MXPA06008885A - Cracking furnace - Google Patents

Abstract

The present invention refers to a novel type of cracking furnaces comprising a firebox provided with cracking coils - the cracking coils having at least one inlet, at least one inlet section, at least one outlet and at least one outlet section - and burners, wherein the parts of the coils are shielded. The invention further relates to a process for cracking hydrocarbon feeds, making use of a furnace according to the invention.

Description

CRAQUEO OVEN Description of the invention The invention relates to a furnace for the cracking (thermally) of a hydrocarbon feed in the vapor phase in the presence of water vapor. The invention also relates to a method for cracking (thermally) a hydrocarbon feed in the vapor phase in the presence of a diluent gas, in particular water vapor. Cracking furnaces are the center of an ethylene plant. In these furnaces, feeds containing one or more types of hydrocarbons are converted into a cracked product gas by cracking the hydrocarbons. Typical examples of hydrocarbon feeds are ethane, propane, butanes, naphthas, kerosenes, and atmospheric and vacuum gas oils. The processes to convert hydrocarbons to a higher temperature have been known for many decades. US 2,182,586, published in 1939, describes a reactor and a process for the pyrolytic conversion of a fluid hydrocarbon oil. Use is made of a single tubular reactor, placed horizontally (the publication refers to "tubes", but these are connected in a flow connection - in series and therefore form a single tube), which leads to relatively long residence times Ref.174834 which are common in the thermal cracking process of liquid hydrocarbon oils to improve the quality of motor fuel such as the separation of viscosities by thermal cracking. The use of the heater described for a process similar to steam cracking or for cracking a feed in the form of steam is not mentioned. Instead, cracking - excessive cracking and excessive gas formation are avoided. US 2,324,553, published in 1943, shows another heater -for the pyrolytic conversion of hydrocarbons, where the tubular reactor is formed of "tubes" connected in series, which are placed horizontally in the heater: In the process described, the oil is passed through the tube at a temperature below an active temperature for cracking. WO 97/28232 describes a cracking furnace for thermally cracking a liquid hydrocarbon feed in a spiral tube. The furnace is said to have a reduced sensitivity to coke formation and an increased residence time of the liquid. The use of the facility for steam cracking is not described. Steam cracking is a specific form of thermal cracking of hydrocarbons in the presence of water vapor with the specific kinetic characteristics of the process and other process characteristics. Here, the hydrocarbon feed is thermally cracked in the vapor phase in the presence of steam. The cracking is carried out at a much higher severity than that applied in the moderate cracking of liquid hydrocarbon oils to improve the quality of the fluid. Steam cracking furnaces comprise at least one combustion chamber (also known as a radiant section), which comprises a number of burners for heating the interior. A number of tubular reactors (known as cracking tubes or cracking coils) through which the feed can pass are placed through the combustion chamber. The steam feed in the tubes is heated to such an elevated temperature that rapid decomposition of the molecules occurs, which produces the desired light olefins such as ethylene and propylene. The mixture of hydrocarbon feed and water vapor is typically introduced to the tubular reactors as a steam at about 600 ° C. In the tubes, the mixture is usually heated to about 850 ° C by the heat released by burning the fuel in the burners. The hydrocarbons react in the heated tubes and are converted into a gaseous product, rich in primary olefins such as ethylene and propylene. In cracking furnaces, tubular reactors can be distributed vertically in one or more passes.

In art, the term cracking coil is also used. One or more of the cracking coils, which may be identical or non-identical, may be present to form the radiant section of the total reactor to a combustion chamber. Conventionally, the ethylene cracking tubes are placed in the combustion chamber in a corridor where the aisle is heated - from both sides by the burners. Such a corridor can be a so-called in-line arrangement by means of which all tubular reactors are placed essentially in the same vertical plane. Alternatively, the tubes in such a passage can be in a so-called stepped arrangement, by means of which the tubes are placed in two essentially vertical parallel planes whereby the tubes are placed in a triangular spacing towards each other. Such triangular spacing can be related to equal sides (ie equilateral triangle spacing) or unequal sides, which is called an extended spacing. Examples of such extended spacing configuration are an isosceles triangle spacing, a triangle spacing of right angles, and any other non-equilateral triangle spacing. An example of such an extended spaced furnace is GK6 ™ (see Figure 1) which characterizes a non-equilateral, isosceles triangle spacing in a double aisle coil arrangement. In the GKß oven, the set of two aisles is heated from both sides by the burners 5 located in the bottom and / or the side wall. The inlet sections (which extend from the inlets 4) and the outlet sections (which extend from the outlets 3) are heated in essentially the same way by the burners 5. It has been found that this leads to less optimal cracking conditions . It is thought that this is due to a non-advantageous heat distribution. The cracking process is an endothermic process and requires the entry of heat into the feed. For the operation (selectivity) of the cracking process it is desirable to maximize the heat input to the inlet section of the cracking coil (tube). The inventors therefore teach a way to alter the heat input in the cracking tubes. In addition, it has been found that the use of a known furnace to crack (thermally) a hydrocarbon vapor in the presence of water vapor, whereby ethylene, propylene and / or one or more other alkenes (also called olefins) are formed. ), leads to less favorable conditions for the mechanical stability of the cracking coil assembly. The inventors understood that "due to the fact that the inlet sections on one side of the stepped hallway have different temperature conditions - and different heat distribution conditions than the outlet sections on the other side of the step hallway, there are different conditions of thermal stress and different thermal deformation conditions between the input sections and the output sections.The deformation is the irreversible expansion that occurs when a metal is heated.The deformation is the result of the thermal stresses inside the metal due to heating. Thermal stress (caused by thermal expansion) is the reversible phenomenon when heating any material, both phenomena have to be carefully considered in the design of the coil and cause the restrictions mentioned above in the mechanical distribution of the cracking coil. Therefore, such a serpent arrangement n stepped usually is considered less suitable in steam cracking furnaces to convert light hydrocarbon gases such as ethane. In the cracking with ethane vapor, due to the rigid nature of the carbon deposit in the inner part of the coil, too great an unbalance in the thermal stresses and thermal deformation can cause buckling of the pipe or even breakage of the coil. Nevertheless, even with an in-line arrangement conventionally applied in the art of ethane cracking, such an arrangement requires a complicated coil support system at the inlet, outlet and bottom, necessary to compensate for thermal stresses and thermal deformation. This is also the case in the cracking of the heavier vapor phase hydrocarbons where a stepped arrangement, extended sufficiently, with a properly designed coil support system with variable adjustment parameters, could be adequate. However, continuous operator attention is required to make the adjustments of the support system in the case of different operating conditions and during the operational life of the furnace because the. The dimensions of the coil and the resistance change as a consequence of the deformation during the course of time. It has been found that the heat input, in a method for cracking (with steam) of a hydrocarbon can be altered by the design of the inlet and outlet sections of the cracking coils in a specific manner. Furthermore, it has been found that the thermal stability of the coils can be improved by the design of the cracking furnace, in particular the inlet and outlet sections of the cracking coils in the combustion chamber of the furnace in a specific manner. Accordingly, the present invention relates to a method for cracking a hydrocarbon feed, comprising the passage of the feed, comprising a hydrocarbon and a diluent gas, in particular water vapor, through at least one cracking coil (in the priority application also referred to as a cracking tube) in a combustion chamber under the cracking conditions, wherein the outlet section of each of the coils is more thermally protected than the inlet section of said coil. In the steam cracking method according to the invention, the feed comprises steam and the hydrocarbon is usually fed to the coil as a vapor or a gas. Unless otherwise specified, the term "vapor", respectively "vaporous" as used herein, includes "gas", respectively "gaseous". Furthermore, the invention relates to a novel cracking furnace, suitable for cracking hydrocarbons, in particular to a method according to the invention. Accordingly, the present invention further relates to a cracking furnace (for steam cracking of a hydrocarbon feed), comprising at least one combustion chamber provided with a plurality of cracking coils, the coils comprise at least one Inlet section and at least one outlet section, the combustion chamber comprises at least one aisle of the exit sections of the cracking coils, at least two aisles of inlet sections of the cracking coils and at least two corridors of burners, wherein at least one aisle of the exit sections is located between at least two aisles of the entry sections, and the aisles of the entry sections are located between at least two burner aisles. The burner aisles are usually essentially parallel to each other. The burners are usually mounted in the bottom and / or the side walls and / or the roof of the combustion chamber. Figure 1 shows schematically a conventional cracking furnace (GK6 ™). Figure 2A shows a typical heat flow profile of a GKß ™ furnace and a profile under similar circumstances for an oven according to the invention (simulated by SPYRO®). Figure 2B shows the process temperature along a coil of a GKß ™ furnace and a profile under similar circumstances for an oven according to the invention (simulated by SPYRO®). Figure 2C shows the temperature of the coil wall along the length of the coil. The figure 3A shows an intersection of the top view of a cracking furnace according to the invention with a structure similar to a fishbone. Figure 3B shows an intersection of the front view of the oven of Figure 3B. Figure 4 shows an alternative arrangement of the same type of serpentine coil and assembly as Figure 3 but with a triangular spacing of right angles between the individual coil sections. Figure 5A shows the top view of an oven according to the invention, where the coils have a distribution of the coil divided into two passes. Figure 5B shows a 3-D view of a single coil as in the oven of Figure 5A. Figure 5C shows a side view of the single coil of Figure 5B. Figure 5D shows a front view of the coil of Figure 5B. Figure 6A shows an oven with a 4-pass coil. Figure ßB shows a coil as in the oven of figure ßA. Figure 7 shows an oven according to the invention wherein the outlet sections are in a stepped configuration. Figure 8A shows an oven according to the invention with a highly symmetrical distribution of the coil 4-1 in a third aisle at the intersection of the top view. Figure 8B shows another furnace with a symmetrical distribution of coils 4-1 (intersection of the top view). Figure 8C shows an intersection of the front view of an oven according to Figure 8A and Figure 8B. Suitable cracking coils (also referred to as cracking tubes) are generally known.

The coils can be formed from one or more cylindrical tubular ducts, preferably with a circular, or oval cross section. The conduits may be connected by connecting devices such as, but not limited to connecting pipes and bends to provide a number of passes, for example as shown in Figure 3B and Figure 6B. A cracking coil may be formed of a plurality of tubular conduits joined together, having for example a "m-like" or "w-like" shape wherein the outer legs represent the inlet sections that are assembled in a single exit section, represented by the central leg of the w / m. Particularly suitable examples wherein the tubes are joined together to form a cracking coil are shown in Figure 5D and in Figure 8 (w-shaped). In art, such cracking coils are commonly known as "split coil" designs. The coils generally each have at least one inlet and at least one outlet. The inlet of the coil is a conduit through which, during use, the feed is introduced to the cracking coil and usually by means of it to the combustion chamber; the outlet is the conduit through which, during use, the product leaves the cracking coil, and through this usually the combustion chamber. The output may be connected with other processing equipment such as, but not limited to, heat exchangers and / or switches. The inlet section of a coil is the first part (in the longitudinal direction) of the coil that is inside the combustion chamber, starting from the inlet of the coil to the combustion chamber. It can be extended until the beginning of the exit section. In particular, it is the part that is less thermally protected than the outlet section. In a preferred embodiment, the inlet section is the part of the coil that thermally protects the outlet section of the coil, when the furnace is operated. The exit section of a coil is the last part (in the longitudinal direction) of the coil that is inside the combustion chamber, ending at the exit of the coil that travels out of the combustion chamber. In particular, it is the part that is more thermally protected than the input section. It may extend to the end of the inlet section or to an intermediate section connecting the inlet section -and- the outlet section (such as return elbows, as will be described later). Usually, a plurality of the cracking tubes are connected to each other to form a flute or parallel path for feeding. Accordingly, in contrast to a design in which the "tubes" are connected in a serial manner and where the feed is introduced to the first "tube", it is partially converted and after that a "tube" is introduced. Subsequently, the present design allows the composition of the current at the inlet of each tube to be essentially the same for each tube. This allows a short residence time and therefore a high performance. If desired, during use, a plurality of the cracking tubes can thus be fed from a single vessel or conduit that is divided into a number of feed streams, each fed to the inlet of a cracking tube and / or the product stream leaving the plurality of tubes through the outlet can be combined again into a single conduit or container. The term that in an entity (such as a section of the coil) is "thermally protected," is defined herein as that heat is prevented from being transferred to the entity. This term is used herein in particular to indicate the degree to which the heat generated by the burners during the operation of the cracking furnace is impeded from being transferred to the protected entity. With regard to the fact that the exit sections of the coils are more thermally protected than the inlet sections of the coils, this means in particular that the transfer of heat to the cracking coils in the outlet section of the coil is displaced in favor of heat transfer to the cracking coils in the inlet section of the coil, during the operation of the burners compared to a coil configuration whereby such protection is not occurring or is occurring at a lower level. The term "essentially vertical" is used herein to indicate that an entity (such as a coil / tube or part thereof, a corridor, a wall, etc.) at least during use is at an angle of more than 45 ° with an horizontal surface (usually the floor of the combustion chamber), in particular at an angle of more than 80 °, preferably at an angle of 90 °. The term essentially horizontal is used herein to indicate that an entity (such as a coil / tube or part thereof, a passageway, a wall, etc.) at least during use is at an angle of less than 45 ° with an horizontal surface (usually the floor of the combustion chamber), in particular at an angle of less than 10 °, preferably at an angle of approximately 0 °. The term essentially parallel (used in the geometric sense) is used herein to indicate that an entity (such as a pipe or part thereof, a corridor, a wall, etc.) at least during use is at an angle of less than 45 ° with another entity with respect to which the entity is said to be essentially parallel, in particular at an angle of less than 10 °, preferably at an angle of about 0 °. The term "about" and the like, as used herein, is defined in particular including a deviation of up to 10%, more particularly up to 5%. A process according to the invention, respectively a furnace of the invention, can offer several advantages. In particular, the outlet section of a coil is thermally protected from the burners by the inlet section which is beneficial, for the reasons described in detail below. Due to the increase in thermal load for the inlet section, which occurs at the expense of the thermal load for the outlet section of a cracking coil, a shorter residence time is needed to achieve a certain feed conversion. This will allow the furnace designer to apply a shorter residence coil design when constructing a furnace by applying the invention. Due to the shorter residence time, the kinetic characteristics of the reaction favor the formation of the desired products such as ethylene at the expense of the formation of unwanted by-products. Accordingly, a smaller amount of the feed is required to produce a given amount of the desired product, for example ethylene. The protection can contribute to a reduction in the formation of the coke in the outlet section of the coil that is a limiting factor in the operation time of the furnace. As a consequence, the furnace can operate for a longer period before it is required to stop the oven cracking operation to make possible the decoking of the furnace. Alternatively, instead of extending the operation of the oven, the capacity of the oven can be increased. The inventors have understood that the protection of the exit sections by the inlet sections, optionally in combination with other factors (as described below), contribute to an improved mechanical stability of the coils, also at elevated temperature, particularly when they use under common conditions for steam cracking, such as heating the coils to a temperature of about 850 ° C or higher (ie the temperature of the external surface of the coil wall). The temperature may still rise to about 1100 ° C or higher, in particular when the furnace is near the end of run conditions and a decoking operation of the furnace becomes necessary. Such a high temperature of the coils is usually relatively close to the melting point of the material from which the coils are made (such as a material of an alloy with high nickel-chromium content). In particular, under such high temperature conditions, the deformation caused by the thermal stresses becomes an important factor, complicating the design of a robust coil assembly in a conventional cracking furnace. Changes in metal temperature as small as 10 ° C are already important design parameters at such high temperatures. Without wishing to be bound by theory, it is contemplated that since the inlet sections are close to the burners, the temperature of the coil wall in the inlet section is increased. With a higher temperature of the inlet section, the "deformation as well as the thermal expansion of the inlet section increases and will become closer to the deformation and thermal expansion of the outlet section of the coils (where the temperature of the wall is "generally higher, than in the entrance section).

Due to the difference in deformation and / or thermal expansion between the inlet sections and the outlet sections, the deformation of the radiating coil during operation is reduced. Preferably, the corridors of the inlet sections of the coils, the exit sections of the coils and the burners in the combustion chamber are geometrically placed essentially parallel to each other. Preferably, the outlet sections and the inlet sections of the tubes are arranged geometrically essentially parallel to one another and placed essentially vertically, at least during their use. It will be understood that in particular (part of) the intermediate sections (such as the return elbows 8, see Figure 8C) of the coils connecting the input section (s) and the section (s) output can be placed essentially non-vertically. Preferably, the cracking coils are distributed in a stepped configuration, in particular an extended or non-extended stepped configuration. The burner aisles are usually substantially parallel to each other. The burners are usually mounted in the bottom and / or the side walls and / or the roof of the combustion chamber. Therefore, all burners can be placed in either the bottom, the side walls or in the ceiling, or the burners can be present in the bottom -'and the side walls, in the bottom and the ceiling, in the side walls and the roof, or the burners may be present on the side walls at the bottom and on the roof. In a preferred furnace, at least a number of the burners are placed on the floor and / or the ceiling. The cracking coils can be adequately arranged in a stepped or stepped arrangement extended in such a way that a high degree of symmetry in the distribution of the coil is obtained. In addition to improved protection and / or improved thermal stability, it is possible to obtain greater cracking capacity per volume of the combustion chamber, because it allows to reduce the space between the tubes, and the configuration of three or more aisles. It is contemplated that in particular an increase in capacity of 10 to 20% may be obtained in the same volume of the combustion chamber with respect to a conventionally designed furnace. Furthermore, it has been found that an oven according to the invention shows good mechanical stability also when exposed to large temperature variations. As a result, much simpler and less sensitive tube supports are required for the operator to secure the tubes to a wall of the combustion chamber. In particular, a furnace in which the inlet sections are arranged essentially symmetrically in relation to the corresponding outlet sections, can be provided with cracking coils which do not need to be supported with assist devices for the bottom guide (when the entrances / exits are in or near the roof of the combustion chamber) respectively in the upper part (when the entrances / exits are in or near the bottom of the combustion chamber). Accordingly, the coils in the combustion chamber can be very adequately self-hanging, or respectively self-stable. For a good mechanical symmetry (and therefore a thermal stability), the combustion chamber comprises cracking coils which are so-called split coils, ie cracking coils comprising several input sections per outlet section, wherein input sections are positioned (approximately) symmetrically relative to the exit sections ^ - - Such split coils are preferably selected from coils comprising an even number of sections per outlet section, wherein a part (preferably half) of the exit sections form the first corridor of the exit sections and another part (preferably the other half) of the exit sections form the second aisle of the exit sections, the aisles are on opposite sides of the aisle of the entry sections. Preferred examples of split coils are cracking coils comprising 2 inlet sections and 1 outlet section (arrangement 2-1 (such as more or less serpentines in the shape of m / serpentines)), and cracking coils comprising 4 input sections and one output section (arrangement 4-1). In the split coil design that applies to the invention, the bending of the coils, caused by the difference in expansion and deformation between the input section (s) and the output section (s) it is reduced, partly because of the protective effect as described above, partly because of the rigidity of the mechanical design that is caused by the coil whereby for each individual coil, the entrance ends are located in the two outer corridors and the The exit section of this coil is located in the interior corridor, which leads to a highly symmetrical coil design. Such a system can thus be operated very well without a guide system for the cracking coils, which are normally used in the art to guide the cracking coil to the floor (in the event that the inlet / outlet is in place). or near the ceiling) or the roof (in case the entrance / exit is on or near the floor). The split coil is preferably designed in such a way that at least two inlet sections are provided substantially uniformly on opposite sides of each outlet section, whereby an essentially symmetrical coil design is obtained (as shown in any of Figures 8A and 8B, which will be described in detail, below). The invention is highly suitable for use in the cracking of a hydrocarbon feed in the presence of water vapor, ie cracking with water vapor. A method according to the invention can be carried out very adequately by mixing the hydrocarbon feed with steam and driving it through the tubes in the aforementioned kiln. It has been found that according to the invention, the hydrocarbon feeds can be cracked very well, if desired, at a higher heat density than in a known furnace. In particular, the invention is very advantageously used in the production of ethylene, with propylene, butadiene, and / or aromatic substances as possible co-products. The hydrocarbon feed to be cracked can be any gaseous hydrocarbon feed, in the form of steam, liquids, or a combination thereof. Examples of suitable feeds include ethane, propane, butanes, naphthas, kerosenes, atmospheric gas oils, vacuum gas oils, heavy distillates, hydrogenated gas oils, gas condensates and mixtures of any of these. The invention is particularly suitable for cracking a gas selected from ethane, propane and mixtures of gaseous hydrocarbons. The invention is also well suited for cracking vaporized heavier feeds such as LPG, naphtha and diesel. It has also been found that a furnace can be operated according to the invention at a much higher heat density relative to a steam cracking furnace known in the art. This is particularly advantageous for the capital costs used as for the same capacity, the dimensions of the combustion chamber can be reduced, or alternatively for the same dimensions, a much higher ethylene production (or other product) can be obtained. , which reduces the number of ovens to power a steam cracking plant on a global scale. For example, it is contemplated that in a world-scale steam cracking plant based on a naphtha feedstock with an annual capacity of 1.4 million metric tons, the number of ovens using conventional art (such as GKß) could be of at least 9 (8 in operation, one of reveal). It is contemplated that 7 ovens according to the invention are sufficient for the same annual ethylene capacity (6 in operation, one in relief). It has been found that an oven according to the invention can be operated with a relatively low temperature difference through the outlet section and therefore has a high degree of isothermicity. In a conventional process in a conventional oven, the elevation of the temperature of the gas through the last tube of the outlet section of the coil in a cracking process is typically about 60-90 ° C, whereas in a similar process carried performed in an oven according to the invention, the rise in temperature is usually lower, typically around 50-80 ° C. Accordingly, the invention allows a reduction of about 10 ° C in raising the temperature, which is energetically advantageous. Accordingly, the process temperature can be relatively high, allowing a relatively short residence time, to give a specific feed conversion, as compared to a comparable oven without the protected output section. For example, the residence time for a GKß ™ furnace is typically 0.20-0.25 seconds, whereas in a comparable process employed in an oven of the present invention the residence time can be reduced to approximately 0.17-0.22 seconds. Accordingly, the present invention allows a reduction in residence time of approximately 15%, to achieve a particular conversion, compared to a GK6 ™ oven. It has also been found that in a furnace according to the invention, respectively with a method according to the invention, a very good selectivity of the reaction is feasible, showing a relatively low tendency to form undesirable byproducts. A typical heat flow profile of a GKß ™ furnace and a profile under the like circumstances for an oven according to the invention, are shown in Figure 2A, (simulated by SPYRO®, a simulation tool widely used in the ethylene industry to simulate the cracking furnace). According to the invention, it has been calculated that the increase in coil capacity in this example (compared to GKß ™) is about 10-15% in performance, 40% in the time interval of the run and / or 1-3% in the selectivity of the olefin when the naphtha is cracked at a total interval at the same severity or cracking conversion. Furthermore, it has been found that an oven according to the invention can be operated with a low tendency of coke formation inside the cracker coil, in comparison with some known ovens, especially at the outlet end of the cracker coil. Accordingly, the invention allows a high availability of the furnace, because the intervals between the subsequent maintenance sessions to remove the coke can be increased. In an oven according to the invention, the outlet sections of the coils are advantageously located in the combustion chamber in at least one passage, where at least one passage is between a first burner passage and a second burner passage. For practical reasons, the aisles are preferably essentially parallel. As indicated above, an oven is well suited where the inlet sections of the coils act as a thermal protection and / or a mechanical stabilizer for the exit sections, such as in a cracking oven where the inlet sections They are placed between the outlet sections and the burners. This configuration has been found very efficient, with respect to the heat distribution, the symmetry and / or the achievement of a desirable thermal profile through the length of the coils. Accordingly, in a very advantageous embodiment, the present invention relates to a cracking furnace comprising a combustion chamber, wherein at least one aisle of the outlet sections of the coils, at least two aisles of the inlet sections of the coils and at least two burner passages are present, in which the combustion chamber of at least one aisle (O) of the exit sections is located between at least two aisles (I) of the entrance sections and the Hallways of the entrance sections are located (such inlet sections act as thermal protection during cracking) between at least one aisle of the exit sections and at least two burner aisles (B). Accordingly, observing from above or below the combustion chamber, this configuration can be represented as a B-I-O-I-B configuration. Examples of highly suitable modalities are shown in Figures 3, 4, 5, ß, 7 and 8. These examples all show a configuration with the inlet and outlet of the coils in or near the ceiling and the burners are placed in the opposite side of the entrance / exit ends of the tubes, on the floor and / or on the side walls. It should be noted that it is also possible to operate an oven that is rotated relative to the configuration shown, in particular an oven where the inlet / outlet ends of the tubes are at or near the bottom of the oven. In this case, the floor burners are preferably replaced by burners placed on or near the ceiling. The arrangement of the exit sections and the inlet sections can advantageously be configured in a fishbone-shaped arrangement. With such modality, a mechanical symmetry and a very effective protection have been found feasible. Figure 3 shows a cracking furnace with a structure similar to a fishbone. In this figure, the cracking coils each comprise an inlet (4, figure 3A) and an outlet (3, figure 3A). The cracking coils are configured essentially vertically in a three aisle assembly. The individual input / output sections are arranged in an isosceles triangle spacing in front of each other. Alternatively, the individual input / output sections can be arranged in an equilateral triangle spacing, or alternatively in a triangular spacing at right angles (FIG. 4) or alternatively, in any form of a scalene or non-scalene triangle spacing. In Figure 3, the burners 5 are shown in -the floor (burners 5a of the floor) and on the side walls (burners 5b of the side wall), although the burners can be placed only on the floor 12 or only on the side walls 9. In general, if the side burners are present in an oven of the invention, these are preferably placed in the upper half of the side walls in the event that the entrance and exit are at or near the ceiling, and placed in the lower half of the side walls in the event that the entrance and exit are on or near the floor . In Figure 3 (where Figure 3A shows an intersection of the top view and Figure 3B shows an intersection of the front view), the cracking coil 2 has its inlet 4 and the outlet 3 in or near the ceiling 11 of the combustion chamber 1. The inlet sections of the coil (ß, figure 3B) typically start at the entrance and extend in this mode to the part of the coil where the inlet section is connected to a return elbow (8, 3B) out of the plane formed by the inlet sections, away from the burners towards the center line of the furnace. The exit sections (7, Fig. 3B) typically start at the end of the return elbow (8, Fig. 3B). Initially, the output section can be extended to the position where the input section ends. More particularly, the outlet section is considered as the part of the coil between the outlet and the part of the coil where the coil is flexed out of the plane formed by the exit end of the coil. Better mechanical stability - is obtained due to the fact that in a parallel aisle arrangement (geometrically) of three or more aisles formed by the sections of the cracking coil, the inlet sections and the exit sections are more isothermal than with an arrangement of a corridor or double aisles. Figure 4 shows an alternative arrangement of the same type of coil and coil assembly as Figure 3 but with a triangle spacing at right angles between the sections of the individual coil. The main distinction with figure 3 is the arrangement of the coils, each coil is now essentially perpendicular with respect to the lines with burners. Figure 5 shows yet another highly advantageous design, the main difference compared to Figures 3 and 4 is the design of the coils, which is now a distribution of split coils of two passes. The coils have two inlets 4 (divided flow) and one outlet 3. Figure 5A shows a top view of such a furnace. Figure 5B shows a 3-D view of a single coil in such furnace. Figure 5C and Figure 5D respectively show a side view and a front view of a single coil. In the front view (figure 5D),; The appearance of the tube (coil) is more or less similar to one or similar to one. In the case of a shape similar to an m, "-the burners are preferably placed on the sides (lower half of them) and / or on the ceiling, instead of on the floor. 4-pass coil Here, the best thermal stability is obtained by a higher level of isothermicity and the protection is carried out in particular by the part of the coil from a to d and the protected section comprises in particular the part of the coil from d to g A furnace with a 4-pass coil, for example as shown in Figure 6, has been found particularly suitable for cracking a raw material which requires a relatively long residence time to obtain a particular conversion, for example for cracking the Ethane Two examples of a highly symmetrical distribution of serpentines 4-1 in a three-pass arrangement applying the invention are shown in Figure 8 (where Figures 8A and 8 B show an intersection of a top view of two modalities and figure 8C shows an intersection of a front view, which is applicable to both of the embodiments of Figure 8A and Figure 8B). In Figure 8A, the individual sections of the coils are placed in an isosceles triangle array facing each other whereby the input sections are positioned not only symmetrically with respect to the output section but also with respect to the line centrally ("through the passageway of the exit sections.) Figure 8B provides the same arrangement of coils 4-1 but with a scaled triangle spacing between the individual tubes." In Figure 8, the cracking coil 2 has four inputs 4 and output 3 (on or near ceiling 11 of combustion chamber 1) The input sections of each coil typically start at the entrance and extend in this mode to the part of the coil where the coil is connected to a return elbow that bends out of the plane formed by the risers, moving away from the burners towards the center line of the furnace. The outlet (7, see Figure 8C) typically begins at the end of the return elbow 8. Initially, the outlet section may extend to the position where the entry section ends. More particularly, the exit section is considered the part of the coil between. the exit of the coil and the end of the return elbow. The section between the outlet section and the inlet section is then referred to as the return elbow 8. In figure 8C, the inlet section ß is placed between the burners 5 and the outlet sections 7, thereby protecting thermally partial the output sections 7. A (mostly) symmetrical distribution of input sections on the opposite sides of the output sections has been found beneficial with respect to the resistance against the damaging deformation of the tubes as a result of the thermal stress and can prolong the life time of the coils. As a result, cracking coils may be present in the combustion chamber without being supported (guided) to the bottom (in the event that the inlet and outlet are not provided at the bottom, but leave the combustion chamber at through the roof or near the ceiling), respectively up to the ceiling (in the event that the entrance and exits are present in the bottom or near the bottom). Consequently, the coils can hang freely, respectively they can be freestanding, in the combustion chamber, without being held by a bottom guide, respectively a roof guide. The skilled person will know how to build an apparatus with the appropriate dimensions, based on the teachings here and the common general knowledge. In the beginning, the design of an apparatus of the present invention may be based on the criteria commonly used when designing a cracking furnace. Examples of such criteria are the distances between the coils, between the burners and between the burners and the coils, the inlets / outlets of the coils, the outlet for the exhaust gases, the design of the combustion chamber, the burners and other parts. Burners that burn gaseous fuel are particularly suitable. The burners can be placed in their place anywhere within the combustion chamber, in the company of the floor and / or the side walls. Very good results have been achieved with such a cracking furnace where the burners are placed on the floor of the combustion chamber and the outlet section (s) of the coil extends (n) through the roof of the chamber of combustion or at least through the side wall, near the ceiling. Optionally, additional burners are present in the side walls, preferably at least in the upper half. It has further been found advantageous that the burners are present on each opposite (radially) side of the two external passages which contain the outlet sections of the coils present in the combustion chamber. This leads to a more isothermal temperature distribution over the length of each coil. For a configuration of symmetrical burning over the width of the combustion chamber, it is further preferred, in an oven according to the invention, that each opposite burner passage during cracking, generates approximately the same amount of heat. Similarly, in a method of the invention, it is preferred that during cracking, each opposite aisle or set of opposite burner aisles have process or mechanical design features that are identical or similar. As cracking coils (cracking tubes), those known in the art can be used. A suitable internal diameter is chosen, for example, in the range of 25-120 mm, depending on the quality of the raw materials and the number of passes per coil. The cracking coils are preferably located essentially vertically in the combustion chamber (ie, preferably the coils are positioned in such a way that a plane through the tube is essentially perpendicular to the floor of the combustion chamber). The coils may be provided with features such as, but not limited to, an extended internal surface, which improve the internal heat transfer coefficient. Examples of such features are already known in the art and are commercially available. The inputs for the feeding in the coils preferably comprise a forward distribution and / or a critical flow venturi. Suitable examples thereof and the suitable ways of using them are already known in the art. The output sections can be properly arranged in an in-line configuration (see for example figures 3, 4, 5 and ß), where the outputs are along a single line along the chamber (typically along, or parallel to, the center line of the camera) or a stepped configuration (e.g., Figure 7). The stepped configuration can be a fully stepped configuration (ie, where three subsequent output sections are placed in a triangular configuration with equal sides (the length of a, b and c is identical, see for example figure 7), also known as a configuration with an equilateral triangle spacing or an extended stepped configuration (i.e. where the output sections are placed in an isosceles delineate spacing formed by the sides a, "b and c (as indicated in figure 7) where the side c is different from the sides a and b and where the sides a and b are equal, or a scalene triangle configuration formed by the sides a, b, c (as indicated in Figure 7) where each of the sides a, b, c (as indicated in Figure 7) of the extended triangle, differs in length from the other sides.For a very effective protection of the exit sections, a configuration In line has been found very suitable In a cracking furnace according to the invention, the ratio of the external diameter / spacing is preferably selected in the range of 1.5 to 10, more preferably in the range of 2 to 6. In this context, the spacing is the distance between the center lines of two adjacent tubes in the same plane ("c" in figure 7) A cracking process according to the invention is usually carried out in the absence of catalysts.

Accordingly, in general the cracking tubes in an oven according to the invention are free of a catalytic material (such as a catalytic bed). The operating pressure in the cracking coil is generally relatively low, in particular less than 10 bar, preferably less than 3 bar. The pressure at the outlet is preferably in the range of 1.1-3 bar, more preferably in the range of 1.5-2.5 bar. The pressure at the inlet is higher than at the outlet and is determined by the difference in pressure. The difference in pressure between the inlet and outlet of the cracking tube (s) is 0.1 to 5 bar, preferably 0.5-1.5 bar. The hydrocarbon feed is usually mixed with water vapor. The ratio of weight to water vapor weight with respect to the hydrocarbon feed can be chosen within wide limits, depending on the feed used. In practice, the ratio is usually at least about 0.2, in particular between about 0.2 and about 1.5. For the cracking of ethane, a value of less than about 0.5 (particularly about 0.4) is preferred. For heavier hydrocarbon feeds, normally a higher ratio is employed. Particularly preferred are: a ratio of approximately 0.6 for naphthas, a ratio of approximately 0.8 for AGO (atmospheric gas oil) and for HVGO (hydrotreated vacuum gas oil) and a ratio of approximately 1 for VGO (vacuum gas oil). The hydrocarbon feed, typically mixed with steam for dilution, is preferably fed to the coil (s), after being heated to a temperature of more than 500 ° C, more preferably to a temperature of 580-700. ° C, even more preferably at a temperature in the range of 590-680 ° C. In the event that a (at least partially) liquid feed is used, this preheating usually leads to the evaporation of the liquid phase. In the cracking coil (s), the feed is preferably heated in such a way that the outlet temperature is up to 950 ° C, more preferably up to an outlet temperature in the range of 800-900 ° C. In the cracking tubes the hydrocarbons are cracked to produce a gas that is enriched in unsaturated compounds, such as ethylene, propylene, other olefinic compounds and / or aromatic compounds. The cracked product leaves the combustion chamber through the outlets and is then sent to the heat exchanger (s), where it is cooled, for example to a temperature of less than 600 ° C, typically at the interval of 450-550 ° C. As a secondary product of cooling, steam can be generated under natural circulation with a steam collector. EXAMPLES A cracking process was simulated for an oven according to the invention and a GKß oven using SPYRO® (See Table 1 for conditions). Figures 2A-2C show the heat flow profiles, the process temperature along the coil and the wall of the pipe along the coil. Applying the invention wherein the dimensions of the furnace coil according to the invention are the same as those of the GKß furnace and whereby all process parameters such as the flow rate, the cracking severity, etc., are maintained identical, the length of the run (maximum operating time without needing to stop the installation for maintenance) is prolonged from 60 to 80 days. The results are tabulated in the "Equal" column. Maintaining the same dimensions of the coil and applying the invention by which all the process parameters except the capacity are kept identical and for which the capacity is increased to maintain the same run length as with GKß, leads to an increase of capacity from 40 to 45 metric tons, therefore 12.5% more ethylene production than with GKß. The results are tabulated in the "Capacity" column. Applying the invention to the furnace containing the coils that are designed in such a way that they process the same amount of feed, operating at the same severity and design for the same run length in this operation, everything compared to GKß, is conducted to a increase in ethylene yield from 27.7 to 28.1% by weight on the hydrocarbon feed, thus saving 1.4% of raw materials for the same amount of the main ethylene and propylene products.

Table 1 It is noted that in relation to this date the best method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (3)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property. A method for cracking a hydrocarbon feed, characterized in that it comprises passing the feed, comprising a hydrocarbon and a diluent gas, in particular water vapor, through a cracking coil in a combustion chamber under cracking conditions , wherein the coil comprises at least one outlet section and at least one inlet section and wherein the outlet section is more thermally protected than the inlet section of the coil. The method according to claim 1, characterized in that the combustion chamber comprises at least one passage of exit sections of the coils, at least two aisles of the inlet sections of coils and at least two burner passageways, in wherein at least one passage of exit sections is located between at least two aisles of entry sections and the aisles of the entry sections are located between at least two burner aisles. 3. The method according to claim 1 or 2, characterized in that the snakes are positioned essentially vertically and essentially parallel to each other. TO . The method according to any of the preceding claims, characterized in that the feed is passed through the coils in a parallel flow in at least part of the coils. The method according to any of the preceding claims, characterized in that the hydrocarbon feed including the diluent gas (water vapor) is heated to a temperature above the vaporization temperature prior to entering the cracker coil or into the cracking coil. The method according to any of the preceding claims, characterized in that the feed comprises a hydrocarbon selected from the group consisting of ethane, propane, butanes, naphthas, kerosenes, atmospheric gas oils, vacuum gas oils, heavy distillates, hydrogenated gas oils, condensates of gas and mixtures thereof. The method according to any of the preceding claims, characterized in that at least one product is formed, selected from the group consisting of ethylene, propylene and butadiene. 8. A cracking furnace, for the steam cracking of a hydrocarbon feed, characterized in that it comprises a combustion chamber provided with a plurality of cracking coils, the combustion chamber comprises at least one corridor of exit sections of the coils , at least two aisles of inlet sections of the coils and at least two burner aisles, wherein at least "one passage of exit sections is located between at least two aisles of entrance sections and the aisles of the entrance sections they are located between at least two burner passages 9. The cracking furnace according to claim 8, characterized in that the passages are essentially parallel to each other 10. The cracking furnace according to claim 8 or 9, characterized in that the exit sections and the inlet sections are placed essentially vertically, at least during use. The cracking furnace according to any of claims 8-10, characterized in that the inlet sections, respectively the outlet sections, in a corridor, are arranged in an in line or staggered array facing each other and in a configuration staggered with respect to the exit sections, respectively the entry sections, present in the aisle or adjacent parallel aisles of the exit sections, respectively the entry sections. 12. The cracking furnace according to claim 11, characterized in that the arrangement of the sections is in an equilateral triangle spacing, an isosceles triangle spacing, a triangular spacing at right angles or a scalene triangle spacing. 13. The cracking furnace according to claim 12, characterized in that the tubes are not guided to the bottom. The cracking furnace according to any of claims 8 to 13, characterized in that at least a number of the burners is placed on the floor and / or the ceiling of the combustion chamber and / or the side walls of the chamber and where the outputs of the coils extend through the roof of the combustion chamber. The cracking furnace according to any of claims 8-14, characterized in that at least part of the cracking coils are placed in an arrangement that allows the parallel flow of the feed through each of the coils, during its use. The cracking furnace according to any of claims 8-15, characterized in that the coils are selected from: - coils comprising two input sections arranged to allow parallel flow during use and an output section in fluid communication with the input sections; and coils comprising four inlet sections positioned to allow parallel flow during use and an outlet section in fluid communication with the inlet sections. 17. The cracking furnace according to any of claims 8-16, characterized in that the exit sections are arranged in an in-line configuration or in a stepped configuration, and wherein the external diameter / spacing is selected in the range of 1.5 to 10, preferably in the range of 2 to 6. 18. A method of cracking a hydrocarbon, optionally a method according to any of claims 1-7, characterized in that a cracking furnace according to any of the claims 8-17 is used.