EP0291408A1 - Steam cracking process in a fluidised-bed reaction zone - Google Patents

Abstract

A method of steam cracking in a reaction zone in a fluid bed of a hydrocarbon feedstock with at least two carbon atoms is described.

According to the method, a mixture of said charge (3) is circulated with water vapor (2) in at least one enclosure (7) in which contact is made with particles of inert solids, heated at (26 ) and (27) at a temperature T1 between 500 and 1800 ° C and flowing globally in co-current from top to bottom or from bottom to top.

After stirring between said particles and the mixture, a first gaseous effluent is separated from the particles which are regenerated in (26) and which are recycled. Said first gaseous effluent is sent to a second part (8) of said reaction zone, in which said effluent is circulated with second particles of cooling solids which are at a temperature T2 at most equal to 800 ° C and lower than T1 . Said cooling particles are then separated in (10) from a second steam cracking effluent which is collected in (15).

Application to petrochemicals and in particular to the production of ethylene and propylene.

Description

The invention relates to an improved process for steam cracking of hydrocarbons in a reaction zone in a fluid bed intended to produce light olefins, and more particularly ethylene and propylene. It also relates to a device for implementing the method.

Steam cracking appeared in 1920 to produce ethylene from ethane, and quickly became a basic process in petrochemicals, using increasingly heavy feedstocks, up to the treatment of gas oils under vacuum.

Its principle is based on the instability at high temperature of paraffins and naphthenes compared to that of olefins and aromatics. The main reactions are the breaking of a C-C bond by a homolytic breaking mechanism to lead to an olefin and a paraffin, and dehydrogenation. These two reactions are endothermic, and therefore favored by a rise in temperature; they also cause an increase in the number of molecules, which means that they are favored by low partial pressures of the hydrocarbons to be treated; this is the reason why this pressure is reduced to the maximum by adding steam to the reaction medium.

However, it was quickly noticed that maintaining a hydrocarbon feedstock at a temperature above 800 ° C. for a period of the order of a few tenths of seconds led to the rapid formation of coke deposits, which is detrimental for several reasons. : decrease in heat transfer between the reactor and the charge to be cracked, significant increase in the reactor skin temperature, reduction in the useful diameter of the reactor resulting in an increase in the pressure drop inside the reactor, which leads to the 'shutdown of the unit to carry out a decoking operation.

The formation of coke is due to side reactions such as the formation of condensed polycyclic aromatic hydrocarbons, as well as to the polymerization of the olefins formed.

This latter reaction stems from the tendency of olefins to polymerize when the temperature is around 500 to 600 ° C; Also, in order to reduce the importance of this secondary reaction, we have to rapidly cool (often called "quenching") the reaction effluents so as to bring them quickly from the temperature where the pyrolysis takes place. a temperature below 500 ° C, generally thanks to an indirect heat exchanger.

It has also been noted that the polymerization of olefins is favored by the presence of nickel on the surface of the metal walls of the heat exchanger, which acts as a heterogeneous polymerization catalyst (M. DENTE et al., "Fouling of transferline exchangers in ethylene plants ", AICHE Meeting of Houston, Texas, March 30, 1983).

Thermodynamic and kinetic studies of the pyrolysis reactions of hydrocarbons therefore lead, in order to increase the selectivity of the reaction towards the production of olefins, to intervene on the following parameters:
- rapid increase in the temperature of the charge up to the optimal pyrolysis temperature for a given charge, and maintaining this temperature as constant as possible in the reaction zone,
- reduction in the charge residence time in the reaction part,
- reduction in the partial pressure of the hydrocarbon charge,
- rapid and effective quenching of the reaction effluents to avoid side reactions of the olefin polymerization type.

• a) un préchauffage de la charge, diluée par de la vapeur d'eau,
• b) un chauffage à haute température de ce mélange dans des fours tubulaires afin de limiter le temps de séjour des hydrocarbures au cours de cette phase de pyrolyse,
• c) une trempe rapide des effluents de réaction.

In terms of technology, these imperatives quickly led to a general process diagram consisting of:

• a) preheating of the load, diluted by steam,
• b) high-temperature heating of this mixture in tubular furnaces in order to limit the residence time of the hydrocarbons during this pyrolysis phase,
• c) rapid quenching of the reaction effluents.

The evolution of the technology has mainly focused on the pyrolysis (b) and quenching (c) phases, in an attempt to satisfy the requirements mentioned above and the diversity of the charges to be treated, which extend to current time from ethane to vacuum diesel.

The evolution of steam cracking furnaces has essentially been geared towards obtaining shorter residence times and a reduction in pressure drop, which has led manufacturers to reduce the length of tubular reactors, and therefore to increase the density of heat flow, especially towards the start of the reactor, on the feed feed side.

In addition, to minimize corrosion, the ovens must be heated from very good quality fuels containing little sulfur, for example natural gas or fuel-gas produced by steam cracking itself, which contributes to increasing the cost price of the process.

With regard to the quenching of the effluent reaction products, the technology is oriented towards temperature exchangers arranged on the transfer lines of the effluents from the pyrolysis reaction (exchangers called TLX, "transfer line exchanger" described for example in U.S. Patent 4,097,544).

The purpose of these exchangers is to carry out the abrupt reduction of the effluent gases from the pyrolysis reactors as quickly as possible to temperatures at which side reactions of the olefin polymerization type do not occur.

However, the temperature to which the effluent is brought to the outlet of the quench exchanger varies as a function of the steam-cracked charge. For example, when treating gas oils under vacuum with an aromatic character, there is, among the steam cracking effluents, a fairly large quantity of condensed polyaromatic fuel oils which cannot be cooled suddenly at low temperature without this producing a excessive fouling of the exchanger, which may limit the operating time of the oven. In this particular case, it is generally preferred to carry out the cooling in two stages, the first being carried out by indirect quenching in the quenching exchanger up to a temperature of the order of 450-500 ° C., the second stage consisting of direct cooling by introducing cold liquids into the effluents of the exchanger.

The manufacturers of “TLX” type exchangers have endeavored to reduce the dead volume existing between the outlets of the tubular reactors and the inlets of the effluents into the quench exchanger, without fully achieving this, which is detrimental to rapid quenching . Furthermore, these exchangers are constructed of refractory steel containing nickel which is a catalyst for the polymerization of olefins.

To our knowledge, none of the techniques for carrying out steam cracking of hydrocarbons is fully satisfactory, in particular, none allows, even with special alloys for the construction of tube furnaces, for example INCOLOY 800 H, to exceed a reaction temperature of about 1100 ° C and therefore very quickly bring the charges to temperatures where the thermal cracking takes place in good conditions. In addition, it is known that the maximum calorific intake must be achieved in the zone of the tubular reactor where the endothermic reactions of cracking of the CC bonds and dehydrogenation take place, which is not carried out satisfactorily in existing processes. .

Furthermore, the need to maintain a high heat flux has led to a reduction in the section of the pyrolysis tubes, which means that their length must be reduced if an acceptable pressure drop is to be maintained. In addition, none of the current methods makes it possible to obtain an approximately constant temperature throughout the reaction zone; on the other hand, the heterogeneity of the heat fluxes leads to significant temperature differences at the circumference of each tube.

In the field of steam cracking processes which do not use tubular reactors, the prior art can be illustrated for example by US patent 4411769 where it is suggested to heat, in an integrated process of coking and steam cracking, the phase gaseous from coking with very hot particles of coke at a temperature sufficient to provide endothermic energy in the steam cracking reaction.

It can also be illustrated by US Pat. No. 4,370,303 which describes a steam cracking process where a charge is cracked in the presence of water vapor by contact with particles of hot solids in a first part of a reaction zone, the effluent obtained is then separated from the hot particles in a primary separation zone and then in a cyclone-type separation zone.

It is then subjected to quenching by contact with cold particles in a second part of the reaction zone which is separate and separated from the first part.

However, the step of bringing the hot particles into contact with the charge, the primary separation step and the cyclone separation step are carried out in different enclosures, which is costly in engineering and which substantially lengthens the total residence time of the effluent before being cooled, not counting that relating to passages in the transfer lines from one enclosure to another.

The technological background is further illustrated by patents EP-A-0.154.385, GB-A-709.583, US-A-2.846.360, FR-2.576.546 and EP-A-0.226.483.

The object of the present invention is to provide a process which overcomes these various drawbacks and allows the steam cracking of a hydrocarbon - or a mixture of hydrocarbons -, comprising at least two carbon atoms, leading to improved yields. ethylene and propylene compared to existing processes.

More particularly, the invention relates to a process for steam cracking in a reaction zone in a fluid bed of a hydrocarbon charge with at least two carbon atoms per molecule, comprising a stage of heating of said charge in a first part of said reaction zone by brought into contact with first particles of hot solids, this heating stage delivering a first gaseous effluent. The method further comprises a stage for cooling said effluent by contacting second particles of cooling solids (or cold particles) in a second part of said reaction zone. This method is characterized in that said first part of the reaction zone comprises at least one enclosure having a central axis and an internal periphery, in that one circulates a mixture of said charge at least partly in vaporized form with steam on the internal periphery of said enclosure in which contact is established between said mixture and said first particles of solids, heated to a temperature T1 of between 500 and 1800 ° C., said mixture and said particles of solids circulating globally in cocurrent from top to bottom or from bottom to top, in that, after mixing between at least the particles of hot solids and said mixing, said particles are separated in said enclosure from at least part of said first gaseous effluent resulting from said mixing, said effluent is sent, at least in part, into the second part of said reaction zone opening into said enclosure substantially along its central axis, a contact is established between said effluent and said second particles of cooling solids which are circulated in said second part of the reaction zone and which are at a temperature T2 at most equal to 800 ° C., said temperature T2 being lower than the temperature T1, said second particles are separated from a second steam cracking effluent resulting from the contacting of said first effluent and said second particles of solids , and collecting said steam cracking effluent.

The process according to the invention has the advantage of bringing the mixture to the optimum steam cracking temperature in a minimum of time, of maintaining this temperature as constant as possible in the enclosure for a very short time and of carrying out rapid quenching and effective steam cracking effluents. The heat flux density is very high insofar as there is direct gas-particle contact, and this, over a very large surface, which is the total surface of all the particles.

Furthermore, the high degree of turbulence inherent in the hydrodynamics of the enclosure (cyclone with generally circular cross-section with helical flow with or without reversal of the spiral) makes it possible to achieve very large transfer coefficients which promote thermal flash of the load and high separation efficiency; generally 0.05 to 0.5% of the flow of hot particles is recovered by the cooling zone. This type of enclosure where various steps are carried out according to the process makes it possible to control and limit the stay of steam-cracked products to very short times, thereby minimizing parasitic steam-cracking reactions. It also allows very rapid contacting of the effluents with the particles intended for quenching, which thus produced according to a stepped profile and not in steps as in the prior art (US 4,370,303).

According to a first mode of implementation of the method, the first particles of hot solids intended for heating the mixture and the second particles of solids, cold, intended for the cooling of the steaming effluent can circulate co-current from top to bottom or from bottom to top; in this first case (see Figure 1 below) the cooling zone is adapted to a downward circulation of cold particles and the gaseous effluent and in the second case, the cooling zone is adapted to an upward circulation, the zone cooling zone and the zone of contact of the hot solid particles with the mixture being generally at vertically opposite poles of the enclosure.

According to a second embodiment of the method, the first particles of hot solids and the second cold particles can circulate against the current. For example, (see Figure 2 below), the particles of hot solids and the mixture circulate first, then the gaseous effluents and the cold particles follow an ascending path in the cooling zone; or the particles of hot solids and the mixture can rise in the enclosure while the gaseous effluents and the cold catalytic particles follow a downward path in the cooling zone. In the two cases of counter-current circulation, the cooling zone and the zone for bringing the mixture into contact with the particles of hot solids are generally situated substantially at the same end of the enclosure.

Generally, the first particles of hot solids and the second particles of cold solids are substantially inert. They have a particle size in general of between approximately 20 and 2000 micrometers, preferably of between approximately 50 and 300 micrometers (1 micrometer = 10 et m) and a density of approximately between 500 and 6000 kg / m³ and preferably between approximately 1500 and 3000 kg / m³. These hot or cold particles are advantageously the same, which has the advantage of not posing a problem of contamination of a population of a loop by that of another loop.

The solid particles intended for heating or cooling generally have a specific surface of less than 100 m² / g (so-called BET method using nitrogen absorption), preferably less than 50 m² / g and even more preferably, less than 30 m² / g. They have a weak catalytic activity (lower than for example around 10%, the value 100% corresponding arbitrarily to the usual average activity of a cracking catalyst). They are of low cost and it is therefore recommended to reject some of them from time to time, and to replace them with the same fresher quantity, if it turns out that they are polluted in the long run.

The particles of solids, hot or cold, are generally chosen from the group formed by calcite, dolomite, limestone, bauxite, baryte, chromite, magnesia, perlite, alumina and silica of small surface specific.

However, according to a particularly advantageous mode, the cold particles may contain an amount of a catalyst representing from 2 to 95%, preferably from 10 to 50% and more particularly from 12 to 45% by weight of the total fraction of cold particles, which makes it possible to control and increase the selectivity in a desired product, in propylene for example.

For example, the catalyst optionally added to the particles intended for cooling the feed could be chosen from catalysts making it possible to carry out the metathesis of an internal olefin with ethylene. These catalysts are generally based on compounds of molybdenum, tungsten, vanadium, niobium, tantalum or rhenium deposited on a matrix of silica, alumina, silica-alumina, zirconium oxide, thorium oxide, etc. This catalyst is suitable for final distribution of steam cracking effluents. These catalytic particles can have a particle size of between approximately 20 and 2000 micrometers and advantageously between 50 and 500 micrometers. They usually have a specific surface greater than 100 m² / g (BET method) and among these, those which have good thermal stability in the presence of water vapor will be advantageously used.

The temperature of the first particles of heating solids is usually between about 500 and 1800 ° C and advantageously between about 800 ° C and 1300 ° C while the temperature of the second cooling particles is usually between about 200 and 800 ° C and advantageously between 300 and 600 ° C.

It is possible to adapt the temperature of the cooling particles in the presence of a catalyst so that the reaction temperature of the gaseous effluent allows the desired selectivity to be obtained, this temperature adaptation obviously being superimposed on the adjustment of other parameters as well. important than the steam / charge ratio or the temperature of heating particles.

According to a characteristic of the invention, the bringing of said mixture into contact with said first particles can be carried out in a zone of said enclosure situated substantially upstream of the inlet of the second part of said reaction zone, that is to say - say the cooling zone of the steam cracking effluent.

In more detail, the method according to the invention comprises the following steps:

Said first hot particles are introduced into a stream of water vapor adapted to generate a particle speed of 10 to 80 m / s, advantageously 15 to 30 m / s and so as to produce a helical flow of said first solid particles in said enclosure. At least part of said charge is injected into said enclosure by spraying or atomizing it, if the charge is still liquid after preheating, or by introducing it, for example, through nozzles if the charge is gaseous, so that the charge exit speed is between approximately 10 and 150 m / s, advantageously between approximately 50 and 100 m / s, the quantity of water vapor entraining said first particles being such that the mass ratio of vapor flow rate d water relative to the charge rate is between about 0.1 and 2, preferably between about 0.3 and 0.8. The mixture thus obtained is left in contact in said enclosure during a residence time of between approximately 0.1 and 2 s at a temperature T3 of between approximately 500 and 1500 ° C., preferably between 700 and 1100 ° C.

Said first particles are separated from said gaseous effluent, said gaseous effluent is sent to the second part of said reaction zone in which said second cooling particles are circulated in a stream of carrier gas (water vapor for example) adapted to generate a particle speed of about 0.5 to 10 m / s and advantageously from 2 to 5 m / s. The said gaseous effluent is left in contact in said second part of the reaction zone during a residence time of between approximately 0.1 and 100 s at a temperature T4 of between 300 and 600 ° C. Said second particles are separated from said second steam cracking effluent and collected.

By separation of the first particles of the gaseous effluent in the enclosure, is meant conventional separation and separation by stripping. The flow of hydrocarbons entrained by these particles during this overall separation step towards the regenerator can represent from 0.01 to 0.5% of the flow rate of the total charge, which is particularly advantageous.

In the case of a downward flow of particles, it is possible possibly to dispense with carrier gas since the particles fall first by gravity and then are entrained by the gaseous effluents to be cooled.

According to another characteristic of the invention, it is possible to carry out at least one regeneration in a fluidized bed of the first particles of solids, which has the double effect of at least partially removing the coke deposited on these particles during the steam cracking reaction. and warm these particles. If necessary, it is possible to carry out an additional heating of said particles by combustion of an auxiliary fuel introduced substantially at the base of a first regeneration zone so as to stagger its combustion all along the zone. This regeneration of the first particles is carried out at a temperature of between 500 and 1800 ° C. in the presence of oxygen or of a gas comprising molecular oxygen, then the major part of the combustion gases is separated from the regenerated particles, at least partially recycles said particles of solids regenerated in said enclosure and at least periodically the particles of hot solids from the regeneration step are removed without returning them to said enclosure.

In another preferred embodiment, said regeneration and said reheating of said first particles can be carried out, in at least two stages, the first in a substantially vertical and elongated tubular zone whose L / D ratio (where L is the length of the tube and D its diameter) is between 20 and 400, at a temperature T5 between 500 and 1500 ° C by means of an oxygen-based carrier gas or a gas comprising molecular oxygen, followed by a second regeneration of heating particles and possibly from the end of the combustion of the auxiliary fuel, in a second zone by means of an oxygen-based carrier gas to a gas comprising molecular oxygen at a temperature T6 of between approximately 700 and 1800 ° C. , T6 being greater than T5.

According to another characteristic of the invention, said second cold particles which have been heated by contact with the gaseous effluent are generally cooled to a temperature of between 200 and 800 ° C. in at least one cooling zone. fluidized bed located downstream of said enclosure, for example in the tubular cooling zone and / or in the separation zone for the steam cracking effluents from the particles.

As fuel capable of bringing the particles of hot solids to a sufficient temperature, it is possible to use fuels having for example initial boiling points of the order of 400 ° C. in particular heavy fuels, atmospheric residues, residues vacuum or asphalt, that is to say fuels which may contain heteroatoms such as sulfur, nitrogen and heavy metals and having a very low price, which is a particularly advantageous advantage compared to the conventional heating of steam cracking processes with good quality fuels suitable for minimizing corrosion of tubular chemical heating reactors. It may be possible to use petroleum cokes, coals or related products: lignite, peat etc ... as well as their mixtures.

In the case where the temperature of the particles allows it and in particular when the regeneration temperature in the storage area is less than 1000 ° C., an adsorbent can be introduced at the same time as the fuel so as to carry out the desulfurization in situ combustion effluents. It can be a compound comprising calcium such as limestone, dolomite, calcite, alone or associated with other inert particles.

The hydrocarbon feedstocks usable within the framework of the invention generally comprise saturated aliphatic hydrocarbons, such as ethane, mixtures of alkanes or petroleum fractions such as naphthas, atmospheric gas oils and vacuum gas oils, the latter can have a final point of distillation of the order of 570 ° C. The petroleum fractions can have, if necessary, undergone a pretreatment such as, for example, a hydrotreatment. It is also possible to use a charge comprising crude oil. The load can be preheated before being brought into contact with the particles, for example at 250 ° C.

The invention also relates to the device for implementing the method. It includes (see Figure 1 commented below):
- at least one enclosure (7) of the cyclone type, comprising a central axis and an internal periphery,
- inlet means (4) (3b fig. 4) of a liquid or gaseous hydrocarbon charge located either upstream and connected to said enclosure or in the enclosure, said inlet means containing spraying means or atomization (50 fig. 4) of said charge towards the inlet of said enclosure when it is liquid or conventional means of introduction such as nozzles when the charge is gaseous,
- inlet means (2) of water vapor upstream of said load inlet means adapted to entrain said first hot particles in said enclosure (7),
- Input means (6) of the first particles of hot solids giving these particles a helical movement at the internal periphery of said enclosure (7) in the direction of the flow resulting from the charge and the water vapor ,
means for separating (7b, 23) solid particles on the one hand and a first gaseous effluent on the other hand in said enclosure (7),
means of transport (25) of the first particles of solids connected to said means of separation towards at least one means of regeneration (26) and optionally of reheating (29) of said particles of solids,
means of storage (27) and recycling (31, 32) of said particles of solids connected to said means of regeneration (26) and optionally of reheating (29) towards said enclosure (7),
means for entering the second particles of cold solids (8a) into said enclosure (7),
- at least one inlet means (9) of said gaseous effluent and of the second particles of cold solids into a cooling reactor which is a column (8) of elongated, tubular and substantially vertical shape opening out inside said substantially enclosure along its central axis, with co-current circulation of the first gaseous effluent and the second particles of cold solids either from top to bottom ("dropper") or from bottom to top ("riser"),
means for separating by stripping (10, 11) a second steam cracking effluent and particles of cold solids, at the end of the column opposite the end by which said gaseous effluent was introduced, and cold solid particles,
- cooling means (12) of said second cold particles in said column and / or in said separation means (10, 11),
- outlet means (15) of the second steam cracking effluent connected to said separation means (10, 11),
means of transport (16) of the second particles of cold solids connected to said means of separation towards a means of storage (18), and
- Recycling means (22) of said second particles at least in part to said cooling reactor.

According to a variant of the apparatus, the cooling zone, for example the ascending zone (or riser) comprises an upper part of diameter R containing said cooling means and a lower part of diameter r opening into the enclosure substantially along its central axis. , such that the ratio R / r is between approximately 1 and 10. This configuration has the advantage of limiting the consumption of steam necessary for entraining the particles up to the entry into the cooling reactor, allowing implantation of exchange surfaces in the reactor, which allows homogeneous cooling of the steam cracking effluent and of the second particles in the same zone and finally of limiting the entrainment of the second particles out of the cooling reactor, which avoids to have to do a significant recycling.

• la figure 1 représente un mode de réalisation du procédé de vapocraquage selon l'invention où un mélange de la charge et de la vapeur d'eau, et des premières particules de solides chaudes circulent globalement ensemble de haut en bas dans le même sens, à co-courant des secondes particules de solides adaptées au refroidissement de l'effluent de vapocraquage,
• la figure 2 représente un autre mode de réalisation où la circulation dans le même sens de haut en bas du mélange ci-dessus et des premières particules est à contre courant des secondes particules,
• la figure 3 illustre une variante de la zone de refroidissement ascendante,
• la figure 4 montre une vue longitudinale de la zone réactionnelle adaptée au refroidissement dans une colonne ascendante,
• les figures 5 et 6 illustrent en coupe transversale suivant un plan AA′ et un plan BB′ de la figure 4 une vue au niveau de l'injection de la charge et des premières particules de solides, chaudes dans l'enceinte et une vue au niveau de l'entrée de l'effluent gazeux dans la zone de refroidissement,
• la figure 7 illustre un autre mode d'admission des particules de solides froides dans la zone de refroidissement.

The invention will be better understood by the description of a few embodiments, given by way of illustration but in no way limiting, which will be made of it below with the aid of the appended figures:

• FIG. 1 represents an embodiment of the steam cracking method according to the invention where a mixture of the load and the water vapor, and of the first particles of hot solids circulate together overall from top to bottom in the same direction, with co-current of the second solid particles suitable for cooling the steam cracking effluent,
• FIG. 2 represents another embodiment where the circulation in the same direction from top to bottom of the above mixture and of the first particles is against the current of the second particles,
• FIG. 3 illustrates a variant of the upward cooling zone,
• FIG. 4 shows a longitudinal view of the reaction zone suitable for cooling in an ascending column,
• Figures 5 and 6 illustrate in cross section along a plane AA 'and a plane BB' of Figure 4 a view at the injection of the charge and the first particles of solid, hot in the enclosure and a view at level of the entry of the gaseous effluent into the cooling zone,
• FIG. 7 illustrates another mode of admission of the particles of cold solids into the cooling zone.

In FIG. 1, the first particles of solid, hot and substantially inert particles which come from the reheating zone are introduced into the ejector 1, where they are suspended and accelerated for a current of water vapor, injected by line 2. The charge, preheated to around 250 ° C. for example, is conveyed by line 3 and introduced into the suspension by a device 4. This device 4 can simply consist of a ring provided with introduction or spraying and surrounding the tube 5 where the vapor-hot particles suspension flows. The heating of the charge very quickly by the hot particles to the steam cracking temperature is generally carried out in line 6 and the steam cracking reaction develops essentially in the upper part 7a of the enclosure 7 which is a Uniflow cyclone, of substantially circular section, with direct passage, with helical flow without reversal of the effluent gas spiral. The reaction temperature is kept substantially constant at this level.

In this same enclosure 7, the particles are gradually separated from the gaseous effluents by centrifugal effect and fall by gravity into the lower part 7b of the cyclone uniflow. A line 23 brings the water vapor which ensures the desorption of the fixed hydrocarbons on the particles in the lower part 7b. The gaseous effluents substantially free of hot particles, pass through inlet means 9 comprising at least one orifice in the upper part of a descending cooling column (or dropper) 8 opening into the enclosure 7 substantially along the axis of said enclosure.

These gaseous effluents are rapidly brought into contact with second particles of cold solids, for example substantially inert, fed to the upper part 8a of the downcomer 8. The effluents circulate co-current with these particles of cold solids in the column 8 .

Its lower end leads to a separation enclosure 10 where the effluent separation of steam cracking-second particles is carried out, essentially by gravity effect and inertial effect.

These particles fall into a fluidized bed 11 located at the bottom of the enclosure 10. This fluidized bed is equipped with exchanger tubes 12 known per se which lower the temperature of the particles. It is fluidized by steam supplied by line 13 under conditions which ensure effective stripping of the hydrocarbons adsorbed on the grains or which could be entrained by the current of cold particles. A cyclone 14 in connection with the enclosure 10 ensures effective gas-solid separation before sending the steam cracking effluents to a further treatment by line 15.

The cooled particles, the flow rate of which is controlled by a valve 22b, are then raised by the lift 16, supplied with driving water vapor by the line 17 in a storage silo 18, which can optionally operate in a fluidized bed and be equipped it too, if necessary, exchange surfaces for further cooling of the particles. These particles pass through a cyclone 19 which separates the particle-entrainment vapor. The storage silo 18 feeds the dropper 8 via the line 20. The control of the flow of cooling solids is ensured by the valve 22, which in particular makes it possible to modify the intensity of the heat exchange.

The first particles of solid, hot, freed from the steam cracking hydrocarbons are directed through a valve 32b, by the line 25 connected to the enclosure 7 towards the lower part of a regeneration and possible reheating assembly which includes a lift 26 and a fluidized storage bed 27. This lift is a substantially vertical tubular column of L / D ratio advantageously between 30 and 200. It allows the combustion in part of the coke deposited or of the hydrocarbons of the feed which have not been steam cracked , thus ensures the regeneration of hot particles and at the same time, the heating of these particles.

If the heat released by the combustion of the coke deposited on the particles is not sufficient to raise their temperature to a sufficient level, it is possible to bring an auxiliary fuel through a line 29 at the base of the lift 26 substantially above of the particle finish line and burn it thanks to the air supply from line 28. This method of introducing the fuel makes it possible to mix it well with the hot particles and to ensure staging of the combustion which is always beneficial in the case of fuels which are difficult to burn.

The lift 26 opens into the enclosure 27 where the combustion of the coke deposited on the grains and the combustion of the auxiliary fuel are terminated, in a fluidized bed, by a supply of air through the line 30. This enclosure 27 also performs a function of storing the particles before they are introduced into the steam cracking zone. This return to the steam cracking zone is ensured by line 31. The flow of heating solids is controlled by valve 32. The heating and regeneration effluents are separated from the hot particles and discharged by line 33 after passing through the cyclone 34 which is connected to the storage and regeneration enclosure.

Figure 2 illustrates the apparatus according to the inventive method where the cooling column is ascending.

The references in FIG. 2 are the same as those in FIG. 1 and correspond to the same means.

The heating loop of the mixture and the regeneration of hot particles is strictly identical to that of Figure 1.

The gaseous effluent, after having been cracked in the upper part 7a of the enclosure 7, is separated from the hot particles in the lower part 7b where the stripping of the particles is also carried out by means of water vapor brought by the line 23. It enters the cooling column 8 by the inlet means 97. The second particles of cooling solids arrive via line 46 of the storage enclosure 10 in the lower part 8a of the column 8 which opens into the lower part of the enclosure 7 along the axis of said enclosure. Water vapor or a recycling of light hydrocarbons supplied by line 41 ensures the fluidization of the particles in the cooling loop. The gaseous effluent is largely cooled in column 8 and enters the separation and storage enclosure 10 where it is separated from the particles and at the same time desorbed from the particles thanks to the supply of water vapor brought by line 13 which also maintains fluidization. A cyclone 14 connected to the storage enclosure refines the separation of the steam cracking effluents which are collected by line 15.

A heat exchanger 12 in and / or around the storage enclosure 10 makes it possible to remove the thermal energy accumulated by the cooling particles during the cooling of the effluent. The particles are then recycled through line 46 and valve 22, the latter making it possible to control their flow in the column.

A variant of the device according to the invention (fig. 3) consists in providing an ascending cooling pipe 8 where its upper part (defined from the outlet of the column of the enclosure (7) of diameter R contains cooling means 42 (for example parietal exchange surfaces described for example in patent FR 2575546). This diameter R is such that it is in a ratio R / r (r being the diameter of said column in the enclosure (7) preferably between 2 and 4. Under these conditions, column 8 ensures the cooling of the The steam cracking effluent as well as that of the cooling particles. This results in a simplification of the apparatus at the same time because it avoids having to handle high flow rates of solid particles in the recycling part.

At the upper part of column 8, a cyclone 43 collects the particles of solids entrained by the stream of hydrocarbons which are brought into the storage silo 45. The steam cracking effluent is collected by line 44 connected to cyclone 43. An additional steam stripping step using a line 47 in the silo 45 makes it possible to desorb the particles and recover these desorbed effluents by another line (not shown in the figure).

The particles are then recycled via line 46 to the cooling column 8 and their flow rate is controlled by valve 22.

In general, the pressures in the various chambers are adjusted so that the pressure of the chamber 7 is greater than the pressure of the cooling zone 8, which limits the passage of cooling, inert and / or catalytic particles in enclosure 7.

FIG. 4 shows a more detailed view of the cyclone 7 at the level of the upper zone 7a where the steam cracking takes place by contact with the particles of hot solids and at the level of the zone of entry of the gaseous effluent into the column upward cooling. According to a particular embodiment, the possibly preheated charge can arrive via line 3b, is divided and injected by means of at least one level of atomizers 50 for atomization or spraying in the case of liquid charges or conventional introduction in the case of gaseous charges, which are known to those skilled in the art and which are arranged on the external wall of the cooling zone 8, for example cylindrical. These injectors can be arranged in a circle perpendicular to the axis of the pipe or in a helix. These injectors are generally placed so as to distribute the charge to be vaporized as uniformly as possible over the hot solids entering the cyclone and which circulate at high speed along its periphery. The size of the droplets when the load is liquid is generally between 10 and 300 micrometers (10 and 300 x 10⁻⁶ m). The speed of entry into the cyclone and the speed of ejection of the charge are usually adjusted so that the droplets are substantially vaporized before they strike the hot solids lining the wall. As shown in Figure 5, the injectors can be arranged in the upper part 7a of cyclone 7, so as to advantageously send the load in the direction of flow of the spiral at an angle of about 0 to 80 ° relative to the radius of the tube passing through the injector and preferably at an angle of approximately 30 to 60 °, at a speed generally varying from 10 to 150 m / s, preferably 30 to 80 m / s towards the particles of hot solids, which open tangentially into cyclone 7 at a speed generally of 10 to 80 m / s, preferably 20 to 40 m / s.

The entry of the gaseous effluents into the cooling zone can be arranged on at least one level 9a and advantageously on at least two levels 9a and 9b of the cooling reactor 8 so that these effluents are better distributed over the cooling particles. The conduit 8a and the reactor 8 are preferably constructed so that their external diameter is substantially the same. According to a preferred but nonlimiting embodiment and if we consider the direction of flow of the cooling particles upwards, the internal diameter of the pipe downstream of level 9a is greater than that located upstream of this same level 9a and less than the diameter located downstream from level 9b. This arrangement facilitates the high speed circulation of the cooling particles and limits their escape into the cyclone. It also ensures better contact of the hydrocarbons with the particles.

According to an advantageous characteristic of the invention, the openings 9a, 9b are cut in a bevel so that the effluents retain at the entrance the circular movement and the high speed which they acquire in the cyclone (FIG. 6) and they are advantageously directed downwards to favor the contact of the two phases.

FIG. 7 shows another mode of admission of the cooling particles into the enclosure 7, which can be applied to the steam cracking loop illustrated in FIG. 1, that is to say in cyclones with or without turning the spiral with a descending cooling reactor 8.

According to FIG. 7, the inlet pipe 8 for the cooling particles passing through the upper part of the cyclone 7 substantially along its central axis is arranged in its interior to channel the particles towards the middle zone 7b of the cyclone where it is placed contact with gaseous effluents. Indeed, the cold particles fall into a bed 51 fluidized by steam or light hydrocarbons (C1 to C3) introduced by distributors (sparged-tubes) 52. It then enters by overflow into troughs 53 distributed uniformly over the section from the fluidized bed, which direct the cooling particles to the inlet means 9 of the reactor 8, the upper end of which is contained in the cyclone 7 substantially at the level of the zone 7b.

The inlet 9a of the reactor 8 and the outlet of the pipe 8a for the cooling particles preferably have a beveled shape allowing high-speed entry of hydrocarbon vapors substantially tangential to the flow of the particles. This arrangement largely avoids the dispersion of cold particles outside the cooling zone. In addition, the particles flowing by gravity from the spouts are accelerated by the vapors of the effluents which enter the reaction zone at high speed, which promotes rapid and homogeneous contact of the vapors with the cooling particles.

The examples which follow are given by way of illustration and show in particular the possibilities of catalytic treatment of the effluents in the cooling loop.

Example 1: With inert cooling particles.

Butane steam cracking was carried out. The operating conditions and the experimental results are presented in the table below.

Example 2: With inert cooling particles and with catalytic particles.

Steam cracking of the same charge was carried out substantially under the same experimental conditions. Catalytic particles were added to the cooling solid particles in order to perform the metathesis of ethylene and butene 2 to obtain propylene. The operating conditions and the experimental results are presented in the table below.

Claims (18)

1 - Steam cracking process in a reaction zone in a fluid bed of a hydrocarbon feedstock with at least two carbon atoms, comprising a stage for heating said feedstock in a first part of said reaction area by contacting with first particles of hot solids, said heating stage delivering a first gaseous effluent, and further comprising a stage of cooling said effluent by contacting second cooling particles in a second part of said reaction zone, characterized in that said first part of the reaction zone comprises at least one enclosure having a central axis and an internal periphery, in that a mixture of said charge is circulated, at least partly in vaporized form, with water vapor at the internal periphery of said enclosure in which contact is established between said mixture and said first particles of solids, heated to a temperature e T1 comprised between 500 and 1800 ° C., said mixture and said particles of solids circulating globally in co-current from top to bottom or from bottom to top, in that, at the end of mixing between at least the particles of solids and said mixture, said particles are separated in said enclosure from at least part of said first gaseous effluent resulting from said mixing, at least partially said effluent is sent into the second part of said reaction zone opening into said enclosure substantially according to its central axis, contact is established between said effluent and the second particles of cooling solids which are circulated in said second part of the reaction zone and which are at a temperature T2 at most equal to 800 ° C., said temperature T2 being lower than the temperature T1, said second particles are separated from a second steam cracking effluent resulting from the contacting of said first effluent and said second particles of solids, and collecting said steam cracking effluent. 2 - The method of claim 1 wherein said first solid particles are substantially inert particles and said second particles can either be substantially inert or contain an amount of catalytic particles, said first and second particles having a particle size between about 20 and 2000 micrometers and a density between approximately 500 and 6000 kg / m³. 3 - Method according to any one of claims 1 to 2, wherein the contacting of said mixture with said first particles is carried out in an area of said enclosure located substantially upstream of the entrance to the second part of said area reactive. 4 - Method according to any one of claims 1 to 3 wherein said first hot particles are introduced into a stream of water vapor adapted to generate a particle speed of 10 to 80 m / s and so as to produce a flow helical of said first solid particles in said enclosure, at least part of said charge is injected either upstream of the enclosure or into said enclosure so that the speed of exit of the charge is between 10 and 150 m / s, the amount of water vapor entrainment of said first particles being such that the mass ratio of water vapor flow rate to load rate is between approximately 0.1 and 2, said mixture is left in contact in said enclosure thus obtained during a residence time of between approximately 0.1 and 2.0 s at a temperature T3 of between approximately 500 and 1500 ° C., said first particles are separated from said gaseous effluent, said gaseous effluent is sent to the second part of said reaction zone in which said second cooling particles are circulated in a current of carrier gas adapted to generate a particle speed of 0.5 and 10 m / s, it is left in contact in said second part of the zone said gaseous effluent during a residence time of between approximately 0.1 and 100 s at a temperature T4 between 300 and 600 ° C, said second particles are separated from said second steam cracking effluent, and said second steam cracking effluent is collected. 5 - Method according to one of claims 1 to 4 wherein at least one regenerates the first solid particles and optionally at least one heating of said particles by combustion of an auxiliary fuel in a fluidized bed at a temperature between 500 and 1800 ° C, in the presence of oxygen or of a gas comprising molecular oxygen, the majority of the combustion gases are separated from the regenerated particles, said particles of regenerated solids are at least partially recycled in said enclosure and at least periodically removing the first particles of solids from the regeneration step without sending them to said enclosure. 6 - The method of claim 5 wherein said regeneration and said reheating is carried out in at least two stages, the first in a substantially vertical and elongated tubular zone whose L / D ratio (where L is the length of the tube and D its diameter ) is between 20 and 400, at a temperature T5 between 500 and 1500 ° C by means of an oxygen-based carrier gas or a gas comprising molecular oxygen, followed by a second regeneration and possibly from the end of the combustion of the auxiliary fuel, in a second zone by means of an oxygen-based carrier gas or of a gas comprising molecular oxygen at a temperature T6 of between approximately 700 and 1800 ° C. , T6 being greater than T5. 7 - Method according to one of claims 5 to 6 wherein said fuel is introduced substantially at the base of the tubular area. 8 - Method according to one of claims 5 to 7 wherein said fuel is selected from the group formed by heavy fuels, atmospheric and / or vacuum residues, asphalts, petroleum cokes, coals, peat, lignites and their mixtures. 9 - Method according to one of claims 1 to 8 wherein the solid particles are chosen from the group formed by calcite, dolomite, limestone, bauxite, baryte, chromite, magnesia, perlite, l alumina and silica of low specific surface. 10 - Method according to one of claims 1 to 9 wherein said second particles are cooled in at least one cooling zone, in a fluidized bed, located downstream of said cooling enclosure, at a temperature between about 200 and 800 ° C, and said cold particles are recycled at least in part in said enclosure. 11 - Method according to one of claims 1 to 10 wherein the temperature of the first particles of hot solids is between about 500 and 1800 ° C and that of the second particles of cold solids is between about 200 and 800 ° C. 12 - Method according to any one of claims 1 to 11 wherein said first solid particles and said mixture generally circulate together either co-current or against the current of said second particles. 13 - Process according to any one of claims in which said second cold particles further contain a catalyst having a matrix chosen from silica, alumina, silica-alumina, zirconium oxide, thorium oxide and containing compounds of molybdenum, tungsten, vanadium, niobium, tantalum or rhenium, said catalyst representing 2 to 95% by weight of the second cold particles. 14 - Apparatus for implementing the method according to claim 1 characterized in that it comprises (fig. 1):
- at least one enclosure (7) of the cyclone type, comprising a central axis and an internal periphery,
- input means (4), (3b, 50, fig. 4) of a liquid or gaseous hydrocarbon feed (3) located either upstream and connected to said enclosure, or in said enclosure,
- inlet means (2) of water vapor upstream of said load inlet means, adapted to entrain said first hot particles in said enclosure (7),
- Input means (6) of the first particles of hot solids giving these particles a helical movement at the internal periphery of said enclosure (7) in the direction of the flow resulting from the charge and the water vapor ,
- means for separating (7b, 23) the solid particles from a first gaseous effluent in said enclosure (7),
means of transport (25) of the first particles of solids connected to said means of separation towards at least one means of regeneration (26) and optionally of reheating (29) of said particles of solids,
means of storage (27) and recycling (31, 32) of said particles of solids connected to said means of regeneration (26) and optionally of reheating (29) towards said enclosure (7),
means for entering second particles of cooling or cold solids (8a) into said enclosure (7),
- At least one input means (9) of said gaseous effluent and of the second particles of cold solids into a cooling reactor which is a column (8) of elongated, tubular and substantially vertical shape opening out inside said substantially enclosure along its central axis, with co-current circulation of the gaseous effluent and the second particles of cold solids either from top to bottom ("dropper") or from bottom to top ("riser"),
means for separating by stripping (10, 11) a second steam cracking effluent and particles of cold solids, at the end of the column opposite the end by which said gaseous effluent was introduced, and cold solid particles,
- cooling means (12) of said second cold particles in said column and / or in said separation means (10, 11),
- outlet means (15) of the second steam cracking effluent connected to said separation means (10, 11),
means of transport (16) of the second particles of cold solids connected to said separation means towards a storage means (18), and
- Recycling means (20, 22) of said second particles at least in part to said cooling reactor. 15 - Apparatus according to claim 14 wherein said regeneration means (26) comprises an elongated tubular column containing at its base injection means (29) of an auxiliary fuel. 16 - Apparatus according to any one of claims 14 to 15 wherein the input means of the first solid particles are located in the upper part (7a) of the enclosure (7) and in which column 8 is adapted to the upward circulation of the gaseous effluent and of the second particles of cold solids. 17 - Apparatus according to any one of claims 14 to 15 wherein the input means of the first solid particles are located in the upper part (7a) of the enclosure and in which the column (8) is adapted to the downward circulation of the gaseous effluent and of the second particles of cold solids. 18 - Apparatus according to any one of claims 14 to 16 wherein the column (8) (Figure 3) comprises an upper part of diameter R containing said cooling means (12) and a lower part of diameter r opening into said enclosure (7) such that the ratio R / r is between approximately 1 and 10 and preferably between approximately 2 and 4.

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