.5 
Sn + Sb 2 14.0 2.5 
4 13.8 1.6 
6 13.8 1.8 
8 13.9 1.8 
12 14.5 4.1 
16 14.6 2.8 
20 15.4 5.4 
23 15.6 7.8 
______________________________________ 
*Run continued for 21.2 hours but no other analyses were obtained because 
of instrumental difficulties. In its second cycle the run was terminated 
after 1.2 hours because sufficient carbon to totally obstruct flow 
collected on the coupon. 
Treatment of 11/4 Cr-1/2Mo steel alloy with solutions of tin or antimony 
separately is considered to be ineffective to reduce the rate of carbon 
deposition under the conditions of these runs. However, when both tin and 
antimony are present, the rate of carbon deposition at the same conditions 
was decreased substantially. 
EXAMPLE 4 
Coupons of Type 304 Stainless Steel were cleaned in the manner described in 
Example 1. A coupon was then treated with solution C of Example 1. The 
treated coupon and a control coupon were then subjected to a cyclic run 
under the conditions set forth in Example 1. The results are summarized in 
Table III. 
TABLE III 
______________________________________ 
Time Weighted Selectivity to CO 
Treating Agent 
Cycle 1 Cycle 2 Cycle 3 
______________________________________ 
None 23.3 74.2 85.4 
Sn 4.1 6.0 8.1 
______________________________________ 
Table III illustrates that tin is an effective antifoulant for Type 304 
Stainless Steel which has an iron content of about 72 weight percent. In 
contrast, Example 3 demonstrates that tin is not an effective antifoulant 
for 11/4Cr-1/2Mo steel alloy which has an iron content of about 98 weight 
percent. It is thus believed that tin is an effective antifoulant for 
steels having relatively high iron contents but the use of tin on steels 
having an iron content of about 98 weight percent or higher should be 
avoided. 
EXAMPLE 5 
The tin antifoulant of the present invention was used in tests on a 
commercial ethylene cracking furnace. The feedstock for the cracking 
furnace was ethane with the exception of a few relatively short intervals 
when propane was used. The cracking tubes were separated from the 
downstream transfer line heat exchanger and a solution containing stannous 
octoate was pumped into the cracking tubes to fill the tubes. The treating 
solution was prepared by diluting stannous octoate, catalyst T-9 from M & 
T Chemicals, Inc., with ten volumes of kerosene. The undiluted compound is 
reported by the manufacturer to contain typically 28 weight percent tin. 
After approximately ten minutes, the solution of stannous octoate was 
drained from the cracking tubes. In addition to treating the tubes, the 
solution of stannous octoate was also applied by spraying to the transfer 
line heat exchanger. 
Operation of a cracking furnace is terminated when the inlet pressure to 
the cracking tubes exceeds a predetermined limit. When the predetermined 
limit is exceeded, the cracking furnace is shut down for oxidative 
burn-out to remove the coke obstruction. In the cracking furnace in which 
the tests were made, the tubes had operated without antifoulant from ten 
to thirty-one days and had been then subjected to an oxidative burn-out to 
remove coke. In three separate tests made with a treatment of antifoulant 
as previously described, operation was maintained for 40, 49 and 47 days 
which is a substantial improvement over the 31-day maximum seen without 
the antifoulant treatment. 
One run was made in which, in addition to treating the tubes of the 
cracking furnace and the transfer line exchanger as previously described, 
the solution of stannous octoate was injected into the ethane before the 
ethane entered the cracking furnace. The concentration of tin in the 
ethane was 23 parts per million. The antifoulant solution was dispersed in 
the ethane by being forced through an orifice at elevated pressure into 
the stream of ethane which was moving with a linear velocity of about 1000 
feet per second. Injection of the antifoulant solution was continued for 
10 of the first 11 days of operation and then terminated. 
In this run, the cracking tube was in use for 60 days before excessive 
inlet pressure necessitated its shut-down. 
All operations in the commercial cracking furnace were made at a 
steam/ethane weight ratio of 0.35:1 and a temperature at the exit of the 
cracking tube of about 843.degree. C. 
EXAMPLE 6 
Using the procedure of Example 1 and solution C of Example 1, three 
separate runs were made at a steam/hydrocarbon molar ratio of 1:1, 2:1 and 
2.5:1 respectively. The results of these tests are illustrated in FIG. 4. 
Referring to FIG. 4, it can be seen that the tin antifoulant performed well 
at a steam/hydrocarbon molar ratio of 1.0. However, the effectiveness 
decreased at higher steam/hydrocarbon molar ratios. 
Reasonable variations and modifications are possible by those skilled in 
the art within the scope of the described invention and the appended 
claims.