Method for restarting operation of a pyrolysis furnace

Abstract

To provide an operation restarting method for a thermal decomposition furnace that can extend a continuous operation period of the thermal decomposition furnace when the thermal decomposition furnace is restarted after decoking a thermal decomposition tube for thermal decomposition treatment of hydrocarbons.SOLUTION: An operation restarting method for a thermal decomposition furnace comprises: a step (1) of continuously supplying a decoking airflow for a predetermined time B1 or more while heating a thermal decomposition tube after hydrocarbon thermal decomposition treatment to a predetermined temperature A1; a step (2) of determining completion of coke removal in the step (1) using indicators related to decoking actually measured in the thermal decomposition tube; a step (3) of continuously supplying a decoking airflow 2 for a predetermined time B2 or less while heating the thermal decomposition tube after the step (2) to a predetermined temperature A2; and a step (4) of restarting the hydrocarbon thermal decomposition treatment under a condition that a rate of increase in differential pressure per unit volume of the thermal decomposition tube after the step (3) becomes a predetermined value or less, wherein the temperature A2 and / or the time B2 are determined in advance so that the rate of the increase in the differential pressure in the step (4) becomes the predetermined value or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for restarting the operation of a pyrolysis furnace, and more specifically, to a method for restarting the operation of a hydrocarbon pyrolysis furnace after decoking the pyrolysis tubes for pyrolysis treatment of hydrocarbons. [Background technology]

[0002] To produce olefins such as ethylene, propylene, and butadiene (hereinafter referred to as "olefin products") by thermally decomposing hydrocarbons such as naphtha and liquefied petroleum gas, a method is employed in which a pyrolysis furnace equipped with a pyrolysis tube is used, the hydrocarbon is heated in the presence of steam to thermally decompose it, and then the desired olefin product is separated and recovered from the decomposition gas. Pyrolysis methods include the tube heating method, the steam cracking method, and the moving bed method (for example, Non-Patent Document 1).

[0003] In processes employing the steam cracking method, a passivation protective coating, such as chromium oxide or aluminum oxide, is usually applied to the inner surface of the pyrolysis tube to prevent corrosion or damage to the tube.

[0004] Incidentally, when hydrocarbon thermal decomposition is carried out continuously for a long period of time, coke accumulates on the inner walls of the thermal decomposition tubes. When a coke layer forms on the inner walls of the thermal decomposition tubes (coking), the chromium and other elements contained in the thermal decomposition furnace materials become hard and brittle chromium carbides due to the carburizing phenomenon. This can cause cracks in the thermal decomposition tubes or increase pressure loss inside the tubes, making stable continuous operation difficult. Furthermore, the coking gradually prevents heat from being transferred to the raw materials (hydrocarbons) inside the pyrolysis tube. If the pyrolysis tube is then heated further, it will deteriorate, such as bending or warping due to thermal expansion.

[0005] Therefore, normally, when the pressure loss in the pyrolysis tube due to the formation of a coke layer exceeds the upper limit, the supply of raw materials to the pyrolysis tube is stopped and the coke layer is removed (decoking) (for example, Patent Documents 1 and 2). The period of continuous operation of the pyrolysis furnace until decoking (hereinafter referred to as the "continuous operation period") is usually 30 to 50 days, although this depends on the type of pyrolysis furnace and the operating conditions. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 1-063791 [Patent Document 2] Japanese Patent Application Publication No. 7-016552 [Non-patent literature]

[0007] [Non-Patent Document 1] Chemical Engineering, Vol. 38, No. 7 (1974), pp. 485-490 [Overview of the project] [Problems that the invention aims to solve]

[0008] Repeated decoking and operation of the pyrolysis furnace can cause corrosion or damage to the protective coating on the inner wall surface of the pyrolysis tube. In such areas, coking may be induced during operation of the pyrolysis furnace, resulting in re-coking of the pyrolysis tube after restarting operation. This can accelerate the increase in pressure loss in the pyrolysis tube and further shorten the continuous operation period. In such cases, it was necessary to replace the pyrolysis tube. Furthermore, the reduction in process output due to the shortened continuous operation period and the associated decoking costs contributed to increased manufacturing costs, making it a challenge to maintain the protective coating in good condition.

[0009] The present invention aims to provide a method for restarting a pyrolysis furnace that allows for the continued use of pyrolysis tubes in which an increase in pressure loss due to coking has been observed, without replacing them with new pyrolysis tubes. In other words, the present invention aims to provide a method for restarting a pyrolysis furnace that suppresses coking of pyrolysis tubes and extends the continuous operation period of the pyrolysis furnace when the operation of the pyrolysis furnace is restarted after decoking. [Means for solving the problem]

[0010] The present inventors, after diligent study to solve the above problems, have come up with a method for restarting the operation of a hydrocarbon pyrolysis furnace after decoking the pyrolysis tube of the hydrocarbon pyrolysis furnace, wherein, when a protective film containing a metal oxide is formed on at least a part of the inner surface of the base material constituting the pyrolysis tube, the pyrolysis tube is heated to a predetermined temperature while a decoking airflow is continuously supplied for a predetermined time or longer to remove coke adhering to the surface of the pyrolysis tube (step (1)); an index related to decoking is measured inside the pyrolysis tube, and when the measured index satisfies predetermined index value conditions, it is determined that step (1) is complete (step (2)); after step (2), the pyrolysis tube is heated to a predetermined temperature while a decoking airflow is continuously supplied for a predetermined time (step (3)); and after step (3), the differential pressure rise rate per unit volume in the pyrolysis tube is 0.15 [kPa / day / m 3 The process includes a step (step (4)) in which the thermal decomposition treatment of hydrocarbons is restarted under the following conditions, wherein the time and / or temperature in step (3) is such that the differential pressure rise rate in step (4) is 0.15 [kPa / day / m 3 By setting the value to be less than or equal to a predetermined value, this invention provides a method for restarting a pyrolysis furnace that solves the problems that arise when restarting the operation of a pyrolysis furnace after decoking and makes it possible to extend the continuous operation period of the pyrolysis furnace.

[0011] In other words, the gist of this invention is as follows: (1) A method for restarting operation of a hydrocarbon pyrolysis furnace after decoking a pyrolysis tube of the hydrocarbon pyrolysis furnace, comprising: a protective coating containing a metal oxide is formed on at least a part of an inner surface of a base material constituting the pyrolysis tube, and A method for restarting operation of a pyrolysis furnace, comprising the following steps (1) to (4): Step (1): While heating the pyrolysis tube after hydrocarbon pyrolysis treatment to a predetermined temperature A1, a first decoking gas stream is continuously supplied for a predetermined time B1 or longer, and coke adhering to the surface of the pyrolysis tube is removed. Step (2): An index related to decoking is actually measured at one or more locations in the pyrolysis tube, and when the actually measured index satisfies a predetermined index value condition, it is determined that coke removal in step (1) is completed. Step (3): After step (2), while heating the pyrolysis tube to a predetermined temperature A2, a second decoking gas stream is continuously supplied for a time not longer than a predetermined time B2. Step (4): After step (3), the differential pressure increase rate per unit volume in the pyrolysis tube is 0.15 [KPa / day / m 3 Hydrocarbon pyrolysis treatment is restarted under the condition of or lower. Provided that in step (3), the temperature A2 and / or the time B2 are predetermined values such that the differential pressure increase rate in step (4) is 0.15 [KPa / day / m 3 or lower.

[0012] (2) The method for restarting operation of a pyrolysis furnace according to (1) above, wherein the time B1 in step (1) and the time B2 in step (3) satisfy B2 < B1. (3) The method for restarting operation of a pyrolysis furnace according to (1) or (2) above, wherein the temperature A1 in step (1) and the temperature A2 in step (3) satisfy A2 ≦ A1. (4) The method for restarting operation of a pyrolysis furnace according to any one of (1) to (3) above, wherein a material constituting a pipe or device located upstream of the pyrolysis tube and directly or indirectly connected to the pyrolysis tube contains iron element in an amount of 90% by weight or more based on the total mass of the material.

[0013] (5) The method for restarting operation of a pyrolysis furnace according to any one of (1) to (4) above, wherein the temperature A1 and the temperature A2 satisfy the following general formula (1). 780°C ≦ A2 ≦ A1 ≦ 920°C Formula (1) (6) The method for restarting operation of a pyrolysis furnace according to any one of (1) to (5) above, wherein the time B1 and the time B2 satisfy the following general formula (2). 0.50 ≦ ln(B1 / B2) ≦ 1.50 Formula (2)

[0014] (7) The method for restarting operation of a pyrolysis furnace according to any one of (1) to (6) above, wherein the first and second decoking gas streams are the same or different from each other, and are air, steam, or a mixture of air and steam. (8) The method for restarting operation of a pyrolysis furnace according to any one of (1) to (7) above, wherein the metal oxide includes at least one selected from the group consisting of chromium oxide, aluminum oxide, iron oxides, nickel oxides, and titanium oxides.

[0015] (9) The method for restarting operation of a pyrolysis furnace according to any one of (1) to (8) above, wherein the base material includes at least one selected from the group consisting of iron, chromium, aluminum, and nickel. (10) The method for restarting operation of a pyrolysis furnace according to any one of (1) to (9) above, wherein the coke is mainly composed of carbide produced by heat treatment of hydrocarbons.

[0016] (11) In the step (3), the temperature A2 is 780°C or higher and 920°C or lower, the time B2 is 6 hours or more and 18 hours or less, The method for restarting operation of a pyrolysis furnace according to any one of (1) to (10) above. (12) In the step (1), the temperature A1 is 780°C or higher and 920°C or lower, the time B1 is 20 hours or more and 45 hours or less, The method for restarting operation of a pyrolysis furnace according to any one of (1) to (11) above. [Advantageous Effects of Invention]

[0017] According to the present invention, when the operation of the pyrolysis furnace is restarted after decoking, the continuous operation period of the pyrolysis furnace can be extended. More specifically, damage to the protective coating during decoking can be suppressed, and the occurrence of coking can be suppressed when the operation of the pyrolysis furnace is restarted, thereby delaying the increase in pressure loss in the pyrolysis tube. As a result, high production volumes can be maintained, and costs associated with decoking and replacing pyrolysis tubes can be reduced, thus suppressing increases in manufacturing costs. [Brief explanation of the drawing]

[0018] [Figure 1] This is a schematic diagram of the overall tubular pyrolysis furnace used in the examples and comparative examples, as well as the operation examples and comparative operation examples. [Figure 2] Figure 1 is a schematic diagram of a tubular pyrolysis furnace. [Figure 3] This graph plots the relationship between the heating time in step (3) and the differential pressure rise rate per unit volume of the pyrolysis furnace for the examples and comparative examples. [Figure 4] This graph plots the relationship between the heating time in step (3) and the number of continuous operation days in step (4) for the examples and comparative examples. [Modes for carrying out the invention]

[0019] Embodiments of the present invention will be described in detail below. The present invention relates to a method for restarting the operation of a hydrocarbon pyrolysis furnace after decoking the pyrolysis tubes of the hydrocarbon pyrolysis furnace. The hydrocarbons that are thermally decomposed by the method of the present invention are not particularly limited, but examples include naphtha and liquefied petroleum gas.

[0020] <Pyrolysis furnace> The structure of a hydrocarbon pyrolysis furnace is not particularly limited as long as it has pyrolysis tubes for pyrolysis treatment of hydrocarbons; conventionally known pyrolysis furnaces can be used. The base material constituting the pyrolysis tube typically includes iron, chromium, aluminum, and nickel, and it is preferable that it includes at least one selected from these. Furthermore, the materials constituting the piping or equipment located upstream of the pyrolysis tube and directly or indirectly connected to it are not particularly limited, and examples include iron and steel materials, but are usually carbon steel or stainless steel, and among these, from a cost standpoint, it is common for the material to contain 90% or more by weight of iron relative to its total mass.

[0021] In this invention, a protective coating containing a metal oxide is formed on at least a portion of the inner surface of the base material constituting the pyrolysis tube. The formation of the protective coating containing a metal oxide has the effect of physically blocking components (oxygen, carbon, etc.) that degrade the base metal. Examples of metal oxides that form the protective coating include chromium oxide, aluminum oxide, iron oxides, nickel oxides, titanium oxides, etc. It is preferable to include at least one selected from these, and among them, aluminum oxide is preferred in terms of the density of the oxide layer.

[0022] The protective coating only needs to be formed on at least a portion of the inner surface of the base material; however, from the viewpoint of physical barriers, it is preferable that the protective coating be formed on 80% or more, more preferably 90% or more, of the inner surface. The method for forming the protective coating is not particularly limited, and conventionally known methods can be employed. For example, one method involves heating while flowing air, steam, or a mixture of air and steam through a pyrolysis tube to form a protective coating containing the above-mentioned metal oxide on at least a portion of the inner surface of the base material.

[0023] <How to resume operation> The present invention's method for restarting a thermal decomposition operation includes the following steps (1) to (4). Step (1): While heating the pyrolysis tube of the hydrocarbon pyrolysis furnace to a predetermined temperature A1, a decoking airflow is continuously supplied for a predetermined time B1 or longer to remove coke adhering to the surface of the pyrolysis tube. Step (2): An indicator related to decoking is measured at one or more locations inside the pyrolysis tube, and it is determined that the removal of coke in step (1) is complete when the measured indicator satisfies predetermined indicator value conditions. Step (3): After step (2), the pyrolysis tube is heated to a predetermined temperature A2 while a decoking airflow is continuously supplied for a predetermined time B2 or less. Step (4): After step (3), the rate of increase in differential pressure per unit volume in the pyrolysis tube is 0.15 [kPa / day / m³]. 3 The hydrocarbon pyrolysis process will be resumed under the following conditions: However, in step (3), time B2 and / or temperature A2 are such that the differential pressure rise rate in step (4) is 0.15 [kPa / day / m 3 It is a predetermined value such that it is less than or equal to ].

[0024] [(A) Process (1)] Step (1) involves heating the pyrolysis tube after the hydrocarbon pyrolysis treatment to a predetermined temperature A1, while continuously supplying a decoking airflow 1 for a predetermined time B1 or longer to remove coke adhering to the surface of the pyrolysis tube (decoking).

[0025] The coke adhering to the surface of pyrolysis tubes typically consists mainly of carbides produced during the heat treatment of hydrocarbons. More specifically, it refers to carbides containing coke precursors that adhere to the inner walls of pyrolysis tubes such as heating tubes and reaction tubes used for heating and decomposing crude oil, light fractions, heavy oil, residue oil, liquefied coal oil, and tar oil in the petroleum refining, petrochemical, and coal chemical industries. Its main component is carbon, but it may also contain other elements such as hydrogen, nitrogen, sulfur, and iron, as well as ash. In process (1), a decoking airflow is continuously supplied to remove the coke.

[0026] The decoking airflow 1 typically consists of air, water vapor, or a mixture of air and water vapor. The velocity of the decoking airflow 1 is typically 50 to 90 m / sec.

[0027] The temperature A1 and time B1 in step (1) can be any temperature and time that allows for decoking, and can be within a known range of decoking temperatures and times. The method for determining the value of time B1 is not particularly limited. It may be the time at which it has been empirically determined that coke can be removed to the required extent by continuously supplying the decoking airflow 1, or it may be the time at which the measured index in process (2) satisfies predetermined index value conditions and it is determined that the removal of coke in process (1) is complete. Typically, the value of time B1 is the time at which it has been empirically determined that coke can be removed to the required extent.

[0028] Temperature A1 is typically between 780°C and 920°C, preferably above 800°C, more preferably above 820°C, while preferably below 900°C, and even more preferably below 880°C. This range allows for efficient removal of coke. Temperature A1 may be maintained at a constant temperature or may fluctuate within the above temperature range. The method for measuring temperature A1 is not particularly limited; for example, the temperature near the inlet, inside, and near the outlet of the pyrolysis tube can be measured using known temperature measuring means. From the viewpoint of temperature control, it is preferable to measure the temperature of the decoking airflow near the outlet of the pyrolysis tube, or at or near the outlet of piping C in the tubular pyrolysis furnace referred to as the "SRT-V type" described later.

[0029] On the other hand, time B1 is usually between 15 hours and 50 hours, preferably between 20 hours and 45 hours, and more preferably between 25 hours and 40 hours.

[0030] [(B) Process (2)] Step (2) is a step in which it is determined that the removal of coke in step (1) is complete. This step involves measuring an index related to decoking at one or more locations inside the pyrolysis tube, and determining that the removal of coke in step (1) is complete when the measured index satisfies predetermined index value conditions.

[0031] Here, the method for determining the completion of coke removal based on indicators related to decoking is not particularly limited, but can be appropriately determined by a person skilled in the art in this field, depending on the specifications and size of the pyrolysis furnace and pyrolysis tubes, as well as the actual measurement conditions of the indicators. The following methods (a) and (b) are specific examples of how to determine the completion of coke removal based on indicators related to decoking. (a) The CO2 concentration near the outlet of the pyrolysis tube is measured, and when this value falls below a predetermined index, for example, 10,000 ppm or less, preferably 5,000 ppm or less, and more preferably 2,000 ppm or less, it is determined that coke removal is complete. The location of the "near the outlet of the pyrolysis tube" where the CO2 concentration is measured is not particularly limited, and for example, in the case of the tube-type pyrolysis furnace called the "SRT-V type" described above, it may be the outlet or near the outlet of pipe C, or the outlet or near the outlet of pipe D. (b) The differential pressure between the inlet and outlet of the pyrolysis tube is measured, and it is determined that coke removal is complete when this value is below a predetermined index, for example, when the difference compared to the differential pressure at the start of operation is 20% or less, preferably 10% or less, and more preferably 5% or less. The location of the "near the outlet of the pyrolysis tube" where the differential pressure is measured is not particularly limited, and for example, in the case of the tubular pyrolysis furnace called the "SRT-V type" described above, it may be the outlet or near the outlet of piping C.

[0032] [(C) Process (3)] Step (3) is a step in which, after step (2), the pyrolysis tube is heated to a predetermined temperature A2 while a decoking airflow 2 is continuously supplied for a predetermined time B2 or less. In other words, even after determining that the removal of coke is complete in step (2), the continuous supply of decoking airflow is continued at a predetermined temperature for a predetermined time. By performing step (3), it becomes possible to form a protective film on the inner surface of the pyrolysis tube.

[0033] Decoking airflow 2 may be the same as decoking airflow 1, but they may be identical or different. Regarding the temperature A2 and time B2 in step (3), normally temperature A2≦A1 and time B2≦B1, preferably B2<B1. Satisfying this range is effective for making the differential pressure increase rate in step (4) described below 0.15 [KPa / day / m 3 or less.

[0034] The temperature A2 is normally 780°C or higher and 920°C or lower, preferably 800°C or higher, more preferably 820°C or higher. On the other hand, it is 900°C or lower, more preferably 880°C or lower. Within this range, coke can be removed efficiently. Note that the temperature A2 may be maintained at a constant temperature or may vary within the above temperature range.

[0035] In addition, the temperature difference between temperature A2 and temperature A1 is not particularly limited. From the perspective of suppressing thermal deterioration of the pyrolysis tube, it is preferably 50°C or less, more preferably 30°C or less, even more preferably 10°C or less, and particularly preferably the same temperature (0°C). Here, when the temperature A2 and temperature A1 are constant, the temperature difference is the temperature difference between them; when each of the temperatures has a range, the temperature difference is determined by the value with the larger difference between the upper limit or the lower limit. The time B2 is normally 6 hours or more and 18 hours or less, preferably 8 hours or more and 16 hours or less, more preferably 10 hours or more and 14 hours or less. Within this range, effective regeneration of the protective coating can be achieved.

[0036] Based on the above, it is preferable that the temperature A1 and the temperature A2 satisfy the following general formula (1). 780°C ≦ A2 ≦ A1 ≦ 920°C Formula (1)

[0037] In addition, it is preferable that the time B1 and the time B2 satisfy the following general formula (2). 0.50 ≦ ln(B1 / B2) ≦ 1.50 Formula (2) The lower limit of the above formula (2) is preferably 0.55, more preferably 0.57, even more preferably 0.59. On the other hand, the upper limit is preferably 1.40, more preferably 1.38, even more preferably 1.34. Satisfying the above formula (2) is preferable because it tends to suppress an increase in differential pressure during operation.

[0038] Provided that the temperature A2 and / or the time B2 are set such that the differential pressure increase rate in step (4) described below is 0.15 [KPa / day / m 3 or less, they are predetermined values. The predetermined values are values within a range that causes no practical problems, and for example, may be values determined empirically using the manufacturing apparatus to be used, or may be values determined theoretically using simulation or the like. In the examples of the present invention, values empirically determined based on the results of an appropriate number of trials using the manufacturing apparatus to be used are employed.

[0039] The differential pressure increase rate is preferably 0.13 [KPa / day / m 3 or less, more preferably 0.10 [KPa / day / m 3 or less, and particularly preferably 0.09 [KPa / day / m 3 or less. The lower the lower limit, the more preferable it is, but it is usually 0.01 [KPa / day / m 3 or more, and preferably 0.03 [KPa / day / m 3 or more. Setting the value as described above tends to make it possible to extend the number of operating days of the hydrocarbon pyrolysis furnace after restart.

[0040] [(D) Step (4)] Step (4) is a step in which, after step (3), the pyrolysis treatment of hydrocarbons is restarted under a condition that the differential pressure increase rate per unit volume in the pyrolysis tube is 0.15 [KPa / day / m 3 or less. Here, the differential pressure increase rate is a numerical value expressed per unit number of days, obtained by measuring a change in the pressure difference between the inlet and the outlet of the pyrolysis tube. The differential pressure increase rate per unit volume is calculated by dividing the above differential pressure increase rate by the volume of the pyrolysis tube. The above differential pressure rise rate is not particularly limited and can be appropriately determined by a person skilled in the art depending on the specifications and size of the pyrolysis furnace and pyrolysis tubes, as well as the pressure measurement conditions. For example, in the case of the tube-type pyrolysis furnace called "SRT-V type" described above, the inlet of pyrolysis tube A is usually the inlet or near the inlet, while the outlet of pyrolysis tube is usually the outlet or near the outlet of pipe C.

[0041] Furthermore, the above differential pressure rise rate is 0.15 [kPa / day / m 3 The method for achieving the above is not particularly limited, but usually, in step (3), predetermined values ​​are adopted as temperature A2 and / or time B2, thereby reducing the differential pressure rise rate to 0.15 [kPa / day / m 3 The values ​​may be as follows. The predetermined values ​​mentioned above are empirically determined values, as explained in step (3) above.

[0042] The temperature for the pyrolysis treatment of hydrocarbons is usually 750°C or higher, preferably 780°C or higher, while it is usually 900°C or lower, preferably 880°C or lower. This range allows for an increase in the yield of the target product. The supply rate of hydrocarbons to the pyrolysis tube is usually 45 m / sec or higher, preferably 80 m / sec. There is no particular upper limit, but it is usually 200 m / sec or lower, preferably 150 m / sec or lower. [Examples]

[0043] The present invention will be described more specifically below with reference to operating examples and comparative operating examples, as well as examples and comparative examples, but the present invention is not limited to these embodiments.

[0044] [Pyrolysis furnace] A tubular pyrolysis furnace called the "SRT-V type," as shown in Figures 1 and 2, was used. In the tubular pyrolysis furnaces shown in Figures 1 and 2, the hydrocarbon raw material gas 1 is mixed with water vapor and air, and before entering the pyrolysis furnace, in manifold (1) 3, it is divided from one supply pipe 2 into 96 (24 pipes x 4 sets) pyrolysis pipes A 4 (pipe diameter 45 mm, heating temperature 815°C) and descends to the bottom of the pyrolysis furnace. That is, the raw material (hydrocarbons) supplied to the pyrolysis furnace is heated and pyrolyzed in the 96 pyrolysis pipes. At manifold (2) 5 located at the bottom of the pyrolysis furnace, these pipes are joined in groups of 12 to form 8 pipes B 6 (pipe diameter 130 mm, heating temperature 815°C). These 8 pipes are then joined in pairs to form 4 pipes C 7 (pipe diameter 190 mm, heating temperature 815°C). Finally, the four pipes merge to form a single pipe D8, and the pyrolysis gas is supplied to a decomposition gas rapid cooling heat exchanger (not shown) located at the outlet of the pyrolysis furnace. In the specification, the volume of the pyrolysis tube (unit: m³) 3 ) can be the volume of the region in the pyrolysis furnace where the pyrolysis reaction of hydrocarbons occurs. For example, in the case of the tubular pyrolysis furnace called "SRT-V type" described above, the volume of the pyrolysis tube is the volume of the pyrolysis tube A, the manifold (2), the piping B, and the piping C (unit: m³). 3 The sum of ) can be used, and 3.3m 3 That was the case.

[0045] [Comparative driving examples 1-4, driving examples 1-2] To confirm the rate of differential pressure rise per unit volume, comparative operation examples and regular operation examples were conducted under the following conditions. For each pyrolysis tube A, a mixture of 12 tons / hour of raw material gas mainly composed of ethane (hereinafter referred to as "ethane") and 5 tons / hour of water vapor was passed through it, and the pyrolysis tube temperature was set to 815°C. The pyrolysis treatment of ethane was carried out continuously for 30 to 60 days.

[0046] Next, the hydrocarbon flow through the tubular pyrolysis furnace is interrupted and the ethane pyrolysis operation is stopped. Then, as step (1), the heating temperature (temperature A1) and heating time (time B1) of the pyrolysis tubes are set to the conditions shown in Table 1, and 1.75 tons of steam and 1450 Nm³ of air are supplied per pyrolysis tube A. 3 We supplied the necessary power and performed decoking. Next, the heating temperature (temperature A2) and heating time (time B2) of the pyrolysis tubes were set to the conditions shown in Table 1, and 1.75 tons of water vapor and 1450 Nm³ of air were supplied per pyrolysis tube. 3 Time was supplied, and process (3) was carried out.

[0047] Next, as step (4), the operation of the hydrocarbon pyrolysis treatment is restarted, and the pressure value that rises when the operation is run continuously for a predetermined number of days from the restart is divided by the number of operating days to determine the differential pressure rise rate [unit: kPa / day], and this is calculated as the volume of the pyrolysis tube (3.3 m³). 3 The rate of differential pressure rise per unit volume divided by [unit: kPa / day / m] 3 The following was calculated and is shown in Table 1. From these results, the inventors infer that the differential pressure rise rate per unit volume depends on the heating temperature (temperature A2) and heating time (time B2) of the pyrolysis tube in step (3), and set the upper limit of the differential pressure rise rate per unit volume to 0.15 [kPa / day / m 3 I set it to ].

[0048] [Examples 1-6, Comparative Examples 1-5] Except for the heating temperature (temperature A1) and heating time (time B1) of the pyrolysis tube in process (1), and the heating temperature (temperature A2) and heating time (time B2) of the pyrolysis tube in process (3) being as shown in Table 2, the pyrolysis furnace was operated under the same conditions as in Operating Example 1. After step (3), the number of days from the restart of the hydrocarbon pyrolysis treatment in step (4) until the pressure loss in the pyrolysis tube reached 80 [kPa] was measured and defined as the number of continuous operating days. The results are shown in Table 2. In this example, the differential pressure between the inlet and outlet of the pyrolysis tube was measured, and it was determined that coke removal was complete when it was confirmed that this value was 20% or less of the differential pressure at the start of operation.

[0049] [Table 1]

[0050] [Table 2]

[0051] In Examples 1 to 6, the operating conditions for step (3) were set such that the differential pressure rise rate per unit volume in step (4) was 0.15 [kPa / day / m³]. 3 The number of consecutive operating days was long because the following conditions were adopted. In Comparative Examples 1 to 5, the operating conditions for process (3) were set such that the differential pressure rise rate per unit volume in process (4) was 0.15 [kPa / day / m³]. 3 Because conditions exceeding [ ] were being used, the number of consecutive operating days was short. [Explanation of symbols]

[0052] 1. Hydrocarbon raw material gas mixed with water vapor and air 2. Supply piping 3 Manifold (1) 4 Pyrolysis tube A 5 Manifold (2) 6 Piping B 7 Piping C 8 Piping D

Claims

1. A method for restarting the operation of a hydrocarbon pyrolysis furnace after decoking the pyrolysis tubes of the hydrocarbon pyrolysis furnace, A protective coating containing a metal oxide is formed on at least a portion of the inner surface of the base material constituting the pyrolysis tube, and A method for restarting a pyrolysis furnace, including the following steps (1) to (4). Step (1): This step involves heating the pyrolysis tube after the hydrocarbon pyrolysis treatment to a predetermined temperature A1 while continuously supplying a decoking airflow 1 for a predetermined time B1 or longer to remove coke adhering to the surface of the pyrolysis tube, wherein the temperature A1 is between 780°C and 920°C. Step (2): An indicator related to decoking is measured at one or more locations inside the pyrolysis tube, and it is determined that the removal of coke in step (1) is complete when the measured indicator satisfies predetermined indicator value conditions. Step (3): After step (2), the pyrolysis tube is heated to a predetermined temperature A2 while a decoking airflow 2 is continuously supplied for a predetermined time B2, wherein the temperature A2 is 780°C or higher and 920°C or lower, and the time B2 is 8 hours or higher and 16 hours or lower. Step (4): After step (3), the rate of increase of differential pressure per unit volume in the pyrolysis tube is 0.15 [kPa / day / m]. ３ The hydrocarbon pyrolysis treatment will be resumed under the following conditions: However, in step (3), the temperature A2 and / or time B2 are such that the differential pressure rise rate in step (4) is 0.15 [kPa / day / m ３ The following are predetermined values:

2. A method for restarting operation of a pyrolysis furnace according to claim 1, wherein the time B1 in process (1) and the time B2 in process (3) satisfy B2 < B1.

3. A method for restarting operation of a pyrolysis furnace according to claim 1 or 2, wherein the temperature A1 in step (1) and the temperature A2 in step (3) satisfy A2 ≤ A1.

4. A method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 3, wherein the material constituting a pipe or equipment located upstream of the pyrolysis pipe and directly or indirectly connected to the pyrolysis pipe contains 90% by weight or more of iron element relative to the total mass of the material.

5. A method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 4, wherein the aforementioned time B1 and time B2 satisfy the following general formula (2). 0.50≦ln(B1 / B2)≦1.50 Formula (2)

6. A method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 5, wherein the decoking airflows 1 and 2 are the same or different from each other and are air, steam, or a mixture of air and steam.

7. A method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 6, wherein the metal oxide includes at least one selected from chromium oxide, aluminum oxide, iron oxides, nickel oxides, and titanium oxides.

8. A method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 7, wherein the base material includes at least one selected from iron, chromium, aluminum, and nickel.

9. A method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 8, wherein the coke mainly consists of carbides produced by the heat treatment of hydrocarbons.

10. In step (3) above, The aforementioned time B2 is 10 hours or more and 14 hours or less. The method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 9.

11. In the above step (1), The aforementioned time B1 is 20 hours or more and 45 hours or less. The method for restarting operation of a pyrolysis furnace according to any one of claims 1 to 10.

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