EP0801670B1 - Steam cracking method and plant comprising powder injection from a single collection point - Google Patents

Description

The steam cracking process is the basic process of petrochemicals. It consists of thermally crack a mixture of hydrocarbons and water vapor to high temperatures of the order of 850 ° C. and then soaking the effluents in a indirect quenching exchanger generally designated by TLX or TLE (transfer line exchanger) and then to fractionate the cooled effluents.

The main operational problem of this process comes from parasitic deposits of coke in the pyrolysis tubes and those of the quench exchanger.

To limit or eliminate this drawback, a method has already been proposed. steam cracking with injection of erosive solid particles (powders), to remove at least part of the coke deposits. The particles are injected "in line "ie either during normal steam cracking operation, or during phases where we temporarily and briefly interrupt (conventionally for a period of less than two hours) feeding in hydrocarbons, the ovens being swept by steam alone, and remaining connected to the downstream cracked gas treatment sections. Operating mode preferred is to inject the particles during normal operation of the oven, possibly by temporarily increasing the gas volume flow cracked, when injecting the powders, to increase their effectiveness.

Generally, and in particular if mineral particles are injected, or metallic, which are not essentially made up of coke, it is necessary to separate the injected powders, at the outlet of the quench exchangers, so as not to pollute the downstream sections of cracked gas treatment. Powders recovered must then be stored, either to be evacuated if the process operates without recycling, or to be recycled, at least in part.

In a typical steam cracking installation, there are a plurality of ovens, each comprising, in general, several quench exchangers (TLE) of effluents; for example, we can have 10 ovens each comprising two TLE, either a total of 20 TLEs operating in parallel.

For reasons of cost and maintenance, it is desirable to have only one very limited number of silos for receiving and storing the recovered powders (used powders), and / or possibly equipment for their treatment before recycling.

For example, we can limit the number of silos of reception and / or treatment modules of the powders recovered for their recycling.

Preferably, a single reception and / or reception module will be chosen if possible. treatment of the recovered powders, common to the entire installation of steam cracking.

According to a first already described variant of the process, the whole can be collected effluents from different TLEs, which is generally carried out in a conventional steam cracking installation without the injection of erosive powders, and separate the powders from the overall effluent from the steam cracking installation, to collect these powders at a single point. This involves installing a cyclone of very large capacity very difficult and expensive to install to treat the whole effluents from the steam cracker, and also to transport solid particles in all of the cracked gas collection circuits.

This technical solution is expensive and presents risks linked to erosion possible numerous bends in the collection circuits; moreover, the effectiveness of a very large cyclone is very poor.

We have also already proposed another variant of the steam cracking process with injection of erosive powders and recovery of these powders at a single point, consisting in sequentially injecting doses of powders into different parts of the installation, for example successively upstream of the various TLEs and / or pyrolysis tube coils, and to orient sequentially, and so coordinated with powder injections the various effluents responsible for particles to a single gas / solid separation module.

For example, one can sequentially direct the effluents from each of the TLEs, time of the corresponding injection of the powders upstream of the TLE considered, towards a unique cyclone for the recovery of particles carried by gases cracked (injected powders and particles of eroded coke from the walls); this allows you to install only a single, medium-sized cyclone, the capacity of which treatment corresponds to cracked gases coming from a single TLE and not from the whole steam cracker TLE, but requires the installation of valve sets of relatively large diameter to direct the effluents of each of the TLE either towards the downstream sections of the steam cracker when no powders are injected upstream of this TLE, i.e. towards the single cyclone when injecting powders upstream of this TLE.

These valves, which must resist erosion, are very expensive for typical dimensions required from 250 to 300 mm gas passage diameter.

This variant process and installation therefore avoids installing a cyclone of very large capacity, inefficient, and often impossible to install on many existing steam crackers, but is however very expensive because it requires a large number of special large diameter valves (e.g. 20 valves for a steam cracker comprising 10 TLE). In addition, the connecting lines between the outputs of the different TLEs and the single cyclone are lines of diameter relatively large (250 to 300 mm also, in general), necessarily made of alloy steel because it typically carries cracked gases of high temperature 450 ° C to 530 ° C at the end of the cycle.

According to patent FR-A-2,706,479, there is described a method and a device for steam cracking comprising the injection of powders upstream of the steam cracker's TLEs, and the separation of these powders from the cracked effluent gas in primary cyclones. Part of the effluent gas supplying the primary cyclones carries the powders separated to a common recovery organ, then to a recovery organ storage. The presence of condensable hydrocarbons in the effluent gas cracked under low pressure prevents good powder transfer, which can clog the installation. The technological background is further illustrated by the demand for WO-A-9012851.

A first objective of the method according to the invention, and of the installation is to offer a technical solution that is both reliable and not very expensive to this problem of collection at a single point of the powders circulating in the steam cracking effluents, when anti-coking agents such as erosive powders.

A second objective of the method according to the invention, and of the corresponding installation, is to solve this same technical problem when injecting another type anti-coking agents, without significant erosive action, but also causing circulation of undesirable solid particles and fragments.

We can indeed introduce a family of very active chemical compounds for catalyze the gasification of coke from pyrolysis tubes by water vapor.

These very active compounds can be injected during decoking phases at the water vapor alone, to greatly accelerate the decoking speed, but also during steam cracking, to reduce the coking speed, or to stop coking, by catalyzing the gasification of coke by dilution water vapor.

However, it appeared that these chemical compounds (therefore without significant erosive capacity) caused a crumbling, probably mechanical, generated by the circulation of gas, probably due to embrittlement of coke by chemical compounds.

We can also note that these crumbled coke fragments then have an action erosive which is not zero on the coke of the quench exchangers located downstream. These fragments may present risks of erosion of the lines downstream of the TLE generate quenching oil pollution problems (clogging of filters, excessively large non-combustible pieces in a conventional burner), and are therefore undesirable.

The object of the invention is therefore to propose a steam cracking process, benefiting a general technical solution, reliable and economical, allowing to separate and recovering solid particles carried by cracked gases at a single point, generated by various types of anti-coking agents.

To this end, the invention therefore proposes a process for steam cracking of hydrocarbons. in an installation comprising at least one steam cracking furnace, this installation comprising a plurality of cracking zones, and a plurality quenching exchangers (TLE) of cracked gases from these cracking zones, the method comprising online injection, at a plurality of points, of decoking generating the circulation of solid particles in the said heat exchangers quenching, and comprising the separation of at least part of these solid particles gases containing them and their transport at a single point to common collection means for this installation.

The method also comprises the recovery, by gravity flow, of at least a portion of the particles thus separated from said primary separators, in a plurality of receiving canisters V1, ..., Vn, each can Vi being associated with at least a primary separator, and the transfer of at least most of the particles contained in the canisters Vi, to said common means of separation and collection by transfer pipes, by means of a transport gas whose flow rate q _i for the evacuation of particles contained in a can Vi is less than or equal to 30% volume of the cracked gas flow passing through the primary separators associated with Vi.

The method is characterized in that each of the receiving canisters Vi is isolated sequentially of the associated separator (s) and in that the transfer of particles to the common means of separation and collection is done using non-coking transport gas with an atmospheric dew point below 110 ° C.

This method, according to the invention, has very significant advantages over previously described processes:

On the one hand, the particles are separated from the cracked gases which contain them or optionally a stream of water vapor alone in a plurality of separators gas / primary solids.

These primary separators therefore have a relatively low unit capacity, which results in reduced device dimensions, facilitating their installation, as well increased efficiency, the effectiveness of a cyclone degrading rapidly with the size, as well as for similar separators.

On the other hand, the particles are no longer routed to the common means of collection by cracked gases but by a "clean" gas, not significantly coking noncondensable at medium temperatures.

As a result, these particle transport lines can be lines relatively cold, not heat-insulated, usually carbon steel, without there are fears of coking problems or condensation of tars. These lines are therefore much less expensive than in the processes previously described. Furthermore, the risks of particle sticking in the presence of condensation of liquid, and clogging of the lines, which is a major advantage.

Finally, the gas flow conveying the particles is disconnected from the gas flow cracked, and can be much lower, for example 30% or even less 20% volume, which allows the use of very small diameter lines: 50 to 100 mm against 250 to 300 mm previously, and to remove the valves special large diameter of the previous process.

According to a preferred variant, characteristic of the invention, the particles transferred from a Vi canister are extracted from this canister by means exclusively pneumatic. This extraction mode, which performs an evacuation pneumatic of all the particles contained in the container (except possible large fragments mechanically blocked by a grid) by pressurizing the container and supplying transport gas at the outlet, is very high reliability, compared to mechanical screw extraction or lock, components that can be blocked by solid fragments of large dimension, or sometimes present flow problems of the "bridging" type, with formation of arches "of powders.

Thus, the functioning of the Vi canisters as an airlock airlock, technique known to those skilled in the art in other industries, with handling of powders and evacuation of particles by the transport gas, increases so important the reliability of the process according to the invention, compared to the process previously described.

According to another characteristic variant, the canisters Vi are heated by thermal means whose temperature level is between 110 ° C and 340 ° C, preferably between 150 and 250 ° C, this level remaining above the point dew of the transport gas at the maximum operating pressure of the canisters Vi. Through temperature level means the condensation temperature of the vapor, when using water vapor tracing, or the maximum level of temperature that can be maintained if using an electrical trace.

This preferred arrangement characteristic of the process, particularly useful when cracking heavy loads such as heavy diesel or distillates under vacuum, goes against the technical provisions previously described, in Patent EP-A-447,527, in which, in particular for cracking charges heavy, the particles are brought to high temperature, higher than the temperature normal output from the TLE, by mixing the TLE effluents containing them with a fraction of uncooled cracked gas from a bypass around the TLE, so spray the traces of tar. We have discovered, surprisingly, that powders which have contacted cracked gases, including cracked gases of pyrolysis of very heavy charges, rich in pyrolysis tars, were found in a non-agglomerated and very slightly sticky state, when they were cooled to temperatures such as those described above (below 340 ° C and even preferably at 250 ° C). This unexpected observation, leading to cool the particles instead of heating them to vaporize the tars, comes probably of the particular nature of pyrolysis tars: made up of heavy products essentially consisting of almost polyaromatic compounds pure, they unexpectedly have extremely high melting points, and they are solid at temperatures of the order of 250 ° C.

The lower limit temperature of said thermal means (110 ° C in general and preferably 150 ° C) aims to avoid any condensation of dilution vapor (dragged with the powder) or fractions of pyrolysis gasoline.

The particles separated in a primary separator, and which are, in this separator, at the outlet temperature of the TLE upstream, fall into a Vi reception, by gravity flow. As the average particle flow on a steam cracking cycle, according to the different process variants according to the invention, are always very low compared to the flow rate of cracked gases (less 1% and generally less than 1 per thousand), the heat capacity of these particles is small, and they are quickly cooled substantially to the temperature of the can Vi, which is determined by the temperature level of the thermal means of heating of Vi. So the particles are stored, temporarily, at a temperature below the melting temperature of pyrolysis tars.

However, it may happen that the particles recovered carry with them, during their fall, gaseous compounds containing condensable vapors, such only vapors of heavy pyrolysis gasoline. To avoid these condensations can cause solidification of the particles collected, provision can be made, typically, to sweep the gas contained in a can Vi by a gas not atmospheric dew point coking (initial condensation temperature at atmospheric pressure) below 110 ° C, before isolating Vi, then transferring the particles contained in Vi. This scanning according to the invention can be carried out also by introducing a barrier gas into Vi or just upstream of Vi, this which is a technical equivalent of said sweep.

In addition to this scanning, according to a preferred and characteristic technical arrangement, percolation of continuous particles can be carried out in a Vi canister, also using a non-coking gas with an atmospheric dew point below 110 ° C, before isolating the Vi canister and transferring the particles contained in Vi.

This percolation (crossing of the bed of particles) by a "dry" gas, makes it possible to “stripping” these particles and better eliminating traces of liquid possibly present. Particularly advantageously, one can perform a final drying of the particles during their pneumatic transfer, in in particular by maintaining the temperature of the particle / transport gas mixture, at the end of the pneumatic transfer, for example in the gas / solid separator secondary, at a value between 40 ° C and 180 ° C, and preferably 80 to 150 ° C. These temperatures can be used when using a transport gas noncondensable at room temperature (e.g. nitrogen or fuel gas), which is prefer. If steam is used as the transport gas, it should be noted these temperatures significantly above the condensation point of water vapor at the pressure of the secondary separator.

This final drying in a fluidized bed circulating during the pneumatic transfer, allows to further improve the flow quality of particles.

It will be very advantageously used when recycling the particles, at least in part.

As has been said, the preferred transport gases are the incondensable gases at normal temperature and pressure, especially those selected from the group of nitrogen, methane, hydrogen, light hydrocarbons comprising from two to four carbon atoms, and mixtures of these compounds.

Available gases such as nitrogen, or fuel gas from the steam cracker (variable mixture methane and hydrogen) are best suited. They allow the use of cold pneumatic transfer lines, generally not insulated.

The decoking agents can be injected during phases where the supply of oil from the cracking zone upstream of a TLE is interrupted (water vapor circulation only).

However, the preferred method variant involves injecting the decoking agents. during normal operation of the installation, i.e. during the steam cracking at normal rate, or possibly momentarily increased from 10 to 50% volume, in the case where erosive solid particles are introduced which wants to increase efficiency.

Regarding the nature of the decoking agents, two main types effective agents can be used:

According to a first variant of the process, the decoking agents comprise erosive solid particles, injected upstream of the quench exchangers, in especially in cracked gas transfer areas between the outlets cracking zones and quench exchangers.

The average particle diameter can be between 0.02 and 4 mm, and preferably between 0.07 and 0.8 mm. When particles are injected at the entrance of cracking zones, the particle size must be reduced, less than 150 micrometers, to get closer to an erosive gas effect. When we inject on the contrary the particles at the entry of the TLE, to strip the coke of these TLE and obtain a very high flexibility of cracking charges ranging from charges very light to very heavy loads, larger particles can be used diameter, typically between 70 and 800 micrometers: indeed, the TLE does not have no bends or changes in direction, but only straight lengths, and there is no fear of impact concentration by particles which may cause local erosion.

The usable particles are very variable, from the moment they have a erosive efficiency. For this reason, it is recommended to use at least 20% of angular (or very irregular) particles. Regarding the composition of these particles, it is possible to use, for example, fluid cracking catalyst (FCC) spent, cement clinker, crushed ores, metallic particles, sand. Particularly interesting particles are mineral particles hard and not very brittle such as silicon carbide, or simple oxides or mixed aluminum, silicon and zirconium. Other particles very interesting are coke particles, especially coke particles stabilized by calcination at 850 ° C or higher, carried out before or after their grinding. These coke particles are more fragile and less effective than mineral particles, and must be injected in increased quantities. On the other hand, these particles being combustible, the risks of pollution of the downstream sections, and in particular ultimately pollution from pyrolysis fuel, are considerably reduced and make it possible to envisage in certain cases (in particular for the cracking of light and medium loads such as kerosene) simplified recovery of these particles (less efficient recovery, but more economical than a cyclone), for example a "coke trap". Such a coke trap can be constituted by a sudden change in direction of the cracked gas flow, for example a simple, non-cyclonic deviation of the flow from an angle between 30 and 180 ° C, for the evacuation of at least most of the cracked gases, and a particle recovery chamber located at the change of direction abrupt, or downstream, connected by a narrowing to a container for receiving particles according to the invention.

Particle injections are preferably carried out sequentially, i.e. say discontinuously. Preferably a dose of particles is injected fixed or variable intervals between 0.3 and 72 hours and preferably between 1 and 20 hours, successively before the various TLEs equipped according to the invention. At the time of injection, the instantaneous quantity of particles, by ratio to cracked gases is between 0.5 and 25% by weight, especially between 1 and 10% by weight. If we compare the total amount of particles injected during of a steam cracking cycle to the total quantity of cracked gases during this cycle, the average levels of particles are then much lower, because we only injects particles for a small fraction of the time. Typically, the average level of solid particles injected during a steam cracking cycle, by compared to cracked gases, is less than 3000 ppm, and generally between 20 and 1500 ppm.

According to a characteristic variant of the invention, at least part is recycled particles recovered in common means of collection, by reintroducing these particles upstream of at least one of the quench exchangers, after having performed a screening operation, carried out at least on this so-called part of the particles recovered in common means. The screening operation can be carried out at atmospheric pressure and under an atmosphere essentially of nitrogen On could also screen the particles without depressurizing them and then recycling them, for example, using fuel oil.

Particle recycling, at least partially, has already been described previously; he reduces consumption of "new" particles. The disposition characteristic of the recycling process according to the invention, consisting in carrying out after the pneumatic transfer of the particles by an incondensable gas, a step of screening at atmospheric pressure, under nitrogen, is of great interest:

Indeed, the invention allows, by this pneumatic transport, to achieve both a drying and cooling of the particles in a circulating fluidized bed. This makes possible the use of existing screens, economical and highly reliable, such as screens, also called screens, centrifuges, or preferably vibrant. Indeed, the flexible connecting cuffs of these devices, made of elastomer possibly reinforced would be incompatible with very high particles temperature (400 ° C or more), which pass through the process filter previously described.

However, this filtration step is essential to avoid the risk of clogging of recycled powder injectors that have a small diameter. In addition, the screening being performed at atmospheric pressure at moderate temperature and under nitrogen, the operations screen maintenance are easy and can be carried out quickly.

Typically, the fragments (coke and foreign bodies) of diameter are eliminated. greater than 3 or 4 mm.

According to another characteristic variant of the invention, the decoking agents include mineral salts catalysts for gasification of coke by steam of water, injected upstream of the cracking zones.

In particular, these mineral salts can comprise at least one salt of an element included in the group of sodium, potassium, lithium, barium and strontium, this salt being active in promoting the gasification of coke.

Very active mineral salts have been discovered to gasify the coke of pyrolysis tubes, comprising salts of alkali and alkaline earth elements, in particular of precursors of oxides or carbonates of these elements.

In particular, mixtures with a melting point lowered below 750 ° C (for example neighboring eutectic example) of sodium carbonate and potassium carbonate have a very effective decoking or coking prevention action.

It is also possible to use mixtures of acetates, for example a mixture equimolar of sodium acetate, potassium acetate, lithium acetate and barium acetate. These compounds, the list of which is not exhaustive, can catalyze the gasification reaction of coke (in particular the reaction of "gas to water ": C + H2O → CO + H2); they can be introduced in powder form or in the form of aqueous solutions, in particular very dilute solutions, atomized in a hot gas, and in particular in the dilution water vapor, or the steam / hydrocarbon mixture at the convection outlet (at a temperature high of the order of 500 to 650 ° C).

The preferred injection method is injection during normal operation of the steam cracking; it is also possible to inject these mineral salts only during steam decoking alone, in particular to accelerate this decoking. The quantity required depends on many factors: nature of the compounds used and the load to crack, severity of cracking and skin temperature of the tubes pyrolysis.

The most suitable amounts are typically between 2 and 200 ppm, and preferably between 5 and 100 ppm, counted by weight of alkaline elements and / or alkaline earth compared to cracked gases.

The invention also relates to a steam cracking installation making it possible to implement the process. More specifically, this installation includes at least a steam cracking oven, a plurality of cracking zones, a plurality quenching exchangers for cracked gases from these cracking zones, this installation also comprising injection means at a plurality of points, decoking agents generating the circulation of solid particles in the quench exchangers, a plurality of gas / solid solids separators, for purifying the effluents from the quench exchangers, each primary separator being connected upstream to at least one quench exchanger associated therewith and comprising a purified gas outlet and an outlet for solid particles, and means for recovery of at least part of these solid particles, these means of recovery including common means of separation and collection gathered at a single point,

• une pluralité de bidons Vi pour la récupération par écoulement gravitaire d'une partie au moins des particules séparées dans les séparateurs primaires, chaque bidon Vi étant connecté à au moins une sortie de particules solides d'au moins un séparateur primaire associé à Vi,
• une pluralité de canalisations de transfert de particules solides, chacune de ces canalisations étant reliée en amont à l'un des bidons Vi et en aval aux dits moyens communs de séparation et de collecte, et des moyens de transfert de particules par un gaz de transport.

The installation also includes:

• a plurality of canisters Vi for the recovery by gravity flow of at least part of the particles separated in the primary separators, each canister Vi being connected to at least one outlet of solid particles of at least one primary separator associated with Vi,
• a plurality of solid particle transfer pipes, each of these pipes being connected upstream to one of the canisters Vi and downstream to said common means of separation and collection, and means of transfer of particles by a transport gas .

The device is characterized in that it comprises sequential isolation means of each of the canisters Vi of the primary separator (s) associated therewith, that the means for the transfer, by the said transfer pipelines, of the most at least a large part of the particles contained in the Vi canisters thus isolated include means for supplying a non-coking and point transport gas atmospheric dew below 110 ° C.

This installation therefore makes it possible to transfer the powders recovered by means of relatively low flow rates of non-fouling transport gas at temperature moderate. Primary separators, individually have a unit capacity relatively low compared to the overall flow of cracked gases from the installation complete, and are therefore effective and easy to install. They carry out a purification effective cracked gases not only during particle injection phases solids, according to the method previously described, but also so permanent, and are therefore also effective against particulate emissions solids after injections of decoking agents. This is useful as well for residual particles circulating after particle injection phases erosives that remained in the dead zones, only when injecting the agents coke gasification chemicals. This installation, which does not include, for what concerns the centralized particle recovery and collection part, no valve additional large diameter, typically greater than 150 mm, is therefore both more efficient (recovery), more reliable (risk of fouling) and more economical due to the use of small diameter transfer lines, temperature moderate, and the absence of special large diameter valves compatible with solid particles. By sequential isolation is meant alternating phases where a receiving canister Vi is in communication with the upstream, and phases where it is isolated from upstream to allow evacuation downstream, towards common means separation and collection. This will preferably, but not necessarily, be done in a coordinated manner for the different Vi canisters, each of these can be isolated successively, so as to stagger the transfers. It is indeed it is also possible to empty several Vi canisters simultaneously.

The common means of collection are typically constituted by a can allowing temporary or prolonged storage of particles, which can possibly include weighing means. A quench exchanger is said associated with a primary separator if this primary separator purifies the effluents from this quench exchanger. Similarly, a primary separator is said to be associated with a receiving can Vi if Vi recovers, by gravity flow, a part at minus the particles separated in this primary separator. So a separator primary may be associated with one or more quench exchangers which it purifies effluents; a receiving can Vi can collect particles from one or more primary separators. According to a characteristic arrangement preferred of the invention, the installation can in fact comprise at least two primary separators associated with the same receiving container, each of these primary separators being connected to this container by a pipe, and comprising control means for sequential shutter means of at least one of the pipes when the other of these pipes is open, the relative arrangement of these primary separators and the receiving container being such that the pipes have a inclination at least 60 degrees from the horizontal.

This arrangement allows the use of a single Vi canister for reception and transfer of particles from several primary separators, and is therefore interesting economically, and from a maintenance point of view. Isolation sequential at least one of the pipes avoids the circulation of gas cracked via the Vi canister from one primary separator to the other, harmful for separation efficiency.

Preferably, the evacuation of the particles contained in a can Vi is carried out by means of drainage, connected to Vi, which are exclusively tires, and use at least one gas source from the group of nitrogen and fuel gas (methane or mixture of methane and hydrogen). These means of emptying pneumatic, by "pneumatic shipping lock", or "pressurized container", and which typically include a pressurization of the can Vi relative to the conditions downstream of the pneumatic transfer line and a gas injection of transport out of Vi, are indeed very intense, and allow to evacuate powders with flow difficulties; they are more effective than means for evacuating the process previously described.

The transport gas flow rate for transferring the particles is only Not more than 30% of the gas flow rate passing through the primary separators associated with Vi during the same period, typically the normal gas flow cracked treated by the primary separator (s) whose particles fall in Vi. The transfer line is therefore much smaller in diameter than that of cracked gas lines (less than or equal to 100 mm against typically 250 to 400 mm).

Preferably, this transport gas is fuel gas or nitrogen, gases which are noncondensable at room temperature, and which will allow drying of the particles during their transfer. Vi cans, according to one characteristic preferred of the invention, are heated by thermal means whose level of temperature is between 110 and 340 ° C, and preferably between 150 and 250 ° C. This temperature level which corresponds to that of the temperature of condensation of the heat tracing, or of the temperature maintained by electrical means is indeed adequate for maintaining pyrolysis tars at solid state.

Typically, the installation comprises means for scanning the gas. contained in Vi cans, by means of a non-coking gas source and point of atmospheric dew below 110 ° C. This scan, which by technical equivalent can be constituted by a barrier gas, has the function of purging Vi of traces possible cracked gases, before evacuation and transfer of particles. According to a preferred characteristic variant, the installation comprises means introduction of a non-coking gas and an atmospheric dew point below 110 ° C, within the particles contained in the Vi canisters, for the percolation of these particles before their evacuation from the canisters Vi.

This makes it possible to carry out a first drying of the particles promoting their evacuation, before that carried out during the transfer itself.

According to a first characteristic variant, the decoking agents comprise erosive solid particles, and means for injecting these particles upstream quench exchangers, and in particular in the transfer zones between the zones cracking and quenching exchangers.

Preferably, all of the solid particles injected are in the zones transfer of cracked gases between the cracking zones and the quench exchangers, in particular in the inlet cones of these exchangers (considered to be part of the transfer areas).

Advantageously, the common means of separation which achieve separation secondary solid particles / transport gas substantially non-condensable, include a purified transport gas outlet connected by a line of connection to a cracked gas circulation line, for the evacuation of this gas clean transport.

This looping back of all of the purified transport gas outlets to a circulation of cracked gases allows to remain under pressure, and in a network "hydrocarbons", for all the different transfers of particles to the common means of separation and collection, which is interesting with regard to safety issues compared to venting the gases from clean transport.

Preferably, the transport gas is then fuel gas, which avoids the consumption of nitrogen and its significant mixture with cracked gases.

According to a characteristic variant, the installation comprises recycling means at least part of the particles recovered in the common means of separation and collection.

According to an equally characteristic arrangement, the installation then comprises a vibrating screen operating under nitrogen pressure at substantially pressure atmospheric and at a temperature below 200 ° C, connected upstream to the means , separation and collection facilities and linked downstream to recycling means for particles.

According to another characteristic variant, the installation comprises means injection of decoking agents, comprising chemical compounds catalyzing gasification of coke with steam, upstream of the cracking zones. In particular such an installation can advantageously include means injection of a solution comprising at least one mineral salt of an element included in the group of sodium, potassium, lithium, barium and strontium, this salt being active in promoting the gasification of steam coke of water, this gasification weakening the coke and causing emissions of pieces of coke, which can be recovered and transferred according to the means of the invention.

When injecting erosive solid particles, this is achieved very generally during short injection phases, representing only one weak part of the time.

Other than these short injection periods, other types of particles like coarse fragments of coke (fragments that can come off walls, naturally, for example during thermal shocks, or favored by injections of chemical gasification catalysts).

To avoid mixing of the two populations of particles, the invention may understand means of sequential injection of erosive particles connected to said transfer zones, means of sequential isolation of each can Vi in outside the particle injection phases upstream of Vi, and means evacuation of particles recovered in the separator (s) associated with Vi in outside these injection phases, without passing through Vi.

In particular, the installation may typically include cans Wi for receiving particles recovered outside the injection phases of particles, and directional switches controlled at one input and two outputs, each switch being connected upstream to a primary separator, and downstream to a Vi receiving container, and a Wi receiving container.

The invention will be better understood and other characteristics, details and advantages will appear more clearly on reading the description which follows, example with reference to the accompanying drawings in which: Figure 1 shows schematically a steam cracking installation according to the invention, comprising several devices relating to different characteristic variants according to the invention.

FIG. 2 schematically represents part of an installation comprising a device characteristic of one of the variants of the invention.

FIG. 3 represents a part of an installation comprising another device characteristic, advantageous for carrying out the invention.

Reference is first made to FIG. 1, where part of two kilns have been shown. steam cracking (1) each comprising a feed (22) of a charge of hydrocarbons and a supply (23) of dilution water vapor. Load overall is preheated, vaporized and superheated to a typical temperature of 500 to 650 ° C in the convection zones of these two ovens, then cracked in two cracking zones (2) constituted by coils of pyrolysis tubes. To the exit from these cracking zones (exit from the oven enclosure), the cracked gases pass through transfer zones (3) to two heat exchangers quenching (4), or "TLE" (Transfer Line Exchanger) allowing to lower suddenly their temperature at around 360 to 630 ° C, and very generally at approximately 360 to 500 ° C, this temperature being measured by indicators of temperature (24). These two streams of cracked cooled gas then pass through two primary gas / solid particle separators (5), for example two cyclones. Each of these primary separators includes a purified gas outlet which joins a line (12) of circulation of cooled cracked gases, for their evacuation and their downstream treatment (primary fractionation, compression, desulfurization, drying, final fractionation).

The two primary separators (5) also each include an outlet for solid particles connected by a pipe (16) to a receiving container (6) maintained at temperature by thermal means (37), for recovery of these solid particles by gravity flow.

The two receiving containers (6) each include isolation means sequential: upstream a controlled valve (7) arranged on the line (16), and downstream a controlled valve (8). These two receiving containers (6) are connected in downstream, each by a transfer pipe (9), to common means of separation and collection of solid particles, comprising a cyclone of separation (10) and a collection container (13). Cyclone effluent gases (10) are introduced into line (12) by a line (11).

This collection container (13) is connected downstream by a line (32) to a screen. vibrating (14) connected to the atmosphere by a line (ATM) operating substantially at atmospheric pressure, under a nitrogen atmosphere, and at moderate temperature compatible with flexible connection sleeves used for screens classic vibes. The container (1), which includes upstream isolation valves and downstream, as well as unrepresented means of depressurization, fulfills the function of particle decompression airlock.

The exit of fine particles from the vibrating screen (14) (particles cleared by example large fragments larger than 3 mm) is connected to a container reception (15), equipped with upstream and downstream controlled valves, as well as means supply, not shown, of nitrogen group gas and fuel gas. In the practical, the vibrating screen (14) will be placed above the can (15), to allow the gravitational flow of powders (this is not the case in Figure 1 for simple drawing reasons).

The can (15) thus equipped can then operate in a pneumatic shipping lock, and constitutes a means of recycling erosive solid particles in the installation. It is connected upstream to a source of transport gas (33) (fuel gas, nitrogen or water vapor), and downstream to different injection means (19) (34) comprising controlled valves and injection pipes for solid particles. The particles can be injected upstream of the cracking zones (2) by the lines (34) in dotted lines, or preferably by lines (19), in the areas of transfer of cracked gases (3), and in particular at the inlet cones of the quench exchangers, cones which by convention form part of the transfer zones (3). In this case, a diffusing impactor (35) will preferably be installed at the interior of each inlet cone. This diffuser impactor has a double purpose: protect the tube plate in the heat exchanger against erosion, and distribute the injected particles more evenly, in the various exchanger tubes (4).

This diffusing impactor (35) advantageously consists of two levels of particle bounce surfaces, offset from each other, so that it is both gas permeable in a plurality of passages and substantially opaque seen from upstream.

The shipping hatch (15) has a particle discharge line (36) worn; we could also send the used particles to a storage silo thanks to a switch arranged on the line (32); a container (18) comprising controlled emptying means (screw or lock) allows to store particles "new", and to replace the used particles.

a) d'injecter un gaz de barrage en amont de la vanne (7), pour s'opposer à la venue de gaz craqués dans le bidon de réception (6),

b) d'injecter grâce à la vanne (25) un tel gaz non cokant et de point de rosée atmosphérique inférieur à 110 °C pour vidanger le gaz contenu dans le bidon de réception Vi (6), avant l'évacuation et le transfert des particules, et de pressuriser le bidon (6) au cours du transfert pneumatique des particules.

c) d'injecter grâce à la vanne (26) un tel gaz au sein même des particules solides, pour réaliser un séchage au moins partiel des traces de liquide éventuelles, par percolation au moyen d'un gaz sec,

d) d'injecter grâce à la vanne (27) un débit contrôlé d'un tel gaz de transport au cours de transfert pneumatique des particules.

The installation also includes pneumatic means enabling particles to be transferred from the Vi canisters (6) to the common means of separation and collection, via the transfer pipes (9): a source (31) non-coking gas, dew point at atmospheric pressure below 110 ° C: water vapor or preferably nitrogen or fuel gas, allows:

a) injecting a barrier gas upstream of the valve (7), to oppose the arrival of cracked gas in the receiving container (6),

b) using the valve (25) to inject such a non-coking gas with an atmospheric dew point below 110 ° C. to drain the gas contained in the receiving canister Vi (6), before evacuation and transfer particles, and pressurize the container (6) during the pneumatic transfer of the particles.

c) injecting, by means of the valve (26), such a gas into the solid particles, to achieve at least partial drying of any traces of liquid, by percolation using a dry gas,

d) injecting through the valve (27) a controlled flow of such a transport gas during pneumatic transfer of the particles.

The installation described in Figure 1 finally includes a programmable controller (17) to control the sequential operation of the installation, in particular valves of the pressure relief airlock and pneumatic shipping airlocks. She also includes injection means (20) upstream of the cracking zone (2) chemical compounds which catalyze the gasification of coke by steam, for example aqueous solutions of an equimolar mixture of sodium carbonate and potassium carbonate, or an equimolar mixture of sodium acetate, potassium acetate, lithium acetate and barium acetate.

These compounds have a surprising anti-smoking efficiency in the areas cracked (2).

We now refer to FIG. 2, where two diagrams are represented quench exchangers (4), or "TLE" whose inlet cones each have a conduit (19) for injecting erosive solid particles. These exchangers are connected downstream to two primary separators (5) connected by lines (16) comprising each a controlled isolation valve (7), with the same receiving container (6), which is one of the Vi canisters of the installation, and is therefore associated with the two primary separators (5) shown. Downstream of the receiving container, a transfer line (9), comprising a controlled valve (8) allows sequentially transfer the particles to common means (10), (13) separation and collection, themselves connected by other pipes of transfer (9), to other receiving canisters Vi, not shown. Canister (6) works in pneumatic airlock, with pressurization of the airlock and removal of particles by a transport gas.

In FIG. 2, the arrangement of the two primary separators (5) is not any, but these dividers are installed sufficiently close together so that the connecting lines (16) with the receiving container single (6) are very inclined and form with the horizontal an angle a at least equal to 60 ° C.

We now refer to FIG. 3, which represents a quench exchanger (4) connected to a primary separator (5), itself connected to a receiving canister Vi (6). This figure 3 also includes other technical elements already described previously and referenced in the same way. In addition, a other container, Wi, (28) for receiving solid particles, also connected to the primary separator (5), and a controlled directional switch (29) (flap, valve or equivalent technical device), allowing the particles to be oriented recovered in the primary separator (5), either to the canister Vi (6), or to the Wi bottle (28).

The installation described in Figure 1 works as follows:
A- Injection of erosive solid particles. Intermittent injections of erosive particles are carried out in the installation, by means (19): controlled valves and injection pipes. When a well known and constant charge is cracked, particles can be injected through the lines (34) upstream of the cracking zones (2); when variable loads are cracked under flexible conditions, the particles are mainly, or exclusively, injected into the transfer zones (3) at the inlet cones of the quench exchangers; it has in fact been found that variable conditions at the level of the charges could lead to coking rates of the pyrolysis tubes which are difficult to predict and unsuitable for controlling particle injections in the cracking zones. On the contrary, the fouling of the quench exchangers, which has unexpectedly turned out to be the only limiting factor in the choice of fillers, in particular heavy loads such as gas oils and vacuum distillates, can be known in a simple manner and reliable by simple measurement of the outlet temperature of this exchanger.

Furthermore, the coke of the quench exchangers is, surprisingly, much easier to erode than cracked areas. It is therefore possible to control the quantities of particles to be injected without realizing of preliminary tests, based on the outlet temperature of the heat exchangers quenching.

Preferably, doses of fine particles are injected discontinuously erosive, each dose corresponding to a weight of particles typically understood between 5 and 150 kg, especially between 20 and 100 kg. Two types of control injections are possible: according to the first type of control, we inject particles, at a given injection point, at a fixed time interval, for example every 3 hours. And we adjust the quantities injected (for example by weighing means, not shown in FIG. 1), so that the increase in the outlet temperature of the relevant quench exchanger located downstream of the point injection remains moderate, for example less than 100 ° C per month and preferably at 30 ° C per month or substantially zero.

According to the other type of control, doses of constant amounts of particles, but at variable time intervals, to limit or cancel increasing the outlet temperature of the quench exchanger.

The particles, typically injected through a conduit (19) comprising typically at its end from 1 to 8 particle injectors in the inlet cone of a quenching exchanger (4) are entrained by cracked gases, rebound on the diffuser impactor (35), and are distributed in an improved manner in the different exchanger tubes (4), where they circulate at speeds included between 20 and 180 m / s and preferably between 35 and 120 m / s, and pickle a part coke or heavy tars deposited on the walls of these tubes.

These particles are then separated in the cyclone (5), and fall by via the line (16) in the receiving canister Vi (6) maintained typically at 150 ° C by thermal means (37). The controlled valve (7) is therefore open during the injection of these particles, to allow their recovery in the container (6); on the other hand the controlled valve (8) is closed during this period. After injection of the particles, which are therefore stored temporarily in the corresponding container (6), injected through the lines (25) and (26) "clean" and dry gas, for example fuel gas or nitrogen from a power supply (31). This allows a first drying of the particles (which may contain traces of liquid), as well as a gas sweep contained in the Vi canister to eliminate any traces of cracked gas. We can then close the controlled valve (7) to isolate the container (6) from upstream, and pneumatically transfer the particles, by pressurizing the balloon (6) by example by the valve (25), by opening the outlet valve (8) and injecting a controlled flow of clean and dry transport gas by the valve (27). This operation of the balloon (6) in a pneumatic airlock can be, without depart from the scope of the invention produced according to several variants, known from those skilled in the art, for example by fluidizing the particles, by opening the valve 8 and injecting transport gas before pressurizing the balloon, using a outlet valve (8) on pipes, for example horizontal, inclined, vertical rising.

The particles are then removed, in dense phase or in diluted phase, by the transfer line (9). The flow qi of transport gas to achieve this transfer is according to the invention much lower than that of cracked gases passing through the primary separator (5). The line (9) is therefore of small diameter, of same as the valves (7) and (8), because we realized, thanks to the gas change conveying the particles: cracked gas → dry clean gas (N2, fuel gas), a decoupling with the cracked gas flow, necessarily very high.

Typically, the line (9) and the valves (7) and (8) are of smaller diameter or equal to 100 mm against 350 mm typically for transfer lines from particles of the process previously described. In addition, line (9) is relatively cold, generally not traced and not insulated on at least part and can be made of carbon steel.

The transfer of the particles according to the invention is therefore particularly economical, and also reliable because it allows the particles to be dried in the Vi canister reception (6) then in a circulating bed, thanks to the transport gas, in the pipeline transfer (9).

This transfer pipeline, the length of which is several meters at minimum, for example between 5 and 100 m allows to cool the particles (the heat exchanger with the cooler walls of the line (9) being favored by circulation in a fluidized bed). This is another advantage of the invention: indeed this will make it possible to be able to use downstream a conventional vibrating screen, very reliable and proven, with flexible connecting cuffs which would not have been compatible with the initial particle temperatures at the primary separator (5).

The particles passing through the pipe (9) are therefore cooled to a typical preferred temperature of 80 to 150 ° C, compatible moderate temperature with the vibrating screen, but sufficient to carry out a possible drying complementary to particles.

These particles reach the common means by the transfer pipe (9) separation and collection.

These common means comprise a cyclone (10) for particle / gas separation. transport, and a container (13) for collecting particles. The purified transport gas is returned by line (11) to line (12) for evacuating cracked gases cooled.

Sequentially, after recovering a dose of particles or more doses of particles, the collection container (13) is isolated from upstream, depressurized by means not shown, and drained via the evacuation line (32). This emptying, for example gravity, is facilitated by the fact that the particles are dry and not sticky. The particles are then screened in the vibrating screen (14), which eliminates fragments larger than 3 mm, and fall into the receiving container (15) whose upstream valve is open and the downstream valve closed.

Fine sieving of the particles is necessary when recycling these particles to avoid clogging of the injectors at the end of the line (19), which are typically of small dimensions (for example 15 mm). A first screening very coarse (15 to 20 mm mesh) can be made using a simple grid in the receiving containers (6) to avoid the risk of obstruction of transfer lines (9).

When the sieved particles are in the can (15), we can then recycle them, by isolating the container (15) from upstream, and by injecting a pressurization gas and a transport gas, according to the same type of operation as the canister (6): evacuation by pneumatic shipping airlock according to several variants of realization, as well as for the airlock (6). Preferred transportation gas is fuel gas, or nitrogen.

Controlled valves, included in the means (19) for injecting particles allow to select the injection site (s) chosen, for example those whose quench exchanger has the highest outlet temperature. The container (15) also comprises means (16) for discharging particles worn, reduced erosive efficiency after a certain number of circulations. The dose of used particles is then replaced by new particles stored in the can (18), and conveyed by a supply of transport gas (33).

The installation in FIG. 1 also makes it possible to inject chemical agents decoking by means (20) which may include a reservoir for a solution active, and a metering pump. These compounds are injected continuously or in discontinuous, finely pulverized in cracked gases.

The installation also includes a control module (17 see Figure 2) such than a programmable controller allowing to operate all actions sequential automatically.

The device described in Figure 2 works like that of Figure 1. Both valves (7) are however never opened simultaneously to avoid parasitic circulation between the two cyclones (5), via the lines (16). The particles are therefore injected into the two exchangers (4) during different phases, the corresponding valve (7) being the only one opened during a injection. The minimum angle a of at least 60 ° ensures flow gravity of the particles recovered.

The device described in FIG. 3 operates as follows:
During the particle injection phases, the directional switch (29) is oriented as shown in the figure to allow the recovery of erosive particles in the receiving canister Vi (6). Outside the injection phases, that is to say for most of the time, the switch is oriented in the opposite direction, so that the particles fall into the receiving Wi container (28). Thus, the particles of coke detached from the walls which can circulate spontaneously in the installation, or resulting from the embrittlement of the coke by the chemical compounds injected, do not mix with the erosive particles, recovered in the container (6). This improves the operation and reliability of the installation. We could also prevent the fall of unwanted particles in the container (6) by closing the valve (7) and then injecting a gas to expel the particles located above this valve.

Examples : Example 1 (comparative):

We consider a steam cracking installation comprising 10 ovens and 20 quench exchangers with a unit capacity of 10,000 kg / h of cracked gas, this installation being equipped with means for injecting erosive particles of coke into upstream of the exchangers.

According to a first variant, the particles contained in the effluents of quench exchangers are conveyed by these effluents to the general network of treatment of cracked gases which comprises a single cyclone. In this installation, there are 20 cracked gas outlet lines that carry the particles to the common cyclone.

There are therefore 20 large diameter lines (350 mm for example) with circulation of particles, each of these lines having a unit capacity of 10,000 kg / h of gas cracked. The common cyclone has a capacity of 20 x 10,000 kg / h, i.e. 200,000 kg / h. It is therefore of considerable size, very difficult to install and little effective. This variant is not in accordance with the invention.

According to a second variant, already described previously, each output of a quench exchanger includes two controlled valves to direct the effluents either to the downstream network for the treatment of cracked gases when does not inject particles, either towards common means of separation and collection.

This known installation makes it possible to transport the particles to a single point, by means of 20 additional cracked gas lines, 350 mm in diameter typical, and includes 20 x 2 i.e. 40 special large diameter valves capable direct the cracked gases towards the appropriate network.

The cyclone, on the other hand, has a reasonable capacity of 10,000 kg / h and is easily implantable and effective.

This expensive installation is not in accordance with the invention.

Example 2, in accordance with the invention:

We also consider a steam cracking installation comprising 10 ovens and 20 quench exchangers with a unit capacity of 10,000 kg / h. This installation comprises 20 primary cyclones (5), each equipped with a receiving container (6). These primary cyclones have a unit capacity of 10,000 kg / h and are therefore efficient and easy to install. Each of the 20 receiving containers (6) is connected by a transfer pipe (9) to a common cyclone (10). So there are 20 transfer pipeline. As the unit flow chosen for the transport gas is 1000 kg / h of fuel gas successively for each of the pipes (9), this flow rate is much lower than the flow rate of 10,000 kg / h of cracked gas passing through a primary separator, in accordance with the invention.

The transfer pipes (9) are therefore of very small diameter (50 to 100 mm), and the cyclone (10) is also very small (capacity 1000 kg / h).

This installation makes it possible to inject erosive particles, for example doses 50 kg of angular coke, or angular silicon carbide, and recover these particles in a common place. It allows, thanks to these injections to be able to avoid fouling of quench exchangers and cracking charges not conventional (kerosene, diesel, condensate) with cycle times more than 1 month, which is not possible without injecting particles.

Preferably, in particular for mineral particles, the most most of the particles recovered.

Example 3:

Consider the installation of Example 2, which also includes 20 separators primary (cyclone (5)), but only 10 receiving Vi tanks (6), arranged according to Figure 2, and 10 capacity transfer lines (9) 1000 kg / h of fuel gas per unit.

This installation, in accordance with the invention, is more economical than that of example 2.

Example 4:

We always consider the same steam cracking installation, in which we installs not 20 but 10 primary separators (5), each separator grouping the effluents from two quench exchangers (1 oven). We use 10 Vi receiving cans and 10 transfer pipes (9) of unit capacity 1000 kg / h of fuel gas. This installation has a recovery efficiency of particles slightly weaker than that of Examples 2 and 3, but is economically inexpensive.

Example 5:

We consider the installation of example 4, supplemented by means (20) injection of 15 to 100 ppm of chemical compounds (by weight of sodium plus potassium) compared to cracked gases, in an aqueous solution of 96% water, equimolar composition of sodium carbonate and potassium carbonate. These compounds promote the gasification of coke from cracked areas, and cause also embrittlement of this coke and emissions of fragments detached from the walls.

In order not to mix these coke fragments with the erosive particles injected (for example silicon carbide), 10 switches (29) and 10 canisters are installed Reception Wi (28) in accordance with Figure 3.

In general, the invention therefore proposes a method and an installation, with several variants, making it possible to use decoking agents effective in authorizing cracking of charges which cannot be cracked in conventional conditions without excessive fouling, and to recover particles solids generated by this implementation, more economically and more reliably than in the processes and installations previously described.

Claims (25)

1. A process for the steam cracking of hydrocarbons in a facility comprising at least one steam cracking furnace, the facility comprising a plurality of cracking zones (2) and a plurality of transfer line exchangers (TLE) (4) for the cracked gases from these cracking zones, the process comprising injection, at a plurality of points, of decoking agents resulting in the circulation of solid particles in said transfer line exchangers, the process comprising separation of at least a portion of said solid particles from the transfer line exchanger effluents in a plurality of primary gas/solid separators (5), and recovery of at least a portion of these solid particles downstream of these transfer line exchangers in common separation and collection means assembled at a single point, the process being characterized in that:

   at least a portion of the separated particles from the primary separators is recovered by gravity flow in a plurality of receiving drums V₁,..., V_n, each drum V_i being associated with at least one primary separator;

   the transfer by channels of at least the majority of the particles contained in the drums V_i to the common separation and collection means, thanks to a transport gas, whose the flow rate qi for evacuating the particles contained in drum V_i is less than or equal to 30 % by volume of the flow rate of the cracked gases passing through the primary separators associated with V_i;

   the process being characterised in that each of the receiving drums V_i is sequentially isolated from the associated primary separator(s),and the particles transfer to said common separation and collection means is carried out by a non coking transport gas with an atmospheric dew point of less than 110°C.
2. A process according to claim 1, characterized in that the particles transferred from drum V_i are extracted from that drum by exclusively pneumatic means.
3. A process according to claim 1 or claim 2, in which receiving drums V_i are heated by heating means whose temperature level is between 110°C and 340°C, preferably between 150°C and 250°C, remaining above the dew point of the transport gas at the maximum operating temperature of drums V_i.
4. A process according to any one of claims 1 to 3, in which the gas contained in drum V_i is flushed by a non coking gas with an atmospheric dew point of less than 110°C before isolating drum V_i then transferring the particles contained in V_i.
5. A process according to any one of claims 1 to 4, in which the particles contained in drum V_i are percolated by a non coking gas with an atmospheric dew point of less than 110°C before isolating drum V_i then transferring the particles contained in V_i.
6. A process according to any one of claims 1 to 5, in which the transport gas is uncondensable at normal temperatures and pressures, and is selected from the group formed by nitrogen, methane, hydrogen, light hydrocarbons containing two to four carbon atoms, and mixtures of these compositions.
7. A process according to any one of claims 1 to 6, in which at least a portion of the decoking agents is injected during normal operation of the facility.
8. A process according to any one of claims 1 to 7, in which the decoking agents comprise solid erosive particles, injected upstream of the transfer line exchangers, in particular into the transfer zones (3) comprised between the outlets to the cracking zones (2) and the transfer line exchangers (4).
9. A process according to any one of claims 1 to 8, in which at least a portion of the particles recovered in the common collection means is recycled to a point upstream of at least one transfer line exchanger, after a jigging operation carried out on at least said portion of particles recovered from the common means, the jigging operation being carried out at atmospheric pressure and in an essentially nitrogen atmosphere.
10. A process according to any one of claims 1 to 9, in which the anticoking compounds comprise mineral salts which catalyse the gasification of coke by steam, injected upstream of the cracking zones (2).
11. A process according to claim 10, characterized in that said mineral salts comprise at least one salt from an element selected from the group formed by sodium, potassium, lithium, barium and strontium, the salt being active in promoting gasification of coke.
12. A steam cracking unit , comprising at least one steam cracking furnace (1), a plurality of cracking zones (2), and a plurality of transfer line exchangers (4) for the cracked gases from the cracking zones, the facility also comprising means for injecting decoking agents, at a plurality of points, resulting in the circulation of solid particles in the transfer line exchangers, a plurality of primary gas/solid separators (5) for purifying the effluents from the transfer line exchangers, each primary separator being connected upstream to at least one transfer line exchanger associated therewith and comprising an outlet for purified gas and an outlet for solid particles, and means for recovering at least a portion of these solid particles, these recovery means comprising common separation and collection means assembled at a single point, the facility being characterized in that it comprises:

    a plurality of drums vi for recovery of at least a portion of the particles separated in the primary separators by gravity flow, each drum V_i being connected to at least one outlet for solid particles from at least one primary separator associated with V_i ;

    a plurality of solid particle transfer channels, each channel being connected upstream to one of the drums V_i and downstream to said common separation and collecting means;

    the facility being characterised in that it comprises:

    means for sequentially isolating each drum V_i from the separator(s) associated therewith, and in that means for transferring, via said transfer channels, at least the major portion of the particles contained in the isolated drums V_i, comprise means for supplying a non coking transport gas with an atmospheric dew point of less than 110°C.
13. A facility according to claim 12, characterized in that each drum V_i is connected to means for evacuating particles, said means being exclusively pneumatic and using at least one source of a transport gas selected from the group formed by nitrogen and fuel gas.
14. A facility according to claim 12 or claim 13, comprising heating means for heating drums V_i.
15. A facility according to any one of claims 12 to 14, comprising means for flushing the gas contained in drums V_i using a source of a non coking gas with an atmospheric dew point of less than 110°C.
16. A facility according to claim 14, comprising means for introducing a non coking gas with an atmospheric dew point of less than 110°C into the particles contained in drums V_i to percolate through the particles before their evacuation from drums V_i.
17. A facility according to any one of claims 12 to 16, in which the decoking agents comprise solid erosive particles, the facility comprising means for injecting said particles upstream of the transfer line exchangers, in particular into the transfer zones between the cracking zones and the transfer line exchangers.
18. A facility according to claim 17, in which the totality of the solid particles is injected into the transfer zones (3) for the cracked gases between the cracking zones (2) and the transfer line exchangers (4), in particular into the inlet cones of the transfer line exchangers.
19. A facility according to any one of claims 12 to 17, in which the common separation means (10), (13) comprise an outlet- for purified transport gas connected via a line (11) to a line (12) for circulation of cracked gases, to evacuate the purified transport gas.
20. A facility according to any one of claims 12 to 19, comprising means for recycling at least a portion of the particles recovered in the common separation and collection means to the transfer zones (3).
21. A facility according to claim 20, comprising a vibrating jig (14) operating in a nitrogen atmosphere substantially at atmospheric pressure and at a temperature of less than 200°C, connected upstream to common separation and collection means (10), (13) and connected downstream to particle recycling means (15), (19).
22. A facility according to any one of claims 12 to 21, comprising at least two primary separators (5) associated with the same receiving drum (6), each primary separator associated with said receiving drum (6) being connected to said drum via a line (16) and comprising control means (17) for means (7) for sequential obstruction of at least one of the lines (16) when the other line is open, the relative disposition of the primary separators (5) and the receiving drum (6) being such that the lines (16) are inclined at an angle of at least 60 degrees to the horizontal.
23. A facility according to any one of claims 12 to 22, comprising means for injecting decoking agents comprising chemical compounds which catalyse the gasification of coke by steam, upstream of the cracking zone (2).
24. A facility according to any one of claims 12 to 23, comprising means for sequential injection of erosive particles connected to the transfer zones (3), means for sequential isolation of each drum V_i outside the phases for injection of particles upstream of V_i, and means for evacuating particles recovered from the separator(s) (5) associated with V_i outside the injection phases without passing through V_i.
25. A facility according to claim 24, comprising drums W_i for receiving the particles recovered outside the particle injection phases, and controlled directional diverters at one inlet and two outlets, each diverter being connected upstream to a primary separator and downstream to a receiving drum V_i, and to a receiving drum W_i.

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