Method for suppressing coking, method for producing light hydrocarbon, and coking inhibitor

Abstract

To provide a coking suppression method that can suppress coking in a heating furnace tube of a delayed coker.SOLUTION: A method for suppressing coking for suppressing coking in a heating furnace tube of a delayed coker according to the invention includes a step of passing a fluid in which a coking inhibitor containing overbased magnesium carbonate is added to a heavy hydrocarbon through the heating furnace tube of the delayed coker.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for inhibiting coking, a method for producing light hydrocarbons, and a coking inhibitor. [Background technology]

[0002] A delayed coker is a device that produces hydrocarbons with various boiling points, such as gas, naphtha, and diesel, and coke by thermally cracking heavy hydrocarbons. A delayed coker generally consists of a heating furnace, a fractionator, a coke drum, and other auxiliary equipment.

[0003] In a delayed coker, heavy hydrocarbons are heated to the thermal cracking temperature in a heating furnace. As a result, coke gradually accumulates in the heating furnace tubes (piping) over the course of operation. If a large amount of coke accumulates, the tubes may become too hot and burst. For this reason, methods have been proposed in which steam is injected into the tubes to increase the flow rate in the tubes and cause turbulence to suppress coke accumulation, and methods have been proposed in which heavy diesel or volatile oil is recirculated to prevent coke accumulation (Patent Documents 1 and 2).

[0004] However, even if the above-mentioned methods are adopted, it is not possible to completely eliminate the coke buildup inside the tubes, so the operation of the delayed coker is periodically stopped and the inside of the heating furnace tubes is cleaned to remove the accumulated coke. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 6-89335 [Patent Document 2] Special Publication No. 6-49866 Summary of the Invention [Problem to be solved by the invention]

[0006] Currently, due to the global oil situation, the need for heavy oil processing is increasing, and there is a stronger demand than ever for an improvement in the processing capacity of delayed cokers. Under these circumstances, in order to improve the processing capacity of delayed cokers, a method is required that can suppress coking in the heater tubes, extend the continuous operation period until the operation of the delayed coker is stopped and the inside of the heater tubes is cleaned, and crack more heavy oil into light oil.

[0007] Therefore, an object of the present invention is to provide a coking inhibition method capable of inhibiting coking in the heating furnace tube of a delayed coker, a method for producing light hydrocarbons, and a coking inhibitor. [Means for solving the problem]

[0008] In order to solve the above problems, the present disclosure provides the following coking inhibition method, method for producing light hydrocarbons, and coking inhibitor.

[0009] [1] A method for inhibiting coking in a furnace tube of a delayed coker, the method comprising the step of passing a fluid in which a coking inhibitor containing overbased magnesium carbonate has been added to heavy hydrocarbons through the furnace tube of the delayed coker. [2] The method for inhibiting coking according to [1] above, wherein the flow rate of the fluid when passing through the heating furnace tube is 1.0 to 4.0 m / s, the residence time of the fluid in the heating furnace is 0.5 to 5 minutes, and the temperature of the fluid at the outlet of the heating furnace is 400 to 580°C. [3] The method for inhibiting coking according to [1] or [2] above, wherein the concentration of the overbased magnesium carbonate in the fluid is 5 ppm by mass or more. [4] A method for producing light hydrocarbons by producing light hydrocarbons from heavy hydrocarbons using a delayed coker, the method comprising a step of passing a fluid, in which a coking inhibitor containing overbased magnesium carbonate has been added to the heavy hydrocarbons, through a heating furnace tube of the delayed coker. [5] The method for producing light hydrocarbons according to [4] above, wherein the flow rate of the fluid when passing through the heating furnace tube is 1.0 to 4.0 m / s, the residence time of the fluid in the heating furnace is 0.5 to 5 minutes, and the temperature of the fluid at the outlet of the heating furnace is 400 to 580°C. [6] The method for producing light hydrocarbons according to [4] or [5] above, wherein the concentration of the overbased magnesium carbonate in the fluid is 5 ppm by mass or more. [7] A coking inhibitor for inhibiting coking in furnace tubes of a delayed coker, the coking inhibitor comprising an overbased magnesium carbonate.

[0010] Coking in the furnace tube of a delayed coker progresses when the coking material in the fluid is concentrated near the inner wall of the furnace tube, and a polymerization reaction occurs when the coking material becomes highly concentrated. In contrast, according to the coking inhibition method described in [1] above, by including a coking inhibitor containing overbased magnesium carbonate in the fluid, the initial minute coking material is coated with the coking inhibitor, and the polymerization reaction between the coking material is suppressed. In addition, the generated coking material has small crystals, is brittle, and easily peels off, so it is easily peeled off from the inner wall of the furnace tube by the turbulent flow of the fluid. Therefore, it is possible to suppress the thick accumulation of coke in the furnace tube and the rise in the surface temperature of the furnace tube. When the surface temperature of the tube of the heating furnace becomes higher than a predetermined temperature, it becomes necessary to stop the operation of the delayed coker and clean the inside of the heating furnace tube. However, according to the above-mentioned method for suppressing coking, the continuous operation period before cleaning of the inside of the heating furnace tube can be extended, and the cumulative amount of oil passing through (the total amount of oil passing through from the start of operation of the delayed coker to the stop of operation) can be increased, thereby improving the processing capacity of the delayed coker.

[0011] According to the coking suppression methods described in [2] and [3] above, the effect of suppressing coking in the heating furnace tube of the delayed coker can be further improved, and the processing capacity of the delayed coker can be further improved.

[0012] According to the method for producing light hydrocarbons described in [4] above, as with the method for suppressing coking described in [1] above, coking in the heating furnace tube of the delayed coker can be suppressed, so that the continuous operation period before cleaning of the heating furnace tube can be extended and the processing capacity of the delayed coker can be improved. As a result, light hydrocarbons can be produced efficiently.

[0013] According to the above-mentioned methods for producing light hydrocarbons described in [5] and [6], the effect of suppressing coking in the heating furnace tube of the delayed coker can be further improved, and the processing capacity of the delayed coker can be further improved, so that light hydrocarbons can be produced more efficiently.

[0014] By adding the coking inhibitor described in [7] above to the heavy hydrocarbons introduced into the heating furnace tube of the delayed coker, coking inside the heating furnace tube can be suppressed, so that the continuous operation period before cleaning inside the heating furnace tube can be extended and the processing capacity of the delayed coker can be improved. Effect of the Invention

[0015] According to the present invention, it is possible to provide a coking inhibition method capable of inhibiting coking in the heating furnace tube of a delayed coker, a method for producing light hydrocarbons, and a coking inhibitor. [Brief description of the drawings]

[0016] [Figure 1] 1 is an overall schematic diagram showing an example of a delayed coker used in the coking inhibition method and the light hydrocarbon production method of the present invention. [Diagram 2] 1 is a polarizing microscope photograph of a cross section of coke produced in Example 3 and Comparative Examples 5 and 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, preferred embodiments of the present invention will be described in detail, with reference to the drawings where necessary.

[0018] [Caulking inhibitor] The coking inhibitor according to the present embodiment is for inhibiting coking in the heating furnace tubes of a delayed coker and contains overbased magnesium carbonate. The overbased magnesium carbonate is an overbased salt of magnesium carbonate. Here, the overbased salt of magnesium carbonate refers to a salt in which magnesium metal is contained in stoichiometric excess over organic acid radicals.

[0019] The coking inhibitor may contain other components in addition to the overbased magnesium carbonate. Examples of the other components include metal salts other than the overbased magnesium carbonate, (phosphite) ester compounds, dispersants, etc. The overbased magnesium carbonate may form a complex with the other components.

[0020] Examples of the metal salt include metal hydroxides, acetates, carbonates, cresolates, naphthenates, acetylacetonates, and mixtures thereof. Examples of metals constituting the metal salt include potassium, sodium, iron, nickel, vanadium, tin, molybdenum, manganese, aluminum, cobalt, calcium, magnesium, and mixtures thereof. These may be used alone or in combination of two or more.

[0021] Examples of the (phosphate) ester compound include phosphonic acid type phosphite ester compounds, phosphorous acid type phosphite ester compounds, diphosphite type phosphite ester compounds, and phosphate ester compounds, etc. These may be used alone or in combination of two or more.

[0022] Examples of the dispersant include polyolefin esters, polyalkenyl-substituted succinic acid esters, etc. These may be used alone or in combination of two or more.

[0023] The coking inhibitor may be a solid or a liquid in which the overbased magnesium carbonate is dissolved or dispersed.

[0024] The content of the overbased magnesium carbonate in the coking inhibitor may be 10 to 100 mass%, preferably 20 to 90 mass%, more preferably 20 to 60 mass%, based on the total amount of the coking inhibitor. When the content of the overbased magnesium carbonate is 10 mass% or more, coking in the heating furnace tube tends to be more sufficiently suppressed.

[0025] [Method for inhibiting coking and method for producing light hydrocarbons] 1 is a schematic diagram showing an example of a delayed coker used in the coking inhibition method and the method for producing light hydrocarbons of the present invention. The delayed coker 1 is composed of a heating furnace 10, a stub tower 12, a fractionator 14, a heat exchanger 20, a switch valve 22, two coke drums 16 and 18, and other auxiliary equipment.

[0026] The feedstock oil is heavy hydrocarbon oil such as bottom oil of atmospheric distillation unit, bottom oil of vacuum distillation unit, decant oil of fluid catalytic cracking unit (FCC), etc. In addition, various heavy hydrocarbon oils generated in the refinery can also be used.

[0027] The initial temperature of the feed oil (temperature in line L1) can be about 100 to 200° C. The feed oil can be preheated to about 250 to 350° C. by heat exchange with heavy gas oil withdrawn from the fractionator 14 via line L13 in the heat exchanger 20. From the viewpoint of reducing the load on the heating furnace 10, it is desirable that the temperature of the feed oil in line L2 is high.

[0028] The preheated feedstock oil is introduced into the stub tower 12. The feedstock oil is separated into light hydrocarbons such as gas, naphtha, diesel, etc., which are low boiling fractions, and heavy hydrocarbons, which are high boiling fractions, in the stub tower 12. If the properties of the feedstock oil are constant, the ratio of the separated light hydrocarbons to the heavy hydrocarbons can usually be constant. The boiling point of the light hydrocarbons may be 30 to 550°C, preferably 40 to 520°C, and more preferably 50 to 480°C. The boiling point of the heavy hydrocarbons may be 450 to 850°C, preferably 465 to 775°C, and more preferably 480 to 700°C.

[0029] The heavy hydrocarbons discharged from the stub tower 12 to the line L3 are added with the coking inhibitor according to the present embodiment in the line L3. The fluid containing the heavy hydrocarbons and the coking inhibitor is then introduced into the heating furnace tube of the heating furnace 10 and heated in the heating furnace 10. The fluid heated in the heating furnace 10 is introduced into the coke drum 16 (or 18) via the line L15 (or L16) by the switch valve 22.

[0030] In the coke drum 16, the heavy hydrocarbons in the heated fluid are thermally cracked to produce cracked hydrocarbons including gas, naphtha, light oil fractions, etc., and coke. The produced cracked hydrocarbons are introduced into the stub tower 12.

[0031] Light hydrocarbons withdrawn from the top of the stub tower 12 through line L10 are introduced into the fractionator 14. The fractionator 14 separates the light hydrocarbons to produce distillate oils. The distillate oils include gas naphtha, light diesel oil, and heavy diesel oil. These distillate oils can be used as blend base stocks for each fuel oil or as feedstocks for subsequent equipment.

[0032] The heavy gas oil withdrawn from the bottom of the fractionator 14 through a line L13 is heat exchanged with the feed oil in a heat exchanger 20 and then sent to a downstream tank or the like.

[0033] <Operation of heating furnace> Next, the operation of the heating furnace will be described. The heating furnace 10 heats a fluid containing heavy hydrocarbons and a coking inhibitor. The temperature of the fluid in the line L4 on the outlet side of the heating furnace 10 is a temperature required for thermal cracking reaction in the subsequent coke drums 16, 18, and can usually be about 400 to 600°C. The temperature of the fluid at the outlet of the heating furnace 10 is adjusted to an optimal value according to the properties of the fluid in order to obtain the desired cracked hydrocarbon yield and coke properties, but from the viewpoint of more sufficiently suppressing coking in the heating furnace tubes, it may be 400 to 580°C, preferably 420 to 560°C, and more preferably 440 to 540°C.

[0034] The flow rate of the fluid passing through the heating furnace tube is adjusted to an optimal value according to the properties of the fluid in order to obtain the desired cracked hydrocarbon yield and coke properties, but from the viewpoint of more fully suppressing coking in the heating furnace tube, it may be 1.0 to 4.0 m / s, preferably 1.2 to 3.7 m / s, and more preferably 2.1 to 3.4 m / s. Here, the flow rate refers to a flow rate calculated by dividing the volumetric flow rate of the fluid introduced into the heating furnace tube at 15.5°C and atmospheric pressure by the flow path cross-sectional area in the heating furnace tube. In addition, when there are cracked product oils or feed oils other than heavy hydrocarbons supplied to the heating furnace tube in addition to the heavy hydrocarbons and the coking inhibitor, the flow rate is calculated including the volumetric flow rates of those.

[0035] The residence time of the fluid in the heating furnace is adjusted to an optimal value depending on the properties of the fluid in order to obtain the desired cracked hydrocarbon yield and coke properties, and from the viewpoint of more sufficiently suppressing coking in the heating furnace tubes, it may be 0.5 to 5 minutes, preferably 1 to 4 minutes, and more preferably 1 to 3 minutes.

[0036] The concentration of overbased magnesium carbonate in the fluid passed through the inside of the heating furnace tube may be 5 ppm by mass or more, preferably 10 ppm by mass or more, more preferably 15 ppm by mass or more, and even more preferably 30 ppm by mass or more, from the viewpoint of more sufficiently suppressing coking in the heating furnace tube. Also, the concentration of overbased magnesium carbonate in the fluid may be 1000 ppm by mass or less, or may be 100 ppm by mass or less, from the viewpoint of economic efficiency.

[0037] The surface temperature of the heating furnace tube gradually increases as the operation time progresses due to the gradual accumulation of coke in the tube of the heating furnace. When this surface temperature reaches an upper limit set by the constraints of the tube material, it is necessary to stop the operation of the delayed coker 1 and clean the tube of the heating furnace 10. According to the coking suppression method and the method for producing light hydrocarbons of the present embodiment, the coking inhibitor of the present embodiment is added to the fluid passing through the heating furnace tube, thereby suppressing the accumulation of coke in the tube of the heating furnace and slowing down the rate of increase in the surface temperature of the heating furnace tube. Therefore, the continuous operation period until the operation of the delayed coker 1 is stopped and the tube of the heating furnace 10 is cleaned can be extended, and the cumulative amount of oil passing (the total amount of oil passing from the start of operation of the delayed coker to the stop of operation) can be increased, thereby improving the processing capacity of the delayed coker.

[0038] <Coke drum operation> Next, the operation of the coke drum will be described. In this embodiment, two coke drums, a coke drum 16 and a coke drum 18, are provided. Fluid heated in a heating furnace 10 is alternately introduced into the two coke drums.

[0039] Fluid heated in the heating furnace 10 is introduced into the coke drum 16, and when a predetermined amount of coke has accumulated, the switch valve 22 is operated to switch to the coke drum 18, which has been waiting for the introduction of fluid. After switching (SW), the coke drum 16 with the accumulated coke is cooled to a predetermined temperature and the coke is removed. When a predetermined amount of coke has accumulated in the coke drum 18, the switch valve 22 is operated to switch to the coke drum 16, which has been waiting for the introduction of fluid. The above coke drum switching operation is repeated. This allows the delayed coker to operate continuously.

[0040] The cracked hydrocarbons produced in the coke drum 16 (18) are introduced from the coke drum 16 (18) through a line L8 (L9) into the stub tower 12. Meanwhile, the condensed oil produced in the coke drum 16 (18) is withdrawn from the bottom of the coke drum 16 (18) through a line L5 (L6) and introduced into the vicinity of the bottom of the fractionator 14 through a line L7. The coking inhibitor is cracked in the heating furnace tubes and the coke drum, and is contained in the product coke.

[0041] <Stub tower and fractionator operation> The cracked hydrocarbons introduced into the stub tower 12 are separated into light and heavy hydrocarbons in the stub tower 12. The light hydrocarbons are withdrawn from the top of the stub tower 12 through a line L10 and introduced into the fractionator 14. On the other hand, the heavy hydrocarbons are appropriately mixed with the feed oil supplied from a line L2, withdrawn from the stub tower 12 through a line L3, and are supplied to the heating furnace 10 after a coking inhibitor is added thereto.

[0042] The light hydrocarbons introduced into the fractionator 14 are separated in the fractionator 14 to produce distillate oil, and gas naphtha is extracted through line L11, and light diesel oil is extracted through line L12. Heavy diesel oil is extracted from the bottom of the fractionator 14 through line L13, exchanges heat with the feed oil in the heat exchanger 20, and is sent to a subsequent tank or the like through line L14.

[0043] Although the present invention has been specifically described above based on the preferred embodiments, the present invention is not limited to the above embodiments.

[0044] Although the delayed coker 1 according to the above embodiment is provided with two coke drums 16 (18), the present invention is also applicable to a case in which three or more coke drums are provided.

[0045] The delayed coker 1 according to the above embodiment includes the stub tower 12, but the stub tower 12 may be omitted. In this case, the feed oil can be preheated in a heat exchanger and then introduced into the bottom of the fractionator, and can be heated in a heating furnace after contacting with cracked hydrocarbons produced in a coke drum.

[0046] The delayed coker 1 according to the above embodiment includes a heat exchanger 20 for heat exchange between the feed oil and heavy diesel oil, but may further include a heat exchanger for heat exchange between the feed oil and light diesel oil and / or gas naphtha. In addition, the fractionator can also produce fractions other than the above distillate oil, such as kerosene. In this case, the delayed coker 1 may further include a heat exchanger for heat exchange between the feed oil and kerosene. EXAMPLES

[0047] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0048] <Effects on coke formation in laboratory equipment> Example 1 50 g of heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) and a fluid (content of overbased magnesium carbonate in the fluid is 50 mass ppm) consisting of a coking inhibitor containing overbased magnesium carbonate were placed in a stainless steel autoclave and heated from room temperature (25°C) to 450°C, and then held at 450°C for 90 minutes to react. The pressure inside the autoclave was kept constant at 1.10 MPa. The mixture was then cooled to room temperature (25°C). The coke generated in the autoclave was removed and its mass was measured, revealing that the amount of coke generated was 10 mg.

[0049] Comparative Example 1 50 g of heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) was placed in a stainless steel autoclave and heated from room temperature (25°C) to 450°C, and then held at 450°C for 90 minutes to react. The pressure inside the autoclave was kept constant at 1.10 MPa. It was then cooled to room temperature (25°C). The coke generated in the autoclave was removed and its mass was measured, revealing that the amount of coke generated was 21 mg.

[0050] Comparative Example 2 50 g of heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) and a fluid (content of phosphite compound in the fluid is 50 mass ppm) consisting of a chemical containing a phosphite compound (phosphonic acid type phosphite compound) were placed in a stainless steel autoclave and heated from room temperature (25°C) to 450°C, and then held at 450°C for 90 minutes to react. The pressure inside the autoclave was kept constant at 1.10 MPa. It was then cooled to room temperature (25°C). The coke generated in the autoclave was removed and its mass was measured, and the amount of coke generated was 15.5 mg.

[0051] <Impact on delayed coker unit operation> Example 2 A fluid was added to heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) to add a coking inhibitor containing overbased magnesium carbonate so that the content of overbased magnesium carbonate was 50 mass ppm. The fluid was continuously passed through the furnace tube of a delayed coker under the following conditions: flow rate 3 m / s, fluid temperature at the furnace outlet 500°C, and reaction time (residence time) in the furnace 2 minutes. The continuous operation period until the surface temperature of the furnace tube reached 630°C was measured and was found to be 240 days. It can be determined that the longer this continuous operation period, the more coking in the furnace tube is suppressed.

[0052] Comparative Example 3 Heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) was continuously passed through the furnace tube of a delayed coker under the following conditions: flow rate 3 m / s, fluid temperature at the furnace outlet 500°C, and reaction time (residence time) in the furnace 2 minutes. The continuous operation period until the surface temperature of the furnace tube reached 630°C was measured and was 90 days.

[0053] Comparative Example 4 A fluid containing a phosphite compound (phosphonic acid type phosphite compound) was added to heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) so that the content of the phosphite compound was 50 ppm by mass. The fluid was continuously passed through the furnace tube of a delayed coker under the following conditions: flow rate 3 m / s, fluid temperature at the furnace outlet 500°C, and reaction time (residence time) in the furnace 2 minutes. The continuous operation period until the surface temperature of the furnace tube reached 630°C was measured and was 90 days.

[0054] <Cross-sectional observation of coke in laboratory equipment> Example 3 50 g of heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) and 50 g of a coking inhibitor containing overbased magnesium carbonate were placed in a stainless steel autoclave and heated from room temperature to 520°C, and then held at 520°C for 30 minutes to react. The pressure inside the autoclave was kept constant at 0.6 MPa. The mixture was then cooled to room temperature. The coke formed in the autoclave was removed, and the cross section of the coke was polished and observed under a polarizing microscope.

[0055] Comparative Example 5 50 g of heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446 to 748°C) was placed in a stainless steel autoclave and heated from room temperature to 520°C, and then held at 520°C for 30 minutes to react. The pressure inside the autoclave was kept constant at 0.6 MPa. It was then cooled to room temperature. The coke formed in the autoclave was removed, and the cross section of the coke was polished and observed under a polarizing microscope.

[0056] Comparative Example 6 50g of heavy hydrocarbon oil (vacuum residual oil with a boiling point of 446-748°C) and 50g of a chemical agent containing a phosphite ester were placed in a stainless steel autoclave, heated from room temperature to 520°C, and then held at 520°C for 30 minutes to react. The pressure inside the autoclave was kept constant at 0.6 MPa. The mixture was then cooled to room temperature. The coke formed in the autoclave was removed, and the cross section of the coke was polished and observed under a polarizing microscope.

[0057] Fig. 2(a) is a cross-section of the coke produced in Example 3, Fig. 2(b) is a cross-section of the coke produced in Comparative Example 5, and Fig. 2(c) is a cross-section of the coke produced in Comparative Example 6. The coke produced in Example 3 had many holes, small crystals, and was brittle, and was easily peeled off from the vessel wall. On the other hand, the cokes produced in Comparative Examples 5 and 6 had few holes, had grown crystals, were dense and hard, and were difficult to peel off from the vessel wall. [Explanation of symbols]

[0058] 1...delayed coker, 10...heating furnace, 12...stub tower, 14...rectification column, 16, 18...coke drum, 20...heat exchanger, 22...switch valve.

Claims

1. 1. A method for inhibiting coking in a furnace tube of a delayed coker, comprising: A method for inhibiting coking, comprising a step of passing a fluid in which a coking inhibitor containing overbased magnesium carbonate has been added to heavy hydrocarbons through the heater tubes of the delayed coker.

2. 2. The method of claim 1, wherein the flow rate of the fluid when passing through the heating furnace tube is 1.0 to 4.0 m / s, the residence time of the fluid in the heating furnace is 0.5 to 5 minutes, and the temperature of the fluid at the heating furnace outlet is 400 to 580° C.

3. 3. The method of claim 1, wherein the concentration of the overbased magnesium carbonate in the fluid is 5 ppm by weight or greater.

4. A method for producing light hydrocarbons, which produces light hydrocarbons from heavy hydrocarbons by a delayed coker, comprising: A method for producing light hydrocarbons, comprising a step of passing a fluid obtained by adding a coking inhibitor containing overbased magnesium carbonate to the heavy hydrocarbons through a heating furnace tube of the delayed coker.

5. 5. The method for producing light hydrocarbons according to claim 4, wherein the flow rate of the fluid when passing through the heating furnace tube is 1.0 to 4.0 m / s, the residence time of the fluid in the heating furnace is 0.5 to 5 minutes, and the temperature of the fluid at the outlet of the heating furnace is 400 to 580° C.

6. 6. The method for producing light hydrocarbons according to claim 4 or 5, wherein the concentration of the overbased magnesium carbonate in the fluid is 5 ppm by mass or more.

7. 1. A coking inhibitor for inhibiting coking in a delayed coker furnace tube, comprising: Coking inhibitors, including overbased magnesium carbonate.

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