Patent Publication Number: US-2004050753-A1

Title: Method for producing diesel fuel by moderate pressure hydrocracking

Description:
[0001] The invention relates to a process with moderate pressure hydrocracking, for the production of very high quality diesel in high yields.  
       [0002] The invention also relates to  
       [0003] a process including said hydrocracking process and to a catalytic cracking process, and to a unit for use in carrying out the process.  
       [0004] The refining industry must now find refinery layouts that can be adapted to the tightening of regulations regarding fuel quality and which will be in force in Europe in 2005. The maximum sulphur content in diesel will be at most 50 ppm. The 95% distillation point (ASTM D-86) for diesel, currently  360 ° C., will probably be reduced, for example by 10° C., which for a refinery currently represents a reduction of 5% in the volume of diesel produced. It is also envisaged that the existing permitted quantities of polyaromatic compounds will be halved from its existing 11% by weight. The cetane number will also be increased to above 51, for example passing from its existing value of 51 to 52.  
       [0005] At the same time, the demand for diesel is constantly increasing; over the next decade, demand is expected to increase by about 20%.  
       [0006] Distilling crude is not sufficient to cover diesel production; diesel is currently produced by high pressure hydrocracking processes (in general at least 120 bars hydrogen partial pressure), treating heavy feeds that are feeds with a T 95  temperature that is usually of the order of at least 500° C., T 95  being the temperature of the 95% by volume obtained by simulated distillation (ASTM-D28 87). Heavy compounds are cracked into lighter compounds, a portion of which is in the middle distillate cut (diesel and kerosine) of the hydrocracking distillation. Such high pressure processes are conventional.  
       [0007] To satisfy the new standards, diesel from crude distillation will have to undergo deep hydrodesulphurisation. Further, high pressure hydrocracking is a solution that can be expensive. Thus, a solution that is more advantageous has been sought that can also be integrated into existing units to optimise the use of existing refinery resources.  
       [0008] The process disclosed in the present application is a hydrocracking process functioning at moderate temperatures (above 70 bars and at most 100 bars of hydrogen partial pressure) that can directly produce diesel satisfying 2005 specifications from relatively light feeds under more economical conditions than those used in high pressure hydrocracking.  
       [0009] More precisely, the invention concerns a process for producing a diesel having a 95% distillation point of less than 360° C., a sulphur content of at most 50 ppm and a cetane number of more than 51, said process treating hydrocarbon feeds with a T 5  temperature in the range 250° C. to 400° C. and a T 95  temperature of at most 470° C., said process comprising hydrotreatment followed by hydrocracking under a hydrogen partial pressure of more than 70 bars and at most 100 bars, at a temperature of at least 320° C., with a H 2 /feed volume ratio of at least 200 Nl/Nl, an hourly space velocity of 0.15-7 h −1  and the process being carried out with a conversion of at least 80% by volume and the liquid effluent obtained by hydrocracking being distilled to separate the diesel. Preferably, the distillation residue is recycled to the process after purging.  
       [0010] Feeds, Hydrotreatment and Hydrocracking  
       [0011] The feeds treated in the process have a T 5  point in the range 250° C. to 400° C., preferably in the range 280° C. to 370° C. The point T 5  represents the temperature of the 5% by volume point obtained by simulated distillation (ASTM-D28 87).  
       [0012] In general, the feeds have a T 5  point in the range 320-400° C. or in the range 320-370° C. Highly advantageously, a diesel fraction can be added to these feeds, for example a heavy diesel from atmospheric crude distillation, which usually has a T 5  point of the order of at least 280° C. This heavy diesel fraction can also be obtained directly in the atmospheric residue.  
       [0013] This arrangement (adding diesel) is particularly advantageous. It allows a heavy portion of the diesel fraction, which is charged with nitrogen-containing and sulphur-containing compounds that are the most difficult to be hydrotreated, to be treated by hydrotreatment followed by moderate pressure hydrocracking. From then on, conventional hydrotreatment can be used to treat the remaining diesel fraction, which needs no expensive investment.  
       [0014] Further, by treating that heavy diesel portion in the process of the invention, the heavy fraction is hydrodesulphurised and at the same time its qualities are improved (cetane number higher than that which would have been obtained by severe hydrotreatment alone).  
       [0015] The feeds that can be used also have a T 5  temperature of at most 470° C., preferably at most 450° C., more preferably in the range 390-430° C., T 95  representing the temperature of the 95% point obtained by simulated distillation (ASTM-D28 87).  
       [0016] Feeds that can be cited are light vacuum distillates, a light fraction of a conventional vacuum gas oil (VGO) (for example, the lightest half), heavy atmospheric gas oils (HGO), mixtures of said feeds or mixtures of said feeds with at least one diesel fraction, for example from crude distillation or from an FCC (catalytic cracking) unit.  
       [0017] The sulphur contents of the treated hydrocarbon feeds are generally 0.2% to 4% by weight, and the nitrogen contents are 100-3500 ppm by weight. Thus, they are generally hydrotreated before being hydrocracked to reduce the organic nitrogen contents (i.e., the nitrogen contained in organic molecules) to below 80 ppm, or preferably to below 50 ppm, more preferably to below 10 ppm, and the amounts of organic sulphur (i.e., sulphur contained in organic molecules) to below 200 ppm, preferably below 50 ppm. These hydrotreated feeds (clean feeds) can then undergo hydrocracking.  
       [0018] The hydrotreatment conditions are generally:  
       [0019] pressure of 5-25 MPa, preferably with a hydrogen partial pressure of more than 70 bars and at most 100 bars, preferably at least 80 bars or at least 85 bars,  
       [0020] temperature of at least 320° C., in general at least 350° C. and at most 450° C., usually at most 430° C.,  
       [0021] H 2 /feed volume ratio of at least 100 N/l, usually between 100-2000 Nl/l or 300-2000 Nl/l,  
       [0022] hourly space velocity of 0.1-10 h −1 , preferably 0.15-7 h −1 , advantageously 0.054 h −1 .  
       [0023] The conversion achieved with hydrotreatment is generally at least 40% by volume, and less than 40% of product boiling below 350° C.  
       [0024] Hydrotreatment can be carried out either in a hydrocracking reactor and in at least one bed preceding the first hydrocracking catalyst bed, in the direction of feed flow, or in an independent reactor preceding the hydrocracking reactor. There may or may not be intermediate separation of the gases regenerated by hydrotreatment. The first mode (same reactor) with no intermediate separation is preferred. The process also includes an implementation in which hydrotreatment is carried out in the refinery a long way upstream of the hydrocracking step; intermediate treatments can also be carried out.  
       [0025] At least a portion of the clean feed is brought into contact with at least one hydrocracking catalyst in the presence of hydrogen under the following operating conditions:  
       [0026] hydrogen partial pressure of more than 70 bars and at most 100 bars, preferably at least 80 bars or at least 85 bars;  
       [0027] temperature of at least 320° C., in general at least 350° C. and at most 450° C., usually at most 430° C.;  
       [0028] H 2 /feed volume ratio of at least 200 Nl/Nl, usually in the range 300-2000 Nl/Nl;  
       [0029] hourly space velocity 0.15-7 h −1 , preferably 0.054 h −1 .  
       [0030] The process can function with or without a recycle of the distillation residue of the hydrocracking residue (unconverted fraction). When present, the recycle is made to the hydrocracking reactor if it is separated from the hydrotreatment step, for example, or to the feed entering the reactor where hydrotreatment and hydrocracking are carried out.  
       [0031] Under these conditions, for the global process, the conversion of products boiling below 350° C. is at least 80% by volume, more generally at least 90% by volume, or at least 95% by volume.  
       [0032] Hydrotreatment Catalysts  
       [0033] Conventional catalysts can be used, which contain at least one amorphous support and at least one hydrodehydrogenating element (generally at least one element from groups VIB and non noble elements from group VIII, and usually at least one element from group VIB and at least one non noble element from group VIII).  
       [0034] Highly advantageously, a hydrotreatment catalyst comprises at least one matrix, at least one hydrodehydrogenating element selected from the group formed by elements from group VIB and from group VIII of the periodic table, optionally at least one promoter element deposited on the catalyst and selected from the group formed by phosphorus, boron and silicon, optionally at least one element from group VIIA (preferably chlorine, fluorine), and optionally at least one element from group VIIB (preferably manganese), and optionally at least one element from group VB (preferably niobium).  
       [0035] In general, the hydrotreatment catalyst contains:  
       [0036] 5% to 40% by weight of at least one element from group VIB and non noble group VIII (5 oxide);  
       [0037] 0-20% of at least one promoter element selected from phosphorus, boron, silicon (% oxide), preferably 0.1-20%; advantageously, boron and/or silicon are present, and optionally phosphorus;  
       [0038] 0-20% of at least one group VIIB element (for example manganese);  
       [0039] 0-20% of at least one group VIIA element (for example fluorine, chlorine);  
       [0040] 0-60% of at least one group VB element (for example niobium);  
       [0041] 0.1-95% of at least one matrix, preferably alumina.  
       [0042] Preferably, this catalyst contains boron and/or silicon as a promoter element, optionally with additional phosphorus as the other promoter element. The boron, silicon and phosphorus contents are 0.1-20%, preferably 0.1-15%, more advantageously 0.1-10%.  
       [0043] Non limiting examples of matrices that can be used alone or as mixture are alumina, halogenated alumina, silica, silica-alumina, clays (for example natural clays such as kaolin or bentonite), magnesia, titanium oxide, boron oxide, zirconia, aluminium phosphates, titanium phosphates, zirconium phosphates, charcoal, aluminates. Preferably, matrices containing alumina are used, in forms known to the skilled person, more preferably aluminas, for example gamma alumina.  
       [0044] The hydrodehydrogenating function is preferably provided by at least one metal or compound of a metal from non noble group VIII and VI, preferably selected from molybdenum, tungsten, nickel and cobalt. Preferably, it is supplied by a combination of at least one element from group VIII (Ni, Co) with at least one element from group VIB (Mo, W).  
       [0045] This catalyst can advantageously contain phosphorus; in the prior art, it is known that this compound endows hydrotreatment catalysts with two advantages: ease of preparation in particular when impregnating nickel and molybdenum solutions, and better hydrogenation activity.  
       [0046] In a preferred catalyst, the total concentration of oxides of metals from groups VI and VIII is in the range 5% to 40% by weight, preferably in the range 7% to 30% by weight, and the weight ratio, expressed as the ratio of metal oxide between the group VIB metal (or metals) and the group VII metal (or metals) is preferably in the range 20 to 1.25, more preferably in the range 10 to 2. The concentration of phosphorous pentoxide P 2 O 5  is less than 15% by weight, preferably 10% by weight.  
       [0047] A further preferred hydrotreatment catalyst that contains boron and/or silicon (preferably boron and silicon) generally comprises, as a % by weight with respect to the total catalyst weight, at least one metal selected from the following groups and in the following amounts:  
       [0048] 3% to 60%, preferably 3% to 45%, more preferably 3% to 30% or at least one group VIB metal;  
       [0049] and optionally  
       [0050] to 30%, preferably 0 to 25%, more preferably 0 to 20% of at least one group VIII metal;  
       [0051] the catalyst further comprising at least one support selected from the following groups and in the following amounts:  
       [0052] 0 to 99%, advantageously 0.1% to 99%, preferably 10% to 98%, and more preferably 15% to 95%, of at least one amorphous or low crystallinity matrix;  
       [0053] said catalyst being characterized in that it further contains:  
       [0054] 0.1% to 20%, preferably 0.1% to 15%, more preferably 0.1% to 10% of boron and/or. 0.1% to 20%, preferably 0.1% to 15%, more preferably 0.1% to 10% of silicon;  
       [0055] and optionally:  
       [0056] 0 to 20%, preferably 0.1% to 15%, more preferably 0.1% to 10% of phosphorus;  
       [0057] and preferably again;  
       [0058] 0 to 20%, preferably 0.1% to 15%, more preferably 0.1% to 10% by weight of at least one element selected from group VIIA, preferably fluorine.  
       [0059] In general, formulae with the following atomic ratios are preferred:  
       [0060] a group VIII metal/group VIB metal atomic ratio in the range 0 to 1;  
       [0061] a B/group VIB metal atomic ratio in the range 0.01 to 3;  
       [0062] an Si/group VIB metal atomic ration in the range 0.01 to 1.5;  
       [0063] a P/group VIB metal atomic ratio in the range 0.01 to 1;  
       [0064] a group VIIA element/group VIB metal atomic ratio in the range 0.01 to 2.  
       [0065] Such a catalyst has a higher activity for hydrogenating aromatic hydrocarbons and for hydrodenitrogenation and hydrodesulphurisation than catalytic formulae containing no boron and/or silicon, and also has a higher activity and selectivity for hydrocracking than known catalytic formulae. The catalyst containing boron and silicon is particularly advantageous. Without wishing to be bound by a particular theory, it appears that the particularly high catalytic activity with boron and silicon is due to a reinforcement of the acidity of the catalyst by the joint presence of boron and silicon in the matrix, which causes both an improvement in the hydrogenating properties, hydrodesulphurisation properties and hydrodenitrogenation properties, and an improvement in the hydrocracking activity compared with catalysts normally used in hydrorefining and hydroconversion reactions.  
       [0066] Preferred catalysts are NiMo and/or NiW on alumina type catalysts, also NiMo and/or NiW on alumina type catalysts doped with at least one element selected from the group formed by phosphorus, boron, silicon and fluorine, or NiMo and/or NiW type catalysts on silicaalumina, or on silica-alumina-titanium oxide doped or otherwise by at least one element selected from the group formed by phosphorus, boron, fluorine and silicon.  
       [0067] A further particularly advantageous catalyst (in particular with an improved activity) for hydrotreatment comprises a partially amorphous Y zeolite that will be described below in the hydrocracking catalyst section.  
       [0068] Prior to injection of the feed, the catalysts used in the process of the present invention preferably undergo a sulphurisation treatment to at least partially transform the metallic species to the sulphide prior to bringing them into contact with the feed to be treated. This sulphurisation activation treatment is well known to the skilled person and can be carried out using any method that has been described in the literature, either in-situ, i.e., in the reactor, or ex-situ.  
       [0069] A conventional sulphurisation method that is well known to the skilled person consists of heating in the presence of hydrogen sulphide (pure or, for example, in a stream of a hydrogen/hydrogen sulphide mixture) at a temperature in the range 150° C. to 800° C., preferably in the range 250° C. to 600° C., generally in a traversed bed reaction zone.  
       [0070] Hydrocracking Catalysts  
       [0071] A preferred catalyst comprised at least one Y zeolite, at least one matrix and at least one hydrodehydrogenating function. Optionally, it can also contain at least one element selected from boron, phosphorus and silicon, at least one group VIIA element (for example chlorine, fluorine), at least one group VIIB element (for example manganese), and at least one group VB element (for example niobium).  
       [0072] The catalyst comprises at least one porous or low crystallinity mineral oxide type matrix. Non limiting examples that can be cited are aluminas, silicas, silica-aluminas, aluminates, alumina-boron oxide, magnesia, silica-magnesia, zirconia, titanium oxide and clay, used alone or as a mixture.  
       [0073] The hydrodehydrogenating function is generally provided by at least one element from group VIB (for example molybdenum and/or tungsten) and/or at least one non noble element from group VIII (for example cobalt and/or nickel) of the periodic table. A preferred catalyst essentially contains at least one group VI metal, and/or at least one non noble group VIII metal, Y zeolite and alumina. A more preferred catalyst essentially contains nickel, molybdenum, Y zeolite and alumina.  
       [0074] The catalyst optionally comprises at least one element selected from the group formed by boron, silicon and phosphorus. Advantageously, the catalyst optionally comprises at least one element from group VIIA, preferably chlorine or fluorine, optionally at least one element from group VIIB (for example manganese) and optionally at least one element from group VB (for example niobium).  
       [0075] Boron, silicon and/or phosphorus can be in the matrix or the zeolite or, as is preferable, deposited on the catalyst and thus principally located on the matrix. A preferred catalyst contains B and/or Si as a promoter element, preferably deposited in addition to the phosphorus promoter. The quantities introduced are 0.1-20% by weight of catalyst, calculated as the oxide.  
       [0076] The element introduced, in particular silicon, principally located on the matrix of the support, can be characterized by techniques such as a Castaing microprobe (distribution profile of the various elements), transmission electron microscopy coupled to X ray analysis of the catalyst components, or by producing a distribution map of the elements present in the catalyst using an electronic microprobe.  
       [0077] In general, a preferred hydrocracking catalyst advantageously contains:  
       [0078] 0.1-80% by weight of Y zeolite;  
       [0079] 0.140% by weight of at least one element from group VIB and group VIII (% oxide);  
       [0080] 0.1-99.8% by weight of matrix (% oxide);  
       [0081] 0-20% by weight of at least one element selected from the group formed by P, B, Si (% oxide), preferably 0.1-20%;  
       [0082] 0-20% by weight of at least one group VIIA element, preferably 0.1-20%;  
       [0083] 0-20% by weight of at least one group VIIB element, preferably 0.1-20%;  
       [0084] 0-60% by weight of at least one group VB element, preferably 0.1-60%.  
       [0085] Regarding the silicon, the range 0-20% only includes the added silicon and not that in the zeolite.  
       [0086] The zeolite can optionally be doped by metallic elements such as metals from the rare earth series, in particular lanthanum and cerium, or noble or non noble group VIII metals such as platinum, palladium, ruthenium, rhodium, iridium, iron and other metals such as manganese, zinc, magnesium.  
       [0087] Different Y zeolites can be used.  
       [0088] A particularly advantageous H-Y acid zeolite is characterized by different specifications: a global SiO 2 /Al 2 O 3  mole ratio in the range about 6 to 70, preferably in the range about 12 to 50; a sodium content of less than 0.15% by weight, determined for the zeolite calcined at 1100° C.; a lattice parameter in the range 24.58×10 −10  m to 24.24×10 −10  m, preferably in the range 24.38×10 −10  m to 24.26×10 −10  m; a CNa sodium ion take-up capacity, expressed as grams of Na per 100 grams of modified zeolite, neutralised then calcined, of more than about 0.85; a specific surface area, determined using the BET method, of more than about 400 m 2 /g, preferably more than 550 m 2 /g, a water vapour absorption capacity at 25° C. for a partial pressure of 2.6 torrs (i.e., 34.6 MPa), of more than about 6%; and advantageously, the zeolite has a pore distribution, determined by nitrogen physisorption, in the range 5% to 45%, preferably in the range 5% to 40% of the total pore volume of the zeolite contained in pores with a diameter located between 20×10 −10  m to 80×10 −10  m, and in the range 5% to 45%, preferably in the range 5% to 40% of the total pore volume of the zeolite contained in pores with a diameter of more that 80×10 −10  m and generally less than 1000×10 −10  m, the remainder of the pore volume being contained in pores with a diameter of less than 20×10 −1   0  m.  
       [0089] A preferred catalyst using this type of zeolite comprises a matrix, at least one dealuminated Y zeolite with a lattice parameter in the range 2.424 nm to 2.455 nm, preferably in the range 2.426 nm to 2.438 nm, a global SiO 2 /Al 2 O 3  mole ratio of more than 8, an amount of alkaline-earth metal or alkali metal cations and/or rare earth cations such that the atomic ratio (n×M n+ )/Al is less than 0.8, preferably less than 0.5 or 0.1, a specific surface area determined by the BET method of more than 400 m 2 /g, preferably more than 550 m 2 /g, and a water adsorption capacity at 25° C. for a P/P 0  value of 0.2 of more than 6% by weight, said catalyst also comprising at least one hydrodehydrogenating metal, and silicon deposited on the catalyst.  
       [0090] In an advantageous implementation of the invention, a catalyst comprising a partially amorphous Y zeolite is used for hydrocracking.  
       [0091] The term “partially amorphous Y, zeolite” means a solid with:  
       [0092] i/ a peak intensity that is less than 0.40, preferably less than about 0.30;  
       [0093] ii/ a crystalline fraction, expressed with respect to a reference Y zeolite in the sodium form (Na), which is less than about 60%, preferably less than about 50%, and determined by X ray diffraction.  
       [0094] Preferably, partially amorphous Y zeolites, solids forming part of the composition of the catalyst of the invention, have at least one (preferably all) of the following characteristics:  
       [0095] iii/ a global Si/Al ratio of more than 15, preferably more than 20 and less than 150;  
       [0096] iv/ a framework Si/Al IV  greater than or equal to the global Si/Al;  
       [0097] v/ a pore volume of at least 0.20 ml/g of solid of which a fraction, in the range 8% to 50%, is constituted by pores with a diameter of at least 5 nm (nanometres), i.e., 50 Å;  
       [0098] a specific surface area of 210-800 m 2 /g, preferably 250-750 m 2 /g, advantageously 300-600 m 2 /g.  
       [0099] The peak intensities and crystalline fractions are determined by X ray diffraction, using a procedure derived from the ASTM D3906-97 method “Determination of relative X-ray diffraction intensities of faujasite-type-containing materials”. Reference should be made to this method for the general conditions of application of the procedure, in particular for the preparation of the samples and references.  
       [0100] A diffractogram is composed of characteristics lines from the crystalline fraction of the sample and a background, caused essentially by diffusion from the amorphous or microcrystalline fraction of the sample (a weak diffusion signal can be linked to the apparatus, air, sample carrier, etc). The peak intensity of a zeolite is the ratio, within a pre-set angular zone (typically 8° to 40°−2θ when using the Kα copper curve (1=0.154 nm), of the area of the zeolite lines (peaks) over the overall area of the diffractogram (peaks+background). This peak/(peak+background) ratio is proportional to the quantity of crystalline zeolite in the material. To estimate the crystalline fraction of a Y zeolite sample, the peak intensity of the sample is compared with that of a reference considered to be 100% crystalline (for example NaY). The peak intensity of a perfectly crystalline NaY zeolite is of the order of 0.55 to 0.60.  
       [0101] The peak intensity of a conventional USY zeolite is 0.45 to 0.55; its crystalline fraction with respect to a perfectly crystalline NaY is 80% to 95%. The peak intensity of the solid used in the present invention is less than 0.4, preferably less than 0.35. Its crystalline fraction is thus less than 70%, preferably less than 60%.  
       [0102] The partially amorphous zeolites are prepared from commercially available Y zeolites, i.e., generally with high crystallinities (at least 80%) using dealumination techniques that are in general use. More generally, it is possible to start from zeolites with a crystalline fraction of at least 60%, or at least 70%.  
       [0103] Y zeolites generally used in hydrocracking catalysts are produced by modifying commercially available Na-Y zeolite. This modification can produce zeolites that are termed stabilised, ultra-stabilised or dealuminated. This modification is carried out by at least one of the dealumination techniques, for example by hydrothermal treatment, or acid attack. Preferably, this modification is carried out by combining three types of operations that are known in the art: hydrothermal treatment, ion exchange and acid attack.  
       [0104] A further particularly advantageous zeolite is a globally non dealuminated and highly acidic zeolite.  
       [0105] The term “globally non dealuminated” means a Y zeolite (structure type FAU, faujasite) in accordance with the nomenclature developed in the “Atlas of zeolite structure types”, W. M. Meier, D. H. Olson and Ch. Baerlocher, 4 Revised edition, 1996, Elsevier. The lattice parameter of this zeolite may be reduced by extracting aluminium from the structure or framework during preparation but the global SiO 2 /Al 2 O 3  ratio is not changed as the aluminium is not chemically extracted. Such a globally non dealuminated zeolite thus has a silicon and aluminium composition, expressed as the global SiO 2 /Al 2 O 3  ratio, equivalent to the starting non dealuminated Y zeolite. The values for the other parameters (SiO 2 /Al 2 O 3  ratio and lattice parameter) are given below. This globally non dealuminated Y zeolite can be in the hydrogen form or it can be at least partially exchanged with metal cations, for example using cations of alkaline-earth metals and/or cations of rare earth metals with atomic number 57 to 71 inclusive. Preferably, a zeolite that is depleted in rare earths and alkaline-earths is used, also for the catalyst.  
       [0106] The globally non dealuminated Y zeolite generally has a lattice parameter of more than 2.438 nm, a global SiO 2 /Al 2 O 3  of less than 8, a framework SiO 2 /Al 2 O 3  ratio of less than 21 and more than the global SiO 2 /Al 2 O 3  ratio. The globally non dealuminated zeolite can be obtained by any treatment that does not extract aluminium from the sample, such as steam treatment, treatment with SiCl 4 , etc. . . .  
       [0107] A further type of advantageous catalyst for hydrocracking contains in acidic amorphous oxide matrix of the alumina type doped with phosphorus, a globally non dealuminated and highly acidic Y zeolite and optionally, at least one element from group VIIA, in particular fluorine.  
       [0108] The invention is not limited to the preferred Y zeolites cited above, but other types of Y zeolites can be used in the process.  
       [0109] Prior to injection of the feed, the catalyst undergoes a sulphurisation treatment to transform at least a portion of the metallic species into the sulphide prior to bringing them into contact with the feed to be treated. This sulphurisation activation treatment is well known to the skilled person and can be carried out using any method that has been described in the literature, either in-situ, i.e., in the reactor, or ex-situ.  
       [0110] A conventional sulphurisation method that is well known to the skilled person consists of heating in the presence of hydrogen sulphide (pure or, for example, in a stream of a hydrogen/hydrogen sulphide mixture) at a temperature in the range 150° C. to 800° C., preferably in the range 250° C. to 600° C., generally in a traversed bed reaction zone.  
       [0111] When the hydrotreatment and hydrocracking catalysts are in the same reactor or in two reactors with no intermediate separation, they are sulphurised at the same time.  
       [0112] Product Separation  
       [0113] The liquid effluent from hydrocracking is then distilled to separate a naphtha cut, a diesel cut, possibly a kerosine cut (which can sometimes include at least a portion of the diesel cut), light LPG gases. A liquid residue remains that can advantageously be recycled to the process generally after purging.  
       [0114] Clearly, the hydrogen has also been separated from the liquid effluent, which can subsequently be stripped before being distilled.  
       [0115] Unexpectedly, it has been seen that with the process of the invention, very high quality diesels are produced that satisfy specifications and no further treatment (severe hydrodesulphurisation, hydrogenation, etc. . . . ) is necessary to improve its qualities.  
       [0116] The process can directly produce a diesel with a 95% by volume distillation point of less than 360° C., and generally, this point is at most 350° C., or even at most 340° C., and has a sulphur content of at most 50 ppm, generally at most 10 ppm, with a cetane number of at least 52 and more generally at least 54, preferably with a polyaromatic compound content of at most 6% and generally at most 1%, preferably a pour point of less than −10° C., preferably an aromatics content of less than 15% by weight.  
       [0117] This process also produces a good quality kerosine with a smoke point of more than 20 mm, preferably more than 22 mm, and with a sulphur content of less than 50 ppm, preferably less than 10 ppm. At least a portion of the kerosine can optionally be sent to the diesel pool, depending on the operator&#39;s requirements.  
       [0118] It is quite remarkable that such diesel quality can be obtained without subsequent treatment and for a much lower investment that high pressure hydrocracking, while enabling upgrading of “light” refinery feeds such as existing gas oils which, because of the tightening of specifications, are either in excess or have to undergo subsequent severe treatments.  
       [0119] Regarding the yields of middle distillate (kerosine+diesel) produced, these are at least 60% by volume, usually at least 65% by volume. The other products formed are light LPG gases (representing at most 10% by weight and more generally at most 5% by weight) and naphtha (generally at least 20% by volume). 
     
    
    
     THE FIGURES  
     [0120] The process will now be described in brief with reference to the Figures.  
     [0121]FIG. 1 shows an implementation of a moderate pressure hydrocracking process.  
     [0122]FIGS. 2B, 2C,  3  and  4  show this process integrated into a catalytic cracking unit, and FIG. 2A shows the prior art.  
    
    
     [0123] The moderate pressure hydrocracking process is shown in FIG. 1 The feed to be treated enters via line  1 , and in the Figure, it is added to the hydrocracking residue recycle via line  2  and hydrogen via line  3 . It passes through a heat exchanger  4  mixed with recycled hydrogen supplied via line  5 , then through a reheater  6  before being introduced into the moderate pressure hydrocracking reactor (or zone)  7  optionally containing upstream hydrotreatment zone(s).  
     [0124] Reactor  7  contains at least one catalytic bed  8  of at least one hydrocracking catalyst. Preferably, it can contain at least one hydrotreatment catalyst upstream of the first bed  8 . The liquid effluent from the reactor leaving via line  9  passes through exchanger  4  then into a gas-liquid separator  10  separating hydrogen, which is recycled to hydrocracking reactor  7  via line  5 .  
     [0125] The separated liquid effluent leaving via line  11  is preferably sent to a stripper  12  that separates naphtha and light gases via line  13  and a resulting effluent leaving via line  14  is distilled in atmospheric distillation column  15 . This arrangement schematically illustrates an embodiment of the distillation. Any other known arrangement that results in separating the same products would also be suitable.  
     [0126] This produces a diesel evacuated via line  16  and a residue recycled to the hydrocracking reactor via line  2 , apart from a purge via line  17 . Kerosine is optionally obtained.  
     [0127] The product separation zone separates a hydrocracking residue with a boiling point of more than at least 535° C., and comprises a line for purging said residue and optionally a line for recycling said purged residue towards the hydrocracking zone or reactor.  
     [0128] The figure does not show the compressors and utilities used, which are known to the skilled person.  
     [0129] The hydrocracking process described here can advantageously be integrated into the refinery into a catalytic cracking process (generally FCC: fluidised bed catalytic cracking).  
     [0130] This results in a combined process producing both good quality diesel and naphtha (for the production of gasoline).  
     [0131] The invention also concerns a unit for carrying out the process described above. This unit comprises:  
     [0132] a column for distilling a hydrocarbon feed to separate at least one fraction with a T 5  temperature in the range 250° C. to 400° C. and a T 95  temperature of at most 470° C.;  
     [0133] at least one zone for hydrotreating said feed or said fraction;  
     [0134] at least one zone for moderate pressure hydrocracking of said fraction, said pressure being more than 70 bars and at most 100 bars;  
     [0135] at least one zone for separating products to obtain a diesel with a 95% distillation point of less than 360° C., a sulphur content of at most 50 ppm and a cetane number of more than 51.  
     [0136] In a particular implementation of the invention, shown below, the unit comprises:  
     [0137] a column for atmospheric distillation of said crude feed to separate at least naphtha, diesel and an atmospheric residue;  
     [0138] a vacuum distillation column to treat said atmospheric residue, and to separate at least one vacuum distillate and a vacuum residue;  
     [0139] in which unit the atmospheric distillation column or vacuum distillation column comprises at least one line recovering a fraction with a T 5  temperature in the range 250° C. to 400° C. and a T 95  temperature of at most 470° C.;  
     [0140] the unit also comprising at least one zone for hydrotreating said fraction followed by at least one moderate pressure hydrocracking zone and at least one zone for separating products to obtain a diesel with a 95% distillation point of less than 360° C., a sulphur content of at most 50 ppm and a cetane number of more than 51.  
     [0141] For a better understanding of the invention and its advantages, FIG. 2A shows the prior art and FIG. 2B shows the invention, and the process of the invention will be described with reference to the Figures.  
     [0142]FIG. 2A shows an existing unit. The crude hydrocarbon feed (or crude oil) arriving via line  20  is distilled in an atmospheric column  21 . A naphtha fraction (line  22 ) (the term “a” in this respect is taken to mean at least one), a fuel fraction (line  23 ) and a diesel fraction (line  24 ) are separated.  
     [0143] The atmospheric residue leaving via line  25  is vacuum distilled in vacuum distillation column  26 . A vacuum distillate (line  27 ) is separated and a vacuum residue remains (line  38 ).  
     [0144] Said vacuum distillate is sent to a catalytic cracking unit  28  (generally a fluidised bed) which produces, inter alia, naphtha evacuated via line  29 , a highly aromatic diesel type fraction (light cycle oil LCO) evacuated via line  30 , and a slurry or residue leaving via line  31 .  
     [0145] Usually, the vacuum residue is treated in a visbreaking unit  39  which, inter alia, produces naphtha (line  40 ) and diesel (line  41 ), both of low quality. Like the slurry, the visbreaking residue (line  42 ) can only be used as fuel; a portion of the LCO can serve to flush the fuel.  
     [0146]FIG. 2B shows the process and unit of the invention, combining moderate pressure hydrocracking with catalytic cracking.  
     [0147] The reference numerals of FIG. 2A are used, and the visbreaker is not shown in order to simplify the figure, although it is generally present in the unit.  
     [0148] In addition to the elements of the prior art (FIG. 2A), the unit of the invention (FIG. 2B), comprises a moderate pressure hydrocracking unit  32  that receives a light fraction from the vacuum distillation step supplied via line  33 .  
     [0149] Unit  32  comprises the moderate pressure hydrocracking reactor(s) or zone(s) and the associated separating means to separate, inter alia, a high quality diesel via line  34 , a naphtha via line  35  and the purge of the hydrocracking residue via line  36 . Normally, unit  32  also comprises a zone for hydrotreatment prior to hydrocracking.  
     [0150] At least a portion of the hydrocracking residue can be sent for catalytic cracking (unit  28 ) but this is not obligatory. The purge from the hydrocracking unit is advantageously sent to unit  28 .  
     [0151] The Figure does not show a recycle of the purged residue to the hydrocracking zone or to the hydrocracking reactor also comprising a hydrotreatment zone. Recycling and passage of the purge to FCC can be carried out separately or together.  
     [0152] Within the context of the process of the invention, as shown, for example, in FIG. 2B with FCC, the cut point for the distilled diesel cut (line  24 ) during atmospheric distillation is selected by the operator.  
     [0153] The atmospheric residue, which thus contains at least a portion of the heavy atmospheric gas oil, is vacuum distilled into at least one light fraction (distillate) and at least one heavy fraction (distillate), and a vacuum residue remains.  
     [0154] Said light fraction to be treated by hydrocracking has a T 5  temperature that is in the range 250° C. to 400° C. and a T 95  temperature that is at most 470° C. It is a light vacuum gas oil (LVGO). The end point is selected by the operator depending on the column that is available and depending on the desired upgrading for the products. Said light fraction has other characteristics of the hydrocarbon feeds treated by the process of the invention and described above.  
     [0155] In general, this light vacuum distillate treatment by moderate pressure hydrocracking can be carried out when the production and/or quality of the diesel is to be increased, regardless of the type of reactor reserved for the heavy distillate(s) and the residue from the vacuum distillation.  
     [0156] In a further implementation, instead of fractionating the atmospheric residue into light and heavy fractions by vacuum distillation and sending the light fraction(s) to the hydrocracking step, the heavy gas oil cut with substantially the same T 5  and T 95  temperatures as regards atmospheric distillation is taken (if the type of column allows it). This mode is shown in FIG. 2C, which shows the same reference numerals as for the preceding figures and where the feed for unit  32  is an atmospheric gas oil supplied via line  37 . In this figure, line  33  no longer exists. In this case, the atmospheric residue boiling above the heavy gas oil is vacuum distilled, vacuum distillation also producing a residue and at least one vacuum distillate, term the heavy distillate in this case.  
     [0157] The vacuum distillation residue (line  38 ), which generally has a T 5  temperature of at least 535° C., preferably at least 550° C., or even at least 565° C. or 570° C., can, for example, undergo visbreaking (shown in FIG. 1) or residue hydroconversion or coking.  
     [0158] In all cases, at least one heavy vacuum distillation fraction located between the light fraction with a T 95  temperature of at most 470° C., and the vacuum residue, undergoes catalytic cracking.  
     [0159]FIG. 3 shows a unit and a process in which prior to said cracking, the heavy fraction from vacuum distillation which undergoes catalytic cracking undergoes hydrotreatment in a zone  43 . The reference numerals of the preceding figures are shown again here.  
     [0160] This hydrotreatment prior to FCC is carried out in the presence of at least one amorphous catalyst. All conventional hydrotreatment catalysts can be used. Catalysts that can be cited include those containing at least one non noble group VIII element (for example Co, Ni) and at least one group VIB element (for example M, W) deposited on a support preferably based on alumina or silica-alumina. The particularity of this step resides in the operating conditions: a hydrogen partial pressure between 25 and 90 bars, preferably less than 85 bars, or more preferably less than 80 bars, or even more preferably less than 70 bars, and a temperature of 350-450° C., preferably 370-430° C. adjusted to maintain a conversion of at least 10%, preferably less than 40% of products boiling below 350° C., preferably 15-30%.  
     [0161] This produces a naphtha (line  44 ) and a diesel (line  45 ) but of medium quality and intended either for use as a domestic fuel or to be sent to the diesel pool.  
     [0162] The hydrotreated effluent then passes into a catalytic cracking unit  28 .  
     [0163]FIG. 4 is a flowchart for an addition of heavy diesel fraction to unit  32  in which moderate pressure hydrocracking is carried out. In this implementation, an atmospheric distillate is obtained, in addition to naphtha cuts (line  22 ), kerosine cuts (line  23 ), a light LGO diesel fraction (line  46 ) and a heavy diesel fraction HGO (line  47 ). This heavy diesel fraction is sent to unit  32  where it undergoes hydrotreatment then moderate pressure hydrocracking.  
     [0164] The present application describes a process for producing diesel and naphtha, diesel production being carried out by a moderate pressure hydrocracking process as described above, and naphtha production being essentially obtained by catalytic cracking. Preferably, the hydrocracking purge is sent to the catalytic cracking step.  
     [0165] We have described a preferred embodiment of this type of process, but other embodiments and arrangements are possible that will produce the same result.  
     [0166] To illustrate the advantage of the invention, we describe below production from an existing refining scheme, and from an existing scheme for producing diesel to 2005 specifications and from a scheme of the invention. In the scheme of FIG. 2A (existing specification 2000 scheme) to treat 10 Mt/yr of North Sea crude, we have (scheme 1):  
                                                      naphtha   3.17 Mt/yr           jet fuel   0.73 Mt/yr           diesel   2.32 Mt/yr           domestic fuel    1.5 Mt/yr                      
 
     [0167] fuel from 565° C.+ residue undergoing visbreaking: 1.42 Mt/yr of 40 cst fuel  
     [0168]  (diluted by LCO and including slurry)  
     [0169] The diesel has a cetane number of 49 and the sulphur content is 2100 ppm. To satisfy specifications, it has to undergo conventional desulphurisation.  
     [0170] Without changing the refinery scheme, but with diesel qualities to 2005 specifications (95% point taken at 340° C.), we have (scheme  2 ):  
                                                      naphtha   3.32 Mt/yr           jet fuel cut   0.82 Mt/yr           diesel   2.13 Mt/yr           domestic fuel   1.36 Mt/yr           40 cst fuel from 565° C. + residue   1.43 Mt/yr                      
 
     [0171] The diesel has a cetane number of only 48 and thus has to undergo extremely severe hydrodesulphurisation and hydrogenation which cannot be achieved with existing units.  
     [0172] The naphtha pool should have a sulphur content of 270 ppm by weight, which would necessitate severe subsequent treatments to reduce it to 10-50 ppm by weight. To avoid expensive investment, the naphtha fraction from FCC will be treated separately by severe hydrodesulphurisation, with the disadvantage of reducing the octane number. The other naphtha fractions (for example from the visbreaker, crude distillation, etc. . . . ) will be sent to the reforming step and possibly to an isomerisation unit prior to hydrotreatment.  
     [0173] Adding a hydrotreatment step prior to catalytic cracking to scheme  2  above (as shown in FIG. 3 but under hydrocracking) produces the following (scheme  3 ):  
                                                      naphtha   3.06 Mt/yr           jet fuel   0.84 Mt/yr           diesel   2.57 Mt/yr           domestic fuel   1.40 Mt/yr           40 cst fuel from 565° C. + residue   1.36 Mt/yr                      
 
     [0174] The cetane number of the diesel remains at about 48. The FCC naphtha has a sulphur content of 15 ppm, and in the naphthene pool the sulphur content can be reduced to as low as about 5.5 ppm, without substantial loss of octane number.  
     [0175] With the preferred scheme  4  of the invention (FIG. 3) including moderate pressure hydrocracking, we obtain (scheme  4 ):  
                                                      naphtha   2.90 Mt/yr           jet fuel   0.86 Mt/yr           diesel   2.82 Mt/yr           domestic fuel   1.44 Mt/yr           40 cst fuel   1.34 Mt/yr                      
 
     [0176] In this case, vacuum distillation separates out a light fraction 350-410° C., a heavy fraction 410-565° C. and a 565° C+ residue.  
     [0177] In hydrocracking, conversion is almost complete (≧98%), the H 2  consumption is 1.85% by weight, and the pressure is 90 bars ppH 2 .  
     [0178] The diesel obtained is as follows:  
                                                      95% point   340° C.           flash point   &gt;60° C.           cetane number    56           sulphur content   &lt;10 ppm           polyarmoatics    &lt;1% by weight                      
 
     [0179] A comparison of these figures shows the excellent quality of the diesel obtained using a scheme of the invention, which quality has never before been produced.  
     [0180] Compared with conventional hydrocracking, for the same product qualities, we have saved 50 bars of hydrogen pressure, which considerably reduces the costs.  
     [0181] In terms of productivity, the quantities of products are similar to those of scheme 3 but in contrast, comparison with scheme 2 (existing refinery but to 2005 specifications) shows important gains, for the same quantity of crude oil, in:  
                                                      diesel   +32.4%           kerosine   4.9%                      
 
     [0182] We could also adjust the diesel/naphtha balance towards high quality diesel, while reducing the 40 cst fuel production (−6.3%) thus adapting the refinery to market needs. Further, the FCC unit does not operate at full capacity, and so the operator can highly advantageously introduce a supplemental feed that can be treated under FCC (such as an atmospheric residue). This addition is preferably made to the feed entering the vacuum distillation column (dotted lines in FIG. 3).