FR2830537A1 - Steam cracking process using two cracking zones, useful for olefin production, comprises recycling C4 hydrocarbons to the second cracking zone - Google Patents

Abstract

The invention relates to the integration on a hydrocarbon (C5 +) steam cracking site of a specific furnace making it possible to crack the residual C4 cut obtained from the steam cracking furnaces of the main cut. The invention consists in using a specific oven with low coking power to treat this C4 cut under conditions of optimized severity which will make it possible to increase the overall ethylene yield of the site and to use the ovens from the main charge to their full capacity.

Description

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The invention relates to the field of steam cracking of a hydrocarbon fraction also called charged in the following text, containing hydrocarbons having more than five carbon atoms in their molecule. It relates more particularly to the integration, on a site of steam cracking of hydrocarbons (C5 +) for example a naphtha or a diesel, of a second steam cracking zone distinct from the first steam cracking zone of the hydrocarbon charge (C5 +) in which we specifically crack a cut from said first zone comprising hydrocarbons with four carbon atoms and often also saturated hydrocarbons with two and three carbon atoms contained in the cracking products of the charge or obtained from this cut by selective hydrogenation of highly unsaturated compounds such as acetylene, propyne or propadiene and separation into saturated compounds and recoverable olefinic compounds such as ethylene and propylene. This second zone comprises at least one oven making it possible to crack the residual C4 cut from the ovens of the first steam cracking zone, possibly without separation. The steam cracking conditions of the first steam cracking zone (sol) are chosen so as to obtain a hydrocarbon fraction having four carbon atoms in their molecule mainly comprising olefinic compounds.

The steam cracking conditions of the second steam cracking zone (S2) are chosen so that at least 80% of the unsaturated compounds contained in the fraction containing saturated and unsaturated hydrocarbons having four carbon atoms in their molecule are cracked.

It is well known to those skilled in the art that steam cracking furnaces for liquid feeds produce significant quantities of C4 cut hydrocarbons. This cut represents between 5 and 20 percent by mass of the cracked products of a steam cracking site for naphtha or diesel. 1-3 butadiene and isobutylene are the predominant products. These products can be used as filler in the rubber industry which conventionally converts them to styrene-butadiene, polybutadiene polyisobutylene and other copolymers. However in many cases this C4 cut has little commercial value. It is therefore totally or partially recycled in the site's steam cracking ovens to increase its market value by transforming it into ethylene and propylene with or without prior valorization of butadiene. As an example of documents

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 previously published mentioning various recycling we can cite various passages from the book entitled MANUFACTURING ETHYLENE published by GREEN EJ et al Tulsa university: Petroleum Publishing Company, 1970 and in particular the diagrams on page 8 showing the evolution of ethylene production units since 1946 where we note that from 1966 we carry out the recycling of ethane and propane either to a single oven which is the oven in which we crack the fresh charge, or to a dedicated oven as mentioned on page 54 and 55 concerning the simultaneous cracking of ethane and naphtha.

In the current configuration shown diagrammatically in FIG. 1, the recycling of the C4 cut supposes the mixing of the said generally olefinic C4 cut with the initial liquid charge and possibly the hydrogenation of the olefins or diolefins contained in the said C4 cut. coking power and avoid limiting the oven cycle times. In addition, the conversion of the C4 cut is low during this recycling because the severity of the cracking is imposed by the main charge of higher molecular weight (C5 +, naphthas or diesel). The low conversion of section C4 therefore results in a significant recycling volume of said section which limits, for a given oven, the capacity for processing the fresh charge itself.

The use of an oven with low coking power such as a ceramic oven or an oven with a non-metallic or refractory wall as described in European patent EP 0 666 104 81 would make it possible to preferentially recycle the C4 cut in this oven, avoiding the dilution in the initial charge and avoiding the possible steps of hydrogenation. Indeed for such a charge containing unsaturated compounds, the coke formed is largely catalytic coke strongly activated by the metals present in the metal alloys of the furnace tubes. The use of ovens containing tubes with ceramic or refractory walls containing few metals (for example less than 4% by mass) or alternatively ovens containing tubes with metal walls coated with a refractory material, makes it possible to limit the formation of catalytic coke and therefore to treat more olefinic charges while keeping coke yields low. By refractory materials is meant mineral materials including refractory concretes, aluminous concretes, porous ceramics and non-porous ceramics. By oven with low coking capacity is also meant ovens with metal tubes having undergone a surface treatment such as that described in the publication Anti coking Coatings for High Temperature

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petrochemical Heactors parue aans la revue un ana uas science ana 1 ecnnology et dans la revue de institut Français du Pétrole, volume 54 (1999) ? 3. pages 375 à 385, (notée référence [2] dans la suite du texte), qui permet de limiter la formation de ce coke catalytique et donc de traiter des charges plus fortement oléfiniques en gardant des rendements en coke faibles. Dans la référence [2] ont été étudiés différents types de revêtements à base de Chrome, d'aluminium, de silicium, de titane, ou d'oxydes de ces composés ainsi que des revêtements à base de carbure de titane (TiC) ou de carbure de silicium (SiC) ou des combinaisons de ces composés. Dans ce type de four à faible pouvoir de cokage on parvient à diminuer d'un facteur 2 à 10 la formation de coke par rapport à un four conventionnel contenant des tubes métalliques du type HK-40 ou HP-40 ou d'un type équivalent.

petrochemical Heactors published in the review un ana uas science ana 1 ecnnology and in the review of institut Français du Pétrole, volume 54 (1999)? 3. pages 375 to 385, (noted reference [2] in the following text), which makes it possible to limit the formation of this catalytic coke and therefore to treat more highly olefinic fillers while keeping coke yields low. In reference [2], different types of coatings based on chromium, aluminum, silicon, titanium, or oxides of these compounds have been studied, as well as coatings based on titanium carbide (TiC) or silicon carbide (SiC) or combinations of these compounds. In this type of oven with low coking power, it is possible to reduce by a factor of 2 to 10 the formation of coke compared to a conventional oven containing metal tubes of the HK-40 or HP-40 type or of an equivalent type. .

For the sake of clarity of the text, a conventional oven will be called a steam cracking oven comprising only metal tubes and an oven with low coking power, any of the types of ovens described above: ceramic oven, oven with refractory coating or with anti-coking coating.

The use of an oven with low coking power in a second zone. cracking therefore makes it possible to optimize the operation of the site by increasing the severity of the oven dedicated to cracking the charge C4 while limiting the coking of the other ovens on the site which no longer have to process this charge. It remains within the scope of the invention using for the main cracking zone, ovens which may be conventional, or for some of them or even for all, with low coking power. In addition, the possible hydrogenation step of the C4 cut can be omitted.

To significantly improve the ethylene yields of the site, it is also proposed to mix the C4 cut with ethane from the fractionation zone and conventionally recycled in a dedicated oven, often called an ethane oven. This co-cracking device makes it possible to significantly improve the cracking yield of the C4 cut due to the interactions with free radicals linked to the cracking of ethane and to the transfer of hydrogen. It is also possible to crack the C4 cut in the dedicated oven in admixture with the propane from the depropanizer, the ethane then being cracked in a dedicated oven separate from the oven treating the C4 cut. In this case, this second cracking zone which we will call in the following second hot section to distinguish it from the main hot section supplied by the fresh charge, comprises at least two

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tours ; un tour ae type cramique ou a parois retraciaires ou a raiDie pouvoir ae cokage, traitant le mélange de la coupe C4 recyclée et du propane et un second four dédié à l'éthane. Ces deux fours qui peuvent être de technologie identique ou différente, c'est à dire de type conventionnel ou à faible pouvoir de cokage, différent par les conditions opératoires et plus précisément par le niveau de sévérité. Le four dédié à l'éthane fonctionne à une sévérité plus élevée que le four traitant le mélange de la coupe C4 et du propane. La sévérité dans le contexte du vapocraquage (steam craking dans la terminologie anglo-saxonne) est généralement définie à partir de la température et du temps de séjour dans la section réactionnelle. On peut en trouver une définition précise dans l'ouvrage ethylene keystone to the petrochemical industry (l'éthylène la clef de l'industrie pétrochimique) de L. Kniel, O. Winter et K.

towers; a lathe with a ceramic type or with retracial walls or with a coking capacity, treating the mixture of the recycled C4 cut and propane and a second oven dedicated to ethane. These two ovens which can be of identical or different technology, that is to say of conventional type or with low coking power, different by the operating conditions and more precisely by the level of severity. The dedicated ethane oven operates at a higher severity than the oven processing the mixture of C4 cut and propane. The severity in the context of steam cracking (steam craking in English terminology) is generally defined from the temperature and the residence time in the reaction section. A precise definition can be found in the book ethylene keystone to the petrochemical industry by L. Kniel, O. Winter and K.

Stork published by Dekker editions in the Chemical Industries series pages 118 and 119. We will note this reference [1] in the rest of the text. It will be noted that the severity increases when the outlet temperature of the oven increases and when the residence time in the reaction zone decreases. It is also possible to send the olefinic C4 cut, the ethane from the specific ethylene-ethane fractionation section and the propane from the specific propylene-propane fractionation section, in a same ceramic or wall furnace. refractory or more generally with low coking power. The second cracking zone can therefore have various configurations depending on whether the olefinic C4 cut is treated there alone or in admixture with ethane and / or propane from the specific separation zones.

The invention will be better understood with reference to Figures 1 and 2 which respectively illustrate the state of the art and the invention.

In FIG. 1, a classic diagram of a steam cracking unit is reproduced comprising a steam charge cracker (naphtha or diesel) forming part of the main hot section (soil) and a separation zone (C 1) The charge naphtha or diesel enters the main hot section (S1) via line E. This main hot section consists of one or more conventional type steam cracking furnaces. At the outlet of the main hot section (S1), the products are sent to a cooling zone (TC) which is often a cryogenic treatment before entering the separation zone (C1). The separation zone (C1) contains a demethanizer in which the methane discharged via the line (14) is generally separated and a certain amount of hydrogen. Hydrogen and

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metnane sont envoyes par cette ligne (14) au reseau Tuel gaz ae la ramène. ruis on effectue la séparation de la fraction C2 issue du déméthaniseur dans un déethaniseur. La dite fraction C2 quittant le déethaniseur par la ligne (3), est hydrogénée dans l'unité d'hydrogénation (H 1) pour assurer la conversion de l'acétylène en éthylène. On sépare alors dans l'unité de séparation éthane/éthylène (C2) la coupe C2 en éthylène sortant par la ligne (10) qui est valorisé et en éthane sortant par la ligne (9) qui est généralement recyclé dans un four spécifique appelé four de craquage d'éthane faisant partie de la seconde section chaude (S2) et travaillant à plus haute sévérité que le ou les fours de la section chaude principale (sol). Les effluents issus de cette seconde section chaude (S2) sont introduits dans la zone de refroidissement (TC) par la ligne 16.

metnane are sent by this line (14) to the Tuel gaz network to bring it back. ruis the separation of fraction C2 from the demethanizer is carried out in a deethanizer. Said fraction C2 leaving the deethanizer via line (3) is hydrogenated in the hydrogenation unit (H 1) to ensure the conversion of acetylene to ethylene. The C2 section is then separated into the ethylene / ethylene separation unit (C2) into ethylene leaving through line (10) which is upgraded and into ethane leaving through line (9) which is generally recycled in a specific oven called an oven. ethane cracking forming part of the second hot section (S2) and working at a higher severity than the furnace (s) of the main hot section (floor). The effluents from this second hot section (S2) are introduced into the cooling zone (TC) via line 16.

The C3 cut from the deethanizer contained in the separation zone (C1) is then separated in a depropanizer and introduced by the line (4) into a hydrogenation unit (H2) where it is hydrogenated to transform the diolefins and acetylenes into less compounds . Unsaturated. The section C3 is then separated in the propane / propylene separation unit (C3) in order to produce the propylene leaving by the line (12), and the propane leaving by the line (11) which is recycled in the second hot section ( S2).

The non-valued C4 cut from the debutanizer contained in the separation zone (C1) is recycled by line 5 possibly via the hydrogenation unit (H3) then line 2 in a conventional oven of the main hot section (floor), the severity is fixed by the main charge, either naphtha or diesel. The low severity of this oven results in a low conversion of the C4 cut and therefore an increase in the recycled C4 cut wheel, which limits the capacity of the ovens compared to the fresh load. The hydrogenation of section C4 can be carried out before passing through the furnace (s) of the main hot section to limit the coking of these ovens, since this coking phenomenon is accelerated by the presence of unsaturated compounds. The C5 + cut, generally called pyrolysis essence, is extracted from the separation zone by the line (15).

The implementation of the present invention is illustrated in FIG. 2. The naphtha or diesel fuel is introduced into the main hot section (soil) which comprises one or more steam cracking ovens of conventional type or with low coking power. Cracked effluents are recovered at the outlet of the hot section

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 main and sent to a cooling zone (TC) which generally consists of a cryogenic treatment. At the outlet of the cooling zone (TC), the effluents are sent to a separation zone (C1) which makes it possible to extract the following five cuts: a cut C1 containing essentially methane and hydrogen, a cut C2 containing saturated and unsaturated hydrocarbon compounds having two carbon atoms in their molecule, a C3 cut containing saturated and unsaturated hydrocarbon compounds having three carbon atoms in their molecule, a C4 cut containing saturated and unsaturated hydrocarbon compounds having four carbon atoms in their molecule their molecule and a C5 + cut containing saturated and unsaturated hydrocarbon compounds having more than four carbon atoms in their molecule, generally called pyrolysis essence. A particular embodiment of this separation zone (C1) which comprises a demethanizer making it possible to separate the methane and a certain quantity of hydrogen which are evacuated via the line (14) is described in the following text. Hydrogen and methane are sent to the refinery's fuel network by this line (14). Then the separation of fraction C2 from the demethanizer is carried out in a deethanizer also forming part of the separation zone. This fraction C2 containing saturated and unsaturated hydrocarbons having two carbon atoms in their molecule resulting from the deethanizer is sent by line (3) in a selective hydrogenation unit (H1) making it possible to obtain a product depleted in acetylene containing l ethylene and ethane. Then separated in the separation unit (C2) a fraction containing essentially ethylene discharged through line (10) which is the desired product and a fraction containing essentially ethane discharged through line (9). This ethane-containing fraction is at least partly recycled to the second hot section (S2). This second hot section (82) usually comprises at least one oven with a low coking power (not shown in FIG. 2) which works with greater severity than the oven or ovens of the main hot section. The effluents from this second hot section (S2) are introduced via line 16 into the cooling zone (TC). The cooling zone (TC) therefore processes both the products from the first steam cracking zone (SI) and the products from the second steam cracking zone (S2). The C3 + section from the deethanizer of the separation zone is then separated in a depropanizer also forming part of the separation zone (C1). The C3 cut from the depropanizer by the

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 line (4) and containing saturated and unsaturated hydrocarbon compounds having three carbon atoms in their molecule is sent to the selective hydrogenation unit (H2) of the polyunsaturated compounds which it contains, making it possible to obtain an effluent depleted in compounds polyunsaturated containing propylene and propane The effluent from the selective hydrogenation unit (H2) is sent by line (7) to the specific separation unit (C3) from which a fraction containing essentially propylene and a fraction essentially containing propane Propylene, the desired product, is evacuated via line (12) and propane is at least partly recycled via line (11) to the second hot section (S2) where it is generally introduced into one of the ovens in this section. The C4 + cut from the depropanizer of the separation zone (C1) is introduced into a debutanizer also forming part of this separation zone. The cut portion C4 not valued is then sent by the line (5) to the second hot section (S2) where it is treated in one of the ovens in this section. A Cs + cut, called pyrolysis gasoline, which is generally sent by line (15) to a hydrogenation unit before being incorporated into the gasoline pool, is extracted from the debutanizer from the separation zone (C1). The possible non-recycled C4 cutting portion is extracted from the unit by the line (17).

It is important to note that, in the process according to the invention, the C4 cut does not need to be previously hydrogenated before its recycling in the second hot section since this last section generally includes at least one oven with low coking capacity of ceramic type or with refractory walls or with anti-coking coating, making it possible to limit the formation of coke. In addition, the main hot section is no longer affected by recycling products and can be optimized for the main load while working at full capacity on this main load. The second hot section, specially dedicated to recycling, is most often composed of at least one oven with low coking power, which makes it possible to process the C4 cut, and possibly as a mixture, the fraction containing essentially the ethane from the specific separation zone (C2) and the fraction essentially containing propane from the specific separation zone (C3). According to another variant, this second hot section will most often comprise a ceramic type oven or with refractory walls or more generally with low coking power for the treatment of the C4 cut in mixture with the

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 propane from the specific separation section (and a tower dedicated to cracking ethane from the specific separation zone (C2). According to a sub-variant of that immediately above, part of the ethane can be sent to the oven with low coking power, the other part being treated in the ethane oven, according to yet another variant, the second hot section (S2) comprises at least two separate ovens, a first oven in which the steam cracking is carried out of the fraction essentially containing propane from the separation zone (C3) and a second furnace, most often with low coking power, in which the steam cracking of the fraction containing saturated and unsaturated hydrocarbons having four carbon atoms is carried out in their molecule, possibly with a mixture of ethane from the separation zone (C2).

In summary, the process according to the invention is a process for steam cracking a hydrocarbon feed containing hydrocarbons having more than five carbon atoms in their molecule comprising: - a first steam cracking zone (S1) and, - a second zone steam cracking (S2), said zones each comprising at least one steam cracking oven, - at least one cooling zone (TC) of the products originating from said first steam cracking zone, and of products originating from the second steam cracking zone, - at least a separation zone (C1) for the products from said cooling zone (TC) in which at least one gaseous fraction containing hydrogen and methane is separated, and at least one fraction containing saturated and unsaturated hydrocarbons having four atoms of carbon in their molecule, in which at least part of the fraction of saturated and unsaturated hydrocarbons having four carbon atoms in their molecule is sent d in said second steam cracking zone (S2).

In the process according to the invention it is optionally possible, in the separation zone (C1), to further separate: - at least one fraction containing saturated and unsaturated hydrocarbons having two carbon atoms in their molecule,

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 at least one traction containing saturated and unsaturated hydrocarbons having three carbon atoms in their molecule, - at least one C5 + fraction containing saturated and unsaturated hydrocarbons having more than four carbon atoms in their molecule.

According to one of the preferred modes of the steam cracking process according to the invention, the fraction containing saturated and unsaturated hydrocarbons having two carbon atoms in their molecule is sent to a zone of selective hydrogenation (H1) of the acetylene contained in said fraction, making it possible to obtain a product depleted in acetylene containing ethylene and ethane which is sent to a specific separation zone (C2) from which a fraction containing essentially ethylene and a fraction is recovered essentially containing ethane. More preferably, the ethane-containing fraction is sent to the second steam cracking zone (S2).

According to another preferred embodiment of the method according to the invention, said second steam cracking zone (S2) comprises at least two separate ovens, a first oven in which the fraction cracking essentially containing ethane is carried out and a second oven in which performs the steam cracking of the fraction containing saturated and unsaturated hydrocarbons having four carbon atoms in their molecule and sends the steam cracking products obtained from each of the two ovens with the products from said first steam cracking zone in said zone cooling (TC), then in said separation zone (C1).

According to another preferred mode of the steam cracking process according to the invention, the fraction containing saturated and unsaturated hydrocarbons having three carbon atoms in their molecule is sent to a zone of selective hydrogenation (H2) of the polyunsaturated compounds which it contains, making it possible to obtain a product depleted in polyunsaturated compounds containing propylene and propane which is sent to a specific separation zone (C3) from which a fraction containing essentially propylene and a fraction containing essentially propane is recovered.

To illustrate the advantages which the invention brings in terms of overall yield of ethylene and of treatment capacity of the ovens of the main cracking section, we give below 3 examples. The yields in each of the examples are calculated using the commercial software CRACKSIM which makes it possible to calculate

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les rendements en sortie ae tours ae vapocraquage a partir ae la composition de la charge et d'un profil de température le long du faisceau de tubes de craquage.

the output yields at steam cracking turns from the composition of the charge and from a temperature profile along the bundle of cracking tubes.

Example 1: A naphtha charge is treated according to the diagram in FIG. 1 in a conventional steam cracking oven working under the following operating conditions: Entering temperature into the oven: 440 C Leaving oven temperature: 854 C Residence time in the reaction zone : 557 ms (1 millisecond = 10-3 seconds) Water vapor on naphtha molar ratio: 0.53 Kg / Kg The C4 cut containing more than 80% of unsaturated compounds is recycled as a mixture with the naphtha charge. The quantity of C4 cut recycled represents 12% by weight of the fresh load.

The ethane and propane produced at the end of the separation zones (C2) and (C3) are recycled in a specific oven called the ethane oven constituting the second hot section. The operating conditions of this oven are as follows: Oven inlet temperature: 150 C Oven outlet temperature: 880 C Residence time: 240 ms (1 millisecond = 10-3 seconds) H2O / HC ratio by weight: 0 , 3 The overall yields obtained according to this scheme are: 30.9% for ethylene In this first example, the steam cracking charge is a naphtha whose main characteristics are given below: Density: 692 Kg / m3 Simulated distillation according to method ASTM D 2887- (97) Point 25%: 54 C Point 50%: 74 C Point 75%: 1020C End point (99, 5%) 179 C The cooling zone (TC) consists of a conventional cryogenic treatment including a description can be found in the work already cited [1] pages

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 93-94. The temperature used is -85 oc and the pressure - is atmospheric pressure. The various separation units of the separation zone (C1) are distillation columns working under conventional conditions and a description can also be found in the work cited in reference [1] pages 93-97.

The hydrogenation units (H1) and (H2) work within the range of conventional operating conditions for this type of unit and use a conventional hydrogenation catalyst comprising palladium. Reference will also be made to reference [1] pages 97-102 for a description of these units. In the present example, the selective hydrogenation of the C2 section is carried out at a temperature of 700C at 25 bars (2.5 MPa) and with a catalyst containing 0.5% by weight of Palladium (Pd) on an alumina support. The selective hydrogenation of section C3 is carried out at a temperature of 40 ° C. under 20 bars (2 MPa) and with a catalyst identical to that of the hydrogenation of section C2.

The specific ethane / ethylene and propane / propylene separation units are distillation columns working at reflux rates of between 2.5 and 4 and containing a hundred trays.

Example 2: (according to the invention) The same naphtha charge is treated in the same conventional oven as in Example 1 and the cut C4 from the separation zone (C1) is treated in a specific oven which is a ceramic technology as described in US Pat. No. 5,554,347. This C4 cut represents 12% by weight of the fresh charge.

The ethane and propane produced at the end of the separation units (C2) and (C3) is, as in Example 1, recycled in an ethane oven operating under the same conditions as those of Example 1. operating conditions for each of the ovens are given below: Main oven: conditions identical to those of Example 1 Specific oven: Entering temperature into the oven: 440 C Leaving temperature from the oven: 940 C Residence time: 300 ms Mass ratio H20 / HC: 0.53 Kg / Kg

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 The overall ethylene yield is: 31.5% The separation units of the separation zone (C1), the hydrogenation units (H1) and (H2) and the ethane / ethylene (C2) and propane separation units / propylene (C3) are identical to those described in Example 1 and work under the same conditions as those of Example 1.

Example 3 (according to the invention) The same naphtha feed is treated in the same conventional oven as in Example 1 and the cut C4 from the separation zone (C1) is treated in mixture with 10% ethane from said separation zone (C2) in a specific oven which is identical to that of Example 2. Cut C4 always represents 12% by weight of the fresh charge.

The remaining 90% of ethane and the propane from the separation units are recycled in an ethane oven working under the same conditions as those of Example 1.

g Temps de séjour : 300 ms (1 ms= 10-3 seconde) Rapport massique H20/HC : 0,53 Kg/Kg Le rendement global en éthylène est de : 32,3 % On voit donc clairement en suivant les trois exemples ci dessus que le fait de traiter la coupe C4 dans un four spécifique de type céramique au lieu de l'envoyer dans le four de craquage principal, permet d'augmenter de 0,6 point le rendement global en éthylène (exemple 2) et que le fait de rajouter en mélange avec la dite coupe C4 une certaine proportion d'éthane (exemple 3) conduit à améliorer encore le rendement en éthylène de 1,4 point. Ce gain sur des unités produisant 50 000 Tonnes/an par four est tout à fait significatif. Par ailleurs, dans les exemples 2 et 3 la zone de craquage principal traite 12 % de charge supplémentaire par rapport à l'exemple 1.The operating conditions of each of the ovens are given below: Main oven: conditions identical to those of Example 1 Specific oven: Entering temperature into the oven: 440 C Leaving temperature from the oven: 945 C

g Residence time: 300 ms (1 ms = 10-3 seconds) H2O / HC mass ratio: 0.53 Kg / Kg The overall ethylene yield is: 32.3% We therefore clearly see by following the three examples below above that the fact of treating the C4 cut in a specific ceramic-type oven instead of sending it to the main cracking oven, makes it possible to increase the overall ethylene yield by 0.6 point (example 2) and that the the fact of adding a mixture of said ethane to the said cut C4 (example 3) leads to further improving the ethylene yield by 1.4 points. This gain on units producing 50,000 tonnes / year per oven is quite significant. Furthermore, in Examples 2 and 3, the main cracking zone processes 12% of additional load compared to Example 1.

Claims (17)

CLAIMS 1. Process for steam cracking a hydrocarbon feed containing hydrocarbons having more than five carbon atoms in their molecule comprising: - a first steam cracking area (sol) and, - a second steam cracking area (S2), said areas comprising each at least one steam cracking oven, - at least one cooling zone (TC) for the products from said first steam cracking zone, and products from the second steam cracking zone, - at least one separation zone (C1) for the products from said cooling zone (TC) in which at least one gaseous fraction containing hydrogen and methane is separated, and at least one fraction containing saturated and unsaturated hydrocarbons having four carbon atoms in their molecule, in which at least part of the fraction of saturated and unsaturated hydrocarbons having four carbon atoms in their molecule is sent to said second vapor zone cracking (S2). 2. Method according to claim 1, in which in the separation zone (C1), there is further separated: - at least one fraction containing saturated and unsaturated hydrocarbons having two carbon atoms in their molecule,

 - at least one fraction containing saturated and unsaturated hydrocarbons having three carbon atoms in their molecule, - at least one C5 + fraction containing saturated and unsaturated hydrocarbons having more than four carbon atoms in their molecule. 3. A steam cracking method according to one of claims 1 or 2 in which the fraction containing saturated and unsaturated hydrocarbons having two carbon atoms in their molecule is sent to a zone of selective hydrogenation (H1) of the acetylene contained in the said fraction, making it possible to obtain a product depleted in acetylene containing ethylene and ethane which is sent to a specific separation zone (C2) from which a fraction containing essentially ethylene is recovered and a fraction essentially containing ethane. <Desc / Clms Page number 14> 4. A steam cracking method according to claim 3 in) eque) ia fraction containing ethane is sent to said second steam cracking zone (S2). 5. The steam cracking method according to claim 4, in which said second steam cracking zone (S2) comprises at least two separate ovens, a first oven in which the fraction essentially containing ethane is steam cracked and a second oven in which steam cracking is carried out on the fraction containing saturated and unsaturated hydrocarbons having four carbon atoms in their molecule and the steam cracking products obtained from each of the two ovens are sent with the products from said first steam cracking zone in said steam cracking zone cooling (TC), then in the said separation zone (C1). 6. A steam cracking method according to claim 1, in which the fraction containing saturated and unsaturated hydrocarbons having three carbon atoms in their molecule is sent to a zone of selective hydrogenation (H2) of the polyunsaturated compounds which it contains, making it possible to obtain a product depleted in polyunsaturated compounds containing propylene and propane which is sent to a specific separation zone (C3) from which a fraction containing essentially propylene and a fraction containing essentially propane is recovered. 7. The steam cracking method according to claim 6, in which the fraction containing propane is sent to said second steam cracking zone (S2). 8. A steam cracking method according to claim 7 wherein said second steam cracking zone (S2) comprises at least two separate ovens, a first oven in which the steam cracking of the fraction containing essentially propane is carried out and a second oven in which one carries out the steam cracking of the fraction containing saturated and unsaturated hydrocarbons having four carbon atoms in their molecule and the steam cracking products obtained are sent from each of the two ovens with the products from said first steam cracking zone in said cooling zone ( TC) then in the separation zone (C1). <Desc / CRUD Page number 15>  9. Method according to one of claims 3 to 7 wherein one sends in said second steam cracking zone (S2) all of the fraction containing saturated and unsaturated hydrocarbons having four carbon atoms in their molecule, the fraction containing mainly ethane from the separation zone (C2) and the fraction containing essentially propane from the separation zone (C3). 10. The method of claim 9 wherein said second steam cracking zone (S2) comprises at least two separate ovens, a first oven in which the fraction cracking essentially containing ethane is carried out and a second oven in which one carries out the steam cracking of the fraction containing propane and the fraction containing saturated and unsaturated hydrocarbons having four carbon atoms in their molecule and the steam cracking products obtained are sent from each of the two ovens with the product from said first zone of steam cracking (soil) in said cooling zone (TC). 11. Method according to one of claims 1 to 10 wherein the conditions of steam cracking in said first steam cracking zone (sol) are chosen so as to obtain a hydrocarbon fraction having four carbon atoms in their molecule mainly comprising olefinic compounds. 12. Method according to one of claims 1 to 11 wherein in said second steam cracking zone (S2) the steam cracking conditions are chosen so that at least 80% of the unsaturated compounds contained in the fraction containing are cracked saturated and unsaturated hydrocarbons having four carbon atoms in their molecule. 13. Method according to one of claims 1 to 12 wherein the charge that is cracked in said first steam cracking area (soil) is selected from the group formed by naphthas, and gas oils. 14. Method according to one of claims 1 to 13 wherein the oven (s) of said second steam cracking zone (S2) are ovens whose walls in contact with the charge to be cracked are made of refractory materials or are covered with a refractory material or a coating with low coking power. <Desc / Clms Page number 16> 15. The method of claim 14 in which the rerractary materials used are chosen from the group formed by mineral refractory materials including refractory concretes, aluminous concretes, porous ceramics and non-porous ceramics. 16. The method of claim 14 wherein the coating with low coking power comprises at least one compound of the elements of the periodic table selected from the group consisting of chromium (Cr), aluminum (AI), silicon (Si ) titanium (Ti). 17. Method according to any one of claims 1 to 16 in which the oven (s) of said second steam cracking zone (S2) operate at a higher level of severity than the oven (s) of said first steam cracking zone ( S1).