Method and ribbed tube for thermally cleaving hydrocarbons

Description of the invention

The invention relates to a finned tube, particularly for the thermal cracking of hydrocarbons and the use of a centrifugally cast tube to manufacture a finned tube.

There are tube furnaces in which a mixture of hydrocarbons with steam is passed through a series of single or meandering pipes (cracking tube coils) at temperatures above 750 ° C, made of heat-resistant chromium-nickel steel alloys with high resistance to oxidation or scaling. and high carburization resistance, which have proved to be suitable for high temperature pyrolysis of hydrocarbons (petroleum derivatives). Pipe coils consist of vertical, straight pipe sections connected to each other by U-shaped pipe bends or are arranged parallel to each other; they are usually heated by burners arranged in the walls and in some cases also by burners located in the bottom and therefore have a so-called light side facing the burners and a so-called dark side offset by 90 ° from it, ie, it runs towards the rows of pipes. The mean metal temperatures of the tubes (TMT) are in some situations over 1000 ° C.

The service life of cracking pipes is very much dependent on the creep resistance and carburization strength, as well as the coking rate of the pipe material. The critical factor of the coking rate, i.e. the build-up of a layer of carbon deposits (pyrolysis coke) on the inner wall of the pipe is, in addition to the type of hydrocarbons used, the temperature of the cracking gas in the area of ​​the inner wall and the so-called intensity of operation, which hides the influence of the pressure in the installation and the residence time in the pipeline for ethylene production . The intensity of exploitation is determined on the basis of the average outlet temperature of cracked gases (e.g. 850 ° C). The higher the temperature of the gas in the vicinity of the inner pipe wall above this temperature, the more intensive the build-up of the pyrolysis coke layer, and the insulating effect of this layer enables an even greater increase in the temperature of the pipe metal. Although chromium-nickel steels containing 0.4% carbon, more than 25% chromium and more than 20% nickel, for example 35% chromium, 45% nickel and, if applicable, 1% niobium, which are used as pipe material, may high carburization strength, carbon diffuses into the pipe wall by damaging the oxide layer, leading to significant carburization, which can result in a carbon content of 1% to 3% at wall depths of 0.5 to 3 mm. This is associated with a significant increase in the brittleness of the pipe material, with the risk of crack formation in the event of fluctuating thermal loads, especially during start-up and shutdown of the furnace.

In order to crush the carbon deposits (coking) on ​​the inner pipe wall, it is necessary to interrupt the cracking operation from time to time and to burn the pyrolysis coke with a steam-air mixture. This requires the plant to be interrupted for approximately 36 hours and therefore has a significant detrimental effect on the economy of the process.

It is also known from GB 969 796 to use cracking pipes with inner ribs. Although internal ribs of this type increase the internal surface area by a good few percent, e.g. 10%, with a corresponding improvement in heat transfer, their use is also associated with the disadvantage that they cause significantly greater pressure losses compared to the pipe. smooth, due to friction on the enlarged inner surface of the pipe. Higher pressure losses require higher system pressures, which inevitably alters the residence time of the feedstock in the system and adversely affects performance. An additional factor is that the known tube materials with high carbon and chromium content can no longer be profiled by cold working, for example by cold drawing. They have the disadvantage that their formability is significantly reduced as the hot strength increases. This leads to high tube metal temperatures of the order of, for example, 1050 ° C, which is desirable from an ethylene efficiency point of view, necessitating the use of centrifugally cast tubes. However, since centrifugally cast tubes can only be produced when they have a cylindrical wall, special shaping processes are needed, for example material removal by electrowinning or by welding shaping if internally ribbed tubes are to be produced.

With this in mind, the invention is based on the problem of improving the economics of the thermal cracking of hydrocarbons in tubular furnaces with externally heated tubes provided with spiral inner ribs.

A finned tube, especially for the thermal cracking of hydrocarbons in the presence of steam, in which the feed mixture is passed through externally heated tubes, wherein

The pipes have helically extending internal ribs, according to the invention, characterized in that the side surface angle of the ribs is 16 [deg.] To 25 [deg.], The ribs extending at an angle of 20 [deg.] To 40 [deg.] With respect to the pipe axis.

Preferably, the ribs extend at an angle of 22.5 ° to 32.5 ° to the axis of the tube.

Preferably, the perimeter of the Up profile is from +5 to -2% of the circumferential circle contacting the valleys between the ribs.

Preferably, the ribs and the valleys between the ribs are designed to be mirror symmetrical in cross-section.

Preferably, the tops of the ribs and the valleys between the ribs merge into each other.

Preferably, the ribs and the valleys between the ribs have the same radius of curvature.

Preferably, the ribs are layer-welded by welding and the valleys between the ribs lie on a common circle.

Preferably, the finned tube has a total of 6 to 12 ribs.

Preferably, the hydraulic diameter of the finned tube is at least equal to the diameter of the inner circle Ri.

Preferably, the ratio of the heat transfer quotients QR / Qo to the pressure loss quotient ΔPR / ΔPo during the water test is 1.4 to 1.5, where R refers to a finned tube and O to a smooth tube.

Preferably, the radius of curvature R of the cross section of the rib is between 3.5 and 20 mm.

Preferably, the rib height H is 1.25 to 3 mm.

Preferably, the clearance cross-section for the perimeter Up of the profile is from 85 to 95% of the area of ​​the envelope circle Fa.

Preferably, the area of ​​the profile Fp is 40% to 50% of the area of ​​the ring between the envelope circle and the inner circle.

Preferably, the centrifugally cast tube comprises a nickel alloy containing 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 in any case up to 0.5% hafnium, titanium, rare earth elements, zirconium and no more than 6% aluminum.

Preferably, the nickel alloy comprises, individually or in combination with each other, at least 0.02% silicon, 0.1% niobium, 0.3% tungsten and 1.5% aluminum.

Preferably, the finned tube is formed of a centrifugally cast tube, the ends of which with ribs parallel to the axial direction are mutually rotated.

Preferably, the inner profile is produced by deformation with a profiling tool.

Preferably, the inner profile is formed by fusing the welded layers.

Preferably, the internal profile is produced by electrowinning.

Use of a centrifugally cast tube to manufacture the finned tube shown above.

Preferably, the centrifugally cast tube comprises a nickel alloy containing 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 in any case up to 0.5% hafnium, titanium, rare earth elements, zirconium and no more than 6% aluminum.

Preferably, the nickel alloy comprises, individually or in combination with each other, at least 0.02% silicon, 0.1% niobium, 0.3% tungsten, and 1.5% aluminum.

The advantageous effects of the invention regarding the hydrocarbon cracking process are achieved by a finned tube in which, in the immediate vicinity of the fins, a vortex flow is created in the preferably centrifugally cast tube, which is transformed into a central zone with a predominant axial flow at a radially increasing distance from ribs. The transition between the outer zone with the eddy flow and the middle zone with the predominant axial flow is gradual, e.g. parabolic in nature.

In the ribbed pipe according to the invention, the heat input, which of course differs between the light side and the dark side around the circumference of the pipe, is compensated by the pipe wall and the inside of the pipe and the heat is quickly dissipated inside the central zone. This is accompanied by a reduction in the risk of local overheating of the processed gas on the pipe wall, with the consequent formation of pyrolysis coke. In addition, the thermal load on the pipe material is reduced due to the temperature compensation between the light side and the dark side, which extends the service life. Finally, in the method according to the invention, the temperature also becomes more uniform across the pipe,

The result is an increase in the yield of olefin production. The reason for this is that, without the radial temperature compensation according to the invention inside the pipe, a re-cracking process and recombination of the cracking products in the center of the pipe could occur on the hot pipe wall.

In addition, in the case of a smooth pipe, a laminar flow layer is created, which is characterized by turbulent flows, with much less heat transfer, which layer is produced with much greater intensity in the case of rib profiles with an inner circumference, which, due to the existence of ribs, is increased by more than 5%, for example At 10 o'clock%. This laminar flow causes more pyrolysis coke formation, as does poor thermal conductivity. The two layers together require more heat input or more burner capacity. This increases the pipe metal temperature (TMT) and shortens the service life accordingly.

The invention overcomes this problem in that the inner circumference of the profile of the finned tube is up to about 5% or less, such as 4% or even 3.5% of the circumference of the circumferential circle abutting the valleys between the ribs. However, the inner circumference can also be up to 2% smaller than the envelope circle. In other words, the field difference in the profile pipe according to the invention, i.e. it has a ribbed inner surface area of ​​5% to 2% or less, relative to a smooth tube of circumferential diameter, or is increased by a ratio of 1.05 to 0.98 relative to that of the smooth tube.

The profile of the finned tube according to the invention allows the use of a lower tube density (kg / m) compared to a finned tube in which the internal circumference of the profile is at least 10% larger than the circumference of the circumferential circle. This was demonstrated by comparing two pipes with the same hydraulic diameter and for the same pressure losses and the same thermal result.

A further advantage of the circumference of the structure of the ribbed profile according to the invention (relative circumference of the profile) in relation to the circumference of the circumferential circle is the faster heating of the feed gas at a reduced temperature of the pipe metal.

The swirl flow according to the invention significantly reduces the extent of the laminar layer; in addition, it is accompanied by a velocity vector directed towards the center of the tube, which reduces the residence time of the cracked radicals and / or the cracked products on the hot tube wall and their chemical or catalytic decomposition to form pyrolysis coke.

Moreover, the temperature differences between the ribs and the rib tips, which are not insignificant in the case of internally profiled tubes with high ribs, are compensated by the eddy flow according to the invention. This increases the time between two necessary coke removal operations. Without the swirling flow according to the invention, there are non-insignificant temperature differences between the rib tips and the bottom of the rib valleys. The residence time of the cracked products, which tend to coke, is reduced in the case of cracking pipes provided with ribs in the individual circuits.

The solution constituting the subject of the invention in the examples of embodiments is illustrated in the drawing, in which Fig. 1 shows a diagram of circumferential velocities on ribbed profiles 3, 4, 6 with different inclinations in relation to the pipe axis, Fig. 2 - distribution of circumferential speeds in the profile 3 of the ribbed tube, Fig. 3 - shapes of the profiles of the finned tube with the fluid temperature profile, respectively, Fig. 4 - comparison of the metal temperature of the tube around the circumference of the tube, from the light to the dark side of the tube, Fig. 5 - temperature distribution on the inner wall of the pipe, on the circumference of the pipe from the light to the dark side of the pipe, Fig. 6 - segment of the finned tube profile to define the mathematical angle β of the side surface and Fig. 7 comparison of the flow velocity, residence time and pressure loss for the same mass flow after traveling a length of 9,950 mm of a heated pipe for various profiles of finned pipes with a smooth pipe.

In the figure, fig. 1 shows:

Profile 6: 16 ° pitch Profile 3: 30 ° pitch Profile 4: 30 ° ribs

These curves clearly show that the higher circumferential speed for the profile 6 with 4.8 mm high fins is consumed in the valleys between the ribs, while the circumferential speed for the profile of the finned tube according to the invention with 2 mm high fins goes to the flow center. Although the circumferential velocity for the profile 4 with only 3 ribs is approximately as high, it does not contribute to any helical acceleration of the flow in the center.

The upper curve showing: The middle curve showing: The lower curve showing:

PL 204 769 B1

In the curves shown in Figure 2, the profile of the finned tube according to the invention influences the helical acceleration in the rib valleys (upper branch of the curve), which covers wide cross-sectional areas of the tube and is therefore responsible for equalizing the temperature in the tube. The lower circumferential velocity at the rib tips (lower branch of the curve) further ensures that turbulent and backward flows do not occur.

Figure 3 shows three test tubes with their data in cross section; these pipes have a profile 3 according to the invention. Each drawing shows the temperature profile along the radius of the tube on the dark and light sides. The comparison of the diagrams shows a smaller temperature difference between the pipe wall and the center of the pipe and a lower temperature difference on the pipe wall in the case of the profile 3 according to the invention.

The swirling flow in the pipe according to the invention ensures that temperature fluctuations on the inner wall along the circumference of the pipe, i.e. between the light side and the dark side, are less than 12 ° C, even when the tubular coils, which are specifically arranged in parallel rows, are heated in the tubular furnace or are acted upon by the flue gases by burners in the side walls, on opposite sides only and therefore each tube has a light side facing the burners and a dark side which is 90 ° offset with respect to it. The average temperature of the pipe metal, i.e. the difference between the temperature of the metal of the pipe on the light side and on the dark side causes internal stresses and therefore determines the service life of the pipes. Therefore, the reduction of the average pipe metal temperature in the pipes according to the invention with eight fins with a pitch of 30 °, with an internal pipe diameter of 38.8 mm and an external pipe diameter of 50.8 mm, i.e. with a difference in height between the valleys between the ribs and the tops of the ribs of 2 mm, by 11 ° compared to smooth pipes of the same diameter, assuming an average service life of 5 years, as can be seen in the diagram shown in Fig. 4, results at operating temperature 1050 ° C, a calculated increase in service life to approximately 8 years.

The temperature distribution between the light side and the dark side for these three profiles shown in Fig. 3 can be found in the graph shown in Fig. 5. There is a noticeable lower level of the temperature curve for profile 3 compared to smooth pipe (profile 0) and a much narrower fluctuation range. for the curve for profile 3 compared to the curve for profile 1.

A particularly suitable temperature distribution arises when isotherms spiral from the inner wall of the tube to the center of flow.

A more uniform temperature distribution across the cross-section occurs, in particular, when the peripheral speed increases over a distance of 2 to 3 m and then remains constant over the entire length of the pipe.

With a view to obtaining high efficiency of olefins on a relatively short section of the finned pipe according to the invention, the cracking process should be controlled in such a way that the temperature homogeneity coefficient in the cross-section and the temperature homogeneity coefficient reduced to the hydraulic diameter is greater than 1 in relation to the homogeneity coefficient for a smooth pipe. (HGo). In this aspect, the homogeneity factors are defined as follows:

Hg0^{[-] (H}After) = ^ΔΤ0 ^dx^{/ ΔΤ}χ ^d0

The flow configuration of the finned tube according to the invention, which includes center flow and swirl flow, can be achieved with a finned tube in which the side surface angle of the fins, which in any case are continuous along the length of the tube cross-section, i.e. the outer angle between the surfaces side ribs and the radius of the tube is 16 ° to 25 °, preferably 19 ° to 21 °. The flank angle of this type, especially in combination with a rib pitch of 20 ° to 40 °, such as 22.5 ° to 32.5 °, ensures that in the rib valleys there is a more or less continuous eddy flow which returns to the valleys between the ribs behind the rib flanks and causes undesirable "vortices" to form in the valleys between the ribs. Instead, the turbulent flow created in the valleys between the fins detaches from the side surfaces of the ribs and is absorbed by the eddy flow. The vortex flow energy excited by the fins accelerates the gas particles and increases the total velocity. This reduces the temperature of the pipe metal, and also leads to its homogenization, as well as leads to a greater homogeneity of the temperature and residence time in the cross-section of the pipe.

The nature of the ribbed pipe according to the invention can be seen in the drawing of the pipe section in Fig. 6 and determined from the following characteristic parameters.

- Hydraulic diameter Dh in mm, Ri <Dh / 2

- Lateral surface angle β

- Rib height H

PL 204 769 B1

- Radius of the envelope circle Ra = Ri + H and Da = 2 x Ra

- Center angle α

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

- Circumference of a hinged circle 2 Π Ra

- Acute triangle angle γ = 180- (α + β)

- Inner radius Ri = 2R (sin γ / sin α) - R

- Rib height H = Ra-Ri

- U-profile circumference_P.= 2 x number of ribs x nR / 180 (2 β + α)

- Rib area FR

- Area of ​​the envelope circle Fa = nda²/4

- The area of ​​the inner circle Fi = n * Di

- Profile area within the boundary circle FP = FR * number of ribs

- U-profile circumference_P.= (1.05 to 0.98) x 2nRa

The ribs and the valleys between the ribs that lie between the ribs may be mirror-cross-sectional and may be adjacent to each other or may form a wavy line at all times for the same radius of curvature. A lateral surface angle is then created between the two tangential radii of curvature at the point of contact and the radius of the tube. In this case, the ribs are relatively shallow; the rib height and flank angle are matched to each other in such a way that the hydraulic diameter of the profile, calculated as 4 x the area / circumference of the profile, is greater than or equal to the inner circle of the profile. Therefore, the hydraulic diameter is on the inner third of the profile height. As a result, the rib height and the number of ribs increase as the diameter increases so that the swirling flow is maintained in the direction and with the intensity needed for the profile to operate.

A higher flow velocity is created between the ribs or in the valleys between the ribs (Fig. 2) causing a self-cleaning effect, i.e. a reduction in the deposited pyrolysis coke.

In a ribbed pipe according to the invention, the swirl flow takes over the detachable turbulent flow on the side surfaces of the ribs, so that turbulent flow is not locally recovered as a continuous circulation flow in the rib valleys. Despite the apparently greater distances traveled by the particles on the helical paths, the mean residence time is shorter than in a smooth tube and, moreover, more uniform in cross-section (see Fig. 7). This is confirmed by the higher total speed in the profiled tube with swirl flow (profile 3) compared to the tube with rectilinear fins (profile 2). This is especially ensured when the swirling flow in the region of the fins, i.e. the ribs, runs at an angle of 20 ° to 40 °, for example 30 °, preferably 25 ° to 32.5 °, to the axis of the tube.

If the ribs are produced by a deposition welding technique or by a centrifugally cast pipe, the wall of the pipe between the individual ribs remains substantially unchanged so that the valleys between the ribs lie on a common circle that corresponds to the inner circumference of the centrifugally cast pipe.

Tests have shown that, irrespective of the inner diameter of the tubes, a total of 8 to 12 fins are sufficient to achieve the flow configuration according to the invention.

In the case of a finned pipe according to the invention, the ratio of the quotient of the heat transfer coefficients Qr / Qo to the quotient of the pressure loss ΔP_R/ ΔP_ABOUT in the water test using and applying the laws of similarity and using Reynolds numbers for a mixture of kerosene and steam, it is preferably from 1.4 to 1.5, where R refers to a finned tube and O refers to a smooth tube.

The perfection of a finned tube according to the invention (profile 3) compared to a smooth tube (profile 0) and a finned tube with eight parallel ribs (profile 1), of which the radial distance between the valleys between the ribs and the tops of the ribs is 4.8 mm, is illustrated by the data presented in the table below. All finned tubes have 8 ribs and the same envelope circle.

<td>PROFILE</td><td> 0</td><td> 1</td><td> 3</td>

<td> 1</td><td> 2</td><td> 3</td><td> 4</td>

<td>Fluid temperature at 9950 mm in the center of T_m [° C]</td><td> 843,6</td><td> 848,1</td><td> 843,0</td>

<td>Fluid temperature at 9950 mm at the T edge_r [° C]</td><td> 888,9</td><td> 894</td><td> 874,8</td>

PL 204 769 B1 cont. table

<td> 1</td><td> 2</td><td> 3</td><td> 4</td>

<td>Temperature range at 9950 mm ΔT = Tr-Tm [° C]</td><td> 45,3</td><td> 45,9</td><td> 31,8</td>

<td>Uniformity factor for smooth pipe H at Hr = ΔTg / ΔTk</td><td> 1</td><td> 0,9869281</td><td> 1,4245283</td>

<td>Hydraulic diameter D_h [m]</td><td> 0,0380</td><td> 0,0256</td><td> 0,0344</td>

<td>Uniformity factor related to diameter hydraulic system based on a smooth pipe Ht0: Ht0 = ΔTo * dχ / ΔTχ * do</td><td> 1</td><td> 0,8477193</td><td> 1,3420556</td>

<td>H classification:</td><td> 2</td><td> 2</td><td> 1</td>

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

Dhydr = 4 * (wetted cross-section) / inner circumference;

Preferably, it corresponds to the inner diameter of a comparable smooth pipe and then gives a uniformity factor of 1.425.

In the water test, the ribbed pipe according to the invention has a heat transfer (QR) that is 2.56 higher than for a smooth pipe, with a pressure loss (/ \ P_R) which was only higher with a coefficient of 1.76.

Fig. 7 compares three different profile pipes, including the pipe according to the invention with 8 ribs with a pitch of 30 ° each, a pipe with a smooth inner wall (smooth pipe). Hydraulic diameter, axial velocity, residence time and pressure loss are given for each cross section.

The starting data used were numerical capacities in a smooth working tube with an internal diameter of 38 mm, which was identical to the hydraulic diameter. Using the laws of similarity (same Reynolds numbers), these data were transformed by calculation for hot water and used as a test basis (see ratio of the quotients for heat transfer and pressure loss for water studies and the reduced homogeneity coefficient for calculation with gases).

Different velocity profiles result from the same numerical throughputs for different hydraulic diameters (inverse relationship).

The speed comparison for profiles 2 and 3, which are identical in cross-section, illustrates the improved speed, acceleration and residence time for pipes according to the invention (profile 3). For the same hydraulic diameter, the velocity component in the circumferential direction due to the whirling motion induced by the ribs causes the flow to be dislodged from the pipe wall and induces a spiraling velocity over the entire cross section.

The directed spiral flow carries heat from the pipe wall into the flow and therefore distributes it more evenly than with normal, non-directional turbulent flow (smooth pipe, profiles 1 and 2). The same is true for the residence time of the particles. The spiral-directed flow distributes the particles more evenly across the cross-section, while acceleration on the side walls of the profile reduces the mean residence time. The higher pressure losses in the case of profile 3 result from the peripheral speed. In the case of profile 1, the cause is a significant reduction in flow and friction losses over a larger inner surface of the profile.

Depending on the material, the finned tubes of the invention can be produced, for example, from a centrifugally cast tube by ending the tube with axially parallel fins turned relative to each other, or by producing an internal profile by deforming the centrifugally cast tube, for example by hot forging, hot drawing or cold working with a profiling tool, for example a floating mandrel or a pin with an outer profile corresponding to the inner profile of a pipe.

Several variants of machine tools for the internal profiling of pipes are known, for example from DE 195 23 280. These machines are also suitable for the production of ribbed pipes according to the invention.

In the case of hot forming, the deformation temperature should be set in such a way that the microstructure grains are partially destroyed in the area of ​​the inner surface and appropriate recrystallization at a later stage under the influence of the operating temperature. The result is a fine-grained microstructure that allows the rapid diffusion of chromium and silicon

And / or aluminum through the austenitic matrix towards the inner surface of the pipe, in those places where there is rapid expansion of the protective oxide layer.

The ribs according to the invention can also be produced by padding (welding); in this case it is not possible to form a curved base of ribs between the individual ribs, but rather to keep at this point substantially the original profile of the inner wall of the tube.

The inner surface of the pipe according to the invention should be as rough as possible; it can therefore be smoothed, for example mechanically polished or electrolytically flattened.

Suitable pipe materials for ethylene production plants are iron and / or nickel alloys containing 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 additives of hafnium, titanium, rare earth elements or zirconium, in each case up to 0.5% and up to 6% aluminum.