PROCESS FOR THE HYDROCONVERSION OF PETROLEUM FEEDSTOCKS VIA SLURRY TECHNOLOGY ALLOWING THE RECOVERY OF METALS FROM THE CATALYST AND FEEDSTOCK USING A LEACHING STEP

Abstract

A process for the hydroconversion of heavy oil feedstocks comprises a step for hydroconversion of the feedstock in at least one reactor containing a catalyst in slurry mode used to recover metals from the residual unconverted fraction, especially those used as catalysts. The process comprises a hydroconversion step, a gas/liquid separation step, a liquid/liquid extraction step, a grinding step, a leaching step, a combustion step, a metals extraction step and a step for the preparation of catalytic solutions which are recycled to the hydroconversion step.

Description

The invention relates to a process for the hydroconversion of heavy oil feedstocks into lighter products, which can be upcycled as fuels and/or raw materials for petrochemicals. More particularly, the invention relates to a process for the hydroconversion of heavy oil feedstocks comprising a step for hydroconversion of the feedstock in at least one reactor containing a catalyst in slurry mode used to recover metals from the residual unconverted fraction, especially those used as catalysts, in order to upcycle them as catalytic solutions and recycle them upstream of the slurry conversion process. The process comprises a hydroconversion step, a gas/liquid separation step, a liquid/liquid extraction step, a grinding step, a leaching step, a combustion step, a metals extraction step and a step for the preparation of catalytic solutions which are recycled to the hydroconversion step.

The conversion of heavy oil feedstocks into liquid products may be carried out by heat treatments or by hydrogenation treatments, also known as hydroconversion. Current research is primarily orientated towards hydroconversion, because heat treatments generally produce mediocre quality products and a quantity of coke which is not negligible.

The hydroconversion of heavy feedstocks comprises conversion of the feedstock in the presence of hydrogen and of a catalyst. Depending on the feedstock, commercial processes use a fixed bed technique, an ebullated bed technique or a slurry technique.

Fixed bed or ebullated bed hydroconversion of heavy feedstocks is carried out using supported catalysts comprising one or more transition metals (Mo, W, Ni, Co, Ru) on silica/alumina or equivalent type supports.

In order to convert heavy feedstocks that are particularly charged with heteroatoms, metals and asphaltenes, fixed bed techniques are generally limited since the contaminants cause rapid deactivation of the catalyst, thereby necessitating too high a frequency of renewal of the catalytic bed; this is too expensive. Ebullated bed processes were developed in order to be able to treat this type of feedstock. However, the degree of conversion in ebullated bed techniques is generally limited to levels below 80% because of the catalytic system employed and the design of the unit.

Hydroconversion techniques operating with slurry technology provide an attractive solution to the disadvantages encountered when using a fixed bed or ebullated bed. Slurry technology can be used to process heavy feedstocks which are heavily contaminated with metals, asphaltenes and heteroatoms, while having degrees of conversion generally of more than 85%.

Techniques for the slurry hydroconversion of residues use a catalyst dispersed in the form of very small particles less than 1 mm in size, preferably a few tens of microns or fewer (generally 0.001 to 100 μm). Because of this small size of the catalysts, the hydrogenation reactions are facilitated by a uniform distribution throughout the reaction zone and coke formation is greatly reduced. The catalysts or their precursors are injected into the inlet to the reactors with the feedstock to be converted. The catalysts pass through the reactors with the feedstocks and the products during conversion, then they are entrained with the reaction products out of the reactors. Following separation, they are found in the heavy residual fraction such as the unconverted vacuum residue. Catalysts used in slurry mode are generally sulphurized catalysts preferably containing at least one element selected from the group formed by Mo, Fe, Ni, W, Co, V and/or Ru. Generally, molybdenum and tungsten have far more satisfactory performances than nickel, cobalt or ruthenium and even more so than vanadium and iron (N Panariti et al, Applied Catalysis A: General 204 (2000), 203-213).

Commercial techniques for the hydroconversion of heavy feedstocks in slurry mode are known. Examples which may be cited are the EST technique licensed by ENI, the VRSH technique licensed by Chevron-Lummus-Global, HDH and HDHPLUS techniques licensed by Intevep, the SRC-Uniflex technique licensed by UOP, the (HC)3 technique licensed by Headwaters, etc.

Although the small size of slurry catalysts means that very high conversions can be obtained, this size proves to be problematic as regards the separation and recovery of the catalyst or catalysts after the hydroconversion reaction. After separation, the catalysts are to be found in the heavy residual fraction such as the unconverted vacuum residue. In some processes, a portion of the vacuum residue containing the unconverted fraction and the catalysts is recycled directly to the hydroconversion reactor to increase the conversion yield. However, those recycled catalysts generally have no activity or very reduced activity compared with that of a fresh catalyst. In addition, the vacuum residue is traditionally used as a fuel for the production of heat, electricity and ash. This ash contains metals and is generally dumped. In this case, then, the metals are not recovered.

In addition, the deactivation of catalysts necessitates regular replacement, thereby creating a demand for fresh catalysts. The treated heavy feedstocks contain a high concentration of metals, essentially vanadium and nickel. Those metals are largely eliminated from the feedstock since they are deposited on the catalysts during the reaction. They are carried away by the particles of catalysts leaving the reactor. Similarly, catalyst deactivation is accentuated by the formation of coke deriving in particular from the high concentration of asphaltenes contained in such feedstocks.

Continuous renewal of the finely dispersed catalytic phase in the reaction zone means that, in contact with hydrogen dissolved in the liquid phase, the injected heavy feedstock can be hydrogenated and hydrotreated. In order to ensure a high degree of conversion and maximum hydrotreatment of the feedstock, the quantity of catalytic solution to be injected is rather large, meaning that operating costs are high on an industrial scale. Thus, slurry hydroconversion processes generally consume a large quantity of catalysts, in particular molybdenum, which is the most active but also the most expensive catalyst. The costs of fresh catalysts, separating catalysts and recovering metals have a major impact on the cost-effectiveness of such processes. Selective recovery of molybdenum and recycling it as a catalyst are two indispensable elements in the industrial upcycling of slurry processes. This recovery is also accompanied by recovering other metals such as nickel (that injected and that recovered in the feedstock) and vanadium recovered in the feedstock, the contents of which are comparable with that of molybdenum and which can be re-sold for metallurgical applications.

Apart from these economic aspects, the recovery of metals is also required for environmental reasons. In fact, the ash from combustion of the residual fraction has been classified in many states as dangerous waste, since the metals contained in the dumped ash pose a danger for ground water.

Thus, there is a genuine need for recovering and recycling metals from catalysts and the heavy feedstock of a slurry hydroconversion process.

PRIOR ART

Processes for recovering metals from slurry processes are known in the art.

U.S. Pat. No. 4,592,827 describes a process for the slurry hydroconversion of heavy feedstocks in the presence of a soluble metallic compound and water comprising, after the hydroconversion reaction, a separation step, a step for deasphalting the vacuum residue fraction using C5 to C8 hydrocarbons and a step for gasification of the asphaltenes, producing hydrogen and ash containing the catalyst. That catalyst then undergoes metal extraction steps; the metals are recycled to the process.

US 2009/0159505 describes a slurry hydroconversion process for heavy feedstocks and the recovery of metals contained in the catalyst by using membrane filtration in the presence of a solvent. After the filtration step, an optional washing step using surfactants is disclosed.

U.S. Pat. No. 4,548,700 describes a slurry hydroconversion process for heavy feedstocks comprising, after the hydroconversion reaction, a step for separating the gaseous fraction, a distillation step, washing the atmospheric residue (650° F.⁺=343° C.⁺) with toluene at atmospheric pressure and ambient temperature, and a step for solid fraction combustion or gasification at temperatures in the range 427-1093° C. (800-2000° F.) to obtain ash containing metals. The metals V and Mo are recovered by an oxalic acid extraction step then recycled to the process.

U.S. Pat. No. 6,511,937 describes a slurry hydroconversion process for heavy feedstocks comprising, after the hydroconversion reaction, a step for separation in a high pressure, low temperature separator to separate a very light fraction, a step for deasphalting the whole of the residual fraction using paraffinic C3 to C5 solvents at ambient temperature, a coking step (427-649° C., no air) and/or a combustion step below 649° C., to produce ash containing the catalyst. That catalyst may then undergo metals extraction steps and be recycled to the process.

SUBJECT MATTER OF THE INVENTION

The characteristic of slurry processes is that the catalyst is finely divided and not supported on a mineral phase; this renders recovery of the metals much more complex than with the traditionally employed supported refining catalysts. The issue in the industrial development of hydroconversion processes using slurry technology is the necessity of recovering and recycling the metals of the catalysts.

The present invention aims to improve processes for the hydroconversion of heavy feedstocks using known slurry technology by allowing a residual unconverted fraction derived from the slurry conversion to be upcycled, this fraction having a high concentration of metals and heteroelements, and also includes the recovery of said metals in said unconverted fraction and the production of catalytic precursors in order to recycle them upstream of the slurry mode conversion process. The process comprises a hydroconversion step, a gas/liquid separation step, a liquid/liquid extraction step, a grinding step, a leaching step, a combustion step, a metals extraction step and a step for the preparation of catalytic solution(s) which is/are recycled to the hydroconversion step.

The research work carried out by the Applicant on the hydroconversion of heavy feedstocks has led to the discovery that surprisingly, this process, comprising a separation used to maximize the light fraction from the hydroconversion reactor and minimize the residual fraction coupled with a liquid/liquid extraction step using a paraffinic solvent and a leaching step to concentrate the metals and a step for moderate combustion, avoiding sublimation of the metals, can be used to carry out the extraction of metals contained in the ash such that very good degrees of recovery of metals that can be recycled to the process are possible. In fact, the critical steps in this recovery are firstly, the concentration of metals on the carbonaceous matrix (via extraction followed by leaching), then the formation of a mineral phase (via the moderate combustion) containing the metallic elements from the catalyst (Mo and Ni), but also from the feedstock (Ni, V and Fe) lacking in carbon.

One advantage of the process of the invention is the re-use of a residual unconverted fraction which is highly concentrated in metals and heteroelements, enabling the recovery of said metals and the production of catalytic precursors in order to recycle them upstream of the conversion process in slurry mode.

Another advantage is the optimization of the hydroconversion step by gas/liquid separation following the hydroconversion, carried out under operating conditions close to those of the reactor and allowing effective separation, in a single step, of a light fraction comprising future fuel bases (gases, naphtha, light gas oil, or even heavy gas oil) from the residual unconverted fraction containing solids such as metals. The light fraction yield is thus maximized at the same time as the unconverted residual fraction is minimized, its reduced quantity thereby facilitating the subsequent concentration of metals. Maintaining the operating conditions during separation also results in economic integration of a subsequent treatment for hydrotreatment and/or hydrocracking of the light fraction without the need for supplemental compressors.

A further advantage is liquid/liquid extraction, followed by a step for leaching the unconverted metal-containing fraction, allowing efficient extraction of insoluble compounds (and thus concentration of the metals).

A further advantage of the process is combustion at moderate temperature in order to separate the organic phase from the inorganic phase containing the metals in order to facilitate subsequent extraction of the metals from the inorganic phase while avoiding vaporization and/or sublimation (and thus loss) of metals during combustion.

The process of the invention can thus be used to optimize the conversion of heavy feedstocks into fuel bases while allowing the recovery of metals with very good levels of recovery.

DETAILED DESCRIPTION

The invention concerns a process for the hydroconversion of heavy oil feedstocks in slurry mode in order to enable the recovery and recycling of metals in the residual unconverted fraction, in particular those used as catalysts.

More particularly, the invention concerns a process for the hydroconversion of heavy oil feedstocks containing metals, comprising:

• • a. a step for hydroconversion of the feedstock in at least one reactor containing a catalyst in the form of a slurry containing at least one metal, and optionally a solid additive;
  • b. a step for separation of the hydroconversion effluent without decompression into a fraction termed the light fraction containing compounds boiling at 500° C. at most and into a residual fraction;
  • b′. an optional step for fractionation, comprising vacuum separation of said residual fraction as obtained in step b) to obtain a vacuum residue which is concentrated in metals;
  • c. a step for liquid/liquid extraction of said residual fraction as obtained in step b) and/or said vacuum residue as obtained in step b′) using a solvent with a saturated nature in order to obtain a solid extract which is concentrated in metals and a raffinate;
  • d. a step for grinding the solid extract which is concentrated in metals obtained from the liquid/liquid extraction step;
  • e. a step for leaching the ground extract in the presence of water, a solvent with a saturated nature and a surfactant in order to obtain a solid extract and a leachate;
  • f. a step for combustion of said solid extract obtained from the leaching step in the presence of oxygen in order to obtain ash which is concentrated in metals;
  • g. a step for extraction of metals from the ash obtained in the combustion step;
  • h. a step for preparing metallic solution(s) containing at least the metal of the catalyst which is/are recycled as the catalyst to the hydroconversion step.

Hydroconversion

The process of the invention comprises a step for hydroconversion of the feedstock in at least one reactor containing a catalyst in slurry mode and optionally a solid additive.

The term “hydroconversion” means hydrogenation, hydrotreatment, hydrodesulphurization, hydrodenitrogenation, hydrodemetallization and hydrocracking reactions.

The heavy feedstocks concerned are oil hydrocarbon feedstocks such as oil residues, crude oils, topped crude oils, deasphalted oils, asphalts or deasphalted tars, oil conversion process derivatives (such as: HCO, FCC slurry, heavy GO/coking VGO, residue from visbreaking or a similar thermal process, etc), bituminous sands or their derivatives, oil shale or their derivatives, or mixtures of such feedstocks. More generally, the term “heavy feedstock” as used here encompasses hydrocarbon feedstocks containing at least 50% by weight of product distilling above 250° C. and at least 25% by weight distilling above 350° C.

The heavy feedstocks used in accordance with the invention contain metals, essentially V and/or Ni, generally in amounts of at least 50 ppm by weight and usually 100-2000 ppm by weight, at least 0.5% by weight of sulphur, and at least 1% by weight of asphaltenes (heptane asphaltenes), usually more than 2% by weight or even more than 5% by weight; quantities of 25% by weight or more of asphaltenes may be obtained; they also contain condensed aromatic structures that may contain heteroelements which are refractory to conversion.

Preferably, the heavy feedstocks concerned are non-conventional oils of the heavy crude type (° API in the range 18 to 25 and a viscosity in the range 10 to 100 cP), extra-heavy crudes (° API in the range 7 to 20 and a viscosity in the range 100 to 10000 cP) and bituminous sands (° API in the range 7 to 12 and a viscosity of less than 10000 cP) present in large quantities in the Athabasca region of Canada and the Orinoco in Venezuela, where reserves are respectively estimated at 1700 Gb and 1300 Gb. These non-conventional oils are also characterized by large quantities of vacuum residue, asphaltenes and heteroelements (sulphur, nitrogen, oxygen, vanadium, nickel, etc), which necessitate specific steps for transformation into commercial gasoline, gas oil or heavy fuel type products.

The heavy feedstock is mixed with a stream of hydrogen and a catalyst which is as dispersed as possible in order to obtain a hydrogenating activity which is also as uniformly distributed as possible in the hydroconversion reaction zone. Preferably, a solid additive favouring the hydrodynamics of the reactor is also added. This mixture supplies the catalytic slurry hydroconversion section. This section is constituted by a preheating furnace for the feedstock and hydrogen and by a reaction section constituted by one or more reactors disposed in series and/or in parallel, depending on the capacity required. In the case of reactors in series, one or more separators may be present on the overhead effluent from each of the reactors. In the reaction section, the hydrogen may supply a single, some or all of the reactors, in equal or different proportions. In the reaction section, the catalyst may supply a single, some or all of the reactors, in equal or different proportions. The catalyst is maintained in suspension in the reactor, moves from bottom to top of the reactor with the gas and the feedstock and is evacuated with the effluent. Preferably, at least one (preferably all) of the reactors is provided with an internal recirculating pump.

The operating conditions of the catalytic slurry hydroconversion section are in general a pressure of 2 to 35 MPa, preferably 10 to 25 MPa, a partial pressure of hydrogen of 2 to 35 MPa, preferably 10 to 25 MPa, a temperature in the range 300° C. to 500° C., preferably 420° C. to 480° C., and a contact time of 0.1 h to 10 h with a preferred duration of 0.5 h to 5 h.

These operating conditions, coupled with the catalytic activity, can be used to obtain conversions per pass of 500° C.⁺ vacuum residue that may be from 20% to 95%, preferably 70% to 95%. The degree of conversion mentioned above is defined as being the fraction by weight of organic compounds with a boiling point of more than 500° C. at the inlet to the reaction section minus the fraction by weight of organic compounds with a boiling point of more than 500° C. at the outlet from the reaction section, this being divided by the fraction by weight of organic compounds with a boiling point of more than 500° C. at the inlet to the reaction section.

The slurry catalyst is in the dispersed form in the reaction medium. It may be formed in situ, but it is preferable to prepare it outside the reactor and in general to inject it continuously with the feedstock. The catalyst promotes hydrogenation of the radicals obtained from thermal cracking and reduces the formation of coke. When coke is formed, it is evacuated by the catalyst.

The slurry catalyst is a sulphurized catalyst, preferably containing at least one element selected from the group formed by Mo, Fe, Ni, W, Co, V, Ru. These catalysts are generally monometallic or bimetallic (by combining, for example, an element from non-noble group VIIIB (Co, Ni, Fe) and an element from group VIB (Mo, W). Preferably, NiMo, Mo or Fe catalysts are used. The catalysts used may be powders of heterogeneous solids (such as natural minerals, iron sulphate, etc), water-soluble dispersed catalysts such as phosphomolybdic acid, ammonium molybdate, or a mixture of Mo or Ni oxide with aqueous ammonia. Preferably, the catalysts used are obtained from precursors which are soluble in an organic phase (oil-soluble dispersed catalysts). The precursors are organometallic compounds such as Mo, Co, Fe or Ni naphthenates or such as multi-carbonyl compounds of these metals, for example Mo or Ni 2-ethyl hexanoates, Mo or Ni acetylacetonates, Mo or W salts of C₇-C₁₂ fatty acids, etc. They may be used in the presence of a surfactant to improve dispersion of the metals when the catalyst is bimetallic. The catalysts are in the form of dispersed particles, which may or may not be colloidal depending on the nature of the catalyst. Such precursors and catalysts that may be used in the process of the invention have been extensively described in the literature.

In general, the catalysts are prepared before being injected into the feedstock. The preparation process is adapted as a function of the state of the precursor and its nature. In all cases, the precursor is sulphurized (ex situ or in situ) to disperse the catalyst in the feedstock. For the preferred case of oil-soluble catalysts, in a typical process, the precursor is mixed with an oil feedstock (which may be a portion of the feedstock to be treated, an external feedstock, a recycled feedstock, etc), the mixture is optionally dried at least in part, and is then or is simultaneously sulphurized by adding a sulphur-containing compound (preferably H₂S) and heated. The preparations of these catalysts have been described in the prior art.

Preferred solid additives are mineral oxides such as alumina, silica, mixed Al/Si oxides, spent supported catalysts (for example on alumina and/or silica) containing at least one element from group VIII (such as Ni, Co) and/or at least one element from group VIB (such as Mo, W). Examples which may be cited are the catalysts described in application US2008/177124. Carbonaceous solids with a low hydrogen content (for example 4% hydrogen), which have optionally been pre-treated, may also be used. It is also possible to use mixtures of such additives. Their preferred particle size is less than 1 mm. The quantity of optional solid additive present at the inlet to the reaction zone of the slurry hydroconversion process is in the range 0 to 10% by weight, preferably in the range 1% to 3% by weight, and the quantity of catalytic solutions is in the range 0 to 10% by weight, preferably in the range 0 to 1% by weight.

Known processes for the hydroconversion of heavy feedstocks using slurry technology are EST and ENI operating at temperatures of the order of 400-420° C., at pressures of 10-16 MPa with a particular catalyst (molybdenite); (HC)3 from Headwaters, operating at temperatures of the order of 400-450° C., at pressures of 10-15 MPa with Fe pentacarbonyl or Mo 2-ethyl hexanoate, the catalyst being dispersed in the form of colloidal particles; HDH and HDHPLUS licensed by Intevep/PDVSA, operating at temperatures of the order of 420-480° C., at pressures of 7-20 MPa, using a dispersed metallic catalyst; CASH from Chevron using a Mo or W sulphurized catalyst prepared by an aqueous method; SRC-Uniflex from UOP, operating at temperatures of the order of 430-480° C., at pressures of 10-15 MPa; VCC developed by Veba and belonging to BP, operated at temperatures of the order of 400-480° C., at pressures of 15-30 MPa, using an iron-based catalyst; Microcat from Exxonmobil, etc.

All of these slurry processes can be used in the process of the invention.

Separation

All of the effluent from the hydroconversion step is directed towards a separation section, generally in a high pressure high temperature (HPHT) separator, which can be used to separate a converted fraction in the gaseous state, termed the light fraction, and an unconverted liquid fraction containing solids, termed the residual fraction. This separation section is preferably implemented under operating conditions close to those of the reactor, which are in general a pressure of 2 to 35 MPa with a preferred pressure of 10 to 25 MPa, a partial pressure of hydrogen of 2 to 35 MPa, preferably 10 to 25 MPa, and a temperature in the range 300° C. to 500° C., preferably 380° C. to 460° C. The residence time for the effluent in this separation section is 0.5 to 60 minutes, preferably 1 to 5 minutes. The light fraction primarily contains compounds boiling at 300° C. at most, or even at 400° C. or 500° C. at most; they correspond to compounds present in the gases, naphtha, light gas oil or even heavy gas oil. It should be pointed out that the cut contains these compounds in the vast majority, as separation is not accomplished at a precise cut point, but is moreover a flash separation. If cut point terms have to be employed, it could be said that it was in the range 200° C. to 400° C. or even 450° C.

Upcycling of the light fraction does not form part of the subject matter of the present invention and these methods are well known to the skilled person. The light fraction obtained after separation may undergo at least one hydrotreatment and/or hydrocracking step, the aim being to bring the various cuts to specification (sulphur content, smoke point, cetane index, aromatics content, etc). The light fraction may also be mixed with another feedstock before being directed to a hydrotreatment and/or hydrocracking section. An external cut generally originating from another process in the refinery or possibly from outside the refinery may be brought in before hydrotreatment and/or hydrocracking; advantageously, the external cut is, for example, VGO from fractionating crude oil (straight run VGO), VGO from conversion, an LCO (light cycle oil) or a HCO (heavy cycle oil), from FCC.

In general, the hydrotreatment and/or hydrocracking after the hydroconversion may be carried out in a conventional manner via a standard intermediate separation section (with decompression) using, after the high pressure high temperature separator, for example, a high pressure low temperature separator and/or atmospheric distillation and/or vacuum distillation. Preferably, the hydrotreatment and/or hydrocracking section is directly integrated with the hydroconversion section without intermediate decompression. In this case, the light fraction is sent directly to the hydrotreatment and/or hydrocracking section without supplemental separation steps and without decompression. This last embodiment can be used to optimize the pressure and temperature conditions, avoid the use of additional compressors and thus minimize supplemental equipment costs.

The residual fraction obtained from the separation (for example via the HPHT separator) and containing metals and a fraction of solid particles used as a possible additive and/or formed during the reaction, may be directed to a fractionation step. This fractionation is optional and comprises a vacuum separation, for example one or more flash vessels and/or, as is preferable, a vacuum distillation, which can be used to concentrate a vacuum residue which is rich in metals at the bottom of the vessels or the column and to recover one or more effluents overhead of the column. Preferably, the residual fraction from the separation step without decompression is fractionated by vacuum distillation into at least one vacuum distillate fraction and a vacuum residue fraction, at least a portion, preferably all, of said vacuum residue fraction being sent to the liquid-liquid extraction step, at least a portion, preferably all, of said vacuum distillate fraction preferably undergoing at least one hydrotreatment and/or hydrocracking step.

A small part of the liquid effluent or effluents from the vacuum distillate fraction produced is/are normally directed towards the slurry hydroconversion unit where they can be recycled directly to the reaction zone, or they may be used in the preparation of catalytic precursors before injection into the feedstock. Another part of the effluent or effluents is directed to the hydrotreatment and/or hydrocracking section, optionally as a mixture with other feedstocks such as, for example, the light fraction obtained from the HPHT separator or a vacuum distillate originating from another unit, in equal or different proportions as a function of the quality of the products obtained. The aim of the vacuum distillation is to increase the yield of liquid effluents for a subsequent treatment by hydrotreatment and/or hydrocracking and thus to increase the yield of fuel bases. At the same time, the quantity of the residual fraction containing the metals is reduced, thereby facilitating concentration of the metals.

Liquid-Liquid Extraction

The residual fraction from the separation without decompression (for example via the HPHT separator, for example) and/or the vacuum residue fraction from the vacuum separation (for example withdrawn from the bottom of the vacuum distillation stage) are then directed towards a liquid/liquid type extraction step. This step is aimed at concentrating the metals in the effluent to be treated subsequently by leaching and by combustion, thereby reducing its quantity, and of maximizing the yield of liquid effluent for the treatment by hydrotreatment and/or hydrocracking.

The liquid/liquid extraction may be carried out in a mixer-decanter or in an extraction column. The general operating conditions are a solvent/feedstock ratio of 1/1 to 10/1, preferably 2/1 to 7/1, and a temperature profile in the range 50° C. to 300° C., preferably in the range 120° C. to 250° C. depending on the solvent under consideration. The solvent used preferably has a saturated nature. It may be a paraffinic solvent, such as butane, pentane, hexane or heptane, mixed or not mixed in equal or different proportions. The solvent may also be a light naphtha (C6 to C10) with a saturated nature, mixed or not mixed in equal or different proportions with the paraffinic solvents cited above. After contacting the solvent with the residual fraction and/or the vacuum residue, two phases are formed, the solid extract being constituted by the portions of the residue which are not soluble in the solvent (and concentrated in metals) and the raffinate being constituted by solvent and the soluble portions of the residue. The solvent is separated by distillation of the soluble parts and recycled internally to the liquid/liquid extraction process; management of the solvent is known to the skilled person.

At least a portion of the soluble fraction after solvent distillation, preferably all of it, is advantageously mixed with the heavy feedstock hydrocarbon upstream of the slurry hydroconversion section. A smaller portion may also be mixed with the light fraction from the separation without decompression for subsequent treatment by hydrotreatment and/or hydrocracking.

The solid extract from the liquid-liquid extraction is sent to a grinding step.

Grinding

The solid extract obtained from the liquid-liquid extraction step is sent to a grinder which can produce the desired granulometry needed for leaching. The grinding step can be used to obtain a solid effluent with a particle size of less than 6 mm, preferably less than 4 mm. The ground solid is directed to a leaching step.

Leaching

The ground solid is directed to a leaching type extraction step. This step is intended to concentrate afresh the metals in the solid to be treated subsequently by combustion by reducing its quantity, and to maximize the yield of liquid effluent for the treatment by hydrotreatment and/or hydrocracking.

The leaching step comprises several sub-steps, especially:

• • a) a step for preparing an emulsion comprising the ground extract from the grinding step, water, a surfactant and a solvent with a saturated nature;
  • b) a step for maturing the emulsion at a temperature in the range 20° C. to 120° C.;
  • c) a decanting step, holding the temperature thereby in order to obtain a solid extract and a leachate.

The leaching step employs a mixture of water, a surfactant and a solvent. The first step consists of preparing an emulsion. A mixture of ground solid is produced with water and a surfactant. The water/feedstock ratio is in the range 0.5/1 to 5/1, preferably in the range 1/1 to 2/1. The surfactant is used in concentrations of 0.05% by weight to 2% by weight with respect to the water, preferably in the range 0.1% by weight to 1% by weight. A solvent is added to the previously prepared solution. The solvent/feedstock ratio is in the range 2/1 to 6/1, preferably in the range 3/1 to 4/1.

The role of the surfactant is to stabilize the dispersion of the extract in the water at the beginning, then to stabilize the solvent-in-water emulsion. Thus, the surfactant has to be sufficiently hydrophilic. The surfactant in the present invention may be an anionic, cationic or non-ionic surfactant.

It is possible to envisage using any conventional anionic surfactant, where the anionic function is:

• • a carboxylate: examples are soaps of alkali metals, alkyl or alkylether carboxylates (for example tall oils or acid derivatives), N-acylaminoacids, N-acylglutamates, and N-acylpolypeptides;
  • a sulphonate: examples are alkylbenzene sulphonates, paraffin sulphonates, olefin sulphonates, petroleum sulphonates, lignosulphonates, sulphosuccinic derivatives, polynaphthylmethane sulphonates, and alkyl taurides;
  • sulphates: examples are alkyl sulphates, alkylether sulphates;
  • a phosphate: examples are monoalkyl phosphates, dialkyl phosphates;
  • a phosphonate.

Examples of cationic surfactants which may be cited are alkylamine salts or quaternary ammonium salts, the nitrogen of which:

• • comprises a fatty chain (for example alkyltrimethyl or triethyl ammonium derivatives, alkyldimethyl derivatives or benzylammonium derivatives);
  • comprises two fatty chains;
  • forms part of a heterocycle (for example pyridinium, imidazolinium, quinolinium, piperidinium, or morpholinium derivatives).

Non-ionic surfactants which may be envisaged for use are any known conventional non-ionic surfactants. The non-ionic surfactants may be classified according to the mode of binding between the hydrophobic part and the hydrophilic part of the molecule. This mode of binding may be an ether bridge, an ester bridge, an amide bridge or something else. It is possible to use:

• • non-ionic derivatives with an ether bridge: examples are oxyethylated fatty alcohols, oxyethylated alkylphenols, oxyethylated-oxypropylated products, and glucose ethers;
  • non-ionic surfactants with an ester bridge: examples are glycerol esters, polyethylene glycols, sorbitan esters and sugar esters;
  • non-ionic surfactants with amide bonds: examples are diethanolamides;
  • other surfactants, examples of which are ethoxylated fatty amines.

It is also possible to cite ethoxylated alkanolamides, ethoxylated amines or ethylene oxide or propylene oxide block copolymers. It is also possible to use a mixture of different surfactants.

Preferably, the surfactant used in the present invention is a mixture of tall oil fatty acid and sodium hydroxide.

The solvent used is preferably a solvent with a saturated nature. It may be a paraffinic solvent, such as hexane or heptane, mixed or not mixed in equal or different proportions. The solvent may also be a light naphtha (C6 to C10) with a saturated nature, mixed or not mixed in equal or different proportions with the paraffinic solvents cited above. Preferably, heptane is used. The solvent used in the leaching step may be identical to the solvent used in the liquid-liquid extraction step and is preferably selected from the group formed by hexane, heptane, a light naphtha (C6 to C10) with a saturated nature, mixed or not mixed and in equal or different proportions in order to facilitate the operation and optimize the process.

The emulsion comprising the ground feedstock, water, the surfactant and the paraffinic solvent is heated to a temperature in the range 20° C. to 120° C., preferably in the range 60° C. to 70° C., and mixed for a period in the range 15 min to 3 hours. The mixture is then directed to a decanting step, maintaining the temperature in order to separate a highly concentrated solid extract at the bottom of the decanter and a leachate which is a light hydrocarbon phase containing the solvent at the head of the decanter. The general operating conditions are a residence time in the range 15 minutes to 30 hours. The leachate is sent to a separation section, for example of the flash type, in order to recycle the solvent upstream of the leaching section and/or to the liquid-liquid extraction unit. The non-evaporated fraction of the leachate may then be mixed with the hydrocarbon feedstock upstream of the slurry hydroconversion section or even mixed in small quantities with the effluent upstream of the hydrotreatment and/or hydrocracking section. The leaching extract, which is highly concentrated in metals, is directed to a moderate combustion step.

The combination of liquid/liquid extraction and leaching can be used to minimize the residual fraction to be treated and thus to concentrate the metals. The combination of liquid/liquid extraction and leaching steps results in a lower ultimate yield of residue for combustion compared with double deasphalting with paraffinic solvents, for example. The degree of extraction by leaching is thus higher than that obtained by double deasphalting.

Combustion

The extract from the leaching is highly concentrated in metals. This extract is directed towards a moderate temperature combustion step. Before being able to recover the metals by conventional metal extraction methods, a preliminary step is necessary in order to separate the organic phase from the inorganic phase containing the metals. Hence, the aim of the combustion step is to obtain ash containing metals that is easily recoverable in the subsequent metal recovery units by burning the organic phase or carbon phase of the extract at a temperature and pressure which limits vaporization and/or sublimation of metals, in particular that of molybdenum (sublimation temperature approximately 700° C. for MoO₃). Hence, the step for reduction of the organic phase consists of a moderate temperature combustion in order to concentrate the metals, without substantial loss by vaporization and/or sublimation into fumes, in a mineral phase which may contain a proportion of organic phase of 0 to 100% by weight, preferably 0 to 40% by weight. The general operating conditions for this combustion are a pressure of −0.1 to 1 MPa, preferably −0.1 to 0.5 MPa, and a temperature of 200° C. to 700° C., preferably 400° C. to 550° C. Combustion is carried out in the presence of oxygen.

The gaseous effluent from combustion requires purification steps in order to reduce emissions of sulphur-containing and nitrogen-containing compounds into the atmosphere. Processes which are conventionally used by the skilled person in the field of air treatment are employed under the operating conditions necessary to satisfy regulations which are in force in states where such a treatment of a hydrocarbon feedstock is carried out.

The solid from combustion is a mineral phase containing all or almost all of the metallic elements contained in the extract, in the form of ash.

The direct treatment of the leaching extract by a metals extraction method as described below without combustion has an insufficient degree of recovery of the metals.

Recovery of Metals

The ash from combustion is sent to a metals extraction step in which the metals are separated from each other in one or more sub step(s). This recovery of metals is necessary, as simply recycling the ash to the hydroconversion step results in a catalytic activity which is very weak. In general, the metals extraction step can be used to obtain a plurality of effluents, each effluent containing a specific metal, for example Mo, Ni or V, generally in the form of a salt or oxide. Each effluent containing a metal of the catalyst is directed to a step for preparing an aqueous or organic solution based on a metal identical to the catalyst or to its precursor, used in the hydroconversion step. The effluent containing a metal from the feedstock which cannot be re-used as a catalyst (for example vanadium) can be re-used outside the process.

The operating conditions, fluids and/or extraction methods used for the various metals are considered to be known to the skilled person and already in industrial use as described, for example, in Marafi et al, Resources, Conservation and Recycling 53 (2008), 1-26, U.S. Pat. No. 4,432,949, U.S. Pat. No. 4,514,369, U.S. Pat. No. 4,544,533, U.S. Pat. No. 4,670,229 or US2007/0025899. The various routes to extraction of metals that are known in general include leaching by acidic and/or basic solutions, by ammonium or ammonium salts, by bio-leaching with microorganisms, by low temperature heat treatment (roasting), by sodium or potassium salts, by chlorination or by recovering metals electrolytically. Leaching with acids may be carried out using inorganic acids (HCl, H₂SO₄, HNO₃) or organic acids (oxalic acid, lactic acid, citric acid, glycolic acid, phthalic acid, malonic acid, succinic acid, salicylic acid, tartaric acid, etc). In general, for basic leaching, ammonia, ammonium salts, sodium hydroxide or Na₂CO₃ is used. In both cases, oxidizing agents (H₂O₂, Fe(NO₃)₃, Al(NO₃)₃, etc) may be present to facilitate extraction. Once the metals are in solution, they may be isolated by selective precipitation (at different pHs and/or with different agents) and/or by extraction agents (oximes, beta-diketone, etc).

Preferably, the step for extraction of metals of the invention comprises leaching with at least one acidic and/or basic solution.

Preparation of Catalytic Solution(s)

The metals recovered after the extraction step are generally in the form of a salt or oxide. The preparation of the catalytic solutions in order to produce organic or aqueous solutions is known to the skilled person and has been described in the hydroconversion section. The preparation of the catalytic solutions primarily involves the metals molybdenum and nickel; vanadium is generally upcycled outside the process as vanadium pentoxide, or in combination with iron to produce ferrovanadium.

The degree of recovery of metals re-used as a catalyst for the slurry hydroconversion process or for the vanadium is at least 50% by weight, preferably at least 65% by weight and more generally 70% by weight.

DESCRIPTION OF FIGURES

The following figures present advantageous embodiments of the invention. The essentials of the unit and the process of the invention will be described. The operating conditions described above will not be reiterated.

FIG. 1 shows a process for the hydroconversion of heavy oil feedstocks integrating a slurry technique without metals recovery;

FIG. 2 describes a process for the hydroconversion of heavy oil feedstocks in accordance with the invention;

FIG. 3 describes the various sub-steps of the leaching step.

In FIG. 1, the feedstock 1 supplies the catalytic slurry hydroconversion section A. This catalytic slurry hydroconversion section is constituted by a preheating furnace for the feedstock 1 and hydrogen 2 and a reaction section constituted by one or more reactors disposed in series and/or in parallel, depending on the capacity required. The catalyst 4 or its precursor is also injected, as well as the optional additive 3. The catalyst 4 is held in suspension in the reactor, moves from bottom to top of the reactor with the feedstock, and is evacuated with the effluent. The effluent 5 obtained from the hydroconversion is directed to a high pressure high temperature separation section B which can be used to separate a converted fraction in the gaseous state 6, termed the light fraction, and a residual unconverted liquid/solid fraction 8. The light fraction 6 may be directed to a hydrotreatment and/or hydrocracking section C. An external cut 7, generally from another process in the refinery or possibly from outside the refinery, may be supplied before the hydrotreatment and/or hydrocracking. The residual unconverted fraction 8 containing the catalyst and a fraction of the solid particles used as an optional additive and/or formed during the reaction is directed to a fractionation step D. The fractionation step D is preferably a vacuum distillation that can be used to concentrate the vacuum residue 10, which is rich in metals, at the bottom of the column and to recover one or more effluents 9 from the head of the column In this layout for re-using a heavy feedstock by a traditional slurry hydroconversion process, the vacuum residue 10 which is rich in metals is re-used as a fuel with a very high viscosity or as a solid fuel after pelletization, for example to produce heat and electricity on-site or offsite or as a cement-works fuel. A priori, the metals are not recovered. A small portion of the effluent or effluents 9 thus produced is/are normally directed via the line 31 to the slurry hydroconversion unit A where they can be recycled directly to the reaction zone, or they may be used in the preparation of catalytic precursors before injection into the feedstock 1; another part may be sent to the hydrotreatment and/or hydrocracking unit C via the line 30 as a mixture with the effluents 6 and/or 7 in equal or different proportions as a function of the quality of the products obtained.

In FIG. 2, the steps (and references) for the hydroconversion, HPHT separation, hydrotreatment and/or hydrocracking and vacuum distillation are identical to FIG. 1. The vacuum residue 10 withdrawn from the bottom of the vacuum distillation D is directed to a liquid/liquid type extraction step E to concentrate the effluent 10. This extraction step E is carried out using a solvent 11 with a saturated nature. The raffinate 12 leaving the extraction unit after evaporation of the solvent is preferably mixed, via the line 33, with the hydrocarbon feedstock 1 upstream of the slurry hydroconversion section A, or mixed via the line 32 with the effluent 6 and/or 7 upstream of the hydrotreatment and/or hydrocracking section C. The solid extract 13, which is highly concentrated in metals, is directed to a grinding step F. This grinding step F can produce a ground solid which is then directed to a single leaching extraction step G in order to concentrate the metals afresh. This leaching step G is carried out in several steps which are described below (FIG. 3). The first step consists of producing a mixture of ground solid 14 with water supplemented with a surfactant 15 and a solvent with a saturated nature 16. This mixture is heated and mixed. The mixture is then directed to a decanting step to separate a solid extract from the bottom of the decanter 18 and a leachate 17 containing the solvent 16 from the head of the decanter. The leachate 17 is sent to a separation section, for example of the flash type, in order to recycle the solvent 16 upstream of the leaching section G and/or to the liquid-liquid extraction unit E. The non-evaporated fraction of the leachate 17 may then be mixed with the hydrocarbon feedstock 1 upstream of the slurry hydrocracking section A via the line 35, or even mixed in small quantities with the effluent 6 and/or 7 upstream of the hydrocracking section via the line 34. The solid extract 18, which is highly concentrated in metals, is directed to a step for reducing the organic phase by moderate temperature combustion H in order to concentrate it very greatly in metals, without substantial losses by vaporization and/or sublimation into fumes. The gaseous effluent obtained from combustion 19 necessitates steps of purification (not shown) in order to reduce the emissions of sulphur-containing and nitrogen-containing compounds into the atmosphere. The product 20 from the combustion H is a mineral phase containing all or nearly all of the metallic elements contained in the extract 18 in the form of ash. The product 20 described below is sent to a step for the extraction of metals I in which the metals are separated from each other in one or more sub step(s). The effluent 21 from the extraction I is composed of a molybdenum type metal in the form of a salt or oxide. This effluent 21 is then directed to a step J for the preparation of an organic or aqueous solution based on molybdenum 23 identical to the catalyst 4 or to its precursor recycled in part or in its entirety to the slurry hydroconversion step A via the line 40. The effluent 22 from the extraction I is composed of a nickel type metal in the form of a salt or oxide. This effluent 22 is then directed to a step K for the preparation of an organic or aqueous solution based on nickel 24 which is identical to the catalyst 4 or to its precursor recycled in part or in its entirety to the slurry hydroconversion step A via the line 41. The effluent 25 from the extraction I is composed of a vanadium type metal in the form of a salt or oxide. This effluent 25 may be upcycled, for example as vanadium pentoxide, or in combination with iron, to produce ferrovanadium.

FIG. 3 describes the various leaching steps. The ground solid 14 is mixed with water supplemented with a surfactant 15. A solvent with a saturated nature 16 is added to the prepared solution to form an emulsion in the step G1. The mixture 101 so formed of the ground feedstock 14, supplemented water 15 and paraffinic solvent 16 is heated to a temperature in the range 20° C. to 120° C., preferably in the range 60° C. to 70° C., and mixed for a period in the range 15 min to 3 hours in the maturation step G2. The mixture 102 is then directed, maintaining the temperature, to a decanting step G3 to separate a solid extract from the bottom of the decanter 18, and a leachate 17, which is a light hydrocarbon phase containing the solvent 16, from the head of the decanter. The leachate 17 is sent to a separation section, for example of the flash type (not shown), in order to recycle the solvent 16 upstream of the leaching section G and/or to the liquid-liquid extraction unit E. The non-evaporated fraction of the leachate 17 may then be mixed with the hydrocarbon feedstock 1 upstream of the slurry hydrocracking section A or mixed in small quantities with the effluent 6 and/or 7 upstream of the hydrotreatment and/or hydrocracking section C. The extract 18 is directed to a combustion step.

In the preferred case of a slurry hydroconversion using a catalyst based on molybdenum and nickel, hydroconversion employs a finely dispersed nickel and molybdenum type catalyst in respective concentrations of 160 ppm by weight and 600 ppm by weight under hydrogen pressure. Assuming that the industrial unit has a capacity of 50000 barrels per day and a 90% per annum usage, the quantity of nickel and molybdenum consumed per annum is thus 0.4 and 1.6 t/year respectively. Assuming the price of nickel is 25k $/t and of molybdenum is 60k $/t, representative of the average prices observed on the metals market over the last 5 years, the operating cost is 100 million dollars per annum.

The process of the invention can be used to re-use a large fraction of the metals nickel and molybdenum present in the unconverted fraction of the effluent from slurry hydroconversion. The degree of recovery of metals re-used as a catalyst for the slurry hydroconversion process is at least 50% by weight, preferably at least 65% by weight, and more generally 70% by weight. This recycling of metals can thus be employed to reduce the operating cost of 100 million dollars per annum to 30 million dollars per annum. This therefore saves 70 million dollars, which can initially be used to offset the additional investment for recovery of these metals. Furthermore, vanadium present in the heavy feedstock in an amount of 400 ppm by weight may be upcycled as ferrovanadium. Assuming a level of recovery of at least 50% by weight, preferably at least 65% by weight, and more generally 70% by weight, assuming an average observed cost of 40k $/t on the metals market over the last 5 years, sales of vanadium can be estimated at 12 million dollars per annum. These sales can also initially be used to offset the supplemental investment required for recovery of these metals.

Recovering these metals in the residual unconverted fraction can be used to reduce the overall quantity of nickel and molybdenum used and to thereby reduce the environmental impact of the slurry hydroconversion process. Assuming a recovery of 70% by weight of metals present at the inlet to the reaction zone, the quantity of makeup catalyst is reduced to 0.1 t/year for nickel and 0.5 t/year for molybdenum, as opposed to 0.4 t/year and 1.6 t/year without a recycle.

Claims

1. A process for the hydroconversion of heavy oil feedstocks containing metals, comprising:

a. a step for hydroconversion of the feedstock in at least one reactor containing a catalyst in the form of a slurry containing at least one metal, and optionally a solid additive;

b. a step for separation of the hydroconversion effluent without decompression into a fraction termed the light fraction containing compounds boiling at 500° C. at most and into a residual fraction;

b′. an optional step for fractionation, comprising vacuum separation of said residual fraction as obtained in step b) to obtain a vacuum residue which is concentrated in metals;

c. a step for liquid/liquid extraction of said residual fraction as obtained in step b) and/or said vacuum residue as obtained in step b′) using a solvent with a saturated nature in order to obtain a solid extract which is concentrated in metals and a raffinate;

d. a step for grinding the solid extract which is concentrated in metals obtained from the liquid/liquid extraction step;

e. a step for leaching the ground extract in the presence of water, a solvent with a saturated nature and a surfactant in order to obtain a solid extract and a leachate;

f. a step for combustion of said solid extract obtained from the leaching step in the presence of oxygen in order to obtain ash which is concentrated in metals;

g. a step for extraction of metals from the ash obtained in the combustion step;

h. a step for preparing metallic solution(s) containing at least the metal of the catalyst which is/are recycled as the catalyst to the hydroconversion step.

2. A process according to claim 1, in which said light fraction obtained from step b) for separation without decompression undergoes at least one hydrotreatment and/or hydrocracking step.

3. A process according to claim 1, in which said residual fraction from the step for separation without decompression is fractionated by vacuum distillation into at least one vacuum distillate fraction and one vacuum residue fraction, at least a portion, preferably all of said vacuum residue fraction being sent to the liquid/liquid extraction step, at least a portion, preferably all of said vacuum distillate fraction undergoing at least one hydrotreatment and/or hydrocracking step.

4. A process according to claim 1, in which the liquid/liquid extraction step is carried out at a temperature in the range 50° C. to 300° C., preferably in the range 120° C. to 250° C.

5. A process according to claim 1, in which the liquid/liquid extraction step is carried out with a solvent/feedstock ratio of 1/1 to 10/1, preferably 2/1 to 7/1.

6. A process according to claim 1, in which the particle size of said solid extract which is concentrated in metals from the liquid/liquid extraction step obtained by grinding is less than 6 mm, preferably less than 4 mm.

7. A process according to claim 1, in which the leaching step comprises:

a. a step for preparing an emulsion comprising the ground extract from the grinding step, said water, said surfactant and said solvent with a saturated nature, the water/feedstock ratio being in the range 0.5/1 to 5/1, preferably in the range 1/1 to 2/1, the solvent/feedstock ratio being in the range 2/1 to 6/1, preferably in the range 3/1 to 4/1, and the concentration of surfactant being in the range 0.05% by weight to 2% by weight, preferably in the range 0.1% by weight to 1% by weight, with respect to the water;

b. a step for maturing said emulsion at a temperature in the range 20° C. to 120° C., preferably in the range 60° C. to 70° C., optionally for a period in the range 15 minutes to 3 hours;

c. a step for decanting, holding the temperature thereby, optionally for a period in the range 15 minutes to 30 hours, in order to obtain a solid extract and a leachate.

8. A process according to claim 1, in which the solvent with a saturated nature used in the liquid/liquid extraction step and in the leaching step is identical and preferably selected from the group formed by hexane, heptane, and a light naphtha with a saturated nature, mixed or not mixed and in equal or different proportions.

9. A process according to claim 1, in which the combustion step is operated at a pressure of −0.1 to 1 MPa, preferably −0.1 to 0.5 MPa and at a temperature of 200° C. to 700° C., preferably 400° C. to 550° C. in the presence of air.

10. A process according to claim 1, in which step g) for extraction of the metals comprises leaching using at least one acidic and/or basic solution.

11. A process according to claim 1, in which the heavy oil feedstock is a hydrocarbon feedstock containing at least 50% by weight of product distilling above 250° C. and at least 25% by weight distilling above 350° C., and contains at least 50 ppm by weight of metals, at least 0.5% by weight of sulphur and at least 1% by weight of asphaltenes (heptane asphaltenes).

12. A process according to claim 1, in which the heavy oil feedstock is selected from oil residues, crude oils, topped crudes, deasphalted oils, deasphalted asphalts, oil conversion process derivatives, bituminous sands or their derivatives, oil shale or their derivatives, or mixtures of such feedstocks.

13. A process according to claim 1, in which the hydroconversion step is operated at a pressure of 2 to 35 MPa, preferably 10 to 25 MPa, a partial pressure of hydrogen of 2 to 35 MPa, preferably 10 to 25 MPa, a temperature in the range 300° C. to 500° C., preferably 420° C. to 480° C. and a contact time of 0.1 h to 10 h, preferably 0.5 h to 5 h.

14. A process according to claim 1, in which the slurry catalyst is a sulphurized catalyst containing at least one element selected from the group formed by Mo, Fe, Ni, W, Co, V and Ru.

15. A process according to claim 1, in which the additive is selected from the group formed by mineral oxides, spent supported catalysts containing at least one element from group VIII and/or at least one element from group VIB, carbonaceous solids with a low hydrogen content or mixtures of said additives, said additive having a particle size of less than 1 mm.