Method for evaluating the anti-coking performance of ethylene cracking furnace tubes

Abstract

The present application relates to the technical field of ethylene cracking furnace tube detection, and particularly relates to an evaluation method for the anti-coking performance of an ethylene cracking furnace tube. The anti-coking performance detection device comprises a tubular heating furnace, a gas inlet device and a gas discharge device. The present application heats the block sample in the ethylene cracking furnace tube in the heating furnace, calculates the average coke formation rate of the material and the thickness of the coking layer of the sample, and then calculates the anti-coking performance function f(x) of the furnace tube material. If f(x)>100, the anti-coking performance is poor, and the decoking period is less than 100 days. If f(x)≤100, the anti-coking performance is good, and the decoking period is greater than or equal to 100 days. The scheme of the present application provides a test and evaluation method for the research and development of a new type of anti-coking furnace tube for an ethylene cracking furnace.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ethylene cracking furnace tube detection, and particularly relates to an evaluation method for the anti-coking performance of an ethylene cracking furnace tube. BACKGROUND

[0002] During service, the outside of the ethylene cracking furnace tube is high-temperature flue gas, and the outer surface is mainly in an oxidation process. Hydrocarbons and mixed steam inside the furnace tube are usually heated to about 700-800 DEG C, and the gaseous medium completes the cracking reaction in the furnace tube, and the cracking products flow out from the outlet, and coke is deposited and attached on the inner wall of the furnace tube as a byproduct of the reaction. Since the coking layer is a poor conductor of heat, the thermal resistance increases, the heat transfer coefficient in the tube decreases, and the furnace tube wall temperature rises. Generally, the tube wall temperature gradually rises from about 900 DEG C, and when the tube wall temperature rises to 1060-1080 DEG C, decoking is needed to remove the coke blocks attached to the inner wall, and the decoking period is generally 40-200 days. According to the type and size of the ethylene cracking furnace, the decoking cost is generally 300-400 thousand yuan / once. In order to prolong the decoking period of the cracking furnace and save operation cost, new type furnace tubes with anti-coking performance have been developed at home and abroad.

[0003] However, there is no method for detecting and evaluating the anti-coking performance of the furnace tube in the prior art, the advantages and disadvantages of the anti-coking performance of the furnace tube cannot be effectively and timely judged, the data obtained through actual use have defects such as long period and many interference factors, the anti-coking performance information of the furnace tube cannot be timely and accurately fed back, and this is not conducive to the upgrading and optimization of the furnace tube. SUMMARY

[0004] One of the purposes of the application is to provide an evaluation method for the anti-coking performance of an ethylene cracking furnace tube, which can quickly and accurately feed back the anti-coking performance of the ethylene cracking furnace tube, and is conducive to the upgrading and optimization of the furnace tube.

[0005] In order to achieve the above-mentioned purposes, the following technical solutions are adopted in the application: An evaluation method for the anti-coking performance of an ethylene cracking furnace tube, comprising the following steps:

[0006] S1, a sample is taken at a position close to the outer wall of the ethylene cracking furnace tube, the surface is polished smooth, and a (5±0.5)×(20±2)×(20±2)mm block-shaped sample is prepared, the block-shaped sample is cleaned and then dried to constant weight at a temperature of 150-200 DEG C, the actual size of the block-shaped sample is measured, and the surface area s of the block-shaped sample is calculated, unit: mm 2 ; and the initial weight m1 of the block-shaped sample is measured, unit: g;

[0007] S2, the block sample is placed in a sealed heating furnace, the heating furnace is heated to 1000-1150℃, vacuumized to a vacuum degree of 1Pa or less, acetylene and nitrogen are introduced into the heating furnace at a volume ratio of 1:5, the reaction time t is 1-3h, then the acetylene is stopped, the nitrogen is kept until the acetylene is replaced, then the heating is stopped, and the sample is taken out after the heating furnace is cooled to room temperature;

[0008] S3, the weight m2 of the sample is weighed, in g, and the average coke formation rate δ of the material is calculated in g / (m 2 ·h), in g / (m 2 ·h);

[0009] S4, the thickness d1 of the coke layer of the sample is observed by using an electron microscope, in μm, according to the average coke formation rate δ and the thickness d1 of the coke layer, the value of f(x) is calculated according to the anti-coking performance function f(x)=a*δ+b*d1 of the furnace tube material, wherein a is 20 and b is 2, and the anti-coking performance is evaluated according to the value of the anti-coking performance function f(x).

[0010] Further improvement of the evaluation method of the anti-coking performance of the ethylene cracking furnace tube:

[0011] Preferably, if f(x)>100, the anti-coking performance is poor, and the decoking period is <100 days;

[0012] If f(x)≤100, the anti-coking performance is good, and the decoking period is ≥100 days.

[0013] Preferably, the polishing method in step S1 is mechanical polishing, and the surface roughness is less than 3.2μm.

[0014] Preferably, the block sample in step S1 is cleaned with alcohol or acetone.

[0015] Preferably, in step S4, a ZEISS Supra40 type electron microscope is used, and the scanning magnification is 100-2000 times.

[0016] Preferably, in step S2, the nitrogen is kept for more than 1 minute until the acetylene is replaced.

[0017] Preferably, in step S1, the number of samples is more than one, and the anti-coking performance function f(x) of the multiple samples is calculated according to the steps S1-S4, and then the average value is obtained.

[0018] Preferably, the test device used in the evaluation process comprises a tubular heating furnace, the furnace body of the tubular heating furnace is respectively provided with an air inlet device, a vacuum system and an air outlet device which are communicated with the inner cavity of the furnace body, the air inlet device is used to adjust the amount of nitrogen and acetylene entering the tubular heating furnace, the vacuum system is used to vacuum the tubular heating furnace, and the air outlet device is used to exhaust the gas in the tubular heating furnace.

[0019] The beneficial effects of the present application compared with the prior art are:

[0020] 1) The present application first proposes an evaluation method for the anti-coking performance of ethylene cracking furnace tube, which provides a test and evaluation method for the research and development of new anti-coking furnace tube of ethylene cracking furnace.

[0021] During the high-temperature service of the ethylene cracking furnace tube, cracking reactions occur inside the ethylene cracking furnace tube, and hydrocarbons are further converted into other products through dehydrogenation, chain scission, disproportionation, condensation and other reactions. The cracking reaction produces by-products, i.e. coke, which forms coking on the inner wall of the furnace tube. The coking mechanism related to the material is mainly catalytic coking, and filamentous coke is formed with metal elements as catalysts. The ethylene cracking furnace tube currently used is Fe-Cr-Ni alloy, and Fe and Ni promote catalytic coking. Hydrocarbon molecules are directly adsorbed or form intermediate products after a series of chemical reactions and then deposited on the surface of the catalytic metal, and carbon atoms or groups are formed at the hot end face of the metal particles with catalytic action. At high temperatures, carbon dissolves into the metal particle phase and diffuses to the end face of the metal particle, and the exposed metal surface continues to catalyze carbon deposition, while the dissolved carbon diffuses through the catalytic metal particles and precipitates at a lower temperature. The continuous process of the above process leads to the continuous growth of the precipitated carbon under the catalytic particles, forming a filamentous carbon column. When the catalytic metal particles at the top of the carbon column are completely covered by the deposited carbon, the filamentous coke stops growing.

[0022] As the service time increases, the thickness of the coking layer increases, the thermal resistance increases, the heat transfer coefficient in the tube decreases, and the wall temperature of the furnace tube increases, so it is necessary to stop the furnace for regular decoking. Therefore, the thickness of the coking layer deposited on the surface of the material represents the anti-coking performance of the material, and the better the anti-coking performance, the longer the decoking period, and the higher the production efficiency of the cracking furnace.

[0023] 2) The present application provides an anti-coking test device, which is simple in structure and convenient for evaluating the anti-coking performance of the ethylene cracking furnace tube. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is the structure diagram of the anti-coking test device used in the embodiment of the present application.

[0025] The meanings of the marks in the drawings are as follows:

[0026] 1. Tubular heating furnace; 2. Air intake device; 3. Vacuum system; 4. Exhaust device; 21. Mixer; 31. Mechanical pump;

[0027] S1, S2, S3---flow meters; V1, three-way valve; V2, vacuum valve. DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0029] Example 1

[0030] like Figure 1 As shown, the anti-coking performance detection device used in the evaluation of the anti-coking performance of the ethylene cracking furnace tube in the present application includes a tubular heating furnace 1, an air intake device 2, a vacuum system 3 and an exhaust device 4. The air intake device 2, the vacuum system 3 and the exhaust device 4 are all arranged on the furnace body of the tubular heating furnace 1 and communicate with the furnace cavity. The air intake device 2 includes nitrogen and acetylene gas, and also includes a flow meter S1 for adjusting the nitrogen flow rate, a flow meter S2 for adjusting the acetylene flow rate, a mixer 21 for mixing nitrogen and acetylene gas, and a flow meter S3 for adjusting the flow rate of the mixed gas in the mixer 21. The nitrogen, acetylene gas and the mixer 21 are connected by a three-way valve V1; the vacuum system 3 includes a mechanical pump 31 and a vacuum valve V2; the exhaust device 4 is used to discharge the gas in the tubular heating furnace 1.

[0031] Example 2

[0032] This embodiment provides a method for evaluating the anti-coking performance of ethylene cracking furnace tubes, comprising the following steps:

[0033] S1. Take a sample from the part near the outer wall of a 25Cr35NiNb heat-resistant alloy furnace tube, polish the surface to a surface roughness of less than 3.2 μm, clean it with alcohol, and dry it at 150°C for 2 hours to constant weight. The actual dimensions of the block sample are measured to be 5.02×19.98×20.03 mm, and its surface area is calculated to be 1202.10 mm. 2 , the initial weight is 15.9523g;

[0034] S2, dry the crucible to constant weight, place the block sample in the crucible, then put them together in a tube furnace, close the door to seal, heat to 1100℃, vacuum the furnace to a vacuum degree of 1 Pa or less. Adjust the flow meter to mix nitrogen and acetylene, make the volume ratio of acetylene to nitrogen 1:5, then pass into the furnace, continuously ventilate for 2 hours, then close the acetylene valve, continue to ventilate with nitrogen for 1 min, then close the nitrogen valve, stop heating, and take out the sample when the furnace cools to room temperature;

[0035] S3, the weight of the sample is 15.9662g, the weight of the generated coke is 0.0139g, and the average coke formation rate δ of the material is calculated as 5.7864g / (m 2 ·h) calculated by coke g / (m 2 ·h);

[0036] S4, the thickness d1 of the coking layer is measured by scanning electron microscopy to be 15μm, and the anti-coking performance function f(x) of the furnace tube material is calculated according to the average coke formation rate δ and the coking layer thickness d1, f(x) = a*δ + b*d1 = 20*5.7864 + 2*15 = 145.728, where a is 20 and b is 2, and f(x) > 100 indicates that the material has poor anti-coking performance at 1100℃, and the decoking period is <100 days.

[0037] Example 3

[0038] The present embodiment provides an evaluation method for the anti-coking performance of an ethylene cracking furnace tube, comprising the following steps:

[0039] S1, take 1 piece of sample from the part near the outer wall of the 35Cr45NiNb heat-resistant alloy furnace tube, polish the surface to a surface roughness of less than 3.2μm, clean with alcohol, and dry at 150℃ for 2h to constant weight. The actual size of the block sample is 4.96×19.99×20.05mm, the surface area is 1198.80mm 2 , and the initial weight is 17.0713g;

[0040] S2, dry the crucible to constant weight, place the block sample in the crucible, then put them together in a tube furnace, close the door to seal, heat to 1050℃, vacuum the furnace to a vacuum degree of 1 Pa or less. Adjust the flow meter to mix nitrogen and acetylene, make the volume ratio of acetylene to nitrogen 1:5, then pass into the furnace, continuously ventilate for 2 hours, then close the acetylene valve, continue to ventilate with nitrogen for 1 min, then close the nitrogen valve, stop heating, and take out the sample when the furnace cools to room temperature;

[0041] S3, the weight of the sample is 17.0840g, the weight of the generated coke is 0.0127g, and the average coke formation rate δ of the material is calculated as 5.7864g / (m 2h) the average coke formation rate δ of the material is 5.2970 g / (m 2 h).

[0042] S4, the coke layer thickness d1 is measured by scanning electron microscope observation, and the coke layer thickness d1 is measured by scanning electron microscope observation. According to the average coke formation rate δ and the coke layer thickness d1, the anti-coking performance function f(x) of the furnace tube material is calculated as f(x) = a*δ + b*d1 = 20*5.2970 + 2*10 = 125.94, wherein a is 20 and b is 2. Since f(x) > 100, the anti-coking performance of the material at 1100℃ is poor, and the decoking period is less than 100 days.

[0043] Example 4

[0044] The present embodiment provides an evaluation method for the anti-coking performance of an ethylene cracking furnace tube, comprising the following steps:

[0045] S1, taking 1 piece of sample from the part near the outer wall of the new aluminum-added heat-resistant alloy furnace tube, polishing the surface to a surface roughness of less than 3.2 μm, cleaning with acetone, drying at 150℃ for 2h to constant weight, measuring the actual size of the block sample as 5.00*19.98*20.03mm, the surface area is 1200.4988mm 2 , the initial weight is 17.2303g;

[0046] S2, dry the crucible to constant weight, place the block sample in the crucible, then together in a tubular heating furnace, close the furnace door to seal, heat to 1150℃, vacuumize the heating furnace to a vacuum degree below 1Pa. Adjust the flow meter to mix nitrogen and acetylene, so that the volume ratio of acetylene gas to nitrogen is 1:5, then pass into the heating furnace, continuously ventilate for 2 hours, then close the acetylene valve, continue to ventilate with nitrogen for 1min, then close the nitrogen valve, stop heating, and take out the sample after the heating furnace cools to room temperature;

[0047] S3, the weight of the sample is 17.2318g, and the weight of the generated coke is 0.0015g. The average coke formation rate δ of the material is calculated as 0.6247 g / (m 2 h) the average coke formation rate δ of the material is 0.6247 g / (m 2 h).

[0048] S4, the coke layer thickness d1 is measured by scanning electron microscope observation, and the coke layer thickness d1 is measured by scanning electron microscope observation. According to the average coke formation rate δ and the coke layer thickness d1, the anti-coking performance function f(x) of the furnace tube material is calculated as f(x) = a*δ + b*d1 = 20*5.2970 + 2*10 = 125.94, wherein a is 20 and b is 2. Since f(x) > 100, the anti-coking performance of the material at 1100℃ is poor, and the decoking period is less than 100 days.

[0049] Those skilled in the art should understand that the above are only several specific embodiments of the present application, but not all embodiments. It should be noted that many modifications and improvements can also be made by those of ordinary skill in the art, and all modifications and improvements that do not exceed the scope of the claims should be considered as the protection scope of the present application.

Claims

1. A method for evaluating the anti-coking performance of ethylene cracking furnace tubes, characterized in that: The steps include: S1. Take a sample from the outer wall of the ethylene cracking furnace tube and polish the surface to make a (5±0.5)×(20±2)×(20±2) mm block sample. After cleaning, dry it at 150-200°C to constant weight. Measure the actual size of the block sample and calculate its surface area s in mm. 2 ; and weigh the initial weight m1 of the block sample in g; S2. Place the block sample in a sealed heating furnace, heat the furnace to 1000-1150°C, evacuate the furnace until the vacuum reaches below 1 Pa, and introduce acetylene and nitrogen into the furnace at a volume ratio of 1:

5. After a reaction time t of 1-3 hours, stop introducing acetylene. Continue introducing nitrogen until acetylene replacement is complete. Stop heating at the same time, and wait for the furnace to cool to room temperature before removing the sample. S3. Weigh the weight of the sample m2, in g, with the value of coke g / (m 2 h) Calculate the average coke formation rate δ of the material, , unit g / (m 2 h); S4. Use an electron microscope to scan and observe the thickness d1 of the coking layer of the sample, in μm. According to the average coke formation rate δ and the coking layer thickness d1, the anti-coking performance function f(x) = a of the furnace tube material is used. δ+b d1 calculates the value of f(x), where a is 20 and b is 2, and the anti-coking performance is evaluated according to the value of the anti-coking performance function f(x); If f(x)>100, the anti-coking performance is poor and the coking cycle is less than 100 days; If f(x)≤100, the anti-coking performance is good and the coking removal cycle is ≥100 days.

2. The method for evaluating the anti-coking performance of ethylene cracking furnace tubes according to claim 1, wherein: The polishing method in step S1 is mechanical polishing until the surface roughness is less than 3.2 μm.

3. The method for evaluating the anti-coking performance of ethylene cracking furnace tubes according to claim 1, wherein: In step S1, alcohol or acetone is used to clean the block sample.

4. The method for evaluating the anti-coking performance of ethylene cracking furnace tubes according to claim 1, wherein: In step S4, a ZEISS Supra40 electron microscope is used, with a scanning magnification of 100-2000 times.

5. The method for evaluating the anti-coking performance of ethylene cracking furnace tubes according to claim 1, wherein: In step S2, nitrogen is kept flowing for more than 1 minute until the acetylene replacement is completed.

6. The method for evaluating the anti-coking performance of ethylene cracking furnace tubes according to claim 1, wherein: In step S1, the number of samples taken is one or more, and after performing operations according to steps S1 to S4 respectively, the anti-coking performance function f(x) finally calculated for the multiple samples is averaged.

7. The method for evaluating the anti-coking performance of ethylene cracking furnace tubes according to claim 1, wherein: The test device used in the evaluation method includes a tubular heating furnace (1), wherein the furnace body of the tubular heating furnace (1) is provided with an air intake device (2), a vacuum system (3) and an exhaust device (4) which are communicated with the inner cavity of the furnace body, respectively. The air intake device (2) is used to adjust the amount of nitrogen and acetylene gas entering the tubular heating furnace (1), the vacuum system (3) is used to evacuate the tubular heating furnace (1), and the exhaust device (4) is used to discharge the gas in the tubular heating furnace (1).

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