GAS / LIQUID OLIGOMERIZATION REACTOR HAVING SUCCESSIVE ZONES OF VARIABLE DIAMETER

Abstract

The invention relates to a gas / liquid oligomerization reactor having successive zones of varying diameters. The invention also relates to a process for the oligomerization of ethylene using a gas / liquid oligomerization reactor having successive zones of varying diameters.

Description

GAS / LIQUID OLIGOMERIZATION REACTOR HAVING SUCCESSIVE ZONES OF VARIABLE DIAMETER RF LQnn / ZZnZ / E / YIAI Field of Invention The present invention relates to a gas / liquid reactor for oligomerization comprising a reaction chamber having zones of decreasing diameter from the bottom to the top of the reactor. The invention also relates to the use of such a gas / liquid reactor in a homogeneous catalytic oligomerization process of ethylene into linear olefins, and in particular of but-l-ene, hex-l-ene, and / or oct-l-ene. Background of the Invention The invention relates to the field of gas / liquid reactors, also known as bubble columns, and their implementation in an ethylene oligomerization process. A drawback encountered during the implementation of this type of reactor in ethylene oligomerization processes is the management of the overhead gas or gaseous headspace in the vessel, corresponding to the upper part of the reactor in a gaseous state. This overhead gaseous headspace includes gaseous compounds that are poorly soluble in the liquid phase, partially soluble but inert compounds in the liquid, and also undissolved ethylene gas. The step Ref. 334175 The movement of gaseous ethylene from the lower liquid portion of the reaction chamber into the upper gaseous space is a phenomenon called penetration. However, the upper gaseous space is purged to remove gaseous compounds. When the amount of gaseous ethylene present in the upper gaseous space is significant, purging it leads to a substantial loss of ethylene, impacting the productivity and cost of the oligomerization process. Furthermore, significant penetration means that a large amount of gaseous ethylene has not dissolved in the liquid phase and therefore cannot react, affecting the productivity and selectivity of the oligomerization process. In order to improve the efficiency of the oligomerization process in terms of productivity and cost, it is essential to limit the ethylene penetration phenomenon to improve its conversion in such a process while maintaining good selectivity of desired linear alpha olefins. Prior art methods that implement a gas / liquid reactor, as illustrated in Figure 1, do not allow limiting the loss of gaseous ethylene, and purging the upper gaseous space leads to an outlet of gaseous ethylene from the reactor that is detrimental to the performance and cost of the process. Processes described in applications WO2019 / 011806 ίαηη / ζζηζ / E / γίΛΐ and WO2019 / 011609 allow for increasing the contact surface area between the upper part of the liquid fraction and the upper gaseous space by means of dispersion or vortices to facilitate the passage of ethylene contained in the upper gaseous space into the liquid phase at the liquid / gas interface. These processes do not limit the penetration phenomenon and are insufficient when the amount of ethylene in the upper gaseous space is significant due to a high penetration rate. Furthermore, this research has revealed that in a reactor operating at a constant flow rate of injected ethylene gas, the amount of dissolved ethylene, and therefore the penetration rate, depends on the dimensions of the reactors implementing the process, and in particular the height of the liquid phase. In fact, the lower the height, the shorter the time it takes for the gaseous ethylene to travel through the liquid phase to dissolve, and the higher the penetration rate. It has been discovered that it is possible to improve the conversion of olefins, maintaining high selectivity for desired linear olefins and in particular for alpha-olefins, by limiting penetration phenomena by means of a gas / liquid reactor that has successive zones of decreasing diameter from the bottom to the top of the reactor. Advantageously, a reactor according to the present invention allows increasing the height of the reactor and therefore the height of the liquid phase without modifying the volume of the reactor or the liquid phase used in an oligomerization reaction, which has the effect of improving the dissolution of oily ethylene and thus limiting the penetration phenomenon for a given volume of liquid phase. The invention thus allows, for a given volume of liquid phase, an increase in the height of the liquid phase with respect to a reactor of constant diameter. The invention also relates to a process for oligomerizing defins and in particular ethylene using the successive zone reactor of decreasing diameter according to the invention. Brief Description of the Invention Therefore, the present invention relates to a qas / liquid reactor with consecutive zones of decreasing diameters comprising: a reaction chamber 1, elongated along the vertical axis, a means of introducing gaseous ethylene 2, located at the bottom of the reaction chamber, - a means 5 for removing a liquid reaction effluent located at the bottom of the reaction chamber, purging means 4 of a gaseous fraction located at the top of said reactor, in which RF LQnn / ZZnZ / E / YIAI ίοηη / ζζηζ / Ε / γίΛΐ the chamber is composed of n consecutive zones that have a decreasing diameter of Dn in the direction from the lower zone to the upper zone of the chamber, - the ratio (Dn / Dn-1) of the diameter of the upper zone, denoted Dn, to the diameter of the adjacent lower zone, denoted Dn-1, is less than or equal to 0.9 for a given zone, the ratio of the volume, denoted as Vn, to the total volume of the reaction chamber, denoted as Vtot, is between 0.2 and 0.8. The n consecutive zones are arranged in series along the vertical axis of the reactor, to define zones in the reaction chamber that have decreasing diameters from bottom to top and thus increase the height of a liquid phase that can be contained in the reaction chamber relative to the height of a reactor of constant diameter. In a preferred mode, the number n of zones is between 2 and 5. In a preferred embodiment, the ratio (Dn / Dn-1) of the diameter of an upper zone n to the diameter of the adjacent lower zone n-1 is between 0.1 and 0.9. In a preferred embodiment, the ratio (Hn / Hn-1) of the height of an upper zone n, called Hn, to the height of the adjacent lower zone n-1, called Hn-1, is between 0.2 and 3.0, preferably between 0.3 and 2.5. In a preferred embodiment, for a given zone, the ratio of the volume indicated as Vn, to the total volume, indicated as Vtot, (indicated as Vn / Vtot) of the reaction chamber corresponding to the sum of the n zones is between 0.2 and 0.8, preferably between 0.25 and 0.75. In a preferred embodiment, the n zones that make up such a chamber are formed by the assembly of cylinders of decreasing diameter. In a preferred embodiment, the n zones that make up such a chamber are formed by interiors placed inside the reaction chamber to reduce its diameter in a given zone. In a preferred embodiment, the reactor further comprises a recirculation circuit comprising an extraction means at the bottom of the reaction chamber, preferably at the bottom, for removing a liquid fraction to one or more heat exchangers suitable for cooling the liquid fraction and a means for introducing the cooled fraction to the top of the reaction chamber. In a preferred embodiment, the reactor also comprises a means for extracting a gaseous fraction at the level of the upper gaseous free space of the reaction chamber and a means for introducing the extracted gaseous fraction in liquid phase to the lower part of the reaction chamber. Another object of the present invention relates to a process for the oligomerization of gaseous ethylene that implements the reactor according to any of the above embodiments. In a preferred embodiment, the process is carried out at a pressure between 0.1 and 10.0 MPa, at a temperature between 30 and 200°C, comprising the following steps: a step a) of introducing a catalytic oligomerization system comprising a metal catalyst and an activating agent into a reaction chamber, a step b) of contacting the catalytic system with gaseous ethylene by introducing the gaseous ethylene into the lower zone of the reaction chamber, a step c) of extracting a liquid fraction, a step d) of cooling the fraction extracted in step c) by passing the fraction through a heat exchanger, a step e) of introducing the fraction cooled in step d) to the upper part of the lower zone of the reaction chamber. In a preferred embodiment, the method further comprises a recycling step of a gaseous fraction extracted from the upper zone of the reaction chamber and introduced at the level of the lower part of the reaction chamber into the liquid phase. DEFINITIONS AND ABBREVIATIONS Throughout this description, the following terms or abbreviations have the following meaning. Lonn / zznz / E / YiAi The term oligomerization refers to any addition reaction of a first olefin to a second olefin, identical or different from the first, and includes dimerization, trimerization, and tetramerization. The olefin thus obtained is of the type CnH2n, where n is greater than or equal to 4. The term olefin designates both a single olefin and a mixture of olefins. The term alpha-olefin designates an olefin, in which the double bond is located at the terminal position of the alkyl chain. The term heteroatom refers to an atom other than carbon and hydrogen. A heteroatom can be chosen from among oxygen, sulfur, nitrogen, phosphorus, silicon, and halides such as fluorine, chlorine, bromine, or iodine. The term hydrocarbon is an organic compound formed exclusively of carbon (C) and hydrogen (H) atoms with the molecular formula CmHp, where m and p are natural whole numbers. The term catalytic system designates a mixture of at least one metallic precursor, at least one activating agent, optionally at least one additive, and optionally at least one solvent. The term alkyl refers to a hydrocarbon chain comprising from 1 to 20 carbon atoms, preferably from 2 to 15 carbon atoms and even more preferably from 2 to 8 carbon atoms, denoted C1-C20 alkyl, saturated or unsaturated, linear or branched, non-cyclic, cyclic, or polycyclic. For example, Ci-Ce alkyl means an alkyl group chosen from methyl, ethyl, propyl, butyl, pentyl, cyclopentyl, hexyl, and cyclohexyl groups. The term aryl is an aromatic group, mono- or polycyclic, fused or not, comprising between 6 and 30 carbon atoms, denoted as C6-C30 aryl. The term alkoxy is a monovalent radical consisting of an alkyl group attached to an oxygen atom, such as the C4H9O- group. The term aryloxy is a monovalent radical consisting of an aryl group attached to an oxygen atom, such as the CgHsO- group. The term liquid phase means the mixture of all compounds that are in a liquid physical state under the temperature and pressure conditions of the reaction chamber; the phase may include gaseous compounds such as ethylene gas. The lower part is understood to be the part of the chamber located at the level of the liquid phase and the phase may comprise gaseous ethylene, reaction products such as the desired linear alpha definite (i.e., 1-butene, 1-hexene, 1-octene), one or more solvents and a catalytic system. The headspace or upper gas space in the chamber is understood as the upper part of the chamber in a gaseous state located at the top of the reaction chamber, that is, directly above the liquid phase and made up of a mixture of compounds that are in a gaseous physical state during the implementation of a reactor in an oligomerization process. The lower side portion of the reaction chamber refers to a part of the reactor reaction chamber enclosure located at the bottom and side. Non-condensable gas is understood to be a species in the form of a physical gas that only partially dissolves in the liquid under the temperature and pressure conditions of the reaction chamber and that can, under certain conditions, accumulate at the top of the reactor (example in this case: ethane). t / h is understood to be the value of a flow expressed in tons per hour and kilograms / second, the value of a flow in kilograms per second. The terms reactor or device designate the set of means that allow the implementation of the oligomerization process according to the invention, such as in particular the reaction chamber and the recirculation circuit. The bottom of the reaction chamber means the lower quarter of the reaction chamber. The top of the reaction chamber means the upper quarter of the reaction chamber. The term bottom zone means the first zone according to the invention located at the bottom of the reaction chamber at the level of the bottom of the chamber. The upper zone is understood to be the last zone according to the invention located at the top of the reaction chamber at the level of the upper part of the chamber. Fresh gaseous ethylene means ethylene external to the process introduced in step b) by the process according to the invention. The penetration phenomenon refers to the passage of undissolved ethylene gas in the liquid phase into the gas at the top. The saturation rate is defined as the percentage of ethylene dissolved in the liquid phase relative to the maximum amount of ethylene that could be dissolved in the liquid phase, as determined by the thermodynamic equilibrium between the partial pressure of gaseous ethylene and the liquid phase. The saturation rate can be measured using gas chromatography. Brief Description of the Figures Figure 1 illustrates a gas / liquid reactor according to the prior art. This device comprises a reaction chamber 1 consisting of a lower portion comprising a liquid phase, an upper portion comprising a gaseous headspace or upper space, and a means for introducing gaseous ethylene 2 through a gas distributor 3 into the liquid phase. The upper portion includes a purge means 4. At the bottom of the reaction chamber 1, there is a conduit for removing a liquid fraction 5. The fraction 5 is divided into two streams: a first main stream 7 sent to a heat exchanger 8 and then introduced through a conduit 9 into the liquid phase, and a second stream 6 corresponding to the effluent sent to a subsequent step. The line 10 at the bottom of the reaction chamber allows for the introduction of the catalytic system. Figure 2 illustrates a gas / liquid reactor with consecutive zones of decreasing diameter according to the invention. The reactor differs from the reactor in Figure 1 in that it comprises two zones of different diameters. Zone 1, located at the bottom of the reaction chamber, has a larger diameter than the zone located at the top of the chamber. The first zone at the bottom is characterized by its diameter, denoted as DI, and its height, H1; these two parameters define the volume, denoted as VI, of the zone. Likewise, the second zone, located at the top, is characterized by its height, denoted as H2, and its diameter, denoted as D2, which is smaller than DI, defining the volume, V2, of the second zone. In this embodiment, the two zones that constitute the reaction chamber 1 are formed of cylinders of decreasing diameter. Figure 3 illustrates another modality that differs from that of Figure 2 in that the second zone located at the top of the reaction chamber 1 is delimited by an internal component 11 located inside the reaction chamber 1. Figure 4 illustrates another modality that differs from that of Figure 2 in that the reaction chamber 1 comprises three consecutive zones of decreasing diameter. Figures 2, 3 and 4 schematically illustrate particular modalities of the subject matter of the present invention without limiting its scope. Detailed Description of the Invention It is specified that, throughout this description, the expression "included between... and..." should be understood to include the aforementioned limits. Within the meaning of the present invention, the various embodiments presented can be used alone or in combination with each other, without limitation of combination when technically feasible. According to the present invention, the various parameter ranges for a given step, such as pressure ranges and temperature ranges, can be used alone or in combination. For example, according to the present invention, a preferred pressure range can be combined with a more preferred temperature range. Therefore, the present invention relates to a RF LQnn / ZZnZ / E / YIAI Lonn / zznz / E / YiAi gas / liquid oligomerization reactor with consecutive zones of decreasing diameter comprising: a reaction chamber 1, elongated along the vertical axis, a means for introducing gaseous ethylene 2, located at the bottom of the reaction chamber, a means 5 for removing a liquid reaction effluent located at the bottom of the reaction chamber, means for purging a gaseous fraction 4 located at the top of said reactor, in which: The chamber is composed of n consecutive zones that have decreasing diameter Dn from the lower zone to the upper zone of the chamber; the ratio (Dn / Dn-1) of the diameter of the upper zone, denoted as Dn, to the diameter of the adjacent lower zone, denoted as Dn-1, is less than or equal to 0.9. - For a given zone, the ratio of the volume, indicated as Vn, to the total volume of the reaction chamber, indicated as Vtot, is between 0.2 and 0.8. Advantageously, a reactor according to the present invention allows increasing the height of the reactor and therefore of the liquid phase without modifying the volume of liquid used in an oligomerization reaction, which has the effect of improving the dissolution of gaseous ethylene and thus limiting the penetration phenomenon for a given volume of phase Lonn / zznz / E / YiAi liquid. A Reaction Chamber The reaction chamber 1 according to the invention thus comprises: In consecutive zones having a decreasing diameter Dn from the lower zone to the upper zone of the chamber, the ratio (Dn / Dn-1) of the diameter of the upper zone, denoted as Dn, to the diameter of the adjacent lower zone, denoted as Dn-1, is less than or equal to 0.9. For a given zone, the volume ratio, denoted as Vn, to the total volume of the reaction chamber, denoted as Vtot, is between 0.2 and 0.8. n consecutive zones according to the invention are arranged in series along the vertical axis of the reactor to define zones in the reaction chamber that have decreasing diameters from bottom to top and thus increase the height of the liquid phase that can be contained in the reaction chamber in relation to the height of a reactor of constant diameter and therefore the time during which the ethylene is present in liquid phase to improve its dissolution. Advantageously, for a given reaction chamber volume and therefore a given liquid volume, the n consecutive zones of decreasing diameter in the reaction chamber allow for an increase in the height of the liquid that can be contained in such a chamber and thus the residence time of the gaseous ethylene introduced into the liquid phase. In this way, the present invention allows for an increase in the amount of ethylene dissolved in the liquid phase and therefore limits the penetration phenomenon. Preferably, the reaction chamber comprises a number n of zones between 2 and 5, preferably between 2 and 4 and preferably n is equal to 2, 3, 4 or 5. The ratio (Dn / Dn-1) of the diameter of an upper zone n, denoted as Dn, to the diameter of the adjacent lower zone n-1, denoted as Dn-1, is less than or equal to 0.9. Preferably, the ratio Dn / Dn-1 is between 0.1 and 0.9, preferably between 0.15 and 0.85, preferably between 0.2 and 0.8, more preferably between 0.25 and 0.75, and most preferably between 0.3 and 0.7. The n zones that make up the reaction chamber have a total height, called Htot, whose sum is equal to the total height of the reaction chamber. Advantageously, the ratio (Hn / Hn-1) of the height of an upper zone n, denoted as Hn, to the height of the adjacent lower zone n-1, denoted as Hn-1, is between 0.2 and 3.0, preferably between 0.3 and 2.5, preferably between 0.4 and 2.0, preferably between 0.5 and 1.5 and most preferably between 0.6 and 1.0. Preferably, for a given area, the ratio of the volume indicated as Vn, to the total volume, indicated as Vtot, The RF LQnn / ZZnZ / E / YIAI (indicated as Vn / Vtot) of the reaction chamber corresponding to the sum of the n zones is between 0.2 and 0.8. Preferably, the ratio (Vn / Vtot) is between 0.25 and 0.75, preferably between 0.3 and 0.7 and more preferably between 0.35 and 0.65. Preferably, the reaction chamber is cylindrical in shape and has a ratio between the total height of the chamber and the diameter of the lower part of the chamber (referred to as Htot / Dl) of between 1 and 17, preferably between 1 and 8 and preferably between 2 and 7. In a particular embodiment shown in Figure 2, the n zones that make up the chamber are formed from cylinders of decreasing diameter. The cylinders are interconnected by walls perpendicular to the vertical axis or forming an angle α of between 90 and 160° with the vertical axis, as shown in Figure 2, to facilitate and, above all, not block the ascent of ethylene gas bubbles into the liquid phase. Preferably, the angle is between 95 and 145°, and more preferably between 100 and 130°. In a second, specific configuration shown in Figure 3, the n zones that make up the chamber are formed by internal components arranged inside the reaction chamber to reduce its diameter in a given area. The internal components can be, for example, solid metal walls. Advantageously, regardless of the method used, the reaction chamber assembly is achieved by securing the cylinders and / or internal components, for example, by welding, gluing, bolting, bolting alone or in combination, or any other similar means. Preferably, the assembly is achieved by welding. Preferably, the reaction chamber also comprises a means for purging non-condensable gases to the level of the upper gas space. Preferably, the reaction chamber also comprises a pressure sensor, allowing the pressure within the reaction chamber to be monitored and, preferably, maintained constant. Preferably, in the event of a pressure drop, the pressure is maintained constant by introducing ethylene gas into the reaction chamber. A Means for Introducing Ethylene Gas According to the invention, the reaction chamber comprises a means for introducing ethylene gas located at the bottom of said chamber, more particularly in the lower side part. Preferably, the means of introducing ethylene is chosen from a duct, a network of ducts, a multi-tubular distributor, a perforated plate, or any other means known to the person experienced in the art. rf Lonn / zznz / E / YiAi In one particular modality, the means of introducing ethylene are located in the recirculation circuit. Preferably, a gas distributor, which is a device that allows the gas phase to disperse uniformly throughout the liquid section, is placed at the end of the introduction means within the reaction chamber. The device comprises a network of perforated tubes, the diameter of which ranges from 1.0 to 12.0 mm, preferably from 3.0 to 10.0 mm, to form millimeter-sized ethylene bubbles in the liquid. An Optional Means of Introducing the Catalytic System According to the invention, the reaction chamber comprises a means of introducing the catalytic system. Preferably, the means of introduction are located at the bottom of the chamber. According to one variant of the modality, the introduction of the catalytic system is carried out in the recirculation circuit. The means of introducing the catalytic system is chosen from any means known to the person experienced in the technique and is preferably a conduit. In the mode where the catalytic system is implemented in the presence of a solvent or a mixture of solvents, the solvent or mixture of solvents is introduced by an introduction means located at the bottom of the reaction chamber or even into the recirculation circuit. An Optional Recirculation Loop Advantageously, the homogeneity of the liquid phase, as well as the regulation of the temperature within the reaction chamber of the reactor according to the invention, can be achieved by using a recirculation circuit comprising an extraction means at the bottom of the reaction chamber, preferably at the bottom, to carry out the extraction of a liquid fraction to one or more heat exchangers that allow the cooling of the liquid fraction and a means for introducing the cooled liquid fraction to the top of the reaction chamber, preferably at the level of the liquid phase. The recirculation circuit allows for good homogenization of concentrations and temperature control in the liquid phase within the reaction chamber. Advantageously, the implementation of a recirculation loop allows inducing a direction of circulation of the liquid phase to the reaction chamber from the top to the bottom of the chamber, which allows increasing the residence time of the ethylene by slowing its ascent to the liquid phase and thus further limiting the penetration phenomenon. The recirculation circuit can be advantageously implemented by any means necessary and known to the RF LQnn / ZZnZ / E / YIAI experienced in the art, such as a pump to extract the liquid fraction, a means capable of regulating the flow of the extracted liquid fraction or another conduit to purge at least part of the liquid fraction. Preferably, the means of extraction and means of introducing the liquid fraction of the reaction chamber consist of a conduit or pipe. The heat exchanger or exchangers capable of cooling the liquid fraction are chosen from any means known to the person experienced in the art. An Optional Overhead Gas Recycling Circuit Advantageously, the gas / liquid oligomerization reactor with consecutive zones of varying diameter also includes a circuit or loop for recycling the overhead gas at the bottom of the reaction chamber at the liquid phase level. The circuit comprises means for extracting a gaseous fraction from the overhead gas level of the reaction chamber and means for introducing the extracted gaseous fraction into the liquid phase at the bottom of the reaction chamber. The recirculation circuit advantageously allows compensating for the penetration phenomenon and preventing the increase in pressure in the reaction chamber, maintaining the saturation of dissolved ethylene in the liquid phase at a desired value. Another advantage of the recycling circuit or loop is to improve the volumetric productivity of the device and therefore RF LQnn / ZZnZ / E / YIAI reduce costs. In a preferred configuration, the recycling circuit also includes a compressor. In one modality, the introduction of the extracted gaseous fraction is carried out through the means of introducing ethylene gas. In another embodiment, the introduction of the extracted gaseous fraction is carried out by means of a gas distributor, a device that allows the gaseous phase to be dispersed uniformly throughout the liquid section and is positioned at the end of the introduction means within the reaction chamber. The device comprises a network of perforated conduits or tubes, with an orifice diameter between 1.0 and 12.0 mm, preferably between 3.0 and 10.0 mm, to form millimeter-sized ethylene bubbles in the liquid. Preferably, the means of introducing the extracted gas fraction is chosen from a conduit or tube, a network of conduits, a multi-tubular distributor, a perforated plate, or any other means known to the person experienced in the art. Oligomerization Process Another object of the present invention covers an oligomerization process that implements the reactor with zones of variable diameter according to the invention as described above. Preferably, in a gas / liquid reactor, the flow The flow of ethylene gas introduced in step b), as defined below, is controlled by the pressure in the reaction chamber. Thus, if the pressure in the reactor increases due to a high rate of ethylene penetration into the upper gas space, the flow of ethylene gas introduced in step b), as defined below, decreases, leading to a decrease in the amount of ethylene dissolved in the liquid phase, and therefore, a decrease in ethylene saturation. This reduction is detrimental to ethylene conversion and is accompanied by a reduction in reactor productivity and, inevitably, in its selectivity. Advantageously, the implementation of the reactor with variable diameter zones according to the invention in an oligomerization process, preferably by homogeneous catalysis, allows a saturation rate of dissolved ethylene in the liquid phase greater than 70.0%, preferably between 70.0 and 100%, preferably between 80.0 and 100%, preferably between 80.0 and 99.0%, preferably between 85.0 and 99.0%, and even more preferably between 90.0 and 98.0%. The saturation rate of dissolved ethylene can be measured by any method known to those experienced in the technique and, for example, by gas chromatographic analysis (commonly called GC) of a fraction of the liquid phase extracted from the reaction chamber. The procedure implemented by the zone reactor RF LQnn / ZZnZ / E / YIAI variable diameter, according to the invention, allows obtaining linear fins and in particular linear alpha-olefins by contacting the fin or olefins with a catalytic system, optionally in the presence of an additive and / or a solvent and by implementing the gas / liquid reactor with variable diameter zones. All catalytic systems known to those experienced in the art and capable of being implemented in dimerization, trimerization, tetramerization processes and more generally in oligomerization processes according to the invention, are part of the field of the invention. Catalytic systems, as well as their implementations, are described in particular in applications FR2984311, FR2552079, FR3019064, FR3023183, FR3042989 or even in application FR3045414. Preferably, catalytic systems comprise, preferably consist of: a metallic precursor preferably based on nickel, titanium or chromium, an activating agent, - optionally an additive and optionally a solvent. The Metallic Precursor The metallic precursor used in the catalytic system is RF LQnn / ZZnZ / E / YIAI chooses from nickel, titanium or chromium-based compounds. In one embodiment, the metallic precursor is nickel-based and preferably comprises nickel with an oxidation degree (+11). Preferably, the nickel precursor is selected from nickel(II) carboxylates such as, for example, nickel 2-ethylhexanoate, nickel(II) phenates, nickel(II) naphthenates, nickel(II) acetate, nickel(II) trifluoroacetate, nickel(II) triflate, nickel(II) acetylacetonate, nickel(II) hexafluoroacetylacetonate, π-allylnickel(II) chloride, π-nickel(II) bromide, TT-allylnickel(II), metalylnickel(II) chloride dimer, η3-allylnickel(II) hexafluorophosphonate, rp-metalylnickel(II) hexafluorophosphate, and nickel(II) 1,5cyclooctadienyl, in their hydrated or dehydrated form, alone or in mixture. In a second embodiment, the metallic precursor is titanium-based and preferably comprises an aryloxy or alkoxy titanium compound. The titanium alkoxy compound advantageously corresponds to the general formula [Ti(OR)4] in which R is a linear or branched alkyl radical. Preferred alkoxy radicals include, but are not limited to, tetraethoxy, tetraisopropoxy, tetra-r|-butoxy, and tetra-2-ethylhexyloxy. The titanium aryloxy compound advantageously corresponds to the general formula [Ti(OR')4] in which R' is an aryl radical substituted or not by alkyl or aryl groups. The R' radical may contain heteroatom-based substituents. The preferred aryloxy radicals are selected from phenoxy, 2-methylphenoxy, 2,6-dimethylphenoxy, 2,4,6-trimethylphenoxy, 4-methylphenoxy, 2-phenylphenoxy, 2,6-diphenylphenoxy, 2,4,6-triphenylphenoxy, 4-phenylphenoxy, 2-tert-butyl-6-phenylphenoxy, 2,4-ditert-butyl-6-phenylphenoxy, 2,6-diisopropylphenoxy, 2,6-ditert-butylphenoxy, 4-methyl-2,6-ditert-butylphenoxy, 2,6-dichloro-4-tert-butylphenoxy and 2,6-dibromo-4tert-butylphenoxy, the biphenoxy radical, binaphthoxy, 1,8-naphthalenedioxy. According to a third embodiment, the metallic precursor is chromium-based and preferably comprises a chromium(II) salt, a chromium(III) salt, or a salt with a different oxidation state, which may comprise one or more identical or different anions, such as halides, carboxylates, acetylacetonates, alkoxy anions, or aryloxy anions. Preferably, the chromium-based precursor is selected from CrCl3, CrCl1(tetrahydrofuran)3, Cr(acetylacetonate)3, Cr(naphthenate)3, Cr(2-ethylhexanoate)3, Cr(acetate)3. The concentration of nickel, titanium, or chromium is between 0.01 and 300.0 ppm by mass of atomic metal with respect to the reaction mass, preferably between 0.02 and 100.0 ppm, preferably between 0.03 and 50.0 ppm, more preferably between 0.5 and 20.0 ppm, and even more preferably between 2.0 and 50.0 ppm by mass of atomic metal with respect to the reaction mass. The Activating Agent Whatever the metal precursor, the catalytic system also comprises one or more activating agents selected from aluminum-based compounds, such as methylaluminum dichloride (MeAlCl₂), dichloroethylaluminum (EtAlCl₂), ethylaluminum sesquichloride (Et₃AlCl), chlorodiethylaluminum (Et₂AlCl), chlorodiisobutylaluminum (iBU₂AlCl₃), triethylaluminum (AlEts), tripropylaluminum (Al(nPr)₃), triisobutylaluminum (Al(i-Bu)₃), diethylethoxyaluminum (Et₂AlOEt), methylaluminoxane (MAO), ethylaluminoxane, and modified methylaluminoxanes (MMAO). The Additive Optionally, the catalytic system comprises one or more additives. When the catalytic system is based on nickel, the additive is selected from nitrogen-type compounds such as trimethylamine, triethylamine, pyrrole, 2,5-dimethylyrol, pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 2-methoxypyridine, 3-methoxypyridine, 4-methoxypyridine, 2fluoropyridine, 3-fluoropyridine, 3-trifluoromethylpyridine, 2 RF LQnn / ZZnZ / E / YIAI phenylpyridine, 3-phenylpyridine, 2-benzylpyridine, 3,5dimethylpyridine, 2,6-diterbutylpyridine and 2,6-diphenylpyridine, quinoline, 1,10-phenanthroline, N-methylpyrrole, N-butylpyrrole Nmethylimidazole, N-butylimidazole, 2,2'-bipyridine, N,N'dimethylethan-1,2-diimine, N,N'-di-t-butyl-etan-1,2-diimine, N,N'-di-t-butyl-butan-2,3-diimine, N,N'-diphenyl-etan-1,2diimine, N,N'-bis-(dimethyl-2,6-phenyl)-etan-1,2-diimine, N,N'bis- (diisopropyl-2,6-phenyl)-ethan-1,2-diimine, N,N'-diphenylbutan-2,3-diimine, N,N'-bis-(dimethyl-2,6-phenyl)-butan-2,3-diimine, N,Ν'-bis-(diisopropyl-2,6-phenyl)-butan-2,3-diimine, RF LQnn / ZZnZ / E / YIAI phosphine-type compounds chosen independently from tributylphosphine, triisopropylphosphine, tricyclopentylphosphine, tricyclohexylphosphine, triphenylphosphine, tris(o-tolyl)phosphine, bis(diphenylphosphine)ethane, trioctylphosphine oxide, triphenylphosphine oxide, triphenylphosphite, or compounds corresponding to general formula (I) or one of the tautomers of such compound: O AR' O^| ^A'R^Rí H(I) where * A and A', identical or different, are independently an oxygen or a single bond between the phosphorus atom and a carbon atom, * the Rlay Rlb groups are chosen independently from the methyl, trifluoromethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, pentyl, cyclohexyl, adamantyl groups, substituted or unsubstituted, containing or not heteroelements; phenyl, o-tolyl, m-tolyl, p-tolyl, mesityl, 3,5-dimethylphenyl, 4-n-butylphenyl, 2-methylphenyl, 4-methoxyphenyl, 2-methoxyphenyl, 3-methoxyphenyl, 4-methoxyphenyl, 2-isopropoxyphenyl, 4-methoxy-3,5-dimethylphenyl, 3,5-ditert-butyl-4-methoxyphenyl, 4-chlorophenyl, 3,5-di(trifluoromethyl)phenyl, benzyl, naphthyl, bisnaphthyl, pyridyl, bisphenyl, furanyl, thiophenyl, * the R2 group is chosen independently from the substituted groups methyl, trifluoromethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, pentyl, cyclohexyl, adamantyl or not, that contain heteroelements or not;phenyl, o-tolyl, m-tolyl, p-tolyl, mesityl, 3,5-dimethylphenyl, 4-n-butylphenyl, 4-methoxyphenyl, 2methoxyphenyl, 3-methoxyphenyl, 4-methoxyphenyl, 2isopropoxyphenyl, 4-methoxy-3,5-dimethyl-phenyl, 3,5-diterbutyl-4-methoxyphenyl, 4-chlorophenyl, 3, 5bis(trifluoromethyl)phenyl, benzyl, naphthyl, bisnaphthyl, pyridyl, bisphenyl, furanyl, thiophenyl.; When the catalytic system is based on titanium, the additive is chosen from diethyl ether, diisopropyl ether, dibutyl ether, diphenyl ether, 2-methoxy-2-metilpropano, 2methoxy-2-metilbutano, 2,2-dimethoxy-propano, di(2ethylhexyloxy)-2,2 propano, 2,5-dihydrofurano, tetrahydrofurano, 2-methoxytetrahydrofurano, 2-metiltetrahydrofurano, 3metiltetrahydrofurano, 2,3-dihydropyrano, tetrahydropyrano, 1,3-dioxolano, 1,3-dioxano, 1,4-dioxano, dimethoxyetano, di (2methoxyetil)éter, benzofurano, glima y diglima tomados solos o en mezcla. When the catalytic system is based on chromium, the additive is selected from nitrogen-type compounds such as trimethylamine, triethylamine, pyrrole, 2,5-dimethylyrrole, pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 2-methoxypyridine, 3-methoxypyridine, 4-methoxypyridine, 2fluoropyridine, 3-fluoropyridine, 3-trifluoromethylpyridine, 2phenylpyridine, 3-phenylpyridine, 2-benzylpyridine, 3, 5dimethylpyridine, 2,6-diterbutylpyridine and 2,6-diphenylpyridine, quinoline, 1,10-phenanthroline, N-methylpyrrole, N-butylpyrrole, N-methylimidazole, N-butylimidazole, 2,2'-bipyridine, N,N'-dimethylethan-1,2-diimine, N,N'-di-t-butyl-etan-1,2-diimine, N,N'-dit-butyl-butan-2,3-diimine, N,N'-diphenyl-ethan-1,2-diimine, N,N'-bis - (dimethyl-2,6-phenyl)-etan-1,2-diimine, N,N'-bis(diisopropyl-2,6-phenyl)-etan-1,2-diimine, N,N'-diphenyl-butan2,3-diimine, N,Ν'-bis-(dimethyl-2,6-phenyl)-butan-2,3-diimine, rf Lonn / zznz / E / YiAi N,N'-bis-(diisopropyl-2,6-phenyl)-butan-2,3-diimine y / o - aryloxy compounds of general formula [M (R3O) 2-nXn] wherein * M is chosen from magnesium, calcium, strontium and barium, preferably magnesium, * R3 is an aryl radical containing from 6 to 30 carbon atoms, X is a halogen or an alkyl radical containing from 1 to 20 carbon atoms, * n is an integer which can take the values ​​of 0 or 1 and * y is an integer between 1 and 10, preferably y is equal to 2, 3 or 4. Preferably, the aryloxy radical R3O is chosen from among 4-phenyl-1-phenoxy, 2-phenylphenoxy, 2,6-diphenylphenoxy, 2,4,6-triphenyl-1-phenoxy, 2,3,5,6-tetraphenyl-1-phenoxy, 2-tert-butyl-6-phenylphenoxy, 2,4-di-tert-butyl-6-phenylphenoxy, 2,6-diisopropyl-phenoxy, 2,6-dimethylphenoxy, 2,6-di-tert-butylphenoxy, 4-methyl-1,2,6-di-tert-butyl-1-phenoxy, 2,6-dichloro-4-tert-butylphenoxy, and 2,6-dibromo-4-tert-butylphenoxy. The two aryloxy radicals may be borne by the same molecule, such as the biphenoxy, binaphthoxy, or 1,8-naphthalenedioxy radicals. Preferably, the aryloxy radical R3O is 2,6-diphenylphenoxy, 2-tert-butyl-6-phenyl-1-phenoxy or 2,4-ditert-butyl-6-phenylphenoxy. Solvent rf Lonn / zznz / E / YiAi In another embodiment according to the invention, the catalytic system optionally comprises one or more solvents. The solvent is chosen from the group consisting of aliphatic and cycloaliphatic hydrocarbons such as hexane, cyclohexane, heptane, butane or isobutane. Preferably, the solvent used is cyclohexane. In one embodiment, a single solvent or a mixture of solvents can be used during the oligomerization reaction. Such a solvent is advantageously chosen independently of the group formed by aliphatic and cycloaliphatic hydrocarbons such as hexane, cyclohexane, heptane, butane, or isobutane. Preferably, the linear alpha-olefins obtained comprise from 4 to 20 carbon atoms, preferably from 4 to 18 carbon atoms, preferably from 4 to 10 carbon atoms, and preferably from 4 to 8 carbon atoms. Preferably, the olefins are linear alpha-olefins, selected from but-l-ene, hex-l-ene, or oct-l-ene. Advantageously, the oligomerization process is implemented at a pressure of between 0.1 and 10.0 MPa, preferably between 0.2 and 9.0 MPa and preferably between 0.3 and 8.0 MPa, at a temperature between 30 and 200°C, preferably between 35 and 150°C and more preferably between 45 and 140°C. Preferably, the catalyst concentration is between 0.01 and 500.0 ppm by mass of atomic metal with respect to the reaction mass, preferably between 0.05 rf Lonn / zznz / E / YiAi and 100.0 ppm, preferably between 0.1 and 50.0 ppm and preferably between 0.2 and 30.0 ppm by mass of atomic metal with respect to the reaction mass. According to another method, the oligomerization process is implemented continuously. The catalytic system, formed as described above, is injected simultaneously with the ethylene into a stirred reactor by conventional mechanical means known to those skilled in the art or by external recirculation and is maintained at the desired temperature. It is also possible to inject the components of the catalytic system separately into the reaction medium. The ethylene gas is introduced through a pressure-controlled inlet valve, which maintains constant pressure in the reactor. The reaction mixture is withdrawn by means of a valve controlled by the liquid level to maintain a constant level. The catalyst is continuously destroyed by any conventional means known to those skilled in the art, and the resulting reaction products, as well as the solvent, are then separated, for example, by distillation.Unconverted ethylene can be recycled back to the reactor. Catalyst residues included in a heavy fraction can be incinerated. Step a) Introduction of the catalytic system The method of implementing the variable diameter successive zone reactor according to the invention comprises a step a) of introducing a catalytic system comprising a metallic catalyst and an activating agent and optionally a solvent or a mixture of solvents, into a reaction chamber comprising a liquid phase in a lower part and an overhead gas in an upper part. Preferably, the introduction of the catalytic system is done in liquid phase to the lower part of the reaction chamber and preferably at the bottom of the reaction chamber. Preferably, the pressure of introduction to the reaction chamber is between 0.1 and 10.0 MPa, preferably between 0.2 and 9.0 MPa and preferably between 0.3 and 8.0 MPa. Preferably the temperature of introduction to the reaction chamber is between 30 and 200°C, preferably between 35 and 150°C and more preferably between 45 and 140°C. Step b) of contact with gaseous ethylene The procedure for implementing the variable-diameter zone reactor according to the invention comprises step b) of contacting the catalytic system introduced in step a) with gaseous ethylene. This gaseous ethylene is introduced into the liquid phase at the bottom of the reaction chamber, preferably at the lower side of the reaction chamber. The introduced ethylene gas comprises fresh ethylene gas, and preferably this fresh ethylene gas is combined with recycled ethylene gas in a separation step following the oligomerization process. During the implementation of the process according to the invention, after the introduction of ethylene gas, the liquid phase comprises undissolved ethylene gas. Therefore, depending on the zones of the reaction chamber, the liquid phase corresponds to a gas-liquid mixture, specifically between the liquid phase and gaseous ethylene. Preferably, the zone at the bottom of the reaction chamber below the ethylene gas introduction level comprises, or preferably consists of, the liquid phase without ethylene gas. Preferably, the gaseous ethylene is dispersed during its introduction into the liquid phase at the bottom of the reaction chamber by a means capable of achieving uniform dispersion throughout the reactor cross-section. Preferably, the dispersion media are selected from a distribution network with a homogeneous distribution of ethylene injection points throughout the reactor cross-section. Preferably, the velocity of the ethylene gas exiting the orifices is between 1.0 and 30.0 m / s. Its surface velocity (volumetric gas velocity divided by the cross-sectional area of ​​the reaction chamber) is between 0.5 and 10.0 centimeters / second and rf Lonn / zznz / E / YiAi preferably between 1.0 and 8.0 cm / s. Preferably, gaseous ethylene is introduced at a rate between 1 and 250 t / h, preferably between 3 and 200 t / h, preferably between 5 and 150 t / h and preferably between 10 and 100 t / h. Preferably, the flow of ethylene gas introduced in step b) is controlled by the pressure in the reaction chamber. According to a particular embodiment of the invention, a flow of hydrogen gas can also be introduced into the reaction chamber, representing a flow of 0.2 to 1.0% by mass of the incoming ethylene flow. Preferably, the hydrogen gas flow is introduced through the conduit implemented for the introduction of ethylene gas. Step c) Extraction of a fraction of the liquid phase The implementation procedure of the variable diameter zone reactor according to the invention comprises a step c) of extracting a fraction of the liquid phase preferably in the lower part of the reaction chamber. The extraction implemented in step c) is preferably carried out at the bottom of the reaction chamber, preferably below the gaseous ethylene injection level and preferably at the bottom of the chamber. The extraction is performed by any suitable means and preferably by means of a pump. RF LQnn / ZZnZ / E / YIAI Preferably, the extraction rate is between 500 and 10,000 t / h, preferably between 800 and 7,000 t / h. In one embodiment, a second stream is extracted from the liquid phase. The second stream corresponds to the effluent obtained at the end of the oligomerization process and can be sent to a separation section located downstream of the device implemented in the process according to the invention. According to a preferred embodiment, the liquid fraction extracted from the liquid phase is divided into two streams. The first stream, called the main stream, is sent to the cooling step (d), and the second stream, the effluent, is sent to the downstream separation section. Advantageously, the flow rate of the second stream is regulated to maintain a constant liquid level in the reactor. Preferably, the flow rate of this second stream is 5 to 200 times lower than the flow rate of the liquid sent to the cooling pass. Preferably, the flow rate of this effluent is 5 to 150 times lower, preferably 10 to 120 times lower, and most preferably 20 to 100 times lower. Step d) Cooling of the liquid fraction The procedure for implementing the reactor with variable diameter rc ίαηη / ζζηζ / E / γίΛΐ zones according to the invention comprises a step d) of cooling the liquid fraction extracted in step c). Preferably, the cooling step is implemented by circulating the main liquid flow extracted in step c), through one or more heat exchangers located inside or outside the reaction chamber and preferably to the outside. The heat exchanger allows the temperature of the liquid fraction to be reduced from 1.0 to 30.0°C, preferably from 2.0 to 20°C, preferably from 2.0 to 15.0°C, preferably from 2.5 to 10.0°C, preferably from 3.0 to 9.0°C, and preferably from 4.0 to 8.0°C. Advantageously, cooling the liquid fraction allows the temperature of the reaction medium to be maintained within the desired temperature ranges. Advantageously, the implementation of the liquid cooling step, through the recirculation circuit, also allows stirring the reaction medium and thus homogenizing the concentrations of the reactive species throughout the liquid volume of the reaction chamber. Step e) Introduction of the cooled liquid fraction The procedure for implementing the reactor with variable diameter zones according to the invention comprises a step e) of introducing the cooled liquid fraction into step d). RF LQnn / ZZnZ / E / YIAI The introduction of the cooled liquid fraction from step d) is carried out in the liquid phase of the reaction chamber, preferably in the upper part of the chamber, by any means known to the person experienced in the art. Advantageously, when the cooled fraction is introduced to the top of the liquid phase contained in the reaction chamber, a downward flow of the liquid phase is induced, which slows the ascent of ethylene gas into the liquid phase and thus improves ethylene dissolution. Therefore, combining this method with the variable-diameter zone reactor according to the invention further limits the penetration phenomenon. Preferably, the rate of introduction of the cooled liquid fraction is between 500 and 10,000 t / h, preferably between 800 and 7,000 t / h. Steps c) through e) constitute a recirculation loop. Advantageously, the recirculation loop allows the reaction medium to be stirred, thus homogenizing the concentrations of the reactive species throughout the liquid volume of the reaction chamber. Step f) Optional recycling of a gaseous fraction extracted from the gas at the top The procedure for implementing the variable diameter zone reactor according to the invention comprises a step f) of recycling a gaseous fraction extracted from the part RF LQnn / ZZnZ / E / YIAI gaseous upper part of the reaction chamber and introduced at the level of the lower part of the reaction chamber in liquid phase, preferably in the lower side of the reaction chamber, preferably at the bottom of the reaction chamber. The lower part designates the lower quarter of the reaction chamber. Step f) of recycling the gaseous fraction is also called the recycling loop. The extraction of the gaseous fraction in step f) is carried out by any means suitable for extraction, and preferably by means of a pump. One advantage of step f) of recycling is to allow simple and economical compensation for the phenomenon of penetration of gaseous ethylene into the upper gaseous space in an oligomerization process, whatever the dimensions of the reactor according to the invention. The penetration phenomenon refers to gaseous ethylene passing through the liquid phase without dissolving and becoming part of the gaseous overhead. When the injected ethylene gas flow rate and the overhead volume are fixed at a given value, penetration causes a pressure increase in the reaction chamber. In a gas / liquid reactor implemented according to a preferred method, the rate of ethylene introduction in step b) is controlled by the pressure in the reaction chamber. Thus, if the reactor pressure increases due to a high rate of ethylene penetration into the overhead, the flow rate of gaseous ethylene introduced in step b) decreases, leading to a decrease in the amount of ethylene dissolved in the liquid phase and therefore a decrease in saturation. This decrease in saturation is detrimental to ethylene conversion and is accompanied by a decrease in reactor productivity.The recycling step of a gaseous fraction according to the invention thus allows for optimizing the saturation of dissolved ethylene and therefore improving the volumetric productivity of the process. The gaseous fraction extracted in step f) can be introduced into the reaction chamber alone or mixed with the gaseous ethylene introduced in step b). Preferably, the gaseous fraction is introduced mixed with the gaseous ethylene introduced in step b). In one particular embodiment, the gaseous fraction extracted in step f) is introduced into the reaction chamber by liquid-phase dispersion at the bottom of the reaction chamber through a means capable of achieving uniform dispersion throughout the reactor cross-section. Preferably, the dispersion media are selected from a distribution network with a homogeneous distribution of RF LQnn / ZZnZ / E / YIAI injection points of the gaseous fraction extracted in the step f) throughout the reactor section. Preferably, the velocity of the gas fraction extracted at the outlet of the orifices is between 1.0 and 30.0 m / s. Its surface velocity (volumetric gas velocity divided by the cross-sectional area of ​​the reaction chamber) is between 0.5 and 10.0 centimeters / second and preferably between 1.0 and 8.0 cm / s. Preferably, the extraction rate of the gaseous fraction is between 0.1 and 100% of the rate of gaseous ethylene introduced in step b), preferably between 0.5 and 90.0%, preferably between 1.0 and 80.0%, preferably between 2.0 and 70.0%, preferably between 4.0 and 60.0%, preferably between 5.0 and 50.0%, preferably between 10.0 and 40.0% and preferably between 15.0 and 30.0%. Advantageously, the extraction rate of the gaseous fraction in step f) is controlled by the pressure inside the reaction chamber, allowing the pressure to be maintained at a desired value or range of values ​​and thus compensating for the phenomenon of ethylene gas penetration into the upper gaseous space. In one particular modality, the gaseous fraction extracted in step f) is divided into two flows, a first flow called main is recycled directly to the reaction chamber and a second gaseous flow. In a preferred embodiment, the second gas flow RF LQnn / ZZnZ / E / YIAI corresponds to a purge of the upper gaseous part, which allows the removal of some of the non-condensable gases. Preferably, the flow rate of the second gas flow is between 0.005 and 1.00% of the flow rate of ethylene introduced in step b), preferably between 0.01 and 0.50%. EXAMPLES The following examples illustrate the invention without limiting its scope. Example 1: Comparison corresponding to figure 1 Example 1 implements a prior art gas / liquid oligomerization reactor, as described in Figure 1, comprising a cylindrical reaction chamber having a diameter of 2.63 m and a liquid height of 4.31 m. Implementation of the ethylene oligomerization process according to the prior art, at a pressure of 7.0 MPa and a temperature of 130°C comprising the following steps: the chromium-based catalytic system, as described in patent FR3019064, is introduced into the liquid phase of the reaction chamber in the presence of cyclohexane as solvent, with a ratio of the incoming solvent mass flow to the incoming ethylene mass flow of 1; the catalytic system is then contacted with gas RC LQnn / ZZnZ / E / YIAI ethylene by introducing ethylene gas to the bottom of such a chamber, the reaction effluent is recovered. The performance of this reactor allows the conversion of 79.6% of the injected gaseous ethylene and a selectivity of 78.8% for hexene-1. This reactor allows a saturation of dissolved ethylene of 60.0% to be obtained, measured by gas chromatographic analysis of a sample of the liquid phase extracted from the reaction chamber. Example 2: according to the invention corresponding to Figure 2 A reactor according to the invention is implemented having two zones of decreasing diameter under the same conditions as Example 1. The following table presents the liquid-phase ethylene saturation results for four reactors with identical total volumes, but with different dimensions (in meters) for the two zones according to the invention. The zone at the bottom of the reaction chamber is designated as 1, and its height, diameter, and corresponding volume are designated H1, D1, and V1, respectively. The zone at the top of the reaction chamber is designated as 2, and its height, diameter, and corresponding volume are designated H2, D2, and V2, respectively. The saturation rate is measured by gas chromatographic rf Lonn / zznz / E / YiAi analysis of a sample of the liquid phase extracted from the reaction chamber. RF LQnn / ZZnZ / E / YIAI Reactor 1 Reactor 2 Reactor 3 Reactor 4 Lower zone height (Hl) 3.017 3.017 1.293 1.293 Lower zone diameter (DI) 2.63 2.63 2.63 2.63 Lower zone volume (VI) 16.39 16.39 7.02 7.02 Upper zone height (H2) 2.02 5.17 4.71 12.07 Upper zone diameter (D2) 2.02 1.315 2.104 1.315 Upper zone volume (V2) 7.02 7.02 16.39 16.39 Total volume 23.4 23.4 23.4 23.4 Saturation rate (%) 77 95 84 97 Ethylene conversion (%) 73.9 67.5 71.5 66.9 Selectivity 1-hexene (%) 83.4 86.9 84.9 87.2 The results presented are obtained for a mass ratio of the injected solvent flow rate to the injected ethylene gas flow rate equal to 1. These results clearly illustrate the performance gain provided by implementing a reactor according to the invention. Thus, a reactor according to the present invention allows for better ethylene saturation of the liquid phase and therefore better selectivity. Lonn / zznz / E / YiAi for the target product, in this case 1-hexene, for the same total reactor volume and for an identical residence time. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. A gas / liquid oligomerization reactor, characterized in that it comprises: a reaction chamber, elongated along a vertical axis, a means for introducing gaseous ethylene, located at the bottom of the reaction chamber, a means for extracting a liquid reaction effluent located at the bottom of the reaction chamber, means for purging a gaseous fraction located at the top of said reactor, wherein: the chamber is composed of n consecutive zones of decreasing diameter Dn in the direction from the lower zone to the upper zone of the chamber, the ratio (Dn / Dn-1) of the diameter of the upper zone Dn to the diameter of the adjacent lower zone Dn-1 is less than or equal to 0.9, for a given zone, the ratio of the volume Vn to the total volume of the reaction chamber, Vtot, (Vn / Vtot) is between 0.2 and 0.8.The n consecutive zones are arranged in series along the vertical axis of the reactor to define zones in the reaction chamber that have decreasing diameters from bottom to top and thus increase the height of a liquid phase that can be contained in the reaction chamber relative to the height of a reactor of constant diameter.

2. The reactor according to claim 1, characterized in that the number n of zones is between 2 and 5.

3. The reactor according to any of the preceding claims, characterized in that the ratio (Dn / Dn-1) of the diameter of an upper zone n to the diameter of the adjacent lower zone n-1 is between 0.1 and 0.

9.

4. The reactor according to any of the preceding claims, characterized in that the ratio (Hn / Hn-1) of the height of an upper zone n, referred to as Hn, to the height of the adjacent lower zone n-1, referred to as Hn-1, is between 0.2 and 3.0, preferably between 0.3 and 2.

5.

5. The reactor according to any of the preceding claims, characterized in that for a given zone, the ratio of the volume referred to as Vn, to the total volume, referred to as Vtot, (referred to as Vn / Vtot) of the reaction chamber corresponding to the sum of the n zones is between 0.25 and 0.

75.

6. The reactor according to any of the preceding claims, characterized in that the n zones that constitute the chamber are formed of cylinders of decreasing diameter.

7. The reactor according to any of claims 1 to 5, characterized in that the n zones that make up the chamber are formed of internal components located inside the reaction chamber to reduce its diameter in a specific zone.

8. The reactor according to any of the preceding claims, characterized in that it further comprises a recirculation circuit comprising an extraction means at the bottom of the reaction chamber, preferably at the bottom, for extracting a liquid fraction to one or more heat exchangers suitable for cooling the liquid fraction and a means for introducing the cooled fraction to the top of the reaction chamber.

9. The reactor according to any of the preceding claims, characterized in that it further comprises means for extracting a gaseous fraction at the level of the upper gaseous space of the reaction chamber and means for introducing the extracted gaseous fraction in liquid phase into the lower part of the reaction chamber.

10. An oligomerization process implementing the reactor according to any of claims 1 to 9, characterized in that the process is implemented at a pressure between 0.1 and 10.0 MPa, at a temperature between 30 and 200°C comprising the following steps: - a step a) introducing a catalytic oligomerization system comprising a metal catalyst and an activating agent into a reaction chamber, b) contacting the catalytic system with gaseous ethylene by introducing the gaseous ethylene into the lower zone of the reaction chamber, c) extracting a liquid fraction, d) cooling the fraction extracted in step c) by passing the fraction through a heat exchanger, e) introducing the cooled fraction in step d) into the upper part of the lower zone of the reaction chamber.

11. The process according to claim 10, characterized in that it further comprises a step of recycling a gaseous fraction extracted from the upper zone of the reaction chamber and introduced at the level of the lower part of the reaction chamber to the liquid phase.