EP0806467B1 - Process and apparatus for the thermal conversion of hydrocarbons into more insaturated aliphatic hydrocarbons by combination of a steam cracking and a pyrolysis step - Google Patents

Description

The invention relates to a process for pyrolysis of a hydrocarbon feedstock with at least two carbon atoms and simultaneously a process for decoking the coke deposited on the walls of the reactor.

It applies in particular to the continuous production of acetylene or of compounds acetylenics such as methyl acetylene.

In high temperature thermal transformation processes of hydrocarbons having minus one carbon atom, for example pyrolysis between 900 and 1500 ° C or a steam cracking around 850 ° C at the end of the heating zone, coke is formed and deposited at the surface of the reactor walls. We then decoke the reactor which is done usually with an air / steam mixture at temperatures most often below 900 ° C, trying in the case of metal ovens to avoid any overheating or points hot damaging the good performance of the metal tubes of the oven. This decoking exothermic therefore involves shutting down the entire unit and above all disconnecting the oven from downstream heat exchangers, which reduces the total productivity of the unit. In addition, safety rules impose the dismantling of the hydrocarbon introduction lines and their replacement by air introduction lines, which requires a very long shutdown of the unit.

When reassembling the unit for the pyrolysis phase, the same drawbacks remain to which is added the need to purge the reaction zone and the lines with an inert gas.

Pyrolysis of hydrocarbons with at least two carbon atoms, making it possible to obtain olefinic or acetylenic hydrocarbon compounds has been described, in particular in Applicant's patent applications FR 2715583 (US-5,554,347), EP-A-733,609 and FR 95/15527 incorporated as references.

The technological background is illustrated by patents EP-A-542,597 and FR 1501836.

In particular, pyrolysis reactors made of ceramic material have been used in which non-watertight partitions advantageously made of ceramic material determine channels where the charge and the reaction effluents circulate. These partitions advantageously have a shape adapted to create turbulence and include, for example, cells or cavities in the level of heating means. These are usually ducts containing a heater electric or a gas burner.

However, these high-tech reactors have a high investment cost and their power supply, in particular electrical power, induces a significant operating cost. We have already described in patent application EP-A-733,609 the possibility of using an effluent from steam cracking, the temperature of which is already around 850 ° C, as feedstock to the pyrolysis since it already contains unsaturated hydrocarbons. The energy needed to conversion of the charge into acetylenic hydrocarbons would then be all the more reduced.

But an industrial steam cracker must be stopped every two or three months to decoker it. A higher temperature pyrolysis oven should be decoked more often, every four to five days for example. During the decoking stage, this oven must be isolated. There is no unfortunately no sealing valves operating between 800 and 900 ° C. Variant then consists in sending the effluent from the steam cracker, cooled after passing through a quench exchanger in the pyrolysis oven, but the benefit of using the gas is lost warm and the gain becomes low. Furthermore, the dead volume of the quench exchanger promotes side reactions at the expense of ethylene yield.

Another drawback is related to the frequency of decoking of the tubes, every two or three month. Indeed, at the end of the cycle, the inside of the tubes is covered with a thick layer of coke. Coke may come off at times and is entrained by the gas flow to speeds of the order of 200 m / s risking damage to the ceramic sheaths of the pyrolysis oven downstream of the steam cracking oven.

An object of the invention is to provide a method for pyrolyzing a charge hydrocarbon without stopping the unit while allowing decoking of this unit.

Another object is to reduce the investment and operating costs of the unit.

Another object of the invention is to maintain the temperature of the installation substantially constant during its walk, to avoid thermal stresses which would not fail to occur, especially when using an oxygen-containing gas for the stage of decoking which implements an exothermic reaction while the pyrolysis step puts in performs an endothermic reaction.

Given the presence of non-watertight bulkheads therefore inexpensive in the area of pyrolysis, we noticed that it was possible to continuously implement a process of pyrolysis of a hydrocarbon feedstock and a process for decoking the reaction zone which are not penalizing.

It has been noticed that by combining a steam cracking furnace working with a high rate of dilution of the charge by steam and at least one very high pyrolysis oven temperature, in the absence of a quenching exchanger between the steam cracking furnace and the furnace of pyrolysis, an excellent selectivity was observed in desired products, for a demand reduced overall enthalpy.

More specifically, the invention relates to a pyrolysis and decoking process in continuous in a reaction zone comprising a pyrolysis zone (40) made of material refractory, elongated in a direction (an axis) having a heating zone and a cooling zone following the heating zone, the heating zone comprising at least two rows (1, 2) substantially parallel to the axis, separated by a partition (70), advantageously not watertight, of refractory material between two rows successive, at least one of said rows (1) receiving hydrocarbons and steam of water, at least one other (2) of said rows essentially receiving water vapor, said rows comprising heating means (8) surrounded by sheaths (7) substantially parallel to each other and substantially perpendicular to the axis of the reactor, coke depositing in the reaction zone, the process being characterized in that a charge is circulated hydrocarbon containing at least one hydrocarbon with at least two carbon atoms to an adequate steam cracking temperature in a steam cracking zone containing at least at least two steam cracking tubes, at least one of said tubes communicating with a feeding the load and steam and being heated so as to crack the load, at at least one other of said tubes communicating with a supply of a fluid consisting essentially in steam and being heated so as to decoker said tube on which is deposited coke, the water vapor flow rate in the steam cracking tube being such that the ratio by weight water vapor / charge is between 0.5 and 20, a gas stream of steam cracking comprising hydrocarbons and water vapor and a gas stream of decoking essentially comprising water vapor and circulating the current of steam cracking in at least one row of the heating zone of the pyrolysis zone (40) which is adjacent to the steam cracking area so as to pyrolyze the steam cracking stream and to have an outlet temperature of said heating zone of at least 850 ° C. and we do circulate the decoking current in at least the other row of the heating zone so at least partially decoking said row and having an exit temperature from said zone heating at least 850 ° C and collecting hydrocarbons comprising at least one acetylene compound for example acetylene and a decoking effluent.

According to a characteristic of the invention, the exit temperature from the steam cracking zone is generally lower than the outlet temperature of the zone heating zone pyrolysis.

The temperature in the steam cracking tube (s) where the steam cracking takes place the charge is advantageously maintained substantially equal to the temperature in the tubes where decoking takes place. Likewise, the temperature in the row or rows where the pyrolysis of the gas stream leaving the steam cracking zone is advantageously maintained approximately equal to the temperature in the row or rows where the decoking.

According to another particularly advantageous characteristic, the outlet temperature of the heating zone relative to hydrocarbons and the exit temperature from the heating relative to the decoking effluent are about 1000 to 1400 ° C.

According to another characteristic, the amount of water vapor introduced into the zone of steam cracking, compared to that of the charge, in other words the steam weight ratio of water on charge, for a determined charge is greater than that corresponding to a conventional steam cracking of the same charge. We generally adopt the one that is the most suitable for the pyrolysis reaction following the steam cracking reaction.

Thus, for a charge consisting essentially of ethane, this ratio is greater than 0.5 while it is usually around 0.2. For a charge of naphtha, the ratio is higher at 0.7 while it is usually around 0.5. For example, also for a charge of diesel, the ratio is greater than 1, for example equal to 2 whereas it is usually close to 1.

Excellent results have been obtained in terms of selectivity with a water vapor to load between 1.5 and 6, the highest value preferably being adapted to the loads the heaviest.

The choice of these ratios combined with a high reaction temperature, both for the steam cracking zone only for the heating zone of the pyrolysis zone facilitates the decoking of tubes and rows since these are sufficiently oxidizing conditions to these temperatures to transform coke and to form carbon monoxide and hydrogen.

This is particularly advantageous in the case of the material pyrolysis reactor. ceramic with bulkheads. Water vapor and hydrogen transfer can therefore be carried out through the wall of the row where decoking is carried out towards the row where pyrolysis takes place.

It has even been observed that a transfer of hydrogen to the pyrolysis row slowed down the deposit of coke on it.

In addition, a transfer of water vapor from the row where decoking takes place to the row where the pyrolysis takes place is not penalizing since the pyrolysis reaction takes place in presence of water vapor. In the other direction, if hydrocarbons pass from the row of pyrolysis towards the row where decoking takes place, they will be in the presence of a significant amount of water and will be pyrolyzed into desired products without making coke additional.

Finally, the choice of a high water vapor to charge ratio has the advantage of reducing the coke deposit. It will not be able to grow significantly since it is planned to decoker every four or five days for example, that is to say at a frequency corresponding to that of decoking the pyrolysis reactor, instead of decoking both at three months in the case of industrial steam crackers.

This decrease in coke deposition, due to the high water vapor content and the frequency decoking, promotes heat transfer through the tubes (the deposit of coke creates a thermal barrier), so we have lower tube skin temperatures than industrial steam crackers, which leads to a gain in selectivity for ethylene compared to these last.

On the other hand, the steam cracker being connected to the pyrolysis furnace by a very short pipe, there is has practically no dead volume whereas when using the effluent from a steam cracker industrial as a pyrolysis reactor charge, having to cool the gas in a heat exchanger creates a large dead volume where secondary products are formed undesirable, by high temperature degradation of ethylene and acetylene.

It has also been found that by adopting a water vapor flow rate in the tube where the decoking 1.1 to 4 times greater than the water vapor flow rate introduced into the tube where the steam cracking, very good decoking results were obtained from these tubes. Of course, the decoking rate of heating rows adjacent to decoking tubes is also improved since this excess steam also circulates in the rows where decoking takes place.

To carry out this decoking step, the hydrocarbon supply in the tube is cut intended to be decoked and the flow of water introduced is substantially increased so as not to not cause excessive thermal disturbance in the gas preheating furnace upstream of the steam cracking zone.

The steam cracking furnace is usually heated by conventional gas burners, type radiant burners. The load is generally preheated between 300 and 400 ° C. The the temperature of the steam cracking zone is usually at most equal to 900 ° C.

The heating means of the pyrolysis reactor can be electrical resistances contained in sheaths as described in the above patents or they may be consisting of sheaths containing a gas burner as described in the patent application of the Applicant (FR 2715583).

Each row may include at least one layer of heating means surrounded by sheaths, substantially parallel to the axis of the reaction zone, these sheaths being substantially perpendicular to said axis.

The characteristics of the heating elements, either electric or comprising burners with gases, their number, the distance between them and their configuration are described in the patents cited above.

It is the same for the sheaths protecting and isolating them from the fluids which circulate in the reactor.

These same heating elements and these same ducts with the same characteristics and same configurations are found both in the pyrolysis zone and in the zone (or row) where steam decoking takes place.

Furthermore, it has been observed that in the presence of electric heating elements contained in relatively porous and inexpensive ceramic sheaths whose sealing is not not perfect, a cladding gas containing hydrogen and / or water vapor and / or carbon monoxide and / or an inert gas could be used and could also diffuse inside to outside of the sheaths without disturbing the pyrolysis reaction and without disturbing the decoking reaction.

According to a first variant, the hydrocarbons collected and the decoking effluent can be mixed before being introduced into the cooling zone.

According to a second variant, the hydrocarbons collected and the decoking effluent are cooled separately in their respective rows, located at the level of the cooling, then possibly mixed.

The cooling zone is usually a zone of direct quenching by a cooling, known to those skilled in the art, advantageously followed by a heat exchanger indirect contact generating steam (TLE: transfer line exchanger).

The installation has the advantage of being safe, reliable and easy to implement. She uses in the pyrolysis zone, refractory materials and more particularly materials ceramics known to those skilled in the art such as cordierite, mullite, silicon nitride or silicon carbide.

• les hydrocarbures aliphatiques saturés tels que l'éthane et des mélanges d'alcanes (LPG), des coupes pétrolières telles que les naphtas, les gazoles atmosphériques et les gazoles sous vide, ces derniers pouvant présenter un point final de distillation de l'ordre de 570 °C.
• les hydrocarbures insaturés tels que l'éthylène, le propylène et le butadiène, des mélanges d'alcanes et d'alcènes tels que éthane + éthylène, les coupes C3, C4 et C5 de craquage catalytique.

The hydrocarbon fillers which can be used are, by way of non-limiting example:

• saturated aliphatic hydrocarbons such as ethane and mixtures of alkanes (LPG), petroleum fractions such as naphthas, atmospheric gas oils and vacuum gas oils, the latter possibly having an end point of distillation of the order of 570 ° C.
• unsaturated hydrocarbons such as ethylene, propylene and butadiene, mixtures of alkanes and alkenes such as ethane + ethylene, cuts C3, C4 and C5 of catalytic cracking.

The invention also relates to a continuous pyrolysis and decoking unit for setting up work in particular of the process according to the invention, comprising a reactor (40) for pyrolysis of elongated shape in a direction (an axis) comprising at least two rows (1, 2) substantially parallel to the axis separated by a partition, (70) advantageously not sealed, of refractory material between two successive rows, each row comprising a plurality of heating means (8) arranged in at least one layer of heating elements surrounded by sheaths (7) of ceramic material substantially parallel to each other and substantially perpendicular to the axis of the reactor, at least one of the rows (1) being adapted to receive hydrocarbons and water vapor, at least one other (2) of said rows being adapted to receive water vapor, said pyrolysis reactor comprising heating control and modulation means connected to the heating means, the pyrolysis reactor further comprising means (47) for cooling the effluents products in each row, said unit being characterized in that it comprises a steam cracking reactor (30) comprising at least two steam cracking tubes (31, 32), each of the tubes (31, 32) being connected at one end to a supply line at a load (11, 12) comprising a control valve (V1, V2) and a supply line (22, 21) in steam comprising a regulating valve (V12, V11), the other end of the tube (31) being connected to the row (1) receiving the hydrocarbons and the water vapor and the other end of the tube (32) being connected to the row (2) receiving the steam, the reactor steam cracking further comprising means for alternately actuating said valves so that one tube (31) is in the steam cracking phase and the other tube (32) is in decoking phase, and heating regulation means so that the temperature of the steam cracking reactor is lower than that of the pyrolysis reactor.

The invention will be better understood by the description of an embodiment, given as purely illustrative but in no way limiting, which will be made of it below with the aid of the figure attached, which represents in cross section a steam cracker followed by a section longitudinal, along a plane parallel to the axis of the reactor (top view) of a reactor pyrolysis.

Hydrocarbon supply lines 11, 12, 13, 14, 15, 16 controlled respectively by valves V1, V2, V3, V4, V5 and V6 introduce into a steam cracker 30 then into a reactor 40 for pyrolysis and decoking of hydrocarbons, for example ethane, from a line 10 mixed with water generally in vapor form provided by a line 60. This line distributes it in lines 17, 18, 19, 20, 21 and 22 controlled respectively by valves V7, V8, V9, V10, V11 and V12.

These various valves V1 to V12 are adapted to allow the circulation of a mixture of hydrocarbons and water vapor in a certain number of tubes of the steam cracker 30 and of adjacent rows of the so-called pyrolysis reactor 40 and only of water vapor in other steam cracker 30 tubes and other adjacent rows of the so-called decoking reactor 40 for removing the coke which has been deposited during the steam cracking and pyrolysis reaction respectively.
Steam cracker tubes 31, 32, 33, 34, 35, 36 transporting the mixture of hydrocarbons and water or transporting water alone, connected respectively to lines 11 and 22, 12 and 21, 13 and 20, 14 and 19, 15 and 18 and finally, 16 and 17, are heated in the steam cracker 30 to a temperature of 850 to 900 ° C so as to crack part of the hydrocarbon charge and are connected respectively to rows 1, 2, 3, 4 , 5 and 6 of the pyrolysis reactor 40.

For example, the valve V1 closing the line 11, it follows that the tube 31 receives only the water vapor supplied by line 22 controlled by valve V12. On the other hand, the tubes 32, 33, 34, 35 and 36 receive the hydrocarbon and water mixture, all the other valves mentioned being open.

All the tubes are preheated to around 400 ° C, essentially by convective heating in the first part of the heating oven, then at around 900 ° C in the second part of oven, essentially by radiative heating, by means of a plurality of burners.

The steam cracking effluent is introduced into the pyrolysis reactor 40 by lines of very short junction, not performing the function of quenching.

The pyrolysis reactor 40 adjacent to the steam cracking reactor 30 is divided into rows longitudinal (1, 2, 3, 4, 5 and 6) substantially parallel to its axis. These rows are separate from each other by partitions, 70, not leaktight in ceramic material, of shape comprising cells adapted to promote turbulence inside the row and therefore to favor the reaction. These rows contain sheaths of ceramic material 7 forming a sheet substantially parallel to the axis of the reactor. These sheaths are substantially parallel between them and substantially perpendicular to the axis of the reactor. They contain, for example, a plurality of electrical resistors 8 bathed in a sheath gas, chosen in the group formed by water vapor, hydrogen, carbon monoxide, an inert gas and a mixture of two or more of these gases.

The tube 31 containing water vapor is connected by a shortest heated line possible with row 1 of reactor 40. Generally, the water vapor flow rate is increased inserted in the tube and in the row where decoking takes place, for example 2 to 3 times that used in the other tubes 32, 33, 34, 35 and the other rows 2, 3, 4, 5 and 6 where the pyrolysis. The outlet temperature of the pyrolysis reactor 40 is heated to around 1200 ° C.

The terminal part of the various rows of reactor 40, intended for pyrolysis or for decoking, receives the pyrolysis or decoking effluents and each row is connected to a direct quenching line 47, comprising a controlled flow injector, for example of ethane if the charge is ethane, which allows these effluents to be cooled. Once cooled to around 800 ° C for example, lines 41, 42, 43, 44, 45 and 46 respectively connected to rows 1, 2, 3, 4, 5 and 6 mix the various effluents which are discharged through a line 50.

According to another mode not illustrated, the effluents can be cooled by circulating through watertight conduits arranged in the end part of the rows by indirect quenching, then mixed as described above.

According to another mode, not illustrated, the pyrolysis and decoking effluents from rows 1, 2, 3, 4, 5 and 6 are collected by lines 41, 42, 43, 44, 45 and 46, then mixed and sent in a direct or indirect quenching zone and once cooled evacuated by line 50.

The heating elements 8 of the pyrolysis reactor are supplied with electrical energy from independently thanks to a pair of electrodes not shown in the figure, probes thermocouple pyrometry not shown are housed in the spaces where the charge and allow to automatically regulate the temperature of each section of heating, by a conventional regulation and modulation device not shown on the figure, depending on the temperature profile chosen which applies equally well to the reaction of pyrolysis than that of decoking the walls of the sheaths.

A temperature regulation device, which may be the same, also makes it possible to control the temperature of the steam cracking reactor burners so that this temperature is lower than the outlet temperature of the oil collected and the final decoking effluent from the pyrolysis reactor.

Example :

A steam cracker-pyrolysis reactor assembly described according to FIG. 1 is used to crack a mixture of ethane and water vapor to produce a mixture of ethylene and acetylene. The weight ratio of water vapor to ethane is 1.8.

The mixture (ethane-water) and the decoking vapor are brought to 900 ° C. in the reactor 30 steam cracking and heated in a substantially linear manner up to 1200 ° C in the pyrolysis reactor at an absolute pressure of 1.3 bar.

The steam cracker has six heating tubes.

The pyrolysis reactor has six heating rows substantially parallel to its axis and separated by partitions in the form of cells and ceramic material such as carbide silicon for example. Each row includes a tablecloth parallel to the axis of elements electric heaters. The ducts perpendicular to the axis of the reactor, surrounding the electrical resistors are made of silicon carbide and contain a sheath gas which is nitrogen.

Five steam cracker heating tubes (nos. 31, 33, 34, 35 and 36) as well as five rows of pyrolysis reactor (n ° 1, 3, 4, 5 and 6) work in pyrolysis while a single tube heating (n ° 32) and only one row (n ° 2) is working on decoking.

258 kg / h of ethane and 464 kg / h of steam are introduced into each steam cracking tube of water, while in the tube operating in decoking, 979 kg / h of steam are introduced by the valve V11, the valve V2 of hydrocarbons being closed.

Steam cracking effluent containing hydrocarbons, hydrogen and water vapor is introduced directly into the appropriate rows of the pyrolysis reactor. The effluent from tube decoking is introduced directly into the row of the pyrolysis reactor subjected to decoking. At the outlet of the pyrolysis reactor, the pyrolysis effluent is cooled to 800 ° C. by direct contact with 91 kg / h of ethane at 16 ° C while the decoking effluent is cooled to 800 ° C by direct contact with 85 kg / h of ethane at 16 ° C.

After 72 hours of pyrolysis in row n ° 1, it is decided to decoke it. For this, the ethane flow is cut by the valve V1 and to avoid a disturbance of the thermal regime of the steam cracker and the pyrolysis oven, the steam flow is increased of water (valve V12) until 979 kg / h is obtained. Simultaneously, the tube 32 is fed again and row n ° 2 with 258 kg / h of ethane and 464 kg / h of water vapor, by opening the valve V2 and valve V11.

The end of decoking is checked by disappearance of carbon monoxide, which is analyzed online by infrared, for example at the outlet of the pyrolysis oven.
It is found that decoking is almost complete after 14 hours in each tube and row where it is carried out and we immediately go back into a steam cracking reaction situation for the tube which has been decoked and pyrolysis for the row which has been decoked.

We therefore have five steam cracking tubes connected to five rows in pyrolysis and one tube in the steam cracking reactor connected to a row in the decoking pyrolysis reactor. 536 kg / h of ethylene are produced constantly and without prolonged shutdown of the unit. 450 kg / h of acetylene. The effluents from the six rows of reactor 40 are mixed and sent by line 50 to the treatments and separations of the products.

Obviously, taking into account the decoking time adapted to the chosen load, we would have could have a reactor with ten rows of pyrolysis and two rows of decoking either adjacent or separated and connected to a steam cracking furnace comprising twelve tubes at total of which two would be simultaneously decoked.

Comparative example :

An industrial steam cracker effluent is used as the pyrolysis hydrocarbon charge. ethane, having operated at a temperature of 900 ° C, this effluent being cooled by quenching indirect at 450 ° C. This charge, introduced by line 10, is divided between five lines (n ° 11, 13, 14, 15 and 16) corresponding as for the example above to the five working rows in pyrolysis (n ° 1, 3, 4, 5 and 6).

In each row of the pyrolysis zone, 258 kg / h of hydrocarbons are introduced and of hydrogen and 86 kg / h of water from the conventional steam cracker and by each line 17, 18, 19, 20 or 22, 378 kg / h of water.

In row 2 of the pyrolysis zone operating in decoking, the valve V2 of hydrocarbons being closed, 979 kg / h of water vapor is sent through valve V11 and line 21.

Of course, in this version the reactor 30 no longer exists and lines 11 to 16 are connected directly to rows 1 to 6 respectively.

The same cycles are used as for the previous example. 510 kg / h of ethylene are produced and 440 kg / h of acetylene.

Claims (13)

1. A continuous pyrolysis and decoking process carried out in a reaction zone comprising a pyrolysis zone (40) which is of refractory material, which is elongate in one direction (one axis), and which comprises a heating zone and a cooling zone following the heating zone, the heating zone comprising at least two rows (1, 2) which are substantially parallel to the axis separated by a wall (70) of refractory material and located between two successive rows, at least one of said rows (1) receiving hydrocarbons and steam, at least one other (2) of said rows receiving essentially steam, said rows comprising heating means (8) surrounded by sleeves (7) which are substantially parallel to each other and substantially perpendicular to the reactor axis, wherein coke is deposited in the reaction zone, the process being characterized in that a hydrocarbon feed comprising at least one hydrocarbon containing at least two carbon atoms is circulated at a temperature which is sufficient for steam cracking in a steam cracking zone containing at least two steam cracking tubes, at least one of said tubes communicating with a supply for feed and steam and being heated so as to crack the feed, at least one other of said tubes communicating with a supply for a fluid consisting essentially of steam and being heated so as to decoke said tube on which coke has been deposited, the flow rate of steam in the steam cracking tube being such that the steam/feed weight ratio is in the range 0 5 to 20, a steam cracking gas stream comprising hydrocarbons and steam and a decoking gas stream comprising essentially steam are obtained and the steam cracking stream is circulated in at least one row of the heating zone in the pyrolysis zone (40) which is adjacent the steam cracking zone so as to pyrolyse the steam cracking stream and to produce a temperature of at least 850°C at the outlet from said heating zone, and the decoking stream is circulated in at least the other row of the heating zone so as to decoke said row at least in part, and to produce a temperature of at least 850°C at the outlet from said heating zone, and in which hydrocarbons comprising at least one acetylenic compound are recovered along with a decoking effluent.
2. A process according to claim 1, in which the temperature at the outlet from the steam cracking zone is lower than the temperature at the outlet from the heating zone in the pyrolysis zone.
3. A process according to claim 1 or claim 2, in which the steam-to-feed ratio for a given feed is higher than the steam-to-feed ratio for conventional steam cracking of said feed
4. A process according to any one of claims 1 to 3, in which the steam-to-feed ratio is more than 0 5 for a feed which consists essentially of ethane.
5. A process according to any one of claims I to 3, in which the steam-to-feed ratio is more than 0.7 for a feed consisting essentially of naphtha
6. A process according to any one of claims 1 to 3, in which the steam-to-feed ratio is more than 1 for a feed consisting essentially of a gas oil
7. A process according to any one of claims 1 to 6, in which the steam flow rate in the tube in which decoking is carried out is 1.1 to 4 times higher than the steam flow rate introduced into the tube in which steam cracking is carried out.
8. A process according to any one of claims 1 to 7, in which the recovered hydrocarbons and the decoking effluent are mixed before being introduced into the cooling zone.
9. A process according to any one of claims 1 to 7, in which the recovered hydrocarbons and the decoking effluent are cooled separately in their respective rows in the cooling zone then are optionally mixed.
10. A process according to any one of claims 1 to 9, in which the recovered hydrocarbons and the decoking effluent are directly cooled.
11. A process according to any one of claims 1 to 10, in which the temperature at the outlet from the heating zone for the hydrocarbons and the temperature at the outlet from the heating zone for the decoking effluent are about 1000°C to 1400°C.
12. A process according to any one of claims 1 to 11, in which the temperature in the steam cracking zone is at most 900°C.
13. A pyrolysis and decoking unit for carrying out the process according to any one of claims 1 to 12, comprising a pyrolysis reactor (40) which is elongate in one direction (one axis) comprising at least two rows (1, 2) which are substantially parallel to the axis separated by a wall (70) of refractory material located between two successive rows, each row comprising a plurality of heating means (8) disposed in at least one layer of heating elements surrounded by sleeves (7) of ceramic material which are substantially parallel to each other and substantially perpendicular to the reactor axis, at least one of the rows (1) being adapted to receive hydrocarbons and steam, at least one other (2) of said rows being adapted to receive steam, said pyrolysis reactor comprising means for heat control and modulation connected to the heating means, the pyrolysis reactor further comprising cooling means (47) for the effluents produced in each row, said unit being characterized in that it comprises a steam cracking reactor (30) comprising at least two steam cracking tubes (31, 32), each of said tubes (31, 32) being connected to an extremity of a supply line for a feed (11, 12) comprising a regulating valve (V1, V2) and a supply line (22, 21) for steam comprising a regulating valve (V12, V11), the other extremity of tube (31) being connected to the row (1) receiving hydrocarbons and steam and the other extremity of tube (32) being connected to the row (2) receiving steam, the steam cracking reactor further comprising means for alternately activating said valves such that one tube (31) is in the steam cracking phase and the other tube (32) is in the decoking phase, and heat regulation means such that the temperature of the steam cracking reactor is lower than that of the pyrolysis reactor.

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