EA010936B1 - Method and ribbed tube for thermally cleaving hydrocarbons - Google Patents

Abstract

The invention relates to a method for thermally cleaving hydrocarbons in the presence of steam, during which the feed mixture is guided through externally heated tubes with helical inner ribs, and a swirling flow is produced inside the gas mixture in order to homogenize the temperature inside the tube wall and over the tube cross-section and to prevent the deposition of pyrolysis coke on the inner wall of the tube. The swirling flow is gradually introduced with a predominantly axial flow into a core zone and at an increasing radial distance from the ribs.

Description

The present invention relates to a method and finned tube for thermal cracking of hydrocarbons in the presence of water vapor, where the feed mixture is passed through externally heated pipes with spiral inner ribs.

Tubular furnaces in which a hydrocarbon / water mixture is passed through a series of individual or bending pipes (tubular coils for a cracking furnace) at temperatures above 750 ° C made of heat-resistant chromium-nickel-steel alloys with high resistance to oxidation or scale formation and high resistance to carburization, are suitable for high-temperature pyrolysis of hydrocarbons (derivatives of crude oil). Tubular coils contain vertically extending straight pipe sections that are connected to each other by ϋ-shaped pipe elbows or parallel to each other; they are usually heated with the help of burners of the side walls, and in some cases also with the help of bottom burners and therefore have, as you know, the bright side facing the burners and, as you know, the dark side that is 90 ° offset from it, those. goes in the direction of the rows of pipes. The average temperature of the pipe metal (TMT) in some cases is above 1000 ° C.

The service life of the pipes for the cracking furnace depends to a large extent on the creep resistance and resistance to carburization, as well as on the degree of coking of the pipe material. The decisive factor for the degree of coking, i.e. the growth of a layer of carbon deposits (pyrolysis coke) on the inner wall of the pipe, in addition to the type of hydrocarbon used, is the temperature of the cracking gas in the region of the inner wall and, as you know, the rigidity of the operating conditions, which hides the effect of system pressure and residence time in the pipe system on the ethylene yield . The severity of the operating conditions is based on the average temperature at the outlet of the cracking gases (for example, 850 ° C). The higher the gas temperature near the inner wall of the above temperature, the more intense the growth of the pyrolysis coke layer becomes, and the insulating effect of this layer allows the temperature of the pipe metal to increase even more. Although chromium-nickel-steel alloys containing 0.4% carbon, more than 25% chromium and more than 20% nickel, for example 35% chromium, 45% nickel and, if appropriate, 1% niobium, which are used as pipe material, have high resistance to carburization, carbon diffuses into the pipe wall through defects in the oxide layer, which leads to significant carburization, which can change the amount of carbon content from 1 to 3% by a wall depth of 0.5-3 mm. This is due to significant embrittlement of the pipe material, with the risk of cracking in the case of unevenly fluctuating thermal loads, in particular when the furnace starts and stops.

For the destruction of carbon deposits (coking) on the inner wall of the pipe, it is necessary to interrupt the cracking operation from time to time in order to burn out the pyrolysis coke using a vapor-air mixture. This requires interruption of work for up to 36 hours and therefore has a significantly worsening effect on the economy of the method.

It is also known from the patent CB 969796 to use pipes for a cracking furnace with internal ribs. Although internal ribs of this type provide an internal surface area that is several percent better, for example 10% more, with a corresponding improvement in heat transfer, they are also associated with a lack of significant pressure loss compared to a smooth pipe due to friction on the increased inner surface of the pipe. A higher pressure loss requires a higher system pressure, which inevitably changes the residence time and has a detrimental effect on the output. An additional factor is that known pipe materials with high carbon and chromium contents can no longer be profiled by cold methods, such as cold drawing. Such methods have the disadvantage that their deformability is significantly reduced, since the hot strength increases. This leads to high temperatures of the pipe metal, for example up to 1050 ° C, which are desirable in terms of ethylene yield, which requires the use of centrifugally cast pipes. However, such centrifugally cast pipes can only be obtained with a cylindrical wall; special molding methods are required, for example, material removal by electrolytic treatment or welding molding if tubes with inner ribs are to be obtained.

In view of this premise, the present invention is based on the problem of improving the economics of thermal cracking of hydrocarbons in tube furnaces with externally heated tubes having spiral inside ribs.

The problem is achieved by a method in which a swirling flow is created in close proximity to the ribs of a preferably centrifugally cast pipe, wherein the swirling flow turns into a core zone with a predominantly axial flow with increasing radial distance from the ribs. The transition between the outer zone with a swirling flow and the core zone with a predominantly axial flow is gradual, for example parabolic.

In the method according to the present invention, the swirling flow perceives flaking turbulence on the sides of the ribs, so that the turbulence does not recycle locally in the form of a continuously circulating flow in the hollows of the ribs. Despite the usually large distances covered by particles through spiral paths, the average residence time is lower than in a smooth pipe and, moreover, more uniform in cross section (see Fig. 7). This is confirmed by more

- 1 010936 high overall speed in a profiled pipe with spiral ribs (profile 3) compared with a pipe with straight ribs (profile 2). This is ensured, in particular, if the swirling flow in the area of the ribs or ribs passes at an angle of 20-40 °, for example 30 °, preferably 25-32.5 ° with respect to the axis of the pipe.

In the method according to the present invention, the heat supply, which inevitably varies around the circumference of the pipe between the light side and the dark side, is aligned in the pipe wall and inside the pipe, and the heat quickly dissipates inward to the core zone. This is due to a decrease in the risk of local overheating of the processed gas on the pipe wall with the formation of pyrolysis as a result of coke. In addition, the thermal load on the pipe material is lower due to equalization of temperature between the light and dark sides, which increases the service life. Finally, in the method according to the present invention, the temperature is also made more uniform in the cross section of the pipe, which gives an improved olefin yield. The reason for this is that without the radial temperature equalization according to the present invention inside the pipe, excessively deep cracking will take place on the hot wall of the pipe, and recombination of the cracking products will take place in the center of the pipe.

In addition, a laminar flow layer, which is a characteristic of turbulent flows, with significantly reduced heat transfer is formed in the case of a smooth pipe and, to a large extent, in the case of rib profiles with an inner circumference, which increases by more than 5%, for example, by 10% by ribs. Laminar flows lead to increased formation of pyrolysis coke, similarly with poor thermal conductivity. The two layers together require more heat input or a higher burner output. This increases the temperature of the pipe metal (TMT) and, accordingly, reduces the service life.

The present invention avoids this by the fact that the inner perimeter of the profile is about 5% greater, for example 4% or even 3.5%, relative to the circumference of the enclosing circle touching the troughs of the ribs. However, the inner perimeter can also be up to 2% less than the enclosing circle. In other words, the relative perimeter of the profile differs at most by 1.05-0.98% from the perimeter of the enclosing circle.

Accordingly, the difference in the area of the profile of the pipe according to the present invention, i.e. its estimated internal surface area with respect to a smooth pipe having a diameter of the female surface is at most 5 to -2% or 1.05-0.98 times the area of a smooth pipe.

The profile of the pipe according to the present invention allows a lower density of the pipe (kg / m) compared with the finned tube, in which the inner perimeter of the profile is at least 10% larger than the perimeter of the enclosing circle. This is shown by comparison between two pipes with the same hydraulic diameter and, accordingly, the same pressure loss and the same thermal result.

An additional advantage of the profile perimeter according to the present invention (relative profile perimeter) with respect to the circumference of the enclosing circle is a more rapid heating of the feed gas at a reduced temperature of the pipe metal.

The swirling flow according to the present invention significantly reduces the degree of the laminar layer, in addition, it is associated with a velocity vector directed to the center of the pipe, which reduces the residence time of cracking radicals and / or cracking products on the hot wall of the pipe and their chemical and catalytic decomposition with the formation of pyrolysis coke . Moreover, the temperature differences between the hollows of the ribs and the ribs, which are significant in the case of internally profiled pipes with high ribs, are aligned by a swirl flow according to the present invention. This increases the time between two necessary coke removal operations. Without a swirling flow according to the invention, there is a significant temperature difference between the tops of the ribs and the base of the hollows of the ribs. The residence time of cracked products, which tend to coke, is shorter in the case of cracked furnace tubes provided with spiral internal ribs. It depends on the nature of the ribs in individual circumstances.

In the diagram shown in FIG. 1 upper curve shows profile 6: slope 16 °;

the middle curve shows profile 3: slope 30 °;

the lower curve shows the profile of the 4: 3 rib with a slope of 30 °.

The curves clearly show that a higher peripheral velocity of profile 6 with ribs 4.8 mm high is consumed in the hollows of the ribs, while the peripheral velocity of the profile according to the present invention with a height of ribs even 2 mm penetrates the core of the stream. Although the peripheral velocity of profile 4, even with 3 ribs, is approximately equally high, it does not give the effect of any spiral acceleration of the core flow.

According to the curves shown in the diagram of FIG. 2, the profile according to the present invention provides helical acceleration in the hollows of the ribs (the upper branch of the curve), which covers wide areas of the cross section of the pipe and is therefore responsible for the homogenization of temperature in the pipe. The lower peripheral speed at the tops of the ribs (the lower branch of the curve), in addition, ensures that turbulence and countercurrents do not occur.

- 2 010936

In FIG. 3 shows a cross-section of three test tubes, including their data; pipes include profile 3 according to the present invention. Each diagram shows the temperature change along the radius of the pipe on the dark and light sides. A comparison of the diagrams shows a lower temperature difference between the wall and the center of the pipe and a lower gas temperature on the pipe wall in the case of profile 3 according to the invention.

The swirling flow according to the present invention ensures that the variation in the wall temperature compared to the periphery of the pipe, i.e. between the light and dark sides, is less than 12 ° C, even though the tubular coils, which are usually arranged in parallel rows, heat up or act on combustible gases by means of burners of the side wall only on opposite sides, and therefore the pipes have each bright side facing the burners and a dark side that is offset 90 ° with respect to it. The average temperature of the pipe metal, i.e. the difference in the temperature of the pipe metal on the light and dark sides leads to internal stresses and therefore determines the service life of the pipes. Therefore, the decrease in the average temperature of the metal of the pipe according to the present invention with eight ribs with an inclination angle of 30 °, an inner diameter of 38.8 mm and an outer diameter of 50.8 mm, i.e. the difference in height between the hollows of the ribs and the tops of the ribs is 2 mm 11 ° compared to a smooth pipe of equal diameter, based on an average life of 5 years, as can be seen in the diagram shown in FIG. 4, gives at an operating temperature of 1050 ° C a calculated increase in service life of up to about 8 years.

The temperature distribution between the light and dark sides for the three profiles shown in FIG. 3 is shown in the diagram shown in FIG. 5. A lower level of the temperature curve for profile 3 compared to a smooth pipe (profile 0) and a significantly narrower range of vibrations for the profile 3 curve compared to the profile 1 curve are noticeable.

A particularly suitable temperature distribution is established if the isotherms go in a spiral form from the inner wall of the pipe to the core of the stream.

A more uniform temperature distribution over the cross section is obtained, in particular, if the peripheral speed increases to 2-3 m and then remains constant over the entire length of the pipe.

From the point of view of achieving a high olefin yield with a relatively short pipe length, the method according to the present invention should work in such a way that the temperature homogeneity factor across the cross section and the temperature homogeneity factor related to the hydraulic diameter is more than 1 relative to the homogeneity factor of the smooth pipe (N _O0 ). In this context, homogeneity factors are defined as follows:

^Η σ0 [ ^- ] ^Η Ρ0 ⁼ ΔΤ0 · άχ / ΔΤ _χ · ά0.

A flow configuration according to the present invention comprising a flow core and a swirling flow can be achieved with a finned tube in which the angle of the side of the ribs, which are in each case continuous along the length of the pipe section, i.e. the external angle between the sides of the ribs and the radius of the pipe is 16-25 °, preferably 19-21 °. An angle of the lateral side of this type, in particular in combination with an angle of inclination of the rib of 20 to 40 °, for example 22.5-32.5 °, ensures that the results in the hollows of the ribs are not more or less continuous swirling flow, which returns to to the hollows of the ribs after the lateral sides of the ribs and leads to the formation of unwanted vortices in the hollows of the ribs. More precisely, the turbulence formed in the hollows of the ribs becomes detachable from the sides of the ribs and is perceived by a swirling flow. The twist energy introduced by the ribs accelerates the gas particles and leads to a higher overall speed. This leads to a decrease in the temperature of the metal of the pipe and also makes the latter more uniform, as well as making the temperature and residence time over the cross section of the pipe more uniform.

The properties of the finned pipe according to the present invention can be seen from showing the pipe segment in FIG. 6 and associated characteristic parameters:

hydraulic diameter E _n (in mm) Β ₁ <Ό _η / 2;

profile angle β; rib height H; radius of the enclosing circle K _n = P _| + H and Ό ₃ = 2χΒ ₃ ; central angle α;

radius of curvature Β = Β ₃ (8ίηα / 28ίηβ + 8ίηα); the perimeter of the enclosing circle 2πΚ. ,,; angle in the oblique triangle γ = 180- (α + β); inner radius Κ. ₁ = 2Β (8ίηγ / δίηα) -Β;

rib height H = B _a -B ₁ ; the perimeter of the profile And _p = 2 is the number of ribs RV / 180 (2B + a);

the surface area of the ribs P _to ;

covering the circumferential area Ρ = πΌ _{3 3} ^2/4;

inner circle area внутренней ₁ = πΌ ₁ ;

the area of the profile in the surrounding circle P _p = P _{to the} number of ribs;

- 3 010936 profile perimeter υ _ρ = (1.05-0.98) · 2πΒ _α .

The ribs and depressions of the ribs that are located between the ribs can be mirror-symmetrical in cross section and adjacent to each other or can form a wave line with the same radii of curvature in each case. The profile angle is then formed between the tangents of the two radii of curvature at the point of contact and the radius of the pipe. In this case, the ribs are relatively gentle; the height of the rib and the angle of the profile are consistent with each other so that the hydraulic diameter of the profile from the ratio of 4x free cross-section / perimeter of the profile is equal to or greater than the inner circumference of the profile. The hydraulic diameter is therefore in the inner third of the profile height. Therefore, the height of the ribs and the number of ribs increase when the diameter becomes larger, so that the swirling flow is maintained in the direction and with the intensity required for the profile.

The high flow rate (Fig. 2) taking place between the ribs or in the hollows of the ribs provides a self-cleaning effect, i.e. reduction in the amount of coke pyrolysis, which is deposited.

If the ribs are formed by surfacing by welding or by welding using a centrifugally cast pipe, the pipe wall between the individual ribs remains essentially unchanged, so that the hollows of the ribs lie on a common circle that corresponds to the inner circumference of the centrifugally cast pipe.

Tests have shown that regardless of the inner diameter of the pipes, a total of 8-12 ribs are sufficient to achieve the flow configuration according to the present invention.

In the case of the finned tube according to the present invention, the ratio of the ratios of the heat transfer coefficients 0ι, / 0 _Ο and the ratios of pressure loss ΔΡ _Β / ΔΡ _Ο in the water test, when applying and observing the similarity rules and using the Reynolds numbers given for the naphtha / water vapor mixture, is preferably from 1.4 to 1.5, where B is a finned tube and Ο is a smooth tube.

The data presented in the table show the advantage of the finned tube according to the present invention (profile 3) compared with a smooth pipe (profile 0) and a finned pipe with 8 parallel ribs (profile 1), in which the radial distance between the hollows of the ribs and the vertices of the ribs is 4 , 8 mm. The finned tubes all have 8 ribs and the same enclosing circumference. __________________________________________________________________________

Profile 0 one 2 The temperature of the liquid at 9950 mm in the center, T _t [° C] 843, 6 848.1 843.0 The temperature of the liquid at 9950 mm at the end, T _g [° C] 888, 9 894 874.8 The temperature range at 9950 mm, DT = T _g -T _et {° C] 45.3 45, 9 31.8 The homogeneity factor for a smooth pipe H at H _± = dT _d / DT _k one 0.9869281 1.4245283 Hydraulic diameter ά., [w] 0,0380 0,0256 0,0344 Homogeneity factor related to hydraulic diameter based on a smooth pipe, Н, .0: Η, 0 = ΔΤο * С1х7ΔΤ, · сЬ one 0.8477193 1.3420556 Classification H 2 2 one

In this context, the hydraulic diameter is defined as follows:

О _hydraulic = 4х (free cross-section) / inner perimeter;

and preferably corresponds to the inner diameter of the comparative smooth pipe and therefore results in a homogeneity factor of 1.425.

- 4 010936

In a water test, the finned tube of the present invention gives heat transfer (O ,,). which is 2.56 times higher. than a smooth pipe. with loss of pressure (ΔΡ _Κ ). which is only 1.76 times higher.

In FIG. 7 shows a comparison of pipes of three different profiles. including a pipe according to the present invention with 8 ribs. with an inclination angle in each case of 30 °. with a pipe with a smooth inner wall (smooth pipe). Hydraulic diameter. axial speed. residence time and pressure loss are given for each cross section.

The initial data used were the quantitative expenditures in a working smooth pipe with an inner diameter of 38 mm. which is identical to the hydraulic diameter. Using similarity rules (identical Reynolds numbers), these data were converted to warm water and used as the basis for testing (see the ratio of the ratios of heat transfer coefficients and pressure loss for testing with water and the relative homogeneity factor for calculation using gases).

Different velocity profiles result from the same quantitative flow rates at different hydraulic diameters (inverse relationship).

Comparison of speeds for profiles 2 and 3. which are identical in cross section. shows improved speed. acceleration and residence time with pipes according to the present invention (profile 3). For the same hydraulic diameter, the velocity component in the peripheral direction. due to twisting. created by ribs. causes the flow to break away from the pipe wall and causes a spiraling speed over the entire cross section.

The directed spiral flow introduces heat from the pipe wall into the stream and therefore distributes it more evenly. than in ordinary non-directional turbulent flow (smooth pipe. profiles 1 and 2). The same applies for the residence time of the particles. The spiral-shaped directed flow distributes the particles more uniformly across the cross section. whereas acceleration on the sides of the profile reduces the average residence time. A higher pressure loss with profile 3 is the result of peripheral speed. In the case of profile 1, the reason is a significant narrowing of the flow and loss of friction on the large internal surface of the profile.

Depending on the material, the finned tubes of the present invention can be obtained. eg. from a centrifugally cast pipe using pipe ends with ribs parallel in the axial direction. rotated in relation to each other. or using an internal profile. obtained by deformation of a centrifugally cast pipe. for example hot forging. hot drawing or cold working with a profiling tool. for example a pulling mandrel or a round mandrel with an external profile. which corresponds to the internal profile of the pipe.

A number of options for cutting machines for internal pipe profiling is known. eg. from German patent 19523280. Such machines are also suitable for producing finned tubes according to the present invention.

In the case of hot molding, the deformation temperature should be set in this way. so that the microstructural grains partially collapse in the zone of the inner surface and. respectively. recrystallized at a later stage under the influence of operating temperature. The result is a fine-grained microstructure. which provides fast diffusion of chromium. silicon and / or aluminum through an austenitic matrix to the inner surface of the pipe. where then the oxygen-protective layer quickly grows.

The fins according to the present invention can also be obtained by welding; in this case, it is impossible to form a curved base of the ribs between the individual ribs. but to a greater extent, the initial profile of the inner wall of the pipe here is actually preserved.

The inner surface of the pipe according to the present invention should have the smallest possible roughness; it can therefore be smoothed. for example mechanically polished or electrolytically aligned.

Suitable pipe materials for use in ethylene plants are iron and / or nickel alloys. containing 0.1-0.5% carbon. 20-35% chromium. 20-70% nickel. up to 3% silicon. up to 1% niobium. up to 5% tungsten and hafnium additives. titanium. rare earth metals or zirconium. in each case up to 0.5%. and up to 6% aluminum.

Claims (20)

CLAIM 1. A finned tube for thermal cracking of hydrocarbons in the presence of water vapor, having a plurality of spiral-shaped inner ribs, characterized in that the ribs extend at an angle of 20-40 ° with respect to the pipe axis, and the outer angle between the sides of the ribs and the radius of the pipe is 16- 25 °. 2. The finned tube according to claim 1, characterized in that the ribs extend at an angle of 22.5-32.5 ° with respect to the axis of the pipe. 3. The finned tube according to claim 1 or 2, characterized in that the perimeter of the profile (Ir) differs by 5 to -2% from the enclosing circle touching the hollows of the ribs. 4. The finned tube according to one of claims 1 to 3, characterized in that said external angle is 16-20 °. 5. The finned tube according to one of claims 1 to 4, characterized in that the ribs and depressions located between the ribs are constructed mirror symmetric in cross section. 6. The finned tube according to one of claims 1 to 5, characterized in that the vertices of the ribs and the depressions of the ribs in each case merge with each other. 7. The finned tube according to one of claims 1 to 6, characterized in that the ribs and depressions of the ribs have the same radius of curvature. 8. The finned tube according to one of claims 1 to 7, characterized in that it has a total of 6-12 ribs. 9. The finned tube according to one of claims 1 to 8, characterized in that the hydraulic diameter of the finned tube is at least equal to the diameter of the inner circle (K ₁ ). 10. A finned tube according to one of claims 1 to 9, characterized in that the ratio of the ratios of the heat transfer coefficients 0, / 0,., And the ratios of pressure loss ΔΡ _Κ / ΔΡ _Ο in the water test is 1.4-1.5, where K is a finned tube and Ο is a smooth tube. 11. The finned tube according to one of claims 1 to 10, characterized in that the radius of curvature (K) of the cross section of the rib is 3.5-20 mm. 12. The finned tube according to one of claims 1 to 11, characterized in that the height of the ribs (H) is 1.25-3 mm. 13. The finned tube according to one of claims 1 to 12, characterized in that the free cross section in the perimeter of the profile (I _p ) is 85-95% of the area of the surrounding circle (E _a ). 14. The finned tube according to one of claims 1 to 13, characterized in that the profile area (E _p ) is 40-50% of the area of the ring between the enclosing circumference and the inner circumference. 15. The finned tube according to one of claims 1 to 14, characterized in that it is a centrifugally cast tube consisting of a nickel alloy containing 0.1-0.5% carbon, 20-35% chromium, 20-70% nickel, up to 3% silicon, up to 1% niobium, up to 5% tungsten, and in each case up to 0.5% hafnium, titanium, rare earth metals, zirconium and up to 6% aluminum. 16. The finned tube of claim 15, wherein the alloy alone or in combination with each other contains at least 0.02% silicon, 0.1% niobium, 0.3% tungsten and 1.5% aluminum. 17. The finned tube according to claims 1-16, characterized in that it is a centrifugally cast tube, the ends of which, after forming parallel in the axial direction of the ribs in the pipe, are turned relative to each other. 18. The finned tube according to claims 1-16, characterized in that the inner profile is obtained by deformation using a profiling tool. 19. The finned tube according to claims 1-16, characterized in that the inner profile is obtained by welding welding. 20. The finned tube according to claims 1-16, characterized in that the inner profile is obtained by electrolytic removal of the material.