HRP20050072A2 - Method and ribbed tube for thermally cleaving 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 process and a finned tube for the thermal cracking of hydrocarbons in the presence of steam, in which the charge mixture passes through externally heated tubes with internal helical fins.

Tube furnaces, in which a mixture of hydrocarbons and steam passes through a series of individual or meandering tubes (cracking tube coils) at temperatures above 7500C, made of heat-resistant chromium-nickel-steel alloys with high resistance to oxidation or scaling and high resistance to carburization, are proven suitable for high-temperature pyrolysis of hydrocarbons (crude oil derivatives). Pipe bundles contain vertically extending, straight pipe sections that are connected to each other via U-shaped pipe elbows or are arranged parallel to each other; they are usually heated with the help of the side burners and in some cases also with the help of the bottom burners and therefore have, as is known, a light side, which faces the burners and, as is known, a dark side, which is the opposite for the 900 with respect to it, i.e. it extends in the direction of the pipe rows. Tube Metal Temperatures (TMT) are in some cases over 10000C.

The service life of a cracking tube depends on a very significant extent of creep resistance and carburization resistance and also on the rate of coke formation of the tube material. The crucial factor for the rate of coke formation, i.e. the growth of the layer of carbon deposit (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 what is known as the intensity of operation, which hides the influence of the system pressure and retention time in the pipe system for the utilization of ethylene. The intensity of the operation is adjusted based on the mean outlet temperature of the cracking gases (eg 850ºC). The higher the temperature of the gas near the inner wall of the tube above this temperature, the stronger the growth of the pyrolysis coke layer becomes, and the insulating effect of this layer allows the temperature of the metal of the tube to increase even further. Although chromium-nickel-steel alloys, containing 0.4% carbon, over 25% chromium and over 20% nickel, for example 35% chromium, 45% nickel and if appropriate 1% niobium, which are used as material for pipes have a high resistance to carburization, carbon diffuses into the pipe wall at defects in the oxide layer, where it leads to significant carburization that can amount to a carbon content of 1% to 3% at wall thicknesses of 0.5 to 3 mm. This is associated with a significant increase in the brittleness of the pipe material, with the risk of cracking in case of fluctuating thermal loads, especially when the furnace is turned on or stopped.

In order to stop the deposition of carbon (coking) on the inner wall of the pipe, it is necessary to stop the cracking work from time to time, so that the pyrolysis coke burns with the help of a mixture of steam and air. This requires work to be stopped for up to 36 hours, and therefore has a significant adverse effect on the economics of the process.

British Patent 969 796 also discloses the use of a cracking tube with internal fins. Although internal fins of this type result in an internal surface area that is a good few percent, for example 10% larger, with a corresponding improvement in heat transfer, they are also associated with the lack of significantly increased pressure loss compared to a smooth tube, at the expense of friction o increased internal pipe surface. A higher pressure loss requires a higher system pressure, which inevitably changes the residence time and has an unfavorable effect on the utilization. An additional factor is that known tube materials with high carbon and chromium content can no longer be profiled by cold working, for example cold drawing. They have the disadvantage that their deformability drops greatly as the strength increases upon heating. This led to high metal temperatures up to, for example, 1050ºC, which are desirable in view of the utilization of ethylene, which requires the use of centrifugally cast pipes. However, since centrifugally cast pipes can only be produced with a cylindrical wall, special forming processes are required, for example material removal by electrolytic machining, welding forming processes, if internally ribbed pipes are to be produced.

In view of this state of the art, the invention is based on the problem of improving the economics of thermal cracking of hydrocarbons in tube furnaces with externally heated tubes having spiral internal fins.

This goal is achieved by a process in which the eddy current is created in the immediate vicinity of the rib, preferably of a centrifugally cast tube, and this eddy current is converted in the core zone to a predominantly axial flow, as the radial distance from the ribs increases. The transition between the outer zone with eddy flow and the core zone with predominantly axial flow is gradual, for example parabolic.

In the process according to the invention, the eddy flow acquires a separating vorticity at the rib flanks, so that the vorticity is not locally recycled in the form of a continuous circulating flow in the rib valleys. Despite the apparently longer distances that the particles have to cover in spiral trajectories, the mean residence time is lower than in a smooth tube and, moreover, more homogeneous across the cross-section (compare with Fig. 7). This is confirmed by the higher overall velocity in the profiled vortex tube (profile 3) compared to the straight finned tube (profile 2). This is particularly ensured if there is a vortex flow in the region of the ribs or if the ribs run at an angle of 200 to 400, for example 300, preferably 25 to 32.50, with respect to the pipe axis.

In the process according to the invention, the heat supply, which inevitably differs across the tube periphery between the bright side and the dark side, is compensated in the tube wall and inside the tube and the heat rapidly spreads inwards to the core zone. This is related to the reduction of the risk of local overheating of the process gas at the pipe wall, with the consequence of the formation of pyrolysis coke. Moreover, the thermal load on the pipe material is lower due to the temperature compensation between the light side and the dark side, which extends the service life. Finally, in the process according to the invention, the temperature is also uniform across the cross-section of the tube, resulting in increased olefin recovery. The reason for this is that without radial temperature compensation in accordance with the invention in the interior of the pipe, excessive cracking would occur at the hot wall of the pipe, and recombination of cracking products would occur in the center of the pipe.

Furthermore, a layer of laminar flow, which is a characteristic of turbulent flows, with a greatly reduced heat transfer is formed in the case of a smooth tube, and to a large extent in the case of ribbed profiles with an inner rim made of ribs, which increases by more than 5%, for example 10% . This laminar flow leads to increased formation of pyrolysis coke, similar to low thermal conductivity. Two layers together require greater heat input or greater burner capacity. This increases the tube metal temperature (TMT) and shortens the service life accordingly.

The invention avoids this due to the fact that the inner circumference of the profile is up to about 5% at most, for example 4% or even 3.5% with respect to the circumference of the inscribed circle touching the valleys of the ribs. However, the inner circumference can also be up to 2% smaller than the inscribed circle. In other words, the relative circumference of the profile is at most 1.05 to 0.98 of the circumference of the inscribed circle. Accordingly, the difference in the surface area of the pipe profile according to the invention, i.e. its developed internal surface area, with respect to a smooth pipe having an inscribed circle diameter, amounts to a maximum of +5% to -2% or 1.05 to 0 .98 times the area of a smooth tube.

The pipe profile according to the invention allows a lower specific mass of the pipe (kg/m) compared to a ribbed pipe where the inner circumference of the profile is at least 10% larger than the circumference of the inscribed circle. This is demonstrated by a comparison between two pipes of the same hydraulic diameter and, accordingly, the same pressure drop and the same thermal result.

A further advantage of the profile rim according to the invention (relative profile rim) with regard to the rim of the inscribed circle is much faster heating of the filled gas at a reduced temperature of the pipe metal.

Eddy flow in accordance with the invention very significantly reduces the extent of the laminar layer; moreover, it is associated with a velocity vector directed towards the center of the tube, which reduces the residence time of cracking radicals and/or cracking products against the hot tube wall, and their chemical and catalytic decomposition to form pyrolysis coke.

In addition, the temperature differences between the valleys of the ribs and the ribs, which are not insignificant in the case of internally profiled pipes with high ribs, are compensated by the vortex flow in accordance with the invention. This increases the time between the two required coking operations. Without the vortex flow in accordance with the invention, a temperature difference between the tops of the ribs and the base of the rib valleys arises, which is not insignificant. The retention time of coke-prone cracking products is shorter in the case of cracking tubes equipped with helical internal fins. This depends on the nature of the ribs in individual circumstances.

In the diagram:

The upper curve shows: profile 6: slope 160

The middle curve shows: profile 3: slope 300

The lower curve shows: profile 4: 3 ribs

with a slope of 300.

The curves clearly show that the higher peripheral speed of the profile 6 with ribs 4.8 mm high is consumed within the valleys of the ribs, while on the contrary the peripheral speed of the profile according to the invention with a rib height of only 2 mm penetrates into the core of the flow. Although the circumferential velocity of profile 4 with only 3 fins is approximately as high, this does not effect any spiral acceleration of the core flow.

According to the curves shown in the diagram presented in Figure 2, the profile according to the invention causes a spiral acceleration in the valleys of the ribs (the upper branch of the curve), which covers wide cross-sectional areas of the tube and is therefore responsible for homogenizing the temperature in the tube. The lower peripheral speed at the tips of the fins (the lower branch of the curve) further ensures that turbulence and backflow do not occur.

Figure 3 illustrates three test tubes, including their data, in cross-section; these pipes include profile 3 according to the invention. Each plot indicates the temperature profile across the tube radius on the dark side and on the bright side. A comparison of the diagrams reveals a lower temperature difference between the pipe wall and the pipe center and a lower gas temperature at the pipe wall in the case of profile 3 according to the invention.

Eddy current in accordance with the invention ensures that the temperature fluctuation inside the wall across the tube circumference, i.e. between the light side and the dark side, is less than 120C, even though the bundles of tubes, which are usually arranged in parallel rows in the tube furnace, are heated and act by combustion gases by means of side burners against the wall only on opposite sides and therefore each tube has a light side facing the burners and a dark side offset by 900 with respect to them. The mean temperature of the pipe metal, i.e. the difference in the temperature of the pipe metal on the light side and the dark side, leads to internal stresses and therefore determines the service life of the pipe. Therefore, the reduction of the mean temperature of the pipe metal in the pipe according to the invention with eight ribs with a pitch of 300, an inner diameter of 38.8 mm and an outer diameter of the pipe of 50.8 mm, i.e. a difference in height between the valleys of the ribs and the tops of the ribs of 2 mm of 110 compared to a smooth pipe of the same diameter, based on an average service life of 5 years, which can be seen from the diagram presented in Figure 4, at an operating temperature of 10500C, results in a calculated increase in service life of approximately 8 years.

The temperature distribution between the bright side and the dark side for the three profiles shown in Figure 3 should be found in the diagram shown in Figure 5. One can notice the lower level of the temperature curve for profile 3 compared to the smooth pipe (profile 0) and a significantly narrower fluctuation range for the curve of profile 3 compared to the curve of profile 1.

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

A more uniform distribution of temperatures across the cross-section results especially if the circumferential velocity increases within 2 to 3 m and then remains constant along the entire length of the pipe.

In order to achieve a high utilization of olefins with a relatively short pipe length, the process according to the invention should be carried out in such a way that the temperature homogeneity factor across the cross section and the temperature homogeneity factor referred to the hydraulic diameter are over 1, in relation to the smooth homogeneity factor pipes (HGØ). In this context, homogeneity factors are defined as follows:

HGØ[-]HPØ = ΔT0·dx/ΔTx·d0

A flow configuration according to the invention, consisting of a core flow and a vortex flow, can be achieved with a finned tube in which the side angle of the fins, which are in any case continuous along the entire length of the tube section, i.e. the outer angle between the flanks of the fins and the radius pipe is 160 to 250, preferably 190 to 210. The side angle of this type, especially in combination with a rib inclination of 200 to 400, for example 22.50 to 32.50, ensures that what results in the valleys of the ribs is not more or less continuous eddy currents that return to the rib valleys behind the rib flanks and lead to the creation of unwanted "twists" in the rib valleys. It is better that the turbulence formed in the valleys of the ribs becomes separated from the flanks of the ribs and is carried out by means of a vortex flow. The vortex energy induced by the fins accelerates the gas particles and leads to a higher overall velocity. This leads to a decrease in the metal temperature of the tube, and also makes the latter more uniform, also making the temperature and residence time across the cross section of the tube more uniform.

The nature of the ribbed pipe according to the invention can be seen from the illustration of the pipe segment in Figure 6 and the associated characteristic parameters

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

- Side angle β

- Rib height H

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

- Central angle α

- Radius of curvature R=Ra (sinα/2 sinβ + sinα)

- Circumference of the inscribed circle 2ΠRa

- Angle in an isosceles triangle γ = 180 - (α+β)

- Inner radius Ri=2R (sinγ/sinα) - R

- Rib height H=Ra-Ri

- Profile circumference Up = 2 x number of ribs x πR/180 (2 β+α)

- Surface area of the FR rib

- Area of the inscribed circle Fa = π·Da2/4

- Area of the inner circle Fi = Π·Di

- Profile area within the inscribed circle

FP = FR · number of ribs

- Profile perimeter Up = (1.05 to 0.98)·2ΠRa

The ribs and rib valleys located between the ribs may be of a mirror-symmetric design in cross-section and join each other or may form a wavy line with the same radii of curvature in each case. The flank angle is then formed between the tangents of the two radii of curvature at the point of contact and the pipe radius. In this case, the ribs are relatively shallow; the height of the ribs and the angle of the side coincide with each other in such a way that the hydraulic diameter of the profile from the ratio 4 x clear cross-section/circumference of the profile is greater than or equal to the inner circle of the profile. The hydraulic diameter is therefore in the inner third of the profile height. As a consequence, the fin height and the number of fins increase as the diameter becomes larger, so that the vortex flow is maintained at the direction and intensity required for the airfoil action.

Between the ribs or in the valleys of the ribs, a higher flow velocity occurs (Figure 2), leading to a self-cleaning effect, i.e. to a reduction in the amount of pyrolysis coke that is deposited.

If the fins are produced by a weld build-up or overlay weld layer, using a centrifugally cast pipe, the pipe wall between the individual fins remains essentially unchanged, so that the valleys of the ribs lie on a common circle that corresponds to the internal circumference of the centrifugally cast pipe.

Tests have shown that, regardless of the inner diameter of the pipe, a total of 8 to 12 ribs is sufficient to achieve the flow configuration according to the invention.

In the case of a finned tube according to the invention, the ratio of the quotients of the heat transfer coefficients QR/Q0 to the pressure loss quotient ΔPR/ΔP0 in the test with water, applying and observing the laws of similarity and using the Reynolds numbers given for the oil/steam mixture, is preferably 1.4 to 1.5, where R indicates a ribbed tube and 0 indicates a smooth tube.

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

[image]

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

Dhidr = 4 x (bright cross-section)/inner circumference;

it conveniently matches the inside diameter of a comparable smooth pipe and then results in a homogeneity factor of 1.425.

In a water test, a finned tube according to the invention gave a heat transfer (QR) that was a factor of 2.56 higher than a smooth tube, with a pressure loss (ΔPR) that was only a factor of 1.76 higher.

Figure 7 compares three different pipe profiles, including a pipe according to the invention, with 8 ribs at a pitch of 300 in each case, 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 initial data used were quantitative passages in a working smooth pipe with an internal diameter of 38 mm, which is identical to the hydraulic diameter. Using laws of similarity (same Reynolds numbers), these data were computationally converted for hot water and used as a basis for tests (compare the ratio of heat transfer coefficients and pressure loss for tests with water and the referenced homogeneity factor for calculations using gases).

Different velocity profiles at different hydraulic diameters resulted from the same quantitative passes (reciprocal relationship).

A comparison of the velocities for Profiles 2 and 3, which are of identical cross-section, illustrates the improved velocity, acceleration and dwell time with tubes in accordance with the invention (Profile 3). For the same hydraulic diameter, the circumferential velocity component induced by fin-induced vorticity causes the flow to separate from the pipe wall and induces a spirally increasing velocity across the entire cross-section.

Directed, spiral flow introduces heat from the pipe wall into the flow and therefore distributes it more evenly than in normal, non-directed turbulent flow (smooth pipe, profiles 1 and 2). The same applies to particle retention time. The spirally directed flow distributes the particles more uniformly over the entire cross-section, while the acceleration at the flanks of the ribs reduces the mean residence time. The higher pressure loss along profile 3 results from the peripheral velocity. In the case of profile 1, the cause is a significant narrowing of the flow and loss of friction at the large inner surface of the profile.

Depending on the material, finned tubes according to the invention can be produced, for example, from a centrifugally cast tube with axially parallel ribs, by means of tube ends that rotate with respect to each other, or by means of an internal profile produced by deformation of a centrifugally cast tube, on for example by hot forging, hot drawing, or cold working using a profiling tool, a flying spindle and a spindle bar with an external profile matching the internal profile of the pipe.

Several variants of cutting machines for the internal profiling of pipes are known, for example from German patent 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 adjusted so that the microstructural grain is partially destroyed in the area of the inner surface and accordingly recrystallizes at a later stage under the influence of the working temperature. The result is a fine-grained microstructure that allows the rapid diffusion of chromium, silicon and/or aluminum through the austenite matrix to the inner surface of the tube, where an oxide protective layer is then rapidly formed.

Ribs according to the invention can also be produced by welding; in this case, it is not possible to form a curved base of the ribs between individual ribs, but the original profile of the inner wall of the pipe is largely maintained.

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

Suitable materials for use in ethylene 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 additions of hafnium, rare earth titanium or zirconium, in any case up to 0.5% and up to 6% aluminium.

Claims (36)

1. A process for thermal cracking of hydrocarbons in the presence of steam, in which the charge mixture is passed through externally heated tubes with spiral internal fins, characterized by the fact that a vortex flow is created immediately next to the fins and in the core zone it turns into a predominantly axial flow, as it increases radial distance from ribs. 2. The process as claimed in claim 1, characterized in that the eddy flow takes on separating turbulence at the flanks of the ribs. 3. The process as claimed in claim 1, characterized in that the circumferential velocity of the gas flow in the valleys of the ribs is higher than at the tops of the ribs. 4. The process as claimed in one of claims 1 to 3, characterized in that the vortex flow at the ribs flows at an angle of 200 to 400, preferably 22.50 to 32.50 with respect to the pipe axis. 5. A process as claimed in one of claims 1 to 4, characterized in that the temperature fluctuation in the inner wall across the pipe circumference is less than 120C. 6. A process as claimed in one of claims 1 to 5, characterized in that the isotherms in the core zone flow in a spiral form. 7. A process as claimed in one of claims 1 to 6, characterized in that the velocity of the vortex flow is enhanced within the first 2 to 3 m of the pipe length and then remains constant. 8. A process as claimed in one of claims 1 to 7, characterized in that the velocity of the vortex flow comprises the entire cross-section after the first 2 to 3 m of the pipe length. 9. The process as claimed in one of claims 1 to 8, characterized in that the temperature homogeneity factor for the entire cross-section and the temperature homogeneity factor referred to the hydraulic diameter is over 1, in relation to the homogeneity factors of the smooth pipe. 10. The process as claimed in one of claims 1 to 9, characterized in that the flow velocity in the boundary layer at the pipe wall is 8 to 12% lower, and the flow velocity in the core zone is 8 to 12% higher, than in the case comparable straight finned tubes of the same type. 11. The process as claimed in claims 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 velocity that is 15 to 20% of the axial velocity in the core zone, and that the peripheral speed behind it remains constant. 12. A process as claimed in claims 1 to 11, characterized in that the sum of the axial velocity and the circumferential velocity is greater than the axial velocity of a comparable straight fin tube of the same type. 13. A process as claimed in claims 1 to 12, characterized in that the gas particles are accelerated at the flanks of the ribs. 14. A ribbed pipe having several spirally flowing internal ribs, indicated by the fact that the circumference of the profile (Up) is from +5 to -2% of the inscribed circle that touches the valleys of the ribs. 15. A finned tube as claimed in claim 14, characterized in that the rib angle is 160 to 250. 16. A ribbed tube as claimed in claim 14 or 15, characterized in that the angle of inclination of the ribs is 200 to 400. 17. A ribbed tube as claimed in one of claims 14 to 16, characterized in that the ribs and valleys located between the ribs are designed to be mirror-symmetrical in cross-section. 18. A ribbed pipe as claimed in one of claims 14 to 17, characterized in that the tops of the ribs and the valleys of the ribs in each case flow into each other. 19. A ribbed pipe as claimed in one of claims 14 to 18, characterized in that the ribs and rib valleys have the same radius of curvature. 20. A ribbed tube as claimed in claim 14 or 15, characterized in that the ribs are welded and that the valleys of the ribs lie on a common circle. 21. A ribbed tube as claimed in one of claims 14 to 20, characterized in that it has a total of 6 to 12 ribs. 22. Ribbed pipe as claimed in one of claims 14 to 21, characterized in that the hydraulic diameter of the ribbed pipe is at least equal to the diameter of the inner circle (Ri). 23. A finned tube as claimed in one of claims 14 to 22, characterized in that the ratio of the quotients of the heat transfer coefficients QR/Q0 to the quotient of pressure losses ΔPR/ΔP0 in the water test is 1.4 to 1.5, wherein R stands for ribbed tube and 0 stands for smooth tube. 24. A ribbed tube as claimed in one of claims 14 to 23, characterized in that the radius of curvature (R) of the cross section of the rib is 3.5 to 20 mm. 25. Ribbed tube as claimed in one of claims 14 to 24, characterized in that the rib height (H) is from 1.25 to 3 mm. 26. Ribbed pipe as claimed in one of claims 14 to 25, characterized in that the clear cross-section within the perimeter of the profile (Up) is 85 to 95% of the area of the inscribed circle. 27. Ribbed tube as claimed in one of claims 14 to 26, characterized in that the profile area (Fp) is 40 to 50% of the annular area between the described circle and the inner circle. 28. A process for producing a ribbed tube as claimed in one of claims 14 to 27, characterized in that the ends of the tube with axially parallel ribs are rotated relative to each other. 29. A process for producing a ribbed tube as claimed in one of claims 14 to 27, characterized in that the inner profile is produced by deformation using a profiling tool. 30. The process as claimed in claim 29, characterized in that during the deformation the microstructural grain is partially broken in the region of the inner surface. 31. A process for the production of a ribbed tube as claimed in one of claims 14 to 27, characterized in that the inner profile is produced by deformation using a profiling tool or by welding. 32. A process for the production of a centrifugally cast pipe as described in claims 14 to 27, characterized in that the inner profile is produced by electrolytic material removal. 33. The process as claimed in one of claims 29 to 32, characterized in that the inner surface of the profiled tube is smoothed. 34. Use of a centrifugally cast pipe, characterized in that it is for the production of a ribbed pipe as required in one of the patent claims 15 to 27. 35. Use as claimed in claim 34, characterized in that the spin-cast tube consists of 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 earths, zirconium and up to 6% aluminum. 36. Use as claimed in claim 35, characterized in that the alloy contains, individually or in combination with each other, at least 0.02% silicon, 0.1% niobium, 0.3% tungsten and 1.5% aluminum .