EP1700900B1 - Hydrocracking process with recycling which includes adsorption of polyaromatic compounds from recycled stream using a silica-alumina based adsorbant with limited macropores concentration - Google Patents

Description

Field of the invention

The invention relates to the elimination of polyaromatic compounds (PNA) in the field of hydrocracking processes.

Description of the prior art

A hydrocracking process is a heavy charge conversion process (higher boiling point of hydrocarbons, generally at 380 ° C) from vacuum distillation. It operates at high temperature and under high pressure of hydrogen and makes it possible to obtain products of very good quality because rich in paraffinic and naphthenic compounds and with very few impurities. However, this method has several drawbacks: it is, by its hydrogen consumption, expensive and it does not have a very high yield (30 to 40% of the unconverted load). It seems interesting to set up a recycling loop. Nevertheless, this recycling leads to an accumulation of polyaromatic compounds (called PNA) which are formed during the passage of the feedstock over the hydrocracking catalyst and ultimately to the formation of coke on the same catalyst. This leads to a loss of capacity or even a total deactivation of the catalyst (poisoning of the adsorption sites and pore blocking). In addition, the larger the size of these molecules, the lower the solubility of these molecules: beyond a certain critical size, they precipitate and settle on the cold parts of installations such as piping and pumps, which generates problems. heat transfer at the exchangers and reduces their efficiency.

To avoid these problems, the simplest solution is to set up a purge of deconcentration on the recycling loop ( US 3,619,407 , US 4,961,839 ). The disadvantage of this technique is that it causes a decrease in the efficiency of the process by a few conversion points. The technical problem is therefore to develop an alternative technique that would ensure the selective elimination, partial or total, of the PNA recycled residue.

The polyaromatic molecules ¹ (or PNA) are molecules consisting of an assembly of aromatic rings (presence also possible of one or more saturated rings), substituted or not by alkyl groups. Because of their high molecular weight, they are very low volatility compounds and often solid at room temperature. Finally, their strong aromaticity and the absence of polar substituents on the rings lead to a very low solubility of these molecules in water or in alkanes. This solubility decreases further when the number and length of the alkyl side chains become lower.
¹ Julius Scherzer; AJGruia Hydrocracking Science and Technology, Marcel Dekker Inc .: New York, 1996; Chapter 11, pp 200-214 .

NAPs are sometimes classified into several categories according to their number of nuclei: light PNAs with 2 to 6 nuclei, heavy PNAs containing 7 to 10 nuclei and finally NAPs with cycles greater than 11. It is generally accepted that the hydrocracker inlet feedstocks mainly contain light PNA. After passing over the hydrocracking catalyst, a higher concentration of these molecules is observed on the one hand, but also the presence of heavy PNAs, which are the most harmful molecules for the hydrocracking process (deposition on the catalyst and on the catalyst). unit / precursors for coke formation). These can be formed either by condensation of two or more light PNAs, or by dehydrogenation of larger polycyclic compounds, or by cyclization of the pre-existing side chains on the PNAs, followed by dehydrogenation. Subsequently reactions of combinations or dimerizations of heavy PNA can take place resulting in the formation of compounds with more than 11 nuclei.

The formation of these heavy PNAs depends on the composition of the filler (the heavier it is and the more it contains heavy PNA precursors) but also the temperature of the reactor. The higher it is, the more the dehydrogenation and condensation will be favored, hence the higher formation of heavy ANP. This effect of temperature is all the more marked as the conversion rate is high.

For the detection and analysis of NAPs, several options are possible ² . However, since they are often mixtures of PNA, it is preferable to separate the different molecules beforehand. For this purpose, liquid chromatography (HPLC) is used. Then, detection, identification and determination of PNA can be done either by UV absorption or by fluorescence. These are specific NAP methods and therefore sensitive, but they do not always detect all NAPs (average quantitative reliability). Direct analyzes by mass spectrometry or IR are also possible but they are more difficult to implement and exploit.
² Milton L. Lee; Milos V.Novotny; Keith D.Bartle Analytical Chemistry of Polycyclic Aromatic Compounds; Academic Press, Inc .: London, 1981 .

Several methods for removing NAP from the recycled fraction have already been proposed in the literature: precipitation followed by filtration, hydrogenation and / or catalytic hydrocracking or adsorption on porous solid.

The precipitation of PNA is caused by the addition of flocculent (patent US5,232,577 ) and / or a drop in temperature (patent US 5,120,426 ) and is followed by decantation or centrifugation and phase separation. This is an effective technique but which seems unsuitable for a hydrocracking process operating continuously because of the high residence times required either for the precipitation itself or for the decantation of PNA and the probable crystallization of paraffins at low temperatures applied.

Catalytic hydrogenation of NAPs ( US 4,411,768 , US 4,618,412 , US5,007,998 and US 5,139,644 ) allows the reduction of PNA content but not their complete elimination. In addition, it requires fairly severe conditions of temperature and pressure. Thus, although compatible with a hydrocracking process operating continuously, it does not, at present, a very cost-effective solution.

Adsorption is an effective method which, depending on the solid and the operating conditions chosen, is compatible with a hydrocracker operating continuously. In fact, it is the most frequently considered solution, as shown by the large number of patents filed on this subject. They cover several process configurations. The adsorption zone may be put in place either before or after the hydrocracker. In the first case, it is a matter of pre-treating the load ( US 4,775,460 ) and eliminate the PNA precursors. Nevertheless, since NAPs are mainly formed during passage over the hydrocracking catalyst, the interest of this solution is limited. It is, on the contrary, useful to seek to reduce or even eliminate the PNA of the fraction that will be recycled on the catalyst to prevent these molecules from growing and accumulating. Here again, several positions of the adsorption zone are conceivable: at the outlet of a first SHP situated before the distillation tower ( US 4,954,242 , US 5,139,646 ) or at the outlet of the distillation tower on a line where all or only part of the recycled fraction passes ( US 4,447,315 , US 4,775,460 , US 5,124,023 , US 5,190,633 , US 5,464,526 , US 6,217,746 / WOO2 / 074,882 ). This second solution is the best. Indeed, by positioning the adsorption zone after and not before the fractionation zone the volume of charge to be treated is much lower. According to the patents, the adsorption zone and in particular the nature of the adsorbent is more or less detailed. In general, all known conventional adsorbents are mentioned: silica gel, activated charcoal, activated or non-activated alumina, silica gel / alumina, clay, polystyrene gel, cellulose acetate, molecular sieve (zeolite). Of all these solids, the most suitable ones seem to be activated carbons, aluminas and amorphous silicas. In addition, it is often mentioned that the selected solids should have a pore volume, a BET surface and the largest possible pore diameter. Some suggest the use of specifically prepared solids, such as porous amorphous silica treated with sulfuric acid ( US 5,464,526 ) in order to improve their adsorption capacity vis-à-vis the PNA. There are also some patents which only concern the adsorbent. So the patent US 3,340,316 proposes the use of active carbons impregnated with fluorinated compounds and the patent application EP 0.274.432 A1 that of an inorganic material supporting a copper-based complex. The patents often specify the operation of the adsorbent bed (fixed or mobile bed, system with two beds in parallel) and the possible regeneration mode for the adsorbent but without too much detail. This is mainly movement of PNA adsorbed by passing a gas stream at elevated temperatures (applicable method as well as in ex situ) or that of a liquid. In the first case, it is possible to use either a low efficiency inert gas or an effective oxidizing gas (burn technique) but which can lead to degradation of the adsorbent especially in the case of activated carbons. One can also consider a steam stripping which allows working at slightly lower temperatures (370-810 ° C) than in the two previous cases. The patent US 5,792,898 proposes the use of a gas rich in hydrogen at a temperature between 149 and 371 ° C to desorb at least partially the aromatic compounds. The exit effluent, once cooled to 16-49 ° C, is then sent to a liquid-vapor separator and the liquid recovered in a distillation column to separate the mono compounds from the polyaromatic compounds. As for the liquid desorbent, it must have a certain affinity with the solid to be able to displace the PNAs and with the PNAs to solubilize them. The best solvents are therefore aromatic compounds alone (toluene, benzene, ethylbenzene, cumene, xylenes) or mixed (light cuts from the FCC reactor) ( US 5,124,023 ). Other types of solvents such as hydrocarbon-halogenated solvents, ketones, alcohols or light hydrocarbons alone or as a mixture ( US 4,732,665 ) were also mentioned.

Adsorption seems to be the most suitable solution for the removal of NAPs in a hydrocracking unit, the optimum positioning of this purification zone being that at the outlet of the distillation tower. This is confirmed by the fact that only this solution has been implemented industrially ³ . These are two beds of 144 m ³ of activated carbon, operating in downflow mode , installed in series. When the first bed needs to be treated (simple "back flush", applicable only three times, or complete renewal of the adsorbent), the second bed works alone. The disadvantage of this method is that it does not provide regeneration of activated carbon and therefore has a high cost.
³ Stuart Frazer; Warren Shirley PTQ 1999, 632, 25-35 .

To make this process economically interesting, it is necessary to find a solid with good adsorption capabilities of PNA and which is simultaneously regenerable. Although activated carbons are the solids with the largest adsorption capacities, they can currently be regenerated only by solvent elution. In addition to the fact that the amount of solvent required is very important, it would be necessary to set up an additional separation system to recycle the solvent. This solution would be much too expensive to implement. In the context of a refinery, the ideal solution would be to regenerate solids by burning. This technique is not applicable to activated carbons. It was therefore a question of identifying sufficiently strong solids with respect to active carbons but more resistant than these. The solids proposed up to now as an alternative to activated carbons had rather low performances, probably because of too small pore size (molecular sieve) or too small a surface (amorphous silica gel meso- and / or macroporous, activated alumina) .

The solid adsorbent must be capable of retaining selectively and in large amounts the PNA with a selectivity greater than 1 and preferably between 2 and 5 for the coronene compared to other lesser PNA such as pyrene (4 aromatic nuclei) or perylene (5 aromatic nuclei). In addition, to be able to optimally use the porosity of the adsorbent, it is necessary that it has free openings (taking into account the Van der Waals radii of the atoms pointing towards the center of the pore) of pores greater than 11.4 Å (calculations from the literature, considering a plane molecule with 1.395 Å for DC, 1.084 Å for CH and 1.2 Å for the van der Waals radius of hydrogen ⁴ ) and preferably greater than 20 Å. This condition excludes so-called microporous solids such as zeolites since the faujasite which is the zeolite with larger pores has tunnels opening 7.4 Å. Conversely, the pore openings must not be too wide in order to prevent the specific surface, the pore volume and therefore the total adsorption capacity from becoming too small. The specific surface must generally be greater than 200 m ² / g and preferably greater than 400 m ² / g. This explains why silica gels and aluminas which often have a BET surface area of less than 200 m ² / g, are not suitable for PNA adsorption. Finally, it is preferable to use a solid whose porous network has branches so as to avoid being in a situation where the adsorption of molecules blocks the entry of pores or tunnels still vacant. This is not the case either for mesostructured materials or bridged clays. In view of these constraints, the solids that seem most suitable for the adsorption of NAPs with the exception of activated carbons are the amorphous mesoporous silica-aluminas. Although having porous volumes, specific surfaces and thus lower adsorption capacities As active carbons, they have the advantage of being prepared at high temperature and therefore of being resistant to burning.
⁴ Henry W. Haynes, Jr .; Jon F.Parcher; Norman E. Heimer Ind.Eng.Chem.Process Des.Dev. 1983, 22, 409 .

Description of the invention

The present invention provides an improved hydrocracking process having a step of removing polyaromatic compounds from at least a portion of the adsorption-recycled fraction on a silica-alumina adsorbent which has good adsorption capacities because of its large specific surface and its pores of sufficient size to be accessible to molecules with more than 4 nuclei. This invention therefore makes it possible to effectively eliminate the PNA from the charge while offering the possibility of using the same adsorbent for several cycles because of its regenerability by burning. In addition, these solids have the advantage of being denser than activated carbon, which partially offsets their lower adsorption capacity to adsorbent iso-mass. In addition to the gain in consumption of solid, this avoids additional investments such as the establishment of a distillation column necessary in the case of solvent regeneration.

• un contenu en sodium inférieur à 0,03 % poids,
• un volume poreux total mesuré par porosimétrie au mercure compris entre 0,45 et 1,2 ml/g,
• une porosité telle que :
  1. i) le volume des mésopores avec un diamètre compris entre 40 et 150 Å et un diamètre moyen poreux compris entre 80 et 140 A (de préférence entre 80 et 120 Å) représente 40-70 % du volume poreux total mesuré par porosimétrie au mercure
  2. ii) le volume des macropores avec un diamètre supérieur à 500 Å représente 30-60 % du volume poreux total mesuré par porosimétrie au mercure
     • une surface spécifique BET comprise entre 200 et 550 m²/g,
     • un diagramme de diffraction X qui contient au moins les raies principales caractéristiques d'au moins une des alumines de transition comprise dans le groupe composé par les alumines alpha, rhô, khi, êta, gamma, kappa, thêta et delta.

More specifically, the invention relates to an improved process for hydrocracking with recycling having a step of removing polyaromatic compounds from at least a portion of the fraction recycled by adsorption on an alumina-silica adsorbent (that is that is, comprising alumina and silica) having a mass content of silica (SiO ₂ ) greater than 5% by weight and less than or equal to 95% by weight; said alumina-silica having:

• a sodium content of less than 0.03% by weight,
• a total pore volume measured by mercury porosimetry of between 0.45 and 1.2 ml / g,
• a porosity such as:
  1. i) the volume of the mesopores with a diameter between 40 and 150 Å and a mean pore diameter of between 80 and 140 Å (preferably between 80 and 120 Å) represents 40-70% of the total pore volume measured by mercury porosimetry
  2. ii) the volume of the macropores with a diameter greater than 500 Å represents 30-60% of the total pore volume measured by mercury porosimetry
     • a BET specific surface area of between 200 and 550 m ² / g,
     • an X-ray diffraction pattern which contains at least the principal characteristic lines of at least one of the transition aluminas included in the group consisting of alpha, rho, khi, eta, gamma, kappa, theta and delta aluminas.

• une étape d'hydrocraquage (l'hydrocraquage étant avantageusement réalisé selon le mode dit en une étape ou selon le mode dit en deux étapes décrits ci-dessous),
• une étape de séparation, généralement dans une tour de distillation atmosphérique, permettant de séparer (en fond de colonne) une fraction non convertie de point de coupe T05 supérieur à 340°C et
• une étape d'adsorption en phase liquide d'une partie ou de la totalité des PNA contenus dans cette fraction non convertie (résidu lourd issu de la distillation).

The method generally comprises the following steps:

• a hydrocracking step (the hydrocracking being advantageously carried out according to the so-called one-step mode or the so-called two-step mode described below),
• a separation step, generally in an atmospheric distillation tower, for separating (at the bottom of the column) an unconverted fraction of cutting point T05 greater than 340 ° C. and
• a step of adsorption in the liquid phase of part or all of the PNA contained in this unconverted fraction (heavy residue from the distillation).

Preferably, the adsorbent undergoes a regeneration treatment by burning after the adsorption step.

The adsorption step can be performed on all or only a portion of the recycled fraction and can operate continuously or discontinuously. Preferably, the adsorption step is performed on the entire recycled fraction.

Detailed description of the invention Step 1: hydrocracking loads

A wide variety of fillers can be processed by the hydrocracking processes described below and generally contain at least 20% by volume and often at least 80% by volume of compounds boiling above 340 ° C.

The feedstock may be, for example, LCOs (light cycle oil - light gas oils from a catalytic cracking unit), atmospheric distillates, vacuum distillates, for example gaszoles obtained from the direct distillation of the crude or from conversion units such as FCC, coker or visbreaking, as well as feedstocks from aromatics extraction units of lubricating oil bases or from solvent dewaxing of lubricating oil bases, or process distillates. desulphurization or hydroconversion in fixed bed or bubbling bed of RAT (atmospheric residues) and / or RSV (vacuum residues) and / or deasphalted oils, or the charge can be a deasphalted oil, or all mixture of the aforementioned fillers. The list above is not exhaustive. In general, the feeds have a boiling point T5 greater than 340 ° C., and more preferably greater than 370 ° C., ie 95% of the compounds present in the feed have a boiling point greater than 340. ° C, and more preferably above 370 ° C.

The nitrogen content of the feedstocks treated in the hydrocracking processes is usually greater than 500 ppm, preferably between 500 and 10,000 ppm by weight, more preferably between 700 and 4000 ppm by weight and even more preferably between 1000 and 4000. ppm. The sulfur content of the feedstocks treated in the hydrocracking processes is usually between 0.01 and 5% by weight, preferably between 0.2 and 4% and even more preferably between 0.5 and 2%.

The charge may optionally contain metals. The cumulative nickel and vanadium content of the feeds treated in the hydrocracking processes is preferably less than 1 ppm by weight.

The asphaltene content is generally less than 3000 ppm, preferably less than 1000 ppm, even more preferably less than 200 ppm.

Cribs

In the case where the feedstock contains resins and / or asphaltenes-type compounds, it is advantageous to first pass the feedstock over a bed of catalyst or adsorbent other than the hydrocracking or hydrotreatment catalyst.

The catalysts or guard beds used are in the form of spheres or extrudates. It is however advantageous that the catalyst is in the form of extrudates with a diameter of between 0.5 and 5 mm and more particularly between 0.7 and 2.5 mm. The shapes are cylindrical (which can be hollow or not), cylindrical twisted, multilobed (2, 3, 4 or 5 lobes for example), rings. The cylindrical shape is preferably used, but any other shape may be used.

In order to overcome the presence of contaminants and / or poisons in the feed, the guard catalysts may, in another preferred embodiment, have more particular geometric shapes in order to increase their void fraction. The void fraction of these catalysts is between 0.2 and 0.75. Their outer diameter can vary between 1 and 35 mm. Among the particular forms possible without this list being exhaustive: hollow cylinders, hollow rings, Raschig rings, serrated hollow cylinders, crenellated hollow cylinders, pentaring carts, multi-hole cylinders, etc.

These catalysts may have been impregnated with an active phase or not. Preferably, the catalysts are impregnated with a hydro-dehydrogenation phase. Very preferably, the CoMo or NiMo phase is used.

These catalysts may have macroporosity. The guard beds can be marketed by Norton-Saint-Gobain, for example the MacroTrap® guard beds. Guard beds can be marketed by Axens in the ACT family: ACT077, ACT935, ACT961 or HMC841, HMC845, HMC941 or HMC945.

It may be particularly advantageous to superpose these catalysts in at least two different beds of variable height. The catalysts having the highest void content are preferably used in the first catalytic bed or first catalytic reactor inlet. It may also be advantageous to use at least two different reactors for these catalysts.

The preferred guard beds according to the invention are HMC and ACT961.

Operating conditions

Operating conditions such as temperature, pressure, hydrogen recycling rate, hourly space velocity, may be very variable depending on the nature of the load, the quality of desired products and facilities available to the refiner. The hydrocracking / hydroconversion or hydrotreating catalyst is generally brought into contact, in the presence of hydrogen, with the charges described above, at a temperature above 200 ° C., often between 250 and 480 ° C., advantageously between 320 and 450 ° C, preferably between 330 and 435 ° C, under a pressure greater than 1 MPa, often between 2 and 25 MPa, preferably between 3 and 20 MPa, the space velocity being between 0.1 and 20h ^{- 1} and preferably 0.1-6h ^-1 , preferably 0.2-3h ^-1 , and the amount of hydrogen introduced is such that the volume ratio by liter of hydrogen / liter of hydrocarbon is between 80 and 5000l / l and most often between 100 and 2000 l / l.

These operating conditions used in the hydrocracking processes generally make it possible to achieve pass conversions, products having boiling points below 340 ° C., and better still below 370 ° C., greater than 15%, and even more so. more preferably between 20 and 95%.

Modes of realization

The hydrocracking / hydroconversion processes using the catalysts according to the invention cover the pressure and conversion ranges from mild hydrocracking to high pressure hydrocracking. Mild hydrocracking is understood to mean hydrocracking leading to moderate conversions, generally less than 40%, and operating at low pressure, generally between 2 MPa and 6 MPa.

The hydrocracking catalyst can be used alone, in one or more fixed bed catalytic beds, in one or more reactors, in a so-called one-step hydrocracking scheme, with or without liquid recycling of the unconverted fraction, optionally in combination with a hydrorefining catalyst located upstream of the hydrocracking catalyst.

The hydrocracking catalyst may be used alone, in one or more bubbling bed reactors, in a so-called one-step hydrocracking scheme, with or without liquid recycling of the unconverted fraction, optionally in combination with a catalyst of hydrorefining located in a fixed bed reactor or bubbling bed upstream of the hydrocracking catalyst.

The bubbling bed operates with removal of spent catalyst and daily addition of new catalyst to maintain stable catalyst activity.

In a two-stage hydrocracking scheme with intermediate separation between the two reaction zones, in a given step, the hydrocracking catalyst may be used in one or both reactors in association or not with a hydrorefining catalyst located upstream of the hydrocracking catalyst.

One-step process

The so-called hydrocracking in one step, comprises firstly and in a general manner advanced hydrorefining which aims to carry out a hydrodenitrogenation and hydrodesulphurization of the feed before it is sent to the hydrocracking catalyst proper , especially in the case where it comprises a zeolite. This extensive hydrorefining of the feed leads only to a limited conversion of the feedstock into lighter fractions, which remains insufficient and must therefore be completed on the more active hydrocracking catalyst. However, it should be noted that no separation occurs between the two types of catalysts. All of the effluent at the outlet of the reactor is injected onto the hydrocracking catalyst proper and only then is separation of the products formed carried out. This version of the hydrocracking, also called "Once Through", has a variant that has a recycling of the unconverted fraction to the reactor for further conversion of the charge.

One-step process in a fixed bed

In the case where the silica-alumina-based catalyst is used upstream of a zeolitic hydrocracking catalyst, for example based on zeolite Y, it will be advantageous to use a catalyst having a high silica content by weight. say with silica weight contents of the support used in the composition of the catalyst between 20 and 80% and preferably between 30 and 60%. It can also be advantageously used in combination with a hydrorefining catalyst, the latter being located upstream of the hydrocracking catalyst.

When the silica-alumina catalyst is used upstream of an alumina-silica or zeolite-based hydrocracking catalyst, in the same reactor in separate catalytic beds or in separate reactors, the conversion is generally (or preferably) less than 50% by weight and preferably less than 40%.

The hydrocracking catalyst may be used upstream or downstream of the zeolite catalyst. Downstream of the zeolite catalyst, it makes it possible to crack the PNA.

One-step process in bubbling bed

The hydrocracking catalyst may be used alone in one or more reactors. In the context of such a process, several reactors in series can advantageously be used, the bubbling-bed reactor (s) containing the hydrocracking catalyst being preceded by one or more reactors containing at least one hydrorefining catalyst. in fixed bed or bubbling bed.

When the silica-alumina catalyst is used downstream of a hydrorefining catalyst, the conversion of the fraction of the feed caused by this hydrorefining catalyst is generally (or preferably) less than 30% by weight and preferred way less than 25%.

One-step process in fixed bed with intermediate separation

The silica-alumina-based catalyst may also be used in a one-step hydrocracking process comprising a hydrorefining zone, a zone allowing the partial elimination of the ammonia, for example by a hot flash, and a zone comprising a hydrocracking catalyst. This process for the hydrocracking of hydrocarbon feeds in one step for the production of middle distillates and optionally of oil bases comprises at least a first reaction zone including hydrorefining, and at least a second reaction zone, in which hydrocracking is carried out. at least a part of the effluent of the first reaction zone. This process also involves an incomplete separation of the ammonia from the effluent leaving the first zone. This separation is advantageously carried out by means of an intermediate hot flash. The hydrocracking performed in the second reaction zone is carried out in the presence of ammonia in an amount less than the amount present in the feed, preferably less than 1500 ppm by weight, more preferably less than 1000 ppm. weight and even more preferably less than 800 ppm nitrogen weight. The hydrocracking catalyst is preferably used in the hydrocracking reaction zone in association or not with a hydrorefining catalyst located upstream of the hydrocracking catalyst. The hydrocracking catalyst may be used upstream or downstream of the zeolite catalyst. Downstream of the zeolite catalyst, it makes it possible in particular to convert the PNAs or the PNA precursors.

The hydrocracking catalyst can be used either in the first reaction zone in pretreatment converting, alone or in combination with a conventional hydrorefining catalyst, upstream of the hydrocracking catalyst, in one or more catalytic beds, in one or more reactors.

One-step hydrocracking process with preliminary hydrorefining on low acid catalyst.

• une première zone réactionnelle d'hydroraffinage dans laquelle la charge est mise en contact avec au moins un catalyseur d'hydroraffinage présentant dans le test standard d'activité un taux de conversion du cyclohexane inférieur à 10 % massique et
• une deuxième zone réactionnelle d'hydrocraquage dans laquelle une partie au moins de l'effluent issu de l'étape d'hydroraffinage est mise en contact avec au moins un catalyseur d'hydrocraquage zéolithique présentant dans le test standard d'activité un taux de conversion du cyclohexane supérieur à 10 % massique, le catalyseur d'hydrocraquage étant présent dans au moins une des deux zones réactionnelles.

The hydrocracking catalyst may be used in a hydrocracking process comprising:

• a first hydrorefining reaction zone in which the feedstock is brought into contact with at least one hydrotreatment catalyst having in the standard activity test a cyclohexane conversion rate of less than 10% by weight and
• a second hydrocracking reaction zone in which at least a portion of the effluent from the hydrorefining stage is brought into contact with at least one zeolitic hydrocracking catalyst having in the standard activity test a conversion rate cyclohexane greater than 10% by weight, the hydrocracking catalyst being present in at least one of the two reaction zones.

The proportion of the catalytic volume of hydrorefining catalyst generally represents 20 to 45% of the total catalytic volume.

The effluent from the first reaction zone is at least partly, preferably completely, introduced into the second reaction zone of said process. An intermediate separation of the gases can be carried out as previously described.

The effluent leaving the second reaction zone is subjected to a so-called final separation (for example by atmospheric distillation optionally followed by vacuum distillation), so as to separate the gases. At least one residual liquid fraction is obtained, essentially containing products whose boiling point is generally greater than 340 ° C., which may be at least partly recycled upstream of the second zone. reaction of the hydrocracking process, and preferably upstream of the hydrocracking catalyst based on alumina-silica, with a view to producing middle distillates.

The conversion to products having boiling points below 340 ° C, or even lower than 370 ° C is at least 50% by weight.

Two-step process

The two-stage hydrocracking comprises a first step whose objective, as in the "one-step" process, is to perform the hydrorefining of the feedstock, but also to achieve a conversion of the latter of the order in general. from 40 to 60%. The effluent from the first step then undergoes separation (distillation), which is often called intermediate separation, which aims to separate the conversion products from the unconverted fraction. In the second step of a two-stage hydrocracking process, only the fraction of the unconverted feedstock in the first step is processed. This separation allows a two-stage hydrocracking process to be more selective in middle distillate (kerosene + diesel) than a one-step process. Indeed, the intermediate separation of the conversion products avoids their "over-cracking" in naphtha and gas in the second step on the hydrocracking catalyst. Furthermore, it should be noted that the unconverted fraction of the feedstock treated in the second stage generally contains very low levels of NH ₃ as well as organic nitrogen compounds, generally less than 20 ppm by weight or less than 10 ppm. weight.

The same configuration of fixed bed or bubbling bed catalytic beds can be used in the first step of a so-called two-stage scheme, whether the catalyst is used alone or in combination with a conventional hydrorefining catalyst. The hydrocracking catalyst may be used upstream or downstream of the zeolite catalyst. Downstream of the zeolite catalyst, it makes it possible in particular to convert the PNAs or the PNA precursors.

For the so-called one-step processes and for the first step of the two-step hydrocracking processes, the preferred hydrocracking catalysts are doped catalysts based on non-noble group VIII elements, even more preferably nickel and tungsten base and the preferred doping element being phosphorus.

The catalysts used in the second stage of the two-stage hydrocracking processes are preferably the noble group-based doped catalysts, more preferably the platinum and / or palladium catalysts and the preferred doping element being phosphorus.

Step 2: separation of the different cuts in a distillation tower

This step consists in separating the effluent from the hydrocracking reactor into different petroleum fractions. After separating the liquid and gaseous streams through high and medium pressure separators, the liquid effluent is injected into an atmospheric distillation column in order to separate and stabilize the sections at desired distillation intervals.

The unconverted fraction which is desired to be treated in the present invention is then obtained in the bottom of an atmospheric distillation column, more specifically in the withdrawal of the reboiler, and corresponds according to the present invention to a fraction of cutting point T05 greater than 340 °. vs.

Given their normal boiling temperature; much greater than 340 ° C, the polyaromatic compounds that the present invention proposes to eliminate are all concentrated in this heavy fraction from the bottom of the column distillation tower (heavy residue).

In the case of the one-step hydrocracking process and one intermediate separation step, the unconverted portion (having a boiling point greater than 340 ° C) is generally at least partially recycled and reinjected either at the inlet hydrorefining catalyst, either at the inlet of the hydrocracking catalyst (preferably).

In the case of the so-called two-stage hydrocracking process, the unconverted part (having a boiling point greater than 340 ° C.) is generally at least partially recycled and reinjected into the second hydrocracking reaction zone.

Step 3: adsorption of the PNA contained in the heavy residue by passing part or all of it in the adsorption zone

This step consists in eliminating all or part of the polyaromatic compounds contained in all or part of the recycled fraction from the bottom of the distillation tower column (fraction 380+ or heavy residue), that is to say The objective is to maintain the content of polyaromatic compounds below a certain critical concentration beyond which there would be a deactivation of the hydrocracking catalyst (deactivation due to accumulation of PNA in the porous network of the hydrocracking catalyst can cause poisoning of the active sites and / or blocking access to these same sites) and a deposit on the cold parts of the process. It is therefore a question of controlling the concentration of PNA in the fraction recycled to the hydrocracking catalyst. Depending on the case, it is therefore possible to limit the load volumes to be treated and thus to minimize the cost of the overall process. Preliminary studies having shown that the molecules most harmful to the hydrocracking catalyst are the compounds having at least 7 fused nuclei (from the coronene), it is a question of controlling mainly the concentration of coronene, which should not exceed that of the fraction recycled in the processes where purging is carried out, that is to say 40 ppm. This concentration limits the deactivation of the catalyst to 2 ° C / month.

The unconverted feedstock derived from the hydrocracker is placed at least in part in contact with a solid adsorbent which is generally capable of selectively and significantly retaining the PNAs with a selectivity greater than 1 and preferably between 2 and 5 for the coronene compared to other, less heavy PNAs such as pyrene (4 aromatic rings) or perylene (5 aromatic rings).

Characteristics of the solid adsorbent that can be used in the process according to the invention

• un pourcentage de silice compris entre 5 et 95 % poids, de préférence entre 10 et 80 %, de manière plus préférée entre 20 et 60 % et de manière très préférée entre 30 et 50 %,
• un contenu en sodium inférieur à 0,03 % poids,
• un volume poreux total mesuré par porosimétrie au mercure compris entre 0,45 et 1,2 ml/g,
• une porosité telle que :
  1. i) Le volume des mésopores dont le diamètre est compris entre 40 Å et 150 Å, et dont le diamètre moyen varie entre 80 et 140 Å (de préférence entre 80 et 120 Å) représente entre 40-70 % du volume poreux total précédemment défini.
  2. ii) Le volume des macropores, dont le diamètre est supérieur à 500 Å, et de préférence compris entre 1000 Å et 10000 Å représente entre 30-60 % du volume poreux total et de manière préférée le volume des macropores représente au moins 35 % du volume poreux total.
• une surface spécifique BET comprise entre 200 et 550 m²/g, de préférence comprise entre 200 et 500 m²/g, de manière préférée inférieure à 350 m²/g et de manière encore plus préférée comprise entre 200 et 350 m2/g,
• un diagramme de diffraction X qui contient au moins les raies principales caractéristiques d'au moins une des alumines de transition comprises dans le groupe composé par les alumines rhô, khi, kappa, êta, gamma, thêta et delta, de manière préférée qui contient au moins les raies principales caractéristiques d'au moins une des alumines de transition compris dans le groupe composé par l'alumine gamma, êta, thêta et delta, et de manière plus préférée qui contient au moins les raies principales caractéristiques de l'alumine gamma et êta, et de manière encore plus préférée qui contient les pics à un d compris entre 1,39 à 1,40 Å et à un d compris entre 1,97 Å à 2,00 Å.

The adsorbent is based on alumina-silica, said alumina-silica having the following characteristics:

• a percentage of silica of between 5 and 95% by weight, preferably between 10 and 80%, more preferably between 20 and 60% and very preferably between 30 and 50%,
• a sodium content of less than 0.03% by weight,
• a total pore volume measured by mercury porosimetry of between 0.45 and 1.2 ml / g,
• a porosity such as:
  1. (i) The volume of mesopores whose diameter is between 40 Å and 150 Å, and whose average diameter varies between 80 and 140 Å (preferably between 80 and 120 Å) represents between 40-70% of the total pore volume previously defined .
  2. ii) The volume of the macropores, whose diameter is greater than 500 Å, and preferably between 1000 Å and 10000 Å, represents between 30-60% of the total pore volume and, preferably, the volume of the macropores represents at least 35% of the total pore volume.
• a BET specific surface area of between 200 and 550 m ² / g, preferably between 200 and 500 m ² / g, preferably less than 350 m ² / g and even more preferably between 200 and 350 m 2 / g ,
• an X-ray diffraction pattern which contains at least the main characteristic lines of at least one of the transition aluminas included in the group consisting of rho, khi, kappa, eta, gamma, theta and delta aluminas, preferred manner which contains at least the main characteristic lines of at least one of the transition aluminas included in the group consisting of gamma, eta, theta and delta alumina, and more preferably which contains at least the main characteristic lines of gamma and eta alumina, and even more preferably which contains the peaks at a d between 1.39 to 1.40 Å and a d ranging from 1.97 Å to 2.00 Å.

Preferably, the alumina-silica comprises from 30 to 50% of Q ² sites, in which one Si atom is bonded to two Si or Al atoms and to two OH groups and also comprises 10-30% of Q sites. ³ in which one Si atom is bonded to three atoms of Si or Al or to an OH group.

• de préférence une teneur en impuretés cationiques inférieure à 0,1 % poids, de manière préférée inférieure à 0,05 % poids et de manière encore plus préférée inférieure à 0,025 % poids. On entend par teneur en impuretés cationiques la teneur totale en alcalins.
• de préférence une teneur en impuretés anioniques inférieure à 1 % poids, de manière préférée inférieure à 0,5 % poids et de manière encore plus préférée inférieure à 0,1 % poids.
• éventuellement au moins un élément hydro-déshydrogénant choisi dans le groupe formé par les éléments du groupe VIB et du groupe VIII de la classification périodique, avec de préférence une teneur massique en métal(aux) du groupe VIB, sous forme métallique ou sous forme oxyde comprise entre 1 et 50 % en poids, de manière préférée entre 1,5 et 35 %, et de manière encore plus préférée entre 1,5 et 30 %, et de préférence une teneur massique en métaux du groupe VIII, sous forme métallique ou sous forme oxyde comprise entre 0,1 et 30 % en poids, de manière préférée entre 0,2 et 25 % et de manière encore plus préférée entre 0,2 et 20 %,
• éventuellement de 0,01 à 6 % de phosphore comme élément dopant déposé sur l'adsorbant (on entend par élément dopant un élément introduit après la préparation de l'adsorbant alumino-silicate décrit précédemment), éventuellement en combinaison avec le bore et/ou le silicium. Ainsi on pourra utiliser comme éléments dopants une combinaison phosphore et bore ou une combinaison phosphore, bore et silicium. Lorsque les éléments bore et/ou silicium sont présents sur l'adsorbant, les teneurs massiques en bore et silicium, calculées sous leur forme oxyde, sont comprises entre 0,01 et 6 %, de préférence entre 0,1 et 4 %, de manière plus préférée entre 0,2 et 2,5 %.
• éventuellement au moins un élément du groupe VIIB (manganèse par exemple et de préférence), et une teneur pondérale comprise entre 0 et 20 %, de préférence entre 0 et 10 % du composé sous forme oxyde ou métal,
• éventuellement au moins un élément du groupe VB (niobium par exemple et de préférence), et une teneur pondérale comprise entre 0 et 40 %, de préférence entre 0 et 20 % du composé sous forme oxyde ou métal.

The adsorbent that can be used in the process according to the invention also comprises:

• preferably a content of cationic impurities of less than 0.1% by weight, preferably less than 0.05% by weight and even more preferably less than 0.025% by weight. The content of cationic impurities means the total content of alkali.
• preferably an anionic impurities content of less than 1% by weight, preferably less than 0.5% by weight and even more preferably less than 0.1% by weight.
• optionally at least one hydro-dehydrogenating element selected from the group consisting of Group VIB and Group VIII elements of the Periodic Table, preferably with a mass content of Group VIB metal (s), in metallic form or in oxide form between 1 and 50% by weight, preferably between 1.5 and 35%, and even more preferably between 1.5 and 30%, and preferably a mass content of metals of group VIII, in metallic form or in oxide form of between 0.1 and 30% by weight, preferably between 0.2 and 25% and even more preferably between 0.2 and 20%,
• optionally from 0.01 to 6% of phosphorus as doping element deposited on the adsorbent (the term "doping element" is understood to mean an element introduced after the preparation of the aluminosilicate adsorbent described above), optionally in combination with boron and / or silicon. Thus, it will be possible to use as doping elements a combination of phosphorus and boron or a combination of phosphorus, boron and silicon. When the boron and / or silicon elements are present on the adsorbent, the boron and silicon mass contents, calculated in their oxide form, are between 0.01 and 6%, preferably between 0.1 and 4%, more preferably between 0.2 and 2.5%.
• optionally at least one element of the group VIIB (manganese for example and preferably), and a weight content of between 0 and 20%, preferably between 0 and 10% of the compound in oxide or metal form,
• optionally at least one member of the group VB (niobium for example and preferably), and a weight content of between 0 and 40%, preferably between 0 and 20% of the compound in oxide or metal form.

According to a preferred embodiment of the invention, the adsorbent consists of alumina-silica alone.

According to another embodiment of the invention, the adsorbent comprises from 1 to 40% by weight of binder. The adsorbent can then result from the mixture of alumina-silica and at least one binder chosen from the group formed by silica, alumina, clays, titanium oxide, boron oxide and zirconia.

In the adsorbent, the proportion of the octahedral Al _VIs determined by the MAS NMR spectral analysis of the solid Al ²⁷ is generally greater than 50%.

The adsorbent may also contain a minor proportion of at least one promoter element selected from the group consisting of zirconia and titanium.

Preferably, prior to use, the adsorbent is subjected to hydrothermal treatment after synthesis, as described later.

Preferably, before use, the adsorbent is subjected to a sulfurization treatment, according to any technique known to those skilled in the art.

The adsorbent according to the invention may contain a zeolite (preferably it does not contain zeolite). The total weight content of zeolite in the adsorbent is generally between 0% and 30%, advantageously between 0.2% and 25%, preferably between 0.3% and 20%, very preferably between 0.5%. and 20% and even more preferably between 1% and 10%.

Depending on the zeolite content introduced, the X-ray diffraction pattern of the adsorbent also generally contains the main characteristic lines of the selected zeolite or zeolites.

The characterization techniques and characteristics of the silica-alumina base of the adsorbent used in the PNA removal process according to the invention are described in the French patent application entitled: "Alumino-silicate doped catalyst and improved process for treating hydrocarbon feedstocks ", filed by the applicant on 22 September 2004, under the number of EN 04 / 09.997.

For practical reasons, the adsorbent may be identical to the catalyst used in the hydrocracking zone.

For practical reasons, the adsorbent may be a regenerated hydrorefining or hydrocracking catalyst.

Characteristics of the adsorption process

The adsorption zone may be designed in various ways: it may consist of one or more fixed beds of adsorbents positioned in series or in parallel.

The choice of two beds in parallel is however the most judicious because it allows a continuous operation. When the first bed is saturated, simply switch to the second to continue adsorption while simultaneously regenerating or replacing the first bed.

It is also conceivable to operate this zone discontinuously, that is to say, to trigger its start-up only when the PNA concentration exceeds the critical concentration set. This makes it possible to minimize the processed charge volumes and therefore the operating costs.

For a good efficiency of the adsorption zone, the operating conditions are generally a temperature of between 50 and 250 ° C., preferably of between 100 and 150 ° C., a pressure of between 1 and 200 bar (according to one embodiment of the invention). preferred, the pressure is between 1 and 10 bar and according to another preferred embodiment, the pressure is between 30 and 200 bars) and a VVH between 0.01 and 500 h ^-1 , preferably between 0.1 and 300, terminals included.

The choice of temperature and pressure is made in order to ensure a good flow of the load (it must be liquid and viscosity not too high) and a good diffusion of the PNA in the porosity of the adsorbent while optimizing the adsorption phenomenon.

The contents of polyaromatic compounds in the feedstock to be recycled are generally between 0 and 500 ppm for coronene, from 0 and 5000 ppm for perylene and for pyrene. At the outlet of the adsorption zone, the contents generally become less than 40, 1000, 1500 ppm, respectively. The determination of the molecules is carried out by liquid chromatography combined with detection by UV absorption.

Step 4: regeneration of the adsorbent of the adsorption zone by burning

This step aims to eliminate the PNA previously adsorbed on the solid of the adsorption zone (step 3) so as to make it reusable for a new adsorption step. Regeneration of the adsorbent by burning is carried out under an N ₂ -based gas stream containing from 0.1 to 21% of O ₂ , preferably from 3 to 6%, at a temperature of between 400 and 650 ° C. preferably between 500 and 550 ° C. This operation can be performed ex situ or in situ.

• On commence par un stripage à chaud avec un gaz inerte tel que l'azote à une température de l'ordre de 200-300°C. Ceci peut être réalisé aussi bien à co-courant qu'à contre-courant. Le but est d'éliminer les hydrocarbures piégés dans les porosités de grains et de lits de l'adsorbant et les éventuelles traces d'hydrogène.
• Brûlage en présence d'air ajouté à l'azote dans une proportion de l'ordre de 5 %, ce mélange est envoyé à co-courant ou à contre-courant sur l'adsorbant. On commence par réaliser cette opération à une température de l'ordre de 400°C pour éliminer les hydrocarbures susceptibles d'être présents dans la porosité de l'adsorbant (réaction exothermique).
• Cette opération est renouvelée à environ 450°C pour s'assurer que toute trace d'hydrocarbures a disparu.
• Quand le système redevient de nouveau athermique, on élève la température à une température comprise entre 500 et 550°C et on la maintient pendant environ 12 heures pour brûler les PNA adsorbés à la surface du solide poreux.

In a preferred manner:

• It starts with a hot stripping with an inert gas such as nitrogen at a temperature of about 200-300 ° C. This can be done both co-current and counter-current. The aim is to eliminate the hydrocarbons trapped in the pores of grains and beds of the adsorbent and any traces of hydrogen.
• Burning in the presence of air added to the nitrogen in a proportion of the order of 5%, this mixture is sent co-current or against the current on the adsorbent. It begins with this operation at a temperature of about 400 ° C to remove hydrocarbons that may be present in the porosity of the adsorbent (exothermic reaction).
• This operation is renewed at approximately 450 ° C to ensure that all trace of hydrocarbons has disappeared.
• When the system becomes athermic again, the temperature is raised to 500-550 ° C and held for about 12 hours to burn adsorbed PNA on the surface of the porous solid.

The mesoporous silica-alumina can undergo these treatments about twenty times before it becomes necessary to renew it.

Description of Figure 1

The invention is described in its embodiment in a step with recycling at the inlet of the first reactor in a nonlimiting manner according to the figure 1 . The charge consisting of saturated compounds, resins and aromatic molecules (mono-, di-, triaromatic and PNA) arriving via a line (1) and a hydrogen flow brought by a line (2) are mixed and introduced into the hydrocracking reactor (4) by a line (3). The output charge of the hydrocracker is conducted via a line (5) to a high pressure distiller (6) whose function is to separate the gaseous and liquid products. The gas corresponds to unreacted hydrogen and is re-injected into the hydrocracking reactor inlet via the lines (8) and (3). The liquid products are conveyed via the line (7) to a fractionation zone (9) where, thanks to the differences in boiling point, the cracked products (lighter compounds) are separated, which are thus recovered at the top of the column by line (10), of those which have not been transformed (residues 380+). These are the bottom of the column and come out through the line (11). Part of this fraction is optionally removed via line (12). The other part is sent into the recycling loop by the line (13). Then, according to the fixed PNA concentration criticality parameters, some or all of the charge is sent to an adsorption zone (17) or (18) via lines (14) and (15) or (16). . At the outlet of this zone, an effluent whose concentration of PNA is low or zero is recovered by the lines (19) or (20) and (21). It is then sent in the line (22), which is the one carrying the part of the untreated charge by adsorption. The mixture of these two fractions is transported by the line (23) to the line containing the fresh charge, that is to say the line (1).

examples Example 1 Preparation of silica-alumina SA 1

The adsorbent SA1 is obtained in the following manner.

The SA1 adsorbent is an alumina-silica which has a chemical composition by weight of 60% of Al ₂ O ₃ and 40% of SiO ₂ . Its Si / Al ratio is 0.6. Its sodium content is of the order of 100-120 ppm by weight. The extrusions are cylindrical with a diameter of 1.6 mm. Its specific surface is 345 m ² / g. Its total pore volume, measured by mercury porosimetry is 0.83 cm ³ / g. The porous distribution is bimodal. In the mesopore domain, a broad peak between 4 and 15 nm with a maximum at 7 nm is observed. For support, macropores, whose diameter is greater than 50 nm, represent about 40% of the total pore volume.

Example 2 Comparison of the Removal of PNAs from an Adsorption Load on Porous Solids

The feed used corresponds to the residues of the bottom of a fractionation column. Its pour point is of the order of 36 ° C and its density at 15 ° C of 0.8357. It contains 95% by weight of saturated compounds (83.6% by weight of paraffinic compounds and 11.4% by weight of naphthenic compounds), 0.5% by weight of resins and 2.9% by weight of aromatic compounds, of which 2.6% by weight monoaromatic compounds, 0.56% by weight of diaromatic compounds, 0.57% by weight of triaromatic compounds, 2704 ppm of pyrene (4 rings), 1215 ppm of perylene (5 rings) and 59 ppm of coronene (7 rings). '

The porous solids tested correspond to a mesoporous solid of the MCM-41 purely silicic type, a SiO ₂ bridged beidellite type clay, a silica gel, an activated alumina, a physically activated carbon derived from a cellulosic precursor and a silica-alumina. according to the invention. They were chosen for their large surface area and their large diameter pores between 20 and 80 Å as appropriate (Table 1) combined with their regenerability by burning. Table 1: BET surface and average pore diameters of different solids. mesoporous Brushed clay Silica gel Activated alumina Charcoal Silica-alumina 1 (SA1) S _BET (m ² / g) 360 403 550 352 1442 345 Φ _pores (Å) 56 26.5 20 50 25 75 + macropores

The filler is contacted with different fixed bed adsorbents with a VVH of 30 at a temperature of 150 ° C and a pressure of 10 bar.

Quand elle est supérieure à 1, cela signifie que l'adsorbant adsorbe davantage le composé i par rapport au composé j. Dans notre cas, comme on calcule les sélectivités du coronène par rapport à des PNA plus légers, ces valeurs doivent être supérieures à 1 car l'objectif principal est d'éliminer préférentiellement les molécules les plus lourdes. On détermine également les volumes de charge par volume d'adsorbant qu'il est possible au maximum de traiter pour que la concentration en coronène dans la charge en sortie ne dépasse pas 2/3 de celle en entrée. Ce rapport permet d'estimer la capacité d'adsorption des solides. L'ensemble de ces résultats est indiqué dans le Tableau 2. Tableau 2 : Sélectivités et volume de charge traitable par volume d'adsorbant pour les différents solides. Mésoporeux Argile pontée Gel de silice Alumine activée Charbon actif Silice-alumine 1 (SA1) α_{coronène/pérylène} 5,5 3,1 1,4 1,5 4,8 5,5 α_{coronène/pyrène} 6,2 6 2,1 2,0 7,6 7,3 V_charge/V_adsorbant (ml/ml) 4 8 6,5 12 38 20 For each of them, the adsorption selectivities of coronene relative to perylene and pyrene are calculated. The selectivity of an adsorbent for two molecules i and j is defined such that: ${\alpha }_{i/j}=\frac{\raisebox{1ex}{$q_{\mathrm{at}ds,i}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{VS}}_i$}\right.}{\raisebox{1ex}{$q_{\mathrm{at}ds,j}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{VS}}_j$}\right.}.$

When it is greater than 1, it means that the adsorbent further adsorbs compound i over compound j. In our case, since the selectivities of coronene are calculated with respect to lighter PNAs, these values must be greater than 1 because the main objective is to preferentially eliminate the heavier molecules. The volume of charge per volume of adsorbent that is maximally treatable is also determined so that the coronene concentration in the output charge does not exceed 2/3 of that at the inlet. This report makes it possible to estimate the adsorption capacity of the solids. All of these results are shown in Table 2. Table 2: Selectivities and volume of treatable load per volume of adsorbent for different solids. mesoporous Brushed clay Silica gel Activated alumina Charcoal Silica-alumina 1 (SA1) α _{coronene / perylene} 5.5 3.1 1.4 1.5 4.8 5.5 α _{coronene / pyrene} 6.2 6 2.1 2.0 7.6 7.3 V _load / V _adsorbent (ml / ml) 4 8 6.5 12 38 20

We note that the best performances are, as expected, those of activated carbon. However, the solid claimed in this patent also has good selectivities and adsorption capabilities. Since it is regenerable several times in a row by burning, its exploitation is more profitable than that of activated carbons.

Example 3: Regeneration by burning of the adsorbent

Regeneration of the adsorbent is carried out by burning with a flow of N ₂ containing 5% O ₂ at 550 ° C. As a result of these operations, 97% of the capacities of the starting solid are recovered.

This operation can be performed about ten times before losing 30% of capacity.

Claims (20)

1. An improved hydrocracking process with a recycle, having a step for eliminating polyaromatic compounds from at least a portion of the recycled portion by adsorption on an adsorbent based on alumina-silica with a mass content of silica (SiO₂) of more than 5% by weight and 95% or less; said alumina-silica having:

   • a sodium content of less than 0.03% by weight;

   • a total pore volume, measured by mercury porosimetry, in the range 0.45 to 1.2 ml/g;

   • a porosity such that:

   i) the volume of mesopores with a diameter in the range 40 Å to 150 Å and a mean pore diameter in the range 80 Å to 140 Å represents 40-70% of the total pore volume measured by mercury porosimetry;

   ii) the volume of macropores with a diameter of more than 500 Å represents 30-60% of the total pore volume measured by mercury porosimetry;

   • a BET specific surface area in the range 200 to 550 m²/g; and

   • an X ray diffraction diagram which contains at least the principal characteristic peaks of at least one of the transition aluminas included in the group composed of alpha, rho, khi, eta, gamma, kappa, theta and delta aluminas.
2. A process according to claim 1, which comprises in succession:

   • a hydrocracking step;

   • a separation step, to separate an unconverted fraction with a T05 cut point of more than 340°C; and

   • a step for liquid phase adsorption of all or part of the PNAs contained in said unconverted fraction from the separation step.
3. A process according to claim 1 or claim 2, in which the hydrocracking step is carried out using a once-through mode, the hydrocracking step comprising firstly, deep hydrorefining before sending it to the more active hydrocracking catalyst, no separation is carried out between the two types of catalyst.
4. A process according to claim 1 or claim 2, in which the hydrocracking step is carried out using a two-step mode, the hydrocracking step comprising a first hydrorefining step, an intermediate separation and a second hydrocracking step, in which only the fraction of feed that is not converted in the first step is treated
5. A process according to one of the preceding claims, in which the adsorbent undergoes a burning regeneration treatment after the adsorption step.
6. A process according to claim 5, in which bum regeneration of the adsorbent is carried out in a stream of gas based on N2 containing 0.1 % to 21 % of 02, at a temperature in the range 400°C to 650°C.
7. A process according to claim 6, in which bum regeneration of the adsorbent is carried out in a stream of gas based on N2 containing 3% to 6% of 02, at a temperature in the range 500°C to 550°C.
8. A process according to one of the preceding claims, in which adsorption is carried out continuously.
9. A process according to one of claims 1 to 7, in which adsorption is carried out batchwise.
10. A process according to one of the preceding claims, in which the adsorption step is carried out on the whole of the recycled fraction.
11. A process according to one of the preceding claims, in which the adsorption step is carried out at a temperature in the range 50°C to 250°C, a pressure in the range 1 to 200 bars and a HSV in the range 0.01 to 500 h^-1.
12. A process according to one of the preceding claims, in which the adsorbent comprises a proportion of octahedral Al_VI determined by solid ²⁷A1 MAS NMR spectral analysis, of more than 50%.
13. A process according to one of the preceding claims, in which the alumina-silica comprises 30% to 50% of Q² sites in which one Si atom is bonded to two Si or Al atoms and to two OH groups and also comprises 10-30% of Q³sites in which one atom of Si is bonded to three atoms of Si or Al or to an OH group.
14. A process according to one of the preceding claims, in which the adsorbent is constituted by alumina-silica.
15. A process according to one of claims 1 to 12, in which the adsorbent comprises 1% to 40% by weight of binder.
16. A process according to claim 14, in which the adsorbent results from mixing alumina-silica and at least one binder selected from the group formed by silica, alumina, clays, titanium oxide, boron oxide and zirconia.
17. A process according to one of the preceding claims, in which the adsorbent comprises a cationic impurities content of less than 0.1% by weight, the cationic impurity content being the total content in alkaline.
18. A process according to one of the preceding claims, in which the adsorbent undergoes a hydrothermal treatment before use.
19. A process according to one of the preceding claims, in which the adsorbent undergoes a sulphurization treatment before use.
20. A process according to one of the preceding claims, in which the adsorbent is identical to the hydrocracking catalyst.

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