EP0698075B1 - Process and device for steam-cracking a light and a heavy hydrocarbon charge - Google Patents

Abstract

The invention relates to a process and device with a convection zone (A) and a radiation zone (B) in which the process has a first stage for the precracking of a charge of light hydrocarbons (1) and a second stage for the subsequent co-cracking of the mixture of said precraked light charge (7) and a charge of heavy hydrocarbons (2). According to the invention, the process involves: the separate preheating of the two charge streams (1, 2) in the convection zone (A) in which the preheating temperature of each charge stream remains below the respective initial cracking temperatures; the precracking (5) of the preheated light hydrocarbons, mixing the precracked light hydrocarbon stream (7) with the preheated and unprecracked heavy hydrocarbon stream (8) to form a mixed stream (9); strongly heating the mixed stream (9) to a higher temperature than the initial cracking temperature, by feeding the mixture into the radiation zone (B) of the furnace (10); co-cracking in the radiation zone (B) of the furnace (10) and cooling the separated gases outside the furnace (10). The two charge streams (1, 2) are preferably preheated to above 300 °C, the preheated light hydrocarbons are precracked (5) at a temperature of 780 and 920 °C and the precracked light hydrocarbons (7) are mixed (9) with the preheated heavy charge (8). The quantity and temperature of the two streams (7, 8) are set in such a way that the temperature of the mixture (9) is higher than 400 °C and lower than the initial cracking temperature. The mixture (9) may be separated into individual streams (12) before entry into the radiation zone (B). The quantity of the hydrocarbon-containing fraction in the light charge (1) is below 50 %, and preferably between 4 and 45 % and even more preferably between 5 and 35 %, of the total quantity of hydrocarbon-containing fractions in both charges (1, 2). The light hydrocarbons (1) preferably have an average molecular weight from 25 to 60 (preferably C2 to C5) and the heavy hydrocarbons (2) have an average molecular weight from 70 to 500 (preferably vacuum gas oils and distillates).

Description

The invention relates to a method for steam cracking hydrocarbons in a cracking furnace with a convection zone and a radiation zone, the method comprising a first stage of pre-cracking a feed of light hydrocarbons and a second stage of final co-cracking of the mixture of this pre-cracked feed of light hydrocarbons and a feed of heavy hydrocarbons. The invention further relates to a device for steam cracking hydrocarbons, a cracking furnace with a convection zone and a radiation zone, at least one preheating tube for feeding light hydrocarbons in the convection zone for preheating this feed, this tube downstream with at least one cracking tube for feeding light hydrocarbons for pre-cracking them in the radiation zone, and includes at least one preheat tube for charging heavy hydrocarbons in the convection zone for preheating these loads.

The steam cracking process is well known and is one of the most important petrochemical processes.

A feed consisting of hydrocarbons and steam is vaporized and preheated in the convection zone of the steam cracking furnace. In the radiation zone of the furnace there is a huge increase in the temperature of this feed beyond the initial cracking temperature and cracking or splitting occurs at high temperature before the cracked gases are cooled and the cracked products are fractionated.

In the context of this invention, “feed” means a mixture of hydrocarbons and water vapor. This applies to both light and heavy loads. The mixture is referred to as feed either before this mixture is cracked or during the cracking of this mixture. A distinction must be made between the feed and the hydrocarbon-containing fraction of the feed (the fraction without water vapor) before the cracking Feed, i.e. the hydrocarbons from the feed or from the mixture of hydrocarbons and water vapor.

The preheat temperatures are usually in the range between 450 and 650 ° C, the cracking temperatures (outlet temperature of the furnace) usually in the range between 780 and 920 ° C.

The high values of the temperature intervals generally concern the lightest loads, the low values the relatively heavy loads. For the feeds, cracking in the convection zone is avoided as far as possible.

In connection with the invention, the "initial cracking temperature" is the temperature above which cracking or splitting of the hydrocarbons occurs or occurs to a significant degree and above which the cracking takes place ever faster. The initial cracking temperatures depend largely on the composition of the feed. The initial crack temperature values are known to those skilled in the art.

In connection with the invention, the following different values apply, for example, to the specified compositions of the feed: Composition of the hydrocarbonaceous fraction of the feed Initial crack temperature C ₂ -C ₃ -C ₄ 720 ° C naphtha 710 ° C Kerosene, atmospheric gas oil 690 ° C Vacuum gas oil 680 ° C

These temperatures are conventional values. Since these temperatures generally do not represent sharp temperature values, temperatures that deviate by 10 to 20 K from the stated values could also be specified. The values given above correspond to very low cracking speeds. In comparison, at higher cracking speeds, the temperatures are normally at least 100 K above the specified temperature values. Such temperatures usually prevail at the outlet of the cracking furnace.

In a manner well known to those skilled in the art, the aim is to greatly increase the temperature of the feed to the initial cracking temperature and beyond, because this is beneficial for the yield. Rapid cooling also favors the yield. The changes in the temperature in the process essentially correspond to a "square" temperature profile.

It is generally avoided to cut too large hydrocarbon cuts which contain very light and very heavy fractions, because the cracking or splitting of this type of mixture leads either to inadequate splitting of the light fractions or to excessive splitting of the heavy fractions. In fact, in a particular furnace, the light, more heat-resistant fractions have to be split at a higher temperature, i.e. with a higher cracking intensity.

The cracking intensity is essentially determined by the cracking conditions or certain parameters such as the dwell time of the feed in the cracking furnace, the temperature and the dilution. At the same time, the cracking intensity reflects the importance of the residence time and the temperature. The crack intensity can be measured using various indices known to the person skilled in the art (for example using the KSF index).

For the purposes of the following, the index of cracking intensity can be defined as the conversion of a feed of normal pentane cracked under the same conditions as temperature, residence time and dilution.

Except in the case where certain furnaces are not available (for example a cracking furnace for decarbonized ethane), co-cracking of very different, mixed fractions with an identical cracking intensity is generally avoided.

“Co-cracking” is understood to mean a process procedure with a combined cracking or splitting of light and heavy hydrocarbon inserts. The heavy hydrocarbon input is usually included in the main feed.

There are numerous methods for co-cracking a light feed and a heavy feed with different cracking intensities for the light Loading and known for heavy loading. Aside from the fact that the effects of too little or too much cracking should be avoided, these methods aim at the energy existing at the high temperature, which is due to pre-cracking of a light and consequently heat-resistant and cracked at a higher temperature Feed is to benefit from cracking the heavy load, particularly to bring a preheated heavy load past its initial crack point by mixing, as it were.

The second goal of certain methods is to use a pre-cracked light feed as a diluent, which at least partially replaces the water vapor used to dilute the heavy feed.

For example, it has been proposed to inject a small amount of a heavy feed (generally a gas oil) into the naphtha fission gases. The amounts of heavy feed that can be fed in this case are greatly reduced (e.g. 10% with respect to naphtha) so that the mixture is at a cracking temperature of the gas oil.

A process called "duocracking" has generally been proposed (see EP-B-0 110 433) in which the heavy load is pre-cracked before it is mixed with the already cracked light load. With this method, the percentage of heavy feed may be slightly increased and the goal of partially replacing the pre-cracked light feed with water vapor can be achieved. The sharp rise in temperature of the heavy feed is undesirable because it is pre-cracked with a very weak dilution (below 0.2). In addition, the additional conversion of the light feed in the course of the final joint cracking is limited, since the heavy feed is already pre-cracked and the final joint cracking can only be carried out with a reduced cracking intensity.

• Die Verwendung einer Menge einer leichten Beschickung, die geringer als die Menge der schweren Beschickung ist, was sich beim Dampfcracken positiv auswirkt, oder ein erneutes Cracken wieder zurückgeführter leichter Fraktionen (C₂ bis C₄) in einer bezogen auf die Hauptbeschickung (beispielsweise Naphtha) geringeren Menge,
• die Bevorzugung eines starken Temperaturanstiegs der schweren Beschickung,
• das Erzielen einer zusätzlichen maximalen Umwandlung der leichten Beschickung während des Co-Crackens, und zwar ohne daß Probleme des Verkokens auftreten.

The invention has for its object to provide a method and an apparatus of the type mentioned, for which all the advantages of the methods described above can be maintained, but at the same time the disadvantages need not be accepted. This means that the invention should enable the following at the same time if possible:

• Using an amount of a light feed less than the amount of the heavy feed, which has a positive effect on steam cracking, or re-cracking recycled light fractions (C ₂ to C ₄ ) in one related to the main feed (e.g. naphtha) lesser amount,
• the preference for a sharp rise in temperature of the heavy feed,
• achieving additional maximum conversion of the light feed during co-cracking without coking problems.

The invention is also based on the object of demonstrating a method and a device of the type mentioned at the outset, wherein the co-cracking in coils can be carried out with a large and uniform throughput. The method and device should also be economical and allow very easy control of the gap parameters.

• a) Ein getrenntes Vorheizen der beiden Beschickungsströme in der Konvektionszone, wobei die Vorheiztemperatur jedes Beschickungsstromes unterhalb jeweils der anfänglichen Spalttemperatur bleibt,
• b) das Vorcracken der vorgeheizten leichten Kohlenwasserstoffe,
• c) das Mischen des vorgecrackten leichten Kohlenwasserstoffstromes mit dem vorgeheizten und nicht vorgecrackten schweren Kohlenwasserstoffstrom unter Bildung eines Mischstromes,
• d) ein starkes Aufheizen des Mischstromes auf eine Temperatur, die höher ist als die anfängliche Spalttemperatur, indem die Mischung in die Strahlungszone des Ofens eingeleitet wird,
• e) das Durchführen eines abschließenden Co-Crackens in der Strahlungszone des Ofens, und
• f) das Abkühlen der beim Co-Cracken entstehenden Spaltgase außerhalb des Ofens.

According to the invention, this object is achieved in that the method comprises the following steps:

• a) separate preheating of the two feed streams in the convection zone, the preheating temperature of each feed stream remaining below the initial gap temperature,
• b) the pre-cracking of the preheated light hydrocarbons,
• c) mixing the pre-cracked light hydrocarbon stream with the preheated and non-pre-cracked heavy hydrocarbon stream to form a mixed stream,
• d) a strong heating of the mixed stream to a temperature which is higher than the initial gap temperature by introducing the mixture into the radiation zone of the furnace,
• e) performing a final co-cracking in the radiation zone of the furnace, and
• f) the cooling of the cracked gases generated during co-cracking outside the furnace.

• a) Ein getrenntes Vorheizen der beiden Beschickungsströme in der Konvektionszone auf Vorheiztemperaturen über 300°C,
• b) das Vorcracken der vorgeheizten leichten Kohlenwasserstoffe bei einer Temperatur im Bereich zwischen 780 und 920°C, vorzugsweise zwischen 800 und 900°C, und
• c) das Mischen des vorgecrackten leichten Kohlenwasserstoffstromes mit dem vorgeheizten und nicht vorgecrackten schweren Kohlenwasserstoffstrom unter Bildung eines Mischstromes, wobei die Menge und die Temperatur jeder der beiden Ströme vor dem Mischen so festgelegt werden, daß die Temperatur des Mischstromes höher als 400°C und niedriger als die anfängliche Spalttemperatur ist.

The method according to the invention is advantageously designed in such a way that method steps a) to c) include:

• a) separate preheating of the two feed streams in the convection zone to preheating temperatures above 300 ° C,
• b) the pre-cracking of the preheated light hydrocarbons at a temperature in the range between 780 and 920 ° C., preferably between 800 and 900 ° C., and
• c) mixing the pre-cracked light hydrocarbon stream with the preheated and non-pre-cracked heavy hydrocarbon stream to form a mixed stream, the amount and temperature of each of the two streams being determined prior to mixing so that the temperature of the mixed stream is higher than 400 ° C and lower than the initial gap temperature.

The method according to the invention has a number of important advantages:

The first advantage is that an important additional (maximum) cracking is achieved for the heavy load: due to the fact that the heavy load is not pre-cracked and the two loads are mixed at a temperature below the initial cracking temperatures, this corresponds to during of the sharp rise in temperature and co-cracking of the mixture reached cracking intensity of the full feed cracking intensity.

In contrast to the known methods, this complete cracking intensity allows an additional maximum cracking of the light feed. This is of particular interest and greatest advantage in view of the very heat-resistant light feeds, which cannot be cracked at more than approximately 60 to 65%, or at least not alone, without major coking problems, such as ethane.

The process allows additional conversion in the course of co-cracking so that approximately 70 to 85% overall conversion can be achieved.

Furthermore, the temperature required for the mixture is relatively low, so that the preheated (relatively cold) heavy feed is much larger Amounts can be used as light feed because their temperature is very high at the end of pre-cracking.

This is very suitable for the case where the light feed from downstream process stages, such as from fractionation, recycled fractions of compounds with 2 to 5 carbon atoms (eg ethane, a C ₄ and / or a C _{5 cut} ) consists. The light feed can also be obtained by cracking a main feed (heavy feed) such as naphtha. In this case, the amount of fractions returned hardly exceeds 15% of the amount of the heavy feed.

According to the invention, the amount of the hydrocarbon-containing light feed is less than 50%, preferably between 4% and 45%, particularly preferably between 5 and 35%, of the total amount of the two hydrocarbon-containing feeds.

The relatively large influence of the heavy feed thus allows a considerable dilution of the light feed during co-cracking, which reduces the coking of the furnace due to the very intensive cracking of the light feed (for example, the coking is very strong if ethane alone when converting from is split more than 65%).

As far as the strong temperature increase of the heavy feed (in mixture) goes beyond the initial cracking temperatures, this temperature increase takes place in contrast to the previously known methods of co-cracking by introducing the mixture into the radiation zone. This increase in temperature is less than that caused by mixing, but it still remains very fast due to the low reactivity of the lightly pre-cracked feed. This low reactivity is achieved by cooling the pre-cracked light feed during mixing. The cooling takes place at least 60 ° C., preferably at least 80 ° C., and particularly preferably at least 100 ° C. It enables a considerable reduction in the amount of fission radicals. The cooled, pre-cracked light feed thus behaves at least in part like a diluent. The temperature increase therefore falls much more than when the heavy feed is pre-cracked alone.

The method allows the use of relatively small amounts of the light feed, a strong additional cracking of this feed, while avoiding the coking problems associated with this heavy cracking because the heavy feed is highly diluted. This heavy feed is brought to its cracking temperature much more quickly due to the presence of the light feed acting as a diluent (effect of mutual dilution).

This preparation according to the invention of a mixture of relatively low temperature, which leads to a cooling of the pre-cracked light feed, is surprising and is in contrast to the processes known in the prior art. Rather, these processes are primarily concerned with using a very high temperature energy source (e.g. 850 ° C) consisting of the pre-cracked light feed. Accordingly, attempts have been made in these known processes to use this heat vector at the highest possible usable heat level, that is to say for the final cracking of the heavy feed (“duocracking process”) or for a strong increase in temperature of this heavy feed by mixing.

It has surprisingly been found in connection with the invention that unexpected advantages can be achieved by restricting the heat level of the energy vector used.

The approach of the method according to the invention differs fundamentally from that of the known methods: Instead of attaching importance to the feeding of the light feed (feed in two respects: energy vector and heavy feed as a diluent), in the method according to the invention the heavy feed functions Feed as a diluent of this light feed tries to achieve a strong additional splitting of the light feed in order to limit its coking. As far as the energy level is concerned, the limitation of the heat level when using the heat supplied through the pre-cracked light feed does not cause any energy loss.

In an embodiment of the method according to the invention, after the mixing, the mixed stream can be divided into a plurality of individual streams immediately before these individual streams are introduced into the radiation zone in order to bring the mixed stream abruptly to its initial gap temperature. In particular, the The mixed stream is advantageously divided into the individual streams at a temperature which is lower than the initial cracking temperature of one of the two feed streams.

According to the invention, the mixed stream circulates at a temperature which is lower than the cracking temperatures of the two feeds. After mixing, the mixed stream is divided into a large number of individual streams. Immediately after the division into individual streams, these individual streams are introduced into the radiation zone in order to bring the mixture abruptly to a temperature which is higher than the initial cracking temperatures of the two feeds. The individual currents then circulate in parallel at least in a first part of the radiation zone.

According to the invention, a plurality of cracking streams can be fed to the cracking zone from one and the same mixing zone or a large number of cracking processes can be carried out. According to known methods, this is done separately or with an at least partial merging of the streams in the end part of the cracking coil according to the so-called "split coil" technique.

In this way, the number of mixing zones of a furnace and thus the number of coils for pre-cracking the light feed can be restricted. This results in a more reliable mode of operation and lower assembly costs.

An important point should be pointed out here:

The mixing zone often consists of distribution nozzles or Venturi tubes. The choice of a low mixing temperature according to the invention essentially prevents coking of the mixing zone, so that the supply of the various streams is not disturbed. Due to the reduced temperature of the mixture, premature cracking in the separation zone can also be prevented, which would have an adverse effect on the yield.

The possibility according to the invention to fractionate the mixture without the problems of separation or coking, and thus to use split coils for co-cracking, makes it possible to use coils with a very large uniform capacity, which the Reduced furnace costs. Any light and heavy feed can be used in accordance with the invention if only the average molecular weight of the hydrocarbon-containing fraction of the light feed is less than that of the hydrocarbon-containing fraction of the heavy feed.

• Ethan, vorzugsweise rückgeführtes Ethan,
• der rohe rückgeführten C₄-Schnitt oder der C₄-Schnitt nach der Extraktion von Butadien oder Isobuten,
• rückgeführte Fraktionen, die Olefine mit 5 Kohlenstoffatomen enthalten und/oder
• gesättigte Fraktionen, beispielsweise der rückgeführte C₄-Schnitt nach der Hydrierung.

The most suitable light feeds are those in which the hydrocarbonaceous fraction of these feeds consist largely of hydrocarbons with 2 to 5 carbon atoms, specifically:

• Ethane, preferably recycled ethane,
• the raw recirculated C _{4 cut} or the C _{4 cut} after the extraction of butadiene or isobutene,
• recycled fractions containing olefins with 5 carbon atoms and / or
• saturated fractions, for example the recirculated C _{4 cut} after the hydrogenation.

These hydrocarbon-containing light feeds are typically in the range of the molecular weight which is preferred according to the invention for the light feed. According to the invention, the hydrocarbon-containing fraction in the feed of light hydrocarbons has an average molecular weight in the range between 25 and 60. This also corresponds to the average molecular weight of the unsaturated fractions recycled.

The mixtures, which mainly consist of hydrocarbons from the group comprising ethane and the unsaturated recycled fractions (for example the C _{4 cut} ), are also particularly suitable as a hydrocarbon-containing fraction according to the invention in the feed of light hydrocarbons, the ethane being the yield in cracking or Cleavage of the unsaturated fraction due to its function as a hydrogen donor, directly or via the intermediate stage of the molecular hydrogen produced during cracking, can improve. A hydrocarbon-containing fraction is therefore preferably used in the feed of light hydrocarbons, the majority of which consists of ethane, preferably of recycled ethane.

The hydrocarbon-containing fractions in the feed of heavy hydrocarbons are preferably in the range of an average molecular weight between 70 and 500. These fractions mainly include naphtha, kerosene and gas oil (atmospheric gas oil or vacuum gas oil).

The process according to the invention can also be carried out with ethane as the hydrocarbon-containing fraction of the light feed and with liquefied gases (saturated or unsaturated C ₃ and / or C ₄ compounds) as the hydrocarbon-containing fraction of the heavy feed.

According to a special variant of the method according to the invention, which is particularly interesting when the hydrocarbon-containing fraction of the light feed consists of ethane, the pre-cracked light feed is subjected to a slight aging in an essentially adiabatic zone in order to reduce its temperature by 10 to 50 ° C before it is mixed with the preheated heavy load.

According to another variant of the process according to the invention, the hydrocarbon-containing fraction of the heavy feed consists mainly of heavy fractions from the group of vacuum gas oils and distillates.

Preferably, especially in the case of a hydrocarbonaceous heavy feed fraction mainly from heavy fractions from the vacuum gas oil and distillate group, the temperatures and amounts of the two feeds are set prior to mixing so that the preheated heavy feed does not evaporate completely and that complete evaporation this loading is done by mixing with at least part of the pre-cracked light loading.

The mixing can optionally also be carried out in several, in particular two, stages: mixing with part of the pre-cracked light feed so that the heavy feed completely evaporates (derivation from the "dry point"), and then mixing with the rest of the pre-cracked light feed . Between the two mixing processes, the heavy charge that has completely evaporated over part of the pre-cracked light charge can, if necessary, be overheated by convection.

Optionally, the portion of the light feed used to completely evaporate the heavy feed may be cooled slightly, for example, by mixing with a small amount of colder water vapor if in the case of heavy loading, excessive temperatures in the mixing zone should be avoided. However, this is not absolutely necessary, and preferably the pre-cracked light feed is not cooled (by an external fluid). Whenever possible, full evaporation of the heavy feed before the mixing zone is preferred.

• eine Mischzone zur Bildung eines Mischstromes mit mindestens einer Einlaßleitung für mindestens einen Teil des vorgecrackten leichten Kohlenwasserstoffstromes, die mit dem stromaufwärtigen Teil des Crackrohres für die Beschickung von leichten Kohlenwasserstoffen verbunden ist, und mit mindestens einer Einlaßleitung für den vorgeheizten und nicht vorgecrackten schweren Kohlenwasserstoffstrom, die mit dem stromaufwärtigen Teil des Vorheizrohres für eine Beschickung von schweren Kohlenwasserstoffen verbunden ist,
• eine Trennzone zum Aufteilen der Mischung in eine Vielzahl von Einzelströmen,
• eine Vielzahl von parallel angeordneten Zirkulationsrohren für die Einzelströme in der Strahlungszone für einen starken Temperaturanstieg der Mischung, und
• mindestens ein Crackrohr für die Mischung, das stromaufwärts mit mindestens einem der Zirkulationsrohre für die Einzelströme und stromabwärts mit Einrichtungen zum Abkühlen der Spaltgase verbunden ist.

The object on which the invention is based is achieved by a device of the type mentioned at the outset, which also comprises the following:

• a mixing zone for forming a mixed stream with at least one inlet line for at least part of the pre-cracked light hydrocarbon stream, which is connected to the upstream part of the cracking tube for the feed of light hydrocarbons, and with at least one inlet line for the preheated and not pre-cracked heavy hydrocarbon stream, the is connected to the upstream part of the preheating pipe for feeding heavy hydrocarbons,
• a separation zone for dividing the mixture into a large number of individual streams,
• a plurality of circulation tubes arranged in parallel for the individual streams in the radiation zone for a sharp rise in the temperature of the mixture, and
• at least one cracking tube for the mixture, which is connected upstream to at least one of the circulation tubes for the individual streams and downstream to devices for cooling the cracked gases.

With this device according to the invention the mixing can take place at a relatively low temperature which is sufficient to avoid premature cracking in the downstream separation zone or considerable coking of this zone.

• eine außerhalb des Ofens gelegene Mischzone zur Bildung eines Mischstromes mit mindestens einer Einlaßleitung für mindestens einen Teil des vorgecrackten leichten Kohlenwasserstoffstromes, die mit dem stromaufwärtigen Teil des Crackrohres für die Beschickung von leichten Kohlenwasserstoffen verbunden ist, und mit mindestens einer Einlaßleitung für den vorgeheizten und nicht vorgecrackten schweren Kohlenwasserstoffstrom, die mit dem stromaufwärtigen Teil des Vorheizrohres für eine Beschickung von schweren Kohlenwasserstoffen verbunden ist,
  • mindestens ein Transportrohr zum Transport der Mischung von der Außenseite des Ofens zum Inneren der Strahlungszone, wobei dieses Rohr stromaufwärts mit der Mischzone und stromabwärts mit mindestens einem Zirkulationsrohr für die Mischung in der Strahlungszone verbunden ist, und
  • mindestens ein Crackrohr für die Mischung, das stromaufwärts mit mindestens einem der Zirkulationsrohre für die Mischung und stromabwärts mit Einrichtungen zum Abkühlen der beim Co-Cracken entstehenden Spaltgase verbunden ist.

According to the invention, a further device is also proposed, this device comprising the following in addition to the features mentioned at the beginning:

• a mixing zone located outside the furnace to form a mixed stream with at least one inlet line for at least part of the pre-cracked light hydrocarbon stream, which with the upstream part of the cracking tube for feeding light ones Is connected to hydrocarbons, and with at least one inlet line for the preheated and not pre-cracked heavy hydrocarbon stream, which is connected to the upstream part of the preheating pipe for feeding heavy hydrocarbons,
  • at least one transport pipe for transporting the mixture from the outside of the furnace to the interior of the radiation zone, this pipe being connected upstream to the mixing zone and downstream to at least one circulation pipe for the mixture in the radiation zone, and
  • at least one cracking tube for the mixture, which is connected upstream with at least one of the circulation tubes for the mixture and downstream with means for cooling the cracked gases formed during co-cracking.

In this device the mixing zone is located outside the furnace, which considerably limits the coking of the furnace, and the mixture can reach the radiation zone undisturbed at a relatively low temperature, where the final co-cracking is carried out without the risk of premature splitting or coking.

According to a characteristic embodiment of the invention, the mixing zone is also the zone in which the heavy feed finally evaporates. A very strong heat vector (the pre-cracked light feed) is used to completely evaporate a very heavy feed such as a vacuum gas oil or distillate with a large safety margin.

• eine außerhalb des Ofens gelegene Mischzone zur Bildung eines Mischstromes mit mindestens einer Einlaßleitung für mindestens einen Teil des vorgecrackten leichten Kohlenwasserstoffstromes, die mit dem stromaufwärtigen Teil des Crackrohres für die Beschickung von leichten Kohlenwasserstoffen verbunden ist, und mit mindestens einer Einlaßleitung für den vorgeheizten und nicht vorgecrackten schweren Kohlenwasserstoffstrom, die mit dem stromaufwärtigen Teil des Vorheizrohres für eine Beschickung von schweren Kohlenwasserstoffen verbunden ist,
• eine Trennzone zum Aufteilen der Mischung in eine Vielzahl von Einzelströmen,
• Transportrohre zum Transport der Einzelströme von der Außenseite des Ofens zum Inneren der Strahlungszone, wobei diese Rohre stromaufwärts mit der Trennzone und stromabwärts mit Zirkulationsrohren für die Mischung in der Strahlungszone verbunden sind,
• eine Vielzahl von parallel angeordneten Zirkulationsrohren für die Einzelströme in der Strahlungszone für einen starken Temperaturanstieg der Mischung, und
• mindestens ein Crackrohr für die Mischung, das stromaufwärts mit mindestens einem der Zirkulationsrohre für die Einzelströme und stromabwärts mit Einrichtungen zum Abkühlen der beim Co-Cracken entstehenden Spaltgase verbunden ist.

Taking advantage of the two devices described above, the device according to the invention can also include the following in addition to the features listed at the beginning:

• a mixing zone located outside the furnace to form a mixed stream with at least one inlet line for at least a portion of the pre-cracked light hydrocarbon stream, which is connected to the upstream part of the cracking tube for the feed of light hydrocarbons, and with at least one inlet line for the preheated and not pre-cracked heavy hydrocarbon stream connected to the upstream part of the preheating tube for heavy hydrocarbon feed,
• a separation zone for dividing the mixture into a large number of individual streams,
• Transport pipes for transporting the individual streams from the outside of the furnace to the inside of the radiation zone, these pipes being connected upstream to the separation zone and downstream to circulation pipes for the mixture in the radiation zone,
• a plurality of circulation tubes arranged in parallel for the individual streams in the radiation zone for a sharp rise in the temperature of the mixture, and
• at least one cracking tube for the mixture, which is connected upstream with at least one of the circulation tubes for the individual streams and downstream with devices for cooling the cracked gases formed during co-cracking.

In an embodiment of the devices according to the invention, two or more circulation tubes can be connected to at least one can in the radiation zone of the furnace.

The devices according to the invention can advantageously be designed in such a way that an adiabatic zone is provided outside the radiation zone between at least one cracking tube for charging light hydrocarbons for pre-cracking them and the inlet line for at least part of the pre-cracked light hydrocarbon stream.

In a further development of the devices, a plurality of can tubes can be connected to a device for cooling the can gases generated during co-cracking.

According to the invention, the preheating tube (s) for preheating the feed of light hydrocarbons in the convection zone within the cracking furnace can be connected to the cracking tube (s) for pre-cracking the light hydrocarbons in the radiation zone.

The devices according to the invention are particularly suitable for carrying out the method according to the invention.

The invention is explained in more detail below with reference to several figures.

Fig. 1

eine schematische Darstellung eines Teils einer Anlage zum Dampfcracken gemäß der Erfindung;

Fig. 2

eine schematische Darstellung der wichtigsten Schritte des Verfahrens gemäß der Erfindung;

Fig. 3

eine schematische Darstellung der verschiedenen Ausführungsformen von Spaltrohrschlangen einer Anlage zum Dampfcracken gemäß der Erfindung;

Fig. 4

eine schematische Ansicht der Mischzone einer Anlage zum Dampfcracken gemäß der Erfindung.

Show:

Fig. 1

a schematic representation of part of a plant for steam cracking according to the invention;

Fig. 2

a schematic representation of the main steps of the method according to the invention;

Fig. 3

is a schematic representation of the various embodiments of canned coils of a plant for steam cracking according to the invention;

Fig. 4

is a schematic view of the mixing zone of a plant for steam cracking according to the invention.

In Fig. 1 , a furnace 10 for steam cracking of hydrocarbons is shown very schematically, which heats by means of convection and radiation and comprises preheating, circulation and cracking tubes or bundles of preheating, circulation and cracking tubes for hydrocarbons in order to preheat and thermally heat them crack.

The furnace 10 shown in FIG. 1 comprises a first part A, in which heating is carried out by convection, and which is connected to a second part B, in which heating is carried out by means of radiation. In part B of the furnace, the very strong heat flow is generally supplied via burners (not shown), the exhaust gases of which then circulate in the first part A of the furnace and provide heating by means of convection. The furnace 10 shown in FIG. 1 can be supplemented by a second furnace half arranged in mirror symmetry and not shown in FIG. 1 but indicated.

The first part A of the furnace (convection zone) comprises one or more preheating tube (s) 4 for a hydrocarbon-containing heavy feed 2 and for water vapor, the hydrocarbons of this feed mainly having at least three carbon atoms (e.g. a naphtha or a gas oil).

Part A of the furnace also includes one or more preheat tube (s) 3 for a light feed consisting, for example, of ethane and water vapor, which tube (s) 3 are shown in phantom in Fig. 1 to remove them from the tubes for the distinguish between heavy loads in which this heavy load flows alone or in a mixture.

The tubes 3 provided in the first part A of the furnace are connected to canned tubes 5 in the second part B of the furnace (radiation zone).

The downstream end of the tubes 5 is connected to an aging zone 6, which consists for example of a tube with a length between 1 and 10 m, the diameter of which is larger than the diameter of the end section of the tubes 5.

When leaving the aging zone 6, the pre-cracked light feed (7) and the preheated and not pre-cracked heavy feed (8) are combined and mixed in a mixing zone with the aid of inlet lines 7 and 8 to form the mixed stream 9. Downstream of the mixing zone, the mixture can be divided into a plurality of individual streams (12) with the aid of a separation zone 11. These currents circulate in the transport pipes 12 and are introduced into the circulation pipes 13 inside the radiation zone B of the furnace 10. The supply of heat to the tubes 13 in the radiation zone B causes the temperature of the mixture to rise sharply above the initial cracking temperatures of the two feeds. The tubes 13 are connected downstream with canned tubes 14. The circulation pipes 13 each merge into canned pipes. A plurality of circulation tubes 13 preferably open into a can 14. The can 14 and also 5 can be arranged in the radiation zone B in different (known per se) ways. In particular, several canned tubes can be combined.

The cracked gases of the final co-cracking are then cooled in the quench cooler 15, preferably in a TLX heat exchanger (Transfer Line eXchanger).

This steam cracking device works in the following way:

The light feed (1), in which the carbon-containing part preferably consists of ethane or a mixture of recycle ethane and unsaturated recycle fractions, which are composed of hydrocarbons with 3 to 6 carbon atoms - for example a mixture of 30 to 70% ethane and additionally an unsaturated one returned C _{4 cut} , is introduced at point 1. This feed is preheated by circulation in the preheating tubes 3 (in one or more parallel passes) to a temperature in the range between 450 ° C and 680 ° C, preferably between 500 ° C and 650 ° C, either to a temperature, which is significantly below the initial cracking temperature, or, for example, to 720 ° C. if this feed is mainly C ₃ and C ₄ hydrocarbons or in equal proportions of ethane and C ₃ and C ₄ compounds contains. If two fractions are present in the same amount, the initial cracking temperature of the heavier feed is taken into account.

The light feed leaves the convection zone A (after point O) without any noticeable start to cracking. It is cracked or split in the radiation zone B by circulation in the tubes 5 at an initial temperature (at point I) in the range between 780 and 920 ° C., preferably between 800 and 900 ° C.

If the hydrocarbon-containing fraction in the light feed is ethane, a conversion in the range between 40 and 65%, preferably between 50 and 65%, can be achieved during the pre-cracking, without causing the tubes 5 to coke too quickly becomes. The conversion can also be significantly higher if the dilution of the light feed (proportion of water vapor) is considerable. The dilution can vary between 0.2 and 1.2 (20 to 120% of the light feed).

If the light feed contains larger fractions of hydrocarbons having 4 or 5 carbon atoms, the ethane is preferably converted during the pre-cracking only in the range between 30 and 55% and particularly preferably in the range between 35 and 50%, around the fractions containing 4 or 5 carbon atoms not cracking too much during the last phase of co-cracking.

The dilution (ratio of steam to hydrocarbon-containing fraction in the feed) of the light feed is generally in the range between 0.2 and 1.2 and preferably between 0.25 and 1.

At the end of the pre-cracking, the light feed is passed through the essentially adiabatic aging zone 6, where a slight cooling takes place between points I and J, for example by 10 to 50 ° C., due to the continuation of the cracking reactions.

This aging zone 6 is particularly important in the case of the use of ethane, since an additional conversion can take place with it, without the yield becoming significantly worse, and since the content of crack radicals before the mixing zone can be reduced. This reduces the risk of the mixture cracking prematurely.

However, the aging zone 6 can also be omitted, in particular for light feeds other than ethane.

The pre-cracked light feed 7 is now mixed with the heavy feed 8 after the latter has been preheated in the tubes 4 of the convection zone A.

The dilution of the heavy feed by the water vapor can vary between 0.05 and 1, preferably between 0.25 and 1.

At point K, just before the mixing zone, the preheating temperature of the heavy feed is in the range between 300 and 650 ° C, preferably between 450 and 650 ° C.

At point J, just before the mixing zone, the temperature of the pre-cracked light feed lies in the range between its initial cracking temperature and 920 ° C., preferably between 750 and 920 ° C.

After the two feeds have been mixed in the mixing zone to form the mixing stream 9, the temperature of the mixture at point L according to the invention is lower than the initial cracking temperatures of the two feeds 1 and 2. This mixture is consequently not very reactive. The mixture can therefore be divided without problems in the separation zone 11 and passed from the outside of the furnace 10 into the interior of the radiation zone B, without the risk of premature cracking or coking of the lines. This advantage of the process is critical because it can maintain optimal yield and avoid imbalance in downstream co-cracking. Such an imbalance could be caused by coking of these lines because they often contain throughput regulating devices such as nozzles or venturi tubes which are particularly sensitive to coking.

• 1) Aufgrund ihres Vorcrackens und der bedeutenden Abkühlung während des Mischens ist die vorgecrackte und abgekühlte leichte Beschickung (abgekühlt um mehr als 60°C, vorzugsweise um mehr als 80°C und besonders bevorzugt um mindestens 100°C, in bezug auf die Temperatur beim Verlassen der Vorcrackzone (Punkt I)) wenig reaktionsfähig und verhält sich somit sehr ähnlich wie ein Verdünnungsmittel. Auf diese Weise läßt sich die schwere Beschickung in den Rohren 13 beachtlich über ihre anfängliche Cracktemperatur hinaus erhitzen, und zwar aufgrund des Effekts der zunehmenden Verdünnung, die der schweren Beschickung zugute kommt. Durch das Abkühlen der leichten Beschickung wird die Menge an Crackradikalen beachtlich eingeschränkt.
• 2) Weil die vorgecrackte leichte Beschickung durch die schwere, noch nicht gecrackte Beschickung stark verdünnt ist, ist ihre Neigung zum Verkoken aufgrund einer noch nicht gecrackten und folglich wenig verkokenden Beschickung herabgesetzt. Somit ist die Neigung zum Verkoken im Punkt L eindeutig schwächer als im Punkt I. Dadurch ist nun eine zusätzliche Umwandlung der leichten Beschickung während des abschließenden Co-Crackens möglich, ohne daß die Phänomene des Verkokens zu ausgeprägt werden. Nach dem erfindungsgemäßen Verfahren können mit dieser zusätzlichen Umwandlung Werte für die endgültige Umwandlung von Ethan im Bereich zwischen 70 % und 85 % erreicht werden, die mit den bekannten Verfahren nicht zu erreichen sind, ohne daß es zu Problemen bezüglich der Verkokung kommt.

The invention also offers other advantages:

• 1) Due to their pre-cracking and significant cooling during mixing, the pre-cracked and cooled light feed (cooled by more than 60 ° C, preferably by more than 80 ° C and particularly preferably by at least 100 ° C, in relation to the temperature at Leaving the Pre-crack zone (point I)) is not very reactive and therefore behaves very similar to a diluent. In this way, the heavy feed in the tubes 13 can be heated considerably beyond its initial cracking temperature due to the effect of increasing dilution that benefits the heavy feed. The amount of crack radicals is considerably reduced by cooling the light feed.
• 2) Because the pre-cracked light feed is heavily diluted by the heavy, not yet cracked feed, its tendency to coke is reduced due to a feed that has not yet cracked and consequently has little coking. Thus, the tendency to coke at point L is clearly weaker than at point I. This means that an additional conversion of the light feed during the final co-cracking is now possible without the phenomena of coking being too pronounced. According to the method according to the invention, values for the final conversion of ethane in the range between 70% and 85% can be achieved with this additional conversion, which values cannot be achieved with the known methods without causing coking problems.

The process according to the invention can also be used to split unsaturated mixtures of ethane and C ₄ hydrocarbons with an increased conversion (60 to 80% for the ethane) and positive effects with regard to the yield of cracking of the unsaturated fractions, which is increased by the higher ratio of H to C benefit from Ethan.

The typical temperatures at point L (mixing temperature) are in the range between 400 and 710 ° C, preferably in the range between 600 and 700 ° C. The very low values (400 to 500 ° C) are used in the final evaporation of a heavy feed (vacuum gas oils and distillates).

After the mixed stream 9 has been heated, the co-cracking takes place beyond the initial cracking temperature in the circulation tubes 13 and in particular in the can 14. In the variant shown in FIG. 1, four parallel ciculation tubes 13 are connected to a can 14 via the collecting point M.

The process temperatures in point I or J (pre-cracking), L (mixing), M and N (co-cracking) can be controlled by changing the heat of the burners and by changing the respective amount of feed. It is also possible, for example, to supply a relatively cold fluid at point O and / or at point I or J (such as, for example, water or slightly heated steam) in order to control the temperature at point J, for example.

2 shows the most important, typical steps of the process and the corresponding temperatures in the case of a heavy naphtha or gas oil feed.

The heavy load is preheated (step 16). The light feed is preheated (step 17), then pre-cracked (step 18). The two feeds are then mixed (step 19). Finally, the mixture is divided and / or passed into the radiation zone (step 20), then brought to a high temperature and cracked (step 21).

In Fig. 3 different geometries for canned coils 3-1 to 3-6 are shown, which can be used for the pre-cracking of the light feed and / or for the strong temperature increase of the mixture and the final co-cracking (tubes 5, 13 and 14 ).

In connection with this invention, the known, conventional coils with 1, 2, 4, 6 or 8 passes (vertical lengths) or so-called split coils can also be used.

In Fig. 4 two embodiments 4-1 and 4-2 of the mixing zone are shown, where the light loading and the heavy loading are fed to a mixing device with an annular space (either for the heavy, relatively cold loading, or for the pre-cracked light loading) the second case 4-2 is preferred when the heavy feed is not completely evaporated.

The invention is not limited to these mixing devices and types of coils. In particular, all types of furnaces (with an internal or external transition point, with a mixing zone arranged inside or outside), pipe coils, mixers, processes for controlling the process temperature, etc. can be used without departing from the scope of the invention.

Claims (19)

1. Process for steam cracking of hydrocarbons in a cracking furnace (10) with a convection zone (A) and a blast zone (B), the process exhibiting a first stage of pre-cracking of a charge of light hydrocarbons (1) and a second stage of final co-cracking of the mixture of this pre-cracked charge of light hydrocarbons (7) and a charge of heavy hydrocarbons (2), characterised in that the process comprises the following steps:

   a) separate pre-heating of the two charge streams (1, 2) in the convection zone (A), the pre-heating temperature of each charge stream remaining below the initial cracking temperature in each case,

   b) pre-cracking (5) of the pre-heated light hydrocarbons,

   c) mixing of the pre-cracked light hydrocarbon stream (7) with the pre-heated but not pre-cracked heavy hydrocarbon stream (8) forming a mixed stream (9),

   d) strong heating of the mixed stream (9) to a temperature which is higher than the initial cracking temperature by introducing the mixture into the blast zone (B) of the furnace (10),

   e) final co-cracking in the blast zone (B) of the furnace (10) and

   f) cooling of (15) the cracking gases formed during the co-cracking outside the furnace (10).
2. Process according to claim 1, characterised in that steps a) to c) comprise:

   a) separate pre-heating of the two charge streams (1, 2) in the convection zone (A) to pre-heating temperatures over 300°C,

   b) pre-cracking (5) of the pre-heated light hydrocarbons at a temperature in the range between 780 and 920°C, preferably between 800 and 900°C, and

   c) mixing of the pre-cracked light hydrocarbon stream (7) with the pre-heated but not pre-cracked heavy hydrocarbon stream (8) forming a mixed stream (9), the quantity and the temperature of each of the two streams (7, 8) prior to mixing being fixed so that the temperature of the mixed stream (9) is higher than 400°C and lower than the initial cracking temperature.
3. Process according to one of claims 1 or 2, characterised in that after mixing, the mixed stream (9) is divided into a plurality of separate streams (12) immediately before these separate streams (12) are introduced into the blast zone (B) in order to bring the mixed stream (13) abruptly to its initial cracking temperature.
4. Process according to claim 3, characterised in that the mixed stream (9) is divided into the separate streams (12) at a temperature which is lower than the initial cracking temperature of one of the two charge streams (7, 8).
5. Process according to one of claims 1 to 4, characterised in that the quantity of the fraction containing hydrocarbons in the charge of light hydrocarbons (1) lies below 50%, preferably between 4 and 45%, particularly preferably between 5 and 35% of the total quantity of the fractions containing hydrocarbons in both charges (1, 2).
6. Process according to one of claims 1 to 5, characterised in that the fraction containing hydrocarbons in the charge of light hydrocarbons (1) has a mean molecular weight between 25 and 60 and predominantly consists of hydrocarbons with 2 to 5 carbon atoms.
7. Process according to claim 6, characterised in that the fraction containing hydrocarbons in the charge of light hydrocarbons (1) essentially comprises a mixture of hydrocarbons of the group consisting of ethane and the recirculated unsaturated fractions.
8. Process according to claim 6 or 7, characterised in that the fraction containing hydrocarbons in the charge of light hydrocarbons (1) consists predominantly of ethane, preferably of recirculated ethane.
9. Process according to one of claims 1 to 8, characterised in that the fraction containing hydrocarbons in the charge of heavy hydrocarbons (2) has a mean molecular weight between 70 and 500.
10. Process according to one of claims 1 to 9, characterised in that the fraction containing hydrocarbons in the charge of heavy hydrocarbons (2) consists predominantly of heavy fractions of the group of vacuum gas oils and distillates.
11. Process according to one of claims 1 to 10, characterised in that the pre-cracked light hydrocarbon stream (7) is subjected to slight ageing in an essentially adiabatic zone (6) in that the temperature of the pre-cracked light hydrocarbon stream (7) is reduced slightly, preferably by 10 to 50°C, before the pre-cracked light hydrocarbon stream (7) is mixed with the pre-heated heavy hydrocarbon stream (8).
12. Process according to one of claims 1 to 11, characterised in that the temperatures and quantities of the two hydrocarbon streams (7, 8) prior to mixing are set so that the pre-heated heavy hydrocarbon stream (8) is not completely evaporated before mixing, but that the complete evaporation of this stream is achieved by mixing with at least one part of the pre-cracked light hydrocarbon stream (7).
13. Device for steam cracking of hydrocarbons comprising:

    a) a cracking furnace (10) with a convection zone (A) and a blast zone (B),

    b) at least one pre-heating pipe (3) for a charge of light hydrocarbons (1) in the convection zone (A) for pre-heating of this charge, this pipe being connected downstream with at least one cracking pipe (5) for the charge of light hydrocarbons (1) for pre-cracking of the same in the blast zone (B), and

    c) at least one pre-heating pipe (4) for a charge of heavy hydrocarbons (2) in the convection zone (A) for pre-heating of this charge,

    characterised in that it also comprises the following:

    d) a mixing zone for formation of a mixed stream (9) with at least one inlet line (7) for at least one part of the pre-cracked light hydrocarbon stream which is connected with the upstream part of the cracking pipe (5) for the charge of light hydrocarbons (1), and with at least one inlet line (8) for the pre-heated but not pre-cracked heavy hydrocarbon stream which is connected with the upstream part of the pre-heating pipe (4) for a charge of heavy hydrocarbons (2),

    e) a separating zone (11) for division of the mixture into a plurality of separate streams,

    f) a plurality of parallel circulating pipes (13) for the separate streams in the blast zone (B) for a sharp increase in the temperature of the mixture, and

    g) at least one cracking pipe (14) for the mixture which is connected upstream with at least one of the circulating pipes (13) for the separate streams and downstream with devices (15) for cooling of the cracking gases.
14. Device for steam cracking of hydrocarbons comprising:

    a) a cracking furnace (10) with a convection zone (A) and a blast zone (B),

    b) at least one pre-heating pipe (3) for a charge of light hydrocarbons (1) in the convection zone (A) for pre-heating of this charge, this pipe being connected downstream with at least one cracking pipe (5) for the charge of light hydrocarbons (1) for pre-cracking of the same in the blast zone (B), and

    c) at least one pre-heating pipe (4) for a charge of heavy hydrocarbons (2) in the convection zone (A) for pre-heating of this charge,

    characterised in that it also comprises the following:

    d) a mixing zone located outside the furnace (10) for formation of a mixed stream (9) with at least one inlet line (7) for at least one part of the pre-cracked light hydrocarbon stream which is connected with the upstream part of the cracking pipe (5) for the charge of light hydrocarbons (1), and with at least one inlet line (8) for the pre-heated but not pre-cracked heavy hydrocarbon stream which is connected with the upstream part of the pre-heating pipe (4) for a charge of heavy hydrocarbons (2),

    e) at least one transporting pipe (12) for transportation of the mixture from the outside of the furnace (10) to the inside of the blast zone (B), this pipe (12) being connected upstream with the mixing zone and downstream with at least one circulating pipe (13) for the mixture in the blast zone (B), and

    f) at least one cracking pipe (14) for the mixture which is connected upstream with at least one of the circulating pipes (13) for the mixture and downstream with devices (15) for cooling of the cracking gases forming during the co-cracking.
15. Device for steam-cracking of hydrocarbons comprising:

    a) a cracking furnace (10) with a convection zone (A) and a blast zone (B),

    b) at least one pre-heating pipe (3) for a charge of light hydrocarbons (1) in the convection zone (A) for pre-heating of this charge, this pipe being connected downstream with at least one cracking pipe (5) for the charge of light hydrocarbons (1) for pre-cracking of the same in the blast zone (B), and

    c) at least one pre-heating pipe (4) for a charge of heavy hydrocarbons (2) in the convection zone (A) for pre-heating of this charge,

    characterised in that it also comprises the following:

    d) a mixing zone located outside the furnace (10) for formation of a mixed stream (9) with at least one inlet line (7) for at least one part of the pre-cracked light hydrocarbon stream which is connected with the upstream part of the cracking pipe (5) for the charge of light hydrocarbons (1), and with at least one inlet line (8) for the pre-heated but not pre-cracked heavy hydrocarbon stream which is connected with the upstream part of the pre-heating pipe (4) for a charge of heavy hydrocarbons (2),

    e) a separating zone (11) for division of the mixture into a plurality of separate streams,

    f) transporting pipes (12) for transportation of the separate streams from the outside of the furnace (10) to the inside of the blast zone (B), these pipes (12) being connected upstream with the separating zone (11) and downstream with circulating pipes (13) for the mixture in the blast zone (B),

    g) a plurality of parallel circulating pipes (13) for the separate streams in the blast zone (B) for a sharp increase in the temperature of the mixture, and

    h) at least one cracking pipe (14) for the mixture which is connected upstream with at least one of the circulating pipes (13) for the separate streams and downstream with devices (15) for cooling of the cracking gases formed during the co-cracking.
16. Device according to one of claims 13 to 15, characterised in that in the blast zone (B) of the furnace (10) two or more circulating pipes (13) are connected with at least one cracking pipe (14).
17. Device according to one of claims 13 to 16, characterised in that outside the blast zone (B) an adiabatic zone (6) is provided between at least one cracking pipe (5) for the charge of light hydrocarbons (1) for pre-cracking of the same and the inlet line (7) for at least one part of the pre-cracked light hydrocarbon stream.
18. Device according to one of claims 13 to 17, characterised in that a plurality of cracking pipes (14) are connected with a device (15) for cooling of the cracking gases formed during the co-cracking.
19. Device according to one of claims 13 to 18, characterised in that the pre-heating pipe or pipes (3) for pre-heating of the charge of light hydrocarbons (1) in the convection zone (A) inside the cracking furnace (10) is or are connected with the cracking pipe or pipes (5) for pre-cracking of the light hydrocarbons (1) in the blast zone (B).

Images