EP3275975B1 - Hydrotreatment method using switchable guard reactors with reversal of the flow direction and parallel arrangement of the reactors - Google Patents

Description

Technical area

The invention relates to fixed bed processes, and more generally to the hydrotreatment processes of heavy hydrocarbon feedstocks, in particular residues or any hydrogenation of petroleum fractions that can cause clogging of the fixed bed.

Prior art

One of the problems with fixed bed heavy-lift catalytic hydrotreatment is clogging, especially in the upper part of the bed. The catalytic beds, more particularly the upper parts of the first catalytic bed in contact with the charge, are likely to clog rather quickly because of the asphaltenes and sediments contained in the charge, which is expressed initially by an increase in the pressure drop (DP) and sooner or later requires a stop of the unit for the replacement of the catalyst.

It is therefore necessary to stop the unit to replace the first deactivated and / or clogged catalytic beds. The hydrotreatment processes for this type of load must therefore be designed so as to allow the longest operating cycle possible without stopping the unit.

Those skilled in the art have sought to overcome these drawbacks fixed bed arrangements in different ways, including using guard beds arranged upstream of the main reactors. The main task of the guard beds is to protect the catalysts of the main hydrotreating reactors downstream by performing part of the demetallation and filtering the particles contained in the charge which can lead to clogging. The guard beds contain at least hydrodemetallization catalyst, optionally supplemented with inert elements and / or pre-beds (grading in English) and are generally integrated in the hydrodemetallation section HDM in a hydrotreating process of charges heavy ones generally including a first hydrodemetallation section, then a second hydrotreating section HDT. Although the guard beds are generally used to carry out a first hydrodemetallation and filtration, other hydrotreatment reactions (HDS hydrodesulfurization, hydrodenitrogenation HDN, etc.) will inevitably take place in these reactors thanks to the presence of hydrogen and water. a catalyst.

The technology called PRS (Permutable Reactors System) is described in the patents FR2681871 and US5417846 of the Applicant. The patent FR2681871 describes a process for the hydrotreatment in at least two stages of a heavy hydrocarbon fraction containing asphaltenes, sulfur impurities and metallic impurities in which, during the first hydrodemetallization stage, the reaction is carried out under conditions of hydrodemetallization, the charge of hydrocarbons and of hydrogen on a hydrodemetallization catalyst, and then during the second subsequent step, the effluent of the first step is passed under hydrodesulphurization conditions over a catalyst of hydrodesulfurization characterized in that the hydrodemetallation step comprises one or more hydrodemetallation zones in fixed beds preceded by at least two hydrodemetallation guard zones also in fixed beds, arranged in series for cyclic use consisting of the successive repetition of steps b) and c) defined hereinafter and in that it comprises the following steps: a) a step, in which the guard zones are used together for a duration at most equal to the deactivation and / or clogging time of one of b) a step, during which the deactivated and / or clogged guard zone is short-circuited and the catalyst contained therein is regenerated and / or replaced by fresh catalyst, and c) a step, during which the guard zones are used together the guard zone whose catalyst has been regenerated during the previous step being reconnected and said step being continued for a time at most equal to the time s of deactivation

This technology allows an extension of the life of the catalysts present in the downstream reactors by a permutation and a change of catalyst without stopping the process completely. This principle has been improved by patents:
US2014001089 , which describes a process for hydrotreating heavy hydrocarbon feedstocks with permutable reactors including at least one step of progressive permutation.

And US2014027351 which describes a process for hydrotreating heavy hydrocarbons with reactive reactors including at least one step of short-circuiting a catalytic bed.

The patent FR2992971 discloses a process for hydrotreatment of heavy charges in a catalytic fixed bed with reversal of the flow direction of the fluids using at least one fixed-bed reactor co-flow descending a gas phase and a liquid phase in which during of the operating cycle of said method, the flow direction of the fluids is inverted to become ascending, the pressure drop threshold triggering the flow reversal is defined from a value DP1 between DP1 and DP1 + (0.5xDP2 ), DP1 being defined as the pressure drop at the beginning of the cycle and DP2 as the pressure loss at the end of the cycle equal to the maximum allowable pressure drop by the process equipment.

The patent FR2970261 discloses a process for hydrotreating a heavy hydrocarbon fraction using a system of stationary fixed bed guard zones including at least one step in which the guard zones operate in parallel.

These systems remain driven by an increase in the overall pressure drop of the bed. When the pressure loss increases too much, it is necessary to unload and recharge the reactor. There is a real loss of catalytic activity because the discharged catalyst is not completely deactivated.

Object of the invention

The invention is an improvement of existing technologies for extending the cycle time of PRS-type reactive reactors, through optimal use of the available fluxes and better use of the catalyst present in the reactors.

For this purpose, a step of inversion of the direction of flow and parallelization of the reactors is inserted each time one of the two reactive reactors starts to clog, so when the pressure drop reaches a value- threshold Dpi defined by those skilled in the art as a function of the maximum allowable pressure drop DPmax in the equipment.

Description of the invention Summary of the invention

1. a) une étape dans laquelle au moins deux réacteurs sont utilisés en série en écoulement descendant jusqu'à l'atteinte d'un seuil de perte de charge Dpi de valeur comprise entre 20 et 70 % de la perte de charge maximale admissible DPmax,
2. b) une étape d'inversion du sens de l'écoulement et de mise en parallèle desdits au moins deux réacteurs jusqu'à l'atteinte de la perte de charge maximale admissible DPmax
3. c) une étape durant laquelle le premier réacteur mis en contact avec la charge à l'étape a) est court-circuité et le catalyseur qu'il contient est régénéré et/ou remplacé par du catalyseur frais, le sens de l'écoulement est modifié et seul le ou les réacteur(s) non encore colmaté(s) fonctionnent en écoulement descendant,
4. d) une étape durant laquelle lesdits au moins deux réacteurs sont utilisés en série, le réacteur dont le catalyseur a été régénéré au cours de l'étape précédente étant reconnecté et ladite étape étant poursuivie jusqu'à l'atteinte d'un seuil de perte de charge Dpi de valeur comprise entre 20 et 70 % de la perte de charge maximale admissible DPmax,
5. e) une étape d'inversion du sens de l'écoulement et de mise en parallèle des deux réacteurs, jusqu'à l'atteinte de la perte de charge maximale admissible DPmax
6. f) une étape durant laquelle le deuxième réacteur mis en contact avec la charge à l'étape a) est court-circuité et le catalyseur qu'il contient est régénéré et/ou remplacé par du catalyseur frais, le sens de l'écoulement est modifié et seul le ou les réacteur(s) non encore colmaté(s) fonctionnent en écoulement descendant; lesdites étapes a), b), c), d), e), f) pouvant être répétées de manière cyclique dans cet ordre.

The invention relates to a process for the catalytic hydrotreatment of a heavy hydrocarbon feedstock, in the presence of hydrogen, comprising a step of prior hydrodemetallation using at least two reactors in fixed bed each comprising at least one catalytic bed, which are used cyclically according to the following steps:

1. a) a step in which at least two reactors are used in series in downward flow until reaching a pressure drop threshold Dpi of value between 20 and 70% of the maximum allowable pressure drop DPmax,
2. b) a step of inversion of the direction of flow and paralleling of the at least two reactors until the maximum allowable pressure drop DPmax is reached;
3. c) a step during which the first reactor contacted with the feedstock in step a) is bypassed and the catalyst contained therein is regenerated and / or replaced with fresh catalyst, the direction of flow is modified and only the reactor (s) not yet clogged (s) operate in downward flow,
4. d) a step during which said at least two reactors are used in series, the reactor whose catalyst has been regenerated during the preceding step being reconnected and said step being continued until a loss threshold is reached a load Dpi with a value between 20 and 70% of the maximum allowable pressure drop DPmax,
5. e) a step of inversion of the direction of flow and paralleling of the two reactors until reaching the maximum permissible pressure drop DPmax
6. f) a step during which the second reactor placed in contact with the feedstock in step a) is short-circuited and the catalyst it contains is regenerated and / or replaced by fresh catalyst, the direction of flow is modified and only the reactor (s) not yet clogged (s) operate in downflow; said steps a), b), c), d), e), f) being repeatable cyclically in this order.

In one embodiment, at least one of the reactors comprises n catalytic beds and the effluent is discharged from said reactor in steps b) and / or e) by lateral withdrawal between a catalytic bed i and a catalytic bed i + 1, n being an integer greater than or equal to 2, i being an integer from 1 to n.

Preferably, said catalytic bed i + 1 denotes the first catalytic bed clogged in the direction of flow.

The at least two permutable reactors may each have at least two catalytic beds.

Preferably, the pressure drop threshold Dpi has a value between 30 and 60% of the maximum allowable pressure drop DPmax.

Very preferably, the pressure drop threshold Dpi has a value between 35 and 50% of the maximum allowable pressure drop DPmax.

The heavy hydrocarbon feed advantageously has an initial boiling temperature of at least 340 ° C and a final boiling temperature of at least 440 ° C.

The feedstock may be chosen from atmospheric residues, vacuum residues from direct distillation, crude oils, crude head oils, deasphalting resins, asphalts or deasphalting pitches, residues resulting from conversion processes, extracts aromatic compounds derived from lubricant base production lines, oil sands or derivatives thereof, bituminous shales or their derivatives, parent rock oils or their derivatives, whether alone or in admixture.

Preferably, the filler is chosen from atmospheric residues or residues under vacuum, or mixtures of these residues.

The hydrotreatment process according to the invention is advantageously carried out at a temperature of between 320 ° C. and 430 ° C., under a hydrogen partial pressure of between 3 MPa and 30 MPa, at a space velocity (VVH) between 0.05 and 5 volume of filler per volume of catalyst per hour, and with a gaseous hydrogen ratio on hydrocarbon liquid feed of between 200 and 5000 normal cubic meters per cubic meter.

The preliminary hydrodemetallization step can be carried out in the presence of a hydrodemetallation catalyst comprising at least one Group VIII metal, and / or at least one Group VIB metal, on a support used chosen from the group formed. by alumina, silica, silica-aluminas, magnesia, clays and mixtures of at least two of these minerals, preferably at a space velocity VVH of each switchable reactor operating between 0.5 and 4 charge volume per volume of catalyst and per hour, very preferably at a space velocity VVH of each operating switchable reactor of between 1 and 2 volume of charge per volume of catalyst per hour.

In general, the maximum allowable pressure drop DPmax is between 0.3 and 1 MPa, preferably between 0.5 and 0.8 MPa.

List of Figures

• Figure 1 : PRS permutable reactor device according to the prior art ( FR2681871 )
• Figure 2 : Reactor device according to the invention
• Figure 3 : Reactor device according to an embodiment with short-circuiting of a catalytic bed according to the invention.
• Figure 4 : Evolution of the pressure loss in each PRS permeable reactor according to the sequence of FR2681871
• Figure 5 : Evolution of the pressure drop in each PRS permeable reactor according to the sequence according to the invention

Detailed description of the invention

The present invention proposes an improvement of existing technologies aimed at lengthening the cycle time of PRS reactors.

The proposed improvement is for both new and existing units with some modifications (lines, valves, reactor internals, distributor down flow and up flow, etc.). The present invention consists in reversing the direction of flow during the operating cycle of the PRS permutable reactor system. When a reactor begins to clog, so when the pressure drop begins to increase and reaches a threshold value Dpi defined by the skilled person according to the maximum allowable pressure drop in the reactors (DPmax), the load is injected simultaneously into the two permutable reactors connected in parallel, so as to seal first the upper layer, then the lower layer of the catalytic bed, which results in a more homogeneous clogging, and gives an increase in the pressure drop more slow, thus leading to an increase in the cycle time.

More particularly, the present invention relates to a process for the hydrotreatment of a heavy hydrocarbon fraction generally containing asphaltenes, sediments, sulfur, nitrogen and metal impurities, in which the hydrotreatment conditions are previously passed under the hydrocarbon and hydrogen feedstock on a hydrotreating catalyst, in at least two fixed-bed permutable reactors each containing at least one catalytic bed,

Surprisingly, it has now been found that it is possible to increase the service life of the reactive reactors before replacement of the catalyst contained in the reactive reactors becomes necessary. The present invention thus improves the performance of the reactive reactors as described by the applicant in the patent FR2681871 , integrating, after the steps in which the charge successively passes through all the reactors, additional steps, in which the direction of flow is reversed and the two reactors in parallel. Thus, during these additional steps, the feed is simultaneously introduced on two reactors. This reversal of the direction of flow makes it possible to delay the rise in the pressure drop of the first reactor put in contact with the charge and thus lengthens its service life.

The introduction of a part of the charge on the second reactor while another part of the charge continues to pass through the first reactor placed in contact with the charge also makes it possible to "relieve" the first reactor and also to use optimally all the catalyst present in the reactor.

The main advantage of this sequencing is not only a better use of the catalyst present in each reactor, but also a better distribution of the liquid flows, the two reactors having not necessarily being clogged in the same proportions (due to the prior operation in series the amount of clogger charged in each of the reactors is significantly different). In addition, the possible fluidization problems related to the change of direction are limited because the flow of fluids is divided by two (which justifies all the more to go into parallel flow), which limits the need for recourse to additional grid-type equipment for containing the expansion of the bed due to fluidization.

The loading / unloading of each of these reactors is done on increase of the pressure loss DP only, when one of the reactors is completely clogged, the active phase of the downstream catalyst being used at best. The downstream reactors of the PRS permutable reactors will have an increased lifetime and a better utilization of the catalytic charge.

Characteristics of the invention

The permutable reactors used as guard zones, especially the first guard zone placed in contact with the charge, progressively load into metals, coke, sediments and other various impurities. When the catalyst or catalysts they contain or the inter-grain space (s) of catalyst is / are substantially saturated with metals and various impurities, the reactors must be disconnected to perform the replacement or regeneration of catalyst (s). Preferably, the catalysts are replaced. This moment is called deactivation and / or clogging time. Although the deactivation and / or clogging time varies depending on the load, the operating conditions and the catalyst (s) used, it is generally expressed by a drop in the catalytic performance (an increase in the concentration metals and / or other impurities in the effluent), an increase in the temperature necessary for the maintenance of a constant hydrotreatment or, in the specific case of a clogging, by a significant increase in the pressure drop. The pressure drop DP, expressing a degree of clogging, is measured continuously throughout the cycle on each of the reactors and may be define by a pressure increase resulting from the passage partially blocked through the reactor.

In order to define a deactivation and / or clogging time, those skilled in the art first define a maximum tolerable value of the pressure loss DP max (hereinafter referred to as the maximum allowable pressure drop) depending on the of the charge to be treated, operating conditions and catalysts chosen, and from which it is necessary to proceed with the disconnection of the switchable reactor

The deactivation and / or clogging time is thus defined as the time when the limit value of the pressure drop DPmax is reached. The maximum allowable pressure drop DPmax is generally confirmed when the reactors are commissioned for the first time. In the case of a process for the hydrotreatment of heavy fractions, the limit value of the maximum allowable pressure drop or pressure drop DPmax is generally between 0.3 and 1 MPa (3 and 10 bar), preferably between 0 and , 5 and 8 MPa (5 and 8 bar).

1. a) une étape dans laquelle au moins deux réacteurs sont utilisés en série en écoulement descendant jusqu'à l'atteinte d'un seuil de perte de charge DPi dont la valeur est comprise entre 20 et 70 % de la perte de charge maximale admissible,
2. b) une étape d'inversion du sens de l'écoulement et de mise en parallèle desdits au moins deux réacteurs pendant une durée au plus égale au temps de colmatage du premier réacteur mis en contact avec la charge, jusqu'à l'atteinte de la perte de charge maximale admissible DPmax.
3. c) une étape durant laquelle ledit premier réacteur colmaté est court-circuité et le catalyseur qu'il contient est régénéré et/ou remplacé par du catalyseur frais, le sens de l'écoulement est modifié et seul le ou les réacteur(s) non encore colmaté(s) fonctionnent en écoulement descendant,
4. d) une étape durant laquelle lesdits au moins deux réacteurs sont utilisés en série, le réacteur dont le catalyseur a été régénéré au cours de l'étape précédente étant reconnecté et ladite étape étant poursuivie pendant une durée au plus égale à une fraction du temps de colmatage du deuxième réacteur mis en contact avec la charge, correspondant à l'atteinte d'un seuil de perte de charge DPi dont la valeur est comprise entre 20 et 70 % de la perte de charge maximale admissible DPmax,
5. e) une étape d'inversion du sens de l'écoulement et de mise en parallèle des deux réacteurs, pendant une durée au plus égale au temps de colmatage du deuxième réacteur mis en contact avec la charge jusqu'à l'atteinte de la perte de charge maximale admissible DPmax
6. f) une étape durant laquelle ledit deuxième réacteur colmaté est court-circuité et le catalyseur qu'il contient est régénéré et/ou remplacé par du catalyseur frais, le sens de l'écoulement est modifié et seul le ou les réacteur(s) non encore colmaté(s) fonctionnent en écoulement descendant ; lesdites étapes a), b), c), d), e), f) pouvant être répétées de manière cyclique dans cet ordre.

The process according to the invention is a process for the catalytic hydrotreatment of a heavy hydrocarbon fraction generally containing asphaltenes, sediments, sulfur, nitrogen and metal impurities in the presence of hydrogen, comprising a step of prior hydrodemetallation. by means of a guard zone comprising at least two fixed bed permutable reactors each comprising at least one catalytic fixed bed, which are used cyclically according to the following steps:

1. a) a step in which at least two reactors are used in series in downward flow until a pressure drop threshold DPi is reached whose value is between 20 and 70% of the maximum allowable pressure drop,
2. b) a step of inversion of the direction of flow and paralleling of said at least two reactors for a period of time at most equal to the clogging time of the first reactor placed in contact with the charge, until reaching the maximum allowable pressure drop DPmax.
3. c) a step during which said first clogged reactor is short-circuited and the catalyst contained therein is regenerated and / or replaced with fresh catalyst, the direction of flow is modified and only the reactor (s) not still clogged (s) operate in downward flow,
4. d) a step during which said at least two reactors are used in series, the reactor whose catalyst has been regenerated during the preceding step being reconnected and said step being continued for a duration at most equal to a fraction of the time of clogging of the second reactor in contact with the load, corresponding to the achievement of a pressure drop threshold DPi whose value is between 20 and 70% of the maximum allowable pressure drop DPmax,
5. e) a step of inversion of the direction of flow and paralleling of the two reactors, for a duration at most equal to the clogging time of the second reactor placed in contact with the charge until the loss is reached; maximum allowable load DPmax
6. f) a step during which said second clogged reactor is short-circuited and the catalyst it contains is regenerated and / or replaced by fresh catalyst, the direction of flow is modified and only the reactor (s) not still clogged (s) operate in downward flow; said steps a), b), c), d), e), f) being repeatable cyclically in this order.

During the upflow during steps b) and e) (Up flow), the two reactors are in parallel, the flow is divided. In all other cases (series reactors: steps a) and d) or single reactor in the case of an intervention on the second reactor: steps c) and f)), the direction of flow is in downflow (down flow).

Depending on the initial configuration (charge flow, nature of the charge, etc.), the Up flow may involve the addition of two-phase distribution internals at the bottom of each reactor and the addition of grid type internals at the top of the reactor to prevent possible expansion of the bed.

loads

The feedstock treated in the hydrotreatment process according to the invention is advantageously a hydrocarbonaceous feed having an initial boiling point of at least 340.degree. C. and a final boiling point of at least 440.degree. Preferably, its initial boiling point is at least 350 ° C., preferably at least 375 ° C., and its final boiling point is at least 450 ° C., preferably at least 460 ° C. C, more preferably at least 500 ° C, and even more preferably at least 600 ° C.

The hydrocarbon feedstock according to the invention may be chosen from atmospheric residues, vacuum residues resulting from direct distillation, crude oils, crude head oils, deasphalting resins, asphalts or deasphalting pitches, process residues. conversion products, aromatic extracts from lubricant base production lines, oil sands or derivatives thereof, oil shales or their derivatives, source rock oils or their derivatives, whether alone or in combination. In the present invention, the fillers being treated are preferably atmospheric residues or vacuum residues, or mixtures of these residues.

The hydrocarbon feedstock treated in the process may contain, among other things, sulfur-containing impurities. The sulfur content may be at least 0.1% by weight, preferably at least 0.5% by weight, preferably at least 1% by weight, more preferably at least 2% by weight. .

The hydrocarbon feedstock treated in the process may contain inter alia metal impurities, especially nickel and vanadium. The sum of the nickel and vanadium contents is generally at least 10 ppm, preferably at least 50 ppm, preferably at least 100 ppm.

These charges can advantageously be used as they are. Alternatively, they can be diluted by co-charging. This co-charge may be a hydrocarbon fraction or a lighter hydrocarbon fraction mixture, which may preferably be chosen from the products resulting from a fluid catalytic cracking (FCC) process according to the English terminology. Saxon), a light cut (LCO or "light cycle oil" according to the English terminology), a heavy cut (HCO or "heavy cycle oil" according to the English terminology), a decanted oil, a residue of FCC, a gas oil fraction, especially a fraction obtained by atmospheric distillation or under vacuum, such as vacuum gas oil, or may come from another refining process such as coking or visbreaking.

The co-charge may also advantageously be one or more cuts resulting from the process of liquefying coal or biomass, aromatic extracts, or any other hydrocarbon cuts, or non-petroleum fillers such as pyrolysis oil. The heavy hydrocarbon feedstock according to the invention may represent at least 50%, preferably 70%, more preferably at least 80%, and even more preferably at least 90% by weight of the total hydrocarbon feedstock treated by the process according to the invention.

Operating conditions

The hydrotreatment process according to the invention, and more particularly the preliminary hydrodemetallation step of the process according to the invention, may advantageously be carried out at a temperature of between 320 ° C. and 430 ° C., preferably 350 ° C. C at 410 ° C, under a hydrogen partial pressure advantageously between 3 MPa and 30 MPa, preferably between 10 and 20 MPa, at a space velocity (VVH) advantageously between 0.05 and 5 volume of charge per volume of catalyst and per hour, and with a hydrogen gas ratio on a hydrocarbon liquid feed advantageously between 200 and 5000 normal cubic meters per cubic meter, preferably 500 to 1500 normal cubic meters per cubic meter.

In the preliminary hydrodemetallization step, the value of the VVH of each operating switchable reactor is preferably between 0.5 and 4 volume of catalyst per volume of charge per hour, and most often between 1 and 2 catalyst volume per volume of charge per hour. The overall VVH value of the reactive reactors and that of each reactor is chosen by those skilled in the art so as to achieve the maximum of hydrodemetallation (HDM) while controlling the reaction temperature (limitation of the exothermicity).

The catalysts used in the hydrotreatment process according to the invention, and in particular in the permutable guard reactors, are preferably known hydrotreatment catalysts and are generally granular catalysts comprising, on a support, at least one metal or compound of metal having a hydrodehydrogenating function. These catalysts are advantageously catalysts comprising at least one Group VIII metal, generally selected from the group consisting of nickel and / or cobalt, and / or at least one Group VIB metal, preferably molybdenum and / or tungsten. . The support used is generally chosen from the group formed by alumina, silica, silica-aluminas, magnesia, clays and mixtures of at least two of these minerals.

Before the charge is injected, the catalysts used in the process according to the present invention are preferably subjected to a sulphurization treatment making it possible, at least in part, to transform the metallic species into sulphide before they come into contact with the charge. treat. This activation treatment by sulfurization is well known to those skilled in the art and can be performed by any method already described in the literature either in-situ, that is to say in the reactor, or ex-situ.

The system of permutable guard zones with reversal of the direction of flow and paralleling of the permutable reactors making it possible to perform a prior hydrodemetallation of the charge advantageously precedes a hydrotreatment section of heavy hydrocarbon feeds in fixed bed or bubbling bed.

Preferably, it precedes a hydrotreatment section comprising at least one hydrodemetallation unit and at least one hydrodesulfurization unit. The permutable reactors are preferably integrated upstream of the hydrodemetallation unit, the permutable reactors being used as guard beds.

The hydrotreating process according to the invention advantageously makes it possible to carry out 50% and more hydrodemetallation of the feedstock at the outlet of the permutable reactors, and more preferably from 50 to 95% of hydrodemetallation), thanks to the speed selected VHV hourly volume and the effectiveness of HDM hydrodemetallization catalyst.

In the case shown on the figure 1 the charge 1 arrives in the reactive reactors or reactors by the pipe 1 and leaves the reactor or these reactors by the pipe 13. The charge leaving the reactor (s) of guard arrives via the pipe 13 in the section d hydrotreatment 14 and more precisely in the hydrodemetallation unit HDM comprising one or more reactors. The effluent from the hydrodemetallation unit HDM 15 is withdrawn via line 16 and then sent to the hydrotreating unit HDT 17 comprising one or more reactors. The final effluent is withdrawn through line 18.

Modes of realization

It is possible to put several reactive reactors upstream, their number being strictly greater than 2, with a change of direction on two or more of them.

According to one embodiment, illustrated by the figure 3 at least one of the permutable reactors comprises at least two catalytic beds. In this configuration, it may be advantageous to short-circuit the upper layer, which has undergone most of the clogging during the upflow stages. Short-circuiting of a clogged bed can be effected externally by lateral withdrawal and a short-circuit line, comprising a valve. It is thus possible for steps b) and / or e) to "bypass" the already clogged catalytic beds, and to discharge the effluent from this reactor via a pipe situated between the catalytic beds in the case of two beds. catalytic or between a bed i and a bed i + 1 in the case where the reactor comprises n catalytic beds, i being an integer ranging from 1 to n. Advantageously, i + 1 denotes the first bed clogged in the direction of flow, the reaction effluent therefore leaves the reactor just before the flow has encountered the clogged zone. This embodiment allows an optimal use of the overall catalytic volume, since the flow does not cross the already clogged portion of the reactor.

In a preferred variant of this embodiment, all the permutable reactors comprise at least two catalytic beds. In this configuration, it is possible for both steps b) and e) to remove the effluent from the reactor being clogged by a pipe located between the catalytic beds in the case of two catalytic beds or between a bed i and a bed i + 1 in the case where the reactor comprises n catalytic beds, i being an integer ranging from 1 to n.

In a still more preferred embodiment, the guard zone comprises two reactive reactors each comprising two catalytic beds. In this configuration, it is possible for both steps b) and e) to remove the effluent from the reactor being clogged by a lateral withdrawal between the two catalytic beds, just before the catalytic zone already clogged.

Description of figures Figure 1: System of reactive reactors according to the prior art

• une première étape (dénommée par la suite "étape a") au cours de laquelle la charge traverse successivement le réacteur R1 a, puis le réacteur R1b,
• une deuxième étape (dénommée par la suite "étape b") au cours de laquelle la charge traverse uniquement le réacteur R1b, le réacteur R1a étant court-circuite pour régénération et/ou remplacement du catalyseur,
• une troisième étape (dénommée par la suite "étape c") au cours de laquelle la charge traverse successivement le réacteur R1b, puis le réacteur R1a,
• une quatrième étape (dénommée par la suite "étape d") au cours de laquelle la charge traverse uniquement le réacteur R1a, le réacteur R1b étant court-circuite pour régénération et/ou remplacement du catalyseur.

The operation of reactive reactors according to FR2681871 is described in the figure 1 having two guard zones (or reactive reactors) R1a and R1 b. This method comprises a series of cycles each comprising four successive steps:

• a first step (hereinafter referred to as "step a") during which the charge successively passes through the reactor R1a, then the reactor R1b,
• a second step (hereinafter referred to as "step b") during which the feed passes only through the reactor R1b, the reactor R1a being bypassed for regeneration and / or replacement of the catalyst,
• a third step (hereinafter referred to as "step c") during which the charge successively passes through the reactor R1b, then the reactor R1a,
• a fourth step (hereinafter referred to as "step d") in which the feed passes only through the reactor R1a, the reactor R1b being bypassed for regeneration and / or replacement of the catalyst.

During step a) of the method, the feedstock is introduced via line 3 and line 21 comprises an open valve V1 towards line 21 'and the guard reactor R1a containing a fixed bed A of catalyst. During this period valves V3, V4 and V5 are closed. The effluent from the reactor R1a is sent via the pipe 23, the pipe 26 comprising an open valve V2 and the pipe 22 'into the guard reactor R1b enclosing a fixed bed B of catalyst. The effluent from the reactor R1b is sent via the pipes 24 and 24 'comprising an open valve V6 and the pipe 13 to the main hydrotreatment section 14. During the stage b) of the process, the valves V1, V2, V4 and V5 are closed and the load is introduced via line 3 and line 22 comprises a valve V3 open to line 22 'and reactor R1b. During this period, the effluent from the reactor R1b is sent via the pipes 24 and 24 'comprising an open valve V6 and the pipe 13 to the main hydrotreatment section 14. During stage c) the valves V1, V2 and V6 are closed and the valves V3, V4 and V5 are open. The charge is introduced via line 3 and lines 22 and 22 'to reactor R1b. The effluent from the reactor R1b is sent via the pipe 24, the pipe 27 comprising an open valve V4 and the pipe 21 'into the guard reactor RI a. The effluent from the reactor R1a is sent via the pipes 23 and 23 'comprising an open valve V5 and the pipe 13 to the main hydrotreatment section 14. During stage d) the valves V2, V3, V4 and V6 are closed and valves V1 and V5 are open. The charge is introduced via line 3 and lines 21 and 21 'to reactor R1a. During this period, the effluent from the reactor R1a is sent via the pipes 23 and 23 'comprising an open valve V5 and the pipe 13 to the main hydrotreatment section 14. The cycle then recommences again. The operations on the valves of the unit 30 allowing the operation of the reactive reactors according to FR2681871 are shown in Table 1. Table 1: Valve operations around commutable reactors according to FR2681871 Cycle stage Intervention V1 V2 V3 V4 V5 V6 at R1A + R1B - O * O F ** F F O b R1B R1A F F O F F O c R1B + R1A - F F O O O F d R1A R1B O F F F O F at R1A + R1B - O O F F F O * O = open, ** F = closed

Figure 2: System according to the invention

• une première étape (dénommée par la suite "étape a") au cours de laquelle la charge traverse successivement le réacteur R1 a, puis le réacteur R1 b, placés en série, l'écoulement est descendant ; jusqu'à atteindre une perte de charge comprise entre 20 et 70 % de la perte de charge maximale admissible par les équipements DPmax, de préférence entre 30 et 60% de DPmax, de manière encore plus préférée entre 35 et 50% de DPmax ;
• une deuxième étape (dénommée par la suite "étape b") d'inversion du sens de l'écoulement et de mise en parallèle des réacteurs R1a et R1b pendant une durée au plus égale au temps de colmatage de R1a, déterminée par l'atteinte de la perte de charge maximale admissible par les réacteurs, la charge traverse alors de manière simultanée le réacteur R1 b et le réacteur R1 a, l'écoulement est ascendant,
• une troisième étape (dénommée par la suite "étape c ») durant laquelle ledit (premier) réacteur colmaté R1a est court-circuité et le catalyseur qu'il contient est régénéré et/ou remplacé par du catalyseur frais, le sens de l'écoulement est modifié et seul le réacteur non encore colmaté R1b fonctionne, la charge traverse uniquement le réacteur RIB de manière descendante.
• une quatrième étape (dénommée par la suite "étape d") au cours de laquelle le premier réacteur R1A est remis en fonctionnement, R1B et R1A fonctionnent en série en écoulement descendant
• une cinquième étape (dénommée par la suite "étape e ») au cours de laquelle on inverse le sens d'écoulement, R1A et R1B fonctionnent en parallèle en courant ascendant
• une sixième étape (dénommée par la suite "étape f ») durant laquelle ledit (deuxième) réacteur colmaté R1B est court-circuité et le catalyseur qu'il contient est régénéré et/ou remplacé par du catalyseur frais, le sens de l'écoulement est modifié et seul le réacteur non encore colmaté R1A fonctionne, la charge traverse uniquement le réacteur R1A de manière descendante, le réacteur R1B étant court-circuite pour régénération et/ou remplacement du catalyseur

The operation of the system of reactive reactors according to the invention is described in a nonlimiting manner in the figure 2 having two guard zones (or reactive reactors) R1a and R1 b. This process comprises a series of cycles each comprising six successive stages:

• a first step (hereinafter referred to as "step a") during which the charge successively passes through the reactor R1a, then the reactor R1b, placed in series, the flow is descending; up to reach a pressure drop of between 20 and 70% of the maximum allowable pressure drop by the DPmax equipment, preferably between 30 and 60% of DPmax, more preferably between 35 and 50% of DPmax;
• a second step (hereinafter referred to as "step b") of inversion of the direction of flow and of paralleling the reactors R1a and R1b for a duration at most equal to the clogging time of R1a, determined by the achievement of the maximum permissible pressure drop by the reactors, the charge then flows simultaneously to the reactor R1b and the reactor R1a, the flow is upward,
• a third step (hereinafter referred to as "step c") during which said (first) clogged reactor R1a is short-circuited and the catalyst it contains is regenerated and / or replaced by fresh catalyst, the direction of flow is modified and only the reactor not yet clogged R1b operates, the load passes only through the RIB reactor downwardly.
• a fourth step (hereinafter referred to as "step d") during which the first reactor R1A is put back into operation, R1B and R1A operate in series in downward flow
• a fifth step (hereinafter referred to as "step e") in which the direction of flow is reversed, R1A and R1B operate in parallel with a rising current
• a sixth step (hereinafter referred to as "step f") during which said (second) clogged reactor R1B is short-circuited and the catalyst contained therein is regenerated and / or replaced by fresh catalyst, the direction of flow is modified and only the reactor not yet clogged R1A operates, the feed passes only through the reactor R1A downwardly, the reactor R1B being bypassed for regeneration and / or replacement of the catalyst

A new cycle can begin, again by step a): R1B is put back into operation, R1A and R1B run in series in downward flow.

In the process according to the invention, illustrated by the figure 2 during step a) of the process, charge 1 is introduced via line 3 and line 21 comprises an open valve V1 towards line 21 'and the guard reactor R1a containing a fixed bed A of catalyst. During this period valves V3, V4 and V5, V8 and V9 are closed. The effluent from the reactor R1a is sent via the pipe 23, the pipe 26 comprising an open valve V2 and the pipe 22 'into the guard reactor R1b enclosing a fixed bed B of catalyst. The reactor effluent R1 b is sent through lines 24 and 24 'comprising an open valve V6 and via line 13 comprising a valve V10 open to the main hydrotreatment section 14. The flow is in downflow.

• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 23' comportant une vanne ouverte V5 vers la ligne 23 et le réacteur R1 A
• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 24' comportant une vanne ouverte V6 vers la ligne 24 et le réacteur R1 B.

During step b) of the process, the direction of flow is reversed in order to flow upwards, the valves V1, V2, V3 and V4 as well as V10 and V11 are closed and the charge is introduced simultaneously with rising current:

• by line 3 and line 28 comprising an open valve V7 and line 23 'comprising an open valve V5 to line 23 and reactor R1 A
• by line 3 and line 28 having an open valve V7 and line 24 'having an open valve V6 to line 24 and reactor R1 B.

During this period, the effluent from the reactor R1b is sent via the pipes 22 'and 30 having an open valve V9 and the pipe 31 to the main hydrotreatment section 14. The effluent from the reactor R1a is sent via the pipes 22 and 30 having an open valve V8 and the pipe 31 to the main hydrotreatment section 14.

During step c) the valves V1, V2, V4, V5, V7, V8, V9 are closed and the valves V3, V6, V10, and V11 are open. The charge is introduced via line 3 and lines 22 and 22 'to reactor R1b. The effluent from the reactor R1b is sent via the pipes 24 and 24 'comprising an open valve V6 and the pipe 13 comprises a valve V10 open towards the main hydrotreatment section 14. The sealed R1a reactor is thus short-circuited in order to allow the intervention on the reactor R1a for the replacement / reloading of the catalyst.

• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 23' comportant une vanne ouverte V5 vers la ligne 23 et le réacteur R1 a.
• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 24' comportant une vanne ouverte V6 vers la ligne 24 et le réacteur R1 b.

During step d) the valves V1, V2, V6, V7, V8, V9 are closed and the valves V3, V4, V5, V10 and V11 are open. The charge is introduced via line 3 and lines 22 and 22 'to reactor R1b, valves V3 and V11 being open. During this period, the effluent from the reactor R1b is sent via the pipes 24 and 27 'comprising an open valve V4 and the pipe 21' to the reactor R1a. The reactor effluent R1a is sent via a line 23 having an open valve V5 and a valve V10 open to the main hydrotreatment section 14. The flow is downward in the reactors R1b and R1a placed in series.
During step e), the direction of flow is reversed in order to flow upwards, the valves V1, V2, V3 and V4 as well as V10 and V11 are closed and the charge is introduced simultaneously with rising current:

• by the line 3 and the line 28 having an open valve V7 and the line 23 'comprising an open valve V5 to the line 23 and the reactor R1 a.
• by the line 3 and the line 28 having an open valve V7 and the line 24 'having an open valve V6 to the line 24 and the reactor R1 b.

During this period, the effluent from the reactor R1b is sent via the pipes 22 'and 30 having an open valve V9 and the pipe 31 to the main hydrotreatment section 14. The effluent from the reactor R1a is sent via the pipes 22 and 30 having an open valve V8 and the pipe 31 to the main hydrotreatment section 14.

During step f), the valves V2, V3, V4, V6, V7, V8, V9 are closed and the valves V1, V5, V10, and V11 are open. The charge is introduced via line 3 and lines 21 and 21 'to reactor R1a. The effluent from the reactor R1a is sent via the pipes 23 and 23 'comprising an open valve V5 and the pipe 13 comprises a valve V10 open to the main hydrotreatment section 14. The sealed reactor R1b is thus short-circuited in order to allow the intervention on the reactor R1b for the replacement / reloading of the catalyst.

The cycle then starts again. The operations on the valves of the unit allowing the operation of the reactive reactors according to the invention are reported in Table 2. Table 2: Operations on the valves around the permutable reactors according to the invention Cycle stage Flow direction Intervention V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 at R1A + R1B Descending - O * O F " F F O F F F O O b R1A + R1B Ascending - F F F F O O O O O F F c R1B Descending R1A F F O F F O F F F O O d R1B + R1A Descending - F F O O O F F F F O O e R1A + R1B Ascending - F F F F O O O O O F F f R1A Descending R1B O F F F O F F F F O O at R1A + R1B Descending - O O F F F O F F F O O * O = open, ** F = closed

Figure 3: Embodiment with shorting

In this embodiment, the figure 3 illustrates a short-circuiting of the catalytic bed already clogged during steps b) and e).

During step a) of the process, charge 1 is introduced via line 3 and line 21 comprises a valve V1 open to line 21 'and the guard reactor R1a containing a fixed bed A of catalyst. During this period the valves V3, V4 and V5, V8, V9, V12, V13 are closed. The effluent from the reactor R1a is sent via the pipe 23, the pipe 26 comprising an open valve V2 and the pipe 22 'into the guard reactor R1b enclosing a fixed bed B of catalyst. The reactor effluent R1 b is sent through lines 24 and 24 'comprising an open valve V6 and via line 13 comprising a valve V10 open to the main hydrotreatment section 14. The flow is in downflow.

• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 23' comportant une vanne ouverte V5 vers la ligne 23 et le réacteur R1 A
• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 24' comportant une vanne ouverte V6 vers la ligne 24 et le réacteur R1 B.

During step b) of the method, the direction of flow is reversed in order to flow upwards, the valves V1, V2, V3 and V4 as well as V10, V11, and V13 are closed and the charge is introduced simultaneously with updraft:

• by line 3 and line 28 comprising an open valve V7 and line 23 'comprising an open valve V5 to line 23 and reactor R1 A
• by line 3 and line 28 having an open valve V7 and line 24 'having an open valve V6 to line 24 and reactor R1 B.

During this period, the effluent from the reactor R1b is sent via the pipes 22 'and 30 having an open valve V9 and the pipe 31 to the main hydrotreatment section 14.

The reactor effluent R1 a is sent through the lines 32 and 30 having an open valve V12 and the pipe 31 to the main hydrotreatment section 14. The valve V8 is closed. The already clogged catalytic bed is thus short-circuited.

During step c) the valves V1, V2, V4, V5, V7, V8, V9, V12, V13 are closed and the valves V3, V6, V10, and V11 are open. The charge is introduced via line 3 and lines 22 and 22 'to reactor R1b. The effluent from the reactor R1b is sent via the pipes 24 and 24 'comprising an open valve V6 and the pipe 13 comprises a valve V10 open to the main hydrotreatment section 14. The sealed reactor R1a is thus bypassed in order to allow the intervention on the reactor R1a for the replacement / reloading of the catalyst.

During step d) the valves V1, V2, V6, V7, V8, V9, V12, V13 are closed and the valves V3, V4, V5, V10 and V11 are open. The charge is introduced via line 3 and lines 22 and 22 'to reactor R1b, valves V3 and V11 being open. During this period, the effluent from the reactor R1b is sent via the pipes 24 and 27 'comprising an open valve V4 and the pipe 21' to the reactor R1a. The reactor effluent R1a is sent via a line 23 having an open valve V5 and a valve V10 open to the main hydrotreatment section 14. The flow is downward in the reactors R1b and R1a placed in series.

• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 23' comportant une vanne ouverte V5 vers la ligne 23 et le réacteur R1 a.
• par la ligne 3 et la ligne 28 comportant une vanne V7 ouverte et la ligne 24' comportant une vanne ouverte V6 vers la ligne 24 et le réacteur R1 b.

Durant cette période l'effluent du réacteur R1 b est envoyé par les conduites 33 et 30 comportant une vanne V13 ouverte et la conduite 31 vers la section d'hydrotraitement principale 14. L'effluent du réacteur R1 a est envoyé par les conduites 22' et 30 comportant une vanne V8 ouverte et la conduite 31 vers la section d'hydrotraitement principale 14.During step e), the direction of flow is reversed in order to flow upwards, the valves V1, V2, V3 and V4 as well as V10, V11 and V12 are closed and the charge is introduced simultaneously with rising current:

• by the line 3 and the line 28 having an open valve V7 and the line 23 'comprising an open valve V5 to the line 23 and the reactor R1 a.
• by the line 3 and the line 28 having an open valve V7 and the line 24 'having an open valve V6 to the line 24 and the reactor R1 b.

During this period, the effluent from the reactor R1b is sent via the pipes 33 and 30 comprising an open valve V13 and the pipe 31 to the main hydrotreatment section 14. The effluent from the reactor R1a is sent via the pipes 22 '. and having an open valve V8 and line 31 to the main hydrotreatment section 14.

During step f), the valves V2, V3, V4, V6, V7, V8, V9, V12, V13 are closed and the valves V1, V5, V10, and V11 are open. The charge is introduced via line 3 and lines 21 and 21 'to reactor R1a. The effluent from the reactor R1a is sent via the pipes 23 and 23 'comprising an open valve V5 and the pipe 13 comprising a valve V10 open towards the section Main hydrotreatment 14. The reactor R1b clogged is thus short-circuited to allow intervention on the reactor R1b for the replacement / reloading of the catalyst.

The cycle then starts again. The operations on the valves of the unit allowing the operation of the reactive reactors according to the invention are reported in Table 3. Step Cycle Flow direction Int. V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12 V13 at R1A + R1B Descending - O * O F ** F F O F F F O O F F b R1A + R1B Ascending - F F F F O O O F O F F O F c R1B Descending R1A F F O F F O F F F O O F F d R1B + R1A Descending - F F O O O F F F F O O F F e R1A + R1B Ascending - F F F F O O O O F F F F O f R1A Descending R1B O F F F O F F F F O O F F at R1A + R1B Descending - O O F F F O F F F O O F F

Examples

The examples below illustrate the invention without limitation.

Example 1 (comparative, not according to the invention, according to FR2681871)

The feed consists of a mixture (70/30% wt) of atmospheric residue (RA) of Middle East origin (Arabian Medium) and a vacuum residue (VR) of Middle East origin (Arabian Light). This mixture is characterized by a high viscosity (0.91 cP) at room temperature, a density of 994 kg / m ³ , high contents of Conradson carbon (14% by weight) and asphaltenes (6% by weight) and a high amount of nickel (22 ppm by weight), vanadium (99 ppm by weight) and sulfur (4.3% by weight).

The hydrotreatment process is carried out according to the process described in FR2681871 and involves the use of two reactive reactors (R1A and R1B of the figure 1 ). Both reactors are charged with a HDM CoMoNi / alumina hydrodemetallization catalyst. A cycle is defined as integrating the steps of a) to d) (see Table 1). The deactivation and / or clogging time is reached when the pressure drop reaches 0.7 MPa (7bar) and / or the average temperature of a bed reaches 405 ° C and / or when the temperature difference on a catalytic bed becomes radially greater than 5 ° C.

The process is carried out under a pressure of 19 MPa, a temperature at the beginning of the cycle of 360 ° C. and at the end of the cycle of 400 ° C., and a VVH = 2h ^-1 per reactor, making it possible to maintain a demetallation rate close to 60%.

The figure 4 shows the evolution of cycles in terms of operating time (in days) for the process according to FR2681871 . A cycle being defined here as the alternative operation of R1a then R1b. So, according to the figure 4 , the curve of the reactor R1a according to the state of the art (base case R1a) shows at the beginning of the cycle an increase in the pressure drop in the first reactor R1a to its maximum allowable value (DPmax = 0.7 MPa or 7 bar) from which the catalyst must be replaced. In the case of the state of the art ( FR2681871 ), the operating time of the reactor R1a is therefore 210 days. When replacing the catalyst of the reactor R1a, the pressure drop in the reactor R1b reached about 3 bars. During the next phase where the charge passes through the reactor R1b, then the reactor R1a containing a new catalyst, the pressure drop of the reactor R1b increases to the maximum allowable value, which is reached after 320 days of operation. A second cycle can be envisaged on these permutable reactors, replacing the reactor catalyst R1b.

The time of deactivation and / or clogging (or the duration of operation) of the first zone is therefore 210 days. In total, there is a cycle time of 320 days for the first cycle and 627 days for two cycles.

It is recalled that at the end of the operating cycle, the process is stopped to replace the catalyst.

Example 2 (in accordance with the invention)

The hydrotreatment process is repeated with the same filler, the same catalyst and under the same operating conditions according to Example 1. The hydrodemetallization rate HDM is maintained at 60%.

The schema used for the example is that presented on the figure 2 . The figure 4 shows that according to the prior art, the cycle time is 320 days for the first cycle. In fact, the pressure loss increases in the catalytic bed until reaching the maximum permissible pressure loss DPmax for the equipment installed in this example of 0.7 MPa (7 bar) after 210 days for the first reactor and 320 days for the second.

On the other hand, by integrating the step of reversing and paralleling the direction of flow according to the invention when the pressure drop reaches a threshold Dpi equal to 0.3 MPa (3 bar), or about 43% of the maximum allowable pressure drop in DPmax equipment ( Figure 5 ), the lifetime for the first reactor is extended to 380 days, for a cycle time of 580 days. It therefore appears in this example that the hydrotreatment process incorporating a step of flow direction change and parallelization of the reactors makes it possible to significantly increase the cycle time of the catalytic hydrodemetallation zone, in particular with a gain of 260 days (80%).

Claims (15)

1. Process for catalytic hydrotreatment of a heavy hydrocarbon-based feedstock, in the presence of hydrogen, comprising a prior hydrodemetallation step by means of at least two fixed bed switchable reactors, each comprising at least one catalytic bed, which are used cyclically according to the following steps:

   a) a step in which at least two reactors are used in series in descending flow mode until a pressure drop threshold Dpi having a value of between 20 and 70% of the maximum admissible pressure drop DPmax is reached,

   b) a step of reversing the direction of the flow and of placing said at least two reactors in parallel until the maximum admissible pressure drop DPmax is reached,

   c) a step during which the first reactor brought into contact with the feedstock in step a) is bypassed and the catalyst that it contains is regenerated and/or replaced with fresh catalyst, the direction of the flow is modified and only the reactor(s) not yet blocked operate in descending flow mode,

   d) a step during which said at least two reactors are used in series, the reactor of which the catalyst was regenerated during the preceding step being reconnected and said step being continued until a pressure drop threshold Dpi having a value of between 20 and 70% of the maximum admissible pressure drop DPmax is reached,

   e) a step of reversing the direction of the flow and of placing the two reactors in parallel, until the maximum admissible pressure drop DPmax is reached,

   f) a step during which the second reactor brought into contact with the feedstock in step a) is bypassed and the catalyst that it contains is regenerated and/or replaced with fresh catalyst, the direction of the flow is modified and only the reactor(s) not yet blocked operate in descending flow mode; said steps a), b), c), d), e) and f) possibly being repeated cyclically in this order.
2. Process according to Claim 1, in which at least one of the reactors comprises n catalytic beds and the effluent is removed from said reactor in steps b) and/or e) by drawing off a sidestream between a catalytic bed i and a catalytic bed i+1, n being an integer greater than or equal to 2, i being an integer ranging from 1 to n.
3. Process according to Claim 2, in which said catalytic bed i+1 denotes the first blocked catalytic bed in the direction of the flow.
4. Process according to Claim 2 or 3, in which said at least two switchable reactors all comprise at least two catalytic beds.
5. Process according to one of Claims 1 to 4, in which the pressure drop threshold Dpi has a value of between 30 and 60% of the maximum admissible pressure drop DPmax.
6. Process according to Claim 5, in which the pressure drop threshold Dpi has a value of between 35 and 50% of the maximum admissible pressure drop DPmax.
7. Process according to one of the preceding claims, in which the heavy hydrocarbon-based feedstock has an initial boiling point of at least 340°C and a final boiling point of at least 440°C.
8. Process according to Claim 7, in which the feedstock is chosen from atmospheric residues, vacuum residues resulting from direct distillation, crude oils, topped crude oils, deasphalting resins, deasphalting tars or asphalts, residues resulting from conversion processes, aromatic extracts resulting from lubricant base production lines, tar sands or derivatives thereof, oil shales or derivatives thereof, source-rock oils or derivatives thereof, taken alone or as a mixture.
9. Process according to Claim 8, in which the feedstock is chosen from atmospheric residues or vacuum residues, or mixtures of these residues.
10. Process according to one of the preceding claims, carried out at a temperature of between 320°C and 430°C, under a hydrogen partial pressure of between 3 MPa and 30 MPa, at an hourly space velocity (HSV) of between 0.05 and 5 volumes of feedstock per volume of catalyst and per hour, and with a hydrogen gas to liquid hydrocarbon-based feedstock ratio of between 200 and 5000 standard cubic metres per cubic metre.
11. Process according to Claim 10, in which the prior hydrodemetallation step is carried out in the presence of a hydrodemetallation catalyst comprising at least one group VIII metal and/or at least one group VIB metal, on a used support chosen from the group made up of alumina, silica, silicas/aluminas, magnesia, clays and mixtures of at least two of these minerals.
12. Process according to Claim 11, in which the prior hydrodemetallation step is carried out at an hourly space velocity (HSV) of each operating switchable reactor of between 0.5 and 4 volumes of feedstock per volume of catalyst and per hour.
13. Process according to Claim 12, in which the prior hydrodemetallation step is carried out at an hourly space velocity (HSV) of each operating switchable reactor of between 1 and 2 volumes of feedstock per volume of catalyst and per hour.
14. Process according to one of the preceding claims, in which the maximum admissible pressure drop DPmax is between 0.3 and 1 MPa.
15. Process according to Claim 14, in which the maximum admissible pressure drop DPmax is between 0.5 and 0.8 MPa.

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