Method and ribbed tube for thermally cleaving hydrocarbons

0001The invention relates to 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.

0002Tube 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 pipe coils consist of vertically running straight pipe sections which are connected to one another via U-shaped pipe bends or are 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.

0003The service life of the cracking tubes depends very much on the creep resistance and the carburization resistance as well as on the coking speed of the tube material. Decisive for the coking rate, 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 inner wall area and the so-called cracking severity, behind which the influence of the system pressure and the dwell 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). The higher the gas temperature in the vicinity of the inner tube wall above this temperature, the more the layer of pyrolysis coke grows, the insulating effect of which further increases the tube 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 due to thermal alternating stress, particularly when the furnace is started up and shut down.

0004In 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.

0005Is known from the British patent <patcit id="pcit0001" dnum="GB969796A"><text>969 796</text></patcit> and the European disclosure document <patcit id="pcit0002" dnum="EP1136541A1"><text>1 136 541 A1</text></patcit> also the use of crack tubes with inner fins. Such inner fins result in 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 considerably increased pressure loss compared to a smooth pipe due to friction on the enlarged pipe inner surface. The higher pressure loss requires a higher system pressure, which inevitably changes the dwell time and worsens the yield. In addition, the known pipe 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., which are desired with regard to the ethylene yield, require the use of centrifugally cast tubes. 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.

0006It is known from the <patcit id="pcit0003" dnum="US5950718A"><text>U.S. Patent 5,950,718</text></patcit> also a whole range of angles of inclination and also distances between the inner ribs, without taking into account the nature of the ribs.

0007Against this background, the object of the invention is to improve the economy of the thermal splitting of hydrocarbons in tube furnaces with externally heated tubes with helical inner fins.

0008The solution to the problem consists in a finned tube according to claim 1.

0009In the finned tube according to the invention, a swirl flow takes up at the vortices detaching the fins, so that there is no local return of the vortices in the manner of a self-contained circular flow into the rip valleys. Despite the obviously longer path of the particles through the spiral paths, the mean dwell time is lower than in the smooth tube and also more homogeneous across the cross-section (cf.<figref idref="f0007">Fig. 7</figref>). 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 if the ribs run at an angle of preferably 25 ° to 32.5 ° with respect to the pipe axis.

0010In the finned tube according to the invention, the inevitably different heat supply in the tube wall and in the tube interior is compensated for over the tube circumference between the sun and shadow sides, and the heat is rapidly dissipated inwards to the core zone. This is associated with a reduction in the risk of local overheating of the process gas on the pipe 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 finned tube according to the invention, the temperature is also made more uniform over the tube 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 middle of the pipe a recombination of fission products would occur.

0011Furthermore, 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 which is characteristic of turbulent flows and has a greatly reduced heat transfer 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.

0012The invention avoids this in that the inner circumference of the profile amounts to a maximum of 5%, for example 4% or also 3.5%, based on the circumference of the enveloping circle touching the rib valleys. In other words: the relative profile circumference is a maximum of 1.05 of the envelope circumference. Accordingly, the surface difference of the profiled tube according to the invention, ie its unwound inner surface, is a maximum of + 5%, based on a smooth tube with the enveloping circle diameter or 1.05 times the smooth tube surface.

0013The tubular 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.

0014Another advantage of the profile circumference (relative profile circumference) based on the circumference of the enveloping circle is that the feed gas heats up more quickly at a reduced tube wall temperature.

0015The swirl flow generated 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 dwell 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.

0016The diagram shows:<tables id="tabl0001" num="0001"><table frame="none"><tgroup cols="3" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26mm" /><colspec colnum="2" colname="col2" colwidth="16mm" /><colspec colnum="3" colname="col3" colwidth="41mm" /><tbody><row><entry>upper curve:</entry><entry>Profile 6:</entry><entry>16 ° slope</entry></row><row><entry>middle curve:</entry><entry>Profile 3:</entry><entry>30 ° slope</entry></row><row><entry>lower curve:</entry><entry>Profile 4:</entry><entry>3rd Ribs with a 30 ° slope</entry></row></tbody></tgroup></table></tables>

0017The course of the curve clearly shows that the higher peripheral speed of the profile 8 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.

0018The profile according to the invention brings about the curve in the diagram of FIG <figref idref="f0002">Fig. 2</figref> a spiral acceleration in the rib valleys (upper curve branch), which covers a wide area of ​​the pipe cross-section and thus causes 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.

0019In <figref idref="f0003">Fig. 3</figref> Three test tubes are shown 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.

0020The swirl flow generated according to the invention ensures that the fluctuation in the inner wall temperature over the circumference of the tube, that is to say 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 only heated or charged with combustion gases on rare sides using rare wall burners, and the tubes 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. So that results from the diagram of the<figref idref="f0004">Fig. 4</figref> apparent reduction in the mean tube wall temperature of a tube according to the invention with eight fins with a pitch of 30 °, an inner tube diameter of 38.8 mm and an outer tube diameter of 50.8 mm, thus a height difference between fin valleys and fin tips of 11 mm compared to one Diameter smooth tube, based on an average lifespan of 5 years, at an operating temperature of 1050 ° C a calculated lifespan increase to about 8 years.

0021The temperature distribution between the sun and shadow side for the three profiles of the <figref idref="f0003">Fig. 3</figref> results from the diagram of the <figref idref="f0005">Fig. 5</figref>. The lower level of the temperature curve for profile 3 compared to the smooth tube (profile 0) and the considerably smaller fluctuation range of the profile 3 curve compared to the profile 1 curve is remarkable.

0022A particularly favorable temperature distribution arises when the isotherms run from the inner tube wall to the core of the flow in a spiral.

0023A 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.

0024In view of a high olefin yield with a comparatively short tube length, the process 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 of the temperature in relation to the hydraulic diameter in relation to the homogeneity factor of a smooth tube (H_GØ) is over 1. The homogeneity factors are defined as follows:<maths id="math0001"><math display="block"><msub><mi mathvariant="normal">H</mi><mi mathvariant="normal">G∅</mi></msub><mfenced open="[" close="]"><mo mathvariant="normal">-</mo></mfenced><mo>⁢</mo><msub><mi mathvariant="normal">H</mi><mi mathvariant="normal">P∅</mi></msub><mo mathvariant="normal">=</mo><mi mathvariant="normal">Δ</mi><mspace width="1em" /><msub><mi mathvariant="normal">T</mi><mn mathvariant="normal">0</mn></msub><mo>•</mo><msub><mi mathvariant="normal">d</mi><mi mathvariant="normal">x</mi></msub><mo mathvariant="normal">/</mo><msub><mi mathvariant="normal">ΔT</mi><mi mathvariant="normal">x</mi></msub><mo>•</mo><msub><mi mathvariant="normal">d</mi><mi mathvariant="normal">O</mi></msub></math><img file="EP1525289B9_D0001.tif" /></maths>

0025The flow pattern of core and swirl flow generated according to the invention can be achieved with a finned tube in which the flank angle of the fins passing through the length of a tube piece in each case, i.e. the outside angle between the finned flanks and the radius of the tube, is 16 ° to 25 °, preferably 19th ° to 21 ° Such a flank angle ensures in conjunction with a rib pitch of 20 ° to 40 °, for example 22.5 ° to 32.5 °, that there is no more or less self-contained vortex flow returning behind the rib flanks in the rib valleys, 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.

0026The nature of the finned tube according to the invention results from the representation of a tube segment in <figref idref="f0006">Fig. 6</figref> and the associated characteristic parameters<ul id="ul0001" list-style="dash" compact="compact"><li>hydraulic diameter Dh in mm, <b>RI ≤ Dh / 2</b></li><li>Flank angle β</li><li>Rib height <b>H</b></li><li>Envelope circle radius <b>Ra = Rl + H and Da = 2 x Ra</b></li><li>Center angle α</li><li>Radius of curvature <b>R = Ra (sin</b> α <b>/ 2 sin</b> β<b>+ sin</b> α<b>)</b></li><li>Envelope circumference <b>2</b> Π <b>Ra</b></li><li>Angle in the oblique triangle γ <b>= 180</b> - <b>(</b>α <b>+</b> β<b>)</b></li><li>Inside radius <b>Ri = 2R (sin</b> γ <b>/ sin</b> α<b>) - R</b></li><li>Rib height <b>H = Ra - Ri</b></li><li>Profile scope <b>U_p = 2 x number of ribs x nR / 180 (2nd</b> β <b>+</b> α<b>)</b></li><li>Rib surface F_R</li><li>Area of ​​the enveloping circle <b>Fa =</b> π <b>There² / 4</b></li><li>Area of ​​the inner circle F_l = Π • DI</li><li>Profile area within the envelope circle F_P = F_R • Number of ribs</li><li>Profile range Up = maximum 1.05 • 2 π Ra</li></ul>

0027The ribs and the rib valleys located between the ribs are mirror-symmetrical in cross section and 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.

0028There is a greater flow velocity between the ribs or in the rib valleys (<figref idref="f0002">Fig. 2</figref>), which leads to a self-cleaning effect, therefore less pyrolysis coke deposits.

0029Tests have shown that - regardless of the inside diameter of the tubes - a total of 8 to 12 fins are sufficient to achieve the flow pattern according to the invention.

0030In the finned tube according to the invention, the ratio of the quotients of the heat transfer coefficients is Q_R / Q₀ to the quotient of the pressure losses ΔP_R/ ΔP₀ in the water test using and observing the laws of similarity and using the Reynolds numbers mediated for a naphtha / water vapor mixture, preferably 1.4 to 1.5, where R denotes a finned tube and 0 a smooth tube.

0031The 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, is illustrated by the data of the following Table. The finned tubes all had 8 fins and the same enveloping circle.<tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="111mm" /><colspec colnum="2" colname="col2" colwidth="15mm" /><colspec colnum="3" colname="col3" colwidth="20mm" /><colspec colnum="4" colname="col4" colwidth="20mm" /><thead><row><entry valign="top">PROFILE</entry><entry align="center" valign="top">0</entry><entry align="center" valign="top">1</entry><entry align="center" valign="top">3</entry></row></thead><tbody><row><entry>Fluid temp. at 9950 mm in the middle T_m[° C]</entry><entry align="center">843,6</entry><entry align="center">848,1</entry><entry align="center">843,0</entry></row><row><entry>Fluid temp. at 9950 mm at the edge T_r[° C]</entry><entry align="center">888,9</entry><entry align="center">894</entry><entry align="center">874,8</entry></row><row><entry>Temperature range at 9950 mm ΔT = T_r-T_m[° C]</entry><entry align="center">45,3</entry><entry align="center">45,9</entry><entry align="center">31,8</entry></row><row><entry>Hormogenicity factor for smooth tube H_t = ΔT_q / ΔT_x</entry><entry align="center">1</entry><entry align="center">0,9869281</entry><entry align="center">1,4245283</entry></row><row><entry>Hydr. Diameter d_H [m]</entry><entry align="center">0,0380</entry><entry align="center">0,0258</entry><entry align="center">0,0344</entry></row><row><entry>related homogeneity factor with respect to hydr. Ø to smooth pipe H_tØ: H_tØ = ΔT₀ · D_x/ ΔTx · d_O</entry><entry align="center">1</entry><entry align="center">0,8477193</entry><entry align="center">1,3420558</entry></row><row><entry>Rank H:</entry><entry align="center">2</entry><entry align="center">2</entry><entry align="center">1</entry></row></tbody></tgroup></table></tables>

0032The hydraulic diameter is defined as follows: <maths id="math0002"><math display="block"><msub><mi mathvariant="normal">D</mi><mi>hydr</mi></msub><mo mathvariant="normal">=</mo><mn mathvariant="normal">4</mn><mspace width="1em" /><mi mathvariant="normal">x</mi><mspace width="1em" /><mfenced><mi>freler cross section</mi></mfenced><mo mathvariant="normal">/</mo><mi>Inner circumference</mi><mo mathvariant="normal">;</mo></math><img file="EP1525289B9_D0002.tif" /></maths>it preferably corresponds to the inside diameter of a comparable smooth tube and then gives a homogeneity factor of 1.425.

0033The finned tube according to the invention results in a heat transfer by the factor 2.56 higher (Q_R) compared to the smooth pipe with a pressure loss increased only by a factor of 1.76 (ΔP_R).

0034In <figref idref="f0007">Fig. 7</figref> are compared to a tube with a smooth inner wall (smooth tube) three different profile tubes, including a tube according to the invention with 8 fins with a gradient of 30 ° each. The hydraulic diameter, the axial speed, the dwell time and the pressure loss are given for each cross-section.

0035The 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 quotient and the pressure loss for tests with water and the related homogeneity factor when calculating with gases).

0036The different speed profiles result from the same throughputs with different hydraulic diameters (reciprocal ratio).

0037The 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.

0038The directional, spiral flow transfers the heat from the pipe wall into the flow and thus distributes it more evenly than in a normal undirected turbulent flow (smooth pipe, profiles 1 and 2). The same applies to the dwell time of the particles. The spiral flow distributes the particles more evenly across 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 reason for profile 1 is the strong constriction of the flow and the loss of friction on the large inner surface of the profile.

0039Depending on the material, the finned tube according to the invention can be produced, for example, from a centrifugal cast tube by rotating the ends of a tube with ribs parallel to the axis, or by the inner profile being preformed by a centrifugally cast tube, for example by hot forging, hot drawing or cold forming using a profile tool, For example, a flying mandrel or a mandrel rod with an outer profile corresponding to the inner profile of the tube is generated.

0040Cutting machines for internally profiling pipes are available in various versions, for example from the German patent specification <patcit id="pcit0004" dnum="DE19523280"><text>195 23 280</text></patcit> known. These machines are also suitable for producing a finned tube according to the invention.

0041During hot forming, the forming temperature should be set so that there is a partial destruction of the grain in the area of ​​the inner surface and consequently later recrystallization under the influence of the operating temperature.The consequence of this is a fine-grained structure that allows rapid diffusion of chromium, silicon and / or aluminum through the austentic matrix to the inner surface of the tube and there leads to the rapid build-up of an oxidic protective layer.

0042The 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.

0043Suitable 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.