EP0322276A1 - Heavy charge catalytic cracking process and apparatus comprising a second stripping in a fluidised bed - Google Patents

Abstract

Process for catalytic cracking of a hydrocarbon feedstock (4) comprising two zones for stripping (8 and 13) spent catalyst particles, and the corresponding device. The second stage of stripping of the spent catalyst (13) is carried out in a fluidised bed in a tubular zone situated between the first stripping zone (8) at the exit of the cracking vessel (1) and the regeneration zone (21), in the presence of at least partially regenerated hot particles (26) travelling concurrently with the flow of the spent particles fluidised with the aid of a stripping gas and oxygen (15). <IMAGE>

Description

The invention relates to a process and a device for catalytic cracking in a fluidized bed of hydrocarbon charges boiling above 400 ° C., comprising in particular an improvement in the stripping of the used catalyst.

It is known that the petroleum industry customarily uses cracking processes, in which molecules of high molecular weight and high boiling point hydrocarbons are split into smaller molecules of lower boiling temperatures and better suited to the intended use. One of the most commonly used processes at present is the so-called fluidized bed catalytic cracking process (in English: Fluid Catalytic Cracking or F.C.C.). In this type of process, the hydrocarbon charge is brought into contact with a hot catalyst which provides the heat input necessary both for the vaporization of said charge and for the cracking of heavy molecules. The catalyst is kept in suspension in the vapors of hydrocarbons, and after having reached the desired product range with the lowering of the corresponding boiling points, the so-called used catalyst because of the deposit of coke which covers its surface, is separated from the hydrocarbon vapors, stripped, regenerated by combustion of the coke deposit, then again returned to the cracking zone.

The cracking reactor is generally in the form of an elongated substantially vertical tube. When the flow of the catalyst and the vaporized charge is ascending, we speak of riser. On the other hand, when the flow is descending, we speak then of dropper.

The charges to be cracked are usually injected into the reaction zone after having been preheated to a temperature of between 80 and 400 ° C. This injection is carried out using one or more devices which spray and disperse the charge on the catalyst. The pressure in the reaction zone is between 0.7 and 3.5 bar. The regenerated catalyst is introduced into the zone of cracking at a temperature of the order of 600 to 950 ° C. The quantity of catalyst injected is regulated by a valve located upstream of the reaction zone. The grains of catalyst initially put into fluidization by appropriate injections of vapor, are then entrained by the hydrocarbon vapors towards the other end of the reactor where the charge opens into an enclosure where the separation of the cracked hydrocarbon vapors takes place. of the catalyst (using ballistic separators and cyclones), as well as the stripping of the catalyst, that is to say essentially the vapor desorption of the hydrocarbons fixed by the grains. The catalyst thus freed from most of the adsorbed hydrocarbons is then sent to one or more regenerators where its catalytic activity is restored by combustion of the coke which has deposited on the grains during the cracking step. During regeneration, the thermal energy released by the combustion of coke is partly given up to the catalyst which heats up. This amount of energy absorbed by the catalyst is then used in the reaction zone to vaporize and crack the charge: the catalyst acts as a coolant between the reactor where endothermic reactions take place and the regenerator or regenerators where reactions take place. exothermic. The FCC process is implemented to ensure the thermal equilibrium of the unit.

The quantity of coke and of hydrocarbons present on the catalyst at the outlet of the stripping stage, is therefore an essential variable of the process since it conditions all of the thermal levels of the unit. We can observe imbalances detrimental to the proper functioning of the installation, when this quantity is too high due to poor stripping or the use of too heavy loads characterized by high contents of asphaltenes, hetero atoms (sulfur , nitrogen, etc ...) and metals. It should be noted that such fillers which are difficult to vaporize are commonly encountered these days; however, poor spraying of the heaviest fractions generally leads to their deposition on the grains of catalyst despite steam stripping. The resulting drawbacks are in particular a reduction in the yields of recoverable products, a release of energy during regeneration exceeding the needs of the unit and leading to final regeneration temperatures that are too high and therefore detrimental to the activity of the catalyst, increased production of fumes and greater air consumption resulting in oversizing of the regeneration unit, as well as high production of steam which is not always useful on site.

In addition, in a conventional catalytic cracking unit, the stripping temperature is substantially the same as that of the reaction zone which itself depends on the conversion which it is desired to achieve for a given charge quality. Obtaining this conversion being the main element taken into consideration for the choice of the temperature of the cracking zone, stripping therefore appears to be an operation on which there are few means of action. At constant temperature, only an increase in the vapor flow rate or the residence time of the catalyst in the stripper currently makes it possible to improve the recovery yield of the absorbed hydrocarbons. In the current state of the art, and more particularly with heavy loads, the impact of a modification of these two operating parameters on the stripping efficiency therefore remains limited.

Furthermore, the combustion of these non-desorbed heavy hydrocarbons not only releases a significant amount of energy which appreciably increases the temperature of the regenerator on the one hand, but also releases appreciable amounts of water vapor due to their high content of hydrogen. However, it is known that the combination of these two factors: high temperature and high partial pressure of water vapor during the regeneration step, significantly accelerates the aging of the catalyst and affects in particular its activity.

Among the most recent means which aim to remedy the aforementioned drawbacks, there are first of all those which aim to reduce the regeneration temperature as well as the water vapor content:
- heat extraction by means of heat exchangers located in the regenerator
- Staging of the regeneration so as to carry out the partial oxidation of the coke and of the non-desorbed hydrocarbons after the stripping step and thus limit the release of energy. This staging also makes it possible to carry out, at relatively low temperature, the preferential oxidation of the hydrocarbons richest in hydrogen and therefore to eliminate most of the hydrogen present on the catalyst, and this, under conditions compatible with good behavior. of the catalyst.

All of these provisions make it possible to preserve the catalyst from premature aging, but do not avoid the loss of significant quantities of carbon which are used solely to generate heat, sometimes in excess. It also involves the installation of complex systems which considerably increase investment.

According to the prior art, it is known that the aforementioned drawbacks can be reduced by eliminating more effectively the adsorbed hydrocarbons, by stripping carried out at a higher temperature than that prevailing in the reaction zone. The increase in the stripping temperature can thus be obtained by injecting hot catalyst from the regeneration stage or stages. US Patent 4,440,632 describes, for example, such a device where hot catalyst taken from the regenerator is introduced into the lower part of the stripper, in the dense phase. This hot catalyst mixes with the used catalyst and leads to a final temperature of the mixture approximately 20 to 150 ° C. higher than that of the reaction zone. US Patent 4,419,221 also describes a similar device where in a unit with two regeneration levels, part of the catalyst of the second stage, that is to say the hottest, is used to reinject it into the lower part of the stripper where the dense phase is located.

It will be noted that the temperature rise of the stripper by recycling partially or totally regenerated and hot catalyst can be much higher than in the case where the combustion gases are recycled as recommended in patent EP-A-184517 and WO 82/04061. Indeed, in the latter case, the ratio of mass flow rate of used catalyst to mass flow rate of smoke is such that one can hardly hope for temperature rises going beyond 15 to 20 ° C.

While these devices provide more efficient desorption of heavy hydrocarbons, there are still some drawbacks. The hot catalyst reinjected near the diluted phase can come into contact with the light hydrocarbons present in the stripping enclosure and cause an overcracking detrimental to the overall performance of the unit. Furthermore, the quantities of stripping vapor used are generally small so as to ensure a substantially piston flow of the catalyst, favorable to stripping and to the countercurrent training of light hydrocarbons. Under these conditions, the mixing of the hot catalyst from the regenerator with the used catalyst is not rapid and leads to hot spots which again favor the overcracking of the hydrocarbons desorbed in the dense phase.

This overcracking can also occur in the case of the process described in patent EP 137998, where a first stripping of the used catalyst is carried out in a separate stripping column in the presence of at least a portion of regenerated catalyst and of a gas. lift, which consists of cracking effluents.

Patent EP 187032 provides some remedies for the aforementioned drawbacks. It describes a cracking process in which the stripping operation is carried out in two stages. In the first stage, a conventional stripping is carried out with steam and at a temperature substantially equivalent to that prevailing in the reaction zone, and in the second stage, a more severe stripping is carried out at a higher temperature, after injecting hot catalyst from the regenerator. The advantage of this process is that the second stripping is carried out so as to desorb the heavy hydrocarbons in a well individualized enclosure where it is easier to adjust the operating parameters (recycled flow rate, gas speed, etc.) without prejudice. for the rest of the vaporized hydrocarbon charge as could be the case with US patents 4,440,632 and 4,419,221. It will be noted in this case as in the previous cases, that obtaining a significant temperature rise in the stripper, requires significant flows of hot catalyst. Furthermore, the effectiveness of stripping in a dense fluidized bed is limited due to the particular characteristics of the catalyst grains and more especially of their small size. A very small fraction of the stripping vapor injected percolates through the grain bed. This vapor passes through the bed essentially in the form of bubbles, and it is easy to understand that the particular hydrodynamics of the dense fluidized bed limits the displacement of the hydrocarbons by the water vapor, especially if the fluidization speed must be maintained relatively weak to ensure a counter-current flow of the catalyst relative to the stripping vapor.

Finally, the prior art is also described in patent EP-A-23402 where, to reduce the amount of carbon monoxide during the regeneration of cracking catalysts, an oxygen-containing gas is introduced into the transfer line due catalyst stripped towards the regenerator.

A first object of the invention is therefore to propose a method and a device which make it possible to overcome the aforementioned drawbacks.

Another object of the invention is to facilitate the desorption of the hydrocarbons attached to the catalyst grains, in particular the heavy hydrocarbons with the aim of increasing the overall yield of the unit and limit the deposits of carbonaceous materials on the catalyst while strictly maintaining the thermal equilibrium of the installation.

The invention therefore relates to a catalytic cracking process in a fluid bed of a hydrocarbon charge boiling above 400 ° C. This method comprises a step of bringing upward or downward flow into contact in an elongated tubular zone, under cracking conditions, of said charge and of the particles of a cracking catalyst, a step of separating the spent catalyst and the charge cracked in a first separation zone at the outlet of said tubular zone, a first stage of stripping of the used catalyst using a first stripping gas injected against the current of this catalyst, a first stage of separation of the catalyst mixture used and effluents resulting from the first stripping step, a second co-current stripping step delivering effluents from the second stripping, a step of regenerating said spent catalyst in at least one regeneration zone to form said hot regenerated catalyst, in conditions for combustion of the coke deposited thereon and a step of recycling at least part of the regenerated catalyst to the supply of said zone tubular. More precisely, after having carried out said first stripping step and said first separation step and before having carried out the regeneration step of said catalyst, the second stripping step is carried out in co-current and in a fluid bed, in a second stripping zone which is an elongated tubular zone, by introducing and mixing, upstream of the tubular zone and advantageously in the immediate vicinity of the end corresponding to the entry into said second tubular zone, at least part said catalyst regenerated and at a higher temperature with the spent catalyst resulting from the first stripping step and a stream of a second pressurized gas containing at least one stripping gas and a gas containing molecular oxygen, and separates effluents from said second stripping zone from the catalyst mixture thus stripped in a second separation zone downstream of this second stripping zone, said second stripping effluents are recovered and the catalyst mixture thus stripped is sent to said regeneration zone.

Advantageously, the regenerated catalyst is introduced in the immediate vicinity of the end, corresponding to the entry into the second tubular zone and preferably substantially above the arrival of the stripping gas, which has the advantage of promoting mixing. particles.

The method according to the invention has the advantage of facilitating better desorption of the hydrocarbons in the second stripping stage, in particular thanks to the introduction of a certain quantity of gas containing molecular oxygen. It follows that the residual deposits of carbonaceous materials before entering the regenerator are limited to strictly maintaining the thermal equilibrium of the unit without leading to unacceptable temperatures and water vapor pressures in the regenerator. for the activity and the service life of the catalyst. Even when the charge treated is heavy, it makes it possible to regenerate the catalyst in a single step, which translates into investment savings.

Another advantage of the invention is that the use of a gas containing molecular oxygen in the stripping gas causes in part the combustion of a fraction of the released hydrocarbons containing hydrogen and a release of heat allowing to maintain a given temperature, while limiting the flow of hot catalyst from the regenerator. This results in simplification and lower cost of installation.

Another advantage of the invention is that the stripping in the second stage is carried out in a fluidized bed, diluted, therefore without diffusion limitation linked to the hydrodynamics of the gas-particle flow. of catalyst. In this case, and unlike the case of conventional dense fluidized beds, the catalyst grains are well individualized within a large volume of gas, so that the partial pressure of hydrocarbons outside the grain is low or all at least much weaker than that prevailing in the dense phase of a fluidized bed (generally called emulsion phase by specialists). On the other hand, in the case of a fluid bed, the sliding speed of the grain relative to the surrounding gas is much greater than in the case of a dense fluidized bed. As a result, the resistance to transfer to the outer surface of the grain is low. These two factors: elimination of the resistance to transfer linked to the gas-particle flow and lowering of the external diffusional resistance at the surface of the grain make the displacement of the hydrocarbons adsorbed by the water vapor much faster in the case of the diluted fluidized bed than in that of the dense fluidized bed.

Another advantage of the invention is that the heating of the used catalyst with hot catalyst from the regenerator is carried out in an enclosure separate from the first stripping zone and makes it possible to avoid any contact of the hot catalyst with light hydrocarbons and eliminates therefore any risk of overcracking of these light hydrocarbons.

Another advantage of the invention relates to the mixing of the used catalyst from the first stripping stage with the hot catalyst of the regenerator. This mixture is produced in a fluid bed, therefore in a much more turbulent medium than the dense fluidized bed, and therefore it is more intimate and faster. This almost instantaneous mixture makes it possible to homogenize the temperatures of the two grain populations very quickly and contributes to significantly improving the efficiency of stripping.

Another advantage of the invention is that the reinjection of hot catalyst at the base of the second stripper makes it possible to rapidly initiate the oxidation of the hydrocarbons. This reinjection also makes it possible to stabilize the combustion in the elevator, even during the transient phases when fluctuations in the flow rate of used catalyst are observed.

Another advantage of the process is that the catalyst after this second stripping is freed from most of the hydrogen initially contained in the coke so that it is possible in most cases to regenerate the catalyst in a single step, even if the temperature is higher than 700 ° C., since the water vapor content of the regeneration effluents is low and therefore does not risk excessively accelerating the aging of the catalyst.

Finally, it has been noted that the invention allows a significant reduction in the sulfur contents in the regeneration effluents, since the heavy hydrocarbons rich in sulfur have been overwhelmingly desorbed from the catalyst grains in this second stripping zone.

According to a characteristic of the invention, the regeneration zone is withdrawn and a mass of regenerated catalyst, generally representing from 5 to 100% by mass, advantageously from 10 to 50%, is recycled into the second tubular stripping zone. mass and preferably between 20 and 40% of the mass of spent catalyst coming from the first stripping stage situated at the end of the cracking reaction zone. The temperature of the regenerated catalyst may be approximately at least 80 ° C higher than the temperature of the spent catalyst at the outlet of the first stripping stage and at most approximately 400 ° C, without however exceeding 700 ° C, which would cause rapid catalyst deactivation.

According to another characteristic of the invention, the hot regenerated catalyst recycled in the second stripping stage can be cleaned beforehand of at least part of the combustion fumes in the connection line leading to the stripping column, preferably by stripping using a fluid (for example, inert gases or water vapor).

According to another characteristic of the invention, the so-called regenerated catalyst recycled in the second stripping stage can come from one or other of the compartments when the regenerator is multi-stage. It can be taken during regeneration in the last compartment, that is to say the one where the regeneration ends and which operates at the highest temperature when it is desired to minimize the recycling rate. But it can also be taken during regeneration in a lower regeneration stage operating at a lower temperature and where the catalyst is partially rid of the deposited coke, if it is feared a catalytic over-cracking or a significant thermal cracking of the hydrocarbons released during of this second stripping step.

According to another characteristic of the invention, the used catalyst from the first stripping stage and the regenerated catalyst from the regenerator are brought into contact with a gas generally containing approximately 0.1 to 30% by mass of oxygen, advantageously from 1 to 10%, and preferably from 2 to 5%, the balance to 100% can be water vapor, nitrogen, hydrogen, light hydrocarbons such as methane, ethane, propane or a mixture of these different gases. Preferably, water vapor and molecular oxygen are used.

The molecular oxygen content is determined in order to carry out the oxidation of the hydrogen released by the second stripping as well as of a minority part of the desorbed hydrocarbons.

According to another characteristic of the invention, the flow rate of gas injected at the base of the tubular stripping zone by means for example of a gas distributor, is adjusted so that the surface speed of the gas in this zone is in generally between approximately 1 and 20 meters per second, and more advantageously between 2 and 10 meters per second, which makes it possible to obtain a entrained bed, to maximize the speed of sliding between the grains of catalyst and said gas of stripping and of ensuring contact times of the catalyst grains with the stripping gas of between 1 and 30 seconds.

In general, the temperature rise of the catalyst in the second stripping zone relative to the first stripping zone, due to the recycling of the hot regenerated catalyst on the one hand and to the partial oxidation of a fraction of the hydrocarbons desorbed from the catalyst, is generally between approximately 10 and 200 ° C. and is preferably between 50 and 100 ° C.

According to another characteristic of the invention, the stripping effluents are separated from the catalyst by means of devices which can, for example, be similar to those used at the outlet of the cracking reaction zone. These effluents which contain inter alia the released hydrocarbons, the products of partial combustion and the stripping water vapor, can be directed like the cracking effluents towards a fractionation tower or any other point of use justified by the quality of said effluents. The catalyst, freed from these effluents, is directed to the regenerator downstream.

In the present invention, the fillers which can be used boil above the range of gasolines and diesel oils, that is to say in general above about 400 ° C. For example, there may be mentioned as fillers, those having initial boiling points of the order of 400 ° C., such as gas oils under vacuum, but also oils heavier hydrocarbons, such as crude and / or de-essential oils, and residues from atmospheric distillation or vacuum distillation. These fillers may have received a preliminary treatment such as, for example, a hydrotreatment in the presence, for example, of cobalt-molybdenum type catalysts. The preferred fillers of the invention will be those containing fractions normally boiling between 400 and 700 ° C and above, which may contain high percentages of asphaltenic products and have a Conradson carbon content of up to 10% and beyond.

The catalysts which can be used in the devices described above include cracking catalysts of the crystalline aluminosilicate type, certain types of silica-aluminas, of silica-magnesia, of silica-zirconium, all having relatively high cracking activities, such as example those described in US Patent 4,405,445.

The invention also relates to a catalytic cracking device in particular for implementing the process. It comprises an elongated tubular enclosure 1, means 4 for injecting pressure under pressure of said charge disposed at one end of said enclosure, means for supplying particles 2 of a cracking catalyst towards said end of said enclosure, first means 6 for separating the cracked charge from the spent catalyst particles at the other end of said enclosure 1 situated in a separation enclosure 5 disposed at the concentric end of the enclosure, a first means of stripping 8 of said spent catalyst by a first stripping gas in said separation enclosure 5, at least one regeneration unit 21 of said spent catalyst by combustion of the coke deposited on it, and means for recycling 27 of at least part of the regenerated catalyst towards said supply means 2.

In more detail, this device comprises an elongated tubular stripping column 13 disposed between said first stripping means 8 and said regeneration unit 21, means for supplying pressure 15 with a second gas comprising at least one stripping gas and a gas containing molecular oxygen, preferably at the inlet of said stripping column 13, means 14 for supplying at least particles of catalyst and regenerated part connected on the one hand to the regeneration unit 21 and on the other hand, preferably, at the inlet of said stripping column 13, means 11 for supplying used catalyst connected to the first stripping means 8 and to the stripping column 13, preferably at its inlet, said supply means 14, 15 being adapted to inject the second gas and the particles of regenerated catalyst into said stripping column, under pressure, in a fluid bed and cocurrently with the flow of the particles of used catalyst coming from the first stripping means 8 and a second separation enclosure 16 for the mixture of stripped catalyst from the effluents of the second stripping connected to the outlet of said tubular column 13 and to said regenerator 21.

The tubular column generally has a length to diameter ratio L / D of between 10 and 300 and advantageously between 25 and 100.

The diameter of the tube is determined so that there is a gas velocity sufficient to entrain the grains of catalyst, this taking into account the total gas flow rate necessary for carrying out the second stripping.

The non-limiting apparatus for implementing the method could be that of FIG. 1 which illustrates the device according to the invention with an ascending cracking enclosure (riser) and an ascending stripping column.

This equipment includes:
a vertical tubular enclosure 1 supplied at its base with hot regenerated catalyst by means 2, themselves supplied by line 27 from the regenerator 21, with fluidization vapor and pre-training of the catalyst by line 3 and with hydrocarbon feedstock by line 4.
a separation enclosure 5 situated at the upper end of the tubular enclosure 1 and concentric therewith and in which the separation of the cracking effluents and of a first stripping described below takes place , used catalyst thanks to an inertial separator 6 placed just above the riser outlet and to one or more levels of cyclones 7 located in the upper part of the enclosure 5, and on the other hand, a first stripping at the vapor of said used catalyst in the lower part 8 of the enclosure 5. The stripping vapor is supplied by the line 8a and the lower part 8 of the enclosure 5 is internally provided with baffles 9 which favor the flow against stream of spent catalyst and said stripping vapor. The hydrocarbon vapors leave the unit via line 10 and are directed to the fractionation tower.
a connecting line 11 between the separation and first stripping enclosure 5.8 and the lower end of a column 13 where the second stripping takes place. This connection line is equipped with a valve 12 for controlling the flow rate of catalyst which is controlled by the level of catalyst in enclosure 5.
a vertical elongated tubular column 13 where the second stripping of the used catalyst takes place in a driven bed. This column is supplied at its base, co-current with the flow, with catalyst used by the line 11, with regenerated catalyst hot by the supply means 14 and with gas containing at least steam and molecular oxygen via line 15. Known type injectors are regularly placed around the column.
an enclosure 16 for separation at the outlet of the column 13 and concentric therewith, where the effluents from the second stripping of the catalyst mixture are separated. The separation is carried out by means of an inertial separator 17 in the immediate vicinity of the column outlet 13 and of one or more levels of cyclones 18 separating the stripped particles from the effluents. These particles are discharged towards the base of the enclosure 16 while the effluents from the second stripping leave the enclosure 16 via line 19 and are directed either to the fractionation tower or to any other point of use justified by the quality of the effluents. The stripped catalyst is sent to the regenerator 21 via line 20 which communicates with the base of the enclosure 16.
a regeneration unit 21 preferably located concentrically with the column 13 and supplied with used catalyst by the line 20 and with gas containing molecular oxygen by the distributor 22, located at the bottom of the unit, itself being supplied by line 23.

The regeneration effluents are dedusted by the cyclone level (s) 24 advantageously arranged outside the unit 21 and leave the installation by line 25. The regenerated catalyst is extracted from the regenerator by line 26 which feeds thanks to supply means 14 for the second stripping column. Line 26, of reduced diameter, is equipped with a valve 28 for adjusting the catalyst flow rate controlled by the temperature prevailing in the second stripping column 13. Means for recycling 27 of the regenerated catalyst connected to the regenerator 21 also supply the cracking enclosure 1 thanks to the supply means 2 and to a valve 29 for adjusting the catalyst flow rate controlled at the desired temperature at the upper end of the cracking enclosure 1.

Figure 2 illustrates the case where the tubular cracking enclosure operates in dropper. There are all the elements of Figure 1, excluding the riser 1 and the inertial separator 6 which have given way to the dropper 30.

The case where only one regeneration unit is used has been illustrated. In the case where the device according to the invention comprises two of them, whether they are downflow or upward flow, the connection line 20 ensures the transfer of the stripped catalytic particles from the second separation enclosure 16 to the first regenerator where s' performs the first regeneration. According to a known scheme described for example in patent application FR 84-18706, the catalyst is then completely regenerated in a second regenerator.

The regenerated catalyst could come from one or other of the regenerators as it was said above, the transfer line 26 can be connected either to the first or to the second regenerator to supply the stripping column 13 while the line 27 leaves the second regenerator to supply the tubular cracking enclosure 1.

Claims (15)

1.- Process for catalytic cracking in a fluid bed of a hydrocarbon charge boiling above about 400 ° C., this process comprising a step of bringing into contact with ascending or descending flow in an elongated tubular zone, under conditions cracking, said charge and the particles of a cracking catalyst, a step of separating the used catalyst and the cracked charge in a first separation zone at the outlet of said tubular zone, a first step of stripping the used catalyst using a first stripping gas injected against the current of this catalyst, a first step of separating the spent catalyst mixture and the effluents resulting from the first stripping step, a second co-current stripping step delivering second stripping effluents, a step of regenerating said spent catalyst in at least one regeneration zone to form said hot regenerated catalyst, under conditions of combustion of deposited coke thereon and a step of recycling at least part of the regenerated catalyst to the supply of said tubular zone, the method being characterized in that, after having carried out said first stripping step and said first separation step and before proceeding to the regeneration step of said catalyst, the second stripping step is carried out in a co-current and in a fluid bed, in a second stripping zone which is an elongated tubular zone, by introducing and mixing, upstream of the tubular zone and advantageously in the immediate vicinity of the end corresponding to the entry into said second tubular zone, at least part of said regenerated catalyst and at a higher temperature with the used catalyst resulting from the first stage of stripping and a stream of a second pressurized gas containing at least one stripping gas and a gas containing molecular oxygen, and the effluents from said second stripping zone are separated from the catalyst mixture thus stripped in a second separation zone downstream of this second stripping zone, said second stripping effluents are recovered and the catalyst mixture thus stripped is sent to said regeneration zone. 2.- Method according to claim 1 wherein the mass of the regenerated catalyst represents approximately from 5 to 100% by mass of the total mass of said used catalyst, and advantageously from 10 to 50% by mass. 3.- Method according to claims 1 or 2 wherein said second gas stream contains about 1 to 10% by mass of oxygen and preferably 2 to 5% by mass of oxygen. 4.- Method according to one of claims 1 to 3 wherein said stripping gas is water vapor, nitrogen, hydrogen, light hydrocarbons alone or in mixture, injected in mass proportion included between about 70 to 99.9% of the flow rate of said gas stream. 5.- Method according to one of claims 1 to 4 wherein the flow rate of said gas stream is between about 1 and 20 m / s and preferably between 2 and 10 m / s. 6.- Method according to one of claims 1 to 5 wherein the temperature of said regenerated catalyst is higher than that of said spent catalyst by at least about 90 ° C and at most about 400 ° C, without exceeding 900 ° C. 7.- Method according to one of claims 1 to 6 wherein the regenerated catalyst is removed from said regenerator zone after having separated at least part of the combustion fumes, preferably by stripping. 8.- Method according to one of claims 1 to 7 wherein said catalyst regenerated during the regeneration of the spent catalyst is taken from the first regeneration zone when the regeneration step comprises two. 9.- Method according to one of claims 1 to 7 wherein said catalyst regenerated during the regeneration of the used catalyst is removed from the second regeneration zone when the regeneration step comprises two. 10.- Cracking device in a fluid bed of hydrocarbon charges comprising an elongated tubular enclosure (1), means for pressure injection (4) of said charge arranged at one end of said enclosure, supply means ( 2) in particles of a cracking catalyst towards said end of said enclosure, a separation enclosure (5) disposed at the other end of the enclosure (1) and concentric therewith and comprising first separation means (6) effluent from spent catalyst particles, a first stripping means (8) of said spent catalyst by a first stripping gas in said separation chamber, at least one regeneration unit (21) of said spent catalyst by combustion of coke deposited thereon, and means for recycling (27) at least part of the regenerated catalyst to said supply means (2), said device being characterized in that it comprises an elongated tubular stripping column ( 13) d arranged between said first stripping means (8) and said regeneration unit (21), means for supplying under pressure (15) a second gas comprising at least one stripping gas and a gas containing molecular oxygen, at the inlet of said stripping column (13), means for supplying (14) at least partially regenerated catalyst particles connected on the one hand to the regeneration unit (21) and on the other hand to the inlet of said stripping column (13), means for supplying (11) spent catalyst connected to the first stripping means (8) and to the stripping column (13), preferably at its inlet, said means feed (11, 14, 15) being adapted to inject the second gas and the particles of regenerated catalyst into said stripping column, under pressure, in a fluid bed and co-current with the flow of spent catalyst particles from the first stripping means, and a second separation enclosure (16) of the catalyst mixture stripped from the effluents of the second stripping connected to the outlet of said tubular column (13) and to said regeneration unit (21). 11.- Device according to claim 10, wherein the tubular column has a length to diameter ratio between 10 and 300. 12.- Device according to claims 10 or 11, wherein the supply means (14, 26) of regenerated catalyst are connected to a first regeneration unit when the device comprises two. 13.- Device according to claims 10 or 11, wherein said means for supplying regenerated catalyst are connected to a second regeneration unit when the device comprises two. 14.- Device according to one of claims 10 to 13, wherein said second separation enclosure (16) comprises an inertial separator (17) located in the vicinity of the end of said stripping column (13) and at least a cyclone 19 connected to means for discharging (19) the effluents from said separation enclosure. 15.- Device according to one of claims 10 to 14 wherein said stripping column (13) is ascending.

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