CA1232856A - Method for on-line decoking of flame cracking reactors - Google Patents

Abstract

METHOD FOR ON-LINE DECOKING
OF FLAME CRACKING REACTORS

ABSTRACT
This invention relates to a method for on-line decoking of flame-crackinq reactors whereby decoking is achieved without interruption of the normal operation of such reactors and without the necessity to change feed equipment and/or disassemble reactor components. While maintaining the temperature of the effluent at 1000°C to 2000°C, the flow of the hydrocarbon feedstock in the reactor is periodically stopped for a time sufficient to reduce the carbon deposits to an acceptable level.

Description

c ~32856 METHOD FOR ONLINE DECO KIN&
OF FLAME Crackling REACTORS

TECHNICAL FIELD OF INVENTION
The present invention relates to a method for toe efficacious decokiDg of flare cracking reactor without interruption of the normal operation of such reactor.
BACKGROUND OF TOE INVENTION
During hydrocarbon cracking processes, carbonaceous apposite are formed on the reactor wall. Eventually, such carbonaceous deposit, if left to build to undesirable levels, can seriously restrict the flow of hydrocarbon vapors through the reaction zone vessel causing toe pressure within the reactor vessel to increase to dinosaur levelfi.
Consequently, when a anger prowar level it reached, the reactor Utah be shut town. Many prowesses have been eloped on the art of hydrocarbon cracking for dealing with this coking ruble.
U.S. Patents Numbers 3,557,241 and 3,365,387 disclose the introduction of sufficient steam and/or valor to at least one tube of the cracking furnace Chile simultaneously reducing the hydrocarbon feed to that tube. The tube it then put D-13,551 back unto service. the treatment of the tube it effected at temperature ranging from a low as OKAY (700F) to about 1100C (2000F). Such heat it supplied by external firing of the reactor tubes. Both patents utilize a separate and distinct feed line for introducing team and/or water lo! the so-called on stream decoying procedure. These lines are controlled by a valve which it put into service on only those occasions when the individual tube in question being decoyed is undergoing such a cleaning operation.
While both patent claim a multiplicity of tubes may be decoyed at one time, U.S. Patent No.
3,557,241, specifically states that it "contemplates the decoying of only a single tube at a Tom' (Column 2, Lines 34-36), which is time consuming.
Utilizing this method the furnace will be decoying during virtually all of its operational time.
Furthermore, utilizing these two method, decoying a multiplicity ox tubes at one time could cause a reduction in the production throughput of the system.
U.S. Patent No. 3,920,537, deals with the coke deposition evolving from hydrocarbon cracking operations by "periodically contacting the coke deposit with a jet of relatively cold, high-pressure water." The patent describes jetting the high-pressure cold water against the coke deposit in an amount sufficient to thermally shock and break up the coke deposit, typically at a pressure in excess of about 5000 pounds per square inch. This type of decoying technique, however, is only particularly useful where the coke deposition occur on surface D-13,551 ~Z3~856 having temperatures of approximately 370C t700~F) to 538C (1,000F).
German Patent Application 2923326 published December 18, 1980 tree European Patent application 0021167) disclose a method for decoying of equipment used in the thermal cracking of hydrocarbons Rich involves a tiptop procedure utilizing steam and oxygen. The first step, involve conducting the gas flow of steam and oxygen through the equipment on an amount 6uçh that the temperature of the coke deposits on the heat exchanging surface of the cracking gas cooler are in the range ox the prevailing ther~o~ra~kinq operating temperature. The fiecona step involves intensifying the gas flow such that the temperature of the coke deposits on the heat exchanging surface of the cracking gas cooler it increased. Though this patent does involve a two step process, the second step merely involves the decoying of a separate piece of equipment, e.g., toe heat exchanger.
U.S. Patent 4,203,778 effects decoying of -furnace tubes by the use of a turbulent stream of impact resistant, non-angular, non-abrasive particle entrained on a gay stream. The particles are entrained at concentration of 0.1 to 1.0 pound per pound of gas and the gas is introduced into the inlet end of the furnace tubes at a gas f lo rate corre~ponaing to an inlet velocity of 14,000 to 20,000 feet per minute.
The prior art decoying procedure in the hydrocarbon cracking field, operate under certain process constraints. The prior art utilize D-13,551 .

-123Z~356 decoying procedures wherein the tractors are made of petal. These processes are operated at reaction temperature not exceeding about 1100C. Because the reactors are jade of metal, the heat for the decoying reactors are transferred through the walls. They usually require taking the reaction train equipment out of service and specially treating that equipment 60 as to reduce or eliminate the coking problem. Furthermore in most caves, these processes require the dismantling of equipment or the addition of equipment in order to effect decoying. Such procedures are exceedingly tire consuming, and add materially to the cost of the operation of the hydrocarbon cracking apparatus.
There have been developed in the art processes for cracking hydrocarbons which utilize a flame cracking reactor. Such a flame cracking reactor system is depicted in U.S. Patent No.
4,136,015. In particular, this patent refers to the Advanced Cracking Reactor" (ARC) process. As characterized in said patent: -"In the 'Advanced Cracking Reactor' (ARC) process, a stream of hot gaseous combustion - products is developed in a first-stage combustion zone. The hot gaseous combustion products may be developed by the burning of a wide variety of fluid fuels (e.g. gaseous, liquid and fluidized solids) in an oxidant and in the presence of super-heated steam. The hydrocarbon feed stock to be cracked is then injected and mixed in a second stage zone into the D-13,551 12328~6 hot gaseous combustion product stream to effect the cracking reaction. Upon quenching in a third stage zone, the combustion and reaction products are then separated from the stream."
The ARC process it de~eribed in varying detail in the following patents: U.S. Patent No.
3,408,417: U.S. Patent No. 3,419,632: U.S. Patent No. 4,136,015; U.S. Patent No. 3,674,679; U.S.
Patent No. 3,795,713: U.S. Patent No. 3,855,339;
U.S. Patent No . 4 ,142, 963; U. 5 . Patent No.
4,150,716; U.S. Patent No. 4,240,898; U.S. Patent No. 4,321,131: U.S. Patent No. 4,134,824; and U.S.
Patent No. 4,264,435.
In addition to the aforementioned patents which are specifically directed to the ARC process, other patents directed to the cracking of hydrocarbons by a flame-cracking process include U.S. Patent No. 2,698,830, U.S. Patent No.
3,565,970, and U.S. Patent No. 2,371,147.
In the operation of such flame-cracking processes for converting hydrocarbons into more volatile component, it it necessary to effect the reaction in a reaction zone that contain a protective surface of a high-temperature resistant material which is also resistant to the products of the reaction. Illustrative of such materials are graphite, silicon carbide, alumina, zircon, magnesia, calcium oxide and the live. All of these materials are extremely resistant to high temperatures but have low thermal conductivity. The continuous operation of the ARC process and a D-13,551 123~85~

flame-cracking reaction process in general, causes coke deposition on the reactor walls. For example, U.S. Patent No. 4.136,015 utilize a reaction zone on which the stream therein is maintained at supersonic velocity flows. Coke formation in a system such as this, will materially alter the nature of the flow, thereby rendering the reaction process less controllable.
There it described herein a process whereby the coking problem can be effectively controlled and which circumvent the physical limitation of the aforementioned ceramic linings, i.e., low thermal conductivity. Furthermore this invention provide a method of decoying without alteration or dismantling of the exaction assembly.
SUMMARY OF THE INVENTION
This invention it an improvement in the continuous process of cracking hydrocarbon feeds in a flame cracking reactor.
This invention involves a method for effecting on-line decoying during a flame-cracking reaction such a that embodied in the ARC process.
Prowesses such as the ACRE involve the combustion of a carbonaceous or hydrogen-containing fuel with oxygen and the resulting combustion product stream is mixed with superheated steam to produce a heat carrier. The heat carrier is contacted with converging hydrocarbon feed stock streams in a zone 3uxtapo~ed and openly connected to the zone in which the flame it formed. The mixture is then pasted into an reaction zone wherein cracking takes place.

D-13,551 ~232856 _ 7 --.

Carbon deposit are formed on the reactor walls during the operation of the reactor.
Twig invention involves periodically stopping the flow of hydrocarbon feed stock streams utilized in the flame cracking reaction process twig.. the ARC process) while maintaining the temperature of the heat carrier flow to the reactor at an appropriate rate and at about 1000C to 20000C
for a period of time sufficient to reduce the carbon deposits to a predetermined level. The combustion gases may be produced by burning a fuel derived from the products of the cracking process or an alternative procesfi fuel. The normal operation of the process may then resume by restarting the flow of feed6toc~ and readjusting the combustion products to their normal flow and temperature.
The removal of certain deposits, commonly referred to as decoying, it carried out by periodically adjusting the fuel rate or the oxygen rate or the fuel to oxygen ratio to produce combustion product which have the desired composition and properties of temperature and velocity. Additionally. the team rate can be changed to modify the operating temperature and velocity. The mixture of combustion products and steam constitute decoying gases.
At the start of the decoying operation, the hydrocarbon feed rate is lowered to a level consistent with the decreased burner flow. The feed stock stream is then completely stopped for a period sufficient to reduce the carbon deposits to an acceptable level for continued operation. once .

D-13,551 the decoying operation it completed , the normal hydrocarbon feed rate it resumed and the cracking operation it continued.
The method of this invention, as oared to conventional decoying methods, has the advantage of allowing the operated machinery to be completely decoyed in a short amount of time, i.e., usually three hour or less. Thus, it doe not require the removal of downstream equipment and the reactor need not be disassembled, mechanically altered, or connected to additional equipment.
DISCUSSION OF THE INVENTION
The operation of the flame-cracking reactions are well described in the references previously cited. It it the purpose of this invention to eliminate a problem which occurs during their continued operation: the coke formation that results during the normal period of operation of these processes.
It is well known that carbon can react with a number of chemical which are present during a - - -high-temperature hydrocarbon cracking reaction.
Carbon, for example, will react with water to form carbon monoxide and hydrogen. It reacts with carbon dioxide to form carbon monoxide. Furthermore, carbon can be hydrogenated to methane by reaction with hydrogen and can be oxidized to carbon dioxide and carbon monoxide by reaction with oxygen. It is the purpose of this invention to utilize all of these known chemical reactions to remove the carbon that has been deposited within an ARC or a typical flame-cracking process reactor.

D-13,551 1232~356 The problem of effectuating carbon removal in the instant case it not as simple a the application of the known chemical reactions stated above During the operation of flame-cracking reactor such as the ARC reactor, in particular, the temperature of the reaction zone range as high as 2000C, and even higher. As the temperature within a reactor of this nature increases, carbon deposition along the wall can become more graphitic in nature and consequently, a layer of carbon which is remarkably resistant to chemical reactions can form. Indeed, such graphitic carbon could be used as an insulating layer for such a reactor. The term graphitic carbon is intended to include carbon which has undergone a sufficient amount of heat treatment such that it crystalline structure becomes either graphite-like or a that of pure graphite.
The deposited coke can be eliminated by flowing a hot steam containing stream (such as steam) over it for a time sufficient to convert at least a portion of said coke to a gaseous material by chemical reaction. This can be best accomplished by controlling the temperature and velocity of the gaseous stream formed by the burning of carbonaceous our hydrocarbon-containing fuel, and transporting those combustion products to the reaction zone.
Simultaneously, the temperature of the reaction zone must be maintained at 1000C to about 2000C for a period of time sufficient to effectively reduce the carbon deposit. Preferably, the velocity of the stream in the reaction zone should be such as to provide a carbon removal rate sufficient to meet the D-13,551 ~232856 - lo _ process requirements of a typical hydrocarbon cracking commercial facility. minimally, the combustion product velocity should be such a to provide for stable combustion of the Caribbean or hydrocarbon-containing fuel. The maximum velocity preferred would be a supersonic velocity within the reaction zone.
In the preferred operation of the process, a higher velocity gas stream it preferred for carbon removal. It it believed that such a high velocity stream enhance the gasification of the carbon and alto enhances the physical removal of particulate carbon from the surface of the reactor wall.
The primary chemical reaction relied upon for carbon removal in the practice of this invention it carbon gasification: the reaction of carbon with team to form carbon monoxide and hydrogen. This reaction to be most effective requires the presence of enough steam within the combustion zone to efficaciously remove the deposited carbon. In the typical case, the amount of team which is present should be at least approximately 10 weight per cent voyeur the weight of the stream fed to the reaction zone. This amount can be reduced should the velocity of that stream be increased. However, the amount of steam can exceed 10 weight per cent and can be as high as 100 per cent of the weight of the stream, e.g., use of hydrogen as a fuel for producing the hot gaseous stream will provide that the stream it all water team This mechanism provide for the actual physical removal of carbon D-13,551 ~2328S6 by polliwog and thermo6hock techniques which will be defocused below.
Where the carbon deposit become graphitic, carbon removal ~iqht necessitate more severe treatment such as, the utilization of gay streams with higher concentrations of team and the operation of such streams at higher velocities and temperatures to induce cracks within the carbon structure which greatly increase the gasification rate by providing more surface area per unit volume. Additionally, these cracks enhance the potential for the flaking away of the carbon deposit from the reactor walls. In the art, this phenomena it referred to a "spelling".
The elimination of carbonaceous deputy by spelling results from the achievement of a thermal gradient across the carbon thickness. A natural temperature gradient exists throughout the coke and the reactor wall and by quickly increasing the temperature of the decoying gases-, this temperature gradient is increased. Gases with higher temperatures and velocities will tend to cause more spelling as well as faster chemical reaction rates.
A high temperature gradient can be achieved by a rapid rise in the temperature of the decoying guy in the reactor, that it, bringing the reactor to a maximum decoying temperature during a short period of time. This greatly enhances the spelling effect, causing thermal stress in the form of cracking within the coke; thereby, allowing it to more readily react with the steam and other reactants present in the decoying gas.

D-13,551 123~856 DETAILED DESCRIPTION OF INVENTION
In order to describe tube invention, references will be wads to the drawings on U.S.
Patent No. 4,136,015 which graphically end schematically depict an ARC reaction assembly. In particular figure 3, thereof, shows a cro~s-sectional Kiev of the critical components typically found in an ARC reactor.
A cracking reactor utilizes the heat of combustion of a carbonaceous or hydrogen-containing fuel with oxygen, either as pure oxygen gas, air, or oxygen mixed with other gazes, Jo heat a hydrocarbon feed stock to its appropriate cracking temperature.
the combustion fuel may coopri6e,-for example, the gases produced by the high-~emperature partial combustion of coal or coke Vito oxygen, or any fluid hydrocarbon material suck as natural gas and/or hydrogen. These fuels and their combustion products are well-known in the art.
The eo~bustion product can be formed by mixing a gaseou6^hydrocarbon, or hydrocarbon mixtures Vito oxygen utilizing a metal burner wit a gas combustion chamber assembly as jet forth in Canadian Patent No. 1,187,395 issued Jay 21, 1985. The combustion gases may be produced by burning a fuel derived from the product of tube cracking process or an alternative ruses fuel. The hydrocarbon feed stock thereafter it introduce into the reactor, in a oiling zone, typically in a affection angular to the flow of the combustion product stream. This admixing occur preferably, on a direction not only D-13,551 angular but countercurrent to the direction of the product stream. The angular introduction of the hydroca bun feed is described in particular in U.S.
Patent No. 4,142,963, U.S. Patent No. 3,674.679.
U.S. Patent No. 3,408,417, U.S. Patent 3,419.632.
U.S. Patent No. 3,855,339, and U.S. Patent No. 4,136,015 both specifically apply to feeding the hydrocarbon feed into the reactor in the form of an atomized spray of liquid droplets in a manner such that said material it linearly injected in a radial direction towards the center axis of the reactor, and counter currently at an angle of 120 to 150 to the passing direction of the heating medium stream which is the combustion gases.
In practicing the preferred embodiment of the ARC process, the hydrocarbon feed to be cracked is enveloped in a steam shroud, which not only enhances the introduction of the feed to the reaction zone but also protects the metal injectors and inhibits carbon deposition at the feed inlet points; The feed and the combustion product stream, are thoroughly intermixed and fed through the constricted throat into the diffuser/reactor portion of the ARC reactor. The velocity of the stream through the throat is preferably sonic velocity and develops supersonic velocity upon exit from the throat in the diffuser/reactor section; all of which it described in considerable detail in U.S. Patent No. 4,136,015. The effluent from the diffuser reactor section as shown in Figure 1 of U.S. Patent 4,136,015 enters the quench zone, whereupon the reaction is stopped and product D-13,551 recovery begins. This is Gore specifically described in U.S. Patent No. 4,150,716.
The fuel which it utilized to for the combustion product stream is typically a mixture of hydrogen and methane. Typically, the oxidant it essentially pure oxygen. This combination it reacted and then moderated by the addition of team delineate to achieve a combustion product stream having a temperature of about 1600 to about 2400C. The combustion product stream it thereafter contacted with the hydrocarbon feed stock which is fed in an essentially countercurrent direction to that of the combustion product streams through a number of injectors which openly connect to the interior of the ASP. Each of these injector it surrounded by concentric annular feed zone which introduce the steam shroud which circumscribes the hydrocarbon feed. The shrouded hydrocarbon feed stock stream mixes with the combustion product stream slightly above a thwarted section within the ACRE This is more specifically described in Figure 3 of U.S. Patent No. 4,136,015 and Figure 1 of U.S.
Patent No. 4,142,963. An illustration of specific injector arrangement utilized for the introduction of the hydrocarbon feed stock and its team shroud can be wound in Figures pa, pa, and their corresponding Figures 3b and 4b of U.S. Patent 4,142,963. The operative conditions by which such a reaction it carried out are fully described in U.S.
Patent 4,136,015.
The mixture of feed stock, combustion product stream and shroud steam flow through the D-13,551 , ~Z32~3S6 thwarted section of the ARC reactor to obtain sons velocity and thereafter i~6ue into the diverging supersonic velocity diffuser/reaction zone wherein the cracking reaction to produce the more volatile products is effected. It it within the expanded reaction zone and the thwarted zone that the carbon deposits develop in quantities sufficient to eventually adversely affect the overall process.
Toe process of this invention most efficiently removes deposited carbon products within the aforementioned zones in a manner which does not require any dismantling of apparatus or the inclusion into the apparatus of other equipment.
The process of this invention allows one to utilize the ARC process, for example, without having to make any change in any of the downstream apparatus normally associated therewith. In the typical case, no uncoupling of downstream equipment is necessary during the decoying operation as herein described.
In carrying out this preferred embodiment, the temperature which is achieved in the combustion -reaction it from about 1000C to about 2400C.
These unusually high temperatures Gould necessitate a lining capable of withstanding these high temperatures.
In the practice of this invention, it is preferable to maintain the highest concentration of oxygen allowable 80 as to enhance the rate of decoying by the reaction of such oxygen with the coke. The concentration of oxygen is limited by safety considerations such a the flammability of the overall mixture.

D-13,551 1232~356 The preferred embodiment of the present invention involve the practice of a two stage method. The decoying it begun by reducing the burner flow capacity to approximately 70% of its usual mast flow rate, while maintaining the reactor at a temperature between approximately 1150C-1200C
for a two hour period. The burner slow capacity it actually the mass flow rate of the high temperature gas used in normal operation.
hen utilizing this preferred embodiment, a steam purge is normally put through a metal team curtain Utah upstream of the Guenther to protect it from high temperatures. Once the inlet pressure it reduced to lower level, indicating that decoying has been completed in the reactor, throat and diffuser the burner flow capacity it raised in the second stage of the process to approximately 90~ and the decoying temperature is increased to 1300~C for a period of one hour. The steam purge to the quencher is then simultaneously decreased. It it this downstream decrease in the steam purge to the Guenther that allows the quench zone to be decoyed;
A the coke deposition increases, the diameter of the throat decreases, and the overall area of the reactor/diffuser section is reduced.
Consequently, it is possible thereby, with reduced velocity in the combustion gas stream to maintain the sonic conditions in the throat and supersonic conditions in the reactor/diffuser section.
Utilizing the combustion products stream which has an extremely high temperature will, of course, enhance the gasification of the coke D-13,551 ~23Z85~;

deposited on the reactor wall. However, such high temperatures can adversely affect the ceramic lining of the reactor and, therefore, in choosing the condition at which the decoying process it operated, it it necessary to take into consideration the issue of mechanical integrity. The ~08t preferred method of effecting coke removal is Jo utilize the most stringent conditions in term of temperature, team concentration, and the like what the particular reactor assembly will accept. This then allow for decoying in the shortest period of time.
An alternate embodiment of the prevent invention involves the practice whereby decoying it achieved by reducing the burner flow capacity to approximately 55%. The reactor temperature is maintained between approximately 1150C-1200C for the entire decoying period. A steam purge is put through the Guenther steam curtain to protect it from high temperature and said steam purge remains at this level throughout the entire decoying process.
Another alternate embodiment of the present invention involves a two stage process whereby different temperature levels are utilized to facilitate the decoying process. The burner flow capacity is reduced to approximately 70% while maintaining the reactor at a temperature between approximately 1350C-1400C. Utilizing this alternate embodiment, a steam purge is put through the Guenther steam curtain to protect it from high temperature. The reactor is maintained at this temperature for a period of time sufficient to D-13,551 detect a noticeable decrease in the pressure, indicating the decoying process is almost at completion, in this instance usually about thirty minutes. The reactor temperature is then elevated to approximately 1450C for the remainder of the decoying period, approximately one hour.
EXAMPLES
Example 1 A pilot-scale flame-cracking ARC reactor, with an ethylene capacity of 250,000 lbs./yr.~ is operated with a whole distillate of Arabian Light crude as the feed stock. A "whole distillate is a blend of the overhead product from the atmospheric and vacuum distillation of a crude oil," i.e., a crude oil minus the residual oil obtained following vacuum distillation. The burner use essentially pure hydrogen and oxygen: steam it added to moderate the temperature of the combustion products. Thus the effluent from the burner consists mainly of superheater steam with small amounts of unconsumed -hydrogen. The reactor exit pressure it kept at 50 prig. At the beginning of the run, the required inlet pressure to the reactor it 59 prig. Over a period of about six hours run time, the inlet pressure gradually increases to about 77 prig, indicating the coke is depositing and is restricting the reactor.
To decode the reactor, the burner is first adjusted to conditions which would result in a reactor temperature of approximately 1200C if no feed were being injected. Feed to the reactor is D-13,551 - Jo 12;~28S6 stopped, and the reactor prowar is reduced to about-30 prig and held constant. total burner effluent it reduced to about 66 percent of normal operating rates. Without feed injection to absorb the endothermic heat of reaction, the reactor temperature rises to about 1200. At the start of the decoying process, the inlet pressure it about 54 prig: after about 10 minutes of decoying, the inlet pressure decreases to about 45 prig, indicating that the coke it being removed. Further operation does not result in another decrease in inlet pressure, indicating that all the coke ha been removed.
At this point the reaction could have been reinstated by reversing the above procedure.
However, the reactor is shut down and disassembled for inspection. The ceramic lining of the reactor is found to be clean and substantially, free of traces of coke. No damage to the reactor resulted from the decoying. Had the reactor been decoyed according to the prior art, the process would have taken at least two days and the coke would not have been removed as completely as accomplished by the invention.

An ARC with an ethylene capacity of 5,000,000 lb8/yr it operating with vacuum gas oil as cracking fistic. The burner fuel is a mixture of gaseous hydrocarbons and hydrogen, which is burned in substantially pure oxygen. Steam is added to moderate the burner temperature. About five percent more than the stoishiometric quantity of fuel it Used, BY the burner effluent consists mainly of high D-13,551 temperature carbon oxides and team, with a small amount of unconsumed fuel. The reactor outlet prowar it kept at about 40 prig. At the chart of the run, inlet pressure it abut 68 prig; during the course of several day& operation, inlet pressure gradually increases to about 74 puke, indicating that coke is depositing in the reactor.
To decode the reactor, first the feed and burner are audited to about half the normal flow rates. The weed it then turned off completely, and the burner adjusted to obtain a temperature in the reactor of about 1150C to 1200C. A steam purge of about 500 lb/hr is put through the quencher Tao curtain to protect it from high temperatures. The reactor is maintained at these condition for approximately three hour. At the end of that time, the process is reversed and the reactor is returned to normal operating conditions. The inlet pressure to the reactor has returned to about 68 prig, demonstrating that the coke has been removed.
- During the decoying process, the downstream processing equipment, such a the gasoline fractionator, it kept in standby mode. The decoying period is BY short that the downstream equipment it easily returned to normal operating condition, with very little upset to the overall process.

The reactor is operated and coking occurs as described in Example 2. The decoying process is conducted similarly, except that the temperature in the reactor it audited to about 1350C to 1400C, and the decoying it only carried out for about 30 D-13,551 minute. The team purge through the Guenther curtain a in Example 2 it used to protect the Guenther. After decoying, the reactor inlet pressure ha again returned to its usual level, demonstrating that the coke has been removed. Coke chip are later discovered in a downstream strainer, indicating that some of the coke ha been removed either by spelling, or by the mechanical force of tube decoying gas stream.

The reactor is operated and coking occurs as described in Examples 2 and 3. The decoying process is conducted at about 1150C to 1200C for about two hour, and then at about 1350C to 1400C
for about one hour. After decoying, the reactor it returned to normal operating conditions by reversing the process, and the inlet pressure returns to its normal level. During this entire process the quencher steam purge remains constant at 500 lbs/hr. No coke chip are discovered in any downstream equipment, indicating that the bulk of the coke way removed by chemical reaction in the first two hours of the process This avoids any possible problem of coke chips clogging downstream equipment. The final hour at a higher temperature ensures that any traces of coke which are especially resistant to chemical reaction are removed, because reaction rate increase greatly with a 200C
increase in temperature.

The reactor is operated as described in D-13,551 ~2328S6 EXAMPLE 2. After several day of operation, eke inlet pressure inquiry eon about 74 pow. In addition, the pressure drop across the quencher increase from its normal value of about 5 pi to about lo pi, indicating that coke is depositing in the quencher. Decoying it begun with a reactor temperature of about 1150 to 1200C. A steam purge of about 500 lb/hr 8 put through the quencher steam curtain to protect it from high temperatures.
At the beginning of the decoying, the pressure drop across the quencher is about 10 pi.
After about two hours of decoying, the pressure drop across the reactor has dropped to a level indicating that the reactor is effectively decoyed. The pressure drop, however, across the quencher remains about lo pi. At this point, eke decoying temperature it increased to about 1350C to 1409C
and the steam purge to the quencher is decreased to 140 lb/hr. This decrease in the steam purge to the quench zone enable said zone to be effectively decoyed; The pressure drop across the quencher -begins to decrease almost immediately, indicating that coke it being removed. The reactor it decoyed for about another hour at these conditions. After a total of about three hours decoying, the process is reversed and the reactor it returned to normal operating conditions. Inlet pressure to the reactor has returned to about 68 esig, and pressure drop across the quencher has decreased to the original 5 psig,-demon~trating that the reactor and Guenther have been effectively decoyed.

D-13,551 Examples 2, 3 and 4 illustrate three possible embodiments of the invention for decoying the reactor. Any of these three methods or sore modification of these methods may be used depending upon the circumstances. Example 2 describe a method which it unlikely to cause excessive reactor wear because the temperatures never exceed about 1200C. Example 3 it effective in a shorter period of time, but causes some coke chip to be carried out of the reactor into the downstream equipment.
This may result in faster reactor wear because of the use of higher temperatures. Example 4 eliminates the problem of the coke chips and because of the increased temperature Thor the last part of the cycle), is very effective at removing the last vestiges of coke. However, this method requires more time than the method of Example 3, and it exposes the reactor to higher temperature than the method of Example 2. -The method of Example 2C it thought to be the most preferred-embodiment at this time, but the other methods are acceptable and may be preferred in Rome circumstances.
Example 5 illustrates how the invention can be extended to the decoying of downstream equipment, which it not normally thought of as part of the main reactor section. The decoying gases are conducted through that equipment and the temperature there it audited. The method of Example 5 is identical to that of Example 4, except that the flow of purge steam through the quencher curtain just upstream of the quencher 8 reduced, allowing the temperature in the quencher to rise to the level necessary for effective decoying.

D-13,551

Claims (6)

Claims

1. In the normal continuous process for cracking hydrocarbon feeds in a flame cracking reactor wherein fuel and oxygen are supplied and reacted to produce a heat carrier which may be mixed with superheated steam and the mixture is contacted with converging hydrocarbon feedstock streams and the mixture is passed to a reaction zone wherein cracking of the feedstock takes place and carbon deposits form on the reactor walls, the improvement which comprises periodically stopping the flow of hydrocarbon feedstock while maintaining the temperature of the heat carrier in a range of from about 1000°C to about 2000°C for a period of time sufficient to reduce the carbon deposits to a predetermined level.

2. The method of Claim 1. wherein the temperature of the heat carrier is maintained in a range of from about 1000°C to about 1250°C.

3. The method of Claim 1, wherein the temperature of the heat carrier is maintained in a range of from about 1250°C to about 1600°C.

4. The method of Claim 1 wherein the temperature of the heat carrier is maintained in a range of from about 1000°C to about 1250°C for a period of time sufficient to remove a substantial portion of the carbon deposits and the temperature of the heat carrier is then raised to a range of from about 1250°C to about 1600°C.

5. The process of Claim 1, wherein the fuel is supplied from a source totally independent from the flame-cracking process.

6. The process of Claim 1, wherein the ratio of fuel to oxygen is adjusted to that the heat carrier includes oxygen.