Pyrolysis tube equipped with a fluid stirring element

Abstract

The present invention provides a pyrolysis tube that can reduce pressure loss while possessing a heat transfer effect. [Solution] The pyrolysis tube 10 of the present invention comprises a tube body 12 through which a fluid flows from the upstream side to the downstream side and which is thermally decomposed by heating from the outside; stirring elements 20 protruding from the inner surface 14 of the tube body and extending in the direction of a spiral S inclined with respect to the tube axis of the tube body, with one or more being arranged within one helical pitch P of the spiral; and slit portions 28 between adjacent stirring elements in the direction of the spiral, wherein the stirring elements have different shapes on the upstream side and the downstream side in the direction of the spiral.

Description

Technical Field

[0001] The present invention relates to a pyrolysis tube used in a pyrolysis reactor for producing ethylene and the like.

Background Art

[0002] Olefins such as ethylene and propylene are produced by flowing a raw material fluid containing hydrocarbons (such as naphtha, natural gas, ethane, etc.) at high speed through a pyrolysis tube heated from the outside, heating the raw material fluid to the reaction temperature range, and pyrolyzing it.

[0003] In order to efficiently carry out the pyrolysis reaction, it is required to heat and raise the temperature of the raw material fluid flowing at high speed to the pyrolysis reaction temperature range at the center in the radial direction of the tube body in a short time.

[0004] Therefore, a pyrolysis tube provided with a stirring element for the flowing fluid on the inner surface of the tube body has been proposed (for example, see Patent Document 1). The stirring element of the pyrolysis tube of Patent Document 1 is a protrusion in the direction of a helix inclined with respect to the tube axis. The fluid flowing at high speed in the tube body is agitated by the stirring element, the boundary layer generated near the inner surface of the tube body is broken, heat transfer is promoted, and the temperature is rapidly raised and heated, so that the pyrolysis is completed in a short time.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Since the stirring element protrudes from the inner surface of the pipe body, increasing the height of the stirring element reduces the cross-sectional area of ​​the flow path. As a result, pressure loss, i.e., the fluid friction coefficient, increases, which can cause the raw material fluid to overheat. Overheating can lead to excessive lightening of hydrocarbons (generation of methane, free carbon, etc.) and polycondensation reactions of decomposition products, which can significantly reduce the yield of the target product. Furthermore, overheating can promote coking (deposition of free carbon on the inner surface of the pipe), leading to a decrease in the heat transfer coefficient of the pipe body, requiring frequent decoking work and reducing operating time.

[0007] In response to this, one might consider lowering the height of the stirring element to increase the cross-sectional area of ​​the flow path, but this would reduce the fluid stirring effect and diminish the heat transfer promotion effect.

[0008] The present invention aims to provide a pyrolysis tube that can reduce pressure loss while possessing a heat transfer effect. [Means for solving the problem]

[0009] The pyrolysis tube of the present invention is A pipe body through which a fluid flows from the upstream to the downstream side, and which is heated from the outside to thermally decompose the fluid, A stirring element is provided, protruding from the inner surface of the pipe body, extending in a spiral direction inclined with respect to the pipe axis of the pipe body, and arranged one or more times within one helical pitch of the spiral, Between adjacent stirring elements in the direction of the helical, there is a slit portion where there is no stirring element, A pyrolysis tube including, The stirring element has different shapes on the upstream and downstream sides in the direction of the helix.

[0010] The upstream side of the stirring element has a rising portion that slopes inward from the inner surface of the pipe body, and the downstream side of the stirring element has a falling portion that slopes toward the inner surface of the pipe body. It is preferable that the shape of the rising portion and the shape of the falling portion are asymmetrically different.

[0011] Preferably, the rising portion of the stirring element has a steeper inclination angle than the falling portion.

[0012] Preferably, the downward-sloping portion of the stirring element has a steeper inclination angle than the upward-sloping portion.

[0013] When the stirring element and the slit portion are viewed in the direction of the pipe axis, it is preferable that the slit portion overlaps with the stirring element at an arbitrary helical pitch of the helix and a downstream helical pitch continuous therewith. [Effects of the Invention]

[0014] The pyrolysis tube of the present invention can achieve a heat transfer effect while minimizing pressure loss. This makes it possible to improve the yield of olefins. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a cross-sectional view of a pyrolysis tube on which a stirring element is formed according to one embodiment of the present invention. [Figure 2] Figure 2 is a view of the pyrolysis tube shown in Figure 1, viewed in the direction of the tube axis. [Figure 3] Figure 3 is an explanatory diagram showing one embodiment of the stirring element. [Figure 4] Figure 4 is an explanatory diagram showing the fluid flow corresponding to the stirring element in Figure 3. [Figure 5] Figure 5 is an explanatory diagram showing another embodiment of the stirring element. [Figure 6] Figure 6 is an explanatory diagram showing the fluid flow corresponding to the stirring element in Figure 5. [Figure 7] Figure 7 is a cross-sectional view of a pyrolysis tube in which a stirring element is formed according to another embodiment of the present invention. [Figure 8] Figure 8 is a view of the pyrolysis tube shown in Figure 7, viewed in the direction of the tube axis. [Figure 9]FIG. 9 is an explanatory view showing further different embodiments of the stirring element in (a) and (b), and (c) is an explanatory view showing a plane V' for these embodiments. [Figure 10] FIG. 10 is an explanatory view of the test pyrolysis tube used in the examples.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, the pyrolysis tube 10 of the present invention will be described with reference to the drawings. Note that the illustrated pyrolysis tube 10 is a straight tube, but generally, pyrolysis tubes 10 made of straight tubes are connected by bent bend tubes and arranged in a pyrolysis furnace in a meandering shape for use.

[0017] FIG. 1 is a cross-sectional view in a plane including the tube axis of the pyrolysis tube 10 of the present invention, and FIG. 2 is a view of the end face seen in the tube axis direction. In FIG. 1, the stirring element 20 on the front side of the cross-section is shown by a dotted line, with the left side of the paper being the upstream side in the fluid flow direction and the right side being the downstream side.

[0018] In the pyrolysis tube 10, a raw material fluid containing hydrocarbons (such as naphtha, natural gas, ethane, etc.) is caused to flow at high speed through the externally heated pyrolysis tube inside the tube body 12, and by heating from the outside of the tube, the fluid flowing inside is pyrolyzed to generate olefins such as ethylene and propylene.

[0019] The tube body 12 can be formed from a heat-resistant alloy material, and examples include 25Cr-Ni (SCH22), 25Cr-35Ni (SCH24), Incoloy (trademark), or an alloy containing Al up to 6.0% by mass. Of course, the material of the tube body 12 is not limited to these, and various heat-resistant alloy materials that can withstand the high-temperature use environment and have the required performance can be used.

[0020] In the pyrolysis tube 10, a stirring element 20 that protrudes inward from the inner surface of the tube body 12 is formed. The stirring element 20 is intermittently arranged so as to extend in the direction of the helix S with respect to the tube axis of the tube body 12. Between the stirring elements 20, there is a slit portion 28 without the stirring element 20.

[0021] First, let's explain the helix S. The helix S is a virtual line in which the stirring elements 20 are intermittently arranged, and it is a spiral shape that is inclined with respect to the pipe axis on the inner surface of the pipe body 12. The helix S can be defined by the helix angle θ and the helix pitch P of the helix S. The helix angle θ is preferably 85° or less, and preferably 30° or less. However, in order to have the shape of a helix S, the helix angle θ should be 5° or more. When the inner diameter of the pipe body 12 is 30mm-150mm, the helix pitch P of the helix S can be 16mm-570mm. Note that the helix angle θ and helix pitch P of the helix S can be the same throughout the pyrolysis pipe 10 from upstream to downstream, or the helix angle θ and helix pitch P can be changed partially or continuously. In Figure 1, there is one helix S, but there may be multiple helixes.

[0022] The stirring element 20 is a projection positioned on the spiral S described above. The stirring element 20 has different shapes on the upstream and downstream sides with respect to the direction of the spiral S. Figures 3(a) and 5(a) are plan views of the stirring element 20 as seen from the center of the pipe axis, and Figures 3(b) and 5(b) are cross-sectional views of the stirring element 20 cut through the center in the width direction along the direction of the spiral S. To make the explanation easier to understand, in Figures 3(a) and 5(a), contour lines C connect parts where the height from the inner surface of the pipe body 12 is the same. In the illustrated embodiment, the stirring element 20 has a rising portion 22 that slopes from the pipe body 12 on the upstream side of the spiral S, and a falling portion 23 that slopes toward the pipe body 12 on the downstream side of the spiral S, and the rising portion 22 and the falling portion 23 have different shapes asymmetrically with respect to the top portion 24. In this invention, "asymmetrically different" means that, as shown in Figures 3 and 5, when the stirring element 20 is cut by a plane V that passes through the top portion 24 and is perpendicular to the helix S, the rising portion 22 and the descending portion 23 are not symmetrical with respect to plane V. In the illustrations, the top portion 24 of the stirring element 20 is eccentric to the upstream or downstream side with respect to the direction of the helix S.

[0023] Since the stirring element 20 is formed along the helix S, a portion of the fluid flowing through the pipe body 12 will swirl along the helix S, thereby enhancing the stirring effect. Furthermore, by making the stirring element 20 have an asymmetrically different shape with respect to the direction of the helix S, the cross-sectional area of ​​the flow path in the pyrolysis pipe 10 can be reduced compared to a stirring element with the same symmetrical shape on the upstream and downstream sides, thereby increasing the fluid stirring effect. In addition, the asymmetrically different shape of the stirring element 20 can reduce pressure loss compared to a symmetrical stirring element.

[0024] In one embodiment, the stirring element 20 is shown in Figure 3(a) and Figure 3(b) As shown, the slope of the rising section 22 can be made steeper than the slope of the descending section 23. That is, the stirring element 20 has a narrower spacing between contour lines C in the rising section 22 than in the descending section 23. Figure 3(b) As shown, when the rising portion 22 and / or the descending portion 23 have a convex shape that bulges toward the pipe axis, the inclination of the rising portion 22 can be defined as the angle α connecting the top 24 of the stirring element 20, which is the highest point toward the pipe axis from the inner surface 14 of the pipe, and the starting point of the rising portion 22, and the inclination of the descending portion 23 can be defined as the angle β connecting the top 24 and the endpoint of the descending portion 23, where α > β.

[0025] Figure 4 is a schematic diagram showing the fluid flow through the stirring element 20 in Figure 3(a), with the fluid flow indicated by arrows. The stirring element 20 has a steep incline at the rising section 22 and a gentle incline at the falling section 23. As shown by the solid arrows, the fluid flowing inside the pipe body 12 experiences increased resistance when it hits the rising section 22, causing it to flow vigorously in an oblique direction along the pipe axis and the inner surface of the pipe body 12, increasing the fluid's stirring energy. As a result, the boundary film that tends to form near the inner surface of the pipe body 12 is broken down, improving heat transfer efficiency. On the other hand, as shown by the dotted arrows, the fluid hitting the gently falling section 23 does not change direction much and the change in flow velocity is small, thus reducing pressure loss. In other words, the rising section 22 promotes heat transfer, and the falling section 23 reduces pressure loss.

[0026] In another embodiment, the stirring element 20 is: Figure 5(a) As shown in Figure 5(b), the slope of the downward section 23 may be steeper than the slope of the upward section 22. That is, the spacing of the contour lines C of the downward section 23 of the stirring element 20 is narrower than the spacing of the contour lines C of the upward section 22. As shown in Figure 5(b), if the upward section 22 and / or the downward section 23 have a convex shape that bulges toward the pipe axis, the slope of the upward section 22 can be defined as the angle α connecting the top 24 of the stirring element 20, which is the highest point toward the pipe axis from the inner surface 14 of the pipe, and the starting point of the upward section 22, and the slope of the downward section 23 can be defined as the angle β connecting the top 24 and the endpoint of the downward section 23, where α < β.

[0027] Figure 6 is a schematic diagram showing the fluid flow when it hits the stirring element 20 in Figure 5(a), with the fluid velocity indicated by the arrows. The stirring element 20 has a gentle slope at the rising section 22 and a steep slope at the falling section 23. As shown by the solid arrows, the fluid flowing inside the pipe body 12 does not flow forcefully toward the pipe axis when it hits the rising section 22, but rather flows gently toward the pipe axis, thus reducing the decrease in pressure loss. On the other hand, as shown by the dotted arrows, the fluid that hits the steep falling section 23 flows forcefully toward the pipe axis side, which is the center of the pipe body 12, and along the inner surface of the pipe body 12 in an oblique direction, increasing the stirring energy of the fluid. This can break down the boundary film that tends to form near the inner surface 14 of the pipe, thereby enhancing the heat transfer effect. In other words, the rising section 22 reduces pressure loss, and the falling section 23 promotes heat transfer.

[0028] As shown in Figures 3 and 5, the height H of the stirring element 20 from the inner surface 14 of the pipe to the top 24 can be 0.2 mm to 6 mm when the inner diameter of the pipe is 30 to 150 mm, and preferably 1 / 60 to 1 / 10 of the inner diameter of the pipe. If the height H of the stirring element 20 is lower than this, the stirring and turbulence generation effects of the fluid may not be fully realized. Also, if the height H of the stirring element 20 is higher than this, the stirring element 20 reduces the cross-sectional area of ​​the flow path, obstructing the flow of fluid and potentially increasing pressure loss. In this invention, since the shape of the stirring element 20 is asymmetrical, the area projected in the direction of the pipe axis can be reduced compared to a symmetrical stirring element with the same height. Therefore, the cross-sectional area of ​​the flow path can be relatively increased, and pressure loss can be reduced.

[0029] The stirring element 20 can be formed from the same heat-resistant alloy material as the pipe body 12, but is not limited to this. For example, the stirring element 20 can be efficiently formed as a build-up bead by build-up welding methods such as powder plasma welding (PTA welding), MIG welding, TIG welding, or laser welding.

[0030] As shown above and in Figure 1, multiple stirring elements 20 are arranged in the direction of the helix S, flanking the slit portion 28. One to 20 stirring elements 20 are arranged per helical pitch P of the helix S, preferably 3 to 19. The length L (length in the arc direction) of the stirring elements 20 along the helix S can be 12 mm to 30 mm when the pipe inner diameter is 30 to 150 mm, and preferably the ratio of the total length of the stirring elements 20 per circumference is 2 / 5 to 4 / 5 of the circumference of the pipe inner diameter. If the length L of the stirring elements 20 is shorter than this, the stirring effect of the stirring elements 20 on the fluid cannot be fully exerted, and if it is longer, the flow path cross-sectional area becomes larger, which may increase pressure loss. On the other hand, the length M (length in the arc direction) of the slit portion 28 along the spiral S can be 1 mm to 30 mm when the inner diameter of the pipe is 30 to 150 mm, and preferably the ratio of the total slit spacing per revolution to the circumference of the inner diameter of the pipe is 1 / 5 to 3 / 5.

[0031] As shown in the cross-sectional view 7 and Figure 8, the stirring elements 20 and the slit portions 28 can be formed so that the period of the arrangement of one helical pitch P of the helix S and the adjacent downstream (or upstream) helical pitch P' are the same. In Figure 7, the stirring elements 20 closer to the viewer than the cross-section are shown by dotted lines. Figure 8 is a view of the end face of Figure 7 as seen in the direction of the pipe axis. Referring to Figure 8, the stirring elements 20 (reference numerals 1-12) with helical pitches P and P' overlap each other in the direction of the pipe axis. Specifically, the stirring elements 20 with reference numerals 1 and 7, 2 and 8, 3 and 9, ... overlap. It can also be seen that the slit portions 28 (reference numeral al) overlap in the direction of the pipe axis. Specifically, the slit portions 28 with reference numerals a and g, b and h, c and i, ... overlap. Even with this arrangement period, the stirring elements 20 have an asymmetric shape, which enhances the heat transfer effect and reduces pressure loss.

[0032] However, to further enhance the heat transfer effect and reduce pressure loss, it is preferable that the agitation elements 20 and slit portions 28 are offset from each other at adjacent helical pitches P and P'. Figure 1 is a cross-sectional view showing the arrangement of agitation elements 20 for two helical pitches P and P' of the helix S, and Figure 2 is a view in the direction of the pipe axis. In Figure 1, the agitation elements 20 closer to the viewer than the cross-section are shown by dotted lines. In Figure 2, the agitation elements 20 for helical pitch P' that are not visible due to overlap are shown by dotted lines. These figures show the most desirable configuration for the arrangement period of agitation elements 20 and slit portions 28 for adjacent helical pitches P and P', where the slit portion 28 for one pitch overlaps with the agitation element 20 for the other pitch within the two pitches P and P'.

[0033] In Figures 1 and 2, 13 stirring elements 20 are formed at two helical pitches P and P', i.e., 6.5 elements per helical pitch P. These stirring elements 20 are numbered 1-13 for identification. The slits 28 between the stirring elements 20 are denoted by the symbol Al. With this arrangement, as shown in Figure 2, a stirring element (symbol 8) at helical pitch P' can be seen between the stirring elements at helical pitch P (symbols 1 and 2). Furthermore, a stirring element 20 (symbol 9) can be seen behind the slit 28 (symbol b) (in Figure 1, the width of the slit 28 at symbol b is indicated by the symbol NN). In this way, by shifting the arrangement cycle of the stirring elements 20 and the slits 28, the fluid flowing around the inner surface of the pipe body 12 in the direction of the pipe axis cannot proceed in a straight line because it will encounter one of the stirring elements 20 between the two pitches P and P', and will receive stirring action from one of the stirring elements 20. Therefore, the heat transfer effect can be dramatically increased.

[0034] In this invention, since the stirring element 20 has an asymmetrical shape on the upstream and downstream sides along the direction of the helix S, the heights of the stirring elements 20 that overlap in the direction of the pipe axis also differ at adjacent helical pitches P and P'. Therefore, the fluid receives a stirring effect with varying speeds from the stirring elements 20 of different heights, which can further enhance the heat transfer effect.

[0035] The above description of the embodiments is for illustrative purposes only and should not be interpreted as limiting or narrowing the scope of the invention as described in the claims. Furthermore, the configuration of each part of the present invention is not limited to the above embodiment, and various modifications are possible within the technical scope described in the claims.

[0036] For example, in Figure 3, the stirring element 20 has a shape with a single apex 24, but as shown in Figure 9(a), the apex may be a flat plane 25, or as shown in Figure 9(b), the apex may be an arc-shaped flat plane 26 similar to the inner surface of the pipe body 12. In these cases, the left side of the planes 25, 26 becomes the rising portion 22, and the right side becomes the falling portion 23. The inclination angles α and β of the rising portion 22 and the falling portion 23 can be defined with the edges of the planes 25, 26 as the apex 24. In this case, the stirring element 20 only needs to be asymmetrical in the part excluding the bent, approximately cylindrical region between the two apex portions 24, 24. Specifically, as shown in Figure 9(a) or Figure 9(b), the stirring element 20 is cut by two planes V, V passing through each apex 24 and perpendicular to the helix S, and as shown in Figure 9(c), the rising portion 22 and the falling portion 23 only need to be symmetrical with respect to the plane V' formed by superimposing these planes V, V.

[0037] For example, the pyrolysis tube 10 can form an oxide film on the inner surface of the tube body 12 and / or on the surface of the stirring element 20 by heating it from the outside while passing an oxygen-containing fluid through it. The formation of the oxide film improves resistance to carburization. [Examples]

[0038] As shown in Figure 8, a test pyrolysis tube 30 was fabricated with a run-up section 31 on the upstream side and either the pyrolysis tube 10 of the inventive example or the pyrolysis tube 40 of the comparative example connected to the downstream side, and the heat transfer effect and pressure loss were compared. The inner diameter of the pyrolysis tubes 10 and 40 is 40 mm.

[0039] The pyrolysis tube 10 of the invention is a pyrolysis tube in which the stirring element 20 has an asymmetric shape on the upstream side and the downstream side in the direction of the helix S, as shown in Figure 3, and the arrangement period of the stirring element 20 and the slit portion 28 is shifted by two helical pitches P and P', as shown in Figures 1 and 2. The helical angle θ is 30°, the height H of the top 24 of the stirring element 20 is 2.1 mm, and the length L along the helix S is 7.0 mm. In addition, the stirring element 20 has an inclination angle α of 26° at the rising portion 22 and an inclination angle β of 12° at the falling portion 23.

[0040] The comparative example pyrolysis tube 40 is a pyrolysis tube in which the stirring element has a symmetrical shape on the upstream and downstream sides in the direction of the helix S. The stirring element and the slit section are arranged to have the same arrangement period at adjacent pitches (see Figures 7 and 8 for the arrangement period). In addition, the height of the top of the stirring element and the length along the helix S are the same as in the inventive example.

[0041] The length of the upstream approach section 31 of the test pyrolysis tube 30 is 1.6 m, and the wall surface is insulated. The approach section 31 connected to the pyrolysis tube 10 of the inventive example has the same stirring element 20 as the pyrolysis tube 10 of the inventive example, and the approach section 31 connected to the pyrolysis tube 40 of the comparative example has the same stirring element as the pyrolysis tube 40 of the comparative example. The length of the pyrolysis tube 10 or 40 is 0.6 m.

[0042] While heating the wall of the test pyrolysis tube 30 with the above configuration, a fluid consisting of 70% by weight of ethane and 30% by weight of water vapor was supplied, and (1) the pressure loss when the heat transfer effect was the same, and (2) the heat transfer effect when the pressure loss was the same were measured for the inventive example and the comparative example.

[0043] As a result, regarding (1), the inventive example showed a reduction in pressure loss of approximately 14%. Also, regarding (2), the inventive example showed an improvement in heat transfer efficiency of approximately 7%. [Explanation of symbols]

[0044] 10 Pyrolysis tube 12 pipe body 14 Tube inner surface 20 stirring elements 22. Rising section 23 Downward section 24 Top 28 Slit section P spiral pitch P' spiral pitch S spiral

Claims

1. A pipe body through which a fluid flows from the upstream to the downstream side, and which is heated from the outside to thermally decompose the fluid, A stirring element is provided, protruding from the inner surface of the pipe body, extending in a spiral direction inclined with respect to the pipe axis of the pipe body, and arranged one or more times within one helical pitch of the spiral, Between adjacent stirring elements in the direction of the helical, there is a slit portion where there is no stirring element, A pyrolysis tube including, The slit portion is made from the inner surface of the pipe body, The stirring element has different shapes on the upstream and downstream sides in the direction of the helix. The upstream side of the stirring element has a rising portion that slopes inward from the inner surface of the pipe body. The downstream side of the stirring element has a downward-sloping portion that is inclined toward the inner surface of the pipe body. The shape of the rising portion and the shape of the falling portion are asymmetrically different. Pyrolysis tube.

2. The stirring element has a rising portion with a steeper inclination angle than the falling portion. The pyrolysis tube according to claim 1.

3. The stirring element has a downward-sloping portion that has a steeper inclination angle than the upward-sloping portion. The pyrolysis tube according to claim 1.

4. When the stirring element and the slit portion are viewed in the direction of the pipe axis, the slit portion is arranged to overlap with the stirring element at an arbitrary helical pitch of the helix and a downstream helical pitch continuous therewith. A pyrolysis tube according to any one of claims 1 to 3.

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