EP0208609B2 - Process and apparatus for the catalytic cracking of hydrocarbons, with control of the reaction temperature - Google Patents

Description

The present invention relates to catalytic cracking of hydrocarbon charges in a fluidized bed. It relates more particularly to a process and a device for such catalytic cracking, with regulation of the reaction temperature by quenching the reaction products.

It is known that, in the so-called catalytic cracking processes (in English Fluid Catalytic Cracking, or FCC process), the hydrocarbon charge is completely vaporized by contacting at high temperature with an appropriate cracking catalyst kept in suspension. After the desired molecular weight range has been reached by cracking, with a corresponding lowering of the boiling points, the catalyst is rapidly separated from the products obtained.

In practice, the catalyst of the FCC process and the feedstock to be treated are brought into contact at the base of a reactor made up of a column known as a "charge lift", which techniques often designate by the English term "riser". . At the top of the column is an enclosure concentric with the elevator. In the upper part of this enclosure and above the elevator is housed a ballistic separation system, to recover the catalyst entrained with the vapors. Cyclonic systems are generally used. The charge is discharged at the top of said enclosure, while the catalyst particles deactivated by coke deposition are collected at the base of the enclosure and meet there a stripping fluid such as water vapor, injected at the base from said enclosure, before being evacuated to a regenerator, in order to restore their activity by burning coke. Combustion air is injected 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 entrained particles of the regenerated catalyst. This is evacuated at 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 , 78105 to 3.5 • 10 ⁵ relative Pascals.

• - la vaporisation de la charge.
• - l'apport de chaleur 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, all the necessary heat being provided by the combustion of the coke deposited during the cracking reaction on the grains of the catalyst. In other words, the supply of regenerated hot catalyst and the injection temperature of the charge to be cracked must be such that the unit can meet the various thermal requirements of the reaction section, namely, in particular:

• - vaporization of the charge.
• - the heat input required by the reactions involved, which, overall, are endothermic.
• - heat losses from the unit.

The pair “charge preheating temperature / regenerated catalyst circulation” is therefore adjusted so as to determine the reaction temperature in any reaction zone, and in particular at the outlet of the riser.

As a variant, the catalyst can be introduced with the charge at the top of an essentially descending or “dropper” reaction column, at the base of which the spent catalyst is recovered, stripped sent for regeneration.

US-A-3,896,024 describes a process for catalytic cracking of a charge, the boiling point of which is between 650 and 720 _° F (343 and 382 _° C). The charge is brought into contact with the catalyst in a rising flow column where the temperature of the mixture can reach at most 1000 FF (538 ° C.). In this mixing zone, the charge is thermally cracked into fractions of lower boiling points. Downstream of this zone, a charge of preheated gas oil is injected into the column to lower the temperature of the reaction mixture between 800 and 900 _°. Mp (426 and 482 _° C). The mixture obtained is then catalytically cracked under milder conditions in the upper part of the column.

The most recent developments in catalytic cracking have, however, only partially resolved the problem of injecting heavy petroleum charges into the catalytic cracking units.

We know, in fact, that the higher the cracked petroleum charge has a higher boiling point, the more it has high molecular weight and metal-rich compounds, which are coke precursors: these compounds, which include asphaltenes, have cyclic or polycyclic in nature and have the particularity of being difficult to vaporize under the usual injection conditions, and consequently of forming significant coke deposits when they are brought into contact with the catalyst grains, in view of the reaction catalytic cracking.

A first solution has therefore been proposed (US Pat. No. 4,427,537), according to which heavy petroleum fillers, in particular having significant quantities of aspaltenes, could be cracked by improving their spraying in the zone of injection of the catalyst, da so as to create, upon contact with the catalyst grains, a sufficient thermal shock to allow thermocracking of the high molecular weight molecules. The thermal cleavage of these molecules into smaller molecules allows the latter to be then catalytically recreated in the gas phase, when they are placed in the presence of the catalyst grains.

Petroleum charges, characterized in that at least 10% of their weight has a boiling point above 500 _° C., have thus been able to be treated in the catalytic cracking units, by virtue of an increase in the temperature in the zone of injecting the catalyst and of the charge to be cracked (or “mixing zone”) and of a good spraying of the charge in this zone. However, the heterogeneity of the temperatures, which results from such an injection, necessitated the development of reactors with short residence times, in order to avoid the occurrence of overcracking or of secondary reactions, such as reactions polymerization of cracked products, as well as to avoid the production of an excess of gases which are difficult to recover.

By continuing its work on the catalytic cracking of heavy petroleum charges, the Applicant has highlighted the fact that this first solution only partially solved the problem of cracking heavy charges which are particularly difficult to recover, for example those of which 10% by weight has a boiling point above 550 ° C, because the heat necessary for the thermocracking of heavy molecules became so great that the reaction rate, linked to the mixing temperature in the injection zone of the charge to be cracked, required time stay limited to only a few seconds, hence very difficult control of the reaction conditions with current means.

• - un choc thermique maximum est effectué à l'injection pendant une durée très courte mais suffisante pour permettre la conversion thermique des molécules à poids moléculaire élevé et notamment des asphaltènes présents dans la charge, et une meilleure vaporisation des molécules destinées à être craqués catalytiquement, puis.
• - la température de la réaction catalytique souhaitée et maintenue et reste indépendante de celle qui résulte de l'injection du catalyseur et de la charge craquer, par introduction, en aval de la zone d'injection de la charge, d'un fluide auxiliaire, dans des conditions telles de débit, de température et de pression, que l'introduction de ce fluide permette à la réaction de craquage catalytique de s'effectuer dans des conditions plus douces et indépendantes de la température de mélange.

However, it has been established by the Applicant that the cracking of heavy loads can be carried out in a simple and effective manner by ensuring appropriate regulation of the temperature in the reaction zone, by means of which:

• a maximum thermal shock is effected on injection for a very short time but sufficient to allow the thermal conversion of the molecules with high molecular weight and in particular of the asphaltenes present in the feed, and better vaporization of the molecules intended to be catalytically cracked, then.
• the temperature of the desired and maintained catalytic reaction and remains independent of that which results from the injection of the catalyst and of the cracked charge, by introduction, downstream of the charge injection zone, of an auxiliary fluid, under conditions such as flow rate, temperature and pressure, that the introduction of this fluid allows the catalytic cracking reaction to be carried out under milder conditions and independent of the mixing temperature.

• - d'une part, l'augmentation substantielle de la température de mélange dans la zone d'injection de la charge lourde sur le catalyseur permet une meilleure vaporisation de celle-ci et un craquage thermique instantané des composés les plus lourds, ce quidiminue la formation de coke sur les grains de catalyseurs, liée en particulier au fait que les composés à haut point d'ébullition, ou à caractère basique (composés aromatiques azotés, hydrocarbures aromatiques condensés etc.), ont tendance à s'adsorber et à neutraliser certains sites acides de catalyseur:
• - d'autres part, si le thermocraquage des molécules les plus lourdes est réalisé de façon quasi- instantanée à haute température, les composés plus légers, en provenance soit de la charge elle-même, soit de la réaction ne thermocraquage, doivent être craqués catalytiquement, par contact avec les sites actifs de grains de catalyseur, au cours d'une réaction plus lente, dont l'optimum de température n'est pas nécesairement celui qui résulte des conditions précédemment évoquées.

This regulation has a double advantage:

• on the one hand, the substantial increase in the mixing temperature in the injection zone of the heavy load on the catalyst allows better vaporization of the latter and instantaneous thermal cracking of the heaviest compounds, which reduces the formation of coke on the grains of catalysts, linked in particular to the fact that compounds with a high boiling point, or with a basic character (aromatic nitrogen compounds, condensed aromatic hydrocarbons etc.), tend to adsorb and neutralize certain acid catalyst sites:
• - on the other hand, if the heavier cracking of the heaviest molecules is carried out almost instantaneously at high temperature, the lighter compounds, coming either from the charge itself, or from the non-cracking reaction, must be cracked catalytically, by contact with the active sites of catalyst grains, during a slower reaction, the optimum temperature of which is not necessarily that which results from the conditions mentioned above.

The temperature regulation carried out according to the invention therefore makes it possible to control the duration of the contact between the molecules to be catalytically cracked and the catalyst, and thus to avoid excessive prolongation of the thermocracking generating coke and light gases, as well as the occurrence of side reactions linked to an unsuitable catalytic cracking temperature.

An object of the present invention is therefore, in a process for catalytic cracking of a charge of hydrocarbons in a fluidized bed, to bring the charge and the catalyst into contact at a high temperature capable of preventing deposits of coke on the catalyst and optimally promote catalytic cracking of this charge.

Another object of the invention is to control the contact time of the catalyst and of the charge at this elevated temperature, so as to decrease: the secondary reactions or to limit the thermocracking liable to occur.

The subject of the invention is therefore a process for the catalytic cracking of a heavy load of hydrocarbons, more than 10% by weight of which has a boiling point above 500 _° C., this process comprising a phase of bringing into contact in a diluted fluidized bed and with short contact time with ascending or descending flow in a column, under cracking conditions, of said charge and of particles of a cracking catalyst, a phase of separation of spent catalyst and of cracked charge downstream of the injection zone of said charge, at least one phase of stripping of the spent catalyst, then a phase of regeneration of said catalyst under conditions of combustion of the coke deposited on it and, finally, a phase of recycling of the catalyst regenerated at l feeding said column, this process being characterized in that injected into said column of regenerated catalyst under conditions such that the equilibrium temperature resulting from the mixture of the vaporized charge and the catalyst is at least greater than 500 _° C and at least 40 to 100 _° C above the final reaction temperature at the end of the column, and in that one injects downstream of this mixing zone, but upstream of the separation phase, an auxiliary fluid vaporizable under the conditions of the injection, so that the temperature of the mixture of the vaporized charge and the particles of the catalyst is lowered by about 10 to 70 _° C. relative to the t initial temperature which results from the contacting of the catalyst and the charge, so as to allow the catalytic cracking reaction to take place under milder conditions and independent of said initial temperature, the final reaction temperature however being higher at the dew point of the heaviest hydrocarbons present, the injection of the auxiliary fluid being carried out at such a distance from the injection of the charge that the resulting lowering of temperature takes place less than a second after the injection of the charge and preferably less than half a second after this injection.

The injected fluid may be a liquid or a gas and, in particular, water, steam, or any hydrocarbon vaporizable under the injection conditions, in particular, coking or visbreaking gas oils , catalytic diluents, heavy aromatic solvents, as well as certain deasphalting cuts extracted with heavy solvent.

The injection of this fluid will therefore have a selective quenching effect on the reaction mixture constituted by the charge and the grains of catalyst, and the injection rate of said fluid must be such that it allows regulation, under optimum conditions, the temperature of the catalytic cracking reaction, without harming the subsequent stripping of the spent catalyst and without causing condensation on the catalyst particles of the heaviest hydrocarbons present, which would result in an increased deposition of coke on the catalyst. The fluid injection conditions will be such that it causes an instantaneous lowering of the temperature of the reaction mixture of the order of 10 to 70 _° C. which, at constant final reaction temperature, therefore results in a substantially increased equivalent of the mixing temperature between the charge to be cracked and the grains of regenerated catalyst.

Thus, as will be explained below in more detail, the process according to the invention can be implemented both in an upward cracking column (riser, also called “riser”) and in a column down flow.

An important advantage of the process according to the invention is that it makes it possible to limit the catalyst regeneration temperature, due to a reduced deposition of coke, and makes it possible to convert heavy charges normally excluded in the catalytic cracking process into fluidized bed.

It is thus possible to maintain, at the entrance to the reaction zone, the rate of recycling of the catalyst and the ratio g between the mass (C) of the catalyst and that (O) of the charge to be cracked at the desired values to ensure a rapid and homogeneous vaporization of the charge approaching "flash" conditions, and obtaining optimum conversion of the products of the charge, by catalytic cracking, into desired products having in particular a better octane number.

To this end, the conditions for injecting the charge and the catalyst will be such that the mixing temperature will be slightly higher than the vaporization temperature of the charge.

Naturally, the conditions of injection of the quenching auxiliary fluid, in particular, the flow rate, the temperature and the pressure, will have to be adjusted as a function of the characteristics of the charge and of the catalyst, as well as of the conditions of catalytic cracking reaction and of the desired temperature profile. that is to say in practice as a function of the mixing temperature and of the desired final reaction temperature.

A preferred embodiment of the process according to the invention therefore consists in regulating the injection of said auxiliary fluid into the cracking column so that the reaction temperature which results from this injection is maintained at a value predetermined and constant, but high enough that the effectiveness of stripping is not substantially impaired.

The invention also relates to a device for implementing the method defined above, this device comprising a cracking column with ascending or descending flow, means for supplying said column under pressure with a hydrocarbon charge and particles of a regenerated cracking catalyst, means for separating the products from the cracked charge and spent catalyst particles, at least one means for stripping spent catalyst particles with at least one fluid, at least one unit for regenerating said catalyst catalyst by combustion of the coke deposited thereon, means for recycling the regenerated catalyst to said supply means and, downstream of the zone of mixing in said column of the hydrocarbon feedstock and the particles of regenerated catalyst, at least one means for injecting an auxiliary fluid under flow, temperature and pressure conditions, such as the injection of this auxiliary fluid decreases the temperature of the reaction zone immediately immediately downstream independently of the mixing temperature, this device being characterized in that it comprises means for regulating the flow rate of said auxiliary fluid so that the temperature of the reaction zone in downstream of the injection of the auxiliary fluid is maintained at a set value which is specific to the nature of the charge to be cracked, to the type of catalyst and to its mode of regeneration.

Preferably, this device will also include means for increasing the flow rate of the hot catalyst when the final reaction temperature tends to decrease relative to the set value.

The means for injecting an auxiliary fluid may advantageously be located in the reaction zone at a distance which may be between 0.1 and 0.8 times the length of said reaction zone and, preferably, between 0.1 and 0.5 times this length, downstream of the injection site of the load to be cracked.

This or these injection means will advantageously consist of an injection device of a type known per se, making it possible to distribute the fluid in a homogeneous manner over the entire section of the reaction zone.

In particular, when the regulating fluid consists of a liquid, the quantities necessary for regulating the mixing temperature will be injected using a spraying device making it possible, in a manner known per se, to spray this liquid in the form very fine fog over the entire section of the zone reacts in such a way as to obtain a heat transfer as instantaneous as possible. Thus, in an industrial unit of the conventional type, an injection of 20 to 150 liters of liquid water per cubic meter of charge treated will make it possible to lower the temperature of the mixing zone located upstream by approximately 10 to 70 _° C.

In addition, when the injected fluid will be a hydrocarbon, such as for example a catalytic diluent, the flow rate necessary to achieve an identical increase in temperature will be from 100 to 700 liters of hydrocarbon per cubic meter of charge treated.

• la figure 1 illustre l'application du procédé selon l'invention à un ensemble de craquage catalytique en lit fluidisé à colonne ascendante ou «riser»;
• la figure 2 illustre l'application de l'invention à un ensemble de craquage à colonne à flux descendant ou «dropper»;
• la figure 3 illustre une variante de la figure 1, avec un autre mode de régulation du procédé.

The invention will be described below in more detail, with reference to the accompanying schematic drawings, in which:

• FIG. 1 illustrates the application of the method according to the invention to a set of catalytic cracking in an ascending column or “riser”;
• FIG. 2 illustrates the application of the invention to a cracking system with a downflow column or “dropper”;
• FIG. 3 illustrates a variant of FIG. 1, with another method of regulating the process.

In FIG. 1, the regenerated catalyst is injected via a pipe 1 at the base of an ascending flow column 2 or “riser”, as well as a carrier gas injected at 3 The load of hydrocarbons to be treated is itself injected in 4 at the base of the column and the hydrocarbons and particles of the catalyst are in contact in zone 2, immediately downstream of the injection of the feed. The temperature of the catalyst-vaporized charge mixture is between approximately 500 and 650 _° C., depending on the nature of the charge and the desired conversion objectives.

At the upper part of the column 1 is provided an enclosure 6 for separating the cracked products and the spent catalyst particles. At the base of this enclosure 6 are arranged injectors 7 of a stripping fluid, generally water vapor, acting against the current of the separated catalyst particles. At the base of enclosure 6, the catalyst particles are removed at 8 to a second stripping device or to a regenerator, while the cracked products are removed at the upper part (not shown) of enclosure 6.

According to the invention, an auxiliary quenching fluid for the cracked products is injected at 9 into part 5 of column 2, downstream of the contact zone between catalyst particles and vaporized charge. This auxiliary fluid - water, water vapor, hydrocarbons, or other - is injected at a rate such that it lowers from 10 to 70 _° C, depending on the desired result, the temperature of the mixture, in order to optimize the reactions catalytic cracking of the charge. The final temperature of the reaction zone thus cooled may be, for example, of the order of 500 _° C, but must be higher than the dew point of the heaviest hydrocarbons present.

To this end, a temperature probe 10 will be provided inside the enclosure 6 to measure the temperature there and to optionally control the valve 12 controlling the flow of the auxiliary fluid in the supply line 13 of the injection means. 9, in such a way that the temperature in the enclosure 6 is maintained above a set temperature specific to the type of load treated. Another temperature probe 14, placed in the column 2 upstream of the means 9 for injecting the auxiliary fluid, that is to say in the zone for bringing the charge into contact with the particles of catalyst, control, d 'a part, the valve 16, thus making it possible to act on the injection flow rate of the regenerated catalyst in the column and therefore on the temperature in the contacting zone and, on the other hand, the valve 12 for regulating the flow rate of the auxiliary fluid in the supply line 13 of the injection means 9.

In the case of FIG. 2, the cracking column 20 is of the so-called “dropper” type, that is to say that the particles of regenerated catalyst 21 are injected into the upper part of the column 20 and fall there by gravity . The load to be cracked is also introduced into the upper part of the column at 22. An enclosure 23 situated at the base of the column 20 makes it possible to separate and evacuate the effluents, namely the products of cracking through the pipe 24, and the spent catalyst particles towards the base of this enclosure 23, in order to strike them, then convey them to the regeneration.

According to the invention, an auxiliary fluid for quenching the cracked products is injected at 26, downstream of the means 22 for injecting the charge. Its flow rate is such that for a given mixing temperature, it lowers the final reaction temperature by approximately 10 to 70 _° C., to allow the catalytic cracking reactions to be carried out under optimal conditions.

As in the previous case, a temperature probe 27, arranged before the pipe 24, makes it possible to control, by a valve 29 arranged on the supply pipe 30, the flow of auxiliary fluid of the injection means 26, in such a way that the rewashed temperature at 27 is constantly equal to or higher than a set temperature, while the probe makes it possible to act on the infection temperature in the reaction zone and on the regulation of the injection rate at 26.

In FIG. 3, the cracking column 2 is again of the riser type and the reference numerals designating the elements of the unit are the same as for those of FIG. 1. This figure presents a simplified and particularly advantageous mode of regulation. , according to the present invention. As soon as the auxiliary fluid is injected at 9, or when the injection volume is increased, the final temperature of the catalytic reaction, measured at 10, will tend to decrease compared to the desired setpoint for the corresponding charge. . The valve 16, acting on the flow of hot regenerated catalyst, will immediately ensure an increase in the mixing temperature which, taking into account the flow injected at 9, will make it possible to bring the final cracking temperature back to the set value. The following two examples illustrate the advantages of the process according to the invention.

Example 1

Is cracked in a cracking pilot device comprising a lift short residence time and catalyst regeneration system in two stages, as described in French patent application _No. 2,574,422, a residual hydrodé-sulfurized filler having the following features:

A commercial catalyst is used for cracking, comprising zeolites with high stability and a matrix capable of cracking the heaviest hydrocarbon molecules under cracking conditions.

Two tests were carried out with the same load. one under usual cracking conditions, the other with injection halfway up the elevator of a quenching fluid consisting of liquid water.

The conditions of the two tests were as follows:

The table above shows the advantage resulting from raising the initial cracking temperature above 550 ° C. and quenching the reaction products, using water, to bring the temperature down. of the mixture at 525 _° C.

For an identical final cracking temperature (525 ° C) and due to the injection temperature (595 ° C instead of 578 ° C), the hydrogen content of the coke is lower in the case of the process according to invention and the A coke is slightly lower, which shows that the heavy fractions have been effectively cracked and do not remain adsorbed on the spent catalyst.

We see here that with a lower temperature for the spent catalyst and with a lower A coke and a lower hydrogen content, it is possible to lower the final regeneration temperature of the catalyst by about 53 _° C. This can be used to increase either the C / O ratio (case of this example), or the temperature of the load, which significantly improves the vaporization of the load.

• On retiendra, essentiellement, une conversion améliorée, une meilleure sélectivité en essence et en distillat, et une production plus faible de gaz secs traduisant une réduction du craquage thermique, et de l'effet du nickel. Par ailleurs, le caractère plus oléfinique et moins aromatique des produits se traduit par un meilleur indice d'octane et un meilleur indice de cétane. Eifin, une meilleure stabilite de catalyseur, due à une température de régénération moins élevée, permet d'en réduire le taux de renouvellement pour maintenir l'activité désirée.

The beneficial effects obtained by the process according to the invention are summarized below:

• We will retain, essentially, an improved conversion, a better selectivity in gasoline and in distillate, and a lower production of dry gases translating a reduction of thermal cracking, and of the effect of nickel. In addition, the more olefinic and less aromatic nature of the products results in a better octane number and a better cetane number. Eifin, better catalyst stability, due to a lower regeneration temperature, makes it possible to reduce the renewal rate to maintain the desired activity.

Example 2

The same device and the same cracking catalyst as in the previous example are used with a filler the characteristics of which are as follows:

The auxiliary fluid this time consists of a recycling of cracking effluents with a boiling point of between 340 ° C. and 460 ° C., carried out 0.2 seconds after injection of the charge into the mixing zone.

The operating conditions are as follows:

In this example, the heavy load considered could not have been treated without resorting to the means used in the present invention, insofar as, according to the prior art, at least 15% by weight of this load could not have been be vaporized, which would have led to a production of coke outside the limits compatible with the satisfaction of the thermal balance of the unit. The mixing temperature was increased by approximately 40 _° C to ensure complete vaporization and the thermal shock conditions necessary for cracking the heaviest compounds present in this charge, after which the spraying of a hydrocarbon in the reaction medium allows reduce the final temperature to 510 _° C and thus ensure optimal conversion.

The process according to the invention therefore makes it possible to improve the performance of a cracking unit for a determined charge of hydrocarbons. It can advantageously be used to treat these heavier and more contaminated loads with greater efficiency. Likewise, this process has an exceptional appeal for the treatment of nitrogenous charges, rich in polar compounds (resins, asphaltenes), where the strong presence of aromatic nitrogen is responsible for a spectacular drop in conversion. On a Nigerian KOLE residue, for example, the conversion gain with the process according to the invention is around 5.5% by volume. This is explained by the higher mixing temperature, which favorably displaces the adsorption balance of the aromatic nitrogen, thereby reducing the neutralization of certain acid sites of the catalyst.

Claims (6)

1. A process for the catalytic cracking of a heavy charge of hydrocarbons, of which more than 10 % by weight has a boiling point above 500 _° C, said process comprising a stage for bringing said charge and particles of a cracking catalyst into contact in a dilute fluidised bed and with a short contact time in ascending or descending flow in a column under cracking conditions, a stage for separating spent catalyst and the cracked charge downstream of the injection zone of said charge, at least one stage for stripping the spent catalyst, then a stage for regenerating said catalyst under combustion conditions of the coke deposited thereon, and, finally, a stage for recycling the regenerated catalyst to the feed of said column, this process being characterised in that regenerated catalyst is injected into said column under conditions such that the equilibrium temperature resulting from the mixing of the vaporised charge and the catalyst is at least higher than 500 °C and at least 40 to 100_°C higher than the final reaction temperature at the end of the column, and in that a vaporisable auxiliary fluid is injected downstream of this mixing zone but upstream of the separating stage under the injection conditions in such a way that the mixing temperature of the vaporised charge and the catalyst particles is lowered by about 10 to 70 _° C in relation to the initial temperature arising from bringing the catalyst and the charge into contact, so as to enable the catalytic cracking reaction to be carried out under conditions which are more gentle and independent of said initial temperature, however, the final reaction temperature being higher than the dew point of the heaviest hydrocarbons present, the injection of the auxiliary fluid being carried out at such a distance from the injection of the charge that the reduction in temperature resulting therefrom takes place less than one second after the injection of the charge and, preferably, less than one half second after this injection.

2. A process according to claim 1, characterized in the auxiliary fluid is water or water vapour injected in a quantity of between 20 and 150 litres per m³ of charge injected into the reaction zone.

3. A process according to claim 1, characterized in that the auxiliary fluid is a hydrocarbon which is vaporisable under the injection conditions and, in particular, a gas-oil, a catalytic diluent, a heavy aromatic solvent or a deasphalting fraction extracted from the heavy solvent, said hydrocarbon being injected in a quantity of between 100 and 700 litres per cubic metre of treated charge.

4. A process according to any one of claims 1 to 3, characterized in that the mixing temperature resulting from the injection of the catalyst and the charge to be cracked is between 500 and 650 _° C.

5. An apparatus for carrying our the process according to claim 1, said apparatus comprising an ascending (2) or descending (20) flow cracking column, means (4, 22; 1, 26) for feeding said column under pressure with a hydrocarbon charge and particles of a regenerated cracking catalyst, a means for separating products of the cracked charge and particles of spent catalyst, at least one means (7) for stripping with at least one fluid the particles of spent catalyst, at least one unit for regenerating said catalyst by combustion of the coke deposited thereon, means for recycling the regenerated catalyst to said feed means and, downstream of the mixing zone in said column of the hydrocarbon charge and the particles of regenerated catalyst, at least one means (9, 26) for injecting an auxiliary fluid under such conditions of flow, temperature and pressure that the injection of this auxiliary fluid reduces the temperature of the reaction zone situated immediately downstream independently of the mixing temperature, said apparatus being characterised in that it comprises means (10, 12; 27, 29) for controlling the flow of said auxiliary fluid and able to maintain the temperature of the reaction zone downstream of the injection of the auxiliary fluid at a rated value which is adapted to the nature of the charge to be cracked, to the type of catalyst used and to its method of regeneration.

6. An apparatus according to claim 5, characterised in that it comprises means (16) for increasing the flow of hot catalyst when the final reaction temperature tends to decrease in relation to the rated value.

Images