Evaluation Method for High-Temperature Service Performance of Anti-Coking Furnace Tubes in Ethylene Cracking Furnaces with Gas Raw Materials

Abstract

The present invention relates to the technical field of evaluation of heat-resistant alloy furnace tubes, and particularly to a method for evaluating the high-temperature service performance of anti-coking furnace tubes for gas-fed ethylene cracking furnaces. The present invention determines the anti-creep performance influencing factors composed of at least micro-alloying elements Ti, Zr, W, and impurity elements S, Pb, Bi; it also determines the anti-carburization and anti-oxidation performance influencing factors composed of alloy element Al, and finally obtains an influence function for judging the quality of furnace tube performance. The present invention for the first time proposes a method for evaluating the high-temperature service performance of anti-coking furnace tubes for gas-fed ethylene cracking furnaces. The present invention provides an evaluation method for ethylene cracking furnace tubes for extending the coking cleaning cycle of gas-fed ethylene cracking furnaces, screens out furnace tubes with better anti-creep, anti-carburization and anti-oxidation performance, thereby realizing the extension of the coking cleaning cycle of ethylene cracking furnaces and saving costs.

Description

Technical Field

[0001] The present invention relates to the technical field of evaluation of heat-resistant alloy furnace tubes, and particularly to a method for evaluating the high-temperature service performance of anti-coking furnace tubes for ethylene cracking furnaces with gaseous raw materials. Background Art

[0002] Heat-resistant alloy furnace tubes are widely used in the radiant section furnace tubes of ethylene cracking furnaces. During service, the outside of the furnace tubes of the ethylene cracking furnace is high-temperature flue gas, and the outer surface is mainly an oxidation process. The hydrocarbons and mixed steam inside the furnace tubes are usually heated to about 700 - 800 °C. The gaseous medium completes the cracking reaction inside the furnace tubes, and the cracking products flow out from the outlet. As a reaction by-product, coke deposits and adheres to the inner wall of the furnace tubes. Since the coking layer is a poor conductor of heat, the thermal resistance increases, the heat transfer coefficient inside the tubes decreases, and the wall temperature of the furnace tubes rises. Generally, the wall temperature gradually rises from about 900 °C. When the wall temperature rises to 1060 - 1080 °C, decoking is required to remove the coke blocks adhering to the inner wall. The decoking cycle is generally 60 - 120 days. According to the type and size of the ethylene cracking furnace, the decoking cost is generally 300,000 - 400,000 yuan per time. Therefore, selecting furnace tubes with better creep resistance, carburization resistance, and oxidation resistance and extending the decoking cycle of the ethylene cracking furnace can reduce costs and have significant economic benefits. Summary of the Invention

[0003] In order to screen out furnace tubes with better creep resistance, carburization resistance, and oxidation resistance, extend the decoking cycle of the ethylene cracking furnace, and reduce costs, one of the purposes of the present invention is to provide a method for evaluating the high-temperature service performance of anti-coking furnace tubes for ethylene cracking furnaces with gaseous raw materials, which can simultaneously screen out furnace tubes with better creep resistance, carburization resistance, and oxidation resistance.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions: A method for evaluating the high-temperature service performance of anti-coking furnace tubes for ethylene cracking furnaces with gaseous raw materials, comprising the following steps:

[0005] Step 1: Analyze the chemical composition of the heat-resistant alloy furnace tube material used in the ethylene cracking furnace with gaseous raw materials, and measure the weight contents of the elements that have an important impact on the creep resistance. The elements include Ti, Zr, W, S, Pb, and Bi;

[0006] Step 2: According to the weight contents of the Ti, Zr, W, S, Pb, and Bi elements obtained in Step 1, determine the creep resistance influence factor f(α) as shown in formula (1):

[0007] f(α) = aω(Ti) + bω(Zr) + cω(W) - dω(S) - eω(Pb) - gω(Bi) (1)

[0008] In formula (1), ω(Ti), ω(Zr), ω(W), and ω(S) are the weight contents of Ti, Zr, W, and S elements in the heat-resistant alloy furnace tube material, with the unit of wt.%; in formula (1), ω(Pb) and ω(Bi) are the weight contents of Pb and Bi elements in the heat-resistant alloy furnace tube material, with the unit of ppm; a, b, c, d, e, and g are coefficients, where the value range of a is (13, 17), the value range of b is (8, 12), the value range of c is (0.18, 0.22), the value range of d is (28, 32), the value range of e is (0.003, 0.007), and the value range of g is (0.08, 0.12); the unit of f(α) is 1.

[0009] As a further improvement to the high-temperature service performance evaluation method of the anti-coking furnace tubes of the above gas feed ethylene cracking furnace:

[0010] Preferably, the anti-creep grade of the heat-resistant alloy furnace tube material is divided according to the value of the anti-creep performance influence factor f(α): if f(α) ≥ 1, the creep rupture time is ≥ 120 h under the conditions of 1100 °C and 17 MPa; if f(α) < 1, the creep rupture time is < 120 h.

[0011] Preferably, in step two, the value of a is 15, the value of b is 10, the value of c is 0.2, the value of d is 30, the value of e is 0.005, and the value of g is 0.10.

[0012] Preferably, in step one, a photometer is used to measure the contents of Pb and Bi elements in the heat-resistant alloy furnace tube material, and a direct-reading spectrometer is used to analyze other chemical elements except Pb and Bi elements.

[0013] Preferably, the elements in step one further include C, Si, Mn, P, Cr, Ni, and Nb, where C: 0.4 - 0.5 wt.%, Si ≤ 1.75 wt.%, Mn ≤ 1.5 wt.%, P ≤ 0.030 wt.%, Cr: 27 - 30 wt.%, Ni: 42 - 48 wt.%, Nb: 0.8 - 1.2 wt.%.

[0014] Preferably, the material of the heat-resistant alloy furnace tube is CrNiFe alloy.

[0015] To achieve the above object, the present invention adopts the following technical solution: a high-temperature service performance evaluation method for the anti-coking furnace tubes of a gas feed ethylene cracking furnace, including the following steps:

[0016] Step one: Analyze the chemical composition of the heat-resistant alloy furnace tube material used in the gas feed ethylene cracking furnace, and measure the weight contents of the elements that have important effects on the carburization resistance and oxidation resistance. The elements include Al.

[0017] Step 2: According to the weight content of Al element obtained in Step 1, determine the anti-carburization and anti-oxidation performance influencing factor f(β) as shown in Equation (2):

[0018] f(β) = hω(Al) (2)

[0019] In Equation (2), ω(Al) is the weight content of Al element in the heat-resistant alloy furnace tube material, with the unit of wt.%; h is a coefficient, and its value range is (0.45, 0.55); the unit of f(β) is 1;

[0020] Step 3: Classify the anti-carburization and anti-oxidation grades of the heat-resistant alloy furnace tube material: If f(β) ≥ 1, the carburized layer thickness after a 200-hour solid carburization test of the material at 1100°C ≤ 1 mm and the oxidation rate of the material at 1100°C < 0.1 g / m 2 ·h; if f(β) < 1, the carburized layer thickness after a 200-hour solid carburization test of the material at 1100°C > 1 mm and the oxidation rate of the material at 1100°C ≥ 0.1 g / m 2 ·h.

[0021] As a further improvement to the high-temperature service performance evaluation method of the anti-coking furnace tubes for ethylene cracking furnaces using gas as raw materials:

[0022] Preferably, the value of h in Step 3 is 0.5.

[0023] Preferably, the heat-resistant alloy furnace tube material includes CrNiFe alloy.

[0024] Preferably, the elements in Step 1 further include C, Si, Mn, P, Cr, Ni, and Nb. Among them, C: 0.4 - 0.5 wt.%, Si ≤ 1.75 wt.%, Mn ≤ 1.5 wt.%, P ≤ 0.030 wt.%, Cr: 27 - 30 wt.%, Ni: 42 - 48 wt.%, Nb: 0.8 - 1.2 wt.%.

[0025] The beneficial effects of the present invention compared with the prior art are as follows:

[0026] 1) Elements that have a significant impact on the anti-creep performance include Ti, Zr, W, S, Pb, and Bi elements.

[0027] The content of Ti element has a significant impact on the high-temperature creep resistance. In the furnace tube, Ti element precipitates at the grain boundaries in the form of (Nb,Ti)C, showing a skeleton-like or short rod-like distribution. When the furnace tube is in service at high temperature, with the increase of Ti content, the transformation amount of niobium carbide to G phase in the furnace tube decreases, and the (Nb,Ti)C phase is distributed in the austenite grain boundaries in a short chain shape. Ti element can delay the transformation of niobium carbide to G phase, which is beneficial to improving the high-temperature creep resistance of the furnace tube. 0.03 - 0.15 wt.% of Ti element should be added to the alloy.

[0028] Zr element is a strong oxide-forming element and easily combines with O element. Zirconia, as a nucleation site, enhances the nucleation of titanium carbide and promotes the precipitation of titanium carbide, which helps to form a uniform and fine distribution of primary carbides on the cross-section of the furnace tube, improving the high-temperature creep resistance of the furnace tube. 0.03 - 0.15 wt.% of Zr element should be added to the alloy, and the content of Ti + Zr should be 0.04 - 0.18 wt.%.

[0029] W element is a solid solution strengthening element, which can improve the high-temperature strength of the alloy and inhibit the diffusion rate of carbon. However, adding an excessive amount will affect the oxidation resistance of the alloy and promote the precipitation of σ phase, reducing the strength and toughness of the alloy. Therefore, the W content in the alloy is 4 - 6 wt.%.

[0030] The content of S element has a significant impact on the high-temperature creep resistance. With the increase of S element, the high-temperature creep resistance of the heat-resistant alloy furnace tube, that is, the furnace tube, decreases. This is because sulfur segregates to the grain boundaries during heat treatment to form sulfides. Due to the poor bonding force between sulfides and the matrix, voids are easily nucleated, and at high temperatures, due to local stress concentration, the voids connect to generate microcracks, leading to the rapid failure of the furnace tube. Usually, the recommended S element content needs to be less than 0.015 wt.%.

[0031] Under high-temperature tensile stress conditions, Pb element segregates to the grain boundaries, reducing the surface energy and promoting the initiation of creep voids, ultimately leading to a decrease in the high-temperature creep resistance. During high-temperature service, Pb element segregates to the grain boundaries. As the local concentration of segregation on the grain boundaries increases, when it is sufficient to reduce the grain boundary surface energy, the nucleation stress will be correspondingly reduced, resulting in an increase in the void nucleation rate, making it easier to initiate voids. Considering the requirements of high-temperature creep resistance and economic cost factors, the content of Pb element in the alloy furnace tube should usually be limited to less than 20 ppm.

[0032] The influence of Bi element on the high-temperature creep resistance is similar to that of Pb. With the increase of Bi element content, the high-temperature creep resistance of the furnace tube decreases significantly. This is because Bi element segregates to the grain boundaries, reducing the grain boundary surface energy and increasing the void nucleation rate. Usually, the content of Bi element in the alloy furnace tube should be limited to less than 1.0 ppm.

[0033] 2) Elements that have a significant impact on carburization resistance and oxidation resistance include Al element.

[0034] At high temperatures, Al element can form Al 2 O 3 oxide film. Compared with conventional Cr 2 O 3 oxide film, Al 2 O 3 oxide film forms preferentially and is not easily exfoliated, forming a dense protective film on the surface of the alloy, improving the carburization resistance and oxidation resistance of the material. 2% - 4% of Al element should be added to the alloy.

[0035] The present invention first proposes a method for evaluating the high-temperature service performance of anti-coking furnace tubes for gas feed ethylene cracking furnaces. The influence functions f(α) and f(β) determined by the above-mentioned various factors that have a significant impact on creep resistance, carburization resistance and oxidation resistance are used to detect and evaluate heat-resistant alloy furnace tubes, and furnace tubes with better creep resistance, carburization resistance and oxidation resistance are selected, so as to extend the coking cycle of the ethylene cracking furnace and save costs. Specific embodiments

[0036] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in combination with embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0037] Embodiment 1

[0038] The composition of a heat-resistant alloy furnace tube for an ethylene cracking furnace is shown in Table 1. ω(Ti) = 0.083 wt.%; ω(Zr) = 0.035 wt.%; ω(W) = 5.29 wt.%; ω(S) = 0.0079 wt.%; ω(Pb) = 3.93 ppm; ω(Bi) = 0.54 ppm; ω(Al) = 3.44 wt.%.

[0039] Table 1 Chemical composition of heat-resistant alloy furnace tubes for ethylene cracking furnaces

[0040]

[0041] The influence function is calculated as follows:

[0042] f(α) = aω(Ti) + bω(Zr) + cω(W) - dω(S) - eω(Pb) - gω(Bi) = 15×0.083 + 10×0.035 + 0.2×5.29 - 30×0.0079 - 0.005×3.93 - 0.10×0.54 = 2.34235 > 1.

[0043] f(β) = hω(Al) = 0.5×3.44 = 1.72 > 1.

[0044] After actual application verification, the creep rupture time of this furnace tube is 160 h under the conditions of 1100 °C and 17 MPa, and its creep resistance is good; after a 200-h solid carburizing test at 1100 °C, the carburized layer thickness is 0.5 mm, and its carburization resistance is good; the oxidation rate of the material is 0.033 g / m 2 ·h at 1100 °C, and its oxidation resistance is good.

[0045] Example 2

[0046] The composition of a heat-resistant alloy furnace tube for an ethylene cracking furnace is shown in Table 2. ω(Ti) = 0.060 wt.%; ω(Zr) = 0.031 wt.%; ω(W) = 5.13 wt.%; ω(S) = 0.0049 wt.%; ω(Pb) = 2.87 ppm; ω(Bi) = 0.22 ppm; ω(Al) = 3.50 wt.%.

[0047] Table 2 Chemical composition of heat-resistant alloy furnace tube for ethylene cracking furnace

[0048]

[0049] The influence function is calculated as follows:

[0050] f(α) = aω(Ti) + bω(Zr) + cω(W) - dω(S) - eω(Pb) - gω(Bi) = 15×0.060 + 10×0.031 + 0.2×5.13 - 30×0.0049 - 0.005×2.87 - 0.10×0.22 = 2.05265 > 1.

[0051] f(β) = hω(Al) = 0.5×3.50 = 1.75 > 1.

[0052] After actual application verification, the creep rupture time of this furnace tube is 172 h under the conditions of 1100 °C and 17 MPa, and its creep resistance is good; after a 200-h solid carburizing test at 1100 °C, the carburized layer thickness is 0.44 mm, and its carburization resistance is good; the oxidation rate of the material is 0.040 g / m 2 ·h at 1100 °C, and its oxidation resistance is good.

[0053] Example 3

[0054] The composition of the heat-resistant alloy furnace tube for ethylene cracking furnace is shown in Table 3. ω(Ti) = 0.053 wt.%; ω(Zr) = 0.022 wt.%; ω(W) = 0.064 wt.%; ω(S) = 0.0063 wt.%; ω(Pb) = 4.15 ppm; ω(Bi) = 0.66 ppm; ω(Al) = 3.80 wt.%.

[0055] Table 3 Chemical composition of the heat-resistant alloy furnace tube for ethylene cracking furnace

[0056]

[0057] The influence function is calculated as follows:

[0058] f(α) = aω(Ti) + bω(Zr) + cω(W) - dω(S) - eω(Pb) - gω(Bi) = 15×0.053 + 10×0.022 + 0.2×0.064 - 30×0.0063 - 0.005×2.87 - 0.10×0.22 = 0.80245 < 1.

[0059] f(β) = hω(Al) = 0.5×3.80 = 1.90 > 1.

[0060] It is confirmed by actual application that the creep rupture time of this furnace tube is 20 h under the conditions of 1100 °C and 17 MPa, and its creep resistance is poor; the carburized layer thickness is 0.30 mm after the solid carburization test at 1100 °C for 200 h, and its carburization resistance is good; the oxidation rate of the material is 0.025 g / m 2 ·h at 1100 °C, and its oxidation resistance is good.

[0061] Example 4

[0062] The composition of the heat-resistant alloy furnace tube for ethylene cracking furnace is shown in Table 4. ω(Ti) = 0.049 wt.%; ω(Zr) = 0.032 wt.%; ω(W) = 0.0070 wt.%; ω(S) = 0.0046 wt.%; ω(Pb) = 1.33 ppm; ω(Bi) = 0.43 ppm; ω(Al) = 0.015 wt.%.

[0063] Table 4 Chemical composition of the heat-resistant alloy furnace tube for ethylene cracking furnace

[0064]

[0065]

[0066] The influence function is calculated as follows:

[0067] f(α) = aω(Ti) + bω(Zr) + cω(W) - dω(S) - eω(Pb) - gω(Bi) = 15×0.080 + 10×0.062 + 0.2×0.0070 - 30×0.0046 - 0.005×2.87 - 0.10×0.22 = 1.64705 > 1.

[0068] f(β) = hω(Al) = 0.5×0.015 = 0.0075 < 1.

[0069] It has been confirmed by actual application that the creep fracture time of the furnace tube under the conditions of 1100 °C and 17 MPa is 158 h, and its creep resistance is good; after a solid carburizing test at 1100 °C for 200 h, the carburized layer thickness is 2 mm, and its carburization resistance is poor; the oxidation rate of the material at 1100 °C is 0.155 g / m 2 ·h, and its oxidation resistance is poor.

[0070] Those skilled in the art should understand that the above are only several specific embodiments of the present invention, rather than all embodiments. It should be noted that many variations and improvements can be made by those of ordinary skill in the art. All variations or improvements that do not exceed the scope of the claims shall be regarded as the protection scope of the present invention.

Claims

1. A method for evaluating the high-temperature service performance of anti-coking furnace tubes in a gas feed ethylene cracking furnace, characterized in that, it comprises the following steps: Step 1: Analyze the chemical composition of the heat-resistant alloy furnace tube material used in the gas feed ethylene cracking furnace, and measure the weight content of the elements that have an important impact on carburization resistance and oxidation resistance. The elements include Al; Step 2: According to the weight content of the Al element obtained in Step 1, determine the carburization resistance and oxidation resistance influence factor f(β) as shown in Equation (2): f(β)=h ω(Al) (2) In Equation (2), ω (Al) is the weight content of the Al element in the heat-resistant alloy furnace tube material, with the unit of wt.%; h is a coefficient, and its value range is (0.45, 0.55); the unit of f(β) is 1; Step 3. Classify the carburization resistance and oxidation resistance grades of the heat-resistant alloy furnace tube material: If f(β) ≥ 1, the carburized layer thickness of the material after a 200-hour solid carburization test at 1100°C is ≤ 1 mm and the oxidation rate of the material at 1100°C is < 0.1 g / m 2 ·h; if f(β) < 1, the carburized layer thickness of the material after a 200-hour solid carburization test at 1100°C is > 1 mm and the oxidation rate of the material at 1100°C is ≥ 0.1 g / m 2 ·h.

2. The method for evaluating the high-temperature service performance of anti-coking furnace tubes in a gas feed ethylene cracking furnace according to Claim 1, characterized in that, in Step 3, the value of h is 0.

5.

3. The method for evaluating the high-temperature service performance of anti-coking furnace tubes in a gas feed ethylene cracking furnace according to Claim 1, characterized in that, the material of the heat-resistant alloy furnace tube is CrNiFe alloy.

4. The method for evaluating the high-temperature service performance of anti-coking furnace tubes in a gas feed ethylene cracking furnace according to Claim 1, characterized in that, the elements in Step 1 further include C, Si, Mn, P, Cr, Ni, and Nb, where C: 0.4~0.5wt.%, Si≤1.75wt.%, Mn≤1.5wt.%, P≤0.030wt.%, Cr: 27~30wt.%, Ni: 42~48wt.%, Nb: 0.8~1.2wt.%.

5. The method for evaluating the high-temperature service performance of anti-coking furnace tubes in a gas feed ethylene cracking furnace according to Claim 1, characterized in that, it further comprises the following steps: Step 1: Analyze the chemical composition of the heat-resistant alloy furnace tube material used in the gas feed ethylene cracking furnace, and measure the weight content of the elements that have an important impact on creep resistance. The elements include Ti, Zr, W, S, Pb, Bi; Step 2: According to the weight content of the Ti, Zr, W, S, Pb, Bi elements obtained in Step 1, determine the creep resistance influence factor f(α) as shown in Equation (1): f(α)=a ω(Ti)+b ω(Zr)+c ω(W)-d ω(S)-e ω(Pb)-g ω(Bi) (1) In formula (1), ω (Ti), ω (Zr), ω (W), and ω (S) are the weight contents of Ti, Zr, W, and S elements in the heat-resistant alloy furnace tube material, with the unit of wt.%; in formula (1), ω (Pb) and ω (Bi) are the weight contents of Pb and Bi elements in the heat-resistant alloy furnace tube material, with the unit of ppm; a, b, c, d, e, and g are coefficients, where the value range of a is (13, 17), the value range of b is (8, 12), the value range of c is (0.18, 0.22), the value range of d is (28, 32), the value range of e is (0.003, 0.007), and the value range of g is (0.08, 0.12); the unit of f(α) is 1.

6. The method for evaluating the high-temperature service performance of the anti-coking furnace tube of the gas raw material ethylene cracking furnace according to claim 5, characterized in that, the anti-creep grade of the heat-resistant alloy furnace tube material is divided according to the value of the anti-creep performance influence factor f(α): if f(α) ≥ 1, the creep rupture time is ≥ 120 h under the conditions of 1100 °C and 17 MPa; if f(α) < 1, the creep rupture time is < 120 h.

7. The method for evaluating the high-temperature service performance of the anti-coking furnace tube of the gas raw material ethylene cracking furnace according to claim 5, characterized in that, in step 2, the value of a is 15, the value of b is 10, the value of c is 0.2, the value of d is 30, the value of e is 0.005, and the value of g is 0.

10.

8. The method for evaluating the high-temperature service performance of the anti-coking furnace tube of the gas raw material ethylene cracking furnace according to claim 5, characterized in that, in step 1, a photometer is used to measure the contents of Pb and Bi elements in the heat-resistant alloy furnace tube material, and a direct-reading spectrometer is used to analyze other chemical elements except Pb and Bi elements.

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