WO2004015029A1

WO2004015029A1 - Method and ribbed tube for thermally cleaving hydrocarbons

Info

Publication number: WO2004015029A1
Application number: PCT/EP2003/004827
Authority: WO
Inventors: Peter WÖLPERT; Benno Ganser; Dietlinde Jakobi; Rolf Kirchheiner
Original assignee: Schmidt + Clemens Gmbh + Co. Kg
Priority date: 2002-07-25
Filing date: 2003-05-08
Publication date: 2004-02-19
Also published as: AU2003227737A1; DE10233961A1; HRP20050072A2; CA2493463C; MXPA05001070A; KR101023668B1; ES2374568T3; EA010936B1; PT1525289E; UA85044C2; EA200500258A1; EP1525289A1; NO20050493L; ATE526385T1; NO337398B1; EP2298850A1; IL166229A; EP1525289B9; KR20050052457A; JP2005533917A

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

Process and finned tube for thermal cracking of hydrocarbons

The invention relates to a method and a finned tube for the thermal splitting of hydrocarbons in the presence of steam, in which the feed mixture is passed through externally heated tubes with helical inner fins.

Tube furnaces have proven themselves for the high-temperature pyrolysis of hydrocarbons (petroleum derivatives), in which a hydrocarbon / water vapor mixture at temperatures above 750 ° C by rows of individual or meandering tubes (cracked tube coils) made of heat-resistant chrome-nickel steel alloys with high oxidation or Scale resistance and high carburization resistance. The coils consist of vertically running straight pipe sections which are connected to one another via U-shaped pipe bends or arranged parallel to one another; They are usually heated with the help of side wall and sometimes also with the help of floor burners and therefore have a so-called sunny side facing the burners and a so-called shadow side that is offset by 90 °, that is to say in the direction of the rows of pipes. The mean pipe wall temperatures (TMT) are sometimes above 1000 ° C.

The service life of the cracking tubes depends very much on the creep resistance and carburization resistance as well as on the coking speed of the tube material. Decisive for the coking speed, i.e. for the growth of a layer of carbon deposits (pyrolysis coke) on the inner pipe wall, in addition to the type of hydrocarbons used, the cracking gas temperature in the area of the inner wall and the so-called cracking severity, behind which the influence of the system pressure and the residence time in the pipe system hides on the ethylene yield. The splitting sharpness is based on the average outlet temperature of the fission gases (e.g. 850 ° C)

BESTATIGUNGSKOPIE 2 -

set. The higher the gas temperature in the vicinity of the inner pipe wall above this temperature, the more the layer of pyrolysis coke grows, the insulating effect of which further increases the pipe wall temperature. Although the chromium-nickel steel alloys used as tube material with 0.4% carbon over 25% chromium and over 20% nickel, for example 35% chromium, 45% nickel and possibly 1% niobium have a high carburization resistance, the carbon diffuses Defects in the oxide layer in the pipe wall and there leads to considerable carburization, which can go up to carbon contents of 1% to 3% in wall depths of 0.5 to 3 mm. Associated with this is a considerable embrittlement of the pipe material with the risk of cracking under alternating thermal loads, especially when the furnace is started up and shut down.

In order to break down the carbon deposits (coking) on the inner wall of the pipe, it is necessary to interrupt the cracking operation from time to time and to burn the pyrolysis coke with the aid of a steam / air mixture. This requires an interruption of operation of up to 36 hours and therefore significantly affects the economics of the process.

The use of cracking tubes with inner fins is also known from British patent specification 969 796. Such inner fins result in a many percent, for example 10% larger inner surface and consequently a better heat transfer; however, they are also associated with the disadvantage of a considerably increased pressure loss due to friction on the enlarged inner surface of the tube compared to a smooth tube. The higher pressure loss requires a higher system pressure, which inevitably changes the dwell time and worsens the yield. In addition, the known tube materials with high carbon and chromium contents can no longer be profiled by cold forming, for example cold drawing. They have the disadvantage that their deformability is greatly reduced with increasing heat resistance. This has led to the fact that the high tube wall temperatures of, for example, up to 1050 ° C require the use of centrifugal cast pipes. However, since centrifugal cast pipes can only be produced with a cylindrical wall, special shaping processes are required, for example an electrolytically abrasive machining or a shaping welding process, in order to produce inner finned pipes.

Against this background, the invention is based on the problem of improving the economy of the thermal splitting of hydrocarbons in tube furnaces with externally heated tubes with helical inner fins.

The solution to this problem consists in a method in which a swirl flow is generated in the immediate vicinity of the fins, preferably a centrifugal cast iron pipe, and with increasing radial distance from the fins, a predominantly axial flow is transferred into a core zone. The transition between the outer zone with the swirl flow and the core zone with the predominantly axial flow takes place gradually, for example parabolically.

In the method according to the invention, the swirl flow absorbs the vertebrae detaching at the rib flanks, so that there is no local return of the vertebrae in the manner of a self-contained circular flow into the rib valleys. Despite the obviously longer path of the particles through the spiral paths, the mean residence time is lower than in the smooth tube and moreover more homogeneous over the cross-section (see Fig. 7). This is confirmed by the higher overall speed in the profile tube with swirl (profile 3) compared to the tube with straight ribs (profile 2). This is ensured in particular when the swirl flow in the area of the fins or the fins extend at an angle of 20 ° to 40 °, for example 30 °, preferably 25 ° to 32.5 ° with respect to the tube axis.

In the method according to the invention, the heat inevitably different over the circumference of the tube between the sun and shadow side is offered balanced in the tube wall and inside the tube and the heat was quickly dissipated inwards to the core zone. This is associated with a reduction in the risk of local overheating of the process gas on the tube wall and the resulting pyrolysis coke. In addition, the thermal stress on the pipe material is lower due to the temperature balance between the sun and shade side, which leads to an extension of the service life. Finally, in the process according to the invention, the temperature is also made more uniform over the pipe cross section, with the result that the olefin yield is better. The reason for this is that without the radial temperature equalization according to the invention inside the pipe inside the hot pipe wall would overcrack and in the pipe center a recombination of fission products would occur.

Furthermore, in the case of smooth tubes and, in the case of rib profiles, with ribs which have an internal circumference increased by more than 5%, for example 10%, a layer of laminar flow with a greatly reduced heat transfer which is characteristic of turbulent flows is formed. It leads to the increased formation of pyrolysis coke, which also has poor thermal conductivity. Both layers together require a higher heat input or a higher burner output. This increases the pipe wall temperature (TMT) and consequently shortens the service life.

The invention avoids this in that the inner circumference of the profile is a maximum of 5%, for example 4% or also 3.5%, based on the circumference of the enveloping circle touching the rib valleys. However, the inner circumference can also be up to 2% smaller than the envelope circle. In other words, the relative profile circumference is at most 1.05 to 0.98% of the circumference of the envelope. Accordingly, the area difference of the profiled tube according to the invention, ie its developed inner surface, is a maximum of + 5% to -2% or 1.05 to 0.98 times the smooth tube surface, based on a smooth tube with the enveloping circle diameter. The tube profile according to the invention allows a lower specific tube weight (kg / m) compared to a finned tube, in which the inner circumference of the profile is at least 10% larger than the circumference of the enveloping circle. This is shown by a comparison of two pipes with the same hydraulic diameter and accordingly the same pressure loss and the same thermal result.

Another advantage of the profile circumference (relative profile circumference) according to the invention, based on the circumference of the enveloping circle, is that the feed gas heats up more quickly at a reduced tube wall temperature.

The swirl flow according to the invention considerably reduces the laminar layer; it is also connected to a velocity vector directed towards the tube center, which reduces the residence time of crack radicals or fission products on the hot tube wall and their chemical and catalytic conversion to pyrolysis coke. In addition, the temperature differences between rib valleys and fins, which are not insignificant in inner profile tubes with high fins, are compensated for by the swirl flow according to the invention. This increases the time interval between two necessary decoctions. Without the swirl flow according to the invention, there is a not inconsiderable temperature difference between the rib tops and the bottom of the rib valleys. The residence time of the fission products which tend to coke is shorter in the case of cracking tubes provided with helical inner fins; In individual cases, this depends on the nature of the ribs.

The diagram shows:

upper curve: profile 6: 16 ° gradient middle curve: profile 3: 30 ° gradient lower curve: profile 4: 3 ribs with 30 ° gradient The course of the curve clearly shows that the higher peripheral speed of the profile 6 with 4.8 mm high ribs is consumed within the rib valleys, while the peripheral speed of the profile according to the invention penetrates into the core of the flow with a rib height of only 2 mm. The peripheral speed of the profile 4 with only 3 ribs is almost as high, but does not cause a spiral acceleration of the core flow.

According to the curve in the diagram in FIG. 2, the profile according to the invention brings about a spiral acceleration in the rib valleys (upper curve branch), which covers large areas of the pipe cross section and thus brings about a homogenization of the temperature in the pipe. The lower peripheral speed at the rib tops (lower curve branch) also ensures that there is no turbulence and backflow.

3 shows three test tubes with their data in cross-section, including the profile 3 according to the invention. The diagrams show the temperature profile over the tube radius (radius) on the shadow and the sun side. A comparison of the diagrams shows the lower temperature difference between the tube wall and the center and the lower gas temperature on the tube wall in the case of the profile 3 according to the invention.

The swirl flow according to the invention ensures that the fluctuation of the inner wall temperature over the circumference of the tube, that is, between the sun and shadow side is below 12 ° C., although the tube coils of a tube furnace, which are usually arranged in parallel rows, are heated or heated with the help of side wall burners only on opposite sides are charged with combustion gases and the pipes therefore each have a sunny side facing the burners and a shadow side offset by 90 °. The mean pipe wall temperature, i.e. the difference in pipe wall temperature between the sunny and the shady side, leads to internal stresses and therefore determines the service life of the pipes. This results in the reduction in the mean, which can be seen from the diagram in FIG. 4 - 7 -

Pipe wall temperature of a pipe according to the invention with eight fins with a pitch of 30 °, a pipe inside diameter of 38.8 mm and a pipe outside diameter of 50.8 mm, thus a height difference between rib valleys and rib tips of 2 mm of 11 ° compared to a diameter-smooth tube to an average lifespan of 5 years, at an operating temperature of 1050 ° C, a calculated lifespan increase to about 8 years.

The temperature distribution between the sun and shadow side for the three profiles in FIG. 3 results from the diagram in FIG. 5. It is noteworthy here that the lower level of the temperature curve for profile 3 compared to the smooth tube (profile 0) is significant smaller fluctuation range of the profile 3 curve compared to the profile 1 curve.

A particularly favorable temperature distribution arises when the isotherms run from the inner tube wall to the core of the flow in a spiral.

A more uniform distribution of the temperature over the cross section results in particular if the peripheral speed builds up within 2 to 3 m and then remains constant over the entire pipe length.

In view of a high olefin yield with a comparatively short tube length, the method according to the invention should be operated in such a way that the homogeneity factor of the temperature over the cross section and the homogeneity factor based on the hydraulic diameter in relation to the homogeneity factor of a smooth tube (H _G0 ) over 1 lies. The homogeneity factors are defined as follows:

H _G0 [-] Hp ₀ = ΔT _o . d _x / ΔT _χ . d _o

The flow pattern of core and swirl flow according to the invention can be achieved with a finned tube in which the flank angle is in each case over the Length of a tube piece of continuous ribs, that is, the outside angle between the rib flanks and the radius of the tube is 16 ° to 25 °, preferably 19 ° to 21 °. Such a flank angle, in particular in connection with a fin pitch of 20 ° to 40 °, for example 22.5 ° to 32.5 °, ensures that there is not a more or less self-contained vortex flow returning into the fin valleys behind the fin flanks results, which leads to the formation of undesirable "twisters" in the rib valleys, that is, of closed vortex braids. Rather, the vortices that arise in the rib valleys separate from the rib flanks and are absorbed by the swirl flow. The swirl energy induced by the ribs accelerates the gas particles and leads to a higher overall speed. This leads to a reduction and equalization of the tube wall temperature and to an equalization of the temperature and the residence time over the tube cross-section.

The nature of the finned tube according to the invention results from the illustration of a tube segment in FIG. 6 and the associated characteristic parameters

hydraulic diameter Dh in mm, Ri <Dh / 2

Flank angle ß rib height H

- Envelope circle radius Ra = Ri + H and Da = 2 x Ra center angle α

Radius of curvature R = Ra (sin α / 2 sin ß + sin α)

- Envelope circumference 2 π Ra

- Angle in the oblique triangle γ = 180 - (α + ß) inner radius Ri = 2R (sin γ / sin α) - R rib height H = Ra - Ri

- Profile circumference U _p = 2 x number of ribs x πR / 180 (2 ß + α) rib area F _R

Area of the enveloping circle Fa = π Da ^2/4 Area of the inner circle F _s = II • Di

Profile area within the envelope circle F _p = F _R • Number of ribs

- Profile range Up = (1.05 to 0.98) • 2 π Ra

The ribs and the rib valleys located between the ribs can be mirror-symmetrical in cross section and adjoin one another or form a wavy line with the same radii of curvature in each case. The flank angle then results between the tangents of the two radii of curvature at the point of contact and the radius of the tube. The ribs are relatively flat; Rib height and flank angle are coordinated so that the hydraulic diameter of the profile from the ratio of 4 x free cross-section / profile circumference is equal to or larger than the inner circle of the profile. The hydraulic diameter is therefore in the inner third of the profile height. The rib height and the number of ribs increase with increasing diameter so that the swirl flow is maintained in the direction and strength required for the effect of the profile.

Between the ribs or in the rib valleys there is a greater flow rate (FIG. 2), which leads to a self-cleaning effect and therefore to less pyrolysis coke deposits.

If the fins are produced by build-up welding or built-up welding using a centrifugal cast tube, the tube wall between the individual fins remains essentially unchanged, so that the rib valleys lie on a common circle that corresponds to the inner circumference of the centrifugal cast tube.

Tests have shown that - regardless of the inside diameter of the pipes

- A total of 8 to 12 ribs are sufficient to achieve the flow pattern according to the invention. 10

In the finned tube according to the invention, the ratio of the quotients of the heat transfer coefficients Q _R / Q ₀ to the quotient of the pressure losses ΔP _R / ΔPo in the water test using and observing the laws of similarity and using the Reynolds numbers mediated for a naphtha / water vapor mixture is preferably 1.4 to 1.5, where R denotes a finned tube and 0 a smooth tube.

The superiority of the finned tube according to the invention (profile 3) compared to a smooth tube (profile 0) and a finned tube with axially parallel fins (profile 1), in which the radial distance between the rib valleys and the fin tips is 4.8 mm, illustrate the data of the following Table. The finned tubes all had 8 fins and the same enveloping circle.

The hydraulic diameter is defined as follows:

D _{h dr} = 4 x (free cross-section) / inner circumference;

it preferably corresponds to the inside diameter of a comparable smooth tube and then gives a homogeneity factor of 1.425. In the water test, the finned tube according to the invention results in a heat transfer (Q _R ) that is higher by a factor of 2.56 compared to the smooth tube with a pressure loss (ΔP _R ) that is only increased by a factor of 76.

In Fig. 7, a pipe with a smooth inner wall (smooth pipe) is compared to three different profile pipes, including a pipe according to the invention with 8 fins, each with a slope of 30 °. The hydraulic diameter, the axial speed, the dwell time and the pressure loss are given for each cross-section.

The starting data were the throughput quantities of a smooth pipe in operation with a 38 mm inner diameter, which is identical to the hydraulic diameter. These data were converted to warm water according to the laws of similarity (same Reynolds numbers) and used as the basis for the tests (see ratio of the heat transfer and pressure loss quotients for tests with water and the related homogeneity factor when calculating with gases).

The different speed profiles result from the same throughputs with different hydraulic diameters (reciprocal ratio).

The comparison of the speeds with the profiles 2 and 3 having the same cross section illustrates the better speed, acceleration and dwell time for the tubes according to the invention (profile 3). With the same hydraulic diameter, the speed component caused by the swirl of the ribs causes the flow to detach from the pipe wall in the circumferential direction and a helically increasing speed in the entire cross section.

The directional, spiral flow transfers the heat from the pipe wall into the flow and thus distributes it more evenly than in one 12 -

normal undirected turbulent flow (smooth tube, profiles 1 and 2). The same applies to the dwell time of the particles. The spiral-shaped flow distributes the particles more evenly over the cross-section, while the acceleration on the profile flanks reduces the average residence time. The higher pressure loss of the profile 3 results from the peripheral speed. The cause of profile 1 is the strong constriction of the flow and the loss of friction on the large inner surface of the profile.

Depending on the material, the finned tubes according to the invention can be produced, for example, from a centrifugally cast tube by rotating the ends of a tube with ribs parallel to the axis, or by deforming a centrifugally cast tube, for example by hot forging, hot drawing or cold forming, using a profile tool, for example a flying one Mandrel or a mandrel rod with an outer profile corresponding to the inner profile of the tube is generated.

Cutting machines for internally profiling pipes are known in various variants, for example from German patent 195 23 280. These machines are also suitable for producing a finned tube according to the invention.

During hot forming, the forming temperature should be set so that there is a partial destruction of the structural grain in the area of the inner surface and consequently later recrystallization under the influence of the operating temperature. The result of this is a fine-grained structure that leads to a rapid diffusion of chromium, silicon and / or aluminum through the austentic matrix to the inner surface of the tube, where it quickly builds up an oxidic protective layer.

The ribs according to the invention can also be produced by cladding; in this case there can be no curved between the individual ribs Rib base arise, but there the original course of the inner wall of the tube is essentially preserved.

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

Suitable pipe materials for use in ethylene plants are iron or nickel alloys with 0.1% to 0.5% carbon, 20 to 35% chromium, 20 to 70% nickel, up to 3% silicon, up to 1% niobium, to 5% tungsten and additions of hafnium, titanium, rare earths or zirconium, each up to 0.5% and up to 6% aluminum.

Claims

- 1 -patent claims:

1. A process for the thermal splitting of hydrocarbons in the presence of steam, in which the feed mixture is passed through externally heated pipes with helical inner ribs, characterized in that a swirl flow is generated in the immediate vicinity of the ribs and with increasing radial distance from the ribs into a core zone predominantly axial flow is transferred.

2. The method according to claim 1, characterized in that the swirl flow absorbs the swirling at the rib flanks.

3. The method according to claim 1 or 2, characterized in that the peripheral speed of the gas flow in the rib valleys is greater than at the rib tops.

4. The method according to any one of claims 1 to 3, characterized in that the swirl flow at the ribs at an angle of 20 ° to 40 °, preferably from 22.5 to 32.5 °, based on the tube axis.

5. The method according to any one of claims 1 to 4, characterized in that the fluctuation in the inner wall temperature over the tube circumference is below 12 ° C.

6. The method according to any one of claims 1 to 5, characterized in that the isotherms are spiral in the core zone.

7. The method according to any one of claims 1 to 6, characterized in that the speed of the swirl flow builds up within the first 2 to 3 m of the tube length and then remains constant. - 15 -

8. The method according to any one of claims 1 to 7, characterized in that the speed of the swirl flow after the first 2 to 3 m tube length covers the entire cross section.

9. The method according to any one of claims 1 to 8, characterized in that the homogeneity factor of the temperature over the cross section and the homogeneity factor of the temperature based on the hydraulic diameter is above 1 in relation to the homogeneity factors of a smooth tube.

10. The method according to any one of claims 1 to 9, characterized in that the flow rate in the boundary layer on the pipe wall is 8 to 12% lower and the flow rate in the core zone is 8 to 12% greater than in a comparable pipe with straight Ribs of the same quality.

11. The method according to claim 1 to 10, characterized in that the gas is accelerated over a distance of 100 to 200 cm, calculated from the gas inlet, to a peripheral speed which is 15 to 20% of the axial speed in the core zone and that thereafter the peripheral speed remains constant.

12. The method according to claim 1 to 11, characterized in that the sum of the axial and peripheral speed is greater than the axial speed of a comparable tube with straight ribs of the same nature.

13. The method according to claim 1 to 12, characterized in that the gas particles are accelerated on the rib flanks. - 16 -

14. Finned tube with several helical inner ribs, characterized in that the profile circumference (Up) makes up + 5 to - 2% of the enveloping circle touching the rib valleys.

15. finned tube according to claim 14, characterized in that the flank angle of the ribs is 16 ° to 25 °.

16. Finned tube according to claim 14 or 15, characterized in that the pitch angle of the ribs is 20 ° to 40 °

17. Finned tube according to one of claims 14 to 16, characterized in that the ribs and the rib valleys located between the ribs are mirror-symmetrical in cross section.

18. Finned tube according to one of claims 14 to 17, characterized in that the rib tops and the rib valleys each merge.

19. Finned tube according to one of claims 14 to 18, characterized in that the ribs and the rib valleys have the same radius of curvature.

20. Finned tube according to claim 14 or 15, characterized in that the ribs are welded on and the rib valleys lie on a common circle.

21. Finned tube according to one of claims 14 to 20, characterized by a total of six to twelve fins.

22. Finned tube according to one of claims 14 to 21, characterized in that the hydraulic diameter of the finned tube is at least equal to the diameter of the inner circle (Ri). - 17 -

23. Finned tube according to one of claims 14 to 22, characterized in that the ratio of the quotients of the heat transfer coefficients Q _R / Q ₀ to the quotient of the pressure losses ΔP _R / ΔP ₀ in the water test is 1.4 to 1.5, where R is a finned tube and 0 denotes a smooth tube.

24. Finned tube according to one of claims 14 to 23, characterized in that the radius of curvature (R) of the fin cross section is 3.5 to 20 mm.

25. Finned tube according to one of claims 14 to 24, characterized by a fin height (H) of 1.25 to 3 mm.

26. Finned tube according to one of claims 14 to 25, characterized in that the free cross section within the profile circumference (Up) is 85 to 95% of the area of the enveloping circle (Fa).

27. Finned tube according to one of claims 14 to 26, characterized in that the profile area (Fp) is 40 to 50% of the ring area between the enveloping circle and the inner circle.

28. A method for producing a finned tube according to one of claims 14 to 27, characterized in that the ends of a tube with axially parallel fins are rotated against each other.

29. A method for producing a finned tube according to one of claims 14 to 27, characterized in that the inner profile is produced by shaping using a profile tool.

30. The method according to claim 29, characterized in that the structural grain is partially shattered in the region of the inner surface during shaping.

31. A method for producing a finned tube according to one of claims 14 to 27, characterized in that the inner profile is produced by shaping using a profile tool or by build-up welding.

32. A method for producing a centrifugal cast pipe according to claims 14 to 27, characterized in that the inner profile is produced by electrolytic removal.

33. The method according to any one of claims 29 to 32, characterized in that the inner surface of the profile tube is smoothed.

34. Use of a centrifugal cast tube for producing a finned tube according to one of claims 15 to 27.

35. Use according to claim 34, wherein the centrifugal casting tube made of a nickel alloy with 0.1 to 0.5% carbon, 20 to 35% chromium, 20 to 70% nickel, to 3% silicon, to 1% niobium, to 5% tungsten and up to 0.5% hafnium, titanium, rare earth metals, zirconium and up to 6% aluminum.

36. Use according to claim 35, wherein the alloy contains individually or side by side at least 0.02% silicon, 0.1% niobium, 0.3% tungsten and 1.5% aluminum.