FR2984914A1 - PROCESS FOR MAXIMIZING AROMATIC PRODUCTION - Google Patents

Abstract

The invention relates to a method for producing a benzene-enriched aromatic petroleum fraction (31) from a naphtha fraction (2) comprising paraffins, aromatic fractions and precursors of said aromatic fractions, comprising the steps of a) separating the paraffins from the naphtha section in a paraffin separation unit (10), b) catalytically reforming the paraffin-reduced fraction obtained in step a) in a catalytic reforming unit (20) , to form a reformate (23) rich in aromatic fractions, and c) separating the aromatic fractions from said reformate (23) from step b) into a unit for separating the aromatic fractions (30), to obtain the aromatic petroleum fraction enriched in benzene (31).

Description

The present invention relates to a method for producing a benzene-enriched aromatic petroleum fraction and an installation for carrying out this method. One of the main aims of the invention is to produce almost pure benzene (at about 99.9% by weight).

Benzene is a molecule with a high value, which requires finding new processes and / or devices to increase production. In addition, the methods typically used today are very energy intensive, since they involve successive distillations. With regard to the refining of petroleum fractions, benzene is mainly produced in catalytic reforming units whose primary purpose, in a refining process, is the production of a high octane gasoline base from a C6-C8 hydrocarbon cut. As the incorporation of benzene into the gasoline pool is limited, the effluent from catalytic reforming is often fractionated to obtain a benzene-rich fraction which is then sent to a unit allowing the production of high purity benzene and which is generally based on liquid-liquid extraction technologies. The evolution of the markets and the balance of production and demand for gasoline sometimes lead to reducing or even stopping catalytic reforming units, which, among other things, has a negative impact on the production of benzene as well as hydrogen. On the other hand, a catalytic reforming unit operating under typical conditions for the production of gasoline has a relatively low yield of benzene and a relatively large cracking of the charge molecules, such as paraffins, producing cracked gases. (from Cl to C4), which negatively affects economic profitability. Therefore, sending directly into a reforming unit charges containing high levels of paraffins is penalizing. Novel processes are proposed commercially to increase the yield of benzene from a majority feed of C6 (C6) paraffins, C6 (N6) naphthenes which are essential benzene precursors, and C6 (C6) aromatics. with the use of a specific catalyst and appropriate operating conditions which impose a particular arrangement of the production units.

U.S. Patent 4,648,961 discloses a process for increasing the aromatic content of a naphtha feedstock by a contact step in a reaction unit with a hydrodecyclization catalyst to produce an aromatic feedstock and a gaseous feed, wherein Aromatic charge is then separated from the gas stream. This document then describes a step of separating the paraffins from the aromatics feed, while the gaseous flow and the paraffins are recycled to the hydrodecyclation reaction unit.

The patent application WO2008 / 092232 discloses a process for recovering aromatic compounds from naphtha feedstocks comprising the steps of desulphurizing said feeds, separating a C6-C11 hydrocarbon fraction, recovering from the C6-C6 fraction. C11 of an aromatic fraction, an aromatic precursor fraction and a raffinate fraction in an aromatics extraction unit, conversion of precursors to aromatic compounds and recovery of aromatic compounds. The precursor fraction comprises naphthenes and C6-C8 paraffins, which paraffins are then separated.

Furthermore, in these new hydrodecyclization processes, the specifications on the pollutant content of the feed are very severe, which requires the implementation of complex and expensive load pretreatment units. The investment cost to build a new unit of this type is therefore high.

There is a need for the production of benzene-rich aromatic cuts with improved yield, by a process and an installation using it, while minimizing the impact of the cracking products, which affect the yield and efficiency of this production.

The invention thus relates to a process for producing a benzene-enriched aromatic petroleum cut from at least one naphtha cut comprising paraffins, aromatic compounds and precursors of said aromatic compounds, comprising the steps of: a) separating the paraffins from the paraffin-rich naphtha section in a paraffin separation unit; b) catalytically reforming the paraffin-reduced fraction obtained in step a) in a catalytic reforming unit to form an enriched reformate in aromatic compounds, and c) separating the aromatic compounds from said reformate of step b) in a separation unit of the aromatic compounds, to obtain the benzene-enriched aromatic petroleum fraction. In order to maximize the production of benzene, the cutting points of the feed are preferably selected so that the concentration and recovery of benzene precursors (mainly benzene, cyclohexane and methyl-cyclopentane) are maximized. These cutting points can be expanded to also recover toluene and toluene precursors which can subsequently be used to produce more benzene from toluene hydrodealkylation (HDA) technologies as will be described later.

Advantageously, the aromatic compounds are benzene derivatives mainly comprising benzene and toluene as a by-product from which more benzene can be produced by means of an HDA unit. This does not exclude the possible presence of other compounds, such as xylene and others.

The process of the invention, comprising an upstream paraffin separation step, has numerous advantages, such as: the increase of the yield of benzene derivatives and preferably of benzene-enriched aromatic petroleum fraction, and of hydrogen with respect to typically achieved yields in conventional Refining-Petrochemical schemes; - The reduction of cracking gases and production in their place of a paraffins stream (n-paraffins and some branched paraffins) which can be efficiently recovered, for example as a charge in a steam cracking unit; - the absence of additional investment costs related to the possibility of implementing existing units; - The improvement of energy efficiency, because the separation of paraffins, preferably by molecular sieve, in addition to its selectivity, has the advantage of requiring only low energy consumption. In comparison, a distillation column would have much lower selectivities with very high energy consumption; and the flexibility of adaptation to existing units and the possibility of treating naphtha feedstocks of various origins, with a limited impact on the operating modes and the severity.

The feedstocks are naphtha cuts more or less rich in paraffins, such as a petroleum cut extracted by direct distillation ("straight-run" naphtha or SR naphtha), or a cut chosen from conversion naphthas, such as than those resulting from cracking, in particular those resulting from catalytic cracking, coking, visbreaking, hydrocracking, pyrolysis naphthas (steam cracking) or those of a bottom effluent from a de-iso-hexanizer column of isomerization unit. These cuts, according to their origin and their point of cut, have variable contents in paraffins and precursors of benzene derivatives. Depending on their composition, their point of entry into the device of the invention may also vary. These fillers should preferably be of a quality compatible with the process, in particular with regard to the content of sulfur and nitrogen, typically less than 0.5 ppm by weight, and to olefins. These fillers must, if necessary, undergo a hydrotreatment or other appropriate purification process, known to those skilled in the art. The precursors of said benzene derivatives are naphthenes, that is to say cyclic alkanes with 5 or 6 carbon atoms. Cyclohexane or methylcyclopentane may be mentioned. The first step of the process (step a)) consists of separating paraffins from the naphtha section in a paraffin separation unit. The Applicant has thus found that the upstream paraffin separation step makes it possible to selectively separate the paraffins which are the molecules which have the lowest reactivity for the production of benzene derivatives in the catalytic reforming unit. Paraffins are also the molecules that have the greatest tendency to crack during the catalytic reforming stage, which leads to a devaluation due to the transformation of the naphtha molecules into C1-C4 gas, but also because cracking consumes hydrogen, which decreases the H2 yield in the reforming unit. Paraffins comprise linear n-paraffins and branched paraffins (iso- or i-paraffins) conventionally present in the naphtha cuts mentioned, and have carbon atoms ranging from C5 to C8. The paraffin contents in the feedstock will be illustrated in the examples described further in this specification. Typically, for some examples of crude oils at the origin of the naphtha cuts of the invention, the contents are as follows: Paraffins, naphthenes and aromatics contents in the C5-145 ° C cut according to WAF DALIA crude% wt 24, 6 72.0 3.4 DALIA ODUDU BLEND% wt 42.0 49.7 8.3 ODUDU BLEND Light Arabic% w 76.9 15.5 7.6 ARAB LIGHT The paraffin separation unit of step a ) is based on known technologies and used in refining and petrochemistry for related purposes, advantageously using suitable adsorbents, such as molecular sieves. During this step, the paraffins are introduced into the pores of the adsorbent and are retained there, while the larger molecules, the precursors of the benzene derivatives and the said derivatives, cross the bed of the adsorbent. Depending on the adsorbent used and the operating conditions, the separated AN molecules are n-paraffins and at least some of the less branched paraffins, such as mono-methyl paraffins, the most branched paraffins not being present. separated precursors and benzene derivatives. The paraffin fraction is then recovered in a desorption stage, for example using a stream of light hydrocarbons, such as n-butane, operating at high temperature and low pressure to extract the pores. The implementation of this technology can be done easily with at least two beds working in parallel, one in adsorption and the other in alternating desorption at the end of each cycle. Another form of implementation uses so-called "simulated moving bed" technologies in which a multi-turn rotary valve of appropriate technology allows different areas of the same column to work simultaneously adsorption and desorption. Typical examples of this latter technology are the processes known under the trade name Molex® (marketed by the US licensor UOP) for the separation of linear light paraffins from branched paraffins, and Parex® (Company UOP) or Eluxyl® (marketed by the French licensor Axens, a subsidiary of IFPEN) that allows the separation of paraxylene from a mixture of the three different isomers of xylene. The separation of paraffins by molecular sieves, in addition to its selectivity, has the advantage of requiring only low energy consumption. In comparison, a distillation column would have much lower selectivities with very high energy consumption. The operating conditions for carrying out such processes are typical, known to those skilled in the art of refining and petrochemistry, and in general not specific to the invention. The paraffin-depleted fraction is then treated by a catalytic reforming step in a suitable unit (step b) to form an aromatics rich reformate.

In general, the objective of catalytic reforming is to convert the precursors of benzene derivatives into these derivatives. The paraffins present in the feedstock may also undergo hydrodecyclization reactions to produce more aromatic compounds, but these reactions are much slower and less complete than those of dehydrogenation of naphthenes (benzene precursors). Paraffins can be cracked to produce light hydrocarbons of the C1-C4 type (cracking gas) but these reactions are much less economically attractive, and in addition, each cracked paraffin molecule consumes one molecule of H2, which decreases the hydrogen yield in the reforming unit. Therefore, the separation of paraffins upstream of this step b) limits the loss of H 2 linked to the cracking of these molecules (light C1-C4 hydrocarbons) and makes it possible to more efficiently recover the paraffins obtained for example in charge of a steam cracker. . Indeed, when the feedstock is concentrated in precursors of benzene derivatives, the required severity for a given conversion is lower, which reduces the cracking reactions and maximize the yield of benzene derivatives and H2. At the same time, a lower severity operation makes it possible to use catalysts which may for example be less stable from the point of view of deactivation but which will be more selective and active with regard to the production of benzene derivatives.

As indicated above, a better H2 yield is obtained in the reforming unit. In addition, with less cracking, the purity of the hydrogenated gas produced is greater.

The paraffins also undergo isomerization reactions during the catalytic reforming step. These reactions are governed by thermodynamic equilibrium. In the reforming or reformate product, the branched paraffins which have not been separated in the paraffin separation unit and unreacted in the catalytic reforming unit will be at thermodynamic equilibrium. Typically, under the typical conditions of catalytic reforming operation, the content of n-hexane in an equilibrium paraffin C6 mixture is of the order of 30-40%. This means that n-paraffins and poorly branched paraffins will be formed from the highly branched paraffins when the former are absent from the feed sent to the reforming unit since they have been removed in the paraffin separation unit.

According to very preferred embodiments, the naphtha section is a section comprising a large majority of C6 compounds (A6, P6, N6) so as to optimize the concentration of precursors to benzene derivatives, such as cyclohexane and methyl-cyclopentane. .

Typical cutting points are of the order of 60-90 ° C (TBP distillation or "True Boiling Point"), but cutting points above 90 ° C, for example 100-110 ° C, can also be considered to increase the production of toluene. In all cases, the end point of this naphtha fraction used according to the invention is typically lower than that of a typical naphtha fraction supplying a catalytic reforming unit, generally 140-160 ° C, which leads to deactivation of the catalyst. lower by much less coke formation. This advantage can be taken advantage of for example by increasing the severity of the reaction and thus promote the production of benzene derivatives. A lower end point also implies less cracking from the heavy molecules, which avoids the hydrogen consumption associated with this cracking. The reforming unit conventionally comprises at least one reactor containing a suitable catalyst conventionally used in reforming (for example recommendations on the choice of catalysts suitable for this purpose are given explicitly on the website of the French company Axens, www.axens .net, or that of the American company UOP also mentioned above, www.UOP.com) and a fractionation unit used to separate the different products at the outlet of the reactor. This catalyst is very sensitive to the presence of sulfur and nitrogen products, so the reforming feedstock must have limited levels of sulfur, nitrogen and their derivatives, or not affecting the activity of the catalyst. The reaction is usually carried out under a pressure of 3 to 20 bar typically and at high temperature, typically of the order of 450 to 550 ° C, with production of hydrogen from the naphthenic molecules. Such operating conditions are known to those skilled in the art and for example detailed in the technical manuals published in the name of Jean-Pierre Wauquier (Oil Refining - Technip Editions) and Pierre Wuithier (Refining and Chemical Engineering - Technip Editions). In general, reference can be made to the contents of these manuals whenever for the implementation of the invention, it is necessary to determine operating conditions which are not specific to the invention and which are not specified in the following. The reformate thus obtained (step b)) is then sent to a separation unit of the aromatic compounds to obtain the benzene-enriched aromatic petroleum cut (step c)). For the purposes of the invention, "aromatic petroleum fraction enriched in benzene" means benzene with a purity greater than 95%, advantageously greater than 98%, in particular between 98 and 99.9% by weight, and, particularly preferably about 99.9% by weight.

The separation or extraction of aromatic compounds with charge the reformate obtained in step b), optionally mixed with other sources of charges, including pyrolysis gasolines rich in benzene precursors, provides a product concentrated in aromatics and a raffinate more or less rich in benzene precursors (essentially naphthenic molecules). Preferably, the starting materials rich in aromatics (content of aromatic compounds greater than about 20-30% aromatics) and low paraffin content will enter the scheme of the invention directly to the aromatic separation unit, which allows on the one hand to reduce the flow to the other units with obvious consequences in the size and in the energy consumption of these, but also to improve the production of aromatic compounds in the reformer because the reduction of the Aromatic content in the feed promotes its production by the effect of thermodynamic equilibria. The raffinate from the aromatics separation unit, rich in aromatic compound precursors and containing paraffins, is recycled to the catalytic reforming unit or, preferably according to the invention, to the paraffin separation unit, according to the paraffin content of this raffinate. Specific devices (washes, traps, hydrotreatments) must be put in place to remove traces of the solvent used in the separation of aromatics which may adversely affect the adsorbent, such as molecular sieves, of the paraffin separation unit or catalysts for catalytic reforming. The aromatics separation unit is based on liquid-liquid extraction or extractive distillation technologies widely known in the industry. The solvent used having a high affinity for the aromatic molecules, thus obtaining an aromatic petroleum fraction enriched in benzene, but which may also contain toluene and / or other aromatics already contained in the fresh feedstock or formed in the catalytic reformer. Depending on the compositions of the feeds and fractions entering and leaving the process and the compounds to be produced, fractionation columns are to be provided upstream or downstream of the aromatic compounds separation unit.

The separation of the aromatic compounds after the catalytic reforming (step b)) makes it possible to obtain a product which is concentrated in benzene derivatives and, advantageously, a raffinate rich in precursors of the benzene derivatives. The separation of paraffins according to step a) makes it possible to reduce the quantities to be recycled and to obtain a raffinate more concentrated in precursors of benzene derivatives, which, on the one hand, appreciably improves the productivity of these derivatives and, on the one hand, on the other hand, reduces energy consumption. According to an optimized embodiment, the process comprises a preferential additional step, step d), of recycling a raffinate from the benzene derivatives separation unit (steps c)), to the paraffin separation unit of step a). This step is carried out by means of specific devices, including in particular washes, traps, hydrotreatments, which must be very advantageously arranged to remove traces of the solvent used in the separation of benzene derivatives and which may affect the functionality of the the adsorbent (molecular sieves) of the paraffin separation unit or that of the catalysts of the catalytic reforming step.

The implementation of this step d) no longer necessarily requires a purge system on the raffinate from the separation step (step c), which prevents the loss of a part of the precursors of benzene derivatives, the flow of the concentrated paraffin fraction from step a) serving as a "selective" purge.

The amount of raffinate recycled to the reformer is much lower than that of the raffinate obtained by conventional methods, not implementing step a) upstream, since it is depleted in paraffins. This will have a significant impact on the size required for the different units but also on the energy balance of the scheme.

The method of the invention may furthermore comprise, to provide an integrated refinery petrochemical scheme, a step e) of recycling the paraffin fraction from step a) to a steam cracking unit where it is very advantageously used. advantageously for the production of olefins and at least one pyrolysis gasoline fraction, called "pygas" fraction, rich in benzene derivatives. In fact, thanks to the invention, the recycling of certain paraffins to an inlet steam cracker makes it possible to increase the production yield of benzene but also of olefins such as ethylene, propylene and butadiene which are valuable products. Moreover, the production of benzene derivatives of the overall scheme is thus increased. This steam reforming or steam-cracking unit can also receive other feeds from other naphtha cuts for the purpose of their treatment. In an advantageous variant, the process comprises, after step e), a step f) of sending the fraction of pyrolysis gasoline produced to the steam cracker to step c) of the process, in addition to the reformate resulting from the step b), advantageously concomitantly with the treatment of a naphtha cut starting with step a).

In this case, this implementation is advantageous because the pyrolysis gasoline fraction is depleted of paraffins and rich in precursors of benzene derivatives and aromatic compounds. According to preferred embodiments, when a first naphtha fraction, as a feedstock available for the invention, contains an amount of aromatic compounds greater than at least 30% by weight based on the total weight of the cup and a a reduced paraffin content, such as an ex-steam reformer naphtha fraction, relative to the naphtha fraction that is sent to the aromatic separation unit, said first naphtha fraction is introduced directly into stage c) of the process, concomitantly with the treatment of a naphtha cut starting with step a).

This latter embodiment offers the advantage, on the one hand, of reducing the flow rates of the feedstock to be treated to the other units (paraffin separation and / or benzene derivatives) with beneficial consequences in the dimensions of the units and in the energy consumption which will be reduced, and, secondly, to improve the production of benzene derivatives in the reforming unit, which is related to the fact that the reduction of the content of benzene derivatives in the feedstock promotes its production thanks to the thermodynamic equilibrium.

Alternatively, when a naphtha fraction which comprises a high content of benzene derivative precursors and a negligible paraffin content (n-paraffins and / or branched paraffins) can be available as feedstock, this is advantageously introduced directly in step b) of the process, also concomitantly with the treatment of a naphtha cut starting with step a). Such a section is, for example, the hydrocracking type naphtha cut. This variant has the same advantages as those described above. The invention also relates to a device or set of processes (or also called integrated scheme (or sequence of processes) refining petrochemical) for the implementation of the method of the invention. This device is arranged for the production of a benzene-enriched aromatic petroleum cut from a naphtha cut comprising paraffins, aromatic compounds and precursors of said aromatic compounds, comprising the following production and / or treatment units: a ) a paraffin separation unit arranged to separate the paraffins from the naphtha fraction, b) a catalytic reforming unit shaped to treat the paraffin-reduced fraction obtained in step a), to form a reformate rich in aromatic compounds and c) an aromatic compound separation unit shaped to separate the aromatic compounds from said reformate of step b) to obtain the benzene-enriched petroleum cut.

The device for producing benzene-enriched petroleum cutter comprising the linking of the three units according to the invention makes it possible to increase the production performances, in particular the increase of the yield of benzene derivatives and preferably of the benzene-enriched aromatic petroleum fraction, and also in H2; the absence of additional investment costs linked to the possibility of implementing existing units (no new technology required); - the improvement of energy efficiency as already described above, and - the flexibility of adapting to the capacities of existing units, by taking advantage of their adaptability to variations in the composition of naphtha cuts at entry or to an enlargement the origin of said cuts, with limited impact on procedures and severity. This sequence makes it possible to optimize the size of the units, the investment costs as well as the energy efficiency of the scheme. Advantageously, the device further comprises a steam reforming unit coupled at the outlet of the paraffin separation unit and whose "pygas" cut produced is treated in the aromatic compound separation unit and then in the paraffin separation unit. , to finally be sent as additional feed to the catalytic reforming unit. Preferably, the device further comprises a hydrodealkylation unit coupled at the outlet of the aromatic compound separation unit and designed to convert toluene to benzene.

This arrangement allows the recovery and treatment of the fraction of the paraffins separated in step (a) in the steam reforming (or steam-cracking) unit, where it is very advantageously used for the production of a gasoline cup. pyrolysis, "pygas" fraction, rich in benzene derivatives, and recoverable olefins with a better yield.

The production of benzene derivatives, and especially aromatic compounds, and advantageously of the benzene-enriched aromatic petroleum fraction, of the overall scheme is thus increased.

The invention is now described with reference to the accompanying non-limiting drawings, in which: FIG. 1 is a simplified schematic view of a device for producing a high purity benzene fraction, and a related method, according to a preferred embodiment, that is to say that this figure represents the overall integrated scheme according to the invention, - FIG. 2 is a schematic view with material, hydrogen and energy balances of a device according to the state of the art of the reformer type centered on a charge C6, coupled to a steam cracker and an extraction of aromatics treating the essence of pyrolysis and the reformat; FIG. 3 is a schematic view according to a variant with material, hydrogen and energy balances of a reformer type device centered on a charge C6, coupled to a steam cracker and an extraction of aromatics treating the reformate and the pyrolysis gasoline, with recycling of the raffinate ex-extraction of aromatics to the reformer; FIG. 4 is a schematic view with material, hydrogen and energy balances of a device according to the invention of reformer type centered on a charge C6 previously separated from the paraffins it contains, coupled with a steam cracker and an extraction of aromatics treating the reformat and pyrolysis gasoline, with recycling of the raffinate ex-extraction of aromatics upstream of paraffin separation; and - FIG. 5 is a schematic view with material, hydrogen and energy balances of a device according to the invention of reformer type centered on a charge C6 previously separated from the paraffins it contains, coupled with a steam cracker and an extraction of aromatics treating the reformate and the pyrolysis gasoline, with recycling of the ex-aromatics raffinate prior to paraffin separation, to which a toluene hydrodealkylation unit has been added, this FIG. 5 corresponding to the simplified diagram of FIG.

Fig.1 shows a diagram of a preferred embodiment of the invention. A starting naphtha feed 2 corresponding to a C6 cut of a typical straight-run naphtha, said SR naphtha, comprising benzene (A6), cyclohexane (N6) as a benzene and naphthene precursor, is fed. C6 paraffins (P6) to the paraffin separation unit (a) 10. Separated paraffins (n-paraffins, certain branched paraffins) 11 are sent to the steam reforming or steam-cracking unit (d) 40, generally a petrochemical unit, producing olefins such as ethylene, propylene and the like, and a section of pyrolysis gasoline 41, or "pygas" fraction rich in aromatics and naphthenes, which is then sent to the unit of separation of aromatic compounds (c) to extract benzene and toluene. When a naphtha section is available which comprises a high content of aromatic compound precursors, such as cyclohexane, and a low content of n-paraffins and / or branched paraffins, this is advantageously introduced directly into the naphtha section. step b) of the process. This case is not shown in Figure 1 but is illustrated in Figure 3. In other words, on this occasion, the step (a) of paraffins separation is omitted. The paraffin-depleted fraction C6 12 ex-separation unit (a) is then sent to the reforming unit (b) 20. This stage allows the production of a reformate 23, hydrogen 21 and cracking gas. 22.

The reformate 23 is then subjected in the aromatic separation unit (c) 30 to extraction so that a benzene-enriched petroleum fraction 31 is recovered. The unit (c) 30 comprises, depending on the case, fillers or fractions entering and leaving this unit 30, upstream or downstream the appropriate fractionation units. These are shown in Figure 4. This step c) also allows the production of a raffinate 32 rich in precursors of aromatic compounds and including, where appropriate, varying amounts of paraffins. This raffinate 32 is recycled to the paraffin separation unit (a) 10. In the diagram shown, at the outlet of the aromatic compound separation unit 30, is placed a purge system 33 to evacuate, if necessary. part of the raffinate 32. In step c), at the outlet of the unit (c) 30, toluene 35 is also produced to a lesser extent than can be used for the purpose of the invention. to produce benzene, maximize and direct to a hydrodealkylation unit called HDA, 50, design and operation known to those skilled in the art. At the outlet of the unit 50, almost pure benzene is recovered and also a little gasoline which is sent to the gasoline "pool" 60. The steam reforming unit (d) 40 also receives other naphtha 43 charges. treatment in this unit, primarily. The other products 42 from this stage, of predominantly olefinic type, are then recovered for their recovery separately. When the naphtha section 3 already contains a large amount of starting aromatic compounds (approximately at least 30% by weight), this is sent directly to the aromatic compounds separation unit (c) at the same time. The pygas section 41. Example 1 is ciphered with reference to FIGS. 1 and 4: The naphtha section 2 of the example is sent to the paraffin separation unit (a) 10, at a rate of 60 Th -1. The naphtha section contains 2% by weight of benzene, 8.6% by weight of C6 naphthenic precursor and 27.5% by weight of C6 paraffins. The paraffin-depleted fraction 12 no longer contains C6 paraffins, or, at the very least, small amounts. The paraffin-depleted fraction 12 is then sent to the reforming unit (b) 20. This step allows the production of the reformate 23 comprising 4.3% by weight of C 6 naphthenes and 21.2% by weight of benzene. In addition, 1.2 Th-1 of hydrogen 21 is recovered. The reformate 23 then undergoes, in the benzene separation unit (c) 30, an extraction so that a benzene cut 31 is recovered, for a flow rate of 24.8 Th-1. Raffinate 32 comprises 20.5% by weight of C6 naphthenes. This raffinate 32 is recycled to the paraffin separation unit (a). By comparison, such a naphtha cut is processed without the upstream paraffin separation step. The reformate 23 then contains 1.9% by weight of C6 naphthenes, 13.4% by weight of benzene and 24.5% by weight of C6 paraffins. In addition, 1.5 T.h -1 of hydrogen 21 and 26.5 T.11-1 of cracked gas are recovered.

The reformate 23 is then subjected to extraction in the benzene separation unit (c) 30 so that a benzene section 31 is recovered at a rate of 24.2 T.h -1. The raffinate 32 comprises 8.2% by weight of C6 naphthenes and 38.9% by weight of C6 paraffins, for a flow rate of 53.6 T.h -1. Part of the raffinate 32 can be purged by a purge system 33 to limit the amount recycled to the paraffin separation unit (a) 10. Example 2, which is encoded with reference to FIG. 5: The interest of the three diagrams represented in FIGS. FIGS. 2 to 4, and in particular FIG. 5, which illustrates an alternative embodiment of FIG. 4 (addition of the unit of HDA 50 according to FIG. 1), is to illustrate the variations of the material balances (or materials in each scheme) for the benzene and hydrogen produced, as well as the associated energy balance changes. The diagram of FIG. 2 represents a state of the art, that of FIG. 3 an intermediate diagram between the state of the art and the invention (excluding the invention), and that of FIG. 4, the preferred embodiment of FIG. the invention. These patterns are self-sufficient because they are directly readable and understandable to those skilled in the art. They will not be described in detail. In general, certain advantages of the invention can be tempered by the relative sizes of the units, for example the steam cracker is a predominant unit in terms of energy consumption and the Applicant has not sought to optimize the size of this unit. . The comparison of the integrated global device production scheme, according to the invention, as illustrated in FIG. 5, with that of the prior art such as that illustrated in FIG. 2, mainly shows: a gain of 62% in benzene recovery; a reduction of 8% of the energy consumption, indicated by "duty" in the diagram of FIG. 5. In more detail, 30 base cases of FIG. 2: state of the art In a site comprising a reformer and a steam cracker and an extraction of aromatics, there are two sources of benzene: the ex-steam-cracker pyrolysis gasoline (pygas) and the reformed naphtha. In order to maximize the production of benzene, a C6-centered reformer charge is produced. Depending on the nature of the crude treated in the distillation, the feed contains more or less paraffins. This cut is processed by the reformer. The reformate thus produced is sent to the extraction of aromatics, like the pyrolytic essence of the steam cracker. This extraction includes a first fractionation step to remove the C5- and a second to produce a C6 cut and a C7 + cut. The toluene content in the C6 cut is adjusted in this second step to satisfy the toluene market of the complex. Cut C5- is sent as a charge to the steam cracker. From the C6 cut are separated aromatics and non-aromatics (raffinate). The aromatics are distilled to produce the benzene and toluene specifications. The C7 + cut, which contains a certain amount of toluene, is sent to the gasoline pool. Case with recycle of the raffinate of FIG. 3: If one looks at the raffinate resulting from the extraction of aromatics, one finds that it contains a not insignificant quantity of naphthenes in C6 (cyclohexane and methylcyclopentane) which are sent to the steam cracker . The interest is to recover this potential by sending this cup to the reformer. The same production of toluene is maintained to meet the needs of the market in this component. By the effect of recycling, the amount that is recycled increases with the situation of the base case. This generates an increase in the amounts of energy to be used, on the one hand for the reformer, but also on fractionation and extraction. At the steam cracker, the charge is decreased which leads to lower energy consumption.

The increase of the reformer treatment increases the production of hydrogen and benzene by 34% and 9% respectively, but by devoting much more energy and increasing the C7 + cut to the gasoline pool, but also the cracking paraffins in the reformer, producing lower value C1-C4. Case with recycle of the raffinate and separation of the paraffins of FIG. 4: By adding a paraffin separation unit upstream of the reformer, a "Paraffin" cut is produced which will be treated with the steam cracker and a "Naphthalene and Aromatic" cut directed towards the reformer. The very branched paraffins remain in the cup "Naphtènes et Aromatiques". Thus, the paraffins are cracked by the reformer (which consumes H2), while recovering the naphthenic potential of the raffinate from the extraction of aromatics. Compared to the previous case of Figure 3, we further increase the production of benzene, while consuming much less energy. The space released to the reformer can make it possible to treat a naphthene-rich and paraffin-poor filler. The recycle to the steam cracker (C5- and Paraffins) increases the production of olefins, which had decreased in the previous case. Case with recycle of raffinate, separation of naphthenes and aromatics and hydro-dealkylation (HDA) of toluene of FIG. 5 which represents the very preferred embodiment of the invention: In order to maximize the production of benzene, it is possible to take an interest in to the potential of toluene. During the C6 cut / C7 cut separation, the amount of toluene which will be treated to extract aromatics is controlled. If the cutting point is adjusted to recover all of the toluene, a portion of the nonaromatic C8 is sent to the extraction. Care must be taken not to allow the xylenes which are likely to cause problems during the benzene / toluene separation to reach the toluene purity specifications, since they will not be separated.

The raffinate needs to be fractionated to remove the C8 + fraction that will crack at the reformer. The C8 + fraction will be sent to the gasoline pool. By treating pure toluene in hydro-dealkylation, benzene is produced by limiting the cracking of impurities. HDA treatment of toluene, however, consumes H2. In the end, we increase much the production of benzene, while lowering the energy expenditure and the amount of molecule for the gasoline pool. The process according to the invention avoids recycling and the systematic presence of paraffins during the treatment, which makes it possible to optimize the overall yield of the integrated scheme. The raffinate is fully recycled to the paraffin separation unit. A major advantage of the device scheme proposed by the Applicant is the flexibility conferred for the injection of the charges which will be possible at different entry points of the scheme chosen according to the composition of said charges. In summary, the invention provides a method and a device for optimizing the production of benzene in a refinery which comprises means for: - separating the paraffins in a paraffin separation unit, - sending the cut resulting from this separation which is rich in aromatic and naphthenes in a catalytic reforming unit, to send the aromatic section from this catalytic reforming unit into an aromatic separation unit, this device being designed so that the n-paraffins and branched paraffins from the unit of separation of the paraffins are recycled as a feedstock of a steam cracker whose pyrolysis gasoline product cut is sent to the aromatic separation unit and that the naphthene stream from the aromatic separation unit, which has not reacted in the catalytic reforming unit is recycled to the additional charge of the paraffin separation unit.

Claims (13)

REVENDICATIONS1. A process for producing a benzene-enriched aromatic petroleum cut (31) from at least one naphtha cut (2) comprising paraffins, aromatic compounds and precursors of said aromatic compounds, comprising the steps of: a) separating the paraffins from the paraffin-rich naphtha section (2) in a paraffin separation unit (10), b) catalytically reforming the paraffin-reduced fraction obtained in step a) in a catalytic reforming unit (20), to form a reformate (23) rich in aromatic compounds, and c) separating the aromatic compounds from said reformate of step b) in a separation unit of the aromatic compounds (30), to obtain the aromatic petroleum fraction enriched in benzene (31). The process according to claim 1, wherein the benzene-enriched aromatic petroleum fraction (31) contains benzene, toluene or a mixture of these two components. 3. Process according to claim 1 or 2, in which the naphtha fraction (2) is a petroleum fraction produced by direct distillation, by conversion, a pyrolytic naphtha resulting from a steam-cracking, or the effluent from the bottom of a die. an isohexanizer of an isomerization unit. 4. Method according to one of claims 1 to 3, wherein the paraffin separation step uses adsorbents molecular sieve type. 5. Method according to one of claims 1 to 4, characterized in that it comprises a d) additional step of recycling a raffinate (32) from the aromatic separation unit (30) (step c) ), to the inlet of the paraffin separation unit (10) of step a). 6. Method according to one of claims 1 to 5, characterized in that it further comprises a step e) recycling the paraffinsissue fraction of step a) to a steam cracking unit (40) for production at least one cut of pyrolysis gasoline (41). 7. Process according to claim 6, characterized in that additional olefins are produced at the outlet of the steam cracker (40). 8. A method according to claim 6 or 7, characterized in that it comprises after step e), a step f) of sending the pyrolysis gasoline cut (41) to step c) of the method , in addition to the reformate (23) from the catalytic reforming unit (20). 9. Method according to one of claims 1 to 8, wherein, when a first naphtha cut (3) contains an amount of aromatics greater than at least 30% by weight relative to the total weight of the cut and a content of paraffins reduced with respect to the naphtha fraction (2) which is sent to the paraffin separation unit (10), said first naphtha fraction (3) is introduced directly into stage c) of the process, concomitantly with the treatment a naphtha cut (2) beginning with step a). 10. A device for the production of a benzene-enriched aromatic petroleum cut (31) from at least one naphtha cut comprising paraffins, aromatic compounds and precursors of said aromatic compounds, comprising the production units and / or processors: a) a paraffin separation unit (10) arranged to separate the paraffins from the naphtha section (2), b) a catalytic reforming unit (20) shaped to process the paraffin-reduced fraction obtained at step a), to form a reformate (23) rich in aromatic compounds, and c) a separation unit of the aromatic compounds (30) shaped to separate the aromatic compounds from said reformate (23) of step b), for obtaining the benzene-enriched oil cut (31). 11. Device according to claim 10, further comprising a steam cracking unit (40) coupled at the outlet of the paraffin separation unit. 12. Device according to one of claims 10 and 11, further comprising a hydrodealkylation unit (50) coupled at the outlet of the aromatic compounds separation unit (40) and designed to convert toluene (35) to benzene . 13. Device for carrying out the process according to claim 8, characterized in that it comprises means for: separating the paraffins in a separation unit of the paraffins (10), sending the cup resulting from this separation which is rich in aromatics and naphthenes in a catalytic reforming unit (20), sending the aromatic section from this catalytic reforming unit (20) to an aromatics separation unit (30), which device is designed for the N and paraffins branched from the paraffin separation unit (10) are recycled as a feedstock of a steam cracker (40) whose pyrolysis gasoline cut produced is sent to the aromatics separation unit (30). and that the naphthene stream from the unreacted aromatics separation unit (30) in the catalytic reforming unit (20) is fed back to the p separation unit araffines (10) .20