Patent Publication Number: US-2004040888-A1

Title: Process for hydrocracking into a stage of hydrocarbon feedstocks

Description:
[0001] This invention relates to a so-called improved process for hydrocracking into a stage of hydrocarbon feedstocks, comprising a first stage that is carried out in a first reaction zone with a hydrotreatment catalyst that exhibits a low acidity according to a standard activity test and a last stage that is carried out in a second reaction zone that is downstream from the first, with an amorphous acid catalyst for hydrocracking that is free of zeolite.  
       [0002] The objective of the process is essentially the production of middle distillates, i.e., fractions with an initial boiling point of at least 150° C. and a final boiling point that goes just up to the initial boiling point of the residue, for example less than 340° C. or else 370° C. and optionally oil bases (residue).  
       [0003] Prior Art  
       [0004] The hydrocracking of heavy petroleum fractions is a very important refining process that makes it possible to produce, from excess heavy feedstocks that cannot be readily upgraded, lighter fractions such as gasolines, jet fuels and light gas oils that the refiner seeks to adapt his production to the structure of the demand. Some hydrocracking processes make it possible also to obtain a strongly purified residue that can provide excellent bases for oils. Relative to the catalytic cracking, the advantage of catalytic hydrocracking is to provide middle distillates, jet fuels and gas oils, of very good quality. Conversely, the gasoline that is produced exhibits an octane number that is much lower than the one that is obtained from the catalytic cracking.  
       [0005] Hydrocracking is a process that draws its flexibility from three main elements that are the operating conditions that are used, the types of catalysts that are used, and the fact that the hydrocracking of hydrocarbon feedstocks can be carried out in one or more stages.  
       [0006] The catalysts that are used in hydrocracking are all of the bifunctional type that combines an acid function with a hydrogenating function. The acid function is provided by large-surface substrates (150 to 800 m 2 .g −1  generally) that exhibit a superficial acidity, such as halogenated aluminas (chlorinated or fluorinated in particular), combinations of boron oxides and aluminum oxides, amorphous silica-aluminas and zeolites. The hydrogenating function is provided either by one or more metals of group VIII of the periodic table, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, or by a combination of at least one metal of group VIB of the periodic table such as molybdenum and tungsten and at least one metal of group VIII.  
       [0007] The equilibrium between the acid and hydrogenating functions is a basic parameter that governs the activity and the selectivity of the catalyst. A weak acid function and a strong hydrogenating function provide low-activity catalysts that work at a generally high temperature (greater than or equal to 390° C.) and at a low feed volumetric flow rate (the VVH that is expressed by volume of feedstock to be treated per unit of volume of catalyst and per hour is generally less than or equal to 2) but equipped with a very good selectivity of middle distillates. Conversely, a strong acid function and a weak hydrogenating function provide active catalysts that exhibit less advantageous selectivities of middle distillates. The search for a suitable catalyst will therefore be centered on a judicious selection of each of the functions to adjust the activity/selectivity pair of the catalyst.  
       [0008] Thus, one of the great advantages of hydrocracking is to exhibit a great flexibility at various levels: flexibility at the level of the catalysts that are used that provides a flexibility of feedstocks to be treated and at the level of the products that are obtained. An easy parameter to control is the acidity of the substrate of the catalyst.  
       [0009] The conventional hydrocracking catalysts can use weakly acidic substrates, such as amorphous silica-aluminas, for example. These systems are more particularly used to produce middle distillates of very good quality and also, when their acidity is very low, oil bases.  
       [0010] In the sparingly acidic substrates, the family of amorphous silica-aluminas is found. A portion of the catalysts of the hydrocracking market are based on amorphous silica-alumina that is combined either with a metal from group VIII or, preferably when the contents of organic compounds that contain sulfur and nitrogen of the feedstock to be treated exceed 0.5% by weight, with a combination of metal sulfides from groups VIB and VIII. These systems have a very good selectivity of middle distillates, and the products that are formed are of good quality. These catalysts, for the less acidic among them, can also produce lubricating bases. The drawback of all of these catalytic systems based on an amorphous substrate is, as has been said, their low activity.  
       [0011] The catalysts that comprise a zeolite, for example an FAU-structural-type Y zeolite, exhibit a catalytic activity that is higher than that of the amorphous silica-aluminas, but exhibit selectivities of light products that are higher.  
       [0012] In the processes where the hydrocracking catalyst is zeolitic, it is necessary to pretreat the feedstock on a hydrotreatment catalyst to eliminate the organic nitrogen that inhibits the activity of the zeolite.  
       [0013] On the other hand, the amorphous hydrocracking catalysts (without a zeolite) readily support the presence of organic nitrogen and consequently prior hydrotreatment to remove the heteroatoms is not used. Thus, these so-called “one stage” processes of the prior art do not comprise hydrotreatment upstream from the hydrocracking since the hydrotreatment takes place on the hydrocracking catalyst.  
       [0014] However, the research work carried out by the applicant led him to discover that, surprisingly enough, in a process of hydrocracking into a stage that uses an amorphous hydrocracking catalyst, a conversion of the hydrocarbon feedstock, a selectivity of middle distillates (kerosene+gas oil) and a cycle time, higher than with the processes in a known stage in the prior art, can be obtained provided that a reaction zone that comprises a hydrorefining catalyst that exhibits a low acidity is introduced upstream from the amorphous hydrocracking catalyst. The addition of this volume of hydrorefining catalyst is carried out without increasing the overall catalytic volume nor reducing the flow rate of the feedstock that is to be treated. Therefore, the improvements that are described above are obtained at a constant feed volumetric flow rate (VVH expressed by volume of feedstock to be treated per unit of volume of the catalyst and per hour) relative to the processes of the prior art using only an acidic amorphous catalyst for hydrocracking.  
       Detailed Description of the Invention  
       [0015] The invention describes a process of hydrocracking, hydrocarbon feedstocks (for example called process “in one stage”) for the production of middle distillates and optionally oil bases that comprise at least a first reaction zone that includes hydrorefining, and at least a second reaction zone , in which the hydrocracking of the effluent that is obtained from the first reaction zone is carried out.  
       [0016] More specifically, the invention is a hydrocracking process that comprises the following stages:  
       [0017] A hydrorefining stage in which the feedstock is brought into contact with at least one hydrorefining catalyst that exhibits in the standard activity test a conversion rate of the methylcyclohexane that is less than 10% by mass;  
       [0018] A hydrocracking stage in which at least a portion of the effluent that is obtained from the hydrorefining stage is brought into contact with at least one non-zeolitic hydrocracking catalyst that in the standard activity test exhibits a conversion rate of the methylcyclohexane that is higher than 10% by mass.  
       [0019] First Reaction Zone  
       [0020] Very varied feedstocks can be treated by the process according to the invention and generally they contain at least 20% by volume and often at least 80% by volume of compounds that boil above 340° C.  
       [0021] The feedstock can form part of, for example, LCO (light cycle oil), atmospheric distillates, vacuum distillates, for example, gas oil that is obtained from direct distillation of crude or conversion units such as the FCC, the coker, or the visbreaking, as well as feedstocks that are obtained from units for extracting aromatic compounds from lubricating oil bases or obtained from solvent dewaxing of lubricating oil bases, or else distillates that are obtained by desulfurization or hydroconversion of RAT (atmospheric residues) and/or RSV (vacuum residues) or else the feedstock can be a desasphalted oil, or else any mixture of the feedstocks cited above. The list above is not limiting. The feedstocks preferably have a boiling point T 5  that is higher than 340° C., and better yet higher than 370° C., i.e., that 95% of the compounds that are present in the feedstock have a boiling point that is higher than 340° C., and better yet higher than 370° C.  
       [0022] The nitrogen content of the hydrocarbon feedstocks that are treated in the process according to the invention is usually higher than 500 ppm and preferably between 500 and 5000 ppm by weight, more preferably between 700 and 4000 ppm by weight and even more preferably between 1000 and 4000 ppm. Generally, the sulfur content is between 0.01 and 5% by weight, more generally between 0.2 and 4%. These feedstocks exhibit very low olefin contents.  
       [0023] In the first reaction zone, the feedstock undergoes at least one hydrorefining cycle (hydrodesulfurization, hydrodenitration, hydrogenation of aromatic compounds).  
       [0024] Standard catalysts can be used that contain at least one amorphous substrate and at least one hydro-dehydrogenating element (generally at least one non-noble element of groups VIB and VIII, and most often at least one element of group VIB and at least one non-noble element of group VIII).  
       [0025] Very advantageously, in the hydrocracking process according to the invention, the feedstock that is to be treated is brought into contact in the presence of hydrogen with a hydrorefining catalyst that comprises at least one matrix, at least one hydro-dehydrogenating element that is selected from the group that is formed by the elements of group VIB and the non-noble group VIII of the periodic table, optionally at least one promoter element that is deposited on the catalyst and selected from the group that is formed by phosphorus, boron and silicon, optionally at least one element of group VIIA (chlorine, fluorine are preferred) and optionally at least one element of group VIIB (manganese is preferred), and optionally at least one element of group VB (niobium is preferred).  
       [0026] The hydrorefining catalysts that are used do not contain zeolite and exhibit a low acidity that is measured by a standard activity test (TSA).  
       [0027] We will now define this test and specify what is meant by low acidity.  
       [0028] The object of the standard activity test is to measure the activity of catalysts (such as those of hydrorefining described above) in the conversion of methylcyclohexane under the following operating conditions:  
       [0029] The catalyst is sulfurized in advance under a pressure of 60 bar, at 350° C. with a so-called reaction mixture that comprises 0.5% by mass of aniline, 1.5% by mass of dimethyl disulfide and 98% by mass of methylcyclohexane, for 4 hours. Then, always under the same reaction flow by adding hydrogen, and under the following reaction conditions: pressure of 60 bar, volumetric flow rate VVh of 1 h −1 , H2/reaction mixture ratio (described above): 1000 Nl of hydrogen/l of liquid reaction mixture (Nl=normal liters), the temperature is gradually brought to a reaction temperature of 380° C.  
       [0030] Under these operating conditions, a catalyst is considered as exhibiting a low acidity and can therefore be used in the first reaction zone if it leads to a conversion rate of methylcyclohexane that is less than 10% by mass and preferably less than 5%.  
       [0031] The conversion of the methylcyclohexane reagent is defined as the transformation of the latter into isomerization products with 7 carbon atoms, such as, for example, the dimethylcyclopentanes, into ring-opening products and into cracking products. The conversion of methyl cyclohexane, as defined, therefore takes into account all of the different products of methyl cyclohexane. Obtaining all of these products requires the presence of a more or less strong acid function on the catalyst.  
       [0032] The hydrorefining catalysts that are used generally contain less than 10% by weight, and preferably at most 5% by weight, of silica. In addition, this silica is preferably brought by doping. The silicon promoter element is then primarily located on the matrix and can be characterized by the Castaing microprobe or another method as described later with the hydrocracking catalyst.  
       [0033] The preferred catalysts do not contain silica.  
       [0034] This catalyst preferably contains boron and/or silicon and/or phosphorus as a promoter element. The contents of boron, silicon and phosphorus are then 0.1-20%, preferably 0.1-15%, even more advantageously 0.1-10%.  
       [0035] The matrices that can be used alone or in a mixture are by way of nonlimiting example alumina, halogenated alumina, clays (selected, for example, from among the natural clays such as kaolin, or bentonite), magnesia, titanium oxide, boron oxide, zirconia, aluminum phosphates, titanium phosphates, zirconium phosphates, carbon, and aluminates. It is preferred to use matrices that contain alumina in all of these forms that are known to one skilled in the art and even more preferably aluminas, for example gamma-alumina.  
       [0036] The role of hydro-dehydrogenating function is preferably filled by at least one metal or metal compound from group VIII that is non-noble and VIB, preferably selected from among molybdenum, tungsten, nickel and cobalt. This role is preferably ensured by the combination of at least one element of group VIII (Ni, Co) with at least one element of group VIB (Mo, W).  
       [0037] This catalyst can advantageously contain phosphorus; actually, it is known in the prior art that this compound provides two advantages to hydrorefining catalysts: a facility for preparation in particular during the impregnation of nickel and molybdenum solutions and a better hydrogenation activity.  
       [0038] In a preferred catalyst, the total content by mass of metal oxides of groups VI and VIII is most often between 5 and 60% and preferably between 7 and 50%, and the ratio by weight that is expressed in terms of metal oxide between group VIB metal (or metals) vs. group VIII metal (or metals) is preferably between 20 and 1.25 and even more preferably between 10 and 2. The content by mass of phosphorus oxide P 2 O 5  will be less than 15% and preferably 10%.  
       [0039] Another preferred catalyst that contains boron and/or silicon (and preferably boron and silicon) generally contains in % by weight relative to the total mass of the catalyst at least one metal that is selected from the following groups and with the following contents:  
       [0040] 3 to 40%, preferably 3 to 35%, and even more preferably 3 to 30% of at least one metal of group VIB and optionally  
       [0041] 0 to 30%, preferably 0 to 25%, and even more preferably 0 to 20% of at least one metal of group VIII,  
       [0042] whereby the catalyst also contains at least one substrate that is selected from the following groups with the following contents:  
       [0043] 0 to 99%, advantageously 0.1 to 99%, preferably 10 to 98%, and even more preferably 15 to 95% of at least one amorphous or poorly crystallized matrix,  
       [0044] whereby said catalyst is characterized in that it also contains  
       [0045] 0.1 to 20%, preferably 0.1 to 15% and even more preferably 0.1 to 10% of boron and/or 0.1 to 15%, preferably 0.1 to less than 10% and even more preferably 0.1 to 5% by weight of silicon,  
       [0046] and optionally  
       [0047] 0 to 20%, preferably 0.1 to 15%, and even more preferably 0.1 to 10% of phosphorus,  
       [0048] and optionally also  
       [0049] 0 to 20%, preferably 0.1 to 15%, and even more preferably 0.1 to 10% of at least one element that is selected from the group VIIA, preferably fluorine.  
       [0050] In general, the formulas that have the following atomic ratios are preferred:  
       [0051] a group VIII metal/group VIB metals atomic ratio of between 0 and 1,  
       [0052] a B/group VIB metals atomic ratio of between 0.01 and 3,  
       [0053] an Si/group VIB metals atomic ratio of between 0.01 and 1.5,  
       [0054] a P/group VIB metals atomic ratio of between 0.01 and 1,  
       [0055] a group VIIA element/group VIB metals atomic ratio of between 0.01 and 2.  
       [0056] The preferred catalysts are the NiMo and/or NiW catalysts on alumina, also the NiMo and/or NiW catalysts on alumina doped with at least one element included in the group of atoms formed by phosphorus, boron, silicon and fluorine.  
       [0057] In general, the hydrorefining catalyst contains:  
       [0058] 5-40% by weight of at least one non-noble element of groups VIB and VIII (% oxide),  
       [0059] 0-20% of at least one promoter element that is selected from among phosphorus, boron, (% oxide), preferably between 0.1-10% and even more preferably between 0.1 and 5% by weight; 0 to less than 10% by weight of promoter silicon, preferably 0.1-5%; advantageously boron and/or silicon are present, and optionally phosphorus.  
       [0060] 0-20% of at least one element of group VIIB (manganese, for example)  
       [0061] 0-20% of at least one element of group VIIA (fluorine, chlorine, for example)  
       [0062] 0-60% of at least one element of group VB (niobium, for example)  
       [0063] 0.1-95% of at least one matrix, and preferably alumina.  
       [0064] The catalysts that are described above are generally used to ensure the hydrorefining that is also called hydrotreatment.  
       [0065] Prior to the injection of the feedstock, the catalysts that are used in the process according to this invention are preferably subjected in advance to a sulfurization treatment that makes it possible to transform, at least in part, metallic radicals into sulfide before they are brought into contact with the feedstock that is to be treated. This treatment of activation by sulfurization is well known to one skilled in the art and can be carried out by any method that is already described in the literature or in situ, i.e., in the reactor, or ex situ.  
       [0066] A standard sulfurization method that is well known to one skilled in the art consists in heating in the presence of hydrogen sulfide (pure or, for example, under a stream of a hydrogen/hydrogen sulfide mixture) to a temperature of between 150 and 800° C., preferably between 250 and 600° C., generally in a flushed-bed reaction zone.  
       [0067] In the first reaction zone of the process, the feedstock is brought into contact, in the presence of hydrogen, with at least one catalyst as described above, at a temperature of between 330 and 450° C., preferably 360-420° C., under a pressure that is higher than 7.5, preferably higher than 8.2 MPa, preferably higher than 9.0 MPa, and even more preferably higher than 11.0 MPa and lower than 20 MPa, whereby the volumetric flow rate is between 0.1 and 6 h −1 , preferably 0.2-3 h −1 , and the amount of hydrogen that is introduced is such that the liter of hydrogen/liter of hydrocarbon volumetric ratio is between 100 and 2000 l/l. Under these conditions, the conversion into products boiling below 340° C. (and even below 370° C.) is most often less than 30% by weight, usually less than 20% and even 15%. A conversion in this stage is not desired.  
       [0068] In the first reaction zone of the process according to the invention, a significant reduction of the content of organic nitrogen-containing and sulfur-containing compounds and of condensed polycyclic aromatic hydrocarbons is obtained. Under these conditions, at least a portion of the nitrogen-containing and sulfur-containing organic products of the feedstock are also transformed into H 2 S and into NH 3 .  
       [0069] The operating conditions under which this hydrorefining is carried out are such that the organic nitrogen content of the feedstock that is obtained from this hydrorefining and that is then admitted to the hydrocracking catalyst bed is less than 200 ppm by weight and preferably less than 100 ppm by weight and even more preferably less than 80 ppm by weight.  
       [0070] The effluent that is obtained from this first reaction zone is at least in part, and preferably completely, introduced into the second reaction zone of the process according to the invention. An intermediate separation of the gases can be carried out.  
       [0071] Second Reaction Zone  
       [0072] The operating conditions that are used in the reactor or reactors that are located downstream from the first reaction zone of the process according to the invention are: a temperature that is higher than 200° C., often between 250-480° C., advantageously between 320 and 450° C., preferably between 330 and 425° C., under a pressure of between 3 MPa and 20 MPa, preferably higher than 7.5, preferably higher than 8.2 MPa, preferably higher than 9.0 MPa, or else higher than 11.0 MPa and less than 20 MPa, whereby the volumetric flow rate is between 0.1 and 20 h −1 , and preferably 0.1-6 h −1 , preferably 0.2-3 h −1 , and the amount of hydrogen that is introduced is such that the liter of hydrogen/liter of hydrocarbon volumetric ratio is between 80 and 5000 l/l and most often between 100 and 2000 l/l. Under these conditions, the overall conversion of the process is generally at least 50% by weight and preferably at least 60% when the objective is to obtain middle distillates.  
       [0073] The process according to the invention is very advantageously operable within 3 ranges of pressure making it possible to obtain different yields and different qualities of products.  
       [0074] It is thus possible to work at low total pressures, generally of at most 7.0 MPa, or high pressures, generally of at least 11 MPa, or within the intermediate range of moderate pressures that are higher than 7 MPa and less than 11 MPa, generally of between 8.2-11 MPa.  
       [0075] Thus, in an advantageous way, within the ranges of low pressures and primarily within ranges of moderate pressures, higher conversion levels are achieved than with single hydrocracking catalysts.  
       [0076] This increase (it is the same at high pressures) is obtained only from the conversion provided by hydrotreatment (which is actually fairly low) but primarily from modification of the feedstock.  
       [0077] These operating conditions that are used in the second reaction zone of the process according to the invention make it possible to achieve conversions per pass into products that have boiling points of less than 340° C. and, better, less than 370° C., greater than 30% by weight and even more preferably between 40 and 95% by weight.  
       [0078] The second reaction zone comprises at least one reactor that contains at least one amorphous catalyst bed of hydrocracking. The hydrocracking catalysts that are used in the hydrocracking processes are all of the bifunctional type combining an acid function with a hydrogenating function. The acid function is provided by large-surface substrates (generally 150 to 800 m 2 .g −1 ) that exhibit a superficial acidity, such as the halogenated aluminas (chlorinated or fluorinated in particular), combinations of boron oxides and aluminum oxides, combinations of titanium oxide, silicon oxide and aluminum oxide, combinations of zirconium oxides, aluminum oxides and silicon oxides, amorphous silica-aluminas, halogenated silica-aluminas (chlorinated or fluorinated in particular). These oxides or combinations of amorphous oxides can be obtained by any of the synthesis methods that are known to one skilled in the art.  
       [0079] The hydrogenating function is provided either by one or more metals of group VIII of the periodic table or by a combination of at least one metal of group VIB of the periodic table and at least one metal of group VIII.  
       [0080] Said catalyst comprises at least one amorphous acid function such as a silica-alumina, and at least one hydro-dehydrogenating function, optionally at least one matrix. Optionally, it can also contain at least one element that is selected from among boron, phosphorus and silicon, at least one element of group VIIA (chlorine, fluorine, for example), at least one element of group VIIB (manganese, for example), and at least one element of group VB (niobium, for example).  
       [0081] According to a preferred method according to the invention, the hydrorefining catalyst and the hydrocracking catalyst are placed in separate reactors. In another method, they are placed in the same reactor but in separate beds, and the entire hydrotreated effluent moves on to hydrocracking.  
       [0082] In all of the cases, the reactor or reactors that contain the hydrorefining catalyst is (are) upstream from the reactor or reactors containing the hydrocracking catalyst. In other words, the hydrocracking catalyst comes from a hydrorefining catalyst with a lower acidity than the hydrocracking catalyst.  
       [0083] Amorphous Catalyst  
       [0084] The non-zeolitic hydrocracking catalyst contains an amorphous acid function, generally a silica-alumina. It also contains a hydro-dehydrogenating function and optionally a matrix. It can also optionally contain at least one promoter element (boron, phosphorus and/or silicon); their content is generally 0-20%, preferably at least 0.1%, advantageously 0.1-15% or else 0.1-10% or 0.1-5%. It optionally contains at least one element of group VIIA (chlorine, fluorine) whose content is generally 0-20%, preferably at least 0.1%, advantageously 0.1-15% or else 0.1-10%; fluorine is preferred. It can also contain at least one element of group VIIB (manganese, for example), and at least one element of group VB (niobium, for example). The element content of group VIIB is 0-20%, preferably at least 0.1%. The element content by weight of group VB is 0-60%, preferably at least 0.1%.  
       [0085] The content by weight of silica of said silica-alumina is between 10 and 95% and preferably between 20 and 90% and even more preferably between 30 and 90%. These silica-aluminas can be prepared by any of the methods that are known to one skilled in the art such as, for example, the methods of cogelation, coprecipitation, . . ..  
       [0086] The amorphous acid function can also be ensured by ternary mixtures of oxides such as titanium silica-alumina-oxide compositions or else zirconia silica-alumina-oxide compositions. The silica-alumina substrates, or titanium silica-alumina-oxide substrates or else zirconia silica-alumina-oxide substrates are prepared by all of the methods that are known to one skilled in the art such as the methods of cogelation, coprecipitation, . . ..  
       [0087] The content by weight of silica of said ternary oxides is between 10 and 90% and preferably between 20 and 90% and even more preferably between 30 and 85%. These ternary oxides can be prepared by any of the methods that are known to one skilled in the art, such as, for example, the methods of cogelation, coprecipitation, . . ..  
       [0088] The role of hydro-dehydrogenating function for the hydrocracking catalyst that comprises at least one acid function, as defined above, is preferably filled by at least one non-noble metal or metal compound of group VIII and of group VIB preferably selected from among molybdenum, tungsten, nickel and cobalt. This role is preferably ensured by the combination of at least one element of group VIII (Ni) with at least one element of group VIB (Mo, W), whereby the total content by weight of said metals is generally 5-40%.  
       [0089] Advantageous amorphous catalysts for hydrocracking are the NiMo and/or NiW catalysts on silica-alumina or on titanium silica-alumina-oxide or else on zirconia silica-alumina-oxide. These catalysts can be prepared by any of the methods that are known to one skilled in the art.  
       [0090] The catalysts that are described above and that are used in the second reaction zone are characterized in that they do not contain zeolite and exhibit a higher acidity than that of catalysts that are used in the first reaction zone upstream. Their acidity is measured by the standard activity test (TSA) that is described above.  
       [0091] Under these operating conditions, a catalyst is considered as exhibiting a sufficient acidity to be used in the second reaction zone if it results in a methylcyclohexane conversion rate that is higher than 10% by mass and preferably higher than 15%.  
       [0092] The substrate on which the metals are deposited can consist of silica-aluminas or ternary oxides as defined in the paragraphs above or result from mixing said silica-aluminas or ternary oxides with a binder such as alumina (Al 2 O), clays, and any mixture of binders cited above. The preferred binder is alumina and even more preferably alumina in all of these forms that are known to one skilled in the art, for example gamma-alumina. The content by weight of binder in the catalyst is such that it makes it possible to obtain a level of acidity as described in the standard activity test (TSA) in the preceding paragraph. The substrate, defined as the mixing of a binder and at least one acid function that is selected from the group that is formed by the silica-aluminas, the ternary oxides such as the titanium silixa-alumina oxides and the zirconia silica-alumina-oxides, most often comprises a content by weight of silica of at least 10% and preferably higher than 20% and less than 95%, or, better, 90%.  
       [0093] The catalysts whose substrate consists only of silica alumina or ternary oxides without any binder are preferred, however; they contain 10-95% by weight of silica.  
       [0094] The substrate can be prepared by shaping silica-alumina or ternary oxides with or without a binder by any technique that is known to one skilled in the art. The shaping can be carried out by, for example, extrusion, pelletizing, the drop (oil-drop) coagulation method, turntable granulation or by any other method that is well known to one skilled in the art. At least one calcination can be carried out after any of the stages of the preparation; it is usually carried out in air at a temperature of at least 150° C., preferably at least 300° C.  
       [0095] The catalysts that comprise at least one silica-alumina or a ternary oxide as described above in the patent, a hydrogenating function that is generally ensured preferably by at least one metal that is selected from the group that is formed by the metals of group VIB and group VIII of the periodic table, also preferably comprise at least one element that is selected from the group that is formed by boron, silicon and phosphorus. The catalyst optionally contains at least one element of group VIIA, preferably chlorine and fluorine, and also optionally at least one element of group VIIB.  
       [0096] Boron, silicon, and/or phosphorus are preferably located on silica-alumina, ternary oxide and/or the substrate in the case where a binder was used for shaping the silica-alumina or ternary oxide used.  
       [0097] The promoter element that is introduced, and in particular silicon, is primarily located on the silica-alumina and/or the substrate and can be characterized by techniques such as the Castaing microprobe (distribution profile of various elements), transmission electron microscopy combined with an X analysis of the components of catalysts, or else by combining distribution mapping of the elements that are present in the catalyst by electronic microprobe.  
       [0098] The metals of group VIB and group VIII of the catalyst of this invention can be present completely or partially in metallic form and/or oxide form and/or sulfide form.  
       [0099] In the case where the acid phase is an amorphous silica-alumina, a usable catalyst, for example, comprises at least one hydro-dehydrogenating element (preferably deposited on the substrate) and a substrate that comprises (or preferably consists of) at least one silica-alumina, whereby said silica-alumina has the following characteristics:  
       [0100] A content by weight of silica SiO 2  of between 10 and 60%, preferably between 20 and 60% and even more preferably between 30 and 50% by weight,  
       [0101] An Na content that is less than 300 ppm by weight and preferably less than 200 ppm by weight,  
       [0102] A total pore volume of between 0.5 and 1.2 ml/g that is measured by mercury porosimetry,  
       [0103] Whereby the porosity of said silica-alumina is as follows:  
       [0104] i/ The volume of mesopores whose diameter is between 40 Å and 150 Å, and whose mean diameter varies between 80 and 120 Å represents between 30 and 80% of the total pore volume defined above and preferably between 40 and 70%.  
       [0105] ii/ The volume of macropores, whose diameter is larger than 500 Å, and preferably between 1000 Å and 10,000 Å, represents between 20 and 80% of the total pore volume and preferably between 30 and 60% of the total pore volume, and even more preferably the volume of the macropores represents at least 35% of the total pore volume.  
       [0106] A specific surface area that is larger than 200 m 2 /g and preferably larger than 250 m 2 /g.  
       [0107] In the case where the catalyst above is used in hydrocracking, a catalyst for hydrotreatment that contains Ni, Mo and P and alumina, or Ni, Mo, phosphorus, alumina and silicon will be preferred; whereby the latter will be brought as a dopant.  
       [0108] Prior to the injection of the hydrocarbon effluent in the second reaction zone of the process according to this invention, the catalyst is subjected to a sulfurization treatment that makes it possible to transform, at least in part, the metallic radicals into sulfide before they are brought into contact with the feedstock that is to be treated. This treatment of activation by sulfurization is well known to one skilled in the art and can be carried out by any method that is already described in the literature or in situ, i.e., in the reactor, or ex situ.  
       [0109] A standard sulfurization method that is well known to one skilled in the art consists in heating in the presence of hydrogen sulfide (pure or, for example, under a stream of a hydrogen/hydrogen sulfide mixture), to a temperature of between 150 and 800° C., preferably between 250 and 600° C., generally in a flushed-bed reaction zone.  
       [0110] The proportion of catalytic volume of the catalyst with low acidity that is present in the first reaction zone represents, according to the cases of 10 to 60% of total catalytic volume, preferably between 15 and 50% and even more preferably between 20 and 45% of the total catalytic volume.  
       [0111] Final Separation  
       [0112] The effluent at the outlet of the second reaction zone of the hydrocracking process according to the invention is subjected to a so-called final separation (for example by atmospheric distillation optionally followed by a vacuum distillation) so as to separate the gases (such as ammonia (NH 3 ) and hydrogen sulfide (H 2 S), as well as the other light gases that are present, hydrogen and conversion products. . . ). At least one residual liquid fraction that essentially contains products whose boiling point is generally higher than 340° C. and that can be at least in part recycled upstream from the second reaction zone of the process according to the invention, and preferably upstream from the hydrocracking catalyst that is based on silica-alumina is obtained in a facility for production of middle distillates.  
       [0113] The conversion into products that have boiling points of less than 340° C. or else less than 370° C. is at least 50% by weight.  
       [0114] The following examples illustrate the invention without, however, limiting its scope.  
     
    
    
     EXAMPLE 1  
     Preparation of Catalysts  
     [0115] Hydrorefining catalyst C 1  is obtained by dry impregnation of a substrate A that consists of cubic gamma-alumina, in the form of cylindrical extrudates with a diameter of 1.6 mm and that have a surface area of 250 m2/g, a pore volume that is measured with mercury of 0.60 ml/g, by an aqueous solution that contains nickel salts, molybdenum salts and phosphoric acid. The nickel salt is nickel nitrate Ni(NO 3 ) 2 .6H 2 O and that of molybdenum is ammonium heptamolybdate (NH 4 ) 6 Mo 7 O 24 .4H 2 O.  
     [0116] After maturation at ambient temperature in a water-saturated atmosphere, the impregnated extrudates are dried at 120° C. and then calcined at 500° C. in dry air. The final content of MoO 3  is 17.1%, and that of NiO is 3.7% by mass and that of P 2 O 5  is 4.1% by mass.  
     [0117] Substrate B is a silica-alumina that has a chemical composition of 40% by weight of SiO 2  and 60% by weight of Al 2 O 3 . Its Si/Al molar ratio is 0.56. Its Na content is on the order of 100-120 ppm by weight. It is in the form of cylindrical extrudates with a diameter of 1.7 mm. Its specific surface area is 320 m 2 /g. Its total pore volume, measured by mercury porosimetry, is 0.83 cc/g. The pore distribution is bimodal. In the domain of mesopores, we observe a broad peak of between 40 and 150 Å with a dV/dD maximum toward 70 Å. On the substrate, macropores that have a size of greater than 500 Å represent about 40% of the total pore volume.  
     [0118] Catalyst C 2  is obtained by dry impregnation of substrate B by an aqueous solution that contains tungsten and nickel salts. The tungsten salt is ammonium metatungstate (NH 4 ) 6 H 2 W 12 O 40 *4H 2 O and that of nickel is nickel nitrate Ni(NO 3 ) 2 *6H 2 O. After maturation at ambient temperature in a water-saturated atmosphere, the impregnated extrudates are dried at 120° C. for one night and then calcined at 500° C. in dry air. The final content of WO 3  is 25% by weight. The final content of NiO is 3.5% by weight.  
     EXAMPLE 2  
     Standard Activity Test (TSA) on Catalysts C 1  and C 2   
     [0119] Catalysts C 1  and C 2  are subjected to a standard activity test (TSA) as follows. The sulfurization stage of the catalysts is carried out at a pressure of 60 bar, at 350° C. with a mixture that comprises 0.5% by mass of aniline, 1.5% by mass of dimethyl disulfide and 98% by mass of methylcyclohexane, for 4 hours.  
     [0120] The catalyst is sulfurized in a fixed-bed reactor at a pressure of 60 bar, at 350° C. by means of a mixture that comprises 0.5% by mass of aniline, 1.5% by mass of dimethyl disulfide, and 98% by mass of methylcyclohexane for 4 hours. Then, still in the same reaction stream and under the following operating conditions: pressure of 60 bar, volumetric flow rate VVh of 1 h −1 , H2/reaction mixture ratio (described above): 1000 Nl of hydrogen/l of liquid reaction mixture (Nl=normal liters), the temperature is brought gradually to 380° C.  
     [0121] Under these conditions, catalyst C 1  leads to a conversion of methylcyclohexane of 6% by weight. It is therefore, as defined above in the text, a catalyst that exhibits a low acidity.  
     [0122] Under the same operating conditions, catalyst C 2  leads to a conversion of methylcyclohexane of 18% by weight. It therefore exhibits an acidity that is higher than that of C 2 .  
     [0123] The conversion of the methylcyclohexane reagent is defined as the transformation of the latter into isomerization products with 7 carbon atoms, such as, for example, the dimethylcyclopentanes, into ring-opening products and into cracking products. The conversion of methylcyclohexane, as defined, therefore takes into account all of the different products of the methylcyclohexane. Obtaining all of these products requires the presence of a more or less strong acid function on the catalyst.  
     EXAMPLE 3  
     Use According to the Invention  
     [0124] The catalysts whose preparations are described in Example 1 are used to carry out the hydrocracking of a vacuum distillate whose main characteristics are provided below:  
                                                   Type of feedstock   Vacuum distillate                                                    Density at 15° C.   0.941           Sulfur, % by weight   2.9           Nitrogen, ppm by weight   1400           Simulated Distillation           DS: 0.5% p° C.   399           DS: 10% p° C.   422           DS: 50% p° C.   494           DS: 90% p° C.   566           DS: Final point ° C.   619                      
 
     [0125] In the case of use according to the process of the invention by using a pilot unit that comprises two flow-through fixed-bed reactors, the fluids circulate from bottom to top (up-flow). In the first reactor (upstream) is placed hydrorefining catalyst C 1  that is described in Example 1, and in the second reactor (downstream) is placed the amorphous hydrocracking catalyst C 2  that is also described in Example 1. The volume of catalyst C 1  represents ⅓ of the total catalytic volume (C 1 +C 2 ) and the volume of catalyst C 2  represents the ⅔ remaining.  
     [0126] The sulfurization of the catalyst is carried out at 120 bar and at 350° C. by means of a direct distillation gas oil diluted with 2% by weight of DMDS.  
     [0127] After sulfurization, the catalytic test is carried out under the following conditions:  
                                                          Total pressure    14 MPa   T = 400° C.           Overall VVH   0.7 h −1                        
 
     [0128] The volumetric flow rate (VVh) is expressed relative to the entire catalytic volume (catalysts C 1 +C 2 ).  
     [0129] The catalytic performance levels are expressed by the net conversion of products that have a boiling point of less than 370° C., by the net selectivity of a middle distillate fraction of 150-370° C., and the ratio of gas oil yield/kerosene yield in the middle distillate fraction. They are expressed from the results of simultaneous distillation.  
     [0130] Net conversion CN is assumed to be equal to:  
     CN 370° C.=[(% of 370° C. −   effluents )−(% of 370° C. −   feedstock )/[100−(% of 370° C. feedstock )] 
     [0131] The net selectivity of middle distillate SN is assumed to be equal to:  
     SN definition=[(fraction of 150-370 effluents )=(fraction of 150-370 fedstock )/[(% of 370° C. effluents )−(% of 370° C. −   feedstock )] 
     [0132] The gas oil yield/kerosene yield (go./ker.ratio) in the middle distillate fraction is assumed to be equal to:  
     Go./ker.ratio=yield of the fraction (250° C.-370° C.) of the effluent/yield of the fraction (150° C.-250° C.) in the effluent.  
     [0133] The catalytic performance levels that are obtained are provided in Table 1 below.  
     EXAMPLE 4  
     Use Not in Accordance with the Invention  
     [0134] In this example, the amorphous hydrocracking catalyst C 2  is not used according to the invention. In this case, catalyst C 2  is used by itself. The sulfurization of the catalyst is carried out at 120 bars, at 350° C. with a direct distillation gas oil that is diluted with 2% by weight of DMDS.  
     [0135] After sulfurization, the catalytic test is carried out under the following conditions:  
                                                          Total pressure    14 MPa   T = 400° C.           Overall VVH   0.7 h −1                        
 
     [0136] The volumetric flow rate (VVh) is expressed relative to the catalytic volume of catalyst C 2 .  
     [0137] The definitions of conversions, selectivities and go./ker.ratio are equivalent to those that are described in Example 3.  
     [0138] The catalytic performance levels that are thus obtained are provided in the tables below.  
               TABLE 1                          Catalytic Results                                                 CN 370° C.           Catalyst   VVh (h −1 )   T° C.   % by weight                       C1 + C2   0.7   400   59           C2   0.7   400   50                      
 
     [0139]               TABLE 2                          Catalytic Results                                             SN % by weight                       Middle   Go./Ker. Ratio               CN 370° C.   Distillate   % by weight/%       Catalyst   VVh (h −1 )   % by weight   (DM)   by weight               C1 + C2   0.7   59   74   1.35       C2   0.7   59   70   1.28                    
     [0140] The results that are noted in Table 1 demonstrate that, for the same volumetric flow rate, amorphous hydrocracking catalyst C 2  leads to a higher net conversion at iso temperature when it is used according to the process of the invention, i.e., with a catalyst C 1  with low acidity upstream, than when it is used by itself.  
     [0141] The results of Table 2 are obtained at the same volumetric flow rate, and the reaction temperature was adjusted to obtain the same net conversion in the cases.  
     [0142] The results that are noted in Table 2 demonstrate that for the same volumetric flow rate and the same net conversion, amorphous hydrocracking catalyst C 2  leads to a higher DM selectivity and a higher go./ker.ratio, when it is used according to the process of the invention, i.e., with a catalyst C 1  with low acidity upstream, than when it is used by itself.  
     [0143] In other words, relative to a process that uses a volume V 1  of a hydrocracking catalyst that is not preceded by a hydrotreatment, the upstream installation of hydrocracking of a volume V 2  of a hydrotreatment catalyst makes it possible to increase the overall conversion and selectivity of the process while offering the possibility of reducing the volume of hydrocracking catalyst, which is often the most expensive catalyst.  
     [0144] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.  
     [0145] In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius, and all parts and percentages are by weight, unless otherwise indicated.  
     [0146] The entire disclosure of all applications, patents and publications, cited herein and of corresponding French Application No. 02/07,046, filed on Jun. 6, 2002, is incorporated by reference herein.  
     [0147] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.  
     [0148] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing form the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.