EP1451269A1

EP1451269A1 - Two-step method for middle distillate hydrotreatment comprising two hydrogen recycling loops

Info

Publication number: EP1451269A1
Application number: EP01274749A
Authority: EP
Inventors: Renaud Galeazzi
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 2001-11-22
Filing date: 2001-11-22
Publication date: 2004-09-01
Anticipated expiration: 2021-11-22
Also published as: DE60130045T2; JP2005509728A; KR100813773B1; ES2292537T3; WO2003044132A1; EP1451269B1; KR20040096494A; ATE370213T1; DE60130045D1; AU2002222019A1; US7384540B2; US20050167334A1

Abstract

The invention concerns a method for hydrotreating of a hydrocarbon feedstock, comprising at least two steps with intermediate stripping of the effluent of the first step with pressurized hydrogen substantially free of unwanted impurities for the second step catalyst, to eliminate in particular part of the H2S formed, each step being performed with a hydrogen recycling loop particular to said step.

Description

PROCESS FOR HYDROTREATING MEDIUM DISTILLATES IN TWO STEPS COMPRISING TWO HYDROGEN RECYCLING LOOPS

The present invention relates to the hydrotreatment of hydrocarbon fractions and for example gasolines or middle distillates, to produce hydrocarbon fractions with low sulfur, nitrogen and aromatic compounds content, usable in particular in the field of fuels for combustion engines. internal. These hydrocarbon fractions include jet fuel, diesel fuel, kerosene and gas oils. The invention makes it possible in particular to treat cuts of middle distillates with a very low cetane index, for example cuts comprising a significant proportion of catalytic cracked diesel (generally designated by LCO or “Light Cycle Oil”), to produce a fuel. with a high cetane number, desulphurized and partially de-aromatized.

Currently cuts of the middle distillate type, whether they come from the direct distillation of crude oil or whether they come from the catalytic cracking process, still contain significant quantities of aromatic compounds, nitrogen compounds and sulfur compounds. . Under the current legislative framework of the majority of industrialized countries, the fuel usable in diesel engines must contain a quantity of sulfur lower than approximately 500 parts per million in weight (ppm), this specification having to be lowered soon to 50 ppm, and probably further reduced in the medium term, for example to 10 ppm or even less. Currently, the aromatic content of diesel fuel is not specified in most countries, but in some cases the aromatic content of the base cuts used to meet the specification must be limited. 'cetane number.

The evolution of the specifications therefore makes it necessary to develop a reliable and efficient process making it possible to obtain from conventional middle distillate cuts of direct distillation or from catalytic cracking (LCO cut) or another conversion process. (coking, visbreaking, residue hydroconversion, etc.) a product having improved characteristics both with regard to the cetane number and the aromatic, nitrogen and, with particular importance, sulfur contents.

The present invention relates to a very efficient hydrotreatment process, which can in particular treat difficult loads and of poor quality to produce high quality fuels. It comprises at least two hydrotreatment stages with intermediate stripping of the effluent from the first stage with hydrogen under pressure substantially free of undesirable impurities for the catalyst of the second stage, in order to eliminate in particular a portion of PH2S formed, each step being carried out with a hydrogen recycling loop specific to this step. This process makes it possible to be able to independently choose the operating pressures of the two stages, to have in the second stage a very low content of pollutants, in particular H2S and water, and to be able to use in the second stage a catalyst based on a noble metal or comprising a noble metal in the best conditions of use of this catalyst, while having a high energy efficiency.

The present invention also relates to the substantially desulphurized and optionally partially de-aromatized hydrocarbon fraction obtained by means of the process according to the invention, and any fuel containing such a fraction.

Definitions and conventions, according to the invention:

Conventionally, the following notations, definitions and conventions will be used in the present description and according to the invention:

- ppm for parts per million, expressed by weight.

- Conventionally, the pressure at a reaction stage is called the pressure at the outlet of the reactor (or last reactor) of this stage. Conventionally, the pressure of a hydrogen-rich gas recycling loop will be called the suction pressure of the recycling compressor.

The term “purity”, applied to a gas containing hydrogen, and designated by the acronym “Pure” will designate a molar percentage of molecular hydrogen: for example a purity Pur1 of 85 of a gas will mean that this gas contains 85 % molar of hydrogen (molecular). Conventionally, we will call gas "rich in hydrogen", or

"Hydrogen" a gas whose molecular hydrogen purity is greater than about 50. Conventionally, the purity of a hydrogen-rich gas recycling loop (or purity of the gas in this loop) is the purity of the gas on suction of the recycling compressor for this loop. - The term “recycling loop”, or “hydrogen recycling loop”, is applied to the recycling of a hydrogen-rich gas to a reactor, after gas / liquid separation downstream of this reactor, and compression of the gas ( or part of the gas) to allow recycling. By extension, we will consider that a "recycling loop" also includes the lines and equipment crossed by the recycled gas, possibly mixed with a liquid phase (such as feed). In particular, a recycling loop comprises at least one recycling compressor, a hydrotreatment reactor, a gas / liquid separation flask downstream of this reactor, and / or possibly the part of a stripping column located above the feed, if this column is located on the recycled gas circuit: A recycling loop therefore includes the lines and equipment located along the path of the recycled gas; it can therefore also include branches and diversions: for example, part of the compressed gas can be taken from the recycled gas stream upstream of the reactor, and feed the reactor and / or a stripping column in the intermediate part, for use as a "quench" gas (according to English terminology, ie quenching) , or direct cooling), or as a stripping gas. The term “recycling loop” is therefore used to describe a closed circuit which may include, downstream of the recycling compressor, branches and parts of the circuit in parallel, following the path of the gas, these parts generally grouping upstream of the or compressors for recycling this loop. On the other hand, the term “recycling loop” excludes open circuits, such as circuits without recycling, or gas evacuation circuits, for example to other uses, for example gas evacuation circuits taken from a recycling loop, such as excess gas or purge gas.

- The charge of the process according to the invention designates a liquid stream of hydrocarbons supplying a hydrotreatment zone, but also a liquid stream recovered downstream from this hydrotreatment zone, which can for example be qualified as partially desulfurized charge , even if this partially desulphurized charge is chemically different from the initial charge, due in particular to the elimination of the sulfur from certain compounds, but also to the saturation of part of the aromatics and the elimination of a fraction of this charge due to the formation of gaseous light products at the level of the reaction stages, not recovered in liquid form - The term “hydrotreatment”, relating to the process according to the invention applies to the treatment of a hydrocarbon charge under a pressure d hydrogen, the total pressure being between approximately 2 and approximately 20 MPa, to preferably carry out one and / or several chemical reactions s from the group consisting of the following reactions: hydro-desulfurization, hydro-denitrogenation, hydro-demetallation (to remove one or more metals such as in particular vanadium, nickel, iron, sodium, titanium, silicon, copper), and hydrodearomatization.

PRIOR ART:

Hydrotreatment processes with at least two steps are already known, the first step generally being a desulphurization step, and the second (or last step) being either an advanced desulphurization step, or a de-aromatization step, or a combination of desulfurization and de-aromatization; each stage can comprise one or more reactors, one or more catalytic zones (or beds), and use identical or different catalysts.

The catalysts used in hydrotreatment (hydrodesulfurization, and / or hydrodemetallization, and / or hydrogenation, in particular of aromatics, and / or hydrodeazotation) generally comprise a porous mineral support, at least one metal or compound of metal from the group VIII of the periodic classification of the elements (this group comprising in particular cobalt, nickel, iron, rhodium, palladium, platinum etc.) and at least one metal or metal compound of group VI B of said periodic classification (this group notably comprising molybdenum, tungsten, etc.).

The sum of metals or metal compounds, expressed by weight of metal relative to the weight of the finished catalyst is often between 0.5 and 50% by weight.

The sum of the metals or metal compounds of group VIII, expressed by weight of metal relative to the weight of the finished catalyst is often between 0.5 and 15% by weight.

The sum of the metals or metal compounds of group VI B, expressed by weight of metal relative to the weight of the finished catalyst is often between 2 and 30% by weight.

The mineral support can comprise, without limitation, one of the following compounds: alumina, silica, zirconia, titanium oxide, magnesia, or two compounds chosen from the preceding compounds, for example silica-alumina or alumina-zirconia, or alumina-titanium oxide, or alumina-magnesia, or even three or more compounds chosen from the preceding compounds, for example silica-alumina-zirconia or silica-alumina-magnesia.

The support may also comprise, in part or in whole, a zeolite.

A frequently used support is alumina, or a support mainly composed of alumina (for example from 80 to 100% of alumina); such a support can also include one or more other promoter elements or compounds, based for example on phosphorus, magnesium, boron, silicon, or comprising a halogen. The support can for example comprise from 0.01 to 20% by weight of B2O3, or of SiO2, or of P2O5, or of a halogen (for example chlorine or of fluorine), or 0.01 to 20% by weight of an association of several of these promoters.

Common catalysts are for example catalysts based on cobalt and molybdenum, or else nickel and molybdenum, or else nickel and tungsten, on an alumina support, this support possibly comprising one or more promoters as mentioned above.

Other catalysts are also frequently used comprising at least one noble metal or compound of a noble metal, this noble metal often being rhodium, palladium or platinum, and most often palladium or platinum (or a mixture of these elements, for example palladium and platinum).

The quantity of noble metal or of noble metals is often comprised, for these catalysts, between 0.01 and approximately 10% by weight relative to the finished catalyst. These noble metal type catalysts are generally more effective than the so-called conventional catalysts, in particular in hydrogenation, and make it possible to operate at lower temperatures and with lower catalytic volumes. They are however more expensive and more sensitive to impurities.

The operating conditions which can be used in hydrotreatment are well known to those skilled in the art: The temperature is typically between approximately 200 and approximately 460 ° C.

The total pressure is typically between approximately 1 MPa and approximately 20 MPa, generally between 2 and 20 MPa, preferably between 2.5 and 18 MPa, and very preferably between 3 and 18 MPa.

The overall hourly space velocity of liquid charge for each catalytic step is typically between approximately 0.1 and approximately 12, and generally between approximately 0.4 and approximately 10.

The purity of the hydrogen in the recycling loop is typically between 50 and 100.

The amount of hydrogen relative to the liquid feed is for each catalytic step typically between approximately 50 and approximately 1200 Nm3 / m3, and often between approximately 100 and approximately 1000 Nm3 / m3 at the outlet of the reactor.

Other elements linked to the operating conditions, to gas purification techniques, and to the catalysts used in hydrotreatment will be found in the documents and patents published, and in particular, without limitation, in the documents or patents cited in the present description. , in particular in European patent application EP A 1 063 275, pages 5 and 6.

For the implementation of the invention, and for each hydrotreatment reactor, those skilled in the art may use one or more catalysts, and operating conditions set out in documents of the prior art, in particular those summarized in this application . The process according to the invention is however not linked to a particular hydrotreatment catalyst or to particular operating conditions, but can be used with any hydrotreatment catalyst (or catalysts) and all hydrotreatment operating conditions, already known. skilled in the art or that would be developed and known in the future.

US Pat. No. 6,221,239 describes the sequence of hydrotreatment with stripping of products with steam, and hydrodearomatization. Striping is a conventional steam stripping. The pressure and the operating conditions of the stripper, in particular the operating pressure, are not specified, and it can therefore be considered that it is a conventional stripper of hydrotreatment products. Such an H2S stripper, typically disposed at the outlet of a hydrotreating unit, operates under a relatively low pressure, conventionally 0.5 to 1.2 MPa. approximately, to promote stripping, and to avoid any condensation of water in the stripper itself. Hydrotreating strippers also operate under relatively low pressure in order to be able to use less steam, and moreover low or medium pressure steam, which is relatively cheaper than high pressure steam. A typical variant of installation according to this process therefore presents the sequence of two successive units: a hydrotreatment with its stripping of H2S with steam, then a de-aromatization unit, each unit having its hydrogen recycling loop. This process uses specific catalysts for the two units, including a noble metal catalyst.

This process makes it possible to obtain a desulphurization and a strong de-nitrogenization at the outlet of the first reaction section, and to use a noble metal catalyst for the hydrogenation section. It also makes it possible to have a high purity of hydrogen in the hydrogenation section, steam stripping possibly allowing elimination of light hydrocarbons, in addition to H2S. However, it has several drawbacks:

The first is linked to energy efficiency, which is not optimal since steam is used for stripping, steam which must be produced with significant energy consumption, then condensed.

In addition, the catalyst comprising a noble metal or a compound of a noble metal is not used under ideal conditions: steam stripping leads to significant water contents in the stripped liquid; however, the water content must be minimized, since water is typically unfavorable for the activity of hydrotreatment catalysts of this type.

The use of a stripper at low pressure requires pumping the stripped product with a high differential pressure. It also requires heating the stripper charge from a temperature of about 50 ° C to about 200 to 300 ° C (in a sequence of two hydrotreating units, the effluent from each unit is typically cooled to about 50 ° C). This requires the installation of numerous exchangers.

Patents are also known concerning hydrotreatment processes with two or more integrated stages with intermediate stripping of IΗ2S with high pressure hydrogen, these processes making it possible to operate in the second and / or last stage with a very low H2S content. :

US patent 5 1 14 562 describes a process for hydrotreating a middle distillate in at least two consecutive stages with a view to producing desulfurized and de-aromatized hydrocarbon cuts comprising a first hydrodesulfurization step, the effluent of which is sent to a zone stripping with hydrogen, in order to eliminate the hydrogen sulfide which it contains. The desulphurized fraction thus obtained is sent to a second reaction zone, in particular hydrogenation, comprising at least two reactors in series, in which the aromatic compounds are hydrogenated. The stripping area does not include reflux; the light hydrocarbons entrained at the top of the stripper, and which are recycled to hydrogenation and then to hydrodesulfurization, reduce the partial pressure of the hydrogen necessary for the reaction. This process uses a single recycling loop, which leads to substantially uniformity of hydrogen purity, in particular the content of light hydrocarbons having from 1 to 4 carbon atoms (C1, C2, C3, C4) in the various reaction zones and to use for these areas fairly similar operating pressures.

US Patent 5,110,444 describes a process comprising hydrotreating a middle distillate in at least three separate steps. The effluent from the first hydrodesulfurization step is sent to a hydrogen stripping zone to remove the hydrogen sulfide it contains. The desulfurized liquid fraction obtained is sent to a first hydrogenation zone, the effluent of which is in turn sent to a second stripping zone separate from the first. Finally, the liquid part from the second stripping zone is sent to a second hydrogenation zone. The stripping zone does not include reflux, and the light hydrocarbons entrained at the top of the stripper, are also recycled to hydrodesulfurization, which causes a reduction in the partial pressure of hydrogen in this stage. This process therefore uses a single recycling loop, with the same consequences as for the previously cited patent.

Patent application EP A 1 063 275 describes a process for hydrotreating a hydrocarbon feedstock comprising a hydrodesulfurization step, a step in which the partially desulfurized effluent is sent to a hydrogen stripping zone, then to a hydrotreatment step making it possible to obtain a partially de-aromatized and substantially desulfurized effluent. In this process, the gaseous effluent from the stripping step is cooled to a temperature sufficient to form a liquid fraction which is returned to the top of the stripping zone. The pressure of the hydrodesulfurization and hydrotreatment stages is between 2MPa and 20MPa. It is not suggested any advantage to operate at different pressures, higher or lower in one of these two stages.

It is also suggested to use one or two recycling compressors, these two compressors aspirating recycling gases from a common line, one of the two compressors aspirating the gas after a drying and desulfurization operation.

Due to their common origin (circulation in a common line), the gases supplying these two compressors have substantially identical compositions with regard to their light hydrocarbon contents (C1, C2, C3, C4). The process thus described therefore uses two recycling loops which mix at a common line, which can be assimilated to a single recycling loop and also leads to standardizing the purity of the gas. recycling at the different reaction zones and using relatively similar pressures.

All these hydrotreatment processes with intermediate stripping with hydrogen under pressure have common elements: The stripping is carried out with hydrogen (typically lean or free of water); the hydrogen after stripping is recycled in the hydrogen loop, which leads to the use of stripping at high pressure (that of the hydrogen loop), and makes it possible not to substantially depressurize the liquid effluent from the first stage before stripping. The two hydrotreatment stages are also strongly integrated with a common or mixed hydrogen loop (mixing of hydrogen from the different circuits).

Compared with the process of US Pat. No. 6,221,239, these processes have several advantages: water vapor is not used for stripping; no additional water is introduced into the stripped liquid; there is no need for a pump or only a relatively low differential pressure pump to transfer the stripped liquid.

However, they also have drawbacks:

The integrated hydrogen loop leads to the use of similar pressures for the two (or more) reaction stages, and prevents independent optimization of the operating pressures according to the specifications required of the final product. For the modification of existing units (whose operating pressure cannot be modified), the addition of a reaction step and an additional reactor, to obtain a product with more stringent specifications, requires designing and dimensioning this stage and this reactor at a pressure close to that of the existing unit, sometimes far from the optimal pressure.

Furthermore, when a catalyst sensitive to impurities is used in step a3), the integrated or mixed recycling loop leads either to treating the hydrogen in this loop by costly means, so as to remove these impurities, or to use a less expensive treatment and accept traces of impurities and conditions of use that are not optimal for the catalyst.

Finally, the integrated or mixed recycling loop leads to the presence of light hydrocarbons in the recycling gas of all the reaction stages, whereas these hydrocarbons are generally produced in large quantities only during the first desulphurization stage. This leads to lowering the purity of the hydrogen and the partial pressure of hydrogen (molecular), which reduces the speed of the hydrotreatment reactions.

The process with stripping with steam, on the one hand, and the integrated processes with intermediate stripping with hydrogen under pressure on the other hand do not have the same technical logic, and are not combinable: indeed , you can't stripper with both steam and hydrogen, neither operate this stripping at both high and low pressure, nor use both a non-integrated process and a highly integrated process.

Applicants have however surprisingly found that it is possible to implement a method having the advantages of the methods described above without having the disadvantages. The method according to the invention allows in particular:

- to be able to independently optimize the operating pressures of the various reaction stages,

- to have a high energy efficiency, not requiring the consumption of steam for the intermediate stripping between reaction stages, with typically a limited cooling and / or reheating of the product between the two reaction stages

(upstream / downstream of the stripping column).

- to be able to operate stripping without significant depressurization, and without requiring a pump to take up stripped liquid at high differential pressure,

- to be able to obtain a recycling gas substantially free of impurities such as in particular H2S and / or H2O in the second reaction step, and therefore to be able to use under optimal conditions efficient catalysts sensitive to impurities, and this without very costly treatment of the all of the hydrogen supplied to the stage in question,

- to be able to greatly reduce the quantity of light hydrocarbons present in the recycling gas supplying the second reaction stage, which increases the purity of this gas and the catalytic activity.

DESCRIPTION:

A typical charge of the process according to the invention is a charge of middle distillates. Within the meaning of the present description, the term middle distillate denotes hydrocarbon fractions boiling in the range from approximately 130 ° C to approximately 410 ° C, generally from approximately 140 ° C to approximately 375 ° C and for example from approximately 150 ° C to about 370 ° C. A charge of middle distillates can also include a diesel or diesel cut, or be designated by one of these names.

The process of the present invention can also find an application in the treatment of hydrocarbon fractions, in particular of direct distillation, having a boiling point located in the range of naphthas: it can be used to produce hydrocarbon cuts usable as solvent or as diluent and preferably containing a reduced content of aromatic and or sulfur compounds; the term naphtha denotes a hydrocarbon fraction ranging from hydrocarbons having 5 carbon atoms to hydrocarbons having a final boiling point of approximately 210 ° C. The process can also be used for hydrotreating and desulfurization of gasoline, in particular gasoline coming from a fluid catalytic cracking unit (unit whose process is known under the name FCC), or other fractions gasoline originating, for example, from coking, visbreaking, or hydroconversion of residue units, the term gasoline denotes a hydrocarbon fraction originating from a cracking unit, boiling between approximately 30 ° C. and approximately 210 ° C.

Another possible charge is kerosene. The term kerosene designates a hydrocarbon fraction boiling in the range 130 ° C to 250 ° C approximately.

The process according to the invention can also be used for hydrotreating heavier cuts such as boiling vacuum distillate in the range 370 ° C to 565 ° C approximately.

The process according to the invention can also be used for hydrotreating heavier cuts than vacuum distillate, in particular deasphalted oil cuts.

The term deasphalted oil denotes a cut boiling above about 565 ° C (or a slightly lower temperature such as about 525 ° C), obtained by deasphalting a heavy residue, for example vacuum residue, by a solvent of propane, butane, pentane, light gasoline type or any other suitable solvent known to a person skilled in the art.

Finally, the process can be used for the hydrotreatment of a wider hydrocarbon fraction, resulting for example (without limitation) from a mixture of at least two of the fractions previously defined.

It is also possible to use residual fillers, for example of vacuum residue, boiling above about 565 ° C., and comprising non-vaporizable asphaltenes.

The method according to the invention comprises the following steps:

A first hydrotreatment stage ai) in which the charge and the excess hydrogen are passed over a first hydrotreatment catalyst, to convert at least part of the sulfur contained in the charge into H2S,

• downstream of step ai), a step a2) of stripping the charge at least partially desulfurized in step ai), in a column under pressure, preferably with liquid reflux, with at least one stripping gas rich in hydrogen, to produce at least one stripping gaseous effluent rich in hydrogen and at least one liquid stripping effluent, the stripping gaseous effluent being at least partly compressed and recycled at the entry of the first step ai) to by means of a first REC1 recycling loop,

• downstream of step a2), a second hydrotreatment step a3) in which P liquid stripping effluent and excess hydrogen are passed over a second catalyst hydrotreatment (different or identical to the first), the effluent from this step a3) being fractionated, in a gas / liquid separation step a4), into a gaseous fraction rich in hydrogen and a hydrotreated liquid fraction, said gaseous fraction in hydrogen being at least partially compressed and recycled at the inlet of P step a3) by means of a second recycling loop REC2, separated from the loop REC1.

By separate loop, it means that the two loops are distinct (unlike a single loop supplying the different hydrotreatment stages in parallel or in series), that they comprise means for compressing different recycling gases, and have no common part or common point or are combined and mixed the recycle gases supplying the two loops; on the other hand this does not exclude connections between the two loops such as one or more lines for discharging purge gas from one loop to the other loop.

If the purge gas is evacuated from the REC2 loop to the REC1 loop, there is no need to treat this purge gas, the impurity content of which in particular in H2S is low, whereas the hydrotreatment step ai) often takes place with a significant H2S content.

If the purge gas is evacuated from the loop REC1 to the loop REC2, it is preferable to ensure that this purge gas does not introduce too large a quantity of undesirable impurities into the loop REC2. It will then generally be possible to carry out a treatment of this purge gas (alone or possibly in admixture with another stream of recycle gas) in order to eliminate in particular most of the PH2S contained in this purge gas, and possibly water if the second stage catalyst is very sensitive to water. Examples of such treatments are given later in this application.

The flow rate of a purge gas being generally relatively low compared to the flow rate of a recycling loop, the treatment of this possible purge gas (from the loop REC1 to the loop REC2) is applied only at a low gas flow rate (typically less than 50% and even more often than 30% of the flow rate of each of the recycling loops REC1 and REC2, (flow rate measured by convention at the level of the recycling compressor).

According to a preferred embodiment, the loop REC2 is supplied with hydrogen independently of the loop REC1, by one or more streams of hydrogen exclusively constituted (s) of stream (s) of auxiliary hydrogen external (s) to the REC1 loop. This makes it possible to obtain in the loop REC2 a recycling gas having a very high purity.

Thus, in the method according to the invention, the loop REC2 is not common, has no common part and has no mixing point with the loop REC1 uniting the two recycling gases. In the preferred embodiment where the REC2 loop is supplied with hydrogen independently of the REC1 loop, it is therefore not polluted by the compounds which may be found in the REC1 loop.

The absence of a mixing point also makes it possible to decouple the pressures of the two loops and in particular to adapt the pressure of the second loop as required. This has an advantage in reaching strict specifications, which may require in certain cases, for example with very low quality fillers, relatively high pressures on one of the hydrotreatment stages (in particular the last), but not necessarily on the two step. One can for example use a pressure in step a3) greater than that of step ai) by at least 1.2 MPa.

In other cases, however, one can use for the second stage a pressure relatively close to that of the first stage, for example a pressure greater than or equal to that of the first stage of approximately 0 to 1.2 MPa, or even a pressure lower than that of the first stage.

The decoupling of the two loops, in combination with stripping, also makes it possible to eliminate pollutants from the REC2 loop which are only present in significant quantities in only part of the recycling (REC1 loop), and to use efficient catalysts but sensitive to certain pollutants in the second or last hydrotreatment stage, without these pollutants being reintroduced in the second or last hydrotreatment stage via a common recycling gas or mixed for the two stages. In the case where an evacuation of purge gas is used from the first loop REC1 to the second loop REC2, the treatment for purifying this purge gas (elimination of H2S and possibly of water) has a limited cost , well below the cost of treating all the loop recycling gas

REC2, due to the fact that the flow rate of this purge gas is typically relatively low (for example at most 50% and in particular at most 30% of the gas flow rate in any one of the two loops).

The stripping column, which preferably operates under a pressure (at the head of the column) close to the pressure of stage ai) (for example under a pressure lower than that of stage ai) of approximately 0 to 1 MPa , and preferably around 0 to 0.6 MPa), essentially makes it possible to carry out an elimination of undesirable compounds in the second reaction step: It is possible, using a stripping gas rich in hydrogen and substantially free of pollutants: H2S, NH3 , (and possibly H2O), carry out a stripping of these pollutants and substantially eliminate them from the bottom product of the column (the liquid stripping effluent), before feeding the second reaction step.

The degree of hydrodesulfurization of step ai) and the efficiency of stripping (linked to the number of theoretical plates of the column) are preferably adjusted so that the H2S content in the REC2 loop is limited, for example to less than 1000 ppm, or even 200 ppm. The preferred contents, in particular if a thioresistant catalyst is not used, are generally less than 100 ppm, or even less than 50 ppm. The most preferred contents are less than 10 ppm, and in particular less than 5 ppm. In the case where a sulfur-resistant catalyst is used in the second reaction step, it is however possible to have more than 500 ppm or even more than 1000 ppm of H2S in the REC2 loop.

The zone of depletion (or of stripping of the liquid from the feed) of the stripping column (that is to say the area situated below the feed) can for example have an efficiency corresponding to from 3 to 60 theoretical plates, generally from 5 to 30 theoretical plates, and often from 8 to 20 theoretical plates, limits included.

A preferred embodiment of the stripping step a2) consists in using a stripping column also comprising a stripping vapor rectification zone (zone situated above the feed), with a liquid reflux (substantially at the head of the zone rectification). The rectification allows substantially all the liquid products, or possibly mainly relatively heavy sulfur-containing products which are difficult to desulfurize, to flow back to the bottom of the column, according to different variants of the process according to the invention, explained below. It makes it possible to recover and submit to the second hydrotreatment step all the compounds for which additional advanced hydrotreatment is sought.

The number of theoretical plates of the rectification zone is generally between 1 and 30, preferably between 2 and 20, and very preferably between 5 and 14 limits included.

Preferably, the total reactor effluent from step ai) feeds the stripping column, the gas contained in the reactor effluent also being subjected to rectification by means of liquid reflux. It is however possible, without departing from the scope of the invention, to separate beforehand the gas contained in the effluent from the reactor of step ai), upstream of the stripping column.

Preferably, the effluent from the reaction step ai) is only partially cooled before entering the stripping column. The inlet temperature into the stripping column is generally at least 140 ° C, often at least 180 ° C, and frequently between 180 ° C and 390 ° C.

Preferably, the liquid reflux is obtained by cooling and partial condensation of the column head vapors then separation of the cooled stream in a gas / liquid separator flask, or reflux separator flask. Preferably, these vapors are cooled to a temperature less than or equal to 80 ° C, for example to about 50 ° C or even to a lower temperature. The cooled gas from this reflux tank then constitutes an "effluent gaseous stripping "which can optionally be treated then is typically compressed and then recycled.

According to a first variant of the process according to the invention, substantially all of the relatively light hydrocarbon liquid phase is returned to the stripping column as internal reflux, and therefore no light stripping liquid effluent is produced at the level reflux flask, or possibly a very small amount generally less than 10% by weight of the initial charge, for example a reduced amount of naphtha or other light products often generated during the first step ai).

In this first variant, it is typically sought to maximize the quantity of the stripping liquid effluent recovered at the bottom of the column, and to prevent compounds boiling in the desired distillation interval from leaving the top of the column, either in the form light stripping liquid effluent, i.e. with the stripping gaseous effluent. A relatively low temperature is therefore preferably used at the reflux tank.

The liquid stripping effluent then represents, in this first variant, preferably at least 90% by weight, and often about 95% by weight or more of the initial charge.

The aim sought in this first variant is to subject the largest possible amount of boiling product in the distillation interval sought in the second hydrotreatment step a3), with the main objective, most often, to carry out hydro-dearomatization important in this second step, applied to as much of the treated section as possible, typically to maximize the number of cetane. Step a3), however, also performs further desulfurization of the feed. To achieve this main objective of advanced hydrogenation in the second reaction step, one therefore generally uses in step a3) one or more catalysts which are very effective in hydrogenation, and in particular preferably a catalyst of the noble metal type, for example of the platinum on alumina or platinum / palladium type on alumina.

In the first case (platinum catalyst in step a3)), very thorough desulfurization is preferably carried out in step ai), for example at about 100 ppm of sulfur, or preferably about 50 ppm and for example at about 10 ppm or less, to limit the amount of residual sulfur in step a3) due to the high sensitivity of this catalyst to sulfur. One can for example use a nickel / molybdenum type catalyst on alumina in step ai). The diagram of the process according to the invention also makes it possible, with the loop REC2 separated from the loop REC1, to practically eliminate the water at the level of the stripping column (with a stripping preferred with the additional hydrogen typically of content of substantially zero water) without reintroducing water at the REC2 loop. Very low water content (e.g. less than about 200 ppm, often less than 100 ppm and generally less than 10 ppm is then easily obtained, which is favorable to the activity of the platinum catalyst. When using a purge gas from the REC1 loop to the REC2 loop, it is very often preferable, with a platinum catalyst, to eliminate IΗ2S from this purge gas and to dehydrate it before introducing it into the loop REC2.

In the second case (platinum / palladium catalyst in step a3)), less desulfurization is optionally carried out in step a1), for example at about 1000 ppm of sulfur, or preferably about 500 ppm and for example at about 100 ppm of sulfur or at a content compatible with good efficiency of the platinum / palladium catalyst used. One can for example use a catalyst of cobalt molybdenum on alumina type in step ai). A higher water content than in the previous case is acceptable in step a3), but in practice an almost total elimination of water can be easily obtained by stripping with additional hydrogen. When using a purge gas from the REC1 loop to the REC2 loop, and a platinum / palladium catalyst in step a3), it is often preferable to remove PH2S from this purge gas and / or to dehydrate it before introducing it into the REC2 loop.

Finally, it is also possible to use in step a3) a conventional catalyst (for example of the nickel / molybdenum type on alumina), less sensitive to impurities but less efficient.

The stripping column supply temperatures that can be used in this first variant are typically between approximately 140 and approximately 270 ° C., and preferably between 180 and 250 ° C.

Two typical operating cases according to the first variant of the method according to the invention are given in the examples (1 and 2).

According to a second variant of the process according to the invention, which can be used in particular for carrying out advanced desulfurization of middle distillates, on the contrary it is sought to produce a light stripping effluent in large quantities (typically between 10 and 70%, in particular between 20% and 60 % by weight relative to the initial charge), which is discharged directly downstream (that is to say without additional hydrotreatment). The design parameters and the operating conditions are chosen so that this light liquid stripping effluent, as well as the liquid reflux, which is typically of identical composition, constitutes a product with very low sulfur content (organic) with the required specifications (less than 50 ppm, often less than 30 ppm, or even less than 10 ppm, for example around 5 ppm sulfur). The severity of desulfurization of the first reaction step ai) must in particular be adapted to this very low sulfur content sought for P light liquid stripping effluent. A point 95% by weight distilled of P light stripping liquid effluent is often chosen between preferably 200 and 315 ° C, and preferably between 235 and 312 ° C to substantially avoid the presence of heavy fractions of the feed, which in many cases include sulfur products relatively refractory to desulfurization, for example dibenzo-thiophenes, which have been refluxed at the bottom of the column.

In this second variant, the second reaction step most often has as its main purpose an advanced desulfurization of the heavy fractions of the feed. Typically, the liquid stripping effluent (feed from step a3)) has a significant sulfur content, for example between 50 and 2000 ppm, and often between 100 and 1000 ppm, most often between 100 and about 500 ppm . The main advantage of the production and evacuation of a light stripping effluent in large quantities, according to this second variant, is that this greatly reduces the flow rate of the charge of the second reaction step a3), therefore the volume required for the reactor in this stage.

The most suitable catalysts for step a3) of this second variant of the process according to the invention are the catalysts suitable for deep desulfurization, in particular catalysts of the platinum / palladium on alumina type. It is also possible to use conventional catalysts, for example of the nickel / molybdenum type on alumina, or even other catalysts which can carry out a final desulfurization (at the same time, generally, that a certain dearomatization of the heavy fraction of the charge, supplied in step a3)).

It is also possible, for this second variant, in the case where a purge gas from the loop REC1 to the loop REC2 is used, carry out a treatment for elimination of H2S and / or dehydration of this purge gas before feed the REC2 loop.

The stripping column supply temperatures which can be used in this second variant are typically between approximately 220 and approximately 390 ° C., preferably between 270 and 390 ° C., and very preferably between 305 and 390 ° C., for example. example between around 315 and 380 ° C. Use is preferably made of a feed temperature of the stripping column having a possible temperature difference of at most 90 ° C, and often at most 70 ° C with the outlet temperature of the first reaction step ai). Generally, the stripping column is fed after limited cooling (at most 90 ° C, and often at most 70 ° C), or without cooling of the reactor effluent from step ai).

This variant of the method according to the invention is the subject of a separate patent application, simultaneous with the present application.

The two variants described above are not limiting, and the method according to the invention can also be used according to other variants and / or objectives of hydrotreatment and / or catalysts and / or operating conditions. In particular, the (or the catalysts) which can be used in stage a1) are not limited to conventional catalysts of the cobalt / molybdenum or nickel / molybdenum on alumina type; and the catalyst (s) that can be used in step a3) are not limited to catalysts of the platinum type on alumina or platinum / palladium on alumina or nickel / molybdenum on alumina. The process according to the invention can be used with any type of hydrotreating catalyst.

For the choice of operating conditions adapted to each stage, and in particular of conditions for deep desulphurization in the first stage ai), severe conditions can be used: space velocity = low VVH, high temperature, high partial pressure of hydrogen, catalyst suitable for the load and under these severe conditions. Adequate operating conditions, and the choice of an adequate catalyst, depend very much on the charge treated, but can be easily determined, for a given charge, by a person skilled in the art.

The embodiments of the process variants can also be diverse:

For example, it is also possible to recycle at the inlet of the stripping column (in particular to control its inlet temperature) another part of the relatively light liquid hydrocarbon phase recovered with the reflux flask. It would also not depart from the scope of the invention if a part of the relatively light hydrocarbon-based liquid phase were recycled at the entrance to step ai).

It is also possible, for the second variant of the process, to cool the column head vapors in two stages:

a first cooling with partial condensation of the column head vapors, followed by a gas / liquid separation in a reflux flask and, for example, all the condensed liquid is returned to the column, the temperature of this flask being for example between 70 and 250 ° C, and such that a significant quantity of hydrocarbons remain contained in the gas from this separator flask,

additional cooling of the gas from the latter separator flask, with or without contacting with a liquid (for example the hydrotreated liquid fraction) capable of absorbing light hydrocarbons, to condense and evacuate the light liquid stripping effluent, alone or in mixed.

The flow rates of liquid reflux and of stripping gas which can be used depend strongly on many parameters, including for example in particular the supply temperature of the stripping column. These parameters may preferably be chosen in a coordinated manner. Generally, a stripping gas flow rate of between 2.5 and 520 Nm ³ / m ³ of charge supplied in stage ai) will be used, and often between 5 and 250 Nm ³ / m ³ of charge supplied in stage have). Preferably, this flow rate will correspond to a hydrogen flow rate between 5% and 150%, and preferably 10% to 100% of the hydrogen flow rate consumed in step ai) (assuming all the hydrogen in the gas of stripping consumed). The amount of liquid reflux will usually between 0.05 and 1.2 kg / kg, and often between 0.15 and 0.6 kg / kg of charge supplied in step ai). Appropriate flow rates of stripping gas and liquid reflux can easily be determined by a person skilled in the art for the desired separation conditions, by simulation of fractionation by computer.

According to a preferred optional arrangement of the method according to the invention, washing water is injected into the overhead vapors of the stripping column, upstream of the cooling exchanger (and / or of an air cooler) , to capture nitrogen compounds, for example ammonia and ammonium sulfide formed in the reactor; the aqueous phase containing a large part of these undesirable compounds is preferably recovered downstream in a gas / liquid separator flask, also acting as decanter, and discharged.

The method according to the invention can also be implemented with various modifications and according to various embodiments.

In the variant of the process where a light liquid stripping effluent is produced, it is very preferably sought to obtain a light product that is substantially desulphurized. Optionally, if the light liquid stripping effluent does not completely meet the required specifications, it is however possible to subject it to a mild, complementary hydrotreatment to bring it to the required specifications.

According to one embodiment of the method, each of the loops REC1 and REC2 is supplied with make-up hydrogen in an amount suitable for the hydrogen consumption of this loop. The two loops can then operate without purge gas.

According to another embodiment of the method according to the invention, there is supplied in step a3), at at least one point of the loop REC2, a flow rate of excess hydrogen in excess of the hydrogen requirements of this step a3), and a hydrogen-rich purge gas flow (corresponding to the excess hydrogen) is extracted from the loop REC2, which is sent to at least one point of the loop REC1. It is advantageously possible to use part or all of this purge gas as stripping gas in step a2): this purging gas, substantially free of impurities since it comes from the loop REC2, provides an additional stripping gas which can advantageously be introduced into the column in the intermediate position, below the effluent inlet of step ai), and above the (optional) inlet of external stripping gas (hydrogen from top-up), of higher purity.

This purging of the REC2 loop can, in this embodiment of the method according to the invention, represent from 10 to 100%, and in particular from 30 to 100% of the hydrogen requirements of the REC1 loop. In the event that this purge represents 100% of the hydrogen requirements of the REC1 loop, the REC1 loop is only supplied with additional hydrogen by purging the REC2 loop.

Finally, in another embodiment, it is possible to supply the REC1 loop with excess hydrogen in excess quantity, and to evacuate the purge gas towards the REC2 loop, preferably after elimination of H2S and often dehydration. .

In general, the method according to the invention may advantageously comprise a step a5) of bringing into contact (or contacting) at least part of the hydrotreated liquid fraction resulting from step a4) with at least part of the stream gas flowing in the REC1 loop. The effluent from this step a5) is then separated into a liquid contact effluent and a gaseous contact effluent; this gaseous contact effluent is then at least partly recycled in the loop REC1.

This contacting makes it possible to use the liquid from the second reaction hydrotreatment step (step a3)), after gas / liquid separation, as an absorbent for light hydrocarbons having from 1 to 4 carbon atoms (C1, C2, C3, C4) contained in the gas of the REC1 loop, and to increase the hydrogen purity of this REC1 loop: Indeed, step a3), whose feed has been stripped, and which produces relatively few compounds light, operates with a high hydrogen purity, and the liquid resulting from this stage constitutes a good absorbent of light hydrocarbons.

The method may also further comprise a treatment for at least partial elimination of PH2S contained in at least part of the gas stream flowing in the loop REC1, this treatment being carried out at a point in the loop REC1 generally located downstream of the step a2) stripping and upstream of step a5) contacting. This treatment can consist of washing the gas with an amine solution, a technique well known to those skilled in the art or removing H2S by another method known to those skilled in the art. Known treatments are, for example, indicated in European patent application EP 1 063 275.

It is also possible with the process according to the invention, in certain cases (for example when the sulfur content of the feed is not too high or if a relatively low space speed is used in step ai) allowing to compensate for a high sulfur content in the recycling gas of the loop REC1), to have no treatment for removing H2S by washing with amines the recycling gases circulating in the installation. In this case, the method however comprises one or more treatments carrying out at least partial elimination of PH2S contained in the recycling gases circulating in the loops REC1 and REC2, in which each of this or these treatments consists of the combination of a step of bringing at least part of the recycling gas circulating in the loop REC1 into contact with a liquid hydrocarbon fraction, making it possible to achieve a limited absorption of H2S by this liquid hydrocarbon fraction, followed by a gas / liquid separation step from the mixture resulting from this contacting and a direct evacuation of at least part liquid thus separated (downstream of the process, ie without additional hydrotreatment). The liquid hydrocarbon fraction capable of absorbing (and evacuating) PH2S may possibly be a part of the relatively light hydrocarbon liquid phase (typically recovered with a reflux flask) and / or often part or all of the hydrotreated liquid fraction. The term treatment (elimination of H2S) must be, according to this process arrangement, understood in a general sense, therefore comprising in particular a simple contact with a liquid phase making it possible to absorb PH2S by liquid / vapor equilibrium, and not just chemical or physico-chemical treatments such as washing with amino acids. The contacting can also be carried out by partial condensation of a liquid hydrocarbon phase, and not only by contacting part or all of the hydrotreated liquid fraction. In other words, this (optional) process arrangement means that two or more stage hydrotreatment can be used with intermediate stripping with hydrogen under pressure, which may optionally include the use of a noble metal catalyst ( for example of the platinum type, or platinum / palladium on alumina) in the second and or last step, without washing the amines with part of the recycling gas. Typically, neither the recycling gas nor the hydrogen makeup (s) is treated by washing with amines according to this optional arrangement of the process according to the invention.

LΗ2S contained in the gas of the loop REC1 (produced in step ai)) is then evacuated thanks to the contact of this gas rich in H2S by the liquid hydrocarbon fraction, making it possible to absorb PH2S which is then evacuated with the liquid product towards a stripper downstream of the process. This absorption by a hydrocarbon liquid internal to the process is less effective than washing with amines and leads to a higher H2S level in the recycling gas of the REC1 loop. By cons it avoids the implementation of an expensive circuit for circulation and regeneration of a solution of amines.

The loop REC2, separated from REC1 in the process according to the invention, makes it possible not to pass on to the loop REC2 a content of H2S which remains relatively high in the loop REC1 in the absence of any washing with amines. A catalyst of the noble metal type can then be used without disadvantage for step a3). The processes of the prior part with two stages of hydrotreatment and integrated hydrogen stripper on the contrary all require washing with amines.

The REC 1 loop often contains water, due to an injection of washing water and / or the presence of water in the load. Because preferential (and often exclusively) make-up hydrogen, generally substantially free of water, such as stripping gas (and / or otherwise a gas with a very low water content, for example less than 5 ppm), a very thorough removal of the water from the liquid stripping effluent is typically obtained. In the case where a purge from the REC1 loop is used towards the REC2 loop, it is preferable to eliminate PH2S from this purge gas, but also more often to dehydrate it, by techniques known to those skilled in the art. loom, for example on a molecular sieve or other solid adsorbent, or by washing with a desiccant liquid, for example diethylene glycol or triethylene glycol, or any other known dehydration process. The need for this dehydration, and the degree of dehydration desirable depends essentially on the water sensitivity of the catalyst of step a3) (typically high for platinum catalysts, medium for platinum / palladium catalysts, and low for conventional catalysts without noble metal). Adequate conditions for dehydration (if any) can easily be determined by a person skilled in the art.

Another object of the stripping step, in addition to at least partial elimination of multiple impurities, is to remove by stripping a significant part of the light hydrocarbons having from 1 to 4 carbon atoms (C1 to C4) present in the effluent. liquid from step ai), before step a3). This makes it possible to obtain a Pur2 purity of hydrogen in the REC2 loop greater than the Pur1 purity of hydrogen in the REC1 loop. We could for example have a Pur2 / Pur1 ratio greater than 1.06, in particular greater than 1.08, and often greater than 1.10.

By way of nonlimiting examples, the following ranges of purity can be obtained:

With a make-up hydrogen purity of 92, it is generally possible to obtain a purity of between approximately 58 and 84, and often between approximately 60 and 80 in the REC1 loop, while the purity of hydrogen of the REC2 loop can generally be between approximately 73 and 90, and often between approximately 75 and 88.

With a make-up hydrogen purity of 99.9, it is generally possible to obtain a purity of between about 73 and 90, and often between 75 and 88 in the REC1 loop, while the purity of hydrogen of the REC2 loop can generally be between approximately 88 and 99.5, and often between 90 and 99.

According to another variant of the method according to the invention, supply of separate hydrogen streams (external to the two loops) separate and of different purity in the two loops REC1 and REC2. Preferably, the loop REC2 is supplied with a stream of make-up hydrogen of higher purity than that of the stream of make-up hydrogen supplying the loop REC1. For example, the make-up hydrogen in the REC1 loop has a purity of about 92, or between 88 and 96, and comes at least in part from a catalytic reforming unit, and the make-up hydrogen in the loop REC2 has a purity of 99.9 or more, and comes from a steam reforming unit followed by a separation (purification) step on a molecular sieve or on another solid adsorbent bed, known to a person skilled in the art under the name "PSA", for "Pressure Swing Adsorption" according to English terminology. The differences in purity can be reduced if one or two mixtures of gases of different purities, for example the aforementioned types, are used as make-up (s), with different percentages for each of the make-ups.

Another technical advantage of the process according to the invention is that it is possible to use loops and reaction steps at possibly very different pressures, which makes it possible to adjust the pressure, in particular that of the second hydrotreatment step a3), at the level optimal wanted.

This is interesting both for new units and for the modification of existing units ("revamping" according to English terminology), for example by adding an additional reaction step a3).

One can in particular use a pressure in step a3) greater by at least 1.2 MPa and at most 12 MPa than that of step ai), in particular greater by at least 1.2 MPa and d '' at most 4.5 MPa to that of step ai).

At the partial hydrogen pressures of the two loops (by convention at the reactor outlet in both cases), it is possible to use a partial hydrogen pressure in step a3) at least 1.35 MPa and at least plus 13.5 MPa to that of stage ai), in particular at least 1.35 MPa and at most 5 MPa greater than that of stage ai).

However, it is also possible to use closer partial pressures of hydrogen, for example a partial pressure of hydrogen in step a3) greater than that of step ai) by a value between 0 and 1.35 MPa, and in particular between 0.1 and 1.35 MPa.

One can also use a pressure in step a3) substantially equal to, or at least 0.1 MPa lower than that of step ai). In such a case, it is often possible to use nevertheless a partial pressure of hydrogen in step a3) higher, for example by at least 0.1 MPa, than that of step ai), due to the higher purity of the REC2 loop.

The invention also relates to any hydrocarbon fraction from the group of gasoline, jet fuel, kerosene, diesel fuel, gas oil, vacuum distillate, and deasphalted oil, containing at least one fraction hydrotreated by the process according to the invention.

Two embodiments, among the preferred embodiments of the method according to the invention are detailed in FIGS. 1 and 2.

We now refer to FIG. 1 which represents a process diagram of a hydrotreatment installation for the production of a first variant of the process according to the invention: The charge of the hydrotreatment installation, for example a cut of the middle distillate type containing a high proportion, or even 100% of catalytic cracked diesel (LCO), is supplied by the line, and added with a gaseous stream of recycling, rich in hydrogen, circulating in line 23. The mixture thus formed circulates in line 2, is reheated in the charge / effluent heat exchanger 3 (as well as often in a heat exchanger with the effluent from the step a3), this exchanger not being shown), then is sent by line 4 to the furnace 5 in which its temperature is brought to the temperature required for the first reaction step ai). At the outlet of the furnace 5, the reaction mixture circulates in line 6, then feeds the hydrotreatment reactor 7, which is typically a reactor with a fixed catalytic bed and downdraft. The effluent from this reactor 7 (carrying out the first reaction step ai)) is then sent via line 8 to the charge / effluent exchanger 3, then feeds, via line 9, the stripping column 10.

This column is also supplied by two sources of stripping gas, rich in hydrogen, which are supplied by lines 34 and 58: The gas supplied by line 34 is typically make-up hydrogen, of medium or possibly high purity , and preferably substantially free of impurities such as in particular H2S and / or water vapor. This gas can, for example, come from a catalytic reforming and / or steam reforming unit;

The second stream of stripping gas, supplied by line 58 is optional. This current comes from a possible excess of hydrogen-rich gas (make-up) supplied in step a3); the excess can then be used (in particular) as stripping gas supplied by the line 58. This use of an excess of make-up hydrogen in reaction step a3) increases the purity of the recycling gas in this step.

The stripping column can also be supplied by other sources of stripping gas not shown in FIG. 1; in particular one could in certain cases supply stripping gas (if it is sufficiently purified) taken from the recycling gas circulating in line 23, and introduce this gas into the column in place of or just above the supply of purge gas from the REC2 loop via line 58.

In general, the stripping gas or gases supplying column 10 may have been previously dried in a dryer (optional, not shown) to substantially remove the water in the stripping step (more particularly if the catalyst step a3) of hydrotreatment contains a noble metal which is sensitive to water).

In general, the stripping gas or gases feeding the column 10 may also have been purified beforehand to eliminate possible traces of H2S, for example by adsorption on a zinc oxide bed (optional, not shown) to eliminate more completely PH2S in the stripping step (more particularly if the catalyst in step a3) of hydrotreatment contains a noble metal which is sensitive to P H2S, for example a platinum catalyst).

This purification of the stripping gas or gases is however generally not necessary if the stripping gas is a make-up hydrogen which comes from a catalytic reformer. In the case where the make-up hydrogen is produced at least partially by steam reforming, it is preferably carried out, after the steam reforming, an almost total elimination of the compounds other than hydrogen, often on molecular sieve (PSA type separation), which allows to obtain hydrogen of very high purity.

The overhead vapors of column 10 circulate in line 11, are added with washing water supplied by line 25, are then cooled with partial condensation in the air cooler 12, then pass through line 13, before being separated in the gas / liquid separator flask 14, also acting as decanter and reflux flask. This balloon 14 separates between three phases: - a gas stream, or “stripping gas effluent”, sent in line 16,

- A relatively light liquid hydrocarbon phase, extracted by line 15. A first fraction of this liquid phase is recycled to column 10, as liquid reflux, still by line 15; the residual liquid fraction (if any), or “light liquid stripping effluent” is discharged via line 27 (optional), preferably downstream (ie it is not treated in the step reaction a3)), an aqueous liquid phase, moreover generally containing nitrogenous impurities, discharged via line 26.

The bottom liquid of column 10, or “stripping liquid effluent” (or “heavy stripping liquid effluent” if there is a light stripping liquid effluent), is sent via line 41 to the second reaction step a3 ).

The stripping gaseous effluent, circulating in the line 16 possibly crosses the equipment 17 for at least partial elimination of PH2S contained in this gas. This equipment 17 could typically be a washer, or absorber of H2S with an amine solution (the inlet and outlet of the amine solution not being shown); it can also be another device for removing H2S, and / or a device for removing H2S and water. This equipment 17 is optional (its usefulness depends on many parameters, in particular on the sulfur content of the feed, and on the space speed used in the first reactor 7, which can, if it is sufficiently low, allow the desired desulfurization in the first step ai) without elimination of H2S by washing with the amines in the loop REC1). At the outlet of this equipment 17, the gas circulates in line 18, is added by a liquid stream called “hydrotreated liquid fraction”, circulating in line 59, then joins the gas / liquid separator flask 20, via line 19 in which an online mixing is carried out. This makes it possible to absorb light hydrocarbons in the liquid phase, and to increase the purity of the REC1 recycling loop.

The liquid effluent from the flask 20, called “contacting liquid effluent” is discharged through line 24, and constitutes a liquid effluent from the hydrotreating installation (another effluent, optional, being the light liquid stripping effluent, possibly evacuated by line 27).

The gas separated in the balloon 20, or "contacting gaseous effluent", is sent by line 21 to the compressor 22 for recycling gas, then recycled to the inlet of the reaction step ai) via line 23.

The liquid stripping effluent circulating in the line 41 is pumped by the pump 40, to bring the pressure to a pressure sufficient for the reaction step a3), this pressure being, in this embodiment, greater than that of the step ai). When the pump 40 is discharged, the hydrogen-rich recycling gas is added to it, supplied by line 56, then circulates in line 43, passes through the charge / effluent exchanger 44, then circulates in line 45, is heated (to new) in the exchanger (or furnace) 46, then joins the reactor 48 of reaction step a3), via line 47. At the outlet of this reactor, the effluent passes through line 49, passes through exchanger 44 , circulates in line 50, is cooled in the air cooler 51, then circulates in line 52 to reach the gas / liquid separator flask 53. The liquid fraction (hydrotreated) is sent in line 59 to be mixed with the circulating gas in line 18 of the loop REC1, and the gaseous fraction is sent by line 54 to the recycling gas compressor 55. Optionally, part of this gaseous fraction (possible excess gas, ie also possible purge gas REC2 loop) is taken and sent by the line (optional) 58 to the lower part of the stripping column 10 (and / or possibly to another point of the loop REC1 by means not shown).

Then added to the residual gas stream flowing in line 54, upstream of the compressor 55, a make-up hydrogen stream supplied by line 33. The recycling gas, after this addition of make-up hydrogen, is then compressed in compressor 55 then recycled at the inlet of reaction step a3) via line 56.

This make-up hydrogen can consist of hydrogen from a catalytic reforming and / or steam reforming unit (often naphtha or natural gas). It is possible optionally to supply via line 33 hydrogen of higher purity than that of hydrogen supplied by line 34. This high purity hydrogen can come from a unit PSA type separation can deliver a hydrogen purity often higher than 99.9. This increases the purity and partial pressure of the hydrogen in the REC2 loop.

In the installation of FIG. 1, the recycling loop REC1 of step ai) comprises the elements referenced below, by "following the path of the gas": 21, 22, 23, 2, 3, 4, 5 , 6, 7, 8, 3, 9, 10 (upper part of the column, located above the supply 9, the gas supplied rising in the column), 11, 12, 13, 14, 16, 17, 18 , 19, 20, and again 21 which closes the loop.

The recycling loop REC2 of step a3) includes the elements referenced below: 54, 55, 56, 57, 43, 44, 45, 46, 47, 48, 49, 44, 50, 51, 52, 53 , and 54 which closes the loop.

It can be seen that, in the method according to the invention, these two recycling loops have no common part, no mixing point. The loop REC2 of the installation of FIG. 1 is supplied exclusively by external make-up hydrogen (via line 33), which avoids its pollution by impurities often present in the loop REC1.

If an excess of hydrogen (hydrogen-rich gas) is added to the loop REC2 via line 33, compared to the hydrogen requirement in reaction step a3), the excess gas can advantageously be used as gas stripping column 10 (evacuated from the loop REC2 by line 58), this gas being substantially free of impurities. The supply of excess make-up gas in the REC2 loop leads to an increase in the purity of the gas in the REC2 loop because the purge extracts from this loop light hydrocarbons having from 1 to 4 carbon atoms as well as traces of Residual H2S.

Optionally, for example if the REC2 loop operates with an amount of purge gas exceeding the stripping gas requirements, a fraction, or even all of the purge gas from the REC2 loop may possibly be sent by means not shown to a point of loop REC1 (for example of line 23), without passing through the stripping column.

In the installation of FIG. 1, the two loops REC1 and REC2 are supplied with make-up hydrogen (hydrogen-rich gas) respectively by lines 34 and 33.

The two make-up hydrogen streams can have a different purity. The hydrogen supplied by line 34 in the REC1 loop can optionally come from a catalytic reformer and have an average purity, for example between 88 and 96. The hydrogen supplied by line 33 in the REC1 loop can optionally and preferably come of a steam reforming unit, followed by a PSA type separation, and having a very high purity, for example 99.9. The two loops REC1 and REC2 could however be supplied from a common line, not shown, by an additional hydrogen of identical purity.

The installation may also include other elements not reproduced in FIG. 1, and for example: - one or more lines of quenching gas, or “quench” gas according to Anglo-Saxon terminology, coming from points on the line 23, and supplying the reactor 7 in the intermediate position (in one or more zones located between two consecutive catalytic beds),

- one or more supplies of stripping gas to column 10 coming from one or more points of line 23 or line 21.

These additional lines would then be included in the REC1 loop.

The installation may also include a pipe for discharging purge gas from a point in the REC1 loop, and / or a pipe for introducing make-up hydrogen at a point in the REC1 loop without passing through the stripping column (for example at line 23).

Similarly, the loop REC2 could include elements not reproduced in FIG. 1, and for example one or more lines of quench gas, coming from points on line 56, and supplying the reactor 48 in the intermediate position (zone between catalytic beds). The installation can also include an evacuation of purge gas from a point of the loop REC2, without this purge gas feeding the loop REC1.

It would not be departing from the scope of the invention to add and / or remove heat exchangers or equivalent equipment and / or to organize the thermal integration of the installation differently. By way of nonlimiting example, the exchanger 46 of the loop REC2 could be the oven 5 itself (or also a part of this oven, in particular a part of the convection zone of the oven). It is also possible and often used to preheat the feed of step ai) and / or the make-up hydrogen with the effluent of step a3), and / or to preheat the recycling gas of the loop REC2 by the effluent from step a3), and / or recovering heat from the effluent at the top of the column 10, upstream from the air cooler 12.

The effluent from the reactor of stage ai) can be cooled in an exchanger (or several exchangers). This cooling (for all the exchangers if several exchangers are used) can be significant, for example often between around 90 ° C and 200 ° C in the first variant of the process according to the invention. It can also be limited to often at most 90 ° C, generally at most 70 ° C in the second variant of the process according to the invention, so as to maintain a high temperature at the inlet of the stripping column. This effluent can also be reheated to a limited extent in an oven before feeding the stripping column. The liquid stripping effluent (or the heavy liquid stripping effluent) can optionally be reheated in a heat exchanger and / or an oven, before feeding the reactor of reaction step a3), or optionally cooled to a limited extent before feeding this reactor. The stripping column 10 can also include a reboiler for the bottom of the column liquid, not shown in FIG. 1. In some cases, the effluent from reactor 7 of step ai) can be fed directly to the stripping column, (without heat exchange), and / or feed the reactor of step a3) directly (without heat exchange) with the liquid stripping effluent,

Those skilled in the art can also use other heat exchanges between several streams circulating in the installation, depending on the respective temperatures of these different streams.

It would not go beyond the scope of the invention if the position of the optional equipment 17 for H2S purification was changed, this equipment then being located downstream of the compressor 22 (on line 23).

Likewise, it would not be departing from the scope of the invention if one, or each of the two reaction stages ai) and a3) was carried out not in one, but in two or even more reactors in series, possibly with intermediate adjustment temperature, or if a reactor had several reaction zones in series, with identical or different catalysts.

Hydrotreatment reactors (7, 48) are typically reactors with a fixed catalytic bed and downdraft for gas and liquid. It would not be departing from the scope of the invention if one or more of the reactors were of another type or of several other types, in particular of the moving bed type, or of the bubbling bed type (due to the introduction gas), or with a fluidized bed (by recycling gas), or with a fixed or mobile bed and upward current for the gas and downward for the liquid.

Reference is now made to FIG. 2 which represents a process diagram of another hydrotreatment installation for the production of a second variant of the process according to the invention, with the same references for the elements common to the two figures 1 and 2:

The first difference with the installation of FIG. 1 relates to the pumping means, linked to the pressures of the various reaction stages. Unlike the installation in Figure 1, the installation in Figure 2 uses a lower pressure in step a3) than in step ai). There is therefore no need for a pump to transfer the liquid stripping effluent through line 41. On the other hand, a pump 60 is used to transfer at least part of the hydrotreated liquid fraction, circulating in line 61 , towards step a5) of contacting, or contacting with the recycling gas of the loop REC1, via line 59. Another part of the hydrotreated liquid fraction can optionally be evacuated directly downstream (without contacting) , via line 62. Another difference concerns the purge gas sampling point (optional) evacuated by line 58. This point is moved downstream of the recycling compressor 55, in order to facilitate the return of purge gas to the loop REC1 (the pressure balance being different in the installation of figure 2). If the pressure in the loop REC2 and in particular at the discharge of the compressor 55 is lower than that of all the points of the loop REC1, it is generally preferable not to send purge gas from the loop REC2 to the loop REC1, which would require an additional compressor (except possibly if a common compressor is used, necessary for another use for example for supplying make-up gas in the loop REC1).

Finally, the loop REC2 comprises, arranged on line 56, an optional equipment 61 for purifying the recycling gas to eliminate if necessary traces of H2S, for example a bed of adsorbent with zinc oxide. This bed can optionally operate at a higher temperature than the outlet temperature of the recycling compressor, using a heater not shown. The absorbent bed 61 could also be integrated into the reactor 48.

In the case where the catalyst of step a3) is not, or is very insensitive to water, for example with a conventional catalyst without noble metal, the optional equipment 61 for purifying the recycling gas can comprise a washing column with an aqueous solution of amines.

These process variants with H2S 61 scrubber are not linked to the staggering of the pressures between steps ai) and a3); they could therefore also be used, in the installation of FIG. 1.

The other elements of figure 2 are identical to those of figure 1. One could also, for the installation of figure 2, use options or technical modifications such as those described for the installation of figure 1 without leaving the part of the invention.

In general, the installation for hydrotreating a hydrocarbon feedstock for the implementation of the process according to the invention comprises:

- A first hydrogen recycling loop REC1, this loop comprising at least a first hydrotreatment reactor (7), connected downstream to a column (10) for stripping under pressure the liquid effluent from the reactor by a rich gas in hydrogen, the head of the column (10) being connected to a means (12) for cooling and partial condensation of the gas stream coming from the column (10), this means of cooling and partial condensation being connected downstream to a first gas / liquid separator (14), itself connected to the suction of a first recycling compressor (22), the discharge of this first compressor being connected to the first hydrotreatment reactor (7), - A second hydrogen recycling loop REC2, separate from the REC1 loop, this loop comprising at least a second hydrotreatment reactor (48), this reactor being connected upstream to the bottom of the stripping column (10), for hydrotreating the liquid effluent from the bottom of the stripping column (10), and connected downstream to a second liquid gas separator (53), itself connected to the suction of a second recycling compressor (55 ), the discharge of this second compressor being connected to the second hydrotreatment reactor (48).

Preferably, the loop REC2 is supplied with hydrogen by one or more supply means (33), each of these supply means (33) being connected upstream exclusively to one or more external hydrogen sources.

The installation can also include at least one conduit (58) for supplying in the loop REC1 a stream of purge hydrogen from the loop REC2, this conduit being connected upstream to a point of the loop REC2 and downstream at a point in the REC1 loop.

The installation may also include a first means (34) for supplying an external hydrogen stream of relatively low purity to the REC1 loop, and a second means (33) for supplying a current d relatively high purity external make-up hydrogen to the REC2 loop.

Preferably, the stripping column includes above its feed a rectification zone having a separation efficiency of at least 1 theoretical plate, for example between 2 and about 20 theoretical plates, and often 4 to 15 theoretical theoretical plates included.

The installation can also include a conduit (27) for discharging downstream a light liquid stripping fraction, this conduit being connected upstream to the first gas / liquid separation means (14).

The various supply and / or discharge means, as mentioned in this description, typically comprise at least one conduit, and may also include one or more valves and / or measuring and / or regulating members, for example flow and / or temperature.

EXAMPLES:

The following examples illustrate, without limitation, operating conditions which can be used in the process of the invention:

Example 1: Charge treated: aromatic catalytic cracking diesel (LCO) with the following characteristics:

Distillation interval (5% -95% distilled): 205-347 ° C

Density: 0.91 Sulfur content: 4000 ppm

Nitrogen content: 800 ppm

Aromatics content: 60% by weight

Number of cetane: 28

Purity of make-up hydrogen: 92.5 Operating conditions of the first stage ai) and first loop REC1:

Catalyst: HR 448 catalyst of Co-Mo type on alumina, sold by the company

AXENS (formerly by PROCATALYSE).

Average reactor temperature: 380 ° C

Reactor outlet pressure: 9.8 MPa Partial H2 reactor outlet pressure: 6.2 MPa

VVH (hourly space speed): 0.6 hr ^"1

Hydrogen content (reactor inlet + quench): 625 Nm ³ / m ³ of charge,

Hydrogen consumed in step ai): 2.03% by weight relative to the charge,

Operating conditions of P step a2): Temperature entering the stripping column: 220 ° C.

Stripping column inlet pressure: 9.5 Mpa

Number of theoretical plates above the food: 9

Number of theoretical plates below the feed: 15

Reflux rate relative to the column charge: 0.25 kg / kg, stripping H2: make-up H2 flow rate corresponding to 95% of hydrogen P consumed in step a1)

Operating conditions of the second reaction step a3) and of the second REC2 loop:

Catalyst: LD 402 platinum type catalyst on alumina, sold by the company

AXENS (formerly by PROCATALYSE). This catalyst has a high efficiency in hydrogenation, and step a3) is essentially a hydrogenation.

Average reactor temperature: 300 ° C,

Reactor outlet pressure: 8.5 MPa

Partial pressure H2 reactor outlet: 6.8 MPa

VVH: 7.0 h ¹ Hydrogen level (inlet + quench): 700 Nm ³ / m ³ of charge from step a3),

Hydrogen consumed in step a3): 1.10% relative to the charge in step ai),

Results: At the end of the first stage, the charge contains 10 ppm of sulfur, 5 ppm of nitrogen, and 27% by weight of aromatics.

At the end of the stripping step, about 99% by weight of the fraction of the charge boiling above 150 ° C. is recovered at the bottom of the stripping column. The installation of Example 1 therefore operates according to the first variant of the method according to the invention.

The final product, split at the end of the installation, has the following characteristics:

Density: 0.84 Sulfur content: 5 ppm Nitrogen content: 1 ppm Aromatics content: 5% by weight Cetane number: 48

Distillation interval (5-95%): 195 - 337 ° C.

Example 1 uses a higher pressure for the first reactor than for the second. This example could therefore be carried out in an installation of the type of that of FIG. 2. The preceding results correspond to an installation diagram such as that of FIG. 2, but keeping step a5) of contacting upstream of the gas / liquid separator 20, as indicated in FIG. 1. In Example 1, all of the relatively light hydrocarbon liquid phase separated in the gas / liquid separator 14 is used as reflux in the stripping column.

The purity of the hydrogen leaving the reactor of the first stage ai) is 63.26, while it is 80 leaving the reactor of the second reaction stage a3). This makes it possible to have a partial pressure of 6.8 MPa at the outlet of the reactor of step a3) greater than the partial pressure of the hydrogen at the outlet of the reactor of stage one ai), which is 6.2 MPa, while c 'is the reverse order for total pressures.

The feed supplied in step a3) is completely rid of impurities such as H2S, NH3, H2O (less than 5 ppm for each of these compounds) and more than 99% by weight of light hydrocarbons having from 1 to 4 carbon atoms present at the end of step ai).

Example 2:

Charge: identical to that of Example 1, The design of the installation is made to obtain the same quality of charge at the end of the first step, and the same final product as in the installation of Example 1 This installation also performs stripping with recovery at the bottom of the column of approximately 99% by weight of the fraction boiling above 150 ° C, and also works according to the first variant of the process according to the invention.

Make-up hydrogen purity: 99,999

Operating conditions of the first step ai) and first loop REC1: Catalyst: HR 448 catalyst of Ni-Mo type on alumina, sold by the company

AXENS (formerly by PROCATALYSE).

Average reactor temperature: 380 ° C,

Reactor outlet pressure: 7.3 MPa

Partial pressure H2 reactor outlet: 5.8 MPa VVH (hourly space speed): 0.55 h ^"1

Hydrogen level (inlet + quench): 625 Nm ³ / m ³ of charge

Hydrogen consumed in step ai): 2.03% by weight relative to the charge,

Operating conditions of P step a2):

Stripping column inlet temperature: 220 ° C Stripping column inlet pressure: 7.0 MPa

Number of theoretical plates above the food: 9

Number of theoretical plates below the feed: 15

Reflux rate in relation to the installation charge (liquid reflux / charge stage ai):

0.25 kg / kg, H2 stripping: 95% P hydrogen consumed in step ai)

Catalyst: LD 402 platinum type catalyst on alumina, sold by the company

AXENS (formerly by PROCATALYSE).

Average reactor temperature: 300 ° C, Reactor outlet pressure: 7.6 MPa

Partial pressure H2 at reactor outlet: 7.2 MPa

VVH: 7.5 hr ^"1

Hydrogen content (input + quench): 700 Nm ³ / m ³ of charge from step a3),

Hydrogen consumed in step a3): 1.10% relative to the charge in step ai),

Example 2 uses a higher pressure for the second reactor than for the first. This example can therefore be carried out in an installation of the type of that of FIG. 1.

The purity of the hydrogen leaving the reactor of the first stage ai) is 79.45 while it is 94.73 leaving the reactor of the second reaction stage a3).

This example is not limiting and one could have, for example, lower pressures in step ai), for example absolute pressures between 3.8 and 6.2 MPa, while the pressure of step a3) could be 1.2 to 4.5 MPa higher than that of step ai). One could also, in particular if the pressure of stage ai) is relatively low, and the feed of stage a3) still contains notable traces of sulfur, to use in stage a3) catalysts with double noble metal, or compound of noble metal, for example of the platinum / palladium on alumina type and / or any type of catalyst resistant to traces of sulfur (and / or to carry out an elimination of these traces of sulfur in the REC2 loop).

The process according to the invention makes it possible, for various loads and product specifications, to eliminate in the most efficient manner possible all the pollutants present in the first hydrotreatment stage, and to optimally use the best available catalysts for the second stage (high hydrogen partial pressure and minimum level of impurities), and this with high energy efficiency, without requiring consumption of stripping vapor.

Claims

1. Method for hydrotreating a hydrocarbon feedstock containing sulfur-containing compounds, comprising the following steps: • a first hydrotreatment step ai) in which said feedstock and excess hydrogen are passed over a first hydrotreatment catalyst , to convert at least part of the sulfur contained in the feed into H2S,

• downstream of step ai), a step a2) of stripping the charge at least partially desulphurized in step ai) in a column under pressure, with at least one stripping gas rich in hydrogen, to produce at least a stripping gaseous effluent rich in hydrogen and at least one liquid stripping effluent, P stripping gaseous effluent being at least partly compressed and recycled at the inlet of the first step ai) by means of a first recycling loop REC1 ,

• downstream of step a2), a second hydrotreatment step a3) in which P liquid stripping effluent and excess hydrogen are passed over a second hydrotreatment catalyst, the effluent from this step a3 ) being fractionated, in a gas / liquid separation step a4), into a gaseous fraction rich in hydrogen and a hydrotreated liquid fraction, said gaseous fraction rich in hydrogen being at least partially compressed and recycled at the inlet of P step a3 ) by means of a second REC2 recycling loop separate from the REC1 loop.

2. The method of claim 1 further comprising a step a5) of contacting at least a portion of said hydrotreated liquid fraction with at least a portion of the gas stream flowing in the loop REC1, the effluent of this step a5 ) being separated into a contacting liquid effluent and a contacting gaseous effluent, said contacting gaseous effluent being at least partly recycled in the loop REC1.

3. Method according to one of claims 1 and 2, wherein the loop REC2 is supplied with hydrogen independently of the loop REC1, by one or more streams of hydrogen exclusively consisting (s) of stream (s) of hydrogen d external backup (s) to the REC1 loop.

4. Method according to one of claims 1 to 3, further comprising one or more treatments carrying out at least partial elimination of PH2S contained in the recycling gases circulating in the loops REC1 and REC2, in which each of this or these treatments consists of the combination of a step of bringing at least part of the recycling gas circulating in the REC1 loop into contact with a liquid fraction hydrocarbon allowing a limited absorption of H2S by this liquid hydrocarbon fraction, followed by a gas / liquid separation step of the mixture resulting from this contacting and a direct evacuation of at least part of the liquid thus separated .

5. Method according to one of claims 1 to 4 wherein the step a2) stripping is carried out in a stripping column comprising a section for rectification of stripping vapors by means of a liquid reflux.

6. Method according to one of claims 1 to 5, in which the head vapors of the stripping column are cooled, in order to condense a relatively light and substantially desulphurized liquid hydrocarbon phase, these cooled vapors are separated from said relatively hydrocarbon liquid phase light in a reflux separator flask, a fraction of this relatively light liquid hydrocarbon phase is taken which is used as liquid reflux from the stripping column, and at least part of the fraction is discharged directly downstream residual of the relatively light hydrocarbon liquid phase.

7. Method according to one of claims 1 to 6, in which there is supplied in step a3) a flow of extra hydrogen over the hydrogen requirements of this step, a current is extracted from the loop REC2 of purge gas rich in hydrogen, and this current is sent to at least one point of the loop REC1.

8. The method of claim 7, wherein at least a portion of said purge gas is used as stripping gas in step a2).

9. Method according to one of claims 1 to 8, in which the hydrotreatment step a3) is carried out with at least one catalyst comprising at least one noble metal or a noble metal compound chosen from the group consisting of palladium and platinum.

10. The method of claim 9, wherein the severity of desulfurization of step ai) is chosen so that the feed of step a3) has a sulfur content compatible with the sulfur tolerance threshold of the catalyst of the step a3).

1 1. Method according to one of claims 1 to 10, wherein the purity Pur2 of hydrogen in the recycling loop REC2 is greater than the purity Pur1 of hydrogen in the recycling loop REC1.

12. Method according to one of claims 1 to 11, in which the loops REC1 and REC2 are supplied by separate make-up hydrogen streams of different purity, the purity of the make-up hydrogen stream feeding the loop REC2 being greater than that of the additional hydrogen current supplying the loop REC1.

13. Method according to one of claims 1 to 12, wherein the pressure of step a3) is at least 1.2 MPa and at most 12 MPa higher than that of step ai).

14. Method according to one of claims 1 to 12, wherein the pressure of step a3) is at least 0.1 MPa lower than that of step ai), and the partial pressure of hydrogen of step a3) is greater than the partial pressure of hydrogen in step ai).

15. Hydrocarbon cut of the group of petrol, jet fuel, kerosene, diesel fuel, gas oil, vacuum distillate, and deasphalted oil, containing at least one fraction hydrotreated by the process according to one of the claims 1 to 14.

16. Installation for hydrotreating a hydrocarbon feedstock for the implementation of the process according to one of claims 1 to 15, comprising:

• A first hydrogen recycling loop REC1, this loop comprising at least a first hydrotreatment reactor (7), connected downstream to a column (10) for stripping under pressure the liquid effluent from the reactor by a rich gas in hydrogen, the head of the column (10) being connected to a means (12) for cooling and partial condensation of the gas stream from the column (10), this means (12) for cooling and partial condensation being connected in downstream to a first gas / liquid separator (14), itself connected to the suction of a first recycling compressor (22), the discharge of this first compressor being connected to the first hydrotreatment reactor (7), • A second hydrogen recycling loop REC2, separate from the loop REC1, this loop comprising at least a second hydrotreatment reactor (48), this reactor being connected upstream to the bottom of the stripping column (10), for the hydrotreatment of effluent li which comes from the bottom of the stripping column (10), and connected downstream to a second gas / liquid separator (53), itself connected to the suction of a second recycling compressor (55), the discharge of this second compressor being connected to the second hydrotreatment reactor (48),

17. Installation according to claim 16 in which the loop REC2 is supplied with hydrogen by one or more supply means (33), each of these supply means (33) being connected upstream exclusively to one or more sources of external hydrogen (s).

18. Installation according to one of claims 16 and 17, comprising at least one conduit (58) for supplying in the loop REC1 a stream of purge hydrogen from the loop REC2, this conduit being connected upstream to a point of the REC2 loop and downstream at a point of the REC1 loop.

19. Installation according to one of claims 16 to 18, comprising a first means (34) for supplying an external hydrogen stream of relatively low purity to the REC1 loop, and a second means (33) supply of a relatively high purity external make-up hydrogen stream to the REC2 loop.

20. Installation according to one of claims 16 to 19, wherein the stripping column comprises above its feed a rectification zone having a separation efficiency of at least 1 theoretical plate.

21. Installation according to one of claims 16 to 20, comprising a conduit (27) for discharging downstream a light liquid stripping fraction, this conduit being connected upstream to the first gas / liquid separator (14) .