EP2298850A1 - Ribbed pipe for thermal catalytic cracking of hydrocarbons - Google Patents

Abstract

In a process to crack crude oil in the presence steam, super-heated gases pass through pipes with helical inner ribs which twist the rising gases, progressively forming a core zone with a primarily axial flow. The helical ribs impart a twist action at their outer margins. The gas speed is faster at the tub roots than at the rib tips. The ribs are set at an angle of 22.5-32.5[deg] w.r.t the pipe axis. The temperature varies within the pipe wall by less than 12[deg]C. The notional isothermal lines in the core are circular. The flow of twisting gases advances in the pipe at a speed of 1.8-2 m/s, representing 7-8% of the free cross sectional area. The ribs and their separation are symmetrical.

Description

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

For the high-temperature pyrolysis of hydrocarbons (petroleum derivatives), tube furnaces have proven in which a hydrocarbon / steam mixture at temperatures above 750 ° C by rows of single or meandering tubes (cracking tubes) made of heat-resistant chromium-nickel steel alloys with high oxidation and scale resistance and high carburization resistance. The coils are made of vertically extending straight pipe sections, which are connected to each other via U-shaped pipe bend or arranged parallel to each other; They are usually heated by means of sidewall and partly also with the aid of bottom burners and therefore have a so-called sun side facing the burners and a 90 ° offset, ie in the direction of the rows of tubes, the so-called shadow side. TMT) partly over 1000 ° C.

The lifetime of the cracking pipes depends very much on the creep resistance and the Aufkohiungsdauerurcheltkelt and the coking rate of the pipe material. Decisive for the rate of coking, that is for the growth of a layer of carbon deposits (pyrolysis) on the pipe inner wall are, in addition to the type of hydrocarbons used, the gap gas temperature in the inner wall and the so-called Crackschärfe, behind the influence of the system pressure and the residence time in the pipe system hides on the Äthylenausbeute. The gap sharpness is determined by the mean outlet temperature of the cracked gases (eg 850 ° C). set. The higher the gas temperature in the vicinity of the tube inner wall is above this temperature, the stronger grows the layer of pyrolysis coke whose insulating effect further increases the tube wall temperature. Although the chromium-nickel steel alloys used as pipe material with 0.4% carbon over 25% chromium and over 20% nickel, for example 35% chromium, 45% nickel and optionally 1% niobium have a high carburization resistance, the carbon diffuses Defects of the oxide layer in the pipe wall and leads there to a considerable carburizing, 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 significant embrittlement of the pipe material with the risk of cracking during thermal cycling especially when starting and stopping the furnace.

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

It is known from the British patent specification 969,796 also the use of cracking tubes with internal ribs. Although such inner ribs provide a many percent, for example, 10% larger inner surface and consequently a better heat transfer; but they are also associated with the disadvantage of a significantly increased pressure loss due to friction on the enlarged tube inner surface compared to a smooth tube. The higher pressure loss requires a higher system pressure, this inevitably changes the residence time and deteriorates the yield. In addition, the known pipe materials with high contents of carbon and chromium 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 meant that the desired in terms of Äthylenausbeute high pipe wall temperatures of, for example to 1050 ° C require the use of centrifugally cast tubes. However, since centrifugally cast tubes can only be produced with a cylindrical wall, special shaping processes are required, for example an electrolytically removing machining or a shaping welding process, in order to produce internal finned tubes.

Against this background, the invention is based on the problem of improving the efficiency of the thermal cracking of hydrocarbons in tubular ovens with externally heated tubes with helical inner ribs.

The solution to this problem consists in a method in which in the immediate vicinity of the ribs preferably a centrifugally cast tube generates a swirl flow and is transferred with increasing radial distance from the ribs in a core zone predominantly axial flow. 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 swirling off at the rib flanks, so that there is no local recycling of the swirls in the manner of a self-contained circular flow into the rib valleys. Despite the obviously longer paths of the particles through the spiral paths, the average residence time is lower than in the smooth tube and also more homogeneous over the cross section (see. Fig. 7 ). This is confirmed by the higher total speed in the profile tube with swirl (profile 3) compared to the tube with straight ribs (profile 2). This is particularly ensured when the swirl flow in the region of the ribs or the ribs at an angle of 20 "to 40 °, for example, 30 °, preferably 25 ° to 32.5 ° relative to the tube axis.

In the method according to the invention is over the pipe circumference between the sun and shadow side inevitably different heat supply balanced in the pipe wall and in the Rohrinnem while the heat dissipated quickly inward to the core zone. Associated with this is a reduction in the risk of local overheating of the process gas on the pipe wall and the resulting formation of pyrolysis coke. In addition, the thermal stress of the pipe material due to the temperature compensation between the sun and shadow side is lower, which leads to an extension of the life. Finally, it comes in the process of the invention also to a homogenization of the temperature over the pipe cross-section, resulting in a better Olefinausbeute. The reason for this is that without the radial temperature compensation according to the invention inside the tube on the hot tube wall there would be an overcracking and in the middle of the tube a recombination of fission products.

Furthermore, in the case of the smooth tube and reinforced in rib profiles with ribs of more than 5%, for example 10%, enlarged inner circumference, a layer of laminar flow characteristic of turbulent flows forms with greatly reduced heat transfer. It leads to increased formation of pyrolysis coke with also poor thermal conductivity. Both layers together require a higher heat input or a higher burner output. This increases the tube wall temperature (TMT) and consequently shortens the life.

The invention avoids this by the fact that the inner circumference of the profile by a maximum of 5%, for example 4% or 3.5%, based on the circumference of the Rippentäler touching enveloping circle. However, the inner circumference can also be up to 2% smaller than the outer circle. In other words, the relative profile perimeter is at most 1.05 to 0.98% of the enveloping circle perimeter. Accordingly, the surface difference of the profile tube according to the invention, ie its unwound inner surface, based on a smooth tube with the outer circle diameter at most + 5% to - 2% or 1.05 to 0.98 times the smooth tube surface.

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 shows a comparison of two pipes with the same hydraulic diameter and accordingly the same pressure loss and the same thermal performance result.

A further advantage of the profile circumference (relative profile circumference) according to the invention, which is based on the enveloping circle circumference, consists in a more rapid heating of the feed gas at a reduced tube wall temperature.

The swirl flow according to the invention considerably reduces the laminar layer; it is also connected to a pipe center directed velocity vector, which reduces the residence time of cracking radicals or fission products on the hot tube wall and their chemical and catalytic conversion to pyrolysis coke. In addition, the not inconsiderable in inner profile tubes with high ribs temperature differences between Rippentälem and ribs are compensated by the swirl flow according to the invention. This increases the time interval between two necessary decoking. Without the swirl flow according to the invention, a not insignificant temperature difference results between the ridge crests and the bottom of the ridge valleys. The residence time of the fouling-prone fission products is shorter in the case of spiral-shaped internal fins; In individual cases, this depends on the nature of the ribs.

The diagram shows: upper curve: Profile 6: 16 ° slope middle curve: Profile 3: 30 ° incline lower curve: Profile 4: 3 ribs with 30 ° slope

The curve clearly shows that the higher peripheral speed of the profile 6 is consumed with 4.8 mm high ribs within the ridge valleys, while the peripheral speed of the inventive profile with a rib height of only 2 mm penetrates into the core of the flow. Although the peripheral speed of the profile 4 with only 3 ribs is approximately as high, but causes no spiral acceleration of the core flow.

The profile of the invention causes according to the curve in the diagram of Fig. 2 a spiral acceleration in the Rippentälern (upper curve branch), which covers wide areas of the pipe cross-section and thus causes a homogenization of the temperature in the pipe. The lower peripheral speed at the rib caps (lower curve branch) also ensures that there is no turbulence and backflow.

In Fig. 3 three test tubes are shown with their data in cross section, including the profile of the invention 3. The diagrams show the temperature profile over the pipe radius (radius) on the shadow and the sunny side again. A comparison of the diagrams shows the lower temperature difference between the pipe wall and center and the lower gas temperature at the pipe wall in 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 pipe, ie between the sun and shade side is below 12 ° C, although the usually arranged in parallel rows of pipe coils of a tube furnace with the help of Seitenwandbrennem heated only on opposite sides or with combustion gases be acted upon and the tubes thus each have a Brennem facing sun side and a 90 ° offset to the dark side. The mean tube wall temperature, ie the difference in the tube wall temperature between the sun and shadow sides leads to internal stresses and therefore determines the service life of the tubes. So the results from the diagram of the Fig. 4 apparent reduction of the mean Tube wall temperature of a pipe according to the invention with eight fins of 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 Rippentälem and -skirts of 2 mm of 11 ° compared to a diameter equal smooth tube, based on an average service life of 5 years, at an operating temperature of 1050 ° C a calculated lifespan increase to about 8 years.

The temperature distribution between sun and shadow side for the three profiles of Figure 3 follows from the diagram of Fig. 5 , Noteworthy here is the lower level of the temperature curve for the profile 3 compared to the smooth tube (profile 0) and the significantly lower fluctuation range of the profile 3 curve compared to the profile 1 curve.

A particularly favorable temperature distribution arises when the isotherms of the tube inner wall to the core of the flow are spiral.

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

The process according to the invention should be operated with a view to a high Olefinausbeute with comparatively short tube length so that the homogeneity _{factor of} the temperature above the cross section and based on the hydraulic diameter homogeneity _{factor of} the temperature in relation to the homogeneity _{factor of} a smooth tube (H _GØ ) is _greater than 1. The homogeneity factors are defined as follows: $${\mathrm{H}}_{\mathrm{GO}}\left[\mathrm{-}\right]{\mathrm{H}}_{\mathrm{PØ}}\mathrm{=}\mathrm{\Delta }{\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">T</figure-callout>}}_{\mathrm{0}}\mathrm{\cdot }{\mathrm{d}}_x\mathrm{/}\mathrm{\Delta }{\mathrm{t}}_x\mathrm{\cdot }{\mathrm{d}}_{\mathrm{O}}$$

The flow pattern according to the invention of core and spin flow can be achieved with a finned tube in which the flank angle of each of the Length of a piece of pipe continuous ribs, that is, the outer angle between the rib edges and the radius of the tube 16 ° to 25 °, preferably 19 ° to 21 °. Such a flank angle ensures, in particular in conjunction with a rib inclination of 20 ° to 40 °, for example 22.5 ° to 32.5 °, that in the Rippentälem not a more or less self-contained, behind the rib flanks in the Rippentäler returning vortex flow results in the Rippentälem to the emergence of unwanted "twisters", that is, of closed vortex pigtails leads. Rather, the resulting in the Rippentälem vortices detach from the rib edges and are absorbed by the swirl flow. The swirl energy induced by the ribs accelerates the gas particles and leads to a higher overall velocity. This leads to a reduction and homogenization of the tube wall temperature and to a homogenization of the temperature and the residence time over the pipe cross-section.

• hydraulischer Durchmesser Dh in mm, Ri ≤ Dh / 2
• Flankenwinkel β
• Rippenhöhe H
• Hüllkreis-Radius Ra=RI+H und Da=2 x Ra
• Zentrumswinkel α
• Krümmungsradius R = Ra (sin α / 2 sin β+ sin α)
• Hüllkreis-Umfang 2 Π Ra
• Winkel im schiefwinkligen Dreieck γ = 180 - (α + β)
• Innen-Radius RI = 2R (sin γ / sin α) - R
• Rippenhöhe H = Ra - RI
• Profil-Umfang U_p = 2 x Rippenzahl x πR /180 (2 β + α)
• Rippenfläche F_R
• Fläche des Hüllkreises Fa = π Da² / 4
• Fläche des Innenkreises F_I = Π · DI
• Profilfläche innerhalb des Hüllkreises F_P = F_R · Rippenzahl
• Profil-Umfang U_P = (1,05 bis 0,98) · 2 Π Ra

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 ie in mm, Ri ≤ Dh / 2
• Flank angle β
• Rib height H
• Envelope radius Ra = RI + H and Da = 2 x Ra
• Center angle α
• Radius of curvature R = Ra (sin α / 2 sin β + sin α )
• Enveloping circle perimeter 2 Π Ra
• Angle in the oblique triangle γ = 180 - ( α + β )
• Internal radius RI = 2R (sin γ / sin α ) - R
• Rib height H = Ra - RI
• Profile perimeter U _p = 2 x fin number x π R / 180 (2 β + α )
• Rib surface F _R
• Area of the enveloping circle Fa = π Da ^2/4
• Area of the inner circle F _I = Π · DI
• Profile surface within the enveloping circle F _P = F _R · number of ribs
• Profile circumference U _P = (1.05 to 0.98) · 2 Π Ra

The ribs and the ridge valleys located between the ribs can be mirror-symmetrical in cross-section and adjoin one another or form a wavy line, each with the same radii of curvature. 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 of the ratio 4 x free cross section / profile circumference is equal to or greater than the inner circle of the profile. The hydraulic diameter is therefore in the inner third of the profile height. Thus, 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 action of the profile.

Between the ribs or in the Rippentälem results in a greater flow velocity ( Fig. 2 ), which leads to a self-cleaning effect, therefore less deposits of pyrolysis coke.

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

Experiments have shown that - regardless of the inner diameter of the tubes - a total of 8 to 12 ribs are sufficient to achieve the flow pattern of the invention.

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 / ΔP ₀ in the water test using the laws of similarity and using the mediated for a naphtha / steam mixture Reynolds numbers, preferably 1.4 to 1.5, where R denotes a finned tube and 0 denotes 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 ribs (profile 1), in which the radial distance between the Rippentälern and the rib caps is 4.8 mm illustrate the data of the following Table. The finned tubes all had 8 ribs and the same envelope circle. PROFILE 0 1 3 Fluidtemp. at 9950 mm in the middle T _m [° C] 843.6 848.1 843.0 Fluidtemp. at 9950 mm at the edge T _r [° C] 888.9 894 874.8 Temperature range at 9950 mm ΔT = T _r -T _m [° C] 45.3 45.9 31.8 Homogeneity factor for smooth tube has H _t = ΔT ₀ / ΔT _x 1 0.9869281 1.4245283 Hydr. Diameter d _n [m] 0.0380 0.0256 0.0344 relative homogeneity factor with respect to hydr. Ø to straight tube H _tø : H _tø = ΔT ₀ • d _x / ΔT _x • d ₀ 1 0.8477193 1.3420556 Rank H: 2 2 1

er entspricht vorzugsweise dem Innendurchmesser eines vergleichbaren Glattrohrs und ergibt dann einen Homogenitätsfaktor von 1 ,425.The hydraulic diameter is defined as follows: $${\mathrm{D}}_{\mathrm{hydr}}\mathrm{=}\mathrm{4}\mathrm{\times }\left(\mathrm{free\; cross\; section}\right)\mathrm{/}\mathrm{inner\; circumference};$$

it preferably corresponds to the inside diameter of a comparable smooth tube and then gives a homogeneity factor of 1.42.

The finned tube according to the invention gives in the water test a higher by a factor of 2.56 heat transfer (Q _R ) compared to the plain tube with only a factor of 1.76 increased pressure drop (ΔP _R ).

In Fig. 7 are a tube with a smooth inner wall (smooth tube) faced three different profile tubes, including a tube according to the invention with 8 ribs with a slope of 30 °. For each cross-section the hydraulic diameter, the axial velocity, the residence time and the pressure loss are indicated.

Output data were the flow rates of a 38 mm internal diameter smooth tube in use, which is identical to the hydraulic diameter. These data were converted to warm water according to the similarity laws (same Reynolds numbers) and based on the experiments (see ratio of heat transfer and pressure drop ratios for tests with water and the related homogeneity factor in the calculation with gases).

The different speed profiles result from equal throughputs at different hydraulic diameters (reciprocal ratio).

The comparison of the speeds in the profiles 2 and 3, which are identical in cross-section, illustrates the better speed, acceleration and residence time in the tubes according to the invention (profile 3). At the same hydraulic diameter, the circumferential velocity component caused by the spin of the ribs causes the flow from the tube wall to detach and a helical rising velocity throughout the cross section.

By the directed, spiral flow, the heat from the pipe wall is introduced into the flow and thus more evenly distributed than in a normal undirected turbulent flow (smooth tube, profiles 1 and 2). The same applies to the residence time of the particles. The spiraling flow distributes the particles more evenly across the cross section while the acceleration at the flanks reduces the average residence time. The higher pressure loss of the profile 3 results from the peripheral speed. In profile 1, the cause 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 twisting the ends of a tube with axially parallel ribs against one another, or by deforming the inner profile by forming a centrifugally cast tube, for example by hot forging. Hot drawing or cold forming over a profile tool, for example, a flying dome or a mandrel with an inner profile of the tube corresponding outer profile is generated.

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

When hot forming solite the forming temperature should be adjusted so that it comes in the region of the inner surface to a partial destruction of the grain structure and therefore later under the influence of the operating temperature to a recrystallization. The result of this is a fine-grained microstructure, which leads to a rapid diffusion of chromium, silicon and / or aluminum through the austenitic matrix to the inner surface of the tube and there for the rapid construction of an oxide protective layer.

The ribs according to the invention can also be produced by build-up welding; in this case, between the individual ribs no curved Ridge bottom arise, but it remains there the original course of the inner wall of the tube is substantially preserved.

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

As pipe material suitable for use in ethylene plants 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, up to 5% tungsten and hafnium additions. Titanium, rare earth, or zirconium, each containing up to 0.5% and up to 6% aluminum.

Claims (15)

Finned tube for thermal cracking of hydrocarbons in the presence of steam, characterized by at an angle of 20 ° to 40 °, relative to the tube axis inclined inner ribs with a flank angle of 16 ° to 25 °. A finned tube according to claim 1, characterized in that the ribs are inclined at 22.5 ° to 32.5 ° with respect to the tube axis. A finned tube according to claim 1 or 2, characterized in that the profile circumference (Up) + makes up 5 to - 2% of the enveloping circle touching the ridge valleys. Finned tube according to one of claims 1 to 3, characterized in that the flank angle (β) of the ribs is 16 to 20 °. Finned tube according to one of claims 1 to 4, characterized in that the ribs and the ridge valleys located between the ribs are of mirror-symmetrical design in cross-section. A finned tube according to any one of claims 1 to 5, characterized in that the ridge crests and the ridge valleys merge into each other. A finned tube according to any one of claims 1 to 6, characterized in that the ribs and the ridge valleys have the same radius of curvature (R). Finned tube according to one of claims 1 to 7, characterized by a total of six to twelve ribs. 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 (Ri). Finned tube according to one of claims 1 to 9, 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 denotes a finned tube and 0 denotes a smooth tube. Finned tube according to one of claims 1 to 10, characterized in that the radius of curvature (R) of the rib cross-section is 3.5 to 20 mm. Finned tube according to one of claims 1 to 11, characterized by a rib height (H) of 1.25 to 3 mm. Finned tube according to one of claims 1 to 12, characterized in that the free cross section within the profile circumference (Up) amounts to 85 to 95% of the area of the enveloping circle (Fa). Finned tube according to one of claims 1 to 13, characterized in that the profile surface (Fp) amounts to 40 to 50% of the annular surface between the enveloping circle and the inner circle. A finned tube according to any one of claims 1 to 14 made by centrifugal casting of a nickel alloy having 0.1 to 0.5% carbon, 20 to 35% chromium, 20 to 70% nickel, up to 3% silicon, up to 1% niobium, up to 5% tungsten and each up to 0.5% hafnium, titanium, rare earth metals, zirconium and up to 6% aluminum, balance iron.

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