EP4565666A1

EP4565666A1 - A method of decoking a cracking furnace

Info

Publication number: EP4565666A1
Application number: EP22800744.9A
Authority: EP
Inventors: Mark B. Davis; Rami H. SABBAH; Quan Yuan; Ernest R. Frank; Liwei Li; Georgios Bellos
Original assignee: Dow Global Technologies LLC
Current assignee: Dow Global Technologies LLC
Priority date: 2022-10-25
Filing date: 2022-10-25
Publication date: 2025-06-11
Anticipated expiration: 2042-10-25
Also published as: CN119855887A; WO2024089443A1; CA3268371A1; EP4565666B1

Abstract

A method of decoking a cracking furnace includes: heating the cracking furnace to a specified average temperature; injecting a gas mixture including air and steam through the at least one tube at a first air to steam ratio; injecting the gas mixture through the at least one tube at a second air to steam ratio upon observing a decrease in the outlet point temperature from the first temperature; and combusting at least a second portion of the coke on the internal surface of the at least one tube. Additionally, the outlet point temperature does not exceed the specified average temperature; the second air to steam ratio is greater than the first air to steam ratio; and the cracking furnace includes at least one tube for transferring a feed, the at least one tube entering the cracking furnace at an inlet point and exiting the cracking furnace at an outlet point.

Description

A METHOD OF DECOKING A CRACKING FURNACE

FIELD

[0001] Embodiments of the present disclosure generally relate to the field of refining and upgrading hydrocarbons, and pertain particularly to a method of decoking a cracking furnace.

BACKGROUND

[0002] Olefins and aromatic compounds, such as ethylene, propylene, butylene, butadiene, benzene, toluene, and xylenes, are basic intermediates for many petrochemical industries. These olefins and aromatic compounds are usually obtained through the thermal cracking (or steam pyrolysis) of petroleum gases and distillates such as naphtha, kerosene, or gas oil. These compounds are also produced through refinery fluidized catalytic cracking (FCC) process where standard heavy feedstocks, such as gas oils or residues, are converted. Typical FCC feedstocks range from hydrocracked bottoms to heavy feed fractions, such as vacuum gas oil and atmospheric residue. However, these feedstocks are limited. Another source for propylene production is currently refinery propylene from FCC units. With the ever-growing demand, FCC unit owners look increasingly to the petrochemicals market to boost their revenues by taking advantage of economic opportunities that arise in the propylene market.

[0003] The worldwide increasing demand for light olefins remains a major challenge for many integrated refineries. In particular, the production of some valuable light olefins such as ethylene, propylene, and butylene has attracted increased attention as pure olefin streams are considered the building blocks for polymer synthesis.

SUMMARY

[0004] The production of olefins and aromatic compounds in conventional methods may also result in the production of by-products such as coke. This coke may build up within and coat the surfaces of treatment units used to upgrade the feed to the treatment unit, such as within the coils or tubes of a steam cracking furnace. Eventually, this buildup of coke may be such that the efficiency of the upgrading units are greatly reduced. At this point, the built-up coke must be otherwise removed through a process known as decoking. Decoking typically involves the introduction of steam and air to the coke, gasifying and oxidizing the coke and resulting in its removal as carbon dioxide and carbon monoxide. The coke, steam, and air may also be heated during this decoking process, increasing the temperature of the reactants, and thus increasing the rate of coke removal. This may be done by using the heating elements of the upgrading unit normally used to conduct the upgrading reactions, which increases the average temperature of the furnace, including the coils.

[0005] During decoking processes, the upgrading units cannot be used to upgrade more of the feed, resulting in non-productive down time. Further, careful control of the decoking process is desired, as temperature swings during the decoking process above the temperature limits of the coils can result in oxide spallation, weakening the coils and reducing their lifetime. This effect is exacerbated when complex decoking processes are used, as the many adjustments to heating temperature and flow rates therein may result in the aforementioned undesired temperature swings. Oxide spallation can also result in safety concerns when the weakening of the coils causes them to rupture during upgrading. Further difficulties can occur when the heating elements of the upgrading unit are used to provide most of the temperature to drive the decoking reactions. This increase in the average temperature can reduce the margin between safe operating temperatures of the coils and oxide spallation causing rates, causing significant oxide spallation and degradation of the coils during the temperature swings.

[0006] Consequently, improved methods of decoking are desired. These methods should reduce the amount of time required to decoke furnaces. At the same time, the methods should be carefully designed to be simple so as not to result in the need to vary multiple factors, which could cause temperature swings that may result in oxide spallation and reduce the lifetime of the coils. Finally, the methods should also limit the energy input to the cracking furnace itself, thereby increasing the margin of safe operating temperatures of the coils and increasing the lifetime of the coils.

[0007] The methods provided herein accomplish the aforementioned goals by providing a decoking process that involves very few steps, decreases decoking times over comparable methods, and limits the surface temperature experienced by tubes/coils of a cracking furnace during the decoking process. This is achieved, in part, by controlling the rates of gasification and oxidation of the deposited coke by adjusting the ratio of injected air to steam in specified steps during the decoking process. An average temperature of the furnace may also be specified and set during the initial heating of the cracking furnace and then left unchanged throughout the remainder of the process, further limiting temperature swings. Adjusting the rates of gasification and oxidation may also operate to increase the severity of the reaction without increasing the average temperature of the furnace and thus the surface temperature of the tubes/coils, increasing the margin of safe operating temperature.

[0008] According to one embodiment, a method of decoking a cracking furnace includes: heating the cracking furnace to a specified average temperature; injecting a gas mixture including air and steam through the at least one tube at a first air to steam ratio, thereby increasing an outlet point temperature to a first temperature and combusting at least a first portion of coke on an internal surface of the at least one tube; injecting the gas mixture through the at least one tube at a second air to steam ratio upon observing a decrease in the outlet point temperature from the first temperature, thereby increasing the outlet point temperature to a second temperature; and combusting at least a second portion of the coke on the internal surface of the at least one tube, and wherein the outlet point temperature does not exceed the specified average temperature; the second air to steam ratio is greater than the first air to steam ratio; and the cracking furnace includes at least one tube for transferring a feed, the at least one tube entering the cracking furnace at an inlet point and exiting the cracking furnace at an outlet point.

[0009] Additional features and advantages of the embodiments described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described, including the detailed description and the claims which are provided infra.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings in which:

[0011] FIG. 1 illustrates a decoking system, according to embodiments herein. [0012] FIG. 2A is a SEM image of a lab coupon, with a similar composition to a cracking furnace coil, after greater than 40 cracking and decoking cycles at an average tube metal temperature of 800 °C;

[0013] FIG. 2B is a SEM image of a lab coupon, with a similar composition to a cracking furnace coil, after greater than 40 cracking and decoking cycles at an average tube metal temperature of 850 °C;

[0014] FIG. 2C is a SEM image of a lab coupon, with a similar composition to a cracking furnace coil, after greater than 40 cracking and decoking cycles at an average tube metal temperature of 900 °C;

[0015] FIG. 3 is an illustration of a method of decoking a cracking furnace, according to one or more embodiments herein;

[0016] FIG. 4 is an illustration of a method of decoking a cracking furnace, according to one or more embodiments herein; and

[0017] FIG. 5 is an illustration of a method of decoking a cracking furnace, according to one or more embodiments herein.

DETAILED DESCRIPTION

[0018] Embodiments described herein relate to methods of decoking a cracking furnace.

[0019] As used herein, “coke” may refer to deposited carbonaceous material, which may include petroleum coke, catalytic coke, pyrolytic coke, or the like.

[0020] As used herein, references to the “severity” of a decoking reaction may refer to the ratio of air to steam of the gas mixture used in the decoking process, the volumetric injection rate of the gas mixture, or both. Accordingly, the severity of a decoking reaction increasing, then, may be generally understood to refer to either the air to steam ratio of the gas mixture increasing or the volumetric injection rate of the gas mixture increasing. [0021] As used herein, the “surface temperature of the at least one tube” may also be generally referred to as a “tube metal temperature.”

[0022] Additional features and advantages of the described embodiments will be set forth in the detailed description, which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the described embodiments, including the detailed description, which follows, as well as the claims.

[0023] Referring initially to FIG. 1 , the cracking furnace 102 may include at least one tube 104 for transferring a feed. The feed may include, but may not be limited to, hydrocarbons, refined hydrocarbons, bio-derived feeds, circular fees, renewable feeds, or combinations thereof. The cracking furnace 102 may also include coke (not shown) that is deposited on the inner surface of the at least one tube 104. The coke may be a by-product of earlier upgrading reactions within the cracking furnace 102.

[0024] The at least one tube 104 may enter the cracking furnace at an inlet point 112 and exit the cracking furnace 102 at an outlet point 114. The at least one tube 104 may also be generally regarded as a furnace coil. The at least one tube 104 or furnace coil may include a metal alloy including aluminum, chromium, iron, nickel, titanium, manganese, or combinations thereof. For example, and in embodiments, the at least one tube 104 may be from 15 wt.% to 45 wt.% chromium, from 20 wt.% nickel to 50 wt.% nickel, from 0.1 wt.% aluminum to 5 wt.% aluminum, or combinations thereof. The at least tube may also include greater than 45 wt.% chromium, greater than 50 wt.% nickel, or both, depending on the application. The at least one tube 104 may also include a coating on the internal surface of the at least one tube 104. The coating may be an inert coating or a catalytic coating, which may itself be dependent on the composition of the coating. The cracking furnace 102 may also include a heating element 106. The heating element 106 may be configured to provide heat to the cracking furnace 102 and the at least one tube 104. The heating element 106 may be a radiant heating element, an electromagnetic inductive heating element, an electric resistor heating element, or combinations thereof. The radiant heating element may include one or more burners, the one or more burners configured to combust a burner gas. The burner gas may be, for example, a flue gas. The flue gas may include a fuel gas, air, hydrogen, oxygen, carbon dioxide, or combinations thereof. The cracking furnace 102 may also include a gas preheating element 108. The gas preheating element 108 may be configured to preheat a gas feed prior to the gas feed’s entry into the cracking furnace 102 through the inlet point 1 12. The gas preheating element 108 may be operable to preheat the gas feed to a temperature of from 20 °C to 800 °C.

[0025] The coke that is deposited on the surface of the at least one tube 104 may be deposited in one or more layers. This phenomenon of layered coke may be referred to as “soft coke” and “hard coke,” with the soft coke commonly understood to overlay the hard coke within the at least one tube 104. In general, soft coke is understood to require less severe decoking reactions to be removed than hard coke. However, in actual observation, the deposited coke may occupy greater than two layers, with the layers deposited farther from the internal tube surface generally being ‘softer,’ or requiring less severe reactions to remove at a given rate, than the layers closer to the internal surface of the at least one tube 104. In this way, decoking reactions may be understood to require increasingly severe reaction conditions to remove all of the coke at a similar rate as the ‘softer’ coke is reacted and removed and ‘harder’ coke is uncovered. Methods herein may remove the one or more layers of increasingly harder coke by progressing the decoking reactions to substantially all oxidation.

[0026] Methods of decoking the cracking furnace 102 discussed herein may include heating the cracking furnace 102 to a specified average temperature. The method may then include injecting a gas mixture including air and steam through the at least one tube 104 at a first air to steam ratio. The first air to steam ratio may operate to increase an outlet point temperature to a first temperature. The first air to steam ratio may also operate to combust at least a first portion of a coke on an internal surface of the at least one tube 104.

[0027] It is contemplated that the one or more observed decreases in the outlet point temperature may result from the exposure of one or more of the layers of harder coke as the softer coke is progressively removed. As previously discussed, the harder coke may require more severe decoking reactions to be removed at a similar rate to the softer coke, and thereby may combust at a lesser rate at the current air to steam ratio that was previously sufficient to combust the softer coke. Thereby, the outlet temperature may be observed to decrease as less coke is combusted at the current reaction severity. Increasing the air to steam ratio, according to embodiments herein, may begin to combust the harder coke at a rate similar to that for the softer coke, and thereby increase the outlet temperature.

[0028] Additionally or alternatively, the outlet temperature may be observed to decrease as localized regions of the coke within the at least one tube 104 are removed at different rates. For example, a localized region of coke within the at least one tube 104 may be completely or substantially composed of softer coke. Thereby, this localized region of coke may be completely removed at lesser reaction severity, while other regions still remain with deposited coke. The localized uncoked region within the at least one tube 104 may then contribute to a decreasing in the outlet temperature as the total surface area of exposed coke within the at least one tube 104 is decreases due to the localized uncoked location.

[0029] One potential advantage of increasing the air to steam ratio only upon observing the decrease in outlet temperature, such as in the defined steps herein, is the avoidance of temperature swings that may damage the at least one tube 104 or lead to oxide spallation. For example, increasing the reaction severity while the softer coke is still present may lead to excessive rates of combustion of the softer coke, thereby leading to sudden increases in the outlet temperature and the temperature of the at least one tube 104.

[0030] As previously discussed, the outlet point temperature may be observed to decrease as harder coke is uncovered and less coke is combusted at the current air to steam ratio. Upon observing the decrease in the outlet temperature, the first air to steam ratio may be modified to a second air to steam ratio. Thereby, the method may further include injecting the gas mixture through the at least one tube 104 at the second air to steam ratio. The second air to steam ratio may be greater than the first air to steam ratio and may thereby operate to combust the uncovered harder coke. This may then operate to increase the outlet point temperature to a second temperature, as well as combust at least a second portion of the coke on the internal surface of the at least one tube 104.

[0031] Similar to above, a second decrease in the outlet temperature may be observed after injecting the gas mixture at the second air to steam ratio. Upon observing the second decrease, the second air to steam ratio may be modified to a third air to steam ratio. Thereby, the method may further include injecting the gas mixture through the at least one tube 104 at the third air to steam ratio. Similar to above, the third air to steam ratio may be greater than the second air to steam ratio and may thereby operate to combust the uncovered harder coke. This may then operate to increase the outlet point temperature to a third temperature, as well as combust at least a third portion of the coke on the internal surface of the at least one tube 104.

[0032] Continuing, the method may also include observing a third decrease in the outlet point temperature from the third temperature, and subsequently injecting the gas mixture at a fourth air to steam ratio, heating the cracking furnace 102 to a second specified average temperature, or both. It is contemplated that this part of the decoking process may be generally referred to as the “air polishing stage.” The air polishing stage may operate to increase the outlet point temperature to a fourth temperature and combust any remaining portion of the coke on the internal surface of the at least one tube 104. This removal, or “polishing,” of the remaining portions of the coke may operate to create a tube surface that is less prone to initial deposition of coke when upgrading is resumed. The polishing may further operate to oxidize the internal surface of the at least one tube 104, particularly metals such as chromium or aluminum, and thereby repair any defects on the interior surface of the at least one tube 104. In this manner, the fourth air to steam ratio for the air polishing stage may also be regarded as the “final air to steam ratio,” in that the at least one tube 104 may be fully decoked after the air polishing stage such that upgrading may resume. The fourth air to steam ratio, regarded as the final air to steam ratio, may also occur with or without the third air to steam ratio. For example, if a substantial portion of coke is removed at the first two steam to air ratios, the decoking method may ‘skip’ the third air to steam ratio and progress instead to the air polishing stage to remove the remainder of the coke.

[0033] The fourth air to steam ratio may be greater than the third air to steam ratio, and may be substantially all air. The second specified average temperature may be greater than the specified average temperature. The second specified average temperature may be achieved in a similar manner as the specified average temperature, which may also be generally regarded as a “first specified average temperature.”

[0034] The specified average temperature may be an average temperature of the cracking furnace 102 recorded at one or more points within the furnace. The average temperature may in turn be calculated by one or more temperature sensors. For example, and in embodiments, the specified average temperature may be recorded at the highest point in the cracking furnace 102, herein referred to as the “arch” of the cracking furnace 102. In this manner, the specified average temperature of the cracking furnace 102 may also be referred to as an “arch temperature.” It is contemplated that measuring at the arch of the cracking furnace 102 may operate to minimize the effect varying reaction temperatures within the at least one tube 104 may have on the observed specified average temperature. It should be understood that the average temperature of the cracking furnace 102 may be monitored by any temperature monitoring system known to a person of ordinary skill in the art.

[0035] In embodiments, heating the cracking furnace 102 may involve activating the heating element 106 of the cracking furnace 102. For example, and in embodiments, if the heating element 106 is a radiant heating element, flue gas may be combusted at the one or more burners. In embodiments, heating the cracking furnace 102 to the specified average temperature may include combusting a first injection rate of flue gas and observing an average temperature within the cracking furnace 102. If the average temperature is greater than or less than the specified average temperature, the rate of flue gas may be lowered or increased, respectively. In embodiments, once the observed average temperature within the cracking furnace 102 is equal to the specified average temperature, the rate of flue gas may be maintained throughout the rest of the decoking processes discussed herein to maintain the specified average temperature.

[0036] In embodiments, the outlet point temperature may be measured as a temperature of the gas mixture exiting the at least one tube 104 at the outlet point. In like manner, an inlet point temperature may be measured as a temperature of the gas mixture entering the at least one tube 104 at the inlet point. As previously discussed, the injection of the gas mixture may operate to start an oxidation reaction, gasification reaction, or both of the coke deposited on the surface of the at least one tube 104. Thereby, the gas mixture exiting the at least one tube 104 may differ from the gas mixture entering the at least one tube 104 in that the exiting gas mixture may additionally include carbon dioxide, carbon monoxide, or both formed from the reactions on the coke. The increase of the temperature within the cracking furnace 102 may operate to increase the severity of the oxidation and gasification reactions, thereby also increasing the rate at which the coke is removed and transformed into carbon dioxide, carbon monoxide, or both. In embodiments, the gas mixture may be injected at a constant mass flow rate. For example, the gas mixture may be injected at a flow rate approximately 850 kilograms per hour (kg/hr). The possible rate of injection of the gas mixture may be limited by a number of factors, including the cross sectional area of the at least one tube 104, the amount of deposited coke, and the number of tubes. For example, and in embodiments, the gas mixture may be injected at a rate of from approximately 300 kg/hr to 2000 kg/hr though each of the at least one tube 104’s. It should be understood that the flow rate of the gas mixture may be monitored or controlled by any flow monitoring or flow metering systems known to a person of ordinary skill in the art.

[0037] As previously mentioned, the gas mixture may include air and steam. However, the gas mixture may further include inert gas. The inert gas may include carbon dioxide or any other gas considered to be chemically inert. Including the inert gas in the gas mixture may operate to increase the heat capacity of the gas mixture through the inclusion of gases that themselves have higher heat capacities. This may in turn improve heat transfer from the gas mixture to the coke.

[0038] In embodiments, the gas mixture may be preheated before being injected into the at least one tube 104. The operations to preheat the gas mixture may be done in a similar manner as to heating the cracking furnace 102, including but not limited to means of radiant heating, electromagnetic inductive heating, electrically resistive heating, or combinations thereof. The gas mixture may be preheated to a temperature of from 550 °C to 750 °C. The gas mixture may also be preheated to a temperature of from 550 °C to 725 °C, from 550 °C to 700 °C, from 550 °C to 650 °C, from 550 °C to 600 °C, from 550 °C to 575 °C, or any smaller range or combination of ranges therein.

[0039] In embodiments, the first air to steam ratio may be from 1 : 10 to 3 : 10 parts air to steam. The air to steam ratios may be measured as the mass flow rate of air to the mass flow rate of steam. The first air to steam ratio may also be from 1 :10 to 6: 10, from 1 :10 to 5: 10, from 1 :10 to 4:10, from 1 :10 to 2:10 parts air to steam, or any smaller range therein, for example from 2: 10 to 3:10 or from 2:10 to 6: 10 parts air to steam. The first air to steam ratio may also be substantially all steam. As previously stated, the second air to steam ratio may be greater than the first air to steam ratio. The second air to steam ratio may be from 1 .1 : 10 to 1 :1 parts air to steam. The second air to steam ratio may also be from 1.1 : 10 to 6: 10, from 1 .1 : 10 to 5 : 10, from 1 .1 : 10 to 4: 10, from 1.1 : 10 to 2:10, from 6:10 to 1.5: 1, from 6:10 to 1.3: 1 , from 6: 10 to 1.1 :1 , from 6:10 to 8:10 parts air to steam, or any smaller range therein, for example from 8:10 to 1 :1 or from 8: 10 to 1.5:1 parts air to steam. The second air to steam ratio may also be substantially all air.

[0040] The third air to steam ratio may be greater than the second air to steam ratio. For example, and in embodiments, the third air to steam ratio may be from 1.2:10 to 1.5:1 parts air to steam. The third air to steam ratio may also be from 6: 10 to 1.5:1 , from 1.2: 10 to 5 : 10, from 1.2: 10 to 4: 10, from 1.2: 10 to 2: 10, from 6: 10 to 1.5: 1, from 6: 10 to 1.3: 1, from 6: 10 to 1.1 :1 , from 6: 10 to 8 : 10, from 6 : 10 to 1.3: 1, from 6 : 10 to 1.1 :1, from 6 : 10 to 8 : 10 parts air to steam, or any smaller range therein, for example from 1 :1 to 1.3:1 or from 8: 10 to 1.5:1 parts air to steam. The third air to steam ratio may also be substantially all air.

[0041] As previously mentioned, injecting the gas mixture at the first air to steam ratio may operate to increase the outlet point temperature of the at least one tube 104 to a first temperature. In embodiments, it is contemplated that increasing the percentage of air within the gas mixture may move the decoking reactions within the at least one tube 104 more towards oxidation from gasification. As the decoking reactions shift towards oxidation, the severity of the reactions increase, which also increases the rate of decoking of the at least one tube 104. As decoking occurs, the temperature within the at least one tube 104 may be observed to increase from the generation of carbon dioxide from the coke. This may in turn be observed as an increase in the outlet temperature.

[0042] In embodiments of the method herein, the outlet point temperature may not exceed the specified average temperature. In like manner, the first temperature, the second temperature, and the third temperature may also not exceed the specified average temperature. In embodiments, the fourth temperature may or may not be greater than the specified average temperature. The outlet point temperature may not exceed the specified average temperature by controlling the first air to steam ratio, the second air to steam ratio, or both. For example, by controlling the air to steam ratios, the amount of oxidation vs. gasification reactions in the at least one tube 104 can be controlled, and thereby the severity of the reactions controlled. [0043] It is contemplated that the outlet point temperature not exceeding the specified average temperature during the methods discussed herein may result in a minimization of the surface temperature of the at least one tube 104 during the decoking process. This is discussed and shown in further detail in the examples section herein. As previously stated, the minimization of the surface temperature of the at least one tube 104 may increase the life of the at least one tube 104 and reduce the oxide spallation rate or degradation of the same.

[0044] In embodiments, the specified average temperature of the cracking furnace 102 may be approximately 840 °C. The specified average temperature may also be from 780 °C to 860 °C, including from 780 °C to 850 °C, from 780 °C to 840 °C, from 780 °C to 830 °C, from 780 °C to 820 °C, from 780 °C to 810 °C, from 780 °C to 790 °C, from 790 °C to 850 °C, from 790 °C to

840 °C, from 790 °C to 830 °C, from 790 °C to 820 °C, from 790 °C to 810 °C, from 810 °C to

850 °C, from 810 °C to 840 °C, from 810 °C to 830 °C, from 810 °C to 820 °C, from 820 °C to

850 °C, from 820 °C to 840 °C, from 820 °C to 830 °C, or any smaller or combination of ranges therein. In embodiments, specified average temperatures on the lower end of the previous range may be more suitable for coke buildups that are primarily soft coke. In like manner, specified average temperatures on the higher end of the previous range may be more suitable for coke buildups that are primarily hard coke.

[0045] In embodiments, the internal surface temperature of the at least one tube 104 may not exceed 890 °C. As previously mentioned, the internal surface temperature of the at least one tube 104 may be minimized, and thus kept below 890 °C, in the methods herein by ensuring that the outlet point temperature does not exceed the specified average temperature. The benefits of minimizing the internal surface temperature of the at least one tube 104 are in lowering the chance of oxide spallation of the at least one tube 104 and reducing the oxidation rate of the same by increasing the margin of safe operating temperatures of the at least one tube 104 during the decoking process. This can be illustrated for example, by reference to FIGS. 1-3. FIGS. 2A-2C illustrate scanning electron microscope photographs of three different chromium alloy tubes, according to embodiments herein. Each of the tubes experienced an equivalent number of upgrading and decoking cycles, with the only difference being the average tube metal temperature experienced during the decoke. FIG. 2A experienced, on average 800 °C, FIG. 2B 850 °C, and FIG. 2C 900 °C. As shown in FIGS. 2A-2C, increases in the tube metal temperature correlated with an increasing degree of chromium depletion and surface degradation of the tubes. Accordingly, minimizing the tube metal temperature can be shown to have beneficial impacts on reducing over-oxidation and oxide spallation and lifetime of the at least one tube 104, according to embodiments herein.

[0046] In embodiments, the internal surface temperature of the at least one tube 104 may be measured in any manner understood to one skilled in the art. For example, and in embodiments, the internal surface temperature of the at least one tube 104 may be measured indirectly through thermal imaging or optical pyrometer measure of the external surface temperature of the at least one tube 104.

EXAMPLES

[0047] Simulations of the decoking process, according to the embodiments herein, were performed using commercially available Computational Fluid Dynamics (CFD) software, along with several custom user defined functions to model the chemistry and coke layers. In addition, a commercially available steam cracker simulation software was used to create boundary conditions to generate the enthalpy flux through the tube walls. Each decoking process was conducted at a constant gas mixture injection rate of 850 kg/hr. The results of each of the simulations are illustrated in FIGS. 3-5. A legend for the abbreviations used in the Figures is provided below in Table 1.

Table 1 : Legend for FIGS. 3-5

[0048] Example 1 : Illustrated by FIG. 3

[0049] A simulation of a decoking process according to embodiments herein, denoted Example 1 (Ex. 1), was analyzed next to a comparative, denoted Comparative Example 1 (CE 1). This is illustrated in FIG. 3. The Comparative Example 1 , indicated by the dashed line, was conducted wherein an average specified temperature was not chosen and the air to steam ratio was adjusted irrespective of the behavior of the outlet point temperature. This included seven different air to steam ratio changes or ‘steps.’ Example 1, indicated by the solid line, included a specified average temperature of 840 °C and first and second air to steam ratios of approximately 0.2:1 and 0.7:1, respectively. The first and second air mass flow rates were 150 kg/hr and 350 kg/hr, respectively. Both the gas mixtures of Example 1 and Comparative Example 1 were also preheated to an initial temperature of 750 °C prior to injection.

[0050] As FIG. 3 illustrates, Example 1 showed a faster decoke time than Comparative Example 1 , at 25 hours to 31 hours, respectively. Example 1 also showed marked reductions to the tube metal temperature over Comparative Example 1 at the later stages of the decoking process. As previously discussed and as shown in FIGS. 2A-2C, this reduction or minimization of tube metal temperature may have a variety of benefits, including reduced oxidation and oxide spallation and extended life of the tube.

[0051] Example 2: Illustrated by FIG. 4

[0052] Another simulation of a decoking process according to embodiments herein, denoted Example 2 (Ex. 2), was analyzed next to another comparative, denoted Comparative Example 2 (CE 2). This is illustrated in FIG. 4. Both were conducted similar to FIG. 3, except that the gas mixture was preheated to 650 °C, rather than 750 °C, to show the effects of preheating on decoke rate.

[0053] As FIG. 4 illustrates, Example 2 showed a faster decoke time than Comparative Example 2, at 20 hours to 24 hours, respectively. Example 2 also showed marked reductions to the tube metal temperature over Comparative Example 2 at the later stages of the decoking process. As previously discussed and as shown in FIGS. 2A-2C, this reduction or minimization of tube metal temperature may have a variety of benefits, including reduced oxide spallation and extended life of the tube. Example 2 showed a faster decoke than Example 1 due to Example 2 starting with approximately 100 kilograms of coke in the at least one tube, whereas Example 1 included approximately 200 kilograms.

[0054] Example 3: Illustrated by FIG. 5

[0055] Another simulation of a decoking process according to embodiments herein, denoted Example 3 (Ex. 3), was analyzed next to another comparative, denoted Comparative Example 3 (CE 3). This is illustrated in FIG. 5. Both were conducted similar to FIG. 4, except that the both the first and second air to steam ratios for Example 3 were greater in FIG. 5. For example, in FIG. 5 the first and second air to steam ratios were approximately 0.26:1 and 1 :1 , respectively. This was done to show the effects of a more aggressive approach to removing the coke, including increasing the severity to a greater degree at each step. FIG. 5 also included a third air to steam ratio of approximately 1.1 :1 as an air polish stage. The air flow rates for the first, second, and third air to steam were 175 kg/hr, 425 kg/hr, and 450 kg/hr, respectively.

[0056] As FIG. 5 illustrates, Example 3 showed a faster decoke time than Comparative Example 2, at 18 hours to 24 hours, respectively. Example 3 also showed marked reductions to the tube metal temperature over Comparative Example 2 at the later stages of the decoking process. As previously discussed and as shown in FIGS. 2A-2C, this reduction or minimization of tube metal temperature may have a variety of benefits, including reduced oxide spallation and extended life of the tube. Example 2 also showed a faster decoke than both Examples 1 and 2, which may be attributable to the increased severity of the decoking reaction at each step. However, the increased severity also resulted in tube metal temperatures that were greater on average and as to the maximum temperature than either Example 1 or 2.

[0057] The summary of the results of Examples 1-3 and Comparative Examples 1-3 are illustrated below in Table 2. As shown in Table 2, the decoking methods according to embodiments herein showed decreased decoking times while also limiting the tube metal temperature and outlet point temperature over the Comparative Examples.

[0058] It is noted that recitations in the present disclosure of a component of the present disclosure being “operable” or “sufficient” in a particular way, to embody a particular property, or to function in a particular manner, are structural recitations, as opposed to recitations of intended use. More specifically, the references in the present disclosure to the manner in which a component is “operable” or “sufficient” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component.

[0059] The singular forms “a,” “an” and “the” include plural referents, unless the context clearly dictates otherwise.

[0060] Throughout this disclosure ranges are provided. It is envisioned that each discrete value encompassed by the ranges are also included. Additionally, the ranges which may be formed by each discrete value encompassed by the explicitly disclosed ranges are equally envisioned. [0061] As used in this disclosure and in the appended claims, the words “comprise,” “has,” and “include” and all grammatical variations thereof are each intended to have an open, nonlimiting meaning that does not exclude additional elements or steps.

[0062] As used in this disclosure, terms such as “first” and “second” are arbitrarily assigned and are merely intended to differentiate between two or more instances or components. It is to be understood that the words “first” and “second” serve no other purpose and are not part of the name or description of the component, nor do they necessarily define a relative location, position, or order of the component. Furthermore, it is to be understood that the mere use of the term “first” and “second” does not require that there be any “third” component, although that possibility is contemplated under the scope of the present disclosure.

[0063] For the purposes of describing and defining the present embodiments it is noted that the terms “substantially” and “approximately” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms “substantially” and “approximately” are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

[0064] According to a first aspect, a method of decoking a cracking furnace includes heating the cracking furnace to a specified average temperature; injecting a gas mixture comprising air and steam through the at least one tube at a first air to steam ratio, thereby increasing an outlet point temperature to a first temperature and combusting at least a first portion of a coke on an internal surface of the at least one tube; injecting the gas mixture through the at least one tube at a second air to steam ratio upon observing a decrease in the outlet point temperature from the first temperature, thereby increasing the outlet point temperature to a second temperature and combusting at least a second portion of the coke on the internal surface of the at least one tube, and wherein the outlet point temperature does not exceed the specified average temperature; the second air to steam ratio is greater than the first air to steam ratio; and the cracking furnace includes at least one tube for transferring a feed, the at least one tube entering the cracking furnace at an inlet point and exiting the cracking furnace at an outlet point. [0065] Another aspect includes any preceding aspect, wherein the outlet point temperature does not exceed the specified average temperature by controlling the first air to steam ratio, the second air to steam ratio, or both.

[0066] Another aspect includes any preceding aspect, wherein the specified average temperature is from 780 °C to 860 °C.

[0067] Another aspect includes any preceding aspect, wherein the internal surface temperature of the at least one tube does not exceed 890 °C.

[0068] Another aspect includes any preceding aspect, wherein injecting the gas mixture is done at a constant mass flow rate.

[0069] Another aspect includes any preceding aspect, wherein the first air to steam ratio is from 1 :10 to 3: 10 parts air to steam; and the second air to steam ratio is from 1.1 :10 to 1 :1 parts air to steam.

[0070] Another aspect includes any preceding aspect, wherein the method further includes injecting the gas mixture at a third air to steam ratio upon observing a second decrease in the outlet point temperature from the second temperature, thereby increasing the outlet point temperature to a third temperature and combusting at least a third portion of the coke on the internal surface of the at least one tube, wherein the third air to steam ratio is greater than the second air to steam ratio.

[0071] Another aspect includes any preceding aspect, wherein the third air to steam ratio is from 1.2:10 to 1.5: 1 parts air to steam.

[0072] Another aspect includes any preceding aspect, wherein the method further includes observing a third decrease in the outlet point temperature from the third temperature; and injecting the gas mixture at a fourth air to steam ratio, wherein the fourth air to steam ratio is substantially all air, thereby increasing the outlet point temperature to a fourth temperature and combusting any remaining portion of the coke on the internal surface of the at least one tube. [0073] Another aspect includes any preceding aspect, wherein the method further includes observing a third decrease in the outlet point temperature from the third temperature; and heating the cracking furnace to a second specified average temperature, wherein the second specified average temperature is greater than the first specified average temperature, thereby increasing the outlet point temperature to a fourth temperature and combusting any remaining portion of the coke on the internal surface of the at least one tube.

[0074] Another aspect includes any preceding aspect, wherein the gas mixture is preheated before being injected into the at least one tube.

[0075] Another aspect includes any preceding aspect, wherein the gas mixture is pre-heated to a temperature of from 550 °C to 750 °C.

[0076] Another aspect includes any preceding aspect, wherein the at least one tube includes a metal alloy including aluminum, chromium, iron, nickel, titanium, or combinations thereof.

[0077] Another aspect includes any preceding aspect, wherein the cracking furnace includes a heating element, the heating element including a radiant heating element, an electromagnetic inductive heating element, an electrically resistive heating element, or combinations thereof; and heating the cracking furnace to the specified average temperature includes generating heat from the heating element.

[0078] Another aspect includes any preceding aspect, wherein the heating element includes the radiant heating element, the radiant heating element including one or more burners, the one or more burners configured to combust flue gas.

[0079] Having described the subject matter of the present disclosure in detail and by reference to specific embodiments, it is noted that the various details disclosed in the present disclosure should not be taken to imply that these details relate to elements that are essential components of the various embodiments described in the present disclosure. Further, it will be apparent that modifications and variations are possible without departing from the scope of the present disclosure, including, but not limited to, embodiments defined in the appended claims.

Claims

1. A method of decoking a cracking furnace, the cracking furnace comprising at least one tube for transferring a feed, the at least one tube entering the cracking furnace at an inlet point and exiting the cracking furnace at an outlet point, the method comprising: heating the cracking furnace to a specified average temperature; injecting a gas mixture comprising air and steam through the at least one tube at a first air to steam ratio, thereby increasing an outlet point temperature to a first temperature and combusting at least a first portion of a coke on an internal surface of the at least one tube; injecting the gas mixture through the at least one tube at a second air to steam ratio upon observing a decrease in the outlet point temperature from the first temperature, thereby increasing the outlet point temperature to a second temperature and combusting at least a second portion of the coke on the internal surface of the at least one tube, and wherein the outlet point temperature does not exceed the specified average temperature; and the second air to steam ratio is greater than the first air to steam ratio.

2. The method of claim 1 , wherein the outlet point temperature does not exceed the specified average temperature by controlling the first air to steam ratio, the second air to steam ratio, or both.

3. The method of any preceding claim, wherein the specified average temperature is from 780 °C to 860 °C.

4. The method of claim 2, wherein the internal surface temperature of the at least one tube does not exceed 890 °C.

5. The method of any preceding claim, wherein injecting the gas mixture is done at a constant mass flow rate.

6. The method of any preceding claim, wherein: the first air to steam ratio is from 1 :10 to 3:10 parts air to steam; and the second air to steam ratio is from 1 .1 : 10 to 1 : 1 parts air to steam.

7. The method of any preceding claim, further comprising injecting the gas mixture at a third air to steam ratio upon observing a second decrease in the outlet point temperature from the second temperature, thereby increasing the outlet point temperature to a third temperature and combusting at least a third portion of the coke on the internal surface of the at least one tube, wherein the third air to steam ratio is greater than the second air to steam ratio.

8. The method of claim 7, wherein the third air to steam ratio is from 1 .2: 10 to 1.5: 1 parts air to steam.

9. The method of either claim 7 or 8, further comprising: observing a third decrease in the outlet point temperature from the third temperature; and injecting the gas mixture at a fourth air to steam ratio, wherein the fourth air to steam ratio is substantially all air, thereby increasing the outlet point temperature to a fourth temperature and combusting any remaining portion of the coke on the internal surface of the at least one tube.

10. The method of any one of claims 7 to 9, further comprising: observing a third decrease in the outlet point temperature from the third temperature; and heating the cracking furnace to a second specified average temperature, wherein the second specified average temperature is greater than the first specified average temperature, thereby increasing the outlet point temperature to a fourth temperature and combusting any remaining portion of the coke on the internal surface of the at least one tube.

11 . The method of any preceding claim, wherein the gas mixture is preheated before being injected into the at least one tube.

12. The method of claim 11, wherein the gas mixture is pre-heated to a temperature of from 550 °C to 750 °C.

13. The method of any preceding claim, wherein the at least one tube comprises a metal alloy comprising aluminum, chromium, iron, nickel, titanium, or combinations thereof.

14. The method of any preceding claim, wherein: the cracking furnace comprises a heating element, the heating element comprising a radiant heating element, an electromagnetic inductive heating element, an electrically resistive heating element, or combinations thereof; and heating the cracking furnace to the specified average temperature comprises generating heat from the heating element.

15. The method of claim 14, wherein the heating element comprises the radiant heating element, the radiant heating element comprising one or more burners, the one or more burners configured to combust flue gas.