EP0184517B1 - Hydrocarbon feed catalytic cracking processes and apparatuses - Google Patents

Description

The present invention relates to catalytic cracking of hydrocarbon charges. It relates more particularly to improvements made to the regeneration of the spent catalyst of such a process, with a view to the use of "load elevators" shorter than those of the prior art.

It is known that the petroleum industry customarily uses cracking methods, in which molecules of high molecular weight and high boiling point hydrocarbons are split into smaller molecules, which can boil in higher temperature ranges. weak, suitable for the intended use.

The most commonly used process for this purpose, at present, is the so-called fluid catalytic cracking process (in English, Fluid Catalytic Cracking, or FCC process). In this type of process, the hydrocarbon charge is simultaneously vaporized and brought into contact at high temperature with a cracking catalyst, which is kept in suspension in the vapors of the charge. After the desired molecular weight range has been reached by cracking, with a corresponding lowering of the boiling points, the catalyst is separated from the products obtained.

In processes of this type, the desired reduction in boiling points results from controlled catalytic and thermal reactions. These reactions take place almost instantaneously when the finely atomized charge is brought into contact with the catalyst. However, it deactivates rapidly, during the short period of time in which it is in contact with the charge, and this is mainly due to an adsorption of hydrocarbons and a deposit of coke on its active sites. It is necessary to continuously strip the spent catalyst, for example with steam, to recover the adsorbed hydrocarbons, and to reactivate it, also continuously, without altering its characteristics, by carrying out a controlled combustion of coke, in a section of regeneration on one or more stages, before recycling the catalyst to the reaction zone.

In practice, the catalyst of the FCC process and the charge to be treated are injected under pressure and at a high temperature at the base of a column known as "charge elevator", which technicians often designate by the English term "riser". . At the top of the column is generally arranged a tank concentric with the elevator. In this tank and above the elevator is housed a ballistic separation system, such as a cyclone, in which the spent catalyst is separated from the cracked charge. This is evacuated at the top of said tank, after passing through cyclones, to reduce the entrainment of dust, while the recovered catalyst particles encounter a stripping gas such as water vapor, injected for example annularly at the base of said tank, before being evacuated to a regenerator. Combustion air is injected, for example annularly, at the base of the regenerator, while at the top of the latter are provided cyclones making it possible to separate the combustion gas from the particles of regenerated catalyst. This is evacuated to the bottom of the regenerator and recycled to the base of the elevator or "riser", where the charge is usually injected at a temperature between 80 ° C and 400 ° C and under a pressure ranging from 0 , 7.10 ⁵ to 3.5.10 ⁵ Relative Pascals.

• - le préchauffage de la charge liquide;
• - la vaporisation de cette charge;
• - l'apport de calories exigé par les réactions impliquées, lesquelles globalement sont endothermiques;
• - les pertes de chaleur de l'unité.

The FCC process is naturally implemented so that the cracking unit is in thermal equilibrium. In other words, the supply of regenerated hot catalyst must be such that it can meet the various thermal requirements of the reaction section, namely, in particular:

• - preheating the liquid charge;
• - the vaporization of this charge;
• - the intake of calories required by the reactions involved, which overall are endothermic;
• - heat losses from the unit.

The amount of coke present on the catalyst at the entrance to the regeneration zone as well as the regeneration mode will determine the final temperature reached in the regeneration zone, since the calories from the combustion of the coke serve both, heat losses, to heat the regeneration fluid (air and / or oxygen) and are shared between the combustion gases and the catalyst particles. Under operating conditions, the quantity of coke produced in the cracking unit will therefore be substantially constant, if the thermal equilibrium is not modified by external constraints.

This quantity of coke is linked to the difference Delta coke between the quantities of coke present on the catalyst at the entry of the regeneration zone and at the exit of this zone by the following relation:

Coke produced = Δcoke x C / 0, where C / 0 denotes the mass ratio of the catalyst and the charge brought into contact with it at the inlet of the reaction zone.

où _TI désigne l'efficacité de l'échange de la chaleur de combustion avec le catalyseur, AH la chaleur de combustion du coke et Cp la chaleur spécifique du catalyseur.Furthermore, the difference between the regeneration temperature, T _r gg _generation . and the temperature at the outlet of the reaction section, T _reactor , is given by the following relation:

where _TI denotes the efficiency of the exchange of combustion heat with the catalyst, AH the heat of combustion of coke and Cp the specific heat of the catalyst.

It is apparent that, if the coke grows, it is necessary, in order to keep the amount of coke produced constant, to reduce the flow rate of the catalyst in circulation. Furthermore, the increase in Δ coke corresponds to a higher regeneration temperature of the catalyst, which may be desirable for vaporizing heavy loads.

Thus, the control of A coke, in a modern cracking unit by the FCC process, where the temperature of regeneration is not limited, appears as one of the fundamental basic variables of the process. WO-A-82/04061 thus describes an FCC process comprising, compared to the conventional process, an additional stripping step by desorption of the catalyst, comprised between the usual stripping step and the regeneration step.

During this stripping, the desorption is carried out using gases originating from the regenerator and whose temperature is at least 100 ° F (38 ° C) higher than that of the catalyst to be desorbed.

The desorbed catalyst is then introduced above the level of the fluidized bed into the first stage of the regenerator.

This additional stripping step makes it possible to separate some of the metals deposited on the catalyst during the reaction and, the gases being hot enough to desorb hydrocarbons with high boiling point, the residual water vapor and hydrogen will necessarily be desorbed .

In the device described, the gas coming from the regenerator is introduced against the current into the stripping device by desorption, while the catalyst is brought to the degenerator by gravity.

Currently, with increasingly severe operating conditions of the FCC process, corresponding to increasingly heavy loads and therefore at high boiling point, there is an increased deposition of coke on the catalyst. To a certain extent this is useful, as this results in a higher temperature of the catalyst at the entrance to the reaction zone, which allows a more complete vaporization of the charge, a controlled thermal cracking of the asphaltenes and an energy higher catalyst activation. However, it is desirable to be able to control and limit the regeneration temperature of the catalyst, in order to preserve its thermal stability and to reduce the harmful effect of certain constituents present such as the stripping vapor entrained and the untreated residues. In addition, it is sometimes desirable to increase the ratio 8 defined above, that is to say the mass ratio of the catalyst in contact with the load at the inlet of the riser "riser", in order to improve the contact of the feed and the catalyst and increase the conversion of the feed, by bringing the latter in the presence of a greater number of active sites of the catalyst.

The Applicant has established that effective desorption of the products entrained by the spent catalyst grains, prior to their regeneration, contributes to obtaining these results.

The stripping of the used catalyst, already used in the conventional technique of FCC processes, aims to displace by a gas, usually steam, the hydrocarbons entrained in the voids separating the catalyst grains and, to a certain extent, the lighter hydrocarbons adsorbed on the surface in the pores of the catalyst. It is known, in fact, that a poorly stripped catalyst before its regeneration has a higher Acoke and a hydrogen concentration on the deposited coke greater than 7% by weight.

It is further known that stripping is better at high temperatures.

In order to obtain a better regeneration of the catalyst, the present invention provides for carrying out, after the conventional stripping of the catalyst, a desorption of the products entrained by the spent catalyst at a temperature at least 25 ° C higher than that of the particles of catalyst having just undergone a stripping. This desorption will advantageously be carried out by injection of combustion gases coming from the regenerator (s) in the current of the catalyst flow. This injection also makes it possible to bring this catalyst to the height required for feeding the regenerator or regenerators, which makes it possible to use a shorter load lifter than in the prior art.

The subject of the present invention is therefore, in a process for the catalytic cracking in the fluid state of a hydrocarbon charge, comprising a phase of contacting in an upward flow in an elevator, under cracking conditions, of said said charge and particles of a cracked catalyst, a phase of separation of the spent catalyst and the cracked charge, downstream of the upper end of said riser, a phase of stripping of the spent catalyst using an injected gas against the flow of this catalyst, a phase of regeneration of said catalyst under conditions of combustion of the coke deposited on it, and a phase of recycling of the regenerated catalyst to the supply of said elevator, the improvement consisting in that, after having undergone said stripping and before being subjected to said regeneration, said catalyst is subjected to a desorption by a gas injected cocurrently with the catalyst at a temperature at least 25 ° C higher than the particle temperature s of catalyst having just undergone said stripping phase, and the resulting mixture is injected into the fluidized part of the regeneration zone which is located above the dense fluidized bed.

The gas used for the desorption phase may be an inert gas or water vapor, but, in a preferred embodiment of the invention, use will be made of the gases originating from the regeneration of the catalyst, which have l advantage of being at a higher temperature than the catalyst to be regenerated, either alone or in mixture with steam.

• - l'élévateur est plus court que dans la technique connue;
• - la vapeur d'eau issue de la zone de strippage et celle formée à partir de l'hydrogène contenu dans le coke ainsi éliminé seront séparées facilement dès le début de la phase de régénération en nt fluidisé; elle sera donc d'autant moins susceptible d'affecter la réactivité du catalyseur;
• - une partie non négligeable des métaux tels que le nickel et le vanadium, qui sont déposés à la surface du catalyseur, est susceptible d'être séparée sous forme de composés volatils éliminables; la durée de vie du catalyseur sera donc améliorée;
• - le gaz de cette phase de désorption servira de gaz élévateur pour le catalyseur provenant du strippage, en vue d'acheminer les grains du catalyseur jusqu'au régénérateur; on exposera plus en détail dans la suite de la présente description l'intérêt de cette fonction auxiliaire du gaz ainsi injecté.

From this co-current and higher temperature desorption phase result a number of advantages, among which:

• - the elevator is shorter than in the known technique;
• - The water vapor from the stripping zone and that formed from the hydrogen contained in the coke thus removed will be easily separated from the start of the regeneration phase in fluidized nt; it will therefore be all the less likely to affect the reactivity of the catalyst;
• - a non-negligible part of the metals such as nickel and vanadium, which are deposited on the surface of the catalyst, is capable of being separated in the form of volatile compounds which can be removed; the service life of the catalyst will therefore be improved;
• - The gas from this desorption phase will serve as a raising gas for the catalyst coming from the stripping, in order to convey the grains of the catalyst to the regenerator; we will explain in more detail below this description describes the advantage of this auxiliary function of the gas thus injected.

The invention also relates to a catalytic cracking device in the fluid state of hydrocarbon charges, comprising a riser type column, means arranged at the base of said elevator for supplying the latter under pressure with a hydrocarbon charge and particles of a cracking catalyst, a means of stripping by a gas of spent catalyst particles in a chamber disposed at the top of said elevator, concentrically thereto, this stripping gas being injected into this chamber countercurrent with particles of spent catalyst, at least one unit for regenerating said catalyst by combustion of the coke deposited thereon, and means for recycling the regenerated catalyst to said supply means, said device being characterized in that it comprises, between said means of stripping and said regeneration unit, a means of desorption by a second gas of the products entrained by the particles of the catalyst, this means of desorption being such that ue the second gas is injected under pressure into the flow of co-current catalyst particles thereof, and that the resulting mixture of spent catalyst and gas is injected into the fluidized part of the regeneration zone which is located above the dense fluidized bed.

Said desorption means will advantageously be placed in the device at a level lower than that of said regeneration unit, said gas thus injected co-current then also serving as carrier gas for said particles, which allows the use of an elevator short.

The desorption gas may or may not be identical to the stripping gas. Advantageously, it will be constituted at least in part by the gases coming from the regeneration unit.

The use of a desorption means provides the catalyst particles with additional driving pressure, which makes it possible to diversify the positioning of the different units of the cracking device, in particular the elevator and the regenerator. It also improves the qualities of the stripping, in particular if the gases coming from the regeneration unit are used as injection gas, which have a higher temperature of about 25 and if possible about 100 ° C. that at which stripping is carried out, which makes it possible to appreciably reduce the Acoke and to limit the latter to the reaction Acoke, with the consequence of a less release of heat on regeneration and a less degraded and more stable catalyst. We can thus reduce the length of the elevator and / or process heavier loads. Finally, it is possible to better control and regularize the supply of spent catalyst to the regenerator, with the advantage of being able to control its temperature by limiting hot spots, which preserves the stability of the catalyst, and thus to obtain a better regenerated catalyst and therefore more active.

The possibility of reducing, thanks to the invention, the length of the elevator must be emphasized. It makes it possible, on the one hand, to obtain better selectivity in cracked products of the gasoline type and light distillates, on the other hand, to raise the temperature of the elevator without increased production of gas and even with a reduction in Acoke. A better conversion of the charge is therefore obtained, with a better octane number of the resulting products and it is possible to process heavier charges which are more difficult to crack. A reduced height lift also lends itself to ultra-short residence times of the load.

As will be described below in more detail, the invention applies equally to cracking assemblies comprising two regeneration units in series as to those comprising a single regeneration unit.

• - la figure 1 est un schéma d'un ensemble conventionnel de craquage catalytique en lit fluide.
• - la figure 2 est un schéma analogue d'un ensemble conforme à l'invention comprenant une unité de régénération du catalyseur à un seul étage.
• - les figures 3 et 4 sont des schémas de deux ensembles conformes à l'invention comprenant chacun une unité de régénération du catalyseur à deux étages.

The accompanying drawings illustrate, in schematic form, a known device for catalytic cracking in the fluid state and various embodiments of the invention. In these drawings:

• - Figure 1 is a diagram of a conventional set of catalytic cracking in a fluid bed.
• - Figure 2 is a similar diagram of an assembly according to the invention comprising a catalyst regeneration unit with a single stage.
• - Figures 3 and 4 are diagrams of two assemblies according to the invention each comprising a catalyst regeneration unit with two stages.

The device for cracking by the FCC process shown in FIG. 1 is of a type known per se. It essentially comprises a column 1 known as a charge riser, or else a "riser", supplied at its base, by line 2, with the charge to be treated and, via line 3, with particles of a cracking catalyst.

Column 1 opens at its apex in an enclosure 4 which is concentric with it and in which, on the one hand, the separation of the cracked charge takes place and, on the other hand, the stripping of the spent catalyst. The treated charge is separated in a cyclone 5, which is housed in the enclosure 4, at the top of which a discharge line 6 of the cracked charge is provided, while the spent catalyst particles are discharged at the base of the enclosure 4. A line 7 supplies stripping gas, generally water vapor, to injectors 8 regularly arranged at the base of enclosure 4. Stripping is therefore preferably carried out in a dense medium against the current of the catalyst.

The spent catalyst particles thus stripped are evacuated at the base of the enclosure 4 to a regenerator 9 via a conduit 10, on which is provided a control valve 11. In the regenerator 9, the coke deposited on the particles of the catalyst are burnt using air, injected at the base of the regenerator by a line 12, which feeds injectors 13 regularly spaced. The particles of the treated catalyst entrained by the combustion gas are separated by cyclones 14, from which the combustion gas is evacuated by a line 15, while the particles of catalyst are discharged towards the base of the regenerator 9, from where they are recycled through line 3, fitted with a control valve 16, to the supply of elevator 1.

• - hauteur de la partie réactionnelle de l'élévateur 1 : 5 à 40 mètres,
• - température de la charge à craquer: 75 à 450° C,
• - débit d'alimentation de l'élévateur 1 en charge à traiter: 1000 à 10 000 tonnes par jour,
• - débit d'alimentation de l'élévateur 1 en catalyseur: 3 à 50 tonnes par minute,
• - temps de séjour de la charge dans l'élévateur 1 : 0,1 à 10 secondes,
• - température de régénération du catalyseur: 650 à 900° C,
• - temps de séjour du catalyseur dans le régénérateur 9 : 5 à 20 mn.

The dimensional and operational characteristics of such a device are usually the following:

• - height of the reaction part of the elevator 1: 5 to 40 meters,
• - temperature of the charge to be cracked: 75 to 450 ° C,
• - feed rate of the elevator 1 in charge to be treated: 1000 to 10 000 tonnes per day,
• - feed rate of elevator 1 in catalyst: 3 to 50 tonnes per minute,
• - residence time of the load in the elevator 1: 0.1 to 10 seconds,
• - catalyst regeneration temperature: 650 to 900 ° C,
• - residence time of the catalyst in the regenerator 9: 5 to 20 min.

FIG. 2 represents a device according to the invention, in which the members already described in relation to FIG. 1 are designated by the same reference numbers assigned to the index '.

In this device, the conduit 10 ', through which the spent catalyst particles are discharged from the enclosure 4' disposed at the upper end of the elevator 1 ', does not open directly into the regenerator 9', but present here a vertical portion 101, communicating through a portion 102 with the regenerator 9 ', and connected by an elbow to the lower end of the conduit 10'.

The base of the duct 101 is supplied with a desorption gas by a line 18. In the case of the drawing, this desorption gas can be constituted by a mixture of steam, brought to the line 18 by the line 19, and of effluent gas from the regenerator 9 ′, derived from line 15 ′ to line 18 by line 20, equipped with the pump 21. It will be noted that the desorption of the catalyst particles is carried out co-current in the vertical part 101 , that the resulting mixture of spent catalyst and gas is injected into the fluidized part of the regeneration zone which is located above the dense fluidized bed, thus allowing good separation of the gases and the grains of catalyst, and that the gas of desorption acts as a carrier gas to raise the particles to the regenerator.

It is thus possible to considerably reduce the residence time of the load to be treated in the elevator 1 and, consequently, the height of the latter. Under these conditions, the enclosure 4 ′ will not be at a sufficient height so that the spent catalyst particles can, by simple gravity, feed the regenerator 9 and, after regeneration, be recycled to the supply of the elevator 1 ′ . The desorption gas, injected at a temperature at least 25 ° C higher than that of the catalyst, enters via line 18 in the section of conduit 101 and therefore advantageously exerts a suitable desorption and a thrust on the particles of spent catalyst for route them to the regenerator.

• - hauteur de la partie réactionnelle de l'élévateur 1': 1 à 30 mètres,
• - temps de séjour de la charge dans l'élévateur 1': 0,05 à 5 secondes,
• - température de régénération du catalyseur: 650 à 900° C,
• - temps de séjour du catalyseur dans le régénérateur 9': 1 à 15 mn,
• - température du catalyseur à la sortie de l'einceinte 5': 480 à 580° C,
• - température du gaz de désorption à l'entrée du conduit 101 : 500 à 900° C.

With the additional system according to the invention, the dimensional and operational characteristics of the cracking device can be modified as follows:

• - height of the reaction part of the elevator 1 ': 1 to 30 meters,
• - residence time of the load in the elevator 1 ': 0.05 to 5 seconds,
• - catalyst regeneration temperature: 650 to 900 ° C,
• - residence time of the catalyst in the regenerator 9 ': 1 to 15 min,
• - catalyst temperature at the outlet of the 5 ′ enclosure: 480 to 580 ° C,
• - temperature of the desorption gas at the inlet of the duct 101: 500 to 900 ° C.

Figures 3 and 4 illustrate two other forms of implementation of the catalytic cracking method according to the invention, in which a two-stage regeneration enclosure is used. In these figures, the members already described in relation to Figures 1 and 2 are designated by the same reference numbers, assigned the indices a and b, respectively.

In the case of FIG. 3, the regenerator 9a is in an upward flow and comprises two stages 91a and 91b. The spent catalyst which has already undergone stripping in the enclosure 4a is conveyed by line 10a, the vertical section of pipe 101a and the horizontal section 102c to the lower stage 91a of the regenerator. A desorption gas is injected under pressure through line 18a at the bottom of the vertical section 101a; this desorption gas comprises a mixture of combustion gases coming from the regeneration enclosure, supplied by line 20a, and optionally steam, supplied by line 19a.

The base of the first combustion stage 91a is supplied with air by the line 12a and the air is distributed by regularly spaced injectors 13a. In this stage, cyclones 14a separate the combustion gas from the partially regenerated catalyst particles. The combustion gas is conveyed to line 20a by a line 115, fitted with a valve 116, making it possible to divert part of the gas flow to a line 117.

The particles of the catalyst having undergone a first regeneration treatment are then transferred to the second stage 91b of the regenerator by the central duct 110a, supplied with air by the line 11a.

The base of the stage 91b is also supplied with air by the line 112a and by the injectors 113a. The particles of the regenerated catalyst are discharged laterally in a buffer enclosure 118a and are recycled through the conduit 3a to the supply of the elevator 1a. The combustion gases discharged to the upper part of stage 91b are treated in an external cyclone 119a, at the base of which the particles of the catalyst are returned by the conduit 120a to stage 91a, while the combustion gases are discharged by lines 121a and 20a to line 18a. A safety valve 122a is provided on line 121a and a valve 123a makes it possible to divert part of the gas to a line 124a.

• - double régénération du catalyseur, permettant une combustion intégrale du coke sans altération des propriétés catalytiques,
• - aucune limitation de la température du second régénérateur, ce qui permet au catalyseur d'acquérir la température requise pour vaporiser et craquer la charge.

This embodiment of the device according to the invention, equipped with a two-stage regenerator with upward flow, has the following advantages:

• - double regeneration of the catalyst, allowing complete combustion of the coke without altering the catalytic properties,
• - no limitation of the temperature of the second regenerator, which allows the catalyst to acquire the temperature required to vaporize and crack the charge.

The embodiment of FIG. 4 also includes a two-stage regenerator 9b 92a and 92b with downward flow.

The catalyst already stripped in the enclosure 4b is conveyed by the conduit 10b, the vertical section 101b and the horizontal section 102b to the upper floor 92b. Desorption gas is injected under pressure into the lower part of the vertical duct 101b via a line 18b. This stripping gas can consist of a mixture of water vapor, supplied by line 19b, and combustion gas, coming from regenerator 9b by line 20b.

Air is injected at the base of stage 92b through line 112b and injectors 113b. Cyclones 14b separate the suspended particles from the combustion gas, which is evacuated by a line 121b, on which a valve 123b makes it possible to divert part of the combustion gas to a line 124b.

The particles treated in the first stage 92b are conveyed by gravity through the conduit 125 to the lower stage 92a of the regenerator, at the base of which air is injected via the line 112b and the injectors 113b. The combustion gas is evacuated to an external cyclone 119b, from where the catalyst particles are returned by the conduit 120b to stage 92a, while the gas is evacuated by line 115b to line 20b. A valve 116b makes it possible to divert part of the gas to an auxiliary line 117b.

The regenerated catalyst is evacuated from the base of stage 92a via line 3b and recycled to the supply of elevator 1b.

• - les deux étages de régénération se font à contre-courant,
• - les cyclones sont externes et leurs supports moins élevés,
• - le régénérateur fonctionnant à la température la plus élevée, et par conséquent le plus lourd, se trouve au niveau inférieur, ce qui simplifie la construction de l'unité.

This embodiment of the cracking device according to the invention, which comprises a two-stage regenerator with downward flow, has, in addition to the advantages described above, the following advantages:

• - the two regeneration stages are carried out against the current,
• - cyclones are external and their supports are lower,
• - the regenerator operating at the highest temperature, and therefore the heaviest, is on the lower level, which simplifies the construction of the unit.

The example which follows is intended to illustrate the invention and is not intended to be limiting.

Example

Two catalytic cracking tests were carried out using the same charge of hydrocarbons in a unit with two regenerators of the type described in FIG. 3. Unlike the first test, the second test was carried out using 'A device comprising a co-current desorption phase according to the present invention with a short elevator.

Type of charge:

Operating conditions:

• - réduction des gaz secs,
• - réduction de l'addition de catalyseur frais de 26 %,
• - augmentation de la conversion,
• - augmentation des rendements en liquides, comme le montre le tableau suivant:
  

It is found that the joint use of a short riser and a co-current desorption with regeneration gas at 595 ° C. leads to a reduction in the delta coke, causing a drop in the regeneration temperature. This allows an increase in the circulation of catalyst (C / O of 5.6) and better stability of the catalyst. The most significant changes to the cracking reactions are:

• - reduction of dry gases,
• - reduction in the addition of fresh catalyst by 26%,
• - increased conversion,
• - increase in liquid yields, as shown in the following table:
  

Claims (9)

1. In a process for the fluid catalytic cracking of a hydrocarbon charge, comprising a stage for bringing said charge and particles of a cracking catalyst into contact in ascending flow in a riser under cracking conditions, a stage for separating the spent catalyst and the cracked charge downstream of the upper end of said riser, a stage for stripping the spent catalyst by means of a gas injected countercurrently to this catalyst, a stage for regenerating said catalyst under combustion conditions of the coke deposited thereon, and a stage for recycling the catalyst regenerated at the feed to said riser, the improvement lying in the fact that, after said catalyst has undergone said stripping and before being subjected to said regeneration, it is subjected to a desorption stage by a gas injected co-currently to the catalyst at a temperature at least 25°C higher than the temperature of the catalyst particles which have just undergone said stripping stage, and in that the resultant mixture of spent catalyst and gas is injected into the fluidised portion of the regeneration zone situated above the dense fluidised bed.

2. A process according to claim 1, characterised in that said desorption gas is composed at least partly of gases originating from the regeneration stage of the catalyst.

3. A process according to either of claims 1 and 2, characterised in that said desorption gas is at a high enough pressure to act as an elevator gas for the purpose of raising the catalyst towards the regenerator or regenerators.

4. An apparatus for the fluid catalytic cracking of hydrocarbon charges, comprising a column (1') of the riser type, means arranged at the base of said riser to feed the latter under pressure with a hydrocarbon charge and particles of a cracking catalyst, a means (7', 8') for stripping with a gas particles of spent catalyst in a vessel (4') disposed at the top of said riser, concentrically thereto, the stripping gas being injected into this vessel countercurrently to the particles of spent catalyst, at least one unit (9') for regenerating said catalyst by combustion of the coke deposited thereon, and means (3') for recycling the regenerated catalyst to said feed means, said apparatus being characterised in that it comprises, between said stripping means and said regeneration unit, a means (13') for the desorption by a second gas of said catalyst particles, this desorption means being such that the second stripping gas is injected under pressure into the flow of catalyst particles co-currently thereto, and in that the resultant mixture of catalyst and gas is injected into the portion of the regeneration zone situated above the dense fluidised bed.

5. An apparatus according to claim 4, in which said regeneration unit (9') comprises a single regeneration stage, characterised in that said desorption gas is injected upstream of said unit into a vertical section (101) of a duct for feeding spent catalyst to said regeneration unit.

6. An apparatus according to claim 5, characterised in that it comprises a line (18) for feeding second stripping gas connected to a line (15') for the removal of combustion gases from the regeneration unit (9').

7. An apparatus according to claim 4, in which said regeneration unit (9a, 9b) comprises two regeneration stages with ascending flow (91a, 91 b) or descending flow (92a, 92b), characterised in that the desorption gas is injected upstream of the upstream stage (91a, 91b) of said unit, into a vertical section (101a, 101b) of the duct for feeding spent catalyst to said regeneration unit.

8. An apparatus according to claim 7, characterised in that it comprises a line (18a, 18b) for feeding desorption gas, connected to a line (20a, 20b) for the removal of combustion gases from the upstream stage (91a, 92b) and/or downstream stage (91b, 92a) of said regeneration unit.

9. An apparatus according to any one of claims 4 to 8, characterised in that it comprises a riser (1'), the reactive part of which is of a height of between 1 and 30 metres.

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