CLIAMS:1. An additive composition for reducing coke formation in a reactor during hydrocarbon cracking; said additive composition comprising:
(a) at least one carbonate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the carbonate salt ranges from 60 to 95% of the total weight of the additive composition; and
(b) at least one acetate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of said acetate salt ranges from 5 to 40% of the total weight of the additive composition.
2. The additive composition as claimed in claim 1, wherein the carbonate salt of group-1A metal is potassium carbonate and the acetate salt of group-1A metal is potassium acetate.
3. The additive composition as claimed in claim 2, wherein the proportion of the amount of potassium carbonate and the amount of potassium acetate is 88:12.
4. A method for reducing coke formation in a reactor during hydrocarbon cracking using an additive composition; said method comprising the following steps,
(1) providing a first mixture comprising the additive composition, a hydrocarbon feed and steam; wherein the steam dilution ratio ranges from 0.1 to 0.4;
(2) heating the first mixture to obtain a second mixture; and
(3) feeding the second mixture to a reactor for cracking of the hydrocarbon feed at a temperature in the range of 800 to 900?C to obtain a product mixture containing olefins;
wherein, the additive composition comprises:
(a) at least one carbonate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the carbonate salt ranges from 60 to 95% of the total weight of the additive composition; and (b) at least one acetate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the acetate salt ranges from 5 to 40% of the total weight of the additive composition; and
wherein, the amount of additive composition in the first mixture ranges from 1 ppmw to 100 ppmw;
wherein, said method is characterized in that during the cracking of the hydrocarbon feed at a temperature in the range of 800 to 900?C, the group-1A metal carbonate is at least partially converted into group-1A metal oxide, and that the group-1A metal carbonate and group-1A metal oxide deposit on the inner surface of the reactor.
5. The method as claimed in claim 4, wherein the carbonate salt of group-1A metal in the additive composition is potassium carbonate and the acetate salt of group-1A metal in the additive composition is potassium acetate.
6. The method as claimed in claim 5, wherein the proportion of the amount of potassium carbonate and the amount of potassium acetate in the additive composition is 88:12.
7. The method as claimed in claim 4, wherein the first mixture of step (1) is obtained by adding the additive composition to water followed by heating at a temperature in the range from 80?C to 100?C and mixing with hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C.
8. The method as claimed in claim 4, wherein the first mixture of step (1) is obtained by adding a solution of the additive composition to steam preheated at a temperature in the range of 80?C to 150?C, followed by mixing with hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C.
9. The method as claimed in claim 8, wherein the solution of the additive composition is prepared in a polar solvent selected from a group consisting of water, ethanol, methanol, propanol, and butanol.
10. The method as claimed in claim 8, wherein the solution of the additive composition is an aqueous solution.
11. The method as claimed in claim 4, wherein the first mixture of step (1) is obtained by adding the additive composition to a mixture of steam and hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C.
12. The method as claimed in claim 4, wherein the first mixture is heated at a temperature in the range of 400?C to 600?C.
13. The method as claimed in claim 4, wherein the cracking of the hydrocarbon feed is carried out for a time period ranging from 84 to 100 hours.
14. The method as claimed in claim 4, wherein the hydrocarbon feed comprises dimethyldisulfide in the range from 80 to 200 ppmw.
15. The method as claimed in claim 4, wherein the reactor is presulfided with dimethyldisulfide; wherein the amount of dimethyldisulfide used for presulfiding ranges from 50 to 150 ppmw.
16. The method as claimed in claim 4, wherein the product mixture containing olefins comprises ethylene. ,TagSPECI:FIELD OF THE DISCLOSURE
The present disclosure relates to a composition for the inhibition of coke formation in a reactor.
BACKGROUND
Steam cracking of hydrocarbons to olefins such as ethylene and propylene is an important process in the petrochemical industry. Hydrocarbons such as ethane, propane, butane, their mixtures and naphtha are cracked to olefins in tubular reactors (cracking reactors) in the presence of steam at higher temperatures in the range from 800-855°C. The production volume of olefins prepared by steam cracking is large and therefore any small improvements in the steam cracking process has large economical and commercially significance.
The inherent problem associated with the material of construction (MOC) of tubular cracking reactors (cracking reactors) is their tendency to promote coke formation on the inner surfaces and transfer line exchangers (TLEs).
A periodic shut down of the cracking reactors is required to burn off the coke by decoking using steam and air at temperatures of around 870°C. Such decoking is required once in 10-80 days depending on the operation mode and the feed composition. During decoking the production of ethylene and other products is stopped for considerable time resulting in a loss of productivity. Further, frequent decoking deteriorates the surface of tubular reactor coil.
Thus, a major challenge in the steam cracking of hydrocarbons is reduction in the coke deposition on the reactor coil and transfer line exchangers (TLEs). An effort to reduce the coke formation and thereby increase the run length between two decoking operations is an important consideration in the petroleum industry.
Several methods have been considered to overcome the deleterious effects of coke build up on reactor surfaces which include (1) expensive offsite metallurgical modification, (2) surface pre-treatment, (3) additive dosing, (4) increased steam dilution ratio, (5) improved control of the operating conditions, and (6) improved feed stock quality.
Coke formation can be reduced to some extent by improving the operating conditions such as increasing the steam dilution ratio and improving the quality of feed stock. However, the cost of making these changes often exceeds its benefits. Metallurgical modifications such as coating require reactor shut down. Further, the metallurgical modifications are often done off site. Furthermore, the coating tends to peel off in course of time. Furthermore, new metallurgical alloys are expensive. Hence, metallurgical modifications are not acceptable.
Inhibition of coke formation can be achieved by additive dosing which is carried out without shutting down the reactor. Further, the additive dosing is economical when carried out using inexpensive reagents.
Additives reduce the coke deposition on the reactor surface by passivating the reactor surface or by catalyzing the coke-steam gasification reaction. An additive can be added to the hydrocarbon feed or to the water stream that are used for the generation of steam. Thus, additive dosing is a simple method that can be used without shutting down the reactor.
Several efforts have been reported for passivating the reactor walls under steam cracking conditions. The formation of metal oxide layers on the reactor surface is reported to passivate the reactor surface and reduce the coke formation. Steam oxidizes certain metals to produce an oxide which forms a layer on the reactor surface, which is more resistant to coke deposition.
Sulfur compounds such as dimethyl sulphide (DMS), dimethyl disulphide (DMDS) and diethyl disulphide (DEDS) and the like have been used widely to passivate the reactor surface either by pre-sulfiding and/or by continuous addition of the sulfur compound to the feed in ppm level. However, the extent of reduction in coke formation obtained by pre-sulfiding operation alone is limited.
Accordingly, in order to achieve higher reduction of the coke formation, the pre-sulfiding technique is combined with other means such as additive dosing. Such combination is reported with phosphorus compounds and silica components as additives. Combinations of alkali and alkaline earth metal salts have been used as additives to enhance the coke gasification reaction which results in reduced coke formation. However, the salts used for this process are expensive.
Accordingly, there is felt a need to provide an inexpensive additive and a simple method to inhibit the coke formation during steam cracking of hydrocarbons to produce olefins. Further, it is desired that such an additive allow the reactor to function for an extended duration before a stoppage is needed for decoking of the reactor surface.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
It is an object of the present disclosure to provide an inexpensive additive composition for reducing the coke formation in a reactor during hydrocarbon cracking.
It is another object of the present disclosure to provide a simple method for using the additive composition for reducing the coke formation in a reactor during hydrocarbon cracking.
Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure.

SUMMARY
In one aspect of the present disclosure there is provided an additive composition for reducing coke formation in a reactor during hydrocarbon cracking. The additive composition comprising (a) at least one carbonate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the carbonate salt ranges from 60 to 95% of the total weight of the additive composition; and (b) at least one acetate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of said acetate salt ranges from 5 to 40% of the total weight of the additive composition.
In accordance with one of the preferred embodiments of the present disclosure, the carbonate salt of group-1A metal is potassium carbonate and the acetate salt of group-1A metal is potassium acetate.
In accordance with one embodiment of the present disclosure, the proportion of the amount of potassium carbonate and the amount of potassium acetate is 88:12.
In another aspect of the present disclosure there is provided a method for reducing coke formation in a reactor during hydrocarbon cracking using an additive composition. The method involves the following steps:
The first step is providing a first mixture comprising the additive composition, a hydrocarbon feed and steam. The steam dilution ratio of the first mixture ranges from 0.1 to 0.4.
The second step is heating the first mixture to obtain a second mixture.
The third step is feeding the second mixture to a reactor for cracking of the hydrocarbon feed at a temperature in the range of 800 to 900?C to obtain a product mixture containing olefins.
The additive composition comprising (a) at least one carbonate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the carbonate salt ranges from 60 to 95% of the total weight of the additive composition; and (b) at least one acetate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the acetate salt ranges from 5 to 40% of the total weight of the additive composition. The amount of additive composition in the first mixture ranges from 1 ppmw to 100 ppmw.
The method is characterized in that during the cracking of the hydrocarbon feed at a temperature in the range of 800 to 900?C, the group-1A metal carbonate is at least partially converted into group-1A metal oxide, and that the group-1A metal carbonate and group-1A metal oxide deposit on the inner surface of the reactor.
In accordance with one of the preferred embodiments of the present disclosure, the carbonate salt of group-1A metal in the additive composition is potassium carbonate and the acetate salt of group-1A metal in the additive composition is potassium acetate.
In accordance with one embodiment of the present disclosure, the proportion of the amount of potassium carbonate and the amount of potassium acetate in the additive composition is 88:12.
The first mixture of first step can be obtained in various ways.
In accordance with one embodiment of the present disclosure, the first mixture of the first step is obtained by adding the additive composition to water followed by heating at a temperature in the range from 80?C to 100?C and mixing with hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C.
In accordance with second embodiment of the present disclosure, the first mixture of the first step is obtained by adding a solution of the additive composition to steam preheated at a temperature in the range of 80?C to 150?C, followed by mixing with hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C. The solution of the additive composition is prepared in a polar solvent selected from a group consisting of water, ethanol, methanol, propanol, and butanol. In accordance with one embodiment of the present disclosure, the solution of the additive composition is an aqueous solution.
In accordance with third embodiment of the present disclosure, the first mixture of the first step is obtained by adding the additive composition to a mixture of steam and hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C.
In the second step, the first mixture is heated at a temperature in the range of 400?C to 600?C to obtain a second mixture.
The cracking of the hydrocarbon feed is carried out for a time period ranging from 84 to 100 hours.
In accordance with one preferred embodiment of the present disclosure, the hydrocarbon feed comprises dimethyldisulfide in the range from 80 to 200 ppmw.
In accordance with one preferred embodiment of the present disclosure, the reactor is presulfided with dimethyldisulfide. In accordance with one embodiment of the present disclosure the amount of dimethyldisulfide used for presulfiding ranges from 50 to 150 ppmw.
The product mixture containing olefins comprises ethylene.
Using the method and the additive composition of the present disclosure, from 50 to 77% reduction in coke deposition was obtained during the cracking operation. The reduction in coke formation results in an increase the run length as high as 2 to 3 times without affecting the downstream units.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The additive composition for inhibition of the coke formation during hydrocarbon cracking will now be described with the help of the accompanying drawings, in which:
Figure 1 illustrates flow-chart of an exemplary process of the present disclosure.
Figure 2 depicts the amount of surface coke in the form of bar graph of base runs (B-221; B-302) and additive composition test runs (R-227, R-305, R-306, R-308) in two reactors; wherein,
B-221 corresponds to the base run corresponds to old Incoloy 800 reactor,
B-302 corresponds to the base run corresponds to new Incoloy 800HT reactor,
R-227 corresponds to the test run with the additive composition with pre-sulfiding in old reactor,
R-305, R-306, R-308 correspond to the reproducibility test runs with additive composition without pre-sulfiding,
R-309 corresponds to the test run with additive composition and pre-sulfiding.
Figure 3 illustrates the spalled coke data of base runs (B-221; B-302) and additive composition test runs (R-306, R-309) in two reactors.
Figure 4 depicts the differential scanning calorimetry - thermogravimetric analysis (DSC-TGA) of potassium carbonate alone.
Figure 5 depicts the differential scanning calorimetry - thermogravimetric analysis (DSC-TGA) of potassium acetate alone.
Figure 6 depicts the differential scanning calorimetry - thermogravimetric analysis (DSC-TGA) of a mixture of potassium carbonate and potassium acetate.

DETAILED DESCRIPTION:
In one aspect of the present disclosure there is provided an additive composition for reducing coke formation in a reactor during hydrocarbon cracking. The additive composition comprising (a) at least one carbonate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the carbonate salt ranges from 60 to 95% of the total weight of the additive composition; and (b) at least one acetate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of said acetate salt ranges from 5 to 40% of the total weight of the additive composition.
In accordance with one of the preferred embodiments of the present disclosure, the carbonate salt of group-1A metal is potassium carbonate and the acetate salt of group-1A metal is potassium acetate.
The proportion of the amount of two salts in the additive composition of the present disclosure is selected to provide maximum reduction in coke formation during cracking of the hydrocarbon feed while maintaining low corrosion level in the reactor.
In accordance with one embodiment of the present disclosure, the proportion of the amount of potassium carbonate and the amount of potassium acetate is 88:12.
In another aspect of the present disclosure there is provided a method for reducing coke formation in a reactor during hydrocarbon cracking using an additive composition. The method involves the following steps.
The first step is providing a first mixture comprising the additive composition, a hydrocarbon feed and steam. The steam dilution ratio of the first mixture ranges from 0.1 to 0.4.
The second step is heating the first mixture to obtain a second mixture.
The third step is feeding the second mixture to a reactor for cracking of the hydrocarbon feed at a temperature in the range of 800 to 900?C to obtain a product mixture containing olefins.
The additive composition comprising (a) at least one carbonate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the carbonate salt ranges from 60 to 95% of the total weight of the additive composition; and (b) at least one acetate salt of group-1A metal selected from the group consisting of potassium, lithium, cesium, and rubidium; wherein the amount of the acetate salt ranges from 5 to 40% of the total weight of the additive composition.
The amount of additive composition in the first mixture ranges from 1 ppmw to 100 ppmw.
In accordance with one embodiment of the present disclosure, the amount of the additive composition in the first mixture is 5 ppmw.
The method is characterized in that during the cracking of the hydrocarbon feed at a temperature in the range of 800 to 900?C, the group-1A metal carbonate is at least partially converted into group-1A metal oxide, and that the group-1A metal carbonate and group-1A metal oxide deposit on the inner surface of the reactor.
In accordance with one of the preferred embodiments of the present disclosure, the carbonate salt of group-1A metal in the additive composition is potassium carbonate and the acetate salt of group-1A metal in the additive composition is potassium acetate.
In accordance with one embodiment of the present disclosure, the proportion of the amount of potassium carbonate and the amount of potassium acetate in the additive composition is 88:12.
The first mixture of first step can be obtained in various ways. The additive composition of the present disclosure can be mixed with the hydrocarbon feed, water, steam or combinations thereof.
In accordance with one embodiment of the present disclosure, the first mixture of the first step is obtained by adding the additive composition to water followed by heating at a temperature in the range from 80?C to 100?C and mixing with hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C.
In accordance with second embodiment of the present disclosure, the first mixture of the first step is obtained by adding a solution of the additive composition to steam preheated at a temperature in the range of 80?C to 150?C, followed by mixing with hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C. The solution of the additive composition is prepared in a polar solvent selected from a group consisting of water, ethanol, methanol, propanol, and butanol. In accordance with one embodiment of the present disclosure, the solution of the additive composition is an aqueous solution.
In accordance with third embodiment of the present disclosure, the first mixture of the first step is obtained by adding the additive composition to a mixture of steam and hydrocarbon feed preheated at a temperature in the range of 80?C to 150?C.
In the second step, the first mixture is heated at a temperature in the range of 400?C to 600?C to obtain a second mixture.
The cracking of the hydrocarbon feed is carried out for a time period ranging from 84 to 100 hours.
It is found that the presence of a sulfur compound in the hydrocarbon feed further reduces the coke deposition on the reactor walls.
In accordance with one preferred embodiment of the present disclosure, the hydrocarbon feed comprises dimethyldisulfide in the range from 80 to 200 ppmw.
In accordance with one preferred embodiment of the present disclosure, the reactor is presulfided with dimethyldisulfide. In accordance with one embodiment of the present disclosure the amount of dimethyldisulfide used for presulfiding ranges from 50 to 150 ppmw.
The product mixture containing olefins comprises ethylene.
Using the method and the additive composition of the present disclosure, from 50 to 77% reduction in coke deposition is obtained during the cracking operation. The reduction in coke formation results in an increase the run length as high as 2 to 3 times without affecting the downstream units.
Potassium carbonate is less corrosive as compared to other salts belonging to group-1A metals.
Potassium acetate has high coke gasification reaction rate among the group 1A metal salts. However, potassium acetate is corrosive in nature and higher amount of potassium acetate may damage the reactor surface.
The proportion of the amount of potassium carbonate and potassium acetate in the additive composition of the present disclosure is optimized to minimize the coke formation during steam cracking operation and to reduce the corrosion of the reactor. These potassium salts are easily available and relatively cheap. It is found that the group-1A metal salts are more reactive for the gasification of coke as compared to the group-2A metal salts such as the calcium salts. Due to their higher reactivity, lesser amount of group-1A salts is needed to achieve requisite reduction in the coke formation.
Thermogravimetric analysis (TGA) of potassium carbonate alone shows that it is stable till 600?C. Potassium carbonate undergoes decomposition at temperature in the range from 650 to1000?C leaving behind 40.31% residue.
Initial decomposition of potassium carbonate produces potassium oxide.
K2CO3 ? K2O + CO2
Further decomposition of potassium carbonate produces elemental potassium, carbon dioxide and oxygen.
2 K2CO3 ? 4 K + 2 CO2 + O2
At temperature in the range from 600 to 840?C, the amount of potassium carbonate decomposed is up to 5% of the total weight.
Thermogravimetric analysis (TGA) of potassium acetate alone showed that potassium acetate is stable till 300?C. Potassium acetate undergoes decomposition at temperature in the range from 300 to 460?C with loss of acetone and loss of 29.64% of the total weight; potassium carbonate is produced as the decomposition product.
2 CH3COOK ? K2CO3 + (CH3)2CO
Thermogravimetric analysis (TGA) of a mixture of potassium carbonate and potassium acetate showed different behavior as compared to the behavior of the individual salts alone.
At least 30% of the total weight of the mixture is vaporized during heating at temperature in the range from 100 to 120?C. Further heating at temperature ranging from 120 to 600?C leads to vaporization of 7 to 10% of the total weight of the mixture.
At temperature ranging from 600 to 850?C, 7 to 10% of the total weight of the additive composition is decomposed to potassium oxide.
The potassium carbonate and potassium oxide deposit on the inner surface of the reactor and passivate the inner surface of the reactor. Potassium carbonate which is deposited on inner surface of the reactor may further decompose and convert to potassium oxide. Potassium oxide catalyzes the coke gasification reaction thereby reducing the coke formation.
A small amount of 1% or less of potassium carbonate and potassium oxide may deposite on transfer line exchanger (TLE) surface which is located downstream to the reactor. Very small amount of a few ppm of elemental potassium can get in to product stream which will be captured by guard bed.
It is found that the additive composition of the present disclosure reduces the coke deposition in the reactor and transfer line exchangers (TLEs). Due to the decrease in coke deposition, the run length of the reactor increases 2 to 3 times before a stoppage is needed for decoking.
Presence of a sulfur compound in the hydrocarbon feed further reduces the coke deposition on the reactor walls. The sulfur compound controls the excess carbon oxides formed during the coke gasification.
An exemplary embodiment of the process of the present disclosure is described with the help of Figure-1. The process comprise a hydrocarbon feed vaporizer (22), water vaporizer (24), mixer (26), cracking reactor (36), cracking furnace (38), hydrocarbon feed tank (10), hydrocarbon feed pump (18), water feed tank (12), water feed pump (20), balances (14 and 16), TLEs (44 and 46), gas-liquid separator (48), inlet for air (30) and inlet for nitrogen (32). All the furnaces are electrically heated.
The hydrocarbon feed (10) and water (12) are stored in two tanks at atmospheric pressure. The tanks are provided with level gauges using which the flow rate of the feeds can be checked regularly. The tanks are placed on two electronic weighing balances (14 and 16). The amount of feed consumed in a run is recorded by these balances. There are two metering pumps (18 and 20) for the pumping of the feeds. The suction is taken from the storage tanks through spiral tubes to minimize pulsations in the feed flow.
There are two vaporizers; hydrocarbon feed vaporizer (22) and water vaporizer (24). The heat is supplied by electrically heated furnaces to vaporize the hydrocarbon feed and water. During a typical run the outlets of vaporizers are sent to a mixer (26) where the temperature of the mixture is raised to 500 to 600?C which is taken as cross over temperature.
The reactor coil (36) is a straight tube having 11 mm inner diameter and 3.01 mm thickness, incoloy 800 tube which is 355 mm long with a provision to measure temperature profile. Thermo well is 260 mm long and 6.35 mm outer diameter, made of SS-316 fixed from the bottom of the rector tube which also serves as a concentric insert. The coil is fixed in an electrically heated furnace (38) with a single zone. The furnace is 360 mm long and 255 mm wide. Temperature can be independently controlled to set any desired temperature profile in the coil. Thermocouple is located inside the reactor coil to measure process gas temperature profile by moving the location. The external wall temperature is measured at a central location. The furnace exit gases are quenched to around 600?C. The hydrocarbon feed flow rate can be varied up to 100g/h. The gases are further cooled in two transfer line heat exchangers (TLEs 44 and 46) connected in series to condense the steam and heaviers in the cracked product mixture. The condensed water and liquid is collected from the gas liquid separator (48) and weighed for mass balance calculations. Non condensed gases were further cooled and measured by a wet gas meter (50). The gaseous mixture is sent for analysis by CO/CO2 analyzer (52), two gas chromatography instruments (54 and 56) and the output of the gas chromatography instruments goes to computers for area integration and processing.
The cracked gas sample is simultaneously analyzed by two gas chromatographic (GC) systems (54 and 56). Hydrogen and methane are detected by a thermal conductivity detector (TCD) in the first GC system. All the hydrocarbons present in the gaseous mixture are analyzed by second GC using flame ionization detector. Peak identification and integration is performed by a commercial integration package. With these the product distribution in terms of weight percentage can be determined. Since the feed flow rate is known, yields of products %weight/weight of hydrocarbon feed and material balance can be calculated.
The present disclosure is further described in light of the following examples which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure.
Test Procedure:
For a typical run, the furnace is turned on and the temperature is slowly increased while nitrogen or air is feed continuously. After desired temperature is achieved, water feed is started. After few minutes, nitrogen or air is discontinued and hydrocarbon feed is fed in hydrocarbon feed line. The flow rates of hydrocarbon feed and water are set in such a way that the desired dilution ratio is maintained. The temperature of the furnace is dropped, as soon as hydrocarbon feed is introduced in to the reactor due to the endothermic reactions. The temperature is increased slowly to reach to the desired temperature and coil outlet temperature. The product gases are analysed by using two gas chromatographs. Typical material balance is performed for one hour period by taking the weights of hydrocarbon feed and water, the amount of liquid product collected, total amount of gas measured through gas flow meter during the one hour period and product gas analysis. During a typical cracking run the product gas is analysed once in 12 hours. After completion of a run, the reactor is cooled down and weight of thermowell is measured to obtain surface coke. The spalled coke collected in the dead leg is measured. After that the thermowell is fixed into the reactor followed by leak test. Then the leak proof reactor is subjected to the decoking in the presence of steam and air.
Comparative Example 1:
Comparative pilot plant runs were conducted for the hydrocarbon – steam feed mixture in a bench scale cracking reactor in a cracker coil made of Incoloy 800 that has been subjected to repeated cracking and decoking conditions. The coil outlet temperature was 852°C and steam dilution ratio was 0.15. The corresponding residence time was 1.5 seconds. The feed olefin content was around 3%.
Blank run was carried out for 100 hours, the furnace was cooled and opened. The surface coke deposited on the surface of thermowell was found to be 0.824 g.
A comparative run was carried out under the similar conditions as above except that an additive was introduced in to the hydrocarbon – steam feed mixture by means of a water based solution. The composition employed during the run was as follows: 98.99% calcium acetate, 0.989% potassium acetate, and 0.0099% phosphate and sulfur component 0.0101 % by weight. The mixture was introduced along with water at a concentration of 5 ppmw and maintained at this level throughout the run.
It was found that for an old cracking coil using a combination of calcium acetate and potassium acetate the surface coke was reduced by 58.7%. ICP analysis of coke and liquid samples showed no evidence of corrosion.
Comparative Example 2:
Comparative pilot plant runs were conducted for hydrocarbon – steam feed mixture in a new cracking reactor made of Incoloy 800HT. The coil outlet temperature was 830°C and steam dilution ratio was 0.32. The residence time was 0.5 seconds. At these conditions blank run without the additive was carried out for 84 hours, the furnace was cooled and opened. The average of surface coke deposited on the surface of thermowell was 0.688 g.
Test run was carried out under the similar conditions as the blank run except that an additive was introduced by means of an aqueous solution in to the hydrocarbon – steam feed mixture after presulfiding with 100 ppmw of DMDS in water for two hours after decoking. The additive composition employed during the run was as follows: 98.98% calcium acetate, 0.99% potassium acetate and sulfur component 0.0107% by weight. The composition was introduced along with water at a concentration of 5 ppmw and maintained at this level throughout the run. Surface coke was reduced by 70%.
Thus, it was found that for a new cracking coil that was pre-sulfided, using a combination of calcium acetate and potassium acetate the surface coke was reduced by 70%. ICP analysis of coke and liquid samples showed no evidence of corrosion.
In the examples 1 and 2, an additive containing a mixture of Group-1A metal salt and a Group-2A metal salt was used for the inhibition of coke formation.
Example 3
A comparative test run was carried out under the similar to the conditions used in the comparative example 2 blank run in reactor coil of example 1 (Incoloy 800 – used coil) with an additive comprising 88% potassium carbonate, 12% potassium acetate by weight. The mixture was introduced along with water at a concentration of 5 ppmw and maintained at this level throughout the run.
It was found that on an old cracking reactor coil without pre-sulfiding, using the composition of the present disclosure, there was 60% reduction in surface coke in the test run. ICP analysis of coke and liquid samples showed no evidence of corrosion.
Example 4
A comparative run was carried out under the similar conditions as the blank run of example 2 except that an additive was introduced by means of an aqueous solution in to the hydrocarbon – steam feed mixture. The additive employed during the run was as follows: 88% potassium carbonate and 12% potassium acetate by weight. The mixture was introduced along with water at a concentration of 5 ppmw and maintained at this level throughout the run.
It was found that using the composition of the present disclosure, without pre-sulfiding of the cracking reactor coil, 77% reduction (an average of three runs) in surface coke was obtained in the test run with composition. No evidence of corrosion is seen by the ICP analysis of coke and liquid samples.
Example 5
Example 4 was repeated with an additional step of pre-sulfiding as mentioned in example 2 before cracking.
It was observed that after pre-sulfiding and using of the additive of the present disclosure, there was 80% reduction in the amount of surface coke. No evidence of corrosion is observed.
Similar experiment using a composition containing 99.98% calcium acetate and 0.99% potassium acetate (example 2) showed a reduction of 70% in coke formation. Thus there was 10% more reduction in coke formation using the composition of the present disclosure.
The combined results of various runs in examples 1-5 are provided in Table 1.

Table 1: The amount of coke reduction during various runs
Run No. Reference % Coke Reduction
B-221 Blank run 0
R-227 Test run 60
B-302 Blank run 0
R-305 Test run 71.9
R-306 Test run 89.97
R-308 Test run 68.4
R-309 Test run 79.5

In Table-1, B-221 corresponds to the base run corresponds to old incoloy 800 reactor. R-227 corresponds to the test run with potassium salts mixture with pre-sulfiding in old reactor. It was found that the amount of coke formed decreased by 60% when cracking was carrying out in the presence of the composition of present disclosure. Similarly the amount of spalled coke also reduced significantly in this case.
In Table-1, B-302 corresponds to the base run corresponds to new Incoloy 800HT reactor. R-305, R-306, R-308 correspond to the reproducibility test runs with mixture of potassium salts without pre-sulfiding. It was found that the amount of coke formed decreased 70-90% when cracking was carrying out in the presence of the composition of present disclosure. Similarly the amount of spalled coke reduced slightly in this case.
R-309 corresponds to the test run with potassium salts mixture and pre-sulfiding wherein 80% decrease in the amount of coke formed was observed. The spalled coke formed is reduced in this case.
The combined results are further illustrated in Figure 2 and Figure 3.
The amounts of the olefins formed during the cracking process with and without the composition of the present disclosure were analyzed. Table 2 gives the yields from base run (R-303) and a test run with composition (R-309).

Table 2: Yields from the base run and the test run
Run No. R-303
base run R-309
test run with composition and pre-sulfiding
CO 0.03 0.038
CO2 0.09 0.02
Methane 13.42 13.86
Ethane 3.44 3.49
Ethylene 30.64 31.90
Propane 0.46 0.47
Propylene 18.30 18.66
iso-Butane 0.42 0.34
n-Butane 0.11 0.10
Propadiene 0.28 0.28
t-2-Butene 0.64 0.60
1-Butene 2.50 2.48
iso-Butene 3.14 3.07
cis-2-Butene 0.50 0.47
iso-Pentane 0.24 0.23
1,2-Butadiene 2.25 2.44
n-Pentane 2.78 2.85
Methyl acetylene 0.36 0.34
1,3-Butadiene 4.79 4.57
H2 0.95 1.01
Total 85.35 87.21

It was observed that the ethylene yield increased by1.26% in the run with the composition of the present disclosure.

ECONOMICAL SIGNIFICANCE AND TECHNICAL ADVANCEMENT
The technical advancements offered by the present disclosure include the realization of:
- The additive of the present disclosure comprises cheap and easily available salts.
- The method of the present disclosure for reduction of coke formation rate using the additive is economical.
- The additive of the present disclosure provides higher reduction in the amount of coke as compared to the additive containing calcium salt.
- The use of the composition of present disclosure in the feed resulted in an increase in the length of run time of the cracking operation by 3-4 times before a stoppage was needed for decoking.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.