EP0282371B1 - Process and apparatus for the catalytic cracking of hydrocarbon feeds - Google Patents

Abstract

The invention relates to a process and apparatus for catalytic cracking in a fluid phase of a hydrocarbon charge. The process comprises contacting, in ascending or descending flow, the charge and grains of a cracking catalyst in a tubular reactor, ballistically separating the spent catalyst and the cracked charge downstream of the end of said reactor, mixing spent catalyst with grains of at least partially regenerated catalyst having a temperature exceeding that of the grains of spent catalyst, stripping spent catalyst in a dense fluidized phase by means of a fluid injected counter-current into this catalyst regenerating the catalyst under conditions to effect combustion of coke deposited thereon, and recycling regenerated catalyst to feed the reactor.

Description

The present invention relates to catalytic cracking of hydrocarbon charges. It relates more particularly to improvements made to the separation of effluents from the cracking reaction and from the spent catalyst.

It is known that, in the so-called catalytic cracking processes (in English Fluid Catalytic Cracking or FCC process), the hydrocarbon charge is 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 the hydrocarbons, with a corresponding lowering of the boiling points, the catalyst is rapidly separated from the lighter products obtained; it is then regenerated by combustion of the coke deposited on its surface during the reaction, then returned with the hydrocarbon charge to the reaction zone.

In practice, the regenerated catalyst (at a temperature generally above 600 ° C.) and the feed to be treated are brought into continuous contact in a vertical or inclined tubular reactor. The latter, when operating in ascending mode, is often designated by specialists by the English term "riser", and designated by the term "dropper" when operating in descending mode. The load, usually preheated to a temperature of 80 to 400 ° C and injected at a pressure between 0.7.10⁵ and 3.5.10⁵ relative Pascals, vaporizes, then cracks on contact with the active sites of the catalyst, while ensuring the pneumatic transport of the grains thereof, the desired average size of which is approximately 70 μm. After a contact time of the order of 0.1 to 10 seconds, the hydrocarbon vapors, at a temperature of the order of 475 to 575 ° C, are separated from the spent catalyst using a separator ballistics disposed at the outlet of the tubular reactor. This separator is located in an area at the top of which the vapors rise hydrocarbons which, after recovery of the catalyst fines by a cyclone, are sent to the fractionation device. The catalyst grains fall by gravity at the bottom of this zone into a medium in a dense fluidized bed where, in order to be completely separated from the hydrocarbons still present in the pores, they are stripped with steam. The stripped catalyst grains are then discharged to a regenerator, in which the catalytic activity is restored by combustion of the coke deposited during the cracking reaction.

During regeneration, the heat of combustion is distributed between the catalyst (around 70%) and the regeneration fumes. The regenerated catalyst is recycled to the reaction zone, where the fraction of the heat of combustion of the coke transmitted to the catalyst in the regenerator is used to vaporize the charge, provide the reaction heat (endothermic) and compensate for the various thermal losses, ensuring thus the thermal equilibrium of the unit. The average cycle time for the catalyst is approximately 15 minutes.

The FCC process is therefore implemented 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 couple "preheating temperature of the charge-circulation of the regenerated catalyst" is thus adjusted so as to obtain the desired reaction temperature throughout the reaction zone and, in particular, at the outlet of the reactor.

• ― c'est notamment le cas lorsque les charges d'hydrocarbures à craquer sont riches en produits lourds tels que des asphaltènes ou des composés à haute teneur en métaux;
• ― c'est aussi ce qui se produit souvent en raison d'une mauvaise qualité de la séparation des effluents de la réaction de craquage ou de la médiocre qualité du strippage des grains de catalyseur provenant de cette réaction.

The quantity of coke deposited on the catalyst is therefore a fundamental variable of the reaction, since it contributes to the supply of heat necessary for the cracking of the hydrocarbons. However, it often happens that the quantity of coke which is deposited on the catalyst grains, during the reaction, is greater than that which is necessary to ensure the thermal equilibrium of the unit:

• - this is particularly the case when the charges hydrocarbons to be cracked are rich in heavy products such as asphaltenes or compounds with high metal content;
• - This is also what often occurs due to a poor quality of the separation of the effluents from the cracking reaction or the poor quality of the stripping of the catalyst grains coming from this reaction.

This excess of coke sent to the regenerator is due, at least in part, to the fact that non-negligible quantities of hydrocarbon residues (the hydrogen content of which can be between 5 and 10% by weight) cannot be separated from the grains of catalyst by the usual means of separation. This leads to a too high regeneration temperature, which is detrimental to the proper functioning of the unit, to the detriment of the quantity of recoverable products recovered in the fractionation zone.

• ― soit en extrayant la chaleur excédentaire du régénérateur, de façon à limiter l'augmentation de la température de régénération,
• ― soit en effectuant la régénération en deux étapes, ce qui permet d'atteindre pour le catalyseur des températures finales de régénération beaucoup plus élevées.

The most recent developments in the field of catalytic cracking have so far sought to remedy the drawbacks mentioned above:

• - either by extracting the excess heat from the regenerator, so as to limit the increase in the regeneration temperature,
• - or by carrying out the regeneration in two stages, which makes it possible to reach for the catalyst much higher final regeneration temperatures.

The object of the present invention is to substantially improve the quality of the separation of the effluents from the cracking reaction and that of the stripping of the catalyst, so as to limit the losses of hydrocarbon residues and to regenerate spent catalyst containing substantially only the coke necessary to ensure the thermal balance of the unit.

In fact, in the effluent separation and stripping chambers of the used catalyst used until now, there are two distinct zones. In a first zone, or disengagement zone, a device ballistics of a type known per se (see for example the American patents 2420558, 4057397, 4478708, or the French patents 2574422 and 2576906) makes it possible to orient the grains of catalyst downwards, while the hydrocarbon vapors rise upwards and are, after separation of the fines using a cyclonic system, sent to the fractionation zone. This operation, which is most often carried out in a dilute fluidized phase, ensures separation that is both rapid and effective between a large portion of the hydrocarbon vapors and the catalyst grains. In a second zone takes place in a dense fluidized phase, below the ballistic separation zone which has just been mentioned, a stripping operation, during which the displacement and recovery of the gaseous hydrocarbons entrained in the suspension of catalyst are ensured by backwashing using a gaseous fluid such as water vapor. It is necessary that the contacting is effective and that any back-mixing is minimized. Generally, the actual stripping takes place in the dense phase, in an enclosure generally characterized by a high height to diameter ratio. This enclosure is often provided with internal baffles, to promote contact of the catalyst suspension with the stripping fluid.

In this second zone, the desorption of the heaviest hydrocarbons trapped on the catalyst is favored by keeping the partial pressure of the hydrocarbons in the vapor phase as low as possible relative to their bubble pressure, therefore by a high temperature and a low pressure. The use of very polar stripping fluids, such as water vapor, more strongly absorbed than hydrocarbons, tends to favor the desorption of hydrocarbons.

The stripping reaction, either by desorption or by displacement of the entrained hydrocarbons, is relatively rapid. It is therefore useless to want to seek a better stripping efficiency by extending the time contact with the stripping fluid, because during the stripping operation, the conditions are also favorable for coking reactions of heavy hydrocarbons, with production of hydrogen and methane in particular; the net result is then a reduction in the hydrogen in the residual coke remaining on the spent catalyst, in favor of the production of light gases.

In the context of its work on catalytic cracking, the Applicant has established that the recovery of the hydrocarbon effluents from the cracking reaction can be considerably improved by practicing, between the two previously described zones, a mixture of the spent catalyst grains coming from the ballistic separation zone, with grains of regenerated or partially regenerated catalyst, having a temperature higher than that of the grains of spent catalyst.

It has already been proposed (see US-A-3886090) to recycle hot regenerated catalyst inside the stripping zone of the spent catalyst, but the injection is not intended to improve stripping and does not allow not significantly increase the temperature of the dense fluidized bed in the vicinity of the interface between it and the dilute fluidized phase disposed above.

Likewise, EP-A-137998 teaches the use of a second stripping chamber in which the spent catalyst is mixed with the hot regenerated catalyst in order to vaporize the heavy hydrocarbons possibly present on the spent catalyst. The recovery of effluents is improved, but it is necessary to use two stripping chambers. Furthermore, according to EP-A-137998, the hot regenerated catalyst is injected at the bottom of the stripping zone of the spent catalyst.

The present invention aims to remedy this drawback and it therefore relates to a process for the catalytic cracking in the fluid state of a hydrocarbon charge, comprising a step of bringing the charge into contact with the upward or downward flow and grains of a cracking catalyst in a tubular reactor, a step of ballistic separation of the spent catalyst and the cracked charge, downstream of the end of said reactor, a step of stripping in dense fluid phase of the spent catalyst thus separated, using a fluid injected against the current of this catalyst, a step of regenerating said catalyst under conditions of combustion of the coke deposited on it, and a step of recycling the regenerated catalyst to the supply of said reactor, characterized in that, between said step of ballistic separation and said stripping step, grains of catalyst at least partially regenerated, having a temperature higher than that of spent catalyst, are dispersed substantially homogeneously, above the dense fluidized phase, along substantially the entire horizontal section of the diluted fluidized phase located above this dense fluidized phase.

The hot catalyst grains coming from the regeneration zone are advantageously mixed in quantity and at a temperature such that the local temperature of the mixture resulting from this dispersion undergoes an increase from 10 to 150 ° C. and, preferably, from 20 to 70 ° C.

This mixture or recycle of catalyst, hot and at least partially regenerated, is preferably itself previously stripped, to eliminate the presence of inert compounds due to entrainment of regeneration gas, so as to avoid an unnecessary overload of the gas compressor cracked.

The amount of catalyst thus recycled will therefore be a function of the temperature difference between the grains of catalyst coming from the regeneration zone and those of the spent catalyst. Depending on the type of regeneration used in the cracking unit, the recycled catalyst can advantageously either be completely regenerated, in the same way as the catalyst supplying the reaction zone, or be only partially regenerated and, in this case, the recycled catalyst in accordance with the present invention may be taken from various points in the area of regeneration and, in particular, in the first regeneration chamber, if the unit has several successive chambers for regenerating the spent catalyst.

• ― soit par gravité, lorsque la section de régénération du catalyseur se trouve en position supérieure à celle de ladite zone d'introduction des grains et, en particulier, lorsque l'unité fonctionne en mode descendant (dropper),
• ― soit par transport pneumatique à l'aide d'un fluide, qui sera de préférence de la vapeur d'eau ou un hydrocarbure léger, seuls ou mélangés.

Depending on the configuration of the unit, the hot and at least partially regenerated catalyst grains will be recycled:

• Either by gravity, when the regeneration section of the catalyst is in a position higher than that of said grain introduction zone and, in particular, when the unit operates in descending mode (dropper),
• - Either by pneumatic transport using a fluid, which will preferably be water vapor or a light hydrocarbon, alone or mixed.

The recycling of catalyst grains is preferably carried out in the immediate vicinity of the surface of the dense fluidized bed of the stripping zone.

The invention also relates to a device for the catalytic cracking of a hydrocarbon feedstock, comprising a cracking column with ascending or descending flow, means for feeding the upstream end of said column with grains of regenerated catalyst, means for introducing into said column a hydrocarbon feedstock, a means of ballistic separation of the products of the cracked feedstock and of the spent catalyst grains, a stripping means in dense fluidized phase of spent catalyst, by at least one fluid, of the 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 the supply of said column, this device being characterized in that it comprises, between said ballistic separation means and said stripping means, at least one means for the distribution, in a substantially homogeneous manner, of catalyst grains at m oins partially regenerated, having a temperature higher than that of the spent catalyst grains, above the dense fluidized phase, along substantially the entire horizontal section of the diluted fluidized phase, located above this dense fluidized phase.

This means of introducing and dispersing the hot regenerated catalyst will be arranged in such a way that the mixing with the spent catalyst will take place in the immediate vicinity of the surface of the dense fluidized bed of the stripping zone.

When the stripping chamber will be of the type provided with internal baffles and having a high height to diameter ratio, the introduction of the hot and at least partially regenerated catalyst may be carried out, in a simple manner and in a manner known per se, by pouring said catalyst immediately above the surface of the dense bed. This type of stripping enclosure in fact promotes immediate homogenization of the catalyst grains at the top of the bed.

In all cases, however, the grains of hot and at least partially regenerated catalyst will therefore be dispersed homogeneously along the entire horizontal section of the diluted fluidized phase, located immediately above the surface of the dense fluidized bed of the zone of stripping, so that the hydrocarbons vaporized during this dispersion are immediately displaced by the rising water vapor. It goes without saying that such a vaporization of the hydrocarbons remaining on the surface of the catalyst grains will be all the better as the ballistic device situated above the outlet of the reaction zone will itself ensure good homogeneous dispersion of the grains of catalyst along the entire section of the enclosure.

• ― alors que, dans les dispositifs usuels de strippage, la température de la suspension de catalyseur usé n'est généralement pas suffisante pour déplacer de façon appréciable l'équilibre de désorption des hydrocarbures restant sur le catalyseur, surtout si les conditions de craquage sont peu sévères, l'augmentation de température qui résulte du recycle de catalyseur chaud au moins partiellement régénéré conduit à une meilleure désorption des hydrocarbures lourds et/ou visqueux aussi bien dans la phase fluidisée dense que dans la phase fluidisée diluée qui la surmonte; ceci se traduit par une récupération améliorée des produits de craquage et, en conséquence, par une température moins élevée de régénération du catalyseur;
• ― le recycle de catalyseur évite, lors de la combustion au régénérateur, l'apparition de points chauds, néfastes à l'activité du catalyseur et liés à la combustion particulièrement exothermique des composés hydrocarbonés restés à la surface des grains de catalyseur;
• ― le mélange de catalyseur usé et de catalyseur au moins partiellement régénéré permet d'augmenter la capacité d'absorption thermique de la chaleur de combustion du coke au régénérateur, ce qui a pour conséquence une meilleure homogénéité de température et de combustion au régénérateur;
• ― le mélange de catalyseur usé et de catalyseur au moins partiellement régénéré permet en outre d'augmenter de façon substantielle la température des grains de catalyseur introduits dans l'unité de régénération, en provenance de la zone de strippage; par conséquent, la cinétique de combustion étant plus rapide que dans les unités de type classique, l'inventaire de catalyseur pourra être substantiellement réduit;
• ― enfin, la Demanderesse a constaté que l'élévation de température de la zone de strippage, due à l'introduction du catalyseur au moins partiellement régénéré, se traduit de façon inattendue par une réduction appréciable des teneurs en oxydes d'azote et de soufre dans les fumées de régénération, cette réduction provenant d'une désorption plus poussée des composés hétérogènes polaires contenant du soufre et de l'azote, ainsi qui d'une réduction-hydrolyse, avec production d'hydrogène sulfuré, des sulfates formés au cours de la régénération.

The invention has numerous advantages over the known catalyst stripping systems usually used in catalytic cracking processes of hydrocarbon charges. Indeed:

• - whereas, in the usual stripping devices, the temperature of the spent catalyst suspension is generally not sufficient to appreciably shift the desorption equilibrium of the hydrocarbons remaining on the catalyst, especially if the cracking conditions are low severe, the increase in temperature which results from the recycling of hot catalyst at least partially regenerated leads to better desorption of heavy and / or viscous hydrocarbons both in the dense fluidized phase and in the diluted fluidized phase which overcomes it; this results in improved recovery of the cracked products and, consequently, in a lower catalyst regeneration temperature;
• The catalyst recycling avoids, during combustion in the regenerator, the appearance of hot spots, harmful to the activity of the catalyst and linked to the particularly exothermic combustion of the hydrocarbon compounds remaining on the surface of the catalyst grains;
• The mixture of spent catalyst and at least partially regenerated catalyst makes it possible to increase the heat absorption capacity of the heat of combustion of the coke at the regenerator, which results in better temperature and combustion homogeneity at the regenerator;
• The mixture of spent catalyst and at least partially regenerated catalyst also makes it possible to substantially increase the temperature of the grains of catalyst introduced into the regeneration unit, coming from the stripping zone; consequently, the combustion kinetics being faster than in conventional type units, the catalyst inventory can be substantially reduced;
• - Finally, the Applicant has found that the rise in temperature of the stripping zone, due to the introduction of the at least partially regenerated catalyst, unexpectedly results in an appreciable reduction in the nitrogen oxides and sulfur contents in the regeneration fumes, this reduction coming from a further desorption of polar heterogeneous compounds containing sulfur and nitrogen, as well as from a reduction-hydrolysis, with production of hydrogen sulfide, sulfates formed during regeneration.

• La figure 1 représente un dispositif de séparation et de strippage du catalyseur usé disposé à la partie supérieure d'une colonne de craquage à flux montant, avec injection par gravité du catalyseur régénéré chaud;
• Les figures 2 et 3 sont des vues analogues, dans le cas d'une injection dans l'enceinte de strippage du catalyseur régénéré en suspension dans un fluide;
• La figure 4 représente un dispositif de séparation et de strippage du catalyseur usé, disposé à la partie inférieure d'une colonne de craquage à flux descendant.

The appended drawings schematically illustrate various forms of implementation of the invention. In these drawings:

• FIG. 1 represents a device for separating and stripping the spent catalyst disposed at the top of a cracking column with rising flow, with gravity injection of the hot regenerated catalyst;
• Figures 2 and 3 are similar views, in the case of an injection into the stripping chamber of the regenerated catalyst suspended in a fluid;
• FIG. 4 represents a device for separating and stripping the spent catalyst, disposed at the bottom of a downflow cracking column.

In FIG. 1, we can see the upper part of an upward cracking column 1, opening into an enclosure 2 which is concentric with it, opposite a ballistic separator 3 promoting disengagement, homogeneous dispersion along the entire section. of enclosure 2 and the fall by gravity of the spent catalyst grains. The cracked products pass through a cyclone 4, where they are separated from the last particles of catalyst, and they are then evacuated by a line 5 to a fractionation zone. The spent catalyst particles collect by gravity at the base of the enclosure 2, where a stripping fluid such as water vapor is introduced against the current by the diffuser 6 while forming a dense fluidized bed of catalyst. The stripped catalyst is then evacuated via line 9 at the base of enclosure 2, to the regeneration device, not shown.

According to the invention, a fraction of the catalyst originating from the regeneration zone, and the temperature of which is greater than that of the spent catalyst grains, is introduced via line 10 into the diluted fluidized bed and is dispersed in a substantially homogeneous manner.

To this end, in the case of the embodiment shown in FIG. 1, the inclined pipe 10 is extended at its downstream end by a part 11 curved upwards, which ensures by gravity the homogeneous distribution of the catalyst along the entire horizontal section of the enclosure 2, immediately above the surface 8 of the dense fluidized bed 7.

The temperature at the outlet of column 1 and in the upper part of enclosure 2 will, for example, be between 480 and 550 ° C., so that by injecting a sufficient quantity of regenerated catalyst at a temperature comprised between 650 and 850 ° C, it will be possible to raise the temperature of the catalyst in dense fluidized phase between 550 and 650 ° C.

By way of a variant and in order to ensure the homogeneity of the dispersion of the recycled catalyst, the curved part 11 extending the pipe 10 may comprise two branches separated by an interval oriented towards the column 1, so as to project two jets of catalyst of on either side of this column.

In FIGS. 2 and 3, which represent alternative embodiments of the invention in the case of a rising cracking column, or "riser", the members already described with reference to FIG. 1, are designated by the same reference figures.

In the embodiment of FIG. 2, the recycled catalyst is no longer distributed by gravity in enclosure 2, but is conveyed by a carrier fluid such as water vapor, a light hydrocarbon or a mixture of the two . In this embodiment, the catalyst supply pipe 14 is an ascending pipe opening laterally into the enclosure 2 and extending at its upper part by a curved part 15, which forms a deflector for the catalyst particles, so as to allow a homogeneous dispersion in the diluted fluidized phase located immediately above the surface 8 of the stripping bed 7.

In the variant of FIG. 3, the recycled catalyst is again conveyed to the enclosure 2 by a carrier fluid circulating in an ascending pipe 14, but this pipe opens out at the center of the enclosure in an annular distributor 16, surrounding the column 1, which distributes the catalyst through lateral openings 17.

The invention also applies to devices with a falling cracking column or "dropper", as shown in FIG. 4.

In this case, the column 18 opens into the enclosure 19 directly above a deflector 20, which distributes the spent catalyst in the enclosure, while ensuring the release of the hydrocarbons: as before, the hydrocarbon vapors pass in a cyclone 21 and are discharged via line 22, while the catalyst grains collect at the bottom of the enclosure. They are kept there in a dense fluidized bed 23 and stripped by steam injected at 24, before being evacuated via line 25 to the regenerator. The recycle catalyst is introduced into the enclosure 19 by an inclined pipe 26, which opens directly above a distributor 27, here disposed in the center of the enclosure 19, below the deflector 20.

In all its forms of implementation, the invention therefore uses simple means, making it possible to appreciably improve the separation of the cracked products and the spent catalyst, as well as the quality of the stripping.

Claims (9)

1. 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 or descending flow in a tubular reactor, a stage for the ballistic separation of the spent catalyst and the cracked charge downstream of the end of said reactor, a stage for stripping the spent catalyst thus separated in a dense fluidised phase by means of a fluid injected countercurrently to said catalyst, a stage for regenerating said catalyst under combustion conditions from the coke deposited thereon, and a stage for recycling the regenerated catalyst to the feed of said reactor, characterised in that, between said ballistic separation stage and said stripping stage, at least partly regenerated catalyst particles having a temperature higher than that of the spent catalyst are dispersed in a substantially homogeneous manner, above the dense fluidised phase, following substantially the entire horizontal section of the dilute fluidised phase situated above this dense fluidised phase.

2. A process according to claim 1, characterised in that the catalyst mixture is used in such a quantity and at such a temperature that it brings about a local rise in temperature of 10 to 150°C and, preferably, 20 to 70°C.

3. A process according to either of claims 1 or 2, characterised in that the at least partly regenerated catalyst is distributed in the immediate vicinity of the surface of the fluidised bed in the stripping zone.

4. A process according to any one of claims 1 to 3, characterised in that the at least partly regenerated catalyst comes from the regeneration zone, after separation of at least part of the combustion gases.

5. A process according to claim 4, characterised in that the separation of at least part of the combustion gases is carried out by stripping using a fluid, such as steam, an inert gas or a mixture of both of them.

6. A process according to claim 4, characterised in that the at least partly regenerated catalyst is removed during the regeneration of the catalyst and, in particular, after it has entered the first combustion chamber, where the regenerator has more than one such chamber.

7. An apparatus for the fluid catalytic cracking of a hydrocarbon charge, comprising a cracking column (1, 18) with ascending or descending flow, means for feeding regenerated catalyst particles at the upstream end of said column, means for introducing a hydrocarbon charge into said column, a ballistic separation means (3, 27) for the products of the cracked charge and the spent catalyst particles, a means (6, 24) for stripping spent catalyst particles in a dense fluidised phase of spent catalyst using at least one fluid, at least one unit for regenerating said catalyst by combustion of the coke deposited thereon, and means for recycling the regenerated catalyst to the feed of said column, said apparatus being characterised in that it comprises, between said ballistic separation means (4, 21) and said stripping means (6, 24), at least one means (10, 11; 14, 15; 14, 16; 26, 27) for the substantially homogeneous distribution of at least partly regenerated catalyst particles, which have a temperature higher than that of the spent catalyst particles, above the dense fluidised phase, following substantially the entire horizontal section of the dilute fluidised phase situated above this dense fluidised phase.

8. An apparatus according to claim 7, characterised in that it comprises a means (10, 11; 14, 15; 14, 16; 26, 27) for dispersing, in a homogeneous manner, the at least partly regenerated catalyst above the surface (8) of the fluidised bed (7, 23) of spent catalyst during the stripping operation.

9. An apparatus according to either of claims 7 or 8, characterised in that it comprises, upstream of the means (10, 11; 14, 15; 14, 16; 26, 27) for introducing and mixing the at least partly regenerated catalyst, a stripping means adapted to eliminate from the partly regenerated catalyst the combustion gases of the regeneration unit.

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