Method for decoking cracking furnace

Abstract

The invention discloses a method for decoking a cracking furnace. The method comprises the following steps: 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: steam ratio; injecting the gas mixture through the at least one tube at a second air: steam ratio when it is observed that the outlet point temperature drops from the first temperature; and combusting at least a second portion of the coke on the inner surface of the at least one tube. In addition, 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 comprises at least one tube for conveying feedstock, the at least one tube entering the cracking furnace at an inlet point and exiting the cracking furnace at an outlet point.

Description

Technical Field

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

[0002] Olefins and aromatic compounds, such as ethylene, propylene, butylene, butadiene, benzene, toluene and xylene, are basic intermediates of many petrochemical industries. These olefins and aromatic compounds are usually obtained by thermal cracking (or steam pyrolysis) of petroleum gas and distillates such as naphtha, kerosene or gas oil. These compounds are also produced by refining fluid catalytic cracking (FCC) process, in which standard heavy raw materials such as gas oil or residue are converted. Typical FCC feedstock ranges from hydrocracking tower bottoms to heavy feed fractions, such as vacuum gas oil and atmospheric residue. However, these raw materials are limited. Another source for propylene production is currently refined propylene from FCC devices. Along with growing demand, FCC device owners are increasingly paying attention to the petrochemical product market, to improve their income by utilizing the economic opportunities that occur in the propylene market.

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

[0004] The production of olefins and aromatic compounds in conventional methods can also lead to the production of by-products such as coke. This coke can accumulate in a processing device (such as in a coil or tube of a steam cracking furnace) for upgrading the feed of the processing device and coat the surface of the processing device. Ultimately, the accumulation of this coke can greatly reduce the efficiency of the upgrading device. At this time, the accumulated coke must be removed by a process called decoking. Decoking generally involves introducing steam and air into the coke, gasifying and oxidizing the coke, and causing it to be removed as carbon dioxide and carbon monoxide. Coke, steam and air can also be heated in the decoking process to increase the temperature of the reactants and thus increase the coke removal rate. This can be done by using a heating element of a upgrading device that is generally used to perform a upgrading reaction, which increases the average temperature of the furnace including the coil.

[0005] During the decoking process, the upgrading unit cannot be used to upgrade more feed, resulting in unproductive downtime. In addition, it is desirable to carefully control the decoking process because temperature fluctuations during the decoking process above the temperature limit of the coils can cause oxide spalling, weakening the coils and reducing their life. This effect is exacerbated when a complex decoking process is used because the many adjustments to the heating temperature and flow rates therein can cause the above-mentioned undesirable temperature fluctuations. Oxide spalling can also lead to safety issues when the weakening of the coils causes them to break during upgrading. Further difficulties can arise when the heating elements of the upgrading unit are used to provide most of the temperature to drive the decoking reaction. This increase in average temperature can reduce the margin between the safe operating temperature of the coils and the rate of oxide spalling initiation, thereby causing significant oxide spalling and coil degradation during temperature fluctuations.

[0006] Therefore, there is a need for improved decoking methods. These methods should reduce the amount of time required to decoke the furnace. At the same time, these methods should be carefully designed to be simple so as not to cause the need to change multiple factors, which can lead to temperature fluctuations, which can cause oxide spalling and reduce the life of the coils. Finally, these methods should also limit the energy input to the cracking furnace itself, thereby increasing the margin for safe operating temperature of the coils and increasing the life of the coils.

[0007] The methods provided herein achieve the above objectives by providing a decoking process that involves very few steps, reduces the decoking time compared to equivalent processes, and limits the surface temperatures experienced by the tubes / coils of the cracking furnace during the decoking process. This is achieved in part by controlling the gasification and oxidation rates of the deposited coke by adjusting the ratio of injected air to steam at specified steps during the decoking process. The average temperature of the furnace can also be specified and set during the initial heating of the cracking furnace and then maintained unchanged during the remainder of the process, further limiting temperature fluctuations. Adjusting the gasification and oxidation rates can also operate to increase the severity of the reaction without increasing the average temperature of the furnace and therefore without increasing the surface temperature of the tubes / coils, thereby increasing the margin for safe operating temperature.

[0008] According to one embodiment, a method for 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:steam ratio, thereby increasing the outlet point temperature to the first temperature and burning at least a first portion of the coke on the inner surface of the at least one tube; when the outlet point temperature is observed to decrease from the first temperature, injecting the gas mixture through the at least one tube at a second air:steam ratio, thereby increasing the outlet point temperature to the second temperature; and burning at least a second portion of the coke on the inner surface of the at least one tube, and wherein the outlet point temperature does not exceed the specified average temperature; the second air:steam ratio is greater than the first air:steam ratio; and the cracking furnace includes at least one tube for conveying a feed, the at least one tube entering the cracking furnace at an inlet point and leaving the cracking furnace at an outlet point.

[0009] Additional features and advantages of the embodiments described herein will be set forth in the following detailed description, and in part will become apparent to those skilled in the art from that description or may be recognized by practicing the described embodiments, including the detailed description and claims provided below. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0011] Figure 1 A decoking system according to embodiments herein is illustrated.

[0012] Figure 2A is an SEM image of a laboratory coupon with a composition similar to that of a cracking furnace coil after more than 40 cracking and decoking cycles at an average tube metal temperature of 800°C;

[0013] Figure 2B is an SEM image of a laboratory specimen with a composition similar to that of a cracking furnace coil after more than 40 cracking and decoking cycles at an average tube metal temperature of 850°C;

[0014] Figure 2C is an SEM image of a laboratory specimen with a composition similar to that of a cracking furnace coil after more than 40 cracking and decoking cycles at an average tube metal temperature of 900°C;

[0015] Figure 3 is a schematic diagram of a method for decoking a cracking furnace according to one or more embodiments herein;

[0016] Figure 4 is a diagrammatic representation of a method for decoking a cracking furnace according to one or more embodiments herein; and

[0017] Figure 5 is a schematic diagram of a method of decoking a cracking furnace according to one or more embodiments herein. DETAILED DESCRIPTION

[0018]

[0013] Embodiments described herein relate to methods of decoking a cracking furnace.

[0019] As used herein, "coke" may refer to deposited carbonaceous materials, which may include petroleum coke, catalytic coke, pyrolytic coke, and the like.

[0020] As used herein, references to the "severity" of the 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. Thus, an increase in the severity of the decoking reaction may generally be understood to refer to an increase in the air:steam ratio of the gas mixture or an increase in the volumetric injection rate of the gas mixture.

[0021] As used herein, the "surface temperature of the at least one tube" may also be generally referred to as the "tube metal temperature".

[0022] Additional features and advantages of the described embodiments will be set forth in the following detailed description, and in part will become apparent to those skilled in the art from the description or may be learned by practicing the described embodiments, including the following detailed description and the claims.

[0023] First reference Figure 1 , the cracking furnace 102 may include at least one tube 104 for conveying feed. The feed may include, but is not limited to, hydrocarbons, refined hydrocarbons, bio-derived feeds, recycled feeds, renewable feeds, or combinations thereof. The cracking furnace 102 may also include coke (not shown) deposited on the inner surface of the at least one tube 104. The coke may be a byproduct of an early upgrading reaction within the cracking furnace 102.

[0024] The at least one tube 104 can enter the cracking furnace at the entry point 112 and leave the cracking furnace 102 at the exit point 114. The at least one tube 104 can also be generally considered as a furnace coil. The at least one tube 104 or furnace coil may include a metal alloy, which includes aluminum, chromium, iron, nickel, titanium, manganese, or a combination thereof. For example, and in an embodiment, the at least one tube 104 can be 15% to 45% by weight of chromium, 20% to 50% by weight of nickel, 0.1% to 5% by weight of aluminum, or a combination thereof. Depending on the application, the at least one tube may also include greater than 45% by weight of chromium, greater than 50% by weight of nickel, or both. The at least one tube 104 may also include a coating on the inner surface of the at least one tube 104. The coating may be an inert coating or a catalytic coating, which itself may depend 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 radiation heating element, an electromagnetic induction heating element, a resistance heating element, or a combination thereof. The radiation heating element may include one or more burners configured to burn a burner gas. The burner gas may be, for example, flue gas. The flue gas may include fuel gas, air, hydrogen, oxygen, carbon dioxide, or a combination thereof. The cracking furnace 102 may also include a gas preheating element 108. The gas preheating element 108 may be configured to preheat the gas feed before the gas feed enters the cracking furnace 102 through the inlet point 112. The gas preheating element 108 may be operable to preheat the gas feed to a temperature of 20°C to 800°C.

[0025] The coke deposited on the surface of the at least one tube 104 can be deposited as one or more layers. This phenomenon of layered coke can be referred to as "soft coke" and "hard coke", where soft coke is generally understood to cover the hard coke inside the at least one tube 104. In general, soft coke is understood to require a less violent decoking reaction to remove than hard coke. However, in actual observations, the deposited coke can occupy more than two layers, and the layers deposited farther from the inner surface of the tube are generally "softer" than the layers closer to the inner surface of the at least one tube 104, or require a less violent reaction to remove at a given rate. In this way, the decoking reaction can be understood as requiring increasingly violent reaction conditions to remove all coke at a similar rate as the "softer" coke is reacted and removed and the "harder" coke is exposed. The method herein can remove one or more layers of increasingly harder coke by conducting the decoking reaction until substantially all of the oxidation is completed.

[0026] The method 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 comprising air and steam through the at least one tube 104 at a first air:steam ratio. The first air:steam ratio may be operated to increase the exit point temperature to a first temperature. The first air:steam ratio may also be operated to burn at least a first portion of the coke on the inner surface of the at least one tube 104.

[0027] It is contemplated that one or more observed decreases in outlet temperature may be caused by exposure of one or more layers of the harder coke layer as the softer coke is gradually removed. As previously discussed, the harder coke may require a more vigorous decoking reaction to be removed at a rate similar to the softer coke, and thus may burn at a lower rate at the current air: steam ratio that was previously sufficient to burn the softer coke. Thus, it may be observed that the outlet temperature decreases as less coke is burned at the current severity of the reaction. According to embodiments herein, increasing the air: steam ratio may begin to cause the harder coke to burn at a rate similar to the softer coke, and thereby increase the outlet temperature.

[0028] Additionally or alternatively, when the local area of ​​coke in the at least one tube 104 is removed at different rates, it can be observed that the outlet temperature decreases. For example, the local area of ​​coke in the at least one tube 104 can be completely or substantially composed of softer coke. Therefore, this local area of ​​coke can be completely removed at a lower reaction severity, while other areas still retain deposited coke. The local uncoked area in the at least one tube 104 can then contribute to the reduction of the outlet temperature because the total surface area of ​​coke exposed in the at least one tube 104 is reduced due to the local uncoked position.

[0029] One potential advantage of increasing the air:steam ratio only when a decrease in outlet temperature is observed (such as in the steps defined herein) is to avoid temperature fluctuations that could damage the at least one tube 104 or cause oxide spalling. For example, increasing the severity of the reaction while softer coke is still present could result in an excessively high burn rate of the softer coke, resulting in a sudden increase in the outlet temperature and the temperature of the at least one tube 104.

[0030] As previously described, when harder coke is exposed and less coke is burned at the current air: steam ratio, a decrease in the exit point temperature may be observed. When a decrease in the exit temperature is observed, the first air: steam ratio may be changed to a second air: steam ratio. Thus, the method may further include injecting the gas mixture through the at least one tube 104 at a second air: steam ratio. The second air: steam ratio may be greater than the first air: steam ratio and may therefore operate to cause the exposed harder coke to burn. This may then operate to increase the exit point temperature to a second temperature and to cause at least a second portion of the coke on the inner surface of the at least one tube 104 to burn.

[0031] Similar to the above, after injecting the gas mixture at the second air: steam ratio, a second decrease in the outlet temperature may be observed. When the second decrease is observed, the second air: steam ratio may be changed to a third air: steam ratio. Thus, the method may further include injecting the gas mixture through the at least one tube 104 at a third air: steam ratio. Similar to the above, the third air: steam ratio may be greater than the second air: steam ratio and may therefore operate to burn the exposed harder coke. This may then operate to increase the outlet point temperature to a third temperature, and to burn at least a third portion of the coke on the inner 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: steam ratio, heating the cracking furnace 102 to the second specified average temperature, or both. Considering that this portion of the decoking process may generally be referred to as an "air polishing stage". The air polishing stage may operate to increase the outlet point temperature to the fourth temperature and burn any remaining portion of the coke on the inner surface of the at least one tube 104. This removal or "polishing" of the remaining portion of the coke may operate to produce a tube surface that is less prone to initial deposition of coke when resuming upgrading. The polishing may further operate to oxidize the inner surface of the at least one tube 104, particularly metals such as chromium or aluminum, and thereby repair any defects on the inner surface of the at least one tube 104. In this way, the fourth air: steam ratio used for the air polishing stage may also be considered a "final air: steam ratio" because the at least one tube 104 may be completely decoked after the air polishing stage so that upgrading can be resumed. The fourth air: steam ratio, which is considered the final air: steam ratio, may also occur with or without the third air: steam ratio. For example, if a majority of the coke is removed at the first two air:steam ratios, the decoking process may "skip" the third air:steam ratio and instead proceed to the air polishing stage to remove the remainder of the coke.

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

[0034] The specified average temperature can be the average temperature of the cracking furnace 102 recorded at one or more points in the furnace. The average temperature can be calculated by one or more temperature sensors. For example, and in an embodiment, the specified average temperature can be recorded at the highest point in the cracking furnace 102 (referred to herein as the "arch" of the cracking furnace 102). In this way, the specified average temperature of the cracking furnace 102 can also be referred to as the "arch temperature". Considering that the measurement at the arch of the cracking furnace 102 can be operated to minimize the effect that the reaction temperature in 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 can be monitored by any temperature monitoring system known to those of ordinary skill in the art.

[0035] In an embodiment, heating the cracking furnace 102 may involve activating a heating element 106 of the cracking furnace 102. For example, and in an embodiment, if the heating element 106 is a radiant heating element, the flue gas may be combusted at the one or more burners. In an embodiment, heating the cracking furnace 102 to a 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 the flue gas may be decreased or increased, respectively. In an embodiment, once the observed average temperature within the cracking furnace 102 is equal to the specified average temperature, the rate of the flue gas may be maintained to maintain the specified average temperature during the remainder of the decoking process discussed herein.

[0036] In embodiments, the outlet point temperature can be measured as the temperature of the gas mixture leaving the at least one tube 104 at the outlet point. Similarly, the inlet point temperature can be measured as the temperature of the gas mixture entering the at least one tube 104 at the inlet point. As previously described, the injection of the gas mixture can be operated to start the oxidation reaction, gasification reaction or both of the coke deposited on the surface of the at least one tube 104. Therefore, the gas mixture leaving the at least one tube 104 can be different from the gas mixture entering the at least one tube 104, because the gas mixture leaving can additionally include carbon dioxide, carbon monoxide or both formed by the reaction of the coke. The increase in temperature in the cracking furnace 102 can be operated to increase the severity of the oxidation and gasification reactions, thereby also increasing the rate of removing coke and converting to carbon dioxide, carbon monoxide or both. In embodiments, the gas mixture can be injected at a constant mass flow rate. For example, the gas mixture can be injected at a flow rate of about 850 kilograms per hour (kg / hr). The possible injection rate of the gas mixture can be limited by a variety of factors, including the cross-sectional area of ​​at least one tube 104, the amount of deposited coke and the number of tubes. For example, in an embodiment, the gas mixture may be injected through each of the at least one tube 104 at a rate of approximately 300 kg / hr to 2000 kg / hr. It should be understood that the flow rate of the gas mixture may be monitored or controlled by any flow monitoring or flow metering system known to those of ordinary skill in the art.

[0037] As previously described, the gas mixture may include air and steam. However, the gas mixture may also include an inert gas. The inert gas may include carbon dioxide or any other gas that is considered chemically inert. The inclusion of an inert gas in the gas mixture may operate to increase the heat capacity of the gas mixture by including a gas that itself has a higher heat capacity. This in turn may improve heat transfer from the gas mixture to the coke.

[0038] In an embodiment, the gas mixture may be preheated before being injected into the at least one tube 104. The operation of preheating the gas mixture may be performed in a manner similar to that of heating the cracking furnace 102, including but not limited to radiation heating, electromagnetic induction heating, resistance heating, or a combination thereof. The gas mixture may be preheated to a temperature of 550°C to 750°C. The gas mixture may also be preheated to a temperature of 550°C to 725°C, 550°C to 700°C, 550°C to 650°C, 550°C to 600°C, 550°C to 575°C, or any smaller range or combination of ranges therein.

[0039] In an embodiment, the first air: steam ratio can be 1: 10 to 3: 10 parts air: steam. The air: steam ratio can be measured as the mass flow rate of air to the mass flow rate of steam. The first air: steam ratio can also be 1: 10 to 6: 10, 1: 10 to 5: 10, 1: 10 to 4: 10, 1: 10 to 2: 10 parts air: steam, or any smaller range therein, such as 2: 10 to 3: 10 or 2: 10 to 6: 10 parts air: steam. The first air: steam ratio can also be substantially all steam. As previously described, the second air: steam ratio can be greater than the first air: steam ratio. The second air: steam ratio can be 1.1: 10 to 1: 1 parts air: steam. The second air: steam ratio may also be 1.1:10 to 6:10, 1.1:10 to 5:10, 1.1:10 to 4:10, 1.1:10 to 2:10, 6:10 to 1.5:1, 6:10 to 1.3:1, 6:10 to 1.1:1,

[0040] 6:10 to 8:10 parts air:steam, or any smaller range therein, such as 8:10 to 1:1 or 8:10 to 1.5:1 parts air:steam. The second air:steam ratio can also be substantially all air.

[0041] The third air: steam ratio can be greater than the second air: steam ratio. For example, in embodiments, the third air: steam ratio can be 1.2: 10 to 1.5: 1 parts air: steam. The third air: steam ratio can also be 6: 10 to 1.5: 1, 1.2: 10 to 5: 10, 1.2: 10 to 4: 10, 1.2: 10 to 2: 10, 6: 10 to 1.5: 1, 6: 10 to 1.3: 1, 6: 10 to 1.1: 1, 6: 10 to 8: 10, 6: 10 to 1.3: 1, 6: 10 to 1.1: 1, 6: 10 to 8: 10 parts air: steam, or any smaller range therein, such as 1: 1 to 1.3: 1 or 8: 10 to 1.5: 1 parts air: steam. The third air: steam ratio can also be substantially all air.

[0042] As previously described, injecting the gas mixture at a first air:steam ratio can be operated to increase the temperature of the outlet point of the at least one tube 104 to a first temperature. In embodiments, it is contemplated that increasing the air percentage within the gas mixture can shift the decoking reaction within the at least one tube 104 more from gasification to oxidation. As the decoking reaction shifts toward oxidation, the severity of the reaction increases, which also increases the decoking rate of the at least one tube 104. As decoking occurs, the temperature within the at least one tube 104 can be observed to increase due to the production of carbon dioxide from the coke. This, in turn, can be observed as an increase in the outlet temperature.

[0043] In embodiments of the methods herein, the outlet point temperature may not exceed a specified average temperature. Similarly, the first temperature, the second temperature, and the third temperature may not exceed a specified average temperature. In embodiments, the fourth temperature may be higher or may not be higher than a specified average temperature. By controlling the first air: steam ratio, the second air: steam ratio, or both, the outlet point temperature may not exceed a specified average temperature. For example, by controlling the air: steam ratio, the amount of oxidation and gasification reaction in the at least one tube 104 may be controlled, and the severity of the reaction may be thereby controlled.

[0044] Considering that the outlet point temperature does not exceed a specified average temperature during the methods discussed herein can lead to minimization of the surface temperature of the at least one tube 104 during the decoking process. This is discussed and illustrated in further detail in the Examples section herein. As previously described, minimization of the surface temperature of the at least one tube 104 can increase the life of the at least one tube 104 and reduce its oxide spallation rate or degradation.

[0045] In an embodiment, the specified average temperature of the cracking furnace 102 may be approximately 840° C. The specified average temperature may also be 780° C. to 860° C., including 780° C. to 850° C., 780° C. to 840° C., 780° C. to 830° C., 780° C. to 820° C., 780° C. to 810° C., 780° C. to 790° C., 790° C. to 850° C., 790° C. to 840° C., 790° C. to 830° C., 790° C. to 820° C., 790° C. to 810° C., 810° C. to 850° C., 810° C. to 840° C., 810° C. to 830° C., 810° C. to 820° C., 820° C. to 850° C., 820° C. to 840° C., 820° C. to 830° C., or any smaller range or combination of ranges therein. In embodiments, the specified average temperature at the lower limit of the aforementioned range may be more suitable for coke accumulation that is mainly soft coke. Likewise, the specified average temperature at the upper limit of the aforementioned range may be more suitable for coke accumulation that is mainly hard coke.

[0046] In an embodiment, the inner surface temperature of the at least one tube 104 may not exceed 890°C. As previously described, in the methods herein, by ensuring that the exit point temperature does not exceed a specified average temperature, the inner surface temperature of the at least one tube 104 may be minimized and thus maintained below 890°C. The benefit of minimizing the inner surface temperature of the at least one tube 104 is to reduce the chance of oxide spalling of the at least one tube 104 and reduce its oxidation rate by increasing the margin of the safe operating temperature of the at least one tube 104 during the decoking process. This can be achieved, for example, by referring to Figures 1 to 3 To give an example. FIG. 2A to FIG. 2CScanning electron microscope photographs of three different chromium alloy tubes according to embodiments herein are illustrated. Each tube was subjected to an equal number of upgrading and decoking cycles, with the only difference being the average tube metal temperature experienced during decoking. Figure 2A The average temperature is 800℃. Figure 2B Experience 850℃, and Figure 2C Experience 900℃. FIG. 2A to FIG. 2C As shown, an increase in tube metal temperature is associated with an increase in chromium loss and surface degradation of the tube. Thus, according to embodiments herein, it can be demonstrated that minimizing tube metal temperature has a beneficial effect on reducing overoxidation and oxide spalling and the life of the at least one tube 104.

[0047] In an embodiment, the inner surface temperature of the at least one tube 104 can be measured in any manner understood by those skilled in the art. For example, and in an embodiment, the inner surface temperature of the at least one tube 104 can be indirectly measured by thermal imaging or optical pyrometry of the outer surface temperature of the at least one tube 104.

[0048] Example

[0049] According to embodiments herein, simulations of the decoking process were performed using commercially available computational fluid dynamics (CFD) software along with several customized user-defined functions to model the chemistry and coke layer. In addition, boundary conditions were created using commercially available steam cracker simulation software to generate enthalpy flux through the tube walls. Each decoking process was performed at a constant gas mixture injection rate of 850 kg / hr. The results of each simulation are illustrated in Figures 3 to 5 A legend for the abbreviations used in the figures is provided in Table 1 below.

[0050] Table 1: Figures 3 to 5 Legend

[0051] abbreviation definition unit TMT Tube metal temperature (tube surface temperature) ℃ COT Coil outlet temperature (outlet point temperature) ℃ <![CDATA[M c ]]> Coke deposited in the tube kilogram XCO2 The ratio of carbon dioxide present in the outlet stream <![CDATA[kg CO2 / kg gas mixture]]> AF Air mass flow rate kg / hr

[0052] Example 1: Figure 3 exemplified

[0053] Following the comparative example denoted as Comparative Example 1 (CE 1), a simulation of a decoking process according to an embodiment herein denoted as Example 1 (Ex. 1) was analyzed. This is illustrated in Figure 3. Comparative Example 1 indicated by the dashed line was performed in which no average specified temperature was selected and the air:steam ratio was adjusted regardless of the behavior of the outlet point temperature. This included seven different air:steam ratio variations or 'steps'. Example 1 indicated by the solid line included a specified average temperature of 840°C and a first air:steam ratio and a second air:steam ratio of approximately 0.2:1 and 0.7:1, respectively. The first air mass flow rate and the second air mass flow rate were 150 kg / hr and 350 kg / hr, respectively. The gas mixture of Example 1 and Comparative Example 1 was also preheated to an initial temperature of 750°C prior to injection.

[0054] like Figure 3 For example, Example 1 shows a faster decoking time than Comparative Example 1, which is 25 hours for Example 1 and 31 hours for Comparative Example 1, respectively. In the late stages of the decoking process, Example 1 also shows a significant reduction in tube metal temperature compared to Comparative Example 1. As previously discussed and as FIG. 2A to FIG. 2C As shown, such reduction or minimization of tube metal temperature can have a variety of benefits, including reduced oxidation and oxide spalling and extended tube life.

[0055] Example 2: Figure 4 exemplified

[0056] Following another comparative example, denoted as Comparative Example 2 (CE 2), another simulation of a decoking process according to an embodiment herein, denoted as Example 2 (Ex. 2), was analyzed. This example is shown in Figure 4 Both are similar to Figure 3 was performed except that the gas mixture was preheated to 650°C instead of 750°C to show the effect of preheating on the decoking rate.

[0057] like Figure 4 For example, Example 2 shows a faster decoking time than Comparative Example 2, which is 20 hours for Example 2 and 24 hours for Comparative Example 2, respectively. In the late stages of the decoking process, Example 2 also shows a significant reduction in tube metal temperature compared to Comparative Example 2. As previously discussed and as FIG. 2A to FIG. 2C As shown, such reduction or minimization of tube metal temperature can have a variety of benefits, including reduced oxide spalling and extended tube life. Example 2 shows faster decoking than Example 1 because Example 2 starts with about 100 kg of coke in the at least one tube, while Example 1 includes about 200 kg.

[0058] Example 3: Figure 5 exemplified

[0059] Following another comparative example, denoted as Comparative Example 3 (CE 3), another simulation of a decoking process according to an embodiment herein, denoted as Example 3 (Ex. 3), was analyzed. This is illustrated in Figure 5 Both are similar to Figure 4 except in Figure 5 The first air: steam ratio and the second air: steam ratio of Example 3 are both greater. Figure 5 The first air: steam ratio and the second air: steam ratio were approximately 0.26: 1 and 1: 1, respectively. This was done to show the effect of a more aggressive decoking process, including a greater increase in severity in each step. Figure 5 A third air:steam ratio of about 1.1: 1 was also included as an air polishing 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.

[0060] like Figure 5 For example, Example 3 shows a faster decoking time than Comparative Example 2, which is 18 hours for Example 3 and 24 hours for Comparative Example 2. In the late stages of the decoking process, Example 3 also shows a significant reduction in tube metal temperature compared to Comparative Example 2. As previously discussed and as FIG. 2A to FIG. 2C As shown, such reduction or minimization of tube metal temperature can have a variety of benefits, including reduced oxide spalling and extended tube life. Example 2 also shows faster decoking than both Example 1 and Example 2, which can be attributed to the increased severity of the decoking reaction in each step. However, the increased severity also results in tube metal temperatures that are, on average, greater than the maximum temperature of either Example 1 or Example 2.

[0061] The results of Examples 1 to 3 and Comparative Examples 1 to 3 are summarized and illustrated in the following Table 2. As shown in Table 2, the decoking method according to the embodiments herein shows a reduced decoking time while also limiting the tube metal temperature and the exit point temperature relative to the comparative examples.

[0062] Example Decoking time (hours) Maximum tube metal temperature (℃) Maximum outlet temperature (℃) Example 1 25 880 850 Example 2 20 875 840 Example 3 18 890 840 Comparative Example 1 31 920 860 Comparative Example 2 24 910 870 Comparative Example 3 24 910 870

[0063] It should be noted that, unlike statements of intended use, statements in the present disclosure that a component of the present disclosure is "operable" or "sufficient" in a particular manner to embody a particular characteristic or function in a particular manner are structural statements. More specifically, references in the present disclosure to the manner in which a component is "operable" or "sufficient" refer to the existing physical condition of the component and, therefore, will be regarded as an explicit statement of the structural characteristics of the component.

[0064] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0065] Ranges are provided throughout the disclosure. It is anticipated that each discrete value encompassed by these ranges is also included. In addition, ranges formed by each discrete value encompassed by the explicitly disclosed ranges are also anticipated.

[0066] As used in this disclosure and the appended claims, the words "comprising," "having," and "including" and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

[0067] As used in this disclosure, terms such as "first" and "second" are arbitrarily designated and are intended only to distinguish between two or more instances or components. It should be understood that the words "first" and "second" have no other purpose and are not part of the component name or description, nor do they necessarily define the relative positioning, location, or order of the components. In addition, it should be understood that the use of the terms "first" and "second" alone does not require the existence of any "third" component, although such a possibility is contemplated within the scope of this disclosure.

[0068] For the purposes of describing and defining embodiments of the present invention, it is noted that the terms "substantially" and "approximately" are used 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 used 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.

[0069] According to a first aspect, a method for decoking a cracking furnace comprises: heating the cracking furnace to a specified average temperature; injecting a gas mixture comprising air and steam through at least one tube at a first air:steam ratio, thereby increasing the outlet point temperature to a first temperature and burning at least a first portion of the coke on the inner surface of the at least one tube; when the outlet point temperature is observed to decrease from the first temperature, injecting the gas mixture through the at least one tube at a second air:steam ratio, thereby increasing the outlet point temperature to a second temperature and burning at least a second portion of the coke on the inner surface of the at least one tube, and wherein the outlet point temperature does not exceed the specified average temperature; the second air:steam ratio is greater than the first air:steam ratio; and the cracking furnace comprises at least one tube for conveying a feed, the at least one tube entering the cracking furnace at an inlet point and leaving the cracking furnace at an outlet point.

[0070] Another aspect includes any of the preceding aspects, wherein by controlling the first air:steam ratio, the second air:steam ratio, or both, the outlet point temperature does not exceed the specified average temperature.

[0071] Another aspect includes any of the preceding aspects, wherein the specified average temperature is 780°C to 860°C.

[0072] Another aspect includes any of the preceding aspects, wherein the inner surface temperature of the at least one tube does not exceed 890°C.

[0073] Another aspect includes any of the preceding aspects, wherein injecting the gas mixture is performed at a constant mass flow rate.

[0074] Another aspect includes any of the preceding aspects, wherein the first air:steam ratio is 1:10 to 3:10 parts air:steam; and the second air:steam ratio is 1.1:10 to 1:1 parts air:steam.

[0075] Another aspect includes any of the preceding aspects, wherein the method further comprises injecting the gas mixture at a third air:steam ratio when the outlet point temperature is observed to decrease a second time 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 inner surface of the at least one tube, wherein the third air:steam ratio is greater than the second air:steam ratio.

[0076] Another aspect includes any of the preceding aspects, wherein the third air:steam ratio is from 1.2:10 to 1.5:1 parts air:steam.

[0077] Another aspect includes any of the preceding aspects, wherein the method further comprises observing a third decrease in the outlet point temperature from the third temperature; and injecting the gas mixture at a fourth air:steam ratio, wherein the fourth air:steam ratio is substantially all air, thereby increasing the outlet point temperature to the fourth temperature and burning any remaining portion of the coke on the inner surface of the at least one tube.

[0078] Another aspect includes any of the preceding aspects, wherein the method further comprises 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 higher than the first specified average temperature, thereby increasing the outlet point temperature to a fourth temperature and burning any remaining portion of the coke on the inner surface of the at least one tube.

[0079] Another aspect includes any of the preceding aspects, wherein the gas mixture is preheated prior to being injected into the at least one tube.

[0080] Another aspect includes any of the preceding aspects, wherein the gas mixture is preheated to a temperature of 550°C to 750°C.

[0081] Another aspect includes any of the preceding aspects, wherein the at least one tube comprises a metal alloy comprising aluminum, chromium, iron, nickel, titanium, or combinations thereof.

[0082] Another aspect includes any of the preceding aspects, wherein the cracking furnace comprises a heating element comprising a radiant heating element, an electromagnetic induction heating element, a resistive heating element, or a combination thereof; and heating the cracking furnace to the specified average temperature comprises generating heat from the heating element.

[0083] Another aspect includes any of the preceding aspects, wherein the heating element comprises a radiant heating element comprising one or more burners configured to combust flue gas.

[0084] The subject matter of the present disclosure has been described in detail and by reference to specific embodiments, it should be noted that the various details disclosed in the present disclosure should not be considered to imply that these details relate to elements that are essential parts of the various embodiments described in the present disclosure. In addition, it is apparent that modifications and changes can be made without departing from the scope of the present disclosure including but not limited to the embodiments defined in the appended claims.

Claims

1. A method of decoking a cracking furnace, the cracking furnace comprising at least one tube for conveying feed, the at least one tube entering the cracking furnace at an inlet point and leaving 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:steam ratio to increase the exit point temperature to a first temperature and to combust at least a first portion of the coke on the inner surface of the at least one tube; When a decrease in the exit point temperature from the first temperature is observed, injecting the gas mixture through the at least one tube at a second air:steam ratio, thereby increasing the exit point temperature to a second temperature and combusting at least a second portion of the coke on the inner surface of the at least one tube, and wherein The outlet point temperature does not exceed the specified average temperature; and The second air:steam ratio is greater than the first air: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:steam ratio, the second air:steam ratio, or both.

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

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

5. A method according to any preceding claim, wherein injecting the gas mixture is performed at a constant mass flow rate.

6. A method according to any preceding claim, wherein: the first air:steam ratio is from 1:10 to 3:10 parts air:steam; and The second air:steam ratio is from 1.1:10 to 1:1 parts air:steam.

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

8. The method of claim 7, wherein the third air:steam ratio is from 1.2:10 to 1.5:1 parts air:steam.

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

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

11. A method according to 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 preheated to a temperature of 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. A method according to any preceding claim, wherein: The cracking furnace includes a heating element, the heating element including a radiation heating element, an electromagnetic induction heating element, a resistance heating element, or a combination thereof; and Heating the cracking furnace to the specified average temperature includes 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 configured to combust flue gas.

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