Improved process for producing middle distillates by oligomerisation of an olefinic feedstock

Abstract

The present invention relates to a process for preparing middle distillates from an olefinic feedstock, comprising: a) a step of heterogeneous oligomerisation fed with the olefinic feedstock, a first recycled substance, a second recycled substance and a third recycled substance, and carried out in the presence of at least one heterogeneous oligomerisation catalyst, to produce a reaction effluent comprising dimers, trimers and oligomers; b) a step of fractionating the reaction effluent into: - a first light fraction comprising at least some of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic feedstock not converted in step a); - a second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction); - an intermediate fraction comprising at least some of the olefins containing between 7 and 9 carbon atoms (C7-C9 fraction); and - a heavy fraction comprising the oligomers; c) a recycling step, comprising preparing a first recycled substance comprising at least some of the light fraction, preparing a second recycled substance, comprising at least some of the second light fraction; preparing a third recycled substance comprising at least some of the intermediate fraction; and transferring the first, second and third recycled substances to step a); d) a step of hydrogenating at least some of the heavy fraction.

Description

[0001] IMPROVED PROCESS FOR PRODUCING MIDDLE DISTILLERIES BY OLIGOMERIZATION OF AN OLEFIN CHARGE

[0002] technical field

[0003] The present invention relates to a process for producing middle distillates, in particular kerosene and / or diesel, meeting the specifications in force, in particular those defined in ASTM D1655 or ASTM D7566 for kerosene and those defined in ASTM D975 or European standard 15940 for diesel, by heterogeneous oligomerization of an olefinic feedstock, in particular a bio-based olefinic feedstock.

[0004] Previous technique

[0005] Airlines have committed to carbon-neutral growth, particularly in commercial aviation, starting in 2021, and US airlines have set a target of reducing CO2 emissions by 50% by 2050 compared to 2005 levels. However, improvements in aircraft and engine efficiency are not proving sufficient to achieve carbon neutrality. Sustainable aviation fuels (SAFs) are therefore critical to reaching this goal.

[0006] It therefore seems necessary to develop methods for manufacturing synthetic kerosene, preferably from bio-sourced feedstocks.

[0007] French patent FR 2926812 thus discloses a process for the oligomerization of olefins, enabling the production of fuel, for example the production of gasoline and / or kerosene from light olefinic feedstocks containing between 2 and 8 carbon atoms (C2-C8), and in particular from light olefinic feedstocks containing high proportions of propylene and / or butenes and / or pentenes and using an oligomerization catalyst based preferably solely on silica-alumina with reduced macropore content.

[0008] Patent EP 1 396 532 describes a process for the valorization of a liquid hydrocarbon feed, comprising: a) the separation from said hydrocarbon feed of a fraction (01) comprising essentially compounds containing 5 carbon atoms (C5) of which at least 2% by weight of pentenes; b) contacting said fraction (01) with a hydrocarbon cut (02) comprising hydrocarbons having a number of carbon atoms between 6 and 10 (C6-C10), of which at least 2% by weight of olefins, in the presence of an acid catalyst promoting the dimerization and alkylation reactions of the species; (c) a separation of the effluent obtained into at least two cuts, including a gasoline cut (a) whose upper distillation point is below 100°C and which includes the majority of the unreacted reagents, and a kerosene cut (P) with a distillation range between 100°C and 300°C.

[0009] Patent EP 1 602 637 describes a process which allows the production of gasoline and diesel to be modulated in a simple and economical way, by transforming the initial charge of hydrocarbons comprising 4 to 15 carbon atoms (C4-C15) into a gasoline fraction with an improved octane rating compared to that of the charge and a diesel fraction with a high cetane rating.

[0010] French patent EP 1 739 069 describes a process for preparing a diesel fraction from a 2-12 carbon (C2-C12) olefinic feedstock, comprising two oligomerization steps with an intervening separation step. The intermediate separation step yields a light C4-C5 olefinic hydrocarbon fraction, an intermediate fraction with a T95 between 180°C and 240°C, and a heavy fraction with a T95 above 240°C. The intermediate fraction is then blended with at least a fraction of the light fraction in a mass ratio (intermediate fraction / light fraction) between 60 / 40 and 80 / 20 and undergoes a second oligomerization.

[0011] Patent EP 2 385 092 describes a process for producing middle distillate hydrocarbon bases from ethanol, more particularly bioethanol.

[0012] EP 2 707462 patent discloses a process for oligomerizing olefins comprising 4 to 6 carbon atoms (C4-C6) into a middle distillate cut having predominantly 10 to 20 carbon atoms (C10-C20). In the process described in EP 2 707462 patent, the starting olefin feed must contain a minimum of branched olefins (or iso-olefins), preferably at least 10% by weight and preferably 20% by weight of iso-olefins relative to the total olefins in the feed.

[0013] French patent FR 2 959 750 describes a process for producing middle distillate hydrocarbon bases, preferably kerosene hydrocarbon base, from an ethanol feedstock derived from biomass, said process comprising the dehydration of ethanol into a predominantly ethylenic effluent, two successive oligomerization steps to obtain a middle distillate effluent.

[0014] French patent FR 3 053 355 describes a process for the oligomerization of light olefinic fillers containing between 2 and 10 carbon atoms (C2-C10) per molecule, using a catalytic system comprising a silica-alumina-based catalyst and a zeolite-based catalyst having pore openings of 10 or 12 oxygen atoms, and carried out at a temperature between 130 and 350°C, at a pressure between 0.1 and 10 MPa and at a WH (volumetric hourly rate) between 0.1 and 5 h' 1 The process of EP 3 053 355 makes it possible to improve the yield in middle distillates and in particular the yield in diesel, compared to an oligomerization process using only one of the catalysts of the catalytic system used, at iso-volume of catalyst.

[0015] US patent 6,372,949 describes the transformation of oxygenated gasoline and distillates (C4-C12 cut) in a single dehydration-oligomerization step using a composite catalyst comprising a one-dimensional 10 MR zeolite selected from the group formed by ZSM 22, ZSM 23, ZSM 35, ZSM 48, ZSM 57 and ferrierite and mixtures thereof, with a multi-dimensional zeolite having an average pore size, such as ZSM-5 zeolite.

[0016] Application WO 2023 / 194337 describes a process for producing middle distillates, in particular kerosene and / or diesel fuel, from light C3 to C6 olefins, preferably C3 to C4, including those derived from bio-based feedstocks, in a highly specific manner and with improved yields of middle distillates, particularly kerosene and / or diesel fuel, by fractionating the oligomerization effluent into a light fraction and an intermediate fraction, and then recycling said light and intermediate fractions upstream of the oligomerization. Such a process improves yields of middle distillates while maintaining sufficiently high feedstock conversion levels.

[0017] However, although technically very efficient, these processes can be improved in terms of energy consumption. Indeed, the effluent fractionation stages are energy-intensive, as they require, in particular, the use of reboilers to heat the effluent and successively vaporize the different fractions that compose it, thus enabling their separation and subsequent recycling.

[0018] We are therefore still looking for less energy-intensive technical solutions to produce middle distillates, in particular kerosene and / or diesel, from light C3 to C6 olefins, especially from bio-based feedstocks, and very specifically with satisfactory yields in middle distillates, especially kerosene and / or diesel, meeting the specifications in force, in particular the specifications of the ASTM D7566 standard and / or the European standard 15940.

[0019] Summary of the invention

[0020] Thus, the present invention relates to a process for preparing middle distillates from an olefinic feedstock, comprising olefins containing between 3 and 6 carbon atoms, comprising: a) a heterogeneous oligomerization step fed with at least the olefinic feedstock, a first recycle, a second recycle and a third recycle, and carried out in the presence of at least one heterogeneous oligomerization catalyst, at a temperature between 20 and 500°C, a pressure between 1.0 and 10 MPa and a WH between 0.1 and 0.5 h' 1 , preferably between 0.2 and 0.3 h' 1 , to produce a reaction effluent comprising dimers, trimers and oligomers; b) a step of fractionating the reaction effluent obtained at the end of step a), into at least:

[0021] - a first light fraction comprising at least some of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge, not converted in step a);

[0022] - a second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction);

[0023] - an intermediate fraction comprising at least some olefins containing between 7 and 9 carbon atoms (C7-C9 fraction); and

[0024] - a heavy fraction comprising the oligomers; c) a recycle step, comprising the preparation of a first recycle comprising at least a part of the light fraction; the preparation of a second recycle, comprising at least a part of the second light fraction; the preparation of a third recycle comprising at least a part of the intermediate fraction; and the transfer of the first, second and third recycles to the oligomerization step a); d) a hydrogenation step of at least a part of the heavy fraction separated in step b) in the presence of hydrogen, to obtain a hydrogenated heavy fraction comprising middle distillates.

[0025] The advantage of the process according to the invention, which proposes an additional separation of the fraction resulting from the oligomerization step, is that it significantly reduces the energy consumption required for the subsequent fractionation of the intermediate and heavy fractions. Indeed, the applicant has shown that separating the C5-C6 fraction from the C3-C4 fraction does not increase energy consumption at this stage. By separately extracting the C5-C6 fraction, it is possible to limit the energy required to separate the intermediate fraction, the volume of which is reduced once it is devoid of the C5-C6 fraction. This results in a process that is just as efficient (in terms of yields of average distillates, particularly kerosene and diesel), but with reduced energy consumption compared to prior processes, thereby limiting the environmental impact of said process.

[0026] Another advantage of the process according to the invention lies in the ability to better control the recycled content of the C3-C4 fractions (first light fraction), C5-C6 fractions (second light fraction), and the intermediate fraction. In the process described in WO 2023 / 194337, where C5-C6 and C8 are recycled simultaneously into a single intermediate fraction, the conversion of C8 is lower. In the present invention, the separation of the different C3-C4 and C5-C6 fractions, as well as the intermediate fraction, allows for control of the proportion of these different fractions during oligomerization, thereby improving the overall feed conversion.

[0027] List of figures

[0028] Figure 1 schematically represents an implementation of a process for preparing middle distillates from an olefinic feedstock, according to the prior art.

[0029] A feedstock 1 rich in C4 and C6 olefins is treated in an oligomerization section (a). The reaction effluent 5 is sent to a separation stage (b) and separated in a two-column system to produce:

[0030] - a C4-rich flux 4 including feed olefins that have not been converted as well as possible feed constituents that do not react (e.g. paraffins) in this boiling range;

[0031] - a stream 3 which includes dimers and trimers, produced during oligomerization but which are too light to be valued in middle distillates;

[0032] - a flow 9 recovered at the bottom of the last column and corresponding to the heavy fraction.

[0033] Stream 4 is at least partly recycled as an input to oligomerization (a). Part of stream 4 can be purged or recovered in another unit (stream 6), either continuously or from time to time, depending on the nature of the feed.

[0034] Stream 3 is sent to the oligomerization stage, at least partially, preferably entirely, as an input to oligomerization (a). Part of stream 3 can also be sent for valorization in a gasoline pool (stream 8). Stream 8 can optionally be sent to hydrogenation (c) and then valorized with stream 11.

[0035] Stream 9 is sent to a hydrogenation section (c). The hydrogenated effluent 10 is then separated in a section (d) into:

[0036] - a flow 11 which is sent to a fuel pool;

[0037] - a stream 13 which is converted into kerosene;

[0038] - a stream 14 comprising the heaviest compounds produced during the process and which can be recovered as diesel fuel. An optional stream 2 comprising a fraction of a kerosene and diesel fuel mixture (stream 12) is sent to the oligomerization step a). It corresponds to an inert stream used to control the exothermicity of the reaction in the reactors of the oligomerization section a).

[0039] Figure 2 schematically represents an implementation of the process for preparing middle distillates from an olefinic feedstock according to the invention.

[0040] A feedstock 1 rich in C4 and C6 olefins is treated in an oligomerization section (a). The reaction effluent 5 is sent to a separation stage (b) and separated in a series of columns to produce:

[0041] - a top flow from the first column rich in C4- compounds including feed olefins that have not been converted as well as possible feed constituents that do not react (e.g. paraffins) in this boiling range;

[0042] - an intermediate drawdown stream from the first column rich in C5 to C6 compounds including some of the C5-C6 olefins from the olefinic feed which have not been converted as well as any constituents of the feed which do not react (e.g. paraffins) in this boiling range;

[0043] - a top stream from the second column which includes at least some of the olefins containing between 7 and 9 carbon atoms (C7-C9), produced during oligomerization a), which have not been separated in the second light fraction and which are too light to be valued in middle distillates;

[0044] - a flow 9 recovered at the bottom of the last column and corresponding to the heavy fraction.

[0045] The overhead and intermediate drawdown streams from the first column are at least partially recycled to the oligomerization feed (a) (streams 4 and 4', respectively). A portion of the overhead and intermediate drawdown streams from the first column can be purged or utilized in another unit (streams 6 and 6'), either continuously or intermittently, depending on the nature of the feedstock. Stream 6' can optionally be sent to hydrogenation (c) and subsequently utilized with stream 11.

[0046] At least a fraction of the overhead stream from the second fractionation column is sent to the oligomerization step, at least partially, preferably entirely, as the oligomerization input (a) (stream 3). A portion of the overhead stream from the second fractionation column may also be sent for valorization in a gasoline pool (stream 8). Stream 8 may optionally be sent to hydrogenation (c) and then valorized with stream 11.

[0047] The stream 9 is sent to a hydrogenation section (c). The hydrogenated effluent 10 is then separated in a section (d), into: - a stream 11 which is sent to a fuel pool;

[0048] - a stream 13 which is converted into kerosene;

[0049] - a stream 14 consisting of the heaviest compounds produced during the process and which can be recovered as diesel fuel.

[0050] An optional stream 2 comprising a fraction of a kerosene and diesel mixture (stream 12) is sent to the oligomerization step a). It corresponds to an inert stream used to control the exothermicity of the reaction in the reactors of the oligomerization section a).

[0051] Description of the implementation methods

[0052] According to the present invention, the expressions "between ... and ..." and "between ... and ..." are equivalent and mean that the limit values ​​of the interval are included within the described range of values. If this is not the case and the limit values ​​are not included within the described range, such clarification will be provided by the present invention.

[0053] In the sense of the present invention, the different parameter ranges for a given step, such as pressure ranges and temperature ranges, can be used alone or in combination. For example, in the sense of the present invention, a preferred pressure range can be combined with a more preferred temperature range.

[0054] In the following, specific embodiments of the invention may be described. They may be implemented separately or in combination with each other, without limitation as to the number of combinations where technically feasible.

[0055] The terms "upstream" and "downstream" are to be understood in relation to the general flow of the flow(s) in question in the process.

[0056] The term "bio-based" means that the material / product / compound it describes is an organic material / product / compound whose carbon comes from CO2 present in the atmosphere, recently fixed (on a human timescale) through solar energy (photosynthesis). On land, this CO2 is captured or fixed by plant life (for example, agricultural crops or forest materials). In the oceans, CO2 is captured or fixed by photosynthetic bacteria or phytoplankton. For example, a bio-based material has an isotopic ratio 14 C / 12 C greater than 0. Conversely, a material of fossil origin has an isotopic ratio 14 C / 12C of approximately 0. The terms "renewable" or "derived from renewables" can also be used. To determine whether a material / product / compound is bio-based or derived from renewables, its modern carbon content (or percent modern carbon, pMC) is measured according to ASTM D 6866-24 ("Determination of Bio-based Content of Natural Range Materials by Radiocarbon and Isotope Ratio Mass Spectrometry Analysis"). The method in this standard measures the isotope ratio. 14 C / 12 C in a sample and compares it to the isotopic ratio 14 C / 12The bio-based content of a standard bio-based reference material is calculated to determine the percentage of bio-based content in the sample. This reference material provides a radiocarbon content approximately equivalent to the atmospheric radiocarbon fraction in 1950. The calculation of bio-based carbon content based on the bio-based reference material (pMC) is given in ASTM 6866-24 ("Determination of Bio-based Content of Solid, Liquid, and Gaseous Samples by Radiocarbon Analysis"). Thus, the bio-based carbon content of the standard bio-based reference material is 100%. The bio-based carbon content of a bio-based material is strictly greater than 0%, for example, greater than or equal to 1%. The bio-based carbon content of a fossil-based material is approximately 0%. Therefore, a current bio-based material could potentially have a bio-based carbon content of 100%.

[0057] In this description, the terms "T95" or "T95 temperature" are interchangeable and refer to the temperature at which 95% by weight of the product in question has evaporated. It is determined according to the ASTM D86 standard method. Similarly, "T5" or "T5 temperature" is the temperature at which 5% by weight of the product in question has evaporated, determined according to the same ASTM D86 standard method.

[0058] In this description, "Cx" refers to compounds containing x carbon atoms. For example, a C3 chemical compound contains 3 carbon atoms. "Cx+" refers to compounds with at least x carbon atoms. For example, C9+ compounds are compounds containing at least 9 carbon atoms (that is, 9 or more carbon atoms). "Cx-" refers to compounds with at most x carbon atoms.

[0059] In the description, a "C5-140°C" cut designates compounds containing at least 5 carbon atoms and a boiling point below 140°C.

[0060] By analogy, a "C6-140°C" cup designates compounds containing at least 6 carbon atoms and a boiling point below 140°C, a "C6-165°C" cup designates compounds containing at least 6 carbon atoms and a boiling point below 165°C, and a "C6-170°C" cup designates compounds containing at least 6 carbon atoms and a boiling point below 170°C.

[0061] Throughout this text, chemical element groups are described according to the new IUPAC classification. For example, groups 9 and 10 correspond to the metals in columns 9 and 10 of the IUPAC classification, respectively, or to the last two columns of group VIIIB of the CAS classification (CRC Handbook of Chemistry and Physics, CRC editor press, editor-in-chief DR Lide, 81st edition, 2000–2001). Similarly, group 6 corresponds to the metals in column 6 of the IIIPAC classification or to the metals in column VIB of the CAS classification.

[0062] According to the present invention, the terms "olefin" and "mono-olefin" are used interchangeably and refer to hydrocarbons comprising a double bond (i.e., a single double bond). Preferably, the olefins in the olefinic feedstock of the process comprise between 3 and 6 carbon atoms (C3-C6). The olefins obtained after oligomerization (step a) preferably comprise between 8 and 30 carbon atoms (C8-C30), preferably between 9 and 25 carbon atoms (C9-C25), in particular between 9 and 16 carbon atoms (C9-C16) or between 10 and 25 carbon atoms (C10-C25).

[0063] According to the present invention, the term "oligomerization" refers to any reaction involving the addition of one olefin to another olefin, resulting in compounds, particularly hydrocarbon compounds, especially olefins, typically containing between 8 and 30 carbon atoms, preferably between 9 and 25 carbon atoms (C9-C25), particularly between 9 and 16 carbon atoms (C9-C16) or between 10 and 25 carbon atoms (C10-C25). Oligomerization differs from polymerization by the addition of a limited number of molecules.Thus, the products obtained are dimers or trimers of the olefins in the olefinic charge, that is, olefinic compounds resulting from the condensation of two and three olefin molecules in the olefinic charge, respectively, or oligomers, which correspond to olefinic compounds resulting from the condensation of several olefin molecules in the olefinic charge (several meaning here more than 3 but preferably less than 10, preferably less than or equal to 5, preferably less than or equal to 4). Typically, from C4 olefins, oligomers are obtained with a number of carbon atoms preferably less than or equal to 30, and preferably between 9 and 25, in particular between 9 and 16 or between 10 and 25.The number of molecules adding up during the oligomerization reaction is in the context of the invention between 2 and 10 inclusive, preferably between 3 and 6, and even more preferably between 3 and 6. The oligomers may however include traces of olefins having been oligomerized with a number of molecules greater than 10; most often, these traces represent less than 5% by weight of the oligomers formed.

[0064] The term "heterogeneous catalysis" defines, in this description, a reaction, in particular oligomerization reactions, where at least two phases coexist. In particular, the oligomerization step of the process according to the invention implements the oligomerization of the olefinic feedstock by heterogeneous catalysis, that is to say, in the presence of a catalyst in solid form, the feedstock and advantageously the products obtained being preferably in the liquid phase.

[0065] According to the present invention, the term "overall feed conversion" is defined as the weight quantity of olefins in the feed that has been converted relative to the overall weight quantity of olefins in the feed.

[0066] According to the present invention, the yield of the process is defined as the ratio between the weight quantity of middle distillates produced and the weight quantity of the olefinic feed entering the process.

[0067] According to the present invention, the selectivity towards kerosene is defined as the ratio between the weight quantity of kerosene produced and the weight quantity of all the products from the process such as diesel and gasoline.

[0068] According to the present invention, the selectivity towards diesel is defined as the ratio between the weight quantity of diesel produced and the weight quantity of all the products from the process such as kerosene and gasoline.

[0069] More particularly, the present invention relates to a process for preparing middle distillates, preferably of a kerosene cut and / or a diesel cut, from an olefinic feed in C3 to C6, preferably in C3 to C4, and in particular in C3 or C3 and C4, or mixtures thereof, comprising, preferably consisting of: a') optionally a pretreatment step of the olefinic feed employing preferably at least one adsorption section, one water washing section, one hydrotreating section and / or one selective hydrogenation section; a”) optionally, a separation step of the olefinic feed to at least partially separate the C5 and C6 compounds present in said olefinic feed;a) an oligomerization step fed at least by the olefinic feedstock, possibly pretreated and / or separated, a first recycle, a second recycle and a third recycle, and operated, preferably in liquid phase, in the presence of at least one heterogeneous oligomerization catalyst, at a temperature preferably between 20 and 500°C, at a pressure preferably between 1.0 and 10 MPa, and a WH preferably between 0.1 and 0.5 h; -1 , to produce a reaction effluent comprising dimers, trimers and oligomers; b) a step of fractionating the reaction effluent obtained at the end of step a), into at least:

[0070] - a first light fraction comprising at least some of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge, not converted in step a);

[0071] - a second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction);

[0072] - an intermediate fraction comprising at least some olefins containing between 7 and 9 carbon atoms (C7-C9 fraction); and

[0073] - a heavy fraction comprising the oligomers; c) a recycle step, comprising: the preparation of a first recycle comprising, preferably consisting of, at least a part of the first light fraction from step b) of fractionation; the preparation of a second recycle comprising, preferably consisting of, at least a part of the second light fraction from step b) of fractionation; the preparation of a third recycle comprising, preferably consisting of, at least a part of the intermediate fraction from step b) of fractionation; and the transfer of the first, second and third recycles to step a) of oligomerization; d) a hydrogenation step of at least a part of the heavy fraction separated in step b) in the presence of hydrogen, to obtain a hydrogenated heavy fraction advantageously comprising at least one middle distillate cut;e) possibly a step of separating the heavy hydrogenated fraction, to obtain at least a middle distillate cut, in particular a kerosene cut and / or a diesel cut.

[0074] The olefin charge

[0075] The feed treated by the process according to the invention is advantageously a so-called light olefinic feed, that is to say comprising hydrocarbon compounds and in particular olefins, comprising at least 80% by weight, preferably at least 85% by weight, preferably at least 90% by weight of olefins containing between 3 and 6 carbon atoms (i.e. C3, C4, C5 and / or C6), preferably 3 carbon atoms (i.e. C3), 4 carbon atoms (i.e. C4) and / or 6 carbon atoms (i.e. C6) (i.e. olefins with 3 carbon atoms, 4 carbon atoms, 6 carbon atoms or mixtures thereof).

[0076] According to a first embodiment, the olefinic charge comprises olefins with 4 carbon atoms (i.e. C4) and 6 carbon atoms (i.e. C6).

[0077] According to a second embodiment, the olefinic filler comprises olefins with 3 carbon atoms (i.e., C3) and 4 carbon atoms (i.e., C4). According to a third embodiment, the olefinic filler comprises olefins with 3 carbon atoms (i.e., C3), 4 carbon atoms (i.e., C4), and 6 carbon atoms (i.e., C6).

[0078] The feed according to the invention may also include olefins heavier than C6 olefins, such as C8 and / or C10 olefins. These C8 and / or C10 olefinic compounds may represent at most 20%, preferably at most 10% by weight of the total feed, without impacting the process.

[0079] Preferably, according to the first embodiment, the olefinic load comprises at least 20% by weight, preferably at least 50% by weight, preferably at least 90% by weight, preferably at least 95% by weight, in particular at least 98% by weight or at least 99% by weight, of C4 and C6 olefins, relative to the total weight of the olefinic load.

[0080] Preferably, according to the second embodiment, the olefin load comprises at least 20% by weight, preferably at least 50% by weight, preferably at least 90% by weight, preferably at least 95% by weight, in particular at least 98% by weight or at least 99% by weight, of C3 and C4 olefins relative to the total weight of the olefin load.

[0081] Preferably, according to the third embodiment, the olefinic load comprises at least 20% by weight, preferably at least 50% by weight, preferably at least 90% by weight, preferably at least 95% by weight, in particular at least 98% by weight or at least 99% by weight, of C3, C4 and C6 olefins relative to the total weight of the olefinic load.

[0082] Advantageously, when the olefinic feedstock includes C4 and C6 olefins, the weight ratio, expressed as a percentage, between the 6-carbon olefins (C6) and the sum of the 4-carbon and 6-carbon olefins (C4 + C6) is preferably greater than 22.0%, preferably between 22.0 and 50.0%, more preferably between 24.0 and 37.0%, and preferably between 28.0 and 34.0%. The yield of step a) is thus improved.

[0083] The olefinic feed may optionally include paraffins, in particular paraffins from the different fractions at the beginning and end distillation points of the feed in question, especially C3 to C6, i.e., fully hydrogenated hydrocarbon compounds, preferably aliphatic, preferably containing between 3 and 6 carbon atoms. Preferably, the olefinic feed may optionally include up to 80% by weight, preferably up to 50% by weight, preferably up to 10% by weight, preferably up to 5% by weight, in particular up to 2% or even up to 1% by weight of paraffins, relative to the total weight of the C3 to C6 olefinic feed, preferably C3, C4 and / or C6. Preferably, the olefinic filler is free of paraffins, i.e., comprises less than 0.5% by weight of paraffins, and preferably less than 0.1% by weight of paraffins, relative to the total weight of the olefinic filler.Treating an olefinic feed containing a low paraffin content, in particular containing paraffins with a content less than or equal to 10% by weight, preferably less than or equal to 5% by weight, preferably less than or equal to 2% by weight, or even an olefinic feed devoid of paraffins, allows the oligomerization step to be carried out at low pressure, in particular lower than that classically used for conventional (or fossil) feeds which generally include paraffin contents greater than 10% by weight and often between 40 and 80% by weight of paraffins.

[0084] An olefinic feedstock particularly suitable for the process according to the invention is an olefinic feedstock essentially in C3 to C6, preferably essentially in C3 and C4 or essentially in C3, C4 and C6 or essentially in C4 and C6, that is to say comprising at least 90% by weight, preferably at least 95% by weight, preferably at least 98% by weight of olefins in C3 and C4 or in C3, C4 and C6 or in C4 and C6 relative to the total weight of olefins contained in the olefinic feedstock. Thus, the olefinic charge advantageously comprises at least 85% by weight of propylene and butenes (in particular isobutene and / or n-butenes) or of propylene, butenes and hexenes or of butenes and hexenes, preferably at least 90% by weight, preferably at least 95% by weight, preferably at least 98% by weight of propylene and butenes, or of propylene, butenes and hexenes or of butenes and hexenes relative to the total weight of the olefinic charge.The olefinic filler can notably be chosen from a "polymer grade" propylene filler ("propylene polymer grade" according to Anglo-Saxon terminology), a filler comprising essentially propylene (i.e. at least 90% weight of propylene) and a small amount of butenes (i.e. less than 10% weight of butenes), and mixtures thereof.

[0085] In the preferred embodiment of the invention, wherein the olefinic filler is a C3 and C4 or C3, C4 and C6 or C4 and C6 olefinic filler, said olefinic filler may also contain C5 compounds. In this case, the C5 compound content is preferably less than or equal to 5% by weight in the olefinic filler, preferably less than or equal to 2% by weight, or even preferably less than or equal to 1% by weight relative to the weight of the olefinic filler. In this case, the C5 olefin content of the olefinic filler is preferably less than or equal to 0.8% by weight, preferably less than or equal to 0.6% by weight. Optionally, the olefinic filler of this preferred embodiment may also include paraffins, in particular propane and / or butane.Preferably, the C3 and C4 olefinic feed, or the C3, C4 and C6 olefinic feed, or the C4 and C6 olefinic feed, is free of propane and / or butane (i.e., it contains less than 0.5 wt% of paraffins, and preferably less than 0.1 wt% of paraffins, relative to the total weight of the olefinic feed), which allows the oligomerization step to be carried out at a lower pressure compared to the oligomerization of a feed containing paraffins. Preferably, the olefinic feed is free of C5 and / or C6 paraffins, i.e., it contains less than 0.5 wt%, preferably less than 0.1 wt% of C5 and / or C6 paraffins, in order to limit the amount of inert compounds introduced, particularly in step a).

[0086] The olefinic filler may also be a C3-C4 olefinic cut (i.e., comprising propylene and butenes), comprising, for example, at least 90% by weight of propylene and up to 10% by weight of butenes, said filler being preferably free of paraffins (i.e., comprising less than 0.5% by weight of paraffins, preferably less than 0.1% by weight of paraffins), the percentages being given in relation to the total weight of the olefinic filler.

[0087] The olefinic charge may also be a C4-C6 olefinic cut (i.e., comprising butenes and hexenes), such that the weight ratio, expressed as a percentage, between olefins having 6 carbon atoms (C6) and the sum of olefins having 4 carbon atoms and olefins having 6 carbon atoms (C4 + C6) is preferably greater than 22.0%, preferably between 22.0 and 50.0%, more preferably between 24.0 and 37.0%, and preferably between 28.0 and 34.0%.

[0088] Preferably, the olefin feedstock is at least partly, and very advantageously entirely, bio-based, in order to produce valuable bio-based products. The C3 to C6 olefin feedstock, preferably C3 and C4 or C4 and C6 or C3, C4 and C6, may in particular come from a Fischer Tropsch unit, an olefin production unit from methanol and / or an alcohol dehydration unit, for example ethanol or isobutanol, in particular from biomass, for example from sugar fermentation.

[0089] The olefinic feedstock can also originate from a conventional unit. In this case, it can preferably be used in a blend with bio-based feedstocks, preferably in weight proportions of 90:10 to 10:90 between conventional and bio-based olefinic feedstocks, preferably between 80:20 and 20:80. Preferably, a conventional olefinic feedstock originates from a steam cracking unit, an FCC (Fluid Catalytic Cracking) unit, a SHU (Selective Hydrogenation of Diolefins) unit, or a paraffin dehydrogenation unit, pure or blended, and / or any other unit leading to the production of light olefins.

[0090] The olefin feedstock treated in the process according to the invention may advantageously undergo a pretreatment step before being sent to step a) of oligomerization. Such a pretreatment step makes it possible to eliminate any compound that could cause poisoning of the oligomerization catalysts, in particular basic nitrogen compounds, water, sulfur derivatives, and basic nitrogen derivatives.

[0091] Preferably, the olefinic feedstock is free of sulfur or sulfur compounds, i.e., it comprises a sulfur content less than or equal to 20 ppm by weight, preferably less than or equal to 12 ppm by weight, preferably less than or equal to 10 ppm by weight relative to the weight of the olefinic feedstock, thus avoiding or at least limiting the poisoning of the oligomerization catalyst in step a). If the olefinic feedstock contains sulfur (i.e., more than 20 ppm by weight), the process advantageously includes a pretreatment step of the olefinic feedstock, located upstream of the oligomerization step a), preferably employing an adsorption section and / or a water washing section and / or a dedicated hydrotreating section, thus protecting the oligomerization catalyst in step a).

[0092] Preferably, the feed for the process according to the invention is free of nitrogen or nitrogen compounds, i.e., comprises a content less than or equal to 0.1 ppm by weight of nitrogen relative to the total weight of the olefinic feed, thus avoiding or at least limiting the poisoning of the oligomerization catalyst in step a). If the olefinic feed contains nitrogen, the process advantageously includes a pretreatment step of the olefinic feed, located upstream of the oligomerization step a), preferably implementing an adsorption and / or water washing and / or hydrotreating section, thus protecting the oligomerization catalyst in step a).

[0093] Preferably, the olefin feedstock treated by the process according to the invention is free of butadiene, in particular 1,3-butadiene, i.e., it comprises a content less than or equal to 0.1% by weight, preferably less than or equal to 500 ppm by weight of butadiene, in particular 1,3-butadiene, relative to the total weight of the olefin feedstock, which protects the oligomerization catalyst. If the olefin feedstock contains butadiene, in particular 1,3-butadiene, the process advantageously includes a pretreatment step of the olefin feedstock, located upstream of the oligomerization step (a), preferably employing a selective hydrogenation section.

[0094] Step a) of oligomerization. The process according to the invention includes an oligomerization step, more particularly implementing a heterogeneous oligomerization reaction (or heterogeneous catalysis reaction), carried out in the presence of at least one oligomerization catalyst, to produce a reaction effluent comprising dimers, trimers, and oligomers. Indeed, this oligomerization step a) yields a hydrocarbon mixture containing olefins with a predominant number of carbon atoms greater than or equal to 8, preferably greater than or equal to 9, the term "predominantly" meaning at least 90% by weight of C8+ hydrocarbons, preferably C9+, relative to the weight of the hydrocarbon mixture obtained. The hydrocarbon mixture obtained may also include unreacted C3 to C6 olefins in the feedstock.

[0095] According to the invention, oligomerization step a) is fed at least by the olefin feedstock, optionally pretreated and / or separated. Preferably, oligomerization step a) is also fed by a first recycle comprising, preferably consisting of, at least a part of the first light fraction from step b) comprising at least a part of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefin feedstock, not converted in step a), said first recycle being advantageously prepared and then transferred to step a) in step c).Preferably, oligomerization step a) is also fed with a second recycle comprising, preferably consisting of, at least a portion of the second light fraction from step b) comprising at least a portion of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction), said second recycle being advantageously prepared and then transferred to step a) in step c). Preferably again, oligomerization step a) is also fed with a third recycle comprising, preferably consisting of, at least a portion of the intermediate fraction from step b), said third recycle being advantageously prepared and then transferred to step a) in step c).

[0096] Advantageously, oligomerization step a) is carried out, preferably in the liquid phase (i.e., the olefinic feedstock and the products formed are in liquid form under the temperature and pressure conditions used), in the presence of an oligomerization catalyst. Preferably, oligomerization step a) is carried out at a temperature between 20 and 500°C, at a pressure between 1.0 and 10 MPa, and with a WH preferably between 0.1 and 0.5 h' 1 , preferably between 0.2 and 0.3 h' 1The WH (or hourly volumetric velocity) is, according to the invention, defined as the ratio between the volumetric flow rate of fresh olefinic feedstock, particularly at 15°C and 1 atm, and the volume of oligomerization catalyst, particularly during operation (also referred to as in operation). The temperature at which oligomerization step a) is carried out, between 20 and 500°C, advantageously corresponds to the inlet temperature of step a), preferably at the inlet of the reactor used in step a). The operating conditions of temperature, pressure, and hourly volumetric velocity can be adjusted by those skilled in the art, particularly according to the composition of the olefinic feedstock and the nature of the oligomerization catalyst used, to maximize yields of middle distillates, especially yields of kerosene or diesel fuel.

[0097] Advantageously, step a) employs at least one heterogeneous oligomerization catalyst, preferably between one and three different oligomerization catalysts, most preferably one oligomerization catalyst. Any type of oligomerization catalyst known to those skilled in the art can be used as an oligomerization catalyst in step a). More particularly, the oligomerization catalyst(s) of step a) can be any type of acid catalyst, especially those chosen from among silica-impregnated phosphoric acid catalysts (supported phosphoric acid, known as SPA-type catalysts), ion-exchange resins, alumina silicas, and pure or alumina-supported zeolites.Preferably, the oligomerization catalyst(s) of step a) is / are chosen from among ion exchange resins, preferably cation exchange resins, alumina silicas (i.e. comprising silica and alumina) and pure or alumina-supported zeolites.

[0098] When the oligomerization catalyst is chosen from SPA type catalysts, step a) is preferably carried out at a temperature, advantageously inlet, between 100 and 300°C, preferably between 160 and 250°C, and at a pressure preferably between 1.5 and 6.5 MPa, preferably between 1.5 and 4.0 MPa.

[0099] Zeolite-based catalysts are particularly suitable for producing linear or weakly branched heavy olefins, which allow in particular the production of high-quality diesel, i.e., after hydrogenation, having a cetane number greater than 45. When the oligomerization catalyst is chosen from among the zeolite-based catalysts, step a) is carried out preferably at a temperature, advantageously at the inlet, between 150 and 500°C, preferably between 200 and 350°C, at a pressure preferably between 2.0 and 10.0 MPa, preferably between 3.0 and 6.5 MPa. Preferably, the zeolite-based oligomerization catalyst comprises at least one zeolite selected from the group consisting of aluminosilicate-type zeolites having an overall Si / Al atomic ratio greater than 10 and a pore structure of 8, 10 or 12MR.The zeolite in question is most preferably selected from the group consisting of the structural zeolites ferrierite, chabazite, Y and US-Y, ZSM-5, ZSM-12, NU-86, mordenite, ZSM-22, NU-10, ZBM-30, ZSM-11, ZSM-57, ZSM-35, IZM-2, ITQ-6 and IM-5, SAPO, and mixtures thereof. Most preferably, the zeolite in question is selected from the group consisting of the zeolites ferrierite, ZSM-5, mordenite, ZSM-22, and mixtures thereof. Even more preferably, the zeolite used is ZSM-5.

[0100] Ion-exchange resin catalysts are chosen for their good mechanical resistance within the temperature and pressure ranges used in step a). When the oligomerization catalyst is selected from ion-exchange resins, step a) is preferably carried out at an inlet temperature between 20°C and 250°C, preferably between 70°C and 180°C, and at a pressure preferably between 2.0 and 10.0 MPa, preferably between 3.0 and 6.5 MPa. Ion-exchange resin catalysts, which are inexpensive and non-regenerable, have the advantage of acceptable cycle times in a fixed-bed operation because they are less sensitive to contaminants than zeolites and alumina silicas.Most preferably, the ion-exchange resin-type catalyst used in step a) is a copolymer of aromatic monovinyl and aromatic polyvinyl, preferably a copolymer of divinyl benzene and styrene, preferably sulfonated, in particular having a degree of crosslinking of between 20 and 45%, preferably between 30 and 40%, and preferably equal to 35%, and an acid strength, representing the number of active sites of said resin, determined by assay, preferably by conductimetry, of the H+ ions released by the acid resin after exchange with Na+ ions (cf. ASTM D4266), of between 1 and 10 mmol H+ equivalent per gram and preferably of between 3.5 and 6 mmol H+ equivalent per gram. For example, the acid oligomerization catalyst, of the ion exchange resin type, used in step a), is a commercial acid resin sold under the reference TA801 by the company Axens.

[0101] According to a particular embodiment of the invention, in which a silica-alumina is used as a catalyst for this oligomerization step a), said silica-alumina enables said oligomerization to be carried out in particular on at least one olefinic feedstock, preferably also on a first recycle comprising, preferably consisting of, at least a part of the first light fraction from step b), preferably also on a second recycle comprising, preferably consisting of, at least a part of the second light fraction from step b), preferably also on a third recycle comprising, preferably consisting of, at least a part of the intermediate fraction from step b), with better control of the reactivity of the olefins, allowing in particular to operate at a low-pass conversion level and very advantageously to optimize the selectivity towards the desired olefins having 8 or more carbon atoms,In particular, those with 9 or more carbon atoms, compared to oligomerization in the presence of other catalysts, for example, zeolites. Furthermore, coke formation appears less significant and less rapid in the presence of silica-alumina than in the presence of zeolites. Thus, silica-alumina requires regeneration at a lower frequency than zeolites.

[0102] Silica-alumina catalysts have the advantage of being regenerable, so that, despite their higher cost compared to resins, substantial savings are achieved in terms of catalyst consumption. When the oligomerization catalyst is chosen from silica-alumina, the temperature at which step a) of oligomerization in the presence of silica-alumina is carried out is preferably between 20 and 300°C, preferably between 30 and 220°C, most preferably between 40 and 200°C, advantageously corresponding to the inlet temperature of step a), preferably at the inlet of the reactor used in step a). The silica-alumina-based oligomerization catalyst(s) is / are amorphous catalyst(s) preferably made of an amorphous mineral material chosen from silica-aluminas and silicified aluminas, and preferably from silica-aluminas.In the silica-alumina based oligomerization catalyst used in step a), the SiCh / AhOs mass ratio is between 0.1 and 10. Preferably, the silica-alumina present in the oligomerization catalyst used in step a) of oligomerization has the following characteristics:.

[0103] - a silica mass content (SiC>2) of between 5% and 95% by weight, preferably between 10% and 80% by weight, more preferably between 20% and 80% by weight and even more preferably between 25% and 75% by weight, relative to the weight of the silica-alumina present in the oligomerization catalyst;

[0104] - a cationic impurity content advantageously less than 0.1% by weight, preferably less than 0.05% by weight and even more preferably less than 0.025% by weight, relative to the weight of the silica-alumina present in the oligomerization catalyst, the cationic impurity content being the total content of alkalis, in particular sodium.

[0105] Catalysts such as those prepared as described in patent FR2926812 may be suitable as oligomerization catalysts for step a).

[0106] According to a particular embodiment of the invention, the oligomerization catalyst used in step a) is made entirely of silica-alumina, that is to say, it is devoid of any other element (that is to say, it comprises less than 0.5% by weight, preferably less than 0.1% by weight of any element other than silica and alumina).

[0107] According to another particular embodiment of the invention, the oligomerization catalyst used in step a) may contain at least one metallic element selected from the metals of groups IVB, VB, VIB, and VIII. Among the metals of group IVB, titanium, zirconium, and / or hafnium may be present in the oligomerization catalyst. Among the metals of group VB, vanadium, niobium, and / or tantalum may be present in the oligomerization catalyst. Among the metals of group VIB, chromium, molybdenum, and / or tungsten may be present in the oligomerization catalyst. Among the metals of group VIII, the metals belonging to the first row of group VIII metals, namely iron, cobalt, and nickel, are preferred. The content of these metals may be up to 10% by weight relative to the weight of the oligomerization catalyst.The oligomerization catalyst may also contain silicon as a dopant element deposited on the silica-alumina.

[0108] Most advantageously, step a) of oligomerization is carried out in the presence of a silica alumina-based catalyst at a pressure between 1.5 and 6.5 MPa, preferably 2.0 to 4.0 MPa.

[0109] According to a particular embodiment, when the olefinic feedstock is a C3 and C4 or C3, C4 and C6 olefinic feedstock, the oligomerization step a) is carried out preferably in the presence of a silica-alumina catalyst, at a temperature, advantageously at the inlet of step a), preferably between 25 and 200°C, preferably between 30 and 190°C, and a pressure between 1.5 and 6.5 MPa, preferably between 2.0 and 4.0 MPa, to preferentially produce kerosene.

[0110] According to another particular embodiment, when the olefinic feedstock is a C4 and C6 olefinic feedstock, the oligomerization step a) is carried out preferably in the presence of a silica-alumina catalyst, at a temperature, advantageously at the inlet of step a), preferably between 35 and 200°C, preferably between 40 and 190°C, and a pressure between 1.5 and 6.5 MPa, preferably between 2.0 and 4.0 MPa, to preferentially produce diesel fuel.

[0111] Preferably, the oligomerization catalyst(s) of oligomerization step a) are in the form of spheres, pellets, or extrudates, preferably extrudates. Advantageously, the oligomerization catalyst(s) are in the form of extrudates with a diameter between 0.5 and 5 mm, and more particularly between 0.7 and 2.5 mm. The extrudates may be cylindrical (hollow or solid), twisted cylindrical, multilobed (2, 3, 4, or 5 lobes, for example), or ring-shaped. Cylindrical and multilobed shapes are preferred, but any other shape may be used. In a very particular embodiment of the invention, the oligomerization step is carried out in the presence of a silica-alumina based oligomerization catalyst, preferably consisting of silica-alumina, which is in the form of trilobed extrudates.Advantageously, the oligomerization step can utilize one or more reactors, preferably at least two, preferably at least three, and up to ten, preferably six, arranged in parallel or in series, preferably in series, comprising one or more different oligomerization catalysts, preferably comprising the same oligomerization catalyst. The operating conditions and oligomerization catalysts described above can be applied to any of the reactors. In a very specific case, the oligomerization step utilizes at least two, or even at least three, reactors in series.To ensure continuous operation of the oligomerization step, at least two reactors or reactor trains can be used. One reactor (or reactor train) can be in the reaction phase, while the other (or reactor train) can be in the regeneration phase, if the level of impurities in the feed leads to rapid deactivation of the catalyst. Optionally, the oligomerization step can also utilize heat exchangers upstream of the reactor(s) to heat the olefinic feed.

[0112] The oligomerization reaction is exothermic. At least part of the temperature increase associated with the exothermicity of the oligomerization reaction can be controlled by the first, second (comprising at least part of the light fraction corresponding at least in part to the unconverted feedstock) and third recycles (comprising at least part of the intermediate fraction), introduced in step a) of oligomerization.Exothermicity can also be controlled, at least in part, by diluting the olefinic feed by adding paraffins from a source external to the process, said paraffins being of the same molecular weight and / or heavier than the olefinic feed, said paraffins being aliphatic or cyclic, and / or by introducing a stream of inerts corresponding to a portion of the hydrogenated heavy fraction obtained at the end of step d), and in particular to a portion of the kerosene and / or diesel cut possibly separated in the optional step e) or a mixture of the kerosene and / or diesel cuts and residue from the optional step e). In this latter case, the stream of added paraffins, in particular the stream of inerts, preferably represents between 0 and 6 times the fresh olefinic feed by weight, preferably between 0.5 and 4, the relative quantities being expressed by weight.

[0113] The oligomerization step a) thus produces a reaction effluent which advantageously comprises dimers, trimers, and oligomers. This reaction effluent is sent, in whole or in part, to a fractionation step b).

[0114] Furthermore, the oligomerization step a) is preferably implemented so that the weight ratio, expressed as a percentage, between olefins with 6 carbon atoms (C6) and the sum of olefins with 4 carbon atoms and olefins with 6 carbon atoms (C4 + C6) is preferably greater than 22.0%, preferably between 22.0 and 50.0%, more preferably between 24.0 and 37.0%, and preferably between 28.0 and 34.0%, thus improving the overall yield of the process for preparing middle distillates.

[0115] Step b) of splitting

[0116] The process according to the invention includes a step of fractionating the reaction effluent obtained at the end of step a), into at least:

[0117] - a first light fraction comprising at least some of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge, not converted in step a);

[0118] - a second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction);

[0119] - an intermediate fraction comprising at least some olefins containing between 7 and 9 carbon atoms (C7-C9 fraction); and

[0120] - a heavy fraction, comprising oligomers present in the reaction effluent from step a), in particular olefinic compounds containing between 8 and 30, preferably between 9 and 25 carbon atoms, and more particularly between 9 and 16 carbon atoms in the case of kerosene or between 10 and 25 carbon atoms in the case of diesel.

[0121] Advantageously, the intermediate fraction comprises at least 50% by weight, preferably at least 70% by weight and preferably at least 85% by weight of olefins containing between 7 and 9 carbon atoms (C7-C9 fraction).

[0122] Preferably, the intermediate fraction comprises less than 50% by weight, preferably less than 30% by weight, and preferably at most 15% by weight of olefins containing 6 carbon atoms.

[0123] The second light fraction comprises at least some of the C5 and C6 olefins from the olefinic feed not converted in step a), when step a) deals with an olefinic feed that includes C5 and / or C6 compounds.

[0124] The second light fraction comprises the products of the 6-carbon oligomerization, when step a) deals with an olefinic charge that includes C3 compounds.

[0125] Advantageously, the second light fraction comprises at least 50% by weight, preferably at least 60% by weight, preferably at least 75% by weight of olefins containing between 5 and 6 carbon atoms (C5-C6 fraction).

[0126] According to one embodiment, step b) may include fractionation to obtain a light fraction and at least one other fraction, preferably the intermediate fraction and the heavy fraction. This light fraction comprises, preferably consists of, at least a portion, preferably all, of the olefinic charge not converted in step a). It comprises at least:

[0127] - a first light fraction comprising at least a portion of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge not converted in step a);

[0128] - a second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction), present in the olefinic charge and not converted in step a), and / or resulting from the dimerization of the C3 olefins of the olefinic charge (in particular the compounds containing 6 carbon atoms of the second light fraction which may be the products of the dimerization of the C3 olefins of the olefinic charge).

[0129] The fractionation of the light fraction is advantageously achieved through vaporization. Separating the C5-C6 fraction from the C3-C4 fraction during vaporization reduces the energy required for this fractionation step and therefore for the entire process. Compared to prior art processes, where the C5-C6 fraction is generally separated along with the intermediate C7-C9 fraction, the volume of the intermediate fraction is reduced because it is devoid of the C5-C6 fraction. This results in a process that is just as efficient, particularly in terms of yields of middle distillates, notably kerosene and diesel fuel, but with reduced energy consumption compared to prior art processes, thus limiting the process's environmental impact. Indeed, energy consumption is reduced by at least 20%, and preferably by at least 30%, compared to prior art processes.

[0130] The first and / or second light fractions may also include non-reacting compounds, i.e., compounds that are not olefins, particularly paraffins already present in the fresh olefin feedstock. These paraffins are inert. Therefore, purging part of the stream may be implemented to prevent their accumulation in step a).

[0131] Preferably, the first and / or second light fractions are free of paraffins, i.e. comprise less than 0.5% by weight, preferably less than 0.1% by weight of paraffins relative to the total weight of the respective fraction.

[0132] The flow rate of the first light fraction and the flow rate of the second light fraction depend on the conversion of the olefinic charge by pass of step a) of oligomerization, as well as the content of compounds that do not react.

[0133] The first and second light fractions are advantageously recycled in whole or in part in step a); they constitute at least a portion of the first and second recycles, respectively, prepared in step c). Optionally, a portion of this first and / or second light fraction can be purged, continuously or discontinuously, particularly when the olefin feedstock contains compounds that do not oligomerize, such as paraffins. In the case of purging, the purged stream can be recovered as liquefied petroleum gas (LPG), for example. Optionally, the first and second light fractions, particularly the first light fraction, may include O1 to O2 compounds, possibly generated during step a) and resulting from cracking and recombination reactions.

[0134] Optionally, following the fractionation step, a gaseous fraction comprising compounds O1 to O2, possibly generated during step a) and resulting from cracking and recombination reactions, can also be separated. The gaseous fraction, if separated in step b), is preferably purged (i.e., removed from the process) continuously or discontinuously, for example, for valorization.

[0135] Most advantageously, the intermediate fraction comprises, preferably consists of, at least a part of, the dimers and trimers advantageously produced in step a).

[0136] Preferably, the intermediate fraction consists of the dimers and trimers produced in step a) that were not separated in the second light fraction. The products it contains, particularly the dimers and trimers, are too light to be recovered as middle distillates, especially kerosene or diesel fuel. The intermediate fraction may also contain paraffins from the olefin feedstock that boil in the same temperature ranges as the olefins in the intermediate fraction, particularly olefins in the O7-O9 range. Preferably, the intermediate fraction is free of paraffins, i.e., it comprises less than 0.5 wt%, preferably less than 0.1 wt%, of paraffins relative to the total weight of the intermediate fraction.Preferably, the intermediate fraction comprises olefin oligomers in the O6+ group and preferably has a T95 temperature below 140°C, particularly below 140°C in the case of kerosene production, or below 165°C, preferably below 170°C, in the case of diesel production. It can therefore also be called C6-140°C or C6-165°C (preferably O6-170°C). The intermediate fraction is advantageously recycled in whole or in part in step a); it constitutes at least part of the third recycle prepared in step c). Optionally, at least part of the intermediate fraction can be recovered and removed from the process (i.e., purged) for further processing or direct valorization, particularly as gasoline.This possible portion purged from the intermediate fraction can undergo a hydrogenation step, and in particular can be sent to step d) of the process or to a separate hydrogenation step of the process according to the invention and operated for example under conditions similar to those described for step d), before being integrated into a gasoline pool.

[0137] The heavy fraction advantageously comprises the oligomers present in the reaction effluent from step a). Advantageously, it comprises, in particular, olefinic compounds containing between 8 and 30, preferably between 9 and 25, carbon atoms. Preferably, the heavy fraction has a T5 greater than or equal to 140°C, in particular greater than or equal to 140°C in the case of kerosene production, or greater than or equal to 165°C, preferably greater than or equal to 170°C, in the case of diesel production. Preferably, the heavy fraction is composed of C9+ olefinic oligomers and, most preferably, boils between 140 and 300°C (also called the 140-300°C fraction) or at a temperature greater than or equal to 165°C, preferably greater than or equal to 170°C. According to a preferred embodiment of the invention, the heavy fraction can correspond to a kerosene fraction with a cutting point enabling a flash point greater than or equal to 38°C to be reached.According to another preferred embodiment of the invention, the heavy fraction can correspond to a diesel fraction with a cutting point enabling a flash point greater than or equal to 55°C to be reached.

[0138] Advantageously, step b) of fractionation employs one or more distillation columns, preferably between one and three. These columns may include reboilers advantageously employing any means known to those skilled in the art, for example, a furnace or a steam exchanger.

[0139] In particular, according to a preferred embodiment, step b) of fractionation is carried out in two steps: a step b1) of fractionation of the reaction effluent obtained at the end of step a), into at least: o a first light fraction comprising at least a part of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge not converted in step a); o a second light fraction comprising at least a part of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction) o an intermediate / heavy fraction comprising at least a part of the olefins containing between 7 and 9 carbon atoms (C7-C9 fraction) and the oligomers; a step b2) of fractionation of the intermediate / heavy fraction obtained at the end of step b1), into at least: o an intermediate fraction comprising at least a part of the olefins containing between 7 and 9 carbon atoms (C7-C9 fraction); and o a heavy fraction comprising the oligomers.

[0140] In this implementation, separating the C5-C6 fraction from the C3-C4 fraction does not increase energy consumption. Furthermore, by removing the C5-C6 fraction, it is possible, compared to prior art processes in which this C5-C6 fraction is generally separated with the C7-C9 fraction, to also limit the energy required to separate the intermediate fraction (whose volume is reduced since it is devoid of the C5-C6 fraction).

[0141] In one particular embodiment, step b) of fractionation uses three separate fractionation columns: the first light fraction comprising at least some of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge not converted in step a) is obtained at the top of the first column, the bottom feeding the second column, the second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction) is obtained at the top of the second column, the bottom feeding a third column, the intermediate fraction comprising at least some of the olefins containing between 7 and 9 carbon atoms (C7-C9 fraction) is obtained at the top of the third column while the heavy fraction comprising the oligomers is obtained at the bottom of this third column.

[0142] Preferably, step b) employs two separate fractionation columns: the first light fraction comprising at least some of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge, not converted in step a), is obtained at the top of the first column, the second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction) is obtained by lateral withdrawal from the first column and the bottom feeds the second column, the intermediate fraction comprising at least some of the olefins containing between 7 and 9 carbon atoms (C7-C9 fraction) is obtained at the top of the second column while the heavy fraction comprising the oligomers is obtained at the bottom of this second column.

[0143] Using two separate columns thus limits the number of pieces of equipment. Advantageously, the lateral discharge from the first column is preferably implemented in the upper part of said first column, advantageously between the column head and the feed tray. Step c) of recycling

[0144] Step c) of recycling the process according to the invention comprises:

[0145] - the preparation of a first recycle which preferably comprises at least a part of the first light fraction, resulting from step b) of fractionation,

[0146] - the preparation of a second recycle which preferably comprises at least part of the second light fraction from step b) of fractionation,

[0147] - the preparation of a third recycle which preferably comprises at least part of the intermediate fraction resulting from step b) of fractionation, and

[0148] - the transfer of the first, second and third recycles to step a) of oligomerization.

[0149] Advantageously, all or part of the first light fraction from separation step b) constitutes the first recycle, which is then recycled to oligomerization step a), preferably directly at the inlet of step a), and preferably upstream of any heat exchangers implemented in step a) upstream of the reactors to heat the olefin feedstock. The first recycle advantageously maximizes the overall conversion and also manages at least part of the exothermicity of the oligomerization reaction in step a). Preferably, the first recycle, which corresponds to the portion of the first light fraction recycled to a), represents a quantity such that the weight ratio between the first recycle and the olefin feedstock, which feeds oligomerization step a), is between 0.3 and 1.5, preferably between 0.5 and 1.2.

[0150] Advantageously, all or part of the second light fraction from separation step b) constitutes the second recycle, which is then recycled to oligomerization step a), preferably directly at the inlet of step a), and preferably upstream of any heat exchangers implemented in step a) upstream of the reactors to heat the olefin feedstock. The second recycle advantageously maximizes overall conversion by optimizing, in particular, the proportion of the C5-C6 fraction in the feedstock subjected to oligomerization in step a). Preferably, the second recycle, which corresponds to the portion of the second light fraction recycled to a), represents a quantity such that the weight ratio between the second recycle and the olefin feedstock supplying oligomerization step a) is between 0.6 and 2.0, preferably between 0.8 and 1.6.

[0151] Advantageously, all or part of the intermediate fraction from separation step b) constitutes the third recycle, which is then recycled to step a), preferably directly and in particular upstream of any heat exchangers implemented upstream of the reactors in step a). Preferably, the intermediate fraction is not cooled before being transferred, in whole or in part, as the third recycle, to oligomerization step a), which contributes in particular to preheating the olefin feedstock entering step a) by simple mixing. Preferably, the third recycle, which corresponds to the portion of the intermediate fraction recycled to a), represents a quantity such that the weight ratio between the third recycle and the olefin feedstock, which feeds oligomerization step a), is between 0.5 and 10.0, preferably between 1.0 and 5.0, and preferably between 1.0 and 4.0.

[0152] Recycling at least part of these three fractions, the first light fraction, the second light fraction and the intermediate fraction, in particular recycling at least part of the intermediate fraction, makes it possible to increase the overall conversion and maximize the yield in target products, in particular in middle distillates, more particularly in kerosene or diesel, by strongly promoting selectivity towards the target products in particular kerosene or diesel.

[0153] Preferably, the heavy fraction comprising oligomers, which is obtained from fractionation step b), is not recycled, and in particular is not recycled to oligomerization step a). This is because the heavy fraction comprises heavy olefinic compounds, particularly those containing between 8 and 30, preferably between 9 and 25, carbon atoms. Thus, if such olefinic compounds were recycled to oligomerization step a), very heavy, undesirable compounds could be generated, reducing selectivity and therefore decreasing yields of middle distillates. The process for preparing middle distillates according to the invention therefore preferably does not involve recycling the heavy fraction obtained in step b). In particular, in the case of kerosene production, the process advantageously does not involve recycling the heavy fraction with a T5 greater than or equal to 140°C.In the case of diesel production, the process is advantageously devoid of recycling of the heavy fraction whose T5 is greater than or equal to 165°C, preferably greater than or equal to 170°C.

[0154] According to a first particular embodiment of the invention, the olefinic filler is composed essentially of propylene, preferably of at least 90% by weight of propylene, possibly up to 10% by weight of butenes and very advantageously less than 0.5% by weight, or even less than 0.1% by weight of paraffins, relative to the total weight of the olefinic filler, and the oligomerization step a) is advantageously carried out in the presence of silica-alumina, preferably at a temperature between 100 and 180°C, preferably between 110 and 170°C, most preferably between 115 and 165°C, at a pressure preferably between 1.5 and 6.5 MPa, most preferably between 2.0 and 4.0 MPa, and a WH preferably between 0.20 and 0.30 h' 1, preferably between 0.20 and 0.25 h' 1In this particular embodiment, a stream of inert materials (i.e., a stream of compounds inert to the oligomerization reaction, i.e., compounds that do not react under the operating conditions of step a)), preferably comprising at least a portion of the hydrogenated heavy fraction obtained at the end of hydrogenation step d), is preferably used to feed step a), so as to control the exothermicity of the oligomerization reaction and thus the reactivity. According to this first particular embodiment, the intermediate fraction separated in step b) advantageously has a T95 below 140°C; the heavy fraction has a T5 greater than or equal to 140°C and preferably boils between 140 and 300°C.According to this first particular embodiment, the third recycle prepared in step c) and which preferably consists of at least a part, preferably all, of the intermediate fraction separated in b), represents a weight quantity such that the weight ratio between the third recycle and the olefinic load at the input of step a) is between 1.0 and 5.0, preferably between 1.5 and 2.0, the third recycle being sent to step a) of oligomerization.

[0155] According to a second particular embodiment of the invention, the olefinic filler is composed essentially (i.e., at least 90% by weight) of n-butenes (but-1-enes and but-2-enes) and hexenes, preferably of at most 70% by weight of n-butenes and at least 30% by weight of C6 olefins and very advantageously less than 0.5% by weight of paraffins such as butane, relative to the total weight of the olefinic filler, and the oligomerization step a) is advantageously carried out in the presence of silica-alumina, preferably at a temperature between 130 and 200°C, preferably between 140 and 190°C, most preferably between 145 and 185°C, at a pressure preferably between 1.5 and 6.5 MPa, most preferably between 2.0 and 4.0 MPa, and a WH preferably between 0.20 and 0.30 h' 1 , preferably between 0.20 and 0.25 h' 1Optionally, an inert stream (i.e., a stream of compounds inert to the oligomerization reaction, i.e., which do not react under the operating conditions of step a)), preferably composed at least in part of a portion of the hydrogenated heavy fraction obtained at the end of hydrogenation step d), can be used to feed step a), so as to control the exothermicity of the oligomerization reaction and thus the reactivity. According to this second particular embodiment, the intermediate fraction separated in step b) advantageously has a T95 below 140°C; the heavy fraction has a T5 greater than or equal to 140°C and preferably boils between 140 and 300°C.According to this second particular embodiment, the third recycle prepared in step c) and which preferably consists of at least a part, preferably all, of the intermediate fraction separated in b), represents a weight quantity such that the weight ratio between the third recycle and the olefinic load at the input of step a) is between 1.0 and 5.0, preferably between 3.5 and 4.0, the third recycle being sent to step a) of oligomerization.

[0156] In these two particular embodiments of the invention, the temperature at which step a) of oligomerization in the presence of silica-alumina is carried out advantageously corresponds to the temperature at the inlet of step a), preferably at the inlet of the reactor carried out in step a).

[0157] Step d) of hydrogenation

[0158] The process according to the invention includes a step of hydrogenating at least part, preferably all, of the heavy fraction separated in step b) in the presence of hydrogen, to obtain a hydrogenated heavy fraction.

[0159] The hydrogenation step allows the olefinic bonds of at least part, preferably all, of the heavy fraction from step c) to be saturated, in order to produce paraffins that can be directly incorporated into fuel pools, in particular the kerosene pool (or jet pool) and very specifically into the SPK jet pool (SPK being the Anglo-Saxon acronym for "Synthetic Paraffinic Kerosene") which meets the specifications of ASTM D7566, Annex 5, or the diesel pool and very specifically into the diesel pool which meets the specifications of European standard 15940. Step d) of hydrogenation of the unsaturated compounds makes it possible in particular to significantly improve the smoke point of the heavy fraction, and in particular of the middle distillates produced, and / or the elimination of any sulfur and / or nitrogen impurities.

[0160] Preferably, step d) of hydrogenation is carried out in the presence of a catalyst preferably comprising at least one Group VIII metal, in particular nickel, palladium, or platinum, deposited on an inert support, such as silica or alumina. Preferably, the hydrogenation step is carried out in the presence of a palladium- or nickel-based catalyst on an alumina support. However, any other catalyst capable of hydrogenating the product of the oligomerization step, and in particular the heavy fraction, especially with C9+ olefins, may be used. For example, a catalyst selected from among NiMo, CoMo, NiCoMo-alumina catalysts, and mixtures thereof, may be used.

[0161] The hydrogenation step (d) is carried out, preferably in the liquid phase, advantageously at a pressure between 0.5 and 5.0 MPa, preferably between 1.0 and 5.0 MPa, and preferably at a temperature between 50 and 300°C, preferably between 60 and 200°C, in the presence of hydrogen preferably at a content between 0.5 and 3% by weight relative to the weight of the portion of the heavy fraction feeding into step (d). Preferably, during step (d), a hydrogenation rate of at least 90%, preferably greater than or equal to 95%, preferably greater than or equal to 99%, is achieved.

[0162] Advantageously, the hydrogenated heavy fraction then obtained comprises, preferably, at least partly consisting of middle distillates, and more particularly of a kerosene cut meeting very advantageously the kerosene specifications of the standards in force, in particular the kerosene specifications of ASTM D7566, in particular ASTM D7566 Annex 5, and / or a diesel cut meeting very advantageously the diesel specifications of the standards in force, in particular the diesel specifications of European standard 15940. The hydrogenated heavy fraction obtained at the end of step d) may optionally be sent in whole or in part to an optional separation step e).

[0163] Optionally, a portion of the hydrogenated heavy fraction obtained at the end of step d) is separated to form an inert stream, which can then be recycled to step a) to help control the exothermicity of the oligomerization reaction in step a), in parallel with the first recycle. Preferably, the weight of the inert stream recycled to step a) represents 0 to 6 times, preferably 0.5 to 4 times by weight, the weight of the fresh olefin feedstock.

[0164] Optional separation step e)

[0165] The process according to the invention includes a step of separating the hydrogenated heavy fraction obtained at the end of d), to obtain at least one middle distillate cut, in particular at least one kerosene cut and / or a diesel fraction, and optionally a gasoline fraction.

[0166] In a very particular way, a kerosene base is separated in step e) and this kerosene base preferably has a final evaporation temperature between 140 and 300°C and advantageously a flash point greater than or equal to 38°C; the separated diesel cut preferably has an evaporation temperature greater than or equal to 165°C, preferably greater than or equal to 170°C, and advantageously a flash point of at least 55°C.

[0167] According to a particular embodiment, the optional separation step e) advantageously allows for obtaining:

[0168] - a kerosene cut whose lower (initial) distillation point is preferably at least 130°C;

[0169] - a gasoline cut whose upper (final) distillation point is preferably below 140°C. In this embodiment, the production of kerosene and gasoline is maximized.

[0170] According to another particular embodiment, the optional separation step e) advantageously allows for obtaining:

[0171] - a head cut which corresponds to a gasoline advantageously comprising hydrocarbons having a number of carbon atoms between 7 and 10;

[0172] - an intermediate cut advantageously comprising hydrocarbons having a number of carbon atoms between 9 and 24, preferably between 9 and 16, and which constitutes a kerosene cut meeting commercial specifications,

[0173] - a so-called residual cut, with an initial boiling point above 300°C, the final cut sought being kerosene, and which advantageously joins the diesel or fuel pool.

[0174] According to another particular embodiment, the optional separation step e) advantageously allows for obtaining:

[0175] - a kerosene cut whose lower (initial) distillation point is preferably at least 130°C, and whose final distillation point is less than or equal to 300°C, which constitutes a kerosene base meeting commercial specifications

[0176] - optionally, a so-called residual cut, with an initial boiling point above 300°C, which advantageously joins the diesel or fuel pool.

[0177] In this embodiment, kerosene production is maximized.

[0178] According to yet another particular embodiment, the optional separation step e) advantageously allows us to obtain:

[0179] - a head cut which corresponds to a gasoline advantageously comprising hydrocarbons having a number of carbon atoms between 7 and 10;

[0180] - an intermediate cut advantageously comprising hydrocarbons having a number of carbon atoms between 10 and 24, and which constitutes a diesel cut meeting commercial specifications.

[0181] In this embodiment, diesel production is maximized.

[0182] Thus, the process according to the invention makes it possible to promote the selectivity of a light olefin oligomerization process towards middle distillates, in particular kerosene and / or diesel fuel, and therefore to maximize yields of middle distillates, while achieving optimal overall olefin loading. The process according to the invention is particularly flexible since those skilled in the art can adapt the selectivity of the oligomerization and the separation of the effluents in order to maximize the production of kerosene and / or diesel fuel, even to the point of producing only diesel fuel or only kerosene. The examples and figures that follow illustrate the invention, in particular specific embodiments of the invention, without limiting its scope.

[0183] Examples

[0184] Example 1 (not in accordance with the invention)

[0185] An olefinic feedstock (1), comprising 64.1 wt% butene, 0.06 wt% n-butane, 27.1 wt% C6 olefinic, 7.2 wt% C8 olefinic, and 1.6 wt% C10+ olefinic, is oligomerized according to the embodiment of the process described in Figure 1, in the presence of a silica-alumina catalyst (commercial catalyst IP 811 from Axens), at a temperature between 140 and 190°C, at a pressure of 3.5 MPa, and at a WH of 0.3 h - 1 The oligomerization reaction is implemented in 3 reactors in series, with an intermediate exchanger between each reactor, allowing cooling before entry into the next reactor.

[0186] The reaction effluent obtained at the end of the oligomerization step is separated by distillation into three fractions:

[0187] 1) a C4 cut (stream 4), comprising the unreacted feed and corresponding to approximately 23.4% by weight of the reaction effluent, said C4 cut being returned to the oligomerization step so that the weight ratio of the C4 cut to the olefinic feed is equal to 1.4. A minimal fraction of this C4 cut is purged in order to limit the accumulation of paraffinic C4;

[0188] 2) a C5-140°C cut (flow 3), constituting the intermediate fraction and corresponding to approximately 61.7% by weight of the reaction effluent, said C5-140°C cut being recycled into the input of the oligomerization step so that the weight ratio of the C5-140°C cut to the olefinic load is equal to 3.6. A minimal fraction of this C5-140°C cut is purged in order to limit the accumulation of paraffinic C5-140°C;

[0189] 3) a 140-300°C cut (flow 9), corresponding to 14.9% by weight of the reaction effluent, which is sent to hydrogenation.

[0190] The fractionation of these three fractions is carried out in two separate fractionation columns. The C4 fraction is obtained at the top of the first column, with the bottom feeding the second column. The C5-140°C fraction is obtained at the top of the second column, while the 140-300°C fraction is obtained at the bottom of this second column. These two columns can operate independent reboilers to achieve the required fractionation qualities.

[0191] Furthermore, the conversion of the olefin feedstock is greater than or equal to 90% by weight. Hydrogenation is carried out in the presence of a nickel catalyst on an alumina support, at 180°C under 3.0 MPa of hydrogen with a WH of 0.5 h -1 and a hydrogen flow rate of 50 NL / h.

[0192] The olefin level observed after hydrogenation is very low (number of bromine < 0.8 g / 100g), i.e. a high hydrogenation rate (greater than 99%).

[0193] The hydrogenation effluent is then sent to a distillation section where it is separated into three cuts:

[0194] - a light essence cut (flux 11) whose upper distillation point is below 140°C with a yield of 7% by weight relative to the weight of olefins in the initial olefinic feed and;

[0195] - a kerosene cut (flux 13), with a distillation range of 140°C-300°C, with a yield of 86% by weight relative to the weight of olefins in the olefinic feed;

[0196] - a residue 300+ (flux 14) corresponding to 7% by weight relative to the weight of olefins in the olefinic load.

[0197] Example 2 (according to the invention)

[0198] An oligomerization unit is implemented in the same way as in example 1, except that the reaction effluent obtained at the end of the oligomerization step is separated by distillation into four cuts (see figure 2):

[0199] 1) a C4 cut (head stream of the first column), comprising the unreacted load and corresponding to approximately 23.4% by weight of the reaction effluent, said C4 cut being sent in part to the oligomerization step (stream 4), so that the weight ratio of stream 4 to the olefinic load is 1.4. A minimal fraction of this C4 cut is purged in order to limit the accumulation of paraffinic C4 (stream 6);

[0200] 2) an additional C5-C6 cut (intermediate draw-off stream from the first column) comprising some of the C5-C6 olefins from the feed that have not been converted as well as any constituents of the feed that do not react (e.g. paraffins) in this boiling range; corresponding to about 25.3% by weight of the reaction effluent, said C5-C6 cut being partly recycled to the inlet of the oligomerization step (stream 4'), so that the weight ratio of stream 4' to the olefinic feed is equal to 1.5; A minimal fraction of this cut is purged in order to limit the accumulation of C5-C6 paraffinic (stream 6');

[0201] 3) a C6-140°C cut (head stream of the second column), constituting the intermediate fraction and corresponding to approximately 36.4% by weight of the reaction effluent, said C6-140°C cut being partly recycled into the inlet of the oligomerization step (stream 3) so that the weight ratio of stream 3 to the olefinic load is equal to 2.2; A minimal fraction of this cut is purged in order to limit the accumulation of paraffinic C6-140°C (stream 8);

[0202] 4) a 140-300°C cut (flow 10), corresponding to 14.9% by weight of the reaction effluent, which is sent to hydrogenation d).

[0203] The fractionation of these four cuts is implemented in the same way as in Example 1, i.e., in two separate columns. The C5-C6 cut is obtained by lateral draw-off from the first column. These two columns can operate reboilers independently, and the energy consumption required for heating and vaporizing the effluents in the second column is reduced by 40% compared to Example 1.

[0204] A kerosene cut is thus obtained with a yield equivalent to or even greater than that of Example 1, in particular greater than or equal to 1% compared to the yield of Example 1. Thus, it is demonstrated that the process according to the invention makes it possible to considerably limit the energy consumption required for the operation of said process, while remaining very efficient, insofar as the yield in middle distillates as well as the selectivity in kerosene are maintained at satisfactory levels.

Claims

Demands 1. A process for preparing middle distillates from an olefinic feed comprising olefins containing between 3 and 6 carbon atoms, the process comprising: a) a heterogeneous oligomerization step fed with at least the olefinic feed, a first recycle, a second recycle and a third recycle, and carried out in the presence of at least one heterogeneous oligomerization catalyst, at a temperature between 20 and 500°C, a pressure between 1.0 and 10 MPa and a WH between 0.1 and 0.5 h' 1 , to produce a reaction effluent comprising dimers, trimers and oligomers; b) a step of fractionating the reaction effluent obtained at the end of step a), into at least: - a first light fraction comprising at least some of the olefins containing between 3 and 4 carbon atoms (C3-C4 fraction) of the olefinic charge, not converted in step a); - a second light fraction comprising at least some of the olefins containing between 5 and 6 carbon atoms (C5-C6 fraction); - an intermediate fraction comprising at least some olefins containing between 7 and 9 carbon atoms (C7-C9 fraction); and - a heavy fraction comprising the oligomers; c) a recycle step, comprising: the preparation of a first recycle comprising at least a part of the first light fraction; the preparation of a second recycle, comprising at least a part of the second light fraction; the preparation of a third recycle comprising at least a part of the intermediate fraction; and the transfer of the first, second and third recycles to the oligomerization step a); d) a hydrogenation step of at least a part of the heavy fraction separated in step b) in the presence of hydrogen, to obtain a hydrogenated heavy fraction comprising middle distillates.

2. A method according to claim 1, wherein the olefinic feedstock comprises olefins containing 3 and 4 carbon atoms (C3 and C4) or 3, 4 and 6 carbon atoms (C3, C4 and C6) or 4 and 6 carbon atoms (C4 and C6).

3. A process according to claim 1 or 2, wherein the olefinic filler is at least partly bio-based.

4. A process according to any one of the preceding claims, wherein the first recycle feeds step a) in a weight ratio of between 0.3 and 1.5, preferably between 0.5 and 1.2, relative to the olefinic feed.

5. A process according to any one of the preceding claims, wherein the second recycle feeds step a) in a weight ratio of between 0.6 and 2.0, preferably between 0.8 and 1.6, relative to the olefinic feed.

6. A process according to any one of the preceding claims, wherein the third recycle feeds step a) in a weight ratio of between 0.5 and 10.0, preferably between 1.0 and 5.0 and preferably between 1.0 and 4.0, relative to the olefinic feed.

7. A process according to any one of the preceding claims, wherein the oligomerization catalyst of step a) is selected from silica-impregnated phosphoric acid-based catalysts, ion exchange resins, alumina silicas and pure or alumina-supported zeolites.

8. A process according to any one of the preceding claims, wherein step a) of oligomerization is carried out in the presence of a silica-alumina-based catalyst, at a temperature between 20°C and 300°C, preferably between 30°C and 220°C, most preferably between 40°C and 200°C.

9. A process according to claim 8, wherein step a) of oligomerization is carried out at a pressure between 1.5 and 6.5 MPa, preferably between 2.0 and 4.0 MPa.

10. A process according to any one of claims 1 to 7, wherein step a) of oligomerization is carried out in the presence of a zeolite-based catalyst, step a) at a temperature between 150 and 500°C, preferably between 200 and 350°C.

11. A process according to claim 10, wherein step a) of oligomerization is carried out at a pressure between 2.0 and 10.0 MPa, preferably between 3.0 and 6.5 MPa.

12. A process according to any one of the preceding claims, wherein the intermediate fraction comprises less than 50% by weight, preferably less than 30% by weight, preferably at most 15% by weight of olefins containing e carbon atoms.

13. A process according to any one of the preceding claims, wherein the second light fraction comprises at least 50% by weight, preferably at least 60% by weight, preferably at least 75% by weight of olefins containing between 5 and 6 carbon atoms (C5-C6 fraction).

14. A process according to any one of the preceding claims, further comprising a step e) of separating the heavy hydrogenated fraction from step d), to separate at least one middle distillate cut, in particular a kerosene cut and / or a diesel cut.

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