FR2605643A1 - METHOD AND DEVICE FOR CATALYTIC CRACKING OF FLUIDIZED BED OF A HYDROCARBON LOAD - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR CATALYTIC CRACKING OF A CHARGE OF HYDROCARBONS. ACCORDING TO THE INVENTION, AT LEAST ONE CUT FROM THE FRACTIONATION OF THE CRACKED CHARGE IS RECYCLED TO A LIQUID STATE BY SPRAYING IN 11 IMMEDIATELY DOWNSTREAM OF THE MIXING AND VAPORIZATION ZONE OF THE CRACKED CHARGE.

Description

Method and apparatus for catalytic cracking

  in a fluidized bed of a hydrocarbon feed.

  The present invention relates to catalytic cracking

  hydrocarbon feeds. It concerns more specifically

  improvements to the processes and devices used for fluidized catalytic cracking of petroleum

heavy loads.

  It is known that, in so-called catalytic cracking processes (in English Fluid Catalytic Cracking, or FCC method), the hydrocarbon feedstock is completely vaporized by contacting at high temperature with a catalyst.

  suitable cracking catalyst maintained 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 regenerated at one time

  generally higher than 6000C and the charge to be treated are put in contact continuously at the base of a vertical tubular reactor or inclined said "charge elevator", which technicians often refer to the term "riser". The charge, usually preheated to a temperature of 80 to 4000 ° C. and injected at a pressure of between 0.7 × 10 5 and 3.5 × 10 5 Pascals, vaporizes and then cracks in contact with the active sites of the catalyst, while ensuring the upward pneumatic transport of the grains. of it, whose average desired size is about 70 fm. After a contact time of the order of 1 to 4 seconds, the hydrocarbon vapors, at a temperature of about 475 to 550 ° C, are separated from the catalyst at the top

  from the reactor, and sent to the fractional

  different products. The spent catalyst, loaded with 0.5 to 2.0% by weight of coke, is first stripped with steam and then discharged to a regenerator, where its activity

  is restored by burning coke. The heat of combustion

  This coke, which can contain from 5 to 10% by weight of hydrogen, is distributed between the catalyst (about 700%) and the regeneration fumes. The regenerated catalyst is recycled to the reaction zone, where the fraction of the coke combustion heat transmitted to the catalyst in the regenerator is used to vaporize the charge,

  provide heat of reaction (endothermic) and compensate

  the various heat losses, thereby ensuring the thermal equilibrium of the unit. The duration of a cycle

  average for the catalyst is about 15 minutes.

  The FCC process is thus carried out in such a way 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 catalyst grains.

  The "preheating temperature of the charge / circulation of the regenerated catalyst" pair is thus adjusted so as to obtain the desired reaction temperature in any reaction azone and, in particular, at the outlet of the reactor. However, when the load becomes heavier,

  temperature levels required for crack reactions.

  In the injection zone, the catalytic equilibrium leads to an equilibrium temperature between the catalyst and the supposed vaporized charge which may become less than the point

dew from it.

  It is known, in fact, that the higher the boiling point, the higher the number of high molecular weight compounds and the asphaltenes, the higher the temperature required for

  vaporize and crack these compounds is high.

  However, in the absence of a complete vaporization of the charge, the non-vaporized fraction, enriched in these heavy compounds, is generating a very large amount of coke. This results in a severe increase in the coke deposition on the catalyst, which leads to an increase in the regeneration temperature, which is detrimental to the stability of the catalyst and is very rapidly constraining from the point of view of mechanical stability. internal organs and thermal equilibrium

of the unit.

  The most recent developments in the field of catalytic cracking of heavy loads have so far addressed the above-mentioned drawbacks, mainly in three ways: either by extracting heat from the regenerator, so as to limit the increase the regeneration temperature, - either by carrying out the regeneration in two stages, which makes it possible to reach much higher temperatures,

  - by improving the efficiency of the information systems

  jection of the charge, so as to minimize the production of coke associated with a bad atomization and a contact of this charge with the catalyst in

bad conditions.

  But, in fact, if these different approaches allow to reduce the limits associated with the treatment of heavy loads, by means of, in the first case, a substantial increase in the total production of coke and, in the second, a possible lowering of the conversion, beyond a certain limit and because of the decrease of the ratio catalyst on charge (C / O), none of them really makes it possible to avoid the unnecessary production of coke associated with a vaporization

incomplete charge.

  The present invention aims to remedy this inconvenience.

  by controlling the temperature of the catalyst mixture and fresh vaporized feed so that it is sufficient to minimize coke production due to the non-vaporization of the heavier fractions, while ensuring an optimum catalytic cracking temperature in the reaction zone. This control and this optimization of the temperature are carried out using a judicious use of various sections of recycles resulting from the fractionation of the effluents

of the cracking reaction.

  Indeed, although the recycling of some of these cuts has been known for a long time (see, for example, US Pat. No. 3,193,494), it has never been envisaged to spray such cups in the liquid state for heat transfers appropriate to the severe cracking conditions (high temperature and short residence time of the feed in a diluted fluidized bed of catalyst grains) which prevail in the modern conversion units, in order to optimize the operation of these units, by regulating their temperature and adapting the formation of coke to the strict quantity necessary for

ensure their thermal equilibrium.

  The invention therefore relates to a process for the catalytic cracking of a hydrocarbon feedstock, said method comprising a column upward or downward contacting phase, under fluidized bed cracking conditions, of said feedstock. feed and grains of a cracking catalyst, a separation phase of the spent catalyst and the feedstock cracked downstream of the reaction zone, at least one stripping phase of the spent catalyst, a regeneration phase of said catalyst under conditions for combusting the coke deposited thereon and finally for recycling the regenerated catalyst to said column, said method being characterized in that:

  the recycled catalyst is introduced into the fluid phase

  said diluted in said column in an amount and a temperature such that the temperature of the mixture of said catalyst and of the finely atomized charge is at least equal to the dew point of the charge to be cracked, at least one cut from the fractionation of the charge The cracked liquid is recycled to the liquid state by spraying, immediately downstream of the mixing and vaporization zone of the charge to be cracked, preferably after a residence time of the vaporized hydrocarbons of this charge at the temperature above -5- dew point less than 0.5 second and preferably 0.3 seconds, the said recycled cup (s) being injected in an amount and at a temperature such that the final reaction temperature measured at the end of the column is always between 475 and and 550 C, said recycled cut being constituted by the lightest cut by

  relative to the most sought after conversion product.

  Among the liquid effluents from catalytic cracking from the fractionation zone and capable of being recycled in this way, the following cuts are usually distinguished: - the gasoline cut, whose boiling range generally extends from the C5 cut to 200-220C; the diesel fuel, often called LCO, by reference to the English term "Light Cycle Oil", whose boiling range generally extends from 160-2200C (beginning of cutting) to 320-380C (end of cut) , this

  cut and the previous one being those which are usually

  For catalytic cracking purposes, the units generally operate either on diesel fuel (maximum diesel production) or on gasoline (maximum gasoline production), as the case may be; - the heavy diluent cup, often called HCO, by reference to the English term "Heavy Cycle Oil", less well

  recovered (low viscosity fuel base) and whose

  boiling range is generally between 300-380 and 460-500oC; - and finally, a section of distillation residue, which contains the heaviest products and the most difficult to crack, and significant amounts of

  catalyst, this residue, which has a point of

  usually greater than 460-5000C (but may also include any distillation residue boiling above 320-380 C), being

  generally called "catalytic slurry".

  The injected recycles at different

  intervals in the cracking column or riser have in particular

  the effect of dividing it into several distinct thermal zones, -6 characterized by different reaction conditions

  and adjustable independently of each other.

  The area between the injection points of the

  fresh load and the first recycle is particularly

  controlled by: an average equilibrium temperature of the catalyst + charge mixture at least equal to the dew point temperature thereof, in order to allow complete vaporization thereof, this high temperature being moreover favorable to the dislocation of the elements heavier

  and the weaker ones, such as asphaltenes, in

  placed smaller, then accessible

  catalytic quage; a high catalyst-to-hydrocarbon ratio, favorable to a fast and efficient heat transfer between the catalyst and the feedstock, in particular making it possible to minimize undesirable thermal cracking reactions and to promote as much as possible the more selective catalytic cracking reactions; - A very short reaction time, less than 0.5 and preferably 0.3 seconds, to control the conversion to the desired level and avoid the development of side reactions detrimental to good selectivity. The objectives pursued in this first cracking zone are therefore in priority a rapid and complete vaporization of the charge and the dislocation of the heavier constituents which, failing that, would generate a large quantity of coke. Moreover, the conjugation

  a temperature and a ratio catalyst on hydro-

  carbides, with a very short residence time, makes it possible to obtain a maximum selectivity of the reaction

cracking.

  On the contrary, the zone or zones further downstream,

  after lowering the temperature due to endother-

  of the reaction itself and of the quenching effect caused by the spraying of the recycles or

  characterized by more conventional cracking conditions

  which is suitable for a fully vaporized product after passing through the first zone. For example, a total contact time of between 0.5 and 5 seconds and preferably between 0.5 and 2 seconds and a final reaction temperature of between 475 and 510 ° C. in the case of a diesel engine can be envisaged.

  maximum, or 500 to 550 C in the case of a maximum fuel

  mum. It should be noted that the invention requires in particular, for maximum efficiency, a very fast vaporization of the load. That is why it is essential that the system of spraying and distributing the charge in the catalyst flow ensures both a fine atomization of this charge and a homogeneous mixture.

with the catalyst.

  Another object of the invention, complementary to the pre-

  this, lies in a further optimization of so-called "maximum diesel" operations, whatever the type

processed loads.

  In this type of operation, conversion by pass

  should be moderate (between 475 and 510 C) or

  rather than gasoline, which implies at least two important differences from a "maximum gasoline" operation: a) the final reaction temperature must be more

  low, which, in accordance with the above-mentioned

  also requires a temperature difference that increases with the initial reaction temperature,

  which must always remain at least equal to the time

  dew point of the feed thereby imposing an increase in the flow of recycled and pulverized products downstream of the fresh feed injection point; b) because of this lower final temperature, the yield of products not converted to the first pass (products heavier than diesel) is higher and it is therefore necessary to increase the flow rate of the pulverized recycles, way to find a conversion level

acceptable.

  On light loads and therefore easier to spray, this recycle could be done in mixture with the charge as is often the case in existing units. In this case, however, the increase in the concentration of aromatics present in large quantities in the recycles has the effect of partially inhibiting the activity of the catalyst and promoting the production of coke, because of the natural affinity of the compounds slightly basic aromatics for the acidic active sites of the catalyst. On the contrary, with the process according to the invention, in which the recycles are sprayed

  downstream of the injection of the charge, these recycles

  the conversion of the load and, at the same time,

  the production of coke is considerably reduced. Fina-

  This allows, on light loads, a recy-

  cleaving a heavier cut until complete extinction

cracking residue.

  The catalytic cracking process object of the present invention

  This invention thus makes it possible to minimize unnecessary coke formations, that is to say those that do not result

not a conversion.

  This method also makes it possible, by a judicious choice of the cracking conditions of the recycling cup (s) resulting from the fractionation of the cracking effluents, to maximize, as the case may be, the production of gasoline or

  diesel fuel, whatever the loads to be

  in the unit, while ensuring thermal equilibrium of the unit by limiting the formation of coke

  the bare necessities for his needs.

  Thus, with so-called heavy loads, that is to say heavily loaded with asphaltenes and metals, having

  more than 50% by volume of compounds whose boiling point

  is greater than 500 ° C., and whose dew point, ie the temperature at which the charge is completely vaporized, is greater than about 550 ° C. or 6000 ° C. under the conditions of cracking, priority should be given to minimizing coke production and, where appropriate, extracting heat. Firstly, a first section of heavy diluent in the liquid state will be recycled by spraying immediately downstream of the injection of the feed, so as to reach the mixing temperature necessary for its vaporization and to ensure in good conditions the conversion of

  this heavy diluent is diluent light, or in essence.

  In addition, it will be possible, if necessary, to perform downstream of the injection of heavy diluent a second injection of light diluent, intended in particular to evacuate any excess heat of the system, in the case of treatment

  less appropriate loads for cracking.

  With lighter loads, less loaded with asphaltenes and polyaromatic compounds and such that the dew point under the cracking conditions is

  less than 5500C, in particular where the cat-

  the lyser used will be unlikely to form coke (so-called octane catalysts), the conditions for injecting the feedstock at high temperature, as well as the first recycle of heavy diluent under the optimal cracking conditions described above, will have this consequence of no longer ensuring coke formation

  sufficient to ensure the thermal equilibrium of the unit.

  This difficulty can then be overcome by spraying

  downstream of the heavy diluent section, catalytic slurry, which will have this time

  given the more refractory nature of this cut, of

  Catalyze the coke needed to achieve thermal equilibrium, while improving the conversion rate of the unit. This second injection will therefore preferably be practiced in the last part of the cracking column. Finally, when the unit will operate in "maximum gasoline" mode, the quantity of the latter can be increased by re-injecting successively into the column of the light diluent which constitutes in this case the

- 10 -

  the lightest cut relative to the heavier wanted conversion product, the heavy diluent or a mixture

  of these, then the catalytic slurry.

  The invention also relates to a device for catalytic cracking of a hydrocarbon feedstock comprising a cracking column with ascending or descending flow of catalyst particles, means for supplying said column under pressure by spraying a feedstock. of hydrocarbons, a means for separating the products from the cracked feedstock and spent catalyst particles, at least one means for stripping with at least one fluid

  spent catalyst grains, at least one regeneration unit

  said catalyst is combusted by burning at high temperature the coke deposited thereon, and means for

  recycling the regenerated catalyst to said feed means

  this device being characterized in that it

  carries a means for fractionating the effluents of the reaction and means for recycling to said column, in the pulverized state, at least one section of liquid coming from

  of this separation into at least one point immediately

  downstream of the charge mixing zone

  fresh and regenerated catalyst.

  The device for recycling by spraying in the cracking column of one or more cracking effluent distillation cups makes it possible to optimize the cracking conditions in view of the desired result.

  (maximum efficiency in gasoline or diesel).

  The injection rates of the recycled section or sections will be slaved to the temperature measured immediately downstream of the injection zone. For this purpose, the device

  include at least one means for regulating the flow of food

  the column is fed into one or more recycled sections in such a way that the temperature of the reaction zone, downstream of the injection zone, is maintained at a set value adapted to the nature of the charge to be cracked, to gasoline walking or desired diesel, the type of catalyst used and its mode

- * 11 -

regeneration.

  Each recycled cup will preferably be injected

  homogeneously throughout the entire section of the reaction zone.

  using one or more spraying devices to ensure thermal transfer

  instantaneous. For this purpose, the injection of the charge with

  quer and the recycles from the fractionation are made using high-performance sprayers, such as impaction sprayers or nozzle sprayers.

  venturi type, so as to give the droplets of

  the smallest possible diameter, generally less than 100 microns and preferably 50 microns; this fineness of spraying makes it possible for the formed droplets to be converted into hydrocarbon vapors almost instantaneously when they are brought into contact with the flow of catalyst grains

at high temperature.

  It is also important that the final temperature

  of reaction is kept stable. For this purpose, the

  positive will include means to regulate the flow of food

  the column in recycled section so that the final reaction temperature is maintained

  equal to a set value adapted to the load

  quer, cracking conditions and the type of market sought (petrol running: final temperature 500-530 C, or

  diesel run: final temperature 480-510oC).

  The temperature of the vaporized charge + cat-

  The lyser can thus be maintained higher than the dew point of the feed while adapting the cracking temperature to the desired selectivity for the cracking effluents. The attached schematic drawing illustrates a form

implementation of the invention.

  In this diagram, which represents a cracking column, it can be seen that the regenerated catalyst is injected via a pipe 1 at the base 2 of an upflow or "lift" column, as well as a carrier gas, introduced at 3

- 12 -

  with a flow rate sufficient for the catalyst to be in a dilute fluidized bed and for the catalyst particles to rise in the column at speeds of between 0.5 and 5 meters per second. The hydrocarbon charge to be cracked is itself injected and sprayed by

  the injector 4, at the base of the column, and the hydrocarbons

  bures, brought into contact with the high temperature catalyst particles in zone 2, immediately downstream of the injection of the charge, vaporize in less than half a second. The temperature resulting from the catalyst + vaporized filler mixture is measured by means of a temperature probe 61 which controls the hot and regenerated catalyst inlet valve 7, so as to maintain the temperature measured at 6 at an equal or greater value.

  to that of the dew point of the charge to crack.

  At the top of the column 1 is provided a chamber 8 for separating cracked products and used catalyst particles. At the base of this enclosure

  8 is arranged a diffuser 9 of stripping fluid, gen-

  water vapor, acting countercurrently

  separate catalyst particles.

  The catalyst particles recovered at the bottom of the chamber 8 are discharged to a second stripping device or to a regenerator, while the cracked products are evacuated to the portion

  upper chamber 8 to a fraction column

  of type known per se, not shown in this figure and for separating gasoline, gas oil, heavy diluent or catalytic slurry fractions. These cuts may, according to the process according to the present invention, be recycled and pulverized in the column 5, downstream of the injectors 4 of the charge to be cracked, using the injector or injectors 11 and 12, and possibly using other

  injectors not shown in this figure.

  According to the invention, the melting temperature

  in the zone immediately downstream of the charge injectors 4 is reduced to the reaction temperature

- 13 -

  catalytically desirable by spraying at 11 a cup containing mainly diluent in the liquid state, which arrives via line 13. Given the endothermic nature of the catalytic cracking reactions, the temperature in the column which results from this spraying will generally be approximately 10 to 30 ° C. higher than that measured at the end of the reaction by the probe 14, disposed in the enclosure 8, at the outlet

cracking effluents.

  Similarly, a section of catalytic slurry arriving via line 17 is sprayed at 12 in the

  column 5, downstream of the injection of the light diluent.

  The probe 14 controls the valves 15 and 16 arranged on the feed lines 13 and 17 of the column 5 in sections from the fractionation zone, in order to adjust the flow rate such that the temperature of the cracking zone located downstream of their injection be kept at the desired value, while maintaining

  at 14 a value equal to or greater than the set value.

  As indicated above, the fraction (s) of

  recycled pes will advantageously be heavy diluent 11 cuts, and thin diluent or catalytic slurry 12 cuts, depending on the nature of the load (heavy or light) and depending on the type of market sought (essences

or diesel).

  It goes without saying that the control systems described above can be the subject of numerous variants for those skilled in the art and that these variants

  are an integral part of the present invention.

  The following examples illustrate the advantages of the invention.

EXAMPLE 1

  We send in a catalytic cracking unit,

  comprising a cracking column and a regeneration system

  catalytic converter type known per se, a charge

  heavy which is here a residue of atmospheric distillation

  than direct (RAT) having the following characteristics:

- 14 -

  Density: 0.939 Sulfur (wt%) 0.95 Conradson Carbon (7 wt.): 5.6 Nickel (wt. Ppm): 12 Vanadium (wt. Ppm): 4 wt.% Not distilling at 500 C: 60

  Dew point of the charge (C): 595.

  For cracking, a commercial catalyst is used.

  cial, comprising ultra-stabilized zeolites and a matrix capable of cracking hydrocarbon molecules

  the heaviest under cracking conditions.

  Two tests were carried out with the same load, one under usual cracking conditions, the other

  with heavy diluent recycle injection placed immediately

  diatement downstream of the charge injection.

  The conditions of the two tests were as follows: Usual process Cracking process according to the invention Injection temperature of fresh feed (C): 131 259

Catalyst temperature

  regenerated (C): 800 753 Ratio C *: 4.8 5.6 o Quantity of heavy diluent (7 of the charge): - Mixture temperature after charge injection (C): 567 598 Contact time of the charge before recycle (s): - 0, 2 Final reaction temperature (C): 520,520 Coke (% by weight on the catalyst) or delta coke: 1,50 1,25 Fuel yield relative to the charge (/ by weight): 43.6 46.2 Yield of diesel compared to the load (5 by weight): 24.8 26.4

- 15 -

  Residual yield in relation to the feed (5 by weight) 9.4 4.4 * 0 C: Catalyst mass (C) and feed (O) ratio at its contact, at the inlet of the reaction zone. It can be seen from this example that, in the usual process, the charge can not be entirely vaporized (dew point significantly greater than the temperature of the mixture), which leads to a very high production of coke (7.2% C relative to load), a relatively low ratio O, a very low charging temperature, close to

  acceptable minimum on this type of load, and a temperature of

  regeneration catalyst very high (800 C), which

  supposes a regeneration in two stages.

  1.5 This charge, because of its high dew point, is at the limit of what can be accepted by C the process. In addition, the relatively low O ratio translates into a lower conversion level than the

  maximum value possible with this load.

  On the contrary, in the process according to the invention,

  this charge can be totally vaporized and the

  coke production associated with the non-vaporized fraction in the

previous case is avoided.

  As a result, the ratio is much higher, the regeneration temperature lower, and the charge temperature significantly higher, resulting in lower viscosity and better atomization quality, which in turn helps to further reduce

  more the tendency to produce coke.

  Finally, the recycling of HCO under the prevailing conditions

  confined, that is to say downstream of the injection of fresh feed, not only does not affect the conversion of

  the latter, but is itself subject to a subsequent conversion.

  ticularly, which ultimately results in a much lower residual yield, mainly in favor of

gasoline and diesel.

- 16 -

EXAMPLE 2

  The same device and the same cracking catalyst as in the previous example are used with a lighter load (of the hydrotreated vacuum distillate type) whose characteristics are as follows: Density: 0.879 Sulfur (% by weight) 0.04 Carbon Conradson (% by weight): 0.88 Nickel (ppm by weight): inf. at 0.1 Vanadium (ppm by weight): inf. 0.2% by weight not distilling at 5000C: 85 Dew point of the charge (C): 460 The first recycle is, as in Example 1, consisting of HC0 whose boiling range is between 340 and 4600C, also practiced 0.2 seconds

  after injection into the mixing zone.

  A second recycle is constituted this time of a recycle of catalytic slurry, whose boiling point

is greater than 4600C.

  The operating conditions are as follows: Usual process Cracking process according to the invention Charge temperature (oc): 301 268 Regenerated catalyst temperature (C): 645 646 C / O ratio: 6.1 6.8 Mixture temperature after injection of charge (C): 521 526 High-severity reaction temperature before injection of HC0 (oc): 506 Injection quantity of HC0 (% of charge): 0 10 Injection temperature of HC0 (C): 220 Contact time of charge before 1st cycle (s): 0.2

- 17 -

  Amount of slurry injected (% of the charge): 0 10 Slurry injection temperature (oC): - 300 Time of contact of the charge between the two recycles (s): - 0.5 Final reaction temperature (oc) 490 485 Coke (% by weight on the catalyst) or delta coke: 0.62 0.66 Yield of gasoline compared to the load (%): 39.3 40.1 Yield of diesel compared to the load (%) : 32.7 37.9 Residual yield with respect to the charge (): 4.5 0 In this example, it can be seen that the temperature which results from the vaporization of the charge to be cracked in the presence of the hot catalyst stream and regenerated is

  significantly higher than the dew point of the charge.

  The quenching effect on the spraying of the heavy diluent (HCO) and then the residue (catalytic slurry) is here relatively moderate, which is related to the fact that with this type of charge to be cracked, the process according to the present invention, unlike the usual cracking process, allows to go until the complete exhaustion of the least valuable product, namely the residue, the latter bringing in addition the additional coke necessary to

  the thermal equilibrium of the unit, taking into account the

  high injection temperature of the regenerated catalyst.

- 18 -

Claims (11)

  A process for the catalytic cracking of a hydrocarbon feedstock, said method comprising a step of upstream or downward contacting in a column (5), under fluidized bed cracking conditions, said feedstock and feedstocks. grains of a cracking catalyst, a separation phase of the spent catalyst and the cracked feedstock downstream from the reaction zone, at least one stripping phase of the spent catalyst, a regeneration phase of said catalyst under combustion conditions of the catalyst; coke deposited thereon and, finally, a phase of recycling the regenerated catalyst to the feed of said column, this process being characterized in that: - the recycled catalyst is introduced (in 1) in the fluidized phase diluted in said column in a quantity and at a temperature such that the temperature of the mixture of this catalyst and of the finely pulverized filler is at least equal to the dew point of the filler to be cracked, at least one cut from the fractionation of the cracked feed is recycled to the liquid state by spraying (at 11) immediately downstream of the mixing and vaporization zone of the feedstock to be cracked, preferably after a residence time of the vaporized hydrocarbons of this feedstock at a temperature above the dew point of less than 0.5 seconds and preferably   0.3 seconds, the recycled cup (s) being   jected in an amount and at a temperature such that the final reaction temperature measured at the end of the column is always between 475 and 550 ° C, said recycled cut being constituted by the lightest cut by   relative to the most sought after conversion product.   2. Process according to claim 1, characterized   in that the catalyst particles in the reaction zone   they are in a diluted fluidized bed and circulate at an ascending or descending rate of between 0.5 and meters per second.   3. Method according to one of claims 1 and 2, - 19 -   characterized in that the injectors (4, 11 12) of the charge to be cracked and the one or more liquid-state cuts from the fractionation of the cracked feed spray into the dilute fluidized phase of catalyst grains, such that the average diameter of the injected droplets is less than 100   microns and preferably at 50 microns.   4. Method according to one of claims 1 to 3,   characterized in that the duration of the catalytic reaction downstream of the spray injection zone of the first recycle cut is less than 5 seconds and preferably at 2 seconds.   5. Method according to one of claims 1 to 4,   characterized in that for loads to be cracked possessed   50p by volume whose boiling point is higher than   at 500 ° C and a dew point greater than 55000 ° C. and when the heavier desired conversion product is diesel, immediately downstream of the injection zone of the feed is sprayed, a cut containing essentially heavy diluent and by maintaining a final reaction temperature of between 475 and 510 C.   6. A process according to one of claims 1 to 4,   characterized in that for loads to be cracked possessed   with a dew point of less than 550 ° C and when the heavier desired conversion product is diesel, it is sprayed immediately downstream   load injection ,. a cup containing essential-   the heavy diluent, then by injection a fractionation residue cut (catalytic slurry), and a final reaction temperature is maintained between 475 and 510 C.   7. The process as claimed in one of claims 1 to 4,   characterized in that for loads to be cracked possessed   at a dew point of less than 550 ° C and when the heavier desired conversion product is gasoline, it is sprayed immediately downstream of the injection zone - 20 -   light thinner, heavy diluent, or   mixture of these, and then by injection of slurry cata-   lytic, and maintains a final reaction temperature between 500 and 5500C.   8. Process according to one of claims 1 to 4,   characterized in that, when the heavier desired conversion product is gasoline, a first cut containing substantially heavy diluent is sprayed immediately downstream of the feed injection zone, and then, downstream, a second thin thinner section, able to evacuate excess heat from the system, in order to maintain the final temperature of reaction between 500 and 550 C.   9. A device for catalytic cracking of a hydrocarbon feedstock comprising a column (5) for cracking upward or downward flow of catalyst particles, means (4) for supplying said column under pressure by spraying a feedstock. hydrocarbons, means (8) for separating the products of the cracked feedstock and spent catalyst grains, at least one stripping means (9), at least one fluid, used catalyst particles, at least one regeneration of said catalyst by combustion at high temperature of the coke deposited thereon, and means for recycling the regenerated catalyst to said feed means, this device being characterized in that it comprises a means for fractionating the effluents of the reaction and a means for for recycling to said column, at the pulverized state, at least one liquid section from said separation at at least one point (11, 12) disposed immediately downstream of the mixing zone; e the charge   fresh and regenerated catalyst.   10.- Device according to claim 9, characterized in that it comprises a plurality of means for recycling to said column a plurality of fractionation cups   of the cracked filler, by spraying appreciably   homogeneously along the whole section of the column, the injection levels (11, 12) of the different sections being offset from upstream to downstream in the direction of movement of the column. load. - 21 -   11.- Device according to one of claims 9 and   , characterized in that it comprises means (14, 15, 16) for regulating the flow rate of the recycled cup (s), such that the final reaction temperature is maintained equal to a set value of between 475 and 5500 C and means (6, 7) for regulating the catalyst flow so as to maintain the temperature of the injection and mixing zone of the feedstock.   crack over the dew point of the charge.