JP2019516942A - Furnace coil remodeling fins - Google Patents

Abstract

The present invention provides thick fins on the surface of a coil or tube of a steam cracking furnace. The fins at their base have a thickness of 1/4 to 3/4 of the radius of the furnace tube. The fins have grooves or ridges on about 10% or more of the major surface. The fins help to increase the radiant heat absorbed by the tubes from the walls and combustion gases in the furnace.

Description

  The present invention relates to the field of cracking paraffins to olefins, and more particularly to solid fins on the outer surface of process coils in the radiant section of a cracking furnace. The fins may be transverse (horizontal) or longitudinal. The fins are open upwards or outwards with a depth which is less than a quarter of the maximum thickness of the fins, in a regular or semi-regular pattern covering at least 10% of the surface area of at least one major surface of the fins And / or an array selected from the group consisting of ridges or ridges having a base not exceeding 10% of the maximum thickness of the fins and a height not exceeding 15% of the maximum thickness of the fins.

  In the field of heat exchanger design, there are many applications of fins to improve heat transfer. Usually this is heat transfer by forced convection mechanism. Heat transfer by forced convection occurs between a moving fluid, which may be gas or liquid, and a solid surface, including the combined effect of conduction and convection. This type of heat transfer occurs in many of the conventional heating systems, whether hot or electric, and in industrial heat exchangers.

In the cracking of a feed containing paraffins such as ethane or naphtha, or mixtures thereof, usually _C2-4 paraffins, the feed is usually a series of pipes or pipes passing through several parts of the furnace with dilution steam It is fed into a cracker equipped with a tube. First, the feed passes through the tubes in the convection section of the furnace where the exhaust gases flowing from the radiant section downstream of the furnace heat the outer surface of the tubes. There, the feed is heated to a temperature at or near a level where decomposition may begin. The feed then flows into the tubes of the radiant section of the furnace, where the tubes are mainly radiation from the refractory wall of the furnace and the combustion gases generated by the burners usually mounted on the floor or walls of the radiant section It is heated by the radiation from. Some of the forced convection heating of the tubes is also provided by the combustion gases. The feed is heated to a temperature of about 800 ° C. to 950 ° C. in a furnace radiant section. At these temperatures, the feed undergoes a number of reactions, including free radical cracking (cracking), reformation of new unsaturated products, and coproduction of hydrogen. These reactions occur during a very short time corresponding to the feed residence time in the coil. The residence time is usually about 0.01 to about 10 seconds, sometimes 0.01 to 2 seconds, sometimes 0.01 to 1 second. The reaction may be heated to a temperature of 750 ° C. to 950 ° C., optionally 800 ° C. to 900 ° C., at a pressure of 200 to 500 kPa, optionally 250 kPa to 550 kPa.

  The interior of the radiant section of the furnace is lined with a heat absorbing / emitting refractory and is usually heated by a gas fueled burner.

  The cracked gas exits the radiant section of the furnace and then passes through a transfer line exchanger to a quencher that rapidly cools the product stream to a temperature at which the reaction stops. The resulting product stream is then separated into various components such as ethylene, propylene and the like.

  Since improving the efficiency of the cracking furnace reduces process costs and emissions of greenhouse gases, a motive force exists to improve the efficiency of the cracking furnace. There are two main approaches to improving efficiency, the first is to improve the heat transfer to the coil of the furnace, ie the heat transfer from the flame, the combustion gases and the refractory wall to the outer surface of the process coil The second is by improving the heat transfer in the coil, i.e. the heat transfer from the inner wall of the coil to the feed material flowing inside the coil.

  One of the methods representative of the second approach is to add inner fins to the inner wall of the coil of the furnace to promote "swirling" or mixing of the feed within the coil. This increases turbulence in the flow of feed material and also increases the heat transfer surface of the hot inner wall of the coil, thereby improving convective heat transfer from the coil wall to the feed material.

  Patent Document 1 provides one example of this type of technology.

  Non-Patent Documents 1 and 2 provide theoretical simulations of the decomposition process with coils finned inward with helical fins and longitudinal fins (more precisely, ridges or ridges). The simulation results are verified by lab scale experiments where hot air flows through these internally finned tubes. The paper shows that tubes with inner spiral fins perform better than tubes with inner longitudinal fins, “the results for tubes with inner spiral fins are very good with industrial observations It is concluded that "it is consistent." However, experimental data to support these conclusions are not provided. No comparison is made with the performance of bare tubes without inner ribs or fins. The author recognizes that one of the potential drawbacks of such a coil with inner fins is that carbon deposits can build up on the fins and the pressure drop in the tube can be large.

  U.S. Pat. No. 5,959,015 teaches a heat exchanger tube having inner and outer fins. There is no teaching or suggestion in U.S. Pat. No. 5,956,015 that the outer fins should have additional grooves in their outer surface. This patent application teaches away from the subject matter of the present application.

  U.S. Pat. No. 5,648,095 teaches an annular outer fin of a cracking furnace coil to increase convective heat exchange into the coil. This patent teaches or suggests that the fins have additional grooves in the main outer surface of the fins.

  U.S. Pat. No. 5,959,095 teaches an externally finned heat exchanger tube in a slurry reaction (Fisher-Tropsch synthesis). In the reaction, a slurry of CO and hydrogen in a hydrocarbyl diluent containing catalyst flows over the outer surface of a heat exchanger tube containing the flowing cooling water. The heat exchanger tube has ribs with an aspect ratio of less than five. There is no teaching or suggestion in this patent that the fin has additional grooves in its major outer surface.

  Patent Document 5 teaches longitudinal fins of a furnace tube of a radiant part of a cracking furnace. The fins do not have grooves on their surface. Paragraph 54 teaches the thickness of the fin at its base. Usually, the fin has a thickness at its base which is 6% to 25% of the diameter of the tube, preferably 7.5% to 15% of the diameter of the furnace tube.

  Patent Document 6 teaches a furnace tube or coil having a raised portion on its surface, which is used in a radiant part of a cracking furnace. This patent does not teach or suggest fins with raised portions on the surface of the coils used in the radiant section of the furnace.

  U.S. Pat. No. 5,959,015 teaches a heat exchanger tube having at least a portion of its major surface, mechanically or shaped and recessed annular fins. Heat exchangers are used to cool gas or air (i.e. air conditioners). Heat exchangers are primarily associated with convective heat exchange, not radiative heat exchange. The heat exchanger can not be compared with the tubes of the cracking furnace. There is no written disclosure of the wall thickness of the heat exchanger tubes, or the thickness of the fins. From the figure, it appears that the depression is about one-half to one-third of the thickness of the fin, which is more than the maximum required by the present invention, which is one-fourth of the thickness of the fin Remarkably large.

  U.S. Pat. No. 5,959,015 teaches a convective heat exchanger fin where the grooves are comparable except that the depth decreases from the base of the fin to the outer edge. This patent teaches that the fin may have a thickness that is about 0.4 to 1 mm at its inner edge (base) and a thickness that is 0.15 to 0.4 mm at its outer edge ( 5th row, 25-30 lines). This patent also teaches that the grooves can have a depth (thickness) that is between 0.4 and 1.5 mm. The grooves appear to have a thickness that is about half that of the fins. Again, these fins are for convective heating and not for radiative heating as in a cracking furnace.

U.S. Patent No. 5,950,718 U.S. Patent Application Publication No. 2003/0015316 U.S. Patent No. 7,128,139 U.S. Patent No. 7,096,931 US Patent Application Publication 2012/0251407 U.S. Patent No. 8,790,602 U.S. Patent No. 7,743,821 U.S. Pat. No. 8,376,033

“Three dimensional coupled simulation of reactor tubes and reactors for the thermal cracking of hydrocarbons”, by T. Detemmerman, G. F. Froment, (Universiteit Gent, Krijgslaan 281, b9000 Gent-Belgium, mars-avri, 1998) "Three dimensional simulation of high internal cracking cracking for olefins production severity", by Jjo de Saegher, T. Detemmerman, G.F.

  The present invention is a thick or sturdy fin for a furnace tube, wherein at least one major surface has a regular or semi-regular pattern covering at least 10% of the surface area of the at least one major surface of said fin An upwardly or outwardly open groove having a depth which is less than a quarter of the thickness of the fin, or a base having a main dimension not exceeding 10% of the maximum thickness of the fin and 15% of the maximum thickness of the fin It seeks to provide a fin having an array selected from the group consisting of ridges having height not exceeding, or both.

  In one embodiment, a furnace tube having one or more thick fins on its outer surface, the fins having a thickness at the base that is 1/4 to 3/4 of the radius of the furnace tube, and the fins Side or parallel side having an upper and lower taper of less than 15 ° with respect to the main axis of the above, and the fins have a regular or semi-regular covering at least 10% of the surface area on at least one major surface Groove outwardly of the basic pattern and having a depth which is less than a quarter of the maximum thickness of the fin, and a base not exceeding 10% of the maximum thickness of the fin, and the maximum thickness of the fin An array selected from the group consisting of ridges having a height not exceeding 15% of, or both of a regular or semi-regular pattern covering at least 10% of the surface area of at least one major surface of said fins Have , The furnace tube is provided.

  In another embodiment, a furnace tube is provided, wherein the groove has a depth of one eighth to one tenth of the maximum thickness of the fin.

  In another embodiment, a furnace tube is provided in which the array of grooves covers one quarter or more of at least one major surface of the fin.

  In another embodiment, the groove is in the form of an outwardly open V, a truncated outwardly open V, an outwardly open U, and an outwardly open parallel-faced channel. A furnace tube is provided.

  In another embodiment, a furnace tube is provided in which the fins form a transverse plate in the form of a circle, an ellipse, or a polygon with N sides.

  In another embodiment, the base of the fin has a thickness that is one third to one half of the radius of the furnace tube.

  In another embodiment, the fins have a cross section in the form of an outwardly extending parabolic, parallelogram or "E" shape (monolith with parallel longitudinal channels), or a rounded "V" A furnace tube is provided, which is a longitudinal fin having.

  In another embodiment, a furnace tube is provided in which the array of grooves covers one quarter or more of at least one major surface of the fin.

  In another embodiment, a furnace tube is provided, wherein the groove has a depth of one eighth to one tenth of the maximum thickness of the fin.

  In another embodiment, the groove is in the form of an outwardly open V, a truncated outwardly open V, an outwardly open U, an outwardly open parallel-faced channel. There is a furnace tube provided.

  In another embodiment, a furnace tube is provided having horizontal fins spaced at least twice the outside diameter of the furnace tube.

  In another embodiment, there is provided a furnace tube having longitudinal fins, the base of said fins ranging from one third to one half of the radius of the furnace tube.

In another embodiment, the array is
i) maximum height, which is 3 to 15% of the base of the fin,
ii) contact surface or base with the fin, the main dimension being 0.1% to 10% of the thickness of the fin,
iii) A furnace tube is provided which comprises a raised portion having a relatively large outer surface with a relatively small volume and a geometric shape.

In another embodiment, the ridges are
tetrahedron,
Johnson's square pyramid,
Pyramid with sides of four isosceles triangles,
Pyramid with sides of isosceles triangle,
Part of a sphere,
A portion of an ellipsoid,
A furnace tube is provided having a shape selected from the group consisting of a teardrop-shaped portion and a parabolic portion.

  In another embodiment, a furnace tube is provided wherein the furnace tube and the fins comprise the same metal composition.

  In another embodiment, about 55 to 65 wt% Ni, about 20 to 10 wt% Cr, about 20 to 10 wt% Co, about 5 to 9 wt% Fe, and one or more. A furnace tube and a fin are provided, including a plurality of trace element residues.

  In another embodiment, 0.2 to 3 wt% Mn, 0.3 to 2 wt% Si, less than 5 wt% titanium, niobium and all other trace metals, 0.75 A furnace tube and a fin are provided, further comprising carbon in an amount of less than% by weight, the sum of the components being 100% by weight.

  In another embodiment, 40 to 65 wt% Co, 15 to 20 wt% Cr, 20 to 13 wt% Ni, less than 4 wt% Fe, one or more trace elements and Furnace tubes and fins are provided which comprise up to 20% by weight of W and a total of 100% by weight of components.

  In another embodiment, 0.2 to 3 wt% Mn, 0.3 to 2 wt% Si, less than 5 wt% titanium, niobium and all other trace metals, 0.75 A furnace tube and a fin are provided, further comprising carbon in an amount of less than% by weight.

  In another embodiment, furnace tubes and fins are provided comprising 20 to 38 wt% chromium and 25 to 48 wt% Ni.

  In another embodiment, 0.2 to 3 wt% Mn, 0.3 to 2 wt% Si, less than 5 wt% titanium, niobium and all other trace metals, 0.75 Furnace tubes and fins are provided, further comprising carbon in an amount of less than weight percent and a balance that is substantially iron.

  In another embodiment, there is provided a cracking furnace comprising a radiant section having a furnace tube as described above.

  Another embodiment is a method of decomposing paraffins, said paraffins in gaseous state in a radiant part of a cracking furnace as described above at a temperature of 600 ° C. to 950 ° C. and for a time of 0.001 to 0.01 seconds. A process is provided which comprises the steps of passing C. and separating the resulting olefin from the feed and by-products.

  The invention also provides any and all combinations of the foregoing embodiments.

FIG. 6 shows a furnace tube with longitudinal fins according to the invention, which is remodeled using surface grooves. FIG. 6 shows a fin of the invention, which is adapted using the ridges of the invention. FIG. 6 is a graph showing the percent increase in surface area of fins remodeled with various ridges of the present invention.

Ranges of Numbers All figures or expressions referring to quantities such as materials, reaction conditions, etc. used in the specification and claims, except where indicated for operating examples or otherwise. It should be understood that, in all cases, it is corrected by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the properties that the invention is desired to obtain. At least, and not intended to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be a fraction of a normal number in light of the number of significant figures reported. It should be interpreted by applying processing techniques.

  Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. However, any numerical value will substantially contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

  Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges contained therein. For example, the range "1 to 10" is all subranges between them, ie 1 or more, including the minimum 1 listed and the maximum 10 listed. It is intended to include all subranges having a minimum value and a maximum value which is less than or equal to ten. As the numerical ranges disclosed are continuous, they include all values between the minimum and maximum values. Unless explicitly stated otherwise, the various numerical ranges specified in the present application are approximate values.

  In fact, all ranges for the compositions presented herein are limited to, and not exceeding, 100 percent (volume percent or weight percent) in total. If a plurality of components can be present in a composition, the amount of components actually used will meet the maximum value of 100 percent, as one skilled in the art would readily understand. The sum of the maximum amounts of each component may exceed 100 percent.

  The term outward, as used herein when referring to a groove, is outward with respect to the main plane of the fin on which the groove rests.

  The height of the fins used herein refers to the distance the fins extend away from the outer surface of the furnace tube.

  According to the invention, the furnace tube has relatively thick fins with high reliability, good stress resistance. Typically, the thickness of the fin at its base is about 33% or more, usually about 40%, desirably about 45% or more of the radius of the furnace tube, and in some embodiments up to 50% of the tube radius become. The fins are thick or stubby. The fins have a height to maximum width ratio of about 0.5 to 5, usually 1 to 3. The sides (edges) of the fins may be parallel or may be slightly tapered inwardly towards the outer edge of the fins. The angle of taper should be only about 15 °, usually about 10 ° or less, inside with respect to the centerline of the fin. The edges of the fins may be flat or pointed (at an angle of 30 ° to 45 ° from each surface) and may have blunt rounded front ends. The fins may have a cross-sectional shape in the form of an outwardly extending parabolic, parallelogram, or blunt "V" shape. In some cases, preferably for the longitudinal fins, the fin cross section may have an "E" shape (a monolith with parallel longitudinal extensions (with parallel grooves)).

  In one embodiment, the at least one major surface of the fin covers at least 10% of the surface area of the at least one major surface of the fin (e.g. the top or bottom in a horizontal fin or the side in a longitudinal fin) It has an array of outwardly open grooves in a regular or semi-regular pattern, said grooves being less than a quarter, and in some cases one eighth to one tenth, of the maximum thickness of the fin Have a depth of The array may cover 25% or more, sometimes 50% or more, preferably more than 75%, most preferably more than 85% and up to 100% of the surface area of one or more major surfaces of the fin . The array may be straight or wavy, parallel lines parallel or at an angle to the main axis of the fins, intersecting lines, wavy lines, squares or rectangles. The groove may take the form of an outwardly open V, a truncated outwardly open V, an outwardly open U, and an outwardly open parallel surfaced channel.

  The fins may be transverse or parallel (e.g. longitudinal) to the main axis of the furnace tube. The transverse fins may be at an angle of about 0 ° to 25 ° off perpendicular to the main axis of the furnace tube. However, it is more expensive and difficult to make the lateral fins at an angle deviated from perpendicular to the main axis of the tube. The lateral fins may have a shape selected from a circle, an ellipse, or an N-sided polygon where N is an integer greater than or equal to three. In some embodiments, N is 4-12. The major surfaces of the lateral fins are the top and bottom surfaces of the fins. The transverse fins should be spaced at least twice, and in some cases three to five times, the outside diameter of the furnace tube.

  The longitudinal fins may have the shape of a parallelogram, an ellipse or a portion of a circle, and about 50% of the length of the furnace tube at the radiant section (sometimes called a path) to the furnace at the radiant section It may have up to 100% of the length of the tube, and any range of length therebetween.

  The base of the longitudinal fins is at least a quarter of the radius of the furnace tube, in some cases 1/4 to 3/4, usually about 1/3 to 3/4, or in some cases 1/0. It may be 3 to 5/8, in other cases 1/3 to 1/2 of the radius of the furnace tube. The fins are thick or stubby. The fins have a height to maximum width ratio of about 0.5 to 5, usually 1 to 3. The sides (edges) of the fins may be parallel or may be slightly tapered inwardly towards the tips of the fins. The angle of taper should be only about 15 ° inside, usually about 10 ° or less, relative to the centerline of the fin. The tip or front edge of the fin may be flat, may be tapered (at an angle of 30 ° to 45 ° from the top and bottom surfaces of the fin), and may have a blunt rounded front end . The leading edge of the longitudinal fins will usually be parallel to the central axis of the furnace tube. If the fins extend less than 100% of the length of the furnace tube, the leading edge of the fins is for the most part parallel to the central axis of the furnace tube and then between about 60 ° and 30 ° , Usually at an angle of 45 °, to the furnace tube wall. In some cases, the fins may end with a flat surface perpendicular to the surface of the tube.

  A furnace tube or path with fluted fins will be described according to FIG. The furnace tube 1 comprises a central channel 2 and an annular wall 3. Fins 4 and 5 in this embodiment are straight and do not curve or taper inwardly to tip 6 and tip 7. The fins carry a series of parallel grooved channels 10 on their surface.

  In another embodiment of the invention, the fins may comprise an array of ridges.

  FIG. 2 shows a fin 20 of the present invention whose surface 21 is covered by one or more ridges. The ridges may be in the form of a square pyramid 23, an equilateral cone 24 or a hemisphere 25.

  The ridges can be applied by casting or machining the fins or by using a knurled roll so that the surface 21 of the fins has a textured surface.

  The array of ridges may cover 10% to 100% (and any range in between) of the outer surface of the fin. In some embodiments of the present invention, the ridges may cover 40 to 100%, usually 50% to 100%, generally 70% to 100% of the outer surface of the fin / radiant coil. If the ridge does not cover the entire surface of the fin, the ridge may be located at the bottom, middle or top of the fin.

The base of the ridge is in contact with the outer coil surface. The area of the base of the ridge is less than 0.1% to 10% of the maximum thickness of the fin. Preferably, the ridges are compared, for example tetrahedrons, pyramids, cubes, cones, parts of spheres (e.g. hemispheres or less), parts of ellipsoids, parts of deformed ellipsoids (e.g. teardrops) etc. Having a geometrical shape with a relatively large outer surface with a small volume. Some useful shapes of ridges are:
A tetrahedron (the base of a triangle and a pyramid with three faces that are equilateral triangles),
Johnson's square pyramid (a pyramid with a square base and sides that are equilateral triangles),
Pyramid with sides of four isosceles triangles,
A pyramid with sides of an isosceles triangle (eg if the pyramid is a pyramid with 4 faces, the base may not be square, it may be rectangular or parallelogram),
A portion of a sphere (eg hemisphere or less than hemisphere),
A portion of an ellipsoid (eg, a portion of a shape or volume formed when an ellipse is rotated about its major or minor axis),
A portion of the teardrop shape (e.g. a portion of the shape or volume formed when the non-uniformly deformed ellipsoid is rotated along the axis of deformation), and a parabola, for example various types of triangular wings Included is a portion of the shape (e.g., a portion of the shape or volume that is formed when the paraboloid is rotated about its major axis (a deformed hemispherical (or less than that spherical)).

  The choice of the shape of the ridges is mainly based on the ease of manufacture of the fins. One of the methods of forming ridges on the fin surface is by casting in a mold having the shape of ridges on the mold wall. This is effective in a relatively simple form. Also, the ridges may be produced by machining the outer surface of the cast fin, such as by using a knurling device, for example a knurled roll.

  The aforementioned raised portion is a closed solid.

The ridges have a height (L _z ) above the surface of the fin, 3% to 15% of the maximum thickness of the fin and any range therebetween, preferably 3% to 10% of the maximum thickness of the fin It can.

  In some embodiments, the density of the ridges is uniform and substantially covers the outer surface of the fin. However, the density may be selected based on the radiant heat flux at the location of the coil path (eg, some locations may have a higher heat flux than others (corners)).

  In the design of the ridges, care must be taken that the ridges absorb more radiant energy than can be emitted. This can be restated that, under the same operating conditions, the transfer of heat from the base of the ridge to the surface of the fin must exceed the heat transferred to the equivalent surface of the bare smooth fin. The bumps may start to reduce heat transfer due to the thermal effect of the excess conduction resistance, if the bumps are overcrowded and their contours are not properly selected, which is according to the invention It ignores the purpose. Properly designed and manufactured ridges increase the net radiant and convective heat transferred from the flowing ambient combustion gases, flames, and furnace refractories to the fins and subsequently to the coils. It will The positive effect of the ridges on radiative heat transfer is not only that more heat can be absorbed through the outer surface of the increased fins, as the contact area between the combustion gas and the fins is increased, as well as the fins It is also due to the fact that the relative heat loss through the radiating fin surface is reduced, since the surface of the is no longer smooth. Thus, when the ridge radiates energy around it, part of this energy is delivered to the other ridge and taken up by the other ridge, so that it is directed again to the fin surface. The increase in the fin outer surface in contact with the flowing combustion gas and the increase in turbulence along the fin surface, thereby reducing the thickness of the gas boundary layer adjacent to the fin surface, The ridges will also increase convective heat transfer to the fins.

  Figure 3 shows an equilateral pyramid 26, a square pyramid 23, a regular cone 24, the ridges having a major dimension "a" (length of a side of the pyramid or diameter for a cone or hemisphere) in mm. And the hemisphere 25 as a percentage increase in the area of the surface 21 of the fin 20.

  The size of the ridges must be carefully selected. Generally, the smaller the size, the higher the surface to volume ratio of the ridges, but it can be more difficult to cast or machine such textures. Furthermore, in the case of excessively small ridges, the precipitation of the various impurities on the fin surface can reduce the benefits of their presence gradually over time. However, the ridges need not be ideally symmetrical. For example, the bottom of the oval may be deformed into a teardrop shaped shape, so shaped that when the coil is positioned in the furnace, preferably the “tail” is of the flue gas flow It may point downward along the general direction.

  Another important advantage of fins with grooves or ridges is that the contact surface of the fins is increased, but their weight can be reduced.

  The fins and furnace tube may comprise the same material. In some embodiments, the fins are easiest to cast as part of the furnace tube. In other embodiments, the fins may be separately cast and welded in place.

  The tubes and fins may comprise about 55 to 65 wt% Ni, about 20 to 10 wt% Cr, about 20 to 10 wt% Co, about 5 to 9 wt% Fe, and one or more It may contain a residual component which is a trace element. The alloy from which the tubes and fins are made is 0.2 to 3 wt% Mn, 0.3 to 2 wt% Si, less than 5 wt% titanium, niobium and all other trace metals. It may further comprise carbon in an amount of less than .75 wt%, and the total of the components may be 100 wt%.

  Furnace tubes and fins may comprise 40 to 65 wt% Co, 15 to 20 wt% Cr, 20 to 13 wt% Ni, less than 4 wt% Fe, one or more trace elements and 20 The sum of the components may be up to 100% by weight, including the balance being W up to weight%. The alloy from which the furnace tubes and fins are made consists of 0.2 to 3 wt% Mn, 0.3 to 2 wt% Si, less than 5 wt% titanium, niobium and all other trace metals It further comprises carbon in an amount of less than 0.75% by weight, and the total of the components may be 100% by weight.

  The furnace tubes and fins may comprise 20 to 38% by weight chromium and 25 to 48% by weight Ni. The alloys from which furnace tubes and fins can be made are: 0.2 to 3% by weight Mn, 0.3 to 2% by weight Si, less than 5% by weight titanium, niobium and all other trace metals The method may further comprise carbon in an amount of less than 0.75% by weight and the balance substantially iron, the total of the components being 100% by weight.

  The grooves or ridges may be machined into the surface of the cast fin. In some embodiments, it is preferable to cold roll the fin (at a temperature below the steel recrystallization temperature) to create a groove / ridge without removing any material. This may be particularly useful if the fins are substantially flat.

  The grooves or ridges may be geometric patterns such as longitudinal or lateral parallel lines, diagonal lines, crosshatch patterns, squares, rectangles, circles, ovals and the like. The pattern may be regular or semi-regular.

  The present invention provides furnace tubes for cracking paraffins to olefins with improved efficiency.

Claims (21)

A furnace tube having one or more thick fins on its outer surface,
The fin has a thickness at its base which is 1/4 to 3/4 of the radius of the furnace tube, and a side or parallel side having an upper inward taper of less than 15 ° with respect to the main axis of the fin Have
The fins are outwardly open grooves of a regular or semi-regular pattern covering at least 10% of the surface area on at least one major surface, which is less than a quarter of the maximum thickness of the fins A groove having a depth, a ridge having a base dimension not exceeding 10% of the maximum thickness of the fin, and a height not exceeding 15% of the maximum thickness of the fin, or at least one major surface of the fin A furnace tube having an array selected from the group consisting of both in a regular or semi-regular pattern covering at least 10% of the surface area of the furnace.   The furnace tube of claim 1, wherein the array covers a quarter or more of at least one major surface of the fin.   The furnace tube according to claim 2, wherein the fin has a thickness at its base which is 1/3 to 1/2 of the radius of the furnace tube.   4. The furnace tube of claim 3, wherein the fins have a cross section in the form of an outwardly extending parabolic, parallelogram, "E" shape, or a rounded "V".   5. The furnace tube of claim 4, wherein the array comprises grooves having a depth of one eighth to one tenth of the maximum thickness of the fins.   Said groove being in the form selected from an outwardly open V, a truncated outwardly open V, an outwardly open U, and an outwardly open parallel surfaced channel; The furnace pipe according to claim 5. The array is
i) a maximum height which is 3 to 15% of the base of the fin,
ii) a contact surface or a base with a fin whose main dimension is 0.1% to 10% of the thickness of the fin,
iii) The furnace tube according to claim 3, comprising a ridge having a geometry with a relatively large outer surface having a relatively small volume. The ridges are
tetrahedron,
Johnson's square pyramid,
Pyramid with sides of four isosceles triangles,
Pyramid with sides of isosceles triangle,
Part of a sphere,
A portion of an ellipsoid,
8. The furnace tube of claim 7, having a shape selected from the group consisting of a teardrop shaped portion and a parabolic shaped portion.   The furnace tube according to claim 5, wherein the fins form a transverse plate in the form of a circle, an ellipse, or an N-side polygon.   The furnace tube according to claim 7, wherein the fins form a transverse plate in the form of a circle, an ellipse or an N-sided polygon.   The furnace tube according to claim 5, wherein the fins are longitudinal fins having a cross section in the form of an outwardly extending parabolic, parallelogram or "E" shape.   The furnace tube according to claim 7, wherein the fins are longitudinal fins having a cross section in the form of an outwardly extending parabolic, parallelogram or "E" shape.   The furnace tube of claim 1, wherein the furnace tube and the fins comprise the same metal composition.   About 55 to 65 wt% Ni, about 20 to 10 wt% Cr, about 20 to 10 wt% Co, about 5 to 9 wt% Fe, and the balance being one or more trace elements 14. The furnace tube of claim 13, including a minute.   0.2 to 3 wt.% Mn, 0.3 to 2 wt.% Si, less than 5 wt.% Titanium, niobium and all other trace metals, and less than 0.75 wt.% Carbon The furnace tube according to claim 14, further comprising a total of 100% by weight of components.   40 to 65 wt% Co, 15 to 20 wt% Cr, 20 to 13 wt% Ni, less than 4 wt% Fe, one or more trace elements and up to 20 wt% W 14. The furnace tube according to claim 13, comprising a certain residue, the sum of the components being 100% by weight.   0.2 to 3 wt.% Mn, 0.3 to 2 wt.% Si, less than 5 wt.% Titanium, niobium and all other trace metals, and less than 0.75 wt.% Carbon The furnace tube according to claim 16, further comprising   14. The furnace tube of claim 13, comprising 20 to 38 wt% chromium and 25 to 48 wt% Ni.   0.2 to 3 wt.% Mn, 0.3 to 2 wt.% Si, less than 5 wt.% Titanium, niobium and all other trace metals, and less than 0.75 wt.% Carbon 19. The furnace tube of claim 18, further comprising: and a balance that is substantially iron.   The decomposition furnace provided with the radiant part which has a furnace pipe of Claim 1.   21. A method of decomposing paraffin, comprising passing the paraffin in gaseous state through the radiant part of the cracking furnace according to claim 20 at a temperature of 600 ° C. to 1000 ° C. for a time of 0.001 to 0.01 second. Method, including.