MXPA99011425A - Pyrolysis furnace with an internally finned u-shaped radiant coil - Google Patents

Abstract

The present invention is directed towards a fired heater for heating a process fluid that uses U tubes provided with increased internal heat transfer surface to reduce tube metal temperatures and coking, and at the same time is not prone to plugging from coke. The fired heater comprises a radiant section enclosure with a number of U tubes in the radiant section. The U tubes are formed by connecting one or more tubular sections, and the U tubes are provided with internal generally longitudinal fins. The invention also is directed towards a process utilizing a fined heater with U tubes as disclosed for producing olefins from hydrocarbon feedstocks.

Description

PIROLYSIS OVEN WITH A U-SHAPED RADIANT SERPENT WITH INTERNAL FINS Field of the Invention The present invention relates to a fire heater for heat processing fluids, for example, process heaters. More specifically, it relates to a fire heater of the type comprising at least one radiant section in which the process fluid flowing through the tubes therein is indirectly heated, preferably, by radiant energy provided by burners. The methods and apparatuses used in accordance with the present invention are particularly suitable and advantageous for the pyrolysis of normally liquid or normally gaseous aromatic and / or aliphatic hydrocarbon feedstocks such as ethane, propane, naphtha or gas oil to produce ethylene and other products secondary ones such as acetylene, propylene, butadiene, etcetera. According to the above, the present invention will be described and explained in the context of hydrocarbon pyrolysis, particularly steam disintegration to produce ethylene. Background of the Invention Vapor disintegration is the predominant commercial method for producing light olefins such as ethylene, propylene and butadiene. Ethylene, propylene and butadiene are fundamental chemical products used in the manufacture of polymeric materials in high volume and commercially important chemical intermediates. The. demand for these fundamental petrochemical products is expected to continue growing in the foreseeable future. Of the products produced in the steam disintegration, ethylene has the highest demand, and is the most expensive to separate and purify. Therefore, improving the yield or selectivity of ethylene is very desirable. The steam disintegration involves a thermal disintegration reaction typically carried out in a tubular reactor with fire7 The selectivity of the reactor to ethylene is favored by short residence time and low partial pressures of hydrocarbons. Hydrocarbon feeds that vary from ethane to vacuum gas oil are used, and the reaction is carried out in the presence of dilution vapor. The complex reactions and the tubular reactor are discussed extensively both in public domain literature and in numerous patents. "Vapor disintegration of typical hydrocarbons has been effected by supplying the feedstock in a vaporized or substantially vaporized form, in admixture with substantial amounts of dilution vapor, to convenient coils in a disintegration furnace. It is conventional to pass the reaction mixture through several coils or parallel tubes which pass through a convection section of the disintegration furnace where the hot combustion gases raise the temperature of the feed and the dilution vapor. Each coil or tube then passes through a radiant section of the disintegration furnace wherein a multiplicity of burners supply the necessary heat to bring the reactants to the desired reaction temperature and effect the desired reaction. ~ The main concern in all configurations of the steam disintegration process is the formation of coke. When the hydrocarbon feed charges are subjected to prevailing heating conditions in a steam disintegrating furnace, coke deposits tend to form on the inner walls of the tubular members forming the disintegration coils. These coke deposits interfere with the flow of heat along the walls of the tube in the reagent stream, which results in higher metal tube temperatures, ultimately reaching the limits of the metallurgy of the tubes. Additionally, coke deposits interfere with the flow of the reaction mixture, resulting in a higher pressure drop, due to the reduced cross-sectional area of the tube. - The optimal way to improve the ethylene selectivity was found to be to reduce the coil volume while maintaining the heat transfer surface area. This has been done by replacing the large diameter coils with a multiplicity of smaller diameter tubes that have a larger surface to volume ratio than tubes of larger diameters. The tubes typically have internal diameters of up to about 7.6 centimeters, usually from 3.0 centimeters to 6.4 centimeters. The desire for shorter residence times has led to the use of shorter coils, reducing the typical lengths over the years from 45 meters to 20 meters - 27 meters, and more recently from 9 meters to 12 meters. Since the coils have been reduced in length, it has been necessary to reduce the diameter of the tube in an effort to reduce the heat flow and therefore the temperatures of the pipe metal. The current disintegration coils are usually constructed of high alloys (25% Cr, 35% Ni, plus additives) of austenitic stainless steels, and are operated at maximum metal temperatures "in the range of 1030-1150 ° C. Despite the significant evolution of the design of the disintegration furnace, the process is still limited by the fact that it produces coke as a secondary product, which is deposited on the inside of the coils, coke acts as an insulator, and It increases the temperature of the coil tube metal.When the metal temperature of the tube reaches the maximum capacity of the material it is necessary to stop production and decarbonize the furnace.This is generally carried out by passing a mixture comprising air and steam to Through coils at high temperature, the coke is removed by a combination of combustion and erosion / chipping, and other industries are also used in the industry. decarbonization techniques, which avoid the use of air. In this case, the coke is removed mainly by erosion / flaking and gasification. Regardless of the technique of decarbonization, some of the chipped coke is in the form of large particles. As the diameters of the tubes have decreased the likelihood of large coke particles clogging the coil before or during decarbonization has increased. Decarbonization typically takes 12 to 48 hours, depending on a variety of factors including: the design of the furnace, the feed that disintegrated, the operating time before decarbonization, and the severity of the disintegration employed. The technology to reduce the temperatures of the metals of the tubes (and therefore the rates of coke formation, or alternatively allow a shorter residence time in the coil to be used) has been much sought after in the industry. Some designers have resorted to multiple inlet leg coils to reduce the heat flow in the outlet tubes (eg.; European patent application EP 0305 799 Al). Others have tried to avoid the formation of the insulating coke layer inside the tube by adding small concentrations of specific elements to the reactor feed. The heat transfer to the very endothermic decay reaction can be represented by the familiar equation Q = UxAx? T. U, the heat transfer coefficient is a function of the gas velocity inside the tube. The higher speeds increase 0, and therefore reduce the? T (temperature difference) thereby reducing the temperature of the metal for a given temperature of the process fluid. However, as the velocities increase, the pressure drop increases, increasing the average partial pressure of the hydrocarbon, eventually the effect of the pressure exceeds the effect of the reduced residence time, and further increases in the speed reduce the selectivity of the reactor to the This represents a maximum practical value for ü - The global area (A) can be increased using multiple small diameter tubes.This trend has been followed by the industry, resulting in reactors with 2.5 cm inner diameter tubes. to 3.8_centimeters.This represents a minimum practical diameter due to manufacturing limitations, and below these diameters the effects of coke accumulation inside the pipe become excessive.The general principle of increasing the area of the inner surface to improve heat transfer is well known in the general heat transfer technique. It started the carbonization services at very high temperatures as steam disintegration is difficult. However, this method for improving the heat transfer to reduce the metal temperatures of the tubes in heat disintegrating furnaces has been proposed in different variants. An example (U.S. Patent No. 4,342,242) uses a specially designed longitudinal insert in a cross section of the formerly circular tube. The insert has a central body and valves that extend outwards which make contact with the interior of the coil. In this particular description the insert is placed only in a portion of the overall tubular coil in the furnace. Another example (Great Britain Patent No. 969, 796) uses internally rounded channels or fins that increase the interior area. The internal profile was smooth to avoid r voltage concentrations and flow disturbances. The specific tubes described in the description make 4 steps through the radiant section and have a relatively large internal diameter of 9,525 centimeters. Variants of these rounded internal channels or finned tube profiles have been applied commercially in specific designs of coils. A work presented before a meeting of the American Institute of Chemical Engineers (AIChET) ("Specialty Furnace Design Steam Reformers and Steam Crackers" by TA Wells, presented at the National Meeting, Spring 1988 of the AIChE, New Orleans, Louisiana, March 6-10, 1988) describes the use of a type of Internal surface tube extended in a one-step tube design. The longer serpentine entrance legs (European patent application 0 305 799 A1) and a reference to this literature, called SRT V (Energy Progress Vol. 8, No. 3, pp. 160-168, Sept. 1988) ) have used extended internal surface. In both of the latter cases the commercial use was based on tubes of approximately 2.5 to 3.8 centimeters of the inner diameter where the section of the tube having the rounded internal channels or fins made only a single pass through the radiant section of the furnace . Another literature reference ("USC Super = U Pyrolysis Concept" by David J. Brown, John R. Brewer and Colin P. Bowen presented at the AIChE National Spring Meeting in Orlando, Florida, March 1990) presents data on tubes _ with internal fins on the entrance leg. This reference speculates that providing fins on the outer leg would be beneficial, - however it does not provide suggestions of what operating or design parameters would be required to successfully demonstrate or allow the use of fins on the outer leg. However, an internal surface design extended so far has not proved feasible in two-step coils typically made of U-shaped tubes. These two-step coils typically have 15 meters to 27 meters in total length, with internal diameters in the range of 3.8 centimeters to 6.4 centimeters. ~ Two-step coils can be as short as 13 meters. One problem is that there is no ability to make a tube with internal fins long enough to form the entire U-shaped tube. A tube with internal fins could be used only for the inlet half of the U-shaped tubes, as described in the European patent application 0 305 799 Al, which uses internal fins, studs or inserts only in the inlet pipes to the furnace -not to the outlet. This reference describes that the inserts placed in the outlet tube would be expected to act as a core for the accumulation of the coke formed during the pyrolysis. However, the higher temperatures of the tube metal occur near the outlet end, so that the advantageous effect of the finned tube is not applied where it is most needed. Applying the finned tube to the exit leg of the coil would be possible, but there is a risk that pieces of coke could be released from the entrance leg and lodged at the beginning of the finned section. Finally, the conventional wisdom of the industry suggested that a section of bended finned tube would be prone to being covered with chipped coke from the inlet leg of the coil. In light of the known deficiencies in heat transfer in steam disintegrating furnaces there is a need for means to increase the heat transfer in the inner part of the tubes to reduce coke accumulation, the temperature of the metal of the tube and improve the selectivity of ethylene. In particular, it would be very desirable to have a design for a 2-step coil or U-shaped tubes that use some means of increasing the internal surface area to reduce the temperature of the tube metal throughout its entire length. SUMMARY OF THE INVENTION The present invention relates to a fire heater ~ to ~ heat a process fluid that provides increased internal heat transfer surface to reduce the temperatures of the pipe metals at the inlet and outlet of a shaped tube. of U and that at the same time is not prone to being covered with coke. The fire heater comprises a housing with radiant section having a plurality of U-shaped tubes disposed therein, an inlet to introduce the process fluid into the U-shaped tubes, burners to expose the external surface of the tubes in U-shape to the radiant heat, an outlet to cool and collect the process fluid from the tubes U-shaped, where the U-shaped tubes are formed by connecting one or more tubular sections; and at least the outlet leg of the U-shaped tubes is provided with generally longitudinal internal fins. In another embodiment the entire length of the U-shaped tubes is provided with generally longitudinal internal fins. Drawings These and other features, aspects and advantages of the present invention will be better understood with respect to the following drawings, description and appended claims. Figure 1 depicts a three-dimensional drawing of a steam disintegration furnace showing a typical arrangement of internal parts. Figure 2 shows a single furnace tube, U-shaped. Figure 3 shows a cross-section of furnace tubes in the form of a finned U-shape. Detailed Description of the Invention The present invention describes a fire heater that heats a process fluid. More particularly the invention is directed towards a fire heater that heats a process fluid which is prone to form coke as a result of chemical reactions that occur as a result of heating. A specific embodiment of the invention is a steam disintegration furnace used in the petrochemical industry to manufacture olefins. Referring to Figure 1 the feed stream 9 enters the convection section 10, through one or more inlet lines 9 where it is preheated preferably to a temperature from about 426 ° C to 816 ° C by combustion gases hot, where these gases are preferably at a temperature of about 816 ° C to about 1316 ° C before the inlet of the radiant section enters 12. From the inlet distributor of the radiant section the preheated feed enters the tubes U-shaped (hereinafter referred to herein as tubes U) which are located inside the housing of the radiant section 16, also known in the art as a radiant box. The housing of the radiant section 16 is generally lined with heat insulating refractory material to preserve the heat energy. The housing of the radiant section includes a plurality of tubes U. The ends of the tubes U which are connected to one or more feed inlet manifolds 12 which introduce the process fluid into the tubes U are called inlet legs 20. The opposite end of each of the tubes U 22 called the output leg is connected to an output head 26 to collect the process fluid after it has been heated and the thermal disintegration reactions have been presented. The temperature of the process fluid is typically from about 816 ° C to about 1093 ° C when leaving the outlet leg of the tube U. From there the process fluid passes to the quench exchanger 27 which cools the process fluid to stop the thermal disintegration reactions. In another embodiment, not shown in Figure 1, the output leg of each of the tubes U is directly connected to an individual heat exchanger to cool the process fluid. The output of each individual tempering exchanger is then connected to an output head. This arrangement is known in the art as a closed coupled transfer line exchanger. In still another embodiment not shown in Figure 1, the outlet leg of each tube U is connected to a tempering point where the process fluid is directly contacted "with a quenching liquid that vaporizes to cool the process fluid. For the purposes of this invention the U-tubes have a "U" -like shape when viewed in a two-dimensional drawing as in Figure 2. A defining feature is that the U-tube effectively passes through the radiant housing 2 passes. The tubes U are composed of an entrance leg 20, an exit leg 22, and a curved or bent portion 21 connecting the entrance leg 20 and the exit leg 22. In other modalities the exit leg can be composed of one or more branched portions In still other preferred embodiments the entry leg 20 may be composed of more than one branched tube.There are a variety of ways in the art of accommodating a plurality of U-tubes in a radiant housing. Those skilled in the art will consider the spatial arrangement, the location of the burners, the location of the input head and the "output elements, and the thermal stresses in the same tubes U when choosing the configuration In some configurations each of the individual U-tubes lies in a single plane. In other configurations the U-tubes are bent out of plane.

All these configurations are contemplated as U-tubes for the purposes of this invention. The "radiant section" housing contains a plurality of burners 28 for exposing the outer surface of the U-shaped tubes to radiant heat A wide variety of types of burners known in the art can be used including raw gas or burners Recent designs have additionally used a variety of gas duct recirculation techniques to reduce NOx formation for environmental reasons.The combustion air source can be from ambient air, preheated air or exhaust from The total length of the U-tubes is preferably from 20 meters to 27 meters, since it is difficult to manufacture internally flared tubes in the desired length of 20 meters to 27 meters, it may be necessary to connect two sections with at least one intermediate weld As described in U.S. Patent No. 4,827,074 intermediate welds are known which are a potential source of deposits. accelerated coke. In a preferred embodiment this potential coke deposit is minimized in U tubes with an intermediate weld in the lower portion of the curved portion of the U and accommodating the U tubes so that the weld is protected from direct radiation by adjacent pipes . In another embodiment, the welded area may be wrapped in insulating material.

The internally finned tubes can be successfully bent to the required radius in the bottom of the U-tube using either well-known cold bending, or heat-induced bending techniques. _ Whether the tubes U ^ are formed by connecting two or more tubular sections or are formed in one piece, preferably the entire length of the finned U tubes is provided with generally longitudinal internal fins. Another modality would provide the fins only on the exit leg. In yet another embodiment the fins are provided in the curved portion of the tube U and the sag leg. Figure 3 shows a cross-sectional view of a tube U provided with fins. The outer diameter of the tube 50 is in the range of 4.4 centimeters to 11.4 centimeters, (preferably from 5 centimeters to 7.6 inches.) The height of the fin 52 defined as the distance between the bottom of the root of the fin 54 and the top of the tip of the fin 56 is in the range of from about 0.13 centimeters to 1 centimeter), preferably from 0.25 centimeters to 0.65 centimeters.) The number of fins around the inner circumference of the tubes is 8 to ~ 24, preferably 10 to 18. The radius of the fin 58 and the tip of the fin 60 are in the range of from 0.13 centimeters to 1.2 centimeters), preferably from 0.25 centimeters to 0.5 centimeters). radius of fin root and radius of tip of fin are equal. The inner diameter 62, defined as the distance through the center of the tube from the root of the fin to the root of the fin is in the range of 3.2 centimeters to 7.6 centimeters, preferably from 3.8 centimeters to 6.4 centimeters, more preferably 5. centimeters to 6.4 centimeters. The ratio of the height of the fin to the internal diameter necessary to provide improved heat transfer, without having excessive pressure drop or propensity to cap is preferably in the range of from 0.05 to 0.20, more preferably in the range of from 0.07 to 0.14 The generally longitudinal fins may be straight along the length of the U-tube or propeller, analogous to the scratching of a rifle barrel.The last longitudinal configuration of fins is also known as longitudinal spiral fins. a section, to form the U-shaped tube, the fins are preferably aligned at each connection to reduce the likelihood of coke particles becoming trapped on the edge of the fins.Examples __ A test program was carried out to determine if the expected limitations could be overcome, and the advantages of the increased internal surface area could be applied to a U-tube vapor disintegration furnace design. Twenty-two internally finned U-tubes were installed in one quadrant of a commercial steam disintegration furnace (88 U-tubes in total). The feed loading furnace was commercial ethane (98% ethane) recovered by natural gas separation facilities. In this way most of the U-tubes in the furnace remained as conventional circular cross-section tubes, while a quarter of the tubes had straight longitudinal fins according to the invention. This provided a direct comparison of the performance of the finned tubes as compared to the conventional circular cross section (plain) tubes. Figure 3 can be used to describe the fin configuration of the U-tubes in the test quadrant of finned tubes. The outer diameter 50 of the U tubes was 6.98 centimeters. The internal diameter 62 of the U tube was 5 centimeters. There were 12 fins. The height of the fin 52 was 0.40 centimeters. The radius of the fin 58 and the radius of the tip of the fin 60 were both 0.40 centimeters. The ratio of the height of the fin to the internal diameter was 0.08. Since it was difficult to isolate the internally flared tubes to the desired length of 20 meters, intermediate welding was required. This intermediate weld was placed in the lower part of each of the U tubes, where they were protected from direct radiation by the adjacent tubes. The fins lined up in this connection. The bent portion of the U coil was not prone to blockages, as has been suggested in the prior art, no increases in sudden pressure drop were observed during the 12-month test program. The internally finned tube reduced the tube metal temperatures. The test coil developed coke deposits at a much slower rate than conventional (smooth) tubes of circular cross-section in the same steam disintegration furnace, with the same feed charge.

T? B? 1 Fall of Yress (Radiant Entrance - Radiant Exit) Days in the Stream? P, bars Conventional Smooth Pipes Finned Tubes 0.5 0.28"~ 0.28 2.5 0.43 ~ 0.36 4.5 0.52 Z 0.38 0.75 = 0.38 11 0.83 0.38 15 0.90 0.40 21 1.48 0.50 Table 1 shows the pressure drop for the U-shaped coils as a function of the days in the stream, this is days since the last decarbonization. The greater the pressure drop, the thicker the coke is formed. The table compares the smooth (conventional) tubes with the finned tubes. As can be seen from the data, the pressure drop increased more significantly during the course of the run for the smooth tubes against the finned tubes, which indicates greater thickness of coke in the smooth tubes. Also the significantly lower pressure drop for the finned tubes clearly indicates that no clogging occurred during the run.

Tg? BLA 2 Tube Metal Temperature Days in the Current Tube Metal Temperature ° C Conventional Smooth Tubes Finned Tubes 0.5 1016 _ 1004 2.5 1031 ~~ - -1003 4.5 1037 1007 8 1048 1016 11 1050 1022 15 1041 _ 1018 21 1056 1028 Average 1040 1014 Table 2 shows the maximum temperature of the tube metal measured with an infrared pyrometer again as a function of the days in the stream. As described above it is critically important to reduce the maximum temperature of the tube metal. The tube metal temperatures were significantly lower during the entire course of the run for the finned tubes against the conventional (flat) tubes, averaging approximately 26 ° C less. Additionally, internally flared tubes required much less time than conventional circular cross section tubes for decarbonization. For the disintegration of ethane the conventional (smooth) tubes required a range of 8 to 10 hours of decarbonization but the finned tubes required a range of 4 to 5 hours. Without wishing to be limited to a specific theory of operation, it appears that the finned U-tubes configured as described in this invention provide fracture zones in the coke layer at the location of each of the fins, so that small pieces of coke They are especially prone to flaking or breaking inside the tube during the decarbonization process. This has two extremely important and unexpected effects compared to conventional smooth tubes. First, it makes the decarbonisation process take less time by allowing the furnace to be put into full productive operation sooner, "thus providing a significant economic benefit to the operator." Second, the fracture zones favor forming only relatively small coke particles, by what has been found that does not cover or block the tubes, even tubes of relatively small diameter in the range of 3.04 centimeters to 6.35 centimeters and even the bent or curved section of the "U" in 2 passes the U tubes. ___ A medium The preferred method of operating a furnace with internally flared U-tubes according to the invention is such that the accumulation of the coke layer is excessive, in order to favor the spalling of small coke particles. The thickness of the coke in an operating pyrolysis furnace can be estimated by approximately 1.5 times the height of the fin. one skilled in the art from operational data of the furnace and knowledge about the disintegration characteristics of the "feed load. The thickness of the coke is calculated based on measured metal tube temperature profiles, measured pressure drop for the tubes within the radiant housing, the known or measured density and the thermal conductivity of the coke. One skilled in the art can use the aforementioned measured parameters in well-known fluid flows and heat transfer equations to estimate the thickness of the coke in an operation oven and schedule the decarbonization operations in accordance with the foregoing. Although the present invention has been defined in considerable detail with reference to certain preferred embodiments, other embodiments are possible. Therefore, the spirit and scope of the invention should not be limited to the preferred embodiments contained herein.

Claims (29)

1. CLAIMS 1. A heater for_ heating a process fluid, comprising: radiant housing means, having disposed therein a plurality of two passage tubes, comprising: (a) at least one "leg of entry in flow communication with (b) at least one exit leg, and (c) curved tubular means for providing flow communication between the entry leg and the exit leg, wherein each exit leg is provided with generally longitudinal internal fins; process fluid to the inlet leg, means for exposing the external surface of the two passage tubes to heat, exit means for cooling and collecting the process fluid from the outlet leg, 2. a heater as defined in FIG. claim 1, wherein the entry leg is provided with generally internal longitudinal fins. A heater as defined in claim 1 or claim 2, wherein the tubular means curved to provide Flow communication between the entrance leg and the exit leg are provided with generally longitudinal internal fins. A heater as defined in claim 1, comprising: radiant section receiving means having a plurality of U-tubes disposed therein, input means for introducing the process fluid into the U-tubes, means for exposing the external surface of the U-tubes to radiant heat, exit means to cool and collect the process fluid from each of the U-tubes, where the length of the U-tubes is provided with generally longitudinal internal fins. A heater as defined in any one of claims 1 to 4, wherein the "inner diameter of the U-tubes is from 3.2 to 7.6 cm 6. A heater combo defined in the claim 5, where the internal diameter of the U-tubes is 3.8 to 6.4 cm. 7. A heater as defined in the claim 6, where the internal diameter of the U-tubes is 5 to 6.4 cm. 8. A heater as defined in any of claims 1 to 7, wherein the U-tubes have a constant diameter. A heater as defined in any of claims 1 to 8, wherein the U-tubes are provided with 8 to 24 generally longitudinal fins spaced around the inner circumference of the U-tubes. 10. A heater as defined in FIG. Claim 9, wherein the U-tubes are provided with 10 to 18 generally longitudinal fins spaced around the inner circumference of the U-tubes. A heater as defined in any of claims 1 to 10, wherein the tubes in U are formed by connecting two or more tubular sections. 12. A heater as defined in claim 11, wherein the fins are aligned essentially at each connection. A heater as defined in claim 11 or claim 12, wherein the U-tube is formed of two tubular sections and a single connection by welding the two tubular sections together and where the weld is substantially shielded from direct radiant heat. A heater as defined in claim 13, wherein the connection alone is made essentially in the lower part of the U. 15. A heater as defined in any of claims 1 to 14, wherein the total length of each of the U-tubes is 13 to 27 m. "" 16. A heater as defined in claim 15, wherein the total length of each of the U-tubes is from 15 to 27 m. A heater as defined in any of claims 1 to 16, wherein the ratio of height to internal fin diameter is in the range of 0.05 to 0.20. 18. A heater as defined in claim 17, wherein the ratio of height to internal fin diameter is in the range of 0.07 to 0.14. 19. A heater as defined in any of claims 1 to 18, wherein the generally longitudinal internal fins have a fin height of 0.13 to 1 cm. 20. A heater as defined in any of claims 1 to 19, where the generally longitudinal internal fins have a fin tip radius in the range of 0.13 to 0.65 cm. 21. A heater as defined in any of claims 1 to 20, wherein the generally longitudinal inner fins have an essentially equal radius for the root and tip of the fin. 22. A process for the manufacture of olefins, comprising: pre-heating a hydrocarbon feedstock, introducing the pre-heated feedstock to a plurality of radiating coils, heating the radiating coils by a plurality of burners to make the hydrocarbon feed material thermally cracked, collect the cracked hydrocarbon feedstock from the radiant coil, cool the cracked hydrocarbon feedstock, recover at least one olefin from the cracked hydrocarbon feedstock, - where each of the coils radiant comprises: (a) at least one leg inlet in flow communication with (b) at least one leg and (c) curved tubular means to provide flow communication between the entrance leg and the exit leg, where each exit leg is provided with generally longitudinal internal fins. 23. The process of claim 22, wherein the entry leg is provided with generally longitudinal internal fins. The process of claim 22 or claim 23, wherein the curved tubular means for providing flow communication between the entrance leg and the exit leg are provided with generally longitudinal internal fins. - - 25. A process as defined in claim 22, comprising: pre-heating a hydrocarbon feedstock, introducing the pre-heated feedstock into a plurality of radiating coils, heating the radiating coils by a plurality of burners to cause that the hydrocarbon feedstock thermally cracks, picks up the cracked hydrocarbon feedstock from the radiant coil, recovers at least one olefin from the cracked hydrocarbon feedstock, decocts the radiant coil from the coke accumulated therein due to cracking reactions thermal, where the radiant coils comprise an inlet leg and an outlet leg connected to form a U-shape, and are provided with generally longitudinal internal fins and where the decoking process of radiating coils is initiated before the thickness of the coke accumulated in it exceeded gives a sufficient thickness to cause large particles of coke to splash from the surface of the tube and plug sections downstream of the radiating coil during the decoking step. 26. The process of claim 25, wherein the decoking process is initiated before the average coke thickness exceeds about 1.5 times the height of the fins. 27. A process as defined in any of claims 22 to 26, wherein the radiating coils are defined as in any of claims 5 to 21. 28. In a fire heater for heating a process fluid, the which heater comprises a radiant section housing with a plurality of U-tubes arranged therein, an inlet head for introducing the process fluid into the tubes, and a plurality of burners for exposing the outer surface of the U-tubes to the radiant heat , the use of U-tubes provided with generally longitudinal inner fins over their entire length 29. The use as defined in claim 28, wherein the U-tubes are as defined in any of the claims. to 21.